arXiv daily: Plasma Physics

arXiv daily: Plasma Physics (physics.plasm-ph)

1.Magnetic flutter effect on validated edge turbulence simulations

Authors:Kaiyu Zhang, Wladimir Zholobenko, Andreas Stegmeir, Konrad Eder, Frank Jenko

Abstract: Small magnetic fluctuations ($B_1/B_0 \sim 10^{-4}$) are intrinsically present in a magnetic confinement plasma due to turbulent currents. While the perpendicular transport of particles and heat is typically dominated by fluctuations of the electric field, the parallel stream of plasma is affected by fluttering magnetic field lines. In particular through electrons, this indirectly impacts the turbulence dynamics. Even in low beta conditions, we find that $E\times B$ turbulent transport can be reduced by more than a factor 2 when magnetic flutter is included in our validated edge turbulence simulations of L-mode ASDEX Upgrade. The primary reason for this is the stabilization of drift-Alfv\'en-waves, which reduces the phase shifts of density and temperature fluctuations with respect to potential fluctuations. This stabilization can be qualitatively explained by linear analytical theory, and appreciably reinforced by the flutter nonlinearity. As a secondary effect, the steeper temperature gradients and thus higher $\eta_i$ increase the impact of the ion-temperature-gradient mode on overall turbulent transport. With increasing beta, the stabilizing effect on $E\times B$ turbulence increases, balancing the destabilization by induction, until direct electromagnetic perpendicular transport is triggered. We conclude that including flutter is crucial for predictive edge turbulence simulations.

2.Local proton heating at magnetic discontinuities in Alfvenic and non-Alfvenic solar wind

Authors:C. A. Gonzalez, J. L. Verniero, R. Bandyopadhyay, A. Tenerani

Abstract: We investigate the local proton energization at magnetic discontinuities/intermittent structures and the corresponding kinetic signatures in velocity phase space in Alfv\'enic and non-Alfv\'enic wind streams observed by Parker Solar Probe. By means of the Partial Variance of Increments method, we find that the hottest proton populations are localized around compressible, kinetic-scale magnetic structures in both types of wind. Furthermore, the Alfv\'enic wind shows preferential enhancements of $T_\parallel$ as smaller scale structures are considered, whereas the non-Alfvenic wind shows preferential $T_\bot$ enhancements. Although proton beams are present in both types of wind, the proton velocity distribution function displays distinct features. Hot beams, i.e., beams with beam-to-core perpendicular temperature up to three times larger than the total distribution anisotropy, are found in the non-Alfv\'enic wind, whereas colder beams in the Alfv\'enic wind. Our data analysis is complemented by 2.5D hybrid simulations in different geometrical setups, which support the idea that proton beams in Alfv\'enic and non-Alfv\'enic wind have different kinetic properties and origins. The development of a perpendicular nonlinear cascade, favored in balanced turbulence, allows a preferential relative enhancement of the perpendicular plasma temperature and the formation of hot beams. Cold field-aligned beams are instead favored by Alfv\'en wave steepening. Non-Maxwellian distribution functions are found near discontinuities and intermittent structures, pointing to the fact that the nonlinear formation of small-scale structures is intrinsically related to the development of highly non-thermal features in collisionless plasmas.

1.Loading loss-cone distributions in particle simulations

Authors:Seiji Zenitani, Shin'ya Nakano

Abstract: Numerical procedures to generate random variates that follow loss-cone velocity distributions in particle simulations are presented. We propose a simple summation algorithm for the Ashour-Abdalla--Kennel-type loss-cone distribution, also known as the subtracted Maxwellian. For the Dory-type loss-cone distribution, we use a random variate for the gamma distribution. Extending earlier algorithms for the kappa and Dory-type distributions, we construct a novel algorithm to generate a popular form of a kappa loss-cone distribution. To better express the loss cone, we discuss another family of loss-cone distributions based on the pitch angle. In addition to the acceptance-rejection method, we propose two transformation algorithms that convert an isotropic distribution into a loss-cone distribution. This allows us to generate loss-cone and kappa loss-cone distributions from the Maxwell and kappa distributions.

2.A linear model of synergetic current drive by low-hybird wave and electron cyclotron wave

Authors:J. N. Chen, S. Y. Chen, M. L. Mou, C. J. Tang

Abstract: A linear model of synergetic current drive(CD) by low-hybrid wave(LHW) and electron cyclotron wave(ECW) is proposed. The SCD efficiency compared to ECCD's is given in parameters space and the existence of synergy effect is also proven quantitatively in smaller $y=2\omega_{c}/\omega$ with fixed ECW parallel refractive index and polodial angle at a given flux surface. The results of kinetic simulation by using a two dimension Fokker-Planck code are consistent with the linear model in trends. The criteria for the occurrence and the sufficiently significance of synergy effect based on the linear properties are presented.

1.On the Effects of Tokamak Plasma Edge Symmetries on Turbulence Relaxation

Authors:Nakia Carlevaro, Giovanni Montani, Fabio Moretti

Abstract: The plasma edge of a tokamak configuration is characterized by turbulent dynamics leading to enhanced transport. We construct a simplified 3D Hasegawa--Wakatani model reducing to a single partial differential equation for the turbulent electric potential dynamics. Simulations demonstrate how the 3D turbulence relaxes on a 2D axisymmetric profile, corresponding to the so-called interchange turbulence. The spectral features of this regime are found to be strongly dependent on the initialization pattern. We outline that the emergence of axisymmetric turbulence is also achieved when the corresponding mode amplitude is not initialized. Then, we introduce the symmetries of the magnetic X-point of a tokamak configuration. We linearize the governing equation by treating the poloidal field as a small correction. We show that it is not always possible to solve the electric potential dynamics following a perturbative approach. This finding, which is due to resonance between the modes of the background and the poloidal perturbation, confirms that the X-point symmetries can alter the properties of turbulent transport in the edge region.

2.Space-time structured plasma waves

Authors:J. P. Palastro, K. G. Miller, R. K. Follett, D. Ramsey, K. Weichman, A. V. Arefiev, D. H. Froula

Abstract: Electrostatic waves play a critical role in nearly every branch of plasma physics from fusion to advanced accelerators, to astro, solar, and ionospheric physics. The properties of planar electrostatic waves are fully determined by the plasma conditions, such as density, temperature, ionization state, or details of the distribution functions. Here we demonstrate that electrostatic wavepackets structured with space-time correlations can have properties that are independent of the plasma conditions. For instance, an appropriately structured electrostatic wavepacket can travel at any group velocity, even backward with respect to its phase fronts, while maintaining a localized energy density. These linear, propagation-invariant wavepackets can be constructed with or without orbital angular momentum by superposing natural modes of the plasma and can be ponderomotively excited by space-time structured laser pulses like the flying focus.

3.Spin-polarized ${}^3$He shock waves from a solid-gas composite target at high laser intensities

Authors:Lars Reichwein, Xiaofei Shen, Alexander Pukhov, Markus Büscher

Abstract: We investigate Collisionless Shock Acceleration of spin-polarized ${}^3$He for laser pulses with normalized vector potentials in the range $a_0 = 100-200$. The setup utilized in the 2D-PIC simulations consists of a solid Carbon foil that is placed in front of the main Helium target. The foil is heated by the laser pulse and shields the Helium from the highly oscillating fields. In turn, a shock wave with more homogeneous fields is induced, leading to highly polarized ion beams. We observe that the inclusion of radiation reaction into our simulations leads to a higher beam charge without affecting the polarization degree to a significant extent.

1.Feature-based Transferable Disruption Prediction for future tokamaks using domain adaptation

Authors:Chengshuo Shen, Wei Zheng, Bihao Guo, Dalong Chen, Xinkun Ai, Fengming Xue, Yu Zhong, Nengchao Wang, Biao Shen, Binjia Xiao, Yonghua Ding, Zhongyong Chen, Yuan Pan, J-TEXT team

Abstract: The high acquisition cost and the significant demand for disruptive discharges for data-driven disruption prediction models in future tokamaks pose an inherent contradiction in disruption prediction research. In this paper, we demonstrated a novel approach to predict disruption in a future tokamak only using a few discharges based on a domain adaptation algorithm called CORAL. It is the first attempt at applying domain adaptation in the disruption prediction task. In this paper, this disruption prediction approach aligns a few data from the future tokamak (target domain) and a large amount of data from the existing tokamak (source domain) to train a machine learning model in the existing tokamak. To simulate the existing and future tokamak case, we selected J-TEXT as the existing tokamak and EAST as the future tokamak. To simulate the lack of disruptive data in future tokamak, we only selected 100 non-disruptive discharges and 10 disruptive discharges from EAST as the target domain training data. We have improved CORAL to make it more suitable for the disruption prediction task, called supervised CORAL. Compared to the model trained by mixing data from the two tokamaks, the supervised CORAL model can enhance the disruption prediction performance for future tokamaks (AUC value from 0.764 to 0.890). Through interpretable analysis, we discovered that using the supervised CORAL enables the transformation of data distribution to be more similar to future tokamak. An assessment method for evaluating whether a model has learned a trend of similar features is designed based on SHAP analysis. It demonstrates that the supervised CORAL model exhibits more similarities to the model trained on large data sizes of EAST. FTDP provides a light, interpretable, and few-data-required way by aligning features to predict disruption using small data sizes from the future tokamak.

2.A Two-dimensional Numerical Study of Ion-Acoustic Turbulence

Authors:Zhuo Liu, Ryan White, Lucio M. Milanese, Nuno F. Loureiro

Abstract: We investigate the linear and nonlinear evolution of the ion-acoustic instability in a collisionless plasma via two-dimensional (2D2V) Vlasov-Poisson numerical simulations. We initialize the system in a stable state and gradually drive it towards instability with an imposed, weak external electric field, thus avoiding super-critical initial conditions that are physically unrealizable. The nonlinear evolution of ion-acoustic turbulence (IAT) is characterized in detail, including the particles' distribution functions, particle heating, (two-dimensional) wave spectrum, and the resulting anomalous resistivity. An important result is that no steady saturated nonlinear state is ever reached in our simulations: strong ion heating suppresses the instability, which implies that the anomalous resistivity associated with IAT is transient and short-lived. Electron-acoustic waves (EAWs) are triggered during the late nonlinear evolution of the system, caused by strong modifications to the particle distribution induced by IAT.

3.Extra invariant and plasma inhomogeneity to improve zonal flow

Authors:Alexander Balk

Abstract: Zonal flows are known to diminish turbulent transport in magnetic fusion. Interstingly, there is an adiabatic invariant that implies the emergence of zonal flow. The paper shows that if this invariant is decreasing (due to some external factors) then the emerging zonal flow is better. It is also shown that the plasma inhomogeneity can lead to the decrease of the adiabatic invariant. A simple condition for such decrease is found.

1.Radiation-dominated injection of positrons generated by the nonlinear Breit-Wheeler process into a plasma channel

Authors:Dominika Maslarova, Bertrand Martinez, Marija Vranic

Abstract: Plasma acceleration is considered a prospective technology for building a compact multi-TeV electron-positron collider in the future. The challenge of this endeavor is greater for positrons than for the electrons because usually the self-generated fields from laser-plasma interaction are not well-suited for positron focusing and on-axis guiding. In addition, an external positron source is required, while electrons are naturally available in the plasma. Here, we study electron-positron pair generation by an orthogonal collision of a multi-PW laser pulse and a GeV electron beam by the nonlinear Breit-Wheeler process. We studied conditions favorable for positron deflection in the direction of the laser pulse propagation, which favors injection into the plasma for further acceleration. We demonstrate using the OSIRIS particle-in-cell framework that the radiation reaction triggered by ultra-high laser intensity plays a crucial role in the positron injection. It provides a suppression of the initial transverse momentum gained by the positrons from the Breit-Wheeler process. For the parameters used in this work, the intensity of at least 2.2x1023 W/cm2 is needed in order to inject more than 1% of positrons created. Above this threshold, the percentage of injected positrons rapidly increases with intensity. Moreover, subsequent direct laser acceleration of positrons in a plasma channel, using the same laser pulse that created them, can ensure a boost of the final positron energy by a factor of two. The positron focusing and guiding on the axis is provided by significant electron beam loading that changes the internal structure of the channel fields.

2.Active learning-driven uncertainty reduction for in-flight particle characteristics of atmospheric plasma spraying of silicon

Authors:Halar Memon, Eskil Gjerde, Alex Lynam, Amiya Chowdhury, Geert De Maere, Grazziela Figueredo, Tanvir Hussain

Abstract: In this study, the first-of-its-kind use of active learning (AL) framework in thermal spray is adapted to improve the prediction accuracy of the in-flight particle characteristics and uses Gaussian Process (GP) ML model as a surrogate that generalises a global solution without necessarily involving physical mechanisms. The AL framework via the Bayesian Optimisation was utilised to: (a) reduce the maximum uncertainty in the given database and (b) reduce local uncertainty around a contrived test point. The initial dataset consists of 26 atmospheric plasma spray (APS) parameters of silicon, aimed at ceramic matrix composites (CMCs) for the next generation of aerospace applications. The maximum uncertainty in the initial dataset was reduced by AL-driven identification of search spaces and conducting six guided spray trails in the identified search spaces. On average, a 52.9% improvement (error reduction) of RMSE and an R2 increase of 8.5% were reported on the predicted in-flight particle velocities and temperatures after the AL-driven optimisation. Furthermore, the Bayesian Optimisation around a contrived test point to predict the best possible characteristics resulted in a three-fold increase in prediction accuracy as compared to the non-optimised prediction. These AL-guided experimental validations not only increase the informativeness of the limited dataset but is adaptable for other thermal spraying methods without necessarily involving physical mechanisms and underlying mechanisms. The use of AL-driven optimisation may drive the thermal spraying towards resource-efficiency and may serve as the first step towards fully digital thermal spraying environments.

1.Average-atom Ziman resistivity calculations in expanded metallic plasmas: effect of mean ionization definition

Authors:Nadine Wetta, Jean-Christophe Pain

Abstract: We present calculations of electrical resistivity for expanded boron, aluminum, titanium and copper plasmas using the Ziman formulation in the framework of the average-atom model. Our results are compared to experimental data, as well as with other theoretical calculations, relying on the Ziman and Kubo-Greenwood formulations, and based on average-atom models or quantum-molecular-dynamics simulations. The impact of the definition of ionization, paying a particular attention to the consistency between the latter and the perfect free electron gas assumption made in the formalism, is discussed. We propose a definition of the mean ionization generalizing to expanded plasmas the idea initially put forward for dense plasmas, consisting in dropping the contribution of quasi-bound states from the ionization due to continuum ones. It is shown that our recommendation for the calculation of the quasi-bound density of states provides the best agreement with measurements.

2.3D simulations of positive streamers in air in a strong external magnetic field

Authors:Zhen Wang, Anbang Sun, Saša Dujko, Ute Ebert, Jannis Teunissen

Abstract: We study how external magnetic fields from 0 to 40 T influence positive streamers in atmospheric air, using 3D PIC-MCC (particle-in-cell, Monte Carlo collision) simulations. When a magnetic field B is applied perpendicular to the background field E, the streamers tend to branch quite symmetrically in the plane spanned by E and B. With a stronger magnetic field the branching angle increases, and for the 40 T case the branches grow almost parallel to the magnetic field. We also observe a deviation in the -ExB direction, which is related to the ExB drift of electrons. Both with a perpendicular and with a parallel magnetic field, the streamer radius decreases with the magnetic field strength.

1.Optical Probing of Ultrafast Laser-Induced Solid-to-Overdense-Plasma Transitions

Authors:Yasmina Azamoum, Georg Alexander Becker, Sebastian Keppler, Guillaume Duchateau, Stefan Skupin, Mickael Grech, Fabrice Catoire, Sebastian Hell, Issa Tamer, Marco Hornung, Marco Hellwing, Alexander Kessler, Franck Schorcht, Malte Christoph Kaluza

Abstract: Understanding the target dynamics during its interaction with a relativistic ultrashort laser pulse is a challenging fundamental multi-physics problem involving at least atomic and solid-state physics, plasma physics, and laser physics. Already, the properties of the so-called pre-plasma formed as the laser pulse's rising edge ionizes the target are complicated to access in experiments and modeling, and many aspects of this laser-induced transition from solid to overdense plasma over picosecond time scales are still open questions. At the same time, applications like laser-driven ion acceleration require precise knowledge and control of the pre-plasma because the efficiency of the acceleration process itself crucially depends on the target properties at the arrival of the relativistic intensity peak of the pulse. By capturing the dynamics of the initial stage of the interaction, we report on a detailed visualization of the pre-plasma formation and evolution. Nanometer-thin diamond-like carbon foils are shown to transition from solid to plasma during the laser rising edge with intensities < 10^16 W/cm^2. Single-shot near-infrared probe transmission measurements evidence sub-picosecond dynamics of an expanding plasma with densities above 10^23 cm^-3 (about 100 times the critical plasma density). The complementarity of a solid-state interaction model and a kinetic plasma description provides deep insight into the interplay of ionization, collisions, and expansion.

2.High-energy acceleration phenomena in extreme radiation-plasma interactions

Authors:J. C. Faure, D. Tordeux, L. Gremillet, M. Lemoine

Abstract: We simulate, using a particle-in-cell code, the chain of acceleration processes at work during the Compton-based interaction of a dilute electron-ion plasma with an extreme-intensity, incoherent gamma-ray flux with a photon density several orders of magnitude above the particle density. The plasma electrons are initially accelerated in the radiative flux direction through Compton scattering. In turn, the charge-separation field from the induced current drives forward the plasma ions to near-relativistic speed and accelerates backwards the non-scattered electrons to energies easily exceeding those of the driving photons. The dynamics of those energized electrons is determined by the interplay of electrostatic acceleration, bulk plasma motion, inverse Compton scattering and deflections off the mobile magnetic fluctuations generated by a Weibel-type instability. The latter Fermi-like effect notably gives rise to a forward-directed suprathermal electron tail. We provide simple analytical descriptions for most of those phenomena and examine numerically their sensitivity to the parameters of the problem.

3.Electron surface scattering kernel for a plasma facing a semiconductor

Authors:F. X. Bronold, F. Willert

Abstract: Employing the invariant embedding principle for the electron backscattering function, we present a strategy for constructing an electron surface scattering kernel to be used in the boundary condition for the electron Boltzmann equation of a plasma facing a semiconducting solid. It takes the microphysics responsible for electron emission and backscattering from the interface into account. To illustrate the approach, we consider silicon and germanium, describing the interface potential by an image-step and impact ionization across the energy gap as well as scattering on phonons and ion cores by a randium-jellium model. The emission yields deduced from the kernel agree sufficiently well with measured data, despite the simplicity of the model, to support its use in the boundary condition of the plasma's electron Boltzmann equation.

4.Influence of the magnetic field's curvature on the radial-azimuthal dynamics of a Hall thruster plasma discharge with different propellants

Authors:Maryam Reza, Farbod Faraji, Aaron Knoll

Abstract: The topology of the applied magnetic field is an important design aspect of Hall thrusters. For modern Hall thrusters, the field topology most often features curved lines with a concave (negative) curvature upstream of the field peak and a convex (positive) curvature downstream. Additionally, the advent of the magnetic shielding technique has resulted in the design of Hall thrusters with non-conventional magnetic fields that exhibit high degrees of concavity upstream of the field's peak. We carry out a rigorous and detailed study of the effects that the magnetic field's curvature has on the plasma properties and the underlying processes in a 2D configuration representative of a Hall thruster's radial-azimuthal cross-section. The analyses are performed for plasma discharges of three propellants: xenon, krypton, and argon. For each propellant, we have carried out high-fidelity reduced-order particle-in-cell (PIC) simulations with various degrees of positive and negative curvatures of the magnetic field. Corresponding 1D radial PIC simulations were also performed for xenon to compare the observations between 1D and 2D simulations. We observed that there are distinct differences in the plasma phenomena between the cases with positive and negative field curvatures. The instability spectra in the cases of positive curvature is mostly dominated by the Electron Cyclotron Drift Instability, whereas the Modified Two Stream Instability is dominant in the negative-curvature cases. The distribution of the plasma properties, particularly the electron and ion temperatures, and the contribution of various mechanisms to electrons' cross-field transport showed notable variations with the field's curvature, especially between the positive and the negative values. Finally, the magnetic field curvature was observed to majorly influence the ion beam divergence along the radial and azimuthal coordinates.

5.Critical roles of edge turbulent transport in the formation of high-field-side high-density front and density limit disruption in J-TEXT tokamak

Authors:Peng Shi, Yuhan Wang, Li Gao, Hongjuan Sun1, Qinghu Yang, Xin Xu, Chengshuo Shen, Yanqiu Chen, Qinlin Tao, Zhipeng Chen, Haosheng Wu, Lu Wang, Zhongyong Chen, Nengchao Wang, Zhoujun Yang, Jingchun Li, Yonghua Ding, Yuan Pan, J-TEXT team

Abstract: This article presents an in-depth study of the sequence of events leading to density limit disruption in J-TEXT tokamak plasmas, with an emphasis on boudary turbulent transport and the high-field-side high-density (HFSHD) front. These phenomena were extensively investigated by using Langmuir probe and Polarimeter-interferometer diagnostics.

6.First data and preliminary experimental results from a new Doppler Backscattering system on the MAST-U spherical tokamak

Authors:P. Shi, R. Scannell, J. Wen, Z. B. Shi, C. Michael, T. Rhodes, V. H. Hall-Chen, Z. C. Yang, M. Jiang, W. L. Zhong

Abstract: A new Doppler backscattering (DBS) system, consisting of Q-band and V-band, has been installed and achieved its first data on the MAST-U spherical tokamak. The Q-band and V-band have separate microwave source systems, but share the same optical front-end components. The Q-band and V-band sources simultaneously generate eight (34, 36, 38, 40, 42, 44, 46 and 48 GHz) and seven (52.5, 55, 57.5, 60, 62.5, 65 and 67.5 GHz) fixed frequency probe beams, respectively. These frequencies provide a large range of radial positions from the low-field-side edge plasma to the core, and possibly to the high-field-side edge, depending on the plasma conditions. The quasi-optical system consists of a remotely-tunable polarizer, a focusing lens and a remotely-steerable mirror. By steering the mirror, the system provides remote control of the probed density fluctuation wavenumber, and allow the launch angle to match the magnetic field. The range of accessible turbulence wavenumbers (k_\theta) is reasonably large with normalized wavenumber k_\theta\rho_s ranging from <0.5 to 9. The first data acquired by this DBS system is validated by comparing with the data from the other DBS system on MAST-U (introduced in Ref. [21]). An example of measuring the velocity profile spanning from the edge to the center in a high-density plasma is presented, indicating the robust capabilities of the integrated Q-band and V-band DBS systems.

1.Suppressing electron disorder-induced heating of ultracold neutral plasma via optical lattice

Authors:HaiBo Wang, Zonglin Yao, Fuyang Zhou, Yong Wu, Jianguo Wang, Xiangjun Chen

Abstract: Disorder-induced heating (DIH) prevents ultracold neutral plasma into electron strong coupling regime. Here we propose a scheme to suppress electronic DIH via optical lattice. We simulate the evolution dynamics of ultracold neutral plasma constrained by three-dimensional optical lattice using classical molecular dynamics method. The results show that for experimentally achievable condition, electronic DIH is suppressed by a factor of 1.3, and the Coulomb coupling strength can reach to 0.8 which is approaching the strong coupling regime. Suppressing electronic DIH via optical lattice may pave a way for the research of electronic strongly coupled plasma.

2.Zonal profile corrugations and staircase formation: transport crossphase modulations

Authors:M. Leconte, T. Kobayashi

Abstract: Recently, quasi-stationary structures called $E \times B$ staircases were observed in gyrokinetic simulations, in all transport channels [Dif-Pradalier et al. Phys. Rev. Lett. 114, 085004 (2015)]. We present a novel analytical theory - supported by plasma fluid simulations - for the generation of density profile corrugations (staircase): Turbulent fluctuations self-organize to generate quasi-stationary radial modulations $\Delta \theta_k(r,t)$ of the transport crossphase $\theta_k$ between density and electric potential fluctuations. The radial modulations of the associated particle flux drive zonal corrugations of the density profile, via a modulational instability. In turn, zonal density corrugations regulate the turbulence via nonlinear damping of the fluctuations.

3.Modeling terahertz emissions from energetic electrons and ions in foil targets irradiated by ultraintense femtosecond laser pulses

Authors:E. Denoual, L. Bergé, X. Davoine, L. Gremillet

Abstract: Terahertz (THz) emissions from fast electron and ion currents driven in relativistic, femtosecond laser-foil interactions are examined theoretically. We first consider the radiation from the energetic electrons exiting the backside of the target. Our kinetic model takes account of the coherent transition radiation due to these electrons crossing the plasma-vacuum interface as well as of the synchrotron radiation due to their deflection and deceleration in the sheath field they set up in vacuum. After showing that both mechanisms tend to largely compensate each other when all the electrons are pulled back into the target, we investigate the scaling of the net radiation with the sheath field strength. We then demonstrate the sensitivity of this radiation to a percent-level fraction of escaping electrons. We also study the influence of the target thickness and laser focusing. The same sheath field that confines most of the fast electrons around the target rapidly sets into motion the surface ions. We describe the THz emission from these accelerated ions and their accompanying hot electrons by means of a plasma expansion model that allows for finite foil size and multidimensional effects. Again, we explore the dependencies of this radiation mechanism on the laser-target parameters. Under conditions typical of current ultrashort laser-solid experiments, we find that the THz radiation from the expanding plasma is much less energetic -- by one to three orders of magnitude -- than that due to the early-time motion of the fast electrons.

4.Machine learning assisted analysis of visible spectroscopy in pulsed-power-driven plasmas

Authors:Rishabh Datta, Faez Ahmed, Jack D Hare

Abstract: We use machine learning models to predict ion density and electron temperature from visible emission spectra, in a high energy density pulsed-power-driven aluminum plasma, generated by an exploding wire array. Radiation transport simulations, which use spectral emissivity and opacity values generated using the collisional-radiative code PrismSPECT, are used to determine the spectral intensity generated by the plasma along the spectrometer's line of sight. The spectra exhibit Al-II and Al-III lines, whose line ratios and line widths vary with the density and temperature of the plasma. These calculations provide a 2500-size synthetic dataset of 400-dimensional intensity spectra, which is used to train and compare the performance of multiple machine learning models on a 3-variable regression task. The AutoGluon model performs best, with an R2-score of roughly 98% for density and temperature predictions. Simpler models (random forest, k-nearest neighbor, and deep neural network) also exhibit high R2-scores (>90%) for density and temperature predictions. These results demonstrate the potential of machine learning in providing rapid or real-time analysis of emission spectroscopy data in pulsed-power-driven plasmas.

1.Development of the Thomson scattering measurement system for cascade arc device with indirectly heated hollow cathode

Authors:K. Yamasaki, K. Okuda, J. Kono, A. Saito, D. Mori, R. Suzuki, Y. Kambara, R. Hamada, S. Namba, K. Tomita, Y. Pan N. Tamura, C. Suzuki H. Okuno

Abstract: We have developed a Thomson scattering measurement system for the cascade arc discharge device designed for the plasma window (PW) application study. The PW is one of the plasma application techniques that sustain the steep pressure gradient between high pressure (10-100 kPa) and a vacuum environment due to the thermal energy of the plasma. Since the plasma thermal energy is the essential parameter for the pressure separation capability of PW, we installed the Thomson scattering measurement system to observe the electron density and temperature within the anode and cathode of the PW for the detailed analysis of the pressure separation capability. The frequency-doubled Nd:YAG laser (532 nm, 200 mJ, 8 ns) was employed for the probe laser. The scattered light was fed to the triple grating spectrometer. The notch filter between the first and second grating eliminated the stray light, realizing a sufficiently high signal-to-noise ratio. The Thomson scattering measurement system successfully obtained the electron density and temperature of the cascade arc plasma at 20 mm downstream from the tip of the cathode. The installed system successfully obtained the Thomson scattering spectrum and showed that the electron density increased from $2\times10^{19} {\rm m}^{-3}$ to $7\times10^{19} {\rm m}^{-3}$ with the discharge power, while the electron temperature was almost constant at about 2 eV. The obtained data successfully contributed to the study of the pressure separation capability of the PW.

2.On the Efficiency of Disorder-induced Heating of Ultracold Plasmas

Authors:Yurii V. Dumin, Anastasiia T. Lukashenko

Abstract: Starting from the beginning of their research in the early 2000's, the ultracold plasmas were considered as a promising tool to achieve considerable values of the Coulomb coupling parameter for electrons. Unfortunately, this was found to be precluded by a sharp spontaneous increase of temperature, which was commonly attributed to the so-called disorder-induced heating (DIH). It is the aim of the present paper to quantify this effect as function of the initial ionic disorder and, thereby, to estimate the efficiency of its mitigation, e.g., by the Rydberg blockade. As a result of the performed simulations, we found that the dynamics of electrons exhibited a well-expressed transition from the case of the quasi-regular arrangement of ions to the disordered one; the magnitude of the effect being about 30%. Thereby, we can conclude that the two-step formation of ultracold plasmas - involving the intermediate stage of the blockaded Rydberg gas - can really serve as a tool to increase the degree of Coulomb coupling, but the efficiency of this method is moderate.

3.Towards Robust Solvers for Nuclear Fusion Simulations Using JOREK: A Numerical Analysis Perspective

Authors:Alex Quinlan, Vandana Dwarka, Ihor Holod, Matthias Hoelzl

Abstract: One of the most well-established codes for modeling non-linear Magnetohydrodynamics (MHD) for tokamak reactors is JOREK, which solves these equations with a B\'ezier surface based finite element method. This code produces a highly sparse but also very large linear system. The main solver behind the code uses the Generalized Minimum Residual Method (GMRES) with a physics-based preconditioner, but even with the preconditioner there are issues with memory and computation costs and the solver does not always converge well. This work contains the first thorough study of the mathematical properties of the underlying linear system. It enables us to diagnose and pinpoint the cause of hampered convergence. In particular, analyzing the spectral properties of the matrix and the preconditioned system with numerical linear algebra techniques, will open the door to research and investigate more performant solver strategies, such as projection methods.

1.Helicity of the magnetic axes of quasi-isodynamic stellarators

Authors:Katia Camacho Mata, Gabriel G. Plunk

Abstract: In this study, we explore the influence of the helicity of the magnetic axis-defined as the self-linking number of the curve-on the quality of quasi-isodynamic stellarator-symmetric configurations constructed using the near-axis expansion method (Camacho Mata et al. 2022; Plunk et al. 2019). A class of magnetic axes previously unexplored within this formalism is identified when analyzing the axis shape of the QIPC configuration (Subbotin et al. 2006): the case of half-helicity (per field period). We show these shapes are compatible with the near-axis formalism and how they can be used to construct near-axis stellarators with up-to 5 field-periods, $\epsilon_{eff} \approx$ 1.3%, and similar rotational transform as existing conventionally optimized designs, without the need of a plasma boundary optimization.

2.Cs evaporation in a negative ion source and Cs cleaning tests by plasma sputtering

Authors:M. Barbisan, R. S. Delogu, A. Pimazzoni, C. Poggi, M. Ugoletti, M. Cavenago

Abstract: The compact radio frequency negative ion source NIO1 (Negative Ion Optimization phase 1) has been designed, built and operated by Consorzio RFX and INFN-LNL in order to study and optimize the production and acceleration of H- ions in continuous operation. In 2020 Cs was evaporated in the source to increase the total extracted ion current. After an initial reduction of extracted electron to ion ratio and subsequently an increase of extracted negative ion current, the source performances progressively worsened, because of the excessive amount of Cs evaporated in the source; the extracted electron to ion ratio increased from below 1 to more than 10, while ion current density reduced from max. 67 A/m2 ion current to not more than 30 A/m2). The paper presents the experimental observations collected during Cs evaporation (reduction of plasma light, Cs emission and H$\beta$/H$\gamma$ ratio, etc.) that can help stopping the process before an excessive amount of Cs is introduced in the source. The paper also reports the cleaning techniques tested to remove the Cs excess by the action of hydrogen or argon plasmas; while argon was predictably more effective in surface sputtering, a 3 h Ar plasma treatment was not sufficient to recover from overcesiation.

1.On learning latent dynamics of the AUG plasma state

Authors:A. Kit, A. E. Järvinen, Y. R. J. Poels, S. Wiesen, V. Menkovski, R. Fischer, M. Dunne, ASDEX-Upgrade Team

Abstract: In this work, we demonstrate the utility of state representation learning applied to modeling the time evolution of electron density and temperature profiles at ASDEX-Upgrade (AUG). The proposed model is a deep neural network which learns to map the high dimensional profile observations to a lower dimensional state. The mapped states, alongside the original profile's corresponding machine parameters are used to learn a forward model to propagate the state in time. We show that this approach is able to predict AUG discharges using only a selected set of machine parameters. The state is then further conditioned to encode information about the confinement regime, which yields a simple baseline linear classifier, while still retaining the information needed to predict the evolution of profiles. We then discuss the potential use cases and limitations of state representation learning algorithms applied to fusion devices.

1.Extremely powerful and frequency-tunable terahertz pulses from a table-top laser-plasma wiggler

Authors:Jie Cai, Yinren Shou, Yixing Geng, Liqi Han, Xinlu Xu, Shuangchung Wen, Baifei Shen, Jinqing Yu, Xueqing Yan

Abstract: The production of broadband, terawatt terahertz (THz) pulses has been demonstrated by irradiating relativistic lasers on solid targets. However, the generation of extremely powerful, narrow-band, and frequency-tunable THz pulses remains a challenge. Here, we present a novel approach for such THz pulses, in which a plasma wiggler is elaborated by a table-top laser and a near-critical density plasma. In such a wiggler, the laser-accelerated electrons emit THz radiations with a period closely related to the plasma thickness. Theoretical model and numerical simulations predict a THz pulse with a laser-THz energy conversion over 2.0$\%$, an ultra-strong field exceeding 80 GV/m, a divergence angle approximately 20$^\circ$, and a center-frequency tunable from 4.4 to 1.5 THz, can be generated from a laser of 430 mJ. Furthermore, we demonstrate that this method can work across a wide range of laser and plasma parameters, offering potential for future applications with extremely powerful THz pulse.

1.Comment on "Modification of Lie's transform perturbation theory for charged particle motion in a magnetic field''

Authors:Alain J. Brizard

Abstract: A recent paper by L.~Zheng [Phys. Plasmas, 30, 042515 (2023)] presented a critical analysis of standard Lie-transform perturbation theory and suggested that its application to the problem of charged-particle motion in a magnetic field suffered from ordering inconsistencies. In the present Comment, we suggest that this criticism is unjustified and that standard Lie-transform perturbation theory does not need to be modified in its application to guiding-center theory.

1.Electron heat flux and propagating fronts in plasma thermal quench via ambipolar transport

Authors:Yanzeng Zhang, Jun Li, Xianzhu Tang

Abstract: The thermal collapse of a nearly collisionless plasma interacting with a cooling spot, in which the electron parallel heat flux plays an essential role, is investigated both theoretically and numerically. We show that such thermal collapse, which is known as thermal quench in tokamaks, comes about in the form of propagating fronts, originating from the cooling spot, along the magnetic field lines. The slow fronts, propagating with local ion sound speed, limit the aggressive cooling of plasma, which is accompanied by a plasma cooling flow toward the cooling spot. The extraordinary physics underlying such a cooling flow is that the fundamental constraint of ambipolar transport along the field line limits the spatial gradient of electron thermal conduction flux to the much weaker convective scaling, as opposed to the free-streaming scaling, so that a large electron temperature and hence pressure gradient can be sustained. The last ion front for a radiative cooling spot is a shock front where cold but flowing ions meet the hot ions.

2.Proton-Boron Fusion Yield Increased by Orders of Magnitude with Foam Targets

Authors:Wen-Qing Wei, Shi-Zheng Zhang, Zhi-Gang Deng, Wei Qi, Hao Xu, Li-Rong Liu, Jia-Lin Zhang, Fang-Fang Li, Xing Xu, Zhong-Min Hu, Ben-Zheng Chen, Bu-Bo Ma, Jian-Xing Li, Xue-Guang Ren, Zhong-Feng Xu, Dieter H. H. Hoffmann, Quan-Ping Fan, Wei-Wu Wang, Shao-Yi Wang, Jian Teng, Bo Cui, Feng Lu, Lei Yang, Yu-Qiu Gu, Zong-Qing Zhao, Rui Cheng, Zhao Wang, Yu Lei, Guo-Qing Xiao, Hong-Wei Zhao, Bing Liu, Guan-Chao Zhao, Min-Sheng Liu, Hua-Sheng Xie, Lei-Feng Cao, Jie-Ru Ren, Wei-Min Zhou, Yong-Tao Zhao

Abstract: A novel intense beam-driven scheme for high yield of the tri-alpha reaction 11B(p,{\alpha})2{\alpha} was investigated. We used a foam target made of cellulose triacetate (TAC, C_9H_{16}O_8) doped with boron. It was then heated volumetrically by soft X-ray radiation from a laser heated hohlraum and turned into a homogenous, and long living plasma. We employed a picosecond laser pulse to generate a high-intensity energetic proton beam via the well-known Target Normal Sheath Acceleration (TNSA) mechanism. We observed up to 10^{10}/sr {\alpha} particles per laser shot. This constitutes presently the highest yield value normalized to the laser energy on target. The measured fusion yield per proton exceeds the classical expectation of beam-target reactions by up to four orders of magnitude under high proton intensities. This enhancement is attributed to the strong electric fields and nonequilibrium thermonuclear fusion reactions as a result of the new method. Our approach shows opportunities to pursue ignition of aneutronic fusion.

1.QScatter: Numerical Framework for Fast Prediction of Particle Distributions in Electron-Laser Scattering

Authors:Óscar Amaro, Marija Vranic

Abstract: The new generation of multi-PetaWatt laser facilities will allow tests of Strong Field QED, as well as provide an opportunity for novel photon and lepton sources. The first experiments are planned to study the (nearly) head-on scattering of intense, focused laser pulses with either relativistic electron beams or high-energy photon sources. In this work, we present a numerical framework that can provide fast predictions of the asymptotic particle and photon distributions after the scattering. The works presented in this manuscript includes multiple features such as spatial and temporal misalignment between the laser and the scattering beam, broadband electron beams, and beam divergence. The expected mean energy, energy spread, divergence or other observables are calculated by combining an analytical description and numerical integration. This method can provide results within minutes on a personal computer, which would otherwise require full-scale 3D QED-PIC simulations using thousands of cores. The model, which has been compiled into an open-source code QScatter, may be used to support the analysis of large-size data sets from high-repetition rate experiments, leveraging its speed for optimization or reconstruction of experimental parameters.

1.3D particle-in-cell simulations of negative and positive streamers in C4F7N-CO2 mixtures

Authors:Baohong Guo, Ute Ebert, Jannis Teunissen

Abstract: We investigate negative and positive streamers in C4F7N-CO2 mixtures through simulations. These mixtures are considered to be more environmentally friendly than the insulating gas SF6 that is widely used in high voltage technology. Simulations are performed using a 3D particle-in-cell model. Negative streamers can propagate when the background field is close to the critical field. We relate this to their short conductive channels, due to rapid electron attachment, which limits their field enhancement. Positive streamers also require a background field close to the critical field, and in addition a source of free electrons ahead of them. In our simulations these electrons are provided through an artificial stochastic background ionization process as no efficient photoionization process is known for these gases. In 3D, we can only simulate the early inception stage of positive discharges, due to the extremely high electric fields and electron densities that occur. Qualitative 2D Cartesian simulations show that the growth of these discharges is highly irregular, resulting from incoming negative streamers that connect to existing channels. The inclusion of a stochastic background ionization process also has an interesting effect on negative discharges: new streamers can be generated behind previous ones, thereby forming a chain of negative streamers.

2.Comparison of saturation rules used for gyrokinetic quasilinear transport modeling

Authors:Scott E. Parker, Calder Haubrich, Qiheng Cai, Stefan Tirkas, Yang Chen

Abstract: Theory-based transport modeling has been widely successful and is built on the foundations of quasilinear theory. Specifically, the quasilinear expression of the flux can be used in combination with a saturation rule for the toroidal mode amplitude. Most transport models follow this approach. Saturation rules are heuristic and difficult to rigorously derive. We compare three common saturation rules using a fairly accurate quasilinear expression for the fluxes computed using local linear gyrokinetic simulation. We take plasma parameters from experimental H-mode profiles and magnetic equilibrium and include electrons, Deuterium, and Carbon species. We find that the various saturation rules give qualitatively similar behavior. This may help explain why the different theory-based transport models can all predict core tokamak profiles reasonably well. Comparisons with nonlinear local and global gyrokinetic simulations are also discussed.

1.Particle acceleration in self-driven turbulent reconnection

Authors:Jian-Fu Zhang Xiangtan, Siyao Xu Princeton, Alex Lazarian Madison, Grzegorz Kowal São Paulo

Abstract: The theoretical prediction that magnetic reconnection spontaneously drives turbulence has been supported by magnetohydrodynamic (MHD) and kinetic simulations. While reconnection with externally driven turbulence is accepted as an effective mechanism for particle acceleration, the acceleration during the reconnection with self-driven turbulence is studied for the first time in this work. By using high-resolution 3D MHD simulations of reconnection with self-generated turbulence, we inject test particles into the reconnection layer to study their acceleration process. We find that the energy gain of the particles takes place when they bounce back and forth between converging turbulent magnetic fields. The particles can be efficiently accelerated in self-driven turbulent reconnection with the energy increase by about 3 orders of magnitude in the range of the box size. The acceleration proceeds when the particle gyroradii become larger than the thickness of the reconnection layer. We find that the acceleration in the direction perpendicular to the local magnetic field dominates over that in the parallel direction. The energy spectrum of accelerated particles is time-dependent with a slope that evolves toward -2.5. Our findings can have important implications for particle acceleration in high-energy astrophysical environments.

2.Characterization of fast magnetosonic waves driven by interaction between magnetic fields and compact toroids

Authors:F. Chu, S. J. Langendorf, T. Byvank, A. L. LaJoie, D. A. Endrizzi, J. Olson, K. J. McCollam

Abstract: Magnetosonic waves are low-frequency, linearly polarized magnetohydrodynamic (MHD) waves that can be excited in any electrically conducting fluid permeated by a magnetic field. They are commonly found in space, responsible for many well-known features, such as heating of the solar corona and acceleration of energetic electrons in Earth's inner magnetosphere. In this work, we present observations of magnetosonic waves driven by injecting compact toroid (CT) plasmas into a static Helmholtz magnetic field at the Big Red Ball (BRB) Facility at Wisconsin Plasma Physics Laboratory (WiPPL). We first identify the wave modes by comparing the experimental results with the MHD theory, and then study how factors such as the background magnetic field affect the wave properties. Since this experiment is part of an ongoing effort of forming a target plasma with tangled magnetic fields as a novel fusion fuel for magneto-inertial fusion (MIF, aka magnetized target fusion), we also discuss a future possible path of forming the target plasma based on our current results.

3.Models of Tokamak Disruptions

Authors:H. R. Strauss

Abstract: Disruptions are a serious issue in tokamaks. In a disruption, the thermal energy is lost by means of an instability which could be a resistive wall tearing mode (RWTM). During precursors to a disruption, the plasma edge region cools, causing the current to contract. Model sequences of contracted current equilibria are given, and their stability is calculated. A linear stability study shows that there is a maximum value of edge $q_a \approx 3$ for RWTMs to occur. This also implies a minimum rational surface radius normalized to plasma radius from RWTMs to be unstable. Nonlinear simulations are performed using a similar model sequence derived from an equilibrium reconstruction. There is a striking difference in the results, depending on whether the wall is ideal or resistive. With an ideal wall, the perturbations saturate at moderate amplitude, causing a minor disruption without a thermal quench. With a resistive wall, there is a major disruption with a thermal quench, if the edge $q_a \le 3.$ There is a sharp transition in nonlinear behavior at $q_a = 3.$ This is consistent with the linear model and with experiments. If disruptions are caused by RWTMs, then devices with highly conducting walls, such as the International Tokamak Experimental Reactor (ITER) will experience much milder, tolerable, disruptions than presently predicted.

4.Towards the demonstration of photon-photon collision with compact lasers

Authors:L. Q. Han, J. Cai, Y. R. Shou, X. D. Liu, J. Q. Yu, X. Q. Yan

Abstract: We report a proposal to observe the two-photon Breit-Wheeler process in plasma driven by compact lasers. A high charge electron bunch can be generated from laser plasma wakefield acceleration when a tightly focused laser pulse transports in a sub-critical density plasma. The electron bunch scatters with the laser pulse coming from the opposite direction and results the emitting of high brilliance X-ray pulses. In a three-dimensional particle-in-cell simulation with a laser pulse of $\sim$10 J, one could produce a X-ray pulse with photon number higher than $3\times10^{11}$ and brilliance above $1.6\times 10^{23}$ photons/s/mm$^2$/mrad$^2$/0.1$\%$BW at 1 MeV. The X-ray pulses collide in the plasma and create more than $1.1\times 10^5$ electron-positron pairs per shot. It is also found that the positrons can be accelerated transversely by a transverse electric field generated in the plasma, which enables the safe detection in the direction away from the laser pulses. This proposal which has solved key challenges in laser driven photon-photon collision could demonstrate the two-photon Breit-Wheeler process on a much more compact device in a single shot.

1.A tutorial overview on the angular scattering models of Electron-Neutral, Ion-Neutral, Neutral-Neutral, and Coulomb Collisions in Monte Carlo collision modeling on low temperature plasma

Authors:Wei Yang

Abstract: Over the past decade, massive modeling practices on low temperature plasma (LTP) reveal that input data such as microscopic scattering cross sections are crucial to output macroscopic phenomena. In Monte Carlo collision (MCC) modeling on LTP, angular scattering model is a non-trivial topic in both natural and laboratory plasma. Conforming to the pedagogical purpose of this overview, the classical and quantum theory of binary scattering including the commonly used Born-Bethe approximation is first introduced. State-of-the-art angular scattering models are derived based on the above theories for electron-neutral, ion-neutral, neutral-neutral, and Coulomb collisions. The tutorial is not aiming to provide accurate cross section data by modern approaches in quantum theory, but to introduce analytical angular scattering models from classical, semi-empirical, and first-order perturbation theory. The reviewed models are expected to be readily incorporated into the MCC codes, in which scattering angle is randomly sampled through analytical inversion instead of numerical accept-reject method. Those simplified approaches are very attractive, and demonstrate in many cases the ability to achieve a striking agreement with experiments. Energy partition models on electron-neutral ionization are also discussed with insight from the binary encounter Bethe theory. This overview is written in a tutorial style, in order to serve as a guide for novice in this field, and at the same time as a comprehensive reference for practitioners in MCC modeling on plasma.

1.Overcoming the Limitation of Long Focal Length with Plasma Telescope for Laser Wakefield Acceleration Beyond Petawatt

Authors:Xuesong Geng, Liangliang Ji, Baifei Shen

Abstract: The compactness of laser wakefield acceleration (LWFA) is limited by its long focal length for high power lasers, e.g., more than 10 meters for 1-peatawatt (PW) laser pulse and up to hundreds of meters for 10-100 PW lasers. The long focal length originates from the low damage threshold of the optical off-axial parabolic (OAP) mirror and consequent large spot size. We propose implementing an OAP plasma mirror (PM) to form a telescope geometry, reducing the beam size and hence constraining the focal length to meter-range for LWFA driven by lasers beyond 1PW. Three-dimensional particle-in-cell simulations are performed to characterize the reflection of a 1-PW laser by the plasma OAP and find that optimal condition is achieved within only 1-m optical length. The new method successfully generates 9GeV electron bunch in the subsequent LWFA stage with consistent acceleration gradients to that of the 1-PW laser via ordinary focusing. The proposed geometry provides a solution of compact LWFAs available for even 100-PW laser systems.

2.RBG-Maxwell Framework: Simulation of Collisional Plasma Systems via Coupled Boltzmann-Maxwell equations on GPU

Authors:Ming-Yan Sun, Peng Xu, Jun-Jie Zhang, Qun Wang, Tai-Jiao Du, Jian-Guo Wang

Abstract: This paper presents the RBG-Maxwell framework, a relativistic collisional plasma simulator on GPUs. We provide detailed discussions on the fundamental equations, numerical algorithms, implementation specifics, and key testing outcomes. The RBG-Maxwell framework is a robust numerical code designed for simulating the evolution of plasma systems through a kinetic approach on large-scale GPUs. It offers easy adaptability to a wide range of physical systems. Given the appropriate initial distributions, particle masses, charges, differential cross-sections, and external forces (which are not confined to electromagnetic forces), the RBG-Maxwell framework can direct the evolution of a particle system from a non-equilibrium state to a thermal state.

3.Vibronic State-Specific Modelling of High-Speed Nitrogen Shocked Flows. Part II: Shock Tube Simulations

Authors:Élio Pereira, Jorge Loureiro, Mário Lino da Silva

Abstract: The conditions of thermochemical and radiative non-equilibrium attained in nitrogen shocked flows were quantified using a vibronic state-specific model. This model, being described in a companion paper, was implemented in Euler one-dimensional simulations for shots $19$, $20$ and $40$ of the EAST's $62^{\textrm{th}}$ campaign. It was found that the peak values of the instrumental radiative intensities were underestimated by one to two orders of magnitude, and sensitivity tests performed on several parameters of the simulations were not successful in getting a reasonable agreement. The shapes of the instrumental radiative intensities' profiles obtained in the low speed shot were correctly predicted, unlike the ones of the medium and high speed shots which revealed non-null plateaus proceeding or coalescing with peaks. These plateaus were not predicted at all. It is suspected that such discrepancies may have resulted from neglecting other shock tube related phenomena, as pointed out by other researchers in the literature: the absorption of radiation emitted by the driver gas and the EAST electric arc, and/or the conduction of heat due to downstream plasma being subjected to a stronger shock wave.

4.Kinetic-Ballooning-Limited Pedestals in Spherical Tokamak Plasmas

Authors:J. F. Parisi, W. Guttenfelder, A. O. Nelson, R. Gaur, A. Kleiner, M. Lampert, G. Avdeeva, J. W. Berkery, C. Clauser, M. Curie, A. Diallo, W. Dorland, J. McClenaghan

Abstract: A theoretical model is presented that for the first time matches experimental measurements of the pedestal width-height Diallo scaling in the low-aspect-ratio high-$\beta$ tokamak NSTX. Combining linear gyrokinetics with self-consistent pedestal equilibrium variation, kinetic-ballooning, rather than ideal-ballooning plasma instability, is shown to limit achievable confinement in spherical tokamak pedestals. Simulations are used to find the novel Gyrokinetic Critical Pedestal constraint, which determines the steepest pressure profile a pedestal can sustain subject to gyrokinetic instability. Gyrokinetic width-height scaling expressions for NSTX pedestals with varying density and temperature profiles are obtained. These scalings for spherical tokamaks depart significantly from that of conventional aspect ratio tokamaks.

1.Polarization Effects in Higher-order Guiding-center Lagrangian Dynamics

Authors:Alain J. Brizard

Abstract: The extended guiding-center Lagrangian equations of motion are derived by Lie-transform method under the assumption of time-dependent and inhomogeneous electric and magnetic fields that satisfy the standard guiding-center orderings for space-time scales. Polarization effects are introduced into the Lagrangian dynamics by the inclusion of the polarization drift velocity in the guiding-center velocity and the appearance of finite-Larmor-radius corrections in the guiding-center Hamiltonian and guiding-center Poisson bracket.

1.Colliding of two high Mach-number quantum degenerate plasma jets

Authors:W. B. Zhang, Y. H. Li, D. Wu, J. Zhang

Abstract: Colliding of two high Mach-number quantum degenerate plasmas is one of the most essential components in the double-cone ignition (DCI) inertial confinement fusion scheme, in which two highly compressed plasma jets from the cone-tips collide along with rapid conversion from the colliding kinetic energies to the internal energy of a stagnated isochoric plasma. Due to the effects of high densities and high Mach-numbers of the colliding plasma jets, quantum degeneracy and kinetic physics might play important roles and challenge the predictions of traditional hydrodynamic models. In this work, the colliding process of two high Mach number quantum degenerate Deuterium-plasma jets with sizable scale ($\sim 1000\ \si{\mu m}$, $\sim 300\ \si{ps}$, $\sim 100\ \si{g/cc}$, $\sim 300\ \si{km/s}$) were investigated with first-principle kinetic simulations and theoretical analyses. In order to achieve high-density compression, the colliding kinetic pressure should be significantly higher than the pressure raised by the quantum degeneracy. This means high colliding Mach numbers are required. However, when the Mach number is further increased, we surprisingly found a decreasing trend of density compression, due to kinetic effects. It is therefore suggested that there is theoretically optimal colliding velocity to achieve the highest density compression. Our results would provide valuable suggestions for the base-line design of the DCI experiments and also might be of relevance in some violent astrophysical processes, such as the merger of two white dwarfs.

2.Collective ion dynamics in Coulomb one-component plasmas within the self-consistent relaxation theory

Authors:Ilnaz I. Fairushin, Anatolii V. Mokshin

Abstract: In this paper, we present the theoretical formalism describing the collective ion dynamics of the nonideal Coulomb classical one-component plasmas on the basis of the self-consistent relaxation theory. The theory is adapted to account for correlations between the frequency relaxation parameters that characterize the three- and four-particle dynamics and the parameters associated with the two-particle dynamics. The dynamic structure factor spectra and dispersion characteristics calculated for a wide range of wave numbers are in agreement with the molecular dynamics simulation data and the results obtained with the theory of the frequency moments. The proposed formalism reproduces all the features inherent to the Coulomb one-component plasmas and requires only knowledge of the coupling parameter and the information about the structure.

3.Nonlinear Landau resonant interaction between whistler waves and electrons: Excitation of electron acoustic waves

Authors:Donglai Ma, Xin An, Anton Artemyev, Jacob Bortnik, Vassilis Angelopoulos, Xiao-Jia Zhang

Abstract: Electron acoustic waves (EAWs), as well as electron-acoustic solitary structures, play a crucial role in thermalization and acceleration of electron populations in Earth's magnetosphere. These waves are often observed in association with whistler-mode waves, but the detailed mechanism of EAW and whistler wave coupling is not yet revealed. We investigate the excitation mechanism of EAWs and their potential relation to whistler waves using particle-in-cell simulations. Whistler waves are first excited by electrons with a temperature anisotropy perpendicular to the background magnetic field. Electrons trapped by these whistler waves through nonlinear Landau resonance form localized field-aligned beams, which subsequently excite EAWs. By comparing the growth rate of EAWs and the phase mixing rate of trapped electron beams, we obtain the critical condition for EAW excitation, which is consistent with our simulation results across a wide region in parameter space. These results are expected to be useful in the interpretation of concurrent observations of whistler-mode waves and nonlinear solitary structures, and may also have important implications for investigation of cross-scale energy transfer in the near-Earth space environment.

1.The equation of state of partially ionized hydrogen and deuterium plasma revisited

Authors:A. V. Filinov, M. Bonitz

Abstract: We present novel first-principle fermionic path integral Monte Carlo (PIMC) simulation results for a dense partially ionized hydrogen (deuterium) plasma, for temperatures in the range $15,000$K $\leq T \leq 400,000$K and densities $7 \cdot 10^{-7}$g/cm$^{3}\leq \rho_H \leq 0.085$ g/cm$^{3}$ ($1.4 \cdot 10^{-6}$g/cm$^{3}\leq \rho_D \leq 0.17$ g/cm$^{3}$), corresponding to $100\geq r_s\geq 2$, where $r_s=\bar r/a_B$ is the ratio of the mean interparticle distance to the Bohr radius. These simulations are based on the fermionic propagator PIMC (FP-PIMC) approach in the grand canonical ensemble [A. Filinov \textit{et al.}, Contrib. Plasma Phys. \textbf{61}, e202100112 (2021)] and fully account for correlation and quantum degeneracy and spin effects. For the application to hydrogen and deuterium, we develop a combination of the fourth-order factorization and the pair product ansatz for the density matrix. Moreover, we avoid the fixed node approximation that may lead to uncontrolled errors in restricted PIMC (RPIMC). Our results allow us to critically re-evaluate the accuracy of the RPIMC simulations for hydrogen by Hu \textit{et al.} [Phys. Rev. B \textbf{84}, 224109 (2011)] and of various chemical models. The deviations are generally found to be small, but for the lowest temperature, $T=15,640$~K they reach several percent. We present detailed tables with our first principles results for the pressure and energy isotherms.

2.Recovering non-Maxwellian particle velocity distribution functions from collective Thomson-scattered spectra

Authors:Bryan C. Foo, Derek B. Schaeffer, Peter V. Heuer

Abstract: Collective optical Thomson scattering (TS) is a diagnostic commonly used to characterize plasma parameters. These parameters are typically extracted by a fitting algorithm that minimizes the difference between a measured scattered spectrum and an analytic spectrum calculated from the velocity distribution function (VDF) of the plasma. However, most existing TS analysis algorithms assume the VDFs are Maxwellian, and applying an algorithm which makes this assumption does not accurately extract the plasma parameters of a non-Maxwellian plasma due to the effect of non-Maxwellian deviations on the TS spectra. We present new open-source numerical tools for forward modeling analytic spectra from arbitrary VDFs, and show that these tools are able to more accurately extract plasma parameters from synthetic TS spectra generated by non-Maxwellian VDFs compared to standard TS algorithms. Estimated posterior probability distributions of fits to synthetic spectra for a variety of example non-Maxwellian VDFs are used to determine uncertainties in the extracted plasma parameters, and show that correlations between parameters can significantly affect the accuracy of fits in plasmas with non-Maxwellian VDFs.

1.Trapped-particle precession and modes in quasi-symmetric stellarators and tokamaks: a near-axis perspective

Authors:E. Rodriguez, R. J. J. Mackenbach

Abstract: This paper presents the calculation of the bounce-averaged drift of trapped particles in a near-axis framework for axisymmetric and quasisymmetric magnetic fields. This analytic consideration provides important insight on the dependence of the bounce-averaged drift on the geometry and stability properties of the field. In particular, we show that, although the maximum-$\mathcal{J}$ property is unattainable in quasisymmetric stellarators, one may approach it through increased plasma $\beta$ and triangular shaping. The description of trapped particles allows us to calculate the available energy of trapped electrons analytically in two asymptotic regimes, providing insight into the behaviour of this measure of turbulence. It is shown that the available energy is intimately related to MHD-stability, providing a potential synergy between this measure of gyrokinetic turbulence and MHD-stability.

2.Dimits shift and impact of collisions using a gyrokinetic moment-based approach

Authors:Hoffmann A. C. D., Frei B. J., Ricci P

Abstract: This study presents a comprehensive benchmark and convergence analysis of the gyromoment (GM) approach in the gyrokinetic local flux-tube limit, focusing on the cyclone base case (CBC) and the Dimits shift. The GM approach demonstrates its efficacy in accurately capturing the nonlinear dynamics of the CBC with fewer velocity space points compared to the GENE code. Increasing velocity dissipation enhances convergence, albeit with a slight discrepancy in the saturated heat flux value. The GM approach successfully reproduces the Dimits shift and effectively captures its width compared to the ITG threshold. In the collisional case, we obtain a good agreement with previous global PIC results on transport. We report that the choice of collision model has a minimal impact both on the ITG growth rate and on the nonlinear saturated heat flux. We attribute this to the adiabatic electron model that impeaches the electron-ion collisions.

3.Moment tracking and their coordinate transformations for macroparticles with an application to plasmas around black holes

Authors:Alexander Warwick, Jonathan Gratus

Abstract: Particle-in-cell codes usually represent large groups of particles as a single macroparticle. These codes are computationally efficient but lose information about the internal structure of the macroparticle. To improve the accuracy of these codes, this work presents a method in which, as well as tracking the macroparticle, the moments of the macroparticle are also tracked. Although the equations needed to track these moments are known, the coordinate transformations for moments where the space and time coordinates are mixed cannot be calculated using the standard method for representing moments. These coordinate transformations are important in astrophysical plasma, where there is no preferred coordinate system. This work uses the language of Schwartz distributions to calculate the coordinate transformations of moments. Both the moment tracking and coordinate transformation equations are tested by modelling the motion of uncharged particles in a circular orbit around a black hole in both Schwarzschild and Kruskal-Szekeres coordinates. Numerical testing shows that the error in tracking moments is small, and scales quadratically. This error can be improved by including higher order moments. By choosing an appropriate method for using these moments to deposit the charge back onto the grid, a full particle-in-cell code can be developed.

1.Electromagnetic conversion into kinetic and thermal energies

Authors:Axel Brandenburg, Nousaba Nasrin Protiti

Abstract: Conversion of electromagnetic energy into magnetohydrodynamic energy occurs when the electric conductivity changes from negligible to finite values. This process is relevant during the epoch of reheating of the early Universe at the end of inflation and before the emergence of the radiation-dominated era. We find that conversion into kinetic and thermal energies is primarily the result of electric energy dissipation and that the magnetic energy plays only a secondary role in this process. This means that, since electric energy dominates over magnetic energy during inflation and reheating, significant amounts of electric energy can be converted into magnetohydrodynamic energy when conductivity emerges early enough, before the relevant length scales become stable.

1.Large amplitude dust-acoustic solitary waves and double layers in nonthermal warm complex plasmas

Authors:N. Alam, A. Mannan, A. A. Mamun

Abstract: Using a Sagdeev pseudopotential approach where the nonlinear structures are stationary in a comoving frame, the arbitrary or large amplitude dust-acoustic solitary waves and double layers have been studied in dusty plasmas containing warm positively charged dust and nonthermal distributed electrons and ions. Depending on the values of the critical Mach number, which varies with the plasma parameter, both supersonic and subsonic dust-acoustic solitary waves are found. It is found that our plasma system under consideration supports both positive and negative supersonic solitary waves, and only positive subsonic solitary waves and negative double layers. The parametric regimes for the existence of subsonic and supersonic dust-acoustic waves and how the polarity of solitary waves changes with plasma parameters are shown. It is observed that the solitary waves and double layers solution exist at the values of Mach number around its critical Mach number. The basic properties (amplitude, width, speed, etc.) of the solitary pulses and double layers are significantly modified by the plasma parameters (viz. ion to positive dust number density ratio, ion to electron temperature ratio, nonthermal parameter, positive dust temperature to ion temperature ratio, etc.). The applications of our present work in space environments (viz. cometary tails, Earth's mesosphere, Jupiter's magnetosphere, etc.) and laboratory devices, where nonthermal ions and electrons species along with positively charged dust species have been observed, are briefly discussed.

1.A Method for Plasma Electron Temperature Evaluation for GOL-NB facility

Authors:S. Polosatkin

Abstract: A new method for measuring the electron temperature of the plasma in GOL-NB facility is proposed. The proposed method is based on measuring the ratio of intensities of spectral lines emitted by fast atoms injected into the plasma. The beams of fast hydrogen atoms used for plasma heating or diagnostics contain atoms with full energy as well as atoms with fractional energies (E/2, E/3, E/18), which arise from the dissociation of molecular ions H+2, H+3, H2O+. Spectral lines of atoms with different energies (especially Ha) can be resolved due to Doppler shift caused by differences in atom velocities. For low-energy atoms, excitation occurs due to collisions with thermal electrons, while for high-energy atoms, collision processes with plasma ions are essential. Therefore, the intensity ratio of line fractions with different energies depends on the electronic temperature of the plasma and can be used for its measurement. At a beam energy of 24 keV, the method can be used to measure electronic temperatures up to 40 eV, which is of interest for experiments on the GOL-NB facility. To measure the temperature with an accuracy of 20 eV, it is necessary to measure the intensity ratio of lines with percentage precision and also measure the same precision of attenuation of the neutral beam passing through the plasma.

2.Theory of electron and ion holes as vortices in the phase-space of collision-less plasmas

Authors:Allen Lobo, Vinod Kumar Sayal

Abstract: This article studies the vortical nature and structure of phase-space holes -- nonlinear B.G.K. trapping modes found in the phase-space collision-free plasmas. A fluid-like outlook of the particles' phase-space is introduced, which makes it convenient to analytically identify electron and ion holes as vortices -- similar to that of ordinary two-dimensional fluids. A fluid velocity and vorticity field is defined for the phase-space of the electrons and ions, Euler equations describing the flow of the phase-space fluid representing the particle system are then developed. Using these equations, electron holes and ion holes are analytically identified as vortices in the phase-space of the plasma. A relation between Schamel's trapping parameter ($\beta$), hole speed ($M$), hole phase-space depth ($-\Gamma$) and hole potential amplitude ($\chi_0$) is derived. The approach introduces a new technique to study the phase-space holes of collision-less plasmas, allowing fluid-vortex-like treatment to these kinetic structures. Phase-space distribution functions for electron hole regions can then be analytically derived from this model, reproducing the schamel-df equations and thus acting as a precursor to the pseudo-potential approach, avoiding the need to assume a solution to the phase-space density.

3.ReMKiT1D -- A framework for building reactive multi-fluid models of the tokamak Scrape-Off Layer with coupled electron kinetics in 1D

Authors:Stefan Mijin, Dominic Power, Ryan Holden, William Hornsby, David Moulton, Fulvio Militello

Abstract: In this manuscript we present the recently developed flexible framework for building both fluid and electron kinetic models of the tokamak Scrape-Off Layer in 1D - ReMKiT1D (Reactive Multi-fluid and Kinetic Transport in 1D). The framework can handle systems of non-linear ODEs, various 1D PDEs arising in fluid modelling, as well as PDEs arising from the treatment of the electron kinetic equation. As such, the framework allows for flexibility in fluid models of the Scrape-Off Layer while allowing the easy addition of kinetic electron effects. We focus on presenting both the high-level design decisions that allow for model flexibility, as well as the most important implementation aspects. A significant number of verification and performance tests are presented, as well as a step-by-step walkthrough of a simple example for setting up models using the Python interface.

4.Interactions between flow fields induced by surface dielectric barrier discharge arrays

Authors:Alexander Böddecker, Maximilian Passmann, Sebastian Wilczek, Lars Schücke, Ihor Korolov, Romuald Skoda, Thomas Mussenbrock, Andrew R. Gibson, Peter Awakowicz

Abstract: This study investigates the flow field induced by a surface dielectric barrier discharge (SDBD) system, known for its efficient pollution remediation of volatile organic compounds (VOCs). We aim to understand the flow dynamics that contribute to the high conversion observed in similar systems. Experimental techniques, including schlieren imaging and particle image velocimetry (PIV), applied with high temporal resolution, were used to analyse the flow field. Complementary, fluid simulations are employed to investigate the coupling between streamer and gas dynamics. Results show distinct fluid field behaviours for different electrode configurations, which differ in geometric complexity. The fluid field analysis of the most basic electrode design revealed behaviours commonly observed in actuator studies. The simulation results indicate the local information about the electron density as well as different temporal phases of the fluid flow. The electrode design with mostly parallel grid line structures exhibits confined vortices near the surface. In contrast, an electrode design also used in previous studies, is shown to promote strong gas transport through extended vortex structures, enhancing gas mixing and potentially explaining the high conversion observed.

1.Experimental validation of particle-in-cell/Monte Carlo collisions simulations in low-pressure neon capacitively coupled plasmas

Authors:Chan-Won Park, Benedek Horváth, Aranka Derzsi, Julian Schulze, J. H. Kim, Zoltán Donkó, Hyo-Chang Lee

Abstract: Plasma simulations are powerful tools for understanding fundamental plasma science phenomena and for process optimization in applications. To ensure their quantitative accuracy, they must be validated against experiments. In this work, such an experimental validation is performed for a 1d3v particle-in-cell simulation complemented with the Monte Carlo treatment of collision processes of a capacitively coupled radio frequency plasma driven at 13.56 MHz and operated in neon gas. In a geometrically symmetric reactor the electron density in the discharge center and the spatio-temporal distribution of the electron impact excitation rate from the ground into the Ne 2p$_1$ state are measured by a microwave cutoff probe and phase resolved optical emission spectroscopy, respectively. The measurements are conducted for electrode gaps between 50 mm and 90 mm, neutral gas pressures between 20 mTorr and 50 mTorr, and peak-to-peak values of the driving voltage waveform between 250 V and 650 V. Simulations are performed under identical discharge conditions. In the simulations, various combinations of surface coefficients characterising the interactions of electrons and heavy particles with the anodized aluminium electrode surfaces are adopted. We find, that the simulations using a constant effective heavy particle induced secondary electron emission coefficient of 0.3 and a realistic electron-surface interaction model (which considers energy-dependent and material specific elastic and inelastic electron reflection, as well as the emission of true secondary electrons from the surface) yield results which are in good quantitative agreement with the experimental data.

2.Drive Asymmetry, Convergence and the Origin of Turbulence in ICF Implosions

Authors:Vincent A. Thomas, Robert J. Kares

Abstract: 2D and 3D numerical simulations with the adaptive mesh refinement Eulerian radiation-hydrocode RAGE are used to investigate hydrodynamic disruption of asymmetrically driven ICF implosions. A central aspect of this phenomenon is the connection between drive asymmetry and the generation of turbulence in the DT fuel. Long wavelength deviations from spherical symmetry in the pressure drive lead to the generation of coherent vortical structures in the DT gas and it is the three dimensional instability of these structures that in turn leads to turbulence and mix. RAGE simulations with spatial resolutions as high as 0.05 {\mu}m in 3D are presented to exhibit the detailed mechanisms of turbulence growth. These simulations suggest that the amplification of small initial surface imperfections by acceleration-induced instabilities is not the only important source of turbulent mix in ICF implosions as is commonly supposed. Rather, the three dimensional instability of coherent vortical structures induced in the gas by the asymmetries in the implosion are an additional important source of turbulent mixing in ICF, perhaps even the dominant source. The effect of convergence on the hydrodynamic disruption of an asymmetrically driven ICF capsule is also considered, demonstrating how higher convergence is expected to lead to greater hydrodynamic disruption by both large scale fingers of pusher material into the fuel as well as the formation of radially outgoing turbulent jets of fuel. Implications of these results for NIF ignition are discussed.

1.Improved Ion Heating in Fast Ignition by Pulse Shaping

Authors:Henry Fetsch, Nathaniel J. Fisch

Abstract: The fast ignition paradigm for inertial fusion offers increased gain and tolerance of asymmetry by compressing fuel at low entropy and then quickly igniting a small region. Because this hotspot rapidly disassembles, the ions must be heated to ignition temperature as quickly as possible, but most ignitor designs directly heat electrons. A constant-power ignitor pulse, which is generally assumed, is suboptimal for coupling energy from electrons to ions. Using a simple model of a hotspot in isochoric plasma, a novel pulse shape to maximize ion heating is presented in analytical form. Bounds are derived on the maximum ion temperature attainable by electron heating only. Moreover, arranging for faster ion heating allows a smaller hotspot, improving fusion gain. Under representative conditions, the optimized pulse can reduce ignition energy by 23%.

2.Relative magnetic helicity in multiply connected domains

Authors:David MacTaggart, Alberto Valli

Abstract: Magnetic helicity is a conserved quantity of ideal magnetohydrodynamics (MHD) that is related to the topology of the magnetic field, and is widely studied in both laboratory and astrophysical plasmas. When the magnetic field has a non-trivial normal component on the boundary of the domain, the classical definition of helicity must be replaced by relative magnetic helicity. The purpose of this work is to review the various definitions of relative helicity and to show that they have a common origin - a general definition of relative helicity in multiply connected domains. We show that this general definition is both gauge-invariant and is conserved in time under ideal MHD, subject only to closed and line-tied boundary conditions. Other, more specific, formulae for relative helicity, that are used frequently in the literature, are shown to follow from the general expression by imposing extra conditions on the magnetic field or its vector potential.

3.High-speed plasma measurements with a plasma impedance probe

Authors:John W. Brooks, Erik M. Tejero, Matthew C. Paliwoda, Michael S. McDonald

Abstract: Plasma impedance probes (PIPs) are a type of RF probe that primarily measure electron density. This work introduces two advancements: a streamlined analytical model for interpreting PIP-monopole measurements and techniques for achieving $\geq 1$ MHz time-resolved PIP measurements. The model's improvements include introducing sheath thickness as a measurement and providing a more accurate method for measuring electron density and damping. The model is validated by a quasi-static numerical simulation which compares the simulation with measurements, identifies sources of error, and provides probe design criteria for minimizing uncertainty. The improved time resolution is achieved by introducing higher-frequency hardware, updated analysis algorithms, and a more rigorous approach to RF calibration. Finally, the new model and high-speed techniques are applied to two datasets: a 4 kHz plasma density oscillation resolved at 100 kHz with densities ranging between $2 \times 10^{14}$ to $3 \times 10^{15}$ m$^{-3}$ and a 150 kHz oscillation resolved at 4 MHz with densities ranging between $4 \times 10^{14}$ to $6 \times 10^{14}$ m$^{-3}$.

1.Preferential acceleration of heavy ions in magnetic reconnection: Hybrid-kinetic simulations with electron inertia

Authors:Neeraj Jain, Jörg Büchner, Miroslav Bárta, Radoslav Bučík

Abstract: Solar energetic particles (SEPs) in the energy range 10s KeV/nucleon - 100s MeV/nucleon originate from Sun. Their high flux near Earth may damage the space borne electronics and generate secondary radiations harmful for the life on Earth and thus understanding their energization on Sun is important for space weather prediction. Impulsive (or ${}^{3}$He-rich) SEP events are associated with the acceleration of charge particles in solar flares by magnetic reconnection and related processes. The preferential acceleration of heavy ions and the extra-ordinary abundance enhancement of ${}^3$He in the impulsive SEP events are not understood yet. In this paper, we study ion acceleration in magnetic reconnection by two dimensional hybrid-kinetic plasma simulations (kinetic ions and inertial electron fluid). All the ions species are treated self-consistently in our simulations. We find that heavy ions are preferentially accelerated to energies many times larger than their initial thermal energies by a variety of acceleration mechanisms operating in reconnection. Most efficient acceleration takes place in the flux pileup regions of magnetic reconnection. Heavy ions with sufficiently small values of charge to mass ratio ($Q/M$) can be accelerated by pickup mechanism in outflow regions even before any magnetic flux is piled up. The energy spectra of heavy ions develop a shoulder like region, a non-thermal feature, as a result of the acceleration. The spectral index of the power law fit to the shoulder region of the spectra varies approximately as $(Q/M)^{-0.64}$. Abundance enhancement factor, defined as number of particles above a threshold energy normalized to total number of particles, scales as $(Q/M)^{-\alpha}$ where $\alpha$ increases with the energy threshold. We discuss our simulation results in the light of the SEP observations.

2.Effect of resonance magnetic perturbation on edge-core turbulence spreading in a tokamak plasma

Authors:Guangzhi Ren, Lai Wei, Zheng-Xiong Wang, Jiquan Li

Abstract: Turbulence spreading from edge to core region with resonance magnetic perturbation (RMP) is investigated using an electromagnetic Landau-fluid model in toroidal geometry. When RMP field with appropriate amplitude are employed in the simulation, long wavelength fluctuations around the resonance surface are excited due to the forced magnetic reconnection. Strong shear flow at the magnetic island separatrix are observed, and break the radial elongated vortex structures of the turbulent fluctuation. The inward flux could be blocked by this shear flow and the saturation level in the core region declines.

3.Magnetic Effect on the Potential Barrier for Nucleosynthesis

Authors:Kiwan Park

Abstract: We demonstrated that a weak magnetic field can increase the permittivity, leading to a reduction in the potential barrier within the Debye sphere consisting of electrons and a nucleus. By solving the Boltzmann equation with the inclusion of the magnetic field, we obtained the magnetized permittivity. The resulting enhanced permittivity field inversely decreases the potential barrier, thereby increasing the reaction rate between two fusing nuclei. We compared this Boltzmann kinetic approach with the Debye potential method. We found that they are qualitatively consistent. Further, we also derived the magnetized Debye potential composed of the conventional term with a new magnetic effect. Both approaches indicate that magnetized plasmas, which have existed since the Big Bang, have ultimately influenced permittivity, potential barrier, and nucleosynthesis.

4.Positron generation and acceleration in a self-organized photon collider enabled by an ultra-intense laser pulse

Authors:K. Sugimoto, Y. He, N. Iwata, I-L. Yeh, K. Tangtartharakul, A. Arefiev, Y. Sentoku

Abstract: We discovered a simple regime where a near-critical plasma irradiated by a laser of experimentally available intensity can self-organize to produce positrons and accelerate them to ultra-relativistic energies. The laser pulse piles up electrons at its leading edge, producing a strong longitudinal plasma electric field. The field creates a moving gamma-ray collider that generates positrons via the linear Breit-Wheeler process -- annihilation of two gamma-rays into an electron-positron pair. At the same time, the plasma field, rather than the laser, serves as an accelerator for the positrons. The discovery of positron acceleration was enabled by a first-of-its-kind kinetic simulation that generates pairs via photon-photon collisions. Using available laser intensities of $10^{22}$$\ $$\rm W/cm^2$, the discovered regime can generate a GeV positron beam with divergence angle of $\sim10^{\circ}$ and total charge of 0.1$\ $pC. The result paves the way to experimental observation of the linear Breit-Wheeler process and to applications requiring positron beams.

1.Identification of the melting line in the two-dimensional complex plasmas using an unsupervised machine learning method

Authors:Hu-Sheng Li, He Huang, Wei Yang, Cheng-Ran Du

Abstract: Machine learning methods have been widely used in the investigations of the complex plasmas. In this paper, we demonstrate that the unsupervised convolutional neural network can be applied to obtain the melting line in the two-dimensional complex plasmas based on the Langevin dynamics simulation results. The training samples do not need to be labeled. The resulting melting line coincides with those obtained by the analysis of hexatic order parameter and supervised machine learning method.

1.Towards practical reinforcement learning for tokamak magnetic control

Authors:Brendan D. Tracey, Andrea Michi, Yuri Chervonyi, Ian Davies, Cosmin Paduraru, Nevena Lazic, Federico Felici, Timo Ewalds, Craig Donner, Cristian Galperti, Jonas Buchli, Michael Neunert, Andrea Huber, Jonathan Evens, Paula Kurylowicz, Daniel J. Mankowitz, Martin Riedmiller, The TCV Team

Abstract: Reinforcement learning (RL) has shown promising results for real-time control systems, including the domain of plasma magnetic control. However, there are still significant drawbacks compared to traditional feedback control approaches for magnetic confinement. In this work, we address key drawbacks of the RL method; achieving higher control accuracy for desired plasma properties, reducing the steady-state error, and decreasing the required time to learn new tasks. We build on top of \cite{degrave2022magnetic}, and present algorithmic improvements to the agent architecture and training procedure. We present simulation results that show up to 65\% improvement in shape accuracy, achieve substantial reduction in the long-term bias of the plasma current, and additionally reduce the training time required to learn new tasks by a factor of 3 or more. We present new experiments using the upgraded RL-based controllers on the TCV tokamak, which validate the simulation results achieved, and point the way towards routinely achieving accurate discharges using the RL approach.

1.Comment on "Evolution Equations of Nonlinearly Permissible, Coherent Hole Structures Propagating Persistently in Collisionless Plasmas''

Authors:I H Hutchinson

Abstract: Recent critical remarks, published in "Annalen der Physik", about the present author's analysis of electron and ion holes and their stability are addressed and shown to be misunderstandings and misrepresentations.

2.Exploration of Non-Resonant Divertor Features on the Compact Toroidal Hybrid

Authors:K. A. Garcia University of Wisconsin - Madison, A. Bader University of Wisconsin - Madison Type One Energy, H. Frerichs University of Wisconsin - Madison, G. J. Hartwell Auburn University, J. C. Schmitt Auburn University Type One Energy, N. Allen Auburn University, O. Schmitz University of Wisconsin - Madison

Abstract: Non-resonant divertors (NRDs) separate the confined plasma from the surrounding plasma facing components (PFCs). The resulting striking field line intersection pattern on these PFCs is insensitive to plasma equilibrium effects. However, a complex scrape-off layer (SOL), created by chaotic magnetic topology in the plasma edge, connects the core plasma to the PFCs through varying magnetic flux tubes. The Compact Toroidal Hybrid (CTH) serves as a test-bed to study this by scanning across its inductive current. Simulations observe a significant change of the chaotic edge structure and an effective distance between the confined plasma and the instrumented wall targets. The intersection pattern is observed to be a narrow helical band, which we claim is a resilient strike line pattern. However, signatures of finger-like structures, defined as heteroclinic tangles in chaotic domains, within the plasma edge connect the island chains to this resilient pattern. The dominant connection length field lines intersecting the targets are observed via heat flux modelling with EMC3-EIRENE. At low inductive current levels, the excursion of the field lines resembles a limited plasma wall scenario. At high currents, a private flux region is created in the area where the helical strike line pattern splits into two bands. These bands are divertor legs with distinct SOL parallel particle flow channels. The results demonstrate the NRD strike line pattern resiliency within CTH, but also show the underlying chaotic edge structure determining if the configuration is diverted or limited. This work supports future design efforts for a mechanical structure for the NRD.

1.Normal mode analysis of fluid discontinuities: numerical method and application to magnetohydrodynamics

Authors:William Béthune

Abstract: Fluid discontinuities such as shock fronts and vortex sheets can reflect waves and become unstable to corrugation. Analytical calculations of these phenomena are tractable in the simplest cases only, while their numerical simulations are biased by truncation errors inherent to discretization schemes. The author lays down a computational framework to study the coupling of normal modes (plane linear waves) through discontinuities satisfying arbitrary conservation laws, as is relevant to a variety of fluid mechanical problems. A systematic method is provided to solve these problems numerically, along with a series of validation cases. As a demonstration, it is applied to magnetohydrodynamic shocks and shear layers to exactly recover their linear stability properties. The straightforward inclusion of nonideal (dispersive, dissipative) effects notably opens a route to investigate how these phenomena are altered in weakly ionized plasmas.

1.Analysis of a cusped helicon plasma thruster discharge

Authors:Pedro Jimenez, Jiewei Zhou, Jaume Navarro, Pablo Fajardo, Mario Merino, Eduardo Ahedo

Abstract: A compact helicon plasma thruster that features a cusp in its internal magnetic field is analyzed with experiments and simulations. A compensated Langmuir probe and a Faraday cup are used in the former, while a hybrid PIC/fluid transport model combined with a frequency-domain electromagnetic field model are used in the latter. Measurements serve to tune the anomalous transport parameters of the model and overall show the same trends as the numerical results, including a secondary peak of electron temperature downstream in the magnetic nozzle, where electron cyclotron resonance conditions for the 13.56 MHz excitation frequency are met. The cusp plays a central role in determining the plasma losses to the walls and the profile of electron temperature, which in turn defines the excitation and ionization losses. While losses to the rear wall are reduced, losses to the lateral wall are increased, which, together with the low production efficiency, limit the performance of the device. Shorter chamber lengths and optimization of antenna and cusp location are suggested as potential ways to improve performance.

1.Towards direct spatial and intensity characterization of ultra-high intensity laser pulses using ponderomotive scattering of free electrons

Authors:A. Longman, S. Ravichandran, L. Manzo, C. Z. He, R. Lera, N. McLane, M. Huault, G. Tiscareno, D. Hanggi, P. Spingola, N. Czapla, R. L. Daskalova, L. Roso, R. Fedosejevs, W. T. Hill

Abstract: Spatial distributions of electrons ionized and scattered from ultra-low pressure gases are proposed and experimentally demonstrated as a method to directly measure the intensity of an ultra-high intensity laser pulse. Analytic models relating the peak scattered electron energy to the peak laser intensity are derived and compared to paraxial Runge-Kutta simulations highlighting two models suitable for describing electrons scattered from weakly paraxial beams ($f_{\#}>5$) for intensities in the range of $10^{18}-10^{21}$Wcm$^{-2}$. Scattering energies are shown to be dependant on gas species emphasizing the need for specific gases for given intensity ranges. Direct measurements of the laser intensity at full power of two laser systems is demonstrated both showing a good agreement between indirect methods of intensity measurement and the proposed method. One experiment exhibited the role of spatial aberrations in the scattered electron distribution motivating a qualitative study on the effect. We propose the use of convolutional neural networks as a method for extracting quantitative information of the spatial structure of the laser at full power. We believe the presented technique to be a powerful tool that can be immediately implemented in many high-power laser facilities worldwide.

1.PIC simulations of stable surface waves on a subcritical fast magnetosonic shock front

Authors:M E Dieckmann, C Huete, F Cobos, A Bret, D Folini, B Eliasson, R Walder

Abstract: We study with particle-in-cell (PIC) simulations the stability of fast magnetosonic shocks. They expand across a collisionless plasma and an orthogonal magnetic field that is aligned with one of the directions resolved by the 2D simulations. The shock speed is 1.6 times the fast magnetosonic speed when it enters a layer with a reduced density of mobile ions, which decreases the shock speed by up to 15\% in 1D simulations. In the 2D simulations, the density of mobile ions in the layer varies sinusoidally perpendicularly to the shock normal. We resolve one sine period. This variation only leads to small changes in the shock speed evidencing a restoring force that opposes a shock deformation. As the shock propagates through the layer, the ion density becomes increasingly spatially modulated along the shock front and the magnetic field bulges out where the mobile ion density is lowest. The perturbed shock eventually reaches a steady state. Once it leaves the layer, the perturbations of the ion density and magnetic field oscillate along its front at a frequency close to the lower-hybrid frequency; the shock is mediated by a standing wave composed of obliquely propagating lower-hybrid waves. We perform three 2D simulations with different box lengths along the shock front. The shock front oscillations are aperiodically damped in the smallest box with the fastest variation of the ion density, strongly damped in the intermediate one, and weakly damped in the largest box. The shock front oscillations perturb the magnetic field in a spatial interval that extends by several electron skin depths upstream and downstream of the shock front and could give rise to Whistler waves that propagate along the shock's magnetic field overshoot. Similar waves were observed in hybrid and PIC simulations and by the MMS satellite mission.

1.Helicon waves in a converging-diverging magnetoplasma

Authors:Félicien Filleul, Antonella Caldarelli, Kazunori Takahashi, Rod Boswell, Christine Charles, John Cater, Nicholas Rattenbury

Abstract: Waves propagating along a converging-diverging rf magnetoplasma having the characteristics of a bounded m=0 helicon mode are reported and characterised. The discharge features a 30 cm separation between the region of radiofrequency energy deposition by a single loop antenna and the region of maximum magnetic field applied by a pair of coils. With 200 W of rf input power, the resulting plasma exhibits a strong axial plasma density gradient peaking at the magnetic mirror throat where an Ar II blue-core is observed. Two dimensional B-dot probe measurements show that the rf magnetic fields are closely guided by the converging-diverging geometry. The wave is characterised as a m=0 mode satisfying the helicon dispersion relation on-axis with radial boundary conditions approximately matching the radii of the plasma column. Analysis of the wave phase velocity and wave axial damping failed to identify collisionless or collisional wave-plasma coupling mechanisms. Instead, the wave axial amplitude variations can be explained by local wave resonances and possible reflections from localised rapid changes of the refractive index. A Venturi-like effect owing to the funnel-shaped magnetoplasma and conservation of the wave energy may also explain some level of amplitude variations.

1.Stationary striations in plasma, created by a short microwave pulse in a waveguide filled with a neutral gas

Authors:Y. Bliokh, Y. Cao, V. Maksimov, A. Chaim, J. G. Leopold, J. G. Leopold, Ya. E. Krasik

Abstract: It was observed experimentally that after crossing a neutral gas filled waveguide, a short powerful microwave pulse leaves a periodic glow of plasma along the waveguide, persisting several tens of nanoseconds. A theoretical model is presented which in combination with numerical simulations proposes a possible explanation of this phenomenon.

2.A self-sustaining mechanism for Internal Transport Barrier formation in HL-2A tokamak plasmas

Authors:W. H. Lin, J. Garcia, J. Q. Li, S. Mazzi, Z. J. Li, X. X. He, X. Yu

Abstract: The formation of Internal Transport Barrier (ITB) is studied in HL-2A plasmas by means of nonlinear gyrokinetic simulations. A new paradigm for the ITB formation is proposed in which different physics mechanisms play a different role depending on the ITB formation stage. In the early stage, fast ions, introduced by Neutral Beam Injection (NBI) ion system, are found to stabilize the thermal-ion-driven instability by dilution, thus reducing the ion heat fluxes and finally triggering the ITB. Such dilution effects, however, play a minor role after the ITB is triggered as electromagnetic effects are dominant in the presence of established high pressure gradients. We define the concept of ITB self-sustainment, as the low turbulence levels found within the fully formed ITB are consequences of large scale zonal flows, which in turn are fed by a non-linear interplay with large scale high frequency electromagnetic perturbations destabilized by the ITB itself.

3.Measurements of dense fuel hydrodynamics in the NIF burning plasma experiments using backscattered neutron spectroscopy

Authors:A. J. Crilly, D. J. Schlossberg, B. D. Appelbe, A. S. Moore, J. Jeet, S. M. Kerr, M. S. Rubery, B. Lahmann, S. O'Neill, C. J. Forrest, O. M. Mannion, J. P. Chittenden

Abstract: The hydrodynamics of the dense confining fuel shell is of great importance in defining the behaviour of the burning plasma and burn propagation regimes of inertial confinement fusion experiments. However, it is difficult to probe due to its low emissivity in comparison to the central fusion core. In this work, we utilise the backscattered neutron spectroscopy technique to directly measure the hydrodynamic conditions of the dense fuel during fusion burn. Experimental data is fit to obtain dense fuel velocities and apparent ion temperatures. Trends of these inferred parameters with yield and velocity of the burning plasma are used to investigate their dependence on alpha heating and low mode drive asymmetry. It is shown that the dense fuel layer has an increased outward radial velocity as yield increases showing burn has continued into re-expansion, a key signature of hotspot ignition. Comparison with analytic and simulation models show that the observed dense fuel parameters are displaying signatures of burn propagation into the dense fuel layer, including a rapid increase in dense fuel apparent ion temperature with neutron yield.

1.Core localized alpha-channeling via low frequency Alfven mode generation in reversed shear scenarios

Authors:Zhiyong Qiu, Shizhao Wei, Tao Wang, Liu Chen, Fulvio Zonca

Abstract: A novel channel for fuel ions heating in tokamak core plasma is proposed and analyzed using nonlinear gyrokinetic theory. The channel is achieved via spontaneous decay of reversed shear Alfv\'en eigenmode (RSAE) into low frequency Alfv\'en modes (LFAM), which then heat fuel ions via collisionless ion Landau damping. The conditions for RSAE spontaneous decay are investigated, and the saturation level and the consequent fuel ion heating rate are also derived. The channel is expected to be crucial for future reactors operating under reversed shear configurations, where fusion alpha particles are generated in the tokamak core where the magnetic shear is typically reversed, and there is a dense RSAE spectrum due to the small alpha particle characteristic dimensionless orbits.

2.Full-F Turbulent Simulation in a Linear Device using a Gyro-Moment Approach

Authors:B. J. Frei, J. Mencke, P. Ricci

Abstract: The first full-F and turbulent simulations based on the Gyro-Moment (GM) are presented by considering a linear device configuration with open and straight field lines. The simulations are based on a simplified version of the gyrokinetic (GK) model proposed by B. J. Frei et al. [J. Plasma Phys. 86, 905860205 (2020)]. By focusing on the electrostatic and long-wavelength limit, a full-F GM hierarchy equation is derived to evolve the ion dynamics, which includes a nonlinear Dougherty collision operator, localized sources, and Bohm sheath boundary conditions. An electron fluid Braginskii model is used to evolve the electron dynamics, coupled to the full-F ion GM hierarchy equation via a vorticity equation. A set of full-F turbulent simulations is performed using the parameters of the LAPD experiments with different numbers of GMs and regimes of collisionality. The GM results (time-averaged profiles and turbulent properties) are compared with those from two-fluid Braginskii simulations, finding good qualitative agreement. Furthermore, the ion distribution function is analyzed, showing the good convergence properties of the GM approach.

3.Inverse cascading for initial MHD turbulence spectra between Saffman and Batchelor

Authors:Axel Brandenburg, Ramkishor Sharma, Tanmay Vachaspati

Abstract: In decaying magnetohydrodynamic (MHD) turbulence with a strong magnetic field, the spectral magnetic energy density increases with time at small wavenumbers $k$, provided the spectrum at low $k$ is sufficiently steep. This is inverse cascading and occurs for an initial Batchelor spectrum, where the magnetic energy per linear wavenumber interval increases like $k^4$. For an initial Saffman spectrum that is proportional to $k^2$, however, inverse cascading is known not to occur. We study here the case of an intermediate $k^3$ spectrum, which may be relevant for magnetogenesis in the early Universe during the electroweak epoch. This case is not well understood in view of the standard Taylor expansion of the magnetic energy spectrum for small $k$. Using high resolution MHD simulations, we show that also in this case there is inverse cascading with a strength just as expected from the conservation of the Hosking integral, which governs the decay of an initial Batchelor spectrum.

1.Species dependence of the impurity injection induced poloidal flow and magnetic island rotation in a tokamak

Authors:Shiyong Zeng, Ping Zhu, Haijun Ren

Abstract: Recent experiments have demonstrated the species dependence of the impurity poloidal drift direction along with the magnetic island rotation in the poloidal plane. Our resistive MHD simulations have reproduced such a dependence of the impurity poloidal flow, which is found mainly determined by a local plasmoid formation due to the impurity injection. The synchronized magnetic island rotation is dominantly driven by the electromagnetic torque produced by the impurity radiation primarily through the modification to the axisymmetric components of current density.

2.First bromine doped cryogenic implosion at the National Ignition Facility

Authors:A. C. Hayes, G. Kyrala, M. Gooden, J. B. Wilhelmy, L. Kot, P. Volegov, C. Wilde, B. Haines, Gerard Jungman, R. S. Rundberg, D. C. Wilson, C. Velsko, W. Cassata, E. Henry, C. Yeamans, C. Cerjan, T. Ma, T. Doppner, A. Nikroo, O. Hurricane, D. Callahan, D. Hinkel, D. Schneider, B. Bachmann, F. Graziani, K. C. Chen, C. Kong, H. Huang, J. W. Crippen, M. Ratledge, N. G. Rice, M. P. Farrell

Abstract: We report on the first experiment dedicated to the study of nuclear reactions on dopants in a cryogenic capsule at the National Ignition Facility (NIF). This was accomplished using bromine doping in the inner layers of the CH ablator of a capsule identical to that used in the NIF shot N140520. The capsule was doped with 3$\times$10$^{16}$ bromine atoms. The doped capsule shot, N170730, resulted in a DT yield that was 2.6 times lower than the undoped equivalent. The Radiochemical Analysis of Gaseous Samples (RAGS) system was used to collect and detect $^{79}$Kr atoms resulting from energetic deuteron and proton ion reactions on $^{79}$Br. RAGS was also used to detect $^{13}$N produced dominantly by knock-on deuteron reactions on the $^{12}$C in the ablator. High-energy reaction-in-flight neutrons were detected via the $^{209}$Bi(n,4n)$^{206}$Bi reaction, using bismuth activation foils located 50 cm outside of the target capsule. The robustness of the RAGS signals suggest that the use of nuclear reactions on dopants as diagnostics is quite feasible.

1.Kinetic theory for spin-polarized relativistic plasmas

Authors:Daniel Seipt, Alec G. R. Thomas

Abstract: The investigation of spin and polarization effects in ultra-high intensity laser-plasma and laser-beam interactions has become an emergent topic in high-field science recently. In this paper we derive a relativistic kinetic description of spin-polarized plasmas, where QED effects are taken into account via Boltzmann-type collision operators under the local constant field approximation. The emergence of anomalous precession is derived from one-loop self-energy contributions in a strong background field. We are interested, in particular, in the interplay between radiation reaction effects and the spin polarization of the radiating particles. For this we derive equations for spin-polarized quantum radiation reaction from moments of the spin-polarized kinetic equations. By comparing with the classical theory, we identify and discuss the spin-dependent radiation reaction terms, and radiative contributions to spin dynamics.

2.Available energy of trapped electrons in Miller tokamak equilibria

Authors:R. J. J. Mackenbach, J. H. E. Proll, G. Snoep, P. Helander

Abstract: Available energy ({\AE}), which quantifies the maximum amount of thermal energy that may be liberated and converted into instabilities and turbulence, has shown to be a useful metric for predicting saturated energy fluxes in trapped-electron-mode-driven turbulence. Here, we calculate and investigate the \AE{} in the analytical tokamak equilibria introduced by \citet{Miller1998NoncircularModel}. The \AE{} of trapped electrons reproduces various trends also observed in experiments; negative shear, increasing Shafranov shift, and negative triangularity can all be stabilising as indicated by a reduction in \AE{}, though it is strongly dependent on the chosen equilibrium. We find that negative triangularity is especially beneficial in vertically elongated configurations with positive shear, or low gradients. We furthermore extract a gradient-threshold like quantity from \AE{} and find that it behaves similarly to gyrokinetic gradient-thresholds: it tends to increase linearly with magnetic shear, and negative triangularity leads to an especially high threshold. We next optimise device geometry for minimal \AE{} and find that the optimum is strongly dependent on equilibrium parameters, e.g. magnetic shear or pressure gradient. If one furthermore investigates the competing effects of increasing the density gradient, pressure gradient, and decreasing the shear, one finds regimes which have steep gradients yet low \AE{}, and that such a regime is inaccessible in negative-triangularity tokamaks. We finally compare \AE{} with saturated heat-flux estimates from the \textsc{tglf} model and find fairly good correspondence.

1.Electromagnetic gyrokinetic instabilities in the Spherical Tokamak for Energy Production (STEP) part II: transport and turbulence

Authors:Maurizio Giacomin, Daniel Kennedy, Francis J Casson, Ajay C. J., David Dickinson, Bhavin S. Patel, Colin M. Roach

Abstract: In this work, we present the results of first-of-their-kind nonlinear local gyrokinetic simulations of electromagnetic turbulence at mid-radius in the burning plasma phase of the conceptual high-$\beta$, reactor-scale, tight-aspect-ratio tokamak STEP (Spherical Tokamak for Energy Production). A prior linear analysis in D.Kennedy et al. submitted to Nucl. Fusion [1] reveals the presence of unstable hybrid kinetic ballooning modes and subdominant microtearing modes at binormal scales approaching the ion- Larmor radius. Local nonlinear gyrokinetic simulations, using three different codes, are in qualitative and quantitative agreement and suggest that hybrid kinetic ballooning modes drive very large turbulent transport in the absence of equilibrium flow shear. The heat flux rises to values that exceed the available heating power by orders of magnitude and the turbulent eddies are highly extended radially so that they may not be well described by the local gyrokinetic model. The saturated transport fluxes are extremely sensitive to equilibrium flow shear, and diamagnetic levels of flow shear can suppress the fluxes to more reasonable values on the chosen surface. Given this sensitivity there is a large uncertainty in the saturated fluxes. The possible transport impact of the subdominant microtearing modes is also analysed in isolation by artificially and unphysically removing compressional magnetic perturbations from nonlinear calculations, to suppress the dominant hybrid kinetic ballooning mode. The microtearing heat flux is found to saturate at negligible values, though we cannot exclude the possibility that microtearing turbulence may be more transport relevant in other regions of parameter space.

2.Electromagnetic gyrokinetic instabilities in the Spherical Tokamak for Energy Production (STEP) part I: linear physics and sensitivity

Authors:Daniel Kennedy, Maurizio Giacomin, Francis J Casson, David Dickinson, William A Hornsby, Bhavin S Patel, Colin M Roach

Abstract: We present herein the results of a linear gyrokinetic analysis of electromagnetic microinstabilites in the conceptual high-$\beta$, reactor-scale, tight-aspect-ratio tokamak STEP (Spherical Tokamak for Energy Production). We examine a range of flux surfaces between the deep core and the pedestal top for the two candidate flat-top operating points of the prototype device (EC and EBW operating points). Local linear gyrokinetic analysis is performed to determine the type of microinstabilities that arise under these reactor-relevant conditions. We find that the equilibria are dominated by a hybrid version of the Kinetic Ballooning Mode (KBM) instability at ion binormal and radial scales, with collisional Microtearing Modes (MTMs) sub-dominantly unstable at very similar binormal scales but different radial scales. We study the sensitivity of these instabilities to physics parameters, and discuss potential mechanisms for mitigating them. The results of this investigation are compared to a small set of similar conceptual reactor designs in the literature. A detailed benchmark of the linear results is performed using three gyrokinetic codes; alongside extensive resolution testing and sensitivity to numerical parameters providing confidence in the results of our calculations, and paving the way for detailed nonlinear studies in a companion article.

3.Stochastic and self-consistent 3D modeling of streamer discharge trees with Kinetic Monte Carlo

Authors:Robert Marskar

Abstract: This paper contains the foundation for a new Particle-In-Cell model for gas discharges, based on Ito diffusion and Kinetic Monte Carlo (KMC). In the new model the electrons are described with a microscopic drift-diffusion model rather than a macroscopic one. We discuss the connection of the Ito-KMC model to the equations of fluctuating hydrodynamics and the advection-diffusion-reaction equation which is conventionally used for simulating streamer discharges. The new model is coupled to a particle description of photoionization, providing a non-kinetic all-particle method with several attractive properties, such as: 1) Taking the same input as a fluid model, e.g. mobility coefficients, diffusion coefficients, and reaction rates. 2) Guaranteed non-negative densities. 3) Intrinsic support for reactive and diffusive fluctuations. 4) Exceptional stability properties. The model is implemented as a particle-mesh model on cut-cell grids with Cartesian adaptive mesh refinement. Positive streamer discharges in atmospheric air are considered as the primary application example, and we demonstrate that we can self-consistently simulate large discharge trees.

1.Electron slingshot acceleration in relativistic preturbulent shocks explored via emitted photon polarization

Authors:Zheng Gong, Xiaofei Shen, Karen Z. Hatsagortsyan, Christoph H. Keitel

Abstract: Electron acceleration mechanisms near the counterstreaming interface of a relativistic collisionless shock (RCS) are investigated using particle-in-cell (PIC) simulations. We identify a slingshot-like injection process induced by the drifting electric field sustained by the flowing focus of backwards-moving electrons, which is distinct from the well-known stochastic acceleration. The flowing focus signifies the plasma kinetic transition from a preturbulent laminar motion to a chaotic turbulence. We find a characteristic correlation between the electron dynamics in the slingshot acceleration and the photon emission features. In particular, the integrated radiation from the RCS exhibits a counterintuitive non-monotonic dependence of the photon polarization degree on the photon energy, which originates from a polarization degradation of relatively high-energy photons emitted by the slingshot-injected electrons. Our results demonstrate the potential of photon polarization as an essential information source in exploring intricate dynamics in RCSs with relevance for earth-based plasma and astrophysical scenarios.

2.Generation of narrow beams of ultrarelativistic positrons (electrons) in the resonant strong electromagnetic field-assisted Breit-Wheeler process

Authors:S. P. Roshchupkin, V. D. Serov, V. V. Dubov

Abstract: The resonant external field-assisted Breit-Wheeler process (Oleinik resonances) for strong electromagnetic fields with intensities less than the critical Schwinger field has been theoretically studied. The resonant kinematics has been studied in detail. The case of high-energy initial gamma quanta and emerging ultrarelativistic electron-positron pairs is studied. The resonant differential cross section is obtained. The generation of narrow beams of ultrarelativistic positrons (for Channel A) and electrons (for Channel B) is predicted with a probability significantly exceeding corresponding to the non-resonant process.

1.Robust stellarator optimization via flat mirror magnetic fields

Authors:J. L. Velasco, I. Calvo, E. Sánchez, F. I. Parra

Abstract: Stellarator magnetic configurations need to be optimized in order to meet all the required properties of a fusion reactor. In this work, it is shown that a flat-mirror quasi-isodynamic configuration (i.e. a quasi-isodynamic configuration with sufficiently small radial variation of the mirror term) can achieve small radial transport of energy and good confinement of bulk and fast ions even if it is not very close to perfect omnigeneity, and for a wide range of plasma scenarios, including low $\beta$ and small radial electric field. This opens the door to constructing better stellarator reactors. On the one hand, they would be easier to design, as they would be robust against error fields. On the other hand, they would be easier to operate since, both during startup and steady-state operation, they would require less auxiliary power, and the damage to plasma-facing components caused by fast ion losses would be reduced to acceptable levels.

2.Evidence of free-bound transitions in warm dense matter and their impact on equation-of-state measurements

Authors:Maximilian P. Böhme, Luke B. Fletcher, Tilo Döppner, Dominik Kraus, Andrew D. Baczewski, Thomas R. Preston, Michael J. MacDonald, Frank R. Graziani, Zhandos A. Moldabekov, Jan Vorberger, Tobias Dornheim

Abstract: Warm dense matter (WDM) is now routinely created and probed in laboratories around the world, providing unprecedented insights into conditions achieved in stellar atmospheres, planetary interiors, and inertial confinement fusion experiments. However, the interpretation of these experiments is often filtered through models with systematic errors that are difficult to quantify. Due to the simultaneous presence of quantum degeneracy and thermal excitation, processes in which free electrons are de-excited into thermally unoccupied bound states transferring momentum and energy to a scattered x-ray photon become viable. Here we show that such free-bound transitions are a particular feature of WDM and vanish in the limits of cold and hot temperatures. The inclusion of these processes into the analysis of recent X-ray Thomson Scattering experiments on WDM at the National Ignition Facility and the Linac Coherent Light Source significantly improves model fits, indicating that free-bound transitions have been observed without previously being identified. This interpretation is corroborated by agreement with a recently developed model-free thermometry technique and presents an important step for precisely characterizing and understanding the complex WDM state of matter.

3.Reduction in benefits of total flux expansion on divertor detachment due to parallel flows

Authors:M. Carpita, O. Février, H. Reimerdes, C. Theiler, B. P. Duval, C. Colandrea, G. Durr-Legoupil-Nicoud, D. Galassi, S. Gorno, E. Huett, J. Loizu, L. Martinelli, A. Perek, L. Simons, G. Sun, E. Tonello. C. Wüthrich, the TCV team

Abstract: The Super-X divertor (SXD) is an alternative divertor configuration leveraging total flux expansion at the outer strike point (OSP). Key features for the attractiveness of the SXD are facilitated detachment access and control, as predicted by the extended 2-point model (2PM). However, parallel flows are not consistently included in the 2PM. In this work, the 2PM is refined to overcome this limitation: the role of total flux expansion on the pressure balance is made explicit, by including the effect of parallel flows. In consequence, the effect of total flux expansion on detachment access and control is weakened, compared to predictions of the 2PM. This new model partially explains discrepancies between the 2PM and experiments performed on TCV, in ohmic L-mode scenarios, where in core density ramps in lower single-null (SN) configuration, the impact of the OSP major radius Rt on the CIII emission front movement in the divertor outer leg - used as a proxy for the plasma temperature - is substantially weaker than 2PM predictions; and in OSP sweeps in lower and upper SN configurations, with a constant core density, the peak parallel particle flux density at the OSP is almost independent of Rt, while the 2PM predicts a linear dependence. Finally, analytical and numerical modelling of parallel flows in the divertor is presented, to support the argument. It is shown that an increase in total flux expansion can favour supersonic flows at the OSP. Parallel flows are also shown to be relevant by analysing SOLPS-ITER simulations of TCV.


Authors:G. S. Bisnovatyi-Kogan, I. A. Kondratyev, S. G. Moiseenko

Abstract: Interaction of plasma flow with a magnetic obstacles is a frequent process in many laser-plasma experiments in the laboratory, and is an important event in many astrophysical objects: X-ray pulsars, AGN, GRB etc. As a result of plasma penetration through the magnetic wall we could expect a formation of MHD shock waves, as well as of electromagnetic ones. To study these processes we need equations following from hydrodynamic and Maxwell equations, which in the limiting situations describe MHD and EM waves, and are valid for the general case, when both phenomena are present. Here we derive a set of equations following from HD and Maxwell equation, without neglecting a displacement current, needed for a formation of EM waves. We find a dispersion equation describing a propagation of a weak linear wave in a magnetized plasma along the $x$ axis, perpendicular to the magnetic field $H_z(x)$, which contains MHD, HD and EM waves in the limiting cases, and some new types of behaviour in a general situation. We consider a plasma with zero viscosity and heat conductivity, but with a finite electro-conductivity with a scalar coefficient.

5.Interparticle-Fields Amplified Radiation Reaction

Authors:Michael J. Quin, Antonino Di Piazza, Christoph H. Keitel, Matteo Tamburini

Abstract: In classical electrodynamics, energy losses due to the emission of electromagnetic radiation can be accounted for by solving the Landau-Lifshitz equation of motion. Analytically, this equation is typically solved while treating each particle independently in an external field; numerically, one often includes a self-consistent mean field, as seen with particle-in-cell (PIC) codes. In both cases, interparticle fields from point-like particles are neglected. By considering the collision of a neutral relativistic electron-positron bunch with an intense laser pulse, we demonstrate that the inclusion of interparticle fields can coherently amplify a broad range of radiated frequencies by orders of magnitude. This corresponds to an amplified energy loss by particles within the bunch, with interparticle fields that feed into the radiation reaction force.

6.Machine learning for advancing low-temperature plasma modeling and simulation

Authors:Jan Trieschmann, Luca Vialetto, Tobias Gergs

Abstract: Machine learning has had an enormous impact in many scientific disciplines. Also in the field of low-temperature plasma modeling and simulation it has attracted significant interest within the past years. Whereas its application should be carefully assessed in general, many aspects of plasma modeling and simulation have benefited substantially from recent developments within the field of machine learning and data-driven modeling. In this survey, we approach two main objectives: (a) We review the state-of-the-art focusing on approaches to low-temperature plasma modeling and simulation. By dividing our survey into plasma physics, plasma chemistry, plasma-surface interactions, and plasma process control, we aim to extensively discuss relevant examples from literature. (b) We provide a perspective of potential advances to plasma science and technology. We specifically elaborate on advances possibly enabled by adaptation from other scientific disciplines. We argue that not only the known unknowns, but also unknown unknowns may be discovered due to an inherent propensity to spotlight hidden patterns in data.

1.Kinetic Simulation of Ion Thruster Plume Neutralization in a Vacuum Chamber

Authors:Keita Nishii, Deborah Levin

Abstract: The electrical environment of a ground vacuum testing chamber creates facility effects for gridded ion thrusters. For example, it is well known that the plume from the thruster generates current paths that are very different from what occurs in space, and the neutralization of this plume is also different. For reasons such as this, it is important to clarify how the experimental testing environment affects plasma flows, but understanding this effect solely through ground experiments is difficult. To that end, this study utilizes particle-in-cell and direct simulation Monte Carlo methods to simulate xenon beam ions and electrons emitted from a neutralizer. First, we compare simulations conducted within the chamber to those conducted in space, demonstrating that grounded chamber walls increase the electric potential and electron temperature. Next, we investigate the impact of the neutralizer's position and the background pressure on the plume in the vacuum chamber. We find that as the neutralizer position moves closer to the location of maximum potential, more electrons are extracted, resulting in increased neutralization of the plume. We also observe that high background pressure generates slow charge-exchange ions, creating ion sheaths on the side walls that alter ion current paths. Finally, we discuss how the potential at the thruster and neutralizer exits affects the plume. The relative potential of the neutralizer to the vacuum chamber wall is observed to significantly influence the behavior of the electrons, thereby altering the degree of plume neutralization. These findings are shown to be consistent with experimental results in the literature and demonstrate the promise of high-performance simulation.

2.Coherent radiation of electrons in an intense laser pulse

Authors:Evgeny Gelfer, Alexander Fedotov, Ondrej Klimo, Stefan Weber

Abstract: We study the conditions for coherent radiation of a group of electrons driven by a strong pulsed electromagnetic plane wave. The required conditions are first analyzed for a couple of neighboring electrons having the same velocities, then generalized to their macroscopic number and the range of parameters is identified to optimize the effect. We show that for a solid density plasma coherence affects the frequencies up to hundreds of keV, and the low-frequency part of the spectrum can be enhanced by many orders of magnitude. Our analytical findings are tested with 3D particle-in-cell simulations of a dense electron bunch passing through a laser pulse, clearly demonstrating how the coherence can essentially modify the observed radiation spectrum.

3.Initial Fulcher band observations from high resolution spectroscopy in the MAST-U divertor

Authors:N. Osborne, K. Verhaegh, M. D. Bowden, T. Wijkamp, N. Lonigro, P. Ryan, B. Lipschultz, V. Soukhanovskii, T. van den Biggelaar, the MAST-U team

Abstract: High resolution Fulcher band spectroscopy was used in the MAST-U divertors during Super-X and elongated conventional divertor density ramps with $\text{D}_{2}$ fuelling from the mid-plane high-field side. In the Super-X case (density ramp from Greenwald fraction 0.12 to 0.24), the upper divertor showed ground state rotational temperatures of the $\text{D}_{2}$ molecules increasing from $\sim$6000 K, starting at the detachment onset, to $\sim$9000 K during deepening detachment. This was correlated with the movement of the Fulcher emission region, which is correlated with the ionisation source. The increase in rotational temperature did not occur near the divertor entrance, where the plasma was still ionising. Qualitative agreement was obtained between the lower and upper divertor. Similar rotational temperatures were obtained in the elongated divertor before the detachment onset, although the increase in rotational temperature during detachment was less clearly observed as less deep detachment was obtained. %In the elongated conventional divertor there was some qualitative agreement of this effect impeded by low signal. The measured vibrational distribution of the upper Fulcher state (first four bands) does not agree with a ground state Boltzmann distribution but shows a different characteristic with an elevated population especially in the $\nu = 2$ and $\nu = 3$ bands. The populations of the $\nu = 2$ and $\nu = 3$ band relative to the $\nu = 0$ band are roughly proportional to the $\textit{rotational}$ temperature.

4.RF plugging of multi-mirror machines

Authors:Tal Miller, Ilan Be'ery, Eli Gudinetsky, Ido Barth

Abstract: One of the main challenges of fusion reactors based on magnetic mirrors is the axial particle loss through the loss cones. In multi-mirror (MM) systems, the particle loss is addressed by adding mirror cells on each end of the central fusion cell. Coulomb collisions in the MM sections serve as the retrapping mechanism for the escaping particles. Unfortunately, the confinement time in this system only scales linearly with the number of cells in the MM sections and requires an unreasonably large number of cells to satisfy the Lawson criterion. Here, it is suggested to reduce the outflow by applying a traveling RF electric field that mainly targets the particles in the outgoing loss cone. The Doppler shift compensates for the detuning of the RF frequency from the ion cyclotron resonance mainly for the escaping particles resulting in a selectivity effect. The transition rates between the different phase space populations are quantified via single-particle calculations and then incorporated into a semi-kinetic rate equations model for the MM system, including the RF effect. It is found that for optimized parameters, the confinement time can scale exponentially with the number of MM cells, orders of magnitude better than a similar MM system of the same length but without the RF plugging, and can satisfy the Lawson criterion for a reasonable system size.

5.Density jump as a function of magnetic field strength for perpendicular collisionless shocks with anisotropic upstream pressure

Authors:Antoine Bret

Abstract: Shock waves are common in astrophysical environments. On many occasions, they are collisionless, which means they occur in settings where the mean free path is much larger than the dimensions of the system. For this very reason, magnetohydrodynamic (MHD) is not equipped to deal with such shocks, be it because it assumes binary collisions, hence temperature isotropy, when such isotropy is not guaranteed in the absence of collisions. Here we solve a model capable of dealing with perpendicular shocks with anisotropic upstream pressure. The system of MHD conservation equations is closed assuming the temperature normal to the flow is conserved at the crossing of the shock front. In the strong shock sonic limit, the behavior of a perpendicular shock with isotropic upstream is retrieved, regardless of the upstream anisotropy. Generally speaking, a rich variety of behaviors is found, inaccessible to MHD, depending on the upstream parameters. The present work can be viewed as the companion paper of MNRAS 520, 6083-6090 (2023), where the case of a parallel shock was treated. Differences and similarities with the present case are discussed.

6.Annihilation-Gamma-based Diagnostic Techniques for Magnetically Confined Electron-Positron Pair Plasma

Authors:J. von der Linden, S. Nißl, A. Deller, J. Horn-Stanja, J. R. Danielson, M. R. Stoneking, A. Card, T. Sunn Pedersen, E. V. Stenson

Abstract: Efforts are underway to magnetically confine electron--positron pair plasmas to study their unique behavior, which is characterized by significant changes in plasma time and length scales, supported waves, and unstable modes. However, use of conventional plasma diagnostics presents challenges with these low-density and annihilating matter-antimatter plasma. To address this problem, we propose to develop techniques based on the distinct emission provided by annihilation. This emission exhibits two spatial correlations: the distance attenuation of isotropic sources and the back-to-back propagation of momentum-preserving 2-$\gamma$ annihilation. We present the results of our analysis of the $\gamma$ emission rate and the spatial profile of the annihilation in a magnetized pair plasma from direct pair collisions, from the formation and decay of positronium, as well as from transport processes. In order to demonstrate the effectiveness of annihilation-based techniques, we tested them on annular $\gamma$ emission profiles produced by a $\beta^+$ radioisotope on a rotating turntable. Direct and positronium-mediated annihilation result in overlapping volumetric $\gamma$ sources, and the 2-$\gamma$ emission from these volumetric sources can be tomographically reconstructed from coincident counts in multiple detectors. Transport processes result in localized annihilation where field lines intersect walls, limiters, or internal magnets. These localized sources can be identified by the fractional $\gamma$ counts on spatially distributed detectors.

1.Nonlinear interaction of Alfvénic instabilities and turbulence via the modification of the equilibrium profiles

Authors:A. Biancalani, A. Bottino, D. Del Sarto, M. V. Falessi, T. Hayward-Schneider, P. Lauber, A. Mishchenko, B. Rettino, J. N. Sama, F. Vannini, L. Villard, X. Wang, F. Zonca

Abstract: Nonlinear simulations of Alfv\'en modes (AM) driven by energetic particles (EP) in the presence of turbulence are performed with the gyrokinetic particle-in-cell code ORB5. The AMs carry a heat flux, and consequently they nonlinearly modify the plasma temperature profiles. The isolated effect of this modification on the dynamics of turbulence is studied, by means of electrostatic simulations. We find that turbulence is reduced when the profiles relaxed by the AM are used, with respect to the simulation where the unperturbed profiles are used. This is an example of indirect interaction of EPs and turbulence. First, an analytic magnetic equilibrium with circular concentric flux surfaces is considered as a simplified example for this study. Then, an application to an experimentally relevant case of ASDEX Upgrade is discussed.

1.On Nonlinear Scattering of Drift Wave by Toroidal Alfven Eigenmode in Tokamak Plasmas

Authors:Liu Chen, Zhiyong Qiu, Fulvio Zonca

Abstract: Using electron drift wave (eDW) as a paradigm model, we have investigated analytically direct wave-wave interactions between a test DW and ambient toroidal Alfv\'en eigenmodes (TAE) in toroidal plasmas, and their effects on the stability of the eDW. The nonlinear effects enter via scatterings to short-wavelength electron Landau damped kinetic Alfv\'en waves (KAWs). Specifically, it is found that scatterings to upper-sideband KAW lead to stimulated absorption of eDW. Scatterings to the lower-sideband KAW, on the contrary, lead to its spontaneous emission. As a consequence, for typical parameters and fluctuation intensity, nonlinear scatterings by TAE have negligible net effects on the eDW stability; in contrast to the ``reverse" process investigated in Ref. [Nuclear Fusion {\bf 62}, 094001 (2022)], where it is shown that nonlinear scattering by ambient eDW may lead to significant damping of TAE.

2.The Effect of Ultrastrong Magnetic Fields on Laser-Produced Gamma-Ray Flashes

Authors:Prokopis Hadjisolomou, Rashid Shaisultanov, Tae Moon Jeong, Petr Valenta, Sergey Vladimirovich Bulanov

Abstract: Laser produced $gamma$-photons can make an important impact on applied and fundamental physics that require high $gamma$-photon yield and strong collimation. We propose addition of a constant magnetic field to the laser-solid interaction to obtain the aforementioned desired $gamma$-photon properties. The $gamma$-ray flash spatial and spectral characteristics are obtained via quantum electrodynamics particle-in-cell simulations. When the constant magnetic field aligns with the laser magnetic field then the $gamma$-ray emission is significantly enhanced. Moreover, the $gamma$a-photon spatial distribution becomes collimated, approximately in the form of a disk.

3.Gyrokinetic theory of toroidal Alfvén eigenmode saturation via nonlinear wave-wave coupling

Authors:Zhiyong Qiu, Liu Chen, Fulvio Zonca

Abstract: Nonlinear wave-wave coupling constitutes an important route for the turbulence spectrum evolution in both space and laboratory plasmas. For example, in a reactor relevant fusion plasma, a rich spectrum of symmetry breaking shear Alfv\'en wave (SAW) instabilities are expected to be excited by energetic fusion alpha particles, and self-consistently determine the anomalous alpha particle transport rate by the saturated electromagnetic perturbations. In this work, we will show that the nonlinear gyrokinetic theory is a necessary and powerful tool in qualitatively and quantitatively investigating the nonlinear wave-wave coupling processes. More specifically, one needs to employ the gyrokinetic approach in order to account for the breaking of the ``pure Alfv\'enic state" in the short wavelength kinetic regime, due to the short wavelength structures associated with nonuniformity intrinsic to magnetically confined plasmas. Using well-known toroidal Alfv\'en eigenmode (TAE) as a paradigm case, three nonlinear wave-wave coupling channels expected to significantly influence the TAE nonlinear dynamics are investigated to demonstrate the strength and necessity of nonlinear gyrokinetic theory in predicting crucial processes in a future reactor burning plasma. These are: 1. the nonlinear excitation of meso-scale zonal field structures via modulational instability and TAE scattering into short-wavelength stable domain; 2. the TAE frequency cascading due to nonlinear ion induced scattering and the resulting saturated TAE spectrum; and 3. the cross-scale coupling of TAE with micro-scale ambient drift wave turbulence and its effect on TAE regulation and anomalous electron heating.

1.Data-driven plasma modelling: Surrogate collisional radiative models of fluorocarbon plasmas from deep generative autoencoders

Authors:Gregory A. Daly, Jonathan E. Fieldsend, Geoff Hassall, Gavin Tabor

Abstract: We have developed a deep generative model that can produce accurate optical emission spectra and colour images of an ICP plasma using only the applied coil power, electrode power, pressure and gas flows as inputs -- essentially an empirical surrogate collisional radiative model. An autoencoder was trained on a dataset of 812,500 image/spectra pairs in argon, oxygen, Ar/O\textsubscript{2}, CF\textsubscript{4}/O\textsubscript{2} and SF\textsubscript{6}/O\textsubscript{2} plasmas in an industrial plasma etch tool, taken across the entire operating space of the tool. The autoencoder learns to encode the input data into a compressed latent representation and then decode it back to a reconstruction of the data. We learn to map the plasma tool's inputs to the latent space and use the decoder to create a generative model. The model is very fast, taking just over 10 s to generate 10,000 measurements on a single GPU. This type of model can become a building block for a wide range of experiments and simulations. To aid this, we have released the underlying dataset of 812,500 image/spectra pairs used to train the model, the trained models and the model code for the community to accelerate the development and use of this exciting area of deep learning. Anyone can try the model, for free, on Google Colab.

1.Modeling of a Liquid Leaf Target TNSA Experiment using Particle-In-Cell Simulations and Deep Learning

Authors:Benedikt Schmitz, Daniel Kreuter, Oliver Boine-Frankenheim

Abstract: Liquid leaf targets show promise as high repetition rate targets for laser-based ion acceleration using the Target Normal Sheath Acceleration (TNSA) mechanism and are currently under development. In this work, we discuss the effects of different ion species and investigate how they can be leveraged for use as a possible laser-driven neutron source. To aid in this research, we develop a surrogate model for liquid leaf target laser-ion acceleration experiments, based on artificial neural networks. The model is trained using data from Particle-In-Cell (PIC) simulations. The fast inference speed of our deep learning model allows us to optimize experimental parameters for maximum ion energy and laser-energy conversion efficiency. An analysis of parameter influence on our model output, using Sobol and PAWN indices, provides deeper insights into the laser-plasma system.

2.Laser induced proton acceleration by resonant nano-rod antenna for fusion

Authors:István Papp, Larissa Bravina, Mária Csete, Archana Kumari, Igor N. Mishustin, Anton Motornenko, Péter Rácz, Leonid M. Satarov, Horst Stöcker, András Szenes, Dávid Vass, Tamás S. Biró, László P. Csernai, Norbert Kroó

Abstract: Recently laser induced fusion with simultaneous volume ignition, a spin-off from relativistic heavy ion collisions, was proposed, where implanted nano antennas regulated and amplified the light absorption in the fusion target. Studies of resilience of the nano antennas were published recently in vacuum and in UDMA-TEGDMA medium. These studies concluded that the lifetime of the plasmonic effect is longer in medium, however, less energy was observed in the UDMA-TEGDMA copolymer, due to the smaller resonant size of gold nanoantenna than in case of Vacuum. Here we show how the plasmonic effect behaves in an environment fully capable of ionization, surrounded by Hydrogen atoms close to liquid densities. We performed numerical simulations treating the electrons of gold in the conduction band as strongly coupled plasma. The results show that the protons close to the nanorod's surface follow the collectively moving electrons rather than the incoming electric field of the light. The results also show that the plasmonic accelerating effect is also dependent on the laser intensity.

3.Contribution of fictitious forces to polarization drag in rotating media

Authors:Julien Langlois, Renaud Gueroult

Abstract: Models for polarization drag - mechanical analog of the Faraday effect - are extended to include inertial corrections to the dielectrics properties of the rotating medium in its rest-frame. Instead of the Coriolis-Faraday term originally proposed by Baranova & Zel'dovich, inertia corrections due to the fictitious Coriolis and centrifugal forces are here derived by considering the effect of rotation on both the Lorentz and plasma dielectric models. These modified rest-frame properties are subsequently used to deduce laboratory properties. Although elegant and insightful, it is shown that the Coriolis-Faraday correction inferred from Larmor's theorem is limited in that it can only capture inertial corrections to polarization drag when the equivalent Faraday rotation is defined at the wave frequency of interest. This is notably not the case for low frequency polarization drag in a rotating magnetized plasma, although it is verified here using the more general phenomenological models that the impact of fictitious forces is in general negligible in these conditions.

1.Slow positron production and storage for the ASACUSA-Cusp experiment

Authors:D. J. Murtagh, C. Amsler, H. Breuker, M. Bumbar, S. Chesnevskaya, G. Costantini, R. Ferragut, M. Giammarchi, A. Gligorova, G. Gosta, H. Higaki, E. D. Hunter, C. Killian, V. Kraxberger, N. Kuroda, A. Lanz, M. Leali, G. Maero, C. Mal\-bru\-not, V. Mascagna, Y. Matsuda, V. Mäckel, S. Migliorati, A. Nanda, L. Nowak, F. Parnefjord Gustafsson, S. Rheinfrank, M. Romé, M. C. Simon, M. Tajima, V. Toso, S. Ulmer, L. Venturelli, A. Weiser, E. Widmann, T. Wolz, Y. Yamazaki, J. Zmeskal

Abstract: The ASACUSA Cusp experiment requires the production of dense positron plasmas with a high repetition rate to produce a beam of antihydrogen. In this work, details of the positron production apparatus used for the first observation of the antihydrogen beam, and subsequent measurements are described in detail. This apparatus replaced the previous compact trap design resulting in an improvement in positron accumulation by a factor of ($52\pm3)$

2.Development of Impedance Sheath Boundary Condition in Stix Finite Element RF Code

Authors:Christina Migliore, Mark Stowell, John Wright, Paul Bonoli

Abstract: Ion cyclotron radio frequency range (ICRF) power plays an important role in heating and current drive in fusion devices. However, experiments show that in the ICRF regime there is a formation of a radio frequency (RF) sheath at the material and antenna boundaries that influences sputtering and power dissipation. Given the size of the sheath relative to the scale of the device, it can be approximated as a boundary condition (BC). Electromagnetic field solvers in the ICRF regime typically treat material boundaries as perfectly conducting, thus ignoring the effect of the RF sheath. Here we describe progress on implementing a model for the RF sheath based on a finite impedance sheath BC formulated by J. Myra and D. A. D'Ippolito, Physics of Plasmas 22 (2015) which provides a representation of the RF rectified sheath including capacitive and resistive effects. This research will discuss the results from the development of a parallelized cold-plasma wave equation solver Stix that implements this non-linear sheath impedance BC through the method of finite elements in pseudo-1D and pseudo-2D using the MFEM library. The verification and comparison of the sheath BC from Stix with results from H. Kohno and J. Myra, Computer Physics Communications 220 129-142 (2017) will also be discussed.

3.Pauli blocking effects on pair creation in strong electric field

Authors:Mikalai Prakapenia, Gregory Vereshchagin

Abstract: The process of electron-positron pair creation and oscillation in uniform electric field is studied, taking into account Pauli exclusion principle. Generally, we find that pair creation is suppressed, hence coherent oscillations occur on longer time scales. Considering pair creation in already existing electron-positron plasma we find that the dynamics depends on pair distribution function. We considered Fermi-Dirac distribution of pairs and found that for small temperatures pair creation is suppressed, while for small chemical potentials it increases: heating leads to enhancement of pair creation.

1.Influence of Specific Energy Inhomogeneity on the CO2 Splitting Performance in a High-Power Plasma Jet

Authors:Hendrik Burghaus, Clemens F. Kaiser, Stefanos Fasoulas, Georg Herdrich

Abstract: Plasma-based CO2 conversion is a promising pathway towards greenhouse gas recycling. In the corresponding research field, various types of plasma reactors are applied for carbon dioxide dissociation. So far, spatial inhomogeneities of the specific energy (SEI) distribution in plasma generators, e.g., induced by non-uniform heating or an inhomogeneous mass distribution, are not the focus of the investigations. In this work, the spatial inhomogeneity of mass-specific enthalpy in the plasma jet of the inductive plasma generator IPG4 at the Institute of Space Systems (IRS) is examined. For this, the mean mass-specific enthalpy as well as the radial distribution of the local enthalpy are measured using plasma probes. Moreover, the influence of the determined specific enthalpy inhomogeneity on the CO2 splitting performance is quantified. It is shown that an inhomogeneous radial distribution of the specific energy can significantly lower the carbon dioxide conversion, compared to a homogeneous case. With regards to IPG4, the performance reduction is 16 %.

1.Simulations of spin/polarization-resolved laser-plasma interactions in the nonlinear QED regime

Authors:Feng Wan, Chong Lv, Kun Xue, Zhen-Ke Dou, Qian Zhao, Mamutjan Ababekri, Wen-Qing Wei, Zhong-Peng Li, Yong-Tao Zhao, Jian-Xing Li

Abstract: Strong-field quantum electrodynamics (SF-QED) plays a crucial role in ultraintense laser matter interactions, and demands sophisticated techniques to understand the related physics with new degrees of freedom, including spin angular momentum. To investigate the impact of SF-QED processes, we have introduced spin/polarization-resolved nonlinear Compton scattering, nonlinear Breit-Wheeler and vacuum birefringence processes into our particle-in-cell (PIC) code. In this article, we will provide details of the implementation of these SF-QED modules and share known results that demonstrate exact agreement with existing single particle codes. By coupling normal PIC with spin/polarization-resolved SF-QED processes, we create a new theoretical platform to study strong field physics in currently running or planned petawatt or multi-petawatt laser facilities.

1.Particle-resolved study of the onset of turbulence

Authors:Eshita Joshi, Markus H Thoma, Mierk Schwabe

Abstract: The transition from laminar to turbulent flow is an immensely important topic that is still being studied. Here we show that complex plasmas, i.e., microparticles immersed in a low temperature plasma, make it possible to study the particle-resolved onset of turbulence under the influence of damping, a feat not possible with conventional systems. We performed three-dimensional (3D) molecular dynamics (MD) simulations of complex plasmas flowing past an obstacle and observed 3D turbulence in the wake and fore-wake region of this obstacle. We found that we could reliably trigger the onset of turbulence by changing key parameters such as the flow speed and particle charge, which can be controlled in experiments, and show that the transition to turbulence follows the conventional pathway involving the intermittent emergence of turbulent puffs. The power spectra for fully developed turbulence in our simulations followed the -5/3 power law of Kolmogorovian turbulence in both time and space. We demonstrate that turbulence in simulations with damping occurs after the formation of shock fronts, such as bow shocks and Mach cones. By reducing the strength of damping in the simulations, we could trigger a transition to turbulence in an undamped system. This work opens the pathway to detailed experimental and simulation studies of the onset of turbulence on the level of the carriers of the turbulent interactions, i.e., the microparticles.

2.Linking Langevin equation to scaling properties of space plasma turbulence at sub-ion scales

Authors:Simone Benella, Mirko Stumpo, Tommaso Alberti, Oreste Pezzi, Emanuele Papini, Emiliya Yordanova, Francesco Valentini, Giuseppe Consolini

Abstract: Current understanding of the kinetic-scale turbulence in weakly-collisional plasmas still remains elusive. We employ a general framework in which the turbulent energy transfer is envisioned as a scale-to-scale Langevin process. Fluctuations in the sub-ion range show a global scale invariance, thus suggesting a homogeneous energy repartition. In this Letter, we interpret such a feature by linking the drift term of the Langevin equation to scaling properties of fluctuations. Theoretical expectations are verified on solar wind observations and numerical simulations thus giving relevance to the proposed framework for understanding kinetic-scale turbulence in space plasmas.

3.Epistemic and Aleatoric Uncertainty Quantification and Surrogate Modelling in High-Performance Multiscale Plasma Physics Simulations

Authors:Yehor Yudin, David Coster, Udo von Toussaint, Frank Jenko

Abstract: This work suggests several methods of uncertainty treatment in multiscale modelling and describes their application to a system of coupled turbulent transport simulations of a tokamak plasma. We propose a method to quantify the usually aleatoric uncertainty of a system in a quasi-stationary state, estimating the mean values and their errors for quantities of interest, which is average heat fluxes in the case of turbulence simulations. The method defines the stationarity of the system and suggests a way to balance the computational cost of simulation and the accuracy of estimation. This allows, contrary to many approaches, to incorporate aleatoric uncertainties in the analysis of the model and to have a quantifiable decision for simulation runtime. Furthermore, the paper describes methods for quantifying the epistemic uncertainty of a model and the results of such a procedure for turbulence simulations, identifying the model's sensitivity to particular input parameters and sensitivity to uncertainties in total. Finally, we introduce a surrogate model approach based on Gaussian Process Regression and present a preliminary result of training and analysing the performance of such a model based on turbulence simulation data. Such an approach shows a potential to significantly decrease the computational cost of the uncertainty propagation for the given model, making it feasible on current HPC systems.

1.Non-collinear polychromatic light for inertial fusion energy

Authors:Yao Zhao, Hongwei Yin, Bin Zhao, Zijian Cui

Abstract: Normal broadband lasers with collinear polychromatic components have immense potential for mitigating laser plasma instabilities (LPIs). However, the projection complexity of collinear polychromatic light (CPL) is a significant challenge owing to the demand for a large bandwidth and beamlet number. Here, we propose a driver scheme for non-collinear polychromatic light (NCPL) with a small angle $\sim4^\circ$ between the double-color beamlets. The frequency difference between the beamlets of each NCPL is 1\%, and the beamlet colors of any two adjacent flanges are different. LPI models of the NCPL in both homogeneous and inhomogeneous plasmas have been developed, which lead to a decoupling threshold for the shared daughter waves under a multibeam configuration. Compared with the CPL, both the growth rate and saturation level of LPIs are greatly reduced by using the NCPL. The two- and three-dimensional simulation results indicate that the NCPL reduces the absolute and convective decoupling thresholds of the CPL and is sufficient to effectively mitigate the reflectivity, hot-electron generation, and intensity of cross-beam energy transfer. A novel design for the efficient generation of ultraviolet NCPL has been presented based on non-collinear sum-frequency generation. The proposed NCPL driver with a compact configuration can significantly enhance the beam-target coupling efficiency, which paves the way towards the realization of robust fusion ignition.

1.Three-dimensional structure and stability of discontinuities between unmagnetized pair plasma and magnetized electron-proton plasma

Authors:M E Dieckmann, D Folini, M Falk, A Bock, P Steneteg, R Walder

Abstract: We study with a 3D PIC simulation discontinuities between an electron-positron pair plasma and magnetized electrons and protons. A pair plasma is injected at one simulation boundary with a speed 0.6$c$ along its normal. It expands into an electron-proton plasma and a magnetic field that points orthogonally to the injection direction. Diamagnetic currents expel the magnetic field from within the pair plasma and pile it up in front of it. It pushes electrons, which induces an electric field pulse ahead of the magnetic one. This initial electromagnetic pulse (EMP) confines the pair plasma magnetically and accelerates protons electrically. The fast flow of the injected pair plasma across the protons behind the initial EMP triggers the filamentation instability. Some electrons and positrons cross the injection boundary and build up a second EMP. Electron-cyclotron drift instabilities perturb the plasma ahead of both EMPs seeding a Rayleigh-Taylor-type instability. Despite equally strong perturbations ahead of both EMPs, the second EMP is much more stable than the initial one. We attribute the rapid collapse of the initial EMP to the filamentation instability, which perturbed the plasma behind it. The Rayleigh-Taylor-type instability transforms the planar EMPs into transition layers, in which magnetic flux ropes and electrostatic forces due to uneven numbers of electrons and positrons slow down and compress the pair plasma and accelerate protons. In our simulation, the expansion speed of the pair cloud decreased by about an order of magnitude and its density increased by the same factor. Its small thickness implies that it is capable of separating a relativistic pair outflow from an electron-proton plasma, which is essential for collimating relativistic jets of pair plasma in collisionless astrophysical plasma.

2.Electronic density response of warm dense hydrogen on the nanoscale

Authors:Tobias Dornheim, Maximilian Böhme, Zhandos Moldabekov, Jan Vorberger

Abstract: The properties of hydrogen at warm dense matter (WDM) conditions are of high importance for the understanding of astrophysical objects and technological applications such as inertial confinement fusion. In this work, we present extensive new \emph{ab initio} path integral Monte Carlo (PIMC) results for the electronic properties in the Coulomb potential of a fixed ionic configuration. This gives us new insights into the complex interplay between the electronic localization around the protons with their density response to an external harmonic perturbation. We find qualitative agreement between our simulation data and a heuristic model based on the assumption of a local uniform electron gas model, but important trends are not captured by this simplification. In addition to being interesting in their own right, we are convinced that our results will be of high value for future projects, such as the rigorous benchmarking of approximate theories for the simulation of WDM, most notably density functional theory.

3.Generation of sub-ion scale magnetic holes from electron shear flow instabilities in plasma turbulence

Authors:Giuseppe Arrò, Francesco Pucci, Francesco Califano, Maria Elena Innocenti, Giovanni Lapenta

Abstract: Magnetic holes (MHs) are coherent structures associated with strong magnetic field depressions in magnetized plasmas. They are observed in many astrophysical environments at a wide range of scales but their origin is still under debate. In this work we investigate the formation of sub-ion scale MHs using a fully kinetic 2D simulation of plasma turbulence initialized with parameters typical of the Earth's magnetosheath. Our analysis shows that the turbulence is capable of generating sub-ion scale MHs from large scale fluctuations via the following mechanism: first, the nonlinear large scale dynamics spontaneously leads to the development of thin and elongated electron velocity shears; these structures then become unstable to the electron Kelvin-Helmholtz instability and break up into small scale electron vortices; the electric current carried by these vortices locally reduces the magnetic field, inducing the formation of sub-ion scale MHs. The MHs thus produced exhibit features consistent with satellite observations and with previous numerical studies. We finally discuss the kinetic properties of the observed sub-ion scale MHs, showing that they are characterized by complex non-Maxwellian electron velocity distributions exhibiting anisotropic and agyrotropic features.

4.Quantum Calculation of Classical Kinetic Equations: A Novel Approach for Numerical Analysis of 6D Boltzmann-Maxwell Equations in Collisionless Plasmas Using Quantum Computing

Authors:Hayato Higuchi, Juan William Pedersen, Akimasa Yoshikawa

Abstract: A novel quantum algorithm for solving the Boltzmann-Maxwell equations of the 6D collisionless plasma is proposed. The equation describes the kinetic behavior of plasma particles in electromagnetic fields and is known for the classical first-principles equations in various domains, from space to laboratory plasmas. We have constructed a quantum algorithm for a future large-scale quantum computer to accelerate its costly computation. This algorithm consists mainly of two routines: the Boltzmann solver and the Maxwell solver. Quantum algorithms undertake these dual procedures, while classical algorithms facilitate their interplay. Each solver has a similar structure consisting of three steps: Encoding, Propagation, and Integration. We conducted a preliminary implementation of the quantum algorithm and performed a parallel validation against a comparable classical approach. IBM Qiskit was used to implement all quantum circuits.

1.Particle-in-cell simulation of a 50~mTorr capacitively coupled argon discharge over a range of frequencies

Authors:Saurabh Simha, Sarveshwar Sharma, Alexander Khrabrov, Igor Kaganovich, Jonathan Poggie, Sergey Macheret

Abstract: The effect of driving frequency in the range of 13.56 MHz to 73 MHz on electron energy distribution and electron heating modes in a 50 mTorr capacitively coupled argon plasma discharge is studied using 1D-3V particle-in-cell simulations. Calculated electron energy probability functions exhibit three distinct ``temperatures'' for low-, mid-, and high-energy electrons. When compared to published experimental data, the calculated probability functions show a reasonable agreement for the energy range resolved in the measurements (about 2 eV to 10 eV). Discrepancies outside this range lead to differences between computational and experimental values of the electron number density determined from the distribution functions, but the predicted effective electron temperature is within 25\% of experimental values. The impedance of the discharge is interpreted in terms of a homogeneous equivalent circuit model and the driving frequency dependence of the inferred combined sheath thickness is found to obey a known, theoretically-derived, power law. The average power transferred from the field to the electrons (electron heating) is computed, and a region of negative heating near the sheath edge, particularly at higher driving frequencies, is identified. Analysis of the electron momentum equation shows that electron inertia, which would average to zero in a linear regime, is responsible for negative values of power deposition near the sheath edge at high driving frequencies due to the highly nonlinear behavior of the discharge.

1.Gamma-Flash Generation in Multi-Petawatt Laser-Matter Interactions

Authors:P. Hadjisolomou, T. M. Jeong, D. Kolenaty, A. J. Macleod, V. Olšovcová, R. Versaci, C. P. Ridgers, S. V. Bulanov

Abstract: The progressive development of high power lasers over the last several decades, enables the study of $\gamma$-photon generation when an intense laser beam interacts with matter, mainly via inverse Compton scattering at the high intensity limit. $\gamma$-ray flashes are a phenomenon of broad interest, drawing attention of researchers working in topics ranging from cosmological scales to elementary particle scales. Over the last few years, a plethora of studies predict extremely high laser energy to $\gamma$-photon energy conversion using various target and/or laser field configurations. The aim of the present manuscript is to discuss several recently proposed $\gamma$-ray flash generation schemes, as a guide for upcoming $\gamma$-photon related experiments and for further evolution of the presently available theoretical schemes.

2.Phase space dynamics of unmagnetized plasmas: collisionless and collisional regimes

Authors:G. Celebre, S. Servidio, F. Valentini

Abstract: Eulerian electrostatic kinetic simulations of unmagnetized plasmas (kinetic electrons and motionless protons) with high-frequency equilibrium perturbations have been employed to investigate the phase space energy transfer across spatial and velocity scales, associated with the resonant interaction of electrons with the self-induced electric field. Numerical runs cover a wide range of collisionless and weakly collisional plasma regimes. An analysis technique based on the Fourier-Hermite transform of the particle distribution function allows to point out how kinetic processes trigger the phase space energy cascade, which is instead inhibited at finer scales when collisions are turned on. Numerical results are presented and discussed for the cases of linear wave Landau damping, nonlinear electron trapping, bump-on-tail and two-stream instabilities. A more realistic situation of turbulent Langmuir fluctuations is also discussed in detail. Fourier-Hermite transform shows an energy spread, highly conditioned by collisions, which involves velocity scales more quickly than the spatial scales, even when nonlinear effects are dominant. This results in anisotropic spectra whose slopes are compatible with theoretical expectations. Finally, an exact conservation law has been derived, which describes the time evolution of the free energy of the system, taking into account the collisional dissipation.

3.Enhanced transport at high plasma $β$ and sub-threshold kinetic ballooning modes in Wendelstein 7-X

Authors:P. Mulholland Eindhoven University of Technology, Eindhoven, The Netherlands, K. Aleynikova Max-Planck-Institut für Plasmaphysik, Greifswald, Germany, B. J. Faber University of Wisconsin-Madison, Madison, USA, M. J. Pueschel Eindhoven University of Technology, Eindhoven, The Netherlands Dutch Institute for Fundamental Energy Research, Eindhoven, The Netherlands, J. H. E. Proll Eindhoven University of Technology, Eindhoven, The Netherlands, C. C. Hegna University of Wisconsin-Madison, Madison, USA, P. W. Terry University of Wisconsin-Madison, Madison, USA, C. Nührenberg Max-Planck-Institut für Plasmaphysik, Greifswald, Germany

Abstract: The effect of plasma pressure $\beta$ on ion-temperature-gradient-driven (ITG) turbulence is studied in the Wendelstein 7-X (W7-X) stellarator, showing that subdominant kinetic ballooning modes (KBMs) are unstable well below the ideal MHD threshold and get strongly excited in the quasi-stationary state. By zonal-flow erosion, these highly non-ideal KBMs affect ITG saturation and thereby enable higher heat fluxes. Controlling these KBMs will be essential in order to allow W7-X and future stellarators to achieve maximum performance.

4.Numerical validation of a volume heated mixed fuel reactor concept

Authors:Hartmut Ruhl, Georg Korn

Abstract: In earlier papers \cite{ruhlkornarXiv,ruhlkornarXiv1,ruhlkornarXiv2} the core elements of a novel direct drive $\text{pBDT}$ mixed fuel reactor without fuel pre-compression have been discussed. The predominant purpose of the mixed fuel is to chemically bind $\text{DT}$. It has been found that the proposed mixed fuel design can reach $Q_T > 1$ with $\text{MJ}$ level external isochoric heating and without fuel pre-compression due to a novel direct drive ultra-fast heating concept. In order to further validate the concept we make use of MULTI, an ICF community code, and show with the help of MULTI simulations that the semi-analytical scaling model presented in a previous paper is capable of making accurate predictions. The MULTI simulations yield $Q_T > 1$ for a $\text{pBDT}$ fuel mix at $\text{MJ}$ level isochoric preheating, which validates our theoretical model involving in-situ compression for $Q_T \gg 1$ at reduced overall heating requirements.

5.Azimuthal ion movement in HiPIMS plasmas -- Part I: velocity distribution function

Authors:S. Thiemann-Monjé, J. Held, S. Schüttler, A. von Keudell, V. Schulz-von der Gathen

Abstract: Magnetron sputtering discharges feature complex magnetic field configurations to confine the electrons close to the cathode surface. This magnetic field configuration gives rise to a strong electron drift in azimuthal direction, with typical drift velocities on the order of \SI{100}{\kilo\meter\per\second}. In high power impulse magnetron sputtering (HiPIMS) plasmas, the ions have also been observed to follow the movement of electrons with velocities of a few \si{\kilo\meter\per\second}, despite being unmagnetized. In this work, we report on measurements of the azimuthal ion velocity using spatially resolved optical emission spectroscopy, allowing for a more direct measurement compared to experiments performed using mass spectrometry. The azimuthal ion velocities increase with target distance, peaking at about \SI{1.55}{\kilo\meter\per\second} for argon ions and \SI{1.25}{\kilo\meter\per\second} for titanium ions. Titanium neutrals are also found to follow the azimuthal ion movement which is explained with resonant charge exchange collisions. The experiments are then compared to a simple test-particle simulation of the titanium ion movement, yielding good agreement to the experiments when only considering the momentum transfer from electrons to ions via Coulomb collisions as the only source of acceleration in azimuthal direction. Based on these results, we propose this momentum transfer as the primary source for ion acceleration in azimuthal direction.

6.Azimuthal ion movement in HiPIMS plasmas -- Part II: lateral growth fluxes

Authors:Steffen Schüttler, Sascha Thiemann-Monje, Julian Held, Achim von Keudell

Abstract: The transport of sputtered species from the target of a magnetron plasma to a collecting surface at the circumference of the plasma is analyzed using a particle tracer technique. A small chromium insert at the racetrack position inside a titanium target is used as the source of tracer particles, which are redeposited on the collecting surface. The azimuthal velocity of the ions along the racetrack above the target is determined from the Doppler shift of the optical emission lines of titanium and chromium. The trajectories are reconstructed from an analysis of the transport physics leading to the measured deposition profiles. It is shown that a simple direct-line-of sight re-deposition model can explain the data for low power plasmas (DCMS) and for pulsed high power impulse magnetron plasmas (HiPIMS) by using the Thompson velocity distribution from the sputter process as starting condition. In the case of a HiPIMS plasma, the drag force exerted on the ions and neutrals by the electron Hall current has to be included causing an azimuthal displacement in \ExB direction. Nevertheless, the Thompson sputter distribution remains preserved for 50\% of the re-deposited growth flux. The implications for the understanding of transport processes in magnetron plasmas are discussed.

1.thermal and stress_strain analysis of the tested iter-like w langmuir probes in east

Authors:Chunyu He, Dahuan Zhu

Abstract: ITER-like tungsten Langmuir probes (W DLPs) have been installed and tested at the lower divertor horizontal target composed of flat-type components in EAST. Due to the non-active cooling, transient thermal and stress\strain analyses considering actual thermal loading and cooling conditions were thus conducted to evaluate the thermal performance and mechanical quality of W DLPs subjected to the long pulse & high plasma flux of EAST. The thermal analysis reveals that the inevitable leading edge induced thermal loading at surrounding area of W DLPs is not ignorable. The thermal performance of W DLPs are largely related to the plasma scenario (Qp: parallel heat flux along magnetic field line, {\alpha}: incline angle of magnetic field line). Under current plasma parameters, melting of W was not occurred in general, but recrystallization as well as the induced cracks may be still possible. And, the interval period (~1000 s) between neighboring shots is sufficient for nature cooling of W DLPs. The stress analysis also tells that the ceramic LLL may be general a crucial weak point of W DLPs, which is expected to not only limit the thermal affordability of long pulse but also cause possible crack problems. Such calculation results can provide important reference for current plasma operation and future improvement of the W DLPs.

2.Reflectionless Plasma Ignition via High-Power Virtual Perfect Absorption

Authors:Théo Delage, Jérôme Sokoloff, Olivier Pascal, Valentin Mazières, Alex Krasnok, Thierry Callegari

Abstract: Plasma ignition is critical in various scientific and industrial applications, demanding an efficient and robust execution mechanism. In this work, we present an innovative approach to plasma ignition by incorporating the analysis of fundamental aspects of light scattering in the complex frequency plane. For the first time, we demonstrate the high-power virtual perfect absorption (VPA) regime, a groundbreaking method for perfectly capturing light within a resonator. By carefully designing the temporal profile of the incident wave, we effectively minimize reflections during the ignition stages, thereby significantly enhancing the efficiency and resilience of the process. Through comprehensive experimental investigations, we validate the viability of this approach, establishing VPA as a powerful tool for reflectionless excitation and optimal control of plasma discharge. By addressing the limitations of conventional plasma ignition methods, this research represents a pivotal step towards transformative advancements in plasma technology, with promising implications for improving the performance and sustainability of numerous applications.

1.Direct Implicit and Explicit Energy-Conserving Particle-in-Cell Methods for Modeling of Capacitively-Coupled Plasma Devices

Authors:Haomin Sun, Soham Banerjee, Sarveshwar Sharma, Andrew Tasman Powis, Alexander V. Khrabrov, Dmytro Sydorenko, Jian Chen, Igor D. Kaganovich

Abstract: Achieving entire large scale kinetic modelling is a crucial task for the development and optimization of modern plasma devices. With the trend of decreasing pressure in applications such as plasma etching, kinetic simulations are necessary to self-consistently capture the particle dynamics. The standard, explicit, electrostatic, momentum-conserving Particle-In-Cell method suffers from tight stability constraints to resolve the electron plasma length (i.e. Debye length) and time scales (i.e. plasma period). This results in very high computational cost, making this technique generally prohibitive for the large volume entire device modeling (EDM). We explore the Direct Implicit algorithm and the explicit Energy Conserving algorithm as alternatives to the standard approach, which can reduce computational cost with minimal (or controllable) impact on results. These algorithms are implemented into the well-tested EDIPIC-2D and LTP-PIC codes, and their performance is evaluated by testing on a 2D capacitively coupled plasma discharge scenario. The investigation revels that both approaches enable the utilization of cell sizes larger than the Debye length, resulting in reduced runtime, while incurring only a minor compromise in accuracy. The methods also allow for time steps larger than the electron plasma period, however this can lead to numerical heating or cooling. The study further demonstrates that by appropriately adjusting the ratio of cell size to time step, it is possible to mitigate this effect to acceptable level.

2.Combining stochastic density functional theory with deep potential molecular dynamics to study warm dense matter

Authors:Tao Chen, Qianrui Liu, Yu Liu, Liang Sun, Mohan Chen

Abstract: In traditional finite-temperature Kohn-Sham density functional theory (KSDFT), the well-known orbitals wall restricts the use of first-principles molecular dynamics methods at extremely high temperatures. However, stochastic density functional theory (SDFT) can overcome the limitation. Recently, SDFT and its related mixed stochastic-deterministic density functional theory, based on the plane-wave basis set, have been implemented in the first-principles electronic structure software ABACUS [Phys. Rev. B 106, 125132(2022)]. In this study, we combine SDFT with the Born-Oppenheimer molecular dynamics (BOMD) method to investigate systems with temperatures ranging from a few tens of eV to 1000 eV. Importantly, we train machine-learning-based interatomic models using the SDFT data and employ these deep potential models to simulate large-scale systems with long trajectories. Consequently, we compute and analyze the structural properties, dynamic properties, and transport coefficients of warm dense matter. The abovementioned methods offer a new approach with first-principles accuracy to tackle various properties of warm dense matter.

1.Two-Dimensional Conformal Plasma Turbulence in the Hasegawa-Mima Equation

Authors:Shigeo Kawata

Abstract: The two-dimensional (2D) conformal field theory (CFT) suggests that the 2D plasma turbulence, governed by the Hasegawa-Mima (H-M) equation, may have multiple exponents of energy spectrum in momentum space. Electrostatic potential driven by drift waves in magnetized 2D plasmas would be described by the H-M equation. On the other hand, the 2D CFT has an infinite-dimensional symmetry. When we focus on minimal models established in 2D CFT, each minimal model provides a different 2D statistical model as presented in fluid turbulence, quantum field theory and string theory, and would provide a specific exponent of the energy spectrum. The CFT analytical results in this work suggests that the H-M plasma turbulence may have multiple exponents of the energy spectrum.

2.Time-resolved optical shadowgraphy of solid hydrogen jets as a testbed to benchmark particle-in-cell simulations

Authors:Long Yang, Lingen Huang, Stefan Assenbaum, Thomas E Cowan, Ilja Goethel, Sebastian Göde, Thomas Kluge, Martin Rehwald, Xiayun Pan, Ulrich Schramm, Jan Vorberger, Karl Zeil, Tim Ziegler, Constantin Bernert

Abstract: Particle-in-cell (PIC) simulations are a superior tool to model kinetics-dominated plasmas in relativistic and ultrarelativistic laser-solid interactions (dimensionless vectorpotential $a_0 > 1$). The transition from relativistic to subrelativistic laser intensities ($a_0 \lesssim 1$), where correlated and collisional plasma physics become relevant, is reaching the limits of available modeling capabilities. This calls for theoretical and experimental benchmarks and the establishment of standardized testbeds. In this work, we develop such a suitable testbed to experimentally benchmark PIC simulations using a laser-irradiated micron-sized cryogenic hydrogen-jet target. Time-resolved optical shadowgraphy of the expanding plasma density, complemented by hydrodynamics and ray-tracing simulations, is used to determine the bulk-electron temperature evolution after laser irradiation. As a showcase, a study of isochoric heating of solid hydrogen induced by laser pulses with a dimensionless vectorpotential of $a_0 \approx 1$ is presented. The comparison of the bulk-electron temperature of the experiment with systematic scans of PIC simulations demostrates that, due to an interplay of vacuum heating and resonance heating of electrons, the initial surface-density gradient of the target is decisive to reach quantitative agreement at \SI{1}{\ps} after the interaction. The showcase demostrates the readiness of the testbed for controlled parameter scans at all laser intensities of $a_0 \lesssim 1$.

3.Modulation of electromagnetic waves in a relativistic degenerate plasma at finite temperature

Authors:S. Roy, A. P. Misra, A. Abdikian

Abstract: We study the modulational instability (MI) of a linearly polarized electromagnetic (EM) wave envelope in an intermediate regime of relativistic degenerate plasmas at a finite temperature $(T\neq0)$ where the thermal energy $(K_BT)$ and the rest-mass energy $(m_ec^2)$ of electrons do not differ significantly, i.e., $\beta_e\equiv K_{B}T/m_{e}c^2\lesssim~(\rm{or}~\gtrsim) 1$, but, the Fermi energy $(K_BT_F)$ and the chemical potential energy $(\mu_e)$ of electrons are still a bit higher than the thermal energy, i.e., $T_F>T$ and $\xi_{e}=\mu_e/K_{B}T\gtrsim1$. Starting from a set of relativistic fluid equations for degenerate electrons at finite temperature, coupled to the EM wave equation and using the multiple scale perturbation expansion scheme, a one-dimensional nonlinear Sch{\"o}dinger (NLS) equation is derived, which describes the evolution of slowly varying amplitudes of EM wave envelopes. Then we study the MI of the latter in two different regimes, namely $\beta_e<1$ and $\beta_e>1$. Like unmagnetized classical cold plasmas, the modulated EM envelope is always unstable in the region $\beta_e>4$. However, for $\beta_e\lesssim1$ and $1<\beta_e<4$, the wave can be stable or unstable depending on the values of the EM wave frequency, $\omega$ and the parameter $\xi_e$. We also obtain the instability growth rate for the modulated wave and find a significant reduction by increasing the values of either $\beta_e$ or $\xi_e$. Finally, we present the profiles of the traveling EM waves in the form of bright (envelope pulses) and dark (voids) solitons, as well as the profiles (other than traveling waves) of the Kuznetsov-Ma breather, the Akhmediev breather, and the Peregrine solitons as EM rogue (freak) waves, and discuss their characteristics in the regimes of $\beta_e\lesssim1$ and $\beta_e>1$.

4.Discharge characteristics of a low-pressure geometrically asymmetric cylindrical capacitively coupled plasma with an axisymmetric magnetic field

Authors:Swati Dahiya Institute for Plasma Research, Bhat, Gujarat, India Homi Bhabha National Institute, Training School Complex, Mumbai, India and, Pawandeep Singh Institute for Plasma Research, Bhat, Gujarat, India Homi Bhabha National Institute, Training School Complex, Mumbai, India and, Yashashri Patil Institute for Plasma Research, Bhat, Gujarat, India, Sarveshwar Sharma Institute for Plasma Research, Bhat, Gujarat, India Homi Bhabha National Institute, Training School Complex, Mumbai, India and, Nishant Sirse Institute of Science and Research and Centre for Scientific and Applied Research, IPS Academy, Indore, India, Shantanu Kumar Karkari Institute for Plasma Research, Bhat, Gujarat, India Homi Bhabha National Institute, Training School Complex, Mumbai, India and

Abstract: We investigate the discharge characteristics of a low-pressure geometrically asymmetric cylindrical capacitively coupled plasma discharge with an axisymmetric magnetic field generating an EXB drift in the azimuthal direction. Vital discharge parameters, including electron density, electron temperature, DC self-bias, and Electron Energy distribution function (EEDF), are studied experimentally for varying magnetic field strength (B). A transition in the discharge asymmetry is observed along with a range of magnetic fields where the discharge is highly efficient with lower electron temperature. Outside this range of magnetic field, the plasma density drops, followed by an increase in the electron temperature. The observed behavior is attributed to the transition from geometrical asymmetry to magnetic field-associated symmetry due to reduced radial losses and plasma confinement in the peripheral region. In this region, the DC self-bias increases almost linearly from a large negative value to nearly zero, i.e., the discharge becomes symmetric. The EEDF undergoes a transition from bi-Maxwellian for unmagnetized to Maxwellian at intermediate B and finally becomes a weakly bi-Maxwellian at higher values of B. The above transitions present a novel way to independently control the ion energy and ion flux in a cylindrical CCP system using an axisymmetric magnetic field with an enhanced plasma density and lower electron temperature operation that is beneficial for plasma processing applications.

5.Fast transport simulations with higher-fidelity surrogate models for ITER

Authors:J. Citrin, P. Trochim, T. Goerler, D. Pfau, K. L. van de Plassche, F. Jenko

Abstract: A fast and accurate turbulence transport model based on quasilinear gyrokinetics is developed. The model consists of a set of neural networks trained on a bespoke quasilinear GENE dataset, with a saturation rule calibrated to dedicated nonlinear simulations. The resultant neural network is approximately eight orders of magnitude faster than the original GENE quasilinear calculations. ITER predictions with the new model project a fusion gain in line with ITER targets. While the dataset is currently limited to the ITER baseline regime, this approach illustrates a pathway to develop reduced-order turbulence models both faster and more accurate than the current state-of-the-art.

6.SDR, EVC, and SDREVC: Limitations and Extensions

Authors:E. D. Hunter, C. Amsler, H. Breuker, M. Bumbar, S. Chesnevskaya, G. Costantini, R. Ferragut, M. Giammarchi, A. Gligorova, G. Gosta, H. Higaki, C. Killian, V. Kraxberger, N. Kuroda, A. Lanz, M. Leali, G. Maero, C. Malbrunot, V. Mascagna, Y. Matsuda, V. Mäckel, S. Migliorati, D. J. Murtagh, A. Nanda, L. Nowak, F. Parnefjord Gustafsson, S. Rheinfrank, M. Romé, M. C. Simon, M. Tajima, V. Toso, S. Ulmer, L. Venturelli, A. Weiser, E. Widmann, Y. Yamazaki, J. Zmeskal

Abstract: Methods for reducing the radius, temperature, and space charge of nonneutral plasma are usually reported for conditions which approximate an ideal Penning Malmberg trap. Here we show that (1) similar methods are still effective under surprisingly adverse circumstances: we perform SDR and SDREVC in a strong magnetic mirror field using only 3 out of 4 rotating wall petals. In addition, we demonstrate (2) an alternative to SDREVC, using e-kick instead of EVC and (3) an upper limit for how much plasma can be cooled to T < 20 K using EVC. This limit depends on the space charge, not on the number of particles or the plasma density.

1.On the capillary discharge in the high repetition rate regime

Authors:P. Sasorov, G. Bagdasarov, N. Bobrova, G. Grittani, A. Molodozhentsev, S. V. Bulanov

Abstract: We investigate the main physical processes that limit the repetition rate of capillary discharges used in laser accelerators of electrons theoretically and with computer simulations. We consider processes in the capillary. We assume that a cooling system independently maintains temperature balance of the capillary, as well as a gas supply system and a vacuum system maintain conditions outside the capillary. The most important factor, determining the highest repetition rates in this case, is the capillary length, which governs a refilling time of the capillary by the gas. For a short capillary, used for acceleration of sub-GeV electron beams, the repetition rate approximately equal to 10 kHz, which is inversely proportional to the square of the capillary length. The effects of the capillary diameter, gas type and the gas density are weaker.

2.Effect of flow shear on the onset of dynamos

Authors:Shishir Biswas, Rajaraman Ganesh

Abstract: Understanding the origin and structure of mean magnetic fields in astrophysical conditions is a major challenge. Shear flows often coexist in such astrophysical conditions and the role of flow shear on dynamo mechanism is only beginning to be investigated. Here, we present a direct numerical simulation (DNS) study of the effect of flow shear on dynamo instability for a variety of base flows with controllable mirror symmetry (i.e, fluid helicity). Our observations suggest that for helical base flow, the effect of shear is to suppress the small scale dynamo (SSD) action, i.e, shear helps the large scale magnetic field to manifest itself by suppressing SSD action. For non-helical base flows, flow shear has the opposite effect of amplifying the small-scale dynamo action. The magnetic energy growth rate ($\gamma$) for non-helical base flows are found to follow an algebraic nature of the form, $\gamma = - aS + bS^\frac{2}{3}$ , where a, b > 0 are real constants and S is the shear flow strength and $\gamma$ is found to be independent of scale of flow shear. Studies with different shear profiles and shear scale lengths for non-helical base flows have been performed to test the universality of our finding.

1.Fast Dynamic 1D Simulation of Divertor Plasmas with Neural PDE Surrogates

Authors:Yoeri Poels, Gijs Derks, Egbert Westerhof, Koen Minartz, Sven Wiesen, Vlado Menkovski

Abstract: Managing divertor plasmas is crucial for operating reactor scale tokamak devices due to heat and particle flux constraints on the divertor target. Simulation is an important tool to understand and control these plasmas, however, for real-time applications or exhaustive parameter scans only simple approximations are currently fast enough. We address this lack of fast simulators using neural PDE surrogates, data-driven neural network-based surrogate models trained using solutions generated with a classical numerical method. The surrogate approximates a time-stepping operator that evolves the full spatial solution of a reference physics-based model over time. We use DIV1D, a 1D dynamic model of the divertor plasma, as reference model to generate data. DIV1D's domain covers a 1D heat flux tube from the X-point (upstream) to the target. We simulate a realistic TCV divertor plasma with dynamics induced by upstream density ramps and provide an exploratory outlook towards fast transients. State-of-the-art neural PDE surrogates are evaluated in a common framework and extended for properties of the DIV1D data. We evaluate (1) the speed-accuracy trade-off; (2) recreating non-linear behavior; (3) data efficiency; and (4) parameter inter- and extrapolation. Once trained, neural PDE surrogates can faithfully approximate DIV1D's divertor plasma dynamics at sub real-time computation speeds: In the proposed configuration, 2ms of plasma dynamics can be computed in $\approx$0.63ms of wall-clock time, several orders of magnitude faster than DIV1D.

1.Phase transition of three-dimensional finite-sized charged dust clusters in a plasma environment

Authors:Hirakjyoti Sarma, Ritupan Sarmah, Nilakshi Das

Abstract: The dynamics of a harmonically trapped three-dimensional Yukawa ball of charged dust particles immersed in plasma is investigated as function of external magnetic field and Coulomb coupling parameter using molecular dynamics simulation. It is shown that the harmonically trapped dust particles organize themselves into nested spherical shells. The particles start rotating in a coherent order as the magnetic field reaches a critical value corresponding to the coupling parameter of the system of dust particles. The magnetically controlled charged dust cluster of finite size undergoes a first-order phase transition from disordered to ordered phase. At sufficiently high coupling and strong magnetic field, the vibrational mode of this finite-sized charged dust cluster freezes, and the system retains only rotational motion.

2.Energy loss enhancement of very intense proton beams in dense matter due to the beam-density effect

Authors:Benzheng Chen, Jieru Ren, Zhigang Deng, Wei Qi, Zhongmin Hu, Bubo Ma, Xing Wang, Shuai Yin, Jianhua Feng, Wei Liu, Zhongfeng Xu, Dieter H. H. Hoffmann, Shaoyi Wang, Quanping Fan, Bo Cui, Shukai He, Zhurong Cao, Zongqing Zhao, Leifeng Cao, Yuqiu Gu, Shaoping Zhu, Rui Cheng, Xianming Zhou, Guoqing Xiao, Hongwei Zhao, Yihang Zhang, Zhe Zhang, Yutong Li, Weimin Zhou, Yongtao Zhao

Abstract: Thoroughly understanding the transport and energy loss of intense ion beams in dense matter is essential for high-energy-density physics and inertial confinement fusion. Here, we report a stopping power experiment with a high-intensity laser-driven proton beam in cold, dense matter. The measured energy loss is one order of magnitude higher than the expectation of individual particle stopping models. We attribute this finding to the proximity of beam ions to each other, which is usually insignificant for relatively-low-current beams from classical accelerators. The ionization of the cold target by the intense ion beam is important for the stopping power calculation and has been considered using proper ionization cross section data. Final theoretical values agree well with the experimental results. Additionally, we extend the stopping power calculation for intense ion beams to plasma scenario based on Ohm's law. Both the proximity- and the Ohmic effect can enhance the energy loss of intense beams in dense matter, which are also summarized as the beam-density effect. This finding is useful for the stopping power estimation of intense beams and significant to fast ignition fusion driven by intense ion beams.

1.A relativistic two-stream instability in an extremely low-density plasma

Authors:Shinji Koide, Masaaki Takahashi, Rohta Takahashi

Abstract: A linear analysis based on two-fluid equations in the approximation of a cold plasma, wherein the plasma temperature is assumed to be zero, demonstrates that a two-stream instability occurs in all cases. However, if this were true, the drift motion of electrons in an electric current over a wire would become unstable, inducing an oscillation in an electric circuit with ions bounded around specific positions. To avoid this peculiar outcome, we must assume a warm plasma with a finite temperature when discussing the criterion of instability. The two-stream instability in warm plasmas has typically been analyzed using kinetic theory to provide a general formula for the instability criterion from the distribution function of the plasma. However, the criteria based on kinetic theory do not have an easily applicable form. Here, we provide an easily applicable criterion for the instability based on the two-fluid model at finite temperatures, extensionally in the framework of special relativity. This criterion is relevant for analyzing two-stream instabilities in low-density plasmas in the universe and in Earth-based experimental devices.

2.Guiding Center Derivation of the Generalized Hasegawa-Mima Equation for Drift Wave Turbulence in Curved Magnetic Fields

Authors:Naoki Sato, Michio Yamada

Abstract: Recently, a generalized Hasegawa-Mima (gHM) equation describing drift wave turbulence in curved magnetic fields has been derived in [N. Sato and M. Yamada, J. Plasma Phys. (2022), vol. 88, 905880319] for an ion-electron plasma modeled as a two-fluid system. In this work, we show that a mathematically equivalent GHM equation can be obtained within the kinetic framework of guiding center motion, and that the relevant drift wave turbulence ordering can be further relaxed, effectively generalizing the applicability of the equation to any magnetic field geometry and electron spatial density, in the sense that no ordering requirements involve spatial derivatives of the magnetic field or the electron spatial density.

3.Hamiltonian Structure and Nonlinear Stability of Steady Solutions of the Generalized Hasegawa-Mima Equation for Drift Wave Turbulence in Curved Magnetic Fields

Authors:Naoki Sato, Michio Yamada

Abstract: The Generalized Hasegawa-Mima (GHM) equation, which generalizes the standard Hasegawa-Mima (HM) equation, is a nonlinear equation describing the evolution of drift wave turbulence in curved magnetic fields. The GHM equation can be obtained from a drift wave turbulence ordering that does not involve ordering conditions on spatial derivatives of the magnetic field or the plasma density, and it is therefore appropriate to describe the evolution of electrostatic turbulence in strongly inhomogeneous magnetized plasmas. In this work, we discuss the noncanonical Hamiltonian structure of the GHM equation, and obtain conditions for the nonlinear stability of steady solutions through the energy-Casimir stability criterion. These results are then applied to describe drift waves and infer the existence of stable toroidal zonal flows with radial shear in dipole magnetic fields.

4.Quantum Description of Free Electron Laser Radiation and Nonlinear Amplitude Equations

Authors:Stephan I. Tzenov, Zhichu Chen

Abstract: A relativistic quantum mechanical model to describe the quantum FEL dynamics has been developed. Neglecting the spin of electrons in the impacting beam, this model is based on the Klein-Gordon equation coupled to the Poisson equation for the space-charge potential and the wave equation for the transverse components of the radiation field. Furthermore, a system of coupled nonlinear envelope equations for the slowly varying amplitudes of the electron beam distribution and the radiation field has been derived. The fundamental system of basic equations have been cast into a suitable hydrodynamic formulation. In the framework of the hydrodynamic representation, a new dispersion relation has been derived and analyzed in both the quantum and the quasi-classical regimes, where the space-charge oscillations of the electron beam are taken into account.

5.Demonstration of tunability of HOFI waveguides via start-to-end simulations

Authors:S. M. Mewes, G. J. Boyle, A. Ferran Pousa, R. J. Shalloo, J. Osterhoff, C. Arran, L. Corner, R. Walczak, S. M. Hooker, M. Thévenet

Abstract: In recent years, hydrodynamic optical-field-ionized (HOFI) channels have emerged as a promising technique to create laser waveguides suitable for guiding tightly-focused laser pulses in a plasma, as needed for laser-plasma accelerators. While experimental advances in HOFI channels continue to be made, the underlying mechanisms and the roles of the main parameters remain largely unexplored. In this work, we propose a start-to-end simulation pipeline of the HOFI channel formation and the resulting guiding properties, and use it to explore the underlying physics and the tunability of HOFI channels. This approach is benchmarked against experimental measurements. HOFI channels are shown to feature excellent guiding properties over a wide range of parameters, making them a promising and tunable waveguide option for laser-plasma accelerators.

6.Nonlinear saturation of resistive tearing modes in a cylindrical tokamak with and without solving the dynamics

Authors:J. Loizu, D. Bonfiglio

Abstract: We show that the saturation of resistive tearing modes in a cylindrical tokamak, as well as the corresponding island width, can be directly calculated with an MHD equilibrium code without solving the dynamics and without considering resistivity. The results are compared to initial value resistive MHD simulations and to an analytical nonlinear theory. For small enough islands, the agreement is remarkable. For sufficiently large islands, the equilibrium calculations, which assume a flat current profile inside the island, overestimate the saturation amplitude. On the other hand, excellent agreement between nonlinear resistive MHD simulations and nonlinear theory is observed for all the considered tearing unstable equilibria.

7.Reduction (or enhancement) of stellarator turbulence by impurities

Authors:J. M. García-Regaña, I. Calvo, F. I. Parra, H. Thienpondt

Abstract: A systematic study of the impact of impurities on the turbulent heat fluxes is presented for Wendelstein 7-X. By means of nonlinear multispecies gyrokinetic simulations, it is shown that impurities, depending on the sign of their density gradient, can significantly enhance or reduce turbulent heat losses. For the relevant scenario of turbulence reduction, heat fluxes have a local minimum at a certain impurity concentration. This result demonstrates the potential of impurities for controlling turbulence and accessing enhanced confinement regimes in stellarators.

8.Magnetized dusty plasma: On issues of its complexity and magnetization of charged dust particles

Authors:Mangilal Choudhary

Abstract: It is possible to excite various linear and non-linear low-frequency modes in dusty plasma which is an admixture of electrons, ions, gas atoms, and negatively charged solid particles. The experimental as well as theoretical study of these low-frequency dynamical modes in dusty plasma is very complex because of the involvement of dynamics of electrons, ions, and neutrals. If the external magnetic field is introduced to dusty plasma then the dynamics of it will be more complex. The complexity of magnetized dusty plasma where plasma species are magnetized is discussed by keeping the experimental observations in magnetized dusty plasma devices in mind. The requirement of theoretical modeling, as well as computation experiments in understanding the dynamics of dusty plasma in the presence of a strong magnetic field, is highlighted in the context of experimental findings. The major challenges to magnetizing charged massive particles in experiments and some expected solutions are discussed in this report.

1.Rotating spokes, potential hump and modulated ionization in radio frequency magnetron discharges

Authors:Liang Xu, Haoming Sun, Denis Eremin, Sathya Ganta, Igor Kaganovich, Kallol Bera, Shahid Rauf

Abstract: In this work, the rotating spoke mode in the radio frequency (RF) magnetron discharge, which features the potential hump and the RF-modulated ionization, is observed and analyzed by means of the two dimensional axial-azimuthal (z-y) particle-in-cell/Monte Carlo collision method. The kinetic model combined with the linear analysis of the perturbation reveals that the cathode sheath (axial) electric field $E_z$ triggers the gradient drift instability (GDI), deforming the local potential until the instability condition is not fulfilled and the fluctuation growth stops in which moment the instability becomes saturated. The potential deformation consequently leads to the formation of the potential hump, surrounding which the azimuthal electric field $E_y$ is present. The saturation level of $E_y$ is found to be synchronized with and proportional to the time-changing voltage applied at the cathode, resulting in the RF-modulation of the electron heating in the $E_y$ due to $\nabla B$ drift. In the instability saturated stage, it is shown that the rotation velocity and direction of the spoke present in the simulations agree well with the experimental observation. In the instability linear stage, the instability mode wavelength and the growth rate are also found to be in good agreement with the prediction of the GDI linear fluid theory.

2.The Nonlinear Evolution of Whistler-Mode Chorus Revisited: Modulation Instability as the Source of Tones

Authors:Daniel Ratliff, Oliver Allanson

Abstract: We review the modulation stability of parallel propagating/field aligned Whistler Mode Chorus waves propagating in a warm plasma from a formal perspective with a focus on wave-particle interactions. The modulation instability criteria is characterised by a curvature of the dispersion relation for Whistler mode waves and a condition on the ratio between the group velocity $c_g$ and the electron sound speed $c_{s,e}$. We also demonstrate the in order to investigate the spatiotemporal evolution of the envelope and the formation of packets, one necessarily needs to account for the motion of ions within the system, leading to an ionic influence on the modulation instability threshold determined by the ion fraction of the plasma. Finally, we demonstrate that chirping may be captured when higher order effects are included within the spatiotemporal evolution of the amplitude. This yields not only an explicit expression for the sweep rate but identifies a possible origin for the power band gap that occurs at half the electron gyrofrequency. Numerical validation demonstrates that the interaction between wave packets is a source for the emergence of tones observed within mission data, and such interactions may be a major source of the electron energisation which Whistler-Mode chorus are responsible for.

1.X-ray Thomson scattering absolute intensity from the f-sum rule in the imaginary-time domain

Authors:Tobias Dornheim, Tilo Döppner, Andrew D. Baczewski, Panagiotis Tolias, Maximilian P. Böhme, Zhandos A. Moldabekov, Divyanshu Ranjan, David A. Chapman, Michael J. MacDonald, Thomas R. Preston, Dominik Kraus, Jan Vorberger

Abstract: We evaluate the f-sum rule on the dynamic structure factor in the imaginary-time domain as a formally exact and simulation-free means of normalizing X-Ray Thomson Scattering (XRTS) spectra. This circumvents error-prone real-time deconvolution of the source function and facilitates calculating the static structure factor from the properly normalized imaginary-time correlation function. We apply our technique to two distinct sets of experimental data, finding that it is effective for both narrow and broad x-ray sources. This approach could be readily adapted to other scattering spectroscopies.

1.Effect of electron-beam energy chirp on signatures of radiation reaction in laser-based experiments

Authors:J. Magnusson, T. G. Blackburn, E. Gerstmayr, E. E. Los, M. Marklund, C. P. Ridgers, S. P. D. Mangles

Abstract: Current experiments investigating radiation reaction employ high energy electron beams together with tightly focused laser pulses in order to reach the quantum regime, as expressed through the quantum nonlinearity parameter $\chi$. Such experiments are often complicated by the large number of latent variables, including the precise structure of the electron bunch. Here we examine a correlation between the electron spatial and energy distributions, called an energy chirp, investigate its significance to the laser-electron beam interaction and show that the resulting effect cannot be trivially ignored when analysing current experiments. In particular, we show that the energy chirp has a large effect on the second moment of the electron energy, but a lesser impact on the first electron energy moment or the photon critical energy. These results show the importance of improved characterisation and control over electron bunch parameters on a shot-to-shot basis in such experiments.

1.Accelerated Bayesian inference of plasma profiles with self-consistent MHD equilibria at W7-X via neural networks

Authors:Andrea Merlo, Andrea Pavone, Daniel Böckenhoff, Ekkehard Pasch, Golo Fuchert, Kai Jakob Brunner, Kian Rahbarnia, Jonathan Schilling, Udo Höfel, Sehyun Kwak, Jakob Svensson, Thomas Sunn Pedersen, the W7-X team

Abstract: High-$\langle \beta \rangle$ operations require a fast and robust inference of plasma parameters with a self-consistent MHD equilibrium. Precalculated MHD equilibria are usually employed at W7-X due to the high computational cost. To address this, we couple a physics-regularized NN model that approximates the ideal-MHD equilibrium with the Bayesian modeling framework Minerva. We show the fast and robust inference of plasma profiles (electron temperature and density) with a self-consistent MHD equilibrium approximated by the NN model. We investigate the robustness of the inference across diverse synthetic W7-X plasma scenarios. The inferred plasma parameters and their uncertainties are compatible with the parameters inferred using the VMEC, and the inference time is reduced by more than two orders of magnitude. This work suggests that MHD self-consistent inferences of plasma parameters can be performed between shots.

1.Laser harmonic generation with tuneable orbital angular momentum using a structured plasma target

Authors:R. M. G. M. Trines, H. Schmitz, M. King, P. McKenna, R. Bingham

Abstract: In previous studies of spin-to-orbital angular momentum (AM) conversion in laser high harmonic generation (HHG) using a plasma target, one unit of spin AM is always converted into precisely one unit of OAM [1, 2]. Here we show, through analytic theory and numerical simulations, that we can exchange one unit of SAM for a tuneable amount of OAM per harmonic step, via the use of a structured plasma target. In the process, we introduce a novel framework to study laser harmonic generation via recasting it as a beat wave process. This framework enables us to easily calculate and visualise harmonic progressions, unify the "photon counting" and "symmetry-based" approaches to HHG and provide new explanations for existing HHG results. Our framework also includes a specific way to analyse simultaneously the frequency, spin and OAM content of the harmonic radiation which provides enhanced insight into this process. The prospects of using our new framework to design HHG configurations with tuneable high-order transverse modes, also covering the design of structured plasma targets, will be discussed.

1.Analysis of ion-acoustic rogue wave in complex magneto-plasmas

Authors:A. Parvez

Abstract: I considered a four-component magnetized plasma medium consisting of opposite polarity ions and super-thermal distributed positrons and electrons to investigate the stable/unstable frequency regimes of modulated ion-acoustic waves (IAWs) in the D-F regions of Earth's ionosphere and laboratory plasmas. A $(3+1)$-dimensional nonlinear Schr\"{o}dinger equation is derived. The parametric regimes for the existence of the MI, first- and second-order rogue waves, and also their basic features (viz., amplitude, width, and speed) are found to be significantly modified by the effect of physical plasma parameters (such as superthermal index and positron to electron temperature ratio) and external magnetic field. It is found that the nonlinearity of the different types of electronegative plasma system depends on the positive to negative ion mass ratio. It is also shown that the presence of super-thermal distributed electrons and positrons modifies the nature of the MI of the modulated IAWs. The implication of our results for the laboratory plasma [e.g., ($Ar^+,~F^-$) electronegative plasma] and space plasma [e.g., ($H^+,~H^-$), ($H^+,~O_2^-$) electronegative plasma in D-F regions of Earth's ionosphere] are briefly discussed.

1.Applicability of the Klein-Gordon equation for pair production in vacuum and plasma

Authors:Haidar Al-Naseri, Gert Brodin

Abstract: In this paper, a phase-space description of electron-positron pair-creation will be applied, based on a Wigner transformation of the Klein-Gordon equation. The resulting theory is similar in many respects to the equations from the Dirac-Heisenberg-Wigner formalism. However, in the former case, all physics related to particle spin is neglected. In the present paper we compare the pair-production rate in vacuum and plasmas, with and without spin effects, in order to evaluate the accuracy and applicability of the spinless approximation. It is found that for modest frequencies of the electromagnetic field, the pair production rate of the Klein-Gordon theory is a good approximation to the Dirac theory, provided the matter density is small enough for Pauli blocking to be neglected, and a factor of two related to the difference in the vacuum energy density is compensated for.

2.X-ray absorption spectroscopy using an ultrafast laboratory-scale laser-plasma accelerator source

Authors:B. Kettle, C. Colgan, E. Los, E. Gerstmayr, M. J. V. Streeter, F. Albert, S. Astbury, R. A. Baggott, N. Cavanagh, K. Falk, T. I. Hyde, O. Lundh, P. P. Rajeev, D. Riley, S. J. Rose, G. Sarri, C. Spindloe, K. Svendsen, D. R. Symes, M. Smid, A. G. R. Thomas, C. Thornton, R. Watt, S. P. D. Mangles

Abstract: The absorption profile of the copper K-edge was measured over a 250 eV window using ultrashort X-rays from a laser-plasma wakefield accelerator. For the first time with a femtosecond probe, Extended X-ray Absorption Fine Structure (EXAFS) features were observed in a single shot, detailing the local atomic structure. This unique capability will allow the investigation of novel ultrafast processes, and in particular probing high energy density matter and physics far-from-equilibrium. A perspective on the additional strengths of a laboratory-based ultrafast X-ray absorption source is presented.

3.Evolution of Coupled Weakly-Driven Waves in a Dissipative Plasma

Authors:N. M. Pham, V. N. Duarte

Abstract: The nonlinear collisional dynamics of coupled driven plasma waves in the presence of background dissipation is studied analytically within kinetic theory. Sufficiently near marginal stability, phase space correlations are poorly preserved and time delays become unimportant. The system is then shown to be governed by two first-order coupled autonomous differential equations of cubic order for the wave amplitudes and two complementary first-order equations for the evolution of their phases. That system of equations can be decoupled and further simplified to a single second-order differential equation of Li\'enard's type for each amplitude. Numerical solutions for this equation are obtained in the general case while analytic solutions are obtained for special cases in terms of parameters related to the spacing of the resonances of the two waves in frequency space, e.g., wave lengths and oscillation frequencies. These parameters are further analyzed to find classes of quasi-steady saturation and pulsating scenarios. To classify equilibrium points, local stability analysis is applied, and bifurcation conditions are determined. When the two waves saturate at similar amplitude levels, their combined signal is shown to invariably exhibit amplitude beating and phase jumps of nearly $\pi$. The obtained analytical results can be used to benchmark simulations and to interpret eigenmode amplitude measurements in fusion experiments.

1.Non-LTE ionization potential depression model for warm and hot dense plasma

Authors:Chensheng Wu, Fuyang Zhou, Yong Wu, Jun Yan, Xiang Gao, Jianguo Wang

Abstract: For warm and hot dense plasma (WHDP), the ionization potential depression (IPD) is a key physical parameter in determining its ionization balance, therefore a reliable and universal IPD model is highly required to understand its microscopic material properties and resolve those existing discrepancies between the theoretical and experimental results. However, the weak temperature dependence of the nowadays IPD models prohibits their application through much of the WHDP regime, especially for the non-LTE dense plasma produced by short-pulse laser. In this work, we propose a universal non-LTE IPD model with the contribution of the inelastic atomic processes, and found that three-body recombination and collision ionization processes become important in determining the electron distribution and further affect the IPD in warm and dense plasma. The proposed IPD model is applied to treat the IPD experiments available in warm and hot dense plasmas and excellent agreements are obtained in comparison with those latest experiments of the IPD for Al plasmas with wide-range conditions of 70-700 eV temperature and 0.2-3 times of solid density, as well as a typical non-LTE system of hollow Al ions. We demonstrate that the present IPD model has a significant temperature dependence due to the consideration of the inelastic atomic processes. With the low computational cost and wide range applicability of WHDP, the proposed model is expected to provide a promising tool to study the ionization balance and the atomic processes as well as the related radiation and particle transports properties of a wide range of WHDP.

2.2D electromagnetic simulations of RF heating via inductive coupling in the SPIDER device

Authors:D. López-Bruna, P. Jain, M. Recchia, B. Zaniol, E. Sartori, C. Poggi, V. Candeloro, G. Serianni, P. Veltri

Abstract: SPIDER is the prototype ion source of MITICA, the full-size neutral beam heating system conceived for the ITER tokamak. It includes eight drivers to heat and sustain the inductively coupled plasma (ICP). Owing to their near cylindrical symmetry, the coupling between the radio-frequency (RF) active currents and the source plasma is studied using a 2D electromagnetic approach with simplified expressions for the plasma electrical conductivity taken from the literature. The power absorbed by the plasma and the effect of the induced plasma currents in lowering the inductance of the driver are based on data from the dedicated S16 experimental campaign (y.~2020) of SPIDER: plasma electron densities on the order of $10^{18}$ m$^{-3}$, electron temperatures $\sim 10$ eV; neutral gas pressure $\sim 0.3$ Pa and up to $50$ kW of net power per driver. It is found that the plasma conductivity cannot be explained by the friction forces associated to local collisional processes alone. The inclusion of an effective collisionality associated to non-local processes seems also insufficient to explain the experimental information. Only when the electrical conductivity is reduced where the RF magnetic field is more intense, can the heating power and driver inductance be acceptably reproduced. We present the first 2D electromagnetic ICP calculations in SPIDER for two types of plasma, without and with the addition of a static magnetic field. The power transfer efficiency to the plasma of the first drivers of SPIDER, in view of these models, is around 50%

3.On propagation of positive and negative streamers in air in uniform electric fields

Authors:G V Naidis, N Yu Babaeva

Abstract: Recently published results of numerical simulations of positive and negative streamers propagating in uniform electric fields in air are analyzed here in the framework of an analytical approach. Obtained approximate relations between the streamer radius, velocity and length, depending on the value of applied electric field, are in reasonable agreement with the results of numerical simulations.

1.Spectral-partitioned Kohn-Sham density functional theory

Authors:Babak Sadigh, Daniel Aberg, John Pask

Abstract: We introduce a general, variational scheme applied to Kohn-Sham density functional theory that allows for partitioning of the ground-state density matrix into distinct spectral domains, each of which spanned by an independent diagonal representation without requirement of mutual orthogonality. It is shown that by generalizing the entropic contribution to the free energy to allow for independent representations in each spectral domain, the free energy becomes an upper bound to the exact one, attaining this limit as representations approach Kohn-Sham eigenfunctions. A numerical procedure is devised for effective calculation of the generalized entropy associated with spectral partitioning of the density matrix. The result is a powerful framework for Kohn-Sham calculations of systems whose occupied subspaces span multiple energy regimes. As a case in point, we apply the proposed framework to warm- and hot-dense matter described by finite-temperature density functional theory, where at high energies the density matrix is represented by that of the free-electron gas, while at low energies it is variationally optimized. We derive expressions for the spectral-partitioned Kohn-Sham Hamiltonian, atomic forces, and macroscopic stresses within the projector-augmented wave (PAW) and the norm-conserving pseudopotential methods. It is demonstrated that at high temperatures, spectral partitioning facilitates accurate calculations at dramatically reduced computational cost. Moreover, as temperature is increased, fewer exact Kohn-Sham states are required for a given accuracy, leading to further reductions in computational cost. Finally, it is shown that standard multi-projector expansions of electronic orbitals within atomic spheres in the PAW method lack sufficient completeness at high temperatures. Spectral partitioning provides a systematic solution for this fundamental problem.

2.The role of magnetic helicity when it is absent on average

Authors:Axel Brandenburg, Gustav Larsson

Abstract: Magnetic helicity plays a tremendously important role when it is different from zero on average. Most notably, it leads to the phenomenon of an inverse cascade. Here, we consider decaying magnetohydrodynamic turbulence as well as some less common examples of magnetic evolution under the Hall effect and ambipolar diffusion, as well as cases in which the magnetic field evolution is constrained by the presence of an asymmetry in the number density of chiral fermions, whose spin is systematically either aligned or anti-aligned with its momentum. In all those cases, there is a new conserved quantity: the Hosking integral. We present quantitative scaling results for the magnetic integral scale as well as the magnetic energy density and its spectrum. We also compare with cases were also a magnetic version of the Saffman integral is initially finite.

1.Elevating 0-D global scaling predictions to self-consistent theory-based simulations

Authors:Tim Slendebroek, Joseph McClenaghan, Orso Meneghini, Brendan C. Lyons, Sterling P. Smith, Tom F. Neiser, Nan Shi, Jeff Candy

Abstract: A new workflow in the OMFIT integrated modelling framework (STEP-0D) has been developed to make theory-based prediction of tokamak scenarios starting from zero-dimensional (0D) quantities. The workflow starts with the PRO-create (profiles creator) module, which generates physically plausible plasma profiles and a consistent equilibrium from the same 0D quantities as the IPB98(y,2) confinement scaling. These results form the starting point for the STEP (Stability, Transport, Equilibrium, and Pedestal) module, which then iterates between state-of-the-art theory-based physics models for the equilibrium, core transport, and pedestal to obtain a self-consistent solution. A systematic validation against the International Tokamak Physics Activity (ITPA) global H-mode confinement database demonstrated that on average STEP-0D is capable of predicting the energy confinement time with a mean relative error (MRE) <19%. The validated workflow was used to predict proposed fusion reactor plasmas finding moderate H-factors between 0.9 and 1.2 its further use will be instrumental for the prediction of the plasma performance of a viable fusion power plant and is being utilized in design studies.

2.3D modeling of positive streamers in air with inhomogeneous density

Authors:Baohong Guo, Ute Ebert, Jannis Teunissen

Abstract: We study the effect of an inhomogeneous gas density on positive streamer discharges in air using a 3D fluid model with stochastic photoionization, generalizing earlier work with a 2D axisymmetric model by Starikovskiy and Aleksandrov (2019 Plasma Sources Sci. Technol. 28 095022). We consider various types of planar and (hemi)spherical gas density gradients, focusing on the case in which streamers propagate from a region of density n0 towards a region of higher gas density, where n0 corresponds to 300 K and 1 bar. We observe streamer branching at the density gradient, with branches growing in a flower-like pattern over the gradient surface. Depending on the gas density ratio, the gradient width and other factors, narrow branches are able to propagate into the higher-density gas. In a planar geometry, we find that such propagation is possible up to a gas density slope of 3.5n0/mm, although this value depends on a number of conditions, such as the gradient angle. Surprisingly, a higher applied voltage makes it more difficult for streamers to penetrate into the high-density region, due to an increase of the primary streamer's radius.

1.Diagnosis of Fast Electron Transport by Coherent Transition Radiation

Authors:Yangchun Liu, Xiaochuan Ning, Dong Wu, Tianyi Liang, Peng Liu, Shujun Liu, Xu Liu, Zhengmao Sheng, Wei Hong, Yuqiu Gu, Xiantu He

Abstract: Transport of fast electron in overdense plasmas is of key importance in high energy density physics. However, it is challenging to diagnose the fast electron transport in experiments. In this article, we study coherent transition radiation (CTR) generated by fast electrons on the back surface of the target by using 2D and 3D first-principle particle-in-cell (PIC) simulations. In our simulations, aluminium target of 2.7 g/cc is simulated in two different situations by using a newly developed high order implicit PIC code. Comparing realistic simulations containing collision and ionization effects, artificial simulations without taking collision and ionization effects into account significantly underestimate the energy loss of electron beam when transporting in the target, which fail to describe the complete characteristics of CTR produced by electron beam on the back surface of the target. Realistic simulations indicate the diameter of CTR increases when the thickness of the target is increased. This is attributed to synergetic energy losses of high flux fast electrons due to Ohm heatings and colliding drags, which appear quite significant even when the thickness of the solid target only differs by micrometers. Especially, when the diagnosing position is fixed, we find that the intensity distribution of the CTR is also a function of time, with the diameter increased with time. As the diameter of CTR is related to the speed of electrons passing through the back surface of the target, our finding may be used as a new tool to diagnose the electron energy spectra near the surface of solid density plasmas.

2.Suppressing stimulated Raman side-scattering by vector light

Authors:Xiaobao Jia, Qing Jia, Rui Yan, Jian Zheng

Abstract: Recently, the verification of stimulated Raman side-scattering (SRSS) in different laser inertial confinement fusion ignition schemes poses an underlying risk of SRSS on ignition. In this paper, we propose a method to use the non-uniform polarization nature of vector light to suppress SRSS and give an additional threshold condition determined by the parameter of vector light. For SRSS at 90 degrees, where the scattered electromagnetic wave travels perpendicular to the density profile, the polarization variation of the pump will change the wave vector of scattered light, thereby reducing the growth length and preventing the scattered electromagnetic wave from growing. This suppressive scheme is verified through three-dimensional particle-in-cell simulations. Our illustrative simulation results demonstrate that for linearly polarized Gaussian light, the SRSS signal occurs in the 90-degree direction fiercely. At the same time, for the vector light, there is few SRSS signal even if the condition dramatically exceeds the threshold. Furthermore, we discuss the impact of vector light on stimulated Raman and Brillouin backscattering, and two-plasma decay.

1.Steady regime of radiation pressure acceleration with foil thickness adjustable within micrometers under 10-100 PW laser

Authors:Meng Liu, Wei-Min Wang, Yu-Tong Li

Abstract: Quasi-monoenergetic GeV-scale protons are predicted to efficiently generate via radiation pressure acceleration (RPA) when the foil thickness is matched with the laser intensity, e.g., $L_{mat}$ at several nm to 100 nm with $10^{19}-10^{22} \rm ~W cm^{-2}$ available in laboratory. However, non-monoenergetic protons with much lower energies than prediction were usually observed in RPA experiments, because of too small foil thickness which is hard to bear insufficient laser contrast and foil surface roughness. Besides the technical problems, we here find that there is an upper-limit thickness $L_{up}$ derived from the requirement that the laser energy density should dominate over the ion source, and $L_{up}$ is lower than $ L_{mat}$ with the intensity below $10^{22} \rm~ W cm^{-2}$, which causes inefficient or unsteady RPA. As the intensity is enhanced to $\geq 10^{23} \rm ~W cm^{-2}$ provided by 10-100 PW laser facilities, $L_{up}$ can significantly exceed $L_{mat}$ and therefore RPA becomes efficient. In this regime, $L_{mat}$ acts as a lower-limit thickness for efficient RPA, so the matching thickness can be extended to a continuous range from $L_{mat}$ to $L_{up}$; the range can reach micrometers, within which foil thickness is adjustable. This makes RPA steady and meanwhile the above technical problems can be overcome. Particle-in-cell simulation shows that multi-GeV quasi-monoenergetic proton beams can be steadily generated and the fluctuation of the energy peaks and the energy conversation efficiency remains stable although the thickness is taken in a larger range with increasing intensity. This work predicts that near future RPA experiments with 10-100 PW facilities will enter a new regime with the adjustable and large-range foil thickness for steady acceleration.

2.Enhancement of Fusion Reactivity under Non-Maxwellian Distributions: Effects of Drift-Ring-Beam, Slowing-Down, and Kappa Super-Thermal Distributions

Authors:Haozhe Kong, Huasheng Xie, Bing Liu, Muzhi Tan, Di Luo, Zhi Li, Jizhong Sun

Abstract: Non-Maxwellian distributions of particles are commonly observed in fusion studies, especially for magnetic confinement fusion plasmas. The particle distribution has a direct effect on fusion reactivity, which is the focus of this study. We investigate the effects of three types of non-Maxwellian distributions, namely drift-ring-beam, slowing-down, and kappa super-thermal distributions, on the fusion reactivities of D-T (Deuterium-Trillium) and p-B11 (proton-Boron) using a newly developed program, where the enhancement of fusion reactivity relative to the Maxwellian distribution is computed while keeping the total kinetic energy constant. The calculation results show that for the temperature ranges of interest to us, namely 5-50 keV for D-T and 100-500 keV for p-B11, these non-Maxwellian distributions can enhance the fusion reactivities. In the case of the drift-ring-beam distribution, the enhancement factors for both reactions are affected by the perpendicular ring beam velocity, leading to decreased enhancement in low temperature range and increased enhancement in high temperature range. However, this effect is favorable for p-B11 fusion reaction and unfavorable for D-T fusion reaction. In the slowing-down distribution, the birth speed plays a crucial role in both reactions, and increasing birth speed leads to a shift in the enhancement ranges towards lower temperatures, which is beneficial for both reactions. Finally, the kappa super-thermal distribution results in a relatively large enhancement in the low temperature range with a small high energy power-law index {\kappa}. Overall, this study provides insight into the effects of non-Maxwellian distributions on fusion reactivity and highlights potential opportunities for enhancing fusion efficiency.

1.Global simulations of kinetic-magnetohydrodynamic processes with energetic electrons in tokamak plasmas

Authors:Jian Bao, Wenlu Zhang, Ding Li, Zhihong Lin, Zhiyong Qiu, Wei Chen, Xiang Zhu, Junyi Cheng, Chao Dong, Jintao Cao

Abstract: The energetic electrons (EEs) generated through auxiliary heating have been found to destabilize various Alfven eigenmodes (AEs) in recent experiments, which in turn lead to the EE transport and degrade the plasma energy confinement. In this work, we propose a global fluid-kinetic hybrid model for studying corresponding kinetic-magnetohydrodynamic (MHD) processes by coupling the drift-kinetic EEs to the Landau-fluid model of bulk plasmas in a non-perturbative manner. The numerical capability of Landau-fluid bulk plasmas is obtained based on a well-benchmarked eigenvalue code MAS [Multiscale Analysis of plasma Stabilities, J. Bao et al. Nucl. Fusion accepted 2023], and the EE responses to the electromagnetic fluctuations are analytically derived, which not only contribute to the MHD interchange drive and parallel current but also lead to the newly kinetic particle compression with the precessional drift resonance in the leading order. The hybrid model is casted into a nonlinear eigenvalue matrix equation and solved iteratively using Newton's method. By calibrating the EE precession frequency against the particle equation of motion in general geometry and applying more realistic trapped particle distribution in the poloidal plane, MAS simulations of EE-driven beta-induced Alfven eigenmodes (e-BAE) show excellent agreements with gyrokinetic particle-in-cell simulations, and the non-perturbative effects of EEs on e-BAE mode structure, growth rate and damping rate are demonstrated. With these efforts, the upgraded MAS greatly improves the computation efficiency for plasma problems related to deeply-trapped EEs, which is superior than initial-value simulations restricted by the stringent electron Courant condition regarding to the practical application of fast linear analysis.

2.Development of the Self-Modulation Instability of a Relativistic Proton Bunch in Plasma

Authors:L. Verra AWAKE Collaboration, S. Wyler AWAKE Collaboration, T. Nechaeva AWAKE Collaboration, J. Pucek AWAKE Collaboration, V. Bencini AWAKE Collaboration, M. Bergamaschi AWAKE Collaboration, L. Ranc AWAKE Collaboration, G. Zevi Della Porta AWAKE Collaboration, E. Gschwendtner AWAKE Collaboration, P. Muggli AWAKE Collaboration

Abstract: Self-modulation is a beam-plasma instability that is useful to drive large-amplitude wakefields with bunches much longer than the plasma skin depth. We present experimental results showing that, when increasing the ratio between the initial transverse size of the bunch and the plasma skin depth, the instability occurs later along the bunch, or not at all, over a fixed plasma length, because the amplitude of the initial wakefields decreases. We show cases for which self-modulation does not develop and we introduce a simple model discussing the conditions for which it would not occur after any plasma length. Changing bunch size and plasma electron density also changes the growth rate of the instability. We discuss the impact of these results on the design of a particle accelerator based on the self-modulation instability seeded by a relativistic ionization front, such as the future upgrade of the AWAKE experiment.

3.Hybrid-VPIC: an Open-Source Kinetic/Fluid Hybrid Particle-in-Cell Code

Authors:Ari Le, Adam Stanier, Lin Yin, Blake Wetherton, Brett Keenan, Brian Albright

Abstract: Hybrid-VPIC is an extension of the open-source high-performance particle-in-cell (PIC) code VPIC incorporating hybrid kinetic ion/fluid electron solvers. This paper describes the models that are available in the code and gives an overview of applications of the code to space and laboratory plasma physics problems. Particular choices in how the hybrid solvers were implemented are documented for reference by users. A few solutions for handling numerical complications particular to hybrid codes are also described. Special emphasis is given to the computationally taxing problem of modeling mix in collisional high-energy-density regimes, for which more accurate electron fluid transport coefficients have been implemented for the first time in a hybrid PIC code.

1.Response of a dusty plasma system to external charge perturbations}

Authors:Hitendra Sarkar, Madhurjya P. Bora

Abstract: The excitation of nonlinear wave structures in a dusty plasma caused by a moving external charge perturbation is examined in this work, which uses a 1-D flux corrected transport simulation. The plasma responds uniquely to different nature of the moving charge, depending on which, for small amplitude perturbations, pinned envelope solitons are generated and electrostatic dispersive ion-acoustic shock waves are formed for a large amplitude perturbation. The presence of dust particles is found to suppress the formation of dispersive shocks at low velocity of the external charge debris. The results are also investigated theoretically as a solution to the generalized Gross-Piteavskii equation, which broadly supports the simulation results.

2.A preliminary analysis for efficient laser wakefield acceleration

Authors:Gaetano Fiore

Abstract: We propose a preliminary analytical procedure in 4 steps (based on an improved fully relativistic plane hydrodynamic model) to tailor the initial density of a cold diluted plasma to the laser pulse profile so as to control wave-breaking (WB) of the plasma wave and maximize the acceleration of small bunches of electrons self-injected by the first WB at the density down-ramp.

1.Numerical study of Weibel instability driven by anisotropic electron temperature in collisionless plasmas

Authors:A. Sladkov, A. Korzhimanov

Abstract: We numerically investigate the process of generating magnetic fields from temperature anisotropy of electrons in collisionless initially uniform plasmas. We use a fully kinetic modeling and compare it against a hybrid modeling which treats ions kinetically and use ten-moment fluid model for electrons. The results of the one-to-one comparison show a good agreement in terms of the maximal magnitude of the self-generated magnetic field and similar trends during the non-linear stage of the instability. Additionally, we performed hybrid modelling of the instability without resolving electron spatial scales. In this case the results are only qualitatively the same however it shows that hydrodynamical approach can be used to some extent for the simulation of the Weibel instability in large-scale systems, including astrophysical environments and laser-produced plasmas.

2.How accurate are flux-tube (local) gyrokinetic codes in modeling energetic particle effects on core turbulence?

Authors:A. Di Siena, T. Hayward-Schneider, P. Mantica, J. Citrin, F. Vannini, A. Bottino, T. Goerler, E. Poli, R. Bilato, O. Sauter, F. Jenko

Abstract: Flux-tube gyrokinetic codes are widely used to simulate drift-wave turbulence in magnetic confinement devices. While a large number of studies show that flux-tube codes provide an excellent approximation for turbulent transport in medium-large devices, it still needs to be determined whether they are sufficient for modeling supra-thermal particle effects on core turbulence. This is called into question given the large temperature of energetic particles (EPs), which makes them hardly confined on a single flux-surface, but also due to the radially broad mode structure of energetic-particle-driven modes. The primary focus of this manuscript is to assess the range of validity of flux-tube codes in modeling fast ion effects by comparing radially global turbulence simulations with flux-tube results at different radial locations for realistic JET parameters using the gyrokinetic code GENE. To extend our study to a broad range of different plasma scenarios, this comparison is made for four different plasma regimes, which differ only by the profile of the ratio between the plasma kinetic and magnetic pressure. The latter is artificially rescaled to address the electrostatic limit and regimes with marginally stable, weakly unstable and strongly unstable fast ion modes. These energetic-particle-driven modes is identified as an AITG/KBAE via linear ORB5 and LIGKA simulations. It is found that the local flux-tube simulations can recover well the global results only in the electrostatic and marginally stable cases. When the AITG/KBAE becomes linearly unstable, the local approximation fails to correctly model the radially broad fast ion mode structure and the consequent global zonal patterns. According to this study, global turbulence simulations are likely required in regimes with linearly unstable AITG/KBAEs. In conditions with different fast ion-driven modes, these results might change.

3.Branching of high-current relativistic electron beam in porous materials

Authors:K. Jiang, T. W. Huang, R. Li, M. Y. Yu, H. B. Zhuo, S. Z. Wu, C. T. Zhou, S. C. Ruan

Abstract: Propagation of high-current relativistic electron beam (REB) in plasma is relevant to many high-energy astrophysical phenomena as well as applications based on high-intensity lasers and charged-particle beams. Here we report a new regime of beam-plasma interaction arising from REB propagation in medium with fine structures. In this regime, the REB cascades into thin branches with local density hundred times the initial value and deposits its energy two orders of magnitude more efficiently than that in homogeneous plasma, where REB branching does not occur, of similar average density. Such beam branching can be attributed to successive weak scatterings of the beam electrons by the unevenly distributed magnetic fields induced by the local return currents in the skeletons of the porous medium. Results from a model for the excitation conditions and location of the first branching point with respect to the medium and beam parameters agree well with that from pore-resolved particle-in-cell simulations.

4.A reaction mechanism for oxygen plasmas

Authors:Tiago C Dias, Chloé Fromentin, Luís L Alves, Antonio Tejero-del-Caz, Tiago Silva, Vasco Guerra

Abstract: This work presents a reaction mechanism for oxygen plasmas, i.e. a set of reactions and corresponding rate coefficients that are validated against benchmark experiments. The kinetic scheme is validated in a DC glow discharge for gas pressures of 0.2-10 Torr and currents of 10-40 mA, using the 0D LisbOn KInetics (LoKI) simulation tool and available experimental data. The comparison comprises not only the densities of the main species in the discharge - $\mathrm{O_2(X^3\Sigma_g^-)}$, $\mathrm{O_2(a^1\Delta_g)}$, $\mathrm{O_2(b^1\Sigma_g^+)}$ and $\mathrm{O(^3P)}$ - but also the self-consistent calculation of the reduced electric field and the gas temperature. The main processes involved in the creation and destruction of these species are identified. Moreover, the results show that the oxygen atoms play a dominant role in gas heating, via recombination at the wall and quenching of $\mathrm{O_2(X^3\Sigma_g^-,v)}$ vibrations and $\mathrm{O_2}$ electronically-excited states. It is argued that the development and validation of kinetic schemes for plasma chemistry should adopt a paradigm based on the comparison against standard validation tests, as it is done in electron swarm validation of cross sections.

1.Quasilinear theory of Brillouin resonances in rotating magnetized plasmas

Authors:J. -M. Rax, R. Gueroult, N. J. Fisch

Abstract: Both spin and orbital angular momentum can be exchanged between a rotating wave and a rotating magnetized plasma. Through resonances the spin and orbital angular momentum of the wave can be coupled to both the cyclotron rotation and the drift rotation of the particles. It is however shown that the Landau and cyclotron resonance conditions which classically describe resonant energy-momentum exchange between waves and particles are no longer valid in a rotating magnetized plasma column. In this case a new resonance condition which involves a resonant matching between the wave frequency, the cyclotron frequency modified by inertial effects and the harmonics of the guiding center rotation is identified. A new quasilinear equation describing orbital and spin angular momentum exchanges through these new Brillouin resonances is then derived, and used to expose the wave driven radial current responsible for angular momentum absorption.

1.Proton acceleration with intense twisted laser light

Authors:Camilla Willim, Jorge Vieira, Victor Malka, Luís O. Silva

Abstract: An efficient approach that considers a high-intensity twisted laser of moderate energy (few J) is proposed to generate collimated proton bunches with multi-10-MeV energies from a double-layer hydrogen target. Three-dimensional particle-in-cell simulations demonstrate the formation of a highly collimated and energetic ($\sim 40$ MeV) proton bunch, whose divergence is $\sim 6.5$ times smaller compared to the proton bunch driven by a Gaussian laser containing the same energy. Supported by theoretical modeling of relativistic self-focusing in near-critical plasma, we establish a regime that allows for consistent acceleration of high-energetic proton bunches with low divergence under experimentally feasible conditions for twisted drivers.

2.Statistics of Pressure Fluctuations in Turbulent Kinetic Plasmas

Authors:Subash Adhikari, William H. Matthaeus, Tulasi N. Parashar, Michael A. Shay, Paul A. Cassak

Abstract: In this study we explore the statistics of pressure fluctuations in kinetic collisionless turbulence. A 2.5D kinetic particle-in-cell (PIC) simulation of decaying turbulence is used to investigate pressure balance via the evolution of thermal and magnetic pressure in a plasma with beta of order unity. We also discuss the behavior of thermal, magnetic and total pressure structure functions and their corresponding wavenumber spectra. The total pressure spectrum exhibits a slope of -7/3 extending for about a decade in the ion-inertial range. In contrast, shallower -5/3 spectra are characteristic of the magnetic pressure and thermal pressure. The steeper total pressure spectrum is a consequence of cancellation caused by density-magnetic field magnitude anticorrelation. Further, we evaluate higher order total pressure structure functions in an effort to discuss intermittency and compare the power exponents with higher order structure functions of velocity and magnetic fluctuations. Finally, applications to astrophysical systems are also discussed.

3.Modelling of the driver transverse profile for laser wakefield electron acceleration at APOLLON Research Facility

Authors:Ioaquin Moulanier, Lewis Dickson, Charles Ballage, Ovidiu Vasilovici, Aubin Gremaud, Sandrine Dobosz Dufrenoy, Nicolas Delerue, Lorenzo Bernardi, Ali Mahjoub, Antoine Cauchois, Arnd Specka, Francesco Massimo, Gilles Maynard, Brigitte Cros

Abstract: The quality of electron bunches accelerated by laser wakefields is highly dependant on the temporal and spatial features of the laser driver. Analysis of experiments performed at APOLLON PW-class laser facility shows that spatial instabilities of the focal spot, such as shot-to-shot pointing fluctuations or asymmetry of the transverse fluence, lead to charge and energy degradation of the accelerated electron bunch. It is shown that PIC simulations can reproduce experimental results with a significantly higher accuracy when the measured laser asymmetries are included in the simulated laser's transverse profile, compared to simulations with ideal, symmetric laser profile. A method based on a modified Gerchberg-Saxton iterative algorithm is used to retrieve the laser electric field from fluence measurements in vacuum in the focal volume, and accurately reproduce experimental results using PIC simulations, leading to simulated electron spectra in close agreement with experimental results, for the accelerated charge, energy distribution and pointing of the electron beam at the exit of the plasma.

1.Bounce-averaged drifts: Equivalent definitions, numerical implementations, and example cases

Authors:R. J. J. Mackenbach, J. M. Duff, M. J. Gerard, J. H. E. Proll, P. Helander, C. C. Hegna

Abstract: In this article we provide various analytical and numerical methods for calculating the average drift of magnetically trapped particles across field lines in complex geometries, and we compare these methods against each other. To evaluate bounce-integrals, we introduce a generalisation of the trapezoidal rule which is able to circumvent integrable singularities. We contrast this method with more standard quadrature methods in a parabolic magnetic well and find that the computational cost is significantly lower for the trapezoidal method, though at the cost of accuracy. With numerical routines in place, we next investigate conditions on particles which cross the computational boundary, and we find that important differences arise for particles affected by this boundary, which can depend on the specific implementation of the calculation. Finally, we investigate the bounce-averaged drifts in the optimized stellarator NCSX. From investigating the drifts, one can readily deduce important properties, such as what subset of particles can drive trapped-particle modes, and in what regions radial drifts are most deleterious to the stability of such modes.

2.Comments on "Volume ignition of mixed fuel" by H. Ruhl and G. Korn (Marvel Fusion, Munich)

Authors:Karl Lackner, Rainer Burhenn, Sina Fietz, Alexander von Müller

Abstract: In the most recent note on Marvel Fusion's concept for a laser driven pB reactor without compression, Ruhl and Korn consider the volumetric energy balance of fusion reactions vs. bremsstrahlung losses in a mixed fuel (DT and pB) environment and claim the satisfaction of this necessary "ideal ignition" condition. Their results are based, however, on improper assumptions about the deposition of fusion energy in the plasma. Correcting for them, we show that the quoted composition of their fuel (a solid boron composite, binding high concentrations of D, T and p) would actually preclude ignition due to the high bremsstrahlung losses associated with the presence of boron. To facilitate ignition, Ruhl and Korn also consider the reduction of the bremsstrahlung losses by confining the radiation in the optically thin fuel region by high Z walls. They suggest to preload this region with radiation so that the radiation temperature equals approximately that of the plasma constituents $T_{r} \approx T_{e} \approx T_{i}$. We show that in this set-up the radiation energy - neglected in these considerations - would, however, vastly exceed the thermal energy of the plasma and actually dominate the ignition energy requirements.

1.Estimating the properties of single positive air streamers from measurable parameters

Authors:Dennis Bouwman, Hani Francisco, Ute Ebert

Abstract: We develop an axial model for single steadily propagating positive streamers in air. It uses observable parameters to estimate quantities that are difficult to measure. More specifically, for given velocity, radius, length and applied background field, our model approximates the ionization degree, the maximal electric field, the channel electric field, and the width of the charge layer. These parameters determine the primary excitations of molecules and the internal currents. We do this by first analytically approximating the electron dynamics in different regions of a uniformly-translating streamer head, then we match the solutions between the different regions and finally we use conservation laws to determine unknown quantities. We find good agreement with numerical simulations for a range streamer lengths and background electric fields, even if they do not propagate in a steady manner. Therefore quantities that are difficult to access experimentally can be estimated from easily measurable quantities and our approximations. The theoretical approximations also form a stepping stone towards efficient axial multi-streamer models.

2.The STAG Code: A Fully Relativistic Super Transition Array Calculation Using Green's Functions

Authors:Nathanael Gill, Christopher Fontes, Charles Starrett

Abstract: Calculating opacities for a wide range of plasma conditions (i.e. temperature, density, element) requires detailed knowledge of the plasma configuration space and electronic structure. For plasmas composed of heavier elements, relativistic effects are important in both the electronic structure and the details of opacity spectra. We extend our previously described superconfiguration and super transition array capabilities [N. M. Gill et al., JPB, 56, 015001 (2023)] to include a fully relativistic formalism. The use of hybrid bound-continuum supershells in our superconfigurations demonstrates the importance of a consistent treatment of bound and continuum electrons in dense plasma opacities, and we expand the discussion of these consequences to include issues associated with equation of state and electron correlations between bound and continuum electrons.

1.Deep Learning assisted microwave-plasma interaction based technique for plasma density estimation

Authors:Pratik Ghosh, Bhaskar Chaudhury, Shishir Purohit, Vishv Joshi, Ashray Kothari

Abstract: The electron density is a key parameter to characterize any plasma. Most of the plasma applications and research in the area of low-temperature plasmas (LTPs) is based on plasma density and plasma temperature. The conventional methods for electron density measurements offer axial and radial profiles for any given linear LTP device. These methods have major disadvantages of operational range (not very wide), cumbersome instrumentation, and complicated data analysis procedures. To address such practical concerns, the article proposes a novel machine learning (ML) assisted microwave-plasma interaction based strategy which is capable enough to determine the electron density profile within the plasma. The electric field pattern due to microwave scattering is measured to estimate the density profile. The proof of concept is tested for a simulated training data set comprising a low-temperature, unmagnetized, collisional plasma. Different types of Gaussian-shaped density profiles, in the range $10^{16}-10^{19}m^{-3}$, addressing a range of experimental configurations have been considered in our study. The results obtained show promising performance in estimating the 2D radial profile of the density for the given linear plasma device. The performance of the proposed deep learning based approach has been evaluated using three metrics- SSIM, RMSLE and MAPE. The favourable performance affirms the potential of the proposed ML based approach in plasma diagnostics.

2.Effect of non-local transport of hot electrons on the laser-target ablation

Authors:Z. H. Chen, X. H. Yang, G. B. Zhang, Y. Y. Ma, H. Xu, S. X. Luan, J. Zhang

Abstract: The non-local heat transport of hot electrons during high-intensity lasers interaction with plasmas can preheat the fuel and limit the heat flow in inertial confinement fusion. It increases the entropy of the fuel and decreases the final compression. In this paper, the non-local electron transport model that is based on the improved SNB algorithm has been embedded into the radiation hydrodynamic code and is benchmarked with two classical non-local transport cases. Then we studied a 2$\omega$ laser ablating a CH target by using the non-local module. It is found that the non-local effect becomes significant when the laser intensity is above $1\times 10^{14} \mathrm{W/cm^{2}} $. The mass ablation rate from the SNB model is increased compared to that of the flux-limited model due to the lower coronal plasma temperature. This non-local model has a better agreement with the experimental results compared to that of the flux-limited model. The non-local transport is strongly dependent on the laser frequency, and the thresholds that the non-local transport should be considered are obtained for lasers of different frequencies. The appropriate flux-limiters that should be employed in the flux-limited model for different lasers are also presented. The results here should have a good reference for the laser-target ablation applications.

1.Stopping power of high-density alpha-particle clusters in warm dense deuterium-tritium fuels

Authors:Z. P. Fu, Z. W. Zhang, K. Lin, D. Wu, J. Zhang

Abstract: The state of burning plasma had been achieved in inertial confinement fusion (ICF), which was regarded as a great milestone for high-gain laser fusion energy. In the burning plasma, alpha particles incident on the cryogenic (warm dense) fuels cannot be simply regarded as single particles, and the new physics brought about by the density effects of alpha particles should be considered. In this work, the collective interaction between them has been considered, namely the effect of the superposition of wake waves. The stopping power of alpha-particle clusters, i.e. the rate of energy loss per unit distance traveled has been calculated using both analytical and simulation approaches. In theory, we obtain the stopping power of alpha clusters in cryogenic (warm dense) fuel by the dielectric function method, which illustrates the importance of the effective interaction between particles. Simulation results using the LAPINS code show that the collective stopping power of the alpha cluster is indeed increased via coherent superposition of excitation fields (the excitation of high-amplitude wake waves). However, the comparison between simulation and theoretical results also illustrates a coherent-decoherent transition of the stopping power of the cluster. The initial conditions with various sizes and densities of the alpha clusters have been considered to verify the condition of decoherence transition. Our work provides a theoretical description of the transport properties of high-density alpha particles in warm dense cryogenic fuel and might give some theoretical guidance for the design of actual fusion processes.

1.A strategy to identify breakdown location in MITICA test facility: results of high voltage test campaign

Authors:Loris Zanotto, Marco Boldrin, Giuseppe Chitarin, Mattia Dan, Tommaso Patton, Francesco Santoro, Vanni Toigo, Hiroyuki Tobari, Atsushi Kojima, Hans Decamps

Abstract: The Acceleration Grid Power Supply of the MITICA test facility in Padova (Italy) is currently under commissioning. The power conversion system, the DC generator, and the High Voltage equipment have been individually commissioned, whereas the integration tests are ongoing. It is a challenging process due to the unconventional application, to the variety of different electrical technologies involved and to the complexity of the interfaces. During the integrated tests of the power supplies the achievement of 700kV stable operation has been demonstrated for the first time in a Neutral Beam Injector, but an unexpected event occurred, most likely a breakdown in the HV part, which resulted in a fault of the DC generator. A subsequent test using an auxiliary power supply was performed to check the voltage withstanding capability of the HV plant, but another breakdown occurred at around 1MV. This paper describes the activity performed to identify the location of the breakdowns affecting the integrated tests. A test campaign has been devised with increased diagnostic capabilities and specific strategy conceived to trigger intentional breakdowns in specific locations and collect measurement patterns for different cases. The results of the campaign will be presented and the current understanding of the issue will be described, with a view on future tests and further improvements of diagnostics.

2.Unsupervised classification of fully kinetic simulations of plasmoid instability using Self-Organizing Maps (SOMs)

Authors:Sophia Köhne, Elisabetta Boella, Maria Elena Innocenti

Abstract: The growing amount of data produced by simulations and observations of space physics processes encourages the use of methods rooted in Machine Learning for data analysis and physical discovery. We apply a clustering method based on Self-Organizing Maps (SOM) to fully kinetic simulations of plasmoid instability, with the aim of assessing its suitability as a reliable analysis tool for both simulated and observed data. We obtain clusters that map well, a posteriori, to our knowledge of the process: the clusters clearly identify the inflow region, the inner plasmoid region, the separatrices, and regions associated with plasmoid merging. SOM-specific analysis tools, such as feature maps and Unified Distance Matrix, provide one with valuable insights into both the physics at work and specific spatial regions of interest. The method appears as a promising option for the analysis of data, both from simulations and from observations, and could also potentially be used to trigger the switch to different simulation models or resolution in coupled codes for space simulations.

1.Self-consistent gyrokinetic modelling of turbulent and neoclassical tungsten transport in toroidally rotating plasmas

Authors:Kyungtak Lim, Xavier Garbet, Yanick Sarazin, Etienne Gravier, Maxime Lesur, Guillaume Lo-Cascio, Timothe Rouyer

Abstract: The effect of toroidal rotation on both turbulent and neoclassical transport of tungsten (W) in tokamaks is investigated using the flux-driven, global, nonlinear 5D gyrokinetic code GYSELA. Nonlinear simulations are carried out with different levels of momentum injection that drive W to the supersonic regime, while the toroidal velocity of the main ions remains in the subsonic regime. The numerical simulations demonstrate that toroidal rotation induces centrifugal forces that cause W to accumulate in the outboard region, generating an in-out poloidal asymmetry. This asymmetry enhances neoclassical inward convection, which can lead to central accumulation of W in cases of strong plasma rotation. The core accumulation of W is mainly driven by inward neoclassical convection. However, as momentum injection continues, roto-diffusion, proportional to the radial gradient of the toroidal velocity, becomes significant and generate outward turbulent flux in the case of ion temperature gradient (ITG) turbulence. Overall, the numerical results from nonlinear GYSELA simulations are in qualitative agreement with the theoretical predictions for impurity transport, as well as experimental observations.

2.Effect of triangularity on plasma turbulence and the SOL-width scaling in L-mode diverted tokamak configurations

Authors:Kyungtak Lim, Maurizio Giacomin, Paolo Ricci, António Coelho, Olivier Février, Davide Mancini, Davide Silvagni, Louis Stenger

Abstract: The effect of triangularity on tokamak boundary plasma turbulence is investigated by using global, flux-driven, three-dimensional, two-fluid simulations. The simulations show that negative triangularity stabilizes boundary plasma turbulence, and linear investigations reveal that this is due to a reduction of the magnetic curvature drive of interchange instabilities, such as the resistive ballooning mode. As a consequence, the pressure decay length $L_p$, related to the SOL power fall-off length $\lambda_q$, is found to be affected by triangularity. Leveraging considerations on the effect of triangularity on the linear growth rate and nonlinear evolution of the resistive ballooning mode, the analytical theory-based scaling law for $L_p$ in L-mode plasmas, derived by Giacomin \textit{et al.} [{Nucl. Fusion}, \href{}{\textbf{61} 076002} (2021)], is extended to include the effect of triangularity. The scaling is in agreement with nonlinear simulations and a multi-machine experimental database, which include recent TCV discharges dedicated to the study of the effect of triangularity in L-mode diverted discharges. Overall, the present results highlight that negative triangularity narrows the $L_p$ and considering the effect of triangularity is important for a reliable extrapolation of $\lambda_q$ from present experiments to larger devices.