Plasma Physics (physics.plasm-ph)

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.

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.

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.

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.

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.

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.