Plasma Physics (physics.plasm-ph)

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.

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.

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.

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.

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.

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.