Plasma Physics (physics.plasm-ph)

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.

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.

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.

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.