Nonlocal transport witnessed in Vlasov-Fokker-Planck simulations of the return current instability

Electron thermal transport in laser produced plasmas is decreased from classical estimates

due to the buildup of an electric field from the rapid motion of superthermal electrons. This

electric field drives a return current in the bulk of the electrons, which can couple to Ion Acoustic

Waves (IAWs), becoming unstable if the electron drift velocity exceeds the sound speed. Several

nonlocal transport models can approximate these kinetic effects, however hydrodynamic codes

often employ constant-valued flux limiters, and an accurate understanding of the transport

processes, including the role of the Return Current Instability (RCI), is lacking.

Kinetic simulations with the code K2 [1], considering Vlasov-Fokker-Planck electrons, and

Vlasov ions show the growth of IAWs, as a result of the RCI. The growth rate and saturation

level of these IAWs is compared to experimental data [2], obtained using Thomson scattering

diagnostics. The effect of laser heating is accounted for, and was shown in steady-state K2

simulations to significantly impact the linear growth rate and threshold, by altering the shape

of the electron distribution function [2].

[1] M. Sherlock, J. P. Brodrick, and C.P. Ridgers, Phys. Plasmas 24, 082706 (2017)

[2] A. L. Milder, J. Zielinski, J. Katz, et al. Submitted for Publication (2022)