Saturation of temperature-gradient driven ion acoustic waves in Vlasov-Fokker-Planck simulations

The generation of Ion Acoustic Waves (IAWs) presents challenges for desirable laser pro-duced plasmas, including laser energy loss via stimulated scattering and limited heat penetration through wave-particle interactions. An important contributor to the latter is the Return Current Instability (RCI), where super-thermal electrons rush out from a high temperature region, drawing a current which balanced by a slow, opposing, electron drift. These modifications to the electron distribution function incur their own heat transfer reductions [1], and IAWs are additionally driven when the return current speed exceeds the sound speed. IAWs saturate through a range of processes, and this investigation is the first to be driven by a realistic, reinforced, electron temperature gradient, modelled from a quasi-stationary period in experiment [1].

We investigate the growth and saturation of RCI-driven IAWs in full-domain (10⁴ λ_De), Vlasov-Fokker-Plank-electron / Vlasov-ion simulations, using K2 [2], spanning up to 800ps (10⁵ ω_pe ). These 1D-3V simulations show several phases of saturation and a consistent linear growth phase. Ion heating, subharmonic generation, electron and ion trapping, nonlinear frequency shifts, and their interrelations are all studied and displayed in thorough detail for simulations with varying noise states and initialization procedures.