Abstract Details
Abstracts
Author: Qile Zhang
Requested Type: Poster
Submitted: 2024-04-20 12:40:09
Co-authors: Xianzhu Tang, Yanzeng Zhang, Qi Tang
Contact Info:
Los Alamos National Laboratory
2290 B 39th St
Los Alamos, NM 87544
United States
Abstract Text:
Runaway electrons can excite plasma kinetic instabilities to significantly modify the distributions where the background plasma
is warm and/or of low density with weak collisional damping. Here we present the first-ever fully kinetic simulations with runaway-driven waves towards nonlinear saturation in a warm plasma (T>100 eV), whose parameters correspond to the tokamak start-up process. The complex
physics uncovered differ greatly from previous quasilinear studies in the literature. Our simulations are initiated with a runaway distribution from the relativistic Fokker-Planck-Boltzmann solver. The slow-X modes can have an order of magnitude larger growth rate than the fastest growing whistler modes. Once the strongest forward-propagating (along the runaway beam) slow-X modes get excited, it can go through parametric decays to produce multiple lower frequency modes including forward whistler waves. This process is much faster than the growth of whistlers directly driven by the runaways. During this short time, the slow-X waves notably diffuse the high energy tail over momentum and pitch sequentially through n=-1,0,1 resonances (with positive n referring to normal Doppler resonances). Meanwhile, the strong slow-X modes can also trigger a series of wave-particle resonances that diffuse forward-going runaways quickly to the backward direction at moderate momentum (p~5me c), involving n=1 resonances of sequentially slow-X, backward and forward whistlers. Fast backward diffusion continues to occur at higher energy (p>10me c) involving sequentially n=-2,-1,0,1,2 resonances of the forward whistlers, and also n=1 resonance of backward whistlers. This reduces almost half of the current carried by high energy runaways. These strong and fast backward diffusion processes occur over a time period orders of magnitude shorter than experimental shot duration or collisional current decay time scale.
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