Abstract Details
Abstracts
Author: Qile Zhang
Requested Type: Consider for Invited
Submitted: 2026-03-01 11:00:37
Co-authors: Qile Zhang, Yanzeng Zhang, Qi Tang, Xianzhu Tang
Contact Info:
University of Maryland, College Park
University of Maryland
College Park, MD 20742
United States
Abstract Text:
Relativistic runaway electrons are efficiently generated in dilute plasmas through strong electric-field acceleration and avalanche multiplication via knock-on collisions. In tokamaks, runaways can damage plasma-facing components during both startup and major disruptions. Understanding the wave instabilities excited by these runaways—and their nonlinear feedback on runaway distributions—is therefore essential.
Here, we identify a nonlinear self-mediation pathway for runaways. Our linear dispersion analysis identifies slow-X waves as the primary instability for runaways during an avalanche: despite stronger collisional damping, they outpace whistler waves in both onset and growth rate. We present fully kinetic simulations of these runaway-driven wave instabilities towards nonlinear saturation in a warm plasma. It is found that the slow-X modes grow an order of magnitude faster than the whistler modes, and undergo parametric decay to produce whistlers much faster than those directly driven by runaways. These parent-daughter waves, as well as secondary and tertiary wave instabilities, initiate a chain of wave-particle resonances that strongly pitch-angle scatter runaways to the counter-streaming direction. This nearly halves the high-energy runaway current, over time scales much shorter than a typical discharge.
The results indicate that runaway saturation is set by nonlinear generation of a multi-mode wave spectrum that opens additional resonant scattering channels. This mechanism may regulate the transport of anisotropic energetic electrons in laboratory, space, and astrophysical plasmas.
Characterization: 1.0
Comments: