Self-mediation of runaway electrons via self-excited wave-wave and wave-particle interactions

TL;DR

This paper presents fully kinetic simulations revealing how runaway electron wave interactions can rapidly diffuse electrons and reduce high-energy runaway currents, with implications for tokamak safety and astrophysics.

Contribution

It introduces the first fully kinetic simulation of runaway-driven wave instabilities leading to nonlinear saturation in warm plasma.

Findings

Fast growth of slow-X modes compared to whistler modes

Parametric decay of slow-X modes into whistlers

Significant reduction of high-energy runaway current within short timescales

Abstract

Nonlinear dynamics of runaway electron induced wave instabilities can significantly modify the runaway distribution critical to tokamak operations. Here we present the first-ever fully kinetic simulations of runaway-driven instabilities towards nonlinear saturation in a warm plasma where collisional damping is subdominant. It is found that the slow-X modes grow an order of magnitude faster than the whistler modes, and they parametrically 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 diffuse runaways to the backward direction. This reduces almost half of the current carried by high-energy runaways, over a time scale orders of magnitude faster than experimental shot duration. These results beyond quasilinear analysis may impact anisotropic energetic electrons broadly in laboratory, space and astrophysics.