Quasi-linear analysis of the extraordinary electron wave destabilized by runaway electrons

TL;DR

This paper investigates how runaway electrons in tokamak plasmas can destabilize the extraordinary electron wave, leading to rapid pitch-angle scattering and potential modifications in synchrotron radiation, with implications for plasma diagnostics.

Contribution

It provides a quasi-linear analysis of the EXEL wave instability driven by relativistic runaway electrons, including simulation results on wave-particle interactions.

Findings

Rapid pitch-angle scattering of runaway electrons within 100-1000 microseconds.

Modification of synchrotron radiation spectrum due to wave-particle interaction.

Potential for experimental detection of runaway electron interactions via radiation spectrum changes.

Abstract

Runaway electrons with strongly anisotropic distributions present in post-disruption tokamak plasmas can destabilize the extraordinary electron (EXEL) wave. The present work investigates the dynamics of the quasi-linear evolution of the EXEL instability for a range of different plasma parameters using a model runaway distribution function valid for highly relativistic runaway electron beams produced primarily by the avalanche process. Simulations show a rapid pitch-angle scattering of the runaway electrons in the high energy tail on the $100-1000\;\rm \mu s$ time scale. Due to the wave-particle interaction, a modification to the synchrotron radiation spectrum emitted by the runaway electron population is foreseen, exposing a possible experimental detection method for such an interaction.