Compressional Alfv\'en eigenmodes excited by runaway electrons

TL;DR

This paper demonstrates that runaway electrons in post-disruption plasma can excite compressional Alfvén eigenmodes, providing insights into kinetic instabilities and potential mitigation strategies in magnetic fusion experiments.

Contribution

It is the first to show that runaway electrons can drive CAE modes, with calculated mode structures, frequencies, and growth rates aligning with experimental observations.

Findings

Runaway electrons can excite CAE modes in post-disruption plasma.

Mode growth rates exceed damping when runaway electron density is high.

CAE excitation offers a new method to mitigate runaway electrons.

Abstract

Compressional Alfv\'en eigenmodes (CAE) driven by energetic ions have been observed in magnetic fusion experiments. In this paper, we show that the modes can also be driven by runaway electrons formed in post-disruption plasma, which may explain kinetic instabilities observed in DIII-D disruption experiments with massive gas injection. The mode-structure is calculated, as are the frequencies which are in agreement with experimental observations. Using a runaway electron distribution function obtained from a kinetic simulation, the mode growth rates are calculated and found to exceed the collisional damping rate when the runaway electron density exceeds a threshold value. The excitation of CAE poses a new possible approach to mitigate seed runaway electrons during the current quench and surpassing the avalanche.