Role of kinetic instability in runaway electron avalanche and elevated critical electric fields

TL;DR

This paper investigates how kinetic whistler-wave instabilities influence runaway electron avalanches in tokamaks, showing they can increase avalanche thresholds and growth rates, aligning models more closely with experimental observations.

Contribution

It introduces a self-consistent quasilinear model demonstrating the role of kinetic instabilities in runaway electron dynamics, improving understanding of experimental phenomena.

Findings

Increased avalanche growth rate and threshold electric field due to wave scattering.

Explanation of fast electron cyclotron emission signal growth.

Identification of spontaneous plasma wave excitation affecting runaway suppression.

Abstract

The effects of kinetic whistler-wave instabilities on the runaway-electron (RE) avalanche is investigated. With parameters from DIII-D experiments, we show that RE scattering from excited whistler waves can explain several poorly understood experimental results seen in a variety of tokamaks. We find an increase of the avalanche growth rate and threshold electric field, bringing the present model much closer to observations than previous results. The excitation of kinetic instabilities and the scattering of resonant electrons are calculated self-consistently using a quasilinear model. We also explain the observed fast growth of electron cyclotron emission (ECE) signals and excitation of very low-frequency whistler modes observed in the quiescent RE experiments at DIII-D [D. A. Spong et al., submitted to Phys. Rev. Lett.]. These results indicate that by controlling the background thermal plasma temperature, the plasma wave can be excited spontaneously in tokamak disruptions and the avalanche generation of runaway electrons may be suppressed.