Experimental investigation of kinetic instabilities driven by runaway electrons in the EXL-50 spherical torus

TL;DR

This paper reports the first experimental observation of high-frequency runaway electron instabilities in a spherical torus, revealing their dependence on plasma density and providing insights into their control and mitigation.

Contribution

It demonstrates the first experimental validation of the relationship between runaway electron density and instability threshold, and observes chirping behaviors consistent with theoretical models.

Findings

Instabilities driven by runaway electrons exhibit exponential dependence on plasma density.

Chirping characteristics of the instabilities align with the Berk-Breizman model.

Increasing plasma density stabilizes the instabilities.

Abstract

In this study, the first observation of high-frequency instabilities driven by runaway electrons has been reported in the EXL-50 spherical torus using a high-frequency magnetic pickup coil. The central frequency of these instabilities is found to be exponentially dependent on the plasma density, similar to the dispersion relation of the whistler wave. The instability frequency displays chirping characteristics consistent with the Berk-Breizman model of beam instability. Theoretically, the excitation threshold of the instability driven by runaway electrons is related to the ratio of the runaway electron density to the background plasma density, and such a relationship is first demonstrated experimentally in this study. The instability can be stabilized by increasing the plasma density, consistent with the wave-particle resonance mechanism. This investigation demonstrates the controlled excitation of chirping instabilities in a tokamak plasma and reveals new features of these instabilities, thereby advancing the understanding of the mechanisms for controlling and mitigating runaway electrons.