Structure and overstability of resistive modes with runaway electrons

TL;DR

This paper develops a new theoretical model to analyze how runaway electron currents influence resistive MHD modes in tokamaks, revealing the emergence of a new runaway current layer affecting mode stability and frequency.

Contribution

It introduces a novel dispersion relation incorporating runaway electron effects and validates it with numerical simulations, advancing understanding of resistive mode behavior in tokamaks.

Findings

Runaway electron current creates a new layer affecting mode properties.

Modes exhibit real frequencies due to runaway current contribution.

Theoretical results agree well with numerical simulations.

Abstract

We investigate the effects of runaway electron current on the dispersion relation of resistive magnetohydrodynamic modes in tokamaks. We present a new theoretical model to derive the dispersion relation, which is based on the asymptotic analysis of the resistive layer structure of the modes. It is found that in addition to the conventional resistive layer, a new runaway current layer can emerge whose properties depend on the ratio of the Alfv\'en velocity to the runaway electron convection speed. Due to the contribution from this layer, both the tearing mode and kink mode will have a real frequency in addition to a growth rate. The derived dispersion relation has been compared with numerical results using both a simplified eigenvalue calculation and a M3D-C1 linear simulation, and good agreement is found in both cases.