Observation of Toroidal Alfven Eigenmodes during Minor Disruptions in Ohmic Plasmas

TL;DR

This paper reports the first observation of Toroidal Alfven Eigenmodes (TAEs) during minor disruptions in a spherical tokamak, linking their occurrence to runaway electrons and plasma density changes.

Contribution

The study provides the first identification of TAEs in ohmically heated plasmas during disruptions, expanding understanding of plasma instabilities without auxiliary heating.

Findings

TAEs observed during minor disruptions in SUNIST tokamak

TAEs have frequencies of 150-500 kHz and specific mode structures

TAEs are associated with runaway electrons and density sweeping

Abstract

Toroidal Alfven eigenmodes (TAEs) excited in purely ohmically heated plasmas without any auxiliary heating have been identified for the first time in the SUNIST spherical tokamak. The TAE modes are observed during minor disruptions and have a frequency range of 150-500 kHz. The mode structure analysis indicates the existence of both m/n=-3/-1 and -4/-1 harmonics, propagating in the electron diamagnetic direction in the laboratory frame of reference. These TAEs appear simultaneously with the generation of runaway electrons in the current quench phase, accompanying with the density sweeping during the minor disruption. Possible driving mechanisms and potential applications of these TAEs are discussed.