Post-Disruptive Runaway Electron Beam in COMPASS Tokamak

TL;DR

This paper reports the first observation of post-disruptive runaway electron beams, called 'runaway plateau', in the COMPASS tokamak, achieved through argon gas injection during plasma disruptions, with implications for ITER-relevant studies.

Contribution

First demonstration of runaway plateau formation after disruption in COMPASS tokamak using argon injection, analyzing parameters influencing its occurrence.

Findings

Runaway plateau observed during plasma current ramp-up.

Disruptions with runaway plateaus meet key parameters for their formation.

Pre-injection plasma current significantly influences runaway plateau occurrence.

Abstract

For ITER-relevant runaway electron studies, such as suppression, mitigation, termination and/or control of runaway beam, obtaining the runaway electrons after the disruption is important. In this paper we report on the first achieved discharges with post-disruptive runaway electron beam, entitled "runaway plateau", in the COMPASS tokamak. The runaway plateau is produced by massive gas injection of argon. Almost all of the disruptions with runaway electron plateaus occurred during the plasma current ramp-up phase. Comparison between the Ar injection discharges with and without plateau has been done for various parameters. Parametrisation of the discharges shows that COMPASS disruptions fulfill the range of parameters important for the runaway plateau occurrence. These parameters include electron density, electric field, disruption speed, effective safety factor, maximum current quench electric field. In addition to these typical parameters, the plasma current value just before the massive gas injection surprisingly proved to be important.