Runaway dynamics in reactor-scale spherical tokamak disruptions

TL;DR

This study investigates runaway electron generation and mitigation in reactor-scale spherical tokamaks, highlighting the importance of mitigation strategies and their impact on energy loss and heat loads during disruptions.

Contribution

It provides a detailed analysis of runaway dynamics and mitigation effectiveness in spherical tokamaks using the DREAM numerical framework, a novel application for this device type.

Findings

Mitigation strategies are often necessary to limit runaway currents.

A deuterium-neon mixture can achieve tolerable runaway and ohmic currents.

Most thermal energy loss occurs via radial transport, risking localized heat loads.

Abstract

Understanding generation and mitigation of runaway electrons in disruptions is important for the safe operation of future tokamaks. In this paper we investigate runaway dynamics in reactor-scale spherical tokamaks. We study both the severity of runaway generation during unmitigated disruptions, as well as the effect that typical mitigation schemes based on massive material injection have on runaway production. The study is conducted using the numerical framework DREAM (Disruption Runaway Electron Analysis Model). We find that, in many cases, mitigation strategies are necessary to prevent the runaway current from reaching multi-megaampere levels. Our results indicate that with a suitably chosen deuterium-neon mixture for mitigation, it is possible to achieve a tolerable runaway current and ohmic current evolution. With such parameters, however, the majority of the thermal energy loss happens through radial transport rather than radiation, which poses a risk of unacceptable localised heat loads.