Confinement of passing and trapped runaway electrons in the simulation of an ITER current quench

TL;DR

This study uses 3D MHD simulations to analyze how stochastic magnetic fields during an ITER disruption influence runaway electron confinement and losses, highlighting the potential for significant RE losses due to field stochasticity.

Contribution

The paper evaluates the impact of stochastic magnetic fields on runaway electron confinement during ITER disruptions using integrated MHD and particle tracing simulations.

Findings

Plasma becomes fully stochastic for 8 ms during disruption

Stochastic fields can deconfine trapped runaway electrons

Significant runaway electron losses are possible due to field stochasticity

Abstract

Runaway electrons (REs) present a high-priority issue for ITER but little is known about the extent to which RE generation is affected by the stochastic field intrinsic to disrupting plasmas. RE generation can be modelled with reduced kinetic models and there has been recent progress in involving losses due to field stochasticity, either via a loss-time parameter or radial transport coefficients which can be estimated by tracing test electrons in 3D fields. We evaluate these terms in ITER using a recent JOREK 3D MHD simulation of plasma disruption to provide the stochastic magnetic fields where RE markers are traced with the built-in particle tracing module. While the MHD simulation modelled only the current quench phase, the case is MHD unstable and exhibits similar relaxation as would be expected during the thermal quench. Therefore, the RE simulations can be considered beginning right after the thermal quench but before the MHD relaxation is complete. The plasma is found to become fully stochastic for 8 ms and the resulting transport is sufficient to overcome RE avalanche before flux surfaces are reformed. We also study transport mechanisms for trapped REs and find those to be deconfined as well during this phase. While the results presented here are not sufficient to assess the magnitude of the formed RE beam, we show that significant RE losses could be expected to arise due to field stochasticity.