Effects of magnetic perturbations and radiation on the runaway avalanche

TL;DR

This paper develops a method to incorporate momentum-dependent spatial transport effects into runaway electron growth models, demonstrating how magnetic perturbations influence runaway dynamics and mitigation in tokamak plasmas.

Contribution

It introduces a perturbative approach to include spatial transport effects in runaway electron models, enhancing simulation accuracy for plasma disruptions.

Findings

Magnetic perturbations reduce runaway current in slow current quench scenarios.

Spatial transport can increase the effective critical electric field for runaway generation.

Localized edge perturbations are less effective in suppressing runaways unless off-axis generation occurs.

Abstract

The electron runaway phenomenon in plasmas depends sensitively on the momentum-space dynamics. However, efficient simulation of the global evolution of systems involving runaway electrons typically requires a reduced fluid description. This is needed for example in the design of essential runaway mitigation methods for tokamaks. In this paper, we present a method to include the effect of momentum-dependent spatial transport in the runaway avalanche growth rate. We quantify the reduction of the growth rate in the presence of electron diffusion in stochastic magnetic fields and show that the spatial transport can raise the effective critical electric field. Using a perturbative approach we derive a set of equations that allows treatment of the effect of spatial transport on runaway dynamics in the presence of radial variation in plasma parameters. This is then used to demonstrate the effect of spatial transport in current quench simulations for ITER-like plasmas with massive material injection. We find that in scenarios with sufficiently slow current quench, due to moderate impurity and deuterium injection, the presence of magnetic perturbations reduces the final runaway current considerably. Perturbations localized at the edge are not effective in suppressing the runaways, unless the runaway generation is off-axis, in which case they may lead to formation of strong current sheets at the interface of the confined and perturbed regions.