Runaway electron current reconstitution after a non-axisymmetric magnetohydrodynamic flush

TL;DR

This paper investigates the physics behind the reconstitution of runaway electron current after a non-axisymmetric MHD event in a reactor similar to ITER, highlighting the roles of trapped runaways and impurity dynamics.

Contribution

It reveals the competing effects of impurity purge and runaway avalanche in current reconstitution post-MHD event in a high-current plasma.

Findings

Trapped runaways dominate seed formation for reconstitution.

Incomplete impurity purge enhances avalanche growth.

A 2-3 MA current step occurs before reconstitution.

Abstract

Benign termination of mega-ampere (MA) level runaway current has been convincingly demonstrated in recent JET and DIII-D experiments, establishing it as a leading candidate for runaway mitigation on ITER. This comes in the form of a runaway flush by parallel streaming loss along stochastic magnetic field lines formed by global magnetohydrodynamic instabilities, which are found to correlate with a low-Z injection that purges the high-Z impurities from a post-thermal-quench plasma. Here we show the competing physics that govern the post-flush reconstitution of the runaway current in a ITER-like reactor where significantly higher current is expected. The trapped ``runaways'' are found to dominate the seeding for runaway reconstitution, and the incomplete purge of high-Z impurities helps drain the seed but produces a more efficient avalanche, two of which compete to produce a 2-3~MA step in current drop before runaway reconstitution of the plasma current.