Electron runaway in ASDEX Upgrade experiments of varying core temperature

TL;DR

This study models runaway electron formation in ASDEX Upgrade experiments, revealing temperature-dependent behaviors and the limited impact of hot-tail mechanisms on post-disruption runaway currents.

Contribution

The paper introduces an enhanced simulation model incorporating hot-tail runaway mechanisms, providing new insights into temperature effects on runaway electron generation.

Findings

Runaway currents are about 3 kA in simulations.

Colder plasmas show insensitivity of runaway current to initial temperature.

Hot-tail runaway can significantly increase in hotter plasmas.

Abstract

The formation of a substantial post-disruption runaway electron current in ASDEX Upgrade material injection experiments is determined by avalanche multiplication of a small seed population of runaway electrons. For the investigation of these scenarios, the runaway electron description of the coupled 1.5D transport solvers ASTRA-STRAHL is amended by a fluid-model describing electron runaway caused by the hot-tail mechanism. Applied in simulations of combined background plasma evolution, material injection, and runaway electron generation in ASDEX Upgrade discharge #33108, both the Dreicer and hot-tail mechanism for electron runaway produce only $\sim$ 3$~$kA of runaway current. In colder plasmas with core electron temperatures $T_\mathrm{e,c}$ below 9$~$keV, the post-disruption runaway current is predicted to be insensitive to the initial temperature, in agreement with experimental observations. Yet in hotter plasmas with $T_\mathrm{e,c} > 10~\mathrm{keV}$, hot-tail runaway can be increased by up to an order of magnitude, contributing considerably to the total post-disruption runaway current. In ASDEX Upgrade high temperature runaway experiments, however, no runaway current is observed at the end of the disruption, despite favourable conditions for both primary and secondary runaway.