Runaway-electron formation and electron slide-away in an ITER post-disruption scenario

TL;DR

This paper investigates runaway-electron formation in ITER post-disruption scenarios using a new non-linear simulation tool, revealing a feedback mechanism that can trigger electron slide-away at lower fields than previously thought.

Contribution

The study introduces the NORSE code for non-linear runaway-electron modeling and uncovers a feedback mechanism leading to electron slide-away at reduced electric fields.

Findings

Feedback mechanism induces slide-away at lower fields.

Entire electron population converts to runaways rapidly.

Time to feedback depends on heat removal processes.

Abstract

Mitigation of runaway electrons is one of the outstanding issues for the reliable operation of ITER and other large tokamaks, and accurate estimates for the expected runaway-electron energies and current are needed. Previously, linearized tools (which assume the runaway population to be small) have been used to study the runaway dynamics, but these tools are not valid in the cases of most interest, i.e. when the runaway population becomes substantial. We study runaway-electron formation in a post-disruption ITER plasma using the newly developed non-linear code NORSE, and describe a feedback mechanism by which a transition to electron slide-away can be induced at field strengths significantly lower than previously expected. If the electric field is actively imposed using the control system, the entire electron population is quickly converted to runaways in the scenario considered. We find the time until the feedback mechanism sets in to be highly dependent on the details of the mechanisms removing heat from the thermal electron population.