Effective Critical Electric Field for Runaway-Electron Generation

TL;DR

This paper investigates the factors affecting the effective critical electric field for runaway-electron generation in plasmas, highlighting the roles of temperature dependence and synchrotron radiation losses, and comparing findings with recent experiments.

Contribution

The study provides a detailed numerical analysis of the effective critical electric field, emphasizing the impact of temperature and radiation losses on runaway-electron thresholds.

Findings

Effective critical field often exceeds previous estimates.

Temperature dependence significantly influences primary runaway generation.

Synchrotron radiation losses raise the threshold for runaway electrons.

Abstract

In this Letter we investigate factors that influence the effective critical electric field for runaway-electron generation in plasmas. We present numerical solutions of the kinetic equation and discuss the implications for the threshold electric field. We show that the effective electric field necessary for significant runaway-electron formation often is higher than previously calculated due to both (1) extremely strong dependence of primary generation on temperature, and (2) synchrotron radiation losses. We also address the effective critical field in the context of a transition from runaway growth to decay. We find agreement with recent experiments, but show that the observation of an elevated effective critical field can mainly be attributed to changes in the momentum-space distribution of runaways, and only to a lesser extent to a de facto change in the critical field.