Trapped-Electron Runaway Effect

TL;DR

This paper investigates the behavior of trapped electrons in tokamaks under strong electric fields, revealing a unique runaway mechanism with distinct signatures that differ from traditional Dreicer runaways.

Contribution

It introduces and quantifies the trapped-electron runaway effect, highlighting their unique signatures and the process of detrapping as they accelerate.

Findings

Trapped electrons can become runaways after detrapping near the tokamak center.

Distinct velocity spectrum and radial extent characterize trapped-electron runaways.

Trapped-electron runaways differ from Dreicer runaways in signature and behavior.

Abstract

In a tokamak, trapped electrons subject to a strong electric field cannot run away immediately, because their parallel velocity does not increase over a bounce period. However, they do pinch towards the tokamak center. As they pinch towards the center, the trapping cone becomes more narrow, so eventually they can be detrapped and run away. When they run away, trapped electrons will have very a different signature from circulating electrons subject to the Dreicer mechanism. The characteristics of what are called trapped-electron runaways are identified and quantified, including their distinguishable perpendicular velocity spectrum and radial extent.