What is the fate of runaway positrons in tokamaks?

TL;DR

This paper investigates the behavior of runaway positrons in tokamaks, revealing they mostly drift out and annihilate on the wall, offering potential diagnostic signatures for plasma runaway dynamics.

Contribution

It introduces a phase space dynamics model for runaway positrons in tokamaks, incorporating geometry, loop voltage, radiation, and collisions, and predicts their drift and annihilation behavior.

Findings

Runaway positrons predominantly drift out of plasma.

In-plasma annihilation probability is less than 0.1%.

Runaway positron dynamics can serve as diagnostic signatures.

Abstract

Massive runaway positrons are generated by runaway electrons in tokamaks. The fate of these positrons encodes valuable information about the runaway dynamics. The phase space dynamics of a runaway position is investigated using a Lagrangian that incorporates the tokamak geometry, loop voltage, radiation and collisional effects. It is found numerically that runaway positrons will drift out of the plasma to annihilate on the first wall, with an in-plasma annihilation possibility less than 0.1\%. The dynamics of runaway positrons provides signatures that can be observed as diagnostic tools.