Use of positrons to study transport in tokamak plasmas (invited)

TLDR

Positron annihilation times in tokamak plasmas are long relative to typical particle confinement times. The study outlines the design of a positron transport experiment, the data it can yield, and the physics questions it may answer. Positrons are injected via bursts of neutral positronium that are ionized in the plasma, and their transport is probed by tracking the time‑dependent gamma radiation from annihilation on a limiter. The technique may also aid transport studies in other magnetic confinement devices like reversed‑field pinches and magnetic mirrors.

Abstract

It now appears feasible to deposit positrons (e+) in a tokamak plasma by injecting bursts of neutral positronium atoms (e+e−), which are then ionized by the plasma. The annihilation time of these positrons in the plasma is long compared with typical particle containment times. Thus the subsequent transport of the positrons can be studied by monitoring the time dependence of the annihilation, gamma radiation produced when the positrons strike a limiter. This paper discusses the design of such an experiment, the kinds of data which can be obtained, and the physics questions which this experiment might address. This diagnostic technique could also be useful in studying transport in other magnetic confinement devices such as reversed-field pinches and magnetic mirrors.