Annihilation-Gamma-based Diagnostic Techniques for Magnetically Confined Electron-Positron Pair Plasma

TL;DR

This paper proposes and demonstrates gamma-ray emission techniques based on annihilation signatures to diagnose magnetically confined electron-positron pair plasmas, overcoming challenges of conventional methods.

Contribution

It introduces annihilation-based diagnostic techniques utilizing gamma-ray emission correlations and demonstrates their effectiveness through experimental tests with radioactive sources.

Findings

Gamma-ray emission profiles can be used to reconstruct plasma spatial structure.

Annihilation signatures enable localization of plasma interactions and boundary effects.

The techniques successfully differentiate between various annihilation processes.

Abstract

Efforts are underway to magnetically confine electron--positron pair plasmas to study their unique behavior, which is characterized by significant changes in plasma time and length scales, supported waves, and unstable modes. However, use of conventional plasma diagnostics presents challenges with these low-density and annihilating matter-antimatter plasma. To address this problem, we propose to develop techniques based on the distinct emission provided by annihilation. This emission exhibits two spatial correlations: the distance attenuation of isotropic sources and the back-to-back propagation of momentum-preserving 2-$\gamma$ annihilation. We present the results of our analysis of the $\gamma$ emission rate and the spatial profile of the annihilation in a magnetized pair plasma from direct pair collisions, from the formation and decay of positronium, as well as from transport processes. In order to demonstrate the effectiveness of annihilation-based techniques, we tested them on annular $\gamma$ emission profiles produced by a $\beta^+$ radioisotope on a rotating turntable. Direct and positronium-mediated annihilation result in overlapping volumetric $\gamma$ sources, and the 2-$\gamma$ emission from these volumetric sources can be tomographically reconstructed from coincident counts in multiple detectors. Transport processes result in localized annihilation where field lines intersect walls, limiters, or internal magnets. These localized sources can be identified by the fractional $\gamma$ counts on spatially distributed detectors.