Gyrokinetic stability theory of electron-positron plasmas

TL;DR

This paper analyzes the linear gyrokinetic stability of electron-positron plasmas in laboratory-relevant conditions, revealing the absence of instabilities in homogeneous fields and conditions for density peaking in dipole configurations.

Contribution

It extends previous gyrokinetic stability analysis to include electromagnetic effects in electron-positron plasmas, identifying conditions for stability and density profile evolution.

Findings

No gyrokinetic instabilities in homogeneous magnetic fields.

Instability thresholds in dipole fields match ideal MHD predictions.

Inward particle flux leads to density peaking under certain temperature gradients.

Abstract

The linear gyrokinetic stability properties of magnetically confined electron-positron plasmas are investigated in the parameter regime most likely to be relevant for the first laboratory experiments involving such plasmas, where the density is small enough that collisions can be ignored and the Debye length substantially exceeds the gyroradius. Although the plasma beta is very small, electromagnetic effects are retained, but magnetic compressibility can be neglected. The work of a previous publication (Helander, 2014) is thus extended to include electromagnetic instabilities, which are of importance in closed-field-line configurations, where such instabilities can occur at arbitrarily low pressure. It is found that gyrokinetic instabilities are completely absent if the magnetic field is homogeneous: any instability must involve magnetic curvature or shear. Furthermore, in dipole magnetic fields, the stability threshold for interchange modes with wavelengths exceeding the Debye radius coincides with that in ideal MHD. Above this threshold, the quasilinear particle flux is directed inward if the temperature gradient is sufficiently large, leading to spontaneous peaking of the density profile.