The universal instability in general geometry

TL;DR

This paper demonstrates analytically that the universal plasma instability can occur in a wide range of magnetic geometries, including toroidal and sheared configurations, and explores its relation to other known modes.

Contribution

It extends the understanding of the universal instability to general geometries and clarifies its connection to trapped-electron modes and residual instabilities in stellarators.

Findings

Universal instability exists in general sheared and toroidal geometries.

In toroidal systems, it relates closely to trapped-electron modes.

Residual instability can persist even when trapped-electron drive is suppressed.

Abstract

The "universal" instability has recently been revived by Landreman, Antonsen and Dorland [1], who showed that it indeed exists in plasma geometries with straight (but sheared) magnetic field lines. Here it is demonstrated analytically that this instability can be present in more general sheared and toroidal geometries. In a torus, the universal instability is shown to be closely related to the trapped-electron mode, although the trapped-electron drive is usually dominant. However, this drive can be weakened or eliminated, as in the case in stellarators with the maximum-$J$ property, leaving the parallel Landau resonance to drive a residual mode, which is identified as the universal instability.