Collisionless microinstabilities in stellarators. IV. The ion-driven trapped-electron mode

TL;DR

This paper investigates a new type of microinstability in stellarators, driven by ions but requiring trapped electrons, which differs from traditional trapped-electron modes and impacts plasma confinement.

Contribution

It identifies and clarifies a previously less understood ion-driven trapped-electron mode in stellarators, expanding knowledge of microinstabilities in magnetic confinement devices.

Findings

The instability is similar to the tokamak 'ubiquitous mode'.

It is driven by ion free energy and requires trapped electrons.

This mode causes less transport than typical trapped-electron modes.

Abstract

Optimised stellarators and other magnetic-confinement devices having the property that the average magnetic curvature is favourable for all particle orbits are called maximum-$J$ devices, and have recently been shown to be immune to trapped-particle instabilities driven by the density gradient. Gyrokinetic simulations reveal, however, that another instability can arise, which is also associated with particle trapping but causes less transport than typical trapped-electron modes. The nature of this instability is clarified here. It is shown to be similar to the "ubiquitous mode" in tokamaks, and is driven by ion free energy but requires trapped electrons to exist.