Resilience of quasi-isodynamic stellarators against trapped-particle instabilities

TL;DR

This paper demonstrates that perfectly quasi-isodynamic stellarators are inherently stable against certain trapped-particle instabilities due to the stabilizing effect of high bounce frequency trapped particles, independent of magnetic geometry details.

Contribution

It proves that in ideal quasi-isodynamic stellarators, high bounce frequency trapped particles stabilize electrostatic and collisionless instabilities, a result derived from energy balance considerations.

Findings

High bounce frequency trapped particles are stabilizing.

Perfect quasi-isodynamic stellarators are stable against specific instabilities.

Stability is independent of magnetic geometry details.

Abstract

It is shown that in perfectly quasi-isodynamic stellarators, trapped particles with a bounce frequency much higher than the frequency of the instability are stabilizing in the electrostatic and collisionless limit. The collisionless trapped-particle instability is therefore stable as well as the ordinary electron-density-gradient-driven trapped-electron mode. This result follows from the energy balance of electrostatic instabilities and is thus independent of all other details of the magnetic geometry.