Critical gradient optimization for quasi-isodynamic stellarators

TL;DR

This paper introduces new methods to optimize quasi-isodynamic stellarator configurations, enhancing ITG mode stability and reducing turbulence-driven transport, with potential improvements over existing designs like W7-X.

Contribution

It develops an updated model for ITG threshold, proposes a configuration that localizes modes in separate wells, and minimizes electron effects to improve stability.

Findings

High critical gradient achieved in a six-field-period QI configuration.

Heat fluxes comparable or lower than W7-X in optimized configurations.

Minimized destabilizing electron effects in the inverse mirror structure.

Abstract

We present new and updated methods for reducing transport caused by electrostatic ion temperature gradient (ITG) driven turbulence in quasi-isodynamic (QI) configurations. We first show an updated model for the threshold (critical) gradient of localized toroidal ITG modes. It is then argued that it is desirable for ITG modes to "split" and localize in separate curvature drift wells, which is leveraged to produce a six-field-period QI configuration with a high critical gradient. We show that the destabilizing effect of kinetic electrons (Costello & Plunk 2025a) on localized ITG modes can be minimized in a magnetic field structure, which we dub "inverse mirror". Applying a general optimization target that improves ITG stability above the critical gradient yields an inverse mirror configuration, which produces heat fluxes below or equal to the W7-X high mirror configuration for a range of applied density gradients.