Coarse-grained gyrokinetics for the critical ion temperature gradient in stellarators

TL;DR

This paper introduces a simplified gyrokinetic model for predicting the ion temperature gradient threshold in stellarators, enabling efficient optimization and design improvements with accurate results.

Contribution

A novel coarse-grained gyrokinetic formalism that accurately estimates the critical gradient with low computational cost for stellarator optimization.

Findings

Good agreement with gyrokinetic solver results across stellarator designs.

Designed a stellarator configuration with nearly double the linear critical gradient of Wendelstein 7-X.

Achieved improved nonlinear critical gradient and reduced ITG mode transport.

Abstract

We present a modified gyrokinetic theory to predict the critical gradient that determines the linear onset of the ion temperature gradient (ITG) mode in stellarator plasmas. A coarse-graining technique is applied to the drift curvature, entering the standard gyrokinetic equations, around local minima. Thanks to its simplicity, this novel formalism yields an estimate for the critical gradient with a computational cost low enough for application to stellarator optimization. On comparing against a gyrokinetic solver, our results show good agreement for an assortment of stellarator designs. Insight gained here into the physics of the onset of the ITG driven instability enables us to devise a compact configuration, similar to the Wendelstein 7-X device, but with almost twice the ITG linear critical gradient, an improved nonlinear critical gradient, and reduced ITG mode transport above the nonlinear critical gradient.