Reduction of electrostatic turbulence in a quasi-helically symmetric stellarator via critical gradient optimization

TL;DR

This paper introduces a stellarator design optimized to significantly increase the critical gradient for ion temperature gradient driven turbulence, resulting in reduced electrostatic turbulence and improved stability.

Contribution

The paper presents a novel stellarator configuration optimized for a high critical gradient, combining drift curvature maximization with quasisymmetry for enhanced stability and low turbulence.

Findings

Largest critical gradient observed in any stellarator.

Gyrokinetic simulations show low turbulence levels above the threshold.

Configuration achieves low neoclassical transport and minimal alpha particle losses.

Abstract

We present a stellarator configuration optimized for a large threshold (``critical gradient'') for the onset of the ion temperature gradient (ITG) driven mode, which achieves the largest critical gradient we have seen in any stellarator. Above this threshold, gyrokinetic simulations show that the configuration has low turbulence levels over an experimentally relevant range of the drive strength. The applied optimization seeks to maximize the drift curvature, leading to enhanced local-shear stabilization of toroidal ITG modes, and the associated turbulence. These benefits are combined with excellent quasisymmetry, yielding low neoclassical transport and vanishingly small alpha particle losses. Analysis of the resulting configuration suggests a trade-off between magnetohydrodynamic (MHD) and ITG stability.