Assessing global ion thermal confinement in critical-gradient-optimized stellarators

TL;DR

This paper evaluates the ion thermal confinement in two optimized stellarator configurations, using gyrokinetic simulations to analyze how magnetic geometry and edge effects influence plasma performance.

Contribution

It introduces a comprehensive simulation framework for assessing confinement in turbulence-optimized stellarators, highlighting the importance of edge modeling.

Findings

HSK and QSTK achieve higher core-to-edge temperature ratios than HSX.

Confinement time is more affected by edge temperature than core temperature.

Optimizing turbulence in the outer core can improve overall plasma performance.

Abstract

We investigate the confinement properties of two recently devised quasi-helically symmetric stellarator configurations, HSK and QSTK. Both have been optimized for large critical gradients of the ion temperature gradient mode, which is an important driver of turbulent transport in magnetic confinement fusion devices. To predict the resulting core plasma profiles, we utilize an advanced theoretical framework based on the gyrokinetic codes GENE and GENE-3D, coupled to the transport code TANGO. Compared to the HSX stellarator, both HSK and QSTK achieve significantly higher core-to-edge temperature ratios, partly thanks to their smaller aspect ratios, with the other part due to more detailed shaping of the magnetic geometry achieved during optimization. The computed confinement time, however, is less sensitive to core temperature than edge temperature, simply due to the disproportionate influence the edge has on stored plasma energy. We therefore emphasize the possible benefits of further optimizing turbulence in the outer core region, and the need to include accurate modelling of confinement in the edge region in order to assess overall plasma performance of turbulence optimized stellarators.