First-principles based plasma profile predictions for optimized stellarators

TL;DR

This paper presents advanced first-principles simulations of plasma profiles in optimized stellarators, integrating turbulence, neoclassical transport, and external sources to better understand energy confinement and guide stellarator design.

Contribution

It introduces a novel coupling framework combining GENE-3D, KNOSOS, and TANGO for comprehensive plasma profile predictions in stellarators, including turbulence and neoclassical effects.

Findings

Inefficient electron-ion thermal coupling in turbulence regimes explains energy confinement degradation.

Direct ion heating can improve on-axis ion temperature in stellarators.

The framework aids in optimizing stellarator design by accurately modeling transport processes.

Abstract

In the present Letter, first-of-its-kind computer simulations predicting plasma profiles for modern optimized stellarators -- while self-consistently retaining neoclassical transport, turbulent transport with 3D effects, and external physical sources -- are presented. These simulations exploit a newly developed coupling framework involving the global gyrokinetic turbulence code GENE-3D, the neoclassical transport code KNOSOS, and the 1D transport solver TANGO. This framework is used to analyze the recently observed degradation of energy confinement in electron-heated plasmas in the Wendelstein 7-X stellarator, where the central ion temperature was "clamped" to $T_i \approx 1.5$ keV regardless of the external heating power. By performing first-principles based simulations, we provide key evidence to understand this effect, namely the inefficient thermal coupling between electrons and ions in a turbulence-dominated regime, which is exacerbated by the large $T_e/T_i$ ratios, and show that a more efficient ion heat source, such as direct ion heating, will increase the on-axis ion temperature. This work paves the way towards the use of high-fidelity models for the development of the next generation of stellarators, in which neoclassical and turbulent transport are optimized simultaneously.