Helically-trapped electron mode in optimized stellarator

TL;DR

This paper identifies a helically-trapped electron mode in the W7-X stellarator, driven by density gradients, which impacts particle confinement and fusion ash removal through nonlinear turbulence processes.

Contribution

It is the first to analyze the HTEM in W7-X using global gyrokinetic simulations, revealing its structure, excitation mechanism, and nonlinear saturation behavior.

Findings

HTEM localizes on the inner side of the torus with unfavorable magnetic curvature.

Mode propagates in the ion direction with a frequency much lower than the diamagnetic frequency.

Steady-state turbulence driven by HTEM results in significant particle flux affecting confinement.

Abstract

Global gyrokinetic simulations find a strong helically-trapped electron mode (HTEM) driven by density gradients in the optimized stellarator W7-X fusion experiment. The eigenmode structure localizes in the inner side of the torus with an unfavorable magnetic curvature and weak magnetic field, where there is a large fraction of helically-trapped electrons. The mode is excited mostly by the ion free energy and propagates in the ion direction with a linear frequency much smaller than the diamagnetic frequency. The instability saturates by nonlinear processes of turbulence spreading in the real space and spectral transfer from unstable to damped regions. The steady state HTEM turbulence drives a large particle flux that may have significant implications for the confinement of fusion fuel and removal of fusion ash in the stellarator reactor.