Suppressing Trapped-Electron-Mode-Driven Turbulence via Optimization of Three-Dimensional Shaping

TL;DR

This study demonstrates that optimizing three-dimensional magnetic configurations can suppress TEM-driven turbulence in stellarators, with implications for reducing turbulent heat transport by considering universal instabilities at different plasma beta levels.

Contribution

The paper introduces optimized 3D-shaped stellarator configurations that suppress TEM-driven turbulence, highlighting the role of universal instabilities and plasma beta in turbulence control.

Findings

TEMs are suppressed in optimized NT and PT configurations.

Heat fluxes from TEMs are reduced in these configurations.

Increasing plasma beta halves heat flux in NT configuration and suppresses it in PT.

Abstract

Turbulent transport driven by trapped electron modes (TEMs) is believed to drive significant heat and particle transport in quasihelically symmetric stellarators. Two three-dimensionally-shaped magnetic configurations with suppressed trapped-electron-mode (TEM)-driven turbulence were generated through optimization that targeted quasihelical symmetry and the available energy of trapped electrons. Initial equilibria have flux surface shapes with a helically rotating negative triangularity (NT) and positive triangularity (PT). In gyrokinetic simulations, TEMs are suppressed in the reduced-TEM NT and PT configurations, showing that negative triangularity does not have the same beneficial turbulence properties over positive triangularity as seen in tokamaks. Heat fluxes from TEMs are also suppressed. Without temperature gradients and with a strong density gradient, the most unstable modes at low $k_y$ were consistent with toroidal universal instabilities (UIs) in the NT case and slab UIs in the PT case. Nonlinear simulations show that UIs drive substantial heat flux in both the NT and PT configurations. A moderate increase in $\beta$ halves the heat flux in the NT configuration, while suppressing the heat flux in the PT geometry. Based on the present work, future optimizations aimed at reducing electrostatic drift wave-driven turbulent transport will need to consider UIs if $\beta$ is sufficiently small.