Stellarator Turbulence Optimization Based on Flux Surface Triangularity*

A important goal of stellarator optimization is to find configurations that reduce turbulent transport using three-dimensional (3D) shaping. Trapped-electron-mode (TEM) turbulence can play a significant role in  quasi-symmetric stellarators [1]. One way to improve the turbulent transport properties of tokamak plasmas is through negative flux-surface triangularity [2]. Nonlinear gyrokinetic simulations suggests that the heat flux of TEM turbulence correlates with the free energy available in background temperature and density gradients, as quantified by the available energy [3]. In this work, we address the possibility of using negative triangularity as a mechanism to reduce TEM turbulence in stellarator plasmas. Towards this end, a new optimization framework is developed using local 3D MHD equilibrium solutions [4]. This approach has been successfully employed to improve the quasi-symmetry properties—a metric for reducing neoclassical transport—while simultaneously reducing the available energy for local 3D MHD equilibria in a stellarator with negative helically-rotating triangularity and in a stellarator with positive helically-rotating triangularity.

[1] B. Faber et al., Phys. Plasmas 22 (2015)

[2] A. Pochelon et al., Nucl. Fusion 47 (1999)

[3] R. J. J . Mackenbach et al., Phys. Rev. Lett. 128 (2022)

[4] J. M. Duff et al., Phys. Plasmas 29 (2022)