Optimizing Trapped Electron Mode Stability in Quasi-Symmetric Stellarators

An important goal of stellarator optimization is to find stellarator configurations with reduced turbulent transport using three-dimensional (3D) shaping. Trapped electron mode (TEM) turbulence is thought to play a prominent role in the confinement properties of quasi-symmetric stellarators [1]. One method for improving the turbulent transport properties of tokamak plasmas is to appeal to negative triangularity. This improvement is in part attributed to precessional drift reversal of trapped electron orbits. In this work, we attempt to use negative 'helical' triangularity as a mechanism to reduce TEM turbulence in quasi-helically symmetric stellarators. A new optimization framework is developed using local 3D MHD equilibrium solutions [2]. Optimization studies using local 3D MHD solutions have successfully found configurations with improved quasi-symmetry. In this work, we use optimization of local 3D MHD solutions to improve TEM stability. The gyrokinetic code GENE is used to assess the local TEM linear stability characteristics. These insights help improve metrics for modeling TEM turbulence in ensuing optimization calculations.