Regulating Trapped Electron Mode Turbulence by Modifying Coil Currents in the HSX Stellarator*

In the Helically Symmetric eXperiment (HSX), heat transport outside the mid-radius is believed to be dominated by Trapped Electron Mode (TEM) turbulence. The growth rates of TEMs depend strongly on the magnetic geometry, thus making them susceptible to adjustments in the main and auxiliary coil currents.  Using the VMEC equilibrium code, we have generated a database of >10⁶ coil-current configurations. Using a hierarchy of models, we aim to predict which configurations will exhibit reduced heat transport. Configurations are first selected for their helical symmetry, low epsilon effective profiles, satisfaction of the Mercier criterion, and avoidance of magnetic resonances. Then, configurations are selected using three flux surface averaged field geometry metrics, including magnetic shear, F_κ and G_ss, where F_κ describes the overlap of magnetic wells with regions bad curvature, and G_ss describes the overlap of the flux surface expansion with regions of bad curvature.  Finally, we use gyrokinetic simulations to calculate TEM growth rates and nonlinear heat fluxes. These results indicate significant flexibility in HSX for modifying both TEM growth rates and the nonlinear heat flux. Work is ongoing to identify 2-3 configurations whose heat flux will be measured experimentally.