TEM turbulence in simulation and experiment in the HSX stellarator

The geometric flexibility of stellarator experiments offers an opportunity to optimize the magnetic field for reduced turbulent transport. The reliability of such optimization depends on the ability of simulations to accurately predict turbulence in existing devices, and few validation studies have been performed for the stellarator. Here, the first comparison of experimental measurements to nonlinear simulations of the TEM at experimental parameters in a stellarator is presented. The magnetic-field flexibility of the Helically Symmetric eXperiment is exploited to investigate Trapped Electron Mode (TEM) turbulence in quasi-helically symmetric (QHS) and degraded-symmetry (Mirror) configurations. The experimental heat flux shows that anomalous transport is larger in the Mirror configuration at the mid-radius when temperature and density profiles are matched. While linear growth rates are not predictive of overall turbulence, general aspects of experimental transport are captured by nonlinear simulations. The heat flux and density fluctuation amplitude in simulation reproduce a stronger dependence on the density gradient, and the simulated heat flux matches measurements within experimental uncertainties. This confirms that ▽n-driven TEM turbulence is the dominant source of anomalous transport in HSX.