Speaker
Description
The Helically Symmetric eXperiment (HSX) has previously demonstrated reduced neoclassical transport and reduced flow damping with quasisymmetry (QHS) as compared to a degraded-symmetry (Mirror) configuration, but the effect of quasisymmetry on the anomalous transport is less clear. Here, experimental heat flux and density fluctuation measurements are compared to gyrokinetic simulations of Trapped Electron Mode (TEM) turbulence in the QHS and Mirror configurations. Density fluctuation amplitudes are measured by a conventional heterodyne O-mode stepped-frequency reflectometer ($15-26\,\mathrm{GHz}$), capable of scanning across the minor radius for densities available in HSX. The reflectometer views the outboard midplane of an up-down symmetric cross-section, matching the center of the flux tube used in simulation and enabling a relatively direct first-order comparison without a complicated synthetic diagnostic.
Instead of trying to reproduce an individual discharge in simulation, a large number of plasma discharges are collected in a database to extract scans across the density and temperature gradient. The gradient dependence of the heat flux and density fluctuation amplitude are reproduced in simulation, and the simulated heat flux matches measurements within experimental uncertainties, confirming that $\nabla n$-driven TEM turbulence is the dominant source of anomalous transport in HSX at the mid-radius. The experimental heat flux also shows that anomalous transport is larger in the Mirror configuration when temperature and density profiles are matched. A shift of turbulence propagation from the electron to the ion direction suggests a target for future investigations of the TEM in HSX.
This work was supported by US DOE grant DE-FG02-93ER54222.
