A Correlation ECE Diagnostic for the HSX-Upgrade Stellarator

Here we present a new correlation ECE diagnostic for the Helically Symmetric eXperiment Upgrade (HSX-U) stellarator. HSX has demonstrated a significant reduction in neoclassical transport compared with conventional stellarators. Anomalous transport, likely due to Trapped Electron Mode turbulence, remains an important loss channel. For operation at 1.0T, a 16-channel heterodyne ECE radiometer was developed for high-resolution electron temperature measurements. However, this system is sensitive only to temperature fluctuations above ${\sim}2.5\%$, well above the level predicted by gyrokinetic simulations. Furthermore, we show that only relatively high-$k$ fluctuations with poor radial localization are accessible with this conventional system. To measure turbulent temperature fluctuations at the level predicted in HSX-U, a correlation ECE technique can be used. Based on electron temperature fluctuation and heat flux predictions from gyrokinetic simulations, we present a new correlation ECE diagnostic for HSX-U. ECE optical depth is estimated assuming representative profiles for HSX-U plasmas over a range of central electron temperature and plasma density and indicates blackbody emission.