Correlation electron cyclotron emission measurements in the HSX stellarator

Long-wavelength k_⊥ρ_s < 1.5 radiation temperature fluctuations are measured within the mid-radius of the Helically Symmetric eXperiment (HSX) stellarator using a 16-channel CECE radiometer. Despite low single-pass optical depth, the X2 emission may be treated as blackbody under certain conditions present in HSX, and CECE measures turbulent electron temperature fluctuations. Outside the ECRH deposition region (r/a > 0.2), fluctuation amplitudes increase with normalized electron temperature gradient. This increase in amplitude coincides with broadening of the fluctuation frequency spectrum at higher normalized electron temperature gradient. To model the core turbulence in HSX, flux-tube gyrokinetic simulations have been performed with the GENE code. Linear simulations suggest that trapped electron mode is the dominant long-wavelength instability. Nonlinear simulations predict that electron temperature fluctuations increase with normalized electron temperature gradient, consistent with CECE measurements. For more detailed comparisons, a synthetic CECE diagnostic has been developed to process the gyrokinetic simulation data and account for diagnostic effects on the temperature fluctuation signal. A comparison of a synthetic and experimental CECE frequency spectrum will be presented.