Comparing HSX Reflectometer Measurements with a Full-Wave Synthetic Diagnostic

A synthetic reflectometer diagnostic has been developed to model the reflectometer used at the Helically Symmetric Experiment (HSX) stellarator. The main purpose of the HSX reflectometer is to measure density fluctuations in order to study turbulence and coherent modes in HSX. A full-wave 2-D synthetic diagnostic modeling the HSX reflectometer has been used to investigate its radial localization, characterize its sensitivities, and relate gyrokinetic simulations to physical measurements. The synthetic diagnostic results show that signal localization peaks slightly outboard of the cutoff surface, with the main localization lobe spanning between 15% and 30% of the minor radius. The poloidal and radial wavenumber sensitivities are k 0 < 1.7 cm-1 and k r < 6 cm-1, respectively, with minimal variation in k r sensitivity across probing regions and density profiles. Conversely, k 0 sensitivity increases to > 2 cm-1 in the core of the device, in accordance with an increase in local flux-surface curvature. Synthetic reflectometer signals scale linearly with density fluctuation amplitude over a large range of fluctuation levels. Multiple density fluctuation level estimators are tested to identify the optimal choice for use in HSX, with an equation from G.D. Conway Plasma Phys. Control. Fusion 1999 consistently performing best overall. Synthetic measurements of rescaled density fluctuations from a gyrokinetic simulation are then compared against experimental measurements, with coherent mode activity at 13-15 kHz and 22 kHz observed in both experimental and synthetic frequency spectra. Synthetic spectra overestimated experimental spectra at low frequencies and underestimated at high frequency.