Edge scanning reflectometry on SPARC

Edge scanning reflectometry (ESRL) on SPARC aims at determining the ICRF evanescent layer location to help assess the ICRF antenna loading and also measuring the H-mode pedestal density profiles. ESRL uses the standard frequency sweep technique and has three bands (Ka-band, U-band and E-band) to cover frequencies from 27 to 90 GHz. Both O-mode and left-hand-cutoff (LHC) X-mode are implemented. For B₀ ~ 12 T, the cutoff density ranges from ~0.1e20 m^-3 at 27 GHz O-mode to ~4e20 m^-3 at 90 GHz LHC X-mode. Each band has one launching horn antenna and two receiving horn antennas for O-mode and X-mode respectively. Vacuum windows are carefully designed for the tritium environment. Considering the relatively short distance (~15 m) from the diagnostic hall to the tokamak, overmoded waveguides are adequate to minimize signal loss while allow system flexibility. The mm-wave and IF electronics have been analyzed and optimized based on synthetic reflectometry data. Choosing a fast frequency sweep rate (full-band sweep at 5 microsecond) and 50 MHz digitizer sampling rate, ESRL is expected to have sufficient spatial and temporal resolution. The horn antennas and other critical parts are optimized with COMSOL modeling. The latest status will be presented.