Optimizing Photon Throughput vs. Spectral Resolution of the NSTX-U FIDA Profile Spectroscopy System using pyFIDASIM

Fast-ion studies are a major part of the scientific goal for NSTX-U for extrapolation to burning plasmas. To this effect, NSTX-U is equipped with a Fast-Ion D-Alpha (FIDA) spectroscopy system to observe the fast-ion distribution function.1 In recent years, the two diffraction grating spectrometers employed in the NSTX-U FIDA system have been further improved by using a different set of focusing lenses and by removing the need of blocking bars via the installation of sCMOS cameras.2 Moreover, the pyFIDASIM code has been used to further optimize the system in terms of its signal-to-noise and spectral resolution. An optimized spectrometer entrance slit width has been determined to provide good spectral resolution to resolve impurity line emissions in the FIDA spectral range, and sufficient signal to noise ratio when operating the new CMOS camera at 5-6 ms time resolution. Additionally, an integrated point-spread function capability has been developed in pyFIDASIM to provide radial localization in FIDA profiles obtained from the NSTX-U spectroscopy system. Finally, a benchmark comparison will be shown between pyFIDASIM and FIDASIM to validate parity between the two codes.