Detector Design and Analysis Technique for Local Electric Field Fluctuation Diagnostic in High Temperature Plasmas Using Spatial Heterodyne Spectroscopy

A novel diagnostic for measuring plasma electric field fluctuations is being developed. It employs high-speed measurements of the spectral separation of the Motional Stark Effect (MSE) split neutral beam emission, where fluctuations in the MSE component separation is proportional to local magnetic and electric field fluctuations. A spatial heterodyne spectrometer (SHS) with high etendue (~5 mm²sr) and resolution (~0.14 nm) has been developed to spectrally resolve the Stark multiplet while collecting all available light to minimize photon noise. An analysis technique based on a least-squares fitting to linear perturbations of the SHS interferogram is used to perform sensitivity studies and to guide the design of new volume phase holographic gratings to maximize the sensitivity of the diagnostic. In preliminary studies, the uncertainty in electric field fluctuations scale with the relative photon noise applied to the interferogram. A low readout noise, high sampling speed (1 MHz), high etendue detector system with modest spatial resolution is in development to record the time-varying interferogram. Both a high-speed, low readout noise CMOS sensor and a multianode microchannel plate photomultiplier are under evaluation.