Spatial Heterodyne Spectroscopy for Motional Stark Effect measurements at DIII-D

One technique to explore the magnetic field structure of fusion experiments is Motional Stark Effect (MSE) spectroscopy, wherein the Stark splitting of the Balmer alpha emission of neutral beams is analyzed. A recent technique to spectrally resolve this radiation is Spatial Heterodyne Spectroscopy (SHS), offering high photon throughput and high spectral resolution. While previous iterations of the SHS system [1] at DIII-D focused on high-speed measurements, the spectrometer has been revised for improved optical performance and the ability to resolve both the splitting and the sigma-to-pi line ratio of the Stark manifold. The revision of the spectrometer entails a reduction in optical components, and a novel technique to compensate for geometric Doppler broadening of the beam emission. Moreover, a calibration technique using a HeNe laser ($lambda$ = 632.8 nm) has been developed that allows one to precisely align the two gratings of the SHS system. To that end, comparisons of the calibration with a newly developed SHS forward modeling code and one-to-one performance-comparisons with conventional Czerny Turner spectrometers will be presented.

[1] M.G. Burke et al.: Rev. Sci. Instrum. 94 (2023)