Examining Fringe Distortions in Spatial Heterodyne Spectroscopy for Motional Stark Effect Measurements*

A well-established technique to probe the magnetic field structure of fusion experiments is Motional Stark Effect (MSE) spectroscopy, where the splitting of the Balmer-alpha (656 nm) emission of neutral beams is analyzed. Spatial Heterodyne Spectroscopy (SHS) is a recently developed diagnostic for fusion plasmas which promises sufficient photon throughput and spectral resolution for MSE fluctuation measurements. First results obtained by a prototype SHS system at DIII-D showed that the performance of the setup was limited by high readout noise at operation above ~1 kHz [1], necessitating image intensification and a reduction in quantum efficiency of the system for higher speed measurements. The SHS has been revised for increased signal-to-noise ratio and enhanced contrast via a modified setup with fewer optical elements, and an attempt to reduce the impact of fringe distortions caused by imperfect elements that have been noted on SHS instruments [1, 2]. The efficacy of vertical demagnification, which increases the photons per pixel incident on the detector and hence reduces the impact of camera readout noise is evaluated with ray-tracing on the reduced distortion setup. The improvements to signal strength and optical efficiency of the renovated system are presented alongside discussions as to the potential future applications of SHS.

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

[2] C.R. Englert et al.: Applied Optics 43(2004)