Development of a polarization-resolved spatial heterodyne spectrometer for high wavelength resolution and high throughput measurement of near-infrared atomic emission lines in magnetically confined toroidal plasmas*

For spectroscopic diagnostics of magnetically confined toroidal plasmas, a viewing-chord-integrated emission line spectrum can be spatially inverted using the correspondence between the given magnetic field profile along the viewing chord and the Zeeman effect appearing on the spectrum. Observation of near-infrared emission lines is advantageous for this method owing to a relative increase in the magnitude of the Zeeman effect compared with the line width. To apply this method to hydrogen atom Paschen-β (wavelength 1282 nm) and helium atom 2³S-2³P (wavelength 1083 nm) emission lines, we designed a novel spatial heterodyne spectrometer (SHS) ^[1]. SHS is an interference spectrometer that can simultaneously achieve high wavelength resolution, high throughput, and no moving parts, so it has been rapidly developed and widely used in the fields of trace components in the atmosphere, environmental gas, and magnetic fusion diagnostics. For enhancing its advantage in weak signal detection, in our design, we reduced light loss from a conventional design by resolving polarization using polarizing beam splitters, quarter waveplates, and double gratings in each interference arm. According to the theoretical evaluation, the expected wavelength resolving power is more than 10⁵. The throughput is approximately 2 times greater than conventional SHS and the etendue is 8 times greater than the Czerny-Turner spectrometer used in measuring helium atom 2³S-2³P emission lines.

 

[1] W Zhang, et al. Microchemical Journal 166 (2021): 106228.