Tunable high resolution spectrometer for H-alpha and visible spectroscopy in ITER

Abstract

A tunable-band high resolution spectrometer (HRS) was designed to study the fine structure of H-alpha and impurity lines in order to separate the divertor stray light (DSL) and the scrape-off layer (SOL) emission required for H-alpha and visible spectroscopy diagnostics in ITER. The spectral resolution not worse than ~0.01 nm is needed to resolve similar Zeeman lines in the SOL and DSL components shifted with a small offset due to some difference of the magnetic field in the divertor and at the ITER first walls. HRS optical design is based on the pseudo-autocollimation scheme with a pair of coplanar holographic gratings and spatially separated back-reflected optical beam. This scheme provides independence of a dispersion versus the wavelength in the whole spectral range, together with acceptable F-number up to f/4.5 of the input/output lenses. Grating and back-reflection mirror units are mount to the motorized rotational platforms with remote control realized in LabView software.The HRS prototype was manufactured and tested to verify the design characteristics. The main parameters: spectral range, inverse linear dispersion, spectral resolution and contrast were measured at 65˚ and 75˚ light incidence angles to the gratings. The inverse linear dispersions are 0.206 nm/mm and 0.13 nm/mm respectively, with 0.01 nm spectral resolution at 65˚ angle measured by scientific-grade PCO edge 4.2 CMOS camera. Close-wavelength contrast of 1:6•10−6 was measured at δλ = 2.5 nm spectral distance from the He-Ne laser line. The total spectral range is 400..700 nm with a tunable sub-band of 1.3÷2.7 nm spectral width defined by the position of holographic gratings.