Development of high-resolution vacuum crystal spectrometer in a compact helical system and large helical device

A vacuum crystal spectrometer that covers a wavelength range of 1.0–2.5 nm has been developed to observe light (oxygen) and heavy (titanium and iron) impurities for large helical device (LHD) diagnostics. A Johann-type spectrometer (2R = 60 cm and 3 m, RAP crystal, 2d = 2.612 nm) was constructed and installed in a compact helical system (CHS). The O VII resonance line was successfully observed through a thin (3.3 μmt) polypropylene film, although the metallic impurity lines were not seen. In the case of a crystal with 2R = 3 m, however, the spectral resolution λ/Δλ was limited to 440, whereas the replacement by the crystal with 2R = 60 cm improved the resolution to 610. The result is discussed with regard to the quality of the bent crystal.