Characteristics of an absolutely calibrated flat-field extreme ultraviolet spectrometer in the 10-130 A range for fusion plasma diagnostics

A flat-field extreme ultraviolet (EUV) spectrometer with a nominal 2400 grooves/mm aberration-corrected ruled grating has been developed to analyze the emission spectrum in the wavelength range of 10 to 130 A from large helical device (LHD) plasmas. Spectral properties such as resolution, sensitivity, contribution of higher-order light, and background stray light have been studied using emission spectra mainly from intrinsic impurities, e.g., C and Fe. It is found that the spectrometer well resolves closely existing spectral lines of highly ionized medium- and high-Z impurities even in a very short wavelength range such as 10 to 20 A. As a result, it allows one to study the charge state distribution of elements in high-temperature fusion plasma. The ruled grating was then replaced by a laminar type holographic grating for the comparative study. The spectral resolution for the ruled grating (Dlambda approximately 0.08 A at 33.73 A) is clearly better than the holographic grating (Dlambda approximately 0.13 A at 33.73 A). Both gratings well suppress the higher-order light, e.g., the second-order light is only less than 11% of the first-order light for C vi(33.73 A). Relative sensitivity calibration with the wavelength has been done using bremsstrahlung continuum from the LHD plasmas. Absolute intensity calibration has been done by comparing the spectral intensities directly with the absolutely calibrated 1200 grooves/mm EUV spectrometer in the overlapping range of 90-120 A due to the absence of a good branching pair in 10-130 A. As a typical result on the present spectrometer well-resolved n=2-3 full transition arrays from Ne- to Li-like ions are measured for Fe and Ti and wavelengths of the spectral array are tabulated for each charge state. Spectroscopic comparison is also made between the 1200 grooves/mm and 2400 grooves/mm gratings in a range of 50-130 A.