A flat-field extreme ultraviolet (EUV) spectrometer with a nominal aberration-corrected ruled grating has been developed to analyze the emission spectrum in the wavelength range of 10 to 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 . 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 ( at ) is clearly better than the holographic grating ( at ). Both gratings well suppress the higher-order light, e.g., the second-order light is only less than of the first-order light for C vi . 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 EUV spectrometer in the overlapping range of due to the absence of a good branching pair in . As a typical result on the present spectrometer well-resolved 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 and gratings in a range of .
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