A flat-field extreme ultraviolet (EUV) spectrometer with a varied line spacing groove grating ($1200\text{grooves}∕\mathrm{mm}$ at grating center) has been developed to study the emission spectra from highly ionized medium $Z$ impurities in large helical device (LHD). It covers a wavelength range of $50–500\mathrm{\AA }$ using a mechanically ruled grating, which was later replaced by a newly developed laminar-type holographic grating for comparative studies. Differences in spectral resolution, intensities of higher order spectra, and sensitivities of the spectrometer were studied between the two gratings by observing the emission spectra of LHD plasmas. Although the achieved resolution was alike between them, i.e., $\Delta \lambda \sim 0.24\mathrm{\AA }$ at $200\mathrm{\AA }$ , the holographic grating was much superior in suppressing the higher order light than the ruled grating. The relative sensitivity between the two gratings was evaluated using continuum radiation from LHD plasmas. As a result, it was found that the holographic grating has a flat response in the full wavelength range, but the sensitivity of the ruled grating drops sharply below $200\mathrm{\AA }$ . A new technique for the absolute calibration of the EUV holographic grating spectrometer was tried by combining the continuum radiation with a branching ratio of $\mathrm{C}\mathrm{IV}$ lines ($3p\text{-}3s$ : $5800\mathrm{\AA }∕3p\text{-}2s$ : $312\mathrm{\AA }$ ), and an accurate absolute sensitivity has been successfully obtained.

A flat-field extreme ultraviolet (EUV) spectrometer with a nominal $2400\text{grooves}/\text{mm}$ aberration-corrected ruled grating has been developed to analyze the emission spectrum in the wavelength range of 10 to $130\text{\hspace{0.17em} Å}$ 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 $20\text{\hspace{0.17em} Å}$. 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 ($\Delta \lambda \sim 0.08\text{\hspace{0.17em} Å}$ at $33.73\text{\hspace{0.17em} Å}$) is clearly better than the holographic grating ($\Delta \lambda \sim 0.13\text{\hspace{0.17em} Å}$ at $33.73\text{\hspace{0.17em} Å}$). 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 $\left(33.73\text{\hspace{0.17em} Å}\right)$. 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 $1200\text{grooves}/\text{mm}$ EUV spectrometer in the overlapping range of $90–120\text{\hspace{0.17em} Å}$ due to the absence of a good branching pair in $10–130\text{\hspace{0.17em} Å}$. 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 $1200\text{grooves}/\text{mm}$ and $2400\text{grooves}/\text{mm}$ gratings in a range of $50–130\text{\hspace{0.17em} Å}$.

Remarkable progress in the physical parameters of net-current free plasmas has been made in the Large Helical Device (LHD) since the last Fusion Energy Conference in Chengdu, 2006 (Motojima et al 2007 Nucl. Fusion 47 S668). The beta value reached 5% and a high-beta state beyond 4.5% from the diamagnetic measurement has been maintained for longer than 100 times the energy confinement time. The density and temperature regimes have also been extended. The central density has exceeded 1 × 10²¹ m⁻³ due to the formation of an internal diffusion barrier. The ion temperature has reached 5.2 keV at the density of 1.6 × 10¹⁹ m⁻³, which is associated with the suppression of ion heat conduction loss. Although these parameters have been obtained in separated discharges, each fusion-reactor relevant parameter has elucidated the potential of net-current free heliotron plasmas. Diversified studies in recent LHD experiments are reviewed in this paper.

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