Fusion Science and Technology / Volume 58 / Number 1 / July 2010 / Pages 394-411
Chapter 8. Diagnostics / Special Issue on Large Helical Device (LHD)
Various types of spectrometers corresponding to different wavelength ranges from X-ray to visible have been developed for the Large Helical Device (LHD). The charge-coupled device is demonstrated to be a suitable solution as a detector for spectral measurements irrespective of the wavelength range. In the ultraviolet (UV)-visible range, an astigmatism-corrected 1.3-m Czerny-Turner-type spectrometer is developed for a simultaneous measurement with 80 lines of sight. Two other UV-visible spectrometers having focal lengths of 0.3 and 0.5 m, respectively, are also prepared for wide wavelength range measurements. An in situ sensitivity calibration is attempted for these spectrometers, for which visible bremsstrahlung from the LHD plasma is utilized. In the vacuum ultraviolet range (30 to 310 nm), a normal incidence spectrometer having a focal length of 3 m is developed for a spatial intensity profile measurement of impurity ions, especially in the plasma boundary region, and for measurements of line broadening of several impurity ions. A number of forbidden emission lines due to magnetic dipole transitions are also identified with this spectrometer. In the extreme ultraviolet range (1 to 50 nm), flat-field spectrometers are developed for measurements of emission lines from high-Z impurities in the plasma core. Two types of gratings, i.e., mechanically ruled and laminar-type holographic, have been tested, and the latter is found to be preferable with respect to the reflectivity and the resolution power. A Johann-type X-ray crystal spectrometer is developed for measurements of the central ion temperature, for which the resonance line of Ar XVII ion (1s2 1S0 - 1s2p 1P1) is mainly used. The central ion temperature is routinely measured with high time resolution.