X-ray crystal spectrometer with a charge-coupled-device detector for ion temperature measurements in the Large Helical Device

Abstract

A Johann-type x-ray crystal spectrometer has been designed using a ray tracing calculation and constructed for ion temperature measurement in the plasma center of a large helical device (LHD) using Doppler broadening of a He-like resonance line of ArXVII, TiXXI, CrXXIII, or FeXXV. Four curved quartz (2020), (2023), (3140), and (2243) crystals are set in a quadrangular rotary crystal holder for measuring the x-ray line from such four different elements. Each of the crystals can be externally selected through a computer network. The curved crystals with a radius of 3 m are fabricated by gluing thin quartz (0.7 mmt) on an accurately ground cylindrical blue glass plate. The curvature of the crystals was checked by scanning a sharp pin along the crystal surface. As a result, an excellent agreement with the designed cylindrical curvature was obtained with an accuracy of 1 μm. A secular change of the curved crystal surface structure was found by measuring the curvature of the curved crystal fabricated 15 years earlier. A charge-coupled-device (CCD) detector is used instead of a usually used proportional counter to increase the count rate and to avoid the influence of magnetic field leakage from the LHD. The influence of magnetic fields on the proportional counter was investigated in a range of B⩽1.5 kG. The use of the CCD brought us a large increment of more than ten times in the count rate compared with the multiwire proportional counter. The ion temperature thus has been successfully observed in a time interval of 20 ms with small error bars. The lowest measured ion temperature was 0.2–0.3 keV for ArXVII and TiXXI, which indicates an excellent spectral resolution of the crystal spectrometer. Finally, the influence of the chord integration on the measured ion temperature is discussed as a function of central electron temperature and density profile with analysis from impurity transport calculations.