Space-resolved 3 m normal incidence spectrometer for edge impurity diagnostics in the large helical device.

A space-resolved vacuum ultraviolet (VUV) spectroscopy using a 3 m normal incidence spectrometer has been developed to measure the impurity profile in the edge ergodic layer composed of stochastic magnetic field by which the edge plasma in the large helical device (LHD) is uniquely characterized. It vertically measures the spatial profile of VUV lines emitted from impurities in the wavelength range of 300-3200 Å. The wavelength interval, Δλ, which can be measured in a single discharge, is about 37 Å. A spectral resolution of 0.153 Å, which results from an entrance slit width of the spectrometer of 20 μm, is adopted. The vertical observation range, ΔZ, can be switched by taking a convex mirror in and out, which enables both the edge profile measurement focused on the ergodic layer and the full profile measurement covering an entire vertical size of the LHD plasma, e.g., 165 ≤ ΔZ ≤ 200 mm and 1000 ≤ ΔZ ≤ 1250 mm for the R(ax)=3.6 m configuration, respectively, which shows a slight wavelength dependence. Precise calibrations on the line dispersion, spectral resolution, vertical range of the observable region, and the spatial resolution have been performed with a unique method. As a preliminary result, the ion temperature profile is obtained for CIV at 1548.20 Å in the second order (denoted as 1548.20 × 2 Å) in high-density helium discharges in addition to the emission profile with a time resolution of 100 ms in a multitrack CCD operation mode. The poloidal flow in the ergodic layer based on the Doppler-shift measurement of CIV at 1548.20 × 2 Å is also observed in high-density hydrogen discharges.