Space-resolved visible spectroscopy for two-dimensional measurement of hydrogen and impurity emission spectra and of plasma flow in the edge stochastic layer of LHD.

A space-resolved visible spectrometer system has been developed for two-dimensional (2D) distribution measurements of hydrogen and impurity emission spectra and of plasma flow in the edge stochastic layer of Large Helical Device (LHD). Astigmatism of the spectrometer has been suppressed by introducing additional toroidal and spherical mirrors. A good focal image at the exit slit is realized in a wide wavelength range (75 nm) as well as in a wide slit height direction (26 mm) with a 300 grooves/mm grating. The capability of the spectrometer optical system for the 2D measurement and further possible improvements are discussed in detail. An optical fiber array of 130 channels with a lens unit is used to spatially resolve the edge plasma into different magnetic field structure components: divertor strike points, divertor legs, X-point of the legs, the stochastic layer, and the last closed flux surface. With a 300 grooves/mm grating, the 2D distributions of several hydrogen and impurity line emissions are simultaneously obtained with absolute intensities. A clear correlation is obtained between the magnetic field structure and the emission intensity. With a 2400 grooves/mm grating with a good spectral resolution (0.03 nm/pixel), the 2D distributions of impurity flow velocity are obtained from the Doppler shift measurement. The wavelength position is accurately calibrated by investigating the wavelength dispersion as well as by correcting a mechanical error of the optical setting in the spectrometer. The uncertainty in the velocity is reduced to less than 10% of a typical impurity velocity ∼10 m/s. A temporal change in the flow directions is observed at different spatial locations in divertor detachment plasma.