A simultaneous measurement of polarization-resolved spectra of neutral helium 23P–33D, 21P–31D and 23P–33S emissions from the periphery of a Large Helical Device plasma

Abstract

We have developed a spectrometer specialized for simultaneous observation of helium emission lines from n = 3 to n = 2 levels, where n is the principal quantum number. The spectrometer is optimized for the light input coupled by 10 optical fibers having 0.4 mm core diameter. The spectral resolutions are 0.016–0.026 nm for the 23P–33D (587.6 nm), 21P–31D (667.8 nm) and 23P–33S (706.5 nm) transition lines for an entrance slit width of 35 µm. The polarization resolved spectra of a Large Helical Device (LHD) plasma are measured with the spectrometer. From the analysis of the spectra with a least-square fit assuming the emission locations to be localized in two positions, inner and outer periphery regions, we deduce the emission locations and intensities of these lines. The deduced results are compared with a one-dimensional penetration model of helium atoms based on collisional–radiative calculation. The systematic dependence of the emission locations and intensities on the transition lines reflects the spatial distribution of the electron density and temperature in the periphery region where neutral helium atoms penetrate from the edge.