Spectroscopic determination of the singly ionized helium density in low electron temperature plasmas mixed with helium in a linear divertor plasma simulator

The spectroscopic method is developed to obtain the He+ ion density n(He+) in low electron temperature, T-e= 5-20 eV, plasmas mixed with He. Plasmas were produced in the PISCES-B linear divertor plasma simulator [R. P. Doerner et al., Phys. Scr. T111, 75 (2004)] where the electron densities are n(e) = (1-15 x 10(18) m(-3) and the ionization degree is similar to 1-10%. In the method, the He I line intensity I-HeI at lambda = 447.1 nm is used, instead of the He II line intensity in the conventional method. The radial confinement time of He+ ions is requisite, and is measured to be at a level of the Bohm confinement time. The He+ ion concentration, n(He+) / n(e), is found to be proportional to I-HeI, and to weakly depend on n(e) and T-e. Because of the higher ionization energy of He than other species (D-2, Ne, and Ar)], the measured n(He+) / n(e) becomes systematically lower than the He gas pressure fraction, and agrees with data from an omegatron mass spectrometer. The omegatron measurement and estimates of the He+ ion loss rates indicate that the influences of vibrationally excited deuterium molecules on the particle balance of He+ ions are small at Te >= 10 eV. (C) 2007 American Institute of Physics.