Determination of the hydrogen and helium ion densities in the initial and final stages of a plasma in the Large Helical Device by optical spectroscopy

Emission line intensities in the UV and visible wavelength range have been measured in the initial and final stages of a discharge in the Large Helical Device (LHD) [O. Motojima et al., Phys. Plasmas 8, 1843 (1999)]. From these results combined with T_e and n_e, which are determined spectroscopically, the ionization and recombination fluxes of hydrogen and helium are estimated in the region of high line emissivities. The temporal behavior of line-averaged n_e which is evaluated from these fluxes is found to be consistent with the results of the interferometer measurement. This implies that in such transient stages of the plasma the temporal variation of n_e is dominated by the atomic ionization-recombination processes rather than the particle transport or diffusion. On the basis of that characteristic in LHD, the line-averaged ion densities for hydrogen and helium just before the termination of the plasma heating are determined. Thus, a method of the density determination of the fueling gas and of the ash is suggested for future fusion devices