Abstract
The equation of radiation transport describes the emission and absorption of a spectral line in a plasma. An apparent decrease in the emission intensity is demonstrated for an optically thick line, which is confirmed with the Balmer−α line observed for hydrogen ice pellet injection into the LHD plasma. The effective decrease in the spontaneous transition probability is expressed in terms of an escape factor, and is incorporated into the collisional-radiative model for neutral helium. For the glow discharge plasma in the LHD, a substantial decrease in the effective A coefficient of the 11S−31P line results in an increase in the upper-level population, leading to a very strong emission of the 21S−31P line; this resolves the last puzzle in the observed spectra. An emission line intensity is proportional to the ionization flux or the recombination flux of the ion species concerned. From the measured line intensities of ionized and neutral helium in the decaying phase of the LHD plasma, densities of the ions, and thus the electron density are estimated, which are in accordance with the electron densities determined by the interferometer. Under the condition of fixed atom and ion densities, starting from a high electron temperature where the plasma is ionizing and an emission line is strong, with a decrease in temperature the line intensity decreases. It takes the minimum at a temperature of ionization balance, and then it increases again in a recombining plasma. This feature interprets various observations. The above conclusion is a substantial generalization of the conventional belief that an emission intensity takes its maximum at the temperature at which the atom and ion densities are approximately equal; this latter statement is valid only under the condition of an ionization balance plasma.
