He I spectroscopy for the determination of plasma parameters is of importance in the high density plasmas and recombining plasmas where electrostatic probe measurement is not applicable. For the neutral pressure of about several mTorr, which is typical in fusion divertor plasma or in industrial high-density discharges, it is necessary that the effect of radiation trapping is taken into account.
In this study, the collisional-radiative (CR) model for neutral helium[1] is applied to the helium Balmer series in a divertor simulator MAP-II. We obtained the population distribution of six term sublevels (n¹S, n¹P, n¹D, n³S, n³P and n³D, where n is the principal quantum number) of neutral helium in the UV to visible region. For the triplet series, the measured intensity was able to be reproduced by using the electron temperature (T_e) and density (n_e) measured by an electrostatic probe in the ionizing plasmas. In addition, in the recombining plasmas, T_e deduced from the CR model agreed with that obtained from the Boltzmann plot method for Rydberg spectra. However, in the singlet series, the CR model result was considerably larger than the spectroscopic result without taking into account the radiation transport. Especially, the difference in 2¹S-n¹P series was an order of magnitude.
Therefore, we have introduced the optical escape factor Λ in our experimental conditions, which yields the effective spontaneous emission coefficient ΛA for the resonant transition 1¹S-n¹P. By calculating it as a function of neutral density, neutral temperature and plasma shape, A values in the CR model were replaced. The result shows that the deviation of the calculated population distributions for each term can be diminished to a considerable extent.

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