Investigation of Excited States of Helium Atoms in a Stationary PIG-Discharge

Abstract

Spectral line intensities emitted by a quiescent PIG-discharge have been measured and the population densities up to a principal quantum number n = 24 have been derived from them. The experimentally determined population densities have been compared with theoretical ones calculated on the basis of a collisional-radiative model in which one accounts for electron and atom collisions. I t is shown that even in the case of different electron and atom temperatures, T_e and T_a, an evaluation of the Boltzmann plot at medium and moderately high quantum numbers always leads to the electron temperature, whereas the Saha-Eggert equation for the same states may yield incorrect electron densities. The theoretical calculations predict an inflection of the slope of the Boltzmann plot from T_e to T_a for very highly excited states. For the plasma para­ meters under which the PIG-discharge was operated (n_e ≅ 2.5 × 10¹² cm^-3, T_e ≅ 1200°K, n₀ ≅ 1.5 × 10¹⁵ cm^-3, 300°K) the change of the slope should become visible for states having principal quantum numbers n > 18. Due to the large experimental error bars it was not possible to check this behaviour.