Nonequilibrium Excitation of Neutral Helium in Plasmas of Moderate Density

Detailed rate equations for the excitation of neutral helium are solved, using a variety of assumed cross sections, and the resulting population densities are compared with measurements on stellarator Ohmic heating discharges and afterglows. It is shown that the experimental results can be satisfactorily reproduced by using for electron impact excitation cross sections for all optically allowed transitions a function (lnU)/U scaled according to the Bethe approximation, where U is the ratio of incident electron energy to the excitation energy. If the assumed cross sections differ from this near the threshold by more than a factor of 1/3, satisfactory agreement with measurements is not obtained. Other items of interest in the analysis of laboratory and astronomical plasmas are also discussed.