ELECTRON VELOCITY DISTRIBUTION FUNCTION AT LOW ELECTRON-DENSITIES IN THE POSITIVE-COLUMN OF A HELIUM GAS-DISCHARGE

The electron velocity distribution function (EVDF) in the positive column of a helium gas discharge is derived taking into account both the spatial electron diffusion as an electron low and associative ionization (AI) as an electron source. The latter is introduced into the kinetic equation for the EVDF and takes into account the typical features of the energy spectrum of electrons ejected during AI, i.e., the energetic structure of the electron source. The derived distribution function permits one to avoid the otherwise very essential discrepancy between theory and experiment, featuring agreement even in the fine details.