Longitudinal structure of the cathode portions of glow discharges

The longitudinal structure of the cathode portions of a glow discharge is examined, including the cathode layer (CL), negative glow regions (NGR), Faraday dark space (FDS), and up to the uniform positive column (PC). All the main factors affecting this structure are taken into account: the nonlocal nature of the electron energy distribution in the CL and NGR, the diffusion and loss of slow electrons, and the existence of two groups of electrons, fast and slow, in the NGR. The temperature of the slow electrons is calculated using a hydrodynamic equation for the energy. In the calculations for the PC we have taken stepwise and associative ionization into account. The calculation is done for helium at a pressure of $1$ torr. The distributions of the electric field, electron and ion densities, average electron energy, and other quantities along the direction of the current are obtained. The results confirm the experimental facts that the field, which is very weak in the NGR and the beginning of the FDS, changes direction and that the electron temperature falls extremely rapidly in the long FDS. It is shown that the transition from the FDS to the uniform positive column is not monotonic.