Monte Carlo simulation of electrons within the cathode fall of a glow discharge in helium

The electron behaviour in the cathode fall region of a glow discharge in helium has been analysed using a one-dimensional Monte Carlo method. It is shown that the electron energy distribution function undergoes a continuous spatial evolution in a way which is incompatible with the requirements for use of the Townsend theory, and that the effective ionization coefficient has a spatial dependence very different from that predicted using the Townsend alpha coefficient in the vicinity of the negative glow. It is believed that the lowest energy group is a result of ionization close to the negative glow where the electric field is insufficient to cause significant acceleration of the ejected electrons.