The mechanism of electron runaway in a gas and a criterion of the self-sustained discharge initiation

The results of simulations using a particle-in-cell technique show that the phenomenon of electron runaway in a gas is determined by the condition that the interelectrode distance must be comparable with the Townsend electron multiplication length, rather than by local criteria commonly accepted at present. For a given gas, a critical voltage between the electrodes, for which the fraction of runaway electrons is relatively large, is a universal function of the product of the electrode spacing and the gas pressure. The same function determines a condition of the self-sustained discharge initiation in which the electron multiplication plays a significant role. This function has, besides the well-known Paschen branch, an additional upper branch describing breakage of the self-sustained discharge operation as a result of a sufficiently rapid application of a high voltage between the electrodes.