Role of neutrals in plasma expansion in ion diodes

An attempt to understand the mechanism of fast anode plasma motion across the magnetic field, which has been observed in recent experiments, is presented. For a fully ionized anode plasma it is seen that electron heating imposes an upper limit on the expansion rate. This limit is much lower than the observed rate of expansion. A model based on a hot, fast-expanding neutral layer is considered. This layer is created by charge exchange processes near the anode and is subsequently ionized by energetic ions and the ensuing electron avalanche. The resulting plasma parameters are calculated and found to be in reasonable agreement with the observations.