A SIMPLIFIED FLUID MODEL OF THE METALLIC PLASMA AND NEUTRAL GAS INTERACTION IN A MULTICATHODE SPOT VACUUM-ARC

A stationary fluid model with spherical symmetry is presented to descr ibe the interaction between metallic plasma ions with neutral gas in t he outer region of a multicathode spot vacuum are operated with a neut ral background gas. It is found that the neutrals penetrate into the m etallic plasma with density values smaller than the initial gas densit y values, but higher than the metallic ion densities. The neutrals are also strongly heated during the transient expansion stage of the meta llic plasma. As a consequence, the ion kinetic energy is gradually del ivered to the neutral gas so that the mean free path for ion-neutral e lastic collisions is larger than the visible plasma ball radius which surrounds the are.