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 describe the interaction between metallic plasma ions with neutral gas in the outer region of a multicathode spot vacuum are operated with a neutral background gas. It is found that the neutrals penetrate into the metallic plasma with density values smaller than the initial gas density values, but higher than the metallic ion densities. The neutrals are also strongly heated during the transient expansion stage of the metallic plasma. As a consequence, the ion kinetic energy is gradually delivered to the neutral gas so that the mean free path for ion-neutral elastic collisions is larger than the visible plasma ball radius which surrounds the arc.