Neutral-gas depletion and repletion in plasmas

Repletion of neutrals, an unexpected increase in the neutral-gas density when ionization is intense, has been recently predicted to occur in a gas discharge when a thermalized neutral gas is collisionless enough, so that its inertia dominates over the drag by collisions with ions. A parameter is identified here that determines which process should occur, neutral-gas repletion or the more expected neutral-gas depletion. The particle and momentum balance equations of the plasma and neutrals are solved, and it is demonstrated how when this parameter is varied, the neutral-gas density exhibits a transition between depletion and repletion.