Ionization layer at the edge of a fully ionized plasma

A model is developed of the ionization layer which separates a thermal plasma close to full ionization from the space-charge sheath adjacent to the surface of an electrode or of an insulating wall. The multifluid description of the plasma is used. Asymptotic solutions are obtained for the cases in which the thickness of the ionization layer is much larger or much smaller than the mean free path for ion-atom collisions. The solution obtained for the latter case describes an interesting new regime which is in some aspects similar to the conventional diffusion regime, though essentially different from the diffusion regime in other aspects. Formulas are derived for the ion flux coming from the ionization layer to the edge of the space-charge sheath. Application of results to atmospheric-pressure argon and mercury plasmas is considered.