THE ION FLUX FROM A THERMAL PLASMA TO A SURFACE

On the basis of a continuum multi-fluid description of a plasma, a the ory is developed of an ionization layer between a plasma in ionization (Saha) equilibrium and the space-charge sheath near a surface of a ca thode or of an insulating wall. The crucial parameter determining the character of the solution is the ratio of the recombination length to the mean free path ion-neutral species; if this ratio is large, then t he developed theory is consistent with a conventional diffusion approa ch. Numerical and asymptotic solutions of a stated problem are obtaine d. It is found that the problem is solvable only if the degree of ioni zation does not exceed a certain value, which varies between approxima tely 0.6 and 1. Beyond this value, the equation of the pressure balanc e cannot be satisfied and the plasma as a whole in the considered laye r cannot stay in static equilibrium. A simple approximate formula for the ion flux is obtained.