The transition from collisionless to collisional active plasma in the fluid model and the relevance of the Bohm criterion to sheath formation

The fluid equations describing the plasma-wall sheath in an electropositive gas are solved numerically with parameters, delta(i) the ratio of the ion collision frequency nu(i) to the ionization rate Z, and alpha the ratio of the ionization rate to the ion plasma frequency omega(pi). From the behavior of the ion speed and from the plasma balance equation relating the plasma dimension L to Z it is possible to identify regions in alpha-delta(i) space in which the plasma is (a) collisionless, (b) transitional, and (c) collisional and in which the boundary layer is (i) collisionless and (ii) collisional. The Bohm criterion ceases to have significance when the layer matching plasma and wall becomes collisional. (C) 2000 American Institute of Physics. [S1070-664X(00)04207-5].