Bohm criterion for the collisional sheath

Using a two-fluid model it is shown analytically that in collisional slightly ionized plasmas, i.e., for many typical low pressure discharges, at the sheath edge the drift velocity of the plasma can be smaller than the ambipolar sound speed of the ions. This basic result was known earlier by numerical calculations but it is demonstrated here using simple analytical expressions. Under these conditions the Bohm criterion, in its usual formulation, is a sufficient condition for the existence of a positive boundary sheath, but it is not a necessary condition. (C) 1996 American Institute of Physics.