Kinetic analysis of the collisional plasma-sheath transition

The subdivision of the plasma boundary layer into a quasi-neutral presheath and a collision-free sheath is strictly valid only in the asymptotic limit lambda(D)/lambda --> 0, where lambda(D) is the electron Debye length and lambda the ion mean free path. This trivial fact requires one to discuss the applicability and correct interpretation of the plasma-sheath concept for finite ratios lambda(D)/lambda. In a former fluid analysis (Riemann K-U 1997 Phys. Plasmas 4 4158) based on the charge exchange model the plasma-sheath transition was investigated on an intermediate scale. The analysis showed that there is no need and no justification for a modification of the Bohm criterion for finite lambda(D). The former results and conclusions are confirmed and improved by a new kinetic analysis. The kinetic analysis of the plasma-sheath transition can be generalized to other presheath mechanisms.