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 λ D /λ → 0, where λ D is the electron Debye length and λ 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 λ D /λ. 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 λ 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.