THE BOHM CRITERION AND BOUNDARY-CONDITIONS FOR A MULTICOMPONENT SYSTEM

In the asymptotic limit lambda(D) --> 0 (small Debye length) the boundary layer problem of a plasma is split up into the separate problems of a quasineutral presheath and a collision free planar sheath, The formation of a stationary sheath requires that the ions fulfill a sheath condition (''Bohm criterion''). In this paper, the boundary layer problem of a multicomponent system is investigated. Both in the frames of a hydrodynamic and of a kinetic theory, a generalized sheath condition is derived accounting for various ion components and arbitrary electron- and ion distributions distributions, It is shown that the sheath condition is usually fulfilled marginally (in the equality form) and provides a boundary condition for the plasma analysis.