Theory of a plasma emitter of positive ions

The results obtained to date by the one-dimensional theory of the emission of positive ions by a plasma are reviewed from a unified point of view. Simple and generalized forms of the Bohm criterion are discussed. The unipolar sheath model and the two-scale theory are outlined. Kinetic and hydrodynamic models for the formation of the Debye sheath and of the ionization zone in the presheath are described. It is shown that, except for some artificial ionization models, the Bohm criterion is fulfilled in the equality form. The emission current from the plasma emitter is calculated in the hydrodynamic and kinetic approximations. Existing numerical codes for the simulation of positive ion sources in two and three dimensions are briefly described.