PHENOMENA AT THE PLASMA-WALL BOUNDARY - KINETICS OF IONS AND DISTRIBUTION OF THE ELECTRIC-POTENTIAL

A consistent theory of the phenomena occurring near the boundary betwe en plasma and a solid surface is proposed. A general expression for th e ion velocity distribution function in the plasma near the wall is fo und by taking into account the charge exchange and ionization (recombi nation) processes. This expression holds for any scattering frequency and takes into account a possible strong distortion of the ion distrib ution function near the wall. Asymptotic dependences of the plasma par ameters (such as the electric potential, plasma density, etc.) far fro m the wall are obtained from the spatial dependence of the ion density , which was reconstructed on the base of the ion distribution function , by assuming electrons to obey the Boltzmann distribution. The obtain ed analytical results are used to formulate a simulation model, and de velop a numerical algorithm. The numerical simulation is performed for the case when the charge-exchange frequency exceeds the ionization (r ecombination) frequency, and the Debye radius is considerably less tha n the ion mean free path. Our numerical experiment shows that the use of simplified models can lead to incorrect results. For example, under certain conditions, the Bohm criterion, based on these models, does n ot hold, and the width of the plasma sheath near the wall, the wall po tential, the electric field at the surface, the velocity of the ion fl ow to the wall, the ion energy near the wall, etc., can considerably d iffer from those obtained from simplified models.