The ion energy distribution in front of a negative wall

The ion energy distribution of ions bombarding a negative wall depends on the sheath potential and on the ion kinetics in the boundary layer. In most discharges the electron Debye length lambda _D is small compared to the ion mean free path lambda _C. For weakly negative walls the sheath is, therefore, nearly collision free. In the case of highly negative walls, however, the sheath thickness increases and collisions in the sheath may become important. The effect of collisions on the ion distribution function and sheath potential variation is investigated theoretically and experimentally. The theory starts from the charge exchange model of ion kinetics, presents a convenient self-consistent description of the unipolar ion sheath and extends a former asymptotic analysis lambda _D/ lambda _C to 0 to the Debye sheath. In the experimental part a retarding field analyser is used to determine the ion energy distribution. Measurements were made in the negative glow of a DC glow discharge at pressures of several Pa in argon, nitrogen and oxygen.