Effects of collisions and finite ion temperature on the sheath structure of cylindrical probes in low-pressure electronegative discharges

Abstract

The spatial distributions of electric potential and velocity and density of positive ions are calculated in the surroundings of a negatively biased cylindrical probe immersed in electronegative plasmas. The model equations are solved on the scale of the electron Debye length. The solutions provide the variation of plasma variables along the distance from the plasma bulk region to the probe surface. The control parameters are the ratio of the negative ion density to the electron density, the ratios of the electron temperature to the positive and negative ion temperatures, and the ratio of the rate coefficient for the momentum transfer collision to that for the ionization. Especially, the effects of collision and finite temperature of positive ions are investigated. As the positive ion temperature increases, the sheath width decreases and the positive ion current collected by the probe increases. As the ratio of the rate coefficient for the momentum transfer collision to that for the ionization increases, the sheath edge approaches the plasma region, and the positive ion current to the probe decreases.