Effect of the positive ion collisions on the positive space-charge in electronegative plasmas

Abstract

An electronegative plasma sheath is studied in the presence of a wide range of collisionality. The dynamic of the positive ions are treated by means of the continuity and momentum equations. It is assumed that the negative species are distributed according to the Boltzmann equation. In the momentum equation, a power law dependence on the ion speed is assumed for the collision cross section. Using Sagdeev potential and appropriate boundary conditions, the appropriate form of Bohm’s criterion is obtained in the collisional electronegative plasma. It is shown that for constant cross section and constant collision frequency models, there are upper and lower limits for the Bohm’s criterion. The profiles of the space charge are obtained for different electronegativity and negative ion temperature in two asymptotic cases of collision cross section. It is shown that the presence of negative ion results in a decrease of the sheath thickness and space charge magnitude and the space charge peak also moves toward the sheath edge. In addition, in the case of constant collision cross section, the space charge peak becomes narrower and sheath thickness increases. In addition, increasing the electron temperature leads to damping of the space charge peak and broadening of the sheath. Finally, the influences of the collision cross section, as well as the electronegativity on the characteristic curves of current-electric potential and floating potential are studied.