Boundary layer structure of the sheath surrounding a cylindrical or spherical probe in electronegative plasmas

Abstract

This paper examines a positive ion sheath close to a cylindrical and/or spherical probe in electronegative plasmas and its transition from electropositive to electronegative behaviours when the relative density and temperature of the negative ions present in the plasma change. We conclude that this transition occurs with continuous variation in the sheath thickness, even when the potential at the sheath edge exhibits a discontinuous jump. We use a two-scale model to describe the plasma–sheath region, which allows us to choose the correct pre-sheath solution for any level of plasma electronegativity, even when there are multiple solutions.