The collection of positive ions by spherical and cylindrical probes in an electronegative plasma

The collection of positive ions by spherical and cylindrical probes is investigated for electronegative plasmas in collisionless, unmagnetized and non-flowing regimes as a function of the ratio of negative ion density to positive ion density. The pre-sheath is given by a solution assuming quasineutrality. The potential in the sheath and the current-voltage characteristics are obtained by solving Poisson's equation with the initial value given by the pre-sheath solution. The calculations are mostly based on the radial motion theory but the orbital motion theory is also considered to take the finite ion temperature effect into account. Particularly, the thin sheath model is considered for the latter case. A method for diagnostics in negative ion plasmas is suggested.