Boundaries and probes in electronegative plasmas

A simple model of a low pressure plasma containing two (or more) negative species (e.g., electron and negative ion) and one positive species is treated. Both negative species are described by assuming a thermal equilibrium between each and any electric potential, giving a two temperature description (T(e) and T(-)). The results have much in common with two electron temperature plasma models (Wickens and Allen, 1978). The positive ion is presumed to be significantly colder than the hotter negative particle and although T(+) may be referred to, positive ion dynamics do not include thermal effects. The main concern is to recover information about the relative particle concentration from currents collected at a probe electrode; the plasma boundary is therefore generally of interest. A criterion for sheath formation (Bohm criterion, (Bohm, 1949)), space potential in the pre-sheath and negative/positive saturation currents is considered.