Characteristics of a Spherical Electrostatic Probe in a Bounded Plasma

Spherical electrostatic probes immersed in a slightly ionized, collision-dominated plasma bounded by a large concentric spherical electrode of radius Rw are treated by asymptotic continuum theory when the sheath thickness is small compared with the probe radius rp but large compared with a mean free path. It is found that when Rw/rp is large, the saturation currents are modified by only O(rp/Rw). However, the maximum slope of the characteristic increases by 20% as Rw/rp increases from 100 to infinity. Since this slope increases similarly as the electron to ion temperature ratio τ increases, it is concluded that reduction of data for measurements in bounded plasmas using continuum theory for unbounded plasmas leads to values of τ (and usually T_) that are smaller than the true values.