Perturbation region near a biased body in a flowing collision-dominated plasma with low ionization density. Current–voltage chracteristics of a langmuir probe

Abstract

The asymptotic theory of flows of high-pressure plasmas past biased bodies (electrostatic probes) is extended to relatively low charge densities, where the Debye radius h is comparable to or exceeds the thickness ▴ of the viscous near- probe boundary layer. Various orders of magnitude of h with respect to a dimension of the probe and various ranges of the probe potential are considered. The governing physical mechanism of current transfer in the perturbation region is identified in each regime, and simple formulae for the probe current–voltage characteristic are obtained whenever possible. In particular, it is shown that an earlier conclusion, valid for h ≪ Δ, on the existence of a linear segment on the electron branch of current-voltage characteristic with a slope depending on the electron density, is also valid in the present situation. It h ≫ Δ, a similar conclusion holds for the ion branch; at large enough probe potentials the jon branch saturates —although for a different physical reason than in the case h ≪ Δ