Probes Dominated by Ion-Neutral Collisions: Oblique Incidence and Saturation

Abstract

Langmuir probes in magnetized plasmas are difficult to treat theoretically because cross-field transport cannot be ignored but is not well understood. Most efforts to date have considered only the case of normal incidence, although it is common practice to use grazing incidence to reduce the power density to the surfaces. In the case that ion-neutral collisions are dominant, cross-field current is governed by an effective conductivity. We here develop a procedure to map solutions onto one another, so that a probe at oblique incidence with an isotropic conductivity can be analyzed in terms of an equivalent probe at normal incidence with isotropic conductivity. It is found that the usual practice of taking the projected shape of the probe is not exactly correct, but is acceptable as a practical matter. Even if a theory based solely on ion-neutral collisions is able to explain the small voltage response of probes in magnetized plasmas, we must point out that it fails in the saturation regime, where some additional mechanism, capable of transporting electrons across the field, must be invoked.