Potential structure near a probe in a flowing magnetoplasma and current collection

Abstract

The distributions of plasma and potential near an electric probe in a relative motion with respect to a magnetized plasma are studied by means of three‐dimensional (3‐D) numerical simulations. The relative motion is simulated by a plasma flowing past the probe across the ambient magnetic field. The plasma flow is imposed by a convection electric field . A probe with a positive potential bias is considered. The prominent features of the potential distribution include (1) wings of positive potential perturbations extending along the magnetic field and swept forward in the direction of the plasma flow and (2) a “fan” shaped structure in planes transverse to the magnetic field in the region where the convection electric field is opposed by space charge electric fields. The wing‐like structure can be interpreted in terms of electrostatic plasma waves belonging to the oblique resonance cone in a magnetized plasma. The relative flow causes the formation of a “bow shock” in front of the probe, where plasma density is enhanced due to the combined effect of the retardation of the flowing ions and the modification in the drift of the electrons in the sheath of a positive probe. The electron collection by the probe is significantly enhanced above the theoretical upper bound current obtained from the conservation of energy and the canonical angular momentum for the case without the relative motion. The current in the plasma, contributing to the collection of electrons by the probe, flows in a magnetic field‐aligned channel in the vicinity of the probe where electric fields parallel to the magneitc field are relativley strong. Electron flux is fed into the channel all along its length by the drift in the self‐consistent electric field, considerably enhancing the current collected by the probe. The field‐aligned current channel is localized near the probe where parallel electric fields dominate; it does not extend to infinity along the probe's magnetic shadow, unlike that for the case of a nonflowing plasma.