Ionization-enhanced currents to an anode in a magnetic field

Abstract

Magnetic-field effects are known to play an important role in determining current-collection characteristics of electrodes in magnetoplasmas. In the Parker-Murphy model of current collection, for example, the magnetic field excludes electrons that originate from outside a characteristic collection radius from reaching the anode. In this paper we show that these excluded electrons can dominate current collection through ionization effects, even at pressures well below the Paschen threshold for ionization. We demonstrate this by releasing a controlled current near a spherical anode in a magnetic field, and measuring the resultant anode current. The use of a spatially localized electron source reveals aspects of current collection that are not evident when a background plasma constitutes the electron source. We find that the anode current can exceed the source current by more than two orders of magnitude. The conditions are such that we can neglect the contribution to the anode current from secondary emission at the chamber wall. We therefore attribute the additional current to the flow of electrons released by ionization of the ambient neutral gas and show how the ionization arises from electron trapping.