Ion Current in Hall Thrusters

Abstract

To match the observed thrust and discharge current in Hall thrusters, computer simulations have typically assumed anomalous electron transport mechanisms, such as Bohm diffusion, to enhance electron mobility across magnetic field lines. Without enhanced electron transport, the simulations predict a much lower discharge current than observed, and too much of the potential drop is downstream of the channel exit. Rather than search for mechanisms to increase the electron scattering frequency, we seek to identify mechanisms that would increase the fraction of the current carried by ions, thus reducing the required electron current. We describe two mechanisms that enhance the current carried by ions. The first mechanism increases the ion current carried in the channel by simply including the effects of multiply charged ions on the plasma response. The importance of using accurate ionization cross sections and the need to include multiply charged ions even for discharge voltages of 300 V are discussed. The second mechanism is a process we term ldquoion reflux.rdquo In this process, current is carried by ions generated downstream of the channel exit. Portions of these ions impact the center of the thruster and are neutralized by cathode electrons. A large fraction of the resultant neutral atoms are reionized as they pass through the main exhaust beam. These newly born ions then carry additional current through the plume. Since neutrals freely move across magnetic field lines, ion reflux effectively enhances the cross-field electron transport. This second mechanism operates downstream of the exit plane and does not enhance electron transport in the acceleration region. Both of these mechanisms, i.e., multiply charged ions and ion reflux, reduce the need to invoke anomalous electron transport mechanisms in Hall thruster computer simulations. However, to date, no Hall thruster simulation has produced results in agreement with data without assuming some anomalous electron transport.