The Effect of Time-Varying Currents on Hollow Cathode Operation

A novel fast-scanning fiber optic probe, high speed swept Langmuir probe and fast ion saturation probe were used to measure the emitter temperature and internal plasma properties in a hollow cathode over a range of magnetic fields, flow rates, and DC currents. Additional measurements were performed using AC currents injected in parallel with the nominal DC current to determine how the hollow cathode accommodates demands for time-varying currents in a Hall thruster. Demands for higher steady state discharge current levels are accommodated primarily by an increase in cathode temperature. The plasma density increases by up to 70% as the current rises from 14.2 to 30.2 A, and the plasma becomes more concentrated at the downstream end. The electron temperature shows no significant changes over this range of currents. The plasma potential is negatively correlated with the discharge current, dropping by 2 - 3 V as the current is increased from 14.2 A to 30.2 A. The mechanism in this case is likely changes in thermionic emission due to temperature changes, with the sheath voltage playing a role in fine-tuning the net current. With kHz-frequency current oscillations, there is no evidence that the cathode temperature adjusts to accommodate the rapid changes. The key difference between changes in DC current level and rapid changes with oscillating currents is in the behavior of the plasma potential. In contrast to the drop in plasma potential observed with increasing DC currents, the potential increases as current increases over a cycle with time-varying currents. This suggests that the mechanism for accommodating time-varying currents is an adjustment in the cathode sheath, modulating the return electron current, which is reflected in the centerline plasma potential variations.