The keeper electrode was believed to be beneficial for improving the hollow cathode performance. However, this paper shows that it can also be hazardous to the discharge if not treated properly. Interactions between the extracted electron beam and the keeper electrode were revealed in an experiment regarding the hollow cathode discharge in the magnetic field environment. It was found that when the electron beam was wider than the keeper orifice, non-negligible electron current was irreversibly lost on the cold keeper wall, inferring from the escalated floating potential of the keeper with respect to the cathode potential. This loss triggered the high-amplitude ionization oscillations in the cathode plume that were highly coherent with the potential fluctuations of keeper itself. The magnetic field that was parallel to the cathode axis could restrict electron diffusion onto the keeper surface and greatly restrain the triggering. Based on the discovered triggering mechanism, several suggestions regarding the cathode design and thruster-cathode coupling were given.

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Inspired by the fact that ionization oscillations in hollow cathode plume are triggered by electron loss on keeper electrode, this paper replaced the material of keeper orifice plate with emissive ones in order to re-compensate the electron loss and checked the influence on plume oscillations. Results show that plume oscillations were indeed sensitive to exterior electron emission. When keeper orifice plate was LaB6 and short-circuited with cathode negative electrode instead of electrically floating, the oscillation amplitude and ion energy dropped by 60% and 50%, respectively, and more under lower flow rates. However, the work function being too low would contrarily increase oscillation amplitude and ion energy. The results recommended the ratio between compensation and loss current to be unity, in which case the main emission mechanism was secondary electron emission due to ion impingement. These results would help increase thruster efficiency and extend lifetime from the perspective of thruster-cathode coupling.

A magnetic focusing type Hall thruster was designed with a cylindrical magnetic seperatrix. During the process of a hollow cathode crossing the separatrix, the variance of plume parameter distribution was monitored. Results show that the ion flux on the large spatial angle is significantly lower when the hollow cathode is located in the inner magnetic field. This convergence effect is preserved even in a distant area. A mechanism was proposed for plume divergence from the perspective of cathode-to-plume potential difference, through which the confinement effect of cylindrical-separatrix-type magnetic field on thruster plume was confirmed and proposed as a means of plume protection for plasma propulsion devices.

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