The Effect of a Hall Thruster-Like Magnetic Field on Operation of a 25-A Class Hollow Cathode

The effect of applying a Hall thruster-like magnetic field to a 25-A class hollow cathode is experimentally characterized. A magnetic simulator that approximated the magnetic field of NASA's HERMeS Hall thruster was used to apply magnetic fields of strengths that varied from 0 to 1.25 of the nominal HERMeS value Bnom. Cathode operation was characterized by parameters such as the discharge voltage, voltage and current oscillation magnitudes, and ion energy spectra. It was found that the application of the magnetic field profoundly affected the operation of the cathode. For the nominal xenon flow rate of 14.7 sccm, when increasing the magnetic field from 0 Bnom to 1.25 Bnom, the discharge voltage increased from 20 V to 40 V and the cathode orifice plate temperature increased from 934 ºC to 987 ºC. Oscillations of the discharge voltage, discharge current, and keeper voltage all remained relatively quiescent. However, the ion energy spectra changed profoundly between conditions, with little ion population above 50 V at 0 Bnom, yet a high-energy ion tail extending above 150 V at the 1.25 Bnom case. These high-energy ions appeared without an increase in the oscillation levels, indicating that previous cathode mode definitions may not apply with the Hall thruster-like magnetic field. The implications of these results on existing cathode operational mode definitions, component-level cathode testing, and operation in a Hall thruster are discussed.