Development of a 50-A heaterless hollow cathode for electric thrusters

Hollow cathodes in electric thrusters normally use an external heater to raise the thermionic electron emitter to emission temperatures. These heaters are a potential single-point failure in the thruster and add a separate power supply to the power processing unit. Heaterless hollow cathodes are attractive for their compact size and potential higher reliability but have only been reliably demonstrated to date in small hollow cathodes capable of discharge currents below around 5 A. A new heaterless LaB₆ hollow cathode has been developed that is capable of discharge currents from 5 to 50 A. The cathode configuration extends the gas feed tube at cathode potential part way into the emitting insert region of the cathode. A high-voltage Paschen discharge is struck from the tube to the keeper that heats the tube tip, which then efficiently heats the insert by radiation. This configuration eliminates the arcing observed in prior large heaterless designs that coupled the high-voltage Paschen discharge to the orifice plate or the insert itself. Discharge current-voltage characteristics show that the presence of the tube does not significantly perturb the insert-region plasma. Startup uses a simple 3 min ignition procedure, and voltage traces of the keeper discharge reveal that much of the present tube-radiator's 100-to-150 W heating power comes from an intermediate thermionic discharge sustained by the tube during the transition between the Paschen discharge and LaB₆ insert thermionic regime. This novel heating mechanism enables an unprecedented class of higher-current heaterless hollow cathodes for the next generation of high-power electric propulsion systems.