High-Current Lanthanum Hexaboride Hollow Cathode for 10-to-50-kW Hall Thrusters

Space missions continue to demand higher power Hall thrusters that provide high thrust and long life. For in-space propulsion applications such as high-power orbit raising and cargo missions, Hall thrusters capable of operating in the range of 10-50 kW are in development. The hollow cathodes for these thrusters will be required to produce discharge currents of 20-100 A with lifetimes well in excess of 10 kh. A lanthanum hexaboride (LaB₆) hollow cathode with these capabilities has been developed that features graphite tubes or sleeves to provide a diffusion boundary that protects the LaB₆ electron emitter from chemical reactions with the refractory metal support structure and includes a long-life heater capable of igniting the high-temperature electron emitter. Cathode operation has been fully characterized at currents of 20-100 A including probe measurements of the plasma parameters inside the LaB₆ insert and energetic ion measurements in the cathode plume. The cathode was also tested at up to 200 A of discharge current to explore the cathode failure mechanisms. While the LaB₆ cathode operates at higher temperatures than conventional barium oxide-impregnated dispenser cathode, LaB₆ provides very high discharge currents, long life, and significantly less sensitivity to propellant impurities and air exposure than conventional dispenser cathodes.