Space missions continue to demand higher power Hall and ion thrusters capable of providing high thrust and long life. For high power Hall thrusters in the range of 20 to 100-kW being developed for future cargo and manned-missions, the hollow cathodes will be required to produce discharge currents of 50 to 400 A with lifetimes in excess of 10 khrs. A lanthanum hexaboride (LaB6) hollow cathode has been previously developed for space applications that features graphite tubes or sleeves to provide a diffusion boundary to protect the LaB6 insert from chemical reactions with the refractory metals, and includes a long life heater capable of igniting the higher temperature emitter. Several versions of this cathode design with different LaB6 insert diameters have been built and tested at up to 250 A of discharge current to demonstrate both the high current capability and ease of handling and gas purity requirements obtained with LaB6 cathodes. While the LaB6 cathode insert operates at a higher temperature than the conventional BaO dispenser cathode, LaB6 offers the capability of very high discharge currents, long life and orders of magnitude less sensitivity to propellant impurities and air exposure than conventional dispenser cathodes.

No abstract

No abstract

NASA is continuing to develop high power Hall thrusters in the range of 20 to 100 kW for future cargo and manned-missions. The cathodes for these thrusters will be required to produce discharge currents in the 50 to likely over 300 A range with lifetimes in excess of 10 khrs. A prototype high current hollow cathode with a 2-cm-dia lanthanum hexaboride (LaB 6 ) insert was previously developed for these applications. The original design featured a graphite cathode tube to interface with the LaB 6 insert and an Al 2 O 3 insulated sheath heater capable of heating the higher temperature emitter to ignition temperatures. A new version of this cathode has been designed and built that uses a refractory metal cathode tube and features a robust design similar to the small LaB 6 cathode used for the H6 Hall thruster. The cathode has been successfully tested at steady-state discharge currents from 25 A to 300 A. This cathode is intended to be used in the X3/80 80-kW Nested Hall thruster being built at the University of Michigan at discharge currents over 250 A. Nomenclature d = variable disk diameter in the gas flow model D o = temperature modified coefficient in the Richardson-Dushman thermionic emission equation e = electron charge k = Boltzman's constant l = disk length in the gas flow model n = neutral density T= temperature T r = temperature normalized to 289.7 K P 1,2 = pressure Q = xenon gas flow rate ζ = gas viscosity φ wf = work function