Ignition and Plume Characteristics of Low-Current Cerium and Lanthanum Hexaboride Hollow Cathodes

This research investigated cerium hexaboride (CeB 6 ) and lanthanum hexaboride (LaB 6 ) as emitters in 6.4-mm-diam hollow cathodes used for low-power electric propulsion applications. Each cathode's ignition performance and plume behavior is presented and discussed. Two identical cathodes, with the exception of the insert material, were assembled and integrated into a cylindrical anode configuration without an applied magnetic field. The experiments tested two orifice geometries and a wide range of xenon flow rates (1.5-4.5 sccm) and anode currents (2-6 A) for each cathode. Also investigated was cathode performance for these conditions with the keeper on and off. A langmuir probe collected plume data to include plasma densities, electron temperature, plasma potential, and floating potential. An oscilloscope monitored the voltage behavior of the keeper electrode and anode. Initially, both cathodes successfully started without excessive keeper voltages (<650 V), heating times (<15 min), or flow rates (<4.5 sccm). The LaB 6 cathode consistently started at these same conditions. However, the CeB 6 cathode demonstrated some degradation after time by requiring higher fiow rates and a longer heating time to start. Both cathodes demonstrated ideal (spot mode) operation for flow rates as low as 1.5 sccm at anode currents as high as 5 A with no heater or keeper power.