American Institute of Aeronautics and Astronautics 1 Extending Hollow Cathode Life for Electric Propulsion in Long-Term Missions Dan M. Goebel * , Ira Katz † , James Polk ‡ , Ioannis G. Mikellides § , Kristina K. Jameson ** , Thomas Liu †† Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 Hollow cathodes are a critical component for the life and performance of most electrostatic and Hall effect ion thrusters. The benchmark life test of a hollow cathode was the Space Station plasma contactor test run at NASA/GRC, in which the cathode operation exceeded 28,000 hours. In a gridded ion thruster, the NSTAR flight-spare thruster cathode operated for over 30,000 hours in tests run at JPL, and the test was stopped without determining the ultimate life of the hollow cathode. This endurance is impressive and exceeds almost all demands on a hollow cathode for solar electric propulsion missions. However, on proposed nuclear electric propulsion missions, thrusters will be required to operate for more than 10 years. Because of this need, we have begun a systematic investigation to understand the processes that can cause hollow cathode performance degradation and failure. This paper will present our present understanding of the hollow cathode barium depletion mechanism and the design required to achieve the required life. The models of the cathode life are based on extensive traveling wave tube impregnated cathode data in the literature, and this data is extended to the hollow cathodes used in ion thrusters. We show that the observed space station cathode life is in agreement with published barium evaporation rates for the vacuum cathodes, indicating that the cathode plasma does not significantly effect on the insert barium depletion rate. Experiments on probing the plasma characteristics in the hollow cathode using ultra-fast scanning probes and directly measuring the surface temperature of the insert by scanning fiber-optic probes provide data for the plasma models that are used to predict the cathode tempearature that ultimately determines the life. Nomenclature A = adatom sublimation coefficient E eff = activation energy for adatom evaporation E o = activation energy for barium evaporation K o = barium sublimation coefficient n o = particle density on the insert surface J D = discharge current J total = flux of material from cathode surface J pl = plasma ion flux to cathode surface L c = insert length m profile = slope of the insert temperature gradient p insert = reservoir insert porosity r c = reservoir cathode radius * Principal Scientist, Advanced Propulsion Technology Group, Senior Member AIAA † Group Supervisor, Advanced Propulsion Technology Group, Senior Member AIAA ‡‡ Principal Scientist, Advanced Propulsion Technology Group, Senior Member AIAA § Member Technical Staff, Advanced Propulsion Technology Group, Member AIAA. ** Graduate Student, Academic Part Time Staff, Advanced Propulsion Technology Group, Member AIAA †† Graduate Student, Summer Intern, Advanced Propulsion Technology Group