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Decoupling the Thermal and Plasma Effects on the Operation of a Xenon Hollow Cathode With Oxygen Poisoning Gas

Capece, Angela M. and Polk, James E. and Shepherd, Joseph E. (2015) Decoupling the Thermal and Plasma Effects on the Operation of a Xenon Hollow Cathode With Oxygen Poisoning Gas. IEEE Transactions on Plasma Science, 43 (9). pp. 3249-3255. ISSN 0093-3813.

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Hollow cathodes are used as the electron source for generating the plasma discharge in electric thrusters. These cathodes contain porous tungsten emitters impregnated with BaO material to achieve a lower surface work function and are operated with xenon propellant. Reactive contaminants such as oxygen in the xenon gas can modify the surface chemistry and morphology of BaO dispenser cathodes and degrade the electron emission properties. Hollow cathodes that operate with reactive impurities in the propellant will experience higher operating temperatures, which increase the evaporation of the emission materials and reduce cathode life. A significant amount of work has been previously done to understand the effects of oxygen poisoning on vacuum cathodes; however, the xenon plasma adds complexity and its role during cathode poisoning is not completely understood. A deeper understanding of the mechanisms initiating cathode failure will improve the thruster operation, increase the lifetime, and ultimately reduce the cost. In this paper, we present the results of experiments in which the cathode was operated with 100 ppm of oxygen gas in the xenon plasma at two different discharge currents. Poisoning was indicated by an increase in the emitter temperature that was measured using a two-color pyrometer and thermocouples. The emitter temperature rose monotonically during poisoning and decayed to its original value when the oxygen flow was turned OFF, indicating that short exposures to oxygen do not result in permanent cathode damage. The experiments show that removal of oxygen adatoms from the cathode surface following poisoning is dominated by thermal desorption rather than xenon ion sputtering. Operation at higher discharge currents improves the resistance to poisoning as a result of the higher operating temperatures at these conditions.

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Shepherd, Joseph E.0000-0003-3181-9310
Additional Information:© 2015 IEEE. Manuscript received March 17, 2015; accepted August 3, 2015. This work was supported by the Jet Propulsion Laboratory, Pasadena, CA, USA. The work described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. The authors would like to thank I. Mikellides for providing the simulation results to help us interpret our experiments.
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Subject Keywords:Barium, cathodes, electron emission, oxygen, poisoning
Issue or Number:9
Record Number:CaltechAUTHORS:20150901-100856359
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Official Citation:Capece, A.M.; Polk, J.E.; Shepherd, J.E., "Decoupling the Thermal and Plasma Effects on the Operation of a Xenon Hollow Cathode With Oxygen Poisoning Gas," in Plasma Science, IEEE Transactions on , vol.43, no.9, pp.3249-3255, Sept. 2015 doi: 10.1109/TPS.2015.2465845
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:60008
Deposited By: Ruth Sustaita
Deposited On:11 Sep 2015 18:50
Last Modified:21 Aug 2020 22:11

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