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Tungsten and barium transport in the internal plasma of hollow cathodes

Show simple item record Polk, James E. Mikellides, Ioannis G. Katz, Ira Capece, Angela M. 2014-12-10T00:42:54Z 2014-12-10T00:42:54Z 2008-07-21
dc.identifier.citation AIAA Joint Propulsion Conference, Hartford, Connecticut, July 21-23, 2008 en_US
dc.identifier.clearanceno 08-2184
dc.description.abstract The effect of tungsten erosion, transport and redeposition on the operation of dispenser hollow cathodes was investigated in detailed examinations of the discharge cathode inserts from an 8200 hour and a 30,352 hour ion engine wear test. Erosion and subsequent re-deposition of tungsten in the electron emission zone at the downstream end of the insert reduces the porosity of the tungsten matrix, preventing the flow of barium from the interior. This inhibits the interfacial reactions of the barium-calcium-aluminate impregnant with the tungsten in the pores. A numerical model of barium transport in the internal xenon discharge plasma shows that the barium required to reduce the work function in the emission zone can be supplied from upstream through the gas phase. Barium that flows out of the pores of the tungsten insert is rapidly ionized in the xenon discharge and pushed back to the emitter surface by the electric field and drag from the xenon ion flow. This barium ion flux is sufficient to maintain a barium surface coverage at the downstream end greater than 0.6, even if local barium production at that point is inhibited by tungsten deposits. The model also shows that the neutral barium pressure exceeds the equilibrium vapor pressure of the impregnant decomposition reaction over much of the insert length,so the reactions are suppressed. Only a small region upstream of the zone blocked by tungsten deposits is active and supplies the required barium. These results indicate that hollow cathode failure models based on barium depletion rates in vacuum dispenser cathodes are very conservative. en_US
dc.description.sponsorship NASA/JPL en_US
dc.language.iso en_US en_US
dc.publisher Pasadena, CA : Jet Propulsion Laboratory, National Aeronautics and Space Administration, 2008 en_US
dc.subject Hollow cathodes en_US
dc.subject material transport en_US
dc.title Tungsten and barium transport in the internal plasma of hollow cathodes en_US
dc.type Preprint en_US

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