Polk, James E. and Mikellides, Ioannis G. and Katz, Ira and Capece, Angela M. (2009) Tungsten and barium transport in the internal plasma of hollow cathodes. Journal of Applied Physics, 105 (11). p. 113301. ISSN 0021-8979 http://resolver.caltech.edu/CaltechAUTHORS:20090825-112808930
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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 8200 h and 30 352 h ion engine wear tests. Erosion and subsequent redeposition of tungsten in the electron emission zone at the downstream end of the insert reduce 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.
|Additional Information:||© 2009 American Institute of Physics. Received 18 September 2008; accepted 2 March 2009; published online 1 June 2009. The authors would like to thank Al Owens, Ray Swindlehurst, and Ron Watkins for their assistance in preparing the test facility and Ron Ruiz and Jim Kulleck for their contributions in electron microscopy. The research 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.|
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|Deposited By:||Tony Diaz|
|Deposited On:||26 Aug 2009 17:03|
|Last Modified:||26 Dec 2012 11:15|
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