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barium depletion in hollow cathode emittersThe 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 ow 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.
Document ID
20150011946
Document Type
Conference Paper
External Source(s)
Authors
Polk, James E.
(Jet Propulsion Lab., California Inst. of Tech. Pasadena, CA, United States)
Capece, Angela M.
(Jet Propulsion Lab., California Inst. of Tech. Pasadena, CA, United States)
Mikellides, Ioannis G.
(Jet Propulsion Lab., California Inst. of Tech. Pasadena, CA, United States)
Katz, Ira
(Jet Propulsion Lab., California Inst. of Tech. Pasadena, CA, United States)
Date Acquired
July 1, 2015
Publication Date
August 3, 2009
Subject Category
Spacecraft Propulsion and Power
Meeting Information
AIAA Joint Propulsion Conference(Denver, CO)
Distribution Limits
Public
Copyright
Other
Keywords
plasma discharges