NASA Logo

NTRS

NTRS - NASA Technical Reports Server

Back to Results
Tungsten and Barium Transport in the Internal Plasma of Hollow CathodesThe 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 pushedback to the emitter surface by the electric field and drag from the xenon ion flow. Thisbarium ion flux is sufficient to maintain a barium surface coverage at the downstream endgreater 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 hollowcathode failure models based on barium depletion rates in vacuum dispenser cathodes are very conservative.
Document ID
20150008618
Acquisition Source
Jet Propulsion Laboratory
Document Type
Conference Paper
External Source(s)
Authors
Polk, James E.
(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)
Capece, Angela M.
(California Inst. of Tech. Pasadena, CA, United States)
Date Acquired
May 20, 2015
Publication Date
July 21, 2008
Subject Category
Spacecraft Propulsion And Power
Plasma Physics
Meeting Information
Meeting: AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit
Location: Hartford, CT
Country: United States
Start Date: July 21, 2008
End Date: July 23, 2008
Sponsors: Society of Automotive Engineers, Inc., American Society for Engineering Education, American Society of Mechanical Engineers, American Inst. of Aeronautics and Astronautics
Distribution Limits
Public
Copyright
Other
Keywords
Hollow cathodes
material transport

Available Downloads

There are no available downloads for this record.
No Preview Available