Oxygen transport in the internal xenon plasma of a dispenser hollow cathode

Journal Article · Mon Apr 21 04:00:00 EDT 2014 · Journal of Applied Physics

DOI:https://doi.org/10.1063/1.4871755· OSTI ID:22273553

Shepherd, Joseph E. ^[1]; Polk, James E.; Mikellides, Ioannis G. ^[2]

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1. California Institute of Technology, Pasadena, California 91125 (United States)
2. Jet Propulsion Laboratory, 4800 Oak Grove Dr., Pasadena, California 91109 (United States)

Reactive gases such as oxygen and water vapor modify the surface morphology of BaO dispenser cathodes and degrade the electron emission properties. For vacuum cathodes operating at fixed temperature, the emission current drops rapidly when oxygen adsorbs on top of the low work function surface. Previous experiments have shown that plasma cathodes are more resistant to oxygen poisoning and can operate with O{sub 2} partial pressures one to two orders of magnitude higher than vacuum cathodes before the onset of poisoning occurs. Plasma cathodes used for electric thrusters are typically operated with xenon; however, gas phase barium, oxygen, and tungsten species may be found in small concentrations. The densities of these minor species are small compared with the plasma density, and thus, their presence in the discharge does not significantly alter the xenon plasma parameters. It is important, however, to consider the transport of these minor species as they may deposit on the emitter surface and affect the electron emission properties. In this work, we present the results of a material transport model used to predict oxygen fluxes to the cathode surface by solving the species conservation equations in a cathode with a 2.25 mm diameter orifice operated at a discharge current of 15 A, a Xe flow rate of 3.7 sccm, and 100 ppm of O{sub 2}. The dominant ionization process for O{sub 2} is resonant charge exchange with xenon ions. Ba is effectively recycled in the plasma; however, BaO and O{sub 2} are not. The model shows that the oxygen flux to the surface is not diffusion-limited; therefore, the high resistance to oxygen poisoning observed in plasma cathodes likely results from surface processes not considered here.

• OSTI ID: 22273553
• Journal Information: Journal of Applied Physics, Journal Name: Journal of Applied Physics Journal Issue: 15 Vol. 115; ISSN JAPIAU; ISSN 0021-8979
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
BARIUM OXIDES
CHARGE EXCHANGE
COMPARATIVE EVALUATIONS
CONCENTRATION RATIO
ELECTRON EMISSION
HOLLOW CATHODES
IONIZATION
OXYGEN
PLASMA
PLASMA DENSITY
TUNGSTEN
WATER VAPOR
WORK FUNCTIONS
XENON
XENON IONS