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Hollow Cathodes 305 Fig. 6-49. Percentage of possible thermionic emission versus partial pressure of oxygen and water, showing the sensitivity of dispenser cathodes relative to LaB6 (from [18]). diode tests of a Type S 4:1:1 dispenser cathode and a LaB6 cathode as a function of the partial pressures of oxygen and water for two different emitter temperatures. Oxygen partial pressures in the 10–7-torr range can completely poison the dispenser cathode at temperatures of 1100 ̊C. In a similar manner, water vapor at partial pressures in the 10–6-torr range will poison dispenser cathodes at temperatures below 1110 ̊C. For typical pressures inside hollow cathodes in excess of 1 torr, partial pressures in this range represent the best purity level that can be achieved by the gas suppliers, resulting in the high “propulsion-grade” purity mentioned above. This is the reason for the stringent purity requirement levied on conventional dispenser hollow cathodes in the U.S. to date. Recent experiments by Polk [56] showed that oxygen poisoning observed in vacuum devices occurred only in hollow cathodes at low plasma densities (low current) and high oxygen levels (>10 PPM), and that the plasma environment inside hollow cathodes tended to mitigate the poisoning of oxygen in dispenser cathodes. However, the plasma may aid in the formation of volatile tungsten oxides and tungstates from the impurity gases that contribute to tungsten migration and redeposition on the insert surface. This modifies the dispenser cathode surface morphology, which may affect the emission capabilities. It is likely that the life of dispenser cathodes can be degraded to some extent by propellant impurities, which has yet to be fully identified and quantified. Lanthanum hexaboride is much less sensitive to impurities that can limit the performance and life of the barium dispenser cathodes. Partial pressures ofPDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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