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254 Chapter 6 has been problematic. However, scandate–BaO–W dispenser cathodes have been successful in several different cathode structures. Because chemistry is involved in the formation of the low work function surface, dispenser cathodes are subject to poisoning that can significantly increase the work function [17]. Some care must be taken in handling the inserts and in the vacuum conditions used during operation of these cathodes to avoid poisoning by impurities in the gas that produce unreliable emission and shorten the lifetime. In addition, impurities in the feed gas that react with the tungsten insert can cause migration and deposition of tungsten or tungstates (compounds of tungsten, barium, and oxygen) on the surface, which change the surface structure and porosity and can reduce the surface coverage of the low work function BaO layer. One of the major drawbacks of using BaO dispenser cathodes in electric propulsion applications is the extremely high feed gas purity specified to avoid these poisoning and tungsten-material transport issues, which has resulted in a special “propulsion-grade” xenon with 99.9995% purity to be specified by some users of these cathodes for flight. Another electron emitter material, lanthanum hexaboride [13], is a crystalline material made by press sintering LaB6 powder into rods or plates and then machining the material to the desired shape. Polycrystalline LaB6 cathodes have a work function of about 2.67 eV, depending on the surface stoichiometry, and will emit over 10 A/cm2 at a temperature of 1650 ̊C, as shown in Fig. 6-7. Since the bulk material is emitting, there is no chemistry directly involved in establishing the low work function surface, and LaB6 cathodes are insensitive to impurities and air exposures that can destroy a BaO dispenser cathode [18]. An LaB6 cathode can withstand gas-feed impurity levels two orders of magnitude higher than dispenser cathodes at the same emission current density. In addition, the cathode life is determined primarily by the low evaporation rate of the LaB6 material at typical operating temperatures. The higher operating temperature of bulk LaB6 and the need to support and make electrical contact with LaB6 with materials that inhibit boron diffusion at the operating temperatures require some careful engineering of the cathode structure. However, LaB6 cathodes are commonly used in Russian Hall thrusters in communications satellite applications [19]. Lanthanum hexaboride was first developed as an electron emitter by Lafferty [13] in the 1950s. The thermionic emission of lanthanum–boron compounds for various surface stoichiometries was extensively studied by several authors [14,15,19]. The first flight of Russian SPT Hall thrusters [20] in 1971, and all subsequent flights, utilized lanthanum hexaboride cathodes. The first reported use of LaB6 in the US in a hollow cathode was by Goebel, et al. [21] in 1978, and the development of a high-current LaB6 cathode for plasma sources thatPDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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