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Hollow Cathodes 255 dealt with supporting and making electrical contact with the material was described by Goebel, et al. [22] in 1985. The lanthanum–boron system can consist of combinations of stable LaB4, LaB6, and LaB9 compounds, with the surface color determined [23] by the dominate compound. The evolution of LaB4 to LaB9 compounds is caused either by preferential sputtering of the boron or lanthanum atoms at the surface by energetic ion bombardment [14] or by preferential chemical reactions with the surface atoms [23]. However, a lanthanum–boride compound, when heated in excess of 1000 ̊C in a reasonable vacuum, will evaporate its component atoms at rates that produce a stable LaB6.0 surface. Dispenser cathodes and LaB6 cathodes offer long lifetimes in thruster applications because the evaporation rate is significantly lower than for refractory metals. Figure 6-8 shows the evaporation rate as a function of the emission current density for a Type S 4:1:1 dispenser cathode [17], LaB6 [24], and tungsten [8] (for comparison). The dispenser cathode and LaB6 cathode evaporation rates are more than one order of magnitude lower when compared with tungsten at the same emission current density. Excessive evaporation of barium and reduced surface coverage usually limit the current density of dispenser cathodes to less than about 20 A/cm2 in continuous operation. In spite of operating at a significantly higher temperature than the barium cathode, the LaB6 has a lower evaporation rate until the emission current exceeds about 15 A/cm2 and can provide longer life. The life of these cathodes is discussed in more detail in Section 6.8. Fig. 6-8. Evaporation rate of Type-S 4:1:1-dispenser cathodes, LaB6 and tungsten.PDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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