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306 Chapter 6 oxygen in the 10–5-torr range are required to degrade the emission of LaB6 at temperatures below 1440 ̊C, which is shown in Fig. 6-49. The curves for water and air poisoning of LaB6 are at much higher partial pressures off the graph to the right. In comparison, LaB6 at 1570 ̊C, where the electron emission current density is nearly the same as for the dispenser cathode at 1100 ̊C, can withstand oxygen partial pressures up to 10–4 torr without degradation in the electron emission. This means that LaB6 can tolerate impurity levels in the feed gas two orders of magnitude higher as compared with dispenser cathodes operating at the same emission current density. For the case of xenon ion thrusters, LaB6 cathodes can tolerate the crudest grade of xenon available ( 99.99% purity) without affecting the LaB6 electron emission or life. LaB6 cathodes also do not require any significant conditioning or activation procedures that are required by dispenser cathodes. The authors have used LaB6 cathodes emitting at currents of 5 to 10 A/cm2 to produce pure oxygen plasmas in background pressures of 10–3 torr of oxygen. In this case, the operating temperature of the cathode had to be increased to just over 1600 ̊C to avoid poisoning of the surface by the formation of lanthanum oxide, consistent with the trends in the published poisoning results shown in Fig. 6-49. The authors have also exposed hot, operating LaB6 cathodes to atmospheric pressures of both air and water vapor. In both cases, the system was then pumped out, the heater turned back on, and the cathodes started up normally. This incredible robustness makes handling and processing electric propulsion devices that use LaB6 cathodes significantly easier than thrusters that use dispenser cathodes. 6.9 Keeper Wear and Life The keeper electrode typically encloses the hollow cathode and serves the functions of facilitating the starting of the cathode by bring a high positive voltage close to the orifice and protecting the cathode from ion bombardment from the cathode plume and thruster plasmas. However, the keeper electrode is biased during normal operation at an intermediate potential between cathode and anode to collect a reduced number of electrons, and since it is below the plasma potential, it is subject to ion bombardment and wear. Cathode orifice plate and keeper electrode erosion rates measured or inferred in various experiments [57,58] and in ion thruster life tests [34,53,59] have been found to be much higher than anticipated. For example, Fig. 6-50 shows the NSTAR cathode before and after the 30,352-hour extended life test [38]. The keeper electrode was completely eroded away by the end of the test, exposing the cathode orifice plate to the thruster discharge chamber plasma, which significantly eroded the cathode orifice plate and the sheath-heater surfaces. These results have been attributed to the high-energy ions bombarding and sputtering the cathode and keeper electrodes.PDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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