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Hollow Cathodes 307 Fig. 6-50. NSTAR discharge cathode before and after the ELT wear test, showing complete sputter erosion of the keeper electrode [38]. A significant effort has been expended trying to understand the mechanism for this rapid erosion. Several organizations have measured the presence of high- energy ions in ion thrusters and in the neighborhood of hollow cathodes using retarding potential analyzers (RPAs) [46,60,61] and laser-induced fluorescence (LIF) [62]. For example, Fig. 6-51 shows the ion energy distribution measured downstream on axis and radially away from the plasma ball for the NSTAR cathode [46]. The high-energy ions are detected in both locations, with varying amounts depending on the position at which they are detected. The energy of some of the ions is greatly in excess of the 26-V discharge voltage, and if these ions were to hit the keeper or cathode orifice, they could cause significant erosion. The source and characteristics of the high-energy ions have been the subject of much research and debate. Models of a direct current (DC) potential hill [63] located inside or just downstream of the cathode orifice, or ion acoustic instabilities in a double layer postulated in the orifice of the cathode [64], have been proposed to explain the production of these ions. However, in probe studies to date [1,6,28,42], there has been no detectable potential hill or unstable double layer at the cathode orifice or in the cathode plume that might explain the mechanisms responsible for the high-energy ions or the electrode wear rates and erosion patterns. High-frequency plasma potential oscillations in the 50- to 1000-kHz range associated with plasma instabilities have been detected in the cathode plume and across the front of the keeper by scanning emissive probes [46] and have been proposed as a mechanism for accelerating ions to high energy. In this case, ions born at the peak potential gain the full radio frequency (rf) potential energy when striking the keeper or cathode surfaces, which can exceed 40 to 80 eV [46]. The fluctuations then produce sufficient ion energy to explain the keeper-face erosion reported in the literature.PDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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