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252 Chapter 6 where E is the electric field at the cathode surface. The Schottky effect often becomes significant inside hollow cathodes where the plasma density is very high and the electric field in the sheath becomes significant. The properties of the material selected for the thermionic emitter or insert determine the required operating temperature of the cathode for a given emission current. The work functions and values of D found in the literature for several common cathode materials are summarized in Table 6-1. Figure 6-7 shows the emission current density calculated using Eq. (6.3-3) for several different emitter materials. The refractory metals are seen to have work functions in excess of 4 eV, and so they must operate at very high temperatures to achieve significant emission current density. The so-called “oxide” cathodes have work functions under about 2 eV and so are capable of producing high emission current densities at temperatures under 1000 ̊C. Oxide layers, such as barium oxide, were first deposited on tungsten or nickel filaments to lower the work function and reduce the heater power required. However, these surface layers evaporate and are easily sputtered by ion bombardment, limiting the life in vacuum applications to thousands of hours and in plasma discharges to tens of hours. This problem was mitigated by the development of dispenser cathodes where a reservoir of the oxide material is fabricated into the tungsten substrate, which continuously re-supplies the low work function surface layer. The most commonly used dispenser cathode in thrusters, the “Phillips Type S,” uses a porous tungsten matrix that is impregnated with an emissive mix of barium and calcium oxides and alumina [16]. Different molar concentrations of the three constituents of the emissive Table 6-1. Work function and Richardson coefficients for several cathode materials. BaO-Scandate [11] BaO-W 411 [12] BaO-W 411 [10] LaB6 [13] LaB6 [14] LaB6 [15] LaB6 [8] Molybdenum [8] Tantalum [8] Tungsten [8] 120 — 120 — — 1.5 — 29 — 110 120 — 120 — — 55 — 37 — 70 8 10–7T 2 – 1.3 103T + 1.96 1.67 + 2.82 – 10–4 T 1.56 2.66 2.87 2.91 2.66 + 1.23 10–4 T 4.2 4.1 4.55 ADPDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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