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Hollow Cathodes 253 100.000 10.000 1.000 0.100 0.010 0.001 Scandate BaO-W 411 LaB6 Tantalum Tungsten 500 1000 1500 Temperature (deg C) 3000 Fig. 6-7. Emission current density versus temperature for various cathode materials. mix are used depending on the required emission current density and life. In ion thrusters, a 4:1:1 emissive mix typically is used, although other mixtures are available [1]. The matrix material containing the impregnate can be directly heated by passing a current through the material, or configured as an insert placed inside hollow cathodes. In dispenser cathodes, chemical reactions in the pores of the matrix or at the surface at high temperatures reduce the emissive material and evolve a barium- oxide dipole attached to an active site on the tungsten substrate. The 4:1:1 Type S cathode has a work function of about 2.06 eV at temperatures in excess of about 800 ̊C. Barium-oxide dispenser cathodes with porous matrix material made of tungsten can provide emission current densities of 10 A/cm2 at surface temperatures of about 1000 ̊C, as shown in Fig. 6-7. The work function can be further reduced by the introduction of small amounts of other refractory materials, such as iridium or osmium, in the tungsten matrix. These “mixed metal matrix” cathodes can have work functions below 1.9 eV, and they typically slow some of the chemical reactions that take place in the cathode. It was also found that the addition of scandium to the surface of the barium-oxide dispenser cathode reduces the work function significantly. This is reflected in Fig. 6-7, where the reported work function [11] at 1100 ̊C is about 1.7 eV, which is significantly less than the 2.06 eV for the BaO dispenser cathode and results in much lower temperatures for a given emission current density. The mechanism for this improvement is not clear to date, and the ability to fabricate stable scandate electron emitters that maintain the low work function over time 2000 2500 Emission Current Density (A/cm2)PDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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