Abstract
Evaluation of the surface ionization of cesium on four porous refractory materials regarding ionization efficiency and critical temperatures is reported. Results from the porous materials are compared with solid surface data. The ionization efficiency of the porous material depends upon the flow rate per pore and, therefore, is directly related to the number of pores per unit area. An optimized emitter has more than 3.106 pores/cm2. In this case the neutral flux at 10 ma/cm2 does not exceed 2.6% under clean surface conditions. The ion engine power efficiency depends upon the critical temperature and current density. Comparison of porous tungsten, rhenium, molybdenum, and tantalum indicates the superiority of tungsten. Power efficiencies for clean and oxygenated porous tungsten at 10 ma/cm2 reach between 80 and 90%. At higher current densities the critical temperature is of less importance, and porous rhenium with its higher ionization efficiency becomes more interesting.
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