A Comparison of the Contact Ionization of Cesium on Tungsten With That of Molybdenum, Tantalum, and Rhenium Surfaces

Surface ionization of cesium on porous tungsten, rhenium, molybdenum and tantalum is reported in light of the ionization efficiency and the critical emitter temperature up to current densities of 10mA/cm{sup 2}. (The experimental method described previously is used.) Results from the porous materials are compared with solid surface data. The neutral detector characteristics are discussed. The power efficiency of the ion engine depends strongly on the critical temperature of the emitter. These temperatures are presented for the four refractory materials and the spectral emissivities of the porous materials are listed. Comparison of porous tungsten, rhenium, molybdenum and tantalum up to ion current densities of 10 mA/ cm{sup 2} indicates the superiority of tungsten with respect to the critical temperature and the related power efficiency. Power efficiencies for clean and oxygenated porous tungsten are computed taking into consideration beam power and radiation losses. By introducing oxygen into the ultra high vacuum system the ionization efficiency can be increased. If carbon is present on the heated parts it can be detected as CO{sub 2} and CO; the time of desorption of oxygen from the emitter surface is short in comparison with the time of engine operation. The proportionality factor between statistically evaluated and measured transmission coefficient may be explained as tortuosity and is connected with the consolidation of the porous material. Monitoring five different porous tungsten pellets at their critical temperatures for 10 mA/cm{sup 2} cesium ion current density, for over 10 months allows preliminary extrapolation of life expectancy.