Thermal Emission Microscope Studies of Cesium Surface Ionization on Porous Refractory Metals

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Abstract
Quantitative thermal emission microscope studies of cesium surface ionization on porous tungsten emitters show under clean surface conditions a surface migration length in the 2-μ range. The migration length from the pore exit is under this condition fairly independent of the flow rate per pore and the emitter temperature. It is therefore concluded that 107 pores/cm2 (traverse counting technique) yield maximum ionization efficiency on porous tungsten. Clean surface conditions were checked by Richardson work function measurements with the built-in Faraday cage. Also, the ion-current transition pattern in the threshold region was used for judgement of the surface condition. In case of a contaminated surface, a smooth transition from high to low ion current with decreasing temperature was observed in contrast to the steep ion-current change-over in the threshold region under clean surface conditions. Also the surface migration length increased on the contaminated surface. Molybdenum, sputtered onto the porous tungsten substrate, showed a uniform work function over the entire emitter area with φ=4.20 eV.