Improved Surface Ionization Efficiency by High Work Function Refractory Metals and Alloys

Abstract

For continuous one-year operation without deterioration of the surface ionization engine characteristic, the neutral efflux should be in the 0.1% range. For the neutral efflux reduction the Saha equation implies the use of high work function metals. Therefore, tungsten-rhenium alloys and iridium filaments were investigated. For completeness, tungsten and rhenium filaments were also studied with regard to their thermionic work function and threshold temperature for cesium surface ionization. The data were measured in an all-metal guard ring diode with directly heated filament under clean surface conditions. The electron work functions and the threshold temperatures of the tungsten-rhenium alloys increase with increasing amount of rhenium in tungsten. For φ-I≤0.5 eV the electron work function governs the surface ionization. One porous tungsten-25% rhenium pellet and two porous tungsten substrates chemically coated with rhenium and iridium yielded work functions below those measured with the corresponding filaments. At 10-mA/cm2 cesium ion current density, the neutral efflux from the rhenium coated pellet was, with 2×106 pores/cm2 (traverse counting technique), 1%±0.1%. Under clean conditions 0.3% neutral efflux may be expected. The increase of the threshold temperature with electron work function leads to increased grain size and lower pore density for long term stability of the porous structure.