Lanthanum Hexaboride Electron Emitter

Abstract

The emitting properties of hot-pressed high-density lanthanum hexaboride rod cathodes have been measured. A current density of 100 A/cm2 is obtained at a true temperature of 1680°C. Lifetime tests have shown a useful life in excess of 200 h with an emission density of more than 50 A/cm2. The initial buildup of emission from the cathode has been found to be a gradual process and the final emission figures are not obtained until approximately 60 min have elapsed. Subsequently however, the full emission is obtained immediately on heating the cathode to operating temperature. The surface structure on the cathode has been examined with a scanning electron microscope both during the initial buildup of emission and during the operating life of the cathode. It is found that surface evaporation reveals sharp crystalline boundaries at a stage which coincides with the full emission density from the cathode. On heating the cathode to operating temperature without drawing emission, the crystalline nature of the material is revealed by the appearance of faceted hillocks on the surface. This structure is etched away when emission is drawn. There is no evidence to support the theory that lanthanum evaporates from selected emission sites. The whole LaB6 surface evaporates during operation and a certain amount of embedded material is revealed. This material is believed to contain tungsten and it accumulates gradually on the surface as operation continues.