A new type of composite all-metal electron emitter for valves and electron-optical devices

Abstract

Tungsten wires coated with a mixture of tungsten powder, metallic thorium and a small addition of zirconium are found to have emission densities an order of magnitude better than thoriated-tungsten emitters at the same temperatures, and currents up to 30 A cm-2 can be drawn. A further improvement is achieved if the tungsten powder is replaced by tungsten carbide. These cathodes can be operated at temperatures up to 1600°C, where a chemical change occurs. Replacing thorium by metallic lanthanum allows the lowering of the temperatures at equal emission by 200 degC. The safe limiting density is about 10 A cm-2 in either case. Replacing zirconium by hafnium allows emission densities of about 100 A cm-2 to be achieved before a chemical change occurs. The new cathodes make it possible to increase the intrinsic brightness in electron-optical devices by an order of magnitude over pure tungsten cathodes, at comparable lives.