Design and optimization of directly heated LaB6 cathode assemblies for electron-beam instruments
Abstract
A description is given of the design and testing of a directly heated, stable, electron source utilizing a single-crystal lanthanum hexaboride (LaB//6) cathode. The emitter mounting fixture consists of an adjustable molybdenum base unit supported on gas-impervious alumina or machinable glass. Single-crystal cathode rods are securely clamped and positioned between vitreous carbon jaws that are resistively heated. The complete assembly is designed to be a direct ″plug-in″ substitute for the conventional tungsten thermionic filaments used in electron-beam instruments. The cathode current density for LT AN BR 110 greater than axial orientations is found to be ten times higher than that for LT AN BR 100 greater than orientations under equivalent conditions, a value of 50 A cm** minus **2 being measured at 1500 degree C with an observed lifetime in excess of 300 h. Optimum vacuum conditions for high lifetime and stable operation are in the range 1 multiplied by 10** minus **6 Torr and lower. Comparison values for the emission at various temperatures from other borides, and tungsten, are also given.
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