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 (LaB6) 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 cathodecurrent density for 〈110〉 axial orientations is found to be ten times higher than that for 〈100〉 orientations under equivalent conditions, a value of 50 A cm−2 being measured at 1500°C with an observed lifetime in excess of 300 h. Optimum vacuum conditions for high lifetime and stable operation are in the range 1×10−6 Torr and lower. Comparison values for the emission at various temperatures from other borides, and tungsten, are also given.