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 cathode current 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.
Highlights
Powered by the California Digital Library University of CaliforniaDesign and optimization of directly heated LaB6 cathode assemblies for electron-beam Instruments
The performance of most e lectron-beam instruments is limited by thc brightness of the electron source, since this in turn sets nn upper bound on the maximum current density which can be produced in any beam formed by the instrument
The cathode current density Jeof a thermionic emitter is given by Richardson's expres.sion as:
Summary
Design and optimization of directly heated LaB6 cathode assemblies for electron-beam Instruments. California 92093 (Rcccivcd 28 December 1977; acceptcd 13 March 1978). A description is given of the design and testing of a directly heated, stable, electron source utilizing a single-crystal lanthanum hexaboride (LaB6) cathode. 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 electronbeam instruments. The cathode current density for ( 110) axial o rientations is found to be ten times higher than that for ( 100) o rientations under equivalent conditions, a value of 50 A cm - 2 being measured at 1500°C with an observed lifetime in excess of 300 h. Comparison values for the emission at various temperatures from other borides, and tungsten, are given
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