Laser heated cathodes for electron beam welding applications

Laser heated cathodes for electron beam welding applications
Colin Ribton, Paul Plumb


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Electron guns for processing applications typically use a thermionic cathode as the source of electrons, where the cathode is a resistively heated refractory metal ribbon. In order to keep the heat input to the gun reasonably low, a thin ribbon is desirable but is prone to distortion during use, and a ribbon of any thickness will evaporate at the temperatures required for electron emission, such that the surface will regress. The beam characteristics are dependent on the maintenance of tight dimensional tolerances around the cathode, so ribbon cathode lifetimes are often limited to some 12 hours. Changing the cathode necessitates equipment down time, and requalification of the equipment to verify its production readiness, leading to lower productivity. TWI have examined an alternative approach by using a button shaped cathode made of lanthanum hexaboride (LaB6). This material has a higher electron emissivity than refractory metals but at a temperature 1000C lower. As a consequence of this and the physical properties of LaB6, evaporation is negligible. The button was heated with a laser. The approach has been applied to a number of different electron guns, from 3 kW triodes at 60 kV up to a 15 0kV, 100 kW diode gun. Modelling and experimental work have been carried out on a number of cathode holder designs. The laser heated cathode system has been fitted to a number of commercially available electron beam systems for welding and powder bed additive manufacturing.

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Ribton C., Plumb P. Laser heated cathodes for electron beam welding applications. Electrotechnica & Electronica (Е+Е), Vol. 57 (5-8), 2022, pp.51-55, ISSN: 0861-4717 (Print), 2603-5421 (Online)