Start date and planned duration: September 2018, 36 months
- Design, build and demonstrate a laser heated cathode gun at 150kV and 60kW in a known geometry, i.e. replace radio frequency (RF) cathode heating with laser cathode heating.
- Design and build an electron gun column incorporating a plasma source and integrate this onto a processing system.
- Develop processing parameters for welding, texturing and cutting using both systems.
Most electron guns used for welding equipment and similar processing use a thermionic emitter as their source of electrons. A refractory material is heated to a high temperature where the electrons can escape the surface and they are then accelerated by an applied electric field. There are a number of inherent problems with this design:
- The cathode dimensions are very critical, and as it runs hot it distorts and evaporates such that these dimensions change.
- The cathode lifetime can be short (e.g. 6 hours weld time on industrially applied equipment) maintenance costs can be high, and with such frequent servicing there is increased risk of incorrect gun set up leading to beam inconsistency.
- During the lifetime of the cathode the beam quality can vary, changing welding performance and requiring re-adjustment of welding parameters to maintain weld quality.
- Cathode lifetime is shortened if the gun vacuum is compromised, as the residual gas reacts with the cathode, degrading performance.
- Cathode wear can be accelerated by ion bombardment – these ions originating from the weld pool.
This project will two types of alternative cathode: laser heated, and plasma.
Indirectly heated cathodes can be more mechanically stable, and can therefore give more consistent beam quality. These type of cathode have been used to generate very high power beams of up to 60kW. However, this approach requires a complex power supply, and only a few commercial systems have been produced. RF coupling addresses this, but the RF circuit requires bespoke components and frequent intervention for servicing and adjustment.
Laser heating of the cathode has been demonstrated at TWI, and is simple, low cost and reliable. This project will demonstrate laser heated cathodes for the 100kW electron gun used in TWI’s high power facility and in the local vacuum equipment.
Plasma cathodes have also been demonstrated at TWI, using ionised low pressure gas as the source of electrons. This offers a beam with high integrity and consistency, low maintenance, rapid switching, and the ability to operate in a coarse vacuum. The project will extend the power range of this technology, integrate it with electron optics and apply the gun to materials processing – primarily welding, texturing and cutting, to develop and demonstrate its capability.
Benefits to Industry
Plasma cathode and laser-heated electron sources will overcome some of the fundamental operating problems encountered in electron-beam welding and will further improve weld quality and productivity.
Research Board Portal