Characterisation of a novel electron beam gun design with a radio frequency excited plasma cathode (TWI Technology Briefing)

摘要

Most material processing by electron beams (EB), such as welding, melting and 3D printing, uses a thermionic triode electron gun design. There are a number of problems that can occur with this type of gun, and these can interrupt processing or cause variations in its welding or manufacturing performance. In thermionic triodes a heated cathode (typically a tungsten ribbon at 2500℃) emits electrons. A bias cup electrode, that is close to the cathode surface and has a potential difference to it of up to -3 kV, controls the beam current level. The sources of the problems that occur are: 1. The cathode itself - as this can distort or wear, and slowly erodes through evaporation - and any dimensional changes of greater than approx. 10 μm will cause significant variation in the intensity and focal position of the beam, and therefore variation in welding or manufacturing performance. 2. The bias potential used to control beam current that can electrically break down to the cathode, which would result in a surge in beam current that is usually arrested by the high voltage power supply control system, but which can cause interruption to the process.