Characterisation of electron beams at different focus settings and work distances in multiple welders using the enhanced modified Faraday cup

Abstract

Using the enhanced modified Faraday cup (EMFC), the differences in the beams produced by two electron beam welders are characterised at different focus settings and work distances. For example, EMFC measurements show that sharply focused beams display different shapes and peak power densities which vary by nearly 20% for the same welding parameters on these two welders. Increases in work distance on each machine were shown to result in decreases in both the peak power density and the resulting weld size and shape. Because of the differences in machine performance, additional differences also arise when comparing the welds produced by each machine. These different weld dimensions are attributed to differences in the beam shape and a 70 mm difference in the theoretical beam crossover location in the upper column of the two welders. The crossover location, which can not be physically measured, is determined using the EMFC by analysing the beam distribution parameters of sharply focused beams over a range of work distances. By combining these results with simplified optics calculations, the magnification of the beam optics can be determined and the machine performance of each welder characterised. The work distance on each machine at which beams with similar peak power density values will be produced can then be determined. With this knowledge, changes in either the beam focus or work distance can be made to attain similar beams from different welders, thus providing a baseline for developing modern weld transfer procedures.