System for measuring the size of high-current density solid electron beams

A measurement system is described for determining the size of a high-curren t density solid electron beam from a Pierce-type electron gun, The apparatu s incorporates ten graphite apertures and two graphite tubes mounted at 30 degrees intervals on an off-axis drum that is rotated by a stepper motor. T he graphite apertures are used to make localized beam-edge current-intercep tion measurements at ten discrete locations at l-cm intervals along the len gth of the beam. The graphite tubes can be used either to produce an integr ated edge-current profile over an 11.4-cm length of the beam, or to measure the net current outside of two specific diameters centered on the beam axi s. The active aperture or tube and its position with respect to the beam ax is are determined by the orientation of the drum, which is read out using a n absolute shaft encoder. This permits the position of each aperture or tub e to be determined to an accuracy of 60 mum.