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

A measurement system is described for determining the size of a high-current density solid electron beam from a Pierce-type electron gun. The apparatus incorporates ten graphite apertures and two graphite tubes mounted at 30° intervals on an off-axis drum that is rotated by a stepper motor. The graphite apertures are used to make localized beam-edge current-interception measurements at ten discrete locations at 1-cm intervals along the length of the beam. The graphite tubes can be used either to produce an integrated 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 axis. The active aperture or tube and its position with respect to the beam axis are determined by the orientation of the drum, which is read out using an absolute shaft encoder. This permits the position of each aperture or tube to be determined to an accuracy of 60 μm.