Aberrations in high perveance electron guns

A detailed experimental investigation was made of the electron beams produced by a 2 µP electron gun designed exactly according to Muller´s charts, and of a similar gun whose design was modified to suit a particular high voltage pulsed klystron. The distribution of current density and profile of the beam were examined either by allowing the electrons to fall on a collector whose face was covered with carbon, or by measuring the current passing through a small pinhole in a screen which could be moved across the beam at a number of axial positions. Purely electrostatic tests showed that the beams were initially annular but eventually became densest near the axis. This is attributed to crossing trajectories, resulting from an anode aperture whose diameter is comparable with the cathode-anode spacing. With magnetic focussing, the current density distribution across the beam varied periodically along the axis to an extent which depended critically on the particular configuration of magnetic field in the accelerating region.