Electron Beam Focusing with Periodic Permanent Magnet Fields

This paper presents experimental and theoretical results on focusing a long electron beam with a permanent magnet structure which produces a periodic magnetic field along the axis of the electron beam. In principle, the periodic permanent magnet made up of N cells will be reduced in weight by a factor of N2 over a permanent magnet producing a uniform field over the same length. The periodic focusing results are comparable to results obtainable with a uniform field. In addition the periodic fields for certain combinations of beam voltage and magnetic field exhibit regions of "stop bands" wherein the beam becomes unstable and no current reaches the collector.