A method of focusing electron beams

The paper describes a method of using reversing magnetic fields to focus electron beams in which considerably smaller permanent-magnet structures are required than with focusing using unidirectional fields. The structures needed are more convenient than those necessary for periodic focusing systems, being applicable to klystrons, for example, and the range of beam parameters over which the method can be used is greater than for periodic focusing systems. It is also probable that beam focusing by reversing-field systems of the type described with few reversals will be less critically dependent on field and beam parameters than in periodic focusing, although the weight-saving advantages of the latter system would be largely retained. The use of an experimental system with one field reversal to focus an electron beam is described.