Compensated reversed field focusing of electron beams

Reversed field focusing of electron beams has advantages over the more commonly used periodic permanent magnet focusing in some applications, such as low-noise traveling-wave tubes, but still permits the use of permanent magnets of reasonably small size and weight. For several reversals, or operation over a considerable voltage range, correction of the defocusing action of the field reversals is necessary near these reversals by providing additional dips or peaks in the magnetic field. Linear approximations to the usual theory permit the effect of such corrections to be evaluated, giving a qualitative understanding of the process, and a design procedure. More exact computer calculations show that the approximations are justified by the accuracy of this procedure. Preliminary results on a low-noise X-band traveling-wave tube using compensated reversed field focusing further confirm the theory.