An improved procedure for the design of periodic-permanent-magnet assemblies for traveling-wave tubes

A detailed analysis of the magnetic-field configurations in periodic focusing structures and new knowledge related to the design of permanent magnets which are to be used in the presence of fields of other magnets have led to the development of a new procedure for the design of periodic-permanent-magnet (ppm) focusing structures for traveling-wave tubes. With this new procedure, the ppm focusing structures can be designed very quickly, and the parameters of the structures produced will approximate the design predictions very closely. This paper describes the derivation and use of a set of design curves which have been used in the design of ppm focusing structures by the RCA Microwave Tube Operations Department for six years. It has been found that the peak axial fields of the structures produced on the basis of these design data are within five percent of the predicted values. Frequently, agreement between actual and predicted values within one percent has been observed. The design procedure requires only a few minutes time, and is applicable to any magnet material whose demagnetization curve is known. A section on the design of the soft-magnetic pole pieces ("shims") is also included.