The mass and bulk of the magnetic focusing structures of traveling wave tubes depend greatly on the details of their design. The authors describe several permanent magnet structures that use no iron pole-pieces and depend upon magnetically rigid permanent magnet materials in novel configurations to produce large longitudinal magnetic fields in the working space with a minimum of leakage to the structural exterior. These configurations are compared with regard to field-mass ratio, field uniformity, and flux leakage. The most mass-effective periodic structure with regard to on-axis field amplitude is the iron-free hybrid structure. For average cross-sectional field strength the triangular hybrid structure is the best choice. Both offer order of magnitude mass and bulk reductions compared to those of the conventional structures which use pole pieces. For short beam paths, high-field, nonperiodic applications, the magic sphere is a good choice. For long beam paths and very uniform fields of modest magnitude (up to 4 kOe), the permanent magnet solenoid should be considered.
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