Research on Solenoidal Permanent Magnet for Guiding and Focusing Annular Electron Beams

The vacuum oscillators generally utilize high current annular relativistic electron beams to generate high-power microwaves. The annular electron beams are usually guided and focused by strong axial magnetic fields. A permanent magnet is developed for the application of annular electron beams. The configuration of the permanent magnet is based on the Halbach structure. A new method is proposed to design this solenoidal permanent magnet. The permanent magnet flux source can generate a 0.88 T axial magnetic field at the interaction region and a peak field of 0.94 T. The diameter of its clear bore is 50 mm. The total weight of the permanent magnet is 320 kg. The design, fabrication, and test results are presented in this article.