Permanent-magnet annular, longitudinal field sources

Some high-power millimeter-wave devices need magnetic fields only near the periphery of a cylindrical region in order to focus an electron beam. Some novel periphery of a cylindrical region in order to focus an electron beam. Some novel permanent-magnet configurations are described which can bring about weight savings by providing a cross-sectionally uniform field in an annular region rather than in an entire cyclinder. This is accomplished by an arrangement of axially and radially oriented magnets that allow almost no flux leakage to the environment.