Abstract: High-power linear-beam electron tubes require an extended uniform magnetic field to focus their beam in an elongated cylinder. When permanent magnets are used to energize the magnet structure, there is inevitably a leakage field outside the main flux-return path. The leakage field can refocus the beam in the tube's collector, damaging it. When the collector has air-cooling fins, it is not practical to shield it completely with magnetic material. In the invention, the leakage field is reduced by making the energizing magnet at the collector end axially magnetized and the magnet at the cathode end radially magnetized. Also, a shield around the outside of the fins may be added.