Abstract: A traveling wave tube adapted for periodic magnetic focusing of the electron beam has a thin-walled, non-magnetic cylinder around the slow-wave circuit portion, forming part of the vacuum envelope. A stack of metal rings surrounding the cylinder has alternating non-magnetic rings and magnetic rings, the latter forming the periodic magnet pole pieces. The rings and the thin cylinder are all brazed together to provide a strong structure. Since the cylinder does not have to be self-supporting, it is made thin enough to allow close spacing between the pole pieces and beam, providing strong magnetic field and good focusing. The brazed joints between rings are not vacuum joints, so the probability of leaks is greatly reduced.