Travelling wave tube with slow wave form - has projections in walls of broad dimension of cavity shut off with coupling unit

The travelling wave tube has a slow wave structure and coupled cavities. The tendency of interference between the coupled cavities is reduced. The traveling wave tube has a stopper, a distribution pole piece with a drift tube and a cavity shut-off element in the form of a rectangular wave-guide with projections operating in conjunction with the shut-off wedge of the travelling wave tube. The projections (20) in the shut-off element (11) are in the broad dimension walls of the cavity shut-off element. Part of the projections are complementary in shape to the shut-off wedge (9). The shut-off wedge lies between the projection and the distribution pole-piece. The cross section of the projections is uniform and match with a cut-out in the wedge.