Abstract: The collector wall axially coextensive with the expanding beam of electrons includes at least portions thereof which extend inwardly to closely approach the periphery of the beam of electrons at several axially spaced planes. In its simplest form this structure consists of a series of annular baffle plates each having a central aperture of a diameter only slightly larger than the diameter of the beam at the axial plane in the collector where the baffle plate is located. These baffle plates have been found to substantially reduce the number of secondary (impact-produced) electrons which return from the collector to the interaction sections of the linear beam microwave tube. As a result spurious signals and noise caused by back-streaming secondary electrons are significantly reduced.