Abstract: A linear electron beam tube comprises an electron gun having a cathode and a grid, and an anode arranged in a first portion of a drift tube. The drift tube is within a vacuum envelope and has first and second portions separated by a gap at which point an electron beam, density modulated with an input RF signal is inductively coupled to an output cavity. The vacuum envelope is partially defined by a cylindrical ceramic wall and a pair of ferromagnetic pole pieces at its ends that form a DC magnetic circuit. The pole pieces extend radially beyond the vacuum envelope. At least those parts of the surface of the pole pieces that are in the RF path are coated with a layer of relatively low RF loss material such as copper. A balance ring separates the ceramic from the pole pieces.

Abstract: An electron beam tube (17) having a longitudinal axis comprises a first component, such as a ceramic wall (5) and a second component such as a drift tube assembly having a mounting plate (3). The tube further comprises means, such as member (18), arranged to allow relative sliding movement of the first component relative to the second component in a radial direction. The invention helps to alleviate mechanical stresses caused by differential thermal expansion of the components.