Abstract: The areas of the walls 1 separating two adjacent cavities that are located on the periphery of the opening 3 for allowing through the electron beam, and whose thermal resistance is greatest, are those comprised between this opening 3 and a cavity coupling opening 2. Owing to the opening 3 for allowing through the electron beam (elliptical, centered on the axis of the line 00', with the large axis of the ellipse merging with the axis of symmetry AA' of the coupling openings 2 intersecting the axis 00'), these areas are the furthest away from the axis of the line 00' along which the electron beam is propagated. The invention is applied to microwave tubes and in particular to travelling-wave tubes.