Travelling wave tube with coupled cavities and focusing by alternating permanent magnets and amplifying system comprising such a tube

A travelling wave tube comprising a cathode system producing an electron beam propagating along an axis and a collector by which the beam is intercepted, and, arranged around said axis between the cathode system and the collector, a delay line formed by successive re-entrant resonator cavities centered on the axis and coupled to one another, said tube being provided with a system for focusing the electron beam having alternating permanent magnets and constituted by magnetized collars, surrounding the outer wall of the cavities, and pole pieces gripped between the cavities and alternating therewith, at a rate of one pole piece every two cavities, said pole pieces being terminated on the side of the axis by a collar centered on said axis, the wall common to two successive cavities being in a material having a thermal conductivity higher than the pole pieces and being itself terminated on the same side by a similar collar made from a magnetic material, these collars forming the re-entrant part of the cavities, whereby transversely with respect to the axis the collars limit the space in which the beam is propagated, wherein the diameter of these collars increases in the beam propagation direction, particularly in the final part of the line, said increase taking place in stages, every other collar, the collar of one pole piece and that of the common wall which follows it having the same diameter.