Abstract: In the last section of a delay line having coupled cavities for a traveling-wave tube, the following parameters are progressively increased in the direction of propagation of the electron beam: the diameter of the opening provided for the passage of the electron beam, thus making it possible to ensure better transmission of the beam and consequently to ensure higher efficiency as well as lower heat build-up; the ratio (D.sub.2 -D.sub.1)/2 of the cavity nozzles; and the thickness of the walls which are common to two adjacent cavities.

Abstract: In a travelling wave tube in which the delay line (1) is constituted by a sequence of double resonator cavities (15) alternating with the pole pieces (3) of the focusing system, the invention provides for the purpose of limiting the interception of electrons by the line an increase in the diameter of the magnetic material collars (30, 40) terminating said pieces, as well as the wall (16) of the cavities on the side of the tube axis. This increase takes place in stages, a collar (40) having the same diameter as the collar (30) which follows it in the beam propagation direction.