Abstract: A coupled-cavity delay line for a traveling-wave tube which includes a device for focusing an electron beam along the axis of the delay line for interaction between the electron beam and an electromagnetic wave propagating along the delay line. The focusing device includes an alternate series of permanent magnets and pole pieces which confine resonant cavities on each side. Each of the pole pieces is common to a pair of adjacent cavities and is provided with a drift tube which is positioned at a center thereof and which has a central aperture for passage of the electron beam. Each of the pole pieces is also pierced by at least one intercavity coupling aperture. The delay line also comprises a heat transfer member provided in at least one of the pole pieces to extend from the drift tube to the outside of the pole piece. This heat transfer member is made of a material such as copper having a heat conductivity higher than that of a pole piece.