The invention relates to a microwave frequency structure for microwave tube comprising a cylindrical vacuum jacket and a device for containing an electron beam in the axis of revolution of the cylindrical jacket. The containing device comprises at least two rows, each containing permanent magnets, each row being aligned either side of and equidistant to the beam-containment axis, the at least two rows containing permanent magnets being of parallelepipedal shapes and having a magnetic polarization parallel to one of its edges in a plane transversal to the axis, their direction of magnetization in the row changing alternately from one containing magnet to another next containing magnet, or preceding containing magnet, to create an alternating periodic magnetic field along the containment axis.