Slow-wave filter for electron discharge device

A filter for suppressing backward wave energy modes propagating along a slow-wave structure. The filter is frequency responsive and comprises a wall having coupling apertures and, in one embodiment, a series of waveguide sections which are joined to the coupling apertures and alternate sections are terminated in matched loads. The apertured wall encloses the slow-wave structure and the apertures are arranged relative to periodic elements of the slow-wave structure such that a forward wave can propagate with substantially no interaction with the filter while the energy in a backward wave is dissipated in the matched loads. The filter is particularly adapted for absorbing backward waves in a travelling wave tube operating at high power and an octave bandwidth.