Backward-wave suppression in a very wide-band helix travelling-wave tube using a slow waveguide filter

In octave bandwidth travelling-wave tubes (TWT), employing helical slow-wave circuits, suppression of the backward-wave, which produces an unwanted oscillation just above the highest frequency of the normal operating band, is usually achieved by means of ´lossy serpentine resonators´ deposited on the helix support rods. This technique is wholly satisfactory when the power levels involved are modest. However, when significant power levels are required, these deposited resonators are insufficiently robust and an alternative procedure is required. This paper describes an external slow-waveguide filter which extracts the backward-wave oscillation (BWO) energy in the output section of a travelling-wave tube through an array of apertures in the helix shield.