The interaction of symmetric and asymmetric modes in a high-power traveling-wave amplifier

A three-dimensional (3-D) model has been developed for the investigation of the coupling of the lowest symmetric and asymmetric modes in a high-power, high-efficiency traveling-wave amplifier. We show that in a uniform structure and for an initially nonbunched beam, the interaction efficiency of the asymmetric mode may be much higher than that of the symmetric mode. It is also shown that the coupling between these two modes is determined by a single parameter that depends on the beam characteristics; its value varies between zero when no coupling exists and unity in case of maximum coupling. For a beam that is uniform at the input end, this parameter varies linearly with the guiding magnetic field. In case of a bunched beam, it decreases linearly with the increasing phase-spread of the bunch. Because of the interaction, the radius of the beam increases linearly with the power associated with the asymmetric mode at the input end; it increases rapidly in the case of an initially uniform beam relative to the case of a prebunched beam. Selective damping to suppress the asymmetric mode is described and analyzed.