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 struct
ure 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 sin
gle parameter that depends on the beam characteristics; its value varies be
tween zero when no coupling exists and unity in case of maximum coupling. F
or 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 li
nearly with the increasing phase-spread of the bunch. Because of the intera
ction, 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. Se
lective damping to suppress the asymmetric mode is described and analyzed.