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The effects of the attenuator and the pitch step and taper on suppressing the backward-wave oscillation (BWO) in high-power broadband helix traveling-wave tubes (TWTs) are analyzed by a nonlinear 1-D beam-wave interaction theory in which the effects of the variation of the angular velocity of the macroparticles along the length of circuit are considered. It is found that the incorporation of an attenuator is an effective way to improve the stability of the TWT input section, while a pitch step or taper can be used to reduce the backward-wave gain in the TWT output section without compromising the inband saturated output power. An optimized design of a high-power broadband TWT with improved BWO suppression and forward-wave saturated output power is presented. The optimized design utilizes an optimally positioned attenuator, along with positive pitch tapers in the input and output TWT sections.