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Study of BWO power holes in helix traveling wave tubes

Slow wave structure employed in a wide-band helix traveling wave tube (TWT) always support backward waves. A backward wave space harmonic of a signal at a harmonic of the drive signal can effectively interact with slow space-charge wave of an electron beam. The minus first space harmonic is in synchronism with the beam at some frequencies. As a consequence, the backward wave can compete with the forward wave and it results in the backward wave oscillation (BWO) power holes under large signal conditions. In this talk, we reported a study of BWO power holes in both simulations and experiments, and developed a step helix pitch method to suppress the BWO power hole.