Suppression of In-Band Power Holes in Helix Traveling-Wave Tubes

Slow-wave structures employed in forward-wave linear beam amplifiers such as wideband helix traveling-wave tubes (TWTs) always support backward waves. A backward-wave space harmonic of a signal at a harmonic of a drive signal can effectively interact with a slow space-charge wave of an electron beam. In this case, the backward wave can compete with a forward wave, which results in backward-wave oscillation (BWO) power holes under large signal conditions. Sufficiently large power holes have serious consequences for the performance of a system in which the TWT is employed. In this paper, we report a study of BWO power holes simulated using a 2.5-D large signal helix TWT code and demonstrate an effective method, the pitch step method, to suppress BWO power holes.