Backward-Wave Suppression Analysis, and Design and Fabrication of a Prototype Millimeter-Wave Ring-Bar Slow-Wave Structure

A challenge for high-power millimeter-wave (mmw) traveling-wave tube (TWT) amplifiers is to realize high-power operation without incurring oscillation from backward-wave (BW) interaction. Conventional wisdom purports that contrawound (including ring bar) helix TWTs are superior to monofilar helix TWTs for stability against BW oscillations due to a zero or at least greatly suppressed BW interaction impedance (K-BW) compared with the forward-wave (FW) interaction impedance (K-FW). We use 3-D electromagnetic (EM) field calculations of a ring-bar helix to compare the BW and FW interaction impedances. Using a sine/cosine basis set and a comparison at a fixed phase velocity (rather than fixed frequency assumed in previous analyses), we show that the BW interaction impedance is not always significantly suppressed in the ring-bar/contrawound helix. In spite of the lack of ubiquitous BW stability, we use EM simulations to illustrate that there remain specific, efficient, high-gain mmw ring-bar designs with K-BW < K-FW. The accuracy of the simulations is validated by experimental measurements that confirm simulation predictions of phase velocity and S-parameters.