Bi-Helix SWS for High-Power TWTs

We have studied the electromagnetic properties of bi-helix slow-wave structures (SWSs) with added azimuthal conductors in the form of: 1) metal rings connecting the helices and 2) an azimuthally conducting screen formed by metal rings with and without capacitive gaps. We show that these added azimuthal conductors suppress backward-wave oscillation and enable stable high-voltage, high-power operation of traveling-wave tubes (TWTs). Prototype circuits were fabricated and the dependence of phase velocities of the in-phase (fundamental) and opposite-phase (BWO) modes on frequency and on the geometrical parameters of the rings were measured and analyzed. Novel, easily fabricated designs of bi-helix SWSs with circular metallization on diaphragm supports are proposed. These practical designs will make it possible to increase the maximum stable operating voltage of wideband TWTs to 30-40 kV, with a corresponding increase in output power without the danger of BWO.