"Comb-Quad" millimeter-wave coupled-cavity TWT interaction structure

The "Comb-Quad" interaction structure has been under study for millimeter-wave space-harmonic TWTs as an electrically comparable, but mechanically very different, substitute for coupled-cavity stacks with "staggered" coupling apertures. Based on four one-piece copper combs, Comb-Quad construction relies on very few piece parts, establishes the periodicity prior to assembly, and leads to a low-loss and mechanically and thermally robust circuit. "Ferrules" are absent, but the interaction-gap capacitance typically is significantly less than that of a ferrule-less conventional cavity stack with the same period, gap length and frequency band. The dispersion has been found readily controllable such that generous "hot" bandwidths may be obtained even at the low beam perveances associated with millimeter-wave TWTs. Topological and electrical equivalents and the determinants of "cold" and "hot" bandwidth have been identified, and the parameters of dispersion, gap capacitance, impedance, and attenuation examined. Secondary passbands and possible extraneous modes were characterized and the impact of dimensional variations and defects charted. Broadband waveguide couplers were easily implemented. Data from an X-band exploratory test vehicle operated at variable low perveance validated the means assembled for predicting small-signal gain and bandwidth and matters of stability.