A high power coaxial TWT for wide-band millimeter wave amplification

To achieve an average power level of ten kilowatts or better at high gain over a wide band in the millimeter range, consideration is given to the coaxial TWT device. A thin, hollow beam, focused by a periodic coaxial permanent magnet array, is passed between helices of a novel contra-wound, multifilar helix circuit. Relief in the requisite magnetic field levels and in the thermal dissipation density level is very significant in circuits of large radii. The space harmonic structure of the multifilar helix circuit makes possible frequency extension of the helix-type circuit into the millimeter regime. Wide band performance (>40 percent) is derivable for multi-start helices deposited upon thin-walled dielectric tubes having metallic wall termination. A generalized modal analysis of practical circuits has indicated favorable dispersion and beam coupling impedance characteristics to be achievable. Adverse band splitting effects stemming from mechanical non-concentricity among the fast symmetric mode and various independent backward wave modes of high azimuthal numbers are believed to be manageable by standard techniques. To achieve seven kilowatts of average power, a multistart circuit of 12.7-cm median diameter interacts with a hollow beam (10A at 8.3 kV) over a 15-cm length, providing 52 dB gain at 35 GHz and 40 percent bandwidth. Standard small and large signal (2- D) analyses, adapted for coaxial interaction and periodic coaxial magnetic focusing has been developed for these design investigations. Profiled loss patterns have been included in these determinations.