Design concepts for an octave-bandwidth coupled-cavity TWT

Abstract

To obtain an octave of instantaneous bandwidth in a coupled-cavity TWT, the circuit must exhibit sufficiently low dispersion over a corresponding "cold" bandwidth. Gain flatness over the "hot" bandwidth must also be assured along with electronic stability. Design concepts capable of meeting these needs, and supported by analytical and cold-test results, are introduced. A coupled-cavity structure meeting the cold-bandwidth and dispersion requirements was developed with little difficulty. Within the band, a 2π-point backward-waye instability requires suppression by nonreciprocal means. Ferrimagnetic spheres internal to the circuit can promote stability through a combination of backward-wave absorption and diminished backward-wave Pierce impedance near the selected frequency. Alternatively, novel external circuitry directionally coupled to the side of the TWT can create an effective "sever" affecting only backward waves at the selected frequency.