Higher order mode excitations in gyro-amplifiers

Abstract

Summary form only given. Minimization of unwanted modes is a key element in the design of gyro-amplifier RV structures. For example, a nonlinear output taper is often employed as the transition, from the near cutoff radius of the interaction circuit to a much larger output waveguide. The tapers are usually designed to avoid passive mode conversion, and thus do not consider the effect of a bunched beam. However, recent simulations,with the self-consistent MAGY code predict that higher order mode interactions with the bunched electron beam can substantially compromise the mode purity of the RF output. The interaction in the taper region is that of a travelling-wave type and is strongly dependent on the beam bunching characteristics resulting from previous interaction with the operating mode in the gyro-device circuit. To verify this prediction, an,experiment was performed to measure the RF output mode content from a Ka-band gyro-klystron at the Naval Research Laboratory. The agreement between salient theoretical and measured RF output characteristics confirms the existence higher order mode excitation in output tapers. Another example of the need to employ a self-consistent theoretical model in the design of gyro-amplifier RF,structures is the phenomenon of beam-induced RF excitation in drift sections, which are cutoff to the operating mode and are used to separate cavities in gyroklystron amplifiers. This non-resonant RF excitation is at the drive frequency and the RF field structure is that of the operating mode. The RF amplitude is found to scale linearly with the bunched beam current. The presence of RF in the drift section has important thermal implications in the design and use of lossy dielectrics in drift-sections, especially for high-average power devices.