Rise and fall time behavior of the gyrotron backward-wave oscillator

Summary form only given. The rise- and fall-time behavior of a pulsed microwave oscillator is of importance to radar and other applications because it can be a transient source of unwanted oscillations and hence the cause of leading/trailing edge jitters during the pulse. Here we present a study of such behavior in the gyrotron backward-wave oscillator (gyro-BWO). The gyro-BWO interaction processes are distinctively different from those of the resonator-based gyrotron oscillator. For the latter device, the oscillation frequency is largely fixed by the resonator structure. Consequently, voltage and pitch-angle variations of the electron beam in the rise and fall portions of the pulse lead to transit angle variations for a number of modes, and each of these modes can be excited over a duration in which its transit angle transitions through the optimum value (~pi). For the gyro-BWO, however, there are no cold resonant modes. The oscillation frequency is free to assume any value over a continuous range and identities of the axial modes are determined entirely by the electron dynamics rather than by the interaction structure. As a result, it has been shown that the (hot) axial modes are linearly characterized by a discrete set of optimum transit angles Theta separated by ~2pi where Theta is the accumulated phase variation of the wave as observed by the electron in traversing the interaction space. The current study reveals theoretically that, during the rise and fall of the beam pulse, the frequency of a given gyro-BWO mode varies in such a way that the transit angle remains at the optimum value for the mode. However, mode competition and mode switching can still occur due to the characteristic asymmetry of the axial mode profiles. A Ka-band gyro-BWO experiment has been carried out to examine the rise/fall-time behavior. Time-frequency analyses of the output pulses show excellent agreement with theoretical predictions. These understandings are expected to provide a useful guide for the design of a stable gyro-BWO operating at a single mode throughout the pulse