Rise and Fall Time Behavior of the Gyrotron Backward-Wave Oscillator

Axial modes of the gyro-BWO are characterized by a discrete set of optimum transit angles separated by ~2pi. We show in theory that, during the rise and fall portions of the beam pulse, the frequency of a given axial mode varies in such a way that its transit angle remains at the characteristic value of the mode. Furthermore, in this transient period, axial mode competition is governed by the distinctive asymmetry of the mode profiles rather than by levels of the start-oscillation currents. 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 good agreement with theoretical predictions.