Stability of gyro backward wave oscillation

Summary form only given, as follows. It was shown theoretically and experimentally that in a conventional slow wave backward wave tube, the oscillation is susceptible to self-modulation or choatic behavior when the beam current exceeds certain critical value. The mechanism responsible for these non-stationary behaviors is attributed to the distributed interaction of the system that leads to a delay in its response to the onset of nonlinearity and to the phenomenon of electron rebunching in the downstream region. The above mentioned two essential characteristics for non-stationary phenomena can also occur in gyro backward wave oscillations (BWO). A time-dependent PIC code is used to investigate the stability of gyro BWO. The result shows that if a uniform waveguide is employed such that the backward oscillation is dominated by a single mode, self-modulation can take place if the beam current is sufficient large. However, if the system consists of a uniform waveguide placed between two tapered waveguides, the oscillation becomes stable due to the excitation of multitude of modes. As a result, the efficiency can exceeds twenty percent.