The performance of a gyrotron backward wave oscillator (gyro-BWO), composed of a uniform waveguide section and a down-tapered waveguide section, is studied by a large-signal self-consistent model. Results show that the efficiency can be drastically enhanced to be 2.5 times higher than that of the gyro-BWO without tapering. Explorations of the physical mechanisms responsible for this enhancement reveal that the effects of the tapered section are twofold. First, the oscillation frequency becomes higher in the tapered case, leading to a higher initial resonance mismatch. As a result, electrons can be bunched deeper in the phase space so that most electrons can be located in the losing energy phase. Second, the reduction of the waveguide radius along the interaction region results in a growth of the coupling between the electron beam and the wave in the region where the wave power decays. In addition, the vulnerability of the efficiency to the velocity spread of the electron beam can be significantly alleviated for the wave of low oscillation frequency by using the tapered waveguide. [References: 18]
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