Theoretical Investigations on Forward-Wave and Backward-Wave Operation of a Frequency-Tunable Gyrotron

Frequency-tunable gyrotron is a promising terahertz (THz) radiation source for many modern applications. Actually, a resonator-based gyrotron oscillator can play dual functions: gyromonotron and reflected-type gyro-backward-wave oscillator (BWO) corresponding to the forward-wave and backward-wave operation. The performances of the gyromonotron and the reflected-type gyro-BWO for a 0.5-THz frequency-tunable gyrotron have been theoretically investigated in detail. The calculated results indicate that the frequency tunability, the field profile of the axial mode, and the beam-wave interaction efficiency of the gyromonotron and the reflected-type gyro-BWO are dramatically different from each other. The gyromonotron state can switch to the reflected-type gyro-BWO state by means of increasing the operating magnetic field. In addition, we have systematically investigated the characteristics of the 0.5-THz frequency-tunable gyrotron with different beam-wave interaction structures, especially for the reflected-type gyro-BWO. It is found that the tapered angle of the output section has a significant effect on the frequency tunability of the reflected-type gyro-BWO, and the multisection cavity can improve the performance of the reflected-type gyro-BWO. The results of this study may be beneficial for getting a deeper insight into the physics of the forward-wave and backward-wave operation of a frequency-tunable gyrotron.