Reflective Gyrotron Backward-Wave Oscillator With Piecewise Frequency Tunability

The gyrotron backward-wave oscillators (gyro-BWOs) feature a continuous frequency tunability in a single transverse mode, while the step-tunable gyrotrons are famous for their ultrawide frequency hopping between various transverse modes. This study proposed a reflective gyro-BWO with a piecewise-continuous and wide tunability. The frequency is tunable in one transverse mode and can be switched to another by changing the applied magnetic field or/and the beam voltage. Meanwhile, the high-order axial modes (HOAMs) inside a cavity with a low-Q factor could broaden the bandwidth of each transverse mode. The interaction structure is optimized for the TE ₂₂ and TE ₀₂ modes that serve as the principal operating modes. The adjacent TE ₅₁ and TE ₆₁ modes could be excited to broaden the overall tuning range by adjusting the operating parameters. The measured magnetic tunings show a wide bandwidth of 36 GHz from 182 GHz at 6.6 Tup to 218 GHz at 8.0 T, which agrees with the numerical simulation predicted from the Computer Simulation Technology (CST). Besides, the measured voltage tunings exhibit a bandwidth of 2 GHz for the TE ₀₂ mode with a low-beam voltage from 12 to 19 kV. This study demonstrated that the reflective gyro-BWO could generate piecewise frequency tuning in four transverse modes and will be an excellent, moderate-power, terahertz source for many applications.