Stability and tunability of gyrotron backward-wave oscillator

The gyrotron backward-wave oscillator (gyro-BWO) features continuous frequency tunability. However, stability issues often forbid smooth tuning over a broad bandwidth. We report theoretical and experimental investigations on beam-voltage and magnetic-field tunings of the gyro-BWO. Stationary and time-dependent simulations indicate that a properly-tapered, short interaction structure permits a very broad single-mode tuning range. A Ka-band experiment based on the simulation results has been conducted. The interaction structure consists of a 3-cm uniform section of radius 2.66 mm connected at each end to a slightly tapered section of 4 cm in length. The structure is immersed in a uniform magnetic field with the exception of the upstream taper which serves to enhance the efficiency and detune the unwanted competing modes. Continuous frequency tunability and smooth power spectra were experimentally demonstrated in both magnetic-field and beam-voltage tunings. A tuning bandwidth of 8.5% with a peak power of 115 kW at 23% efficiency has been achieved.