[[abstract]]Summary form only given. The stability issue in the gyrotron backward wave-oscillator (gyro-BWO) is the major limitation of demonstrating its capability of broadband and continuous tunability. A time-dependent code is employed to examine the stability of gyro-BWO for either magnetic-field or beam-voltage tuning. Numerical results indicate that shortening the interaction length stabilizes the operation of gyro-BWO without sacrificing the efficiency for a tapered waveguide structure. Besides, the up-tapered magnetic field can also enhance the efficiency and, more importantly, suppress the unwanted oscillation at the upstream section. A Ka-band gyro-BWO experiment is conducted. The interaction structure consists of a 3 cm uniform section connected at each end to a slightly tapered section of 4 cm in length. The circuit is immersed in a uniform magnetic-field with the exception of the upstream waveguide tapered section. Appreciable efficiency with continuous frequency tunability is experimentally demonstrated for both magnetic-field and beam-voltage tunings. A peak power of 145 kW at 29% efficiency with a smooth tuning range of 1.8 GHz has been achieved
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