Tapered tube, microsecond electron beam gyrotron backward-wave-oscillators

Experiments have been performed to test microwave efficiency enhancement and pulselength extension of the gyrotron-backward-wave-oscillator (gyro-BWO) through the use of a tapered interaction tube. The MELBA accelerator has been utilized to generate electron beams with parameters: V=-0.7 to -0.8 MV, I=1-4 kA, and pulselengths from 0.5-1 μs. The microwave frequency is magnetically tunable in the range from 4.6-6 GHz. MAGIC code modeling has guided the experimental selection of taper magnitudes to enhance efficiency and pulselength. The optimal taper magnitude found in both the experiment and model has been a 10% downtapered tube, which gave a significant increase in both microwave tube power (factor of ~2, up to about 80 MW) and pulselength (≈30 % average increase up to 0.38 μs) over uniform tubes. Integrated microwave pulse-energy is also maximized for the 10% downtapered tube. Taper magnitudes larger than 10% gave reduced microwave power and energy