Tapered gyrotron-backward-wave-oscillators for high power, long-pulse microwave generation

Summary form only given. Experiments have been performed to investigate the effects of tapered interaction tubes on the gyro-BWO (backward wave oscillator) microwave power and pulse length for an intense microsecond electron beam. The MELBA microsecond electron beam accelerator is operated in these experiments at parameters: V = - 0.8 MV, I = 1-4 kA, and pulse length = 0.5-1 /spl mu/s. Peak microwave tube power is about 8 MW for the untapered case, with tunable frequency in the range from 4.5 to 6 GHz. MAGIC code simulations and experiments were conducted at a number of different tapered-tube magnitudes, 10%, 23% and 43%, and orientations (uptaper or downtaper). Of these, only the 10% downtapered tube was predicted by the MAGIC code to give a gyro-BWO microwave power enhancement for a flat voltage pulse. Experiments have shown significant increases in both microwave power and pulse length for a 10% downtapered tube.