Magnetic Tapering Of Intense E-beam, Long-pulse Gyro-bwos

Experiments have been performed to investigate the effects of tapered magnetic fields on the high power microwave emission from the gyrotron-backward-wave-oscillator (gyro-BWO). Tapering trimcoils have been utilized to compare gyro-BWO microwave emission for uniform versus tapered magnetic fields in the range of {Delta}B/B{sub 0} = 10--15%, where B{sub 0} = 4--6 kG. These magnetic tapering experiments employed a TE{sub 11} mode at tunable-frequency of 4.5--6 GHz, with a uniform interaction tube radius. Initial experimental results show that the peak microwave power and integrated energy are nearly the same for the uniform magnetic field, compared to the 11% downtapered B field. Uptapered magnetic fields ({approximately}15%) gave lower powers and energies. MAGIC code simulations have been run for the gyro-BWO with a scaled TE{sub 01} mode to model various magnetic tapering magnitudes and both directions. Future gyro-BWO experiments will combine tapered tube radius with magnetic tapering to optimize microwave emission power, pulselength, and energy.