EFFECTS OF TAPERING ON GYROTRON BACKWARD-WAVE OSCILLATORS

Computer modeling has been utilized to guide gyrotron backward-wave os cillator (gyro-BWO) experiments at the University of Michigan over a w ide range of tapered interaction regions and tapered magnetic fields. E-GUN code is used to examine beam and diode characteristics, while MA GIC is used to analyze the dynamics of the problem, such as particle k inematics and microwave power production. Several innovative technique s are used to create matching boundary conditions for a backward propa gating wave, MAGIC simulations predict optimum performance of the gyro -BWO operating in a TE(01) mode within a combination of uniform intera ction region and a tapered axial magnetic field which increases 7.5% i n the direction of beam propagation, Experiments have been performed t o investigate the effects of tapering magnetic fields and tapered inte raction region radii on the high-power microwave emission from the gyr o-BWO over the frequency range from 4.0 to 6.0 GHz. These experiments were performed on the Michigan Electron Long-Pulse Accelerator (MELBA) with parameters: V = -0.7 to -0.9 MV, I-diode = 1-10 kA, I-tube = 1-4 kA, tau(e-beam) = 0.4-1.0 mu s. Tapered interaction regions of 37%, 2 3%, 9.4%, and 6.4% were built and tested to determine their effect on microwave power, pulselength, and inferred energy compared to the unif orm interaction region, Magnetic tapering trim coils with a range of - 10.6% < Delta B/B-o < + 15.0% were constructed which allow the orienta tion of the held taper to be changed without breaking the vacuum, The peak microwave power from individual shots was from 30 to 55 MW. Exper iments on magnetic held tapering indicate that positive tapered fields improve microwave power and energy output.