EFFECTS OF TAPERED TUBES ON LONG-PULSE MICROWAVE EMISSION FROM INTENSE E-BEAM GYROTRON-BACKWARD-WAVE-OSCILLATORS

Experiments are reported on tapered-tube versus uniform-tube, gyrotron -backward-wave-oscillators (4.5-6 GHz) driven by an intense electron b eam with parameters: 0.8 MV, 1-4 kA, and pulselength (0.5-1 mu s). Res ults show that, compared to a uniform interaction tube, a gyro-BWO wit h a 10% downtapered tube produces the following effects: 1) highest mi crowave peak-power (up to about 100 MW in internal tube), a factor of 2 higher than the uniform tube, 2) more reproducible long-pulse (400-5 00 ns) emission, and 3) the largest inferred-integrated energy (factor of 2.5-3 increase). Experiments show high power microwave spikes with lower power plateaus. Experimental observations are in qualitative ag reement with MAGIC code simulations of uniform and tapered-tube gyro-B WO's.