EFFICIENCY ENHANCEMENT OF HIGH-POWER VACUUM BWOS USING NONUNIFORM SLOW-WAVE STRUCTURES

The Sinus-6, a high-power relativistic repetitively-pulsed electron be am accelerator, is used to drive various slow wave structures in a BWO configuration in vacuum. Peak output power of about 550 MW at 9.45 GH z was radiated in an 8-ns pulse. We describe experiments which study t he relative efficiencies of microwave generation from a two-stage nonu niform amplitude slow wave structure and its variations without an ini tial stage. Experimental results are compared with 2.5 D particle-in-c ell computer simulations. Our results suggest that prebunching the ele ctron beam in the initial section of the nonuniform BWO results in inc reased microwave generation efficiency. Furthermore, simulations revea l that, in addition to the backward propagating surface harmonic of th e TM(01) mode, backward and forward propagating volume harmonics with phase velocity twice that of the surface harmonic play an important ro le in high-power microwave generation and radiation.