Advances in the understanding of efficient operation of high power backward-wave oscillators

Summary form only given. Recent work has shown that the efficiency of converting electron beam energy to microwave energy in a backward wave oscillator (BWO) depends very strongly on reflections within the slow wave structure (SWS), and on the interaction of the electron beam with the forward travelling harmonics. The latter result differs from earlier work where it was indicated that the electron beam cannot interact with forward travelling harmonics. Experiments were performed at UNM using the Sinus-6 relativistic electron beam accelerator and an X-band BWO comprised of uniform amplitude corrugations. Sections of smooth-walled waveguide having different lengths were inserted between the BWO and the upstream cutoff neck. Results from these experiments show that the radiated power and oscillation frequency are periodic functions of the length of the smooth-walled waveguide. Particle-in-cell simulations of the experiments predict similar behaviour. Simulations show that the transverse magnetic wave is dominated by fast, backward and forward travelling space harmonics, and the slow, backward travelling harmonic associated with the corrugated structure. The periodic variation of output power and oscillation frequency with the length of the smooth-walled waveguide is well correlated with the constructive and destructive interference of the dominant space harmonics, which form a standing wave in the region between the cutoff neck and SWS.