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Effects of end reflections on the performance of relativistic backward wave oscillators

In a backward wave oscillator (BWO) the primary energy exchange occurs between the electron beam and the backward traveling wave within the slow wave structure (SWS). In most BWO configurations, the backward traveling wave is reflected by a cutoff neck at the entrance to the SWS and the radiation is extracted in the direction of the electron beam. Studies have investigated the effect of the forward traveling wave on the energy exchange between the electron bunches and the backward wave, and the effect of multiple end reflections and cavity resonances. In our experiments, we have observed changes in BWO performance as the phase of reflections at both ends of the SWS were changed. This was accomplished at the entrance of the SWS by placing a section of smooth-walled circular waveguide between the beginning of the SWS and the cutoff neck. At the exit of the SWS, smooth waveguide was added between the SWS and an additional reflection ring. The conversion efficiency from beam power to radiated power and the RF frequency were measured as a function of the length of smooth-walled waveguide introduced and is a measure of the influence of the forward wave on electron bunching as well as the cavity effects of the SWS due to end reflections. These results were compared with TWOQUICK computer simulations. These simulations included both cold tests of the electromagnetic response of the SWS configuration and comparisons of RF output. There is good overall agreement between the experimental results and numerical simulations.