Overmoded backward-wave oscillator: a comparison of experimental results with non-linear analysis

Summary form only given, as follows. Overmoded structures have the potential for increasing the power handling capability of linear beam, high power microwave (HPM) devices, reducing internal rf electric fields to below field-emission breakdown levels. However, potential detrimental effects include multi-mode generation and a reduction in efficiency and power. To study the feasibility of using overmoded structures in HPM generation, we conducted a series of experiments with an overmoded (D//spl lambda/-3), relativistic backward-wave oscillator (BWO) in the 5.4 to 5.6 GHz frequency range. The experiment, which was designed to operate near the edge of the passband of the TM/sub 01/ mode, produced highly coherent radiation (/spl Delta/f/f/spl les/0.5%) with a maximum power of 320 MW. Power, efficiency, and frequency were measured as a function of both electron beam parameters and electrodynamic structure length. In parallel, we developed theoretical models of BWOs operating near cutoff, which included end-reflections and finite magnetic field effects. For most of the experiment, the measured frequencies were within 5% of the model, which correctly tracked the trend of efficiency with structure length. We present a comparison of the theoretical and experimental results and an interpretation of the physical processes.