Experimental studies of overmoded relativistic backward-wave oscillators

Internal field-emission breakdown in the electrodynaniic structures of high-power microwave (HPM) devices can seriously limit the device's output power and pulse duration. Increasing the diameter of the electrodynamic structure to several times an electromagnetic wavelength can reduce these internal fields to below critical breakdown levels, but may introduce mode competition as an unwanted side effect. This paper presents the design and results of experiments with overmoded (D/\ ~ 3), sinusoidally corrugated backward-wave oscillators (BWO's) that successfully produced TM®j, high-power microwave radiation in the frequency range of 5.2-5.7 GHz. Overmoded BWO's reproducibly generated ~200 MW of peak power with corresponding efficiencies of ~4%. Pulse shortening was not observed in any of the experiments. The radiation generated by the devices was highly coherent (typically, A/// < 0.5%) and corresponded to a fundamental TMjfj-mode interaction. The experimental results were compared with calculations made with recently developed nonlinear models; the measured data are shown to agree favorably with theory. The results of the experiments and modeling demonstrate that overmoded electrodynamic structures can be used to decrease internal electric field stresses while avoiding multimode generation and maintaining good spectral purity.