EXPERIMENTAL STUDIES OF OVERMODED RELATIVISTIC BACKWARD-WAVE OSCILLATORS

Internal field-emission breakdown in the electrodynamic structures of high-power microwave (HPM) devices can seriously limit the device's ou tput power and pulse duration. Increasing the diameter of the electrod ynamic structure to several times an electromagnetic wavelength can re duce these internal fields to below critical breakdown levels, but may introduce mode competition as an unwanted side effect. This paper pre sents the design and results of experiments with overmoded (D/lambda s imilar to 3), sinusoidally corrugated backward-wave oscillators (BWO's ) that successfully produced TM0.1.; high-power microwave radiation in the frequency range of 5.2-5.7 GHz. Overmoded BWO's reproducibly gene rated similar to 200 MW of peak power with corresponding efficiencies of similar to 4%, Pulse shortening was not observed in any of the expe riments. The radiation generated by the devices was highly coherent (t ypically, Delta f/f < 0.5%) and corresponded to a fundamental TR0.1.-m ode interaction, The experimental results were compared with calculati ons made with recently developed nonlinear models; the measured data a re shown to agree favorably with theory. The results of the experiment s and modeling demonstrate that overmoded electrodynamic structures ca n be used to decrease internal electric field stresses while avoiding multimode generation and maintaining good spectral purity.