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Significant efficiency enhancements in high power backward wave oscillators using inhomogeneous slow wave structures

We present a high efficiency backward wave oscillator (BWO) using an inhomogeneous slow wave structure (SWS). This SWS is based on a novel unit cell design that features mode dominance reversal and complete isolation of passbands. Inhomogeneity is introduced by varying the periods of the SWS cells in each section. The resulting SWS is then fabricated and cold-tested to determine its dispersion curve. Results from this cold test demonstrate higher order dispersion in the form of multiple secondary inflection points (MSIPs). Current simulations with commercial particle-in-cell (PIC) software show that a 3-section inhomogeneous SWS can singificantly enhance power efficieny by over 20%. PIC simulations using this SWS design predict a peak output power of 8.25 MW at 2.62 GHz, corresponding to a power conversion efficiency of 70%.