Novel Concepts for Slow Wave Structures used in High Power Backward Wave Oscillator

Backward wave oscillators (BWOs) are high power microwave sources that can generate megawatts (MW) to gigawatts (GW) of electromagnetic radiation. They achieve this by converting the energy of a high power electron beam into electromagnetic wave energy via Cerenkov radiation. A key issue with BWOs is their poor energy conversion efficiency which ranges from 15-40%. An integral component of BWOs is the slow wave structure (SWS) which serves as the medium for the beam-to-mode energy conversion. In this work, novel SWS designs and concepts are presented and shown to significantly improve the efficiency and performance of high power BWOs. A novel slow wave structure design is presented. This design features a deeply corrugated cylindrical waveguide with cavity recessions and metallic ring insertions. A new technique for mode control in waveguides is also presented. In addition to demonstrating mode control in slow wave structures, the key aspects of the presented design are mode dominance reversal and a 100% improvement in interaction impedance. A novel and cost effective fabrication technique for the SWS is presented. Fabrication and testing results are also presented to experimentally validate the dispersion properties of the novel SWS. For the first time, we experimentally demonstrated mode dominance reversal in SWSs. Particle in cell (PIC) simulations using this SWS design predict an output power of 5.92 MW at 27 GHz with 58% peak power efficiency for a homogeneous SWS. To further improve the energy conversion efficiency of BWOs, we conduct an extensive study of inhomogeneous SWSs to determine the optimum inhomogeneous SWS design. Results from this study show that a 3-section inhomogeneous SWS yielded the highest efficiency. Fabrication and testing of this 3-section inhomogeneous SWS are presented. Results from this experiment show higher order dispersion in the form multiple secondary inflection points (MSIPs). Further S-band simulations using the inhomogeneous SWS predict 8.25 MW output power at 2.62 GHz with 70% peak power efficiency.