Characteristics Study of 3π Stopbands of Folded Waveguide Slow-Wave Structures in V-Band Traveling-Wave Tubes

This paper presents a characteristics study of 3 pi stopbands generated by three types of loading (beam tunnels, bends, and straight segments) in a V-band folded waveguide (FWG) slow-wave structure (SWS). CST Microwave Studio was used to simulate the dispersion characteristics and the instability at 3 pi phase shift. For loading of beam tunnels, horizontal offset and radius give rise to a negligible width of 3 pi stopbands, whereas vertical offset (0.01 mm) results in 3 pi stopband instability. Simulation results also show that the loading of both bends and straight segments generates a wide range of widths of 3 pi stopbands, for which it is believed that the loading of every geometrical period of the FWG SWS is responsible.