Design and Simulation of 10-GHz Bandwidth Phase-Velocity Tapers Folded Waveguide for 108-GHz Continuous-Wave Traveling-Wave Tube

Folded waveguide (FW) traveling-wave tubes (TWT) is a typical kind of vacuum electron device (VED) with some unique characteristics of high frequency, high power, and moderate bandwidth, which are widely used in radars, communication systems, electronic countermeasure, and so on. For the application to the demonstrations of ocean communication with the frequency of over 100 GHz and continuous-wave power with 15 W, an FWTWT operating at the frequency of 108 GHz and the CW power with 50 W is designed. The optimizations of dispersion characteristics and structural parameters of this FWTWT are optimized through a detailed theoretical analysis and a particle-in-cell (PIC) simulation, respectively. For the improvements of communication quality by the increase of output power, a novel method of phase-velocity tapered (PVT) FW slow wave structure (SWS) is used. Comparing with the conventional FWSWS, the output power with the proposed scheme is enhanced 1.5 times. Moreover, the effects of angle and kinetic spreads of electron beam velocity on output power are first investigated using PIC simulations. On the other hand, for obtaining the optimum signal transmission, a tapered and modified pill-box window is presented and used as the transmission structure, which is the characteristics of the broad bandwidth and low attenuation of transmission. With the help of optimized parameters, the CW FWTWT operates at the frequency of 108 GHz, and the maximum output power, gain, and bandwidth are obtained at 78.6 W, 31.9 dB, and 11 GHz, respectively. The optimum results are beneficial to the development of CW FWTWT.