220GHz Folded Waveguide Slow Wave Structure Design

Terahertz refers to electromagnetic waves with frequencies between 0.1THz ~ 10 THz,located between millimeter wave and infrared.With unique properties and great scientific values,it can be used for object imaging,environment monitoring,medical diagnosis,satellite communication,military radar systems and so on.Low radiation power of terahertz source,is still one of the major bottlenecks in the development of terahertz technology.Therefore,it is necessary to develop an ideal terahertz amplifier.Traveling wave tube is one of the most widely used vacuum electronic amplifiers,which can efficiently amplify the signals for terahertz source.Slow wave structure is the core component of a traveling wave tube,among which the folded waveguide slow wave structure holds the advantages such as high heat dissipation capacity,large bandwidth,easy processing and low loss in high frequency,due to its all-metal structure.Accordingly,the folded waveguide traveling wave tube(FWGTWT)has become the focus of THz devices research globally,showing great development potential and attracting attentions from large numbers of research institutions.220GHz is an atmospheric window of the terahertz frequency band,which is of great scientific value and attracts many countries into the competition.In response to the urgent demand of broadband terahertz radiation sources,a 220 GHz folded waveguide traveling wave tube is designed and simulated in CST in this dissertation.Also,the fabrication of the slow wave structure is discussed and initial processing has been carried out by Micro-Wire cut Electrical Discharge Machining(WEDM).The major research efforts and achievements are as follows:(1)The dispersion characteristics,interaction impedance and loss characteristics of the folded waveguide slow wave structure are analyzed theoretically and simulated in CST microwave studio.The simulation results show that the dispersion characteristic is relatively flat and the interaction impedance is about 2? at 220 GHz.(2)Based on the theory of interaction between electron beam and wave,the interaction circuit is designed and optimized.The nonlinear large signal numerical simulations of folded waveguide traveling wave tube are analyzed in CST particle studio.The relationship between gain and interaction parameters(beam current,electron's emitting angle,beam energy spread,etc.)is discussed.Finally,FWTWT in length of 60 periods for interaction has been simulated and the output signal in time domain and frequency domain,power level,gain,bandwidth and other indices are obtained and discussed.(3)In order to solve the problem that the cross-sectional dimension of the 220 GHz folded waveguide slow wave structure is inconsistent with the standard waveguide,the transition section of the structure to the standard waveguide is designed by linear gradient method and simulated in CST Microwave StudioTM.The standing-wave ratio meets the requirements of the whole tube design.(4)Based on the micro processing technology,the fabrication of 220 GHz folded waveguide slow wave structure is discussed and initial processing is carried out by WEDM.Problems in the fabrication process are discussed and corresponding solutions are put forward for further processing.In summary,the theoretical analysis of 220 GHz folded waveguide slow wave structure,the integrated design,the nonlinear large signal numerical simulations and the exploration of micro-machining process will lay a solid foundation for further practical developing of the 220 GHz folded waveguide traveling wave tube.