Study Of Beam-wave Interaction In Folded Waveguide Traveling Wave Tubes And Novel Devices

Folded waveguide(FWG)traveling wave tubes(TWTs)are characterized by high power,wide bandwidth,robust structure,and easy processing,and have promising applications in communication,imaging,electronic countermeasures,etc.To cope with new opportunities and challenges,this dissertation focuses on the study of beam-wave interaction model of FWG TWT,novel devices and the design methodology of FWG TWT on terahertz power source and communication.The specific work and innovation are as follows:1.Research on nonlinear beam-wave interaction of FWG TWTs.Considering the rapid increase of the coupling impedance near the cutoff region of FWG could lead to the inaccurate beam-wave interaction calculation,this dissertation analyzes the characteristic impedance of FWG and replaces the coupling impedance with the characteristic impedance in the proposed model.The loss characteristics of FWG is also discussed.When FWG operates near the cutoff region,the attenuation constant is replaced by the quality factor.Compared with high-frequency characteristic results calculated by HFSS,the results of the proposed model are consistent with HFSS.Moreover,the proposed model takes much less time in calculating the high-frequency characteristics.Then,a nonlinear cavity-cascaded beam-wave interaction model for FWG TWTs is proposed in this dissertation.The slow-wave structure(SWS)is modeled as a sequence of coupled resonant cavities.The electron beam is treated as a current source that can induce electromagnetic wave in the cavity.The beam-wave interacting process is modeled as the active matrix.Compared with conventional beam-wave interaction analysis model,the proposed model can even analyze beam-wave interaction of FWG TWT in cutoff region.2.Program realization and experimental verification of cavity cascade model.Based on the analysis and theory mentioned above,the proposed model is implemented in a code and calculation process of code is described in detail.To validate the calculation accuracy of cavity cascade model,an E-band FWG TWT is designed,and the predicted results are compared with the experimental results.Meanwhile,the results of a Q-band FWG TWT calculated by the proposed model are compared with its experimental results.The comparison results show that the difference between calculation results and experimental results is less than 10%.To verify the performance of the proposed model near-cutoff region,an FWG TWT with three-staged SWS is designed and the beam-wave interaction of the FWG TWT is analyzed by the proposed model and CST,respectively.The comparison results show that simulation results in the proposed model are consistent with those in CST.The analysis above proves that the proposed model can effectively guide accurate design of FWG TWTs.3.Research on THz High-harmonic TWT(HHTWT).To overcome the limitation of the THz signal source with high cost and low power,an FWG HHTWT is proposed.HHTWT is designed based on the theory that electron beam modulated by electromagnetic(EM)waves can generate high harmonic signals.When an E-band signal is input,HHTWT can output high-power terahertz signal by amplifying the fourth harmonic of the input signal.The proposed model and the particle simulation method are applied to analyze the beam-wave interaction of the HHTWT.The results show that when the input signals are 200 m W from 85 GHz to 87 GHz,the output power of HHTWT is over 100 m W,and the corresponding frequency range is 340 GHz-348 GHz.The peak power can reach 350 m W.The simulation results demonstrate that amplifying high-order harmonic signals is a promising idea for obtaining high-power terahertz sources.4.Research on dual-beam cascade schemes for HHTWTs.Based on the HHTWT,two novel dual-beam schemes for HHTWT,named cascaded enhanced HHTWT(CEHHTWT)and pre-amplified HHTWT(PA-HHTWT),are proposed to further improve output power.The high-frequency characteristics and the beam-wave interaction of these two devices are analyzed by the proposed model and particle simulation method.The results show that when the voltage and current of the two electron beams are the same,the power of the input signals are 200 m W from 85 GHz-87 GHz,the output power of CE-HHTWT with entire bandwidth is over 400 m W,peak power of 1100 m W is achieved at 346 GHz.In PA-HHTWT,the frequency range of the input signal is 84.5 GHz-87.5GHz,the power of the input signal is 200 m W.The power of the output signal within 338GHz-350 GHz is over 600 m W.The 3-d B bandwidth reaches 16.5 GHz.5.Research on gain equalization design method for FWG TWT.To improve gain equalization characteristics of FWG TWT and make it meet the requirements of high-rate communications on low bit error rate(BER),a gain equalization design method is proposed.By introducing an enhanced positive-dispersion section,gain equalization can be implemented by improving gain at high frequency and decreasing gain at low frequency.By utilizing this method,a Q-band 3-stage FWG-TWT is designed by the proposed model.Compared with the conventional FWG TWT with uniform SWS,the gain variation of the proposed 3-stage FWG-TWT within whole bandwidth can be less than 0.3 d B.The 3-stage FWG-TWT also can extend the 3-d B bandwidth by more than～45%.It provides a beneficial solution for improving the gain equalization characteristics of the device.