| As the most classical type of vacuum electronic devices in the microwave and millimeter-wave band,traveling-wave tubes(TWTs)are widely used for communications,radar,remote sensing,radio astronomy,electronic countermeasures,etc.The slow wave structure(SWS)is the core component of beam-wave interaction in TWT,where can achieve the efficient energy exchange process.However,as the operating frequency increases to the V band,the SWS is encountering key technical issues about of strong reflection and hightransmission loss.In order to adapt to the development of the higher frequency,efforts are underway to find new types of SWS to realize the improved TWTs.In these structures,the sine waveguide(SWG)combines the characteristics and advantage of the folded waveguide and staggered double grating,possesses some unique properties,including and low reflection and low propagation loss,as well as its easy fabrication,thus it can be considered as a SWS in millimeter wave and terahertz regime.Meanwhile,the effective degree of interaction between electromagnetic wave and electron beam directly determines the work of the traveling wave tube bandwidth,output power,etc.Therefore,it is necessary to carry out the study on the characteristics of the beam-wave interaction of the TWT,which can provide reliable theoretical guidance for the design and processing of the TWT,and can also quickly optimize the structure size of the TWT.At present,due to the researches on the SWG TWT is still in the primary stage,researchers usually require the use of particle-in-cell(PIC)simulation software as a reference in the design of SWG TWT.However,due to the long computing time and require a large amount of memory,the computation efficiency of PIC simulation is not conductive to shorten the design cycle.Therefore,it is urgent to provide an accurate theoretical basis for the practical design of the SWG TWT.In this dissertation,the characteristics of beam-wave interaction of SWG TWT are studied.At first,the high frequency characteristics of the SWG SWS including dispersion characteristics,interaction impedance and attenuation have been analyzed by using rigorous field analysis,which lays the foundation for subsequent research?Then,the linear analysis and nonlinear analysis of the beam–wave interaction in SWG TWT are presented,which can shorten the design period of SWG TWT while lay a foundation for the further theoretical research of SWG TWT in the future at the meantime.Besides the effect of metal's processing roughness on transmission loss of SWG is carried out.The main work and innovations of this paper are as follows:1.A theoretical model for calculation of the high frequency characteristics in the sine waveguide is presented.According the structure characteristics of SWG,the SWG are considered as a whole system instead of divided into n regions,so the model was greatly simplified.The dispersion equations is obtained by rigorous electromagnetic analysis,which reflect accurately the propagation characteristics of electromagnetic waves in the SWG.And on this basis,the formulas of interaction impedances and attenuation are obtained.The theoretical model is used to numerically calculate the high frequency characteristics of SWG,and the simulation results are compared with the computed results using HFSS code,indicating the reliability of the model.The effects of the structure dimentsions on the dispersion?interaction impedances and attenuation are numerically calculated,it is shown that the practical design should make adjustments according to the needs of the performance index requirements.2.A theoretical model is proposed for linear analysis of the beam-wave interactions of sine waveguide traveling-wave tubes.The field expressions and "hot" dispersion equation describing linear beam-wave interaction are derived by means of field matching.The fields' distribution are illustrated and the effects of structure dimentsions and the beam parameters on the linear gain and bandwidth are calculated.It is shown that the SWG SWS could realize the beam-wave synchronization in a relatively wide frequency band and have large power capacity by selecting reasonable structural parameters and electron-beam dimensions.These obtained results are useful for designing wide band and large power in millimeter TWT.3.A 1D frequency-domain nonlinear model for illustrating the beam-wave interactions of SWG TWTs is studied.Based on the periodicity of structure,poynting theorem,electron motion equation and relativity principle of TWT,a complete set of nonlinear theoretical equations is derived and established in detail.Referring to Tien's 1D disc model,The sheet beam was split into a series of rectangular electron plates in the axial direction and the space-charge field was calculated.A 220 GHz SB-TWT based on a SWG SWS was chosen to verify the validity of the model presented.The first step is to focus on energy conservation in order to verify model accuracy,then the nonlinear characteristics of the power saturation,as well as the electron velocity and phase space distributions are obtained.And the output power and gain results were in good agreement with those from the CST 3D particle-in-cell simulation.4.The transmission loss due to the surface roughness of metal is analyzed.The effect of surface roughness is reflected in the modification of conductivity.The equivalent electrical conductivities is used instead of the ideal conductivity to calculate the transmission loss.Based on the commonly metal surface roughness model: the H-B model and Groiss model,the effective conductivity and transmission loss with frequency with frequency and surface roughness variation degree are further investigated.Then the effect of surface roughness on gain and bandwidth of TWT is studied.Then,a more reasonable roughness model is established based on fractal geometry theory to solve the problems of local surface roughness in the machining process,such as solder accumulation and burr,etc.The surface roughness causes the propagation path of electromagnetic wave on the inner surface of SWG,the transmission loss is increased.According to the actual rough surface data of the test,the relevant parameters are obtained.The influence of surface roughness on the transmission loss on SWG is obtained by using the boundary condition of the mathematical model of surface roughness. |
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