Research On High Frequency Characteristics Of 0.67THz Folded Waveguide Traveling Wave Tube

In recent years,terahertz traveling wave tube(THz-TWT)becomes a promising source of producing high power terahertz wave.With the advantages of large power capacity,high coupled impedance and easy to manufacture,the folded waveguide slow-wave circuit has become the most common technique proposal.Deep theoretical analysis and numerical simulation on high frequency characteristics of folded waveguide slow-wave circuit has been carried out,a design of slow-wave circuit and energy coupling structure is completed for 0.67 THz traveling wave tube to improve performance.In the first,two theoretical model including the simple model and equivalent-circuit model have been established to analyze dispersion characteristics.The formula calculating coupled impedance is derived according to the basic properties of slow-wave modes in periodic structure.A theoretical model calculating the loss of folded waveguide slow-wave circuit is presented,and the attenuation coefficient of the bend waveguide is derived.The results of theoretical model and simulation software show perfectly accordant,which indicates the theoretical model has great precision.Then,numerical simulation analytical method of folded waveguide slow-wave circuit has been introduced on the basis of theoretical model.According to the Floquet theory,building the quasiperiodic model instead of the entire model can reduce the calculating area and improve efficiency remarkably.The influence of the variation of structure parameter on the high frequency characteristics is also studied,the results have been optimized by the software.As to the energy coupling structure,a box window transmitting wave power has been designed and optimized by equivalent-circuit model.The equation calculating the reflection coefficient is derived by the small reflection and impedance transform theory,with which the reflection coefficients of tapered,hyperbolic and laddered transition waveguide structure have been computed and compared.Finally,beam-wave interaction has been investigated to verify the design of slow-wave circuit.The equation of small signal gain is derived with the consideration of the lossy wall effect,synchronization effect and space charge effect.The nonlinear electronics and excitation equation is also derived by Lagrange variable.The TWT gain is calculated using the slow-wave circuit structure parameter designed as input.The results show the nonlinear gain is very close to the PIC simulation,which indicates the theoretical model is appropriate.Compared with PIC simulation,the theoretical model improves computing efficiency remarkably.