Low-power Millimeter Wave Mode Converter

As a very important high power microwave source,gyrotron plays an irreplaceable role in aerospace,military,medicine,communication and other fields,it is one of the research focuses in the world.Gyrotron is complex in structure,precise in assembly,expensive in price.Its key part,which is called quasi-optical mode converter,have to be tested in low power condition before use,it is also called cold test,in order to prevent unexpected accidents happen in high power condition.So that a corresponding high order rotary mode converter need to be designed to simulate the output of gyrotron,and its purity has a great influence on the accuracy of test results.Therefore,the design of high order rotary mode converter with high purity can promote the development of gyrotron industry.In this paper,a theoretical analysis and design of spiral waveguide and coaxial-spiral waveguide mode converters are carried out by using the theory of coupling wave,spiral waveguide,radius gradient coupling theory,mode suppression technology,coaxial cavity mode selection,etc.The main work and research results are as follows:1.The development and current situation of the international millimeter wave technique,gyrotron and cold test,and high order mode converter are systematically described.2.Firstly,the discrete coupling and its coupling wave difference equations in the basic theory of coupling wave are analyzed.Then,the coupling wave differential equations with continuous coupling of modes are obtained by the relationship between discrete coupling and continuous coupling.And a detailed derivation process is given.Then the spiral waveguide theory is analyzed,and the coupling equations of the helical waveguide and its coupling coefficient are given.Finally,the theory of radius gradual coupling wave is introduced,the corresponding coupling wave equations and coupling coefficients are also given,as well as the contour function obtained by phase matching technique.3.By using the electromagnetic simulation software to get the best optimization parameters of cavity.The simulation results show that:In the center frequency 30.5 GHz,the purity of this mode converter is up to 99.9%,the output conversion efficiency is also above 99.85%,the bandwidth of the output conversion and purity higher than 95%is4.4%;In this bandwidth,the output mode is the circular polarized TE₁₁ mode.The amplitude difference of the two mutually linear TE₁₁ modes is less than 0.02,and the phase difference is around 90±2°.The axial ratio is about 1d B,it showing good circular polarization characteristics.The design has the advantages of high purity,high efficiency and the output mode is circular polarized mode,which lays a foundation for the design of high order rotary mode converter.Based on the successful design of the low order TE₁₁mode converter,in this thesis,a high order rotary TE₆₂ mode converter is designed.Similarly,in the center frequency of 94 GHz,the output TE₆₂ mode purity and conversion efficiency is up to 92.52%and 80%,respectively.The amplitude difference between TE₆₂mode and its cross-polarized mode is less than 0.05,and the phase difference is also close to 90°,so the axial ratio is around 1.3d B.Compare with other researcher's design,this mode converter can generate the high order rotary mode directly,which does not include the generation of transition mode and achieves the function of compact structure.In this study,a structure that varies both azimuthal and radical at the same time is used,namely helical waveguide,its function and theory is integrally introduced,and the process of mode conversion is analyzed in detail,and the influence of perturbation amplitude and period to mode conversion is emphatically introduced.Besides,a mode suppressor is designed to improve the output mode purity,the influence of its dimension and position to the mode converter are also analyzed.This design has the high purity,compact and circular polarization characteristics.4.In this thesis,the most important breakthrough is to realize the combination of helical waveguide and coaxial cavity,the high order rotary mode is generated directly by changing the outer wall in a coaxial cavity.This coaxial cavity is sidewall-fed by a rectangular TE₁₀ mode waveguide,and then by using the characteristic of a coaxial cavity can choose mode to select TE₆₁ mode.And change the outer wall in the coaxial cavity to a helical surface at the same time,get the optimum parameters by using Computer Simulation Technology?CST?Microwave Studio,finally the coaxial cavity is gradually changed to circular waveguide,thus the circular waveguide rotary mode is generated.In this thesis,the theory of coaxial cavity generate TE₆₁ mode is introduced in detail,and the influence of the ratio of coaxial cavity inner radius and outer radius to resonant frequency is also analyzed,the simulation results show that the output TE₆₂ mode purity is up to91.31%.This design also has the characteristics of high purity,compact and output is rotary mode.