Research On High-power Millimeter Wave Quasi-optical Mode Excitter

Millimeter wave source is a very important part of high power millimeter wave system.The output power of millimeter wave source will directly determine the operating efficiency of high power millimeter wave system.Gyrotron is a millimeter wave source capable of generating high power radiation,which has very important applications in industry and national defense.In this thesis,the quasi-optical mode exciter required for the cold test of the quasi-optical mode converter in the gyrotron is carried out.Firstly,the theory of Gaussian beam is derived,which lays a theoretical foundation for the generation and transmission of Gaussian beam.Then,some relevant theories of coaxial resonator are derived,such as the transmission line equation,boundary conditions and mode selection of coaxial resonator.At the same time,the characteristic equation of coaxial resonator is obtained.By solving the characteristic equations,the relevant characteristic parameters of coaxial resonator are obtained,such as quality factor and resonant frequency.Finally,the mode coupled wave theory used by both Gaussian horn and coaxial resonator is derived and the mode coupled coefficient of radius gradient circular waveguide and coaxial resonator are obtained.Based on the above theory and geometric optics theory,the specific design process of quasi-optical mode exciter is introduced.Firstly,the feed-source,namely Gaussian horn,is designed through the mode coupling theory,and the Gaussian beam which meets the subsequent requirements is obtained.Secondly,the specific structure of the quasi-parabolic mirror is solved by geometric optics theory,which ensures that the Gaussian beam can radiate into the cavity of the coaxial resonator.Then,the design of coaxial resonator,according to the coaxial resonator theory,the structure parameters,resonant frequency and quality factor of the resonant cavity are obtained by numerical calculation.The waveguide coaxial resonator with rectangular waveguide feed is simulated by the simulation software,and then the resonant cavity required by the quasi-optical mode actuator is designed by using the side wall of the coaxial resonator.Finally,the designed Gaussian horn,quasi-parabolic mirror and quasi-optical coaxial cavity drilled on the side wall are combined to form a complete quasi-optical mode exciter.The simulation results show that the mode purity of the 34 GHz quasi-optical mode exciter reaches 98.7%,which meets the working requirements.Then the quasi-optical power synthesis system is briefly studied.Firstly,the design of quasi-optical devices is introduced,including 140 GHz Gaussian horn,ellipsoid mirror and parabolic mirror.Secondly,the single-channel test of the combination of aligned optical devices is carried out.The simulation results show that the transmission efficiency of the single channel reaches 97.8%.By rotating the single channel 90 degrees each time,a four-channel quasi-optical power synthesis system is formed.The simulation results show that the power synthesis efficiency of the four-channel quasi-optical power synthesis system reaches 81.9%,basically achieving the expected effect.