Research On Mode Generator For High Power Cyclotron Cold Measurement

With the development of high-power gyrotrons,the wave-interacting cavity is getting larger and larger,and the mode order of the output is getting higher and higher.These high-order modes cannot be directly used,and it is necessary to use the quasi-optical mode converter,which can be converted to a Gaussian mode.In order to verify the reliability of the quasi-optical mode converter,it must be tested before installation,and the key to the test is to design the source of the quasi-optical mode converter.At present,there are two different design methods at home and abroad for generating such high-order mode sources,one is a coaxial waveguide mode generator,and the other is a coaxial quasi-optical mode generator.In order to research the coaxial resonant cavity in depth,the surface impedance matching theory is used to analyze the selection ability of the coaxial resonant cavity.Compared with the hollow cylindrical resonant cavity,it has stronger mode selection capability.Then,based on Maxwell's equations and transmission line theory,the one-dimensional linear differential equations satisfying the coaxial cavity are derived.By writing a Python program to solve this system of equations,the magnitude of the electric field along the axis of the cavity,the resonant frequency of the cavity,and the quality factor can be obtained.The program can quickly optimize the optimized parameters of the cavity,which is unmatched by traditional electromagnetic simulation software.Through these theories,three types of coaxial resonators are designed to generate theTE₆₂ mode.The operating frequencies are 93.81 GHz,93.82 GHz and94.08 GHz,respectively.The output purity is 93.27%,80.39%,and 96.29%.Based on the coaxial resonant cavity,a quasi-optical mode generator is designed.The quasi-optical mode generator is composed of a coaxial cavity with a side wall perforation,a quasi-light parabolic mirror surface,a Gaussian horn and a convex lens.The quasi-light parabolic mirror surface is designed by the geometric optics theory,the Gaussian horn is designed using the coupled waveguide theory,and the sidewall punching is optimized by the electromagnetic simulation software.Through the overall simulation optimization,theTE₆₂ mode purity of the output is 97.81%,and the bandwidth is above 1 GHz,but the conversion efficiency is not as high as that of the coaxial waveguide mode exciter,and the overall conversion efficiency is only 8.0%.This is because the cavity size is too small,and the number of sidewall openings is limited,and the Gaussian beam energy is not concentrated,resulting in a rapid decrease when the energy radiated into the cavity.