Research On Key Technologies Of Broadband Quasi-Optical Mode Conversion Of High-Power Millimeter Wave

With the rapid development of radar,missile guidance,electronic countermeasures,controlled nuclear fusion,microwave biology,material manufacture and other technologies and applications,millimeter-wave has gradually become a hot topic in civilian and military fields.The development trend of millimeter-wave towards high frequency and high power.The gyrotron traveling-wave tube?gyro-TWT?,based on electron cyclotron maser,has obvious advantages over other high power sources and amplifiers in bandwidth,output power and gain.At present,the techniques of gyro-TWTs are relatively mature and reach industrial utilizations.However,the output modes of gyro-TWTs are unsuitable for direct use and must be transformed into a spatial Gaussian field distribution before accessing the antenna system.The high ohmic attenuation and low power handling capacity of standard metallic waveguide supporting the fundamental mode are prohibitive for high power millimeter-wave.Therefore,the quasi-optical mode conversion based on oversized waveguide is proposed for high power millimeter-wave system.In oversized waveguide,any small boundary perturbations will excite the parasitic modes,resulting a decreasing efficiency,and the generated parasitic modes will interfere with the later systems.Moreover,due to the large quantity of mesh-grids,the simulation software has a very slow calculation,and is unsuitable for the design or optimization of oversized waveguide structure.Based on the theory of coupling-wave,this dissertation researches the beam control by changing the waveguide boundary,and programs for fast design and optimization of oversized mode converters.This dissertation mainly focuses on the broadband quasi-optical mode conversion technique,which is used for converting the high power millimeter-wave from gyro-TWTs into Gaussian-like distributions,and transmitting it to the antenna feeding system.More details are as follows:?1?The characteristics of modes coupling in circular waveguide are analyzed,and the results reveal that the narrow bandwidth of circular waveguide bend is caused by the degeneration between TE₀₁ mode and TM₁₁ mode.Analyzing the dispersion characteristics of elliptical waveguide,a typical section-perturbed circular waveguide,the results show that the efficient and broadband waveguide bend can be realized by elliptical waveguide bend as the degeneration between TE₀₁ mode and TM₁₁ mode is removed in elliptical waveguide.To avoid the difficult of using simulation software to calculate the oversized waveguide devices,the coupling wave equation and coupling coefficient of elliptical waveguide are derived,based which a program is developed for fast calculation of elliptical waveguide bends.Using the program,an oversized elliptical waveguide bend is designed,which has advantages of broad bandwidth and high power capacity.Furthermore,the coupling rules in elliptical waveguide bend are analyzed and summarized,and an oversized dual-mode and dual-band waveguide bend is designed according to the analysis results.?2?The mechanism of TE_0n-to-TM_1n power conversion in axis-bending circular waveguide is studied,and it is found that the direction of power conversion depends on the relative phase between TE_0n mode and TM_1n mode.According to the analysis results,the phase rematching method is proposed to control the mode conversion process by adjusting the relative phase.A 90-degree oversized TE₀₁-to-TM₁₁ mode converter is designed based the proposed phase rematching method,which removes the acute bend caused by traditional TE₀₁-to-TM₁₁ mode converter and facilitates the installation of the whole transmission line.Besides,this structure has the functions of waveguide bend and mode converter,and could improve the efficiency of whole transmission line by reducing system components.Furthermore,the dependence of the curvature distribution of the waveguide bend and the content of parasitic mode is analyzed,and a universal solution to reduce the parasitic mode content at the output port of the waveguide-bending structure is proposed.The curvature is compressed to the front half of the waveguide,by which the curvature near the output port is reduced and thus the parasitic mode at the output port is reduced.Combing the axis curvature optimization and the phase rematching method,a 90-degree TE₀₁-to-TM₁₁ mode converter with sinusoidal square axis is designed with improved bandwidth and conversion efficiency.?3?The working principle of the iteration method for mode converter is analyzed,and a multi-frequency iteration method is proposed for the design of broadband mode converter.The iteration method uses the curvature of waveguide axis as optimization parameter,and the correction of curvature during iteration process is with a simple and explicit formulation.The explicit formulation of the correction of curvature brings the iteration method a fast convergence,and offers a possibility to achieve multi-frequencies optimization.A piecewise weight function is introduced to guide the corrections of curvature corresponding to different frequencies into different regions,by which the interference between the corrections of different frequencies are avoided,and the convergence speed of the calculation is improved.Furthermore,the coaxial control technology is proposed to solve the defect that the input port and the output port designed by iteration method is not coaxial.With the coaxial control technology,the iteration method is suitable for system requiring high installation accuracy.Using the improved iteration method,two TE₀₁-to-TE₁₁ mode converters respectively working in Ka-band and W-band are designed.?4?The working mechanism of broadband smooth-walled horn with input port only supporting fundamental mode is analyzed.Inspired by the analysis results,this dissertation proposes a synthesis mechanism for broadband oversized smooth-walled horn.By controlling the inner taper of the horn to adjust the content of TM₁₁ mode,the relative phase is modulated,which means the relatively fast-growing relative phase is given a smaller initial value.Owing to phase modulation,the deviation between the target value and the relative phase at the output port is reduced,thus the broadband characteristics is achieved by the oversized horn.Based on the proposed method,a Ka-band broadband oversized smooth-walled horn is designed.