Study Of High-order Mode Generator Of A Barrel-shaped Cavity

Gyrotron,as the main source of high power microwave systems,has been one of the international research focuses for its extensive applications in electron cyclotron resonance heating(ECRH),deep-space millimeter-wave radar and millimeter wave communication.Modern gyrotron usually adopts high-order rotating volume mode as its operating mode,which is then transformed by a built-in quasi-mode converter into a linearly polarized quasi-optical mode for practical applications.In order to verify the quality of a design of the quasi-optical mode converter of a gyrotron before assembly,usually,low power tests of the mode converter is of vital necessity.In such tests,the mode purity of the rotating high-order mode generator plays an extremely important role in the credibility of the tests.Therefore,it is of great significance to study the higher-order mode generator with high mode purity.At the same time,circular polarizer is an important part of the mode generator system,which directly determines the performance of the mode generator.Thus the design of compact circular polarizer should be implemented simultaneously.In this dissertation,the design of high-order mode generator in a barrel-shaped cavity and compact circular polarizer is theoretically analyzed,numerically calculated and simulated based on the hole diffraction theory,coaxial cavity selection characteristic and loading phase shifting technique.The mainly work and study results are listed as follows:1.Numerical calculations are written to analyze the characteristics of open coaxial cavities.By using this program,the field vertical distribution,resonance frequency and quality factor of the high-order TE5,3 mode in the three-stage coaxial cavity are analyzed.The optimized parameters of the cavity are calculated,providing a certain basis for the subsequent research of high-order mode generator in the barrel-shaped cavity.Simulation results of the commercial software HFSS are in good agreement with the calculations.2.A method for designing the higher-order mode generator is proposed in [1],however,the mode purity,which is the most important performance indicator of the generator,is not mentioned.Even more unfortunately,the overall progress of their design is apparently ambiguous and over-simplified.Thus,an improving design of the rotating high-order mode generator based on the helically-distributed perforation(HDP)technique is presented.The design principle of the generator with HDP is studied in detail,and the whole process of mode conversion is completely analyzed.The paper focuses on how to determine the parameters of the coaxial cavity structure by using the numerical calculation program,and studies the arrangement of HDP as well as the working principle of the mode suppressor.Numerical calculations and simulation results indicate that the mode purity of TE5,3 mode generator is as high as 97.4%,and the conversion efficiency is 96.1% near the center frequency of 30 GHz.Compared with its old version,the generator has the obvious advantages of high mode purity and compactness.3.The research on compact circular polarizer(CP).According to the requirement of the rotating high-order mode exciter studied above,the paper is focused on its compactness.Two types of circular polarizers in the Ka-band based on phase-shifting element loading technology,including CP with a ladder-shaped dielectric membrane and a fan-shaped groove loading,are designed.Simulation results show that these circular polarizers are designed with compactness,low axial ratio and simple structure.Finally,the designed CP with a fan-shaped groove loading is applied to the rotating high-order mode generator of a barrel-shaped cavity.The results of the joint simulation indicate that the designed high-order mode generator and CP have practical applications.