Study Of A Quasi-optical Mode Converter For220GHz Whispering-gallery Mode

In order to meet the requirements of high frequency-high power gyrotron, thecavity modes with higher order and lower loss are often used as the operating modes.There will be serious diffraction and polarization loss while the higher order cavitymodes transmit in the over mode structure, so it is necessary to transform the higherorder cavity modes to the free space Gauss beam or the lower order waveguide modes.Because traditional mode converters have a lot of problems to transform the higherorder modes to the lower order modes in the high-frequency band, the Vlasovquasi-optical mode converter can be used to convert the operating mode to the Gaussbeam (TEM00). This thesis focuses on the design and analysis of a220GHz Vlasovquasi-optical mode converter operating atTE52mode. Another focus is themanufacture and experiment of a220GHzTE10TE01side-wall coupling modeconverter.Firstly, the basic principle of the Vlasov mode converter is detailed analyzedusing the geometrical optics theory, and the design formula of the radiator structure isobtained. According to the principle of the light transmission, the space mirror systemand the output window position are gained by moving the coordinate system. By thevector diffraction theory, the process of the electromagnetic beam transformation isdescribed. In order to calculate the radiation field, the integral aperture of helical cut isdivided into two parts. The shielding effect of helical cut is analyzed, and the reflectionat the helical cut is calculated by the iterative method. Finally, the numerical simulationof a220GHzTE52quasi-optical mode converter, the cylindrical waveguide radius ofwhich is5mm, is carried out by Matlab codes. Simulation results show that, at theoutput window, the efficiency of power conversion is78.24%, the Gauss componentcontent is94.09%and the waist radius is6mm.In the last part of the thesis, a220GHz side-wall coupling model converter, thatconverts theTE10mode of rectangular waveguide to theTE01mode of cylindricalwaveguide, is designed by CST software and tested. Results indicate that the conversionefficiency is more than99%at220GHz, the efficiency is more than98.5%with the4 GHz bandwidth, and the loss of noise mode is less than-45dB from210GHz to230GHz. The simulation results can meet the design requirements. The model is tested byusing220GHz solid state source and PM4power meter test equipment. Theexperimental result show that the structure is feasible.