Research Of THz Quasi Optical Mode Converter

As an important high power terahertz source, gyrotron causes widespread publicconcern for its excellent performing. With the development of gyrotron into higherpower and higher frequency, the operating mode turn from circular TE0nmodes towhispering gallery TEmnmodes. As a key part of gyrotron, the mode converter can notsatisfy the change in adoption of traditional waveguide. The inside quasi optical modeconverter become a research focus for this problem. Compared with the traditionalmethod, the inside quasi optical mode converter is simpler to fabricate, can suppressrefletion, realize the division of the electron beam and wave beam, and with higherefficiency, etc. At present, a lot of in-depth study by foreign research institutions hasbeen done into the inside quasi optical mode converter, and progress can also be seen athome. Under the circumstances, we study and design the inside quasi optical modeconverter on the basis of existing achievement.Firstly, we use optical equivalence for the transmission mode in the overmodedstructure, and use geometrical optics to explore the general working principles of quasioptical mode converter. Equivalent optical model is established to introduce andanalysis each component in quasi optical mode converter, including waveguide launcherand mirror system, at the same time, we use geometrical optics to calculate thelauncher’s dimension. As for the advanced waveguide launcher--Denisov launcher, weuse the coupled wave theory to analysis and calculate the influence on the modecoupling and distribution in the launcher made by the wall loaded spatially periodicperturbed structure.Secondly, vector diffraction theory is adopted to analysis and calculate the radiatedfield on each aperture, as a reference to optimize the quasi optical structure, aimed at theformation of fundamental Gaussian mode on the output window.In the design of mirrorsystem,phase correction mirror is adopted as the conventional quadratic mirror to adjustoutput wave phase.The KSA algorithm is used to caculate phase correction mirrorstructure.Finally,the Matlab programs are written for caculating and designing quasi optical mode converter. By using these programs,a Vlasov quasi optical mode converter and aDenisov quasi optical mode converter are designed for94GHz,TE62mode gyrotron.Thestructure size and the radiated field on each aperture are given. A Vlasov quasi opticalmode converter and a Denisov launcher are also designed for420GHz,TE17,4modegyrotron.By using the3D EM simulation software,the caculation result of420GHzDenisov launcher is verified.