Study On The Quasi-optical Mode Converter For High Power Terahertz Gyrotron

Currently,as one of the most important international academic fields,terahertz?THz?science and technology has been preferentially developed and become the scientific commanding height.It is very significant to improve the national scientific and technological strength,maintain the homeland security and social stability.But lacking of efficient and stable high power THz radiation source restricts the development of THz science and technology.Gyrotron,as the radiation source which can generate high output power in THz frequency region,has been received wide attention and made great progress.It is also called electron cyclotron maser and it is one kind of fast wave devices,which is based on the mechanism of electron cyclotron stimulated radiation in the longitudinal magnetic field.With the efforts of all countries,gyrotron is developing towards higher output power and frequency,and high-order mode is always chosen as its operating mode to increase the power capacity and reduce the cavity ohmic loss.Yet,the high-order mode should be converted into low-order mode in waveguide or Gaussian beam in free space for the disadvantage of remote transmission and application.When the traditional mode converter is adopted,there are serious mode competition and loss,difficulty for processing and large size for converting which goes against the compact design.However,the quasi-optical mode converter has the advantages of simple structure,compact design and convenient for application and has been widely used.Therefore,it is very important to investigate the high power THz gyrotron quasi-optical mode converter.Theoretical and experimental investigations about 0.42THz-TE26 mode Vlasov-type and 0.42THz-TE_17,4 mode Denisov-type quasi-optical mode converters are presented in this paper.Then two phase-correcting mirrors improving the quality of output wave beam has been designed.At las the study about the quasi-optical mode converter for a dual-frequency operation coaxial gyrotron with two electron beams and a method to separate radiations from the dual-frequency operation gyrotron are proposed.Specific research contents are as follows:1.The electromagnetic wave propagation in the waveguide is analyzed by using basic Maxwell equations,geometrical optics theory and filed theory of guide waves;based on vector diffraction theory and iterative method,the field of 0.42THz-TE26 mode Vlasov-type launcher is calculated and the numerical results agree well with the simulated results calculated by the full-wave electromagnetic field analysis software FEKO.2.Based on transmission line theory and impedance perturbation theory,the coupled wave equations considering the backward waves are deduced.The field distribution and mode coupled situation are analyzed and the numerical results agree well with the FEKO simulated results.3.The equations of quasi-elliptic and parabolic mirrors which meet the requirements of focusing the wave beam are derived by using geome trical optics theory;based on KSA algorithm and vector diffraction theory,two phase-correctors are designed for improving the quality of output wave beam and the numerical results agree well with the FEKO simulated results.4.Experimental investigations about 0.42THz-TE26 mode Vlasov-type and 0.42THz-TE_17,4 mode Denisov-type quasi-optical mode converters are carried out,the experimental test results are in good agreement with those obtained by the theoretical predictions.5.Simulation results and measurements about the dual-frequency operation coaxial gyrotron with two electron beams are presented,which verifies that it can operate at fundamental and second harmonic simultaneously;then the situation that two output modes converted by the same quasi-optical mode converter is analyzed;at last,a method to separate radiations from the dual-frequency operation gyrotron by using a spatial filter basedon frequency selective surface base on are proposed,and the preliminary test with a terahertz time-domain spectrometer,about the transmission coefficient of spatial filter based on frequency selective surface is presented.