Research On Third Harmonic Confocal Waveguide Gyrotron Oscillator

Terahertz wave has a wide range of application prospects and has received more and more attention from most of the countries in the world.As one of the highest output power terahertz radiation sources at present,the gyrotron has unique advantages in high frequency and high power.With the increase of operating frequency,the traditional struc-ture gyrotron faces problems such as serious mode competition and excessive operating magnetic field.In order to solve the above problems,the third harmonic confocal waveg-uide gyrotron is investigated.The specific contents are as follows:The theoretical method of confocal waveguide is derived in details,and the mode characteristics and transverse diffraction characteristics of confocal waveguide are ana-lyzed based on this theory.TE_0,20mode is chosen as the operating mode of the third har-monic confocal waveguide gyrotron,and the transverse field distribution and transverse diffraction loss of the operating mode are analyzed.Based on the open resonator theory,the longitudinal diffraction loss and ohmic loss of the confocal waveguide gyrotron are analyzed.According to the transverse diffraction loss,longitudinal diffraction loss and ohmic loss,as well as the field distribution charac-teristics of TE_0,20mode,the optimized design of confocal waveguide gyrotron resonator is completed.On this basis,the linear theory of gyrotrons is used to calculate the start-ing current of the third harmonic confocal waveguide gyrotron,and the mode competition analysis is carried out.The self-consistent nonlinear theory based on the high-frequency structure of the con-focal waveguides is used to analyze the beam-wave interaction efficiency in the confocal waveguide gyrotron.The optimal operating parameters of the third harmonic confocal waveguide gyrotron are set through parameter optimization:operating voltage 30.2k V,operating current 2.5A,operating magnetic field 11.91T.Under these optimal parameters,the injection interaction efficiency of the gyrotron is 3.35%,and the operating frequency is 949GHz.Based on the theory of electron optics and the principle of adiabatic compression,the preliminary design of the dual anode magnetron injection electron gun was completed,and the electron gun is further optimized with the particle simulation software MAGIC.When the operating voltage is 30.2k V and the operating current is 2.5A,the guiding center radius of the electron beam is 1.05mm,and the speed spread is 5.76%.The electron gun can well meet the requirements of the third harmonic confocal waveguide gyrotron.Through the combination of theoretical analysis and simulation,the output window design of the third harmonic confocal gyrotron is completed.When the thickness of the output window is 0.256mm,the reflection coefficient is better than-25d B,which meets the design requirements.