Research On Beam-wave Interaction In THz Electron Cyclotron Maser With High Order Mode

It is well known that terahertz wave?THz?is sandwiched between millimeter wave and infrared wave,thus terahertz wave possesses both characteristics of millimeter and light.Since the particularity of terahertz wave,teraherz wave has significant application prospect and value,thus the researches on terahertz science and technology have become the hotspots and emphases in the scientific research field.But the basis and premise of terahertz science and technology research is the study on terahertz radiation sources,the gyrotron referring to the mechanism of electron cyclotron maser?ECM?has been demonstrated to be the most likely to achieve high power source of terahertz radiation with high efficiency,thus the developments on gyrotron for high power terahertz source are very meaningful.According to the theory of gyrotron,to achieve high power,the gyrotron has to operate necessarily at high-order mode to maintain reasonable cavity transverse dimensions at the operating frequency,which in turn alleviates the problem of wall heating and beam interception on the cavity wall.Meanwhile,to overcome the limitation caused by the achievable strength of external magnetic field,the gyrotron is designed to operate at high harmonic of the cyclotron frequency.But gyrotron operating at high harmonic and high order mode brings a very serious problem-mode competition,which is the major factor to hinder gyrotron to achieve high power terahertz radiation with high efficiency.Therefore,an in-depth study on the mode competition in the terahertz gyrotron with high order mode is presented in this thesis.Based on the national“Twelve·Five”project“The research on electronic cyclotron maser terahertz source with XXX”,a 0.42 THz second harmonic complex cavity gyrotron with high order mode is designed in this paper,and the influence of the size of complex cavity on the cold cavity characteristic is carefully analyzed,also including the influences of beam parameters,magnetic field,and the beam quality on the beam-wave interaction.According to the results of nonlinear simulation,the mode competition in the designed0.42 THz complex cavity gyrotron is investigated on the basis of time-dependent,quasi self-consistent,nonlinear theory,the influences of window reflection,Ohmic loss and beam quality on the mode competition are discussed.Finally,the designed 0.42 THz complex cavity gyrotron is manufactured and tested in the Terahertz Science and Technology Research Center,the experimental results can well satisfy the requirement of the project.The follows are the main contents and contributions of the thesis:1.Based on the coupled wave theory in the cicular waveguide,a cold cavity simulation code to study the cold cavity field distribution in the gradually tapered complex cavity is written,which has been demonstrated by comparison with HFSS,CST.Meanwhile the influences of cavity size on the cold cavity characteristic of the complex cavity are analyzed in detailed with the help of the self-development code,and a complex cavity operating at 0.42 THz with the operating mode of TE_17.4 is designed preliminarily.2.Starting current is the prerequisite of mode competition investigation.In this paper,the expression of starting current in the complex cavity is derived from the original definition of the starting current,and a corresponding code to simulate the starting current in the complex cavity is written.With the help of this code,the influences of beam quality,Ohmic loss and the phase difference of the operating mode on the starting current are carefully investigated.3.Based on the transmission line equations with source term and relativistic electron motion equation,the beam-wave interaction equations in the complex cavity are derived in detailed.At the same time,a corresponding simulation code is self-developed,which is demonstrated by comparison with CST-particle simulation software.With the help of the self-development code,the influences of beam voltage,beam current,and magnetic field on the beam-wave interaction are discussed in detailed,Meanwhile,the influences of beam quality such as beam width and velocity spread are also studied.Finally,a 0.42 THz second harmonic gyrotron with complex cavity is designed,which can generate 78.78 kW terahertz radiation with the frequency of 420.0210 GHz.4.A numerical code to simulate the mode competition in the complex cavity is self-developed on the basis of the time-dependent,quasi self-consistent,nonlinear model.With the help of the code,the mode competition in the designed 0.42 THz complex cavity gyrotron is investigated,and the influences of beam quality and Ohmic loss are discussed.The simaltion results show that the operating mode TE_17.4 can be excited in the regime of selected operating parameters and competing modes are well suppressed in the region of beam-wave interation,meanwhile the complex cavity has an obvious advantage in suppressing mode competition.5.Based on numerous simulation on linear theory,nonlinear theory and mode competition,the designed 0.42 THz complex cavity gyrotron is manufactured and tested in the Terahertz Science and Technology Research Center.The experimental results show that the designed gyrotron can generate 19.3 kW terahertz radiation with the frequency of421.645 GHz.Due to that the output power and operating frequency are deviated from the designed values,the influences of manufacture error,beam quality,startup and the overshoot on the beam-wave interaction are investigated.The results show that the experimental results can be consistent with the numerical ones when considering suitable influence factor.