Research On Terahertz Quasi-optical Electron Cyclotron Maser

Terahertz?THz?science and technology is one of the most important cutting-edge field in international academic research,and it is of great value in scientific research and is of great significance to economic development and national defense.The terahertz radiation source is the foundation of the development of terahertz science and technology.Up till now,gyro-device,based on the principle of electron cyclotron maser?ECM?,is the most powerful radiation source in millimeter-wave and terahertz band,and has been widely applied in many fields,especially in high-resolution radar,long distance imaging,satellite communication,electromagnetic countermeasure and directed energy weapons.Motivated by developing the novel high power THz vacuum electronic devices?VEDs?,the comprehensive theoretical analysis and experimental tests results of THz gyro-devices with a novel interaction structure,which is based on the quasi-optical waveguide,are presented in this dissertation.The main contents and results as follows:1.Theoretical investigations on quasi-optical waveguide:Combining with electromagnetic field theory and geometrical optical approach,the characteristics of quasi-optical waveguide,such as field distribution,dispersion relation,mode spectra and diffraction losses are theoretical analyzed and validated by the results of electromagnetic simulation software.It establishes a good foundation to design the interaction structures for THz quasi-optical gyro-devices.2.Based on the ECM principle,a kind of self-consistent nonlinear theory for gyrotron traveling wave amplifier?gyro-TWA?with special cross-section interaction waveguide is developed in this dissertation.The beam-wave interaction is investigated by this new numerical nonlinear theory.Effects of electron beam parameters,velocity dispersion and mirror seperation distance on the output power and gain are illustrated by calculation results.The production and suppression of gyro-TWA self-oscillations are discussed.The calculation results suggest that the multi-section interaction circuit with optical severs can significant suppress the self-oscillations but has no effect on forward wave amplification.Finally,it is indicated that the proposed 220GHz quasi-optical gyro-TWA consists of three sections separated by two severs could produce a saturation power of 46.7kW with a gain of 32 dB and a 3-dB band of 4.8 GHz.3.A nonlinear theory for gyrotron oscillation with special cross-section interaction cavity is introduced,and three types of gyrotron cavity based on quasi-optical waveguide are studied by this theory.A new frequency tuning mechanism based on the mechanically adjustment of mirror separation distance is proposed and simulated by a 3D Particle-In-Cell?PIC?code.The simulation results demonstrate that this new mechanism has broad tuning range,smoothly continuous frequency tunability,high output power and fast tuning rate.It provides a possibility to design a new type high power and broadband continuously frequency-tunable THz gyrotron.Based on harmonic gyrotron theory,a 400GHz second harmonic quasi-optical cavity is also designed and simulated.A novel interaction cavity based on dual confocal waveguide is proposed firstly.Due to its high interaction efficiency,this new interaction structure can be utilized to high power THz gyrotron.4.A 220 GHz quasi-optical gyro-TWA prototype tube and a 400 GHz second harmonic gyrotron oscillator prototype tube,respectively,are manufactured and tested by dynamic THz gyro-device testing platform,which is built up to shorten the gyro-device design period.The gyro-TWA experiment get the zero-drive stable region for this gyro-TWA prototype tube.Besides,by adjusting the operating magnetic field,the self-oscillation of TE₀₆ mode could generate a maximum output power of 20 kW with a continuous frequency tuning range of 205.66²08.97 GHz.Moreover,the experimental results illustrate that the second harmonic gyrotron prototype tube could generate 6.44kW of output power at 395.35 GHz,which corresponds to an electron efficiency of 3.4%.This is the first experiment results of quasi-optical THz gyrotron operated at harmonic cyclotron.5.A high-order mode input coupler and an output quasi-optical mode converter?QOMC?for 220 GHz confocal gyro-TWA are proposed,simulated,and demonstrated by experimental tests.The input coupler is designed to excite confocal TE₀₆ mode from rectangle waveguide TE₁₀ mode with high mode purity.Simulation results predict that a-6 dB bandwidth of 7.1 GHz with a minimum conversion loss of 2.72 dB.Considering of the gyrotron interaction theory,an effective bandwidth of 5 GHz is achieved,which satisfies the design requirement of gyro-TWA.Cold test results of the output field pattern measurement and the vector network analyzer both keep good agreements with simulation results.Both simulation and experimental results illustrate that the reflection S11 at input port is sensitive to the perpendicular separation of two mirrors.It provides an engineering possibility for estimating the assembly precision quantitatively,and contributes to a stronger ability in manufacturing THz quasi-optical gyro-devices.In addition,the out QOMC is proposed to convert confocal TE₀₆ mode into Gaussian beam and the output scalar Gaussian content exceed 90%over a 6.5 GHz frequency band with a maximum of 97.67%.The results of QOMC testing indicate that all the measured scalar Gaussian content are higher than 90%in the 6GHz range that we could test.It is demonstrated that both the design of input coupler and output QOMC satisfy the design requirement of gyro-TWA.