Study Of THz Gyrotron Traveling Wave Amplifier Based On A Quasi-optical Waveguide

The vacuum electronic THz sources can produce high power radiation,they have very important applications in detection and imaging,radar,electronic and information countermeasures,interstellar communications,biomedical diagnosis and treatment technology,etc.Gyrotron traveling wave tubes（Gyro-TWTs）are the fast wave devices based on the relativistic electron cyclotron resonance in a magnetic field.In the microwave and THz bands,due to its high power and wide band,Gyro-TWT is one of the most promising high mean power or peak power vacuum electronics radiating sources.Due to the scale effect,the size of the beam-wave interaction circuits is inversely proportional to the operating frequency of the Gyro-TWTs.With the increase of the operating frequency of the low-order mode Gyro-TWTs,the size of the interaction circuit becomes smaller and the power capacity becomes lower.Therefore,the development of the Gyro-TWTs operating at a high-order mode is important to expand their operating frequency to a higher frequency band,and also is important to improve their output power.However,for a high-order mode Gyro-TWT,the backward-wave oscillation from the low-order parasitic modes becomes an important obstacle,the key to solve this problem is to find the high-frequency circuits with the ability to improve the mode competition.The Massachusetts Institute of Technology（MIT）proposed to use the single-confocal waveguide as the high-frequency circuit of the gyro-TWTs.The mode density of the single-confocal waveguide will not increase with the increase of the mode index.At the same circuit size,the mode density of the single-confocal waveguide is far lower than that of the circular waveguide.The single-confocal waveguide is a transversely open waveguide,the intrinsic diffraction loss can be utilized to suppress the low-order parasitic modes,therefore,the single-confocal waveguide Gyro-TWT can operate at a high-order mode without severe mode competition problems.The drawback of the single-confocal waveguide is the relatively low beam-wave interaction efficiency because the field strength at the open side of the waveguide is weak.In order to alleviate this problem,the double-confocal waveguide is proposed as the interaction circuit of the gyro-TWTs.With the support of the project in the research group,this thesis focuses on the theoretical investigation,optimization design and preliminary experiment of the Gyro-TWTs based on a double-confocal waveguide The details are as follows:1.Based on the diffraction theory,the electromagnetic characteristics of the double-confocal waveguides are analyzed in details,including the field distribution,the dispersion characteristics and the diffraction loss.Double-confocal waveguides have similar mode selection characteristics to single-confocal waveguides,so they have the potential to operate in higher order modes,and the field distribution of double-confocal waveguides is more uniform than that of single-confocal waveguides,so they also have the potential to improve the efficiency of beam-wave interaction.2.The kinetic theory,the single particle nonlinear theory of the gyro-TWTs,and the corresponding numerical calculation code are developed to investigate the power amplification and the parasitic oscillation in the process of the beam-wave interaction.Under the same operating parameters,the beam-wave interaction efficiency,saturation output power and gain of double-confocal gyro-TWT are higher than the single-confocal gyro-TWT.The starting current of the self-excited oscillation of the operating mode and the starting length of the back-wave oscillation of the parasitic modes are analyzed.By loading attenuating ceramics on the outside of the quasi-optical waveguide,the stray field between the high-frequency structure and the metal wall is suppressed.Finally,CST-PIC software is used to simulate the electromagnetic characteristics of the high-frequency circuit of the multi-section concentrated attenuation structure.3.An input system for the 220 GHz gyro-TWT is developed,including the low-loss transmission line,the input window and the input coupler.Specifically,based on the scattering matrix theory,the TE₁₀-HE₁₁ mode converter is designed.The loss characteristics of the quasi-optical miter bend are analyzed.The reflection and transmission characteristics of the whole transmission line are measured.The measured results of the whole transmission line show that the loss is less than-4 d B in 210 GHz～230GHz.A broadband,low reflection pill-box input window has been designed,and the test results show that it has good transmission performance in 210 GHz～230 GHz.Two kinds of double-confocal waveguide input couplers are designed and measured.The input coupler based on the power-division network structure has a wide bandwidth but a complex structure,while the input coupler based on the coaxial resonator structure has a narrow bandwidth but a simple structure.4.A double-anode magnetron injection electron gun for the 220 GHz gyro-TWT is optimized.The optimized electron gun has good stability when the parameters change in a certain range,and is suitable for the preliminary experiment of the gyro-TWT.Based on the transfer matrix theory,the output window of a multilayer structure and the output window based on metamaterial structure are analyzed and designed.The designed three-layer structure output window and metamaterial structure output window have a reflection of less than-20 d B in the 60 GHz range.The output window designed completely meets the requirements of the gyro-TWT.In the preliminary test,the performance of the metamaterial output window is obviously better than the single layer window.5.A 220 GHz double confocal waveguide gyrotron traveling wave tube was fabricated,assembled,and preliminary experimentally explored.The output power of the instable oscillation is 283 W at an operating voltage of 40 k V and a current of 3.15 A.By reducing the operating current to 1.9 A or decreasing the pitch-factor of the electron beam,the output power of the unstable oscillation is zero.By combining experiments with theory,the causes of unstable oscillation were analyzed and simulated,and a later improvement plan was proposed.