Research On Terahertz Gyrotron Traveling Wave Tube

Terahertz (THz) science and technology has significant application prospect, and is one of the major hotspots in the scientific research field. Meanwhile, the THz radiation source is the key of THz science and technology research. Having the characteristic of high power and high efficiency, the electron cyclotron maser (ECM) is an important kind among the THz sources. Up to now, United States, Russia, Europe, and Japan are the pacemaker in the theoretical and technical study of the electron cyclotron maser, and our country has a long way to catch up them.Gyrotron traveling wave tube (gyro-TWT) is one specific kind of the electron cyclotron maser devices, and it has wide applications in high-power radar, microwave and millimeter-wave communication, precision weapon guiddance, particle accelerator and so on. However, gyro-TWT is lagging behind when compared with gyrotron oscillator and gyroklystron, as gyro-TWT is easy to be affected by the parasitic oscillations caused by various kind of instability. Especially when the operating frequency increases to submillimeter and THz band, higher order modes are often adopted as the operating modes to maintain the power capacity. At the same time,operating at higher order modes often leads to more severe mode competition. Hence,investigation on the high frequency structure to effectively suppress mode competition is an important matter.In this dissertation, based on the physical principles of gyro-TWT, the key issues in the design of gyro-TWT are reviewed systematically. And the development history of the high frequency structure of gyro-TWT is summarized. A high frequency structure of mode-select properties, that is to say the quasi-optical waveguide, is adopted in the design of gyro-TWT. The field distribution and the diffraction loss are studied in the quasi-optical waveguide, especially the confocal waveguide. The dispersion relationship of circular waveguide and quasioptical waveguide are compared, and it's found that the quasioptical waveguide has a lower mode density compared to the circular waveguide.Whereafter, based on the single-particle theory, the linear theory of quasioptical gyro-TWT is developed. The preliminary operation parameters are obtained by the linear analysis, including the linear gain and bandwidth. Instabilities and launching loss in gyro-TWT are introduced, including absolute instability, backward wave oscillation instability,reflective instability. Backward wave oscillation instabilities caused by the major potential competition modes are studied. The starting current of the potential competition mode is investigated numerically, and the results are compared with that of the circular waveguide. The results show that quasioptical gyro-TWT has greater starting current than circular gyro-TWT, and this means that the quasioptical gyro-TWT will be more stable. At the same time, due to a lower mode density in the quasioptical waveguide, the mode competition will reduces greatly.Next, the nonlinear theory of the quasioptical gyro-TWT in the rectangular coordinate system is derived, including the kinematic equation of relativistic electron and the transmission line equation. The obtained stationary, multi-mode nonlinear equation set can be solved numerically when applied to a certain model. In the numercial calculations, single one mode is adopted. Taking the specific boundary condition, initial condition and velocity spread into consideration, the nonlinear equation set of 0.34 THz quasioptical gyro-TWT is calculated using a MATLAB routine.In the beam-wave interaction numerical calculation, the diffraction loss and ohmic loss of the quasioptical waveguide are taken into consideration, and conservation of energy is used in the process to insure the validity. On account of the characteristic of quasioptical waveguide, the possibility of adopting new electron gun is discussed. The numerical results are compared with that of traditional electron gun, and results show that the new electron gun is capable of increasing efficiency and shortening the interaction length.In the end, in view of the existing experimental facilities, theoretical and simulation investigation of the 0.17 THz quasioptical gyro-TWT is introduced,including the simulation work of input coupler, output coupler, and the new electron gun. In the simulation of electron gun, influence brought by the emission band in the new electron gun in discussed. The simulation results show that, input coupler and output coupler of the 0.17 THz quasioptical gyro-TWT are both of good performance.In the simulation of electron gun, the change of emission band has little influence on the electron gun, and the new electron has some practical potential in the design of quasioptical gyro-TWT.