Nonlinear Multimode Studies Of Terahertz Coaxial Cavity High-Order Mode Gyrotron Oscillators

Terahertz (THz) wave, which means the electromagnetic radiation between far infrared and microwave region, occupies a special position in the electromagnetic spectrum and consequently has a lot of unique characteristics. It has widely potential applications including radar, communications, material processing, imaging and so on. With its applications more and more widely, it becomes a research hotspot in this field for many countries in the world to develop an effective THz radiation source. Gyrotron—one of the fastwave gyro-devices, based on the mechanism of ECRM, has been considered to be the most promising THz radiation source to generate high power (over100kilowatt).Because of the low-order operating mode, the transverse size of the traditional cylindrical cavity THz gyrotron is small, which causes problems in machining, ohmic heating and further increasing of output power. Meanwhile, as far as the coaxial cavity is concerned, the high-order mode could be employed due to the suppression of competition modes even with much denser mode spectrum, which could increase the cavity diameter and output power.Currently, the main aim of the coaxial cavity gyrotron experiments is to provide the microwave radiation source for ITER, the highest operating frequency is0.17THz and the corresponding power is2.2MW. So far the research on the coaxial cavity gyrotron with higher operating frequency has never been reported.To generate the high power THz radiation by employing coaxial cavity gyrotron, two main problems have to be solved:1) Increasing the transverse size of the cavity as large as possible on benefit of machining and heating;2) Decreasing the working magnetic field as low as possible to reduce the engineering cost. This paper aims to use the nonlinear method to verify the possibility of single-mode high power THz radiation based on the coaxial cavity gyrotron with high-order modes, high harmonics and low magnetic field.This dissertation is organized as follows:In Chapter1, the detailed states of the arts of THz coaxial cavity gyrotron and existing problems are introduced, the contents and value of the work are also presented.In Chapter2, the multimode startup model in the coaxial cavity gyrotron oscillator is established. Firstly, the model of the cold-coaxial-cavity analysis is established, and the corresponding program is completed by employing FORTRAN language, by which the resonant frequency, diffractive factor and cold-cavity field distribution of each mode could be obtained with provided cavity parameters. Secondly, the multimode startup model with assumed cold distribution is given, the ohmic losses on inner rod and outer wall of each mode provided shallow slot depth are derived, the numerical mothed is presented and the time-dependent, multimode code with the ohmic losses taken into account is developed.In Chapter3, by use of the parameters of the fundermental TE34,19,0.17THz coaxial cavity experiments in KIT, the numerical calculations are performed to simulate the multimode startup scenario. The results agree well with the experimental report, which demonstrates the validity of the multimode startup model and program.In Chapter4, a coaxial cavity gyrotron with0.22THz operating frequency is designed in detail. The cavity parameters are selected and the corresponding competiting modes are confirmed. The influence of proper corrugated inner condunctor with slope on the mode competition is analyzed. The results indicate that it can rarefy the mode spectrum and decrease the diffractive factor of the competiting modes, which leads to the rise of the starting current. Under the optimal electron guiding-center radius, the multimode startup simulations with the different rotating modes taken into account show that the maximum output power of0.8MW could be attained for the expected mode within the region of65kV-75kV, and the maximum heating losses are under the limitation.In Chapter5, an ultra high-order mode, high harmonic(TE43,4,3rd harmonic) coaxial-cavity gyrotron oscillator with over100kW output power is designed. It is found that the conventional3rd high-orde mode could hardly startup because of the fierce mode competition and weak interaction with electron beam. The problem could be solved to some degree by employing the whispering gallery mode. Through the analysis of the influence of the slot depth on the resonance frequency, it is found that the mode spectrum between the operating mode and the most important competiting fundermental mode could be rarefied greatly by the proper selection of the slot depth. Nonlinear simlulation shows that the maxim output power of163kW for the operating mode is obtained, and the single-mode operation is achieved in a comparatively narrow region of accelerating voltage.