| Gyrotron are important sources for millimeter and sub-millimeter wave with the characteristics of high peak and average power, high efficiency and suitability bandwidth and so on. It can be used as the next generation source for the millimeter wave radar system and weapons system, millimeter wave communication, Plasma heating. There are two main parts in the Gyrotron:the Electronic gun and the High frequency structure. This thesis focuses on these two parts.As for electronic gun, this thesis focuses on the MIG (Magnetic injection gun); the quality of gyro-beam it produced has great influence on the performance of Gyrotron. The velocity spread is one of the most important factors of the beam quality. Many groups have focus on this aspect, recently a new mechanism, which cause velocity spread has been observe in experiments:The Low Frequency Oscillation.Supported by the Office of Fusion Energy of the US Department of Energy, this thesis has analyzed the low frequency oscillation deeply and proposed the methods to suppress the low frequency oscillation, which are confirmed by the experiments. The main works as follows:1. A static model is built to describe the electron moving in the external electromagnetic fields. In the static model, a potential well is introduced and the electrons emission from cathode is catalog into two groups, one group is transited electron another one is trapped electron.2. An analytical model is considered which allows one to apply the space-charge wave theory to the analysis of such oscillations. In the framework of the small signal theory, the regions of low frequency oscillations, the oscillation frequency and the temporal and spatial growth rates of low frequency oscillations are determined in the parameter space. These results are consistent with some experimental dates. They also allow recommending some means for suppressing low frequency oscillations.3. A version of a one-dimensional numerical simulation method is developed to analyze the self-consistent accumulation and bunching caused by the space charge of these trapped electrons. The velocity spread and the sink mechanism for the trap electrons are taken into account. The threshold conditions for excitation of LFOs, the spatial character, the origin and evolution of the LFOs are analyzed and interpreted. Numerical simulation results agree with the experimental data.As for the High frequency structure, in this thesis the Ka bands Gyro-twt has been studied in theory and experiment.Main efforts are as follows:1. An approximate physical model which using theoretical analysis of the input coupler is described in detail, combining this analytical model and the software of HFSS, the high transform efficiency input cavity with pure mode are obtained.2. Using the small signal linear theory to analyze the amplification of the working and competition mode. After the analyze, the optimum design parameter is given by this small signal linear theory.3. Nonlinear theory is used to simulate the beam-wave interaction of Gyro-twt. The influence of the velocity spread and the dielectric load is taken into consideration in this model, which can provide a high efficient method to design the gyro-twt.4. The output section between the beam-wave interaction section and the output window is studied to obtain the low reflector coefficiency of the working and competition mode. The characteristic of the output window, Such as the reflector and absorber of high average power are analyzed. An output window for Ka band, TEoi high power gyro-twt amplifiers is designed with the reflectivity less than 1%, and absorbing power is about 2W while the output power is lOkW.5. A gyro-twt has been designed and fabricated which achieved 160kW peak plused output power, and 38dB saturated gain with 1.5 GHz bandwidth centered at 34GHz for a 70 kV,10A electron beam.
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