Research On Electromagnetic System Of 263GHz Gyrotron Oscillator

Gyrotron oscillators are a new type of important millimeter wave and sub-millimeter wave devices with the characteristics of high efficiency,high power and so on.These devices have vast application vistas in many fields such as controllable nuclear fusion,nuclear magnetic resonance?NMR?,material processing and other systems.Due to the excellent resolution of NMR spectra,NMR has evolved as the preferred spectroscopic approach in many areas of science,including physics,chemistry,biology,medicine,geology and materials science.Dynamic nuclear polarization?DNP?experiments are a technique used to increase the sensitivity of nuclear magnetic resonance?NMR?experiments by transferring the higher polarization of unpaired electrons spins to nuclear spins.DNP significantly improves the sensitivity of NMR spectra,thus reducing the acquisition time in NMR experiments.DNP experiments have been proven to be one of the most effective and versatile methods in its ability to polarize a wide range of NMR spectroscopy.However,extending DNP to higher fields proved challenging due to a lack of high-power RF sources.Gyrotrons are the appropriate for use with high-field DNP because they can reliably generate high power necessary for DNP experiments at terahertz frequencies.THz DNP-NMR is a rapidly developing,exciting area of THz science and technology.This thesis made a deep critical technical and problems research on 263GHz gyrotron oscillator of 400MHz DNP-NMR.The main works of this dissertation are listed as follows:1.The state-of-art and application of gyrotron oscillators are reviewed in detail.The configuration and operating mechanism of the gyrotron oscillators,operating mechanism of NMR are briefly introduced.The influence of DNP on NMR and RF requirement of DNP-NMR are given,and current status and technical analysis of RF sources for THz DNP-NMR applications are surveyed.Some remaining critical technical problems are discussed and some technical ways that may solve the problems are also presented.2.This paper is based on linear and nonlinear theory of gyrotron,and a cold cavity and self-consistent nonlinear code of gyrotron are developed along.The effect of various factors,such as ohmic losses,tapering the magnetic field and velocity spread can be taken into account in the single-mode nonlinear code,which is also applied to different structural cavity and harmonic-multiplying operating mode.Firstly,the preliminary design of a 263GHz gyrotron oscillator operates in the TE₀₃ mode at fundamental cyclotron harmonic has been carried out.Some important parameters of the gyrotron oscillator are determined,output power and efficiency of cavity are analyzed.Next,the characteristics of the cold cavity and PIC?Partical In Cell?of the hot cavity are calculated using electromagnetic simulation software.Finally,a barrel cavity is analyzed systematically,the results demonstrate that the barrel cavity can be used as an effective structure for increasing diffractive Q factor,lowering its start current,and the interaction effciency can be increased significantly in gyrotron oscillators.Otherwise,by the programs,the studies of the gyrotron of 140GHz has been carried out and contrasted with experimental results,further verity the reliability of the code.The maximum output power is 430kW,corresponding to an effciency of22.6%in frequency of 140.2GHz.3.The reliability of the codes and the validity of the gyrotron single-mode theory are effectively verified.In order to ensure the reliability of colde cavity and self-consistent nonlinear code,much validation work has been done.Besides using the professional electromagnetic simulation software to analyze the cavity of gyrotron,the simulation model from the classical papers is calculated by cold and hot cavity codes,and the results are analyzed and contrasted to prove the correction of the codes.4.The key technology of quasi-optical?QO?output system for short millimeter wave was systematically studied,and a Vlasov mode converter of 263GHz gyrotron has been designed.Firstly,the basic theory and analysis method of the QO mode converter are introduced,the schematic diagram of the circularly polarized mode Vlasov QO mode converter and reflect mirror system are given.Then,based on the geometric optics and vector diffraction theory,a simulation code for QO mode converter capable of calculating Vlasov and Denisov mode converter is developed.The results computed by the code are in good agreement with the simulated results in authority papers,so the contrast ensures the reliability of the simulation colde.Finally,the Vlasov mode converter of 263GHz gyrotron is optimized using the code,the electric field distribution of the mode converter can be determined.5.According to the requirement of Gaussian mode output window for 263GHz gyrotron oscillator,the size of the Gaussian mode output window is designed.Firstly,sappire is chosen as the output window material,Based on the law of Fresnel,the design goal for 263GHz Gaussian mode output window is determinated by theoretical analysis.Then,we alse show the influence on the transmission of Gaussian mode and else parameters using theoretical analysis and FEKO software.The design parameters of Gaussian mode output window are obtained.Finally,the steady-state thermal analysis of the situation of output window has been done by the Thermodynamic finite element analysis software.Otherwise,based on the theory of output window,the gaissian mode output windows for 140GHz gyrotron is designed and developed,the experimental results are in good agreement with the theoretical predictions.