| Nuclear magnetic resonance refers to the phenomenon that atoms with non-zero magnetic moments absorb electromagnetic waves of a specific frequency under the action of an external magnetic field.Modern scientists have widely used nuclear magnetic resonance in fields such as chemistry,biology,and geology,providing a solid foundation for the development of modern science and technology.However,nuclear magnetic resonance is essentially an insensitive spectroscopic technology.For decades,scientists have developed a variety of methods to improve the sensitivity of nuclear magnetic resonance.Dynamic nuclear polarization is an important method used to increase the sensitivity of nuclear magnetic resonance.With this method,the sensitivity of nuclear magnetic resonance can be improved by the phenomenon of polarization transfer of electrons and nuclei under the condition that atoms are irradiated with electromagnetic waves of a specific frequency.Nowadays,with the development of various cross-cutting fields,the resolution of nuclear magnetic resonance,the applied magnetic field and the radiation frequency required to achieve dynamic nuclear polarization is getting much higher than before.At present,the radiation for DNP-NMR has reached the millimeter wave or terahertz frequency band.In the terahertz band,the gyrotron is supposed to be the best source for dynamic nuclear polarization because of the advantage of long working hours,stability,and high power output.Under an external magnetic field,the electrons perform a gyrating motion in the gyrotron,exciting a specific electromagnetic mode in the cavity.At the output of the gyrotron,the electromagnetic mode in the circular waveguide is converted into a quasi-Gaussian beam by a quasi-optical mode converter.The beam can be transmitted through free space and can also be efficiently coupled into a corrugated waveguide.The transmission line for DNP-NMR is designed to realize the function of transmitting the Gaussian beam output from the gyrotron to the sample in the nuclear magnetic resonance spectrometer with high quality and efficiency,and can make flexible adjustment.The existing model of transmission line still needs improvement in the scheme and specific design.According to the requirements for the 0.5THz DNP-NMR transmission line,this paper makes a detailed description of the theory,and designs and optimizes the model.The main work is as follows:1.With the basic theory of Gaussian beams and quasi-optical transmission,the analysis theory and design method applied to quasi-optical transmission are introduced.According to the specific requirements of DNP-NMR system,a preliminary design scheme for the model of long-distance transmission Gaussian beam is proposed.2.Starting from the basic theory of circular waveguides,the transmission characteristics,characteristic equations and field distribution of over-mode corrugated waveguides are analyzed using the boundary function analysis method.The optimal coupling between Gaussian beam and the corrugated waveguide is discussed,and a new structure for improving beam quality is proposed to improve coupling efficiency.3.According to the preliminary scheme,the transmission line model is designed and optimized in Matlab,and verified by modeling and simulation in FEKO.According to the specific numerical model,the layout is made on the optical breadboard,the specific model of the mirror surface is designed,and the engineering drawing of the mirror surface model is drawn to prepare for the subsequent experiments. |
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