Investigations On A THz Transmission And Focusing System For DNP-NMR Spectroscopy

Nuclear magnetic resonance(NMR)is a physical phenomenon in which an atomic nuclei can absorb and re-emit electromagnetic radiation.This characteristic of NMR allows observation of the quantum mechanical magnetic properties of the nucleus.Utilizing the phenomenon of nuclear magnetic resonance,many science and technology use nuclear magnetic resonance spectroscopy to study molecular physics,crystalline and non-crystalline materials.The advanced medical imaging technology-magnetic resonance imaging(MRI)is based on the principle of nuclear magnetic resonance.Dynamic nuclear polarization(DNP)is an important method in nuclear magnetic resonance spectroscopy.It can achieve polarization transfer between electrons and nuclei and pass higher polarizations of electron spins to nuclear spins to increase the sensitivity of nuclear magnetic resonance.The increase in sensitivity of nuclear magnetic resonance can greatly improve measurement accuracy and data acquisition speed.With the gradual improvement of the NMR resolution level,the required magnetic field also increases(up to 23T),and the frequency required for dynamic nuclear polarization also gets to the terahertz frequency band(0.14-0.6 THz).In addition to higher magnetic fields,NMR experiments are required to output stable high-power terahertz microwave.The gyrotron,which is a high-power vacuum tube with megawatts of electromagnetic waves generated by the electron cyclotron resonance in a strong magnetic field,is an indispensable device for the development of enhanced nuclear magnetic resonance(DNP-NMR)experiments.The gyrotron has achieved continuous and stable operation at high frequencies and high power.A highly efficient DNP-NMR system requires that the beam is an approximately ideal Gaussian beam(quasi-Gaussian beam),because this type of beam can still guarantee the quality of microwave transmission during the coupling of high-power microwave transmission lines and microwaves to the sample.DNP-NMR system's high requirements on transmission quality depend on a transmission and focusing system with a compact structure,high transmission efficiency and low loss.However,the existing transmission and focusing systems still have a worthwhile improvement in terms of implementation scheme,structure and performance.This dissertation combines the needs of the scientific research team to design a 0.263 THz DNP-NMR system and focuses on the basic theory of transmission and focusing in the system.On this basis,the software is also used for numerical simulation analysis and model simulation to optimize the structural parameters.The main work is as follows:The design framework is applied to the DNP-NMR transmission and focusing system.Based on the basic theory of electromagnetic field electromagnetic wave and principle of guided wave,a combination of numerical calculation and simulation analysis is used to analyze the quasi-Gaussian from the gyrotron.The beam is transmitted and focused in the system and finally reaches the sample.Modeling and analysis softwares,MATLAB and FEKO,are used to analyze the performance and parameters of the system,to design a transmission and focusing system based on 0.263 THz DNP-NMR.