Design And Research Of Short Dead Time RF Coil And RF Switch For Low Field NMR

With the development of nuclear magnetic resonance(NMR)technology,low field magnetic resonance is attracting more and more applications due to its unique advantages,especially in energy,geology,geotechnical,cement and other industries.Under low field conditions,physical property evaluation,structural analysis,and related kinetic processes are mainly dependent on the relaxation or diffusion characteristics of the sample.However,the structural uniformity of porous materials such as cores and cement-based materials is poor.Due to the structural heterogeneity of the sample itself,the distribution of internal 1H relaxation time is not uniform.During actual testing and analysis,the longer the dead time of the spectrometer,the more loss of relaxation information collected,which brings about practical application problems such as decreased signal sensitivity,undetectable trace samples and increased experimental error.As a key component of low field Nuclear Magnetic Resonance spectrometer,RF coils determine the signal resolution and directly affect the performance of the entire system.In order to realize the direct NMR measurement of short relaxation time samples,the development of NMR coils with short dead time and fast transmission and reception switching has important application value.This thesis briefly introduces the NMR experimental problems caused by the long ring-down time of the coil.Through the basic theoretical analysis behind these practical application problems,we firstly presented a design scheme of transmitting and receiving separated coils composed of Loop-Gap Resonator(LGR)coil and solenoid coil in the low field NMR system,and complete the electrical performance test and NMR experimental test.The main research contents are as follows:(1)Based on the analysis of the dead time generation mechanism,a short dead time RF coil design scheme with LGR coil as the transmitter coil and solenoid coil as the receiver coil is proposed,and the feasibility of 10 ?s dead time is analyzed from a theoretical level.(2)By using electromagnetic simulation software,the design of LGR coil size structure was optimized.(3)A balanced tuning and matching network of the coil was designed.(4)According to NMR experiment requirements PIN diode switch circuit and driver was design and simulate.(5)By the electrical performance test and NMR experiment test of the short dead time RF coil,the dead time can reduced to 10 ?s,which verifies the feasibility of the design scheme.