The Study Of 2MHz Halbach NMR Magnet And Gradient Coils

Nuclear Magnetic Resonance(NMR)technology is a method which can analyze materials’ function and nature from the micro level.It developed into a powerful technology soon since its discovery,and has been widely used in physics,chemistry and other fields.The study of oil detection and analysis of rock properties also obtained the huge development.In 1980 s,Halbach magnet structure which is used in large particle accelerator was proposed,and it provided a new type of magnet structure for NMR technology.Combined with core analysis and Halbach NMR sensor,this paper studied a method for the design of low-field Halbach NMR magnet structure and gradient coils which can be used for core analysis.The content includes the following parts:(1)Design and optimization of Halbach NMR structure.In this study,the traditional Halbach magnet structure was designed and optimized by means of electromagnetic simulation software.After two-step design and two-step optimization,the NMR magnet composed of three-stage Halbach rings was designed.In order to further reduce the nonuniformity of the main magnetic field caused by the machining error,the passive shimming was adopted by the method of pasting the ferromagnetic shimming sheets on the inner wall of the Halbach magnet.And the self-made magnetic field measurement tool is used to measure the main magnetic field distribution in the target area accurately.Finally,the Halbach NMR magnet was Φ300mm × H176 mm,and after passive shimming,the uniformity of the main magnetic field was increased from 2418 ppm to 350 ppm in the spherical target area with the center diameter of 40 mm.(2)Design of coils in Halbach NMR system.Due to the particularity of the main magnetic field direction of the Halbach magnet which is vertical to the axial direction,the shimming and gradient coils designed for the superconducting NMR system using the conventional target field method is no longer applicable.Therefore,This paper discusses Halbach shimming target field gradient coils design method and discrete stream function field goal design method,and design gradient coils and part of shimming coils using the first method,using second methods to design some other shimming coils,and the gradient magnetic field generated by the gradient coil was measured.Similarly,due to the particularity of the direction of the Halbach magnet magnetic field,the simple solenoid coil can be used as the RF coil.This paper also optimized the solenoid coil.(3)Experimental study on Halbach NMR sensor.The following experiments were carried out using the Halbach NMR sensor: 1)Measurement of T2 of CuSO4 · 5H2 O solution.The Halbach NMR sensor designed in this paper was used to measure the T2 of CuSO4 · 5H2 O in different concentrations.And the signal to noise ratio and the measurement accuracy were tested 2)Measurement of T2 of the pore medium.In this study,glass bottles filled with different sizes of quartz sand were filled with water to simulate the porous media samples and the transverse relaxation time T2 measurements were carried out using Halbach NMR sensors.It can be concluded that the signal to noise ratio of Halbach sensor is 66.2 through the T2 experiment of CuSO4 · 5H2 O solution which the conclusion is that the transverse relaxation time T2 decreased with the increase of solution concentration.However,there is a big difference between the numerical value and the standard value.The T2 measurement experiment of the porous media samples made from quartz sand of different sizes can be concluded that T2 is gradually increased as the size of the quartz sand increases,that is,the pore size of the sample increases.The measurement results approximated the theory of porosity assessment of rock samples in NMR logging.The results showed that the 2MHz Halbach NMR sensor developed in this paper can be used to analyze the porous media.But there are still many areas to be improved,for example,to further improve the uniformity of Halbach magnets,to further optimize the structures of gradient coils and RF coil,to test the performance of Halbach sensor,and to measure the pore size and diffusion coefficient of the core sample.