Study On High-uniformity NMR Halbach Magnets

Nuclear Magnetic Resonance?NMR?technology is a research method that can analyze the functions and properties of the material from a microscopic prospective.Since its discovery in 1946,it has been a wide concern in the scientific community.Developing so rapidly,it has been widely used in different areas of physics and chemistry.Since the appearance of the Halbach permanent magnet structure,it provided a new type of permanent magnet structure for the research in the field of NMR,and it also expanded a new research direction.Compared with the traditional C-shaped and rectangular-ambulatory-plane permagnets,the Halbach magnet structure has the advantage of small volume and lighter weight,but the disadvantage of its comparatively low magnetic field homogeneity.Besides that,the research of shimming method for Halbach is still notmaturely enough.This paper mainly focuses on the improvement of the homogeneity of the Halbach magnets,to overcome its existing limitations for more extensive applications.The research content of high-homogeneity NMR Halbach magnets mainly includes two major aspects,namely Halbach NMR magnet structure and shimming methods.The specific contents include the following aspects:?1?Halbach NMR magnet structure design and optimization.In this study,electromagnetic finite element simulation of the electromagnetic field is used to optimize the structure of the traditional Halbach magnet.Through the influence of the structural parameters of the magnet on the uniformity of the magnetic field in the region of interest?ROI?,an 8-ring Halbach NMR magnet was designed with a size of?350mm×H580 mm,a center frequency of 9.28 MHz,and a simulation uniformity of 137ppm and the measured uniformity of 388ppm.?2?Magnetic field harmonic analysis and passive shimming.Firstly,according to the theory of electromagnetic field and mathematical derivation,the magnetic field is analyzed by harmonics.The essence of shimming work is to remove higher harmonics.Because the magnetic field inhomogeneity is caused by processing error and other factors,the passive shimming method of the ferromagnetic shim iron plate on the inner wall of the Halbach magnet.The sphere with a diameter of 40 mm after shimming The uniformity of the magnetic field in the ROI increased from 388 ppm to 62 ppm.?3?Halbach NMR magnet coil design.Because the direction of the main magnetic field of the Halbach magnet is perpendicular to the axial direction,the traditional method design for the coil of the superconducting magnet system is no longer applicable to this study.Therefore,this article first introduces the concept of the target field method and the stream function,and introduces two typical coil design methods:harmonic coefficient method and magnetic dipole method.For the research situation in this paper,the harmonic coefficient method was chosen to design and simulate the shimming coils.The eight-way shim coils X?Y?Z?XY?XZ?YZ?X²-Y² and 2Z²-X²-Y²were produced and produced.The magnetic field was measured.In addition,according to the direction of the main magnetic field of the Halbach magnet,a simple solenoid coil was selected for the RF coil,and the solenoid coil was also optimized for design.?4?Experimental study.Firstly,the composition and sample selection of the Halbach NMR magnet system are introduced briefly.Then the following confirmatory experiments were carried out:1)Spectrum measurement of nuclear magnetic resonance signals of pure water samples.With using the Halbach NMR magnet,the NMR spectrum signal of the pure water sample was measured before the shimming,after the passive shimming,and after the active shimming.2)T₂ measurement of pure water sample.In this study,the CPMG attenuation signal of the pure water sample was measured before the shimming,the passive shimming,and the active shimming,and the change of the transverse relaxation time T₂ was obtained.?1?Through pure water NMR spectrum signal measurement experiments,it can be found that the first-order coils in the active shimming have a greater impact on the overall spectrum,and the second-order coils have a greater impact on the spectral details.After adding 8 shimming coils,Halbach magnet uniformity increased from 388ppm to 27 ppm.?2?By measuring the CPMG echo attenuation signal,the transverse relaxation time T₂ can be fitted.With the increase of the magnetic field uniformity,the transverse relaxation time T₂ of the sample gradually increases and the echo signal decays more slowly.The above verification experiments show that the high homogeneity Halbach NMR magnet developed in this paper has superior performance,and the shimming method is feasible and effective.However,there are still many areas for improvement.For example,the structure of shimming coils and RF coils need to be further optimized,and Halbach magnets are used for in-depth analysis research.