| As an advanced imaging technology,magnetic resonance imaging(MRI)has been widely used in the medical field.With the development of superconducting materials,superconducting magnets have played an important role in MRI systems.In recent years,high-temperature superconducting(HTS)develop rapidly and HTS magnets have become a research hotspot.The stability of HTS magnets used in magnetic resonance systems is studied in this thesis.The high temperature superconducting tape YBCO has the characteristics of high critical temperature,strong current carrying capacity and low loss,so it has broad application prospects and research value.The magnets studied in this thesis use YBCO superconducting materials.On the one hand,the spatial stability of the magnetic field of the magnet is studied.On the other hand,the time stability of the magnetic field of the magnet is studied.The stability in space of the superconducting magnet is analyzed.The finite element analysis of the magnet is carried out using OPERA software.The magnetic field strength and homogeneity of the superconducting magnet and the permanent magnet before the shimming are mainly analyzed.According to foreign research results,the homogeneity analysis is carried out using a superconducting magnet with a central magnetic field strength of 1.5T made of YBCO superconducting material.The uniformity of a specific area of the center from the horizontal and vertical directions is analyzed.At the same time,the model of C-type magnet made of permanent magnetic material is analyzed.Comparing the analysis results of superconducting magnets and permanent magnets to explain the advantages of superconducting magnet.The low n-index and superconducting joint resistance of HTS will cause the current of HTS magnets to decay,which will affect the time stability of the magnetic field of the magnet.In this thesis,a HTS pulsed magnetic flux pump(MFP)is used to compensate the current attenuation of the HTS magnet and improve the stability of the magnetic field of the magnet in time.In order to further study the practical value of the magnetic flux pump,liquid nitrogen is used as a low temperature experimental environment.An YBCO superconducting coil is produced,and the MFP is used to excite the YBCO superconducting coil.The joint resistance of YBCO coil is calculated based on the pump current curve.The effects of different supply voltages and operating frequencies on the saturation pump current are studied,and the output power of the magnetic flux pump used in this experiment under ideal operating conditions is calculated.The effects of core loss and superconducting band dynamic resistance on pump current at different frequencies are analyzed.A small superconducting magnet made of 130m long YBCO superconducting tape is used as a load to study the excitation effect of the MFP on the magnet.The magnetic field of the magnet can continuously increase during the process of exciting the magnet by the MFP,but the increase is small.By calculation,it would take 9.8 hours to reach 1.5T.The current pumped by the flux pump used in this thesis is very small,so it is more suitable for magnetizing the magnet after the magnetization is completed.In the experiment,the magnet is magnetized so that it has a certain initial current and magnetic field.Then the change of the magnetic field of the magnet with or without the flux pump is measured.It is found that the magnetic field of the magnet is basically stable during the operation of the magnetic flux pump,and the magnetic flux pump can well perform current compensation for the high temperature superconducting magnet. |
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