Research On Applications Of The Magnetic Flux Pump For Superconducting MRI Magnets

High temperature superconductors(HTS)have been developed for decades.Compared with the low temperature superconductors(LTS),HTS have higher critical temperature.This gives HTS significant advantages such as simpler cryogenic system in practical applications,lower operating costs,and so on.HTS materials have been commercialized gradually.It is a good choice that using HTS coil as superconducting magnet in Magnetic Resonance Imaging(MRI)system.However,HTS own a natural property of low n-index.Superconducting joint is also difficult to be realized for HTS.So it is hard to make HTS coil working in persistent current mode for a long time range.Large-scale applications for HTS are limited.In the excitation stage of MRI magnet,current leads as a general method of power,have inevitable disadvantages such as heat leak to the cryogenic system.Magnetic flux pump(MFP)as a new power supply for magnets,can replace current leads to charge the superconducting MRI magnets,compensate the current decay caused by the low n-index and non-superconducting joint.Combined with the latest researches of the MFP national and international,this thesis presents some further researches on the pulse type MFP.The pulse type MFP uses a relatively small DC voltage source to excite the superconducting coil,and sustain the stability of the current and magnetic field in coil.This thesis described the structure and working principle of the pulse type MFP in detail.Using the pulse type MFP to excite the HTS coil made by different materials.In the structure design aspect,it is optimized by adding a cold frame on the basis of the original flux pump to fasten the HTS pump bridge.Through the analysis of the temperature measurement at each region in the cryogenic system,it can be concluded that the cold frame can effectively reduce the thermal transmission between the magnetic poles and the pump bridge.In this experiment,a Bi-2223 double pancake coil,a MgB2 coil and a YBCO single ring are prepared.The winding and joint processing methods are simply introduced.The saturated current and the current pumping rate are investigated by changing the input voltage,working frequency.and the temperature of the pump bridge.The joint resistance is calculated by fitting the measuring results of the current decay.The ripple is also analyzed in this thesis.When the temperature of the pump bridge is 20 K,the input voltage is 5 V,the working frequency is 20 Hz,the 76?H Bi-2223 double pancake coil could reach a saturated current of 200 A.The current of the 8 mH MgB2 coil could reach 0.95 A after the MFP persistent working 300 s.When the temperature of the pump bridge is 55 K,the input voltage is 6 V,the working frequency is 20 Hz,the YBCO single ring could reach a saturated current of 156 A.The performance of pulse type MFP in actual MRI magnet is deduced from the experimental results in small load.The feasibility of its application in the superconducting MRI magnet is analyzed.The excitation current and the current ripple in Bi-2223 coil are measured to analogize those in actual MRI coil.The calculation results show that the pulse type MFP is satisfied with the requirement of stability of MRI magnet to the ppm orders of magnitude.It could be concluded that the pulse type MFP is promising to be a power supply for MRI system to compensate the decay of the persistent current.