Investigation Of HTS No-insulation Magnet Excitation By Rotating Flux Pump

The High Magnetic Field system can effectively induce the spin and orbital order of electrons,and change the electronic structure to a new material state.Therefore,the magnetic field intensity is an important parameter to evaluate the performance of the High Magnetic Field system.The main magnet of the general High Magnetic Field system is large in volume,not easy to move,and the magnetic field strength is insufficient and the stability is low.High temperature superconducting(HTS)no-insulation(NI)magnet has the characteristics of high current density,high field stability and small volume,which is in accordance with the development trend of MRI /NMR equipment for main magnet.On the one hand,the charging and discharging speed of the magnet is affected by the turn-to-turn contact resistance of HTS NI magnet.On the other hand,the electrothermal stability and self-protection ability of the main magnet are improved in the process of over-current and quench.In addition,the excitation mode of non-contact rotating HTS flux pump is adopted to overcome the current fluctuation issue caused by the traditional power supply mode and meet the high stability requirements of the main magnet in MRI / NMR equipment.In this thesis,the investigation on HTS NI magnet excited by rotating flux pump is carried out theoretically and experimentally,which mainly includes the following aspects:(1)The transient mathematical model of turn to turn contact resistance of HTS coil is established.The model is used to reveal the variation of turn to turn contact resistance with time.It is found that the resistance value increases obviously in a short time at the beginning of charging and at the end of discharging,and decreases to a stable value with the stability of charging and discharging.It is also found that the resistance increases obviously during the over-current experiment and tends to be stable with the stability of the over-current.The results verify the self-protection ability and thermal stability of NI coil when the current is too large.Charging,discharging and over-current processes of each coil in the magnet under variable current ramp rate are studied based on the transient mathematical model of turn to turn contact resistance of HTS coil.The distribution of the tangential current to the superconducting layer and the radial current to the turn-to-turn contact resistance layer are found.The results show that the tangential current ramp rate of the coil on both sides of the magnet is faster,and the radial current ramp rate of the coil near the centre of the magnet is faster which provides a supplement to the related experimental research for the subsequent design and development of HTS NI magnet.(2)The relationship between the dynamic resistance and the current ramp rate is proposed.The open circuit experimental platform of rotating HTS flux pump is designed and set up.The variation of the output characteristic of the flux pump was found through the research on the current ramp rates,the number of rotor permanent magnets,the clearance between rotor and stator and the rotation speed.In order to investigate the nonlinear phenomenon of the V-I characteristics of rotating flux pump,the variation trend of critical current of magnetic and non-magnetic substrate tape in self field and external field with different current rising rate is studied.The influence of the hysteresis characteristics of magnetic substrate on the critical current of HTS tapes is revealed.The results show that the non-magnetic substrate tape has better external magnetic field performance.It is helpful for researchers to further study the magnetization behavior of HTS tapes in traveling magnetic field.(3)In order to solve the problem that the quench of HTS materials affects the output performance of flux pump and the stability of magnet.The quench detection platform of HTS NI magnet excited by rotating flux pump is designed and built.The quench of the device is achieved by triggering the heating resistor attached to the stator side of the flux pump and the magnet.The quench process of HTS flux pump tape and NI magnet are analyzed.The effects of Joule heat and operation time of heating plate on the MQE and NZPV are studied.It is found that the loop current decreases rapidly due to the quench of the stator side lead of the flux pump.It is revealed that the quench of the magnet changes the internal current distribution.This research complements the related experimental research for the protection of HTS NI magnet excited by rotating flux pump.