Study On Screening Current Characteristics Of HTS Magnet

The second generation of high temperature superconducting materials have the advantages of high critical magnetic field,high critical temperature and high current density,which promote the wide application of high temperature superconducting magnets in nuclear magnetic resonance,magnetic resonance imaging,medical accelerators and other high field equipment.Greatly broadens the application prospects of high temperature superconducting magnets.However,when high temperature superconducting magnets are electrically excited or placed in the background magnetic field,screening currents are generated,and the screening currents seriously damage the homogeneity and stability of the magnetic field,affecting the application of high temperature superconducting magnets in high precision equipment such as nuclear magnetic resonance and magnetic resonance imaging.Therefore,there is a great need to carry out research on the characteristics of screening currents in high temperature superconducting magnets.Based on the T-A equation,current density of high temperature superconducting magnets is derived from the critical current density of high temperature superconducting tapes,and further modeled and simulated in the multi-physics field analysis finite element software COMSOL to systematically study the screening current characteristics in high temperature superconducting strips under different control conditions.Mainly include:(1)Only apply alternating background magnetic field;(2)Only apply alternating conduction;(3)High temperature superconducting tape with alternating conduction current is simultaneously in the alternating background magnetic field.The influence of the conduction current and the amplitude of the background magnetic field on the current density distribution of the superconducting tape is further analyzed.The magnetic field attenuation caused by the screening current of the high temperature superconducting magnet is numerically analyzed,and the magnetic field generated by the center of the high temperature superconducting double-cake coil under the screening current and the uniform current density distribution is simulated and calculated,and then the screening current caused Magnetic field attenuation,the magnetic field attenuation and hysteresis caused by screening current are studied.The magnetic field attenuation caused by the screening current of the high temperature superconducting magnet was experimentally studied,the high temperature superconducting double-cake coil was wound,and the experimental test platform under the condition of liquid nitrogen 77 K was built to measure the central magnetic field of the coil and the magnetic field constant of the coil.The attenuation and hysteresis of the magnetic field caused by the screening current are analyzed and compared with the simulation results to verify the correctness of the simulation results and the accuracy of the experiment.The method of eliminating the screening current is summarized and introduced.Considering the limited space structure of the coil,the previous vibrating coil method is improved.Only the coaxial coil is added outside the coil,and the experimental platform of the improved vibrating coil method is built,and the vibration is added to the measurement.The magnetic field at the center of the coil before and after the coil is improved by the vibration coil method to eliminate the influence of the screening current,and the influence of the amplitude of the alternating current on the elimination of the screening current is further studied.