Study On Electromagnetic And Mechanical Behavior And Quench Characteristics Of Superconducting Structures

The theory of accidental release of energy indicates that the physical nature of safety accidents is the loss of energy,such as mechanical energy,electric energy,magnetic energy,heat energy and other forms of energy.As a control object in man-machine-environment system,superconducting structure itself has a large amount of electric energy and magnetic energy.At the same time,it is likely to cause safety accidents when operating in the extremely low temperature environment created by liquid helium or liquid nitrogen.Among many materials for preparing superconducting structures,REBCO high temperature superconducting material is widely used in the fields of energy,electric power and transportation due to its advantages of high current-carrying capacity,low cost,good mechanical properties and high critical temperature.However,defects such as bubbles,holes and micro-cracks were inevitably introduced in the preparation process due to technological processes and operational techniques,which was extremely unfavorable to the mechanical stability of REBCO material and structure.At the same time,the critical current of REBCO superconducting material is greatly affected by the strain,which shows the decrease of current carrying performance.In this paper,superconducting electromagnetic solid mechanics theory and COMSOL finite element simulation method are used to study the mechanical and thermal stability of superconductors with defects,high temperature superconducting strip winding structure and 25.7 T high and low temperature mixed superconducting magnets.The main contents are as follows:The governing equation of H method suitable for calculation of electromagnetic field of superconductors is derived according to Maxwell equations.Since superconducting thin film is often used as superconducting cable for current transmission by superconducting composite strip,superconducting block is usually applied to flywheel energy storage and maglev technology after magnetization.Therefore,the current carrying model of superconducting thin film with cracks and the excitation model of superconducting block with holes are established respectively.The influences of the position,size and Angle of crack defects on the current density of superconducting thin film under current carrying condition are analyzed.The influences of the shape,position and size of hole defects on the electromagnetic characteristics and stress of superconducting block under excitation condition are analyzed.The results show that the presence of defects will affect the uneven distribution of nearby current density and cause local stress concentration,but can improve the magnetic flux trapping capacity of the superconductor.For superconducting thin film and superconducting block,the material filling method and the reinforced ring method are used respectively to obtain good protection effect.(2)Based on the study of current-carrying conditions of simple superconducting thin films,a two-dimensional axisymmetric geometric model of the 60-turn high temperature superconducting strip winding structure was established.Based on the shunt model,the superconductivity characteristics and mechanical behaviors of the electromagnetic,heat-force multiple physical fields were studied when the current was loaded.The accuracy and validity of the simulation results were verified by comparing the quench propagation velocity(NZPV)with the literature data.The temperature diffusion and distribution in the quench propagation process caused by thermal disturbance were expounded.The minimum quench energy(MQE)that can cause the superconducting strip winding structure was analyzed by applying thermal disturbance of different energies.The stress distribution of the winding structure of superconducting strip after loss of lead and the influence of local defects on the stress of the winding structure of superconducting strip are analyzed.The results show that the temperature distribution value at the position of thermal disturbance is always the largest.If the heat there can not be diffused in time,it is easy to cause the safety accident.Although the thermal strain generated by the loss time produces a compressive stress region near the heat source,which is conducive to reducing the tensile failure risk of the superconducting strip,the overall stress is obviously greater than that when the loss time is acted by electromagnetic force only.(3)By comparing with the H method,the accuracy and efficiency of T-A method in calculating the winding structure of superconducting strip with small turns are analyzed,and the mechanical parameters of superconducting composite strip are equivalent by using the representative volume element method(RVE),which is applied to the numerical study of the structure of superconducting magnet with large turns.The simulation results of 25.7 T superconducting magnet were compared between the two models.Based on the strain sensitivity,the shielding current and stress-strain under reversible and irreversible prestrain were analyzed,and the corresponding suppression measures for the magnetic field attenuation and stress increase caused by the shielding current were proposed.The results show that the contribution magnetic field increases by 0.4 T when the electromagnetic and force coupling calculation model is adopted.Although the shielding current peak value increases overall,it greatly reduces the penetration of the superconducting pancake.The stress in the coil wafer changes little in the reversible prestrain range,while the stress in the superconducting wafer decreases in the irreversible prestrain range.