| The demand for strong magnetic fields in human production and life drives the development of high-field magnet technology.Compared with traditional water-cooled magnets,superconducting magnets have superior advantages such as low energy consumption,compact structure,light weight,and high spatial uniformity of the magnetic field,and thus they show great potential application in the fields of energy,medical,scientific research,etc.The insert magnet made of high-temperature superconducting(HTS)tape has successfully broken the record of the steady-state strong magnetic field.And meanwhile,the HTS tape is regarded as the most promising conductor in the application of ultra high-field magnets because of the large currentcarrying capacity in high magnetic field and high mechanical strength.With the improvement of strong magnetic field,there are still some significant challenges for the further advancement of HTS magnets,such as quench and degradation of electromechanical performance.The electromagnetic and mechanical research of core component is directly related to the safety and functionality of the magnets.According to the actual operating conditions of superconducting devices,this dissertation carries out the numerical simulations on the electromagnetic response,quench behavior and electromechanical properties of HTS coils made of second-generation HTS tape.The main contents are as follows:1)Based on the self-consistent model and the H-formulation,the electromagnetic behaviors of the HTS coil under DC and AC current conditions are studied respectively.The distributions of the current density and the magnetic flux density are calculated.Meanwhile,the effects of the magnetic substrate and magnetic flux diverter on the critical current and AC loss of the HTS coil under different external magnetic field conditions are discussed.The numerical results indicate that the magnetic flux diverter can change the distribution of the magnetic flux density and makes the magnetic flux lines deflect toward the parallel direction of the superconducting tape.Consequently,both the perpendicular field and AC loss of superconducting coil decrease.In addition,from the perspective of reducing the AC loss of HTS coil,the size of the magnetic flux diverter,the magnetic saturation peoperty and its relative position to the HTS coils are analyzed in detail.2)Based on the aforementioned transient electromagnetic model and the nonlinear heat conduction equations,a two-dimensional electromagnetic-thermal coupled model of HTS coil is built with consideration of the actual configuration of HTS tape.To characterize the mechanical response during the quench,the elastic-plastic mechanical model of HTS coil is established as well.Both numerical models are used to simulate the variations of the coil temperature,current density,and the stress and strain in the process of quench propagation.The numerical results show that the variation trends of local temperature and radial strain are basically consistent,which confirms the theoretical feasibility of the strain-based quench detection scheme.In addition,the influences of substrate materials,ambient temperature,coil size and external magnetic field on the quench and mechanical behaviors are discussed.It is pointed out that the HTS coil is likely to undergo delamination behavior due to the large radial tensile stress during the quench propagation process.3)The screening current and electromechanical properties of large-scale HTS magnets are numerically studied in this part.Firstly,the electromagnetic model of HTS magnet is constructed by combining the T-A formulation and the homogenization technique.With the aid of the electromagnetic model,the screening current and the associated electromagnet force of HTS magnet during the charging process are numerically calculated.Secondly,an elastic-plastic mechanical analysis is performed for the dry-wound HTS coil under a Lorentz load.The stress-strain distributions of HTS coil at the fully charged state are analyzed.Then,according to the preparation and operation sequence of HTS magnets,the elastic-plastic deformation and electromechanical degradation of HTS coils under the combined effects of winding tension,thermal stress and electromagnet force are simulated.The simulation results show that the screening current is a key factor that disturbs the mechanical stability and the critical current of HTS magnets.Finally,the screening current effects of HTS magnets under different current ramping paths are given.4)Based on the homogenized T-A formulation and the elastic-plastic mechanical model involving turn-to-turn contact behavior,the coupled electromagneticmechanical model of HTS coils is established with consideration of the structural deformation and the electromechanical degradation of HTS tape.Besides,the coupled model is extended from two-dimensional to three-dimensional model to characterize the longitudinal in-homogenous critical current of a HTS tape.The reliability of the coupled model is verified by comparing the numerical results with the experimental data.On this basis,the three-dimensional coupled model is then utilized to study the influences of the critical current non-uniformity and the strain sensitivity of HTS tape on the maximum current-carrying capacity of HTS coils under different external fields.During the electromagnetic excitation process,the screening current and the stress-strain distributions of HTS coils are explored as well.The simulation results indicate that the in-homogeneous critical current of the HTS tape has a great impact on the current-carrying capacity of the HTS coils under the conditions of self-field and low external field.While the external field is at a high level,the electro-mechanical property of commercial HTS tape is found to strongly affect the critical current property of high-field HTS coils.In addition,the influencing factors such as the mechanical stiffness of co-wound materials and local defects are investigated to understand their role in the mechanical stability of HTS coils. |
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