Quantitative Study Of Thermal Stability And Mechanical Behaviors In High-temperature Superconducting Coils

High-temperature superconducting(HTS)materials have remarkable advantage in the application of high-field magnet because of their special electromagnetic properties.Many cutting-edge researches involved in the Medium and Long-term Plan for National Major Science and Technology Infrastructure Construction(2012-2030)rely on the realization of high magnetic field.Therefore,the success of this technology is of great significance for the promotion of national advanced science and technology,which is embodied in the development of magnetic constraint nuclear fusion reactor,nuclear magnetic resonance(NMR),high-energy particle accelerators,fundamental research and so on.However,the development of high-field superconducting magnets in the long term is faced to two key problems: firstly,it is very difficult for the quench detection and protection in HTS coils due to the slow quench propagation velocity of HTS.This severely reduces the thermal stability of the magnet.Secondly,in high field,HTS coils not only suffer from huge electromagnetic stress,but also may experience large thermal stress induced by thermal disturbance.This will increase the risk of magnet damage.Therefore,the study of the thermal stability and mechanical behaviors of HTS coils in high-field superconducting magnets is the basis for ensuring the success of this technology.From the perspective of the real working condition of the magnet,the thesis studies the electromagnetic-thermo-mechanical behaviors of HTS coils by extending and establishing the multiphysics coupling model.At first,the self-consistent model is extended to investigate the critical current of Bi-2212 round strand.When the fitted parameters based on single strand are used to calculate the critical current of the multi-strand cable,the numerical results are in agreement with the experiment.The results indicate that when the magnetic field is applied in reverse,the outermost turn of the coil reaches the critical state firstly.For the large superconducting coils,in order to reduce the amount of computation,the equivalent models are proposed to estimate their critical currents and central magnetic fields.Afterwards,multi-layer current loading method is used to enhance effective utilization of the coil,and the central field of coil also increases.Because of HTS magnet often embedded in low temperature superconducting(LTS)magnet,the model is further extended to study the critical current of HTS coils and total central field of the hybrid magnet.Secondly,after considering the heat generated by the contact resistance at the interface between two adjacent turns,a multiphysics coupling model is built by combining finite difference method and finite element method to clarify the thermal stability and mechanical behaviors in NI single-pancake coil and NI layer-wound coil.The numerical results indicate that NI coils can avoid local temperature rise by radial shunting due to the low turn-to-turn contact resistance during the thermal quench,which also reveals the reason of the high thermal stability in NI coils.However,in high contact resistivity,the secondary permanent quench may appear in NI coils.When the metallic cladding method is used in the layer-wound coil,its field delay effect can be effectively alleviated.Due to relatively small electromagnetic stress of the coil in self-field,the mechanical behaviors are mainly caused by temperature rise.Under different inner boundaries,the distributions of stress and strain in a single-pancake coil are different,and the change of its hoop stress is much larger than that of radial stress.In a layer-wound coil,its hoop and axial stresses both have an obvious change.Finally,an electromagnetic-thermo-mechanical coupling bulk model is built to study multiphysical field properties during the charging process of the NI magnets consisted of many double-pancake coils.The results of the bulk model are verified by comparing the numerical results of real structure and bulk models.In self-field,the radial compression stress causes the change of current distribution of the magnet for both the ideal cooling boundary and the adiabatic boundary,which leads to the increase of field delay time of magnet and non-unniform change of contact loss.The numerical results also reveal the importance of electro-mechanical coupling behaviors in evaluating the complex multi-field characteristics of magnets.In high field,the magnet is stretched along the radial direction under the action of the large electromagnetic stress,and its hoop stress is much larger than the radial stress and is the largest at both ends of the magnet.