Study On The Electromagnetic-Thermal Characteristics Analysis And Optimization Method Of The No-Insulation High Temperature Superconducting Magnet

High temperature superconducting(HTS)materials have the characteristics of high current carrying capacity,high critical magnetic field and wide working temperature range.Its applications in accelerator magnet,magnetic resonance imaging magnet and background field magnet have achieved phased research results.HTS magnet has a bright development prospect in the field of steady-state high-magnetic field.However,the quench propagation speed of HTS material is low.The energy of quench can not spread rapidly,leading to local temperature rise which affects the stability of the whole superconducting magnet.The lack of thermal stability of magnet limits its application.In order to improve the stability of HTS magnet,some scholars put forward the scheme of no-insulation HTS magnet.The magnet is wound by superconducting wire without insulation layer.When quench occurs in the magnet,the current can flow to the adjacent turns through turn-to-turn contact to avoid local hot spots.Based on the development states of no-insulation HTS magnet and the change rules of turn-to-turn contact resistance,this thesis analyzes the electromagnetic-thermal characteristics and related mechanism of no-insulation magnet.In order to enhance the excitation rate,the stability and the magnetic field intensity,a series of researches on the analysis and design methods of no-insulation magnet are carried out.A no-insulation HTS magnet is designed and manufactured for experimental verification.This thesis mainly contents the following aspects:(1)Based on the characteristics of the turn-to-turn contact resistivity,the curve of turn-to-turn contact resistivity varies with stress and temperature is obtained from experiment.In view of the radial stress distribution in the coil,a method for calculating the turn-to-turn contact resistance is proposed.Compared with the traditional average evaluation method of turn-to-turn contact resistivity based on the measurement value,the calculation method considering the change characteristics of turn-to-turn contact resistivity can effectively improve the calculation accuracy.The existing calculation method of the pre-tightening stress is not suitable for the non-central-symmetry structure(such as racetrack coil).A 3D numerical model for stress calculation is established using the function of element birth and death in the ANSYS.The simulation of superposition behavior of each turn during the winding process is achieved.Combined with the contact analysis module,the calculations of the winding pre-tightening stress,thermal stress and electromagnetic stress of various structural coils are realized.The radial stress distribution of the coil and the contact resistance distribution between turns are calculated through the finite element model,and the theoretical calculation results are compared with the experimental results to verify the effectiveness of the model.(2)In order to solve the problems of high degree of freedom and poor convergence of conventional methods for electromagnetic characteristic simulation of superconducting materials,a method using uniform current density ratio to calculate the resistance of superconducting layer is proposed.The ratio of current density to critical current density isthe main factor affecting the resistivity of superconducting layer.According to the over-current characteristics of no-insulation superconducting coil,the current density distribution and critical current density distribution in the tape are studied.The results show that the ratio tends to be uniformly distributed under the over-current operation.This method can realize the fast electromagnetic analysis of the 3D model of the non-central-symmetrical structure magnet.Combined with the structure analysis model and the heat conduction analysis model,a multi-physics coupling analysis model can be established to simulate the dynamic process of the no-insulation superconducting magnet during over-current operation.(3)The excitation time,excitation loss,over-current operation ability,thermal stability and other characteristics of no-insulation HTS magnet are studied.The influence of turn-toturn contact resistance on the electromagnetic-thermal characteristics of no-insulation HTS coil is analyzed.In view of the incompleteness of design method of no-insulation superconducting magnet and the lack of optimization of turn-to-turn contact resistivity,a design method of no-insulation magnet considering the optimization of turn-to-turn contact resistance is proposed.The method uses genetic algorithm to impose non-uniform pre-tightening force in order to control the turn-to-turn contact resistivity.The results show that the optimized scheme is helpful to improve the excitation performance and thermal stability of no-insulation magnet.(4)A prototype of conduction cooling racetrack no-insulation HTS magnet is designed,manufactured and tested.The electro-magnetic,stress and thermal characteristics of the magnet in the excitation process and the over-current thermal runaway process are simulated.The experimental results and numerical simulation results are compared,and the feasibility of the analysis method is verified.The results show that the cooling uniformity of the no-insulation magnet without impregnation is poor,and the turn-to-turn loss caused by faster excitation rate will lead to the quench of the magnet.No-insulation magnet with more turns and under higher magnetic field shows lower over-current operation ability than a single no-insulation coil.Considering the nonlinear change of turn-to-turn contact resistivity caused by the rise of temperature and the decrease of magnetic field in the study of thermal runaway,the results show that the turn-to-turn contact resistance increases significantly,which will further aggravate the coil heating.The study verifies that it is necessary to consider the variation characteristics of turn-to-turn contact resistivity in the assessment of thermal stability of magnet.