Research On Key Technologies Of Heat Treatment Of Large Nb₃Sn Superconducting Coil

Magnetic confinement tokamak is one of the most promising devices for the controllable utilization of fusion energy,and it is an effective way to solve the problem of future energy shortage.The China Fusion Engineering Test Reactor（CFETR）project is a major scientific project oriented to the world’s scientific and technological frontiers,major national needs and frontier areas of engineering.The superconducting magnet system is the key subsystem of the tokamak device,and the magnetic field strength directly determines the confinement performance and stable operation performance of the plasma.In order to ensure the smooth development of CFETR superconducting magnets,China took the lead in initiating the pre-research work of CFTER central solenoid model coil（CSMC）and toroidal field coil（TF）.The design of the CFETR magnet system is based on Nb₃Sn superconducting coils.The technical route of "wind and react" is generally used for the development of largesize Nb₃Sn superconducting coils.Heat treatment is an important link in the formation of superconducting phase by solid-state diffusion reaction.The quality of heat treatment directly determines the superconducting properties of magnets.In the process of heat treatment research of the first large Nb₃Sn superconducting coil for fusion in China,there are two major problems:lack of heat treatment equipment for large magnets and lack of heat treatment experience.It is necessary to solve the key technical problems such as heat treatment equipment development,aging process research and heat treatment reliability issues.Firstly,this paper focuses on the scientific research of Nb₃Sn conductor materials.Based on the diffusion mechanism of Nb₃Sn material and the experience of magnet heat treatment at home and abroad,the heat treatment evolution curve suitable for large Nb₃Sn CICC coil is formulated.In addition,the effects of conductor forming process and aging treatment on the mechanical properties of jacket materials are deeply analyzed and evaluated.Meanwhile,the thermal conductivity and specific heat capacity of the conductor and its components are measured,which provides thermophysical parameters basis for subsequent heat transfer analysis and simulation.Secondly,the basic equations of heat conduction,convection and radiation for multi-cake and multi-turn CICC coil heat treatment are deduced based on the theory of heat transfer.A fine two-dimensional heat transfer model of the coil heat treatment was established and the material thermophysical parameters under actual working conditions were applied to verify the heat treatment temperature uniformity.In addition,theoretical analysis and experimental research were carried out on the flow resistance characteristics of the CICC jacket under aging conditions.According to the geometric size of the coil and the technical requirements of heat treatment,the heat treatment method of the full argon shielded coil is proposed,and a large-scale coil shielded atmosphere heat treatment system is developed.A large circulation structure design of hot air in the furnace is proposed to improve the temperature uniformity.A method of creating an oxygen-free environment in a furnace by gas replacement is proposed,which ensures the good condition of the material inside and outside the magnet and reduces the risk of non-superconductivity and low insulation of the magnet.Based on Hazard and Operability（HAZOP）method,the reliability of heat treatment system and process is analyzed.The fault simulation is carried out for the weak links of heat treatment,and the emergency measures are formulated.At the same time,the performance of Nb₃Sn strands under the extreme failure state of heat treatment was studied,and a heat treatment failure rescue method of "interruption and reannealing" was proposed.The results show that any interruption is acceptable,Finally,after system installation and debugging,a set of heat treatment program specification for large Nb₃Sn CICC coil is gradually developed.The heat treatment of CSMC coil was successfully completed.The breakthrough of key heat treatment technology has laid the foundation for the development and process research of largescale superconducting magnet heat treatment system in the future.