Design And Performance Study Of Bi-2212 CICC Conductor

The controlled nuclear fusion technology using magnetic confinement is an effective way to solve the future energy crisis,and the magnetic confinement tokamak is one of the most promising devices for achieving fusion energy utilization.In the early development of tokamak devices,conventional conductors were usually used to construct the magnet system.In order to achieve stable andoverenergy operation of controlled nuclear fusion devices,superconducting magnets gradually replaced conventional conductors in the construction of tokamak devices.China is the first country in the world to successfully build a fully superconducting tokamak device,and as an important participating country,it is actively carrying out the construction of the International Thermonuclear Experimental Reactor(ITER).Based on the digestion and absorption of ITER’s existing technology,China’s next-generation magnetic confinement nuclear fusion device,the China Fusion Engineering Test Reactor(CFETR),is steadily advancing its design and development work.Superconducting magnets for magnetic confinement fusion are usually made by winding cable-in-conduit conductors(CICC),which provide a strong magnetic field environment that is a key element for stable operation of tokamak devices.CICC conductors can operate under high current and high magnetic field conditions,and their structure can provide strong support and protection for superconducting cables,achieve lower energy loss,and internal liquid helium forced flow cooling,making them the preferred structure for fusion devices.Currently,the preparation technology of NbTi CICC conductors for low-temperature superconducting materials is very mature,and Nb3Sn CICC conductors have been developed and applied tooverfield superconducting magnets in fusion reactors.In the future,the highest magnetic field of the central solenoid coil in China’s fusion reactor CFETR will exceed 15 T,and the operating current will reach 60 kA.Low-temperature superconducting materials based on NbTi and Nb3Sn can no longer meet the application requirements,and there is an urgent need to develop superconducting conductor technology with high field and high current to achieve stable operation under high magnetic field.So far,high temperature superconducting materials with simultaneous multi-strand structure,high upper critical magnetic field,low AC loss,and can be prepared into isotropic round wires for strong magnetic field applications only have Bi-2212,which is an ideal material for developing large current superconducting conductors and large-scale magnets under strong magnetic fields in the future.Bi-2212 superconducting round wires need to undergo high temperature,overpressure,and oxygen-rich heat treatment to achieve excellent current-carrying performance.However,during the development of Bi-2212 CICC conductors,there are still three key problems:1.Lack of performance parameters of commercial Bi-2212 superconducting strands;2.Theoretical and empirical research on Bi-2212 CICC conductor preparation and heat treatment processes are extremely scarce;3.Lack of design and preparation experience for Bi-2212 CICC conductors that can be referenced.This article focuses on the application needs ofoverfield,large-current,high temperature superconducting conductors,and focuses on conductor structure design and process research.It conducts theoretical analysis and experimental research on the magnetic field and temperature dependence of Bi-2212 wire performance,heat treatment process,and conductor structure design based on Bi-2212 material characteristics and CICC conductor design principles,combined with the influence of heat treatment on Bi-2212 superconducting wires.When the Bi-2212 wire is subjected tooverpressure heat treatment,the diameter of the wire decreases by 4-5%,which causes the gap rate to increase after the conductor is prepared,resulting in performance degradation of the superconducting wire due to bending deformation during conductor operation.This article proposes a pre-over-pressure+overpressure heat treatment process suitable for Bi-2212 wires and CICC conductors,which solves the problem of excessive gap rate caused by the decrease in superconducting wire diameter during conductor heat treatment and provides a new idea for Bi-2212 conductor preparation.Firstly,the critical performance of two commercial Bi-2212 superconducting strand products produced by Oxford Instruments(OST)and the Northwest Nonferrous Metal Research Institute(NIN)is systematically characterized for their dependence on magnetic field and temperature,providing basic parameters for subsequent Bi-2212 CICC conductor design.Based on the Bi-2212 heat treatment process and theoretical foundation,a large-diameter and long-temperature-zone heat treatment system design and heat transfer analysis suitable for Bi-2212 full-size conductors were carried out;the constant temperature zone of the heat treatment system was calibrated and sample tested to verify the reliability of the system function.The feasibility of pre-overpressure+overpressure heat treatment on superconducting long wires was verified using the heat treatment system,and a slow temperature rise and multi-platform heat treatment system suitable for large-sized magnets was proposed,and preliminary feasibility verification was carried out using superconducting wires,laying the foundation for the heat treatment of future large-scale magnets.Secondly,based on the characteristics of Bi-2212 materials and CICC conductor design principles,combined with the influence of heat treatment on Bi-2212 superconducting wires,aoverstability Bi-2212 CICC conductor design scheme with metal tube reinforcement was proposed,and preliminary design parameters were given,and Bi-2212 cables and conductor samples were successfully prepared.Finally,based on the 5.8 T background field test platform,a comprehensive performance study of Bi-2212 cables and CICC conductor samples at low temperatures was systematically carried out,including DC performance,current sharing temperature,electromagnetic load,and quench characteristics.The experimental results show that when the conductor passes a current of 18 kA under a background magnetic field of 5.8 T,the current sharing temperature reaches 17.8 K;the performance of the prepared Bi-2212 cables and conductors exceeds 90%of the critical current value of the total amount of the same superconducting strand lines,which is the best performance retention rate among the cables or conductor samples prepared internationally;the feasibility of conductor preparation and pre-overpressure+overpressure heat treatment process in Bi-2212 CICC conductor development is verified;after more than 100 electromagnetic loads and six quenches,the critical performance of the conductor did not show a significant decline,verifying the low sensitivity of the conductor to electromagnetic cycling and quenching.