Investigation On Several Key Mechanical Subjects In CICC Application For The ITER Project

"The International Thermonuclear Experimental Reactor(ITER)program is of the largest and the most influential international cooperation project,and its purpose is to verify the scientific and technical feasibility of the magnetic constraint nuclear fusion reactor.The high temperature plasma with billion degrees Celsius can be constrained in a magnetic cage,which can provide a strong magnetic field by a Tokamak device wounded by CICC(Cable in Conduit Conductor)conductors.In 2014,Dr.A.Devred a chairman of ITER magnet Project pointed out that the ITER CICC conductor is mainly faced with three challenges: 1,During insertion into the jacket assembly,the cable exhibits a tendency to rotate under the action of the pulling force.That may increase the twist pitch,especially for the final one.Cable twist pitches must be controlled to prevent excessive AC losses in the CICC,which is the threaten to the stability of CICC.The elongation of the twist pitch must be settled.2,The CICC’s current sharing temperature(Tcs)showing degradation after the electro-magnetic(EM)and thermal cycling load,that means the Tokamak can only run thousands of times,less than the original design of 30,000 times,raised fears at the fusion project.3,Find the way to fabricate the high-performance superconducting wire and the low resistance joint.As we all know,the central and the toroidal field solenoid superconducting cable of the Tokamak were made of Nb3 Sn superconducting strands,and the superconducting cable will working at the mechanical – thermal – electrical-magnetic fields environment.Previous studies have shown that Nb3 Sn superconductivity is sensitive to the mechanical deformation.Therefore,it’s important for the design of the Tokamak magnet system to investigate the equivalent mechanical parameters of the cable and its mechanical behavior under the action of multiple fields.In this paper,several key mechanical problems such as the equivalent mechanical parameters of the superconducting cable,the untwisting behavior in the process of insertion,the buckling behavior of the superconducting wire under the action of the thermal-electromagnetic static load and the Tcs degradation under the thermo-electromagnetic cyclic loads have been studied.The main contents of this dissertation are as follows:Firstly,the stress-strain curve of the triplet was analyzed based on the thin-rod model and the tensile stiffness model of the triplet was established.Due to the shortcomings of the preparation process,there were two kinds of defects: residual strain and voids in the specimens.In this paper,the spring model and modified thin rod model were established for the triplet tensile stiffness on these two cases,respectively.The results of the models are in good agreement with the experimental results.Then,the thermal strain was introduced into the thin rod model for calculate the effective coefficient of thermal expansion(CTE)of the twisted structure,and we tested the CTE of the carbon fiber strand and the Kevlar strand.The theoretical data are consistent with the experimental results.The influence of the twisted pitch on the axial CTE was discussed.Finally,on the basis of the triplet model,a complex model of the tensile stiffness and the equavalent CTE of the 3*3 strand was build,and the detailed derivation process was provided.Secondly,the untwisting behavior of the cable in CICC fabrication was investgated.The bending stiffness model of the petal under the wrapping was established,and the tensile untwisting model of the superconducting cable was also build.The accuracy of the model was verified by compared with the elongation experiments of the superconducting cable.The effects of parameters such as material stiffness,wrapping force,wrapping rate,final tension and insert force on the petal’s bending stiffness and the cable torsion were studied,respectively.The carve of the untwist angle with the axial tension force was given.Thirdly,under the action of the thermo-electromagnetic static load,the influences of the wrapping parameters,friction coefficient and the triplet pitch on the buckling critical value of the superconducting strands were studied.The theoretical results show that the pitch is the key parameters for the buckling critical value of the wires.The smaller the pitch is,the more difficult the buckling occurs.The probability of the buckling of superconducting strands can be reduced by increasing the wrapping rate and bandage stiffness.In addition,the larger surface friction coefficient of the strands is beneficial to the stability of the strands,and it is also an effective methods to improve the axial compression stiffness of the superconducting cable.Finally,a mechanical model was established for the Tcs degradation mechanism of TF and CS CICC conductors under the action of thermo-electromagnetic cyclic load.Based on the compatible relationship between the transverse compressive strain and the axial elongation of the superconducting cable,the impacts of axial compressive stiffness and surface friction coefficient on the axial compression strain of the cable under the thermo-electromagnetic cyclic load were studied.Then the relationship between the axial strain and Tcs of the cable was introduced into the theoretical model,and we can get the relationship between the Tcs and the number of electromagnetic cycles.The results show that the stiffness of the cable plays a significant role in the performance of the Tcs.The larger the stiffness,the smaller the axial compressive strain of the cable,and the lower Tcs degradation is.In addition,the SULTAN test facility with the high and low magnetic field distribution characteristics should be responsible for the Tcs enhancement in the short pitch CS conductor measurements.All the theoretical results are agree well with the experimental results tested by SULTAN facility.