Analysis On The Contact Force Behaviors Among Strands Of CICC Conductor Cross Section

The superconducting magnet?also known as the tokamak?is the core of the International Thermonuclear Experimental Reactor?ITER?.Its main function is to generate a strong magnetic field to restrict the movement of high temperature plasma,in order to achieve controllable thermonuclear fusion.Superconducting magnets are usually made of cable-in-conduit conductors of complex structure which is made up of thousands of NbTi/Cu wires or Nb₃Sn/Cu wires twisted in four or five stages.When the superconducting device works,the superconducting cable runs in the extreme environment of ultra-low temperature,high current,strong magnetic field,etc.The superconducting strands are subjected to huge Lorentz forces,which causes the deformation of the superconducting strands by squeezing each other,leads to the degradation of the superconducting material properties,and even leads to the fracture of the superconducting strands.Meantime,it is seriously affecting the safety and stability of magnet devices.Therefore,it is of great significance for the functional and safety design of ITER device to carry out research on the contact force characteristics among the superconducting strands,especially the contact force behavior of the strands caused by the action of electromagnetic forces.In recent years,the researches on the contact forces among superconducting strands mainly include the following two directions:the two-dimensional photoelastic particle model is used for the experiment,which gives the equivalent contact force characteristics.The main problem of this method is that it is difficult to apply the experimental results directly to CICC.In the numerical study,the discrete element method?DEM?is used to simplify the cross section of the strands into rigid particles so that the contact force distribution characteristics of the strands are calculated.The deformations of the particles are not considered in the calculation process of this method,which is obviously inconsistent with the experimental results.Therefore,how to give the contact forces among the strands on the cross section of real CICC conductor becomes a difficult or a challenge in this field.In 2012,Jose and Carlos proposed the granular element method?GEM?considering the contact deformation of particles.This method can give the contact force behaviors of particles which are opaque and have no photoelastic effect.However,the analysis theory of the contact force behaviors when particles bear the body forces is not given in this model.It is one of the core characteristics of ITER CICC that the particles simplified from superconducting strands bear the electromagnetic forces.In this dissertation,firstly,the contact force characteristics of the real CICC under the transverse compression are given by using GEM and digital image correlation method?DICM?.Then,the 2D GEM theoretical model with the body forces?or under the body forces?is deduced.And the contact force characteristics of ferromagnetic particles subjected to electromagnetic body force are given in combination with experiments.Finally,the 2D GEM model is extended to 3D,which provides the basis for the solution of the contact forces among superconducting strands under the electromagnetic forces.?1?Real CICC conductor bears single direction of electromagnetic forces in the process of operation.In the experiment,the unidirectional compression is first used to be equivalent to the unidirectional electromagnetic force.The speckle patterns before and after transverse compression is obtained by spraying speckle on the surface of the real CICC conductor,and then the deformation characteristics of the cross section are obtained.Then,the deformation characteristics of cross section were input into the self-developed GEM calculation program as the intermediate parameters,and the statistical characteristics of the contact force,force chain distribution and contact angle of the real CICC conductor section in the case of transverse compression were given for the first time.?2?A 2D GEM theoretical framework under body forces is established.Through theoretical derivation,the GEM equations with body forces are given,and an experimental platform composed of ferromagnetic particles is built,which is put into the unidirectional magnetic field environment.The contact characteristics of particles under magnetic field conditions are calculated by DIC.Finally,the contact characteristics are input into the GEM theory with body forces,and the statistical law of the contact characteristics of particles under electromagnetic forces is obtained.?3?The 2D GEM theory with body forces is extended to 3D.