Experimental Investigation On The Contact Mechanical Characteristics Of Superconducting Strands In The CICC Cross-section

Tokamak magnet system is the most important part of International Thermonuclear Experimental Reactor,or ITER.The main function of Tokamak is to limit the high temperature plasma in the magnet cavity to achieve controllable thermonuclear fusion.Therefore,the configuration of the magnetic field generated by the superconducting magnet which is winded by cable-in-conductor directly determines the reaction time(fusion function)of the ITER device,and becomes the guarantee of the safety and functionality of the device.CICC conductor in ITER device is usually made up of thousands of superconducting composite strands through multi-level cable,the conductor is carrying up to 68 kA of the current in the process of operation,and the magnetic field environment is close to 10 T.Therefore,superconducting strands endure huge electromagnetic force and compress tightly in the direction of the electromagnetic force.The close contact between the strands often leads to buckling and fracture of superconducting strand,hence reducing the current carrying capacity of conductors,which brings security risks to the superconducting magnet.Therefore,the study of the contact force behavior between superconducting strands is of great significance to the safety and functional design of the large magnet devices.This paper uses a simplified model,which simplifies the three-dimensional superconducting strand to a two-dimensional disc,using one-way load instead of electromagnetic force in real environment.It adopts the method of photoelasticity with digital image correlation method and finds a new algorithm which is to identify the contact position,and finally obtains the distribution characteristics of strand contact force.First of all,this paper develops an algorithm which can automatically identify the contact points of granular.The main procedures include: first,template matching method is used to extract each kind of photoelastic disk,so the image matrix transformation is carried out.Then use principal component analysis to analyze the image and extract the principal components which is used to recover the original image.The major function is to eliminate the influence of the zero level fringe on the contact point identification.Finally,the vertical gray level gradient of transformed image is calculated,and the gradient curve is described and the peak value of the curve is extracted to identify the contact point(position).Compared with the original identification methods of the contact point of the granular,this method not only realizes the automatic identification of particle contact position,but significantly reduces labor costs required for recognition of photoelastic disk or the time for machine finding contact position pixel by pixel,and it has high recognition accuracy.Next,this paper designs a system based on the photoelastic mechanics principle.This system can research the characteristic of contact mechanics between particles of the two-dimensional granular system under one-way loading.So as to simulate the contact force between superconducting strands in CICC conductor under the action of electromagnetic force.Let the particles have similar boundary condition to superconducting strands and apply the uniform force,then use the contact position recognition algorithm which is proposed in this paper to obtained the contact forces(including the distribution of force chains)and contact angle distribution between particles.The experimental results show that there are significant anisotropic in the distribution of contact force between strands.In this paper,the probability density function(PDF)is introduced to analyze the anisotropic.At the same time,according to the deformation characteristics of the particles,we get the energy dissipation of this system under uniaxial compression.In the end,the paper uses high-speed photography technology to conduct a preliminary exploration on the contact force characteristics of particle system under impact load,and mainly gets the distribution of stress wave propagation,the time evolution of the force chain and contact angle distribution.