Investigations Of AC Loss Of Joint In High Temperature Superconducting Coated Conductors

YBa₂Cu₃O_7-x coated conductor（YBCO CC） is one of the most promising superconductors in many applications due to high critical current density and high critical temperature and good mechanical performance. However, one limiting factor for the coated conductor in engineering applications is the manufacturing length. It is difficult to manufacture a large length conductor and ensure their electromagnetic and mechanical properties. It is reported that the length of conductor can reach about one kilometer using advanced manufacturing technique. However, it is still difficult to satisfy the length requirement for superconducting magnet coil and power transmission and so on. In order to get a longer length of the CC, the joint are used in most superconducting devices. In addition, the joint has contact resistances which can lead to degradation of current-carrying capacity. Therefore, we need to consider the stability of joint structure in the superconducting devices.Many researchers have paid attentions to the joining techniques of conductors to meet the demand of application. The main problem of joint are the resistance and the degradation of critical current. The mechanical properties of joint are also important and the strain and stress can degrade the electrical properties of joint. Most of researches mainly studied joint structure experimentally. With the development of the numerical simulation of high temperature superconductor, some numerical methods have been proposed. A H-formulation method has been successfully implemented with the commercial FEM packages. H-formulation has become a popular method used to investigate the electromagnetic properties of superconducting wires, cables and coils. In order to give a detail information of the electromagnetic response of joint, a numerical simulation for joint will be a convenient way to study the influence of the geometry and optimize the structure of joint.Firstly, the AC loss and the electromagnetic characteristics of the joint under self-field are investigated with 3D finite-element model. Based on the nonlinear E-J power law and H-formulation, we consider the effect of lap length and thickness. It is obvious that increasing the lap length and decreasing the solder layer thickness both have a beneficial impact on reducing of the total AC loss of joint. The loss has a concentration at the both ends of joint overlap part which will influence the reliability of joint structure. The lap length has less effect upon the distribution of AC loss in the SC part than in the solder part of joint. Furthermore, the increase of the solder layer thickness directly leads to an increase of the solder layer resistance which has a larger influence upon the AC loss in the solder.In practice, the welding quality of joint might be not able to get a good guarantee. There will exist defect in the solder part. And, in the complex superconducting operating conditions, the joint will inevitably be subject to external load, thus, the joint may have some loose weld area in the solder part. Both of the two situations will have an impact on the performance of the joint. Therefore, this paper investigated the joint AC loss by considering the effects of the defect and the loose weld in the solder part. We have found that both of the two situation were degradating the joint eletromagnetic properties which should be avoided.