The Study Of Inhomogeneity Of Critical Current Of HTS Tape And Its Effect On Quench Behavior

The specific properties of HTS (high temperature superconductor) tapes able to carry strong current under very high magnetic induction offer an exceptional opportunity to go beyond 25T, the actual limit of LTS (low temperature superconductor) wire. Electric transport property of YBCO tape will be affected by bending strain, tensile stress and lamination stress in the manufacture and operation process of HTS coils, which may cause quench. The inhomogeneity of critical current of HTS tape has become one of the most important issues to prevent heat spots in high field magnet. In the mechanical characteristic test of YBCO tape, the average h is often measured by four-probe method. Although the average Ic is accurate enough for evaluating the overall current-carrying performance, the local Ic degradation caused by cracks is averaged duo to relative large resolution of four-probe method, which is in the scale of several centimeters. Another disadvantage of four-probe method is that the average value of Ic seems too optimistic in the simulation process of heat spot generation. In this thesis, the uniformity of Ic under bending strain and tensile stress is well studied by a HSA (hall sensor array) system with high resolution. The thermoelectric coupling model is built combined with the inhomogenity of Ic to provide a reasonable forecast of hot spot and quench behavior in the operation of HTS tapes in the mechanical strain.Firstly, a continuous HSA system has been designed in order to research the inhomogeneity of Ic in the different mechanical stress and strain conditions. The system uses the hall sensor array for detecting the remanent field or shielded field around the tape surface,combines with the inverse operation algorithm and gets the distribution of the Ic inside the superconductors, at the same time the in-situ mechanical strain is applied. The spatial resolution of the system is 0.4mm at the measurement speed of 10mm/s. By controlling the offset in the measurement process, the measurement error caused by mechanical disturbance is greatly reduced. The COV (coefficient of variation) of lateral critical current density on the cross section of conductor is used as a parameter describing the lateral inhomogeneity of critical current density. The result shows that the COV of the segment increase with the increase of the amount and size of defects. The Ic distribution of commercial HTS wire, such as YBCO tape, Bi2223 tape and Bi2212 wire/tape is measured by HSA system and the reliability and stability of the system is proved. The relationship between the remanent field and Ic is calibrated by FEM (finite element method). The result shows that the FEM is an effective method for evaluating the Ic distribution of the YBCO tape with ferromagnetic substrate.The strain of YBCO tape in distorted bending mode is studied. Only when the equivalent bending radius was smaller than the critical bending radius, the Ic dropped sharply and cannot be restored. The inhomogeneity of Ic of YBCO HTS tapes under bending strain of 0 ?0.7% is studied by the HSA system. The results show that when the bending strain is more than 0.6%, the inhomogeneity of Ic increases rapidly, the reason might be that the structure of the superconducting layer is destroyed with the increase of bending strain. The inhomogeneity of Ic of YBCO tape in the axial tension of 0 ?250 N is also studied by the HSA system in situ. When the tensile stress is beyond 221N, the deformation of YBCO tape goes into plastic stage, the defects are existed in superconducting layer and the inhomogeneity of Ic increases. The tensile strength can be improved by pre-compression process of the HTS tape. The three-parameter weibull distribution model is established to obtain the Ic distribution of HTS tape under mechanical stress and strain, which could be used for the Monte Carlo simulation of overall critical current.The thermoelectric coupling model is used to investigate the effect of the inhomogeneity of Ic on the quench behavior of YBCO tape under mechanical strain. The heat spot, NZPV (normal zone propagation velocity) and MQE (minimum quench energy) are studied. The results show that the higher the inhomogeneity of Ic of HTS tape, the heat spots appear more frequently. When the bending strain is more than 0.6% or the tensile stress is beyond 221N, the rate of the temperature of the hot spots reaches the quench temperature gets higher, which means that the quench protection system need a smaller delay time constant to ensure the superconductor work in the safe temperature range. The temperature of heat spot is determined not only by local Ic of defect and but also by the size of the defect,The MQE is significantly decreased and the NZPV is slowly increased in the defect area.The YBCO tape with higher inhomogneneity of Ic is more easily quenched by a small thermal disturbance at the same applied current. The weibull distribution model is available on predicting the quench behavior of YBCO tape in mechanical strain. The calculation will help the defect recognition of the HTS tape and the quench protection design of superconducting magnet.