Teruou Takayama

Detection of Critical Current Distribution of YBCO-Coated Conductors Using Permanent Magnet Method

We developed a non-destructive and contactless system for measuring the critical current (I_c) in YBa₂Cu₃O_7-δ (YBCO)-coated conductors by using a permanent magnet (Sm₂Co₁₇). This I_c measurement method is based on the repulsive force (F_r) between the magnet and the shielding current in high-temperature superconductor-coated conductors. We measured F_r using a high-resolution load sensor and found that accurate F_r could be determined without the effect of thick copper film on the YBCO thin film and Hastelloy tape of the substrate. We can determine I_c from F_r0, which is the maximum repulsive force determined from an extrapolated value of the F_r vs. L curve for L = 0 mm, described in our previous paper. We show that the permanent magnet method can be used to determine the longitudinal I_c distribution and large-scale defects in YBCO-coated conductors. In addition, it turned out from this experiment that the permanent magnet method was effective to rapidly measure the longitudinal I_c distribution of long-scale coated conductors.

Analysis of Measurement Method for Critical Current Density by Using Permanent Magnet

The Permanent Magnet method for measuring critical current density in high-temperature superconducting thin film is numerically and experimentally investigated. Numerical results show that the critical current density is approximately proportional to the maximum repulsive force. Furthermore, the maximum repulsive force increases as the value of diameter of permanent magnet increases. These tendency is also observed in the experimental investigation. Finally, the spatial distributions of the shielding current density are reproduced by the system experimentally.

Numerical Simulation of Inductive Method and Permanent-Magnet Method for Measuring Critical Current Density

Two types of contactless methods for measuring the critical current density have been investigated numerically. For the purpose of reproducing the experimental results numerically, the behavior of the shielding current density is formulated by use of the current-vector-potential method. A numerical code has been developed for solving an initial-boundary-value problem of the resulting integral-differential equations and, by use of the code, both the inductive method and the permanent-magnet method are simulated successfully.

Virtual Voltage Method for Analyzing Shielding Current Density in High-Temperature Superconducting Film With Cracks/Holes

An accurate numerical method is proposed for calculating the shielding current density in a high-temperature superconducting film containing defects. If the initial-boundary-value problem of the shielding current density is formulated by the

Weight Function Control of Moving Least-Squares Interpolants: Application to Axisymmetric Shielding Current Analysis in HTS

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Numerical Investigation on Crack Detection in HTS Film: Accuracy of Scanning Permanent Magnet Method

-method, integral forms of Faradays law on defect surfaces are also imposed as natural boundary conditions. However, the conditions are not satisfied exactly by a numerical solution and their residuals develop intolerably with a decrease in the film thickness. In order to resolve this problem, the following method is proposed: virtual voltages be applied along the defect surfaces as to have the natural boundary conditions numerically satisfied. A numerical code is developed on the basis of the proposed method, and the influence of a crack on the inductive method or the permanent-magnet method is numerically investigated.

Numerical Simulation of Shielding Current Density in High-Temperature Superconducting Film: Influence of Film Edge on Permanent Magnet Method

The modified basis functions of element-free Galerkin (EFG) method generated by weight function control of moving least-squares interpolants for an axisymmetric problem are investigated. In the axisymmetric problem, a physical quantity of rotating angle direction on the symmetry axis should be zero. By using standard EFG, however, the values on symmetric axis are non zero because the axisymmetric is not considered in basis function. In order to clear up the above problem, the modified basis function of EFG is proposed. The numerical simulation of shielding current density in axisymmetric high-temperature superconductor is adopted for practical applications of modified axisymmetric EFG. The results of computations show that the shielding current density on symmetric axis vanish in both cases. In addition, the experimental critical current density measurement method is reproduced numerically by using the numerical code.

Evaluation of Parallelized Meshless Approach: Application to Shielding Current Analysis in HTS

By simulating two types of the contactless methods for measuring the critical current density, the resolution of the crack detection in a high-temperature superconducting (HTS) film has been investigated. To this end, a numerical code has been developed for analyzing the shielding current density in an HTS film with a crack. By using the code, the scanning permanent magnet method and the inductive one have been reproduced numerically. The results of computations show that the scanning permanent magnet method shows a relatively poor accuracy for the crack detection. If the inductive method is applied to the region obtained by the scanning method, the detection accuracy of the crack is drastically improved. The crack can be detected with high speed and high accuracy by combining the inductive method and the scanning method.

Numerical Simulation of Noncontact Methods for Measuring of Critical Current Density: Permanent Magnet Method and Inductive Method

A numerical method is developed for analyzing the shielding current density in a high-temperature superconducting (HTS) film. When an HTS film contains a crack, an additional boundary condition is imposed on the crack surface and it can be incorporated into the weak form. Although the weak form can be numerically solved with the essential boundary conditions, the resulting solution does not exactly satisfy Faradays law on the crack surface. In order to resolve this problem, the following method is proposed: a virtual voltage is applied around the crack so as to make Faradays law satisfied numerically. A numerical code for analyzing the shielding current density is developed on the basis of the proposed method and, by means of the code, the permanent magnet method is investigated numerically. Especially, the influence of a film edge or a crack on accuracy is assessed.

Numerical Simulation of Permanent Magnet Method: Relation Between Critical Current Density and Maximum Repulsive Force

Parallelized meshless approach using OpenMP is evaluated. The meshless approach does not require finite elements. However, it takes tremendous CPU time to generate the coefficient matrix of linear system instead of unnecessary mesh generation procedure. The code for Poisson problem and shielding current analysis in high-temperature superconductor (HTS) by using element-free Galerkin method is developed and parallelize the code using OpenMP. Results of the computation shows that the CPU time of 4PUs (processing units) is 3.8 times faster than that of 1PU in case of Poisson problem. However, speedup ratio does not show good performance in case of the shielding current analysis in HTS.