Time-Evolution Simulation of Shielding Current Density in High-Temperature Superconductor

众所周知，临界电流密度是高温超导体工程应用中最重要的参数之一。在这个意义上，非破坏性和非接触式的方法已被期望用于测量高温超导薄膜的临界电流密度。Claassen等人提出了测量高温超导薄膜中临界电流密度的电感法。当在一个小线圈上施加交流电时，他们监测线圈中感应电压的谐波。结果表明，当且仅当线圈电流的幅值超过一定限度时，才会检测到三次谐波电压。这意味着可以通过测量阈值电流来估计临界电流密度。另一方面，Ohshima等人提出的永磁体法与电感法有很大的不同。当将永磁体靠近高温超导薄膜时，他们测量了作用在薄膜上的电磁力。结果，他们发现最大排斥力大约与临界电流密度成正比。这一趋势表明，临界电流密度可以通过测量磁体与高温超导薄膜之间的电磁相互作用来确定。在本研究中，开发了用于分析高温超导薄膜中屏蔽电流密度随时间演变的数值代码，并利用该代码对电感法和永磁体法进行了数值再现。本研究的结论总结如下：1)对于临界电流密度低于0.25 MA/cm^2的情况，电感法的精度不大于3%。否则，其准确性将显著降低。2)永磁体法的数值模拟表明，最大斥力与临界电流密度近似成正比。这些数值结果与Ohshima等人的实验结果定性一致。少

As is well known, the critical current density is one of the most important parameters for engineering applications of the high-temperature superconductor (HTS). In this sense, the nondestructive and noncontact method has been desired for measuring the critical current density of the HTS thin film.Claassen et al. proposed the inductive method for measuring the critical current density in HTS thin films. While applying an ac current in a small coil placed just above a film, they monitored harmonics of the voltage induced in the coil. As a result, they showed that the third-harmonic voltage is detected if and only if amplitude of the coil current exceeds a certain limit. This result means that the critical current density can be estimated by measuring the threshold current.On the other hand, Ohshima et al. have proposed the permanent magnet method that is widely different from the inductive method. While bringing a permanent magnet closer to a HTS thin film, they measured the electromagn … More etic force acting on the film. As a result, they found that the maximum repulsive force is approximately proportional to the critical current density. This tendency implies that the critical current density can be determined by measuring the electromagnetic interaction between the magnet and the HTS film.In the present study, the numerical code has been developed for analyzing the time evolution of the shielding current density in the HTS thin film and, by use of the code, both the inductive method and the permanent magnet method have been reproduced numerically. Conclusions obtained in the present study are summarized as follows.1)For the case where the critical current density is below 0.25 MA/cm^2, an accuracy of the inductive method is not more than 3%. Otherwise, its accuracy is remarkably degraded.2)The numerical simulation of the permanent magnet method shows that the maximum repulsive force varies in approximate proportion to the critical current density. These numerical results agree qualitatively with the experimental ones by Ohshima et al. Less