The high spatial homogeneity of critical current density, Jc, in HTS tape is one of the most important requirements to realize practical superconducting tapes. However, it is hardly possible to detect local Jc distribution using conventional characterization techniques because the length scale of such Jc variation is several orders smaller than that of the conventional techniques. We have succeeded in developing novel techniques such as low-temperature laser scanning microscopy and reel-toreel Hall probe scanning microscopy for spatially resolved measurements of local Jc in HTS tapes of multiple lengths and operating conditions. In this paper, I summarize these techniques together with other useful methods including magneto-optical imaging and the Hall sensor array technique.
Practical superconducting wires are designed with a composite structure to meet the desired engineering characteristics by expert selection of materials and design of the architecture. In practice, the local strain exerted on the superconducting component influences the electromagnetic properties. Here, recent progress in methods used to measure the local strain in practical superconducting wires and conductors using quantum beam techniques is introduced. Recent topics on the strain dependence of critical current are reviewed for three major practical wires: ITER-Nb3Sn strand, DI-BSCCO wires and REBCO tapes.
This article reviews a method for characterizing the local critical current density (Jc) distribution in superconducting tapes and wires based on scanning Hall-probe microscopy (SHPM). This method is very powerful for (1) finding the bottleneck that limits the global performance of a superconductor, (2) investigating the local inhomogeneity that may become the origin of local quench, especially in HTS applications, (3) establishing the processes for narrow and/or multifilamentary HTS conductors for reducing the magnetization in magnet applications and the AC losses in power applications. The principle and the functions of this technique are introduced by referring to characterization examples of REBCO coated conductors and an MgB2 wire.
Nondestructive, AC inductive methods detecting third-harmonic voltages are widely used to measure the distribution
of local critical current densities Jc of superconducting films deposited on large-area single-crystal substrates. We have extended
this method to determine the electric field E versus current density J relation and the n-value (index of the power-law E-J
characteristics) by evaluating the dependence of Jc on measurement frequency. The method to determine Jc with an electric-field
criterion has been established as an international standard. This convenient method can also be applied to coated conductors with
metallic substrates. Magnetic-field angular dependent Jc measurements are possible, and such measurements are of practical importance in applying coated conductors to superconducting coils. They are also useful in investigating the flux pinning mechanism. In this focused review, we briefly introduce the measuring method that has been based on our long years of research, and describe the precautionary points when recent coated conductors with high critical currents per unit width are measured.
Recently, quantum beams such as neutrons or X-rays of synchrotron radiation are being used to measure the internal strain of practical superconducting wires. These quantum beams can observe the superconducting materials in practical wires directly. The neutron and X-ray diffractions are useful methods for observing the internal strains in various practical superconducting wires. However, large experimental facilities must be visited in order to use neutron or synchrotron radiation. If it is possible to evaluate superconducting materials in practical wires at our own laboratory, it would be very useful. In this paper, I discuss the possibility of the internal strain evaluation of coated conductors using laboratory X-rays. The experiment of measuring the internal strain of a coated conductor was performed using Mo Kα. The diffraction peaks from a GdBCO in the coated conductor were observed. The behavior of the internal strain of a GdBCO in the coated conductor under tensile strain was estimated using laboratory X-rays. I found that this experimental technique is very useful for evaluating the internal strain of coated conductors and other thin superconducting tapes.