The electromagnetic response of superconducting tapes exposed to transport currents and/or applied magnetic fields is the key issue for electric power application of superconducting tapes. In this article, theoretical expressions on the current distributions, magnetic-field distributions, and ac losses of superconducting tapes are presented. Those expressions are derived on the basis of Bean's critical state model, and the thickness/width ratio of the superconducting layers is regarded as an infinitesimal. The analytical expressions for assembled conductors using superconducting tapes (i.e., stacks, coplanar arrays, and polygonally arranged tapes) are also presented.
Theories on the summation of randomly directed individual pinning forces to estimate the pinning force density are described based on the historical understanding of the pinning phenomena. The statistical theory clarified that the non-zero pinning force density is closely associated with the hysteresis nature of the pinning loss. The result of the dynamic theory was found to reduce to that of the statistical theory in the static limit, indicating the ergodic nature of random systems. However, the two theories failed to explain the experimental result that the threshold value of elementary pinning force to bring about non-zero pinning force density does not actually exist. Larkin and Ovchinnikov showed that the statistical summation theory assuming the long-range order of flux line lattice is not correct because of the finite pinning correlation length. However, since the pinning force density is quite simply estimated using the concept of collective pinning in the Larkin-Ovchinnikov theory, more exact theoretical estimation was needed. Next, the coherent potential approximation theory was proposed based on the mean field theory for random systems. In this theory it was shown that the threshold pinning force is not zero but always smaller than the elementary pinning force. Thus, the threshold problem was actually resolved and it was shown that the pinning loss is of a hysteresis nature even for very weak pinning centers. Observed pinning force density is compared with the predictions of the Larkin-Ovchinnikov theory and the coherent potential approximation theory.
Cubic perovskite structure has been found to play an important role for the nano-rod formation in REBa2Cu3O7-δ films. BaWO4,with a sheelite structure, and BaNb2O6, with a tungsten bronze structure, were doped into REBa2Cu3O7-δ targets. Laser-deposited, these materials formnano-rods in REBa2Cu3O7-δ films accompanied by Ln elements, resulting in the composition of a pseudo-cubic perovskite structure. This was confirmed by selected area electron diffraction patterns (SADP) and composition mapping using energy-dispersive X-ray spectroscopy scanning transmission electron microscope (EDS-STEM) analysis. BaWO4 with a sheelite structure, and BaNb2O6 with a tungsten bronze structure, doped into targets no longer retain their structures, but canform pseudo-cubic perovskite structures in laser-deposited REBa2Cu3O7-δ films. The perovskite crystal structure is thought to beimportant for nano-rod formation in the laser deposited REBa2Cu3O7-δ film.
The separation method using electromagnetic force is receiving particular attention as an elimination method of inclusions in molten metal. In this study, low-melting metal is used as a model metal and separation control by electromagnetic force is discussed. The motion of a levitated insulated lead ball in convective molten metal was studied by calculation and through experimentation. It was found that the force by convection is weaker than the electromagnetic force under experimental conditions. It was successfully demonstrated that the motion of the insulated lead balls injected into the molten metal could be controlled using an electromagnetic field.
High-temperature superconductor (HTS) cables have been intensively studied because they are more compact compared with conventional copper cables. Since it is strongly expected that HTS cables will replace conventional power lines, some HTS cables have been designed, manufactured, installed in power grids and tested to demonstrate full-time operation. Recently, a triaxial cable composed of three concentric phases was developed because of the use of fewer HTS tapes, small leakage field and low heat loss when compared to three-coaxial HTS cables. The layers inside the triaxial cable are subject to azimuthal fields applied from inner layers and axial fields applied from outer layers with different phase from their transport currents. These out-of-phase magnetic fields should be calculated under the condition of the three-phase balanced distribution of the triaxial cable, and thereby AC losses should be evaluated. In this paper, the AC loss in the triaxial HTS cable consisting of one layer per phase is theoretically treated for simplicity. The AC losses in the cable are calculated as functions of the twist pitches of HTS tapes. It is found that the AC losses rapidly decrease with increasing twist pitch.