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Abstract: As one of the programs in the Ministry of Economy, Trade and Industry (METI) project regarding R&D on superconducting coated conductor, three-phase superconducting fault current limiter (SFCL) for 6.6kV application was developed and successfully tested. The developed SFCL was mainly comprised three-phase set of current limiting coils installed in a sub-cooled nitrogen cryostat with a GM cryocooler, circuit breakers and a sequence circuit. The whole system was installed in a cubicle. Two tapes of coated conductor were wound in parallel in each coil to obtain the rated current of 72Arms. After developing the whole SFCL system, short circuit experiments were implemented with a short circuit generator. In a three-line ground fault test, the SFCL successfully restricted the prospected short circuit current over 1.6kA to about 800A by the applied voltage of 6.6kV. The SFCL was installed in a user field and connected with a gas engine generator, followed by a consecutive operation. In this program, 600A class FCL coil, with which four coated conductor tapes were wound, was also developed. The coil showed sufficiently low AC loss at the rated current. With these results, the program attained the planned target of the fundamentals for the 6.6kV/600A SFCL. [Copyright &y& Elsevier]

Abstract: One of the programs in the Ministry of Economy and Trade and Industry (METI) project regarding R & D on YBCO conductor is to evaluate the applicability of the developed conductor toward several applications. This paper focuses on a fault current limiter (FCL) as one of the expected power applications. YBCO tape conductors with ion beam assisted deposition (IBAD) substrate are used in this work. In order to obtain high resistance of the conductor, which is preferable to an FCL, the thickness of the protecting layer made of silver was decreased as possible. Then high-resistive metal stabilizing layer is attached on the silver layer to improve stability. Obtaining the relevant current limiting performance on short sample experiments, model coils were developed to aim the 6.6kV-class FCL. Short circuit experiments were implemented with a short circuit generator. The coil successfully restricted the short circuit current over 17kA to about 700A by the applied voltage of 3.8kV, which is nominal phase-to-ground voltage. The experimental results show good agreement with computer analyses and show promising toward the application. [Copyright &y& Elsevier]

The magnetic drug delivery system (MDDS) is a key technology to reduce the side effects in the medical applications, and the magnetic force control is a very important issue in MDDS. In general, the high magnetic field along the axial direction and high magnetic field gradient along the longitudinal direction are very useful for MDDS. So, we proposed the new magnetic force control system that consists of superconducting magnet, high temperature superconductors (HTS) bulks or tapes and ferromagnetic substances. In this new system, the shielding currents in HTS bulks and tapes due to diamagnetism of the superconductor generate the magnetic field with same direction of external magnetic field, and we can control the magnetic field gradient along the longitudinal direction by the arrangement of the HTS bulks, tapes and ferromagnetic substances. In this study, the stacked GdBCO tapes without stabilizer were used in experiments and electromagnetic analysis based on finite element method (FEM) was carried out to optimize the arrangement of the superconductors and the irons. It was confirmed that the control of magnetic field gradient along the longitudinal direction was possible by arranging the HTS bulks, tapes and irons on the superconducting magnet and then it was possible to obtain a magnetic field higher than the external field. [ABSTRACT FROM AUTHOR]

Highlights: [•] We reveal the mechanism of “self-healing” effect of uninsulated REBCO coils. [•] Uninsulated REBCO pancake coil shows “self-healing” from thermal runaway. [•] Current mode transition from “multi-turn” mode to “single-turn” triggers this effect. [Copyright &y& Elsevier]

Highlights: [•] We developed a 4μm-thick ultra-thin polyimide coating on REBCO conductors. [•] The developed coating improves the fraction of conductor in the coil to 90%. [•] The coating dramatically increases the overall current density of REBCO coils. [Copyright &y& Elsevier]

Abstract: Degradation of the epoxy impregnated YBCO coil performance is due to tensile radial stress concentration on the outer edge of the conductor during cool down. This stress acts as a cleavage stress and opens the conductor edge, fracturing the YBCO layer. The fracture propagates to another edge of the conductor, resulting in degradation of the coil performance. Degradation of the epoxy impregnated YBCO coil is eliminated, if we use a polyimide-electrodeposited YBCO-coated conductor: tensile radial stress concentration on the outer edge of the conductor is reduced due to plastic deformation of the ductile polyimide. Polyimide electrodeposition onto the YBCO-coated conductor is reliable, uniform, easy to apply, and can be extended to larger YBCO magnets, removing the risk of coil degradation. [Copyright &y& Elsevier]

Abstract: Cleavage strength for an YBCO-coated conductor at 77K was investigated with a model experiment. The nominal cleavage strength for an YBCO-coated conductor is extremely low, typically 0.5MPa. This low nominal cleavage strength is due to stress concentration on a small part of the YBCO-coated conductor in cleavage fracture. Debonding by the cleavage stress occurs at the interface between the buffer layer and the Hastelloy substrate. The nominal cleavage strength for a slit edge of the conductor is 2.5-times lower than that for the original edge of the conductor; cracks and micro-peel existing over the slit edge reduce the cleavage strength for the slit edge. Cleavage stress and peel stress should be avoided in coil winding, as they easily delaminate the YBCO-coated conductor, resulting in substantial degradation of coil performance. These problems are especially important for epoxy impregnated YBCO-coated conductor coils. It appears that effect of cleavage stress and peel stress are mostly negligible for paraffin impregnated YBCO-coated conductor coils or dry wound YBCO-coated conductor coils. [Copyright &y& Elsevier]

Abstract: Now that YBCO-coated conductors have been commercialized, a number of YBCO coils have been developed. However, their basic performances have not been systematically investigated so far. Here, we demonstrate that of a YBCO double pancake coil. The critical current of an epoxy impregnated YBCO double pancake coil was substantially degraded, i.e. the normal voltage appears above 8A, only 18% of that for the dry coil. It was inferred that degradation occurs if the cumulative radial stress developed during cool down exceeds the critical transverse stress for the YBCO-coated conductor (typically 10MPa). Under these conditions, the conductor was debonded at the interface between the buffer layer and YBCO layers, or fractured in the YBCO layer itself, causing cracks on the YBCO layer, resulting in a significant decline of the critical current. These negative effects are suppressed if the coils are dry wound or impregnated with paraffin, as the bonding strengths between turns are negligible and therefore turns are separated if the cumulative radial stress tends to be tensile. For non-circular coils in which epoxy impregnation is inevitable, degradation due to cumulative tensile transverse stress is still the major problem. [ABSTRACT FROM AUTHOR]

Abstract: The effect of current sweep reversal on the temporal drift in magnetic field intensity for a Bi-2223 solenoid was investigated by experiment and using numerical simulation. Current sweep reversal, by as small as 1% of the peak current, was found to stabilize the drift in magnetic field intensity for a Bi-2223 tape solenoid. The field drift was due to flux creep in the Bi-2223 tape and the current sweep reversal formed a barrier for flux entrance at the upper and lower surface of the conductor, preventing flux creep. With a current reversal of several% of the peak current, the barrier formation extended over half of the solenoid and the magnetic field intensity became constant with time. The current sweep reversal technique should prove useful to stabilize an ultra-high field low/high-temperature superconducting nuclear magnetic resonance magnet operated at frequencies (field intensities) beyond 1GHz (23.5T). [Copyright &y& Elsevier]

Abstract: The screening current-induced magnetic field in the (Bi,Pb)2Sr2Ca2Cu3O x (Bi-2223) insert coil proposed for a beyond 1GHz nuclear magnetic resonance (NMR) spectrometer may generate a long-term field drift, resulting in a loss of field-frequency lock operation and an inability to make high resolution NMR measurements. The measured screening current-induced magnetic field of a Bi-2223 double-pancake coil exhibits a hysteresis effect at 4.2K that is reproduced by a numerical simulation based on a finite thickness rectangular superconductor bar model. The screening current-induced field at the coil center is of opposite polarity to that generated by the coil current, and thus the apparent field intensity shows a positive drift with time. On the contrary, the field at a coil end is of the same polarity as the coil field, and the apparent field intensity decreases with time. If we wait for ∼1000h after coil excitation, the field drift rate approaches the field decay rate of the persistent current of 10−8 h−1, suitable for a long-term NMR measurement in a beyond 1GHz NMR spectrometer. [Copyright &y& Elsevier]

Abstract: We investigated current transport properties in MgB2 wires subjected to transient application of magnetic field, and then discussed their applicability to magnetically controlled switching elements. These wires were fabricated by powder-in-tube (PIT) technique, and Cu–Ni as well as stainless was utilized for their sheath material. A short sample of the wire was installed in a Gifford–McMahon (GM) cryocooler, and was impregnated with solid nitrogen for the sake of thermal stabilization. After stabilizing the temperature of the sample at a certain bias current, the voltage and the temperature in between potential taps were monitored with a transient application of magnetic field. Such measurements were repeated systematically as functions of bias current, operating temperature and magnetic field intensity. It was shown that the sample quickly generated resistance without temperature increment by applying the transient and weak magnetic field. This indicates the applicability of MgB2 wires to persistent current switches (PCS) with quick response. [Copyright &y& Elsevier]