A high-strength pancake-coil structure called “Yoroi-coil" (Y-based oxide superconductor and reinforcing outer integrated coil), in which superconducting wire and the reinforcing outer plates of the coil withstand electromagnetic force, was developed for high magnetic field applications. The double-pancake (DP) coil of this coil structure was prepared and verified for durability against electromagnetic forces using the hoop stress test. The coil achieved 1.5 kA transporting at 4.2 K in an 8 T backup magnetic field without degradation of transport properties. Maximum hoop stress at the hoop stress test reached 1.74 GPa, based on the calculations. This result confirmed that the “Yoroi-coil" structure has the capability to withstand large hoop stress, which exceeds the tensile strength of Y-based coated conductors, not only owing to the strength of superconducting wire but also owing to the integration of the whole coil structure itself. Additionally, the strengthening effect of the structure was verified by analysis.
REBa2Cu3O7 (REBCO)-coated conductors can raise the operating temperature of magnets equipped for use in maglev applications. High operating temperature enables cryogen-free cooling and downsizing of the power source. A REBCO magnet has been fabricated to demonstrate that magnetic flux density of 5 T, which is the same level as that of the existing magnet equipped, is achievable at high temperatures above 40 K. The demonstration magnet has no radiation shield. Therefore, the gap between the superconducting coil and the cryostat is only 5 mm long. In the case of maglev applications, this corresponds to an increase in interlinkage magnetic flux.
REBa2Cu3Ox (REBCO, RE = rare-earth) coated conductors are expected to show high performance in superconducting applications because of their high mechanical strength and high current density in magnetic fields. More characteristics data of REBCO coils with larger diameters are required for use of these conductors in practical superconducting applications. We fabricated a cryo-cooled high-temperature superconducting (HTS) magnet, which was composed of 24 pancake coils with an inner diameter of 260 mm, using REBCO coated conductors. The stored energy of the magnet was 426 kJ. The total length of the REBCO coated conductors was approximately 7.2 km. These conductors were laminated with a copper stabilizer and were fabricated by Fujikura Ltd. using ion-beam-assisted-deposition (IBAD) and pulsed-laser-deposition (PLD) methods. Before fabricating the magnet, all pancake coils were evaluated in terms of their V-I characteristics in liquid nitrogen in order to confirm the characteristics without degradation. After the fabrication, the magnet was cooled down to 24 K using a GiffordMcMahon (GM) cryocooler. We were successful in excitation of the magnet up to 5 T. Furthermore, we confirmed that a 1% current reversal of the coil current could stabilize the axial central magnetic field drift of the magnet.
We developed a cryocooled YBCO vector magnet system for X-ray magnetic circular dichroism (XMCD) measurements. This system can generate a two-dimensional vector-controllable magnetic field in any chosen direction using two external power supplies and vary the photon polarization. The magnet mounted in the system is cryogen-free and conduction-cooled, and consists of two-axial split coils. The material of the coils is YBCO tape conductor. The peak vector magnetic field of 1.2 T is obtained. In this report, we describe specifications of the system, cooling and operating test results and XMCD measurement results using the system.
We are developing conduction-cooled REBCO superconducting coils. We first observed thermal runaway currents of the coils during the process of making impregnated single-pancake coils with an outer diameter of 300 mm with REBCO- coated conductors and conduction-cooling them using a GM cryocooler. In addition, we performed a numerical simulation of the electromagnetic and thermal behavior of the coils with the critical current properties of the REBCO tapes through use of a threedimensional heat conduction equation. Comparing the observed results with the theoretical prediction, we confirmed the theoretical predictability of the thermal runaway currents of the single-pancake coils. These results suggest that the winding and impregnation were successfully carried out without degradation of the properties of the prepared REBCO tapes. Further, in testing conducted on an impregnated small coil in a 4 T background magnetic field, we confirmed that the impregnated coils could withstand hoop stress as high as the mechanical strength of the REBCO tape itself.