高尾 智明

<jats:title>Abstract</jats:title> <jats:p>This paper describes the first persistent-mode medium magnetic field (400 MHz; 9.39 T) nuclear magnetic resonance (NMR) magnet which uses superconducting joints between high-temperature superconductors (HTSs). As the ultimate goal, we aim to develop a high-resolution 1.3 GHz (30.5 T) NMR magnet operated in the persistent-mode. The magnet requires superconducting joints between HTSs and those between an HTS and a low-temperature superconductor (LTS). Towards this goal, we have been developing persistent-mode HTS inner coils to be operated in a 400 MHz (9.39 T) NMR magnet and here we present the first prototype inner coil wound with a single piece (RE = rare earth)Ba<jats:sub>2</jats:sub>Cu<jats:sub>3</jats:sub>O<jats:sub>7−<jats:italic>x</jats:italic> </jats:sub> (REBCO) conductor. The coil and a REBCO persistent current switch are connected with intermediate grown superconducting joints with high critical currents in external magnetic fields. To evaluate the performance of the joints in an ultimately stable and homogeneous magnetic field, the coil is operated in the persistent-mode, generating 0.1 T, in a 9.3 T background magnetic field of a persistent-mode LTS outer coil. The magnetic field drift over two years of the 400 MHz LTS/REBCO NMR magnet is as small as ∼1 ppm, giving high-resolution NMR spectra. The magnetic field drift rate over the second year was 0.03 × 10<jats:sup>−3</jats:sup> ppm h<jats:sup>−1</jats:sup>, which is more than three orders of magnitude smaller than that required for an NMR magnet, demonstrating that the superconducting joints function satisfactorily in a high-resolution NMR system. The corresponding joint resistance is inferred to be &lt;10<jats:sup>−14</jats:sup> Ω.</jats:p>

The superconducting Magnetic Energy Storage (SEMS) application still has a great potential to stabilize the utility grid when the uncontrollable power generation from renewable sources increases and power flows change rapidly due to the broad introduction of high-speed response semiconductor switching devices. Along with the development of liquid hydrogen supply chain, the SMES system using MgB₂ conductors also attracts great attention at this point. Although the MgB₂ wires which have critical temperature of around 39 K have been commercially available with more affordable prices, their bending strain sensitivity is an issue to be solved for fabricating large-scale conductors and coils. The experience of constructing a 10-kJ SMES system using Bi2223 tapes and the successful demonstration of compensating very fast electric power fluctuations in the previous project will help us to develop a larger-scale MgB₂ SMES system by investigating conductor and coil design while considering its bending strain sensitivity and mechanism of critical current deterioration to maximize its performance as one of the most promising energy storage devices, following the movement toward a CO₂-free environment.

This project aims to establish key hard technologies for large-scale high-temperature superconducting (HTS) rotating machines for a ship propulsion system. Using HTS technology, compact, right weighted and highly efficient ship propulsion motors can be realized. The objectives of the project are R&D of robust and reliable HTS field coils with low loss,and compact cooling systems integrated to a rotor and design technology for a 20 MW-class ship propulsion motor. This project is now in the first year of the stage III; stage II was completed in the last Japanese fiscal year. This report reviews the R&D results in stage II and the plans in stage III.

The 20^th Power and Energy Society Annual Conference was held on August 18-20, 2009 at Shibaura Institute of Technology. The total number of technical papers was 352, and technical sessions were 47 (46 oral sessions and 1 poster session). An invited lecture, a panel discussion, technical exhibitions and two technical tours were organized. All events were very well attended and the final enrollment attained to 881 registrations. The conference was successfully closed by the great contribution of all participants. The outline of the conference is reported in this article.

We have developed a novel critical current and stability measurement experimental setup, which utilizes a closed electric circuit with a multi-strand superconducting cable. The feature of this setup is mechanical loading applied to the multi-strand cable in the transverse direction. It was reported that Lorentz forces caused degradation in the critical current of the ITER-TFMC conductor. Furthermore, these phenomena were mainly observed in the ITER full-size conductors with large Lorentz forces under high magnetic fields. The advantage of our setup is critical current measurement under mechanical stresses comparable to those in the full-size conductor under high magnetic fields. By employing an inductive critical current measurement technique, we conducted an experiment with a transport current of about 10 kA without any power supply or current leads. In our experiments, we observed significant degradation in critical currents due to a compressive stress of about 30 MPa. We applied an innovative technique to mitigate the critical current degradation in mechanically loaded Nb₃Sn superconducting multi-strand cables. We molded one such cable with ice and tested it. No degradation occurred in the icemolded cable. In addition, stability was also ensured due to the large thermal conductivity of ice. Thus, we have successfully mitigated the degradation in the critical current of the Nb₃Sn conductor by ice molding.