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.

REBCO coated conductors are now available from several industrial manufacturers and are expected to be promising conductors for high-field-magnet applications. Using these conductors, the development of solenoids capable of generating high magnetic fields of 20–30 T is ongoing in major high-field laboratories in the world. In addition, CERN recently launched a conceptual design study for the Future Circular Collider, in which a 20-T dipole magnet is listed as a candidate for the bending magnet of the main ring. However, there has been limited research published on the electrical transport properties of commercially available REBCO conductors in a high-field, low-temperature environment. For magnet designers, the transport properties are of the highest importance in choosing a suitable conductor, and the data form the bases for high-field magnet development. Therefore, in this work, a new sample holder, which allows the measurements of full-width conductors to be carried out relatively easily, was developed, and the transport properties of commercial REBCO conductors from seven manufacturers (AMSC, Fujikura, Shanghai Superconductor, SuNAM, SuperOx, SuperPower, and SWCC Showa) were investigated at 4.2 K in perpendicular fields of up to 18 T. The results show that the Ic values at 4.2 K clearly vary to some extent among these commercial conductors and the higher-current 4-mm-wide conductors have Ic values in the range of 230–305 A at 18 T and in the range of 320–424 A at 12 T.