LAUSR

Advanced Technology Research Laboratory, Nippon Steel Corporation, 20-1 Sintomi, Futtsu-shi, Chiba 293-8511, Japan E-mail: [email protected] This is a viewpoint on the letter by S Namba et al 2019 (Supercond. Sci. Technol. 32 12LT03). High-Tc bulk superconductors such as RE-Ba-Cu-O (RE: a rare earth element) and MgB2 can trap high magnetic fields at low temperatures. The magnetized bulk superconductors can be used as a compact, high strength trapped field magnet (TFM) for many practical applications. However, the trapped field is limited experimentally by the poor mechanical strength of the brittle ceramic material. A large Lorentz force is developed in the bulk during the magnetization process, which sometimes results in crack formation and propagation, leading to eventual mechanical failure. Remarkable progress has been made in increasing trapped fields with mechanical reinforcement using epoxy resin or shrink-fit stainless steel. Up to now, trapped magnetic fields over 17 T are achieved in RE-Ba-Cu-O disk pairs through field-cooling magnetization (FCM) [1, 2]. The group of Iwate University and University of Cambridge has been working on the magnetization techniques of bulk superconductors. Recently, Fujishiro et al [3] suggested the possibility to achieve a trapped field over 20 T in the RE-Ba-Cu-O disk pair by FCM method using numerical simulation. However, FCM requires a high applied field generated by a superconducting magnet, which has limitations for practical applications. Pulsed-field magnetization (PFM) is the most practical method to magnetize bulk superconductors since it is a faster magnetizing process than FCM and is an inexpensive and mobile experimental setup using a copper pulsed magnet. A trapped field of 5.2 T by PFM is reported in an HTS bulk [4]. A flux jump assisted multi-pulse magnetization has recently been reported led to a trapped field of 5.3 T in Gd-Ba-Cu-O bulk [5]. However, the maximum trapped field by PFM is still significantly lower than that achieved by FCM. The magnetic lens was reported by Matsumoto et al for the first time in 2003 [6]. In this device, magnetic flux concentration was carried out by using hollow cylinders made of lowand high-temperatures superconductors. It is important to develop compact high magnetic field systems. A magnetic lens made of a Gd-Ba-Cu-O bulk was placed in a superconducting magnet. An external field of 2 T was amplified to 5.65 T at the center of the lens, demonstrating magnetic flux concentration by a Gd-Ba-Cu-O magnetic lens [7]. A concentrated field of Bc = 12.42 T has been obtained at 20 K for a background field of 8 T using a Gd-Ba-Cu-O bulk magnetic lens, and Bc = 30.35 T at 4.2 K achieved for a background field of 28.3 T [8].