Superconductor Science and Technology

VIEWPOINT • OPEN ACCESS

Realization of compact hybrid trapped field magnet above 10 T with 7 T
applied fieldTo cite this article: In-Gann Chen 2021 Supercond. Sci. Technol. 34 110501

View the article online for updates and enhancements.

This content was downloaded from IP address 121.111.60.188 on 23/09/2021 at 12:12

Superconductor Science and Technology
Supercond. Sci. Technol. 34 (2021) 110501 (3pp)

https://doi.org/10.1088/1361-6668/ac19f2

Viewpoint

Realization of compact hybrid trappedfield magnet above 10 T with 7 T appliedfieldIn-Gann Chen
Department of Materials Science andEngineering, National Cheng KungUniversity, Tainan 70101, Taiwan
E-mail: ingann@ncku.edu.tw

Abstract

In this paper, Takahashi et al reported a successful case to reach this goal with ahybrid trapped field magnet (i.e. inner lens and outer cylinder trapped fieldmagnet). The inner lens were cooled from room temperature to about 20 K inzero field cooling followed by applied field (e.g. 7 T) that the flux density will beconcentrated to above 10 T. And in the same time the outer trapped field magnet
will be field cooled from room temperature to about 20 K in this above 10 T field.
This is a viewpoint on the letter by Keita Takahashi et al (2021 Supercond. Sci.
Technol. 34 05LT02).
Following the discovery of Y–Ba–Cu–O high temperature superconductor (HTS)in 1987 by Wu and Chu with superconducting transition temperature Tc higher
than that of liquid nitrogen boiling temperature (77 K) [1], many studies have tried
to use this Y–Ba–Cu–O HTS for high magnetic field applications. Soon, it wasdiscovered that most of the rare earth elements can replace Y in the Y–Ba–Cu–O
compound, and have similar Tc near 90 K [2]. This family of compounds are
known as rare-earth barium copper oxides, or REBCO. Other series ofcooper-oxides HTS compounds, i.e. Bi–Sr–Ca–Cu–O and Tl–Ba–Ca–Cu–O witheven higher Tc were also discovered later. Nevertheless, two basic intrinsic
materials problems related to the critical current density Jc(H,T) of HTS limited
the progress of HTS high-field applications. First, the weak magnetic flux pinning
force associated with Bi–Sr–Ca–Cu–O HTS limits the operating temperature ofhigh magnetic field applications to below 30 K [3]. Secondly, the weak link effectof the grain boundary limits the Y (or RE)–Ba–Cu–O HTS material to be in theform of an epitaxial film or single-grained monolith [4]. Single-grained monoliths
are typically referred to simply as bulks and are fabricated using a seeded
melt-growth process to obtain the required texture. The requirement of biaxial
texture has driven the development of so-called second generation (2G) or coatedconductor REBCO tapes to achieve the required Jc performance in long lengthsof conductor’. Using this 2G HTS tape, prototype hybrid magnets running at 4 K
with a magnetic field exceeding 30 T have been demonstrated in recent years [5].
This hybrid magnet consists of a 15 T low-temperature superconductor (made ofNbTi and Nb3 Sn) outer coil and 17 T inner coil (made of 2G-HTS), both of which
are operated at 4 K, weigh close to 2 tons [6].
On the other hand, single-grained monoliths have also shown significant
progress. Two RE–Ba–Cu–O monoliths, each with a diameter of 2.65 cm and athickness of 1.5 cm, cooled in the field of an 18 T magnet demonstrated theOriginal content from this work may be used under the terms of the Creative Commons
Attribution 4.0 licence. Any further distribution of this work must maintain attribution to theauthor(s) and the title of the work, journal citation and DOI.
1361-6668/21/110501+3$33.00

1© 2021 The Author(s). Published by IOP Publishing Ltd

Printed in the UK

Supercond. Sci. Technol. 34 (2021) 110501

Viewpointcapability to trap magnetic flux density exceeds 17 T at 29 K. In two reports[7, 8], a trapped field close to 10 T can be achieved at 45 K–50 K. In the report by
Tomita and Murakami [7], RE–Ba–Cu–O was impregnated with Wood’s metal
together with resin reinforced carbon fiber. And in the other report by Durrell
et al [8], the monoliths were reinforced with shrink-fit steel. REBCO monoliths
can perform either magnetic flux concentration/shielding by zero field cooling(ZFC) or flux trapping by field cooling (FC), and Durrell and Cardwell haveshown that different types of portable strong field sources can be realized [9].
The paper by Takahashi et al [10] described the detailed experimental
procedures and results of a hybrid trapped field magnet with four pieces of
RE–Ba–Cu–O bulks. A compact quasi-permanent magnetic field source near 10 T
can be realized through a 7 T external applied field together with a 20 K cryostat.
The paper by Takahashi et al is the third in a series of reports to validate theconcept of hybrid trapped field magnet lens (HTFML) in which a trapped fieldabove 10 T can be achieved using an externally applied field less than 10 T (7 T in
this report). In previous papers [11, 12], the experiment of RE–Ba–Cu–O
monoliths at 77 K and a hybrid magnet with MgB2 lens at ∼20 K andRE–Ba–Cu–O cylinder at 50 K were reported. This article also summarizes thesedevelopments.
In this latest work, Takahashi et al have successfully reached this goal with ahybrid trapped field magnet consisting of an inner lens and outer cylinder trappedfield magnet. The inner lens was cooled from room temperature to about 20 K in
ZFC followed by application of a 7 T field, resulting in a flux density
concentrated to above 10 T. At the same time, the outer trapped field magnet was
FC from room temperature to about 20 K in this >10 T field.
Since a RE–Ba–Cu–O monolith can be used as either a magnetic flux
concentration/shield by ZFC or as a flux trap by FC, the inner part of this device
acts as a lens to amplify the magnetic flux strength, and the outer part acts as anexternally applied magnetic flux source. Therefore, it is possible to realize theinternal total magnetic flux density greater than the externally applied magnetic
flux density. By appropriately controlling the cooling temperature of theRE–Ba–Cu–O inner lens to be lower than the temperature of the outer cylinder,
Takahashi’s team have realized the concept of HTFML above 10 T. Thisdifferential cooling speeds between inner and outer pieces is achieved by a special
experimental setup to have different thermal conduction between the cryostat andthe working pieces.
The authors also conducted an experiment in attempt to trap magnetic field
higher than 10 T, but failed due to RE–Ba–Cu–O monolith fracture caused by flux
quench at about 12 T. Detailed procedures and failure analysis were also reported,
indicating the need for further improvements in structural strengthening andcooling procedures.
ORCID iD

In-Gann Chen  https://orcid.org/0000-0001-7466-4238
References[1] Wu M K, Ashburn J R, Torng C J, Hor P H, Meng R L, Gao L, Huang Z J, Wang Y Q and
Chu C W 1987 Superconductivity at 93 K in a new mixed-phase Y-Ba-Cu-O compoundsystem at ambient pressure Phys. Rev. Lett. 58 908[2] Hor P H, Meng R L, Wang Y Q, Gao L, Huang Z J, Bechtold J, Forster K and Chu C W 1987
Superconductivity above 90 K in the square-planar compound system ABa2Cu3O6+x with
A = Y, La, Nd, Sm, Eu, Gd, Ho, Er and Lu Phys. Rev. Lett. 58 1891
[3] Larbalestier D, Gurevich A, Feldmann D M and Polyanskii A 2001 High transition
temperature superconducting materials for electric power applications Nature 414 368–77
2Supercond. Sci. Technol. 34 (2021) 110501

Viewpoint[4] Dimos D, Chaudhari P, Mannhart J and LeGoues F K 1988 Orientation dependence ofgrain-boundary critical currents in YBa2Cu3O7−δ bicrystals Phys. Rev. Lett. 61 219[5] Weijers H W et al 2016 Progress in the development and construction of a 32 Tsuperconducting magnet IEEE Trans. Appl. Supercond. 26 4300807[6] Weijers H W National MagLab, Oxford Instrument, SuperPower Inc. 2017 (available at:https://nationalmaglab.org/news-events/news/new-world-record-magnet-fulfillssuperconducting-promise)[7] Tomita M and Murakami M 2003 High-temperature superconductor bulk magnets that cantrap magnetic fields of over 17 tesla at 29 K Nature 421 517[8] Durrell J H et al 2014 A trapped field of 17.6 T in melt-processed, bulk Gd-Ba-Cu-O
reinforced with shrink-fit steel Supercond. Sci. Technol. 27 082001[9] Durrell J H, Ainslie M D, Zhou D, Vanderbemden P, Bradshaw T, Speller S, Filipenko M andCardwell D A 2018 Bulk superconductors: a roadmap to applications Supercond. Sci.
Technol. 31 103501[10] Takahashi K, Fujishiro H, Namba S and Ainslie M D 2021 Experimental realization of anall-(RE)BaCuO hybrid trapped field magnet lens generating a 9.8 T concentrated magnetic
field from a 7 T external field Supercond. Sci. Technol. 34 05LT02[11] Takahashi K, Fujishiro H and Ainslie M D 2018 A new concept of a hybrid trapped fieldmagnet lens Supercond. Sci. Technol. 31 044005[12] Namba S, Fujishiro H, Naito T, Ainslie M D and Takahashi K 2019 Experimental realizationof a hybrid trapped field magnet lens using a GdBaCuO magnetic lens and MgB2 bulk
cylinder Supercond. Sci. Technol. 32 12LT03

3