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Experimental realization of an all-(RE)BaCuO hybrid trapped fieldmagnet lens generating a 9.8 T concentrated magnetic field from a 7 T
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Superconductor Science and Technology
Supercond. Sci. Technol. 34 (2021) 05LT02 (9pp)

https://doi.org/10.1088/1361-6668/abeb01

LetterExperimental realization of anall-(RE)BaCuO hybrid trapped fieldmagnet lens generating a 9.8 T
concentrated magnetic field from a
7 T external field
Keita Takahashi1,∗, Hiroyuki Fujishiro1, Sora Namba1 and Mark D Ainslie2
1

Department of Physical Science and Materials Engineering, Faculty of Science and Engineering, Iwate
University, Morioka 020-8551, Japan2
Bulk Superconductivity Group, Department of Engineering, University of Cambridge, Cambridge CB2
1PZ, United Kingdom
E-mail: s3119004@iwate-u.ac.jp and fujishiro@iwate-u.ac.jp
Received 11 December 2020, revised 12 February 2021
Accepted for publication 1 March 2021
Published 19 March 2021
Abstract

In this work, we have verified experimentally an all-(RE)BaCuO hybrid trapped field magnet
lens (HTFML) using only one cryocooler and a special technique named the ‘loose contact
method’. In the experimental setup, only the inner magnetic lens was tightly connected to thecold stage and cooled at all times, and the outer trapped field magnet (TFM) cylinder was
loosely connected to the cold stage before the magnetizing process by introducing a gap
between the outer TFM and cold stage of the cryocooler. As a result, the superconducting statefor zero-field cooled magnetization of the inner magnetic lens and the non-superconducting(normal) state for field-cooled magnetization of the outer TFM cylinder can co-exist at the sametime. A maximum concentrated field of Bc = 9.8 T was achieved for the magnetizing processwith an applied field of Bapp = 7 T in the present HTFML, consistent with the numericalestimation in our previous conceptual study. These results validate the HTFML concept as acompact and desktop-type magnet device that can provide 10 T-class magnetic fieldenhancement from the viewpoint of the magnetizing method. However, during magnetization
with a higher Bapp of 10 T, thermal instability of the outer stacked TFM cylinder caused flux
jumps to occur, resulting in mechanical fracture of multiple bulks. These results suggest that thefurther development of a practical cooling method that can realize a stable and controllable∗

Author to whom any correspondence should be addressed.

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

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Supercond. Sci. Technol. 34 (2021) 05LT02

cooling process for each part of the HTFML is necessary based on fundamental studies relating
to the thermal stability of the large stacked TFM cylinder.
Keywords: hybrid trapped field magnet lens, bulk superconductors, trapped field magnets,
magnetic lens, vortex pinning effect, magnetic shielding effect, loose contact method(Some figures may appear in colour only in the online journal)

1. Introduction

be used to control the temperature of each bulk part individually, such as the use of two cryocoolers, a thermal (heater)
method or a mechanical switch [11]. Hence, the magnetizing
sequence of the HTFML includes two magnetizing methods:
ZFCM for the inner magnetic lens and FCM for the outer TFM
cylinder. As a first step, we have realized the HTFML based on
the same (RE)BaCuO bulk material for both parts using liquid
nitrogen, in which the inner magnetic lens and outer TFM cylinder were housed in separate containers with liquid nitrogen
poured into each container sequentially according to the corresponding magnetizing sequence. As a result, Bc = 1.83 T
was obtained at 77 K after magnetization with Bapp = 1.80 T[12]. However, to best exploit the trapped field capability of theouter (RE)BaCuO cylinder and realize a higher Bc over 10 T,
the all-(RE)BaCuO HTFML should be cooled to a lower temperature below 50 K and magnetized, where the J c (B, T) characteristics have a higher value under higher magnetic fields.
Furthermore, if the HTFML device can be realized using one
cold stage of a cryocooler, the whole system would be morecost-effective, and provide both a higher magnetic field andmagnetic field gradient in an open bore space outside thevacuum chamber [13].
In this work, we have verified experimentally an all(RE)BaCuO HTFML magnetized under 50 K using only one
cryocooler by a special technique, in which only the innermagnetic lens was tightly connected to the cold stage andcooled at all times, and the outer TFM cylinder was loosely
connected to the cold stage before the magnetizing process byintroducing a gap between the outer TFM cylinder and cold
stage. During the cooling process before magnetization, theouter TFM cylinder then cooled much slower than the innermagnetic lens. As a result, the non-superconducting (normal)
state for the outer TFM cylinder and the superconducting state
for the inner magnetic lens can be simultaneously realized.
A maximum concentrated field of Bc = 9.8 T was achieved
for the magnetizing process from Bapp = 7 T in the present
HTFML, consistent with the numerical estimation in our previous conceptual study [9]. The concentrated field values arecompared for other HTFML devices reported. These results
validate the HTFML as a compact and desktop-type magnet
device that can provide 10 T-class magnetic field enhancementfrom the viewpoint of the magnetizing method.

To provide a higher magnetic field in more cost-effective
way, large single-grain bulk superconductors such as the(RE)BaCuO (RE: rare earth elements or Y) family of materials are known to be a promising for generating magnetic
fields of several Tesla or more as so-called trapped field magnets (TFMs). This TFM can provide the trapped field quasipermanently without any additional current source once it ismagnetized by exploiting the ‘vortex pinning effect’, and only
needs to be cooled below a superconducting transition temperature, T c . To date, the highest trapped field of BT = 17.6 T
has been achieved in a two-stack GdBaCuO bulk pair magnetized by field-cooled magnetization (FCM) [1, 2]. According to Bean’s critical state model, where the critical current
density of the bulk, J c , is assumed to be field-independent, thetrapped field of a disc-shaped TFM by FCM increases in proportional to its diameter and J c . Hence, many of conventional
approaches for the trapped field enhancement have focused on
the crystallization process, resulting in a higher and uniformJ c of the bulk material [3, 4]. In this sense, it is expected thatthese compact and strong bulk magnets would replace a conventional magnetic field source in applications such as rotatingmachines [5], Lorentz force velocimetry [6] and nuclear magnetic resonance apparatus [7]. On the other hand, the magnetic
lens utilizing the same bulk material exploits its ‘diamagnetic
shielding effect’ nature to concentrate a magnetic field from anexternal coil magnet. Zhang et al achieved a concentrated fieldof Bc = 13 T at 20 K using a GdBaCuO bulk lens by zero-fieldcooled magnetization (ZFCM) with an external applied field of
7 T [8].
In 2018, the authors proposed a new concept of a hybrid
TFM lens (HTFML), in which the inner bulk magnetic lens canconcentrate the trapped field provided from the outer TFM cylinder, resulting in a concentrated trapped field higher than theexternal magnetizing field required for magnetization that persists even after removal of the external field [9]. In the paper,a higher trapped field of Bc = 13.49 T was predicted at 20 K
for the applied field of Bapp = 10 T using inner (RE)BaCuO
lens and the outer (RE)BaCuO TFM cylinder, and Bc = 4.73 T
was predicted at 20 K for Bapp = 3 T using inner (RE)BaCuO
lens and the outer MgB2 TFM cylinder. This concept was first
verified experimentally using different bulk materials, i.e. an
inner GdBaCuO lens (T c = 92 K) and an outer MgB2 TFM
cylinder (T c = 39 K) and exploiting the difference in T c . For
this design, a maximum concentrated field of Bc = 3.55 T wasachieved reliably with an applied field of Bapp = 2 T at 20 K
[10]. In the case of the HTFML using an inner (RE)BaCuO
lens and outer (RE)BaCuO cylinder, a special technique must

2. Experimental setup and magnetizing sequence
Figure 1(a) presents the experimental setup of the HTFML,
in which the inner magnetic lens and the outer TFM cylinder
are encapsulated in a stainless steel (SS) holder for mechanical2Supercond. Sci. Technol. 34 (2021) 05LT02

Figure 1. (a) Experimental setup of the HTFML, in which the inner magnetic lens and outer TFM cylinder, both made from (RE)BaCuO,

are encapsulated in a stainless steel (SS) holder for mechanical reinforcement. The side and top views of the actual setup are also presented
in the photographs. Schematic views of (b) the outer TFM cylinder and (c) the inner magnetic lens.

reinforcement. Side and top views of the actual setup are alsopresented in the photographs. Figures 1(b) and (c), respectively, show the side and top views of each bulk componentof the outer TFM cylinder and the inner magnetic lens. Allof the bulks were fabricated by the QMGTM method (Nippon Steel Corporation, Japan) [14]. The outer TFM cylinder was constructed using three stacked EuBaCuO ring bulks(top bulk, middle bulk and bottom bulk). The dimensions of
the EuBaCuO cylinder are a 60 mm outer diameter (OD),
36 mm inner diameter (ID) and 54 mm height (H). Each of the
EuBaCuO ring bulks was reinforced by an Al alloy ring 5 mmin thickness (OD = 70 mm, ID = 60 mm) adhered by a thinlayer of epoxy resin. These bulks were also encapsulated in the
SS holder with a ‘hat structure’ 5 mm in thickness for mechanical reinforcement against the Lorentz force generated during magnetization. Such mechanical reinforcement is necessary to avoid fracture of the bulks, particularly for appliedfields as high as 10 T. The effect of the ‘hat structure’ of theouter SS holder on the mechanical reinforcement was investigated by numerical simulation, which can provide a compressive stress around 100 MPa from the cooling process due tothe difference in the thermal expansion coefficient between
the bulk material and SS holder [15]. The effect was experimentally confirmed to avoid fracture of a ring bulk for FCM
from Bapp = 10 T in [16]. As shown in figure 1(c), the innermagnetic lens (OD = 36 mm, ID = 10 mm, ID2 = 26 mm,
IH = 8 mm, OH = 30 mm) was constructed using two pieces ofconical-shaped GdBaCuO bulks, in which a slit with 200 µm
width is inserted in the diagonal direction so that the magnetic
flux can be concentrated into the central bore through the slit.
The TFM cylinder and magnetic lens are the same used in theprevious experiment at 77 K [12]. Note that only the inner lenscomponent, including the SS holder and a copper spacer, wastightly connected to the cold stage through a thin indium sheet.
The temperature of the TFM cylinder, magnetic lens and thecold stage of the GM-cycle helium cryocooler was monitored
by two CernoxTM thermometers and a Pt–Co thermometer,
as shown in figure 1(a). The whole device was enclosed in a
vacuum chamber. A cryocooled 10 T superconducting magnet(JASTEC, JMTD-10T100) was utilized to ramp up/down theexternal magnetizing field. The concentrated field was monitored at the center of the lens by an axial-type Hall sensor(F.W. Bell, BHA921), and the external magnetic field was
calculated by measuring the electric current through a shunt
resistor.
Figure 2 shows the time sequence of the temperature forboth bulk parts—the inner magnetic lens, T L , and the outer
TFM cylinder, T T —and the external field, Bex , and the concentrated magnetic field, Bc , at the center of the HTFML, which
has been revised for these experiments in comparison to theoriginal conceptual study [9]. To delay the cooling rate of theouter TFM cylinder quasi-independently even using one cold
stage of the cryocooler, a special technique named the ‘loose
contact method’ has been designed. Figure 3 shows the conceptual view of the ‘loose contact method’, exploiting two
kinds of experimental configurations of the HTFML in thevacuum chamber, aligned in (a) the horizontal direction during
the cooling process and (b) the vertical direction during magnetizing process. In the initial preparation of the experimental
setup of the HTFML on the cold stage, the outer TFM cylinder
was loosely connected to the cold stage—with a 1 mm gap—
by screws and then installed in the vacuum chamber. During3Supercond. Sci. Technol. 34 (2021) 05LT02

constructed from the following process from (1) to (5), whichis special for the HTFML exploiting the gap for the outer TFM
cylinder.
(1) During the cooling process, where the whole HTFML
device is aligned in the horizontal direction: the temperature of the inner magnet lens, T L , (as well as that of thecold stage, T s ) is lowered from 300 K to the lowest temperature as possible (roughly, a minimum of T L ∼= 20 Kin the present experiment). At the end of this stage, theinner magnet lens is in the superconducting state, but theouter TFM cylinder is in the normal state (>T c ) due to its
delayed cooling speed.
(2) Ascending stage of the magnetizing process (while thecooling process proceeds), where the whole HTFML
device is stood upright and aligned in the vertical direction, and then inserted in the superconducting magnet: theexternal magnetic field, Bex , is ramped up linearly to Bappat a certain rate (+0.1 T min−1 . in the present experiment),
which corresponds to the ascending stage of ZFCM for the
inner magnetic lens. The magnetic field is concentrated
in the bore of the magnetic lens, which would be higher
than Bapp because of the shielding effect by the magnetic
lens. The temperature of the outer TFM cylinder should
be kept over T c until Bapp completely penetrates the outer
TFM cylinder. It is desirable that the difference in temperature between the outer TFM cylinder, T T , and T L ,∆T (=T T −T L ), be over 50–100 K just before magnetization, which enables the separation of the magnetization
processes of ZFCM for the magnetic lens and FCM for
the TFM cylinder in the later stage.
(3) Cooling process in a static magnetic field: T T is thendecreased gradually to the lowest T T ∼= 50 K, below T c
but higher than T L , due to the imperfect thermal contact tothe cold stage through the gap ∼=0 mm. The Bc value canbe maintained reliably if T L is kept at 20 K.
(4) Descending stage of the magnetizing process at the lowest
temperature: Bex is decreased linearly down to zero at a
certain rate (−0.1 T min−1 , or −0.002 T min−1 . for higher
Bapp over 7 T, in the present study). During this process, theouter TFM cylinder is magnetized by FCM and a magnetic
field is trapped inside the TFM cylinder within its trappedfield capability based on its J c (B, T) characteristics (if thiscan be done successfully without the occurrence of large
flux jumps caused by the resultant heat generation and/or
mechanical fracture of the bulk material).
(5) As a result, the HTFML can reliably generate Bc higher
than the trapped field of the single cylindrical TFM, as wellas Bapp , even after Bex = 0.

Figure 2. Time sequence of (a) the temperature T (the outer TFM

cylinder, T T , and the inner magnetic lens, T L ) and (b) the magnetic
field B (external field, Bex , and concentrated magnetic field, Bc at thecenter of the present HTFML using all-(RE)BaCuO bulks) during
the magnetizing process divided into steps from (1) to (5). The
magnetizing applied field, Bapp , corresponds to the maximum valueof Bex .

the cooling stage of (1) in figure 2, the cooling process wasstarted, and several hours later, the whole HTFML device wastilted 90◦ along the horizontal direction using a hand-held rod
attached to the bottom of the apparatus. In this stage, as shownin figure 3(a), a non-uniform gap, i.e. 0–1 mm, was introduced
at the interface between the TFM cylinder and the cold stage
due to its own weight. This method worked sufficiently as todelay the cooling rate of the TFM cylinder in contrast to that of
the inner magnetic lens, which is connected to the stage tightly
without a gap. Before proceeding to the magnetization in theascending stage of (2) in figure 2, the whole HTFML device
was aligned vertically, where the gap between the TFM cylinder and the cold stage would be ∼=0 mm and the TFM cylinder
would contact loosely with the cold stage by its own weight,
as shown in figure 3(b). At stage (2), the inner magnetic lensis in the superconducting state (T L < T c ) and ZFCM is performed, but the outer TFM cylinder must be in the normal
state (T T > T c ). As mentioned before, the inner lens component was tightly connected to the cold stage and the temperature can be precisely controlled. In the magnetizing process,the whole HTFML device in the vacuum chamber was lifted
up and inserted into the bore of the superconducting magnet
as shown in figure 3(c).
In figure 2, the magnetizing applied field, Bapp , corresponds
to the maximum value of Bex . The magnetizing sequence was

Figure 4 shows the typical experimental results for the cooling process of the present HTFML exploiting the ‘loose contact method’, in which the HTFML device set in the vacuum
chamber was reclined up after t = +11 h, from the horizontal
direction to the vertical direction. The maximum ∆T ∼= 100 K
was obtained at t = +8 h after beginning the cooling process.

4Supercond. Sci. Technol. 34 (2021) 05LT02

Figure 3. Conceptual view of the ‘loose contact method’, exploiting two kinds of experimental configurations of the HTFML in the vacuum

chamber, aligned in (a) the horizontal direction during the cooling process and (b) the vertical direction during magnetizing process. (c) The
whole HTFML device in the vacuum chamber lifted up and inserted into the bore of the superconducting magnet in the magnetizing process.

TFM cylinder, T T , and the cold stage, T s —and the concentrated field, Bc , and the external field, Bex , during magnetization with several applied fields, Bapp , of (a) 3 T, (b) 5 T and (c)7 T, respectively. The lowest value of T L and T T for magnetization process is included in the bottom panel of each figure.
In the case of Bapp = 3 T, as shown in figure 5(a), the Bex
value was ramped up at t = +8.5 h, when the maximum temperature difference ∆T ⩾ 100 K was obtained reliably after
cooling to T L = 40 K and T T = 150 K. The lowest temperatures were, respectively, T L = 24 K for ZFCM and T L = 57 K
for FCM. Resultantly, Bc = 5.5 T was achieved with little flux
creep caused by the non-linear electrical property of the bulk
material, even well after FCM process at t = +24 h. Compared with the higher Bapp cases of 5 T and 7 T, as shown infigures 5(b) and (c), it was confirmed that the ∆T value variedin the range of 50–100 K, and the lowest temperatures of T L
and T T were not consistent because the non-uniform thermal
contact through the gap during the initial cooling stage is notcontrollable precisely. It is also noteworthy that the present
system does not require the precise adjustment of the temperature, but only needs the outer TFM cylinder to be kept above
its T c until the ascending stage of ZFCM for the inner magnetic lens is completed. Higher Bc values of 7.9 T and 9.8 T
were achieved for Bapp = 5 T and 7 T, respectively. Theseexperimental results successfully verified the numerical estimations for the 10 T-class HTFML, up to Bapp = 7 T, for the
first time [9]. These values will be compared with conventional HTFMLs at the end of this section. It should be notedthat the experimental T T value was not constant and gradually
increased due to the heat generation caused by the movement

Figure 4. Typical experimental results of cooling process of thepresent HTFML exploiting the ‘loose contact method’, in which the
HTFML device set in the vacuum chamber was reclined up at
t = +11 h from the horizontal direction to the vertical direction.

It was also confirmed that ∆T would change depending on the
degree of inhomogeneous contact through the gap. The lowest
temperatures were T T = 54 K, T L = 24 K and T s = 14 K foreach point on the outer TFM cylinder, inner magnetic lens andunder the cold stage, respectively.
3. Experimental results
Figure 5 shows the time dependence of the temperatures measured at each position—the inner magnetic lens, T L , the outer5Supercond. Sci. Technol. 34 (2021) 05LT02

Figure 5. Time dependence of the temperatures measured at each position—the outer TFM cylinder, TT , the inner magnetic lens, T L , andthe cold stage, T s —and the concentrated field, Bc , and the external field, Bex , during magnetization with several applied fields, Bapp , of(a) 3 T, (b) 5 T and (c) 7 T, respectively.

of magnetic flux during FCM of the outer TFM cylinder, which
was remarkably large for higher Bapp . This thermal instability
might come from the possibility of larger heat generation in
the stacked TFM cylinder, as well as the slower cooling speed
and efficiency of the loose contact method in the present setup.
Figures 6(a) and (b), respectively, show similar results ofthe time dependence of the temperature and magnetic field
during magnetization with an applied field of Bapp = 10 T.
During the ZFCM process until t = +14 h, Bc = 13.9 T was
generated at the center of the HTFML device with a background field of Bex = 10 T, which means that the maximum of
Bc might be over 13 T, if the outer TFM cylinder could replace
external magnetizing magnet after the FCM process. Unfortunately, during the descending stage of the FCM process, theT T value increased and stayed as high as 68 K, and then the Bc
value suddenly dropped at Bex = 5 T due to the occurrence ofa large flux jump at t = +18 h, at which the temperatures also
abruptly increased to above T c . After that, the Bc gradually
decreased with decreasing Bex and the final Bc value became
negative. Such a negative Bc value is typically obtained in thecase for the single magnetic lens, in which the magnetic flux
is trapped in a part of magnetic lens after completing the conventional ZFCM process [8]. These results indicate that thethermal instability of the outer TFM cylinder could result in aflux jump during FCM and/or the mechanical fracture of thebulk material. To realize a Bc value over 10 T, further investigations are required relating to the following issues: thermal
stability of the large, stacked TFM cylinder, and developmentof a more practical cooling method that can achieve a stable
and controllable cooling process for each part of HTFML.

Figure 7 summarizes the magnetization curves of the
HTFML magnetized from Bapp = 3, 5, 7 and 10 T, which cancompare the difference of the lens effect during the ascending stage, and the flux creep and the final Bc value in the descending stage. In the ascending stage, for every Bapp value, theBc −Bex relation shows an identical trend, which indicates thegood reliability of the shielding effect by the magnetic lens.
However, in the descending stage for Bapp values of 7 T and
10 T, the flux creep seems clearly due to the large temperature rise during FCM (up to 68 K) in the stacked TFM cylinder. In the case of Bapp = 10 T, a flux jump occurred during
the descending stage that may be of mechanical or thermal
nature, which will be discussed later. If this flux jump did notoccur under a constant and stable temperature during magnetization from Bapp = 10 T, it is expected that Bc = 13 T would
be achieved.
To find out whether mechanical fracture occurred after themagnetization of the HTFML from Bapp = 10 T, the innermagnetic lens and outer TFM cylinder were magnetized separately again. Figure 8 shows the magnetization curve of themagnetic lens by itself during the ascending stage of ZFCM
with Bapp = 3 T and T L = 30 K, after the occurrence ofthe flux jump during the magnetization of the HTFML from
Bapp = 10 T (labeled ‘After’). A similar result before theflux jump is shown for comparison (labeled ‘Before’; seefigure 5(a)). The measured Bc was enhanced (with respect toBex ) by the magnetic lens and increased in proportional to Bex ,consistent with the ‘Before’ results. This result clearly showsthat there was no fracture in the inner magnetic lens after theflux jump.
6Supercond. Sci. Technol. 34 (2021) 05LT02

Figure 8. Magnetization curve of the single magnetic lens during

the ascending stage of ZFCM with the applied field of Bapp = 3 T,
after the happening of the flux jump during the magnetization of the
HTFML from Bapp = 10 T, labeled as ‘After’. The result before theflux jump is referred as ‘Before’ from figure 5(a).

top surface view of the bottom bulk, in which the crack penetrates along the 10 o’clock direction as depicted. In addition,
it was confirmed that there is an identical trace of burning at
the interface between the top surface of the bottom bulk andthe bottom surface of the middle bulk, as marked in each picture. The degree of fracture behavior was determined fromthe results of the trapped field profile into three-levels: notbroken (top bulk), partially broken (middle bulk), and completely broken (bottom bulk), respectively. The bottom bulk, inwhich the crack destroyed the circumferential current, showed
the so-called ‘C-shaped’ magnetic field profile on both surfaces of the bulk and there is no longer the remnant trappedfield inside the central region, i.e. BT = 0 T. In contrast, only
the top bulk showed a comparative, uniform field profile with apeak value of BT = ± 0.32 T at the center of both surfaces. This
profile might deserve to be described as ‘not broken’. In the
middle bulk, evaluated as ‘partially broken’, the bulk showed
an inhomogeneous trapped field profile that is not identicalfor each surface, and the trapped magnetic field is as small as
BT = ± 0.02 T at the center. These results offer evidence thatthe thermal instability of the stacked TFM cylinder caused aflux jump during the FCM process, which resulted in the mechanical fracture of multiple bulks at the same time, even though
the mechanical reinforcement using the SS support with the‘hat structure’ was applied. It is therefore recommended thatthe stacked bulk cylinder should be kept at a constant temperature with adequate cooling to avoid flux jumps when it is magnetized with higher Bapp ∼= 10 T. In this sense, fundamental
studies related to the suppression of the thermal instability of
the HTFML should be performed in more detail.
Finally, figure 10 summarizes the concentrated field, Bc ,at the center of the HTFML, as a function of Bapp , comparedwith previously reported experimental results [10, 12]. Since
2018, the feasibility of the HTFML has been proved particularly for lower Bapp up to 3 T for two cases: (a) exploiting
an inner GdBaCuO bulk lens and outer MgB2 TFM cylinder,in which a maximum concentrated field of Bc = 3.55 T wasachieved for Bapp = 2 T at 20 K, and (b) replacing the outer

Figure 6. Time dependence of the temperatures measured at eachposition—the outer TFM cylinder, T T , the inner magnetic lens, T L ,and the cold stage, T s —and the concentrated field, Bc , and theexternal field, Bex , during magnetization with an applied field,
Bapp = 10 T.

Figure 7. Magnetization curves of the HTFML magnetized withapplied fields, Bapp = 3, 5, 7 and 10 T. The lens effect during theascending stage, and the flux creep and the final trapped field in thedescending stage are compared.

Figure 9 shows the trapped field profiles for the (a) top
surfaces and (b) bottom surfaces of each TFM ring bulk (topbulk, middle bulk, and bottom bulk), magnetized by FCM from
Bapp = 1 T in liquid nitrogen after the flux jump, in which
the trapped field value was measured 3 mm above the surfacesof each ring bulk. Photographs of each bulk were also taken
after the measurement. The fracture point can be seen in the7Supercond. Sci. Technol. 34 (2021) 05LT02

Figure 9. Trapped field profiles for (a) the top and (b) the bottom surface of each TFM ring bulk (top bulk, middle bulk, and bottom bulk),magnetized by FCM with the applied field of Bapp = 1 T in liquid nitrogen after the happening of the flux jump. The trapped field value wasmeasured 3 mm above the surfaces. Photographs of each bulk were also taken after the measurement.

this study, the HTFML exploiting the ‘loose contact method’
using one cold stage of a cryocooler realized such a realistic
configuration which could work in a range of Bapp from 3 T to
7 T. A maximum Bc = 9.8 T value was achieved successfully
when magnetizing with Bapp = 7 T, although the flux creep
was larger compared to that of lower Bapp values. The Bc −Bapprelation obtained from this experiment is consistent with ournumerical estimations (indicated by the dotted line), but notfor higher Bapp due to the larger flux creep for Bapp over 7 T
and the mechanical fracture of the bulk materials over 10 T.
This issue would be resolved by use of a more practical cooling method that can provide proper individual cooling of eachpart of the HTFML using one (or two) cold stages, combinedwith a switch that would work based on its thermal property
and/or mechanical function. Furthermore, fundamental studies
to understand and improve the thermal and mechanical stability of the stacked TFM cylinder during its magnetization
would have an important role in improving the practical designof the HTFML.

Figure 10. The concentrated field, Bc , at the center of the HTFML(labelled ‘All-RE [This study]’), as a function of Bapp , comparedwith previously reported experimental results. In the previous
experiments, the HTFMLs were constructed with the combinationof an outer MgB2 TFM cylinder and inner GdBaCuO magnetic lens(labeled ‘MgB2 -RE: 20 K’) [10], as well as an all-(RE)BaCuO
design at 77 K (labeled ‘All-RE: 77 K’) [12]. The numericalestimations for the HTFML with all-(RE)BaCuO bulks are also
included (labeled ‘All-RE (Sim.): 20 K’) [9].

4. Conclusion
MgB2 cylinder with an EuBaCuO one, in which Bc = 1.83 T
was achieved for Bapp = 1.80 T at 77 K. However, these configurations are not appropriate for magnetization with higher Bappover 3 T, because of (a) the inferior J c (B, T) characteristicsof the MgB2 bulk and (b) the operating temperature of 77 K.
An original concept of the HTFML that could generate over
Bc = 10 T requires that both the inner and outer (RE)BaCuO
bulks are utilized at lower temperatures below 50 K [9]. In

We have verified experimentally an all-(RE)BaCuO HTFML
magnetized under 50 K, using only one cryocooler and a
special technique named the ‘loose contact method’, where
the outer TFM was loosely connected to the cold stage
before magnetizing process by introducing a gap between thecold stage. A maximum concentrated field of Bc = 9.8 T
was achieved after magnetization with an applied field of8Supercond. Sci. Technol. 34 (2021) 05LT02

Bapp = 7 T, which is twice as superior as the other HTFML
devices to date. The experimental Bc values, as a functionof Bapp , were consistent with the numerical estimation reported in our previous conceptual study. These results validate
the HTFML concept as a compact and desktop-type magnet
device that can provide 10 T-class magnetic field enhancementfrom the viewpoint of the magnetizing method.
However, during magnetization with a higher Bapp of 10 T,
the Bc value suddenly dropped due to the occurrence of a large
flux jump that resulted in the mechanical fracture of the bottom bulk in the stacked TFM cylinder, even though mechanical reinforcement using the SS support with the ‘hat structure’
was applied. If the flux jump did not occur during magnetization from Bapp = 10 T, it is predicted that Bc = 13 T would beachieved. To realize a Bc value of over 10 T, further investigations are needed relating to the thermal stability of the stacked
TFM cylinder and the development of a more practical cooling method that can achieve a stable and controllable coolingprocess for each part of HTFML.

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Acknowledgments
The authors thank Mr Y Yanagi of IMRA Material R&D Co.,
Ltd, Japan, and Mr Y Takeda of Iwate University for theirvaluable experimental supports. This research is supported by
JSPS KAKENHI Grant No. 19K05240, and by Adaptable andSeamless Technology transfer Program through Target-driven
R&D (A-STEP) from Japan Science and Technology Agency(JST), Grant Nos. VP30218088419 and JPMJTM20AK.
M D Ainslie would like to acknowledge financial support from an Engineering and Physical Sciences ResearchCouncil (EPSRC) Early Career Fellowship, EP/P020313/1.
All data are provided in full in the results section ofthis paper.
ORCID iDs
Keita Takahashi  https://orcid.org/0000-0002-8278-2688
Hiroyuki Fujishiro  https://orcid.org/0000-0003-1483835X
Sora Namba  https://orcid.org/0000-0001-7268-5326
Mark D Ainslie  https://orcid.org/0000-0003-0466-3680
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Bradshaw T, Speller S, Filipenko M and Cardwell D A
2018 Bulk superconductors: a roadmap to applications
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