Superconductor Science and Technology
Supercond. Sci. Technol. 33 (2020) 085002 (10pp)

https://doi.org/10.1088/1361-6668/ab9542

A record-high trapped field of 1.61 T in
MgB2 bulk comprised of copper plates
and soft iron yoke cylinder using
pulsed-field magnetization
Tatsuya Hirano, Yuhei Takahashi, Sora Namba, Tomoyuki Naito andHiroyuki Fujishiro
Department of Physical Science and Materials Engineering, Faculty of Science and Engineering, Iwate
University, Morioka 020-8551, Japan
E-mail: fujishiro@iwate-u.ac.jp and thirano1995@gmail.com
Received 10 March 2020, revised 3 May 2020
Accepted for publication 21 May 2020
Published 18 June 2020
Abstract

A trapped field of BT = 1.61 T was experimentally achieved at the central surface of an MgB2
bulk composite (60 mm in diameter and 20 mm in height) at 20 K by double pulsed-fieldmagnetization (PFM) using a split-type coil. The composite bulks consisted of two MgB2 ringbulks sandwiched by thin copper ring plates, which were then stacked, and a soft iron yoke
cylinder was inserted in the central bore of the rings. The copper ring plates delayed the rise timeand duration of the magnetic pulse due to eddy currents. The inserted soft iron yoke attracted themagnetic flux and enhanced the trapped field strength mainly due to its large permeability. As aresult, the trapped field was enhanced from BT = 0.34 T for the single MgB2 ring bulk without
the copper plates and soft iron yoke to BT = 1.00 T for the composite with both copper plates
and the soft iron yoke. The inserted soft iron yoke can be exploited to enhance the trapped field
because the intrinsic BT of the single MgB2 ring bulk was smaller than the saturation field of theyoke (∼2 T). Using an optimized second pulse application after suitable flux trapping from thefirst pulse application, the trapped field was enhanced considerably to BT (2nd) = 1.61 T, which
is a record-high trapped field for an MgB2 bulk by PFM to date. The combination of the longer
magnetic pulse application by the copper plates, the enhancement of the effective applied fieldby the inserted soft iron yoke, and the double pulse application using split-type coil is aneffective technique to enhance the trapped field in the MgB2 bulk using PFM.
Keywords: MgB2 , pulsed field magnetization, trapped field, soft iron yoke cylinder, double pulse
application, copper plate stack, rise time elongation(Some figures may appear in colour only in the online journal)

1. Introduction

systems and compact cryogen-free nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) systems[2–6]. A record high trapped field of BT (FCM) = 17.6 T at
26 K has been achieved in the GdBaCuO disk bulk pair activated by field-cooled magnetization (FCM) [7, 8]. During FCM,
the bulk is cooled below the transition temperature, T c = 92 K,
under the applied field, Bex , using superconducting magnet(SM) and then the magnetic field decreases to zero.

Large, single-grain RE-Ba-Cu-O (REBaCuO, RE: a rare earth
element or Y) bulk superconductors can trap higher magnetic
field by strong vortex pinning effect and are a promising material for use as a compact, high-strength trapped field magnet(TFM) [1] for various practical applications, such as rotating machines, magnetic separation, flywheel energy storage1361-6668/20/085002+10$33.00

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On the other hand, MgB2 bulk superconductors have also
promising potential as TFMs, such as being rare-earth-free,lightweight and presenting a homogeneous trapped field distribution [9], which are in clear contrast with REBaCuO bulks.
The better and larger MgB2 polycrystalline TFMs can berealized because of their long coherence length, ξ, below
T c = 39 K [10]. MgB2 bulks with high critical current density, J c , have been fabricated by various methods and usuallyactivated by FCM [11–15]. A record-high trapped field of
BT (FCM) = 5.4 T has been achieved at 12 K on a surface of a
MgB2 disk bulk 20 mm in diameter [16]. In this way, FCM canachieve the maximum trapped field of the bulk. However, since
the SM is expensive and heavy, FCM is generally ill-suited forwide practical applications.
Pulsed-field magnetization (PFM) is another magnetizing
method to magnetize bulk superconductors [17–21], which ismobile, inexpensive experimental setup with no use of SM.
During PFM, the bulk is cooled below T c , and the magnetic
pulsed field with the rise time of milliseconds is applied using acopper coil and condenser bank. However, the trapped field byPFM, BT (PFM), is generally lower than that by FCM becauseof a large temperature rise caused by the dynamics of the magnetic flux during the magnetic pulse application [22].
Multi-pulse techniques are effective to enhance the BT
value for REBaCuO bulks due to the reduction of the temperature rise [23–26]. We have achieved the trapped field of
BT (PFM) = 5.20 T on a GdBaCuO bulk 45 mm in diameterat 29 K using a modified multi-pulse technique with stepwise
cooling (MMPSC) [17], which is a record-high BT (PFM) for
REBaCuO bulk by PFM to date. The PFM technique has also
been applied to MgB2 bulks [27–32]. BT (PFM) = 0.81 T wasachieved at 14 K for a high-J c MgB2 bulk fabricated by the
hot isostatic pressing (HIP) method magnetized using a copper solenoid coil, in which BT (FCM) = 2.23 T was trapped at
16 K by FCM [29]. However, flux jumps took place frequently
during PFM in the high-J c MgB2 bulks and consequently the
final BT (PFM) value decreased for larger pulsed fields. The
multi-pulse application would be to enhance the trapped field
for the MgB2 bulk.
The effectiveness of the split-type coil (or vortex-type coil)during PFM has been clarified for REBaCuO bulks experimentally [33, 34] and numerically [19, 35]. The magnetic flux
starts to intrude not from the periphery, but mainly from theboth surfaces of the disk bulk, and is trapped in the centerof the bulk, even for lower pulsed fields. It was shown thatthe maximum BT could be enhanced in comparison to thatobtained using the solenoid-type coil. In this case, the temperature rise during PFM for the split-type coil was smaller thanthat for the solenoid-type coil. We have achieved a trappedfield of BT (PFM) = 1.1 T on a high-J c MgB2 bulk at 13 K
without flux jumps by PFM using a split-type coil, in which apair of soft iron yokes were inserted in the bore of the splittype coil. BT (PFM) = 1.1 T has been the record high trappedfield by PFM for MgB2 bulk since 2016 [36]. We have performed many PFM experiments and numerical simulations for
REBaCuO and MgB2 bulks, in which the following effectiveness has been suggested to enhance the trapped field; the useof split-type coil [19, 33–35], the multi-pulse application for

the REBaCuO bulk [17, 23, 24], the use of soft iron yoke inthe pulse coil [19] and the long pulse application [37].
In this study, we have investigated the trapped field properties of MgB2 ring bulk composites inserted by soft iron yokeat 20 K by single and double pulsed field applications using
a split-type coil. Furthermore, the effect of thin copper plates,
which sandwiched the MgB2 ring bulks, was investigated to
expect the longer magnetic pulse application. As a result, a
new record-high trapped field of BT = 1.61 T was experimentally achieved at 20 K for an MgB2 ring bulk comprised of bothcopper plates and a soft iron yoke using double pulsed-field
application by split-type coil.

2. Experimental details
We performed the PFM experiments using three types of bulk
modules. An MgB2 ring bulk (20 mm in inner diameter (I.D.),
60 mm in outer diameter (O.D.) and 19 mm in height (H))
was fabricated by an in-situ infiltration method [38], in which
BT (FCM) = 1.57 T was trapped by FCM at 20 K. We abbreviate this as ‘single bulk’, as shown in figure 1(a). The single
bulk was sliced in half (H = 9 mm), and each ring bulk was
sandwiched by oxygen-free copper plates 0.5 mm in thickness using Apiezon-NTM grease, and then stacked, as shownin figure 1(b). We abbreviate this as ‘composite (w/o yoke)’.
A soft iron yoke cylinder (20 mm in diameter and 20 mm in H)
was inserted in the composite (w/o yoke), as shown in figure1(c). We abbreviate this as ‘composite (w/yoke)’.
Figure 2 shows the experimental setup for PFM using the
split-type coil. Each MgB2 bulk module was fastened in a brassholder using a stainless steel (SS) bolt and nut with thin indium
foil and connected to the cold stage of a Gifford–McMahon(GM) cycle helium refrigerator in a vacuum chamber. In theprevious study, a copper holder was used to fasten the disk
bulk for the split-type coil [19], in which non-negligible temperature rise was observed due to the eddy current flowing in
the holder during PFM. To eliminate the temperature rise in theholder, a brass holder was used in this study, which has lower
electrical conductivity. A Hall sensor (HG-106 C; ASAHIKASEI) was placed to the center of the bulk surface, and athermometer (CernoxTM ) was connected to the brass holder.
The split-type coil (72 mm in I.D., 124 mm in O.D., 35 mm inH), which was submerged in liquid nitrogen, was placed outside the vacuum chamber, in which a pair of Ni-plated soft ironyokes (60 mm in diameter and 65 mm in height) was inserted
in the central bores of the coil.
The strength of the magnetic pulse was estimated using thefollowing methods. Figure 3 shows the experimental method
to estimate the magnitude of pulsed field, Bex (shunt) andBex (Hall). The time evolution of magnetic field, Bex (shunt)(t),was estimated by observing the current, I(t), flowing throughthe shunt resistor from the pulse current source (condenserbank) using a digital oscilloscope (YOKOGAWA Electric,
DL1640). For example, to achieve Bex (shunt)max = 1.5 T at
the center of the split coil at T s = 40 K (>T c of the MgB2bulk), I peak = 440 A. The time evolution of the magnetic field,
Bex (Hall)(t), was also simultaneously monitored at the central2Supercond. Sci. Technol. 33 (2020) 085002

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Figure 1. Three types of MgB2 bulk modules used for the PFM experiments: (a) ‘single bulk’, (b) ‘composite (w/o yoke)’, where two sliced

ring bulks were sandwiched by oxygen-free copper plates and are stacked, and (c) ‘composite (w/yoke)’, where a soft iron yoke cylinder isinserted in (b).

Figure 2. Experimental setup of the PFM for the three types of bulk modules using split-type coil with soft iron yoke.

surface of the single MgB2 ring bulk and the composite (w/oyoke) by measuring the Hall voltage of the Hall sensor using
the digital oscilloscope. For the composite (w/yoke), the time
evolution of magnetic field, Bex (Hall)(t), which is defined as
Bex (yoke)(t) hereafter, was monitored by measuring the Hall
voltage at the central surface of the soft iron yoke.
The initial temperature, T s , of the bulk was set to 20 K, anda single magnetic pulse, Bex (Hall)(t), with a peak up to 2.0 T
was applied via a pulsed current flowing in the coil, as shown infigure 4(a). The PFM experiments were also performed using
double pulse application, as shown in figure 4(b). The first
pulse of Bex1 (Hall)max = 1.20 or 1.32 T was applied at 20 K.
Next, once the temperature of the bulk returned to 20 K, thesecond pulse, Bex2 (Hall)max , ranging from 1.1 to 1.6 T, wasapplied to the bulk.
During PFM, the time evolution of the temperature, T(t),and the trapped field, BT , which was defined as the final valueof BT (t) at 500 ms, were measured. After the removal of the3Supercond. Sci. Technol. 33 (2020) 085002

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Figure 3. The experimental method to estimate the magnitude of pulsed field, Bex (shunt) and Bex (Hall).

Figure 4. Schematic illustration of (a) single-pulse and (b) double-pulse application during PFM at T s = 20 K.

split-type coil (15 min after the each pulse application), twodimensional trapped field profiles were mapped at 5 mm above
the bulk surface (on the outer surface of the vacuum chamber)by scanning a Hall sensor (BHA 921; F W Bell) using an x–ystage controller.

(w/yoke) cases are quite different with those for the single
bulk case.
Figure 5(b) shows the relationship between the maximum values (Bex (Hall)max and Bex (shunt)max ) of the curves
Bex (Hall)(t) and Bex (shunt)(t) for each case at T s = 40 K.
The maximum value of the Bex (Hall)(t) curve, named as
Bex (Hall)max for the composite (w/o yoke) was about 20%
smaller than that of the Bex (shunt)(t), named as Bex (shunt)max ,because of eddy currents flowing in the high-conductive copper plates along the direction to avoid the magnetic flux
intrusion. On the other hand, the Bex (Hall)max value for thecomposite (w/yoke) was much larger than Bex (shunt)max ,and its rate increases with increasing Bex (shunt)max . The
Bex (Hall)max enhancement for the composite (w/yoke) mainly
results from flux concentration in the ferromagnetic yoke cylinder, although the induced current flowing in the yoke acts to
reduce the flux intrusion.
Figure 5(c) shows the rise time, trise (Hall), measured by
the Hall sensor and trise (shunt), as a function of Bex (shunt)max
at T s = 40 K. The magnitude of trise (Hall) for the composite (w/o yoke) increased due to the copper plates, and that forthe composite (w/yoke) increased more due to the existence ofthe yoke, compared with that for the single bulk. The detailed
study using the copper plates is planned in the next paper.

3. Results and discussion
3.1. Effect of the copper plates and soft iron yokeFirst, we clarify the effects of the copper plate stack and softiron yoke insertion on the magnetic pulse, Bex (t). Figure 5(a)shows the time dependence of Bex (shunt)(t) for an appliedmagnetic field of 1.03 T, which was measured for the ‘single
bulk’ case at T s = 40 K. The rise time, trise , was about 20 ms,
and the pulse duration was about 400 ms, which were determined by the coil inductance and the existence of the ferromagnetic soft iron yoke inserted in the bore of the split-type
coil. In the figure, the Bex (Hall)(t) profiles for the single bulk,
composite (w/o yoke) and composite (w/yoke) cases are also
shown, which were measured by the adhered Hall sensors.
Bex (Hall)(t) for the single bulk was slightly delayed, compared with Bex (shunt)(t). However, the magnitude, rise timeand duration of Bex (Hall)(t) for the composites (w/o yoke) and4

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Figure 5. (a) The time dependence of Bex (shunt)(t) for an applied magnetic field of 1.03 T, measured for the single MgB2 bulk case, andBex (Hall)(t) for the single bulk, composite (w/o yoke) and composite (w/yoke) cases at T s = 40 K. (b) The relationship between the
maximum values (Bex (Hall)max and Bex (shunt)max ) of the curves Bex (shunt)(t) and Bex (Hall)(t) for each case at T s = 40 K. (c) The rise time,trise (Hall) and trise (shunt), as a function of Bex (shunt)max at T s = 40 K.

yoke) was slightly larger than that for the single bulk case.
For the composite (w/yoke), the magnetic flux started to penetrate into the center around Bex (Hall)max = 1.03 T, took amaximum of BT = 1.00 T at Bex (Hall)max = 1.20 T, and thendecreased with increasing Bex (Hall)max . It should be noted thatfor the composite (w/yoke), the BT value was enhanced, but theBex (Hall)max value at which BT value took a maximum wasnearly the same as that for the composite (w/o yoke). These
results suggest that the enhancement of the BT value resulted
mainly from the yoke cylinder and that the Bex (Hall)max value
at which BT value takes a maximum was mainly determined by
the copper plates, although we did not perform PFM experiments for the MgB2 bulk with only the yoke without the copper
plates.
Figure 7(a) shows the maximum temperature rise, T max ,during PFM for each composite, as a function of Bex (Hall)max
at 20 K. For the single bulk, T max abruptly increases at
Bex (Hall)max = 0.85 T due to the pinning loss of the flux trap[39], at which the magnetic flux reached the center of the bulk,
and BT = 0.34 T was trapped. T max increased with increasing Bex (Hall)max due to the viscous loss of the flux movement[39]. For the composites (w/yoke) and (w/o yoke) cases, T max
linearly increased with increasing Bex (Hall)max , even though
the magnetic flux was not trapped the center of the bulk for3.2. Results for single pulse application

The PFM experiments were performed for the three types of
MgB2 modules at T s = 20 K (<T c of the MgB2 bulk). Figure 6
shows the trapped field, BT , which is defined as the steadyvalue of the curves BT (t) at t = 500 ms, at the central surfaceof each MgB2 module, as a function of Bex (Hall)max at 20 K.
The trapped field profiles for each case, when the maximum
BT value was achieved, are also shown. For the single MgB2
bulk case, the magnetic flux started to penetrate into the center at Bex (Hall)max = 0.85 T, and a maximum of BT = 0.34 T
was trapped at the central surface. For larger Bex (Hall)max ,the BT value decreased due to a larger temperature rise. The
trapped field profile is shown, for which a trapezoidal profile was measured 5 mm above the bulk surface. For the composite (w/o yoke), the magnetic flux started to penetrate into
the center at Bex (Hall)max = 1.23 T, and then decreased withincreasing Bex (Hall)max . A maximum BT = 0.44 T was trappedat the central surface. The Bex (Hall)max value for the composite (w/o yoke), at which the BT value took a maximum,
was larger than that for the single MgB2 bulk. Here, a larger
magnetic pulse is necessary to trap the magnetic flux at thecenter due to the existence of the copper plates, as shown infigure 5(b). The maximum BT value for the composite (w/o5Supercond. Sci. Technol. 33 (2020) 085002

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Figure 6. The trapped field, BT , at the central surface of each module, as a function of Bex (Hall)max at 20 K. The trapped field profiles foreach case, when the maximum BT value was achieved, are also shown.

Bex (Hall)max < 1.2 T. These results indicate that the heat generation took place mainly in the copper plates due to the eddy
currents in addition to the heat generation due to the flux trap.
For Bex (Hall)max > 1.2 T, for which the magnetic flux wastrapped, the temperature rise due to the flux trap was superimposed and T max was larger than that of the single bulk. Becauseof the additional heat generation from the eddy current in theinserted yoke, the temperature rise of the composite (w/yoke)
was larger than that of the composite (w/o yoke) for higher
Bex (Hall)max .
Figure 7(b) shows an example of the time dependenceof the temperature, T(t), in the composite (w/o yoke) forBex (Hall)max = 0.91, 1.23 and 1.29 T, which was measured
on the surface of the brass holder. T(t) took a maximum
at t = 2 ∼ 3 s and then linearly decreased with increasing time. The maximum temperature increased with increasing Bex (Hall)max . Similar trends were also observed for othercases. The results in figure 7 reflect indirectly the trapped field
properties during PFM, even though the thermometer was notadhered directly on the bulk surface.
Figure 8 shows the time dependence of the applied field,
Bex (t), and the trapped field, BT (t), for each case and foreach single pulse application of Bex (Hall)max . Figures 8(a) to(c) show the results for the single MgB2 ring bulk, where
Bex (shunt)(t) and Bex (Hall)(t) are also shown. The magnetic
flux cannot penetrate well to the center of the bulk due to its
magnetic shielding for Bex (Hall)max = 0.83 T (figure 8(a)).
With increasing Bex (Hall)max , the magnetic flux was able topenetrate into the center, and the BT (t = 500 ms) value
took a maximum at Bex (Hall)max = 0.85 T (figure 8(b)). For
Bex (Hall)max = 0.88 T, the BT (t = 500 ms) value decreased
due to a large flux flow out of the bulk (figure 8(c)). Figures
8(d) to (f) show the results for the composite (w/o yoke). Here,similar trends to the single bulk were observed. The BT value
took a maximum of BT = 0.42 T at Bex (Hall)max = 1.23 T(figure 8(e)), and then decreased at Bex (Hall)max = 1.30 T. For

higher Bex (Hall)max , as shown in figure 8(f), a long gradualdecay in BT (t) can be seen at 100 ms < t < 250 ms. This isa characteristic decay in MgB2 bulk during PFM [30], which
was also reported by other researcher [32], but has not beenclarified the reason. Figures 8(g) to (i) show the results forthe composite (w/yoke), where Bex (yoke)(t) is also shown, in
addition to Bex (shunt)(t) and Bex (Hall)(t). The BT value took amaximum of 1.00 T at Bex (Hall)max = 1.21 T (figure 8(h)) andthen decreased at Bex (Hall)max = 1.32 T. The trapped field of
the composite (w/yoke) was higher than that of the composite (w/o yoke), because the iron yoke has a large permeability.
The period of the characteristic BT (t) decay for the composite(w/yoke) becomes longer at 150 ms < t < 400 ms. These results
indicate that the composite bulk is considered to be pseudolong pulsed field magnetization technique.

3.3. Results for double pulse application

In the previous subsection, the effects of the copper plates
on the MgB2 bulk surface and the insertion of soft ironyoke cylinder on the trapped field enhancement were clearly
confirmed for the single pulse application. In this subsection, the effect of the double pulse application is presented.
Figure 9(a) shows the trapped field, BT (2nd), on the central
surface of the composite (w/yoke) at T s = 20 K, as a function of the second applied pulsed field, Bex2 (Hall)max , after
the first pulsed field, Bex1 (Hall)max , of 1.20 T or 1.32 T wasapplied and the magnetic flux was already trapped. Afterthe application of Bex1 (Hall)max = 1.32 T, the final BT (2nd)value was 0.65 T, as shown in figure 8(i). For the second
pulse application, BT (2nd) sharply increased, took a maximum of 1.61 T at Bex2 (Hall)max = 1.26 T, and then decreased
with increasing Bex2 (Hall)max . The BT (2nd) = 1.61 T is arecord high BT value for an MgB2 bulk by PFM to date. The
trapped field profile 5 mm above the bulk surface was fairly

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Figure 7. The maximum temperature rise, T max , during PFM for each composite, as a function of Bex (Hall)max at 20 K. (b) An example ofthe time dependence of the temperature, T(t), for the composite (w/o yoke) for Bex (Hall)max = 0.91, 1.23 and 1.29 T at 20 K.

Figure 8. The time dependence of the applied fields, Bex (t), and the trapped field, BT (t), for single pulse application of Bex (Hall)max for(a)–(c) the single bulk, (d)–(f) the composite (w/o yoke), and (g)–(i) the composite (w/yoke).

cone shaped. The highest BT value comes from the reduction of temperature rise due to the already trapped magnetic
flux during the 1st magnetic pulse [17, 40]. For the case of
Bex1 (Hall)max = 1.20 T, similar BT (2nd) vs Bex2 (Hall)max behavior can be observed. However, the maximum BT (2nd) value
was smaller than that for Bex1 (Hall)max = 1.32 T. These results suggest that an optimum trapped field and BT (1st) profile

exists to maximize the BT (2nd) value. Figure 9(b) shows the
maximum temperature rise, T max , of the composite (w/yoke)
at T s = 20 K, as a function of second applied pulsed field,
Bex2 (Hall)max . T max for the single pulse application of the composite (w/yoke) at T s = 20 K is also shown, which was presented in figure 7(a). It should be noted that T max after the second
pulse application is about 2 K smaller than that for the single

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Figure 9. (a) The trapped field, BT (2nd), on the central surface of the composite (w/yoke) at T s = 20 K, as a function of the second

applied pulsed field, Bex2 (Hall), after the first pulsed fields, Bex1 (Hall)max , of 1.20 T and 1.32 T were applied. The trapped field profiles of
the composite (w/yoke) measured 5 mm above the bulk surface at 20 K, for which the maximum BT (2nd) was achieved for
Bex1 (Hall)max = 1.20 T and 1.32 T. (b) The maximum temperature rise, T max , of the composite (w/yoke) at T s = 20 K, as a function of
Bex2 (Hall)max . T max for the single pulse application for the composite (w/yoke) at T s = 20 K is also shown.

Figure 10. The time dependence of the applied fields, Bex2 (t), and the trapped field, BT (2nd)(t) for the second pulse application,

Bex2 (Hall)max , after the first pulse application of (a)–(c) Bex1 (Hall)max = 1.32 T and (d)–(f) Bex1 (Hall)max = 1.21 T at T s = 20 K.

at T s = 20 K for Bex2 (Hall)max = 1.17, 1.26 and 1.50 T, after
the first pulse of Bex1 (Hall)max = 1.32 T was applied. The
time dependence of BT (1st)(t) for Bex1 (Hall)max = 1.32 T wasshown in figure 8(i), in which BT (1st) = 0.65 T was trapped.
For Bex2 (Hall)max = 1.26 T, the maximum BT (2nd) of 1.61 T

pulse application. The reduction of temperature rise results inthe enhancement of BT after the second pulse.
Figures 10(a)–(c), respectively, show the time dependenceof the applied fields, Bex2 (shunt)(t), Bex2 (Hall)(t) for w/o yoke
and Bex2 (yoke)(t) for w/yoke, and the trapped field, BT (2nd)(t),8Supercond. Sci. Technol. 33 (2020) 085002

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was achieved. Figures 10(d)–(f), respectively, show the timedependence of applied fields and the trapped field, BT (2nd)(t),
at T s = 20 K for Bex2 (Hall)max = 1.21, 1.26 and 1.52 T, after
the first pulse of Bex1 (Hall)max = 1.20 T was applied. The
time dependence of BT (1st)(t) for Bex1 (Hall)max = 1.21 T wasshown in figure 8(h), in which BT (1st) = 1.00 T was trapped.
As shown in figure 9(a), a similar trend can be seen after
the Bex1 (Hall)max application of 1.32 and 1.20 T, althougha slight difference in the maximum value of BT (2nd) exists.
However, the BT (t) behavior was different after the identical
Bex2 (Hall)max = 1.26 T application, as shown in figures 10(b)and (e). In the case of Bex1 (Hall)max = 1.32 T, which was
higher than the optimum Bex1 (Hall)max shown in figure 6,
BT (1st) was as low as 0.65 T. For the 2nd pulse applicationof Bex2 (Hall)max = 1.26 T, a clear BT (2nd)(t) enhancement up
to 1.61 T can be observed. On the other hand, in the case of
Bex1 (Hall)max = 1.20 T, which was the optimum Bex1 (Hall)max
shown in figure 6, BT (1st) was 1.00 T. For the 2nd pulse application of Bex2 (Hall)max = 1.26 T, the BT (2nd)(t) enhancement
was relatively small. These results suggest that the enhancement of BT (2nd) is sensitive to the trapped field profile inthe bulk after the 1st pulse application. A similar trend in theenhancement of BT (2nd) was observed in the results from the
MMPSC method for the GdBaCuO bulk in [17].

(4) The combination of the longer magnetic pulse by the copper plates, the enhancement of the effective applied fieldby the inserted soft iron yoke and the double pulse application using the split-type coil is an effective technique toenhance the trapped field in the MgB2 bulk using PFM.
This composite structure with copper plates and an ironyoke may also be applicable to REBaCuO bulk trappedfield magnets magnetized by PFM.
Acknowledgments
The authors thank Dr Mark D Ainslie of University of Cambridge for his valuable discussions and suggestions. This
research is supported by Adaptable and Seamless Technology transfer Program through Target-driven R&D (A-STEP)
from Japan Science and Technology Agency (JST), Grant
No. VP30218088419 and by JSPS KAKENHI Grant No.
19K05240.
ORCID iDs
Tatsuya Hirano  https://orcid.org/0000-0003-1658-914X
Sora Namba  https://orcid.org/0000-0001-7268-5326
Tomoyuki Naito  https://orcid.org/0000-0001-7594-3466
Hiroyuki Fujishiro  https://orcid.org/0000-0003-1483835X

4. Conclusion
We have investigated the pulsed field magnetization (PFM) of
three types of MgB2 bulk modules (single ring bulk, composite (w/o yoke) and composite (w/yoke)) using a split-type coil(also inserting a soft iron yoke) at T s = 20 K. The composite(w/o yoke) consisted of two MgB2 ring bulks sandwiched bythin oxygen-free copper ring plates, which were then stacked.
The composite (w/yoke) module included a soft iron yoke cylinder that was inserted in the composite (w/o yoke). We alsostudied the effect of the copper plates and soft iron yoke toenhance the trapped field, BT . The important results and conclusions are summarized as follows.

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the magnitude and shape of the effective pulsed field, Bex .
(2) The trapped field was enhanced from BT = 0.34 T for the
single MgB2 ring bulk without both the copper plates andsoft iron yoke to BT = 0.44 T for the composite (w/o yoke)
and, finally to BT = 1.00 T for the composite (w/yoke).
The inserted soft iron yoke can be exploited to enhance thetrapped field because the intrinsic BT of the single MgB2
ring bulk was smaller than the saturation field of the yoke.
(3) Using the optimized second pulse application after
appropriate flux trapped, BT (1st), by the first pulse,
Bex1 , the trapped field was enhanced considerably to
BT (2nd) = 1.61 T, which is a record-high trapped field for
MgB2 bulk by PFM to date.
9Supercond. Sci. Technol. 33 (2020) 085002

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