3762

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005

Effect of Metal Ring Setting Outside HTSC Bulk
Disk on Trapped Field and Temperature Rise inPulse Field Magnetizing
Hiroyuki Fujishiro, Kazuya Yokoyama, Masahiko Kaneyama, Tetsuo Oka, and Koshichi Noto

Abstract—In order to enhance the trapped field in cryo-cooled
HTSC bulks using pulse field magnetizing (PFM), a metal ring(stainless steel 304 and/or Al) has been tightly set onto the SmBaCuO bulk disk and the relation between the total trapped fluxP , the trapped field P , and the temperature rise, has beenTT
investigated as a function of the applied pulse field ex . The PTP values are enhanced about 10 20% by the metal ring dueand T. These results suggest
to the reduction in the temperature risethat a part of the generated heat due to the flux motion in the peripheral region promptly transfers to the metal ring and the heattransfer to the cold stage is improved by the ring setting.

11Index Terms—High c bulk superconductors, metal ring setting,pinning and viscous loss, pulse field magnetizing, temperature
measurement.

I. INTRODUCTION

IN VIEW of the practical applications of high- bulk superconductors (HTSCs) as a high strength bulk magnet fora magnetic levitation system and so on, pulse field magnetizing(PFM) as well as the static field-cooled magnetizing (FCM) has
been intensively investigated because of the relatively compactand inexpensive setup. The field trapped by PFM is, however,lower than that attained by FCM at temperatures below 77 K.
The main reason has been attributed to the large heat generation due to the dynamic motion of the magnetic flux against
and the viscous force . We havethe vortex pinning force
and trapped fieldon thestudied the temperature risesurface of cryo-cooled YBaCuO and SmBaCuO bulks during
reduction for
PFM and pointed out the importance of theenhancement [1]–[4]. The total generated heat was
theof the bulk and the maxestimated using the specific heat
. It was pointed out that thevalues as functions
imumof the initial temperature and applied pulse field
can beby FCM vs.
understood on the basis of the trapped field
Manuscript received October 3, 2004. This work was supported in part by
Japan Science and Technology Corporation under the Joint-research Project for
Regional Intensive in the Iwate Prefecture on Development of practical applications of magnetic field technology for use in the region and in everyday living.
H. Fujishiro and M. Kaneyama are with the Faculty of Engineering,
Iwate University, Morioka 020-8551, Japan (e-mail: fujishiro@iwate-u.ac.jp;t2203004@iwate-u.ac.jp).
K. Yokoyama is with the National Institute for Materials Science, Tsukuba,
305-0003, Japan (e-mail: Yokoyama.Kazuya@nims.go.jp).
T. Oka is with IMRA Material R&D Co., LTD., 5-50 Hachiken-cho, Kariya,448-0021, Japan (e-mail: toka@ai-i.aisin.co.jp).
K. Noto was with the Faculty of Engineering, Iwate University. He is now
with Iwate Industrial Promotion Center, Morioka 020-0852, Japan (e-mail:notokoshichi@yahoo.co.jp).
Digital Object Identifier 10.1109/TASC.2005.849425

diagram, i.e., by the decrease of the critthe temperatureassociated with the temperature rise. Inical current density
andvalues by PFM, the reduction
order to enhance theis an indispensable issue. The iteratively magnetizingof thepulsed-field method with reducing amplitude (IMRA) [5], locating yoke pieces around a bulk [6], a use of vortex-type coils[7] and a multi-pulse technique with step-wise cooling (MPSC)[8] have been attempted to suppress the heat generation duringof 17.24 T at 29 K in YBaCuO bulk has been rePFM. A
alized by FCM, enhancing the thermal conduction of the bulk
by impregnating a high thermal conductivity alloy into drilledholes [9].
In this study, we set a metal ring (Al and/or stainless steel304) tightly on a SmBaCuO bulk disk. The following effects
are expected from the metal ring set on to the HTSC bulk disk.
First, since the heat generation due to PFM occurs mainly inthe peripheral region, the metal ring attached to the bulk disk
periphery can easily and promptly receive the heat. Second,is far larger thansince the -plane thermal conductivity
the -axis , the generated heat can easily reach the metalring along the radial paths of the disk, then transferring to thecold stage through the highly conductive ring. These two effectsreduction of the bulk. Finally, theshould contribute to themetal ring setting helps to mechanically reinforce the bulk disk
[10].
II. EXPERIMENTAL PROCEDURE
A highly -axis oriented SmBaCuO bulk superconductor(Dowa Mining Co., Ltd) with 45 mm diameter and 15 mm
thickness was used. This consisted of four growth sector
regions (GSRs), divided by the growth sector boundaries (GSBs) [2]. The bulk is composed of(Sm123) and(Sm211) with the molar ratio of, 15.0 wt.%
powder, and0.5 wt.% Pt powder. The bulk was uniformly impregnatedby epoxy resin in vacuum and then the epoxy resin layer on
the surface of the bulk disk was removed. The stainless steel(SUS304) or the aluminum (Al) ring with 4 mm in thickness
and 15 mm in height, was fixed onto the bulk disk using
apiezon-N grease. We call the ring-attached sample as SUS-Smor Al-Sm. Since a large heat generation due to the eddy current
at 100 K), the ring wastook place in the Al ring (cut with a gap of 1 mm as shown in Fig. 1. Resultantly,
was diminished to within 0.2 K. Since the Al ring did notcontact tightly with the bulk due to the gap, another SUS ring1051-8223/$20.00 © 2005 IEEE

FUJISHIRO et al.: EFFECT OF METAL RING SETTING OUTSIDE HTSC BULK DISK ON TRAPPED FIELD IN PFM

3763

Fig. 1. Photograph of the SmBaCuO-bulk disk fitted with theW(Al+SUS)-ring. The positions of the temperature and magnetic fieldmeasurements (P0P4, PR and PH) are shown.

TABLE I
SUMMARY OF SIZES AND MATERIAL PARAMETERS OF Sm-BULK AND RINGSwith 1 mm in thickness was set outside the Al-ring. Hereafter,
we denote this sample with the Al+SUS (W) ring the W-Sm.
Table I summarizes the sizes, the volume of each metal ring,, the heat capacity (J/K)
and the specific heatat 40 K of the Sm-bulk and each metal ring, and the ratio of theheat capacity of the metal ring to that of the Sm-bulk. The heat
capacity increases by 30%, 18% and 27% for the SUS-ring,Al-ring and W-ring, respectively, relative to that for the bareSm bulk. The -plane and -axis thermal conductivity at 40
K of the Sm-bulk and that of SUS and Al are also shown [11].
and were measured for the present specimens [12].
The bulk disk was tightly stacked on the sapphire plate (45mm in diameter and 20 mm in thickness) attached to the cold
stage of a helium refrigerator. The initial stage temperature
was kept at 40 K. The temperatures,at the center of theat
were monitored by finebulk (P0) and,chromel-constantan thermocouples adhered to the upper bulk
were situated on the censurface by GE7031 varnish.
tral radial lines of each GSR by 9 mm apart from P0. The temwas also measured at PR. The totalperature of the metal ringand the distribution of the trapped
trapped magnetic flux

Fig. 2. Examples of the time evolution of temperatures T (t) of the Sm-bulk
without ring, with SUS-ring, with Al-ring and with W(SUS+Al)-ring at (a) T 3,(b) T 0 and (c) T R after applying the No1 and No5 pulse fields of B =4:70 T.

magnetic flux density
were measured using an axial typeHall sensor (F.W. Bell, model BHA 921), which scanned 3 mmabove the bulk surface stepwise with a pitch of 1.2 mm. The
on the bulk surface was measured by the Hall
trapped fieldsensor adhered to the position at PH with a 2.5 mm distance) with thefrom P0. Five iterative magnetic pulses (from 3.83 T to 6.04 T (rise time: 12 ms)same amplitude,
andwere applied sequentially after re-cooling to .
were measured at each stage.
III. RESULTS AND DISCUSSION
Fig. 2(a) shows the time dependence of temperatureat.
P3 after applying the No1 and No5 pulse of
For the bare Sm-bulk (B-Sm) without the ring, the magnetic

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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005

111 4Fig. 3. The maximum temperature rises TBand T R
at the bulk
and the ring for (a) the No1 and (b) No5 pulses as a function of B . Theto T.
T B values are the averaged ones over T1

1 0

flux preferentially intrudes into the bulk from the GSRs containing P2, P3 and P4 and the sharp and large temperature rises
take place within 3 s at these positions.
reafter 15 20 min.
ofcovers to the initial temperaturemaximum, although
SUS-Sm decreases faster after thethe maximum temperature rise is similar to that of B-Sm. This
result means that a part of the generated heat promptly transvalue atdecreases
fers to the SUS-ring. The maximum
to 20 K for Al-Sm and W-Sm. The rate of temperature deafter the peak is small for Al-Sm but thatcreasefor the W-Sm is large and almost the same as the SUS-Sm. As
is the largest forfor the iterative pulse field applications,
the No1 pulse and decreases for the succeeding pulses [2]. Theis also the largest for theincrement of the trapped field
No1 pulse, followed by a gradual increase for the No2 and No3
peak disappears
pulses. For the No5 pulse application, theis slightly smaller for the SUS-Sm andfor all the settings.
W-Sm than for the bare bulk and Al-ring setting.
at P0 for each metal ring setting.
Fig. 2(b) presents therises up latest among
because the distancefrom the major heat source is usually the longest [2]. For thereaches a maximum faster than that
SUS-Sm and W-Sm,for B-Sm. These results mean that the flux motion is enhancedin the central region of the bulk by the SUS-Sm and W-Sm andthe magnetic fluxes are accumulated faster in the bulk center.
The anomalously long time constant for the temperature risefor the Al-ring setting may result from the high thermal contact
resistance between the bulk and Al-ring.
for each metal ring setting. TheFig. 2(c) shows themaximum
values for SUS-Sm and W-Sm are 12 K and10 K, respectively, and the time constant for the temperature
is longer for the SUS-Sm and W-Sm than that ofrise of. The temperature rise in the metal ring is mainly due tothe heat conduction from the bulk and the temperature rise in theAl-ring is slower because of the high thermal contact resistance.
Figs. 3(a) and (b) summarize the maximum temperature rise
and the ringafter the No1 andof the bulk
No5 pulse as a function of.
, the averaged value, increases with increasingfor both
atof B-Sm is the largest and is
No1 and No5 pulses.
reduced by the metal ring setting. Especially, the reduction ofFig. 4. The total trapped flux 8 for each metal ring setting after (a) the No1and (b) the No5 pulse application, as a function of B . The typical conicalare displayed for the SUS-ring setting andtrapped field distributions B
bare Sm-bulk.

theis the largest for W-Sm and Al-Sm; e.g., from.
23 K to 18 K for the No1 pulse ofalso increases with increasing
for both No1 and No5 pulses.
is the largest forand is about 50 to 80% of
the SUS-ring and the smallest for the W-ring.
after theFigs. 4(a) and 4(b) present the total trapped flux
No1 and No5 pulse as a function of. For the No1 pulse,of B-Sm is smallest for eachand thevalue is enhancedby the metal ring setting. Especially, for the SUS-Sm and W-Sm,values after the 4.70 T and 5.53 T pulses increase about
the20% compared with those of the bulk without ring. The increase
may mainly come from the decrease of the temperature rise dueto the ring setting, which results in the increase of the effective
enhancement is observed for
critical current density. TheAl-Sm, which suggests that the good thermal contact between
the ring and the Sm-bulk is of vital importance. The trapped fieldmeasured at PH is also enhanced by the metal ring setting;for W-Sm is about 10% larger thanfor example,
that of B-Sm
after the No5 pulse of.
are shown in theThe typical trapped field distributions
inset of Fig. 4(a) in which a conical field distribution can beconfirmed.
Neglecting the heat drained to the cold stage, the generated
heat by the PFM operation can be estimated using the following equation,(1)

whereandare the specific heat, andandare the volume of the Sm-bulk and the metalring, respectively. Fig. 5 shows the estimated values as a function offor each metal ring setting. For the No1 pulse, thevalue of the bulk with each metal ring is smaller than thatof B-Sm. This result suggests that the heat transferred from thebulk is partially drained to the cold stage through the ring. For

FUJISHIRO et al.: EFFECT OF METAL RING SETTING OUTSIDE HTSC BULK DISK ON TRAPPED FIELD IN PFM(3)3765bulk. The good thermal conductance between the bulkand toperiphery and ring is necessary to reduce
andvalues.
enhance theThe metal ring with a high thermal conductivity alsoenhances the thermal drain for the generated heat totransfer to the cold stage.
ACKNOWLEDGMENT

The authors would like to thank Prof. M. Ikebe of Iwate University for the valuable suggestions.
REFERENCES

QBFig. 5. The estimated generated heat as a function offor each metalring setting. The inset indicates the heat capacity of the Sm-bulk, SUS304and Al.

C

the No5 pulse, The value of B-Sm is nearly the same as thetotal of the bulk and the metal ring, irrespective of the species
and volume of the metal ring. This result means that the heat isgenerated mainly in the bulk and is transferred to the metal ringnearly adiabatically for the No5 pulse. The heat flow to the cold
is not so large.
stage may remain small in caseIV. SUMMARY
In order to enhance the trapped field in cryo-cooled HTSC
bulks by the pulse field magnetizing (PFM) method, the effectof a metal ring set around the SmBaCuO bulk disk has been in, the trapped
vestigated by measuring the total trapped flux
and the temperature risefor the successive apfieldwith various strengths. Important
plication of pulse fields
experimental results and conclusions obtained in this study aresummarized as follows.
(1) Theandvalues are enhanced about 10 20%
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the reduction of
the heat generation mostly takes place in the peripheral region of the bulk disk in PFM, the metal ring set
around the bulk is more effective than the insertion ofa high-thermal-conductive alloy on the bulk center.
(2) The SUS- and W(Al+SUS)-rings behave as a good heat
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