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Development of a superconducting magnet for nuclear magnetic resonance using
bulk high-temperature superconducting materials
Article in Concepts in Magnetic Resonance · January 2006
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Development of a Superconducting Magnet
for Nuclear Magnetic Resonance Using Bulk
High-Temperature Superconducting Materials
TAKASHI NAKAMURA,1 YOSHITAKA ITOH,2 MASAAKI YOSHIKAWA,2 TETSUO OKA,2
JUN UZAWA1
1
2
RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
IMRA Material R&D Co., Ltd., 5-50 Hachiken-cho, Kariya, Aichi 448-0021, Japan
ABSTRACT: We developed a trapped-field magnet system using a bulk high-temperature superconducting (HTS) material (Sm-Ba-Cu-O; abbreviated as Sm123) and used it to
observe a signal of 1H nuclear magnetic resonance (NMR). The compact magnet system
can fit on a table and needs no conventional refrigerant such as liquid helium. The magnet has a c-axis-oriented single-domain Sm123 bulk superconductor containing 20 wt.%
Ag and was synthesized in the melt-textured process. The magnet is constructed of two
cylindrical pieces, 36-mm outer and 7-mm inner diameters, respectively, and 16-mm
thickness. This configuration results in magnetic field stability and suitable homogeneity
for observation of NMR signal. The magnet was magnetized to 3 T by an ordinary NMR
superconducting magnet by using a field cooling (FC) method. The bulk magnet was magnetized at 60K and has been kept at the temperature under 58K by using a pulse tube
refrigerator. We observed 1H NMR signal from the silicon rubber with the size of 1.2 mm
diameter and 1.5 mm length at 2.89 Tesla (123 MHz) and 228 kHz peak width.
Ó 2007
Wiley Periodicals, Inc.
KEY WORDS:
bulk HTS
Concepts Magn Reson Part B (Magn Reson Engineering) 31B: 65–70, 2007
magnet; superconducting magnet; high temperature superconductor; HTS;
ducting (HTS) materials. Unfortunately, HTS magnets are not available for any practical use at present
because there are difficulties in fabricating the HTS
wire to withstand the huge electromagnetic hoop
stress generated in their practical operation. Recently,
however, so-called conduction magnets, which can
be driven without any use of liquid helium, were
realized in the practical market with HTS as current
leads to feed the electrical current to the superconducting coil. A third application uses bulk HTS material as a successful substitute for permanent magnets.
Bulk LTS cannot hold static magnetic field as a bulk
HTS can because the flux avalanche so-called quench
frequently occurs in the superconducting bulk LTS,
the transition temperature Tc of LTS is low, and the
specific heat is small at the temperature near Tc. On
the contrary, bulk HTS samples can generate high
magnetic fields by trapping the applied magnetic
INTRODUCTION
There are two main applications for low-temperature
superconductors (LTS). One is the superconducting
wire for solenoid-type magnets that we use for the
conventional NMR apparatus. Another is the thin
film in electric devices. Therefore, NMR offers two
major applications for high-temperature superconReceived 5 October 2006; revised 27 November
2006; accepted 27 November 2006
Correspondence to: Takashi Nakamura; E-mail: takashi.nakamura@
riken.jp
Concepts in Magnetic Resonance Part B (Magnetic Resonance
Engineering), Vol. 31B(2) 65–70 (2007)
Published online in Wiley InterScience (www.interscience.wiley.
com). DOI 10.1002/cmr.b.20083
Ó 2007 Wiley Periodicals, Inc.
65
66
NAKAMURA ET AL.
flux. Recently, Tomita and Murakami (1) developed
bulk HTS magnets that can trap magnetic fields of
over 17 Tesla (T) at 29K. RE-Ba-Cu-O (where RE
presents rare-earth elements of Y, Sm, Nd, etc.) bulk
superconductors have a microstructure consisting of
a REBa2Cu3Oy matrix, including small particles of
nonsuperconductive RE2BaCuO5.
The magnitude of the trapped field is proportional
to the critical current density and the volume of the
superconductor (2). RE-Ba-Cu-O bulk magnets have
been found to fracture during high field activation
owing to electromagnetic hoop stress. Some mechanical improvements have addressed this problem. For
example, the addition of silver improves the mechanical properties (3). Moreover, mechanical support to
the sample using a stainless steel ring has also been
found to be effective (4). Therefore, an HTS bulk
magnet is capable of trapping high magnetic field.
MAGNETIZATION TECHNIQUE
Because the bulk HTS magnets could not be energized by simply feeding electric currents to them,
they need to be magnetized by external magnetic
fields. One method is to expose it in the pulsed field
generated by feeding discharging currents stored in
the capacitor banks to a coil. This magnetization
process is called pulse field magnetization (PFM).
And another is to use a static field B0, field cooling
(FC), in which the bulk HTS samples immersed in
static field B0 generated by using a superconducting
magnet (SCM), which is used for the conventional
NMR spectrometer, and cooled below Tc. We chose
the latter method to get higher strength and more homogeneous field than that obtained by PFM.
REFRIGERATOR
A higher magnetic field gives a stronger NMR signal.
The magnitude of the trapped field is proportional to
the critical current density of bulk HTS sample. The
critical electric current density increases when it is
cooled to low temperatures. We have made the permanent magnet devices with the use of pulse tube
(PT) refrigerators or Gifford-McMahon (GM) refrigerators. GM refrigerators have higher cooling ability
than PT refrigerators, but they cause intolerable mechanical vibrations. The cooling ability of PT refrigerators is inferior to GM, but the vibration is greatly
suppressed. PT refrigerators use helium gas instead
of the mechanical piston used in GM. Typically, Tc
of REBa2Cu3Oy HTS is about 90K. The field-trap-
Figure 1 Temperature dependence of the trapped flux
density measured at 0.55 mm above the surface of the
sample by a Hall sensor (6). Symbols are as follows: (~)
123:211 ¼ 3:2, 10 wt.% Ag2O 30 mmf; (*) 123:211 ¼ 3:1,
15 wt.% Ag2O 30 mmf; (!) 123:211 ¼ 3:1, 10 wt.%
Ag2O 30 mmf; (l) 123:211 ¼ 3:1, 10 wt.% Ag2O 36
mmf; (n) 123:211 ¼ 3:1, 20 wt.% Ag2O 30 mmf.
ping ability of the bulk HTS samples depends on
their temperature, as shown in Fig. 1. The PT refrigerator that we chose can cool the bulk sample to 58K
by feeding the electric power of 0.8 kVA.
MAGNET MATERIAL AND DESIGN
We synthesized the c-axis-oriented single-domain
SmBa2Cu3Oy (Sm123) bulk superconductor containing 20 wt.% Ag with 36 mm in diameter under the
Ar gas flow by using Nd123 as a seed crystal. We
found the trapped magnetic field was 9 T at 25K (5).
Figure 2 shows trapped magnetic field distribution
above the surface of the magnet. The field distribution indicates a maximum field of about 0.6 T with a
single peak, which means the sample had no serious
cracks or defects in it and was successfully fabricated
as a single domain. Mizutani and coworkers (6) noted
NMR signal detection using a bulk HTS magnet. The
magnet needs high field stability and high field homogeneity for application to NMR. To get the highest and most homogeneous magnetic field, we drilled
a 7-mm hole through the center of the bulk HTS
(Fig. 3) and placed the probe in the hole. Two bulk
magnets were stacked with their seeded surfaces
faced toward each other. We selected two bulk samples having similar field distribution abilities as
Concepts in Magnetic Resonance Part B (Magnetic Resonance Engineering) DOI 10.1002/cmr.b
SUPERCONDUCTING MAGNET FOR NUCLEAR MAGNETIC RESONANCE
67
Figure 2 Trapped magnetic field distribution at 58 K for Sm123 sample 36 mm in diameter
with 20 wt.% Ag2O addition. The axial component Bz was measured by scanning a Hall sensor
at 2 mm above the surface of the HTS bulk magnet.
determined by measuring field mapping. Figure 4
shows an apparatus of the bulk HTS NMR magnet.
The bulk HTS magnets were cooled by the cold head
of cryostat in a vacuum chamber. A refrigerator conveys the heat in the copper membrane connected to a
cold head in a vacuum chamber and cools the bulk
HTS samples. We prepared the SCM (JASTEC
JMTC-400/89/SS), which is capable of generating
9.4 T (400 MHz) with a room temperature bore
89-mm inner diameter, for the magnetization process
in the field cooling. The SCM of a conventional
NMR spectrometer has the large homogeneous field
that we need. A cold head is extended so that HTS
came to the homogeneous region of the SCM when
the magnetizing operation was performed. The size
of our novel magnet was nearly equal to a conventional NMR probe, which contains the sample in the
room temperature bore of SCM. Our magnet contains
a room-temperature bore with 5-mm outer diameter
and 4.2-mm inner diameter of the glass tube inserted
through the top surface of the vacuum chamber of
the NMR magnet. The room-temperature bore
extends to the area in which the magnetic field is sufficiently uniform to generate NMR signals.
Figure 3 A photograph of the bulk superconductor used
in the magnet. HTS are 36 mm in outer and 7 mm in
inner diameters, respectively, with stainless steel bandage.
Figure 4 Schematic diagram of implementation of the
bulk HTS magnet.
Concepts in Magnetic Resonance Part B (Magnetic Resonance Engineering) DOI 10.1002/cmr.b
68
NAKAMURA ET AL.
a simple solenoid coil with four turns and 3-mm
outer diameter. All NMR measurements were carried
out on the CMX-Infinity spectrometer. We selected a
solid-state NMR spectrometer for its faster analogto-digital converter (ADC) because solid samples
show broad signals in spectra. We used the simple
Hahn spin-echo experiment. The length of a typical
90-degree pulse was 1.2 ms and echo time was 20 ms.
RESULTS AND DISCUSSIONS
Figure 5 Trapped magnetic field distribution of the bulk
HTS magnet. The axial component Bz was measured
along the z axis of magnet center by scanning a Hall
sensor.
EXPERIMENT
We energized the HTS bulk magnet by FC methods.
We chose the magnetization field of 3 T as a target
because it is greater than the value achieved with
electromagnets or permanent magnets. In addition, a
field of 3 T does not damage the bulk samples by the
electromagnetic forces generated by trapping a strong
field in the FC process. The bulk HTS magnet at
room temperature was inserted in the room-temperature bore of the SCM and carefully adjusted at the
magnetic field center. The bulk HTS was cooled by
the PT refrigerator, and temperature was controlled
by an electric heater at 60 K. After the temperature
equalized, the SCM was gradually demagnetized.
When the applied field goes to zero, the bulk HTS
captures the flux and maintains the magnetic field.
When the demagnetization process finished, the bulk
HTS temperature control was turned off, and the
temperature fell to 55 K without any temperature
control. A magnetic field is trapped by the bulk HTS
magnet, which acts as a quasi-permanent magnet.
Figure 5 shows the field distribution map measured
by a Hall sensor along the z axis. The magnetic field
distribution of data shows that the space regions from
79 mm to 81 mm in its vertical position are best and
have the best field homogeneity. Because we need a
small sample for our small magnet, we chose silicon
rubber (7) as a sample for 1H measurements. We
constructed a home-built probe for this magnet using
Figure 6 shows 1H NMR spectrum obtained from the
silicon rubber sample at various z-axis positions. The
resonance frequency was 123 MHz, which indicates
the trapped magnetic field was 2.89 T. It shows that
we have a problem in the magnetization process that
allowed the trapped magnetic field to decay to 2.89 T
from 3 T. That is attributed to the heat generated during the magnetization process. The best resolution
point of 228 kHz full width at half-maximum
(FWHM) was obtained at 83 mm from the edge of
magnet room-temperature bore. We could obtain the
spectra at the five positions from 81 mm to 85 mm
along center of the z axis in the HTS magnets.
Although the homogeneity of the magnetic field was
not enough for NMR application, the basic design of
the magnet was valid because several points of signals were identified in the z-axis direction. The evaluation of magnetic field stability was difficult because of low spectrum resolution. The magnet
field stability was within 60.3 mT throughout the
5 months they were measured by a Hall sensor. It
shows that magnetic field stability was excellent for
the long term.
CONCLUSIONS
The bulk HTS magnet was successfully energized by
FC method using SCM at 3T. We indicated that FC
method was effective in generating homogeneous
magnetic field on bulk HTS magnet for application
to an NMR spectrometer. The magnetic field homogeneity at the center of the bulk magnet has reached
2.4 103 in the space of 1.2 mm diameter and
5 mm length. The magnet field has been stable for a
few months. We must improve the magnetization
process so that heat generation does not occur. We
have to compare the distributions of the magnetic
field of a bulk HTS magnet and the original static
field of SCM, and to examine it in detail. An NMR
signal will give us a good guideline. This magnet is
very small and requires only electricity. Because the
Concepts in Magnetic Resonance Part B (Magnetic Resonance Engineering) DOI 10.1002/cmr.b
SUPERCONDUCTING MAGNET FOR NUCLEAR MAGNETIC RESONANCE
69
Figure 6 1H NMR spectra measured at 81–85 mm position from the magnet top surface. The
spectrum was obtained with 4,000 scans times accumulation, 1 sec pulse delay, t ¼ 20 ms, 90degree pulse width 1.2 ms.
diameter of the bulk magnet is small, 36 mm, the
strayed magnetic fields are weak at the fringe of the
magnet. We attached a UPS battery system to preserve the cooler operation from electric power failures, which also allows it to operate where no electric
power supply exists. It is expected that practical
application of an unprecedented magnet will be created. We must improve the homogeneity of this magnet through improvement of the magnetization process, magnet configuration, and HTS material. That
will lead to a new high magnetic field apparatus and
allow multiple applications in various industries in
the near future.
ACKNOWLEDGMENTS
Support of this work by RIKEN President’s Special
Research Grant is gratefully acknowledged. T.N.
thanks Professor Bernhard Blümich for helpful discussions and Dr. James S. Frye for critical reading
of the manuscript.
Concepts in Magnetic Resonance Part B (Magnetic Resonance Engineering) DOI 10.1002/cmr.b
70
NAKAMURA ET AL.
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Concepts in Magnetic Resonance Part B (Magnetic Resonance Engineering) DOI 10.1002/cmr.b
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