a 1.5-t magic-angle spinning nmr magnet: 4.2-k...
TRANSCRIPT
John Voccio, Seungyong Hahn, Youngjae Kim, Jungbin Song, Kazuhiro Kajikawa, Juan Bascuñán, So Noguchi, and Yukikazu Iwasa
October, 2014
Francis Bitter Magnet Laboratory, Plasma Science and Fusion Center
Massachusetts Institute of Technology Cambridge MA 02139 USA
This work was supported by the National Institute of Biomedical Imaging and Bioengineering of the National Institutes of Health under Award Number R01EB013231. Reproduction, translation, publication, use and disposal, in whole or in part, by or for the United States government is permitted.
Submitted to IEEE Trans. Appl. Supercond.
PSFC/JA-15-36
A 1.5-T Magic-Angle Spinning NMR Magnet: 4.2-K Performance and Field Mapping Test Results
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A 1.5-T Magic-Angle Spinning NMR Magnet:
4.2-K Performance and Field Mapping Test Results
John Voccio, Seungyong Hahn, Youngjae Kim, Jungbin Song, Kazuhiro Kajikawa, So Noguchi, Juan Bascuñán,
and Yukikazu Iwasa
Abstract— We present results of full-current testing at 4.2 K of
a z-axis 0.866-T solenoid and an x-axis 1.225-T dipole coil that
comprise a 1.5-T/75-mm RT (room temperature) bore MAS
(magic-angle-spinning) NMR (nuclear magnetic resonance)
magnet developed at the MIT Francis Bitter Magnet Laboratory.
Also included in the paper are results of the magnet performance
when the magnet assembly is immersed, to enhance its thermal
mass, in solid nitrogen, and operated in the temperature range of
4.2 to 4.3 K.
Index Terms—magic-angle-spinning superconducting magnet,
nuclear magnetic resonance (NMR)
I. INTRODUCTION
1.5 T superconducting magic-angle spinning (MAS)
magnet [1-4], comprised of a 1.2247-T dipole field and a
0.8660-T solenoid field, has been completed and
successfully tested. Phase 1 of this project had two specific
aims: (1) build a superconducting magnet system comprising
a z (axial)-field solenoid (Bz) and an x-y dipole (Bx), whose
combined magic-angle field, Bma, of NMR-quality and 1.5 T
points at an angle of 54.74o (magic angle) from its spinning (z)
axis; and (2) demonstrate an innovative cryogenic system that
houses this magnet. Both of these aims have been
demonstrated, and ferroshimming design is now underway.
II. MAGNET DESIGN
Table I summarizes the coil design parameters for this
magnet. The 18-filament NbTi conductor purchased from
Supercon Inc. (Shrewsbury, MA) has insulated dimensions of
1.6 mm x 0.8 mm with a copper-to-superconducting ratio of
7:1. As depicted in Fig. 1, the magnet assembly consists of:
Manuscript received August 10, 2014. This work was supported by the
National Institute of Biomedical Imaging and Bioengineering of the National
Institutes of Health under Award Number R01EB013231.
J. Voccio is with the MIT Francis Bitter Magnet Laboratory (FBML),
Plasma Science and Fusion Center (PSFC), Cambridge, MA 02139 USA
(phone: 617-869-2830; email: [email protected]).
S. Hahn, Y. Kim, J. Ling, J. Song, J. Bascuñán and Y. Iwasa are also with
the MIT FBML, PSFC, Cambridge, MA 02139 USA.
K. Kajikawa was a visiting scientist at FBML, 4/1/2013-3/31/2015.
Currently, he is at the Research Institute of Superconductor Science and
Systems, Kyushu University, Fukuoka 819-0395, Japan.
S. Noguchi was a visiting scientist at FBML. Currently, he is at the
Graduate School of Information Science and Technology, Hokkaido
University, Kita 14 Nishi 9, Kita-ku, Sapporo 060-0814, Japan.
(1) a solenoid coil wound on a central tube; (2) dipole coils
mounted onto same tube; and (3) outer iron yoke.
Fig. 1. Solenoid, dipole iron yoke assembly (left to right).
TABLE I
MAS MAGNET SUMMARY
Item Units Value
Center Magic-Angle Field (Bma) [T] 1.5
NbTi Conductor Width; Thickness [mm] 1.60; 0.85
Copper: Superconductor Ratio 7:1
Est. Critical Current @ 2T, 5.5 K [A] 400
Solenoid (axial, z) Field (Bz) [T] 0.8660
Dipole (x-y) Field (Bx) [T] 1.2247
Center Field Orientation [degree
]
54.74
Axial RT Bore [mm] 75
Iron Yoke Inside Diameter [mm] 150
Iron Yoke Outside Diameter [mm] 275
Iron Yoke Stack Height [mm] 600
Operating Current (dipole/solenoid) [A] 225.0 / 228.3
Rotation Frequency [Hz] 6
Operating Mode w/ LHe Transfer persistent
Temperature range w/o LHe transfer [K] 4.2 to 5.5
Operation duration w/o LHe transfer [hr] ~2
Target homogeneity @ 10Φ X 20 MM [ppm] < 0.1
III. MAGNET FABRICATION
Previously, we reported results from testing the first two
layers of the dipole [5]. The purpose of that test was to
confirm the winding technique and quench behavior. That
test showed no issues with quench, so we proceeded to
complete the dipole coil by winding a second double pancake
on top of the underlying coil. Table II provides a summary of
the dipole winding parameters, and a photograph of the dipole
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coil assembly prior to overbanding is provided in Fig. 2. Table
III provides a summary of the solenoid winding parameters,
and a photograph of the completed solenoid coil is shown in
Fig. 3. After completing the solenoid winding, we applied
layers of fiberglass and epoxy to the outer surface of the coil
and then sanded to make it flush with the outer diameter of the
stainless steel tube. This helped provide mechanical stability
to the solenoid to minimize quench by preventing wire motion
while also providing a smooth, continuous surface onto which
the dipole coils could be mounted.
TABLE II
MAS DIPOLE WINDING SUMMARY
Item Units Value
Dipole Winding ID; OD [mm] 125.0; 137.4
Dipole Overall Length [mm] 486
Total Number of Turns (Turns/one pancake) 320 (80)
Number of Layers 4
Winding Section on Circumference [deg] 120 + 120
Winding Pitch (Conductor Center to Center) [mm] 1.55
Total Conductor Length [m] 740
Operating Current (air-core/iron yoke) [A] 369.24 (225.0)
Field @ Magnet Center [T] 1.2247
Homogeneity (φ10-mm; 20-mm long cylinder) [ppm] <100 (bare)
Estimated Inductance (with Iron Yoke) [H] 0.12 (0.23)
Stored Energy (with Iron Yoke) [kJ] 3.0 (5.8)
TABLE III
MAS SOLENOID WINDING SUMMARY
Parameter
Coil 1
Coils 2-1; 2-2
Winding ID./OD/Length [mm] 105.0/111.0/49.0 105.0/113.0/57.0
Coil Midplane Location [mm] 0.0 +/- 57.5
Turns per Layer/Layers per Coil 49/3 57/4
Wire Length per Coil [m] 49.9 78.1
Total Wire Length [m] 206
Operating current, Iop [A] 257.0 (228.3)
Field @ Magnet Center [T] 0.866
Est. Inductance (w/ iron yoke) [mH] 18.2 (19.5)
Stored Energy [J] 600 (552)
Homogeneity [ppm] <100 (bare)
IV. SYSTEM ASSEMBLY
The two dipole coils were assembled onto the central
solenoid tube in a clam-shell configuration as shown in Fig. 2.
Then, the coil assembly was overbanded with 2 layers of 1mm
diam. 316 L stainless wire to manage the electromagnetic
stresses.
Next, the superconducting joints and PCS switches were
installed and fixed to a G-10 support which resides directly
above the magnet assembly.
Finally, this coil assembly was integrated with the 600 mm
high iron yoke, which consists of ~540 1.1 mm thick, 280 mm
OD and 178 mm ID plates made from 1008 steel and also with
the 75 mm RT insert and the copper tube heat exchanger. This
entire assembly is shown in Fig. 4.
Fig. 2. Photograph of completed dipole coil.
Fig. 3. Photograph of solenoid coil.
Fig. 4. MAS magnet final assembly. The magnet assembly is surrounded by
iron laminations for shielding which reduces the magnet operating current..
A copper tube heat exchanger is wound around the iron plate stack for cooling
the solid nitrogen to 4.2 K with cold helium forced through the copper coil.
V. CRYOGENIC DESIGN
The innovative cryogenic design concept discussed in
previous literature [7-11] developed at MIT was employed as
shown in Fig. 5, in which the magnet is immersed in solid
nitrogen (SN2). This all-solid cold body ameliorates thermo-
fluid issues associated with liquid under rotation. Also,
solid nitrogen ensures a uniform temperature throughout the
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windings and provides a large thermal mass, enabling the
magnet to maintain its operating field over a time period even
when a flow of liquid helium (LHe), its primarily cooling
source, is shut off. The Phase 1 cryostat, due to the addition
of the 10-liter SN2, had a warm-up time period, from 4.5-K to
5.5-K, of ~2 hr. as shown in Fig. 6.
Fig. 5. Solid nitrogen cooling configuration.
Fig. 6. Solid nitrogen warm-up curve.
VI. QUENCH PROTECTION
Since the mutual inductance between the dipole and
solenoid coils is very small, each coil had its own separate
protection circuit. The dipole coils were protected [6] in both
driven-mode and persistent-mode operation using the quench
protection circuit shown in Fig. 7. While the coil is energized
with the PCS switch open, the coil is protected using an IGBT
switch circuit. Once the differential voltage, D1 minus D2,
exceeds an allowable value, which in this case was 1 mV, the
switch opens disconnecting the power supply and dumping the
energy across the external dump resistor.
Fig. 7. Dipole quench protection circuit, showing external IGBT switch, dump
resistor, internal shunt resistors, R1 and R2, and internal (dotted line)
persistent-mode circuit.
In persistent-mode, the dipole coils are protected by internal
resistors that shunt across each half of the dipole. These
resistors were designed for mutual communication between
sections D1 and D2 in order to help limit the hot spot
temperature to < 200 K. A similar circuit was used to protect
the solenoid coil. The dipole stored energy is ~6 kJ, while the
solenoid stores only ~1 kJ.
VII. TESTING
Liquid Helium, Partial Field Testing
Both coils were first tested separately in driven-mode in
liquid helium without the iron yoke at 4.2 K without the
persistent current switches (PCS). The dipole was ramped at
20 A/s beyond its full-field operating current to 275 A without
quenching, and the solenoid was ramped at 30 A/s to 275 A
also without quenching.
After this successful individual coil testing, the PCS’s were
installed for both coils. This required making superconducting
joints (2 per coil) between the NbTi/Cu wire and the
NbTi/CuNi in each PCS. Then, each coil was tested again at
4.2 K, including the first combined coil test with both coils
energized to 200 A for a combined magic angle field of ~0.8
T. Again, there was no quench.
Solid Nitrogen, Full Field Testing
After assembling the coils with the iron yoke structure and
copper heat exchanger (shown previously in Fig. 4), we
performed the solid nitrogen testing. The following procedure
was used for this cooldown:
1. Precool magnet with liquid nitrogen to 77 K;
2. Use vacuum pump to solidify nitrogen and lower
temperature to ~55 K;
3. Force cold helium gas thru heat exchanger to cool the
magnet-iron yoke-SN2 assembly to 4.2 K; and
4. Add additional liquid helium on top of SN2 block to
provide cooling for the vapor-cooled leads, which are
only being used in Phase 1.
With the coils embedded in the SN2 block in the 4.2-4.3 K
temperature range, the full-field testing was performed using
the following test sequence:
1. The dipole was ramped at 20 A/min to it full field of
1.225 T at a current of 225.0 A.
2. The dipole was put into persistent mode.
3. The solenoid was ramped to its full field of 0.866 T at a
current of 228.3 A.
4. The solenoid was put into persistent mode.
This created the combined magic-angle field of 1.5 T
pointing an angle of 54.7o from the axis. Once again, no
quench occurred, showing that this magnet is stable. At this
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point, the field mapping was conducted before de-energizing
both coils.
In the next phase of this project, we will use a Cryomech
PT815 cryocirculator to cool the solid nitrogen to ~10 K
before using cold helium gas to achieve 4.2 K. Dr. Kazuhiro
Kajikawa demonstrated this capability cooling a ~10 liter
volume of SN2 to ~10 K over a period of ~24 hrs. using this
cryocirculator.
VIII. FIELD MAPPING
Using a Hall sensor array goniometer (Fig. 8), field
mapping was conducted. Fig. 9 shows the dipole field at the
magnet midplane (z = 0). Sensors P1 and P3 show good
agreement between the field measured at radial positions of 10
and 15 mm, respectively, while Sensor P2 shows the error
field at 5 mm. P4 measures the axial field.
Fig. 10 shows good agreement between the dipole field
measured at z = -5, 0 and +5 mm for Sensor P3 at radial
position of 15 mm.
Lastly, Fig. 11 shows the field mapping of the solenoid,
showing ~500 ppm drop from center field value of 0.866 T at
z-axis positions of -5 and +5 mm.
In general, these field mapping measurements show
uniformity at or below the Hall sensor array resolution of
~0.1%, or 1000 ppm.
Fig. 8. Hall sensor array goniometer used for field mapping.
Fig. 9. Dipole field mapping results at z = 0 at 4.2 K in solid nitrogen showing
good agreement between measured values and pure cosine field profile.
Fig. 10. Dipole field mapping at various z-axis positions (-5, 0 and +5 mm)
using sensor P3 (r = 15 mm).
Fig. 11. Solenoid field mapping profile.
VII. CONCLUSION
The construction and testing of a 1.5-T MAS magnet
system has been completed and successfully tested to its full
field. Testing includes operation in persistent mode at 4.2 K
in liquid helium and in the range 4.2-4.3 K with solid nitrogen
cooling. Initial field mapping results look promising. Next,
more accurate field mapping will be conducted with an NMR-
quality field probe. Then, we will shim the field with a
combination of LTS shim coils, ferromagnetic shims and RT
electroshims designed to withstand 6-Hz when the entire
system will be housed in a rotation-proof cryostat and rotated
at 6 Hz. Dr. So Noguchi has developed a micro genetic
algorithm (µGA) for designing the ferromagnetic tile layout
for this magnet [12, 13]. This approach will be applied once
we have higher-quality NMR field mapping capability later
this year.
ACKNOWLEDGMENT
This work was supported by the National Institute of
Biomedical Imaging and Bioengineering of the National
Institutes of Health under Award Number R01EB013231. The
authors would like to thank Julio Colque and Peter Allen for
their assistance in the laboratory.
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