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

1LOr2A-05 1

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: jvoccio@mit.edu).

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

A

1LOr2A-05

2

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

1LOr2A-05

3

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

1LOr2A-05

4

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.

1LOr2A-05

5

REFERENCES

[1] R.A. Wind and J.Z. Hu, “In vivo and ex vivo high-resolution 1H NMR

in biological systems uses low-speed magic angle spinning,” Prig. NMR

Spectrosc. 49, 207 (2006).

[2] C.A. Meriles, D. Sakellariou, A. Moule, T.F. Budinger, and A. Pines,

“High-resolution NMR of static sample by rotation of the magnetic

field,” J. Magn. Reson. 169, 13 (2004).

[3] D. Sakellariou, C. A. Meriles, R.W. Martin, A. Pines, “NMR in rotating

magnetic fields: magic-angle field spinning,” Magn. Reson. Imag. 23,

295 (2005).

[4] R.C. Jachmann, D. Trease, L.S. Bouchard, D. Sakellariou, R. Martin,

R.D. Schlueter, T.F. Budinger, A. Pines, “Multipole shimming of

permanent magnets using harmonic corrector rings,” Rev. Sci. Instr. 78,

035115 (2007).

[5] John Voccio, Seungyong Hahn, Dong Keun Park, Jiayin Ling, Youngjae

Kim, Juan Bascuñán, and Yukikazu Iwasa, “Magic-Angle-Spinning

NMR Magnet Development: Field Analysis and Prototypes.” IEEE

Tran. Appl. Supercond. 23, 1051 (2013).

[6] Yukikazu Iwasa, “Case Studies in Superconducting Magnets,”

2nd edition, 2009, pp. 254-258.

[7] Weijun Yao, Juan Bascunan, Woo-Seok Kim, Seungyong Hahn, Haigun

Lee, and Yukikazu Iwasa, “A solid nitrogen cooled MgB2

‘demonstration’ coil for MRI applications,” IEEE Tran. Appl.

Supercond. 18, 912 (2008).

[8] Benjamin J. Haid, Haigun Lee, Yukikazu Iwasa, “Thermal behavior of a

solid nitrogen impregnated high-temperature superconducting pancake

test coil under transient heating,” IEEE Trans. Appl. Supercond. 11,

1852 (2001).

[9] Benjamin J. Haid, Haigun Le, Yukikazu Iwasa, Sang-Soo Oh, Young-

Kil Kwon, and Kang-Sik Ryu, “Design analysis of a solid heat capacitor

cooled ‘permanent’ high-temperature superconducting magnet system,”

Cryogenics 42, 229 (2002).

[10] Y. Iwasa, J. Bascunan, J. Jankowski, H.M. Kim, H. Lee, “Thermal and

magnetic responses of a solid nitrogen/magnetized YBCO disk system

undergoing temperature cycles in the range of 8-60 K,” IEEE Trans.

Appl. Supercond. 14, 1727 (2004).

[11] J. Bascunan, S. Hahn, M. Ahn and Y. Iwasa, “Construction and test of a

500 MHz/200 mm RT bore solid nitrogen cooled Nb3Sn MRI magnet,”

Adv. Cryogenic Tech. (2010), Vol. 1218, pp. 523-530. [12] S. Noguchi, “Formulation of the spherical harmonic coefficients of the

entire magnetic field components generated by magnetic moment and

current for shimming,” Journal of Applied Physics 115, 163908 (2014)

[13] S. Noguchi, et. al., “Passive Shimming for Magic-Angle-Spinning

NMR,” IEEE Transactions on Applied Superconductivity, Vol. 24, No.

3, June 2014.