Numerical Study to Obtain the Improved Field Homogeneity and Enlarged Inner Diameter of HTS Bulk Magnet for Compact NMR

Available online at www.sciencedirect.com

ScienceDirect Physics Procedia 65 (2015) 245 – 248

27th International Symposium on Superconductivity, ISS 2014

Numerical study to obtain the improved field homogeneity and enlarged inner diameter of HTS bulk magnet for compact NMR D. Miyazawa, S.B. Kim*, H. Kitamura, D. Ishizuka, K. Hojo Graduate School of Natural Science and Technology, Okayama University, 3-1-1, Tsushima-Naka, Kita-ku, Okayama 700-8530, Japan

Abstract We have been studying the compact magnet for NMR device that consists of a stacked high temperature superconducting (HTS) GdBCO bulk annuli. We can generate the trapped magnetic field over 1.5 T at 77.4 K and 150 ppm/cm3 on inner diameter of 20 mm HTS bulks using field compensation methods. However, it is necessary to enlarge the inner diameter of the HTS bulk magnet because the diameter of commercial NMR probe is larger than 20 mm. In this paper, we studied an optimal shape of the stacked HTS bulk magnet to obtain the enlarged inner diameter using 3-D FEM based analysis. We was able to enlarge the inner diameter of the HTS bulk magnet from 20 mm to 34 mm remaining magnetic field strength of 1.5 T and magnetic field homogeneity of 666 ppm/cm3 by proposed passive field compensation method.

© Published by by Elsevier Elsevier B.V. B.V. This is an open access article under the CC BY-NC-ND license © 2015 2015 The The Authors. Authors. Published (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the ISS 2014 Program Committee. Peer-review under responsibility of the ISS 2014 Program Committee Keywords: NMR relaxometry; HTS bulk magnet; field homogeneity; enlarged inner diameter; field compensation

1. Introduction We have been developing the NMR relaxometry device. The strength and homogeneity of the magnetic field required for the NMR relaxometry device were 1.5 T and 150 ppm/cm3 respectively, these values were much lower than a conventional NMR device. In our previous works, we can generate the trapped magnetic field over 1.5 T at 77.4 K using the stacked HTS bulks with 80 mm height, and 150 ppm/cm3 field homogeneity was obtained using the fabricated field compensation methods on inner diameter of 20 mm HTS bulks [1-4]. However, we needed to enlarge the inner diameter of the HTS bulk magnet because the diameter of the commercial NMR probe was larger than 20 mm. In this paper, we designed the optimized shape of HTS bulk magnet for NMR relaxometry.

*Corresponding author. Tel: +81-86-251-8116; fax: +81-86-251-8110. E-mail address: [email protected]

1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the ISS 2014 Program Committee doi:10.1016/j.phpro.2015.05.135

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2. Design concept of NMR relaxometry Fig.1 (a) shows the magnetized HTS bulk model with the superconducting magnet (SCM) and 3-D NMR relaxometry model is shown in Fig (b). This NMR relaxometry does not require a current source after magnetized, it is possible to freely carry and downsizing. Fig.2 is scaled schematic drawing of the conceptual model of NMR relaxometry. The detail design concept of the HTS bulk magnet for NMR relaxometryy will be described. Super conducting magnet

Probe space ((20 to 26 mm))

HTS bunk (OD 60 mm)

Vaccum (5 mm)

B

Stainlesss (1 mm))

insulating layer (7 mm)

Fig. 1. The schematic draw of magnetization process by super conducucting magnet (SCM) and compact HTS bulk NMR magnet with trapped field.

Fig. 2. Scaled cross-sectional view of HTS bulk NMR magnet with L.N2 bath.

2.1. Required inner diameter of HTS bulk magnet The inner diameter of the commercial NMR probe which used current NMR is more than 20 mm. We think 20 mm diameter NMR probe can be used for NMR relaxometry, therefore the room temperature bore with 20 mm diameter is required. Next, the HTS bulk magnet is placed in a low temperature vessel in order to prevent heat penetration by radiation heat. It is necessary to provide the heat insulating layer. The required thickness of insulation layer is 14 mm including 5 mm vacuum space and 2 mm stainless path. Accordingly, the required inner diameter of the HTS bulk magnet is at least 34 mm, and over 40 mm inner diameter needs to insert the NMR probe smoothly. However, the micro NMR probe with 8.5 mm diameter was developed and reported. Therefore, the room temperature bore with 24 mm diameter is reasonable when we use the micro NMR probe with 10 mm diameter. 2.2. Required height of HTS bulk magnet

3

Field homogeneity (ppm/cm )

SCM wound with low temperature superconductors (NbTi & Nb3Sn), and have a 100 mm room temperature bore size and 10 T was used in the experiments. The field homogeneity of our SCM is 610 ppm in the centre region along the 5 mm axial direction. To investigate the distributions of magnetic field in the SCM, the commercial software based on a FEM was used for analysis. Fig.3 shows the scaled schematic draws of SCM and cylinder HTS bulk model with various heights and inner diameter when the outer diameter HTS bulk was fixed at 60 mm. In this study, the superconducting current in the HTS bulk during the FC process were induced by the Bean’s critical state model and nvalue model, the trapped magnetic fields of HTS bulk magnets were obtained by field cooling method at 77.4 K and the critical current density of HTS bulk with 2.01×108 A/m2 was used at 1.5 T and 77.4 K. In proposed our NMR relaxometry, the required magnetic properties were the magnetic field strength of 1.5 T, the magnetic field homogeneity of 150ppm at spherical sample space with 10 mm diameter. Therefore, we discuss the field homogeneity along the axial and radial directions in the sample space with ±5 mm.

Fig. 3. Scaled schematic draws of SCM and cylinder HTS bulk model with various height and inner diameter when the outer diameter HTS bulk was fixed at 60 mm.

8000 7000 6000 5000 40 mm 4000 30 mm 3000 2000 220 mm m 1000 0 40 60 80 100 120 140 160 180 200 220

Height of HTS bulk (mm)

Fig. 4. Calculated the field homogeneity of the centre region along the 5 mm z-axis direction of the cylinder HTS bulk magnet models as a function of height value when the maximum trapped field of 1.5T at 77.4 K.

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In our previous works, it has been found that the height of HTS bulk had no effect on the trapped magnetic field over SCM height, so the maximum height of HTS bulk was determined by 200 mm. Fig.4 shows the calculated field homogeneity of the centre region along the ±5 mm z-axis direction of the cylinder HTS bulk magnet models as a function of height of HTS bulk when the maximum trapped field of 1.5 T at 77.4 K. The field homogeneity of inner diameter of 40 mm was saturated with height 140 mm. Also, the field homogeneity of inner diameter 20 mm was saturated with height 80 mm. So, the required height of the HTS bulk magnet when ID 20 mm and 40 mm were 80 mm and 140 mm respectively. 2.3. Achieved field homogeneity by various field compensation methods The comparison for field homogeneity along the ±5 mm axial diameter in each field compensation methods when ID and OD were fixed by 20 mm and 60 mm was shown in Table 1. From Table.1, the field homogeneity of HTS bulk magnet was improved by various field compensation methods. The best compensated field homogeneity of 9 ppm/cm3 was obtained by reapplied method, and effect of field compensation of active shimming was not good compared with another methods. On the other hand, the passive shimming method is very simple and can be obtain the high compensation ratio up to 29. Therefore, we can obtain the 10 times improved field homogeneity using by passive shimming and/or combine the active and passive field compensation method. So, in this study, the target field homogeneity of enlarged inner diameter HTS bulk magnet is determine 1500 ppm/cm3 without any field compensation method. Table 1. The comparison for field homogeneity along the ±5 mm axial diameter in each field compensation methods when ID and OD ware fixed by 20 mm and 60 mm Compensation methods

Bulk height (mm)

Before (ppm/cm3)

After (ppm/cm3)

Compensation ratio (Before/After)

Active shimming

75

700

250

2.8

Active shimming and passive shimming

75

700

68.7

10

Passive shimming [3]

70

780

126

6.2

Passive shimming [3]

80

664

23

29

Reapplied [4]

80

664

9

73

3. Enlarged inner diameter of HTS bulk magnet

3

1.005 1.000

ID 20 mm 24 mm 28 mm 32 mm 34 mm 36 mm 40 mm

0.995 0.990 0.985 0.980 0.975 0.970 -15

Field homogeneity (ppm/cm )

Normalized B (Bz/Bzmax)

Fig.5 shows the calculated normalized magnetic field distributions along the z-axis direction as a function of ID value when the maximum trapped field in center position was 1.5 T. The calculated field homogeneity of the centre region along the ±5 mm z-axis direction of the cylinder HTS bulk magnet models as a function of ID value when the maximum trapped field of 1.5 T was shown in Fig.6. From Fig.5, the trapped magnetic field was reduced with increasing the inner diameter of the HTS bulk because the total volume of HTS bulk was decreased with inner diameter. From Fig.6, the magnetic field uniformity was deteriorated because the presence area of the magnetic flux was enlarged by expanding inner diameter. The field homogeneity of 140 mm height was obtained under 1500 ppm/cm3 at each inner diameter. The field homogeneity of 80 mm height was 1220 ppm/cm3 at ID 34 mm and 1502 ppm/cm3 at ID 36 mm.

-10

-5

0

5

z axis (mm)

10

15

Fig. 5. Calculated normalized the magnetic field distributions along the z-axis direction as a function of ID value when the maximum trapped field in centre position was 1.5 T.

2400 2000 80 mm

1600 1200 800 400 16

140 mm 20

24

28

32

36

40

44

Inner diameter of HTS bulk (mm)

Fig. 6. The calculated field homogeneity of the centre region along the 5 mm z-axis direction of the cylinder HTS bulk magnet models as a function of ID value when the maximum trapped field of 1.5T.

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4. Passived field compensation by iron rings

Magnetic flux density (T)

Fig.7 shows the scaled schematic draws of SCM, iron rings and cylinder HTS bulk with two kinds of inner diameter. 20 mm height iron ring was fixed at the central position in the axial direction, and 13 mm iron rings are located on the upper and bottom side. The best field homogeneity of HTS bulk magnet with 20 mm inner diameter and 80 mm height was improved in this position. The magnetic property of the iron rings (Shin Nippon Steel: 35H210) as shown in Fig.8 was used the analysis. Fig.9 shows the calculated normalized magnetic field distributions along the z-axis and r-axis directions as function of ID value when the maximum trapped field in center position was 1.5 T. The calculated field homogeneity along the z-axis direction as function of ID value when the maximum trapped field in center position was 1.5 T was shown in Fig.10. From Figs.9 and 10, the normalized magnetic field strengths at centre region became flat because the applied magnetic field by SCM was concentrated in the iron rings. Therefore, the field homogeneity of HTS bulk magnet with various inner diameters was improved by field compensation method using the iron rings. The field homogeneity of HTS bulk magnet with 80 mm height and 34 mm ID was improved from 1220 ppm/cm3 to 666 p method. ppm/cm3 by passive compensation 1.8 1.6 Shin Nippon Steel : 35H210 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 10 100 1000

10000

Magnetic field strength (A/m)

z axis (mm)

r axis (mm)

Fig. 9. Calculated normalized the magnetic field distributions i along l the (a) zaxis and (b) r-axis direction as a function of ID value when the maximum trapped field in centre position was 1.5 T.

Field homogeneity (ppm)

Normalized B (Bz/Bzmax)

Normalized B (Br/Brmax)

Fig. 8. The magnetic property of the iron rings (Shin Fig. 7. Scaled l d schematic h i ddraws off SCM, iiron rings and Nippon Steel : 35H210) used the analysis. cylinder HTS bulk with two kinds of inner diameter. 1400 r 1.002 ID 34mm 1.00035 SCM + Iron ring ID 20 mm 1200 SCM only (b) 1.00030 1.000 Passive shimming 1000 ID 34 mm 1.00025 ID 20 mm 0.998 SCM 800 SCM only 1.00020 (a) 0.996 Passive shimming ID 34 mm m 600 1.00015 SCM 0.994 400 1.00010 0.992 200 1.00005 ID 20 mm 0.990 0 1.00000 SCM + Iron ring i 0.988 -200 0.99995 -15 -10 -5 0 5 10 15 -6 -4 -2 0 2 4 -6 -4 -2 0 2 4 6

z axis (mm)

6

Fig. 10. The calculated field hhomogeneity i along the z-axis direction as function a of ID value when the maximum trapped field in centre position was 1.5 T.

5. Conclusion We have been developing the NMR relaxometry consists of HTS bulk annuli and operating at liquid nitrogen temperature. The required magnetic field strength and homogeneity of NMR relaxometry of 1.5 T and 150 ppm/cm3 were achieved using the stacked HTS bulks and the fabricated field compensation methods. But we need to enlarge the inner diameter of HTS bulk due to the necessary NMR probe and insulation layer. In this study, we studied the optimal shape of HTS bulk magnet to obtain the enlarged inner diameter when the outer diameter was fixed by 60 mm. The value of less than 1500 ppm/cm3 was obtained when the HTS bulk magnet was 80 mm height and 34 mm ID. We has been attempted to correct 1500 ppm to 150 ppm by passive compensation method, but the field homogeneity of HTS bulk magnet with 80 mm height and 34 mm ID was improved from 1220 ppm/cm3 to 666 ppm/cm3 by passive compensation method. References [1] S.B. Kim, R. Takano, T. Nakano, M. Imai, S.Y. Hahn, Physica C 469 (2009) 1811 [2] S.B. Kim, M. Imai, R. Takano, J.H. Joo, S. Hahn, IEEE Trans, Appl. Supercond. 21 (2011) 2080 [3] H. Kitamura, S.B. Kim, N. Hayashi, D. Ishizuka, D. Miyazawa, Physica C 58 (2014) 298 [4] S.B. Kim, N. Hayashi, H. Kitamura, I. Eritate, IEEE Trans, Appl. Supercond. 24 (2014) 4301405