- Email: [email protected]
The characteristics of spatial homogeneity and strength of magnetic field for compact NMR magnets using stacked HTS bulks with various gap lengths
Physica C 471 (2011) 1454–1458
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
The characteristics of spatial homogeneity and strength of magnetic field for compact NMR magnets using stacked HTS bulks with various gap lengths S.B. Kim a,⇑, T. Kimoto a, M. Imai a, Y. Yano a, J.H. Joo a, S. Hahn b, Y. Iwasa b, M. Tomita c a
Department of Electrical and Electronic Engineering, Okayama University, 3-1-1, Tsushima Naka, Kita-ku, Okayama 700-8530, Japan Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, 170 Albany Street, Cambridge, MA 02139, USA c Railway Technical Research Institute, 2-8-38, Hikari-cho, Kokubunji-shi, Tokyo 185-8540, Japan b
a r t i c l e
i n f o
Article history: Available online 14 May 2011 Keywords: Compact NMR HTS bulk annuli Axial gap length
a b s t r a c t A compact nuclear magnetic resonance (NMR) magnet is one of the new applications after a technique to enhance maximum trapped field of an high temperature superconducting (HTS) bulk. In design of a compact NMR magnet which consists of the stacked HTS bulk annuli, the issues of strength, spatial homogeneity and temporal stability by trapped magnetic fields are very important. This paper presents a study on the effects of magnetization field strength and gap length between stacked bulks for the compact HTS bulk NMR applications. Four-stacked HTS bulk magnet with ID 20 mm and OD 60 mm was prepared to investigate the optimized configuration. The thickness of each HTS bulk is 5 mm, and the gap lengths from 0 mm to 10 mm were used as parameters in analysis and experiment, respectively. Four-stacked HTS bulk magnets with various gap lengths were tested at two different background magnetic fields of 0.5 T and 2 T at 77 K. The optimized axial gap length was found out by analytical results, and the better magnetic field homogeneity and temporal decay property of trapped magnetic field were obtained by lower magnetization field in this experiments. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, the performance of high temperature superconducting (HTS) bulks in terms of mechanical strength, size and critical current density are greatly enhanced, thus their trapped magnetic field capability [1] expanded from the conventional HTS bulk applications. On the other hand, the nuclear magnetic resonance (NMR) spectroscopy becomes an indispensable measurement device in organic compound analysis and has been paid attention in food and drug industries as an effective tool for a non-destructive testing of nucleic acids. The analysis precision of the NMR is in proportion to strength of the magnetic field, so superconducting magnets wound with superconducting wire has been generally used as NMR magnets. However, many researchers still suffer from a lack of laboratory space as well as high fixed cost to install additional NMR units in their research facilities because the conventional NMR magnets still occupies significant spaces. The new type compact NMR magnet consisted of a stack of ring-shaped HTS bulks where a magnetic fields are trapped by field cooling method was suggested [2–4]. Since this magnetically charged HTS bulk magnet for NMR device does not need a power supply and additional coolant supply system, so it is expected that the new NMR device can achieve not only the compactness but also cost-efficiency. We have been devel⇑ Corresponding author. Tel.: +81 86 251 8116; fax: +81 86 251 8259. E-mail address: [email protected] (S.B. Kim). 0921-4534/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2011.05.215
oping a new prototype compact NMR magnet with stacked HTS bulk annuli and fundamental studies for the optimized configuration were carried out experimentally and analytically [5–8]. The proposed compact NMR magnet consisted of stacked HTS bulk annuli was magnetized by an external superconducting magnet with field cooling method, then magnetized HTS bulk annuli can be used with cryogenic container as a compact personal NMR magnet without additional superconducting magnets and any power sources. In our previous work, the spatial homogeneity and field strength of HTS bulk magnet were analytically investigated as a function of gap length against various thicknesses of HTS bulk. It was clarified that the spatial homogeneity in the HTS bulk magnet would be improved by reducing a thickness of the HTS bulk and enlarging a gap length between the bulks [8]. Therefore, in this study, we prepared the four-stack HTS bulk magnet with each HTS bulk thickness 5 mm. The numerical analysis based on 3-D FEM was carried out firstly in order to obtain an optimized axial gap length, then, the characteristics of spatial homogeneity and strength of magnetic field of stacked HTS bulk magnet with various axial gap lengths were investigated experimentally. 2. Analysis results To investigate the characteristics of trapped magnetic field in HTS bulk magnet, the commercial software based on a finite ele-
1455
S.B. Kim et al. / Physica C 471 (2011) 1454–1458
60 mm
z
20 mm +z
5 mm 269
Gap 0 -10mm
HTS bulk
5 mm Gap 0 -10mm
z=0 5 mm
x
Gap 0 -10mm -z
5 mm
Fig. 1. To-scaled schematic drawing of four stacked HTS bulk magnet with various axial gap lengths between bulks.
20 60 130 270 Fig. 2. To-scaled schematic analytical models of the four stacked HTS bulk magnet and superconducting magnet for magnetization.
1.0 0.9
Magnetic flux density (T)
ment method was used for simulation. The details of simulation method including governing equations were shown in previous work [7]. In this study, four GdBCO HTS bulks with 5 mm thickness were used, and Fig. 1 shows a scaled-schematic drawing of four stacked HTS bulk magnet. The inner and outer diameters of HTS bulk annulus are 20 and 60 mm, respectively. In analysis, the axial gap lengths between stacked HTS bulks were used as parameters and it was changed from 0 to 10 mm. Fig. 2 shows a schematic drawing of the analytical model including four-stacked HTS bulk magnet and external superconducting magnet. In this analysis, the magnetization field of 1 T was applied to study the characteristics of spatial homogeneity, and linearly decayed to 0 T in 250 s. Fig. 3 shows the calculated magnetic field distributions along the z-axis of the four-stacked HTS bulk magnet as a function of gap length (0–10 mm) between the HTS bulks. Fig. 4 provides an enlarged view of the calculated magnetic field error in ppm at near the center of axial direction (up to +10 mm). As shown in Figs. 3 and 4, the maximum field strength was decreased with increasing of the gap length, but the spatial homogeneity was improved as the gap length increasing up to 9 mm. However, the spatial field homogeneity becomes worse when the gap length was above 10 mm. The overall height of stacked HTS bulk magnet was enlarged with increasing the gap length, so, the spatial homogeneity was improved according to the overall height of HTS bulk magnet increasing until the gap length was below two times larger than thickness of HTS bulk of 5 mm. When the gap length became larger than thickness of HTS bulk, the leakage of magnetic field would be too large. From these results, when designing the compact NMR using four-stacked HTS bulk with each thickness is 5 mm, we need to fix gap length below 10 mm. Thus, we need to optimize the gap length to obtain the both of spatial homogeneity and field strength for compact NMR magnet.
(unit: mm)
0.8 0.7 0.6
Gap 0.1mm Gap 2mm Gap 4mm Gap 6mm Gap 8mm Gap 10mm
0.5 0.4 0.3 -25
-20
-10
-5
0
5
10
15
20
25
z axis (mm) Fig. 3. Calculated magnetic field distributions along the z-axis of the four stacked bulk magnet.
100000
Gap 0.1mm Gap 1mm Gap 2mm Gap 3mm Gap 4mm Gap 5mm Gap 6mm Gap 7mm Gap 8mm Gap 9mm Gap 10mm
90000 80000
Field error (ppm)
3. Experiment and results Fig. 5 shows the photographs of the top view of single GdBCO HTS bulk annulus and four-stacked bulk magnet. Inner diameter, outer diameter and thickness of HTS bulks are 20, 60 and 5 mm, respectively, and each HTS bulk was surrounded by the stainless steel ring with 2 mm thickness to improve mechanical strength. The maximum strengths of the trapped magnetic field of three HTS bulks are 0.55 T, and the other one is 0.44 T. The HTS bulk annuli were magnetized by the field cooling method at 77 K, and axial and radial components of the trapped magnetic field on the surface and in the bore of stacked annuli were scanned by a Hall probe. The Hall probe with a 25 25 lm2 sensing area was mounted on a computer controlled x–y–z stage with the spatial resolution of 1 lm. The schematic drawing of the stacked HTS bulk magnet with spacer (Bakelite plate) and measurement system were shown in
-15
Gap 1mm Gap 3mm Gap 5mm Gap 7mm Gap 9mm
70000 60000 50000 40000 30000 20000 10000 0
0
1
2
3
4
5
6
7
8
9
10
z axis, DSV (mm) Fig. 4. Calculated spatial field error profiles along z-axis of the four stacked bulk magnet.
1456
S.B. Kim et al. / Physica C 471 (2011) 1454–1458
Fig. 5. Photograph of (a) a single GdBCO bulk annulus, and (b) the four-stacked HTS bulk magnet.
Fig. 6. The temporal decay property of the trapped field was estimated just at the time when the external magnetic field was removed, and field distribution property was measured after removing the external magnetic field and then 8 h passing. The measured magnetic field profiles along the z-axis of the four-stacked HTS bulk magnet as a function of gap length (0, 5, 10 mm) was shown in Fig. 7 when applied magnetic field was 2 T at 77 K, and inset figure shows the normalized profiles. When the applied magnetic field was 2 T, the maximum trapped magnetic field of the HTS bulk magnet without gap was almost 1.3 T. And the maximum strength of the trapped magnetic field was decreased due to increasing of the gap length, however the spatial homogeneity near the center in axial direction was obviously improved according to increasing of the gap length because the overall height of HTS bulk magnet has been enlarged. In Fig. 7, the homogeneity of the trapped filed along the z-axis shows asymmetry, it means the physical center of axial direction and measured position have maximum strength are not same. In this experiment, the HTS bulk with the lowest critical current density was located at most upper position, and this stack configuration bring on moving peak position. Fig. 8 (left side figures) presents x–y plane field mapping (scan area 7 7 mm2) of the four-stacked HTS bulk magnet at center of z-axis, and right side graphs show the measured field profiles in radial direction (white line in x–y plane field mapping) at various axial positions (0, +2, +5 mm) with various gap lengths (0, 5, 10 mm) when applied magnetic field was 2 T at 77 K. Inset figures in Fig. 8 show the enlarged field profiles in radial direction. From Fig. 8, we found that the homogeneity of radial direction at
same z-axis position was improved as increasing of the gap length, and the convex of the magnetic field profiles in radial direction was reversed when the gap length was 10 mm since this gap length far larger than thickness of HTS bulk. Fig. 9 shows the normalized axial magnetic field distributions with various gap lengths (0 and 5 mm) when applied magnetic field were 0.5 T and 2 T at 77 K in order to investigate the relationships between the spatial homogeneity and strength of magnetization field. Inset figure shows the measured magnetic field distributions along z-axis when 0.5 T applied. From Fig. 9, we could easily find a remarkable improvement of spatial homogeneity when the gap length was 5 mm and 0.5 T applied. To investigate the temporal stability of the trapped field, the field decays at axial center position with different applied fields were measured for over 8 h. The measured characteristics of temporal decays of the four stacked HTS bulk magnet as functions of gap length and applied field were shown in Fig. 10. And inset figure shows the normalized decay profiles based on the value when 100 min passed after removing the applied magnet field, since we wanted to evaluate decay property after the initial field decay by the intense flux creep was removed. According to Fig. 10, in case of 2 T applied, the trapped magnetic field of both HTS bulk magnets with gap lengths are 0 and 5 mm were decreased about 4% in 8 h while there were rarely decay in 0.5 T applied. Thus, it is confirmed that the temporal stability of trapped field depends on the strength of applied magnetic field. As the trapped field becomes smaller, the temporal stability gets well. Therefore, to obtain the higher spatial
1.3
Hall sensor
Liquid N2
3D-axial stage
Bakelite plate
1.1 1.0 0.9 1.05
0.8
1.00
Normalized Bz
7mm
7mm
HTS bulk
Gap=0mm @2T Gap=5mm @2T Gap=10mm @2T
1.2
Magnetic flux dencity (T)
Measurement area
0.7 0.6
0.95 0.90 0.85 0.80 0.75
Gap=0mm Gap=5mm Gap=10mm
0.70 0.65 0.60 0.55 -20
0.5
-15
-10
-5
0
5
10
15
20
z axis (mm)
-20
-15
-10
-5
0
5
10
15
20
z axis (mm) Fig. 6. The schematic drawing of the stacked HTS bulk magnet with spacer (Bakelite plate) and measurement system.
Fig. 7. Measured magnetic field profiles along the z-axis of the four-stacked HTS bulk magnet as a function of gap length (0, 5, 10 mm) when applied magnetic field was 2 T at 77 K, and inset figure shows the normalized profiles.
1457
S.B. Kim et al. / Physica C 471 (2011) 1454–1458
1.4 1.3 1.2 1.1 1.0
z=0mm z=+2mm z=+5mm
0.9 0.8
Magntetic flux density (T)
(a) Magnetic flux density (T)
7 mm
7 mm
1.30 1.28 1.26 1.24 1.22 1.20 1.18 1.16
-4
-3
-2
-1
0
1
2
3
4
x axis (mm)
-4
-3
-2
-1
0
1
2
3
4
x axis (mm) 1.4 Magnetic flux density (T)
Magnetic flux density (T)
(b)
z=0mm z=+2mm z=+5mm
1.3 1.2
1.105 1.100 1.095 1.090 1.085 1.080 1.075 1.070 1.065 1.060
1.1
-4
-3
-2
-1
0
1
2
3
4
1.0 0.9 0.8
-4
-3
-2
-1
0
1
2
3
4
1.4 1.3
z=0mm z=+2mm z=+5mm
1.2 1.1
Magnetic flux density (T)
(c) Magnetic flux density (T)
x axis (mm)
0.896 0.894 0.892 0.890 0.888 0.886 0.884 0.882 0.880 0.878 0.876 0.874
-4
-3
-2
-1
0
1
2
3
4
x axis (mm)
1.0 0.9 0.8
-4
-3
-2
-1
0
1
2
3
4
x axis (mm) Fig. 8. Left figures; x–y plane field mapping (scan area 7 7 mm2) of the four-stacked HTS bulk magnet at center of z-axis, Right side graphs; Measured field profiles in radial direction (white line in x–y plane field mapping) at various axial positions when applied magnetic field was 2 T at 77 K (a) gap length = 0 mm, (b) gap length = 5 mm, (c) gap length = 10 mm. Inset figures show the enlarged field profiles in radial direction.
homogeneity and temporal stability of compact HTS bulk NMR magnet, we have to optimize the field cooling procedure.
4. Conclusions To develop a compact NMR magnet using HTS bulk annuli, experimental and analytic studies on trapped magnetic field char-
acteristics have been performed using the four stacked GdBCO HTS bulk magnet (thickness of bulk is 5 mm). In this study, the gap length between bulks and strength of the magnetization field were used as functions of parameters. The optimized axial gap length (below 10 mm) was found out by analytical results, and the better magnetic field homogeneity and temporal decay property of trapped magnetic field was obtained by lower magnetization filed (0.5 T) in this study. Therefore, to obtain the higher spatial homo-
1458
S.B. Kim et al. / Physica C 471 (2011) 1454–1458
geneity and temporal stability of compact HTS bulk NMR magnet, we have to optimize the field cooling procedure. In our future work, it is necessary to increase the number of HTS bulks to develop the compact NMR magnet that has higher magnetic field strength and the spatial homogeneity.
1.0
Magnetic flux density (T)
Normalized Bz
0.9
0.8
0.7
References
0.50 0.48 0.46 0.44
Gap=0mm @0.5T Gap=5mm @0.5T
0.42
Gap=0mm @2T Gap=5mm @2T Gap=0mm @0.5T Gap=5mm @0.5T
0.40
0.6
-20
-15
-10
-5
0
5
10
15
20
z axis (mm)
-10
-5
0
5
10
z axis (mm) Fig. 9. Measured magnetic field profiles along the z-axis in the stacked bulk annuli near the center with various gap lengths when applied magnetic field was 2 T and 0.5 T at 77 K. Inset figure shows measured magnetic field distribution when applied magnetic field was 0.5 T.
2.0
Magnetic flux density (T)
1.8 1.6
Normalized Bz
1.00
Gap=0mm @2T Gap=5mm @2T Gap=0mm @0.5T Gap=5mm @0.5T
Gap=0mm @2T Gap=5mm @2T Gap=0mm @0.5T Gap=5mm @0.5T
0.99
0.98
0.97
0.96
1.4
50
100
150
200
250
300
350
400
450
500
Time (min)
1.2 1.0 0.8 0.6 0
100
200
300
400
500
Time (min) Fig. 10. Measured characteristics of temporal decays of the four stacked HTS bulk magnet as functions of gap length (0 and 5 mm) and applied field (0.5 and 2 T), and inset figure shows the normalized decay profiles.
[1] M. Tomita, M. Murakami, Nature 421 (2003) 517. [2] T. Nakamura, Y. Itoh, M. Yoshikawa, T. Oka, J. Iwasa, Concept. Magnetic Res. Part B (2006) 65. [3] T. Oka, Physica C 463–465 (2007) 7. [4] Y. Iwasa, Physica C 445–448 (2006) 1008. [5] S.B. Kim, R. Takano, M. Imai, S.Y. Hahn, Physica C 469 (2009) 1844. [6] S.B. Kim, T. Nakano, R. Takano, S.Y. Hahn, IEEE Trans. Appl. Supercond. 19 (2009) 2273. [7] S.B. Kim, M. Imai, R. Takano, K. Kashima, S. Hahn, Physica C 470 (2010) 1740. [8] S.B. Kim, M. Imai, R. Takano, J.H. Joo, S. Hahn, IEEE Trans. Appl. Supe 21 (2011) 2080.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
The characteristics of spatial homogeneity and strength of magnetic field for compact NMR magnets using stacked HTS bulks with various gap lengths S.B. Kim a,⇑, T. Kimoto a, M. Imai a, Y. Yano a, J.H. Joo a, S. Hahn b, Y. Iwasa b, M. Tomita c a
Department of Electrical and Electronic Engineering, Okayama University, 3-1-1, Tsushima Naka, Kita-ku, Okayama 700-8530, Japan Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, 170 Albany Street, Cambridge, MA 02139, USA c Railway Technical Research Institute, 2-8-38, Hikari-cho, Kokubunji-shi, Tokyo 185-8540, Japan b
a r t i c l e
i n f o
Article history: Available online 14 May 2011 Keywords: Compact NMR HTS bulk annuli Axial gap length
a b s t r a c t A compact nuclear magnetic resonance (NMR) magnet is one of the new applications after a technique to enhance maximum trapped field of an high temperature superconducting (HTS) bulk. In design of a compact NMR magnet which consists of the stacked HTS bulk annuli, the issues of strength, spatial homogeneity and temporal stability by trapped magnetic fields are very important. This paper presents a study on the effects of magnetization field strength and gap length between stacked bulks for the compact HTS bulk NMR applications. Four-stacked HTS bulk magnet with ID 20 mm and OD 60 mm was prepared to investigate the optimized configuration. The thickness of each HTS bulk is 5 mm, and the gap lengths from 0 mm to 10 mm were used as parameters in analysis and experiment, respectively. Four-stacked HTS bulk magnets with various gap lengths were tested at two different background magnetic fields of 0.5 T and 2 T at 77 K. The optimized axial gap length was found out by analytical results, and the better magnetic field homogeneity and temporal decay property of trapped magnetic field were obtained by lower magnetization field in this experiments. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, the performance of high temperature superconducting (HTS) bulks in terms of mechanical strength, size and critical current density are greatly enhanced, thus their trapped magnetic field capability [1] expanded from the conventional HTS bulk applications. On the other hand, the nuclear magnetic resonance (NMR) spectroscopy becomes an indispensable measurement device in organic compound analysis and has been paid attention in food and drug industries as an effective tool for a non-destructive testing of nucleic acids. The analysis precision of the NMR is in proportion to strength of the magnetic field, so superconducting magnets wound with superconducting wire has been generally used as NMR magnets. However, many researchers still suffer from a lack of laboratory space as well as high fixed cost to install additional NMR units in their research facilities because the conventional NMR magnets still occupies significant spaces. The new type compact NMR magnet consisted of a stack of ring-shaped HTS bulks where a magnetic fields are trapped by field cooling method was suggested [2–4]. Since this magnetically charged HTS bulk magnet for NMR device does not need a power supply and additional coolant supply system, so it is expected that the new NMR device can achieve not only the compactness but also cost-efficiency. We have been devel⇑ Corresponding author. Tel.: +81 86 251 8116; fax: +81 86 251 8259. E-mail address: [email protected] (S.B. Kim). 0921-4534/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2011.05.215
oping a new prototype compact NMR magnet with stacked HTS bulk annuli and fundamental studies for the optimized configuration were carried out experimentally and analytically [5–8]. The proposed compact NMR magnet consisted of stacked HTS bulk annuli was magnetized by an external superconducting magnet with field cooling method, then magnetized HTS bulk annuli can be used with cryogenic container as a compact personal NMR magnet without additional superconducting magnets and any power sources. In our previous work, the spatial homogeneity and field strength of HTS bulk magnet were analytically investigated as a function of gap length against various thicknesses of HTS bulk. It was clarified that the spatial homogeneity in the HTS bulk magnet would be improved by reducing a thickness of the HTS bulk and enlarging a gap length between the bulks [8]. Therefore, in this study, we prepared the four-stack HTS bulk magnet with each HTS bulk thickness 5 mm. The numerical analysis based on 3-D FEM was carried out firstly in order to obtain an optimized axial gap length, then, the characteristics of spatial homogeneity and strength of magnetic field of stacked HTS bulk magnet with various axial gap lengths were investigated experimentally. 2. Analysis results To investigate the characteristics of trapped magnetic field in HTS bulk magnet, the commercial software based on a finite ele-
1455
S.B. Kim et al. / Physica C 471 (2011) 1454–1458
60 mm
z
20 mm +z
5 mm 269
Gap 0 -10mm
HTS bulk
5 mm Gap 0 -10mm
z=0 5 mm
x
Gap 0 -10mm -z
5 mm
Fig. 1. To-scaled schematic drawing of four stacked HTS bulk magnet with various axial gap lengths between bulks.
20 60 130 270 Fig. 2. To-scaled schematic analytical models of the four stacked HTS bulk magnet and superconducting magnet for magnetization.
1.0 0.9
Magnetic flux density (T)
ment method was used for simulation. The details of simulation method including governing equations were shown in previous work [7]. In this study, four GdBCO HTS bulks with 5 mm thickness were used, and Fig. 1 shows a scaled-schematic drawing of four stacked HTS bulk magnet. The inner and outer diameters of HTS bulk annulus are 20 and 60 mm, respectively. In analysis, the axial gap lengths between stacked HTS bulks were used as parameters and it was changed from 0 to 10 mm. Fig. 2 shows a schematic drawing of the analytical model including four-stacked HTS bulk magnet and external superconducting magnet. In this analysis, the magnetization field of 1 T was applied to study the characteristics of spatial homogeneity, and linearly decayed to 0 T in 250 s. Fig. 3 shows the calculated magnetic field distributions along the z-axis of the four-stacked HTS bulk magnet as a function of gap length (0–10 mm) between the HTS bulks. Fig. 4 provides an enlarged view of the calculated magnetic field error in ppm at near the center of axial direction (up to +10 mm). As shown in Figs. 3 and 4, the maximum field strength was decreased with increasing of the gap length, but the spatial homogeneity was improved as the gap length increasing up to 9 mm. However, the spatial field homogeneity becomes worse when the gap length was above 10 mm. The overall height of stacked HTS bulk magnet was enlarged with increasing the gap length, so, the spatial homogeneity was improved according to the overall height of HTS bulk magnet increasing until the gap length was below two times larger than thickness of HTS bulk of 5 mm. When the gap length became larger than thickness of HTS bulk, the leakage of magnetic field would be too large. From these results, when designing the compact NMR using four-stacked HTS bulk with each thickness is 5 mm, we need to fix gap length below 10 mm. Thus, we need to optimize the gap length to obtain the both of spatial homogeneity and field strength for compact NMR magnet.
(unit: mm)
0.8 0.7 0.6
Gap 0.1mm Gap 2mm Gap 4mm Gap 6mm Gap 8mm Gap 10mm
0.5 0.4 0.3 -25
-20
-10
-5
0
5
10
15
20
25
z axis (mm) Fig. 3. Calculated magnetic field distributions along the z-axis of the four stacked bulk magnet.
100000
Gap 0.1mm Gap 1mm Gap 2mm Gap 3mm Gap 4mm Gap 5mm Gap 6mm Gap 7mm Gap 8mm Gap 9mm Gap 10mm
90000 80000
Field error (ppm)
3. Experiment and results Fig. 5 shows the photographs of the top view of single GdBCO HTS bulk annulus and four-stacked bulk magnet. Inner diameter, outer diameter and thickness of HTS bulks are 20, 60 and 5 mm, respectively, and each HTS bulk was surrounded by the stainless steel ring with 2 mm thickness to improve mechanical strength. The maximum strengths of the trapped magnetic field of three HTS bulks are 0.55 T, and the other one is 0.44 T. The HTS bulk annuli were magnetized by the field cooling method at 77 K, and axial and radial components of the trapped magnetic field on the surface and in the bore of stacked annuli were scanned by a Hall probe. The Hall probe with a 25 25 lm2 sensing area was mounted on a computer controlled x–y–z stage with the spatial resolution of 1 lm. The schematic drawing of the stacked HTS bulk magnet with spacer (Bakelite plate) and measurement system were shown in
-15
Gap 1mm Gap 3mm Gap 5mm Gap 7mm Gap 9mm
70000 60000 50000 40000 30000 20000 10000 0
0
1
2
3
4
5
6
7
8
9
10
z axis, DSV (mm) Fig. 4. Calculated spatial field error profiles along z-axis of the four stacked bulk magnet.
1456
S.B. Kim et al. / Physica C 471 (2011) 1454–1458
Fig. 5. Photograph of (a) a single GdBCO bulk annulus, and (b) the four-stacked HTS bulk magnet.
Fig. 6. The temporal decay property of the trapped field was estimated just at the time when the external magnetic field was removed, and field distribution property was measured after removing the external magnetic field and then 8 h passing. The measured magnetic field profiles along the z-axis of the four-stacked HTS bulk magnet as a function of gap length (0, 5, 10 mm) was shown in Fig. 7 when applied magnetic field was 2 T at 77 K, and inset figure shows the normalized profiles. When the applied magnetic field was 2 T, the maximum trapped magnetic field of the HTS bulk magnet without gap was almost 1.3 T. And the maximum strength of the trapped magnetic field was decreased due to increasing of the gap length, however the spatial homogeneity near the center in axial direction was obviously improved according to increasing of the gap length because the overall height of HTS bulk magnet has been enlarged. In Fig. 7, the homogeneity of the trapped filed along the z-axis shows asymmetry, it means the physical center of axial direction and measured position have maximum strength are not same. In this experiment, the HTS bulk with the lowest critical current density was located at most upper position, and this stack configuration bring on moving peak position. Fig. 8 (left side figures) presents x–y plane field mapping (scan area 7 7 mm2) of the four-stacked HTS bulk magnet at center of z-axis, and right side graphs show the measured field profiles in radial direction (white line in x–y plane field mapping) at various axial positions (0, +2, +5 mm) with various gap lengths (0, 5, 10 mm) when applied magnetic field was 2 T at 77 K. Inset figures in Fig. 8 show the enlarged field profiles in radial direction. From Fig. 8, we found that the homogeneity of radial direction at
same z-axis position was improved as increasing of the gap length, and the convex of the magnetic field profiles in radial direction was reversed when the gap length was 10 mm since this gap length far larger than thickness of HTS bulk. Fig. 9 shows the normalized axial magnetic field distributions with various gap lengths (0 and 5 mm) when applied magnetic field were 0.5 T and 2 T at 77 K in order to investigate the relationships between the spatial homogeneity and strength of magnetization field. Inset figure shows the measured magnetic field distributions along z-axis when 0.5 T applied. From Fig. 9, we could easily find a remarkable improvement of spatial homogeneity when the gap length was 5 mm and 0.5 T applied. To investigate the temporal stability of the trapped field, the field decays at axial center position with different applied fields were measured for over 8 h. The measured characteristics of temporal decays of the four stacked HTS bulk magnet as functions of gap length and applied field were shown in Fig. 10. And inset figure shows the normalized decay profiles based on the value when 100 min passed after removing the applied magnet field, since we wanted to evaluate decay property after the initial field decay by the intense flux creep was removed. According to Fig. 10, in case of 2 T applied, the trapped magnetic field of both HTS bulk magnets with gap lengths are 0 and 5 mm were decreased about 4% in 8 h while there were rarely decay in 0.5 T applied. Thus, it is confirmed that the temporal stability of trapped field depends on the strength of applied magnetic field. As the trapped field becomes smaller, the temporal stability gets well. Therefore, to obtain the higher spatial
1.3
Hall sensor
Liquid N2
3D-axial stage
Bakelite plate
1.1 1.0 0.9 1.05
0.8
1.00
Normalized Bz
7mm
7mm
HTS bulk
Gap=0mm @2T Gap=5mm @2T Gap=10mm @2T
1.2
Magnetic flux dencity (T)
Measurement area
0.7 0.6
0.95 0.90 0.85 0.80 0.75
Gap=0mm Gap=5mm Gap=10mm
0.70 0.65 0.60 0.55 -20
0.5
-15
-10
-5
0
5
10
15
20
z axis (mm)
-20
-15
-10
-5
0
5
10
15
20
z axis (mm) Fig. 6. The schematic drawing of the stacked HTS bulk magnet with spacer (Bakelite plate) and measurement system.
Fig. 7. Measured magnetic field profiles along the z-axis of the four-stacked HTS bulk magnet as a function of gap length (0, 5, 10 mm) when applied magnetic field was 2 T at 77 K, and inset figure shows the normalized profiles.
1457
S.B. Kim et al. / Physica C 471 (2011) 1454–1458
1.4 1.3 1.2 1.1 1.0
z=0mm z=+2mm z=+5mm
0.9 0.8
Magntetic flux density (T)
(a) Magnetic flux density (T)
7 mm
7 mm
1.30 1.28 1.26 1.24 1.22 1.20 1.18 1.16
-4
-3
-2
-1
0
1
2
3
4
x axis (mm)
-4
-3
-2
-1
0
1
2
3
4
x axis (mm) 1.4 Magnetic flux density (T)
Magnetic flux density (T)
(b)
z=0mm z=+2mm z=+5mm
1.3 1.2
1.105 1.100 1.095 1.090 1.085 1.080 1.075 1.070 1.065 1.060
1.1
-4
-3
-2
-1
0
1
2
3
4
1.0 0.9 0.8
-4
-3
-2
-1
0
1
2
3
4
1.4 1.3
z=0mm z=+2mm z=+5mm
1.2 1.1
Magnetic flux density (T)
(c) Magnetic flux density (T)
x axis (mm)
0.896 0.894 0.892 0.890 0.888 0.886 0.884 0.882 0.880 0.878 0.876 0.874
-4
-3
-2
-1
0
1
2
3
4
x axis (mm)
1.0 0.9 0.8
-4
-3
-2
-1
0
1
2
3
4
x axis (mm) Fig. 8. Left figures; x–y plane field mapping (scan area 7 7 mm2) of the four-stacked HTS bulk magnet at center of z-axis, Right side graphs; Measured field profiles in radial direction (white line in x–y plane field mapping) at various axial positions when applied magnetic field was 2 T at 77 K (a) gap length = 0 mm, (b) gap length = 5 mm, (c) gap length = 10 mm. Inset figures show the enlarged field profiles in radial direction.
homogeneity and temporal stability of compact HTS bulk NMR magnet, we have to optimize the field cooling procedure.
4. Conclusions To develop a compact NMR magnet using HTS bulk annuli, experimental and analytic studies on trapped magnetic field char-
acteristics have been performed using the four stacked GdBCO HTS bulk magnet (thickness of bulk is 5 mm). In this study, the gap length between bulks and strength of the magnetization field were used as functions of parameters. The optimized axial gap length (below 10 mm) was found out by analytical results, and the better magnetic field homogeneity and temporal decay property of trapped magnetic field was obtained by lower magnetization filed (0.5 T) in this study. Therefore, to obtain the higher spatial homo-
1458
S.B. Kim et al. / Physica C 471 (2011) 1454–1458
geneity and temporal stability of compact HTS bulk NMR magnet, we have to optimize the field cooling procedure. In our future work, it is necessary to increase the number of HTS bulks to develop the compact NMR magnet that has higher magnetic field strength and the spatial homogeneity.
1.0
Magnetic flux density (T)
Normalized Bz
0.9
0.8
0.7
References
0.50 0.48 0.46 0.44
Gap=0mm @0.5T Gap=5mm @0.5T
0.42
Gap=0mm @2T Gap=5mm @2T Gap=0mm @0.5T Gap=5mm @0.5T
0.40
0.6
-20
-15
-10
-5
0
5
10
15
20
z axis (mm)
-10
-5
0
5
10
z axis (mm) Fig. 9. Measured magnetic field profiles along the z-axis in the stacked bulk annuli near the center with various gap lengths when applied magnetic field was 2 T and 0.5 T at 77 K. Inset figure shows measured magnetic field distribution when applied magnetic field was 0.5 T.
2.0
Magnetic flux density (T)
1.8 1.6
Normalized Bz
1.00
Gap=0mm @2T Gap=5mm @2T Gap=0mm @0.5T Gap=5mm @0.5T
Gap=0mm @2T Gap=5mm @2T Gap=0mm @0.5T Gap=5mm @0.5T
0.99
0.98
0.97
0.96
1.4
50
100
150
200
250
300
350
400
450
500
Time (min)
1.2 1.0 0.8 0.6 0
100
200
300
400
500
Time (min) Fig. 10. Measured characteristics of temporal decays of the four stacked HTS bulk magnet as functions of gap length (0 and 5 mm) and applied field (0.5 and 2 T), and inset figure shows the normalized decay profiles.
[1] M. Tomita, M. Murakami, Nature 421 (2003) 517. [2] T. Nakamura, Y. Itoh, M. Yoshikawa, T. Oka, J. Iwasa, Concept. Magnetic Res. Part B (2006) 65. [3] T. Oka, Physica C 463–465 (2007) 7. [4] Y. Iwasa, Physica C 445–448 (2006) 1008. [5] S.B. Kim, R. Takano, M. Imai, S.Y. Hahn, Physica C 469 (2009) 1844. [6] S.B. Kim, T. Nakano, R. Takano, S.Y. Hahn, IEEE Trans. Appl. Supercond. 19 (2009) 2273. [7] S.B. Kim, M. Imai, R. Takano, K. Kashima, S. Hahn, Physica C 470 (2010) 1740. [8] S.B. Kim, M. Imai, R. Takano, J.H. Joo, S. Hahn, IEEE Trans. Appl. Supe 21 (2011) 2080.