- Email: [email protected]
Magnetic evaluation of a solar panel using HTS-SQUID
Physica C 494 (2013) 195–198
Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Magnetic evaluation of a solar panel using HTS-SQUID Toshihiko Kiwa ⇑, Yohei Fukudome, Shingo Miyazaki, Mohd Mawardi Saari, Kenji Sakai, Keiji Tsukada Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushimanaka, Kitaku, Okayama 700-8530, Japan
a r t i c l e
i n f o
Article history: Received 14 January 2013 Received in revised form 23 April 2013 Accepted 26 April 2013 Available online 7 May 2013 Keywords: HTS-SQUID Solar panel Magnetic imaging Pick-up coil
a b s t r a c t The magnetic evaluation system of a solar panel using HTS-SQUID has been proposed and developed. A normal pick-up coil was applied to detect the tangential magnetic field to the panel surface. Since the detected field could be related to the currents of the solar panels, the electric properties of the solar panels could be evaluated. In this work, the evaluation of the electric properties of the commercial solar panels as well as the electric circuits made by the discrete devices on the circuit board was visualized. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The use of sustainable energy, including solar power, wind power, and fuel cell energy, has become crucial because of the on-going depletion of fossil energy sources. Among a various power generators, solar panels are one of the promised devices because solar panels have less restriction to build the power plants compared to other natural power generators. However, the production cost of the electric power is relatively higher than the conventional energy sources, and thus, the high effective solar cells are still demanded. Up to now, a various types of non-destructive evaluation system to diagnose the solar cells has been proposed and developed. For example, the microwave technique and the photo-excited method have been developed for evaluation of semiconductor parameters of solar cells, which include carrier lifetime, carrier density, mobility and conductivity [1,2]. An electro or a photo-luminescence [3,4] measurement system [5,6] is widely used to measure the electric properties of each cell and pn junction in the solar panels. A laser-SQUID system is also expected as one of the most promised systems for evaluating micro-cracks and photocurrent distributions [7–10]. In our group, the high-TC superconducting quantum interference devices (HTS-SQUIDs) unit with a normal pick-up coil has been developed to visualize the magnetic field distribution. This type of systems can easily customize to suit measuring samples [11–13]. Recently, the magnetic imaging of the solar panel surface with applying the AC voltages to the panel was carried out. Since the magnetic field from the panels can be related to the currents
⇑ Corresponding author. Tel./fax: +81 86 251 8130. E-mail address: [email protected] (T. Kiwa). 0921-4534/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2013.04.080
of the cells, the distribution of the electric properties of the solar panels could be evaluated [11]. In this work, we applied the normal pick-up coil which can detect the two independent components of the tangential magnetic field to the panel surface and demonstrate the evaluation of the electric properties of commercial solar panels. 2. Experimental The schematic diagram of the developed HTS-SQUID system is shown in Fig. 1. A HTS-SQUID unit was separated from a solar panel measurement unit. Thus, the HTS-SQUID units can share for various types of measurement systems. The HTS-SQUID units consist of the liquid nitrogen Dewar, the cylindrical magnetic shield and the HTS-SQUID with the input coil mounted on the chip. The ramp-edge type Josephson junctions fabricated by ISTEC-SRL, Japan was applied. The base and counter superconducting electrodes were La0.1Er0.95Ba1.95Cu3Oy and SmBa2Cu3Oy, respectively. The fabrication process and the structure of the HTS-SQUID were reported elsewhere [14,15]. The planer input coil was mounted on the one side of the gradiometer of the HTS-SQUID with flip-chip configuration [16]. The inductance of the input coil was about 110 lH and the mutual inductance between the input coil and the HTS-SQUID was about 1.88 nH. The HTS-SQUID was driven by the commercial flux-locked loop (FFL) circuit. In this experiment, the input coil was connected to the pick-up coils in the solar panel measurement unit. The solar panel was mounted on the x–y movable stage in the magnetic shielding box made from a parmalloy metals. The solar panel was connected to the voltage source which can apply the AC voltages with the offset voltage. The pick-up coils were fixed on the electrolytic cell. The directions of the coils were set to be along x and y axis, thus the coils could detect the tangential components
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T. Kiwa et al. / Physica C 494 (2013) 195–198
Fig. 1. The schematic diagram of the HTS-SQUID system.
of the magnetic field generated by the currents in the solar panel. The pick-up coil had 200 turns and the inner size was 5 mm. The distance between the coil and cell was about 1 mm and therefore the distance between the coil and electrolyte was about 2 mm. The detected signals were transferred to the input coil of the HTS-SQUID unit and lock-in amplified. When we applied the following bias voltage V(t) to the solar panel,
VðtÞ ¼ V ac cos xt þ V dc ;
ð1Þ
the magnetic field B(V) generated from the solar panel could be expanded as
BðVÞ ¼ Bðcos xt þ V dc Þ 2
2
¼ BðV dc Þ þ dB=dV V ac cos xt þ 1=2d B=dV ðV ac cos xtÞ2 ;
ð2Þ
where Vac and Vdc are the amplitude of the bias voltage and the offset voltage, respectively, and x represents the modulation frequency given by 2p 1.7 kHz. Lock-in amplification allowed us to obtain the amplitude of the second term of Eq. (2), dB/dV Vac as the output signal, which is proportional to the differential conductivity of the solar panel at the location of the pick-up coil. Thus, the differential conductivity imaging of the solar panel could be realized by scanning the pick-up coil across the solar panel surface.
Fig. 2. The schematic diagram of the fabricated circuit. R1, R2, and C were respectively 200 X, 2 kX and 0.47 lF.
3. Results and discussion As a first demonstration of the magnetic evaluations, the electric circuit made by the discrete devices on the circuit board was visualized. Fig. 2 shows the schematic diagram of the fabricated circuit. R1, R2, and C were respectively 200 X, 2 kX and 0.47 lF. A conventional LED was used in this circuit. By applying the AC voltage with peak-to-peak voltage of 0.3 V with offset voltage of 2.0 V to the circuit, the distribution of the dB/dV signals above the surface of the circuit board was measured using developed HTSSQUID system. The signals along x-axis and y-axis were vector synthesized and then mapped as shown in Fig. 3. The magnitudes of the signals were distributed depending on the types and/or the values of the discrete devices and estimated the dB/dV signals to be 13.1, 4.2, 10.3, and 1.5 for R1, R2, LED, and C, respectively. We simulated the current–voltage curves of the circuits using a simulation program with integrated circuit emphasis (SPICE) as indicated in
Fig. 3. The dB/dV signals measured by the developed system.
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T. Kiwa et al. / Physica C 494 (2013) 195–198
1
R1 R2
Current (mA)
20
LED C
(a) Y-axis
10
2 0
X-axis
1
2
3
4
5
Voltage (V) Fig. 4. The current–voltage curves of the circuits indicated in Fig. 3. simulated by SPICE.
Fig. 4. The differential conductance dI/dV at 2.0 V were estimated from the simulated date between 1.85 V and 2.15 V and the values were 4.90 103, 4.89 103, 1.84 105, and 9.80 1021, respectively, for R1, R2, LED, and C. These values were qualitatively consistent with the obtained date in Fig. 3. This result indicates that the developed system could visualize the distribution of the electric properties. To demonstrate the visualization of electric properties of solar panel, the commercial poly-crystal solar panel was measured. Fig. 5a shows the photograph of measured area of the panel, and the Fig. 5b and c shows the dB/dV signals obtained by sweeping the offset voltages. The circles 1 and 2 in Fig. 5a indicate the position of the pick-up coil when the data shown in Fig. 5b and c was measured, respectively. Although the amplitude of the signal along y-axis was larger than that of along x-axis at position 1, the amplitude of the signal along y-axis was smaller than that of along x-axis at position 2. This fact implies that the currents around position 1 and position 2 flow x-axis and y-axis respectively. Fig. 6 shows the dB/dV mapping of the commercial solar panel. One can see that the signals were emphasized along the cells. The amplitude of the signal of each solar cell was different from each other, which may suggests that the variation of the electric properties of each cell. When the sample is thin and the depth profile of the current is negligible, the vector of a local current I(t) can be given by,
IðtÞ / By ðtÞ ex þ BxðtÞ ey ;
Signal along x-axis Signal along y-axis
4
2
0 0
1
2
3
4
5
4
5
Voltage (V)
(c) 6 dB/dV signal (arb. units)
0
dB/dV signal (arb. units)
(b) 6
Signal along x-axis Signal along y-axis
4
2
0
0
1
2
3
Voltage (V) Fig. 5. (a) The photograph of measured area of the panel, and (b) and (c), respectively indicate the dB/dV signals at the circles 1 and 2 indicated in Fig. 5a.
ð3Þ
where Bx(t) and By(t) represent the x and y components of the measured magnetic fields above the currents, ex and ey are the unit vectors along x and y axis [17]. In our experiment, since the lock-in detection technique was applied, the vector component of the differential conductivity dI/dV can be obtained.
dI=dV / ðdBy ðtÞ=dVÞ ex þ ðBx ðtÞ=dVÞ ey :
ð4Þ
The dI/dV has the magnitude proportional to the differential conductivity and the direction of the vector is parallel to the current. The arrows indicated in Fig. 6 represent the approximate dI/dV calculated at where the signal was obtained. Although the spatial resolution of the system was still poor and the image and the arrow map was enlarged compared to the sample size, this result suggests that the magnetic evaluation of the solar panels could be possible. The magnetic imaging with higher resolution is now under way. Fig. 6. The dB/dV mapping of the commercial solar panel. The arrows indicate the approximate currents estimated from the signals.
4. Summary The magnetic evaluation system of a solar panel using HTSSQUID was developed. As the demonstration of imaging of the electric properties, the circuits with discrete devices on the circuit
board was measured. The signal above each device was qualitatively consistent with the result estimated from the circuit simulation. The electric properties of the solar panel could be also
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T. Kiwa et al. / Physica C 494 (2013) 195–198
visualized. These results suggest that the developed system can be used as one of the evaluation tools to develop solar panels. Acknowledgements Authors thank to A. Tsukamoto, S. Adachi, and K Tanabe from ISTEC-SRL, Japan for the HTS-SQUID fabrications and the fruitful discussion on the sensing system. This work is supported by ‘‘Strategic Promotion of Innovative R&D’’ in Japan Science and Technology Agency (JST). References
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
[1] [2] [3] [4]
S. Bothra, J.M. Borrego, S.K. Ghandhi, Sol. Cells 27 (1989) 437–444. S.N. Singh, N.K. Arora, N.P. Singh, Sol. Cells 13 (1985) 271–275. G.H. Bauer, L. Gutay, Chimia 61 (2007) 801–805. J.I. Pankove, Sol. Cells 2 (1980) 443–449.
[17]
M. Daibo, T. Kotaka, A. Shikoda, Physica C 357 (2001) 1483–1487. M. Daibo, A. Shikoda, M. Yoshizawa, Physica C 372 (2002) 263–266. H. Itozaki, T. Hayashi, J. Korean Phys. Soc. 48 (2006) 1090–1094. K. Kojima, S. Suda, X.Y. Kong, H. Itozaki, Physica C 445 (2006) 979–981. X.Y. Kong, K. Kojima, K. Sakuta, H. Itozaki, Physica C 463 (2007) 1048–1051. K. Nikawa, IEICE Trans. Electron. E85c (2002) 746–751. T. Kiwa, T. Maeda, S. Miyake, N. Kataoka, A. Tsukamoto, S. Adachi, et al., Physica C 471 (2011) 1238–1241. K. Sakai, M.M. Saari, T. Kiwa, A. Tsukamoto, S. Adachi, K. Tanabe, et al., Supercond. Sci. Technol. 25 (2012) 045005. M.M. Saari, K. Sakai, T. Kiwa, A. Tsukamoto, S. Adachi, K. Tanabe, et al., Jpn. J. Appl. Phys. 51 (2012) 046601. S. Adachi, K. Hata, T. Sugano, H. Wakana, T. Hato, Y. Tarutani, et al., Physica C 468 (2008) 1936–1971. H. Wakana, S. Adachi, K. Hata, T. Hato, Y. Tarutani, K. Tanabe, IEEE Trans. Appl. Supercond. 19 (2009) 782–785. A. Tsukamoto, S. Adachi, Y. Oshikubo, K. Tanabe, K. Enpuku, Supercond. Sci. Technol. 26 (2013) 015013. K. Tsukada, T. Kiwa, IEEE Trans. Appl. Magn. 42 (2006) 3315–3317.
Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Magnetic evaluation of a solar panel using HTS-SQUID Toshihiko Kiwa ⇑, Yohei Fukudome, Shingo Miyazaki, Mohd Mawardi Saari, Kenji Sakai, Keiji Tsukada Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushimanaka, Kitaku, Okayama 700-8530, Japan
a r t i c l e
i n f o
Article history: Received 14 January 2013 Received in revised form 23 April 2013 Accepted 26 April 2013 Available online 7 May 2013 Keywords: HTS-SQUID Solar panel Magnetic imaging Pick-up coil
a b s t r a c t The magnetic evaluation system of a solar panel using HTS-SQUID has been proposed and developed. A normal pick-up coil was applied to detect the tangential magnetic field to the panel surface. Since the detected field could be related to the currents of the solar panels, the electric properties of the solar panels could be evaluated. In this work, the evaluation of the electric properties of the commercial solar panels as well as the electric circuits made by the discrete devices on the circuit board was visualized. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The use of sustainable energy, including solar power, wind power, and fuel cell energy, has become crucial because of the on-going depletion of fossil energy sources. Among a various power generators, solar panels are one of the promised devices because solar panels have less restriction to build the power plants compared to other natural power generators. However, the production cost of the electric power is relatively higher than the conventional energy sources, and thus, the high effective solar cells are still demanded. Up to now, a various types of non-destructive evaluation system to diagnose the solar cells has been proposed and developed. For example, the microwave technique and the photo-excited method have been developed for evaluation of semiconductor parameters of solar cells, which include carrier lifetime, carrier density, mobility and conductivity [1,2]. An electro or a photo-luminescence [3,4] measurement system [5,6] is widely used to measure the electric properties of each cell and pn junction in the solar panels. A laser-SQUID system is also expected as one of the most promised systems for evaluating micro-cracks and photocurrent distributions [7–10]. In our group, the high-TC superconducting quantum interference devices (HTS-SQUIDs) unit with a normal pick-up coil has been developed to visualize the magnetic field distribution. This type of systems can easily customize to suit measuring samples [11–13]. Recently, the magnetic imaging of the solar panel surface with applying the AC voltages to the panel was carried out. Since the magnetic field from the panels can be related to the currents
⇑ Corresponding author. Tel./fax: +81 86 251 8130. E-mail address: [email protected] (T. Kiwa). 0921-4534/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2013.04.080
of the cells, the distribution of the electric properties of the solar panels could be evaluated [11]. In this work, we applied the normal pick-up coil which can detect the two independent components of the tangential magnetic field to the panel surface and demonstrate the evaluation of the electric properties of commercial solar panels. 2. Experimental The schematic diagram of the developed HTS-SQUID system is shown in Fig. 1. A HTS-SQUID unit was separated from a solar panel measurement unit. Thus, the HTS-SQUID units can share for various types of measurement systems. The HTS-SQUID units consist of the liquid nitrogen Dewar, the cylindrical magnetic shield and the HTS-SQUID with the input coil mounted on the chip. The ramp-edge type Josephson junctions fabricated by ISTEC-SRL, Japan was applied. The base and counter superconducting electrodes were La0.1Er0.95Ba1.95Cu3Oy and SmBa2Cu3Oy, respectively. The fabrication process and the structure of the HTS-SQUID were reported elsewhere [14,15]. The planer input coil was mounted on the one side of the gradiometer of the HTS-SQUID with flip-chip configuration [16]. The inductance of the input coil was about 110 lH and the mutual inductance between the input coil and the HTS-SQUID was about 1.88 nH. The HTS-SQUID was driven by the commercial flux-locked loop (FFL) circuit. In this experiment, the input coil was connected to the pick-up coils in the solar panel measurement unit. The solar panel was mounted on the x–y movable stage in the magnetic shielding box made from a parmalloy metals. The solar panel was connected to the voltage source which can apply the AC voltages with the offset voltage. The pick-up coils were fixed on the electrolytic cell. The directions of the coils were set to be along x and y axis, thus the coils could detect the tangential components
196
T. Kiwa et al. / Physica C 494 (2013) 195–198
Fig. 1. The schematic diagram of the HTS-SQUID system.
of the magnetic field generated by the currents in the solar panel. The pick-up coil had 200 turns and the inner size was 5 mm. The distance between the coil and cell was about 1 mm and therefore the distance between the coil and electrolyte was about 2 mm. The detected signals were transferred to the input coil of the HTS-SQUID unit and lock-in amplified. When we applied the following bias voltage V(t) to the solar panel,
VðtÞ ¼ V ac cos xt þ V dc ;
ð1Þ
the magnetic field B(V) generated from the solar panel could be expanded as
BðVÞ ¼ Bðcos xt þ V dc Þ 2
2
¼ BðV dc Þ þ dB=dV V ac cos xt þ 1=2d B=dV ðV ac cos xtÞ2 ;
ð2Þ
where Vac and Vdc are the amplitude of the bias voltage and the offset voltage, respectively, and x represents the modulation frequency given by 2p 1.7 kHz. Lock-in amplification allowed us to obtain the amplitude of the second term of Eq. (2), dB/dV Vac as the output signal, which is proportional to the differential conductivity of the solar panel at the location of the pick-up coil. Thus, the differential conductivity imaging of the solar panel could be realized by scanning the pick-up coil across the solar panel surface.
Fig. 2. The schematic diagram of the fabricated circuit. R1, R2, and C were respectively 200 X, 2 kX and 0.47 lF.
3. Results and discussion As a first demonstration of the magnetic evaluations, the electric circuit made by the discrete devices on the circuit board was visualized. Fig. 2 shows the schematic diagram of the fabricated circuit. R1, R2, and C were respectively 200 X, 2 kX and 0.47 lF. A conventional LED was used in this circuit. By applying the AC voltage with peak-to-peak voltage of 0.3 V with offset voltage of 2.0 V to the circuit, the distribution of the dB/dV signals above the surface of the circuit board was measured using developed HTSSQUID system. The signals along x-axis and y-axis were vector synthesized and then mapped as shown in Fig. 3. The magnitudes of the signals were distributed depending on the types and/or the values of the discrete devices and estimated the dB/dV signals to be 13.1, 4.2, 10.3, and 1.5 for R1, R2, LED, and C, respectively. We simulated the current–voltage curves of the circuits using a simulation program with integrated circuit emphasis (SPICE) as indicated in
Fig. 3. The dB/dV signals measured by the developed system.
197
T. Kiwa et al. / Physica C 494 (2013) 195–198
1
R1 R2
Current (mA)
20
LED C
(a) Y-axis
10
2 0
X-axis
1
2
3
4
5
Voltage (V) Fig. 4. The current–voltage curves of the circuits indicated in Fig. 3. simulated by SPICE.
Fig. 4. The differential conductance dI/dV at 2.0 V were estimated from the simulated date between 1.85 V and 2.15 V and the values were 4.90 103, 4.89 103, 1.84 105, and 9.80 1021, respectively, for R1, R2, LED, and C. These values were qualitatively consistent with the obtained date in Fig. 3. This result indicates that the developed system could visualize the distribution of the electric properties. To demonstrate the visualization of electric properties of solar panel, the commercial poly-crystal solar panel was measured. Fig. 5a shows the photograph of measured area of the panel, and the Fig. 5b and c shows the dB/dV signals obtained by sweeping the offset voltages. The circles 1 and 2 in Fig. 5a indicate the position of the pick-up coil when the data shown in Fig. 5b and c was measured, respectively. Although the amplitude of the signal along y-axis was larger than that of along x-axis at position 1, the amplitude of the signal along y-axis was smaller than that of along x-axis at position 2. This fact implies that the currents around position 1 and position 2 flow x-axis and y-axis respectively. Fig. 6 shows the dB/dV mapping of the commercial solar panel. One can see that the signals were emphasized along the cells. The amplitude of the signal of each solar cell was different from each other, which may suggests that the variation of the electric properties of each cell. When the sample is thin and the depth profile of the current is negligible, the vector of a local current I(t) can be given by,
IðtÞ / By ðtÞ ex þ BxðtÞ ey ;
Signal along x-axis Signal along y-axis
4
2
0 0
1
2
3
4
5
4
5
Voltage (V)
(c) 6 dB/dV signal (arb. units)
0
dB/dV signal (arb. units)
(b) 6
Signal along x-axis Signal along y-axis
4
2
0
0
1
2
3
Voltage (V) Fig. 5. (a) The photograph of measured area of the panel, and (b) and (c), respectively indicate the dB/dV signals at the circles 1 and 2 indicated in Fig. 5a.
ð3Þ
where Bx(t) and By(t) represent the x and y components of the measured magnetic fields above the currents, ex and ey are the unit vectors along x and y axis [17]. In our experiment, since the lock-in detection technique was applied, the vector component of the differential conductivity dI/dV can be obtained.
dI=dV / ðdBy ðtÞ=dVÞ ex þ ðBx ðtÞ=dVÞ ey :
ð4Þ
The dI/dV has the magnitude proportional to the differential conductivity and the direction of the vector is parallel to the current. The arrows indicated in Fig. 6 represent the approximate dI/dV calculated at where the signal was obtained. Although the spatial resolution of the system was still poor and the image and the arrow map was enlarged compared to the sample size, this result suggests that the magnetic evaluation of the solar panels could be possible. The magnetic imaging with higher resolution is now under way. Fig. 6. The dB/dV mapping of the commercial solar panel. The arrows indicate the approximate currents estimated from the signals.
4. Summary The magnetic evaluation system of a solar panel using HTSSQUID was developed. As the demonstration of imaging of the electric properties, the circuits with discrete devices on the circuit
board was measured. The signal above each device was qualitatively consistent with the result estimated from the circuit simulation. The electric properties of the solar panel could be also
198
T. Kiwa et al. / Physica C 494 (2013) 195–198
visualized. These results suggest that the developed system can be used as one of the evaluation tools to develop solar panels. Acknowledgements Authors thank to A. Tsukamoto, S. Adachi, and K Tanabe from ISTEC-SRL, Japan for the HTS-SQUID fabrications and the fruitful discussion on the sensing system. This work is supported by ‘‘Strategic Promotion of Innovative R&D’’ in Japan Science and Technology Agency (JST). References
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
[1] [2] [3] [4]
S. Bothra, J.M. Borrego, S.K. Ghandhi, Sol. Cells 27 (1989) 437–444. S.N. Singh, N.K. Arora, N.P. Singh, Sol. Cells 13 (1985) 271–275. G.H. Bauer, L. Gutay, Chimia 61 (2007) 801–805. J.I. Pankove, Sol. Cells 2 (1980) 443–449.
[17]
M. Daibo, T. Kotaka, A. Shikoda, Physica C 357 (2001) 1483–1487. M. Daibo, A. Shikoda, M. Yoshizawa, Physica C 372 (2002) 263–266. H. Itozaki, T. Hayashi, J. Korean Phys. Soc. 48 (2006) 1090–1094. K. Kojima, S. Suda, X.Y. Kong, H. Itozaki, Physica C 445 (2006) 979–981. X.Y. Kong, K. Kojima, K. Sakuta, H. Itozaki, Physica C 463 (2007) 1048–1051. K. Nikawa, IEICE Trans. Electron. E85c (2002) 746–751. T. Kiwa, T. Maeda, S. Miyake, N. Kataoka, A. Tsukamoto, S. Adachi, et al., Physica C 471 (2011) 1238–1241. K. Sakai, M.M. Saari, T. Kiwa, A. Tsukamoto, S. Adachi, K. Tanabe, et al., Supercond. Sci. Technol. 25 (2012) 045005. M.M. Saari, K. Sakai, T. Kiwa, A. Tsukamoto, S. Adachi, K. Tanabe, et al., Jpn. J. Appl. Phys. 51 (2012) 046601. S. Adachi, K. Hata, T. Sugano, H. Wakana, T. Hato, Y. Tarutani, et al., Physica C 468 (2008) 1936–1971. H. Wakana, S. Adachi, K. Hata, T. Hato, Y. Tarutani, K. Tanabe, IEEE Trans. Appl. Supercond. 19 (2009) 782–785. A. Tsukamoto, S. Adachi, Y. Oshikubo, K. Tanabe, K. Enpuku, Supercond. Sci. Technol. 26 (2013) 015013. K. Tsukada, T. Kiwa, IEEE Trans. Appl. Magn. 42 (2006) 3315–3317.