- Email: [email protected]
CVD-YBCO tapes for pancake-type coils
Physica C 468 (2008) 1723–1726
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Basic AC loss properties of IBAD/CVD-YBCO tapes for pancake-type coils K. Funaki a,*, T. Sueyoshi a, M. Iwakuma a, K. Shikimachi b, N. Hirano b, S. Nagaya b a b
Research Institute of Superconductor Science and Systems, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan Chubu Electric Power Co., Inc., 20-1 Kitasekiyama, Ohdaka-cho, Midori-ku, Nagoya 459-8522, Japan
a r t i c l e
i n f o
Article history: Available online 1 July 2008 PACS: 74.25.Ha 74.72.Bk Keywords: YBCO coated conductor AC loss measurement Saddle-shaped pickup coil Temperature scaling Cu stabilizer Perpendicular magnetic field configuration
a b s t r a c t We are experimentally studying basic AC loss properties of IBAD/CVD-YBCO coated conductors with a copper layer for stabilizing, especially the temperature dependence of perpendicular field loss in alternating electromagnetic environments. We prepared two types of short specimens with and without a copper layer and measured AC losses by a saddle-shaped pickup coil in an alternating magnetic field perpendicular to the wide surfaces at liquid helium temperature. In the ranges of the amplitude up to 4 T and the frequency up to 0.2 Hz, the AC losses both of the two specimens are hardly dependent upon the frequency. The results show that hysteresis loss is a major component of the AC loss in the specimens and the effects of the copper layer can be negligible. We also measured AC losses for the specimens with the copper layer at liquid nitrogen temperature to estimate the dependence on measurement temperature. The results suggested that the AC loss vs. the amplitude of applied field can be scaled by a critical current at a zero magnetic field. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Since the critical temperature of YBCO oxide superconductors exceed liquid nitrogen temperature and the high-field properties in the current capacity are well improved in comparison with other high-temperature superconductors such as BSCCO oxide superconductors, the future feasibility is expected in various industrial fields such as electric power devices, magnetic energy storage systems, magnetic levitation and so on. Several groups have reported on the conductor fabrication of the YBCO coated conductors using advanced technology to deposit a thin superconducting textured film on 100 m-class substrates. Fabrication techniques of IBAD (Ion Beam Assisted Deposition)/ PLD (Pulse Laser Deposition) by Fujikura and EHTS, IBAD/MOCVD (Metal Organic Chemical Vapor Deposition) by SuperPower [1] and Chubu Electric Power [2], IBAD/TFA-MOD (Trifluoro Acetate Metal Organic Deposition) by SWCC and SRL, RABiTS (Rolling Assisted Biaxially Textured Substrates)/MOD by American Superconductor, RABiTS/PLD by Sumitomo, and so on have been making significant progress in both Ic and long length which are required for power applications. IBAD/PLD-GdBCO coated conductor fabricated by Fujikura achieved the world record of the Ic, 305 A and the length, 368 m [3].
* Corresponding author. Tel.: +81 92 802 3830; fax: +81 92 802 3829. E-mail address: [email protected] (K. Funaki). 0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2008.05.183
For the extended applications in AC electromagnetic environments, especially, AC loss property of the conductors has been also studied as one of the important design factors. From the viewpoint of the applications in a wide range of temperature, for example, it was shown experimentally that the AC loss property of a single YBCO coated conductor can be scaled by the zero-field critical current for temperature [4]. Geometrical effects on the AC loss properties were also evaluated theoretically for the piled conductors to simulate the practical arrangements in future applications [5]. We designed and fabricated YBCO coated conductors by IBAD/ CVD method to evaluate the electromagnetic properties for the future application to advanced windings of magnetic energy storage systems in electric power grids. In the present paper, we prepared two types of short specimens composed of laminated coated conductors with and without a copper layer to estimate the effects on the AC loss properties and measured their AC losses by a saddle-shaped pickup coil in an alternating magnetic field perpendicular to the wide surfaces at liquid helium temperature. We also measured AC losses in the specimens with the copper layer at liquid nitrogen temperature and discussed on temperature scaling in the AC loss properties. 2. Preparation of short specimens Two types of coated conductors were fabricated by multi-stage CVD for AC loss measurement. A multi-stage CVD technique is useful for a rapid fabrication of long YBCO coated conductor. The YBCO
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K. Funaki et al. / Physica C 468 (2008) 1723–1726
layers were deposited on substrates prepared by a combination of IBAD and PLD processes. The IBAD-GZO buffer layers were deposited on the flexible metal tapes of Hastelloy with 100 lm in thickness and 10 mm in width. The PLD-CeO2 cap layers were formed on them. The thicknesses of the IBAD-GZO layer and PLD-CeO2 cap layer were about 1 lm and 0.4 lm, respectively. About 20 lm thick Ag layers, as protection and stabilizing layers, were deposited on the YBCO layers by sputtering. Subsequently, oxygen anneal was carried out. Their Ic values were measured by the conventional four-probe method with the Ic criterion of 1 lV/cm and the Ic of specimens A and B were 120 A and 115 A, respectively. Only specimen B was soldered with 0.1 mm thick Cu tape for stabilization. The main characteristics are listed in Table 1. Specimens A and B are composed of 21 and 14 laminated conductors cut with a length of 60 mm, respectively, where Kapton sheets are put between the conductors for electrical insulation. A total height is 5.2 mm for specimen A and 4.4 mm for specimen B. A photograph of the two specimens is given in Fig. 1.
Fig. 1. Layered short specimens for AC loss measurements.
Bm= 0.4T (specimen A)
10
10
B = 0.4T (specimen B) m
B = 0.8T (specimen A) m
AC losses in layered short specimens of the IBAD/CVD-YBCO tapes were measured in liquid helium or liquid nitrogen by means of a saddle-shaped pickup coil method [6]. The external sinusoidal magnetic field was applied in the direction parallel or perpendicular to the flat surfaces of the layered specimen. The angle of the external magnetic field was adjusted to the flat surface within an error of a few degrees. The measurements were performed in the ranges of the frequency from 0.01 Hz to 0.2 Hz and the amplitude of external magnetic field between 0.01 T and 4.0 T. The experimental setup is the same in principle as that of the usual concentric pickup coil method, which has been standardized to measure the AC losses of round superconducting wires in liquid helium [7], except for the shape of the pickup coils and their arrangement.
AC Loss [J/m3/cycle]
B = 0.8T (specimen B)
3. AC loss measurements
m
B = 2.0T (specimen A) m
9
10
B = 2.0T (specimen B) m
8
10
7
10
-2
10
10 Frequency [Hz]
-1
Fig. 2. Frequency dependence of AC loss for layered YBCO tapes with and without Cu layer.
4. Results and discussion
The frequency dependence of the AC losses for two sets of the layered specimen in Table 1 was observed in liquid helium and in perpendicular magnetic field configuration, where the effect of the Cu layer may be remarkable compared with other configuration. The results in the frequency range of 0.01–0.2 Hz are shown in Fig. 2 for the amplitude Bm of AC magnetic field, 0.4 T, 0.8 T and 2.0 T. The AC losses observed both for the specimens are scarcely dependent upon the frequency. This means that the hysteresis loss of the YBCO layer is predominant in the specimens A and B, and that the effect of the Cu layer is negligible in the specimen B in the range of the measurement frequency. Table 1 Characteristics of layered short specimens of IBAD/CVD-YBCO tapes Specimen A Length (mm) Width (mm) Thickness of each layer (lm) Cu Ag YBCO CeO2 GZO Hastelloy Layered number Total height (mm) Critical current at s.f. and 77 K (A) Criterion: 1 lV/cm
Specimen B
60 10
60 10
– 20 1 0.4 1 100 21 5.2 120
100 20 1.1 0.4 1 100 14 4.4 115
Fig. 3 shows the dependence of the AC loss on Bm for the specimens A and B in the parallel and perpendicular field configurations. The experimental results in the frequency range from 0.01 Hz to 0.2 Hz are superposed for each specimen in both field configurations, which are on a common curve because the AC loss
10
10
9
10
8
AC Loss [J/m3/cycle]
4.1. Effect of Cu layer on AC loss property in liquid helium
10
7
10
6
10
specimen B (perpendicular) specimen B (parallel) specimen A (perpendicular) specimen A (parallel)
5
10
4
10
3
10
2
10
-2
10
10
-1
10
0
1
10
Bm [T] Fig. 3. AC loss vs. Bm in parallel and perpendicular field configurations for layered YBCO tapes with and without Cu layer.
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K. Funaki et al. / Physica C 468 (2008) 1723–1726
is almost independent of the frequency. A turning point in each common curve must indicate the condition of full flux penetration. It is found in Fig. 3 that the penetration field of the specimen A is a little higher than that of the specimen B. In a practical range of Bm more than about 1 T, the AC losses in the perpendicular field configuration are extraordinary larger than those in the parallel one. 4.2. AC loss in liquid nitrogen and the temperature scaling In liquid nitrogen, the AC losses of the specimen B with Cu layer were also measured in the perpendicular field configuration. The AC losses observed are plotted by squares for the amplitude of applied magnetic field in Fig. 4 in a similar way to Fig. 3, where the experimental results in liquid helium are plotted again by circles for comparison. The penetration field at liquid nitrogen temperature is about one tenth smaller than that at liquid helium one. Supposing such temperature dependence in the AC loss property can be predicted by a theoretical consideration, it must be an advantageous tool to design superconducting windings used in alternating
electromagnetic environment at a wide range of temperature. In this way, it may be useful to indicate that the experimental results obtained in Fig. 4 are scaled with respect to temperature as follows [6]. For a superconducting tape with a width of h exposed to a perpendicular magnetic field Be, the critical current Ic(Be, T) is approximately estimated from the magnetization M(Be, T) on the master hysteresis curve at measurement temperature T by [6]
Ic ðBe ; TÞ ¼ 4hjMðBe ; TÞj
where the Be dependence of the magnetization can be obtained in the AC loss measurement. An example for the Be dependence of the critical current at liquid helium temperature is given in Fig. 5. It is shown in this figure that the critical current asymptically approaches to a constant Ico in decreasing Be, and a power function of Be in increasing it. Supposing this relation of the critical current with Be can be extended to a wide range of temperature, Ic(Be, T) is approximately expressed by
10
10
10
10
9
AC Loss / μ I (T)2 [J/TAm4/cycle]
8
10
7
10
6
10
9
10
8
10
7
10
6
10
5
10
4
0 c0
AC Loss [J/m3/cycle]
10
10
5
10
specimen B, 4.2 K (perpendicular) specimen B, 77 K (perpendicular)
4
10
-2
10
-1
0
10
1
10
specimen B, 4.2 K specimen B, 77 K
3
10
ð1Þ
1
2
10
10
3
10
10
10
4
Bm / μ0Ic0(T) [1/m]
Bm [T] Fig. 4. AC loss vs. Bm in perpendicular configuration for YBCO tapes with Cu layer at liquid helium and nitrogen temperatures.
Fig. 6. Normalized AC loss vs. Bm for YBCO tapes with Cu layer.
0.0005
1
0.0004
0.8
0.0003
0.6
0.0002
0.4
0.0001
0.2
2000
Bb(T) / Ic0(T) [T/A]
1000 900 800 700
γ (T)
Ic [A]
Bb
600 500 400 0.01
0.1
1 Be [T]
Fig. 5. Critical current vs. perpendicular magnetic field in logarithmic scales at liquid helium temperature.
0
0
10
20
30
40 T [K]
50
60
70
0 80
Fig. 7. The temperature dependence of characteristic parameters.
1726
Ic ðBe ; TÞ ¼ Ico ðTÞ;
K. Funaki et al. / Physica C 468 (2008) 1723–1726
cðTÞ1 Be Be 6 BbðTÞ ¼ Ico ðTÞ ; Bb ðTÞ
Bb ðTÞ 6 Be ð2Þ
where Bb(T) is obtained from an intersection between the two asymptotic lines as shown in Fig. 5. Considering the AC loss is calculated by integrating the magnetization with respect to Be over the whole cycle, it is suggested from Eqs. (1) and (2) in the same manner as shown for a single IBAD/PLD YBCO tape in Ref. [6] that the AC loss normalized by l0Ico(T)2 can be scaled for Be normalized by Ico(T) under the following conditions; (i) The exponent c(T) in Eq. (2) is independent of T, (ii) The ratio Bb(T)/Ico(T) is independent of T. The normalized AC losses on the basis of the characteristic parameters Ico and c estimated in Fig. 5 are plotted in Fig. 6, where the symbols circles and squares indicate the results at liquid helium and liquid nitrogen temperatures, respectively. The two groups of the normalized AC losses overlap with each other and form a scaled master curve. The estimated values of Ico and c from each magnetization curve at liquid helium or liquid nitrogen temperature are shown in Fig. 7, where open and closed circles indicate Bb(T)/Ico(T) and c(T), respectively. As can be seen in this figure, the characteristic parameters are scarcely dependent upon the temperature. In the present case, the conditions for the scaling are approximately satisfied. 5. Concluding remarks We studied AC loss properties of layered short specimen of IBAD/CVD-YBCO tapes with a Cu stability layer by means of a saddle-shaped pickup coil. The AC loss properties obtained are summarized in the following. (i) In a practical range of the magnetic field amplitude more than about 1 T, the AC losses in the perpendicular field configuration are extraordinary larger than those in the parallel one.
(ii) The effect of the Cu layer on the AC loss property can be negligible in the perpendicular field configuration in ranges of the amplitude less than a few T and the frequency less than several Hz. (iii) In the perpendicular field configuration, the AC loss normalized by l0Ico(T)2 can be scaled for Be normalized by Ico(T) between liquid helium and liquid nitrogen temperatures. Ico(T) is an asymptotic critical current in the region of lower external field, which can be estimated from the magnetization curve. Acknowledgements This work was supported by the New Energy and Industrial Technology Development Organization (NEDO), under the Research and Development of Superconducting Magnetic Energy Storage System sponsored by Agency of Natural Resources and Energy, Ministry of Economy, Trade and Industry (METI). In addition, YBCO coated conductors used in this work were produced by Chubu Electric Power Co., supported by NEDO through ISTEC, as the Collaborative Research and Development of Fundamental Technologies for Superconductivity Applications. References [1] V. Selvamanickam, Y. Chen, X. Xiong, Y.Y. Xie, J.L. Reeves, X. Zhang, Y. Qiao, K.P. Lenseth, R.M. Schmidt, A. Rar, D.W. Hazelton, K. Tekletsadik, IEEE Trans. Appl. Supercond. 17 (2007) 3231. [2] T. Watanabe, N. Kashima, N. Suda, M. Mori, S. Nagaya, S. Miyata, A. Ibi, Y. Yamada, T. Izumi, Y. Shiohara, IEEE Trans. Appl. Supercond. 17 (2007) 3386. [3] Y. Shiohara, Presented as a plenary speech on 20 Septemper at EUCAS (2007). [4] M. Iwakuma, M. Nigo, D. Inoue, N. Miyamoto, K. Funaki, Y. Iijima, T. Saitoh, Y. Yamada, T. Izumi, Y. Shiohara, Supercond. Sci. Technol. 19 (2006) 350. [5] Y. Mawatari, IEEE Trans. Appl. Supercond. 7 (1997) 1216. [6] M. Iwakuma, M. Nanri, M. Fukui, Y. Fukuda, K. Kajikawa, K. Funaki, Supercond. Sci. Technol. 15 (2003) 545. [7] IEC61788-8 Superconductivity-Part 8: AC loss measurements – Total AC loss measurement of Cu/NbTi composite superconducting wires exposed to a transverse alternating magnetic field by a pickup coil method, First edition 2003-4.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Basic AC loss properties of IBAD/CVD-YBCO tapes for pancake-type coils K. Funaki a,*, T. Sueyoshi a, M. Iwakuma a, K. Shikimachi b, N. Hirano b, S. Nagaya b a b
Research Institute of Superconductor Science and Systems, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan Chubu Electric Power Co., Inc., 20-1 Kitasekiyama, Ohdaka-cho, Midori-ku, Nagoya 459-8522, Japan
a r t i c l e
i n f o
Article history: Available online 1 July 2008 PACS: 74.25.Ha 74.72.Bk Keywords: YBCO coated conductor AC loss measurement Saddle-shaped pickup coil Temperature scaling Cu stabilizer Perpendicular magnetic field configuration
a b s t r a c t We are experimentally studying basic AC loss properties of IBAD/CVD-YBCO coated conductors with a copper layer for stabilizing, especially the temperature dependence of perpendicular field loss in alternating electromagnetic environments. We prepared two types of short specimens with and without a copper layer and measured AC losses by a saddle-shaped pickup coil in an alternating magnetic field perpendicular to the wide surfaces at liquid helium temperature. In the ranges of the amplitude up to 4 T and the frequency up to 0.2 Hz, the AC losses both of the two specimens are hardly dependent upon the frequency. The results show that hysteresis loss is a major component of the AC loss in the specimens and the effects of the copper layer can be negligible. We also measured AC losses for the specimens with the copper layer at liquid nitrogen temperature to estimate the dependence on measurement temperature. The results suggested that the AC loss vs. the amplitude of applied field can be scaled by a critical current at a zero magnetic field. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Since the critical temperature of YBCO oxide superconductors exceed liquid nitrogen temperature and the high-field properties in the current capacity are well improved in comparison with other high-temperature superconductors such as BSCCO oxide superconductors, the future feasibility is expected in various industrial fields such as electric power devices, magnetic energy storage systems, magnetic levitation and so on. Several groups have reported on the conductor fabrication of the YBCO coated conductors using advanced technology to deposit a thin superconducting textured film on 100 m-class substrates. Fabrication techniques of IBAD (Ion Beam Assisted Deposition)/ PLD (Pulse Laser Deposition) by Fujikura and EHTS, IBAD/MOCVD (Metal Organic Chemical Vapor Deposition) by SuperPower [1] and Chubu Electric Power [2], IBAD/TFA-MOD (Trifluoro Acetate Metal Organic Deposition) by SWCC and SRL, RABiTS (Rolling Assisted Biaxially Textured Substrates)/MOD by American Superconductor, RABiTS/PLD by Sumitomo, and so on have been making significant progress in both Ic and long length which are required for power applications. IBAD/PLD-GdBCO coated conductor fabricated by Fujikura achieved the world record of the Ic, 305 A and the length, 368 m [3].
* Corresponding author. Tel.: +81 92 802 3830; fax: +81 92 802 3829. E-mail address: [email protected] (K. Funaki). 0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2008.05.183
For the extended applications in AC electromagnetic environments, especially, AC loss property of the conductors has been also studied as one of the important design factors. From the viewpoint of the applications in a wide range of temperature, for example, it was shown experimentally that the AC loss property of a single YBCO coated conductor can be scaled by the zero-field critical current for temperature [4]. Geometrical effects on the AC loss properties were also evaluated theoretically for the piled conductors to simulate the practical arrangements in future applications [5]. We designed and fabricated YBCO coated conductors by IBAD/ CVD method to evaluate the electromagnetic properties for the future application to advanced windings of magnetic energy storage systems in electric power grids. In the present paper, we prepared two types of short specimens composed of laminated coated conductors with and without a copper layer to estimate the effects on the AC loss properties and measured their AC losses by a saddle-shaped pickup coil in an alternating magnetic field perpendicular to the wide surfaces at liquid helium temperature. We also measured AC losses in the specimens with the copper layer at liquid nitrogen temperature and discussed on temperature scaling in the AC loss properties. 2. Preparation of short specimens Two types of coated conductors were fabricated by multi-stage CVD for AC loss measurement. A multi-stage CVD technique is useful for a rapid fabrication of long YBCO coated conductor. The YBCO
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K. Funaki et al. / Physica C 468 (2008) 1723–1726
layers were deposited on substrates prepared by a combination of IBAD and PLD processes. The IBAD-GZO buffer layers were deposited on the flexible metal tapes of Hastelloy with 100 lm in thickness and 10 mm in width. The PLD-CeO2 cap layers were formed on them. The thicknesses of the IBAD-GZO layer and PLD-CeO2 cap layer were about 1 lm and 0.4 lm, respectively. About 20 lm thick Ag layers, as protection and stabilizing layers, were deposited on the YBCO layers by sputtering. Subsequently, oxygen anneal was carried out. Their Ic values were measured by the conventional four-probe method with the Ic criterion of 1 lV/cm and the Ic of specimens A and B were 120 A and 115 A, respectively. Only specimen B was soldered with 0.1 mm thick Cu tape for stabilization. The main characteristics are listed in Table 1. Specimens A and B are composed of 21 and 14 laminated conductors cut with a length of 60 mm, respectively, where Kapton sheets are put between the conductors for electrical insulation. A total height is 5.2 mm for specimen A and 4.4 mm for specimen B. A photograph of the two specimens is given in Fig. 1.
Fig. 1. Layered short specimens for AC loss measurements.
Bm= 0.4T (specimen A)
10
10
B = 0.4T (specimen B) m
B = 0.8T (specimen A) m
AC losses in layered short specimens of the IBAD/CVD-YBCO tapes were measured in liquid helium or liquid nitrogen by means of a saddle-shaped pickup coil method [6]. The external sinusoidal magnetic field was applied in the direction parallel or perpendicular to the flat surfaces of the layered specimen. The angle of the external magnetic field was adjusted to the flat surface within an error of a few degrees. The measurements were performed in the ranges of the frequency from 0.01 Hz to 0.2 Hz and the amplitude of external magnetic field between 0.01 T and 4.0 T. The experimental setup is the same in principle as that of the usual concentric pickup coil method, which has been standardized to measure the AC losses of round superconducting wires in liquid helium [7], except for the shape of the pickup coils and their arrangement.
AC Loss [J/m3/cycle]
B = 0.8T (specimen B)
3. AC loss measurements
m
B = 2.0T (specimen A) m
9
10
B = 2.0T (specimen B) m
8
10
7
10
-2
10
10 Frequency [Hz]
-1
Fig. 2. Frequency dependence of AC loss for layered YBCO tapes with and without Cu layer.
4. Results and discussion
The frequency dependence of the AC losses for two sets of the layered specimen in Table 1 was observed in liquid helium and in perpendicular magnetic field configuration, where the effect of the Cu layer may be remarkable compared with other configuration. The results in the frequency range of 0.01–0.2 Hz are shown in Fig. 2 for the amplitude Bm of AC magnetic field, 0.4 T, 0.8 T and 2.0 T. The AC losses observed both for the specimens are scarcely dependent upon the frequency. This means that the hysteresis loss of the YBCO layer is predominant in the specimens A and B, and that the effect of the Cu layer is negligible in the specimen B in the range of the measurement frequency. Table 1 Characteristics of layered short specimens of IBAD/CVD-YBCO tapes Specimen A Length (mm) Width (mm) Thickness of each layer (lm) Cu Ag YBCO CeO2 GZO Hastelloy Layered number Total height (mm) Critical current at s.f. and 77 K (A) Criterion: 1 lV/cm
Specimen B
60 10
60 10
– 20 1 0.4 1 100 21 5.2 120
100 20 1.1 0.4 1 100 14 4.4 115
Fig. 3 shows the dependence of the AC loss on Bm for the specimens A and B in the parallel and perpendicular field configurations. The experimental results in the frequency range from 0.01 Hz to 0.2 Hz are superposed for each specimen in both field configurations, which are on a common curve because the AC loss
10
10
9
10
8
AC Loss [J/m3/cycle]
4.1. Effect of Cu layer on AC loss property in liquid helium
10
7
10
6
10
specimen B (perpendicular) specimen B (parallel) specimen A (perpendicular) specimen A (parallel)
5
10
4
10
3
10
2
10
-2
10
10
-1
10
0
1
10
Bm [T] Fig. 3. AC loss vs. Bm in parallel and perpendicular field configurations for layered YBCO tapes with and without Cu layer.
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K. Funaki et al. / Physica C 468 (2008) 1723–1726
is almost independent of the frequency. A turning point in each common curve must indicate the condition of full flux penetration. It is found in Fig. 3 that the penetration field of the specimen A is a little higher than that of the specimen B. In a practical range of Bm more than about 1 T, the AC losses in the perpendicular field configuration are extraordinary larger than those in the parallel one. 4.2. AC loss in liquid nitrogen and the temperature scaling In liquid nitrogen, the AC losses of the specimen B with Cu layer were also measured in the perpendicular field configuration. The AC losses observed are plotted by squares for the amplitude of applied magnetic field in Fig. 4 in a similar way to Fig. 3, where the experimental results in liquid helium are plotted again by circles for comparison. The penetration field at liquid nitrogen temperature is about one tenth smaller than that at liquid helium one. Supposing such temperature dependence in the AC loss property can be predicted by a theoretical consideration, it must be an advantageous tool to design superconducting windings used in alternating
electromagnetic environment at a wide range of temperature. In this way, it may be useful to indicate that the experimental results obtained in Fig. 4 are scaled with respect to temperature as follows [6]. For a superconducting tape with a width of h exposed to a perpendicular magnetic field Be, the critical current Ic(Be, T) is approximately estimated from the magnetization M(Be, T) on the master hysteresis curve at measurement temperature T by [6]
Ic ðBe ; TÞ ¼ 4hjMðBe ; TÞj
where the Be dependence of the magnetization can be obtained in the AC loss measurement. An example for the Be dependence of the critical current at liquid helium temperature is given in Fig. 5. It is shown in this figure that the critical current asymptically approaches to a constant Ico in decreasing Be, and a power function of Be in increasing it. Supposing this relation of the critical current with Be can be extended to a wide range of temperature, Ic(Be, T) is approximately expressed by
10
10
10
10
9
AC Loss / μ I (T)2 [J/TAm4/cycle]
8
10
7
10
6
10
9
10
8
10
7
10
6
10
5
10
4
0 c0
AC Loss [J/m3/cycle]
10
10
5
10
specimen B, 4.2 K (perpendicular) specimen B, 77 K (perpendicular)
4
10
-2
10
-1
0
10
1
10
specimen B, 4.2 K specimen B, 77 K
3
10
ð1Þ
1
2
10
10
3
10
10
10
4
Bm / μ0Ic0(T) [1/m]
Bm [T] Fig. 4. AC loss vs. Bm in perpendicular configuration for YBCO tapes with Cu layer at liquid helium and nitrogen temperatures.
Fig. 6. Normalized AC loss vs. Bm for YBCO tapes with Cu layer.
0.0005
1
0.0004
0.8
0.0003
0.6
0.0002
0.4
0.0001
0.2
2000
Bb(T) / Ic0(T) [T/A]
1000 900 800 700
γ (T)
Ic [A]
Bb
600 500 400 0.01
0.1
1 Be [T]
Fig. 5. Critical current vs. perpendicular magnetic field in logarithmic scales at liquid helium temperature.
0
0
10
20
30
40 T [K]
50
60
70
0 80
Fig. 7. The temperature dependence of characteristic parameters.
1726
Ic ðBe ; TÞ ¼ Ico ðTÞ;
K. Funaki et al. / Physica C 468 (2008) 1723–1726
cðTÞ1 Be Be 6 BbðTÞ ¼ Ico ðTÞ ; Bb ðTÞ
Bb ðTÞ 6 Be ð2Þ
where Bb(T) is obtained from an intersection between the two asymptotic lines as shown in Fig. 5. Considering the AC loss is calculated by integrating the magnetization with respect to Be over the whole cycle, it is suggested from Eqs. (1) and (2) in the same manner as shown for a single IBAD/PLD YBCO tape in Ref. [6] that the AC loss normalized by l0Ico(T)2 can be scaled for Be normalized by Ico(T) under the following conditions; (i) The exponent c(T) in Eq. (2) is independent of T, (ii) The ratio Bb(T)/Ico(T) is independent of T. The normalized AC losses on the basis of the characteristic parameters Ico and c estimated in Fig. 5 are plotted in Fig. 6, where the symbols circles and squares indicate the results at liquid helium and liquid nitrogen temperatures, respectively. The two groups of the normalized AC losses overlap with each other and form a scaled master curve. The estimated values of Ico and c from each magnetization curve at liquid helium or liquid nitrogen temperature are shown in Fig. 7, where open and closed circles indicate Bb(T)/Ico(T) and c(T), respectively. As can be seen in this figure, the characteristic parameters are scarcely dependent upon the temperature. In the present case, the conditions for the scaling are approximately satisfied. 5. Concluding remarks We studied AC loss properties of layered short specimen of IBAD/CVD-YBCO tapes with a Cu stability layer by means of a saddle-shaped pickup coil. The AC loss properties obtained are summarized in the following. (i) In a practical range of the magnetic field amplitude more than about 1 T, the AC losses in the perpendicular field configuration are extraordinary larger than those in the parallel one.
(ii) The effect of the Cu layer on the AC loss property can be negligible in the perpendicular field configuration in ranges of the amplitude less than a few T and the frequency less than several Hz. (iii) In the perpendicular field configuration, the AC loss normalized by l0Ico(T)2 can be scaled for Be normalized by Ico(T) between liquid helium and liquid nitrogen temperatures. Ico(T) is an asymptotic critical current in the region of lower external field, which can be estimated from the magnetization curve. Acknowledgements This work was supported by the New Energy and Industrial Technology Development Organization (NEDO), under the Research and Development of Superconducting Magnetic Energy Storage System sponsored by Agency of Natural Resources and Energy, Ministry of Economy, Trade and Industry (METI). In addition, YBCO coated conductors used in this work were produced by Chubu Electric Power Co., supported by NEDO through ISTEC, as the Collaborative Research and Development of Fundamental Technologies for Superconductivity Applications. References [1] V. Selvamanickam, Y. Chen, X. Xiong, Y.Y. Xie, J.L. Reeves, X. Zhang, Y. Qiao, K.P. Lenseth, R.M. Schmidt, A. Rar, D.W. Hazelton, K. Tekletsadik, IEEE Trans. Appl. Supercond. 17 (2007) 3231. [2] T. Watanabe, N. Kashima, N. Suda, M. Mori, S. Nagaya, S. Miyata, A. Ibi, Y. Yamada, T. Izumi, Y. Shiohara, IEEE Trans. Appl. Supercond. 17 (2007) 3386. [3] Y. Shiohara, Presented as a plenary speech on 20 Septemper at EUCAS (2007). [4] M. Iwakuma, M. Nigo, D. Inoue, N. Miyamoto, K. Funaki, Y. Iijima, T. Saitoh, Y. Yamada, T. Izumi, Y. Shiohara, Supercond. Sci. Technol. 19 (2006) 350. [5] Y. Mawatari, IEEE Trans. Appl. Supercond. 7 (1997) 1216. [6] M. Iwakuma, M. Nanri, M. Fukui, Y. Fukuda, K. Kajikawa, K. Funaki, Supercond. Sci. Technol. 15 (2003) 545. [7] IEC61788-8 Superconductivity-Part 8: AC loss measurements – Total AC loss measurement of Cu/NbTi composite superconducting wires exposed to a transverse alternating magnetic field by a pickup coil method, First edition 2003-4.