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Improvement in stability of superconducting coil by different mechanical properties of bobbins
Physica C 392–396 (2003) 1205–1209 www.elsevier.com/locate/physc
Improvement in stability of superconducting coil by different mechanical properties of bobbins N. Sekine a,*, T. Takao a, Y. Kojo a, Y. Yamaguchi a, S. Tada a, M. Takeo b, S. Sato b, A. Yamanaka c, S. Fukui d a
Department of Electrical and Electronic Engineering, Sophia University, Takao Laboratory, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan b Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan c Toyobo Co., Ltd, 2-1-1 Katata, Otsu, Shiga 520-0292, Japan d Niigata University, 8050 Ninocho, Igarashi, Niigata 950-2181, Japan Received 13 November 2002; accepted 15 January 2003
Abstract The Dyneemaâ fiber reinforced plastic (DFRP) expands to a direction of the Dyneema fibers during cooling process. Therefore, a winding angle in Dyneema fibers of the DFRP bobbin manufactured by the filament winding method makes possible to control thermal expansion/contraction properties to the circumferential direction on the bobbin. Moreover, DFRPÕs frictional coefficients are considerably low. These properties are extremely different from those of GFRP which is generally used as the spacers and the bobbins of superconducting coils. In this paper, the coils fabricated with DFRP and GFRP bobbins are examined in DC and AC operating experiments. When the coil was operated under DC current, the low frictional properties of the coils having DFRP bobbins improved the instability due to the wire motion, on the other hand, in case of AC operating test, the thermal expansion properties of DFRP bobbin could reduce the mechanical losses between the wire and the bobbin. Ó 2003 Elsevier B.V. All rights reserved. PACS: 74.25.Fy; 74.25.Ld Keywords: Stability; Friction; Fiber reinforced plastic; Frictional coefficient
1. Introduction Glass fiber reinforced plastics (GFRPs), which is generally used as structural materials of the superconducting coils, have a property of contrac-
*
Corresponding author. Tel.: +81-3-3238-3326; fax: +81-33238-3321. E-mail address: [email protected] (N. Sekine).
tion to the fiber direction during cooling down from room temperature (RT) to liquid helium temperature (LHeT). When the coils are cooled, the winding tensions of the coils whose bobbins are GFRP at RT are smaller than the ones at LHeT; it is described that GFRP is inappropriate as the bobbin. To improve of the stability against the friction, we selected the Dyneemaâ fiber reinforced plastics (DFRPs) as the bobbin of the coil [1–5]. Since the Dyneema fiber expands to the fiber
0921-4534/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-4534(03)01126-2
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direction during cooling down, when DFRP is used as the bobbin for superconducting coils, we can control the thermal expansion/contraction properties by the winding angle of the fiber. Using those characteristics to the bobbin, the winding tension of the coils at LHeT can be determined. The relationship between the winding tension at LHeT and the friction are considerably close. Moreover, the frictional coefficient of DFRP is approximately a half value of GFRPÕs coefficient [6]. The main factor of instabilities of the coils is the abrupt and local wire motion and the heat generated owing to the friction under DC operating. When an AC current is supplied to the coil, the superconducting winding vibrates, and the friction occurs. We think that this mechanical loss is one of various factors of AC losses. This mechanical loss under AC operating is so called frictional loss. It is expected that the thermal and mechanical properties of DFRP fit to improve the stability against these frictions. Therefore, we evaluated the performances of several kinds of the coils with different winding tensions and the bobbins (DFRP and GFRP) in DC/AC experiments, and experimentally confirmed that the thermal and mechanical properties of DFRP bobbin were effective to improve the stability of the superconducting coil.
2. Experimental procedure A superconducting wire in this experiment is the three-component wire whose materials are NbTi, Cu and CuNi. The specifications of the superconducting wire and the sample coils are listed in Tables 1 and 2 respectively. Three kinds of the coilÕs bobbins were prepared, which were one kind of GFRP bobbin and two kinds of DFRP bobbins whose winding angles of the Dyneema fiber are 40°
Table 1 Specifications of wire Material ratios Strand diameter (mm) Filament diameter (lm) DC critical current (A)
NbTi:Cu:CuNi ¼ 1:0.35:4.3 0.25 0.1 250 (at 0 T, 4.2 K)
Table 2 Specifications of sample coil Inner/outer diameters (mm) Height (mm) Materials of bobbins
Winding tensions (N) Shape of groove
40/55 100 GFRP 55° (contraction) DFRP 40° (contraction) DFRP 50° (expansion) 0.5, 3.5, 5.0 U-shape groove (Radius of curvature ¼ 0.5 mm)
and 50° to the axis of the coil. We name the coils having the GFRP bobbin as G55-coil, the coils with the DFRP bobbins (winding angles ¼ 40° and 50°) as D40-coil and D50-coil respectively. The G55-coil and the D40-coil contract on circumference of the bobbins during cooling down, on the contrary, the D50-coil expands circumferentially. In DC experiments, the sample coils were excited with a constant ramp rate of the current and no external magnetic field until the coils were quenched. In AC experiments, the sample coils were operated (frequency ¼ 60 Hz) in liquid helium with the DC magnetic fields (0.75, 0.50, 0.25 T). A sample coilÕs voltage to a coil current is measured by the voltage between the coilÕs taps, and a parallel component of the voltage to the current and a slight phase error in the signal are compensated in a lock-in amplifier. Base on the parallel component of the voltage from the lock-in amplifier and the current measured by the shunt resistor we estimated the sample coilÕs AC loss [7].
3. Experimental results and discussion 3.1. DC experiments In Fig. 1, a typical example of the training behavior measurements of some sample coils. The training effect was not observed in this experiment. Quench currents of all G55-coils fluctuate, and the D40-coils and the D50-coils have high quench currents and the quite stable behavior. As compared with the D40-coils and the D50-coils, the G55-coils were clearly unstable, but a few high
N. Sekine et al. / Physica C 392–396 (2003) 1205–1209
Fig. 1. Training behaviors of sample coils (winding tension at RT ¼ 0.5 N).
quench current of the G55-coil was obtained. Therefore, the coilsÕ performance can not be discussed based on only the maximum value of the quench currents; we used a standard deviation of quench currents to evaluate the performance of the coils quantitatively (see Fig. 2). The vertical axis indicates the average of quench currents, and the horizontal axis is the standard deviation of the currents. In the figure, the upper left is the good location, on the contrary the lower right is the bad one. All data of the D40-coils and the D50-coils
Fig. 2. Relation between average value of quench currents and standard deviation of quench currents.
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are plotted at the upper left area, that is to say, they are both high quench currents and stable. The D40-coils and the D50-coils are high performance coils. On the other hand, all data of G55-coils are plotted at the right part, that is, G55-coils are unstable. Although there is a few data plotted at the upper region, a rare high quench current occurs in the fluctuating currents of the training. We think that the unstable reason of the G55-coils is the local temperature rise due to the wire motion. The performances of the two kinds of the coils having DFRP bobbins (D40-coil and D50-coil) are higher than those of the coils having GFRP bobbins, and difference of the performance of the D40coils and the D50-coils are not clear. The reason is the DFRPÕs low frictional coefficient. Even if the wire motion occurs, Joule heat generated on the superconducting wire become small, and hence, the coils having DFRP coils show very stable characteristics without quenching at the low current level. 3.2. AC experiments An example of the measured AC losses of the sample coils with three different coils is shown in Fig. 3. These losses were measured under the same experimental conditions, that is to say, hysteresis losses and coupling losses in the coils are same in each coil. However, the different AC losses in the sample coils were measured. Moreover, the
Fig. 3. Measured AC losses of sample coils (winding tension at RT ¼ 0.5 N, background field ¼ 0.75 T).
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difference of AC losses in the coils hardly exists without external field in this experiment. In case of applying external fields, the difference of AC losses occurs, and the difference becomes large with increasing the external fields. These facts indicate that the origin of the difference of AC losses is different frictional losses on the contact between the wires and the bobbins. The frictional losses depend on both the thermal expansion and contraction properties of the bobbins and the winding tensions of the coils. We compared the measured AC losses of the nine coils in same experimental conditions (the coils current ¼ 31Apeak , the external field ¼ 0.75 T). In Fig. 4, the compared results are summarized as the horizontal axis of the winding tension at RT. The AC losses of the G55-coils are larger than those of the D40-coils and the D50-coil. And the smallest AC loss was obtained in case of the combination of the D50-coil and the winding tension of 0.5 N at RT. Comparing with the G55-coils, the D50-coil could reduce the loss of approximately 17%. The losses of the D40-coils, which contract circumferentially during cooling down, are almost larger than those of D50-coils whose property is expansion. Therefore, the difference of the losses in the bobbins of the same material indicates that the difference of the frictional losses depends on the thermal expansion and contraction properties of each bobbin rather than the frictional coefficient of the bobbin materials. Since the G55-coil and the
Fig. 4. Summary of AC losses of sample coils plotted against the winding tension at RT.
D40-coil contract to circumferential direction on the bobbins during cooling, the winding tensions at LHeT are smaller than the tensions at RT. Thus, the frictional losses increase owing to the insufficient winding tension, and even if the winding tension becomes large at RT, the enough effects cannot be acquired. On the other hand, in the D50coil, the AC loss become large with the gain of the winding tension at RT. When the winding tension at LHeT is too large because of the expansion of the bobbins, AC losses become large owing to the degradation of the wire. In consequence, we think that the combination of D50-coil and the winding tension at RT of 0.5 N obtain the good tension against the frictional losses at LHeT in nine sample coils prepared for this experiment.
4. Concluding remarks We showed the potency of DFRP as the bobbins of superconducting coils in this study. In experiment, the sample coils were fabricated with one kind of GFRP and two kinds of DFRP bobbins, which had contraction and expansion properties to the circumferential direction during cooling down. The performances of the sample coils were compared in DC/AC experiments. Conclusions of the work are as follows: (1) In the DC experiments, to compare the performance of the coils having DFRP and GFRP bobbins, we used the relation between the average value of quench currents and the standard deviation of the currents. As the result, the coils whose bobbin is DFRP have high quench currents and high stability. Even if the wire motion occurs, the coils whose bobbins are DFRP have high quench currents and high stability because of small frictional heat generating. (2) In the AC experiments, the coils with the expansion bobbins can reduce the frictional losses, because the sufficient winding tension at LHeT against the wireÕs vibration is obtained by the expansion property of the DFRP bobbin. However, when the winding tension at RT is large, the degradation of the wire occurs owing to too large tension at LHeT. Hence, we must pay attention to design of the coils with DFRP.
N. Sekine et al. / Physica C 392–396 (2003) 1205–1209
(3) The results of the DC and AC experiments showed that the coils fabricated with DFRP bobbins are effective to improve the instability caused by the friction between the wire and the bobbin. It is demonstrated that DFRP has advantage compared with GFRP as the bobbin material of superconducting coils. Acknowledgements This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References [1] N. Sekine, T. Takao, Y. Kojo, Y. Yamaguchi, S. Tada, T. Higuchi, M. Takeo, S. Sato, A. Yamanaka, S. Fukui, in: ASC 2002, no. 5LF05, Houston, USA, 2002.
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[2] T. Takao, T. Suzuki, N. Sekine, H. Taniguchi, K. Nakamura, T. Kashima, A. Yamanaka, M. Takeo, S. Sato, S. Fukui, in: ASC 2000, no. 1LJ07, Virginia Beach, USA, 2000. [3] N. Sekine, T. Takao, T. Kashima, A. Yamanaka, M. Takeo, S. Sato, S. Fukui, in: Proc. 63rd Meeting of Cryogenics and Superconductivity, p. 109, Kumamoto, Japan, 2000 (in Japanese). [4] N. Sekine, Y. Kojo, Y. Yamaguchi, S. Tada, T. Takao, M. Takeo, S. Sato, A. Yamanaka, in: Proc. 2002 National Convention Record I.E.E. Japan, p. 80, Tokyo, Japan, 2002 (in Japanese). [5] T. Kashima, A. Yamanaka, S. Nishijima, T. Okada, Thermal strain of pipes composed with high strength polyethylene fiber reinforced plastics at cryogenic temperatures, Advances in Cryogenic Engineering 42 (1996) 147. [6] T. Takao, T. Kashima, A. Yamanaka, Frictional properties on surfaces of high strength polymer fiber reinforced plastics, Advances in Cryogenic Engineering 46A (2000) 127. [7] S. Fukui, M. Ikeda, T. Yoshida, T. Sato, M. Yamaguchi, Investigation of AC loss characteristics of Bi2223 twisted multifilamentary tape, IEEE Transactions on Applied Superconductivity 12 (1) (2002) 1612.
Improvement in stability of superconducting coil by different mechanical properties of bobbins N. Sekine a,*, T. Takao a, Y. Kojo a, Y. Yamaguchi a, S. Tada a, M. Takeo b, S. Sato b, A. Yamanaka c, S. Fukui d a
Department of Electrical and Electronic Engineering, Sophia University, Takao Laboratory, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan b Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan c Toyobo Co., Ltd, 2-1-1 Katata, Otsu, Shiga 520-0292, Japan d Niigata University, 8050 Ninocho, Igarashi, Niigata 950-2181, Japan Received 13 November 2002; accepted 15 January 2003
Abstract The Dyneemaâ fiber reinforced plastic (DFRP) expands to a direction of the Dyneema fibers during cooling process. Therefore, a winding angle in Dyneema fibers of the DFRP bobbin manufactured by the filament winding method makes possible to control thermal expansion/contraction properties to the circumferential direction on the bobbin. Moreover, DFRPÕs frictional coefficients are considerably low. These properties are extremely different from those of GFRP which is generally used as the spacers and the bobbins of superconducting coils. In this paper, the coils fabricated with DFRP and GFRP bobbins are examined in DC and AC operating experiments. When the coil was operated under DC current, the low frictional properties of the coils having DFRP bobbins improved the instability due to the wire motion, on the other hand, in case of AC operating test, the thermal expansion properties of DFRP bobbin could reduce the mechanical losses between the wire and the bobbin. Ó 2003 Elsevier B.V. All rights reserved. PACS: 74.25.Fy; 74.25.Ld Keywords: Stability; Friction; Fiber reinforced plastic; Frictional coefficient
1. Introduction Glass fiber reinforced plastics (GFRPs), which is generally used as structural materials of the superconducting coils, have a property of contrac-
*
Corresponding author. Tel.: +81-3-3238-3326; fax: +81-33238-3321. E-mail address: [email protected] (N. Sekine).
tion to the fiber direction during cooling down from room temperature (RT) to liquid helium temperature (LHeT). When the coils are cooled, the winding tensions of the coils whose bobbins are GFRP at RT are smaller than the ones at LHeT; it is described that GFRP is inappropriate as the bobbin. To improve of the stability against the friction, we selected the Dyneemaâ fiber reinforced plastics (DFRPs) as the bobbin of the coil [1–5]. Since the Dyneema fiber expands to the fiber
0921-4534/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-4534(03)01126-2
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N. Sekine et al. / Physica C 392–396 (2003) 1205–1209
direction during cooling down, when DFRP is used as the bobbin for superconducting coils, we can control the thermal expansion/contraction properties by the winding angle of the fiber. Using those characteristics to the bobbin, the winding tension of the coils at LHeT can be determined. The relationship between the winding tension at LHeT and the friction are considerably close. Moreover, the frictional coefficient of DFRP is approximately a half value of GFRPÕs coefficient [6]. The main factor of instabilities of the coils is the abrupt and local wire motion and the heat generated owing to the friction under DC operating. When an AC current is supplied to the coil, the superconducting winding vibrates, and the friction occurs. We think that this mechanical loss is one of various factors of AC losses. This mechanical loss under AC operating is so called frictional loss. It is expected that the thermal and mechanical properties of DFRP fit to improve the stability against these frictions. Therefore, we evaluated the performances of several kinds of the coils with different winding tensions and the bobbins (DFRP and GFRP) in DC/AC experiments, and experimentally confirmed that the thermal and mechanical properties of DFRP bobbin were effective to improve the stability of the superconducting coil.
2. Experimental procedure A superconducting wire in this experiment is the three-component wire whose materials are NbTi, Cu and CuNi. The specifications of the superconducting wire and the sample coils are listed in Tables 1 and 2 respectively. Three kinds of the coilÕs bobbins were prepared, which were one kind of GFRP bobbin and two kinds of DFRP bobbins whose winding angles of the Dyneema fiber are 40°
Table 1 Specifications of wire Material ratios Strand diameter (mm) Filament diameter (lm) DC critical current (A)
NbTi:Cu:CuNi ¼ 1:0.35:4.3 0.25 0.1 250 (at 0 T, 4.2 K)
Table 2 Specifications of sample coil Inner/outer diameters (mm) Height (mm) Materials of bobbins
Winding tensions (N) Shape of groove
40/55 100 GFRP 55° (contraction) DFRP 40° (contraction) DFRP 50° (expansion) 0.5, 3.5, 5.0 U-shape groove (Radius of curvature ¼ 0.5 mm)
and 50° to the axis of the coil. We name the coils having the GFRP bobbin as G55-coil, the coils with the DFRP bobbins (winding angles ¼ 40° and 50°) as D40-coil and D50-coil respectively. The G55-coil and the D40-coil contract on circumference of the bobbins during cooling down, on the contrary, the D50-coil expands circumferentially. In DC experiments, the sample coils were excited with a constant ramp rate of the current and no external magnetic field until the coils were quenched. In AC experiments, the sample coils were operated (frequency ¼ 60 Hz) in liquid helium with the DC magnetic fields (0.75, 0.50, 0.25 T). A sample coilÕs voltage to a coil current is measured by the voltage between the coilÕs taps, and a parallel component of the voltage to the current and a slight phase error in the signal are compensated in a lock-in amplifier. Base on the parallel component of the voltage from the lock-in amplifier and the current measured by the shunt resistor we estimated the sample coilÕs AC loss [7].
3. Experimental results and discussion 3.1. DC experiments In Fig. 1, a typical example of the training behavior measurements of some sample coils. The training effect was not observed in this experiment. Quench currents of all G55-coils fluctuate, and the D40-coils and the D50-coils have high quench currents and the quite stable behavior. As compared with the D40-coils and the D50-coils, the G55-coils were clearly unstable, but a few high
N. Sekine et al. / Physica C 392–396 (2003) 1205–1209
Fig. 1. Training behaviors of sample coils (winding tension at RT ¼ 0.5 N).
quench current of the G55-coil was obtained. Therefore, the coilsÕ performance can not be discussed based on only the maximum value of the quench currents; we used a standard deviation of quench currents to evaluate the performance of the coils quantitatively (see Fig. 2). The vertical axis indicates the average of quench currents, and the horizontal axis is the standard deviation of the currents. In the figure, the upper left is the good location, on the contrary the lower right is the bad one. All data of the D40-coils and the D50-coils
Fig. 2. Relation between average value of quench currents and standard deviation of quench currents.
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are plotted at the upper left area, that is to say, they are both high quench currents and stable. The D40-coils and the D50-coils are high performance coils. On the other hand, all data of G55-coils are plotted at the right part, that is, G55-coils are unstable. Although there is a few data plotted at the upper region, a rare high quench current occurs in the fluctuating currents of the training. We think that the unstable reason of the G55-coils is the local temperature rise due to the wire motion. The performances of the two kinds of the coils having DFRP bobbins (D40-coil and D50-coil) are higher than those of the coils having GFRP bobbins, and difference of the performance of the D40coils and the D50-coils are not clear. The reason is the DFRPÕs low frictional coefficient. Even if the wire motion occurs, Joule heat generated on the superconducting wire become small, and hence, the coils having DFRP coils show very stable characteristics without quenching at the low current level. 3.2. AC experiments An example of the measured AC losses of the sample coils with three different coils is shown in Fig. 3. These losses were measured under the same experimental conditions, that is to say, hysteresis losses and coupling losses in the coils are same in each coil. However, the different AC losses in the sample coils were measured. Moreover, the
Fig. 3. Measured AC losses of sample coils (winding tension at RT ¼ 0.5 N, background field ¼ 0.75 T).
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N. Sekine et al. / Physica C 392–396 (2003) 1205–1209
difference of AC losses in the coils hardly exists without external field in this experiment. In case of applying external fields, the difference of AC losses occurs, and the difference becomes large with increasing the external fields. These facts indicate that the origin of the difference of AC losses is different frictional losses on the contact between the wires and the bobbins. The frictional losses depend on both the thermal expansion and contraction properties of the bobbins and the winding tensions of the coils. We compared the measured AC losses of the nine coils in same experimental conditions (the coils current ¼ 31Apeak , the external field ¼ 0.75 T). In Fig. 4, the compared results are summarized as the horizontal axis of the winding tension at RT. The AC losses of the G55-coils are larger than those of the D40-coils and the D50-coil. And the smallest AC loss was obtained in case of the combination of the D50-coil and the winding tension of 0.5 N at RT. Comparing with the G55-coils, the D50-coil could reduce the loss of approximately 17%. The losses of the D40-coils, which contract circumferentially during cooling down, are almost larger than those of D50-coils whose property is expansion. Therefore, the difference of the losses in the bobbins of the same material indicates that the difference of the frictional losses depends on the thermal expansion and contraction properties of each bobbin rather than the frictional coefficient of the bobbin materials. Since the G55-coil and the
Fig. 4. Summary of AC losses of sample coils plotted against the winding tension at RT.
D40-coil contract to circumferential direction on the bobbins during cooling, the winding tensions at LHeT are smaller than the tensions at RT. Thus, the frictional losses increase owing to the insufficient winding tension, and even if the winding tension becomes large at RT, the enough effects cannot be acquired. On the other hand, in the D50coil, the AC loss become large with the gain of the winding tension at RT. When the winding tension at LHeT is too large because of the expansion of the bobbins, AC losses become large owing to the degradation of the wire. In consequence, we think that the combination of D50-coil and the winding tension at RT of 0.5 N obtain the good tension against the frictional losses at LHeT in nine sample coils prepared for this experiment.
4. Concluding remarks We showed the potency of DFRP as the bobbins of superconducting coils in this study. In experiment, the sample coils were fabricated with one kind of GFRP and two kinds of DFRP bobbins, which had contraction and expansion properties to the circumferential direction during cooling down. The performances of the sample coils were compared in DC/AC experiments. Conclusions of the work are as follows: (1) In the DC experiments, to compare the performance of the coils having DFRP and GFRP bobbins, we used the relation between the average value of quench currents and the standard deviation of the currents. As the result, the coils whose bobbin is DFRP have high quench currents and high stability. Even if the wire motion occurs, the coils whose bobbins are DFRP have high quench currents and high stability because of small frictional heat generating. (2) In the AC experiments, the coils with the expansion bobbins can reduce the frictional losses, because the sufficient winding tension at LHeT against the wireÕs vibration is obtained by the expansion property of the DFRP bobbin. However, when the winding tension at RT is large, the degradation of the wire occurs owing to too large tension at LHeT. Hence, we must pay attention to design of the coils with DFRP.
N. Sekine et al. / Physica C 392–396 (2003) 1205–1209
(3) The results of the DC and AC experiments showed that the coils fabricated with DFRP bobbins are effective to improve the instability caused by the friction between the wire and the bobbin. It is demonstrated that DFRP has advantage compared with GFRP as the bobbin material of superconducting coils. Acknowledgements This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References [1] N. Sekine, T. Takao, Y. Kojo, Y. Yamaguchi, S. Tada, T. Higuchi, M. Takeo, S. Sato, A. Yamanaka, S. Fukui, in: ASC 2002, no. 5LF05, Houston, USA, 2002.
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[2] T. Takao, T. Suzuki, N. Sekine, H. Taniguchi, K. Nakamura, T. Kashima, A. Yamanaka, M. Takeo, S. Sato, S. Fukui, in: ASC 2000, no. 1LJ07, Virginia Beach, USA, 2000. [3] N. Sekine, T. Takao, T. Kashima, A. Yamanaka, M. Takeo, S. Sato, S. Fukui, in: Proc. 63rd Meeting of Cryogenics and Superconductivity, p. 109, Kumamoto, Japan, 2000 (in Japanese). [4] N. Sekine, Y. Kojo, Y. Yamaguchi, S. Tada, T. Takao, M. Takeo, S. Sato, A. Yamanaka, in: Proc. 2002 National Convention Record I.E.E. Japan, p. 80, Tokyo, Japan, 2002 (in Japanese). [5] T. Kashima, A. Yamanaka, S. Nishijima, T. Okada, Thermal strain of pipes composed with high strength polyethylene fiber reinforced plastics at cryogenic temperatures, Advances in Cryogenic Engineering 42 (1996) 147. [6] T. Takao, T. Kashima, A. Yamanaka, Frictional properties on surfaces of high strength polymer fiber reinforced plastics, Advances in Cryogenic Engineering 46A (2000) 127. [7] S. Fukui, M. Ikeda, T. Yoshida, T. Sato, M. Yamaguchi, Investigation of AC loss characteristics of Bi2223 twisted multifilamentary tape, IEEE Transactions on Applied Superconductivity 12 (1) (2002) 1612.