- Email: [email protected]
Novel AC loss measurement of high Tc superconducting long wire
Physica C 314 Ž1999. 285–290
Novel AC loss measurement of high Tc superconducting long wire J.X. Jin a
a,)
, S.X. Dou a , T. Hardono b, C. Cook
b
Institute for Superconducting and Electronic Materials, UniÕersity of Wollongong, Wollongong, NSW 2522, Australia b Department of Electric Power, UniÕersity of Wollongong, Wollongong, NSW 2522, Australia Received 22 April 1998; received in revised form 26 January 1999; accepted 29 January 1999
Abstract A novel method for AC loss measurement of a long length high Tc superconducting ŽHTSC. wire has been investigated. An 8 m long ŽBi,Pb. 2 Sr2 Ca 2 Cu 3 O10qx Ag clad multifilament wire was selected to prepare a special sample in which the inductive voltage induced by AC transport currents can be cancelled. Consequently the resistive voltage of the HTSC long wire can be measured. The HTSC sample has been tested at 77 K with AC transport currents up to 3 Ic and at various frequencies from 10 Hz to 1 kHz. The loss of the ŽBi,Pb. 2 Sr2 Ca 2 Cu 3 O10qx Ag clad wire related to its zero field AC transport current characteristics has been obtained experimentally, and results also prove that this method is effective and useful for AC loss measurement of a long length HTSC wire. q 1999 Elsevier Science B.V. All rights reserved. PACS: 7460J; 7460M; 7490 Keywords: AC loss; HTSC wire; HTSC coil; Magnetic field; Coil inductance
1. Introduction AC loss of a high Tc superconducting ŽHTSC. wire is an important issue related to its practical applications, and the principle of the HTSC AC loss has been discussed w1–3x. With regard to AC loss measurement, there are various techniques which have been practically used for HTSC short samples. Indirect measurements, such as magnetisation or susceptibility, can be used to calculate the HTSC AC )
Corresponding author. Tel.: q61-02-4221-5769; Fax: q6102-4221-5731; E-mail: [email protected]
hysteretic loss in theory w4x. For direct AC loss measurement, a lock-in amplifier is normally used to obtain a short HTSC sample resistive signal by controlling phase-shift angle of the lock-in loop w5– 9x. Other direct methods used are calorimetric or bolometric methods w10–13x. In this paper a novel method is introduced which can be used to obtain the resistive voltage drop and therefore real power loss of a long length HTSC sample directly. A long length ŽBi,Pb. 2 Sr2 Ca 2Cu 3 O 10qx Ag clad HTSC wire has been selected to prepare a sample with special configuration for the test, in which the inductive component of AC volt-
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 1 8 4 - 7
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age drop induced by an AC transport current can be eliminated. Consequently the HTSC resistive AC loss can be detected directly. The AC losses for the ŽBi,Pb. 2 Sr2 Ca 2 Cu 3 O 10qx Ag clad HTSC wire have been obtained for 77 K operation, and the measurement was under the zero field condition with AC currents passing through the superconducting sample in a frequency range from f s 10 Hz to f s 1 kHz.
2. Principle and sample configuration In normal DC critical current Ž Ic . measurements of superconductors, a common high sensitivity voltage meter can readily detect a voltage signal down to 0.1 mV which satisfies the normal DC transport current criterion 1 mVrcm. However, for an AC transport current Ž IAC ., the induced voltage signal VAC contains an resistive component VR , and an inductive component V L which is caused by the circuit inductance and can be much higher than the resistive component for a superconductor. To measure resistive loss Q R s IAC VR rf of a HTSC sample, the component V L must be separated from VAC . As a novel method, a reliable direct measurement has been developed which requires only a voltage meter to detect the AC resistive signal. By using a special configuration, the sample inductive compo-
nent VL can be balanced by itself. Therefore the AC loss of the HTSC sample can be measured directly. To prepare a zero inductance coil, the HTSC long wire is wound in the form of a coil in which the conductor contains two adjacent wires having opposite transport currents. Therefore the magnetic fields generated cancel.
3. Sample preparation A ŽBi,Pb. 2 Sr2 Ca 2 Cu 3 O 10qx Ag clad 27-core multifilament HTSC wire was selected. The HTSC wire is 8.08 m in length, having cross-section 3.4 mm = 0.28 mm. The area ratio of HTSCrŽHTSCq Ag. is ; 27%. The sample used for the measurement was wound under the normal react and wind procedure using the HTSC wire which is used in a configuration of double wires in parallel and each wire was insulated with Teflon tapes and insulation papers. The sample has a pancake coil form with 15 = 2 turns, and it has internal and external diameters of f in s 5 cm and fout s 11.5 cm, respectively. The measured DC critical current Ic of the HTSC sample in liquid nitrogen is shown in Fig. 1, where 1
Fig. 1. Critical current of the HTSC wire.
J.X. Jin et al.r Physica C 314 (1999) 285–290 Table 1 Characteristics of the high Tc superconducting wire and the testing sample HTSC wire
ŽBi,Pb. 2 Sr2 Ca 2 Cu 3 O10qx Ag clad 27filament composite tape HTSC wire cross-section CS tape s 3.4 mm=0.28 mm, CS HT S rCS tape s 0.27 Sample wire length 8.08 m f in s 5 cm, fout s11.5 cm, Sample dimension hs 0.4 cm, with 15=2 turns Sample critical current Ic s 7.2 A Ž1 mVrcm. at 77 K
mVrcm criterion is used. Table 1 shows the HTSC sample characteristics.
4. Experimental results and analysis 4.1. AC resistiÕe Õoltage The sample was measured at 77 K with AC transport currents, varying in both frequency ŽHz. and amplitude ŽA rms., and the results are shown in Fig. 2. Sinusoidal transport currents having frequencies from 10 Hz to 1 kHz were used, and results show that the AC resistive voltage drops VR for currents less than Ic become significant when the
287
Table 2 Resistive voltages of the sample IAC Žrms.
VR ŽmVrcm.
dVR rd IAC wmVrŽcm A.x
Ic r2 Ic 2 Ic
0.24 1.75 48.7
0.14 1.3 22.3
frequency increases to above 250 Hz. For lower frequencies, the sample AC I–VR curve is similar to its DC I–V curve, which shows clearly the normal resistive region and the superconducting region having AC losses by comparing with Fig. 1 within the critical current region. At 10 Hz, the measured AC resistive voltage drops are shown in Table 2, where VR s 1.75 mVrcm and dVR rd IAC ; 1.3 mVrŽcm A. for IAC Žrms. s Ic . The sample IAC –VR curves obtained in the low frequency range show good agreement with previous results from the lock-in measurement w14x. 4.2. Sample configuration analysis The HTSC sample configuration arranged as shown in Fig. 3 has been analysed. In Fig. 3, Ž1. shows a normal pancake coil form; Ž2. shows a pair
Fig. 2. AC transport currents for various frequencies.
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Fig. 3. Sample configuration analysis arrangement.
of double pancakes having the same transport currents but oppositely generated magnetic fields; and Ž3. shows the configuration of the sample used for this measurement, which has no self-magnetic fields. The sign ‘q’ and ‘y’ used in Fig. 3 represent the direction of the transport current. A normal coil inductance L is related to its flux linkage c as c s NSB s LI, where N is the number of turns of a normal coil, S is its area enclosed, and
B is the magnetic flux density generated. The magnetic flux density of the sample prepared was analysed in comparison with a normal coil. Fig. 4 shows the calculation results of the generated magnetic fields for different configurations along lines O–L as shown in Fig. 3. The transport current I s 7.2 A was selected. The L used in Fig. 4 corresponds to the distance from the point O shown in Fig. 3, and the original point O is at the coil centre. As shown in
Fig. 4. Magnetic field generated by different coil configurations.
J.X. Jin et al.r Physica C 314 (1999) 285–290
289
Fig. 5. Resistive loss for various frequencies.
Fig. 4, the magnetic field generated by the sample has been effectively cancelled by this configuration, and therefore the sample has no inductance. 4.3. AC losses AC losses of a HTSC wire include a hysteretic loss in its HTSC core, a normal eddy current loss related to AC currents flowing in its composite matrix, and a normal ohmic resistive loss. These losses are in the form of resistive voltage drops across the sample. In the case of low frequency and zero field, the hysteretic loss is the principal loss when the AC transport current is less than Ic . The sample resistive voltage drops in Fig. 2 are converted to Joule loss per cycle, Q R s IAC VR rf, and the results are shown in Fig. 5. This is the total resistive loss of the ŽBi,Pb. 2 Sr2 Ca 2 Cu 3 O 10qx Ag clad 27-core multifilament HTSC wire at 77 K with various AC transport currents under zero field. At 50 Hz, the AC losses for the HTSC wire used as a conductor for 77 K operation are Q R s 0.07 = 10y3 mJrcycle cm for IAC Žrms. s Icr2, Q R s 0.45 = 10y3 mJrcycle cm for IAC s Ic , and Q R s 17 = 10y3 mJrcycle cm for IAC s 2 Ic . From Fig. 5, when f - 250 Hz, the loss could be tolerated for engineering applications if IAC - Ic . In the lower frequency region, the loss increases with the increased frequency at the superconducting stage IAC - Ic , and the Q R –IAC curves
are merged together when IAC ) 2 Ic , where the Ag sheath is assumed to become the main path of the transport current. When f increases up to 1 kHz, the skin effect becomes significant since the loss is much higher, both in the superconducting and the normal state, compared with those having lower frequencies.
5. Conclusions A novel method for HTSC AC loss measurement has been introduced, and the method has been applied to a ŽBi,Pb. 2 Sr2 Ca 2 Cu 3 O 10qx Ag clad 27-core long HTSC wire. This method has been theoretically and practically proved in this work to be a simple and effective method for AC loss measurement of HTSC wires in long lengths. By using a specially prepared HTSC sample, the ŽBi,Pb. 2 Sr 2 Ca 2 Cu 3 O 10qx Ag clad 27-core HTSC wire has been measured at 77 K with regard to its AC transport current characteristics and AC loss by using this novel method. The AC resistive voltage VR and AC loss Q R s IAC VR rf of the HTSC long wire have been obtained with AC transport currents up to 3 Ic by using this direct measurement in the frequency range from 10 Hz–1 kHz. Examples of the results obtained include V R Ž10 Hz. s 0.24 mVrcm and Q RŽ50 Hz. s 0.07 = 10y3 mJrcycle cm for
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IAC Žrms. s Icr2. The measured IAC –VR curve using this method agrees with a previous measurement with the lock-in technique. Consequently this is an effective and useful method for HTSC long wire AC loss measurements.
Acknowledgements The authors would like to acknowledge Australian Research Council and Metal Manufactures for financial support to the HTSC wire project.
References w1x A.M. Campbell, IEEE Trans. Appl. Supercond. 5 Ž1995. 682. w2x S.A. Boggs, E.W. Collings, M.V. Parish, IEEE Trans. Appl. Supercond. 2 Ž1992. 117. w3x M. Ciszek, B.A. Glowacki, S.P. Ashworth, A.M. Campbell, J.E. Evetts, IEEE Trans. Appl. Supercond. 5 Ž1995. 709.
w4x J.R. Clem, A. Sanchez, Phys. Rev. B 50 Ž1994. 9355. w5x Y. Yang, T. Hughes, C. Beduz, D.M. Spiller, Z. Yi, R.G. Scurlock, IEEE Trans. Appl. Supercond. 5 Ž1995. 701. w6x J.J. Gannon Jr., A.P. Malozemoff, M.J. Minot, F. Barenghi, P. Metra, G. Vellego, J. Orehotsky, M. Suenaga, Adv. Cryo. Eng. 40 Ž1994. 45. w7x S.A. Awan, S. Sali, C.M. Friend, T.P. Beales, IEEE Trans. Appl. Supercond. 7 Ž1997. 335. w8x J. Paasi, M. Polak, P. Kottman, D. Suchon, M. Lahtinen, J. Kokavec, IEEE Trans. Appl. Supercond. 5 Ž1995. 713. w9x M. Ciszek, A.M. Campbell, B.A. Glowacki, Physica C 233 Ž1994. 203. w10x M. Sugimoto, A. Kimura, M. Mimura, Y. Tanaka, H. Ishii, S. Honjo, Y. Iwata, Physica C 279 Ž1997. 225. w11x D.E. Daney, H.J. Boenig, M.P. Maley, D.E. McMurry, B.G. DeBlanc, IEEE Trans. Appl. Supercond. 7 Ž1997. 310. w12x N. Chakraborty, A.V. Volkozub, A.D. Caplin, Paper presented in the International Workshop on Critical Currents in Superconductors for Practical Applications, 6–8 March, 1997, Xi’an, China. w13x T. Hardono, C.D. Cook, J.X. Jin, Supercond. Sci. Technol. 11 Ž1998. 1087. w14x J.X. Jin, C. Grantham, Y.C. Guo, J.N. Li, R. Bhasale, H.K. Liu, S.X. Dou, Physica C 278 Ž1997. 85.
Novel AC loss measurement of high Tc superconducting long wire J.X. Jin a
a,)
, S.X. Dou a , T. Hardono b, C. Cook
b
Institute for Superconducting and Electronic Materials, UniÕersity of Wollongong, Wollongong, NSW 2522, Australia b Department of Electric Power, UniÕersity of Wollongong, Wollongong, NSW 2522, Australia Received 22 April 1998; received in revised form 26 January 1999; accepted 29 January 1999
Abstract A novel method for AC loss measurement of a long length high Tc superconducting ŽHTSC. wire has been investigated. An 8 m long ŽBi,Pb. 2 Sr2 Ca 2 Cu 3 O10qx Ag clad multifilament wire was selected to prepare a special sample in which the inductive voltage induced by AC transport currents can be cancelled. Consequently the resistive voltage of the HTSC long wire can be measured. The HTSC sample has been tested at 77 K with AC transport currents up to 3 Ic and at various frequencies from 10 Hz to 1 kHz. The loss of the ŽBi,Pb. 2 Sr2 Ca 2 Cu 3 O10qx Ag clad wire related to its zero field AC transport current characteristics has been obtained experimentally, and results also prove that this method is effective and useful for AC loss measurement of a long length HTSC wire. q 1999 Elsevier Science B.V. All rights reserved. PACS: 7460J; 7460M; 7490 Keywords: AC loss; HTSC wire; HTSC coil; Magnetic field; Coil inductance
1. Introduction AC loss of a high Tc superconducting ŽHTSC. wire is an important issue related to its practical applications, and the principle of the HTSC AC loss has been discussed w1–3x. With regard to AC loss measurement, there are various techniques which have been practically used for HTSC short samples. Indirect measurements, such as magnetisation or susceptibility, can be used to calculate the HTSC AC )
Corresponding author. Tel.: q61-02-4221-5769; Fax: q6102-4221-5731; E-mail: [email protected]
hysteretic loss in theory w4x. For direct AC loss measurement, a lock-in amplifier is normally used to obtain a short HTSC sample resistive signal by controlling phase-shift angle of the lock-in loop w5– 9x. Other direct methods used are calorimetric or bolometric methods w10–13x. In this paper a novel method is introduced which can be used to obtain the resistive voltage drop and therefore real power loss of a long length HTSC sample directly. A long length ŽBi,Pb. 2 Sr2 Ca 2Cu 3 O 10qx Ag clad HTSC wire has been selected to prepare a sample with special configuration for the test, in which the inductive component of AC volt-
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 1 8 4 - 7
286
J.X. Jin et al.r Physica C 314 (1999) 285–290
age drop induced by an AC transport current can be eliminated. Consequently the HTSC resistive AC loss can be detected directly. The AC losses for the ŽBi,Pb. 2 Sr2 Ca 2 Cu 3 O 10qx Ag clad HTSC wire have been obtained for 77 K operation, and the measurement was under the zero field condition with AC currents passing through the superconducting sample in a frequency range from f s 10 Hz to f s 1 kHz.
2. Principle and sample configuration In normal DC critical current Ž Ic . measurements of superconductors, a common high sensitivity voltage meter can readily detect a voltage signal down to 0.1 mV which satisfies the normal DC transport current criterion 1 mVrcm. However, for an AC transport current Ž IAC ., the induced voltage signal VAC contains an resistive component VR , and an inductive component V L which is caused by the circuit inductance and can be much higher than the resistive component for a superconductor. To measure resistive loss Q R s IAC VR rf of a HTSC sample, the component V L must be separated from VAC . As a novel method, a reliable direct measurement has been developed which requires only a voltage meter to detect the AC resistive signal. By using a special configuration, the sample inductive compo-
nent VL can be balanced by itself. Therefore the AC loss of the HTSC sample can be measured directly. To prepare a zero inductance coil, the HTSC long wire is wound in the form of a coil in which the conductor contains two adjacent wires having opposite transport currents. Therefore the magnetic fields generated cancel.
3. Sample preparation A ŽBi,Pb. 2 Sr2 Ca 2 Cu 3 O 10qx Ag clad 27-core multifilament HTSC wire was selected. The HTSC wire is 8.08 m in length, having cross-section 3.4 mm = 0.28 mm. The area ratio of HTSCrŽHTSCq Ag. is ; 27%. The sample used for the measurement was wound under the normal react and wind procedure using the HTSC wire which is used in a configuration of double wires in parallel and each wire was insulated with Teflon tapes and insulation papers. The sample has a pancake coil form with 15 = 2 turns, and it has internal and external diameters of f in s 5 cm and fout s 11.5 cm, respectively. The measured DC critical current Ic of the HTSC sample in liquid nitrogen is shown in Fig. 1, where 1
Fig. 1. Critical current of the HTSC wire.
J.X. Jin et al.r Physica C 314 (1999) 285–290 Table 1 Characteristics of the high Tc superconducting wire and the testing sample HTSC wire
ŽBi,Pb. 2 Sr2 Ca 2 Cu 3 O10qx Ag clad 27filament composite tape HTSC wire cross-section CS tape s 3.4 mm=0.28 mm, CS HT S rCS tape s 0.27 Sample wire length 8.08 m f in s 5 cm, fout s11.5 cm, Sample dimension hs 0.4 cm, with 15=2 turns Sample critical current Ic s 7.2 A Ž1 mVrcm. at 77 K
mVrcm criterion is used. Table 1 shows the HTSC sample characteristics.
4. Experimental results and analysis 4.1. AC resistiÕe Õoltage The sample was measured at 77 K with AC transport currents, varying in both frequency ŽHz. and amplitude ŽA rms., and the results are shown in Fig. 2. Sinusoidal transport currents having frequencies from 10 Hz to 1 kHz were used, and results show that the AC resistive voltage drops VR for currents less than Ic become significant when the
287
Table 2 Resistive voltages of the sample IAC Žrms.
VR ŽmVrcm.
dVR rd IAC wmVrŽcm A.x
Ic r2 Ic 2 Ic
0.24 1.75 48.7
0.14 1.3 22.3
frequency increases to above 250 Hz. For lower frequencies, the sample AC I–VR curve is similar to its DC I–V curve, which shows clearly the normal resistive region and the superconducting region having AC losses by comparing with Fig. 1 within the critical current region. At 10 Hz, the measured AC resistive voltage drops are shown in Table 2, where VR s 1.75 mVrcm and dVR rd IAC ; 1.3 mVrŽcm A. for IAC Žrms. s Ic . The sample IAC –VR curves obtained in the low frequency range show good agreement with previous results from the lock-in measurement w14x. 4.2. Sample configuration analysis The HTSC sample configuration arranged as shown in Fig. 3 has been analysed. In Fig. 3, Ž1. shows a normal pancake coil form; Ž2. shows a pair
Fig. 2. AC transport currents for various frequencies.
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J.X. Jin et al.r Physica C 314 (1999) 285–290
Fig. 3. Sample configuration analysis arrangement.
of double pancakes having the same transport currents but oppositely generated magnetic fields; and Ž3. shows the configuration of the sample used for this measurement, which has no self-magnetic fields. The sign ‘q’ and ‘y’ used in Fig. 3 represent the direction of the transport current. A normal coil inductance L is related to its flux linkage c as c s NSB s LI, where N is the number of turns of a normal coil, S is its area enclosed, and
B is the magnetic flux density generated. The magnetic flux density of the sample prepared was analysed in comparison with a normal coil. Fig. 4 shows the calculation results of the generated magnetic fields for different configurations along lines O–L as shown in Fig. 3. The transport current I s 7.2 A was selected. The L used in Fig. 4 corresponds to the distance from the point O shown in Fig. 3, and the original point O is at the coil centre. As shown in
Fig. 4. Magnetic field generated by different coil configurations.
J.X. Jin et al.r Physica C 314 (1999) 285–290
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Fig. 5. Resistive loss for various frequencies.
Fig. 4, the magnetic field generated by the sample has been effectively cancelled by this configuration, and therefore the sample has no inductance. 4.3. AC losses AC losses of a HTSC wire include a hysteretic loss in its HTSC core, a normal eddy current loss related to AC currents flowing in its composite matrix, and a normal ohmic resistive loss. These losses are in the form of resistive voltage drops across the sample. In the case of low frequency and zero field, the hysteretic loss is the principal loss when the AC transport current is less than Ic . The sample resistive voltage drops in Fig. 2 are converted to Joule loss per cycle, Q R s IAC VR rf, and the results are shown in Fig. 5. This is the total resistive loss of the ŽBi,Pb. 2 Sr2 Ca 2 Cu 3 O 10qx Ag clad 27-core multifilament HTSC wire at 77 K with various AC transport currents under zero field. At 50 Hz, the AC losses for the HTSC wire used as a conductor for 77 K operation are Q R s 0.07 = 10y3 mJrcycle cm for IAC Žrms. s Icr2, Q R s 0.45 = 10y3 mJrcycle cm for IAC s Ic , and Q R s 17 = 10y3 mJrcycle cm for IAC s 2 Ic . From Fig. 5, when f - 250 Hz, the loss could be tolerated for engineering applications if IAC - Ic . In the lower frequency region, the loss increases with the increased frequency at the superconducting stage IAC - Ic , and the Q R –IAC curves
are merged together when IAC ) 2 Ic , where the Ag sheath is assumed to become the main path of the transport current. When f increases up to 1 kHz, the skin effect becomes significant since the loss is much higher, both in the superconducting and the normal state, compared with those having lower frequencies.
5. Conclusions A novel method for HTSC AC loss measurement has been introduced, and the method has been applied to a ŽBi,Pb. 2 Sr2 Ca 2 Cu 3 O 10qx Ag clad 27-core long HTSC wire. This method has been theoretically and practically proved in this work to be a simple and effective method for AC loss measurement of HTSC wires in long lengths. By using a specially prepared HTSC sample, the ŽBi,Pb. 2 Sr 2 Ca 2 Cu 3 O 10qx Ag clad 27-core HTSC wire has been measured at 77 K with regard to its AC transport current characteristics and AC loss by using this novel method. The AC resistive voltage VR and AC loss Q R s IAC VR rf of the HTSC long wire have been obtained with AC transport currents up to 3 Ic by using this direct measurement in the frequency range from 10 Hz–1 kHz. Examples of the results obtained include V R Ž10 Hz. s 0.24 mVrcm and Q RŽ50 Hz. s 0.07 = 10y3 mJrcycle cm for
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IAC Žrms. s Icr2. The measured IAC –VR curve using this method agrees with a previous measurement with the lock-in technique. Consequently this is an effective and useful method for HTSC long wire AC loss measurements.
Acknowledgements The authors would like to acknowledge Australian Research Council and Metal Manufactures for financial support to the HTSC wire project.
References w1x A.M. Campbell, IEEE Trans. Appl. Supercond. 5 Ž1995. 682. w2x S.A. Boggs, E.W. Collings, M.V. Parish, IEEE Trans. Appl. Supercond. 2 Ž1992. 117. w3x M. Ciszek, B.A. Glowacki, S.P. Ashworth, A.M. Campbell, J.E. Evetts, IEEE Trans. Appl. Supercond. 5 Ž1995. 709.
w4x J.R. Clem, A. Sanchez, Phys. Rev. B 50 Ž1994. 9355. w5x Y. Yang, T. Hughes, C. Beduz, D.M. Spiller, Z. Yi, R.G. Scurlock, IEEE Trans. Appl. Supercond. 5 Ž1995. 701. w6x J.J. Gannon Jr., A.P. Malozemoff, M.J. Minot, F. Barenghi, P. Metra, G. Vellego, J. Orehotsky, M. Suenaga, Adv. Cryo. Eng. 40 Ž1994. 45. w7x S.A. Awan, S. Sali, C.M. Friend, T.P. Beales, IEEE Trans. Appl. Supercond. 7 Ž1997. 335. w8x J. Paasi, M. Polak, P. Kottman, D. Suchon, M. Lahtinen, J. Kokavec, IEEE Trans. Appl. Supercond. 5 Ž1995. 713. w9x M. Ciszek, A.M. Campbell, B.A. Glowacki, Physica C 233 Ž1994. 203. w10x M. Sugimoto, A. Kimura, M. Mimura, Y. Tanaka, H. Ishii, S. Honjo, Y. Iwata, Physica C 279 Ž1997. 225. w11x D.E. Daney, H.J. Boenig, M.P. Maley, D.E. McMurry, B.G. DeBlanc, IEEE Trans. Appl. Supercond. 7 Ž1997. 310. w12x N. Chakraborty, A.V. Volkozub, A.D. Caplin, Paper presented in the International Workshop on Critical Currents in Superconductors for Practical Applications, 6–8 March, 1997, Xi’an, China. w13x T. Hardono, C.D. Cook, J.X. Jin, Supercond. Sci. Technol. 11 Ž1998. 1087. w14x J.X. Jin, C. Grantham, Y.C. Guo, J.N. Li, R. Bhasale, H.K. Liu, S.X. Dou, Physica C 278 Ž1997. 85.