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Development and quality control of the superconductors for the helical coils of LHD
Fusion Engineering and Design 41 (1998) 241 – 246
Development and quality control of the superconductors for the helical coils of LHD N. Yanagi *, T. Mito, S. Imagawa, K. Takahata, T. Satow, J. Yamamoto 1, O. Motojima, The LHD Group National Institute for Fusion Science, Toki, Gifu 509 -52, Japan
Abstract A composite-type superconductor with NbTi/Cu compacted strands and aluminum/copper stabilizers has been developed for the pool-cooled helical coils of the Large Helical Device. The internal configuration of the conductor was determined from the viewpoint of high stability performance and fabrication reliability, by incorporating some newly developed techniques, such as a Cu–2%Ni-clad pure aluminum stabilizer and electron beam welding of the half-hard copper sheath. The conductor has been fabricated over the total length of 36 km under careful inspections both on the component materials and on the final full-conductors from the viewpoint of quality control. Short sample tests of the conductors showed that the measured critical currents and the recovery currents satisfied the specified values for the Phase I operation condition of LHD, although they showed some scattering around their mean values. © 1998 Elsevier Science S.A. All rights reserved.
1. Introduction The Large Helical Device (LHD) is a fusion experimental device which is now under construction with all superconducting coil systems [1]. The two helical coils have the major radius of 3.9 m and the average minor radius of 0.975 m with a poloidal pole number of 2 and a toroidal pitch number of 10. They will be immersed in liquid helium of 4.4 K in the Phase I operation condition of LHD and will generate a heliotron magnetic configuration with the central toroidal field strength of 3 T. Superfluid helium will be later utilized in the Phase II operations at the reduced * Corresponding author. 1 Sadly passed away on 5 January 1997.
temperature of 1.8 K in order to raise the central field up to 4 T. The stored magnetic energy will reach 1.6 GJ in the 4 T operation mode. Pool-cooled composite-type superconductors of NbTi/Cu compacted strands with copper and pure aluminum stabilizers were selected to be wound up as helical coils, incorporating the advantage of their high cryogenic stability and the mechanical flexibility. It took one and a half years for the on-site winding process of the helical coils (900 turns) to be successfully completed in May, 1996. In this paper, the characteristics of the developed aluminum-stabilized superconductors are briefly reviewed and the inspection results of the produced conductors including the full-conductor short sample tests are discussed from the viewpoint of quality control.
0920-3796/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0920-3796(98)00118-5
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N. Yanagi et al. / Fusion Engineering and Design 41 (1998) 241–246
2. Development of the superconductor The conductor development started in 1989 aiming at a pool-cooled type NbTi superconductor which should satisfy cryogenic stable condition with the nominal transport current of 21.2 kA under the maximum magnetic field of 6.9 T and the liquid helium temperature of 4.4 K. Several types of superconductors with cross-sections of 19 mm square but with various internal structures were proposed and experimentally examined using short sample tests [2]. One of the most difficult problems with this conductor development was the finding of unexpected low recovery currents observed in most of the conductors, which were later attributed to the unexpected enhancement of the magnetoresistivity of copperclad aluminum stabilizers. We finally concluded that this observation could be explained by the ‘Hall current’ generation model [3,4] of metalmetal composites with different conductive materials. From the nature of this effect, we had expected that a resistive layer around a pure aluminum would reduce the ‘Hall current’ and hence enhance the cryogenic stability of the superconductors [5]. The final superconductor was developed in 1993. In this phase, the conductor size was reduced and became 12.5×18.0 mm with the nominal current of 13.0 kA and 17.3 kA for the Phase I and II operation conditions, respectively. The main specifications of the superconductor are listed in Table 1 and the cross-sectional configuration is shown in Fig. 1. The reduction of the Table 1 Main specifications of the superconductor Conductor size Superconducting material Copper to NbTi ratio Diameter of filament Number of filaments Diameter of strand Number of strands Nominal current at phase I (@ 4.4 K, 6.9 T) Nominal current at Phase II (@ 1.8 K, 9.2 T)
12.5×18.0 mm NbTi 0.9 47 mm 726 1.74 mm 15 13.0 kA 17.3 kA
Fig. 1. Cross-sectional view of the superconductor developed and used for the helical coils of LHD.
conductor size has not only improved the mechanical flexibility of the conductor to facilitate the complicated on-site winding process but also improved the cryogenic stability due to the increase of the surface to volume ratio of the conductor. Fifteen superconducting strands are twisted and formed into a Rutherford-type flat cable. Instead of conventional OFC, Cu–2%Ni (resistivity: 2.5×10 − 8 Vm at 7 T) was selected for the clad material around the pure (5N) aluminum stabilizer to compromise the conflicting demands for effectively insulating the ‘Hall current’ as well as for maintaining the smooth current transfer from the superconducting strands to the aluminum. Another important technique developed with this superconductor is the electron beam welding of the half-hard copper sheath, which has drastically enhanced the mechanical toughness of the conductor not only against the compressional stress of up to 100 MPa acting during excitations but also against the plastic deformation during the three dimensional helical winding process.
3. Fabrication of the superconductors The superconductor has been fabricated for the windings of one pair of the helical coils (each consists of three blocks and twenty layers) with
N. Yanagi et al. / Fusion Engineering and Design 41 (1998) 241–246
the total length of 36 km (unit length: 600–1100 m, divided into 38 units). During the fabrication, the component materials of the superconductors were intensively inspected in many items by sampling the end regions of each product. The most important inspection items were the critical currents of superconducting strands before and after the electron beam welding process, the magnetoresistivity of stabilizers (aluminum, copper and Cu–2% Ni), the heat transfer coefficient from the oxidized (blackened) copper surface to liquid helium, and the mechanical toughness of the whole conductor. Fig. 2 shows some of these inspection results as a function of the product number of 1 to 38. As is seen in Fig. 2(a), the critical current density of superconducting strands shows some scattering (standard deviation: 2%) around their mean value of 1360 A mm2 at 7 T, but no degradation was found during the whole fabrication processes. The magnetoresistivity of the stabilizer materials as well as the effective heat flux from the conductor surface also show stable production results as shown in Fig. 2(b).
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field. Since four conductors were connected in series (as shown in Fig. 3) and the transport current was common, the data points of the critical currents were restricted to those which showed normal transitions first among the four of them before shutting down the power supplies after a quench. As is seen in Fig. 4(a), the measured critical currents are rather in good agreement with the predicted values based on the critical currents measured for single strands.
4. Short sample tests for quality control As the final inspection procedure of the superconductors before being supplied to the on-site winding process, full-conductor short samples were taken from the end region of each product and were examined using the superconductor test facilities with a 9 T split coil, 100 kA current leads and 75 kA DC power supplies. A short sample consisting of four conductors, each 2 m long, soldered together both at the top and at the bottom as shown in Fig. 3, was vertically inserted into the split coil which has a 90% flat top field region of about 250 mm [2,6]. A number of voltage taps, resistive thermometers and stainless steel resistive heaters were attached on the conductor surfaces imbedded beneath the GFRP spacers which define the exposure rate of the conductors as 50% in the standard tests. Fig. 4 shows the main results obtained in the short sample tests, which are the critical currents and the recovery currents of the superconductors, both plotted as a function of the bias magnetic
Fig. 2. Inspection test results of, (a) the critical current density of superconducting strands before and after the electron beam welding processes under the bias magnetic field of 7 T, and (b) the magnetoresistivity of stabilizers (aluminum, copper and Cu – 2%Ni) at 7 T and the effective heat flux from the oxidized (blackened) copper surface to the liquid helium. Each data is plotted as a function of the product number of 1 to 38, both in (a) and (b). All data, courtesy of Hitachi Ltd. and Hitachi Cable Ltd.
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Fig. 3. Schematic drawing of a short sample with four superconductors connected in series by solder both at the top and at the bottom.
The self-field effect due to the large transport current in the conductors as well as the three dimensional distribution of the bias magnetic field were taken into account, and the critical current of a whole conductor at each bias field was calculated by adopting an iteration method. As for the measured critical currents of the conductors, the scattering of the data seems a little larger (standard deviation: 3% at 7 T) than that measured for single strands (Fig. 2a). Stability tests were carried out by initiating a normal zone with resistive heaters and the recovery current was measured by reducing the transport current after a normal zone showed stagnation. In this case, each conductor was examined separately without affecting the neighboring conductors since the transport current was much lower than the critical current. It should be noted that as is seen in Fig. 4(b) all the measured recovery currents exceeded the specified value set as 13.0 kA at 7.0 T based on the nominal current for the Phase I operation condition. The scattering of the measured recovery currents seems rather large, especially in the higher field region (e.g. standard deviation: 7% at 7 T). The most important parameter that defines the recovery current should be the longitudinal resistivity of the stabilizer after the normal transition. In this connection, the longitudinal resistivity evaluated from the voltage signals near the conductor center after normal transitions is plot-
ted in Fig. 5 as a function of the bias magnetic field. Fig. 5 suggests that the scatter of the recovery currents can be explained mainly due to the variation of the resistivity of the stabilizers, especially in the higher field region. As shown in Fig. 2(b), the deviation of the magnetoresistivity of each component material, such as aluminum, copper and Cu–2%Ni was well kept in the small range from their mean values, and thus the large scatter observed in the effective longitudinal resistivity of the conductors after normal transitions cannot be explained by the variation of the resistivity of each material. The most probable explanation to this problem is that the variation of the contact resistance between the aluminum and the Cu–2%Ni clad or between the Cu– 2%Ni clad and the surrounding solder may drastically enhance the variation of the effective longitudinal resistivity of the stabilizer through the ‘Hall current’ generation [6]. It should be noted that the finite length effect of the bias magnetic field in the split coil should be carefully taken into account to evaluate the real recovery current under the uniform field distribution [7]. Moreover, the exposure rate of the conductors inside the helical coils of LHD is varied by adjusting the spacer width depending on the position so that the whole conductors will satisfy the cryogenic stable condition while maintaining sufficient mechanical toughness against the electromagnetic stress during excitations [8].
N. Yanagi et al. / Fusion Engineering and Design 41 (1998) 241–246
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Fig. 5. Effective longitudinal resistivity of the short sample conductors as a function of the bias magnetic field.
5. Conclusions Pool-cooled composite-type superconductors with NbTi/Cu compacted strands and copper and pure aluminum stabilizers have been developed and fabricated for the helical coils of LHD. Each product conductor was carefully inspected in many items including the full-conductor short sample tests from the viewpoint of quality control. The measured critical currents and the recovery currents satisfied the specified values although they showed some scattering around their mean values.
Acknowledgements
Fig. 4. (a) Critical currents and (b) recovery currents measured in the short sample tests of the product conductors, both plotted against the bias magnetic field. In (a), the dashed-dotted line corresponds to the value determined as the measured critical current of single strands multiplied by the number of strands, 15, whereas the dashed line shows the expected critical current of a conductor calculated by taking into account the self-field effect of the transport current. In (b), the dashed – dotted curve shows the expected recovery current based on Maddock’s equal area theorem under the uniform field distribution. Used in the calculation is the longitudinal resistivity evaluated from the voltage signals which were observed after the normal transitions in the cases that give minimum recovery currents.
The authors are grateful to Hitachi Ltd. and Hitachi Cable Ltd. for their great effort in developing many new techniques as well as in the fabrication of the superconductors with very high quality. The authors also appreciate the great support given by many staffs of NIFS to our conductor R&D programs, including many short sample experiments.
References [1] O. Motojima, K. Akaishi, K. Fujii, et al., Physics and engineering design studies on the Large Helical Device, Fusion Eng. Des. 20 (1993) 3 – 14.
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N. Yanagi et al. / Fusion Engineering and Design 41 (1998) 241–246
[2] T. Mito, K. Takahata, N. Yanagi, et al., Short sample tests of aluminum-stabilized superconductors for Large Helical Device, Fusion Eng. Des. 20 (1993) 233 – 242. [3] P.W. Eckels, N.C. Iyer, A. Patterson, et al., Magnetoresistance; the Hall effect in composite aluminum cryoconductors, Cryogenics 29 (1989) 748–752. [4] H. Kaneko, N. Yanagi, Enhancement of magnetoresistance due to Hall current in aluminum-copper composite, Cryogenics 32 (1992) 1114–1120. [5] H. Kaneko, Insulation of highly conductive metal in composite stabilizer for reduction of Hall current across surface, Cryogenics 33 (1993) 1077–1085.
.
[6] N. Yanagi, T. Mito, K. Takahata, et al., Experimental observation of anomalous magnetoresistivity in 10 – 20 kA class aluminum-stabilized superconductors for the Large Helical Device, Adv. Cryogen. Eng. Mater. 40 (1994) 459 – 468. [7] S. Imagawa, N. Yanagi, T. Mito, et al., Evaluation of recovery current of the helical coil for LHD, ICEC 16, Kita-Kyushu, paper OC7-5, 1996. [8] S. Imagawa, N. Yanagi, T. Satow, et al., Optimization of wetted surface fraction of helical coil for LHD, Cryogenics 34 (1994) 701 – 704.
Development and quality control of the superconductors for the helical coils of LHD N. Yanagi *, T. Mito, S. Imagawa, K. Takahata, T. Satow, J. Yamamoto 1, O. Motojima, The LHD Group National Institute for Fusion Science, Toki, Gifu 509 -52, Japan
Abstract A composite-type superconductor with NbTi/Cu compacted strands and aluminum/copper stabilizers has been developed for the pool-cooled helical coils of the Large Helical Device. The internal configuration of the conductor was determined from the viewpoint of high stability performance and fabrication reliability, by incorporating some newly developed techniques, such as a Cu–2%Ni-clad pure aluminum stabilizer and electron beam welding of the half-hard copper sheath. The conductor has been fabricated over the total length of 36 km under careful inspections both on the component materials and on the final full-conductors from the viewpoint of quality control. Short sample tests of the conductors showed that the measured critical currents and the recovery currents satisfied the specified values for the Phase I operation condition of LHD, although they showed some scattering around their mean values. © 1998 Elsevier Science S.A. All rights reserved.
1. Introduction The Large Helical Device (LHD) is a fusion experimental device which is now under construction with all superconducting coil systems [1]. The two helical coils have the major radius of 3.9 m and the average minor radius of 0.975 m with a poloidal pole number of 2 and a toroidal pitch number of 10. They will be immersed in liquid helium of 4.4 K in the Phase I operation condition of LHD and will generate a heliotron magnetic configuration with the central toroidal field strength of 3 T. Superfluid helium will be later utilized in the Phase II operations at the reduced * Corresponding author. 1 Sadly passed away on 5 January 1997.
temperature of 1.8 K in order to raise the central field up to 4 T. The stored magnetic energy will reach 1.6 GJ in the 4 T operation mode. Pool-cooled composite-type superconductors of NbTi/Cu compacted strands with copper and pure aluminum stabilizers were selected to be wound up as helical coils, incorporating the advantage of their high cryogenic stability and the mechanical flexibility. It took one and a half years for the on-site winding process of the helical coils (900 turns) to be successfully completed in May, 1996. In this paper, the characteristics of the developed aluminum-stabilized superconductors are briefly reviewed and the inspection results of the produced conductors including the full-conductor short sample tests are discussed from the viewpoint of quality control.
0920-3796/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0920-3796(98)00118-5
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N. Yanagi et al. / Fusion Engineering and Design 41 (1998) 241–246
2. Development of the superconductor The conductor development started in 1989 aiming at a pool-cooled type NbTi superconductor which should satisfy cryogenic stable condition with the nominal transport current of 21.2 kA under the maximum magnetic field of 6.9 T and the liquid helium temperature of 4.4 K. Several types of superconductors with cross-sections of 19 mm square but with various internal structures were proposed and experimentally examined using short sample tests [2]. One of the most difficult problems with this conductor development was the finding of unexpected low recovery currents observed in most of the conductors, which were later attributed to the unexpected enhancement of the magnetoresistivity of copperclad aluminum stabilizers. We finally concluded that this observation could be explained by the ‘Hall current’ generation model [3,4] of metalmetal composites with different conductive materials. From the nature of this effect, we had expected that a resistive layer around a pure aluminum would reduce the ‘Hall current’ and hence enhance the cryogenic stability of the superconductors [5]. The final superconductor was developed in 1993. In this phase, the conductor size was reduced and became 12.5×18.0 mm with the nominal current of 13.0 kA and 17.3 kA for the Phase I and II operation conditions, respectively. The main specifications of the superconductor are listed in Table 1 and the cross-sectional configuration is shown in Fig. 1. The reduction of the Table 1 Main specifications of the superconductor Conductor size Superconducting material Copper to NbTi ratio Diameter of filament Number of filaments Diameter of strand Number of strands Nominal current at phase I (@ 4.4 K, 6.9 T) Nominal current at Phase II (@ 1.8 K, 9.2 T)
12.5×18.0 mm NbTi 0.9 47 mm 726 1.74 mm 15 13.0 kA 17.3 kA
Fig. 1. Cross-sectional view of the superconductor developed and used for the helical coils of LHD.
conductor size has not only improved the mechanical flexibility of the conductor to facilitate the complicated on-site winding process but also improved the cryogenic stability due to the increase of the surface to volume ratio of the conductor. Fifteen superconducting strands are twisted and formed into a Rutherford-type flat cable. Instead of conventional OFC, Cu–2%Ni (resistivity: 2.5×10 − 8 Vm at 7 T) was selected for the clad material around the pure (5N) aluminum stabilizer to compromise the conflicting demands for effectively insulating the ‘Hall current’ as well as for maintaining the smooth current transfer from the superconducting strands to the aluminum. Another important technique developed with this superconductor is the electron beam welding of the half-hard copper sheath, which has drastically enhanced the mechanical toughness of the conductor not only against the compressional stress of up to 100 MPa acting during excitations but also against the plastic deformation during the three dimensional helical winding process.
3. Fabrication of the superconductors The superconductor has been fabricated for the windings of one pair of the helical coils (each consists of three blocks and twenty layers) with
N. Yanagi et al. / Fusion Engineering and Design 41 (1998) 241–246
the total length of 36 km (unit length: 600–1100 m, divided into 38 units). During the fabrication, the component materials of the superconductors were intensively inspected in many items by sampling the end regions of each product. The most important inspection items were the critical currents of superconducting strands before and after the electron beam welding process, the magnetoresistivity of stabilizers (aluminum, copper and Cu–2% Ni), the heat transfer coefficient from the oxidized (blackened) copper surface to liquid helium, and the mechanical toughness of the whole conductor. Fig. 2 shows some of these inspection results as a function of the product number of 1 to 38. As is seen in Fig. 2(a), the critical current density of superconducting strands shows some scattering (standard deviation: 2%) around their mean value of 1360 A mm2 at 7 T, but no degradation was found during the whole fabrication processes. The magnetoresistivity of the stabilizer materials as well as the effective heat flux from the conductor surface also show stable production results as shown in Fig. 2(b).
243
field. Since four conductors were connected in series (as shown in Fig. 3) and the transport current was common, the data points of the critical currents were restricted to those which showed normal transitions first among the four of them before shutting down the power supplies after a quench. As is seen in Fig. 4(a), the measured critical currents are rather in good agreement with the predicted values based on the critical currents measured for single strands.
4. Short sample tests for quality control As the final inspection procedure of the superconductors before being supplied to the on-site winding process, full-conductor short samples were taken from the end region of each product and were examined using the superconductor test facilities with a 9 T split coil, 100 kA current leads and 75 kA DC power supplies. A short sample consisting of four conductors, each 2 m long, soldered together both at the top and at the bottom as shown in Fig. 3, was vertically inserted into the split coil which has a 90% flat top field region of about 250 mm [2,6]. A number of voltage taps, resistive thermometers and stainless steel resistive heaters were attached on the conductor surfaces imbedded beneath the GFRP spacers which define the exposure rate of the conductors as 50% in the standard tests. Fig. 4 shows the main results obtained in the short sample tests, which are the critical currents and the recovery currents of the superconductors, both plotted as a function of the bias magnetic
Fig. 2. Inspection test results of, (a) the critical current density of superconducting strands before and after the electron beam welding processes under the bias magnetic field of 7 T, and (b) the magnetoresistivity of stabilizers (aluminum, copper and Cu – 2%Ni) at 7 T and the effective heat flux from the oxidized (blackened) copper surface to the liquid helium. Each data is plotted as a function of the product number of 1 to 38, both in (a) and (b). All data, courtesy of Hitachi Ltd. and Hitachi Cable Ltd.
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N. Yanagi et al. / Fusion Engineering and Design 41 (1998) 241–246
Fig. 3. Schematic drawing of a short sample with four superconductors connected in series by solder both at the top and at the bottom.
The self-field effect due to the large transport current in the conductors as well as the three dimensional distribution of the bias magnetic field were taken into account, and the critical current of a whole conductor at each bias field was calculated by adopting an iteration method. As for the measured critical currents of the conductors, the scattering of the data seems a little larger (standard deviation: 3% at 7 T) than that measured for single strands (Fig. 2a). Stability tests were carried out by initiating a normal zone with resistive heaters and the recovery current was measured by reducing the transport current after a normal zone showed stagnation. In this case, each conductor was examined separately without affecting the neighboring conductors since the transport current was much lower than the critical current. It should be noted that as is seen in Fig. 4(b) all the measured recovery currents exceeded the specified value set as 13.0 kA at 7.0 T based on the nominal current for the Phase I operation condition. The scattering of the measured recovery currents seems rather large, especially in the higher field region (e.g. standard deviation: 7% at 7 T). The most important parameter that defines the recovery current should be the longitudinal resistivity of the stabilizer after the normal transition. In this connection, the longitudinal resistivity evaluated from the voltage signals near the conductor center after normal transitions is plot-
ted in Fig. 5 as a function of the bias magnetic field. Fig. 5 suggests that the scatter of the recovery currents can be explained mainly due to the variation of the resistivity of the stabilizers, especially in the higher field region. As shown in Fig. 2(b), the deviation of the magnetoresistivity of each component material, such as aluminum, copper and Cu–2%Ni was well kept in the small range from their mean values, and thus the large scatter observed in the effective longitudinal resistivity of the conductors after normal transitions cannot be explained by the variation of the resistivity of each material. The most probable explanation to this problem is that the variation of the contact resistance between the aluminum and the Cu–2%Ni clad or between the Cu– 2%Ni clad and the surrounding solder may drastically enhance the variation of the effective longitudinal resistivity of the stabilizer through the ‘Hall current’ generation [6]. It should be noted that the finite length effect of the bias magnetic field in the split coil should be carefully taken into account to evaluate the real recovery current under the uniform field distribution [7]. Moreover, the exposure rate of the conductors inside the helical coils of LHD is varied by adjusting the spacer width depending on the position so that the whole conductors will satisfy the cryogenic stable condition while maintaining sufficient mechanical toughness against the electromagnetic stress during excitations [8].
N. Yanagi et al. / Fusion Engineering and Design 41 (1998) 241–246
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Fig. 5. Effective longitudinal resistivity of the short sample conductors as a function of the bias magnetic field.
5. Conclusions Pool-cooled composite-type superconductors with NbTi/Cu compacted strands and copper and pure aluminum stabilizers have been developed and fabricated for the helical coils of LHD. Each product conductor was carefully inspected in many items including the full-conductor short sample tests from the viewpoint of quality control. The measured critical currents and the recovery currents satisfied the specified values although they showed some scattering around their mean values.
Acknowledgements
Fig. 4. (a) Critical currents and (b) recovery currents measured in the short sample tests of the product conductors, both plotted against the bias magnetic field. In (a), the dashed-dotted line corresponds to the value determined as the measured critical current of single strands multiplied by the number of strands, 15, whereas the dashed line shows the expected critical current of a conductor calculated by taking into account the self-field effect of the transport current. In (b), the dashed – dotted curve shows the expected recovery current based on Maddock’s equal area theorem under the uniform field distribution. Used in the calculation is the longitudinal resistivity evaluated from the voltage signals which were observed after the normal transitions in the cases that give minimum recovery currents.
The authors are grateful to Hitachi Ltd. and Hitachi Cable Ltd. for their great effort in developing many new techniques as well as in the fabrication of the superconductors with very high quality. The authors also appreciate the great support given by many staffs of NIFS to our conductor R&D programs, including many short sample experiments.
References [1] O. Motojima, K. Akaishi, K. Fujii, et al., Physics and engineering design studies on the Large Helical Device, Fusion Eng. Des. 20 (1993) 3 – 14.
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N. Yanagi et al. / Fusion Engineering and Design 41 (1998) 241–246
[2] T. Mito, K. Takahata, N. Yanagi, et al., Short sample tests of aluminum-stabilized superconductors for Large Helical Device, Fusion Eng. Des. 20 (1993) 233 – 242. [3] P.W. Eckels, N.C. Iyer, A. Patterson, et al., Magnetoresistance; the Hall effect in composite aluminum cryoconductors, Cryogenics 29 (1989) 748–752. [4] H. Kaneko, N. Yanagi, Enhancement of magnetoresistance due to Hall current in aluminum-copper composite, Cryogenics 32 (1992) 1114–1120. [5] H. Kaneko, Insulation of highly conductive metal in composite stabilizer for reduction of Hall current across surface, Cryogenics 33 (1993) 1077–1085.
.
[6] N. Yanagi, T. Mito, K. Takahata, et al., Experimental observation of anomalous magnetoresistivity in 10 – 20 kA class aluminum-stabilized superconductors for the Large Helical Device, Adv. Cryogen. Eng. Mater. 40 (1994) 459 – 468. [7] S. Imagawa, N. Yanagi, T. Mito, et al., Evaluation of recovery current of the helical coil for LHD, ICEC 16, Kita-Kyushu, paper OC7-5, 1996. [8] S. Imagawa, N. Yanagi, T. Satow, et al., Optimization of wetted surface fraction of helical coil for LHD, Cryogenics 34 (1994) 701 – 704.