Study on performance improvements of a high-temperature superconducting coil with a lattice-shape cross section

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Abstract

For designing inexpensive high-temperature superconducting (HTS) coils, it is essential to obtain large magnetic fields and stored energy with shorter lengths of HTS tape. To improve the performance of a coil, it is necessary to improve its transport-current performance. The critical current and n-value of an HTS tape were measured at various magnetic field magnitudes and angles at 77 K. The HTS tape employed in the coil was Bi-2223/Ag tape. From this measured data, fitting equations of the critical current and n-values were obtained. With these fitting equations, coil critical currents were analyzed according to our analytical model. The analysis showed that relatively large electric fields are generated at coil edges, inhibiting improvement of the transport-current performance of the coil. To solve this problem, we propose an HTS coil, which is produced by cutting and displacing the central portion of the rectangular cross section. By this rearrangement, the magnetic field distribution changes, resulting in an improvement in the coil critical current. We calculated performances of the proposed coil by varying the magnitude of displaced coils while maintaining a constant total HTS tape length. We found that there is an optimum cross section shape of the proposed coil, which provides improvements in the stored energy and the central magnetic field. In particular, the stored energy improves by approximately 43% compared with a rectangular cross section coil employing the same HTS tape length.

Introduction

High-temperature superconducting (HTS) tapes used in HTS coils are expensive. Therefore, to reduce the costs, we need to improve coil performances such as central magnetic fields or stored energy, to be able to use a shorter length of HTS tape. Under constant temperature, the critical current and n-value of an HTS tape depend on the magnitudes and angles of magnetic fields applied to the tape [1], [2]. To analyze transport-current performance of a coil, it is necessary to obtain these dependencies. We measured critical currents and n-values at magnetic fields with various magnitudes and angles, and obtained fitting equations to calculate coil critical currents. Coil critical current analysis showed that relatively large electric fields are generated at coil edges [3], which prevent improvement of the coil transport-current performance. If this coil critical current is improved, then the parameters of the coil, such as the stored energy or the central magnetic field, would also improve.

To solve this problem, the parts of the coil where the electric field is generated were removed, resulting in an increase in the critical current [4]. However, the cross section shape became complex. This is not desirable from a coil manufacturing viewpoint.

In this study, we cut and displaced central portions of the cross section of a coil with a rectangular cross section derived from the Fabry factor constant curve [5]. The resultant cross section consisted of four small coils. Magnetic field distribution analysis showed that the magnetic fields at each tape in the cross section decreased. Accordingly, the coil critical current increased.

Next, we analyzed the coil performances by varying the magnitude of displaced coils, while maintaining a constant total tape length. We found that there was an optimum cross section shape of the proposed coil, for which the critical current, stored energy, and central magnetic field achieved maximum value. At this optimum shape, the central magnetic field and the stored energy increased by approximately 5% and 43%, respectively, compared with those for a rectangular cross section coil with the same HTS tape length.

Section snippets

Fitting equations for an HTS tape

To analyze the transport-current performance of an HTS coil, it is necessary to obtain the magnetic field dependencies of critical current and n-value of an HTS tape. Therefore, we measured critical currents and n-values of a Bi-2223/Ag tape at various magnetic field magnitudes and angles at a constant temperature 77 K. From these data, fitting equations were derived with the magnetic field magnitude and angle as parameters. The critical current was defined as 1 μV/cm. The width of the tape was

Analysis of the transport-current performance of an HTS coil

First, the magnetic field magnitudes and angles to each tape of a coil cross section are calculated. These magnitudes and angles are substituted in the fitting equations (1), (2), (3), (4). Then, voltages at each tape are calculated. All voltages within the entire cross section of the coil are added to calculate a coil voltage for each current to obtain the current–voltage characteristic of the coil. The specifications of the coil analyzed in this study are listed in Table 1. This coil has the

Analysis of an HTS coil with a lattice-shaped cross section

As discussed in the previous section, for improving the transport-current performance of an HTS coil, the magnetic field magnitudes or angles at each tape in the cross section need to be reduced. In this study, we propose an HTS coil produced by cutting and displacing the central portion of the rectangular cross section. The schematic of the method for designing this coil is shown in Fig. 5. As shown in this figure, the total HTS tape length did not change even after this rearrangement. We

Conclusion

For designing inexpensive optimum HTS coils, it is necessary to obtain large magnetic fields and stored energy with shorter HTS tape length. To improve coil performances, it is essential to increase the coil critical current. Our analytical model for calculating coil critical currents showed that relatively large electric fields were generated at the coil edges. These large electric fields prevent improvement of the transport-current performance. In this study, we proposed rearrangement of

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Cited by (3)

  • Performance improvements and optimization of new high-temperature superconducting coil with circle vacancies at edge portions

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    Citation Excerpt :

    Considering the above background, it is necessary to create an HTS coil which generates large magnetic fields and stored energy using a shorter HTS tape length. Thus far, we have created a variety of new HTS coils which satisfy the above needs [5]. In general, the critical current and n-value of an HTS tape depend on magnetic field magnitudes and angles at constant temperature [6].

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