Michitaka Ono

Ramp-Rate limitation due to current imbalance in a large cable-in-conduit conductor consisting of chrome-plated strands

The current distribution in the conductor, consisting of chrome-plated strands, was analysed assuming asymmetric strand transposition. The results show the circulation current is induced through the electrical joints at both ends of the conductor and electrical contact among the strands in the conductor. The current imbalance is produced as a result of the superimposition of the circulation and transport currents and becomes larger as the ramping rate increases. It was also found that the large current induced in the strands during a pulse charge cannot sufficiently be reduced at normal generation because of the induced voltage on these strands. The current flowing in the normal-state strands becomes larger for faster ramping. In addition, the effect of the non-uniform current distribution on the stability was experimentally investigated. The stability margin deteriorated when the current distribution in the conductor was not uniform. Moreover, the quench process in the ramp-rate limitation was considered. Since the coolant temperature is locally raised around the normal-state strands in the laminar-state coolant flow, the generation of the laminar flow region affects the ramp-rate limitation as a result of the current imbalance. From these results, it can be concluded that the current imbalance in the conductor has a very strong influence on the ramp-rate limitation.

Development of MJ-class HTS SMES for bridging instantaneous voltage dips

MJ-class HTS SMES has been developed for bridging instantaneous voltage dips using newly-developed Bi-2212 cable. The developed Bi-2212 wire for SMES coils achieves high-performance conductive characteristics that do not deteriorate in high magnetic fields beyond 10 T, which enable compactly arranged SMES coils to be operated in a high magnetic field, and SMES coils of the Bi-2212 wire can be adequately insulated due to a high temperature margin. Therefore it is possible for the SMES coils to enhance dielectric strength and output power. The insulating, cooling and conductive characteristics of 4 unit coils (Outer Diameter: 700 mm, Height: 127 mm, Stored Energy: 90 kJ) were checked under a variety of conditions. Moreover, fundamental performance tests were done on bridging instantaneous voltage dips, using a 125 kW resistance and 50 kW motor as imitation loads. Testing showed that the HTS SMES operated reliably. Up to the present, 11 unit coils (Outer Diameter: 700 mm, Height: 390 mm, Stored Energy: 560 kJ) have been stacked and tested.

Development of a 1 MJ cryocooler-cooled split magnet with Ag-sheathed Bi2223 tapes for Si single-crystal growth applications

A project to develop a high-temperature superconducting split magnet for Si single-crystal growth applications began in October 1997 and is scheduled to be completed for three years. The project is being executed on the basis of collaboration between Toshiba Corp., Sumitomo Electric Industries Ltd. and Shin-Etsu Handotai Co. Ltd., and is partially funded by Ministry of International Trade and Industry (MITI) of Japan. The purpose of this project is to confirm the energy-saving performance and high reliability of a large HTS split magnet (1 MJ) using Bi2223 tapes. This split coil system is composed of 2 coils, each consisting of 18 pancakes, and the total length of HTS tapes is approximately 80 km. The magnet is to be cooled to below 20 K by a highly efficient GM-type cryocooler in order to make overall current density of the magnet close to the density of metal superconducting magnets. In the first year of this project, a conceptual design was established and R&D of the fragile HTS tapes was carried out. In the second year, the design, fabrication, testing and evaluation of an experimental magnet, incorporating pancake coils of the same size as those of the actual magnet, has been accomplished. This work will contribute to the improvement of the design and fabrication of the full-scale magnet in the final year of this project.

A cryocooler-cooled 19 T superconducting magnet with 52 mm room temperature bore

This paper describes a design of a cryocooler-cooled 19 T superconducting magnet. The technical features of the magnet are a Bi2223 insert coil composed of 25 double pancake coils, Nb/sub 3/Sn coils using a Nb/sub 3/Sn wire reinforced with Nb-Ti-Cu compound, and a cooling structure using two types of cryocoolers. Coil protection from quenching was confirmed by numerical analysis. A preliminary experiment was carried out in order to investigate the influence of the bending strain upon a maximum permissible hoop stress of Ag-sheathed Bi2223 tape conductor.

Test results of the 100 kWh SMES model coil – AC loss performance

Abstract In order to establish a technology needed for a small-scale 100 kWh SMES device, an SMES model coil was fabricated. Performance tests were carried out at the Japan Atomic Energy Research Institute (JAERI) in 1996. After that, the coil was installed in the Lawrence Livermore National Laboratory (LLNL) facility and tested in 1998, in collaboration between Japan and the United States. The AC losses measured at LLNL were in good agreement with those measured at JAERI. It was reconfirmed that the coupling loss of the coil could be expressed in two components: one with a short and another with a long coupling time constant. We found out from the Hall probe signals that the loop currents with long decay times were induced in the CIC conductor by varying magnetic field. These currents resulted in additional AC loss in the coil. To develop a concept of CIC with low AC loss, we made a sub-scale CIC conductor of strands coated with CuNi. We fabricated a small coil out of this conductor and measured the AC loss. The measured AC loss in this coil was about 1/6 of that in the SMES model coil conductor per strand volume. Thus, the CuNi coating of the strands was demonstrated to be effective to reduce the AC loss in the coil.

Homogeneous current distribution in a coaxial superconductor with and without return current path

The authors have developed a theoretical method based on magnetic flux conservation between adjacent layers. One of the advantages of this method is that one can directly obtain twisting pitch and radius for realizing homogeneous current distribution in coaxial multi-layer superconductor. A set of the obtained twisting pitch and radius was employed in a sample three-layer conductor comprised of silver-sheathed multi-filamentary BSCCO-2223 tapes and the current distribution was measured by a Rogowski coil. Agreement between the experiment and the theory on current distribution is quite remarkable. Using this theory, the authors analytically investigated the influence of the manufacturing error of twisting pitch and radius on current distribution. The results revealed that the manufacturing errors of twisting pitch and radius have large effect on current distribution and a suitable set of twisting pitch and radius against manufacturing error can be found. They also investigated the relationship between twisting pitch and current distribution in coaxial six-layer conductor with return current path. The characteristics of twisting pitch in the conductor with return current path are different from those of the conductor without return current path.

Influence of current distribution on conductor performance in coaxial multi-layer HTS conductor

We have developed a simulation method based on magnetic flux conservation between two filaments of adjacent layers to estimate current distribution in coaxial multi-layer HTS conductor. Using this method, we have demonstrated homogeneous current distribution and verified that current distribution was controllable directly by the conductor parameters of layer radius, twisting pitch and twisting direction. Although it has been considered that homogeneous current distribution is effective for reducing AC loss, the most suitable conductor parameters and operating condition have not been investigated sufficiently yet. Therefore, we improved our developing method to estimate current distribution more rigorously considering the nonlinear voltage-current characteristic of HTS tape. To verify the validity of the simulation method, we measured current distribution using coaxial two-layer conductors. Agreement of current distribution between the experiment and the analysis was good.

Design of a 30 T Superconducting Magnet Using a Coated Conductor Insert

A program to develop a 30 T superconducting magnet based on novel concepts is now in progress at the High Field Laboratory for Superconducting Materials (HFLSM) at Tohoku University and the Tsukuba Magnet Laboratory (TML) at the National Institute for Materials Science. A 30 T superconducting magnet comprising a high-temperature superconducting (HTS) insert and a low-temperature superconducting (LTS) outsert was conceptually designed. For the high-field HTS insert, a YBCO coated conductor tape was adopted because of its high critical current density in high fields and its high mechanical strength. A relatively high tolerance limit of hoop stress in the insert coil can be assumed in the coil design according to its mechanical properties. The critical current density of the YBCO tape was analytically predicted as a function of temperature and magnetic field. To withstand a large electromagnetic force, the LTS outsert was composed of CuNb/Nb3Sn and NbTi coils. The CuNb/Nb3Sn coil was designed using high-strength cable consisting of internally reinforced Nb3Sn strands with a CuNb reinforcing stabilizer subjected to repeated bending treatment. The results of this design study show the potential for a compact high-field magnet employing an insert coil formed of YBCO coated conductor.

Test results of the SMES model coil—pulse performance

A model coil for superconducting magnetic energy storage (SMES model coil) has been developed. To establish the technology needed for a small-scale 100 kW h SMES device, a SMES model coil was fabricated and tested in 1996. The coil was successfully charged up to about 30 A and down to zero at the designed magnetic-field ramp rate for the SMES. Alternating current (AC) losses in the coil were measured by an enthalpy method. The results were analyzed and compared with the test results from a short sample. The measured hysteresis loss is in good agreement with that estimated from the short sample results. It was found that the coupling loss of the coil could be described as consisting of two components with different coupling time constants. One has a short time constant of about 220 ms, which is in agreement with the test result of a short conductor. The other has a long time constant of about 30 s, which was not expected from the test results for the short sample.

Critical current measurements using 13-T split coils and 100-kA superconducting transformer (for FER)

A description is given of a large scale superconductor test facility composed of a 13-T magnetic field and a 100-kA sample current. A superconductor transformer with a 100-kA secondary conductor was fabricated as a current amplifier in order to supply the 100-kA sample current. Superconducting split coils with 100-mm clear bore diameter were fabricated, and a 13-T available field was generated by these coils. Both the 100-kA superconducting transformer and the 13-T superconducting split coils were installed in a 2-m-diameter FRP dewar for the purpose of testing large-scale superconductors. A description is given of the performance of the 100-kA superconducting transformer and the 13-T superconducting split coils as well as the results from critical current measurements of prototype conductors for toroidal coils.