Kei Koyanagi

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.

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.

Progress of Research and Development of Fundamental Technologies for Accelerator Magnets Using Coated Conductors

A project to develop the fundamental technologies for accelerator magnets using coated conductors is in progress. A coil-dominated magnet and an iron-dominated magnet were designed, based on the conceptual design of spiral sector fixed field alternating gradient accelerator for carbon cancer therapy and that for accelerator-driven subcritical reactor, respectively. The required winding technologies were clarified through designing the magnets. The R&D of winding technologies for coils with three-dimensional shape and those with negative bend have been carried out. The influence of the magnetization of coated conductors on the field quality of magnets was studied experimentally.

Development of Saddle-Shaped Coils Using Coated Conductors for Accelerator Magnets

A project to develop fundamental technologies for accelerator magnets using high-Tc superconductors is currently underway. The primary applications in this project are fixed field alternating gradient (FFAG) accelerators for carbon cancer therapy systems and accelerator-driven subcritical reactors. One of the challenging issues in this study is to accomplish the complicated coil shape required for the accelerator magnets using YBa2Cu3Ox (YBCO) coated conductors. An asymmetric, three-dimensional winding structure has been designed for the main ring of an FFAG accelerator, but the anisotropic bending flexibility of coated conductors is constrained because of their tape-like shape. To demonstrate a suitable winding technology for a three-dimensional coil using coated conductors, a prototype winding machine with a two-axis motion mechanism was constructed. Based on the present design of the FFAG magnet, a saddle-shaped coil wound on a curved surface with a radius of 700 mm was designed. A 100-turn coil was wound by the winding machine using an approximately 90 m length of 3 mm-wide YBCO coated conductor made by the IBAD-MOCVD method. The coil was impregnated with epoxy resin, and the voltage-current (V-I) characteristics were evaluated in a liquid nitrogen environment. From the V-I characteristics in an electric field region of 10-9V/cm, an index value of above 20 was obtained, which means that the superconducting properties were not degraded.

Temporal behaviour of multipole components of the magnetic field in a small dipole magnet wound with coated conductors

To study the influence of coated-conductor magnetization on the field quality of accelerator magnets, we made a small dipole magnet consisting of four racetrack coils wound with GdBCO coated conductors and measured its magnetic field in liquid nitrogen by using rotating pick-up coils. We focused on the dipole and sextupole components (coefficients) of the magnetic field, which vary with time owing to the decay of the magnetization of the coated conductors. About 50 min (3055 s) after the current was ramped up to 50 A, the dipole coefficient normalized by the design value of the dipole component, i.e., the value calculated with the designed coil shape and the uniform current distribution in the coated conductors, increased by 7.4 × 10−4, and the sextupole coefficient normalized by the design value of the dipole component increased by 1.8 × 10−4. The magnitudes of the dipole and sextupole components depended on the excitation history of the magnet. Electromagnetic field analyses were carried out to calculate the current distributions in coated conductors, considering their superconducting properties; the dipole and sextupole coefficients were then determined from the calculated current distributions. Although the analyses were based on the two-dimensional approximation of the cross-section of the magnet, the temporal behaviours as well as the hysteretic characteristics of the calculated dipole and sextupole coefficients agree qualitatively with those of the dipole and sextupole coefficients measured in the magnet.

Design of a High Energy-Density SMES Coil With Bi-2212 Cables

This report describes a basic design of an HTS-SMES coil wound with large current capability Bi-2212 cables. The objective of this development is to evaluate fundamental properties of the coil in a high magnetic field. The coil is comprised of an HTS inner coil wound with Bi-2212 Rutherford cables and an LTS outer backup-field coil wound with NbTi conductors. The Bi-2212 coil is designed to generate a maximum field of 8 T at 4.2 K within the backup field at an operating current of 1584 A. In this study, general specifications of the coil were discussed with some important issues; winding dimensions for a peak field, current density limitation, elasticity of the winding, and protection from the thermal runaway. The coil is fabricated in FY 2006, and testing will be performed in FY 2007. In the experiments, superconducting characteristics of the coil in a high magnetic field will be evaluated

Compact Design of a 30 T Superconducting Magnet Incorporating YBa2Cu3O7 Coated Conductor Tapes and Pre-reacted Nb3Sn Strand Cables

We set out to design as compactly as possible a 30 T superconducting magnet consisting of an YBa2Cu3O7 (Y123) insert coil and a CuNb-reinforced Nb3Sn (CuNb/Nb3Sn) background coil. The engineering current density Je for an Y123-coated conductor tape is larger at 14–15 T and 4.2 K than that for a CuNb/Nb3Sn strand. Adopting Y123-coated conductor tapes in the high field region above 14–15 T proves highly effective for the fabrication of a very compact 30 T superconducting magnet. A 30 T, 52 mm-room-temperature bore superconducting magnet can be constructed in the coil grading of a 16 T Y123 insert and a 14 T CuNb/Nb3Sn background coil, and the stored magnetic energy of 32 MJ successfully lies in the 1/3 level in comparison with the 30 T nuclear-magnetic-resonance (NMR) superconducting magnet reported previously.

Design study of superconducting magnets for uniform and high magnetic force field generation

Magnetic force is one of the most promising tools to realize a virtual microgravity environment on earth. It has been found that the growth of protein crystals might be affected by microgravity owing to the suppression of convectional flow. We started the development of superconducting magnets for the generation of uniform and high magnetic force fields to suppress convectional flow, as it was not clear what configuration of superconducting coils could generate most effectively high magnetic force fields, while they maintain their uniformity. For this purpose, we used a nonlinear programming method. The results obtained clarified that a magnet whose inner coil is longer than the outer one can generate more uniform and higher magnetic force fields in a long sample space. A superconducting magnet generating a magnetic force field of 240 T/sup 2//m has already been constructed with NbTi conductors at the Tsukuba Magnet Laboratory. We have also completed the design of a superconducting magnet composed of Nb/sub 3/Sn and NbTi conductors to generate uniform magnetic force fields up to 882 T/sup 2//m.

Cryocooler directly cooled 6 T NbTi superconducting magnet system with 180 mmroom temperature bore

Abstract This paper describes a cryocooler cooled NbTi superconducting magnet system. The technical features of this magnet system are a 4K-Gifford-McMahon (GM) refrigerator using magnetic regenerator material and a high-Tc Bi 2 Sr 2 CaCu 2 O y superconducting current lead. The NbTi coil was directly cooled by the 4K-GM refrigerator without liquid helium and it took about 21 hours for the NbTI coil to be cooled from room temperature to below 4 K. The stable magnetic field of 6 T at 3.5 K and the maximum magnetic field of 6.45 T were obtained in the 180 mm room temperature bore.

Fabrication of YBCO Small Test Coils for Accelerator Magnet Development

A research program for studying coil winding technologies using YBa2Cu3Ox (YBCO) coated conductors is now in progress. In this program, we are attempting to develop high-Tc superconducting magnets for highly functional, efficient, and compact accelerator systems. In designing superconducting coils with thin tape conductors, we must be concerned about magnetization of the superconducting layer. In order to experimentally evaluate the influence of the additional field resulting from the magnetization current, prototype racetrack coils were constructed with YBCO coated conductors. A 62 m length of coated conductor was wound to form each of the coils, which were then impregnated with epoxy resin. For precise analysis of the additional magnetic field, it is essential to fabricate superconducting coils without degradation because the voltage generated at such a degraded part would affect the behavior of the additional magnetic field. In a liquid nitrogen environment, the racetrack coils were energized to evaluate the voltage-current (V-I) characteristics. It was found that there was no degradation in the superconductivity. The V-I characteristics showed sufficiently high index values of 28 and 34 throughout an electric field range of 10-9 V/cm. Magnet design derived from accelerator design, as well as construction of an experimental reduced-scale coil in a typical shape, are also discussed.