Superconducting magnets are effective for obtaining a highly stable, strong magnetic field for magnetic resonance imaging (MRI). Current MRI superconducting magnets require cooling in liquid helium at 4.2 K in order to use NbTi superconducting wire. The development of a high-temperature superconducting (HTS) coil that can be used without liquid helium cooling is greatly desired. In order to develop liquid helium-free MRI magnets using a high-temperature superconductor, the author has prototyped a model magnet that is able to generate extremely uniform, highly stable magnetic fields. This particular development includes such subjects as developing a coil structure where the superconducting characteristics do not deteriorate, working on a method of producing precision coils, designing coils that generate extremely uniform magnetic fields, and having high-temperature superconducting coils generate highly stable magnetic fields. MR imaging was carried out to verify the uniformity and stability of the magnetic fields.
A project for developing of RE1Ba2Cu3O7-δ (REBCO) magnets to be utilized in ultrahigh-field magnetic resonance imaging (MRI) was started in 2013. Our final targets are 9.4 T MRI systems for whole-body and brain imaging. In this project, two different development approaches towards the final target were planned. One is a small REBCO coil that can generate 10 Tclass magnetic fields, which is the same level as the target magnetic field. The other is a conduction-cooled 1.5 T REBCO MRI magnet that has a room-temperature bore of 396 mm, which is as large as those of mid-sized model magnets. These results were reflected in the design of a conduction-cooled 9.4 T REBCO magnet for whole-body MRI systems.
A research and development project to reduce the weight of a rotating gantry for carbon ions using high-temperature superconducting (HTS) magnets started in 2013, supported by the Japanese Ministry of Economy, Trade and Industry (METI) and the Japan Agency for Medical Research and Development (AMED). In this project, we aim to develop fundamental technologies for designing and fabricating a HTS gantry, and fabricate a small model of a HTS dipole magnet and evaluate its performance. The results of the project are described in this paper.
The major results of conducting research and development on the common core technologies for HTS coils are reviewed. The theme has three sub-themes that are necessary for advanced superconducting devices. For the sub-theme of long coated conductors (CCs) with a high in-field Ic, a 93 m-long CC with a high Ic (min.) value of 124 A/cm-w at 77 K under 3 T was successfully fabricated by PLD. Concerning the sub-theme of generating CCs at low heat, Ic uniformity and scribing technique were both improved and uniform filament Ic values were confirmed. Additionally, lower a joint resistance than 3 nΩ was achieved using a paste that included nano-sized metals. In the sub-theme regarding the fundamental evaluation of CCs and coils, the effect of CC scribing on the decay of the shielding current was confirmed both in CCs and coils. Furthermore, a new operation process for the MRI magnet was proposed based on the above-mentioned knowledge.
We propose a new structure for layer-wound coil manufactured using a high-temperature superconducting (HTS) wire in the form of a tape to reduce the central magnetic field produced by screening currents induced on the broad surfaces of HTS tapes. Reducing this screening current-induced field is based on the abnormal transverse-field effect observed generally in the mixed state of Type-II superconductors. The structure proposed enables the inner bore of special coil required nuclear magnetic resonance systems to be used effectively. However, it cannot eliminate the screening current-induced fields completely because the innermost and outermost parts of the HTS winding are always exposed to magnetic fields weaker than the full penetration fields. We carry out a series of experiments to confirm the effectiveness of reducing the screening current-induced fields in a HTS coil fabricated with a commercially available coated conductor.
Austenite stainless steel is used for liquid natural tanks and superconducting facilities since it has a face-centered cubic lattice, which is less likely to decrease its toughness at cryogenic temperatures. The structural materials of the ITER toroidal field coil structure (TFCS) are required to have high fracture toughness at cryogenic temperature (4 K) in order to prevent unstable fracturing by the huge electromagnetic force. Yield strength at 4 K can be accurately predicted pragmatically. However, the estimation method for fracture toughness at 4 K is not yet well developed. In this study, the authors investigated the correlation between several material properties and 4 K fracture toughness of actual sized ITER TFCS materials. As a result, there is a low correlation between 4 K fracture toughness and the parameters (i.e., 4 K yield strength, nitrogen content and grain size), which were thought to be well matched for fracture toughness as reported in previous studies. In contrast, 4 K tensile strength and Md30 are in good correlation with fracture toughness because local transformation into martensite occurring at the crack tip affects fracture toughness. Md30 is used as an index of stability in the austenite phase. The authors therefore established a new method that simplifies controlling 4 K fracture toughness of austenite stainless steel using Md30. In addition, it is demonstrated that this method is effective for actual TFCS materials. The views and opinions expressed herein do not necessarily reflect those of the ITER organization.