Osamu Ozaki

Operation of a 920-MHz high-resolution NMR magnet at TML

A 920-MHz high-resolution NMR spectrometer has been operating at the Tsukuba Magnet Laboratory (TML) since April 2002. It has proved its effectiveness by determining the 3-D structures of protein molecules. To accelerate studies in structural biology and solid-state NMR, a second high-field NMR magnet was developed and installed at TML. Although its basic design was the same as that of the first magnet, some improvements were made. For the innermost coil, a 16%Sn-bronze-processed (Nb,Ti)/sub 3/Sn conductor was employed. The increase in the critical current density above that of a 15%Sn-bronze-processed (Nb,Ti)/sub 3/Sn conductor made it possible to reduce the conductor size from 3.5 mm /spl times/ 1.75 mm in the first magnet to 2.80 mm /spl times/ 1.83 mm in the second. At the same operating current of the first magnet, the second magnet is expected to operate at 930 MHz. The liquid helium reservoir and the superfluid helium cooler, which were separated in the first system, were united in the same chamber in the new magnet. The latter magnet was energized up to 21.9 T without quenching in March 2004 and has operated in a persistent-mode at that field. It will be utilized mainly for solid-state NMR measurements.

Persistent-mode operation of a 920 MHz high-resolution NMR magnet

Development of a high-field NMR magnet has been underway at the Tsukuba Magnet Laboratory of the National Institute for Materials Science. The magnet succeeded in a persistent-mode operation at 21.17 T in December 1999. A 283-day long-term operation was carried out from October 2000 to August 2001. It included a persistent operation at 21.6 T (920 MHz) for 108 days. This was the highest field that the superconducting magnets have ever achieved in a persistent operation. Field decay was less than 2 Hz/h. Field homogeneity after correcting with superconducting shim coils were less than 0.1 ppm in a sample volume. These results confirmed that this magnet had been successfully developed as a high-resolution NMR magnet.

Development and operation of superconducting NMR magnet beyond 900 MHz

As a milestone in the 1-GHz NMR magnet project being carried out at the Tsukuba Magnet Laboratory, a 900-MHz class NMR magnet was successfully manufactured and operated in December 1999. The developed magnet is made of 15%Sn-bronze-processed (Nb,Ti)/sub 3/Sn, Ta-reinforced (Nb,Ti)/sub 3/Sn, and NbTi conductors. The room temperature bore of the cryostat is 54 mm is diameter. All the coils are cooled with pressurized superfluid helium. The magnet generated a field of 21.20 T in a driven mode and then operated in a persistent mode at 21.17 T corresponding to a proton NMR frequency of 902 MHz. The field may be raised to the range of 21.6 T (920 MHz) in the near future.

Development of a Bi-2223 HTS Magnet for 3T MRI System for Human Brains

We are developing a cryogen-free high temperature superconducting (HTS) magnet system for a compact 3T MRI system for human brains. In the conceptual design, the magnet system consists of 5 main coils that are layer windings of Bi-2223 tapes. The magnet system will have 500 mm room temperature bore and be operated at 20 K using G-M cryocoolers. The target field is 3 T ±5 ppm for 250 mm (dia,) and 200 mm (length) volume. All the coils will be connected in series and operated in driven mode. Controlled overpressure (CT-OP) processed Bi-2223/Ag tapes which are reinforced with Cu-alloy laminations are to be used for these coils. We investigated Ic-B-T performance in detail for short samples of the tape. We fabricated and tested five small layer-wound coils using the tape (38.4 ~ 46.2 m piece for each coil). Each small coil could be energized up to its expected current that was calculated using the short sample performance and the coil parameters. The maximum electromagnetic force (hoop stress) reached 137 MPa, and caused no degradation in the coil performance. These results show that our layer-winding techniques and the conductor performance (Ic-B-T and homogeneity along length) can be applicable and suitable for our 3T MRI magnet.

Newly Designed 3 T MRI Magnet Wound With Bi-2223 Tape Conductors

We have designed and fabricated a 3T magnetic resonance imaging magnet system for the human brain, which was wound with Bi-2223 tape conductors. Cooled by a Gifford-McMahon cryocooler, it was operated at 20 K with a stored energy of 2.3 MJ. A magnetic-field homogeneity of 5 ppm was attained at 1.5 T, which was our target value. Using this Bi-2223 high-temperature superconducting magnet, we obtained magnetic resonance images in 1.5 T at 8.5 K. The system was successfully magnetized to 3 T, which is the final target field in our project. This work demonstrates the potential of the high-temperature superconducting magnet for use in human magnetic resonance imaging experiments.

Examination of an Interior Permanent Magnet Type Axial Gap Motor for the Hybrid Electric Vehicle

Hybrid electric vehicles (HEVs) that emit less carbon dioxide have attracted much attention and rapidly become widespread, but further popularization of HEVs requires further technical advancement of mounted traction motors. Accordingly, our research group focuses on axial gap motors that can realize high torque density. In this paper, an axial gap motor with a novel interior permanent magnet (IPM) rotor structure is proposed and an examination of the proposed motor at the actual motor size of an HEV is presented. For comparison, we selected the newest radial gap-type 60 kW IPM synchronous motor equipped in the third-generation Toyota Prius. Under the condition that the size of the proposed motor be the same as the comparison motor, we confirmed through three-dimensional finite-element analysis that the proposed motor could output twice the maximum torque of the comparison motor. In addition, a comparison was made with a previously reported conventional IPM-type axial gap motor, and the proposed motor was found to be more effective in generating reluctance torque. Moreover, the proposed motor exhibited sufficient durability to irreversible demagnetization of the permanent magnets, to stress caused by rotating the rotor, and to unbalanced electromagnetic forces caused by axial rotor eccentricity.

Present status of 920 MHz high-resolution NMR spectrometers

The first 920 MHz high-resolution NMR (nuclear magnetic resonance) magnet was successfully installed at the Tsukuba Magnet Laboratory of the National Institute for Materials Science. A persistent operation at 21.6 T has continued since April 2002 without any problems. Excellent field stability of 0.31 Hz/h was observed from December 2002 to January 2003. Since July 2002, the magnet was used as an essential part of the only 920 MHz high-resolution NMR spectrometer in the world. The signal-to-noise ratio of 0.1% ethylbenzene using a proton-selective probe was as high as 2981. An HCN triple-resonance probe is now attached to the spectrometer to contribute to the National Project on Protein Structural and Functional Analysis in Japan. The completion of the second 920 MHz-class NMR spectrometer is scheduled for spring 2004.

Superconducting magnets for generating uniform magnetic force field

We report a new application of high magnetic fields to structural biology. We usually design and fabricate a magnet to achieve uniform magnetic field as well as uniform magnetic field gradient. In this new application we adopt a uniform magnetic force field. It has been found that the growth of protein crystals is affected by the presence of magnetic force. Development of uniform magnetic force field magnets is now in progress at the Tsukuba Magnet Laboratory of the National Research Institute for Metals. These magnets are superconducting magnets because they must be continuously run for several days to grow protein crystals. The first magnet wound with NbTi is now under installation. This magnet is designed to generate a uniform force field of 240 T/sup 2//m in a cylindrical space of 10 mm in diameter and 10 mm in height, and the magnetic force field fluctuation along z-direction is better than 0.4%. In liquid helium, it could achieve the design current.

Analysis of an Abnormal Event in a 3-T MRI Magnet Wound With Bi-2223 Tape Conductors

We have designed and fabricated a 3-T Magnetic Resonance Imaging (MRI) magnet system for the human brain, which was wound with Bi-2223 tape conductors. The magnet was successfully energized to 1.5 T and then up to 3.0 T. The magnet experienced more than 60 ramp-up and down processes to 1.5 T with no trouble. A magnetic field of 3.0 T (184.8 A) could be maintained for longer than 3 h without any problems. However, abnormal voltage behavior was observed during the ramp down for the third trial to 3.0 T, and the temperature rapidly increased with a ramping rate of 1.7 K/min. To investigate the cause, we opened the cryostat and inspected the inside of the magnet. In addition, we have analyzed the details of this event using the recorded current, voltage, and temperature data. The purpose of this paper is to report the investigation of this abnormal event in the magnet.

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.