Superconductor technology: apparatus – material – process – High temperature devices – systems – apparatus – com- ponents,... – Superconducting wire – tape – cable – or fiber – per se
Reexamination Certificate
2000-05-26
2003-07-08
Bryant, David P. (Department: 3726)
Superconductor technology: apparatus, material, process
High temperature devices, systems, apparatus, com- ponents,...
Superconducting wire, tape, cable, or fiber, per se
C029S599000, C505S705000
Reexamination Certificate
active
06591120
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an apparatus for generating a strong magnetic field by utilizing an oxide superconductor. More particularly, the present invention relates to a new oxide superconducting wire which is suitable for those apparatuses which need a strong, uniform magnetic field, such as scientific instruments, NMR analyzers, and medical MRI apparatus. It relates also to a solenoid coil and a magnetic field generating apparatus with said oxide superconducting wire and a process for producing said oxide superconducting wire.
BACKGROUND OF THE INVENTION
Superconducting magnetic apparatuses generally find use in two fields: one in which no consideration need be given to the uniformity and stability of the magnetic field so long as a magnetic field is generated, and the other in which the quality of the magnetic field (or the uniformity and stability of the magnetic field) is a matter of important concern. A highly uniform, highly stable magnetic field is usually required for research work, such as measurements of physical properties and magnetic field generating apparatuses for medical use. It is essential for the technology of high-quality superconducting magnets. Such superconducting magnets have conventionally been produced with accurately worked wires made of a metal superconductor, such as a niobium-titanium superconductor and a niobium-tin superconductor. Superconducting wires are wound into a solenoid under stringent quality control. The resulting superconducting magnet is run with an extremely stable power supply or in a permanent current mode. The thus generated magnetic field is highly stable time-wise and space-wise. Recent years have seen a remarkable advance in magnets generating a highly uniform magnetic field. Magnets capable of generating as high an intensity as 20T have appeared to meet the need for a NMR apparatus. Unfortunately, the intensity of the magnetic field that can be generated by the conventional metal superconductor is limited to about 20T on account of the critical magnetic field inherent in the material used. In order to exceed this limit, it is essential to resort to an oxide superconductor.
A magnetic field generating apparatus has been developed with an oxide superconductor in which the coil is formed by winding a tape-like wire into a double pancake shape, as reported in Journal of Applied Physics, Vol. 35, 1996, Part 2, L623 to 626. This coil is suitable for generating a magnetic field stronger than 22T, which has never been attained with conventional metal superconductors. However, this coil suffers the disadvantage of being unable to generate a uniform magnetic field. The reason for this it that the conductor is in the form of a tape, and the tape is wound into a coil of pancake shape in such a way that more than one coil is placed on top of the other. One way to address this problem is to use a conductor with a round cross section and wind it into a solenoid coil for the magnet, as reported in Journal of Material Science, Vo. 30, 1995, pp. 3200 to 3206. Such a conductor generates a comparatively strong, uniform magnetic field; however, it cannot generate a desired magnetic field because its critical current density (Jc) is only one-fifth that of the tape-like wire. It is possible to secure a strong magnetic field easily by winding a tape-like wire into a solenoid, but it is difficult to secure a sufficiently uniform magnetic field in this way on account of the outer dimensional accuracy of tape-like wire.
A wire with a round cross section is usually produced by drawing a metal through a die, and it has a higher accuracy (of the order of microns) compared with a tape-like wire. Therefore, such a wire is suitable for the generation of uniform magnetic fields. By contrast, a tape-like wire is produced by rolling, and its working accuracy (thickness and width) is usually limited to the order of 10 microns. Therefore, when it is wound in thousands of turns, the number of turns varies from one place to another because of the uneven working accuracy. This results in an uneven magnetic field. For this reason, there has been a demand for an oxide superconducting wire which meets requirements for both high current density and mechanical working accuracy so that it generates a strong, uniform magnetic field.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an oxide superconducting wire which has high dimensional accuracy and generates a strong, uniform magnetic field. It is another object of the present invention to provide a solenoid coil formed from the oxide superconducting wire. The term “uniform magnetic field” means that the intensity of the magnetic field varies less than 0.1%, preferably less than 0.01%, and the term “strong magnetic field” means that the intensity of the magnetic field is higher than 22T. In the past, it was difficult to generate such a strong, uniform magnetic field; it was only possible to achieve it by means of a magnet formed from an oxide superconducting wire.
The basic reason for this is that an oxide superconductor is a greatly anisotropic substance. When it is made into a tape-like wire by rolling, the resulting wire permits a coil current density of about 50 to 100 A/mm
2
in a magnetic field stronger than 10T. However, when it is made into a round wire by drawing, the resulting wire permits a current density of only 10 to 20 A/mm
2
.
An object of the present invention is to provide an oxide superconducting wire which has high dimensional accuracy and generates a strong, uniform magnetic field with a high critical current density. Another object of the present invention is to provide a solenoid coil and a magnetic field generating apparatus formed from the oxide superconducting wire.
According to the present invention, it is possible to produce an oxide superconducting wire which has high dimensional accuracy without a decrease in current density. The first aspect of the present invention is directed to an oxide superconducting wire which is characterized in that the wire has an approximately round cross section perpendicular to its lengthwise direction, the cross section is composed of several units, each unit being composed of a plurality of tape-like oxide superconductors, the tapes in each unit being laminated stepwise on top of the other in the direction perpendicular to the lengthwise direction at an angle of about 60 degrees with respect to the tape surface within the cross section, each unit having an approximately rhombic shape within the cross section, the cross section having at least three different units which are arranged such that adjacent units have a rotational symmetry through about 120 degrees with respect to the direction of tape lamination and at least one side of the rhombic shape of a unit is in contact with an adjacent unit.
In other words, the oxide superconducting wire of the present invention is characterized in that it has a round cross section perpendicular to its lengthwise direction and the cross section is composed of oxide cores which are geometrically arranged at rotationally symmetric positions. This rotational symmetry may be established by 3 rotations, 4 rotations, or 6 rotations within the round cross section. For the oxide to be packed most closely, it is desirable to arrange the cores in triangular symmetry. It follows, therefore, that the most efficient packing ratio (or the ratio of the sectional area of the oxide to the total sectional area) is achieved when three rhombi are arranged in rotational symmetry, each rhombus being composed of two regular triangles.
According to the present invention, the oxide superconductor has a tape-like shape, and these tapes are laminated on top of the other to form a conductor. However, it is very difficult to obtain a conductor of ideal configuration because of the limitation accuracy in tape rolling and tape assembling. In view of this, the angle of lamination direction should be about 60 degrees or 120 degrees. The closer to ideal the shape is, the better the performance will be. As
Fukushima Keiji
Okada Michiya
Tanaka Kazuhide
Wakuda Tsuyoshi
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