Permanent current switch

This application is a divisional application of U.S. patent application Ser. No. 11/654,008, filed Jan. 17, 2007, which claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2006-041800, filed Feb. 20, 2006, the entire disclosures of which are herein expressly incorporated by reference.

(1) Field of the Invention

The present invention relates to a permanent current switch having a coiled superconducting wire and a heater wire, which is prepared using, in the superconducting wire, a magnesium diboride superconducting (hereinafter abbreviated as MgB₂) wire (hereinafter referred to as the MgB₂ wire) that exhibits a superconducting property in an environment not in excess of a critical temperature.

(2) Description of Related Art

The MgB₂ wire has a critical temperature of 39 K, which is about 20 K or more higher than that of an ordinary metal-based superconducting material. The wire also has a high critical magnetic field, and this is said to be about 50 T on a thin film. In addition, since the wire has a remarkably small anisotropic magnetic field and an excellent flexural property, it has a very few differences from the ordinary metal-based superconducting material in preparing and handling of a wire material.

A permanent current switch is one of applications utilizing a very high thermal stability of this MgB₂ wire. A permanent current switch using this MgB₂ wire is described in JP-A-2003-37303.

The permanent current switch is generally formed using an NbTi wire in which a CuNi alloy is used as a stabilization material. However, NbTi has a critical temperature as low as about 9 K, and has a temperature margin of only about 5 K in liquid helium having a critical temperature of 4.2 K. Therefore, the NbTi wire may be easily transferred to a normal conducting state even when small disturbance energy is applied. For such a reason, a permanent current switch has been developed using the MgB₂ wire having a temperature margin of 35 K in liquid helium.

However, there has been a problem that even if the permanent current switch is prepared simply using the MgB₂ wire, the switch has a resistance of about 1×10⁻¹⁰Ω, and a highly reliable permanent current switch having a less resistance value cannot be prepared.

An object of the present invention is, therefore, to provide a permanent current switch which has a high temperature margin and which is thermally stable and which ensures a switching operation between a superconducting state and a normal conducting state of a superconducting wire.

A permanent current switch of the present invention has a coiled superconducting wire and a heater wire which switches the superconducting wire between a superconducting state and a normal conducting state, the superconducting wire being a magnesium diboride superconducting wire having a high-resistance metal on an outer side and a magnesium diboride superconducting portion on an inner side and prepared by forming a superconducting metal on a layer between the high-resistance metal and the magnesium diboride superconducting portion, the permanent current switch further comprising: a superconductive connecting section superconductively connected to a lead wire guided from the superconducting wire and a superconducting wire for a wiring line.

Moreover, the superconducting wire for use in the permanent current switch of the present invention is a multi-core magnesium diboride superconducting wire having a plurality of magnesium diboride superconducting cores formed by coating magnesium diboride superconducting portions with a superconducting metal and prepared by arranging the plurality of magnesium diboride superconducting cores in a high-resistance metal.

Furthermore, it is preferable that the superconductive connecting section superconductively connects the magnesium diboride superconducting portion or the superconducting metal with which the magnesium diboride superconducting portion is coated to a superconducting core portion of the superconducting wire for the wiring line via a low-melting superconducting metal or a low-melting superconducting alloy.

Moreover, it is preferable that the high-resistance metal is an alloy containing Cu as a main component. It is to be noted that examples of the alloy containing Cu as the main component preferably include CuNi, CuSn, CuZn, CuMn, CuMg, CuIn, CuCo and CuCr.

Furthermore, it is preferable that the superconducting metal is an alloy containing Nb as a main component. It is to be noted that examples of the alloy containing Nb as the main component include Nb, NbTi and NbTa.

In addition, it is preferable that the low-melting superconducting metal or the low-melting superconducting alloy is made of at least one or more metals of Sn, Mg, In, Ga, Pb, Te, Tl, Zn, Bi and Al.

Moreover, a superconducting magnet of the present invention can realize a thermally stable permanent current operation by use of these permanent current switches.

Furthermore, to achieve the object of the present invention, it is possible to apply a method of manufacturing a permanent current switch, comprising: winding a magnesium diboride superconducting wire or a multi-core magnesium diboride superconducting wire and a heater wire to form a coil shape; and superconductively connecting a lead wire guided from the superconducting wire to a superconducting wire for a wiring line.

In addition, according to a predetermined embodiment, it is possible to provide a highly reliable permanent current switch which allows a magnetic field to decay.