1.  Field of the Invention
The present invention relates to a superconducting circuit having superconducting joints and a production method of the superconducting joints, and further relates to a superconducting magnet provided with the superconducting circuit and a production method of the superconducting magnet. 
2.  Description of Related Art
A superconducting magnet is capable of generating a high intensity and low attenuation magnetic field.  Therefore, the superconducting magnet is used for a nuclear magnetic resonance analysis equipment (hereinafter, referred to as NMR apparatus: Nuclear Magnetic Resonance apparatus) and a medical magnetic resonance diagnostic equipment (hereinafter, referred to as MRI apparatus: Magnetic Resonance Imaging apparatus).  Especially, with respect to an apparatus utilizing a nuclear magnetic resonance phenomenon such as the NMR apparatus and the MRI apparatus, analytical sensitivity and imaging quality can be improved by increasing the magnetic field intensity and the magnetic field time stability.  Then, application of the superconducting magnet, which is capable of generating a high intensity/low attenuation magnetic field, to these apparatuses is becoming a main stream. 
A superconducting magnet generates a high intensity/low attenuation magnetic field under the persistent current mode by a large current flowing in a superconducting coil without supplying electric power from outside.  However, the magnetic field slightly attenuates even in the persistent current mode.  A loss in a superconducting joint that connects superconducting wires with each other is considered one of reasons for the magnetic field attenuation, and a method for jointing a superconducting wire has been proposed, for example, in U.S. Pat.  No.  4,907,338 and JP H05-152045. 
In U.S. Pat.  No.  4,907,338, a method for jointing a superconducting wire having a structure where a filament made of niobium titanium (NbTi) is arranged in a matrix made of copper (Cu) is described.  In the jointing method, the matrix on the end portion of the superconducting wire is replaced with tin (Sn), the tin is further replaced with lead-bismuth alloy (PbBi) that is a superconductor, and each end portion of the superconducting wire is inserted into a metal tube where melted lead-bismuth alloy is filled to joint the superconducting wire with each other. 
In JP H05-152045, there is a description on a jointing method, where a matrix on the end portion of a superconducting wire is dissolved to expose a filament, and the exposed filament of the superconducting wire is inserted into a sleeve to be pressed together with the sleeve.