Along with improvements in performance of the superconducting wires and advances in coil manufacturing techniques using such wires, as well as technical developments in related apparatuses such as heat-insulating containers and refrigerators, various types of superconducting magnets and application apparatuses employing such magnets have been created. Among these, there is a type which is operated in a persistent current mode. Superconducting magnet apparatuses for magnetic resonance imaging systems (MRI) and for magnetically levitated vehicles (Maglev) are examples of this type which have already been put into practical use. These superconducting magnet apparatuses supply an electric current from an external excitation power source to a coil that is cooled to an extremely low temperature. While a required magnetic field is produced, winding start and end portions of the coil are shunted by a superconducting switch, and this makes the apparatus run in a persistent current mode in which the electric current continues to flow into the coil without a power supply. FIG. 5 shows an example of an excitation circuit in these conventional superconducting magnet apparatuses.
As shown in FIG. 5, superconducting switch 140 is connected to each end of the winding start portion and the winding end portion of a superconducting coil 110 in a conventional superconducting magnet apparatus 101 and the superconducting coil 110 is placed in an extremely low-temperature area (about 4.2K) inside the superconducting magnet apparatus 101. A normal conducting current lead 132 having a low thermal conductivity is also connected to each end of this superconducting coil 110. The other end of this normal conducting current lead 132 extending to the outer surface of the superconducting magnet apparatus 101 is connected to an external excitation power source 151 in a normal-temperature domain (about 300K).
Conventionally, a thermal superconducting switch, whose resistance is zero when it is on and which has a simple structure, has been mainly used as an aforementioned superconducting switch 140. However, a mechanical superconducting switch, and a superconducting switch comprising a thermal superconducting switch 141 and a mechanical superconducting switch 142 connected in parallel as shown in FIG. 5, have been proposed (see patent document 1 and non-patent document 1, for example).
Patent Document 1
    Unexamined Japanese Patent Publication No. 6-350148Non-Patent Document 1    “Handbook of Research and Development of Superconductivity”, International Superconductivity Technology Center, published by Ohmsha, Ltd., P 160–163, (1991)
However, conventional superconducting magnet apparatuses have the following problems, when each superconducting switch is adopted in the excitation circuit.
In the case of using a thermal superconducting switch alone, there is a disadvantage that it takes time to cool or to heat between ON-state (superconducting state) and OFF-state (normal conduction state). In other words, it takes time to change from ON-state to OFF-state and vice versa, since it utilizes thermal phenomena. Especially in the case of using superconducting coils having a high superconducting critical temperature and also employing a material having a high superconducting critical temperature for a superconducting switch in order to keep the superconducting state in the persistent current mode, it takes a longer time to change from the ON-state to the OFF-state and vice versa, since the temperature difference between ON-state and OFF-state is larger and the thermal capacity is larger. As a consequence, during excitation when the superconducting magnet apparatus is switched into the persistent current mode, the energized time of the current lead becomes longer and the Joule heat increases, thus causing a problem of an increase in the load of an external refrigerator which cools inside of the superconducting magnet apparatus.
In the case of employing a mechanical superconducting switch alone, it is possible to turn it on and off instantly, but it is difficult to decrease contact resistance sufficiently in an ON-state, so the contact resistance causes problems such as current decay and heat generation when the switch is connected to the superconducting coil. Furthermore, there is a disadvantage in that considerable invasion heat from the drive mechanism that drives the contact in a contact or non-contact state increases heat load of the superconducting coil.
As shown in FIG. 5, in the case of connecting a thermal superconducting switch and a mechanical superconducting switch in parallel, as in the case of employing a mechanical superconducting switch alone, there is the same disadvantage that invasion heat from the drive mechanism that drives the contact increases the heat load of the superconducting coil. There also is a problem that the contact resistance of the mechanical superconducting switch causes heat generation until the thermal superconducting switch has completed switching when turned on.
One object of the present invention, which was made in view of the above problems, is to provide a superconducting magnet apparatus capable of preventing or controlling heat invasion into the inside of the apparatus at the time of changeover of the switching, thereby reducing the cooling load of the external refrigerator, and capable of quick changeover operation.