Abstract:
In the related-art methods for mechanically switching the heat transfer path to a permanent current switch, a heat transfer rod is made to contact a cooling stage simply by the force of a drive unit. These methods have a problem that an excessive load acts on a support rod which supports the cooling stage from the normal temperature side. There are conflicting problems that while it is difficult to thicken the support rod in view of the amount of heat penetration, a predetermined contact pressure is necessary in order to connect the heat transfer rod to the cooling stage. If a sufficient contact pressure cannot be achieved, the refrigeration capability needs to be increased, contributing greater cost. In view thereof a permanent current switch device of a refrigerator cooling-type superconducting magnet is provided so that a permanent current mode can be realized efficiently.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a superconducting magnet device and particularly to a superconducting magnet employing a refrigerator cooling system. 
         [0003]    2. Description of the Related Art 
         [0004]    Japanese Patent No.3,117,173 is a background art in this technical field. This literature discloses a technique in which a permanent current switch is arranged on a cooling stage on a high temperature side thermally connected to a refrigerator, whereas a superconducting coil is arranged on a cooling stage thermally connected to a low temperature side of the refrigerator, so as to enable collection of generated heat when the permanent current switch is off. 
         [0005]    JP-A-10-247753 is another background art. This literature discloses “comprising a unit to which superconductive device is thermally connected, a separation/connection unit thermally connected to this cooling unit, and a permanent current switch thermally connected to a part that is not connected to the cooling unit, of the separation/connection unit”. 
         [0006]    Japanese Patent No.3,020,140 is still another background art. This literature discloses “a structure comprising a heat transfer rod thermally connected to a permanent current switch, a drive unit which mechanically moves the heat transfer rod, a two-stage refrigerator, and cooling stages connected to a high temperature side and a low temperature side of the refrigerator, wherein the drive unit is controlled to thermally connect the heat transfer rod to the cooling stage on the high temperature side or the cooling stage on the low temperature side”. 
         [0007]    JP-A-8-138928 is still another background art. This literature discloses a unit which mechanically disconnects thermal connection between a permanent current switch and a refrigerator, as in JP-A-10-247753. 
         [0008]    In the methods for mechanically switching the heat transfer path to the permanent current switch, disclosed in Japanese Patent No.3,117,173, JP-A-10-247753, Japanese Patent No.3,020,140, and JP-A-8-138928, the heat transfer rod is made to contact the cooling stage simply by the force of the drive unit. These methods have a problem that an excessive load acts on a support rod which supports the cooling stage from the normal temperature side. There are conflicting problems that while it is difficult to thicken this support rod in view of the amount of heat penetration, a predetermined contact pressure or above is necessary in order to connect the heat transfer rod to the cooling stage. If a sufficient contact pressure cannot be achieved, refrigeration capability needs to be increased excessively, contributing to a rise in cost. 
       SUMMARY OF THE INVENTION 
       [0009]    In view of the foregoing problems, an object of the invention is to provide a permanent current switch device of a refrigerator cooling-type superconducting magnet so that a permanent current mode can be realized efficiently. 
         [0010]    To solve the foregoing problems, according to an aspect of the invention, a permanent current switch device of a refrigerator cooling-type superconducting magnet includes: a superconducting coil cooled by solid thermal conduction; and a permanent current switch. A part of a structure thermally connected to a refrigerator is structured in such a way that this part can be inserted into an axis part of a former of the permanent current switch. 
         [0011]    According to another aspect of the invention, a refrigerator cooling-type superconducting magnet device includes: a superconducting coil cooled by solid thermal conduction; a permanent current switch; and a cooling stage connected to a refrigerator. The superconducting coil, the permanent current switch, and the cooling stage are contained in a vacuum container. A structure thermally connected to the permanent current switch is in the form of a threaded bolt. A nut is arranged on the bolt-shaped structure. The cooling stage is arranged between the nut and a head of the bolt. The bolt-shaped structure is structured to be rotatable from an atmospheric side of the vacuum container. 
         [0012]    Using the permanent current switch device of the refrigerator cooling-type superconducting magnet device enables reduction in thermal resistance between the cooling stage and the permanent current switch device. Therefore, the permanent current switch can be cooled more efficiently. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  shows a superconducting magnet device according to a first example. 
           [0014]      FIG. 2  shows a superconducting magnet device according to a second example. 
           [0015]      FIG. 3  is an enlarged view showing a part of the second example. 
           [0016]      FIG. 4  shows a superconducting magnet device according to a comparative example. 
           [0017]      FIG. 5  shows an electrical circuit and a thermal circuit of the superconducting magnet device according to the comparative example. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    Hereinafter, examples will be described with reference to the drawings. 
       EXAMPLE  1   
       [0019]      FIG. 1  is a cross-sectional view of a refrigerator cooling-type superconducting magnet device according to a first example of the invention. 
         [0020]    A superconducting magnet device  1  mainly includes a superconducting coil  2 , a permanent current switch  3 , a refrigerator  4  which cools the superconducting coil  2  and the permanent current switch  3 , and a vacuum container  5  which contains the superconducting coil  2  and the permanent current switch  3 . The inside of the vacuum container  5  is kept in high vacuum for thermal insulation, and a vacuum container lid  6  is arranged on the top of the vacuum container  5 . A test space  7  is prepared in order to use a magnetic field generated by the superconducting magnet device  1 . The superconducting coil  2  is wound on a bobbin  8 . The bobbin  8  is thermally connected to low temperature-side cooling stage  10  (hereinafter, cooling stage  10 ) of the refrigerator  4  via a highly elastic good conductor  9 . 
         [0021]    In order to reduce the amount of heat penetration from outside as much as possible, the cooling stage  10  is supported from the vacuum container lid  6  by a support rod  11  made from FRP or the like with a low thermal conductivity. In this case, the cooling stage  10  is fixed to the support rod  11  with a bolt  12  arranged to sandwich the cooling stage  10  vertically. A current to the superconducting coil  2  is supplied from a DC power source  13  via a superconducting wire  14  (power lead). The superconducting wire  14  may be thermally connected to the refrigerator  4  according to need, in order to maintain a superconducting state. 
         [0022]    The permanent current switch  3  is configured in the form of a superconducting wire wound on a former  15 . A heater  16  that is necessary to turn off the permanent current switch is wound on the outside of the permanent current switch  3 . The heater  16  may be arranged on the former  15  side of the permanent current switch  3 . In any case, the heater  16  has the function of heating the permanent current switch  3 . 
         [0023]    A superconducting wire  17  connected to the permanent current switch  3  is electrically connected to the superconducting wire  14 , in such a way that the superconducting coil  2  and the permanent current switch  3  are connected in parallel to each other, as viewed from the DC power source  13 . The heater  16  is connected to a power source  19  with a sufficient capacity, via a normal conducting wire  18 , and the current thereto is on/off-controlled by a controller, not shown. The former  15  of the permanent current switch  3  is supported from the cooling stage  10  by a thermal insulation support  20  of FRP or the like with a low thermal conductivity. 
         [0024]    Before explaining the cooling structure of the permanent current switch  3  in this example, a comparative example will be described.  FIG. 4  is a cross-sectional view of a superconducting magnet device according to a comparative example. 
         [0025]    In the comparative example, the former  15  is thermally connected to a heat transfer rod  23  that can be moved up and down by a drive unit  21  via a drive support rod  22 , and to a highly elastic good conductor  24 . Under the control of the drive unit  21 , the heat transfer rod  23  can contact the cooling stage  10  or a high temperature-side cooling stage  25  (hereinafter, cooling stage  25 ). The cooling stage  25  thermally connected to a highly elastic good conductor  26  and the high temperature side of the refrigerator  4 . 
         [0026]      FIG. 5  shows an equivalent circuit of the electrical circuit and the thermal circuit shown in  FIG. 4 . The reference numbers in  FIG. 5  are the same as described with reference to  FIG. 4  and therefore will not be described further. Using  FIG. 5 , startup of the superconducting coil current and shift to a permanent current mode will be described. 
         [0027]    First, when injecting a current to the superconducting coil  2 , the permanent current switch  3  needs to be in a normal conducting state. Therefore, a current is supplied to electrify the heater  16  of the permanent current switch  3  by the power source  19 , and the permanent current switch is thus heated. At the same time, in order to collect the generated heat at the cooling stage  25 , the drive unit  21  is controlled to connect the heat transfer rod  23  to the cooling stage  25  (the circuit state of  FIG. 5 ). 
         [0028]    At this point, the electrification of the heater  16  is not necessary if the critical temperature of the superconducting wire material used for the permanent current switch  3  is set below the temperature of the cooling stage  25  of the refrigerator  4 . Then, the DC power source  13  is controlled to increase the current until a predetermined current flows through the permanent current switch  3 . As the current reaches a predetermined value, the electrification of the heater  16  is stopped and the drive unit  21  is controlled to connect the heat transfer rod  23  to the cooling stage  10 , thus cooling the permanent current switch  3 , in order to shift the permanent current switch  3  to a superconducting state. When the permanent current switch  3  is cooled sufficiently, the voltage of the DC power source  13  is lowered, thus shifting to a permanent current mode. 
         [0029]    Meanwhile, the former  15  of the permanent current switch in the present example is supported form the low temperature-side cooling stage  10 , using the thermal insulation support  20 . However, in this example, unlike the comparative example of  FIG. 4 , the permanent current switch  3  is arranged in such a way that the axis of the former  15  thereof is parallel to the vertical direction. The former  15  is a hollow tubular member and may have not only a circular cross sectional but also various cross-sectional shapes. The inner-diameter cross section of the former  15  mentioned below refers to a cross section of the hollow part in the tube in the case where the former  15  is sliced on a plane perpendicular to the vertical direction. 
         [0030]    On the drive unit support  22 , a high temperature-side heat transfer member  27  (first heat transfer member) and a low temperature-side heat transfer member  28  (second heat transfer member) are fixed. The high temperature-side heat transfer member  27  (hereinafter, heat transfer member  27 ) is thermally connected to the cooling stage  25  via a highly elastic good conductor  29 . The low temperature-side heat transfer member (hereinafter, heat transfer member  28 ) is thermally connected to the cooling stage  10  via a highly elastic good conductor  30 . The heat transfer members  27  and  28  are shaped in such a way that these members can be inserted in the hollow part of the former  15 . 
         [0031]    Here, as the material of the heat transfer members  27  and  28 , a material with a smaller coefficient of thermal expansion than the material of the former  15  of the permanent current switch  3  is chosen. For example, it is preferable to use copper for the heat transfer members  27  and  28 , and aluminum or the like for the former  15 . As the critical temperature of the superconducting wire used for the permanent current switch  3 , a lower temperature than the temperature of the high temperature-side cooling stage is employed. 
         [0032]    Next, the operation at the time of starting up the current in the superconducting coil  2  in this example will be described. First, at the time of starting up the current in the superconducting coil  2 , the heater  16  of the permanent current switch  3  is electrified to thermally expand the former  15  of the permanent current switch  3 , as explained with reference to  FIG. 5 . The permanent current switch  3  may be heated by the heater  16  at this point, since the permanent current switch  3  may be in the normal conducting state under the circumstance where the superconducting magnet device  1  is not operating in the permanent current mode. 
         [0033]    Subsequently, the drive unit  21  is controlled to lower the drive unit support  22  and thus move the heat transfer member  27  so that the heat transfer member  27  is situated inside the former  15 . Then, the electrification of the heater  16  is stopped. Thus, the former  15  is cooled by heat radiation and therefore deforms by thermal contraction to tightly bind the heat transfer member  27  in the state of being placed in the center, and thus tightly contacts the heat transfer member  27 . This deformation of the former  15  secures a contact pressure between the heat transfer member  27  and the former  15 . The permanent current switch  3  reaches the same temperature as the cooling stage  25  and exceeds the critical temperature and therefore enters into the normal conducting state. 
         [0034]    Next, at the time of shifting to the permanent current mode, the heater  16  is electrified. This causes the former  15  with a greater coefficient of thermal expansion than the heat transfer member  27  to expand more, and therefore enables the heat transfer member  27  to operate up and down. After the heat transfer member  27  becomes operable, the drive unit  21  is controlled to lift the drive unit support  22  and thus move the heat transfer member  28  so that the heat transfer member is situated inside the former  15 . After that, the electrification of the heater  16  is stopped. This causes the former  15  to tightly bind the heat transfer member  28  in the state of being placed in the center, by thermal contraction via heat radiation. Therefore, a predetermined contact pressure is secured at the contact surface between the heat transfer member  28  and the former  15 , and the permanent current switch  3  is cooled efficiently. Thus, the permanent current mode can be maintained stably. 
         [0035]    In view of securing a contact pressure between the heat transfer members  27  and  28  and the former  15 , it is desirable that a cross-sectional shape formed by slicing the heat transfer members  27  and  28  on a plane perpendicular to the vertical direction is a similar figure to the inner-diameter cross section of the former  15  and is equal to or smaller than the inner-diameter cross section of the former  15  when thermally expanding and greater than the inner-diameter cross section of the former  15  when thermally contracting. This is because, by having a larger cross section than the cross-sectional shape of the former  15  when thermally contracting, a higher contact pressure can be expected when the heat transfer members are tightly bound. 
         [0036]    As a matter of course, the cross-sectional shape of the heat transfer members  27  and  28  is not limited to a similar figure to the inner-diameter cross section of the former  15 , and the cross-sectional shape of the heat transfer members  27  and  28  and the shape of the inner-diameter cross section of the former  15  can be freely chosen within a range where a predetermined contact pressure can be secured by thermal contraction. 
         [0037]    Also, if the superconducting wire  17  of the permanent current switch  3  has a sufficient length, another embodiment that can achieve similar effects to the above example can be formed by a structure that holds the low temperature-side heat transfer member  28  in the state of being situated inside the former  15  without using the drive unit  21 . That is, if the former  15  is separated from the low temperature-side heat transfer member  28  by the heater  16 , the superconducting wire  17  is the only cooling path of the permanent current switch  3 . If the superconducting wire  17  has a sufficient length, it is equivalent to securing thermal resistance. Therefore, the permanent current switch  3  can be maintained in the normal conducting state by the heater  16 . 
         [0038]    Thus, as the superconducting wire  17  has a sufficient length, there is no need to provide the drive unit  21  and the drive support rod  22  and there is no heat input via these members, either. This forms an example in which the permanent current mode with higher stability can be maintained. 
       EXAMPLE  2   
       [0039]    A second example of the invention will be described with reference to  FIG. 2 . The configurations of the permanent current switch  3  and its former  15  and heater  16  are the same as in  FIG. 1 . The difference is that drive units  21 - 1  and  21 - 2  are installed for the high temperature-side and low temperature-side heat transfer members, respectively. Also, supports  22 - 1  and  22 - 2  are installed for the heat transfer members, respectively. Each of the supports  22 - 1  and  22 - 2  has a double structure, as described below. 
         [0040]    Next, details of the configuration will be described with reference to  FIG. 3 .  FIG. 3  is an enlarge view of the part denoted by  31  in  FIG. 2 . 
         [0041]    The drive support  22 - 2  has a double structure, as described above, and includes a support  22 - 2 - 1  having a threaded portion at a part in the center or over the entire support, and a fixing support  22 - 2 - 2  on the outer periphery of the support  22 - 2 - 1 . An end portion  34  of the drive support  22 - 2 - 1  that is opposite to an end connected to the drive unit  21 - 2  has the shape of a disk or flat plate and forms an unified structure with the drive support  22 - 2 - 1 , like the head of a bolt. Although  FIG. 3  illustrates an example in which the end portion  34  is in the shape of a disk or flat plate, the shape of the end portion  34  is not limited to this as long as the drive support  22 - 2 - 1  has a threaded portion at a part thereof, forming a structure (stopper) that can tightly bind the cooling stage  25  with a nut  33 . The cooling stage  25  has a penetration hole or slit or the like through which the drive support  22 - 2 - 1  passes and which is narrower than the stopper portion. 
         [0042]    As the supports  22 - 1  and  22 - 2 , low thermal conductivity members of FRP or the like are employed. On the cooling stage  25  side of the end portion  34 , a heat transfer member  35  formed by a good conductor with a high thermal conductivity is arranged and thermally connected to the highly elastic good conductor  29 . The nut  33  is arranged on the side of the cooling stage  25  that is opposite to the end portion  34  (drive unit side). 
         [0043]    The fixing support  22 - 2 - 2  has an opening that is greater than a hypothetical circle having a diameter equal to the diagonal length of the nut  33 , and has a protrusion  32  (prevention part) arranged at a position that is approximately at the height of the nut  33  so that nut  33  will not move toward the drive unit. The protrusion  32  may be a pawl-like protrusion or a constriction as long as it can prevent the nut  33  from moving toward the drive unit. 
         [0044]    According to these configurations, as the drive unit  21 - 1  is made to operate, the drive support  22 - 2 - 1  rotates, narrowing the distance between the nut  33  and the support end portion  34 , which in turn tightly bind the cooling stage  25 . Thus, a predetermined contact pressure that is necessary for thermal connection between the heat transfer member  35  and the cooling stage  25  can be secured. In the case of separating the heat transfer member  35  and the cooling stage  25 , the drive unit  21 - 1  can be rotated backward. In the case of securing thermal connection between the cooling stage  10  and the former  15  of the permanent current switch  3 , it is possible to secure a contact pressure by the same principle as above, by causing the drive unit  21 - 2  to operate. Also, using the measures in this example, it is possible to secure thermal connection between the permanent current switch  3  and the refrigerator  4  without causing load concentration on the support rod  11  connecting the cooling stages  10  and  25  and the vacuum container lid  6 , since the fixing support  22 - 2 - 2  is joined to the cooling stage  25 . That is, efficient cooling of the permanent current switch can be realized and the structurally robust superconducting magnet device  1  can be realized. 
         [0045]    In the above description, the supports  22 - 1  and  22 - 2  are connected to the drive units  21 - 1  and  21 - 2 . However, a device that is rotationally operable from the atmospheric side (outside of the vacuum container), for example, a handle-like member to be manually operated, and the supports  22 - 1  and  22 - 2  may be connected together, instead of installing a device having a drive force. In this case, since the structure is simplified, the manufacturing cost can be restrained. 
         [0046]    It is also possible to install only the support  22 - 1  and the drive unit  21 - 1  to connect to the cooling stage  10  for low temperature cooling. That is, the permanent current switch  3  is shifted to normal conduction, heated by the heater  16 , and in the case of shifting to the permanent current mode, the cooling stage  10  and the former  15  are connected together via the heat transfer member  35 , making a shift to the superconducting state. In this case, to stop the permanent current mode, the heater  16  can be made to operate to heat the permanent current switch  3 , and the drive unit  21 - 1  can be rotated to pull the drive support  22 - 1  out of the nut  33 . Thus, the heat transfer member  35  and the cooling stage  10  are separated from each other, enabling quick cancelation of the permanent current mode. 
         [0047]    Also, according to such an embodiment, the drive unit  21 - 2  and the support  22 - 2  to connect the cooling stage  25  and the former  15  need not be provided, and the heat penetration paths are reduced. Therefore, the superconducting magnet device  1  operable in a more stable permanent current mode can be provided. 
         [0048]    While the invention has been described above with reference to the drawings, the invention is not limited to the configurations described in the above embodiments and the configurations can be changed according to need, without departing from the scope of the invention described in the accompanying claims. The above embodiment examples are described in detail in order to explain the invention intelligibly. The invention is not necessarily limited to embodiment examples having all the configurations described above.