Patent Publication Number: US-9887028-B2

Title: Superconducting magnet

Description:
TECHNICAL FIELD 
     The present invention relates to a superconducting magnet and more particularly to a superconducting magnet with a fixed-type refrigerator and a fixed-type current lead which are non-removable. 
     BACKGROUND ART 
     A prior art document, Japanese Patent Laying-Open No. 8-159633 (PTD 1), discloses a cryogenic apparatus for a superconducting magnet where liquid helium is used as cryogenic coolant, the cryogenic apparatus uniformly cooling radiation shields so as to suppress an increase in amount of heat intrusion into a cryogenic coolant tank. In the cryogenic apparatus disclosed in PTD 1, a radiation-shield cooling pipe is composed of a plurality of radiation-shield cooling pipes constituted of parallel flow paths. Each of the radiation-shield cooling pipes constituted of parallel flow paths is provided with a flow rate control valve around its exit which is varied in position with a change in temperature of the coolant flowing in the flow path. 
     A prior art document, Japanese Patent Laying-Open No. 2000-105072 (PTD) 2), discloses a multi-circulation type liquid helium recondensation apparatus that cools a helium storage tank with sensible heat of helium gas. A prior art document, Japanese Utility Model Laying-Open No. 3-88366 (PTD 3), discloses a superconducting magnetic shield that cools a radiation shield with sensible heat of helium gas. 
     In the multi-circulation type liquid helium recondensation apparatus disclosed in PTD 2, a major part of the helium gas is cooled to about 40 K by a first heat exchanger of a small-sized refrigerator while positions of flow rate regulating valves are being adjusted, and the cooled helium gas is supplied into the liquid helium storage tank through a flow path. The rest of the helium gas is liquefied through the first heat exchanger and a second heat exchanger of the small-sized refrigerator, and the liquid helium is supplied into the liquid helium storage tank through a flow path. 
     In the superconducting magnetic shield disclosed in PTD 3, a heat transfer plate is disposed on the outer side of a liquid helium container in such a way as to surround the liquid helium container, and pipes for the passage of helium gas are provided in contact with the outer peripheral surface of the heat transfer plate. 
     CITATION LIST 
     Patent Document 
     PTD 1: Japanese Patent Laying-Open No. 8-159633 
     PTD 2: Japanese Patent Laying-Open No. 2000-105072 
     PTD 3: Japanese Utility Model Laying-Open No. 3-88366 
     SUMMARY OF INVENTION 
     Technical Problem 
     With regard to the superconducting magnet disclosed in PTD 1, when there is no electric power supply, e.g., during a power breakdown or transportation, the flow rate control valves do not work and it is difficult to suppress heat intrusion into the coolant container. With regard to the multi-circulation type liquid helium recondensation apparatus disclosed in PTD 2, when there is no electric power supply, e.g., during a power breakdown or transportation, the flow rate regulating valves do not work and it is difficult to suppress heat intrusion into the coolant container. With regard to the superconducting magnetic shield disclosed in PTD 3, no consideration is given to the adjustment of a flow rate of helium gas. 
     The present invention has been made in view of the above problems. An object of the present invention is to provide a superconducting magnet that can suppress heat intrusion into a coolant container with ability to maintain a flow rate ratio of helium gas between a plurality of flow paths while electric power supply is stopped. 
     Solution to Problem 
     A superconducting magnet according to the present invention includes: a superconducting coil; a coolant container containing the superconducting coil in a state where the superconducting coil is immersed in liquid coolant; a radiation shield surrounding the coolant container; a vacuum container containing the superconducting coil, the coolant container, and the radiation shield; a refrigerator for cooling the radiation shield and the inside of the coolant container; a current lead electrically connected to the superconducting coil; a first pipe passing through the vacuum container and the radiation shield and leading to the inside of the coolant container to form a flow path of vaporized coolant, the first pipe including a mounting opening in which the refrigerator is inserted and fixed; a second pipe passing through the vacuum container and the radiation shield and leading to the inside of the coolant container to form another flow path of vaporized coolant, the second pipe including a lead-out opening through which the current lead passes to be led out; and a flow rate ratio maintaining mechanism connected to at least one of a downstream side of the mounting opening of the first pipe and a downstream side of the lead-out opening of the second pipe, the flow rate ratio maintaining mechanism allowing the vaporized coolant to flow through the first pipe and the second pipe at a constant flow rate ratio. 
     Advantageous Effects of Invention 
     According to the present invention, a flow rate ratio of helium gas between a plurality of flow paths is maintainable while electric power supply is stopped and thus heat intrusion into a coolant container can be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view showing a structure of a superconducting magnet according to Embodiment 1 of the present invention. 
         FIG. 2  is a cross-sectional view showing a structure of a flow rate ratio maintaining mechanism in the superconducting magnet according to Embodiment 1 of the present invention. 
         FIG. 3  is a cross-sectional view showing a structure of a flow rate ratio maintaining mechanism in a superconducting magnet according to Embodiment 2 of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter superconducting magnets according to embodiments of the present invention are described with reference to the figures. In the description of the embodiments below, identical or equivalent parts are identically denoted in the figures and redundant explanations are not repeated. 
     Note that although a description of a hollow cylindrical superconducting magnet is given in the following embodiments, the present invention is not necessarily limited to a hollow cylindrical superconducting magnet but may also be applied to an open superconducting magnet. 
     Embodiment 1 
       FIG. 1  is a cross-sectional view showing a structure of a superconducting magnet according to Embodiment 1 of the present invention.  FIG. 1  shows a cross section of only the upper part of the superconducting magnet. As shown in  FIG. 1 , in a superconducting magnet  100  according to Embodiment 1 of the present invention, a hollow cylindrical vacuum container  110  is disposed on the outermost side. Vacuum container  110  is formed of, for example, a non-magnetic material such as stainless-steel or aluminum for vacuum insulation between the inner and outer sides of vacuum container  110 . 
     The inside of vacuum container  110  is reduced in pressure by a pressure reducing device (not shown) to form a vacuum. In vacuum container  110 , a hollow cylindrical radiation shield  120  is disposed that is substantially similar to vacuum container  110  in shape. 
     Radiation shield  120  is formed of, for example, a non-magnetic material having a high light reflectance, such as aluminum. A multi-layer heat insulating material  150  (superinsulation) is disposed so as to cover the outer side of radiation shield  120 . Multi-layer heat insulating material  150  may be attached to the surface of radiation shield  120 . 
     In radiation shield.  120 , a hollow cylindrical coolant container  130  is disposed that is substantially similar to radiation shield  120  in shape. Radiation shield  120  serves as a heat insulator between coolant container  130  and vacuum container  110  as surrounding coolant container  130 . Coolant container  130  is formed of a non-magnetic material such as stainless-steel or aluminum. 
     In coolant container  130 , a superconducting coil  140  is contained. Superconducting coil  140  is wound around a reel  132  formed of a non-magnetic material such as stainless-steel or aluminum. Reel  132  is supported on a support (not shown) and fixed in coolant container  130  with a gap lying between reel  132  and coolant container  130 . Note that superconducting coil  140  may be wound around the bottom of coolant container  130 , in which case no reel  132  is provided. 
     The inside of coolant container  130  is filled with liquid helium  160  which is liquid coolant. Superconducting coil  140  is cooled as being immersed in liquid helium  160 . Superconducting coil  140  is made up, for example, by winding a superconducting wire formed of a copper matrix with a niobium-titanium alloy embedded in its center part. 
     Vacuum container  110  thus contains superconducting coil  140 , coolant container  130 , and radiation shield  120 . Vacuum container  110  is connected to one end of each of support rods  131  made of, for example, glass epoxy. Each support rod  131  is connected to radiation shield  120  and coolant container  130 . That is, each of radiation shield  120  and coolant container  130  is fixed to vacuum container  110  with support rods  131 . 
     Although liquid helium  160  is used as coolant in the present embodiment, the type of coolant is not limited to liquid helium but may be any other coolant that can bring superconducting coil  140  into a superconducting state, e.g., liquid nitrogen. 
     Superconducting magnet  100  includes a refrigerator  170  for cooling radiation shield  120  and the inside of coolant container  130 . A Gifford-McMahon refrigerator or a pulse tube refrigerator each having two refrigeration stages may be used as refrigerator  170 . 
     A first refrigeration stage  171  of refrigerator  170  is indirectly connected to radiation shield  120  with a heat transfer plate  121  interposed therebetween. Heat transfer plate  121  is made of, for example, copper and passes through a part of the peripheral wall of a first pipe  180  which is described later. A second refrigeration stage  172  of refrigerator  170  is located in the upper part in coolant container  130  and re-liquefies vaporized helium gas  161 . 
     Refrigerator  170  is inserted through a mounting opening  182  of first pipe  180  which is described later, and is fixed thereto. Refrigerator  170  is sealed with, for example, an O-ring (not shown) so that a gap may not be created between the upper surface of mounting opening  182  of first pipe  180  and the lower surface of a flange of refrigerator  170  while refrigerator  170  is disposed through mounting opening  182 . Refrigerator  170  according to the present embodiment is a non-removable fixed-type refrigerator. 
     Superconducting magnet  100  includes a current lead  141  electrically connected to superconducting coil  140 . Current lead  141  is led out from a lead-out opening  183  through the inside of a second pipe  181  which is described later. Current lead  141  is led out from lead-out opening  183  in an airtight fashion. The tip of led-out current lead  141  is connected to a power source (not shown) that supplies electric power. Current lead  141  according to the present embodiment is a non-removable fixed-type current lead. A material for current lead  141  contains phosphorous-deoxidized copper as a major component. The major component of the material for current lead  141 , however, is not limited to phosphorous-deoxidized copper but may also be, for example, brass or electrolytic copper. 
     Superconducting magnet  100  includes first pipe  180  passing through vacuum container  110  and radiation shield  120  and leading to the inside of coolant container  130  to form a flow path of vaporized helium gas  161 , first pipe  180  including mounting opening  182  for refrigerator  170  to be inserted therethrough and to be fixed thereto. First pipe  180  is formed of carbon fiber reinforced plastic (CFRP). The material for first pipe  180  is, however, not limited to CFRP but may be any other material that has a low heat conductivity. 
     At a part where refrigerator  170  is inserted, a continuous gap serving as a flow path of helium gas  161  is provided between the inner peripheral surface of first pipe  180  and the outer peripheral surface of refrigerator  170 . 
     Superconducting magnet  100  includes second pipe  181  passing through vacuum container  110  and radiation shield  120  and leading to the inside of coolant container  130  to form another flow path of vaporized helium gas  161 , second pipe  181  including lead-out opening  183  through which current lead  141  passes to be led out. Second pipe  181  is formed of CFRP. The material for second pipe  181  is, however, not limited to CFRP but may be any other material that has a low heat conductivity. 
     Superconducting magnet  100  includes a flow rate ratio maintaining mechanism  190  that is connected to the downstream side of mounting opening  182  of first pipe  180  and to the downstream side of lead-out opening  183  of second pipe  181 , flow rate ratio maintaining mechanism  190  allowing helium gas  161  to flow through first pipe  180  and second pipe  181  at a constant flow rate ratio. 
       FIG. 2  is a cross-sectional view showing a structure of the flow rate ratio maintaining mechanism in the superconducting magnet according to Embodiment 1 of the present invention.  FIG. 2  shows a state where a vent valve  193  which is described later is open. 
     As shown in  FIG. 2 , flow rate ratio maintaining mechanism  190  in superconducting magnet  100  according to the present embodiment is composed of hand valves provided on first pipe  180  and second pipe  181 , respectively. Specifically, flow rate ratio maintaining mechanism  190  is composed of a first hand valve  191  provided on first pipe  180  and a second hand valve  192  provided on second pipe  181 . 
     A ratio between a flow rate of helium gas  161   a  in first pipe  180  and a flow rate of helium gas  161   b  in second pipe  181  can be maintained constant through adjustment of positions of first hand valve  191  and second hand valve  192 . 
     Vent valve  193  for venting helium gas  161  is provided downstream of flow rate ratio maintaining mechanism  190 . In the present embodiment, helium gas  161  that has flowed through first pipe  180  and second pipe  181  is vented together from one vent valve  193 . This embodiment is, however, not limited as such but two vent valves  193  may be provided to vent helium gas  161  that has flowed through first pipe  180  and helium gas  161  that has flowed through second pipe  181  separately from each other. 
     In the present embodiment, superconducting magnet  100  further includes a first thermometer  184  that is disposed in radiation shield  120  to measure a temperature of first pipe  180 , and a second thermometer  185  that is disposed in radiation shield  120  to measure a temperature of second pipe  181 . As first thermometer  184  and second thermometer  185 , platinum resistance temperature detectors are used, which have good measurement accuracy for a cryogenic range. The thermometer is, however, not limited as such but thermocouples, for example, may also be used. Note that superconducting magnet  100  does not necessarily have to include first thermometer  184  and second thermometer  185 . 
     Hereinafter operations of superconducting magnet  100  are described. 
     In superconducting magnet  100 , the outside of vacuum container  110  is at a room temperature around 300 K. The lower end part of each of first pipe  180  and second pipe  181  is cooled to around 4 K substantially the same as a temperature of superconducting coil  140 . First pipe  180  and second pipe  181  which extend from the outside of vacuum container  110  to coolant container  130  act as pathways of intrusion of heat into coolant container  130 . 
     When heat intrudes into coolant container  130  through first pipe  180  and second pipe  181 , liquid helium  160  vaporizes and helium gas  161  is generated. When refrigerator  170  is in operation, helium gas  161  is re-liquefied by second refrigeration stage  172  of refrigerator  170 . 
     When there is no electric power supply, e.g., during a power breakdown or transportation of superconducting magnet  100 , refrigerator  170  does not operate and helium gas  161  is not re-liquefied, which results in increase in pressure of helium gas  161  with the vaporization of liquid helium  160 . When a pressure of helium gas  161  exceeds a threshold, vent valve  193  opens and helium gas  161  is vented to the outside. 
     Helium gas  161  flows through first pipe  180  and second pipe  181  and is vented from vent valve  193 . A ratio between a flow rate of helium gas  161  in first pipe  180  and that in second pipe  181  is determined by a ratio between degrees of opening of first hand valve  191  and second hand valve  192 . 
     For example, in a case where a degree of opening of first hand valve  191  is made twice as large as a degree of opening of second hand valve  192 , a flow rate of helium gas  161  in first pipe  180  is substantially twice as large as a flow rate of helium gas  161  in second pipe  181 . 
     Helium gas  161  cools first pipe  180  and second pipe  181  with the sensible heat while flowing through first pipe  180  and second pipe  181 . A higher flow rate of helium gas  161  makes a better cooling effect with the sensible heat. Thus, a ratio between a flow rate of helium gas  161  in first pipe  180  and that in second pipe  181  determines a cooling ratio between first pipe  180  and second pipe  181 . 
     As described above, first pipe  180  and second pipe  181  act as pathways of intrusion of heat into coolant container  130 . Amounts of heat intrusion from first pipe  180  and second pipe  181  into coolant container  130  can be estimated from materials, shapes, and dimensions etc. of first pipe  180  and second pipe  181 . 
     First pipe  180  to which refrigerator  170  is attached is larger in diameter and larger in volume of the part located outside vacuum container  110  than second pipe  181  through which current lead  141  is led out. Accordingly, an amount of heat that intrudes into coolant container  130  through first pipe  180  is larger than an amount of heat that intrudes into coolant container  130  through second pipe  181 . 
     Thus, by setting first hand valve  191  to be higher in degree of opening than second hand valve  192 , more helium gas  161  can flow into first pipe  180  larger in amount of heat intrusion than second pipe  181 . First pipe  180  and second pipe  181  can be cooled with effective use of the sensible heat of helium gas  161  by determining a ratio between a flow rate of helium gas  161  in first pipe  180  and that in second pipe  181  in correspondence with a ratio between an amount of heat intrusion in first pipe  180  and an amount of heat intrusion in second pipe  181 . 
     As a result, amounts of heat intrusion into coolant container  130  through first pipe  180  and second pipe  181  can be effectively reduced. Note that first hand valve  191  and second hand valve  192  do not require electric power. Accordingly, a ratio between a flow rate of helium gas  161  in first pipe  180  and that in second pipe  181  can be maintained and the heat intrusion into coolant container  130  and thus vaporization of liquid helium  160  can be suppressed when there is no electric power supply, e.g., during a power breakdown or transportation of superconducting magnet  100 . 
     Although the hand valve is provided on each of first pipe  180  and second pipe  181  in superconducting magnet  100  according to the present embodiment, the hand valve may be provided only on second pipe  181  smaller in amount of heat intrusion. That is, flow rate ratio maintaining mechanism  190  only needs to be connected to at least a pipe smaller in amount of heat intrusion. 
     As described above, superconducting magnet  100  according to the present embodiment includes first thermometer  184  and second thermometer  185 . Accordingly, a ratio between an amount of heat intrusion through first thermometer  184  and that through second thermometer  185  may be confirmed based on measurement values of first thermometer  184  and second thermometer  185 , and a ratio between a flow rate of helium gas  161  in first pipe  180  and that in second pipe  181  may be determined based on the results. 
     That is, a position of first hand valve  191  and a position of second hand valve  192  may be adjusted based on the results of comparison between the measurement values of first thermometer  184  and second thermometer  185 . This allows determination of a ratio between a flow rate of helium gas  161  in first pipe  180  and that in second pipe  181  according to the current conditions, and accordingly can more reliably suppress heat intrusion into coolant container  130 . 
     Hereinafter a superconducting magnet according to Embodiment 2 of the present invention is described. The superconducting magnet according to this embodiment is different from superconducting magnet  100  according to Embodiment 1 only in feature of a flow rate ratio maintaining mechanism, and thus the explanations of other features are not repeated. 
     Embodiment 2 
       FIG. 3  is a cross-sectional view showing a structure of the flow rate ratio maintaining mechanism in the superconducting magnet according to Embodiment 2 of the present invention.  FIG. 3  shows a state where vent valve  193  is open. 
     As shown in  FIG. 3 , a flow rate ratio maintaining mechanism  190   a  in the superconducting magnet according to Embodiment 2 of the present invention includes of orifices provided in first pipe  180  and second pipe  181 , respectively. Specifically, flow rate ratio maintaining mechanism  190   a  includes a first orifice  191   a  provided in first pipe  180  and a second orifice  192   a  provided in second pipe  181 . 
     Controlling an aperture diameter d 1  of first orifice  191   a  and an aperture diameter d 2  of second orifice  192   a  can maintain a constant ratio between a flow rate of helium gas  161   a  in first pipe  180  and a flow rate of helium gas  161   b  in second pipe  181 . 
     A ratio between a flow rate of helium gas  161  in first pipe  180  and that in second pipe  181  is determined by a ratio between aperture diameters of first orifice  191   a  and second orifice  192   a.    
     For example, in a case where aperture diameter d 1  of first orifice  191   a  is made twice as large as aperture diameter d 2  of second orifice  192   a , a flow rate of helium gas  161  in first pipe  180  is substantially twice as large as a flow rate of helium gas  161  in second pipe  181 . 
     First orifice  191   a  and second orifice  192   a  do not require electric power. Accordingly, a ratio between a flow rate of helium gas  161  in first pipe  180  and that in second pipe  181  can be maintained and the heat intrusion into coolant container  130  and thus vaporization of liquid helium  160  can be suppressed when there is no electric power supply, e.g., during a power breakdown or transportation of the superconducting magnet. 
     Although the orifice is provided in each of first pipe  180  and second pipe  181  in the superconducting magnet according to the present embodiment, the orifice may be provided only in second pipe  181  smaller in amount of heat intrusion. That is, flow rate ratio maintaining mechanism  190   a  only needs to be connected to at least a pipe smaller in amount of heat intrusion. 
     The embodiments disclosed herein are illustrative in every respect, and do not serve as a basis for limitative interpretation. Therefore, the technical scope of the present invention should not be interpreted only based on the embodiments described above, but is defined based on the description in the scope of the claims. Further, any modification within the scope and meaning equivalent to the scope of the claims is included. 
     REFERENCE SIGNS LIST 
       100 : superconducting magnet;  110 : vacuum container;  120 : radiation shield;  121 : heat transfer plate;  130 : coolant container;  131 : support rod;  132 : reel;  140 : superconducting coil;  141 : current lead;  150 : multi-layer heat insulating material;  160 : liquid helium;  161 ,  161   a ,  161   b : helium gas;  170 : refrigerator;  171 : first refrigeration stage;  172 : second refrigeration stage;  180 : first pipe;  181  second pipe;  182 : mounting opening;  183 : lead-out opening;  184 : first thermometer;  185 : second thermometer;  190 ,  190   a : flow rate ratio maintaining mechanism;  191 : first hand valve;  191   a : first orifice;  192 : second hand valve;  192   a : second orifice;  193 : vent valve; d 1 , d 2 : aperture diameter