Abstract:
A cryogenic refrigerator includes : a axially extending cylinder; an axially reciprocating displacer provided inside the cylinder, at a gap between an inner circumferential surface of the cylinder and an outer circumferential surface of the displacer, the displacer shifting to create an expansion space between the displacer and a first axial end portion of the cylinder; a regenerator built in the displacer; and a sleeve disposed along the inner circumferential surface of the first axial end portion of the cylinder, encompassing the expansion space. A first passage for guiding the refrigerant gas from the regenerator to the gap is provided in the displacer, and a second passage for guiding the refrigerant gas from the gap to the expansion space is provided between the first axial end of the cylinder and the sleeve, and/or is provided between the outer surface and the inner surface of the sleeve.

Description:
RELATED APPLICATIONS 
       [0001]    Priority is claimed to Japanese Patent Application No. 2014-206156, filed Oct. 7, 2014, the entire content of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    Certain embodiments of the invention relate to a cryogenic refrigerator. 
         [0004]    2.Description of Related Art 
         [0005]    Cryogenic refrigerators are used to cool a refrigeration article down to temperatures in a range of, for example, from about 100 K (Kelvin) to about 4 K. Examples of cryogenic refrigerators include Gifford-McMahon (GM) refrigerators, pulse tube refrigerators, Stirling refrigerators, and the Solvay refrigerator. Cryogenic refrigerators are used, for example, for cooling superconducting magnets or detectors, or in cryopumps. 
       SUMMARY 
       [0006]    According to a certain embodiment of the invention, there is provided a cryogenic refrigerator including: an axially extending cylinder; an axially reciprocating displacer provided inside the cylinder, at a gap between an inner circumferential surface of the cylinder and an outer circumferential surface of the displacer, for shifting to create an expansion space for refrigerant gas between the displacer and an axial end portion of the cylinder; a regenerator built into the displacer; and a sleeve disposed along the inner circumferential surface of the axial end portion of the cylinder, encompassing the expansion space. A first passage for guiding the refrigerant gas from the regenerator to the gap is provided in the displacer, and a second passage for guiding the refrigerant gas from the gap to the expansion space is provided between the axial end portion of the cylinder and the sleeve, and/or is provided between the outer surface and the inner surface of the sleeve. 
         [0007]    According to embodiments of the invention, it is possible to enhance heat exchanging efficiency of a cryogenic refrigerator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic diagram illustrating a cryogenic refrigerator according to an embodiment of the invention. 
           [0009]      FIG. 2  is a schematic top view of a sleeve according to an embodiment of the invention. 
           [0010]      FIG. 3  is a schematic top view of a sleeve according to another embodiment of the invention. 
           [0011]      FIG. 4  is a schematic top view of a sleeve according to still another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    The need for improving heat exchanging efficiency has been felt in the art of cryogenic refrigerators. 
         [0013]    Hereinafter, embodiments of the invention will be described in detail with reference to the drawings. In addition, the same reference signs are assigned to the same components in the description, and the duplicated description is appropriately omitted. The configuration described below is an example, and does not limit the scope of the invention. 
         [0014]      FIG. 1  is a schematic diagram illustrating a cryogenic refrigerator according to an embodiment of the invention. The cryogenic refrigerator is, for example, a GM refrigerator  10 . The illustrated GM refrigerator  10  is a single-stage refrigerator. The GM refrigerator  10  uses a helium gas, for example, as a refrigerant gas. 
         [0015]    The regenerator-type cryogenic refrigerator such as the GM refrigerator  10  includes a regenerator  12 , an expander  14 , and a compressor  16 . As illustrated in  FIG. 1 , the regenerator  12  is provided in the expander  14 , and is configured to pre-cool the high-pressure refrigerant gas that is supplied from the compressor  16  to the expander  14 . The expander  14  includes an expansion space  18  of the refrigerant gas. The refrigerant gas that is pre-cooled by the regenerator  12  is expanded in the expansion space  18  and is further cooled. The regenerator  12  is configured to be cooled by the refrigerant gas that is cooled by the expansion. The compressor  16  is configured to collect the refrigerant gas from the regenerator  12 , to compress the refrigerant gas, and to supply the refrigerant gas to the regenerator  12  and the expansion space  18  again. 
         [0016]    The expander  14  includes a cold head including a cylinder  20 , a cooling stage  22 , and a displacer  24 . The cylinder  20  is an air-tight container of the refrigerant gas, and is a hollow member that extends in an axial direction Q. The cylinder  20  has, for example, a cylindrical shape. 
         [0017]    The cooling stage  22  is thermally connected to the cylinder  20  by surrounding the expansion space  18 . The cooling stage  22  is formed to have, for example, a bottomed cylindrical shape, and is attached to the outer side of the cylinder  20 . The cooling stage  22  functions as a heat exchanger that performs heat exchange between the refrigerant gas and a cooling object such as an external heat source. The cooling stage  22  may be called a thermal load flange. 
         [0018]    The displacer  24  is arranged on the same axis as the cylinder  20 . The regenerator  12  is built in the displacer  24 . The displacer  24  has, for example, a cylindrical shape having a diameter that is slightly smaller than that of the cylinder  20 . A gap is provided between the inner circumferential surface of the cylinder  20  and the outer circumferential surface of the displacer  24 . This gap is referred to as a first clearance  26  below. The outer circumferential surface of the displacer  24  is a side surface of the displacer  24 , and the inner circumferential surface of the cylinder  20  is the surface of the cylinder  20  facing the side surface of the displacer  24 . 
         [0019]    The displacer  24  is a piston that divides the internal space of the cylinder  20  into the expansion space  18  and a room temperature space  28 . The expansion space  18  is formed on one side of the cylinder  20  with respect to the displacer  24 , and the room temperature space  28  is formed on the other side of the cylinder  20  with respect to the displacer  24 . Therefore, one end portion of the cylinder  20  (or the displacer  24 ) in the axial direction Q can be called a low temperature end, and the other end of the cylinder  20  (or the displacer  24 ) in the axial direction Q can be called a high temperature end. Accordingly, the expansion space  18  is formed between the low temperature end of the displacer  24  and the low temperature end of the cylinder  20 , and the room temperature space  28  is formed between the high temperature end of the displacer  24  and the high temperature end of the cylinder  20 . 
         [0020]    Hereinafter, for the convenience of description, the relative positional relationship between elements may be described by representing the room temperature side as “upper” and the low temperature side as “lower”. For example, it is possible to describe that the room temperature space  28  is present at the upper portion of the displacer  24  and the expansion space  18  is present at the lower portion of the displacer  24 . 
         [0021]    The displacer  24  is provided in the cylinder  20  so as to move in the axial direction Q in a reciprocating manner. A driving unit  25  is connected to the high temperature end of the displacer  24  for the reciprocating movement of the displacer  24 . By the reciprocating movement of the displacer  24 , the volumes of the expansion space  18  and the room temperature space  28  are complementarily changed. 
         [0022]    A displacer upper opening  30  is provided to the high temperature end of the displacer  24  in order to cause the refrigerant gas to flow between the room temperature space  28  and the regenerator  12 . The displacer upper opening  30  is formed along the axial direction Q. A displacer lower opening  32  is provided to the low temperature end of the displacer  24  in order to cause the refrigerant gas to flow between the regenerator  12  and the expansion space  18 . The displacer lower opening  32  is a passage that guides the refrigerant gas from the low temperature end of the regenerator  12  to the first clearance  26 . The displacer lower opening  32  is formed along a radial direction that is orthogonal to the axial direction Q. 
         [0023]    A seal  34  may be provided at the upper portion of the first clearance  26 . The flow of the gas that has passed through the first clearance  26  is blocked by the seal  34 . Accordingly, the flow of the refrigerant gas between the room temperature space  28  and the expansion space  18  passes through the regenerator  12 . In a case where the seal  34  is a contact seal such as a seal ring, the seal  34  may be provided to the high temperature end of the displacer  24 . The seal  34  may be a non-contact seal. In addition, in a certain embodiment, the flow or leaking of the refrigerant gas that has passed through the first clearance  26  may be allowed. 
         [0024]    In addition, the expander  14  includes a sleeve  36  that is arranged around the expansion space  18  at the inside of the low temperature end of the cylinder  20 . The sleeve  36  is arranged on the same axis as the cylinder  20 . The sleeve  36  is mounted to the low temperature end of the cylinder  20 . Therefore, at least one contacting portion (not illustrated) that is in contact with the inner surface of the cylinder  20  may be provided on the outer surface of the sleeve  36 . The sleeve  36  may be formed of the same material (for example, stainless steel) as the cylinder  20 . 
         [0025]    The sleeve  36  defines the passage that guides the refrigerant gas from the first clearance  26  to the expansion space  18 . This gas passage is a gap formed between the low temperature end of the cylinder  20  and the sleeve  36 . Hereinafter, this gap is called a second clearance  38 . The second clearance  38  is narrower than the first clearance  26 . That is, the width of the second clearance  38  in the radial direction is smaller than the width of the first clearance  26  in the radial direction. The sleeve  36  configures a flow velocity increasing mechanism for the refrigerant gas in the cooling stage  22 . 
         [0026]      FIG. 2  is a schematic top view of the sleeve  36  according to a certain embodiment of the invention. As illustrated in  FIGS. 1 and 2 , the sleeve  36  includes a sleeve cylindrical portion  40  that faces the inner circumferential surface of the cylinder  20 , and a sleeve bottom plate  42  that faces the bottom portion of the cylinder  20 . The sleeve cylindrical portion  40  extends in the axial direction Q along the inner circumferential surface of the cylinder  20  at the low temperature end of the cylinder  20 . The sleeve bottom plate  42  extends from the sleeve cylindrical portion  40  toward the inside in the radial direction. In this manner, the sleeve  36  is formed to have a bottomed cylindrical shape. The sleeve cylindrical portion  40  is, for example, a short cylinder that extends in the axial direction Q, and has a diameter that is slightly smaller than the inner diameter of the cylinder  20 . The sleeve bottom plate  42  is a disk that is attached to a lower end of the sleeve cylindrical portion  40 . 
         [0027]    As illustrated in  FIG. 1 , the second clearance  38  includes a lateral gap  44 , which is formed between the sleeve cylindrical portion  40  and the inner circumferential surface of the cylinder  20 , and a bottom gap  46 , which is formed between the sleeve bottom plate  42  and the bottom portion of the cylinder  20  and is connected to the lateral gap  44 . The sleeve bottom plate  42  has a through hole  48  at the center thereof, the through hole  48  allows the bottom gap  46  to communicate with the expansion space  18 . In this manner, the flow path of the refrigerant gas can be extended to the through hole  48 . 
         [0028]    The position of a sleeve upper end  50  in the axial direction is substantially the same as that of a cooling stage upper end  23  in the axial direction. Accordingly, a gas inlet from the first clearance  26  to the second clearance  38  is provided at substantially the same height as that of the cooling stage upper end  23 . The gas inlet may be provided at a height different from the height of the cooling stage upper end  23 . In addition, a gas outlet (that is, the through hole  48 ) from the second clearance  38  to the expansion space  18  is provided at the same position as that of a bottom center  49  of the cooling stage  22  in the radial direction. The gas outlet may be provided at a position different from the position of the bottom center  49 . 
         [0029]    In this manner, the sleeve  36  forms the flow path of the refrigerant gas between the cooling stage  22  and the sleeve  36 . This flow path reaches the bottom center  49  of the cooling stage  22  from the cooling stage upper end  23  along the inner surface of the cylinder  20 . The sleeve  36  provides the flow path that causes the refrigerant gas to flow in parallel with the inner surface of the cooling stage  22  in almost the entire area of the inner surface of the cooling stage  22 . In  FIG. 1 , the flow of the refrigerant gas in the lateral gap  44  is indicated by arrows A, and the flow of the refrigerant gas in the bottom gap  46  is indicated by arrows B. In addition, the flow of gas passing through the through hole  48  is indicated by an arrow C. 
         [0030]    In the movable range in the axial direction (hereinafter, referred to as a stroke) of the displacer  24 , the displacer lower opening  32  is usually positioned at the upper portion of the sleeve upper end  50  in the axial direction Q. The displacer lower opening  32  is usually positioned at the upper portion of the second clearance  38  and does not enter the inside of the sleeve  36 . Therefore, the displacer lower opening  32  is not hidden by the sleeve  36  from the cylinder  20  (or the cooling stage  22 ). In addition, in a certain embodiment, in at least a portion of the stroke (for example, when the displacer  24  is at the bottom dead center), the displacer lower opening  32  may be positioned at the lower portion of the sleeve upper end  50  in the axial direction Q. 
         [0031]    The sleeve upper end  50  defines an opening that receives the low temperature end of the displacer  24 . In the stroke of the displacer  24 , the low temperature end of the displacer  24  is usually inserted into the sleeve  36 . In other words, the movable range of a displacer bottom surface  33  is in the sleeve  36 . The sleeve upper end  50  is inserted into the lower portion of the first clearance  26 , and the sleeve cylindrical portion  40  surrounds the low temperature end of the displacer  24 . In addition, in a certain embodiment, in at least a portion of the stroke (for example, when the displacer  24  is at the top dead center) or entire stroke, the displacer bottom surface  33  may be at the outside of the sleeve  36 . 
         [0032]    A gap in the radial direction, which is formed between the sleeve cylindrical portion  40  and the low temperature end of the displacer  24  when the displacer  24  is inserted into the sleeve  36 , is narrower than the lateral gap  44 . That is, the width of the gap in the radial direction is smaller than the width of the lateral gap  44  in the radial direction. In this manner, it is possible to increase the flow rate of the gas passing through the lateral gap  44 . 
         [0033]    The sleeve  36  may provide a seal between the low temperature end of the displacer  24  and the sleeve  36 . The seal may be a contact seal or a non-contact seal. A direct gas flow from the first clearance  26  to the expansion space  18  is blocked by the seal. Accordingly, all the flow of the refrigerant gas between the first clearance  26  and the expansion space  18  passes through the second clearance  38 . In this case, the inner surface of the sleeve cylindrical portion  40  may be in contact with the outer circumferential surface of the low temperature end of the displacer  24 . Otherwise, the inner surface of the sleeve cylindrical portion  40  may be in non-contact with the outer circumferential surface of the low temperature end of the displacer  24  by providing a slight gap therebetween. According to the reciprocating movement of the displacer  24 , the low temperature end of the displacer  24  moves in a sliding manner or in a non-contact manner with respect to the sleeve  36 . 
         [0034]    In addition, the GM refrigerator  10  includes a piping system  52  that connects the compressor  16  to the expander  14 . In the piping system  52 , a high pressure valve  54  and a low pressure valve  56  are provided. The piping system  52  is connected to the high temperature end of the cylinder  20 . The GM refrigerator  10  is configured to supply the high-pressure refrigerant gas from the compressor  16  to the expander  14  via the high pressure valve  54  and the piping system  52 . In addition, the GM refrigerator  10  is configured to discharge the low-pressure refrigerant gas from the expander  14  to the compressor  16  via the piping system  52  and the low pressure valve  56 . 
         [0035]    The GM refrigerator  10  includes a valve driving unit (not illustrated) that selectively closes and opens the high pressure valve  54  and the low pressure valve  56  in synchronization with the reciprocating movement of the displacer  24 , and switches between the supply and the discharge of the refrigerant gas with respect to the expansion space  18 . The valve driving unit may be the driving unit  25  described above. The high pressure valve  54 , the low pressure valve  56 , and the valve driving unit may be incorporated in the expander  14 . 
         [0036]    Next, the operation of the GM refrigerator  10  is described. When the displacer  24  is positioned at the bottom dead center or in the vicinity of the bottom dead center of the cylinder  20 , the high pressure valve  54  is opened. The high-pressure refrigerant gas is supplied from the compressor  16  to the cylinder  20  via the high pressure valve  54  and the piping system  52 . The refrigerant gas flows into the regenerator  12  from the room temperature space  28  via the displacer upper opening  30 . The refrigerant gas is cooled while passing through the regenerator  12 . The refrigerant gas flows into the expansion space  18  via the displacer lower opening  32 , the first clearance  26 , and the second clearance  38 . While the refrigerant gas flows into the expansion space  18 , the displacer  24  moves toward the top dead center of the cylinder  20 . In this manner, the volume of the expansion space  18  is increased. Accordingly, the expansion space  18  is filled with the high-pressure refrigerant gas. 
         [0037]    When the displacer  24  is positioned at the top dead center or in the vicinity of the top dead center of the cylinder  20 , the high pressure valve  54  is closed. At the same timing as, or slightly after, the high pressure valve  54  is closed, the low pressure valve  56  is opened. The refrigerant gas of the expansion space  18  is expanded and cooled. The refrigerant gas absorbs the heat from the cooling stage  22 . 
         [0038]    The low-pressure refrigerant gas is collected in a reversed route. The refrigerant gas flows into the regenerator  12  from the expansion space  18  via the second clearance  38 , the first clearance  26 , and the displacer lower opening  32 . The refrigerant gas cools the regenerator  12  while passing through the regenerator  12 . The refrigerant gas is discharged from the cylinder  20  via the displacer upper opening  30  and the room temperature space  28 . The refrigerant gas is collected by the compressor  16  via the low pressure valve  56  and the piping system  52 . While the refrigerant gas flows out from the expansion space  18 , the displacer  24  moves toward the bottom dead center of the cylinder  20 . In this manner, the volume of the expansion space is decreased, and the low-pressure refrigerant gas is discharged from the expansion space  18 . 
         [0039]    One cooling cycle in the GM refrigerator  10  is described above. The GM refrigerator  10  repeatedly performs this cooling cycle, and therefore, the cooling stage  22  is cooled to a desired temperature. In this manner, the GM refrigerator  10  can absorb the heat from the cooling object (not illustrated) that is thermally connected to the cooling stage  22  and can cool the cooling object. The cooling stage  22  may be cooled to a target temperature selected from a range of, for example, about 10 K to about 30 K. Otherwise, the cooling stage  22  may be cooled to a target temperature selected from a range of, for example, about 50 K to 100 K. 
         [0040]    As described above, according to the embodiment, the passage of the refrigerant gas from the first clearance  26  to the expansion space  18  (that is, the second clearance  38 ) is defined by providing the sleeve  36  to the inside of the cylinder  20  to be adjacent to the cooling stage  22 . By defining the gas passage in this manner, the lowering of the velocity component in a direction along the surface of the cooling stage  22  is suppressed compared to a case in the related art in which the gas is directly blown from the low temperature end of the displacer  24  to the expansion space  18 . Since the velocity can be increased compared to that in the related art, it is possible to enhance the heat exchanging efficiency of the cooling stage  22 . 
         [0041]    The second clearance  38  is narrower than the first clearance  26 . Specifically, the gas passage defined in an outside region of the sleeve  36  by the sleeve  36  is narrower than the gap between the cylinder  20  and the displacer  24  in the radial direction. Accordingly, when the gas flows in the gas passage from the gap, the velocity is increased, and therefore, it is possible to enhance the heat exchanging efficiency. According to a trial calculation, if the velocity of the refrigerant gas flowing in the expansion space  18  is doubled, the refrigerating capacity of the refrigerator is improved by about 5% to about 10%. Therefore, as the refrigerator is a large-sized refrigerator having a high refrigerating capacity, the increasing amount of the refrigerating capacity by the application of the sleeve  36  according to the embodiment becomes large. Typically, such a large-sized refrigerator is a single-stage refrigerator. Accordingly, the embodiment is preferable for a single-stage refrigerator having a high capacity (for example, a single-stage refrigerator having a refrigerating capacity of 100 W to 300 W at 10 K, or a single-stage refrigerator having a refrigerating capacity of 500 W to 1 kW at 70 K) . 
         [0042]    In addition, according to the embodiment, it is possible to enhance the heat exchanging efficiency of the refrigerator by a relatively simple operation such as mounting of the sleeve  36  to the cylinder  20 . By adding the sleeve  36  to the existing refrigerator, it is possible to easily enhance the heat exchanging efficiency of the refrigerator. 
         [0043]    Herein before, the invention is described based on the embodiments. The invention is not limited to the embodiments described above. Those skilled in the art can understand that various changes in design and various modification examples are possible, and such modification examples are in the scope of the invention. 
         [0044]    It is not essential that the sleeve  36  includes the sleeve bottom plate  42 . In a certain embodiment, the sleeve  36  includes only the sleeve cylindrical portion  40 . It can be said that the diameter of the through hole  48  at a sleeve lower end is equal to the diameter of the sleeve cylindrical portion  40 . 
         [0045]      FIG. 3  is a schematic top view of a sleeve  136  according to another embodiment of the invention. As illustrated in the drawing, the unevenness may be formed on the outer surface of the sleeve  136  (for example, the sleeve cylindrical portion). In this case, a convex portion  142  may be in contact with the inner surface of the cylinder  20  (or the cooling stage  22 ), and a refrigerant gas passage  146  may be formed between a concave portion  144  and the inner surface of the cylinder  20 . The refrigerant gas passage  146  may be provided along the axial direction of the cylinder  20 . The inner surface of the cylinder  20  is illustrated by a broken line. 
         [0046]    Similarly, the unevenness may be formed on the bottom surface of the sleeve bottom plate. In this case, a gas passage formed between the sleeve bottom plate and the cylinder may be provided along the radial direction. 
         [0047]    As an alternative, the unevenness may be formed on the inner surface of the cylinder. In this case, a convex portion may be in contact with the outer surface of the sleeve, and a passage of the refrigerant gas may be formed between a concave portion and the outer surface of the sleeve. 
         [0048]      FIG. 4  is a schematic top view of a sleeve  236  according to still another embodiment of the invention. The sleeve  236  (for example, sleeve cylindrical portion) may define a gas passage between an outer surface  238  and an inner surface  240  of the sleeve  236 . This gas passage may be a through hole  242  formed in the sleeve  236 . The through hole  242  may be provided along the axial direction of the cylinder. Such a through hole may be provided to a sleeve bottom plate, and in this case, the through hole may provided along the radial direction. 
         [0049]    In a certain embodiment, the gas passage defined between the cylinder and the sleeve (for example, the gas passage illustrated in  FIG. 1 or 3 ) may be used in combination with the gas passage defined between the outer surface and the inner surface of the sleeve (for example, the gas passage illustrated in  FIG. 4 ). For example, a gas passage is defined between the sleeve cylindrical portion and the cylinder, a gas passage connected to the gas passage may be formed on the sleeve bottom plate as a through hole. Otherwise, a through hole is formed on a sleeve cylindrical portion, and a gas passage connected to the through hole may be defined between the sleeve bottom plate and the cylinder. 
         [0050]    In a certain embodiment, in a case where the sleeve is necessarily accommodated, the outer diameter of the low temperature end of the displacer may be slightly smaller than that of the high temperature end. Otherwise, the inner diameter of the low temperature end of the cylinder or the inner diameter of the cooling stage may be slightly greater than that of the high temperature end of the cylinder. 
         [0051]    In a certain embodiment, the sleeve may be provided at the low temperature end of at least a stage in a two-stage (or multiple-stage) refrigerator. 
         [0052]    In the above embodiment, the GM refrigerator  10  is described as an example, but it is not limited thereto. In a certain embodiment, a sleeve may be provided in another type of refrigerator that includes a displacer in which a regenerator is built, and a cylinder that accommodates the displacer. 
         [0053]    The GM refrigerator  10  or another refrigerator including the sleeve according to the embodiment may be used as cooling means or liquefying means in a superconducting magnet, a cryopump, an X-ray detector, an infrared sensor, a quantum photon detector, a solid state detector, a dilution refrigerator, an He3 refrigerator, an insulated demagnetized refrigerator, a helium liquefier, and a cryostat. 
         [0054]    It should be understood that the invention is not limited to the above-described embodiment, but may be modified into various forms on the basis of the spirit of the invention. Additionally, the modifications are included in the scope of the invention.