Abstract:
A cryogenic refrigerator includes a compressor for installation on a stationary component, an expander for installation on a rotating component, and a rotary joint fluidly coupling the compressor with the expander. The rotary joint includes: a rotor fixed to the rotating component and coaxial with its rotational axis; a stator disposed adjacent to the rotor to form a clearance between the rotor and the stator, and fixed to the stationary component; a first high-pressure flowpath and a second high-pressure flowpath, extending between the rotor and stator through the clearance, and a working-gas sealing area dividing the clearance into a first high-pressure section communicating with the first high-pressure flowpath, and into a second high-pressure section communicating with the second high-pressure flowpath.

Description:
RELATED APPLICATIONS 
       [0001]    Priority is claimed to Japanese Patent Application No. 2015-038220, filed Feb. 27, 2015, 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 refrigerators, and particularly, to cryogenic refrigerators for chilling rotating objects. In addition, certain embodiments of the invention relate to a rotary joint suited to such cryogenic refrigerators. 
         [0004]    2. Description of Related Art 
         [0005]    Cryogenic cooling devices that realize cryogenic temperatures in continuously rotating systems are known. The cooling devices include a cryogenic refrigerator and a compressor that circulates helium gas to the cryogenic refrigerator. The helium gas circulates through the refrigerator and the compressor via a rotary coupler for helium gas. 
       SUMMARY 
       [0006]    One embodiment of the present invention affords a cryogenic refrigerator for installation on an apparatus, which includes a stationary portion and a rotating portion supported by the stationary portion and axially rotatable, the refrigerator including: a compressor, installed on the stationary portion; an expander installed on the rotating portion; and a rotary joint fluidly coupling the compressor with the expander. The rotary joint includes: a rotor fixed to the rotating portion and coaxial with the rotational axis of the rotating portion; a stator disposed adjacent to the rotor to form a clearance between the rotor and the stator, and which is fixed to the stationary portion; a first high-pressure flowpath, being a flowpath for a first high-pressure working gas having a first high pressure that is higher than ambient pressure of the cryogenic refrigerator and extending from the stator to the rotor through the clearance; a second high-pressure flowpath, being a flowpath for a second high-pressure working gas having a second high pressure that is higher than ambient pressure and is lower than the first high pressure, and extending from the rotor through the clearance to the stator; and a working gas sealing area dividing the clearance into a first high-pressure section communicating with the first high-pressure flowpath, a second high-pressure section adjacent to the first high-pressure section and communicating with the second high-pressure flowpath, and a pressurization section adjacent to the first high-pressure section along an end thereof opposite from its second high-pressure section end. The pressurization section has an intermediate pressure that is higher than ambient pressure and that is lower than the first high pressure. 
         [0007]    Another embodiment of the present invention affords a rotary joint for fluidly coupling a cryogenic-refrigerator compressor and expander, the compressor being installed on a stationary component and the expander being installed on a rotating component axially rotatably supported by the stationary component. The joint includes: a rotor fixed to the rotator and coaxial with the rotator&#39;s rotational axis; a stator disposed adjacent to the rotor to form a clearance between the rotor and the stator, and fixed to the stationary component; a flowpath for working gas having a pressure higher than ambient pressure of the cryogenic refrigerator, the working gas flowpath extending from the stator through the clearance to the rotor; and a working-gas sealing area dividing the clearance into a high-pressure section communicating with the working gas flowpath, a first pressurization section adjacent to the high-pressure section, and a second pressurization section adjacent to the high-pressure section along an end thereof opposite its first pressurization section end. The first pressurization section has a first intermediate pressure that is higher than ambient pressure and is lower than the high pressure section, and the second pressurization section has a second intermediate pressure that is higher than ambient pressure and is lower than the high pressure section. 
         [0008]    Still another embodiment of the present invention affords a cryogenic refrigerator for installation on an apparatus including a stationary portion and a rotating portion axially rotatably supported by the stationary portion. The refrigerator comprises: a compressor installed on the stationary portion; an expander installed on the rotating portion; and a rotary joint fluidly coupling the compressor with the expander. The rotary joint includes: a rotor fixed to the rotating portion and coaxial with the rotating portion&#39;s rotational axis; a stator disposed adjacent to the rotor to form a clearance between the rotor and the stator, and fixed to the stationary portion; a first high-pressure flowpath being a flowpath for a first high-pressure working gas having a first high pressure that is higher than ambient pressure of the cryogenic refrigerator, and extending from the stator through the clearance to the rotor; a second high-pressure flowpath, being a flowpath for a second high-pressure working gas having a second high pressure that is higher than ambient pressure and is lower than the first high pressure, and extending from the rotor through the clearance to the stator; and a working-gas sealing area dividing the clearance into a first high-pressure section communicating with the first high-pressure flowpath, and a second high-pressure section communicating with the second high-pressure flowpath. The rotor includes a radially extending annular flat surface surrounding the rotational axis and being perpendicular to the rotational axis. The working-gas sealing area includes a seal sleeve disposed about the rotational axis to seal the second high pressure section from ambient pressure, and the seal disk has a sealing surface contacting the annular flat surface. 
         [0009]    Yet another embodiment of the present invention affords a cryogenic refrigerator for installation on an apparatus including a stationary portion and a rotating portion axially rotatably supported by the stationary portion. The refrigerator comprises: a compressor installed on the stationary portion; an expander installed on the rotating portion; and a rotary joint fluidly coupling the compressor with the expander. The rotary joint includes: a rotor fixed to the rotating portion and coaxial with the rotating portion&#39;s rotational axis; a stator disposed adjacent to the rotor to form a clearance between the rotor and the stator, and fixed to the stationary portion; a first high-pressure flowpath, being a flowpath for a first high-pressure working gas having a first high pressure that is higher than ambient pressure of the cryogenic refrigerator, and extending from the stator through the clearance to the rotor; a second high-pressure flowpath, being a flowpath for a second high-pressure working gas having a second high pressure that is higher than ambient pressure and is lower than the first high pressure, and extending from the rotor through the clearance to the stator; and a working-gas sealing area dividing the clearance into a first high-pressure section communicating with the first high pressure flowpath, and a second high-pressure section communicating with the second high-pressure flowpath. The working-gas sealing area has a first diameter in the first high-pressure section, and a second diameter in the second high-pressure section, and the first diameter is smaller than the second diameter. 
         [0010]    A still further embodiment of the present invention affords a cryogenic refrigerator for installation on an apparatus including a stationary portion and a rotating portion axially rotatably supported by the stationary portion. The refrigerator comprises: a compressor installed on the stationary portion; an expander installed on the rotating portion; and a rotary joint fluidly coupling the compressor with the expander. The rotary joint includes: a rotor fixed to the rotating portion and coaxial with the rotating portion&#39;s rotational axis; a stator disposed adjacent to the rotor to form a clearance between the rotor and the stator, and fixed to the stationary portion; a first high-pressure flowpath, being a flowpath for a first high-pressure working gas having a first high pressure that is higher than ambient pressure of the cryogenic refrigerator, and extending from the stator through the clearance to the rotor; a second high-pressure flowpath, being a flowpath for a second high-pressure working gas having a second high pressure that is higher than ambient pressure and is lower than the first high pressure, and extending from the rotor through the clearance to the stator; and a working-gas sealing area dividing the clearance into a first high-pressure section communicating with the first high pressure flowpath, and a second high-pressure section communicating with the second high-pressure flowpath. The rotor includes a rotor base portion fixed to the rotating portion, and a rotor axial end located on the rotor&#39;s axially opposite side from the rotor base portion, and where between the stator and the rotor axial end the first high-pressure section is formed. The stator includes a stator bottom wall portion forming a semi-enclosed section surrounding the rotor axial end. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a view schematically showing a cryogenic refrigerator according to an embodiment of the present invention. 
           [0012]      FIG. 2  is a view schematically showing a cryogenic refrigerator according to another embodiment of the present invention. 
           [0013]      FIG. 3  is a view schematically showing the cryogenic refrigerator according to another embodiment of the present invention. 
           [0014]      FIG. 4  is a view schematically showing the cryogenic refrigerator according to another embodiment of the present invention. 
           [0015]      FIG. 5  is a view schematically showing the cryogenic refrigerator according to another embodiment of the present invention. 
           [0016]      FIG. 6  is a view schematically showing a cryogenic refrigerator according to still another embodiment of the present invention. 
           [0017]      FIG. 7  is a view schematically showing the cryogenic refrigerator according to still another embodiment of the present invention. 
           [0018]      FIG. 8  is a view schematically showing a cryogenic refrigerator according to still another embodiment of the present invention. 
           [0019]      FIG. 9  is a view schematically showing a cryogenic refrigerator according to still another embodiment of the present invention. 
           [0020]      FIG. 10  is a view schematically showing a superconducting wind-power generator according to still another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    O-rings for sealing helium gas are provided in a rotary joint. One O-ring seals helium gas, which flows into a refrigerator, from atmospheric pressure. In addition, one other O-ring seals the helium gas, which is re-circulated to a compressor, from atmospheric pressure. In general, since the helium gas used in the refrigerator has a pressure that is significantly higher than atmospheric pressure, it is difficult to completely prevent the helium gas from leaking to an atmospheric environment with only one O-ring. 
         [0022]    It is desirable to improve seal efficiency of a working gas in a cryogenic refrigerator that cools a rotating object. 
         [0023]    According to certain embodiments of the present invention, it is possible to improve seal efficiency of a working gas in a cryogenic refrigerator that cools a rotating object. 
         [0024]    Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. 
         [0025]    In addition, in descriptions, the same reference numerals are assigned to the same elements, and overlapping descriptions thereof are appropriately omitted. In addition, the configurations described below are only examples, and do not limit the scope of the present invention. 
       First Embodiment 
       [0026]      FIG. 1  is a view schematically showing a cryogenic refrigerator  10  according to a first embodiment of the present invention. The cryogenic refrigerator  10  includes a compressor  12 , which compresses a working gas (for example, helium gas), and an expander  14 , which cools the working gas by adiabatic expansion. The cryogenic refrigerator  10  is an arbitrary regenerative type refrigerator such as a Gifford-McMahon (GM) refrigerator. 
         [0027]    The cryogenic refrigerator  10  is installed on an apparatus that includes a stationary portion  50  and a rotating portion  52 . The rotating portion  52  is supported by the stationary portion  50  so as to be axially rotatable. That is, the rotating portion  52  is supported by the stationary portion  50  so as to be rotatable around a predetermined rotational axis. The direction of the rotational axis is up-down in  FIG. 1 . The cryogenic refrigerator  10  cools a rotating object  68  provided on the rotating portion  52 . For example, the object  68  is a superconducting device (for example, a superconducting coil). Accordingly, the expander  14  is installed on the rotating portion  52 . The compressor  12  is installed on the stationary portion  50 . 
         [0028]    The rotating portion  52  includes a rotating table  54  and a vacuum vessel  56 , and the stationary portion  50  includes a support surface  58 , a support  60 , and a support frame  62 . The rotating table  54  is supported by the support  60  via a bearing  64  so as to be rotatable around the rotational axis of the rotating portion  52 . The vacuum vessel  56  is attached to the rotating table  54 . The expander  14  is attached to the vacuum vessel  56  so that the low temperature portion of the expander is accommodated in the vacuum vessel  56 . The object  68  to be cooled by the cryogenic refrigerator  10  is also accommodated in the vacuum vessel  56 . The object  68  is thermally connected to the low temperature portion of the expander  14 , and is supported on the rotating table  54  by an object supporting member  69 . For example, the object supporting member  69  is formed of a material (for example, glass fiber reinforced plastic (GFRP)) having low thermal conductivity. In addition, the rotating table  54  includes a table opening  66 , which has the rotational axis of the rotating portion  52  as the center. Meanwhile, the support  60  is supported by the support surface  58 . The support frame  62  is also supported by the support surface  58 . The compressor  12  is attached to the support surface  58 . 
         [0029]    The compressor  12  supplies a first high pressure working gas to the expander  14 . The first high pressure working gas has a first high pressure Ph, which is higher than the ambient pressure Pa of the cryogenic refrigerator  10 . For example, the ambient pressure Pa of the cryogenic refrigerator  10  is atmospheric pressure or 0.1 MPa. For example, the first high pressure Ph is greater than ten times the pressure (1 MPa) of the ambient pressure Pa or twenty times the pressure (2 MPa) of the ambient pressure Pa. 
         [0030]    The first high pressure working gas is reduced in pressure to become the second high pressure working gas by adiabatic expansion in the expander  14 . The second high pressure working gas has a second high pressure Pl that is higher than the ambient pressure Pa of the cryogenic refrigerator  10  and that is lower than the first high pressure Ph. For example, the second high pressure Pl is smaller than ten times the pressure (1 MPa) of the ambient pressure Pa or twenty times the pressure (2 MPa) of the ambient pressure Pa. The compressor  12  recovers the second high pressure working gas from the expander  14 . The second high pressure working gas is pressurized so as to become the first high pressure working gas again by the compressor  12 . 
         [0031]    The cryogenic refrigerator  10  includes an working gas line  16  that connects the compressor  12  to the expander  14  so that the first high pressure working gas is discharged from the compressor  12  to the expander  14  and the second high pressure working gas is recirculated from the expander  14  to the compressor  12 . The working gas line  16  is a piping system for circulating the working gas between the compressor  12  and the expander  14 . The working gas line  16  includes a first stationary portion gas line  18 , a second stationary portion gas line  20 , a first rotating portion gas line  22 , a second rotating portion gas line  24 , and a rotary joint  100 . The rotary joint  100  is provided so as to perform fluid connection between the compressor  12  and the expander  14 , and includes a first high pressure flowpath  102 , and a second high pressure flowpath  104 . 
         [0032]    The first stationary portion gas line  18  and the second stationary portion gas line  20  are disposed in the stationary portion  50 . The first stationary portion gas line  18  connects a gas discharging port of the compressor  12  to the first high pressure flowpath  102  of the rotary joint  100 . The second stationary portion gas line  20  connects a gas receiving port of the compressor  12  to the second high pressure flowpath  104  of the rotary joint  100 . 
         [0033]    The first rotating portion gas line  22  and the second rotating portion gas line  24  are disposed in the rotating portion  52 . The first rotating portion gas line  22  connects a gas receiving port of the expander  14  to the first high pressure flowpath  102  of the rotary joint  100 . The second rotating portion gas line  24  connects a gas discharging port of the expander  14  to the second high pressure flowpath  104  of the rotary joint  100 . The first rotating portion gas line  22  and the second rotating portion gas line  24  are respectively connected to the first high pressure flowpath  102  and the second high pressure flowpath  104  through the table opening  66 . 
         [0034]    Accordingly, the compressor  12  supplies the first high pressure working gas to the expander  14  through the first stationary portion gas line  18 , the first high pressure flowpath  102 , and the first rotating portion gas line  22 . In addition, the compressor  12  recovers the second high pressure working gas from the expander  14  through the second rotating portion gas line  24 , the second high pressure flowpath  104 , and the second stationary portion gas line  20 . 
         [0035]    The rotary joint  100  includes a rotor  106  and a stator  108 . The rotor  106  is a shaft member that is disposed so as to be coaxial with the rotational axis of the rotating portion  52 . The rotor  106  includes a cylindrical outer circumferential surface, which has the rotational axis of the rotating portion  52  as the center axis. The stator  108  is a non-rotating member that is fixed to the stationary portion  50 . The stator  108  is a hollow member that is disposed so as to be coaxial with the rotational axis of the rotating portion  52  and surrounds the rotor  106 . The stator  108  includes a cylindrical inner circumferential surface, which has the rotational axis of the rotating portion  52  as the center. 
         [0036]    A clearance  110  is formed between the rotor  106  and the stator  108 . The stator  108  is disposed to be adjacent to the rotor  106  so as to form the clearance  110 . The clearance  110  is a slight gap in a radial direction for allowing a rotational movement of the rotor  106  with respect to the stator  108 , and an inner diameter of the stator  108  is slightly larger than an outer diameter of the rotor  106 . 
         [0037]    The rotor  106  includes a rotor base portion  112  that is fixed to the rotating portion  52 . The rotor  106  extends from the rotor base portion  112  along the rotational axis of the rotating portion  52  and terminates at a rotor axial end  114 . The rotor axial end  114  is positioned in the vicinity of the support frame  62  with a narrow gap in the axial direction between the rotor axial end  114  and the support frame  62 . The stator  108  includes a stator base portion  116  that is fixed to the support frame  62  and surrounds the rotor axial end  114 . The stator  108  extends from the stator base portion  116  along the rotational axis of the rotating portion  52  and terminates at a stator end  118 . The stator end  118  is positioned in the vicinity of the rotor base portion  112  with a narrow gap in the axial direction between the rotor base portion  112  and the stator end  118 . In this way, the clearance  110  is formed between the rotor base portion  112  and the stator end  118 , and between the rotor axial end  114  and the stator base portion  116 . 
         [0038]    In the present specification, for easy explanation, a side close to the rotor base portion  112  and the stator end  118  in the axial direction may be referred to as an “upper side”, and a side close to the rotor axial end  114  and the stator base portion  116  in the axial direction may be referred to as a “lower side”. The definitions do not limit the disposition of the cryogenic refrigerator  10 . The rotational axis of the rotating portion  52  may be positioned in an arbitrary direction. For example, the rotational axis may be parallel with a horizontal surface, and the rotor  106  may extend right and left in actual disposition of the cryogenic refrigerator  10 . 
         [0039]    The rotary joint  100  includes a working-gas sealing area  120 , which divides the clearance  110  into at least three sections in an axial direction. The working-gas sealing area  120  divides the clearance  110  into an upper second high pressure section  124 , a first high pressure section  126 , and a lower second high pressure section  128 . The first high pressure section  126  is positioned at the center in the axial direction, and two second high pressure sections are adjacent to both sides of the first high pressure section  126 . The first high pressure section  126  communicates with the first high pressure flowpath  102 . The upper second high pressure section  124  and the lower second high pressure section  128  communicate with the second high pressure flowpath  104 . 
         [0040]    In addition, an upper ambient pressure section  122  and a lower ambient pressure section  130  are respectively provided on the outsides in the axial direction of the upper second high pressure section  124  and the lower second high pressure section  128 . Accordingly, the operation gas seal portion  120  divides the clearance  110  into five sections, that is, the upper ambient pressure section  122 , the upper second high pressure section  124 , the first high pressure section  126 , the lower second high pressure section  128 , and the lower ambient pressure section  130 . 
         [0041]    In order to perform the section division, the working-gas sealing area  120  includes four seals that are disposed so as to be separated in the axial direction. Each seal is a ring-shaped or annular seal member, which extends between the outer circumferential surface of the rotor  106  and the inner circumferential surface of the stator  108  around the rotational axis, for example, a seal ring such as an O-ring. As the four seals, a first seal  132 , a second seal  134 , a third seal  136 , and a fourth seal  138  are disposed from the upper side to the lower side in the drawings. 
         [0042]    The first seal  132  defines a first boundary between the upper ambient pressure section  122  and the upper second high pressure section  124 . The second seal  134  defines a second boundary between the upper second high pressure section  124  and the first high pressure section  126 . The third seal  136  defines a third boundary between the first high pressure section  126  and the lower second high pressure section  128 . The fourth seal  138  defines a fourth boundary between the lower second high pressure section  128  and the lower ambient pressure section  130 . That is, the first seal  132  and the fourth seal  138  are a pair of outer seals that partition off pressurization sections from the surrounding environment. The second seal  134  and the third seal  136  are a pair of inner seals that form the first high pressure section  126  in the midportion of the pressurization sections. 
         [0043]    The first high pressure flowpath  102  is a gas flow passage of the rotary joint  100  that extends from the stator  108  to the rotor  106  through the first high pressure section  126  of the clearance  110 . The first high pressure flowpath  102  includes a first outlet  140 , which is open on the rotor base portion  112 , a first inlet  142 , which is open to the first high pressure section  126 , and a first connection path  144 , which connects the first inlet  142  to the first outlet  140 . The first inlet  142  is an annular groove that is formed on the outer circumferential surface of the rotor  106  around the rotational axis. The direction of the first connection path  144  changes from the direction perpendicular to the rotational axis to the direction parallel to the rotational axis midway. The first high pressure working gas flows from the first stationary portion gas line  18  to the first rotating portion gas line  22  through the first high pressure section  126 , the first inlet  142 , the first connection path  144 , and the first outlet  140 . 
         [0044]    The second high pressure flowpath  104  is another gas flow passage that is formed in parallel with the first high pressure flowpath  102  and extends from the rotor  106  to the stator  108  through the upper second high pressure section  124  and the lower second high pressure section  128  of the clearance  110 . The second high pressure flowpath  104  includes a second inlet  146 , which is open on the rotor base portion  112 , a second upper outlet  148 , which is open to the upper second high pressure section  124 , a second lower outlet  150 , which is open to the lower second high pressure section  128 , and a second connection path  152 , which branches the second inlet  146  into the second upper outlet  148  and the second lower outlet  150 . The second upper outlet  148  and the second lower outlet  150  are annular grooves that are respectively formed on the outer circumferential surface of the rotor  106  around the rotational axis. The direction of the second connection path  152  changes from the direction parallel to the rotational axis to the direction perpendicular to the rotational axis midway. 
         [0045]    The second high pressure working gas flows from the second rotating portion gas line  24  to the second stationary portion gas line  20  through the second inlet  146 , the second connection path  152 , the second upper outlet  148 , and the upper second high pressure section  124 . In addition, the second high pressure working gas flows from the second rotating portion gas line  24  to the second stationary portion gas line  20  through the second inlet  146 , the second connection path  152 , the second lower outlet  150 , and the lower second high pressure section  128 . Two flows of the second high pressure working gas that branch in the second connection path  152  are combined in the second stationary portion gas line  20 . 
         [0046]    According to the first embodiment, the first high pressure section  126  is interposed between the two second high pressure sections. Accordingly, the intermediate pressure sections are positioned on both sides of the first high pressure section  126 . Therefore, unlike a case where the first high pressure section  126  is directly adjacent to the ambient pressure Pa, it is possible to decrease leakage of the first high pressure working gas from the first high pressure section  126  to the surrounding environment. 
         [0047]    In an embodiment, the working-gas sealing area  120  may divide the clearance  110  into the first high pressure section  126 , the upper second high pressure section  124 , and a lower pressurization section. Alternatively, the working-gas sealing area  120  may divide the clearance  110  into the first high pressure section  126 , an upper pressurization section, and the lower second high pressure section  128 . Alternatively, the working-gas sealing area  120  may divide the clearance  110  into the first high pressure section  126 , the upper pressurization section, and the lower pressurization section. Here, the upper pressurization section and the lower pressurization section are intermediate pressure regions that are adjacent to the first high pressure section  126  but are independent from the second high pressure flowpath  104 . The upper pressurization section and the lower pressurization section respectively have a first intermediate pressure and a second intermediate pressure that are higher than the ambient pressure Pa and are lower than the first high pressure Ph. The first intermediate pressure may be equal to or be different from the second intermediate pressure. At least one of the first intermediate pressure and the second intermediate pressure may be equal to the second high pressure Pl. 
         [0048]    In another embodiment, at least one second high pressure section may not be adjacent to the first high pressure section  126  and may be separated from the first high pressure section  126  in the axial direction. In this case, the working-gas sealing area  120  may divide the clearance  110  into the second high pressure section, the upper pressurization section, and the lower pressurization section. Here, the upper pressurization section and the lower pressurization section respectively have a first intermediate pressure and a second intermediate pressure that are higher than the ambient pressure Pa and are lower than the second high pressure Pl. 
         [0049]    The second high pressure Pl, the first intermediate pressure, and/or the second intermediate pressure may be selected from a range of 0.11 MPa to 0.2 MPa. Accordingly, when the ambient pressure Pa is atmospheric pressure, a differential pressure between the second high pressure section (and/or the pressurization section) and the ambient pressure section can decrease so as to be 0.1 MPa or less. In this case, magnetic fluid seals can be adopted as the pair of outer seals that divide the pressurization section from the surrounding environment. 
         [0050]    In still another embodiment, the working gas line  16  may include a first rotary joint, which connects the first stationary portion gas line  18  to the first rotating portion gas line  22 , and a second rotary joint, which connects the second stationary portion gas line  20  to the second rotating portion gas line  24 . The first rotary joint may include a first rotor and a first stator, and a first clearance may be formed between the first rotor and the first stator. The first rotary joint may include a first working-gas sealing area, which divides the first clearance into a first high pressure section, an upper pressurization section, and a lower pressurization section. The second rotary joint may include a second rotor and a second stator, and a second clearance may be formed between the second rotor and the second stator. The second rotary joint may include a second working-gas sealing area that divides the second clearance into a second high pressure section, an upper pressurization section, and a lower pressurization section. 
       Second Embodiment 
       [0051]      FIG. 2  is a view schematically showing a cryogenic refrigerator  10  according to a second embodiment of the present invention. The cryogenic refrigerator  10  according to the second embodiment is different from the cryogenic refrigerator  10  according to the first embodiment in that each of two buffer sections is added on each of both sides of two second high pressure sections (or upper pressurization section and lower pressurization section). That is, a buffer section is formed between each of the second high pressure sections and each of the ambient pressure sections. 
         [0052]    In the working-gas sealing area  120 , a first buffer section  154  and a second buffer section  156  are formed in the clearance  110 . The first buffer section  154  is adjacent to the upper second high pressure section  124  on a side opposite to the first high pressure section  126  in the axial direction. The second buffer section  156  is adjacent to the lower second high pressure section  128  on a side opposite to the first high pressure section  126  in the axial direction. In addition, the first buffer section  154  includes a first annular groove  155  that is formed on the outer circumferential surface of the rotor  106  around the rotational axis. The second buffer section  156  includes a second annular groove  157  that is formed on the outer circumferential surface of the rotor  106  around the rotational axis. 
         [0053]    The first buffer section  154  and the second buffer section  156  respectively have a first buffering pressure and a second buffering pressure that are higher than the ambient pressure Pa and are lower than the second high pressure Pl. The first buffering pressure and the second buffering pressure may be equal to or be different from each other. 
         [0054]    The working-gas sealing area  120  includes a first auxiliary seal  158  and a second auxiliary seal  160  in addition to the first seal  132 , the second seal  134 , the third seal  136 , and the fourth seal  138 . The six seals are separated from one another in the axial direction and are disposed in the clearance  110 . Similarly to other seals, each of the first auxiliary seal  158  and the second auxiliary seal  160  is a ring-shaped or annular seal member that extends between the outer circumferential surface of the rotor  106  and the inner circumferential surface of the stator  108  around the rotational axis, for example, a seal ring such as an O-ring. 
         [0055]    The first auxiliary seal  158  defines a boundary between the upper ambient pressure section  122  and the first buffer section  154 . The second auxiliary seal  160  defines a boundary between the second buffer section  156  and the lower ambient pressure section  130 . The first auxiliary seal  158  and the second auxiliary seal  160  are a pair of outermost seals that divide the buffer sections from the surrounding environment. The first seal  132  and the fourth seal  138  are positioned between the outermost seals. Accordingly, the first seal  132  defines a boundary between the first buffer section  154  and the upper second high pressure section  124 . The fourth seal  138  defines a boundary between the lower second high pressure section  128  and the second buffer section  156 . 
         [0056]    According to the second embodiment, the first high pressure section  126  is interposed between the two second high pressure sections, and the two second high pressure sections are interposed between the two buffer sections. In this way, there are two intermediate pressure sections between the first high pressure section  126  and the ambient pressure Pa on both sides of the first high pressure section  126 . Accordingly, unlike the case where the first high pressure section  126  is directly adjacent to the ambient pressure Pa, it is possible to decrease leakage of the first high pressure working gas from the first high pressure section  126  to the surrounding environment. In addition, unlike the case where the second high pressure section is directly adjacent to the ambient pressure Pa, it is possible to decrease leakage of the second high pressure working gas from the second high pressure section to the surrounding environment. 
         [0057]    The first buffering pressure and/or the second buffering pressure may be selected from a range of 0.11 MPa to 0.2 MPa. Accordingly, when the ambient pressure Pa is atmospheric pressure, the differential pressure between the buffer section and the ambient pressure section can decrease so as to be 0.1 MPa or less. In this case, magnetic fluid seals can be adopted as the pair of outermost seals that divide the buffer section from the surrounding environment. In addition, a general operating pressure of the cryogenic refrigerator  10  can be used as the second high pressure Pl. In this case, for example, the second high pressure Pl may be a pressure that is selected from a range that is higher than 0.2 MPa and less than 1 MPa. 
         [0058]    In another embodiment, as shown in  FIG. 3 , the rotary joint  100  may include a buffer volume  162  or a pressure control volume. For example, the buffer volume  162  is a buffer tank. The buffer volume  162  is installed on the stationary portion  50 , and is connected to the first buffer section  154  and the second buffer section  156  using communication paths  164 . The buffer volume  162  has a pressure that is higher than the ambient pressure Pa and that is lower than the second high pressure Pl, for example, a pressure that is selected from a range of 0.11 MPa to 0.2 MPa. Even when gas leaks to the buffer section through the first seal  132  and the fourth seal  138 , it is possible to prevent pressure of the first buffer section  154  and the second buffer section  156  from increasing using the buffer volume  162 . 
         [0059]    In still another embodiment, the buffer volume  162  may be installed on the rotating portion  52 , and in this case, the buffer volume  162  may be connected to the first buffer section  154  and the second buffer section  156  through a rotor inner flowpath serving as the communication path  164 . In addition, in still another embodiment, the buffer volume  162  may include a first buffer volume that is connected to the first buffer section  154  and a second buffer volume that is connected to the second buffer section  156 . 
         [0060]    In still another embodiment, as shown in  FIG. 4 , the rotary joint  100  may include a pressure control valve  166  in the communication path  164 . The buffer volume  162  is connected to the first buffer section  154  and the second buffer section  156  via the pressure control valve  166 . The pressure control valve  166  is mechanically opened and closed by a differential pressure between the buffer volume  162  and the buffer section. When the pressure of the buffer section increases and the differential pressure exceeds a predetermined value, the pressure control valve  166  is opened. Accordingly, pressure of the first buffer section  154  and the second buffer section  156  can be released to the buffer volume  162  through the pressure control valve  166 . Accordingly, it is possible to prevent the pressure of the first buffer section  154  and the second buffer section  156  from increasing. 
         [0061]    In still another embodiment, the rotary joint  100  may include a first pressure control valve and a second pressure control valve. The buffer volume  162  may be connected to the first buffer section  154  via the first pressure control valve, and be connected to the second buffer section  156  via the second pressure control valve. 
         [0062]    Instead of the buffer volume  162 , in still another embodiment, as shown in  FIG. 5 , the rotary joint  100  may include an auxiliary compressor  168  in the communication path  164 . A pressure sensor  170 , which measures pressure of the buffer section, and the communication path  164  may be provided. In addition, a control unit  172 , which controls the auxiliary compressor  168  based on the pressure measured by the pressure sensor  170 , may be provided. The control unit  172  may be a portion of a controller for controlling the cryogenic refrigerator  10 . 
         [0063]    The first buffer section  154  and the second buffer section  156  are connected to the upper secondhigh pressure section  124  via the auxiliary compressor  168 . The control unit  172  determines whether or not the pressure of the buffer section exceeds a predetermined value, based on the pressure measured by the pressure sensor  170 . When the pressure of the buffer section does not exceed the predetermined value, the control unit  172  does not operate the auxiliary compressor  168 . Meanwhile, when the pressure of the buffer section exceeds the predetermined value, the control unit  172  operates the auxiliary compressor  168 . In this way, the auxiliary compressor  168  recovers and compresses the gas from the first buffer section  154  and the second buffer section  156 , and supplies the gas to the upper second high pressure section  124 . Accordingly, it is possible to prevent pressure of the first buffer section  154  and the second buffer section  156  from increasing. 
         [0064]    In still another embodiment, the first buffer section  154  and the second buffer section  156  may be connected to the first high pressure section  126 , the lower second high pressure section  128 , or arbitrary other high pressure regions in the cryogenic refrigerator  10  via the auxiliary compressor  168 . Moreover, in still another embodiment, the auxiliary compressor  168  may include a first auxiliary compressor and a second auxiliary compressor. The first buffer section  154  may be connected to the upper second high pressure section  124  or arbitrary other high pressure regions via the first auxiliary compressor. The second buffer section  156  may be connected to the lower second high pressure section  128  or arbitrary other high pressure regions via the second auxiliary compressor. In still another embodiment, the rotary joint  100  may include both the buffer volume  162  and the auxiliary compressor  168 . 
         [0065]    In still another embodiment, in the working-gas sealing area  120 , only one of the first buffer section  154  and the second buffer section  156  may be formed in the clearance  110 . In still another embodiment, a gas different from the working gas of the cryogenic refrigerator  10 , for example, the same gas as the gas of the surrounding environment may be sealed in the first buffer section  154  and/or the second buffer section  156 . 
       Third Embodiment 
       [0066]      FIG. 6  is a view schematically showing a cryogenic refrigerator  10  according to a third embodiment of the present invention. With respect to a low-pressure side seal structure, the cryogenic refrigerator  10  according to the third embodiment is different from the cryogenic refrigerators  10  according to the first and second embodiments. 
         [0067]    Similarly to the cryogenic refrigerators  10  according to the first and second embodiments, also in the cryogenic refrigerator  10  according to the third embodiment, the clearance  110  is formed between the rotor  106  and the stator  108 . The stator  108  is disposed so as to be adjacent to the rotor  106  to form the clearance  110 . 
         [0068]    The rotor  106  includes a rotor base portion  112  that is fixed to the rotating portion  52 , and a rotor shaft portion  174  that coaxially extends from the rotor base portion  112  along the rotational axis of the rotating portion  52 . The rotor base portion  112  includes an annular flat surface  175  that extends in the radial direction around the rotational axis and is perpendicular to the rotational axis. The rotor shaft portion  174  is thinner than the rotor base portion  112 . The rotor shaft portion  174  protrudes from the center portion of the annular flat surface  175  and terminates at the rotor axial end  114 . 
         [0069]    According to the shape of the rotor, the stator  108  includes a stator small diameter portion  176  and a stator large diameter portion  178 . The stator  108  includes a stator step portion  177  that connects between the stator small diameter portion  176  and the stator large diameter portion  178  in the radial direction. The stator small diameter portion  176  coaxially surrounds the rotor axial end  114 , and the stator large diameter portion  178  coaxially surrounds the rotor shaft portion  174 . 
         [0070]    In addition, the stator  108  includes a stator bottom wall portion  179  that is fixed to the support frame  62 . The stator bottom wall portion  179  closes the lower end of the stator small diameter portion  176 , and forms a semi-enclosed section that surrounds the rotor axial end  114 . Instead of inserting a seal between two members adjacent to each other, in the stator  108 , airtightness is structurally held on the lower end of the stator  108 . 
         [0071]    The stator small diameter portion  176  extends from the stator bottom wall portion  179  to the stator step portion  177  along the rotational axis of the rotating portion  52 . The stator large diameter portion  178  extends from the stator step portion  177  to the stator end  118  along the rotational axis of the rotating portion  52 . 
         [0072]    The working-gas sealing area  120  divides the clearance  110  into the first high pressure section  126  that communicates with the first high pressure flowpath  102 , and the second high pressure section  127  that communicates with the second high pressure flowpath  104 . The first high pressure section  126  and the second high pressure section  127  are adjacent to each other. The ambient pressure section  123  is adjacent to the second high pressure section  127  on the side opposite to the first high pressure section  126 . Accordingly, the working-gas sealing area  120  includes three seals that are disposed so as to be separated from each other in the axial direction, that is, a first seal  132 , a second seal  134 , and a seal sleeve  180 . 
         [0073]    The seal sleeve  180  is disposed around the rotational axis of the rotating portion  52  so as to seal the second high pressure section  127  from the ambient pressure Pa. The seal sleeve  180  is surrounded by the stator large diameter portion  178 . In addition, the seal sleeve  180  is a tubular member that coaxially surrounds the rotor shaft portion  174 . One end surface of the tubular portion is a ring-shaped seal surface  182  that comes into contact with the annular flat surface  175 . The other end surface of the tubular portion is a pressure receiving surface  184  facing the second high pressure section  127 . The pressure receiving surface  184  receives the second high pressure Pl, and accordingly, the seal sleeve  180  is pressed to the annular flat surface  175 . 
         [0074]    In addition, the working-gas sealing area  120  includes a pressing member  186  that connects the seal sleeve  180  to the stator  108  and presses the seal sleeve  180  to the annular flat surface  175 . For example, the pressing member  186  is a push spring. One end of the pressing member  186  is attached to the stator step portion  177 , and the other end is attached to the pressure receiving surface  184 . The seal sleeve  180  is pressed to the annular flat surface  175  by not only the second high pressure Pl but also the pressing member  186 . 
         [0075]    Since the seal sleeve  180  is connected to the stator  108 , similarly to the stator  108 , the seal sleeve  180  is a non-rotating member. However, the seal sleeve  180  is connected to the stator  108  so as to have play in the axial direction. A guide pin  188  is provided in the stator large diameter portion  178 , and the seal sleeve  180  includes a guide pin hole  190  that receives the guide pin  188 . The guide pin  188  protrudes inward in the radial direction from the inner circumferential surface of the stator large diameter portion  178 . The guide pin hole  190  is formed at a position in the outer circumferential surface of the seal sleeve  180  facing the guide pin  188 . There is a movement freedom in the axial direction between the guide pin  188  and the guide pin hole  190 , and when the seal sleeve  180  is pressed to the annular flat surface  175 , the seal sleeve  180  can slightly move in the axial direction. 
         [0076]    The seal surface  182  defines a boundary between the ambient pressure section  123  and the second high pressure section  127 . The first seal  132  also defines the boundary between the ambient pressure section  123  and the second high pressure section  127 . However, the first seal  132  is disposed between the stator  108  and the seal sleeve  180 . A gap between the inner circumferential surface of the stator large diameter portion  178  and the outer circumferential surface of the seal sleeve  180  circumferentially extends around the rotational axis due to the first seal  132 . The second seal  134  defines a boundary between the second high pressure section  127  and the first high pressure section  126 . A gap between the inner circumferential surface of the stator small diameter portion  176  and the outer circumferential surface of the rotor shaft portion  174  circumferentially extends around the rotational axis due to the second seal  134 . The first high pressure section  126  is formed between the rotor axial end  114  and the stator bottom wall portion  179 . 
         [0077]    The first high pressure flowpath  102  includes a first rotor flowpath  192  and the first stator flowpath  194 . The first rotor flowpath  192  penetrates the rotor shaft portion  174  in the axial direction from the rotor base portion  112  to the rotor axial end  114 . The first stator flowpath  194  penetrates the stator small diameter portion  176  in the radial direction. The first high pressure working gas flows from the first stationary portion gas line  18  to the first rotating portion gas line  22  through the first stator flowpath  194 , the first high pressure section  126 , and the first rotor flowpath  192 . 
         [0078]    The second high pressure flowpath  104  includes a second rotor flowpath  196  and a second stator flowpath  198 . The second rotor flowpath  196  penetrates the rotor base portion  112  in the axial direction. The second stator flowpath  198  penetrates the stator large diameter portion  178  in the radial direction. The second high pressure working gas flows from the second rotating portion gas line  24  to the second stationary portion gas line  20  through the second rotor flowpath  196 , the second high pressure section  127 , and the second stator flowpath  198 . 
         [0079]    According to the third embodiment, the working-gas sealing area  120  forms an axial contact seal between the seal sleeve  180  and the rotor base portion  112 . By sufficiently increasing the area of the seal surface  182 , it is possible to remarkably decrease leakage of the second high pressure operation gas from the second high pressure section  127  to the surrounding environment. Since the first seal  132  is provided between two non-rotating members, unlike a case where the first seal  132  is provided between the rotating portion  52  and the stationary portion  50 , it is possible to easily realize improved seal efficiency. 
         [0080]    In addition, since the lower end of the stator  108  is sealed by the stator bottom wall portion  179 , it is possible to prevent the first high pressure working gas from leaking from the first high pressure section  126  to the surrounding environment. 
         [0081]    The working-gas sealing area  120  has a first diameter in the first high pressure section  126  and a second diameter in the second high pressure section  127 , and the first diameter is smaller than the second diameter. Accordingly, the length in the circumferential direction of the second seal  134  is shorter than the length in the circumferential direction of the first seal  132 . By using the shorter seal in the higher pressure side, it is possible to decrease leakage of gas from the high pressure side. 
         [0082]    In an embodiment, as shown in  FIG. 7 , the working-gas sealing area  120  may include a first auxiliary seal  158  that is disposed between the seal sleeve  180  and the rotor  106  and that extends around the rotational axis of the rotating portion  52 . 
         [0083]    The rotor shaft portion  174  may include an enlarged diameter portion  174   a  for attaching the first auxiliary seal  158 . A gap between the inner circumferential surface of the seal sleeve  180  and the outer circumferential surface of the enlarged diameter portion  174   a  of the rotor shaft portion  174  may extend in the circumferential direction around the rotational axis due to the first auxiliary seal  158 . The first auxiliary seal  158  may be disposed on the inner side in the radial direction of the first seal  132  at the same axial position as the first seal  132 . 
         [0084]    The clearance  110  may include a buffer section  153  that is formed between the first auxiliary seal  158  and the seal surface  182 . The buffer section  153  may have a buffering pressure that is higher than the ambient pressure Pa and that is lower than the second high pressure Pl. In this way, it is possible to decrease leakage of the second high pressure working gas from the second high pressure section  127  to the surrounding environment. 
       Fourth Embodiment 
       [0085]      FIG. 8  is a view schematically showing a cryogenic refrigerator  10  according to a fourth embodiment of the present invention. Similarly to the cryogenic refrigerator  10  according to the third embodiment, in the cryogenic refrigerator  10  according to the fourth embodiment, the working-gas sealing area  120  includes the first diameter in the first high pressure section  126  and the second diameter in the second high pressure section  127 , and the first diameter is smaller than the second diameter. 
         [0086]    However, unlike the cryogenic refrigerator  10  according to the third embodiment, the cryogenic refrigerator  10  according to the fourth embodiment does not include the seal sleeve. Similarly to the cryogenic refrigerators  10  according to the first and second embodiments, the cryogenic refrigerator  10  according to the fourth embodiment includes the first seal  132 , the second seal  134 , and the third seal  136 . 
         [0087]    Since the diameter of the first high pressure section  126  is smaller than the diameter of the second high pressure section  127 , the lengths in the circumferential direction of the second seal  134  and the third seal  136  are shorter than the length in the circumferential direction of the first seal  132 . By using the shorter seal in the higher pressure side, it is possible to decrease leakage of gas from the high pressure side. In other words, in the clearance  110 , the sectional area of the flowpath on the high pressure side is smaller than the sectional area of the flowpath on the low pressure side. In this way, it is possible to decrease leakage of the first high pressure working gas from the first high pressure section  126  to the surrounding environment. 
         [0088]    In an embodiment, the thickness in the axial direction of the third seal  136  may be thicker than the thickness in the axial direction of the first seal  132  and/or the second seal  134 . 
       Fifth Embodiment 
       [0089]      FIG. 9  is a view schematically showing a cryogenic refrigerator  10  according to a fifth embodiment of the present invention. Similarly to the cryogenic refrigerator  10  according to the third embodiment, in the cryogenic refrigerator  10  according to the fifth embodiment, the stator bottom wall portion  179  forms a semi-enclosed section, which surrounds the rotor axial end  114 . However, unlike the cryogenic refrigerator  10  according to the third embodiment, the cryogenic refrigerator  10  according to the fifth embodiment does not include the seal sleeve. In this way, it is possible to prevent the first high pressure working gas from leaking from the first high pressure section  126  to the surrounding environment. 
         [0090]    In addition, similarly to the cryogenic refrigerator  10  according to the fourth embodiment, in the cryogenic refrigerator  10  according to the fifth embodiment, the working-gas sealing area  120  has the first diameter in the first high pressure section  126  and the second diameter in the second high pressure section  127 , and the first diameter is smaller than the second diameter. The length in the circumferential direction of the second seal  134  is shorter than the length in the circumferential direction of the first seal  132 . 
       Sixth Embodiment 
       [0091]      FIG. 10  is a view schematically showing a superconducting wind-power generator  200  according to a sixth embodiment of the present invention. The superconducting wind-power generator  200  includes a support column  204 , which stands upright on a foundation  202 , a nacelle  206  that is installed on the upper end of the support column  204 , and a rotary head  208  that is rotatably assembled on the nacelle  206 . A plurality of windmill blades  210  are attached to the rotary head  208 . In the inner portion of the nacelle  206 , a superconducting generator  212  is connected to the rotary head  208 . 
         [0092]    The superconducting generator  212  includes a stationary portion  50 , a rotating portion  52 , and a connection mechanism  214  that connects the stationary portion  50  to the rotating portion  52 . The rotating portion  52  includes a superconducting coil. The connection mechanism  214  includes the rotary joint  100  according to any one of the first to fifth embodiments. In addition, the cryogenic refrigerator  10  according to any one of the first to fifth embodiments is installed on the superconducting generator  212 . As described above, the compressor  12  is installed on the stationary portion  50 , and the expander  14  is installed on the rotating portion  52 . The superconducting coil of the rotating portion  52  is cooled by the expander  14 . In this way, it is possible to provide the cryogenic refrigerator  10 , which cools the rotating portion  52  of the superconducting generator  212 . 
         [0093]    It should be understood that the invention is not limited to the above-described embodiments, and 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.