Abstract:
A cryocooler includes: a housing furnished with a housing bottom surface; a displacer furnished with a displacer upper surface between the housing bottom surface and which an upper gas chamber is formed, and being enabled to reciprocate axially with respect to the housing; a housing gas flow path formed in the housing and opening onto the upper gas chamber; a displacer upper gas flow path formed in the displacer and opening onto the upper gas chamber; and a gas-guiding flow channel formed in at least either the housing bottom surface or the displacer upper surface constituting a portion of the upper gas chamber, and interconnecting the housing gas flow path and the displacer upper gas flow path when the displacer is positioned at top-dead center in its axial reciprocation.

Description:
Related Applications 
       [0001]    Priority is claimed to Japanese Patent Application No. 2016-108964, filed May 31, 2016, the entire content of which is incorporated herein by reference. 
     
    
     BACKGROUND 
     Technical Field 
       [0002]    Certain embodiments of the present invention relate to cryocoolers. 
       Description of Related Art 
       [0003]    Cryocoolers, typified by Gifford-McMahon (GM) refrigerators, include working-gas (also called refrigerant-as) expanders and compressors. Expanders for the most part include a displacer that is axially reciprocated by a driving means, and a regenerator that is built into the displacer. The displacer is accommodated in a cylinder that guides its reciprocation. The variable volume that by the relative movement of the displacer with respect to the cylinder is formed between the two is employed as the working-gas expansion chamber. Appropriate synchronizing of the expansion-chamber volume change and pressure change enables the expander to produce coldness. 
       SUMMARY 
       [0004]    The present invention in one embodiment affords a cryocooler including: a housing including a housing bottom surface; a displacer including a displacer upper surface between the housing bottom surface and which an upper gas chamber is formed, and being enabled to reciprocate axially with respect to the housing; a housing gas flow path formed in the housing and opening onto the upper gas chamber; a displacer upper gas flow path formed in the displacer and opening onto the upper gas chamber; and a gas-guiding flow channel formed in at least either the housing bottom surface or the displacer upper surface constituting a portion of the upper gas chamber, and interconnecting the housing gas flow path and the displacer upper gas flow path when the displacer is positioned at top-dead center in its axial reciprocation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a view schematically showing an entire configuration of a cryocooler according to one embodiment. 
           [0006]      FIG. 2  shows a portion of a configuration of a working gas flow path of an expander according to the one embodiment. 
           [0007]      FIG. 3  shows a portion of the configuration of the working gas flow path of the expander according to the one embodiment. 
           [0008]      FIG. 4  shows a portion of the configuration of the working gas flow path of the expander according to an embodiment. 
           [0009]      FIG. 5  shows a portion of the configuration of the working gas flow path of the expander according to an embodiment. 
           [0010]      FIG. 6  is a view schematically showing an entire configuration of a cryocooler according to another embodiment. 
           [0011]      FIG. 7  shows a portion of a configuration of a working gas flow path of an expander according to another embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    A housing which can accommodate the driving means of the displacer is fixed to a cylinder on a side opposite to the expansion chamber in an axial direction. Another gas space is formed between the displacer and the housing in order to secure a stroke of axial reciprocation of the displacer. Unlike the expansion chamber, this gas space is a dead volume which does not contribute to generation of coldness. Accordingly, preferably, the dead volume is as small as possible as long as the stroke of the reciprocation of the displacer is appropriately secured. 
         [0013]    The gas space formed between the displacer and the housing can have another role of configuring a portion of a working gas flow path in the cryocooler. In a case where the gas space is excessively narrow, particularly, when the displacer is positioned at a top dead center in the reciprocation, an excessive pressure drop may occur in a flow of gas flowing through the gas space. As a result, refrigerating capacity of the cryocooler may decrease. 
         [0014]    It is desirable to prevent an excessive increase of the dead volume while decreasing the pressure drop in the working gas flow path of the cryocooler. 
         [0015]    According to the present invention, it is possible to prevent an excessive increase of a dead volume while decreasing a pressure drop in a working gas flow path of a cryocooler. 
         [0016]    Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, in descriptions thereof, the same reference numerals are assigned to the same elements, and overlapping descriptions are appropriately omitted. Moreover, configurations described below are exemplified and do not limit the scope of the present invention. 
         [0017]      FIG. 1  is a view schematically showing a cryocooler  10  according to an embodiment. The cryocooler  10  includes a compressor  12  which compresses a working gas and an expander  14  which cools the working gas by adiabatic expansion. For example, the working gas is helium gas. The expander  14  may be also referred to as a cold head. A regenerator  16  which pre-cools the working gas is included in the expander  14 . The cryocooler  10  includes a gas pipe  18  which includes a first pipe  18   a  and a second pipe  18   b  which are respectively connected to the compressor  12  and the expander  14 . The shown cryocooler  10  is a single-stage GM cryocooler. 
         [0018]    As is well known, a working gas having a first high-pressure is supplied from a discharging port  12   a  of the compressor  12  to the expander  14  through the first pipe  18   a . The pressure of the working gas is decreased from the first high-pressure to a second high-pressure which is lower than the first high-pressure due to adiabatic expansion in the expander  14 . The working gas having the second high-pressure is returned from the expander  14  to a suction port  12   b  of the compressor  12  through the second pipe  18   b . The compressor  12  compresses the returned working gas having the second high-pressure. Accordingly, the pressure of the working gas increases to the first high-pressure again. In general, the first high-pressure and the second high-pressure are significantly higher than the atmospheric pressure. For convenience of descriptions, the first high-pressure and the second high-pressure are simply referred to as a high pressure and a low pressure, respectively. Typically, for example, the high pressure is 2 to 3 MPa, and the low pressure is 0.5 to 1.5 MPa. For example, a difference between the high pressure and the low pressure is approximately 1.2 to 2 MPa. 
         [0019]    The expander  14  includes an expander movable portion  20  and an expander stationary portion  22 . The expander movable portion  20  is configured so as to reciprocate in an axial direction (up-down direction in  FIG. 1 ) with respect to the expander stationary portion  22 . The movement direction of the expander movable portion  20  is indicated by an arrow A in  FIG. 1 . The expander stationary portion  22  is configured so as to support the expander movable portion  20  to be reciprocated in the axial direction. In addition, the expander stationary portion  22  is configured of an airtight container in which the expander movable portion  20  is accommodated along with a high-pressure gas (including first high-pressure gas and second high-pressure gas). 
         [0020]    The expander movable portion  20  includes a displacer  24  and a displacer drive shaft  26  which reciprocates the displacer  24 . A regenerator  16  is built in the displacer  24 . The displacer  24  includes a displacer member  24   a  which surrounds the regenerator  16 . An internal space of the displacer member  24   a  is filled with a regenerator material. Accordingly, the regenerator  16  is formed inside the displacer  24 . For example, the displacer  24  has a substantially columnar shape which extends in the axial direction. The displacer member  24   a  includes an outer diameter and an inner diameter which are substantially constant in the axial direction. Accordingly, the regenerator  16  also has a substantially columnar shape which extends in the axial direction. 
         [0021]    In addition, the displacer  24  includes a displacer upper surface  24   b . The displacer upper surface  24   b  is a substantially circular region which is perpendicular to the axial direction. One end of the displacer drive shaft  26  is fixed to the center of the displacer upper surface  24   b . 
         [0022]    The expander stationary portion  22  approximately has two configurations which includes a cylinder  28  and a drive mechanism housing (hereinafter, simply referred to as a housing)  30 . The upper portion of the expander stationary portion  22  in the axial direction is the housing  30 , the lower portion of the expander stationary portion  22  in the axial direction is the cylinder  28 , and the housing  30  and the cylinder  28  are firmly connected to each other. The cylinder  28  is configured to guide the reciprocation of the displacer  24 . The cylinder  28  extends in the axial direction from the housing  30 . The cylinder  28  has an inner diameter which is substantially constant in the axial direction, and accordingly, the cylinder  28  has a substantially cylindrical inner surface which extends in the axial direction. The inner diameter is slightly greater than the outer diameter of the displacer member  24   a.    
         [0023]    The housing  30  includes a housing bottom surface  30   a . The housing bottom surface  30   a  is a portion of the surface of the housing  30  and faces the displacer upper surface  24   b . The housing bottom surface  30   a  is parallel to the displacer upper surface  24   b  and, similarly to the displacer upper surface  24   b , is a substantially circular region which is perpendicular to the axial direction. However, the center of the housing bottom surface  30   a  is penetrated by the displacer drive shaft  26 . The displacer  24  can reciprocate in the axial direction with respect to the housing  30 . 
         [0024]    Moreover, the expander stationary portion  22  includes a cooling stage  32 . The cooling stage  32  is fixed to the terminal of the cylinder  28  on the side opposite to the housing  30  in the axial direction. The cooling stage  32  is provided so as to transmit coldness generated by the expander  14  to other objects. The objects are attached to the cooling stage  32 , and are cooled by the cooling stage  32  during the operation of the cryocooler  10 . 
         [0025]    In the present specification, for convenience of the description, terms such as an axial direction, a radial direction, and a circumferential direction are used. As shown by an arrow A, the axial direction indicates the movement direction of the expander movable portion  20  with respect to the expander stationary portion  22 . The radial direction indicates a direction (a lateral direction in the drawing) perpendicular to the axial direction, and the circumferential direction indicates a direction which surrounds the axial direction. An element of the expander  14  being close to the cooling stage  32  in the axial direction may be referred to “down, ” and the element being far from the cooling stage  32  in the axial direction may be referred to as “up.” Accordingly, a high-temperature portion and a low-temperature portion of the expander  14  are respectively positioned on the upper portion and the lower portion in the axial direction. The expressions are used so as to only assist understanding of a relative positional relationship between elements of the expander  14 . Accordingly, the expressions are not related to the disposition of the expander  14  when the expander  14  is installed in site. For example, in the expander  14 , the cooling stage  32  may be installed upward and the drive mechanism housing  30  may be installed downward. Alternatively, the expander  14  may be installed such that the axial direction coincides with a horizontal direction. 
         [0026]    During the operation of the cryocooler  10 , the regenerator  16  includes a regenerator high-temperature portion  16   a  on one side (upper side in the drawing) in the axial direction, and a regenerator low-temperature portion  16   b  on the side (lower side in the drawing) opposite to the regenerator high-temperature portion  16   a . In this way, the regenerator  16  has a temperature distribution in the axial direction. Similarly, other components (for example, displacer  24  and cylinder  28 ) of the expander  14  which surrounds the regenerator  16  also have axial temperature distributions. Accordingly, the expander  14  includes a high-temperature portion on one side in the axial direction and a low-temperature portion on the other side in the axial direction during the operation of the expander  14 . For example, the high-temperature portion has a temperature such as an approximately room temperature. The cooling temperatures of the low-temperature portion are different from each other according to the use of the cryocooler  10 , and for example, the low-temperature portion is cooled to a temperature which is included in a range from approximately 10 K to approximately 100 K. The cooling stage  32  is fixed to the cylinder  28  to enclose the low-temperature portion of the cylinder  28 . 
         [0027]    A configuration of a working gas flow path in the expander  14  is described.  FIGS. 2 and 3  show a portion of the configuration of the working gas flow path of the expander  14  according to the embodiment.  FIGS. 2 and 3  show the displacer upper surface  24   b , the housing bottom surface  30   a , and the working gas flow path around these. 
         [0028]    The expander  14  includes a valve portion  34 , a housing gas flow path  36 , an upper gas chamber  37 , a displacer upper gas flow path  38 , a displacer lower gas flow path  39 , a gas expansion chamber  40 , and a low-pressure gas chamber  42 . A high-pressure gas flows from the first pipe  18   a  into the gas expansion chamber  40  via the valve portion  34 , the housing gas flow path  36 , the upper gas chamber  37 , the displacer upper gas flow path  38 , the regenerator  16 , and the displacer lower gas flow path  39 . The gas returned to the gas expansion chamber  40  flows to the low-pressure gas chamber  42  via the displacer lower gas flow path  39 , the regenerator  16 , the displacer upper gas flow path  38 , the upper gas chamber  37 , the housing gas flow path  36 , and the valve portion  34 . 
         [0029]    Although it is described below in detail, the upper gas chamber  37  includes a gas-guiding flow channel  43 . The gas-guiding flow channel  43  is formed on the housing bottom surface  30   a  and configures a portion of the upper gas chamber  37 . The gas-guiding flow channel  43  causes the housing gas flow path  36  to communicate with the displacer upper gas flow path  38  when the displacer  24  is positioned at a top dead center in the axial reciprocation. In  FIG. 2 , a cross section of the expander  14  along a plane perpendicular to the axial direction in the upper gas chamber  37  is schematically shown.  FIG. 3  is an enlarged view showing a portion of the gas-guiding flow channel  43  shown in  FIG. 1 . 
         [0030]    The valve portion  34  is configured to control the pressure of the gas expansion chamber  40  to be synchronized with the reciprocation of the displacer  24 . The valve portion  34  functions as a portion of a supply path for supplying a high-pressure gas to the gas expansion chamber  40 , and functions as a portion of a discharging path for discharging a low-pressure gas from the gas expansion chamber  40 . The valve portion  34  is configured to end the discharging of the low-pressure gas and to start the supply of the high-pressure gas when the displacer  24  passes a bottom dead center or the vicinity thereof. The valve portion  34  is configured to end the supply of the high-pressure gas and to start the discharging of the low-pressure gas when the displacer  24  passes a top dead center or the vicinity thereof. In this way, the valve portion  34  is configured to switch the supply function and the discharging function of the working gas to be synchronized with the reciprocation of the displacer  24 . 
         [0031]    The housing gas flow path  36  is formed so as to penetrate the housing  30  such that gas flows between the expander stationary portion  22  and the upper gas chamber  37 . The housing gas flow path  36  is formed in the housing  30  and is open to the upper gas chamber  37 . The housing gas flow path  36  starts from the valve portion  34  and terminates at the upper gas chamber  37 . That is, one end of the housing gas flow path  36  is connected to a gas passage of the valve portion  34  and the other end of the housing gas flow path  36  is connected to the upper gas chamber  37 . 
         [0032]    The upper gas chamber  37  is formed between the expander stationary portion  22  and the displacer  24  on the regenerator high-temperature portion  16   a  side. More specifically, the upper gas chamber  37  is interposed between the housing bottom surface  30   a  and the displacer upper surface  24   b  in the axial direction, and is surrounded by the cylinder  28  in the circumferential direction. The upper gas chamber  37  is adjacent to the low-pressure gas chamber  42 . The upper gas chamber  37  is also referred to as a room temperature chamber. The upper gas chamber  37  is a variable volume which is formed between the expander movable portion  20  and the expander stationary portion  22 . 
         [0033]    The displacer upper gas flow path  38  is formed in the displacer  24  and is open to the upper gas chamber  37 . The displacer upper gas flow path  38  is at least one hole of the displacer member  24   a  which is formed to cause the regenerator high-temperature portion  16   a  to communicate with the upper gas chamber  37 . 
         [0034]    Specifically, the displacer upper gas flow path  38  includes a plurality of holes which are formed on the displacer upper surface  24   b . The plurality of holes penetrate the displacer member  24   a  in the axial direction from the displacer upper surface  24   b  to the regenerator high-temperature portion  16   a . In addition, the holes are arranged on the displacer upper surface  24   b  so as to surround the displacer drive shaft  26 . The plurality of holes are disposed at equal angle intervals in the circumferential direction on a circumference which has the central axis of the displacer as a center. For example, four holes are formed every 90° on the displacer upper surface  24   b , and the four holes are equidistant from the center of the displacer upper surface  24   b.    
         [0000]    For understanding, the displacer upper gas flow path  38  is shown by dashed lines in  FIG. 2 . 
         [0035]    Among the plurality of holes, one hole is disposed at the same position (that is, immediately below the housing gas flow path  36 ) as the position of the housing gas flow path  36  in a cross section along a plane perpendicular to the axial direction. Among the plurality of holes, remaining holes are formed at positions different from the position of the housing gas flow path  36  in the cross section along the plane perpendicular to the axial direction. 
         [0036]    As shown in  FIG. 1 , the displacer lower gas flow path  39  is at least one hole of the displacer member  24   a  which is formed to cause the regenerator low-temperature portion  16   b  to communicate with the gas expansion chamber  40 . 
         [0037]    A seal portion  44  which seals a clearance between the displacer  24  and the cylinder  28  is provided on the side surface of the displacer member  24   a . The seal portion  44  may be attached to the displacer member  24   a  so as to surround the displacer upper gas flow path  38  in the circumferential direction. 
         [0038]    The gas expansion chamber  40  is formed between the cylinder  28  and the displacer  24  on the regenerator low-temperature portion  16   b  side. Similarly to the upper gas chamber  37 , the gas expansion chamber  40  is a variable volume which is formed between the expander movable portion  20  and the expander stationary portion  22 , and the volume of the gas expansion chamber  40  is complementarily changed with the volume of the upper gas chamber  37  by the relative movement of the displacer  24  with respect to the cylinder  28 . Since the seal portion  44  is provided, a direct gas flow (that is, the flow of gas which bypasses the regenerator  16 ) between the upper gas chamber  37  and the gas expansion chamber  40  is not generated. 
         [0039]    The low-pressure gas chamber  42  is defined inside the housing  30 . The second pipe  18   b  is connected to the housing  30 . Accordingly, the low-pressure gas chamber  42  communicates with the suction port  12   b  of the compressor  12  through the second pipe  18   b . Therefore, the low-pressure gas chamber  42  is always maintained to a low pressure. 
         [0040]    A drive configuration of the expander  14  will be described. As shown in  FIG. 1 , the displacer drive shaft  26  protrudes from the displacer  24  to the low-pressure gas chamber  42  through the upper gas chamber  37 . The expander stationary portion  22  includes a pair of drive shaft guides  46   a  and  46   b  which support the displacer drive shaft  26  in the axial direction in a movable manner. Each of the drive shaft guides  46   a  and  46   b  is provided in housing  30  so as to surround the displacer drive shaft  26 . The drive shaft guide  46   b  positioned on the lower side in the axial direction or the lower end section of the housing  30  is airtightly configured. Accordingly, the low-pressure gas chamber  42  is separated from the upper gas chamber  37 . The direct gas flow between the low-pressure gas chamber  42  and the upper gas chamber  37  is not generated. 
         [0041]    The expander  14  includes a drive mechanism  48  which drives the displacer  24 . The drive mechanism  48  is accommodated in the low-pressure gas chamber  42  and includes a motor  48   a  and a scotch yoke mechanism  48   b . The displacer drive shaft  26  forms a portion of the scotch yoke mechanism  48   b . In addition, the scotch yoke mechanism  48   b  includes a crank pin  49  which extends to be parallel to the output shaft of the motor  48   a  and is eccentric to the output shaft. The displacer drive shaft  26  is connected to the scotch yoke mechanism  48   b  to be driven in the axial direction by the scotch yoke mechanism  48   b . Accordingly, the displacer  24  is reciprocated in the axial direction by the rotation of the motor  48   a . The scotch yoke mechanism  48   b  is interposed between the drive shaft guides  46   a  and  46   b , and the drive shaft guides  46   a  and  46   b  are positioned at different positions from each other in the axial direction. 
         [0042]    The valve portion  34  is connected to the drive mechanism  48  and is accommodated in the housing  30 . The valve portion  34  is a rotary valve type valve portion which includes a valve rotor  34   a  and a valve stator  34   b . The valve rotor  34   a  and the valve stator  34   b  are disposed in the low-pressure gas chamber  42 . The valve rotor  34   a  is connected to the output shaft of the motor  48   a  so as to be rotated by the rotation of the motor  48   a . The valve rotor  34   a  is in surface-contact with the valve stator  34   b  so as to rotationally slide on the valve stator  34   b . The valve stator  34   b  is fixed to the housing  30 . The valve stator  34   b  is configured so as to receive the high-pressure gas which enters the housing  30  from the first pipe  18   a.    
         [0043]    The gas-guiding flow channel  43  will be described in detail. As shown in the drawings, the gas-guiding flow channel  43  is formed on the housing bottom surface  30   a  to face the plurality of holes configuring the displacer upper gas flow path  38 . In addition, the housing gas flow path  36  is open to the gas-guiding flow channel  43 . That is, an outlet of the housing gas flow path  36  is disposed on a bottom surface of the gas-guiding flow channel  43 . According to this configuration, even when the displacer upper surface  24   b  is very close to the housing bottom surface  30   a  such as a case where the displacer  24  is positioned at the top dead center, a volume which allows a gas flow between the housing gas flow path  36  and the displacer upper gas flow path  38  is secured by the gas-guiding flow channel  43 . In addition, since the gas-guiding flow channel  43  is relatively easily processed on the housing bottom surface  30   a , a new load to a manufacturing process of the expander  14  is small. 
         [0044]    The gas-guiding flow channel  43  is formed such that a volume of the gas-guiding flow channel  43  is equal to or less than half of a volume of the upper gas chamber  37  when the displacer  24  is positioned at the top dead center. According to this configuration, since the volume of the gas-guiding flow channel  43  is relatively formed small, it is possible to prevent a dead volume from excessively increasing due to the formation of the gas-guiding flow channel  43 . Along with this, a pressure drop in the upper gas chamber  37  decreases when the displacer  24  is positioned at the top dead center and a decrease in refrigeration capacity of the cryocooler  10  is prevented. 
         [0045]    Specifically, a height, a width, and a length of the gas-guiding flow channel  43  are determined such that the volume of the gas-guiding flow channel  43  is equal to or less than half of the volume of the upper gas chamber  37  when the displacer  24  is positioned at the top dead center. Here, for example, the height, the width, and the length of the gas-guiding flow channel  43  respectively are an axial dimension, a radial dimension, and a circumferential dimension of the gas-guiding flow channel  43 . 
         [0046]    As shown in  FIG. 3 , the gas-guiding flow channel  43  may be formed such that an axial height D 1  of the gas-guiding flow channel  43  from the housing bottom surface  30   a  is larger than an axial gap D 2  from the housing bottom surface  30   a  to the displacer upper surface  24   b  when the displacer  24  is positioned at the top dead center. The axial gap D 2  corresponds to a minimum distance from the housing bottom surface  30   a  to the displacer upper surface  24   b  in the reciprocation of the displacer  24 . In addition, the gas-guiding flow channel  43  may be formed such that the axial height D 1  of the gas-guiding flow channel  43  from the housing bottom surface  30   a  is smaller than a radial width D 3  of the gas-guiding flow channel  43 . Even in this way, it is possible to prevent an excessive increase of the dead volume in the upper gas chamber  37  and it is possible to decrease a pressure drop in the upper gas chamber  37 . 
         [0047]    As shown in  FIG. 2 , the gas-guiding flow channel  43  extends around the central axis (for example, the displacer drive shaft  26 ) of the displacer  24 . For example, the gas-guiding flow channel  43  is an annular groove which has the central axis of the displacer  24  as a center. In this way, it is possible to relatively easily process the gas-guiding flow channel  43 . 
         [0048]    As shown in the drawings, for example, a sectional shape of the gas-guiding flow channel  43  is rectangular. However, the present invention is not limited to this, and the gas-guiding flow channel  43  may have an arbitrary sectional shape. 
         [0049]    The operation of the cryocooler  10  having the above-described configuration will be described. When the displacer  24  moves to the bottom dead center of the cylinder  28  or the position around the bottom dead center, the valve portion  34  is switched to connect the discharging port of the compressor  12  to the gas expansion chamber  40 . Since the displacer  24  is positioned at the bottom dead center of the cylinder  28  or around the bottom dead center, the upper gas chamber  37  is wide. The high-pressure gas easily flows into the regenerator high-temperature portion  16   a  through the housing gas flow path  36 , the gas-guiding flow channel  43 , the upper gas chamber  37 , and the displacer upper gas flow path  38  from the valve portion  34 . 
         [0050]    The gas is cooled while passing through the regenerator  16  and enters the gas expansion chamber  40  through the displacer lower gas flow path  39  from the regenerator low-temperature portion  16   b . While the gas flows into the gas expansion chamber  40 , the displacer  24  moves toward the top dead center of the cylinder  28 . Accordingly, the volume of the gas expansion chamber  40  increases. In this way, the gas expansion chamber  40  is filled with a high-pressure gas. 
         [0051]    When the displacer  24  moves to the top dead center of the cylinder  28  or the position around the top dead center (refer to  FIGS. 1 and 3 ), the valve portion  34  is switched so as to connect the suction port of the compressor  12  to the gas expansion chamber  40 . The high-pressure gas is expanded and cooled in the gas expansion chamber  40 . The expanded gas enters the regenerator  16  through the displacer lower gas flow path  39  from the gas expansion chamber  40 . The gas is cooled while passing through the regenerator  16 . The gas is returned from the regenerator  16  to the compressor  12  via the displacer upper gas flow path  38 , the gas-guiding flow channel  43 , the housing gas flow path  36 , the valve portion  34 , and the low-pressure gas chamber  42 . While the gas flows out from the gas expansion chamber  40 , the displacer  24  moves toward the bottom dead center of the cylinder  28 . Accordingly, the volume of the gas expansion chamber  40  decreases and a low-pressure gas is discharged from the gas expansion chamber  40 . 
         [0052]    The above-described process is one-time cooling cycle in the cryocooler  10 . The cryocooler  10  repeats the cooling cycle and cools the cooling stage  32  to a desired temperature. Accordingly, the cryocooler  10  can cool an object which is thermally connected to the cooling stage  32  to a cryogenic temperature. 
         [0053]    When the displacer  24  is positioned at the top dead center, the axial gap between the housing bottom surface  30   a  and the displacer upper surface  24   b  is narrow, and for, example, is several millimeters (for example, approximately 1 to 3 mm). This is to decrease the dead volume. As described above, the gas-guiding flow channel  43  is formed on the housing bottom surface  30   a  and forms a portion of the upper gas chamber  37 . The gas-guiding flow channel  43  causes the housing gas flow path  36  to communicate with the displacer upper gas flow path  38  when the displacer  24  is positioned at the top dead center in the axial reciprocation. Accordingly, even when the displacer  24  is positioned at the top dead center, the flow of the working gas between the housing gas flow path  36  and the displacer upper gas flow path  38  is secured. The gas can smoothly and uniformly flow in the regenerator  16 . The pressure drop is decreased, and a decrease of the refrigeration capacity in the cryocooler  10  is prevented. 
         [0054]    In an embodiment, as shown in  FIG. 4 , all of the plurality of holes of the displacer upper gas flow path  38  may be formed at positions different from the position of the housing gas flow path  36  in a cross section along the plane perpendicular to the axial direction (that is, in this case, the displacer upper gas flow path  38  does not exist immediately below the housing gas flow path  36 ). This configuration contributes to uniformity of the flow of the working gas. 
         [0055]    In an embodiment, the gas-guiding flow channel  43  may not extend over the entire circumference around the central axis of the displacer  24  as long as the gas-guiding flow channel  43  faces the plurality of holes. As shown in  FIG. 5 , for example, the gas-guiding flow channel  43  may be a C shaped groove. The C shaped gas-guiding flow channel  43  is open on a side opposite to the housing gas flowpath  36  and the hole serving as the displacer upper gas flow path  38  is disposed at each of two end portions of the gas-guiding flow channel  43 . In this way, the length of the gas-guiding flow channel  43  decreases (compared to the case where the gas-guiding flow channel  43  is formed over the entire circumference), and it is possible to prevent an excessive increase of the dead volume due to the gas-guiding flow channel  43 . 
         [0056]      FIG. 6  is a view schematically showing an entire configuration of a cryocooler  10  according to another embodiment.  FIG. 7  shows a portion of a configuration of a working gas flow path of an expander  14  according to another embodiment. 
         [0057]    In the embodiment described with reference to  FIGS. 1 to 3 , the gas-guiding flow channel  43  is formed on the housing bottom surface  30   a . However, the present invention is not limited to this. As shown in  FIGS. 6 and 7 , the gas-guiding flow channel  43  may be formed on the displacer upper surface  24   b . The gas-guiding flow channel  43  is formed on the displacer upper surface  24   b  so as to face the housing gas flow path  36 . The displacer upper gas flow path  38  includes a plurality of holes which are open to the gas-guiding flow channel  43 . For understanding, the housing gas flow path  36  is shown by a dashed line in  FIG. 7 . 
         [0058]    Even in this way, it is possible to prevent an excessive increase of the dead volume in the upper gas chamber  37  and it is possible to decrease the pressure drop in the upper gas chamber  37 . In addition, it is possible to relatively easily process the gas-guiding flow channel  43 , and manufacturability is improved. 
         [0059]    Hereinbefore, the present invention is described based on the embodiments. The present invention is not limited to the embodiment, and a person skilled in the art understands that various design modifications can be applied, various modification examples can be applied, and the modification examples are also included in the scope of the present invention. 
         [0060]    The gas-guiding flow channel  43  may be formed on both the housing bottom surface  30   a  and the displacer upper surface  24   b  forming a portion of the upper gas chamber  37 . That is, a groove-shaped recess portion formed on the housing bottom surface  30   a  and a groove-shaped recess portion formed on the displacer upper surface  24   b  may be combined to form the gas-guiding flow channel  43 . 
         [0061]    As shown in the drawings, one housing gas flow path  36  is provided, and one outlet of the housing gas flow path  36  which is open to the upper gas chamber  37  is provided. However, in an embodiment, the housing gas flow path  36  may be divided in the housing  30  so as to have a plurality of outlets which are open to the upper gas chamber  37 , or a plurality of housing gas flow paths  36  may be formed in the housing  30 . A plurality of outlets of the housing gas flow path  36  which are open to the upper gas chamber  37  may be disposed on the housing bottom surface  30   a  (or the gas-guiding flow channel  43 ) so as to surround the displacer drive shaft  26 . 
         [0062]    In the above-described embodiments, the embodiments are described in which the cryocooler is a single-stage GM cryocooler. However, the present invention is not limited to this, and the configurations of the working gas flowpath according to the embodiments can be applied to a two-stage or a multiple-stage GM cryocooler, or can be applied to other cryocoolers such as a pulse tube cryocooler. 
         [0063]    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.