Patent Publication Number: US-2021180834-A1

Title: Cryocooler

Description:
RELATED APPLICATIONS 
     The contents of Japanese Patent Application No. 2018-167725, and of International Patent Application No. PCT/JP2019/031007, on the basis of each of which priority benefits are claimed in an accompanying application data sheet, are in their entirety incorporated herein by reference. 
    
    
     BACKGROUND 
     Technical Field 
     Certain embodiments of the present invention relate to a cryocooler. 
     Description of Related Art 
     A Gifford-McMahon (GM) cryocooler as one representative example of cryocoolers is roughly classified into two types such as a motor-driven type and a gas-driven type, depending on a drive source of a displacer. In the motor-driven type, the displacer is mechanically connected to a motor, and is driven by the motor. In the gas-driven type, the displacer is driven by a gas pressure. 
     SUMMARY 
     According to an embodiment of the present invention, there is provided a cryocooler including a cylinder, a displacer disposed inside the cylinder and driven to reciprocate by a gas pressure, a collar rigidly connected to the displacer to reciprocate together with the displacer, a collar chamber divided into an upper section and a lower section by the collar, a second seal portion provided between the displacer and the cylinder to seal the lower section, a lower bumper provided in the lower section to mitigate interference between the displacer and the cylinder when the displacer is located at a bottom dead center, and a communication passage formed in the collar or in the collar chamber to ensure communication between the upper section and the lower section when the displacer is located at a bottom dead center. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view schematically illustrating a cryocooler according to one embodiment. 
         FIG. 2  is a view schematically illustrating the cryocooler according to the embodiment. 
         FIG. 3  is a view schematically illustrating a collar and a bumper according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The present inventors have recognized the following facts, as a result of intensive research on a gas-driven cryocooler. In the gas-driven cryocooler in the related art, a displacer moves due to a gas pressure until the displacer interferes (for example, collides) with a cylinder end portion. The interference may cause vibration and noise. A design called a “collar bumper” may be adopted to prevent the interference between the displacer and the cylinder end portion and to reduce the vibration and the noise. However, in the gas-driven cryocooler adopting a collar bumper type, when the displacer reaches a bottom dead center, a low pressure sealed region is formed on one side of a collar. Consequently, due to a differential pressure from a high pressure region on the other side of the collar, a movement of the displacer toward a top dead center may be hindered. 
     It is desirable to facilitate the movement of the displacer from the bottom dead center toward the top dead center in the gas-driven cryocooler adopting the collar bumper type. 
     Any desired combinations of the above-described components or those in which components or expressions according to the present invention are substituted with each other in methods, devices, and systems may be effectively adopted as an aspect of the present invention. 
     According to the present invention, it is possible to facilitate the movement of the displacer from the bottom dead center to the top dead center in the gas-driven cryocooler adopting the collar bumper type. 
     Hereinafter, embodiments according to the present invention will be described in detail with reference to the drawings. The same reference numerals will be assigned to the same or equivalent components, members, and processes in the description and the drawings, and repeated description will be appropriately omitted. Scales or shapes of respectively illustrated elements are set for convenience in order to facilitate the description, and are not to be interpreted in a limited manner unless otherwise specified. The embodiments described below are merely examples, and do not limit the scope of the present invention at all. All features or combinations thereof described in the embodiments are not necessarily essential to the invention. 
       FIGS. 1 and 2  are views schematically illustrating a cryocooler  10  according to one embodiment. For example, the cryocooler  10  is a gas-driven GM cryocooler. 
     The cryocooler  10  includes a compressor  12  which compresses working gas (for example, helium gas) and a cold head  14  which cools the working gas through adiabatic expansion. The compressor  12  has a compressor discharge port  12   a  and a compressor suction port  12   b . The compressor discharge port  12   a  and the compressor suction port  12   b  respectively function as a high pressure source and a low pressure source of the cryocooler  10 . The cold head  14  is also called an expander. 
     As will be described in detail later, the compressor  12  supplies high pressure (PH) working gas from the compressor discharge port  12   a  to the cold head  14 . The cold head  14  includes a regenerator  15  which pre-cools the working gas. The precooled working gas is further cooled through expansion inside the cold head  14 . The working gas is recovered to the compressor suction port  12   b  through the regenerator  15 . The working gas cools the regenerator  15  when the working gas passes through the regenerator  15 . The compressor  12  compresses the recovered low pressure (PL) working gas, and supplies the working gas to the cold head  14  again. 
     The illustrated cold head  14  is a single stage type. However, the cold head  14  may be a multi-stage type. 
     The cold head  14  includes an axially movable body  16  serving as a free piston driven by a gas pressure, and a cold head housing  18  configured to be hermetic and accommodating the axially movable body  16 . The cold head housing  18  supports the axially movable body  16  to be capable of reciprocating in an axial direction, and is configured to serve as a pressure vessel for the working gas. Unlike a motor-driven type GM cryocooler, the cold head  14  does not have a motor for driving the axially movable body  16  and a connecting mechanism (for example, a scotch yoke mechanism). 
     The axially movable body  16  includes a displacer  20  capable of reciprocating in the axial direction (upward-downward direction in  FIG. 1 , indicated by an arrow C), and a drive piston  22  coaxially connected to the displacer  20  to drive the displacer  20  in the axial direction. The drive piston  22  is rigidly connected to the displacer  20  so that the displacer  20  reciprocates in the axial direction integrally with the drive piston  22 . The drive piston  22  has a dimension smaller than that of the displacer  20 . An axial length of the drive piston  22  is shorter than that of the displacer  20 , and a diameter of the drive piston  22  is smaller than that of the displacer  20 . 
     The cold head housing  18  includes a displacer cylinder  26  which accommodates the displacer  20 , and a piston cylinder  28  which accommodates the drive piston  22 . The piston cylinder  28  is located coaxially with and adjacent to the displacer cylinder  26  in the axial direction. Although details will be described later, a drive part of the cold head  14  which is the gas-driven type is configured to include the drive piston  22  and the piston cylinder  28 . A volume of the piston cylinder  28  is smaller than that of the displacer cylinder  26 . The axial length of the piston cylinder  28  is shorter than that of the displacer cylinder  26 , and the diameter of the piston cylinder  28  is smaller than that of the displacer cylinder  26 . 
     Axial reciprocation of the displacer  20  is guided by the displacer cylinder  26 . In general, the displacer  20  and the displacer cylinder  26  are cylindrical members which respectively extend in the axial direction, and an inner diameter of the displacer cylinder  26  coincides with or slightly larger than an outer diameter of the displacer  20 . Similarly, the axial reciprocation of the drive piston  22  is guided by the piston cylinder  28 . In general, the drive piston  22  and the piston cylinder  28  are cylindrical members which respectively extend in the axial direction, and the inner diameter of the piston cylinder  28  coincides with or slightly larger than the outer diameter of the drive piston  22 . 
     The displacer  20  and the drive piston  22  are rigidly connected to each other. Accordingly, an axial stroke of the drive piston  22  is equal to an axial stroke of the displacer  20 , and both of these integrally move over all strokes. A position of the drive piston  22  with respect to the displacer  20  is unchanged during the axial reciprocation of the axially movable body  16 . 
     A first seal portion  32  is provided between the drive piston  22  and the piston cylinder  28 . The first seal portion  32  is mounted on one of the drive piston  22  and the piston cylinder  28 , and slides on the other of the drive piston  22  and the piston cylinder  28 . For example, the first seal portion  32  is formed of a sealing member such as a slipper seal or an O-ring. The piston cylinder  28  is configured to be hermetic with respect to the displacer cylinder  26  by the first seal portion  32 . Since the first seal portion  32  is provided, there is no direct gas circulation between the piston cylinder  28  and the displacer cylinder  26 . An internal pressure of the piston cylinder  28  and an internal pressure of the displacer cylinder  26  can have different magnitudes. 
     The displacer cylinder  26  is divided into an expansion chamber  34  and a room temperature chamber  36  by the displacer  20 . The displacer  20  forms the expansion chamber  34  with the displacer cylinder  26  in one end in the axial direction, and forms the room temperature chamber  36  with the displacer cylinder  26  in the other end in the axial direction. The room temperature chamber  36  can also be called a compression chamber. In addition, the cold head  14  is provided with a cooling stage  38  fixed to the displacer cylinder  26  so as to wrap the expansion chamber  34 . 
     The regenerator  15  is incorporated in the displacer  20 . An upper lid portion of the displacer  20  has an inlet flow path  40  through which the regenerator  15  communicates with the room temperature chamber  36 . In addition, a cylinder portion of the displacer  20  has an outlet flow path  42  through which the regenerator  15  communicates with the expansion chamber  34 . Alternatively, the outlet flow path  42  may be provided in a lower lid portion of the displacer  20 . In addition, the regenerator  15  includes an inlet retainer  41  inscribed in the upper lid portion and an outlet retainer  43  inscribed in the lower lid portion. A regenerator material may be a copper wire mesh, for example. The retainer may be a wire mesh which is coarser than the regenerator material. 
     A second seal portion  44  is provided between the displacer  20  and the displacer cylinder  26 . For example, the second seal portion  44  is a slipper seal, and is mounted on the cylinder portion or the upper lid portion of the displacer  20 . A clearance between the displacer  20  and the displacer cylinder  26  is sealed by the second seal portion  44 . Accordingly, there is no direct gas circulation (that is, a gas flow bypassing the regenerator  15 ) between the room temperature chamber  36  and the expansion chamber  34 . 
     The working gas flows into the regenerator  15  from the room temperature chamber  36  through the inlet flow path  40 . More precisely, the working gas flows into the regenerator  15  from the inlet flow path  40  through the inlet retainer  41 . The working gas flows into the expansion chamber  34  from the regenerator  15  by way of the outlet retainer  43  and the outlet flow path  42 . When the working gas returns to the room temperature chamber  36  from the expansion chamber  34 , the working gas passes a reverse path thereof. That is, the working gas returns to the room temperature chamber  36  from the expansion chamber  34  through the outlet flow path  42 , the regenerator  15 , and the inlet flow path  40 . The working gas trying to flow into the clearance after bypassing the regenerator  15  is blocked by the second seal portion  44 . 
     The cold head  14  is installed in an illustrated direction at a job site where the cold head  14  is used. That is, the cold head  14  is vertically installed by disposing the displacer cylinder  26  below in the vertical direction and disposing the piston cylinder  28  above in the vertical direction, respectively. In this way, the cryocooler  10  has highest cooling capacity when the cooling stage  38  is installed by adopting a downward facing posture in the vertical direction. However, disposition of the cryocooler  10  is not limited thereto. Conversely, the cold head  14  may be installed by adopting a posture in which the cooling stage  38  faces upward in the vertical direction. Alternatively, the cold head  14  may be installed sideways or in any other direction. The cold head  14  can perform a cooling operation even when the cold head  14  is installed by adopting any posture. 
     An end of the reciprocating stroke of the displacer  20  on the expansion chamber  34  side will be referred to as a bottom dead center of the displacer  20 , and an end of the reciprocating stroke of the displacer  20  on the room temperature chamber  36  side will be referred to as a top dead center of the displacer  20 . A movement of the displacer  20  toward the top dead center may be referred to as an upward movement, and a movement of the displacer  20  toward the bottom dead center may be referred to as a downward movement. However, these terms do not limit the posture of the cold head  14 . 
     When the displacer  20  moves in the axial direction, the expansion chamber  34  and the room temperature chamber  36  complementarily increase and decrease respective volumes. That is, when the displacer  20  moves downward, the expansion chamber  34  is narrowed, and the room temperature chamber  36  is widened. And vice versa. Therefore, when the displacer  20  is located at the bottom dead center, the volume of the expansion chamber  34  is minimized (volume of the room temperature chamber  36  is maximized). When the displacer  20  is located at the top dead center, the volume of the expansion chamber  34  is maximized (the volume of the room temperature chamber  36  is minimized). 
     Furthermore, the cryocooler  10  includes a working gas circuit  52  which connects the compressor  12  to the cold head  14 . The working gas circuit  52  is configured to generate a pressure difference between the piston cylinder  28  and the displacer cylinder  26  (that is, the expansion chamber  34  and/or the room temperature chamber  36 ). The pressure difference causes the axially movable body  16  to move in the axial direction. When the pressure of the displacer cylinder  26  is lower than that of the piston cylinder  28 , the drive piston  22  moves downward, and consequently, the displacer  20  also moves downward. Conversely, when the pressure of the displacer cylinder  26  is higher than that of the piston cylinder  28 , the drive piston  22  moves upward, and consequently, the displacer  20  also moves upward. 
     The working gas circuit  52  includes a valve portion  54 . The valve portion  54  may be disposed adjacent to the piston cylinder  28  to be integrated with the cold head housing  18 , and may be connected to the compressor  12  by using a pipe. The valve portion  54  may be disposed outside the cold head housing  18 , and may be connected to each of the compressor  12  and the cold head  14  by using a pipe. 
     The valve portion  54  includes an expansion chamber pressure switching valve (hereinafter, also referred to as a main pressure switching valve)  60  and a drive chamber pressure switching valve (hereinafter, also referred to as an auxiliary pressure switching valve)  62 . The main pressure switching valve  60  has a main intake on-off valve V 1  and a main exhaust on-off valve V 2 . The auxiliary pressure switching valve  62  has an auxiliary intake on-off valve V 3  and an auxiliary exhaust on-off valve V 4 . 
     The working gas circuit  52  includes a high pressure line  13   a  and a low pressure line  13   b  which connect the compressor  12  to the valve portion  54 . The high pressure line  13   a  extends from the compressor discharge port  12   a , branches in an intermediate portion, and is connected to the main intake on-off valve V 1  and the auxiliary intake on-off valve V 3 . The low pressure line  13   b  extends from the compressor suction port  12   b , branches in an intermediate portion, and is connected to the main exhaust on-off valve V 2  and the auxiliary exhaust on-off valve V 4 . 
     In addition, the working gas circuit  52  includes a main communication passage  64  and an auxiliary communication passage  66  which connect the cold head  14  to the valve portion  54 . The main communication passage  64  connects the displacer cylinder  26  to the main pressure switching valve  60 . The main communication passage  64  extends from the room temperature chamber  36 , branches in an intermediate portion, and is connected to the main intake on-off valve V 1  and the main exhaust on-off valve V 2 . The auxiliary communication passage  66  connects the piston cylinder  28  to the auxiliary pressure switching valve  62 . The auxiliary communication passage  66  extends from the piston cylinder  28 , branches in an intermediate portion, and is connected to the auxiliary intake on-off valve V 3  and the auxiliary exhaust on-off valve V 4 . 
     The main pressure switching valve  60  is configured so that the compressor discharge port  12   a  or the compressor suction port  12   b  selectively communicates with the room temperature chamber  36  of the displacer cylinder  26 . In the main pressure switching valve  60 , the main intake on-off valve V 1  and the main exhaust on-off valve V 2  are respectively and exclusively opened. That is, the main intake on-off valve V 1  and the main exhaust on-off valve V 2  are inhibited from being opened at the same time. The main intake on-off valve V 1  and the main exhaust on-off valve V 2  may be temporarily closed together. 
     When the main intake on-off valve V 1  is open, the main exhaust on-off valve V 2  is closed. The working gas flows from the compressor discharge port  12   a  to the displacer cylinder  26  through the high pressure line  13   a  and the main communication passage  64 . As described above, the working gas flows from the room temperature chamber  36  to the expansion chamber  34  through the regenerator  15 . In this way, the working gas having a high pressure PH is supplied from the compressor  12  to the expansion chamber  34 , and the expansion chamber  34  is pressurized. Conversely, when the main intake on-off valve V 1  is closed, the supply of the working gas from the compressor  12  to the expansion chamber  34  is stopped. 
     On the other hand, when the main exhaust on-off valve V 2  is open, the main intake on-off valve V 1  is closed. First, the working gas having the high pressure PH is expanded and decompressed in the expansion chamber  34 . The working gas flows from the expansion chamber  34  to the room temperature chamber  36  through the regenerator  15 . The working gas flows from the displacer cylinder  26  to the compressor suction port  12   b  through the main communication passage  64  and the low pressure line  13   b . In this way, the working gas having a low pressure PL is recovered from the cold head  14  to the compressor  12 . When the main exhaust on-off valve V 2  is closed, the recovery of the working gas from the expansion chamber  34  to the compressor  12  is stopped. 
     The auxiliary pressure switching valve  62  is configured so that the compressor discharge port  12   a  or the compressor suction port  12   b  selectively communicates with the piston cylinder  28 . The auxiliary pressure switching valve  62  is configured so that the auxiliary intake on-off valve V 3  and the auxiliary exhaust on-off valve V 4  are respectively and exclusively opened. That is, the auxiliary intake on-off valve V 3  and the auxiliary exhaust on-off valve V 4  are inhibited from being opened at the same time. The auxiliary intake on-off valve V 3  and the auxiliary exhaust on-off valve V 4  may be temporarily closed together. 
     When the auxiliary exhaust on-off valve V 4  is open, the auxiliary intake on-off valve V 3  is closed. The working gas flows from the compressor discharge port  12   a  to the piston cylinder  28  through the high pressure line  13   a  and the auxiliary communication passage  66 . In this way, the working gas having the high pressure PH is supplied from the compressor  12  to the piston cylinder  28 , and the piston cylinder  28  is pressurized. When the auxiliary intake on-off valve V 3  is closed, the supply of the working gas from the compressor  12  to the piston cylinder  28  is stopped. 
     On the other hand, when the auxiliary exhaust on-off valve V 4  is open, the auxiliary intake on-off valve V 3  is closed. The working gas is recovered from the piston cylinder  28  to the compressor suction port  12   b  through the auxiliary communication passage  66  and the low pressure line  13   b , and the piston cylinder  28  is decompressed to the low pressure PL. When the auxiliary exhaust on-off valve V 4  is closed, the recovery of the working gas from the piston cylinder  28  to the compressor  12  is stopped. 
     In this way, the main pressure switching valve  60  generates periodic pressure fluctuations of the high pressure PH and the low pressure PL in the expansion chamber  34 . In addition, the auxiliary pressure switching valve  62  generates periodic pressure fluctuations of the high pressure PH and the low pressure PL in the piston cylinder  28 . 
     The auxiliary pressure switching valve  62  is configured to control the pressure of the piston cylinder  28  so that the drive piston  22  drives the displacer  20  to reciprocate in the axial direction. Typically, the pressure fluctuations in the piston cylinder  28  are generated in a substantially opposite phase to and in the same cycle as that of the pressure fluctuations in the expansion chamber  34 . When the expansion chamber  34  has the high pressure PH, the piston cylinder  28  has the low pressure PL, and the drive piston  22  can move the displacer  20  upward. When the expansion chamber  34  has the low pressure PL, the piston cylinder  28  has the high pressure PH, and the drive piston  22  can move the displacer  20  downward. 
     The valve portion  54  may adopt a form of a rotary valve. A group of valves (V 1  to V 4 ) is incorporated in the valve portion  54 , and the valves are synchronously driven. The valve portion  54  is configured so that the valves (V 1  to V 4 ) are properly switched therebetween by rotational sliding of a valve disc (or a valve rotor) with respect to a valve main body (or a valve stator). The group of valves (V 1  to V 4 ) is switched in the same cycle during an operation of the cryocooler  10 . In this manner, four on-off valves (V 1  to V 4 ) periodically changes opened and closed states. The four on-off valves (V 1  to V 4 ) are opened and closed in respectively different phases. 
     The cryocooler  10  may include a rotation drive source  56  connected to the valve portion  54  to rotate the valve portion  54 . The rotation drive source  56  is mechanically connected to the valve portion  54 . The rotation drive source  56  is a motor, for example. However, the rotation drive source  56  is not mechanically connected to the axially movable body  16 . In addition, the cryocooler  10  may include a control unit  58  that controls the valve portion  54 . The control unit  58  may control the rotation drive source  56 . 
     In a certain embodiment, the group of valves (V 1  to V 4 ) may adopt a form of a plurality of individually controllable valves. Each of the valves (V 1  to V 4 ) may be an electromagnetic on-off valve. In this case, the rotation drive source  56  is not provided, and each of the valves (V 1  to V 4 ) is electrically connected to the control unit  58 . The control unit  58  may control the opening and closing of each of the valves (V 1  to V 4 ). 
       FIG. 1  illustrates a state where the displacer  20  is located at the bottom dead center, and  FIG. 2  illustrates a state where the displacer  20  is located at the top dead center. 
     The cryocooler  10  adopts a collar bumper type. Accordingly, the cold head  14  includes a collar  70  and a collar chamber  72  divided into an upper section  72   a  and a lower section  72   b  by the collar  70 . The collar  70  is rigidly connected to the displacer  20  to reciprocate together with the displacer  20 , and forms a portion of the axially movable body  16 . As will be described later, the reciprocating stroke of the collar  70  in the collar chamber  72  determines the reciprocating stroke of the displacer  20 . 
     The displacer cylinder  26  includes a cylinder flange  26   a  that defines a cylinder upper opening. The cylinder flange  26   a  extends outward in the radial direction from an upper end of the displacer cylinder  26  in the axial direction. The cold head housing  18  includes a top plate  30  and a sleeve  73 . The piston cylinder  28  and the sleeve  73  are fixed to the top plate  30 , and the valve portion  54  is mounted on the top plate  30 . The cylinder flange  26   a  is connected to the top plate  30  via the sleeve  73 . The sleeve  73  is disposed outside the piston cylinder  28  to surround the piston cylinder  28 . 
     The collar  70  includes a cylindrical main body  70   a  and a collar upper end  70   b . The main body  70   a  has an outer diameter substantially the same as that of the displacer  20 , and extends upward from the room temperature chamber  36  side of the displacer  20 . An inner diameter of the main body  70   a  is larger than an outer diameter of the piston cylinder  28 . The collar upper end  70   b  exists outside the outer diameter of the displacer  20 . The collar chamber  72  is divided into an upper section  72   a  and a lower section  72   b  by the collar upper end  70   b . The collar chamber  72  communicates with the room temperature chamber  36 . When the displacer  20  reciprocates inside the displacer cylinder  26 , the collar  70  reciprocates in the collar chamber  72  without rubbing against the displacer cylinder  26  and the piston cylinder  28 . The collar  70  does not rub against an inner peripheral surface of the sleeve  73 . 
     In addition, the cold head  14  includes an upper bumper  74  provided in the upper section  72   a  to mitigate interference between the displacer  20  and the displacer cylinder  26  when the displacer  20  is located at top dead center. The upper bumper  74  is installed on an upper surface of the collar chamber  72 , and has an upper cushioning material  74   a  and an upper retainer  74   b . For example, the upper bumper  74  is attached to the sleeve  73 . For example, the upper cushioning material  74   a  is a resin-made annular member such as an O-ring, and is pinched between the upper surface of the collar chamber  72  and the upper retainer  74   b . For example, the upper retainer  74   b  is formed of a resin material. The upper retainer  74   b  may not be provided. 
     The upper bumper  74  comes into contact with the collar  70  when the displacer  20  is located at the top dead center, and prevents the displacer  20  and the displacer cylinder  26  from colliding with each other on the room temperature chamber  36  side. The collar upper end  70   b  engages with the upper bumper  74  inside the collar chamber  72  before the displacer  20  collides with the piston cylinder  28  when the displacer  20  moves upward. In this case, the collar upper end  70   b  comes into contact with the upper retainer  74   b , and the upper cushioning material  74   a  is compressed to absorb an impact. 
     The cold head  14  includes a lower bumper  76  provided in the lower section  72   b  to mitigate interference between the displacer  20  and the displacer cylinder  26  when the displacer  20  is located at the bottom dead center. The lower bumper  76  is installed on a lower surface of the collar chamber  72  and has a lower cushioning material  76   a  and a lower retainer  76   b . For example, the lower bumper  76  is attached to the cylinder flange  26   a . The lower bumper  76  may be attached to the sleeve  73 . For example, the lower cushioning material  76   a  is a resin-made annular member such as an O-ring, and is pinched between the lower surface of the collar chamber  72  and the lower retainer  76   b . For example, the lower retainer  76   b  is formed of a resin material. The lower retainer  76   b  may not be provided. 
     The lower bumper  76  comes into contact with the collar  70  when the displacer  20  is located at the bottom dead center, and prevents the displacer  20  and the displacer cylinder  26  from colliding with each other on the expansion chamber  34  side. The collar upper end  70   b  engages with the lower bumper  76  inside the collar chamber  72  before the displacer  20  collides with the displacer cylinder  26  on the expansion chamber  34  side when the displacer  20  moves downward. In this case, the collar upper end  70   b  comes into contact with the lower retainer  76   b , and the lower cushioning material  76   a  is compressed to absorb the impact. 
     The upper section  72   a  communicates with the room temperature chamber  36 . A first gap  78   a  is formed between the outer peripheral surface of the piston cylinder  28  and the inner peripheral surface of the collar  70 , and the working gas can flow between the room temperature chamber  36  and the upper section  72   a  through the first gap  78   a.    
     The lower section  72   b  communicates with the upper section  72   a . A second gap  78   b  is formed between the inner peripheral surface of the sleeve  73  and the outer peripheral surface of the collar upper end  70   b , and the working gas can flow between the upper section  72   a  and the lower section  72   b  through the second gap  78   b . However, when the displacer  20  is located at the bottom dead center, the collar upper end  70   b  comes into contact with the lower bumper  76 , and communication between the lower section  72   b  and the upper section  72   a  through the second gap  78   b  is blocked. When the displacer  20  is located at the top dead center, the collar upper end  70   b  comes into contact with the upper bumper  74 , and the communication between the lower section  72   b  and the upper section  72   a  through the second gap  78   b  is blocked. Therefore, when the displacer  20  is located at an intermediate position between the top dead center and the bottom dead center, the lower section  72   b  communicates with the room temperature chamber  36  through the upper section  72   a , and the working gas can flow between the room temperature chamber  36  and the lower section  72   b . In addition, the lower section  72   b  is sealed by the second seal portion  44 . Accordingly, the lower section  72   b  does not communicate with the expansion chamber  34 . 
     In addition, the cold head  14  includes a communication passage  80  that ensures the communication between the upper section  72   a  and the lower section  72   b  when the displacer  20  is located at the bottom dead center. The communication passage  80  is formed in the collar  70  so that the upper section  72   a  communicates with the lower section  72   b  in a state where the collar upper end  70   b  is in contact with the lower bumper  76 . The communication passage  80  may be formed to penetrate the collar  70  (for example, the collar upper end  70   b ) from the upper section  72   a  to the lower section  72   b , and at least one communication passage  80  may be in a circumferential direction. As illustrated, when the collar upper end  70   b  extends outward in the radial direction from the main body  70   a  of the collar  70 , the communication passage  80  is formed in the collar upper end  70   b  at a position inside the lower bumper  76  in the radial direction. The communication passage  80  may be formed to penetrate the main body  70   a  of the collar  70 . 
     The first gap  78   a , the second gap  78   b , and the communication passage  80  function as flow path resistance. Therefore, when the displacer  20  reciprocates, the upper section  72   a  and the lower section  72   b  can respectively generate a gas spring force. The displacer  20  moves upward, and the collar upper end  70   b  also moves upward so that the upper section  72   a  is narrowed. In this case, the gas of the upper section  72   a  is compressed, and the pressure increases. The pressure in the upper section  72   a  acts downward on the upper surface of the collar upper end  70   b . Therefore, the upper section  72   a  generates a gas spring force acting against an upward movement of the collar  70  and the displacer  20 . Similarly, when the displacer  20  moves downward, the lower section  72   b  generates a gas spring force acting against a downward movement of the collar  70  and the displacer  20 . The upper section  72   a  and the lower section  72   b  may be respectively referred to as an upper gas spring chamber and a lower gas spring chamber. The gas spring force is helpful in reducing the vibration and the noise which are generated when the collar  70  comes into contact with the upper bumper  74  and the lower bumper  76 . 
     An operation of the cryocooler  10  will be described. When the displacer  20  is located at or in the vicinity of the bottom dead center, an intake process of the cryocooler  10  starts. The main intake on-off valve V 1  is opened, and the main exhaust on-off valve V 2  is closed. The working gas is supplied from the compressor discharge port  12   a  to the displacer cylinder  26  of the cold head  14  through the main intake on-off valve V 1 , and the expansion chamber  34  and the room temperature chamber  36  have the high pressure PH. The exhaust process of the piston cylinder  28  is performed simultaneous with an intake process of the expansion chamber  34 . The auxiliary intake on-off valve V 3  is closed, and the auxiliary exhaust on-off valve V 4  is opened. The working gas is discharged from the piston cylinder  28  to the compressor suction port  12   b  through the auxiliary exhaust on-off valve V 4 , and the piston cylinder  28  is decompressed to the low pressure PL. 
     Therefore, in the intake process, a driving force generated by a differential pressure (PH-PL) between the piston cylinder  28  and the expansion chamber  34  acts upward on the drive piston  22 . As a result, the displacer  20  moves together with the drive piston  22  from the bottom dead center toward the top dead center. In this way, a volume of the expansion chamber  34  increases, and the expansion chamber  34  is filled with the high pressure gas. 
     The collar  70  also moves upward together with the displacer  20 . The collar  70  comes into contact with the upper bumper  74  before the displacer  20  collides with a high temperature end portion (for example, the piston cylinder  28 ) of the displacer cylinder  26 . The upper cushioning material  74   a  is compressed to absorb the impact. While the collar  70  moves upward, the upper section  72   a  communicates with the room temperature chamber  36  through the first gap  78   a , and the lower section  72   b  communicates with the upper section  72   a  through the second gap  78   b  and the communication passage  80 . Thereafter, the upper section  72   a  and the lower section  72   b  have the high pressure PH as in the room temperature chamber  36 . 
     When the displacer  20  is located at or in the vicinity of the top dead center, the exhaust process of the cryocooler  10  starts. The main exhaust on-off valve V 2  is opened, and the main intake on-off valve V 1  is closed. The high pressure gas is expanded and cooled in the expansion chamber  34 . The expanded gas is recovered to the compressor suction port  12   b  through the room temperature chamber  36  while cooling the regenerator  15 . The expansion chamber  34  and the room temperature chamber  36  have the low pressure PL. The intake process of the piston cylinder  28  is performed simultaneous with the exhaust process of the expansion chamber  34 . The auxiliary exhaust on-off valve V 4  is closed, and the auxiliary intake on-off valve V 3  is opened. The working gas is supplied from the compressor discharge port  12   a  to the piston cylinder  28  through the auxiliary intake on-off valve V 3 , and the piston cylinder  28  is pressurized to a high pressure PH. 
     Therefore, in the exhaust process, a driving force generated by the differential pressure (PH-PL) between the piston cylinder  28  and the expansion chamber  34  acts downward on the drive piston  22 . Therefore, the displacer  20  moves together with the drive piston  22  from the top dead center toward the bottom dead center. In this way, the volume of the expansion chamber  34  decreases, and the low pressure gas is discharged. 
     The collar  70  moves downward together with the displacer  20 . The collar  70  comes into contact with the lower bumper  76  before the displacer  20  collides with a low temperature end portion of the displacer cylinder  26 . The lower cushioning material  76   a  is compressed to absorb the impact. While the collar  70  moves downward, the upper section  72   a  communicates with the room temperature chamber  36  through the first gap  78   a , and the lower section  72   b  communicates with the upper section  72   a  through the second gap  78   b  and the communication passage  80 . Thereafter, the upper section  72   a  and the lower section  72   b  have the low pressure PL as in the room temperature chamber  36 . 
     The cryocooler  10  cools the cooling stage  38  by repeating a refrigeration cycle (that is, a GM cycle) in this way. In this manner, the cryocooler  10  can cool an object to be cooled (not illustrated) thermally coupled to the cooling stage  38 . 
     The cryocooler  10  adopts the collar bumper type. Accordingly, it is possible to reduce the vibration and the noise by preventing the interference (for example, collision) between the displacer  20  and the displacer cylinder  26  which is caused by the contact between the collar  70  and the bumpers ( 74  and  76 ). 
     Incidentally, a gas-driven cryocooler adopting a typical collar bumper type does not have the communication passage  80 , unlike the above-described embodiment. In this case, when the collar  70  is located at the bottom dead center, the working gas having the low pressure PL may be sealed in the lower section  72   b . In this state, if the upper section  72   a  is pressurized to the high pressure PH when the intake process starts, the collar upper end  70   b  may be pressed against the lower bumper  76  due to the differential pressure (PH-PL). This differential pressure power may hinder the upward movement of the displacer  20 . 
     However, the cryocooler  10  according to the embodiment includes the communication passage  80  formed in the collar  70  to ensure the communication between the upper section  72   a  and the lower section  72   b  when the displacer  20  is located at the bottom dead center. Therefore, even when the collar  70  is located at the bottom dead center and the collar upper end  70   b  is in contact with the lower bumper  76 , the lower section  72   b  communicates with the upper section  72   a  through the communication passage  80 . The lower section  72   b  is not sealed. The differential pressure that may be generated between the upper section  72   a  and the lower section  72   b  is reduced or eliminated through the communication passage  80 . Accordingly, the upward movement of the displacer  20  is not hindered. Therefore, the displacer  20  can move from the bottom dead center toward the top dead center. 
     The communication passage  80  is formed in the collar  70 . In this case, it is easy to form the communication passage  80  in terms of manufacturing. 
       FIG. 3  is a view schematically illustrating a collar and a bumper according to another embodiment. As illustrated, the communication passage  80  may be formed in the collar chamber  72  without being formed in the main body  70   a  of the collar  70  or the collar upper end  70   b . For example, the communication passage  80  may be formed in the lower bumper  76 . For example, the communication passage  80  may be a groove formed on the upper surface of the lower retainer  76   b  on a side opposite to the lower cushioning material  76   a . The communication passage  80  formed in the collar chamber  72  illustrated in  FIG. 3  is applicable to the cryocooler  10  illustrated in  FIGS. 1 and 2  or other gas-driven cryocoolers adopting the collar bumper type. 
     Even in this case, the communication passage  80  can ensure the communication between the upper section  72   a  and the lower section  72   b  when the displacer  20  is located at the bottom dead center. The differential pressure that may be generated between the upper section  72   a  and the lower section  72   b  is reduced or eliminated through the communication passage  80 . Accordingly, it is possible to facilitate the movement of the displacer  20  from the bottom dead center toward the top dead center. 
     In another example in which the communication passage  80  is formed in the collar chamber  72 , the communication passage  80  may be a flow path formed in the cold head housing  18 . For example, the communication passage  80  may extend from the upper section  72   a  to the lower section  72   b  by way of the sleeve  73  and the cylinder flange  26   a . Even in this case, the communication passage  80  can ensure the communication between the upper section  72   a  and the lower section  72   b  when the displacer  20  is located at the bottom dead center. 
     Hitherto, the present invention has been described based on the embodiments. The present invention is not limited to the above-described embodiments. It may be understood by those skilled in the art that various design changes can be made, various modification examples can be adopted, and the modification examples also fall within the scope of the present invention. Various features described with regard to a certain embodiment are also applicable to other embodiments. A new embodiment acquired from the combination compatibly achieves respective advantageous effects of the combined embodiments. 
     In the above-described embodiment, the collar upper end  70   b  is provided outside the displacer  20  in the radial direction. However, this specific shape is not essential. For example, the collar upper end  70   b  may extend inward in the radial direction from the main body  70   a , and may exist inside the outer diameter of the displacer  20 . In this case, the collar chamber  72  is formed on the piston cylinder  28  side without being formed on the sleeve  73  side as described above. 
     In the above-described embodiment, the upper bumper  74  is attached to the upper surface of the collar chamber  72  and is disposed in the upper section  72   a . The lower bumper  76  is attached to the lower surface of the collar chamber  72 , and is disposed in the lower section  72   b . However, the upper bumper  74  and the lower bumper  76  may be attached to the collar  70 . For example, the upper bumper  74  may be attached to the upper surface of the collar upper end  70   b , and may be disposed in the upper section  72   a . The lower bumper  76  may be attached to the lower surface of the collar upper end  70   b , and may be disposed in the lower section  72   b . Even in this way, it is possible to reduce the vibration and the noise by preventing the interference (for example, collision) between the displacer  20  and the displacer cylinder  26  which is caused by the contact between the collar chamber  72  and the bumpers ( 74  and  76 ). 
     In the above-described embodiments, the GM cryocooler has been described as an example. However, the above-described design of the collar bumper type having the communication passage  80  is also applicable to other gas-driven cryocoolers. In that case, the terms “displacer” and “drive piston” in the above description may respectively mean a “first piston” and a “second piston”. 
     The present invention can be used in a field of cryocoolers. 
     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.