Patent Publication Number: US-11384963-B2

Title: GM cryocooler

Description:
RELATED APPLICATIONS 
     Priority is claimed to Japanese Patent Application No. 2016-232916, filed Nov. 30, 2016 and Japanese Patent Application No. 2017-134376, filed Jul. 10, 2017, and International Patent Application No. PCT/JP2017/042656, the entire content of each of which is incorporated herein by reference. 
     BACKGROUND 
     Technical Field 
     Certain embodiments of the present invention relate to a Gifford-McMahon (GM) cryocooler. 
     Description of Related Art 
     GM cryocoolers are roughly divided into two types, a motor driven type and a gas driven type depending on drive sources thereof. In the motor driven type, a displacer is mechanically coupled 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 invention, a GM cryocooler includes a displacer that is reciprocatable in an axial direction; a displacer cylinder that houses the displacer; a drive piston that is coupled to the displacer so as to drive the displacer in the axial direction; and a piston cylinder that houses the drive piston and that includes a drive chamber of which a pressure is controlled to drive the drive piston, and a gas spring chamber which is airtightly formed with respect to the displacer cylinder and is partitioned from the drive chamber by the drive piston. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view illustrating a GM cryocooler related to a first embodiment. 
         FIG. 2  is a view illustrating an example of the operation of the GM cryocooler. 
         FIG. 3  is a schematic view illustrating a GM cryocooler related to a second embodiment. 
         FIG. 4  is a schematic view illustrating a GM cryocooler related to a third embodiment. 
         FIG. 5  is a schematic view illustrating the GM cryocooler related to the third embodiment. 
         FIG. 6  is a schematic view illustrating the GM cryocooler related to the third embodiment. 
         FIG. 7  is a schematic view illustrating the GM cryocooler related to the third embodiment. 
         FIG. 8  is a schematic view illustrating the GM cryocooler related to the third embodiment. 
         FIG. 9  is a schematic view illustrating the GM cryocooler related to the third embodiment. 
         FIGS. 10A and 10B  are schematic views illustrating the GM cryocooler related to the third embodiment. 
         FIG. 11  is a schematic view illustrating the GM cryocooler related to the third embodiment. 
         FIG. 12  is a schematic view illustrating the GM cryocooler related to the third embodiment. 
         FIG. 13  is a schematic view illustrating the GM cryocooler related to the third embodiment. 
         FIG. 14  is a schematic view illustrating the GM cryocooler related to the third embodiment. 
         FIG. 15  is a schematic view illustrating the GM cryocooler related to the third embodiment. 
         FIG. 16  is a schematic view illustrating the GM cryocooler related to the third embodiment. 
         FIGS. 17A and 17B  are schematic views illustrating the GM cryocooler related to the third embodiment. 
         FIG. 18  is a schematic view illustrating the GM cryocooler related to the third embodiment. 
         FIG. 19  is a schematic view illustrating a GM cryocooler related to a fourth embodiment. 
         FIG. 20  is a schematic view illustrating a GM cryocooler related to a fourth embodiment. 
         FIG. 21  is a schematic view illustrating the GM cryocooler related to the fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the case of the motor driven type, a stroke of the displacer is determined by a coupling mechanism. Therefore, it is easy to design the motor-driven GM cryocooler so as for the displacer not collide against a cylinder. For example, if a slight gap is provided between a bottom dead center of the displacer and a bottom surface of the cylinder, a collision between the displacer and the cylinder is avoided. Meanwhile, in typical gas-driven GM cryocoolers, the displacer continues moving due to the action of the gas pressure until the displacer collides against or come into contact with the bottom surface of the cylinder. The collision or contact of the displacer with the cylinder may become a cause of vibration or abnormal noise. 
     It is desirable to reduce vibration or abnormal noise of a gas-driven GM cryocooler. 
     In addition, optional combinations of the above constituent elements and those obtained by substituting the constituent elements or expressions of the invention with each other among methods, devices, systems, and the like are also effective as aspects of the inventions. 
     According to the invention, vibration or abnormal noise of the gas-driven GM cryocooler can be reduced. 
     Hereinafter, embodiments for carrying out the invention will be described in detail. In addition, the configuration to be described below is merely exemplary and does not limit the range of the invention at all. Additionally, in the description of the drawing, the same elements will be designated by the same reference signs, and the duplicate description thereof will be appropriately omitted. Additionally, in the drawings to be referred to in the following description, the size and thickness of respective constituent members are for convenience of description, and do not necessarily indicate actual dimensions and ratios. 
     First Embodiment 
       FIG. 1  is a schematic view illustrating a GM cryocooler  10  related to a first embodiment. 
     The GM cryocooler  10  includes a compressor  12  that compresses a working gas (for example, helium gas), and a cold head  14  that cools the working gas by adiabatic expansion. The cold head  14  is also referred to as an expander. As will be described below in detail, the compressor  12  supplies a high-pressure working gas to the cold head  14 . The cold head  14  is provided with a regenerator  15  that pre-cools the working gas. The pre-cooled working gas is further cooled due to expansion within the cold head  14 . The working gas is collected in the compressor  12  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 collected working gas and supplies the compressed working gas to the cold head  14  again. 
     The cold head  14  illustrated is of a single stage type. However, the cold head  14  may be of a multistage type. 
     The cold head  14  is of a gas driven type. Therefore, the cold head  14  includes an axial movable body  16  serving as a free piston to be driven by a gas pressure, and a cold head housing  18  that is airtightly configured and houses the axial movable body  16 . The cold head housing  18  supports the axial movable body  16  so as to be reciprocatable in an axial direction. Unlike a motor-driven GM cryocooler, the cold head  14  does not have a motor that drives the axial movable body  16 , and a coupling mechanism (for example, a scotch yoke mechanism). 
     The axial movable body  16  includes a displacer  20  that is reciprocatable in the axial direction (an upward-downward direction, indicated by an arrow C illustrate  FIG. 1 ), and a drive piston  22  coupled to the displacer  20  such that the displacer  20  is driven in the axial direction. The drive piston  22  is disposed coaxially with the displacer  20  and apart therefrom in the axial direction. 
     The cold head housing  18  includes a displacer cylinder  26  that houses the displacer  20 , and a piston cylinder  28  that houses the drive piston  22 . The piston cylinder  28  is disposed coaxially with the displacer cylinder  26  and adjacent thereto in the axial direction. 
     Although described below in detail, a drive unit of the cold head  14  that is of the gas driven type is configured to include the drive piston  22  and the piston cylinder  28 . Additionally, the cold head  14  includes a gas spring mechanism that acts on the drive piston  22  so as to alleviate or prevent a collision or contact between the displacer  20  and the displacer cylinder  26 . 
     Additionally, the axial movable body  16  includes a coupling rod  24  that rigidly couples the displacer  20  to the drive piston  22  such that the displacer  20  reciprocates in the axial direction integrally with the drive piston  22 . The coupling rod  24  also extends from the displacer  20  to the drive piston  22  coaxially with the displacer  20  and the drive piston  22 . 
     The drive piston  22  has dimensions smaller than the displacer  20 . The axial length of the drive piston  22  is shorter than that of the displacer  20 , and the diameter of the drive piston  22  is also smaller than that of the displacer  20 . The diameter of the coupling rod  24  is smaller than that of the drive piston  22 . 
     The 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 also smaller than that of the displacer cylinder  26 . 
     In addition, a dimensional relationship between the drive piston  22  and the displacer  20  is not limited to the above-described one, and may be different from that. Similarly, the dimensional relationship between the piston cylinder  28  and the displacer cylinder  26  is not limited to the above-described one, and may be different from that. 
     The axial reciprocation of the displacer  20  is guided by the displacer cylinder  26 . Typically, the displacer  20  and the displacer cylinder  26  are respectively cylindrical members that extend in the axial direction, and the internal diameter of the displacer cylinder  26  coincides with or is slightly larger than the external diameter of the displacer  20 . Similarly, the axial reciprocation of the drive piston  22  is guided by the piston cylinder  28 . Typically, the drive piston  22  and the piston cylinder  28  are respectively cylindrical members that extend in the axial direction, and the internal diameter of the piston cylinder  28  coincide with or is slightly larger than the external diameter of the drive piston  22 . 
     Since the displacer  20  and the drive piston  22  are rigidly coupled to each other by the coupling rod  24 , the axial stroke of the drive piston  22  is equal to the axial stroke of the displacer  20 , and both the displacer and the drive piston move integrally over the entire stroke. The position of the drive piston  22  with respect to the displacer  20  is invariable during the axial reciprocation of the axial movable body  16 . 
     Additionally, the cold head housing  18  includes a coupling rod guide  30  that connects the displacer cylinder  26  to the piston cylinder  28 . The coupling rod guide  30  extends from the displacer cylinder  26  to the piston cylinder  28  coaxially with the displacer cylinder  26  and the piston cylinder  28 . The coupling rod  24  passes through the coupling rod guide  30 . The coupling rod guide  30  is configured as a bearing that guides the axial reciprocation of the coupling rod  24 . 
     The displacer cylinder  26  is airtightly coupled with the piston cylinder  28  via the coupling rod guide  30 . In this way, the cold head housing  18  is configured as a pressure vessel for the working gas. In addition, the coupling rod guide  30  may be regarded as being a portion of the displacer cylinder  26  or the piston cylinder  28 . 
     A first seal part  32  is provided between the coupling rod  24  and the coupling rod guide  30 . The first seal part  32  is mounted on any one of the coupling rod  24  or the coupling rod guide  30 , and slides on the other of the coupling rod  24  or the coupling rod guide  30 . The first seal part  32  is constituted of, for example, a seal member, such as a slipper seal or an O-ring. The piston cylinder  28  is airtightly configured with respect to the displacer cylinder  26  by the first seal part  32 . In this way, the piston cylinder  28  is fluidly isolated from the displacer cylinder  26 , and a direct gas flow between the piston cylinder  28  and the displacer cylinder  26  is not generated. 
     The displacer cylinder  26  is partitioned into an expansion chamber  34  and a room temperature chamber  36  by the displacer  20 . The displacer  20  forms the expansion chamber  34  between the displacer  20  and the displacer cylinder  26  at one axial end thereof, and forms the room temperature chamber  36  between the displacer  20  and the displacer cylinder  26  at the other axial end thereof. The expansion chamber  34  is disposed on a bottom dead center LP 1  side, and the room temperature chamber  36  is disposed on a top dead center UP 1  side. Additionally, the cold head  14  is provided with a cooling stage  38  anchored to the displacer cylinder  26  so as to envelop the expansion chamber  34 . 
     The regenerator  15  is built in the displacer  20 . The displacer  20  has an inlet flow path  40 , which allows the regenerator  15  to communicate with the room temperature chamber  36 , at an upper lid part thereof. Additionally, the displacer  20  has an outlet flow path  42 , which allows the regenerator  15  to communicate with the expansion chamber  34 , at a tube part thereof. Alternatively, the outlet flow path  42  may be provided at a lower lid part of the displacer  20 . In addition, the displacer  20  includes an inlet flow straightener  41  inscribed on the upper lid part, and an outlet flow straightener  43  inscribed on the lower lid part. The regenerator  15  is sandwiched between a pair of such flow straighteners. 
     A second seal part  44  is provided between the displacer  20  and the displacer cylinder  26 . The second seal part  44  is, for example, a slipper seal and is mounted on the tube part or the upper lid part of the displacer  20 . Since a clearance between the displacer  20  and the displacer cylinder  26  is sealed by the second seal part  44 , there is no direct gas flow (that is, a gas flow that bypasses the regenerator  15 ) between the room temperature chamber  36  and the expansion chamber  34 . 
     When the displacer  20  moves in the axial direction, the expansion chamber  34  and the room temperature chamber  36  are complementarily increased or decreased in volume. That is, when the displacer  20  moves downward, the expansion chamber  34  becomes narrow and the room temperature chamber  36  becomes wide. The reverse is also the same. 
     The working gas flows from the room temperature chamber  36  through the inlet flow path  40  into the regenerator  15 . More exactly, the working gas flows from the inlet flow path  40  through the inlet flow straightener  41  into the regenerator  15 . The working gas flows from the regenerator  15  via the outlet flow straightener  43  and the outlet flow path  42  into the expansion chamber  34 . When the working gas returns from the expansion chamber  34  to the room temperature chamber  36 , the working gas passes through a reverse route. That is, the working gas returns from the expansion chamber  34  through the outlet flow path  42 , the regenerator  15 , and the inlet flow path  40  to the room temperature chamber  36 . The working gas to bypass the regenerator  15  and flow through the clearance is blocked by the second seal part  44 . 
     The piston cylinder  28  includes a drive chamber  46  of which the pressure is controlled so as to drive the drive piston  22 , and a gas spring chamber  48  partitioned from the drive chamber  46  by the drive piston  22 . The drive piston  22  forms the drive chamber  46  between the drive piston  22  and the piston cylinder  28  at one axial end thereof, and forms the gas spring chamber  48  between the drive piston  22  and the piston cylinder  28  at the other axial end thereof. When the drive piston  22  moves in the axial direction, the drive chamber  46  and the gas spring chamber  48  are complementarily increased or decreased in volume. 
     The drive chamber  46  is disposed opposite to the displacer cylinder  26  in the axial direction with respect to the drive piston  22 . The gas spring chamber  48  is disposed on the same side as the displacer cylinder  26  in the axial direction with respect to the drive piston  22 . In other words, the drive chamber  46  is disposed on a top dead center UP 2  side, and the gas spring chamber  48  is disposed on a bottom dead center LP 2  side. An upper surface of the drive piston  22  receives the gas pressure of the drive chamber  46 , and a lower surface of the drive piston  22  receives the gas pressure of the gas spring chamber  48 . 
     The coupling rod  24  extends from the lower surface of the drive piston  22  through the gas spring chamber  48  to the coupling rod guide  30 . Moreover, the coupling rod  24  extends to the upper lid part of the displacer  20  through the room temperature chamber  36 . The gas spring chamber  48  is disposed on the same side as the coupling rod  24  with respect to the drive piston  22 , and the drive chamber  46  is disposed opposite to the coupling rod  24  with respect to the drive piston  22 . 
     A third seal part  50  is provided between the drive piston  22  and the piston cylinder  28 . The third seal part  50  is, for example, a slipper seal and is mounted on a side surface of the drive piston  22 . Since a clearance between the drive piston  22  and the piston cylinder  28  is sealed by the third seal part  50 , there is no direct gas flow between the drive chamber  46  and the gas spring chamber  48 . Additionally, since the first seal part  32  is provided, there is also no gas flow between the gas spring chamber  48  and the room temperature chamber  36 . In this way, the gas spring chamber  48  is airtightly formed with respect to the displacer cylinder  26 . The gas spring chamber  48  is sealed by the first seal part  32  and the third seal part  50 . 
     When the drive piston  22  moves downward, the gas spring chamber  48  becomes narrow. In this case, the gas of the gas spring chamber  48  is compressed, and the pressure thereof is increased. The pressure of the gas spring chamber  48  acts on the lower surface of the drive piston  22  upward. Therefore, the gas spring chamber  48  generates a gas spring force that resists the downward movement of the drive piston  22 . 
     On the contrary, when the drive piston  22  moves upward, the gas spring chamber  48  becomes wide. The pressure of the gas spring chamber  48  drops, and the gas spring force acting on the drive piston  22  also becomes small. In addition, in this case, the drive chamber  46  becomes narrow. Therefore, while a second intake valve V 3  and a second exhaust valve V 4  are closed, the drive chamber  46  can also be regarded as a second gas spring chamber that generates a downward gas spring force that resists the upward movement of the drive piston  22 . 
     The cold head  14  is installed in the illustrated orientation in a field where the cold head  14  is to be used. That is, the cold head  14  is installed in a vertical orientation such that the displacer cylinder  26  is disposed on a vertically lower side and the piston cylinder  28  is disposed on a vertically upper side. In this way, when the cooling stage  38  is installed in a posture that faces the vertically lower side, the cryocooling capacity of the GM cryocooler  10  becomes the highest. However, the disposition of the GM cryocooler  10  is not limited to this. On the contrary, the cold head  14  may be installed in a posture in which the cooling stage  38  faces the vertically upper side. Alternatively, the cold head  14  may be installed sideways or in other orientations. 
     Moreover, the GM cryocooler  10  includes a working gas circuit  52  that connects the compressor  12  to the cold head  14 . The working gas circuit  52  is configured so as to cause a pressure difference between the piston cylinder  28  (that is, the drive chamber  46 ) and the displacer cylinder  26  (that is, the expansion chamber  34  and/or the room temperature chamber  36 ). The axial movable body  16  moves in the axial direction due to the pressure difference. If the pressure of the displacer cylinder  26  is lower than that of the piston cylinder  28 , the drive piston  22  moves downward, and the displacer  20  also moves downward along with this. On the contrary, if the pressure of the displacer cylinder  26  is higher than that of the piston cylinder  28 , the drive piston  22  moves upward, and the displacer  20  also moves upward along with this. 
     The working gas circuit  52  includes a valve unit  54 . The valve unit  54  includes a first intake valve V 1 , a first exhaust valve V 2 , the second intake valve V 3 , and the second exhaust valve V 4 . The second intake valve V 3  and the second exhaust valve V 4  may also be respectively referred to as a high-pressure valve and a low-pressure valve for driving the drive piston  22 . 
     The valve unit  54  may be disposed in the cold head housing  18  and may be connected to the compressor  12  by piping. The valve unit  54  may be disposed outside the cold head housing  18  and may be connected to the compressor  12  and the cold head  14 , respectively, by piping. 
     The valve unit  54  may take a rotary valve type. That is, the valve unit  54  may be configured such that the valves V 1  to V 4  are appropriately switched depending on rotational sliding of a valve disc with respect to a valve body. In that case, the valve unit  54  may include a rotational driving source  56  for rotationally driving the valve unit  54  (for example, the valve disc). The rotational driving source  56  is a motor. However, the rotational driving source  56  is not connected to the axial movable body  16 . Additionally, the valve unit  54  may include a control unit  58  that controls the valve unit  54 . The control unit  58  may control the rotational driving source  56 . 
     In a certain embodiment, the valve unit  54  includes a plurality of individually controllable valves V 1  to V 4 , and the control unit  58  may control opening and closing of the respective valves V 1  to V 4 . In this case, the valve unit  54  may not include the rotational driving source  56 . 
     The first intake valve V 1  is disposed in a first intake flow path  60  that connects a discharge port of the compressor  12  to the room temperature chamber  36  of the cold head  14 . The first exhaust valve V 2  is disposed in a first exhaust flow path  62  that connects an intake port of the compressor  12  to the room temperature chamber  36  of the cold head  14 . As illustrated, a portion of the first exhaust flow path  62  may be shared with the first intake flow path  60  on the room temperature chamber  36  side, and the remaining portion of the first exhaust flow path  62  may branch from the first intake flow path  60  on the valve unit  54  side. 
     The second intake valve V 3  is disposed in a second intake flow path  64  that connects the discharge port of the compressor  12  to the drive chamber  46  of the piston cylinder  28 . As illustrated, a portion of the second intake flow path  64  may be shared with the first intake flow path  60  on the compressor  12  side. The second exhaust valve V 4  is disposed in a second exhaust flow path  66  that connects the intake port of the compressor  12  to the drive chamber  46  of the piston cylinder  28 . As illustrated, a portion of second exhaust flow path  66  may be shared with the second intake flow path  64  on the drive chamber  46  side, and the remaining portion of the second exhaust flow path  66  may branch from the second intake flow path  64  on the valve unit  54  side. Additionally, a portion of second exhaust flow path  66  may be shared with the first exhaust flow path  62  on the compressor  12  side. 
       FIG. 2  is a view illustrating an example of the operation of the GM cryocooler  10 . Since one cycle of the axial reciprocation of the axial movable body  16  is represented in correspondence with 360 degrees in  FIG. 2 , 0 degree corresponds to a start point of the cycle, and 360 degrees corresponds to an endpoint of the cycle. 90 degrees, 180 degrees, and 270 degrees correspond to ¼ cycle, half cycle, and ¾ cycle, respectively. 
     In addition, valve timings illustrated in  FIG. 2  are also applicable to those of second to fourth embodiments to be described below as well as the first embodiment. 
     A first intake period A 1  and a first exhaust period A 2  of the cold head  14  and a second intake period A 3  and a second exhaust period A 4  of the drive chamber  46  are illustrated in  FIG. 2 . The first intake period A 1 , the first exhaust period A 2 , the second intake period A 3 , and the second exhaust period A 4  are determined by the first intake valve V 1 , the first exhaust valve V 2 , the second intake valve V 3 , and the second exhaust valve V 4 , respectively. 
     In the first intake period A 1  (that is, when the first intake valve V 1  is open), the working gas flows from the discharge port of the compressor  12  to the room temperature chamber  36 . Conversely, when the first intake valve V 1  is closed, supply of the working gas from the compressor  12  to the room temperature chamber  36  is stopped. In the first exhaust period A 2  (that is, when the first exhaust valve V 2  is open), the working gas flows from the room temperature chamber  36  to the intake port of the compressor  12 . When the first exhaust valve V 2  is closed, the collection of the working gas from the room temperature chamber  36  to the compressor  12  is stopped. 
     In the second intake period A 3  (that is, when the second intake valve V 3  is open), the working gas flows from the discharge port of the compressor  12  to the drive chamber  46 . When the second intake valve V 3  is closed, the supply of the working gas from the compressor  12  to the drive chamber  46  is stopped. In the second exhaust period A 4  (that is, when the second exhaust valve V 4  is open), the working gas flows from the drive chamber  46  to the intake port of the compressor  12 . When the second exhaust valve V 4  is closed, the collection of the working gas from the drive chamber  46  to the compressor  12  is stopped. 
     In an example illustrated in  FIG. 2 , the first intake period A 1  and the second exhaust period A 4  are within a range of 0 degree to 135 degrees, and the first exhaust period A 2  and the second intake period A 3  are within a range of 180 degrees to 315 degrees. The first intake period A 1  alternates with and does not overlap the first exhaust period A 2 , and the second intake period A 3  alternates and does not overlap the second exhaust period A 4 . The first intake period A 1  overlaps the second exhaust period A 4 , and the first exhaust period A 2  overlaps the second intake period A 3 . At 0 degree, the displacer  20  and the drive piston  22  are located at or near the bottom dead centers LP 1  and LP 2 , respectively, and at 180 degrees, the displacer  20  and the drive piston  22  are located at or near the top dead centers UP 1  and UP 2 , respectively. 
     The operation of the GM cryocooler  10  having the above configuration will be described. When the displacer  20  is located at or near the bottom dead center LP 1 , the first intake period A 1  is started (0 degree of  FIG. 2 ). The first intake valve V 1  is opened, and a high-pressure gas is supplied from the discharge port of the compressor  12  to the room temperature chamber  36  of the cold head  14 . The gas is cooled while passing through the regenerator  15 , and enters the expansion chamber  34 . 
     The second exhaust period A 4  is also started simultaneously with the first intake period A 1  (0 degree of  FIG. 2 ). The second exhaust valve V 4  is opened, and the drive chamber  46  of the piston cylinder  28  is connected to the intake port of the compressor  12 . Therefore, the drive chamber  46  has a pressure lower than the room temperature chamber  36  and the expansion chamber  34 . The drive piston  22  moves from the bottom dead center LP 2  toward the top dead center UP 2 . 
     The displacer  20  also moves from the bottom dead center LP 1  toward the top dead center UP 1  together with the drive piston  22 . The first intake valve V 1  is closed, and the first intake period A 1  is ended (135 degrees of  FIG. 2 ). The second exhaust valve V 4  is closed, and the second exhaust period A 4  is ended (135 degrees of  FIG. 2 ). The drive piston  22  and the displacer  20  continue moving toward the top dead centers UP 1  and UP 2 , respectively. In this way, the expansion chamber  34  is increased in volume and filled with the high-pressure gas. 
     When the displacer  20  is located at or near the top dead center UP 1 , the first exhaust period A 2  is started (180 degrees of  FIG. 2 ). The first exhaust valve V 2  is opened, and the cold head  14  is connected to the intake port of the compressor  12 . The high-pressure gas is expanded by the expansion chamber  34  and is cooled. The expanded gas is collected in the compressor  12  through the room temperature chamber  36  while cooling the regenerator  15 . 
     The second intake period A 3  is also started together with the first exhaust period A 2  (180 degrees of  FIG. 2 ). The second intake valve V 3  is opened, and a high-pressure gas is supplied from the discharge port of the compressor  12  to the drive chamber  46  of the piston cylinder  28 . Therefore, the drive chamber  46  has a pressure higher than the room temperature chamber  36  and the expansion chamber  34 . The drive piston  22  moves from the top dead center UP 2  toward the bottom dead center LP 2 . 
     The displacer  20  also moves from the top dead center UP 1  toward the bottom dead center LP 1  together with the drive piston  22 . The first exhaust valve V 2  is closed, and the first exhaust period A 2  is ended (315 degrees of  FIG. 2 ). The second intake valve V 3  is closed, and the second intake period A 3  is ended (315 degrees of  FIG. 2 ). The drive piston  22  and the displacer  20  continue moving toward the bottom dead centers LP 1  and LP 2 . In this way, the low-pressure gas is discharged while the volume of the expansion chamber  34  is decreased. 
     The cold head  14  cools the cooling stage  38  by repeating such a cooling cycle (that is, a GM cycle). Accordingly, the GM cryocooler  10  can cool a superconducting device or other objects to be cooled (not illustrated) that are thermally combined with the cooling stage  38 . 
     As described above, since the cold head  14  is installed in a posture in which the cooling stage  38  faces the vertical lower side, gravity acts downward as indicated by an arrow D. For that reason, the weight of the axial movable body  16  acts to assist in the downward driving force of the drive piston  22 . A larger driving force acts on the drive piston  22  during the downward movement compared to during the upward movement. Therefore, in the typical gas-driven GM cryocooler, a collision or contact between a displacer and a displacer cylinder easily occurs at a bottom dead center of the displacer. 
     However, the cold head  14  is provided with the gas spring chamber  48 . The gas stored in the gas spring chamber  48  is compressed when the drive piston  22  moves downward, and the pressure thereof is increased. Since this pressure acts in a direction opposite to gravity, the driving force that acts on the drive piston  22  becomes small. The speed just before the drive piston  22  reaches the bottom dead center LP 2  can be reduced. 
     In this way, a contact or collision between the drive piston  22  and the piston cylinder  28  and/or between the displacer  20  and the displacer cylinder  26  can be avoided. Alternatively, since collision energy is reduced due to speed reduction of the drive piston  22 , for example, even if a collision has occurred, collision sound is suppressed. 
     Second Embodiment 
       FIG. 3  is a schematic view illustrating a GM cryocooler  10  related to a second embodiment. The GM cryocooler  10  related to the second embodiment is the same as the GM cryocooler  10  related to the first embodiment except that a flow path resistance part  68  that allows the gas spring chamber  48  to communicate with the drive chamber  46  is added. 
     The GM cryocooler  10  includes a pressure release path  70  that allows the gas spring chamber  48  to communicate with the drive chamber  46  such that the gas pressure is released from the gas spring chamber  48  to the drive chamber  46 . The pressure release path  70  is provided in the piston cylinder  28  so as to shunt the gas spring chamber  48  to the drive chamber  46 . The flow path resistance part  68 , such as an orifice, is disposed in the middle of the pressure release path  70 . 
     In addition, as indicated by a dashed line in  FIG. 3 , the pressure release path  70  and the flow path resistance part  68  may be provided in the drive piston  22 . 
     Even in this way, similarly to the first embodiment, the gas stored in the gas spring chamber  48  is compressed when the drive piston  22  moves downward, and the pressure thereof is increased. A contact or collision between the axial movable body  16  and the cold head housing  18  is suppressed, and vibration or abnormal noise of the GM cryocooler  10  can be reduced. 
     Since the flow path resistance part  68  is provided, in a case where the drive piston  22  excessively moves downward and the pressure of the gas spring chamber  48  is excessively raised, the pressure can be released from the gas spring chamber  48  to the drive chamber  46 . Therefore, the piston cylinder  28  is protected. 
     Third Embodiment 
       FIGS. 4 to 16  are schematic views illustrating a GM cryocooler  10  related to a third embodiment. The GM cryocooler  10  related to the third embodiment is the same as the GM cryocooler  10  related to the first embodiment except that the clearance between the drive piston  22  and the piston cylinder  28  is utilized as a flow path resistance part. Therefore, the third seal part  50  is not provided unlike the first embodiment. The gas spring chamber  48  is not sealed. 
     As illustrated in  FIG. 4 , the GM cryocooler  10  includes a radial clearance  72  serving as the flow path resistance part. The gas spring chamber  48  is allowed to communicate with the drive chamber  46  through the radial clearance  72 . The radial clearance  72  is formed between the drive piston  22  and the piston cylinder  28 . That is, the radial clearance  72  is a radial gap that is determined depending on the external diameter of the drive piston  22  and the internal diameter of the piston cylinder  28 . The radial clearance  72  is made constant in the axial direction. Even in this way, similarly to the like above-described respective embodiments, vibration or abnormal noise of the GM cryocooler  10  can be reduced. 
     As illustrated in  FIG. 5 , the piston cylinder  28  may include a tubular guide member  28   a , for example, a guide bush. As the drive piston  22  slides along an inner peripheral surface of the guide member  28   a , the guide member  28   a  can guide the drive piston  22  in the axial direction. In order to realize excellent slidability with the drive piston  22 , the guide member  28   a  is formed of, for example, an appropriate resin material. The guide member  28   a  may be disposed in the piston cylinder  28  so as to guide the drive piston  22  over the entire axial stroke of the drive piston  22 . The guide member  28   a  surrounds the gas spring chamber  48 . The gas spring chamber  48  is formed by the drive piston  22  and the guide member  28   a.    
     In order for the radial clearance  72  to function as an effective seal between the drive piston  22  and the piston cylinder  28  (or the guide member  28   a ), it is desirable that the radial width of the radial clearance  72  is 0.1 mm or less. From a viewpoint of easy manufacture, it is desirable that the radial width of the radial clearance  72  is 0.01 mm or more. 
     The radial clearance  72  may vary continuously or stepwise in the axial direction. Accordingly, the flow path resistance of the radial clearance  72  may vary depending on the axial position of the drive piston  22  with respect to the piston cylinder  28 . Generally, the value of the flow path resistance is uniquely determined mainly from the shapes and dimensions of flow paths. 
     For example, the radial clearance  72  may have a first flow path resistance R 1  when the drive piston  22  is at a first axial position (for example, the bottom dead center LP 2 ), and may have a second flow path resistance R 2  when the drive piston  22  is at a second axial position (for example, the top dead center UP 2 ). Here, the first axial position may be closer to the bottom dead center LP 2  of the drive piston  22  than the second axial position, and the first flow path resistance R 1  may be larger than the second flow path resistance R 2 . In this way, a flow path resistance when the drive piston  22  is located at or near the bottom dead center LP 2  can be made larger than a flow path resistance when the drive piston  22  is located at or near the top dead center UP 2 . As a result, the gas spring chamber  48  can more effectively generate the gas spring force that resists the downward movement of the drive piston  22 , at or near the bottom dead center LP 2  of the drive piston  22 . 
     As illustrated in  FIG. 6 , the radial clearance  72  may become stepwise narrower axially downward. Therefore, an inner peripheral surface of the piston cylinder  28  may be formed in a conical shape. In this way, the radial clearance  72  may vary continuously in the axial direction. 
     As illustrated in  FIG. 7 , the radial clearance  72  includes a radial clearance upper part  72   a  having a second flow path resistance R 2 , and a radial clearance lower part  72   b  having the first flow path resistance R 1 . As described above, the first flow path resistance R 1  is larger than the second flow path resistance R 2 . The radial clearance lower part  72   b  is adjacent to the radial clearance upper part  72   a  in the axial direction. Therefore, the gas spring chamber  48  is allowed to communicate with the drive chamber  46  through the radial clearance upper part  72   a  and the radial clearance lower part  72   b . The radial widths of the radial clearance upper part  72   a  and the radial clearance lower part  72   b  are, for example, within a range of 0.01 to 0.1 mm. 
     The piston cylinder  28  includes a stepped part  74  to be a boundary between the radial clearance upper part  72   a  and the radial clearance lower part  72   b . The piston cylinder  28  has a first internal diameter axially above the stepped part  74 , and the piston cylinder  28  has a second internal diameter smaller than the first internal diameter, axially below the stepped part  74 . Both the first internal diameter and the second internal diameter are larger than the external diameter of the drive piston  22 . Therefore, the radial width of the radial clearance lower part  72   b  is narrower than the radial width of the radial clearance upper part  72   a . In this way, the radial clearance  72  may vary stepwise in the axial direction. 
     As illustrated in  FIG. 8 , the drive piston  22  may include a communication path  76  that allows the gas spring chamber  48  to communicate with the radial clearance  72 . The communication path  76  is a through-hole formed in the drive piston  22 , and has an outlet  76   a  directed to the inner peripheral surface of the piston cylinder  28 . 
     The communication path  76  is formed in the drive piston  22  so as to allow the gas spring chamber  48  to communicated with the radial clearance upper part  72   a  therethrough when the drive piston  22  is at the bottom dead center LP 2  and allow the gas spring chamber  48  to communicate with the radial clearance lower part  72   b  therethrough when the drive piston  22  is at the top dead center UP 2 . In other words, the outlet  76   a  is disposed so as to be located below the stepped part  74  in the axial direction when the drive piston  22  is at the bottom dead center LP 2  and be located above the stepped part  74  in the axial direction when the drive piston  22  is at the top dead center UP 2 . 
     In this case, the drive piston  22  can also be considered to constitute a flow rate control valve in cooperation with the piston cylinder  28 . When the outlet  76   a  is located below the stepped part  74 , the gas spring chamber  48  is allowed to communicate with the drive chamber  46  through the radial clearance lower part  72   b  (and the radial clearance upper part  72   a ). Since the flow path resistance of the radial clearance lower part  72   b  is large, the flow rate from the gas spring chamber  48  to the drive chamber  46  is limited. On the contrary, when the outlet  76   a  is located above the stepped part  74 , the gas spring chamber  48  is allowed to communicate with the drive chamber  46  through the radial clearance upper part  72   a . Since the flow path resistance of the radial clearance upper part  72   a  is small, the flow rate from the gas spring chamber  48  to the drive chamber  46  is increased. 
     It is desirable that the timing at which the outlet  76   a  passes by the stepped part  74  during the downward movement of the drive piston  22  is in a central region B of the first intake period A 1  (an arrow indicated by  FIG. 2 ). The central region B may be, for example, ¼ to ¾ of the first intake period A 1 . In this way, the gas spring force can be increased between the top dead center UP 2  and the bottom dead center LP 2  of the drive piston  22 . 
     As illustrated in  FIG. 9 , the communication path  76  may be a longitudinal groove formed in an outer peripheral surface of the drive piston  22 . The longitudinal groove extends in the axial direction from the gas spring chamber  48  to a central part of the drive piston  22 . 
     In  FIGS. 8 and 9 , the radial clearance lower part  72   b  may be extremely narrow or may be omitted. The third seal part  50  illustrated in  FIG. 1  may be provided at the radial clearance lower part  72   b . Additionally, although the number of communication paths  76  is one in the above-described example, a plurality of the communication paths  76  may be provided in the drive piston  22 . In that case, the communication paths  76  may be formed at equal intervals of angles in a circumferential direction of the drive piston  22 . 
     As illustrated in  FIG. 10A , the radial clearance  72  serving as a flow path resistance part may include a buffer volume part  96  that communicates with the radial clearance  72 . The buffer volume part  96  is formed between the piston cylinder  28  and the drive piston  22 . 
     The buffer volume part  96  is a groove or recess formed over the entire circumference on the side surface (outer peripheral surface) of the drive piston  22 . A depth Dl of the buffer volume part  96  is larger than a radial width t of the radial clearance  72 . For example, the depth D 1  of the buffer volume part  96  may be 10 or more times the radial width t of the radial clearance  72 . 
     The buffer volume part  96  is disposed at an axial intermediate part on the side surface of the drive piston  22 , and communicates with the radial clearance upper part  72   a  and the radial clearance lower part  72   b . The radial clearance upper part  72   a  and the radial clearance lower part  72   b  communicate with each other via the buffer volume part  96 . In this example, although the radial widths of the radial clearance upper part  72   a  and the radial clearance lower part  72   b  are equal to each other, this is not essential, and the radial widths may be different from each other. 
     In this way, the buffer volume part  96  is connected to each of the drive chamber  46  and the gas spring chamber  48  through the radial clearance  72 . The buffer volume part  96  is not directly connected to the drive chamber  46  and the gas spring chamber  48 . 
     Since the buffer volume part  96  communicates with the drive chamber  46  and the gas spring chamber  48  through the radial clearance  72 , the buffer volume part  96  can take an intermediate pressure between the drive chamber  46  and the gas spring chamber  48 . When the drive chamber  46  is at a high pressure, gas may flow from the drive chamber  46  through the radial clearance upper part  72   a  into the buffer volume part  96 . While the intermediate pressure of the buffer volume part  96  is lower than the high pressure of the drive chamber  46 , the buffer volume part  96  can receive and temporarily store an incoming gas. Therefore, compared to a case where there is no buffer volume part  96 , the flow rate of the gas that passes through the radial clearance  72  from the drive chamber  46  to the gas spring chamber  48  is suppressed. On the contrary, when the gas spring chamber  48  is at a high pressure, the buffer volume part  96  can receive the gas that flows in from the gas spring chamber  48  through the radial clearance lower part  72   b . Compared to a case where there is no buffer volume part  96 , the flow rate of the gas that passes through the radial clearance  72  from the drive chamber  46  to the gas spring chamber  48  is suppressed. 
     In this way, the buffer volume part  96  has an effect of suppressing the flow rate of the gas that passes through the radial clearance  72 . Hence, the buffer volume part  96  can reduce the influence on sealing performance resulting from the fluctuation of the radial width of the radial clearance  72 . Even if the radial width of the radial clearance  72  slightly deviates from design dimensions due to a manufacturing error, the fluctuation of the sealing performance of the radial clearance  72  is alleviated. It is easy to ensure the robustness of the radial clearance  72  when the GM cryocooler  10  is manufactured as a mass-produced product. 
     The shape of the buffer volume part  96  is optional. The buffer volume part  96  may be a groove or recess of any shape formed on the side surface of the drive piston  22 . For example, as illustrated in  FIG. 10B , the buffer volume part  96  may be a plurality of grooves formed on the side surface of the drive piston  22 . These grooves extend parallel to each other over the entire circumference on the side surface of the drive piston  22 . The buffer volume part  96  is connected to the drive chamber  46  and the gas spring chamber  48  through the radial clearance  72 . In this way, the plurality of buffer volume parts  96  may be aligned in the axial direction on the side surface of the drive piston  22 . Alternatively, instead of the plurality of grooves, the buffer volume part  96  may be one or a plurality of spiral grooves that are formed on the side surface of the drive piston  22 . The buffer volume part  96  may not essentially extend over the entire circumference of the drive piston  22 . For example, a plurality of recesses formed on the side surface of the drive piston  22  may be arranged in the circumferential direction. 
     As described with reference to  FIGS. 8 and 9 , in a case where the drive piston  22  is provided with the communication path  76 , the buffer volume part  96  is formed so as not to communicate with the communication path  76 . The buffer volume part  96  and the communication path  76  are separate gas spaces formed in the drive piston  22 . Therefore, there is no direct gas flow between the buffer volume part  96  and the communication path  76 . Therefore, the buffer volume part  96  is disposed on the side surface of the drive piston  22  so as to avoid the outlet  76   a  of the communication path  76 . For example, in a case where a plurality of outlets  76   a  is provided, the plurality of buffer volume parts  96  and the plurality of outlets  76   a  may be disposed alternately in the circumferential direction. Alternatively, the buffer volume part  96  may be disposed at a location different from the outlet  76   a  in the axial direction. 
     It is not essential that the buffer volume part  96  is provided in the drive piston  22 . The buffer volume part  96  may be provided in the piston cylinder  28  or may be provided, for example, on the inner peripheral surface of the guide member  28   a  illustrated in  FIG. 5 . 
     As illustrated in  FIG. 11 , the radial clearance  72  serving as a flow path resistance part may have the first flow path resistance R 1  when the drive piston  22  is at the first axial position (for example, the bottom dead center LP 2 ), may have the second flow path resistance R 2  when the drive piston  22  is at the second axial position (for example, the top dead center UP 2 ), and may have a third flow path resistance R 3  when the drive piston  22  is at a third axial position. Here, the third axial position may be located between the first axial position and the second axial position, and may be, for example, a midpoint MP between the bottom dead center LP 2  and top dead center UP 2 . That is, an axial distance from the bottom dead center LP 2  to the midpoint MP is equal to an axial distance from the top dead center UP 2  to the midpoint MP. 
     The third flow path resistance R 3  is smaller than the first flow path resistance R 1  and smaller than the second flow path resistance R 2 . Although the first flow path resistance R 1  may be larger than the second flow path resistance R 2  as described above, this is not essential, and the first flow path resistance R 1  may be smaller than the second flow path resistance R 2 . 
     In this way, when the drive piston  22  is located at or near the bottom dead center LP 2 , the gas spring chamber  48  can generate the gas spring force that resists the downward movement of the drive piston  22 . Additionally, when the drive piston  22  is located at or near the top dead center UP 2 , the drive chamber  46  serving as the second gas spring chamber can generate the gas spring force that resists the upward movement of the drive piston  22 . 
     In a case where the gas spring force is excessively strong, the upward and downward movements of the drive piston  22  are suppressed, and the stroke of the drive piston  22  becomes small. Along with this, the stroke of the displacer  20  also becomes small. This may lower PV (pressure-volume) work in the expansion chamber  34 , and thus, may affect the cryocooling capacity of the GM cryocooler  10 . As one of the measures of suppressing such an adverse effect, it is considered the stroke of the drive piston  22  is enlarged while lengthening the axial length of the piston cylinder  28 . As a result, however, the size of the GM cryocooler  10  may become large. 
     By making the third flow path resistance R 3  small as described above, the gas spring force acting on the drive piston  22  when the drive piston  22  moves by an intermediate part of the stroke thereof can be made small. Accordingly, the driving force of the displacer  20  resulting from the drive piston  22  becomes large, the stroke of the displacer  20  is maintained, and a decrease in cryocooling capacity of the GM cryocooler  10  can be suppressed. 
     As illustrated in  FIG. 11 , the radial clearance  72  may become stepwise wider axially downward from the drive chamber  46 . As illustrated in  FIG. 11 , the radial clearance  72  may become stepwise wider axially upward from the gas spring chamber  48 . In this way, the radial clearance  72  may vary continuously in the axial direction. 
     As illustrated in  FIG. 12 , the radial clearance  72  includes the radial clearance upper part  72   a  having the second flow path resistance R 2 , the radial clearance lower part  72   b  having the first flow path resistance R 1 , and a radial clearance intermediate part  72   c  having the third flow path resistance R 3 . The top dead center UP 2  of the drive piston  22  is located at the radial clearance upper part  72   a , the bottom dead center LP 2  of the drive piston  22  is located at the radial clearance lower part  72   b , and the midpoint MP of the drive piston  22  is located at the radial clearance intermediate part  72   c.    
     As described above, the third flow path resistance R 3  is smaller than the first flow path resistance R 1  and smaller than the second flow path resistance R 2 . The radial clearance intermediate part  72   c  is adjacent to the radial clearance upper part  72   a  in the axial direction. The radial clearance lower part  72   b  is adjacent to the radial clearance intermediate part  72   c  in the axial direction. Therefore, the gas spring chamber  48  is allowed to communicate with the drive chamber  46  through the radial clearance upper part  72   a , the radial clearance intermediate part  72   c , and the radial clearance lower part  72   b.    
     The piston cylinder  28  includes a first stepped part  92   a  to be a boundary between the radial clearance upper part  72   a  and the radial clearance intermediate part  72   c , and a second stepped part  92   b  to be a boundary between the radial clearance intermediate part  72   c  and the radial clearance lower part  72   b . The piston cylinder  28  has a first internal diameter axially below the second stepped part  92   b , has a second internal diameter axially above the first stepped part  92   a , and has a third internal diameter between the first stepped part  92   a  and the second stepped part  92   b . The third internal diameter is larger than the first internal diameter and larger than the second internal diameter. Any of the first internal diameter, the second internal diameter, and the third internal diameter is larger than the external diameter of the drive piston  22 . Therefore, the radial width of the radial clearance intermediate part  72   c  is larger than the radial width of the radial clearance upper part  72   a  and larger than the radial width of the radial clearance lower part  72   b . In this way, the radial clearance  72  may vary stepwise in the axial direction. 
     A stroke S of the drive piston  22  illustrated in  FIG. 12  is illustrated in  FIG. 13 . The drive piston  22  when being located at the top dead center UP 2  is illustrated by a solid line, the drive piston  22  when being located at the bottom dead center LP 2  is illustrated by a dashed line, and the drive piston  22  when being located in midpoint MP is illustrated by a one-dot chain line. As illustrated, the radial clearance upper part  72   a  has a first radial width t 1 , the radial clearance lower part  72   b  has a second radial width t 2 , and the radial clearance intermediate part  72   c  has a third radial width t 3 . The first radial width t 1  is, for example, within a range of 0.01 to 0.1 mm, the second radial width t 2  is, for example, within a range of 0.01 to 0.1 mm, and the third radial width t 3  is, for example, within a range of 0.15 to 1.0 mm. 
     Additionally, the radial clearance upper part  72   a  has a first axial length L 1 , the radial clearance lower part  72   b  has a second axial length L 2 , and the radial clearance intermediate part  72   c  has a third axial length L 3 . The third axial length L 3  of the radial clearance intermediate part  72   c  may be longer than half of the stroke S of the drive piston  22 . The second axial length L 2  of the radial clearance lower part  72   b  may be longer than the first axial length L 1  of the radial clearance upper part  72   a . Determining the axial length of the radial clearance  72  in this way helps to relatively shorten the axial length of the piston cylinder  28  while maintaining the stroke of the drive piston  22 . 
     As illustrated in  FIG. 14 , the drive piston  22  may include the communication path  76  that allows the gas spring chamber  48  to communicate with the radial clearance  72 . The communication path  76  may be a through-hole formed in the drive piston  22 . The communication path  76  functions similarly to the embodiment illustrated in  FIG. 8 . Additionally, as required, the drive piston  22  may include another communication path  94  that allows the drive chamber  46  to communicate with the radial clearance  72 . 
     As illustrated in  FIG. 15 , the communication path  76  may be the longitudinal groove formed in the outer peripheral surface of the drive piston  22 . The longitudinal groove extends in the axial direction from the gas spring chamber  48  to the central part of the drive piston  22 . The communication path  76  functions similarly to the embodiment illustrated in  FIG. 9 . Additionally, the other communication path  94  may also be a longitudinal groove. 
     Instead of providing the radial clearance  72  with the radial clearance intermediate part  72   c , as illustrated in  FIG. 16 , the GM cryocooler  10  may include the pressure release path  70  together with the radial clearance  72 . As described above, the pressure release path  70  is provided in the piston cylinder  28  so as to shunt the gas spring chamber  48  to the drive chamber  46 . The flow path resistance part  68 , such as an orifice, is disposed in the middle of the pressure release path  70 . The pressure release path  70  includes a first outlet  70   a  on an axially upper side thereof, and includes a second outlet  70   b  on an axially lower side thereof. 
     In this way, when the drive piston  22  is located at or near the bottom dead center LP 2  (that is, when the drive piston  22  is located axially below the second outlet  70   b ), the gas spring chamber  48  can generate the gas spring force that resists the downward movement of the drive piston  22 . Additionally, when the drive piston  22  is located at or near the top dead center UP 2  (that is, when the drive piston  22  is located axially above the first outlet  70   a ), the drive chamber  46  serving as the second gas spring chamber can generate the gas spring force that resists the upward movement of the drive piston  22 . 
     When the drive piston  22  moves in the axial direction between the first outlet  70   a  and the second outlet  70   b , the gas spring chamber  48  and the drive chamber  46  are allowed to communicate with each other through both the radial clearance  72  and the pressure release path  70 . Hence, the gas spring force acting on the drive piston  22  when the drive piston  22  moves by an intermediate part of the stroke thereof can be made small. Accordingly, the driving force of the displacer  20  resulting from the drive piston  22  becomes large, the stroke of the displacer  20  is maintained, and a decrease in cryocooling capacity of the GM cryocooler  10  can be suppressed. 
     In addition, in  FIG. 16 , although the radial clearance  72  is constant in the axial direction, this is not essential. The radial clearance  72  may include the radial clearance upper part  72   a , the radial clearance lower part  72   b , and the radial clearance intermediate part  72   c . In this case, the first outlet  70   a  may be provided at the radial clearance upper part  72   a . The second outlet  70   b  may be provided at the radial clearance lower part  72   b . Alternatively, the first outlet  70   a  and the second outlet  70   b  may be provided at the radial clearance intermediate part  72   c.    
     As illustrated in  FIG. 17A , a drive piston projection  22   a  may protrude in the axial direction from the upper surface of the drive piston  22 . The drive piston projection  22   a  is disposed so as to be insertable into an outlet  64   a  of the second intake flow path  64  and advance into and retreat from the outlet  64   a  together with the axial reciprocation of the drive piston  22 . The outlet  64   a  of the second intake flow path  64  is also an outlet of the second exhaust flow path  66 . The outlet  64   a  is a gas inlet/outlet of a drive chamber for controlling the pressure of the drive chamber  46 , and gas flows between the compressor  12  and the drive chamber  46  through the outlet  64   a . The outlet  64   a  is formed to pass through an upper surface of the drive chamber  46  (that is, the piston cylinder  28 ). 
     The drive piston projection  22   a  is inserted into the outlet  64   a  of the second intake flow path  64  when the drive piston  22  is located at or near the top dead center UP 2 . The inserted drive piston projection  22   a  completely or partially blocks the outlet  64   a , and thereby, the gas flow of the outlet  64   a  is hindered, or the flow rate of the gas that passes through the outlet  64   a  is limited. The drive piston projection  22   a  is withdrawn above the outlet  64   a  of the second intake flow path  64  when the drive piston  22  is separated from the top dead center UP 2  or its vicinity. Therefore, the drive piston projection  22   a  is not inserted into the outlet  64   a  of the second intake flow path  64  but is located out of the outlet  64   a  when the drive piston  22  is located at or near the bottom dead center LP 2 . Since the drive piston projection  22   a  is out of the outlet  64   a , the gas flow of the outlet  64   a  is recovered. 
     Hence, when the drive piston  22  moves upward toward the top dead center UP 2 , the drive piston projection  22   a  enters the outlet  64   a  of the second intake flow path  64 , and as the drive piston  22  further moves upward and the drive chamber  46  becomes narrow, the pressure of the drive chamber  46  increases effectively. When the drive piston  22  is located at or near the top dead center UP 2 , the drive chamber  46  serving as the second gas spring chamber can generate the gas spring force that resists the upward movement of the drive piston  22 . For example, even if either the second intake valve V 3  or the second exhaust valve V 4  is released, the gas flow rate of the outlet  64   a  is reduced due to the insertion of the drive piston projection  22   a  into the outlet  64   a  of the second intake flow path  64 , and the drive chamber  46  can generate the gas spring force. In this way, a contact or collision between the axial movable body  16  and the cold head housing  18  is suppressed, and vibration or abnormal noise of the GM cryocooler  10  can be reduced. 
     In addition, as illustrated in  FIG. 17B , a projection  28   b , which protrudes in the axial direction from an upper surface of the piston cylinder  28 , may be formed so as to surround the outlet  64   a  of the second intake flow path  64 , and a recess  22   b  capable of receiving the projection  28   b  may be formed on the upper surface of the drive piston  22 . Even in this way, the projection  28   b  of the piston cylinder  28  is received in the recess  22   b  of the drive piston  22  when the drive piston  22  is located at or near the top dead center UP 2 . Accordingly, the outlet  64   a  is at least partially blocked by the drive piston  22  when the drive piston  22  is at the top dead center UP 2 . In this way, the gas flow of the outlet  64   a  is hindered, or the flow rate of the gas that passes through the outlet  64   a  is limited. Therefore, the drive chamber  46  can generate the gas spring force that resists the upward movement of the drive piston  22 . 
     As illustrated in  FIG. 18 , the outlet  64   a  of the second intake flow path  64  may be disposed on the side surface of the drive chamber  46  (that is, the piston cylinder  28 ). 
     When the drive piston  22  is located at or near the top dead center UP 2  (that is, when the drive piston  22  is located axially above the outlet  64   a ), the side surface of the drive piston  22  faces the outlet  64   a , and thereby, the gas flow of the outlet  64   a  is hindered, or the gas flow rate that passes through the outlet  64   a  is limited. Additionally, when the drive piston  22  moves downward, the outlet  64   a  is exposed to the drive chamber  46 , and the gas flow of the outlet  64   a  is recovered. Even in this way, when the drive piston  22  is located at or near the top dead center UP 2 , the drive chamber  46  serving as the second gas spring chamber can effectively generate the gas spring force that resists the upward movement of the drive piston  22 . 
     In addition, in  FIGS. 17A, 17B, and 18 , although the radial clearance  72  is constant in the axial direction, this is not essential. Similar to the embodiment illustrated in  FIGS. 7 to 9 , the radial clearance  72  may include the radial clearance upper part  72   a  and the radial clearance lower part  72   b . In this case, the outlet  64   a  may be provided at the radial clearance upper part  72   a . Similar to the embodiment illustrated in FIGS.  11  to  15 , the radial clearance  72  may include the radial clearance upper part  72   a , the radial clearance lower part  72   b , and the radial clearance intermediate part  72   c . The outlet  64   a  may be provided at the radial clearance upper part  72   a  or the radial clearance intermediate part  72   c.    
     Fourth Embodiment 
       FIGS. 19 to 21  are schematic views illustrating a GM cryocooler  10  related to a fourth embodiment. The GM cryocooler  10  related to the fourth embodiment is the same as the GM cryocooler  10  related to the first embodiment except that a check valve  78  is provided with a shunt path  80 . 
     As illustrated in  FIG. 19 , the check valve  78  is disposed between the gas spring chamber  48  and the drive chamber  46  so as to resist the outflow of gas from the gas spring chamber  48  to the drive chamber  46 . The piston cylinder  28  includes the shunt path  80  that shunts the gas spring chamber  48  to the drive chamber  46 . The check valve  78  is disposed in the middle of the shunt path  80 . 
     In this way, when the drive piston  22  moves downward, the check valve  78  is closed. Therefore, the drive piston  22  can compress the gas stored in the gas spring chamber  48 . Similar to the first embodiment, a contact or collision between the axial movable body  16  and the cold head housing  18  is suppressed, and vibration or abnormal noise of the GM cryocooler  10  can be reduced. 
     As illustrated in  FIG. 20 , a second check valve  82  that allows the gas spring chamber  48  and the drive chamber  46  to communicate with each other may be provided in parallel with the check valve (hereinafter referred to as the first check valve)  78 . However, the second check valve  82  is provided in an orientation reverse to the check valve  78 , and resists the outflow of gas from the drive chamber  46  to the gas spring chamber  48 . A set differential pressure for opening the first check valve  78  is open is smaller than a set differential pressure for opening the second check valve  82 . Even in this way, vibration or abnormal noise of the GM cryocooler  10  can be reduced. Additionally, an excessive pressure in the gas spring chamber  48  can be released to the drive chamber  46 . 
     As illustrated in  FIG. 21 , a flow path resistance part may be connected in series with a check valve. A first flow path resistance part  84  is connected in series with the first check valve  78 , and a second flow path resistance part  86  is connected in series with the second check valve  82 . The first flow path resistance part  84  has a smaller low flow path resistance than the second flow path resistance part  86 . The set differential pressure for opening the first check valve  78  may be equal to the set differential pressure for opening the second check valve  82 . Even in this way, the same effects as those of the configuration illustrated in  FIG. 20  can be exhibited. 
     The invention has been described above on the basis of the embodiments. It should be understood by those skilled in the art that the invention is not limited to the above embodiments, that various design changes are possible and various modification examples are possible, and that such modification examples are also within the scope of the invention. 
     In a certain embodiment, a flow path resistance part  90  may be provided between the drive chamber  46  and the valve unit  54 . The flow path resistance part  90  may be provided between the drive chamber  46  and the second intake valve V 3  in the second intake flow path  64 . In this way, in an exhaust process (the first exhaust period A 2  illustrated in  FIG. 2 ) of the cold head  14 , a delay occurs in the pressure rising (the second intake period A 3  illustrated in  FIG. 2 ) of the drive chamber  46 . Accordingly, rising of a downward driving force that acts on the drive piston  22  can be delayed. This helps to suppress a contact or collision between the axial movable body  16  and the cold head housing  18  and reduce vibration or abnormal noise of the GM cryocooler  10 . 
     In a case where the GM cryocooler  10  is designed so as to be upwardly installed, the disposition of the drive chamber  46  and the gas spring chamber  48  may be reversed. The gas spring chamber  48  may be disposed axially opposite to the displacer cylinder  26  with respect to the drive piston  22 , and the drive chamber  46  may be disposed on the same axial side as the displacer cylinder  26  with respect to the drive piston  22 . 
     Various features described in relation to the embodiments can also be applied to other embodiments. New embodiments created by combination have the effects of respective combined embodiments in combination. For example, the check valve described in relation to the fourth embodiment may be applied to the first embodiment to the third embodiment. 
     The invention is applicable to the field of the GM cryocooler. 
     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.