Abstract:
A GM cryocooler is furnished with: a first cold head including a first displacer and a first cylinder; a second cold head including a second displacer and a second cylinder and being disposed opposing the first cold head; a common drive mechanism for driving axial reciprocation of the first displacer and the second displacer; and a working gas circuit for generating between the first cold head and the second cold head a pressure differential that assists the common drive mechanism.

Description:
INCORPORATION BY REFERENCE 
       [0001]    Priority is claimed to Japanese Patent Application No. 2015-208614, filed Oct. 23, 2015, and Japanese Patent Application No. 2016-116329, filed Jun. 10, 2016, the entire content of each of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    Technical Field 
         [0003]    The present invention in particular embodiments relates to Gifford-McMahon (GM) cryocoolers. 
         [0004]    Description of Related Art 
         [0005]    GM cryocoolers, which are typifying examples of cryogenic refrigerators, generate extremely low temperatures using the GM cycle. That means that GM cryocoolers are configured so as to appropriately synchronize periodic pressure fluctuations in the expansion space—deriving from intake of the working gas into, its adiabatic expansion in, and its exhausting from, the expansions space—with periodic variation in volume of the expansion space due to the reciprocating movement of the displacer. 
       SUMMARY 
       [0006]    One embodiment of the present invention affords a GM cryocooler including: a first cold head including an axially reciprocatory first displacer, and a first cylinder between the first displacer and which a first gas chamber is formed; a second cold head including a second displacer disposed coaxially with respect to the first displacer and axially reciprocatory unitarily with the first displacer, and a second cylinder between the second displacer and which a second gas chamber is formed, and disposed opposing the first cold head; a common drive mechanism connected to the first displacer and the second displacer such as to drive axial reciprocation of the first displacer and the second displacer; and a working gas circuit connected to the first cold head and the second cold head such as to generate between the first gas chamber and the second gas chamber a pressure differential assisting the common drive mechanism. 
         [0007]    Another embodiment of the present invention affords a GM cryocooler including; a first cold head including an axially reciprocatory first displacer, and a first cylinder between the first displacer and which a first gas chamber is formed; and a second cold head including a second displacer disposed coaxially with respect to the first displacer and axially reciprocatory unitarily with the first displacer, and a second cylinder between the second displacer and which a second gas chamber is formed, and disposed opposing the first cold head. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a sectional view schematically showing a GM cryocooler according an embodiment of the present invention. 
           [0009]      FIG. 2  is an outline view schematically showing the GM cryocooler shown in  FIG. 1 . 
           [0010]      FIG. 3  is a view showing an example of an operation of the GM cryocooler shown in  FIG. 1 . 
           [0011]      FIG. 4  is a sectional view schematically showing a GM cryocooler according to another embodiment of the present invention. 
           [0012]      FIG. 5  is a sectional view schematically showing a GM cryocooler according to still another embodiment of the present invention. 
           [0013]      FIG. 6A  shows an upward assist force which acts on a Scotch yoke when a displacer connector shown in  FIG. 5  moves upward, and  FIG. 6B  shows a downward assist force which acts on the Scotch yoke when the displacer connector moves downward. 
           [0014]      FIG. 7  is a sectional view schematically showing a GM cryocooler according to still another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    A general basic configuration of a GM cryocooler includes one compressor and one expander (that is, combination between one displacer and a drive portion thereof). As a configuration example derived from this basic configuration, a cryocooler is suggested which includes two displacers which are disposed for one displacer drive portion in parallel and in which intake operations to expansion spaces corresponding to the two displacers are alternately performed. The alternate intake operations of two expanders decrease the pressure fluctuation in a compressor, and improve the efficiency of the compressor. Accordingly, this contributes to improvement in the efficiency of the cryocooler. 
         [0016]    However, in order to drive two displacers by one drive portion, a relatively large drive portion which generates a corresponding drive torque is required. In addition, an area of floor for installation of the cryocooler is liable to be increased due to the parallel disposition of the two expanders. 
         [0017]    In a GM cryocooler having a plurality of displacers, it is desirable to realize improvement in the efficiency of a compressor while decreasing a drive torque of the displacers. 
         [0018]    In addition, arbitrary combinations of the above-described components, or components or expression of the present invention may be replaced by each other in methods, devices, systems, or the like, and these replacements are also included in aspects of the present invention. 
         [0019]    According to the present invention, in a GM cryocooler having a plurality of displacers, it is possible to realize improvement in the efficiency of a compressor while decreasing drive torque of the displacers. 
         [0020]    Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, in descriptions, the same reference numerals are assigned to the same elements, and overlapping descriptions thereof are appropriately omitted. Moreover, configurations described below are exemplified, and do not limit the scope of the present invention. 
         [0021]      FIG. 1  is a sectional view schematically showing a GM cryocooler  10  according to an embodiment of the present invention.  FIG. 2  is an outline view schematically showing the GM cryocooler  10  shown in  FIG. 1 .  FIG. 3  is a view showing an example of the operation of the GM cryocooler  10  shown in  FIG. 1 . 
         [0022]    The GM cryocooler  10  includes a compressor  12  which compresses a working gas (for example, helium gas), and a plurality of cold heads which are cooled by adiabatic expansion of the working gas. The cold head is referred to as an expander. As described in detail below, the compressor  12  supplies a high-pressure working gas to the cold heads. A regenerator which pre-cools the working gas is provided in the cold head. The pre-cooled working gas is cooled by expansion in the cold head again. The working gas is recovered to the compressor  12  through the regenerator. When the working gas passes through the regenerator, the regenerator is cooled. The compressor  12  compresses the recovered working gas, and supplies the compressed working gas to the expander again. 
         [0023]    The GM cryocooler  10  includes a first cold head  14   a  and a second cold head  14   b  which are disposed so as to face each other. In addition, the GM cryocooler  10  includes a common drive mechanism  40  for the first cold head  14   a  and the second cold head  14   b . The first cold head  14   a  is disposed on one side with respect to the common drive mechanism  40 , and the second cold head  14   b  is disposed on the other side with respect to the common drive mechanism  40 . In addition, the GM cryocooler  10  includes a working gas circuit  70  which connects the compressor  12  to the first cold head  14   a  and the second cold head  14   b.    
         [0024]    The first cold head  14   a  is a single staged cold head. The first cold head  14   a  includes a first displacer  16   a  which can axially reciprocate, and a first cylinder  18   a  which accommodates the first displacer  16   a . The axial reciprocation of the first displacer  16   a  is guided by the first cylinder  18   a . In general, each of the first displacer  16   a  and the first cylinder  18   a  is a cylindrical member which axially extends, and the inner diameter of the first cylinder  18   a  is slightly greater than the outer diameter of the first displacer  16   a . Here, an axial direction is an upward-downward direction in  FIG. 1  (arrow C). 
         [0025]    A first expansion chamber  20   a  is formed between the first displacer  16   a  and the first cylinder  18   a  on one end in the axial direction, and a first room-temperature chamber  22   a  is formed between the first displacer  16   a  and the first cylinder  18   a  on the other end in the axial direction. The first room-temperature chamber  22   a  is positioned near the common drive mechanism  40 , and the first expansion chamber  20   a  is positioned far from the common drive mechanism  40 . This means that the first room-temperature chamber  22   a  is formed on a proximal end of the first cold head  14   a  and the first expansion chamber  20   a  is formed on a distal end of the first cold head  14   a . A first cooling stage  24   a , which is fixed to the first cylinder  18   a  so as to enclose the first expansion chamber  20   a , is provided on the distal end of the first cold head  14   a.    
         [0026]    When the first displacer  16   a  axially moves, the first expansion chamber  20   a  and the first room-temperature chamber  22   a  complementarily increase and decrease the volume. That is, when the first displacer  16   a  moves upward, the first expansion chamber  20   a  is widened, and the first room-temperature chamber  22   a  is narrowed, and vice versa. 
         [0027]    The first displacer  16   a  includes a first regenerator  26   a  which is built therein. The first displacer  16   a  includes a first inlet flow path  28   a , which allows the first regenerator  26   a  to communicate with the first room-temperature chamber  22   a , on the upper lid portion of the first displacer  16   a . In addition, the first displacer  16   a  includes a first outlet flow path  30   a , which allows the first regenerator  26   a  to communicate with the first expansion chamber  20   a , on the tubular portion of the first displacer  16   a . Alternatively, the first outlet flow path  30   a  may be provided on the lower lid portion of the first displacer  16   a . Moreover, the first displacer  16   a  includes a first inlet flow-straightener  32   a  which is in inner-contact with the upper lid portion, and a first outlet flow-straightener  34   a  which is in inner-contact with the lower lid portion. The first regenerator  26   a  is interposed between the pair of flow-straighteners. 
         [0028]    The first cold head  14   a  includes a first seal portion  36   a  which blocks a clearance formed between the first cylinder  18   a  and the first displacer  16   a . For example, the first seal portion  36   a  is a slipper seal, and is mounted on the tubular portion or the upper lid portion of the first displacer  16   a.    
         [0029]    In this way, the first seal portion  36   a  is positioned near the common drive mechanism  40 , and the first outlet flow path  30   a  is away from the common drive mechanism  40  and is positioned near the first cooling stage  24   a . In other words, the first seal portion  36   a  is attached to a proximal portion of the first displacer  16   a , and the above-described first outlet flow path  30   a  is formed in a distal portion of the first displacer  16   a.    
         [0030]    The working gas flows from the first room-temperature chamber  22   a  into the first regenerator  26   a  through the first inlet flow path  28   a . More specifically, the working gas flows from the first inlet flow path  28   a  into the first regenerator  26   a  through the first inlet flow-straightener  32   a . The working gas flows from the first regenerator  26   a  into the first expansion chamber  20   a  via the first outlet flow-straightener  34   a  and the first outlet flow path  30   a . The working gas goes through a reverse pathway with respect to the above-described pathway when the working gas is returned from the first expansion chamber  20   a  to the first room-temperature chamber  22   a . That is, the working gas is returned from the first expansion chamber  20   a  to the first room-temperature chamber  22   a  through the first outlet flow path  30   a , the first regenerator  26   a , and the first inlet flow path  28   a . The working gas, which bypasses the first regenerator  26   a  and flows into the clearance, is interrupted by the first seal portion  36   a.    
         [0031]    As described above, the second cold head  14   b  is disposed on the side opposite to the first cold head  14   a  with respect to the common drive mechanism  40 . Except for this, the configuration of the second cold head  14   b  is similar to that of the first cold head  14   a . Accordingly, similarly to the first cold head  14   a , the second cold head  14   b  is a single staged cold head, and has the shape and size similar to those of the first cold head  14   a.    
         [0032]    The second cold head  14   b  includes a second displacer  16   b  which is coaxially disposed with respect to the first displacer  16   a  and is able to axially reciprocate integrally with the first displacer  16   a , and a second cylinder  18   b  which accommodates the second displacer  16   b . The axial reciprocation of the second displacer  16   b  is guided by the second cylinder  18   b . In general, each of the second displacer  16   b  and the second cylinder  18   b  is a cylindrical member which axially extends, and the inner diameter of the second cylinder  18   b  is slightly greater than the outer diameter of the second displacer  16   b.    
         [0033]    A second expansion chamber  20   b  is formed between the second displacer  16   b  and the second cylinder  18   b  on one end in the axial direction, and a second room-temperature chamber  22   b  is formed between the second displacer  16   b  and the second cylinder  18   b  on the other end in the axial direction. The second room-temperature chamber  22   b  is positioned near the common drive mechanism  40 , and the second expansion chamber  20   b  is positioned far from the common drive mechanism  40 . This means that the second room-temperature chamber  22   b  is formed on a proximal end of the second cold head  14   b  and the second expansion chamber  20   b  is formed on a distal end of the second cold head  14   b . A second cooling stage  24   b , which is fixed to the second cylinder  18   b  so as to enclose the second expansion chamber  20   b , is provided on the distal end of the second cold head  14   b.    
         [0034]    When the second displacer  16   b  axially moves, the second expansion chamber  20   b  and the second room-temperature chamber  22   b  complementarily increase and decrease the volume. That is, when the second displacer  16   b  moves upward, the second expansion chamber  20   b  is widened, and the second room-temperature chamber  22   b  is narrowed, and vice versa. 
         [0035]    The second displacer  16   b  includes a second regenerator  26   b  which is built therein. The second displacer  16   b  includes a second inlet flow path  28   b , which allows the second regenerator  26   b  to communicate with the second room-temperature chamber  22   b , on the upper lid portion of the second displacer  16   b . In addition, the second displacer  16   b  includes a second outlet flow path  30   b , which allows the second regenerator  26   b  to communicate with the second expansion chamber  20   b , on the tubular portion of the second displacer  16   b . Alternatively, the second outlet flow path  30   b  may be provided on the lower lid portion of the second displacer  16   b . Moreover, the second displacer  16   b  includes a second inlet flow-straightener  32   b  which is in inner-contact with the upper lid portion, and a second outlet flow-straightener  34   b  which is in inner-contact with the lower lid portion. The second regenerator  26   b  is interposed between the pair of flow-straighteners. 
         [0036]    The second cold head  14   b  includes a second seal portion  36   b  which blocks a clearance formed between the second cylinder  18   b  and the second displacer  16   b . For example, the second seal portion  36   b  is a slipper seal, and is mounted on the tubular portion or the upper lid portion of the second displacer  16   b.    
         [0037]    In this way, the second seal portion  36   b  is positioned near the common drive mechanism  40 , and the second outlet flow path  30   b  is away from the common drive mechanism  40  and is positioned near the second cooling stage  24   b . In other words, the second seal portion  36   b  is attached to a proximal portion of the second displacer  16   b , and the above-described second outlet flow path  30   b  is formed in the distal portion of the second displacer  16   b.    
         [0038]    The working gas flows from the second room-temperature chamber  22   b  into the second regenerator  26   b  through the second inlet flow path  28   b . More specifically, the working gas flows from the second inlet flow path  28   b  into the second regenerator  26   b  through the second inlet flow-straightener  32   b . The working gas flows from the second regenerator  26   b  into the second expansion chamber  20   b  via the second outlet flow-straightener  34   b  and the second outlet flow path  30   b . The working gas goes through a reverse pathway with respect to the above-described pathway when the working gas is returned from the second expansion chamber  20   b  to the second room-temperature chamber  22   b . That is, the working gas is returned from the second expansion chamber  20   b  to the second room-temperature chamber  22   b  through the second outlet flow path  30   b , the second regenerator  26   b , and the second inlet flow path  28   b . The working gas, which bypasses the second regenerator  26   b  and flows into the clearance, is interrupted by the second seal portion  36   b.    
         [0039]    The GM cryocooler  10  is installed in the shown direction in the use site thereof. That is, the first cold head  14   a  is disposed downward in the vertical direction, the second cold head  14   b  is disposed upward in the vertical direction, and thus, the GM cryocooler  10  is installed in a longitudinal direction. The second cold head  14   b  is installed with a posture inverted to that of the first cold head  14   a . The second expansion chamber  20   b  is disposed upward in the vertical direction in the second cold head  14   b  while the first expansion chamber  20   a  is disposed downward in the vertical direction in the first cold head  14   a . Alternatively, the GM cryocooler  10  may be installed in a horizontal direction or in other directions. 
         [0040]    The common drive mechanism  40  includes a reciprocation drive source  42  which drives the axial reciprocation of the first displacer  16   a  and the second displacer  16   b . The reciprocation drive source  42  includes a rotation drive source  44  (for example, motor) having a rotation output shaft  46 , and a Scotch yoke  48  which is connected to the rotation output shaft  46  so as to convert the rotation of the rotation output shaft  46  into axial reciprocation. 
         [0041]    The common drive mechanism  40  includes a first connection rod  50   a  and a second connection rod  50   b . The first connection rod  50   a  axially extends from the reciprocation drive source  42  and connects the reciprocation drive source  42  to the first displacer  16   a . The second connection rod  50   b  axially extends from the reciprocation drive source  42  on the side opposite to the first connection rod  50   a  and connects the reciprocation drive source  42  to the second displacer  16   b . The first displacer  16   a , the first connection rod  50   a , the second connection rod  50   b , and the second displacer  16   b  are coaxially disposed with respect to each other. 
         [0042]    More specifically, the first connection rod  50   a  axially extends from the Scotch yoke  48  to the first displacer  16   a  and connects the Scotch yoke  48  to the first displacer  16   a . The first connection rod  50   a  rigidly connects the proximal portion of the first displacer  16   a  to the Scotch yoke  48 . The first connection rod  50   a  is supported by a first bearing portion  38   a  so as to be movable in the axial direction. The first bearing portion  38   a  is disposed between the Scotch yoke  48  and the first displacer  16   a.    
         [0043]    The second connection rod  50   b  axially extends from the Scotch yoke  48  to the second displacer  16   b  and connects the Scotch yoke  48  to the second displacer  16   b . The second connection rod  50   b  rigidly connects the proximal portion of the second displacer  16   b  to the Scotch yoke  48 . The second connection rod  50   b  is supported by a second bearing portion  38   b  so as to be movable in the axial direction. The second bearing portion  38   b  is disposed between the Scotch yoke  48  and the second displacer  16   b.    
         [0044]    As shown in  FIG. 2 , the common drive mechanism  40  includes a drive mechanism housing  52 . The first cylinder  18   a  is fixed to one side of the drive mechanism housing  52 , and the second cylinder  18   b  is fixed to the other side of the drive mechanism housing  52 . The second cylinder  18   b  is coaxially disposed with respect to the first cylinder  18   a . Moreover, for simplification, in  FIG. 2 , the compressor  12  is not shown. 
         [0045]    The reciprocation drive source  42  and the Scotch yoke  48  shown in  FIG. 1  are accommodated in the drive mechanism housing  52 . Similarly to the Scotch yoke  48 , the proximal ends of the first connection rod  50   a  and the second connection rod  50   b  are accommodated in the drive mechanism housing  52 . Similarly to the first displacer  16   a  and the second displacer  16   b , the distal ends of the first connection rod  50   a  and the second connection rod  50   b  are respectively accommodated in the first cylinder  18   a  and the second cylinder  18   b . The first bearing portion  38   a  is disposed at the boundary between the first cylinder  18   a  and the drive mechanism housing  52  and in the vicinity thereof. The second bearing portion  38   b  is disposed at the boundary between the second cylinder  18   b  and the drive mechanism housing  52  and in the vicinity thereof. The first bearing portion  38   a  and the second bearing portion  38   b  are configured as seal portions which hold airtightness of the first cylinder  18   a  and the second cylinder  18   b  with respect to the drive mechanism housing  52 . 
         [0046]    In this way, the common drive mechanism  40  is connected to the first displacer  16   a  and the second displacer  16   b  so as to drive the axial reciprocation of the first displacer  16   a  and the second displacer  16   b . The first displacer  16   a  and the second displacer  16   b  configure a single displacer connector  16  which is fixedly connected to each other. A relative position of the second displacer  16   b  with respect to the first displacer  16   a  is not changed during the axial reciprocation of the first displacer  16   a  and the second displacer  16   b.    
         [0047]    Accordingly, the axial reciprocation of the first displacer  16   a  and the axial reciprocation of the second displacer  16   b  have phases opposite to each other. When the first displacer  16   a  is positioned at the top dead center (that is, the dead center on the proximal end side), the second displacer  16   b  is positioned at the bottom dead portion (that is, the dead center on the distal end side). When the first displacer  16   a  moves from the top dead center to the bottom dead center (that is, when the first displacer  16   a  moves from the proximal end of the first cold head  14   a  to the distal end thereof so as to narrow the first expansion chamber  20   a ), the second displacer  16   b  moves from the bottom dead center to the top dead center (that is, the second displacer  16   b  moves from the distal end of the second cold head  14   b  to the proximal end thereof so as to widen the second expansion chamber  20   b ). 
         [0048]    As shown in  FIG. 2 , a refrigerant circulation circuit  54  is provided in the GM cryocooler  10 . The GM cryocooler  10  cools a refrigerant (for example, liquid nitrogen) which flows through the refrigerant circulation circuit  54 . The refrigerant cooled by the GM cryocooler  10  is supplied to an object to be cooled (not shown) through the refrigerant circulation circuit  54 . The refrigerant used so as to cool the object to be cooled is recovered through the refrigerant circulation circuit  54 , and is re-cooled by the GM cryocooler  10 . 
         [0049]    The refrigerant circulation circuit  54  includes a first refrigerant cooling unit  54   a  which is thermally coupled to the first cold head  14   a , a second refrigerant cooling unit  54   b  which is thermally coupled to the second cold head  14   b , and a connection refrigerant pipe  54   c  which connects the first refrigerant cooling unit  54   a  to the second refrigerant cooling unit  54   b . In addition, the refrigerant circulation circuit  54  includes a supply pipe  54   d  and a recovery pipe  54   e . Each of the first refrigerant cooling unit  54   a  and the second refrigerant cooling unit  54   b  is a spiral refrigerant pipe which is wound around the first cooling stage  24   a  and the second cooling stage  24   b . The first refrigerant cooling unit  54   a  is cooled by the first cooling stage  24   a , and the second refrigerant cooling unit  54   b  is cooled by the second cooling stage  24   b . The connection refrigerant pipe  54   c  is connected to one end of the first refrigerant cooling unit  54   a , and the supply pipe  54   d  is connected to the other end thereof. The connection refrigerant pipe  54   c  is connected to one end of the second refrigerant cooling unit  54   b , and the recovery pipe  54   e  is connected to the other end thereof. 
         [0050]    A detachable connection mechanism  54   f  is provided in the connection refrigerant pipe  54   c . Accordingly, when the connection mechanism  54   f  is removed, the portion of the connection refrigerant pipe  54   c  on the first refrigerant cooling unit  54   a  side and the portion of the connection refrigerant pipe  54   c  on the second refrigerant cooling unit  54   b  side are separated from each other. According to the connection mechanism  54   f , disassembly of the refrigerant circulation circuit  54  is easily performed. This contributes to an increase in efficiency of maintenance work of the GM cryocooler  10 . 
         [0051]    Flow directions of the refrigerant in the refrigerant circulation circuit  54  are shown by arrows. The refrigerant flows from the recovery pipe  54   e  to the supply pipe  54   d  through the second refrigerant cooling unit  54   b , the connection refrigerant pipe  54   c , and the first refrigerant cooling unit  54   a . In this way, first, the refrigerant is cooled by the second refrigerant cooling unit  54   b , and thereafter, is cooled by the first refrigerant cooling unit  54   a.    
         [0052]    The cold head has a highest freeze capacity when the cold head is installed in a posture in which the expansion chamber is positioned downward in the vertical direction. As described above, the first cold head  14   a  has first expansion chamber  20   a  on the lower side in the vertical direction. However, the second cold head  14   b  does not have the second expansion chamber on the lower side in the vertical direction. Accordingly, the temperature of the second cooling stage  24   b  is higher than the temperature of the first cooling stage  24   a . According to the above-described refrigerant circuit configuration, first, the recovered refrigerant having a relatively high temperature is cooled by the second cold head  14   b  having a high temperature, and thereafter, is cooled by the first cold head  14   a  having a low temperature. Accordingly, it is possible to improve heat exchange efficiency between the refrigerant and the GM cryocooler  10 . 
         [0053]    In addition, the GM cryocooler  10  includes an auxiliary vacuum vessel  56  in which the second cold head  14   b  and the second refrigerant cooling unit  54   b  are accommodated, and a flanged portion  60  for attaching the first cold head  14   a  to a main vacuum vessel  58  separated from the auxiliary vacuum vessel  56 . The first cold head  14   a  and the first refrigerant cooling unit  54   a  are accommodated in the main vacuum vessel  58 . 
         [0054]    The auxiliary vacuum vessel  56  is attached to the proximal end of the second cylinder  18   b , and the flanged portion  60  is attached to the proximal end of the first cylinder  18   a . The auxiliary vacuum vessel  56  is connected to the flanged portion  60  by a connection pipe  62  which allows the auxiliary vacuum vessel  56  to airtightly communicate with the main vacuum vessel  58 . The connection pipe  62  provided a passage through which the supply pipe  54   d  and the connection refrigerant pipe  54   c  are introduced from the main vacuum vessel  58  to the auxiliary vacuum vessel  56 . The connection pipe  62  has a bellows portion midway. 
         [0055]    The second cold head  14   b  and the second refrigerant cooling unit  54   b  are covered with the auxiliary vacuum vessel  56 , and only the first cold head  14   a  and the first refrigerant cooling unit  54   a  are exposed. Therefore, in an operation in which the GM cryocooler  10  is attached to the main vacuum vessel  58 , an operator can handle the GM cryocooler  10  as a general GM cryocooler having a single cold head. 
         [0056]    The working gas circuit  70  shown in  FIG. 1  is configured so as to generate a pressure difference between a first gas chamber (that is, first expansion chamber  20   a  and/or first room-temperature chamber  22   a ) and a second gas chamber (that is, second expansion chamber  20   b  and/or second room-temperature chamber  22   b ). The pressure difference acts on the displacer connector  16  so as to assist the common drive mechanism  40 . In  FIG. 1 , when the displacer connector  16  moves downward (that is, when the first (second) displacer  16   a  ( 16   b ) moves from the top (bottom) dead center to the bottom (top) dead center), the working gas circuit  70  increases the pressure of the second gas chamber with respect to the first gas chamber. In this way, it is possible to assist the downward movement of the displacer connector  16  by the pressure difference between the first gas chamber and the second gas chamber, and vice versa. 
         [0057]    The working gas circuit  70  includes a valve portion  72 . The valve portion  72  includes a first intake valve V 1 , a first exhaust valve V 2 , a second intake valve V 3 , and a second exhaust valve V 4 . The valve portion  72  is accommodated in the drive mechanism housing  52  shown in  FIG. 2 . The valve portion  72  may be a rotary type valve. In this case, the valve portion  72  may be connected to the rotation output shaft  46  so as to be rotationally driven by the rotation of a rotation drive source  44 . Alternatively, the valve portion  72  may include a plurality of control valves which are individually controllable, and a controller which controls the control valve. 
         [0058]    The first intake valve V 1  is configured so as to determine a first intake period A 1  of the first cold head  14   a . The first intake valve V 1  is disposed in a first intake flow path  74   a  which connects a discharge port of the compressor  12  to the first room-temperature chamber  22   a  of the first cold head  14   a . In the first intake period A 1  (that is, when the first intake valve V 1  opens), the working gas flows from the discharge port of the compressor  12  into the first room-temperature chamber  22   a . Inversely, when the first intake valve V 1  is closed, the supply of the working gas from the compressor  12  to the first room-temperature chamber  22   a  is stopped. 
         [0059]    The first exhaust valve V 2  is configured so as to determine a first exhaust period A 2  of the first cold head  14   a . The first intake valve V 2  is disposed in a first exhaust flow path  76   a  which connects a suction port of the compressor  12  to the first room-temperature chamber  22   a  of the first cold head  14   a . In the first exhaust period A 2  (that is, when the first exhaust valve V 2  opens), the working gas flows from the first room-temperature chamber  22   a  into the suction port of the compressor  12 . When the first exhaust valve V 2  is closed, the recovery of the working gas from the first room-temperature chamber  22   a  to the compressor  12  is stopped. As shown in  FIG. 1 , a portion of the first exhaust flow path  76   a  and the first intake flow path  74   a  may share each other on the first room-temperature chamber  22   a  side. 
         [0060]    Similarly, the second intake valve V 3  is configured so as to determine a second intake period A 3  of the second cold head  14   b . The second intake valve V 3  is disposed in a second intake flow path  74   b  which connects the discharge port of the compressor  12  to the second room-temperature chamber  22   b  of the second cold head  14   b . In the second intake period A 3  (that is, when the second intake valve V 3  opens), the working gas flows from the discharge port of the compressor  12  into the second room-temperature chamber  22   b . When the second intake valve V 3  is closed, the supply of the working gas from the compressor  12  to the second room-temperature chamber  22   b  is stopped. As shown in  FIG. 1 , a portion of the second intake flow path  74   b  and the first intake flow path  74   a  may share each other on the compressor  12  side. 
         [0061]    The second exhaust valve V 4  is configured so as to determine a second exhaust period A 4  of the second cold head  14   b . The second exhaust valve V 4  is disposed in a second exhaust flow path  76   b  which connects the suction port of the compressor  12  to the second room-temperature chamber  22   b  of the second cold head  14   b . In the second exhaust period A 4  (that is, when the second exhaust valve V 4  opens), the working gas flows from the second room-temperature chamber  22   b  to the suction port of the compressor  12 . When the second exhaust valve V 4  is closed, the recovery of the working gas from the second room-temperature chamber  22   b  to the compressor  12  is stopped. As shown in  FIG. 1 , a portion of the second exhaust flow path  76   b  and the second intake flow path  74   b  may share each other on the second room-temperature chamber  22   b  side. Moreover, a portion of the second exhaust flow path  76   b  and the first exhaust flow path  76   a  may share each other on the compressor  12  side. 
         [0062]    In  FIG. 3 , 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 exemplified. In  FIG. 3 , one period in the axial reciprocation of the displacer connector  16  is shown so as to correspond to 360°,  00  corresponds to a starting time of the period, and 360° corresponds to an end time of the period. 90°, 180°, and 270° respectively correspond to a ¼ period, a half period, and a ¾ period. 
         [0063]    The first intake period A 1  and the second exhaust period A 4  are within a range from 0° to 135°, and the first exhaust period A 2  and the second intake period A 3  are within a range from 180° to 315°. The first intake period A 1  and the first exhaust period A 2  are alternately positioned to each other, and the second intake period A 3  and the second exhaust period A 4  are alternately positioned to each other. The first (second) displacer  16   a  ( 16   b ) is positioned at the bottom (top) dead center or in the vicinity thereof at  00 , and the first (second) displacer  16   a  ( 16   b ) is positioned at the top (bottom) dead center or in the vicinity thereof at  1800 . 
         [0064]    The operation of the GM cryocooler  10  having the above-described configuration will be described. When the first displacer  16   a  is positioned at the bottom dead center of the first cylinder  18   a  or in the vicinity thereof, the first intake period A 1  starts (0° in  FIG. 3 ). The first intake valve V 1  opens, and a high-pressure gas is supplied from the discharge port of the compressor  12  to the first room-temperature chamber  22   a  of the first cold head  14   a . Gas is cooled while passing through the first regenerator  26   a , and enters the first expansion chamber  20   a . While the gas flows into the first cold head  14   a , the first displacer  16   a  moves from the bottom dead center toward the top dead center. The first intake valve V 1  is closed, and the first intake period A 1  ends (135° in  FIG. 3 ). The first displacer  16   a  continuously moves toward the top dead center. In this way, the volume of the first expansion chamber  20   a  increases, and the first expansion chamber  20   a  is filled with a high-pressure gas. 
         [0065]    When the first displacer  16   a  positioned at the top dead center or in the vicinity thereof, the first exhaust period A 2  starts (180° in  FIG. 3 ). The first exhaust valve V 2  opens and the first cold head  14   a  is connected to the suction port of the compressor  12 . A high-pressure gas is expanded in the first expansion chamber  20   a  and is cooled. The expanded gas is recovered to the compressor  12  via the first room-temperature chamber  22   a  while cooling the first regenerator  26   a . While the gas flows out from the first cold head  14   a , the first displacer  16   a  moves from the top dead center toward the bottom dead center. The first exhaust valve V 2  is closed, and the first exhaust period A 2  ends (315° in  FIG. 3 ). The first displacer  16   a  continuously moves toward the bottom dead center. In this way, the volume of the first expansion chamber  20   a  decreases, and a low-pressure gas is discharged. 
         [0066]    The first cold head  14   a  repeats the cooling cycle (that is, GM cycle), and thus, the first cooling stage  24   a  is cooled. Accordingly, the refrigerant is cooled by the first refrigerant cooling unit  54   a.    
         [0067]    Simultaneously with the above-described operation of the first cold head  14   a , the second cold head  14   b  is operated. When the second displacer  16   b  positioned at the top dead center or in the vicinity thereof, the second exhaust period A 4  starts (0° in  FIG. 3 ). The second exhaust valve V 4  opens and the second cold head  14   b  is connected to the suction port of the compressor  12 . A high-pressure gas is expanded in the second expansion chamber  20   b  and is cooled. The expanded gas is recovered to the compressor  12  via the second room-temperature chamber  22   b  while cooling the second regenerator  26   b . While the gas flows out from the second cold head  14   b , the second displacer  16   b  moves from the top dead center toward the bottom dead center (upward in the  FIG. 1 ). The second exhaust valve V 4  is closed, and the second exhaust period A 4  ends (135° in  FIG. 3 ). The second displacer  16   b  continuously moves toward the bottom dead center. In this way, the volume of the second expansion chamber  20   b  decreases, and a low-pressure gas is discharged. 
         [0068]    When the second displacer  16   b  positioned at the bottom dead center of the second cylinder  18   b  or in the vicinity thereof, the second intake period A 3  starts (180° in  FIG. 3 ). The second intake valve V 3  opens, and a high-pressure gas is supplied from the discharge port of the compressor  12  to the second room-temperature chamber  22   b  of the second cold head  14   b . Gas is cooled while passing through the second regenerator  26   b , and enters the second expansion chamber  20   b . While the gas flows into the second cold head  14   b , the second displacer  16   b  moves from the bottom dead center toward the top dead center (downward in  FIG. 1 ). The second intake valve V 3  is closed, and the second intake period A 3  ends (135° in  FIG. 3 ). The second displacer  16   b  continuously moves toward the top dead center. In this way, the volume of the second expansion chamber  20   b  increases, and the second expansion chamber  20   b  is filled with a high-pressure gas. 
         [0069]    In this way, in the second cold head  14   b , the cooling cycle (that is, GM cycle) which has a phase opposite to the phase of the first cold head  14   a  but is similar to the cycle of first cold head  14   a  is repeated. Accordingly, the second cooling stage  24   b  is cooled, and the refrigerant is cooled by the second refrigerant cooling unit  54   b.    
         [0070]    In the expander of the GM cryocooler, there is a technology referred to as so-called “gas assist” using a gas pressure in order to decrease the drive torque. Typical gas assist is realized by distributing a portion of the supplied working gas to a gas assist chamber inside the expander separated from the expansion space. The working gas supplied to the gas assist chamber cannot contribute to PV work in the expansion space. Accordingly, in the gas assist, there is a disadvantage that a decrease in the PV work may occur, that is, a decrease in freezing capacity may occur. 
         [0071]    However, in the above-described embodiment, the first intake period A 1  overlaps the second exhaust period A 4 . Accordingly, when gas is supplied from the compressor  12  to the first cold head  14   a , the gas is recovered from the second cold head  14   b  to the compressor  12 . In this case, the pressure of the first expansion chamber  20   a  is higher than the pressure of the second expansion chamber  20   b , and this pressure difference biases the displacer connector  16  upward in the  FIG. 1 . Since the direction of the biasing force coincides with the movement direction of the displacer connector  16 , it is possible to assist the common drive mechanism  40  by the pressure difference. 
         [0072]    In addition, since the first exhaust period A 2  overlaps the second intake period A 3 , when gas is recovered from the first cold head  14   a , gas is supplied to the second cold head  14   b , and the pressure of the first expansion chamber  20   a  is lower than the pressure of the second expansion chamber  20   b . This pressure difference biases the displacer connector  16  downward in  FIG. 1 . Accordingly, similarly to the first intake period A 1 , in the first exhaust period A 2 , it is possible to assist the common drive mechanism  40  by the pressure difference. 
         [0073]    Accordingly, operations of the first cold head  14   a  and the second cold head  14   b  themselves provide the gas assist to the displacer connector  16 . As the above-described typical gas assist configuration, the working gas is not consumed in the dedicated gas assist chamber, and thus, loss of the PV work does not occur. Therefore, it is possible to decrease the drive torque generated by the common drive mechanism  40  to drive the displacer connector  16 , and thus, a decrease in a size of the drive mechanism can be obtained. 
         [0074]    In order to obtain the above-described advantages, the first intake period A 1  and the second exhaust period A 4  may not correctly coincide with each other. The second exhaust period A 4  may at least partially overlap the first intake period A 1 . Similarly, the first exhaust period A 2  and the second intake period A 3  may not correctly coincide with each other. The second intake period A 3  may at least partially overlap the first exhaust period A 2 . 
         [0075]    In the above-described embodiment, the second intake period A 3  does not overlap the first intake period A 1 . In addition, the second exhaust period A 4  does not overlap the first exhaust period A 2 . In this way, the intake-exhaust cycle from the compressor  12  to the first cold head  14   a  is completely deviated from the intake-exhaust cycle from the compressor  12  to the second cold head  14   b . Accordingly, variation between a high pressure and a low pressure of the compressor  12  decreases, and thus, it is possible to improve efficiency of the compressor  12 . 
         [0076]    In order to obtain the advantages, the intake-exhaust cycles of the two cold heads need not be completely deviated from each other. Preferably, the second intake period A 3  may be later than first intake period A 1  by 150° or more. Along with this, or instead of this, preferably, the second exhaust period A 4  may be later than the first exhaust period A 2  by 150° or more. 
         [0077]    In addition, lengths of the first intake period A 1  and the second exhaust period A 4  may be different from each other. Similarly, lengths of the first exhaust period A 2  and the second intake period A 3  may be different from each other. For example, the difference between the intake period and the exhaust period may be within 20° or 5°. In this way, the difference between freezing capacities of the first cold head  14   a  and the second cold head  14   b  may be adjusted. 
         [0078]    In addition, the lengths of the first intake period A 1  and the first exhaust period A 2  may be different from each other. Similarly, the lengths of the second intake period A 3  and the second exhaust period A 4  may be different from each other. In this case, for example, the difference between the intake period and the exhaust period may be within 20° or 5°. 
         [0079]    Moreover, in the above-described embodiment, since the GM cryocooler  10  is installed such that the two cold heads disposed to face each other are positioned in the longitudinal direction, it is possible to reduce the area of floor for installation of the GM cryocooler  10 . 
         [0080]    In the GM cryocooler  10  described with reference to  FIGS. 1 to 3 , the common drive mechanism  40  is assisted by the working gas circuit  70 . However, it is possible to drive the displacer connector  16  by only the pressure difference between the two cold heads. That is, as shown in  FIG. 4 , the GM cryocooler  10  may not have the common drive mechanism  40 . 
         [0081]      FIG. 4  is a sectional view schematically showing the GM cryocooler  10  according to another embodiment of the present invention. The GM cryocooler  10  includes the first connection rod  50   a  and the second connection rod  50   b , and the first connection rod  50   a  and the second connection rod  50   b  are axially connected to each other. The first displacer  16   a  is connected to the second displacer  16   b  via the first connection rod  50   a  and the second connection rod  50   b  such that the axial reciprocation of the first displacer  16   a  has the phase opposite to the phase of the axial reciprocation of the second displacer  16   b . The relative position of the second displacer  16   b  with respect to the first displacer  16   a  is not changed during the axial reciprocation of the first displacer  16   a  and the second displacer  16   b . The first displacer  16   a , the first connection rod  50   a , the second connection rod  50   b , and the second displacer  16   b  are coaxially disposed with respect to each other. 
         [0082]    The first connection rod  50   a  and the second connection rod  50   b  configure a single connection rod  50  which is fixedly connected to each other. Alternatively, the first connection rod  50   a  and the second connection rod  50   b  may be fixedly connected to each other via an intermediate member. 
         [0083]    The first connection rod  50   a  has a first cross-sectional area S 1  in a plane perpendicular to the axial direction, and the second connection rod  50   b  has a second cross-sectional area S 2  in a plane perpendicular to the axial direction. The first cross-sectional area S 1  is the same as the second cross-sectional area S 2 . For example, the first connection rod  50   a  may have a circular cross-section having a first diameter, and the second connection rod  50   b  may have a circular cross-section having a second diameter which is the same as the first diameter. Typically, the first connection rod  50   a  and the second connection rod  50   b  have the same cross-sectional shape as each other. However, both may have cross-sectional shapes different from each other. 
         [0084]    The working gas circuit  70  is configured so as to drive the axial reciprocation of the first displacer  16   a  and the second displacer  16   b . The working gas circuit  70  is connected to the first cold head  14   a  and the second cold head  14   b  so as to generate the pressure difference between the first gas chamber and the second gas chamber. 
         [0085]    Similarly to the GM cryocooler  10  shown  FIG. 1 , in the GM cryocooler  10  shown in  FIG. 4 , the valve timing shown in  FIG. 3  is adopted. 
         [0086]    The first intake period A 1  overlaps the second exhaust period A 4 . Accordingly, when gas is supplied from the compressor  12  to the first cold head  14   a , the gas is recovered from the second cold head  14   b  to the compressor  12 . In this case, the pressure of the first expansion chamber  20   a  is higher than the pressure of the second expansion chamber  20   b . In this way, it is possible to move the displacer connector  16  upward by the pressure difference. 
         [0087]    In addition, the first exhaust period A 2  overlaps the second intake period A 3 . Gas is supplied to the second cold head  14   b  when gas is recovered from the first cold head  14   a , and thus, the pressure of the first expansion chamber  20   a  is lower than the pressure of the second expansion chamber  20   b . It is possible to move the displacer connector  16  downward by the pressure difference. 
         [0088]    In this way, it is possible to provide the GM cryocooler  10  which does not have the common drive mechanism  40 . The GM cryocooler  10  is configured of a gas differential-pressure drive type cryocooler. In addition, in a case where the valve portion  72  is configured of a rotary valve, as described above, the GM cryocooler  10  may include a drive source (for example, rotation drive source  44 ) which is connected to a rotary valve so as to rotationally drive the rotary valve. 
         [0089]    In addition, in the GM cryocooler  10  shown in  FIG. 1 , the first connection rod  50   a  has a first cross-sectional area in a plane perpendicular to the axial direction, and the second connection rod  50   b  has a second cross-sectional area in a plane perpendicular to the axial direction. The first cross-sectional area S 1  is the same as the second cross-sectional area S 2 . For example, the first connection rod  50   a  may have a circular cross-section having a first diameter, and the second connection rod  50   b  may have a circular cross-section having a second diameter which is the same as the first diameter. 
         [0090]      FIG. 5  is a sectional view schematically showing the GM cryocooler  10  according to still another embodiment of the present invention. In the GM cryocooler  10  described with reference to  FIGS. 1 to 4 , the first connection rod  50   a  and the second connection rod  50   b  have the same cross-sectional area as each other. However, as shown in  FIG. 5 , the first connection rod  50   a  and the second connection rod  50   b  may have cross-sectional areas different from each other. 
         [0091]    The first connection rod  50   a  has the first cross-sectional area S 1  in a plane perpendicular to the axial direction, and the second connection rod  50   b  has the second cross-sectional area S 2  in a plane perpendicular to the axial direction. The first cross-sectional area S 1  is different from the second cross-sectional area S 2 . For example, the first cross-sectional area S 1  is greater than the second cross-sectional area S 2 . For example, the first connection rod  50   a  has a circular cross-section having a first diameter, and the second connection rod  50   b  has a circular cross-section having a second diameter. The second diameter is smaller than the first diameter. 
         [0092]    Accordingly, the working gas circuit  70  can generate a pressure difference assisting the common drive mechanism  40 . The operations of the first cold head  14   a  and the second cold head  14   b  themselves provide the gas assist to the displacer connector  16 . 
         [0093]    Moreover, the GM cryocooler  10  shown in  FIG. 5  has an asymmetrical gas assist configuration in which the first cross-sectional area S 1  is different from the second cross-sectional area S 2 . Different assist forces are applied to the displacer connector  16  according to the movement directions of the displacer connector  16 . 
         [0094]      FIG. 6A  shows a upward assist force Fup which acts on the Scotch yoke  48  when the displacer connector  16  shown in  FIG. 5  moves upward, and  FIG. 6B  shows a downward assist force Fdown which acts on the Scotch yoke  48  when the displacer connector  16  moves downward. 
         [0095]    The Scotch yoke  48  is accommodated in an internal space  53  of the drive mechanism housing  52 . As described above, the first bearing portion  38   a  and the second bearing portion  38   b  respectively seal the first room-temperature chamber  22   a  and the second room-temperature chamber  22   b  from the internal space  53 . The internal space  53  communicates with the discharge port of the compressor  12  shown in  FIG. 1 , and accordingly, is always maintained to a low pressure PL. 
         [0096]    When the displacer connector  16  moves upward, since the first room-temperature chamber  22   a  is a high pressure PH and the second room-temperature chamber  22   b  is a low pressure PL, the upward assist force Fup is represented by Fup=(PH−PL) S 1 . Meanwhile, when the displacer connector  16  moves upward, since the first room-temperature chamber  22   a  is a low pressure PL and the second room-temperature chamber  22   b  is a high pressure PH, the downward assist force Fdown is represented by Fdown=(PH−PL) S 2 . Accordingly, in a case where the first cross-sectional area S 1  is greater than the second cross-sectional area S 2 , the upward assist force Fup is greater than the downward assist force Fdown. 
         [0097]    The GM cryocooler  10  is installed in the shown direction in the use site thereof. That is, the first cold head  14   a  is disposed downward in the vertical direction, the second cold head  14   b  is disposed upward in the vertical direction, and thus, the GM cryocooler  10  is installed in a longitudinal direction. In this case, the load of the drive source (for example, rotation drive source  44 ) may be different from each other according to the movement directions of the displacer connector  16 . For example, due to the weight of the displacer connector  16  itself, the load of the drive source (for example, the rotation drive source  44 ) when the displacer connector  16  moves upward may be greater than the load of the drive source when the displacer connector  16  moves downward. 
         [0098]    The GM cryocooler  10  shown in  FIG. 5  adopts the asymmetrical gas assist configuration, and thus, it is possible to uniformize drive loads. For example, the first cross-sectional area S 1  is greater than the second cross-sectional area S 2 , and thus, the upward assist force Fup is greater than the downward assist force Fdown. Accordingly, it is possible to at least partially eliminate influences of the ownweight of the displacer connector  16 . This contributes to uniformization of freezing performance of the first cold head  14   a  and the second cold head  14   b . In addition, since a peak value of the drive load decreases due to uniformization of the drive load, the asymmetrical gas assist configuration contributes to a decrease in size of the drive source. 
         [0099]    In an embodiment, the internal space  53  of the drive mechanism housing  52  may be maintained to a predetermined pressure different from the low pressure PL. Similarly, it is possible to apply assist forces different from each other to the displacer connector  16  according to the movement direction of the displacer connector  16 . 
         [0100]    In an embodiment, the first cross-sectional area S 1  of the first connection rod  50   a  may be smaller than the second cross-sectional area S 2  of the second connection rod  50   b . For example, the first connection rod  50   a  has a circular cross-section having a first diameter, the second connection rod  50   b  has a circular cross-section having a second diameter, and the first diameter may be smaller than the second diameter. In this way, the upward assist force Fup can be smaller than the downward assist force Fdown. 
         [0101]      FIG. 7  is a sectional view schematically showing a GM cryocooler  10  according to still another embodiment of the present invention. Similarly to the GM cryocooler  10  shown in  FIG. 4 , the GM cryocooler  10  shown in  FIG. 7  does not have the common drive mechanism  40 . 
         [0102]    The GM cryocooler  10  includes the first connection rod  50   a  and the second connection rod  50   b , and the first connection rod  50   a  and the second connection rod  50   b  are axially connected to each other. The first displacer  16   a  is connected to the second displacer  16   b  via the first connection rod  50   a  and the second connection rod  50   b  such that the axial reciprocation of the first displacer  16   a  has the phase opposite to the phase of the axial reciprocation of the second displacer  16   b . The relative position of the second displacer  16   b  with respect to the first displacer  16   a  is not changed during the axial reciprocation of the first displacer  16   a  and the second displacer  16   b.    
         [0103]    The first connection rod  50   a  and the second connection rod  50   b  configure a single connection rod  50  which is fixedly connected to each other. Alternatively, the first connection rod  50   a  and the second connection rod  50   b  may be fixedly connected to each other via an intermediate member. 
         [0104]    The first connection rod  50   a  has the first cross-sectional area S 1  in a plane perpendicular to the axial direction, and the second connection rod  50   b  has the second cross-sectional area S 2  in a plane perpendicular to the axial direction. The first cross-sectional area S 1  is different from the second cross-sectional area S 2 . For example, the first cross-sectional area S 1  is greater than the second cross-sectional area S 2 . For example, the first connection rod  50   a  has a circular cross-sectional area having a first diameter, and the second connection rod  50   b  has a circular cross-section having a second diameter. The second diameter is smaller than the first diameter. 
         [0105]    Similarly to the GM cryocooler  10  shown  FIG. 1 , in the GM cryocooler  10  shown in  FIG. 7 , the valve timing shown in  FIG. 3  is adopted. 
         [0106]    In this way, the GM cryocooler  10  can be configured of a gas differential-pressure drive type cryocooler. In addition, it is possible to apply drive forces different from each other to the displacer connector  16  according to the movement direction of the displacer connector  16 . Accordingly, the upward movement and the downward movement of the displacer connector  16  can be symmetrized to each other. It is possible to uniformize the freezing performance of the first cold head  14   a  and the second cold head  14   b.    
         [0107]    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. 
         [0108]    For example, two cold heads may have configurations different from each other. The first cold head  14   a  and the second cold head  14   b  have sizes different from each other, and thus, may have freezing capacities different from each other. Alternatively, one or both of the first cold head  14   a  and the second cold head  14   b  may be multiple-staged cold head (for example, two-staged cold head). 
         [0109]    The reciprocation drive source  42  may have a linear motor which drives the axial reciprocation of the first displacer  16   a  and the second displacer  16   b.