Patent Publication Number: US-8991196-B2

Title: Regenerator, GM refrigerator, and pulse tube refrigerator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application filed under 35 U.S.C. 111(a) claiming benefit under 35 U.S.C. 120 and 365(c) of International Application PCT/JP2011/056045, filed on Mar. 15, 2011, and designated the U.S., which claims priority to Japanese Patent Application No. 2010-065037, filed on Mar. 19, 2010. The entire contents of the foregoing applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to regenerators, and more particularly to a regenerator usable in regenerative refrigerators and to regenerative refrigerators using the regenerator. 
     2. Description of the Related Art 
     Regenerative refrigerators such as Gifford-McMahon (GM) refrigerators and pulse tube refrigerators are capable of producing cold temperatures from low temperatures of approximately 100 K (kelvin) to cryogenic temperatures of approximately 4 K, and may be used for cooling superconducting magnets and detectors and in cryopumps. 
     For example, in GM refrigerators, working gas such as helium gas compressed in a compressor is introduced into a regenerator to be pre-cooled by a regenerator material in the regenerator. Further, after producing cold temperatures corresponding to work of expansion in an expansion chamber, the working gas again passes through the regenerator to return to the compressor. At this point, the working gas passes through the regenerator while cooling the regenerator material in the regenerator for working gas to be introduced next. Cold temperatures are periodically produced based on this process as one cycle. 
     In such regenerative refrigerators, a magnetic material such as HoCu 2  is used as the regenerator material of the regenerator as described above in the case of producing cryogenic temperatures lower than 30 K. 
     Further, lately, studies have been made of using helium gas as a regenerator material of regenerators. Such regenerators are also referred to as helium-cooling type regenerators. For example, Japanese Laid-Open Patent Application No. 11-37582 illustrates using multiple thermally conductive capsules filled with helium gas as a regenerator material for a regenerator. 
       FIG. 1  is a graph illustrating changes in the specific heat of helium gas and the specific heat of a HoCu 2  magnetic material relative to temperature.  FIG. 1  clearly illustrates that at cryogenic temperatures around approximately 10 K, the specific heat of helium gas of a pressure of approximately 1.5 MPa is higher than the specific heat of the HoCu 2  magnetic material. Accordingly, in such a temperature range, using helium gas in place of the HoCu 2  magnetic material makes it possible to perform heat exchange more efficiently. 
     Practically, however, it is not easy to manufacture the capsule as illustrated in Japanese Laid-Open Patent Application No. 11-37582. For example, a pressure of approximately 160 MPa at room temperature is necessary in order for the helium gas in the capsule to have a pressure of approximately 1.5 MPa. A capsule filled with such high-pressure helium cannot be easily manufactured. Further, the formation of such a capsule resistant to high pressure inevitably results in an increase in the thickness of the capsule, thus reducing its thermal conductivity. 
     Therefore, lately, there has been a report of a helium-cooling type regenerator configured by providing multiple containers with holes inside the regenerator and causing helium gas used as the working gas of an apparatus to flow through the containers through the holes. (See Japanese Patent No. 2650437.) 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, a helium-cooling type regenerator configured to retain cold temperatures of working gas includes a first section through which the working gas flows; a second section configured to accommodate helium gas as a regenerator material; and a regenerator material pipe connected to the second section and to a helium source. 
     According to an aspect of the present invention, a Gifford-McMahon refrigerator includes the helium-cooling type regenerator as set forth above; and a compressor configured to feed the working gas to an expansion chamber via the helium-cooling type regenerator and to collect the working gas from the expansion chamber via the helium-cooling type regenerator, wherein the regenerator material pipe is connected to the compressor as the helium source. 
     According to an aspect of the present invention, a pulse tube refrigerator includes the helium-cooling type regenerator as set forth above; and a compressor configured to feed the working gas to a pulse tube via a regenerator tube and to collect the working gas from the pulse tube via the regenerator tube, wherein the helium-cooling type regenerator is provided in the regenerator tube, and the regenerator material pipe is connected to the compressor as the helium source. 
     According to an aspect of the present invention, a pulse tube refrigerator includes the helium-cooling type regenerator as set forth above; a compressor configured to feed the working gas to a pulse tube via a regenerator tube and to collect the working gas from the pulse tube via the regenerator tube; and a buffer tank connected to the pulse tube, wherein the helium-cooling type regenerator is provided in the regenerator tube, and the regenerator material pipe is connected to the buffer tank as the helium source. 
     The object and advantages of the embodiments will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a graph illustrating changes in the specific heat of helium gas and the specific heat of a HoCu 2  magnetic material relative to temperature; 
         FIG. 2  is a schematic diagram illustrating a configuration of a common Gifford-McMahon (GM) refrigerator; 
         FIG. 3  is a schematic diagram illustrating a configuration of a conventional helium-cooling type regenerator; 
         FIG. 4  is a schematic cross-section view of a helium-cooling type regenerator, illustrating a configuration thereof, according to an embodiment of the present invention; 
         FIG. 5  is a diagram illustrating a configuration of a GM refrigerator including a regenerator according to an embodiment of the present invention; 
         FIG. 6  is a diagram illustrating a configuration of a pulse tube refrigerator including a regenerator according to an embodiment of the present invention; 
         FIG. 7  is a diagram illustrating a configuration of a pulse tube refrigerator including a regenerator according to an embodiment of the present invention; and 
         FIG. 8  is a diagram illustrating a configuration of a pulse tube refrigerator including a regenerator according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the above-described helium-cooling type regenerator of Japanese Patent No. 2650437, the regenerator is implemented by helium gas, also serving as working gas, flowing into and out of the containers through the holes formed in the containers. However, when such inflow and outflow of helium gas into and from the containers frequently occurs, a variation in the pressure of helium gas working as a regenerator material in the containers increases. Further, this destabilizes the temperature of helium gas, which is a regenerator material, thus making it difficult for the regenerator to maintain stable regeneration performance. 
     According to an aspect of the present invention, a helium-cooling type regenerator is provided that is capable of maintaining regeneration performance more stably than those of the conventional system, and a refrigerator is provided that includes the regenerator. 
     First, for a better understanding of embodiments of the present invention, a description is given of a common regenerative refrigerator including a helium-cooling type regenerator. 
       FIG. 2  is a schematic diagram illustrating a GM refrigerator as an example of the regenerative refrigerator. 
     Referring to  FIG. 2 , a GM refrigerator  1  includes a gas compressor  3  and a two-stage cold head  10  that operates as a refrigerator. The cold head  10  includes a first-stage cooling part  15  and a second-stage cooling part  50 . These cooling parts  15  and  50  are so connected to a flange  12  to be concentric with each other. 
     The first-stage cooling part  15  includes a hollow first-stage cylinder  20 , a first-stage displacer  22 , a first-stage regenerator  30 , a first-stage expansion chamber  31 , and a first-stage cooling stage  35 . The first-stage displacer  22  is so provided in the first-stage cylinder  20  as to be reciprocatable in axial directions. The first-stage regenerator  30  fills in the first-stage displacer  22 . The first-stage expansion chamber  31  is provided inside the first-stage cylinder  20  on the side of a low-temperature end  23   b . The volume of the first-stage expansion chamber  31  changes as the first-stage displacer  22  reciprocates. The first-stage cooling stage  35  is provided on the first-stage cylinder  20  near its low-temperature end  23   b . A first-stage seal  39  is provided between the inner wall surface of the first-stage cylinder  20  and the outer wall surface of the first-stage displacer  22 . 
     Multiple first-stage high-temperature-side flow passages  40 - 1  are formed in the first-stage displacer  22  on the side of a high-temperature end  23   a  of the first-stage cylinder  20  so as to allow helium gas to flow into and out of the first-stage regenerator  30 . Further, multiple first-stage low-temperature-side flow passages  40 - 2  are formed in the first-stage displacer  22  on the side of the low-temperature end  23   b  of the first-stage cylinder  20  so as to allow helium gas to flow into and out of the first-stage regenerator  30  and the first-stage expansion chamber  31 . 
     The second-stage cooling part  50  has substantially the same configuration as the first-stage cooling part  15 . The second-stage cooling part  50  includes a hollow second-stage cylinder  51 , a second-stage displacer  52 , a second-stage regenerator  60 , a second-stage expansion chamber  55 , and a second-stage cooling stage  85 . The second-stage displacer  52  is so provided in the second-stage cylinder  51  as to be reciprocatable in axial directions. The second-stage regenerator  60  fills in the second-stage displacer  52 . The second-stage expansion chamber  55  is provided inside the second-stage cylinder  51  on the side of a low-temperature end  53   b . The volume of the second-stage expansion chamber  55  changes as the second-stage displacer  52  reciprocates. The second-stage cooling stage  85  is provided on the second-stage cylinder  51  near its low-temperature end  53   b . A second-stage seal  59  is provided between the inner wall surface of the second-stage cylinder  51  and the outer wall surface of the second-stage displacer  52 . 
     A second-stage high-temperature-side flow passage  40 - 3  is formed in the second-stage displacer  52  on the side of a high-temperature end  53   a  of the second-stage cylinder  51  so as to allow helium gas to flow into and out of the second-stage regenerator  60 . Further, multiple second-stage low-temperature-side flow passages  54 - 2  are formed in the second-stage displacer  52  on the side of the low-temperature end  53   b  of the second-stage cylinder  51  so as to allow helium gas to flow into and out of the second-stage expansion chamber  55 . 
     In the GM refrigerator  1 , high-pressure helium gas is fed from the gas compressor  3  to the first-stage cooling part  15  via a valve (intake valve)  5  and a pipe  7 . Further, low-pressure helium gas is discharged from the first-stage cooling part  15  to the gas compressor  3  via the pipe  7  and a valve (exhaust valve)  6 . The first-stage displacer  22  and the second-stage displacer  52  are caused to reciprocate by a drive motor  8 . In conjunction with this reciprocation, the valve  5  and the valve  6  are opened and closed to control the timing of taking in and discharging helium gas. 
     The high-temperature end  23   a  of the first-stage cylinder  20  is, for example, at room temperature. The low-temperature end  23   b  of the first-stage cylinder  20  is, for example, at 20 K through 40 K. The high-temperature end  53   a  of the second-stage cylinder  51  is, for example, at 20 K through 40 K. The low-temperature end  53   b  of the second-stage cylinder  51  is, for example, at 4 K. 
     Next, a brief description is given of an operation of the GM refrigerator  1  of this configuration. 
     First, it is assumed that the first-stage displacer  22  and the second-stage displacer  52  are at their respective bottom dead ends inside the first-stage cylinder  20  and the second-stage cylinder  51  with the valve  5  and the valve  6  being closed. 
     In this state, opening the valve  5  with the valve  6  being closed causes high-pressure helium gas to flow from the gas compressor  3  into the first-stage cooling part  15 . The high-pressure helium gas flows into the first-stage regenerator  30  through the first-stage high-temperature-side flow passages  40 - 1  to be cooled to a predetermined temperature by the regenerator material of the first-stage regenerator  30 . The cooled helium gas flows into the first-stage expansion chamber  31  through the first-stage low-temperature-side flow passages  40 - 2 . 
     Part of the high-pressure helium gas that has flown into the first-stage expansion chamber  31  flows into the second-stage regenerator  60  through the second-stage high-temperature-side flow passage  40 - 3 . This helium gas is further cooled to a lower predetermined temperature by the regenerator material of the second-stage regenerator  60  to flow into the second-stage expansion chamber  55  through the second-stage low-temperature-side flow passages  54 - 2 . As a result, the pressure increases inside the first-stage expansion chamber  31  and the second-stage expansion chamber  55 . 
     Next, as the first-stage displacer  22  and the second-stage displacer  52  move to their respective top dead ends, the valve  5  is closed, and the valve  6  is opened. As a result, the helium gas inside the first-stage expansion chamber  31  and the second-stage expansion chamber  55  is reduced in pressure and increases in volume (expands), so that low temperatures are produced in the first-stage expansion chamber  31  and the second-stage expansion chamber  55 . Further, this cools the first-stage cooling stage  35  and the second-stage cooling stage  85 . 
     Next, the first-stage displacer  22  and the second-stage displacer  52  are caused to move toward their respective bottom dead ends. With this movement, the low-pressure helium gas travels back the above-described route to return to the gas compressor  3  through the valve  6  and the pipe  7  while cooling the first-stage regenerator  30  and the second-stage regenerator  60 . Thereafter, the valve  6  is closed. 
     By employing the above-described operation as one cycle and repeatedly performing the above-described operation, in the first-stage cooling stage  35  and the second-stage cooling stage  85 , it is possible to absorb heat from objects of cooling (not graphically illustrated) thermally coupled to the first-stage cooling stage  35  and the second-stage cooling stage  85 , respectively, so that it is possible to cool the objects of cooling. 
     Here, for example, in the case of producing cryogenic temperatures lower than 30 K in the second-stage cooling stage  85 , a magnetic material such as HoCu 2  is used as the regenerator material of the second-stage regenerator  60 . 
     Further, lately, it has been proposed to use a so-called helium-cooling type regenerator that uses helium gas as the regenerator material of the regenerator. 
       FIG. 3  is a schematic diagram illustrating a configuration of a conventional helium-cooling type regenerator  60 A along with members on its periphery. The helium-cooling type regenerator  60 A is used as the second-stage regenerator  60  of the GM refrigerator  1  illustrated in  FIG. 2 . In  FIG. 3 , the same members as those in  FIG. 2  are referred to by the same reference numerals as in  FIG. 2 . 
     As illustrated in  FIG. 3 , the conventional helium-cooling type regenerator  60 A is used as a second-stage regenerator in the second-stage displacer  52  illustrated in  FIG. 2 . 
     The helium-cooling type regenerator  60 A includes multiple containers  62 . Each of these containers  62  has an elongated bar shape, and is elongated (extends) along the vertical directions of the regenerator  60 A (that is, for example, along the second-stage cylinder  51  in a direction from its high-temperature end  53   a  to its low-temperature end  53   b ). Each of the containers  62  has a hole  65  formed at its end on the low-temperature end  53   b  side of the second-stage cylinder  51 . Helium gas  68  serving as a regenerator material is present in the containers  62 . 
     In general, helium gas is higher in specific heat than magnetic materials such as HoCu 2  around 10 K. Using helium gas as a regenerator material makes it possible to more efficiently cool working gas (helium gas) flowing through the regenerator  60 A. 
     However, according to the regenerator  60 A of the above-described configuration, the helium gas  68 , which is also working gas, easily flows into and out of the containers  62  through the holes  65  provided in the containers  62 . When such inflow and outflow of the helium gas  68  into and from the containers  62  frequently occur, a greater variation is caused in the pressure of the helium gas  68  working as a regenerator material in the containers  62 . Further, this destabilizes the temperature of the helium gas  68 , which is a regenerator material, thus making it difficult for the regenerator  60 A to maintain stable regeneration performance. 
     In order to solve these problems, according to an aspect of the present invention, a helium-cooling type regenerator includes a first section through which working gas flows and a second section that stores helium gas as a regenerator material, and the second section is connected to a regenerator material pipe connected to a helium source. According to this regenerator, when the pressure of helium gas decreases in the second section, high-pressure helium gas is introduced into the second section through the regenerator material pipe so as to compensate for the decrease in the pressure of helium gas. Therefore, according to the helium-cooling type regenerator of the aspect of the present invention, it is possible to reduce or eliminate such a problem of the pressure variation and associated temperature instability of a regenerator material (helium gas) in a container as in the conventional helium-cooling type regenerator  60 A. 
     A description is given below, with reference to the accompanying drawings, embodiments of the present invention. 
       FIG. 4  is a diagram illustrating a helium-cooling type regenerator according to an embodiment of the present invention. 
     As illustrated in  FIG. 4 , a helium-cooling type regenerator  160  according to this embodiment may be provided in, for example, the second-stage displacer  52  of the above-described GM refrigerator  1  ( FIG. 2 ). 
     The regenerator  160  includes multiple hollow tubes  165  and a space part  175 . The space part  175  corresponds to a region where the hollow tubes  165  are absent in the regenerator  160 . The positions of the hollow tubes  165  are fixed by upper and lower flanges  164 . The flanges  164  interrupt communication between the space part  175  and the inside of the hollow tubes  165 . 
     In the example of  FIG. 4 , the inside of the hollow tubes  165  may correspond to a first section of the regenerator  160 . Working gas such as helium flows through the hollow tubes  165 . Further, in the example of  FIG. 4 , the space part  175  may correspond to a second section of the regenerator  160 . This space part  175  serves as a part that contains (accommodates) helium gas, which is a regenerator material. The regenerator  160  further includes a first passage  161  and a second passage  162  for working gas. The first and second passages  161  and  162  communicate with the first section. 
     The regenerator  160  further includes a regenerator material pipe  170 . The regenerator material pipe  170  has a first end connected to the space part  175  of the regenerator  160 , and has a second end connected to a so-called “helium source” (not graphically illustrated). 
     In its concept, the “helium source” includes any part that stores high-pressure helium gas and/or liquid helium. For example, when the regenerator  160  is used for a regenerator tube of a GM refrigerator, the “helium source” may be a compressor that feeds and collects working gas. Further, when the regenerator  160  is used for a regenerator tube of a pulse tube refrigerator, the “helium source” may be a compressor that feeds and collects working gas or a buffer tank connected to a pulse tube. 
     According to the regenerator  160  configured as illustrated in  FIG. 4 , working gas flows along mainstream directions P. That is, working gas enters the first passage  161  and passes through the hollow tubes  165  to be let out (discharged) through the second passage  162 , or moves in the reverse direction. 
     Meanwhile, helium gas regenerator material is introduced into the space part  175  from the helium source through the regenerator material pipe  170 . Here, the pressure of the regenerator material inside the space part  175  is substantially equal to the pressure of the helium source immediately after the start of the operation of the regenerator  160 . Thereafter, as the temperature inside the regenerator  160  starts to decrease with the operation of the regenerator  160 , the pressure of the regenerator material inside the space part  175  decreases with the temperature decrease. However, in response to this pressure decrease, helium gas is supplementally fed from the helium source into the space part  175  through the regenerator material pipe  170 . Accordingly, a change in temperature does not cause so great a change in the pressure of the regenerator material inside the space part  175 . Therefore, it is possible for the regenerator  160  of this embodiment to maintain stable regeneration performance during its operation. 
     In the example of  FIG. 4 , in the regenerator  160 , the first section is defined by the first passage  161 , the internal spaces of the hollow tubes  165 , and the second passage  162 , and the second section is defined by the space part  175 . That is, working gas flows through the hollow tubes  165 , and a regenerator material is accommodated in the spacer part  175 . However, according to this embodiment, the regenerator  160  is not limited to this configuration. For example, the first section and the second section may be opposite to the configuration of  FIG. 4 . That is, a regenerator may be formed by accommodating a regenerator material inside the hollow tubes  165  and causing working gas to flow through the space part  175 . In this case, the regenerator material tube  170  is connected to the hollow tubes  165 . 
     Further, in the example of  FIG. 4 , the inside of the regenerator  160  is divided into two sections by the inside of the hollow tubes  165  and the space part  175 . Alternatively, however, a regenerator may be divided into two sections by other methods. For example, the inside of a regenerator may be divided by a container having an internal space and a space part around the container. 
     In the above description, a description is given of configurations and their effects according to the embodiment of the present invention, taking, as an example, a case where a regenerator material inside a regenerator is composed of helium gas alone. According to embodiments of the present invention, a regenerator material in a regenerator may be composed of multiple regenerator materials. For example, it is possible to use a HoCu 2  magnetic material on the high-temperature side and helium on the intermediate and low-temperature side in a single regenerator. It is also possible to further use a magnetic material such as Gd 2 O 2 S as a third regenerator material on the yet lower-temperature side. 
     A helium-cooling type regenerator according to embodiments of the present invention may be applied to various kinds of regenerative refrigerators such as GM refrigerators and pulse tube refrigerators. A description is given below of a configuration of a regenerative refrigerator to which a helium-cooling type regenerator may be applied according to an embodiment of the present invention. 
       FIG. 5  is a diagram illustrating a configuration of a GM refrigerator  100  including the regenerator  160  according to an embodiment of the present invention. The GM refrigerator  100  has the same basic configuration as the GM refrigerator  1  illustrated in  FIG. 2 , and accordingly, the basic configuration of the GM refrigerator  100  is not described in detail below. Further, in the GM refrigerator  100 , the same members as those of the GM refrigerator  1  illustrated in  FIG. 2  are referred to by the same reference numerals as in  FIG. 2 . 
     The GM refrigerator  100  includes the regenerator  160  of the above-described embodiment inside the second-stage displacer  52 . Further, according to this embodiment, the second-stage cylinder  51  is connected to the high-pressure side of the compressor  3  through the regenerator material pipe  170  ( FIG. 4 ). Accordingly, the gap between the second-stage cylinder  51  and the second-stage displacer  52  communicates with the regenerator material pipe  170 . Further, the second-stage displacer  52  is provided with small holes  179 . A space containing a regenerator material inside the regenerator  160  (the space part  175  in  FIG. 4 ) and the gap communicate with each other through these small holes  179 . An additional seal  159  is provided in this gap. This additional seal  159  prevents a regenerator material flowing through the regenerator material tube  170  from mixing with working gas. 
     When the temperature of the regenerator  160  decreases so that the pressure of the space part  175  containing a regenerator material inside the regenerator  160  decreases during the operation of the GM refrigerator  100 , helium gas is fed from the compressor  3  into the space part  175  through the regenerator material pipe  170 . Accordingly, as described above, the regenerator material inside the regenerator  160  is less likely to be subject to a great pressure change so that it is possible for the regenerator material to maintain stable regeneration performance during the operation of the regenerator  160 . Accordingly, it is possible for the GM refrigerator  100  of this embodiment to stably produce cold temperatures in the second-stage cooling stage  85 . 
     Here, the compressor  3 , which may be a common compressor, includes an internal bypass valve for releasing pressure. Accordingly, when the pressure increases inside the space part  175  and the regenerator material pipe  170  of the regenerator  160  at the time of stoppage of the GM refrigerator  100 , this bypass valve starts to operate to allow a generator material to flow from the high-pressure side to the low-pressure side inside the compressor  3 . Therefore, according to the GM refrigerator  100  of this embodiment, no member for releasing a high-pressure regenerator material is newly required in particular in the regenerator  160 . 
     In the example of  FIG. 5 , the regenerator material pipe  170  is connected to the high-pressure side of the compressor  3 . Alternatively, the regenerator material pipe  170  may be connected to the low-pressure side of the compressor  3 . 
       FIG. 6  is a diagram illustrating a configuration of a pulse tube refrigerator including a regenerator according to an embodiment of the present invention. 
     As illustrated in  FIG. 6 , a pulse tube refrigerator  200  is a two-stage pulse tube refrigerator. 
     The pulse tube refrigerator  200  includes a compressor  212 , a first-stage regenerator tube  240 , a second-stage regenerator tube  280 , a first-stage pulse tube  250 , a second-stage pulse tube  290 , a first pipe  256 , a second pipe  286 , an orifice  260 , an orifice  261 , and opening and closing valves V 1 , V 2 , V 3 , V 4 , V 5  and V 6 . 
     The first-stage regenerator tube  240  includes a high-temperature end  242  and a low-temperature end  244 . The second-stage regenerator tube  280  includes the high-temperature end  244  (corresponding to the low-temperature end  244  of the first-stage regenerator tube  240 ) and a low-temperature end  284 . The first-stage pulse tube  250  includes a high-temperature end  252  and a low-temperature end  254 . The second-stage pulse tube  290  includes a high-temperature end  292  and a low-temperature end  294 . Heat exchangers are provided at the high-temperature ends  252  and  292  and the low-temperature ends  254  and  294  of the first-stage and second-stage pulse tubes  250  and  290 . The low-temperature end  244  of the first-stage regenerator tube  240  is connected to the low-temperature end  254  of the first-stage pulse tube  250  via the first pipe  256 . Further, the low-temperature end  284  of the second-stage regenerator tube  280  is connected to the low-temperature end  294  of the second-stage pulse tube  290  via the second pipe  286 . 
     A refrigerant passage on the high-pressure side (the outlet or discharge side) of the compressor  212  branches off in three directions at Point A. First, second, and third refrigerant feed channels H 1 , H 2 , and H 3  are formed in these three directions, respectively. The first refrigerant feed channel H 1  forms a channel that connects the high-pressure side of the compressor  212 , a first high-pressure-side pipe  215 A provided with the opening and closing valve V 1 , a common pipe  220 , and the first-stage regenerator tube  240 . The second refrigerant feed channel H 2  forms a channel that connects the high-pressure side of the compressor  212 , a second high-pressure-side pipe  225 A provided with the opening and closing valve V 3 , a common pipe  230  provided with the orifice  260 , and the first-stage pulse tube  250 . The third refrigerant feed channel H 3  forms a channel that connects the high-pressure side of the compressor  212 , a third high-pressure-side pipe  235 A provided with the opening and closing valve V 5 , a common pipe  299  provided with the orifice  261 , and the second-stage pulse tube  290 . 
     A refrigerant passage on the low-pressure side (the intake or collection side) of the compressor  212  branches off in three directions into first, second, and third refrigerant collection channels L 1 , L 2 , and L 3 . The first refrigerant collection channel L 1  forms a channel that connects the first-stage regenerator tube  240 , the common pipe  220 , a first low-pressure-side pipe  215 B provided with the opening and closing valve V 2 , Point B, and the compressor  212 . The second refrigerant collection channel L 2  forms a channel that connects the first-stage pulse tube  250 , the common pipe  230  provided with the orifice  260 , a second low-pressure-side pipe  225 B provided with the opening and closing valve V 4 , Point B, and the compressor  212 . The third refrigerant collection channel L 3  forms a channel that connects the second-stage pulse tube  290 , the common pipe  299  provided with the orifice  261 , a third low-pressure-side pipe  235 B provided with the opening and closing valve V 6 , Point B, and the compressor  212 . 
     A general principle of operation of the pulse tube refrigerator  200  having this configuration is known to a person having ordinary skill in the art, and accordingly, a description of the principle of operation of the pulse tube refrigerator  200  is omitted. 
     According to the pulse tube refrigerator  200  of this embodiment, a regenerator  265  having the same configuration as the regenerator  160  illustrated in  FIG. 4  is provided in the second-stage regenerator tube  280 . Further, a space part containing a regenerator material inside the regenerator  265  is connected to the high-pressure side of the compressor  212  via a regenerator material pipe  270  including a flow resistance  275 . The flow resistance  275  may be omitted. 
     According to this embodiment, when the temperature of the regenerator  265  decreases so that the pressure of the space part containing a regenerator material inside the regenerator  265  decreases during the operation of the pulse tube refrigerator  200 , helium gas is fed into the space part from the compressor  212  through the regenerator material pipe  270 . As a result, as described above, the regenerator material inside the regenerator  265  is less likely to be subject to a great pressure change so that it is possible for the regenerator material to maintain stable regeneration performance during the operation of the regenerator  265 . Accordingly, it is possible for the pulse tube refrigerator  200  as well to stably produce cold temperatures at the low-temperature end  294  of the second-stage pulse tube  290 . 
     In the example of  FIG. 6 , the regenerator material pipe  270  may include another flow resistance such as a valve between the regenerator  265  and the compressor  212 . In this case, it is possible to control the flow rate of helium gas fed into the space part of the regenerator  265  containing a regenerator material during the operation of the pulse tube refrigerator  200 . 
     Further, in the example of  FIG. 6 , the regenerator material pipe  270  is connected to the high-pressure side of the compressor  212 . Alternatively, the regenerator material tube  270  may be connected to the low-pressure side of the compressor  212 . 
       FIG. 7  is a diagram illustrating a configuration of a pulse tube refrigerator including a regenerator according to another embodiment of the present invention. 
     A pulse tube refrigerator  300  illustrated in FIG.  7  basically has substantially the same configuration as the pulse tube refrigerator  200  illustrated in  FIG. 6 . In  FIG. 7 , the same members as those illustrated in  FIG. 6  are referred to by the same reference numerals as in  FIG. 6 . 
     According to this embodiment, the pulse tube refrigerator  300  includes a buffer tank  366 . The buffer tank  366  is connected to the high-temperature end  252  of the first-stage pulse tube  250  via a pipe  362  including an orifice  364 . According to the pulse tube refrigerator  300 , the regenerator  265  having the same configuration as the regenerator  160  illustrated in  FIG. 4  is connected to the buffer tank, instead of the compressor  212 , through a regenerator material pipe  370 . 
     According to this embodiment, when the temperature of the regenerator  265  decreases so that the pressure of the space part containing a regenerator material inside the regenerator  265  decreases during the operation of the pulse tube refrigerator  300 , helium gas is fed into the space part containing a regenerator material from the buffer tank  366  through the regenerator material pipe  370 . As a result, as described above, the regenerator material inside the regenerator  265  is less likely to be subject to a great pressure change so that it is possible for the regenerator material to maintain stable regeneration performance during the operation of the regenerator  265 . Accordingly, it is possible for the pulse tube refrigerator  300  as well to stably produce cold temperatures at the low-temperature end  294  of the second-stage pulse tube  290 . 
       FIG. 8  is a diagram illustrating a configuration of a pulse tube refrigerator including a regenerator according to yet another embodiment of the present invention. 
     A pulse tube refrigerator  400  illustrated in  FIG. 8  basically has substantially the same configuration as the pulse tube refrigerator  200  illustrated in  FIG. 6 . In  FIG. 8 , the same members as those illustrated in  FIG. 6  are referred to by the same reference numerals as in  FIG. 6 . 
     According to this embodiment, the pulse tube refrigerator  400  includes a regenerator material pipe  470  that connects a second section (a space containing a regenerator material) inside the regenerator  265  provided in the second-stage regenerator tube  280  to the high-pressure side of the compressor  212 . 
     The regenerator material pipe  470  includes a first part  470 A, a second part  470 B, and a third part  470 C. The first part  470 A of the regenerator material pipe  470  is connected to the high-pressure side of the compressor  212 . For example, in the example of  FIG. 8 , the first part  470 A is connected to the second high-pressure-side pipe  225 A at Point C. Further, the second part  470 B of the regenerator material pipe  470  is provided around the first-stage regenerator tube  240 . Further, the third part  470 C of the regenerator material pipe  470  is connected to the regenerator  265  of the second-stage regenerator tube  280 . 
     According to this configuration, during the operation of the pulse tube refrigerator  400 , when the temperature of the regenerator  265  decreases so that the pressure of the space part containing a regenerator material inside the regenerator  265  decreases, helium gas flows from the compressor  212  to the third part  470 C of the regenerator material tube  470  through the second high-pressure-side pipe  225 A. This helium gas is pre-cooled by the first-stage regenerator tube  240  when passing through the second part  470 B of the regenerator material pipe  470 . Accordingly, the pre-cooled helium gas is introduced into the regenerator  265  of the second-stage regenerator tube  280  through the third part  470 C of the regenerator material pipe  470 . Therefore, according to this configuration, it is possible to more effectively control a possible temperature increase caused by the introduction of a regenerator gas into the regenerator  265 . 
     All examples and conditional language provided herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority or inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.