REGENERATIVE REFRIGERATOR

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based upon and claims the benefit of priority of Japanese Patent Application No. 2009-063608, filed on Mar. 16, 2009, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to regenerative refrigerators. The present invention more particularly relates to a regenerative refrigerator capable of attaining cryogenic temperatures with the reciprocating motions of a displacer filled with a regenerator material in a cylinder.

2. Description of the Related Art

Examples of refrigerators widely used in cryogenic ranges include a regenerative refrigerator. The regenerative refrigerator includes a regenerative heat exchanger called a regenerator. The regenerator contains a heat exchange material called a regenerator material in the container.

A material having high specific heat at a target temperature is used as the regenerator material. The refrigerator is used in a wide temperature range of room temperature to approximately 4.2 K. Accordingly, it is desirable to select a material that has as high specific heat as possible over the entire range. The temperature dependence of specific heat varies greatly from material to material, and no single material can support the entire temperature range. Accordingly, an optimum combination of materials is used in accordance with temperature.

Further, refrigerators include those with a lowest attainable temperature of 4.2 K used for condensing liquid helium and those used at 10 K in cryopumps and the like. Those of a two-stage type having two regenerators are often used.

Usually, 10 K refrigerators use a wire mesh of copper or stainless steel for a first-stage regenerator and lead spheres for a second-stage regenerator. Lead has been widely used because it is higher in specific heat than other materials and has a certain degree of structural strength at temperatures lower than or equal to 50 K and is also inexpensive. (See, for example, Japanese Laid-Open Patent Application No. 3-99162.)

In the member states of the European Union, however, due to its effect on the environment, the use of lead has been strictly restricted by the Restriction of Hazardous Substances Directive or RoHS, which took effect on Jul. 1, 2006. Therefore, regenerative refrigerators using lead as a regenerator material may be subject to this restriction. Accordingly, various kinds of regenerator materials have been proposed as replacements for lead as a regenerator material used in regenerative refrigerators. (See, for example, Japanese Laid-Open Patent Application No. 2004-225920.)

Japanese Laid-Open Patent Application No. 2004-225920 describes an alloy of indium, bismuth, and a third material as a regenerator material to substitute for lead. Indium has the specific heat next highest to that of lead at temperatures lower than or equal to 50 K. The idea is to take advantage of this characteristic of indium.

Indium, however, is a very soft metal and cannot be used as a regenerator material as it is. Therefore, indium is made into an alloy with bismuth and another metal to have a hardness required for a regenerator material, but is still insufficient in hardness to be practically used as a regenerator material. Further, there is also a problem in that indium, whose price is approximately three times the price of lead, is too expensive to be used as a regenerator material. In response to this, bismuth or an alloy of bismuth and antimony has been proposed as a regenerator material to replace lead. (See, for example, Japanese Laid-Open Patent Application No. 2006-242484.)

As a regenerator material to replace lead, bismuth, which is also used as a material for cosmetics, is believed to be highly safe and free of concern for environmental pollution, and is also inexpensive. Bismuth, however, is lower in specific heat than lead. In particular, in a cryogenic environment at or below 15 K, the specific heat of bismuth is significantly reduced. Therefore, although bismuth has good characteristics in terms of safety and burdens on the environment as described above, it has been believed difficult to use bismuth as a regenerator material in regenerative refrigerators for achieving cryogenic temperatures.

In order to solve this problem, it has been proposed to mix bismuth with other regenerator materials. (See, for example, Japanese Laid-Open Patent Application No. 2006-242484.)

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a regenerative refrigerator includes a cylinder formed of a material having a low thermal conductivity and a high airtightness, the cylinder having a cylindrical interior circumferential surface; a displacer provided in the cylinder so as to be reciprocatable in axial directions thereof with an expansion space formed between one end of the cylinder and the displacer, the displacer having an exterior circumferential surface along a cylindrical shape of the interior circumferential surface of the cylinder, the exterior circumferential surface being slightly smaller in diameter than the interior circumferential surface; a groove pattern formed on one of the exterior circumferential surface of the displacer and the interior circumferential surface of the cylinder so as to form a first gas passage connecting a first end and a second end of the one of the exterior circumferential surface of the displacer and the interior circumferential surface of the cylinder, the groove pattern including a groove having at least a part thereof extending along a direction to cross the axial directions of the displacer so as to cause a gas flowing from one to another of the first end and the second end of the one of the exterior circumferential surface of the displacer and the interior circumferential surface of the cylinder in a gap between the exterior circumferential surface of the displacer and the interior circumferential surface of the cylinder to actively exchange heat with the cylinder and the displacer; a second gas passage through which the gas is supplied to and collected from the expansion space; and a regenerator material formed of bismuth granules and provided in at least a part of the second gas passage, wherein a lowest attainable temperature of the regenerative refrigerator is in a range of cryogenic temperatures higher than or equal to 5 K and lower than or equal to 15 K in an unloaded state.

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 as claimed.

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 cross-sectional view of a regenerative refrigerator according to a first embodiment of the present invention, illustrating a basic configuration of the regenerative refrigerator;

FIG. 2 is a cross-sectional view of a two-stage regenerative refrigerator according to a second embodiment of the present invention, illustrating a schematic configuration of the two-stage regenerative refrigerator;

FIG. 3 is a partial cross-sectional view of a second-stage displacer according to the second embodiment of the present invention, illustrating a configuration of the second-stage displacer;

FIG. 4 is a graph illustrating the volumetric specific heats of materials forming regenerator materials;

FIGS. 5A and 5B illustrate the characteristics of the two-stage regenerative refrigerator according to the second embodiment of the present invention and the characteristics of a conventional two-stage regenerative refrigerator in comparison (with no load at a compressor operating frequency of 50 Hz), where FIG. 5A illustrates first-stage temperature characteristics and FIG. 5B illustrates second-stage temperature characteristics;

FIGS. 6A and 6B illustrate the characteristics of the two-stage regenerative refrigerator according to the second embodiment of the present invention and the characteristics of the conventional two-stage regenerative refrigerator in comparison (loaded at a compressor operating frequency of 50 Hz), where FIG. 6A illustrates first-stage temperature characteristics and FIG. 6B illustrates second-stage temperature characteristics;

FIGS. 7A and 7B illustrate the characteristics of the two-stage regenerative refrigerator according to the second embodiment of the present invention and the characteristics of the conventional two-stage regenerative refrigerator in comparison (with no load at a compressor operating frequency of 60 Hz), where FIG. 7A illustrates first-stage temperature characteristics and FIG. 73 illustrates second-stage temperature characteristics;

FIGS. 8A and 8B illustrate the characteristics of the two-stage regenerative refrigerator according to the second embodiment of the present invention and the characteristics of the conventional two-stage regenerative refrigerator in comparison (loaded at a compressor operating frequency of 60 Hz), where FIG. 8A illustrates first-stage temperature characteristics and FIG. 83 illustrates second-stage temperature characteristics;

FIG. 9 is a graph illustrating a relationship between the bismuth size and the refrigerating capacity of a bismuth regenerator material according to the second embodiment of the present invention;

FIG. 10 is a partial cross-sectional view of the second-stage displacer of the two-stage regenerative refrigerator according to the second embodiment of the present invention, illustrating another configuration of the second-stage displacer;

FIG. 11 is a partial cross-sectional view of a first-stage displacer of the two-stage regenerative refrigerator according to the second embodiment of the present invention, illustrating a configuration of the first-stage displacer;

FIGS. 12A through 12H are schematic developments illustrating groove patterns formed on displacer surfaces according to the second embodiment of the present invention; and

FIG. 13 is a cross-sectional view of the regenerative refrigerator according to the first embodiment of the present invention, illustrating another basic configuration of the regenerative refrigerator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

However, in the case of mixing bismuth with other regenerator materials as described above, it is difficult to determine the mixture ratio of bismuth and other regenerator materials. Further, there is also the problem of an increase in the prices of regenerative refrigerators because other regenerator materials, which may be used as normal regenerator materials, are expensive.

According to one aspect of the present invention, a regenerative refrigerator may be provided that is capable of attaining cryogenic temperatures lower than or equal to 15 K while using bismuth as a regenerator material.

A description is given below, with reference to the accompanying drawings, of embodiments of the present invention.

FIG. 1 is a diagram illustrating a basic configuration of a regenerative refrigerator according to a first embodiment of the present invention. The regenerative refrigerator includes a cylinder 1 and a cylindrical displacer 2 provided in the cylinder 1. The cylinder 1 is formed of a rigid material low in thermal conductivity and high in airtightness, such as stainless steel. A helical gas passage 4 is foamed on the cylindrical exterior circumferential surface of the displacer 2. The helical gas passage 4 includes one or more helical groove patterns 2a connecting the upper end surface and the lower end surface of the displacer 2.

The displacer 2 has a hollow structure. A gas passage 3 is formed inside the displacer 2. A regenerator material 5 is contained in the gas passage 3. The regenerator material 5 has a high heat capacity at operating temperatures. Bismuth is used as the regenerator material 5. An expansion space 6 is defined between the displacer 2 and the lower end of the cylinder 1.

A refrigerant gas supplied from above is supplied to the expansion space 6 through the gas passage 3 inside the displacer 2. Further, part of the refrigerant gas diverges from the gas passage 3 to flow through a gap between the displacer 2 and the cylinder 1. This diverged (part of the) gas flows downward through the helical gas passage 4 provided on the exterior circumferential surface of the displacer 2 while exchanging heat with the surfaces of the displacer 2 and the cylinder 1, to be supplied to the expansion space 6.

The refrigerant gas is expanded and cooled in the expansion space 6 with the (upward) movement of the displacer 2. When the refrigerant gas is collected, part of the cooled refrigerant gas flows through the gas passage 3 to cool the bismuth regenerator material 5. A remaining part of the refrigerant gas cooled in the expansion space 6 diverges to flow upward through the helical gas passage 4 while exchanging heat with the surfaces of the displacer 2 and the cylinder 1, and thereafter merges with the refrigerant gas that has flowed through the gas passage 3.

As described above, the regenerative refrigerator according to this embodiment uses the bismuth regenerator material 5. As described above, bismuth, which is also used as a material for cosmetics, is believed to be highly safe and free of concern for environmental pollution, and is also inexpensive. Therefore, the bismuth regenerative material 5 is preferable in terms of safety and environmental burdens.

However, bismuth is lower in specific heat than lead, and in particular, the specific heat of bismuth is significantly reduced in a cryogenic environment at or below 15 K. Therefore, it has been believed difficult to use bismuth as a regenerator material in regenerative refrigerators that attain cryogenic temperatures lower than or equal to 15 K.

According to one aspect of the present invention, at the same time that bismuth is used as the regenerator material 5, the helical gas passage 4 including the one or more groove patterns 2a is provided on the exterior circumferential surface of the displacer 2 so as to allow a refrigerant gas to pass through the helical gas passage 4. As a result, compared with a configuration where a refrigerant gas flows through only a gas passage inside a displacer, the refrigerant gas comes into sufficient contact with the surfaces of the displacer 2 and the cylinder 1. This allows more heat exchange between the surfaces of the gas passage and the refrigerant gas.

As a result, even when bismuth, which is lower in specific heat than conventionally-used lead, is used as the regenerator material 5 at cryogenic temperatures lower than or equal to 15 K, it is possible to improve thermal efficiency with respect to the bismuth regenerator material 5 and to improve refrigeration performance.

Next, a description given of a regenerative refrigerator according to a second embodiment of the present invention, which is based on the above-described first embodiment. In the following description, a two-stage Gifford-McMahon cycle refrigerator (hereinafter referred to as “two-stage GM refrigerator”) is taken as an example of the regenerative refrigerator of this embodiment. FIG. 2 is a schematic diagram illustrating a configuration of the two-stage GM refrigerator, which attains cryogenic temperatures of approximately 4.2 K to approximately 10 K. In the following, a description is given of the case of attaining a cryogenic temperature of approximately 10 K.

Referring to FIG. 2, a helium compressor 10 compresses helium gas to approximately 20 Kgf/cm², and supplies high-pressure helium gas. The high-pressure helium gas is supplied into a first-stage cylinder 11 through an intake valve V1 and a gas passage 16. A second-stage cylinder 12 is joined to the first-stage cylinder 11.

A first-stage displacer 13 and a second-stage displacer 14, which are joined to each other, are contained in the first-stage cylinder 11 and the second-stage cylinder 12, respectively. A shaft member S extends upward from the first-stage cylinder 11 to be joined to a crank mechanism 15 which is in turn joined to a drive motor M.

The first-stage displacer 13 and the second-stage displacer 14 are hollow and have internal spaces (cavities), in which regenerator materials 17 and 18, respectively, are contained. Further, the first-stage displacer 13 and the second-stage displacer 14 have gas passages 23a and 23b and gas passages 24a and 24b, respectively. The gas passages 23a and 23b connect the internal space and the outside of the first-stage displacer 13, and the gas passages 24a and 24b connect the internal space and the outside of the second-stage displacer 14. Further, a first-stage expansion space 21 is defined between the first-stage displacer 13 and the first-stage cylinder 11, and a second-stage expansion space 22 is defined between the second-stage displacer 14 and the second-stage cylinder 12.

Usually, the first-stage cylinder 11 and the second-stage cylinder 12 are formed of a material having sufficient strength, low thermal conductivity, and a capability to sufficiently block or prevent leaking of helium gas, such as stainless steel (for example, SUS304 of Japanese Industrial Standards). Further, the first-stage displacer 13 and the second-stage displacer 14 are formed of a material having low specific gravity, sufficient wear resistance, relatively high strength, and low thermal conductivity, such as fabric-containing phenolic resin (Bakelite).

The high-pressure helium gas supplied through the intake valve V1 from the helium compressor 10 is supplied into the first-stage cylinder 11 through the gas passage 16 to be further supplied to the first-stage expansion space 21 through the gas passage 23a, the regenerator material 17 for the first stage, and the gas passage 23b. The regenerator material 17 is formed of a wire mesh or the like. The compressed helium gas in the first-stage expansion space 21 is further supplied to the second-stage expansion space 22 through the gas passage 24a, the regenerator material 18 for the second stage, and the gas passage 24b. The regenerator material 18 is formed of bismuth, and may be hereinafter referred to as “bismuth regenerator material 18.” In FIG. 2, the gas passages 23a and 23b and the gas passages 24a and 24b are functionally illustrated to explain the flow of the refrigerant gas, and may have actual structures different from the illustrated structures.

When the intake valve V1 is closed and an exhaust valve V2 is opened, the high-pressure helium gas in the second-stage cylinder 12 and the first-stage cylinder 11 follows the intake path in the reverse direction to be collected into the helium compressor 10 through the gas passage 16 and the exhaust valve V2.

When the two-stage GM refrigerator of FIG. 2 is in operation, the drive motor M rotates to vertically reciprocate the first-stage displacer 13 and the second-stage displacer 14 as indicated by a double-headed arrow in FIG. 2. When the first-stage displacer 13 and the second-stage displacer 14 are driven downward, the intake valve V1 becomes open to allow high-pressure helium gas to be fed into the first-stage cylinder 11 and the second-stage cylinder 12.

When the first-stage displacer 13 and the second-stage displacer 14 are driven upward by the drive motor M, the intake valve V1 becomes closed and the exhaust valve V2 becomes open so that the helium gas is collected into the helium compressor 10, and the pressure of the first-stage expansion space 21 in the first-stage cylinder 11 and the pressure of the second-stage expansion space 22 in the second-stage cylinder 12 are reduced. At this point, the helium gas expands to generate coldness in the first-stage expansion space 21 and the second-stage expansion space 22. The cooled helium gas passes through the second-stage displacer 14 and the first-stage displacer 13 to be collected. During this process, the cooled helium gas cools the regenerator materials 18 and 17. (A detailed description is given below of this cooling process.)

The high-pressure helium gas supplied in the next intake process is cooled by being supplied through the regenerator materials 17 and 18. The cooled helium gas is further cooled through its expansion. In a steady state, the first-stage expansion space 21 of the first-stage cylinder 11 is maintained at temperatures of approximately 40 K to approximately 70 K, and the second-stage expansion space 22 of the second-stage cylinder 12 is maintained at cryogenic temperatures of approximately 9.5 K to approximately 15 K, for example.

A first-stage heat station 19 is provided around the bottom part of the first-stage cylinder 11 to be thermally coupled to the first-stage cylinder 11. A second-stage heat station 20 is provided around the bottom part of the second-stage cylinder 12 to be thermally coupled to the second-stage cylinder 12. The first-stage heat station 19 is joined to, for example, a cryopanel to cause gas molecules to be adsorbed to the cryopanel. Further, the second-stage heat station 20 is joined to, for example, an adsorption tower containing an adsorbent such as activated carbon to adsorb remaining gas molecules. A cryopump having such a configuration is used to form a clean vacuum in sputtering apparatuses and the like.

FIG. 3 is a diagram illustrating a configuration of the second-stage displacer 14 of the two-stage GM refrigerator of FIG. 2. The second-stage displacer 14 includes a cylindrical member 30 formed of fabric-containing phenolic resin. The cylindrical member 30 has a cylindrical shape open at its upper and lower ends. For example, if the second-stage cylinder 12 illustrated in FIG. 12 is 35 mm in inside diameter, the cylindrical member 30 is slightly smaller than 35 mm in outside diameter and 30 mm in inside diameter. The second-stage displacer 14 is, for example, approximately 200 mm long in an axial direction. A lid member 31 formed of a material such as fabric-containing phenolic resin is inserted into and bonded to the cylindrical member 30 at its lower end. A wire mesh 32 is placed on the lid member 31, and a felt plug 33 is placed on the wire mesh 32.

The bismuth regenerator material 18 consisting of bismuth is placed on the felt plug 33. A felt plug 34 is placed on the bismuth regenerator material 18. Thus, the bismuth regenerator material 18 fills in the space between the felt plugs 33 and 34. A perforated metal 35 is placed on the felt plug 34. The perforated metal 35 is fixed to a stepped part provided circumferentially on the internal surface of the cylindrical member 30 at its upper end portion. A joining mechanism 36 for joining the second-stage displacer 14 to the first-stage displacer 13 is attached to the upper end of the cylindrical member 30.

Openings 37 forming a gas passage are provided in the sidewall of the cylindrical member 30 at the same lengthwise position as the wire mesh 32 in a lengthwise direction of the cylindrical member 30. That is, the positions of the openings 37 are level with the position of the wire mesh 32 in a vertical direction. A helical gas passage 38 of a single helical groove connecting the positions of the openings 37 and the upper end of the cylindrical member 30 is formed on the cylindrical exterior circumferential surface of the cylindrical member 30 above the openings 37. For example, this groove may be approximately 2 mm in width and approximately 0.6 mm in depth, and may have a pitch of approximately 4 mm.

The cylindrical member 30 is slightly smaller in diameter below the openings 37 than above the openings 37. The gap formed between the cylindrical member 30 and the second-stage cylinder 12 (FIG. 2) below the openings 37 forms a gas passage connecting the inside of the cylindrical member 30 and the second-stage expansion space 22 illustrated in FIG. 2.

The gap (distance) between the exterior circumferential surface of the cylindrical member 30 and the cylindrical interior circumferential surface of the second-stage cylinder 12 (FIG. 2) is preferably greater than or equal to 0.01 mm for stable reciprocation of the second-stage displacer 14. Further, above the openings 37, the gap (distance) between the exterior circumferential surface of the cylindrical member 30 and the interior circumferential surface of the second-stage cylinder 12 (FIG. 2) is preferably smaller than or equal to 0.03 mm in order to prevent leaking gas from flowing linearly in the axial directions.

The two-stage GM refrigerator configured as described above uses the bismuth regenerator material 5 as the regenerator material of the second-stage displacer 14 that generates cryogenic temperatures of approximately 5 K to approximately 10 K. As described above, bismuth is a regenerator material suitable in terms of safety and environmental burdens, but is lower in specific heat than lead. FIG. 4 is a graph illustrating the volumetric specific heats of materials used as regenerator materials including bismuth. As illustrated in FIG. 4, the specific heat of bismuth is lower than the specific heat of lead, and is significantly reduced in a cryogenic environment at or below 10 K in particular. Accordingly, it has been believed difficult to use bismuth for regenerative refrigerators that attain cryogenic temperatures lower than or equal to 10 K.

The inventors of the present invention have diligently studied a regenerative refrigerator that attains cryogenic temperatures lower than or equal to 10 K while using bismuth as a regenerator material, and have succeeded in attaining cryogenic temperatures lower than or equal to 15 K while using bismuth as a regenerator material by forming the helical gas passage 38 on one of the cylindrical exterior circumferential surface of the second-stage displacer 14 and the cylindrical interior circumferential surface of the second-stage cylinder 12 (FIG. 2), the helical gas passage 38 connecting both ends of the exterior circumferential surface or the interior circumferential surface.

Letting the inside of the second-stage displacer 14 through which helium gas (a refrigerant gas) flows be a main gas passage, the helical gas passage 38 forms an auxiliary gas passage. Further, the helical gas passage 38 includes a groove pattern formed on the exterior circumferential surface of the second-stage displacer 14 or on the interior circumferential surface of the second-stage cylinder 12 (FIG. 2). This groove pattern includes a groove having at least part thereof extending along a direction to cross the axial directions (vertical directions in FIG. 2 and FIG. 3) of the second-stage displacer 14 so as to cause the helium gas to actively exchange heat with the second-stage cylinder 12 and the second-stage displacer 14 in the gap between the second-stage displacer 14 and the second-stage cylinder 12, the helium gas flowing from one end to the other end of the exterior circumferential surface or the interior circumferential surface. FIG. 2 and FIG. 3 illustrates a case where the helical gas passage 38 is formed on the exterior circumferential surface of the of the second-stage displacer 14 of the two-stage GM refrigerator.

FIGS. 5A and 5B, 6A and 6B, 7A and 7B, and 8A and 8B illustrate the cooling characteristics of the two-stage GM refrigerator according to this embodiment (Example) and a conventional two-stage GM refrigerator (Comparative Example) in comparison. The conventional two-stage GM refrigerator used employs lead as a regenerator material and has a sealing ring to control gas flow provided between the second-stage cylinder and the second-stage displacer. In FIGS. 5A through 6B, the two-stage GM refrigerator according to this embodiment is indicated as “Bi+helix,” and the conventional two-stage GM refrigerator is indicated as “Pb+sealing ring.”

Further, FIGS. 5A through 6B illustrate characteristics at an operating frequency of 50 Hz, and FIGS. 7A through 8B illustrate characteristics at an operating frequency of 60 Hz. Further, FIGS. 5A and 5B and 7A and 7B illustrate characteristics at the time of no load on either the first-stage heat station or the second-stage heat station. FIGS. 6A and 6B and 8A and 8B illustrate characteristics at the time of applying a load of 12 W on the first-stage heat station and a load of 3 W on the second-stage heat station. Further, FIGS. 5A, 6A, 7A, and 8A illustrate the temperature characteristics of the first-stage heat stations, and FIGS. 5B, 6B, 7B, and 8B illustrate the temperature characteristics of the second-stage heat stations.

In no-load operations, the first-stage temperatures of the Example and the Comparative Example are substantially the same as illustrated in FIG. 5A and FIG. 7A. On the other hand, as illustrated in FIG. 5B and FIG. 7B, while the second-stage temperature of the Comparative Example is 6.5 K to 7.2 K, the second-stage temperature of the Example is 5.3 K to 5.5 K. Thus, there is improvement in the temperature characteristic of Example with respect to the second stage.

In the loaded operations (at 50 Hz) illustrated in FIGS. 6A and 6B, the first-stage temperature of the Comparative Example is 71 K to 80 K, while the first-stage temperature of the Example is 65 K to 66 K as illustrated in FIG. 6A. Thus, there is improvement in the temperature characteristic of the Example with respect to the first stage. Further, the second-stage temperature of the Comparative Example is 10.1 K to 11.0 K, while the second-stage temperature of the Example is 9.5 K to 9.8 K as illustrated in FIG. 6B. Thus, there is improvement in the temperature characteristic of the Example with respect to the second stage.

In the loaded operations (at 60 Hz) illustrated in FIGS. 8A and 8B, the first-stage temperature of the Comparative Example is 65 K to 78 K, while the first-stage temperature of the Example is 62 K to 63 K as illustrated in FIG. 8A. Thus, there is improvement in the temperature characteristic of the Example with respect to the first stage. Further, the second-stage temperature of the Comparative Example is 9.8 K to 10.7 K, while the second-stage temperature of the Example is 9.2 K to 9.4 K as illustrated in FIG. 8B. Thus, there is improvement in the temperature characteristic of the Example with respect to the second stage.

It is believed to be for the following reason that the two-stage GM refrigerator according to this embodiment shows a good cooling characteristic relative to the Comparative Example.

According to the two-stage GM refrigerator according to this embodiment, a groove pattern forming the helical gas passage 38 is formed on, for example, the exterior circumferential surface of the second-stage displacer 14. This causes helium gas (a refrigerant gas) to diverge from the main gas passage passing through the second-stage displacer 14 so as to flow through the helical gas passage 38 formed between the second-stage displacer 14 and the second-stage cylinder 12.

The groove pattern forming the helical gas passage 38 is so formed as to include a groove along a direction to cross the axial directions of the second-stage displacer 14 so as to cause the helium gas flowing through the groove to actively exchange heat with the second-stage displacer 14 and the second-stage cylinder 12.

Therefore, when the helium gas, which is a refrigerant gas, flows from the lower-temperature side to the higher-temperature side, the helium gas cools the second-stage displacer 14 and the second-stage cylinder 12 more efficiently than conventionally. As a result, the bismuth regenerator material 18 filling in the second-stage displacer 14 is cooled with more efficiency than in the conventional configuration without the helical gas passage 38. On the other hand, when the diverged helium gas flows from the higher-temperature side to the lower-temperature side, the helium gas is more cooled than in the case of flowing directly in the axial direction. Accordingly, it is believed that it is possible to improve the cooling efficiency by providing the helical gas passage 38 even if bismuth, which is lower in specific heat than lead, is used as the regenerator material 18 at cryogenic temperatures lower than or equal to 15 K.

FIG. 9 is a graph illustrating a relationship between the bismuth size (grain size) and the refrigerating capacity of the bismuth regenerator material 18. FIG. 9 shows that if the grain size is less than 0.14 mm, the second-stage displacer 14 is filled with bismuth with excessively high density so as to cause a sharp increase in the resistance to passage of helium gas, which is a refrigerant gas. On the other hand, if the grain size exceeds 1.6 mm, there may be a significant decrease in the efficiency of heat exchange between the bismuth regenerator material 18 and the helium gas and the second-stage displace 14. Accordingly, the bismuth granules are desirably more than or equal to 0.14 mm and less than or equal to 1.6 mm in grain size.

FIG. 10 is a diagram illustrating another configuration of the second-stage displacer 14 according to this embodiment. According to this configuration, the cylindrical member 30 includes a cylindrical stainless steel tube 39 and a wear-resistant resin member 40 fixed onto the surface of the stainless steel tube 39. The wear-resistant resin member 40 is formed of fabric-containing phenolic resin.

For example, the wear-resistant resin member 40 is slightly smaller than 35 mm in outside diameter and 32 mm in inside diameter, and the stainless steel tube 39 is 30 mm in inside diameter. The stainless steel tube 39 having high mechanical strength is provided inside the wear-resistant resin member 40 to control the thermal contraction of the wear-resistant resin member 40 at the time of cooling. As a result, the heat distortion properties of the second-stage displacer 14 approach the heat distortion properties of the stainless steel tube 39.

A lid member 41 having a circular ring shape is inserted into the cylindrical member 30 at its upper open end, but otherwise the configuration of the second-stage displacer 14 is the same as the configuration illustrated in FIG. 3. The configurations as illustrated in FIG. 3 and FIG. 10 make it unnecessary for the second-stage displacer 14 to contain a sealing ring. Accordingly, it is possible to reduce the thickness of the sidewall of the cylindrical member 30.

This means a possible increase in the space for containing the bismuth regenerator material 18 in the second-stage displacer 14. An increase in the amount of bismuth leads to an increase in the refrigerating capacity. In particular, in the case of using bismuth, which is lower in specific heat than lead, as the regenerator material 18, this increase in the bismuth regenerator material 18 is advantageous in terms of improving the cooling capacity.

Although a description is given above of the case of providing the helical gas passage 38 only on the second-stage displacer 14 in the regenerative refrigerator illustrated in FIG. 2, it is also possible to provide a helical gas passage on the first-stage displacer 13. FIG. 11 illustrates a configuration of the first-stage displacer 13 having a helical gas passage 55 provided on its cylindrical exterior circumferential surface.

Referring to FIG. 11, the first-stage displacer 13 includes a cylindrical member 50 formed of fabric-containing phenolic resin. The cylindrical member 50 has a cylindrical shape with an upper lid (not graphically illustrated), and is open at its lower end. A flange 51, whose diameter is slightly smaller than the outside diameter of the cylindrical member 50, is attached to the upper surface of the upper lid of the cylindrical member 50. An opening 52 that forms a gas passage is provided through the flange 51 and the upper lid of the cylindrical member 50. The drive shaft S for driving the cylindrical member 50 in the vertical directions indicated by a double-headed arrow in FIG. 11 is attached to the upper surface of the flange 51.

In the cylindrical member 50, the regenerator material 17 such as a wire mesh of copper fills in a space between an upper wire mesh and a lower wire mesh (both of which are not graphically illustrated). That is, the upper wire mesh is placed on the regenerator material 17, and the lower wire mesh is placed under the regenerator material 17. Openings 53 for forming a gas passage are formed in the sidewall of the cylindrical member 50 at the same vertical position as where the lower mesh wire is placed under the regenerator material 17.

Further, a lid member 54 formed of fabric-containing phenolic resin or the like is inserted into and bonded to the cylindrical member 50 at its open lower end. The lid member 54, which is a blank cap, hermetically seals the lower-end opening of the cylindrical member 50. Further, a recess for attaching the joining mechanism 36 (FIG. 3) for connection to the second-stage displacer 14 is formed on the lower surface of the lid member 54.

The helical gas passage 55 formed of a single helical groove is formed on the exterior circumferential (circumferential) surface of the cylindrical member 50 from its upper end to a vertical position where the openings 53 are formed. The cylindrical member 50 is slightly smaller in outside diameter below the vertical position of the openings 53 than above the vertical position of the openings 53. Accordingly, a gap is formed between the cylindrical interior circumferential surface of the first-stage cylinder 11 (FIG. 2) and the exterior circumferential surface of the cylindrical member 50 below the vertical position of the openings 53. This gap serves as a gas passage connecting the inside of the cylindrical member 50 and the first-stage expansion space 21 illustrated in FIG. 2.

The diameter of the flange 51 is slightly smaller than the outside diameter of the cylindrical member 50. Therefore, a gap is formed between the exterior circumferential surface of the flange 51 and the interior circumferential surface of the first-stage cylinder 11. This gap serves as a gas passage connecting the openings 53 (gas passage) and the upper space inside the first-stage cylinder 11 illustrated in FIG. 2. Thus, providing the first-stage displacer 13 with the helical gas passage 55 makes it possible to improve the cooling characteristic in the first stage for the same reasons as the cooling characteristic is improved in the second stage as described above.

Further, in the above-described embodiments, a description is given of the case of forming the helical gas passage (4, 38, 55) on the surface of the displacer (2, 14, 13). However, the shape of the helical gas passage is not limited to a helical shape and may be other shapes as long as the shapes allow helium gas to flow through a gap between the cylinder (1, 12, 11) and the displacer (2, 14, 13) while exchanging heat sufficiently with the surface of the gas passage. A description is given below, with reference to FIGS. 12A through 12H, of other gas passage shapes.

FIGS. 12A through 12H are schematic diagrams illustrating circumferentially developed groove patterns formed on the exterior circumferential surfaces of the displacers. FIGS. 12A through 12H illustrate the characteristics of the shapes of groove patterns, and do not illustrate a groove pitch or a groove inclination relative to the axial directions.

FIG. 12A illustrates the case where a single helical groove is formed on the exterior circumferential surface of a displacer from its one end to the other end. Alternatively, multiple helical grooves may be provided as illustrated in FIG. 12B. FIG. 12B illustrates the case where four grooves are formed substantially parallel to one another. Further, the helical groove may be formed in a wavy line as illustrated in FIG. 12C or a zigzag line as illustrated in FIG. 12D. Further, as illustrated in FIG. 12E, the helical groove may be formed in a step-like zigzag line by combining straight lines parallel to and straight lines perpendicular to the axial directions of the displacer. Further, as illustrated in FIG. 12F, a wavy line and a zigzag line may be combined.

As illustrated in FIG. 12G, two or more helixes that spiral in directions opposite to each other may be combined so that helical grooves cross each other. Further, as illustrated in FIG. 12H, multiple grooves may be formed circumferentially on the exterior circumferential surface of a displacer, and connection grooves that connect adjacent circumferential grooves may be formed. In this case, in order to form as long a gas passage as possible, it is preferable to provide the connection grooves, formed vertically between the circumferential grooves, at different circumferential positions. Further, it is preferable to position the connection grooves axisymmetrically.

Thus, a groove pattern is formed so that at least part of one or more grooves thereof extends along a direction to cross the axial directions of a displacer. As a result, gas flows through a longer passage than in the case of flowing parallel to the axial directions. This allows heat exchange to be performed with more efficiency between the gas and the displacer and the cylinder.

The cross section of the gas passage formed on the exterior circumferential surface of a displacer may be rectangular, triangular, semicircular, or of other shapes. Further, in order to increase the heat exchange efficiency of the gas flowing through the gas passage formed on the exterior circumferential surface of a displacer, a regenerator material may be stuck to the exterior circumferential surface of the displacer or the internal surface of the gas passage. Further, the gas passage may be filled with a regenerator material.

In the above-described embodiments, a description is given of the case of forming a groove pattern on the exterior circumferential surface of a displacer (2, 13, 14). However, the same effects may be produced by forming a groove pattern on the interior circumferential surface of a cylinder (1, 11, 12). In this case, the groove pattern may be formed to connect both ends of a cylindrical region of the interior circumferential surface of the cylinder, the cylindrical region including at least a range over which the displacer reciprocates.

FIG. 13 is a diagram illustrating a basic configuration of the cylinder 1 and the displacer 2 in the case where a groove pattern is formed on the cylindrical interior circumferential surface of the cylinder 1. A helical gas passage 4a is formed on the interior circumferential surface of the cylinder 1 in place of the helical gas passage 4 formed on the exterior circumferential surface of the displacer 2 (FIG. 1). Otherwise, the configuration of FIG. 13 is the same as the basic configuration of FIG. 1. Various groove patterns such as those illustrated in FIGS. 12A through 12H may be formed in place of the helical groove pattern.

According to one aspect of the present invention, a regenerative refrigerator uses bismuth as a regenerator material. Accordingly, it is possible to reduce burdens on the environment.

Further, a groove pattern may be formed on the cylindrical exterior circumferential surface of a displacer, so that gas that diverges from a main gas passage containing a regenerator material flows along this groove pattern in a gap between the displacer and a cylinder. This groove pattern is formed to include a groove extending along a direction to cross the axial directions of the displacer so as to allow the gas flowing in the groove to actively exchange heat with the displacer and the cylinder.

Accordingly, when the diverged gas flows from the higher-temperature side to the lower-temperature side, the gas is more cooled than in the case of flowing directly in the axial direction. On the other hand, when the diverged gas flows from the lower-temperature side to the higher-temperature side, the gas cools the displacer and the cylinder more. Accordingly, it is possible to ensure attainment of cryogenic temperatures lower than or equal to 15 K even with bismuth, which is lower in specific heat than lead conventionally used as a regenerator material.

All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventors to furthering the art, and are to be construed as being without limitation 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 the embodiments of the present inventions have been described in detail, the present invention is not limited to those specifically disclosed embodiments. For example, the present invention is applicable to not only GM refrigerators but also other refrigerators using regenerators, such as Stirling refrigerators and Solvay cycle refrigerators.

Further, the above description is given, taking a two-stage displacer configuration as an example. However, the present invention may also be applied to the case of using a single stage displacer or three or more stage displacers. Further, in other configurations, the present invention may be applied to regenerative refrigerators using displacers at low temperatures. It should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.

REGENERATIVE REFRIGERATOR

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