Cryocooler

Abstract

To suppress performance deterioration of a cryocooler in which a ceramic-based magnetic cold storage material is incorporated.SOLUTION: A cryocooler 10 includes: a second cylinder 16b that has first thermal conductivity and extends in an axial direction; a second cooling stage 35 that has second thermal conductivity higher than the first thermal conductivity and includes a stage end portion 35a and a stage tubular portion 35b connecting the stage end portion 35a to the second cylinder 16b in the axial direction; a second displacer 18b that is capable of reciprocating in the axial direction in the second cylinder 16b and forms a second expansion chamber 34 with the stage end portion 35a and in which the second expansion chamber 34 takes maximum volume at a top dead center; and a ceramic-based magnetic cold storage material 28c that is accommodated in the second displacer 18b and of which the axial position in the second displacer 18b is determined so as to overlap the stage tubular portion 35b in the axial direction when the second displacer 18b is at the top dead center.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a cryogenic refrigerator. [Background technology]

[0002] In general, cryogenic refrigerators that provide cooling to extremely low temperatures below about 10K (for example, the liquid helium temperature of about 4.2K) are equipped with a regenerator equipped with a magnetic regenerator material that has a large peak of specific heat in this temperature range associated with a magnetic phase transition. Magnetic regenerator materials are useful for improving the cooling capacity of cryogenic refrigerators at such extremely low temperatures. There are two main types of magnetic regenerator materials: metal-based magnetic regenerator materials such as HoCu2, and ceramic-based magnetic regenerator materials such as Gd2O2S (also known as GOS). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2015-183970 A Summary of the Invention [Problem to be solved by the invention]

[0004] In cryocoolers, such as the Gifford-McMahon (GM) refrigerator, in which a displacer containing a regenerator moves in the axial direction, the temperature of the regenerator may fluctuate slightly due to the axial temperature distribution in the refrigerator itself as the displacer moves in the axial direction. In many ceramic magnetic regenerators, including GOS, the specific heat is quite sensitive to temperature, that is, the specific heat peak is very sharp. Therefore, even a slight temperature fluctuation in a ceramic magnetic regenerator can lead to a noticeable fluctuation in the specific heat, for example a large decrease. This may affect the regenerative performance of the ceramic magnetic regenerator and, ultimately, the performance of the cryocooler.

[0005] An exemplary object of an embodiment of the present invention is to suppress a decrease in performance of a cryogenic refrigerator incorporating a ceramic magnetic regenerator material. [Means for solving the problem]

[0006] According to one embodiment of the present invention, a cryogenic refrigerator comprises: a cylinder having a first thermal conductivity and extending in an axial direction; a cooling stage having a second thermal conductivity higher than the first thermal conductivity, the cooling stage including a stage end portion and a stage cylindrical portion axially connecting the stage end portion to the cylinder; a displacer capable of reciprocating axially within the cylinder, forming an expansion space between the stage end portion and the stage end portion, the expansion space having a maximum volume at top dead center; and a ceramic-based magnetic regenerator material contained in the displacer, the axial position of the ceramic-based magnetic regenerator material being determined so that the displacer axially overlaps with the stage cylindrical portion when the displacer is at top dead center. Effect of the Invention

[0007] According to the present invention, it is possible to suppress the deterioration of the performance of a cryogenic refrigerator incorporating a ceramic magnetic regenerator material. [Brief description of the drawings]

[0008] [Figure 1] 1 is a diagram illustrating a schematic diagram of a cryogenic refrigerator according to an embodiment; [Diagram 2] 1 is a diagram illustrating a schematic diagram of a cryogenic refrigerator according to an embodiment; [Diagram 3] FIG. 2 is a diagram showing the temperature change of specific heat of a typical magnetic regenerator material. [Figure 4] FIG. 2 is a diagram illustrating a portion of a cryogenic refrigerator according to a comparative example. [Diagram 5] 5(a) and 5(b) are diagrams illustrating the axial reciprocating motion of the displacer assembly according to the embodiment. [Figure 6] FIG. 10 is a diagram illustrating an example of a displacer cap according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, the embodiments for carrying out the present invention will be described in detail with reference to the drawings. In the description and drawings, the same or equivalent components, members, and processes are given the same reference numerals, and duplicated descriptions are omitted as appropriate. The scale and shape of each part shown in the drawings are set for convenience in order to facilitate the description, and are not to be interpreted as being limiting unless otherwise specified. The embodiments are merely examples, and do not limit the scope of the present invention in any way. All features and combinations thereof described in the embodiments are not necessarily essential to the invention.

[0010] 1 and 2 are schematic diagrams illustrating a cryogenic refrigerator 10 according to an embodiment. The cryogenic refrigerator 10 is, as an example, a two-stage Gifford-McMahon (GM) refrigerator. FIG. 1 shows the exterior of the cryogenic refrigerator 10, and FIG. 2 shows the internal structure of the cryogenic refrigerator 10.

[0011] The cryogenic refrigerator 10 includes a compressor 12 and an expander 14. The compressor 12 is configured to recover the working gas of the cryogenic refrigerator 10 from the expander 14, pressurize the recovered working gas, and supply the working gas again to the expander 14. The working gas, also called a refrigerant gas, is typically helium gas, although other suitable gases may be used.

[0012] The expander 14 includes a refrigerator cylinder 16, a displacer assembly 18, and a refrigerator housing 20. The refrigerator housing 20 is coupled to the refrigerator cylinder 16, thereby forming an airtight container that houses the displacer assembly 18. Note that the internal volume of the refrigerator housing 20 may be connected to the low pressure side of the compressor 12 and maintained at low pressure.

[0013] The refrigerator cylinder 16 has a first cylinder 16a and a second cylinder 16b extending in the axial direction. The first cylinder 16a and the second cylinder 16b are, for example, members having a cylindrical shape, and the second cylinder 16b has a smaller diameter than the first cylinder 16a. The first cylinder 16a and the second cylinder 16b are arranged coaxially, and the lower end of the first cylinder 16a is rigidly connected to the upper end of the second cylinder 16b.

[0014] 2, the displacer assembly 18 has a first displacer 18a and a second displacer 18b. The first displacer 18a and the second displacer 18b are, for example, cylindrical members, and the second displacer 18b has a smaller diameter than the first displacer 18a. The first displacer 18a and the second displacer 18b are arranged coaxially.

[0015] The first displacer 18a is accommodated in the first cylinder 16a, and the second displacer 18b is accommodated in the second cylinder 16b. The first displacer 18a can reciprocate in the axial direction along the first cylinder 16a, and the second displacer 18b can reciprocate in the axial direction along the second cylinder 16b. The first displacer 18a and the second displacer 18b are connected to each other and move together.

[0016] In this document, in order to explain the positional relationship between the components of the cryogenic refrigerator 10, for convenience, the side closer to the top dead center of the axial reciprocating motion of the displacer will be referred to as "top" and the side closer to the bottom dead center as "bottom." The top dead center is the position of the displacer where the volume of the expansion space is maximum, and the bottom dead center is the position of the displacer where the volume of the expansion space is minimum. During operation of the cryogenic refrigerator 10, a temperature gradient occurs in which the temperature decreases from the top to the bottom in the axial direction, so the top side can also be called the high temperature side and the bottom side the low temperature side.

[0017] The first displacer 18a accommodates the first regenerator 26. The cylindrical body of the first displacer 18a serves as a first regenerator container extending in the axial direction. The first regenerator 26 is formed by filling the body of the first displacer 18a with a wire mesh such as copper or other suitable first regenerator material. The upper and lower lids of the first displacer 18a may be provided as separate members from the body of the first displacer 18a, and the upper and lower lids of the first displacer 18a may be fixed to the body by suitable means such as fastening or welding, thereby accommodating the first regenerator material in the first displacer 18a.

[0018] Similarly, the second displacer 18b accommodates the second regenerator 28. The cylindrical body of the second displacer 18b serves as a second regenerator container extending in the axial direction. The upper and lower lids of the second displacer 18b may be provided as separate members from the body of the second displacer 18b, and the upper and lower lids of the second displacer 18b may be fixed to the body by appropriate means such as fastening or welding, thereby accommodating the second regenerator material in the second displacer 18b. Hereinafter, for convenience of explanation, the lower lid of the second displacer 18b will be referred to as a displacer cap 46.

[0019] The second regenerator 28 includes at least two types of regenerator materials, in this example, three types of regenerator materials, which are stacked in the axial direction of the second displacer 18b. As shown in Fig. 2, the second regenerator 28 includes, in the axial direction from the high temperature side to the low temperature side, a non-magnetic regenerator material 28a, a metal-based magnetic regenerator material 28b, and a ceramic-based magnetic regenerator material 28c. In order to increase the refrigeration capacity of the second stage of the cryogenic refrigerator 10, materials having large specific heats at different temperatures are selected as the regenerator materials.

[0020] The non-magnetic regenerator material 28a is disposed on the high temperature side of the second displacer 18b, i.e., axially adjacent to the top cover of the second displacer 18b, within the second displacer 18b. The non-magnetic regenerator material 28a may be formed of a material having a relatively high volumetric specific heat (e.g., a volumetric specific heat higher than that of copper) at the temperature of the high temperature side of the second displacer 18b, for example, in a temperature range of 10K to 50K or at least a part thereof. The non-magnetic regenerator material 28a may have a volumetric specific heat higher than that of copper, which is a regenerator material typically used in the first displacer 18a as described above, in this temperature range. The non-magnetic regenerator material 28a may also have a volumetric specific heat higher than that of a magnetic regenerator material described below in this temperature range.

[0021] The non-magnetic heat accumulating material 28a may be made of a non-magnetic metal material such as bismuth, zinc, lead, or an alloy containing at least one of these. Two types of materials may be laminated and used as the non-magnetic heat accumulating material 28a. For example, a zinc heat accumulating material may be arranged axially adjacent to the upper cover of the second displacer 18b, and a bismuth heat accumulating material may be arranged axially adjacent to the zinc heat accumulating material.

[0022] The metallic magnetic regenerator material 28b is disposed in the middle of the second displacer 18b, i.e., axially adjacent to the non-magnetic regenerator material 28a, in the second displacer 18b. The metallic magnetic regenerator material 28b may be formed of a material having a relatively high volumetric specific heat at the temperature of the axial middle of the second displacer 18b, for example, in a temperature range of 5K to 10K or at least a part of the temperature range. Typically, the metallic magnetic regenerator material 28b has a peak of volumetric specific heat associated with a magnetic phase transition in this temperature range. The metallic magnetic regenerator material 28b may be formed of a metallic magnetic material, for example, HoCu2, Er3Ni, etc.

[0023] The ceramic magnetic regenerator material 28c is disposed in the low-temperature portion of the second displacer 18b, that is, axially adjacent to the displacer cap 46 and within the second displacer 18b. The ceramic magnetic regenerator material 28c is disposed between the metallic magnetic regenerator material 28b and the displacer cap 46. The ceramic magnetic regenerator material 28c may be directly adjacent to the displacer cap 46. Alternatively, the ceramic magnetic regenerator material 28c and the displacer cap 46 may be adjacent to each other with a rectifying layer interposed therebetween.

[0024] The ceramic magnetic regenerator material 28c may be formed of a magnetic regenerator material having a relatively high volumetric specific heat in the temperature range of, for example, 1 K to 5 K or at least a part thereof, which is the temperature of the low-temperature side of the second displacer 18b (i.e., the displacer low-temperature end where the temperature is the lowest in the displacer assembly 18). Typically, the ceramic magnetic regenerator material 28c has a peak of volumetric specific heat associated with a magnetic phase transition in this temperature range.

[0025] The ceramic magnetic regenerator material 28c has the general formula R x O2S or (R 1-y R′ y ) x O2S (R and R′ are at least one kind of rare earth element, 0.1 ≦ x ≦ 9, 0 < y < 1), or may contain at least one kind of magnetic regenerator material represented by Gd x Al 2-x O3 (1 ≦ x < 2). The rare earth elements R and R′ may be, for example, yttrium Y, lanthanum La, cerium Ce, praseodymium Pr, neodymium Nd, promethium Pm, samarium Sm, europium Eu, gadolinium Gd, terbium Tb, dysprosium Dy, holmium Ho, erbium Er, thulium Tm, or ytterbium Yb.

[0026] Therefore, the ceramic magnetic regenerator material 28c is Gd2O2S (also called GOS), Tb2O2S (also called TBOS), (Gd 1-y Tb y)2O2S (where 0 < y < 1, also known as GdTbOS), or it may be GdAlO3 (also known as GAP), or it may contain at least one of these.

[0027] As is known, the ceramic-based magnetic regenerator material 28c having such a composition has a high (e.g., several times higher) volumetric specific heat, for example, in the temperature range of 1K to 5K or at least a part thereof, compared to metal magnetic regenerator materials such as HoCu2. Therefore, the ceramic-based magnetic regenerator material 28c is suitable for enhancing the refrigeration capacity of the cryogenic refrigerator 10 that provides cryogenic cooling in this temperature range.

[0028] In addition, in order to improve the mechanical strength of the magnetic regenerator material, additives may be added to the magnetic regenerator material. The additive may be, for example, at least one of zirconium Zr, aluminum Al, and alumina (Al2O3). The weight ratio of the additive to the magnetic regenerator material may be, for example, 20% or less, or 15% or less. In this way, without significantly changing the volumetric specific heat of the magnetic regenerator material (i.e., without significantly affecting the refrigeration capacity of the cryogenic refrigerator 10), the hardness of the magnetic regenerator material can be increased, and the risk of damage or pulverization due to impact can be reduced.

[0029] The non-magnetic regenerator material 28a, the metal-based magnetic regenerator material 28b, and the ceramic-based magnetic regenerator material 28c may be granular. The granules constituting such a granular regenerator material may be formed into granules having a size of, for example, 0.01 mm or more and 3 mm or less and filled in the second displacer 18b. The particle size of the granules may be, for example, 0.14 mm or more and 1.6 mm or less, preferably 0.15 mm or more and 1.4 mm or less, and more preferably 0.22 mm or more and 1.3 mm or less. The proportion of granules having a particle size of 0.14 mm or more and 1.6 mm or less may be 70% by weight or more based on all the granules. When the particle size is less than 0.14 mm, the density when filling the regenerator becomes too high, and the flow resistance of the working gas (e.g., helium gas) may increase rapidly. Also, when the particle size exceeds 1.6 mm, there is a concern that the heat exchange efficiency between the granules and the working gas may be significantly reduced.

[0030] Moreover, the ratio of the maximum diameter to the minimum diameter of the granules (aspect ratio) is 5 or less in any three-dimensional direction, preferably 3 or less, more preferably 2 or less, and further preferably as close to a sphere as possible. If the aspect ratio exceeds 5, the granules are prone to mechanical deformation and destruction, and it becomes difficult to pack them at a high density, which reduces the cooling efficiency. Therefore, the proportion of granules with a ratio of the major axis to the minor axis of 5 or less may be 70% by weight or more of the total granules.

[0031] To protect such particles, the surfaces may be covered with a coating (such as alumina (Al2O3), fluororesin, etc.) having a thickness of, for example, 1 to 50 μm.

[0032] In order to separate the non-magnetic heat accumulating material 28a from the metal-based magnetic heat accumulating material 28b, a partition member such as a wire mesh may be disposed at a boundary 29a between the non-magnetic heat accumulating material 28a and the metal-based magnetic heat accumulating material 28b. Also, if necessary, a partition member may be disposed at a boundary 29b between the metal-based magnetic heat accumulating material 28b and the ceramic-based magnetic heat accumulating material 28c.

[0033] The displacer assembly 18 defines an upper chamber 30, a first expansion chamber 32, and a second expansion chamber 34 within the refrigerator cylinder 16. The expander 14 includes a first cooling stage 33 and a second cooling stage 35 for heat exchange with a desired body or medium to be cooled by the cryogenic refrigerator 10. The cooling stages are sometimes referred to as heat load flanges.

[0034] The first cylinder 16a and the second cylinder 16b are formed of a material having a first thermal conductivity, whereas the first cooling stage 33 and the second cooling stage 35 are formed of a material having a second thermal conductivity higher than the first thermal conductivity. For example, the first cylinder 16a and the second cylinder 16b may be formed of a metal material having a relatively low thermal conductivity, such as stainless steel, and the first cooling stage 33 and the second cooling stage 35 may be formed of a metal material such as copper (e.g., pure copper such as oxygen-free copper or tough pitch copper) or other material having a high thermal conductivity.

[0035] The upper chamber 30 is formed between an upper cover portion of the first displacer 18a and an upper portion of the first cylinder 16a. The first expansion chamber 32 is formed between a lower cover portion of the first displacer 18a and a first cooling stage 33. The first cooling stage 33 is fixed to the lower portion of the first cylinder 16a so as to surround the first expansion chamber 32.

[0036] The second cooling stage 35 includes a stage end portion 35a and a stage tube portion 35b that axially connects the stage end portion 35a to the second cylinder 16b. The second cooling stage 35 may be provided as a single component having the stage end portion 35a and the stage tube portion 35b. Alternatively, the stage end portion 35a and the stage tube portion 35b may be provided as separate members that are connected to each other to form the second cooling stage 35.

[0037] The second expansion chamber 34 is formed between a lower lid portion of the second displacer 18b, i.e., a displacer cap 46, and the second cooling stage 35. More specifically, the displacer cap 46 faces the stage end portion 35a across the second expansion chamber 34, and the stage tube portion 35b is fixed to the lower portion of the second cylinder 16b so as to surround the second expansion chamber 34.

[0038] The first regenerator 26 is connected to the upper chamber 30 through a working gas passage 36a formed in the upper lid of the first displacer 18a, and is connected to the first expansion chamber 32 through a working gas passage 36b formed in the lower lid of the first displacer 18a. The second regenerator 28 is connected to the first regenerator 26 through a working gas passage 36c formed from the lower lid of the first displacer 18a to the upper lid of the second displacer 18b. The second regenerator 28 is also connected to the second expansion chamber 34 through a working gas passage 36d formed in the displacer cap 46.

[0039] The working gas flow passage 36d may have an axial flow passage 36d1 and a radial flow passage 36d2. The axial flow passage 36d1 extends axially downward in the displacer cap 46 from the axial lower end of the ceramic magnetic regenerator material 28c. The radial flow passage 36d2 extends radially in the displacer cap 46 from the lower end of the axial flow passage 36d1. The radial flow passage 36d2 is directed toward the stage cylinder portion 35b of the second cooling stage 35. In order to blow the working gas evenly from the displacer cap 46 in the circumferential direction, a plurality of radial flow passages 36d2 may extend from the axial flow passage 36d1. Thus, the working gas that has entered the axial flow passage 36d1 from the ceramic magnetic regenerator material 28c blows out from the radial flow passage 36d2 toward the stage cylinder portion 35b, and flows into the second expansion chamber 34 through the radial clearance between the displacer cap 46 and the stage cylinder portion 35b.

[0040] A first seal 38a and a second seal 38b may be provided so that the working gas flow between the first expansion chamber 32, the second expansion chamber 34 and the upper chamber 30 is guided to the first regenerator 26 and the second regenerator 28, instead of through the clearance between the refrigerator cylinder 16 and the displacer assembly 18. The first seal 38a may be attached to an upper cover of the first displacer 18a so as to be disposed between the first displacer 18a and the first cylinder 16a. The second seal 38b may be attached to an upper cover of the second displacer 18b so as to be disposed between the second displacer 18b and the second cylinder 16b.

[0041] The expander 14 also includes a pressure switching valve 40 and a drive motor 42. The pressure switching valve 40 is accommodated in the refrigerator housing 20, and the drive motor 42 is attached to the refrigerator housing 20.

[0042] As shown in FIG. 2, the pressure switching valve 40 includes a high-pressure valve 40a and a low-pressure valve 40b, and is configured to generate periodic pressure fluctuations in the refrigerator cylinder 16. The working gas discharge port of the compressor 12 is connected to the upper chamber 30 via the high-pressure valve 40a, and the working gas intake port of the compressor 12 is connected to the upper chamber 30 via the low-pressure valve 40b. The high-pressure valve 40a and the low-pressure valve 40b are configured to selectively and alternately open and close (i.e., when one is open, the other is closed). High-pressure (e.g., 2 to 3 MPa) working gas is supplied from the compressor 12 to the expander 14 through the high-pressure valve 40a, and low-pressure (e.g., 0.5 to 1.5 MPa) working gas is collected from the expander 14 to the compressor 12 through the low-pressure valve 40b. For ease of understanding, the flow direction of the working gas is indicated by arrows in FIG. 2.

[0043] The drive motor 42 is provided to drive the reciprocating motion of the displacer assembly 18. The drive motor 42 is connected to a displacer drive shaft 44 via a motion conversion mechanism 43, such as a Scotch yoke mechanism. The motion conversion mechanism 43 is housed in the refrigerator housing 20, as is the pressure switching valve 40. The displacer drive shaft 44 extends from the motion conversion mechanism 43 through the refrigerator housing 20 into the upper chamber 30 and is fixed to the upper cover of the first displacer 18a. A third seal 38c is provided to prevent leakage of the working gas from the upper chamber 30 to the refrigerator housing 20 (which may be maintained at a low pressure as described above). The third seal 38c may be attached to the refrigerator housing 20 so as to be disposed between the refrigerator housing 20 and the displacer drive shaft 44.

[0044] When the drive motor 42 is driven, the rotational output of the drive motor 42 is converted into axial reciprocating motion of the displacer drive shaft 44 by the motion conversion mechanism 43, and the displacer assembly 18 reciprocates in the axial direction inside the refrigerator cylinder 16. In addition, the drive motor 42 is connected to the high pressure valve 40a and the low pressure valve 40b so as to selectively and alternately open and close these valves.

[0045] When the compressor 12 and the drive motor 42 are operated, the cryogenic refrigerator 10 generates periodic volume fluctuations and synchronous pressure fluctuations of the working gas in the first expansion chamber 32 and the second expansion chamber 34, thereby forming a refrigeration cycle, and the first cooling stage 33 and the second cooling stage 35 are cooled to a desired cryogenic temperature. The first cooling stage 33 can be cooled to a first cooling temperature in the range of, for example, about 30K to about 80K. The second cooling stage 35 can be cooled to a second cooling temperature lower than the first cooling temperature, for example, 1K to 20K. The second cooling temperature may be the liquid helium temperature of about 4.2K or a temperature lower than that.

[0046] FIG. 3 is a diagram showing the temperature change of the specific heat of a representative magnetic regenerator material, and more specifically, a graph showing the temperature change of the specific heat of HoCu2 and GOS in the cryogenic temperature region. As shown in FIG. 3, HoCu2 and GOS have a peak where the specific heat is maximum in the temperature range of about 4K to about 10K, that is, in the temperature range of the region in which HoCu2 and GOS are accommodated in the second regenerator 28. For example, the specific heat of HoCu2 is maximum at two points where the temperatures are about 6K and about 9K. In addition, the specific heat of GOS has a very sharp peak at about 5K. Many other types of ceramic magnetic regenerator materials also have a sharp peak at a certain temperature below about 5K, similar to GOS.

[0047] FIG. 4 is a schematic diagram of a part of a cryogenic refrigerator according to a comparative example. FIG. 4 shows the lowest temperature part of the cryogenic refrigerator, that is, the second cooling stage 135, the low temperature side of the second cylinder 116b extending axially upward from the second cooling stage 135, and the low temperature side of the second displacer 118b including the displacer cap 146. The second cooling stage 135 includes an end portion 135a and a cylindrical portion 135b that axially connects the end portion 135a to the second cylinder 116b. A magnetic regenerator material, for example, a ceramic-based magnetic regenerator material 128c, is accommodated on the low temperature side of the second displacer 118b. The displacer cap 146 is formed with a working gas flow passage 136d having an axial flow passage 136d1 and a radial flow passage 136d2. The ceramic-based magnetic regenerator material 128c and the second expansion chamber 134 are connected through the working gas flow passage 136d.

[0048] In the state shown in FIG. 4, the second displacer 118b is located at the top dead center. Therefore, the displacer cap 146 is at the uppermost position in the axial direction, and the second expansion chamber 134 has the maximum volume. Typically, in the existing design of the working gas passage 136d of the displacer cap 146, when the second displacer 118b is located at the top dead center, the axial position of the radial passage 136d of the working gas passage 136d, which is the outlet of the working gas, is the same as the axial upper end of the cylindrical portion 135b of the second cooling stage 135 (or the axial lower end of the second cylinder 16b), as shown in the figure. In this way, the axial height of the radial passage 136d and the upper end of the cylindrical portion 135b of the second cooling stage 135 are aligned.

[0049] Therefore, when the second displacer 118b is at the top dead center, the ceramic magnetic regenerator material 128c adjacent to the displacer cap 146 in the axial direction is located axially above the cylindrical portion 135b of the second cooling stage 135 and is surrounded by the second cylinder 116b. Thus, at the top dead center of the second displacer 118b, the ceramic magnetic regenerator material 128c is located outside the space 148 surrounded by the second cooling stage 135, as shown by the dashed line in FIG.

[0050] During operation of the cryogenic refrigerator, the displacer cap 146 and the lower end of the ceramic magnetic regenerator 128c adjacent thereto are at the lowest temperature in the cryogenic refrigerator, as is the second cooling stage 135. On the other hand, the second cylinder 116b has an axial temperature distribution in which the temperature increases toward the upper side in the axial direction. Therefore, when the second displacer 118b is at the top dead center, the part of the second cylinder 116b surrounding the ceramic magnetic regenerator 128c has a somewhat higher temperature than the ceramic magnetic regenerator 128c. Due to the influence of the temperature difference between the ceramic magnetic regenerator 128c and the second cylinder 116b surrounding it, a periodic temperature fluctuation may occur in the ceramic magnetic regenerator 128c with the axial reciprocating motion of the second displacer 118b.

[0051] As shown in Fig. 3, the specific heat of the ceramic magnetic regenerator 128c, for example, GOS, is very sensitive to temperature. Even a slight temperature fluctuation in the ceramic magnetic regenerator 128c can lead to a significant fluctuation, for example, a significant decrease, in the specific heat. Due to this, in the cryogenic refrigerator of the comparative example, the cold storage performance of the ceramic magnetic regenerator 128c and thus the performance of the cryogenic refrigerator may be affected.

[0052] Figures 5(a) and 5(b) are diagrams according to an embodiment, which diagrammatically show the axial reciprocating motion of the displacer assembly 18. Figure 5(a) diagrammatically shows the expander 14 when the displacer assembly 18 is located at the top dead center, and Figure 5(b) diagrammatically shows the expander 14 when the displacer assembly 18 is located at the bottom dead center. In Figure 2, the displacer assembly 18 is in an intermediate position between the top dead center and the bottom dead center.

[0053] As described above, when the first displacer 18a and the second displacer 18b are at the top dead center, the first expansion chamber 32 and the second expansion chamber 34 have the maximum volume, respectively, and when the first displacer 18a and the second displacer 18b are at the bottom dead center, the first expansion chamber 32 and the second expansion chamber 34 have the minimum volume, respectively. When the first displacer 18a is at the bottom dead center, the axial clearance between the first displacer 18a and the first cooling stage 33 is ideally zero. However, in actual design, the axial clearance is set to, for example, about 0.1 mm to about 1 mm to avoid contact between the two. Similarly, the axial clearance between the second displacer 18b and the stage end portion 35a of the second cooling stage 35 when the second displacer 18b is at the bottom dead center can be set to, for example, within a range of about 0.1 mm to about 1 mm.

[0054] Conversely, the upper chamber 30 has a minimum volume when the first displacer 18a is at the top dead center, and a maximum volume when the first displacer 18a is at the bottom dead center. The axial height of the upper chamber 30 when the first displacer 18a is at the top dead center, i.e., the axial clearance between the refrigerator housing 20 and the first displacer 18a, is ideally zero, and can be set in the range of, for example, about 0.1 mm to about 1 mm in practice. This makes it possible to minimize the dead volume in the upper chamber 30.

[0055] In this embodiment, as shown in Fig. 5(a), the axial position of the ceramic magnetic regenerator material 28c in the second displacer 18b is determined so as to overlap with the stage cylinder portion 35b in the axial direction when the second displacer 18b is at the top dead center. Therefore, when the second displacer 18b is at the top dead center, at least a part of the ceramic magnetic regenerator material 28c, for example, the axial lower part of the ceramic magnetic regenerator material 28c, is located axially lower than the upper end 50 of the stage cylinder portion 35b of the second cooling stage 35 and is surrounded by the stage cylinder portion 35b. Similarly, the displacer cap 46 is also surrounded by the stage cylinder portion 35b.

[0056] In this way, at the top dead center of the second displacer 118b, the lower end of the second displacer 18b, i.e., at least a portion of the ceramic magnetic regenerator material 128c and the displacer cap 46, are positioned within the space 48 surrounded by the second cooling stage 35, as shown by the dashed line in Figure 5(a).

[0057] 5(b), when the second displacer 18b is at the bottom dead center, the second displacer 18b enters further into the space 48 surrounded by the second cooling stage 35. At the bottom dead center of the second displacer 118b, a larger portion of the ceramic magnetic regenerator material 28c, together with the displacer cap 46, is surrounded by the stage cylinder portion 35b and is located in the space 48.

[0058] Therefore, by determining the axial position of the ceramic magnetic regenerator material 28c within the second displacer 18b so that it axially overlaps with the stage cylinder portion 35b when the second displacer 18b is at top dead center, at least a portion of the ceramic magnetic regenerator material 128c is retained within the space 48 surrounded by the second cooling stage 35 throughout the entire stroke length S of the axial reciprocating motion of the second displacer 18b.

[0059] During operation of the cryogenic refrigerator, the displacer cap 46 and the lower end of the ceramic magnetic regenerator material 28c adjacent thereto are at the lowest temperature in the cryogenic refrigerator, as is the second cooling stage 35. Therefore, unlike the comparative example shown in Fig. 4, in this embodiment, the ceramic magnetic regenerator material 28c can be stabilized in temperature by being surrounded by the second cooling stage 35. This makes it possible to suppress the influence of temperature fluctuations that may occur in the ceramic magnetic regenerator material 28c on the regenerator performance and, ultimately, on the performance of the cryogenic refrigerator 10.

[0060] In this embodiment, the axial position of the metal-based magnetic regenerator material 28b in the second displacer 18b is determined so that the metal-based magnetic regenerator material 28b does not overlap with the stage cylinder 35b in the axial direction when the second displacer 18b is at the top dead center. Therefore, when the second displacer 118b is at the top dead center, as shown in FIG. 5(a), the metal-based magnetic regenerator material 28b is located axially above the stage cylinder 35b and surrounded by the second cylinder 16b. Thus, at the top dead center of the second displacer 18b, the metal-based magnetic regenerator material 28b is located outside the space 48 surrounded by the second cooling stage 35.

[0061] The cryocooler 10 may be used under a strong magnetic field, for example, for cooling a superconducting magnet. When the metallic magnetic heat accumulator 28b moves in a magnetic field distribution, an eddy current is induced inside the metallic magnetic heat accumulator 28b, and the eddy current can be converted into heat by the resistance of the metallic magnetic heat accumulator 28b itself. Therefore, by disposing the metallic magnetic heat accumulator 28b at a position axially away from the second cooling stage 35, it is possible to suppress a temperature rise in the second cooling stage 35 due to heat generation by the metallic magnetic heat accumulator 28b.

[0062] In addition, the axial position of the metal-based magnetic regenerator material 28b in the second displacer 18b may be determined so as not to overlap with the stage cylinder portion 35b in the axial direction when the second displacer 18b is at the bottom dead center. Therefore, when the second displacer 118b is at the bottom dead center, the metal-based magnetic regenerator material 28b may be located axially above the stage cylinder portion 35b, surrounded by the second cylinder 16b, and outside the space 48, as shown in FIG. 5(b).

[0063] To achieve this, the axial position of the ceramic magnetic refrigerant storage material 28c within the second displacer 18b may be determined so as to satisfy Hs>S-Hc, where S is the stroke length of the reciprocating movement of the second displacer 18b in the axial direction, Hc is the axial height of the displacer cap 46, and Hs is the axial height from the displacer cap 46 to the upper end (i.e., boundary 29b) of the ceramic magnetic refrigerant storage material 28c.

[0064] In this way, the metal-based magnetic regenerator material 28b is held outside the space 48 surrounded by the second cooling stage 35 over the entire stroke length S of the axial reciprocating motion of the second displacer 18b. Therefore, it is possible to suppress a temperature rise of the second cooling stage 35 due to heat generation of the metal-based magnetic regenerator material 28b.

[0065] The speed of the axial reciprocating motion of the second displacer 18b is slow near the top dead center and the bottom dead center, and is fast at the axial intermediate portion. The faster the moving speed of the second displacer 18b, the larger the time change of the magnetic field acting on the metal-based magnetic regenerator material 28b becomes, and the larger the amount of heat generated by eddy currents may become. The axial intermediate portion corresponds to, for example, a range of 1 / 4 to 3 / 4 of the stroke length S. Therefore, in order to arrange the metal-based magnetic regenerator material 28b while avoiding this range, the axial position of the ceramic-based magnetic regenerator material 28c in the second displacer 18b may be determined to satisfy Hs>(3 / 4)S-Hc.

[0066] The displacer cap 46 may be formed of a material having a higher electrical resistivity than the second cooling stage 35. In this way, when the second displacer 18b moves in the magnetic field distribution, it is possible to reduce eddy currents that may be induced inside the displacer cap 46, and to reduce heat generation from the displacer cap 46 due to the eddy currents.

[0067] As described above, the second cooling stage 35 is typically made of copper. Therefore, the displacer cap 46 may be made of a metal having a higher electrical resistivity than copper, such as stainless steel, a nickel alloy such as nichrome or Inconel (registered trademark), or a titanium alloy. Alternatively, the displacer cap 46 may be made of a synthetic resin material such as phenolic resin. Alternatively, the displacer cap 46 may be made of a ceramic magnetic regenerator material such as GOS.

[0068] FIG. 6 is a diagram illustrating an example of the displacer cap 46 according to the embodiment. The displacer cap 46 may include a main body 46a and a cold storage body 46b. As described above, the main body 46a may be made of a material having a higher electrical resistivity than the second cooling stage 35, such as stainless steel. The main body 46a may have a working gas flow path 36d formed therein. The cold storage body 46b may be made of a ceramic magnetic cold storage material, such as GOS. In this way, by providing the cold storage body 46b in the displacer cap 46, the temperature rise of the displacer cap 46 can be suppressed and stabilized.

[0069] The cold storage body 46b may be exposed to the second expansion chamber 34. For example, the cold storage body 46b may be fixed to the outer surface of the displacer cap 46 so as to face the stage end portion 35a. The main body 46a may have a recess for receiving the cold storage body 46b, and the cold storage body 46b may be bonded to the main body 46a using an adhesive for cryogenic temperatures such as Nitofix (registered trademark). Alternatively, a screw may be formed on the outer peripheral surface of the cold storage body 46b, a screw hole corresponding to the main body 46a may be formed, and the cold storage body 46b may be screwed into the screw hole of the main body 46a and fixed.

[0070] The present invention has been described above based on examples. It will be understood by those skilled in the art that the present invention is not limited to the above-described embodiments, that various design changes are possible, that various modifications are possible, and that such modifications are also within the scope of the present invention. Various features described in relation to one embodiment can also be applied to other embodiments. A new embodiment resulting from a combination has the combined effects of each of the combined embodiments.

[0071] In the above embodiment, the GM refrigerator is described as an example, but the present invention is not limited thereto. In an embodiment, the cryogenic refrigerator 10 may be another type of cryogenic refrigerator in which a displacer containing a ceramic magnetic regenerator material reciprocates in the axial direction, such as a Solvay refrigerator.

[0072] The present invention has been described using specific terms based on the embodiments, but the embodiments merely show one aspect of the principles and applications of the present invention, and many modifications and changes in arrangement are permitted to the embodiments without departing from the spirit of the present invention as defined in the claims. [Explanation of symbols]

[0073] 10 cryogenic refrigerator, 16 refrigerator cylinder, 16a first cylinder, 16b second cylinder, 18 displacer assembly, 18a first displacer, 18b second displacer, 28b metal-based magnetic regenerator material, 28c ceramic-based magnetic regenerator material, 32 first expansion chamber, 33 first cooling stage, 34 second expansion chamber, 35 second cooling stage, 35a stage end portion, 35b stage tube portion, 46 displacer cap.

Claims

1. an axially extending cylinder having a first thermal conductivity; a cooling stage having a second thermal conductivity higher than the first thermal conductivity, the cooling stage including a stage end portion and a stage tube portion connecting the stage end portion to the cylinder in the axial direction; a displacer capable of reciprocating in the axial direction within the cylinder, forming an expansion space between the displacer and an end portion of the stage, the expansion space having a maximum volume at a top dead center; a ceramic-based magnetic regenerator material accommodated in the displacer, the ceramic-based magnetic regenerator material having an axial position within the displacer determined so as to overlap with the stage cylinder portion in the axial direction when the displacer is at the top dead center.

2. 2. The cryogenic refrigerator according to claim 1, further comprising a metallic magnetic regenerator material accommodated in the displacer, the metallic magnetic regenerator material having an axial position within the displacer determined so as not to overlap with the stage cylinder in the axial direction when the displacer is at the top dead center.

3. 3. The cryogenic refrigerator according to claim 1, wherein the displacer includes a displacer cap that faces an end portion of the stage across the expansion space.

4. 4. The cryogenic refrigerator according to claim 3, wherein the displacer cap is made of a material having a higher electrical resistivity than the cooling stage.

5. 4. The cryogenic refrigerator according to claim 3, wherein the displacer cap includes a cold storage body made of a ceramic magnetic cold storage material.

6. The cryogenic refrigerator according to claim 5, wherein the cold storage body is exposed to the expansion space.

7. the ceramic magnetic regenerator material is disposed within the displacer adjacent to the displacer cap in the axial direction; The cryogenic refrigerator described in claim 3, characterized in that the axial position of the ceramic magnetic regenerator material within the displacer is determined so as to satisfy Hs>S-Hc, where S is the stroke length of the reciprocating movement of the displacer in the axial direction, Hc is the axial height of the displacer cap, and Hs is the axial height from the displacer cap to the upper end of the ceramic magnetic regenerator material.

8. 8. The cryogenic refrigerator according to claim 7, wherein the axial position of the ceramic magnetic regenerator material in the displacer is determined so as to satisfy Hs>(3 / 4)S-Hc.

Images