Cryogenic refrigerator

Abstract

The purpose of the present invention is to suppress a reduction in performance of a cryogenic refrigerator in which a ceramic-based magnetic cold storage material is incorporated. A cryogenic refrigerator (10) is provided with: a second cylinder (16b) having a first thermal conductivity and extending in the axial direction; a second cooling stage (35) having a second thermal conductivity higher than the first thermal conductivity and provided with a stage tip section (35a) and a stage tube section (35b) connecting the stage tip section (35a) to the second cylinder (16b) in the axial direction; a second displacer (18b) capable of reciprocating in the axial direction inside the second cylinder (16b), forming a second expansion chamber (34) between the second displacer (18b) and the stage tip (35a), the second expansion chamber having a maximum volume when the second displacer (18b) is positioned at a top dead center; and a ceramic-based magnetic cold storage material (28c) housed in the second displacer, the axial position of the ceramic-based magnetic cold storage material (28c) in the second displacer being determined so as to overlap the stage cylinder section in the axial direction when the second displacer is located at the top dead center.

Description

[0001] This application claims priority based on Japanese Patent Application No. 2023-168492 filed on September 28, 2023. The entire contents of the Japanese Patent Application are incorporated herein by reference. Technical Field

[0002] The invention relates to an ultra-low temperature refrigerator. Background Art

[0003] Typically, in a cryogenic refrigerator that provides ultra-low temperature cooling below about 10K (for example, a liquid helium temperature of about 4.2K), a regenerator is mounted, which has a magnetic regenerator material with a large specific heat peak in the temperature region accompanying a magnetic phase transition. The magnetic regenerator material helps to improve the refrigeration capacity of the cryogenic refrigerator at such ultra-low temperatures. There are roughly two types of magnetic regenerator materials, known as metal-based magnetic regenerator materials such as HoCu2 and ceramic-based magnetic regenerator materials such as Gd2O2S (also known as GOS).

[0004] Patent Document 1: Japanese Patent Application Publication No. 2015-183970

[0005] For example, in an ultra-low temperature refrigerator such as the Gifford-McMahon (GM) refrigerator, in which a displacer with a built-in cold storage material moves axially, the temperature of the cold storage material will change slightly as the displacer moves axially due to the axial temperature distribution generated by the refrigerator itself. In many ceramic magnetic cold storage materials, such as GOS, the specific heat is very sensitive to temperature, that is, the peak value of the specific heat is very sharp. Therefore, even if the temperature change in the ceramic magnetic cold storage material is small, it may cause a significant change in the specific heat, such as a sharp drop. As a result, it may affect the cold storage performance of the ceramic magnetic cold storage material and even the performance of the ultra-low temperature refrigerator. Summary of the invention

[0006] One of the exemplary purposes of one embodiment of the present invention is to suppress the performance degradation of a cryogenic refrigerator having a built-in ceramic-based magnetic cold storage material.

[0007] According to one embodiment of the present invention, an ultra-low temperature refrigerator comprises: a cylinder body having a first thermal conductivity and extending axially; a cooling stage having a second thermal conductivity higher than the first thermal conductivity, the cooling stage having a stage end portion and a stage barrel portion axially connecting the stage end portion to the cylinder body; a displacer capable of axially reciprocating in the cylinder body, forming an expansion space between the displacer and the stage end portion, and when the displacer is at the top dead center, the expansion space has a maximum volume; and a ceramic magnetic cold storage material contained in the displacer, and its axial position in the displacer is determined to overlap with the stage barrel portion axially when the displacer is at the top dead center.

[0008] According to the present invention, it is possible to suppress the performance degradation of a cryogenic refrigerator having a built-in ceramic-based magnetic cold storage material. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 It is a diagram schematically showing a cryogenic refrigerator according to an embodiment.

[0010] Figure 2 It is a diagram schematically showing a cryogenic refrigerator according to an embodiment.

[0011] Figure 3 This is a diagram showing the temperature change of the specific heat of a typical magnetic cold storage material.

[0012] Figure 4 This is a diagram schematically showing a part of a cryogenic refrigerator according to a comparative example.

[0013] Figure 5 (a) and (b) are diagrams schematically showing the axial reciprocating movement of the displacer assembly according to the embodiment.

[0014] Figure 6 This is a diagram schematically showing an example of a displacer cover according to the embodiment.

[0015] In the figure: 10-ultra-low temperature refrigerator, 16-refrigerator cylinder, 16a-first cylinder, 16b-second cylinder, 18-displacer assembly, 18a-first displacer, 18b-second displacer, 28b-metallic magnetic cold storage material, 28c-ceramic magnetic cold storage material, 32-first expansion chamber, 33-first cooling stage, 34-second expansion chamber, 35-second cooling stage, 35a-stage end portion, 35b-stage barrel portion, 46-displacer cover. DETAILED DESCRIPTION

[0016] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description and drawings, identical or equivalent components, parts and processes are marked with the same symbols, and repeated descriptions are appropriately omitted. For ease of description, in each of the drawings, the scale or shape of each part is appropriately set, and unless otherwise specifically stated, it is not interpreted as a limitative one. The embodiments are illustrative and do not limit the scope of the present invention in any way. All features or combinations thereof described in the embodiments are not necessarily the essence of the invention.

[0017] Figure 1 and Figure 2 1 is a diagram schematically showing a cryogenic refrigerator 10 according to an embodiment. As an example, the cryogenic refrigerator 10 is a two-stage Gifford-McMahon (GM) refrigerator. Figure 1FIG. 4 shows the appearance of the ultra-low temperature refrigerator 10. Figure 2 The internal structure of the cryogenic refrigerator 10 is shown in FIG.

[0018] 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, increase the pressure of the recovered working gas, and then supply the working gas to the expander 14 again. The working gas is also called a refrigerant gas, and is usually helium, but other suitable gases may also be used.

[0019] The expander 14 includes a refrigerator cylinder 16, a displacer assembly 18, and a refrigerator housing 20. The refrigerator housing 20 is combined with the refrigerator cylinder 16 to form an airtight container for accommodating the displacer assembly 18. In addition, the internal volume of the refrigerator housing 20 can also be connected to the low-pressure side of the compressor 12 to maintain it at a low pressure.

[0020] The refrigerator cylinder 16 has a first cylinder 16a and a second cylinder 16b extending in the axial direction. As an example, the first cylinder 16a and the second cylinder 16b are cylindrical components, and the diameter of the second cylinder 16b is smaller than the diameter of the first cylinder 16a. The first cylinder 16a and the second cylinder 16b are coaxially arranged, and the lower end of the first cylinder 16a and the upper end of the second cylinder 16b are rigidly connected together.

[0021] like Figure 2 As shown, the displacer assembly 18 includes a first displacer 18a and a second displacer 18b. As an example, the first displacer 18a and the second displacer 18b are both cylindrical components, and the diameter of the second displacer 18b is smaller than that of the first displacer 18a. The first displacer 18a and the second displacer 18b are coaxially arranged.

[0022] 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.

[0023] In this specification, in order to explain the positional relationship between the components of the ultra-low temperature refrigerator 10, for the sake of convenience, the side close to the upper dead point of the axial reciprocating movement of the displacer is marked as "up", and the side close to the lower dead point is marked as "down". The upper dead point is the position of the displacer when the volume of the expansion space becomes the maximum, and the lower dead point is the position of the displacer when the volume of the expansion space becomes the minimum. When the ultra-low temperature refrigerator 10 is running, a temperature gradient is generated in which the temperature decreases from the upper axial side to the lower axial side, so the upper side can also be called the high temperature side, and the lower side can be called the low temperature side.

[0024] The first displacer 18a accommodates the first cold storage device 26. The cylindrical body of the first displacer 18a becomes the first cold storage device container extending in the axial direction. The first cold storage device 26 is formed by filling the body of the first displacer 18a with a metal wire mesh such as copper or other appropriate first cold storage material. The upper cover and the lower cover of the first displacer 18a can be provided as components different from the body of the first displacer 18a, and the upper cover and the lower cover of the first displacer 18a can be fixed to the body by an appropriate method such as fastening and welding, so that the first cold storage material is accommodated in the first displacer 18a.

[0025] Similarly, the second displacer 18b accommodates the second cold storage material 28. The cylindrical body of the second displacer 18b becomes the second cold storage material container extending in the axial direction. The upper cover and the lower cover of the second displacer 18b can be provided as components different from the body of the second displacer 18b, and the upper cover and the lower cover of the second displacer 18b can be fixed to the body by appropriate methods such as fastening and welding, so that the second cold storage material is accommodated in the second displacer 18b. Hereinafter, for the sake of convenience, the lower cover of the second displacer 18b is referred to as a displacer cover 46.

[0026] The second regenerator 28 includes at least two types of regenerator materials, and in this example, includes three types of regenerator materials, and these regenerator materials are stacked in the axial direction of the second displacer 18b. Figure 2 As shown, from the high temperature side toward the low temperature side in the axial direction, the second cold storage device 28 has a non-magnetic cold storage material 28a, a metal-based magnetic cold storage material 28b and a ceramic-based magnetic cold storage material 28c in sequence. In order to improve the second stage refrigeration capacity of the ultra-low temperature refrigerator 10, materials having large specific heat at different temperatures are selected as these cold storage materials.

[0027] The non-magnetic cold storage material 28a is arranged on the high temperature side of the second displacer 18b, that is, arranged in the second displacer 18b in a manner adjacent to the upper cover portion of the second displacer 18b in the axial direction. The non-magnetic cold storage material 28a can be made of a material having a high volumetric specific heat (for example, a volumetric specific heat higher than the volumetric specific heat of copper) in the temperature range of 10K to 50K or at least a part thereof at the high temperature side of the second displacer 18b. The non-magnetic cold storage material 28a can have a higher volumetric specific heat than the volumetric specific heat of the cold storage material (that is, copper) typically used in the first displacer 18a as described above in this temperature range. In addition, the non-magnetic cold storage material 28a can have a higher volumetric specific heat than the volumetric specific heat of the magnetic cold storage material described later in this temperature range.

[0028] The non-magnetic cold storage material 28a can be made of a non-magnetic metal material such as bismuth, zinc, lead, or an alloy containing at least one of them. As the non-magnetic cold storage material 28a, two materials can be stacked and used. For example, a zinc cold storage material can be arranged adjacent to the upper cover portion of the second displacer 18b in the axial direction, and a bismuth cold storage material can be arranged adjacent to the zinc cold storage material in the axial direction.

[0029] The metal-based magnetic cold storage material 28b is arranged in the middle part of the second displacer 18b, that is, it is arranged in the second displacer 18b adjacent to the non-magnetic cold storage material 28a in the axial direction. The metal-based magnetic cold storage material 28b can be made of a material having a high volumetric specific heat in the temperature range of the axial middle part of the second displacer 18b, for example, 5K to 10K or at least a part thereof. Typically, the peak value of the volumetric specific heat of the metal-based magnetic cold storage material 28b accompanying the magnetic phase change is within this temperature range. The metal-based magnetic cold storage material 28b can be made of a metal magnetic material such as HoCu2, Er3Ni, etc.

[0030] The ceramic magnetic cold storage material 28c is arranged in the low temperature part of the second displacer 18b, that is, arranged in the second displacer 18b adjacent to the displacer cover 46 in the axial direction. The ceramic magnetic cold storage material 28c is arranged between the metal magnetic cold storage material 28b and the displacer cover 46. The ceramic magnetic cold storage material 28c may be directly adjacent to the displacer cover 46. Alternatively, the ceramic magnetic cold storage material 28c and the displacer cover 46 may be adjacent to each other via a rectifying layer.

[0031] The ceramic magnetic cold storage material 28c can be made of a magnetic cold storage material having a high volumetric specific heat in the temperature range of, for example, 1K to 5K or at least a part thereof, at the low temperature side of the second displacer 18b (i.e., the low temperature end of the displacer where the temperature becomes the lowest in the displacer assembly 18). Typically, the peak value of the volumetric specific heat accompanying the magnetic phase change of the ceramic magnetic cold storage material 28c is within this temperature range.

[0032] The ceramic magnetic cold storage material 28c may include a general formula R x O2S or (R 1-y R′ y ) x O2S (R, R' are at least one rare earth element, 0.1≤x≤9, 0<y<1) or Gd x Al 2-x At least one of the magnetic cold storage materials represented by O3 (1≤x<2). The rare earth elements R, 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.

[0033] Therefore, the ceramic magnetic cold storage material 28c can be Gd2O2S (also called GOS), Tb2O2S (also called TbOS), (Gd 1-y Tb y )2O2S (0<y<1, also known as GdTbOS) or GdAlO3 (also known as GAP), or may also contain at least one of them.

[0034] As is well known, the ceramic magnetic cold storage material 28c having such a composition has a higher (e.g., several times higher) volumetric specific heat than a metal magnetic cold storage material such as HoCu2, for example, in a temperature range of 1K to 5K or at least a portion thereof. Therefore, the ceramic magnetic cold storage material 28c is suitable for improving the refrigeration capacity of the ultra-low temperature refrigerator 10 that provides ultra-low temperature cooling in this temperature range.

[0035] In addition, in order to improve the mechanical strength of the magnetic cold storage material, additives can be added to the magnetic cold storage material. The additive can be, for example, at least one of zirconium Zr, aluminum Al, and aluminum oxide (Al2O3). The weight ratio of the additive to the magnetic cold storage material can be, for example, less than 20% or less than 15%. In this way, the hardness of the magnetic cold storage material can be increased without significantly changing the volumetric specific heat of the magnetic cold storage material (that is, without having a significant impact on the refrigeration capacity of the ultra-low temperature refrigerator 10), thereby reducing the risk of breakage or pulverization caused by impact.

[0036] The non-magnetic cold storage material 28a, the metal-based magnetic cold storage material 28b and the ceramic-based magnetic cold storage material 28c can be granular. The granules constituting these granular cold storage materials can be formed into granules of a size of, for example, 0.01 mm or more and 3 mm or less and filled into the second displacer 18b. The particle size of the granules can 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 with a particle size of 0.14 mm or more and 1.6 mm or less relative to all granules can be 70% by weight or more. When the particle size is less than 0.14 mm, the density becomes too high when filled in the cold storage device, and the resistance to the passage of the working gas (for example, helium) may increase sharply. In addition, when the particle size is greater than 1.6 mm, the heat exchange efficiency between the granules and the working gas may decrease significantly.

[0037] Furthermore, the ratio of the maximum diameter to the minimum diameter (aspect ratio) of the granules is 5 or less, preferably 3 or less, more preferably 2 or less, and further preferably as close to a sphere as possible in any three-dimensional direction. When the aspect ratio exceeds 5, mechanical deformation and damage are likely to occur, and it is difficult to pack at a high density, so the cooling efficiency decreases. Therefore, the ratio of granules having a major diameter to minor diameter ratio of 5 or less to all granules can be 70% by weight or more.

[0038] In order to protect these particles, their surfaces may be coated with a coating layer (eg, alumina (Al 2 O 3 ), fluororesin, etc.) with a thickness of, for example, 1 to 50 μm.

[0039] In order to separate the non-magnetic cold storage material 28a from the metal-based magnetic cold storage material 28b, a partition member such as a metal mesh may be arranged on the boundary 29a between the non-magnetic cold storage material 28a and the metal-based magnetic cold storage material 28b. In addition, a partition member may be arranged on the boundary 29b between the metal-based magnetic cold storage material 28b and the ceramic-based magnetic cold storage material 28c as needed.

[0040] The displacer assembly 18 forms an upper chamber 30, a first expansion chamber 32, and a second expansion chamber 34 inside the refrigerator cylinder 16. In order to exchange heat with a desired object or medium to be cooled by the cryogenic refrigerator 10, the expander 14 includes a first cooling stage 33 and a second cooling stage 35. The cooling stage is sometimes also called a heat load flange.

[0041] 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 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 and tough copper) or other material having a high thermal conductivity.

[0042] The upper chamber 30 is formed between the upper cover of the first displacer 18a and the upper part of the first cylinder 16a. The first expansion chamber 32 is formed between the lower cover of the first displacer 18a and the first cooling stage 33. The first cooling stage 33 is fastened to the lower part of the first cylinder 16a so as to surround the first expansion chamber 32.

[0043] The second cooling stage 35 includes a stage end portion 35a and a stage tube portion 35b that connects the stage end portion 35a to the second cylinder 16b in the axial direction. The second cooling stage 35 can 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 can also be provided as separate components, and these components can be connected to each other to form the second cooling stage 35.

[0044] The second expansion chamber 34 is formed between the lower cover portion of the second displacer 18b (i.e., the displacer cover 46) and the second cooling stage 35. More specifically, the displacer cover 46 is opposed to the stage end portion 35a across the second expansion chamber 34, and the stage barrel portion 35b is fastened to the lower portion of the second cylinder 16b in a manner surrounding the second expansion chamber 34.

[0045] The first regenerator 26 is communicated with the upper chamber 30 through a working gas flow path 36a formed on the upper cover portion of the first displacer 18a, and is communicated with the first expansion chamber 32 through a working gas flow path 36b formed on the lower cover portion of the first displacer 18a. The second regenerator 28 is communicated with the first regenerator 26 through a working gas flow path 36c formed from the lower cover portion of the first displacer 18a to the upper cover portion of the second displacer 18b. Furthermore, the second regenerator 28 is communicated with the second expansion chamber 34 through a working gas flow path 36d formed on the displacer cover 46.

[0046] The working gas flow path 36d may include an axial flow path 36d1 and a radial flow path 36d2. The axial flow path 36d1 extends axially downward from the axial lower end of the ceramic magnetic cold storage material 28c in the displacer cover 46. The radial flow path 36d2 extends radially from the lower end of the axial flow path 36d1 in the displacer cover 46. The radial flow path 36d2 faces the table barrel portion 35b of the second cooling stage 35. In order to evenly blow the working gas from the displacer cover 46 in the circumferential direction, a plurality of radial flow paths 36d2 may extend from the axial flow path 36d1. Therefore, the working gas that enters the axial flow path 36d1 from the ceramic magnetic cold storage material 28c is blown from the radial flow path 36d2 toward the table barrel portion 35b, and flows toward the second expansion chamber 34 through the radial gap between the displacer cover 46 and the table barrel portion 35b.

[0047] In order to prevent the working air flow between the first expansion chamber 32, the second expansion chamber 34 and the upper chamber 30 from being introduced into the gap between the refrigerator cylinder 16 and the displacer assembly 18 and being introduced into the first cold storage device 26 and the second cold storage device 28, a first seal 38a and a second seal 38b may be provided. The first seal 38a may be installed on the upper cover of the first displacer 18a in a manner arranged between the first displacer 18a and the first cylinder 16a. The second seal 38b may also be installed on the upper cover of the second displacer 18b in a manner arranged between the second displacer 18b and the second cylinder 16b.

[0048] Furthermore, the expander 14 includes a pressure switching valve 40 and a drive motor 42. The pressure switching valve 40 is accommodated in the refrigerator casing 20, and the drive motor 42 is attached to the refrigerator casing 20.

[0049] like Figure 2 As shown, the pressure switching valve 40 is constructed to include a high-pressure valve 40a and a low-pressure valve 40b and to generate periodic pressure fluctuations in the refrigeration machine cylinder 16. The working gas outlet of the compressor 12 is connected to the upper chamber 30 via the high-pressure valve 40a, and the working gas intake 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 constructed to selectively open and close alternately (that is, while one valve is open, the other valve is closed). High-pressure (for example, 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 (for example, 0.5 to 1.5 MPa) working gas is recovered from the expander 14 to the compressor 12 through the low-pressure valve 40b. For ease of understanding, in Figure 2 The arrows in the figure indicate the flow direction of the working gas.

[0050] The drive motor 42 is provided to drive the displacer assembly 18 to reciprocate. The drive motor 42 is connected to the displacer drive shaft 44 via a motion conversion mechanism 43 such as a scotch yoke mechanism. Similar to the pressure switching valve 40, the motion conversion mechanism 43 is accommodated in the refrigerator housing 20. The displacer drive shaft 44 extends toward the upper chamber 30 after passing through the refrigerator housing 20 from the motion conversion mechanism 43, and is fixed to the upper cover portion of the first displacer 18a. In order to prevent leakage of the working gas from the upper chamber 30 to the refrigerator housing 20 (sometimes maintained at a low pressure as described above), a third seal 38c is provided. The third seal 38c can be installed in the refrigerator housing 20 in a manner arranged between the refrigerator housing 20 and the displacer drive shaft 44.

[0051] When the drive motor 42 is driven, the rotation output of the drive motor 42 is converted by the motion conversion mechanism 43 into axial reciprocating movement of the displacer drive shaft 44, and the displacer assembly 18 reciprocates in the axial direction in the refrigerator cylinder 16. In addition, the drive motor 42 is connected to the high-pressure valve 40a and the low-pressure valve 40b in a manner that selectively opens and closes these valves alternately.

[0052] When the compressor 12 and the drive motor 42 are running, the ultra-low temperature refrigerator 10 generates periodic volume changes and pressure changes of the working gas synchronized therewith 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 the desired ultra-low 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 (for example, 1K to 20K) lower than the first cooling temperature. The second cooling temperature can be a liquid helium temperature of about 4.2K or a temperature lower than it.

[0053] Figure 3 : is a graph showing the temperature change of the specific heat of a typical magnetic cold storage material, more specifically, a graph showing the temperature change of the specific heat of HoCu2 and GOS in the ultra-low temperature region. Figure 3 As shown in FIG. 1 , in the temperature range of about 4K to about 10K (i.e., the temperature range of the region in the second regenerator 28 containing HoCu2 or GOS), HoCu2 and GOS have peaks where the specific heat becomes maximum. For example, the specific heat of HoCu2 becomes maximum at two locations at temperatures of about 6K and about 9K. In addition, the specific heat of GOS has a very sharp peak at about 5K. Similar to GOS, most other types of ceramic-based magnetic regenerator materials also have sharp peaks at a certain specific temperature below about 5K.

[0054] Figure 4 FIG. 1 is a diagram schematically showing a portion of a cryogenic refrigerator according to a comparative example. Figure 4, the lowest temperature part of the ultra-low temperature refrigerator, i.e., 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 cover 146 are shown. The second cooling stage 135 includes a terminal portion 135a and a cylindrical portion 135b that connects the terminal portion 135a to the second cylinder 116b in the axial direction. A magnetic cold storage material (e.g., a ceramic magnetic cold storage material 128c) is accommodated on the low temperature side of the second displacer 118b. A working gas flow path 136d having an axial flow path 136d1 and a radial flow path 136d2 is formed on the displacer cover 146. The ceramic magnetic cold storage material 128c is connected to the second expansion chamber 134 through the working gas flow path 136d.

[0055] exist Figure 4 In the state shown, the second displacer 118b is located at the top dead center. Therefore, the displacer cover 146 is located at the top in the axial direction, and the second expansion chamber 134 has the maximum volume. Typically, in the conventional design of the working gas flow path 136d of the displacer cover 146, when the second displacer 118b is located at the top dead center, as shown in FIG. Figure 4 As shown in FIG. 1 , the axial position of the outlet of the working gas (i.e., the radial flow path 136d2 of the working gas flow path 136d) is the same as the axial upper end of the cylinder portion 135b of the second cooling stage 135 (or the axial lower end of the second cylinder body 116b). Thus, the axial height of the radial flow path 136d2 is consistent with the axial height of the upper end of the cylinder portion 135b of the second cooling stage 135.

[0056] Therefore, when the second displacer 118b is at the top dead center, the ceramic magnetic cold storage material 128c adjacent to the axial upper side of the displacer cover 146 is located axially above the cylinder portion 135b of the second cooling stage 135 and is surrounded by the second cylinder body 116b. Figure 4 The outside of the space 148 surrounded by the second cooling stage 135 is indicated by a dotted line.

[0057] During the operation of the cryogenic refrigerator, similarly to the second cooling stage 135, the lower end of the displacer cover 146 and the ceramic magnetic cold storage material 128c adjacent thereto also become the lowest temperature in the cryogenic refrigerator. On the other hand, an axial temperature distribution is formed in the second cylinder 116b, in which the temperature becomes higher as it moves axially upward. Therefore, when the second displacer 118b is located at the top dead center, the portion of the second cylinder 116b surrounding the ceramic magnetic cold storage material 128c has a slightly higher temperature than the ceramic magnetic cold storage material 128c. Affected by the temperature difference between the ceramic magnetic cold storage material 128c and the second cylinder 116b surrounding it, periodic temperature changes may occur in the ceramic magnetic cold storage material 128c as the second displacer 118b moves axially back and forth.

[0058] like Figure 3 As shown, the specific heat of the ceramic magnetic cold storage material 128c (e.g., GOS) is very sensitive to temperature. Even if the temperature change in the ceramic magnetic cold storage material 128c is small, it may cause a significant change in the specific heat (e.g., a significant decrease). Therefore, in the ultra-low temperature refrigerator of the comparative example, the cold storage performance of the ceramic magnetic cold storage material 128c and even the performance of the ultra-low temperature refrigerator may be affected.

[0059] Figure 5 (a) and (b) are diagrams schematically showing the axial reciprocating movement of the displacer assembly 18 according to the embodiment. Figure 5 FIG. 1( a ) schematically shows the expander 14 when the displacer assembly 18 is at top dead center. Figure 5 (b) schematically shows the expander 14 when the displacer assembly 18 is at the bottom dead center. Figure 2 In the embodiment of the present invention, the displacer assembly 18 is located at an intermediate position between the top dead center and the bottom dead center.

[0060] As described above, when the first displacer 18a and the second displacer 18b are located at the top dead center, the first expansion chamber 32 and the second expansion chamber 34 respectively become the maximum volume, and when the first displacer 18a and the second displacer 18b are located at the bottom dead center, the first expansion chamber 32 and the second expansion chamber 34 respectively become the minimum volume. The axial clearance between the first displacer 18a and the first cooling stage 33 when the first displacer 18a is located at the bottom dead center is ideally zero. However, in actual design, in order to avoid contact between the two, an axial clearance of, for example, about 0.1 mm to about 1 mm is set. 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 located at the bottom dead center can be set, for example, in the range of about 0.1 mm to about 1 mm.

[0061] On the contrary, the upper chamber 30 has the minimum volume when the first displacer 18a is at the top dead center, and has the 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 gap between the refrigerator housing 20 and the first displacer 18a) is ideally zero, but in practice, it can be set within a range of about 0.1 mm to about 1 mm, for example. Thus, the dead volume in the upper chamber 30 can be minimized.

[0062] In this embodiment, if Figure 5 As shown in (a), the axial position of the ceramic magnetic cold storage material 28c in the second displacer 18b is determined so as to overlap with the table tube portion 35b in the axial direction when the second displacer 18b is located at the top dead center. Therefore, when the second displacer 18b is located at the top dead center, at least a portion of the ceramic magnetic cold storage material 28c (for example, the axial lower portion of the ceramic magnetic cold storage material 28c) is located axially lower than the upper end 50 of the table tube portion 35b of the second cooling stage 35 and is surrounded by the table tube portion 35b. Similarly, the displacer cover 46 is also surrounded by the table tube portion 35b.

[0063] Thus, when the second displacer 18b is located at the top dead center, the lower end of the second displacer 18b (that is, at least a portion of the ceramic magnetic cold storage material 28c and the displacer cover 46) is located at Figure 5 In (a), the space 48 surrounded by the second cooling stage 35 is indicated by the dotted line.

[0064] Furthermore, when the second displacer 18b is at the bottom dead center, Figure 5 As shown in (b), the second displacer 18b further enters the space 48 surrounded by the second cooling stage 35. When the second displacer 18b is located at the bottom dead center, more of the ceramic magnetic cold storage material 28c is surrounded by the stage tube portion 35b together with the displacer cover 46 and is located in the space 48.

[0065] Therefore, by determining the axial position of the ceramic magnetic cold storage material 28c in the second displacer 18b so as to overlap with the stage tube portion 35b in the axial direction when the second displacer 18b is located at the top dead center, at least a portion of the ceramic magnetic cold storage material 28c is maintained in the space 48 surrounded by the second cooling stage 35 during the entire stroke length S of the axial reciprocating movement of the second displacer 18b.

[0066] During the operation of the cryogenic refrigerator, similarly to the second cooling stage 35, the lower end of the displacer cover 46 and the ceramic magnetic cold storage material 28c adjacent thereto becomes the lowest temperature in the cryogenic refrigerator. Figure 4Unlike the comparative example shown, in this embodiment, the temperature of the ceramic magnetic cold storage material 28c can be stabilized because it is surrounded by the second cooling stage 35. As a result, the influence of temperature fluctuations that may occur in the ceramic magnetic cold storage material 28c on the cold storage performance and even the performance of the cryogenic refrigerator 10 can be suppressed.

[0067] Furthermore, in this embodiment, the axial position of the metal-based magnetic regenerator material 28b in the second displacer 18b is determined so as not to overlap the table tube portion 35b in the axial direction when the second displacer 18b is located at the top dead center. Figure 5 As shown in (a), when the second displacer 18b is at the top dead center, the metal-based magnetic regenerator 28b is located axially above the stage tube portion 35b and is surrounded by the second cylinder 16b. Thus, when the second displacer 18b is at the top dead center, the metal-based magnetic regenerator 28b is located outside the space 48 surrounded by the second cooling stage 35.

[0068] The cryogenic refrigerator 10 is sometimes used in a strong magnetic field (for example, for cooling superconducting magnets). When the metal-based magnetic cold storage material 28b moves in the magnetic field distribution, eddy currents are induced inside the metal-based magnetic cold storage material 28b, and the eddy currents are converted into heat based on the resistance of the metal-based magnetic cold storage material 28b itself. Therefore, by arranging the metal-based magnetic cold storage material 28b at a position away from the second cooling stage 35 in the axial direction, the temperature rise of the second cooling stage 35 caused by the heat generated by the metal-based magnetic cold storage material 28b can be suppressed.

[0069] Furthermore, the axial position of the metal-based magnetic cold storage material 28b in the second displacer 18b can be determined so as not to overlap the table tube portion 35b in the axial direction when the second displacer 18b is located at the bottom dead center. Figure 5 As shown in (b), the metallic magnetic cold storage material 28b may be located axially above the table tube portion 35b and surrounded by the second cylinder 16b, and located outside the space 48.

[0070] To achieve this, when the stroke length of the reciprocating movement of the second displacer 18b in the axial direction is set to S, the axial height of the displacer cover 46 is set to Hc, and the axial height from the displacer cover 46 to the upper end of the ceramic magnetic cold storage material 28c (i.e., the boundary 29b) is set to Hs, the axial position of the ceramic magnetic cold storage material 28c in the second displacer 18b can be determined to satisfy Hs>S-Hc.

[0071] Thus, throughout the stroke length S of the axial reciprocating movement of the second displacer 18b, the metal-based magnetic regenerator 28b is held outside the space 48 surrounded by the second cooling stage 35. Therefore, the temperature rise of the second cooling stage 35 caused by the heat generated by the metal-based magnetic regenerator 28b can be suppressed.

[0072] The speed of the axial reciprocating movement of the second displacer 18b becomes slower near the top dead center and the bottom dead center, and becomes faster in the axial middle portion. The faster the movement speed of the second displacer 18b, the greater the temporal change of the magnetic field acting on the metal-based magnetic cold storage material 28b, and the greater the heat generated by the eddy current. The axial middle portion corresponds to, for example, a range of 1 / 4 to 3 / 4 of the stroke length S. Therefore, in order to configure the metal-based magnetic cold storage material 28b to avoid this range, the axial position of the ceramic-based magnetic cold storage material 28c in the second displacer 18b can be determined to satisfy Hs>(3 / 4)S-Hc.

[0073] The displacer cover 46 may be made of a material having a resistivity greater than that of the second cooling stage 35. In this way, when the second displacer 18b moves in the magnetic field distribution, eddy currents that may be induced inside the displacer cover 46 can be reduced, and the heat generated by the displacer cover 46 can be reduced.

[0074] As described above, the typical second cooling stage 35 is made of copper. Therefore, the displacer cover 46 can be made of a metal having a resistivity greater than that of copper, such as stainless steel, nickel-chromium alloy or nickel alloy such as Inconel (registered trademark), titanium alloy, etc. Alternatively, the displacer cover 46 can also be made of a synthetic resin material such as phenolic resin. Alternatively, the displacer cover 46 can also be made of a ceramic magnetic cold storage material such as GOS.

[0075] Figure 6 : is a diagram schematically showing an example of a displacer cover 46 involved in the embodiment. The displacer cover 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 resistivity greater than that of the second cooling stage 35, such as stainless steel. A working gas flow path 36d may be formed on the main body 46a. 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 on the displacer cover 46, the temperature rise of the displacer cover 46 can be suppressed to make it stable.

[0076] 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 cover 46 in a manner opposite to the stage end portion 35a. A recessed portion for accommodating the cold storage body 46b may be formed on the main body 46a, and the cold storage body 46b may be bonded to the main body 46a using an ultra-low temperature adhesive such as Nitofix (registered trademark). Alternatively, screws may be formed on the outer peripheral surface of the cold storage body 46b, and corresponding screw holes may be formed on the main body 46a, so that the cold storage body 46b is screwed into the screw holes of the main body 46a for fixing.

[0077] The present invention has been described above based on the embodiments. The present invention is not limited to the above-mentioned embodiments. It should be understood by those skilled in the art that the present invention can be subjected to various design changes, and various modifications can exist, and these modifications are also included in the scope of the present invention. Various features described in one embodiment can also be applied to other embodiments. The new embodiment generated by the combination has the effects of each of the combined embodiments.

[0078] In the above embodiment, a GM refrigerator is used as an example, but the present invention is not limited thereto. In one embodiment, the ultra-low temperature refrigerator 10 may be another type of ultra-low temperature refrigerator in which a displacer with a ceramic magnetic cold storage material built in reciprocates along the axial direction, such as a Solvay refrigerator.

[0079] The present invention has been described above based on the implementation mode and using specific terms, but the implementation mode merely illustrates one aspect of the principle and application of the present invention. Without departing from the scope of the idea of ​​the present invention specified in the technical solution, the implementation mode may allow for a variety of variations or configuration changes.

Claims

1. A cryogenic refrigerator, characterized in that: have: A cylinder body 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 comprising a stage end portion and a stage barrel portion connecting the stage end portion to the cylinder in the axial direction; and a displacer capable of reciprocating along the axial direction in the cylinder body, forming an expansion space between the displacer and the end portion of the stage, wherein the expansion space has a maximum volume when the displacer is located at the top dead center; and A ceramic magnetic cold storage material is accommodated in the displacer, and its axial position in the displacer is determined so as to overlap with the table tube portion in the axial direction when the displacer is located at the top dead center.

2. The ultra-low temperature refrigerator according to claim 1, characterized in that: A metallic magnetic cold storage material is further provided. The metallic magnetic cold storage material is accommodated in the displacer, and the axial position of the metallic magnetic cold storage material in the displacer is determined so as not to overlap with the table tube portion in the axial direction when the displacer is located at the top dead center.

3. The ultra-low temperature refrigerator according to claim 1 or 2, characterized in that: The displacer includes a displacer cover facing the stage end portion across the expansion space.

4. The ultra-low temperature refrigerator according to claim 3, characterized in that: The displacer cover is made of a material having a resistivity greater than that of the cooling stage.

5. The ultra-low temperature refrigerator according to claim 3, characterized in that: The displacer cover includes a cold storage body made of a ceramic-based magnetic cold storage material.

6. The ultra-low temperature refrigerator according to claim 5, characterized in that: The cold storage body is exposed to the expansion space.

7. The ultra-low temperature refrigerator according to claim 3, characterized in that: The ceramic magnetic cold storage material is arranged adjacent to the displacer cover along the axial direction in the displacer. When the stroke length of the reciprocating movement of the displacer in the axial direction is set to S, the axial height of the displacer cover is set to Hc, and the axial height from the displacer cover to the upper end of the ceramic magnetic cold storage material is set to Hs, the axial position of the ceramic magnetic cold storage material in the displacer is determined to satisfy Hs>S-Hc.

8. The ultra-low temperature refrigerator according to claim 7, characterized in that: The axial position of the ceramic magnetic cold storage material in the displacer is determined to satisfy Hs>(3 / 4)S-Hc.

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