Cryogenic refrigerator

Abstract

A cryogenic refrigerator (10) is provided with: a first cooling section (33) that is cooled to a first cooling temperature; a second cooling section (35) that is cooled to a second cooling temperature lower than the first cooling temperature; a second cylinder (16b) connecting the first cooling section (33) and the second cooling section (35); a second displacer (18b) provided with a magnetic cold storage material (28b) and housed in the second cylinder (16b) so as to be capable of reciprocating in the axial direction inside the second cylinder (16b); and a magnetic shield (50) that is thermally connected to a portion of the second cylinder (16b) that is separated from the first cooling section (33) and the second cooling section (35) in the axial direction so as to be cooled to an intermediate temperature between the first cooling temperature and the second cooling temperature, and that extends along the second cylinder (16b) on the outside of the second cylinder (16b) within at least a partial range of the reciprocating stroke of the magnetic cold storage material (28b).

Description

Technical Field

[0001] The invention relates to an ultra-low temperature refrigerator. Background Art

[0002] Ultra-low temperature refrigerators, such as the Gifford-McMahon (GM) refrigerator, are used to cool superconducting magnet systems such as magnetic resonance imaging (MRI) devices. Typically, in such ultra-low temperature refrigerators, magnetic cold storage materials are installed to achieve cooling to a liquid helium temperature of about 4.2K or less.

[0003] Magnetic cold storage materials can be magnetized under strong magnetic fields generated by magnetic field generating sources such as superconducting magnets. When the magnetic cold storage material moves in a magnetic field environment, magnetic noise may be generated. For example, in the case of a GM refrigerator, the magnetic cold storage material is contained in a displacer. As the displacer moves back and forth, the magnetic cold storage material also moves, which can generate magnetic noise around the GM refrigerator. Magnetic noise can cause the magnetic field around the ultra-low temperature refrigerator to change, which may affect the measurement accuracy of the MRI device or other measuring devices. Therefore, in order to reduce the impact of magnetic noise on the surrounding environment, it has been proposed to provide a magnetic shielding part on the ultra-low temperature refrigerator. The magnetic shielding part is installed in the secondary cooling section of the ultra-low temperature refrigerator, for example, cooled to liquid helium temperature.

[0004] Previous technical literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2019-15466 Summary of the invention

[0007] Technical problem to be solved by the invention

[0008] In the case of recondensation of ultra-low temperature fluids such as liquid helium used to cool superconducting magnets in an ultra-low temperature refrigerator, the secondary cooling section of the ultra-low temperature refrigerator and the above-mentioned magnetic shielding parts may be exposed to the vaporized ultra-low temperature fluid. At this time, the heat that flows from the gas environment into the magnetic shielding parts through convective heat transfer will enter the secondary cooling section through heat transfer using the magnetic shielding parts as the heat transfer path. The secondary refrigeration capacity must not only cope with the recondensation of the ultra-low temperature fluid, but also cope with the heat input caused by convective heat transfer. Therefore, the recondensation capacity of the ultra-low temperature refrigerator is reduced from the original refrigeration capacity of the secondary by the size of the heat input.

[0009] One of the exemplary purposes of an embodiment of the present invention is to suppress a decrease in recondensation capacity in a cryogenic refrigerator having a magnetic shield.

[0010] Means for solving technical problems

[0011] According to one embodiment of the present invention, an ultra-low temperature refrigerator comprises: a first cooling section, which is cooled to a first cooling temperature; a second cooling section, which is cooled to a second cooling temperature lower than the first cooling temperature; a cylinder body, which connects the first cooling section and the second cooling section; a second displacer, which comprises a magnetic cold storage material and is accommodated in the cylinder body in a manner capable of axially reciprocating in the cylinder body; and a magnetic shielding member, which is thermally connected to a portion of the cylinder body axially away from the first cooling section and the second cooling section so as to be cooled to an intermediate temperature between the first cooling temperature and the second cooling temperature, and which extends along the cylinder body on the outside of the cylinder body within at least a portion of the reciprocating stroke of the magnetic cold storage material.

[0012] Effects of the Invention

[0013] According to the present invention, it is possible to suppress a decrease in the recondensation capability in a cryogenic refrigerator equipped with a magnetic shield. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a diagram schematically showing a cryogenic system according to an embodiment.

[0015] Figure 2 It is a schematic diagram that can be applied to Figure 1 A diagram of a cryogenic refrigerator of a cryogenic system is shown.

[0016] Figure 3 This is a graph showing changes in resistivity with temperature of an example of the material of the magnetic shield according to the embodiment.

[0017] Figure 4 (a) and Figure 4 (b) is a diagram schematically showing a modification of the magnetic shield according to the embodiment.

[0018] Explanation of symbols:

[0019] 10-ultra-low temperature refrigerator, 16-expansion machine cylinder, 16a-first cylinder, 16b-second cylinder, 18-displacer assembly, 18a-first displacer, 18b-second displacer, 28b-magnetic cold storage material, 33-first cooling section, 35-second cooling section, 50-magnetic shielding member, 50a-joint portion, 50b-first shielding portion, 50c-second shielding portion. DETAILED DESCRIPTION

[0020] Hereinafter, with reference to the accompanying drawings, embodiments of the present invention will be described in detail. 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 proportions or shapes of each part are appropriately set, and unless otherwise specifically stated, they are not interpreted as limiting. 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.

[0021] Figure 1 It is a diagram schematically showing a cryogenic system 100 according to an embodiment. Figure 2 It is a schematic diagram that can be applied to Figure 1 FIG. 1 is a diagram of a cryogenic refrigerator 10 of a cryogenic system 100. The appearance of the cryogenic refrigerator 10 is shown in FIG. Figure 1 As shown in FIG. 1 , the internal structure of the ultra-low temperature refrigerator 10 is Figure 2 As an example, the cryogenic refrigerator 10 is a two-stage Gifford-McMahon (GM) refrigerator.

[0022] The ultra-low temperature system 100 is configured to cool the object to be cooled 102 by immersion cooling. That is, the object to be cooled 102 is cooled to an ultra-low temperature by heat exchange with an ultra-low temperature liquid refrigerant 104. The entire or a part of the object to be cooled 102 is immersed in the liquid refrigerant 104 and is in direct contact with the liquid refrigerant 104. Alternatively, a flow path and / or piping through which the liquid refrigerant 104 flows can be provided inside and / or around the object to be cooled 102, and the liquid refrigerant 104 and the object to be cooled 102 can exchange heat via the flow path and / or piping.

[0023] In this embodiment, the ultra-low temperature system 100 may be, for example, a magnetic resonance imaging (MRI) system or a part of a superconducting system having a superconducting device such as a superconducting electromagnet, and the cooled object 102 may be, for example, a superconducting coil. The liquid refrigerant 104 may be, for example, liquid helium. By being immersed in the liquid refrigerant 104, the superconducting coil is cooled to an ultra-low temperature below the critical temperature of superconductivity.

[0024] The ultra-low temperature system 100 includes a vacuum container 110 and an ultra-low temperature refrigerator 10. The vacuum container 110 may be, for example, a cryostat, which is configured to provide an ultra-low temperature vacuum environment inside. The vacuum container 110 accommodates a cooled object 102 and a liquid refrigerant 104, and maintains them in the ultra-low temperature vacuum environment. The ultra-low temperature refrigerator 10 is mounted in the vacuum container 110 for cooling and recondensing the vaporized liquid refrigerant 104. The ultra-low temperature refrigerator 10 can indirectly cool the cooled object 102 using the liquid refrigerant 104.

[0025] The vacuum container 110 includes an outer tank 112 and an inner tank 114. A vacuum insulation layer 116 is formed between the outer tank 112 and the inner tank 114. The outer tank 112 is configured to separate the vacuum insulation layer 116 from the surrounding environment of the ultra-low temperature system 100 (for example, the room temperature atmospheric pressure environment). In the vacuum insulation layer 116, for example, an insulating structure such as a multilayer insulation (MLI) can be provided. In addition, the inner tank 114 is configured to accommodate a cooled object 102 and a liquid refrigerant 104 therein, and to separate these cooled objects 102 and the liquid refrigerant 104 from the vacuum insulation layer 116. The outer tank 112 and the inner tank 114 are made of metal materials such as stainless steel or other suitable high-strength materials, for example, to withstand the pressure difference between the inside and the outside.

[0026] 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.

[0027] The expander 14 includes an expander cylinder 16, a displacer assembly 18, and an expander casing 20. The expander casing 20 is combined with the expander cylinder 16 to form an airtight container for accommodating the displacer assembly 18. The expander cylinder 16 and the expander casing 20 are made of, for example, a metal material such as stainless steel or other suitable high-strength materials.

[0028] The expander 14 is installed in the vacuum container 110 in a state where the expander cylinder 16 is inserted into the inner groove 114 of the vacuum container 110 and the expander housing 20 is installed on the outside of the vacuum container 110. As an example, the expander 14 is installed in the upper part of the vacuum container 110 in such a manner that its central axis is aligned with the vertical direction. However, the installation position and installation posture of the expander 14 are not limited to this. For example, the expander 14 can be installed in the lower part of the vacuum container 110. In addition, the expander 14 can be installed in a desired posture, and can be installed in the vacuum container 110 so that the central axis is aligned with the inclined direction or the horizontal direction.

[0029] The expander cylinder 16 has a Figure 1 and Figure 2 The first cylinder body 16a and the second cylinder body 16b extend in the vertical direction (in the vertical direction). The second cylinder body 16b is arranged in series with the first cylinder body 16a in the axial direction. As an example, the first cylinder body 16a and the second cylinder body 16b are cylindrical components, and the diameter of the second cylinder body 16b is smaller than the diameter of the first cylinder body 16a. The first cylinder body 16a and the second cylinder body 16b are coaxially arranged, and the lower end of the first cylinder body 16a and the upper end of the second cylinder body 16b are rigidly connected together.

[0030] 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 members, 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.

[0031] 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.

[0032] In this specification, in order to explain the positional relationship between the components of the cryogenic refrigerator 10, for the sake of convenience, the side close to the top dead center of the axial reciprocating movement of the displacer is marked as "up", and the side close to the bottom dead center is marked as "down". The top dead center is the position of the displacer when the volume of the expansion space is the largest, and the bottom dead center is the position of the displacer when the volume of the expansion space is the smallest. When the cryogenic refrigerator 10 is running, a temperature gradient is generated in which the temperature decreases from the upper axial direction to the lower axial direction, so the upper side can also be called the high temperature side, and the lower side can be called the low temperature side.

[0033] The first displacer 18a accommodates the first cold storage device 26. The first cold storage device 26 is formed by filling a metal wire mesh such as copper or other appropriate first cold storage material in the cylindrical main body of the first displacer 18a. The upper cover and the lower cover of the first displacer 18a can be provided as components different from the main body of the first displacer 18a, and the upper cover and the lower cover of the first displacer 18a can be fixed to the main body by an appropriate method such as fastening or welding, so that the first cold storage material is accommodated in the first displacer 18a.

[0034] Similarly, the second displacer 18b accommodates the second cold storage device 28. The second cold storage device 28 is formed by filling the cylindrical main body of the second displacer 18b with a non-magnetic cold storage material 28a and a magnetic cold storage material 28b. The upper cover and the lower cover of the second displacer 18b can be provided as components different from the main body of the second displacer 18b, and the upper cover and the lower cover of the second displacer 18b can be fixed to the main body by a suitable method such as fastening or welding, so that the second cold storage material is accommodated in the second displacer 18b.

[0035] The second regenerator 28 is divided into a non-magnetic regenerator material 28a and a magnetic regenerator material 28b. The non-magnetic regenerator material 28a is located in the high temperature side region of the second regenerator 28 and is composed of a secondary regenerator material made of a non-magnetic material such as lead or bismuth. The magnetic regenerator material 28b is located in the low temperature side region of the second regenerator 28 and is composed of a secondary regenerator material such as HoCu. 2 The magnetic regenerator 28b is a regenerator made of a magnetic material such as a second regenerator. The magnetic regenerator 28b is a regenerator made of a magnetic body whose specific heat increases with a magnetic phase transition at ultra-low temperatures. The regenerator material of the second regenerator 28 may be formed in a granular form.

[0036] The displacer assembly 18 forms an upper chamber 30, a first expansion chamber 32, and a second expansion chamber 34 inside the expander cylinder 16. In order to perform heat exchange with the desired object or medium to be cooled by the ultra-low temperature refrigerator 10, the expander 14 has a first cooling section 33 and a second cooling section 35. 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 section 33. The second expansion chamber 34 is formed between the lower cover of the second displacer 18b and the second cooling section 35. The first cooling section 33 is fixed to the lower part of the first cylinder 16a in a manner of surrounding the first expansion chamber 32, and the second cooling section 35 is fixed to the lower part of the second cylinder 16b in a manner of surrounding the second expansion chamber 34. The first cooling section 33 and the second cooling section 35 are made of, for example, pure copper (eg, oxygen-free copper, tough pitch copper, etc.) or other metals with high thermal conductivity.

[0037] 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 lower cover portion of the second displacer 18b.

[0038] In order to make the working airflow between the first expansion chamber 32, the second expansion chamber 34 and the upper chamber 30 not be introduced into the gap between the expander cylinder 16 and the displacer assembly 18, but be 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 portion 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 portion of the second displacer 18b in a manner arranged between the second displacer 18b and the second cylinder 16b.

[0039] Furthermore, the expander 14 includes a pressure switching valve 40 and a drive motor 42. The pressure switching valve 40 is accommodated in the expander casing 20, and the drive motor 42 is attached to the expander casing 20.

[0040] 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 expander 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 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.

[0041] 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 movement conversion mechanism 43 such as a scotch yoke mechanism. Similar to the pressure switching valve 40, the movement conversion mechanism 43 is accommodated in the expander housing 20. The displacer drive shaft 44 extends from the movement conversion mechanism 43 through the expander housing 20 toward the upper chamber 30 and is fixed to the upper cover portion of the first displacer 18a. In order to prevent the working gas from leaking from the upper chamber 30 to the expander housing 20 (sometimes maintained at a low pressure as described above), a third seal 38c is provided. The third seal 38c can be installed on the expander housing 20 in a manner arranged between the expander housing 20 and the displacer drive shaft 44.

[0042] When the drive motor 42 is driven, the rotation output of the drive motor 42 is converted by the movement 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 expander 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.

[0043] 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 section 33 and the second cooling section 35 are cooled to the desired ultra-low temperature. The first cooling section 33 can be cooled to a first cooling temperature in the range of, for example, about 20K to about 40K. The second cooling section 35 can be cooled to a second cooling temperature lower than the first cooling temperature (for example, a liquid helium temperature of about 1K to about 4K).

[0044] However, when the object to be cooled 102 is a superconducting coil, the cryogenic refrigerator 10 may be exposed to a high magnetic field generated by the superconducting coil. In a high magnetic field environment, the magnetic cold storage material 28b can be magnetized. The movement of the magnetic cold storage material 28b accompanied by the reciprocating movement of the second displacer 18b will produce magnetic field fluctuations. The magnetic field fluctuations may affect the operation of the cryogenic system 100 (for example, the measurement of an MRI device) as magnetic noise.

[0045] Therefore, in order to reduce the influence of the magnetic noise generated by the movement of the magnetic cold storage material 28 b on the cryogenic system 100 , it has been proposed to provide a magnetic shield 50 on the cryogenic refrigerator 10 .

[0046] The magnetic shield 50 is made of a non-magnetic conductive material such as an electrical conductor such as pure copper or pure aluminum. The magnetic flux interlinked with the magnetic shield 50 changes due to the movement of the magnetic cold storage material 28b. In order to offset the change in the magnetic flux, an electric current flows through the magnetic shield 50. The direction of the current flow varies depending on the configuration of the ultra-low temperature refrigerator 10 and the magnetic field acting on it. For example, the current flows along the axial direction of the expansion machine cylinder 16, or rotates around the axial direction, or flows in other directions. Through this current, the magnetic field changes in the area further outside than the magnetic shield 50 are eliminated. As a result, the magnetic field changes caused by the movement of the magnetic cold storage material 28b can be suppressed from being transmitted to the space further outside than the magnetic shield 50. That is, magnetic noise can be reduced.

[0047] Therefore, the working principle of the magnetic shield 50 is different from that of a conventional magnetic shield made of a magnetic material such as iron. Conventional magnetic shields can shield static magnetic fields, but the magnetic shield 50 is made of a non-magnetic material and therefore does not shield static magnetic fields. The magnetic shield 50 can reduce or prevent changes in the magnetic field from being transmitted from the inside of the magnetic shield 50 to the outside (or from the outside to the inside).

[0048] The magnetic shield 50 is cooled to a cryogenic temperature by the cryogenic refrigerator 10. The cooling reduces the resistivity of the material forming the magnetic shield 50, and current flows more easily in the magnetic shield 50, thereby improving the shielding performance of the magnetic shield 50 against a fluctuating magnetic field.

[0049] As an indicator of the electrical conductivity of a material at ultra-low temperatures, the residual resistance ratio (RRR) can be used. When pure copper or pure aluminum is used as the material of the magnetic shield 50, the RRR of the magnetic shield material at a reference cooling temperature (e.g., 4K) is preferably at least 300 (i.e., above 300). In addition, considering the ease of obtaining the material, the RRR of the magnetic shield material at the reference cooling temperature is preferably below 30,000.

[0050] In a typical design, the magnetic shield 50 is mounted on the second cooling section 35 and is cooled to the second cooling temperature (e.g., liquid helium temperature) by the second cooling section 35. However, the inventors are aware that problems may arise at this time. In the case where the ultra-low temperature refrigerator 10 is used to recondense the vaporized liquid refrigerant 104, the second cooling section 35 and the magnetic shield 50 mounted thereon may be exposed to the refrigerant gas environment. At this time, the heat that flows from the gas environment into the magnetic shield 50 through convective heat transfer enters the second cooling section 35 through heat transfer with the magnetic shield 50 as the heat transfer path. In this way, the secondary refrigeration capacity of the ultra-low temperature refrigerator 10 not only needs to deal with the recondensation of the liquid refrigerant 104, but also needs to deal with the heat input caused by convective heat transfer. Therefore, the recondensation capacity of the ultra-low temperature refrigerator 10 is reduced from the original secondary refrigeration capacity by the size of the heat input.

[0051] In contrast, in this embodiment, the magnetic shield 50 is cooled to a temperature higher than the second cooling temperature. Specifically, the magnetic shield 50 is cooled to an intermediate temperature between the first cooling temperature of the first cooling section 33 and the second cooling temperature of the second cooling section 35. As described later, the intermediate temperature may be, for example, 20K or less or 10K or less. Furthermore, the intermediate temperature may be, for example, 8K or more.

[0052] Figure 3 Graph showing the change in resistivity of an example of the material of the magnetic shield 50 according to the embodiment with respect to temperature. Figure 3 2 shows the resistivity of pure copper with an RRR of 300 as a function of temperature. As can be understood from this figure, the resistivity changes with temperature with a limit of about 20K. As the temperature of the material decreases from 100K to 20K, the resistivity of the material also decreases significantly (for example, the resistivity is less than 10K at 100K). -2 μΩm, less than 10 at 20K -4 On the other hand, even if the temperature of the material drops further from 20K, the resistivity does not change and remains basically constant.

[0053] As described above, the shielding performance of the magnetic shield 50 against a fluctuating magnetic field depends on the resistivity of the material forming the magnetic shield 50 . Therefore, as long as the magnetic shield 50 is cooled to 20 K or less, the cooling temperature does not affect the shielding performance of the magnetic shield 50 .

[0054] It is known that this change in resistivity is the same even when the RRR is larger. It is also known that the change in resistivity is the same even when the material is not pure copper but pure aluminum.

[0055] Therefore, in order to maximize the shielding performance of the magnetic shield 50 against the changing magnetic field, it is not necessary for the magnetic shield 50 to be cooled to the second cooling temperature of the second cooling stage 35. By cooling the magnetic shield 50 to any temperature below 20K, the magnetic shield 50 can provide sufficient performance (performance equivalent to the case of being cooled to 4K or below).

[0056] In order to cope with heat input to the magnetic shield 50 and / or temperature fluctuation (temperature increase) of the magnetic shield 50 due to other factors, the magnetic shield 50 may be cooled to, for example, approximately 10 K or less.

[0057] It is known that in addition to the first cooling section 33 and the second cooling section 35, the expander 14 can also absorb heat on the second cylinder 16b. This heat absorbing part is cooled to a cooling temperature based on the axial temperature distribution on the second cylinder 16b, and can provide a certain refrigeration capacity at this cooling temperature. The cooling temperature of the heat absorbing part becomes an intermediate temperature between the first cooling temperature of the first cooling section 33 and the second cooling temperature of the second cooling section 35. In the normal use scenario of the ultra-low temperature refrigerator 10, the object to be cooled is thermally connected to either the first cooling section 33 or the second cooling section 35 according to the expected cooling temperature. Nothing is connected to the heat absorbing part on the second cylinder 16b, and the refrigeration capacity of the heat absorbing part is not utilized.

[0058] Therefore, the magnetic shield 50 is thermally connected in the axial direction to a portion of the second cylinder 16b that is distant from the first cooling stage 33 and the second cooling stage 35. Thus, the magnetic shield 50 can be cooled to an intermediate temperature between the first cooling temperature and the second cooling temperature.

[0059] The axial temperature distribution on the second cylinder 16b is higher as the axial position is closer to the first cooling section 33, and lower as the axial position is closer to the second cooling section 35. By selecting the axial position of the magnetic shield 50 on the second cylinder 16b, the cooling temperature of the magnetic shield 50 can be adjusted between the first cooling temperature and the second cooling temperature.

[0060] The closer the portion of the second cylinder 16b to which the magnetic shield 50 is coupled is to the second cooling section 35 in the axial direction, the more likely the heat input from the refrigerant gas environment to the magnetic shield 50 will affect the secondary refrigeration capacity of the cryogenic refrigerator 10. Therefore, the portion of the second cylinder 16b to which the magnetic shield 50 is coupled is preferably spaced a certain distance from the second cooling section 35 in the axial direction. From this point of view, the coupling position of the magnetic shield 50 and the second cylinder 16b can be selected to be a cooling temperature at which the magnetic shield 50 is cooled to about 8K or more. In other words, by coupling the magnetic shield 50 to a portion on the second cylinder 16b that is about 8K or more, the heat input to the magnetic shield 50 is absorbed by the second cylinder 16b, thereby effectively suppressing the influence on the secondary refrigeration capacity of the cryogenic refrigerator 10.

[0061] In order to facilitate cooling the magnetic shield 50 to an appropriate cooling temperature, for example, a temperature of 8K or more and 20K or less, the magnetic shield 50 may be further combined with the second cylinder 16b at the axial middle portion of the second cylinder 16b. The axial middle portion of the second cylinder 16b is, for example, in the range of 1 / 4 to 3 / 4 at a normalized axial position on the second cylinder 16b (i.e., a dimensionless axial position in which the positions of the first cooling section 33 and the second cooling section 35 are respectively set to 0 and 1). In many cases, the axial position on the second cylinder 16b that is cooled to 10K is located approximately in the middle of the second cylinder 16b (the normalized axial position is 1 / 2), and the axial position with an intermediate temperature of 8K to 20K may be included in the axial middle portion.

[0062] An exemplary shape of the magnetic shield 50 is described. The magnetic shield 50 extends along the second displacer 18b outside the second cylinder 16b in at least a portion of the reciprocating stroke of the magnetic cold storage material 28b. The magnetic shield 50 has a cylindrical shape with a larger diameter than the second cylinder 16b and surrounds the entire circumference of the second cylinder 16b.

[0063] The magnetic shield 50 includes a coupling portion 50a, a first shielding portion 50b, and a second shielding portion 50c. The coupling portion 50a is fixed to a portion of the second cylinder 16b that is away from the first cooling section 33 and the second cooling section 35 in the axial direction, thereby thermally connecting the magnetic shield 50 to the portion of the second cylinder 16b. The magnetic shield 50 extends up and down from the coupling portion 50a in the axial direction. That is, the first shielding portion 50b extends from the coupling portion 50a toward the first cooling section 33, and the second shielding portion 50c extends from the coupling portion 50a toward the second cooling section 35.

[0064] In order to improve the shielding performance of the magnetic shield 50 against the changing magnetic field, the magnetic shield 50 may extend along the second displacer 18b outside the second cylinder 16b within the entire range of the reciprocating stroke of the magnetic regenerator 28b. The upper end of the first shield portion 50b may extend to the same position as or above the upper surface of the magnetic regenerator 28b when the second displacer 18b is located at the top dead center. The lower end of the second shield portion 50c may extend to the same position as or below the lower surface of the magnetic regenerator 28b when the second displacer 18b is located at the bottom dead center.

[0065] The magnetic shield 50 is arranged in non-contact with the first cooling segment 33. Therefore, the upper end of the first shield portion 50b does not reach the first cooling segment 33. The magnetic shield 50 is separated from the first cooling segment 33, and direct heat transfer does not occur therebetween.

[0066] Similarly, the magnetic shield 50 is arranged in non-contact with the second cooling segment 35. Therefore, the lower end of the second housing portion 50c is not connected to the second cooling segment 35. The magnetic shield 50 is separated from the second cooling segment 35, and direct heat transfer does not occur therebetween.

[0067] The coupling portion 50a may be an inner flange formed at the lower end of the first shielding portion 50b (or the upper end of the second shielding portion 50c), and the inner flange may be mounted on the outer peripheral surface of the second cylinder 16b. The second cylinder 16b may also be provided with an outer flange, and the coupling portion 50a may also be mounted on the outer flange. The upper end of the second shielding portion 50c (or the lower end of the first shielding portion 50b) may be mounted on the coupling portion 50a, thereby assembling the magnetic shield 50.

[0068] Alternatively, the joint portion 50a may be a portion (eg, an outer flange) formed integrally with the second cylinder 16b. The magnetic shield 50 can be assembled by attaching the first shield portion 50b and the second shield portion 50c to the joint portion 50a.

[0069] The magnetic shield 50 (for example, the first shielding part 50b or the second shielding part 50c) may have a split structure. For example, the magnetic shield 50 may be formed into a cylindrical shape by combining a plurality of arc-shaped parts that are divided in the circumferential direction. For example, the magnetic shield 50 may be formed into a cylindrical shape by combining two semi-cylindrical parts of the same diameter. In this case, each part may be separately mounted on the second cylinder 16b.

[0070] As described above, according to the embodiment, in order to achieve good magnetic shielding performance, the magnetic shield 50 can be cooled to an ultra-low temperature of about 10 K. Compared with the case where the magnetic shield 50 is mounted on the second cooling stage 35 and cooled to the second cooling temperature as in the conventional design, the influence of heat input from the refrigerant gas environment to the magnetic shield 50 on the ability of the ultra-low temperature refrigerator 10 to recondense the refrigerant gas can be reduced. Therefore, it is possible to provide an ultra-low temperature refrigerator 10 suitable for use in a refrigerant gas environment.

[0071] 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.

[0072] In the above embodiment, the magnetic shield 50 extends upward and downward from the connection portion with the second cylinder 16 b , but the magnetic shield 50 may extend only upward or only downward from the connection portion with the second cylinder 16 b .

[0073] Figure 4 (a) and Figure 4 (b) is a diagram schematically showing a modified example of the magnetic shield 50 involved in the embodiment. The magnetic shield 50 is thermally connected to a portion of the second cylinder 16b that is axially away from the first cooling section 33 and the second cooling section 35, and is cooled to an intermediate temperature between the first cooling temperature and the second cooling temperature. The magnetic shield 50 extends along the second cylinder 16b on the outside of the second cylinder 16b within at least a portion of the reciprocating stroke of the magnetic cold storage material 28b.

[0074] like Figure 4 As shown in (a), the magnetic shield 50 may extend downward from the joint portion with the second cylinder 16b. In this case, the magnetic shield 50 may be joined to the second cylinder 16b near the first cooling section 33 in the axial direction (for example, above the above-mentioned axial middle portion).

[0075] like Figure 4 As shown in (b), the magnetic shield 50 may extend upward from the joint portion with the second cylinder 16b. In this case, the magnetic shield 50 may be joined to the second cylinder 16b near the second cooling section 35 in the axial direction (for example, below the above-mentioned axial middle portion).

[0076] 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.

[0077] Industrial Applicability

[0078] The present invention can be applied to the field of ultra-low temperature refrigerators.

Claims

1. A cryogenic refrigerator, characterized in that: have: A first cooling section, cooled to a first cooling temperature; A second cooling section, cooled to a second cooling temperature lower than the first cooling temperature; A cylinder body, connecting the first cooling section and the second cooling section; a displacer having a magnetic cold storage material and housed in the cylinder body so as to be reciprocable in the axial direction within the cylinder body; and A magnetic shield is thermally connected to a portion of the cylinder body that is away from the first cooling section and the second cooling section in the axial direction so as to be cooled to an intermediate temperature between the first cooling temperature and the second cooling temperature, and extends along the cylinder body on the outside of the cylinder body within at least a portion of the reciprocating stroke of the magnetic cold storage material.

2. The ultra-low temperature refrigerator according to claim 1, characterized in that: The magnetic shield is arranged in non-contact with the second cooling stage.

3. The ultra-low temperature refrigerator according to claim 1, characterized in that: The magnetic shield is arranged in non-contact with the first cooling stage.

4. The ultra-low temperature refrigerator according to claim 1, characterized in that: The magnetic shield is thermally connected to the axial middle portion of the cylinder body, The normalized axial position of the axial middle portion of the cylinder body on the cylinder body is within a range of 1 / 4 to 1 / 3.

5. The ultra-low temperature refrigerator according to claim 1, characterized in that: The intermediate temperature is below 20K.

6. The ultra-low temperature refrigerator according to claim 1, characterized in that: The intermediate temperature is below 10K.

7. The ultra-low temperature refrigerator according to claim 1, characterized in that: The intermediate temperature is above 8K.

8. The ultra-low temperature refrigerator according to claim 1, characterized in that: The magnetic shielding member is made of non-magnetic material.

9. The ultra-low temperature refrigerator according to any one of claims 1 to 8, characterized in that: The magnetic shield includes: a first shield portion extending from the portion of the cylinder toward the first cooling stage; and a second shield portion extending from the portion of the cylinder toward the second cooling stage.

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