CRYOCOOLER

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No. 2023-168492, filed on Sep. 28, 2023, which is incorporated by reference herein in its entirety.

BACKGROUND
Technical Field

Certain embodiments of the present invention relates to relates to a cryocooler.

Description of Related Art

In general, on a cryocooler that provides cooling to a cryogenic temperature of approximately 10K or less (for example, a liquid helium temperature of approximately 4.2K), a regenerator including a magnetic regenerator material having a great specific heat peak associated with a magnetic phase transition in this temperature range is mounted. The magnetic regenerator material is useful in improving the cooling capacity of the cryocooler at such a cryogenic temperature. There are two types of magnetic regenerator materials, and a metal-based magnetic regenerator material such as HoCu₂and a ceramic-based magnetic regenerator material such as Gd₂O₂S (also called GOS) are known (for example, the related art).

For example, in a cryocooler in which a displacer in which a regenerator material is built moves in an axial direction, such as a Gifford-McMahon (GM) cryocooler, the temperature of the regenerator material may slightly fluctuate with the axial movement of the displacer by being affected by an axial temperature distribution generated in the cryocooler itself. In many ceramic-based magnetic regenerator materials including GOS, specific heat is considerably sensitive to temperature, that is, a specific heat peak is extremely sharp. For this reason, a temperature fluctuation in the ceramic-based magnetic regenerator material may lead to, even if the fluctuation is slight, a significant fluctuation of the specific heat, for example, a substantial decrease. Accordingly, the cryogenic performance of the ceramic-based magnetic regenerator material and ultimately the performance of the cryocooler may be affected.

It is desirable to suppress a performance decrease of a cryocooler in which a ceramic-based magnetic regenerator material is built.

SUMMARY

According to an aspect of the present invention, there is provided a cryocooler including a cylinder that has a first thermal conductivity and that extends in an axial direction, a cooling stage that has a second thermal conductivity higher than the first thermal conductivity and that includes a stage end portion and a stage tubular portion connecting the stage end portion to the cylinder in the axial direction, a displacer that is capable of reciprocating in the axial direction in the cylinder, that forms an expansion space with the stage end portion, and in which the expansion space takes a maximum volume at a top dead center, and a ceramic-based magnetic regenerator material that is accommodated in the displacer and of which an axial position in the displacer is determined to overlap the stage tubular portion in the axial direction when the displacer is at the top dead center.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing a cryocooler according to an embodiment.

FIG. 2 is a view schematically showing the cryocooler according to the embodiment.

FIG. 3 is a graph showing a temperature change of specific heat of a representative magnetic regenerator material.

FIG. 4 is a view schematically showing a part of a cryocooler according to a comparative example.

FIGS. 5A and 5B are views schematically showing axial reciprocation of a displacer assembly according to the embodiment.

FIG. 6 is a view schematically showing an example of a displacer cap according to the embodiment.

DETAILED DESCRIPTION

Hereinafter, an embodiment for carrying out the present invention will be described in detail with reference to the drawings. In the description and drawings, the same or equivalent components, members, and processing will be assigned with the same reference symbols, and redundant description thereof will be omitted as appropriate. The scales and shapes of shown parts are set for convenience in order to make the description easy to understand and are not to be understood as limiting unless stated otherwise. The embodiment is merely an example and does not limit the scope of the present invention. All characteristics and combinations to be described in the embodiment are not necessarily essential to the invention.

FIGS. 1 and 2 are views schematically showing a cryocooler 10 according to the embodiment. The cryocooler 10 is, for example, a two-stage type Gifford-McMahon (GM) cryocooler. FIG. 1 shows an appearance of the cryocooler 10, and FIG. 2 shows an internal structure of the cryocooler 10.

The cryocooler 10 includes a compressor 12 and an expander 14. The compressor 12 is configured to collect a working gas of the cryocooler 10 from the expander 14, to pressurize the collected working gas, and to supply the working gas to the expander 14 again. The working gas is also called a refrigerant gas, and other suitable gases may be used although a helium gas is typically used.

The expander 14 includes a cryocooler cylinder 16, a displacer assembly 18, and a cryocooler housing 20. The cryocooler housing 20 is coupled to the cryocooler cylinder 16, thereby forming a hermetic container that accommodates the displacer assembly 18. The internal volume of the cryocooler housing 20 may be connected to a low pressure side of the compressor 12 and be maintained at a low pressure.

The cryocooler cylinder 16 includes a first cylinder 16a and a second cylinder 16b that extend in an axial direction. The first cylinder 16a and the second cylinder 16b each are, for example, a member that has a cylindrical shape, and the second cylinder 16b has a diameter smaller than the first cylinder 16a. The first cylinder 16a and the second cylinder 16b are coaxially disposed, and a lower end of the first cylinder 16a is strongly connected to an upper end of the second cylinder 16b.

As shown in FIG. 2, the displacer assembly 18 includes a first displacer 18a and a second displacer 18b. The first displacer 18a and the second displacer 18b each are, for example, a member that has a cylindrical shape, and the second displacer 18b has a diameter smaller than the first displacer 18a. The first displacer 18a and the second displacer 18b are coaxially disposed.

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 integrally.

In the present specification, in order to describe a positional relationship between components of the cryocooler 10, for convenience of description, a side close to a top dead center of axial reciprocation of a displacer will be referred to as “up” and a side close to a bottom dead center will be referred to as “down”. The top dead center is the position of the displacer at which the volume of an expansion space is maximized, and the bottom dead center is the position of the displacer at which the volume of the expansion space is minimized. Since a temperature gradient in which the temperature drops from an upper side to a lower side in the axial direction is generated during the operation of the cryocooler 10, the upper side can also be called a high temperature side and the lower side can also be called a low temperature side.

The first displacer 18a accommodates a first regenerator 26. A tubular main body of the first displacer 18a is a first regenerator container extending in the axial direction. The first regenerator 26 is formed by filling the main body of the first displacer 18a with, for example, a wire mesh made of, such as copper, or other appropriate first regenerator material. An upper lid portion and a lower lid portion of the first displacer 18a may be provided as members separate from the main body of the first displacer 18a, or the first regenerator material may be accommodated in the first displacer 18a by fixing the upper lid portion and the lower lid portion of the first displacer 18a to the main body through appropriate means such as fastening and welding.

Similarly, the second displacer 18b accommodates a second regenerator 28. A tubular main body of the second displacer 18b is a second regenerator container extending in the axial direction. An upper lid portion and a lower lid portion of the second displacer 18b may be provided as members separate from the main body of the second displacer 18b, or a second regenerator material may be accommodated in the second displacer 18b by fixing the lower lid portion and the upper lid portion of the second displacer 18b to the main body through appropriate means such as fastening and welding. Hereinafter, for convenience of description, the lower lid portion of the second displacer 18b will be called a displacer cap 46.

The second regenerator 28 includes at least two types of regenerator materials, in this example, three types of regenerator materials, and these regenerator materials are stacked in the axial direction of the second displacer 18b. As shown in FIG. 2, the second regenerator 28 includes a non-magnetic regenerator material 28a, a metal-based magnetic regenerator material 28b, and a ceramic-based magnetic regenerator material 28c in order from the high temperature side to the low temperature side in the axial direction. In order to increase the cooling capacity of a second stage of the cryocooler 10, materials having a great specific heat at different temperatures are selected as the regenerator materials.

The non-magnetic regenerator material 28a is disposed on the high temperature side of the second displacer 18b, that is, adjacent to the upper lid portion of the second displacer 18b in the axial direction in the second displacer 18b. The non-magnetic regenerator material 28a may be formed of a material having a relatively high specific heat by volume (for example, a specific heat by volume higher than that of copper) at a temperature on the high temperature side of the second displacer 18b, for example, a temperature range of 10K to 50K or at least a part thereof. As described above, the non-magnetic regenerator material 28a may have a higher specific heat by volume than copper, which is a regenerator material typically used in the first displacer 18a, in this temperature range. In addition, the non-magnetic regenerator material 28a may have a higher specific heat by volume than a magnetic regenerator material (to be described later) in this temperature range.

The non-magnetic regenerator material 28a may be formed of a non-magnetic metal material such as an alloy containing, for example, bismuth, zinc, lead, or at least one of these. Two types of materials may be stacked and used as the non-magnetic regenerator material 28a. For example, a regenerator material of zinc may be disposed adjacent to the upper lid portion of the second displacer 18b in the axial direction, and a regenerator material of bismuth may be disposed adjacent to the regenerator material of zinc in the axial direction.

The metal-based magnetic regenerator material 28b is disposed in an intermediate portion of the second displacer 18b, that is, adjacent to the non-magnetic regenerator material 28a in the axial direction in the second displacer 18b. The metal-based magnetic regenerator material 28b may be formed of a material having a relatively high specific heat by volume at a temperature of an axial intermediate portion of the second displacer 18b, for example, a temperature range of 5K to 10K or at least a part thereof. The metal-based magnetic regenerator material 28b typically has a peak of the specific heat by volume associated with a magnetic phase transition in this temperature range. The metal-based magnetic regenerator material 28b may be formed of a metal magnetic material, such as HoCu₂and Er₃Ni.

The ceramic-based magnetic regenerator material 28c is disposed in a low-temperature section of the second displacer 18b, that is, adjacent to the displacer cap 46 in the axial direction in the second displacer 18b. The ceramic-based magnetic regenerator material 28c is disposed between the metal-based magnetic regenerator material 28b and the displacer cap 46. The ceramic-based magnetic regenerator material 28c may be directly adjacent to the displacer cap 46. Alternatively, the ceramic-based magnetic regenerator material 28c and the displacer cap 46 may be adjacent to each other with a flow straightening layer interposed therebetween.

The ceramic-based magnetic regenerator material 28c may be formed of a magnetic regenerator material having a relatively high specific heat by volume at a temperature on the low temperature side of the second displacer 18b (that is, a low temperature end of the displacer in which the temperature is the lowest in the displacer assembly 18), for example, a temperature range of 1K to 5K or at least a part thereof. The ceramic-based magnetic regenerator material 28c typically has a peak of the specific heat by volume associated with a magnetic phase transition in this temperature range.

The ceramic-based magnetic regenerator material 28c may include at least one type of magnetic regenerator material represented by the general formula R_xO₂S or (R_1-yR′_y)_xO₂S (where R and R′ are at least one type of rare earth element, 0.1≤x≤9, 0<y<1), or Gd_xAl_2-xO₃(1≤x<2). The rare earth elements R and R′ may be, for example, yttrium Y, lanthanum La, cerium Ce, praseodymium Pr, neodymium Nd, promethium Pm, samarium Sm, europium Eu, gadolinium Gd, terbium Tb, dysprosium Dy, holmium Ho, erbium Er, thulium Tm, or ytterbium Yb.

Accordingly, the ceramic-based magnetic regenerator material 28c may be Gd₂O₂S (also called GOS), Tb₂O₂S (also called TBOS), (Gd_1-yTb_y)₂O₂S (0<y<1, also called GdTbOS), or GdAlO₃(also called GAP) or may contain at least one type of these.

As is known, the ceramic-based magnetic regenerator material 28c having such a composition has a higher (for example, several times higher) specific heat by volume in a temperature range of, for example, 1K to 5K or at least a part thereof, compared to a metal magnetic regenerator material such as HoCu₂. Accordingly, the ceramic-based magnetic regenerator material 28c is suitable for increasing the cooling capacity of the cryocooler 10 that provides cryogenic cooling in this temperature range.

In order to improve the mechanical strength of the magnetic regenerator material, an additive may be added to the magnetic regenerator material. For example, the additive may be at least one type of zirconium Zr, aluminum Al, and alumina (Al₂O₃). For example, the weight ratio of the additive with respect to the magnetic regenerator material may be 20% or less or 15% or less. In this manner, the hardness of the magnetic regenerator material is increased without significantly changing the specific heat by volume of the magnetic regenerator material (that is, without significantly affecting the cooling capacity of the cryocooler 10), and the risk of damage and powdering caused by impact can be reduced.

The non-magnetic regenerator material 28a, the metal-based magnetic regenerator material 28b, and the ceramic-based magnetic regenerator material 28c may be in a granular form. The granules forming such a granular regenerator material may be formed in a granular form having a size of, for example, 0.01 mm or more and 3 mm or less and may fill the second displacer 18b. For example, the particle size of the granules may be 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 ratio of the granules having a particle size of 0.14 mm or more and 1.6 mm or less may be 70% by weight or more with respect to the total granules. In a case where the particle size is less than 0.14 mm, the density at the time of filling the regenerator is extremely high, and the passage resistance of the working gas (for example, a helium gas) may be rapidly increased. In addition, in a case where the particle size exceeds 1.6 mm, there is a concern that heat exchange efficiency between the granules and the working gas may be significantly lowered.

In addition, the ratio (aspect ratio) of a maximum diameter with respect to a minimum diameter of the granules is, in any three-dimensional direction, 5 or less, preferably 3 or less, and more preferably 2 or less. Furthermore, achieving a shape as close to a spherical shape as possible is preferable. In a case where the aspect ratio exceeds 5, deformation and fracture are likely to occur mechanically, and filling at a high density is difficult, so that the cooling efficiency is lowered. Accordingly, the ratio of the granules in which the ratio of the major axis to the minor axis is 5 or less may be 70% by weight or more with respect to the total granules.

In order to protect the granules, a surface thereof may be coated with a coating having a thickness of, for example, 1 to 50 μm (for example, alumina (Al₂O₃), a fluororesin, or the like).

In order to separate the non-magnetic regenerator material 28a and the metal-based magnetic regenerator material 28b, a partition member such as a wire mesh may be disposed at a boundary 29a between the non-magnetic regenerator material 28a and the metal-based magnetic regenerator material 28b. In addition, if necessary, the partition member may be disposed at a boundary 29b between the metal-based magnetic regenerator material 28b and the ceramic-based magnetic regenerator material 28c.

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

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

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

The second cooling stage 35 includes a stage end portion 35a and a stage tubular portion 35b that connects the stage end portion 35a to the second cylinder 16b in the axial direction. The second cooling stage 35 may be provided as a single component including the stage end portion 35a and the stage tubular portion 35b. Alternatively, the stage end portion 35a and the stage tubular portion 35b may be provided as separate members and may be connected to each other to form the second cooling stage 35.

The second expansion chamber 34 is formed between the lower lid portion of the second displacer 18b, that is, the displacer cap 46 and the second cooling stage 35. More specifically, the displacer cap 46 faces the stage end portion 35a with the second expansion chamber 34 interposed therebetween, and the stage tubular portion 35b is fixed to a lower portion of the second cylinder 16b to surround the second expansion chamber 34.

The first regenerator 26 is connected to the upper chamber 30 through a working gas flow path 36a formed in the upper lid portion of the first displacer 18a and is connected to the first expansion chamber 32 through a working gas flow path 36b formed in the lower lid portion of the first displacer 18a. The second regenerator 28 is connected to the first regenerator 26 through a working gas flow path 36c formed from the lower lid portion of the first displacer 18a to the upper lid portion of the second displacer 18b. In addition, the second regenerator 28 is connected to the second expansion chamber 34 through a working gas flow path 36d formed in the displacer cap 46.

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 a lower end of the ceramic-based magnetic regenerator material 28c in the axial direction inside the displacer cap 46. The radial flow path 36d2 extends in a radial direction from a lower end of the axial flow path 36d1 inside the displacer cap 46. The radial flow path 36d2 is directed to the stage tubular portion 35b of the second cooling stage 35. In order to uniformly blow the working gas from the displacer cap 46 in a circumferential direction, a plurality of radial flow paths 36d2 may extend from the axial flow path 36d1. Accordingly, the working gas that has entered the axial flow path 36d1 from the ceramic-based magnetic regenerator material 28c is blown from the radial flow paths 36d2 toward the stage tubular portion 35b and flows into the second expansion chamber 34 through a radial clearance between the displacer cap 46 and the stage tubular portion 35b.

A working gas flow between the first expansion chamber 32, the second expansion chamber 34, and the upper chamber 30 is not a clearance between the cryocooler cylinder 16 and the displacer assembly 18, and a first seal 38a and a second seal 38b may be provided in order to guide the working gas flow to the first regenerator 26 and the second regenerator 28. The first seal 38a may be mounted on the upper lid portion of the first displacer 18a to be disposed between the first displacer 18a and the first cylinder 16a. The second seal 38b may be mounted on the upper lid portion of the second displacer 18b to be disposed between the second displacer 18b and the second cylinder 16b.

In addition, the expander 14 includes a pressure switching valve 40 and a driving motor 42. The pressure switching valve 40 is accommodated in the cryocooler housing 20, and the driving motor 42 is attached to the cryocooler housing 20.

As shown in FIG. 2, the pressure switching valve 40 is configured to include a high pressure valve 40a and a low pressure valve 40b and to generate periodic pressure fluctuations in the cryocooler cylinder 16. A working gas discharge port of the compressor 12 is connected to the upper chamber 30 via the high pressure valve 40a, and a working gas suction port of the compressor 12 is connected to the upper chamber 30 via the low pressure valve 40b. The high pressure valve 40a and the low pressure valve 40b are configured to open and close selectively and alternately (that is, such that when one is open, the other is closed). A 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 a low pressure (for example, 0.5 to 1.5 MPa) working gas is collected from the expander 14 to the compressor 12 through the low pressure valve 40b. To facilitate understanding, a direction in which the working gas flows is shown with arrows in FIG. 2.

The driving motor 42 is provided to drive reciprocation of the displacer assembly 18. The driving motor 42 is connected to a displacer drive shaft 44 via a motion conversion mechanism 43 such as a Scotch yoke mechanism. The motion conversion mechanism 43 is accommodated in the cryocooler housing 20 like the pressure switching valve 40. The displacer drive shaft 44 extends from the motion conversion mechanism 43 into the upper chamber 30 through the cryocooler housing 20 and is fixed to the upper lid portion of the first displacer 18a. A third seal 38c is provided in order to prevent leakage of the working gas from the upper chamber 30 to the cryocooler housing 20 (which is maintained at a low pressure in some cases as described above). The third seal 38c may be mounted on the cryocooler housing 20 to be disposed between the cryocooler housing 20 and the displacer drive shaft 44.

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

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

FIG. 3 is a graph showing a temperature change of specific heat of a representative magnetic regenerator material, and more specifically, is a graph showing a temperature change of specific heat of HoCu₂and GOS in a cryogenic temperature region. As shown in FIG. 3, HoCu₂and GOS have a peak at which the specific heat is maximized in a range in which a temperature is approximately 4K to approximately 10K, that is, a temperature range of a region of the second regenerator 28 in which HoCu₂or GOS is accommodated. For example, the specific heat of HoCu₂is maximized at two locations where the temperature is approximately 6K and approximately 9K. In addition, the specific heat of GOS has an extremely sharp peak at approximately 5K. Other types of ceramic-based magnetic regenerator materials also have a sharp peak at a specific temperature of approximately 5K or less, as in GOS.

FIG. 4 is a view schematically showing a part of a cryocooler according to a comparative example. FIG. 4 shows a lowest temperature section of the cryocooler, that is, a second cooling stage 135, a low temperature side of a second cylinder 116b extending upward in the axial direction from the second cooling stage 135, and a low temperature side of a second displacer 118b including a displacer cap 146. The second cooling stage 135 includes a terminal end portion 135a and a tubular portion 135b connecting the terminal end portion 135a to the second cylinder 116b in the axial direction. A magnetic regenerator material, for example, a ceramic-based magnetic regenerator material 128c is accommodated on the low temperature side of the second displacer 118b. A working gas flow path 136d including an axial flow path 136d1 and a radial flow path 136d2 is formed in the displacer cap 146. The ceramic-based magnetic regenerator material 128c and a second expansion chamber 134 are connected to each other through the working gas flow path 136d.

In the state shown in FIG. 4, the second displacer 118b is positioned at the top dead center. Accordingly, the displacer cap 146 is located at its uppermost position in the axial direction, and the second expansion chamber 134 takes a maximum volume. Typically, in existing design of the working gas flow path 136d of the displacer cap 146, as shown, when the second displacer 118b is positioned at the top dead center, an axial position of the radial flow path 136d2 of the working gas flow path 136d, which is an outlet of the working gas, is the same as an axial upper end of the tubular portion 135b of the second cooling stage 135 (or an axial lower end of the second cylinder 116b). In this manner, axial heights of the radial flow path 136d2 and an upper end portion of the tubular portion 135b of the second cooling stage 135 are aligned.

For this reason, the ceramic-based magnetic regenerator material 128c adjacent to an upper side of the displacer cap 146 in the axial direction is positioned above the tubular portion 135b of the second cooling stage 135 in the axial direction and is surrounded by the second cylinder 116b when the second displacer 118b is positioned at the top dead center. In this manner, at the top dead center of the second displacer 118b, the ceramic-based magnetic regenerator material 128c is positioned outside a space 148 surrounded by the second cooling stage 135, which is shown by a broken line in FIG. 4.

During the operation of the cryocooler, the displacer cap 146 and a lower end portion of the ceramic-based magnetic regenerator material 128c adjacent thereto are at the lowest temperature in the cryocooler, as in the second cooling stage 135. On the other hand, an axial temperature distribution in which the temperature becomes higher as going upward in the axial direction is formed in the second cylinder 116b. For this reason, a portion of the second cylinder 116b surrounding the ceramic-based magnetic regenerator material 128c has a temperature somewhat higher than that of the ceramic-based magnetic regenerator material 128c when the second displacer 118b is at the top dead center. Due to such an effect of a temperature difference between the ceramic-based magnetic regenerator material 128c and the second cylinder 116b surrounding the ceramic-based magnetic regenerator material 128c, a periodic temperature fluctuation may occur in the ceramic-based magnetic regenerator material 128c with axial reciprocation of the second displacer 118b.

As shown in FIG. 3, the specific heat of the ceramic-based magnetic regenerator material 128c, for example, GOS is extremely sensitive to temperature. A temperature fluctuation in the ceramic-based magnetic regenerator material 128c may cause a significant fluctuation of the specific heat, for example, a substantial decrease, even if the decrease is slight. Consequently, in the cryocooler of the comparative example, the cryogenic performance of the ceramic-based magnetic regenerator material 128c, and ultimately the performance of the cryocooler may be affected.

FIGS. 5A and 5B are views schematically showing axial reciprocation of the displacer assembly 18 according to the embodiment. FIG. 5A schematically shows the expander 14 when the displacer assembly 18 is positioned at the top dead center, and FIG. 5B schematically shows the expander 14 when the displacer assembly 18 is positioned at the bottom dead center. In FIG. 2, the displacer assembly 18 is at an intermediate position between the top dead center and the bottom dead center.

As described above, when the first displacer 18a and the second displacer 18b are at the top dead center, each of the first expansion chamber 32 and the second expansion chamber 34 has its maximum volume, and when the first displacer 18a and the second displacer 18b are at the bottom dead center, each of the first expansion chamber 32 and the second expansion chamber 34 has its minimum volume. An axial clearance between the first displacer 18a and the first cooling stage 33 when the first displacer 18a is at the bottom dead center is ideally zero. However, in actual design, for example, an axial clearance of approximately 0.1 mm to approximately 1 mm is set in order to avoid contact between the two. Similarly, an axial clearance between the second displacer 18b and the stage end portion 35a of the second cooling stage 35 when the second displacer 18b is at the bottom dead center may be set within a range of, for example, approximately 0.1 mm to approximately 1 mm.

On the contrary, the upper chamber 30 has a minimum volume when the first displacer 18a is at the top dead center and has a maximum volume when the first displacer 18a is at the bottom dead center. The axial height of the upper chamber 30 when the first displacer 18a is at the top dead center, that is, an axial clearance between the cryocooler housing 20 and the first displacer 18a is ideally zero and may be set, for example, within a range of approximately 0.1 mm to approximately 1 mm in practice. Accordingly, a dead volume in the upper chamber 30 can be minimized.

In the embodiment, as shown in FIG. 5A, the axial position of the ceramic-based magnetic regenerator material 28c in the second displacer 18b is determined to overlap the stage tubular portion 35b in the axial direction when the second displacer 18b is at the top dead center. Accordingly, when the second displacer 18b is positioned at the top dead center, at least a part of the ceramic-based magnetic regenerator material 28c, for example, an axially lower portion of the ceramic-based magnetic regenerator material 28c is positioned below an upper end 50 of the stage tubular portion 35b of the second cooling stage 35 in the axial direction and is surrounded by the stage tubular portion 35b. Similarly, the displacer cap 46 is also surrounded by the stage tubular portion 35b.

In this manner, at the top dead center of the second displacer 18b, a lower end portion of the second displacer 18b, that is, at least a part of the ceramic-based magnetic regenerator material 28c and the displacer cap 46 are positioned in a space 48 surrounded by the second cooling stage 35, which is shown by a broken line in FIG. 5A.

In addition, when the second displacer 18b is at the bottom dead center, as shown in FIG. 5B, the second displacer 18b further enters the space 48 surrounded by the second cooling stage 35. At the bottom dead center of the second displacer 18b, more portions of the ceramic-based magnetic regenerator material 28c are surrounded by the stage tubular portion 35b together with the displacer cap 46 and are positioned in the space 48.

Therefore, the axial position of the ceramic-based magnetic regenerator material 28c in the second displacer 18b when the second displacer 18b is at the top dead center is determined to overlap the stage tubular portion 35b in the axial direction. Consequently, at least a part of the ceramic-based magnetic regenerator material 28c is held in the space 48 surrounded by the second cooling stage 35 over an entire stroke length S of axial reciprocation of the second displacer 18b.

During the operation of the cryocooler, the displacer cap 46 and a lower end portion of the ceramic-based magnetic regenerator material 28c adjacent thereto are at the lowest temperature in the cryocooler, as in the second cooling stage 35. Therefore, unlike the comparative example shown in FIG. 4, in this embodiment, the ceramic-based magnetic regenerator material 28c can stabilize the temperature by being surrounded by the second cooling stage 35. Accordingly, an increase in an effect on the cryogenic performance depending on a temperature fluctuation that may occur in the ceramic-based magnetic regenerator material 28c and ultimately on the performance of the cryocooler 10 can be suppressed.

In addition, in the present embodiment, the axial position of the metal-based magnetic regenerator material 28b in the second displacer 18b is determined not to overlap the stage tubular portion 35b in the axial direction when the second displacer 18b is at the top dead center. Accordingly, when the second displacer 18b is positioned at the top dead center, as shown in FIG. 5A, the metal-based magnetic regenerator material 28b is positioned above the stage tubular portion 35b in the axial direction and is surrounded by the second cylinder 16b. In this manner, at the top dead center of the second displacer 18b, the metal-based magnetic regenerator material 28b is positioned outside the space 48 surrounded by the second cooling stage 35.

The cryocooler 10 is used under a strong magnetic field in some cases, such as an application of cooling a superconducting magnet. When the metal-based magnetic regenerator material 28b moves in a magnetic field distribution, an eddy current is induced inside the metal-based magnetic regenerator material 28b, and the eddy current may be converted into heat by a resistance of the metal-based magnetic regenerator material 28b itself. Therefore, the metal-based magnetic regenerator material 28b is disposed at a location separated away from the second cooling stage 35 in the axial direction. Consequently, a temperature rise of the second cooling stage 35 caused by heat generation of the metal-based magnetic regenerator material 28b can be suppressed.

In addition, the metal-based magnetic regenerator material 28b may have an axial position in the second displacer 18b which is determined not to overlap the stage tubular portion 35b in the axial direction when the second displacer 18b is at the bottom dead center. Accordingly, when the second displacer 18b is positioned at the bottom dead center, as shown in FIG. 5B, the metal-based magnetic regenerator material 28b may be positioned above the stage tubular portion 35b in the axial direction, may be surrounded by the second cylinder 16b, and may be positioned outside the space 48.

In order to realize this, the axial position of the ceramic-based magnetic regenerator material 28c in the second displacer 18b may be determined so that, when the stroke length of reciprocation of the second displacer 18b in the axial direction is denoted by S, the axial height of the displacer cap 46 is denoted by Hc, and an axial height from the displacer cap 46 to an upper end (that is, the boundary 29b) of the ceramic-based magnetic regenerator material 28c is denoted by Hs, Hs>S−Hc is satisfied.

In this manner, the metal-based magnetic regenerator material 28b is held outside the space 48 surrounded by the second cooling stage 35 over the entire stroke length S of axial reciprocation of the second displacer 18b. Therefore, a temperature rise of the second cooling stage 35 caused by heat generation of the metal-based magnetic regenerator material 28b can be suppressed.

The speed of axial reciprocation of the second displacer 18b is slow in the vicinity of the top dead center and the bottom dead center and is fast in the axial intermediate portion. The faster the movement speed of the second displacer 18b, the larger a temporal change of a magnetic field acting on the metal-based magnetic regenerator material 28b, and the larger the amount of heat generated by an eddy current. The axial intermediate portion is, for example, in a range of ¼ to ¾ of the stroke length S. Therefore, in order to dispose the metal-based magnetic regenerator material 28b to avoid this range, the axial position of the ceramic-based magnetic regenerator material 28c in the second displacer 18b may be determined to satisfy Hs>(¾)S−Hc.

The displacer cap 46 may be formed of a material having a high electrical resistivity compared to the second cooling stage 35. In this manner, when the second displacer 18b moves in a magnetic field distribution, an eddy current that may be induced inside the displacer cap 46 can be reduced, and heat generation from the displacer cap 46 caused by the eddy current can also be reduced.

The second cooling stage 35 is typically formed of copper as described above. Accordingly, the displacer cap 46 may be formed of a metal having a higher electrical resistivity than that of copper, for example, stainless steel, a nickel alloy such as nichrome and Inconel (registered trademark), and a titanium alloy. Alternatively, the displacer cap 46 may be formed of, for example, a synthetic resin material such as a phenol resin. Alternatively, the displacer cap 46 may be formed of, for example, a ceramic-based magnetic regenerator material such as GOS.

FIG. 6 is a view schematically showing an example of the displacer cap 46 according to the embodiment. The displacer cap 46 may include a main body 46a and a regenerator body 46b. As described above, the main body 46a may be formed of a material having a high electrical resistivity compared to the second cooling stage 35, such as stainless steel. The working gas flow path 36d may be formed in the main body 46a. The regenerator body 46b may be formed of a ceramic-based magnetic regenerator material, such as GOS. In this manner, as the displacer cap 46 is provided with the regenerator body 46b, a temperature rise of the displacer cap 46 can be suppressed and stabilized.

The regenerator body 46b may be exposed to the second expansion chamber 34. For example, the regenerator body 46b may be fixed to an outer surface of the displacer cap 46 to face the stage end portion 35a. The main body 46a may have a recess that receives the regenerator body 46b, and the regenerator body 46b may be bonded to the main body 46a using a cryogenic adhesive such as Nitofix (registered trademark). Alternatively, a screw may be formed on an outer peripheral surface of the regenerator body 46b, a screw hole, which corresponds to the main body 46a, may be formed, and the regenerator body 46b may be screwed and fixed into the screw hole of the main body 46a.

The present invention has been described hereinbefore based on the examples. It is clear for those skilled in the art that the present invention is not limited to the embodiment, various design changes are possible, various modification examples are possible, and such modification examples are also within the scope of the present invention. Various characteristics described in relation to one embodiment are also applicable to other embodiments. A new embodiment generated through combination also has effects of each of the combined embodiments.

In the embodiment described above, a GM cryocooler has been described as an example, but the present invention is not limited thereto. In one embodiment, the cryocooler 10 may be another type of cryocooler, such as a solvay cryocooler, in which a displacer in which a ceramic-based magnetic regenerator material is built reciprocates in the axial direction.

Although the present invention has been described using specific phrases based on the embodiment, the embodiment merely shows one aspect of the principles and applications of the present invention, and many modification examples and changes in disposition are allowed without departing from the concept of the present invention specified in the claims.

It should be understood that the invention is not limited to the above-described embodiment, but may be modified into various forms on the basis of the spirit of the invention. Additionally, the modifications are included in the scope of the invention.

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