Cryogenic refrigerator and buffer device

Abstract

A cryogenic refrigerator (10) comprises: a main compressor (12) that increases the pressure of a working gas; an expander (14) that generates cold by expansion of the working gas; a gas line (62) that connects the main compressor (12) and the expander (14) so as to circulate the working gas between the main compressor (12) and the expander (14); and a buffer line (68) that is configured to supply and discharge the working gas to the gas line (62). The buffer line (68) comprises: a sub-compressor (70) that increases the pressure of the working gas; and a buffer volume (72) that is connected between the discharge side of the sub-compressor (70) and the gas line (62), or between the gas line (62) and the suction side of the sub-compressor (70), and that stores the working gas.

Description

Cryogenic refrigerator and buffer device

[0001] The present invention relates to a cryogenic refrigerator and a buffer device.

[0002] Cryogenic refrigerators are used to cool a variety of objects, such as superconducting equipment, measuring instruments, and samples, that are used in cryogenic environments. To cool an object with a cryogenic refrigerator, the cryogenic refrigerator must first be started and cooled from an initial temperature, such as room temperature, to the desired cryogenic temperature. This initial cooling of a cryogenic refrigerator is also called cool-down.

[0003] International Publication No. 2022 / 230770

[0004] The initial cooling of a cryogenic refrigerator is merely a preparation for starting to cool an object. To minimize the increase in the time required for the initial cooling, it has been proposed to optimize the operation of the cryogenic refrigerator by replenishing the working gas from a buffer tank to the cryogenic refrigerator during the initial cooling. However, since the buffer tank has a certain size, it requires a considerable amount of space when installed together with the cryogenic refrigerator. The larger the cryogenic refrigerator, the larger the buffer tank. Furthermore, when multiple cryogenic refrigerators are installed side by side, multiple buffer tanks may be required accordingly. Depending on the space constraints at the installation site, it may not be possible to use a buffer tank together with the cryogenic refrigerator.

[0005] One exemplary objective of certain aspects of the present invention is to reduce the buffer volume used with a cryogenic refrigerator.

[0006] According to one aspect of the present invention, a cryogenic refrigerator includes a main compressor that pressurizes a working gas, an expander that generates refrigeration by expanding the working gas, a gas line that connects the main compressor and the expander so as to circulate the working gas between the main compressor and the expander, and a buffer line that is configured to supply and discharge the working gas to and from the gas line. The buffer line includes a sub-compressor that pressurizes the working gas, and a buffer volume that is connected between the discharge side of the sub-compressor and the gas line or between the gas line and the suction side of the sub-compressor and stores the working gas.

[0007] According to one aspect of the present invention, there is provided a buffer device connectable to a cryogenic refrigerator. The cryogenic refrigerator includes a main compressor that pressurizes a working gas, an expander that expands the working gas to generate refrigeration, and a gas line that connects the main compressor and the expander so as to circulate the working gas between them. The buffer device includes a buffer line that is configured to supply and discharge the working gas to and from the gas line. The buffer line includes a sub-compressor that pressurizes the working gas, and a buffer volume that is connected between the discharge side of the sub-compressor and the gas line or between the gas line and the suction side of the sub-compressor and stores the working gas.

[0008] Any combination of the above components or mutual substitution of the components or expressions of the present invention between methods, devices, systems, etc. are also valid aspects of the present invention.

[0009] According to the present invention, the buffer volume used in conjunction with the cryogenic refrigerator can be reduced.

[0010] FIG. 1 is a diagram schematically showing a cryogenic refrigerator according to an embodiment. FIG. 2 is a diagram schematically showing a cryogenic refrigerator according to an embodiment. FIG. 3 is a flowchart illustrating a control method for a cryogenic refrigerator according to an embodiment. FIG. 4 is a flowchart illustrating a control method for a cryogenic refrigerator according to an embodiment. FIG. 5 is a diagram schematically showing another example of a cryogenic refrigerator according to an embodiment. FIG. 6 is a diagram schematically showing another example of a cryogenic refrigerator according to an embodiment. FIG. 7 is a diagram schematically showing another example of a cryogenic refrigerator according to an embodiment. FIG. 8 is a diagram schematically showing another example of a cryogenic refrigerator according to an embodiment. FIG. 9 is a diagram schematically showing another example of a cryogenic refrigerator according to an embodiment.

[0011] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the description and drawings, identical or equivalent components, parts, and processes are designated by the same reference numerals, and redundant explanations will be omitted as appropriate. The scale and shape of each part shown in the drawings are set for convenience to facilitate explanation, and should not be interpreted as limiting unless otherwise specified. The embodiments are merely examples and do not limit the scope of the present invention in any way. All features and combinations thereof described in the embodiments are not necessarily essential to the invention.

[0012] 1 and 2 are diagrams schematically illustrating a cryogenic refrigerator 10 according to an embodiment. The cryogenic refrigerator 10 is, for example, a two-stage Gifford-McMahon (GM) refrigerator. Fig. 1 schematically illustrates a main compressor 12 and an expander 14 that constitute the cryogenic refrigerator 10, along with a control device 100. Fig. 2 illustrates the internal structure of the expander 14 of the cryogenic refrigerator 10.

[0013] The main 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 supply the working gas to the expander 14 again. The main compressor 12 and the expander 14 form the refrigeration cycle of the cryogenic refrigerator 10, thereby enabling the cryogenic refrigerator 10 to provide the desired cryogenic cooling. The expander 14 is also referred to as a cold head. The working gas, also referred to as a refrigerant gas, is typically helium gas, although other suitable gases may be used. For ease of understanding, the flow direction of the working gas is indicated by arrows in FIG. 1.

[0014] Generally, the pressure of the working gas supplied from the main compressor 12 to the expander 14 and the pressure of the working gas recovered from the expander 14 to the main compressor 12 are both significantly higher than atmospheric pressure and can be referred to as a first high pressure and a second high pressure, respectively. For ease of explanation, the first high pressure and the second high pressure are also simply referred to as a high pressure and a low pressure, respectively. Typically, the high pressure is, for example, 2 to 3 MPa. The low pressure is, for example, 0.5 to 1.5 MPa, e.g., approximately 0.8 MPa. For ease of understanding, the flow direction of the working gas is indicated by arrows.

[0015] The expander 14 includes a refrigerator cylinder 16 and a displacer assembly 18. The refrigerator cylinder 16 guides the linear reciprocating motion of the displacer assembly 18, and forms expansion chambers (32, 34) for the working gas between the refrigerator cylinder 16 and the displacer assembly 18. The expander 14 also includes a pressure switching valve 40 that determines the timing at which the working gas starts to be drawn into the expansion chamber and the timing at which the working gas starts to be exhausted from the expansion chamber.

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

[0017] The refrigerator cylinder 16 includes a first cylinder 16a and a second cylinder 16b. The first cylinder 16a and the second cylinder 16b are, for example, cylindrical members, and the second cylinder 16b has a smaller diameter than the first cylinder 16a. The first cylinder 16a and the second cylinder 16b are arranged coaxially, and the lower end of the first cylinder 16a is rigidly connected to the upper end of the second cylinder 16b.

[0018] The displacer assembly 18 includes a first displacer 18a and a second displacer 18b that are connected to each other and move together. The first displacer 18a and the second displacer 18b are, for example, cylindrical members, and the second displacer 18b has a smaller diameter than the first displacer 18a. The first displacer 18a and the second displacer 18b are arranged coaxially.

[0019] 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 is reciprocatingly movable in the axial direction along the first cylinder 16a, and the second displacer 18b is reciprocatingly movable in the axial direction along the second cylinder 16b.

[0020] 2, the first displacer 18a accommodates a first regenerator 26. The first regenerator 26 is formed by filling a cylindrical main body of the first displacer 18a with a first regenerator material, such as a wire mesh made of copper or other suitable material. The upper and lower covers of the first displacer 18a may be provided as separate members from the main body of the first displacer 18a, and the upper and lower covers of the first displacer 18a may be fixed to the main body by suitable means such as fastening or welding, thereby accommodating the first regenerator material in the first displacer 18a.

[0021] Similarly, the second displacer 18b accommodates a second regenerator 28. The second regenerator 28 is made of a non-magnetic regenerator material such as bismuth, HoCu, etc., housed in a cylindrical body of the second displacer 18b. ２ The second displacer 18b is formed by filling it with a magnetic regenerator material such as a refrigerant or other suitable second regenerator material. The second regenerator material may be formed in a granular form. The upper and lower lids of the second displacer 18b may be provided as separate members from the main body of the second displacer 18b, and the upper and lower lids of the second displacer 18b may be fixed to the main body by suitable means such as fastening or welding, thereby containing the second regenerator material in the second displacer 18b.

[0022] The displacer assembly 18 defines a room-temperature chamber 30, a first expansion chamber 32, and a second expansion chamber 34 inside the refrigerator cylinder 16. The expander 14 includes a first cooling stage 33 and a second cooling stage 35 for heat exchange with a desired object or medium to be cooled by the cryogenic refrigerator 10. The room-temperature chamber 30 is defined between the upper cover of the first displacer 18a and the top of the first cylinder 16a. The first expansion chamber 32 is defined between the lower cover of the first displacer 18a and the first cooling stage 33. The second expansion chamber 34 is defined between the lower cover of the second displacer 18b and the second cooling stage 35. The first cooling stage 33 is fixed to the lower part of the first cylinder 16a to surround the first expansion chamber 32, and the second cooling stage 35 is fixed to the lower part of the second cylinder 16b to surround the second expansion chamber 34.

[0023] The first regenerator 26 is connected to the room-temperature chamber 30 through a working gas passage 36a formed in the upper lid of the first displacer 18a, and is connected to the first expansion chamber 32 through a working gas passage 36b formed in the lower lid of the first displacer 18a. The second regenerator 28 is connected to the first regenerator 26 through a working gas passage 36c formed from the lower lid of the first displacer 18a to the upper lid of the second displacer 18b. The second regenerator 28 is also connected to the second expansion chamber 34 through a working gas passage 36d formed in the lower lid of the second displacer 18b.

[0024] A first seal 38a and a second seal 38b may be provided so that the flow of working gas between the first expansion chamber 32, the second expansion chamber 34 and the room temperature chamber 30 is directed to the first regenerator 26, the second regenerator 28, rather than through the clearance between the refrigerator cylinder 16 and the displacer assembly 18. The first seal 38a may be attached to an upper cover of the first displacer 18a so as to be disposed between the first displacer 18a and the first cylinder 16a. The second seal 38b may be attached to an upper cover of the second displacer 18b so as to be disposed between the second displacer 18b and the second cylinder 16b.

[0025] 1, the expander 14 includes a refrigerator housing 20 that houses a pressure switching valve 40. The refrigerator housing 20 is coupled to the refrigerator cylinder 16, thereby forming an airtight container that houses the pressure switching valve 40 and the displacer assembly 18.

[0026] 2, the pressure switching valve 40 includes a high-pressure valve 40a and a low-pressure valve 40b, and is configured to generate periodic pressure fluctuations in the refrigerator cylinder 16. The working gas discharge port of the main compressor 12 is connected to the room-temperature chamber 30 via the high-pressure valve 40a, and the working gas inlet port of the main compressor 12 is connected to the room-temperature 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 (i.e., when one is open, the other is closed).

[0027] The pressure switching valve 40 may take the form of a rotary valve. That is, the pressure switching valve 40 may be configured so that the high-pressure valve 40a and the low-pressure valve 40b are alternately opened and closed by the rotational sliding of a valve disc relative to a stationary valve body. In this case, the expander motor 42 may be connected to the pressure switching valve 40 so as to rotate the valve disc of the pressure switching valve 40. For example, the pressure switching valve 40 is arranged so that the valve rotation axis is coaxial with the rotation axis of the expander motor 42.

[0028] Alternatively, the high pressure valve 40 a and the low pressure valve 40 b may be valves that can be controlled individually, in which case the pressure switching valve 40 does not need to be connected to the expander motor 42 .

[0029] The expander motor 42 is connected to a displacer drive shaft 44 via a motion conversion mechanism 43, such as a Scotch yoke mechanism. The expander motor 42 is attached to the refrigerator housing 20. The motion conversion mechanism 43 is housed in the refrigerator housing 20, similar to the pressure switching valve 40. The motion conversion mechanism 43 converts the rotational motion output by the expander motor 42 into linear reciprocating motion of the displacer drive shaft 44. The displacer drive shaft 44 extends from the motion conversion mechanism 43 into the room-temperature chamber 30 and is fixed to the upper lid of the first displacer 18a. The rotation of the expander motor 42 is converted into axial reciprocating motion of the displacer drive shaft 44 by the motion conversion mechanism 43, and the displacer assembly 18 reciprocates linearly in the axial direction within the refrigerator cylinder 16.

[0030] The expander 14 may also include a temperature sensor 46 that measures the temperature of the second cooling stage 35 (and / or the first cooling stage 33) and outputs a measured temperature signal indicative of the measured temperature.

[0031] The main compressor 12 includes a high-pressure gas outlet 50, a low-pressure gas inlet 51, a high-pressure flow path 52, a low-pressure flow path 53, a first pressure sensor 54, a second pressure sensor 55, a bypass line 56, a compressor body 57, and a compressor housing 58. The high-pressure gas outlet 50 is provided in the compressor housing 58 as a working gas discharge port of the main compressor 12, and the low-pressure gas inlet 51 is provided in the compressor housing 58 as a working gas suction port of the main compressor 12. The high-pressure flow path 52 connects the discharge port of the compressor body 57 to the high-pressure gas outlet 50, and the low-pressure flow path 53 connects the low-pressure gas inlet 51 to the suction port of the compressor body 57. The compressor housing 58 houses the high-pressure flow path 52, the low-pressure flow path 53, the first pressure sensor 54, the second pressure sensor 55, the bypass line 56, and the compressor body 57. The main compressor 12 is also referred to as a compressor unit.

[0032] The compressor body 57 is configured to compress the working gas drawn in through its intake port and discharge the compressed gas from its discharge port. The compressor body 57 may be, for example, a scroll type, a rotary type, or any other type of pump that pressurizes the working gas. In this embodiment, the compressor body 57 is configured to discharge a fixed, constant flow rate of the working gas. Alternatively, the compressor body 57 may be configured to vary the flow rate of the working gas discharged. The compressor body 57 may also be referred to as a compression capsule.

[0033] The first pressure sensor 54 is disposed in the high-pressure flow path 52 to measure the pressure of the working gas flowing through the high-pressure flow path 52. The first pressure sensor 54 is configured to output a first measured pressure signal PH representing the measured pressure. The second pressure sensor 55 is disposed in the low-pressure flow path 53 to measure the pressure of the working gas flowing through the low-pressure flow path 53. The second pressure sensor 55 is configured to output a second measured pressure signal PL representing the measured pressure. Therefore, the first pressure sensor 54 and the second pressure sensor 55 can also be referred to as a high-pressure sensor and a low-pressure sensor, respectively. In this specification, the term "pressure sensor" may be used to refer to either the first pressure sensor 54 or the second pressure sensor 55, or to collectively refer to both.

[0034] Pressure sensors such as the first pressure sensor 54 and the second pressure sensor 55 do not necessarily need to be provided in the main compressor 12, and may be provided at any location where pressure can be measured, such as the gas line 62 or the expander 14. For example, the first pressure sensor 54 may be provided at any location in the high-pressure line 63, and the second pressure sensor 55 may be provided at any location in the low-pressure line 64.

[0035] The bypass line 56 connects the high-pressure line 52 to the low-pressure line 53 so as to return the working gas from the high-pressure line 52 to the low-pressure line 53, bypassing the expander 14. The bypass line 56 is provided with a relief valve 60 for opening and closing the bypass line 56 or for controlling the flow rate of the working gas flowing through the bypass line 56. The relief valve 60 is configured to open when a differential pressure equal to or greater than a set pressure acts between its inlet and outlet. The relief valve 60 may be an on / off valve or a flow control valve, such as a solenoid valve. The set pressure can be set appropriately based on the designer's empirical knowledge or experiments or simulations. This prevents the differential pressure between the high-pressure line 63 and the low-pressure line 64 from exceeding the set pressure and becoming excessive. Furthermore, excessive pressure in the high-pressure line 63 can be prevented.

[0036] The relief valve 60 may be configured to operate as a so-called safety valve, i.e., it may mechanically open when a differential pressure between the inlet and outlet exceeds a set pressure. Alternatively, the relief valve 60 may be opened and closed under the control of the control device 100. The control device 100 may compare the measured differential pressure between the high-pressure line 63 and the low-pressure line 64 with a set pressure and control the relief valve 60 to open when the measured differential pressure is equal to or greater than the set pressure and close when the measured differential pressure is less than the set pressure. The control device 100 may obtain the measured differential pressure between the high-pressure line 63 and the low-pressure line 64 based on a first measured pressure signal PH from the first pressure sensor 54 and a second measured pressure signal PL from the second pressure sensor 55. As another example, the control device 100 may compare the measured pressure in the high-pressure line 63 with an upper limit pressure based on the first measured pressure signal PH and control the relief valve 60 to open when the measured pressure is equal to or greater than the upper limit pressure and close when the measured pressure is less than the upper limit pressure.

[0037] The main compressor 12 may have various other components. For example, the high-pressure flow path 52 may be provided with an oil separator, an adsorber, etc. The low-pressure flow path 53 may be provided with a storage tank or other components. The main compressor 12 may also be provided with an oil circulation system that cools the compressor body 57 with oil, a cooling system that cools the oil, etc.

[0038] The cryogenic refrigerator 10 also includes a gas line 62 that circulates working gas between the main compressor 12 and the expander 14. The gas line 62 includes a high-pressure line 63 that connects the main compressor 12 to the expander 14 to supply working gas from the main compressor 12 to the expander 14, and a low-pressure line 64 that connects the main compressor 12 to the expander 14 to return working gas from the expander 14 to the main compressor 12. The refrigerator housing 20 of the expander 14 is provided with a high-pressure gas inlet 22 and a low-pressure gas outlet 24. The high-pressure gas inlet 22 is connected to the high-pressure gas outlet 50 by a high-pressure pipe 65, and the low-pressure gas outlet 24 is connected to the low-pressure gas inlet 51 by a low-pressure pipe 66. The high-pressure line 63 consists of the high-pressure pipe 65 and the high-pressure flow path 52, and the low-pressure line 64 consists of the low-pressure pipe 66 and the low-pressure flow path 53. The bypass line 56 may be considered to be part of the gas line 62. The bypass line 56 connects the high pressure line 63 to the low pressure line 64 so as to return the working gas from the high pressure line 63 to the low pressure line 64 , bypassing the expander 14 .

[0039] Therefore, the working gas recovered from the expander 14 to the main compressor 12 passes from the low-pressure gas outlet 24 of the expander 14 through the low-pressure piping 66 to enter the low-pressure gas inlet 51 of the main compressor 12, then passes through the low-pressure flow path 53 to return to the compressor main body 57, and is compressed and pressurized by the compressor main body 57. The working gas supplied from the main compressor 12 to the expander 14 passes from the compressor main body 57 through the high-pressure flow path 52 to exit the high-pressure gas outlet 50 of the main compressor 12, and then passes through the high-pressure piping 65 and the high-pressure gas inlet 22 of the expander 14 to be supplied to the expander 14.

[0040] Furthermore, the cryogenic refrigerator 10 includes a buffer line 68 configured to supply and discharge the working gas to and from the gas line 62. The buffer line 68 may be housed in a housing separate from the main compressor 12 and the expander 14, and may be connected to the cryogenic refrigerator 10 as a buffer device separate from the main compressor 12 and the expander 14.

[0041] The buffer line 68 includes a sub-compressor 70 that pressurizes the working gas, and a buffer volume 72 that stores the working gas and is connected between the discharge side of the sub-compressor 70 and the gas line 62. Similar to the compressor body 57 of the main compressor 12, the sub-compressor 70 is configured to compress the working gas drawn in through its intake port and discharge it from its discharge port. The buffer volume 72 may be, for example, the internal volume of a buffer tank 74.

[0042] The buffer line 68 branches off from the high-pressure line 63 and merges with the low-pressure line 64 so as to recover the working gas from the high-pressure line 63 and supply it to the low-pressure line 64. The buffer line 68 has an intake passage 68a branching off from the high-pressure line 63 and a discharge passage 68b that merges with the low-pressure line 64. The intake side of the sub-compressor 70 is connected to the high-pressure line 63 by the intake passage 68a, and the discharge side of the sub-compressor 70 is connected to the low-pressure line 64 by the discharge passage 68b. A buffer tank 74 is provided on the discharge passage 68b, and a buffer volume 72 is connected between the discharge side of the sub-compressor 70 and the low-pressure line 64.

[0043] The buffer line 68 is provided with a supply valve 76. The supply valve 76 connects the buffer volume 72 to the low pressure line 64. The supply valve 76 may be an on-off valve or a flow control valve, for example, a solenoid valve.

[0044] The buffer line 68 also includes a buffer pressure sensor 78 connected to the buffer volume 72 to measure the pressure in the buffer volume 72. The buffer pressure sensor 78 is electrically connected to the control device 100 and configured to output a measured buffer pressure signal PB to the control device 100, the measured buffer pressure signal PB representing the measured pressure.

[0045] Therefore, when the supply valve 76 is open during operation of the cryogenic refrigerator 10, the working gas is supplied from the buffer volume 72 to the low-pressure line 64 through the supply valve 76. When the supply valve 76 is closed, the supply of working gas from the buffer volume 72 to the low-pressure line 64 is stopped. Furthermore, when the sub-compressor 70 is operating, the working gas is drawn into the sub-compressor 70 from the high-pressure line 63, pressurized by the sub-compressor 70, and recovered in the buffer volume 72. When the sub-compressor 70 is stopped, the recovery of the working gas from the high-pressure line 63 to the buffer volume 72 is stopped. In this way, the amount of working gas circulating through the gas line 62 can be adjusted by the operation of the sub-compressor 70 and the supply valve 76, and as a result, the pressures of the high-pressure line 63 and the low-pressure line 64 can also be controlled.

[0046] 1, the control device 100 that controls the cryogenic refrigerator 10 includes a controller 110 that controls the buffer line 68. The controller 110 is electrically connected to the first pressure sensor 54 and the second pressure sensor 55 to acquire a first measured pressure signal PH and a second measured pressure signal PL. The controller 110 is electrically connected to the buffer pressure sensor 78 to acquire a measured buffer pressure signal PB. The controller 110 is also electrically connected to the temperature sensor 46 to acquire a measured temperature signal from the temperature sensor 46.

[0047] In the illustrated example, the control device 100 is provided separately from the main compressor 12 and the expander 14 and connected to them, but this is not limiting. The control device 100 may be mounted on the main compressor 12. The control device 100 may also be mounted on the expander 14, for example, by being mounted on the expander motor 42.

[0048] The control device 100 is realized as a hardware configuration by elements and circuits such as a computer CPU and memory, and as a software configuration by a computer program, etc., but is depicted as functional blocks realized by the cooperation of these elements in Figure 1. Those skilled in the art will understand that these functional blocks can be realized in various ways by combining hardware and software.

[0049] When the main compressor 12 and the expander motor 42 are operating, the cryogenic refrigerator 10 generates periodic volume fluctuations and synchronized pressure fluctuations of the working gas in the first expansion chamber 32 and the second expansion chamber 34. Typically, during the intake stroke, the low-pressure valve 40b closes and the high-pressure valve 40a opens, causing high-pressure working gas to flow from the main compressor 12 through the high-pressure valve 40a into the room-temperature chamber 30, be supplied to the first expansion chamber 32 through the first regenerator 26, and be supplied to the second expansion chamber 34 through the second regenerator 28. In this way, the pressures of the first expansion chamber 32 and the second expansion chamber 34 are increased from low to high. At this time, the displacer assembly 18 is moved upward from bottom dead center to top dead center, increasing the volumes of the first expansion chamber 32 and the second expansion chamber 34. The intake stroke ends when the high-pressure valve 40a closes.

[0050] During the exhaust stroke, the high-pressure valve 40a closes and the low-pressure valve 40b opens, thereby opening the high-pressure first and second expansion chambers 32, 34 to the working gas intake port of the main compressor 12. This causes the working gas to expand in the first and second expansion chambers 32, 34, and the resulting low-pressure working gas is discharged from the first and second expansion chambers 32, 34 through the first and second regenerators 26, 28 to the room-temperature chamber 30. At this time, the displacer assembly 18 is moved downward from top dead center to bottom dead center, reducing the volumes of the first and second expansion chambers 32, 34. The working gas is recovered from the expander 14 to the main compressor 12 through the low-pressure valve 40b. The exhaust stroke ends when the low-pressure valve 40b closes.

[0051] In this manner, a refrigeration cycle such as a GM cycle is configured, and the first cooling stage 33 and the second cooling stage 35 are cooled to a desired cryogenic temperature. The first cooling stage 33 can be cooled to a first cooling temperature, for example, in the range of about 20 K to about 40 K. The second cooling stage 35 can be cooled to a second cooling temperature (for example, about 1 K to about 4 K) that is lower than the first cooling temperature.

[0052] The cryogenic refrigerator 10 can perform initial cooling and steady-state operation following the initial cooling. Initial cooling is an operating mode of the expander 14 in which the expander 14 rapidly cools from an initial temperature to a cryogenic temperature upon startup of the cryogenic refrigerator 10. Steady-state operation is an operating mode of the expander 14 in which the expander 14 maintains the cryogenically cooled state achieved by the initial cooling. The initial temperature may be ambient temperature (e.g., room temperature). The expander 14 is cooled to a standard cooling temperature during the initial cooling, and during steady-state operation, the expander 14 is maintained within an allowable cryogenic temperature range that includes the standard cooling temperature. The standard cooling temperature varies depending on the application and settings of the cryogenic refrigerator 10, but is typically approximately 4.2 K or less for applications such as cooling superconducting devices. In other cooling applications, the standard cooling temperature may be, for example, approximately 10 K to 20 K, or even 10 K or less. As described above, the initial cooling can also be referred to as a cool-down.

[0053] Prior to operation (e.g., initial cooling) of the cryogenic refrigerator 10, a working gas recovery operation to the buffer line 68 may be performed. In this working gas recovery operation, the sub-compressor 70 is operated with the supply valve 76 closed. As a result, working gas is recovered from the gas line 62 through the sub-compressor 70 to the buffer volume 72, and the buffer volume 72 is pressurized. Note that the working gas recovery operation may be performed while the main compressor 12 and the expander 14 of the cryogenic refrigerator 10 are operating. Alternatively, the working gas recovery operation may be performed with the main compressor 12 and the expander 14 stopped.

[0054] The working gas recovery operation may be performed while monitoring the pressure of the buffer volume 72. In this case, the buffer pressure sensor 78 measures the pressure of the buffer volume 72 and outputs a measured buffer pressure signal PB representing the measured pressure of the buffer volume 72. The controller 110 receives the measured buffer pressure signal PB and acquires the measured pressure of the buffer volume 72. The measured pressure is compared with a target pressure value for the buffer volume 72. The controller 110 compares the measured pressure of the buffer volume 72 with the target pressure value for the buffer volume 72 and operates the sub-compressor 70 when the measured pressure of the buffer volume 72 is lower than the target value. The controller 110 stops the sub-compressor 70 when the measured pressure of the buffer volume 72 recovers to the target value, i.e., when the measured pressure of the buffer volume 72 is equal to or higher than the target value. In this way, working gas is supplied from the gas line 62 through the sub-compressor 70 to the buffer volume 72, thereby increasing the pressure of the buffer volume 72 to the target value.

[0055] Therefore, the sub-compressor 70 is configured to pressurize the working gas to a target pressure value for the buffer volume 72. The target pressure value for the buffer volume 72 may be, for example, a pressure higher than the average pressure of the working gas in the high-pressure line 63 and the low-pressure line 64. Preferably, the target pressure value for the buffer volume 72 may be a pressure higher than the pressure of the working gas in the high-pressure line 63. The target pressure value for the buffer volume 72 may be at least twice the appropriate pressure range for the high-pressure line 63 (described later), and may be, for example, 5 MPa or more. The target pressure value for the buffer volume 72 may be within three times the appropriate pressure range for the high-pressure line 63, and may be, for example, 7 MPa or less. The target pressure value for the buffer volume 72 can be set as appropriate based on the designer's empirical knowledge or on experiments, simulations, or the like.

[0056] In the existing configuration, the sub-compressor 70 is not provided in the buffer line 68, and therefore the pressure in the buffer volume 72 is the pressure at which the working gas is charged to the cryogenic refrigerator 10, in other words, the average pressure of the pressure in the high-pressure line 63 and the pressure in the low-pressure line 64. In contrast, according to the embodiment, the sub-compressor 70 is used to increase the pressure in the buffer volume 72 to a higher level. Therefore, more working gas can be stored in a smaller buffer volume. This means that the buffer volume 72 can be made smaller than in the existing configuration. Because the buffer volume 72 has a reasonable size, reducing the size of the buffer volume 72 helps reduce the installation space for the cryogenic refrigerator 10. This is particularly advantageous for saving space when the cryogenic refrigerator is large or when multiple cryogenic refrigerators are installed side by side.

[0057] During initial cooling, the density of the working gas increases in the expander 14 as the temperature drops from the initial temperature to a cryogenic temperature. Accordingly, the amount of working gas accumulated in the expander 14 increases, and the working gas is absorbed into the expander 14 from the gas line 62. As a result, as the cooling of the expander 14 progresses, the pressure of the working gas circulating through the gas line 62 gradually decreases. The decrease in working gas pressure reduces the refrigeration capacity of the cryogenic refrigerator 10, which may lengthen the time required for initial cooling. Because initial cooling is merely preparation for the cryogenic refrigerator to begin cooling an object, it is desirable for the required time to be as short as possible.

[0058] To address this problem, in this embodiment, the controller 110 controls the supply valve 76 during initial cooling to maintain the pressure in the high-pressure line 63 within a preset appropriate pressure range, based on the pressure in the high-pressure line 63 measured by the first pressure sensor 54. More specifically, during initial cooling, the controller 110 may compare the measured pressure in the high-pressure line 63 with a lower limit value Pc of the appropriate pressure range, and operate the supply valve 76 to repeatedly open and close the supply valve 76 so that the pressure in the high-pressure line 63 does not fall below the lower limit value Pc.

[0059] 3 is a flowchart illustrating a method for controlling the cryogenic refrigerator 10 according to the embodiment. This method is repeatedly executed by the controller 110 at a predetermined cycle during the initial cooling of the cryogenic refrigerator 10. Note that this method may be executed continuously not only during the initial cooling but also during steady operation of the cryogenic refrigerator 10.

[0060] First, the pressure in the high-pressure line 63 is measured (S10). The first pressure sensor 54 measures the pressure in the high-pressure line 63 and outputs a first measured pressure signal PH that represents the measured pressure in the high-pressure line 63. The controller 110 receives the first measured pressure signal PH and acquires the measured pressure in the high-pressure line.

[0061] Next, the measured pressure in the high-pressure line 63 is compared with an appropriate pressure range (S12). The lower limit Pc of the appropriate pressure range is set so that the cryogenic refrigerator 10 provides sufficient refrigeration capacity. The upper limit Pd of the appropriate pressure range is set so that excessive pressure is not generated in the high-pressure line 63. The upper limit Pd of the appropriate pressure range may be set to a pressure value lower than the above-mentioned set pressure at which the relief valve 60 opens. The appropriate pressure range can be set as appropriate based on the designer's empirical knowledge or experiments or simulations conducted by the designer. The appropriate pressure range may be stored in advance in the controller 110 as an initial setting for the cryogenic refrigerator 10, or may be set in the controller 110 by a user before operating the cryogenic refrigerator 10.

[0062] As an example, the upper limit Pd and lower limit Pc of the appropriate pressure range may be selected from a range of, for example, 2 MPa to 3 MPa, or a range of 2.1 MPa to 2.7 MPa. The width of the appropriate pressure range, i.e., the difference between the upper limit Pd and the lower limit Pc of the appropriate pressure range, may be set to a value within 0.5 MPa, 0.3 MPa, or 0.1 MPa. For example, the appropriate pressure range may be set to 2.45±0.05 MPa, in which case the width of the appropriate pressure range is 0.1 MPa, the upper limit Pd is 2.5 MPa, and the lower limit Pc is 2.4 MPa.

[0063] The controller 110 compares the measured pressure in the high-pressure line 63 with the lower limit Pc of the appropriate pressure range, and if the measured pressure in the high-pressure line 63 is below the lower limit Pc (PH<Pc), opens the supply valve 76 (S14). This causes the working gas to be supplied from the buffer volume 72 to the low-pressure line 64 through the supply valve 76. Since the amount of working gas circulating in the gas line 62 increases, the pressure in the high-pressure line 63 is restored.

[0064] When the measured pressure in the high-pressure line 63 returns to the appropriate pressure range, the controller 110 closes the supply valve 76 (S16). For example, the controller 110 may compare the measured pressure in the high-pressure line 63 with a lower limit value Pc of the appropriate pressure range, and close the supply valve 76 when the measured pressure in the high-pressure line 63 exceeds the lower limit value Pc (PH>Pc, or PH≧Pc). When the supply valve 76 is closed, the supply of working gas from the buffer volume 72 to the low-pressure line 64 is stopped. This ends the method, and the method is executed again in the next control cycle.

[0065] The pressure threshold value for closing the supply valve 76 may be different from the lower limit value Pc of the appropriate pressure range, and may be greater than the lower limit value Pc, for example. This pressure threshold value may be set so as not to exceed the upper limit value Pd of the appropriate pressure range. For example, the pressure threshold value may be a value obtained by adding a predetermined percentage of the width of the appropriate pressure range (upper limit value Pd - lower limit value Pc) to the lower limit value Pc. The predetermined percentage may be, for example, 50% or less, 30% or less, or 10% or less.

[0066] As described above, the temperature drop of the expander 14 during initial cooling increases the density of the working gas in the expander 14, which has the effect of lowering the pressure PH in the high-pressure line 63. Therefore, even if the pressure PH in the high-pressure line 63 recovers once, it may fall below the lower limit Pc again. In this case, the supply valve 76 opens again, the pressure in the high-pressure line 63 recovers, and the supply valve 76 closes. In this way, the supply valve 76 operates to repeatedly open and close so as to maintain the pressure PH in the high-pressure line 63 within the appropriate pressure range.

[0067] If the working gas were not supplied to the gas line 62 during the initial cooling, the pressure PH in the high-pressure line 63 would drop significantly due to a drop in the temperature of the expander 14. Since the refrigeration capacity of the cryogenic refrigerator 10 is correlated with the pressure PH in the high-pressure line 63, the refrigeration capacity of the cryogenic refrigerator 10 would decrease as the initial cooling progresses. This could be a factor that lengthens the time required for the initial cooling.

[0068] In contrast, according to the embodiment, the pressure PH in the high-pressure line 63 can be maintained within an appropriate pressure range by controlling the supply valve 76 during the initial cooling. Therefore, the refrigeration capacity of the cryogenic refrigerator 10 can be maintained appropriately, and an increase in the initial cooling time can be suppressed. Furthermore, by maintaining the pressure PH in the high-pressure line 63 approximately constant, the cryogenic refrigerator 10 can provide a stable refrigeration capacity.

[0069] In this embodiment, since the bypass line 56 and the relief valve 60 are provided, when the pressure PH in the high-pressure line 63 increases, the working gas can be released from the high-pressure line 63 to the low-pressure line 64 through the bypass line 56, thereby preventing excessive pressure rise. However, such a bypass flow reduces the flow rate of the working gas supplied from the main compressor 12 to the expander 14, which may result in a decrease in the refrigeration capacity of the cryogenic refrigerator 10. However, this embodiment is advantageous in that it does not need to rely on the bypass flow because the buffer volume 72 can be used to maintain the pressure PH in the high-pressure line 63 within an appropriate pressure range.

[0070] Furthermore, in this embodiment, during the initial cooling, the controller 110 may control the sub-compressor 70 so as to maintain the pressure in the high-pressure line 63 within an appropriate pressure range, based on the pressure in the high-pressure line 63 measured by the first pressure sensor 54. More specifically, during the initial cooling, the controller 110 may compare the measured pressure in the high-pressure line 63 with an upper limit value Pd of the appropriate pressure range, and operate or stop the sub-compressor 70 so that the pressure in the high-pressure line 63 does not exceed the upper limit value Pd.

[0071] Fig. 4 is a flowchart illustrating a method for controlling the cryogenic refrigerator 10 according to the embodiment. This method is repeatedly executed by the controller 110 at a predetermined cycle during the initial cooling of the cryogenic refrigerator 10. This method may be executed in parallel with the method shown in Fig. 3. Note that this method may be executed continuously not only during the initial cooling but also during steady operation of the cryogenic refrigerator 10.

[0072] First, the pressure in the high-pressure line 63 is measured using the first pressure sensor 54 (S20). The controller 110 receives the first measured pressure signal PH from the first pressure sensor 54 and acquires the measured pressure in the high-pressure line.

[0073] Next, the measured pressure in the high-pressure line 63 is compared with an appropriate pressure range (S22). The controller 110 compares the measured pressure in the high-pressure line 63 with an upper limit value Pd of the appropriate pressure range, and if the measured pressure in the high-pressure line 63 exceeds the upper limit value Pd (PH>Pd), operates the sub-compressor 70 (S24). As a result, working gas is collected from the high-pressure line 63 through the sub-compressor 70 into the buffer volume 72, and the pressure in the high-pressure line 63 decreases.

[0074] When the measured pressure in the high-pressure line 63 returns to the appropriate pressure range, the controller 110 stops the sub-compressor 70 (S26). For example, the controller 110 may compare the measured pressure in the high-pressure line 63 with an upper limit value Pd of the appropriate pressure range, and stop the sub-compressor 70 when the measured pressure in the high-pressure line 63 falls below the upper limit value Pd (PH<Pd, or PH≦Pd). This stops the recovery of the working gas from the high-pressure line 63 to the buffer volume 72. This ends the method, and the method is executed again in the next control cycle.

[0075] In this way, it is possible to avoid excessive pressure buildup in the high-pressure line 63. The main compressor 12 may be configured to stop for safety reasons when the pressure in the gas line 62 becomes too high, but it is possible to reduce the risk of such an emergency shutdown of the main compressor 12. In addition, the buffer volume 72 is pressurized by recovering the working gas, and this can be effectively used to supply working gas from the buffer volume 72 to the low-pressure line 64.

[0076] The pressure threshold value for stopping the sub-compressor 70 may be different from the upper limit value Pd of the appropriate pressure range, for example, may be smaller than the upper limit value Pd. This pressure threshold value may be selected from the appropriate pressure range, i.e., may be larger than the lower limit value Pc of the appropriate pressure range.

[0077] The appropriate pressure range may be changed during operation of the cryogenic refrigerator 10. For example, the appropriate pressure range for initial cooling may be different from the appropriate pressure range for steady operation, and may be higher than the appropriate pressure range for steady operation. For example, the lower limit value Pc for initial cooling may be higher than the lower limit value Pc for steady operation, and / or the upper limit value Pd for initial cooling may be higher than the upper limit value Pd for steady operation.

[0078] In this case, the switch from initial cooling to steady operation and the change of the appropriate pressure range may be controlled by the control device 100. For example, the control device 100 may compare the measured temperature of the second cooling stage 35 (and / or the first cooling stage 33) with the above-mentioned standard cooling temperature based on the measured temperature signal from the temperature sensor 46. Furthermore, the control device 100 may perform initial cooling when the measured temperature is higher than the standard cooling temperature, and may transition from initial cooling to steady operation when the measured temperature is equal to or lower than the standard cooling temperature. The controller 110 may change the appropriate pressure range in conjunction with the transition from initial cooling to steady operation.

[0079] Another possible method is to control the sub-compressor 70 and the supply valve 76 based on the pressure in the low-pressure line 64 in order to maintain the pressure PH in the high-pressure line 63 within the appropriate pressure range. The pressure in the low-pressure line 64 is affected by the cooling temperature of the expander 14 (it fluctuates depending on the cooling temperature). Therefore, in practice, it is necessary to set the appropriate pressure range for the low-pressure line 64 to different values ​​depending on the cooling temperature, which makes the control design complicated. Furthermore, even if the low-pressure line 64 is within the appropriate pressure range, the pressure in the high-pressure line 63 may become excessively high depending on the cooling temperature. Therefore, the method based on the pressure in the high-pressure line 63 as in the embodiment is advantageous in that it alleviates or prevents such problems.

[0080] However, if possible, the controller 110 may control the sub-compressor 70 and the supply valve 76 during the initial cooling period to maintain the pressure in the low-pressure line 64 within a predetermined appropriate pressure range, based on the pressure in the low-pressure line 64 measured by the second pressure sensor 55. Alternatively, the controller 110 may control the sub-compressor 70 and the supply valve 76 during the initial cooling period to maintain the differential pressure between the high-pressure line 63 and the low-pressure line 64 within a predetermined appropriate pressure range, based on the pressure in the high-pressure line 63 measured by the first pressure sensor 54 and the pressure in the low-pressure line 64 measured by the second pressure sensor 55.

[0081] 5 to 11 , several modified examples of the buffer line 68 will be described below. These modified examples also enable the buffer volume 72 to be pressurized to a high pressure using the sub-compressor 70, as in the above-described embodiment. Therefore, the buffer volume 72 can be made smaller. Furthermore, by replenishing and recovering the working gas from the buffer volume 72 to the gas line 62 during initial cooling, the operating pressure of the cryogenic refrigerator 10 can be optimized, and an increase in the time required for initial cooling can be suppressed.

[0082] 5 is a diagram schematically illustrating another example of a cryogenic refrigerator 10 according to an embodiment. Similar to the above-described embodiment, the cryogenic refrigerator 10 includes a main compressor 12, an expander 14, a gas line 62, and a buffer line 68. The gas line 62 includes a high-pressure line 63 and a low-pressure line 64.

[0083] The buffer line 68 includes a sub-compressor 70, a buffer volume 72, a supply valve 76, and a buffer pressure sensor 78. An intake flow path 68a of the buffer line 68 branches off from the high-pressure line 63 and is connected to the intake side of the sub-compressor 70. A discharge flow path 68b of the buffer line 68 connects the discharge side of the sub-compressor 70 to the low-pressure line 64. The buffer volume 72 may be, for example, the internal volume of a buffer tank 74. The buffer tank 74 and the supply valve 76 are provided on the discharge flow path 68b, and the buffer volume 72 is connected between the discharge side of the sub-compressor 70 and the supply valve 76.

[0084] 5, an accumulator 80 may be provided on the suction passage 68a. The accumulator 80 is a gas volume, such as a storage tank, and is connected to the suction side of the sub-compressor 70. The accumulator 80 stabilizes the working gas pressure in the suction passage 68a of the buffer line 68 and helps to suppress pulsations in the working gas pressure flowing from the high-pressure line 63 to the sub-compressor 70.

[0085] In addition to or instead of the accumulator 80, a recovery valve 82 may be provided on the suction passage 68a. As shown in FIG. 5 , the recovery valve 82 may connect the high-pressure line 63 to the accumulator 80. Alternatively, the recovery valve 82 may be connected between the accumulator 80 and the sub-compressor 70. The recovery valve 82 may be an on / off valve or a flow control valve, for example, a solenoid valve. When the recovery valve 82 is open, the working gas flows from the high-pressure line 63 through the recovery valve 82 into the buffer line 68. When the recovery valve 82 is closed, the flow of working gas from the high-pressure line 63 into the buffer line 68 is blocked.

[0086] The controller 110 (see FIG. 1 ) may open and close the recovery valve 82 in synchronization with the on / off of the sub-compressor 70. That is, the controller 110 may open the recovery valve 82 when the sub-compressor 70 is operated, and close the recovery valve 82 when the sub-compressor 70 is stopped.

[0087] 6 is a diagram schematically illustrating another example of a cryogenic refrigerator 10 according to an embodiment. Similar to the above-described embodiment, the cryogenic refrigerator 10 includes a main compressor 12, an expander 14, a gas line 62, and a buffer line 68. The gas line 62 includes a high-pressure line 63 and a low-pressure line 64.

[0088] The buffer line 68 includes a sub-compressor 70, a buffer volume 72, a supply valve 76, and a buffer pressure sensor 78. An intake flow path 68a of the buffer line 68 branches off from the high-pressure line 63 and is connected to the intake side of the sub-compressor 70. A discharge flow path 68b of the buffer line 68 connects the discharge side of the sub-compressor 70 to the low-pressure line 64. The buffer volume 72 may be, for example, the internal volume of a buffer tank 74. The buffer tank 74 and the supply valve 76 are provided on the discharge flow path 68b, and the buffer volume 72 is connected between the discharge side of the sub-compressor 70 and the supply valve 76.

[0089] 6, the buffer line 68 may be provided with an intermediate valve 84 in addition to the recovery valve 82. The recovery valve 82 is connected to the suction side of the sub-compressor 70, while the intermediate valve 84 is connected between the discharge side of the sub-compressor 70 and the buffer volume 72.

[0090] Both the recovery valve 82 and the intermediate valve 84 may be check valves that are provided in the buffer line 68 so as to allow the working gas to flow in a specified flow direction of the buffer line 68, i.e., in this example, from the high-pressure line 63 to the low-pressure line 64, and to block the working gas from flowing in the opposite direction.

[0091] Therefore, the recovery valve 82 opens when the pressure on the suction side of the sub-compressor 70 decreases due to operation of the sub-compressor 70, causing the pressure difference between the high-pressure line 63 and the suction side of the sub-compressor 70 to exceed the valve opening pressure of the recovery valve 82. The recovery valve 82 also closes when the pressure difference between the high-pressure line 63 and the suction side of the sub-compressor 70 falls below the valve opening pressure of the recovery valve 82, for example, when the sub-compressor 70 is stopped. The intermediate valve 84 opens when the pressure on the discharge side of the sub-compressor 70 increases due to operation of the sub-compressor 70, causing the pressure difference between the discharge side of the sub-compressor 70 and the buffer volume 72 to exceed the valve opening pressure of the intermediate valve 84. The intermediate valve 84 also closes when the pressure difference between the discharge side of the sub-compressor 70 and the buffer volume 72 falls below the valve opening pressure of the intermediate valve 84.

[0092] In the example shown in FIG. 6 , similarly to the example shown in FIG. 5 , the buffer line 68 may include an accumulator 80 connected to the suction side of the sub-compressor 70 .

[0093] 7 is a diagram schematically illustrating another example of a cryogenic refrigerator 10 according to an embodiment. Similar to the above-described embodiment, the cryogenic refrigerator 10 includes a main compressor 12, an expander 14, a gas line 62, and a buffer line 68. The gas line 62 includes a high-pressure line 63 and a low-pressure line 64.

[0094] The buffer line 68 includes a sub-compressor 70, a buffer volume 72, a supply valve 76, and a buffer pressure sensor 78. However, in the example shown in Fig. 7, the sub-compressor 70 and the buffer volume 72 are arranged in the opposite direction to the above example, and the buffer volume 72 is connected between the gas line 62 and the suction side of the sub-compressor 70.

[0095] An intake passage 68a of the buffer line 68 branches off from the high-pressure line 63 and is connected to the intake side of the sub-compressor 70. A buffer tank 74 and a recovery valve 82 are provided on the intake passage 68a, and the buffer volume 72 is connected between the recovery valve 82 and the intake side of the sub-compressor 70. A discharge passage 68b of the buffer line 68 connects the discharge side of the sub-compressor 70 to the low-pressure line 64.

[0096] Even in this case, the working gas stored in the buffer volume 72 can be pressurized by the sub-compressor 70 and replenished into the gas line 62. In addition, by opening the recovery valve 82, the working gas can be recovered into the buffer volume 72 from the high-pressure line 63.

[0097] The supply valve 76 may be provided on the discharge flow path 68b. The controller 110 may open and close the supply valve 76 in synchronization with the on / off of the sub-compressor 70. That is, the controller 110 may open the supply valve 76 when the sub-compressor 70 is operated and close the supply valve 76 when the sub-compressor 70 is stopped. Alternatively, the supply valve 76 may be a check valve, similar to the intermediate valve 84 shown in FIG. 6 .

[0098] 8 is a diagram schematically illustrating another example of a cryogenic refrigerator 10 according to an embodiment. Similar to the above-described embodiment, the cryogenic refrigerator 10 includes a main compressor 12, an expander 14, a gas line 62, and a buffer line 68. The gas line 62 includes a high-pressure line 63 and a low-pressure line 64.

[0099] The buffer line 68 includes a sub-compressor 70, a buffer volume 72, a supply valve 76, and a buffer pressure sensor 78. However, in the example shown in Figure 8, the buffer line 68 branches off from the high-pressure line 63 and rejoins the high-pressure line 63. A discharge flow path 68b of the buffer line 68 is connected to the high-pressure line 63, not the low-pressure line 64. The supply valve 76 connects the buffer volume 72 to the high-pressure line 63. The buffer line 68 can recover working gas from the high-pressure line 63 and supply it to the high-pressure line 63.

[0100] 9 is a diagram schematically illustrating another example of a cryogenic refrigerator 10 according to an embodiment. As in the above-described embodiment, the cryogenic refrigerator 10 includes a main compressor 12, an expander 14, a gas line 62, and a buffer line 68. The gas line 62 includes a high-pressure line 63 and a low-pressure line 64.

[0101] The buffer line 68 includes a sub-compressor 70, a buffer volume 72, a supply valve 76, and a buffer pressure sensor 78. The buffer volume 72 may branch off from the buffer line 68. As shown in FIG. 9 , the buffer volume 72 may be connected to the discharge flow path 68b via a supply valve 76. The supply valve 76 may be switchable between a recovery state, which allows working gas to flow from the sub-compressor 70 to the buffer volume 72, and a supply state, which allows working gas to flow from the buffer volume 72 to the gas line 62. The supply valve 76 may be, for example, a three-way valve. In this manner, the buffer volume 72 may be connected between the discharge side of the sub-compressor 70 and the gas line 62.

[0102] 8 and 9, similarly to the example shown in Fig. 5, the buffer line 68 may include an accumulator 80 and / or a recovery valve 82 connected to the suction side of the sub-compressor 70. Similar to the example shown in Fig. 6, the buffer line 68 may include an intermediate valve 84. Similar to the example shown in Fig. 7, the sub-compressor 70 and the buffer volume 72 may be arranged in reverse. Also, similarly to the example shown in Fig. 8, similarly to the example shown in Fig. 8, similarly to the example shown in Fig. 8, the buffer line 68 may branch off from the high-pressure line 63 and rejoin the high-pressure line 63.

[0103] 10 is a diagram schematically illustrating another example of a cryogenic refrigerator 10 according to an embodiment. As in the above-described embodiment, the cryogenic refrigerator 10 includes a main compressor 12, an expander 14, a gas line 62, and a buffer line 68. The gas line 62 includes a high-pressure line 63 and a low-pressure line 64.

[0104] The buffer line 68 includes a sub-compressor 70, a buffer volume 72, a supply valve 76, and a buffer pressure sensor 78. As shown in Figure 10, the buffer line 68 may branch off from the low-pressure line 64 and merge with the high-pressure line 63 so as to recover the working gas from the low-pressure line 64 and supply it to the high-pressure line 63.

[0105] An intake passage 68a of the buffer line 68 branches off from the low-pressure line 64 and is connected to the intake side of the sub-compressor 70. A discharge passage 68b of the buffer line 68 connects the discharge side of the sub-compressor 70 to the high-pressure line 63. The buffer volume 72 may be, for example, the internal volume of a buffer tank 74. The buffer tank 74 and a supply valve 76 are provided on the discharge passage 68b, and the buffer volume 72 is connected between the discharge side of the sub-compressor 70 and the supply valve 76.

[0106] Therefore, when the supply valve 76 is open during operation of the cryogenic refrigerator 10, the working gas is supplied from the buffer volume 72 to the high-pressure line 63 through the supply valve 76. When the supply valve 76 is closed, the supply of working gas from the buffer volume 72 to the high-pressure line 63 is stopped. Furthermore, when the sub-compressor 70 is operating, the working gas is drawn into the sub-compressor 70 from the low-pressure line 64, pressurized by the sub-compressor 70, and recovered in the buffer volume 72. When the sub-compressor 70 is stopped, the recovery of working gas from the low-pressure line 64 to the buffer volume 72 is stopped. In this way, the amount of working gas circulating through the gas line 62 can be adjusted by the operation of the sub-compressor 70 and the supply valve 76, and as a result, the pressures of the high-pressure line 63 and the low-pressure line 64 can also be controlled.

[0107] 10 , as in the above-described embodiment, the sub-compressor 70 can be used to boost the pressure in the buffer volume 72 to a high level (for example, a pressure higher than that of the high-pressure line 63). This allows the buffer volume 72 to be made smaller. Furthermore, by replenishing and recovering the working gas from the buffer volume 72 to the gas line 62 during initial cooling, the operating pressure of the cryogenic refrigerator 10 can be optimized and an increase in the time required for initial cooling can be suppressed.

[0108] 10 , the buffer line 68 may branch off from the low-pressure line 64 and rejoin the low-pressure line 64. The discharge flow path 68b of the buffer line 68 may be connected to the low-pressure line 64 instead of the high-pressure line 63. The supply valve 76 may connect the buffer volume 72 to the low-pressure line 64. In this way, the buffer line 68 may recover the working gas from the low-pressure line 64 and supply it to the low-pressure line 64.

[0109] 5 to 9 may also be applied to the example shown in Fig. 10. For example, the buffer line 68 may include an accumulator 80 and / or a recovery valve 82 connected to the suction side of the sub-compressor 70, as in the example shown in Fig. 5. The buffer line 68 may include an intermediate valve 84, as in the example shown in Fig. 6. The sub-compressor 70 and the buffer volume 72 may be arranged in reverse, as in the example shown in Fig. 7.

[0110] 11 is a diagram schematically illustrating another example of a cryogenic refrigerator 10 according to an embodiment. Similar to the above-described embodiment, the cryogenic refrigerator 10 includes a main compressor 12, an expander 14, a gas line 62, and a buffer line 68. The gas line 62 includes a high-pressure line 63 and a low-pressure line 64. The buffer line 68 includes a sub-compressor 70, a buffer volume 72, a supply valve 76, and a buffer pressure sensor 78.

[0111] 11 , the buffer line 68 includes a looped flow path 86 that includes the sub-compressor 70 and the buffer volume 72, and a connecting flow path 88 that connects the looped flow path 86 to the high-pressure line 63. The looped flow path 86 has an intake flow path 68a that connects the connecting flow path 88 to the intake side of the sub-compressor 70, and a discharge flow path 68b that connects the discharge side of the sub-compressor 70 to the connecting flow path 88.

[0112] A recovery valve 82 is provided in the suction flow path 68a. The discharge flow path 68b is provided with an intermediate valve 84, a buffer volume 72, and a supply valve 76. The buffer volume 72 may be, for example, the internal volume of the buffer tank 74. The intermediate valve 84 is connected between the discharge side of the sub-compressor 70 and the buffer volume 72, and the supply valve 76 is connected between the buffer volume 72 and the connecting flow path 88. The supply valve 76 may be an on / off valve or a flow control valve, for example, a solenoid valve. Both the recovery valve 82 and the intermediate valve 84 may be check valves. These check valves are provided in the loop flow path 86 to allow the working gas to flow in a specified flow direction in the loop flow path 86 (clockwise in the illustrated example) and to block the working gas from flowing in the opposite direction (counterclockwise in this example).

[0113] During operation of the cryogenic refrigerator 10, when the supply valve 76 is open, the working gas is supplied from the buffer volume 72 to the high-pressure line 63 through the supply valve 76. When the supply valve 76 is closed, the supply of working gas from the buffer volume 72 to the high-pressure line 63 is stopped. Furthermore, when the sub-compressor 70 is operating, the working gas is drawn into the sub-compressor 70 from the high-pressure line 63 through the recovery valve 82, is pressurized in the sub-compressor 70, and is recovered in the buffer volume 72 through the intermediate valve 84. When the sub-compressor 70 is stopped, the recovery of the working gas from the high-pressure line 63 to the buffer volume 72 is stopped. In this manner, the amount of working gas circulating through the gas line 62 can be adjusted.

[0114] 11 , similarly to the above-described embodiment, the sub-compressor 70 can be used to boost the pressure in the buffer volume 72 to a high level (for example, a pressure higher than that of the high-pressure line 63). This allows the buffer volume 72 to be made smaller. Furthermore, by replenishing and recovering the working gas from the buffer volume 72 to the gas line 62 during initial cooling, the operating pressure of the cryogenic refrigerator 10 can be optimized and an increase in the time required for initial cooling can be suppressed.

[0115] 1 to 10, the buffer line 68 is connected to the gas line 62 by two flow paths, i.e., an intake flow path 68a and a discharge flow path 68b, whereas in the example shown in Fig. 11, the buffer line 68 can be connected to the gas line 62 by a single connection flow path 88. This advantageously simplifies the piping connection between the cryogenic refrigerator 10 and the buffer device.

[0116] 11 , the connecting flow path 88 may be connected to the low-pressure line 64 instead of the high-pressure line 63. In this manner, the buffer line 68 may recover the working gas from the low-pressure line 64 and supply it to the low-pressure line 64.

[0117] The present invention has been described above based on examples. It will be understood by those skilled in the art that the present invention is not limited to the above-described embodiments, and that various design changes and modifications are possible, and that such modifications are also within the scope of the present invention. Various features described in relation to one embodiment can also be applied to other embodiments. A new embodiment created by combining embodiments will have the combined effects of the respective combined embodiments.

[0118] In the above-described embodiment, the buffer line 68 is arranged outside the main compressor 12 and the expander 14, but this is not limiting. For example, the buffer line 68 may be arranged inside the main compressor 12.

[0119] Although the above embodiment has been described with reference to an example in which the cryocooler 10 is a two-stage GM refrigerator, the cryocooler 10 is not limited to this. The cryocooler 10 may be a single-stage or multi-stage GM refrigerator, or may be any other type of cryocooler, such as a pulse tube refrigerator.

[0120] The present invention has been described above based on examples. It will be understood by those skilled in the art that the present invention is not limited to the above-described embodiments, and that various design changes and modifications are possible, and that such modifications are also within the scope of the present invention.

[0121] The present invention can be used in the fields of cryogenic refrigerators and buffer devices.

[0122] 10 cryogenic refrigerator, 12 main compressor, 14 expander, 62 gas line, 63 high-pressure line, 64 low-pressure line, 68 buffer line, 70 sub-compressor, 72 buffer volume, 86 loop-shaped flow path, 88 connecting flow path.

Claims

1. A cryogenic refrigerator comprising: a main compressor that pressurizes a working gas; an expander that generates refrigeration by expanding the working gas; a gas line connecting the main compressor and the expander so as to circulate the working gas between the main compressor and the expander; and a buffer line configured to supply and discharge the working gas to and from the gas line, the buffer line comprising: a sub-compressor that pressurizes the working gas; and a buffer line connected between the discharge side of the sub-compressor and the gas line, or between the gas line and the suction side of the sub-compressor, and having a buffer volume for storing the working gas.

2. The cryogenic refrigerator according to claim 1, wherein the buffer volume is connected between the discharge side of the sub-compressor and the gas line.

3. The cryogenic refrigerator according to claim 1 or 2, characterized in that the gas line comprises: a high-pressure line connecting the main compressor to the expander so as to supply the working gas from the main compressor to the expander; and a low-pressure line connecting the main compressor to the expander so as to recover the working gas from the expander to the main compressor; and the sub-compressor is configured to pressurize the buffer volume to a pressure higher than the average pressure of the working gas in the high-pressure line and the working gas in the low-pressure line.

4. The cryogenic refrigerator of claim 3, wherein the sub-compressor is configured to pressurize the buffer volume to a pressure higher than the pressure of the working gas in the high-pressure line.

5. The cryogenic refrigerator according to claim 4, wherein the suction side of the sub-compressor is connected to the high-pressure line.

6. The cryogenic refrigerator according to claim 5, wherein the buffer line includes an accumulator connected between the high-pressure line and the suction side of the sub-compressor.

7. The cryogenic refrigerator of claim 3, wherein the expander is capable of performing initial cooling to cool from an initial temperature to a cryogenic temperature and steady operation to maintain the cryogenic temperature following the initial cooling, and the cryogenic refrigerator further comprises: a first pressure sensor that measures the pressure of the high-pressure line; and a controller configured to control the buffer line based on the pressure of the high-pressure line measured by the first pressure sensor during the initial cooling so as to maintain the pressure of the high-pressure line within a predetermined appropriate pressure range.

8. The cryogenic refrigerator according to claim 3, wherein the buffer line branches off from the high-pressure line and merges with the low-pressure line so as to recover the working gas from the high-pressure line and supply it to the low-pressure line.

9. The cryogenic refrigerator according to claim 8, wherein the buffer line is provided with at least one check valve that allows the flow of working gas in a flow direction from the high-pressure line to the low-pressure line and blocks the flow of working gas in the opposite direction.

10. The cryogenic refrigerator according to claim 3, wherein the buffer line branches off from the low-pressure line and merges with the high-pressure line so as to recover the working gas from the low-pressure line and supply it to the high-pressure line.

11. The cryogenic refrigerator according to claim 3, characterized in that the buffer line comprises: a loop-shaped flow path including the sub-compressor and the buffer volume; and a connecting flow path that connects the loop-shaped flow path to either the high-pressure line or the low-pressure line.

12. A buffer device connectable to a cryogenic refrigerator, the cryogenic refrigerator comprising: a main compressor that pressurizes a working gas; an expander that generates refrigeration by expanding the working gas; and a gas line connecting the main compressor and the expander so as to circulate the working gas between the main compressor and the expander; the buffer device comprising: a buffer line configured to supply and discharge the working gas to and from the gas line, a sub-compressor that pressurizes the working gas, and a buffer line connected between the discharge side of the sub-compressor and the gas line, or between the gas line and the suction side of the sub-compressor, and having a buffer volume for storing the working gas.

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