Cryogenic refrigerator and method of operating a cryogenic refrigerator

Abstract

A cryogenic refrigerator (10) comprises: an expander (14) capable of initial cooling to cool from an initial temperature to a cryogenic temperature and steady operation to maintain the cryogenic temperature following the initial cooling; a high pressure line (63) that is connected to the expander (14) and through which the working gas to be sucked into the expander (14) flows; a low pressure line (64) that is connected to the expander (14) and through which the working gas exhausted from the expander (14) flows; a first pressure sensor (54) that measures the pressure in the high pressure line (63); a buffer volume (70) that stores the working gas; a supply valve (72) that connects the buffer volume (70) to the low pressure line (64); and a controller (110) that controls the supply valve (72) to keep the pressure in the high pressure line (63) within a preset proper pressure range on the basis of the pressure in the high pressure line (63) measured by the first pressure sensor (54) during initial cooling.

Description

Technical Field

[0001] The present invention relates to a cryogenic refrigerator and a method for operating a cryogenic refrigerator.

Background Art

[0002] Cryogenic refrigerators are used to cool various objects such as superconducting devices, measuring devices, and samples used in cryogenic environments. In order to cool an object with a cryogenic refrigerator, first, the cryogenic refrigerator must be started up, and the cryogenic refrigerator must be cooled from an initial temperature such as room temperature to the target cryogenic temperature. Such initial cooling of a cryogenic refrigerator is also referred to as cool-down.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In order to cool an object with a cryogenic refrigerator, first, the cryogenic refrigerator must be started up, and the cryogenic refrigerator must be cooled from an initial temperature such as room temperature to the target cryogenic temperature. Such initial cooling of a cryogenic refrigerator is also referred to as cool-down. Since the initial cooling is only a preparation for starting to cool the object, it is desirable that the required time be as short as possible.

[0005] One exemplary object of an aspect of the present invention is to shorten the initial cooling time of a cryogenic refrigerator.

Means for Solving the Problems

[0006] According to one aspect of the present invention, the cryogenic refrigerator includes an expander capable of performing initial cooling, which cools from an initial temperature to a cryogenic temperature, and steady-state operation, which maintains the cryogenic temperature following the initial cooling; a high-pressure line connected to the expander through which working gas drawn into the expander flows; a low-pressure line connected to the expander through which working gas exhausted from the expander flows; a pressure sensor for measuring the pressure in the high-pressure line; a buffer volume for storing working gas; a supply valve connecting the buffer volume to the low-pressure line; and a controller that controls the supply valve to maintain the pressure in the high-pressure line within a preset appropriate pressure range based on the pressure in the high-pressure line measured by the pressure sensor during initial cooling.

[0007] According to one aspect of the present invention, a method for operating a cryogenic refrigerator is provided. The cryogenic refrigerator comprises an expander, a high-pressure line connected to the expander through which working gas is drawn into the expander, a low-pressure line connected to the expander through which working gas is exhausted from the expander, a pressure sensor for measuring the pressure in the high-pressure line, a buffer volume for storing working gas, and a supply valve connecting the buffer volume to the low-pressure line. The method comprises performing initial cooling to cool the expander from an initial temperature to a cryogenic temperature, and performing steady-state operation following the initial cooling to maintain the expander at a cryogenic temperature. During the initial cooling, the supply valve is controlled to maintain the pressure in the high-pressure line within a preset appropriate pressure range based on the pressure in the high-pressure line measured by the pressure sensor.

[0008] Furthermore, any combination of the above components, or any substitution of components or expressions of the present invention between methods, apparatus, systems, etc., is also valid as an embodiment of the present invention. [Effects of the Invention]

[0009] According to the present invention, the initial cooling time of a cryogenic refrigerator can be shortened. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic diagram showing a cryogenic refrigerator according to an embodiment. [Figure 2] This is a schematic diagram showing a cryogenic refrigerator according to an embodiment. [Figure 3] This is a flowchart illustrating the control method for a cryogenic refrigerator according to an embodiment. [Figure 4] This is a flowchart illustrating the control method for a cryogenic refrigerator according to an embodiment. [Figure 5] This graph shows an example of temperature and pressure changes during operation of a cryogenic refrigerator according to an embodiment. [Figure 6] Figures 6(A) and 6(B) are graphs illustrating an example of pressure changes during operation of a cryogenic refrigerator, relating to an embodiment. [Figure 7] This graph shows an example of temperature and pressure changes during operation of a cryogenic refrigerator according to an embodiment. [Figure 8] This is a schematic diagram showing a cryogenic refrigerator according to an embodiment. [Modes for carrying out the invention]

[0011] The embodiments for carrying out the present invention will be described in detail below with reference to the drawings. In the description and drawings, identical or equivalent components, members, and processes are denoted by the same reference numerals, and redundant descriptions will be omitted as appropriate. The scale and shape of the illustrated parts are set for convenience to facilitate the explanation and are not to be interpreted restrictively unless otherwise specified. The embodiments are illustrative and do not limit the scope of the present invention in any way. Not all features or combinations thereof described in the embodiments are necessarily essential to the invention.

[0012] Figures 1 and 2 are schematic diagrams showing a cryogenic refrigerator 10 according to an embodiment. The cryogenic refrigerator 10 is, as an example, a two-stage Gifford-McMahon (GM) refrigerator. Figure 1 schematically shows the compressor 12 and expander 14 that constitute the cryogenic refrigerator 10 together with the control device 100, and Figure 2 shows the internal structure of the expander 14 of the cryogenic refrigerator 10.

[0013] The compressor 12 is configured to recover the working gas from the cryogenic refrigerator 10 from the expander 14, pressurize the recovered working gas, and supply it back to the expander 14. The compressor 12 and the expander 14 constitute 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 called the cold head. The working gas is also called the refrigerant gas, and is usually helium gas, but other suitable gases may be used. For understanding, the direction of flow of the working gas is indicated by arrows in Figure 1.

[0014] Generally, the pressure of the working gas supplied from the compressor 12 to the expander 14 and the pressure of the working gas recovered from the expander 14 to the compressor 12 are both considerably higher than atmospheric pressure and can be called the first high pressure and the second high pressure, respectively. For convenience of explanation, the first high pressure and the second high pressure are also simply called high pressure and 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, or approximately 0.8 MPa. For understanding, the direction of the working gas flow is indicated by arrows.

[0015] The expander 14 comprises 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 itself and the displacer assembly 18. The expander 14 also includes a pressure switching valve 40 that determines the timing for the start of intake of working gas into the expansion chamber and the timing for the start of exhaust of working gas from the expansion chamber.

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

[0017] The refrigerator cylinder 16 has a first cylinder 16a and a second cylinder 16b. The first cylinder 16a and the second cylinder 16b are, for example, members having a cylindrical shape, and the second cylinder 16b has a smaller diameter than the first cylinder 16a. The first cylinder 16a and the second cylinder 16b are coaxially arranged, 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 connected to each other, and they move integrally. The first displacer 18a and the second displacer 18b are, for example, members having a cylindrical shape, and the second displacer 18b has a smaller diameter than the first displacer 18a. The first displacer 18a and the second displacer 18b are coaxially arranged.

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

[0020] As shown in FIG. 2, the first displacer 18a houses the first regenerator 26. The first regenerator 26 is formed by filling, for example, a wire mesh made of copper or other appropriate first regenerative material into the cylindrical main body portion of the first displacer 18a. The upper lid portion and the lower lid portion of the first displacer 18a may be provided as separate members from the main body portion of the first displacer 18a, and the upper lid portion and the lower lid portion of the first displacer 18a are fixed to the main body by appropriate means such as fastening and welding, whereby the first regenerative material may be housed in the first displacer 18a.

[0021] Similarly, the second displacer 18b houses the second regenerator 28. The second regenerator 28 is formed by filling the cylindrical body of the second displacer 18b with a non-magnetic regenerator such as bismuth, a magnetic regenerator such as HoCu2, or other suitable second regenerator. The second regenerator may be formed into granules. The upper and lower lids of the second displacer 18b may be provided as separate components from the body of the second displacer 18b, and the lower lid of the upper lid of the second displacer 18b may be fixed to the body by suitable means such as fastening or welding, thereby housing the second regenerator in the second displacer 18b.

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

[0023] The first regenerator 26 is connected to the room temperature chamber 30 through an operating gas passage 36a formed in the upper lid of the first displacer 18a, and to the first expansion chamber 32 through an operating 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 an operating 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 an operating 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 to ensure that the working gas flow between the first expansion chamber 32, the second expansion chamber 34 and the room temperature chamber 30 is directed to the first regenerator 26 and the second regenerator 28, rather than to the clearance between the chiller cylinder 16 and the displacer assembly 18. The first seal 38a may be mounted on the top cover of the first displacer 18a so as to be positioned between the first displacer 18a and the first cylinder 16a. The second seal 38b may be mounted on the top cover of the second displacer 18b so as to be positioned between the second displacer 18b and the second cylinder 16b.

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

[0026] The pressure switching valve 40, as shown in Figure 2, comprises a high-pressure valve 40a and a low-pressure valve 40b, and is configured to generate periodic pressure fluctuations within the refrigeration cylinder 16. The working gas outlet of the compressor 12 is connected to the room temperature chamber 30 via the high-pressure valve 40a, and the working gas inlet of the 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 such that the high-pressure valve 40a and the low-pressure valve 40b are alternately opened and closed by the rotational sliding of the valve disc relative to the 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 may be positioned such that the valve rotation axis is coaxial with the rotation axis of the expander motor 42.

[0028] Alternatively, the high-pressure valve 40a and the low-pressure valve 40b may each be individually controllable valves, 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 the displacer drive shaft 44 via a motion conversion mechanism 43, such as a Scotch yoke mechanism. The expander motor 42 is mounted in the refrigerator housing 20. The motion conversion mechanism 43, like the pressure switching valve 40, is housed in the refrigerator housing 20. 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 top cover of the first displacer 18a. The rotation of the expander motor 42 is converted by the motion conversion mechanism 43 into axial reciprocating motion of the displacer drive shaft 44, causing the displacer assembly 18 to reciprocate linearly in the axial direction within the refrigerator cylinder 16.

[0030] The expander 14 may also be equipped with 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 indicating the measured temperature.

[0031] The compressor 12 comprises a high-pressure gas outlet 50, a low-pressure gas inlet 51, a high-pressure passage 52, a low-pressure passage 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 installed in the compressor housing 58 as the working gas discharge port of the compressor 12, and the low-pressure gas inlet 51 is installed in the compressor housing 58 as the working gas intake port of the compressor 12. The high-pressure passage 52 connects the discharge port of the compressor body 57 to the high-pressure gas outlet 50, and the low-pressure passage 53 connects the low-pressure gas inlet 51 to the intake port of the compressor body 57. The compressor housing 58 houses the high-pressure passage 52, the low-pressure passage 53, the first pressure sensor 54, the second pressure sensor 55, the bypass line 56, and the compressor body 57. The 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 inlet and discharge it from its outlet. The compressor body 57 may be, for example, a scroll type, a rotary type, or another pump that pressurizes the working gas. In this embodiment, the compressor body 57 is configured to discharge a fixed, constant flow rate of working gas. Alternatively, the compressor body 57 may be configured to make the discharged flow rate of working gas variable. The compressor body 57 is sometimes referred to as a compression capsule.

[0033] The first pressure sensor 54 is positioned in the high-pressure channel 52 to measure the pressure of the working gas flowing through the high-pressure channel 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 positioned in the low-pressure channel 53 to measure the pressure of the working gas flowing through the low-pressure channel 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 called the high-pressure sensor and the low-pressure sensor, respectively. In this document, the term "pressure sensor" may also be used to refer to either the first pressure sensor 54 or the second pressure sensor 55, or to refer to both collectively.

[0034] The bypass line 56 connects the high-pressure passage 52 to the low-pressure passage 53 so as to recirculate the working gas from the high-pressure passage 52 to the low-pressure passage 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 controlling the flow rate of the working gas flowing through the bypass line 56. The relief valve 60 is configured to open when a pressure difference exceeding a set pressure acts between its inlet and outlet. The relief valve 60 may be an on / off valve or a flow control valve, and may be a solenoid valve, for example. The set pressure can be set appropriately based on the designer's empirical knowledge or experiments and simulations conducted by the designer. This prevents the pressure difference between the high-pressure line 63 and the low-pressure line 64 from exceeding this set pressure. It also prevents the pressure in the high-pressure line 63 from becoming excessive.

[0035] The relief valve 60 may be configured to act as a so-called safety valve, that is, it may be mechanically opened when a differential pressure greater than or equal to a set pressure acts between the inlet and outlet. Alternatively, the relief valve 60 may be opened and closed by control of the control device 100. The control device 100 may compare the differential pressure between the high-pressure line 63 and the low-pressure line 64 to be measured with a set pressure and control the relief valve 60 to open if the measured differential pressure is greater than or equal to the set pressure, and to close the relief valve 60 if the measured differential pressure is less than the set differential pressure. The control device 100 may acquire 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 a first pressure sensor 54 and a second measured pressure signal PL from a second pressure sensor 55. As another example, the control device 100 may compare the measured pressure of 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 if the measured pressure is greater than or equal to the upper limit pressure, and to close the relief valve 60 if the measured pressure is less than the upper limit pressure.

[0036] Furthermore, the compressor 12 may have various other components. For example, the high-pressure passage 52 may be provided with an oil separator, an adsorber, etc. The low-pressure passage 53 may be provided with a storage tank or other components. In addition, the compressor 12 may be provided with an oil circulation system for cooling the compressor body 57 with oil, and a cooling system for cooling the oil.

[0037] The cryogenic refrigerator 10 also includes a gas line 62 for circulating working gas between the compressor 12 and the expander 14. The gas line 62 includes a high-pressure line 63 connecting the compressor 12 to the expander 14 to supply working gas from the compressor 12 to the expander 14, and a low-pressure line 64 connecting the compressor 12 to the expander 14 to recover working gas from the expander 14 to the 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 high-pressure piping 65, and the low-pressure gas outlet 24 is connected to the low-pressure gas inlet 51 by low-pressure piping 66. The high-pressure line 63 consists of high-pressure piping 65 and a high-pressure flow path 52, and the low-pressure line 64 consists of low-pressure piping 66 and a low-pressure flow path 53. The bypass line 56 may be considered 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 recirculate the working gas from the high-pressure line 63 to the low-pressure line 64, bypassing the expander 14.

[0038] Therefore, the working gas recovered from the expander 14 to the compressor 12 enters the low-pressure gas inlet 51 of the compressor 12 through the low-pressure piping 66 from the low-pressure gas outlet 24 of the expander 14, and then returns to the compressor body 57 via the low-pressure flow path 53, where it is compressed and pressurized by the compressor body 57. The working gas supplied from the compressor 12 to the expander 14 exits the high-pressure gas outlet 50 of the compressor 12 through the high-pressure flow path 52 from the compressor body 57, and is then supplied to the expander 14 via the high-pressure piping 65 and the high-pressure gas inlet 22 of the expander 14.

[0039] Furthermore, the cryogenic refrigerator 10 includes a buffer volume 70, a supply valve 72, and a recovery valve 74. The buffer volume 70 is a volume for storing working gas and may be, for example, a buffer tank. The supply valve 72 connects the buffer volume 70 to the low-pressure line 64, and the recovery valve 74 connects the buffer volume 70 to the high-pressure line 63. The supply valve 72 and the recovery valve 74 may be on / off valves or flow control valves, and may be, for example, solenoid valves.

[0040] The pressure in the buffer volume 70 is the pressure at which the working gas is supplied to the cryogenic refrigerator 10 when the cryogenic refrigerator 10 is stopped. When the cryogenic refrigerator 10 is in operation (for example, during initial cooling or steady-state operation), the pressure in the buffer volume 70 is an intermediate pressure between the pressure of the high-pressure line 63 and the pressure of the low-pressure line 64 (for example, the average pressure of the high and low pressures).

[0041] Therefore, during operation of the cryogenic refrigerator 10, when the supply valve 72 is open, working gas is supplied from the buffer volume 70 to the low-pressure line 64 through the supply valve 72. When the supply valve 72 is closed, the supply of working gas from the buffer volume 70 to the low-pressure line 64 is stopped. Also, when the recovery valve 74 is open, working gas is recovered from the high-pressure line 63 to the buffer volume 70 through the recovery valve 74. When the recovery valve 74 is closed, the recovery of working gas from the high-pressure line 63 to the buffer volume 70 is stopped. In this way, by opening and closing the supply valve 72 and the recovery valve 74, the amount of working gas circulating in the gas line 62 can be adjusted, and as a result, the pressures of the high-pressure line 63 and the low-pressure line 64 can also be controlled.

[0042] As shown in Figure 1, the control device 100 that controls the cryogenic refrigerator 10 includes a controller 110 that controls the supply valve 72 and the recovery valve 74. 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. As will be described later, the controller 110 receives the first measured pressure signal PH from the first pressure sensor 54 and is configured to open and close the supply valve 72 and the recovery valve 74 based on the measured pressure of the high-pressure line 63 indicated by the first measured pressure signal PH. The controller 110 is also electrically connected to the temperature sensor 46 to acquire a measured temperature signal from the temperature sensor 46.

[0043] In the illustrated example, the control device 100 is provided separately from and connected to the compressor 12 and the expander 14, but this is not limited to the case. The control device 100 may be mounted on the compressor 12. The control device 100 may be provided on the expander 14, such as being mounted on the expander motor 42. The controller 110 may be provided on the supply valve 72, or on the recovery valve 74, or on the supply valve 72 and the recovery valve 74 respectively.

[0044] The control device 100 is implemented in hardware form by components and circuits such as the CPU and memory of a computer, and in software form by computer programs, etc., but in Figure 1, it is depicted as a functional block realized through the coordination of these components as appropriate. Those skilled in the art will understand that these functional blocks can be realized in various forms by combinations of hardware and software.

[0045] When the compressor 12 and expander motor 42 of the cryogenic refrigerator 10 are in operation, periodic volume fluctuations and synchronized pressure fluctuations of the working gas occur in the first expansion chamber 32 and the second expansion chamber 34. Typically, during the intake process, the low-pressure valve 40b closes and the high-pressure valve 40a opens, causing high-pressure working gas to flow from the compressor 12 into the room-temperature chamber 30 through the high-pressure valve 40a, and is supplied to the first expansion chamber 32 through the first regenerator 26 and to the second expansion chamber 34 through the second regenerator 28. In this way, the first expansion chamber 32 and the second expansion chamber 34 are pressurized from low pressure to high pressure. 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 process ends when the high-pressure valve 40a closes.

[0046] In the exhaust process, the high-pressure valve 40a closes and the low-pressure valve 40b opens, opening the high-pressure first expansion chamber 32 and second expansion chamber 34 to the low-pressure working gas inlet of the compressor 12. As a result, the working gas expands in the first and second expansion chambers 32 and 34, and the resulting low-pressure working gas is discharged from the first and second expansion chambers 32 and 34 through the first and second regenerators 26 and 28 to the room temperature chamber 30. At this time, the displacer assembly 18 moves downward from top dead center to bottom dead center, reducing the volume of the first and second expansion chambers 32 and 34. The working gas is recovered from the expander 14 to the compressor 12 through the low-pressure valve 40b. The exhaust process ends when the low-pressure valve 40b closes.

[0047] In this way, 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 20K to about 40K. The second cooling stage 35 can be cooled to a second cooling temperature lower than the first cooling temperature (for example, about 1K to about 4K).

[0048] The cryogenic refrigerator 10 is capable of initial cooling and steady-state operation following initial cooling. Initial cooling is the operating mode of the expander 14, which rapidly cools the cryogenic refrigerator 10 from an initial temperature to a cryogenic temperature when the cryogenic refrigerator 10 is started up, while steady-state operation is the operating mode of the expander 14, which maintains the cryogenic temperature achieved by initial cooling. The initial temperature may be the ambient temperature (e.g., room temperature). The expander 14 is cooled to a standard cooling temperature by initial cooling and maintained within an allowable cryogenic temperature range, including this standard cooling temperature, during steady-state operation. The standard cooling temperature varies depending on the application and settings of the cryogenic refrigerator 10, but is typically about 4.2K or less for cooling applications such as superconducting devices. For other cooling applications, the standard cooling temperature may be, for example, about 10K to 20K, or even 10K or less. Initial cooling can also be called cool-down, as described above.

[0049] Incidentally, during initial cooling, as the temperature drops from the initial temperature to the cryogenic temperature, the density of the working gas in the expander 14 increases. Consequently, the amount of working gas accumulating in the expander 14 increases, and in a sense, the working gas is absorbed from the gas line 62 into the expander 14. As a result, as the cooling of the expander 14 progresses, the pressure of the working gas circulating in the gas line 62 gradually decreases. This decrease in working gas pressure leads to a decrease in the cooling capacity of the cryogenic refrigerator 10, which is a concern as it may increase the time required for initial cooling. Since initial cooling is merely preparation for the cryogenic refrigerator to begin cooling the object, it is desirable that the time required for it be as short as possible.

[0050] To address these issues, in this embodiment, the controller 110 controls the supply valve 72 to maintain the pressure of the high-pressure line 63 within a preset appropriate pressure range, based on the pressure of the high-pressure line 63 measured by the first pressure sensor 54 during initial cooling. More specifically, the controller 110 may compare the measured pressure of the high-pressure line 63 with a lower limit Pc of the appropriate pressure range during initial cooling and operate the supply valve 72 to repeatedly open and close the supply valve 72 so that the pressure of the high-pressure line 63 does not fall below the lower limit Pc.

[0051] Furthermore, in this embodiment, the controller 110 controls the recovery valve 74 to maintain the pressure of the high-pressure line 63 within an appropriate pressure range based on the pressure of the high-pressure line 63 measured by the first pressure sensor 54 during initial cooling. More specifically, the controller 110 may compare the measured pressure of the high-pressure line 63 with the upper limit Pd of the appropriate pressure range during initial cooling and operate the recovery valve 74 to repeatedly open and close so that the pressure of the high-pressure line 63 does not exceed the upper limit Pd.

[0052] Figure 3 is a flowchart illustrating the control method for the cryogenic refrigerator 10 according to the embodiment. This method is repeatedly executed at a predetermined cycle by the controller 110 during the initial cooling of the cryogenic refrigerator 10. This method may be continuously executed not only during the initial cooling but also during the steady-state operation of the cryogenic refrigerator 10.

[0053] First, the pressure of the high-pressure line 63 is measured (S10). The first pressure sensor 54 measures the pressure of the high-pressure line 63 and outputs a first measured pressure signal PH representing the measured pressure of the high-pressure line 63. The controller 110 receives the first measured pressure signal PH and obtains the measured pressure of the high-pressure line.

[0054] Next, the measured pressure of the high-pressure line 63 is compared with the appropriate pressure range (S12). The lower limit Pc of the appropriate pressure range is set so that the cryogenic refrigerator 10 provides sufficient cooling capacity. The upper limit Pd of the appropriate pressure range is set so as not to generate excessive pressure in the high-pressure line 63. The upper limit Pd of the appropriate pressure range may be set to a pressure value smaller than the aforementioned 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 and simulations conducted by the designer. The appropriate pressure range may be pre-saved in the controller 110 as the initial setting of the cryogenic refrigerator 10, or it may be set in the controller 110 by the user before operation of the cryogenic refrigerator 10.

[0055] For example, the upper limit Pd and lower limit Pc of the appropriate pressure range may be selected from, for example, a range of 2 MPa to 3 MPa or a range of 2.1 MPa to 2.7 MPa. The width of the appropriate pressure range, that is, the difference between the upper limit Pd and the lower limit Pc of the appropriate pressure range, may be set to a certain value, for example, within 0.5 MPa, within 0.3 MPa, or within 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.

[0056] The controller 110 compares the measured pressure of the high-pressure line 63 with the lower limit value Pc of the appropriate pressure range, and when the measured pressure of the high-pressure line 63 is lower than the lower limit value Pc (PH<Pc), it opens the supply valve 72 (S14). As a result, the working gas is supplied from the buffer volume 70 to the low-pressure line 64 through the supply valve 72. Since the amount of the working gas circulating in the gas line 62 increases, the pressure in the high-pressure line 63 is restored.

[0057] When the measured pressure of the high-pressure line 63 is restored to the appropriate pressure range, the controller 110 closes the supply valve 72 (S16). For example, the controller 110 may compare the measured pressure of the high-pressure line 63 with the lower limit value Pc of the appropriate pressure range, and when the measured pressure of the high-pressure line 63 is higher than the lower limit value Pc (PH>Pc, or Ph≧Pc), it may close the supply valve 72. When the supply valve 72 is closed, the supply of the working gas from the buffer volume 70 to the low-pressure line 64 is stopped. Thus, this method ends and is executed again in the next control cycle.

[0058] Note that the pressure threshold for closing the supply valve 72 may be different from the lower limit value Pc of the appropriate pressure range. For example, it may be larger than the lower limit value Pc. This pressure threshold may be set so as not to exceed the upper limit value Pd of the appropriate pressure range. For example, the pressure threshold may be a value obtained by adding a predetermined ratio of the width of the appropriate pressure range (upper limit value Pd - lower limit value Pc) to the lower limit value Pc. The predetermined ratio may be, for example, 50% or less, 30% or less, or 10% or less.

[0059] FIG. 4 is a flowchart for explaining the control method of 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 continuously executed not only during the initial cooling but also during the steady operation of the cryogenic refrigerator 10.

[0060] First, the pressure of 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 of the high-pressure line.

[0061] Next, the measured pressure of the high-pressure line 63 is compared with the appropriate pressure range (S22). The controller 110 compares the measured pressure of the high-pressure line 63 with the upper limit value Pd of the appropriate pressure range. When the measured pressure of the high-pressure line 63 exceeds the upper limit value Pd (PH > Pd), the recovery valve 74 is opened (S24). Thereby, the working gas is recovered from the high-pressure line 63 through the recovery valve 74 into the buffer volume 70, and the pressure of the high-pressure line 63 decreases.

[0062] The controller 110 closes the recovery valve 74 when the measured pressure of the high-pressure line 63 returns to the appropriate pressure range (S26). For example, the controller 110 may compare the measured pressure of the high-pressure line 63 with the upper limit value Pd of the appropriate pressure range and close the recovery valve 74 when the measured pressure of the high-pressure line 63 is below the upper limit value Pd (PH < Pd or PH ≤ Pd). When the recovery valve 74 is closed, the recovery of the working gas from the high-pressure line 63 to the buffer volume 70 stops. Thus, this method ends and is executed again in the next control cycle.

[0063] Note that the pressure threshold for closing the recovery valve 74 may be different from the upper limit value Pd of the appropriate pressure range. For example, it may be smaller than the upper limit value Pd. This pressure threshold is selected from the appropriate pressure range, that is, it may also be larger than the lower limit value Pc of the appropriate pressure range.

[0064] The appropriate pressure range may be changed during the operation of the cryogenic refrigerator 10. For example, the appropriate pressure range during initial cooling may be different from that during steady operation. For example, it may be higher than the appropriate pressure range during steady operation. For example, the lower limit value Pc during initial cooling may be higher than the lower limit value Pc during steady operation, and / or the upper limit value Pd during initial cooling may be higher than the upper limit value Pd during steady operation.

[0065] In this case, the switching from initial cooling to steady-state operation and the change in 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 standard cooling temperature based on the measured temperature signal from the temperature sensor 46, and perform initial cooling if the measured temperature is higher than the standard cooling temperature, and transition from initial cooling to steady-state operation if the measured temperature is lower than or equal to the standard cooling temperature. The controller 110 may change the appropriate pressure range in conjunction with the transition from initial cooling to steady-state operation.

[0066] Furthermore, as will be described later with reference to Figures 7 and 8, the switching from initial cooling to steady-state operation and the change in the appropriate pressure range may be performed based on the pressure of the buffer volume 70 or based on the differential pressure between the high-pressure line 63 and the low-pressure line 64. In this way, the control device 100 can complete the initial cooling of the cryogenic refrigerator 10 without relying on the temperature sensor 46.

[0067] Here, in order to ensure a reliable supply of working gas from the buffer volume 70, we consider the desired conditions for the buffer volume 70. From the ideal gas law, when the cryogenic refrigerator 10 is shut down (i.e., before initial cooling), PI(VH+VL+VB)=nRT (1) The following holds true. Here, PI (MPa) is the operating gas filling pressure of the cryogenic refrigerator 10 at temperature T (K), VH (L) is the volume of the high-pressure line 63, VL (L) is the volume of the low-pressure line 64, VB (L) is the volume of the buffer volume 70, n (mol) is the amount of operating gas in the cryogenic refrigerator 10, and R is the gas constant.

[0068] Similarly, during steady-state operation of the cryogenic refrigerator 10, PHVH + PLVL + PBVB = nRT (2) The following holds true. Here, PH (MPa) represents the pressure of the high-pressure line 63 during steady-state operation at temperature T, PL (MPa) represents the pressure of the low-pressure line 64 during steady-state operation at temperature T, and PB (MPa) represents the pressure of the buffer volume 70 during steady-state operation at temperature T.

[0069] From equations (1) and (2), PI(VH+VL+VB)=PHVH+PLVL+PBVB (3) This is the result.

[0070] To supply working gas from buffer volume 70 to low-pressure line 64 at any time during operation of cryogenic refrigerator 10, for any temperature T in the temperature range from the initial temperature to the cryogenic temperature of cryogenic refrigerator 10, PL ≤ PB (4) It should satisfy this condition.

[0071] Solving equation (3) for PB and substituting it into equation (4), we obtain the following relationship. VB≧VH(PH-PI) / (PI-PL)-VL (5)

[0072] Therefore, in order to ensure the supply of working gas from the buffer volume 70 to the low-pressure line 64, it is preferable that the buffer volume 70 satisfies equation (5) for any temperature in the temperature range from the initial temperature to cryogenic temperatures.

[0073] Similarly, to ensure gas recovery into the buffer volume 70, we consider the desired conditions for the buffer volume 70. In this case, in order to supply working gas from the buffer volume 70 to the high-pressure line 63 at any time during the operation of the cryogenic refrigerator 10, for any temperature T in the temperature range from the initial temperature to the cryogenic temperature of the cryogenic refrigerator 10, PB ≤ PH (6) It should satisfy this condition.

[0074] Solving equation (3) for PB and substituting it into equation (6), we obtain the following relationship. VB≧-VH+VL(PI-PL) / (PH-PI) (7)

[0075] Therefore, in order to ensure the recovery of working gas from the high-pressure line 63 to the buffer volume 70, it is preferable that the buffer volume 70 satisfies equation (7) for any temperature in the temperature range from the initial temperature to cryogenic temperatures.

[0076] Figure 5 is a graph showing an example of the time variation of temperature and pressure during operation of the cryogenic refrigerator 10 according to an embodiment. The pressure changes shown were obtained experimentally. The upper part of Figure 5 shows the pressure PH of the high-pressure line 63 measured by the first pressure sensor 54 and the pressure PL of the low-pressure line 64 measured by the second pressure sensor 55. The lower part of Figure 5 shows the temperature T1 of the first cooling stage 33 and the temperature T2 of the second cooling stage 35. The horizontal axis represents time.

[0077] Before the cryogenic refrigerator 10 is started (time 0), the pressure PH of the high-pressure line 63 and the pressure PL of the low-pressure line 64 are both the sealing pressure PI, and the temperature T1 of the first cooling stage 33 and the temperature T2 of the second cooling stage 35 are both room temperature (approximately 300K). When the cryogenic refrigerator 10 is started and initial cooling begins, the compressor 12 and the expander 14 operate, the pressure PH of the high-pressure line 63 increases from the sealing pressure PI, and the pressure PL of the low-pressure line 64 decreases from the sealing pressure PI. Due to the initial cooling, the temperature T1 of the first cooling stage 33 and the temperature T2 of the second cooling stage 35 decrease. When the first cooling stage 33 and the second cooling stage 35 are cooled to the above-mentioned standard cooling temperatures (for example, T1 ≤ 30K, T2 ≤ 4K), initial cooling is completed and steady-state operation begins.

[0078] Figure 6(A) schematically shows an enlarged view of section A shown in Figure 5, and Figure 6(B) schematically shows an enlarged view of section B shown in Figure 5. Figure 6(A) shows the pressure PH of the high-pressure line 63 immediately after the start of initial cooling, along with the open / closed state of the recovery valve 74, and Figure 6(B) shows the pressure PH of the high-pressure line 63 after section A, along with the open / closed state of the supply valve 72.

[0079] As shown in Figure 6(A), when the pressure PH of the high-pressure line 63 exceeds the upper limit Pd of the appropriate pressure range, the recovery valve 74 opens. Since the working gas is recovered from the high-pressure line 63 into the buffer volume 70 through the recovery valve 74, the pressure PH of the high-pressure line 63 decreases. When the pressure PH of the high-pressure line 63 falls below the upper limit Pd, the recovery valve 74 closes. In this way, excessive pressure increase in the high-pressure line 63 can be avoided. The risk of emergency shutdown of the compressor 12 due to excessive pressure increase is reduced. In addition, since the buffer volume 70 is pressurized by the recovery of the working gas, it can be effectively used to supply working gas from the buffer volume 70 to the low-pressure line 64.

[0080] As shown in Figure 6(B), when the pressure PH of the high-pressure line 63 falls below the lower limit Pc of the appropriate pressure range, the supply valve 72 opens. Working gas is supplied from the buffer volume 70 to the low-pressure line 64 through the supply valve 72. As the amount of working gas circulating in the gas line 62 increases, the pressure in the high-pressure line 63 is restored. When the pressure PH of the high-pressure line 63 exceeds the lower limit Pc, the supply valve 72 closes.

[0081] As described above, the decrease in temperature of the expander 14 during initial cooling increases the density of the working gas within the expander 14, which has the effect of lowering the pressure PH of the high-pressure line 63. Therefore, even if the pressure PH of the high-pressure line 63 recovers once, it falls below the lower limit Pc again. The supply valve 72 opens again, the pressure in the high-pressure line 63 recovers, and the supply valve 72 closes. In this way, the supply valve 72 operates by repeatedly opening and closing to maintain the pressure PH of the high-pressure line 63 within the appropriate pressure range.

[0082] If the working gas were not supplied to the gas line 62 during the initial cooling, the pressure PH of the high-pressure line 63 could drop significantly due to the temperature decrease of the expander 14. Since the cooling capacity of the cryogenic refrigerator 10 is correlated with the pressure PH of the high-pressure line 63, the cooling capacity of the cryogenic refrigerator 10 may decrease as the initial cooling progresses. This could be a factor that prolongs the time required for initial cooling.

[0083] In contrast, according to this embodiment, the pressure PH of the high-pressure line 63 can be maintained within an appropriate pressure range by controlling the supply valve 72 during the initial cooling. Therefore, the cooling capacity of the cryogenic refrigerator 10 can be properly maintained, and the increase in the initial cooling time can be suppressed. Furthermore, by keeping the pressure PH of the high-pressure line 63 approximately constant, the cryogenic refrigerator 10 can provide a stable cooling capacity.

[0084] In this embodiment, since a bypass line 56 and a relief valve 60 are provided, when the pressure PH of the high-pressure line 63 increases, excessive pressure rise can be suppressed by releasing the working gas from the high-pressure line 63 to the low-pressure line 64 through the bypass line 56. However, such bypass flow reduces the flow rate of the working gas supplied from the compressor 12 to the expander 14, which can lead to a decrease in the cooling capacity of the cryogenic refrigerator 10. However, in this embodiment, the pressure PH of the high-pressure line 63 can be maintained within an appropriate pressure range by utilizing the buffer volume 70, so there is no need to rely on bypass flow, which is advantageous.

[0085] To maintain the pressure PH of the high-pressure line 63 within the appropriate pressure range, a method of controlling the supply valve 72 and the recovery valve 74 based on the pressure of the low-pressure line 64 is also conceivable. The pressure of the low-pressure line 64 is affected by the cooling temperature of the expander 14 (it fluctuates depending on the cooling temperature). Therefore, it is practically essential to set the appropriate pressure range of the low-pressure line 64, i.e., the pressure threshold of the low-pressure line 64 for opening and closing the supply valve 72 and the recovery valve 74, to different values ​​depending on the cooling temperature, which complicates the control design. Furthermore, even if the low-pressure line 64 is within the appropriate pressure range, depending on the cooling temperature, the pressure of the high-pressure line 63 may become excessively high. Therefore, the method based on the pressure of the high-pressure line 63, as in the embodiment, is advantageous in that it mitigates or prevents such problems.

[0086] Figure 7 is a graph showing an example of temperature and pressure changes during operation of the cryogenic refrigerator 10 according to the embodiment. Figure 8 is a schematic diagram showing the cryogenic refrigerator 10 according to the embodiment.

[0087] Similar to the embodiment described above, the cryogenic refrigerator 10 includes a compressor 12, an expander 14, a buffer volume 70, and a control device 100. During initial cooling, the controller 110 controls the supply valve 72 to maintain the pressure of the high-pressure line 63 within a preset appropriate pressure range based on the pressure of the high-pressure line 63 measured by the first pressure sensor 54. The controller 110 also controls the recovery valve 74 to maintain the pressure of the high-pressure line 63 within an appropriate pressure range based on the pressure of the high-pressure line 63 measured by the first pressure sensor 54 during initial cooling.

[0088] The cryogenic refrigerator 10 includes a buffer pressure sensor 76 connected to the buffer volume 70 to measure the pressure in the buffer volume 70. The buffer pressure sensor 76 is electrically connected to the control device 100 and is configured to output a measured buffer pressure signal PB representing the measured pressure to the control device 100.

[0089] The upper part of Figure 7 shows the pressure PH of the high-pressure line 63 and the pressure PL of the low-pressure line 64, as shown in Figure 5, as well as the pressure PB of the buffer volume 70 measured by the buffer pressure sensor 76. The lower part of Figure 7 shows the temperature T1 of the first cooling stage 33 and the temperature T2 of the second cooling stage 35. As can be seen from Figure 7, once the cryogenic refrigerator 10 is sufficiently cooled upon completion of the initial cooling and the temperatures of the first cooling stage 33 and the second cooling stage 35 stabilize, the pressure PH of the high-pressure line 63 and the pressure PL of the low-pressure line 64 also stabilize. At this time, both the supply valve 72 and the recovery valve 74 are closed, and the buffer volume 70 is disconnected from the gas line 62. Therefore, the pressure PB of the buffer volume 70 also becomes constant (the final buffer pressure PF shown in Figure 7).

[0090] Therefore, the completion of initial cooling can be determined by detecting the stabilization of the pressure PB of the buffer volume 70. If the operating gas filling pressure PI and operating conditions (e.g., high pressure PH, low pressure PL, temperatures T1, T2, etc.) of the cryogenic refrigerator 10 are known, the final pressure of the buffer volume 70 at the completion of initial cooling can be predicted. In this case, the controller 110 may compare the predicted value of the final buffer pressure with the measured pressure PB of the buffer volume 70 and determine, based on the comparison result, whether the measured pressure PB of the buffer volume 70 is equal to the predicted value of the final buffer pressure. The controller 110 may complete initial cooling if the state in which the measured pressure PB of the buffer volume 70 is equal to the predicted value of the final buffer pressure continues for a predetermined time (e.g., several minutes).

[0091] Alternatively, the controller 110 may determine the completion of initial cooling by calculating the difference between the measured pressure PB of the buffer volume 70 and the reference pressure during initial cooling, and detecting the stabilization of the calculated pressure difference. The reference pressure may be the previously measured pressure of the buffer volume 70, or for example, the maximum value PM of the pressure of the buffer volume 70 measured during initial cooling. As can be seen from Figure 7, the pressure of the buffer volume 70 increases from the sealing pressure PI to the maximum value PM immediately after the start of initial cooling.

[0092] The controller 110 may compare the calculated pressure difference (i.e., the difference between the measured pressure PB of the buffer volume 70 and the reference pressure) with the target pressure difference value and, based on the comparison result, determine whether the calculated pressure difference is equal to the target pressure difference value. The controller 110 may complete the initial cooling if the state in which the calculated pressure difference is equal to the target pressure difference value continues for a predetermined time. The predetermined time may be selected from, for example, a range of 1 minute or more and 10 minutes or less. If the difference between the calculated pressure difference and the target pressure difference value is within a predetermined value (e.g., 0.05 MPa), the calculated pressure difference can be considered equal to the target pressure difference value. Since this target pressure difference value does not depend on the sealing pressure PI, the completion of initial cooling can be determined even if the sealing pressure PI is unknown.

[0093] As another example of a reference pressure, the pressure PH of the high-pressure line 63 (or the pressure PL of the low-pressure line 64), measured at the same time as the measured pressure PB of the buffer volume 70, may be used. The controller 110 may determine the completion of initial cooling by calculating the difference between the measured pressure PB of the buffer volume 70 and the measured pressure PH of the high-pressure line 63 (or the measured pressure PL of the low-pressure line 64) and detecting the stabilization of the calculated pressure difference. The controller 110 may also compare the calculated pressure difference with a target pressure difference value, as in the example above, and complete initial cooling if the calculated pressure difference is equal to the target pressure difference value for a predetermined period of time.

[0094] As a further alternative, the controller 110 may determine the completion of initial cooling by calculating the difference between the measured pressure PH of the high-pressure line 63 and the measured pressure PL of the low-pressure line 64, and detecting the stabilization of the calculated pressure difference.

[0095] The present invention has been described above based on examples. Those skilled in the art will understand that the present invention is not limited to the above embodiments, that various design changes are possible, and that various modifications are possible, and that such modifications also fall within the scope of the present invention. Various features described in relation to one embodiment are applicable to other embodiments. New embodiments resulting from combinations will possess the combined effects of each of the embodiments combined.

[0096] In the above-described embodiment, the case in which the expander motor 42 operates at a constant operating frequency (motor rotation speed) (i.e., the case in which the expander motor 42 operates at the same operating frequency during initial cooling and steady-state operation) is described as an example, but the present invention is not limited to this. The cryogenic refrigerator 10 may be equipped with an expander motor 42 with a variable operating frequency, and so-called accelerated cooling may be performed during initial cooling by operating the expander motor 42 at a higher operating frequency than that of steady-state operation. In this case, the switching from initial cooling (accelerated cooling) to steady-state operation and the change in operating frequency may be performed based on the pressure of the buffer volume 70 or based on the differential pressure between the high-pressure line 63 and the low-pressure line 64, as described above with reference to Figures 7 and 8. By performing accelerated cooling, the initial cooling time can be further shortened.

[0097] The pressure sensors, such as the first pressure sensor 54 and the second pressure sensor 55, are not necessarily required to be installed in the compressor 12, but may be installed in 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 installed in any location on the high-pressure line 63, and the second pressure sensor 55 may be installed in any location on the low-pressure line 64.

[0098] In the above-described embodiment, the supply valve 72 and the recovery valve 74 are provided as separate valves, each connected to the buffer volume 70, but the present invention is not limited thereto. For example, the supply valve 72 and the recovery valve 74 may be integrated, or they may be a three-way valve connected to the buffer volume 70. By switching the three-way valve, it may be possible to switch between a supply state in which the buffer volume 70 is connected to the low-pressure line 64 and a recovery state in which the buffer volume 70 is connected to the high-pressure line 63.

[0099] In the embodiments described above, the buffer volume 70 is a single buffer tank, but in some embodiments, the buffer volume 70 may consist of multiple buffer tanks. One buffer tank may be connected to the low-pressure line 64 by a supply valve 72, and another buffer tank may be connected to the high-pressure line 63 by a recovery valve 74. Also, in the embodiments described above, the buffer volume 70 is located outside the compressor 12 and the expander 14, but is not limited to this. For example, the buffer volume 70 may be located inside the compressor 12.

[0100] The above-described embodiment is explained using the case where the cryogenic refrigerator 10 is a two-stage GM refrigerator as an example, but is not limited to this. The cryogenic refrigerator 10 may be a single-stage or multi-stage GM refrigerator, or even another type of cryogenic refrigerator such as a pulse tube refrigerator.

[0101] The present invention has been described above based on examples. Those skilled in the art will understand that the present invention is not limited to the above embodiments, that various design changes are possible, and that various modifications are possible, and that such modifications also fall within the scope of the present invention. [Industrial applicability]

[0102] The present invention can be used in the fields of cryogenic refrigerators and methods for operating cryogenic refrigerators. [Explanation of symbols]

[0103] 10 Cryogenic refrigerators, 14 Expanders, 63 High-pressure lines, 64 Low-pressure lines, 70 Buffer volume controls, 72 Supply valves, 74 Recovery valves, 76 Buffer pressure sensors, 110 Controllers.

Claims

1. An expander capable of performing initial cooling, which cools the temperature from an initial temperature to an extremely low temperature, and steady-state operation, which follows the initial cooling and maintains the extremely low temperature, A high-pressure line connected to the aforementioned expander, through which the working gas drawn into the expander flows, A low-pressure line connected to the aforementioned expander, through which the working gas exhausted from the expander flows, A pressure sensor for measuring the pressure in the aforementioned high-pressure line, The buffer volume for storing the working gas, A supply valve connected to the buffer volume in the low-pressure line, During the initial cooling, a controller controls the supply valve to maintain the pressure in the high-pressure line within a preset appropriate pressure range, based on the pressure in the high-pressure line measured by the pressure sensor. The system includes a buffer pressure sensor for measuring the pressure of the buffer volume, The cryogenic refrigerator is characterized in that the controller completes the initial cooling based on the pressure of the buffer volume measured by the buffer pressure sensor.

2. The cryogenic refrigerator according to claim 1, characterized in that the controller, during the initial cooling, compares the measured pressure of the high-pressure line with the lower limit of the appropriate pressure range, and operates the supply valve to repeatedly open and close the supply valve so that the pressure of the high-pressure line does not fall below the lower limit.

3. The cryogenic refrigerator according to claim 1, characterized in that, when the buffer volume is denoted as VB, the volume of the high-pressure line as VH, the volume of the low-pressure line as VL, the operating gas filling pressure at a certain temperature as PI, the pressure of the high-pressure line during steady-state operation at that temperature as PH, and the pressure of the low-pressure line during steady-state operation at that temperature as PL, the buffer volume satisfies VB ≥ VH(PH - PI) / (PI - PL) - VL for any temperature in the temperature range from the initial temperature to the cryogenic temperature.

4. The buffer volume is further provided with a recovery valve connected to the high-pressure line, The cryogenic refrigerator according to claim 1, characterized in that the controller controls the recovery valve to maintain the pressure of the high-pressure line within the appropriate pressure range.

5. The cryogenic refrigerator according to claim 4, wherein, when the buffer volume is denoted as VB, the volume of the high-pressure line as VH, the volume of the low-pressure line as VL, the operating gas filling pressure at a certain temperature as PI, the pressure of the high-pressure line during steady-state operation at that temperature as PH, and the pressure of the low-pressure line during steady-state operation at that temperature as PL, the buffer volume satisfies VB ≥ -VH + VL(PI - PL) / (PH - PI) for any temperature in the temperature range from the initial temperature to the cryogenic temperature.

6. A method for operating a cryogenic refrigerator, wherein the cryogenic refrigerator comprises an expander, a high-pressure line connected to the expander through which working gas drawn into the expander flows, a low-pressure line connected to the expander through which working gas exhausted from the expander flows, a pressure sensor for measuring the pressure in the high-pressure line, a buffer volume, and a supply valve connecting the buffer volume to the low-pressure line, and the method is as follows: The expansion device is subjected to initial cooling to cool it from its initial temperature to an extremely low temperature, The system comprises performing a steady-state operation to maintain the expander at the cryogenic temperature, following the initial cooling. The supply valve is controlled during the initial cooling process to maintain the pressure in the high-pressure line within a preset appropriate pressure range, based on the pressure in the high-pressure line measured by the pressure sensor. The aforementioned method, During the initial cooling process, the pressure of the buffer volume is measured, A method further comprising completing the initial cooling based on the measured pressure of the buffer volume.

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