CRYOGENIC REFRIGERATOR, AND METHOD FOR COOLING DOWN CRYOGENIC REFRIGERATOR

Abstract

A cryocooler includes: a cold head including a first cooling stage and a second cooling stage cooled to a lower temperature than the first cooling stage; a heater thermally coupled to the first cooling stage; a first temperature sensor that measures a first temperature of the first cooling stage; and a controller configured to perform a cool-down operation of the cold head to cool the first cooling stage from an initial temperature to a first final target temperature and to cool the second cooling stage from the initial temperature to a second final target temperature lower than the first final target temperature, in which the controller is configured to, during the cool-down operation, acquire the first temperature from the first temperature sensor, and control the heater such that the first temperature follows an intermediate target temperature lower than the initial temperature and higher than the first final target temperature.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a bypass continuation of International PCT Application No. PCT/JP2024/000833, filed on Jan. 15, 2024, which claims priority to Japanese Patent Application No. 2023-014704, filed on Feb. 2, 2023, which are incorporated by reference herein in their entirety.

BACKGROUND

Technical Field

Certain embodiments of the present invention relate to a cryocooler and a cool-down method for a cryocooler.

Description of Related Art

A cryocooler is used to cool various objects such as superconducting devices, measuring instruments, and samples used in a cryogenic environment.

SUMMARY

According to an aspect of the present invention, there is provided a cryocooler including: a cold head including a first cooling stage and a second cooling stage cooled to a lower temperature than the first cooling stage; a heater thermally coupled to the first cooling stage; a first temperature sensor that measures a first temperature of the first cooling stage; and a controller configured to perform a cool-down operation of the cold head to cool the first cooling stage from an initial temperature to a first final target temperature and to cool the second cooling stage from the initial temperature to a second final target temperature which is lower than the first final target temperature. The controller is configured to, during the cool-down operation, acquire the first temperature of the first cooling stage from the first temperature sensor, and control the heater such that the first temperature of the first cooling stage follows an intermediate target temperature which is lower than the initial temperature and higher than the first final target temperature.

According to another aspect of the present invention, there is provided a cool-down method for a cryocooler, the cool-down method including: performing a cool-down operation of a cold head to cool a first cooling stage of the cold head from an initial temperature to a first final target temperature and to cool a second cooling stage of the cold head from the initial temperature to a second final target temperature which is lower than the first final target temperature; and during the cool-down operation, applying a heat load to the first cooling stage such that a temperature of the first cooling stage follows an intermediate target temperature which is lower than the initial temperature and higher than the first final target temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

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

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

FIG. 3 is a diagram showing an example of temperature changes in a first cooling stage and a second cooling stage in a cool-down method according to a comparative example.

FIG. 4 is a flowchart showing a cool-down method according to the embodiment.

FIG. 5 is a diagram showing an example of temperature changes in a first cooling stage and a second cooling stage in the cool-down method according to the embodiment.

FIG. 6 is a diagram showing another example of temperature changes in the first cooling stage and the second cooling stage in the cool-down method according to the embodiment.

DETAILED DESCRIPTION

In order to cool an object with the cryocooler, first, the cryocooler has to be activated to cool the cryocooler from an initial temperature to a target cryogenic temperature. Such initial cooling of the cryocooler is also referred to as cool-down. Since the cool-down is only a preparation for starting the cooling of the object, it is desirable that a time required for the cool-down is as short as possible.

It is desirable to shorten a cool-down time of a cryocooler.

Hereinafter, an embodiment for carrying out the present invention will be described in detail with reference to the drawings. In the description and the drawings, the same or equivalent components, members, and processes are denoted by the same reference numerals, and overlapping description is omitted as appropriate. The scale and the shape of each of parts illustrated in the drawings are set for convenience to make the description easy to understand, and are not to be interpreted as limiting unless stated otherwise. The embodiment is merely an example and does not limit the scope of the present invention. All features described in the embodiment or combinations thereof are not necessarily essential to the present invention.

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

The cryocooler 10 can provide cryogenic cooling for various applications. For example, the cryocooler 10 may be used to cool a superconducting coil of a superconducting magnet device. The superconducting magnet device can be mounted on a high magnetic field utilization device as a magnetic field source of, for example, a single crystal pulling-up device, a nuclear magnetic resonance (NMR) system, a magnetic resonance imaging (MRI) system, an accelerator such as a cyclotron, a high energy physics system such as a nuclear fusion system, or another high magnetic field utilization device (not shown) to generate a high magnetic field required for the high magnetic field utilization device.

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

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

The cryocooler cylinder 16 includes a first cylinder 16a and a second cylinder 16b. As an example, the first cylinder 16a and the second cylinder 16b each are 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 disposed, and a lower end of the first cylinder 16a is rigidly connected to an upper end of the second cylinder 16b.

The displacer assembly 18 includes a first displacer 18a and a second displacer 18b. As an example, the first displacer 18a and the second displacer 18b each are 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 disposed.

The first displacer 18a is accommodated in the first cylinder 16a, and the second displacer 18b is accommodated in the second cylinder 16b. The first displacer 18a can reciprocate in an axial direction along the first cylinder 16a, and the second displacer 18b can reciprocate in the axial direction along the second cylinder 16b. The first displacer 18a and the second displacer 18b are connected to each other, and move integrally.

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

The first displacer 18a accommodates a first regenerator 26. The first regenerator 26 is formed by filling a tubular main body of the first displacer 18a with a metal mesh made of copper or the like or other suitable first regenerator materials. In addition, the second displacer 18b accommodates the second regenerator 28. The second regenerator 28 is formed by filling a tubular main body portion of the second displacer 18b for example, with a non-magnetic regenerator material such as bismuth, a magnetic regenerator material such as HoCu₂, or other suitable second regenerator materials. The second regenerator material may be formed in a granular shape.

The displacer assembly 18 forms an upper chamber 30, a first expansion chamber 32, and a second expansion chamber 34 inside the cryocooler cylinder 16. The cold head 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 cryocooler 10. The first cooling stage 33 is fixed to a lower portion of the first cylinder 16a to surround the first expansion chamber 32, and the second cooling stage 35 is fixed to a lower portion of the second cylinder 16b to surround the second expansion chamber 34. The upper chamber 30 is formed between an upper end portion of the first displacer 18a and an upper portion of the first cylinder 16a. The first expansion chamber 32 is formed between a lower end portion of the first displacer 18a and the first cooling stage 33. The second expansion chamber 34 is formed between a lower lid portion of the second displacer 18b and the second cooling stage 35.

The first cooling stage 33 and the second cooling stage 35 are formed of, for example, a metal material such as copper or other materials having high thermal conductivity. The cryocooler cylinder 16 is usually formed of a metal material having a lower thermal conductivity than the first cooling stage 33 and the second cooling stage 35, for example, stainless steel.

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

A first seal 38a and a second seal 38b may be provided so that working gas flows between the first expansion chamber 32, the second expansion chamber 34 and the room temperature chamber 30 are guided to the first regenerator 26 and the second regenerator 28, rather than to a clearance between the cryocooler cylinder 16 and the displacer assembly 18. The first seal 38a may be mounted on the upper end portion of the first displacer 18a to be disposed between the first displacer 18a and the first cylinder 16a. The second seal 38b may be mounted on the upper end portion of the second displacer 18b to be disposed between the second displacer 18b and the second cylinder 16b.

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

As shown in FIG. 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 cryocooler cylinder 16. A working gas discharge port of the compressor 12 is connected to the upper chamber 30 via the high pressure valve 40a, and a working gas suction port of the compressor 12 is connected to the upper chamber 30 via the low pressure valve 40b. The high pressure valve 40a and the low pressure valve 40b are configured to open and close selectively and alternately (that is, such that when one is open, the other is closed). A high pressure (for example, 2 to 3 MPa) working gas is supplied from the compressor 12 to the cold head 14 through the high pressure valve 40a, and a low pressure (for example, 0.5 to 1.5 MPa) working gas is collected from the cold head 14 to the compressor 12 through the low pressure valve 40b. To facilitate understanding, a direction in which the working gas flows is indicated by arrows in FIG. 2.

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

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

When the compressor 12 and the drive motor 42 are operated, the cryocooler 10 generates periodic volume fluctuations in the first expansion chamber 32 and the second expansion chamber 34 and pressure fluctuations of the working gas synchronized therewith, thereby forming a refrigeration cycle, and the first cooling stage 33 and the second cooling stage 35 are cooled to a desired cryogenic temperature. The first cooling stage 33 is cooled to a first cooling temperature in a first temperature range, and the second cooling stage 35 is cooled to a second cooling temperature in a second temperature range. The first temperature range may be, for example, a temperature range of about 30 K to about 80 K. The second temperature range is lower than the first temperature range, and may be, for example, a temperature range of about 3 K to about 20 K. The first cooling temperature and the second cooling temperature can be selected to various temperature values according to the use of the cryocooler 10. For example, in a case where the cryocooler 10 is used to cool a superconducting coil, the second cooling temperature is typically about 4.2 K or lower.

In this embodiment, the cold head 14 includes a heater 46 thermally coupled to the first cooling stage 33. As shown in FIG. 1, the heater 46 may be mounted on a surface of the first cooling stage 33 or inside the first cooling stage 33. The heater 46 is configured to apply a heat load corresponding to an output thereof to the first cooling stage 33 and heat the first cooling stage 33. The heater 46 may be a heating device such as an electric heater.

The cold head 14 includes a first temperature sensor 48a and a second temperature sensor 48b. The first temperature sensor 48a is configured to measure a first temperature of the first cooling stage 33 and to generate a first measured temperature signal S1 indicating the first temperature. The second temperature sensor 48b is configured to measure a second temperature of the second cooling stage 35 and to generate a second measured temperature signal S2 indicating the second temperature. As shown in FIG. 1, the first temperature sensor 48a may be attached to the surface of the first cooling stage 33 or to the inside of the first cooling stage 33. The second temperature sensor 48b may be attached to a surface of the second cooling stage 35 or to an inside of the second cooling stage 35.

In addition, a controller 50 that controls the cryocooler 10 is provided. The controller 50 may be configured to switch on and off the cryocooler 10 (that is, on and off the compressor 12 and the drive motor 42 of the cold head 14).

The controller 50 is electrically connected to the first temperature sensor 48a and the second temperature sensor 48b to acquire the first measured temperature signal S1 and the second measured temperature signal S2. In addition, the controller 50 is electrically connected to the heater 46 to switch on and off the heater 46 and/or to control an output of the heater 46. As will be described later, the controller 50 may be configured to switch on and off the heater 46 and/or control the output of the heater 46, based on the measured temperature of the first temperature sensor 48a and/or the second temperature sensor 48b.

In the shown example, the controller 50 is provided separately from the compressor 12 and the cold head 14 and is connected to the compressor 12 and the cold head 14, but the controller 50 is not limited thereto. The controller 50 may be mounted on the compressor 12. Alternatively, the controller 50 may be provided in the cold head 14, for example, by being mounted on the drive motor 42.

The controller 50 is implemented as a hardware configuration by elements and circuits including a central processing unit (CPU) and a memory of a computer, and as a software configuration by a computer program or the like. However, in the drawings, the configurations are illustrated as functional blocks implemented through cooperation therebetween. Those skilled in the art may understand that these functional blocks can be implemented in various forms by combining hardware and software.

The cryocooler 10 can perform a steady operation and a cool-down operation prior to the steady operation. The cool-down operation is an operation mode of the cryocooler 10 in which the cold head 14 is rapidly cooled from an ambient temperature (for example, room temperature) to a desired cryogenic temperature as preparation for the steady operation, and can also be referred to as initial cooling of the cryocooler 10. The steady operation is an operation mode of the cryocooler 10 in which the cryocooler 10 maintains a state of being cooled to the cryogenic temperature by the cool-down operation.

Therefore, an object to be cooled by the cryocooler 10 is cooled from the ambient temperature to a cryogenic temperature at which the object to be cooled can operate by the cool-down operation of the cryocooler 10, and then operates while being maintained at the cryogenic temperature by the steady operation of the cryocooler 10. For example, in the case of the superconducting magnet device, the superconducting coil is cooled to a cryogenic temperature equal to or lower than a superconducting transition temperature from the ambient temperature by a cool-down operation. Thereafter, during the steady operation of the cryocooler 10, the superconducting coil cooled to the cryogenic temperature is powered and capable of generating a strong magnetic field.

In the cool-down operation, the cold head 14 is operated to cool the first cooling stage 33 from an initial temperature to a first final target temperature and to cool the second cooling stage 35 from the initial temperature to a second final target temperature which is lower than the first final target temperature. The controller 50 is configured to perform a cool-down operation of the cold head 14. The initial temperature may be the ambient temperature (for example, room temperature) of the cryocooler 10. The first final target temperature may be selected from the above-described first temperature range, and the second final target temperature may be selected from the above-described second temperature range. The first final target temperature and the second final target temperature may be set in advance and stored in the controller 50.

Incidentally, the present inventor has recognized that there is a case where the first cooling stage 33 is cooled more rapidly than the second cooling stage 35 during the cool-down operation of the cold head 14, and in a case where such pre-cooling of the first cooling stage 33 occurs, the cooling of the second cooling stage 35 is delayed, resulting in an increase in time required for the cool-down.

In general, a cooling capacity in the first cooling stage 33 is larger than a cooling capacity in the second cooling stage 35 that is cooled to a lower temperature. In addition to such a basic difference in cooling capacity, a cooling rate of each of the first cooling stage 33 and the second cooling stage 35 is also influenced by a difference in weight (more accurately, heat capacity) of an object to be cooled in each cooling stage. Therefore, in a case where an object to be cooled by the second cooling stage 35 is heavier than an object to be cooled by the first cooling stage 33, the time required for cooling of the second cooling stage 35 is longer than the time required for cooling the first cooling stage 33. That is, as illustrated in FIG. 3, during the cool-down operation, a first temperature T1 of the first cooling stage 33 is cooled and lowered earlier than the second temperature T2 of the second cooling stage 35.

In addition, a density of the working gas (for example, helium gas) in the first expansion chamber 32 is increased due to the pre-cooling of the first cooling stage 33. The increase in the density of the working gas promotes accumulation of the working gas supplied to the cold head 14, in the first expansion chamber 32, and has an effect of absorbing the working gas into the first expansion chamber 32, so to speak. That is, this means that the amount of the working gas distributed to the second expansion chamber 34 by passing through the first expansion chamber 32 is reduced. Therefore, in a case where the first cooling stage 33 is cooled prior to the second cooling stage 35, the cooling capacity of the second cooling stage 35 may be lowered due to the increase in the density of the working gas in the first expansion chamber 32. In this manner, there is a concern that the cool-down time is further extended.

In a case where the cryocooler 10 is used to cool the superconducting magnet device, the superconducting coil is cooled by the second cooling stage 35. A cooling load of the second cooling stage 35 tends to be larger than a cooling load of the first cooling stage 33. Therefore, the problem of an increase in the cool-down time described above is more likely to become apparent.

Therefore, in this embodiment, a cool-down method for the cryocooler 10 includes performing the cool-down operation of the cold head 14 and applying a heat load to the first cooling stage 33 such that the temperature of the first cooling stage 33 follows an intermediate target temperature during the cool-down operation. As described above, by performing the cool-down operation, the first cooling stage 33 is cooled from the initial temperature to the first final target temperature, and the second cooling stage 35 is cooled from the initial temperature to the second final target temperature which is lower than the first final target temperature. The intermediate target temperature is lower than the initial temperature and higher than the first final target temperature. The heater 46 is used to apply a heat load to the first cooling stage 33.

In this way, the first temperature of the first cooling stage 33 is temporarily maintained at or near the intermediate target temperature during the cool-down operation of the cold head 14. That is, the first cooling stage 33 is intentionally maintained at a temperature which is higher than the first final target temperature, and as a result, the cooling rate of the first cooling stage 33 is reduced. Due to such an intentional cooling delay of the first cooling stage 33, an imbalance in the cooling rate between the first cooling stage 33 and the second cooling stage 35 can be eliminated or alleviated, and thus the increase in the cool-down time described above can be coped with.

FIG. 4 is a flowchart illustrating the cool-down method according to the embodiment. This method is performed by the controller 50. As shown in FIG. 4, first, the first temperature T1 of the first cooling stage 33 and the second temperature T2 of the second cooling stage 35 are measured (S10). The first measured temperature signal S1 is output from the first temperature sensor 48a to the controller 50, and the second measured temperature signal S2 is output from the second temperature sensor 48b to the controller 50. The controller 50 receives the first measured temperature signal S1 from the first temperature sensor 48a, and acquires the first temperature T1 from the first measured temperature signal S1. In addition, the controller 50 receives the second measured temperature signal S2 from the second temperature sensor 48b, and acquires the second temperature T2 from the second measured temperature signal S2.

Next, a condition for ending the cool-down is determined (S12). The condition for ending the cool-down is that the first temperature T1 of the first cooling stage 33 is equal to or lower than the first final target temperature and the second temperature T2 of the second cooling stage 35 is equal to or lower than the second final target temperature. The controller 50 is configured to compare the first temperature T1 to the first final target temperature, compare the second temperature T2 to the second final target temperature, and determine whether or not the condition for ending the cool-down is satisfied, based on comparison results.

In a case where the condition for ending the cool-down is satisfied (Yes in S12), the controller 50 ends the cool-down operation. In this case, as described above, the cryocooler 10 may transition to the steady operation following the end of the cool-down operation.

In a case where the condition for ending the cool-down is not satisfied (No in S12), the cool-down operation is continued. In this case, the controller 50 may compare the second temperature T2 of the second cooling stage 35 to a predetermined temperature threshold Tx (S14). This comparison is performed to determine whether or not the second cooling stage 35 is sufficiently cooled toward the second final target temperature. For example, the temperature threshold Tx may be determined in advance to be a temperature value close to the first final target temperature of the first cooling stage 33, and may be, for example, 50 K or lower. The temperature threshold Tx may be equal to or lower than the first final target temperature. The temperature threshold Tx may be higher than the second final target temperature.

In a case where the second temperature T2 of the second cooling stage 35 exceeds the above-described temperature threshold Tx (No in S14), as will be described later, the controller 50 controls the heater 46 such that the first temperature T1 of the first cooling stage 33 follows an intermediate target temperature Tm.

In order to control the heater 46, the controller 50 first sets the intermediate target temperature Tm (S16). The intermediate target temperature Tm is selected from a temperature range which is lower than the initial temperature of the cool-down and higher than the first final target temperature.

Plain text: The controller 50 may be configured to change the intermediate target temperature Tm during the cool-down operation. The cooling rate of the first cooling stage 33 during the cool-down operation can be controlled by changing the intermediate target temperature Tm.

For example, the controller 50 may be configured to set the intermediate target temperature Tm based on the second temperature T2 of the second cooling stage 35. In this way, the intermediate target temperature Tm can be set to be close to the second temperature T2, and as a result, it is possible to avoid the first temperature of the first cooling stage 33 largely deviating from the second temperature T2 of the second cooling stage 35.

In this case, the intermediate target temperature Tm may be set to coincide with the second temperature T2 of the second cooling stage 35, or may be set to a temperature which is higher than the second temperature T2 (for example, a temperature value obtained by adding a margin value to the second temperature T2). The margin value may be, for example, a value within 10 K. In this way, it is possible to prevent the first temperature of the first cooling stage 33 from falling below the second temperature T2 of the second cooling stage 35, or to reduce such a probability.

The intermediate target temperature Tm may be set to a temperature which is lower than the second temperature T2 of the second cooling stage 35 as long as a difference in the cooling rate of the first cooling stage 33 from the second cooling stage 35 and an influence on the cool-down time due to the difference are within practically acceptable ranges. In this case, a lower limit of the intermediate target temperature Tm may be a temperature value obtained by subtracting a predetermined temperature value (for example, a value within 10 K) from the second temperature T2. Therefore, the intermediate target temperature Tm may be selected from a predetermined temperature range (for example, a range of the second temperature T2±10 K) including the second temperature T2.

Next, the controller 50 compares the first temperature T1 of the first cooling stage 33 to the intermediate target temperature Tm (S18). In a case where the first temperature T1 falls below the intermediate target temperature Tm (Yes in S18), the controller 50 turns on the heater 46 (S20). Accordingly, the first cooling stage 33 is heated by the heater 46. Excessive cooling of the first cooling stage 33 can be avoided, and the temperature of the first cooling stage 33 can be raised to the intermediate target temperature Tm.

On the other hand, in a case where the first temperature T1 exceeds the intermediate target temperature Tm (No in S18), the controller 50 turns off the heater 46 (S22). In this case, the first cooling stage 33 is cooled by the cool-down operation of the cold head 14. The temperature of the first cooling stage 33 can be lowered to the intermediate target temperature Tm.

Subsequent to such control of the heater 46 in which the first temperature of the first cooling stage 33 follows the intermediate target temperature Tm, as shown in FIG. 4, the measurement (S10) of the first temperature T1 and the second temperature T2 and the determination (S12) of the end of the cool-down are performed again. Then, as necessary, the heater 46 is controlled again such that the first temperature of the first cooling stage 33 follows the intermediate target temperature Tm.

In addition, in a case where the second temperature T2 of the second cooling stage 35 falls below the above-described temperature threshold Tx (Yes in S14), the controller 50 turns off the heater 46 (S22). This is because the second cooling stage 35 is being sufficiently cooled toward the second final target temperature, and therefore, it is not necessary to apply a heat load to the first cooling stage 33. In this way, the controller 50 can end the control of the heater 46 in which the first temperature of the first cooling stage 33 follows the intermediate target temperature Tm. Also in this case, as shown in FIG. 4, the measurement (S10) of the first temperature T1 and the second temperature T2 and the determination (S12) of the end of the cool-down are performed again.

In the above-described cool-down method, instead of turning on the heater 46, the controller 50 may increase the output of the heater 46. In addition, instead of turning off the heater 46, the controller 50 may decrease the output of the heater 46. Even in this way, the heater 46 can be controlled so that the first temperature of the first cooling stage 33 follows the intermediate target temperature Tm.

FIG. 5 is a diagram showing an example of temperature changes in the first cooling stage 33 and the second cooling stage 35 in the cool-down method according to the embodiment. A vertical axis and a horizontal axis of FIG. 5 represent temperature and time, respectively. In addition, ON and OFF states of the heater 46 are also shown in a lower part of FIG. 5. The first cooling stage 33 and the second cooling stage 35 start to be cooled from an initial temperature TO.

As described above, the heater 46 is turned on when the first temperature T1 of the first cooling stage 33 falls below the intermediate target temperature Tm, and is turned off when the first temperature T1 exceeds the intermediate target temperature Tm (in this embodiment, the intermediate target temperature Tm changes from moment to moment, and thus is not explicitly shown in FIG. 5). When the heater 46 is turned on, a heat load is applied to the first cooling stage 33 by the heater 46. The first cooling stage 33 is maintained at the intermediate target temperature Tm or is cooled moderately according to a balance between the heat load from the heater 46 and the cooling capacity of the cold head 14 in the first cooling stage 33. When the heater 46 is turned off, the heat load from the heater 46 to the first cooling stage 33 is removed, and thus the first cooling stage 33 is rapidly cooled according to the cooling capacity of the cold head 14. In this way, the first temperature T1 of the first cooling stage 33 decreases stepwise as the heater 46 is repeatedly turned on and off. Since the intermediate target temperature Tm is set to or to be close to the second temperature T2 of the second cooling stage 35, the first temperature T1 of the first cooling stage 33 decreases while following the second temperature T2 of the second cooling stage 35.

When the cool-down progresses and the second temperature T2 of the second cooling stage 35 falls below the above-described temperature threshold Tx, the heater 46 is turned off, and the temperature control of the first cooling stage 33 based on the intermediate target temperature Tm is ended. Therefore, the first temperature T1 of the first cooling stage 33 is cooled toward a first final target temperature T1a, and the second temperature T2 of the second cooling stage 35 is cooled toward a second final target temperature T2a. In this way, when the first cooling stage 33 is cooled to the first final target temperature T1a or lower and the second cooling stage 35 is cooled to the second final target temperature T2a or lower, the cool-down is ended.

In this way, the first temperature T1 of the first cooling stage 33 can be controlled so as not to fall below the second temperature T2, preferably by referring to the second temperature T2 of the second cooling stage 35 during the cool-down of the cryocooler 10.

According to the embodiment, the first temperature T1 of the first cooling stage 33 can be caused to follow the intermediate target temperature Tm by using the heat load from the heater 46, and the first cooling stage 33 can be temporarily maintained in a state higher than the first final target temperature during the cool-down operation of the cold head 14. In this manner, an excessive temperature decrease in the first cooling stage 33 relative to the second cooling stage 35 can be suppressed, and preferably, a pace of the temperature decrease between the first cooling stage 33 and the second cooling stage 35 can be kept. Therefore, it is possible to suppress an increase in the density of the working gas in the first expansion chamber 32 caused by an excessive temperature decrease in the first cooling stage 33, and to properly distribute the working gas between the first expansion chamber 32 and the second expansion chamber 34. In this way, it is possible to suppress a decrease in the cooling capacity of the second cooling stage 35 that may occur due to the pre-cooling of the first cooling stage 33 during the cool-down, and to shorten the cool-down time of the cryocooler 10.

In the above-described embodiment, the intermediate target temperature Tm is changed during the cool-down, based on the second temperature T2 of the second cooling stage 35. Alternatively, the intermediate target temperature Tm may be changed in accordance with an intermediate target temperature profile determined in advance. Plain text: For example, the controller 50 may include an intermediate target temperature profile that determines a relationship between an elapsed time from the start of the cool-down operation and the intermediate target temperature Tm. The intermediate target temperature profile may determine the relationship between the elapsed time from the start of the cool-down operation and the intermediate target temperature Tm such that the intermediate target temperature Tm decreases stepwise (or continuously) as time passes.

Alternatively, the controller 50 may be configured to maintain the intermediate target temperature Tm constant during the cool-down operation. In this case, the intermediate target temperature Tm is not changed during the cool-down operation. Such an example is shown in FIG. 6. Even in this way, as in the above-described embodiment, it is possible to suppress the pre-cooling of the first cooling stage 33 and the resulting delay in the cooling of the second cooling stage 35, and to shorten the cool-down time of the cryocooler 10.

In this case, the controller 50 may predict a time Δt required for the first cooling stage 33 to be cooled from the intermediate target temperature Tm to the first final target temperature T1a. The controller 50 turns off the heater 46 prior to a timing the of completion of the cool-down of the second cooling stage 35 by the predicted time Δt. In this manner, the control of the heater 46 can be ended so that the timings of the completion of the cool-down of the first cooling stage 33 and the second cooling stage 35 are aligned with each other. The timing the of the completion of the cool-down of the second cooling stage 35 may be acquired based on empirical knowledge of a designer of the cryocooler 10, or on a preliminary experiment or simulation.

The controller 50 may estimate heat capacities of the first cooling stage 33 and an object to be cooled in order to predict the time Δt required for cooling the first cooling stage 33 from the intermediate target temperature Tm to the first final target temperature T1a. The controller 50 may estimate a heat capacity of a first stage based on a temperature gradient 52 of the first cooling stage 33 from the initial temperature TO of the cool-down to the intermediate target temperature Tm. The intermediate target temperature Tm may be selected from, for example, a range of about 300 K to about 250 K.

The present invention has been described hereinbefore based on the examples. It will be understood by those skilled in the art that the present invention is not limited to the above-described embodiment, various design changes are possible, various modification examples are possible, and such modification examples are also within the scope of the present invention. Various features described in relation to a certain embodiment are also applicable to other embodiments. A new embodiment generated through combination also has the effects of each of the combined embodiments.

In the above-described embodiment, a case where the heater 46 is attached to the first cooling stage 33 has been described as an example. However, the present invention is not limited thereto. In a certain embodiment, the heater 46 may be attached to, for example, another part of the cold head 14 such as the first cylinder 16a, and may be thermally coupled to the first cooling stage 33 via the part. Alternatively, the heater 46 may be attached to the compressor 12 or a working gas pipe connecting the compressor 12 and the cold head 14 so as to heat the working gas supplied to the cold head 14. The heated working gas is supplied to the cold head 14, and can apply a heat load to the first cooling stage 33. In other words, the heater 46 may be thermally coupled to the first cooling stage 33 via the working gas.

In the above-described embodiment, a case where the cryocooler 10 is a GM cryocooler has been described as an example. However, the present invention is not limited thereto. In a certain embodiment, the cryocooler 10 may be another type of cryocooler, such as a Solvay cryocooler, a Stirling cryocooler, or a pulse tube cryocooler.

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

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

Claims

1. A cryocooler comprising:

a cold head including a first cooling stage and a second cooling stage cooled to a lower temperature than the first cooling stage;

a heater thermally coupled to the first cooling stage;

a first temperature sensor that measures a first temperature of the first cooling stage; and

a controller configured to perform a cool-down operation of the cold head to cool the first cooling stage from an initial temperature to a first final target temperature and to cool the second cooling stage from the initial temperature to a second final target temperature which is lower than the first final target temperature,

wherein the controller is configured to, during the cool-down operation, acquire the first temperature of the first cooling stage from the first temperature sensor, and control the heater such that the first temperature of the first cooling stage follows an intermediate target temperature which is lower than the initial temperature and higher than the first final target temperature.

2. The cryocooler according to claim 1,

wherein the controller is configured to change the intermediate target temperature during the cool-down operation.

3. The cryocooler according to claim 2, further comprising:

a second temperature sensor that measures a second temperature of the second cooling stage,

wherein the controller is configured to, during the cool-down operation, acquire the second temperature of the second cooling stage from the second temperature sensor, and set the intermediate target temperature based on the second temperature of the second cooling stage.

4. The cryocooler according to claim 3,

wherein the controller is configured to set the intermediate target temperature to the second temperature or a temperature which is higher than the second temperature.

5. The cryocooler according to claim 1,

wherein the controller is configured to compare the first temperature of the first cooling stage to the intermediate target temperature, turn on the heater when the first temperature falls below the intermediate target temperature, and turn off the heater when the first temperature exceeds the intermediate target temperature.

6. The cryocooler according to claim 1, further comprising:

a second temperature sensor that measures a second temperature of the second cooling stage,

wherein the controller is configured to, during the cool-down operation, acquire the second temperature of the second cooling stage from the second temperature sensor, and end the control of the heater in which the first temperature of the first cooling stage follows the intermediate target temperature when the second temperature of the second cooling stage falls below a predetermined temperature threshold.

7. The cryocooler according to claim 1,

wherein the controller is configured to maintain the intermediate target temperature constant during the cool-down operation.

8. A cool-down method for a cryocooler, the cool-down method comprising:

performing a cool-down operation of a cold head to cool a first cooling stage of the cold head from an initial temperature to a first final target temperature and to cool a second cooling stage of the cold head from the initial temperature to a second final target temperature which is lower than the first final target temperature; and

during the cool-down operation, applying a heat load to the first cooling stage such that a temperature of the first cooling stage follows an intermediate target temperature which is lower than the initial temperature and higher than the first final target temperature.