CRYOGENIC SYSTEM AND CONTROL METHOD FOR CRYOGENIC SYSTEM

Abstract

A cryogenic system includes a cryocooler and a controller. The cryocooler includes a first cylinder, a second cylinder including a heat absorbing portion, a first temperature sensor measuring a first measurement temperature, a second temperature sensor measuring a second measurement temperature, and a third temperature sensor measuring a third measurement temperature. The controller is configured to acquire a first measurement temperature from the first temperature sensor, a second measurement temperature from the second temperature sensor, and a third measurement temperature from the third temperature sensor, to set an upper limit temperature of the heat absorbing portion, based on the first measurement temperature, the second measurement temperature, and an axial position of the heat absorbing portion, and to control the heat source such that the third measurement temperature is equal to or lower than the upper limit temperature of the heat absorbing portion.

Description

BACKGROUND

Technical Field

Certain embodiments of the present invention relate to a cryogenic system and a control method for a cryogenic system.

Description of Related Art

In the related art, a cryocooler represented by a Gifford-McMahon (GM) cryocooler is known. The cryocooler is used to cool various cryogenic systems.

SUMMARY

According to an embodiment of the present invention, there is provided a cryogenic system including a cryocooler and a controller. The cryocooler includes a first cylinder, a second cylinder provided in series with the first cylinder in an axial direction and including a heat absorbing portion thermally connected to a heat source in an axial intermediate portion of the second cylinder, a first temperature sensor measuring a first measurement temperature in one axial end portion of the second cylinder close to the first cylinder, a second temperature sensor measuring a second measurement temperature in the other axial end portion of the second cylinder far from the first cylinder, and a third temperature sensor measuring a third measurement temperature in the heat absorbing portion. The controller is configured to acquire the first measurement temperature from the first temperature sensor, the second measurement temperature from the second temperature sensor, and the third measurement temperature from the third temperature sensor, to set an upper limit temperature of the heat absorbing portion, based on the first measurement temperature, the second measurement temperature, and an axial position of the heat absorbing portion, and to control the heat source such that the third measurement temperature is equal to or lower than the upper limit temperature of the heat absorbing portion.

According to another embodiment of the present invention, there is provided a control method for a cryogenic system. The cryogenic system includes a cryocooler including a first cylinder and a second cylinder which are provided in series in an axial direction, and the second cylinder includes a heat absorbing portion thermally connected to a heat source in an axial intermediate portion of the second cylinder. The method includes measuring a first measurement temperature in one axial end portion of the second cylinder close to the first cylinder, measuring a second measurement temperature in the other axial end portion of the second cylinder far from the first cylinder, measuring a third measurement temperature in the heat absorbing portion, setting an upper limit temperature of the heat absorbing portion, based on the first measurement temperature, the second measurement temperature, and an axial position of the heat absorbing portion, and controlling the heat source such that the third measurement temperature is equal to or lower than the upper limit temperature of the heat absorbing portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically showing a cryogenic system according to an embodiment.

FIG. 2 is a diagram schematically showing a cryocooler applicable to the cryogenic system shown in FIG. 1.

FIGS. 3A and 3B are graphs showing experimental results obtained by the present inventor according to the embodiment of the present invention.

FIGS. 4A and 4B are graphs showing experimental results obtained by the present inventor according to the embodiment.

FIG. 5 is a flowchart showing a control method for a cryogenic system according to the embodiment.

FIG. 6 is a flowchart showing an example of a control process of a refrigerant gas line shown in FIG. 5.

FIG. 7 is a diagram schematically showing a cryogenic system according to another embodiment.

DETAILED DESCRIPTION

It is desirable to efficiently cool a cryogenic system.

Any desired combination of the above-described components, and those in which the components or expressions according to the present invention are substituted from each other in methods, devices, or systems are effectively applicable as an embodiment of the present invention.

Hereinafter, embodiments according to the present invention will be described in detail with reference to the drawings. The same reference numerals will be assigned to the same or equivalent components, members, and processes in the description and the drawings, and repeated description will be appropriately omitted. A scale or a shape of each shown element is set for convenience in order to facilitate the description, and is not to be interpreted in a limited manner unless otherwise specified. The embodiments are merely examples, and do not limit the scope of the present invention at all. All features or combinations thereof which are described in the embodiments are not necessarily essential to the invention.

FIG. 1 is a diagram schematically showing a cryogenic system 100 according to an embodiment. FIG. 2 is a diagram schematically showing a cryocooler 10 applicable to the cryogenic system 100 shown in FIG. 1. An appearance of the cryocooler 10 is shown in FIG. 1, and an internal structure of the cryocooler 10 is shown in FIG. 2. As an example, the cryocooler 10 is a two-stage type Gifford-McMahon (GM) cryocooler.

In the present embodiment, the cryogenic system 100 can be used as a cryogenic temperature liquid storage device. Therefore, in addition to the cryocooler 10, for example, the cryogenic system 100 includes a vacuum chamber 110 for storing liquid helium or other cryogenic temperature liquid 102. The cryocooler 10 cools the stored cryogenic temperature liquid 102 to a cryogenic temperature equal to or lower than a liquefaction temperature thereof (in a case of the liquid helium, approximately 4 K).

The vacuum chamber 110 includes an outer tank 112 and an inner tank 114. A vacuum insulation layer 116 is formed between the outer tank 112 and the inner tank 114, and the outer tank 112 is configured to separate the vacuum insulation layer 116 from an ambient environment (for example, a room temperature atmospheric pressure environment) of the cryogenic system 100. For example, a heat insulation structure such as a multilayer insulation (MLI) may be provided in the vacuum insulation layer 116. In addition, the inner tank 114 is configured to internally accommodate the cryogenic temperature liquid 102 and to separate the cryogenic temperature liquid 102 from the vacuum insulation layer 116. For example, the outer tank 112 and the inner tank 114 are formed of a metal material such as stainless steel or other suitable high-strength materials to withstand a pressure difference between the inside and the outside.

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

The expander 14 includes a cryocooler cylinder 16, a displacer assembly 18, and a cryocooler housing 20. The cryocooler housing 20 is coupled to the cryocooler cylinder 16, thereby forming a hermetic container that accommodates the displacer assembly 18. For example, the cryocooler cylinder 16 and the cryocooler housing 20 are formed of a metal material such as stainless steel or other suitable high-strength materials.

The expander 14 is installed in the vacuum chamber 110 in a state where the cryocooler cylinder 16 is inserted into the inner tank 114 of the vacuum chamber 110 and the cryocooler housing 20 is attached to the outside of the vacuum chamber 110. As an example, the expander 14 is installed in an upper portion of the vacuum chamber 110 such that a center axis thereof coincides with a vertical direction. However, an attachment location and an attachment posture of the expander 14 are not limited thereto. For example, the expander 14 may be installed in a lower portion of the vacuum chamber 110. In addition, the expander 14 can be installed in a desired posture, and may be installed in the vacuum chamber 110 such that the center axis coincides with an oblique direction or a horizontal direction.

The cryocooler cylinder 16 includes a first cylinder 16a and a second cylinder 16b which extend in an axial direction (up-down direction in FIGS. 1 and 2). The second cylinder 16b is provided in series with the first cylinder 16a in the axial direction. 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 diameter smaller than the first cylinder 16a. The first cylinder 16a and the second cylinder 16b are coaxially disposed, and a lower end of the first cylinder 16a is strongly connected to an upper end of the second cylinder 16b.

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 diameter smaller than the first displacer 18a. The first displacer 18a and the second displacer 18b are coaxially disposed.

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

In the present specification, in order to describe a positional relationship between components of the cryocooler 10, for convenience of description, a side close to a top dead center of axial reciprocation of the 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 maximized, and the bottom dead center is a position of the displacer at which the volume of the expansion space is minimized. Since a temperature gradient in which the temperature decreases from an upper side to a lower side in the axial direction is generated during an operation of the cryocooler 10, the upper side can be called a high temperature side, and the lower side can be called 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 portion of the first displacer 18a with a wire mesh such as copper or other appropriate first regenerator materials. An upper lid portion and a 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 may be fixed to the main body by appropriate means such as fastening or welding. In this manner, the first regenerator material may be accommodated in the first displacer 18a.

Similarly, the second displacer 18b accommodates a 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 appropriate second regenerator materials. The second regenerator material may be formed in a granular shape. The upper lid portion and the lower lid portion of the second displacer 18b may be provided as separate members from the main body portion of the second displacer 18b, and the lower lid portion and the upper lid portion of the second displacer 18b may be fixed to the main body by appropriate means such as fastening or welding. In this manner, the second regenerator material may be accommodated in the second displacer 18b.

The displacer assembly 18 forms an upper chamber 30, a first expansion chamber 32, and a second expansion chamber 34 inside the cryocooler cylinder 16. In order to exchange heat with a desired object or medium to be cooled by the cryocooler 10, the expander 14 includes a first cooling stage 33 and a second cooling stage 35. The upper chamber 30 is formed between the upper lid portion of the first displacer 18a and an upper portion of the first cylinder 16a. The first expansion chamber 32 is formed between the lower lid portion of the first displacer 18a and the first cooling stage 33. The second expansion chamber 34 is formed between the lower lid portion of the second displacer 18b and the second cooling stage 35. The first cooling stage 33 is fixed to the lower portion of the first cylinder 16a to surround the first expansion chamber 32, and the second cooling stage 35 is fixed to the lower portion of the second cylinder 16b to surround the second expansion chamber 34. For example, the first cooling stage 33 and the second cooling stage 35 are formed of pure copper (for example, oxygen-free copper, tough pitch copper, or the like) or other high thermal conductive metal.

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

A first seal 38a and a second seal 38b may be provided such that a working gas flow between the first expansion chamber 32, the second expansion chamber 34, and the upper chamber 30 is guided to the first regenerator 26 and the second regenerator 28, without being guided to a clearance between the cryocooler cylinder 16 and the displacer assembly 18. The first seal 38a may be mounted on the upper lid portion of the first displacer 18a to be disposed between the first displacer 18a and the first cylinder 16a. The second seal 38b may be mounted on the upper lid portion of the second displacer 18b to be disposed between the second displacer 18b and the second cylinder 16b.

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

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

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

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

When the compressor 12 and the driving motor 42 are operated, the cryocooler 10 generates periodic volume fluctuations in the first expansion chamber 32 and the second expansion chamber 34 and pressure fluctuations of the working gas synchronized therewith, thereby forming a refrigeration cycle, and the first cooling stage 33 and the second cooling stage 35 are cooled to a desired cryogenic temperature. The first cooling stage 33 can be cooled to a first cooling temperature in a range of approximately 20 K to approximately 40 K, for example. The second cooling stage 35 can be cooled to a second cooling temperature (for example, approximately 1 K to approximately 4 K) lower than the first cooling temperature.

Incidentally, in addition to the first cooling stage 33 and the second cooling stage 35, for example, it is known that the expander 14 can absorb heat on the cryocooler cylinder 16 in the axial intermediate portion of the second cylinder 16b. This heat absorbing portion 46 is cooled to a cooling temperature, for example, based on the axial temperature distribution on the cryocooler cylinder 16 on the second cylinder 16b and the axial position of the heat absorbing portion 46, and can provide some cooling capacity at the cooling temperature. The cooling temperature of the heat absorbing portion 46 is a temperature between the first cooling temperature of the first cooling stage 33 and the second cooling temperature of the second cooling stage 35. The heat absorbing portion 46 may be in a range of ¼ to ¾, for example, at a standardized axial position (that is, a non-dimensional axial position where positions of the first cooling stage 33 and the second cooling stage 35 are each set to 0 and 1) on the second cylinder 16b.

In a general use scene of the cryocooler 10, an object to be cooled is thermally connected to and cooled by either the first cooling stage 33 or the second cooling stage 35 depending on a desired cooling temperature. Nothing is connected to the heat absorbing portion 46 on the second cylinder 16b, and cooling capacity of the heat absorbing portion 46 is not used.

When the cooling capacity of the heat absorbing portion 46 can be used in addition to the two cooling stages, this configuration can lead to more efficient cooling of the cryogenic system 100. However, input heat to the heat absorbing portion 46 may affect the cooling temperature of the cooling stage, for example, the second cooling stage 35 (large input heat to the heat absorbing portion 46 may cause a temperature rise of the second cooling stage 35). Usually, in the cryogenic system 100, in order to maintain the object to be cooled at a desired cooling temperature, it is required to operate the cryocooler 10 such that a cooling stage temperature does not exceed a predetermined limit temperature. The reason that cooling using the heat absorbing portion 46 is not used so far is as follows. There is a possibility of risks in that the temperature of the cooling stage rises due to the input heat to the heat absorbing portion 46.

The present inventor has found the followings. As will be described in detail below, the temperature rise of the second cooling stage 35 can be prevented or minimized by maintaining the temperature of the heat absorbing portion 46 at a certain upper limit temperature or lower. From this viewpoint, in the present embodiment, the cryogenic system 100 is configured to control a heat source (that is, the object to be cooled) to optimize the input heat to the heat absorbing portion 46 from the heat source. The optimization of the input heat is realized by maintaining the temperature of the heat absorbing portion 46 to be equal to or lower than the upper limit temperature.

FIGS. 3A and 3B are graphs showing experimental results obtained by the present inventor according to the embodiment. In this experiment, a heater for simulating the input heat to the heat absorbing portion 46 is installed in the heat absorbing portion 46. A position of the heater, in other words, a position of the heat absorbing portion 46 is approximately 0.54 on a slightly lower temperature side of an axial center, specifically, at the standardized axial position on the second cylinder 16b (where the first cooling stage 33 is set to 0 and the second cooling stage 35 is set to 1 as described above). In addition, a heater is also installed to apply a heat load (for example, a rated heat load of 1.5 W) to the second cooling stage 35. The cryocooler 10 is operated to maintain the first cooling stage 33 at a constant temperature (for example, 43 K) while the rated heat load is applied to the second cooling stage 35.

FIG. 3A shows a relationship between a standardized heater output representing the input heat to the heat absorbing portion 46 and a temperature rise of the second cooling stage 35. The temperature rise of the second cooling stage 35 represents the temperature rise with respect to a target cooling temperature (for example, 4 K) of the second cooling stage 35. FIG. 3B shows a relationship between a standardized axial position and the temperature on the second cylinder 16b, that is, how an axial temperature distribution on the second cylinder 16b is changed depending on the standardized heater output. In FIGS. 3A and 3B, temperature measurement results when the standardized heater output is zero (that is, no input heat to the heat absorbing portion 46) are shown by a circle symbol, and temperature measurement results when the standardized heater output is 0.085, 0.31, and 0.45 are each shown by square, triangle, and rhombus symbols.

Referring to FIG. 3A, the following can be understood. The temperature rise of the second cooling stage 35 is 0 K (circle symbol) when the standardized heater output is zero, and the temperature rise of the second cooling stage 35 increases as the standardized heater output increases to 0.085, 0.31, and 0.45 (square, triangle, and rhombus symbols). Specifically, the standardized heater output is 0.085, the temperature rise is approximately 0.04 K (square symbol), the standardized heater output is 0.31, the temperature rise is approximately 0.09 K (triangle symbol), the standardized heater output is 0.45, and the temperature rise is approximately 0.17 K (rhombus symbol).

Based on the knowledge and the experience of the present inventor, in some applications of the cryocooler 10, for example, such as liquid helium cooling in which the target cooling temperature of the second cooling stage 35 is set to approximately 4 K, in many cases, it is desirable that an allowable value for the temperature rise of the second cooling stage 35 is set to approximately 0.1 K, and it is required that the allowable value is set to approximately 0.2 K at most.

It is understood from FIG. 3A that a standardized heater output threshold Th1 indicated by a broken line in FIG. 3A represents one important boundary. When the input heat to the heat absorbing portion 46 is smaller than the threshold Th1, the temperature rise of the second cooling stage 35 is suppressed to a relatively small value within approximately 0.1 K or smaller. On the other hand, when the input heat to the heat absorbing portion 46 exceeds the threshold Th1, the temperature rise of the second cooling stage 35 is changed in phase, increases proportionally in conjunction with the increase in the standardized heater output, and easily exceeds 0.2 K.

Referring to FIG. 3B, it is possible to identify a relationship between the standardized heater output and the temperature of the heat absorbing portion 46 (temperature at a standardized axial position of approximately 0.54). The following can be understood. When the standardized heater output is zero, the temperature of the heat absorbing portion 46 is approximately 8 K (circle symbol), and the temperature of the heat absorbing portion 46 increases as the standardized heater output increases to 0.085, 0.31, and 0.45 (square, triangle, and rhombus symbols). More specifically, the standardized heater output is 0.085 and the temperature of the heat absorbing portion 46 is approximately 13 K (square symbol), the standardized heater output is 0.31 and the temperature of the heat absorbing portion 46 is approximately 20 K (triangle symbol), and the standardized heater output is 0.45 and the temperature of the heat absorbing portion 46 is approximately 24 K (rhombus symbol).

A broken line L1 shown in FIG. 3B represents a straight line connecting a measurement temperature of the first cooling stage 33 and a measurement temperature of the second cooling stage 35. Referring to FIGS. 3A and 3B, the following can be importantly understood. The temperature of the heat absorbing portion 46 at the standardized heater output threshold Th1 is located on the straight line L1 or in the vicinity thereof (between the square symbol and the rhombus symbol in FIG. 3B).

For this reason, when the input heat to the heat absorbing portion 46 is large (that is, exceeds the threshold Th1), and as a result, the temperature of the heat absorbing portion 46 exceeds an upper limit temperature of the straight line L1 or in the vicinity thereof, the temperature may significantly rise in the second cooling stage 35 due to the input heat to the heat absorbing portion 46. In addition, when the input heat to the heat absorbing portion 46 has such a magnitude that the temperature of the heat absorbing portion 46 does not exceed the upper limit temperature, the temperature rise in the second cooling stage 35 which is caused by the input heat to the heat absorbing portion 46 does not substantially occur. However, the temperature rise may be within an allowable range (for example, within 0.2 K or preferably within 0.1 K).

FIGS. 4A and 4B are graphs showing experimental results obtained by the present inventor according to the embodiment. The experimental results shown in FIGS. 4A and 4B are acquired by changing the axial position of the heat absorbing portion 46 from the experimental results in FIGS. 3A and 3B, and other experimental conditions are common. In FIGS. 4A and 4B, compared to FIGS. 3A and 3B, the position of the heat absorbing portion 46 is approximately 0.35 on the high temperature side, specifically, at the standardized axial position on the second cylinder 16b.

As in FIG. 3A, FIG. 4A shows the relationship between the standardized heater output representing the input heat to the heat absorbing portion 46 and the temperature rise of the second cooling stage 35. As in FIG. 3B, FIG. 4B shows the relationship between the standardized axial position and the temperature on the second cylinder 16b. In FIGS. 4A and 4B, the temperature measurement results when the standardized heater output is zero (that is, no input heat to the heat absorbing portion 46) are shown by circle symbols, and furthermore, the temperature measurement results when the standardized heater output is 0.31, 0.55, and 0.72 are each shown by square, triangle, and rhombus symbols.

Referring to FIG. 4A, the following can be understood. When the standardized heater output is zero, there is no temperature rise of the second cooling stage 35 (circle symbol), and the temperature rise of the second cooling stage 35 increases as the standardized heater output increases. Specifically, the standardized heater output is 0.31and the temperature rise is approximately 0.02 K (square symbol), the standardized heater output is 0.55 and the temperature rise is approximately 0.07 K (triangle symbol), and the standardized heater output is 0.72 and the temperature rise is approximately 0.09 K (rhombus symbol). In FIG. 4A, the heat absorbing portion 46 is located on the high temperature side compared to FIG. 3A. Therefore, the temperature rise of the second cooling stage 35 which is caused by the increase in the input heat to the heat absorbing portion 46 decreases.

In FIG. 4A as well, a standardized heater output threshold Th2 indicated by a broken line is an important boundary. When the input heat to the heat absorbing portion 46 is smaller than the threshold Th2, the temperature rise of the second cooling stage 35 is suppressed to a very small value of approximately 0.02 K or smaller. On the other hand, when the input heat to the heat absorbing portion 46 exceeds the threshold Th2, the temperature rise of the second cooling stage 35 is changed in phase, and proportionally increases in accordance with the standardized heater output.

Referring to FIG. 4B, the relationship between the standardized heater output and the temperature of the heat absorbing portion 46 (temperature at the standardized axial position of approximately 0.35) can be identified. When the standardized heater output is zero, the temperature of the heat absorbing portion 46 is approximately 16 K (circle symbol), and the temperature of the heat absorbing portion 46 increases as the standardized heater output increases. Specifically, the standardized heater output is 0.31 and the temperature of the heat absorbing portion 46 is approximately 27 K (square symbol), the standardized heater output is 0.55 and the temperature of the heat absorbing portion 46 is approximately 33 K (triangle symbol), and the standardized heater output is 0.72 and the temperature of the heat absorbing portion 46 is approximately 38 K (rhombus symbol).

A broken line L2 shown in FIG. 4B represents a straight line connecting the measurement temperature of the first cooling stage 33 and the measurement temperature of the second cooling stage 35. Referring to FIGS. 4A and 4B, it is considered that the temperature of the heat absorbing portion 46 at the standardized heater output threshold Th2 is located on the straight line L2 or in the vicinity thereof (between the square symbol and the triangle symbol in FIG. 4B).

Therefore, when the input heat to the heat absorbing portion 46 is large (that is, exceeds the threshold Th2), and as a result, the temperature of the heat absorbing portion 46 exceeds the upper limit temperature of the straight line L2 or in the vicinity thereof, the temperature may significantly rise in the second cooling stage 35 due to the input heat to the heat absorbing portion 46. In addition, when the input heat to the heat absorbing portion 46 has such a magnitude that the temperature of the heat absorbing portion 46 does not exceed the upper limit temperature, the temperature rise in the second cooling stage 35 which is caused by the input heat to the heat absorbing portion 46 does not substantially occur. However, it is considered that the temperature rise may be within the allowable range.

In addition to the experimental results shown in FIGS. 3A to 4B, the present inventor performs the same experiment by changing the axial position of the heat absorbing portion 46 and the temperature of the first cooling stage 33 in various ways. As a result, the present inventor confirms that the same action as that in the above-described result is observed in both the relationship between the input heat to the heat absorbing portion 46 and the temperature rise of the second cooling stage 35 and the relationship between the standardized axial position and the temperature on the second cylinder 16b.

Therefore, the upper limit temperature of the heat absorbing portion 46 for preventing or minimizing the temperature rise of the second cooling stage 35 which is caused by the input heat to the heat absorbing portion 46 can be set, based on the measurement temperatures in both ends of the second cylinder 16b and the axial position of the heat absorbing portion 46.

In one exemplary method, the upper limit temperature of the heat absorbing portion 46 may be set, based on the reference temperature distribution of the second cylinder 16b and the axial position of the heat absorbing portion 46. Here, the reference temperature distribution of the second cylinder 16b may be a temperature distribution having a first measurement temperature in one axial end portion (for example, the first cooling stage 33) of the second cylinder 16b, and having a second measurement temperature in the other axial end portion (for example, the second cooling stage 35) of the second cylinder 16b and linearly changed depending on the axial position in the second cylinder 16b.

An exemplary configuration for optimizing the input heat to the heat absorbing portion 46 will be described below. Referring to FIG. 1 again, the cryocooler 10 includes a first temperature sensor 51, a second temperature sensor 52, a third temperature sensor 53, and a controller 60.

The first temperature sensor 51 measures a first measurement temperature T1 in one axial end portion of the second cylinder 16b close to the first cylinder 16a. The first temperature sensor 51 may be provided in the first cooling stage 33, and may measure the first measurement temperature T1 in the first cooling stage 33. The second temperature sensor 52 measures a second measurement temperature T2 in the other axial end portion of the second cylinder 16b far from the first cylinder 16a. The second temperature sensor 52 may be provided in the second cooling stage 35, and may measure the second measurement temperature T2 in the second cooling stage 35. The third temperature sensor 53 is provided in the heat absorbing portion 46, and measures a third measurement temperature T3 in the heat absorbing portion 46.

The controller 60 is communicably connected to the temperature sensors to respectively receive the first measurement temperature T1, the second measurement temperature T2, and the third measurement temperature T3 from the first temperature sensor 51, the second temperature sensor 52, and the third temperature sensor 53.

an internal configurations of the controller 60 is realized by elements or circuits such as a central processing unit (CPU) and a memory of a computer as a hardware configuration, and is realized by a computer program or the like as a software configuration. However, in the drawings, the internal configuration is appropriately shown as functional blocks realized through the cooperation therebetween. Those skilled in the art may understand that these functional blocks can be realized in various forms including the combination of hardware and software.

The cryogenic system 100 includes a refrigerant gas line 118 cooled by the cryocooler 10. A refrigerant gas of the refrigerant gas line 118 is condensed by cooling, and the cryogenic temperature liquid 102 is generated. The refrigerant gas line 118 includes a supply line 120 for supplying the cryogenic temperature liquid 102 to the inner tank 114 of the vacuum chamber 110 and a return line 122 for the cryogenic temperature liquid 102 (that is, the refrigerant gas) vaporized in the inner tank 114. The supply line 120 and the return line 122 may be rigid or flexible pipes through which the refrigerant gas flows.

The supply line 120 includes a first heat exchanger 124, a second heat exchanger 126, and a recondensing unit 128, and the refrigerant gas flows through the first heat exchanger 124, the second heat exchanger 126, and the recondensing unit 128 in this order in the supply line 120.

The first heat exchanger 124 is provided outside the first cooling stage 33 inside the inner tank 114, and is configured to pre-cool the refrigerant gas to the first cooling temperature through heat exchange between the first cooling stage 33 and the refrigerant gas. The second heat exchanger 126 is provided outside the second cylinder 16b inside the inner tank 114, and is configured to further pre-cool the refrigerant gas to the cooling temperature of the heat absorbing portion 46 through heat exchange between the heat absorbing portion 46 of the second cylinder 16b and the refrigerant gas.

The recondensing unit 128 is an outlet of the supply line 120 to the inner tank 114. The recondensing unit 128 is provided outside the second cooling stage 35 inside the inner tank 114, cools the refrigerant gas to the second cooling temperature through heat exchange between the second cooling stage 35 and the refrigerant gas, and is configured to recondense the refrigerant gas into the cryogenic temperature liquid 102. The recondensed cryogenic temperature liquid 102 is stored in the inner tank 114 as described above. As shown in the drawing, the recondensing unit 128 may be integrated with the second cooling stage 35, or may have a fin-shaped protrusion or an uneven portion to increase a surface area in contact with the refrigerant gas or the cryogenic temperature liquid 102. For example, as in the second cooling stage 35, the recondensing unit 128 is formed of pure copper (for example, oxygen-free copper, tough pitch copper, or the like) or other high thermal conductive metal.

In addition, the refrigerant gas line 118 includes a flow rate regulator 130 configured to regulate a flow rate of the refrigerant gas in the refrigerant gas line 118. In the shown example, the flow rate regulator 130 is provided on the refrigerant gas line 118 to connect the return line 122 to the supply line 120. The flow rate regulator 130 receives the refrigerant gas from the return line 122, regulates the flow rate of the refrigerant gas, and feeds the refrigerant gas to the supply line 120 at the regulated flow rate. The flow rate regulator 130 may be a circulation flow generator, for example, such as a pump and a compressor, or a mechanism that regulates the gas flow rate, for example, such as a flow rate control valve and a variable orifice.

As shown in the drawing, the flow rate regulator 130 may be disposed outside the vacuum chamber 110, or may be disposed inside the vacuum chamber 110. In addition, the flow rate regulator 130 may be provided at any place on the refrigerant gas line 118, may be provided on the supply line 120, or may be provided on the return line

As will be described later, under the control of the controller 60, the flow rate regulator 130 can regulate the flow rate of the refrigerant gas circulating through the refrigerant gas line 118. The input heat from the refrigerant gas line 118 to the heat absorbing portion 46 is controlled by the flow rate regulation.

The refrigerant gas line 118 serves as a heat source for the cryocooler 10. The heat source includes a first portion (for example, the first heat exchanger 124), a second portion (for example, the recondensing unit 128), and a third portion (for example, the second heat exchanger 126) connecting the first portion and the second portion. One axial end portion (for example, the first cooling stage 33) of the second cylinder 16b is thermally connected to the first portion of the heat source, and the other axial end portion (for example, the second cooling stage 35) of the second cylinder 16b is thermally connected to the second portion of the heat source. The heat absorbing portion 46 is thermally connected to the third portion of the heat source. The refrigerant gas line 118 passes through the first portion, the third portion, and the second portion of the heat source in this order.

FIG. 5 is a flowchart showing a control method for the cryogenic system 100 according to the embodiment. The present method includes acquiring the first measurement temperature T1 from the first temperature sensor 51, the second measurement temperature T2 from the second temperature sensor 52, and the third measurement temperature T3 from the third temperature sensor 53 (S10), setting the upper limit temperature of the heat absorbing portion 46, based on the first measurement temperature T1, the second measurement temperature T2, and the axial position of the heat absorbing portion 46 (S20), and controlling the heat source, that is, the refrigerant gas line 118, such that the third measurement temperature T3 is equal to or lower than the upper limit temperature of the heat absorbing portion 46 (S30).

In S10, the first measurement temperature T1 is measured by the first temperature sensor 51 in one axial end portion (for example, the first cooling stage 33) of the second cylinder 16b close to the first cylinder 16a. A signal indicating the first measurement temperature T1 is input from the first temperature sensor 51 to the controller 60. In addition, the second measurement temperature T2 is measured by the second temperature sensor 52 in the other axial end portion of the second cylinder 16b far from the first cylinder 16a. A signal indicating the second measurement temperature T2 is input from the second temperature sensor 52 to the controller 60. The third measurement temperature T3 is measured by the third temperature sensor 53 in the heat absorbing portion 46. A signal indicating the third measurement temperature T3 is input from the third temperature sensor 53 to the controller 60.

In S20, first, the controller 60 determines the reference temperature distribution of the second cylinder 16b, based on the first measurement temperature T1 and the second measurement temperature T2. The reference temperature distribution is a temperature distribution having the first measurement temperature T1 in one axial end portion of the second cylinder 16b, and having the second measurement temperature T2 in the other axial end portion of the second cylinder 16b and which is linearly changed depending on the axial position in the second cylinder 16b. The reference temperature distribution is a straight line (Y=(T2−T1)X+T1) passing through two points (0, T1) and (1, T2) on an XY plane when the standardized axial position on the second cylinder 16b is set as an X-axis and the temperature is set as a Y-axis.

The controller 60 sets the upper limit temperature of the heat absorbing portion 46, based on the reference temperature distribution and the axial position of the heat absorbing portion 46. Since the axial standardized position of the heat absorbing portion 46 is substituted into the above-described expression representing the reference temperature distribution, a candidate value of the upper limit temperature of the heat absorbing portion 46 is obtained. The controller 60 may adopt the candidate value as the upper limit temperature.

Alternatively, when it is important to reliably prevent the temperature rise of the second cooling stage 35 which is caused by the input heat to the heat absorbing portion 46, the controller 60 may adopt a value somewhat smaller than the candidate value (for example, a value selected from a range of 70% to 100%, or 80% to 100%, or 90% to 100% of the candidate value) as the upper limit temperature of the heat absorbing portion 46.

Alternatively, when the temperature rise of the second cooling stage 35 which is caused by the input heat to the heat absorbing portion 46 is allowed to some extent, the controller 60 may adopt a value somewhat greater than the candidate value (for example, a value selected from a range of 100% to 130%, or 100% to 120%, or 100% to 110% of the candidate value) as the upper limit temperature of the heat absorbing portion 46.

In S30, the controller 60 controls the flow rate of the refrigerant gas of the refrigerant gas line 118 such that the third measurement temperature T3 is equal to or lower than the upper limit temperature of the heat absorbing portion 46. An example of the control will be described later with reference to FIG. 6.

The controller 60 may update the upper limit temperature of the heat absorbing portion 46 in accordance with the first measurement temperature T1 and the second measurement temperature T2 by periodically repeating a process shown in FIG. 5. In addition, when the cryocooler 10 is operated to maintain a cooling temperature of the first cooling stage 33 (and/or the second cooling stage 35) at a certain target temperature, the reference temperature distribution is substantially unchanged unless the target temperature is changed. In this case, the upper limit temperature of the heat absorbing portion 46 which is set once may be continuously used thereafter.

FIG. 6 is a flowchart showing an example of a control process (S30) of the refrigerant gas line 118 shown in FIG. 5. In this process, the controller 60 controls the flow rate regulator 130 of the refrigerant gas line 118, based on a comparison between the third measurement temperature T3 and the upper limit temperature of the heat absorbing portion 46, thereby controlling the flow rate of the refrigerant gas in the refrigerant gas line 118. This process is repeatedly performed by the controller 60 at a predetermined cycle during an operation of the cryocooler 10.

Therefore, as shown in FIG. 6, the controller 60 first compares the third measurement temperature T3 with an upper limit temperature T_lim of the heat absorbing portion 46 (S31). The third measurement temperature T3 is measured by the third temperature sensor 53 as described above (S10 in FIG. 5). The upper limit temperature of the heat absorbing portion 46 is set, based on the measurement temperatures in both ends of the second cylinder 16b and the axial position of the heat absorbing portion 46 (S20 in FIG. 5).

The controller 60 compares the third measurement temperature T3 with the upper limit temperature T_lim of the heat absorbing portion 46, and outputs a magnitude relationship between both temperatures as a comparison result. That is, the comparison result obtained by the controller 60 represents any one of the following three states, (i) where the third measurement temperature T3 is higher than the upper limit temperature T_lim, (ii) where the third measurement temperature T3 is lower than the upper limit temperature T_lim, and (iii) where the third measurement temperature T3 is equal to the upper limit temperature T_lim.

The controller 60 controls the flow rate regulator 130, based on the comparison result. Specifically, (i) when the third measurement temperature T3 is higher than the upper limit temperature T_lim, the controller 60 controls the flow rate regulator 130 to decrease the flow rate of the refrigerant gas (S32). In this manner, the input heat from the second heat exchanger 126 to the heat absorbing portion 46 can be reduced, and the third measurement temperature T3 can be lowered. (ii) When the third measurement temperature T3 is lower than the upper limit temperature T_lim, the controller 60 controls the flow rate regulator 130 to increase the flow rate of the refrigerant gas (S33). In this manner, the input heat from the second heat exchanger 126 to the heat absorbing portion 46 can be increased, and the refrigerant gas line 118 can be more efficiently cooled. (iii) When the third measurement temperature T3 is equal to the upper limit temperature T_lim, it is not necessary to increase or decrease the flow rate of the refrigerant gas. Therefore, the controller 60 controls the flow rate regulator 130 to maintain a current flow rate of the refrigerant gas. The case of (iii) may be included in either (i) or (ii).

As described above, according to the embodiment, the refrigerant gas line 118 can be cooled by using cooling capacity of the heat absorbing portion 46 on the second cylinder 16b in addition to the first cooling stage 33 and the second cooling stage 35. The cryogenic system 100 can be more efficiently cooled, compared to a typical cooling configuration that does not use the heat absorbing portion 46.

In addition, in the embodiment, the upper limit temperature T_lim of the heat absorbing portion 46 is set, based on the first measurement temperature T1, the second measurement temperature T2, and the axial position of the heat absorbing portion 46, and the heat source (for example, the refrigerant gas line 118) for the cryocooler 10 in the cryogenic system 100 is controlled such that the third measurement temperature T3 is equal to or lower than the upper limit temperature T_lim. In this way, the temperature of the heat absorbing portion 46 is maintained at the upper limit temperature T_lim or lower. In this manner, the temperature rise of the second cooling stage 35 can be prevented or minimized.

Furthermore, in the embodiment, the upper limit temperature T_lim is set, based on the above-described linear reference temperature distribution and the axial position of the heat absorbing portion 46. In this case, even when the temperatures of the first cooling stage 33 and the second cooling stage 35 are changed depending on various operation states of the cryocooler 10, or even when the heat absorbing portion 46 is provided at various axial positions, the upper limit temperature T_lim can be clearly and easily determined.

FIG. 7 is a diagram schematically showing the cryogenic system 100 according to another embodiment. The cryogenic system 100 shown in FIG. 7 is different from the cryogenic system 100 shown in FIG. 1 in the object to be cooled, and the remaining configurations are generally common. Hereinafter, different configurations will be mainly described, and common configurations will be briefly described or omitted.

In the above-described embodiment, a case where the cryogenic system 100 is the storage device for the cryogenic temperature liquid 102 has been described as an example. Meanwhile, other configurations can also be adopted. For example, the cryogenic system 100 may be applied to superconductive equipment, and the cryocooler 10 may be used to cool a superconducting coil 150 disposed inside the vacuum chamber 110 and the current lead 152 for supplying power to the superconducting coil 150. The superconducting coil 150 is cooled by the second cooling stage 35, and the current lead 152 is cooled by the first cooling stage 33, the heat absorbing portion 46, and the second cooling stage 35.

The current lead 152 electrically connects a power supply 154 disposed outside the vacuum chamber 110 to the superconducting coil 150, and serves as a current path from the power supply 154 to the superconducting coil 150. Therefore, the current lead 152 can generate heat when energized. Accordingly, the current lead 152 serves as the heat source for the cryocooler 10. The current lead 152 includes a first portion 152a, a second portion 152b, and a third portion 152c connecting the first portion 152a and the second portion 152b.

One axial end portion (for example, the first cooling stage 33) of the second cylinder 16b is thermally connected to the first portion 152a of the current lead 152, and the other axial end portion (for example, the second cooling stage 35) of the second cylinder 16b is thermally connected to the second portion 152b of the current lead 152. The heat absorbing portion 46 is thermally connected to the third portion 152c of the current lead 152.

As an example of thermal connection, the first cooling stage 33 may be connected to the first portion 152a of the current lead 152 by a first heat transfer member 156, and the second cooling stage 35 may be connected to the second portion 152b of the current lead 152 by a second heat transfer member 158. In addition, the second cooling stage 35 may be connected to the superconducting coil 150 by the second heat transfer member 158. The heat absorbing portion 46 of the second cylinder 16b may be connected to the third portion 152c of the current lead 152 by a heat bridge 160.

The controller 60 is configured to control the current of the current lead 152 such that the third measurement temperature T3 is equal to or lower than the upper limit temperature. The controller 60 may control the power supply 154, based on a comparison between the third measurement temperature T3 and the upper limit temperature of the heat absorbing portion 46, and in this manner, may control the current of the current lead 152.

As an example, (i) when the third measurement temperature T3 is higher than the upper limit temperature T_lim, the controller 60 controls the power supply 154 to decrease the current of the current lead 152. In this manner, the input heat from the second heat exchanger 126 to the heat absorbing portion 46 can be reduced, and the third measurement temperature T3 can be lowered. (ii) When the third measurement temperature T3 is lower than the upper limit temperature T_lim, the controller 60 controls the power supply 154 to increase the current of the current lead 152. In this manner, the input heat from the second heat exchanger 126 to the heat absorbing portion 46 can be increased, and the current lead 152 can be more efficiently cooled. (iii) When the third measurement temperature T3 is equal to the upper limit temperature T_lim, it is not necessary to increase or decrease the current of the current lead 152. Therefore, the controller 60 controls the power supply 154 to maintain the current at present.

In this case, the current lead 152 can be cooled by using the cooling capacity of the heat absorbing portion 46 on the second cylinder 16b in addition to the first cooling stage 33 and the second cooling stage 35. The cryogenic system 100 can be more efficiently cooled, compared to a typical cooling configuration that does not use the heat absorbing portion 46. Since the temperature of the heat absorbing portion 46 is maintained at the upper limit temperature T_lim or lower, the temperature rise of the second cooling stage 35 can be prevented or minimized.

Hitherto, the present invention has been described based on the embodiments. The present invention is not limited to the above-described embodiments. It may be understood by those skilled in the art that various design changes can be made, various modification examples can be adopted, and the modification examples also fall within the scope of the present invention. Various features described with reference to a certain embodiment are also applicable to other embodiments. A new embodiment acquired from a combination of the embodiments compatibly achieves each advantageous effect of the combined embodiments.

In the above-described embodiment, one temperature sensor (third temperature sensor 53) is provided to measure the temperature of the heat absorbing portion 46. Meanwhile, other configurations can also be adopted. In a certain embodiment, the temperature of the heat absorbing portion 46 may be measured at a plurality of temperature measurement positions. Therefore, a plurality of temperature sensors (for example, two third temperature sensors 53) may be provided in the heat absorbing portion 46. The temperature sensors are provided at positions different from each other in the axial direction on the second cylinder 16b. As an example, one third temperature sensor 53 is disposed on the high temperature side with respect to the other third temperature sensor 53.

In this case, the controller 60 may set the upper limit temperature, based on the measurement temperatures in both ends of the second cylinder 16b for each temperature measurement position (temperature sensor). The controller 60 may control the heat source such that any measurement temperature of the plurality of measurement temperatures is equal to or lower than the corresponding upper limit temperature.

In the above-described embodiment, a case where the cryocooler 10 is the two-stage type GM cryocooler has been described as an example. Meanwhile, other configurations can also be adopted. For example, the cryocooler 10 may be a single-stage type GM cryocooler. Alternatively, for example, the cryocooler 10 may be another type of the cryocoolers such as a Solvay cryocooler, a Stirling cryocooler, and a pulse tube cryocooler.

The present invention has been described by using specific terms and phrases, based on the embodiments. However, the embodiments show only one aspect of principles and applications of the present invention. The embodiments allow many modification examples or disposition changes within the scope not departing from the idea of the present invention defined in the appended claims.

The present invention can be used in a field of a cryogenic system and a control method for a cryogenic system.

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 cryogenic system comprising: a cryocooler including a first cylinder,a second cylinder provided in series with the first cylinder in an axial direction and including a heat absorbing portion thermally connected to a heat source in an axial intermediate portion of the second cylinder,a first temperature sensor measuring a first measurement temperature in one axial end portion of the second cylinder close to the first cylinder,a second temperature sensor measuring a second measurement temperature in the other axial end portion of the second cylinder far from the first cylinder, anda third temperature sensor measuring a third measurement temperature in the heat absorbing portion; anda controller configured to: acquire the first measurement temperature from the first temperature sensor, the second measurement temperature from the second temperature sensor, and the third measurement temperature from the third temperature sensor,set an upper limit temperature of the heat absorbing portion, based on the first measurement temperature, the second measurement temperature, and an axial position of the heat absorbing portion, andcontrol the heat source such that the third measurement temperature is equal to or lower than the upper limit temperature of the heat absorbing portion.

2. The cryogenic system according to claim 1, wherein the controller is configured to: determine a reference temperature distribution of the second cylinder having the first measurement temperature in the one axial end portion of the second cylinder and having the second measurement temperature in the other axial end portion of the second cylinder and linearly changed depending on an axial position in the second cylinder, andset the upper limit temperature of the heat absorbing portion, based on the reference temperature distribution and the axial position of the heat absorbing portion.

3. The cryogenic system according to claim 1, wherein the one axial end portion of the second cylinder is thermally connected to a first portion of the heat source, the other axial end portion of the second cylinder is thermally connected to a second portion of the heat source, andthe heat absorbing portion is thermally connected to a third portion of the heat source connecting the first portion and the second portion.

4. The cryogenic system according to claim 3, wherein the heat source includes a refrigerant gas line that passes through the first portion, the third portion, and the second portion, in this order, andthe controller is configured to control a refrigerant gas flow rate of the refrigerant gas line such that the third measurement temperature is equal to or lower than the upper limit temperature.

5. The cryogenic system according to claim 3, wherein the heat source includes a current lead that passes through the first portion, the third portion, and the second portion in this order, andthe controller is configured to control a current of the current lead such that the third measurement temperature is equal to or lower than the upper limit temperature.

6. A control method for a cryogenic system, the cryogenic system including a cryocooler including a first cylinder and a second cylinder which are provided in series in an axial direction, the second cylinder including a heat absorbing portion thermally connected to a heat source in an axial intermediate portion of the second cylinder, the method comprising: measuring a first measurement temperature in one axial end portion of the second cylinder close to the first cylinder;measuring a second measurement temperature in the other axial end portion of the second cylinder far from the first cylinder;measuring a third measurement temperature in the heat absorbing portion;setting an upper limit temperature of the heat absorbing portion, based on the first measurement temperature, the second measurement temperature, and an axial position of the heat absorbing portion; andcontrolling the heat source such that the third measurement temperature is equal to or lower than the upper limit temperature of the heat absorbing portion.