Cryogenic system and method for controlling the same

Abstract

To efficiently cool a cryogenic system.SOLUTION: A cryogenic system 100 comprises a cryogenic refrigerator 10 and a controller 60. The cryogenic refrigerator 10 comprises a first cylinder 16a, a second cylinder 16b comprising a heat absorbing part 46 thermally connected to a heat source in an axial intermediate part, a first temperature sensor 51 for measuring a temperature in one axial end part of the second cylinder 16b, a second temperature sensor 52 for measuring a temperature in the other axial end part of the second cylinder 16b, and a third temperature sensor 53 for measuring a temperature in the heat absorbing part 46. The controller 60 acquires a first measurement temperature, a second measurement temperature, and a third measurement temperature from the first temperature sensor 51, the second temperature sensor 52, and the third temperature sensor 53 respectively, sets an upper limit temperature of the heat absorbing part 46 on the basis of the first measurement temperature, the second measurement temperature, and the axial position of the heat absorbing part 46, and controls the heat source so that the third measurement temperature becomes equal to or lower than the upper limit temperature of the heat absorbing part 46.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a cryogenic system and a method for controlling a cryogenic system. [Background technology]

[0002] Cryogenic refrigerators, such as the Gifford-McMahon (GM) refrigerator, are well known and are used to cool various cryogenic systems. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-116379 Summary of the Invention [Problem to be solved by the invention]

[0004] One exemplary objective of certain aspects of the present invention is to efficiently cool a cryogenic system. [Means for solving the problem]

[0005] According to one embodiment of the present invention, a cryogenic system includes a cryogenic refrigerator and a controller. The cryogenic refrigerator includes a first cylinder, a second cylinder axially connected in series with the first cylinder and including a heat absorption portion at an axially intermediate portion of the second cylinder thermally connected to a heat source, a first temperature sensor for measuring a first measured temperature at one axial end of the second cylinder closer to the first cylinder, a second temperature sensor for measuring a second measured temperature at the other axial end of the second cylinder farther from the first cylinder, and a third temperature sensor for measuring a third measured temperature at the heat absorption portion. The controller is configured to acquire the first measured temperature from the first temperature sensor, the second measured temperature from the second temperature sensor, and the third measured temperature from the third temperature sensor, set an upper limit temperature of the heat absorption portion based on the first measured temperature, the second measured temperature, and the axial position of the heat absorption portion, and control the heat source so that the third measured temperature is equal to or less than the upper limit temperature of the heat absorption portion.

[0006] According to one aspect of the present invention, there is provided a method for controlling a cryogenic system. The cryogenic system includes a cryogenic refrigerator having a first cylinder and a second cylinder arranged in series in an axial direction, the second cylinder having a heat absorption section at an axially intermediate portion thereof that is thermally connected to a heat source. The method includes measuring a first measured temperature at one axial end of the second cylinder that is closer to the first cylinder, measuring a second measured temperature at the other axial end of the second cylinder that is farther from the first cylinder, measuring a third measured temperature at the heat absorption section, setting an upper limit temperature of the heat absorption section based on the first measured temperature, the second measured temperature, and the axial position of the heat absorption section, and controlling the heat source so that the third measured temperature is equal to or less than the upper limit temperature of the heat absorption section.

[0007] Any combination of the above components or mutual substitution of the components or expressions of the present invention between methods, devices, systems, etc. are also valid aspects of the present invention. [Effects of the Invention]

[0008] According to the present invention, it is possible to efficiently cool a cryogenic system. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a diagram illustrating a schematic view of a cryogenic system according to an embodiment. [Figure 2] FIG. 2 is a diagram schematically illustrating a cryogenic refrigerator that can be applied to the cryogenic system shown in FIG. [Figure 3] 3(a) and 3(b) are graphs showing the results of experiments conducted by the inventors according to the embodiment. [Figure 4] 4(a) and 4(b) are graphs showing the results of experiments conducted by the inventors according to the embodiment. [Figure 5] 3 is a flowchart illustrating a method for controlling a cryogenic system according to an embodiment. [Figure 6] 6 is a flowchart showing an example of a control process (S30) of the refrigerant gas line shown in FIG. [Figure 7] FIG. 10 is a schematic diagram of a cryogenic system according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the description and drawings, identical or equivalent components, parts, and processes are designated by the same reference numerals, and redundant explanations will be omitted as appropriate. The scale and shape of each part shown in the drawings are set for convenience to facilitate explanation, and should not be interpreted as limiting unless otherwise specified. The embodiments are merely examples and do not limit the scope of the present invention in any way. All features and combinations thereof described in the embodiments are not necessarily essential to the invention.

[0011] Fig. 1 is a diagram that schematically illustrates a cryogenic system 100 according to an embodiment. Fig. 2 is a diagram that schematically illustrates a cryogenic refrigerator 10 that can be applied to the cryogenic system 100 shown in Fig. 1. The external appearance of the cryogenic refrigerator 10 is shown in Fig. 1, and the internal structure of the cryogenic refrigerator 10 is shown in Fig. 2. The cryogenic refrigerator 10 is, for example, a two-stage Gifford-McMahon (GM) refrigerator.

[0012] In this embodiment, the cryogenic system 100 can be used as a cryogenic liquid storage device. Thus, in addition to the cryogenic refrigerator 10, the cryogenic system 100 includes a vacuum vessel 110 for storing, for example, liquid helium or other cryogenic liquid 102. The cryogenic refrigerator 10 cools the stored cryogenic liquid 102 to an extremely low temperature below its liquefaction temperature (approximately 4 K in the case of liquid helium).

[0013] The vacuum vessel 110 includes an outer vessel 112 and an inner vessel 114. A vacuum insulation layer 116 is formed between the outer vessel 112 and the inner vessel 114, and the outer vessel 112 is configured to separate the vacuum insulation layer 116 from the ambient environment of the cryogenic system 100 (e.g., room temperature and atmospheric pressure environment). The vacuum insulation layer 116 may be provided with an insulating structure such as a multilayer insulation (MLI). The inner vessel 114 is configured to contain the cryogenic liquid 102 therein and separate the cryogenic liquid 102 from the vacuum insulation layer 116. The outer vessel 112 and the inner vessel 114 are formed of a metallic material such as stainless steel or other suitable high-strength material to withstand the pressure difference between the inside and outside of the vessel.

[0014] The cryogenic refrigerator 10 includes a compressor 12 and an expander 14. The compressor 12 is configured to recover the working gas of the cryogenic refrigerator 10 from the expander 14, increase the pressure of the recovered working gas, and supply the working gas again to the expander 14. The working gas, also referred to as a refrigerant gas, is typically helium gas, although other suitable gases may be used.

[0015] The expander 14 includes a refrigerator cylinder 16, a displacer assembly 18, and a refrigerator housing 20. The refrigerator housing 20 is coupled to the refrigerator cylinder 16 to form an airtight container that houses the displacer assembly 18. The refrigerator cylinder 16 and the refrigerator housing 20 are formed from a metallic material, such as stainless steel, or other suitable high-strength material.

[0016] The expander 14 is installed in the vacuum vessel 110 with the refrigerator cylinder 16 inserted into the inner vessel 114 of the vacuum vessel 110 and the refrigerator housing 20 attached to the outside of the vacuum vessel 110. As an example, the expander 14 is installed in the upper part of the vacuum vessel 110 with its central axis aligned vertically. However, the installation location and orientation of the expander 14 are not limited to this. For example, the expander 14 may be installed in the lower part of the vacuum vessel 110. Furthermore, the expander 14 can be installed in any desired orientation, and may be installed in the vacuum vessel 110 with its central axis aligned diagonally or horizontally.

[0017] The refrigerator cylinder 16 has a first cylinder 16a and a second cylinder 16b that extend in the axial direction (the vertical direction in FIGS. 1 and 2). The second cylinder 16b is provided in series with the first cylinder 16a in the axial direction. The first cylinder 16a and the second cylinder 16b are, for example, cylindrical members, and the second cylinder 16b has a smaller diameter than the first cylinder 16a. The first cylinder 16a and the second cylinder 16b are arranged coaxially, and the lower end of the first cylinder 16a is rigidly connected to the upper end of the second cylinder 16b.

[0018] The displacer assembly 18 has a first displacer 18a and a second displacer 18b. The first displacer 18a and the second displacer 18b are, for example, cylindrical members, and the second displacer 18b has a smaller diameter than the first displacer 18a. The first displacer 18a and the second displacer 18b are arranged coaxially.

[0019] The first displacer 18a is housed in the first cylinder 16a, and the second displacer 18b is housed in the second cylinder 16b. The first displacer 18a is capable of reciprocating in the axial direction along the first cylinder 16a, and the second displacer 18b is capable of reciprocating 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 together.

[0020] In this document, for convenience in explaining the positional relationships between the components of the cryocooler 10, the side closer to the top dead center of the displacer's axial reciprocating motion will be referred to as "top" and the side closer to the bottom dead center as "bottom." The top dead center is the position of the displacer where the volume of the expansion space is maximum, and the bottom dead center is the position of the displacer where the volume of the expansion space is minimum. During operation of the cryocooler 10, a temperature gradient occurs in which the temperature decreases from top to bottom in the axial direction, so the top side can also be referred to as the high-temperature side and the bottom side as the low-temperature side.

[0021] The first displacer 18a accommodates a first regenerator 26. The first regenerator 26 is formed by filling a cylindrical main body of the first displacer 18a with a wire mesh such as copper or other suitable first regenerator material. The upper and lower lids of the first displacer 18a may be provided as separate members from the main body of the first displacer 18a, and the upper and lower lids of the first displacer 18a may be fixed to the main body by suitable means such as fastening or welding, thereby accommodating the first regenerator material in the first displacer 18a.

[0022] Similarly, the second displacer 18b accommodates the second regenerator 28. The second regenerator 28 is formed by filling the cylindrical main body of the second displacer 18b with a non-magnetic regenerator material such as bismuth, a magnetic regenerator material such as HoCu2, or another suitable second regenerator material. The second regenerator material may be formed in a granular form. The upper and lower covers of the second displacer 18b may be provided as separate members from the main body of the second displacer 18b, and the upper and lower covers of the second displacer 18b may be fixed to the main body by suitable means such as fastening or welding, thereby accommodating the second regenerator material in the second displacer 18b.

[0023] The displacer assembly 18 defines an upper chamber 30, a first expansion chamber 32, and a second expansion chamber 34 within the refrigerator cylinder 16. The expander 14 includes a first cooling stage 33 and a second cooling stage 35 for heat exchange with the desired object or medium to be cooled by the cryogenic refrigerator 10. The upper chamber 30 is defined between the upper cover of the first displacer 18a and the top of the first cylinder 16a. The first expansion chamber 32 is defined between the lower cover of the first displacer 18a and the first cooling stage 33. The second expansion chamber 34 is defined between the lower cover of the second displacer 18b and the second cooling stage 35. The first cooling stage 33 is fixed to the lower part of the first cylinder 16a to surround the first expansion chamber 32, and the second cooling stage 35 is fixed to the lower part of the second cylinder 16b to surround the second expansion chamber 34. The first cooling stage 33 and the second cooling stage 35 are formed of, for example, pure copper (for example, oxygen-free copper, tough pitch copper, etc.) or other highly thermally conductive metal.

[0024] The first regenerator 26 is connected to the upper chamber 30 through a working gas passage 36a formed in the upper lid of the first displacer 18a, and is connected to the first expansion chamber 32 through a working gas passage 36b formed in the lower lid of the first displacer 18a. The second regenerator 28 is connected to the first regenerator 26 through a working gas passage 36c formed from the lower lid of the first displacer 18a to the upper lid of the second displacer 18b. The second regenerator 28 is also connected to the second expansion chamber 34 through a working gas passage 36d formed in the lower lid of the second displacer 18b.

[0025] A first seal 38a and a second seal 38b may be provided so that the flow of working gas between the first expansion chamber 32, the second expansion chamber 34, and the upper chamber 30 is directed to the first regenerator 26 and the second regenerator 28, rather than through the clearance between the refrigerator cylinder 16 and the displacer assembly 18. The first seal 38a may be attached to an upper cover of the first displacer 18a so as to be positioned between the first displacer 18a and the first cylinder 16a. The second seal 38b may be attached to an upper cover of the second displacer 18b so as to be positioned between the second displacer 18b and the second cylinder 16b.

[0026] The expander 14 also includes a pressure switching valve 40 and a drive motor 42. The pressure switching valve 40 is housed in the refrigerator housing 20, and the drive motor 42 is attached to the refrigerator housing 20.

[0027] 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 within the refrigerator cylinder 16. The working gas discharge port of the compressor 12 is connected to the upper chamber 30 via the high-pressure valve 40a, and the working gas inlet 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 selectively and alternately open and close (i.e., one is open while the other is closed). High-pressure (e.g., 2 to 3 MPa) working gas is supplied from the compressor 12 to the expander 14 through the high-pressure valve 40a, and low-pressure (e.g., 0.5 to 1.5 MPa) working gas is returned from the expander 14 to the compressor 12 through the low-pressure valve 40b. For ease of understanding, the flow direction of the working gas is indicated by arrows in FIG. 2.

[0028] The drive motor 42 is provided to drive the reciprocating motion 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, like the pressure switching valve 40, is housed in the refrigerator housing 20. The displacer drive shaft 44 extends from the motion conversion mechanism 43 through the refrigerator housing 20 into the upper chamber 30 and is fixed to the top cover of the first displacer 18a. A third seal 38c is provided to prevent leakage of the working gas from the upper chamber 30 to the refrigerator housing 20 (which may be maintained at a low pressure as described above). The third seal 38c may be attached to the refrigerator housing 20 so as to be disposed between the refrigerator housing 20 and the displacer drive shaft 44.

[0029] When the drive motor 42 is driven, the rotational output of the drive motor 42 is converted into axial reciprocating motion of the displacer drive shaft 44 by the motion conversion mechanism 43, and the displacer assembly 18 reciprocates in the axial direction within the refrigerator cylinder 16. The drive motor 42 is also connected to the high-pressure valve 40a and the low-pressure valve 40b so as to selectively and alternately open and close these valves.

[0030] When the compressor 12 and the drive motor 42 are operated, the cryogenic refrigerator 10 generates periodic volume fluctuations and synchronized pressure fluctuations of the working gas in the first expansion chamber 32 and the second expansion chamber 34, thereby forming a refrigeration cycle and cooling the first cooling stage 33 and the second cooling stage 35 to a desired cryogenic temperature. The first cooling stage 33 can be cooled to a first cooling temperature in the range of, for example, about 20 K to about 40 K. The second cooling stage 35 can be cooled to a second cooling temperature (for example, about 1 K to about 4 K) that is lower than the first cooling temperature.

[0031] In addition to the first cooling stage 33 and the second cooling stage 35, the expander 14 can also absorb heat at the refrigerator cylinder 16, for example, at the axially intermediate portion of the second cylinder 16b. Such a heat absorption portion 46 is cooled to a cooling temperature based on the axial temperature distribution on the refrigerator cylinder 16, for example, the second cylinder 16b, and the axial position of the heat absorption portion 46, and can provide some refrigeration capacity at this cooling temperature. The cooling temperature of the heat absorption 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 absorption portion 46 may be located, for example, in a range from 1 / 4 to 3 / 4 of the normalized axial position on the second cylinder 16b (i.e., a dimensionless axial position where the positions of the first cooling stage 33 and the second cooling stage 35 are 0 and 1, respectively).

[0032] In a typical use scenario of the cryogenic refrigerator 10, the object to be cooled is thermally connected to either the first cooling stage 33 or the second cooling stage 35 depending on the desired cooling temperature. Nothing is connected to the heat absorption part 46 on the second cylinder 16b, and the refrigeration capacity of the heat absorption part 46 is not utilized.

[0033] If the refrigeration capacity of the heat absorption unit 46 could be utilized in addition to the two cooling stages, this could lead to more efficient cooling of the cryogenic system 100. However, heat input to the heat absorption unit 46 could affect the cooling temperature of a cooling stage, such as the second cooling stage 35 (large heat input to the heat absorption unit 46 could cause the second cooling stage 35 to heat up). Typically, in a cryogenic system 100, in order to maintain the object to be cooled at a desired cooling temperature, it is required to operate the cryogenic refrigerator 10 so that the cooling stage temperature does not exceed a predetermined limit temperature. Cooling using the heat absorption unit 46 has not been used until now because of concerns about the risk of the cooling stage heating up due to heat input to the heat absorption unit 46.

[0034] The inventors have found that, as will be described in detail below, by maintaining the temperature of the heat absorption section 46 at or below a certain upper limit temperature, it is possible to prevent or minimize a temperature rise in the second cooling stage 35. From this perspective, in this embodiment, the cryogenic system 100 is configured to control the heat source (i.e., the object to be cooled) so as to optimize the heat input from the heat source to the heat absorption section 46. The optimization of the heat input is achieved by maintaining the temperature of the heat absorption section 46 at or below the upper limit temperature.

[0035] 3(a) and 3(b) are graphs showing experimental results by the inventors according to an embodiment of the present invention. In this experiment, a heater was installed in the heat absorption section 46 to simulate heat input to the heat absorption section 46. The heater, i.e., the position of the heat absorption section 46, was located slightly lower than the axial center, specifically at approximately 0.54°C in terms of the normalized axial position on the second cylinder 16b (where the first cooling stage 33 is 0 and the second cooling stage 35 is 1, as described above). A heater was also installed to apply a heat load (e.g., a rated heat load of 1.5 W) to the second cooling stage 35. The cryocooler 10 was operated to maintain the first cooling stage 33 at a constant temperature (e.g., 43 K) while applying the rated heat load to the second cooling stage 35.

[0036] FIG. 3(a) shows the relationship between the normalized heater power, which represents the heat input to the heat absorption section 46, and the temperature rise of the second cooling stage 35. The temperature rise of the second cooling stage 35 represents the temperature rise relative to the target cooling temperature (e.g., 4 K) of the second cooling stage 35. FIG. 3(b) shows the relationship between the normalized axial position and temperature on the second cylinder 16b, i.e., how the axial temperature distribution on the second cylinder 16b changes depending on the normalized heater power. In FIGS. 3(a) and 3(b), the temperature measurement results when the normalized heater power is zero (i.e., no heat input to the heat absorption section 46) are indicated by circles, and the temperature measurement results when the normalized heater power is 0.085, 0.31, and 0.45 are indicated by squares, triangles, and diamonds, respectively.

[0037] 3(a), when the normalized heater output is zero, the temperature rise of the second cooling stage 35 is 0 K (circle symbol), and as the normalized heater output increases to 0.085, 0.31, and 0.45, the temperature rise of the second cooling stage 35 also increases (square, triangle, and diamond symbols). Specifically, when the normalized heater output is 0.085, the temperature rise is approximately 0.04 K (square symbol), when the normalized heater output is 0.31, the temperature rise is approximately 0.09 K (triangle symbol), and when the normalized heater output is 0.45, the temperature rise is approximately 0.17 K (diamond symbol).

[0038] Based on the inventor's knowledge and experience, in some applications of the cryogenic refrigerator 10, such as liquid helium cooling, where the target cooling temperature of the second cooling stage 35 is approximately 4 K, it is often desirable to limit the allowable temperature rise of the second cooling stage 35 to approximately 0.1 K, and a maximum of approximately 0.2 K is required.

[0039] 3(a) shows that the normalized heater output threshold value Th1, indicated by the dashed line in FIG. 3(a), represents an important boundary. If the heat input to the heat absorption unit 46 is below this threshold value Th1, the temperature rise of the second cooling stage 35 is kept to a relatively small value of approximately 0.1 K or less. On the other hand, if the heat input to the heat absorption unit 46 exceeds this threshold value Th1, the temperature rise of the second cooling stage 35 changes and increases proportionally with the increase in normalized heater output, easily exceeding 0.2 K.

[0040] Referring to Figure 3(b), the relationship between the normalized heater power and the temperature of the heat absorption section 46 (the temperature at a normalized axial position of approximately 0.54) can be seen. When the normalized heater power is zero, the temperature of the heat absorption section 46 is approximately 8 K (circle symbol). It can be seen that as the normalized heater power increases to 0.085, 0.31, and 0.45, the temperature of the heat absorption section 46 also increases (square, triangle, and diamond symbols). More specifically, when the normalized heater power is 0.085, the temperature of the heat absorption section 46 is approximately 13 K (square symbol), when the normalized heater power is 0.31, the temperature of the heat absorption section 46 is approximately 20 K (triangle symbol), and when the normalized heater power is 0.45, the temperature of the heat absorption section 46 is approximately 24 K (diamond symbol).

[0041] The dashed line L1 shown in Figure 3(b) represents a line connecting the measured temperature of the first cooling stage 33 and the measured temperature of the second cooling stage 35. From Figures 3(a) and 3(b), it is important to note that the temperature of the heat absorption section 46 at the normalized heater output threshold value Th1 is located on or in the vicinity of this line L1 (between the square symbol and the triangle symbol in Figure 3(b)).

[0042] For this reason, if the heat input to the heat absorption portion 46 is large (i.e., exceeds the threshold value Th1) and, as a result, the temperature of the heat absorption portion 46 exceeds the upper limit temperature on or near this line L1, a significant temperature rise can occur in the second cooling stage 35 due to the heat input to the heat absorption portion 46. Furthermore, if the heat input to the heat absorption portion 46 is large enough that the temperature of the heat absorption portion 46 does not exceed this upper limit temperature, the temperature rise of the second cooling stage 35 due to the heat input to the heat absorption portion 46 will not occur substantially or will be within an acceptable range (for example, within 0.2 K, or preferably within 0.1 K).

[0043] 4(a) and 4(b) are graphs showing experimental results by the inventors of the present invention. The experimental results shown in FIGS. 4(a) and 4(b) were obtained by changing the axial position of the heat absorption portion 46 compared to the experimental results shown in FIGS. 3(a) and 3(b), but the other experimental conditions were the same. In FIGS. 4(a) and 4(b), the position of the heat absorption portion 46 is on the higher temperature side compared to FIGS. 3(a) and 3(b), specifically, at approximately 0.35° in normalized axial position on the second cylinder 16b.

[0044] 4(a), like FIG. 3(a), shows the relationship between the normalized heater output, which represents the heat input to the heat absorption section 46, and the temperature rise of the second cooling stage 35. FIG. 4(b), like FIG. 3(b), shows the relationship between the normalized axial position on the second cylinder 16b and the temperature. In FIGS. 4(a) and 4(b), the temperature measurement results when the normalized heater output is zero (i.e., no heat input to the heat absorption section 46) are shown with a circle, and the temperature measurement results when the normalized heater output is 0.31, 0.55, and 0.72 are shown with a square, triangle, and diamond, respectively.

[0045] 4(a), it can be seen that when the normalized heater output is zero, there is no temperature rise in the second cooling stage 35 (circle symbol), and as the normalized heater output increases, the temperature rise in the second cooling stage 35 also increases. Specifically, when the normalized heater output is 0.31, the temperature rise is approximately 0.02 K (square symbol), when the normalized heater output is 0.55, the temperature rise is approximately 0.07 K (triangle symbol), and when the normalized heater output is 0.72, the temperature rise is approximately 0.09 K (diamond symbol). In FIG. 4(a), the heat absorption portion 46 is on the higher temperature side compared to FIG. 3(a), so the temperature rise in the second cooling stage 35 due to increased heat input to the heat absorption portion 46 is weaker.

[0046] 4(a), the normalized heater output threshold value Th2 indicated by the dashed line is also an important boundary. If the heat input to the heat absorption unit 46 is below this threshold value Th2, the temperature rise of the second cooling stage 35 is kept to a very small value of approximately 0.02 K or less. On the other hand, if the heat input to the heat absorption unit 46 exceeds this threshold value Th2, the temperature rise of the second cooling stage 35 changes and increases proportionally to the normalized heater output.

[0047] Referring to Figure 4(b), the relationship between the normalized heater power and the temperature of the heat absorption section 46 (the temperature at a normalized axial position of approximately 0.35) can be seen. When the normalized heater power is zero, the temperature of the heat absorption section 46 is approximately 16 K (circle symbol), and as the normalized heater power increases, the temperature of the heat absorption section 46 also increases. Specifically, when the normalized heater power is 0.31, the temperature of the heat absorption section 46 is approximately 27 K (square symbol), when the normalized heater power is 0.55, the temperature of the heat absorption section 46 is approximately 33 K (triangle symbol), and when the normalized heater power is 0.72, the temperature of the heat absorption section 46 is approximately 38 K (diamond symbol).

[0048] The dashed line L2 shown in Figure 4(b) represents a straight line connecting the measured temperature of the first cooling stage 33 and the measured temperature of the second cooling stage 35. From Figures 4(a) and 4(b), it is considered that the temperature of the heat absorption part 46 at the normalized heater output threshold value Th2 is located on or in the vicinity of this straight line L2 (between the square symbol and the triangle symbol in Figure 4(b)).

[0049] Therefore, if the heat input to the heat absorption section 46 is large (i.e., exceeds the threshold value Th2), and as a result the temperature of the heat absorption section 46 exceeds this line L2 or the upper limit temperature nearby, a significant temperature rise can occur in the second cooling stage 35 due to the heat input to the heat absorption section 46. Furthermore, if the heat input to the heat absorption section 46 is of an amount that does not cause the temperature of the heat absorption section 46 to exceed this upper limit temperature, it is thought that the temperature rise of the second cooling stage 35 due to the heat input to the heat absorption section 46 will not occur substantially or will be within an acceptable range.

[0050] 3(a) to 4(b), the inventors have performed the same experiments by varying the axial position of the heat absorption section 46 and the temperature of the first cooling stage 33. As a result, the inventors have confirmed that behavior similar to the above results is observed for both the relationship between the heat input to the heat absorption section 46 and the temperature rise of the second cooling stage 35, and the relationship between the normalized axial position on the second cylinder 16b and the temperature.

[0051] Therefore, the upper limit temperature of the heat absorption section 46 for preventing or minimizing the temperature rise of the second cooling stage 35 due to heat input to the heat absorption section 46 can be set based on the measured temperatures at both ends of the second cylinder 16b and the axial position of the heat absorption section 46.

[0052] In one exemplary method, the upper limit temperature of the heat absorption portion 46 may be set based on a reference temperature distribution of the second cylinder 16b and the axial position of the heat absorption portion 46. Here, the reference temperature distribution of the second cylinder 16b may be a temperature distribution that has a first measured temperature at one axial end of the second cylinder 16b (e.g., the first cooling stage 33) and a second measured temperature at the other axial end of the second cylinder 16b (e.g., the second cooling stage 35), and that varies linearly depending on the axial position in the second cylinder 16b.

[0053] The following describes an exemplary configuration for optimizing the heat input to the heat absorption section 46. Referring again to Figure 1, the cryogenic refrigerator 10 includes a first temperature sensor 51, a second temperature sensor 52, a third temperature sensor 53, and a controller 60.

[0054] The first temperature sensor 51 measures a first measured temperature T1 at one axial end of the second cylinder 16b that is closer to the first cylinder 16a. The first temperature sensor 51 may be provided in the first cooling stage 33 and may measure the first measured temperature T1 at the first cooling stage 33. The second temperature sensor 52 measures a second measured temperature T2 at the other axial end of the second cylinder 16b that is farther from the first cylinder 16a. The second temperature sensor 52 may be provided in the second cooling stage 35 and may measure the second measured temperature T2 at the second cooling stage 35. The third temperature sensor 53 is provided in the heat absorption portion 46 and measures a third measured temperature T3 at the heat absorption portion 46.

[0055] The controller 60 is communicatively connected to the first temperature sensor 51, the second temperature sensor 52, and the third temperature sensor 53 so as to receive the first measured temperature T1, the second measured temperature T2, and the third measured temperature T3 from these temperature sensors, respectively.

[0056] The internal configuration of the controller 60 is realized as a hardware configuration by elements and circuits such as a computer CPU (Central Processing Unit) and memory, and as a software configuration by a computer program, etc., but in the figure it is depicted as functional blocks realized by the cooperation of these elements. Those skilled in the art will understand that these functional blocks can be realized in various ways by combining hardware and software.

[0057] The cryogenic system 100 includes a refrigerant gas line 118 that is cooled by a cryogenic refrigerator 10. The refrigerant gas in the refrigerant gas line 118 is cooled and condensed to produce a cryogenic liquid 102. The refrigerant gas line 118 includes a supply line 120 that supplies the cryogenic liquid 102 to the inner tank 114 of the vacuum vessel 110, and a return line 122 for the cryogenic liquid 102 (i.e., the refrigerant gas) that has vaporized in the inner tank 114. The supply line 120 and the return line 122 may each be rigid or flexible piping through which the refrigerant gas flows.

[0058] The supply line 120 includes a first heat exchanger 124, a second heat exchanger 126, and a recondenser 128, and in the supply line 120, the refrigerant gas flows through the first heat exchanger 124, the second heat exchanger 126, and the recondenser 128 in this order.

[0059] The first heat exchanger 124 is provided in the inner tank 114 outside the first cooling stage 33 and configured to pre-cool the refrigerant gas to a first cooling temperature by heat exchange between the first cooling stage 33 and the refrigerant gas. The second heat exchanger 126 is provided in the inner tank 114 outside the second cylinder 16b and configured to further pre-cool the refrigerant gas to the cooling temperature of the heat absorption portion 46 by heat exchange between the refrigerant gas and the heat absorption portion 46 of the second cylinder 16b.

[0060] The recondensing section 128 serves as an outlet for the supply line 120 to the inner vessel 114. The recondensing section 128 is provided outside the second cooling stage 35 within the inner vessel 114 and is configured to cool the refrigerant gas to a second cooling temperature through heat exchange between the second cooling stage 35 and the refrigerant gas and recondense the refrigerant gas into cryogenic liquid 102. The recondensed cryogenic liquid 102 is stored in the inner vessel 114 as described above. The recondensing section 128 may be integral with the second cooling stage 35 as shown in the figure, and may have fin-like protrusions or irregularities to increase the surface area in contact with the refrigerant gas or the cryogenic liquid 102. Like the second cooling stage 35, the recondensing section 128 is formed of, for example, pure copper (e.g., oxygen-free copper, tough pitch copper, etc.) or another highly thermally conductive metal.

[0061] The refrigerant gas line 118 also includes a flow regulator 130 configured to regulate the flow rate of the refrigerant gas in the refrigerant gas line 118. In the illustrated example, the flow regulator 130 is provided on the refrigerant gas line 118 to connect the return line 122 to the supply line 120. The flow regulator 130 receives the refrigerant gas from the return line 122, regulates the flow rate of the refrigerant gas, and delivers the refrigerant gas at the regulated flow rate to the supply line 120. The flow regulator 130 may be a circulation flow generator such as a pump or a compressor, or a mechanism for regulating the gas flow rate such as a flow control valve or a variable orifice.

[0062] The flow regulator 130 may be located outside the vacuum vessel 110, as shown, or may be located within the vacuum vessel 110. The flow regulator 130 may also be located anywhere on the refrigerant gas line 118, on the supply line 120, or on the return line 122.

[0063] As will be described later, under the control of the controller 60, the flow regulator 130 can adjust the flow rate of the refrigerant gas circulating through the refrigerant gas line 118. This flow rate adjustment controls the heat input from the refrigerant gas line 118 to the heat absorption unit 46.

[0064] The refrigerant gas line 118 serves as a heat source for the cryogenic refrigerator 10. This heat source includes a first portion (e.g., the first heat exchanger 124), a second portion (e.g., the recondenser 128), and a third portion of the heat source (e.g., the second heat exchanger 126) connecting the first and second portions. One axial end of the second cylinder 16b (e.g., the first cooling stage 33) is thermally connected to the first portion of the heat source, and the other axial end of the second cylinder 16b (e.g., the second cooling stage 35) is thermally connected to the second portion of the heat source. The heat absorption portion 46 is thermally connected to the third portion of the heat source. The refrigerant gas line 118 passes through the first, third, and second portions of the heat source in this order.

[0065] 5 is a flowchart showing a control method for cryogenic temperature system 100 according to the embodiment. This method includes the steps of acquiring a first measured temperature T1 from first temperature sensor 51, a second measured temperature T2 from second temperature sensor 52, and a third measured temperature T3 from third temperature sensor 53 (S10), setting an upper limit temperature of heat absorption unit 46 based on first measured temperature T1, second measured temperature T2, and the axial position of heat absorption unit 46 (S20), and controlling the heat source, i.e., refrigerant gas line 118, so that third measured temperature T3 is equal to or lower than the upper limit temperature of heat absorption unit 46 (S30).

[0066] In S10, a first measured temperature T1 is measured by a first temperature sensor 51 at one axial end (e.g., first cooling stage 33) of second cylinder 16b that is closer to first cylinder 16a. A signal indicating the first measured temperature T1 is input from first temperature sensor 51 to controller 60. A second measured temperature T2 is measured by a second temperature sensor 52 at the other axial end of second cylinder 16b that is farther from first cylinder 16a. A signal indicating the second measured temperature T2 is input from second temperature sensor 52 to controller 60. A third measured temperature T3 is measured by a third temperature sensor 53 at heat absorption portion 46. A signal indicating the third measured temperature T3 is input from third temperature sensor 53 to controller 60.

[0067] In S20, the controller 60 first determines a reference temperature distribution for the second cylinder 16b based on the first measured temperature T1 and the second measured temperature T2. This reference temperature distribution has the first measured temperature T1 at one axial end of the second cylinder 16b and the second measured temperature T2 at the other axial end of the second cylinder 16b, and is a temperature distribution that varies linearly depending on the axial position on the second cylinder 16b. The reference temperature distribution is represented by a straight line (Y = (T2 - T1)X + T1) passing through two points, (0, T1) and (1, T2), on an XY plane with the normalized axial position on the second cylinder 16b as the X axis and the temperature as the Y axis.

[0068] The controller 60 sets the upper limit temperature of the heat absorption unit 46 based on the reference temperature distribution and the axial position of the heat absorption unit 46. A candidate value for the upper limit temperature of the heat absorption unit 46 is obtained by substituting the normalized axial position of the heat absorption unit 46 into the above equation representing the reference temperature distribution. The controller 60 may use this candidate value as the upper limit temperature.

[0069] Alternatively, if emphasis is placed on reliably preventing the temperature of the second cooling stage 35 from rising due to heat input to the heat absorption section 46, the controller 60 may adopt a value somewhat smaller than this candidate value (for example, a value selected from the 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 absorption section 46.

[0070] Alternatively, if a slight temperature rise in the second cooling stage 35 due to heat input to the heat absorption section 46 is permitted, the controller 60 may adopt a value somewhat larger than this candidate value (for example, a value selected from the 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 absorption section 46.

[0071] In S30, the controller 60 controls the flow rate of the refrigerant gas in the refrigerant gas line 118 so that the third measured temperature T3 is equal to or lower than the upper limit temperature of the heat absorption portion 46. An example of this control will be described later with reference to FIG.

[0072] 5, the controller 60 may update the upper limit temperature of the heat absorption unit 46 in accordance with the first measured temperature T1 and the second measured temperature T2. Furthermore, when the cryogenic refrigerator 10 is operated so as to maintain the cooling temperature of the first cooling stage 33 (and / or the second cooling stage 35) at a certain target temperature, the reference temperature distribution will remain substantially unchanged unless the target temperature is changed. In this case, the upper limit temperature of the heat absorption unit 46, once set, may continue to be used thereafter.

[0073] Fig. 6 is a flowchart showing an example of the control process (S30) of the refrigerant gas line 118 shown in Fig. 5. In this process, the controller 60 controls the flow regulator 130 of the refrigerant gas line 118 based on a comparison between the third measured temperature T3 and the upper limit temperature of the heat absorption unit 46, thereby controlling the refrigerant gas flow rate in the refrigerant gas line 118. This process is repeatedly executed by the controller 60 at a predetermined interval while the cryogenic refrigerator 10 is in operation.

[0074] Therefore, as shown in FIG. 6, the controller 60 first calculates the third measured temperature T3 as the upper limit temperature T lim The third measured temperature T3 is measured by the third temperature sensor 53 as described above (S10 in FIG. 5). The upper limit temperature of the heat absorption portion 46 is set based on the measured temperatures at both ends of the second cylinder 16b and the axial position of the heat absorption portion 46 (S20 in FIG. 5).

[0075] The controller 60 sets the third measured temperature T3 to the upper limit temperature T lim The controller 60 compares the third measured temperature T3 with the upper limit temperature T4 and outputs the comparison result to determine which is larger. lim (ii) the third measured temperature T3 is higher than the upper limit temperature T lim (iii) the third measurement temperature T3 is lower than the upper limit temperature T lim or is equal to.

[0076] The controller 60 controls the flow regulator 130 based on the comparison result. Specifically, (i) when the third measured temperature T3 is equal to or lower than the upper limit temperature T lim If the third measured temperature T3 is higher than the upper limit temperature T, the controller 60 controls the flow regulator 130 to reduce the refrigerant gas flow rate (S32). This reduces the heat input from the second heat exchanger 126 to the heat absorption unit 46, thereby reducing the third measured temperature T3. (ii) If the third measured temperature T3 is higher than the upper limit temperature T limIf the third measured temperature T3 is lower than the upper limit temperature T, the controller 60 controls the flow regulator 130 to increase the refrigerant gas flow rate (S33). This increases the heat input from the second heat exchanger 126 to the heat absorption unit 46, allowing the refrigerant gas line 118 to be cooled more efficiently. (iii) If the third measured temperature T3 is lower than the upper limit temperature T lim If the value of (iii) is equal to (i) or (ii), there is no need to increase or decrease the refrigerant gas flow rate, and the controller 60 controls the flow regulator 130 to maintain the current refrigerant gas flow rate. Note that the case of (iii) may be included in either (i) or (ii).

[0077] As described above, according to the embodiment, the refrigerant gas line 118 can be cooled by utilizing the refrigeration capacity of the heat absorption portion 46 on the second cylinder 16b in addition to the first cooling stage 33 and the second cooling stage 35. Compared to a typical cooling configuration that does not utilize the heat absorption portion 46, the cryogenic system 100 can be cooled more efficiently.

[0078] In the embodiment, the upper limit temperature T of the heat absorption portion 46 is calculated based on the first measured temperature T1, the second measured temperature T2, and the axial position of the heat absorption portion 46. lim is set, and the third measurement temperature T3 is set to this upper limit temperature T lim The heat source (for example, the refrigerant gas line 118) for the cryogenic refrigerator 10 in the cryogenic system 100 is controlled so that the temperature of the heat absorption portion 46 is set to the upper limit temperature T lim By keeping the temperature at or below this value, it is possible to prevent or minimize the temperature rise of the second cooling stage 35.

[0079] Furthermore, in the embodiment, the upper limit temperature T lim is based on the linear reference temperature distribution and the axial position of the heat absorption portion 46. In this way, even if the temperatures of the first cooling stage 33 and the second cooling stage 35 change according to various operating conditions of the cryocooler 10, or even if the heat absorption portion 46 is provided at various axial positions, the upper limit temperature T lim can be clearly and easily determined.

[0080] Fig. 7 is a diagram schematically illustrating a cryogenic system 100 according to another embodiment. The cryogenic system 100 shown in Fig. 7 differs from the cryogenic system 100 shown in Fig. 1 in terms of the object to be cooled, but the rest of the cryogenic system is generally the same. Below, the different configurations will be mainly described, and the common configurations will be briefly described or will not be described at all.

[0081] Although the above-described embodiment has been described by way of example in which the cryogenic system 100 is a storage device for the cryogenic liquid 102, other configurations are also possible. For example, the cryogenic system 100 may be applied to superconducting equipment, and the cryogenic refrigerator 10 may be used to cool a superconducting coil 150 disposed in a vacuum vessel 110 and a 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 absorption unit 46, and the second cooling stage 35.

[0082] The current lead 152 electrically connects a power supply 154 disposed outside the vacuum vessel 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, and therefore serves as a heat source for the cryogenic refrigerator 10. The current lead 152 has a first portion 152a, a second portion 152b, and a third portion 152c connecting the first portion 152a and the second portion 152b.

[0083] One axial end of the second cylinder 16b (for example, the first cooling stage 33) is thermally connected to the first portion 152a of the current lead 152, and the other axial end of the second cylinder 16b (for example, the second cooling stage 35) is thermally connected to the second portion 152b of the current lead 152. The heat absorption portion 46 is thermally connected to the third portion 152c of the current lead 152.

[0084] 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. Furthermore, the second cooling stage 35 may be connected to the superconducting coil 150 by the second heat transfer member 158. The heat absorption 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.

[0085] The controller 60 is configured to control the current in the current lead 152 so that the third measured temperature T3 is equal to or less than the upper temperature limit. The controller 60 may control the power supply 154 based on a comparison between the third measured temperature T3 and the upper temperature limit of the heat sink 46, thereby controlling the current in the current lead 152.

[0086] As an example, (i) the third measured temperature T3 is the upper limit temperature T lim If the third measured temperature T3 is higher than the upper limit temperature T, the controller 60 controls the power supply 154 to reduce the current in the current lead 152. This reduces the heat input from the second heat exchanger 126 to the heat absorption unit 46, thereby reducing the third measured temperature T3. lim If the third measured temperature T3 is lower than the upper limit temperature T, the controller 60 controls the power supply 154 to increase the current in the current lead 152. This increases the heat input from the second heat exchanger 126 to the heat absorption section 46, allowing the current lead 152 to be cooled more efficiently. lim If equal to , there is no need to increase or decrease the current in current lead 152, so controller 60 controls power supply 154 to maintain the current.

[0087] In this way, the current lead 152 can be cooled by utilizing the refrigeration capacity of the heat absorption portion 46 on the second cylinder 16b in addition to the first cooling stage 33 and the second cooling stage 35. This allows the cryogenic system 100 to be cooled more efficiently than a typical cooling configuration that does not utilize the heat absorption portion 46.lim By keeping the temperature at or below this value, it is possible to prevent or minimize the temperature rise of the second cooling stage 35.

[0088] The present invention has been described above based on examples. It will be understood by those skilled in the art that the present invention is not limited to the above-described embodiments, and that various design changes and modifications are possible, and that such modifications are also within the scope of the present invention. Various features described in relation to one embodiment can also be applied to other embodiments. A new embodiment created by combining embodiments will have the combined effects of the respective combined embodiments.

[0089] In the above-described embodiment, one temperature sensor (third temperature sensor 53) is provided to measure the temperature of the heat absorption portion 46, but other configurations are possible. In one embodiment, the temperature of the heat absorption portion 46 may be measured at multiple temperature measurement positions. Therefore, multiple temperature sensors (e.g., two third temperature sensors 53) may be provided in the heat absorption portion 46. These temperature sensors are provided at different axial positions on the second cylinder 16b. For example, one third temperature sensor 53 is located on the higher temperature side relative to the other third temperature sensor 53.

[0090] In this case, the controller 60 may set an upper limit temperature for each temperature measurement position (temperature sensor) based on the measured temperatures at both ends of the second cylinder 16b and the temperature measurement positions. The controller 60 may control the heat source so that all of the measured temperatures are equal to or lower than the corresponding upper limit temperature.

[0091] In the above embodiment, the cryocooler 10 is described as a two-stage GM refrigerator, but other configurations are also possible. For example, the cryocooler 10 may be a single-stage GM refrigerator. Alternatively, the cryocooler 10 may be another type of cryocooler, such as a Solvay refrigerator, a Stirling refrigerator, or a pulse tube refrigerator.

[0092] The present invention has been described using specific terms based on the embodiments, but the embodiments merely illustrate one aspect of the principles and applications of the present invention, and many modifications and changes in arrangement are permitted to the embodiments as long as they do not deviate from the concept of the present invention as defined in the claims. [Explanation of symbols]

[0093] 10 cryogenic refrigerator, 16a first cylinder, 16b second cylinder, 46 heat absorption part, 51 first temperature sensor, 52 second temperature sensor, 53 third temperature sensor, 60 controller, 100 cryogenic system, 118 refrigerant gas line, 152 current lead.

Claims

1. A first cylinder; a second cylinder provided in series with the first cylinder in the axial direction, the second cylinder having a heat absorption portion in an axially intermediate portion thereof that is thermally connected to a heat source; a first temperature sensor for measuring a first measured temperature at one axial end of the second cylinder proximate to the first cylinder; a second temperature sensor for measuring a second measured temperature at the other axial end of the second cylinder remote from the first cylinder; a third temperature sensor that measures a third measurement temperature at the heat absorption portion; obtaining the first measured temperature from the first temperature sensor, the second measured temperature from the second temperature sensor, and the third measured temperature from the third temperature sensor; setting an upper limit temperature of the heat absorption unit based on the first measured temperature, the second measured temperature, and an axial position of the heat absorption unit; a controller configured to control the heat source so that the third measured temperature is equal to or less than an upper limit temperature of the heat absorption portion.

2. The controller determining a reference temperature distribution for the second cylinder, the reference temperature distribution having the first measured temperature at the one axial end of the second cylinder and the second measured temperature at the other axial end of the second cylinder, the reference temperature distribution varying linearly with axial position on the second cylinder; The cryogenic system according to claim 1 , wherein the system is configured to set an upper limit temperature of the heat absorption portion based on the reference temperature distribution and the axial position of the heat absorption portion.

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

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

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

6. A method for controlling a cryogenic system, the cryogenic system including a cryogenic refrigerator including a first cylinder and a second cylinder arranged in series in an axial direction, the second cylinder including a heat absorption part at an axial intermediate part thereof that is thermally connected to a heat source, the method comprising: measuring a first temperature at one axial end of the second cylinder proximate to the first cylinder; measuring a second temperature at another axial end of the second cylinder remote from the first cylinder; Measuring a third measurement temperature at the heat absorption portion; setting an upper limit temperature of the heat absorption unit based on the first measured temperature, the second measured temperature, and an axial position of the heat absorption unit; and controlling the heat source so that the third measured temperature is equal to or less than an upper limit temperature of the heat absorption portion.

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