EP3477225B1

EP3477225B1 - Cryogenic system

Info

Publication number: EP3477225B1
Application number: EP18201006.6A
Authority: EP
Inventors: Katsuhiro Narasaki
Original assignee: Sumitomo Heavy Industries Ltd
Current assignee: Sumitomo Heavy Industries Ltd
Priority date: 2017-10-25
Filing date: 2018-10-17
Publication date: 2022-03-02
Anticipated expiration: 2038-10-17
Also published as: EP3477225A1; JP6975015B2; JP2019078481A

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a cryogenic system.

Description of Related Art

In the past, a cryogenic system, which includes a Joule-Thomson (JT) cryocooler, has been used to cool highly-sensitive electromagnetic wave detection elements, which are used for astronomical observation and the like, or other objects to be cooled to a desired very low temperature. The cryogenic system usually includes a precooler that precools the JT cryocooler. A two-stage mechanical cryocooler, such as a two-stage Stirling cryocooler or a two-stage Gifford-McMahon (GM) cryocooler, is typically used as the precooler. Such a cryogenic system can cool an object to be cooled to the temperature range of, for example, 1K to 4K (see Japanese Unexamined Patent Application Publication No. 4-44202 ).
JP 2003 194428 A discloses a cryogenic system having two single-stage GM type pulse tube refrigerators and a JT cryocooler.

SUMMARY OF THE INVENTION

An exemplary object of an aspect of the invention is to improve the efficiency of a cryogenic system.
According to the invention, a cryogenic system is defined by claim 1 and includes a single-stage precooler that includes a first-stage cooling stage, a two-stage precooler that includes a first-stage cooling stage and a second-stage cooling stage, and a JT cryocooler that includes a first-stage precooling unit to be cooled by the first-stage cooling stage of the single-stage precooler and a second-stage precooling unit to be cooled by the second-stage cooling stage. The single-stage precooler is a single-stage Stirling cryocooler or a single-stage Stirling type pulse tube cryocooler and is operated at a first operating frequency. The two-stage precooler is a two-stage Stirling cryocooler or a two-stage Stirling type pulse tube cryocooler and is operated at a second operating frequency lower than the first operating frequency. The first-stage precooling unit is thermally coupled to the first-stage cooling stage of the single-stage precooler and is thermally isolated from the first-stage cooling stage of the two-stage precooler.
According to the invention, the efficiency of a cryogenic system can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically showing a cryogenic system according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the invention will be described in detail below with reference to the drawings. The same or equivalent components, members, and processing in the description and the drawing are denoted by the same reference numerals and the repeated description thereof will be appropriately omitted. The scale and shape of each part to be shown are conveniently set to facilitate description, and is not interpreted in a limited way as long as not particularly mentioned. The embodiment is exemplary, and does not limit the scope of the invention which is solely defined by the appended claims.
FIG. 1 is a diagram schematically showing a cryogenic system 10 according to an embodiment of the present invention. The cryogenic system 10 includes a single-stage precooler 12, a two-stage precooler 14, a JT cryocooler 16, a vacuum chamber 18, a first-stage radiation shield 20, a second-stage radiation shield 22, and a cryogenic cooling unit 26 that cools an object 24 to be cooled.
The single-stage precooler 12, the two-stage precooler 14, and the JT cryocooler 16 are installed on the vacuum chamber 18 so that a normal-temperature portion of each of the single-stage precooler 12, the two-stage precooler 14, and the JT cryocooler 16 is disposed outside the vacuum chamber 18, and a low-temperature portion of each of the single-stage precooler 12, the two-stage precooler 14, and the JT cryocooler 16 is disposed in the vacuum chamber 18. The exemplary structures of the single-stage precooler 12, the two-stage precooler 14, and the JT cryocooler 16 will be described later.
The vacuum chamber 18 is a cryogenic vacuum chamber, such as a cryostat, and receives the first-stage radiation shield 20 and the second-stage radiation shield 22. A heat insulating material, such as a multi-layered heat insulating material, may be disposed between the vacuum chamber 18 and the first-stage radiation shield 20. The first-stage radiation shield 20 is disposed in the vacuum chamber 18 so as to surround the second-stage radiation shield 22, the object 24 to be cooled, and the cryogenic cooling unit 26, and suppresses the transfer of radiant heat to the second-stage radiation shield 22, the object 24 to be cooled, and the cryogenic cooling unit 26. The second-stage radiation shield 22 is disposed in the vacuum chamber 18 (specifically, in the first-stage radiation shield 20) so as to surround the object 24 to be cooled and the cryogenic cooling unit 26, and suppresses the transfer of radiant heat to the object 24 to be cooled and the cryogenic cooling unit 26.
For example, the object 24 to be cooled is a detection element that detects infrared rays, submillimeter waves, X-rays, or other electromagnetic waves, and such a detection element is a component of an observation device that is used for astronomical observation. Each of the vacuum chamber 18, the first-stage radiation shield 20, and the second-stage radiation shield 22 is provided with an observation window 28 through which electromagnetic waves to be detected by the detection element pass. Accordingly, electromagnetic waves can be incident on the detection element from the outside of the cryogenic system 10 through the observation windows 28.
The cryogenic cooling unit 26 is cooled by the JT cryocooler 16. The cryogenic cooling unit 26 is also called the cooling stage of the JT cryocooler 16. The object 24 to be cooled is in physical contact with the cryogenic cooling unit 26 and is thermally coupled to the cryogenic cooling unit 26, or is thermally coupled to the cryogenic cooling unit 26 through a heat transfer member. The cryogenic cooling unit 26 can be cooled to a temperature range lower than, for example, 4K (for example, the range of 1K to 4K). Accordingly, the cryogenic cooling unit 26 can cool the object 24 to be cooled to the temperature range.
The cryogenic system 10 is adapted to be capable of being mounted on, for example, a spacecraft, such as an artificial satellite, together with an observation device that includes the above-mentioned electromagnetic wave detection element. The cryogenic system 10 may be mounted on a ground facility including such an observation device. The cryogenic system 10 may be mounted on a spacecraft or a ground facility together with, for example, a superconducting device or other devices for which a cryogenic environment is desirable.
The single-stage precooler 12 is a single-stage Stirling cryocooler. The single-stage precooler 12 includes a first compressor 30, a single-stage cold head 32 as an expander, and a first connecting pipe 34 that connects the first compressor 30 to the single-stage cold head 32. The first connecting pipe 34 provides a gas flow channel that circulates a refrigerant gas (for example, a helium gas) between the first compressor 30 and the single-stage cold head 32. The single-stage cold head 32 includes a first-stage cooling stage 32a. The normal-temperature portion of the single-stage precooler 12 includes the first compressor 30 and the first connecting pipe 34, and the low-temperature portion of the single-stage precooler 12 includes the first-stage cooling stage 32a.
The first compressor 30 is adapted to generate the pressure oscillation of the refrigerant gas. The generated pressure oscillation is transmitted to the single-stage cold head 32 through the first connecting pipe 34. The single-stage cold head 32 is adapted to induce pressure oscillation where the pressure oscillation transmitted from the first compressor 30 has a phase difference at the same frequency as the pressure oscillation in the single-stage cold head 32. Accordingly, a refrigeration cycle (specifically, reverse Stirling cycle) is formed between the first compressor 30 and the single-stage cold head 32.
In this way, the first-stage cooling stage 32a of the single-stage precooler 12 is cooled to a first-stage cooling temperature. The first-stage cooling temperature of the single-stage precooler 12 is selected from the temperature range of, for example, 50Ktol50K. The first-stage cooling temperature may be in the temperature range of, for example, 80K to 120K, and may be, for example, about 100K.
The two-stage precooler 14 is a two-stage Stirling cryocooler. The two-stage precooler 14 includes a second compressor 36, a two-stage cold head 38 as an expander, and a second connecting pipe 40 that connects the second compressor 36 to the two-stage cold head 38. The second connecting pipe 40 provides a gas flow channel that circulates a refrigerant gas (for example, a helium gas) between the second compressor 36 and the two-stage cold head 38. The two-stage cold head 38 includes a first-stage cooling stage 38a and a second-stage cooling stage 38b. The normal-temperature portion of the two-stage precooler 14 includes the second compressor 36 and the second connecting pipe 40, and the low-temperature portion of the two-stage precooler 14 includes the first-stage cooling stage 38a and the second-stage cooling stage 38b. The second-stage cooling stage 38b is disposed inside the first-stage radiation shield 20.
The second compressor 36 is adapted to generate the pressure oscillation of a refrigerant gas. The generated pressure oscillation is transmitted to the two-stage cold head 38 through the second connecting pipe 40. The two-stage cold head 38 is adapted to induce pressure oscillation where the pressure oscillation transmitted from the second compressor 36 has a phase difference at the same frequency as the pressure oscillation in the two-stage cold head 38. Accordingly, a refrigeration cycle (specifically, reverse Stirling cycle) is formed between the second compressor 36 and the two-stage cold head 38.
In this way, the first-stage cooling stage 38a of the two-stage precooler 14 is cooled to a first-stage cooling temperature, and the second-stage cooling stage 38b is cooled to a second-stage cooling temperature. The first-stage cooling temperature of the two-stage precooler 14 is selected from the temperature range of, for example, 50K to 150K. The first-stage cooling temperature maybe in the temperature range of, for example, 80K to 120K, and may be, for example, about 100K. The second-stage cooling temperature is lower than the first-stage cooling temperature. The second-stage cooling temperature is selected from the temperature range of, for example, 10K to 25K. The second-stage cooling temperature may be in the temperature range of, for example, 15K to 20K, and may be, for example, about 15K.
The first-stage cooling temperature of the two-stage precooler 14 may be equal to the first-stage cooling temperature of the single-stage precooler 12. Alternatively, the first-stage cooling temperature of the two-stage precooler 14 may be different from the first-stage cooling temperature of the single-stage precooler 12. For example, the first-stage cooling temperature of the two-stage precooler 14 may be lower than the first-stage cooling temperature of the single-stage precooler 12. In this case, the first-stage cooling stage 38a of the two-stage precooler 14 may be disposed inside the first-stage radiation shield 20.
The first-stage radiation shield 20 is thermally coupled to the first-stage cooling stage 32a of the single-stage precooler 12, and is thermally isolated from the first-stage cooling stage 38a of the two-stage precooler 14. The first-stage radiation shield 20 is in physical contact with the first-stage cooling stage 32a of the single-stage precooler 12 and is thermally coupled to the first-stage cooling stage 32a, or is thermally coupled to the first-stage cooling stage 32a through a heat transfer member. The first-stage radiation shield 20 is disposed so as to be apart from the first-stage cooling stage 38a of the two-stage precooler 14, and is not in physical contact with the first-stage cooling stage 38a. The first-stage radiation shield 20 may be supported by the first-stage cooling stage 38a with a heat insulating member therebetween. In this way, the first-stage radiation shield 20 is cooled to the first-stage cooling temperature of the single-stage precooler 12 by the first-stage cooling stage 32a of the single-stage precooler 12.
The second-stage radiation shield 22 is thermally coupled to the second-stage cooling stage 38b of the two-stage precooler 14. The second-stage radiation shield 22 is in physical contact with the second-stage cooling stage 38b and is thermally coupled to the second-stage cooling stage 38b, or is thermally coupled to the second-stage cooling stage 38b through a heat transfer member. In this way, the second-stage radiation shield 22 is cooled to the second-stage cooling temperature of the two-stage precooler 14 by the second-stage cooling stage 38b.
The JT cryocooler 16 includes a JT compression system 46, a heat exchanger group 48, a first-stage precooling unit 50, a second-stage precooling unit 52, a JT valve 54, and a refrigerant circulation line 56 that connects these components. A refrigerant, which is circulated in the JT cryocooler 16, is, for example, helium (helium-3 or helium-4). The heat exchanger group 48 includes a series of counterflow heat exchangers (48a to 48c). The refrigerant circulation line 56 includes a refrigerant supply line 56a that connects the discharge side of the JT compression system 46 to the supply side of the cryogenic cooling unit 26, and a refrigerant collection line 56b that connects the collection side of the cryogenic cooling unit 26 to the suction side of the JT compression system 46. The refrigerant circulation line 56 is isolated from both the single-stage precooler 12 and the two-stage precooler 14 with respect to fluid.
The JT compression system 46 is adapted to increase the pressure of a refrigerant gas, which is collected from the refrigerant collection line 56b, and to send the refrigerant gas to the refrigerant supply line 56a. The JT compression system 46 functions as a refrigerant source that circulates a refrigerant in the refrigerant circulation line 56. The JT compression system 46 is disposed outside the vacuum chamber 18.
For example, the JT compression system 46 has a two-stage compression structure that includes a first-stage JT compressor 46a and a second-stage JT compressor 46b connected in series. A refrigerant gas having low pressure equal to, for example, about the atmospheric pressure is collected to the JT compression system 46 from the refrigerant supply line 56a. The first-stage JT compressor 46a can increase the pressure of the collected refrigerant gas to, for example, about several atmospheres. The second-stage JT compressor 46b can increase the pressure of the refrigerant gas, of which the pressure is increased by the first-stage JT compressor 46a, to, for example, about several tens of atmospheres. A high-pressure refrigerant gas, which is obtained in this way, is sent to the refrigerant supply line 56a from the JT compression system 46. The JT compression system 46 may have other multi-stage compression structures, and may have a single-stage compression structure.
The heat exchanger group 48 is disposed on the refrigerant circulation line 56 between the JT compression system 46 and the cryogenic cooling unit 26. The heat exchanger group 48 has a three-stage structure that includes a first heat exchanger 48a, a second heat exchanger 48b, and a third heat exchanger 48c. The first heat exchanger 48a cools a high-temperature (for example, a normal temperature, for example, about 300K) refrigerant gas that flows into the vacuum chamber 18 from the outside of the vacuum chamber 18. The second heat exchanger 48b further cools the refrigerant that is cooled by the first heat exchanger 48a and the first-stage precooling unit 50. The third heat exchanger 48c further cools the refrigerant that is cooled by the second heat exchanger 48b and the second-stage precooling unit 52. The heat exchanger group 48 may have other multi-stage structures.
The refrigerant supply line 56a includes a high pressure-side flow channel of each of the first heat exchanger 48a, the second heat exchanger 48b, and the third heat exchanger 48c, and the refrigerant collection line 56b includes a low pressure-side flow channel of each of the first heat exchanger 48a, the second heat exchanger 48b, and the third heat exchanger 48c. A refrigerant, which flows through the high pressure-side flow channel, can be cooled in each heat exchanger by heat exchange between the high pressure-side flow channel and the low pressure-side flow channel. The high pressure-side flow channel and the low pressure-side flow channel can also be called a high temperature-side flow channel and a low temperature-side flow channel, respectively.
The first heat exchanger 48a is disposed between the vacuum chamber 18 and the first-stage radiation shield 20, that is, in a space inside the vacuum chamber 18 and outside the first-stage radiation shield 20. The second heat exchanger 48b is disposed between the first-stage radiation shield 20 and the second-stage radiation shield 22, that is, in a space inside the first-stage radiation shield 20 and outside the second-stage radiation shield 22. The third heat exchanger 48c is disposed inside the second-stage radiation shield 22.
The first-stage precooling unit 50 is thermally coupled to the first-stage cooling stage 32a of the single-stage precooler 12, and is thermally isolated from the first-stage cooling stage 38a of the two-stage precooler 14. A refrigerant, which flows through the first-stage precooling unit 50, is cooled by heat exchange with the first-stage cooling stage 32a of the single-stage precooler 12. The first-stage precooling unit 50 is disposed on the refrigerant supply line 56a between the first and second heat exchangers 48a and 48b.
The second-stage precooling unit 52 is thermally coupled to the second-stage cooling stage 38b of the two-stage precooler 14. A refrigerant, which flows through the second-stage precooling unit 52, is cooled by the second-stage cooling stage 38b. The second-stage precooling unit 52 is disposed on the refrigerant supply line 56a between the second and third heat exchangers 48b and 48c.
The JT valve 54 is disposed on the refrigerant supply line 56a between the heat exchanger of the last stage (in this embodiment, the third heat exchanger 48c) of the heat exchanger group 48 and the cryogenic cooling unit 26. The JT valve 54 is, for example, a fixed orifice.
Further, the JT cryocooler 16 includes a bypass channel 58 that bypasses the JT valve 54, and the bypass channel 58 includes a bypass valve 60. For example, the bypass channel 58 is branched from the refrigerant supply line 56a between the second-stage precooling unit 52 and the third heat exchanger 48c, and joins the refrigerant supply line 56a between the JT valve 54 and the cryogenic cooling unit 26 again. The bypass valve 60 is, for example, a gas pressure-driven type on-off valve. The bypass valve 60 may be an electromagnetic on-off valve, a mechanical on-off valve, a manual on-off valve, or other driven-type on-off valves.
In a case in which the bypass valve 60 is opened, a refrigerant flows through the bypass channel 58. In a case in which the bypass valve 60 is closed, a refrigerant flows through the JT valve 54. The bypass valve 60 is opened to precool the JT cryocooler 16 by using the single-stage precooler 12 and the two-stage precooler 14 at the time of the start of the JT cryocooler 16. The bypass valve 60 is closed during the normal operation of the JT cryocooler 16.
A refrigerant flows through the refrigerant circulation line 56 as described below during the normal operation of the JT cryocooler 16. A high-pressure refrigerant, which is compressed by the JT compression system 46, is supplied to the high pressure-side flow channel of the first heat exchanger 48a first. The high-pressure refrigerant, which flows through the high pressure-side flow channel of the first heat exchanger 48a, is cooled by exchanging heat with a return low-pressure refrigerant that flows through the low pressure-side flow channel of the first heat exchanger 48a. The high-pressure refrigerant, which is cooled by the first heat exchanger 48a, flows into the first-stage precooling unit 50 through the refrigerant supply line 56a.
The high-pressure refrigerant is cooled in the first-stage precooling unit 50 by the first-stage cooling stage 32a of the single-stage precooler 12, and is sent to the high pressure-side flow channel of the second heat exchanger 48b. The high-pressure refrigerant, which flows through the high pressure-side flow channel of the second heat exchanger 48b, is cooled by exchanging heat with a return low-pressure refrigerant that flows through the low pressure-side flow channel of the second heat exchanger 48b. The high-pressure refrigerant, which is cooled by the second heat exchanger 48b, flows into the second-stage precooling unit 52 through the refrigerant supply line 56a.
The high-pressure refrigerant is cooled in the second-stage precooling unit 52 by the second-stage cooling stage 38b of the two-stage precooler 14, and is sent to the high pressure-side flow channel of the third heat exchanger 48c. The high-pressure refrigerant, which flows through the high pressure-side flow channel of the third heat exchanger 48c, is cooled by exchanging heat with a return low-pressure refrigerant that flows through the low pressure-side flow channel of the third heat exchanger 48c. The high-pressure refrigerant is cooled to a temperature equal to or lower than a temperature at which Joule-Thomson effect is expected in this way, and is sent to the JT valve 54.
When passing through the JT valve 54, the cooled high-pressure refrigerant becomes a mist-like low-pressure refrigerant, which is in a state where a gas and liquid are mixed, due to Joule-Thomson effect. Accordingly, cooling capacity in the temperature range of a liquefied refrigerant is generated. The mist-like low-pressure refrigerant is sent to the cryogenic cooling unit 26. In a case in which the refrigerant is helium as described above, the cryogenic cooling unit 26 is cooled to the temperature range of liquid helium and the object 24 to be cooled can be cooled to the temperature by the cryogenic cooling unit 26.
The mist-like low-pressure refrigerant is evaporated and is vaporized again in a case in which the mist-like low-pressure refrigerant cools the cryogenic cooling unit 26. A refrigerant, which is not liquefied at the JT valve 54, and a refrigerant, which is vaporized by evaporation, return to the low pressure-side flow channel of the third heat exchanger 48c. The low-pressure refrigerant flows through the refrigerant collection line 56b in the order of the second heat exchanger 48b and the first heat exchanger 48a. In this case, the temperature of the low-pressure refrigerant rises while the low-pressure refrigerant cools the high-pressure refrigerant at each of the heat exchangers (48c, 48b, and 48a) as described above. The low-pressure refrigerant, which returns to the normal temperature in this way, gets out of the vacuum chamber 18, is collected to the JT compression system 46, and is compressed again.
The JT cryocooler 16 is not limited to the above-mentioned specific structure, and can appropriately employ various typical structures.
In this way, the cryogenic system 10 can cool the cryogenic cooling unit 26 and the object 24 to be cooled to a temperature lower than the second-stage cooling temperature of the two-stage precooler 14, for example, a desired temperature lower than 4K (for example, a temperature of 1K to 4K).
The number of times of a refrigeration cycle per unit time in the single-stage precooler 12, that is, the frequency of the pressure oscillation of a refrigerant gas in the single-stage cold head 32 (or the first compressor 30) is referred to as a "first operating frequency" in this specification. The first operating frequency is based on the dynamic characteristics of the single-stage precooler 12. Accordingly, in a case in which the single-stage precooler 12 has a certain design, the first operating frequency has a fixed value based on the design.
Further, the number of times of a refrigeration cycle per unit time in the two-stage precooler 14, that is, the frequency of the pressure oscillation of a refrigerant gas in the two-stage cold head 38 (or the second compressor 36) is referred to as a "second operating frequency" in this specification. The second operating frequency is based on the dynamic characteristics of the two-stage precooler 14. Accordingly, in a case in which the two-stage precooler 14 has a certain design, the second operating frequency has a fixed value based on the design.
Incidentally, a first stage and a second stage of a cold head of a two-stage regenerative cryocooler, such as the two-stage cold head 38 of the two-stage precooler 14, are filled with different regenerator materials. Regenerator materials, which are optimal for the first-stage cooling temperature and the second-stage cooling temperature, can be used. The efficiency of heat exchange with a refrigerant gas and a pressure loss vary at the first stage and the second stage due to a difference in a cooling temperature and regenerator material. For this reason, an optimal operating frequency at which refrigeration efficiency (for example, refrigeration capacity per unit power consumption) is improved originally varies at the first stage and the second stage. However, since a two-stage cryocooler is adapted so that a first stage is driven in synchronization with a second stage, it is difficult for the operating frequencies at the first and second stages to be made different from each other due to the structure of the two-stage cryocooler. Accordingly, it is difficult to individually optimize the operating frequencies at the first and second stages in one two-stage cryocooler. For example, in a case in which an operating frequency optimal for the first stage is selected, the operating frequency is not an operating frequency optimal for the second stage.
A high operating frequency is preferred to improve the refrigeration efficiency of the first stage, and a low operating frequency is preferred to improve the refrigeration efficiency of the second stage. The main cause thereof is that the time constant of heat conduction, that is, thermal responsiveness of the regenerator material of the first stage is different from that of the regenerator material of the second stage. The time constant of heat conduction of the regenerator material of the second stage suitable for a lower temperature is generally larger than that of the regenerator material of the first stage, and the thermal responsiveness of the regenerator material of the second stage suitable for a lower temperature is generally more gentle than that of the regenerator material of the first stage. For this reason, even though the two-stage cryocooler is operated at an excessively high operating frequency, it is difficult for the second cooling stage to be cold.
Since the single-stage precooler 12 and the two-stage precooler 14 are formed of individual cryogenic cryocoolers, the single-stage precooler 12 and the two-stage precooler 14 can be operated independently of each other. Since the refrigerant gas circuit of the single-stage precooler 12 and the refrigerant gas circuit of the two-stage precooler 14 are isolated from each other with respect to fluid, the pressure oscillation of a refrigerant gas in the single-stage precooler 12 and the pressure oscillation of a refrigerant gas in the two-stage precooler 14 do not affect each other. Accordingly, the first operating frequency of the single-stage precooler 12 and the second operating frequency of the two-stage precooler 14 can be set to values different from each other.
Therefore, in this embodiment, the second operating frequency of the two-stage precooler 14 is lower than the first operating frequency of the single-stage precooler 12. In this case, refrigeration efficiency at the second-stage cooling stage 38b of the two-stage precooler 14 can be improved, and refrigeration efficiency at the first-stage cooling stage 32a of the single-stage precooler 12 can also be improved. Since the single-stage precooler 12 and the two-stage precooler 14 are used together, the first operating frequency and the second operating frequency can be individually optimized.
In a case in which the single-stage precooler 12 is a single-stage Stirling cryocooler, the first operating frequency is selected from the frequency range of, for example, 35 Hz to 100 Hz. Since the lower limit of the first operating frequency is set to 35 Hz, it is easy to ensure the refrigeration capacity of the single-stage precooler 12 at a desired first-stage cooling temperature. As the first operating frequency is higher, it is easier to ensure refrigeration capacity but it is difficult to ensure the mechanical reliability of the single-stage precooler 12. For example, there is a concern that the mechanical strength of a movable component, which is provided in the single-stage precooler 12, and the mechanical strength of the peripheral structure thereof may be easily reduced due to fatigue. Accordingly, it is desirable that the upper limit of the first operating frequency is set to 100 Hz.
The first operating frequency may be in the frequency range of, for example, 45 Hz to 65 Hz or the frequency range of, for example, 50 Hz to 60 Hz, and may be, for example, about 50 Hz. In this case, refrigeration capacity can be ensured, and the single-stage precooler 12 can be operated at an optimal operating frequency that is determined considering mechanical reliability.
In a case in which the two-stage precooler 14 is a two-stage Stirling cryocooler, the second operating frequency is selected from the frequency range of, for example, 1 Hz to 30 Hz. Since a range from which the second operating frequency can be selected is set to the range of 1 Hz to 30 Hz, it is easy to ensure the refrigeration capacity of the two-stage precooler 14 at a desired second-stage cooling temperature.
The second operating frequency may be in the frequency range of, for example, 10 Hz to 20 Hz, and may be, for example, about 15 Hz. In this case, refrigeration efficiency at the second-stage cooling stage 38b of the two-stage precooler 14 can be further improved.
Since the second operating frequency is relatively low, refrigeration capacity at the first-stage cooling stage 38a of the two-stage precooler 14 can be suppressed. However, the first-stage cooling stage 38a is thermally isolated from the first-stage radiation shield 20 and the first-stage precooling unit 50. The first-stage cooling stage 38a is used for only the cooling of the second-stage cooling stage 38b. For example, the thermal load of the first-stage cooling stage 38a is small, and the cooling of the second-stage cooling stage 38b can be covered even by the relatively small refrigeration capacity of the first-stage cooling stage 38a.
Since the two-stage precooler 14 can be operated independently of the single-stage precooler 12, the influence of the deterioration of performance of the single-stage precooler 12 on the performance of the two-stage precooler 14 is minor even if the refrigeration capacity of the single-stage precooler 12 deteriorates with time. Further, since the second operating frequency is lower than the first operating frequency, there is also an advantage that it is more difficult for the first-stage refrigeration capacity of the two-stage precooler 14 to deteriorate with time than the refrigeration capacity of the single-stage precooler 12.
Lastly, the test calculation of the improvement of efficiency performed by the inventor will be introduced. Further, a case in which only a two-stage Stirling cryocooler is used as a precooler for the JT cryocooler will be considered as Comparative Example. For example, in a case in which an operating frequency is 15 Hz and power consumption is 80 W, the first-stage refrigeration capacity of a certain typical two-stage Stirling cryocooler is 1W@100K and the second-stage refrigeration capacity thereof is 0.2W@20K. Assuming that the temperature of the normal-temperature portion is 300K, Carnot efficiency η1 of the first-stage portion satisfies "η1=100K/(300K-100K)=0.5" and Carnot efficiency η2 of the second-stage portion satisfies "η2=20K/(300K-20K)=0.0714". Ideal work in this case satisfies "W1=1W/0.5=2W"and"W2=0.2W/0.0714=2.8W". Accordingly, aratio of refrigeration capacity per unit power consumption satisfies "(2W+2.8W)/80W=0.06".
For example, the case of Example will be described below. For example, in a case in which an operating frequency is 52 Hz and power consumption is 30 W, the first-stage refrigeration capacity of a single-stage Stirling cryocooler as the single-stage precooler 12 is 2W@100K. In this case, since Carnot efficiency satisfies "η1=100K/(300K-100K)=0.5" as described above, ideal work satisfies "W1=2W/0.5=4W". In a case in which the first-stage portion of a two-stage Stirling cryocooler as the two-stage precooler 14 is simply substituted with this single-stage Stirling cryocooler, a ratio of refrigeration capacity per unit power consumption satisfies "(4W+2.8W)/80=0.085". Accordingly, a ratio of refrigeration capacity per unit power consumption in Example is improved by about 41.7% as compared to that in Comparative Example (0.085/0.06=1.417) .
According to the cryogenic system 10 of this embodiment, it is possible to build a more efficient cooling system as described above. It is possible to improve refrigeration efficiency in a temperature range lower than, for example, 4K (for example, the range of 1K to 4K) under certain given power consumption.
The invention has been described above on the basis of Example.
The single-stage precooler 12 is not limited to a single-stage Stirling cryocooler. The single-stage precooler 12 may be a single-stage Stirling type pulse tube cryocooler. The two-stage precooler 14 is not limited to a two-stage Stirling cryocooler. The two-stage precooler 14 may be a two-stage Stirling type pulse tube cryocooler. In this case, the optimal values of the first and second operating frequencies can be different from those in a case in which the single-stage precooler 12 is a single-stage Stirling cryocooler and the two-stage precooler 14 is a two-stage Stirling cryocooler. Accordingly, even though the single-stage precooler 12 is a single-stage Stirling type pulse tube cryocooler and the two-stage precooler 14 is a two-stage Stirling type pulse tube cryocooler, it is expected that refrigeration efficiency is improved in a case in which the second operating frequency is set to be lower than the first operating frequency.
Even in a case in which the single-stage precooler 12 is a single-stage Stirling type pulse tube cryocooler and the two-stage precooler 14 is a two-stage Stirling type pulse tube cryocooler, there is also a possibility that optimal operating frequencies may be the same as those in a case in which these two precoolers are Stirling cryocoolers. Accordingly, in a case in which the single-stage precooler 12 is a single-stage Stirling type pulse tube cryocooler, the first operating frequency may be selected from the frequency range of, for example, 35 Hz to 100 Hz. The first operating frequency may be in the frequency range of, for example, 45 Hz to 65 Hz or the frequency range of, for example, 50 Hz to 60 Hz, and may be, for example, about 50 Hz. In a case in which the two-stage precooler 14 is a two-stage Stirling type pulse tube cryocooler, the second operating frequency may be selected from the frequency range of, for example, 1 Hz to 30 Hz. The second operating frequency may be in the frequency range of, for example, 10 Hz to 20 Hz, and may be, for example, about 15 Hz.
Further, in a case in which there is a margin in the refrigeration capacity of the first-stage cooling stage 38a of the two-stage precooler 14, the first-stage cooling stage 38a maybe used for the cooling of anyone of the first-stage precooling unit 50 and the first-stage radiation shield 20. For example, the first-stage precooling unit 50 may be thermally coupled to the first-stage cooling stage 32a of the single-stage precooler 12 and may be thermally isolated from the first-stage cooling stage 38a of the two-stage precooler 14, and the first-stage radiation shield 20 maybe thermally isolated from the first-stage cooling stage 32a of the single-stage precooler 12 and may be thermally coupled to the first-stage cooling stage 38a of the two-stage precooler 14. Alternatively, the first-stage radiation shield 20 may be thermally coupled to the first-stage cooling stage 32a of the single-stage precooler 12 and may be thermally isolated from the first-stage cooling stage 38a of the two-stage precooler 14, and the first-stage precooling unit 50 may be thermally isolated from the first-stage cooling stage 32a of the single-stage precooler 12 and may be thermally coupled to the first-stage cooling stage 38a of the two-stage precooler 14. As described above, in the embodiment, the first-stage precooling unit 50, the first-stage radiation shield 20, or both the first-stage precooling unit 50 and the first-stage radiation shield 20 may be thermally coupled to the first-stage cooling stage 32a of the single-stage precooler 12 and may be thermally isolated from the first-stage cooling stage 38a of the two-stage precooler 14.

Brief Description of the Reference Symbols

10:: cryogenic system
12:: single-stage precooler
14:: two-stage precooler
16:: JT cryocooler
18:: vacuum chamber
20:: first-stage radiation shield
22:: second-stage radiation shield
24:: object to be cooled
26:: cryogenic cooling unit
32a:: first-stage cooling stage
38a:: first-stage cooling stage
38b:: second-stage cooling stage
50:: first-stage precooling unit
52:: second-stage precooling unit

Claims

A cryogenic system (10) comprising:
a single-stage precooler (12) that includes a first-stage cooling stage (32a);

a two-stage precooler (14) that includes a first-stage cooling stage (38a) and a second-stage cooling stage (38b); and

a JT cryocooler (16) that includes a first-stage precooling unit (50) configured to be cooled by the first-stage cooling stage (32a) of the single-stage precooler (12) and a second-stage precooling unit (52) configured to be cooled by the second-stage cooling stage (38b),

wherein the single-stage precooler (12) is a single-stage Stirling cryocooler or a single-stage Stirling type pulse tube cryocooler and is configured to be operated at a first operating frequency,

the two-stage precooler (14) is a two-stage Stirling cryocooler or a two-stage Stirling type pulse tube cryocooler and is configured to be operated at a second operating frequency lower than the first operating frequency, and

the first-stage precooling unit (50) is thermally coupled to the first-stage cooling stage (32a) of the single-stage precooler (12) and is thermally isolated from the first-stage cooling stage (38a) of the two-stage precooler (14).
The cryogenic system (10) according to claim 1, further comprising:
a first-stage radiation shield (20) that surrounds the second-stage cooling stage (38b) and the second-stage precooling unit (52) and is cooled by the first-stage cooling stage (32a) of the single-stage precooler (12),

wherein the first-stage radiation shield (20) is thermally coupled to the first-stage cooling stage (32a) of the single-stage precooler (12) and is thermally isolated from the first-stage cooling stage (38a) of the two-stage precooler (14).
The cryogenic system (10) according to claim 1 or 2,
wherein the first operating frequency is selected from a range of 35 Hz to 100 Hz and the second operating frequency is selected from a range of 1 Hz to 30 Hz.
The cryogenic system (10) according to claim 3,
wherein the first operating frequency is selected from a range of 50 Hz to 60 Hz and the second operating frequency is selected from a range of 10 Hz to 20 Hz.
The cryogenic system (10) according to claim 1,
wherein the two-stage precooler (14) is filled with different regenerator materials in a first stage and a second stage.