JOULE-THOMSON CRYOCOOLER

Abstract

A Joule-Thomson cryocooler includes a pre-cooling cryocooler that includes a pre-cooling stage, a refrigerant circuit that includes a heat exchanger and a refrigerant pipe extending from the heat exchanger and cooled by the pre-cooling stage, and a heat conduction path that is provided separately from the refrigerant pipe and connects the pre-cooling stage to the heat exchanger to enable conductive cooling of the heat exchanger through the pre-cooling stage.

Claims

1. A Joule-Thomson cryocooler comprising: a pre-cooling cryocooler that comprises a pre-cooling stage; a refrigerant circuit that comprises a heat exchanger and a refrigerant pipe extending from the heat exchanger and cooled by the pre-cooling stage; and a heat conduction path that is provided separately from the refrigerant pipe and connects the pre-cooling stage to the heat exchanger to enable conductive cooling of the heat exchanger through the pre-cooling stage.

2. The Joule-Thomson cryocooler according to claim 1, wherein the pre-cooling cryocooler comprises a cylinder extending from the pre-cooling stage, and the heat conduction path comprises: a first heat conduction member thermally coupled to the pre-cooling stage and extending from the pre-cooling stage to a side opposite to the cylinder, and a second heat conduction member that thermally couples the first heat conduction member to the heat exchanger.

3. The Joule-Thomson cryocooler according to claim 1, wherein the pre-cooling cryocooler comprises a first pre-cooling stage and a second pre-cooling stage that is cooled to a lower temperature than the first pre-cooling stage, the refrigerant circuit comprises a first heat exchanger, a second heat exchanger that further cools a refrigerant cooled by the first heat exchanger, and a third heat exchanger that further cools the refrigerant cooled by the second heat exchanger, and the refrigerant pipe extends from the second heat exchanger via the second pre-cooling stage to the third heat exchanger, and the heat conduction path connects the second pre-cooling stage to at least one of the second heat exchanger and the third heat exchanger to enable conductive cooling of the at least one of the second heat exchanger and the third heat exchanger through the second pre-cooling stage.

4. The Joule-Thomson cryocooler according to claim 3, wherein the heat conduction path connects the second pre-cooling stage to the second heat exchanger to enable conductive cooling of the second heat exchanger through the second pre-cooling stage.

5. The Joule-Thomson cryocooler according to claim 3, wherein the pre-cooling cryocooler comprises a cylinder that connects the second pre-cooling stage to the first pre-cooling stage, and the heat conduction path comprises: a stage extension component thermally coupled to the pre-cooling stage and extending from the pre-cooling stage to a side opposite to the cylinder, and a heat conduction plate that thermally couples the stage extension component to the at least one of the second heat exchanger and the third heat exchanger.

6. The Joule-Thomson cryocooler according to claim 5, wherein the at least one of the second heat exchanger and the third heat exchanger and the stage extension component are disposed on the same side of the heat conduction plate.

7. The Joule-Thomson cryocooler according to claim 1, wherein the heat conduction path comprises a thermal resistance element.

8. The Joule-Thomson cryocooler according to claim 2, wherein the heat conduction path comprises a thermal resistance element between the heat exchanger and the second heat conduction member, and a thermal conductivity of the thermal resistance element is smaller than a thermal conductivity of the second heat conduction member.

9. The Joule-Thomson cryocooler according to claim 3, wherein the heat conduction path connects the second pre-cooling stage to the third heat exchanger via a thermal resistance element.

10. The Joule-Thomson cryocooler according to claim 5, wherein the heat conduction path comprises a thermal resistance element between the third heat exchanger and the heat conduction plate, and a thermal conductivity of the thermal resistance element is smaller than a thermal conductivity of the heat conduction plate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1 is a diagram schematically showing a cryogenic cooling device according to an embodiment.

[0006] FIG. 2 is a diagram schematically showing an exemplary configuration of a heat conduction path according to the embodiment.

[0007] FIG. 3 is a diagram schematically showing a modification example of the heat conduction path according to the embodiment.

DETAILED DESCRIPTION

[0008] When a JT cryocooler is started, so-called cool-down is performed in which the JT cryocooler is cooled from an environmental temperature (for example, a room temperature of about 300 K) to a target cryogenic temperature. A refrigerant gas supplied to a JT valve is cooled from the environmental temperature to a temperature equal to or lower than an inversion temperature by the cool-down. Since the cool-down is merely a preparation for the JT cryocooler to cool a desired object to be cooled to a cryogenic temperature, it is desirable that a time required for the cool-down is as short as possible.

[0009] It is desirable to shorten a cool-down time of the JT cryocooler.

[0010] Hereinafter, an embodiment for carrying out the present invention will be described in detail with reference to the drawings. In the description and the drawings, the same or equivalent components, members, and processes are denoted by the same reference numerals, and overlapping description is omitted as appropriate. The scale and the shape of each of parts illustrated in the drawings are set for convenience to make the description easy to understand, and are not to be interpreted as limiting unless stated otherwise. The embodiment is merely an example and does not limit the scope of the present invention. All features described in the embodiment or combinations thereof are not necessarily essential to the present invention.

[0011] FIG. 1 is a diagram schematically showing a cryogenic cooling device 10 according to an embodiment. The cryogenic cooling device 10 includes a vacuum chamber 12, a radiation shield 14, and a JT cryocooler 18 for cooling an object 16 to be cooled.

[0012] The vacuum chamber 12 may be, for example, a cryostat, and is configured to provide a cryogenic vacuum environment therein. The vacuum chamber 12 is formed of a metal material such as stainless steel or other suitable high-strength material to withstand ambient pressure (for example, atmospheric pressure). The radiation shield 14, a low-temperature section of the JT cryocooler 18, and the object 16 to be cooled are disposed in the vacuum chamber 12.

[0013] The radiation shield 14 is disposed in the vacuum chamber 12 to surround the low-temperature section of the JT cryocooler 18 and the object 16 to be cooled, and suppresses inflow of radiant heat from the vacuum chamber 12 to the JT cryocooler 18 and the object 16 to be cooled. The radiation shield 14 is formed of a high thermal conductivity metal material such as copper (for example, pure copper). An insulator such as a multi-layer insulator may be disposed between the vacuum chamber 12 and the radiation shield 14.

[0014] The object to be cooled 16 may be, for example, a superconducting device such as a superconducting coil, a measuring device that operates better at a cryogenic temperature, or other devices for use at a cryogenic temperature. Alternatively, the object 16 to be cooled may be, for example, a cryogenic fluid such as liquid helium, and the JT cryocooler 18 may be used for recondensing a vaporized cryogenic fluid.

[0015] The JT cryocooler 18 includes a pre-cooling cryocooler 20 and a refrigerant circuit 40 including a JT valve 30 and a final heat exchanger 32. A refrigerant flowing through the refrigerant circuit 40 is pre-cooled by the pre-cooling cryocooler 20, further cooled by JT expansion in the JT valve 30, and is supplied to the final heat exchanger 32. The object 16 to be cooled is cooled by heat exchange with the final heat exchanger 32. The cooled refrigerant is collected from the final heat exchanger 32, pressurized by a compressor described below, pre-cooled by the pre-cooling cryocooler 20 again, and is supplied to the JT valve 30. In this way, the refrigerant circulates through the refrigerant circuit 40. The JT cryocooler 18 is capable of cooling the final heat exchanger 32 to a temperature range of, for example, about 4 K or lower (for example, 1 K to 4 K), and thus, the object 16 to be cooled can be cooled to the temperature range.

[0016] The pre-cooling cryocooler 20 is, for example, a two-stage GM cryocooler. The pre-cooling cryocooler 20 includes a first compressor 21 and an expander 22 also referred to as a cold head. The expander 22 includes a drive unit 23, a first cylinder 24, a first pre-cooling stage 25, a second cylinder 26, and a second pre-cooling stage 27. The first compressor 21 is disposed in an ambient environment (for example, room temperature and atmospheric pressure environment), that is, outside the vacuum chamber 12. The expander 22 is installed in the vacuum chamber 12 such that the drive unit 23 is disposed outside the vacuum chamber 12 and the cylinders and the pre-cooling stages are disposed inside the vacuum chamber 12.

[0017] The first cylinder 24 connects the first pre-cooling stage 25 to the drive unit 23, so that the first pre-cooling stage 25 is structurally supported by the drive unit 23. The second cylinder 26 connects the second pre-cooling stage 27 to the first pre-cooling stage 25, so that the second pre-cooling stage 27 is structurally supported by the first pre-cooling stage 25. The first cylinder 24 and the second cylinder 26 extend coaxially, and the drive unit 23, the first cylinder 24, the first pre-cooling stage 25, the second cylinder 26, and the second pre-cooling stage 27 are linearly arranged in a line in this order. Typically, the first pre-cooling stage 25 and the second pre-cooling stage 27 are formed of a high thermal conductivity metal material such as copper (for example, pure copper), and the first cylinder 24 and the second cylinder 26 are formed of other metal materials such as stainless steel.

[0018] A first displacer and a second displacer (not shown) are disposed reciprocally in an inside of each of the first cylinder 24 and the second cylinder 26. A first regenerator and a second regenerator (not shown) are incorporated into the first displacer and the second displacer, respectively. In addition, the drive unit 23 includes a drive mechanism (not shown) such as a motor for reciprocating the first displacer and the second displacer. The drive mechanism includes a flow path switching mechanism that switches a flow path of a refrigerant gas to periodically repeat supply and exhaust of the refrigerant gas to and from an inside of the expander 22. A refrigerant gas of the pre-cooling cryocooler 20 is usually a helium gas, but other appropriate gases may be used.

[0019] The first compressor 21 is configured to collect the refrigerant gas from the expander 22, to pressurize the collected refrigerant gas, and to supply the refrigerant gas to the expander 22 again. By circulating the refrigerant gas between the first compressor 21 and the expander 22 with an appropriate combination of a pressure fluctuation and a volume fluctuation of the refrigerant gas in the expander 22, a thermodynamic cycle (for example, a GM cycle) for cold generation is configured, and the expander 22 can provide cryogenic cooling.

[0020] The first pre-cooling stage 25 is cooled to a first cooling temperature, and the second pre-cooling stage 27 is cooled to a second cooling temperature lower than the first cooling temperature. The first cooling temperature may be selected from a temperature range of, for example, 50 K or higher and 150 K or lower. The second cooling temperature may be selected from a temperature range of, for example, 10 K or higher and 25 K or lower.

[0021] The radiation shield 14 is thermally coupled to the first pre-cooling stage 25 by being in physical contact with the first pre-cooling stage 25, or is thermally coupled to the first pre-cooling stage 25 via a heat transfer member. Therefore, the radiation shield 14 is cooled to the first cooling temperature by the first pre-cooling stage 25.

[0022] The refrigerant circuit 40 includes a second compressor 41, a heat exchanger group 42, and a refrigerant supply line 44 and a refrigerant collection line 46 that connect these components in addition to the JT valve 30 and the final heat exchanger 32. A refrigerant gas that circulates through the refrigerant circuit 40 is usually a helium gas, but other appropriate gases may be used. The refrigerant circuit 40 is not limited to the specific configuration described herein and can adopt various typical configurations as appropriate.

[0023] The second compressor 41 is configured to pressurize the refrigerant gas collected from the refrigerant collection line 46 and to send the collected refrigerant gas to the refrigerant supply line 44. For understanding, a direction in which the refrigerant flows is indicated by an arrow in FIG. 1. The second compressor 41 serves as a refrigerant source that circulates the refrigerant in the refrigerant circuit 40. The second compressor 41 is disposed outside the vacuum chamber 12.

[0024] The heat exchanger group 42 in the refrigerant circuit 40 is disposed between the second compressor 41 and the final heat exchanger 32. The heat exchanger group 42 is composed of a series of counterflow heat exchangers (42a to 42c), and has a three-stage configuration of a first heat exchanger 42a, a second heat exchanger 42b, and a third heat exchanger 42c in the present embodiment. The first heat exchanger 42a is disposed between the vacuum chamber 12 and the radiation shield 14, that is, in a space inside the vacuum chamber 12 and outside the radiation shield 14. The second heat exchanger 42b, the third heat exchanger 42c, and the final heat exchanger 32 are disposed inside the radiation shield 14.

[0025] The first heat exchanger 42a cools a high-temperature (for example, a room temperature, for example, approximately 300 K) refrigerant gas flowing into the vacuum chamber 12 from the outside of the vacuum chamber 12. The second heat exchanger 42b further cools the refrigerant cooled by the first heat exchanger 42a and the first pre-cooling stage 25. The third heat exchanger 42c further cools the refrigerant cooled by the second heat exchanger 42b and the second pre-cooling stage 27.

[0026] The refrigerant supply line 44 connects a discharge side of the second compressor 41 to a refrigerant inlet of the final heat exchanger 32, and the refrigerant collection line 46 connects a refrigerant outlet of the final heat exchanger 32 to a suction side of the second compressor 41. The refrigerant supply line 44 includes a high pressure side flow path of each of the first heat exchanger 42a, the second heat exchanger 42b, and the third heat exchanger 42c, and the refrigerant collection line 46 includes a low pressure side flow path of each of the first heat exchanger 42a, the second heat exchanger 42b, and the third heat exchanger 42c. The refrigerant flowing through the high pressure side flow path can be cooled by heat exchange between the high pressure side flow path and the low pressure side flow path in each heat exchanger. The high pressure side flow path and the low pressure side flow path may be called a high temperature side flow path and a low temperature side flow path, respectively.

[0027] In addition, the refrigerant supply line 44 includes a first refrigerant pipe 44a and a second refrigerant pipe 44b. The refrigerant pipes are formed of, for example, a high thermal conductivity metal material such as copper (for example, pure copper).

[0028] The first refrigerant pipe 44a extends from the first heat exchanger 42a via the first pre-cooling stage 25 to the second heat exchanger 42b. The first refrigerant pipe 44a connects the high pressure side flow path of the first heat exchanger 42a to the high pressure side flow path of the second heat exchanger 42b. The first refrigerant pipe 44a is thermally coupled to the first pre-cooling stage 25, and the refrigerant flowing through the first refrigerant pipe 44a is cooled by the first pre-cooling stage 25. The first refrigerant pipe 44a may be fixed to the first pre-cooling stage 25 in a state of being wound around an outer peripheral surface of the first pre-cooling stage 25.

[0029] The second refrigerant pipe 44b extends from the second heat exchanger 42b via the second pre-cooling stage 27 to the third heat exchanger 42c. The second refrigerant pipe 44b connects the high pressure side flow path of the second heat exchanger 42b to the high pressure side flow path of the third heat exchanger 42c. The second refrigerant pipe 44b is thermally coupled to the second pre-cooling stage 27, and the refrigerant flowing through the second refrigerant pipe 44b is cooled by the second pre-cooling stage 27. The second refrigerant pipe 44b may be fixed to the second pre-cooling stage 27 in a state of being wound around an outer peripheral surface of the second pre-cooling stage 27.

[0030] The JT valve 30 is disposed between the last heat exchanger of the heat exchanger group 42 (in the present example, the third heat exchanger 42c) and the final heat exchanger 32 in the refrigerant supply line 44. The high pressure side flow path of the third heat exchanger 42c is connected to the refrigerant inlet of the final heat exchanger 32 via the JT valve 30. The JT valve 30 is a fixed orifice in the present embodiment. However, the JT valve 30 may be a variable orifice of which an opening degree is adjustable.

[0031] In the steady operation of the JT cryocooler 18, the refrigerant flows through the refrigerant circuit 40 as follows. The high-pressure refrigerant compressed by the second compressor 41 is first supplied to the high pressure side flow path of the first heat exchanger 42a. The high-pressure refrigerant flowing through the high pressure side flow path of the first heat exchanger 42a is cooled by heat exchange with the returning low-pressure refrigerant flowing through the low pressure side flow path of the first heat exchanger 42a. The high-pressure refrigerant cooled by the first heat exchanger 42a flows into the first refrigerant pipe 44a.

[0032] The high-pressure refrigerant flowing through the first refrigerant pipe 44a is cooled by the first pre-cooling stage 25 of the pre-cooling cryocooler 20 and is sent into the high pressure side flow path of the second heat exchanger 42b. The high-pressure refrigerant flowing through the high pressure side flow path of the second heat exchanger 42b is cooled by heat exchange with the returning low-pressure refrigerant flowing through the low pressure side flow path of the second heat exchanger 42b. The high-pressure refrigerant cooled by the second heat exchanger 42b flows into the second refrigerant pipe 44b.

[0033] The high-pressure refrigerant flowing through the second refrigerant pipe 44b is cooled by the second pre-cooling stage 27 of the pre-cooling cryocooler 20 and is sent into the high pressure side flow path of the third heat exchanger 42c. The high-pressure refrigerant flowing through the high pressure side flow path of the third heat exchanger 42c is cooled by heat exchange with the returning low-pressure refrigerant flowing through the low pressure side flow path of the third heat exchanger 42c. In this way, the high-pressure refrigerant is cooled to a temperature (that is, a temperature equal to or lower than an inversion temperature) at which the JT effect is expected, and is sent to the JT valve 30.

[0034] When the cooled high-pressure refrigerant passes through the JT valve 30, the cooled high-pressure refrigerant becomes a low-pressure refrigerant in a mist-like gas-liquid mixed state due to the Joule-Thomson effect, and generates a cooling capacity in a temperature range of the liquefied refrigerant. The mist-like low-pressure refrigerant is sent to the final heat exchanger 32. As described above, in a case where the refrigerant is helium, the final heat exchanger 32 can be cooled to a liquid helium temperature range. The final heat exchanger 32 can cool the object 16 to be cooled to the temperature through heat exchange with the object 16 to be cooled.

[0035] When cooling the final heat exchanger 32, the mist-like low-pressure refrigerant evaporates and vaporizes again. The unliquefied refrigerant and the refrigerant that is vaporized by evaporation in the JT valve 30 are returned to the low pressure side flow path of the third heat exchanger 42c. The low-pressure refrigerant flows through the refrigerant collection line 46 in the order of the third heat exchanger 42c, the second heat exchanger 42b, and the first heat exchanger 42a. In this case, the low-pressure refrigerant is heated while cooling the high-pressure refrigerant in each of the heat exchangers (42c, 42b, and 42a) as described above. In this way, the low-pressure refrigerant that has returned to the room temperature exits the vacuum chamber 12, is collected in the second compressor 41, and is compressed again.

[0036] In this way, the cryogenic cooling device 10 can cool the object 16 to be cooled to a desired temperature lower than the second cooling temperature of the pre-cooling cryocooler 20, for example, about 4 K or lower (for example, 1 K to 4 K).

[0037] Incidentally, when the JT cryocooler 18 is started, the JT cryocooler 18 is cooled from the environmental temperature (for example, a room temperature of about 300 K) to a target cryogenic temperature (for example, a minimum attainable temperature of less than 4 K). This initial cooling is also called cool-down. The refrigerant gas supplied to the JT valve 30 is cooled from the environmental temperature to a temperature equal to or lower than the inversion temperature by cool-down. Since the cool-down is merely a preparation for the JT cryocooler 18 to cool the object 16 to be cooled to a cryogenic temperature, it is desirable that a time required for the cool-down is as short as possible.

[0038] The JT valve 30 is designed to provide an optimal JT flow rate at a cryogenic temperature. Therefore, a flow rate of the refrigerant gas that can pass through the JT valve 30 in an initial stage of the cool-down where the refrigerant gas temperature is high can be considerably small. This can significantly increase a time required for the cool-down.

[0039] In a case where the JT valve 30 is the variable orifice, the JT valve 30 is opened wider than an optimal opening degree at the cryogenic temperature at the start of the cool-down to secure a large flow rate, and then the opening degree of the JT valve 30 is reduced as the cooling proceeds, thereby shortening the cool-down time. However, this measure is time-consuming. For example, a service technician may need to adjust the opening degree of the JT valve 30 to an appropriate level. In addition, in a case where the JT valve 30 is the fixed orifice, this measure cannot be adopted (as a result, it may take more than twice as long to cool the JT cryocooler 18 to a minimum attainable temperature (for example, approximately 4 K) compared to a case of the variable orifice).

[0040] In a cryocooler of an existing design, in general, a pre-cooling cryocooler and a heat exchanger of a JT refrigerant circuit are disposed so as not to be thermally connected to each other in order to prevent heat from entering the JT refrigerant circuit from the pre-cooling cryocooler during a normal operation. A pre-cooling stage of the pre-cooling cryocooler is supported by a surrounding structure with an insulating material such as fiber reinforced plastic, and there is substantially no heat conduction path from the pre-cooling stage to the heat exchanger.

[0041] This embodiment aims to shorten the time required for the cool-down by using the pre-cooling cryocooler 20. The JT cryocooler 18 includes a heat conduction path 48 that connects at least one pre-cooling stage and at least one heat exchanger of the pre-cooling cryocooler 20 to enable conductive cooling of at least one heat exchanger of the heat exchanger group 42 through the pre-cooling cryocooler 20. The heat conduction path 48 may connect the second pre-cooling stage 27 to at least one of the second heat exchanger 42b and the third heat exchanger 42c to enable conductive cooling of at least one of the second heat exchanger 42b and the third heat exchanger 42c through the second pre-cooling stage 27.

[0042] For example, the JT cryocooler 18 may include a first heat conduction path 48a that connects the second pre-cooling stage 27 to the second heat exchanger 42b to enable conductive cooling of the second heat exchanger 42b through the second pre-cooling stage 27. In addition, or instead of this, the JT cryocooler 18 may include a second heat conduction path 48b that connects the second pre-cooling stage 27 to the third heat exchanger 42c to enable conductive cooling of the third heat exchanger 42c through the second pre-cooling stage 27.

[0043] In order to prevent a heat load from being transferred from a high-temperature section to a low-temperature section of the JT cryocooler 18 when the cool-down is completed and the JT cryocooler 18 is in the normal operation, the heat conduction path 48 is installed such that a temperature difference between both ends of the heat conduction path 48 is as small as possible during the normal operation of the JT cryocooler 18. The heat conduction path 48 may be installed such that the temperature difference between both ends of the heat conduction path 48 is, for example, less than 5 K or less than 3 K during the normal operation of the JT cryocooler 18.

[0044] As an example, as shown in FIG. 1, the first heat conduction path 48a may be connected to a low temperature side of the second heat exchanger 42b. In the exemplary JT cryocooler 18, it is expected that the low temperature side of the second heat exchanger 42b and the second pre-cooling stage 27 of the pre-cooling cryocooler 20 have similar temperatures (for example, about 12 K) during the normal operation of the JT cryocooler 18.

[0045] On the other hand, a high temperature side of the second heat exchanger 42b is assumed to have a temperature similar to that of the first pre-cooling stage 25 of the pre-cooling cryocooler 20. Assuming that the first heat conduction path 48a connects the high temperature side of the second heat exchanger 42b and the second pre-cooling stage 27 of the pre-cooling cryocooler 20, during the normal operation of the JT cryocooler 18, a temperature difference corresponding to a temperature difference between the first pre-cooling stage 25 and the second pre-cooling stage 27 occurs between both ends of the first heat conduction path 48a, which increases a heat load on the second pre-cooling stage 27 and may have an undesirable effect on the refrigeration performance of the JT cryocooler 18.

[0046] For similar reasons, as shown in FIG. 1, the second heat conduction path 48b may be connected to a low temperature side of the third heat exchanger 42c. Instead of this, or in addition to this, the second heat conduction path 48b may be connected to a high temperature side of the third heat exchanger 42c.

[0047] The refrigerant pipe (for example, the first refrigerant pipe 44a or the second refrigerant pipe 44b) that constitutes the refrigerant circuit 40 structurally connects the pre-cooling stage and the heat exchanger, but a cross-sectional area perpendicular to a pipe axial direction is considerably small, so that it is not possible to achieve sufficient heat conduction to enable reduction in the cool-down time.

[0048] Therefore, the heat conduction path 48 is provided separately from the refrigerant pipe that constitutes the refrigerant circuit 40. That is, the first heat conduction path 48a is provided separately from the second refrigerant pipe 44b, and thermally couples the second pre-cooling stage 27 and the second heat exchanger 42b to each other. The second heat conduction path 48b is provided separately from the second refrigerant pipe 44b, and thermally couples the second pre-cooling stage 27 and the third heat exchanger 42c to each other.

[0049] The heat conduction path 48 is formed of one or more heat conduction members. Such a heat conduction member is formed of, for example, a metal material having a high thermal conductivity, such as copper (for example, pure copper or a copper alloy) or aluminum (for example, pure aluminum or an aluminum alloy), or other high thermal conductivity material. The heat conduction member may be formed of, for example, a material having a higher thermal conductivity than stainless steel (for example, SUS304).

[0050] FIG. 2 is a diagram schematically showing an exemplary configuration of the heat conduction path according to the embodiment. As shown in the figure, the first heat conduction path 48a includes a stage extension component 50 and a heat conduction plate 52 as an example of the heat conduction member.

[0051] The stage extension component 50 is a tubular (for example, cylindrical) heat conduction member formed of a high thermal conductivity material (for example, pure copper), and is thermally coupled to the second pre-cooling stage 27 at one end of the stage extension component 50 and is thermally coupled to the heat conduction plate 52 at the other end. For example, a flange formed at one end of the stage extension component 50 may be fixed to the second pre-cooling stage 27 by a fastening member such as a bolt, and a flange formed at the other end of the stage extension component 50 may be fixed to the heat conduction plate 52.

[0052] The heat conduction plate 52 is a flat plate-shaped heat conduction member formed of a high thermal conductivity material (for example, an aluminum alloy), and the stage extension component 50 and a low temperature end of the second heat exchanger 42b are thermally coupled by the heat conduction plate 52. The stage extension component 50 and the second heat exchanger 42b may be disposed on the same side of the heat conduction plate 52 and fixed to the same surface of the heat conduction plate 52.

[0053] The second heat exchanger 42b may have a typical configuration including a tubular casing. The second heat exchanger 42b may include a mandrel coaxially disposed inside the casing, and a pipe wound around an outer peripheral surface of the mandrel and disposed in a tubular cavity between the mandrel and the casing. The pipe may be used as a high pressure side flow path of the second heat exchanger 42b, and the tubular cavity may be used as the low pressure side flow path of the second heat exchanger 42b.

[0054] As shown in the figure, the second refrigerant pipe 44b and the refrigerant collection line 46 penetrate a low temperature side end plate of the casing of the second heat exchanger 42b. As described with reference to FIG. 1, the second refrigerant pipe 44b is connected to the high pressure side flow path of the second heat exchanger 42b, and the refrigerant collection line 46 is connected to the low pressure side flow path of the second heat exchanger 42b. In addition, the second refrigerant pipe 44b is attached to the second pre-cooling stage 27 by being wound around the second pre-cooling stage 27 to enable heat exchange with the second pre-cooling stage 27.

[0055] Although not shown in FIG. 2, the third heat exchanger 42c may be thermally coupled to the second pre-cooling stage 27 via the heat conduction plate 52 and the stage extension component 50 in the same manner. The third heat exchanger 42c may also have a tubular outer shape similarly to the second heat exchanger 42b, and a low temperature end thereof may be fixed to the heat conduction plate 52. As an exemplary disposition, the third heat exchanger 42c may be disposed on a front side or a rear side of the second heat exchanger 42b with respect to a paper surface of FIG. 2, in parallel with the second heat exchanger 42b.

[0056] As described above, according to the embodiment, the second heat exchanger 42b and the third heat exchanger 42c are connected to the second pre-cooling stage 27 of the pre-cooling cryocooler 20 by the heat conduction path 48. During the cool-down, cooling of the heat exchanger group 42 of the JT cryocooler 18 can be facilitated by using the conductive cooling via the heat conduction path 48 in addition to the refrigerant gas circulating through the refrigerant circuit 40. Therefore, the cool-down time of the JT cryocooler 18 can be shortened.

[0057] According to the verification of the present inventor, the existing design in which the heat conduction path 48 is not provided requires approximately 24 hours to cool down, whereas in the embodiment in which the heat conduction path 48 is provided, it is possible to complete the cool-down in approximately 12 hours.

[0058] FIG. 3 is a diagram schematically showing a modification example of the heat conduction path according to the embodiment. As shown in the figure, the third heat exchanger 42c may be thermally coupled to the second pre-cooling stage 27 by the second heat conduction path 48b. Note that the second heat conduction path 48b may include a thermal resistance element 60 in addition to the heat conduction member (for example, the stage extension component 50 and the heat conduction plate 52). The second heat conduction path 48b may connect the second pre-cooling stage 27 to the third heat exchanger 42c via the thermal resistance element 60.

[0059] The thermal resistance element 60 may be a spacer formed of a metal material or other material having a lower thermal conductivity than the heat conduction member, and the third heat exchanger 42c may be fixed to the heat conduction plate 52 so as to sandwich the thermal resistance element 60 between the low temperature end of the third heat exchanger 42c and the heat conduction plate 52. For example, in a case where the heat conduction member is formed of copper or aluminum as described above, the thermal resistance element 60 may be formed of stainless steel (for example, SUS304).

[0060] As shown in the figure, the refrigerant supply line 44 and the refrigerant collection line 46 are connected to an end plate on the low temperature side of the casing of the third heat exchanger 42c. As described with reference to FIG. 1, the refrigerant supply line 44 is connected to the high pressure side flow path of the third heat exchanger 42c, and the refrigerant collection line 46 is connected to the low pressure side flow path of the third heat exchanger 42c. In addition, the refrigerant supply line 44 is connected to the JT valve 30.

[0061] Even in this way, during the cool-down, cooling of the third heat exchanger 42c can be facilitated by the conductive cooling via the second heat conduction path 48b from the second pre-cooling stage 27, and the cool-down time of the JT cryocooler 18 can be shortened.

[0062] On the other hand, during the normal operation of the JT cryocooler 18, a certain degree of temperature difference may occur between the low temperature end of the third heat exchanger 42c and the second pre-cooling stage 27. For example, while the low temperature end of the third heat exchanger 42c is cooled to approximately 5 K, the second pre-cooling stage 27 can be cooled to approximately 12 K as described above. In this case, the second pre-cooling stage 27 serves as a heat source for the third heat exchanger 42c, and the heat entering the third heat exchanger 42c from the second pre-cooling stage 27 via the second heat conduction path 48b may reduce the refrigeration performance of the JT cryocooler 18.

[0063] However, in the present embodiment, since the thermal resistance element 60 is provided in the second heat conduction path 48b, it is possible to limit the heat input to the third heat exchanger 42c via the second heat conduction path 48b that may occur during the normal operation of the JT cryocooler 18. Therefore, it is possible to alleviate or prevent the above-described problem.

[0064] In addition, if necessary, the thermal resistance element 60 may be provided in the first heat conduction path 48a. The first heat conduction path 48a may connect the second pre-cooling stage 27 to the second heat exchanger 42b via the thermal resistance element 60.

[0065] The present invention has been described above based on the examples. It will be understood by those skilled in the art that the present invention is not limited to the above-described embodiment, various design changes are possible, various modification examples are possible, and such modification examples are also within the scope of the present invention. Various characteristics described in relation to one embodiment are also applicable to other embodiments. A new embodiment generated through combination also has the effects of each of the combined embodiments.

[0066] The pre-cooling cryocooler 20 is not limited to the GM cryocooler. The pre-cooling cryocooler 20 may be a cryocooler of another type such as a pulse tube cryocooler or a Stirling cryocooler.

[0067] In the above-described embodiment, a case where the heat exchanger group 42 includes the first to third heat exchangers has been described as an example, but the heat exchanger group 42 may have other multi-stage configurations. For example, the JT cryocooler 18 may include an additional heat exchanger (that is, a fourth heat exchanger) between the third heat exchanger 42c and the final heat exchanger 32.

[0068] The JT valve 30 described above may be disposed between the last heat exchanger of the heat exchanger group 42 (that is, the fourth heat exchanger) and the final heat exchanger 32 in the refrigerant supply line 44. Alternatively, the JT cryocooler 18 may adopt a two-stage JT expansion system, in which a first JT valve is disposed between the third heat exchanger 42c and the fourth heat exchanger in the refrigerant supply line 44, and a second JT valve is disposed between the fourth heat exchanger and the final heat exchanger 32 in the refrigerant supply line 44.

[0069] The JT cryocooler 18 may include a third heat conduction path that connects the second pre-cooling stage 27 to the fourth heat exchanger to enable conductive cooling of the fourth heat exchanger through the second pre-cooling stage 27. The third heat conduction path may also be the stage extension component 50 and the heat conduction plate 52 as with the first heat conduction path 48a and the second heat conduction path 48b. That is, the fourth heat exchanger may be thermally coupled to the second pre-cooling stage 27 via the heat conduction plate 52 and the stage extension component 50.

[0070] Although the present invention has been described using specific words and phrases based on the embodiment, the embodiment merely shows one aspect of the principle and application of the present invention, and various modifications and improvements can be made within the scope of the present invention described in claims.

[0071] It should be understood that the invention is not limited to the above-described embodiment, but may be modified into various forms on the basis of the spirit of the invention. Additionally, the modifications are included in the scope of the invention.