Pulse tube refrigerator and superconducting magnet device

Abstract

The pulse tube refrigerator (10) of the present invention comprises: a pulse tube (18, 24); a cold accumulator (16, 22) arranged in parallel with the pulse tube; and a thermal bridge (50) having a thermal switch (52) and connecting the pulse tube and the cold accumulator via the thermal switch (52). The thermal switch (52) functions as a heat insulating element when the cold accumulator side of the thermal bridge (50) is in a first temperature zone, and functions as a heat transfer element when the cold accumulator side of the thermal bridge (50) is in a second temperature zone lower than the first temperature zone.

Description

Technical Field

[0001] The present invention relates to a pulse tube refrigerator and a superconducting magnet device. Background Art

[0002] Pulse tube refrigerators are typically deployed in a vacuum environment, but are sometimes also deployed in a gas atmosphere, for applications such as helium recondensation. The compression of the working gas within the pulse tube during the refrigeration cycle heats the pulse tube while the regenerator cools to a very low temperature. Pulse tube refrigerators typically employ a parallel arrangement of the pulse tube and regenerator. The temperature difference between the pulse tube and regenerator causes natural convection of the ambient gas between them, and the resulting heat input to the regenerator reduces the cooling capacity of the pulse tube refrigerator. Therefore, a proposal has been made to connect the regenerator and pulse tube via a thermal bridge, cooling the pulse tube from the regenerator through the thermal bridge to match the axial temperature distribution of the pulse tube with that of the regenerator. This approach suppresses heat input from the pulse tube to the regenerator through natural convection of the ambient gas, thereby reducing the loss of cooling capacity of the pulse tube refrigerator.

[0003] Previous technical literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Laid-Open No. 2006-214717 Summary of the Invention

[0006] Technical issues to be solved by the invention

[0007] The present inventors conducted research on the aforementioned pulse tube refrigerator and discovered the following issues. The aforementioned technology achieves the desired purpose when the pulse tube refrigerator has been cooled to an ultra-low temperature and is in a stable operating state. However, during initial cooling (also known as cooling down) from ambient temperature (e.g., room temperature) to an ultra-low temperature when starting the pulse tube refrigerator, heat transferred from the pulse tube to the cold storage via a thermal bridge increases the time required to cool the cold storage, thus prolonging the cooling time. Cooling down is necessary to prepare the pulse tube refrigerator for cooling operation, and therefore is desirably completed as quickly as possible.

[0008] One of the exemplary purposes of an embodiment of the present invention is to suppress an increase in the temperature drop time and to improve the refrigeration capacity of a pulse tube refrigerator.

[0009] Means for solving technical problems

[0010] According to one embodiment of the present invention, a pulse tube refrigerator comprises: a pulse tube; a regenerator arranged in parallel with the pulse tube; and a thermal bridge having a thermal switch and connecting the pulse tube and the regenerator via the thermal switch. The thermal switch functions as a thermal insulation element when the regenerator side of the thermal bridge is in a first temperature zone, and functions as a heat transfer element when the regenerator side of the thermal bridge is in a second temperature zone lower than the first temperature zone.

[0011] According to one embodiment of the present invention, a superconducting magnet device includes: a superconducting coil; a cryostat having a liquid refrigerant tank for accommodating the superconducting coil and liquid refrigerant; and the pulse tube refrigerator of the above embodiment, which is arranged in the cryostat and recondenses the liquid refrigerant.

[0012] Effects of the Invention

[0013] According to the present invention, it is possible to improve the refrigeration capacity of a pulse tube refrigerator while suppressing an increase in the temperature drop time. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a diagram schematically showing a pulse tube refrigerator according to an embodiment.

[0015] Figure 2 It is a diagram schematically showing a refrigerant gas recondensing device and a superconducting magnet device according to an embodiment.

[0016] Figure 3 (a) is a schematic diagram showing an example of a thermal bridge with a thermal switch according to an embodiment. Figure 3 (b) is a graph showing exemplary characteristics of a thermal switch incorporated in the thermal bridge according to the embodiment.

[0017] Figure 4 This is a schematic diagram showing another row of thermal bridges with thermal switches according to the embodiment. DETAILED DESCRIPTION

[0018] The following describes embodiments of the present invention in detail with reference to the accompanying drawings. In the following description, identical elements are denoted by identical reference numerals, and duplicate descriptions are omitted where appropriate. Furthermore, the structures described below are examples and are not intended to limit the scope of the present invention. Furthermore, in the drawings referenced in the following description, the sizes and thicknesses of the various structural components are appropriately set for ease of description and do not necessarily represent actual dimensions and ratios.

[0019] Figure 1 This is a diagram schematically showing a pulse tube refrigerator 10 according to an embodiment. Figure 2 It is a diagram schematically showing a refrigerant gas recondensing device 100 and a superconducting magnet device 200 according to an embodiment.

[0020] The pulse tube refrigerator 10 serves as a cooling source for the refrigerant gas recondensing device 100 and can be installed in a liquid refrigerant tank 102 that stores liquefied refrigerant gas. Liquid refrigerant 104, for example, cools a superconducting device, sensor, or other object to ultra-low temperatures, thereby vaporizing it. The vaporized refrigerant is then recondensed by the pulse tube refrigerator 10. Helium is a widely used refrigerant gas, for example. Therefore, the helium is recondensed into liquid helium by the pulse tube refrigerator 10. However, other suitable refrigerants, such as nitrogen, may also be used.

[0021] Figure 2 The illustrated superconducting magnet device 200 can be used, for example, as a magnetic field source for an NMR (Nuclear Magnetic Resonance) system. The superconducting magnet device 200 includes a superconducting coil 202 and a cryostat 204 having a liquid refrigerant tank 102. The superconducting coil 202 and the liquid refrigerant 104 are housed in the liquid refrigerant tank 102. The pulse tube refrigerator 10 is installed in the cryostat 204 and recondenses the liquid refrigerant 104.

[0022] In addition, the superconducting magnet device 200 can also be mounted on a high magnetic field utilizing device (not shown) as a magnetic field source for, for example, a single crystal pulling device, an MRI (Magnetic Resonance Imaging) system, an accelerator such as a cyclotron, a high energy physics system such as a nuclear fusion system, or other high magnetic field utilizing device (not shown), and can generate the high magnetic field required by the device.

[0023] like Figure 1 As shown, the pulse tube refrigerator 10 is, as an example, a GM (Gifford-McMahon) type four-valve two-stage pulse tube refrigerator.

[0024] The pulse tube refrigerator 10 includes a cold head 11 and a compressor 12. The cold head 11 includes a main pressure switching valve 14, a first-stage regenerator 16, a first-stage pulse tube 18, and optionally a first-stage phase control mechanism including a first secondary pressure switching valve 20 and a first flow rate adjustment element 27a. The compressor 12 and the main pressure switching valve 14 constitute the oscillating flow generation source of the pulse tube refrigerator 10. The cold head 11 also includes a second-stage regenerator 22, a second-stage pulse tube 24, and optionally a second-stage phase control mechanism including a second secondary pressure switching valve 21 and a second flow rate adjustment element 27b. The oscillating flow generation source, the first-stage phase control mechanism, and the second-stage phase control mechanism share the compressor 12.

[0025] In this specification, the terms longitudinal direction A and lateral direction B are used to facilitate description of the positional relationship between the structural elements of the pulse tube refrigerator 10. Generally, the longitudinal direction A and lateral direction B correspond to the axial direction and radial direction of the pulse tubes 18 and 24 and the regenerators 16 and 22, respectively. However, the longitudinal direction A and lateral direction B do not need to be strictly orthogonal, as long as they are approximately orthogonal to each other. Furthermore, the longitudinal direction A and lateral direction B do not limit the orientation in which the pulse tube refrigerator 10 is installed at its location of use. The pulse tube refrigerator 10 can be installed in any desired orientation. For example, it can be installed with the longitudinal direction A and lateral direction B oriented in the vertical direction and the horizontal direction, respectively. Conversely, it can be installed with the longitudinal direction A and lateral direction B oriented in the horizontal direction and the vertical direction, respectively. Alternatively, it can be installed with the longitudinal direction A and lateral direction B oriented in different inclined directions.

[0026] The regenerators 16 and 22 and the pulse tubes 18 and 24 are arranged in parallel. The two regenerators 16 and 22 are connected in series and extend in the longitudinal direction A. The two pulse tubes 18 and 24 each extend in the longitudinal direction A. The first-stage regenerator 16 is arranged in parallel with the first-stage pulse tube 18 in the transverse direction B, and the second-stage regenerator 22 is arranged in parallel with the second-stage pulse tube 24 in the transverse direction B. The first-stage pulse tube 18 has approximately the same length in the longitudinal direction A as the first-stage regenerator 16, and the second-stage pulse tube 24 has approximately the same length in the longitudinal direction A as the combined length of the first-stage regenerator 16 and the second-stage regenerator 22. The regenerators 16 and 22 and the pulse tubes 18 and 24 are arranged substantially parallel to each other.

[0027] In addition, Figure 1 In the figure, the first stage pulse tube 18 and the second stage pulse tube 24 are arranged on both sides of the cold accumulators 16 and 22, but this is only shown for the convenience of illustration. Generally, the cold accumulators 16 and 22, the first stage pulse tube 18 and the second stage pulse tube 24 can be arranged to form a triangle when viewed in the longitudinal direction A.

[0028] The compressor 12 is configured with a compressor discharge port 12a and a compressor intake port 12b. It compresses recovered low-pressure PL working gas to generate high-pressure PH working gas. Working gas is supplied from the compressor discharge port 12a to the first-stage pulse tube 18 via the first-stage regenerator 16, and is then recovered from the first-stage pulse tube 18 via the first-stage regenerator 16 to the compressor intake port 12b. Furthermore, working gas is supplied from the compressor discharge port 12a to the second-stage pulse tube 24 via the first-stage regenerator 16 and the second-stage regenerator 22, and is then recovered from the second-stage pulse tube 24 via the second-stage regenerator 22 and the first-stage regenerator 16 to the compressor intake port 12b.

[0029] The compressor outlet 12a and compressor intake 12b function as the high-pressure source and low-pressure source, respectively, for the pulse tube refrigerator 10. The working gas, also known as refrigerant gas, is, for example, helium. In general, both the high-pressure (PH) and low-pressure (PL) are significantly higher than atmospheric pressure.

[0030] The main pressure switching valve 14 includes a main intake on-off valve V1 and a main exhaust on-off valve V2. The first auxiliary pressure switching valve 20 includes a first auxiliary intake on-off valve V3 and a first auxiliary exhaust on-off valve V4. The second auxiliary pressure switching valve 21 includes a second auxiliary intake on-off valve V5 and a second auxiliary exhaust on-off valve V6.

[0031] The pulse tube refrigerator 10 is equipped with a high-pressure pipeline 13a and a low-pressure pipeline 13b. High-pressure PH working gas flows from the compressor 12 to the cold head 11 through the high-pressure pipeline 13a. Low-pressure PL working gas flows from the cold head 11 to the compressor 12 through the low-pressure pipeline 13b. The high-pressure pipeline 13a connects the compressor outlet 12a to the intake valves V1, V3, and V5. The low-pressure pipeline 13b connects the compressor intake 12b to the exhaust valves V2, V4, and V6.

[0032] The first-stage regenerator 16 has a first-stage regenerator high-temperature end 16a and a first-stage regenerator low-temperature end 16b, and extends along a longitudinal direction A from the first-stage regenerator high-temperature end 16a toward the first-stage regenerator low-temperature end 16b. The first-stage regenerator high-temperature end 16a and the first-stage regenerator low-temperature end 16b can also be referred to as the first end and the second end of the first-stage regenerator 16, respectively. Similarly, the second-stage regenerator 22 has a second-stage regenerator high-temperature end 22a and a second-stage regenerator low-temperature end 22b, and extends along a longitudinal direction A from the second-stage regenerator high-temperature end 22a toward the second-stage regenerator low-temperature end 22b. The second-stage regenerator high-temperature end 22a and the second-stage regenerator low-temperature end 22b can also be referred to as the first end and the second end of the second-stage regenerator 22, respectively. The first-stage regenerator low-temperature end 16b is in communication with the second-stage regenerator high-temperature end 22a.

[0033] The first-stage pulse tube 18 has a first-stage pulse tube high-temperature end 18 a and a first-stage pulse tube low-temperature end 18 b, and extends from the first-stage pulse tube high-temperature end 18 a toward the first-stage pulse tube low-temperature end 18 b in a longitudinal direction A. The first-stage pulse tube high-temperature end 18 a and the first-stage pulse tube low-temperature end 18 b may also be referred to as a first end and a second end of the first-stage pulse tube 18, respectively.

[0034] Similarly, the second-stage pulse tube 24 has a second-stage pulse tube high-temperature end 24 a and a second-stage pulse tube low-temperature end 24 b, and extends from the second-stage pulse tube high-temperature end 24 a toward the second-stage pulse tube low-temperature end 24 b along the longitudinal direction A. The second-stage pulse tube high-temperature end 24 a and the second-stage pulse tube low-temperature end 24 b may also be referred to as the first end and the second end of the second-stage pulse tube 24, respectively.

[0035] In the illustrated configuration, the regenerators 16 and 22 are cylindrical tubes filled with a regenerator material, and the pulse tubes 18 and 24 are cylindrical tubes with a hollow interior.

[0036] Rectifiers can be installed at each end of pulse tubes 18 and 24 to even out or adjust the working gas flow velocity distribution in a plane perpendicular to the pulse tube's axis. These rectifiers also function as heat exchangers. Furthermore, buffer volumes can be connected to the high-temperature ends of pulse tubes 18 and 24 for phase control.

[0037] The cold head 11 includes a first cooling stage 28 and a second cooling stage 30. Figure 2 As shown, the neck tube 206 is disposed above the cryostat 204 and the liquid refrigerant tank 102, the cold head 11 of the pulse tube refrigerator 10 is inserted into the neck tube 206, and the first cooling stage 28 and the second cooling stage 30 are disposed within the neck tube 206. The internal volume of the neck tube 206 forms a portion of the liquid refrigerant tank 102, and the refrigerant vaporized in the liquid refrigerant tank 102 contacts the second cooling stage 30 and is recondensed.

[0038] Reference again Figure 1 The first-stage regenerator 16 and the first-stage pulse tube 18 extend in the same direction from the first cooling stage 28, and the first-stage regenerator high-temperature end 16a and the first-stage pulse tube high-temperature end 18a are arranged on the same side relative to the first cooling stage 28. In this way, the first-stage regenerator 16, the first-stage pulse tube 18, and the first cooling stage 28 are arranged in a U-shape. Similarly, the second-stage regenerator 22 and the second-stage pulse tube 24 extend in the same direction from the second cooling stage 30, and the second-stage regenerator high-temperature end 22a and the second-stage pulse tube high-temperature end 24a are arranged on the same side relative to the second cooling stage 30. In this way, the second-stage regenerator 22, the second-stage pulse tube 24, and the second cooling stage 30 are arranged in a U-shape.

[0039] The first-stage pulse tube low-temperature end 18b and the first-stage regenerator low-temperature end 16b are structurally connected to each other and are thermally connected to each other via a first cooling stage 28. A first-stage communication path 29 is formed in the first cooling stage 28. This first-stage communication path 29 connects the first-stage regenerator low-temperature end 16b and the first-stage pulse tube low-temperature end 18b, thereby allowing the working gas to flow between the first-stage regenerator low-temperature end 16b and the first-stage pulse tube low-temperature end 18b.

[0040] Similarly, the second-stage pulse tube low-temperature end 24b and the second-stage regenerator low-temperature end 22b are structurally connected to each other via a second cooling stage 30, thereby being thermally connected to each other. A second-stage communication path 31 is formed in the second cooling stage 30. This second-stage communication path 31 connects the second-stage regenerator low-temperature end 22b and the second-stage pulse tube low-temperature end 24b, thereby allowing the working gas to flow between the second-stage regenerator low-temperature end 22b and the second-stage pulse tube low-temperature end 24b.

[0041] The cooling stages 28 and 30 are made of a metal material with high thermal conductivity, such as copper. The cylindrical portions of the regenerators 16 and 22 and the pulse tubes 18 and 24 are made of a material with lower thermal conductivity than the cooling stages (28 and 30), such as a metal material such as stainless steel.

[0042] The flow rate adjustment elements 27a and 27b include, for example, flow path resistances such as orifices and throttle valves. The flow path resistances may be fixed or adjustable.

[0043] On the other hand, the first-stage regenerator high-temperature end 16a, the first-stage pulse tube high-temperature end 18a, and the second-stage pulse tube high-temperature end 24a are connected together by a flange portion 36. The flange portion 36 is mounted on a support portion 38, such as a support table or support wall, on which the pulse tube refrigerator 10 is installed. The support portion 38 may also be a wall material or other portion of an insulating container or vacuum container that accommodates the cooling stages 28 and 30 and the object to be cooled.

[0044] The pulse tubes 18, 24 and the regenerators 16, 22 extend from one main surface of the flange portion 36 toward the cooling stages 28, 30. A valve portion 40 is provided on the other main surface of the flange portion 36. The valve portion 40 houses the primary pressure switching valve 14, the first secondary pressure switching valve 20, and the second secondary pressure switching valve 21. Therefore, if the support portion 38 constitutes part of an insulating container or a vacuum container, when the flange portion 36 is attached to the support portion 38, the pulse tubes 18, 24, the regenerators 16, 22, and the cooling stages 28, 30 are housed within the container, while the valve portion 40 is positioned outside the container.

[0045] Furthermore, the valve portion 40 does not need to be directly mounted on the flange portion 36. The valve portion 40 can be disposed separately from the cold head 11 of the pulse tube refrigerator 10 and connected to the cold head 11 via rigid or flexible piping. In this manner, the phase control mechanism of the pulse tube refrigerator 10 can be disposed separately from the cold head 11.

[0046] The main pressure switching valve 14 is configured to alternately connect the first-stage regenerator high-temperature end 16a to the compressor discharge port 12a and the compressor intake port 12b to generate pressure fluctuations within the pulse lines 18 and 24. The main pressure switching valve 14 is configured so that when one of the main intake on-off valve V1 and the main exhaust on-off valve V2 is open, the other valve remains closed. The main pressure switching valve 14 is connected to the first-stage regenerator high-temperature end 16a via the regenerator communication passage 32. The main intake on-off valve V1 connects the compressor discharge port 12a to the first-stage regenerator high-temperature end 16a, while the main exhaust on-off valve V2 connects the compressor intake port 12b to the first-stage regenerator high-temperature end 16a.

[0047] While the main intake valve V1 is open, working gas is supplied from the compressor outlet 12a to the regenerators 16 and 22 via the high-pressure line 13a, the main intake valve V1, and the regenerator communication passage 32. Working gas is further supplied from the first-stage regenerator 16 to the first-stage pulse line 18 via the first-stage communication passage 29, and from the second-stage regenerator 22 to the second-stage pulse line 24 via the second-stage communication passage 31. Meanwhile, while the main exhaust valve V2 is open, working gas is recovered from the pulse lines 18 and 24 to the compressor intake 12b via the regenerators 16 and 22, the main exhaust valve V2, and the low-pressure line 13b.

[0048] The first secondary pressure switching valve 20 alternately connects the first-stage pulse tube high-temperature end 18a to the compressor discharge port 12a and the compressor intake port 12b via the first pulse tube communication passage 34. The first secondary pressure switching valve 20 is configured so that when one of the first secondary intake on-off valve V3 and the first secondary exhaust on-off valve V4 is open, the other remains closed. The first secondary intake on-off valve V3 connects the compressor discharge port 12a to the first-stage pulse tube high-temperature end 18a, while the first secondary exhaust on-off valve V4 connects the compressor intake port 12b to the first-stage pulse tube high-temperature end 18a.

[0049] While the first auxiliary intake valve V3 is open, working gas is supplied from the compressor discharge port 12a to the first-stage pulse pipe 18 via the high-pressure line 13a, the first auxiliary intake valve V3, the first pulse pipe communication path 34, and the first-stage pulse pipe high-temperature end 18a. Meanwhile, while the first auxiliary exhaust valve V4 is open, working gas is recovered from the first-stage pulse pipe 18 to the compressor intake port 12b via the first-stage pulse pipe high-temperature end 18a, the first auxiliary exhaust valve V4, and the low-pressure line 13b.

[0050] Similarly, the second secondary pressure switching valve 21 alternately connects the second-stage pulse pipe high-temperature end 24a to the compressor outlet 12a and the compressor intake 12b via the second pulse pipe communication passage 35. The second secondary pressure switching valve 21 is configured so that when one of the second secondary intake on-off valve V5 and the second secondary exhaust on-off valve V6 is open, the other valve remains closed. The second secondary intake on-off valve V5 connects the compressor outlet 12a to the second-stage pulse pipe high-temperature end 24a, and the second secondary exhaust on-off valve V6 connects the compressor intake 12b to the second-stage pulse pipe high-temperature end 24a. While the second secondary intake on-off valve V5 is open, working gas is supplied from the compressor outlet 12a to the second-stage pulse pipe 24. While the second secondary exhaust on-off valve V6 is open, working gas is recovered from the second-stage pulse pipe 24 to the compressor intake 12b.

[0051] By adopting this structure, the pulse tube refrigerator 10 generates working gas pressure oscillations of high pressure PH and low pressure PL in the pulse tube. Synchronously with the pressure oscillations and with an appropriate phase delay, displacement oscillations of the working gas (i.e., reciprocating movement of the gas piston) are generated in the pulse tube. The movement of the working gas that periodically moves up and down in the pulse tube while maintaining a certain pressure is generally referred to as a "gas piston," which is often used in descriptions of the operation of the pulse tube refrigerator 10. When the gas piston is located at or near the high-temperature end of the pulse tube, the working gas expands at the low-temperature end of the pulse tube, thereby generating cold. By repeating this refrigeration cycle, the pulse tube refrigerator 10 is able to cool the cooling stage. The first cooling stage 28 is cooled to a first cooling temperature (e.g., 30K to 80K), and the second cooling stage 30 is cooled to a second cooling temperature (e.g., 3K to 20K) that is lower than the first cooling temperature.

[0052] As described above, when the pulse tube refrigerator 10 is used to recondense refrigerant gas, it can cool the gas or liquid in contact with the second cooling stage 30. Furthermore, the working gas of the pulse tube refrigerator 10 and the refrigerant gas recondensed by the pulse tube refrigerator 10 can be the same gas (e.g., helium), but they are isolated from each other. The cold head 11 is an airtight container, and the working gas within it cannot leak outside the cold head 11, and therefore cannot mix with the recondensed refrigerant gas.

[0053] When the pulse tube refrigerator 10 is used for other purposes, an object to be cooled (not shown) is thermally connected to the second cooling stage 30. The object can be placed directly on the second cooling stage 30, or it can be thermally connected to the second cooling stage 30 via a rigid or flexible heat transfer component. The pulse tube refrigerator 10 can cool the object through conductive cooling from the second cooling stage 30. Furthermore, the object cooled by the pulse tube refrigerator 10 can be a superconducting electromagnet or other superconducting device, or an infrared imaging element or other sensor, but is not limited thereto.

[0054] Furthermore, it is natural that the first cooling stage 28 can be used to cool an object different from the object cooled by the second cooling stage 30. For example, a radiation shield for reducing or preventing heat intrusion into the second cooling stage 30 can be thermally connected to the first cooling stage 28.

[0055] In this embodiment, the pulse tube refrigerator 10 includes at least one thermal bridge 50. The thermal bridge 50 includes a thermal switch 52, which connects the pulse tube and the cold accumulator via the thermal switch 52. The thermal bridge 50 also includes a cold accumulator connection portion 54, which connects the thermal switch 52 to the cold accumulator, and a pulse tube connection portion 56, which connects the thermal switch 52 to the pulse tube. The cold accumulator connection portion 54 and the pulse tube connection portion 56 are made of a metal material with a high thermal conductivity, such as copper. Alternatively, one end of the thermal switch 52 can be directly connected to the pulse tube, while the other end can be directly connected to the cold accumulator.

[0056] As an example, a thermal bridge 50 may be provided connecting the second-stage pulse tube 24 and the second-stage regenerator 22. Additionally or alternatively, another thermal bridge 50 may be provided connecting the first-stage pulse tube 18 and the first-stage regenerator 16. Additionally or alternatively, another thermal bridge 50 may be provided connecting the second-stage pulse tube 24 and the first-stage regenerator 16. The thermal bridge 50 may also be provided at other locations, for example, connecting the second-stage pulse tube 24 and the first cooling stage 28.

[0057] The compression of gas within the pulse tube during the refrigeration cycle of the pulse tube refrigerator 10 may cause the middle portion of the pulse tube (e.g., 1 / 4 to 3 / 4 or 1 / 3 to 2 / 3 of the axial length of the pulse tube) to be heated more strongly than the end portions of the pulse tube. Therefore, the thermal bridge 50 may be connected to the middle portion of the first-stage pulse tube 18 or the middle portion of the second-stage pulse tube 24.

[0058] like Figure 1 As shown, the thermal bridge 50 is in the axial direction ( Figure 1 The pulse tube and the cold accumulator are connected at the same position in the longitudinal direction A). The temperature at the axial position connected by the thermal bridge 50 can be close to (or equal to) that in the pulse tube and the cold accumulator, which is conducive to matching the axial temperature distribution of the pulse tube and the cold accumulator.

[0059] Alternatively, the thermal bridge 50 may connect the pulse tube and the regenerator at different locations in the axial direction. This allows, for example, adjustment of the axial temperature distribution of the pulse tube and the regenerator. Alternatively, the thermal bridge 50 may be installed at different locations in the pulse tube and the regenerator to facilitate installation of the thermal bridge 50.

[0060] The thermal switch 52 is configured to function as a thermal insulation element when the regenerator is in a first temperature zone T1, and as a heat transfer element when the regenerator is in a second temperature zone T2, which is lower than the first temperature zone T1. The thermal switch 52 operates by switching between on and off according to temperature changes. At relatively low temperatures (i.e., the second temperature zone T2), the regenerator and the pulse tube are thermally connected (i.e., connected). At relatively high temperatures (i.e., the first temperature zone T1), the thermal connection between the regenerator and the pulse tube is cut off (i.e., disconnected).

[0061] The pulse tube refrigerator 10 is capable of performing steady-state operation and a cool-down operation performed prior to steady-state operation. As described above, the cool-down operation can also be referred to as initial cooling. The cool-down operation is an operating mode in which the pulse tube refrigerator 10 is rapidly cooled from the initial temperature to a very low temperature when starting up, while the steady-state operation is an operating mode in which the pulse tube refrigerator 10 maintains the state cooled to the very low temperature by the cool-down operation. The initial temperature can be ambient temperature (e.g., room temperature). The pulse tube refrigerator 10 is cooled to a standard cooling temperature by the cool-down operation, and during steady-state operation, it is maintained within an allowable temperature range of the very low temperature, including the standard cooling temperature. The standard cooling temperature varies depending on the application and settings of the pulse tube refrigerator 10. For example, in the cooling application of superconducting devices, it is typically below approximately 4.2K. In other cooling applications, the standard cooling temperature can be, for example, approximately 10K to 20K, or below 10K. Switching from the cool-down operation to the steady-state operation can be achieved by controlling the valve unit 40 to change the number of refrigeration cycles per unit time (the frequency of the refrigeration cycle). For example, the pulse tube refrigerator 10 may be operated with a high-frequency refrigeration cycle in the cool-down operation, and may be operated with a lower-frequency refrigeration cycle in the steady-state operation.

[0062] In this embodiment, the first temperature zone T1 is a temperature range higher than the set temperature Tc, and the second temperature zone T2 is a temperature range lower than the set temperature Tc. The set temperature Tc is selected, for example, from the temperature range of 4 K to 100 K. The set temperature Tc can be determined based on the first cooling temperature (the target cooling temperature of the first cooling stage 28) described above. For example, it can be determined to be a temperature higher than the first cooling temperature by a predetermined temperature (for example, within 5 K or within 10 K).

[0063] Thus, the thermal switch 52 can be turned off at the start of the cool-down operation of the pulse tube refrigerator 10, and can be switched from off to on during (e.g., at the end of) the cool-down operation or when transitioning from the cool-down operation to the steady-state operation. During the steady-state operation, the thermal switch 52 can be turned on.

[0064] Figure 3 (a) is a schematic diagram showing an example of a thermal bridge 50 with a thermal switch 52 according to the embodiment. Figure 3(b) is a graph showing exemplary characteristics of the thermal switch 52 incorporated in the thermal bridge 50 according to the embodiment.

[0065] like Figure 3 As shown in (a), the thermal bridge 50 includes a heat pipe serving as a thermal switch 52. Working gas 60, which vaporizes in a first temperature zone T1 and liquefies in a second temperature zone T2, is enclosed within the heat pipe. One end of the heat pipe is connected to a regenerator connection portion 54, thermally connecting the heat pipe to a regenerator (e.g., the second-stage regenerator 22) via the regenerator connection portion 54. Furthermore, the other end of the heat pipe is connected to a pulse tube connection portion 56, thermally connecting the heat pipe to a pulse tube (e.g., the second-stage pulse tube 24) via the pulse tube connection portion 56.

[0066] like Figure 3 As shown in (b), the heat switch 52 has a first thermal conductivity α1 in the first temperature zone T1, and a second thermal conductivity α2 greater than the first thermal conductivity α1 in the second temperature zone T2 which is lower than the first temperature zone T1. The set temperature Tc which is the boundary between the first temperature zone T1 and the second temperature zone T2 is equivalent to the boiling point of the sealed working gas 60. In the first temperature zone T1 and the second temperature zone T2, the change in thermal conductivity is small and stable compared with the transition temperature zone (including the set temperature Tc) between them. In the transition temperature zone, there is a large change from the first thermal conductivity α1 to the second thermal conductivity α2. In addition, in Figure 3 In (b), for convenience, the first thermal conductivity α1 in the first temperature zone T1 and the second thermal conductivity α2 in the second temperature zone T2 are shown as constant values, but the values ​​of these thermal conductivities may vary slightly depending on the temperature.

[0067] When both ends of the thermal switch 52 (i.e., the heat pipe) are in the first temperature zone T1, the enclosed working gas 60 is completely gaseous, and therefore the thermal switch 52 conducts virtually no heat. Ideally, the first thermal conductivity α1 is zero, but in practice, it assumes a relatively small value corresponding to the thermal conductivity of the working gas 60 in its gaseous state. Furthermore, the thermal conductivity of the walls of the thermal switch 52, where the working gas 60 is enclosed, also contributes to the first thermal conductivity α1. However, because the first thermal conductivity α1 is sufficiently smaller than the second thermal conductivity α2, the thermal switch 52 can be considered an insulating element in the first temperature zone T1, blocking heat input from the pulse tube to the regenerator.

[0068] In contrast, in the second temperature zone T2, the working gas 60 is liquefied. When one end of the thermal switch 52 (the side of the regenerator connection portion 54) is in the second temperature zone T2, the working gas 60 in contact with its inner wall surface imparts heat to the wall surface and is cooled to become liquid. The droplets 62 of the working gas 60 are transported in the thermal switch 52 toward the other end (the side of the pulse tube connection portion 56), for example, by the action of gravity or by utilizing a capillary phenomenon. The transported droplets 62 absorb heat and are vaporized again. Thus, the thermal switch 52 acts as a heat transfer element that conducts heat from the side of the pulse tube connection portion 56 to the side of the regenerator connection portion 54, thereby acting as a heat bridge 50. Thus, the other end of the thermal switch 52 (the side of the pulse tube connection portion 56) can be cooled to the set temperature Tc.

[0069] The set temperature Tc is selected by selecting the type of gas of the working gas 60 sealed in the thermal switch 52. The working gas 60 may include, for example, at least one of helium (approximately 4.2K), hydrogen (approximately 20.4K), neon (approximately 27.1K), nitrogen (approximately 77.4K), oxygen (approximately 90.2K), and argon (approximately 87.3K). Here, the portion marked in parentheses after the gas type is the boiling point of each gas type at atmospheric pressure. Therefore, for example, when neon is sealed as the working gas 60 in the thermal switch 52 at atmospheric pressure, the set temperature Tc can be set to approximately 27.1K. The working gas 60 may also be sealed in the thermal switch 52 at a high pressure higher than atmospheric pressure (for example, within 5 atmospheres or within 10 atmospheres). The set temperature Tc can be adjusted by adjusting the sealing pressure of the working gas 60.

[0070] The working gas 60 may be, for example, air (about 78.8K), other mixed gases including nitrogen, mixed gases including helium, or other mixed gases. The set temperature Tc can be adjusted by adjusting the composition of the mixed gas.

[0071] As described at the beginning of this specification, by connecting the regenerator and the pulse tube using a thermal bridge 50, the regenerator cools the pulse tube through the thermal bridge 50, thereby matching the axial temperature distribution of the pulse tube with that of the regenerator (i.e., reducing the temperature difference between the pulse tube and the regenerator at the same axial position). This reduces the loss of cooling capacity of the pulse tube refrigerator 10 caused by natural convection of the ambient gas. This thermal bridge 50 is effectively implemented during steady-state operation of the pulse tube refrigerator 10 by activating the thermal switch 52.

[0072] During the pulse tube refrigerator 10's cooling operation, the thermal switch 52 is initially open, and the thermal bridge 50 acts as an insulating element, blocking heat transfer from the pulse tube to the regenerator through the thermal bridge 50. Compared to a case where heat transfer from the pulse tube to the regenerator through the thermal bridge 50 is not disconnected, the pulse tube refrigerator 10 can cool the regenerator and the cooling stage more quickly, shortening the cooling time required. As cooling progresses, the regenerator temperature drops. As described above, the thermal switch 52 switches from open to open, and the thermal bridge 50 transitions from an insulating element to a heat transfer element, allowing the pulse tube refrigerator 10 to transition to steady-state operation.

[0073] Therefore, according to the embodiment, the refrigeration capacity of the pulse tube refrigerator 10 can be improved while suppressing an increase in the temperature drop time.

[0074] Figure 4 Schematic diagram showing another row of thermal bridges 50 with thermal switches 52 according to an embodiment. Thermal bridge 50 includes thermal switches 52, which connect a pulse tube (e.g., the second-stage pulse tube 24) and a cold accumulator (e.g., the second-stage cold accumulator 22) via thermal switches 52. Thermal bridge 50 also includes a cold accumulator connection portion 54 for connecting thermal switches 52 to the cold accumulator, and a pulse tube connection portion 56 for connecting thermal switches 52 to the pulse tube.

[0075] The thermal switch 52 includes an airtight container 64, a gas supply unit 66 for supplying gas to the airtight container 64, and a gas discharge unit 68 for discharging gas from the airtight container 64. The gas supplied to the airtight container 64 may be, for example, the same gas as the working gas sealed in the heat pipe, or any other suitable gas may be used.

[0076] The airtight container 64 is a cylindrical pressure vessel, one end of which is connected to the regenerator connection part 54, and the other end is connected to the pulse tube connection part 56. The gas supply part 66 includes a gas supply source 66a, a supply valve 66b and a supply pipe 66c. The gas supply source 66a is connected to the airtight container 64 through the supply pipe 66c. The supply valve 66b is provided on the supply pipe 66c, and when the supply valve 66b is opened, gas is supplied from the gas supply source 66a to the airtight container 64. The gas discharge part 68 includes a discharge pump 68a, a discharge valve 68b and a discharge pipe 68c. The discharge pump 68a is connected to the airtight container 64 through the discharge pipe 68c. The discharge valve 68b is provided on the discharge pipe 68c, and when the discharge pump 68a is operated and the discharge valve 68b is opened, gas is discharged from the airtight container 64 to the discharge pump 68a, so that the airtight container 64 can be set to a vacuum. The gas supply source 66 a , the supply valve 66 b , the discharge pump 68 a , and the discharge valve 68 b may be arranged outside the vacuum container in which the pulse tube refrigerator 10 is installed.

[0077] The thermal switch 52 can be switched on and off by supplying gas to and exhausting gas from the airtight container 64. When gas is supplied to the airtight container 64, the thermal switch 52 is turned on by heat conduction of the gas. When the gas is exhausted from the airtight container 64 to create a vacuum, the thermal switch 52 is turned off by vacuum insulation.

[0078] Therefore, the thermal switch 52 can be operated so as to function as a heat insulating element when the regenerator side is in the first temperature zone T1, and as a heat transfer element when the regenerator side is in the second temperature zone T2, which is lower than the first temperature zone T1. The thermal switch 52 can include, for example, a temperature sensor 70 that measures the temperature of the regenerator connection portion 54, the regenerator, or the cooling stage, and can be switched between on and off manually or automatically based on the measured temperature.

[0079] When it is expected that the difference in thermal conductivity can be increased by turning the thermal switch 52 on and off, the above-mentioned heat pipe type thermal switch 52 is more advantageous, but the gas supply and exhaust type thermal switch 52 is advantageous in that the operating temperature (set temperature Tc) of the thermal switch 52 for switching on and off can be determined at will.

[0080] The present invention has been described above based on embodiments. Those skilled in the art will appreciate that the present invention is not limited to the aforementioned embodiments, and that various design changes and variations are possible, and that these variations are also within the scope of the present invention. Various features described in one embodiment may also be applicable to other embodiments. New embodiments created through combination have the effects of each of the combined embodiments.

[0081] In the above embodiment, a GM-type four-valve two-stage pulse tube refrigerator was used as an example. However, the thermal bridge with a thermal switch of the present invention can be applied to various pulse tube refrigerators in which the pulse tube and the regenerator are arranged in parallel. The thermal bridge with a thermal switch can also be applied to single-stage or multi-stage pulse tube refrigerators, such as three-stage. The thermal bridge with a thermal switch can also be applied to GM-type pulse tube refrigerators with different phase control mechanisms than the four-valve type, such as the two-way air intake type and the active buffer type. The thermal bridge with a thermal switch can also be applied to pulse tube refrigerators with different oscillating flow generation methods than the GM type, such as the Stirling type.

[0082] The present invention has been described above based on the implementation method and using specific terms, but the implementation method only represents one aspect of the principle and application of the present invention. In the implementation method, various modifications or configuration changes are allowed without departing from the scope of the idea of ​​the present invention defined in the technical solution.

[0083] Industrial applicability

[0084] The present invention can be used in the fields of pulse tube refrigerators and superconducting magnet devices.

[0085] Explanation of symbols

[0086] 10-Pulse tube refrigerator, 16-First-stage cold accumulator, 18-First-stage pulse tube, 22-Second-stage cold accumulator, 24-Second-stage pulse tube, 50-Thermal bridge, 52-Thermal switch, 60-Working gas, 64-Airtight container, 66-Gas supply unit, 68-Gas exhaust unit, 200-Superconducting magnet device, 202-Superconducting coil, 204-Cryostat.

Claims

1. A pulse tube refrigerator, characterized in that: have: blood vessels; A cold storage device, arranged in parallel with the pulse tube: and a thermal bridge having a thermal switch and connecting the pulse tube and the cold accumulator via the thermal switch, The thermal bridge includes a heat pipe as the thermal switch, wherein a working gas that vaporizes in a first temperature zone and liquefies in a second temperature zone lower than the first temperature zone is sealed inside the heat pipe. The thermal switch functions as a heat insulating element when the regenerator side of the thermal bridge is in the first temperature zone, and functions as a heat transfer element that conducts heat through the liquid droplets of the working gas when the regenerator side of the thermal bridge is in the second temperature zone.

2. The pulse tube refrigerator according to claim 1, wherein The working gas includes at least one of helium, hydrogen, neon, nitrogen, oxygen and argon.

3. The pulse tube refrigerator according to claim 1 or 2, characterized in that: The working gas is a mixed gas.

4. The pulse tube refrigerator according to any one of claims 1 to 3, characterized in that The first temperature zone is a temperature range higher than a set temperature, and the second temperature zone is a temperature range lower than the set temperature. The set temperature is selected from a temperature range of 4K to 100K.

5. A superconducting magnet device, characterized in that: have: superconducting coils; a cryostat having a liquid refrigerant tank for accommodating the superconducting coil and liquid refrigerant; and The pulse tube refrigerator according to any one of claims 1 to 4, wherein the pulse tube refrigerator is provided in the cryostat and recondenses the liquid refrigerant.

Images