Thermal switch and displacer replacement method

Abstract

To provide a thermal switch excellent in safety and high in reproducibility.SOLUTION: A thermal switch 5 includes: a first connection member 13 connected to a low-temperature portion; a first thermal conductor 11 connected to the low-temperature portion via the first connection member 13; a second connection member 14 connected to a cooled portion; and a second thermal conductor 12 connected to the cooled portion via the second connection member 14, wherein at least one of the first connection member 13 and the second connection member 14 is constituted by a flexible heat transfer member having flexibility and heat conductivity, the second thermal conductor 12 is fitted to the first thermal conductor 11, and a contact thermal resistance between the first thermal conductor 11 and the second thermal conductor 12 changes due to a difference in a thermal shrinkage rate between the first thermal conductor 11 and the second thermal conductor 12, thereby switching between an ON state in which heat is conducted between the first thermal conductor 11 and the second thermal conductor 12 and an OFF state in which the contact thermal resistance is larger than that in the ON state.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] SUMMARY OF THE INVENTION Embodiments of the present invention relate to a thermal switch and a method for replacing a displacer. [Background technology]

[0002] Generally, superconducting magnets must be cooled to extremely low temperatures using cooling methods such as liquid helium or cryogenic refrigerators. In recent years, the number of superconducting magnets cooled by cryogenic refrigerators, which are easy to operate, has been increasing. Such cryogenic refrigerator-cooled superconducting magnets require cryogenic refrigerator maintenance approximately once a year. One method of maintaining a cryogenic refrigerator involves removing the cryogenic refrigerator's displacer from the cylinder while the superconducting coil remains cryogenically cooled, heating the inside of the cylinder to room temperature with a heater, and then inserting a replacement displacer. The removed displacer is then sent to a factory, where consumables such as seals are replaced. With this method, when the cylinder is heated, heat flows through the superconducting coil, which is connected by a heat transfer plate, causing the temperature of the superconducting coil to rise, and it takes more than 24 hours to recool it.

[0003] One way to shorten this re-cooling time is to block the heat that flows to the superconducting coil via a heat transfer plate when the cylinder is heated. For example, a known technique involves placing a sleeve on the outside of the refrigerator cylinder and cooling the superconducting coil through the sleeve. This technique thermally separates the cylinder from the sleeve before heating the inside of the cylinder, preventing heat from reaching the coil. However, thermal resistance occurs between the sleeve and the cylinder. There is also a risk to the reproducibility of the attachment and detachment operation. The mechanism that thermally switches ON / OFF, as in the attachment and detachment operation of the cylinder and sleeve in this technique, is called a thermal switch. In particular, the method of attaching and detaching the cylinder and sleeve is called a mechanical method.

[0004] On the other hand, there is also a type of thermal switch called a gas type. This is a method of switching between a state in which gas is placed between two heat transfer members and a vacuum state. Because this method uses gas, there is a risk that the gas will leak into the vacuum vessel if a crack occurs in the gas pipe. There is also a risk that the pipe that takes in and out the gas will become blocked and the thermal switch will burst. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-53068 Summary of the Invention [Problem to be solved by the invention]

[0006] The use of a thermal switch can reduce the amount of heat flowing into the superconducting coil during heating and shorten the time it takes to cool it down again. However, mechanical thermal switches have difficulty in reproducibility, and gas-type thermal switches have the risk of gas leaks or explosions.

[0007] The problem to be solved by the present invention is to provide a thermal switch that is highly safe and highly reproducible, and by using this thermal switch, to shorten the re-cooling time after replacing a displacer in a cryogenic refrigerator. [Means for solving the problem]

[0008] A thermal switch according to an embodiment of the present invention comprises a first connecting member connected to a low-temperature portion, a first thermal conductor connected to the low-temperature portion via the first connecting member, a second connecting member connected to a portion to be cooled, and a second thermal conductor connected to the portion to be cooled via the second connecting member, wherein at least one of the first connecting member and the second connecting member is made of a flexible heat transfer member that is flexible and heat conductive, and the second thermal conductor is fitted into the first thermal conductor, and the difference in thermal contraction rate between the first thermal conductor and the second thermal conductor changes the contact thermal resistance between the first thermal conductor and the second thermal conductor, thereby switching between an ON state in which heat is conducted between the first thermal conductor and the second thermal conductor, and an OFF state in which the contact thermal resistance is greater than in the ON state. [Effects of the Invention]

[0009] According to an embodiment of the present invention, a thermal switch with excellent safety and high reproducibility is provided, and by using this thermal switch, the re-cooling time after replacing a displacer in a cryogenic refrigerator can be shortened. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view showing a superconducting device during operation. [Figure 2] FIG. 10 is a cross-sectional view showing the superconducting device when the displacer is removed. [Figure 3] FIG. 1 is a cross-sectional view showing a superconducting device with a heater inserted. [Figure 4] FIG. 10 is a cross-sectional view showing the superconducting device when inserting a displacer. [Figure 5] FIG. [Figure 6] FIG. 4 is a perspective view showing a thermal switch in an ON state. [Figure 7] FIG. 4 is a perspective view showing the thermal switch in the OFF state. [Figure 8] Graph showing the temperature dependence of the thermal shrinkage rate of aluminum and copper. [Figure 9] 6 is a graph showing the relationship between the inner and outer diameter ratio of the first thermal conductor and the maximum circumferential stress and surface pressure. [Figure 10] 1 is a graph showing the relationship between the OFF / ON ratio of the thermal switch and the ratio of the re-cooling time. [Figure 11] Graph showing the relationship between stress and plastic strain applied to high-purity aluminum. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the thermal switch and the displacer replacement method will be described in detail with reference to the drawings.

[0012] 1, reference numeral 1 denotes a superconducting device of this embodiment. This superconducting device 1 includes a vacuum vessel 2, a heat shield 3, a superconducting coil 4, a thermal switch 5, and a cryogenic refrigerator 10. The cryogenic refrigerator 10 is composed of a displacer 6, a drive unit 7, and a cylinder unit 8.

[0013] The vacuum vessel 2 is a vessel whose interior is evacuated. Inside the vacuum vessel 2, a superconducting coil 4 and a thermal switch 5 are provided, surrounded by a thermal shield 3. The superconducting coil 4 is an electromagnet that is cooled to an extremely low temperature to cause a superconducting phenomenon. This superconducting coil 4 is the cooled part in this embodiment.

[0014] A cylinder section 8 is provided in the vacuum vessel 2. This cylinder section 8 is a cylindrical member that extends from the top of the vacuum vessel 2 to the inside of the heat shield 3. The inside of the cylinder section 8 is in communication with the outside of the vacuum vessel 2, and a displacer 6 is inserted into this cylinder section 8. The cylinder section 8 is filled with helium gas. A drive section 7 is connected to the top of the displacer 6.

[0015] The superconducting coil 4 is connected to a displacer 6 via a thermal switch 5. When a driving unit 7 is driven, the superconducting coil 4 is cooled via the displacer 6 and the thermal switch 5. The displacer 6 serves as the low-temperature part of this embodiment.

[0016] When the superconducting device 1 is in use, the displacer 6 connected to the driving unit 7 is inserted into the cylinder portion 8 of the vacuum vessel 2. Here, the displacer 6 is thermally connected to the thermal switch 5. When the driving unit 7 starts to operate, the driving unit 7 cools the thermal switch 5 via the displacer 6, thereby turning the thermal switch 5 into the ON state. In this ON state, the superconducting coil 4 is cooled by the driving unit 7.

[0017] Maintenance is performed on the drive unit 7 and the displacer 6 after each period of use. As shown in Fig. 2, when the displacer 6 is replaced, that is, when maintenance is performed, the drive unit 7 and the displacer 6 are pulled out from the cylinder unit 8 while the superconducting coil 4 remains cooled to an extremely low temperature.

[0018] Here, the cylinder section 8 is cooled to an extremely low temperature, so if air gets inside the cylinder section 8, the moisture will freeze and the air will liquefy. Therefore, as shown in Figure 3, a heater 9 is inserted into the cylinder section 8 as a heating element instead of the displacer 6. This heater 9 heats the inside of the cylinder section 8 to room temperature and dries it. Then, maintenance of the displacer 6 is performed, and consumable parts such as seals are replaced. Thereafter, the displacer 6 is inserted into the cylinder section 8 again, as shown in Figure 4.

[0019] During maintenance, if heater 9 is inserted inside cylinder 8 and heated to room temperature, and the heat is transferred to superconducting coil 4, it will take a long time to re-cool superconducting coil 4. Therefore, thermal switch 5 prevents heat transfer between cylinder 8 and superconducting coil 4, thereby maintaining superconducting coil 4 at an extremely low temperature.

[0020] In this embodiment, when the heater 9 is driven, the thermal switch 5 is turned OFF while heating the thermal switch 5 to room temperature, and in this OFF state, the superconducting coil 4 is thermally isolated from the outside, thereby maintaining the superconducting coil 4 at a low temperature.

[0021] As shown in FIG. 5, the thermal switch 5 has a first thermal conductor 11, a second thermal conductor 12, a first connecting member 13, and a second connecting member .

[0022] The first connecting member 13 is connected to the cylinder portion 8 into which the displacer 6 serving as the low-temperature portion is inserted. The first thermal conductor 11 is connected to the cylinder portion 8 via the first connecting member 13. The second connecting member 14 is connected to the superconducting coil 4 serving as the portion to be cooled via a coil cooling stage 15. The second thermal conductor 12 is connected to the superconducting coil 4 via the second connecting member 14. The superconducting coil 4 and the coil cooling stage 15 are supported by predetermined members not shown.

[0023] The white arrows in Fig. 5 indicate the flow of heat. When the driving unit 7 (Fig. 1) is driven, the heat of the superconducting coil 4 is transferred to the displacer 6 via the thermal switch 5, and the superconducting coil 4 is cooled.

[0024] 6, the first thermal conductor 11 is a cylindrical member. The second thermal conductor 12 is a columnar member. The second thermal conductor 12 is fitted into a central hole of the first thermal conductor 11.

[0025] The first thermal conductor 11 is made of aluminum with a purity of 99.99% or higher. The second thermal conductor 12 is made of copper (oxygen-free copper) with a purity of 99.96% or higher. Both are made of materials with high thermal conductivity at cryogenic temperatures.

[0026] The thermal switch 5 is switched between an ON state and an OFF state by a change in the contact thermal resistance between the first thermal conductor 11 and the second thermal conductor 12 due to the difference in thermal contraction rate between the first thermal conductor 11 and the second thermal conductor 12. In the ON state, heat is conducted between the first thermal conductor 11 and the second thermal conductor 12. In the OFF state, the contact thermal resistance is greater than in the ON state, and heat conduction between the first thermal conductor 11 and the second thermal conductor 12 is suppressed.

[0027] The graph in Figure 8 shows the relationship between the thermal shrinkage rate and temperature for aluminum and copper. As shown in this graph, there is a difference in the thermal shrinkage rate between aluminum and copper. The thermal shrinkage rate of aluminum is greater than that of copper. The difference between the two is 0.07% when comparing room temperature and an extremely low temperature (for example, 4K). For example, if the diameter of the second thermal conductor 12 made of copper is 40 mm, the difference between the two is 28 μm.

[0028] For example, a gap G (FIG. 7) of 10 μm is provided between the first thermal conductor 11 (aluminum) and the second thermal conductor 12 (copper) at room temperature. In other words, a difference in diameter of 20 μm is provided between the inner diameter D1 of the first thermal conductor 11 and the outer diameter D3 of the second thermal conductor 12. In this way, the first thermal conductor 11 (cylinder) clamps the second thermal conductor 12 (column) with an interference of 4 μm at cryogenic temperatures.

[0029] At room temperature, a gap G (Fig. 7) of 10 μm on average occurs in the thermal switch 5, and the thermal switch 5 is in an OFF state with high thermal resistance. On the other hand, at extremely low temperatures, the first thermal conductor 11 (the inner surface of the cylinder) and the second thermal conductor 12 (the outer surface of the cylinder) come into close contact with each other, and the thermal switch 5 is in an ON state with low thermal resistance.

[0030] Since high-purity aluminum is used for the first thermal conductor 11 (cylinder), if stress increases in the ON state, there is a possibility that the first thermal conductor 11 will deform. Therefore, in the thermal switch 5 of this embodiment, the outer diameter D2 of the first thermal conductor 11 is made sufficiently large to reduce stress.

[0031] The graph in Fig. 9 shows the relationship between the inner / outer diameter ratio of the first thermal conductor 11 and the maximum circumferential stress. For example, as shown in Fig. 6, the ratio of the inner diameter D1 to the outer diameter D2 of the first thermal conductor 11 is the inner / outer diameter ratio (D2 / D1). In the ON state, the inner diameter D1 of the first thermal conductor 11 and the outer diameter D3 of the second thermal conductor 12 are the same. On the other hand, as shown in Fig. 7, in the OFF state, a gap G is generated between the first thermal conductor 11 and the second thermal conductor 12.

[0032] 9, the gap G is set to 2.5% of the inner diameter D1 of the first thermal conductor 11. As shown in this graph, in order to keep high-purity aluminum in its elastic deformation region, it is sufficient for the inner / outer diameter ratio of the first thermal conductor 11 to be 2.5 or greater. The thermal switch 5 of this embodiment is designed so that the ratio of the inner diameter D1 to the outer diameter D2 of the cylinder in the first thermal conductor 11 is 2.5 or greater.

[0033] Furthermore, to maintain high-purity aluminum in its elastic deformation region, the stress must be 20 MPa or less. For example, as shown in FIG. 11, the inventors measured the plastic strain generated by applying a predetermined stress to various test specimens made of high-purity aluminum. As a result, it was found that if the stress applied to high-purity aluminum is 20 MPa or less, almost no plastic strain occurs. Therefore, the thermal switch 5 of this embodiment is designed so that the maximum circumferential stress of the cylinder in the first thermal conductor 11 is 20 MPa or less.

[0034] Next, a configuration for actually using the thermal switch 5 will be described. As shown in FIG. 5, the first thermal conductor 11 (aluminum) of the thermal switch 5 is thermally connected to the displacer 6 of the low-temperature section (refrigerator side) via a first connecting member 13 serving as a heat transfer plate. The second thermal conductor 12 (copper) of the thermal switch 5 is thermally connected to the superconducting coil 4 of the cooled section (coil side) via a second connecting member 14 serving as a heat transfer plate and a coil cooling stage 15. The superconducting coil 4 is cooled by the coil cooling stage 15, which is cooled by the cryogenic refrigerator 10 (FIG. 1) via the thermal switch 5. In this configuration, during steady-state cooling, the thermal switch 5 is cooled to a cryogenic temperature (e.g., 4 K) and is in an ON state with low thermal resistance, so the temperature difference between the cryogenic refrigerator 10 and the superconducting coil 4 is small.

[0035] Next, we will explain what happens during maintenance of the drive unit 7 and displacer 6. Until the drive unit 7 (Fig. 2) is stopped and the displacer 6 is pulled out, the temperature rise of the thermal switch 5 is small and it remains in the ON state. At this time, if air enters the inside of the cylinder unit 8, heat will enter from the cylinder unit 8 and a small amount of heat will flow toward the superconducting coil 4.

[0036] Next, when heater 9 is inserted into cylinder portion 8 and heating begins, the temperature of thermal switch 5 rises rapidly. At this time, first thermal conductor 11 (aluminum) expands (thermally expands) more than second thermal conductor 12 (copper), creating a gap G (FIG. 7) at the contact point (engaged portion) between them, increasing the thermal contact resistance and turning thermal switch 5 OFF. When thermal switch 5 turns OFF, the amount of heat transferred to superconducting coil 4 is significantly reduced. As a result, the temperature rise of superconducting coil 4 is smaller than when thermal switch 5 is not present, and the re-cooling time when restarting operation is shorter.

[0037] However, although the thermal switch 5 of this embodiment has an average gap G of 10 μm at room temperature, the first thermal conductor 11 and the second thermal conductor 12 are partially in contact with each other and transfer heat, so heat cannot be completely blocked and a small amount of heat is transferred from the outside to the superconducting coil 4.

[0038] The graph in Figure 10 shows the results of calculating the relationship between the ratio of thermal resistance when the thermal switch 5 is in the ON state and when it is in the OFF state (ON / OFF ratio) and the re-cooling time. The vertical axis is the ratio when the re-cooling time when the thermal switch 5 is not present is set to 1. The horizontal axis is the ON / OFF ratio. This graph shows the effect of the heat transfer amount when the thermal switch 5 is in the OFF state.

[0039] This graph shows that even if the ON / OFF ratio is increased, the re-cooling time converges to about 0.4, so an ON / OFF ratio of about 10 is sufficient. In fact, the inventors prototyped a thermal switch 5 with an inner diameter of 40 mm and a gap of about 10 μm, and measured the ON / OFF ratio, finding it to be about 100 to 1000, which satisfied the above-mentioned condition.

[0040] Furthermore, because the thermal switch 5 of this embodiment has no moving parts, it has good reproducibility of ON / OFF operation and its thermal resistance when in the ON state is almost constant. The inventors' actual measurements showed that the thermal resistance at 4.2 K was within ±10%. This result is difficult to achieve with a mechanical thermal switch. Furthermore, because the thermal switch 5 of this embodiment does not use gas, there is no risk of gas leaks or explosions.

[0041] Therefore, this embodiment can provide a thermal switch 5 that requires no operation, has good reproducibility, and is free from the risk of gas leaks or explosions. Furthermore, by using this thermal switch 5, the re-cooling time when the displacer 6 of the cryogenic refrigerator 10 is replaced can be significantly reduced.

[0042] As shown in FIG. 5, both the first connecting member 13 and the second connecting member 14 serving as heat transfer plates are made of flexible heat transfer members that are flexible and heat transferable.

[0043] The flexible heat transfer member of this embodiment is a plate-shaped member in which high-purity aluminum sheets (foils) are laminated and the edges are welded. For example, aluminum sheets of about 0.5 mm are laminated.

[0044] In a side view (FIG. 5), first connection member 13 is at least partially curved and bent to form an S-shape, and second connection member 14 is at least partially curved and bent to form a C-shape.

[0045] Because the first connecting member 13 and the second connecting member 14 are made of a flexible heat transfer material, axial misalignment of the first connecting member 13 and the second connecting member 14 is less likely to occur when thermal contraction or thermal expansion occurs due to temperature changes. If the first connecting member 13 and the second connecting member 14 were made of a rigid material, axial misalignment would occur and uneven contact would occur when thermal contraction or thermal expansion occurs. This would result in uneven stress being applied to the contact surfaces of the first connecting member 13 and the second connecting member 14. In contrast, the thermal switch 5 of this embodiment can suppress axial misalignment and uniformly apply stress to the contact surfaces of the first connecting member 13 and the second connecting member 14.

[0046] In the above-described embodiment, both the first connecting member 13 and the second connecting member 14 are made of flexible heat transfer members, but other configurations are also possible. For example, only the first connecting member 13 may be made of a flexible heat transfer member, and only the second connecting member 14 may be made of a rigid member. Alternatively, only the second connecting member 14 may be made of a flexible heat transfer member, and only the first connecting member 13 may be made of a rigid member.

[0047] According to the embodiment described above, at least one of the first connecting member 13 and the second connecting member 14 is made of a flexible heat transfer member that is flexible and heat conductive, thereby providing a thermal switch 5 that is safe and highly reproducible. Furthermore, by using this thermal switch 5, the re-cooling time after replacing the displacer 6 in the cryogenic refrigerator 10 can be shortened.

[0048] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, modifications, and combinations can be made without departing from the spirit of the invention. These embodiments and modifications thereof are intended to be included within the scope and spirit of the invention, as well as within the scope of the invention described in the claims and their equivalents. [Explanation of symbols]

[0049] 1...superconducting device, 2...vacuum vessel, 3...heat shield, 4...superconducting coil, 5...thermal switch, 6...displacer, 7...drive unit, 8...cylinder unit, 9...heater, 10...ultra-low temperature refrigerator, 11...first thermal conductor, 12...second thermal conductor, 13...first connecting member, 14...second connecting member, 15...coil cooling stage, D1...inner diameter, D2, D3...outer diameter, G...gap.

Claims

1. a first connection member connected to the low-temperature portion; a first thermal conductor connected to the low-temperature portion via the first connecting member; a second connection member connected to the cooled part; a second heat conductor connected to the cooled portion via the second connecting member; and At least one of the first connecting member and the second connecting member is made of a flexible heat transfer member having flexibility and heat transfer properties, the second thermal conductor is fitted to the first thermal conductor; A difference in thermal contraction rate between the first thermal conductor and the second thermal conductor changes the contact thermal resistance between the first thermal conductor and the second thermal conductor, thereby switching between an ON state in which heat is conducted between the first thermal conductor and the second thermal conductor and an OFF state in which the contact thermal resistance is larger than in the ON state. Thermal switch.

2. the first thermal conductor has a cylindrical shape and is made of aluminum with a purity of 99.99% or more; The second thermal conductor has a cylindrical shape and is made of copper with a purity of 99.96% or more. The thermal switch of claim 1 .

3. The first thermal conductor has a cylindrical shape, and a maximum circumferential stress of the cylinder in the first thermal conductor is 20 MPa or less. The thermal switch of claim 1 .

4. The first thermal conductor has a cylindrical shape, and the ratio of the inner diameter to the outer diameter of the cylinder of the first thermal conductor is 2.5 or more. The thermal switch of claim 1 .

5. A method using the thermal switch according to any one of claims 1 to 4, the cooled part and the thermal switch connected to the cooled part are housed in a vacuum container, a displacer serving as the low-temperature part is inserted into the cylinder part of the vacuum vessel to connect the displacer to the thermal switch; The displacer is driven and the thermal switch is cooled via the displacer, whereby the thermal switch is turned on, and in the on state, the cooled portion is cooled by the displacer, When replacing the displacer, the displacer is pulled out from the cylinder portion, A heater is inserted into the cylinder portion instead of the displacer, The heater is driven to heat the thermal switch, thereby turning the thermal switch to the OFF state, and in the OFF state, the cooled part is thermally isolated from the outside, thereby maintaining the cooled part at a low temperature. How to replace a displacer.

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