Thermal switch

Abstract

To reduce thermal resistance during ON operation. [Solution] A thermal switch 30 that turns ON in the initial cooling stage of a superconducting coil 23 to cool the superconducting coil by the first-stage cooling stage 25 and the second-stage cooling stage 26 of a two-stage refrigerator 24, comprising: a first heat transfer member 31 thermally connected to the first-stage cooling stage and having a first contact portion 36; a second heat transfer member 32 thermally connected to the second-stage cooling stage 26 and having a second contact portion 38; and a drive mechanism 33 that drives the first heat transfer member to turn ON by bringing the first contact portion into contact with the second contact portion, wherein the first contact surface 36A of the first contact portion and the second contact surface 38A of the second contact portion are held parallel to each other while the first heat transfer member is driven by the drive mechanism.

Description

Technical Field

[0001] Embodiments of the present invention relate to a thermal switch.

Background Art

[0002] Superconducting magnets need to be cooled to extremely low temperatures. In the past, they were cooled using liquid helium. In recent years, since small cryogenic refrigerators capable of cooling down to the liquid helium temperature (4K) have been put into practical use, superconducting magnet devices cooled only by small cryogenic refrigerators have become widespread. In particular, since the resource problem of helium has been attracting attention due to a temporary shortage of helium, the shift to the refrigerator cooling method has been accelerating.

[0003] Such a superconducting magnet device with a refrigerator cooling method has many advantages compared to the conventional liquid helium cooling method. Its operation is very simple, no special skills required for handling liquid helium are needed, and there is no need for replenishment of liquid helium in the first place. On the other hand, there are also demerits. One of them is that the time (precooling time) required to cool the superconducting magnet from room temperature to 4K can be long. For example, what took several hours with the cooling method using liquid helium may take several weeks with the refrigerator cooling method.

[0004] As a countermeasure, a method has been adopted in which pipes are made to crawl on the superconducting coil and liquid nitrogen is passed through these pipes to cool the superconducting coil, thereby shortening the precooling time of the superconducting coil. However, in an environment where liquid nitrogen cannot be used, there are cases where it is cooled only by a refrigerator, and thus shortening the precooling time only by a refrigerator has again become an important issue. [[ID=!]]

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

[0006] Another method for shortening the pre-cooling time is to use a thermal switch. Examples of such thermal switches are provided in Patent Documents 1, 2, and 3. Patent Document 1 discloses a thermal switch that operates ON / OFF by inletting and outletping gas. Patent Documents 2 and 3 disclose mechanical thermal switches that can be attached and detached mechanically.

[0007] For example, the thermal switch (pre-cooling thermal contact mechanism 7) described in Patent Document 2 is installed between a radiant shield 2 thermally connected to the first-stage cooling stage of a refrigerator and a superconducting coil 1 thermally connected to the second-stage cooling stage. In other words, the thermal switch (pre-cooling thermal contact mechanism 7) has a contact plate 17 connected to the superconducting coil 1, a contact element 14 connected to the radiant shield 2 via a heat transfer plate (thermal anchor 15), and an adiabatic drive body 12 that drives the contact element 14 up and down. This thermal switch (pre-cooling thermal contact mechanism 7) is thermally turned ON / OFF by attaching and detaching the contact element 14 to the contact plate 17 using the adiabatic drive body 12.

[0008] During pre-cooling, the thermal switch (pre-cooling thermal contact mechanism 7) is turned ON to thermally connect the contact element 14 (first-stage cooling stage) and the contact plate 17 (second-stage cooling stage), and the superconducting coil 1 is cooled using the cooling capacity of both cooling stages. Next, when the superconducting coil 1 has cooled sufficiently and is close to the cooling temperature of the first-stage cooling stage, the thermal switch (pre-cooling thermal contact mechanism 7) is turned OFF, and the superconducting coil 1 is cooled to 4K using only the second-stage cooling stage.

[0009] Here, we will specifically examine the characteristics required for a thermal switch. When a superconducting coil is cooled by conduction using a refrigerator, a temperature difference occurs between the refrigerator and the superconducting coil. This temperature difference reduces the apparent cooling capacity of the refrigerator as seen from the superconducting coil. Figure 8 shows the relationship between the superconducting coil temperature and the apparent cooling capacity, calculated using the thermal resistance between the refrigerator and the superconducting coil as a parameter.

[0010] Taking the case where the superconducting coil temperature is 100K as an example, if the thermal resistance is 0.5K / W, the apparent cooling capacity as seen from the superconducting coil is 63W, whereas if the thermal resistance is 1.5K / W, the apparent cooling capacity drops to 38W. Therefore, reducing the thermal resistance between the refrigerator and the superconducting coil is important. For this reason, research has been conducted on the thermal resistance of thermal switches. The inventor has also researched the thermal resistance of the contact part in a mechanical thermal switch and has been able to reduce it to 0.1K / W.

[0011] However, thermal resistance between the refrigerator and the superconducting coil occurs not only in the thermal switch section but also in the heat transfer plate (for example, the thermal anchor 15 in Reference 2) and the cooling stage of the refrigerator, and reducing these is also important. Since the thermal resistance of the heat transfer plate can be reduced by increasing its cross-sectional area, it was conventionally considered a design problem. However, in order to actually reduce the thermal resistance of the heat transfer plate to a level comparable to that of the thermal switch, it is necessary to set the cross-sectional area of ​​the heat transfer plate to a considerably large size. In that case, the contact surface of the thermal switch tilts due to the weight of the heat transfer plate, and this uneven contact surface causes a problem in which the thermal resistance of the thermal switch increases significantly.

[0012] Embodiments of the present invention have been made in consideration of the above circumstances, and aim to provide a thermal switch that can reduce thermal resistance during ON operation. [Means for solving the problem]

[0013] The thermal switch in an embodiment of the present invention is configured such that, when a first object to be cooled and a second object to be cooled are placed in a vacuum container, the first object to be cooled is thermally connected to a first cooling source and cooled by the first cooling source, and the second object to be cooled is thermally connected to a second cooling source and cooled to a lower temperature than the first object to be cooled by the second cooling source, the thermal switch is activated ON in the initial stages of cooling of the second object to be cooled to cool the second object by the first and second cooling sources, and comprises: a first heat transfer member disposed in the vacuum container and thermally connected to the first cooling source and having a first contact portion; a second heat transfer member disposed in the vacuum container and thermally connected to the second cooling source and having a second contact portion; and a drive mechanism that drives the first heat transfer member to activate the ON operation by bringing the first contact portion into contact with the second contact portion, wherein the first contact surface of the first contact portion and the second contact surface of the second contact portion are held parallel to each other while the first heat transfer member is driven by the drive mechanism. [Effects of the Invention]

[0014] According to embodiments of the present invention, thermal resistance during ON operation can be reduced. [Brief explanation of the drawing]

[0015] [Figure 1] A schematic diagram showing a superconducting magnet device to which a thermal switch according to the first embodiment is applied. [Figure 2] A side view showing the thermal switch in Figure 1. [Figure 3] A cross-sectional view along the line III-III in Figure 2. [Figure 4] A partial side view showing the main part of the thermal switch according to the second embodiment. [Figure 5] This graph shows the contact thermal resistance when the contact surfaces of the first and second contact portions in Figure 4 are in direct contact with each other, when the material surface is in contact with the coating layer, and when the coating layer is in contact with the release layer. [Figure 6] A schematic diagram showing a superconducting magnet device to which a thermal switch according to the third embodiment is applied. [Figure 7] Side view showing the thermal switch of FIG. 6. [Figure 8] Graph showing the relationship between the temperature of the superconducting coil and the apparent refrigeration capacity of the refrigerator.

Embodiments for Carrying Out the Invention

[0016] Hereinafter, embodiments for carrying out the present invention will be described based on the drawings. [A] First Embodiment (FIGS. 1 to 3) FIG. 1 is a configuration diagram showing an outline of a superconducting magnet device to which the thermal switch according to the first embodiment is applied. In the superconducting magnet device 20 shown in FIG. 1, a thermal shield 22 as a first cooled object is disposed in a vacuum vessel 21, and a superconducting coil 23 as a second cooled object is disposed in the thermal shield 22. Further, a two-stage GM refrigerator (hereinafter referred to as a two-stage refrigerator 24) is installed in the vacuum vessel 21.

[0017] The thermal shield 22 and current leads (not shown) are thermally connected to a first cooling stage 25 as a first cooling source in the two-stage refrigerator 24, and are conduction-cooled to an extremely low temperature (for example, around 20K) by this first cooling stage 25. Also, the superconducting coil 23 is installed on a coil stage 27, and this coil stage 27 is thermally connected to a second cooling stage 26 as a second cooling source in the two-stage refrigerator 24 via a conduction conductor 28 and a second heat transfer plate 37 (described later). Thereby, the superconducting coil ∗23 is conduction-cooled to an extremely low temperature (for example, around 4K) lower than that of the thermal shield 22 or the like by the second cooling stage 26 of the two-stage refrigerator 24. The above-described thermal shield ∗22, superconducting coil ∗23, and current leads are vacuum-insulated by being installed in the vacuum vessel 21.

[0018] Incidentally, the first-stage cooling stage 25 and the second-stage cooling stage 26 of the two-stage refrigerator 24 are installed inside the vacuum vessel 21. Of these, the second-stage cooling stage 26 has a low cooling target temperature of about 4K, but a cooling capacity of about 1W. In contrast, the first-stage cooling stage 25 has a high cooling target temperature of about 20K, but a cooling capacity of several tens of watts. Since a high cooling temperature is acceptable during the initial cooling (pre-cooling) of the superconducting coil 23, the cooling capacity of the first-stage cooling stage 25 is utilized during the initial cooling of the superconducting coil 23 by turning on the thermal switch 30 described later.

[0019] As shown in Figures 1 and 2, the aforementioned thermal switch 30 is activated ON during the initial cooling of the superconducting coil 23, and cools the superconducting coil 23 using the first-stage cooling stage 25 and the second-stage cooling stage 26 of the two-stage refrigerator 24. It is configured to include a first heat transfer member 31, a second heat transfer member 32, and a drive mechanism 33.

[0020] The first heat transfer member 31 is placed inside the vacuum container 21 and is vacuum insulated, and is thermally connected to the first cooling stage 25 of the two-stage refrigerator 24, and comprises a first heat transfer plate 34, a flexible conductor 35, and a first contact portion 36.

[0021] The first heat transfer plate 34 is thermally directly connected to the first cooling stage 25 of the two-stage refrigerator 24. As shown in Figures 2 and 3, the flexible conductor 35 is provided between the first heat transfer plate 34 and the first contact portion 36 and is configured to be flexible. Multiple flexible conductors 35 are arranged around the drive rod 39 (described later) of the drive mechanism 33 and symmetrically with respect to the drive rod 39, and are also arranged vertically.

[0022] The first heat transfer plate 34, flexible conductor 35, and first contact portion 36 constituting the first heat transfer member 31 are made of high-purity aluminum or high-purity copper, which have high thermal conductivity at low temperatures. Of these, the flexible conductor 35 is made of a flat braided wire of high-purity aluminum or high-purity copper, or a laminate of thin sheets of high-purity aluminum or high-purity copper.

[0023] As shown in Figures 1 and 2, the second heat transfer member 32 is placed inside the vacuum container 21 and is vacuum-insulated, and is thermally connected to the second cooling stage 26 of the two-stage refrigerator 24, and comprises a second heat transfer plate 37 and a second contact portion 38. These second heat transfer plate 37 and second contact portion 38 are made of high-purity aluminum or high-purity copper, which have high thermal conductivity at low temperatures.

[0024] The second heat transfer plate 37 is thermally directly connected to the second cooling stage 26 of the two-stage refrigerator 24. Furthermore, the second heat transfer plate 37 is thermally connected to the conductive conductor 28, and through this conductive conductor 28 and the coil stage 27, is thermally connected to the superconducting coil 23.

[0025] The second contact portion 38 is fixed to the second heat transfer plate 37 and includes a second contact surface 38A. This second contact surface 38A, together with the first contact surface 36A formed on the first contact portion 36 of the first heat transfer member 31, is arranged horizontally and formed, for example, in a planar shape. The first contact surface 36A and the second contact surface 38A are configured to be able to come into contact or separate by the operation of the drive mechanism 33.

[0026] As shown in Figures 2 and 3, the drive mechanism 33 drives the first heat transfer member 31 by pushing it vertically downward in order to bring the first contact surface 36A of the first contact portion 36 of the first heat transfer member 31 into contact with the second contact surface 38A of the second contact portion 38 of the second heat transfer member 32, thereby turning on the heat switch 30. The drive mechanism 33 comprises a drive rod 39, a retaining plate 40, and a spring 41 as an elastic member.

[0027] As shown in Figures 1 to 3, the drive rod 39 extends vertically from the room temperature outside the vacuum vessel 21 into the vacuum vessel 21. The retaining plate 40 is tiltably connected to the end of the drive rod 39 inside the vacuum vessel 21 using, for example, a ball joint, and transmits the force of the drive rod 39 to the spring 41. Multiple springs 41 are installed in parallel between the first contact portion 36 of the first heat transfer member 31 and the retaining plate 40. These retaining plates 40 and springs 41 are placed inside the vacuum vessel 21 and are vacuum-insulated together with the portion of the drive rod 39 placed inside the vacuum vessel 21.

[0028] Furthermore, because the pressing plate 40 and the drive rod 39 are connected in a way that allows them to tilt freely, even when the drive rod 39 is slightly tilted relative to the vertical and the pressing plate 40 is pressed down, the parallel state between the pressing plate 40 and the first contact portion 36 of the first heat transfer member 31 is maintained.

[0029] As the drive rod 39 is pushed downward vertically, the first contact portion 36 of the first heat transfer member 31 is pressed downward vertically via the retaining plate 40 and spring 41. This first contact portion 36 then contacts the second contact portion 38 of the second heat transfer member 32, causing the thermal switch 30 to turn ON. As a result, as shown in Figure 1, the first cooling stage 25 of the two-stage refrigerator 24 is thermally connected to the superconducting coil 23 via the thermal switch 30, the conductive conductor 28, and the coil stage 27. The superconducting coil 23 is cooled by the first cooling stage 25 as well as the second cooling stage 26 of the two-stage refrigerator 24 during the initial cooling phase (pre-cooling). This shortens the pre-cooling time of the superconducting coil 23.

[0030] When the superconducting coil 23 reaches the cooling target temperature of the first-stage cooling stage 25 (for example, 20K), the drive rod 39 of the drive mechanism 33 is pulled vertically upward to turn off the thermal switch 30, thereby blocking heat intrusion from the first-stage cooling stage 25 to the superconducting coil 23. As a result, the superconducting coil 23 is cooled to, for example, 4K by only the second-stage cooling stage 26 of the two-stage refrigerator 24.

[0031] As shown in Figures 2 and 3, the flexible conductors 35 of the first heat transfer member 31 are arranged in multiple positions around the drive rod 39 of the drive mechanism 33 and symmetrically with respect to the drive rod 39. Therefore, when the drive rod 39 is pushed downward in the vertical direction and driven, the force acting on the flexible conductors 35 is symmetrical with respect to the drive rod 39. Consequently, the first contact surface 36A of the first contact portion 36 of the first heat transfer member 31 and the second contact surface 38A of the second contact portion 38 of the second heat transfer member 32 are held parallel to each other, and the first contact surface 36A and the second contact surface 38A make uniform surface contact without uneven contact.

[0032] Furthermore, when the drive rod 39 of the drive mechanism 33 is driven by being pushed vertically downward while the first contact surface 36A of the first contact portion 36 and the second contact surface 38A of the second contact portion 38 are in contact, the spring force of the multiple springs 41 of the drive mechanism 33 becomes equal in the contact state between the first contact surface 36A and the second contact surface 38A. Therefore, even if the first contact surface 36A of the first contact portion 36 and the second contact surface 38A of the second contact portion 38 are slightly misaligned from a parallel state, this slight misalignment is corrected by the springs 41, ensuring uniform surface contact between the first contact surface 36A and the second contact surface 38A.

[0033] Furthermore, even if the drive rod 39 of the drive mechanism 33 is pushed downward in the vertical direction and tilts slightly relative to the retaining plate 40, causing the first contact surface 36A of the first contact portion 36 and the second contact surface 38A of the second contact portion 38 to deviate from their parallel state, the aforementioned multiple springs 41 correct the deviation from the parallel state between the first contact surface 36A and the second contact surface 38A, thereby ensuring uniform surface contact between the first contact surface 36A and the second contact surface 38A.

[0034] As configured as described above, this first embodiment provides the following effects (1) and (2). (1) As shown in Figures 2 and 3, the flexible conductors 35 of the first heat transfer member 31 are arranged in multiple positions symmetrically with respect to the drive rod 39 of the drive mechanism 33. Therefore, when the first heat transfer member 31 is driven vertically downward by the drive mechanism 33, the first contact surface 36A of the first contact portion 36 of the first heat transfer member 31 and the second contact surface 38A of the second contact portion 38 of the second heat transfer member 32 are held parallel to each other, and as a result, the first contact surface 36A and the second contact surface 38A make uniform surface contact without uneven contact. As a result, when the thermal switch 30, which is activated when the first contact surface 36A of the first contact portion 36 and the second contact surface 38A of the second contact portion 38 make contact, is activated, the thermal resistance of the thermal switch 30 can be reduced.

[0035] (2) When the thermal switch 30 that pushes down the drive rod 39 of the drive mechanism 33 is turned ON, the parallel state of the first contact surface 36A of the first contact portion 36 of the first heat transfer member 31 and the second contact surface 38A of the second contact portion 38 of the second heat transfer member 32 may be slightly misaligned, or the drive rod 39 of the drive mechanism 33 may be slightly tilted with respect to the retaining plate 40. In such cases, since multiple springs 41 are arranged in parallel between the retaining plate 40 of the drive mechanism 33 and the first contact portion 36 of the first heat transfer member 31, the slight misalignment from the parallel state of the first contact surface 36A and the second contact surface 38A and the slight tilt of the drive rod 39 can be corrected, and thus uniform surface contact between the first contact surface 36A and the second contact surface 38A can be ensured. As a result, even when the thermal switch 30 repeatedly switches between ON and OFF states, uniform surface contact between the first contact surface 36A and the second contact surface 38A is ensured, improving the reproducibility of the thermal resistance reduction of the thermal switch 30.

[0036] [B] Second embodiment (Figures 4 and 5) Figure 4 is a partial side view showing the main parts of the thermal switch according to the second embodiment. In this second embodiment, parts that are the same as those in the first embodiment are denoted by the same reference numerals as in the first embodiment, thereby simplifying or omitting their description.

[0037] The thermal switch 45 shown in Figure 4 has the same configuration as the thermal switch 30 of the first embodiment, but with a coating layer 46 formed on either the first contact surface 36A of the first contact portion 36 of the first heat transfer member 31 or the second contact surface 38A of the second contact portion 38 of the second heat transfer member 32, and a release layer 47 formed on the other.

[0038] The coating layer 46 is made of a soft and highly thermally conductive metal (e.g., indium) and is formed on the first contact surface 36A or the second contact surface 38A (e.g., the first contact surface 36A) by plating, cold spraying, or welding. The release layer 47 is made of a material that facilitates removal from the coating layer 46 and is formed on the first contact surface 36A or the second contact surface 38A (e.g., the second contact surface 38A) by, for example, coating.

[0039] Figure 5 shows the results of measuring the contact thermal resistance per unit area of ​​the first contact surface 36A and the second contact surface 38A, with temperature as the parameter, when various treatments are performed on the first contact surface 36A and the second contact surface 38A. Treatment condition A is when the material surfaces of the first contact surface 36A and the second contact surface 38A are in contact with each other, treatment condition B is when the indium plating is in contact with the material surface of the first contact surface 36A or the second contact surface 38A, and treatment condition C is when the indium plating is in contact with the release layer. The contact thermal resistance under treatment condition B is more than an order of magnitude (10 times) lower than the contact thermal resistance under treatment condition A at temperatures below 10K, and is about 30% lower even at 60K.

[0040] However, under treatment condition B, which involves a combination of indium plating and the material surface of the first contact surface 36A or the second contact surface 38A, the adhesive strength becomes too strong, and if left for a long time, it may become difficult to remove the first contact surface 36A and the second contact surface 38A. In contrast, it has been confirmed that this problem does not occur under treatment condition C.

[0041] In this case, if the release layer 47 is thick, the thermal resistance between the first contact surface 36A and the second contact surface 38A will increase, so it needs to be thin. On the other hand, durability is also required so that the release layer 47 does not break during the release operation. A spray-type coating agent consisting of an alkylsilane coupling agent and a fluororesin can be used as a release layer 47 that satisfies these conditions. Furthermore, a release layer 47 with a thinner film thickness than this coating agent may be formed by applying a fluororesin or silicon-based organic material after plasma irradiation.

[0042] As configured as described above, this second embodiment provides the same effects as the first embodiment (1) and (2), as well as the following effect (3).

[0043] (3) Since a coating layer 46 made of a soft and highly thermally conductive metal is formed on either the first contact surface 36A of the first contact portion 36 of the first heat transfer member 31 or the second contact surface 38A of the second contact portion 38 of the second heat transfer member 32, the thermal resistance when the thermal switch 45 is ON, in which the first contact surface 36A and the second contact surface 38A are in contact via the coating layer 46, can be further reduced. Moreover, since a release layer 47 is formed on the other of the first contact surface 36A of the first contact portion 36 or the second contact surface 38A of the second contact portion 38, the first contact surface 36A and the second contact surface 38A can be separated well in the presence of the coating layer 46 when the thermal switch 45 is OFF, thus ensuring the reproducibility of the ON and OFF operations of the thermal switch 45.

[0044] [C] Third embodiment (Figures 6 and 7) Figure 6 is a schematic diagram showing a superconducting magnet device to which the thermal switch according to the third embodiment is applied. In this third embodiment, parts that are the same as in the first embodiment are denoted by the same reference numerals as in the first embodiment, and their explanation is simplified or omitted.

[0045] In the superconducting magnet device 50 shown in Figure 6, the first cooling source is the cooling stage 53 of a single-stage GM refrigerator (hereinafter referred to as the single-stage refrigerator 51). This cooling stage 53 is thermally connected to the heat shield 22, and the heat shield 22 is conduction-cooled by the single-stage refrigerator 51 to an extremely low temperature, for example, 20K.

[0046] Furthermore, the second cooling source is a two-stage cooling stage 54 of a two-stage refrigerator 52, similar to the two-stage refrigerator 24 of the first embodiment. This two-stage cooling stage 54 is thermally connected to the superconducting coil 23 via the second heat transfer plate 37, conduction conductor 28 and coil stage 27 of the second heat transfer member 32, and conduction cooling of the superconducting coil 23 to an extremely low temperature, for example, 4K, lower than the heat shield 22.

[0047] The thermal switch 55 applied to this superconducting magnet device 50, as shown in Figure 7, is configured similarly to the thermal switch 30 of the first embodiment, having a first heat transfer member 31, a second heat transfer member 32, and a drive mechanism 33. In this thermal switch 55, the first heat transfer plate 34 of the first heat transfer member 31 is thermally connected to the cooling stage 53 of the single-stage refrigerator 51. In addition, the second heat transfer plate 37 of the second heat transfer member 32 is thermally connected to the second-stage cooling stage 54 of the two-stage refrigerator 52.

[0048] Therefore, this third embodiment also produces the same effects as those of the first embodiment (1) and (2).

[0049] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be carried out in various other forms, and various omissions, substitutions, modifications, and combinations can be made without departing from the spirit of the invention, and such substitutions, modifications, and combinations are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Explanation of symbols]

[0050] 20...Superconducting magnet device, 21...Vacuum vessel, 22...Heat shield (first object to be cooled), 23...Superconducting coil (second object to be cooled), 24...Two-stage refrigerator, 25...Single-stage cooling stage (first cooling source), 26...Two-stage cooling stage (second cooling source), 30...Heat switch, 31...First heat transfer member, 32...Second heat transfer member, 33...Drive mechanism, 34...First heat transfer plate, 35...Flexible conductor, 36...First contact part, 36A...First contact surface, 37...Second heat transfer plate, 38...Second contact part, 38A...Second contact surface, 39...Drive rod, 40...Pressing plate, 41...Spring (elastic member), 45...Thermal switch, 46...Coating layer, 47...Release layer, 50...Superconducting magnet device, 51...Single-stage refrigerator, 52...Two-stage refrigerator, 53...Cooling stage (first cooling source), 54...Two-stage cooling stage (second cooling source), 55...Thermal switch.

Claims

1. When a first object to be cooled and a second object to be cooled are placed in a vacuum container, the first object to be cooled is thermally connected to a first cooling source and cooled by the first cooling source, and the second object to be cooled is thermally connected to a second cooling source and cooled to a lower temperature than the first object to be cooled by the second cooling source, A thermal switch that turns ON in the initial stages of cooling the second object to be cooled, and cools the second object to be cooled by the first cooling source and the second cooling source, A first heat transfer member is disposed within the vacuum vessel and thermally connected to the first cooling source, and is equipped with a first contact portion. A second heat transfer member is disposed within the vacuum container and thermally connected to the second cooling source, and is equipped with a second contact portion. The device includes a drive mechanism that drives the first heat transfer member to bring the first contact portion into contact with the second contact portion and turn it ON, A thermal switch characterized in that the first contact surface of the first contact portion and the second contact surface of the second contact portion are held in parallel while the first heat transfer member is driven by the drive mechanism.

2. The first heat transfer member comprises a first heat transfer plate thermally connected to a first cooling source, a first contact portion, and a flexible conductor provided between the first heat transfer plate and the first contact portion. The first contact surface of the first contact portion and the second contact surface of the second contact portion of the second heat transfer member are arranged horizontally and formed in a planar shape, and the first heat transfer member is driven vertically by a drive mechanism. The thermal switch according to claim 1, characterized in that the flexible conductors are arranged in a plurality at symmetrical positions with respect to the drive mechanism and are also arranged in the vertical direction.

3. The thermal switch according to claim 1, characterized in that the drive mechanism comprises a drive rod extending from a room temperature area outside the vacuum vessel into the vacuum vessel, a retaining plate connected to the end of the drive rod inside the vacuum vessel, and a plurality of elastic members provided in parallel between the first contact portion of the first heat transfer member and the retaining plate.

4. The thermal switch according to claim 1, characterized in that a coating layer made of a soft and highly thermally conductive metal is formed on either the first contact surface of the first contact portion or the second contact surface of the second contact portion by plating, cold spraying, or welding, and a release layer that can be peeled off from the coating layer is formed on the other by coating application.

5. The thermal switch according to claim 1, characterized in that the first cooling source is a first-stage cooling stage in a two-stage refrigerator, and the second cooling source is a second-stage cooling stage in the two-stage refrigerator.

6. The thermal switch according to claim 1, characterized in that the first cooling source is a single-stage refrigerator and the second cooling source is a two-stage cooling stage in a two-stage refrigerator.

7. The thermal switch according to claim 1, characterized in that the first heat transfer member and the second heat transfer member are made of high-purity aluminum or high-purity copper.

8. The thermal switch according to claim 2, characterized in that the flexible conductor is made of a flat braided wire of high-purity aluminum or high-purity copper, or a laminate of thin sheets of high-purity aluminum or high-purity copper.

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