Cryogenic heat switch, method for manufacturing cryogenic heat switch, and continuous nuclear adiabatic demagnetization refrigeration unit

Abstract

One aspect of the present invention provides a cryogenic heat switch. The cryogenic heat switch includes at least one switching portion, a first heat conducting portion, a second heat conducting portion, and a restricting portion. The at least one switching portion is configured to switch between a superconducting state and a normal conducting state upon application of a magnetic field equal to or greater than a critical magnetic field. The at least one switching portion in the normal conducting state has a higher thermal conductivity than the at least one switching unit in the superconducting state. The first heat conducting portion comprises a first base portion coupled to a first member and at least one first switching coupling portion. The at least one first switching coupling portion extends from the first base portion in a first direction and is coupled to a first end of the at least one switching portion. The at least one first switching coupling portion has a rigidity lower than that of the first base portion. The second heat conducting portion comprises a second base portion coupled to a second member and at least one second switching coupling portion. The at least one second switching coupling portion extends from the second base portion in a second direction and is coupled to a second end of the at least one switching portion. The at least one second switching coupling portion has a rigidity lower than that of the second base portion. The restricting portion is coupled to the first base portion and the second base portion without the at least one switching portion being interposed therebetween. The thermal conductivity of the restricting portion is lower than the thermal conductivity of the at least one switching portion in the superconducting state.

Description

Cryogenic heat switch, method of manufacturing the same, and continuous nuclear adiabatic demagnetization refrigeration unit

[0001] The present invention relates to a cryogenic heat switch, a method for manufacturing a cryogenic heat switch, and a continuous nuclear adiabatic demagnetization refrigeration unit.

[0002] Patent Document 1 discloses a technique relating to a refrigeration device that can continuously maintain a cooling state for a long period of time.

[0003] The refrigeration device described in Patent Document 1 comprises a front-stage refrigerator that cools a first cooling stage, and two adsorption refrigerators that are arranged in parallel between the first cooling stage and the second cooling stage and cool the second cooling stage. The adsorption refrigerators have an adsorber having an adsorbent material that can adsorb and desorb a refrigerant and a heater, a condensation-evaporation vessel that is connected to the adsorber through a tube and in which the refrigerant condenses and evaporates, an upper heat switch that switches between a normal conducting state and a superconducting state between the first cooling stage and the adsorber, and a lower heat switch that switches between a normal conducting state and a superconducting state between the condensation-evaporation vessel and the second cooling stage.

[0004] JP 2014-134334 A

[0005] However, there is room for further improvement in the technology of cryogenic thermal switches implemented at cryogenic temperatures.

[0006] According to one aspect of the present invention, there is provided a cryogenic thermal switch. The cryogenic thermal switch includes at least one switching unit, a first thermally conductive unit, a second thermally conductive unit, and a restriction unit. The at least one switching unit is configured to switch between a superconducting state and a normal-conducting state upon application of a magnetic field equal to or greater than a critical magnetic field. The at least one switching unit in the normal-conducting state has a higher thermal conductivity than the at least one switching unit in the superconducting state. The first thermally conductive unit includes a first base coupled to a first member and at least one first switching coupling unit. The at least one first switching coupling unit extends from the first base in a first direction and is coupled to a first end of the at least one switching unit. The at least one first switching coupling unit has lower rigidity than the first base. The second thermally conductive unit includes a second base coupled to a second member and at least one second switching coupling unit. The at least one second switching coupling portion extends from the second base portion along the second direction and is coupled to a second end of the at least one switching portion. The at least one second switching coupling portion has lower rigidity than the second base portion. The restricting portion couples the first base portion and the second base portion without the at least one switching portion. The restricting portion has a thermal conductivity lower than the thermal conductivity of the at least one switching portion in the superconducting state.

[0007] This configuration provides a superior cryogenic thermal switch technology that can be implemented at cryogenic temperatures.

[0008] 8 is a diagram for explaining a configuration example of a continuous nuclear adiabatic demagnetization refrigeration unit 1. FIG. 9 is a diagram illustrating a configuration example of a cryogenic thermal switch unit 3. FIG. 10 is a diagram illustrating a configuration example of a cryogenic thermal switch unit 3 equipped with a regulating portion 6. FIG. 11 is a perspective view of a cryogenic thermal switch 32 equipped with a regulating portion 6. FIG. 12 is a diagram illustrating the cryogenic thermal switch 32 as viewed in a plan view from the thickness direction D1 in FIG. 4. FIG. 13 is a diagram illustrating the cryogenic thermal switch 32 as viewed in a plan view from the width direction D3 in FIG. 4. FIG. 14 is a diagram illustrating the cryogenic thermal switch 32 as viewed in a plan view from the width direction D3 in FIG. 15. FIG. 16 is a diagram illustrating the cryogenic thermal switch 32 as viewed in a plan view from the width direction D3 in FIG. 16. FIG. 17 is an enlarged view of the coupling portion between the switching portion 4 and the heat conducting portion 5 in FIG. 17. FIG. 18 is a diagram illustrating the dependency of the sum 2Rc of the contact resistances of the first switching coupling portion and the second switching coupling portion and the volume V of the switching portion 4 on the second length L2. FIG. 19 is a flowchart illustrating a manufacturing process of a cryogenic thermal switch 32.

[0009] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the accompanying drawings. Various features shown in the following embodiments can be combined with each other.

[0010] 1. Configuration Example of Continuous Nuclear Adiabatic Demagnetization Refrigeration Unit 1 This section describes a configuration example of the continuous nuclear adiabatic demagnetization refrigeration unit 1. Figure 1 is a diagram for explaining a configuration example of the continuous nuclear adiabatic demagnetization refrigeration unit 1.

[0011] The continuous nuclear adiabatic demagnetization refrigeration unit 1 is an example of a cryogenic refrigeration unit capable of freezing a sample S to a cryogenic temperature and maintaining that temperature for a substantially unlimited period of time. Here, cryogenic temperature is defined as a temperature range that is difficult to freeze using conventional refrigerators such as dilution refrigerators. Specifically, it can be defined as a temperature range below 10 mK, below 1 mK, or below 0.1 mK. In particular, the temperature range in which the heat switch for the adsorption refrigerator described in Patent Document 1 operates is up to about 0.3 K, and temperatures lower than this can be defined as cryogenic. The continuous nuclear adiabatic demagnetization refrigeration unit 1 is connected to a refrigerator D through a heat conduction path P capable of conducting heat. The refrigerator D is configured to absorb heat contained in the continuous nuclear adiabatic demagnetization refrigeration unit 1 down to a predetermined temperature. The continuous nuclear adiabatic demagnetization refrigeration unit 1 comprises at least two nuclear adiabatic demagnetization refrigeration devices 2 (a first nuclear adiabatic demagnetization refrigeration device 2a and a second nuclear adiabatic demagnetization refrigeration device 2b), at least two cryogenic temperature thermal switch units 3 (a first cryogenic temperature thermal switch unit 3a and a second cryogenic temperature thermal switch unit 3b), and at least two thermometers 22.

[0012] The nuclear adiabatic demagnetization refrigerator 2 is a device capable of freezing an object using nuclear adiabatic demagnetization. The nuclear adiabatic demagnetization refrigerator 2 is configured to control the entropy based on the nuclear spin of the object by controlling the object with an external magnetic field, thereby enabling the object to be cooled to extremely low temperatures using the resulting thermal cycle. Examples of materials that effectively contain nuclear spin include pure metals such as the most standard Cu, as well as intermetallic compounds known as enhanced nuclear magnets, such as PrNi5. By using PrNi5, the object temperature can be reduced to less than 1 mK simply by applying a significantly weaker external magnetic field (specifically, an external magnetic field one-twelfth the magnitude) than when using Cu. The temperature at which the nuclear spin system of Cu undergoes magnetic ordering and loses entropy is 60 nanokelvin, whereas the magnetic ordering temperature of PrNi5 is as high as 0.4 mK, so the minimum temperature generated is only 0.3 mK. However, if the goal is to generate and maintain sub-mK temperatures of less than 1 mK, using PrNi5 allows the size of the superconducting magnet that applies the external magnetic field to be significantly smaller than when using Cu, and leakage magnetic field shielding can also be effectively performed in close proximity to the magnet, significantly reducing the overall size of the continuous nuclear adiabatic demagnetization refrigeration unit 1. This allows for a device of a realistic size in which two nuclear stages 21 can be installed in close proximity, and by assigning each nuclear stage 21 the role of absorbing external heat leaks and measurement heat and the role of transporting that heat to an external refrigerator, a continuous nuclear adiabatic demagnetization refrigerator that can continuously generate and maintain sub-mK temperatures without any restrictions can be realized. The continuous nuclear adiabatic demagnetization refrigeration unit 1 can continuously generate and maintain not only sub-mK temperatures, but also temperatures between 1 mK and 10 mK without any restrictions, which are difficult to freeze using an external refrigerator D such as a dilution refrigerator.

[0013] Each nuclear adiabatic demagnetization refrigerator 2 includes a nuclear stage 21, a thermometer 22, and a thermal stage 23. The nuclear stage 21 accommodates a material that effectively contains nuclear spins and is configured to allow application of an external magnetic field to the material. The thermometer 22 is connected to the thermal stage 23, thereby measuring the temperature of the nuclear stage 21, i.e., information regarding the freezing temperature of the nuclear adiabatic demagnetization refrigerator 2. The thermometer 22 may be configured to measure cryogenic temperatures of 10 mK or less, or even 1 mK or less. Such a thermometer 22 may be implemented, for example, as a platinum nuclear magnetic resonance thermometer, a thermal noise thermometer, or a superfluid helium-3 viscosity thermometer. In addition to the thermometer 22, each nuclear adiabatic demagnetization refrigerator 2 may also include a thermometer T, such as a resistance thermometer, that can easily measure temperatures above 10 mK. Complementary use of the thermometer 22 and the thermometer T enables accurate measurement over a wider temperature range. The nuclear stage 21 and the thermometer 22 are connected to the thermal stage 23. The thermal stage 23 is also configured to be connectable to the outside via a thermal switch, thereby enabling heat exchange between the nuclear adiabatic demagnetization refrigerator 2 and the outside.

[0014] In the second nuclear adiabatic demagnetization refrigerator 2b of this embodiment, a sample S to be measured at cryogenic temperatures may be connected to the thermal stage 23. The nuclear adiabatic demagnetization refrigerator 2 can freeze the sample S set on the thermal stage 23 using the nuclear stage 21. The thermometer 22 of the second nuclear adiabatic demagnetization refrigerator 2b can measure a temperature close to that of the sample S. Note that the nuclear adiabatic demagnetization refrigerator 2 is not limited to those intended for measuring the physical properties of the sample S at cryogenic temperatures, but may also be those intended simply for cooling, for example, quantum bit devices, nanomechanics devices, or microcalorimeters. Furthermore, the second nuclear adiabatic demagnetization refrigerator 2b may include a heater H for heating the thermal stage 23. For convenience of explanation, the thermal stage 23 of the first nuclear adiabatic demagnetization refrigerator 2a may be referred to as the first thermal stage 23a, and the thermal stage 23 of the second nuclear adiabatic demagnetization refrigerator 2b may be referred to as the second thermal stage 23b.

[0015] Each of the cryogenic thermal switch units 3 is configured so that the thermal conductivity between the first thermal stage 23a and the heat conduction path P and the thermal conductivity between the second thermal stage 23b and the first thermal stage 23a can be controlled by an external magnetic field. This allows each of the cryogenic thermal switch units 3 to switch between a closed state, in which heat conduction is easy, and an open state, in which heat conduction is difficult. Details of the cryogenic thermal switch units 3 will be described later.

[0016] In this embodiment, the first cryogenic heat switch unit 3a is connected between the heat conduction path P leading to the external refrigerator D and the thermal stage 23 of the first nuclear adiabatic demagnetization refrigerator 2a. The second cryogenic heat switch unit 3b is connected between the first nuclear adiabatic demagnetization refrigerator 2a and the second nuclear adiabatic demagnetization refrigerator 2b. By using these units and repeating the following four thermal processes (1) to (4), it is possible to continuously maintain the sample S at a constant sub-mK temperature without any restrictions.

[0017] (1) First, when the second cryogenic heat switch unit 3b is in the open state, the first cryogenic heat switch unit 3a is closed, a constant high magnetic field is applied to the first nuclear adiabatic demagnetization refrigerator 2a, and the magnetization heat is selectively released to the external refrigerator D side rather than the second nuclear adiabatic demagnetization refrigerator 2b side, thereby cooling to a constant cryogenic temperature (pre-cooling process of the first nuclear adiabatic demagnetization refrigerator 2a). During this process, the entropy of the nuclear spin system in the first nuclear adiabatic demagnetization refrigerator 2a can be reduced.

[0018] (2) Next, the first cryogenic thermal switch unit 3a is opened, and the second cryogenic thermal switch unit 3b reduces the magnetic field applied to the nuclear adiabatic demagnetization refrigerator 2, thereby cooling the temperature to a temperature slightly lower than that of the nuclear adiabatic demagnetization refrigerator 2 (the adiabatic demagnetization cooling process of the first nuclear adiabatic demagnetization refrigerator 2a). Because this demagnetization process is carried out almost adiabatically, the entropy of the first nuclear adiabatic demagnetization refrigerator 2a remains almost unchanged during this process, maintaining a low entropy state. During this process, the magnetic field applied to the second nuclear adiabatic demagnetization refrigerator 2b is constantly and slowly reduced, maintaining the sample temperature at a constant sub-mK temperature even in the presence of external heat inflow and heat generated during measurement. This causes the entropy of the nuclear spin system in the second nuclear adiabatic demagnetization refrigerator 2b to continue increasing.

[0019] (3) Next, the second cryogenic heat switch unit 3b is closed, the magnetic field applied to the second nuclear adiabatic demagnetization refrigerator 2b is increased, and the magnetization heat is reduced by the magnetic field applied to the first nuclear adiabatic demagnetization refrigerator 2a. This allows the entropy accumulated in the second nuclear adiabatic demagnetization refrigerator 2b to be transferred to the first nuclear adiabatic demagnetization refrigerator 2a while maintaining a constant sample temperature (entropy transfer process).

[0020] (4) Finally, the second cryogenic heat switch unit 3b is opened, and a constant high magnetic field is applied to the first nuclear adiabatic demagnetization refrigerator 2a. Then, at an appropriate time, the first cryogenic heat switch unit 3a is closed, and the first nuclear adiabatic demagnetization refrigerator 2a is pre-cooled (1) (the magnetization process of the first nuclear adiabatic demagnetization refrigerator 2a).

[0021] The thermometer 22 and the thermometer T are configured to be able to measure the temperatures of the thermal stage 23 of the first nuclear adiabatic demagnetization refrigerator 2 a and the second nuclear adiabatic demagnetization refrigerator 2 b. The second cryogenic thermal switch unit 3 b can be configured to switch between a closed state and an open state based on the temperatures of both stages and the magnetic fields applied to the two nuclear stages 21 a and 21 b.

[0022] 2. Configuration Example of Cryogenic Temperature Thermal Switch Unit 3 In this section, a configuration example of the above-mentioned cryogenic temperature thermal switch unit 3 will be described. Figures 2 and 3 are diagrams showing a configuration example of the cryogenic temperature thermal switch unit 3. Note that components common to the first cryogenic temperature thermal switch unit 3a and the second cryogenic temperature thermal switch unit 3b will be given common component numbers and their description may be omitted. The cryogenic temperature thermal switch unit 3 includes a magnetic field application unit 31 and a cryogenic temperature thermal switch 32.

[0023] The magnetic field application unit 31 is configured to apply an external magnetic field B to the cryogenic thermal switch 32 (particularly the switching unit 4), which will be described later, and is configured, for example, by a superconducting solenoid coil. The external magnetic field B is a magnetic field applied to the cryogenic thermal switch 32 from the outside and is also referred to as an applied magnetic field. The direction of the external magnetic field B is along the extension direction D2. The extension direction D2 is the direction in which the cryogenic thermal switch 32, which will be described later, extends and is perpendicular to the thickness direction D1. The thickness direction D1 is along the direction that defines the thickness of the switching unit 4, which will be described later. The magnetic field application unit 31 is configured so that the magnitude of the external magnetic field B is maximum (Bmax) in the region indicated by the center line C and decreases with increasing distance from the center line C. The magnetic field application unit 31 is configured to control the magnetic field applied to the switching unit 4, which will be described later, based on, for example, the temperatures of each thermal stage 23 measured by the thermometers 22 and T. For example, the magnetic field application section 31 of the second cryogenic heat switch unit 3b is configured to control the magnetic field applied to the switching section 4 of the cryogenic heat switch 32 described later based on the temperature of the nuclear adiabatic demagnetization refrigerator 2a and the temperature of the second nuclear adiabatic demagnetization refrigerator 2b.

[0024] The cryogenic temperature thermal switch 32 includes at least one switching unit 4, two heat conducting units 5, and two regulating units 6. Hereinafter, for convenience of explanation, when distinguishing between the two heat conducting units 5, they may be referred to as a first heat conducting unit 5a and a second heat conducting unit 5b, respectively.

[0025] Each of the switching units 4 is configured to switch between a superconducting state and a normal conducting state upon application of a magnetic field equal to or greater than the critical magnetic field. The switching unit 4 in the normal conducting state has a higher thermal conductivity than the switching unit 4 in the superconducting state. For example, the thermal conductivity of the switching unit 4 in the normal conducting state at 30 mK is 10,000 times or more, 100,000 times or more, or 10,000,000 times or more than the thermal conductivity of the switching unit 4 in the superconducting state. The cryogenic thermal switch 32 of this embodiment includes multiple switching units 4 as at least one switching unit 4.

[0026] The switching unit 4 is made of a material that becomes superconducting at cryogenic temperatures and can undergo a phase transition to a normal conducting state upon application of a magnetic field. The material used for the switching unit 4 is a metallic material that exhibits a type-I superconducting state, such as simple metals such as lead (Pb), titanium (Ti), tin (Sn), indium (In), aluminum (Al), zinc (Zn), and cadmium (Cd). In particular, at least one switching unit 4 may be made of zinc (Zn) to achieve various conflicting requirements: a transition temperature of approximately 0.3 K or higher to provide sufficient thermal insulation in the open state; a critical magnetic field as low as possible to suppress the generation of latent heat (magnetic latent heat) due to magnetic energy repelled by perfect diamagnetism during the transition from the normal conducting state to the superconducting state; and a low environmental impact. The magnitude of the magnetic latent heat is proportional to the volume of the superconductor and also proportional to the square of the critical magnetic field. Although aluminum is also a useful material, zinc has a lower critical magnetic field than aluminum. Therefore, using zinc for the switching unit 4 reduces the amount of latent heat generated and further improves the temperature stability and cooling efficiency of the nuclear adiabatic demagnetization refrigerator 2. Zinc has a superconducting transition temperature of 0.85 K. The material used for at least one switching unit 4 can be configured to be able to switch from a superconducting state to a normal conducting state at temperatures below 1 mK by applying an external magnetic field of 5.4 mT or more, and conversely, be able to switch from a normal conducting state to a superconducting state by removing the external magnetic field. This configuration allows for smooth entropy cycling at cryogenic temperatures. In this case, for example, when opening the cryogenic thermal switch 32, the magnetic field application unit 31 may be controlled so that an external magnetic field below the superconducting critical magnetic field is applied to the switching unit 4.

[0027] The heat conductive portion 5 is configured to conduct heat between the switching unit 4 and the outside (e.g., the heat conduction path P or the thermal stage 23). The heat conductive portion 5 is preferably configured from a material with high thermal conductivity regardless of whether an external magnetic field is applied, such as a precious metal such as silver or copper. In this embodiment, the heat conductive portion 5 is made of silver. The first heat conductive portion 5a and the second heat conductive portion 5b are each configured to conduct heat from the outside of the cryogenic thermal switch 32 to the switching unit 4. As shown in FIGS. 2 and 3 , the heat conductive portion 5 (first heat conductive portion 5a) of the second cryogenic thermal switch unit 3b is coupled to the first thermal stage 23a. The heat conductive portion 5 (second heat conductive portion 5b) of the second cryogenic thermal switch unit 3b is coupled to the second thermal stage 23b along the extension direction D2. Therefore, the second cryogenic thermal switch unit 3b is configured to guide heat from the second thermal conductive portion 5b to the first thermal conductive portion 5a. This cools the first nuclear stage 21 and the sample S. The first thermal stage 23a is an example of a first member. The second thermal stage 23b is an example of a second member. The temperature of the first thermal stage 23a is always higher than the temperature of the second thermal stage 23b, except during the thermal process (3).

[0028] The restricting portion 6 bridges the first heat-conducting portion 5a and the second heat-conducting portion 5b, thereby restricting changes in the positional relationship between the first heat-conducting portion 5a and the second heat-conducting portion 5b. The restricting portion 6 may have any shape as long as it can absorb mechanical loads on the switching portion 4. For example, the restricting portion 6 may be formed as a rectangular plate extending in the extension direction D2 along the first heat-conducting portion 5a, the switching portion 4, and the second heat-conducting portion 5b. The thermal conductivity of the restricting portion 6 is lower than the thermal conductivity of at least one switching portion 4 in the superconducting state. This prevents heat from leaking between the first heat-conducting portion 5a and the second heat-conducting portion 5b through the restricting portion 6 as a heat conduction path when the cryogenic thermal switch 32 is in the open state. For example, the thermal conductivity of the restricting portion 6 may be 1 / 10 or less of the thermal conductivity of the entire plurality of switching portions 4 in the superconducting state. The material constituting the regulating portion 6 is not particularly limited. For example, the regulating portion 6 may be formed primarily from a non-metallic material. This configuration suppresses a decrease in heat-blocking properties when the switching portion 4 is in a superconducting state due to the low thermal conductivity of the non-metallic material, while the rigidity of the non-metallic material allows the load on each heat-conducting portion and the switching portion 4 to be absorbed. Therefore, it is possible to both suppress a decrease in heat-blocking properties and maintain strength. Specifically, the regulating portion 6 may be formed using a resin member. The resin member may be made of any material, including, for example, a polymer resin such as polyimide, epoxy, polycarbonate, or acrylonitrile butadiene styrene (ABS), and even better, one filled with carbon powder or other materials with low thermal conductivity. However, a glass-epoxy material, which is an inexpensive and readily available material made by filling glass fiber into epoxy resin, provides sufficient strength and thermal insulation. For example, when viewed in a plan view from the width direction D3, the restricting portion 6 bridges between the first heat conductive portion 5a and the second heat conductive portion 5b so as to cover each of the switching portions 4. This makes it possible to prevent mechanical loads from being concentrated on the switching portions 4. The restricting portion 6 may have higher rigidity than the switching portions 4 and a switching connecting portion 53 described later.

[0029] 3. Details of the Cryogenic Thermal Switch 32 This section provides a more detailed description of the cryogenic thermal switch 32 described in the previous section. FIG. 4 is a perspective view of the cryogenic thermal switch 32 equipped with a restriction portion 6. FIG. 5 is a view of the cryogenic thermal switch 32 as viewed from the thickness direction D1 in FIG. 4 . FIG. 6 is a view of the cryogenic thermal switch 32 as viewed from the width direction D3 in FIG. 4 . FIG. 7 is a perspective view of the cryogenic thermal switch 32 without the restriction portion 6. FIG. 8 is a view of the cryogenic thermal switch 32 as viewed from the width direction D3 in FIG. 7 . FIG. 9 is an enlarged view of the joint between the switching portion 4 and the heat conducting portion 5 in FIG. 8 . In the cryogenic thermal switch 32 described in this section, the first heat conducting portion 5a and the second heat conducting portion 5b have similar structures, and therefore, both will be collectively referred to as the heat conducting portion 5.

[0030] As shown in FIGS. 7 to 9 , each of the switching portions 4 is formed, for example, in a plate or strip shape. In this embodiment, the switching portion 4 has a surface (principal surface) with the largest area in the thickness direction D1, extends in the extension direction D2, and is formed in a rectangular membrane shape (in other words, a rectangular strip shape) with a width in the width direction D3, making it deformable when subjected to external force. This allows the switching portion 4 to absorb loads imposed by external mechanical forces. The thickness direction D1, extension direction D2, and width direction D3 are mutually different, specifically, perpendicular directions. The principal surface of the switching portion 4 is mirror-polished and can be bonded to other mirror-polished surfaces using a bonding method such as thermal diffusion fusion. As shown in FIG. 9 , the switching portion 4 has a first end 41 and a second end 42. The first end 41 and the second end 42 are end portions spaced apart from each other in the extension direction D2. The switching sections 4 are arranged in the form of multiple quadrangles at intervals SP in the thickness direction D1. This increases the flexibility of the switching sections 4 compared to when the switching sections 4 are made of a single, thicker member with the same cross-sectional area perpendicular to the extension direction D2, making it more difficult for external mechanical forces to affect the joint surface between the switching sections 4 and the switching joint section 53. This also significantly reduces the amount of heat generated by eddy currents when controlling (sweeping) the external magnetic field.

[0031] 4 to 9 , each of the two heat conductive portions 5 includes a base portion 51, a joint portion 52, and a plurality of switching coupling portions 53. The switching coupling portion 53 of the first heat conductive portion 5a is an example of a first switching coupling portion, and the switching coupling portion 53 of the second heat conductive portion 5b is an example of a second switching coupling portion. That is, the cryogenic temperature thermal switch 32 includes a plurality of switching coupling portions 53 of the first heat conductive portion 5a as the switching coupling portion 53 of at least one first heat conductive portion 5a, and a plurality of switching coupling portions 53 of the second heat conductive portion 5b as the switching coupling portion 53 of at least one second heat conductive portion 5b.

[0032] The base 51 is a columnar member extending in the extension direction D2. The specific shape of the base 51 is arbitrary, but may be, for example, a rectangular columnar shape. The base 51 has a first surface 51a and a second surface 51b. The first surface 51a is a surface perpendicular to the width direction D3. The second surface 51b extends from an end of the first surface 51a in the width direction D3 and is a surface perpendicular to the thickness direction D1. The first surface 51a includes a first region 51c and a second region 51d. Two member connecting portions 511 are provided in the first region 51c, and two restricting fastening portions 512 are provided in the second region 51d. The base 51 of the first heat conductive portion 5a corresponds to a first base, and the base 51 of the second heat conductive portion 5b corresponds to a second base.

[0033] Each of the member coupling portions 511 is configured to be coupleable with a target of heat exchange (a so-called first member or second member) via the switching portion 4, such as the heat conduction path P or the thermal stage 23. The member coupling portion 511 of the first heat conduction portion 5a corresponds to the first member coupling portion, and the member coupling portion 511 of the second heat conduction portion 5b corresponds to the second member coupling portion. In this embodiment, the member coupling portions 511 are fastening holes into which bolts can be inserted. Members such as the thermal stage 23 are fastened to the base 51 with bolts and nuts inserted into the member coupling portions 511. This enables heat exchange between the thermal stage 23 and the base 51. The multiple member coupling portions 511 are spaced apart from one another in the extension direction D2. This allows the fastened members, such as the thermal stage 23, to be more firmly fixed.

[0034] In this embodiment, at least a portion of the second region 51d constitutes a contact area that contacts the member (first member or second member) connected to the member connecting portion 511. In this contact area, the surfaces of the base 51, the first member, and the second member are covered with a film made of a material that is resistant to deterioration in air at room temperature, such as gold (Au). This gold film may be formed, for example, by chemical plating (gold plating) or by physical deposition, such as vacuum deposition. This reduces the contact thermal resistance of the contact area and suppresses a decrease in thermal conductivity through the cryogenic thermal switch 32. The thickness of this film is arbitrary, but specific examples include 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 μm, and may be within a range between any two of the values ​​exemplified here. To extend the life of the film against wear during removal of the cryogenic thermal switch 32, the thickness of the film may be 4 μm or more. This configuration prevents oxidation and sulfurization of the contact surface in the ambient air at room temperature, allowing for a long-term maintenance of good contact. Furthermore, the gold film deforms due to pressure applied when removing and attaching the cryogenic heat switch 32 from the first and second members, making it easier to maintain a good contact. The film may also cover areas other than the contact area included in the first region 51c, such as the entire second region 51d or the second surface 51b.

[0035] The restricting fastening portion 512 is a structure for fastening the restricting portion 6 (described later) to the base portion 51 and is configured, for example, by a fastening hole into which a screw, bolt, or the like can be inserted. At least a portion of the first surface 51a corresponds to a coupled portion onto which the restricting portion 6, which is fastened via the restricting fastening portion 512, is overlapped. The region of the first surface 51a corresponding to the coupled portion is planar and parallel to the contact surface of the restricting portion 6. The two restricting fastening portions 512 are spaced apart from each other in the extension direction D2. This prevents the restricting portion 6 from rotating in the thickness direction D1 due to external forces, etc., and forms a stronger bond between the base portion 51 and the restricting portion 6. Note that the number of restricting fastening portions 512 is not limited to two and may be three or more. This configuration allows the restricting portion 6 to more firmly restrict the movement of each heat conduction portion, thereby more effectively reducing the load on each switching coupling portion 53 and the switching portion 4.

[0036] The joint portion 52 is a member for joining the switching joint portion 53 (described later) to the base portion 51, and is formed, for example, by a plurality of fins before welding. Each of the fins extends in the extension direction D2 from the surface of the tip of the base portion 51 in the extension direction D2, and is formed in a quadrangular shape having a width in the width direction D3 and a thickness in the thickness direction D1. The fins are arranged at intervals from each other in the thickness direction D1, and are made of the same material as the base portion 51. The surfaces of the fins perpendicular to the thickness direction D1 are mirror-polished before welding.

[0037] The switching coupling portion 53 is a member that couples the base portion 51 and the switching portion 4 (more specifically, the joint portion 52 and the switching portion 4), and is formed from the same material (silver in this embodiment) as the base portion 51 and the joint portion 52. The switching coupling portion 53 of the first heat conductive portion 5a corresponds to the first switching coupling portion, and the switching coupling portion 53 of the second heat conductive portion 5b corresponds to the second switching coupling portion. Like the joint portion 52, each of the switching coupling portions 53 extends in the extension direction D2 of the base portion 51 (more specifically, from the joint portion 52 after welding), has a width in the width direction D3, and is formed in a plate or strip shape (more specifically, a quadrangular membrane shape) having a thickness in the thickness direction D1.

[0038] Each of the switching coupling portions 53 is inserted between the fins of the joint portion 52. Thereafter, the switching coupling portions 53 and the joint portions 52 are subjected to electron beam welding or the like, thereby integrally forming the joint portions 52 and the switching coupling portions 53. As a result, the base portion 51 and the switching coupling portions 53 are joined via the joint portions 52, and the switching coupling portions 53 are configured to protrude from the joint portions 52 in the extension direction D2. Therefore, the joint portions 52 themselves are not limited to being fin-shaped, but may be block-shaped similar to the base portion 51. Note that the specific form of the joint portions 52 is not limited thereto and is arbitrary, and the joint portions 52 may be configured so that the fins remain without being welded or the like. For ease of explanation, the end of the switching coupling portion 53 protruding from the joint portion 52 that is coupled to the joint portion 52 will be referred to as the first end 53a of the switching coupling portion 53, or simply as the first end 53a, and the end located opposite the first end 53a in the extension direction D2 will be referred to as the second end 53b of the switching coupling portion 53, or simply as the second end 53b.

[0039] In this embodiment, the first direction, which is the direction in which the switching coupling portion 53 of the first heat conductive portion 5a extends, and the second direction, which is the direction in which the switching coupling portion 53 of the second heat conductive portion 5b extends, are both extension direction D2. In other words, the switching coupling portion 53 of at least one first heat conductive portion 5a is formed in a plate or strip shape extending in the first direction, and the switching coupling portion 53 of at least one second heat conductive portion 5b is formed in a plate or strip shape extending in the second direction. The second direction is aligned with the first direction. In other words, each of the multiple first switching coupling portions is coupled to the first base portion from the first direction. Similarly, each of the multiple second switching coupling portions is coupled to the second base portion from the second direction.

[0040] As shown in FIG. 9 , the multiple switching coupling portions 53 include two outer edge coupling portions 531 and at least one inner coupling portion 532. The outer edge coupling portion 531 is a switching coupling portion 53 located at the outer edge of the switching coupling portion 53 in the thickness direction D1. The inner coupling portion 532 is a switching coupling portion 53 located between the two outer edge coupling portions 531. Each outer edge coupling portion 531 is coupled to the base portion 51 of the first heat conductive portion 5a, closer to the inside than the outer edge of the base portion 51 in the thickness direction D1. In other words, the distance t1 between the outer edge coupling portion 531 and the outer edge of the base portion 51 in the thickness direction D1 is arbitrary as long as it is shorter than the distance between the outer edges of the base portion 51 in the thickness direction D1 minus the thickness of one switching coupling portion 53 and the thickness of one superconductor sheet of the switching portion 4. This forms a step portion 52a around the coupling region between the joint portion 52 and the switching coupling portion 53. With this configuration, the switching joint 53 including the outer edge joint 531 is less likely to come into contact with the regulating part 6 when manufacturing or installing the cryogenic heat switch 32, so when the cryogenic heat switch 32 is in the open state, heat can be prevented from being transmitted between the first heat conducting part 5a and the second heat conducting part 5b using the regulating part 6 as a heat conduction path.

[0041] The switching coupling portion 53 of the first heat conductive portion 5a is coupled to the first end 41 of the switching portion 4, and the switching coupling portion 53 of the second heat conductive portion 5b is coupled to the second end 42 of the switching portion 4. Specifically, the switching coupling portion 53 of the first heat conductive portion 5a and the switching coupling portion 53 of the second heat conductive portion 5b are each coupled to at least one switching portion 4 such that the thickness direction of the switching coupling portion 53 of the second heat conductive portion 5b is aligned with the thickness direction of at least one switching portion 4. This configuration makes it easier to absorb the load applied to each switching coupling portion 53 through deformation of the switching coupling portion 53 and the switching portion 4, thereby preventing deterioration of the coupling state between the switching portion 4 and each heat conductive portion. More specifically, the multiple switching coupling portions 53 are arranged at intervals from each other in the thickness direction D1. In other words, the multiple first switching coupling portions and the multiple second switching coupling portions are arranged at intervals from each other in the thickness direction D1. In this embodiment, the surface of the switching coupling portion 53 in the thickness direction D1 is mirror-polished and is coupled to the mirror-polished surface of the switching portion 4 by thermal diffusion fusion. As a result, each switching portion 4 is coupled to a different switching portion 4 at the surface in the thickness direction D1. As a result, a gap SP is provided between adjacent switching portions 4 in the thickness direction D1. As a result, each of the multiple switching coupling portions of the first heat conductive portion 5a and the second heat conductive portion 5b is configured so that the switching portions 4 are coupled in parallel between their respective base portions 51. This configuration significantly reduces eddy current heating when an external magnetic field is swept due to the opening and closing operation of the thermal switch.

[0042] Each of the switching coupling portions 53 has lower rigidity than the base portion 51. In other words, each of the switching coupling portions 53 is more easily deformed in the thickness direction D1 or the extension direction D2 than the base portion 51 to which it is coupled. For example, the thickness of the switching coupling portion 53 in the thickness direction D1 is shorter than the length of the base portion 51 in the thickness direction D1. The ratio of the thickness of the switching coupling portion 53 in the thickness direction D1 to the length of the base portion 51 in the thickness direction D1 is, for example, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1, and may be within a range between any two of the numerical values ​​exemplified here. With this configuration, the restricting portion 6 restricts deformation of the first switching coupling portion and the second switching coupling portion, and the deformation of the switching coupling portion 53 and the thin plate of the switching portion 4 reduces or distributes the load applied to the coupling surface of the switching portion 4 when manufacturing or installing the cryogenic thermal switch 32. Therefore, it is possible to suppress deterioration of the coupling state between the switching portion 4 and each heat conduction portion.

[0043] 4 to 6 , the restriction portion 6 is coupled to each base portion 51 without passing through at least one switching portion 4. Specifically, at least a portion of the restriction portion 6 is coupled to each of the base portion 51 of the first heat conductive portion 5a and the base portion 51 of the second heat conductive portion 5b so as to extend along the switching coupling portion 53 of the first heat conductive portion 5a, at least one switching portion 4, and the switching coupling portion 53 of the second heat conductive portion 5b. With this configuration, the restriction portion 6 can more easily absorb the load on the switching portion 4, etc., and therefore, deterioration of the coupling state between the switching portion 4 and each shaft portion can be further suppressed.

[0044] The restricting portion 6 is coupled to the base 51 of the first heat conductive portion 5a and the base 51 of the second heat conductive portion 5b by fastening the restricting fastening portion 512 of the first heat conductive portion 5a and the restricting fastening portion 512 of the second heat conductive portion 5b using fastening members 61. With this configuration, coupling the restricting fastening portions 512 of the first heat conductive portion 5a and the restricting fastening portions 512 of the second heat conductive portion 5b to the plurality of first heat conductive portion 5a and the plurality of second heat conductive portion 5b facilitates restricting rotational movement around the fastening portions as the rotation axis. This effectively reduces the load on each switching coupling portion 53 and the switching portion 4. Examples of the fastening members 61 include screws, bolts, and nuts. The fastening members 61 may be integrally formed with the restricting portion 6. For example, if the restricting fastening portions 512 are formed with recesses and projections into which the restricting portion 6 can be fitted, the fastening members 61 can be implemented by forming a portion of the restricting portion 6 into a shape that can be fitted into the recesses and projections.

[0045] As shown in FIG. 6 , in this embodiment, when the cryogenic heat switch 32 is viewed from above in the width direction D3, the restricting portion 6 is positioned to cover the switching coupling portion 53 of the first heat conductive portion 5a, the switching portion 4, and the switching coupling portion 53 of the second heat conductive portion 5b. This allows the restricting portion 6 to absorb external forces in the width direction D3 that would otherwise deform the switching portion 4 or each switching coupling portion 53, thereby preventing significant deformation, buckling, or breakage that would impair the thermal conductivity of the switching portion 4 or each switching coupling portion 53. This also reduces the possibility of objects interfering with the switching portion 4 and each switching coupling portion 53 when attaching the cryogenic heat switch 32 to the thermal stage 23, etc. The restricting portion 6 in this embodiment is a rectangular plate extending in the extension direction D2.

[0046] In the continuous nuclear adiabatic demagnetization refrigeration unit 1 and the cryogenic heat switch 32 described above, the restricting member 6 restricts deformation of the switching joint 53 of the first heat conductive member 5a and the switching joint 53 of the second heat conductive member 5b, thereby distributing the load on the switching joint 53 of the first heat conductive member 5a and the switching joint 53 of the second heat conductive member 5b when installing the cryogenic heat switch 32. This prevents deterioration of the connection between the switching member 4 and the heat conductive members 5a and 5b.

[0047] In this embodiment, when the base 51 of the first heat conductive unit 5a is coupled to the first thermal stage 23a and the base 51 of the second heat conductive unit 5b is coupled to the second thermal stage 23b, the regulating unit 6 is configured to be detachable from the base 51 of the second heat conductive unit 5b. This configuration prevents heat from the regulating unit 6 or heat conducted from the first nuclear adiabatic demagnetization refrigeration device 2a via the regulating unit 6 during thermal process (1) in which the first nuclear adiabatic demagnetization refrigeration device 2a reaches its highest temperature (maximum of approximately 30 mK) from propagating through the base 51 of the second heat conductive unit 5b to the second thermal stage 23b and further to the second nuclear adiabatic demagnetization refrigeration device 2b. The term "detachable" means that the force required for removal is sufficient to induce only an acceptable level of deformation in the switching coupling unit 53 of the first heat conductive unit 5a, the switching coupling unit 53 of the second heat conductive unit 5b, and the switching unit 4, so that deterioration of the coupling state is within an acceptable range. For example, the restricting portion 6 may be configured to be removable by adjusting the fastening force of the fastening member 61 (screw and nut). Specifically, the fastening member 61 may be fastened so that the force with which the fastening member 61 fastens the restricting fastening portion 512 of the second heat conductive portion 5b to the restricting portion 6 (hereinafter referred to as fastening force N2) is smaller than the force with which the fastening member 61 fastens the restricting fastening portion 512 of the first heat conductive portion 5a to the restricting portion 6 (hereinafter referred to as fastening force N1). Note that the method for making the restricting portion 6 removable is not limited to adjusting the fastening force of the screw and nut, and may be any method, such as adjusting the shape of the restricting portion 6 or the base 51 and utilizing elastic deformation of the restricting portion 6.

[0048] In particular, it is preferable that the restriction portion 6 be configured to be detachable from the base 51 of the second heat conductive portion 5b. This makes it possible to adjust the coupling mode of the restriction portion 6 depending on the situation, such as coupling the restriction portion 6 to both bases 51 when attaching the cryogenic heat switch 32 to the cryogenic heat switch unit 3, removing the restriction portion 6 from the base 51 of the second heat conductive portion 5b when operating the cryogenic heat switch unit 3, and coupling the restriction portion 6 to the base 51 of the second heat conductive portion 5b again when removing the cryogenic heat switch 32 from the cryogenic heat switch unit 3, thereby improving maintainability while maintaining the performance of the cryogenic heat switch 32.

[0049] 7 and 8, the cryogenic temperature thermal switch 32 can achieve the same effect even if the restricting portion 6 is removed after installation in the cryogenic temperature thermal switch unit 3. That is, the base 51 of the first heat conductive portion 5a only needs to include a restricting fastening portion 512 of the first heat conductive portion 5a that is fastened to the first end 41 of the restricting portion 6 that is different from the first member and the second member, and the base 51 of the second heat conductive portion 5b only needs to include a restricting fastening portion 512 that is fastened to the second end 42 of the restricting portion 6. According to this configuration, by fastening the highly rigid restricting member 6 to the restricting fastening members 512 provided on the base 51 of the first heat conductive member 5a and the base 51 of the second heat conductive member 5b so as to bridge the base 51 of the first heat conductive member 5a and the base 51 of the second heat conductive member 5b, the load applied to the switching joint 53 of the first heat conductive member 5a and the switching joint 53 of the second heat conductive member 5b when attaching the cryogenic temperature thermal switch 32 to the first member and the second member can be distributed to the restricting member 6. Therefore, it is possible to suppress deterioration of the connection between the switching member 4 and each heat conductive member 5a, 5b.

[0050] 5. Characteristic Length of the Cryogenic Thermal Switch 32: The magnitude of the thermal resistance represented by the pair of switching junctions 53 and switching unit 4 depends at least on the length characterizing the structure of the switching junctions 53 and switching unit 4 in the extension direction D2. When the thermal resistance is large, the temperature difference between the second nuclear adiabatic demagnetization refrigerator 2b and the first nuclear adiabatic demagnetization refrigerator 2a required to transfer the same amount of entropy as heat from the second nuclear adiabatic demagnetization refrigerator 2b to the first nuclear adiabatic demagnetization refrigerator 2a becomes larger. This increases the irreversibility of the thermal process, thereby increasing the amount of entropy increase. As a result, the amount of entropy that can be transferred from the second nuclear adiabatic demagnetization refrigerator 2b to the first nuclear adiabatic demagnetization refrigerator 2a within a certain period of time decreases, i.e., the cooling power of the continuous nuclear adiabatic demagnetization refrigerator unit 1 decreases. Therefore, reducing this thermal resistance makes it easier to achieve lower cryogenic temperatures. Such a characteristic length can be, for example, the first length L as shown in FIG. 9 . １ and the second length L ２ and the third length L ３ The first length L １is the distance (particularly the shortest distance) from the first end 53a to the joint between the switching joint 53 and the switching unit 4. ２ is the length of the connecting region between the switching connecting portion 53 and the switching portion 4 in the extending direction D2. ３ is the distance between the pair of switching coupling portions 53 in the extension direction D2, in other words, the length in the extension direction D2 of the region of the switching portion 4 that is not coupled to the switching coupling portion 53. １ is the thermal resistance R １ is a parameter corresponding to the second length L ２ is a parameter that defines the contact thermal resistance between the switching coupling portion 53 and the switching portion 4, and the third length L ３ is the thermal resistance R of the switching unit 4 ２ is a parameter that defines the value of the residual electrical resistance at 4K. According to the Wiedemann-Franz law, the magnitude of the thermal resistance of a pure metal at extremely low temperatures is proportional to the magnitude of the residual electrical resistance at 4K, and the proportionality constant (Lorentz number) is a constant that does not depend on the type of material. Since the heat conduction paths of the continuous nuclear adiabatic demagnetization refrigeration unit 1 are all made of pure metals, hereinafter, the electrical resistance at 4K, which is easier to measure and more familiar to the user, i.e., the residual electrical resistance, will sometimes be used instead of the thermal resistance, or these may simply be referred to as "resistance". The first length L １ Resistance R corresponding to １ is further defined as the resistivity ρ of the material that constitutes the switching coupling portion 53. １ and thickness t ３ The third length L ３ Resistance R corresponding to ２ is the resistivity ρ of the material constituting the switching portion 4 ２ and thickness t ４ In addition, the contact resistance R between the switching coupling portion 53 and the switching portion 4 is determined based on the following. ｃ is the contact resistivity ρ, which is the contact resistance per unit contact area ｃ Therefore, when discussing resistance, the second length L ２ Hereinafter, for the sake of convenience, the total thickness of all the switching coupling portions 53 and the switching portion 4 included in the cryogenic thermal switch 32 is defined as t ａｌｌIf the number of switching units 4 is n, then t ａｌｌ is n(t ３ +t ４ ) is approximately equal to the contact resistance R ｃ can be expressed as follows:

[0051] However, x and t in formula (1) ｚ , L ０ are expressed as follows, and the first length L １ , second length L ２ , third length L ３ is an independent parameter.

[0052] The first term on the right side of equation (1) is the second length L ２ On the other hand, the second and third terms on the right side of the equation (1) increase linearly with the increase of the second length L ２ As a result, the contact resistance R ｃ The second length L ２ is approximately 2L ０ This becomes:

[0053] On the other hand, the thickness t of the switching portion 4 ４ As the volume V decreases, the resistance R of the switching unit 4 ２ While there is a disadvantage that the thickness t of the switching portion 4 increases, ４ When V increases, the size of the cryogenic thermal switch 32 as a whole increases, which is a disadvantage. Furthermore, the magnetic latent heat generated when the switching unit 4 is switched to the superconducting state increases in proportion to V, which hinders stable maintenance of cryogenic temperatures. Therefore, the thickness t ４ 10 shows the relationship between the contact resistance of the first switching coupling portion and the contact resistance of the second switching coupling portion, 2R ｃ and the second length L of the volume V of the switching portion 4 ２ The line 100 is a graph showing the dependence of the contact resistance 2R ｃ and line 200 represents the volume V. In this case, the second length L ２ is the 2nd round ｃ The second length L is preferably 3 mm or more, at which point the rate of decrease of the second length L begins to decrease.２ Specifically, V and 2R are, for example, 3, 4, 5, 6, 7, 8, 9, and 10 mm, and may be within a range between any two of the values ​​exemplified here. ｃ In order to achieve both of these, the second length L ２ It is particularly preferable that the third length L ３ The amount of heat leakage when the cryogenic heat switch 32 is open is determined by the resistance R ２ The third length L ３ A graph showing the dependency is created, and the third length L is set in advance to achieve both in the same way as in FIG. ３ = 4 mm and n = 6. Figure 10 shows the results based on these factors.

[0054] In order for the switching coupling portion 53 to flexibly deform in response to an external force, the first length L １ On the other hand, the first length L １ As the length of the resistor R １ In view of this, the first length L １ is the second length L ２ and the third length L ３ It is preferable that the third length L ３ The third length L may be two, three, four, or five times the third length L. ３ When the first length L is 2 to 4 mm, １ Specifically, the first length L is, for example, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 mm, and may be within a range between any two of the values ​​exemplified here. １ is preferably 10 to 14 mm.

[0055] For example, the thickness t ２ , t ３Under the condition that the thickness is made thin within the tolerance range of durability and processing accuracy, the first length L １ , second length L ２ , third length L ３ By setting -５ W / K or more, preferably 1.2 × 10 -４ W / K or more, more preferably 1.7 × 10 -４ W / K or more, and at 30 mK, the superconducting state is 7 × 10 -７ W / K or less, preferably 2×10 -７ W / K or less, more preferably 7×10 -８ At least one switching unit is coupled to the first switching coupling unit and the second switching coupling unit so that the temperature difference is equal to or less than W / K. With this configuration, it is possible to smoothly realize an entropy cycle at extremely low temperatures.

[0056] 6. Example of Manufacturing Method of Cryogenic Thermal Switch 32 This section describes an example of a manufacturing method of the cryogenic thermal switch 32. FIG.

[0057] First, in step S1, a first precursor of the heat conductive part 5 is prepared. The heat conductive part 5 may be prepared in any manner. For example, the base 51 is formed by processing the raw material of the heat conductive part 5 (silver in this embodiment), and multiple fins constituting the joints 52 are formed at the tip of the base 51. Alternatively, the fins may be formed by bonding a film-like material serving as a precursor of the fins to the tip of the base by electron beam welding or the like. Furthermore, the base 51 is provided with a member joint 511 and a restricting fastening part 512 in advance. This produces a first precursor of the heat conductive part 5 having the base 51 and the joints 52.

[0058] Next, in step S2, a member (e.g., a silver strip) capable of forming the switching coupling portion 53 is bonded to the fin of the joint portion 52 in the first precursor prepared in step S1. This forms the heat conductive portion 5 including the base portion 51, the joint portion 52, and the switching coupling portion 53. For example, a member capable of forming the switching coupling portion 53 is inserted into the fin of the joint portion 52, and then the entire fin of the joint portion 52 into which the switching coupling portion 53 has been inserted is pressed from the thickness direction D1 using a jig or the like. The pressing then causes the switching coupling portion 53 to be crimped to the fin. An electron beam is irradiated onto the fin of the joint portion 52 to which the switching coupling portion 53 is crimped, performing electron beam welding. This results in the base portion 51, the joint portion 52, and the switching coupling portion 53 being integrally formed. In this manner, the first heat conductive portion 5a and the second heat conductive portion 5b are manufactured. In particular, by performing ion beam welding while crimping with a jig made of the same material as the heat conductive portion 5, contamination of the joint 52 can be suppressed and an increase in the electrical resistance of the heat conductive portion 5 can be prevented. In this case, unnecessary portions of the jig can be cut off after welding. Furthermore, when crimping with a jig made of a different material from the heat conductive portion 5, it is preferable to interpose a protective film on the contact surface between the jig and the joint 52 to suppress contamination between the jig and the joint 52. As the protective film, a material with a higher melting point than the melting point of the joint 52 is preferred to prevent thermal fusion during electron beam welding, and a film made of tantalum (Ta), which has high chemical stability and hardness, is particularly preferred. Furthermore, of the components that can constitute the switching coupling portion 53, the surfaces that are coupled to the joint 52 and the switching portion 4 are mirror-polished in advance.

[0059] Next, in step S3, the first precursor with the switching coupling portion 53 bonded thereto is heat-treated to enhance crystallinity and reduce thermal resistance. Heat-treating a high-purity metal under appropriate conditions can increase its resistance ratio from approximately 100-500 before heat treatment to approximately 2000-3000, significantly reducing thermal resistance at cryogenic temperatures. Here, the resistance ratio is the electrical resistance at room temperature divided by the residual electrical resistance at 4 K. According to the Wiedemann-Franz law, the thermal conductivity of a metal at cryogenic temperatures increases in proportion to the resistance ratio.

[0060] Appropriate heat treatment conditions vary depending on the type and size (particularly the minimum size) of the metal used to form the switching joint 53. For example, in the case of silver, the heat treatment temperature is set to a value between 600°C and 850°C, and the temperature is maintained for a period of 4 to 24 hours. The container containing the metal sample is first evacuated to a vacuum of approximately 0.001 Pa using a vacuum pump. During the heat treatment, oxygen gas is continuously supplied from the outside to maintain a pressure of 0.1 to 0.01 Pa inside the container. This allows magnetic impurities, which are randomly distributed in the metal and cause a decrease in the resistance ratio, to selectively oxidize with oxygen atoms that thermally diffuse into the metal, resulting in a nonmagnetic oxide. Microclusters of these oxides then migrate to the grain boundaries via thermal diffusion, thereby improving crystallinity. This allows for a further reduction in resistivity.

[0061] Next, in step S4, the switching coupling portions 53 and the switching portion 4 are coupled together. Specifically, first, the first end 41 of the switching portion 4 is positioned between the switching coupling portions 53 of the first heat conductive portion 5a, and the second end 42 of the switching portion 4 is positioned between the switching coupling portions 53 of the second heat conductive portion 5b. The first end 41 and the second end 42 are mirror-polished in advance. As a result, the switching coupling portions 53 of the first heat conductive portion 5a and the first end 41 of the switching portion 4 are alternately stacked in the thickness direction D1 so that their mirror-polished surfaces face each other. Similarly, the switching coupling portions 53 of the second heat conductive portion 5b and the second end 42 of the switching portion 4 are alternately stacked in the thickness direction D1 so that their mirror-polished surfaces face each other. Next, the switching joint 53 and the switching unit 4, stacked in the thickness direction D1, are pressed from the thickness direction D1 using a jig or the like and heated for a certain period of time, thereby thermally diffusing the switching joint 53 and the switching unit 4 together. This produces a second precursor of the cryogenic thermal switch 32, which includes the switching unit 4, the first thermal conductive portion 5a, and the second thermal conductive portion 5b. The appropriate conditions for thermal diffusion bonding vary depending on the materials of the switching joint 53 and the switching unit 4. For example, if the former is silver and the latter is zinc, the temperature is 220°C and the time is 3 hours. The second precursor is an example of a cryogenic thermal switch 32 before the regulating portion 6 is bonded. Steps S1 to S4 can also be considered a manufacturing method for a cryogenic thermal switch 32 without the regulating portion 6. Finally, the aforementioned gold film is formed on the surface of the contact region included in the first region 51c.

[0062] Next, in step S5, the restricting portion 6 is bonded to the second precursor prepared in advance by, for example, manufacturing in step S4. In this manner, the cryogenic thermal switch 32 including the restricting portion 6 can be manufactured.

[0063] The cryogenic heat switch 32 thus manufactured is inserted into the magnetic field application unit 31, and the member connection unit 511 is connected to the thermal stage 23 and other components at a predetermined position in the extension direction D2. This allows the cryogenic heat switch 32 to be incorporated into the continuous nuclear adiabatic demagnetization refrigeration unit 1 as part of the cryogenic heat switch unit 3. The predetermined position can be any position as long as it allows the magnetic field of the magnetic field application unit 31 to be applied to the switching unit 4. However, it is preferable that the predetermined position be configured so that the position at which the maximum magnetic field Bmax can be applied by the magnetic field application unit 31 overlaps with the first end 41 (see FIG. 9 ). This allows the second end 42 to enter the superconducting state before the first end 41 when the magnitude of the applied magnetic field is reduced below the critical magnetic field of the superconductor to place the switching unit 4 in the superconducting state. This allows the magnetic latent heat of the switching unit 4, which is released when the switching unit 4 transitions to a superconducting state, to be selectively transported to the first nuclear adiabatic demagnetization refrigerator 2a via the first end 41, the first heat conducting unit 5a connected to the first end 41, and the thermal stage 23, thereby suppressing an increase in the temperature of the sample S connected to the second end 42. In other words, the cryogenic heat switch 32 is preferably arranged so that the magnetic field applied to the switching unit 4 by the magnetic field application unit 31 is asymmetric in the extension direction D2. In particular, the cryogenic heat switch 32 is preferably arranged so that the magnetic field applied to the second end 42 by the magnetic field application unit 31 is smaller than the magnetic field applied to the first end 41 by the magnetic field application unit 31.

[0064] 7. Others The continuous nuclear adiabatic demagnetization refrigeration unit 1 or the cryogenic heat switch 32 according to the above embodiment may employ the following aspects.

[0065] The member coupling portion 511 does not need to be implemented by a fastening hole, but may be a connector that can be coupled by pressing the heat stage 23 or the like and the base portion 51 with a strong external force in some way.

[0066] The region of the first surface 51 a corresponding to the to-be-coupled portion is not limited to a single surface, but may be formed of multiple surfaces having irregularities, etc. For example, the irregularities may be configured to be able to position the restricting portion 6 and the heat conducting portion 5.

[0067] The position of the outer edge joining portion 531 is arbitrary, and for example, the outer edge joining portion 531 may protrude beyond the base portion 51 in the thickness direction D1.

[0068] In the above-described cryogenic heat switch 32, the first direction defining the extension direction of the switching coupling portion 53 of the first heat conductive portion 5a and the second direction defining the extension direction of the switching coupling portion 53 of the second heat conductive portion 5b are both along the extension direction D2, but the relationship between the first direction and the second direction is not limited thereto. For example, the first direction and the second direction may be configured perpendicular to each other. In this case, the cryogenic heat switch 32 is not on a straight line extending along the extension direction D2, but is L-shaped, bending 90 degrees at the switching portion 4. Furthermore, the number of heat conductive portions 5 connected to the switching portion 4 may be three or more.

[0069] The number of switching coupling parts 53 is not limited to a plurality and may be one. Accordingly, the number of switching parts 4 may be one. In other words, it is sufficient that the first heat conducting part 5a includes the base part 51 of the first heat conducting part 5a and at least one switching coupling part 53 of the first heat conducting part 5a, and it is sufficient that the second heat conducting part 5b includes the base part 51 of the second heat conducting part 5b and at least one switching coupling part 53 of the second heat conducting part 5b.

[0070] The switching coupling 53 is not limited to use for the continuous nuclear adiabatic demagnetization refrigeration unit 1, but can be used for any unit that requires control of heat flow at extremely low temperatures of about 700 mK or less.

[0071] It may be provided in the following manner.

[0072] (1) A thermal switch for cryogenic use, comprising at least one switching unit, a first heat conducting unit, a second heat conducting unit, and a regulating unit, wherein the at least one switching unit is configured to switch between a superconducting state and a normal conducting state by application of a magnetic field equal to or greater than a critical magnetic field, wherein the at least one switching unit in the normal conducting state has a higher thermal conductivity than the at least one switching unit in the superconducting state, the first heat conducting unit comprises a first base portion coupled to a first member, and at least one first switching coupling portion, the at least one first switching coupling portion extends from the first base portion along a first direction, and the a second heat conducting portion including a second base portion coupled to a second member and at least one second switching coupling portion, the second heat conducting portion being coupled to a first end of at least one switching portion and having lower rigidity than the first base portion, the second heat conducting portion including a second base portion coupled to a second member and at least one second switching coupling portion extending from the second base portion along a second direction and coupled to a second end of the at least one switching portion and having lower rigidity than the second base portion; the regulating portion couplings the first base portion and the second base portion without the at least one switching portion being interposed therebetween, and the thermal conductivity of the regulating portion being lower than the thermal conductivity of the at least one switching portion in the superconducting state.

[0073] According to this configuration, the restricting portion restricts deformation of the first and second switching coupling portions, so that the load applied to the first and second switching coupling portions when the cryogenic thermal switch is installed can be distributed, thereby preventing deterioration of the coupling state between the switching portions and each heat conduction portion.

[0074] (2) In the cryogenic temperature thermal switch described in (1) above, the at least one switching part is formed in a plate shape or a strip shape, the at least one first switching coupling part is formed in a plate shape or a strip shape extending in the first direction, the at least one second switching coupling part is formed in a plate shape or a strip shape extending in the second direction, the second direction is along the first direction, and the first switching coupling part and the second switching coupling part are each coupled to the at least one switching part such that the thickness direction of the at least one first switching coupling part and the at least one second switching coupling part is along the thickness direction of the at least one switching part.

[0075] With this configuration, the load applied to each switching joint can be more easily absorbed by deformation of the switching joint, thereby preventing deterioration of the joint between the switching joint and each heat conduction part.

[0076] (3) In the cryogenic temperature thermal switch described in (2) above, the cryogenic temperature thermal switch includes a plurality of switching units as the at least one switching unit, a plurality of first switching coupling units as the at least one first switching coupling unit, and a plurality of second switching coupling units as the at least one second switching coupling unit, wherein each of the plurality of first switching coupling units and the plurality of second switching coupling units is arranged at intervals in the thickness direction and is coupled to different switching units on its surface, and the switching units are coupled in parallel between the first base and the second base.

[0077] With this configuration, the superconducting state and the normal conducting state are switched at multiple switching sections, so that the high thermal conductivity in the normal conducting state can be utilized as much as possible while suppressing the decrease in thermal conductivity in the superconducting state.

[0078] (4) In the cryogenic heat switch described in (3) above, each of the plurality of first switching coupling parts is coupled to the first base from the first direction, and among the plurality of first switching coupling parts, each of the outer edge coupling parts, which is a first switching coupling part located on the outer edge in the thickness direction, is coupled to the inside of the first base from the outer edge of the first base in the thickness direction.

[0079] With this configuration, the first switching joint, including the outer edge joint, is less likely to interfere when the cryogenic heat switch is moved, etc., thereby reducing the possibility of load being applied to the joint area between the first switching joint and the first base.

[0080] (5) In the cryogenic temperature thermal switch described in any one of (2) to (4) above, at least a portion of the regulating portion is coupled to each of the first base and the second base so as to extend along the first switching coupling portion, the at least one switching portion, and the second switching coupling portion.

[0081] According to this configuration, the restricting portion can more easily absorb the load on the switching portion and the like, and therefore deterioration of the connection between the switching portion and each shaft portion can be further suppressed.

[0082] (6) In the cryogenic temperature thermal switch described in (5) above, each of the first base portion and the second base portion has a planar coupled portion on which the restricting portion is superimposed.

[0083] According to this configuration, the movement of each heat conduction portion can be more firmly restricted by the restricting portion, and therefore the load on each switching coupling portion and switching portion can be more effectively reduced.

[0084] (7) In the cryogenic heat switch described in any one of (1) to (5) above, the first base has a first restricting fastening portion, the second base has a second restricting fastening portion, and the restricting portion is coupled to the first base and the second base by being fastened to each of the first restricting fastening portion and the second restricting fastening portion using a fastening member.

[0085] With this configuration, by connecting the first restricting fastening parts and the second restricting fastening parts, it becomes easier to restrict the rotational movement around the fastening parts as the rotation axis, thereby more effectively reducing the load on each switching coupling part and switching part.

[0086] (8) In the thermal switch for use at cryogenic temperatures described in any one of (1) to (7) above, the regulating portion is formed mainly from a non-metallic material.

[0087] With this configuration, the low thermal conductivity of the non-metallic material prevents a decrease in heat insulation performance when the switching unit is in a superconducting state, while the rigidity of the non-metallic material allows the load on each heat conductive unit and the switching unit to be absorbed. Therefore, it is possible to both prevent a decrease in heat insulation performance and maintain strength.

[0088] (9) In the cryogenic heat switch described in any one of (1) to (8) above, the contact area of ​​the first base with the first member and the contact area of ​​the second base with the second member are each covered with a gold (Au) film, and the film thickness of the film is 4 μm or more.

[0089] With this configuration, the gold film deforms due to the pressure applied when attaching the cryogenic heat switch to the first and second members, making it easier to form a good contact state, thereby reducing contact resistance.

[0090] (10) In the thermal switch for use at cryogenic temperatures described in any one of (1) to (9) above, the at least one switching portion is made of zinc (Zn).

[0091] In this configuration, zinc has a lower critical magnetic field than materials that become superconductors at low temperatures, so by using zinc for the switching section, the amount of heat generated due to the magnetic field energy that is repelled by perfect diamagnetism during the transition from the normal conducting state to the superconducting state can be reduced, further improving cooling efficiency.

[0092] (11) In the cryogenic thermal switch according to any one of (1) to (10), the composite value of the thermal conductivity of the entire at least one switching unit is 1.7 × 10 in a normal conduction state at 1 mK. -４ W / K or more, and at 30 mK, the superconducting state is 7 × 10 -８ each of the at least one switching portion is coupled to the first switching coupling portion and the second switching coupling portion such that a temperature difference between the first switching coupling portion and the second switching coupling portion is equal to or less than W / K.

[0093] With this configuration, the entropy cycle can be smoothly realized at extremely low temperatures.

[0094] (12) A cryogenic thermal switch, configured to be interposed between a first member and a second member to control heat exchange between the first member and the second member, comprising a switching unit, a first heat conducting unit, and a second heat conducting unit, wherein the switching unit is configured to switch between a superconducting state and a normal conducting state by application of a magnetic field equal to or greater than a critical magnetic field, wherein the switching unit in the normal conducting state has a higher thermal conductivity than the switching unit in the superconducting state, and the first heat conducting unit comprises a first base, a first member connecting unit, and a first switching connecting unit, wherein the first member connecting unit is connected to the first member, and the first switching connecting unit is connected to a second base. a second heat conduction portion including a second base, a second member coupling portion, and a second switching coupling portion, the second member coupling portion being configured to be coupleable with the second member, the second switching coupling portion extending from the second base along a second direction and coupled to a second end of the switching portion, the second member coupling portion having lower rigidity than the second base, the first base including a first restricting fastening portion fastened to a first end of a restricting portion different from the first member and the second member, and the second base including a second restricting fastening portion fastened to a second end of the restricting portion.

[0095] According to this configuration, by fastening the highly rigid restricting portion to the restricting fastening portion provided on each of the first and second bases so as to bridge the first and second bases, the load applied to the first and second switching coupling portions when attaching the cryogenic thermal switch to the first and second members can be distributed to the restricting portion, thereby suppressing deterioration of the coupling state between the switching portion and each heat conduction portion.

[0096] (13) A method for manufacturing a cryogenic heat switch according to any one of (1) to (12) above, comprising the following steps: in the bonding step, bonding the regulating portion to a pre-prepared precursor, which is a cryogenic heat switch before bonding the regulating portion.

[0097] With this configuration, the regulating portion regulates deformation of the first switching coupling portion and the second switching coupling portion, so that a cryogenic heat switch can be provided that easily distributes the load applied to the first switching coupling portion and the second switching coupling portion when installing the cryogenic heat switch.

[0098] (14) A continuous nuclear adiabatic demagnetization refrigeration unit comprising: a cryogenic heat switch according to any one of (1) to (12) above; a nuclear adiabatic demagnetization refrigeration device; a second member; and a magnetic field application unit, wherein the nuclear adiabatic demagnetization refrigeration device is coupled to the first heat conduction unit, the second member is coupled to the second heat conduction unit, and the magnetic field application unit is configured to control the magnetic field applied to the switching unit based on the temperature of the nuclear adiabatic demagnetization refrigeration device and the temperature of the second member.

[0099] With this configuration, the cryogenic thermal switch can be switched appropriately according to the temperature of the connected member, thereby improving the cooling efficiency especially in a sub-millikelvin environment.

[0100] Finally, while various embodiments of the present disclosure have been described, they are presented as examples and are not intended to limit the scope of the invention. The novel embodiments may be embodied in various other forms, and various omissions, substitutions, and modifications may be made without departing from the spirit of the invention. Such embodiments and modifications are intended to be included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as defined in the claims.

[0101] 1: Continuous nuclear adiabatic demagnetization refrigeration unit, 2: Nuclear adiabatic demagnetization refrigeration device, 2a: First nuclear adiabatic demagnetization refrigeration device, 2b: Second nuclear adiabatic demagnetization refrigeration device, 3: Cryogenic heat switch unit, 3a: First cryogenic heat switch unit, 3b: Second cryogenic heat switch unit, 4: Switching unit, 5: Heat conduction unit, 5a: First heat conduction unit, 5b: Second heat conduction unit, 6: Restricting unit, 21: Nuclear stage, 22: Thermometer, 23: Heat stage, 23a: First heat stage, 23b: Second heat stage, 31: Magnetic field application unit, 32: Cryogenic heat switch switch, 41: first end, 42: second end, 51: base, 51a: first surface, 51b: second surface, 51c: first region, 51d: second region, 52: joint, 52a: step, 53: switching joint, 53a: first end, 53b: second end, 61: fastening member, 511: member joint, 512: restricting fastening portion, 531: outer edge joint, 532: internal joint, C: center line, D1: thickness direction, D2: extension direction, D3: width direction, H: heater, P: heat conduction path, S: sample, T: thermometer, V: volume, t1: distance, t2: distance, t3: thickness, t4: thickness

Claims

1. A thermal switch for cryogenic use, comprising at least one switching unit, a first thermal conduction unit, a second thermal conduction unit, and a regulating unit, wherein the at least one switching unit is configured to switch between a superconducting state and a normal conducting state by application of a magnetic field equal to or greater than a critical magnetic field, and wherein the at least one switching unit in the normal conducting state has a higher thermal conductivity than the at least one switching unit in the superconducting state, the first thermal conduction unit comprises a first base connected to a first member and at least one first switching coupling unit, the at least one first switching coupling unit extends from the first base unit in a first direction, is coupled to a first end of the at least one switching unit, and has a lower rigidity than the first base, the second thermal conduction unit comprises a second base connected to a second member, and at least one second switching coupling unit, A thermal switch for cryogenic temperatures, the thermal switch extending from the second base in a second direction and coupled to a second end of the at least one switching portion and having a lower rigidity than the second base, the restricting portion coupling the first base and the second base without passing through the at least one switching portion, and the thermal conductivity of the restricting portion being lower than the thermal conductivity of the at least one switching portion in the superconducting state.

2. A thermal switch for cryogenic temperatures as described in claim 1, wherein the at least one switching portion is formed in a plate or band shape, the at least one first switching coupling portion is formed in a plate or band shape extending in the first direction, the at least one second switching coupling portion is formed in a plate or band shape extending in the second direction, the second direction is along the first direction, and each of the first switching coupling portion and the second switching coupling portion is coupled to the at least one switching portion such that the thickness directions of the at least one first switching coupling portion and the at least one second switching coupling portion are along the thickness direction of the at least one switching portion.

3. A thermal switch for cryogenic use as claimed in claim 2, comprising a plurality of switching parts as the at least one switching part, a plurality of first switching coupling parts as the at least one first switching coupling part, and a plurality of second switching coupling parts as the at least one second switching coupling part, each of the plurality of first switching coupling parts and the plurality of second switching coupling parts being arranged at intervals in the thickness direction and being coupled to different switching parts at their surfaces, and the switching parts being configured to be coupled in parallel between the first base part and the second base part.

4. A thermal switch for cryogenic use as described in claim 3, wherein each of the plurality of first switching coupling parts is coupled to the first base from the first direction, and each of the outer edge coupling parts, which is a first switching coupling part located at the outer edge in the thickness direction among the plurality of first switching coupling parts, is coupled to the inside of the first base from the outer edge of the first base in the thickness direction.

5. A thermal switch for use at cryogenic temperatures as claimed in claim 2, wherein at least a portion of said regulating portion is coupled to each of said first base portion and said second base portion so as to extend along said first switching coupling portion, said at least one switching portion, and said second switching coupling portion.

6. A thermal switch for use at cryogenic temperatures according to claim 5, wherein each of the first base portion and the second base portion is provided with a planar coupled portion on which the restricting portion is superimposed.

7. A thermal switch for use at cryogenic temperatures as described in claim 1, wherein the first base comprises a first restrictive fastening portion, the second base comprises a second restrictive fastening portion, and the restrictive portion is coupled to the first base and the second base by being fastened to the first restrictive fastening portion and the second restrictive fastening portion using fastening members, respectively.

8. A thermal switch for use at cryogenic temperatures according to claim 1, wherein the regulating portion is formed mainly from a non-metallic material.

9. A thermal switch for use at cryogenic temperatures as claimed in claim 1, wherein the contact area of ​​the first base that comes into contact with the first member and the contact area of ​​the second base that comes into contact with the second member are each covered with a gold (Au) film, the film having a thickness of 4 μm or more.

10. The cryogenic thermal switch of claim 1, wherein said at least one switching portion is made of zinc (Zn).

11. In the thermal switch for cryogenic use according to claim 1, the composite value of the thermal conductivity of the entire at least one switching section is 1.7×10 in a normal conductive state at 1 mK. -４ At 30 mK, the superconducting state is 7×10 -８ each of the at least one switching portion is coupled to the first switching coupling portion and the second switching coupling portion such that a thermal resistance of the at least one switching portion is equal to or less than W / K.

12. A thermal switch for cryogenic use, configured to be interposed between a first member and a second member and thereby capable of controlling heat exchange between the first member and the second member, comprising a switching unit, a first thermal conductive unit, and a second thermal conductive unit, the switching unit configured to switch between a superconducting state and a normal conductive state by application of a magnetic field equal to or greater than a critical magnetic field, wherein the switching unit in the normal conductive state has a higher thermal conductivity than the switching unit in the superconducting state, the first thermal conductive unit comprising a first base, a first member coupling unit, and a first switching coupling unit, the first member coupling unit being coupled to the first member, the first switching coupling unit extending from the first base in a first direction and coupled from the first base to a first end of the switching unit, and having a lower rigidity than the first base, a second heat conducting portion including a second base, a second member connecting portion, and a second switching connecting portion, the second member connecting portion being configured to be connectable to the second member, the second switching connecting portion extending from the second base along a second direction and connected to a second end of the switching portion and having lower rigidity than the second base, the first base including a first regulating fastening portion fastened to a first end of a regulating portion different from the first member and the second member, and the second base including a second regulating fastening portion fastened to a second end of the regulating portion.

13. A method for manufacturing a heat switch for use at cryogenic temperatures according to any one of claims 1 to 12, comprising the following steps: in the bonding step, the regulating portion is bonded to a precursor, which is a heat switch for use at cryogenic temperatures prior to bonding of the regulating portion, which has been prepared in advance.

14. A continuous nuclear adiabatic demagnetization refrigeration unit comprising: a cryogenic heat switch according to any one of claims 1 to 12; a nuclear adiabatic demagnetization refrigeration device; a second member; and a magnetic field application unit, wherein the nuclear adiabatic demagnetization refrigeration device is coupled to the first heat conductive member; the second member is coupled to the second heat conductive member; and the magnetic field application unit is configured to control the magnetic field applied to the switching unit based on a temperature of the nuclear adiabatic demagnetization refrigeration device and a temperature of the second member.

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