阅读说明：本技术 具有依赖于温度的热分流器的低温冷却系统 (Cryogenic cooling system with temperature dependent thermal shunt ) 是由 T·E·阿姆托尔 M·富德勒 G·B·J·米尔德 C·洛斯勒 P·福斯曼 P·A·门特乌尔 于 2016-11-24 设计创作，主要内容包括：一种低温冷却系统(10)包括低温恒温器(12)、两级低温冷头(24)和至少一个热连接构件(136；236；336；436),该至少一个热连接构件被构造成提供热传递路径(138；238；338；438)的从两级低温冷头(24)的第二级构件(30)到第一级构件(26)的至少一部分。该热传递路径(138；238；338；438)被布置在冷头(24)的外部。热传递路径(138；238；338；438)的所提供的至少一部分在第二低温温度下的热阻大于热传递路径(138；238；338；438)的所提供的至少一部分在第一低温温度下的热阻。(A cryogenic cooling system (10) includes a cryostat (12), a two-stage coldhead (24), and at least one thermal connection member (136; 236; 336; 436) configured to provide at least a portion of a heat transfer path (138; 238; 338; 438) from a second stage member (30) to a first stage member (26) of the two-stage coldhead (24). The heat transfer path (138; 238; 338; 438) is arranged outside the cold head (24). At least a portion of the heat transfer path (138; 238; 338; 438) is provided with a thermal resistance at the second cryogenic temperature that is greater than a thermal resistance of at least a portion of the heat transfer path (138; 238; 338; 438) at the first cryogenic temperature.)

1. A cryogenic cooling system (10), comprising:

-a cryostat (12) having an outer housing (14) and at least one heat shield (16) disposed within the outer housing (14), the at least one heat shield (16) defining an interior region (18);

wherein a heat insulation area (20) is defined by the at least one heat shield (16) and the outer cover (14) and between the at least one heat shield (16) and the outer cover (14),

-a cryogenic cold head (24) having

A first stage component (26) at least partially disposed in the insulation region (20), wherein the first stage component (26) is configured to operate at a first cryogenic temperature in a quiescent state and includes a thermally conductive link member (28) thermally connected to the at least one heat shield (16),

a second stage component (30) disposed at least partially in the interior region (18), wherein the second stage component (30) is configured to operate at a second cryogenic temperature that is lower than the first cryogenic temperature in a quiescent state, an

At least one thermal connection member (136; 236; 336; 436) configured to provide at least a portion of a heat transfer path (138; 238; 338; 438) from the second stage member (30) to the first stage member (26) in at least one operating state of the cryogenic cooling system (10), wherein the heat transfer path (138; 238; 338; 438) is arranged outside of the cryogenic cold head (24) and a thermal resistance of the provided at least a portion of the heat transfer path (138; 238; 338; 438) at the second cryogenic temperature is greater than a thermal resistance of the provided at least a portion of the heat transfer path (138; 238; 338; 438) at the first cryogenic temperature;

wherein the at least one thermal connection member (236; 336; 436) comprises a bimetal member (252; 352; 452) having a first end (254; 354; 454) and a second end (256; 356; 456), wherein,

the first end (254; 354; 454) is fixedly attached and thermally connected to the second stage member (30),

the second end (256; 356; 456) is configured to apply a mechanical surface pressure greater than zero toward a thermally conductive member (46) thermally connected to the first stage member (26) or the first stage member (26) if the temperature of the second stage member (30) is greater than the first cryogenic temperature, and

the second end (256; 356; 456) is configured to apply zero mechanical surface pressure towards a thermally conductive member (46) thermally connected to the first stage member (26) or the first stage member (26) if the temperature of the second stage member (30) is less than the first cryogenic temperature.

2. The cryogenic cooling system (10) recited in claim 1, wherein the cryogenic cooling system includes a plurality of thermal connection members (236; 336; 436), each thermal connection member (236; 336; 436) including a bimetal member (252; 352; 452) having a first end (254; 354; 454) and a second end (256; 356; 456), wherein the cryogenic cooling system includes a plurality of thermal connection members (236; 336; 436), wherein

The first end (254; 354; 454) is fixedly attached and thermally connected to the second stage member (30);

the second end (256; 356; 456) being configured to apply a mechanical surface pressure greater than zero toward a thermally conductive member (46) thermally connected to the first stage member (26) or the first stage member (26) if the temperature of the second stage member (30) is greater than the first cryogenic temperature,

the second end (256; 356; 456) is configured to apply zero mechanical surface pressure towards the thermally conductive member (46) or the first stage member (26) that is thermally connected to the first stage member (26) if the temperature of the second stage member (30) is below the first cryogenic temperature.

3. The cryogenic cooling system (10) recited in claim 2, wherein at least one of the plurality of thermal connection members (336) or the at least one thermal connection member (336) further comprises a plurality of carbon fibers (340), each carbon fiber (340) having a first end (342) and a second end (344), wherein

The first end (342) of the carbon fiber (340) of the plurality of carbon fibers (340) is permanently thermally connected to the second stage member (30), and

the second end (344) of the carbon fiber (340) of the plurality of carbon fibers (340) is disposed between the second end (356) of the bimetal member (352) and one of the first stage member (26) and the thermally conductive member (46) that is thermally connected to the first stage member (26).

4. The cryogenic cooling system (10) according to claim 2 or 3, wherein at least one of the plurality of thermal connection members (236; 336; 436) or the at least one thermal connection member (236; 336; 436) further comprises a plurality of carbon fibers (340), each carbon fiber (340) having a first end (342) and a second end (344), wherein

The first end (342) of the carbon fiber (340) of the plurality of carbon fibers (340) is permanently thermally connected to the second stage member (30), and

the second end (344) of the carbon fiber (340) of the plurality of carbon fibers (340) is attached to the second end (356) of the bimetal member (352).

5. The cryogenic cooling system (10) according to claim 2, wherein at least one of the plurality of thermal connection members (236; 336; 436) or the at least one thermal connection member (236; 336; 436) comprises a second bimetal member (452') having a first end (454') and a second end (456'), and the two bimetal members (452, 452') are arranged opposite to each other, wherein

The second bimetal member (452') is thermally connected to the first stage member (26) with the first end (454'),

the second ends (456, 456') of the two bimetal members (452, 452') are configured to cooperate with each other and exert a mechanical surface pressure greater than zero towards each other if the temperature of the second stage member (30) is higher than the first cryogenic temperature, and

the second ends (456, 456') of the two bimetal members (456, 456') are configured to apply zero mechanical surface pressure towards each other if the temperature of the second stage member (30) is lower than the first cryogenic temperature.

6. The cryogenic cooling system (10) according to any of claims 1 to 3, wherein a total thickness of the at least one bimetal member (252, 352, 452) is selected to be in a range between 0.1mm and 2 mm.

7. The cryogenic cooling system (10) according to any of claims 1 to 3, wherein the provided at least a portion of the heat transfer path (138; 238; 338; 438) has a thermal resistance at the second cryogenic temperature that is at least 10 times greater than a thermal resistance at the first cryogenic temperature of the provided at least a portion of the heat transfer path (138; 238; 338; 438).

8. The cryogenic cooling system (10) according to any one of claims 1 to 3, wherein the cryogenic cooling system further comprises superconducting magnet coils (22) configured to provide a quasi-static magnetic field and adapted for use in a magnetic resonance examination apparatus, wherein the superconducting magnet coils (22) are arranged within the inner region (18) and are thermally connected to the second stage member (30), and the second cryogenic temperature is below a critical temperature of the superconducting magnet coils (22).

Technical Field

The present invention relates to a cryogenic cooling system having a two-stage cold head and in particular comprises a superconducting magnetic coil for use in a magnetic resonance examination apparatus.

Background

Two-stage cryocoolers are frequently used as cooling sources for cooling devices to cryogenic temperatures. Representative examples of commercially available two-stage cryocoolers using helium as the working fluid are the Gifford-mcmahon (gm) refrigeration system and the Pulse Tube (PT) refrigeration system. They allow cooling of samples, equipment and other equipment without the inconvenience and expense of using liquid helium. In particular, such devices may comprise superconducting materials which exhibit their superconducting properties when cooled to a particular temperature below what is known as the critical temperature. A representative example of such an apparatus is a superconducting magnet system intended to generate a static magnetic field when operating in a persistent mode, as is well known in the art.

The first stage of a two-stage cryocooler is typically maintained at a temperature between 50K and 100K and may be thermally connected to a thermal radiation shield surrounding an interior region configured to receive a device to be cooled to a lower temperature (e.g., to 4K). The apparatus is thermally coupled to the second stage of the two-stage cryocooler.

Typically, the cooling capacity of the first stage is one or two orders of magnitude greater than the cooling capacity of the second stage. Thus, when cooling is initiated from room temperature, the time required to cool the interior region to the rated temperature of the second stage is much longer than the time required to cool the interior region to the rated temperature of the first stage.

Patent document US 5,111,667a describes a two-stage cryopump having a refrigerator including a first stage, a second stage that is cooler than the first stage, and a condensing member having a condensing surface. The first coupling is configured to connect the condensing member to the second stage in a thermally conductive manner. An adsorption member including an adsorption surface is spaced apart from the condensing member. The second coupling is configured to connect the adsorbent member to the second stage in a thermally conductive manner. A heater for heating the adsorption member during a period of regenerating the adsorption member is also provided. The second coupling is designed such that it sufficiently insulates the adsorption member from the second stage and from the condensation member at least during a heating cycle of the adsorption member for preventing heating of the condensation member by the heater.

Disclosure of Invention

It is therefore an object of the present invention to provide a cryogenic cooling system having efficient operation and reduced time for cooling the ambient temperature to cryogenic temperatures.

In one aspect of the invention, this object is achieved by a cryogenic cooling system comprising a cryostat having an outer housing and at least one thermal shield disposed within the outer housing. The at least one heat shield defines an interior region, and a heat insulation region is defined by and between the at least one heat shield and the outer cover.

The cryogenic cooling system further includes a cryocooler having:

a first stage member at least partially disposed in the insulating region, wherein the first stage member is configured to operate at a first cryogenic temperature in a quiescent state and comprises a thermally conductive link member thermally connected to at least one heat shield,

at least a second stage component disposed at least partially in the interior region, wherein the second stage component is configured to operate at a second cryogenic temperature that is lower than the first cryogenic temperature in a quiescent state, and

at least one thermal connection component configured to provide at least a portion of a heat transfer path from the second stage component to the first stage component in at least one operating state of the cryogenic cooling system, wherein the heat transfer path is disposed outside of the cold head and the provided thermal resistance of the at least a portion of the heat transfer path at the second cryogenic temperature is greater than the provided thermal resistance of the at least a portion of the heat transfer path at the first cryogenic temperature.

The term "thermally connected to the first (second) stage member" as used in this application shall be understood in particular as being thermally connected to at least one of the heat conducting members which in turn is thermally connected to the first (second) stage member and directly connected to the first (second) stage member.

The term "thermal connection" as used in this application shall be understood in particular as a mechanical connection capable of heat transfer by conduction of heat.

The term "heat transfer path" as used in this application shall be understood in particular as a path along which heat is transferred via heat conduction, and shall expressly exclude heat transfer paths by radiation.

The term "thermal resistance" as used in this application is to be understood in particular as the ratio of the temperature difference between two locations along the heat transfer path to the thermal power (thermal energy per time) being transferred between the two locations.

The term "cryogenic temperature" as used in this application shall be understood specifically as a temperature below 100K.

The operation of a cryostat system is typically based on a closed loop expansion cycle using helium as the working fluid. A complete cryostat system comprises two main components: a compressor unit that compresses a working fluid and removes heat from the system, and a cold head configured to obtain the working fluid through an expansion cycle to cool it to a cryogenic temperature. The term "cold head" as used in this application should be understood in this sense in particular.

It is noted herein that the terms "first," "second," and the like are used for distinguishing purposes only and are not intended to be used in any way to indicate a sequence or priority.

Since the provided at least part of the heat transfer path has a lower thermal resistance at a first cryogenic temperature than at a second cryogenic temperature, the second stage can be cooled faster and in a more efficient manner via the provided at least one thermal connection means, in which case the thermal resistance of the provided at least part of the heat transfer path can be designed to be large enough to prevent an unacceptably high thermal load on the second stage means. In this manner, the higher cooling efficiency of the first stage component of the cryogenic cooling head may be used to remove a significant amount of heat from the second stage component at the beginning of the cooling process. The time for cooling the interior region from ambient to cryogenic temperatures can be advantageously reduced.

In a preferred embodiment, the thermal resistance of the provided at least a portion of the heat transfer path at the second cryogenic temperature is at least 10 times greater than the thermal resistance of the provided at least a portion of the heat transfer path at the first cryogenic temperature.

More preferably, the thermal resistance at the second cryogenic temperature is at least 100 times greater than the thermal resistance at the first cryogenic temperature, and most preferably at least 1000 times greater.

In this way, a substantial reduction in the time for cooling the inner region from ambient temperature to cryogenic temperature can be achieved.

In another preferred embodiment, the at least one thermal connecting member comprises a plurality of carbon fibers, each carbon fiber having two ends, and wherein one end of a carbon fiber of the plurality of carbon fibers is thermally connected to the first stage member and the other end of a carbon fiber of the plurality of carbon fibers is thermally connected to the second stage member.

The term "plurality" as used in this application should be understood to mean at least two in number.

At temperatures above 50K, carbon fibers may exhibit exceptionally high thermal conductivity. The thermal conductivity can be as high as 1000W/(m × K) at room temperature, much higher than that of copper. Thus, a low thermal resistance between the two first stage components and the second stage component can be achieved, and the more powerful and more efficient first stage component can directly cool the second stage component and its thermal load, thereby rapidly reducing its temperature.

The thermal conductivity of carbon fibers decreases rapidly at lower temperatures compared to other good thermally conductive materials. The thermal conductivity of graphite, which is comparable to the thermal conductivity of carbon fibers in the longitudinal direction, is shown in the following fig. 1 as a dashed line (a low temperature thermal conductivity database from Woodcraft et al, CP1185, low temperature detector LTD 13, conference on international conference 13, AIP 2009). In the relevant temperature range for cryocoolers (from about 300K to 4K), the thermal conductivity is reduced by about four orders of magnitude.

When the instantaneous temperature of at least one thermally connected component is being reduced during cooling from ambient temperature, its thermal conductivity therefore drops sharply until the first and second stage components are nearly thermally disconnected. At a temperature below the first cryogenic temperature, the second stage component may then further cool the interior region to a temperature below the first cryogenic temperature.

Preferably, the carbon fibers of the plurality of carbon fibers are not mechanically connected to each other, for example by using a resin, and are also not encapsulated, so that no additional conductive heat transfer through other materials takes place. Thereby, a beneficial large difference in thermal resistance of the provided at least part of the heat transfer path at the first cryogenic temperature and at the second cryogenic temperature may be achieved.

Pure carbon fibers are commercially available, for example as yarns typically comprising between 1000 ("1K", 67 tex 67g/1,000m) and 48000 ("48K", 3200 tex) filaments/yarns and as weave.

In one embodiment, the plurality of carbon fibers are thermally connected to at least one of the first stage component and the second stage component by at least one force-locking connection. In this way, a low thermal resistance of the interface between the plurality of carbon fibers and the respective stage component may be achieved.

In some embodiments, this may be advantageously achieved when the plurality of carbon fibers are thermally connected to at least one of the first stage member and the second stage member by at least one adhesive joint.

In another preferred embodiment of the cryogenic cooling system, the at least one thermal connection member comprises a bimetallic member. The bimetal member has a first end and a second end. The first end is fixedly attached and thermally connected to the second stage member. The second end is configured to apply a mechanical surface pressure greater than zero toward at least one of the first stage component and a thermally conductive member thermally coupled to the first stage component if a temperature of the second stage component is greater than the first cryogenic temperature. The second end is configured to apply zero mechanical surface pressure toward both the thermally conductive member connected to the first stage component and the first stage component if the temperature of the second stage component is below the first cryogenic temperature.

In this way, the thermal resistance of the provided at least a portion of the heat transfer path at the second cryogenic temperature is infinite, and the first stage member and the second stage member may be thermally disconnected at the second cryogenic temperature, while at the first cryogenic temperature, at least a portion of the heat transfer path from the second stage member to the first stage member may be provided with a low thermal resistance. In a temperature region between the first cryogenic temperature and the second cryogenic temperature, the thermal resistance of the interface of the second end of the bimetal member and the first stage member beneficially increases from a particular value at the first cryogenic temperature to an infinite value at the second cryogenic temperature due to the variable surface pressure exerted by the bimetal member on a contact location with at least one of the thermally conductive member and the first stage member thermally connected to the first stage member.

If the cryogenic cooling system includes a plurality of thermally coupled components, a multiplicative effect may be achieved with respect to the time required to cool the interior region from ambient to cryogenic temperatures. Each thermal connection member comprises a bimetal member. Each bimetal member has a first end and a second end. The first end is fixedly attached to the second stage member,

-the second end is configured to apply a mechanical surface pressure greater than zero towards at least one of the first stage component and a thermally conductive member thermally connected to the first stage component if the temperature of the second stage component is higher than the first cryogenic temperature, an

The second end is configured to apply zero mechanical surface pressure towards both the thermally conductive member thermally connected to the first stage member and the first stage member if the temperature of the second stage member is below the first cryogenic temperature.

In one embodiment, at least one of the at least one thermal connection member or the plurality of thermal connection members other than the bimetal member further comprises a plurality of carbon fibers. Each carbon fiber has a first end and a second end. A first end of a carbon fiber of the plurality of carbon fibers is permanently thermally connected to the second stage member. The second end of the carbon fiber of the plurality of carbon fibers is disposed between the thermally conductive member thermally connected to the first stage member and the second end of the bimetal member.

In this way, each bimetal member may advantageously exert a temperature-dependent surface pressure on the plurality of carbon fibres at a contact location of one of the plurality of carbon fibres with the plurality of carbon fibres at the thermally conductive member thermally connected to the first stage member and the first stage member. Furthermore, tolerance requirements for assembling the at least one thermal connection member or at least one of the plurality of thermal connection members may be reduced.

It is important that the plurality of carbon fibers be permanently thermally connected to the second stage member while having a bimetallic pressure dependent attachment at the first stage member. When the second stage component is at the second cryogenic temperature, the thermal resistance of the interface between the plurality of carbon fibers and the first stage component is greater than the thermal resistance in the warm state, i.e., at a temperature greater than the first cryogenic temperature. Thus, the bimetal helps to maintain the plurality of carbon fibres at a temperature close to the second cryogenic temperature, so that they are not thermally conductive over virtually their entire length.

In one embodiment, at least one of the at least one thermal connection member or the plurality of thermal connection members other than the bimetal member further comprises a plurality of carbon fibers. Each carbon fiber has a first end and a second end. A first end of a carbon fiber of the plurality of carbon fibers is permanently thermally connected to the second stage member. A second end of a carbon fiber of the plurality of carbon fibers is attached to the second end of the bimetal member.

In this manner, the plurality of carbon fibers are attached to the bimetal member at a second end thereof, which is disposed proximate to the first stage member. The thermal conductivity across the plurality of carbon fibers (i.e., over the distance from the first stage member to the bimetal member) is relatively low, resulting in a low thermal load on the second stage member at the second cryogenic temperature.

Preferably, the second end of the carbon fiber of the plurality of carbon fibers is attached to the second end of the bimetal member by using an adhesive.

In another preferred embodiment, the at least one thermal connection member or at least one of the plurality of thermal connection members comprises two bimetal members, each bimetal member having a first end and a second end arranged opposite to each other.

One of the two bimetal members is thermally connected to the first stage member with a first end. The other of the two bimetal members is thermally connected to the second stage member with the first end. The second ends of the two bimetal members are configured to mate with each other and apply a mechanical surface pressure greater than zero toward each other if the temperature of the second stage member is greater than the first cryogenic temperature. The second ends of the two bimetal members are configured to apply zero mechanical surface pressure towards each other if the temperature of the second stage member is below the first cryogenic temperature.

Thereby, if the temperature of the second stage member is higher than the first cryogenic temperature, a beneficially large contact area between the second ends of the two bimetal members may be achieved and the requirements regarding assembly tolerances for the at least one thermal connection member or at least one of the plurality of thermal connection members may be reduced.

Preferably, the total thickness of the at least one bimetal member is selected to be in the range between 0.1mm and 2 mm. In this way, a sufficiently low thermal resistance of the provided at least part of the heat transfer path may be provided at the first cryogenic temperature, so as to produce a substantial effect of a reduced time for cooling the interior region from ambient temperature to cryogenic temperature. Further, a sufficient amount of bending of the bimetal member can be achieved to form an infinite thermal resistance for the interface of the second end of the bimetal member and the first stage member at the second cryogenic temperature, and a heat transfer path from the second stage member to the first stage member can be achieved with a low thermal resistance at the first cryogenic temperature for a wide variety of widely used cryostat sizes.

Furthermore, the thermo-mechanical shear forces which are present between the two metals of the bimetal member and which are required for bending the bimetal member are kept within reasonable limits, so that material fatigue or material damage can be avoided.

In another aspect of the invention, the cryogenic cooling system further comprises superconducting magnetic coils configured to provide a quasi-static magnetic field and adapted for use in a magnetic resonance examination apparatus. The superconducting magnetic coil is disposed within the interior region and thermally connected to the second stage member, and the second cryogenic temperature is below a critical temperature of the superconducting magnetic coil. Thereby, a superconducting magnetic coil for magnetic resonance examination may be provided, which may be cooled from ambient temperature to the second cryogenic temperature in a fast and efficient manner.

Drawings

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. Such embodiments do not necessarily represent the full scope of the invention, however, and reference is made to the claims and herein for interpreting the scope of the invention.

In the drawings:

fig. 1 shows the thermal conductivity of graphite in the cryogenic temperature range compared to other selected materials,

figure 2 shows a schematic view of a cryogenic cooling system according to the invention,

FIG. 3 is a schematic view of a two-stage coldhead including a thermal coupling member of the cryogenic cooling system according to FIG. 1,

figure 4 is a schematic view of an alternative embodiment of a thermal connection member,

FIG. 5 is a schematic view of another alternative embodiment of a thermal connection member, an

FIG. 6 is a schematic view of yet another alternative embodiment of a thermal connection member.

Detailed Description

FIG. 1 is a graphical representation of thermal conductivity as a function of temperature.

FIG. 2 shows a schematic diagram of a cryogenic cooling system 10 according to the present invention. The cryogenic cooling system 10 includes a cryostat 12, the cryostat 12 having an outer housing 14 and a thermal shield 16 disposed within the outer housing 14. The thermal shield 16 defines an interior region 18 within which interior region 18 is disposed a superconducting magnetic coil 22 of the cryogenic cooling system 10. The superconducting magnetic coil 22 is configured to provide a quasi-static magnetic field having a magnetic field strength of several tesla and is suitable for use in a magnetic resonance examination apparatus. Superconducting magnetic coil 22 is designed for nominal operation at a temperature of 4K, which is well below the 10K critical temperature of the niobium-titanium (NbTi) superconducting wire forming the windings of superconducting magnetic coil 22.

The insulated region 20 of the cryostat 12 is defined by the heat shield 16 and the outer cover 14 and is defined between the heat shield 16 and the outer cover 14. The insulated area 16 may include an insulating material, such as a widely used multi-layer insulation Material (MLI).

The cryogenic cooling system 10 also includes a two-stage coldhead 24. The two-stage coldhead 24 has a first stage component 26 disposed in the insulated region 20. The first stage member 26 is configured to operate at a first cryogenic temperature of 70K in a quiescent state and includes a thermally conductive link member 28 formed from a connecting metal flange that is thermally connected to the first stage member 26 and the thermal shield 16. In addition, the two-stage coldhead 24 has a second stage component 30 disposed in the interior region 18. The second stage member 30 is configured to operate at a second cryogenic temperature of 4K in the quiescent state, which is lower than the first cryogenic temperature and corresponds to a temperature for rated operation of the superconducting magnetic coil 22. The superconducting magnetic coil 22 is thermally connected to the second stage member 30 by another heat conductive member formed by a metal flange 32 made of copper.

The two-stage cryocooler 24 may be connected to an electric compressor unit 34, the electric compressor unit 34 being configured to provide a compressed working fluid formed from gaseous helium to the two-stage cryocooler 24 via a gas line. This part of the technology is well known in the art and therefore need not be described in more detail herein. Two-stage cryogenic cold head 24 is capable of cooling superconducting magnetic coil 22 from an ambient temperature of about 300K to a second cryogenic temperature of 4K.

Fig. 3 is a schematic view of the two-stage coldhead 24 of the cryogenic cooling system 10 according to fig. 2, and illustrates a thermal connection member 136, the thermal connection member 136 being configured to provide a heat transfer path 138 in an operational state of the cryogenic cooling system 100 that cools the superconducting magnetic coil 22 from an ambient temperature of about 300K to a second cryogenic temperature of 4K, the heat transfer path 138 being disposed external to the two-stage coldhead 24 from the second stage member 30 to the first stage member 26.

The thermal connection member 136 includes a plurality of carbon fibers 140 formed into a 12K yarn. Each carbon fibre has two ends 142, 144, and one end 142 of a carbon fibre 140 of the plurality of carbon fibres 140 is thermally connected to the first stage member 26 via the heat-conducting link member 28 by a force-locking connection formed as a threaded connection, the end 142 of the carbon fibre 140 being compressed between the metal plate 58 and the connecting metal flange (left hand bottom side of fig. 3) by the force-locking connection. The other end 144 of the carbon fiber 140 of the plurality of carbon fibers 140 is thermally connected to the second stage member 30 by an adhesive joint (right hand bottom side of fig. 3) via the connecting copper flange 32. To this end, the connecting copper flange 32 includes a tapered cutout 148 filled with a thermally well-conducting epoxy 150, the ends 144 of the plurality of carbon fibers 140 having been placed into the tapered cutout 148 during curing of the epoxy 150. The conical shape of the cut-outs 148 has an increased surface area, which results in a lower thermal contact resistance between the ends 142, 144 of the carbon fibers 140 and the connecting copper flange 32.

Although in this particular embodiment the ends 142, 144 of the plurality of carbon fibers 140 are thermally connected to the first stage member 26 by a force-lock connection and the other ends 144 of the plurality of carbon fibers 140 are thermally connected to the second stage member 30 by an adhesive joint, it is also contemplated to provide an adhesive joint for thermally connecting the plurality of carbon fibers to the first stage member and to provide a force-lock connection for thermally connecting the ends of the plurality of carbon fibers to the second stage member or to provide a force-lock connection at both ends of the plurality of carbon fibers or to provide an adhesive joint at both ends of the plurality of carbon fibers.

Due to the thermally conductive properties of the plurality of carbon fibers 140, the thermal resistance of the provided heat transfer path 138 at the second cryogenic temperature is greater than the thermal resistance of the provided heat transfer path 138 at the first cryogenic temperature.

By the thermally conductive properties of the carbon fiber provided in fig. 1 ("graphite AXM-5Q") at a first cryogenic temperature of 70K and at a second cryogenic temperature of 4K, it can be estimated that the thermal resistance of the provided heat transfer path 138 at the second cryogenic temperature is more than 2,000 times greater than the thermal resistance of the provided heat transfer path 138 at the first cryogenic temperature. That is, at a first cryogenic temperature, an effective heat transfer path 138 is provided from the second stage component 30 to the first stage component 26, while at a second cryogenic temperature, the first and second stage components 26, 30 are thermally disconnected from a practical perspective.

In the following, several alternative embodiments of the thermal connection member according to the invention are disclosed. Various alternative embodiments are described with reference to particular figures and are identified by the prefix number for the particular alternative embodiment starting with "1". Features that are functionally identical or substantially identical in all embodiments are identified by reference numerals that consist of a prefix number to the alternative embodiment to which it relates, followed by the reference numeral for the feature. Reference should be made to the description of the previous embodiment if no features of the alternative embodiment are described in the corresponding graphical description.

Fig. 4 is a schematic view of an alternative embodiment of a thermal connection member 236. The thermal connection member 236 includes a bimetal member 252 formed as a rectangular sheet having a first end 254 and a second end 256. The bimetal member 252 has a total thickness of 0.5 mm. In this particular embodiment, the bimetal member 252 comprises a sheet side made of copper and an opposite sheet side made of stainless steel. However, other combinations of metals that appear suitable to those skilled in the art are also contemplated.

The first end 254 of the bimetal member 252 is fixedly attached and thermally connected to the second stage member 30 via the connecting copper flange 32. The heat conductive member 46, which is formed as a metal plate made of copper, is fixedly attached to and thermally connected to the first stage member 26, and protrudes from the heat conductive link member 28 toward the second end 256 of the bimetal member 252. The top of fig. 4 shows the thermal connection member 236 at a temperature higher than the first cryogenic temperature. In this case, the copper side of the second end 256 of the bimetal member 252 is in mechanical contact with the side of the metal plate and applies a temperature dependent surface pressure greater than zero towards the side of the heat conducting member 46. Thus, a heat transfer path 238 with low thermal resistance is provided from the second stage component 30 to the first stage component 26.

During the cooling process from the ambient temperature to the second cryogenic temperature, when the instantaneous temperature of the second stage member 30 becomes equal to the first cryogenic temperature, the second end 256 of the bimetal member 252 applies zero mechanical surface pressure towards the thermally conductive member 46. When the instantaneous temperature of the second stage member 30 is below the first cryogenic temperature, there is a gap between the copper side of the second end 256 of the bimetal member 252 and the thermally conductive member 46 and the thermal resistance of the provided heat transfer path 238 becomes infinite. This is shown in the bottom of fig. 4.

Without further explanation, those skilled in the art will readily appreciate that cryogenic cooling system 10 may include a plurality of thermal connection members 236, wherein some of thermal connection members 236 may include such bimetallic component 252 as previously described. In this manner, a plurality of heat transfer paths 238 arranged in parallel may be provided from the second stage component 30 to the first stage component 26 when the instantaneous temperature of the second stage component 30 is higher than the first cryogenic temperature. At transient temperatures of the second stage component 30 below the first cryogenic temperature, the thermal resistance of the parallel heat transfer paths 238 provided will be infinite.

Fig. 5 is a schematic view of another alternative embodiment of a thermal connection member 336. Alternative embodiments of thermal connection member 336 will be exemplarily described with respect to a single sample. However, as previously described, the cryogenic cooling system 10 may include one thermal connection member 336 or a plurality of thermal connection members 336.

In addition to the bimetal member 352 having a first end 354 and a second end 356, the thermal connection member 336 includes a plurality of carbon fibers 340 formed as 24K yarns. The first end 354 of the bimetal member 352 is fixedly attached and thermally connected to the connecting metal flange 32 made of copper, which connecting metal flange 32 is in turn thermally connected to the second stage member 30. The second end 356 of the bent bimetal member 352 is directed towards the thermally conductive link member 28 formed as a metal flange that is thermally connected to the first stage member 26. The carbon fiber 340 has a first end 342 and a second end 344. The first end 342 of a carbon fiber 340 of the plurality of carbon fibers 340 is permanently thermally connected to the connecting metal flange 32, which connecting metal flange 32 is in turn thermally connected to the second stage member 30. For example, the thermal connection may be formed by a clamping joint (not shown). The second end 344 of the carbon fiber 340 of the plurality of carbon fibers 340 is adhesively attached to the second end 356 of the bimetal member 352 and is disposed between the second end 356 of the bimetal member 352 and the thermally conductive link member 28.

Fig. 5 shows the situation where the instantaneous temperature of the second stage component 30 has dropped to a first cryogenic temperature below 70K during the cooling process from ambient temperature (300K) to a second cryogenic temperature of 4K. The bi-metallic member 352 has been bent far enough to move the plurality of carbon fibers 340 away from the thermally conductive link member 28 such that the thermally conductive transfer path 338 between the first stage member 26 and the second stage member 30₁、338₂Is infinite. For transient temperatures of the second stage member 330 between ambient and the first cryogenic temperature, the bimetal member 352 is more straightened and the second end 356 of the bimetal member 352 applies a temperature dependent mechanical surface pressure greater than zero towards the plurality of carbon fibers 340 and the thermally conductive link member 28 to provide a heat transfer path 338 having low thermal resistance from the second stage member 30 to the first stage member 26.

Fig. 6 is a schematic view of another alternative embodiment of a single thermal connection member 436, the single thermal connection member 436 including two bimetal members 452, 452 'formed as rectangular sheets, each bimetal member 452, 452' including a sheet side made of copper and an opposite sheet side made of stainless steel. Again, the cryogenic cooling system 10 may include one thermal connection member 436 or a plurality of thermal connection members 436.

The two bimetal members 452, 452' are arranged opposite to each other. The first end 454 of the first bimetal member 452 is fixedly attached and thermally connected to the copper flange 32, which copper flange 32 is in turn thermally connected to the second stage member 30. A first end 454 'of the second bimetal member 452' is fixedly attached and thermally connected to the thermally conductive link member 28 formed as a metal flange which is in turn thermally connected to the first stage member 26.

If the instantaneous temperature of the second stage component 30 is higher than the first cryogenic temperature, the second ends 456, 456 'of the two bimetal components 452, 452' are configured to mate with their copper sides and exert a mechanical surface pressure greater than zero towards each other. A heat transfer path 438 having a low thermal resistance is provided from the second stage component 30 to the first stage component 26. This situation is shown in fig. 6. If the temperature of the second stage member 30 is lower than the first cryogenic temperature, the second ends 456, 456' of the two bimetal members 452, 452' are configured to exert zero mechanical surface pressure towards each other by further bending of the bimetal members 452, 452 '.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the term "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims shall not be construed as limiting the scope.