CRYOGENIC COOLING SYSTEM

Description

TECHNICAL FIELD

The present disclosure relates to cryogenic cooling, and more particularly to a cryogenic cooling system.

BACKGROUND

In cryogenic cooling systems, a plurality of cryocoolers, such as pulse tubes, can be used to increase the cooling power and thus reduce the cooling time of the system. For example, multiple pulse tubes can be used to cool the 4-kelvin stage and higher temperature stages of a cryogenic cooling system. Other means, such as a dilution refrigerator, can be used to cool lower temperature stages of the system to, for example, millikelvin temperatures. If all cryocoolers are used to cool the 4-kelvin stage and other parts of the system as well as the helium of the dilution refrigerator, the temperature of the helium can increase, which can affect the performance of the dilution refrigerator and may make it unstable. On the other hand, if some of the cryocoolers are dedicated to cooling the helium, cooling of the cryogenic cooling system can take longer.

SUMMARY

This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

It is an objective to provide a cryogenic cooling system. The foregoing and other objectives are achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

According to a first aspect, a cryogenic cooling system comprises: a vacuum enclosure and a working region within the vacuum enclosure, a first cryocooler having a first cooling stage, the first cooling stage being located in the vacuum enclosure, a first conduit for passing a stream of fluid cooling medium towards the working region, at least one thermal coupling of the first conduit and the first cooling stage for cooling the fluid cooling medium on its way towards the working region, a second cryocooler having a second cooling stage, the second cooling stage being located in the vacuum enclosure, a first cold stage located in the vacuum enclosure, at least one thermal coupling of the second cooling stage and the first cold stage for cooling the first cold stage, and a first thermal coupling of the first cooling stage and the first cold stage, provided by a coupling element, for cooling the first cold stage, wherein the coupling element comprises a controlled thermal coupling for reducing the first thermal coupling when a relevant temperature of the cryogenic cooling system is below a threshold temperature.

In an implementation form of the first aspect, the working region is arranged to receive an object to be cooled.

In another implementation form of the first aspect, the first cold stage is thermally coupled to a heat radiation shield surrounding the working region or is a part of a heat radiation shield surrounding the working region.

In another implementation form of the first aspect, the first cold stage is a 4K stage of the cryogenic cooling system.

In another implementation form of the first aspect, the first cryocooler comprises a first pulse tube, the second cryocooler comprises a second pulse tube, the first cooling stage is a lower temperature stage of the first pulse tube, and the second cooling stage is a lower temperature stage of the second pulse tube.

In another implementation form of the first aspect, the coupling element comprises a thermal conductor and the controlled thermal coupling comprises an actuator and a controller for controlling the actuator, the thermal conductor being arranged to be moved between a contact state and a non-contact state by the actuator and the controller, wherein in the contact state the thermal conductor is in thermal contact with the first cold stage and the first cooling stage, and in the non-contact state the thermal conductor is not in thermal contact with the first cooling stage and/or the first cold stage.

In another implementation form of the first aspect, the actuator comprises a piezoelectric actuator.

In another implementation form of the first aspect, the controlled thermal coupling comprises a first material, wherein a thermal conductivity of the first material is greater at a higher reference temperature above the threshold temperature than at a lower reference temperature below the threshold temperature.

In another implementation form of the first aspect, the first material comprises a superconducting material wherein a critical temperature of the superconducting material is higher than the threshold temperature.

In another implementation form of the first aspect, the superconducting material is niobium.

In another implementation form of the first aspect, the controlled thermal coupling further comprises a second material wherein the second material is in thermal contact with the first cooling stage and the first material is in thermal contact with the first cold stage, and the first material and the second material are thermally coupled.

In another implementation form of the first aspect, the second material is a thermal conductor at all temperatures.

In another implementation form of the first aspect, the second material comprises copper or silver and the first material comprises stainless steel, graphite, and/or niobium.

In another implementation form of the first aspect, the controlled thermal coupling comprises a heat exchange fluid.

In another implementation form of the first aspect, the cryogenic cooling system further comprises a gas-tight sleeve enclosing the first cooling stage, wherein the sleeve is provided with a thermally conductive interface coupled to the first cold stage and the cryogenic cooling system further comprises a pump for pumping the heat exchange fluid into and out of the sleeve.

In another implementation form of the first aspect, the controlled thermal coupling has a first thermal conductance above the threshold temperature and a second thermal conductance below the threshold temperature and the first thermal conductance is greater than the second thermal conductance.

In another implementation form of the first aspect, a ratio between the second thermal conductance and the first thermal conductance is 0.1 or less.

In another implementation form of the first aspect, the fluid cooling medium is helium-3, helium-4, or a mixture of helium-3 and helium-4.

In another implementation form of the first aspect, the fluid cooling medium is helium in a dilution refrigerator.

Many of the attendant features will be more readily appreciated as they become better understood by reference to the following detailed description considered in connection with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

In the following, example embodiments are described in more detail with reference to the attached figures and drawings, in which:

FIG. 1 illustrates a schematic representation of a cryogenic cooling system according to an embodiment;

FIG. 2 illustrates a schematic representation of a cryogenic cooling system according to another embodiment;

FIG. 3 illustrates a schematic representation of a controlled thermal coupling implemented using an actuator according to an embodiment;

FIG. 4 illustrates a schematic representation of a controlled thermal coupling implemented using a first material according to an embodiment;

FIG. 5 illustrates a schematic representation of a controlled thermal coupling implemented using a first and a second material according to an embodiment;

FIG. 6 illustrates a plot representation of thermal conductivity of various materials as a function of temperature;

FIG. 7 illustrates a schematic representation of a controlled thermal coupling implemented using a heat exchange fluid according to an embodiment; and

FIG. 8 illustrates a schematic representation of a controller according to an embodiment.

In the following, like reference numerals are used to designate like parts in the accompanying drawings.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanying drawings, which form part of the disclosure, and in which are shown, by way of illustration, specific aspects in which the present disclosure may be placed. It is understood that other aspects may be utilised, and structural or logical changes may be made without departing from the scope of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense, as the scope of the present disclosure is defined by the appended claims.

For instance, it is understood that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if a specific method step is described, a corresponding device may include a unit to perform the described method step, even if such unit is not explicitly described or illustrated in the figures. On the other hand, for example, if a specific apparatus is described based on functional units, a corresponding method may include a step performing the described functionality, even if such step is not explicitly described or illustrated in the figures. Further, it is understood that the features of the various example aspects described herein may be combined with each other, unless specifically noted otherwise.

FIG. 1 illustrates a schematic representation of a cryogenic cooling system according to an embodiment.

According to an embodiment, a cryogenic cooling system 100 comprises a vacuum enclosure and a working region 101 within the vacuum enclosure.

In the embodiment of FIG. 1, the vacuum enclosure is not illustrated for clarity of illustration.

The working region 101 may comprise, for example, a mixing chamber stage/flange 130 of the cryogenic cooling system 100. The mixing chamber stage/flange 130 can also be referred to as a base temperature stage/flange, a mixing plate, or similar. The mixing chamber of a dilution refrigerator may be thermally coupled to the mixing chamber stage/flange 130 for cooling the working region 101 to millikelvin temperatures.

The cryogenic cooling system 100 may further comprise a first cryocooler 102 having a first cooling stage 103, the first cooling stage 103 being located in the vacuum enclosure.

The attribute “first”, as well as corresponding attributes “second”, “third”, etc. in the continuation, are used in herein solely for the purpose of unambiguous reference, without any limiting indication to any numerical order.

The first cryocooler 102 may comprise any number of cooling stages and the first cooling stage 103 may be any cooling stage of the first cryocooler 102.

Herein, a cryocooler may also be referred to as a mechanical pre-cooler, a mechanical refrigerator, or similar.

The cryogenic cooling system 100 may further comprise a first conduit 104 for passing a stream of fluid cooling medium towards the working region 101.

For example, in some embodiments, the working region 101 may be cooled by a mixing chamber 131 of a dilution refrigerator, in which case the fluid cooling medium may be helium-3 or a mixture of helium-3 and helium 4 and the first conduit 104 may be part of the circulation required for proper operation of the dilution refrigerator. The working region may be a surface of the mixing chamber 131 or a thermally conductive object in thermal contact with the mixing chamber 131. In other embodiments, the fluid cooling medium may cool the working region by absorbing heat from the working region and/or a payload connected to the working region. The absorbed heat may cause a condensed cooling fluid to evaporate, or the flow of the cooling fluid may transfer the heat away from the working region.

The cryogenic cooling system 100 may further comprise at least one thermal coupling 105 of the first conduit 104 and the first cooling stage 103 for cooling the fluid cooling medium on its way towards the working region 101.

Herein, a thermal coupling between two objects may refer to any coupling that can be used to transfer heat sufficiently efficiently between the two objects. For example, the two objects may be in mechanical contact, a heat exchange gas may be arranged to transfer heat between the two objects, some other material(s) may be arranged to transfer heat between the two objects, or the thermal coupling may be implemented in some other manner.

The at least one thermal coupling 105 of the first conduit 104 and the first cooling stage 103 may be implemented in various ways. For example, at least some part of the first conduit 104 may be in mechanical contact with the first cooling stage 103. Further, the first conduit 104 may be thermally coupled also to other components, such as other cooling stages of the first cryocooler 102. For example, in some embodiments, other cooling stages of the first cryocooler 102 may be used to cool the fluid cooling medium before further cooling it via the first cooling stage 103.

The cryogenic cooling system 100 may further comprise a second cryocooler 106 having a second cooling stage 107, the second cooling stage 107 being located in the vacuum enclosure.

The second cryocooler 106 may comprise any number of cooling states and the second cooling stage 107 may be any cooling stage of the second cryocooler 106.

The cryogenic cooling system 100 may further comprise a first cold stage 108 located in the vacuum enclosure.

The first cold stage 108 may comprise, for example, a 4-kelvin (4K) flange/stage of the cryogenic cooling system 100.

The cryogenic cooling system 100 may further comprise at least one thermal coupling 110 of the second cooling stage 107 and the first cold stage 108 for cooling the first cold stage 108.

The second cryocooler 106 may be configured to cool the first cold stage 108 via the at least one thermal coupling 110 of the second cooling stage 107 and the first cold stage 108.

The cryogenic cooling system 100 may further comprise a first thermal coupling 111 of the first cooling stage 103 and the first cold stage 108, provided by a coupling element 109, for cooling the first cold stage 108.

The coupling element 109 may comprise a controlled thermal coupling for reducing the first thermal coupling when a relevant temperature of the cryogenic cooling system 100 is below a threshold temperature.

The coupling element 109 may, for example, substantially thermally decouple the first cooling stage 103 and the first cold stage 108 when the relevant temperature of the cryogenic cooling system 100 is below the threshold temperature.

The relevant temperature of the cryogenic cooling system 100 may comprise any temperature based on which the coupling element 109 can be operated for reducing the first thermal coupling.

The relevant temperature of the cryogenic cooling system 100 may comprise, for example, a temperature of the mixing chamber stage/flange 130, a temperature of the first cold stage 108, a temperature of a still stage/flange the cryogenic cooling system 100, a temperature of the first cooling stage 103, or a temperature of the coupling element 109. In some embodiments, the relevant temperature can be an estimated relevant temperature, for example estimated based on the cooling time of the cryogenic cooling system 100. The cooling time may be measured, for example, from when the cryogenic cooling system 100 started to cool from room temperature.

The threshold temperature is a predefined/preconfigured temperature. For example, in some embodiments, the threshold temperature can be defined by the properties, such as the selection of materials and the geometry, of the coupling element 109. In other embodiments, the threshold temperature may be a temperature set by the operator of the cryogenic cooling system 100. In other embodiments, the threshold temperature can be an estimate of the temperature of the system after certain time has passed since the cooling started.

According to an embodiment, the relevant temperature of the cryogenic cooling system 100 is the temperature of the mixing chamber stage/flange 130 and the threshold temperature is 100 mK. Thus, the coupling element 109 may reduce the first thermal coupling 111, such as substantially decouple the first cooling stage 103 and the first cold stage 108, when the temperature of the mixing chamber stage/flange 130 is below 100 mK.

According to an embodiment, the relevant temperature of the cryogenic cooling system 100 is the temperature of the first cold stage 108 and the threshold temperature is 2.2K, 4K, 6K, 10 K, or 15K. Thus, the coupling element 109 may reduce the first thermal coupling 111, such as substantially decouple the first cooling stage 103 and the first cold stage 108, when the temperature of the first cold stage 108 is below 10 K.

According to an embodiment, the relevant temperature of the cryogenic cooling system 100 is the temperature of the still stage of the cryogenic cooling system 100 and the threshold temperature is 1K, 2.2K, 4K, 6K, 10K, or 15 K. Thus, the coupling element 109 may reduce the first thermal coupling 111, such as substantially decouple the first cooling stage 103 and the first cold stage 108, when the temperature of the still stage is below 15 K.

According to an embodiment, the first cold stage 108 is a 4K stage of the cryogenic cooling system 100.

According to an embodiment, the controlled thermal coupling has a first thermal conductance above the threshold temperature and a second thermal conductance below the threshold temperature and the first thermal conductance is greater than the second thermal conductance.

According to an embodiment, a ratio between the second thermal conductance and the first thermal conductance is 0.1 or less.

In any embodiment, the cryogenic cooling system 100 may comprise any number of cryocoolers in addition to the first cryocooler 102 and the second cryocooler 106. The additional cryocoolers can be used to, for example, further cool the first cold stage 108.

With the cryogenic cooling system 100, both the first cryocooler 102 and the second cryocooler 106 can be used to cool the first cold stage 108 when the relevant temperature of the cryogenic cooling system 100 is above the threshold temperature. When the relevant temperature of the cryogenic cooling system 100 is below the threshold temperature, the coupling element 109 can be used to reduce the first thermal coupling 111 between the first cooling stage 103 and the first cold stage 108 and the first cooling stage 103 can be used to, for example, only cool the fluid cooling medium. Thus, the cooling time of the first cold stage 108 can reduced by using both first cryocooler 102 and the second cryocooler 106 to cool the first cold stage 108 and the working region can be cooled to a lower temperature using the fluid cooling medium.

In some embodiments, the cryogenic cooling system 100 can be cooled in two phases. For example, during a first phase, the first cryocooler 102 and the second cryocooler 106 can be used to cool the first cold stage 108 to or close to the threshold temperature. Since the cooling power of the first and second cryocooler 102, 106 can be used to cool the first cold stage 108 during the first phase, cooling time of the system can be reduced. Then, during a second phase, the first cryocooler 102 can be substantially thermally decoupled from the first cold stage 108 and used to cool the fluid cooling medium. The fluid cooling medium can be used to cool the working region 101 to, for example, millikelvin temperatures. When the first cooling stage 103 is substantially thermally decoupled from the first cold stage 108, the first cold stage 108 can be allowed to warm up to, for example, 10 K, where the cooling power of the cryocoolers is higher, without influencing the cryogenic cooling of the working region 100.

If the first thermal coupling 111 was not reduced, the temperature of the first cold stage 108 could be one limiting factor of the cryogenic cooling system 100. If the temperature of the fluid cooling medium, after having been cooled by the first cooling stage 103 increases too much, the dilution refrigerator can become unstable, and its performance can suffer. This can occur, for example, when the temperature of the first cooling stage 103 exceeds 3.2K and/or the temperature of the first cold stage 108 exceeds 3.6K. For many use cases, this 3.6K limit is an unnecessary restriction for other components of the cryogenic cooling system 100. For example, many components thermally coupled to the first cold stage 108 can be operated, for example, temperatures up to 4-10 K. For example, the first cold stage 108 may be used to thermalize, for example, wiring going to the working region 101. Various experimental apparatus can also be located at the first cold stage 108.

FIG. 2 illustrates a schematic representation of a cryogenic cooling system according to another embodiment.

The outermost structure of the cryogenic cooling system 100 can be a vacuum enclosure 121, which is shown with dashed lines in FIG. 2. The topmost flange 122 is the lid of the vacuum enclosure. The room temperature stages 123, 124 of the first and second cryocooler 102, 106 can be attached to the topmost flange 122.

According to an embodiment, the first cryocooler 102 comprises a first pulse tube, the second cryocooler 106 comprises a second pulse tube, the first cooling stage 103 is a lower temperature stage of the first pulse tube, and the second cooling stage 107 is a lower temperature stage of the second pulse tube.

For example, in the embodiment of FIG. 2, the first cryocooler 102 comprises a first pulse tube and the second cryocooler 106 comprises a second pulse tube. The first pulse tube and the second pulse tube further comprise a higher temperature stage 125, 126 attached to a second cold stage 127. It should be appreciated that the naming of the cold stages herein may not reflect the ordering of the cold stages, but only the order in which they are introduced herein. For example, in the embodiment of FIG. 2, the second cold stage 127 is a higher temperature stage than the first cold stage 108.

In other embodiments, the first cryocooler 102 and/or the second cryocooler 106 may comprise any other type of cryocooler, such as a Gifford-McMahon cryocooler, a Joule - Thomson cryocooler, a Stirling cryocooler, a regenerative heat exchanger, or a recuperative heat exchanger.

During operation of the cryogenic cooling system 100, the higher temperature stage 125, 126 may reach a temperature of about 30 to 50 K, for example. The lower temperature stages 103, 107 may reach a temperature of about 2 to 10 K, such as 2.2K, 4K, 6K, or 10K, for example.

The second cold stage 127 and the first cold stage 108 may also be referred to as the 50 K stage/flange and the 4 K stage/flange, respectively, reflecting their approximate temperatures during operation of the cryogenic cooling system 100.

The cryogenic cooling system 100 may further comprise a still stage/flange 128 to which a still 129 of a dilution refrigerator can be attached.

According to an embodiment, the fluid cooling medium is helium-3, helium-4, or a mixture of helium-3 and helium-4.

According to an embodiment, the fluid cooling medium is helium in a dilution refrigerator.

The cryogenic cooling system 100 may further comprise a base temperature stage/flange 130. A mixing chamber of the dilution refrigerator can be attached to the base temperature stage/flange 130. The base temperature stage/flange 130 may comprise a target region 132 for a payload that is to be refrigerated. The payload may also be referred to as a sample.

According to an embodiment, the first cold stage 108 is thermally coupled to a heat radiation shield surrounding the working region 101 or is a part of a heat radiation shield surrounding the working region 101.

For example, the cryogenic cooling system 100 may further comprise cylindrical radiation shields, which are not shown in FIG. 2 for graphical clarity. The cylindrical radiation shields can be attached to the stages/flanges in a nested configuration.

The cryogenic cooling system 100 may further comprise other intermediate stages/flanges like a so-called 100 mK stage/flange between the still stage/flange 128 and the base temperature stage/flange 130.

According to an embodiment, the working region 101 is arranged to receive an object to be cooled.

For example, the stages/flanges of the cryogenic cooling system 100 may comprise aligned apertures to provide a so-called line-of-sight port to the target region 132 via which the object to be cooled can be inserted.

FIG. 3 illustrates a schematic representation of a controlled thermal coupling implemented using an actuator according to an embodiment.

According to an embodiment, the coupling element 109 comprises a thermal conductor and the controlled thermal coupling comprises an actuator and a controller for controlling the actuator, the thermal conductor being arranged to be moved between a contact state and a non-contact state by the actuator and the controller, wherein in the contact state the thermal conductor is in thermal contact with the first cold stage and the first cooling stage, and in the non-contact state the thermal conductor is not in thermal contact with the first cooling stage and/or the first cold stage.

According to an embodiment, the actuator comprises a piezoelectric actuator.

For example, in the embodiment of FIG. 3, the coupling element 109 comprises a first thermal conductor 202 and a second thermal conductor 203 and the controlled thermal coupling comprises a piezoelectric actuator 201. The piezoelectric actuator 201 can be electrically coupled to a controller for controlling the piezoelectric actuator 201. The first and second thermal conductors 202, 203 are arranged to be moved between a contact state, illustrated on the right side of FIG. 3, and a non-contact state, illustrated on the left side of FIG. 3, by the piezoelectric actuator 201 and the controller. In the contact state, the first thermal conductor 202 is in thermal contact with the first cold stage 108 via the second thermal conductor 203 and the second thermal conductor 203 is in thermal contact with the first cooling stage 103 via the first thermal conductor 202. In the non-contact state, the first thermal conductor 202 is not in thermal contact with the first cold stage 108 and the second thermal conductor 203 is not in thermal contact with the first cooling stage 103. In the contact state and in the non-contact state, the first thermal conductor 202 is in contact with the first cooling stage 103 and the second thermal conductor 203 is in contact with the first cold stage 108.

In other embodiment, the piezoelectric actuator 201 can be replaced with any other kind of actuator.

The thermal conductor can comprise, for example, copper.

The piezoelectric actuator 201 can provide mechanical motion and force induced by a voltage applied to the piezoelectric actuator 201 without significant amount of heating to the cryogenic cooling system 100.

The controller may be configured to monitor the relevant temperature of the cryogenic cooling system 100 and, in response to the relevant temperature of the cryogenic cooling system 100 being less than the threshold temperature, control the actuator to move the coupling element 109 from the contact state to the non-contact state.

In some embodiments, the controller can monitor the relevant temperature of the cryogenic cooling system 100 by estimating the relevant temperature of the cryogenic cooling system 100 based on cooling time. For example, based on previous measurements, the operator of the cryogenic cooling system 100 may have determined that it takes a specific amount of time for the relevant temperature of the cryogenic cooling system 100 to reach the threshold temperature. Based on this, the operator can configure the controller to control the actuator to move the coupling element 109 from the contact state to the non-contact state after this amount of time.

In other embodiments, the operator may perform operations of the controller manually. For example, the operator can monitor the relevant temperature of the cryogenic cooling system 100 by estimating the relevant temperature of the cryogenic cooling system 100 based on cooling time and control the actuator to move the coupling element 109 from the contact state to the non-contact state based on the estimated relevant temperature of the cryogenic cooling system.

In some embodiments, the piezoelectric actuator 201 can comprise a plurality of layers of a piezoelectric material. Cryogenic long range moving piezoelectric actuators can generate forces up to approximately one newton. Piezoelectric actuators comprising a plurality of layers of a piezoelectric material, also referred to as piezoelectric stacks, can produce up to several thousand newtons. In some embodiments, leveraging can be used to increase the force generated by the piezoelectric actuator 201.

Long range stepping piezoelectric actuators can produce movement ranges up to centimetres. Piezoelectric actuators comprising a plurality of layers of a piezoelectric material typically produce movement ranges of less than one millimetre.

The piezoelectric actuator 201 can comprise, for example, lead zirconate titanate (PZT) and/or zinc oxide (ZnO).

When the coupling element 109 is implemented using an actuator, the controlled thermal coupling can be controlled with a high degree of customizability. For example, the controller can be configured to control the actuator based on various relevant temperatures of the cryogenic cooling system 100 and/or the relevant temperature does not need to be measured directly but can be estimated based on cooling time. Further, since the thermal conductor 202, 203 can be moved to the non-contact state, good thermal isolation can be achieved between the first cooling stage 103 and the first cold stage 108.

It should be appreciated that the embodiment of FIG. 3 only illustrates the operating principle of a coupling element 109 comprising a piezoelectric actuator 201 in a simplified manner. Various aspects of the coupling element 109, such as the geometry, may be modified to better apply to a specific application.

FIG. 4 illustrates a schematic representation of a controlled thermal coupling implemented using a first material according to an embodiment.

According to an embodiment, the controlled thermal coupling comprises a first material 301, wherein a thermal conductivity of the first material 301 is greater at a higher reference temperature above the threshold temperature than at a lower reference temperature below the threshold temperature.

The higher reference temperature may refer to any temperature above the threshold temperature but equal to or below room temperature.

The lower reference temperature may refer to any temperature below the threshold temperature.

In some embodiments, a thermal conductivity of the first material may be greater at all temperatures above the threshold temperature than at temperatures below the threshold temperature.

In some embodiments, the threshold temperature may be 10 K and the higher reference temperature may be a temperature greater than 100 K.

In some embodiments, the higher reference temperature can be any temperature above, for example, 10K, such as 50K or 100K and the lower reference temperature can be any temperature below 10K, such as 4K or 2K.

According to an embodiment, a thermal conductivity of the first material 301 is 20 times greater at the higher reference temperature above the threshold temperature than at the lower reference temperature below the threshold temperature.

According to an embodiment, the first material 301 comprises a superconducting material wherein the critical temperature of the superconducting material is higher than the threshold temperature.

According to an embodiment, the superconducting material is niobium.

Niobium undergoes superconducting transition at 9.3 K, below which its thermal conductivity decreases exponentially. Therefore, when the first cold stage 108 reaches a temperature of, for example, 3-6 K due to the cooling of the first cooling stage 103 and the second cooling stage 107, the residual thermal conductivity of niobium can be so small that the first cooling stage 103 and the first cold stage 108 effectively become thermally isolated.

In some embodiments, the first material 301 may be single-crystal niobium, which may improve thermal conductivity above the threshold temperature.

When the coupling element 109 is implemented using a first material 301, the controlled thermal coupling can be controlled in a passive manner. Thus, structure of the coupling element 109 can be simplified since other components are not needed for controlling the controlled thermal coupling.

It should be appreciated that the embodiment of FIG. 4 only illustrates a coupling element 109 comprising the first material 301 in a simplified manner. Various aspects of the coupling element 109, such as the geometry, may be modified to better apply to a specific application.

FIG. 5 illustrates a schematic representation of a controlled thermal coupling implemented using a first and a second material according to an embodiment.

According to an embodiment, the controlled thermal coupling further comprises a second material 401 wherein the second material 401 is in thermal contact with the first cooling stage 103 and the first material 301 is in thermal contact with the first cold stage 108, and the first material 301 and the second material 401 are thermally coupled.

Alternatively, the second material 401 may be in thermal contact with the first cold stage 108, the first material 301 may be in thermal contact with the first cooling stage 103, and the first material 301 and the second material 401 are thermally coupled.

According to an embodiment, the second material 401 is a thermal conductor at all temperatures.

In cryogenic technology, a high thermal conductivity may be for example at least 100 W/(m*K) at or above 10 K, at least 10 W/(m*K) at 1 K, at least 1 W/(m*K) at 0.1 K, or at least 0.1 W/(m*K) at 0.01 K. A material may be considered a thermal conductor when the thermal conductivity of the material is high.

In cryogenic technology, low thermal conductivity may be for example less than 50 W/(m*K) at 100 K, less than 5 W/(m*K) at 10 K, less than 0.75 W/(m*K) at 1 K, less than 0.075 W/(m*K) at 0.1 K, and less than 0.0075 W/(m*K) at 0.01 K.

According to an embodiment, the second material 401 comprises copper or silver and the first material 301 comprises stainless steel, graphite, and/or niobium.

According to an embodiment, the second material 401 comprises copper and the

first material 301 comprises stainless steel.

According to an embodiment, the second material 401 comprises copper and the first material 301 comprises graphite.

According to an embodiment, the second material 401 comprises copper and the first material 301 comprises niobium.

According to an embodiment, the second material 401 comprises silver and the first material 301 comprises stainless steel.

According to an embodiment, the second material 401 comprises silver and the first material 301 comprises graphite.

According to an embodiment, the second material 401 comprises silver and the first material 301 comprises niobium.

Thermal conductivity of the first material 301 below the threshold temperature can be less than the thermal conductivity of the second material 401 below the threshold temperature.

Thermal conductivity of the first material 301 above the threshold temperature can be less than the thermal conductivity of the second material 401 above the threshold temperature.

The first material 301 can comprise, for example, a material with a sharp decline in thermal conductivity between 50 K and 4 K and the second material 401 can comprise a material with a high thermal conductivity, such as copper or silver.

By using the first material 301 and the second material 401, the threshold temperature and the thermal conductivity of the first thermal coupling 111 can be finetuned by, for example, modifying the dimensions of the first material 301 and of the second material 401 and/or the geometry of the first material 301 and of the second material 401.

Thermal combined conductance K_s of the first material 301 and the second material 401 is

K s = K 1 ⁢ K 2 K 1 + K 2 ,

where K₁ is the thermal conductance of the first material 301 and K₂ is the thermal conductance of the second material 401. K₁ and K₂ can be altered by modifying, for example, the geometry of first material 301 and of the second material 401. For example, the thermal conductance of either material can be increased by increasing the cross-sectional surface area of the material or by decreasing the thickness of the material.

When the coupling element 109 is implemented using the first material 301 or a combination of the first material 301 and the second material 401, the relevant temperature may be the temperature of the first material 301 and/or of the second material 401. The first material 301 or the combination of the first material 301 and the second material 401 can thus be used to implement the controlled thermal coupling for reducing the first thermal coupling when the relevant temperature of the cryogenic cooling system 100 is below the threshold temperature in a passive manner.

When the coupling element 109 is implemented using a combination of the first material 301 and the second material 401, the controlled thermal coupling can be controlled in a passive manner. Thus, structure of the coupling element 109 can be simplified since other components are not needed for controlling the controlled thermal coupling. Further, the controlled thermal coupling can be controlled with a high degree of customizability by choosing the materials of the first material 301 and of the second material 401 and/or the dimensions of the first material 301 and of the second material 401.

FIG. 6 illustrates a plot representation of thermal conductivity of various materials as a function of temperature.

Curve 501 correspond to copper, curve 502 corresponds to pure aluminium, curve 503 corresponds to niobium, curve 504 corresponds to 6061 aluminium alloy, curve 505 corresponds to titanium, curve 506 corresponds to 304 stainless steel, curve 507 corresponds to invar, and curve 508 corresponds to G10, i.e. garolite.

FIG. 7 illustrates a schematic representation of a controlled thermal coupling implemented using a heat exchange fluid according to an embodiment.

According to an embodiment, the controlled thermal coupling comprises a heat exchange fluid.

The heat exchange fluid may comprise, for example, a heat exchange gas and/or a heat exchange liquid, such as gaseous or liquid helium.

According to an embodiment, the cryogenic cooling system 100 further comprises a gas-tight sleeve 601 enclosing the first cooling stage 103, wherein the sleeve 601 is provided with a thermally conductive interface 602 coupled to the first cold stage 108 and the cryogenic cooling system 100 further comprises a pump for pumping the heat exchange fluid into and out of the sleeve 601.

For example, in the embodiment of FIG. 7, the first cryocooler 102 comprises a first pulse tube and the first cooling stage 103 is a lower temperature stage of the first pulse tube. A gas-tight sleeve 601 encloses the first cooling stage 103 and the sleeve 601 is provided with a thermally conductive interface 602 coupled to the first cold stage 108.

The pulse tube may not be mechanically coupled to the first cold stage 108. Rather, the thermal coupling between the first cooling stage 103 and the first cold stage 108 can be achieved via, for example, gaseous or liquid helium by filling the sleeve 601 with it using the pump 603.

In some embodiments, the cryogenic cooling system 100 may further comprise a heater for evacuating the heat exchange fluid from the sleeve 601. The heater can make evacuating the sleeve 601 more efficient.

The fluid cooling medium should be cooled even when the heat exchange fluid is evacuated from the gas-tight sleeve 601. Thus, there should still be the at least one thermal coupling 105 of the first conduit 104 and the first cooling stage 103 even when the heat exchange fluid is evacuated from the gas-tight sleeve 601. This can be achieved by, for example, routing the first conduit 104 inside the gas-tight sleeve 601 and in thermal contact with the first cooling stage 103 even when there is no heat exchange fluid.

In the embodiment of FIG. 7, the pulse tube further comprises a higher temperature stage 125 and the cryogenic cooling system 100 further comprises a second gas-tight sleeve 604 enclosing the higher temperature stage 125. The second gas-tight sleeve 604 may be used to control the thermal coupling between, for example, the higher temperature stage 125 and the second cold stage 127.

The use of the gas-tight sleeves 601, 604 can reduce the number of interfaces between the cooling stages 103, 125 of the pulse tube and the cold stages/flanges 108, 127.

The use of the gas-tight sleeves 601, 604 can make the swapping of the pulse tubes easier.

The use of the gas-tight sleeves 601, 604 can provide good vibration isolation between the cooling stages 103, 125 of the pulse tube and the cold stages/flanges 108, 127.

In other embodiments, the gas-tight sleeve 601 may be replaced with a different type of container for the heat exchange fluid having a different type of geometry. The container may be provided with a thermally conductive interface coupled to the first cold stage 108 and the pump 603 may be arranged to pump the heat exchange fluid into and out of the container.

When the coupling element 109 is implemented using a gas-tight sleeve 106 and a heat exchange fluid, the controlled thermal coupling can be controlled with a high degree of customizability. For example, a controller can be configured to control the pumping of the heat exchange fluid based on various relevant temperatures of the cryogenic cooling system 100 and/or the relevant temperature does not need to be measured directly but can be estimated based on cooling time. Further, since there may not be mechanical contact between the first cooling stage 103 and the first cold stage 108, good thermal isolation can be achieved. Moreover, the use of a gas-tight sleeve 106 and a heat exchange fluid can make the removal/swapping of the first cryocooler 102 easier.

FIG. 8 illustrates a schematic representation of a controller according to an embodiment.

The controller 800 may comprise at least one processor 801. The at least one processor 801 may comprise, for example, one or more of various processing devices, such as a co-processor, a microprocessor, a digital signal processor (DSP), a processing circuitry with or without an accompanying DSP, or various other processing devices including integrated circuits such as, for example, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a microprocessor unit (MCU), a hardware accelerator, a special-purpose computer chip, or the like.

The controller 800 may further comprise a memory 802. The memory 802 may be configured to store, for example, computer programs and the like. The memory 802 may comprise one or more volatile memory devices, one or more non-volatile memory devices, and/or a combination of one or more volatile memory devices and non-volatile memory devices. For example, the memory 802 may be embodied as magnetic storage devices (such as hard disk drives, floppy disks, magnetic tapes, etc.), optical magnetic storage devices, and semiconductor memories (such as mask ROM, PROM (programmable ROM), EPROM (erasable PROM), flash ROM, RAM (random access memory), etc.).

The controller 800 may further comprise other components not illustrated in the embodiment of FIG. 8. The controller 800 may comprise, for example, an input/output bus for connecting the controller 800 the coupling element 109 and/or other components, such as one or more temperature sensors for measuring the relevant temperature of the cryogenic cooling system 100. Further, a user may control the controller 800 via the input/output bus. The user may, for example, control the operation of the coupling element 109 using the controller 800. For example, the operator of the cryogenic cooling system 100 may save the threshold temperature to the memory 802 of the controller 800 and the memory 802 may comprise program code configured to control the coupling element 109 according to the threshold temperature.

When the controller 800 is configured to implement some functionality, some component and/or components of the controller 800, such as the at least one processor 801 and/or the memory 802, may be configured to implement this functionality. Furthermore, when the at least one processor 801 is configured to implement some functionality, this functionality may be implemented using program code comprised, for example, in the memory 802.

The controller 800 may be implemented using, for example, a computer, some other computing device, or similar.

In some embodiments, the controller 800 may be electrically coupled to the piezoelectric actuator 201. The electrical coupling may comprise, for example, a boost converter. The controller 800 may be configured to control the boost converter and the boost converter may be configured to provide a high voltage needed for driving the piezoelectric actuator 201. The controller 800 may be configured to, in response to the relevant temperature of the cryogenic cooling system 100 being below the threshold temperature, move the thermal conductor 202, 203 from the contact state to the non-contact state using the piezoelectric actuator 201.

In some embodiments, the piezoelectric actuator 201 may be replaced with any other type of actuator and the controller 800 may be electrically coupled to the actuator.

In some embodiments, the controller 800 may be electrically coupled to the pump 603. The controller 800 may be configured to, in response to the relevant temperature of the cryogenic cooling system 100 being below the threshold temperature, control the pump 603 to pump the heat exchange fluid out of the sleeve 601.

Any range or device value given herein may be extended or altered without losing the effect sought. Also any embodiment may be combined with another embodiment unless explicitly disallowed.

Although the subject matter has been described in language specific to structural features and/or acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as examples of implementing the claims and other equivalent features and acts are intended to be within the scope of the claims.

It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. It will further be understood that reference to ‘an’ item may refer to one or more of those items.

The steps of the methods described herein may be carried out in any suitable order, or simultaneously where appropriate. Additionally, individual blocks may be deleted from any of the methods without departing from the spirit and scope of the subject matter described herein. Aspects of any of the embodiments described above may be combined with aspects of any of the other embodiments described to form further embodiments without losing the effect sought.

The term ‘comprising’ is used herein to mean including the method, blocks or elements identified, but that such blocks or elements do not comprise an exclusive list and a method or apparatus may contain additional blocks or elements.

It will be understood that the above description is given by way of example only and that various modifications may be made by those skilled in the art. The above specification, examples and data provide a complete description of the structure and use of exemplary embodiments. Although various embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this specification.