Air Gap Thermal Switch Construction

Abstract

A cryogenic cooling system is provided, comprising: a cooling plate (2) thermally coupled to a cryogenic refrigerator (9), a thermal switch assembly, and a target assembly (5). The target assembly (5) comprises a target refrigerator (12), the target refrigerator being configured to obtain a base temperature lower than that of the cryogenic refrigerator (9). The thermal switch assembly (18) comprises one or more air gap thermal switches, the thermal switch assembly (18) having a first end thermally coupled to the cooling plate (2) and a second end thermally coupled to the target assembly (5). An adsorption pump (22) is configured to control the thermal conductivity of the thermal switch assembly (18) according to the temperature of the adsorption pump (22). The adsorption pump (22) is thermally coupled to the cryogenic refrigerator (9) via a thermal link 46 extending from the cooling plate (2) to the thermal switch assembly (18). The adsorption pump (22) is arranged at a position along the thermal link (46) between the thermal switch assembly 18 and the cooling plate (2).

Description

Technical Field

[0001] The present invention relates to a cryogenic cooling system and a method of operating the same. Background Art

[0002] Cryogenic systems generally include multiple stages enclosed in a vacuum chamber, which are maintained at different temperatures during steady-state operation of the system. To facilitate cooling from room temperature to cryogenic temperatures, it is desirable to be able to thermally connect the stages during the cooling process and to thermally isolate the stages during steady-state operation. This can be accomplished using a thermal switch. For example, an air gap thermal switch can be controlled to transfer a heat load from one end of the switch to the other end or to isolate a heat load between one end and the other end of the switch.

[0003] The air gap thermal switch includes two conductors separated from each other in a chamber where gas can be introduced. When the switch is closed, the gas in the chamber contributes to the heat transfer between the conductors by conduction. The switch is opened by exhausting the gas from the chamber so that the heat transfer path is no longer available. The thermal conductivity of the chamber containing the conductor is low compared to the conductor at the system operating temperature. Therefore, when the thermal conductivity between the two conductors exceeds the thermal conductivity of the chamber, the air gap thermal switch can be considered "closed". In the "open" state, the thermal conductivity is determined by the geometry, material and temperature of the chamber, and the thermal conductivity in the "open" state is several orders of magnitude lower than the thermal conductivity in the "closed" state. An adsorption pump can be used to control the opening and closing of the switch. When the temperature of the pump is reduced to below the threshold transition temperature, the adsorption pump adsorbs gas molecules from the switch, thereby opening the switch. Conversely, when the temperature of the pump is increased to above the transition temperature, the pump desorbs these gas molecules so as to reintroduce these gas molecules into the chamber and close the switch.

[0004] During the process of cooling the system from room temperature to cryogenic temperatures, the air gap thermal switch is usually kept closed to allow heat transfer between each stage. However, once the adsorption pump has cooled below the transition temperature, the switch will open to substantially thermally isolate the stages connected to opposite ends of the switch. This enables each stage to subsequently attain different temperatures through continued operation of the system.

[0005] It will be appreciated that the adsorption pump facilitates the automatic control of the air gap thermal switch without the need for dedicated pumping lines and associated equipment. Therefore, the adsorption pump can reduce the complexity of the system and improve the performance of the system. However, the prior art systems incorporating such adsorption pumps are generally also limited in terms of the possible operating modes of the system. For example, the temperature of a single stage of the system cannot be raised to a threshold temperature above the adsorption pump without desorbing the gas from the adsorption pump. This may cause the switch to transition to a closed state, thereby affecting system performance and limiting the possible temperatures that can be achieved at each stage. In the context of addressing this problem, the present invention is proposed. Summary of the invention

[0006] A first aspect of the present invention provides a low-temperature cooling system, comprising: a cooling plate thermally connected to a low-temperature refrigerator; a target component, wherein the target component includes a target refrigerator, the target refrigerator being configured to obtain a base temperature lower than that of the low-temperature refrigerator; a thermal switch component comprising one or more air gap thermal switches, the thermal switch component having a first end thermally connected to the cooling plate and a second end thermally connected to the target component; and an adsorption pump configured to control the thermal conductivity of the thermal switch component according to the temperature of the adsorption pump, wherein the adsorption pump is thermally connected to the low-temperature refrigerator via a thermal link extending from the cooling plate to the thermal switch assembly, wherein the adsorption pump is arranged at a position along the thermal link between the thermal switch assembly and the cooling plate.

[0007] The thermal link advantageously provides a thermal connection between the cooling plate and the sorption pump that is independent of the thermal switch assembly. This can better control the temperature of the sorption pump, thereby better controlling the thermal conductivity of the thermal switch assembly. For example, a cryogenic refrigerator can be used to maintain the temperature of the sorption pump to be lower than the temperature of one or more air gap thermal switches connected to the sorption pump. Therefore, the temperature of the target component can be increased without accidentally heating the sorption pump, otherwise the thermal switch assembly can be converted from an open state to a closed state. Therefore, a new operating mode of the cryogenic system is possible. For example, the temperature of the target component can be increased to a temperature higher than the temperature of the cooling plate. In addition, because the thermal switch assembly can be maintained in an open state during the heating of the target component, the heating of the target component does not affect the temperature of the cooling plate.

[0008] It will be understood that the adsorption pump is generally configured to close one or more air gap thermal switches of the thermal switch assembly in response to the temperature of the adsorption pump exceeding the nominal transition temperature. The nominal transition temperature may depend on the application, such as the selection of the working fluid in the air gap thermal switch and the adsorption material, however, the nominal transition temperature is generally between 4 Kelvin and 30 Kelvin, and preferably between 15 Kelvin and 25 Kelvin. As previously described, the open state of the air gap thermal switch corresponds to a state in which the gas has been substantially or completely removed from the air gap thermal switch, thereby substantially reducing the thermal conductivity between the first end and the second end of the thermal switch assembly. In contrast, the closed state corresponds to a state in which the gas is contained in the air gap thermal switch, and contributes to a higher thermal conductivity between the first end and the second end. The thermal link is preferably arranged so that the adsorption pump can be maintained below the nominal transition temperature of the adsorption pump during the operation of the cryogenic refrigerator. In particular, the adsorption pump is preferably thermally coupled to the cooling plate, thereby maintaining the temperature of the adsorption pump below the nominal transition temperature without relying on the target component temperature.

[0009] The target assembly typically includes a target heater such as an electrically controlled resistive heater. The target heater optionally forms a part of the target refrigerator, for example in the case where the target refrigerator includes a distiller or mixing chamber of a dilution refrigerator. The target assembly may include a target plate thermally coupled to the target heater and the target refrigerator. The sorption pump is preferably thermally coupled to the cooling plate so that the temperature of the sorption pump is maintained below the nominal transition temperature of the sorption pump during operation of the target heater. The operation of the target heater may raise the temperature of the target assembly to a temperature above the nominal transition temperature of the sorption pump.

[0010] As previously mentioned, even when the gas is removed from the air gap heat switch, the operation of the target heater also causes heat to be conducted along the heat switch assembly and conducted to the adsorption pump. The adsorption pump is usually connected to the heat switch assembly by a conduit (also referred to as a capillary), thereby providing fluid communication between the adsorption pump and the heat switch assembly. The conduit is preferably formed by a low conductivity material, but inevitably provides a thermal connection between the heat switch assembly and the adsorption pump. In the case where the heat link does not extend between the cooling plate and the adsorption pump, particularly when one end of the heat switch assembly itself is heated to a nominal transition temperature higher than the adsorption pump, the heat input from the target heater along the conduit may cause the gas to desorb from the pump. This may cause the heat switch assembly to be unnecessarily converted to a closed state. However, the heat link advantageously ensures that any heat introduced from the target assembly to the adsorption pump is conducted out by the operation of the cryogenic refrigerator. This contributes to the high temperature operation of the target assembly.

[0011] Typically, during the use of the cryogenic system, the cryogenic refrigerator will operate continuously. Nevertheless, it is still desirable to be able to use an adsorption pump to transition the thermal switch assembly from an open state to a closed state. As described above, this typically requires raising the temperature of the adsorption pump to a nominal transition temperature higher than the adsorption pump. Therefore, preferably, the system also includes an adsorption heater configured to apply local heating to the adsorption pump. For example, the adsorption heater may include an electrically operated resistance heater disposed at the adsorption pump, and the electrically operated resistance heater may be operated to raise the temperature of the adsorption pump to a temperature higher than the nominal transition temperature. Therefore, the adsorption heater may be enabled, typically using an electrical control system, so that the cryogenic refrigerator is thermally coupled to the target component when needed.

[0012] The thermal link typically includes a first connecting member and a second connecting member, wherein the first connecting member extends between the thermal switch assembly and the adsorption pump, and wherein the second connecting member extends between the adsorption pump and the cooling plate. The first connecting member typically includes a conduit for conveying gas between the adsorption pump and one or more air gap thermal switches of the thermal switch assembly. The first connecting member is preferably formed of a low conductivity material, and the first connecting member typically has a thermal conductivity lower than the thermal conductivity of the second connecting member. Nevertheless, high temperature operation of the target component can result in heat being transferred to the adsorption pump by conduction along the thermal switch assembly and the first connecting member. The second connecting member advantageously provides a thermal conduction path along which this heat can be removed from the adsorption pump by operation of the cryogenic refrigerator. Unlike the first connecting member, the thermal connection formed by the second connecting member is "desirable" in the sense that the second connecting member prevents accidental desorption of gas from the adsorption pump.

[0013] Although a certain degree of heat transfer is expected along the second connecting member, the thermal conductivity of the second connecting member is preferably low enough so that the temperature of the sorption pump can be controlled separately. For example, it can be expected that the temperature of the sorption pump will be raised to a temperature higher than the temperature of the cooling plate by the operation of the sorption heater. Therefore, the second connecting member preferably forms a weak thermal link. It is particularly desirable that the second connecting member has a thermal conductivity between 1 milliwatt per Kelvin and 50 milliwatts per Kelvin, and preferably between 5 milliwatts per Kelvin and 10 milliwatts per Kelvin. Therefore, the sorption pump can be maintained below the nominal transition temperature of the sorption pump until the time when the thermal switch assembly is expected to be closed by the operation of the sorption heater.

[0014] The thermal switch assembly may include a plurality of air gap thermal switches. For example, one or more stages may be arranged between the cooling plate and the target assembly, wherein each stage is thermally coupled to one or more air gap thermal switches of the thermal switch assembly. Each of the stages may be arranged to obtain a corresponding base temperature during operation of the cryogenic cooling system. In addition, an adsorption pump may be configured to open and close each of the air gap thermal switches of the thermal switch assembly. Alternatively, each of the air gap thermal switches of the thermal switch assembly may be controlled using a corresponding adsorption pump, and each of the adsorption pumps may be thermally coupled to the cooling plate via a thermal link. The system may further include a gas reservoir fluidly coupled to the thermal switch assembly. The gas reservoir may be controlled to introduce a predetermined amount of gas into each of the air gap thermal switches of the thermal switch assembly before the system is cooled to a cryogenic temperature.

[0015] It is conceivable that the system is a "wet" system that relies on a liquid cryogen to cool the cooling plate. For example, the cryogenic refrigerator may include a Dewar of liquid nitrogen or liquid helium. However, it is particularly desirable that the cryogenic refrigerator is a mechanical refrigerator selected from a group including the following groups, the group including: a pulse tube refrigerator, a Stirling refrigerator, and a Gifford-McMahon refrigerator, which refrigerators have the advantage of not requiring a liquid cryogen. The target refrigerator typically has a cooling power lower than that of a cryogenic refrigerator. The target refrigerator may include any of a helium-3 refrigerator, a dilution refrigerator, a distiller or a mixing chamber, and a 1 kelvin pot.

[0016] A second aspect of the present invention provides a method for operating a cryogenic cooling system according to the first aspect, wherein the adsorption pump is configured to thermally couple a cryogenic refrigerator to a target component in response to the temperature of the adsorption pump exceeding a nominal transition temperature, the method comprising the following steps: (a) raising the temperature of the target component from a first temperature below the nominal transition temperature to a second temperature above the nominal transition temperature; wherein the adsorption pump is thermally coupled to the cryogenic refrigerator using a thermal link so as to maintain the temperature of the adsorption pump below the nominal transition temperature throughout step (a).

[0017] The second aspect has similar advantages as those described in relation to the first aspect. Any features described in relation to the first aspect are also applicable to the second aspect, and vice versa.

[0018] As previously mentioned, the cryogenic cooling system preferably also includes an adsorption heater thermally attached to the adsorption pump. In this case, the method also preferably includes the following steps: (b) operating the adsorption heater to increase the temperature of the adsorption pump to above the nominal transition temperature, thereby thermally coupling the cryogenic refrigerator to the target component. As will be understood, step (b) will be performed after step (a). The operation of the cryogenic refrigerator and the subsequent target refrigerator will cause the system's stages to return to the corresponding base temperatures of the various stages.

[0019] It is usually desirable that the target component further comprises a target heater, in which case step (a) can be performed by operating the target heater. After performing step (a), the method preferably further comprises the step of reducing or eliminating the heat generated by the target heater.

[0020] The first temperature and the second temperature may be selected depending on the application, however, typically the first temperature is typically below 5 Kelvin and the second temperature is typically above 20 Kelvin. The second temperature is preferably at least 30 Kelvin and may be at least 100 Kelvin. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

[0022] Figure 1 is a schematic diagram of a low temperature cooling system according to a first embodiment of the present invention;

[0023] Figure 2 is a schematic diagram of a low temperature cooling system according to a second embodiment of the present invention;

[0024] Figure 3 is a graph illustrating a first exemplary temperature distribution of components of a cryogenic cooling system during operation according to a first embodiment of the present invention;

[0025] Figure 4 is a graph showing a second exemplary temperature distribution of components of the cryogenic cooling system during operation according to the first embodiment of the present invention; and

[0026] Figure 5 It shows Figure 4 A graph of the temperature distribution area between 3.5 hours and 5.5 hours. DETAILED DESCRIPTION

[0027] Now refer to Figure 1 A first embodiment of the cryogenic cooling system is described. Figure 1 A cross-sectional view of the interior of a cryogen-free cooling system is depicted. The system comprises a plurality of thermal stages 1 to 5 and an outer stage 6 forming a layered assembly in which the plurality of thermal stages are aligned along a central axis and are spatially dispersed. The thermal stages 1 to 5 are contained within a cryostat 7 mounted to the outer stage 6. The cryostat 7 is typically evacuated when in use. An outer surface 8 of the outer stage 6 is exposed to an ambient environment at room temperature and atmospheric pressure and is typically formed of aluminum.

[0028] The cryogenic cooling system includes a cooling device that cools the system from room temperature to an operational base temperature. Each thermal stage 1 to 5 is formed of a high conductivity material such as copper and has a different operational base temperature. In the present embodiment, cooling is achieved by using a mechanical refrigerator and a dilution unit. The mechanical refrigerator can be a pulse tube refrigerator (PTR), a Stirling refrigerator, or a Gifford-McMahon refrigerator. In the present embodiment, the mechanical refrigerator is a PTR 9. PTR 9 includes a first PTR stage 10 thermally coupled to the first thermal stage 1 and a second PTR stage 11 thermally coupled to a cooling plate in the form of a second thermal stage 2. As an alternative to a two-stage PTR 9 (as shown in the figure), the mechanical refrigerator may optionally have only one cooling stage or more than two cooling stages. In the present embodiment, the second PTR stage 11 forms the lowest temperature stage of the PTR 9. As Figure 1 As is the case in FIG. 2 , the sorption pump 22 is typically thermally connected to the lowest temperature stage of the PTR.

[0029] The third thermal stage 3, the fourth thermal stage 4 and the fifth thermal stage 5 are thermally coupled to the dilution unit 12. The third thermal stage 3 is thermally coupled to a distiller 13 forming part of the dilution unit 12. The fifth thermal stage 5 is thermally coupled to a mixing chamber 14 of the dilution unit 12. Cooling of the third thermal stage 3, the fourth thermal stage 4 and the fifth thermal stage 5 is achieved by operation of the dilution unit, wherein an operating fluid circulates around a cooling circuit 24 to reduce the temperature. The operating fluid is typically a mixture of helium-3 and helium-4. The operating fluid is pumped around the cooling circuit 24 including a condensation line 25 and a distillation pumping line 26 using a compressor pump 27 and a turbomolecular pump 28. The operating fluid may be stored in a first storage container 29, and the operating fluid is supplied to the cooling circuit 24 using a supply line 30.

[0030] In use, each of the five thermal stages 1 to 5 is configured to reach a different operating base temperature. Thermal radiation shields can be attached to thermal stages 1 to 5, each of which surrounds a corresponding one of the remaining lower base temperature components to reduce any unnecessary thermal connectivity between thermal stages 1 to 5. This allows the thermal stages to reach different operating base temperatures to represent the lowest temperature obtainable by a given component during steady-state operation of the system. In the present embodiment, the first thermal stage 1 is configured to reach an operating base temperature of about 50 Kelvin to 70 Kelvin and is mounted to the first thermal radiation shield 15. The operating base temperature of the second thermal stage 2 is about 3 Kelvin to 5 Kelvin. The second thermal radiation shield 16 is mounted to the second thermal stage 2. The operating base temperature of the third thermal stage 3 is typically 0.5 Kelvin to 2 Kelvin. The third thermal radiation shield 17 is mounted to the third thermal stage 3. The operating base temperature of the fifth thermal stage 5 is typically 3 milliKelvin to 30 milliKelvin. The fourth thermal stage 4 forms an intermediate stage between the third thermal stage 3 and the fifth thermal stage 5 and has an operating base temperature of about 50 millikelvin to 200 millikelvin.

[0031] Cryogenic cooling systems such as these can be used to perform measurements at low temperatures. Typically, the sample 35 is mounted to the coldest thermal stage of the system, in this case the fifth thermal stage 5. The dilution unit 12 is used to obtain a millikelvin temperature at the fifth thermal stage 5. As part of normal operation, it will be understood that the resistive heater forms part of the distiller and the mixing chamber. In this embodiment, the resistive heater of the mixing chamber 14 provides a target heater 36 that is thermally coupled to the fifth thermal stage 5 and can be used to heat the sample 35. This advantageously facilitates the controlled measurement of the sample 35 with respect to temperature. As described later, the target heater 36 can be a resistive heater and can be used to increase the temperature of the fifth thermal stage 5 from the operating base temperature to about 30 Kelvin. In an alternative embodiment, the temperature of the fifth thermal stage can be increased to more than 100 Kelvin by using an additional target heater that is thermally coupled to the fifth thermal stage and can be separated from the mixing chamber. As described later, in the high temperature operating mode of the cryogenic cooling system, the temperature of the fifth thermal stage 5 can be controllably increased and then can be cooled back to the operating base temperature of the fifth thermal stage 5.

[0032] The fifth thermal stage 5, the mixing chamber 14, the sample 35 and the target heater 36 form a target assembly 42. Therefore, the fifth thermal stage 5 is also referred to as a "target plate" herein. The mixing chamber 14 forms a "target refrigerator" that is configured to cool the target assembly 42 to a temperature lower than that of the second PTR stage 11. The thermal switch assembly 18 provides a selectively connectable thermal connection between the second thermal stage 2 and the target assembly 42. The thermal switch assembly 18 is formed by three air gap thermal switches 19 to 21. The first air gap thermal switch 19 has an upper end connected to the second thermal stage 2 and a lower end connected to the third thermal stage 3. The second air gap thermal switch 20 has an upper end connected to the third thermal stage 3 and a lower end connected to the fourth thermal stage 4. The third air gap thermal switch 21 has an upper end connected to the fourth thermal stage 4 and a lower end connected to the fifth thermal stage 5.

[0033] Each air gap thermal switch consists of two conductors separated from each other in a low conductivity chamber into which a conductive gas can be introduced. The chamber is an annular housing formed of stainless steel, and the conductive gas used is helium. When the gas is removed from the chamber, the switch opens and the thermal stages are thermally isolated. In practice, the air gap thermal switch has a thermal conductivity equal to that of the chamber in the open state, which is approximately 0.01 Wcm at 10 Kelvin. -1 K -1 When conductive gas is present within the air gap thermal switch, the switch is closed and the air gap thermal switch is thermally coupled to a respective thermal stage connected at each end of the air gap thermal switch. The thermal conductivity of the closed switch is at least two orders of magnitude higher than the thermal conductivity of the open switch.

[0034] The operation of the switch between opening and closing is usually achieved using an adsorption pump and is temperature dependent. The adsorption pump includes an adsorbent material (usually activated carbon or molecular sieve). When the temperature of the adsorption pump 22 is reduced to below the threshold transition temperature (in this case, about 20 Kelvin), the adsorption pump 22 adsorbs gas molecules from the switch, thereby opening the switch. On the contrary, when the temperature of the adsorption pump 22 is increased to above the transition temperature, the adsorption pump 22 desorbs these gas molecules, thereby reintroducing the gas molecules into the chamber and closing the switch. In the present embodiment, the adsorption pump 22 is equipped with a temperature sensor (not shown) for monitoring purposes.

[0035] The adsorption pump 22 is fluidly connected to each of the air gap thermal switches 19 to 21 by a conduit referred to herein as a first connecting member 45. In addition to facilitating gas flow, the first connecting member 45 inevitably provides a thermally conductive path between the thermal switches 19 to 21 and the adsorption pump 22. Another part of the adsorption pump 22 is thermally connected to the second thermal stage 2 by a second connecting member 46, and the second connecting member 46 provides a mechanical connection between the adsorption pump 22 and the second thermal stage 2. The second connecting member 46 can, for example, form a copper support for the adsorption pump. In another embodiment, the adsorption pump can be mounted to the second thermal stage using a material with very low thermal conductivity such as stainless steel. In this case, the thermal connection between the adsorption pump and the second thermal stage will be supplemented by a second connecting member with relatively high conductivity (such as one or more copper wires extending from the adsorption pump to the second thermal stage).

[0036] The first connection member 45 and the second connection member 46 together form a thermal link between the thermal switch assembly 18 and the second thermal stage 2, along which the sorption pump 22 is located. As described later, the thermal conduction path provided by the second connection member 46 is particularly advantageous because the second connection member 46 thermally couples the high cooling power of the second PTR stage 11 with the sorption pump 22, so that any unwanted heat introduced via the first connection member 45 can be removed from the sorption pump 22. With this arrangement, the temperature of the target assembly 42 can be raised to above the transition temperature of the sorption pump 22 without shutting down the thermal switch assembly 18.

[0037] A local heat source in the form of an adsorption heater 31 is thermally coupled to the adsorption pump 22, the operation of which causes the gas to desorb from the adsorption pump 22. The desorption of the gas closes the air gap thermal switches 19 to 21 and thermally couples the second thermal stage 2 to the fifth thermal stage 5. In the present embodiment, the adsorption heater 31 is a resistive heater.

[0038] The second connection member 46 provides a weak thermal connection between the sorption pump 22 and the second thermal stage 2. The thermal conductivity of the second connection member 46 must be low enough to ensure effective heating of the sorption pump 22 by the sorption heater 31. In addition, the thermal conductivity of the second connection member 46 must be high enough to ensure that the cooling of the sorption pump 22 by the PTR 9 is achieved within a reasonable time frame. For example, it takes less than 2 hours, preferably less than 30 minutes, to cool the sorption pump from 30 Kelvin to 5 Kelvin.

[0039] The second storage container 32 is configured to supply a thermally conductive gas to each of the air gap thermal switches 19 to 21 via a gas supply line 33. The supply is controlled by operating a gas pump 34 or under a pressure applied by the second storage container 32. The choice of thermally conductive gas affects the transition temperature of the air gap thermal switch. In this example, helium-4 gas is used, and the transition temperature is typically between 4 Kelvin and 30 Kelvin, or preferably between 10 Kelvin and 20 Kelvin. In other embodiments, helium-3, hydrogen, or neon may be used.

[0040] The control system 37 can be used to control Figure 1 The control system 37 controls each part of the system, including the operation of the PTR 9, dilution unit 12, heaters 31 and 36, pumps 27, 28, 34 and associated valves, monitoring of sensors and operation of other auxiliary equipment, to carry out the desired procedure. This control is achieved using a suitable computer system, although manual control is also contemplated.

[0041] Figure 2 is a schematic diagram of a cryogenic cooling system according to a second embodiment. Primer-marked reference numerals are used to indicate similar device features. The system includes a plurality of thermal stages 1' to 4' arranged in a manner similar to the thermal stages of the first embodiment. A thermal switch assembly 18' is formed by two air gap thermal switches 19' and 20' connected to a second thermal stage 2', a third thermal stage 3', and a fourth thermal stage 4'. In this embodiment, the fourth thermal stage 4' can be considered a "target board" of a target assembly 42', and the thermal switch assembly 18' can be used to thermally connect the fourth thermal stage 4' to the second thermal stage 2'. A target heater 36' is mounted to the fourth thermal stage 4'.

[0042] In the present embodiment, the thermal switch assembly 18' is pre-filled with helium-4 gas. The chamber connecting the thermal switch assembly 18' and the adsorption pump 22' is a closed system, so in the absence of any leakage, no gas filling pipeline is required. In other embodiments, multiple adsorption pumps can be provided, and one or more of the multiple adsorption pumps can be thermally coupled to the second thermal stage 2' using a connecting member.

[0043] The cooling of the third thermal stage 3' and the fourth thermal stage 4' is achieved using a helium refrigerator including a first helium reservoir 40' and a second helium reservoir 41'. The first helium reservoir 40', which is typically at an operating base temperature of about 1.3 Kelvin, is thermally coupled to the third thermal stage 3' and is configured to contain liquid helium-4. The first helium reservoir 40' is also commonly referred to as a "1 Kelvin tank". The second helium reservoir 41' is thermally coupled to the fourth thermal stage 4'. The second helium reservoir 41', which is typically at an operating base temperature of about 0.3 Kelvin, forms part of the "target refrigerator" in this embodiment and is configured to contain liquid helium-3. The first helium reservoir 40' and the second helium reservoir 41' are respectively coupled to the first cooling circuit 38' and the second cooling circuit 39' of pumped helium. The fluid can be stored in and supplied from external storages 29' and 29".

[0044] Although Figure 1 and Figure 2 The embodiments described use a cryogenic refrigerator 9, 9' without refrigerant, but alternative embodiments may instead involve a "wet" system incorporating a reservoir of liquid refrigerant to apply cooling to the hot stage of the sorption pump heat link.

[0045] Figure 3 1 is a graph showing the temperature variation over time during a first high temperature experiment performed on a low temperature cooling system according to a first embodiment of the present invention. The graph shows the temperature variation of the fifth thermal stage 5, the second thermal stage 2 and the second PTR stage 11 during the high temperature experiment. Figure 1 The components of the system are described for the operation of the cryogenic cooling system during high temperature experiments.

[0046] See also Figure 3 , between the elapsed time of 0 seconds to 6000 seconds, the system is in steady state operation with each component at its operating base temperature. During this period, the air gap thermal switches 19 to 21 are open, so the second thermal stage 2 is thermally isolated from the fifth thermal stage 5. From the similarity of the temperature distribution between the second PTR stage 11 and the second thermal stage 2 over the entire measurement period of 0 seconds to 20000 seconds, it can be seen that the second PTR stage 11 is thermally coupled to the second thermal stage 2. The fifth thermal stage 5 is thermally coupled to the mixing chamber 14, and the operating fluid circulates around the cooling circuit 24 of the dilution unit 12 to provide cooling for the fifth thermal stage 5. Although the temperature of the fifth thermal stage 5 is shown as approximately 1.5 Kelvin between the elapsed time of 0 seconds to 6000 seconds, this is only because the actual temperature is lower than the lowest temperature that can be reliably recorded by the temperature sensor. In fact, the temperature of the fifth thermal stage 5 will be its operating base temperature, which is approximately 3 milliKelvin to 30 milliKelvin.

[0047] The low temperature cooling system switches from steady state operation to high temperature operation in an elapsed time of about 6000 seconds. In particular, the control system 37 is used to operate the target heater 36 to increase the temperature of the fifth thermal stage 5 to 15 Kelvin. During the high temperature operation, the operating fluid circulating around the cooling circuit 24 during the steady state base temperature operation is collected and stored in the first storage container 29. It is worth noting that although the operation of the target heater 36 causes the temperature of the fifth thermal stage 5 to rise sharply, the temperature of the second thermal stage 5 and the second PTR stage 11 is basically not disturbed by such heating. This is because the adsorption pump 22 is maintained below the transition temperature by the cooling power of the second PTR stage 11, wherein the second PTR stage 11 is thermally connected to the adsorption pump 22 by the second connecting member 46. By maintaining the adsorption pump 22 below the transition temperature, the air gap thermal switches 19 to 21 are maintained in an open state.

[0048] At an elapsed time of approximately 7500 seconds, the temperature of the fifth thermal stage 5 was maintained at 15 Kelvin for approximately 1500 seconds. An advantage of the optional high temperature operation of the cryogenic cooling system is the ability to perform measurements over a wide temperature range, such as measurements on the sample 35. The cryogenic cooling system of the first embodiment enables the operator to increase the temperature of the target component 42 from milliKelvin temperatures to tens of Kelvin in a controllable manner while maintaining operation of the cryogenic refrigerator. Prior art systems generally do not cover this range, and therefore the cryogenic cooling system provides additional flexibility in performing measurements at low temperatures. Suitable experiments performed over a range of temperatures include transport measurements and pressure sensor experiments. Typically, these experiments can be performed based on changing the electric and magnetic fields while the temperature is controlled. In order to be able to perform repeatable experiments, the system must be configured to controllably adjust the sample temperature. As Figure 3 As illustrated, this may entail maintaining the temperature at a selected value for a period of time during which measurements of the sample 35 may be taken.

[0049] At an elapsed time of about 8800 seconds, the temperature of the fifth thermal stage 5 rises to 26 Kelvin, which remains approximately constant between 11100 seconds and 12500 seconds. Temperature control is achieved by the operation of the target heater 36. During this period, although the thermal switch assembly 18 is in the open state, some heat will inevitably be conducted to the adsorption pump 22 along the chambers of the air gap thermal switches 19 to 21 and the first connecting member 45. In the absence of the second connecting member 46, this heat may raise the temperature of the adsorption pump to the transition temperature (about 20 Kelvin) or above of the adsorption pump 22. Therefore, gas molecules will desorb from the adsorption pump 22, and the thermal switch assembly 18 will begin to transition to the closed state. The second connecting member 46 advantageously provides a thermal conduction path along which the "unnecessary" heat can be removed from the adsorption pump 22. This "unnecessary" heat removal is achieved by thermally connecting the adsorption pump 22 to the high cooling power provided by the second PTR stage 11. This prevents gas desorption and maintains the thermal switch assembly 18 in the open state as long as necessary.

[0050] To return the cryogenic cooling system to its operating base temperature, the adsorption heater 31 is operated at an elapsed time of about 12,500 seconds, causing the thermal switches 19 to 21 to close and thermally coupling the second thermal stage 2 with the third thermal stage 3, the fourth thermal stage 4, and the fifth thermal stage 5. The target heater 36 is also deactivated. The deactivation of the target heater is typically performed simultaneously with the operation of the adsorption heater. The initial heat load applied by the target assembly on these components results in an increase in the temperature of the second PTR stage 11 and the second thermal stage 2 between the time period of 12,500 seconds and 14,500 seconds.

[0051] As shown between the time period of 12500 seconds to 14500 seconds, turning off the thermal switch assembly causes the temperature of the fifth thermal stage 5 to decrease rapidly. Operation of the target heater is terminated at approximately 12500 seconds, thereby removing the heat input to the target assembly and reducing the cooling time. In this embodiment, the adsorption heater 31 is operated to maintain the thermal switch assembly 18 in the off state until the temperature of the target assembly decreases to the temperature of the second PTR stage 5. This occurs at approximately 14500 seconds. At this time, the adsorption pump 22 is allowed to cool below the transition temperature so as to thermally isolate the second thermal stage 2 from the fifth thermal stage 5.

[0052] Although the second connecting member 46 provides an important thermal connection between the adsorption pump 22 and the second thermal stage, the second connecting member 46 itself is formed to have a relatively low thermal conductivity through the selection of materials and geometry. For example, if a high conductivity material such as copper is used, a small area to length ratio is selected to reduce the thermal conductivity, and if a low conductivity material such as stainless steel or brass is used, a larger area to length ratio is selected. The second connecting member 46 can have a thermal conductivity of 8 mW / K at a temperature of about 4 Kelvin. This ensures that the temperature of the adsorption pump 22 can vary independently of the temperature of the second thermal stage 2, as occurs during operation of the adsorption heater 31. Despite the low thermal conductivity, the second PTR stage 11 will cool the adsorption pump 22 to below the transition temperature of the adsorption pump 22 in the absence of any warming effects from the adsorption heater 31. At a temperature of 4 Kelvin, the cooling power of the second PTR stage 11 is typically in excess of 1 Watt. Operation of the adsorption heater 31 will not typically increase the temperature of the second thermal stage 2 by more than 0.2 Kelvin. However, as Figure 3 As shown, when the thermal switch assembly 18 is transitioned to the closed state, the heat load from the fifth thermal stage 5 may cause the temperature of the second thermal stage 2 to further increase.

[0053] The temperature of the adsorption pump 22 is maintained above its nominal transition temperature until the second thermal stage 2 to the fifth thermal stage 5 cool to about 5 Kelvin. The control system 37 is used to monitor the temperature of each stage and automatically deactivate the adsorption heater 31 and reintroduce the operating fluid into the cooling circuit 24 at that stage. The operating fluid then circulates through the dilution unit 12 to continue cooling the third thermal stage 3, the fourth thermal stage 4, and the fifth thermal stage 5 to their respective operating base temperatures. After the adsorption heater 31 is deactivated, the adsorption pump 22 cools from 33 Kelvin to 5 Kelvin in about 0.5 hours.

[0054] Figure 4 is a graph showing the temperature variation over time during a second high temperature experiment performed on a cryogenic cooling system according to a first embodiment of the present invention. The operation of the system is basically as follows Figure 3 However, Figure 4 Further details of the temperatures of the fifth thermal stage 5, the sorption pump 22, the second thermal stage 2, the third thermal stage 3 and the fourth thermal stage 4 during this use are provided.

[0055] exist Figure 4In the example of FIG. 5 , the temperature of the fifth thermal stage 5 is gradually increased between 0 hours and 3 hours so that measurements can be obtained from the target component at each temperature interval. The size and timing of the temperature steps can be determined by the system user. In one embodiment, these values ​​can be pre-programmed and the high temperature experiment can be automatically run using a control system 37. The operation of the target heater 36 is controlled based on feedback data obtained from the temperature sensor on the fifth thermal stage 5 to achieve the desired temperature control of the fifth thermal stage 5. The second PTR stage 11 is thermally coupled to the second thermal stage 2 and provides continuous cooling throughout the method.

[0056] Shortly after the experiment began, the temperature of the fifth thermal stage was increased to 5 Kelvin and maintained for about 0.8 hours. Then, the temperature of the fifth thermal stage 5 was again increased to 10 Kelvin and maintained between 0.8 hours and 1.4 hours. Then, over a period of about 0.2 hours, the temperature of the fifth thermal stage 5 was increased to 20 Kelvin and maintained between 1.6 hours and 2.2 hours. Then, over a period of about 0.6 hours, the temperature of the fifth thermal stage 5 was increased to 30 Kelvin and maintained at this temperature for an additional 0.8 hours. Once again, although the temperature of the target component was increased to significantly above the transition temperature of the adsorption pump 22, the adsorption pump 22 was maintained below the transition temperature via the second connecting member 46 due to the cooling influence of the second PTR stage 11. This keeps the thermal switch assembly 18 in the open state, so that the operation of the target heater 36 does not affect the temperature of the second thermal stage 2. The temperatures of the third thermal stage 3 and the fourth thermal stage 4 did not change between the elapsed times of 0 hours and 3.5 hours. Figure 4 This is shown in , because the corresponding temperature sensors are not enabled (inactive) during this time. However, it is expected that the third thermal stage 3 and the fourth thermal stage 4 will remain in the range of 0.5 Kelvin to 2.5 Kelvin.

[0057] To start the cooling process after high temperature operation of the cryogenic cooling system, the control system 37 is used to increase the temperature of the sorption pump 22 to about 33 Kelvin by operating the sorption heater 31. This occurs at about 3.5 hours of elapsed time, when the temperature sensors of the third thermal stage 3 and the fourth thermal stage 4 are also enabled. The operation of the target heater 36 is terminated substantially simultaneously or later, thereby allowing the fifth thermal stage 5 to cool. In another embodiment, the target heater 36 is turned off before the temperature of the sorption pump 22 is increased.

[0058] The operation of the adsorption heater 31 and the resulting thermal connection between the target assembly and the remaining thermal stages rapidly increases the temperature of each of the second thermal stage 2, the third thermal stage 3, and the fourth thermal stage 4 to a maximum value of approximately 4.5 Kelvin, 14 Kelvin, and 18 Kelvin, respectively, at an elapsed time of approximately 3.6 hours. Thereafter, when the thermal switch assembly 18 is closed, the temperature of each of these thermal stages decreases under the cooling influence of the second PTR stage 11.

[0059] When the temperature of each of the third, fourth and fifth thermal stages 3, 4 and 5 reaches about 5 Kelvin, the adsorption heater 31 is turned off. Then, the adsorption pump 22 is cooled to below the transition temperature by the second PTR stage 11 to turn the thermal switch assembly 18 to the open state.

[0060] As the target refrigerator loop starts up and the initial condensate of the helium mixture returns to the dilution refrigerator, an increase in the temperature of the fifth thermal stage 5 can be observed at 4.4 hours. Once the mixture condenses, the temperature of the target refrigerator is subsequently cooled to below the temperature of the second PTR stage 11. This causes the temperature of the third thermal stage 3, the fourth thermal stage 4, and the fifth thermal stage 5, which are thermally coupled to the dilution unit 12, to decrease until the corresponding operating base temperatures are reached.

[0061] Figure 5 It shows Figure 4 A graph of the temperature distribution between 3.5 hours and 5.5 hours of elapsed time for the experiment shown. Figure 4 different, Figure 5 The y-axis is shown on a logarithmic temperature scale to more clearly show the temperatures of different thermal stages spanning several orders of magnitude. Figure 5 The temperature of each component of the system is shown to decrease as it returns to its respective operating base temperature.

[0062] In summary, it will be appreciated that an improved cryogenic cooling system is thus provided in which a sorption pump is thermally coupled to a cryogenic refrigerator. Since the operation of the air gap thermal switch is dependent upon the temperature of the sorption pump, the air gap thermal switch can be controllably opened and closed independently of any heat conducted from the thermal switch assembly to the sorption pump. Furthermore, a target assembly connected to one end of the thermal switch assembly can be operated at an elevated temperature without raising the temperature of the remaining thermal stages of the system.

Claims

1. A cryogenic cooling system, include: a cooling plate thermally coupled to the cryogenic refrigerator; a target assembly, wherein the target assembly includes a target refrigerator configured to obtain a base temperature lower than that of the cryogenic refrigerator; a thermal switch assembly comprising one or more air gap thermal switches, the thermal switch assembly having a first end thermally coupled to the cooling plate and a second end thermally coupled to the target assembly; and a sorption pump configured to control the thermal conductivity of the thermal switch assembly based on a temperature of the sorption pump, wherein the sorption pump is thermally coupled to the cryogenic refrigerator via a thermal link extending from the cooling plate to the thermal switch assembly, wherein the sorption pump is disposed at a location along the thermal link between the thermal switch assembly and the cooling plate; wherein the thermal link comprises a first connecting member and a second connecting member, wherein the first connecting member extends between the thermal switch assembly and the sorption pump, and wherein the second connecting member extends between the sorption pump and the cooling plate, the second connecting member having a thermal conductivity of between 1 milliwatt per Kelvin and 50 milliwatts per Kelvin at 4 Kelvin; and A sorption heater is configured to apply localized heating to the sorption pump.

2. The cryogenic cooling system according to claim 1, in, The sorption pump is configured to close one or more air gap thermal switches of the thermal switch assembly in response to a temperature of the sorption pump exceeding a nominal transition temperature, and wherein the sorption pump is thermally coupled to the cooling plate to cool the temperature of the sorption pump to below the nominal transition temperature during operation of the cryogenic refrigerator.

3. The cryogenic cooling system according to claim 2, in, The sorption pump is thermally coupled to the cooling plate such that the temperature of the sorption pump is maintained below the nominal transition temperature independent of the temperature of the target component.

4. The cryogenic cooling system according to claim 2, in, The nominal transition temperature is between 4 Kelvin and 30 Kelvin.

5. The cryogenic cooling system according to claim 4, in, The nominal transition temperature is between 15 Kelvin and 25 Kelvin.

6. The cryogenic cooling system according to claim 2, in, The target assembly includes a target plate thermally coupled to a target heater and the target refrigerator, wherein the sorption pump is thermally coupled to the cooling plate to maintain a temperature of the sorption pump below the nominal transition temperature during operation of the target heater.

7. The cryogenic cooling system according to claim 6, in, The operation of the target heater raises the temperature of the target plate above the nominal transition temperature.

8. The cryogenic cooling system according to any one of claims 1 to 4, in, The target assembly includes a target plate thermally coupled to a target heater and the target cooler.

9. The cryogenic cooling system according to any one of claims 1 to 7, in, The sorption pump is configured to open and close each of the air gap thermal switches of the thermal switch assembly.

10. The cryogenic cooling system according to any one of claims 1 to 7, in, The first connection member includes a conduit for conveying gas between the sorption pump and one or more air gap thermal switches of the thermal switch assembly.

11. The cryogenic cooling system according to any one of claims 1 to 7, in, The second connection member has a thermal conductivity of between 5 milliwatts per Kelvin and 10 milliwatts per Kelvin at 4 Kelvin.

12. The cryogenic cooling system according to any one of claims 1 to 7, further comprising one or more stages arranged between the cooling plate and the target component, in, Each stage is thermally coupled to one or more air gap thermal switches of the thermal switch assembly.

13. The cryogenic cooling system according to any one of claims 1 to 7, in, The cryogenic refrigerator is a mechanical refrigerator selected from the group consisting of a pulse tube refrigerator, a Stirling refrigerator and a Gifford-McMahon refrigerator.

14. The cryogenic cooling system according to any one of claims 1 to 7, in, The target refrigerator includes any one of a helium-3 refrigerator, a distiller or a mixing chamber of a dilution refrigerator, and a 1 Kelvin tank.

15. A method of operating a cryogenic cooling system, the system include: a cooling plate thermally coupled to the cryogenic refrigerator; a target assembly, wherein the target assembly includes a target refrigerator configured to obtain a base temperature lower than that of the cryogenic refrigerator; a thermal switch assembly comprising one or more air gap thermal switches, the thermal switch assembly having a first end thermally coupled to the cooling plate and a second end thermally coupled to the target assembly; and A sorption pump configured to control the thermal conductivity of the thermal switch assembly based on the temperature of the sorption pump, wherein the sorption pump is thermally coupled to the cryogenic refrigerator via a thermal link extending from the cooling plate to the thermal switch assembly, wherein the sorption pump is arranged at a position along the thermal link between the thermal switch assembly and the cooling plate, wherein the sorption pump is configured to thermally couple the cryogenic refrigerator to the target assembly in response to the temperature of the sorption pump exceeding a nominal transition temperature, the method comprising the steps of: a) increasing the temperature of the target component from a first temperature below the nominal transition temperature to a second temperature above the nominal transition temperature; wherein the sorption pump is thermally coupled to the cryogenic refrigerator using the thermal link so as to maintain the temperature of the sorption pump below the nominal transition temperature during step (a).

16. The method according to claim 15, in, The cryogenic cooling system further comprises an adsorption heater thermally coupled to the adsorption pump, and the method further comprises performing the following steps after step (a): b) operating the sorption heater to raise the temperature of the sorption pump above the nominal transition temperature to thermally couple the cryocooler to the target component.

17. The method according to claim 15 or 16, in, The target assembly further includes a target heater, and wherein step (a) is performed by operating the target heater.

18. The method according to claim 15 or 16, in, The first temperature is below 5 Kelvin.

19. The method according to claim 15 or 16, in, The second temperature is above 20 Kelvin.

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