Dilution refrigeration apparatus and method

Abstract

The invention relates to a dilution refrigeration device comprising a working circuit in the form of a loop containing a circulating fluid comprising a mixture of helium-3 (3He) and helium-4 (4He), the working circuit comprising a mixing chamber, an evaporator and a transport member, these components being arranged in series and fluidically connected via a set of lines, the circuit being configured to connect the outlet of the mixing chamber to the inlet of the evaporator and to connect the outlet of the evaporator to the inlet of the transport member, the circuit also being configured to connect the outlet of the transport member to the inlet of the mixing chamber, the device further comprising at least one cooling member in heat exchange with the working circuit and configured to deliver cold energy to the circulating fluid, the at least one cooling member comprising a cryogenic refrigerator of the Gifford-McMahon type or of the pulse tube type for example, the device comprising a support, a fluidically sealed enclosure (9).

Description

[0001] This invention relates to a dilution refrigeration apparatus and method.

[0002] More specifically, the present invention relates to a dilution refrigeration apparatus comprising a working loop in the form of a loop containing a circulating fluid comprising a mixture of helium-3 (3He) and helium-4 (4He), the working loop comprising a mixing chamber, an evaporator, and a transfer member, the components being arranged in series and fluidly connected via a set of pipelines, the loop being configured to connect the outlet of the mixing chamber to the inlet of the evaporator and the outlet of the evaporator to the inlet of the transfer member, the loop being further configured to connect the outlet of the transfer member to the inlet of the mixing chamber, the apparatus further comprising at least one cooling member that exchanges heat with the working loop and is configured to transfer cold energy to the circulating fluid, the at least one cooling member comprising, for example, a Gifford-McMahon type or a pulse tube type cryogenic refrigerator, the apparatus comprising a support and a fluid-sealed housing (9).

[0003] The present invention particularly relates to a cryogenic refrigeration apparatus and method for low or extremely low temperatures (meaning temperatures that can be reduced to the range of 1 mK to about 100 mK).

[0004] In fact, the conventional means of obtaining refrigeration power at temperatures ranging from about 1 mK to about 100 mK is a helium-3 / helium-4 dilution refrigerator.

[0005] Document FR 2914050 A1 describes a cooling system of a cryogenic refrigerator type used in a dilution refrigeration apparatus.

[0006] However, connecting a cryogenic refrigerator to a dilution refrigeration unit (thermally and mechanically) can transmit vibrations to the dilution refrigeration unit. These vibrations are harmful because they can lead to excessive power consumption or overheating, as well as parasitic noise on objects cooled by the dilution refrigeration unit.

[0007] In addition, due to the need for a large number of fluid loops, the known devices are relatively complex and bulky.

[0008] The purpose of this invention is to overcome all or some of the defects in the prior art.

[0009] Therefore, the essential features of the device according to the invention, which in other respects also conforms to its general definition given in the preceding preamble, are that the open end of the housing is mechanically connected to the support via a fluid-sealed flexible bellows, the upper end of the cryogenic cooler is fixed to the support, the device further includes a fluid-sealed sheath housed within the housing, the lower end of the cryogenic cooler extending within the sheath and inside the bellows, such that the housing and sheath are at least partially mechanically isolated from the vibrations generated by the cryogenic cooler, the operating circuit includes a fluid injection line that connects the outlet of the transmission member to the inlet of the mixing chamber via a fluid-sealed passage in the sheath, the injection line being mechanically disengaged from the cryogenic cooler and the support, that is, the injection line being at least partially mechanically isolated from the vibrations generated by the cryogenic cooler, and the operating circuit includes a set of return lines that connect the outlet of the evaporator to the inlet of the transmission member outside the housing via a circulating gas passage in the sheath, the circulating fluid flowing into the passage and circulating within the volume of the sheath in contact with the lower end of the cryogenic cooler.

[0010] Therefore, the device according to the invention operates together with a cryogenic refrigerator without subjecting the cold dilution portion to vibration from the cryogenic refrigerator or making it overly sensitive to vibration.

[0011] Furthermore, the system is compact and can use only the same single fluid as both the working fluid and the atmosphere in the jacket.

[0012] Furthermore, embodiments of the present invention may include one or more of the following features: - The jet line exchanges heat with the circulating fluid that circulates in the jacket between the cryogenic refrigerator and the outlet of the evaporator and the inlet of the transfer component. This set of return lines includes a first return line and a second return line. The first return line extends into the lower end of the sheath to inject recirculated gas into it, and the lower end of the second return line connects to the upper end of the sheath to collect the recirculated gas. The lower end of the cryogenic refrigerator includes at least one heat exchanger for cooling the circulating gas. A first-section return line extends below the heat exchanger to direct the circulating fluid injected into the jacket into contact with the at least one heat exchanger. - The at least one heat exchanger includes one or more passages, such as orifices and / or fins, within its thickness, the one or more passages being oriented vertically to accommodate at least a portion of the circulating gas flow exiting via a first-section return line. - This set of return lines is also mechanically disconnected from the cryogenic refrigerator and its support components, meaning that this set of return lines is at least partially mechanically isolated from the vibrations generated by the cryogenic refrigerator. - The injection line is fixed to the inner wall of the sheath and / or suspended within the sheath. - The mixing chamber and evaporator are installed in a shell and / or container located within the outer casing, while the transfer components are located outside the outer casing. In the operating configuration, the lower end of the cryogenic refrigerator includes a first portion and a second portion. The first portion is cooled to a first temperature between 4 K and 100 K, for example, equal to 50 K, and the second portion is cooled to a second temperature between 2 K and 8 K, for example, equal to 4 K. - The at least one cooling component includes another cooling system located upstream of the evaporator inlet, such as a Joule-Thomson type cooler. - The support includes a first base, which is mounted on a first set of outriggers, for example, three outriggers. - The support includes a second base located below the first base and mounted on a second set of legs, such as three legs, with the two ends of the bellows (10) connected to the first base and the second base, respectively. The outer casing is mechanically connected to the second base.

[0013] The present invention also relates to a dilution refrigeration method using an apparatus according to any of the features described above or below, the method comprising: a step of refrigerating by a cryogenic refrigerator; a step of circulating a circulating fluid in a working circuit, the method further comprising at least one of the following steps: adjusting the amount of gas in the jacket, adjusting the pressure in the jacket to a predetermined value, for example close to atmospheric pressure, particularly between 0 bar and 2 bar, and adjusting the pressure in the jacket to a pressure corresponding to the liquefaction pressure of helium-4 at the lowest temperature of the cryogenic refrigerator.

[0014] The present invention may also relate to any alternative apparatus or method that includes any combination of the features described above or below within the scope of the claims.

[0015] Further specific features and advantages will become apparent from the following description with reference to the accompanying drawings, in which: [ Figure 1 A schematic partial view illustrating a possible example of the structure and operation of a device according to the invention is shown. [ Figure 2 A simplified schematic partial view is shown, illustrating another example of a support structure used for such a device.

[0016] [ Figure 3 A simplified schematic partial top view of an example of the structure of the heat exchanger of the demonstration device is shown.

[0017] [ Figure 1The dilution refrigeration device 1 shown includes a working loop 2 in the form of a loop containing a circulating fluid. This circulating fluid includes a mixture of helium-3 (3He) and helium-4 (4He).

[0018] This working loop 2 forms, for example, a closed loop and includes a mixing chamber 3, an evaporator 5, and a transfer component 6 (e.g., a pump or other device), which are arranged in series and fluidly connected via a set of lines 12, 13, 130. This set of lines in the working loop 2 is specifically configured to connect the outlet of the mixing chamber 3 to the inlet of the evaporator 5 and the outlet of the evaporator 5 to the inlet of the transfer component 6.

[0019] Furthermore, this set of pipelines in loop 2 is configured to connect the outlet of the transmission component 6 to the inlet of the mixing chamber 3. Typically, the heat exchange section (not shown for simplicity) can be located between the countercurrent pipelines between the mixing chamber 3 and the evaporator 5.

[0020] The device 1 further includes at least one cooling component that exchanges heat with the working circuit 2, and the at least one cooling component is configured to transfer cold energy to the circulating fluid (that is, to cool the circulating fluid by supplying cooling power to it).

[0021] This dilution system allows for the generation of very low temperatures in mixing chamber 3. This can typically be used for cooling applications (schematically shown with reference numeral 22 in the attached figure), and its power dissipation can vary over time.

[0022] In particular, it can achieve temperatures in the range between 1 mK and 100 mK.

[0023] The cooling components include, for example, Gifford-McMahon type or pulse tube type cryogenic refrigerators 4 (but any other suitable cryogenic unit can be envisioned).

[0024] The device 1 includes support members 7 and 8 and a fluid-sealed housing 9.

[0025] The housing 9 includes, for example, a fluid-sealed canister (typically made of stainless steel, aluminum, or any other suitable material) whose open end is mechanically and fluid-sealed to the supports 7, 8 via a fluid-sealed flexible bellows 10. For example, the flexible element is formed by corrugating hydroformed stainless steel. Of course, any other type of vibration damping and / or filtering connection can replace or complement the aforementioned bellows 10, such as edge-welded bellows or elastomeric sleeves. For example, the supports include a base 8 that is horizontal in the use position and mounted on a first set of legs 7, for example, three legs 7.

[0026] The cryogenic refrigerator 4 is fixed to the support members 7 and 8 by means of threaded connection, for example.

[0027] As shown, the upper end of the cryogenic cooler 4 can be mounted on the base 8 of the support (especially through the base 8 in a fluid-tight manner).

[0028] The device 1 further includes a fluid-tight sheath 90, which is housed within the housing 9 and also forms a fluid-tight closed volume.

[0029] For example, the internal volume of the outer casing 9 is under vacuum (under pressure lower than the external pressure).

[0030] For example, the sheath 90 comprises, or is constituted by, a fluid-sealed metal canister or shell. The sheath 90 is, for example, fixed to the upper end of the housing 9. For example, the open upper end of the sheath 90 is suspended or connected to the upper end of the housing 9, if necessary, via a connector or vibration damping element. The lower end of the cryogenic refrigerator 4 extends inside the bellows 10, within the sheath 90 (and thus within the housing 9 that houses these components).

[0031] According to this arrangement, the outer casing 9 and the sheath 90 (and the components contained therein) are at least partially (especially through the bellows 10) mechanically isolated from the vibrations generated by the cryogenic refrigerator 4.

[0032] The mixing chamber 3 and the evaporator 5 can be housed / installed in a fluid-sealed shell 900 located in the housing 9, and in particular in a container 300 housed in this shell 900.

[0033] The transmission component 6 is located outside the housing 9. For example, the volume formed by the volume of the housing 900 may or may not be in communication with the volume of the sheath 90.

[0034] Preferably, the volume of the outer shell 9, the volume of the shell 900, and the volume of the container 300 are under vacuum and are in communication with each other but not with the volume of the sheath 90, the volume of the sheath being in an independent atmosphere (the volume of the sheath 90 includes at least one of the following: helium 3, helium 4).

[0035] The working circuit 2 includes a circulating fluid jet line 12 (which may be, for example, a cupronickel capillary or a copper capillary) that connects the outlet of the transmission member 6 to the inlet of the mixing chamber 3 via a (fluid-tight) passage in the sheath 90 and the housing 9. The jet line 12 exchanges heat with a cryogenic cooler 4 (which cools the jet line). For example, the cryogenic cooler 4 includes a plate and / or a cooling exchanger in contact with the cold portion of the cryogenic cooler 4.

[0036] For example, in an operating configuration, the lower end of the cryogenic cooler 4 in the sheath 90 includes a first portion (first platform) and a second portion (second platform), the first portion being cooled to a first temperature, for example, between 4 K and 100 K (e.g., equal to 50 K), and the second portion being cooled to a second temperature, between 2 K and 8 K (e.g., equal to 4 K).

[0037] Furthermore, the injection line 12 is mechanically disconnected from the cryogenic refrigerator 4 and the supports 7 and 8, meaning that the injection line 12 is at least partially mechanically isolated from the vibrations generated by the cryogenic refrigerator 4.

[0038] Therefore, the injection line 12 is not directly mechanically connected or fixed to the cryogenic refrigerator 4 or any components associated with it, thus avoiding damping from the latter's vibrations. For example, the injection line 12 is fixed to the housing 9 without contacting the supports 7, 8. Alternatively or cumulatively, the injection line 12 is fixed to components within the sheath 90, which are mechanically isolated from the vibrations of the cryogenic refrigerator 4. For example, the injection line 12 passes through the support 8 via an insulated passage having one or more vibration isolation systems (seals, dampers, etc.) without touching the support and / or the housing 9 and / or the sheath 90. Within the sheath 90, the injection line 12 may be fixed (e.g., by welding and / or gluing) to the inner wall of the sheath 90 and / or may be suspended within the sheath 90. In particular, the injection line 12 does not make mechanical contact at the height of the plates (multiple cold platforms or exchangers) of the cryogenic refrigerator 4.

[0039] The injection line 12 is thermally connected to the cryogenic refrigerator 4 via gas present in the housing 90 that houses the cryogenic refrigerator 4. The resulting continuous heat exchanger allows for effective pre-cooling of the mixture constituting the circulating gas while avoiding the transmission of vibrations to the lower platform of the device 1.

[0040] As shown, at least one circulating gas cooling component may further include another cooling system located upstream of the inlet of the evaporator 5, such as a Joule-Thomson type cooler 50 (or any other suitable system, such as an I-Kpot).

[0041] The working loop 2 further includes a set of return lines 13, 130, which connect the outlet of the evaporator 5 to the inlet of the transfer member 6 outside the cold box via a circulating gas passage in the sheath 90, into which circulating fluid flows and circulates within the volume of the sheath 90 that is in direct contact with the lower end of the cryogenic refrigerator 4.

[0042] For example, this set of return lines includes a first return line 13 and a second return line 130. The first return line extends to the lower end of the sheath 90 to inject recirculated gas into it, and the lower end of the second return line communicates with the upper end of the sheath 90 to collect the recirculated gas.

[0043] The return lines 13 and 130 are also mechanically disconnected from the cryogenic refrigerator 4 and the supports 7 and 8, meaning that the return lines are at least partially mechanically isolated from the vibrations generated by the cryogenic refrigerator 4.

[0044] Therefore, the circulating fluid (a mixture of H3-He4 gases) circulates from bottom to top and directly exchanges heat with the injection line 2 and the heat exchangers 16, 17 of the cryogenic refrigerator 4. The heat exchangers 16, 17 are preferably thermally induced (i.e., cooled) on an intermediate platform between 4 K and 50 K, thereby restoring the cold energy of the working gas returning from the evaporator 5 and optimizing the pre-cooling of the working fluid circulating in the opposite direction from top to bottom. Therefore, the atmosphere surrounding the heat exchangers 16, 17 in the jacket 90 is not composed of static gas. This facilitates heat exchange. This allows for the use of less gas and makes it easier to lower temperatures (below 4 K, for example, equal to 2.8 K). This is achieved by maintaining a gaseous atmosphere in the volume of the jacket 90 surrounding the cold portion of the cryogenic refrigerator 4.

[0045] Alternatively, the gas from this atmosphere can be liquefied and can form a bath at the bottom of the sheath 90.

[0046] This structure helps to reduce or eliminate mechanical contact with the cold part of the cryogenic refrigerator 4, and therefore does not transmit vibration.

[0047] In addition, the same single type of gas can be used as the working gas and this gaseous atmosphere in the jacket 90.

[0048] The lower end of the cryogenic refrigerator 4 may include at least one heat exchanger 17 for cooling circulating gas, particularly circulating gas in the atmosphere within the volume of the jacket 90. A first section of the return line 13 may advantageously extend below the heat exchanger 17 so that the rising flow of circulating fluid is directed to contact this heat exchanger 17.

[0049] This heat exchanger 17 may include a structure provided with multiple passages, such as orifices 170 and / or fins, these passages being oriented vertically to accommodate at least a portion of the circulating gas flow exiting via the first return line 13 (see […]). Figure 3 This improves the efficiency of heat exchange between different parts (especially the fluid).

[0050] For example, device 1 can be configured to maintain the pressure in jacket 90 at a predetermined value, such as close to atmospheric pressure, particularly between 0 bar and 2 bar. For example, the target pressure is the liquefaction pressure of helium at the lowest temperature (e.g., 5 K, 4 K, 2.5 K, etc.) of cryogenic refrigerator 4.

[0051] The support may include a first base 8, which is horizontally mounted on a first set of legs 7, such as three legs (a balanced system).

[0052] like[ Figure 2 As schematically shown in the diagram, the support may include a second base 20 located below the first base 8 and mounted on a second set of legs 21, such as three legs. The two ends of the bellows 10 may be connected to the first base 8 and the second base 20, respectively. The second base 20 is connected to the first base 8, for example, via the bellows 10, and the housing 9 is mechanically connected to the second base 20, for example.

[0053] This structure with a dual-balance frame allows for better decoupling of reaction forces and vibrations in device 1.

[0054] Groups 7, 8 and 20, 21 can be fixed to the same reference surface or to two corresponding different reference surfaces.

[0055] The bellows 10 may also be provided with columns or supports in parallel (e.g., three of them) to avoid any potential damage to the bellows 10 and other components during transport of the device 1 and during installation, maintenance, and operation of the device. The horizontal clearance (in the XY plane) between each column and the corresponding flange can be used to verify that the installation is correctly aligned.

[0056] The vertical clearance (on the Z-axis) can be eliminated or employed (e.g., in "transport" mode or "operation" mode). In "operation" mode, the vertical clearance allows for the release / optimization of the operation of the bellows 10 while maintaining the safety function of preventing the bellows 10 from being crushed in the event of failure.

[0057] This device has other advantages over existing technologies. For example, it improves the ease of maintenance. Specifically, by disassembling (screws or other) at 300 K from the cryogenic cooler to the other cold elements of the device 1, it is sufficient to perform work on the cryogenic cooler 4 on the support.

[0058] In addition to or in place of an external pump, the device may include a pump or cryogenic compressor for circulating the working fluid flow, for example at a first cold plateau in the loop (e.g., at a temperature of 77 K, 50 K, or 4 K), in order to increase the pressure in the working loop 2 and maintain a more efficient heat exchange mechanism.

Claims

1. A dilution refrigeration apparatus comprising a working circuit (2) in the form of a loop containing a circulating fluid comprising a mixture of helium-3 (3He) and helium-4 (4He), the working circuit (2) comprising a mixing chamber (3), an evaporator (5), and a transfer member (6), the components being arranged in series and fluidly connected via a set of pipelines, the circuit (2) being configured to connect the outlet of the mixing chamber (3) to the inlet of the evaporator (5) and the outlet of the evaporator (5) to the inlet of the transfer member (6), the circuit (2) being further configured to... The outlet of the transfer member (6) is connected to the inlet of the mixing chamber (3). The device (1) further includes at least one cooling member (4, 50), which exchanges heat with the working circuit (2) and is configured to transfer cold energy to the circulating fluid. The at least one cooling member (4, 50) includes a cryogenic refrigerator (4). The device (1) includes a support member (7, 8) and a fluid-sealed housing (9). The open end of the housing (9) is mechanically connected to the support member (7, 8) via a fluid-sealed flexible bellows (10). The upper end of the cryogenic refrigerator (4) is fixed. To the support members (7, 8), the device (1) further includes a fluid-sealed sheath (90) housed in the housing (9), the lower end of the cryogenic refrigerator (4) extending within the sheath (90) and inside the bellows (10), such that the housing (9) and the sheath (90) are at least partially mechanically isolated from the vibrations generated by the cryogenic refrigerator (4), the working circuit (2) includes a fluid injection line (12) that connects the outlet of the transmission member (6) to the inlet of the mixing chamber (3) via a fluid-sealed passage in the sheath (90), the injection line ( 12) The cryogenic cooler (4) and the support are mechanically disconnected, that is, the injection line (12) is at least partially mechanically isolated from the vibrations generated by the cryogenic cooler (4), and the working circuit (2) includes a set of return lines (13, 130) that connect the outlet of the evaporator (5) to the inlet of the transmission member (6) outside the housing (9) via a circulating gas passage in the sheath (90), in which the circulating fluid flows into the passage and circulates within the volume of the sheath (90) in contact with the lower end of the cryogenic cooler (4).

2. The apparatus as claimed in claim 1, characterized in that, The injection line (12) exchanges heat with the circulating fluid circulating in the sheath (90) between the outlet of the cryogenic refrigerator (4) and the inlet of the transfer member (6).

3. The apparatus as described in claim 1, characterized in that, The set of return lines (13, 130) includes a first return line (13) and a second return line (130). The first return line extends to the lower end of the sheath (90) to inject the recirculated gas into it, and the lower end of the second return line communicates with the upper end of the sheath (90) to collect the recirculated gas.

4. The apparatus as claimed in claim 2, characterized in that, The set of return lines (13, 130) includes a first return line (13) and a second return line (130). The first return line extends to the lower end of the sheath (90) to inject the recirculated gas into it, and the lower end of the second return line communicates with the upper end of the sheath (90) to collect the recirculated gas.

5. The apparatus as described in claim 3, characterized in that, The lower end of the cryogenic refrigerator (4) includes at least one heat exchanger (17) for cooling circulating gas, and the first part of the return line (13) leads to the lower part of the heat exchanger (17) to direct the circulating fluid flow injected into the sheath (90) to contact the at least one heat exchanger (17).

6. The apparatus as claimed in claim 4, characterized in that, The lower end of the cryogenic refrigerator (4) includes at least one heat exchanger (17) for cooling circulating gas, and the first part of the return line (13) leads to the lower part of the heat exchanger (17) to direct the circulating fluid flow injected into the sheath (90) to contact the at least one heat exchanger (17).

7. The apparatus as claimed in claim 5, characterized in that, The at least one heat exchanger (17) includes one or more passages (170) within its thickness, the one or more passages being oriented vertically to accommodate at least a portion of the circulating gas flow emerging via the first section return line (13).

8. The apparatus of claim 7, wherein the one or more passages (170) are orifices and / or fins.

9. The apparatus as claimed in claim 6, characterized in that, The at least one heat exchanger (17) includes one or more passages (170) within its thickness, the one or more passages being oriented vertically to accommodate at least a portion of the circulating gas flow emerging via the first section return line (13).

10. The apparatus of claim 9, wherein the one or more passages (170) are orifices and / or fins.

11. The apparatus of any one of claims 1 to 10, wherein the cryogenic cooler (4) is of the Gifford-McMahon type or the pulse tube type.

12. The apparatus according to any one of claims 1 to 10, characterized in that, The return lines (13, 130) are mechanically disconnected from the cryogenic refrigerator (4) and the support (7, 8), meaning that the return lines are at least partially mechanically isolated from the vibrations generated by the cryogenic refrigerator (4).

13. The apparatus according to any one of claims 1 to 10, characterized in that, The injection line (12) is fixed to the inner wall of the sheath (90) and / or suspended in the sheath (90).

14. The apparatus according to any one of claims 1 to 10, characterized in that, The mixing chamber (3) and the evaporator (5) are installed in a shell (900) and / or container (300) located in the housing (9), and the transfer member (6) is located outside the housing (9).

15. The apparatus according to any one of claims 1 to 10, characterized in that, In the operating configuration, the lower end of the cryogenic cooler (4) includes a first part and a second part, the first part being cooled to a first temperature between 4 K and 100 K, and the second part being cooled to a second temperature between 2 K and 8 K.

16. The apparatus as claimed in claim 15, characterized in that, The first temperature is equal to 50 K.

17. The apparatus as claimed in claim 15, characterized in that, The second temperature is equal to 4 K.

18. The apparatus according to any one of claims 1 to 10, 16, and 17, characterized in that, The at least one cooling component includes another cooling system located upstream of the inlet of the evaporator (5).

19. The apparatus as claimed in claim 18, characterized in that... The other cooling system is a Joule-Thomson type cooler (50).

20. The apparatus according to any one of claims 1 to 10, 16, 17, and 19, characterized in that, The support includes a first base (8) which is mounted on a first set of legs (7).

21. The apparatus as claimed in claim 20, characterized in that, The first set of outriggers (7) consists of three outriggers.

22. The apparatus as claimed in claim 20, characterized in that, The support includes a second base (20) located below the first base (8) and mounted on a second set of legs (21), with the two ends of the bellows (10) connected to the first base (8) and the second base (20), respectively.

23. The apparatus as claimed in claim 22, characterized in that, The second set of outriggers (21) consists of three outriggers.

24. The apparatus as claimed in claim 22, characterized in that, The outer casing (9) is mechanically connected to the second base (20).

25. The apparatus as claimed in claim 23, characterized in that, The outer casing (9) is mechanically connected to the second base (20).

26. A dilution refrigeration method using the apparatus of any one of claims 1 to 25, the method comprising the steps of refrigeration by the cryogenic refrigerator (4); and the step of circulating the circulating fluid in the working circuit (2), the method further comprising at least one of the following steps: adjusting the amount of gas in the sheath (90); adjusting the pressure in the sheath (90) to a predetermined value; and adjusting the pressure in the sheath (90) to a pressure corresponding to the liquefaction pressure of helium-4 at the lowest temperature of the cryogenic refrigerator (4).

27. The method of claim 26, wherein the pressure in the sheath (90) is adjusted to near atmospheric pressure.

28. The method of claim 26, wherein the pressure in the sheath (90) is adjusted to between 0 bar and 2 bar.

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