HEAT EXCHANGER FOR A CRYOGENIC COOLER, ...DILUTION REFRIGERATOR, AND METHODS OF FORMING SAME - Patent application

Abstract

To provide a heat exchanger suitable for a simpler assembly process that can be semi-automated or fully automated. A heat exchanger for a cryogenic cooling device is provided having a first conduit, a second conduit, and a chamber, the chamber being positioned to receive a fluid from the first conduit, the second conduit being thermally coupled to an exterior of the chamber, the chamber having a first region and a second region, the first region being separated from the second region by a plate extending through the chamber, the plate including one or more openings that permit flow of the fluid from the first region to the second region.

Description

[Technical Field]

[0001] The present invention relates to a heat exchanger for a cryogenic cooling device. In a particularly advantageous embodiment, the heat exchanger forms part of a dilution refrigerator. [Background technology]

[0002] Many applications require cooling to millikelvin temperatures. Such temperatures can be achieved by operating a dilution refrigerator. A dilution unit will form part of the dilution refrigerator and will include a still and a mixing chamber connected by a set of heat exchangers. During operation, a working fluid formed of a helium-3 / helium-4 mixture circulates through the dilution unit. Cooling is obtained in the mixing chamber from the enthalpy of mixing as helium-3 is diluted with helium-4. The mixing chamber is thereby operable to obtain the lowest temperature of any part of the dilution refrigerator. Helium-3 boils in the still, which removes energy through the latent heat of vaporization. A cold plate is positioned between the still and the mixing chamber and typically obtains a temperature between these two components during use.

[0003] Heat exchangers are a key element of dilution refrigerator design and are used to combine the "cold" Helium3 leaving the mixing chamber with the "warm" Helium3 returning to the mixing chamber. The quality of this exchange determines, for example, the minimum temperature that can be reached. There are two basic types of heat exchangers used today (on all dilution refrigerators): the so-called "continuous" exchangers and "step" exchangers.

[0004] An example of a prior art dilution unit is shown in Figure 1. The working fluid flows between a still 102 and a mixing chamber 105 through two countercurrent paths within the heat exchange unit. The heat exchange unit includes a continuous heat exchanger 101 disposed between the still 102 and a cold plate 103. The continuous heat exchanger 101 includes a helically configured coaxial unit through which two paths travel in opposite directions, with the inner path surrounded by the outer path in a coiled configuration (not shown). Continuous heat exchangers can generally be used to obtain temperatures up to approximately 30 millikelvin. Dilution refrigerators using only continuous heat exchangers are limited to operating at approximately 30 millikelvin due to the increased Kapitza (thermal boundary) resistance between liquid helium and metal at low temperatures. At lower temperatures, continuous heat exchangers do not provide sufficient surface area to overcome the increased thermal boundary resistance. Step heat exchangers can be used to obtain even lower temperatures by using large-surface-area sinters to overcome the Kapitza resistance. Two step heat exchangers 104 are arranged in a stack between the cooling plate 103 and the mixing chamber 105. Each step heat exchanger forms a substantially disk-shaped structure with two paths separated by a foil. The number of step heat exchangers provided can be selected to suit the application.

[0005] In common use, there are two geometries of step heat exchangers: "counterflow block" and "semi-continuous." Counterflow block has two countercurrent flows of helium-3 thermally coupled by sinter through a support medium, which can be a thin membrane. Semi-continuous heat exchangers generally have a coiled geometry similar to continuous heat exchangers. However, the internal piping is made up of a set of individual sinters joined together and enclosed within an outer tube. If the outer tube is exposed, the semi-continuous heat exchanger can have the appearance of a continuous heat exchanger. Alternatively, the outer tube can be housed within a welded box, giving it the appearance of a step heat exchanger.

[0006] Assembly of a dilution unit, particularly the step heat exchangers described above, is typically a complex and labor-intensive procedure that requires hundreds of hours of highly trained technician completion. Dilution refrigerator performance is sensitive to minor variations in the assembly process, and a high reliance on manual assembly techniques means that the final performance of a dilution refrigerator cannot always be accurately guaranteed in advance. It would be desirable to reduce the standard deviation between the performance of a heat exchanger manufactured according to a specific process and that of the dilution refrigerator. It would also be desirable to provide a simpler method for constructing these devices that is amenable to automation. The present invention is defined in the context of solving these problems. Summary of the Invention [Means for solving the problem]

[0007] A first aspect of the present invention provides a heat exchanger for a cryogenic cooling apparatus, the heat exchanger comprising a first conduit, a second conduit and a chamber, the chamber being arranged to receive a fluid from the first conduit, the second conduit being thermally coupled to an exterior of the chamber, the chamber having a first region and a second region, the first region being separated from the second region by a plate extending through the chamber, the plate comprising one or more openings to allow fluid flow from the first region to the second region.

[0008] The heat exchanger configuration lends itself to a simpler assembly process that can be semi-automated or fully automated. Therefore, the repeatability of the heat exchanger's performance is improved compared to some prior art heat exchangers. The plates are preferably positioned to obstruct fluid flow through the chamber. Thus, the openings can be positioned relative to the first conduit so that the fluid follows a non-linear path through the chamber. Because the second conduit is thermally coupled to the exterior of the chamber, the non-linear path increases thermal coupling between the fluid from the first conduit and the fluid in the second conduit within the chamber.

[0009] The chamber can be disposed along the first conduit. In other words, the chamber can be disposed to receive fluid directly from a first portion of the first conduit, and the second portion of the first conduit can be disposed to receive fluid directly from the chamber. The first conduit is typically fluidly coupled to the chamber interior at a first location within the first region and a second location within the second region, and one or more openings are laterally offset from the first location and / or the second location in a direction along the plate. This improves thermal coupling between the fluid in the chamber and the fluid in the second conduit. The heat exchanger typically has a central axis extending through the center of the chamber, and the first conduit is coupled to the chamber at two locations disposed along the central axis. Thus, one or more openings can be radially dispersed from the central axis. Having the first conduit extend along the central axis ensures that the heat exchanger is properly supported and facilitates easier assembly. The heat exchanger is preferably rotationally symmetric about the central axis. This further simplifies the assembly method because, for example, if a welding process needs to be used to form the joint, the heat exchanger can be rotated about its central axis during the welding process.

[0010] The purpose of the heat exchanger is to thermally couple the fluid in the first conduit with the fluid in the second conduit during use. To ensure that these two conduits are effectively thermally coupled, the first conduit is preferably positioned inside the second conduit. Similarly, the chamber is preferably positioned inside the second conduit. The fluid inside the second conduit will be in direct contact with the outside of the first conduit and the chamber.

[0011] The chamber preferably has a first end and a second end, each forming opposite sides of the chamber. The first end is connected to the second end by a flow deflector. The flow deflector includes a collar separating the first end from the second end, and the plate extends across the collar to form a portion of the flow deflector. These components can be fused together as described below. Typically, one or both of the first and second ends includes a first surface disposed inside the chamber, a second surface disposed outside the chamber, and a foil member disposed between the first and second surfaces, each of which includes a sintered material applied to the foil member. The sintered material can be a metal powder such as silver, copper, or titanium, and is typically a metal used for foil members. The sintered material is porous and has a large effective surface area to ensure adequate heat exchange between the fluid in the chamber and the fluid in the second conduit. However, sintered materials are generally not compatible with the high temperatures that can result from a welding or fusing process. Therefore, a peripheral support member is preferably disposed around the periphery of each foil member, and the peripheral support member is fused to the collar by a localized heating process, such as, for example, laser welding or electron beam welding.

[0012] The first and / or second surfaces can be contoured so that the thickness of the sinter on the foil member increases with radial distance from the central axis. This is particularly advantageous when the heat exchanger includes a central axis extending through the center of the chamber and the first conduit is coupled to the chamber at two locations along the central axis. Contouring the sinter in this manner typically reduces viscous heating within the heat exchanger. Typically, both the first and second end portions are contoured in a similar manner. Any shape or contour that can be machined into a press tool can be used to apply the contoured sinter. For example, the sinter on the first and second end portions can be contoured so that the distance between the first surface and the plate decreases, typically linearly, with radial offset from the central axis. Similarly, the sinter on the first and second end portions can be contoured so that the distance between the second surface and the second conduit decreases, typically linearly, with radial offset from the central axis. The thickness of the sinter applied to the foil member is typically 0.1 to 3.0 mm, preferably 0.2 to 2.0 mm, at any location along the first and second surfaces where the sinter is applied. For example, the thickness of the sinter may vary from a minimum of 0.5 mm near the center of the foil member to 1 mm near the edges. The specific value can be selected depending on the operating temperature of the heat exchanger. The maximum separation distance between the sinter on the first surface and the plate is typically 0.1 to 5.0 mm, preferably 0.1 to 3.0 mm, and more preferably 0.2 to 1.50 mm (measured along the central axis of the chamber). This corresponds to the "chamber depth" or "channel depth" within the chamber.

[0013] The same material is typically used to form the collar and the peripheral support member. For example, the collar and the peripheral support member can each be formed of stainless steel. The sintered material and the foil member are preferably formed of the same material, such as silver, copper, or titanium. The thermal conductivity of the foil member and / or sintered material is preferably substantially higher than that of the peripheral support member and / or collar. For example, the thermal conductivity of the foil member and / or sintered material can be at least 20 times greater than that of the peripheral support member and / or collar at a temperature of 300 K. The thermal conductivity of a material generally depends on its temperature, but in this case, the manufacturing process is typically performed at nominal "room temperature." At 300 K, the thermal conductivity of copper is approximately 392 W / m / K, and the thermal conductivity of stainless steel is approximately 15 W / m / K. The lower thermal conductivity of the peripheral support member ensures that the heat input from fusing the end portion to the collar is not transferred to the sintered material effectively enough to cause unwanted liquefaction of the sintered material. The first end can be configured similarly to the second end and can be fused to a collar to form a chamber to form a simple and effective heat exchanger suitable for low temperature applications.

[0014] The chamber can define a flow path that transports a fluid through the first and second regions. For example, the fluid can flow from the chamber inlet to the chamber outlet through an internal volume defined by the separation distance between the first face of the end portion and the plate. Alternatively, the flow path can be partially or completely formed within the sinter applied to the first and second end portions, particularly within the sinter applied to the "first face" (facing the plate) of each end portion. The flow path can include one or more flow passages through the chamber, where the one or more flow passages are formed by the sinter applied to the first and second end portions. Thus, the flow passages can be imprinted into the sintered material to define one or more paths along which the fluid flows through the first and second regions. This controls the flow direction of the working fluid, which can enable better heat distribution through the chamber and improved thermal performance of the heat exchanger. Alternatively, the one or more flow passages can be imprinted into the sinter applied to the "second face" of the end portion that forms a portion of the second conduit. This improves thermal coupling throughout the heat exchanger. The depth of the channels may decrease with radial distance from the central axis to balance the impact of viscous heating against the helium-3 demand.

[0015] Further aspects of the present invention that share similar advantages as discussed above are described below, and any feature described in relation to one aspect is equally applicable to the remaining aspects.

[0016] The heat exchanger of the first aspect is particularly suited to replacing prior art step heat exchangers that use liquid helium, but can be applied to a variety of cryogenic cooling systems. A second aspect of the invention provides a cryogenic cooling apparatus comprising a target refrigerator and a heat exchanger according to the first aspect, wherein a first conduit is arranged to convey a working fluid to the target refrigerator and a second conduit is arranged to convey a working fluid from the target refrigerator. The working fluid conveyed along the first conduit is typically in a different state, and usually at a different temperature, than the working fluid conveyed along the second conduit.

[0017] A third aspect of the present invention provides a dilution refrigerator comprising a still, a mixing chamber and a heat exchanger according to the first aspect, wherein a first conduit is arranged to pass working fluid from the still to the mixing chamber and a second conduit is arranged to pass working fluid from the mixing chamber to the still, and the heat exchanger is configured to thermally couple the working fluid in the first conduit with the working fluid in the second conduit.

[0018] The mixing chamber typically includes a sintered mass, a first conduit having an open end portion extending around a portion of the sintered mass to contact the portion of the sintered mass with the working fluid, and a second conduit extending around the end portion and the sintered mass to convey the working fluid away from the sintered mass. The dilution refrigerator is preferably configured such that operation of the dilution refrigerator generates a phase boundary in the working fluid at a location inside the end portion of the first conduit. This phase boundary typically refers to the boundary between rich and lean phases of helium-3 that occurs in the mixing chamber of the dilution refrigerator. The incoming rich phase is typically conveyed by the first conduit from the location of the fractionator to the mixing chamber, where it is in thermal contact with the outgoing lean phase conveyed by the second conduit along the fractionator. It should be understood that the rich and lean phases typically do not mix in the fractionator.

[0019] Heat exchangers are typically simpler in structure than prior art step heat exchangers and are therefore suitable for low-temperature applications. Accordingly, particular advantages are achieved when the heat exchanger is arranged to obtain a temperature of less than 30 mK during operation of the dilution refrigerator. For example, the dilution refrigerator may further include a cold plate arranged between the still and the mixing chamber, the cold plate arranged to obtain a reference temperature between the still and the mixing chamber during operation of the dilution refrigerator, the dilution refrigerator further including a chamber assembly including one or more chambers arranged along a portion of a first conduit extending between the cold plate and the mixing chamber, each of the chambers arranged to receive the working fluid from the first conduit, and a second conduit thermally coupled to the outside of each of the chambers.

[0020] In steady-state operation, the total fluid flow rate through all chambers is equal. The temperature of the fluid from the first conduit typically decreases as it progresses through the chamber assembly to lower-temperature regions. Often, the viscosity of the fluid increases as the temperature is reduced, and viscous fluid flow can cause unwanted heating and reduce the efficiency of the heat exchanger. To mitigate this, so-called "flow channels" can be introduced to provide a low-impedance path for the fluid to flow. The size of these channels is generally controlled to provide the required fluid flow rate while reducing the total volume of fluid in the chambers (resulting in a reduction in the amount of helium-3 required for operation, a scarce and expensive resource). While many existing dilution refrigerators rely on custom-made, uniquely sized or shaped components, mass production and automation facilitate parts standardization. Thus, preferably, each chamber includes one or more flow channels that transport fluid through the respective first and second regions, each flow channel formed in the sinter, the chamber assembly being arranged along a thermal gradient during operation of the dilution refrigerator, with the first chamber being arranged to obtain a higher base temperature than the second chamber, and the diameter of the one or more flow channels in the first chamber being smaller than the diameter of the one or more flow channels in the second chamber. The flow channel diameters can thereby be controlled to achieve a desired balance between flow rate and total fluid volume, thereby improving thermal performance throughout the heat exchanger. Additionally, the second conduit can include flow channels formed in the sinter on the exterior of the chamber to further improve the thermal performance of the heat exchanger. Furthermore, imprinting the flow channels into the sinter allows for efficient and repeatable mass production of the flow channels.

[0021] The chamber assembly can form a step heat exchanger, with each heat exchanger corresponding to a respective step and configured to obtain a respective temperature during operation of the dilution refrigerator. The chamber assembly can include a first heat exchanger and a second heat exchanger, where the first heat exchanger is disposed between a cooling plate and a second heat exchanger chamber, and the depth of the chamber of the second heat exchanger and / or the number / size of openings through the plate of the second heat exchanger are greater than those of the first heat exchanger. This allows fluid flow through the chamber assembly to be optimized to improve system performance, as described above.

[0022] To further simplify the assembly method, the chamber assembly and the mixing chamber are preferably rotationally symmetric about an axis extending through the first conduit. Furthermore, the second conduit preferably forms the exterior of the heat exchanger and includes multiple modules fused together. Similarly, the first conduit is preferably formed from multiple modules fused together. This fusion process can be achieved by electron beam welding or laser beam welding, creating a reliable joint without requiring complex and time-consuming manual processes.

[0023] A fourth aspect of the present invention is a method of forming a heat exchanger for a cryogenic refrigerator, the method comprising the steps of: providing a first conduit, a second conduit, a first end portion, a second end portion, and a flow deflector, the flow deflector including a collar and a plate, the plate extending across the collar; providing the first end portion comprises fusing a first peripheral support member to an outer periphery of a first foil member and thereafter applying a sintering material to an opposite surface of the first foil member; and wherein the thermal conductivity of the first peripheral support member is greater than or equal to that of the first foil member at a temperature of 300K. The thermal conductivity of the material is at least 20 times lower, and the method further includes fusing the first peripheral support member to the collar to form a chamber, the chamber having a first region separated from a second region by a plate, the plate being disposed between the first end and the second end, the first conduit being positioned to convey fluid into the first region and out of the second region, the plate having one or more openings that allow fluid flow from the first region to the second region, and the second conduit being thermally coupled to the exterior of the chamber.

[0024] This method is substantially easier to implement than the complex bonding processes typically required to assemble prior art step heat exchangers. It is also more amenable to automation. As a result, the assembly time for the heat exchanger is shorter and the standard deviation in performance between different heat exchangers manufactured according to the same technique is reduced. A sintering material (typically formed from a metal powder such as silver or copper) is applied to the foil member to increase the surface area for heat exchange between the fluid in the chamber and any fluid in the second conduit during use. The sintering material tends to melt when exposed to high temperatures. Therefore, the peripheral support member is fused to the foil member before applying the sintering material. Furthermore, the peripheral support member is selected to have a lower thermal conductivity than the sintering material and, preferably, the foil member. Once the sintering material is applied, the peripheral support member can then be fused to the collar to form the chamber without the risk of the sintering material melting.

[0025] The first portion of the first conduit is preferably fused to the first foil member to facilitate fluid flow through the first foil member. This is typically done before applying the sintering material and can be done simultaneously when the first peripheral support member is fused to the first foil member. The first portion of the first conduit, and preferably the first peripheral support member as well, are fused to the first foil member, preferably by welding or vacuum brazing. For example, the parts can be assembled together and then fired in a vacuum chamber to fuse them together. A similar process can then be performed to form the second end portion. For example, providing the second end portion can include fusing a second peripheral support member to the outer periphery of the second foil member and then applying a sintering material to the opposite surface of the second foil member, where the thermal conductivity of the second peripheral support member is at least 20 times lower than that of the second foil member at a temperature of 300 K. Forming the chamber can further include fusing the second peripheral support member to the collar. The method can further include fusing a second portion of the first conduit to the second foil member to facilitate fluid flow through the second foil member, where the second portion of the first conduit is preferably fused to the second foil member by welding or vacuum brazing. Typically, the second portion of the first conduit is fused to the second foil member before the sintering material is applied to the second foil member, preferably at the same time as the second peripheral support member is fused to the second foil member. The second portion of the first conduit, and preferably also the second peripheral support member, are preferably fused to the second foil member by welding or vacuum brazing. The second peripheral support member is preferably fused to the collar simultaneously with the first peripheral support member. Each of the peripheral support members is preferably fused to its respective foil member by vacuum brazing. In contrast, each of the support members is preferably fused to the collar by a localized heat source, such as laser or electron beam welding. A localized heating step is preferred because the sintering material has already been applied to the foil member at this stage, and it is desirable to reduce the amount of heat transferred to the sinter.

[0026] A fifth aspect of the present invention is a method of forming a dilution refrigerator, the method comprising the steps of providing an evaporator and a mixing chamber and forming a heat exchanger according to any of the first to fourth aspects, wherein a first conduit is arranged to pass working fluid from the evaporator to the mixing chamber and a second conduit is arranged to pass working fluid from the mixing chamber to the evaporator.

[0027] The first conduit preferably includes an end portion arranged to receive the working fluid from the chamber, and the step of providing a mixing chamber then includes the steps of: disposing the end portion around a portion of the sintered mass so that the portion of the sintered mass is in contact with the working fluid; and disposing a second conduit around the end portion and the sintered mass to convey the working fluid away from the sintered mass. The sintered mass can be formed of sintered material or from several smaller sintered masses and is configured to be received by the end portion of the first conduit. The method can then further include sealing the second conduit to a support to which the sintered mass is attached. This closes the distal end of the second conduit on the support, which can be the lowest temperature thermal stage of the dilution refrigerator.

[0028] Particular advantages can be achieved when the first and second conduits are formed from multiple modules for assembly, and the method further includes fusing a first module of the first conduit together with a second module of the first conduit at a location between the chamber and the end portion, and / or fusing a first module of the second conduit together with a second module of the second conduit at a location between the chamber and the end portion, with the resulting assembly having a fully welded structure, which ensures that the joint is reliably formed according to a fast and highly reproducible process.

[0029] The heat exchanger of the first aspect is particularly suitable for use at low temperatures, including below 30 milliKelvin. The cold plate of a dilution refrigerator typically has a reference temperature of 40 to 150 milliKelvin, more preferably 40 to 60 milliKelvin, while the mixing chamber typically has a reference temperature of less than 2.5 milliKelvin, preferably less than 10 milliKelvin, during use. Accordingly, the method further comprises the steps of: positioning the cold plate between the still and the mixing chamber such that the reference temperature is obtained between the still and the mixing chamber during operation of the dilution refrigerator; and providing a plurality of said chambers arranged along a portion of a first conduit extending between the cold plate and the mixing chamber, each said chamber being arranged to receive working fluid from the first conduit, and a second conduit being thermally coupled to the exterior of each said chamber. As described above, the first and second conduits are preferably formed from multiple modules for assembly, and the method further includes fusing a first module of the first conduit with a second module of the first conduit at a location between the two chambers, and / or fusing a first module of the second conduit with a second module of the second conduit at a location between the two chambers. These chambers are typically similarly formed and will have the features described in connection with the first aspect. The modules are preferably fused using a localized heat source, preferably by electron beam welding, thereby reducing heat input to the sinter. Furthermore, to ensure effective heat transfer between the working fluids in the first and second conduits, the portion of the first conduit extending from the cooling plate to the mixing chamber is preferably located inside the second conduit. The chamber is also preferably located substantially inside the second conduit.

[0030] A sixth aspect of the present invention is a dilution refrigerator comprising: a still and a mixing chamber; a first conduit arranged to transport working fluid from the still to the mixing chamber; a second conduit arranged to transport working fluid from the mixing chamber to the still; and a heat exchanger arranged to thermally couple the working fluid in the first conduit with the working fluid in the second conduit at a location between the still and the mixing chamber, the heat exchanger including one or more chambers arranged along a portion of the first conduit, each said chamber having a first region and a second region, the first region separated from the second region by a plate extending through the chamber, the plate including one or more openings allowing flow of working fluid from the first region to the second region, the second conduit arranged around the outside of each said chamber.

[0031] Preferably, a plurality of chambers are provided, the second conduit being formed of a plurality of modules welded together between each of the chambers, and the first conduit being preferably formed of a plurality of modules welded together between each of the chambers, each of the chambers preferably comprising a first end and a second end forming opposite sides of the chamber, the first end being coupled to the second end by a flow deflector, the flow deflector comprising a collar separating the first end from the second end, the plate extending across the collar, each of the first and second ends having a first face disposed inside the chamber and a second face disposed outside the chamber, the first and second faces being formed from a sintered material applied to a foil member disposed between the first and second faces, and each of the first and second ends further comprising a respective outer support member extending around a respective outer periphery of the foil member, the outer support member being fused to the collar. The thermal conductivity of the foil member is typically at least 20 times that of the outer support member at a temperature of 300K.

[0032] An embodiment of the present invention will be described with reference to the accompanying drawings. [Brief explanation of the drawings]

[0033] [Figure 1] FIG. 1 is an illustration of a prior art dilution unit. [Figure 2] 1 is a perspective view of a foil member forming part of a first embodiment of the present invention; FIG. [Figure 3] FIG. 2 is a perspective view of a first portion of a first conduit forming part of a first embodiment of the present invention; [Figure 4] 1 is a perspective view of a peripheral support member forming part of a first embodiment of the present invention; [Figure 5] 1 is a perspective view of a first end portion forming part of a first embodiment of the present invention prior to application of a sintering material; FIG. [Figure 6] 1 is a perspective view of a flow deflector forming part of a first embodiment of the present invention; FIG. [Figure 7] 1 is a first cross-sectional view of a chamber forming part of a first embodiment of the present invention; [Figure 8] FIG. 2 is a second cross-sectional view of a chamber forming part of a first embodiment of the present invention. [Figure 9] 1 is a first cross-sectional view of a mixing chamber forming part of a first embodiment of the present invention; FIG. [Figure 10] FIG. 2 is a second cross-sectional view of a mixing chamber forming part of a first embodiment of the present invention. [Figure 11] FIG. 1 is a schematic diagram of a dilution refrigerator according to a first embodiment. [Figure 12] 1 is a flowchart illustrating a method according to an embodiment of the present invention. [Figure 13] FIG. 2 is a cross-sectional view of a chamber forming part of a second embodiment of the present invention. [Figure 14] FIG. 10 is an illustration of a foil member forming part of the second embodiment. [Figure 15] FIG. 10 is a cross-sectional view of a chamber forming part of a third embodiment of the present invention. [Figure 16] FIG. 10 is a cross-sectional view of a chamber forming part of a fourth embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0034] A method of assembling a heat exchanger and dilution refrigerator according to a first embodiment of the present invention will now be described. The method begins at step 201 (FIG. 12), at which point first and second end pieces are fabricated to form a portion of a heat exchanger. A first foil member 10 (FIG. 2) is formed from a highly thermally conductive material, such as silver or copper, typically having a thermal conductivity greater than 300 W / m / K at 300 K. In this case, the first foil member 10 is a substantially planar silver disk having a central opening that receives an inlet tube 12, which, in use, forms a first portion of a first conduit (described below) that carries a working fluid. The first foil member 10 has a diameter of approximately 45 mm, but more typically, is typically between 20 and 100 mm, depending on the application. A first peripheral support member 14 (FIG. 4) is provided and is formed from a relatively poorly thermally conductive material, such as stainless steel, typically having a thermal conductivity less than 15 W / m / K at 300 K. The first peripheral support member 14 is ring-shaped and configured to support the first foil member 10. The first peripheral support member 14 extends around the exterior of the first foil member 10 and contacts the outer periphery of the first foil member 10 and an outer portion of one of the opposite faces of the first foil member 10. The first foil member 10, inlet tube 12, and peripheral support member are assembled as shown in FIG. 5 and then fused together, for example, by welding or vacuum brazing.

[0035] Next, the material to be sintered is applied as a powder to the major surfaces of the first foil member 10. The sintering material is a highly thermally conductive material, typically the same material as used for the first foil member 10. Pressure is applied to form sinter 15 on the two opposite surfaces of the first foil member 10. For silver powder, pressure alone is sufficient for this operation, but for copper powder, firing is typically also required during this operation. Using an appropriate tool, the powder can be pressed onto both sides of the first foil member 10 in a single operation. Sinter 15 is typically applied to the entire surfaces of the two major surfaces of the first foil member 10, but not to the peripheral support member 14. This produces a first end portion 22 for the heat exchanger. This process is then repeated using the second foil member 11, the second peripheral support member, and the outlet tube 32 to form the second end portion 24.

[0036] The first end portion 22 and the second end portion 24 are configured to fit onto opposite ends of the flow deflector 16, as shown in FIG. 6 . The flow deflector 16 includes a collar 17, which is an annular element having approximately the same circumference as the peripheral support member. A plate 18 extends radially throughout the flow deflector 16. The plate 18 is approximately centrally located within the collar 17 and subdivides the flow deflector 16 into upper and lower portions located on opposite sides of the plate 18 inside the collar 17. The plate 18 includes a plurality of openings 20 for fluidly connecting the upper and lower portions. In FIG. 6 , these openings are distributed at a constant radius around the plate 18, with a substantially constant spacing maintained between adjacent openings. In this embodiment, the flow deflector 16 is formed as a unitary member. Specifically, the flow deflector 16 is a “machined body,” and the openings can be added by a process such as electrical discharge machining. Alternatively, the flow deflector 16 can be formed, for example, from a foil element with openings etched into it, which is then welded between two annular supports to form the flow deflector 16.

[0037] The method proceeds to step 202, where the heat exchange chamber 30 is formed. The first and second end portions 22, 24 are positioned against opposite ends of the flow deflector 16, as shown in FIG. 7, and sinter 15 is applied to the distal surface of each end portion positioned inside the collar 17, with the peripheral support member contacting the opposing ends of the collar 17. A highly controlled, localized heating process, such as electron beam welding, laser beam welding, or tungsten inert gas (TIG) welding, is then used to fuse the peripheral support member to each end of the collar 17, thereby forming the chamber 30. The localized heating process used to fuse the first and second end portions 22, 24 to the collar 17, combined with the relatively low thermal conductivity of the stainless steel peripheral support member, protects the sinter 15 from the heat of the welding process. While the location of the electron beam welds in this embodiment is shown in FIG. 7, it should be understood that welding typically occurs around the circumference of the peripheral support member. Therefore, it is advantageous for the inlet pipe 12 and the outlet pipe 32 to extend along the central axis of the flow deflector 16 and the chamber 30 because this allows the assembly to be rotated about the inlet pipe 12 and the outlet pipe 32 when performing the welding procedure, without the need to move the heating elements. This process lends itself to automation and ensures that a reliable joint is welded.

[0038] The formed chamber 30 has a first region 26 separated from a second region 28 by a plate 18, with an inlet pipe 12 arranged to flow fluid into the first region and an outlet pipe 32 arranged to flow fluid out of the second region. The inlet pipe 12 and outlet pipe 32 form first and second portions, respectively, of a first conduit 46 arranged to flow fluid through the chamber 30. When used in a dilution refrigerator, the first conduit 46 and chamber 30 accommodate the flow of a sodium-3 rich phase working fluid from the still (including from outside the still) to the dilution refrigerator's mixing chamber 45 during steady-state operation. The first conduit 46 is also commonly referred to as the "rich phase flow path" of the dilution refrigerator. Arrows are included in FIG. 8 (not to scale) to indicate the direction of fluid flow through the interior of the chamber 30. As shown, the arrangement of apertures 20 radially distributed from a central axis along which inlet tube 12 and outlet tube 32 are disposed ensures that the fluid follows a non-linear flow path inside chamber 30. This, combined with the use of sintered material 15 on first and second end portions 22, 24, ensures that a large effective surface area is provided for heat exchange between the fluid inside chamber 30 and another fluid in contact with the exterior of chamber 30. The origin and flow of this ambient fluid is described below with reference to steps 203 and 204 of FIG.

[0039] The method continues to step 203, at which point a mixing chamber 45 for the dilution refrigerator is formed. A mass of sinter 36 is formed directly on or attached to a highly thermally conductive support 8 that forms the coldest stage of the dilution refrigerator. The material forming the mass of sinter 36 is typically the same material (e.g., silver and / or copper) as that applied to the first and second foil members 10, 11. A terminal portion 40 of a first conduit 46 is provided, the terminal portion 40 having a first region 42 and a second region 44, the second region 44 having a larger diameter than the first region 42. The terminal portion 40 is positioned such that the first region 42 is configured to receive the fluid flow from the outlet tube 32, and the second region 44 is positioned such that a proximal portion of the sinter mass is disposed within the second region 44 and a distal portion of the sinter mass is disposed outside the terminal portion 40. Thus, the end portion 40 is positioned relative to the sinter mass 36 so that the phase boundary of the working fluid between the helium-3 rich phase and the helium-3 lean phase lies inside the end portion 40, preferably inside the second region 44, as shown by the dashed line in FIG. 9. In FIG. 9, arrows are provided to indicate the direction of fluid flow along the first region 42 of the end portion 40, from around the sinter mass 36 to the region surrounding the mixing chamber 45. Of course, this flow direction is only possible when the dilution refrigerator is fully assembled and operational. During use, concentrated vapor is typically contained inside the end portion 40, so the end portion 40 can also be referred to as a "concentrated vapor cap."

[0040] FIG. 10 shows a second conduit 48 formed around the first conduit 46. The second conduit 48 is also referred to as a "dilute phase flow path" that returns fluid from the mixing chamber 45 to the still. The second conduit 48 is coaxially disposed around the outside of the first conduit 46. Each of the first and second conduits 46 and 48 is formed from a series of modules that are welded together in step 204 to form the heat exchanger assembly. With the end portion 40 positioned above the mass of sinter 36 (as described with reference to FIG. 9), a first portion 50 of the second conduit 48 is disposed above and around the end portion 40 and attached to the support 8. Typically, the first portion 50 of the second conduit 48 is sealed to the support 8 with an indium seal, although a ConFlat (CF) flange can alternatively be used to achieve this attachment. The first portion 50 of the second conduit 48 is thereby positioned to receive the flow of working fluid from the end of the first conduit 46 .

[0041] The distal end of the outlet tube 32 is then welded to the proximal end of the first region 42 of the end portion 40. This fluidly couples the inlet tube 12 to the mixing chamber 45 and the second conduit 48. The distal end of the second portion 52 of the second conduit 48 is then fused to the proximal end of the first portion 50 of the second conduit 48. This bond is made about the central axis of the assembly, at a location between the chamber 30 and the mass of sinter 36, typically along the first region 42 of the end portion 40 of the first conduit 46. Although Figures 7 to 10 are schematic and therefore not to scale, a substantially constant separation distance is maintained between the inner wall of the second conduit 48 and the outer wall of the first conduit 46. The second conduit 48 conforms around the shape of the chamber 30 to form the steps of the step heat exchanger 53. In normal operation of a dilution refrigerator, helium-3 evaporates from the still and is removed by a pumping system. This causes a flow of helium-3 atoms across the phase boundary in mixing chamber 45 (from rich to lean) to replenish the helium-3 in the still. The dilution of helium-3 into the lean phase causes cooling in mixing chamber 45. Thus, the lean phase of helium flowing along second conduit 48 will be cooler than the incoming rich phase of helium-3 transported along first conduit 46. The relatively large surface area of ​​chamber 30 forms an effective heat exchanger, so the fluid in first conduit 34 and chamber 30 is further cooled before reaching mixing chamber 45.

[0042] The heat exchanger assembly may comprise a plurality of step heat exchangers 53 or "steps," each step formed by a chamber disposed along the first conduit 46 and surrounded by a portion of the second conduit 48 (as described with reference to Figures 7 and 8). Figure 10 shows such a second chamber 130 with a corresponding portion of the first conduit 46 disposed above the proximal end of the inlet tube 12. The portions of each of the first and second conduits are fused together about a central axis in a stepped manner, as described above, to form the completed assembly.

[0043] The arrangement of the heat exchanger assembly within the dilution refrigerator is shown in the schematic diagram of Figure 11 and described below. A cryostat 1 is provided, typically comprising a large hollow cylinder formed from stainless steel or aluminum, with an outer vacuum vessel 5. Within the cryostat 1 are multiple spatially distributed stages, including a first stage 6, a second stage 7, and a third stage 8. Each stage provides a platform formed from a highly conductive material (e.g., copper) and is separated from the remaining stages by low thermal conductivity rods (not shown). The second stage 7, commonly referred to as the "cold plate," provides an intermediate heat sink between the first stage 6 and the third stage 8. A sample holder 55 is shown attached to the third stage 8, which forms the coldest stage during steady-state operation of the system.

[0044] Cryostat 1 in this example is substantially cryogen-free (also referred to in the art as "dry") and is not primarily cooled by contact with a reservoir of cryogenic fluid. Cooling of the cryostat is instead achieved through the use of a mechanical refrigerator, which may be a Stirling refrigerator, a Gifford-McMahon (GM) refrigerator, or a pulse tube refrigerator (PTR). However, despite being substantially cryogen-free, some cryogenic fluid is typically present within the cryostat during use to facilitate normal operation of the dilution unit. The primary cooling power for cryostat 1 is provided in this embodiment by PTR 2. The PTR generates cooling by controlling the compression and expansion of a working fluid supplied at high pressure from an external compressor. The first PTR stage typically has a relatively high cooling power compared to the second PTR stage. In this case, PTR 2 cools first PTR stage 3 to approximately 50 to 70 Kelvin and second PTR stage 4 to approximately 3 to 5 Kelvin. The second PTR stage 4 therefore forms the lowest temperature stage of PTR2.

[0045] Various thermal radiation shields are provided within the outer vacuum vessel 5, each enclosing a respective one of the remaining cryogenic components. The first PTR stage 3 is thermally coupled to a first radiation shield 19, and the second PTR stage 4 is thermally coupled to a second radiation shield 54. The first radiation shield 19 surrounds the second radiation shield 54, which in turn surrounds each of the first, second, and third stages 6-8. Furthermore, the first and second stages 6 and 7 could, in theory, be coupled to their respective thermal radiation shields to reduce unwanted heat transfer between the stages.

[0046] The dilution refrigerator still 9 is operable to cool the first stage 6 to a reference temperature of 0.5-2 Kelvin. The mixing chamber 45 is attached to the third stage 8 and is operable to cool the third stage 8 to a reference temperature below 10 milliKelvin. In use, the second stage 7 obtains a reference temperature between that of the first stage 6 and the third stage 8, typically 40-150 milliKelvin.

[0047] Still 9 is fluidly coupled to a storage vessel 50 by a cooling circuit 37. Storage vessel 50 is located outside of cryostat 1 and contains a working fluid in the form of a mixture of helium-3 and helium-4 isotopes. Various pumps 17, 39 are also located outside of cryostat 1 along the conduits of cooling circuit 37, which control the flow of working fluid around the circuit, as indicated by the solid arrows. Cooling circuit 37 includes a feed line 41 that provides a conduit for facilitating the flow of working fluid from storage vessel 50 to a condenser line 46′. This fluid can then be conveyed along condenser line 46′ to still 9, where it comes into thermal contact with a dilute phase of helium inside still 9. Condenser line 46′ then continues to a rich-phase flow path 46 from still 9 to a mixing chamber 45. Condenser line 46′ and rich-phase flow path 46 further include one or more impedances (not shown) that reduce the temperature of the working fluid as it flows toward mixing chamber 45 due to the Joule-Thomson effect. A compressor pump 13 is positioned along condensate line 46' to supply this flow at a pressure of 0.5 to 2 bar. A lean phase flow path 48 is positioned to convey the working fluid from the mixing chamber 45 through the still 9, where it is transported by the still pump line 48' to a location external to the cryostat 1. From this location the working fluid can then be circulated back to the condensate line 46'. A turbomolecular pump 39 is positioned along the still pump feed line 48' to provide a high vacuum (e.g., less than 0.1 mbar) on the low pressure side of the circuit, thus enabling the flow of working fluid away from the still 9.

[0048] As noted above, the rich-phase flow path 46 and the lean-phase flow path 48 form the first and second conduits of the heat exchanger, respectively. For clarity, these conduits are not explicitly shown between the first stage 6 and the third stage 8 in the schematic diagram of FIG. 11 . The first and second conduits 46, 48 are arranged to form a continuous heat exchanger 26 disposed between the first stage 6 and the second stage or “cold plate” 7. Within the continuous heat exchanger 26, the first conduit 46 is arranged in a coil, and the second conduit 48 is wound around the first conduit 46. This ensures that the helium-3 rich phase of the fluid flowing along the first conduit 46 is cooled by the helium-3 lean phase of the fluid flowing along the second conduit 48. Continuous heat exchangers are typically only effective at temperatures above 30 millikelvin. Thus, a step heat exchanger assembly comprising multiple step heat exchangers 53, 53', 53" (as described above with reference to Figures 2-0) is positioned in the cold region between the second stage 7 and the third stage 8. Fluid in the first conduit 46 flows from the second stage 7 through multiple chambers comprising flow deflectors to the mixing chamber 45. The helium-3 lean phase of the fluid flows counter-currently from the mixing chamber 45 along the second conduit surrounding the chamber. The outgoing fluid in the second conduit directly contacts the outer wall of the first conduit and the chamber to further cool the incoming fluid in the first conduit.

[0049] Fluid viscosity may increase as temperature decreases, and viscous fluid flow can cause unwanted heating and reduce the efficiency of the heat exchanger. To mitigate this, the chamber depth and / or the number or size of openings inside the chamber may be increased for chambers located at lower temperatures. For example, the chamber depth (along the central axis of the assembly) may be minimized for the top heat exchanger (to reduce the total volume of helium-3 required for operation) and maximized for the bottom heat exchanger (to reduce viscous heating). This has been shown to improve the thermal performance of the system by optimizing the balance between viscous heating and total fluid volume.

[0050] The heights of first region 26 and second region 28 are depicted relatively large in Figures 7 and 8 for ease of illustration, but are preferably on the order of 0.1 to 5.0 mm, and more preferably 0.2 to 1.5 mm, particularly for use in a dilution refrigerator. This height may vary depending on the operating temperature of the heat exchanger, as determined by the location along first conduit 46 (as discussed above). In general, the shapes and dimensions of first region 26 and second region 28 are selected to facilitate circulation of helium-3, reduce viscous heating, allow osmotic pressure to develop within still 9 and mixing chamber 45, reduce the amount of helium-3 used, and reduce hydrodynamic instabilities and convection. Similar considerations apply to surrounding second conduit 48.

[0051] Figures 13 and 14 show elements of a heat exchanger according to a second embodiment of the invention. Figure 13 is a cross-sectional view equivalent to Figure 7, with primed reference numerals used to indicate similar features. In this embodiment, the flow deflector plate 18' is a foil element welded between two annular supports. The chamber 30' is otherwise formed essentially as described for the first embodiment with reference to Figures 2 to 8, except that in this case a dedicated flow passage 21' is formed in the sinter 15' to convey the helium-3 rich-phase fluid from the inlet tube 12' through the opening and back to the outlet tube 32' (it should be understood that similar features may also apply to the sinter 15' on the opposite side of the foil members 10', 11' to convey the dilute-phase fluid). Although the sinter 15' is applied to both surfaces of the first and second foil members 10', 11', the first and second foil members 10', 11' are disposed within the chamber 30' such that the sinter 15 applied to the first surface of the first foil member 10' and the first surface of the second foil member 11' are separated from the opposite surface of the flow deflector plate 18' by a small gap, typically 0.1-1.0 mm, preferably 0.2-0.6 mm (depending on configuration). Optionally, the opposite surface of the flow deflector plate 18' instead abuts the sinter 15' on the first surface of the first and second foil members 10', 11', leaving no such separation. The flow channels 21' are typically imprinted into the sinter during step 201 and can take a variety of different patterns for controlling the flow direction of the working fluid. One or more channels may be provided within the chamber 30' to transport the rich phase of the fluid through a first region to an opening in the plate 18' and then through a second region of the chamber 30' to the outlet tube 32'. One or more channels may also be provided to transport the dilute phase of the fluid through a second conduit on the exterior of the chamber 30'. Any number of different patterns may be applied, including radial or spiral. Figure 14 is a perspective view of the sinter 15' taken on the plane X-X' from Figure 13. In Figure 14, the channels 21' are bifurcated to place a majority of the sinter 15' in intimate contact with the working fluid, enhancing heat exchanger performance.

[0052] The shape of the channels 21' may be limited by what can be machined into the press die, but can be semicircular, oval, triangular, rectangular, etc. The channels 21' will typically have a width of 0.5 to 1.0 mm. The velocity of the fluid flow will depend on the number and width of the channels 21' (at a given total flow rate). Thus, the width of the channels may vary depending on the relative placement of the heat exchanger chambers 30' within the step heat exchanger assembly, with the width increasing at lower temperatures to optimize the balance between viscous heating and fluid volume within the assembly. This further improves the thermal performance of the system.

[0053] In the first and second embodiments, the first and second regions within the chamber generally have a constant height across the plate (typically between 0.5 and 4 mm). As a result, the fluid flow rate typically decreases as the fluid spreads radially outward across a wider area. This means that more viscous heating occurs toward the central axis, which can limit the performance of the cryogenic system in which the heat exchanger is installed. FIG. 15 shows elements of a heat exchanger according to a third embodiment of the present invention. Double prime reference numerals are used to represent similar components. This embodiment is similar to the first and second embodiments, except that the sinter 15″ on the inside of the chamber 30″ is contoured so that the heights of the first and second regions 26″ and 28″ continuously decrease with increasing radius from the central axis. This contouring is achieved by pressing the sinter with a forming tool in step 201 (FIG. 12). Contouring the sinter in this manner results in “deeper” flow passages at smaller radii, thereby compensating for the effects described above and reducing viscous heating. The maximum depth of the first and second regions 26" and 28" is typically about 1.5 mm, and the minimum depth is typically about 0.2 mm. However, the specific parameters may vary depending on the operating temperature of the heat exchanger, as determined by, for example, the location of such heat exchanger within the stack.

[0054] In the example of FIG. 15 , the sinter 15″ applied to the opposing first major surfaces of the first and second foil members 10″, 11″ is contoured so that the thickness of the sinter 15″ increases linearly with radius. In contrast, the thickness of the sinter 15″ applied to the opposing second major surfaces of the first and second foil members 10″, 11″ remains generally constant. However, remember that the sinter 15″ on the outside of the chamber 30″ comes into contact with fluid conveyed along a second conduit surrounding the chamber (as illustrated by FIG. 10 ). From a dilution refrigerator perspective, this is typically a dilute phase of helium-3. Additional unwanted viscous heating can occur within the second conduit, which can be mitigated by contouring the sinter applied to the second major surfaces of the first and second foil members 10″, 11″. This contouring is typically performed so that the thickness of the sinter applied to the first and second foil members 10″, 11″ also increases linearly with radius. In practice, one or both major surfaces of the first and second foil members may be contoured to control any viscous heating within the heat exchanger. Figure 16 shows a portion of a heat exchanger according to a fourth embodiment, in which the surfaces of both the first and second foils are contoured to reduce any viscous heating and thereby improve the performance of the cryogenic cooling system in which the heat exchanger is installed. Figures 13 through 16, although not to scale, show examples of various shapes that can be pressed into the sintered product to control fluid flow through the heat exchanger.

[0055] The result is an effective heat exchanger capable of operating at low temperatures, thereby ensuring reliable operation of the cryogenic cooling system. The heat exchanger design is relatively simple and lends itself to welding and automated manufacturing processes, ensuring highly repeatable thermal performance. Consequently, lower temperatures can be obtained in cryogenic cooling systems, such as dilution refrigerators, incorporating the heat exchanger, with the minimum temperature achievable being determined by the performance of the heat exchanger. Furthermore, such processes can be used to reduce the time required to manufacture a cryogenic cooling system. [Explanation of symbols]

[0056] 10 First foil member 11 Second foil member 12 Inlet pipe 14 First peripheral support member 15 Sintered materials 16 Flow deflector 22 first end portion 24 Second end 26 First Area 28 Second Area 30 Heat Exchange Chamber 32 Outlet pipe

Claims

1. 1. A heat exchanger for a cryogenic cooling system, comprising: a first conduit (46), a second conduit (48), and a chamber (30); the chamber is positioned to receive the fluid from the first conduit, the second conduit is thermally coupled to an exterior of the chamber, the chamber having a first region (26) and a second region (28), the first region being separated from the second region by a plate (18) extending through the chamber, the plate including one or more openings (20) that allow the fluid to flow from the first region to the second region; the chamber (30) includes a first end (22) and a second end (24) forming opposite sides of the chamber, the first end being coupled to the second end by a flow deflector (16), the flow deflector including a collar (17) separating the first end from the second end, the plate extending across the collar to form a part of the flow deflector.

2. 2. The heat exchanger of claim 1, wherein one or both of the first end portion (22) and the second end portion (24) includes a first surface disposed inside the chamber, a second surface disposed outside the chamber, and a foil member (10, 11) disposed between the first surface and the second surface, each of the first surface and the second surface including a sintered material (15) applied to the foil member.

3. 3. A heat exchanger according to claim 2, wherein a peripheral support member (14) is disposed around the outer periphery of each of said foil members (10, 11), said peripheral support member being fused to a collar (17).

4. 4. A heat exchanger according to claim 3, wherein the thermal conductivity of the foil members (10, 11) and / or the sintered material (15) is at least 20 times greater than the thermal conductivity of the peripheral support member (14) and / or the collar (17) at a temperature of 300K.

5. the heat exchanger has a central axis extending through the center of the chamber (30'), the first conduit being coupled to the chamber at two locations disposed along the central axis; 3. The heat exchanger of claim 2, wherein the first surface and / or the second surface are contoured such that a thickness of the sintered material (15′) on the foil members (10″, 11″) increases with radial distance from the central axis.

6. 2. The heat exchanger of claim 1, wherein the chamber (30) defines a flow path (21') that conveys the fluid through the first region and the second region, the flow path being formed in sinter (15') applied to the first end (22) and the second end (24).

7. the heat exchanger has a central axis extending through the center of the chamber (30''), the first conduit being coupled to the chamber at two locations disposed along the central axis; the chamber (30'') defines a flow path that conveys the fluid through the first region (26'') and the second region (28''); The heat exchanger of claim 2 , wherein the depth of the flow passages decreases with radial distance from the central axis.

8. A cryogenic cooling device (1), comprising: A refrigerator and A heat exchanger (53) according to any one of claims 1 to 7; Equipped with The first conduit (46) is arranged to convey a working fluid to the refrigerator, and the second conduit (48) is arranged to convey the working fluid from the refrigerator.

9. A dilution refrigerator, comprising: a still (9) and a mixing chamber (45); A heat exchanger (53) according to any one of claims 1 to 7; Equipped with the first conduit (46) is arranged to pass working fluid from the still to the mixing chamber, and the second conduit (48) is arranged to pass working fluid from the mixing chamber to the still; The heat exchanger is configured to thermally couple the working fluid in the first conduit with the working fluid in the second conduit.

10. 10. The dilution refrigerator of claim 9, wherein the mixing chamber (45) comprises a mass of sinter (36), the first conduit has an end portion (40) that is open and extends around a portion of the sinter mass to contact the portion of the sinter mass with the working fluid, and the second conduit extends around the end portion and the sinter mass to convey the working fluid away from the sinter mass.

11. 10. The dilution refrigerator of claim 9, further comprising a cold plate (7) disposed on the first conduit (46) between the still (9) and the mixing chamber (45), the cold plate acting as an intermediate heat sink between the still (9) and the mixing chamber, the dilution refrigerator further comprising a chamber assembly including one or more of the chambers (30, 130) disposed along a portion of the first conduit extending between the cold plate and the mixing chamber, each of the chambers positioned to receive the working fluid from the first conduit (46), and the second conduit (48) thermally coupled to an exterior of each of the chambers.

12. 12. The dilution refrigerator according to claim 11, wherein the chamber assembly includes a first chamber (130) and a second chamber (30), the first chamber being disposed between the cooling plate (7) and the second chamber, and the depth of the second chamber and / or the number of openings penetrating the plate of the second chamber or the size of the openings penetrating the plate of the second chamber are greater than those of the first chamber.

13. 12. The dilution refrigerator of claim 11, wherein each of the chambers (30′) includes one or more flow paths (21′) that transport the fluid through the respective first region and the respective second region, each of the flow paths being formed in sinter, the chamber assembly including a first chamber and a second chamber, the first chamber and the second chamber being arranged along a thermal gradient during operation of the dilution refrigerator, the first chamber being arranged along a portion of the first conduit at a position between the cold plate (7) and the second chamber, and a diameter of the one or more flow paths in the first chamber being smaller than a diameter of the one or more flow paths in the second chamber.

14. 1. A method of forming a heat exchanger for a cryogenic cooling system, comprising: providing a first conduit (46), a second conduit (48), a first end (22), a second end (24), and a flow deflector (16), the flow deflector including a collar (17) and a plate (18), the plate extending across the collar; Including, The step of providing the first end portion (201) comprises: a first peripheral support member (14) fused around the periphery of a first foil member (10) and thereafter applying a sintering material (15) to an opposite surface of said first foil member, said first peripheral support member having a thermal conductivity at least 20 times lower than that of said first foil member at a temperature of 300K; The method further comprises: fusing the first peripheral support member (14) to the collar (17) to form a chamber (30), the chamber having a first region (26) separated from a second region (28) by the plate (18), the plate being disposed between the first end and the second end; Including, the first conduit (46) is positioned to convey a fluid into the first region (26) and out of the second region (28), and the plate (18) includes one or more openings (20) that allow the fluid to flow from the first region to the second region; A second conduit (48) is thermally coupled to the exterior of the chamber (30).

15. 1. A method of forming a dilution refrigerator, comprising: Providing a still (9) and a mixing chamber (45); forming (202) a heat exchanger according to any one of claims 1 to 7; Including, the first conduit (46) is arranged to pass working fluid from the still to the mixing chamber; The second conduit (48) is positioned to pass the working fluid from the mixing chamber to the still.

16. The first conduit (46) includes an end portion (40) arranged to receive the working fluid from the chamber (30), and the step (203) of providing the mixing chamber (45) comprises: placing the end portion (40) around a mass of sinter (36) so that a portion of the sinter mass is in contact with the working fluid; disposing the second conduit (48) around the end portion (40) and a portion of the sinter (36) mass to convey the working fluid away from the sinter mass; 16. The method of claim 15, comprising:

17. placing a cooling plate (7) at a position on the first conduit between the still (9) and the mixing chamber (45), the cooling plate acting as an intermediate heat sink between the still (9) and the mixing chamber; providing a plurality of said chambers (30, 130) disposed along a portion of said first conduit (46) extending between said cold plate and said mixing chamber; each of the chambers is positioned to receive the working fluid from the first conduit; a second conduit thermally coupled to an exterior of each of the chambers; the first conduit and the second conduit are formed from a plurality of modules for assembly; The method comprises:

16. The method of claim 15, further comprising the step of fusing a module of the first conduit with another module of the first conduit at a position between two of the chambers, and / or the step of fusing a module of the second conduit with another module of the second conduit at a position between two of the chambers.

18. The method of claim 17, wherein the modules are fused together using a localized heat source.

19. A dilution refrigerator, comprising: A still (9), a mixing chamber (45); a first conduit (46) arranged to pass working fluid from said still (9) to said mixing chamber (45); a cooling plate (7) located on the first conduit between the still (9) and the mixing chamber (45), the cooling plate (7) acting as an intermediate heat sink between the still (9) and the mixing chamber; a second conduit (48) arranged to pass the working fluid from the mixing chamber (45) to the still; a heat exchanger assembly comprising one or more heat exchangers (53, 53', 53'') disposed along a portion of the first conduit extending between the cold plate and the mixing chamber; Equipped with Each of the heat exchangers includes a chamber (30) positioned to receive the working fluid from the first conduit (46); the second conduit (48) is thermally coupled to the exterior of the chamber, the chamber having a first region (26) and a second region (28), the first region separated from the second region by a plate (18) extending through the chamber, the plate (18) including one or more openings (20) that allow the fluid to flow from the first region (26) to the second region (28), and the heat exchanger configured to thermally couple the working fluid in the first conduit (46) with the working fluid in the second conduit (48). Dilution refrigerator.

20. 20. The dilution refrigerator of claim 19, wherein the mixing chamber (45) comprises a mass of sinter (36), the first conduit (46) has an end portion (40) that is open and extends around a portion of the sinter mass to contact the portion of the sinter mass with the working fluid, and the second conduit extends around the end portion and the sinter mass to convey the working fluid away from the sinter mass.

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