Heat exchangers for cryogenic cooling systems

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] There are many applications that require cooling to millikelvin temperatures. Such temperatures can be obtained by the operation of a dilution refrigerator. A dilution unit will form part of the dilution refrigerator, the dilution unit comprising a still and a mixing chamber connected by a set of heat exchangers. In operation, a working fluid formed of a helium-3 / helium-4 mixture circulates in the dilution unit. Cooling is obtained in the mixing chamber from the enthalpy of mixing when helium-3 is diluted into 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 by the latent heat of vaporization. A cold plate is placed between the still and the mixing chamber and generally obtains the temperature between these two components in use.

[0003] The heat exchanger is a key element of dilution refrigerator design and is used to combine the "cold" Helium 3 leaving the mixing chamber with the "warm" Helium 3 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 FIG. 1. The working fluid flows between the still 102 and the mixing chamber 105 through two countercurrent paths in the heat exchange unit. The heat exchange unit comprises a series heat exchanger 101 disposed between the still 102 and the cold plate 103. The series heat exchanger 101 comprises a helically configured coaxial unit through which the two paths go in opposite directions, with the inner path surrounded by the outer path in a coiled configuration (not shown). Series heat exchangers can generally be used to obtain temperatures up to about 30 millikelvin. Dilution refrigerators using only series heat exchangers are limited to operate at about 30 millikelvin due to the increased Kapitza (thermal boundary) resistance between liquid helium and metal at low temperatures. At lower temperatures, series heat exchangers do not provide enough 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-like structure with two paths separated by a foil. The number of step heat exchangers provided can be selected to suit the application.

[0005] There are two geometries of step heat exchangers in common use: "counterflow block" and "semi-continuous". For counterflow block, there are two countercurrent flows of helium-3 that are 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 tubing is made up of a set of individual sinters that are bonded 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] The assembly of the dilution unit, and in particular the above step heat exchangers, is usually a complex and labor-intensive procedure that requires hundreds of hours of highly trained technicians to complete. The performance of the dilution refrigerator is sensitive to minor differences in the assembly process, and the high reliance on manual assembly techniques means that the final performance of the dilution refrigerator cannot always be guaranteed accurately in advance. It is desirable to reduce the standard deviation between the heat exchangers manufactured according to a specific process and the performance of the dilution refrigerator. It is also desirable to provide a simpler method of building these devices that is amenable to automation. The present invention is defined in the context of solving such problems. Summary of the Invention [Means for solving the problem]

[0007] A first aspect of the 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 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 separated from the second region by a plate extending through the chamber, the plate comprising one or more openings to allow flow of fluid 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. Thus, the repeatability of the heat exchanger's performance is improved compared to some prior art heat exchangers. The plates are preferably positioned to impede the flow of fluid through the chamber. Thus, the openings can be positioned relative to the first conduit such that the fluid follows a non-linear path through the chamber. Since the second conduit is thermally coupled to the exterior of the chamber, the non-linear path increases the thermal coupling between the fluid from the first conduit and the fluid in the second conduit within the chamber.

[0009] The chamber can be arranged along the first conduit. In other words, the chamber can be arranged to receive the fluid directly from a first portion of the first conduit, and the second portion of the first conduit can be arranged to receive the fluid directly from the chamber. The first conduit is typically fluidly coupled to the chamber interior at a first location in the first region and at a second location in the second region, with the one or more openings being 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 comprises 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, the one or more openings can be radially distributed from the central axis. The extension of the first conduit 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 engaged, the first conduit is preferably located inside the second conduit. Similarly, the chamber is preferably located inside the second conduit. The inner fluid of the second conduit will be in direct contact with the outside of the first conduit and the chamber.

[0011] The chamber preferably comprises a first end and a second end, each of which forms an opposing side of the chamber, the first end being joined to the second end by a flow deflector, the flow deflector comprising a collar separating the first end from the second end, and the plate extending across the collar to form a portion of the flow deflector. These components may be fused together as described below. Typically, one or both of the first end and the second end comprises a first face disposed inside the chamber, a second face disposed outside the chamber, and a foil member disposed between the first and second faces, each of the first and second faces comprising a sintered material applied to the foil member. The sintered material may be a metal powder such as silver, copper, titanium, etc., typically a metal such as that used for the foil member. The sintered body is porous and has a large effective surface area to provide 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 faces can be contoured such that the thickness of the sinter on the foil member increases with the radial separation from the central axis. This is particularly advantageous when the heat exchanger comprises a central axis extending through the center of the chamber and the first conduit is coupled to the chamber at two locations located along the central axis. By contouring the sinter in this manner, viscous heating in the heat exchanger is typically reduced. Typically, both the first and second ends 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 ends can be contoured such that the separation between the first face and the plate decreases, typically linearly, with the radial offset from the central axis. Similarly, the sinter on the first and second ends can be contoured such that the separation between the second face and the second conduit decreases, typically linearly, with the 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 faces 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 ends. The particular value may be selected depending on the operating temperature of the heat exchanger. The maximum separation between the sinter on the first face and the plate is typically 0.1 to 5.0 mm, preferably 0.1 to 3.0 mm, 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" inside the chamber.

[0013] The same material is typically used to form the collar and the peripheral support member. For example, each of the collar and the peripheral support member may be formed of stainless steel. The sintered material and the foil member are preferably formed of the same material, for example, silver, copper, or titanium. The thermal conductivity of the foil member and / or the sintered material is preferably substantially higher than that of the peripheral support member and / or the collar. For example, the thermal conductivity of the foil member and / or the sintered material may be at least 20 times greater than that of the peripheral support member and / or the collar at a temperature of 300K. The thermal conductivity of a material generally depends on its temperature, but in this case the manufacturing process is typically carried out at a nominal "room temperature". At 300K, the thermal conductivity of copper is about 392 W / m / K and that of stainless steel is about 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 conveys the fluid through the first and second regions. For example, the fluid can flow from an inlet of the chamber to an outlet of the chamber through an internal volume defined by the separation distance between the first face of the end and the plate. Alternatively, the flow path can be partially or completely formed in the sinter applied to the first and second end, specifically in the sinter applied to the "first face" (facing the plate) of each of the end. The flow path can comprise one or more flow paths through the chamber, the one or more flow paths being shaped by the sinter applied to the first and second end. Thus, the flow path can be imprinted into the sintered material to define one or more paths along which the fluid flows through the first and second regions. Thereby, the flow direction of the working fluid is controlled, which can allow for better heat distribution through the chamber and improved thermal performance of the heat exchanger. Also, the one or more flow paths can be imprinted into the sinter applied to the "second face" of the end forming 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 effects of viscous heating against the helium-3 demand.

[0015] Further aspects of the 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 suitable for replacing prior art step heat exchangers using liquid helium, but may be applied in a variety of cryogenic cooling systems. A second aspect of the invention provides a cryogenic cooling device comprising a target refrigerator and a heat exchanger according to the first aspect, a first conduit arranged to convey a working fluid to the target refrigerator and a second conduit 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 also 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 flow working fluid from the still to the mixing chamber and a second conduit is arranged to flow 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 comprises a mass of sinter, a first conduit with an end portion that is open and extends around a portion of the mass of sinter to contact the portion of the mass of sinter with the working fluid, and a second conduit that extends around the end portion and the mass of sinter to convey the working fluid away from the mass of sinter. The dilution refrigerator is preferably configured such that operation of the dilution refrigerator creates a phase boundary in the working fluid at a location inside the end portion of the first conduit. This phase boundary typically refers to a boundary between rich and dilute 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, such that it is in thermal contact with the outgoing dilute phase conveyed by the second conduit in the fractionator and along the first conduit. It should be understood that the rich and dilute phases typically do not mix in the fractionator.

[0019] The heat exchanger is typically simpler in structure than the prior art step heat exchangers, and therefore more suitable for low temperature applications. Thus, a particular advantage is 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 comprise a cold plate arranged between the still and the mixing chamber, the cold plate being arranged to obtain a reference temperature between the still and the mixing chamber during operation of the dilution refrigerator, the dilution refrigerator further comprising 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 being arranged to receive the working fluid from the first conduit, and the second conduit being thermally coupled to the outside of each of the chambers.

[0020] In steady state operation, the total flow rate of fluid through all chambers will be equal. The temperature of the fluid from the first conduit typically decreases as it progresses through the chamber assembly to regions of lower temperature. Often the viscosity of the fluid increases as the temperature is reduced, and the flow of viscous fluids can cause unwanted heating and reduce the efficiency of the heat exchanger. To mitigate this, so-called "flow paths" can be introduced to provide a low impedance path for the fluid to flow. The size of these flow paths is generally controlled to provide the required fluid flow rate while reducing the total amount of fluid in the chambers (resulting in a reduction in the amount of Helium-3 required for operation, a scarce and expensive resource). Many existing dilution refrigerators rely on custom, uniquely sized or shaped parts, but mass production and automation aid in parts commonality. Thus, preferably, each chamber comprises one or more flow passages for conveying fluid through the respective first region and the respective second region, each flow passage formed in the sinter, the chamber assembly being arranged along a thermal gradient during operation of the dilution refrigerator, the first chamber being arranged to obtain a higher reference temperature than the second chamber, and the diameter of the one or more flow passages in the first chamber being smaller than the diameter of the one or more flow passages in the second chamber. The diameter of the flow passages can thereby be controlled to achieve a desired balance between flow rate and total fluid volume, thereby improving thermal performance throughout the heat exchanger. The second conduit can also comprise flow passages formed in the sinter on the outside of the chamber to further improve the thermal performance of the heat exchanger. Also, by imprinting the flow passages in the sinter, the flow passages can be mass-produced efficiently and repeatably.

[0021] The chamber assembly may form step heat exchangers, each heat exchanger corresponding to a respective step and configured to obtain a respective temperature during operation of the dilution refrigerator. The chamber assembly may comprise a first said heat exchanger and a second said heat exchanger, the first said heat exchanger being disposed between a cold plate and a second heat exchanger chamber, the depth of the chamber of the second said heat exchanger and / or the number / size of the openings through the plate of the second said heat exchanger being greater than that of the first said heat exchanger. Thereby, the flow of fluids through the chamber assembly is optimized to improve the performance of the system, as described above.

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

[0023] A fourth aspect of the 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, the step of providing the first end portion comprising 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, the first peripheral support member having a thermal conductivity greater than or equal to that of the first foil member for 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, a first conduit being positioned to transport fluid into and out of the first region and out of the second region, the plate having one or more openings to permit fluid flow from the first region to the second region, and the second conduit being thermally coupled to an exterior of the chamber.

[0024] The method is substantially easier to implement than the complicated bonding processes typically required to assemble prior art step heat exchangers. Also, the method is more amenable to automation. As a result, the assembly of the heat exchanger takes less time and the standard deviation in the performance of 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 in use. The sintering material has a tendency 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 as well. 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 also the first peripheral support member 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 done to form the second end portion. For example, providing the second end portion may include fusing a second peripheral support member to an outer periphery of the second foil member and then applying a sintering material to an opposite surface of the second foil member, the thermal conductivity of the second peripheral support member being at least 20 times lower than the thermal conductivity of the second foil member at a temperature of 300K, and forming the chamber further includes fusing the second peripheral support member to the collar. The method may further include fusing a second portion of the first conduit to the second foil member to facilitate fluid flow through the second foil member, the second portion of the first conduit being 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 that 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 said peripheral support member is preferably fused to the respective foil member by vacuum brazing. In contrast, each said support member 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 at this stage the sintering material has already been applied to the foil member 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 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 flow working fluid from the evaporator to the mixing chamber and a second conduit is arranged to flow working fluid from the mixing chamber to the evaporator.

[0027] The first conduit preferably comprises 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 placing the end portion around a portion of the sintered mass to bring the portion of the sintered mass into contact with the working fluid, and placing the 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 formed to be received by the end portion of the first conduit. The method can then further include the step of 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, the method further comprising 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, 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 the 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. The method thus further comprises the steps of arranging the cold plate between the still and the mixing chamber such that a 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 the working fluid from the first conduit, and a second conduit being thermally coupled to the exterior of each said chamber. As before, the first and second conduits are preferably formed from a plurality of modules for assembly, the method further comprising fusing a first module of the first conduit together with a second module of the first conduit at a position between two of said chambers, and / or fusing a first module of the second conduit together with a second module of the second conduit at a position between two of said chambers. These chambers will typically be formed similarly and have the features described in relation to the first aspect. The modules are preferably fused using a localized heat source, preferably by electron beam welding, which reduces the heat input to the sinter. Furthermore, to ensure an effective heat transfer between the working fluids in the first and second conduits, the part of the first conduit that extends from the cold plate to the mixing chamber is preferably arranged inside the second conduit. Also, the chamber is preferably arranged 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, and 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 said chambers, and the first conduit being preferably formed of a plurality of modules welded together between each of said chambers, each of said 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 end and second end having a first face disposed inside the chamber and a second face disposed outside the chamber, the first face and second face being formed from a sintered material applied to a foil member disposed between the first face and the second face, each of the first and second end further comprising a respective outer support member extending around a respective 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] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to the accompanying drawings. [Brief description of the drawings]

[0033] [Figure 1] FIG. 1 is an illustration of a prior art dilution unit. [Diagram 2] FIG. 2 is a perspective view of a foil member forming part of a first embodiment of the present invention; [Diagram 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; [Diagram 5] FIG. 2 is a perspective view of a first end forming part of a first embodiment of the present invention prior to application of a sintering material; [Figure 6] FIG. 1 is a perspective view of a flow deflector forming part of a first embodiment of the present invention; [Figure 7] FIG. 2 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] FIG. 2 is a first cross-sectional view of a mixing chamber forming part of a first embodiment of the present invention. [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 flow chart illustrating a method according to an embodiment of the present invention. [Figure 13] FIG. 4 is a cross-sectional view of a chamber forming part of a second embodiment of the present invention. [Figure 14] FIG. 11 is an illustration of a foil member forming part of the second embodiment; [Figure 15] FIG. 11 is a cross-sectional view of a chamber forming part of a third embodiment of the present invention. [Figure 16] FIG. 11 is a cross-sectional view of a chamber forming part of a fourth embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] A method of assembling a heat exchanger and dilution refrigerator according to a first embodiment of the invention will now be described. The method begins at step 201 (Figure 12) at which point first and second end pieces are manufactured to form part of a heat exchanger. A first foil member 10 (Figure 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 300K. In this case, the first foil member 10 is a substantially planar silver disc having a central opening which receives an inlet tube 12 which, in use, forms a first portion of a first conduit (described below) which carries a working fluid. The first foil member 10 has a diameter of about 45mm, but more typically will be between 20 and 100mm depending on the application. A first peripheral support member 14 (Figure 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 300K. 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 opposing faces of the first foil member 10. The first foil member 10, inlet tube 12, and peripheral support member are assembled as shown in Figure 5 and then fused together, for example, by welding or vacuum brazing.

[0035] The material to be sintered is then applied as a powder to the main faces of the first foil member 10. The sintering material is a highly thermally conductive material and is typically the same as the material used for the first foil member 10. Pressure is applied to form sinters 15 on the two opposite faces of the first foil member 10. In the case of silver powder, this operation requires only pressure, but in the case of copper powder, it is typically also necessary to sinter during this operation. With a suitable tool, the powder can be pressed onto both sides of the first foil member 10 in one operation. The sinters 15 are typically applied to the entire surfaces of the two main faces of the first foil member 10, but not to the peripheral support member 14. Thereby, a first end portion 22 for the heat exchanger is produced. The process is then repeated with 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 22 and the second end 24 are configured to fit against opposite ends of the flow deflector 16, as shown in FIG. 6. The flow deflector 16 comprises a collar 17, which is an annular element having approximately the same circumference as the peripheral support member. A plate 18 extends radially across the flow deflector 16. The plate 18 is approximately centrally located within the collar 17 and subdivides the flow deflector 16 into an upper portion and a lower portion located on opposite sides of the plate 18 inside the collar 17. The plate 18 comprises a number of openings 20 for fluidly coupling the upper and lower portions. In FIG. 6, the openings are distributed at a constant radius around the plate 18, with a substantially constant spacing maintained between each adjacent opening. In this embodiment, the flow deflector 16 is formed as a unitary member. In particular, 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 may be formed, for example, from a foil element with openings etched into the foil element that 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 the sinter 15 is applied to a 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 welding in this embodiment is shown in FIG. 7, it should be understood that the welding is typically performed 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 chamber 30 formed has a first region 26 separated from a second region 28 by a plate 18, with an inlet tube 12 arranged to flow fluid into the first region and an outlet tube 32 arranged to flow fluid out of the second region. The inlet tube 12 and outlet tube 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 correspond to the flow of a lithium-3 rich phase working fluid from the still (including from outside the still) to a mixing chamber 45 of the dilution refrigerator 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 show the direction of fluid flow through the interior of the chamber 30. As shown, the arrangement of the apertures 20 radially distributed from a central axis along which the inlet tube 12 and outlet tube 32 are located ensures that the fluid follows a non-linear flow path inside the chamber 30. This, combined with the use of sintered material 15 on the first and second ends 22, 24, ensures that a large effective surface area is provided for heat exchange between the fluid inside the chamber 30 and another fluid in contact with the exterior of the chamber 30. The origin and flow of this surrounding 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. An end portion 40 of a first conduit 46 is provided, the end 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 end portion 40 is positioned such that the first region 42 is configured to receive a 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 inside the second region 44 and a distal portion of the sinter mass is disposed outside the end portion 40. Thus, the end portion 40 is positioned relative to the mass of sinter 36 such that the phase boundary of the working fluid between the helium-3 rich phase and the helium-3 lean phase is 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 presented to show the direction of fluid flow along the first region 42 of the end portion 40, from around the mass of sinter 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. In use, the 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 called the "lean phase flow path" that is arranged to return fluid from the mixing chamber 45 to the still. The second conduit 48 is arranged coaxially around the outside of the first conduit 46. Each of the first conduit 46 and the second conduit 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 positioned 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 by an indium seal, but alternatively, a Conflat (CF) flange can be used to achieve this attachment. A 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 around 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 is maintained between the inner wall of the second conduit 48 and the outer wall of the first conduit 46. The second conduit 48 fits 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 is evaporated from the still and removed by a pumping system. This causes a flow of helium-3 atoms to cross the phase boundary in the 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 the mixing chamber 45. Thus, the lean phase of helium flowing along the second conduit 48 will be cooler than the incoming rich phase of helium-3 conveyed along the first conduit 46. The relatively large surface area of ​​the chamber 30 forms an effective heat exchanger, so that the fluid in the first conduit 34 and chamber 30 is further cooled before reaching the mixing chamber 45.

[0042] The heat exchanger assembly may comprise a plurality of step heat exchangers 53 or "steps", each step being formed with 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 FIG. 11 and described below. A cryostat 1 is provided comprising a large hollow cylinder, typically made of stainless steel or aluminum, with an outer vacuum vessel 5. Within the cryostat 1 are a number of spatially distributed stages including a first stage 6, a second stage 7, and a third stage 8. Each stage provides a platform made of a highly conductive material (e.g., copper) and is spaced from the remaining stages by low thermal conductivity rods (not shown). The second stage 7 is commonly referred to as the "cold plate" and 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 lowest temperature 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 of 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 the first PTR stage 3 to about 50 to 70 Kelvin and the second PTR stage 4 to about 3 to 5 Kelvin. The second PTR stage 4 thus 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. Additionally, the first and second stages 6 and 7 could theoretically be coupled to their own 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. A 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] The still 9 is fluidly coupled to a storage vessel 50 by a cooling circuit 37. The storage vessel 50 is located outside the cryostat 1 and contains a working fluid in the form of a mixture of helium-3 and helium-4 isotopes. The various pumps 17, 39 are also located outside the cryostat 1 along with the conduits of the cooling circuit 37 that control the flow of the working fluid around the circuit, as indicated by the solid arrows. The cooling circuit 37 comprises a feed line 41 that provides a conduit for facilitating the flow of the working fluid from the storage vessel 50 to a condensation line 46'. This fluid can then be conveyed along the condensation line 46' to the still 9, where the fluid is in thermal contact with the dilute phase of helium inside the still 9. The condensation line 46' then continues to a rich phase flow path 46 from the still 9 to a mixing chamber 45. The condensation line 46' and the rich phase flow path 46 further comprise one or more impedances (not shown) that reduce the temperature of the working fluid due to the Joule-Thomson effect as it flows towards the mixing chamber 45. The compressor pump 13 is positioned along the condensate line 46' which delivers 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 conveyed by the still pump line 48' to a location external to the cryostat 1. From this location the working fluid may 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 mentioned 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. These conduits are not explicitly shown between the first stage 6 and the third stage 8 in the schematic diagram of FIG. 11 for the sake of clarity. The first and second conduits 46, 48 are arranged to form a continuous heat exchanger 26, which is arranged between the first stage 6 and the second stage or "cold plate" 7. In 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 a plurality of step heat exchangers 53, 53', 53'' (as described above with reference to Figures 2-0) is disposed 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 a plurality of 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 depth of the chamber and / or the number or size of openings inside the chamber may be increased for chambers located at lower temperatures. For example, the depth of the chamber (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 found to improve the thermal performance of the system by optimizing the balance between viscous heating and total fluid volume.

[0050] The heights of the first and second regions 26 and 28 are depicted in Figures 7 and 8 as being relatively large for ease of illustration, but are preferably on the order of 0.1 to 5.0 mm, more preferably 0.2 to 1.5 mm, particularly when used in a dilution refrigerator. This height may vary depending on the operating temperature of the heat exchanger, as determined by the location along the first conduit 46 (as discussed above). In general, the shapes and dimensions of the first and second regions 26 and 28 are selected to facilitate circulation of the helium-3, reduce viscous heating, allow osmotic pressure to develop in the still 9 and mixing chamber 45, and reduce the amount of helium-3 used to reduce hydrodynamic instabilities and convection. Similar considerations apply to the 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 plate 18' for the flow deflector is a foil element welded between two annular supports. The chamber 30' is otherwise essentially formed as described for the first embodiment with reference to Figures 2 to 8, but 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 an opening and back to the outlet tube 32' (it will be appreciated that similar features can also be applied to the sinter 15' on the opposite side of the foil members 10', 11' to convey the dilute phase fluid). The sinter 15' is applied to both faces of the first and second foil members 10', 11', but as the first and second foil members 10', 11' are disposed within the chamber 30', the sinter 15 applied to the first face of the first foil member 10' and the first face of the second foil member 11' are separated from the opposite face of the flow deflector 18' by a small gap, typically 0.1-1.0 mm, preferably 0.2-0.6 mm (depending on configuration). Optionally, the opposite face of the flow deflector 18' abuts instead the sinter 15' on the first face of the first and second foil members 10', 11', leaving no such separation. The flow passages 21' are typically imprinted into the sinter during step 201 and can take a variety of different patterns to control the flow direction of the working fluid. One or more channels may be provided within the chamber 30' to convey the dense phase of the fluid through a first region to the 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 convey the dilute phase of the fluid through a second conduit on the outside of the chamber 30'. Any number of different patterns may be applied, including radial or spiral. FIG. 14 is a perspective view of the sinter 15' taken on the plane X-X' from FIG. 13. In FIG. 14, the channels 21' are branched to bring most of the sinter 15' into intimate contact with the working fluid to enhance the performance of the heat exchanger.

[0052] The shape of the channels 21' may be limited by what can be machined into the press die, but can be semicircular, elliptical, 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). The width of the channels may therefore 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 optimise 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 in the chamber generally have a constant height across the plate (typically between 0.5 and 4 mm). As a result, the flow rate of the fluid typically decreases as the fluid spreads radially outward across a wider area. This means that more viscous heating occurs towards 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 invention. Double primed reference numbers 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 such that the height of the first region 26″ and the second region 28″ continuously decreases 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 way results in “deeper” flow passages at smaller radii, which compensates for the above effects and reduces viscous heating. The maximum depth of the first region 26'' and the second region 28'' is typically about 1.5 mm, and the minimum depth is typically about 0.2 mm. However, the particular parameters may vary depending on the operating temperature of the heat exchanger, for example as determined by the location of such heat exchanger within a 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 such 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 a fluid conveyed along a second conduit that surrounds the chamber (as shown by FIG. 10). From the perspective of a dilution refrigerator, this is typically a dilute phase of helium-3. Further 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 such 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 therefore improve the performance of the cryogenic cooling system in which the heat exchanger is installed. Figures 13 to 16, although not to scale, show examples of various shapes that can be pressed into the sinter to control the flow of fluid through the heat exchanger.

[0055] As a result, an effective heat exchanger is provided that can operate at low temperatures, thereby ensuring reliable operation of the cryogenic cooling system. The heat exchanger design is of relatively simple construction and lends itself to welding and automated manufacturing processes, ensuring high repeatability in terms of thermal performance. Thus, lower temperatures can be obtained in cryogenic cooling systems, such as dilution refrigerators, that incorporate the heat exchanger, with the minimum obtainable temperature being determined by the performance of the heat exchanger. Such processes can also 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 24 Second end 26 First Area 28 The Second Region 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 fluid from the first conduit, the second conduit being 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) permitting flow of the fluid 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 portion 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) include 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 as claimed in 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 members being fused to a collar (17).

4. 4. A heat exchanger as claimed in 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 a center of the chamber (30'), and the first conduit is 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 a sinter (15') applied to the first end (22) and the second end (24).

7. The heat exchanger has a central axis extending through a center of the chamber (30''), and the first conduit is 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 target refrigerator (45); A heat exchanger (53) according to any one of claims 1 to 7; Equipped with A cryogenic cooling apparatus, wherein the first conduit (46) is arranged to convey a working fluid to the target refrigerator and the second conduit (48) is arranged to convey the working fluid from the target 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 flow the working fluid from the still to the mixing chamber, and the second conduit (48) is arranged to flow the 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 comprises 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 between the still (9) and the mixing chamber (45), the cold plate being arranged to obtain a reference temperature between the still and the mixing chamber during operation of the dilution refrigerator, 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 being arranged to receive the working fluid from the first conduit (46), and the second conduit (48) being 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 a depth of the second chamber and / or a number of openings penetrating the plate of the second chamber or a size of the openings penetrating the plate of the second chamber is greater than that 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 convey the fluid through a respective one of the first regions and a respective one of the second regions, each of the flow paths being formed in sinter, the chamber assembly being arranged along a thermal gradient during operation of the dilution refrigerator, a first chamber being arranged to obtain a higher base temperature than a 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 refrigerator, comprising the steps of: 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 a first end portion (201) includes: the step of fusing a first peripheral support member (14) around an outer 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 said first peripheral support member (14) to said collar (17) to form a chamber (30), said chamber having a first region (26) separated from a second region (28) by said plate (18), said plate being disposed between said first end and said 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), the plate (18) including one or more openings (20) that permit flow of the fluid 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); Producing (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 flow 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 of providing (203) the mixing chamber (45) comprises: disposing said end portion (40) about a mass of sinter (36) so as to bring a portion of said mass of sinter into contact with said 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 cold plate (7) between the still (9) and the mixing chamber (45) so as to obtain a reference temperature between the still and the mixing chamber during operation of the dilution refrigerator; 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 location 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 location 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 cooling plate (7) arranged between the still (9) and the mixing chamber (45), the cooling plate (7) being arranged to obtain a reference temperature between the still and the mixing chamber during operation of the dilution refrigerator; a first conduit (46) arranged to pass working fluid from the still (9) to 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) disposed to receive the working fluid from the first conduit (46); the second conduit (48) is thermally coupled to an 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) permitting the flow of the fluid from the first region (26) to the second region (28), and the heat exchanger is 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) comprises 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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