HEAT EXCHANGER FOR CRYOGENIC COOLING APPARATUS

Abstract

A heat exchanger for a cryogenic cooling apparatus is provided. The heat exchanger has a first conduit, a second conduit and a chamber, wherein the chamber is arranged to receive a fluid from the first conduit, and wherein second conduit is thermally coupled to the outside of the chamber. The chamber has 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 apertures for allowing a flow of the fluid from the first region to the second region.

Claims

1-48. (canceled)

49. A heat exchanger for a cryogenic cooling apparatus, comprising: a first conduit, a second conduit and a chamber, wherein the chamber is arranged to receive a fluid from the first conduit, and wherein the second conduit is thermally coupled to the outside 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 apertures for allowing a flow of the fluid from the first region to the second region; wherein the chamber comprises a first end piece and a second end piece forming opposing sides of the chamber respectively, the first end piece coupled to the second end piece by a flow deflector, the flow deflector comprising a collar separating the first end piece from the second end piece, wherein the plate extends across the collar to form part of the flow deflector.

50. A heat exchanger according to claim 49, wherein one or both of the first end piece and the second end piece comprises a first face arranged inside the chamber, a second face arranged outside the chamber and a foil member arranged between the first face and the second face, wherein the first face and the second face each comprise a sintered material applied to the foil member.

51. A heat exchanger according to claim 50, wherein a peripheral support member is arranged around the perimeter of each said foil member, the peripheral support member being fused to the collar.

52. A heat exchanger according to claim 51, wherein the thermal conductivity of the foil member and/or the sintered material is at least twenty times larger than that of the peripheral support member and/or the collar when at a temperature of 300 K.

53. A heat exchanger according to claim 50, wherein the heat exchanger comprises a central axis extending through the centre of the chamber, and wherein the first conduit is coupled to the chamber at two positions arranged along the central axis; wherein the first face and/or the second face is profiled so that the thickness of the sinter on the foil member increases with the radial distance from the central axis.

54. A heat exchanger according to claim 49, wherein the chamber defines a flow channel for conveying the fluid through the first region and the second region, wherein the flow channel is formed within a sinter applied to the first end piece and the second end piece.

55. A heat exchanger according to claim 50, wherein the heat exchanger comprises a central axis extending through the centre of the chamber, and wherein the first conduit is coupled to the chamber at two positions arranged along the central axis; wherein the chamber defines a flow channel for conveying the fluid through the first region and the second region, and wherein the depth of the flow channel decreases with the radial distance from the central axis.

56. A cryogenic cooling apparatus comprising: a target refrigerator; and a heat exchanger according to claim 49, wherein the first conduit is arranged to convey an operational fluid to the target refrigerator and the second conduit is arranged to convey the operational fluid from the target refrigerator.

57. A dilution refrigerator comprising: a still, a mixing chamber and a heat exchanger according to claim 49, wherein the first conduit is arranged to flow an operational fluid from the still to the mixing chamber and the second conduit is arranged to flow the operational fluid from the mixing chamber to the still, the heat exchanger configured to thermally couple the operational fluid in the first conduit with the operational fluid in the second conduit.

58. A dilution refrigerator according to claim 57, wherein the mixing chamber comprises a mass of sinter, and the first conduit comprises an end portion that is open and extends around a portion of the mass of sinter so as to bring said portion of the mass of sinter into contact with the operational fluid, the second conduit extending around the end portion and the mass of sinter so as to convey the operational fluid in a direction away from the mass of sinter.

59. A dilution refrigerator according to claim 57, further comprising a cold plate arranged between the still and the mixing chamber, the cold plate arranged to obtain a base temperature between that of the still and the mixing during operation of the dilution refrigerator, the dilution refrigerator further comprising a chamber assembly comprising one or more said chambers arranged along a portion of the first conduit extending between the cold plate and the mixing chamber, each said chamber being arranged to receive the operational fluid from the first conduit, and wherein second conduit is thermally coupled to the outside of each said chamber.

60. A dilution refrigerator according to claim 59, wherein the chamber assembly comprises a first said chamber and a second said chamber, the first said chamber being arranged between the cold plate and the second said chamber, wherein the depth of the second said chamber and/or the number of apertures through the plate of the second said chamber or the size of the apertures through the plate of the second said chamber is higher than that of the first said chamber.

61. A dilution refrigerator according to claim 59, wherein each said chamber comprises one or more flow channels for conveying the fluid through the respective first region and the respective second region, wherein each said flow channel is formed within a sinter, wherein the chamber assembly is arranged along a thermal gradient during operation of the dilution refrigerator so that a first said chamber is arranged to obtain a higher base temperature than a second said chamber, and wherein the diameter of the one or more flow channels in the first chamber is lower than the diameter of the one or more flow channels in the second chamber.

62. A method of forming a heat exchanger for a cryogenic refrigerator, the method comprising: providing a first conduit, a second conduit, a first end piece, a second end piece and a flow deflector, the flow deflector comprising a collar and a plate, the plate extending across the collar; wherein providing the first end piece comprises: fusing a first peripheral support member around the perimeter of a first foil member, and then applying a sintered material to opposing faces of the first foil member, the thermal conductivity of the first peripheral support member being at least twenty times lower than that of the first foil member when at a temperature of 300 K; and fusing the first peripheral support member to the collar so as to form a chamber, the chamber having a first region separated from a second region by the plate, the plate arranged between the first end piece and the second end piece; wherein the first conduit is arranged to convey a fluid into the first region and out from the second region, and wherein the plate comprises one or more apertures for allowing a flow of the fluid from the first region to the second region; and wherein the second conduit is thermally coupled to the outside of the chamber.

63. A dilution refrigerator comprising: a still; a mixing chamber; a cold plate arranged between the still and the mixing chamber, the cold plate arranged to obtain a base temperature between that of the still and the mixing chamber during operation of the dilution refrigerator; a first conduit arranged to flow an operational fluid from the still to the mixing chamber; a second conduit is arranged to flow the operational fluid from the mixing chamber to the still; and a heat exchanger assembly comprising one or more heat exchangers arranged along a portion of the first conduit extending between the cold plate and the mixing chamber; wherein each said heat exchanger comprises: a chamber arranged to receive the operational fluid from the first conduit, wherein the second conduit is thermally coupled to the outside 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 apertures for allowing a flow of the fluid from the first region to the second region, the heat exchanger configured to thermally couple the operational fluid in the first conduit with the operational fluid in the second conduit.

64. A dilution refrigerator according to claim 63, wherein the mixing chamber comprises a mass of sinter, and the first conduit comprises an end portion that is open and extends around a portion of the mass of sinter so as to bring said portion of the mass of sinter into contact with the operational fluid, the second conduit extending around the end portion and the mass of sinter so as to convey the operational fluid in a direction away from the mass of sinter.

65. A dilution refrigerator according to claim 63, wherein a said heat exchanger is arranged to obtain a temperature below 30 mK in use.

66. A dilution refrigerator according to claim 63, wherein the heat exchanger assembly comprises a first said heat exchanger and a second said heat exchanger, the first said heat exchanger being arranged between the cold plate and the second said heat exchanger, wherein the depth of the chamber of the second said heat exchanger and/or the number of apertures through the plate of the second said heat exchanger or the size of the apertures through the plate of the second said heat exchanger is higher than that of the first said heat exchanger.

67. A dilution refrigerator according to claim 63, wherein each said chamber comprises one or more flow channels for conveying the fluid through the respective first region and the respective second region, wherein each said flow channel is formed within a sinter, wherein the heat exchanger assembly is arranged along a thermal gradient during operation of the dilution refrigerator so that a first said heat exchanger is arranged to obtain a higher base temperature than a second said heat exchanger, and wherein the diameter of the one or more flow channels in the first heat exchanger is lower than the diameter of the one or more flow channels in the second heat exchanger.

68. A dilution refrigerator according to claim 63, wherein each said chamber comprises a first end piece and a second end piece forming opposing sides of the chamber respectively, the first end piece coupled to the second end piece by a flow deflector, the flow deflector comprising a collar separating the first end piece from the second end piece, wherein the plate extends across the collar to form part of the flow deflector.

69. A dilution refrigerator comprising: a still, a mixing chamber and a heat exchanger, wherein the heat exchanger comprises: a first conduit, a second conduit and a chamber, wherein the first conduit is arranged to flow an operational fluid from the still to the mixing chamber and the second conduit is arranged to flow the operational fluid from the mixing chamber to the still, the heat exchanger configured to thermally couple the operational fluid in the first conduit with the operational fluid in the second conduit; wherein the second conduit forms the exterior of the heat exchanger, wherein the chamber is arranged to receive the operational fluid from the first conduit, and wherein second conduit is thermally coupled to the outside 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 apertures for allowing a flow of the operational fluid from the first region to the second region.

70. A dilution refrigerator according to claim 69, wherein the first conduit and/or the second conduit is formed from a plurality of modules that are fused together.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Embodiments of the invention will now be discussed with reference to the accompanying drawings in which:

[0033] FIG. 1 is an illustration of a prior art dilution unit;

[0034] FIG. 2 is an illustration of a perspective view of a foil member forming part of a first embodiment of the invention;

[0035] FIG. 3 is an illustration of a perspective view of a first portion of a first conduit forming part of the first embodiment of the invention;

[0036] FIG. 4 is an illustration of a perspective view of a peripheral support member forming part of the first embodiment of the invention;

[0037] FIG. 5 is an illustration of a perspective view of a first end piece forming part of the first embodiment of the invention prior to applying a sintered material;

[0038] FIG. 6 is an illustration of a perspective view of a flow deflector forming part of the first embodiment of the invention;

[0039] FIG. 7 is a first cross-sectional illustration of a chamber forming part of the first embodiment of the invention;

[0040] FIG. 8 is a second cross-sectional illustration of a chamber forming part of the first embodiment of the invention;

[0041] FIG. 9 is a first cross-sectional illustration of a mixing chamber forming part of the first embodiment of the invention;

[0042] FIG. 10 is a second cross-sectional illustration of a mixing chamber forming part of the first embodiment of the invention;

[0043] FIG. 11 is a schematic illustration of a dilution refrigerator according to the first embodiment;

[0044] FIG. 12 is a flow chart illustrating a method according to an embodiment of the invention;

[0045] FIG. 13 is a cross-sectional illustration of a chamber forming part of a second embodiment of the invention;

[0046] FIG. 14 is an illustration of a foil member forming part of the second embodiment;

[0047] FIG. 15 is a cross-sectional illustration of a chamber forming part of a third embodiment of the invention; and

[0048] FIG. 16 is a cross-sectional illustration of a chamber forming part of a fourth embodiment of the invention.

DETAILED DESCRIPTION

[0049] A method for assembling a heat exchanger and a dilution refrigerator according to a first embodiment of the invention will now be discussed. The method begins at step 201 (FIG. 12), at which point first and second end pieces are produced for forming part of a heat exchanger. A first foil member 10 (FIG. 2) is provided that is formed from a high thermal conductivity material such as silver or copper, typically with 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 disc having a central aperture for receiving an inlet tube 12 forming a first portion of a first conduit (to be discussed) that conveys an operational fluid in use. The first foil member 10 has diameter of around 45 mm but more generally is usually between 20-100 mm depending on the application. A first peripheral support member 14 (FIG. 4) is provided, formed from a relatively low thermal conductivity material such as stainless steel, typically with a thermal conductivity below 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 outside of the first foil member 10, contacting the perimeter 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 FIG. 5, and then fused together, for example by welding or vacuum brazing.

[0050] A material to be sintered is next applied as a powder to the major faces of the first foil member 10. The sinter material is a high thermal conductivity material and typically the same material as used as the first foil member 10. Pressure is applied to form a sinter 15 on the two opposing faces of the first foil member 10. In the case of a silver powder pressure alone is sufficient for this operation, however copper powders typically also need to be baked during this operation. With the appropriate tooling, the powder can be pressed onto both sides of the first foil member 10 in one operation. The sinter 15 is typically applied to the entire surface of the two major faces of the first foil member 10 but not to the peripheral support member 14. A first end piece 22 for a heat exchanger is thereby produced. This process is then repeated with a second foil member 11, second peripheral support member and an outlet tube 32 to form a second end piece 24.

[0051] The first end piece 22 and the second end piece 24 are configured to fit against opposing ends of a flow deflector 16, which is shown by FIG. 6. The flow deflector 16 comprises a collar 17, which is an annular element having approximately the same circumference as the peripheral support members. A plate 18 extends across the flow deflector 16 in a radial direction. The plate 18 is arranged approximately centrally within the collar 17, subdividing the flow deflector 16 into an upper portion and a lower portion that are provided on opposite sides of the plate 18, inside the collar 17. The plate 18 comprises a plurality of apertures 20 to fluidly couple the upper and lower portions. In FIG. 6 these apertures are dispersed at a constant radius around the plate 18, with approximately a constant separation kept between each neighbouring aperture. In the present embodiment the flow deflector 16 is formed as a unitary member. In particular, the flow deflector 16 is a machined body and the apertures may be added by a process such as electrical discharge machining. Alternatively, the flow deflector 16 can be formed from a foil element with apertures formed in the foil element, for example by etching. This foil element would then be welded between two annular supports to form the flow deflector 16.

[0052] The method proceeds to step 202, at which point the heat exchanger chamber 30 is formed. The first and second end pieces 22, 24 are arranged against opposing ends of the flow deflector 16, as shown in FIG. 7, with the sinter material 15 applied to the distal face of each end piece arranged inside the collar 17, and the peripheral support members contacting opposing ends of the collar 17. A highly controlled, localised heating process, such as electron-beam welding, laser beam welding or Tungsten Inert Gas (TIG) welding, is then used to fuse the peripheral support members to the respective ends of the collar 17 and thereby form a chamber 30. The localised heating process used to fuse the first and second end pieces 22, 24 to the collar 17, combined with the relatively low thermal conductivity of the stainless steel peripheral support members, protects the sinter 15 from the heat of the welding process. The location of the electron-beam welds for the present embodiment are indicated in FIG. 7, however it will be appreciated that the weld is typically made around the circumference of the peripheral support members. It is advantageous therefore that the inlet tube 12 and the outlet tube 32 extend along the central axis for the flow defector 16 and the chamber 30 because the assembly can then be rotated about the inlet tube 12 and the outlet tube 32 when performing the welding procedure, without needing to move the heating element. This process is amenable to automation and ensures that a reliable joint is welded.

[0053] The chamber 30 formed has a first region 26 separated from a second region 28 by the plate 18, with the inlet tube 12 arranged to flow a fluid into the first region, and the outlet tube 32 arranged to flow a fluid out of the second region. The inlet tube 12 and the outlet tube 32 form first and second portions of a first conduit 46 respectively, the first conduit 46 being arranged to flow a fluid through the chamber 30. When used within a dilution refrigerator the first conduit 46 and the chamber 30 will accommodate the flow of helium-3 rich phase of operational fluid during steady state operation from a still (including from the outside of the still) to a mixing chamber 45 of the dilution refrigerator. The first conduit 46 is also commonly referred to as the concentrated phase flow channel in a dilution refrigerator. Arrows are included to FIG. 8 (which is not to scale) to indicate the direction of flow of the fluid through the inside of the chamber 30. As shown, the arrangement of the apertures 20, which are radially dispersed from the central axis along which the inlet and outlet tubes 12, 32 are arranged, ensures that the fluid follows a non-linear flow path inside the chamber 30. This, combined with the use of the sintered material 15 on the first and second end pieces 22, 24, ensures that a large effective surface area is provided for heat exchange between a fluid inside the chamber 30 and another fluid in contact with the outside of the chamber 30. The origin and flow of this surrounding fluid will be now discussed with reference to steps 203 and 204 from FIG. 12.

[0054] The method proceeds to step 203, at which point a mixing chamber 45 for the dilution refrigerator is formed. A mass of sinter 36 is formed directly onto, or mounted to a high thermal conductivity support 8, which forms the lowest temperature stage of the dilution refrigerator. The material forming the mass of sinter 36 is typically the same material as was applied to the first and second foil members 10, 11 (e.g. silver and/or copper). An end portion 40 of the 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 arranged so that the first region 42 is configured to receive a flow of the fluid from the outlet tube 32 and the second region 44 is arranged so that a proximal portion of the mass of sinter is arranged inside the second region 44 and a distal portion of the mass of sinter is outside the end portion 40. The end portion 40 is thus arranged relative to the mass of sinter 36 so that a phase boundary of the operational fluid between a helium-3 rich phase and a helium-3 poor phase exists inside the end portion 40 and preferably inside the second region 44, as shown by the broken line in FIG. 9. Arrows are provided in FIG. 9 to show the direction of flow of the fluid from along the first region 42 of the end portion 40 and around the mass of sinter 36 into a region surrounding the mixing chamber 45. Of course, this direction of flow is only possible when the dilution refrigerator is fully assembled and operational. In use, the concentrated steam is typically contained inside the end portion 40 and so the end portion 40 may also be referred to as a concentrated steam cap.

[0055] FIG. 10 shows a second conduit 48 formed surrounding the first conduit 46. The second conduit 48 is also referred to as the dilute phase flow channel that is arranged to return the fluid from the mixing chamber 45 to the still. The second conduit 48 is co-axially arranged around the outside of the first conduit 46. The first conduit 46 and the second conduit 48 are each formed from a series of modules that are welded together at step 204 to form the heat exchanger assembly. With the end portion 40 arranged over the mass of sinter 36 (as discussed with reference to FIG. 9), a first portion 50 of the second conduit 48 is arranged over and around the end portion 40 and mounted to the support 8. Typically, the first portion 50 of the second conduit 48 is sealed to the support 8 by an indium seal, alternatively though a ConFlat (CF) flange may be used to achieve this mounting. The first portion 50 of the second conduit 48 is thereby arranged to receive a flow of operational fluid from the end of the first conduit 46.

[0056] A distal end of the outlet tube 32 is then welded to a proximal end of the first region 42 of the end portion 40. This fluidly couples the inlet tube 12 with the mixing chamber 45 and the second conduit 48. A distal end of a second portion 52 of the second conduit 48 is then fused to a proximal end of the first portion 50 of the second conduit 48. This joint is made around the central axis of the assembly and at a position 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. FIGS. 7-10 are schematic illustrations and thus not to scale, however, an approximate constant separation is maintained between the inner walls of the second conduit 48 and the outer walls of the first conduit 46. The second conduit 48 conforms around the shape of the chamber 30 to form a step in a 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 drives a flow of helium-3 atoms to cross the phase boundary at the mixing chamber 45 (from the concentrated to dilute phases) to replenish the helium-3 in the still. The dilution of helium-3 into the dilute phase causes cooling at the mixing chamber 45. The dilute phase of helium that flows along the second conduit 48 will therefore be cooler than the incoming concentrated 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 prior to arriving at the mixing chamber 45.

[0057] The heat exchanger assembly may comprise a plurality of step heat exchangers 53 or steps, each step formed of a chamber (as described with reference to FIGS. 7 and 8) arranged along the first conduit 46, and being surrounded by a portion of the second conduit 48. FIG. 10 shows a second such chamber 130 together with corresponding portions of the first conduit 46 arranged above the proximal end of the inlet tube 12. The respective portions of the first and second conduits would be fused together about the central axis, as discussed above, in a step-wise manner to form the completed assembly.

[0058] The arrangement of the heat exchanger assembly within a dilution refrigerator is shown by the schematic illustration in FIG. 11, which will now be discussed. A cryostat 1 is provided comprising a large hollow cylinder, typically formed from stainless steel or aluminium, which comprises an outer vacuum vessel 5. A plurality of spatially dispersed stages is provided within the cryostat 1, comprising a first stage 6, a second stage 7 and a third stage 8. Each stage provides a platform formed from high conductivity material (e.g. copper) and is spaced apart 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 intermediary heat sink between the first stage 6 and the third stage 8. A sample holder 55 is shown mounted to the third stage 8, which forms the lowest temperature stage during steady state operation of the system.

[0059] The cryostat 1 in the present example is substantially cryogen-free (also referred to in the art as dry) in that it is not principally cooled by contact with a reservoir of cryogenic fluid. The cooling of the cryostat is instead achieved by 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 when in use to facilitate normal operation of the dilution unit. The main cooling power of cryostat 1 is provided in this embodiment by a PTR 2. PTRs generate cooling by controlling the compression and expansion of a working fluid which is supplied at high pressure from an external compressor. The first PTR stage will typically have a relatively high cooling power in comparison with the second PTR stage. In the present case, the PTR 2 cools a first PTR stage 3 to about 50 to 70 kelvin and a second PTR stage 4 to about 3 to 5 kelvin. The second PTR stage 4 therefore forms the lowest temperature stage of the PTR 2.

[0060] Various heat radiation shields are provided inside the outer vacuum vessel 5, wherein each shield encloses each of the remaining lower base-temperature 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 and the second radiation shield 54 surrounds each of the first, second and third stages 6-8. Additionally, the first and second stages 6 and 7 could in theory be connected to respective heat radiation shields, in order to reduce any unwanted thermal communication between the stages.

[0061] The still 9 of the dilution refrigerator is operable to cool the first stage 6 to a base temperature of 0.5-2 kelvin. The mixing chamber 45 is mounted to the third stage 8 and is operable to cool the third stage 8 to a base temperature below 10 millikelvin. In use, the second stage 7 obtains a base temperature between that of the first stage 6 and the third stage 8, typically of 40-150 millikelvin.

[0062] The still 9 is fluidly coupled to a storage vessel 50 by a cooling circuit 37. The storage vessel 50 is arranged outside the cryostat 1 and contains an operational fluid in the form of a mixture of helium-3 and helium-4 isotopes. Various pumps 17, 39 are also arranged outside the cryostat 1, along conduits of the cooling circuit 37 for controlling a flow of the operational fluids around the circuit, as indicated by the solid arrowheads. The cooling circuit 37 comprises a supply line 41 which provides a conduit to facilitate a flow of operational fluid from the storage vessel 50 to a condensing line 46. This fluid may then be conveyed along the condensing line 46 to the still 9 whereupon it is in thermal contact with the dilute phase of helium inside the still 9. The condensing line 46 then continues into a concentrated phase flow channel 46 from the still 9 to the mixing chamber 45. The condensing line 46 and the concentrated phase flow channel 46 further comprise one or more impedances (not shown) for reducing the temperature of the operational fluid due to the Joule-Thomson effect as it flows towards the mixing chamber 45. A compressor pump 13 is arranged along the condensing line 46 for providing this flow at a pressure of 0.5-2 bar. A dilute phase flow channel 48 is arranged to convey the operational fluid from the mixing chamber 45 through the still 9, whereupon this fluid is conveyed to a position exterior to the cryostat 1 by a still pumping line 48. The operational fluid may then be circulated from this position back into the condensing line 46. A turbomolecular pump 39 is arranged along the still pumping line 48 for providing a high vacuum on the low pressure side of the circuit (for example less than 0.1 mbar), and so enables the flow of the operational fluid away from the still 9.

[0063] The concentrated phase flow channel 46 and dilute phase flow channel 48 form the first and second conduits respectively of the heat exchanger, as earlier discussed. These conduits are not explicitly shown between the first stage 6 and the third stage 8 in the schematic illustration of FIG. 11 for sake of clarity. The first and second conduits 46, 48 are arranged to form a continuous heat exchanger 26 positioned 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 wrapped around the first conduit 46. This ensures that the helium-3 concentrated phase of fluid flowing along the first conduit 46 is cooled by the helium-3 dilute phase of fluid flowing along the second conduit 48. Continuous heat exchangers are only are only typically effective at temperatures above 30 millikelvin. Therefore a step heat exchanger assembly comprising a plurality of step heat exchangers 53, 53, 53 (as earlier discussed with reference to FIGS. 2-10) is arranged at the lower temperature region between the second stage 7 and the third stage 8. The fluid in the first conduit 46 flows from the second stage 7 to the mixing chamber 45 via a plurality of chambers comprising flow deflectors. The helium-3 dilute phase of fluid flows from the mixing chamber 45 back along the second conduit that encloses the chambers. The outgoing fluid in the second conduit is directly in contact with the outer walls of the first conduit and the chambers to further cool the incoming fluid in the first conduit.

[0064] The viscosity of a fluid may increase as the temperature is reduced and the flow of a viscous fluid can lead to unwanted heating, reducing the efficiency of the heat exchanger. To mitigate against this, the depth of the chambers and/or the number or size of apertures inside the chamber may increase for chambers that are arranged at lower temperatures. For example, the depth of the chamber (along the central axis of the assembly) may be smallest for the uppermost step heat exchanger (to reduce the total volume of helium-3 required for operation) and largest for the lowermost step heat exchanger (to reduce viscous heating).

[0065] It has been found that this increases thermal performance of the system by optimising the balance between viscous heating and total fluid volume.

[0066] The height of the first region 26 and the second region 28 is depicted as being relatively large in FIGS. 7 and 8 for ease of explanation but, in particular where used within a dilution refrigerator, is preferably of the order of 0.1-5.0 mm, preferably still 0.2-1.5 mm. This height may vary depending on the operating temperature of the heat exchanger, as determined by its position along the first conduit 46 (as described above). In general the shape and dimensions of the first region 26 and second region 28 are selected to promote circulation of the helium-3, reduce viscous heating, allow for an osmotic pressure to develop in the still 9 and mixing chamber 45, reduce the amount of helium-3 used and reduce hydrodynamic instabilities and convection. Similar considerations apply for the surrounding second conduit 48.

[0067] FIGS. 13 and 14 illustrate parts of a heat exchanger according to a second embodiment of the invention. FIG. 13 is a cross-sectional illustration equivalent of that of FIG. 7 in which primed reference numerals have been used to show like 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 formed essentially as described in the first embodiment with reference to FIGS. 2-8 but in this case dedicated flow channels 21 are formed within the sinter 15 for conveying the helium-3 concentrated phase of fluid from the inlet tube 12 through the apertures and returning it to the outlet tube 32 (where it is understood that similar features could also be applied to the sinters 15 on the opposite sides of the foil members 10, 11 for conveying the dilute phase fluid). Sinter 15 is applied to both faces of the first and second foil members 10, 11, however the first and second foil members 10, 11 are arranged within the chamber 30 so that the sinter 15 applied to a first face of the first foil member 10 and a first face of the second foil member 11 is separated from the opposing faces of the flow deflector plate 18 by a small gap, typically of 0.1-1.0 mm, preferably 0.2-0.6 mm (depending on the configuration). Optionally opposite sides of the flow deflector plate 18 instead abut against the sinter 15 on the first face of the first and second foil members 10, 11 to leave 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 direction of flow of the operational fluid. One or more channels may be provided inside the chamber 30 for conveying the concentrated phase of fluid through the first region to the apertures of the plate 18 and then through the second region of the chamber 30 to the outlet tube 32. One or more channels may further be provided for conveying the dilute phase of fluid through the second conduit on the outside of the chamber 30. Any number of different patterns could be applied including radial or spiralling. FIG. 14 is a perspective view of the sinter 15 taken through the plane X-X from FIG. 13. In FIG. 14 the flow channels 21 bifurcate to bring a greater portion of the sinter 15 into close contact with the operational fluid and so enhance the performance of the heat exchanger.

[0068] The profile of the flow channel 21 may be limited by what can be machined into the press tool, but could be semi-circular, elliptical, triangular, rectangular etc. The flow channel 21 will typically have a width in the range of 0.5-1.0 mm. The velocity of the fluid flow will (at a given total flow rate) depend on the number and width of the flow channels 21. The width of the flow channel may therefore vary depending on the relative placement of the heat exchanger chamber 30 within a 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 increases the thermal performance of the system.

[0069] In the first and second embodiments, the first and second regions within the chamber generally have a constant height in the direction across the plate (generally between 0.5 to 4 mm). Consequently, the fluid flow rate typically decreases as the fluid spreads radially outwards over a wider area. This may mean 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 illustrates parts of a heat exchanger according to a third embodiment of the invention. Double primed reference numerals are used to depict like components. The embodiment is similar to the first and second embodiments except that the sinter 15 on the inside of the chamber 30 is profiled so that the height of the first region 26 and second region 28 continually decreases at larger radiuses from the central axis. This profiling is achieved by pressing the sinter with a shaped tool in step 201 (FIG. 12). By profiling the sinter in this way, a deeper flow channel is provided at smaller radiuses, which compensates the above effect and reduces any viscous heating. The maximum depth of the first region 26 and second region 28 is typically around 1.5 mm and the minimum depth is typically around 0.2 mm. The specific parameters may be varied, however, according to the operating temperature of the heat exchanger, for example as determined by its position in a stack of such heat exchangers.

[0070] In the example of FIG. 15 the sinter 15 applied to the opposing first major faces of the first and second foil members 10, 11 is profiled so that the thickness of sinter 15 linearly increases with the radius. In contrast, the thickness of the sinter 15 applied to the opposite second major faces of the first and second foil members 10, 11 remains generally constant. It should be remembered, however, that the sinter 15 on the outside of the chamber 30 is in contact with fluid conveyed along the second conduit surrounding the chamber (as shown by FIG. 10). In the context of a dilution refrigerator, this is typically the dilute phase of helium-3. Additional unwanted viscous heating can arise within the second conduit that may be mitigated by profiling the sinter applied to the second major faces of the first and second foil members 10, 11. This profiling is typically again performed so that the thickness of the sinter applied to the first and second foil members 10, 11 linearly increases with the radius. In practice one or both major surfaces of the first and second foil members may be profiled to control any viscous heating within the heat exchanger. FIG. 16 illustrates part of a heat exchanger according to a fourth embodiment in which both surfaces of the first and second foil are profiled to reduce any viscous heating and so improve the performance of the cryogenic cooling system in which the heat exchanger is installed. FIGS. 13-16 are not to scale but show examples of different shapes that can be pressed into the sinter to control the flow of the fluids through the heat exchanger.

[0071] An effective heat exchanger is thereby provided, operable at low temperatures and which ensures reliable operation of a cryogenic cooling system. The design of the heat exchanger has a relatively simple construction, which lends itself well to welding and automated manufacturing processes that can guarantee a high degree of repeatability in terms of thermal performance. Lower temperatures may therefore be obtained in cryogenic cooling systems such as dilution refrigerators incorporating the heat exchanger, where the lowest obtainable temperature is dependent on the performance of the heat exchanger. Such processes may also be used to speed up the time required to manufacture the cryogenic cooling systems.