Attachment mechanism for two-stage cold head

Abstract

An attachment mechanism for connecting a first stage displacer of a two-stage cold head to a second stage displacer of the two-stage cold head. The attachment mechanism comprises a plug that is attachable to an end of the first stage displacer. The plug comprises a recess for receiving an end of the second stage displacer. The attachment mechanism further comprises a holding means which is configured to be mounted at least partially around the end of the second stage displacer and retain the end of the second stage displacer within the recess of said plug by engaging at least one formation extending from a radially outer surface of the second stage displacer. The attachment mechanism further comprises means for securing the holding means to the plug.

Description

[0001] ATTACHMENT MECHANISM FOR TWO-STAGE COLD HEAD

[0002] FIELD OF THE INVENTION

[0003] The field of the invention relates to an attachment mechanism for connecting a first stage displacer to a second stage displacer of a two-stage cold head.

[0004] BACKGROUND

[0005] A two-stage cold head, such as those used in Gifford McMahon refrigerators, comprises a first stage displacer and a second stage displacer. The first and second stage displacers contain a regenerator. The displacers are configured to move within a housing to pass refrigerant through the regenerator to cool the refrigerant. An attachment mechanism is used to attach the second stage displacer to the first stage displacer.

[0006] Conventionally, a two-pin joint attachment mechanism has been used to couple the first stage displacer to the second stage displacer. A two-pin joint attachment mechanism comprises a first pin received through a hole defined in the first stage displacer such that the first pin extends across the diameter of the first stage displacer. A second pin of the attachment mechanism is received through a hole defined in the second stage displacer such that the second pin extends across the diameter of the second stage displacer. The end of the second stage displacer may be received within the end of the first stage displacer such that the first pin may pass through a hole defined in the second pin to lock the first stage displacer to the second stage displacer. US11156214 B2 discloses examples of two-pin attachment mechanisms as known in the art.

[0007] Whilst conventional two-pin joint attachment mechanisms are simple and have a long life, they can occupy a large volume of space between the first and second stage displacers where the refrigerant, such as helium, is flowing. Therefore, the two-pin joint attachment mechanism contributes to hydraulic losses and leads to a significant dead volume between the first displacer stage and the second displacer stage. As a result, the efficiency of the two-stage cold head may be reduced.

[0008] It would be beneficial to provide an improved attachment mechanism for a two- stage cold head.

[0009] SUMMARY

[0010] According to an aspect, there is provided an attachment mechanism for connecting a first stage displacer of a two-stage cold head to a second stage displacer of said two-stage cold head, said attachment mechanism comprising: a plug attachable to an end of said first stage displacer, said plug comprising a recess for receiving an end of said second stage displacer; a holding means configured to be mounted at least partially around said end of said second stage displacer and retain said end of said second stage displacer within said recess of said plug by engaging at least one formation extending from a radially outer surface of said second stage displacer; and means for securing said holding means to said plug.

[0011] This attachment mechanism may be advantageous because the holding bracket is mountable around the outside of the end of the second stage displacer to attach the second stage displacer to the plug. In this way, all the components of the attachment mechanism are positioned radially outside the second stage displacer. In contrast, a two-pin joint comprises two interlocking pins extending across the diameter of the second stage displacer taking up space within the second stage displacer and obstructing the refrigerant entering the second stage displacer, thereby reducing its efficiency. The proposed attachment mechanism removes such an obstruction, thereby allowing for improved refrigerant flow into the second stage displacer. It also avoids taking up volume within the second stage displacer allowing for more regenerator material to be contained within the second stage displacer where, in the past, the interlocking pins would be positioned. If more regenerator is not desired, a more compact second stage displacer may be provided without sacrificing performance. In some embodiments, said holding means comprises a holding ring. In this way, the holding means may extend around the entire circumference of the second stage displacer. This may help provide an evenly distributed force to retain the second stage displacer.

[0012] In some embodiments, said securing means comprises at least one pin, and wherein said holding means and said plug each comprise at least one respective hole for receiving said at least one pin. This provides an easy to manufacture and cost-effective way to secure the holding means to the plug.

[0013] In some embodiments, said at least one pin of said securing means is secured to said holding means and / or said plug by press fit or by thread. For example, the securing means may comprise at least one dowel configured to connect the holding means to the plug. For example, the securing means may comprise at least one screw.

[0014] In some embodiments, said securing means comprises glue. Glue requires no additional components for the attachment mechanism, so it provides a simple yet effective way to secure the holding means to the plug. Glue may be used in addition to another securing means such as the at least one pin.

[0015] In some embodiments, when said holding means is secured to said plug, said at least one respective hole of said holding means and said plug are aligned with a radially central axis defined by said plug. In some embodiments, said at least one respective hole of said holding means and said plug extend substantially parallel to said axis. In this way, when the holding means is secured to the plug, the at least one pin and the at least one respective hole are positioned axially rather than radially. In use in a cold head, they align with the axis defined by the first and second displacer. Such an arrangement may allow for a compact securing means which is entirely located around the outside of the second stage displacer in contrast to the known two-pin joint mechanism. In some embodiments, said at least one pin comprises a plurality of pins and said at least one respective hole comprises a plurality of respective holes. Having multiple pins may provide a more secure connection.

[0016] In some embodiments, said plurality of respective holes are equidistantly circumferentially spaced around said axis on said plug and said holding means.

[0017] In this way, the securing means is evenly spaced around the second stage displacer.

[0018] In some embodiments, when said holding means is secured to said plug, said holding means is positioned within said recess of said plug. In use, this may provide for a compact attachment mechanism.

[0019] In some embodiments, said holding means comprises one or more through holes to allow refrigerant for cooling said cold head into said recess of said plug. In this way, refrigerant flowing from the first stage displacer to the second stage displacer is permitted to flow through the holding means.

[0020] In some embodiments, said one or more through holes comprises a plurality of through holes. This may reduce obstruction to refrigerant flow which can improve the displacer efficiency.

[0021] In some embodiments, said plurality of through holes are equidistantly circumferentially spaced around said holding means. In this way, an evenly distributed flow of refrigerant around the end of the second stage displacer may be provided. This can improve refrigerant flow to the second stage displacer, thereby improving its efficiency.

[0022] In some embodiments, said attachment mechanism further comprises a spacer configured to be mounted at least partially around said end of said second stage displacer to space said end of said second stage displacer from an end surface of said recess of said plug. In this way, there is a space between the end surface of the recess of the plug and the end of the second stage displacer. This space allows a smooth flow of refrigerant into the second stage displacer in an axial direction.

[0023] In some embodiments, said spacer at least partially defines at least one passage, said at least one passage being configured to allow refrigerant for cooling said cold head into a region of said recess directly adjacent said end of said second stage displacer. In this way, obstruction by the displacer of refrigerant flow to the second stage displacer may be reduced or minimised.

[0024] In some embodiments, said at least one passage comprises a plurality of passages.

[0025] In some embodiments, said plurality of passages are equidistantly circumferentially spaced around said spacer. This may provide an evenly distributed flow of refrigerant into the space created by the spacer from around the second stage displacer.

[0026] In some embodiments, said at least one passage extends perpendicularly to said axis.

[0027] In some embodiments, said spacer comprises a first spring configured to be mounted around said end of said second stage displacer to bias said at least one formation away from an end surface of said recess of said plug. In this way, the spacer naturally comprises gaps defining passages for refrigerant to flow through whilst giving a degree of flexibility to the joint between the first and second stage displacers. This flexibility may allow for some axial offset between the axes of the first and the second stage displacers. This flexibility also provides some axial freedom allowing the first stage displacer and the second stage displacer to axially compress when the second stage displacer engages an end of the second cylinder of the cold head. In some embodiments, said attachment mechanism comprises a second spring configured to be mounted around said end of said second stage displacer to bias said holding means away from said at least one formation. Having two springs on either side of the at least one formation can allow the second stage displacer to have some axial degree of freedom in case this is needed for the operation of the cold head, for example, to help reduce vibrations.

[0028] In some embodiments, said recess of said plug comprises a bore for permitting refrigerant to pass between said end of said first stage displacer and said end of said second stage displacer. The bore allows refrigerant to pass directly between the first and second stage displacers in an axial direction. In some embodiments, the bore is defined through the end wall of the plug defining the recess. In some embodiments, the plug is less than 5mm in axial length and, in some embodiments, less than 3mm in axial length. In some embodiments, the bore is less than 1 mm in axial length. In some embodiments, the end wall of the plug is less than 1 mm thick.

[0029] In some embodiments, said bore is defined in said end surface of said recess.

[0030] In some embodiments, said bore is positioned at a radial centre of said plug.

[0031] In some embodiments, said attachment mechanism further comprises a washer configured to be mounted around said end of said second stage displacer. The washer may be configured to be positioned between the plug and the at least one formation. When the attachment mechanism comprises a spacer, the washer may be configured to be positioned between the spacer and the at least one formation. The washer may help reduce wear of the components of the attachment mechanism and / or of the second stage displacer.

[0032] In some embodiments, said washer comprises an angled surface adapted to correspond to a sloped surface of said at least one formation. In this way, the washer may be adapted to adjust for any angled surface on the at least one formation such that a robust engagement between the plug or the spacer and the at least one formation may be achieved.

[0033] According to another aspect, there is provided a two-stage cold head comprising: a first cylinder housing a first stage displacer; and a second cylinder, attached to an end of said first cylinder, housing a second stage displacer, said second stage displacer comprising at least one formation extending from a radially outer surface of said second stage displacer, wherein said first stage displacer and said second stage displacer each contain a regenerator and are configured to move within said first cylinder and said second cylinder respectively to pass refrigerant through each of said regenerators to cool said refrigerant, wherein said second stage displacer is attached to said first stage displacer by an attachment mechanism according to any preceding claim, said plug being attached to said end of said first stage displacer, said holding means being mounted around said end of said second stage displacer and secured to said plug such that it is engaged with said at least one formation to retain said end of said second stage displacer within said recess of said plug.

[0034] In this way, a cold head is provided with an attachment mechanism in which no joint pins penetrate the displacer body and the helium flow path, thereby reducing the dead volume and hydraulic losses can be reduced. Such an attachment mechanism further allows the second stage displacer to be filled with more regenerator for a displacer with the same overall length. Alternatively, if more regenerator is not desired, the length of the displacer and corresponding cylinder can be reduced giving a more compact size whilst achieving the same performance.

[0035] In such a cold head, the tolerance of the attachment mechanism allows for some axial alignment offset between the first and second displacer. This is particularly relevant for horizontally operated two-stage cold heads where the displacers may have a slight axial offset due to the weight of the displacers acting perpendicular to the axis of the cold head.

[0036] In some embodiments, a portion of said plug defining said recess is received within a cavity of said first stage displacer. In this way, the axial space between the first and second stage displacer taken up by the attachment mechanism may be reduced, thereby providing a compact attachment mechanism.

[0037] In some embodiments, said plug is attached to said end of said first stage displacer by glue. This may be the only attachment means used or may be in addition to at least one pin. Glue requires no additional components for the cold head and therefore provides a simple yet effective way to attach the plug to the first stage displacer.

[0038] In some embodiments, said plug is attached to said end of said first stage displacer by at least one pin extending between respective recesses formed in said plug and in said first stage displacer. This provides an easy to manufacture and cost-effective way to secure the plug to the first stage displacer.

[0039] In some embodiments, said at least one pin comprises a plurality of pins and said respective recess comprises a plurality of respective recesses. A plurality of pins may improve the connection between the plug and the first stage displacer.

[0040] In some embodiments, said at least one pin is secured to said first stage displacer and / or said plug by press fit or by thread. For example, the securing means may comprise at least one dowel configured to connect the holding means to the plug. For example, the securing means may comprise at least one screw.

[0041] In some embodiments, said at least one formation comprises a formation extending partially or entirely around a circumference of said end of said second stage displacer. In some embodiments, said at least one formation comprises a plurality of formations.

[0042] In some embodiments, said at least one formation comprises at least one protrusion integrally formed with said second stage displacer. Integrally forming the at least one formation may simplify the manufacturing process of the displacer as it requires no additional components or assembly.

[0043] In some embodiments, said at least one formation comprises at least one pin extending from at least one respective recess or through hole defined in said second stage displacer. In contrast to an integrally formed formation, using a separate component to make the formation may allow the formation to be retrofitted onto existing displacers.

[0044] In some embodiments, said at least one pin is secured to said second stage displacer by press fit or by thread. For example, said at least one pin comprises at least one dowel. For example, said at least one pin comprises at least one screw.

[0045] In some embodiments, said at least one formation comprises at least one screw extending from a through hole defined in said second stage displacer.

[0046] In some embodiments, said at least one formation comprises a rectangular cross section or a frustoconical cross section. A rectangular cross section may be simple to manufacture and allow for a better engagement between the holding means and the formation compared to a frustoconical cross section. A frustoconical cross section may allow for a slight angle or offset between the axis of the first displacer and the axis of the second stage displacer. This may be particularly useful for a cold head configured for horizontal operation where, under the wight of the second stage displacer, it may not align perfectly with the first stage displacer. In some embodiments, when said at least one formation comprises a frustoconical cross section, edges of said conical cross section of said at least one formation define an angle of between 0 degrees and 10 degrees from the radial direction.

[0047] In some embodiments, said first stage displacer is configured to work in an approximate temperature range of room temperature to 15K. In some embodiments, said second stage displacer is configured to work in an approximate temperature range of 20K to 1 K.

[0048] In some embodiments, when said attachment mechanism comprises said spring, said spring is mounted around said end of said second stage displacer between said at least one formation and said end surface of said recess of said plug to bias said second stage displacer away from said end surface of said recess of said plug. This may be a bias towards a cold end of said second cylinder. In some embodiments, when said attachment mechanism comprises said second spring, said spring is mounted around said end of said second stage displacer between said at least one formation and said holding means to bias said second stage displacer away from said end surface of said recess of said plug. This may be a bias towards a cold end of said second cylinder.

[0049] In some embodiments, said cold head further comprises: a first diamagnetically magnetised magnetic ring mounted in a rotationally fixed manner around said end of said second stage displacer and a second diamagnetically magnetised magnetic ring rotationally mounted around said second cylinder, said first magnetic ring and said second magnetic ring being orientated such that opposite poles of said first and second magnetic rings are aligned; and an actuator configured to rotate said second magnetic ring.

[0050] According to a further aspect, there is provided a Gifford-McMahon (GM) refrigerator comprising a cold head according to the another aspect. According to one aspect, there is provided a cryopump, a utility for liquefaction of gas, or a utility for cooling a quantum computer comprising a GM refrigerator according to the further aspect.

[0051] According to one other aspect, there is provided a method of attaching a first stage displacer of a two-stage cold head to a second stage displacer of said two- stage cold head using an attachment mechanism, said method comprising: attaching a plug of said attachment mechanism to an end of said first stage displacer; mounting a holding means of said attachment mechanism around an end of said second stage displacer; securing said holding means to said plug using means for securing of said attachment mechanism such that said end of said second stage displacer is retained within a recess of said plug, said holding means being engaged with at least one formation extending from a radially outer surface of said second stage displacer.

[0052] Whilst this method may be implemented during manufacture of a cold head, the method may also be implemented to retrofit the attachment mechanism on a first and second stage displacer of an existing cold head to provide the advantages discussed above in relation to aspects.

[0053] Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.

[0054] Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

[0056] Figure 1 shows a cold head according to an embodiment;

[0057] Figure 2 shows an exploded view and an assembled view of a displacer for a cold head comprising an attachment mechanism according to an embodiment; Figure 3 shows an enlarged view of the attachment mechanism of Figure 2;

[0058] Figure 4 shows a cross section of a plug in a plane perpendicular to the axis of the cold head;

[0059] Figure 5 shows a cross section of a holding means in a plane perpendicular to the axis of the cold head;

[0060] Figure 6 shows an exploded view and an assembled view of a displacer for a cold head comprising an attachment mechanism according to an embodiment; Figure 7 shows an enlarged view of the attachment mechanism of Figure 6; Figure 8 shows an exploded view, an assembled view, and a section of a displacer for a cold head comprising an attachment mechanism according to an embodiment;

[0061] Figure 9 shows an enlarged view of the attachment mechanism of Figure 8;

[0062] Figure 10 shows an exploded view, an assembled view and a section of a displacer of a cold head comprising an attachment mechanism according to an embodiment;

[0063] Figure 11 shows an enlarged view of the attachment mechanism of Figure 10;

[0064] Figure 12 shows an attachment mechanism according to an embodiment;

[0065] Figure 13 illustrates steps in a method according to an embodiment.

[0066] DESCRIPTION OF THE EMBODIMENTS

[0067] Before discussing the embodiments in any more detail, first an overview will be provided.

[0068] The two-pin joint is a widely used coupling means for joining a first stage displacer to a second stage displacer. In this joint, the two pins are orthogonal to each other and allow for movements in the joint along each pin in the plane perpendicular to the displacer axis. The two-pin joint has the disadvantage that it requires a comparably large working volume in a very important region of the connection between the cold head stages where the helium refrigerant is flowing between the two stages. Also, the pins extend across and within the displacer stages, thereby obstructing refrigerant flow. Thus, this kind of coupling contributes to hydraulic losses and leads to an increased dead volume between the first and the second stages. Dead volume and hydraulic losses are known to reduce the efficiency of a cold head of a GM refrigerator.

[0069] Embodiments provide a coupling means between subsequent stages in a two- stage cold head for GM refrigerators. The coupling means, or attachment mechanism, comprises a plug configured to sit within the cavity of at the end of the first stage displacer. The plug comprises a recess for receiving the second stage displacer. A holding means or holding ring is configured be mounted around the outside of the second stage displacer and be secured to the plug. The holding means engages a formation on the second stage displacer to retain the second stage displacer within the recess of the plug attached to the first stage displacer, thereby coupling the second stage displacer to the first stage displacer. The construction is configured to allow for some lateral movement of the displacers relative to each other. In the proposed construction, no joint parts are present in the main flow path of the Helium between first and second stages of the GM cryocooler. Thus, compared to the conventional 2-pin joint, the dead volume and hydraulic losses in this region can be reduced. The space which was taken up by the pins may at least partially accommodate more regenerator. Therefore, for displacers of the same dimensions, more regenerator may be contained within the displacers compared to conventional displacers. In case there is no need to increase the amount of the regenerator, the length of the displacer and of the corresponding cylinder may be reduced whilst generating the same performance, thereby allowing for a more compact cold head which is advantageous for some applications. Figure 1 shows a cross-section of a two-stage cold head 10 according to an embodiment. The cold head 10 comprises a first cylinder 11 housing a first stage displacer 12 and a second cylinder 13, attached to an end of said first cylinder 11 , housing a second stage displacer 14. The first stage displacer 12 and the second stage displacer 14 each contain a regenerator 15 and are configured to move within the first cylinder 11 and the second cylinder 13 respectively to pass refrigerant through the regenerators 15 to cool the refrigerant as is known. A motor 18 is configured to drive the movement of the first and second stage displacers 12, 14. The second stage displacer 14 is attached to the first stage displacer 12 by an attachment mechanism 20 according to an embodiment.

[0070] Figures 2 and 3 illustrate the attachment mechanism 20 in more detail. For simplicity, the regenerator 15 and the first and second cylinders 11 , 13 have been omitted but it should be assumed that, in use, the displacers in all figures are contained within a cold head housing, such as the first and second cylinders, and are to be filled with regenerator in a similar way to the displacers shown in Figure 1 . The left-hand diagram in Figures 2 and 3 shows an exploded view of the cold head 10 including the attachment mechanism 20. The right-hand diagram shows the cold head 10 and the attachment mechanism 20 in an assembled state. The attachment mechanism 20 comprises a plug 21 , a holding means 24, a spacer 27 and means for securing the holding means 24 to the plug 21 .

[0071] The plug 21 is attachable to an end of the first stage displacer 12. In this embodiment, a portion of the plug 21 defining a recess 22 for receiving an end of the second stage displacer 14 is received within a cavity 16 of the first stage displacer 14.

[0072] The plug 21 may be attached to the first stage displacer 12 by any appropriate attachment means. For example, the plug 21 may be attached to the first stage displacer 12 by one or more pins 35 as shown in Figure 12 which extend between respective recesses or holes formed in the plug 21 and the first stage displacer 12. The one or more pins may be secured to the plug 21 and / or the first stage displacer 12 by press fit or by thread. For example, screws may be used to secure the plug 21 to the first stage displacer 12. Alternatively or additionally, the plug 21 may be glued to the first stage displacer 12.

[0073] The holding means 24 comprises a holding ring which is configured to be mounted around the outside of the second stage displacer 14. When assembled, the holding means 24 engages at least one formation 30 defined by the second stage displacer 14 to retain the second stage displacer 14 within the recess 22 of the plug 21. The holding means 24 is secured to the plug 21 by securing means, for example, the pins 29 shown in the left-hand diagram of Figure 3 configured to connect the holding means 24 to the plug 21 as discussed below. It will be appreciated that there may be one or there may be multiple pins. The one or more pins may be secured to the plug 21 and / or the holding means 24 by press fit or by thread. Alternatively or additionally, the holding means 24 may be glued to the plug 21 .

[0074] Figure 4 shows a cross section of the plug 21 in a lateral plane - i.e. , a plane perpendicular to the axis of the cold head 10. Holes 25a are configured to receive a respective one of the plurality of pins 29 to secure the holding means 24 to the plug 21 . The holes are evenly distributed circumferentially around the plug 21 within the recess 22.

[0075] The plug 21 further comprises a radially centrally positioned bore 23 for allowing refrigerant to flow directly from the end of the first stage displacer 12 to the second stage displacer 14. The bore 23 is optional and, in some embodiments, is not included. In this case, in use, refrigerant may flow out of the first stage displacer 12 via holes 17 spread circumferentially about the end of the second stage displacer 14. The refrigerant then flows around the outside of the displacers (but within the cylinders 11 , 13) and into the second stage displacer 14 via the attachment mechanism 20 as shown by the arrows in the right-hand diagram of Figure 3. Specifically, the refrigerant flows through one or more axially orientated through holes 26 defined in the holding means 24 and then through one or more lateral passages 28 defined between the spacer 27 and the end surface of the recess 22 of the plug 21 to reach the second stage displacer 14 and the regenerator 15 contained within.

[0076] Figure 5 shows a cross section of the holding means 24 in a lateral plane - i.e., a plane perpendicular to the axis of the cold head 10. The holding means 24 comprises holes 25b for receiving pins 29. Thus, pins extend from holes 25b of the holding means 24 to holes 25a of the plug 21 to secure the holding means 24 to the plug 21 . In this embodiment, the holes 25b comprise a countersink to accommodate the head of a screw. It will be appreciated that this is optional. When the holding means 24 is secured to the plug 21 , the holding means 24 is positioned within the recess 22 of the plug 21 . In this way, a compact attachment mechanism may be provided.

[0077] The holding means 24 further comprises through holes 26 which allow refrigerant for cooling the cold head to access the recess of the plug and subsequently the second stage displacer 14 as described above.

[0078] Returning to Figures 2 and 3, the attachment mechanism 20 comprises a spacer 27. The spacer 27 comprises a ring-shaped spacer configured to be mounted around the outside of the second stage displacer 14 such that it is positioned between the plug 21 and the formation 30 formed on the second stage displacer 14. The spacer 27 maintains a gap between the end surface of the recess 22 of the plug 21 and the end of the second stage displacer 14 such that refrigerant can flow between the plug 21 and the second stage displacer 14. The spacer 27, together with the end surface of the recess 22 of the plug 21 , define multiple passages 28 for allowing refrigerant to pass through the spacer 27 as described above.

[0079] In this embodiment, the formation 30 of the second stage displacer 14 comprises a ring extending radially outwardly around the entire circumference of the second stage displacer and is integrally formed with the second stage displacer 14. The formation 30 comprises a rectangular cross-section. However, it will be appreciated that the formation 30 may vary form. In some embodiments, the formation 30 comprises a plurality of formations distributed circumferentially around the second stage displacer 14. In some embodiments, the at least one formation is not integrally formed as discussed in more detail below.

[0080] Figures 6 and 7 illustrate a first stage displacer and a second stage displacer coupled via an attachment mechanism in similar fashion to Figures 2 and 3 except that the formation 30b comprises a frustoconical cross-section and the attachment mechanism comprises a washer 31 positioned between the spacer 27 and the at least one formation 30. The washer 31 may help reduce wear of the spacer 27 and the formation 30. A second washer (not shown) may be provided between the holding means 24 and the formation 30b. It will be appreciated that the washers may be implemented in other embodiments, for example, the embodiment shown in Figures 2 and 3.

[0081] The frustoconical cross-section may have sides that are sloped up to 15 degrees, up to 10 degree or up to 5 degrees from the plane perpendicular to the axis of the displacer. The frustoconical cross-section of the formation 30b may allow the axis of the second stage displacer 14 to be slightly misaligned with the axis defined by the first stage displacer 12. In other words, the sloped sides may compensate for any tilt of the displacers in case the second stage displacer 14 is not fully parallel to the first stage. This allowance may be particularly helpful for a cold head configured for horizontal operation because the weight of the displacers or other mounted components may cause a misalignment between the displacers.

[0082] Alternatively, misalignment may result from manufacturing tolerances. In some embodiments, the axial end surfaces of the washer are not parallel. For example, the end surface of the washer configured to engage the spacer 27 may extend perpendicular to the axis of the displacer and the opposite end surface configured to engage the formation 30b may be sloped at an angle which matches the slope of the formation 30b to help provide a secure engagement between the spacer 27 and the formation 30b. Figures 8 and 9 illustrate a first stage displacer and a second stage displacer coupled via an attachment mechanism in similar fashion to previous embodiments except that the attachment mechanism does not comprise a spacer and the at least one formation comprises pins 30c. The pins 30c extend radially outwardly from respective holes 32 in the second stage displacer 14 and are configured to engage a shoulder defined in the recess 22 of the plug 21 . The shoulder may comprise holes for receiving screws for securing the holding means 24 to the plug 21 . A washer 31 is positioned between the shoulder and the pins 30c. The pins 30c may be secured to the second stage displacer 14 by press fit or by thread. It will be appreciated that any number of pins may be used to create the formation of the second stage displacer. For example, one pin may be used or a plurality of pins may be used.

[0083] Figures 10 and 11 illustrate a first stage displacer and a second stage displacer coupled via an attachment mechanism in similar fashion to Figures 8 and 9 except that there is no washer and the pins 30c do not extend all the way through the second stage displacer wall. Instead, the pins 30c are received in recesses 33 formed in the second stage displacer 14 and the holding means 24. The pins 30c may be secured to the second stage displacer 14 by press fit or by thread. In some embodiments, there is only one pin. In some embodiments, there are two pins. In some embodiments, there may be more than two pins distributed circumferentially around the second stage displacer.

[0084] Figure 12 illustrates an embodiment of an attachment mechanism in which the spacer comprises a spring 34. The spring 34 is configured to be mounted around the outside of the end of the second stage displacer 14 to bias the formation 30 away from the end surface of the recess 22 of the plug 21 . The biasing of the spring 34 creates passages for the refrigerant to flow through and also provides a degree of flexibility to the joint between the first and second stage displacers. This flexibility may help reduce vibrations transmitted through the attachment mechanism and help allow for the axis of the second stage displacer 14 to be offset slightly from the axis of the first stage displacer 12.

[0085] In some embodiments, the attachment mechanism comprises a second spring mounted around the outside of the second stage displacer 14 to bias the holding means away from the formation 30. By providing two springs on either side of the formation 30, the second stage displacer 14 may have some axial degree of freedom in case this is needed for the operation of the cold head, for example, to help reduce vibrations.

[0086] In the embodiment shown in Figure 12, the plug 12 is attached to the first stage displacer 12 by pins 35. It will be appreciated that only one pin may be used or a plurality of pins. The pins 35 may be secured to the first stage displacer 12 and / or the plug 21 by press fit or by thread. Glue may be used to secure the plug 21 to the first stage displacer in addition to the pins 35.

[0087] In some embodiments, the cold head further comprises a first diamagnetically magnetised magnetic ring mounted in a rotationally fixed manner around the outside of the end of the second stage displacer and a second diamagnetically magnetised magnetic ring rotationally mounted around the second cylinder. The first magnetic ring and the second magnetic ring are oppositely orientated, i.e. , opposite poles of the first and second magnetic rings are aligned along the axis of the cold head. An actuator is used to rotate the second magnetic ring. The first magnetic ring is attracted to the second magnetic ring and so will rotate with the second magnetic ring. Given that the first magnetic ring is rotationally fixed to the second stage displacer, the actuator is able to rotate the second stage displacer. This may be useful if there is increased friction between the second stage displacer and the second cylinder at certain orientations of the displacer. The actuator may be used to rotate the second displacer away from these orientations to reduce or minimise friction. Note that the friction of the second stage displacer may be observed by measuring the temperature of the second stage of the cold head. Figure 13 shows the steps of a method of attaching a first stage displacer to a second stage displacer of a two-stage cold head using an attachment mechanism according to an embodiment.

[0088] In step S10, the method comprises attaching a plug of the attachment mechanism to an end of said first stage displacer.

[0089] In step S20, the holding means of the attachment mechanism is mounted around an end of the second stage displacer.

[0090] In step S30, the holding means is secured to the plug using means for securing, such as screws, such that the end of the second stage displacer is retained within a recess of the plug, and the holding means is engaged with at least one formation extending from a radially outer surface of the second stage displacer.

[0091] The embodiments described above may provide the following advantages:

[0092] • no joint pins penetrate the displacer bodies;

[0093] • less dead volume between stages;

[0094] • more regenerator filling may be put inside the displacers compared to a cold head with a two-pin joint for displacers having the same overall length; or

[0095] • displacer and cylinder length may be decreased without compromising performance because additional regenerator can be used for the same length of displacer compared to a cold head with a two-pin joint.

[0096] Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents. REFERENCE SIGNS

[0097] 10 Two-Stage Cold Head

[0098] 11 First Cylinder

[0099] 12 First Stage Displacer

[0100] 13 Second Cylinder

[0101] 14 Second Stage Displacer

[0102] 15 Regenerator

[0103] 16 Cavity

[0104] 17 Holes

[0105] 18 Motor

[0106] 20 Attachment Mechanism

[0107] 21 Plug

[0108] 22 Recess

[0109] 23 Bore

[0110] 24 Holding means

[0111] 25a, b Holes

[0112] 26 Through Holes

[0113] 27 Spacer

[0114] 28 Passage

[0115] 29 Pins

[0116] 30 Formation

[0117] 31 Washer

[0118] 32 Holes

[0119] 33 Recesses

[0120] 34 Spring

[0121] 35 Pin

Claims

CLAIMS1 . An attachment mechanism for connecting a first stage displacer of a two- stage cold head to a second stage displacer of said two-stage cold head, said attachment mechanism comprising: a plug attachable to an end of said first stage displacer, said plug comprising a recess for receiving an end of said second stage displacer; a holding means configured to be mounted at least partially around said end of said second stage displacer and retain said end of said second stage displacer within said recess of said plug by engaging at least one formation extending from a radially outer surface of said second stage displacer; and means for securing said holding means to said plug.

2. An attachment mechanism according to claim 1 , wherein said securing means comprises at least one pin, and wherein said holding means and said plug each comprise at least one respective hole for receiving said at least one pin.

3. An attachment mechanism according to claim 2, wherein said at least one pin of said securing means is secured to said holding means and / or said plug by press fit or by thread.

4. An attachment mechanism according to any preceding claim, wherein said holding means comprises one or more through holes to allow refrigerant for cooling said cold head into said recess of said plug.

5. An attachment mechanism according to any preceding claim, further comprising a spacer configured to be mounted at least partially around said end of said second stage displacer to space said end of said second stage displacer from an end surface of said recess of said plug.

6. An attachment mechanism according to claim 5, wherein said spacer at least partially defines at least one passage, said at least one passage beingconfigured to allow refrigerant for cooling said cold head into a region of said recess directly adjacent said end of said second stage displacer.

7. An attachment mechanism according to claim 5 or claim 6, wherein said spacer comprises a first spring configured to be mounted around said end of said second stage displacer to bias said at least one formation away from an end surface of said recess of said plug.

8. An attachment mechanism according to any preceding claim, wherein said recess of said plug comprises a bore for permitting refrigerant to pass between said end of said first stage displacer and said end of said second stage displacer.

9. A two-stage cold head comprising: a first cylinder housing a first stage displacer; and a second cylinder, attached to an end of said first cylinder, housing a second stage displacer, said second stage displacer comprising at least one formation extending from a radially outer surface of said second stage displacer, wherein said first stage displacer and said second stage displacer each contain a regenerator and are configured to move within said first cylinder and said second cylinder respectively to pass refrigerant through each of said regenerators to cool said refrigerant, wherein said second stage displacer is attached to said first stage displacer by an attachment mechanism according to any preceding claim, said plug being attached to said end of said first stage displacer, said holding means being mounted around said end of said second stage displacer and secured to said plug such that it is engaged with said at least one formation to retain said end of said second stage displacer within said recess of said plug.

10. A cold head according to claim 9, wherein said plug is attached to said end of said first stage displacer by glue.

11. A cold head according to claim 9 or claim 10, wherein said plug is attached to said end of said first stage displacer by at least one pin extending between respective recesses formed in said plug and in said first stage displacer.

12. A cold head according to any one of claims 9 to 11 , wherein said at least one formation comprises at least one protrusion integrally formed with said second stage displacer.

13. A cold head according to any one of claims 9 to 11 , wherein said at least one formation comprises at least one pin extending from at least one respective recess or through hole defined in said second stage displacer.

14. A cold head according to any one of claims 9 to 13, wherein said at least one formation comprises a rectangular cross section or a frustoconical cross section.

15. A Gifford-McMahon (GM) refrigerator comprising a cold head according to any one of claims 9 to 14.

16. A cryopump, a utility for liquefaction of gas, or a utility for cooling a quantum computer comprising a GM refrigerator according to claim 15.

17. A method of attaching a first stage displacer of a two-stage cold head to a second stage displacer of said two-stage cold head using an attachment mechanism, said method comprising: attaching a plug of said attachment mechanism to an end of said first stage displacer; mounting a holding means of said attachment mechanism around an end of said second stage displacer; securing said holding means to said plug using means for securing of said attachment mechanism such that said end of said second stage displacer is retained within a recess of said plug, said holding means being engaged with atleast one formation extending from a radially outer surface of said second stage displacer.

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