Abstract:
An arrangement for mounting a two stage cryogenic refrigerator ( 17 ) into a cryostat, the arrangement comprising a vacuum sock ( 15 ) for accommodating at least a part of the refrigerator, attachment means ( 32, 34 ) for attaching an upper part of the refrigerator to a surface ( 14 ) of the cryostat around an opening of the vacuum sock, a thermally conductive portion ( 26 ) of a wall of the vacuum sock which, in use, is thermally and mechanically in contact with a second cooling stage ( 24 ) of the refrigerator, and arrangements ( 40, 42 ) are provided for thermally connecting a first stage ( 22 ) of the refrigerator to a thermal radiation shield ( 16 ) of the cryostat.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to improved arrangements for providing thermal connection between a cryogenic refrigerator and cooled components, wherein the refrigerator is removable, and the thermal connection must be capable of being broken and re-made without discernable increase in thermal resistance. 
         [0003]    2. Description of the Prior Art 
         [0004]    The present invention is described in the context of a cryogenic refrigerator cooling to temperatures of about 4.2K for re-condensing helium in a cryostat used for cooling superconducting magnets for MRI systems. 
         [0005]      FIG. 1  shows a conventional arrangement of a cryostat including a cryogen vessel  12 . A cooled superconducting magnet  10  is provided within cryogen vessel  12 , itself retained within an outer vacuum chamber (OVC)  14 . One or more thermal radiation shields  16  are provided in the vacuum space between the cryogen vessel  12  and the outer vacuum chamber  14 . In some known arrangements, a refrigerator  17  is mounted in a refrigerator sock  15  located in a turret  18  provided for the purpose, toward the side of the cryostat. Alternatively, a refrigerator  17  may be located within access turret  19 , which retains access neck (vent tube)  20  mounted at the top of the cryostat. The refrigerator  17  provides active refrigeration to cool cryogen gas within the cryogen vessel  12 , in some arrangements by recondensing it into a liquid. The refrigerator  17  may also serve to cool the radiation shield  16 . As illustrated in  FIG. 1 , the refrigerator  17  may be a two-stage refrigerator. A first cooling stage  22  is thermally linked to the radiation shield  16 , and provides cooling to a first temperature, typically in the region of 80-100K. A second cooling stage  24  provides cooling of the cryogen gas to a much lower temperature, typically in the region of 4-10K. 
         [0006]    A negative electrical connection  21  a is usually provided to the magnet  10  through the body of the cryostat. A positive electrical connection  21  is usually provided by a conductor passing through the vent tube  20 . 
         [0007]    Typically, the cryogenic refrigerator will be a two-stage refrigerator, providing high-power cooling to a first cryogenic temperature and lower-power cooling to a much lower cryogenic temperature, as illustrated in  FIG. 1 . In current cryogenic refrigerators, the first stage may provide about 44 W of cooling to 50K and about 1 W of cooling at about 4K. Typically, a first stage heat exchanger  22  is in thermal contact with the thermal radiation shield  16  as illustrated in  FIG. 1 . 
         [0008]    In some conventional systems, a second stage heat exchanger is exposed to a gaseous cryogen environment in the present example, gaseous cryogen. The second stage is cooled to a temperature below the boiling point of the cryogen, which condenses onto the second stage heat exchanger. Such arrangements provide direct contact between cryogen and second stage heat exchanger, but care must be taken when removing and replacing the refrigerator, since air will tend to be drawn into the cryogen vessel, where it will freeze onto surfaces, and may cause dangerous blockages. The service operation of removing and replacing the refrigerator with the magnet at field is also a hazardous operation, as a quench could take place while the refrigerator is absent, placing a service technician at risk from exposure to liquid and gaseous cryogen. 
         [0009]      FIG. 2  schematically illustrates a conventional arrangement in which the cryogenic refrigerator is housed within an enclosure  15 , colloquially known as a “vacuum sock”, sealed from the interior of the cryogen vessel  12 . In this case, the second stage heat exchanger  24  is in thermal contact with gaseous cryogen in the cryogen vessel  12  through a part  26  of a wall of the vacuum sock  15 . A heat exchanger surface  28  may be provided on the cryogen vessel side of this part  26  of the wall, to enhance thermal transfer, for example having a finned and/or textured surface. Cooling in this way, by conduction through a wall of the vacuum sock, introduces thermal resistance between the second stage  24  of the refrigerator and the cryogen gas, but provides the advantage that the cryogenic refrigerator  17  may be removed and replaced without exposing the interior of the cryogen vessel  12  to air. Air may enter the vacuum sock  15 , but this will solidify inside the vacuum sock when the refrigerator is in use, and does not pose a risk of dangerous blockage. The thermal connection between the first cooling stage  22  and the thermal radiation shield  16  may be provided by a tapered cooling stage  22  and a tapered interface block  30 . 
         [0010]    It is of course important to ensure effective thermal transfer between the first cooling stage  22  of the refrigerator  17  and the thermal radiation shield  16 . This may be achieved, as illustrated, using a tapered first cooling stage  22  and a tapered interface block  30  which is thermally and mechanically joined to the thermal radiation shield  16 . The first cooling stage  22  and the interface block  30  are each typically of copper, and the taper angle a is chosen to be narrow enough to ensure a high enough pressure between the surfaces of the first cooling stage  22  and the interface block  30  to ensure good thermal conductivity, but not so narrow an angle that the refrigerator  17  becomes difficult to remove. At an upper end of the refrigerator  17 , a flange  32  is bolted  34  to the surrounding surface of the cryostat OVC  14 . The dimensions of the various components are carefully calculated such that the first cooling stage  22  and the interface block  30  are driven together with an appropriate force as the refrigerator is tightened into position by bolts  34 . Some flexibility in the mounting of the interface block  30  restricts the maximum force to an appropriate level, and allows for some tolerance in the respective dimensions. 
         [0011]    Thermal connection must also be provided from the second cooling stage  24  through the wall of the vacuum sock  15 . Typically, a part  26  of the wall which contacts the second stage  24  will be of a thermally conductive material such as copper, and may be profiled to provide an enhanced surface  28  for heat exchange on the side which is exposed to the interior of the cryogen vessel. For example, that surface may be finned and/or textured. In certain known arrangements, the various components are dimensioned such that the second cooling stage  24  presses in to wall part  26  with appropriate force and the tapered first cooling stage  22  meets the tapered interface block  30  with appropriate force as the flange  32  is bolted  34  on to the surrounding surface of the cryostat OVC  14 . Conventionally, a compliant interface material, typically an indium washer, may be placed between mating surfaces of the wall part  26  and the second cooling stage  24  to allow effective thermal connection while allowing some tolerance in mechanical position. A difficulty with such an arrangement is that the indium washer is destroyed when the refrigerator is removed, and it is difficult to remove all traces of an old indium washer from the inside of the vacuum sock  15 . Any remaining traces of an old indium washer will degrade the thermal interface provided by a replacement indium washer. 
         [0012]    In the prior art arrangements as discussed above, efficient thermal interfaces between the refrigerator and cooled components have relied upon precise mechanical dimensions. Mechanical force applied when bolting  34  the flange  32  of the refrigerator  17  in place is shared between sealing the refrigerator to the surrounding surface of the cryostat OVC  14 , and interface forces between the first cooling stage  22  and the interface block  30 ; in some embodiments, also interface forces between the second cooling stage  24  and the part  26  of the wall of the vacuum sock. This sharing of forces means that any mechanical tolerance in respective dimensions will change the proportions of force applied at each interface, resulting in unpredictable thermal resistances of the various interfaces. This is usually overcome at the first stage by adding additional thermal links with braids and an axial spring mechanism to allow for build tolerances at the expense of less efficient thermal transfer, caused by an increased number of thermal joints. 
       SUMMARY OF THE INVENTION 
       [0013]    An object of the present invention is to address these problems by providing mounting arrangements for a cryogenic refrigerator wherein an interface force is applied to the first cooling stage  22  in a direction perpendicular to a direction of insertion of the refrigerator  17 . The interface force applied to the first cooling stage  22  is thereby independent of the mechanical arrangements for mounting the refrigerator. Accordingly, the mounting force and the interface force may be independently optimized for their respective functions. When the second cooling stage  24  is also subjected to an interface force, the interface force applied to the first thermal interface is in a direction perpendicular to an interface force applied to the second cooling stage. The first stage thermal interface force is independent from the second stage thermal interface force. Increasing one thermal interface force will have no effect on the other thermal interface force. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  shows a cross-section through a conventional cryogenically cooled magnet for MRI imaging. 
           [0015]      FIG. 2  shows a conventional mounting arrangement for a refrigerator in a cryostat. 
           [0016]      FIG. 3  shows a mounting arrangement for a refrigerator in a cryostat, according to an embodiment of the present invention. 
           [0017]      FIG. 4  shows a detail of another mounting arrangement for a refrigerator in a cryostat, according to an embodiment of the present invention. 
           [0018]      FIG. 5  shows an axial view of certain features of another embodiment of the present invention. 
           [0019]      FIG. 6  shows an axial view of certain features of another embodiment of the present invention. 
           [0020]      FIG. 7  shows a detail of a mounting arrangement for a refrigerator in a cryostat, according to an embodiment of the present invention. 
           [0021]      FIG. 8  shows a mounting arrangement for a refrigerator in a cryostat, according to an embodiment of the present invention. 
           [0022]      FIG. 9  illustrates a detail of a bellows arrangement used in an embodiment of the present invention. 
           [0023]      FIG. 10  illustrates a detail of an alternative bellows arrangement used in another embodiment of the present invention. 
           [0024]      FIG. 11  illustrates an optional feature which may be employed with bellows arrangements according to certain embodiments of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    According to an aspect of the present invention, a thermal link between the thermal radiation shield and the first cooling stage  22  of the cryogenic refrigerator  17  is retracted when a refrigerator is inserted or removed, and the thermal link is pressed into contact with the first cooling stage  22  when in operation by a force acting perpendicular to a force applied to the refrigerator to seal it to the cryostat. 
         [0026]      FIG. 3  shows a generic illustration of a first embodiment of the present invention. A vacuum sock  15  is provided, and second cooling stage  24  is in thermal and mechanical contact with a part  26  of the wall of the sock  15 . In use, the refrigerator  17  is inserted into the sock  15 , and is sealed and mounted in place to the surrounding surface of the cryostat OVC  14 . The interior volume of the vacuum sock  15  may be evacuated through a valve  36  or may effectively be evacuated by cryo-pumping when operational: as the refrigerator cools, any air components in the vacuum sock will freeze onto the second cooling stage  24 . The vacuum load on the refrigerator: that is, atmospheric pressure acting on the exposed surfaces of the refrigerator, acts to make a firm joint between the second cooling stage  24  and the part  26  of the wall of the vacuum sock  15 . None of that load is used to make a thermal contact between the first cooling stage  22  and the thermal radiation shield  16 . As a relatively high force is available for making the joint between the second cooling stage  24  and the part  26  of the wall of the vacuum sock  15 , it has been found that an effective thermal joint may be made without the use of an indium washer or similar between the second cooling stage  24  and the part  26  of the wall. The entire vacuum load on the top of the refrigerator is available to make the joint between the second cooling stage  24  and the part  26  of the wall. The refrigerator may be bolted  34  or similarly attached to the surrounding surface of the cryostat OVC  14 . The force applied to the refrigerator by the bolts  34  or similar fasteners will add to the vacuum load and may contribute to the pressure at the contact surface between the second cooling stage  24  and the part  26  of the wall of the vacuum sock  15 . If the vacuum force, and the force applied by bolts, is too high for the second thermal interface, it may be reduced by adding springs under the flange  32 , for example around the bolts  34 . 
         [0027]    According to a feature of this embodiment of the invention, bellows  40  are provided, containing an amount of cryogen which remains gaseous at the temperature of operation of the first cooling stage  22 . The cryogen may be helium. The bellows  40  themselves are of thermally conductive material, such as copper or brass, and carry a contact piece  42  of copper or brass at an extremity nearest the refrigerator  17 . At the other extremity, the bellows is thermally linked to the thermal radiation shield  16 , either by thermal conduction through a thermal plate  43  attached to a wall of the vacuum sock (as illustrated), or through an aperture in the wall of the vacuum sock, the aperture being closed by the bellows and/or an arrangement thermally linking the bellows to the thermal radiation shield. Preferably, a plurality of sets of bellows is provided, equally spaced around the first cooling stage  22 . The interface block  30  of  FIG. 2  may not be required, representing a valuable saving in cost and weight. 
         [0028]    When the vacuum sock  15  is opened to atmosphere, the bellows  40  are driven into a retracted position by atmospheric pressure, as illustrated. The refrigerator  17  may be installed or removed without interfering with the bellows  40 . When the vacuum sock is evacuated, either by pumping out through a valve  36  or by cryo-pumping by the refrigerator in use, the pressure within the vacuum sock  15  will fall, and the pressure of the cryogen gas within the bellows  40  will cause the bellows to expand, pressing the contact piece  42  into contact with the first stage  22  of the refrigerator. A thermally conductive path is accordingly established between the thermal radiation shield  16  and the first cooling stage  22 . The pressure applied by the bellows  4   0  onto the first cooling stage  22  is determined by the characteristics of the bellows and the quantity and nature of cryogen gas sealed into the bellows. The vacuum load acting on the top of the refrigerator plays no part in determining the pressure between the contact piece  42  and the first cooling stage  22 . As the first cooling stage thermal interface pressure is independent from the second cooling stage thermal interface pressure, both are easy to control. This is advantageous because controlled thermal interface pressure enables accurate calculation and provision of effective thermal contact which is derived from pressure and surface area. 
         [0029]      FIG. 4  shows a detailed view of an alternative bellows arrangement. Here, the contact piece  42  is connected by a flexible thermally conductive joint  44 , such as an aluminum or copper braid or laminate, to a block  46 , thermally linked to the thermal radiation shield  16 . In such embodiments, no thermal conduction need take place through the material of the bellows  40 . The bellows may then be of any material of suitable mechanical properties, without being constrained to materials of high thermal conductivity. 
         [0030]    If required, mating surfaces of the contact piece  42  and the first cooling stage  22  may have a thin coating of thermally conductive grease or an indium contact to reduce thermal resistance between the two pieces. 
         [0031]      FIG. 5  shows an example embodiment, which may be an embodiment as shown in  FIG. 3 , or an embodiment as shown in  FIG. 4 , when viewed in direction V. 
         [0032]    As shown, the first stage  22  of the refrigerator is circular in plan, as is conventional. The contact pieces  42  are provided with a corresponding concave surface  50  to increase a contact surface area with the first stage  22 . As shown, multiple contact pieces and corresponding multiple bellows may be provided to increase the contact area with the first stage, and reduce the thermal resistance between the first stage  22  and the thermal radiation shield  16  by providing multiple thermal paths in parallel. At  52  are represented through-holes, into which thermally conductive braids may be attached, for example by soldering, in embodiments such as shown in  FIG. 4 . 
         [0033]      FIG. 6  shows a similar view to that shown in  FIG. 5 , of an alternative embodiment. Here, instead of using contact pieces which are shaped to match the surface of the first cooling stage  22 , thermally conductive blocks  54 , for example of copper or aluminum are attached to the first cooling stage, for example by bolting or similar. The thermally conductive blocks  54  each provide a flat mating surface  56  for pressed contact with a corresponding contact piece  42  carried by a bellows  40 . Such an arrangement may be found easier to manufacture than the profiled contact blocks shown in  FIG. 5 . 
         [0034]      FIG. 7  illustrates another version of the present invention. Here, thermal contact between the first cooling stage  22  and the first stage interface block  30 , which is thermally connected to the thermal radiation shield  16 , is provided by a thermal bus bar  58  which is provided with a flexible part, here shown as a joggle  60  in the profile of the bus bar  58 . As shown, the vacuum sock  15  is at atmospheric pressure, the bellows are retracted, and the bus bar  58  is not in contact with the first cooling stage. The refrigerator  17  may be inserted or removed without interfering with the bus bar  58 , bellows  40  or any of the thermal paths to the thermal radiation shield  16 . In use, the vacuum sock  15  will be evacuated. The pressure of cryogen enclosed within the bellows  40  will cause the bellows to elongate. The bellows  40  and contact piece  42  will bear upon the thermal bus bar  58  and bend it about its flexible portion  60  such that it enters into thermal contact with the first cooling stage  22 . Such embodiments are advantageous in that no modification needs to be made to the refrigerator  17  itself: there is complete freedom in choice of material of the bellows, as no thermal conduction need take place through the bellows. The material cross-sectional area of the thermal path through the bus bar  58  including its flexible part  60  may be significantly greater than the material cross-sectional area of the bellows, braiding or laminate  44  used to conduct heat in the other embodiments discussed above. 
         [0035]      FIG. 8  shows another embodiment of the present invention. Here, the first stage thermal intercept block  30  is a relatively close fit around the first cooling stage  22  of the refrigerator  17 . An upper surface  62  of the first stage thermal interface block  30  is preferably tapered to assist installation of the refrigerator  17 . When correctly aligned, the first cooling stage  22  passes unimpeded through the first stage thermal interface block  30 . As shown, one or more recesses or ports  64  are provided in the material of the first thermal interface block  30 . A bellows  40  is provided in each of the recesses or ports  64 . Each of the bellows  40  may be arranged according to any of the embodiments described above, or any similar arrangement. In the embodiment schematically represented in  FIG. 8 , the bellows may correspond to the embodiment of  FIG. 3 : the bellows are of a thermally conductive material and the first cooling stage  22  cools the thermal radiation shield  16  by conduction through contact piece  42 , bellows  40  and interface block  30  to the shield  16 . As in the case with the other embodiments described, the interface force of the thermal contact with the first cooling stage  22  is directed radially to an elongate axis A-A of the refrigerator, and perpendicular to an interface force of the thermal contact with the second cooling stage, which is directed parallel to the elongate axis A-A of the refrigerator. 
         [0036]      FIG. 9  shows a detailed representation of a bellows arrangement of a particular embodiment of the invention. Here, a flexible thermal conductor  64  is provided, such as a copper or aluminum braid. One end of the flexible conductor  64  is affixed to an interface piece  42  provided at a radially inner end of the bellows  40 . The flexible conductor extends the length of the bellows  40  to a fitting (not shown), in thermal contact with the thermal radiation shield  16 . The bellows is naturally extended in its “rest” state and is forced into a retracted position when the vacuum sock is at atmospheric pressure. 
         [0037]      FIG. 10  shows a detailed representation of another bellows arrangement of the present invention. This arrangement is somewhat similar to the arrangement of  FIG. 9 , in that a flexible thermal conductor  64  extends through the bellows  40 , and is joined to interface piece  42  at a surface within the bellows  40 . At the radially outer end of the bellows, a similar interface piece  68  is provided, and sealed to a wall of the vacuum sock  15  with an end plate  66 . Another thermal link (not illustrated) will be provided between the interface piece  68  and the thermal radiation shield  16 . The force and pressure applied by the bellows at the first thermal interface may be varied by design of the bellows and the interface piece  42 . Increasing the cross-sectional area of the bellows will increase the interface force, as will increasing the length of bellows in the radial direction. Reducing a surface area of the interface piece  42  will not change the interface force, but will raise the interface pressure. 
         [0038]      FIG. 11  schematically represents an improved embodiment of the present invention. Here, a small bore pipe  70  is shown, communicating with an interior volume of each of the bellows  40 . Another end of the pipe  70  passes through the wall of the cryostat to a supply of a gas. Rather than relying simply on a difference in pressure between the interior of the vacuum sock  15  and the interior of the bellows  40  which contains a fixed mass of cryogen gas, this embodiment allows an increased pressure to be provided within the bellows  40  by adding a gas such as neon, argon or helium once the vacuum sock is at vacuum. This may allow improved thermal conductivity between the interface piece  42  and the first cooling stage  22  by increasing the contact pressure between the interface piece  42  and the first cooling stage. Some heat transfer also takes place through the gas in the bellows. Preferably, gas is removed through pipe  70  from the bellows  40  when the refrigerator  17  is to be removed, allowing the bellows to retract away from the first cooling stage, providing clearance for removal of the refrigerator. 
         [0039]      FIGS. 12-16  represent a series of further embodiments, in which thermal interface pieces are arranged to rotate about certain axes into contact with refrigerator first stage  22  when the refrigerator is in position and under vacuum in the sock  15 , and to rotate out of contact with the refrigerator first stage  22  when the interior of the sock  15  is at atmospheric pressure during servicing operations. In some embodiments, one or more bellows is used, which contains a sealed mass of cryogen gas, such that the bellows will elongate when the sock  15  is at vacuum, and will retract when the interior of the sock is at atmospheric pressure. In other embodiments, one or more bellows are provided with a pipe  70 , as described with reference to  FIG. 11 , which allows the pressure within the respective bellows to be controlled at will. 
         [0040]      FIG. 12  schematically illustrates an axial view of a radially outer surface of the refrigerator first stage  22  with clamping contact pieces  72  in position, in contact with the refrigerator first stage. Clamping contact pieces  72  pivot about axle  74 . In their closed position, illustrated, radially inner surfaces  76  of the clamping contact pieces  72  are pressed into contact with a radially outer surface  78  of the refrigerator first stage  22 . The radially inner surfaces  76  of the clamping contact pieces  72  are shaped to provide a large contact surface area between the clamping contact pieces and the refrigerator first stage  22 . Part of a wall of sock  15  is schematically shown. According to this embodiment of the invention, sealed bellows units  40  are provided, each between an actuator  80  attached to, or forming part of, each clamping contact piece  72  and a bearing surface  82  mechanically restrained in a fixed position with respect to the sock  15 . Axle  74  is preferably also restrained in position with respect to the sock  15  to carry some of the weight of the clamping contact pieces  72 .  FIG. 12  illustrates the assembly in the case that the sock  15  is evacuated. A predetermined mass of a cryogen gas is sealed into each bellows  40 . When the sock is evacuated, the pressure of the cryogen within the bellows causes it to elongate, and to bear against the respective actuator  80  and bearing surface  82 . The bellows accordingly presses the contact pieces  72  into thermal and mechanical contact with the first stage of the refrigerator. A thermal link, such as any of those described above with reference to other embodiments may be used to provide a cooling path from the contact pieces  72  to the first stage of the sock, and so to the thermal radiation shield. The bellows may be adapted in length, and diameter to provide an appropriate force to clamp the contact pieces  72  against the first stage of the refrigerator. 
         [0041]    When the sock is at atmospheric pressure, the pressure differential between the sock and the bellows will reduce and may even reverse in sign. This will cause the bellows to compress. The ends of the bellows  40  are respectively attached to the actuator  80  and the bearing surface  82 , and the contracting bellows disengage the surfaces  76  of the contact pieces  72  from the first stage  22  of the refrigerator. The contact pieces  72  are shaped in the region of the axle  74  to ensure that an uninterrupted clearance space is provided around the first stage of the refrigerator when the bellows are compressed: when the sock is at atmospheric pressure. This allows the refrigerator to be removed and replaced unimpeded. 
         [0042]      FIG. 16  shows a possible arrangement of the contact pieces  72  adjacent to the axle  74 , in the direction XVI shown in  FIG. 12 . Ends of the contact pieces interlock and axle  74  passes through both of them. A thermal connector  84  may conveniently be attached to the contact pieces at the axle  74 . 
         [0043]    In alternative embodiments, fewer or more than two contact pieces may be provided, each associated with a bellows  40 , and axle  74  and a bearing surface  82 . In other embodiments, the actuator  80  may be dispensed with, the bellows  40  arranged essentially radially to bear against a part of each contact piece preferably distant from the corresponding axle  74 .  FIGS. 12A ,  12 B and  12 C schematically illustrate such embodiments. 
         [0044]      FIG. 13  schematically illustrates another type of embodiment, wherein the contact pieces are assembled in a manner similar to a pair of pliers, in that a part which contacts the first stage  22  of the refrigerator is arranged on one side of an axle  74 , while an extension piece  88  extends on the opposite side of the axle and is used to actuate the arrangement. In the illustrated embodiment, each contact piece is actuated with a corresponding bellows  40 . When the sock  15  is at atmospheric pressure, each bellows  40  is compressed, and the surface  76  of each contact piece is pulled away from the surface  78  of the first stage of the refrigerator  22 . A spring (not shown) may be provided to push the contact pieces away from the first stage  22  of the refrigerator. When the sock is at vacuum, the cryogen gas within each bellows causes the bellows to elongate, and the contact pieces  72  to press into contact with the first stage  22  of the refrigerator. Other features shown in  FIG. 13  correspond to features of  FIG. 12 . 
         [0045]    In variants of the embodiment of  FIG. 13 , more or fewer than two contact pieces may be provided, each with their own axle.  FIG. 13A  illustrates one of these variants. 
         [0046]      FIGS. 14 ,  15 ,  17  illustrate embodiments in which a pipe  70  is provided to introduce or remove cryogen gas from each bellows. Such an arrangement has been discussed above with reference to  FIG. 11 . In such arrangements, cryogen gas is introduced into the bellows  40  when the sock is at atmospheric pressure and drives the contact pieces  72  out of contact with the first stage  2  of the refrigerator. When the sock is at vacuum, cryogen gas is withdrawn from the bellows, to pull the contact pieces  72  into contact with the first stage  22  of the refrigerator. Numerous variations of such embodiments are possible within the scope of the invention, as will be apparent to those skilled in the art in a manner similar to the variants shown in  FIGS. 12-13A .