Abstract:
A mechanical actuation arrangement for remotely applying a force to a cryogenically-cooled device has a mechanical actuator composed of multiple parts. In use, the parts bear against one another to enable a force to be applied to the device by an actuator device, and when not in use, the parts separate.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to arrangements for remote actuation of devices in a cryogenic environment. In particular, the present invention provides arrangement for actuation at room temperature of a mechanical or electromechanical device which is at a cryogenic temperature, which has a limited thermal conductivity between the room temperature actuator and the electromechanical device at cryogenic temperature. 
         [0003]    The present invention will be particularly described with reference to an application to superconducting magnets retained within a cryostat, but may be applied to other systems, as will be apparent to those skilled in the art. 
         [0004]    2. Description of the Prior Art 
         [0005]    In cryogenically cooled systems, such as superconducting magnet systems, it is frequently required to apply an actuation force to a variety of devices such as thermal links, electrical switches, other electrical devices. 
         [0006]    Conventionally, such actuation forces have been applied by numerous arrangements such as electrical drives, gas pressure in expanding bellows, pistons or the like, or mechanically through an access port such as a neck tube in a cryogen vessel. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides an alternative to these existing arrangements for applying actuation forces, which employs mechanical actuation without introducing an excessive thermal conduction into the cryogenic environment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  schematically illustrates an embodiment of the present invention in a first state. 
           [0009]      FIG. 2  schematically illustrates the same embodiment of the present invention in a first state. 
           [0010]      FIGS. 3-4  schematically illustrate further embodiments of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0011]    The present invention will be particularly described with reference to a cryostat comprising an inner, cryogen cooled vessel, tank or pipework or similar contained within an outer vacuum container (OVC), with a thermal radiation shield placed within the OVC, shielding the cryogen cooled component from radiant heat from the OVC, which is typically itself at ambient temperature. 
         [0012]      FIG. 1  schematically illustrates an embodiment of the present invention. The drawing represents a fragment of a cryostat wall, comprising a cryogen vessel  10  within an outer vacuum container OVC  12 , with a thermal radiation shield  14  located between them, shielding the cryogen vessel  10  from radiant heat emitted by the OVC  12 . The cryogen vessel  10 , OVC  12  and thermal radiation shield  14  are all retained in respective positions by mechanical retention means, not shown, and other apparatus, such as a cryogenic refrigerator and/or volume of liquid cryogen, is provided, as will be apparent to those skilled in the art. 
         [0013]    According to this embodiment of the invention, a device  16  to be actuated is attached to the cryogen vessel  10 , either on its outer surface as shown in  FIG. 1 , or on its inner surface, as will be discussed in more detail below, in the context of a further embodiment of the present invention. An actuator device  18  is mounted to an external surface of the OVC. Actuator device  18  comprises an output tube  19  and serves to drive a first push-rod  20  through the output tube  19  inwards or outwards of the OVC, towards or away from the device  16 . Actuator device  18  may itself be electrically, pneumatically, hydraulically or manually mechanically operated. 
         [0014]    A second push-rod  22  traverses the radiation shield  14  through a hole  30 . A thermal intercept  32  may be provided to ensure that the second push-rod  22  is cooled to the temperature of the thermal radiation shield  14 . The second push-rod is supported and mechanically biased to the illustrated rest position. 
         [0015]    Second push-rod  22  is mounted to the thermal radiation shield  14 . The mounting arrangement should provide thermal connection between second push-rod  22  and thermal radiation shield  14 , should block thermal radiation from OVC  12  to cryogen vessel  10  and should urge the second push-rod  22  into a defined rest position. In the illustrated embodiment, second push-rod  22  passes through a guide bushing  62 , which may be a plastic moulding. The plastic moulding may be loaded with metal or carbon powder to increase its thermal conductivity. Guide bushing  62  comprises a bore  64  for passage of the second push-rod  22  therethrough, and otherwise covers hole  30  in the thermal radiation shield  14 . The guide bushing  62  is mechanically mounted onto the thermal radiation shield and provides mechanical support to the second push-rod  22 . A collar, enlarged head or similar protrusion  66  provided on the second push-rod near an end nearest device  16  retains the second push-rod  22  in the guide bushing  62  and may serve to close any radiation path through the bore  64  between the second push-rod  22  and the guide bushing  62 . Preferably, as illustrated, the collar  66  is thermally linked to the thermal radiation shield  14  by a thermally conductive braid, laminate or other flexible, thermally conductive path  32 . A second collar, enlarged head or similar protrusion  68  provided on the second push-rod near an end furthest from device  16  retains the second push-rod  22  in the guide bushing  62 . A spring  70  or equivalent resilient member bears between second collar, enlarged head or similar protrusion  68  and the guide bushing  62  or thermal radiation shield  14 . The combination of spring  70  and first and second collar, enlarged head or similar protrusion  66 ,  68  operate to bias the second push-rod to a rest position in its range of travel at a location furthest from device  16 . Other equivalent mounting arrangements may be provided, but preferably provide the functions of mechanically mounting and restraining the second push-rod while biasing it to a defined rest position and providing thermal conductivity between second push-rod  22  and thermal radiation shield  22 . 
         [0016]    Device  16  is, in this embodiment, mounted on an outside surface of the cryogen vessel  10 . An actuator rod  24  is provided. In operation, the actuator rod  24  must be actuated by mechanical pressure from actuator device  18 . Actuator rod  24  may have a form similar to that of first- and/or second- push-rods  20 ,  22 . According to its type, the device  16  will change status in response to pressure applied to the actuator rod  24 . 
         [0017]    Actuator device  18  may be mounted onto an access hatch  34  which is demountable for ease of servicing, removal or replacement of the arrangement of the present invention, or any component of it. Such access hatch  34  may be attached to the rest of the OVC  12  by removable fasteners  36  such as bolts screwed into blind threaded holes  38 . A seal  40  such as a polymer gasket may be provided to prevent influx of air into the vacuum region  42 . 
         [0018]    Output tube  19  may be sealed  44 , for example with a polymer gasket, to prevent air influx at the interface between first push-rod  20  and the access hatch  34  or OVC  12 . In an alternative arrangement, seal  44  may bear upon the first push-rod  20 . In such case, output tube  19  may be omitted. 
         [0019]      FIG. 1  shows the arrangement of this embodiment of the invention in “rest” mode. The actuator device  18  causes the first push-rod  20  to displace away from device  16 , outwards from the OVC. Contact between the first push-rod  20 , second push-rod  22  and actuator rod  24  is broken. No force is being applied to actuator rod  24  and second push-rod  22  is displaced to its rest position, out of contact with both the first push-rod  20  and the actuator rod  24 . 
         [0020]      FIG. 2  shows the arrangement of the embodiment of  FIG. 1  in an “active” mode. Features corresponding to features shown in  FIG. 1  carry corresponding reference labels. In this mode, actuator device  18  has caused first push-rod  20  to be displaced towards the device  16 . First push-rod  20  has entered into contact with second push rod  22  and displaced it, against the mechanical bias provided by spring  70  or equivalent, into contact with actuator rod  24 . First push-rod  20  has displaced second push-rod  22  sufficiently to apply pressure to the actuator rod  24 , causing a change in status of the device  16 , according to the type of device it is. Preferably, first push-rod  20 , second push-rod  22  and actuator rod  24  are constructed of a material of low thermal conductivity, such as hollow resin-impregnated fiber glass tube. Second push-rod  22  should not have a clear bore through it, as that would allow thermal radiation from the OVC  12  to the cryogen vessel  10 . Second push-rod  22  may be solid, or may have a bore which is closed off at one or both ends, or at another location along its length. In the “active” mode illustrated in  FIG. 2 , a solid thermal path exists between actuator device  18  and OVC  14  at ambient temperature and the device  16  attached to the cryogen vessel  10 . By constructing first push-rod  20 , second push-rod  22  and actuator  24  of material of low thermal conductivity, the transfer of heat from ambient temperature to cryogen vessel  10  is limited. At the end of the “active” mode, actuator device  18  retracts first push-rod  20  away from device  16 , outwards of the OVC. The arrangement reverts to the “rest” mode shown in  FIG. 1 . The second push-rod  22  reverts to its biased rest position out of contact with both the first push-rod  20  and the actuator rod  24 . 
         [0021]    Although not illustrated in the drawings, it is conventional to provide solid insulation between the OVC  12  and the thermal radiation shield  14 , for example in the form of multi-layered aluminised polyester sheets. Preferably, such solid insulation is provided around at least the second push-rod  22  to reduce any transmission of heat from the OVC to the cryogen vessel  10  by radiation through hole  30 . 
         [0022]    While the invention has been described above with reference to a limited number of specific embodiments, numerous modifications and variations are possible, and are provided by the present invention. Some of these modifications and variations are described below. 
         [0023]      FIG. 3  illustrates an actuation arrangement according to another embodiment of the present invention. Features corresponding to features shown in  FIGS. 1 and 2  carry corresponding reference numerals. 
         [0024]    The embodiment of  FIG. 3  corresponds to the embodiment of  FIG. 1  except in that output tube  19  of the actuator device  18  is sealed to the OVC  12  or access hatch  34  by a bellows  46  instead of the polymer seal  44  shown in  FIGS. 1 and 2 . Bellows  46  may be a stainless steel bellows brazed, soldered or welded to the OVC  14  or access hatch  34  and the output tube  19  of the actuator device  18 . The bellows may alternatively be bonded by an appropriate adhesive or attached and sealed by any other appropriate arrangement. First push-rod  20  is driven through output tube  19  by actuator device  18  as described with reference to  FIGS. 1 and 2 . In an alternative arrangement, bellows  46  may be sealed to the first push-rod  20 . In such case, output tube  19  may be omitted. 
         [0025]      FIG. 4  illustrates an actuation arrangement according to another embodiment of the present invention. Features corresponding to features shown in  FIGS. 1-3  carry corresponding reference numerals. 
         [0026]    The embodiment of  FIG. 4  corresponds to the embodiment of  FIG. 3  except in that device  16  is mounted inside the cryogen vessel  10 . Actuator rod  24  protrudes through a hole  48  in the cryogen vessel  10  and is sealed to the cryogen vessel by a bellows  50 . Bellows  50  may be a stainless steel bellows brazed, soldered or welded to the cryogen vessel  10 . The bellows may alternatively be bonded by an appropriate adhesive or attached and sealed by any other appropriate arrangement. Actuator rod  24  is driven by second push-rod  22  as described in relation to other embodiments, and bellows  50  is compressed or expands in response to force applied to the actuator rod  24  by second push-rod  22  and also to the difference in gas pressure between the interior of the cryogen vessel  10  and the vacuum region  42 . 
         [0027]    In various embodiments of the invention, the actuator device  18  may be operated electrically, hydraulically, pneumatically or manually, among others. The device  16  may be an electromechanical switch, a mechanical thermal linkage, or other electrical device, as examples. 
         [0028]    Actuator device  18  may be located inside the OVC, but in that case it will be necessary to transmit commands or actuation force to the actuator device  18  through the wall of the OVC  12 , so a suitable sealing arrangement would need to be provided. 
         [0029]    By providing a mechanical linkage between actuator device  18  and device  16 , the present invention allows a higher force to be applied to the device  16  than might be possible in the case of, for example, pneumatic or electrical actuation of actuator rod  24  of device  16 . 
         [0030]    By placing actuator device  18  on the outside of the OVC, or on a demountable access panel  34 , replacement and servicing is simplified. In the case of demountable access panel  34 , access to second push rod  22  is simplified. It would also be possible to mount second push rod  22  on a demountable access panel (not illustrated) in the thermal radiation shield  14 , making it relatively easy to access device  16 . 
         [0031]    In the “rest” mode, as illustrated in  FIG. 4 , gaps between first push-rod  20 , second push-rod  22  and actuator rod  24  limit thermal influx by conduction through the arrangement of the present invention. The second push-rod  22  is preferably thermally linked to the thermal radiation shield, and thermally stabilises at the temperature of the thermal radiation shield when in “rest” mode. 
         [0032]    Other modifications and variations are also possible within the scope of the present invention as defined in the appended claims.