Abstract:
An arrangement for cryogenic cooling comprising a cryogen tank ( 14 ), a cryogenic recondensing refrigerator ( 12 ) arranged to cool a heat exchanger which is exposed to the interior of the cryogen tank ( 14 ) and an arrangement ( 16; 26 ) for conducting heat from a cooled article ( 10 ) to the cryogen tank. A further cryogen tank ( 20 ) is provided below the heat exchanger and arranged to receive cryogen liquid recondensed on the heat exchanger.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0001]    The present invention relates to the cryogenic cooling of sensitive equipment, particularly in the context of superconducting magnets for MRI (Magnetic Resonance Imaging) systems. 
       Description of the Prior Art 
       [0002]    As is well known to those skilled in the art, superconducting magnets comprise coils of superconductive wire which require cooling to cryogenic temperature appropriate to the material of the superconducting wire to maintain their superconducting properties. This is commonly achieved by at least partially immersing coils of superconducting wire in liquid cryogen at its boiling point. 
         [0003]    Different superconducting materials are known, and the cryogen must be chosen to have a boiling point below the superconducting transition temperature of the appropriate material. Liquid helium is often used. It has the lowest boiling point of all, about 4K, but is increasingly scarce and expensive. 
         [0004]    To reduce consumption of helium, pipe cooled magnet systems have become available.  FIG. 1  schematically illustrates the principle of a pipe cooled magnet. The pipe cooled arrangement may also be referred to as a thermosiphon. 
         [0005]    The present invention will be particularly described with reference to cylindrical superconducting magnets, which have a number of superconducting coils  10  aligned along a horizontal axis A. Such magnets are also referred to as “solenoidal” magnets, even if their construction is not a solenoid in its true sense. However, the present invention is not limited to such magnets, and extends to other types of superconducting magnet as will be apparent to those skilled in the art. 
         [0006]    In  FIG. 1 , a magnet structure  10  has axially aligned coils of superconducting wire retained in position by conventional means, such a thermosetting resin impregnation onto a mechanical support structure. Refrigerator  12  is a cryogenic recondensing refrigerator. It acts to cool a heat exchanger  13  which is exposed to the interior of a cryogen tank  14 . At least one cooling pipe  16  encircles the magnet structure  10  and is in thermal contact with each coil. An inlet end of the cooling pipe  16  is connected to the cryogen tank  14  near a lower extremity thereof, and an outlet end of the cooling pipe is connected to the cryogen tank  14  nearer an upper extremity thereof. 
         [0007]    The apparatus illustrated in  FIG. 1  would be enclosed within an evacuated outer vacuum container (OVC) (not illustrated). Thermal radiation shields are typically provided located inside the OVC, surrounding the coils  10 , pipe  16  and cryogen tank  14 . 
         [0008]    In operation, a liquid cryogen  15  is introduced into cryogen tank  14  at its boiling point. Cryogenic refrigerator  12  cools boiled-off cryogen vapor back to a liquid and maintains a stable temperature within the cryogen vessel. Heat generated in coils  10 , or removed from the coils to cool them, causes boiling of cryogen within the cooling pipe  16 . Boiled off cryogen vapor rises in the cooling pipe  16  to leave through the outlet end into the cryogen tank. The cryogen vapor is recondensed by refrigerator  12  into liquid cryogen  15 . Cryogen thereby circulates into the inlet end of the cooling pipe  16 , out of the outlet end of the cooling pipe and back into the cryogen vessel  14 . In this way, the cooling effect of refrigerator  12  is distributed around the circumference of the coils  10 . 
         [0009]    A relatively small cryogen tank  14 , with a relatively small mass of cryogen  15  is found sufficient to cool the magnet coils  10 . However, such an arrangement has certain drawbacks. 
         [0010]    Although not represented in the schematic illustration of  FIG. 1 , various electrical components such as superconducting joints, a superconducting switch, electrical connections and diagnostic sensors, dissipate energy as heat during various stages of the systems operation, and need to be cooled by the refrigerator  12 . In arrangements such as represented in  FIG. 1 , the components can suffer from insufficient cooling. In some arrangements, the components may be mechanically and thermally attached to the exterior of cryogen tank  14  or cooling pipe  16 , or may be attached to a surface of coils  10 . The pipe cooling arrangement of  FIG. 1  must be retained in a vacuum to enable effective operation. However, cooling of the components through mechanical contact has been found to be inefficient in a vacuum, as compared to cooling in contact with a cryogen liquid or gas as there is no cryogen to bridge any small contact gaps. 
         [0011]    The superconducting magnet coils  10  have a high thermal conductivity, and so are easy to cool and to keep cool by cooling even just a small percentage of their surface area, in this example by pipes  16  which contain cold cryogens and are in thermal contact with at least certain regions of the surface of each coil. 
         [0012]    When a magnet is energized, the coils must be cooled below the superconducting transition temperature of the relevant wire. Associated superconducting switches must be open to allow energization of the magnet. This involves heating the switches above the superconducting transition temperature of the relevant wire. If the coils  10  are not sufficiently cooled, this switch heating may reach the coils  10  of the magnet and prevent them achieving superconductive status. Once the magnet is energized, the switches must be rapidly cooled to regain their superconducting properties to enable the magnet to become persistent. 
         [0013]    One possible arrangement for ensuring effective cooling of such components in a pipe cooled magnet such as represented in  FIG. 1  provides the components inside the cryogen tank  14 , in contact with the liquid cryogen  15  to cool the components. While this ensures effective cooling, this arrangement does have some drawbacks. For example, the placing of such components within the cryogen tank  14  reduces the available volume of liquid cryogen available for cooling the magnet coils  10 . 
         [0014]    In the event of excessive cryogen boil off for any reason, the level of liquid cryogen  15  may drop below the level of the components. This may allow the components to heat above their superconducting transition temperature, which in turn could cause the magnet to quench. 
         [0015]    During a quench, the liquid cryogen  15  in the cryogen tank  14  could be boiled off or expelled out of the cryogen tank  14 . This may allow the components to heat above their superconducting transition temperature. When a superconducting switch needs to be “open”, heat must be applied. 
         [0016]    If this switch, and therefore also the heat, is contained within cryogen tank  14 , more liquid cryogen  15  is evaporated. This could interrupt the flow of cryogen through the cooling pipe(s)  16 . 
         [0017]    The present invention may also be applied to other types of magnet, for example where coils  10  are cooled by thermal conduction through a thermal link, such as a metal braid, laminate or thermal busbar. In such arrangements, the liquid cryogen  15  does not circulate, but serves to maintain a constant temperature of the cryogen tank  14 . 
       SUMMARY OF THE INVENTION 
       [0018]    The present invention therefore addresses the above problems, to provide an arrangement for effective cooling of components in a pipe cooled or contact cooled superconducting magnet system, which avoids or reduces the effects of the above-mentioned drawbacks. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  schematically represents a conventional pipe cooled superconducting magnet. 
           [0020]      FIGS. 2-5  schematically represent pipe cooled superconducting magnets according to embodiments of the present invention. 
           [0021]      FIG. 6  shows a feature of certain embodiments of the present invention. 
           [0022]      FIG. 7  represents a conduction cooled superconducting magnet according to an embodiment of the invention. 
           [0023]      FIGS. 8-9  schematically represent cryogen vessels comprising a constriction, as employed in certain embodiments of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    As recognized in conventional arrangements, the most effective method for cooling an item to a stable cryogenic temperature is to submerge the item in a cryogenic fluid at its boiling point. The cryogen is held in a closed vessel at saturation temperature and pressure. When heat is generated in the item, the cryogen that is closest to the heat absorbs the heat by vaporizing. The gaseous cryogen is much less dense than the liquid surrounding it so is displaced by other liquid cryogen and hence the heat is removed. 
         [0025]    The difficulties discussed above with respect to placing cooled components within cryogen tank  14  arise because the cryogen in a same cryogen tank must serve to cool the electrical components and to provide cooling to the magnet coils  10 . The cryogen may be expelled from cryogen tank  14  during a quench event. 
         [0026]    In an embodiment of the present invention, a further cryogen tank is provided for housing the components within a quantity of liquid cryogen. This quantity of liquid cryogen and the further cryogen tank are in communication with the cryogen tank  14  through a constriction. Cryogen tank  14  and liquid cryogen  15  are used for cooling the magnet coils  10  as discussed above. Such arrangement allows excellent cooling of the components by direct contact with liquid cryogen, but avoids any of the difficulties associated with the use of a single cryogen volume for both cooling of magnet coils and cooling of the components. 
         [0027]    The arrangement of the present invention does not require reduction of the liquid cryogen capacity of the cryogen tank  14 . 
         [0028]    The further cryogen tank is preferably arranged below the cryogen tank  14 , to ensure that the components would be completely covered in liquid cryogen even when the level of liquid cryogen  15  within the cryogen tank  14  is low. 
         [0029]      FIG. 2  schematically illustrates a first embodiment of the invention. Features common with  FIG. 1  show common reference numerals. In this embodiment, cooling pipe  16  is divided into an upstream portion  16   a  and a downstream portion  16   b . Further cryogen tank  20  houses components  21 . Electrical connections  22  are made between components  21  and the magnet coils  10 . Electrical feedthroughs  23  are provided to allow an electrical path to the components  21  to extend through the wall of the further cryogen tank  20 . Further cryogen tank  20  forms part of the cooling loop arrangement, in that a circulation path of cryogen flows from cryogen vessel  14 , through inlet end of pipe  16 , through further cryogen tank  20  and through outlet end of the pipe  16  back to the cryogen tank  14 . Downstream portion  16   b  of the cooling pipe is attached to the further cryogen tank  20  near an upper extremity thereof, and to cryogen tank  14  near an upper extremity thereof. Upstream portion  16   a  of the cooling pipe  16  is connected to cryogen tank  14  and further cryogen tank  20  at locations below the respective connection of downstream portion  16   b . Further cryogen tank  20  is located below cryogen tank  14 , and may preferably be located at the lower extremity of cooling pipe  16   a / 16   b . Further cryogen tank  20  is accordingly in communication with cryogen tank  14  through a constriction defined by cooling pipe  16 . 
         [0030]    In operation, further cryogen tank  20  fills preferentially with liquid cryogen. Under the influence of gravity, liquid cryogen  15  will fill further cryogen tank  20  first, and only once that is full will the cooling pipe  16  and cryogen tank  14  fill with liquid cryogen. The cooling loop itself will operate as described with reference to  FIG. 1 : heat from coils  10  will cause boiling of cryogen into vapor which will circulate through the pipe  16   a / 16   b , entering the cryogen tank  14  though downstream portion  16   b  to be recondensed by the refrigerator  12  back into liquid cryogen  15  for recirculation through upstream portion  16   a  of the pipe. Any heat generated by the components  21 , such as heat provided to open a superconducting switch, may cause boiloff of liquid cryogen, and the resulting cryogen vapor will rise and circulate through downstream portion  16   b  back to cryogen tank  14 . 
         [0031]    In some embodiments of the invention, multiple further cryogen vessels  20  may be provided, each accommodating sub-sets of the components  21  to be cooled and each in communication with cryogen tank  14  through a constriction. Similarly, multiple pipes  16 ,  16   a / 16   b  may be provided, or which one or more may be connected to a further cryogen tank  20 . 
         [0032]      FIG. 3  schematically represents another embodiment of the present invention. In this arrangement, further cryogen tank  20  is located beneath cryogen tank  14 , not at the lowest extremity of cooling pipe  16 . A relatively short connecting pipe  24  extends essentially vertically between cryogen tank  14  and further cryogen tank  20  defining a constriction which ensures communication between cryogen tank  14  and further cryogen tank  20 . The remaining features of this embodiment are as discussed with reference to  FIG. 2 . 
         [0033]    In such embodiments, a lower part of pipe  16  will fill first with added liquid cryogen  15 , and then the further cryogen tank  20  will fill, before connecting pipe  24  and then cryogen tank  14 . Any heat generated within further cryogen tank  20  may cause cryogen to boil, and the resulting cryogen vapor will rise upwards through the constriction to cryogen tank  14 , where it will be recondensed by refrigerator  12 . 
         [0034]      FIG. 4  shows another embodiment of the present invention. Here, the further cryogen tank  20  is fed from a tee  27  in the pipe  16 . Further cryogen vessel  20  is in communication with cryogen vessel  14  through a constriction defined by pipe  16  and tee  27 . While the further cryogen tank  20  will preferentially fill with liquid cryogen, the cryogen circulating in the pipe  16  will not necessarily pass through the further cryogen tank  20 . Preferably, the further cryogen tank  20  is located such that cryogen vapor generated in the further cryogen tank  20  will rise through the constriction towards the cryogen tank  14  in the normal direction of circulation of cryogen in pipe  16 . 
         [0035]    Other arrangements may be found, but it is preferred that the further cryogen tank  20  should be positioned below the cryogen tank  10 , in fluid communication therewith. The fluid communication through a constriction between cryogen tank  14  and further cryogen tank  20  need not form part of the cooling loop (thermosiphon) path. 
         [0036]      FIG. 5  illustrates an embodiment in which further cryogen tank  20  is connected to cryogen tank  14  by connecting pipe  24 , separately from the cooling loop path of pipe  16 , defining a constriction which provides fluid communication between further cryogen tank  20  and cryogen tank  14 . As with other embodiments, further cryogen tank  20  will fill preferentially, before cryogen tank  14 . A baffle  28  may be provided, as desired, to ensure that either the pipe  16 , or the further cryogen tank  20 , fills before the other. This may be arranged simply by selecting the relative position of the heat exchanger of the cryogenic refrigerator with respect to the baffle. 
         [0037]    In yet other arrangements, as shown in  FIG. 6 , the cryogenic refrigerator  12  may provide liquid cryogen into a recondensing chamber  30 , fluidly connected to both cryogen tank  14  and further cryogen tank  20 . The recondensing chamber divides the liquid cryogen between the cryogen tank  14  and the further cryogen tank  20 . The geometry of the recondensing chamber may be adjusted to determine which of the cryogen tank  14  and the further cryogen tank  20  will fill preferentially. Here, constrictions are provided by pipes linking further cryogen tank  20  and cryogen tank  14  to recondensing chamber  30 . 
         [0038]      FIG. 7  schematically represents a further series of embodiments of the present invention. In  FIG. 7 , no cooling loop (thermosiphon) is provided. Instead of a pipe  16  carrying cryogen in a circuit to cool the coils, a solid thermal conductor  26  is provided, in thermal contact with the cryogen tank  14  and the coils  10 . The solid thermal conductor  26  may be of any conventional type, such as a laminate, braid or thermal busbar of high purity aluminum, copper of other suitable material such as a composite containing aluminum or copper. In use, heat is transferred from coils  10  to cryogen tank  14  through solid thermal conductor  26 . The heat will cause boiling of the liquid cryogen  15  in the cryogen tank  15 , and the cryogen vapor will be recondensed by cryogenic refrigerator  12 . In the illustrated embodiment, the further cryogen tank  20  is located beneath cryogen tank  14 , connected by connecting pipe  24  defining a constriction, such that further cryogen tank  20  is preferentially filled with liquid cryogen. The remaining features in  FIG. 7  are as described with reference to the earlier drawings. In some embodiments, a cooling loop may be provided in addition to the solid thermal conductor. 
         [0039]    The variant illustrated in  FIG. 6 —where a recondensing chamber is provided to distribute liquid cryogen between cryogen tank  14  and further cryogen tank  20 —may be applied to the arrangement of  FIG. 7 . 
         [0040]      FIGS. 8 and 9  illustrate features of further embodiments of the present invention. In these embodiments, the cryogen tank  14  and further cryogen tank  20  are provided by subdivisions of a single vessel, and are linked by a constriction  32 . In the arrangement of  FIG. 8 , the constriction is provided by shaping of the single vessel  34 . In the arrangement of  FIG. 9 , the constriction is provided by a baffle arrangement. 
         [0041]    Numerous other variants will be apparent to those skilled in the art, within the scope of the present invention as defined in the appended claims. Although described with reference to cooling of superconducting coils  10 , the present invention may find application in the cryogenic cooling of other types of cooled article. Although the components  21  housed within the further cryogen tank have been described as particular types of electrical component, other types of electrical component, and indeed other types of component, may be cooled by placement within the further cryogen tank of the present invention.