Abstract:
A superconducting magnet structure has a thermally conductive former with a former body having a former surface and a channel in said former surface that is open at said former surface, a thermally conductive tube disposed in the channel and configured to receive a circulating coolant therethrough, and the former body has at least one deformable retaining element integrally formed as a part of said former body and projecting from said surface of the former body next to the channel and being deformed over said tube in the channel to cover the tube in said channel and retain the tube in the channel.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to cryogenic cooling equipment, and particularly relates to cryogenic cooling equipment for cooling magnet coils to superconducting temperatures. 
         [0003]    2. Description of the Prior Art 
         [0004]      FIG. 1  shows a typical arrangement of superconducting magnet coils  12  wound onto a former  10 . The former may be of any structural material, but is preferably of a composite such as fiberglass reinforced resin, or a thermally conductive material such as aluminum. Stainless steel is also commonly used for the coil former. 
         [0005]    The magnet comprising former  10  and coils  12  is held within a cryogen tank  14 . The cryogen tank  14  is at least partially filled with a liquid cryogen, such as liquid helium. The liquid cryogen boils, holding the magnet at a steady temperature, being the boiling point of the cryogen. For helium, this is approximately 4K. In normal operation, boiled off cryogen is recondensed back into liquid by a recondensing refrigerator located within the service neck  20 . 
         [0006]    An outer vacuum chamber  16 , surrounds the cryogen vessel. The space between the cryogen vessel  14  and the outer vacuum chamber  16  is evacuated, to provide thermal insulation. Thermal shields  18  may be placed in the space between the cryogen vessel and the outer vacuum chamber, to reduce heat influx to the cryogen vessel by thermal radiation from the outer vacuum chamber. 
         [0007]    The cryogen tank holds a relatively large volume of cryogen. The provision and maintenance of such a large volume of cryogen is costly. The required volume of the cryogen tank also determines, to a significant degree, the final size of the cryostat containing the magnet. 
         [0008]    An object of the present invention is to provide an apparatus and methods for cooling superconducting magnets while reducing or avoiding the need for immersion of the magnet in a tank of liquid cryogen. 
         [0009]    The above object is achieved in accordance with the present invention by a superconducting magnet structure having a number of superconducting coils mounted on a thermally conductive former, the former being cooled by a cooling arrangement that includes a thermally conducting tube at least substantially contained within a channel in the body of the former, the thermally conductive tube being in thermal and mechanical contact with the body of the former and being configured to receive a circulating coolant therethrough, and wherein the former body has at least one deformable retention element integrally formed in the body at a side of the channel on opposite sides of the channel, the retention element being deformed over the thermally conductive tube in the channel to retain the thermally conductive tube in the channel. 
         [0010]    The retention element can be a retention strip or a retention lug. 
         [0011]    The above object is also achieved in accordance with the present invention by a method for manufacturing a superconducting magnet structure that includes the steps of forming a channel in a former body of a thermally conductive former and integrally, forming at least one deformable retaining element at a side of the channel, placing a thermally conductive tube in the channel, and thereafter deforming the retaining element onto the tube in the channel, to retain the tube in the channel in mechanical contact with the channel surface. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  shows a typical arrangement of a superconducting magnet within a cryostat. 
           [0013]      FIG. 2  shows a superconducting magnet within a cryostat, modified according to the present invention. 
           [0014]      FIG. 3  schematically illustrates an arrangement for causing the liquid cryogen to circulate around the cryogen tubes. 
           [0015]      FIG. 4  shows a cryogen tube housed within a channel, according to a feature of the present invention. 
           [0016]      FIG. 5  shows a cryogen tube housed within a channel, according to a feature of another embodiment of the present invention. 
           [0017]      FIG. 6  illustrates a process of retaining a cryogen tube within a channel, according to a feature of an embodiment of the present invention. 
           [0018]      FIG. 7  illustrates a cryogen tube retained within a channel as a result of the process illustrated in  FIG. 6 . 
           [0019]      FIG. 8  shows a tool, according to an aspect of the present invention, useful for performing the process illustrated in  FIG. 6 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    According to the present invention, the cryogen tank  14  of  FIG. 1  is dispensed with. A tube of thermally conductive material is provided, in thermal contact with the former  10 , which is also of thermally conductive material. 
         [0021]    Preferably, as shown in  FIG. 2 , a cryogen tube  20  is provided, following a circumference near each end of the former. In use, liquid cryogen circulates around the cryogen tubes. A refrigerator is provided, to supply cryogen at about its boiling point. For example, the cryogen may be liquid helium at a temperature of about 4K. The liquid cryogen circulates through the cryogen tubes  20  and absorbs heat from the former. The heat is carried to the refrigerator, where the heat is removed. The cooled former  10 , in turn, cools the coils  12 , holding them in a superconducting state, below their critical temperature. 
         [0022]      FIG. 3  schematically illustrates an arrangement for causing the liquid cryogen  78  to circulate around the cryogen tubes  20 . A relatively small cryogen tank  80  is provided in the cryogen tube circuit. A recondensing refrigerator  82  is also provided. In operation, some of the liquid cryogen  78  in cryogen tube  20  will absorb heat from the cryogen tube  20 , and thus from the former  10 . This will cause some of the liquid cryogen  78  to boil into a gaseous state. The boiled-off cryogen gas  84  will rise toward the top of the cryogen tube circuit, and will enter the recondensing refrigerator  82 . The recondensing refrigerator  82  operates to cool the cryogen gas  84 , recondensing it into liquid cryogen  78 , and removing heat from the system. As illustrated in  FIG. 3 , boiling of the liquid cryogen will take place substantially on the right-hand side of the circuit as illustrated, and will rise to the recondensing refrigerator  82 . The recondensed liquid cryogen supplied by refrigerator  82  will descend through the left hand side of tube  20 , as illustrated. Hence, this arrangement provides continuous circulation of the cryogen, and effective cooling. Although a cryogen tank  80  is required, the volume of liquid cryogen  78  required is very much reduced as compared to cryogen tanks  14  of the prior art, which allowed immersion of the magnet in a bath of liquid cryogen. 
         [0023]    In a preferred embodiment, the tube  20  is a stainless steel tube, held in position by mechanical deformation of lugs or retaining strips formed in the material of the former. In certain embodiments, channels are formed in the material of the former to house the tube. The tube may be of other materials of high thermal conductivity, such as copper. 
         [0024]    In the case of an aluminum former, it has been found that the thermal expansion of a stainless steel tube is sufficiently similar to the thermal expansion of the former. The material chosen for the tube must be sufficiently mechanically strong to withstand the pressure of the cryogen. 
         [0025]    If the cryogen tube  20  is to be retained by mechanical deformation, then this process may be performed after the magnet coils are wound onto the former, if preferred. 
         [0026]    A particularly preferred embodiment is illustrated in  FIG. 4 . According to this embodiment, a channel  30  is machined in the material of the former  10  to house the tube  20 . The channel  30  may be formed with a profile which is complementary to the cross-section of the tube  20 . Two lugs or retaining strips  32  are also machined into the surface of the retainer  10 . As illustrated in  FIG. 4 , this may be achieved by machining three adjacent channels  34 ,  30 ,  38  into the material of the former, with the lugs or retaining strips  32  being formed by the material of the former left between the channels. 
         [0027]    In an alternative embodiment, illustrated in  FIG. 5 , a single channel  30  is formed to house the tube, and retaining strips or lugs  32  are formed projecting from the surface of the former. 
         [0028]    Preferably, the channel  30  formed for housing the tube  20  is an interference fit, such that the tube may be pressed into position by machine or by hand, and will be retained in position by frictional interaction with the walls of the channel. 
         [0029]    As illustrated in  FIG. 6 , the tube is retained in position by deforming the lugs or retaining strips  32  towards each other, over the tube in the directions of two of the arrows shown. The material of the former should be chosen so that it is malleable yet rigid at room temperature. Certain grades of aluminum and stainless steel have appropriate properties. In this way, the tube  20  is retained in stable position and in good thermal and mechanical contact with the former  10 , while requiring no welding or braising step. Since the process uses only machining techniques, the tubes  20  may be installed during the manufacture of the former, resulting in a low cost process. 
         [0030]      FIG. 7  illustrates the structure after the lugs or retaining strips  32  have been deformed over the tube  30 . The tube  30  is protected from damage, for example during handling, by being embedded within the material of the former. It is held in intimate thermal and mechanical contact with the former  10 . 
         [0031]      FIG. 8  illustrates a tool  70  which may be used to deform the lugs or retaining strips  32  over the tube  30  and so retain the tube in position. The tool  70  comprises a pair of angled forming wheels  72 , mounted axially  74  on a spindle  76 . The spindle is retained on a tool body  78  which may itself be mounted to a handle for manual use, or may be mounted on a machine for automated or power assisted use. In use, the angled forming wheels  72  are brought to bear on the lugs or retaining strips  32  which run alongside the channel  30  holding the tube  20 . Pressure is imparted onto the tool in a direction substantially perpendicular to the surface of the former  10 , generally in the direction of the upper arrow shown in  FIG. 5 . The surfaces of the angled forming wheels  72  are so angled that the pressure they impart on the lugs or retaining strips causes the lugs or retaining strips  32  to be deformed to turn inwards towards each other over the tube  20 , as shown in  FIG. 7 . 
         [0032]    The cooling tubes and retaining means according to the present invention provides a cost effective means for cooling equipment such as magnet formers, and so cooling the magnet coils themselves. Such magnet coils and formers may be employed in Nuclear Magnetic Resonance or Magnetic Resonance Imaging. By arranging the cooling of the magnet according to the present invention, the volume of liquid cryogen required may be significantly reduced. For example, a magnet for an MRI imaging system may be cooled according to the present invention with as little as 80-100 liters of cryogen provided to circulate in the tubes  20  according to the arrangement described with reference to  FIG. 3 . This compares very favorably with present systems which typically require a volume of 2000 liters of cryogen in cryogen tank  14 . 
         [0033]    For the apparatus cooled according to the present invention, there is no requirement for a cryogen tank  14  enveloping the former  10  and coils  12 , so the outer vacuum container may be reduced in size, resulting in a smaller overall system. 
         [0034]    Although the present invention has been described with reference to a limited number of specific embodiments, those skilled in the art will recognize that numerous modifications and variations may be made to the present invention, within the scope of the appended claims. 
         [0035]    For example, while the present invention may usefully be applied to cooling a superconducting magnet for use in an MRI system, the present invention may be applied to any apparatus which requires cooling. 
         [0036]    While a certain particular tool has been described for deforming the lugs or retaining strips, other tools may of course be used to perform this task. 
         [0037]    While the invention, has been particularly described in relation to retention of the tube by two lugs or retaining strips  32 , the present invention may be embodied by arrangements having lugs or retaining strip along only one side of channel  30 .