Abstract:
A method for assembling a thermal shield suspension assembly including a plurality of straps, wherein the straps are arranged in an alternately cross hatch arrangement such that a plurality of first straps extend at a first orientation with regards to the cryogenic vessel and a plurality of second straps extend at a second orientation with regards to the cryogenic vessel, and wherein the first orientation is different from the second orientation and each second strap is disposed between the adjacent first strap is provided, the method comprising securing the first strap including the tensioning block to a thermal shield flange and a cryogenic vessel flange and securing the second strap to the thermal shield flange and the cryogenic vessel flange.

Description:
BACKGROUND OF INVENTION 
     This invention relates generally to superconductive magnets and more particularly, to thermal shield suspensions for use in superconductive magnets. 
     Superconductive magnets include superconductive coils which generate uniform and high strength magnetic fields, such as are employed in magnetic resonance imaging (MRI) systems for medical diagnostics. Within at least some known magnets, the superconductive coil assemblies are enclosed in a thermal shield surrounded by a vacuum enclosure. Various designs of tube suspension systems are employed to support the superconductive coil assembly such that the magnet is maintained a pre-determined distance from both the thermal shield and the vacuum enclosure. 
     The suspension systems include a plurality of straps and bolts rigidly connected together to support the superconductive coil assembly and the associated components, i.e. the thermal shield and vacuum enclosure. The suspension systems are subjected to stresses from supporting the weight of the components and during operation, such systems may also be subjected to thermal stresses induced between components. Additionally, vibrational stresses may also be induced into the suspension system during operation. Overtime, continued operation with such stresses may cause premature failure. 
     To facilitate preventing such failures, at least some known suspension systems include extensive damping systems to minimize the effects of the stresses. However such damping systems are expensive and increase the overall cost of manufacture. 
     SUMMARY OF INVENTION 
     In one aspect a method for assembling a thermal shield suspension assembly including a plurality of straps, wherein the straps are arranged in an alternately cross hatch arrangement such that a plurality of first straps extend at a first orientation with regards to the cryogenic vessel and a plurality of second straps extend at a second orientation with regards to the cryogenic vessel, and wherein the first orientation is different from the second orientation and each second strap is disposed between the adjacent first strap is provided, the method comprising securing the first strap including the tensioning block to a thermal shield flange and a cryogenic vessel flange and securing the second strap to the thermal shield flange and the cryogenic vessel flange. 
     In another aspect a superconductive magnet is provided that includes a superconductive coil assembly including a cryogenic vessel, wherein the cryogenic vessel includes at least a flange, a thermal shield enclosing the coil assembly, wherein the thermal shield includes at least one flange, a vacuum enclosure enclosing the thermal shield, and a thermal shield suspension assembly positioned between the cryogenic vessel and the thermal shield. The thermal shield suspension assembly includes a plurality of straps configured to facilitate damping vibrational forces induced to the magnet. The straps are arranged in an alternately cross hatch arrangement such that a plurality of first straps extend at a first orientation with regards to the cryogenic vessel and a plurality of second straps extend at a second orientation with regards to the cryogenic vessel, wherein the first orientation is different from the second orientation and each second strap is disposed between the adjacent first strap. 
     In a further aspect a thermal shield suspension assembly for a superconductive magnet is provided, the assembly includes a superconductive coil assembly including a cryogenic vessel, wherein the cryogenic vessel includes at least a flange, a thermal shield enclosing the coil assembly, wherein the thermal shield includes at least one flange, and a plurality of straps. The straps are positioned between the cryogenic vessel and the thermal shield and configured to dampen vibrational and hyper-sonic sound responses induced in the magnet, the straps are arranged in an alternately cross hatch arrangement such that a plurality of first straps extend at a first orientation with regards to the cryogenic vessel and a plurality of second straps extend at a second orientation with regards to the cryogenic vessel, wherein the first orientation is different from the second orientation and each second strap is disposed between the adjacent first strap. 
     In yet another aspect, a plurality of thermal shield suspension straps are provided, wherein the straps are arranged in an alternately cross hatch arrangement. A first strap includes a first end, a second end, and a unitary elongate body extending therebetween, the first end includes a tensioning block, and the first strap extends at a first orientation with regards to a cryogenic vessel. A second strap includes a first end, a second end, and a unitary elongate body extending therebetween, and the second strap extends at a second orientation with regards to the cryogenic vessel, wherein the first orientation is different from the second orientation and each second strap is disposed between the adjacent first strap. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic side view of an open magnet including the thermal shield suspension straps. 
     FIG. 2 is a schematic fragmentary view of a portion of the magnet in FIG. 1 including the thermal shield suspension straps. 
     FIG. 3 is a plan view of a first thermal shield suspension strap. 
     FIG. 4 is side view of first thermal suspension strap. 
     FIG. 5 is a plan view of a second thermal shield suspension strap. 
     FIG. 6 is a side view of second thermal shield suspension strap. 
     FIG. 7 is an enlarged cross-sectional view of a first thermal shield fastener. 
     FIG. 8 is an enlarged cross-sectional view of a second thermal shield fastener. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a schematic side view of an open magnet  10 . Magnet  10  includes a first assembly  12  which includes a plurality of first and second thermal shield suspension straps  14  and  16 , respectively. In one embodiment, magnet  10  is a superconductive magnet. First assembly  12  has a centrally-located longitudinal axis  18  and defines a bore  20  therein. 
     First assembly  12  includes a superconductive coil assembly  22  at a cryogenic temperature, a thermal shield  24  that encloses coil assembly  22 , and a vacuum enclosure  26  at an ambient temperature and enclosing thermal shield  24 . Coil assembly  22  includes a cryogenic vessel  28  containing cryogenic fluid  30  and superconductive coils  32 . In the exemplary embodiment, cryogenic vessel  28 , thermal shield  24  and vacuum enclosure  26  are substantially toroidal-shaped and radially spaced from one another with reference to axis  18 . Furthermore, in the exemplary embodiment, cryogenic vessel  28 , thermal shield  24  and vacuum enclosure  26  are coaxially aligned with axis  18  such that thermal shield  24  is positioned between, and spaced apart from cryogenic vessel  28  and vacuum enclosure  26 . In another embodiment, thermal shield  24  is formed from aluminum. In one embodiment, vacuum enclosure  26  and cryogenic vessel  28  are formed from non-magnetic stainless steel or aluminum. 
     First assembly  12  also includes a tube suspension assembly  34  including a plurality of tubes (not shown) extending between cryogenic vessel  28 , thermal shield  24 , and vacuum enclosure  26 . Tube suspension assembly  34  is under tension and mechanically supports components within the vacuum enclosure  26  against magnetic forces generated by magnet  10 . 
     In one embodiment, magnet  10  includes a second assembly  36  that is connected to first assembly  12  by a support member  38 . More specifically, support member  38  extends longitudinally between first assembly  12  and second assembly  36 . Second assembly  36  is a mirror image of first assembly  12  with respect to a plane  40  positioned longitudinally and equi-distant between first assembly  12  and second assembly  36 . Plane  40  is substantially perpendicular to axis  18 . In one embodiment, support member  38  is a nonmagnetizable support member consisting of non-magnetic stainless steel. 
     FIG. 2 is a schematic fragmentary view of a portion of magnet  10  including a first thermal shield strap  14  and a second thermal shield strap  16 . Straps  14  and  16  are both positioned within thermal shield  24  to interconnect thermal shield  24  and cryogenic vessel  28 . Although only one first thermal shield strap  14  and only one second thermal shield strap  16  are shown, a plurality of thermal shield straps  14  and  16 , described in more detail below, are substantially coaxially and equally spaced circumferentially about axis  18 . In one embodiment, each magnet assembly  10  includes eight pairs of straps  14  and  16 ; In another embodiment, thermal shield straps  14  and  16  are spaced about two inches apart. Thermal shield  24  and cryogenic vessel  28  are variably spaced apart by a jacking bolt  42 . 
     First thermal strap  14  has an elongate body  50  including a first strap end  52  and a second strap end  54 . First strap end  52  is secured to thermal shield  24  by a fastener  56  and a tensioning block  58 . Second strap end  54  is secured to cryogenic vessel  28  by a fastener  60 . Fasteners  56  and  60  are described in more detail below. 
     Second thermal shield strap  16  has an elongate body  62  including a first strap end  64  and a second strap end  66 . First end  64  is secured to cryogenic vessel  28  by a fastener  68 . Second end  66  is secured to thermal shield  24  by fastener  68 . 
     FIG. 3 is a plan view of a thermal shield suspension strap  14 . FIG. 4 is a side view of thermal shield suspension strap  14  including elongate body  50 , first end  52 , and second end  54 . Strap  14  is unitary, rigid, and substantially flat. Accordingly, body  50 , first end  52 , and second end  54  are substantially co-planar. In one embodiment, strap  14  is fabricated from Inconel® 718 which is commercially available from Special Materials, Huntington, W.Va. Alternatively, strap  14  is fabricated from a metallic material such as, but not limited to, titanium or stainless-steel. 
     In the exemplary embodiment, elongate body  50  is substantially rectangular in shape and includes a thickness  70 , a length  72 , and a width  74 . In one embodiment, thickness  70  is approximately 0.040 inches, length  72  is approximately 23.50 inches, and width  74  is approximately 0.3125 inches. Thickness  70  is substantially uniform throughout body  50  along length  72 . First end  52  is substantially square and includes a top surface  76  and an opposite bottom surface  78 . First end  52  has a length  80  and a width  81 . In one embodiment, length  80  and width  81  are each approximately 0.75 inches. Second end  54  is also substantially square and includes a top surface  82  and bottom surface  84 . Second end  54  also has length  80  and width  81 . Strap  14  has a length  86  measured between first and second ends  52  and  54 , respectively. In one embodiment, length  86  is approximately 25.00 inches. 
     Elongate body  50  also includes an axis of symmetry  88  extending from first end  52  to second end  54 . First end  52  includes an axis of symmetry  90  that is perpendicular to axis  88 , and second end  54  includes an axis of symmetry  92  that is perpendicular to axis  88 . A first aperture  94  is disposed within first end  52  and a second aperture  96  is disposed within second end  54 . Each aperture  94  and  96  include a center  98  and  100  respectively. Centers  98  and  100  are each positioned along axis  88 . In one embodiment, aperture  94  has a diameter of approximately 0.375 inches, and aperture  96  has a diameter of approximately 0.25 inches. 
     Tensioning block  58  is secured to first end bottom surface  78 . In one embodiment, tensioning block  58  is welded to surface  78 . Tensioning block  58  is substantially cubicle and includes a thickness  110 , a length  112 , and a width (not shown). In one embodiment, thickness  110  is approximately 0.50 inches, and length  112 , and the width are each approximately 0.75 inches. A block aperture  114  is disposed within tensioning block  58  and concentrically aligned with respect to first end aperture  94  and axis  88 . Block aperture  114  is threaded and sized to receive fastener  56  therein. In one embodiment, block aperture  114  has a diameter of approximately 0.375 inches. 
     FIG. 5 is a plan view of a thermal shield suspension strap  16  and FIG. 6 is a side view of thermal shield suspension strap  16  including elongate body  150 , first end  152 , and second end  154 . Strap  16  is unitary, rigid, and substantially flat. Accordingly, body  150 , first end  152 , and second end  154  are substantially co-planar. In one embodiment, strap  16  is fabricated from Inconel® 718 which is commercially available from Special Materials, Huntington, W.Va. Alternatively, strap  16  is fabricated from a metallic material, such as, but not limited to, titanium or stainless-steel. 
     Elongate body  150  is substantially rectangular in shape and includes a thickness  170 , a length  172 , and a width  174 . In one embodiment, thickness  170  is approximately 0.040 inches, length  172  is approximately 21.50 inches, and width  174  is approximately 0.3125 inches. First end  152  is substantially square and includes a top surface  176  and an opposite bottom surface  178 . First end  152  has a length  180  and a width  181 . In one embodiment, length  180  and width  181  are approximately 0.75 inches. Second end  154  is also substantially square and includes a top surface  182  and bottom surface  184 . Second end  154  also has length  180  and width  181 . Strap  16  has a length  186  measured between first and second ends  152  and  154 , respectively. In one embodiment, length  186  is approximately 23.00 inches. 
     Elongate body  150  also includes an axis of symmetry  188  extending from first end  152  to second end  154 . First end  152  includes an axis of symmetry  190  that is perpendicular to axis  188 , and second end  154  includes an axis of symmetry  192  that is perpendicular to axis  188 . A first aperture  194  is disposed within first end  152  and a second aperture  196  is disposed within second end  154 . Each aperture  194  and  196  include a center  198  and a center  200 . Centers  198  and  200  are each positioned along axis  188 . In one embodiment, aperture  194  has a diameter of approximately 0.375 inches, and aperture  196  has a diameter of approximately 0.25 inches. 
     FIG. 7 is an enlarged cross-sectional view of thermal shield fastener  56  including tensioning block  58 , thermal shield  24 , and first thermal suspension strap  14 . Strap  14  is secured to a thermal shield flange  220  by fastener  56  and block  58  such that a gap  222  is defined between strap  14  and shield flange  220 . Fastener  56  is inserted through thermal shield aperture  224  and strap aperture  94  into threaded tension block aperture  114 . 
     FIG. 8 is an enlarged cross-sectional view of thermal shield fastener  60  including cryogenic vessel  28 , and first thermal suspension strap  14 . Strap  14  is secured to a cryogenic vessel flange  226  by fastener  60 . Fastener is inserted through strap aperture  96  into a threaded aperture  228  in cryogenic vessel  28 . 
     During assembly, second thermal suspension strap  16  is installed with thermal shield  24  deformed by approximately 0.0625 inches. Thermal shield  24  is deformed by adjusting jacking bolt  42 . First strap  14  is installed with gap  220  set at approximately 0.125 inches. Then jacking bolt  42  is removed and straps  14  and  16  are tensioned by tightening fastener  56  to reduce gap  220  to zero. Once the straps are tensioned, the 0.0625 inch deformation of thermal shield  24  is removed. 
     Securing the ends of the thermal shield suspension straps  14  and  16  is accomplished by forming a rounded rim in the ends of the straps, some of such ends being attached to thermal shield  24  and cryogenic vessel  28 , as appropriate and as can be appreciated by an artisan. It is noted that straps  14  and  16  are under tension and provide a superior system for mechanically supporting the magnet components within the vacuum enclosure thermal shield  24  against the magnetic forces generated by magnet  10 , as can be understood by those skilled in the art. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.