Abstract:
A structure for superconducting magnets is provided. The structure includes a thermally conductive electrically resistive composite bobbin, a superconducting coil disposed around the thermally conductive electrically resistive composite bobbin for conducting current in a superconductive state. The structure also includes an electrically open cryogenic coil disposed on the thermally conductive composite electrically resistive bobbin, which can receive a flow of cryogenic fluid to maintain the superconducting coil in the superconductive state by transfer of heat from the superconducting coil to the electrically open cryogenic coil through the thermally conductive electrically resistive composite bobbin.

Description:
BACKGROUND 
   The invention relates generally to superconducting magnets, and more particularly to a low-AC loss cold mass structure for superconducting magnets and a process for manufacturing the cold mass structure. 
   A number of applications exist for superconducting magnets. For example, magnetic resonance imaging (MRI) systems utilize superconducting magnets to generate a strong, uniform magnetic field within which a patient or other subject is placed. Magnetic gradient coils and radio-frequency transmit and receive coils then influence gyromagnetic materials in the subject to provoke signals that can be used to form useful images. Other systems that use such coils include spectroscopy systems, magnetic energy storage systems, and superconducting generators. 
   In many superconducting magnet assemblies, a superconducting magnet is disposed in a vacuum vessel that insulates the magnet from the environment during operation. The vacuum vessel of MRI and similar magnets is generally made of components that are welded together during assembly of the magnet to form a pressure boundary. The function of the vacuum vessel of an MRI magnet is to provide a reliable pressure boundary for maintaining proper vacuum operation. Vacuum vessels known in the art are usually made of metals such as stainless steel, carbon steel and aluminum. Although, metal vacuum vessels are strong enough to resist vacuum forces, they generate eddy currents and unwanted field distortions in the imaging volume when exposed to an AC field. 
   The cold mass of a conventional superconducting magnet consists of one or several superconducting coils, a coil support structure and a helium vessel. The helium vessel is a pressure vessel located within the vacuum vessel for thermal isolation. Typically, liquid helium in the helium vessel provides cooling for the coils and maintains the cold mass at a temperature of around 4.2 Kelvin, for superconducting operation. The coils themselves are wrapped around the coil support structure. 
   Metals, such as stainless steel or aluminum, are usually used to make the helium vessel. When the magnet is operated in an AC field environment, eddy currents will be induced in those metal components, generating AC losses. The AC losses add to the total heat load for the refrigeration system because the eddy currents generate heat at cryogenic temperatures, which is expensive to remove. For certain superconducting magnet applications, these AC losses can be significant and requires to be minimized or eliminated if possible. 
   Thus, there is a need for reducing field effect losses from eddy currents, while providing desired cooling for superconducting magnets. 
   SUMMARY 
   In accordance with one aspect of the present technique, a structure for superconducting magnets is provided. The structure includes a thermally conductive electrically resistive composite bobbin, a superconducting coil disposed around the thermally conductive electrically resistive composite bobbin for conducting current in a superconductive state. The structure also includes an electrically open cryogenic coil disposed on the thermally conductive composite electrically resistive bobbin, which can receive a flow of cryogenic fluid to maintain the superconducting coil in the superconductive state by transfer of heat from the superconducting coil to the electrically open cryogenic coil through the thermally conductive electrically resistive composite bobbin. A method for manufacturing the structure for superconducting magnets is also provided. Additionally, a system for reducing eddy current losses in a magnetic resonance (MR) system is also provided. 
   These and other advantages and features will be more readily understood from the following detailed description of preferred embodiments of the invention that is provided in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a magnet assembly in accordance with aspects of the present technique; 
       FIG. 2  is a cross-sectional view of the magnet assembly of  FIG. 1  taken along line II—II of  FIG. 1 ; 
       FIG. 3  is an exploded view of an outer shell (vessel) of the magnet assembly of  FIG. 1  in accordance with aspects of the present technique; 
       FIG. 4  is a cross-sectional view of an alternative embodiment of the outer shell (vessel) taken along line II—II of  FIG. 1  in accordance with aspects of the present technique; 
       FIG. 5  is an exploded view of an alternative embodiment of an outer shell for use in a magnet assembly in accordance with aspects of the present technique; 
       FIG. 6  is a diagrammatic view of a horizontally-oriented superconducting magnet in accordance with aspects of the present technique; 
       FIG. 7  is a diagrammatic view of a vertically-oriented superconducting magnet in accordance with aspects of the present technique; 
       FIG. 8  is a diagrammatic representation of a closed MR system illustrating a superconducting magnet assembly disposed within the MR system in accordance with aspects of the present technique; and 
       FIG. 9  is a diagrammatic representation of an open MR system illustrating a superconducting magnet assembly disposed within the MR system in accordance with aspects of the present technique. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   In the subsequent paragraphs, an approach for manufacturing a superconducting coil assembly will be explained in detail. The approach described hereinafter produces a superconducting magnet structure with reduced eddy current and field effect losses, such as for magnetic resonance (MR) applications, including magnetic resonance imaging or magnetic resonance spectroscopy. The various aspects of the present technique will be explained, by way of example only, with the aid of figures hereinafter. 
   Referring generally to  FIG. 1 , manufacturing techniques will be described by reference to a magnet assembly designated generally by numeral  10 . It should be appreciated, however, that the magnet assembly may find application in a range of settings and systems, and that its use in the MR application discussed is but one such application. The magnet assembly  10  includes an outer shell (vessel)  12  surrounding a bore  14 . As will be appreciated by those skilled in the art, in an imaging application, a subject is placed in the bore  14  for imaging. Thus, the bore  14  provides access to the imaging volume for a subject. The outer shell  12  includes an outer lining  16 , an inner lining  18 , and two annular end lining flanges  20  and  22 . The outer lining  16 , inner lining  18 , and the two annular end lining flanges  20  and  22  together form a closed composite structure that encloses an evacuated volume as described below. 
   Referring now to  FIG. 2 , a cross-sectional view of the magnet assembly  10  of  FIG. 1  taken along line II—II of  FIG. 1 , is illustrated. As illustrated, the magnet assembly  10  includes the outer shell  12  surrounding the bore  14 . The outer shell  12  is constructed to enclose a vacuum volume or a vacuum cavity  24 . Within the vacuum cavity  24 , a superconducting magnet assembly  26  is disposed. The outer shell  12  is constructed by disposing a composite outer cylinder  28  over a composite inner cylinder  30  in a concentric fashion. The composite outer cylinder  28  and the composite inner cylinder  30  are closed via two annular flanges  32  and  34 , to form a closed composite structure. The composite outer cylinder  28 , the composite inner cylinder  30 , and the two flanges  32  and  34  may be made of a plastic or fiber material, such as but not limited to, a fiberglass material, a ceramic material, or a synthetic plastic material. Therefore, the two flanges  32  and  34  may be either thermally fused together or even separably joined with the composite outer cylinder  28  and the composite inner cylinder  30  at corners shown generally by reference numeral  36 . 
   The closed composite structure thus formed is then surrounded and sealed by thin metallic sheets that form an external lining over the closed composite structure. An outer metallic lining  16  is disposed proximate to the composite outer cylinder  28 , while an inner metallic lining  18  is disposed proximate to the composite inner cylinder  30 . Two annular end linings  20  and  22  are disposed proximate to the flanges  32  and  34 , respectively. The outer metallic lining  16 , the inner metallic lining  18 , and the two annular end linings  20  and  22  may be made of metal, such as stainless steel, carbon steel, or aluminum. These components  16 ,  18 ,  20 , and  22  may be welded together at the edges, as designated generally by reference numeral  38 . Thus, the outer shell  12  is a sealed vacuum vessel enclosing the vacuum volume  24 , which withstands vacuum forces shown generally by arrows  40 . 
   It may be noted that the magnetic field of MR magnet assembly  10 , particularly important within bore  14 , is not influenced to a large extent by metallic lining at the outer periphery  16 . Therefore, the outer metallic lining  16  may be thicker than the inner metallic lining  18 . This is because the superconducting magnet assembly  26  provides a magnetic field that is directed into the bore  14 . 
   The superconducting magnet assembly  26  is disposed within the outer shell  12  in the vacuum volume  24  via mechanically support structures that are not shown for clarity. The superconducting magnet assembly  26  includes a composite bobbin-shaped structure  42 , which includes a plurality of recesses  44 . The composite bobbin  42  may be made of thermally conductive strands, such as copper, that may be co-wound, intertwined, with fiberglass strands and reinforced with, for example, epoxy to form a composite body. 
   In each of the recesses  44 , is disposed a superconducting coil  46 , which may be made of a coil of metallic or ceramic wires, such as of Niobium-Titanium wires. The superconducting coil  46  wound in each recess  44  may be interlinked with that disposed in another proximate recess  44 , via electrical couplers or jumpers. A cryogenic coil  48  is wound or disposed over the composite bobbin  42 , such that the cryogenic coil  48  is proximate to the composite bobbin  42  in locations not including the recesses  44 . 
   As previously described, the superconducting coil  46  is wound in the recesses  44  of the composite bobbin  42 . Each segment of the superconducting coil  46  disposed in each recess  44  may be disposed over an insulating liner  50  that prevents the superconducting coil  46  to be electrically coupled to the composite bobbin  42 . The insulating liner  50  may be an epoxy liner, or other electrically insulating material. It may be noted that the wires of the superconducting coil  46  may also be coated with an insulating material. The structure thus formed is coated with a potting material  52  that forms a uniform overlayer. Leads of the superconducting coil  46 , shown generally by reference numeral  54 , and conduits of the cryogenic coil  48 , shown generally by reference numeral  56 , may be conducted out of the potting  52  for electrical coupling with magnet operation control circuitry and cryogen feed mechanism (not shown), respectively. 
   Turning now to  FIG. 3 , an exploded view of an outer shell  12  of the magnet assembly  10  of  FIG. 1  is shown. As illustrated, the outer shell  12  includes a composite outer cylinder  28  and a composite inner cylinder  30  that are arranged concentric to each other with respect to their central axes. Two annular flanges  32  and  34  are also arranged axially to the composite cylinders  28  and  30 , such that the annular flanges  32  and  34  and the composite cylinders  28  and  30  together form the closed composite structure enveloping an annular inner volume. It may be noted that the diameter of the composite outer cylinder  28 , and the outer diameters of the annular flanges  32  and  34  are the same. Similarly, the diameter of the composite inner cylinder  30 , and the inner diameters of the annular flanges  32  and  34  are the same. 
   A thin outer metallic lining  16  having diameter substantially equal to the outer diameter of the composite outer cylinder  28  is arranged radially over the composite outer cylinder  28 . Another thin inner metallic lining  18  having diameter substantially equal to the inner diameter of the composite inner cylinder  30  is also arranged radially within the composite inner cylinder  30 . These metallic linings  16  and  18  are then welded together with two annular end linings  20  and  22 , which are also arranged axially to the metallic linings  16  and  18 . As noted above, the outer metallic lining  16  may be thicker than the inner metallic lining  18 . Moreover, welding of these metallic sheets  16 ,  18 ,  20 , and  22  ensures the outer shell thus formed to be vacuum-sealed. Because the metallic sheets alone are not sufficiently strong to withstand the forces resulting from the pressure difference across the vessel wall when a vacuum is drawn within the vessel, the underlying composite material provides the necessary strength. At the same time, the lining provides an air-tight boundary to prevent leakage into the vessel through the composite material. The use of thin metal reduces the influence of AC fields on the overall structure. 
     FIG. 4  is a cross-sectional view of an alternative embodiment of an outer shell  58  for use in the magnet assembly of  FIG. 1 . As illustrated, the outer shell  58  includes a composite inner cylinder  30 , and an outer metallic cylinder  60 . The composite inner cylinder  30  and the outer metallic cylinder  60  are joined using annular flanges  32  and  34 . An inner lining  18 , two annular metallic linings  20  and  22  are then welded together at the joints or corners  38 . The outer shell thus formed includes a bore  14 , as shown. Although, the outer cylinder  60  is made of a metal, the metal cylinder does not interfere with the strong magnetic field generated by the superconducting magnet assembly  26  that is disposed within the vacuum cavity  24 . This is because the subject to be scanned or imaged is located within the bore  14 , and the magnetic field is directed towards the centre into the bore. The outer cylinder  60  is thus relatively spaced from the more important portion of the field. 
   Referring generally to  FIG. 5 , an exploded view of an alternative embodiment of an outer shell  62  for use in a magnet assembly is shown. The outer shell  62  is constructed using a composite cylinder  64 , which is joined together via two flanges, an upper composite flange  66 , and a lower composite flange  68 . The composite cylinder  64  and the composite flanges  66  and  68  may be made of a fiber or plastic material, such as fiberglass, ceramic, or plastic. These components  64 ,  66 , and  68  may be thermally fused or joined to form a closed pancake-shaped structure. A thin metallic outer lining  70  having an inner diameter substantially equal to the outer diameter of the composite cylinder  64  is then welded with two metallic discs, an upper metallic lining  72  and a lower metallic lining  74 . The outer shell  62  thus formed includes a vacuum volume or a vacuum cavity, within which is disposed a superconducting magnet assembly. Such a magnet assembly may be utilized for an open MR system, such as an open MRI or open magnetic resonance spectroscopy systems. 
     FIG. 6  is a diagrammatic view of a superconducting magnet that may be disposed horizontally within an outer shell of a magnet assembly in an MR system. As shown, the composite bobbin  42  of the superconducting magnet assembly  26  includes recesses  44 . Although only two recesses  44  are shown, the composite bobbin  42  may include more recesses. The recesses  44  are wound with a superconducting coil  46 . The superconducting coil  46  in the two recesses  44  are joined together via jumpers or electrical coupling that runs over the composite bobbin  42 . Leads  54  of the superconducting coil  46  are conducted to electrical coupling with a magnet operation control circuitry. A cryogen coil  76  is arranged over the composite bobbin  42  in an anti-vapor-locking configuration, as shown. For example, in a horizontally-oriented superconducting magnet structure  26 , the cryogen coil  76  may be disposed in a commonly known refrigerator cooling coil configuration. This refrigerator cooling coil configuration in the horizontally-oriented superconducting magnet structure  26  prevents vapor-locking of the cryogen, as will be appreciated by those skilled in the art. The cryogen feed mechanism feeds liquid helium, or other cryogen known in the art, into the cryogen coil  76  in a direction shown generally by reference numeral  78 . The cryogen flows down into the bottom  80  of the cryogen coil  76 . As the cryogen cools the superconducting magnet  26 , the cryogen vaporizes and passes through the serpentine cryogen coil  76 , without any cryogen vapor being locked in the cryogen coil  76 . The vaporized cryogen escapes into the cryogen feed mechanism in the direction generally shown by reference numeral  82 . 
   The composite bobbin  42 , made of thermally conductive material, as described previously helps in conducting heat away from the superconducting coils thus maintaining superconducting operation. The generated heat is conducted away towards the cryogenic coil  48 . The thermally conductive composite bobbin  42  therefore reduces the thermal gradient between the superconducting coil  46  and the cryogenic coil  48 . 
     FIG. 7  is a diagrammatic view of a superconducting magnet that may be disposed vertically within an outer shell of a magnet assembly in an MR system.  FIG. 7  shows the composite bobbin  42  of the superconducting magnet assembly  26  including the recesses  44 . As shown, cryogen coil  84  is arranged over the composite bobbin  42  in an anti-vapor-locking configuration. In this vertically-oriented superconducting magnet structure  26 , the cryogen coil  84  is disposed in a helical configuration. Again, this helical configuration in the vertically-oriented superconducting magnet structure  26  prevents vapor-locking of the cryogen. The cryogen feed mechanism feeds cryogen into the cryogen coil  84  in a direction shown generally by reference numeral  86 . The cryogen cools the superconducting magnet  26  and vaporizes through the spiral cryogen coil  84 . The vaporized cryogen escapes into the cryogen feed mechanism in the direction generally shown by reference numeral  88 . 
   The forgoing structures may be used in a range of systems and applications, such as for magnetic resonance imaging. Referring generally to  FIG. 8 , a magnetic resonance (MR) system  90 , such as for a magnetic resonance imaging or magnetic resonance spectroscopy application, is shown in which the forgoing structures are incorporated. The MR system  90  shows a closed MR system, having a bore  92  for receiving a subject  94 . Subject  94  may lie over a patient table  96  that may be introduced into the bore  92 . A magnet assembly  10  including a superconducting magnet assembly  26  made via the techniques discussed above may be utilized to generate the magnetic field for the MR system  90 . The superconducting operation may be controlled via an imaging control circuitry  98 . 
   Referring now to  FIG. 9 , an open MR system  100  is shown. The magnetic field for the open MR system  100  may be generated by a magnet assembly  10 . The magnet assembly  10  may include an outer shell  62 , constructed in accordance with the teachings of  FIG. 5 . Within the outer shell  62 , a superconducting magnet assembly  26  may be disposed, which may have a thick disc shape without a bore. Again, the superconducting operation may be controlled via an imaging control circuitry  98 . 
   Although the embodiments illustrated and described hereinabove represent only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. For example, the magnet assembly  10  including the outer shell  12  that encloses the superconducting magnet assembly  26 , may be constructed in a conventional patient bore configuration, an open MRI configuration, a long-U configuration, among others. 
   Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.