Abstract:
A freely oriented portable superconducting magnet is disclosed. Coolant is supplied to the superconducting magnet from a repository separate from the magnet, enabling portability of the magnet. A plurality of support assemblies structurally anchor and thermally isolate the magnet within a thermal shield. A plurality of support assemblies structurally anchor and thermally isolate the thermal shield within a vacuum vessel. The support assemblies restrain movement of the magnet resulting from energizing and cooldown, as well as from changes in orientation, enabling the magnet to be freely orientable.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   The United States Government has rights in this invention pursuant to Contract No. DE-AC52-06NA25396 between the U.S. Department of Energy National Nuclear Security Administration and the Los Alamos National Security LLC for the operation of the Los Alamos National Laboratory. 

   CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not Applicable 
   REFERENCE TO A “SEQUENCE LISTING”, A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON COMPACT DISC AND AN INCORPORATION-BY-REFERENCE OF THE MATERIAL ON THE COMPACT DISC 
   Not Applicable 
   BACKGROUND OF THE INVENTION 
   1) Field of the Invention The present invention relates generally to a superconducting magnet apparatus, and particularly to a superconducting magnet apparatus that is freely orientable and portable. 
   2) Background of the Need 
   For many current superconducting magnet applications, weight is typically not a major design criterion because they do not need to be mobile. Existing superconducting magnet systems often have the liquid cryogens (and storage means) as an integral part of the magnet cryostat. Systems that have the cryogens stored separately from the magnets are typically stationary and may not allow free orientation of the magnet with respect to gravity. 
   As superconducting materials mature, (e.g. higher temperature operation and current density, smaller size), their weight and cost will decrease thereby opening up new or expansion of existing opportunities. A light-weight, portable magnet (“field deployable”) that could be oriented in any direction could have applications for temporary deployment of military or civilian medical facilities for use with MRI or NMR machines. For example, such field-deployed MRI or NMR machines could be used at the scene of accidents or in homes where a patient is not movable. A concern with current MRI machines is the claustrophobic effect where some patients simply can&#39;t endure the procedure without sedation. Children become terrified and are inconsolable by parents. There is a need for an open framed MRI machine. Multiple (flowed cryogens) coils could allow for an open frame MRI configuration. The open frame alleviates claustrophobia, permits contact with the patient, and permits diversions such as television or movies. A freely orientable high-field magnet with separate cryogens would allow cranes, specialized laboratory lifting devices, and machining chucks to connect (magnetic coupling) to metal objects. A freely orientable portable magnet could be moved to and provide an external magnetic field to magnetize various ferretic objects. Two opposing magnets could be coupled together to produce lifting devices for large objects in emergencies or specialized industrial applications. There is a need for a superconducting high field magnet that can be operated in any orientation relative to gravity, and is portable (preferably operable and each subassembly transportable by one person due to its low weight and size). 
   3) Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98 
   Superconducting magnets require cooling systems to maintain the conductors at superconducting temperatures. Cooling may occur via one or more methods: thermal conduction, radiation or convection. Systems that utilize coolant for convection cooling generally fall into two categories. One category provides for immersing the conductors in a pool of coolant. This mode of cooling limits orientation of the magnet. The second category provides for circulating coolant through or in channels near the conductors. The coolant may be single phase (gaseous utilizing sensible heat) or two phase (utilizing latent and sensible heat). 
   U.S. Pat. No. 4,048,437 discloses cooling superconducting magnets via circulating coolant in conduits in which the conductors are disposed. U.S. Pat. No. 4,692,560 discloses a superconducting coil apparatus wherein the superconducting wires are cooled via coolant flowing through the conduit in which the wires are disposed. U.S. Pat. No. 5,317,879 discloses using cables to cool superconducting magnets via thermal conduction. U.S. Pat. No. 5,402,648 discloses a system for cooling superconducting magnets via circulating coolant from a dewar. U.S. Pat. No. 5,442,928 discloses cooling superconducting magnets by circulating liquid helium. U.S. Pat. No. 5,461,873 discloses natural convection cooling of a superconducting magnet by circulating helium gas. U.S. Pat. No. 5,485,730 discloses cooling superconducting magnets via circulating coolant from a remotely located cryocooler. U.S. Pat. No. 5,495,718 discloses cooling superconducting magnets via circulating coolant from a closed dewar. U.S. Pat. No. 5,848,532 discloses cooling superconducting magnets via circulating coolant remotely from a remote cryocooler. U.S. Pat. No. 6,107,905 discloses a superconducting magnet apparatus wherein cooling occurs utilizing the latent heat of vaporization of liquid helium. 
   Superconducting magnet apparatus require support structures to prevent movement of the conductors that may create friction causing quenching of the superconducting state. 
   U.S. Pat. No. 5,446,433 discloses a support structure for a superconducting magnet which is resistant to shock and vibration forces. U.S. Pat. No. 6,078,234 discloses structural members to resist forces generated within and between pairs of superconducting magnetic assemblies. U.S. Pat. No. 6,011,454 discloses a suspension system, comprising frustro-conical surfaces, for a superconducting magnet assembly. 
   BRIEF SUMMARY OF THE INVENTION 
   In the present invention, the magnet is a solenoid preferably of rectangular or circular cross-section to allow the most compact insulation and vacuum vessel. The magnet coil is shaped as a toroid providing a warm (ambient air temperature) bore open section enabling objects to be passed through as in an MRI. The magnet may have circular, elliptical, rectangular, square or other polygonal cross-section shape. The magnet is disposed on a bobbin assembly (together hereinafter referred to as magnet/bobbin subassembly) either by winding directly on the bobbin or attachment post winding of the coil. Inner support assemblies (in at least three locations, preferably generally equally spaced circumferentially) support (thermally isolate) and anchor (restrain movement) the magnet/bobbin subassembly within a thermal shield with a toroidal or other cross-sectional shape. In preferred embodiments, a bobbin bridge (shown as two half sections) is interposed between each inner support assembly and the magnet/bobbin subassembly to improve thermal isolation. The inner support assemblies include a plurality of restraint members (straps, rods/links, beams or combination of the foregoing), which provide structural connections and thermal isolation between the magnet/bobbin subassembly and support ring portions of the inner support assemblies. Outer support assemblies (in at least three locations, preferably generally equally spaced circumferentially) support (thermally isolate) and anchor (restrain movement) the thermal shield within a toroid shaped vacuum vessel. In preferred embodiments, a thermal shield bridge (shown as two half sections) is interposed between each outer support assembly and the thermal shield to improve thermal isolation. The outer support assemblies include a plurality of restraint members (straps, rods/links, beams, or combination of the foregoing), which provide structural connections and thermal isolation between the thermal shield and support ring portions of the outer support assemblies. Bobbin and thermal shield bridges may also be used to provide appropriate surfaces for strap restraint members to bear against. With this configuration, movement of the magnet/bobbin subassembly in all directions, heat leaks and size of components are thereby minimized. In other embodiments, the actively cooled thermal shield is absent, in which case the magnet/bobbin subassembly is anchored/supported directly within the vacuum vessel. 
   “Straps” refer to restraint members that provide tensile resistance, “Rods/links” refer to restraint members that provide compressive and tensile resistance. Rods/links would be pinned or threaded connections that provide some moment (bending) resistance. “Beams” refer to restraint members that provide tensile, compressive, torsion and bending resistance. In any orientation, the restraint members restrain movement of the magnet due to energizing and cooldown and react the magnet weight and magnetic forces induced during operation, while providing the most compact magnet, thermal shield, and vacuum vessel arrangement for weight minimization. Restraint members would preferably be comprised of materials with low thermal conductivity, and high strength. Since straps require a certain degree of flexibility to install, a fiber/polymer composite is preferred. Thin stainless steel or titanium straps could also be used where there may be an insulating material between them and the magnet/bobbin subassembly or the thermal shield, such as the bridges described previously. With respect to rods/links and beams, hollow titanium rods may be used, and hollow stainless steel if the higher thermal losses can be tolerated, or bridges used (as previously described). Composites (polymers strengthened with high tensile strength fibers) are preferred due to their low thermal conductivity. In a preferred embodiment using beams, G10 composite (glass fiber reinforced epoxy) beams are used. Bobbin and thermal shield bridges would preferably be comprised of machinable, composite material(s) with low thermal conductivity, low permeability, and low density such as nylon or G10. 
   Coolant is stored in one or more repositories (e.g. closed dewars) separate from the magnet, reducing the weight of the magnet assembly. In the context of this invention, the terms “cryogen” and “coolant” are to be understood as being interchangeable. To overcome the limitation on orientation in the prior art where the magnet is immersed in a bath, in the present invention coolant is circulated from the repository(s) to (and from) the magnet in one or more (preferably flexible) transfer lines allowing the magnet to be oriented in any direction. In a preferred embodiment, the transfer line has a concentric tube design wherein liquid flows in the inner tube and vapor returns in the outer tube enabling intercept of heat leaks. In other embodiments, separate lines are used for supply and return of the coolant. Vapor is vented away from the magnet assembly, preferably nearer to the repository(s). Pumping force for the cryogen results from one or more pumps, or preferably vapor generated pressurization to minimize weight. Cooling is provided to the magnet via flow of coolant in fluid channels, tubing thermally connected with the magnet/bobbin subassembly, or a combination of the foregoing. In certain embodiments, cooling is provided to the thermal shield via flow of coolant in fluid channels, tubing thermally connected with the thermal shield, or a combination of the foregoing. In certain embodiments, cooling is provided to the current leads. In certain embodiments, multiple coolants are used. Coolant supplied to bobbin may be supplied in a single path coiled around the bobbin, or in parallel paths around the bobbin. Similarly, coolant supplied to the thermal shield may be a single coiled path or in parallel paths. 
   In one embodiment, liquid nitrogen is supplied to a thermal shield surrounding the magnet, while liquid helium is supplied to the magnet/bobbin subassembly. Thermal energy conducted from the magnet/bobbin subassembly vaporizes the helium which is vented. Effluent vapor is used to cool the current leads as well. In another embodiment, only helium is used as the coolant. Liquid helium is supplied to the magnet/bobbin subassembly, and thermal energy conducted from the magnet/bobbin subassembly vaporizes the helium. Vaporized helium is circulated through the thermal shield surrounding the magnet/bobbin subassembly prior to being vented. In another embodiment, wherein the actively cooled thermal shield is absent, liquid helium cools the magnet/bobbin subassembly and the vaporized helium is vented. Although less preferred, it is to be understood that the present invention also encompasses embodiments wherein the coolant vapor is retained and re-liquefied. It is to be understood that the use of nitrogen and helium is exemplary and that the choice of coolant depends on the temperature threshold requirement for the specific superconducting magnet material. Persons with ordinary skill in the art would be able to specify the appropriate coolant(s) for specific applications. 
   In the present invention, the magnet is wound so as to make it a rigid body (such as through the use of epoxy bonding). This precludes problems associated with quenching due to movement of wires during energizing and damage due to thermal shock during cooldown. The design allows for a quick cooldown (using liquid helium). When the magnet cools it contracts more than the thermal shield which in turn contracts more than the vacuum vessel. This tightens the concentric straps, providing better stiffness when the magnet is energized. The coolant is forced through a channel (or tube) that is structurally connected to the magnet to minimize thermal resistance. Two-phase flow of the liquid coolant presents a constant temperature to the magnet around its circumference with passage of the coolant. This is beneficial from a thermal stress point of view, as well as providing thermal capacity for cooling of hot spots. Temperature gradients across the magnet could cause undesirable thermal stresses and quenching. 
   In the present invention, bayonets or permanent transfer line connections, power lead feed throughs, vents and other interconnecting coolant plumbing in the vacuum space are housed in a vacuum bulkhead. The vacuum bulkhead may interface with the vacuum vessel toroid in a method that facilitates assembly and coolant tubing connections and is structurally sound due to external pressure and provides for a compact assembly depending on the application. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       FIG. 1  illustrates a basic configuration of the present invention  1 . In the figures, like or similar elements (such as vacuum vessel  9 ) utilize the same reference characters throughout the various views. 
       FIG. 2  illustrates an embodiment of the invention wherein a single coolant is supplied to the magnet/bobbin subassembly, and subsequently supplied to the thermal shield. 
       FIG. 3  illustrates an embodiment of the invention wherein coolant is supplied to the magnet/bobbin subassembly and does not have an actively cooled thermal shield. 
       FIG. 4  illustrates an embodiment of the invention wherein one coolant is supplied to the thermal shield, and a second coolant is supplied to the magnet/bobbin subassembly. 
       FIG. 5A  illustrates an embodiment of the invention wherein coolant is provided through a single coaxial penetration.  FIG. 5B  illustrates an embodiment of the invention wherein coolants are provided via separate inlet and outlet penetrations. 
       FIGS. 6A-6F  illustrate aspects of the invention with respect to components comprising an outer support assembly (OSA). 
       FIGS. 7A-7D  illustrate aspects of the invention with respect to the magnet, bobbin subassembly, outer support assemblies, vacuum vessel, and the thermal shield. 
       FIGS. 8A-8C  illustrate embodiments of the invention wherein rods/links, or beam restraint members anchor/support the thermal shield within the vacuum vessel. 
       FIGS. 9A-9E  illustrate aspects of the invention with respect to thermal shield coolant paths. 
       FIGS. 10A-10E  illustrate aspects of the invention with respect to the magnet, bobbin, coolant tubing, and bobbin bridge. 
       FIG. 11A  illustrates aspects of the invention with respect to the thermal shield bridge.  FIG. 11B  illustrates aspects of the invention with respect to the bobbin bridge. 
       FIGS. 12A-12D  illustrate aspects of the invention with respect to the inner support assemblies, magnet/bobbin subassembly, thermal shield, and bobbin bridge. 
       FIGS. 13A-13F  illustrate aspects of the invention with respect to components comprising an inner support assembly (ISA). 
       FIGS. 14A-14D  illustrate embodiments of the invention wherein rods/links, or beam restraint members anchor/support the magnet/bobbin subassembly within the thermal shield. 
       FIGS. 15A-15F  illustrate aspects of the invention with respect to various embodiments for bobbin coolant paths. 
       FIG. 16  illustrates an embodiment of the invention having ferromagnetic material in the center bore for shielding or focusing of the magnetic field. 
       FIGS. 17A-17C  illustrate embodiments for concentricity and off-centricity of the thermal shield and thermal shield bridge with the vacuum vessel with respect to hoop cross-section and the major bore axis of the vacuum vessel. 
       FIGS. 18A-18C  illustrate embodiments for concentricity and off-centricity of the magnet, bobbin and bobbin bridge with the thermal shield with respect to hoop cross-section, and concentricity of these same components with respect to the major bore axis of the thermal shield. 
       FIGS. 19A-19E  illustrate various embodiments for circular and non-circular hoop cross-section shapes for the vacuum vessel and/or thermal shield. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A basic embodiment of the present invention is described hereunder with reference to  FIG. 1 . A single coolant  49  stored in dewar  3  is transported to/from a vacuum vessel  9  to cool a superconducting magnet  43  which is anchored/supported within vacuum vessel  9 . Coolant and electrical connections are made via vacuum vessel bulkhead  7 . Coolant  49  is transported through transfer line  5  to vacuum vessel  9  as saturated liquid, and preferably returns in vapor (gaseous) form, preferably vented via vapor vent  4 . In a further embodiment, a plurality of support points  84  provide anchoring points for positioning and/or storage hardware. 
   Referring to  FIG. 2 , coolant  57  (e.g. helium) contained in dewar  3  is transported via transfer line  59  to bobbin  41  which is in thermal contact with superconducting magnet  43 . Coolant  57  flows to and actively cools thermal shield  13 , and then exits vacuum vessel  9  via transfer line  61  (preferably primarily as vapor). Current leads  73  are cooled by coolant  57  via heat conduction path  71 . 
   Referring to  FIG. 3 , coolant  57  (e.g. helium) contained in dewar  3  is transported via transfer line  59  to bobbin  41  which is in thermal contact with superconducting magnet  43 . Coolant  57  exits passively cooled thermal shield  13 , and then exits vacuum vessel  9  via transfer line  61  (preferably primarily as vapor). Current leads  73  are cooled by coolant  57  via heat conduction path  71 . 
   Referring to  FIG. 4 , dewar  3  contains coolant  57  (e.g. helium) and coolant  63  (e.g. nitrogen). Coolant  57  is transported via transfer line  59  to bobbin  41  which is in thermal contact with superconducting magnet  43 . Coolant  57  is subsequently vented via return transfer line  61 . Coolant  63  is transported via transfer line  65  to thermal shield  13  and then exits vacuum vessel  9  via transfer line  67  (preferably primarily as vapor). Current leads  73  are cooled by coolants  57  and  63  via heat conduction path  71 . In other embodiments, the current leads are cooled by a single coolant only. 
     FIG. 5A  illustrates a single coolant embodiment with a coaxial coolant feedthru (exemplified by helium bayonet  51 ) and hermetic electrical feedthrus  55  in vacuum vessel bulkhead  7 .  FIG. 5B  illustrates a two coolant embodiment with coolant feedthrus (exemplified by supply/return helium bayonets  51  and supply/return nitrogen bayonets  53 ) and hermetic electrical feedthrus  55  in vacuum vessel bulkhead  7 . Preferred materials for thermal shield  13 , vacuum vessel bulkhead  7 , and vacuum vessel  9  would be those having lightweight, high strength, high thermal conductivity and relative permeability of 1 (to preclude interference with the magnetic field). Aluminum would be preferred over copper where weight is an issue, and preferred over titanium if cost is an issue. It is to be understood that references to various elemental metals represent their alloys and material conditions (e.g. heat treatment) as well. 
     FIGS. 6A to 6E  illustrate components of an embodiment of outer support assembly  15  utilizing strap restraint members. Referring to  FIG. 6E , a plurality (at least three) of straps  23  ( FIG. 6D ) are spaced generally equally around the circumference of the support ring  17 . The two ends of each strap  23  are anchored to support ring  17  ( FIG. 6B ) by tension assembly  19  ( FIG. 6A ) at support ring section  21  ( FIG. 6C ). Preferred materials for support ring  17  and tension assembly  19  would be those having lightweight, high thermal conductivity and relative permeability of 1 (to preclude interference with the magnetic field). Aluminum would be preferred over copper where weight is an issue, and preferred over titanium if cost is an issue. 
     FIGS. 7A to 7D  further illustrate orientation of the plurality (three shown) of outer support assemblies (OSAs)  15  (utilizing straps  23 ) with respect to vacuum vessel  9 , thermal shield  13 , and magnet/bobbin subassembly  25  located within thermal shield  13 .  FIG. 7B  presents a side view of thermal shield  13  and plurality of outer support assemblies  15 .  FIG. 7A  presents a cross-section view of the thermal shield taken at a point away from an outer support assembly  15 .  FIG. 7C  presents a cutaway perspective view of vacuum vessel  9  with vacuum vessel bulkhead  7  removed for clarity; thermal shield  13  is anchored and supported within vacuum vessel  9  by a plurality (at least three) of outer support assemblies  15 . The outer support assemblies  15  thermally isolate and minimize physical movement of the thermal shield  13  within vacuum vessel  9 . Outer support assemblies  15  may be secured to vacuum vessel  9  by means well known to those skilled in the art, including but not limited to, welding and mechanical fastening.  FIG. 7D  presents a cross-section view of the thermal shield at the location of an outer support assembly  15 .  FIG. 7D  also illustrates the presence of thermal shield bridge  27  ( FIG. 11A ) disposed between thermal shield  13  and straps  23  to provide thermal isolation of thermal shield  13  from vacuum vessel  9  (not shown). Straps  23  (two shown) are concentric with thermal shield bridge  27  and thermal shield  13 . The straps keep the thermal shield concentric with vacuum vessel  9  with respect to hoop cross-section ( FIG. 17A ) and with respect to the major bore axis of the vacuum vessel  9  ( FIG. 17B ). The location of the OSAs to ISAs with respect to each other depends on the application and forces involved. 
     FIGS. 8A to 8C  illustrate embodiments wherein rods/links ( FIGS. 8A ,  8 B) or beams ( FIG. 8C ) are used to anchor/support thermal shield  13  to vacuum vessel  9 .  FIG. 8A  (end view) illustrates a plurality of rods/links restraint members  22  emanating radially, that anchor/support thermal shield  13  to vacuum vessel  9  via attachment to thermal shield bridge  27 .  FIG. 8B  (side view) illustrates how the plurality of rods/links restraint members  22  emanate azimuthally, to anchor/support thermal shield  13  to vacuum vessel  9  via attachment to thermal shield bridge  27 .  FIG. 8C  (end view) illustrates a plurality of beam restraint members  22  emanating radially, that anchor/support thermal shield  13  to vacuum vessel  9  via attachment to thermal shield bridge  27 . It should be understood that the beam restraint members  22  also emanate azimuthally (not shown), analogous to the rods/links shown in  FIG. 8B . 
     FIGS. 9A to 9D  illustrate an embodiment of the thermal shield  13  comprising toroid half shells, having coolant tubing  14  attached.  FIGS. 9A ,  9 B and  9 C present a side cutaway view, end view, and perspective view, respectively, of thermal shield  13  and coolant tubing  14 .  FIG. 9D  presents a cross-section view illustrating coolant tubing  14  that begin and end at appropriate points and methods to access the vacuum bulkhead  7 . In preferred embodiments, the tubing is square or rectangular in cross-section to maximize thermal contact with the thermal shield surfaces. Other shapes such as circular, elliptical, or other polygons may be useful as a particular situation dictates. Preferred materials for the tubing would be those with high thermal conductivity, such as copper or aluminum. Attachment methods include brazing (for copper), welding (for aluminum), and mechanical fastening using indium as a gasket material.  FIG. 9E  presents a cross-section view for an embodiment with thermal shield  13  having coolant channels  12 . 
     FIGS. 10A to 10E  illustrate orientation of superconducting magnet  43  to bobbin  41  (magnet/bobbin subassembly  25 ).  FIG. 10B  presents a side view of magnet/bobbin subassembly  25  and a plurality (three shown) of bobbin bridges  29  ( FIG. 11B ).  FIG. 10A  illustrates an embodiment wherein coolant tubing  45  is attached to bobbin  41 .  FIG. 10C  presents a perspective view of magnet/bobbin subassembly  25 , bobbin bridges  29  and coolant tubing  45 .  FIG. 10D  presents a cross-section view of magnet/bobbin subassembly  25  taken at the location of a bobbin bridge  29 .  FIG. 10E  presents a cross-section view of magnet/bobbin subassembly  25  taken at a location away from bobbin bridges  29 .  FIGS. 10A to 10E  illustrate the relationship of the bobbin  41  and magnet  43  to the bobbin bridges  29  for an embodiment encompassing coolant tubing and a round cross-section magnet. It should be understood that the present invention encompasses other embodiments as well, with respect to other magnet cross-section shapes, coolant channels, and magnet/bobbin orientations as illustrated in  FIGS. 15A to 15F  and other support techniques as illustrated in  FIGS. 8A to 8C  that may or may not require a bobbin bridge  29 . 
     FIG. 11A  illustrates a thermal shield bridge  27  embodiment comprising half-shell pieces that conform as needed to coolant tubing geometry.  FIG. 11B  illustrates a bobbin bridge  29  embodiment comprising half-shell pieces that conform as needed to the magnet and bobbin. It should be understood that the invention includes other bridge embodiments wherein the sections conform as needed for other coolant geometries. 
     FIGS. 12A to 12D  further illustrate orientation of the plurality (three shown) of inner support assemblies  31  (utilizing straps  39 ) with respect to magnet/bobbin subassembly  25 .  FIGS. 12A and 12B  present side and perspective views, respectively, of magnet/bobbin subassembly  25 , and a plurality of inner support assemblies  31  (three shown).  FIG. 12C  presents a side view of magnet/bobbin subassembly  25 , inner support assemblies  31  and thermal shield  13 . Inner support assemblies  31  may be secured to thermal shield  13  by means well known to those skilled in the art, including but not limited to, welding and mechanical fastening.  FIG. 12D  presents a cross-section view taken at the location of an inner support assembly  31 . Bobbin bridge  29  is disposed between inner support assembly  31  and magnet/bobbin subassembly  25  to provide thermal isolation. Straps  39  (two shown) are concentric with magnet/bobbin subassembly  25 . In a preferred embodiment, the straps keep the magnet  43 , bobbin  41  and bobbin bridge  29  concentric with thermal shield  13  with respect to hoop cross-section ( FIG. 8A ) and with respect to the major bore axis of the thermal shield  13  ( FIG. 18B ). In another embodiment, the straps  39  and/or bobbin bridge  29  hold the magnet  43 , bobbin  41  and bobbin bridge  29  off-centric with thermal shield  13  with respect to hoop cross-section ( FIG. 18C ) and concentric with respect to the major bore axis of the thermal shield  13  ( FIG. 18B ). 
     FIGS. 13A to 13F  illustrate components of an embodiment of inner support assembly  31  utilizing strap restraint members. Referring to  FIG. 13E , a plurality (at least three) of straps  39  ( FIG. 13D ) are spaced generally equally around the circumference of the support ring  33 . The two ends of each strap  39  are anchored to support ring  33  ( FIG. 13B ) by tension assembly  37  ( FIG. 13A ) at support ring section  35  ( FIG. 13C ) via tension assembly ramps  40  ( FIG. 13F ). In preferred embodiments, tension assembly ramps  40  would be an integral part of support ring  33 . Preferred materials for support ring  33 , tension assembly  37 , and tension assembly ramps  40 , would be those having lightweight, high thermal conductivity and relative permeability of 1 (to preclude interference with the magnetic field). Aluminum would be preferred over copper where weight is an issue, and preferred over titanium if cost is an issue. 
     FIGS. 14A ,  14 B,  14 C, and  14 D illustrate embodiments wherein rods/links ( FIGS. 14A ,  14 B, and  14 C) or beams ( FIG. 14D ) are used to anchor/support magnet  43  and bobbin  41  to thermal shield  13 .  FIG. 14A  (end view) illustrates a plurality of rods/links (restraint members  38 ) emanating radially, that anchor/support circular cross-section magnet  43  and bobbin  41  to support ring  33  via attachment to bobbin bridge  29 .  FIG. 14B  (side view) illustrates the plurality of rods/links (restraint members  38 ) emanating azimuthally, anchoring/supporting magnet  43  and bobbin  43  to thermal shield  13  via attachment to bobbin bridge  29 .  FIG. 14C  (end view) illustrates a plurality of rods/links (restraint members  38 ) emanating radially, that anchor/support square cross-section magnet  43  and bobbin  41  to support ring  33  via attachment to bobbin bridge  29 .  FIG. 14D  (end view) illustrates a plurality of beams (restraint members  38 ) emanating radially, that anchor/support square cross-section magnet  43  and bobbin  41  to support ring  33  via attachment to bobbin bridge  29 . It should be understood that the beam restraint members  38  emanate azimuthally (not shown), analogous to the beam restraint members shown in  FIG. 14B . In embodiments where thermal shield  13  is present, support ring  33  is attached to thermal shield  13 . In embodiments where thermal shield  13  is absent, support ring  33  would be attached directly to vacuum vessel  9 . In the embodiments shown, bobbin  41  and magnet  43  are cooled by coolant flowing in coolant channel  47 . 
     FIGS. 15A to 15F  present cross-section views that illustrate various embodiments for supplying coolant  49  to bobbin  41  and magnet  43  cross-sections.  FIG. 15A  illustrates an embodiment wherein square cross-section magnet  43  is cradled in square shaped bobbin  41 , and coolant  49  flows in coolant channel(s)  47 .  FIG. 15B  illustrates an embodiment wherein square cross-section magnet  43  is disposed on flat bobbin  41 , and coolant  49  flows in coolant channel  47 .  FIG. 15C  illustrates an embodiment wherein round cross-section magnet  43  is cradled in round shaped bobbin  41 , and coolant  49  flows in coolant channel(s)  47 .  FIG. 15D  illustrates an embodiment wherein square cross-section magnet  43  is cradled in square shaped bobbin  41 , and coolant  49  flows in coolant tubes  45 .  FIG. 15E  illustrates an embodiment wherein square cross-section magnet  43  is disposed on flat bobbin  41 , and coolant  49  flows in coolant tubes  45 .  FIG. 15F  illustrates an embodiment wherein round cross-section magnet  43  is cradled in round shaped bobbin  41 , and coolant  49  flows in coolant tubes  45 . It should be understood that rectangular cross-sections and shapes for the magnet and bobbin could be used instead of square cross-sections. In preferred embodiments, the tubing is square or rectangular in cross-section to maximize thermal contact with the bobbin surfaces. Other shapes such as circular, elliptical, or other polygons may be useful as a particular situation dictates. Preferred materials for the bobbin and tubing would be those with high thermal conductivity, such as copper or aluminum. Attachment methods include brazing (for copper), welding (for aluminum), and mechanical fastening using indium as a gasket material. It is also understood that the cross-sections represent situations in which the bobbin  41  is on the inner as well as outer diameter of the magnet  43  coil. In cases where they represent the outer diameter of the magnet  43 , then the magnet  43  is wound separately on a different bobbin and then applied to the final assembly bobbin  41 . 
     FIG. 16  presents an end view illustrating an embodiment of the invention having ferromagnetic core  83  in the center bore enabling shielding or focusing of the magnetic field. Ferromagnetic core  83  is held in place by radial supports (spokes)  81 , secured to toroidal shaped vacuum vessel  9 . Magnet  43 , bobbin  41  thermal shield  13 , and vacuum vessel  9  are all as described previously. Ferromagnetic core  83  allows focusing of the magnetic field closer to the center line axis of the magnet, possibly allowing less magnet material or smaller diameter. Less magnet material would reduce cost, and smaller diameter would increase portability. A tradeoff would need to be made as to weight of the ferromagnetic core  83  vs. weight savings from the magnet or as the situation dictates. 
     FIGS. 17A and 17B  illustrate an embodiment for concentricity of the thermal shield  13  and thermal shield bridge  27  with vacuum vessel  9  with respect to hoop cross-section ( FIG. 17A ) and the major bore axis ( FIG. 17B ) of vacuum vessel  9 .  FIGS. 17C and 17B  illustrate an embodiment for off-centricity of the thermal shield  13  and thermal shield bridge  27  with vacuum vessel  9  with respect to hoop cross-section ( FIG. 17C ) and concentricity of these same components with respect to the major bore axis ( FIG. 17B ) of vacuum vessel  9 . 
     FIGS. 18A  and B illustrate an embodiment for concentricity of the magnet  43 , bobbin  41  and bobbin bridge  29  with the thermal shield  13  with respect to hoop cross-section ( FIG. 18A ), and concentricity of these same components with respect to the major bore axis ( FIG. 18B ) of the thermal shield  13 .  FIGS. 18C and 18B  illustrate an embodiment for off-centricity of the magnet  43 , bobbin  41  and bobbin bridge  29  with the thermal shield  13  with respect to hoop cross-section ( FIG. 18C ), and concentricity of these same components with respect to the major bore axis ( FIG. 18B ) of the thermal shield  13 . 
     FIGS. 19A to 19D  illustrate embodiments for circular and non-circular hoop cross-section shapes for vacuum vessel  9  and/or thermal shield  13 .  FIG. 19A  illustrates a circular cross-section shape.  FIG. 19B  illustrates a cross-section shape having a portion that is dished and another portion that is circular.  FIG. 19C  illustrates a dished cross-section shape.  FIG. 19D  illustrates a dished cross-section shape with portions that are flat. The hoop cross-section shape ( FIG. 19E ) can affect the distance from the center plane of the magnet to the face of the vacuum vessel ( FIG. 19E ). A non-circular hoop cross-section vacuum vessel and/or thermal shield as shown in  FIG. 19E  reduces the distance from the center plane of the magnet to the face of the vacuum vessel thereby increasing the projection of the usable magnetic field. This would allow less magnet material and hence a lighter magnet for the same projected field as compared to a circular hoop cross-section. 
   Obviously numerous modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described therein.