Support structure for superconducting coil

Abstract

A support structure for a superconducting coil, the support structure extending axially beyond a radially outer surface of the coil.

Description

Technical Field

[0001] The present invention relates to a support structure for a superconducting coil, and in particular to a support structure for a lightweight, flexible superconducting coil. Background Art

[0002] Conventionally, many superconducting magnets such as those used in magnetic resonance imaging (MRI) or nuclear magnetic resonance (NMR) systems include a main magnet coil that generates a strong magnetic field, a static magnetic field, and a shielding coil having a diameter larger than the diameter of the main magnet coil, which usually transmits current in the opposite direction to the current in the main magnet coil. The shielding coil reduces the amount of stray fields emitted by the magnet. For the sake of effectiveness, the shielding coil tends to be radially separated from the main magnet coil by a considerable radial gap. In order to effectively shield the stray field, the shielding coil must be accurately positioned. In use, such a shielding coil may be subjected to many tons of axial body force. Therefore, it is necessary to provide a support structure for such a shielding coil, which can resist such axial loads and can accurately and repeatedly keep the shielding coil in the desired position. The support structure also needs to be lightweight and cost-effective in order to meet the current drive to minimize the inventory and consumption of liquid refrigerants, minimize transportation costs and material consumption.

[0003] Conduction cooled magnets that do not rely on a bath or circulating mass of cryogen require that the "cold mass" of the superconducting magnet be kept low. The "cold mass" includes all components of the superconducting magnet that are in use at the operating temperature of the superconducting coils. Thus, the "cold mass" would include the superconducting coils themselves, the support structure, the electrical connections, and the equipment attached to the support structure.

[0004] Where the cold mass is confined, the material of the coil is reduced in wire size and the conventional coil former may be reduced in size or eliminated entirely. As a result, the coil becomes very thin and loses much of the inherent stiffness of the coil, making it more difficult to support such a coil. Issues such as transportation, interaction between coils, and iron near the magnets require more consideration than with conventional magnet structures. Since the reduced mass coil may be flexible to some extent, it is preferred that the coil should not be subject to mechanical point loads. That is, the coil should not be confined at only a few locations around the circumference of the coil, as mechanical point loads generated during operation can cause the coil to deform.

[0005] Current and future designs of superconducting magnets are also likely to feature low mass, flexible superconducting coils as main magnet coils, and therefore the methods and structures of the present invention may be applied to both main magnet coils and shield magnet coils. Summary of the invention

[0006] Thus, the present invention provides a support structure for a superconducting coil and a method for supporting a superconducting coil that minimizes material consumption and material consumption, minimizes mechanical interaction with other components, and enables the coil to be supported at many locations around the circumference of the coil. Using the method and apparatus of the present invention, a relatively thin and flexible coil can be mounted to a rigid support structure without worrying about the operational reliability of the coil.

[0007] Conventional mounting and reinforcement arrangements, such as rigid coil formers or large cross-section wires, are not suitable for superconducting magnet structures that seek to minimize mass, material consumption and cooling requirements.

[0008] Accordingly, the present invention provides methods and apparatus as defined in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The above and other objects, features and advantages of the present invention will become more apparent from the following description of certain embodiments given by way of example only and the accompanying drawings, in which:

[0010] Figures 1 to 3 Various example embodiments of the invention are shown in partial radial cross-section, partial axial cross-section; and

[0011] Figures 4 to 6 There is schematically represented various stages in a manufacturing process for making a coil by means of a flexible mounting tape according to an embodiment of the invention, which manufacturing process itself is according to an embodiment of the invention.

[0012] Figure 7 With Figure 1 Schematically represents an alternative embodiment of the present invention in a similar manner; and

[0013] Figure 8 and Fig. 9 Indicates the use for manufacturing Figure 7 The steps in the method of the product represented in FIG. DETAILED DESCRIPTION

[0014] The present invention provides methods and apparatus for joining thin flexible coils to rigid support structures in a manner that avoids stress concentrations and provides continuous or near-continuous support around the circumference of the supported coil. This is achieved with a structure of very low mass and may find effective application in superconducting magnet arrangements that include no or little cryogen material, where the low structural mass enables relatively rapid cooling and re-cooling to operating temperature.

[0015] Figure 1A partial axial cross section, a partial radial cross section of an embodiment of the invention is schematically represented. The structure is substantially rotationally symmetric about an axis AA (schematically represented) and reflectionally symmetric about a centerline CC (schematically represented). In this specification, references to "axial" and similar terms indicate directions parallel to the axis AA, while references to "radial" and similar terms indicate directions perpendicular to the axis AA.

[0016] The coil 10 is a thin flexible coil. The coil 10 may be a shield coil and in this specification may be referred to as a shield coil although the invention extends to embodiments incorporating a main magnet coil and indeed any magnet coil.

[0017] According to a feature of the present invention, a flexible mounting band 12 is attached to a radially outer surface 14 of the coil 10 and extends axially beyond the surface of the coil. In the illustrated embodiment, the flexible mounting band 12 extends over the entire radially outer surface of the coil 10. In other embodiments of the present invention, the band does not extend over the entire radially outer surface, but extends over a sufficient width to provide the necessary mechanical interface strength. In the illustrated embodiment, a plurality of holes 18 are formed through the flexible mounting band 12, preferably near the axial end of the flexible mounting band 12, for the purpose of mounting the flexible mounting band 12 to the support structure, for example, by rivets or bolts. In other embodiments not illustrated separately, the flexible mounting band 12 can be alternatively or additionally bonded or mechanically clamped to the support structure in an adhesive manner rather than riveted or bolted to the support structure. These methods will provide continuous support and will further minimize stress concentrations that may occur in such an embodiment, in which the support structure is intermittently supported, for example, by bolts or rivets.

[0018] exist Figure 1 In an embodiment, a plurality of holes 18 are formed near the axial inner end 16 of the flexible mounting band 12. A fastener 20, such as a rivet or bolt, is provided through each hole 18 and is used to attach the mounting band 12 to a support structure 22 at a plurality of locations. The number of holes 18 and fasteners 20 is selected to be at least sufficient to prevent the coil 10 from being subjected to point loads sufficient to cause deformation during use. In certain embodiments, a reinforcing band 24 is provided on a portion of the flexible mounting band 12 sufficient to cover the axial extent of the hole 18 and the area surrounding the hole, thereby providing support for the fastener 20. The hole 18 extends through the reinforcing band. If the reinforcing band is strong enough, and / or the holes 18 and fasteners 20 are sufficient, it may be found that the reinforcing band 24 is unnecessary. However, when the flexible mounting band 12 is lightweight, the advantages of the present invention are best obtained.

[0019] The support structure 22 may be any conventional or suitable support structure. In the case where the coil 10 is a shielded coil, the support structure 22 may be mounted to a main magnet structure, a cryogen container, or a vacuum container (outer vacuum chamber "OVC"). In the case where the coil 10 is a main magnet coil, the support structure 22 may be mounted to a cryogen container or an OVC.

[0020] In use, the coil 10 is subject to significant axial forces, as illustrated by arrows 26. Depending on the magnet design, the axial forces indicated at 26 may act towards or away from an axial mid-plane CC of the magnet.

[0021] In use, the coil 10 and the remaining cold mass are cooled to a superconducting operating temperature, which is typically 20 K or lower, typically about 4 K. The axial force 26 experienced by the coil 10 together with the hoop stresses and thermal expansion differences experienced due to tension in the annular coil 10 will cause the coil 10 and the flexible mounting strap 12 to shift relative to the support structure 22 to an active position 10' shown in dashed lines. Figure 1 As shown in , forces acting on the coil 10 may cause the coil 10 to deform, in the illustrated example by barreling outwardly. In other embodiments, the coil 10 may barrel inwardly, or deform conically. During the design of a superconducting magnet, the extent and location of such deformation and movement may be modeled by computer modeling software known to those skilled in the art, and the operation of the magnet may be simulated with the coil 10 in its "active" position 10'. Preferably, the flexible mounting band 12 is relatively rigid in the axial direction but relatively compliant in the radial direction. One or more additional reinforcing bands, not shown, may be added along the axial length of the flexible mounting band to resist buckling of the flexible mounting band under axial compressive loads while still maintaining radial compliance.

[0022] In embodiments of the invention in which the coil 10 is cooled by contact with a liquid refrigerant and / or a gaseous refrigerant, it may be found that the structure of the invention is advantageous because, since the flexible mounting strip 12 may be relatively thin and thermally conductive, the entire surface of the coil 10 is cooled by contact with the refrigerant.

[0023] The flexible mounting strap 12 is relatively thin and flexible under the forces experienced by the coil 10. The flexible mounting strap 12 is bonded to the radially outer surface of the coil 10 and serves as a means of attaching the coil 10 to a support structure. In other embodiments described in more detail below, the coil 10 may be attached to one or more other coils, which are also provided with similar flexible mounting straps, by the flexible mounting strap 12. In other alternative embodiments, also described in more detail below, the flexible mounting strap 12 is bonded to the radially inner surface of the coil 10.

[0024] The radial compliance of the flexible mounting strap 12 allows the coil 10 to expand relative to the support structure 22 without introducing excessive interface stresses between the coil 10 and the flexible mounting strap 12. Because the flexible mounting strap 12 is connected at many points, or continuously connected, and the coil 10 is continuously connected to the flexible mounting strap 12, the coil is held in a circular shape by the support structure 22. Such coil supports have been found to be able to resist considerable lateral forces caused by shipping shocks and electromagnetic (EM) loads, such as from nearby iron shielding.

[0025] In certain embodiments of the invention, the flexible mounting strip 12 is attached to the support structure 22 by a continuous mechanical connection. This may be done by means of a resin impregnated covering of glass or carbon fiber cloth or by welding, brazing or the like, or adhesive bonding, depending on the material used.

[0026] A quench in a superconducting magnet is a very severe load condition, especially for thin coils, wherein the coil 10 can heat up substantially relative to the support structure 22. Figure 1 In the case of the present invention represented by the embodiment of FIG. 1 , the radial compliance provided by the flexible mounting band 12 can accommodate relative changes in position due to differential thermal expansion without applying mechanical stress to the coil 10 .

[0027] The mechanical interface between the coil 10 and the support structure 22 is effectively located at the fastener 20, away from the coil. Any mechanical movement of the coil relative to the support structure 22 will be borne by the movement between the flexible mounting strip 12 and the support structure 22, which is away from the coil 10. Conversely, any mechanical movement between the flexible mounting strip 12 and the support structure 22 for any other reason will also occur away from the coil 10, and the heat generated by friction in any such movement will not reach the coil 10 and will not cause the coil to quench.

[0028] The flexible mounting strap 12 is a radially compliant curved support that avoids the introduction of stress concentrations into the coil 10 due to events such as cooling, energization, or quenching. The flexible mounting strap 12 is rigid in the axial and azimuthal directions and is therefore able to resist EM and transport loads.

[0029] Thus, the present invention provides a coil support method that is particularly suitable for very thin coils. The coil support of the present invention supports the coil at many azimuthal positions, thereby preventing the coil from becoming out of round or buckling. In preferred embodiments, the coil support method adds very little cost or structural material to the superconducting magnet. Stick-slip induced quenching is avoided by the structure of the present invention, because any stick-slip event will occur away from the coil itself.

[0030] Figure 2 Another embodiment of the present invention is shown, in which the main magnet coils 50 are each connected to the reference Figure 1 The embodiment described in the embodiment of the present invention is attached to the flexible mounting strip 52 in a similar manner. In the case of an end coil, such as Figure 2 As shown on the left hand side of FIG. 5 , the flexible mounting strip 52 extends axially beyond the axially inner end of the associated coil 50 . In the case of the other (inner) coil, the flexible mounting strip 52 extends axially beyond both axial ends of the coil 50 .

[0031] Thus, the first coil element 60 includes an end coil 50 and an associated flexible mounting band 52 extending axially beyond the axial inner end of the associated end coil 50. The second coil element 62 and the third coil element 64 each include an inner coil 50 and an associated flexible mounting band 52 extending axially beyond both axial ends of the coil 50.

[0032] Annular connectors 54 are provided between the coils, and these annular connectors 54 provide mechanical connections between the coils, because corresponding flexible mounting bands 52 are attached to each annular connector 54 to link adjacent coils. The annular connector 54 can be a composite material, machined, or formed into an aluminum extrusion in an annular shape, or other suitable materials that will be apparent to those skilled in the art of superconducting magnet design. The annular connector 54 will be more rigid than the flexible mounting bands 52. The annular connector 54 therefore provides mechanical strength for the final structure and ensures that each coil is firmly supported at one or more axial ends of the associated flexible mounting bands 52. The annular connector 54 can be used as a load-bearing support to keep the magnet structure in a cryostat. Support rods or the like can be attached to the annular connector 54 and used to carry the weight of the cold substance.

[0033] Figure 3 Another embodiment is shown, namely Figure 2 In another embodiment, two or more coils 50 are attached to a single common flexible mounting strip 52', wherein there is no annular connector 54 between the coils. Accordingly, a multi-coil element 66 is provided, which has two or more coils 50, which are axially spaced apart but connected to a common flexible mounting strip 52'.

[0034] In particular, Figure 2 and Figure 3The arrangement of the coil support structure of the present invention is shown, which can be used to build a complete very lightweight low-cost coil system in a modular manner. The ring connector 54 can be purchased from any of a large number of suppliers and maintained as a stock item. The coil 50 can be manufactured in a desired range of designs and maintained in stock. By having all flexible mounting bands 52, 52' have a maximum desired axial dimension, magnets of different designs can be manufactured by trimming the flexible mounting bands 52, 52' to the desired axial dimension before manufacturing a specific design.

[0035] Figures 4 to 6 Stages in an exemplary method for manufacturing the coil support structure of the present invention are illustrated. These methods themselves form embodiments of the present invention.

[0036] like Figure 4 As shown in FIG. 1 , an annular, detachable mold 70 is provided. The mold 70 in the illustrated example comprises a plurality of parts 70a, 70b, 70c which are detachable from one another in order to facilitate removal of the complete coil structure from the mold.

[0037] When assembled, mold 70 defines a first annular cavity 72 of the correct size for winding the desired superconducting coil and a second annular cavity 74 of the correct size at least partially overlapping the first annular cavity and for enclosing flexible mounting straps 12, 52. Optionally, the second annular cavity may be further defined to accommodate reinforcing strap 24.

[0038] In the first stage of the manufacturing process, Figure 4 As illustrated in , the coils 10, 50 are formed in a first annular cavity 72 by winding a plurality of turns of superconducting wire. The wire may be dry wound for later resin impregnation, or the wire may be wet wound, that is, coated with an uncured resin so that the coil becomes resin impregnated as it is formed. In other embodiments, the coil may be wound from wire coated with a bondable thermoplastic coating that fuses together when heated, or from bondable wire coated with a material that fuses together when exposed to ultraviolet light, or the wire layers may be interlaced with a carrier material that is pre-impregnated with an uncured resin, such composite materials being colloquially referred to as "prepreg materials." Once the coil is fully wound, the process proceeds as shown. Figure 5 The next stage is shown in Figure .

[0039] exist Figure 5In the embodiment of the present invention, material is wound into the second annular cavity 74 to form the flexible mounting band 12, 52, 52'. The material is wound over at least a portion of the radially outer surface of the coil 10, 50 and extends over an axially adjacent portion of the mold 70a so that the material forms a flexible mounting band that extends axially away from the coil 10, 50 on at least one side. In embodiments of the present invention in which the flexible mounting band 52' extends axially away from the coil 10, 50 on both axial sides, the second annular cavity 74 extends axially away from the first annular cavity 72 on both axial sides to define a volume in which the flexible mounting band 52' is formed.

[0040] The material used to form the flexible mounting band 12, 52, 52' can be glass fiber cloth or glass fiber filaments, or carbon fiber cloth or carbon fiber filaments. The cloth or filaments can be dry wound so as to be impregnated with resin later, or the cloth or filaments can be wet wound, that is, coated with uncured resin, so as to form a resin-impregnated flexible mounting band as the cloth or filaments are wound, or the cloth or filaments can be wound by other arrangements, such as the arrangements outlined above. Alternatively, a metal strip, such as a 1 mm thick stainless steel strip, can be used as the material for the flexible mounting band. The strip can extend the entire circumference of the coil 10, 50, and the ends of the strip will preferably overlap to a certain extent; or the ends of the strip can be welded, brazed or otherwise connected into a single annular structure placed above the coil 10, 50. This embodiment may require adding or removing parts of the mold 70 to allow access to place the single annular structure on the coil. In other embodiments, a glass fiber cloth or carbon fiber cloth coated with uncured resin may be first wound around the coil 10, 50, and then a metal tape or similar tape may be wound around the cloth layer. Once cured, the resin will serve to bond the tape to the coil and begin to address any inconsistencies in the surface of the tape and coil. Another layer of glass fiber cloth or carbon fiber cloth coated with uncured resin may be wound around the tape to hold the tape in place and provide the required mechanical rigidity where appropriate. If desired, a reinforcing tape 24 may be formed, typically by winding a strip of glass fiber cloth or carbon fiber cloth of appropriate width around the end of the flexible mounting tape 12, 52 axially away from the coil 10, 50 to form the reinforcing tape 24.

[0041] In the next step, the resin is cured and hardened into a unitary structure of material surrounding the coil windings and the flexible mounting straps.

[0042] Next, the coil and flexible mounting tape impregnated in the unitary resin impregnated structure are removed from the mold 70 by disassembling the mold into its component parts 70a, 70b, 70c. As will be familiar to those skilled in the art of superconducting coil fabrication, a mold release agent may be provided to assist this step.

[0043] Figure 6 The finished structure is shown removed from the mould. The coils 10, 50 are provided with flexible mounting strips 12, 52, 52' and holes 18 are formed at intervals around the circumference of the flexible mounting strips. Such holes 18 pass through the reinforcing strips 24, if present. Figure 6 The structure is ready for use, for example Figure 2 and Figure 3 In the embodiment shown in .

[0044] exist Figure 7 As shown in and similar to Figure 1 In an alternative embodiment shown in FIG. 1 , the flexible mounting band 12 is attached to the radially inner surface of the coil 10 instead of Figure 1 The radially outer surface 14 employed in the embodiment of FIG. The flexible mounting band 12 is attached to the radially inner surface 82 of the coil 10 , wherein the optional reinforcement band 24 located on the radially outer surface of the flexible mounting band 12 is axially displaced from the coil 10 . Figure 8 and Fig. 9 The steps in the method of manufacturing such a structure are shown, similar to those described above with reference to Figures 4 to 6 The method steps are described in Figure 8In the step shown in , the annular cavity 84 is defined by a removable mold tool 86 including parts 86a, 86b, 86c. Material is wound into the annular cavity 84 to form a flexible mounting belt 12, 52, 52'. The material forms a flexible mounting belt, which will extend axially away from the coil 10, 50 on at least one side in the completed assembly. The material used to form the flexible mounting belt 12, 52, 52' can be glass fiber cloth or glass fiber filaments, or carbon fiber cloth or carbon fiber filaments. The cloth or filaments can be dry wound so that they are impregnated with resin later, or the cloth or filaments can be wet wound, that is, coated with uncured resin, so that a resin-impregnated flexible mounting belt is formed as the cloth or filaments are wound, or the cloth or filaments are wound by other arrangements, such as arrangements as outlined above. Alternatively, metal strips, such as 1mm thick stainless steel strips, can be used as the material of the flexible mounting belt. The strip can extend the entire circumference of the coil 10, 50, and the ends of the strip will preferably overlap to a certain extent; or the ends of the strip can be welded, brazed or otherwise connected into a single annular structure. This embodiment may require adding or removing parts of the mold 86 to allow access to place the single annular structure in the annular cavity 84. In other embodiments, a glass fiber cloth or carbon fiber cloth coated with uncured resin can be first wound in the annular cavity 84, and then a metal tape or similar tape is wound on the cloth layer. Once cured, the resin will be used to bond the tape to the coil and begin to process any inconsistencies in the surface of the tape and the coil. Another layer of glass fiber cloth or carbon fiber cloth coated with uncured resin can be wrapped around the tape to keep the tape in place and provide the required mechanical rigidity where appropriate. If necessary, a reinforcing tape 24 can be formed, usually by winding a strip of glass fiber cloth or carbon fiber cloth of appropriate width around the end of the flexible mounting tape 12, 52 axially away from the coil 10, 50 in the completed assembly to form a reinforcing tape 24.

[0045] After the material of the flexible mounting strip 12, 52, 52' is located within the annular cavity 84, one or more other removable portions 86d of the mold are installed to define a coil cavity 88 in which the coil 10, 50 will be wound around the flexible mounting strip 12, 52, 52'.

[0046] The coils 10, 50 are then wound into the coil cavity 88 of the superconducting wire, which may be wet wound, subsequently impregnated, wound with a "prepreg material" or wound by any other suitable method, such as those described above. The resulting structure is as Fig. 9 . A strip material 90 may be placed over the components of mold portion 86d to hold the components of mold portion 86d in place.

[0047] The impregnated material is then cured or allowed to cure and then removed from the mold tool by disassembling the mold tool. Figure 7 As will be familiar to those skilled in the art, a release layer or coating may be provided between the coil and the mold tool surface to assist in this process. Figure 7 The structure is assembled into Figures 2 to 3 A magnet structure similar to the magnet structure shown in FIG.

[0048] In an embodiment of the present invention in which the flexible mounting band 52' extends axially away from the coils 10, 50 on two axial sides, the coil cavity 88 is defined by placing other mold pieces similar to the mold piece shown at 86d on the two axial sides of the annular cavity 84, and the coil cavity 88 is correspondingly formed axially away from the two axial ends of the annular cavity 84.

[0049] In other embodiments, a flexible mounting strip 12 , 52 , 52 ′ may alternatively or additionally be attached to an axially outer surface of the or each coil 10 .

[0050] Thus, the present invention provides a coil support structure that is very simple and can be manufactured by employing conventional coil manufacturing techniques. The structure and method of the present invention provide for minimizing the mass of the coil support structure and also minimizing undesirable mechanical interactions between the coil and the support structure, thereby reducing the risk of quenching. The structure and method of the present invention provide a support structure that is highly optimized for supporting thin superconducting coils while avoiding the introduction of stress concentrations.

Claims

1. A component for supporting a superconducting coil, the component for supporting a superconducting coil comprising: - a support structure for a superconducting coil, the support structure comprising a flexible mounting strap attached to a surface (14) of the coil and extending axially beyond a radially outer surface of the coil, wherein the flexible mounting strap extends over the entire radially outer surface of the coil; - the flexible mounting strap is attached to the support structure (22) at a plurality of locations; - wherein said coil is attached via said flexible mounting strip to one or more other coils, such other coils being attached to respective flexible mounting strips.

2. The assembly for supporting a superconducting coil according to claim 1, wherein: Each of the flexible mounting straps extends across the radially outer surface of the corresponding coil.

3. The assembly for supporting a superconducting coil according to claim 1 or 2, wherein: Apertures (18) extend through the flexible mounting strip, and a fastener (20) is provided through each aperture to the support structure (22).

4. The assembly for supporting a superconducting coil according to claim 3, further comprising a reinforcing tape (24) provided on a portion of the flexible mounting tape sufficient to cover the axial extent of the hole (18) and an area surrounding the hole.

5. The assembly for supporting a superconducting coil according to claim 1 or 2, wherein: The coil is a shielded coil and the support structure (22) is mounted to a main magnet structure, a cryogen container, or an outer vacuum chamber.

6. The assembly for supporting a superconducting coil according to claim 1 or 2, wherein: The coil is a main magnet coil and the support structure (22) is mounted to a cryogen vessel or an outer vacuum chamber.

7. The assembly for supporting a superconducting coil according to claim 1 or 2, wherein: The flexible mounting band is relatively rigid in the axial direction but relatively compliant in the radial direction.

8. The assembly for supporting a superconducting coil according to claim 3, wherein: The aperture is disposed adjacent an axial end of the flexible mounting band.

9. The assembly for supporting a superconducting coil according to claim 1 or 2, wherein: The flexible mounting strap is attached to the support structure (22) by a continuous mechanical connection.

10. The assembly for supporting a superconducting coil according to claim 1 or 2, wherein: In the case of the other coils, the flexible mounting strip extends beyond both axial ends of the associated coil.

11. The assembly for supporting a superconducting coil according to claim 1 or 2, wherein: An annular connector (54) is provided between the coils, and a respective flexible mounting band is connected to the respective annular connector (54) at an axial end of the flexible mounting band.

12. The assembly for supporting a superconducting coil according to claim 11, wherein: The annular connector (54) acts as a load bearing support to hold the magnet structure including the coil within the cryostat.

13. The assembly for supporting a superconducting coil according to claim 1 or 2, wherein: Two or more coils are attached to a single common flexible mounting strap.

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