The present invention relates to structures of superconducting magnets such as used in magnetic resonance imaging (MRI) systems. It relates to the type of system in which a superconducting magnet is provided, composed of several essentially annular coils aligned along a common axis. In such magnets, it is common to have a series of "main" or "inner" coils (referred to as main coils hereafter), which generate most of the magnetic flux of the magnetic field produced within the bore of the magnet, and a number of "shield" or "outer" coils (referred to as shield coils hereafter), of greater diameter than the main coils, placed coaxially with the main coils. The shield coils usually overlap with the main coils, in an axial direction. Such magnets produce a magnetic field within their bore which is very homogeneous, within an imaging region. The imaging region is typically a sphere of about 50 cm diameter within the bore, at the geometric centre of the magnet.

Throughout the present description, the term "axial" will be used to describe a direction parallel to the common axis of the coils, and the term "radial" will be used to describe any direction perpendicular to, and passing through, the common axis of the coils.

In all such magnets, arrangements must be provided to retain the main and shield coils in their correct respective positions. Various arrangements have conventionally been employed. Fig. 5A schematically illustrates a conventional arrangement in which main coils 10 are wound into journals 12 machined into a solid former 14 which may be of aluminium or a composite material of glass fibre embedded in epoxy resin. Typically, the coils are themselves impregnated with epoxy resin, for example. Shield coils 16 are held within similar formers or "journals" 18. The structure is essentially axisymmetrical about axis A-A. The journals 18 are typically retained in position relative to the main coils by aluminium webs 20, bolted to the former 14 at positions lying between main coils 10.

The webs 20 supporting the shield coils may only be attached to the former 14 at positions between main coils. The different materials of the former and the coils will contract at different rates when cooled to operating temperature, which may cause the coils to move on the former, possibly causing quench or degrading the homogeneity of the resultant magnetic field.

In use, stresses will be generated within the coils, as a result of the interaction of currents flowing in the coils with the generated magnetic field.

The interaction between the coils and the former can be a mechanism for quenching due to heat generated at the slip plane interface.

The webs 20 supporting the shield coils may only be attached to the support tube 22 at positions between main coils 10. In use, stresses will be generated within the coils, as a result of the interaction of currents flowing in the coils with the generated magnetic field.

The limited possibilities for placement of webs 20 means that the interaction of the webs with the former may not assist in diminishing the effect of stresses on the coils.

Fig. 5B shows an alternative conventional arrangement, in which main coils 10 are attached to an inner surface of a support tube 22 by their radially outer surfaces. The coils' radially outer surfaces are conventionally referred to as A2 surfaces, and this type of arrangement may be known as A2 bonding. The tube 22 is typically of a composite material, for example filament wound glass fibre or glass fibre cloth impregnated with epoxy resin. Journals 18 may be provided, as described with reference to Fig. 5A, and these journals are attached to the support tube 22 by webs 20 bolted to the support tube.

In such arrangement, forces due to interaction with the magnetic field, and differences in thermal contraction between coils 10 and support tube 22 will cause shear stresses at the bonded interfaces. These stresses may lead to cracking of resin impregnant within the coils, or bonding the coils to the support tube. In turn, this cracking may realease sufficient energy to cause a quench, or allow part of the coil to move sufficiently to cause a quench.

Fig. 5C shows another alternative arrangement, in which the main coils 10 are attached to a support tube 22 by A2 bonding. In this example, shield coils 16 are bonded to another support tube 24 by A2 bonding, and the two support tubes 22, 24 are held in fixed relative positions by attachment to a pair of end discs 26. These end discs may be of a non-metallic composite material such as glass fibre cloth impregnated with epoxy resin, or may form part of a cryostat housing the magnet.

As the support tubes 22, 24 are retained only by their ends, there is scope for the tubes to flex, under the influence of gravity and/or stresses within the coils, so that the coils are no longer coaxial. This will reduce the homogeneity of the resultant field generated by the magnet. This arrangement will suffer from shear stress which may cause quench, as discussed in relation to Fig. 5B. Recently, magnet structures have been proposed in which the main coils do not require any mechanical supporting former, but the main coils are used as structural pieces themselves, and their relative positions are determined by spacer elements bonded between the coils. Such structurally self-supporting magnets may be referred to as "serially bonded magnets". The spacer elements may be separate components which are bonded between completed coils, or may be permeable pieces which are included in an impregnation step during which the coils and the permeable pieces are impregnated together forming a monolithic structure of epoxy resin within which coils and permeable pieces are embedded. Fig. 5D illustrates an example arrangement of this type, having a coil layout similar to that of Fig. 5C.

However, although such structures have found to be mechanically robust and to avoid the problems of mechanical interaction with support structures during operation or cooling, difficulty has been experienced in finding a suitable arrangement for mounting shield coils to the main magnet coils formed in this way. The arrangement of Fig. 5D is not preferred, as it causes an increase in length of the magnet.

The present invention accordingly provides superconducting magnet structures in which shield coils are mounted to main coils formed as serially bonded magnets. Furthermore, the structures of the present invention enable control over resultant stresses within the coils structures in use, according to the design and construction of intermediate coil support structures which retain the shield coils on the main coils. The present invention provides methods and apparatus as defined in the appended claims. The above, and further, objects, characteristics and advantages of the present invention will become more apparent from the following description of certain embodiments thereof, in conjunction with the accompanying drawings, wherein:

Fig. 1 shows an arrangement for mounting shield coils to main magnet coils according to an embodiment of the invention;

Fig. 2 shows results of modelling of stresses in a main coil and shield coil, which are not mounted together, as if in use;

Fig. 3 shows results of modelling of stresses in a main coil and shield coil, which are mounted together according to an embodiment of the invention;

Fig. 4 shows results of modelling of stresses in a main coil and shield coil, which are mounted together according to another embodiment of the invention;

Figs. 5A-5C show conventional arrangements for mounting shield coils to main coils; and

Fig. 6 shows an arrangement for mounting shield coils to main magnet coils according to another embodiment of the invention.

Fig. 1 shows a schematic part cross-section through an arrangement according to the present invention. An example main coil 10 is shown, part of a structurally self- supporting magnet structure. Similarly, an example shield coil 16 is shown, mounted within a journal 18. An intermediate coil support structure 31 is provided, comprising webs 30 and preferably also comprising partial oversleeve 32, joining the shield coil 16 to the radially outer surface of main coil 10. The structurally self-supporting magnet structure comprising main coil 10 is therefore the support structure for the shield coils 16. The intermediate coil support structure is attached to the structurally self-supporting magnet structure, such as by an adhesive bond, resin impregnation or bolting.

According to certain features of selected embodiments of the present invention, and as schematically illustrated in Fig. 1 , a support structure 31 is provided for mounting shield coils 16 onto main coils 10. Preferably, such support structure is arranged to reduce the localised hoop stress of the main coils, and to make use of structural properties of main coils to support the shield coils. Structures of the present invention may also be arranged to at least partially offset large hoop stresses which may be generated during energisation of magnets of high field strength, or de-energising them, for example during a quench. According to the present invention, a structurally self-supporting magnet of annular coils 10 and spacer elements bonded to them is used as a structural base onto which the shield coils 16 are mounted by an intermediate coil support structure 31. According to certain embodiments of the present invention, for example as illustrated in Fig. 6, an intermediate coil support structure 31 comprises a web 30 directly bonded onto main coils 10 of a structurally self-supporting magnet structure. While this is preferably achieved by an adhesive bond both to main coil 10 and to shield coil 16, other techniques such as bolting the webs 30 to the structurally self-supporting magnet may be employed, in embodiments of the invention. The intermediate coil support structure 31 may be found most effective when bonded to axially outer coils 10 of the structurally self-supporting magnet, although the invention allows the support structure 31 to be bonded to other coils if desired. Typically, best reduction in hoop stresses may be achieved by bonding the webs 30 to the main coils 10 at a region of highest hoop stress.

Advantageously, thermal contraction and structural properties of the intermediate coil support arrangement joining the shield coils to the main coils locally reduce hoop stress in the main coils.

Preferably, in an arrangement analogous to the structurally self-supporting magnet of the main coils, structurally self-supporting shield coils may be used.

Conventional approaches to reducing hoop stresses have included an oversleeve over the entire radially outer surfaces of the affected coils. This may result in significant interface stresses between the coils and the oversleeve, once cooled, due to differences in thermal contraction between the coils and the oversleeve. As discussed above, such stresses may lead to quench of the magnet. According to certain embodiments of the present invention, the intermediate coil support structure 31 includes a hoop stress restraining feature in the form of partial oversleeve 32, placed between main coil 10 and web 30, axially located only over the areas of maximum hoop stress. This provides effective strain relief, without large stresses due to axial thermal mismatch. Even if the same materials are used, the reduced axial length of the oversleeve means that a reduced stress will result. Partial oversleeve 32 need only cover a selected part of the axial length of main coil 10. This contrasts with conventional stress-relief arrangements in which an oversleeve, such as an overbinding, typically of resin-impregnated glass fibre or resistive wire, or an over-sleeve of a material of high tensile strength such as stainless steel, is applied over the whole surface of the main coil. During cooling of a superconducting magnet to operating temperature, coils tend to contract in the axial dimension rather more than materials such as resin impregnated glass fibre. In arrangements such as shown in Figs. 5B and 5C, the resultant axial contraction of the coils exceeds the axial contraction of the support tube, leading to unwanted stresses in the coils, typically in the region where axial extremities of the coils meet the support tube. In embodiments such as shown in Fig. 1 , partial oversleeve 32 extends axially only part of the length of the coil, and so any stress due to differences in axial thermal contraction will have minimal effect, as the axial length of the partial oversleeve 32 is much reduced.

A composite material may be designed to match the thermal contraction of the coils. Even in this case, there is risk of thermally induced shear stresses, for example during a quench event, since the current-carrying turns of the coils will suddenly heat, and expand thermally, while the material of the oversleeve will not heat in the same way, and so will not expend to the same extent. As discussed above, these thermally induced interface stresses may lead to quench.

In operation, hoop stresses in the main coils 10 tend to urge the main coils to expand. Conventionally, these stresses have been restrained by support tubes 22 in A2 bonded structures, or have been largely unrestrained in former structures such as shown in Fig. 5A. According to an aspect of the present invention, annular partial oversleeve 32 or discrete patches of oversleeve structure may be placed as required, wherever on the surface of the main magnet coils that localised stresses can usefully be minimised.

More effective stress relief may be provided by positioning the partial oversleeve to restrain particular regions of the main coil which would otherwise have been under a relatively large stress. Annular partial oversleeve 32 which extends around the entire circumference of main coil 10 is effective at restraining hoop stress. Positioning of individual patches of oversleeve structure with webs structures 30 may relieve stress at particular locations around the circumference of the main coil 10, but is not effective at restraining hoop stress.

In use, although hoop stresses on main magnet coils 10 tend to cause those coils to expand, such stresses may not affect the shield coils 16 to the same extent because they are in a region of lower field strength. The hoop stress in a shield coil is a function of the cross-section of the coil, the cross-sectional area of the wire used, and the local strength of the background magnetic field. By careful selection of the position of partial oversleeve 32 and the angle of webs 30, useful compensation of localised stress may be achieved by bracing selected locations on main coils 10 against the former 18 of shield coils 16. In other embodiments, not illustrated, the shield coils may be self-supporting, similar to the arrangement of Fig. 5D, in which case selected locations on main coils 10 are braced against the shield coil assembly. The angle of inclination a of the webs 30 relative to the radially outer surface of main coil 10 may be determined to allow partial oversleeve 32 to be positioned at an optimal location on the main coil 10 for local stress relief. There may be differences in thermal characteristics of the material of main coil 10, shield coil 16, partial oversleeve 32 and webs 30 which mean that the relative positions of the main coil 10 and shield coil 16 change during cooling of the magnet, which needs to be taken into account when designing the magnet so that all coils end up in the required relative positions when cooled. The angle a may be chosen to optimise the stress relief effects of the present invention, to ensure correct relative positioning of shield coils once cooled. The angle a may be greater than 90°, less than 90°, or exactly 90°, depending on the desired effects. The correct angle to use in any particular structure may be determined by simulation. Webs 30 need not be straight, as illustrated, but may be angled or curved if required, for example to provide the correct relative positioning of shield coils 16 and main coils 10 when cooled, to enable partial oversleeve 32 to be positioned at a desired axial location, or to provide appropriate stress relief force onto the main coil 10. In certain embodiments of the invention, the material of webs 30 may bend as it is cooled, with the result that the angle a when at operating temperature may be controlled, to counteract any delamination effect of changing a on cooling. Savings in material cost may be achieved as compared to conventional stress- relieving arrangements due to the reduced axial length of partial oversleeve 32 as compared to conventional oversleeve and elimination of complex structures conventionally required to mount the shielding coils to the main magnet. The present invention allows efficient use of material to support the shield coils 16.

While the intermediate support structure 31 may comprise several individual webs 30 positioned around the circumference of main coil 10, it may alternatively comprise a single conical web, which may have the advantage of ensuring that the restraining effect of the support structure, braced against the shield coil, is applied equally around the circumference of the main coil 10. Such a conical web may be made up of sections assembled around the main coil. Gaps may be left between such sections to provide an intermittent conical support structure. The use of multiple discrete webs 30 may be simpler in terms of both design and assembly, although a conical web 30 provides the advantages of radial strength for retaining hoop stress and stiffness to mechanically support the shield coil 16. Any thermal mismatch between the material of a conical web and the material of coil 10 will cause greater thermally-induced stresses than would be the case if multiple discrete webs 30 were used.

While the present invention is particularly advantageous when applied to shield coils which are structurally self-supporting, similar to the main coils of a serially-bonded magnet, the present invention may also be applied to shield coils formed within a journal 18, such as shown in Fig. 1 , or shield coils supported by a support tube, either in an A2-bonded arrangement or the alternative structure in which shield coils may be supported on a support tube by their radially inner surfaces, so-called A1- bonding, such as shown in Figs. 5B and 5C.

The annular partial oversleeve 32 may be formed by overbinding the coil 10 with resistive wire, for example of steel, aluminium or copper, and impregnating that wire structure with epoxy resin. This may be achieved either by winding the annular partial oversleeve over a completed structurally self-supporting magnet structure and impregnating the oversleeve, thereby bonding it onto the coil 10; or by winding the oversleeve over the coil windings prior to the impregnation step, followed by impregnating the coil and oversleeve in one step. In an alternative process, the partial oversleeve 32 may be formed separately from the main coils 10, raised to a temperature higher than that of the main coil 10, slid over the main coil 10 while at its higher temperature. As the temperatures of coil 10 and partial oversleeve 32 equalise, the oversleeve will grip onto the coil. In such embodiments, the material chosen for the oversleeve preferably has a greater coefficient of thermal expansion than the coil 10. Example materials which may be used for the partial oversleeve include resin-impregnated materials such as glass fibre, aluminium wire, steel wire, copper wire. A combination of such materials may be employed to obtain desired thermal properties. The use of copper wire is attractive as it is thermally matched to typical superconducting wire. However, copper is not optimal for restraining hoop stress, and in arrangements where copper wire is used as the partial oversleeve, much of the strength required to compensate for hoop stress will need to be provided by rigidity in the web 30, braced against shield coil 16.

The web 30 is preferably attached to partial oversleeve by an adhesive bond, for example using the resin used for impregnation. Alternatively, however, threaded inserts may be included in the partial oversleeve, and the web may be bolted to these inserts.

Fig. 2 shows an example of modelled stresses in a structurally self-supporting main coil assembly 10 and shield coil 16, such as illustrated in Fig. 1 , carrying currents as if in use. In this theoretical arrangement, no support structures are provided between the main coil assembly 10 and the shield coil 16. The region of maximum stress in main coil 10 is labelled MX, and the region of minimum stress is labelled MN. A region of high stress is clearly shown.

Fig. 3 illustrates the results of simulation of an embodiment of the present invention, showing internal stresses in the same manner as used for Fig. 2. In this arrangement, both the main coil 10 and the shield coil 16 are formed as self- supporting structures, as in serially-bonded magnets. The partial oversleeve 32 may be formed of materials similar to the material of the coils. This may be resin- impregnated glass fibre, resin-impregnated glass beads, or resin-impregnated overwound wire. Composite structures comprising combinations of resin, fibres such as glass fibre, wires, glass beads, may be found appropriate. In the illustrated embodiment, the web 30 is a cone of aluminium. As can clearly be seen in the figure, the region MX of maximum stress shown on Fig. 2 is relieved. While regions MX and MN of maximum and minimum stress, respectively, are shown in Fig. 3, the maximum level of stress encountered in the structure is moved, and reduced in value. In this example, the maximum hoop stress encountered is reduced, typically by several tens of MPa, which could reduce the coil stress below that of the yield stress of the wire, thereby avoiding any plastic deformation of the wire.

Fig. 4 illustrates the results of simulation of another embodiment of the present invention, showing internal stresses in the same manner as used for Figs. 2-3. This embodiment differs from the embodiment of Fig. 3 in that the intermediate coil support structure 31 comprises discrete aluminium webs 30 mounted onto a partial oversleeve 32 formed of resin-impregnated overwound copper wire. As illustrated, the different material properties of the support structure mean that the resulting stresses are different from those of the embodiment of Fig. 3.

The point stresses which arise at the axial ends of the partial oversleeve 32 and illustrated in the embodiments of Figs. 3 and 4 are similar to stresses encountered at axial extremities of coils in A2 bonded or serially bonded structures. The effect is, however, minimised by the limited axial extent of the partial oversleeve. Detailed design of the interface between partial oversleeve 32 and coil 10, aided by modelling of the designed structures, may enable the magnitude of these point stresses to be minimised.

As the magnet structure is cooled to operating temperature, thermal contraction of the various components may take place at differing rates, allowing compensating negative hoop stress to be "built in" to the magnet structure, for example by having an annular partial oversleeve 32 which contracts more than the coil 10 it is attached to. By modelling, and selecting the angle a, material and circumferential extent of the web 30, the axial location and axial extent of partial oversleeve 32, the structure may be optimised to achieve a desired reduction in hoop stress. Webs bonded to patches of material similar to that described for use as annular partial oversleeve, placed on the main coil 10 may be designed and provided to alleviate local stresses. Fig. 6 shows a simple embodiment of the present invention. In this embodiment, no partial oversleeve is provided. Rather, web 30 is bonded directly to the radially outer surface of a coil 10. Stress relief may be provided to coil 10 at positions of bonding with web 20, by bracing coil 10 against shield coil 16. Web 20 may comprise a cone, in which case stress relief may be provided circumferentially around main coil 10, or the web 30 may be intermittent, made up of discrete and separate parts, in which case stress relief is only provided to those parts where the web 30 is bonded to the coil 10. The present invention provides a novel structure of main coils and shield coils in a superconducting electromagnet. Intermediate support structure 31 provides the combined functions of supporting the shield coil and reducing the hoop stress in a main coil by using the shield coils to modify localised hoop stress in the main coil 10. While the web 30 is constructed of a metal in both of the specific examples described, it may be preferred to construct the webs from a composite material. For example, the webs may be glassfibre rods, for example constructed of resin- impregnated glassfibre cloth, chopped glassfibre or glass beads. These latter two options may be amenable to processing by injection moulding. In preferred embodiments, a fibre-reinforced composite structure may be devised in which optimised fibre lay-up provides optimised thermal and mechanical properties. One or more ribs of another material may be enclosed within each web to increase stiffness of the resulting web. If a cloth material is used, the strength and rigidity of webs in any direction can be controlled by appropriately arranging the orientation of fibres in the cloth used.

It is even possible to use a combination of materials joined together such that they deform on cooling, rather in the manner of the well-known bimetallic strip. In certain embodiments, the whole structure of self-supporting inncer coil assembly, self-supporting shield coil assembly and intermediate support structure may be formed in a single impregnation step, with all coils wound into a mould which are also provided with materials for forming the intermediate support structure, and the whole cavity impregnated in a single step to form a monolithic resin impregnated structure of inner coils, shield coils and intermediate support structure.