Coil support

A superconducting magnet assembly may include a main magnet assembly having at least one annular coil arranged about an axis, and at least one shield coil, of greater diameter than the main magnet assembly, arranged about the axis. At least one support may be provided, attached to the shield coil and to the main magnet assembly.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a U.S. National Stage Patent Application of PCT/EP2020/070141, filed Jul. 16, 2020, which claims priority to Great Britain (GB) Patent Application No. 1913853.6, filed Sep. 26, 2019. Each of these applications is incorporated herein by reference in its entirety.

BACKGROUND

Field

The present disclosure relates to superconducting magnets, and in particular to superconducting magnets for use in Magnetic Resonance Imaging (MRI) systems and Nuclear Magnetic Resonance (NMR) systems.

Related Art

Typically, such superconducting magnets incorporate active shielding to reduce stray magnetic field. Active shielding comprises counter-running coils positioned outside of the main magnet coils. For an efficient design, there should be a substantial radial gap between the shielding coils and the inner magnet. In operation, the shield coils typically experience axial body forces of many tonnes, and must be accurately positioned. It may be problematic to support these shield coils in an efficient and cost-effective manner.

The present disclosure addresses this issue and aims to provide arrangements to support these shield coils in an efficient and cost-effective manner.

DETAILED DESCRIPTION

FIG.1shows a conventional superconducting magnet for use in an MRI system, as described in WO2013/102509. Self-supporting main magnet assembly1may comprise at least one annular superconducting main coil9. A number of shield coils6of greater diameter than the main coils are provided, placed coaxially with the main coils about axis A-A (schematically represented). Intermediate coil support structures3are affixed to the self-supporting main magnet assembly1and to journals8retaining the shield coils6, which may be impregnated in epoxy resin13, to retain the main and shield coils in their correct respective positions.

Former-less structures1are particularly suited to very lightweight magnet structures such as those required for cryogen-free magnets requiring fast cooldown times. Other conventional arrangements become impractical with very thin coils. It may become impractical to suspend the “cold-mass”; being the self-supporting main magnet assembly1as shown inFIG.1, directly from the vacuum vessel as an unacceptable level of deformation of the magnet assembly may result.

The above problem, including the solution proposed by WO2013/102509, becomes difficult to solve when it is required to keep the “cold-mass” low, that is to say, the mass of the equipment which is held at the operating temperature of the superconducting magnet, which is typically lower than 20K, commonly about 4K. Recent developments include conduction-cooled magnets, which are not provided with a bath of liquid cryogen. To minimise the cooldown time for such structures, the coils and their formers, where provided, tend to become quite thin, and have reduced inherent stiffness due to the drive to reduce mass. Despite this, it is required to allow the suspension of the cold-mass within the cryostat without excessive distortion, and it becomes difficult to provide arrangements for supporting the shield coils from the main magnet assembly.

Former-less all-bonded solutions, where coils are bonded to sides of annular spacers of similar radial extent, may be employed for very light structures such as those required for cryogen-free magnets with fast cool-down times. The known coil support solutions become increasingly difficult, or impossible, to implement as the coils get thinner. The cold-mass may need to be stiffened significantly to allow the magnet suspension to be connected directly to the cold-mass. A lightweight and stiff structure is required which supports the shield coils around their entire circumference without subjecting any coils to concentrated mechanical loading, which might otherwise cause deformation of the coils.

The present disclosure aims to provide thin, lightweight supports for mounting thin, lightweight shield coils onto a thin, lightweight main magnet assembly. The supports of the present disclosure add relatively little mass to the cold-mass, which maintains efficiency of cooling for conduction-cooled magnets. The resulting cold-mass structure has a high inherent stiffness when completely assembled, minimising distortion of the coils when suspended, along with minimising thermal and electromagnetic loading.

In use, each superconducting magnet coil is subjected to electromagnetic loading comprising radial and axial forces due to the interaction of the magnetic field generated by the superconducting coil with the magnetic field generated by the superconducting magnet as a whole. Each superconducting coil experiences an axial body force which is typically outwards away from the mid-plane, but may be inwards towards the mid-plane, depending on magnet design, and is also subjected to a substantial radial force which is typically in an outwards radial direction but can also be radially inwards in some cases, depending on magnet design. For thin coils, this combination of forces due to the electromagnetic loading can cause the superconducting coil to buckle if supported at only a few points and, at least, can cause local deformation which gives rise to high local stresses and poor uniformity in the generated magnetic field. Conventionally, such stress concentrations have been addressed by making the superconducting coil structure heavier and more expensive. The present disclosure provides continuous support around the circumference of a superconducting coil such that stress concentrations are virtually eliminated, distortion is minimised and buckling modes are avoided. In turn, this minimises any resultant degradation of magnetic field uniformity.

The present disclosure seeks to provide a light and stiff structure which supports the shield coils around their entire circumference, without subjecting any coils to concentrated mechanical loading.

FIG.2shows a partial cross-section of an embodiment of the present disclosure, which essentially has rotational symmetry about axis A-A and reflective symmetry about centre plane C-C.

As illustrated by the example embodiment schematically represented inFIG.2, the disclosure provides one or more conical annular supports20to connect a thin and flexible inner magnet (“main magnet assembly”)10to thin shield coils16. While all of these components are flexible and relatively mechanically weak prior to assembly, when connected together, a very stiff and extremely light structure is formed.FIG.3shows a magnified view of certain features ofFIG.2.

InFIG.2, a main magnet assembly10, not illustrated in detail, comprises at least one superconducting coil12, which may be impregnated in resin13. Shield coils16are provided, of greater diameter than the main magnet assembly10. In the illustrated embodiment, an end-ring18is provided, bonded to the axially outer end of each respective shield coil16. This may be of a resin-impregnated fibre material.

According to a feature of the present disclosure, a conical annular support20is provided. A radially inner circumference22of this conical annular support20is attached to the main magnet assembly10. In the illustrated example, this is achieved by bolts24through an inner flange26in the material of the conical annular support20, and directed axially inwards from the radially inner circumference22of the annular support20either into a tapped hole in the material of the spacer14, or into a threaded insert itself retained within the material of the spacer14. A radially outer circumference28of the conical annular support20is attached to one of the shield coils16. In the illustrated embodiment, this is achieved by bolts30directed axially through a peripheral radially-extending outer flange32in the material of the conical annular support20, either into a tapped hole in the material of the end-ring18, or into a threaded insert itself retained within the material of the end-ring18.

In an exemplary embodiment, a similar arrangement is provided at the other axial extremity of the main magnet assembly10, providing support to the axially outer extremity of the other shield coil.

In an exemplary embodiment, the or each conical annular support20is formed from a thin metal sheet, for example 1 mm thick stainless steel, aluminium or aluminium alloy, although each conical annular support20could be made of other material such as composite materials such as Carbon-fiber-reinforced polymers (CFRP) or Glass Reinforced Plastic (GRP).

In some embodiments, such as illustrated inFIG.2, one or more suspension element, such as a pillar leg supports34, may be attached to the main magnet assembly10. As illustrated, and conveniently, the pillar leg support34may be attached to the main magnet assembly10by one or more of the bolts24which attach the conical annular support20to the main magnet assembly10. The other end of the pillar leg support34may be attached to an inner surface of an Outer Vacuum Container (OVC) (not illustrated inFIG.2) which defines a vacuum region around the magnet structure. In such a manner, the weight of the main magnet assembly10may be borne by the OVC through tension in the pillar leg supports34. The weight of the shield coils16may be borne by the OVC through tension in the pillar leg supports34and tension and compression in the conical annular supports20.

In use, shield coils16are subjected to axially outward forces of many tonnes, which may be sufficient to deform the or each conical annular support20. In the view shown inFIG.2, this deformation would appear in that the shield coil16rotates to some extent about the radially inner circumference22of the annular support20with deformation of the material of the conical annular support20. This in turn would mean that the radially-extending outer flange32is no longer radially-directed, and may lead to deformation of the shape of the shield coil(s)16. To resist such deformation, in certain embodiments of the present disclosure, axial rods36may be provided, extending between shield coil support structures at opposite ends of the magnet structure. For example, eight such rods may be provided, distributed circumferentially around the magnet structure. The axial rods36provide additional stiffness in the axial direction and thus prevent excessive distortion of the conical annular support20. This additional stiffness provided by the axial rods36may allow a thinner material to be used to form the conical annular support20. In exemplary embodiments, a conical annular support20is provided near each axial extremity of the main magnet assembly10, and the axial rods36extend between the two conical annular supports20. In the embodiment illustrated inFIG.2, flattened regions38are provided at circumferential intervals near to the outer flange32, and respective axial rods36are mounted to respective flattened regions38by bolting, or by attaching nuts to a threaded part of the axial rod36itself.

In the embodiment ofFIG.2, each axial rod36is of a solid material, such as stainless steel. End regions, at least, of each axial rod are threaded and located through corresponding holes in respective conical annular supports20. Nuts are threaded onto the threaded regions either side of the conical annular support20and washers may be placed between each nut and the associated conical annular support20.

In an alternative arrangement, illustrated inFIG.2A, the axial rod40is a hollow rod, with a threaded bolt42or similar mounted in each end, to enable it to be mounted to conical annular supports20in the manner of the axial rod36ofFIG.2.

The presence of such axial rods prevents, or at least limits, deformation of the conical annular supports20and so reduces the tendency of the shield coils16to deform or change position under electromagnetic loading.

The conical annular supports20are attached to the main magnet assembly10at least at intervals around the circumference of the main magnet assembly, for example by bolts24. In some embodiments, the conical annular supports20may be continuously attached to the main magnet assembly10, for example by a resin-impregnated glass band which overlaps the inner flange26and the main magnet assembly10. In alternative embodiments, the conical annular supports20are bonded to the main magnet assembly10or are clamped to the main magnet assembly10by a mechanical compression band. Discreet fixings such as the bolts24illustrated in the drawing simplify manufacture, but in embodiments where the conical annular supports20are of a resin-impregnated composite material, a bonded joint may be found advantageous.

In the radial direction, the conical annular supports20provide rigid support and retain the annular shape of the main magnet assembly10. This is required to enable adequate uniformity of the magnetic field.

The conical annular supports20are attached to the shield coils16at least at intervals around the circumference of the main magnet assembly, for example by bolts30. In some embodiments, the conical annular supports20may be continuously attached to the main magnet assembly10, for example by a resin-impregnated glass band which overlaps the inner flange26and the main magnet assembly. In the radial direction, the conical annular supports20provide rigid support and retain the annular shape of the shield coils16. This ensures that the shield coils remain accurately positioned with respect to each other and with respect to the main magnet assembly10. By supporting the shield coils around their entire circumference, out-of-plane bending of the shield coils and stress concentrations may be avoided, although both of these occur with conventional approaches which use discrete tension elements to support shield coils.

The conical annular supports20provide axial and radial stiffness to resist mechanical, thermal and electromagnetic loads.

The conical annular supports20may also provide mounting locations for a tensile suspension system, discussed below with reference toFIG.5, or a tension or compression suspension system such as using pillar supports34mounted to the inner flanges22of the conical annular supports20as discussed above with reference toFIG.2. The pillar supports34may attach to the conical annular supports20, as illustrated inFIGS.2and3, but may alternatively be mounted to the main magnet assembly10, such as near to a conical annular support20. The conical annular supports20retain the main magnet assembly10round, and thereby assist in preventing the pillar support from distorting the inner magnet.

The constraint of the main magnet assembly10and shield coils16provided by the conical annular supports20may be balanced against requirements for axial stiffness, by appropriately selecting parameters of the conical annular supports20, such as the cone material, thickness and rake angle β. Further tuning of the mechanical properties of the structure may be achieved by adjusting the stiffness, number and position of axial rods36;40, where provided.

In certain embodiments of the disclosure, the conical annular supports20may be made of a material of relatively high thermal conductivity, such as copper or aluminium. This will provide a high-conductivity thermal path between the main magnet assembly10and the shield coils16. This will assist with thermal uniformity, particularly in conduction-cooled magnets.

Conventionally, a termination area is provided on a main magnet assembly10such as shown at39inFIG.2. There, ends of superconducting wires making up the main magnet coils and the shield coils are brought together, and electrical connections made to a superconducting persistent switch, also known as a magnet switch, and to a power supply, a run-down load and so on. Conveniently, such termination area may be provided on a radially outer surface of the main magnet assembly10. The conical annular supports20of the present disclosure allow such region to be particularly accessible. The accessibility of such regions is improved yet further in case tension rods36,40are omitted, for example, in embodiments where the conical annular supports20themselves provide sufficient stiffness to locate and retain the shield coils.

The conical annular supports20can be cost effectively manufactured by fabrication, spinning or pressing of sheet metal or by composite lay-up. The support structure provided by the present disclosure has a much lower part count and complexity compared to conventional support structures for shield coils, and magnet structure supports.

FIG.4shows detail of an alternative embodiment of the disclosure, in which shield coils16are not provided with an end ring (18,FIG.2). Instead, an overbinding41of a composite material such as resin-impregnated glass fibre, or resin-impregnated carbon fibre is provided, at least partially over the radially outer surface of each shield coil16. The overbinding41protrudes axially beyond the axially outer end of the respective shield coil16. In the illustrated embodiment, the conical annular support20has a radially outer extremity turned axially towards the axial centre of the magnet, to form an outer axially-directed flange42. The overbinding41is attached to the outer axially-directed flange42, in the illustrated embodiment by fasteners such as rivets44. Such fasteners may be provided at intervals around the circumference of the shield coil16. The number of fasteners, and hence their spacing, should be determined in order to ensure that the shield coil16does not deform appreciably during normal use. That determination should also consider the mechanical strength of the overbinding41, which will provide annular support to the shield coil.

Similar to the embodiment ofFIG.2, axial rods36or40may be attached to respective flattened regions46provided near to the outer flange42. In either embodiment, the flattened region may be provided axially inboard or outboard of the cone of the conical annular support20. In some embodiments, due to the use of a relatively thin material for the conical annular support20, flattened regions46for attachment of axial rods36or40are provided with reinforcement such as plates of material, effectively locally increasing the thickness of the material of the conical annular support20, or pressed-in or spun-in features in the material of the conical annular support20.

In other embodiments, not specifically illustrated, the orientation of the conical annular support20is reversed, that is to say that the conical annular support20reaches the axially inner edges of the shield coils and slopes axially outwards towards the surface of the main magnet assembly10. In further embodiments, not specifically illustrated, each shield coil16may be provided on the opposite axial side of the conical annular support20to that proposed hitherto. Conical annular support20may be replaced with annular supports of other shapes, in order to improve desired structural properties.

FIG.5illustrates a further embodiment of the present disclosure. In this embodiment, the conical annular support20is adapted to support the weight of the main magnet assembly10by support rods70braced against outer vacuum container (OVC)50. In an exemplary embodiment, the support rods70preferably also support the weight of a thermal radiation shield52. OVC50provides an evacuated volume which encompasses the superconducting magnet. The thermal radiation shield52(at about 50K) must of course be retained and mechanically supported in a manner which ensures that it is thermally insulated both from the OVC (at about 300K) and the superconducting magnet (at about 4K).

Thermal radiation shield52is located between the superconducting magnet and the OVC. It prevents thermal radiation from the interior surface of the OVC—typically at about 300K—from reaching the superconducting magnet—typically at about 4K. The thermal radiation shield52is typically cooled to a temperature of about 50K. Cryogenic refrigerators typically provide much greater cooling power at 50K than at 4K, so it is useful to remove any thermal influx at 50K rather than trying to remove it at 4K. Thermal radiation which reaches the superconducting magnet from the thermal radiation shield52has therefore only been emitted at 50K, and so carries much less energy than thermal radiation emitted at 300K, allowing the thermal influx which reaches the superconducting magnet from the thermal radiation shield52to be removed by the cryogenic refrigerator at 4K.

FIG.5illustrates an embodiment of the present disclosure which enables such retention and mechanical support. As is conventional in itself, multi-layer insulation (MLI)54may be provided in a space between the OVC and the thermal radiation shield. MLI typically comprises multiple layers of aluminised polyester sheet. It is illustrated intermittently for ease of representation, but in fact will essentially enclose the thermal radiation shield52. Its purpose is to reflect thermal radiation from the OVC, and to establish a stable thermal gradient between the OVC and the thermal radiation shield.

The embodiment ofFIG.5shares many features with other embodiments of the present disclosure, and those features carry identical reference numbers to the reference numbers carried in earlier drawings.

A number of rod bosses62are introduced into the conical annular support60. For example, four such rod bosses may be provided distributed circumferentially around each conical annular support60. In an exemplary embodiment, each of the rod bosses is of a material such as resin-impregnated glass fibre or carbon fibre; stainless steel; or titanium and includes a through-hole68directed radially, inclined to the axis A-A to accommodate a corresponding support rod70. Each rod boss62may be shaped, or otherwise arranged, to be retained in place mounted in the conical annular support60.

In an exemplary embodiment, eight such bosses are provided, four on each of two conical annular supports60, located at respective ends of the superconducting magnet assembly. In an exemplary embodiment, the rake angle β of each conical annular support corresponds to the orientation of the support rods70mounted to the rod bosses62. Each of eight support rods70, each mounted to a respective one of the rod bosses62, is arranged at a compound angle between the OVC and the respective conical annular support60to provide mechanical support for the weight of the superconducting magnet structure, through the material of the conical annular support60to the OVC. By providing the tension rods70which follow the cone angle, torsion and bending of the material of the conical annular support60is minimised. Provision may thereby be made for support of the weight of the superconducting magnet without requiring a lengthening of the structure as a whole.

In the illustrated embodiment, both ends of support rod70are threaded. At the radially inner end of the support rod, the support rod70passes through the rod boss62. At the radially outer end of the support rod, the support rod passes through a hole or notch in a mounting point72attached to the inner surface of the OVC, by welding or some similar permanent attachment. In an exemplary embodiment, both threaded ends of the support rod70are respectively fastened in place with a nut and also an optional washer. The support rods70are respectively brought into tension by tightening of the respective nuts, so as to restrain and support the superconducting magnet structure within the OVC.

A further feature of the embodiment ofFIG.5is that the weight of thermal radiation shield52is borne by the support rods70. To this end, a shield support74is mounted on the support rod70. In an exemplary embodiment, at least one shield support74is mounted on at least one support rod. In an exemplary embodiment, at least one shield support74is mounted on each support rod70. Each shield support74is attached to the respective support rod at an appropriate location, such that, in a stable thermal condition, the temperature of the support rod at the location of the shield support74would be approximately the same as the temperature of the thermal radiation shield, so that, in use, little thermal transfer occurs between the thermal radiation shield and the support rod70. Shield support74is attached to the thermal radiation shield, so that the weight of the thermal radiation shield is at least partially borne by the shield support74.

In certain embodiments of the present disclosure, such as that illustrated inFIG.6by way of example, the conical annular supports20;60form parts of a “light-tight” thermal radiation shield. For example, an opaque, lightweight tube80, such as of thin conductive metal such as aluminium or copper foil or a composite substrate covered in foil, may be provided between radially outer extremities of conical annular supports20;60of any of the embodiments of the present disclosure, so as to form a radiation shield for the magnet cold mass comprising main magnet assembly10and shield coils16. In this manner, conical annular supports20;60, lightweight tube80and main magnet assembly10form an isothermal volume in which termination parts39can be accommodated without additional thermal shielding. The main magnet assembly10may be brought within the isothermal volume by addition of a thermally conductive layer80, for example of aluminium or copper foil, over surfaces of the main magnet assembly which are otherwise exposed outside of the isothermal volume.

The present disclosure accordingly provides a superconducting magnet assembly comprising a main magnet assembly10comprising at least one annular superconducting coil arranged about an axis A-A, and at least one shield coil16, of greater diameter than the main magnet assembly10, arranged about the axis A-A, wherein at least one annular support is provided, attached to the shield coil16and to the main magnet assembly10.

In an exemplary embodiment, the annular support may be conical. In other embodiments, the support may be described as a thin conical section spanning a radial gap between inner (main) and outer (shield) coils.

Such an arrangement provides a lightweight and mechanically stiff shield coil support that is suitable for supporting very thin coils. The mechanical characteristics of the shield coil support can easily be tuned by changing the cone geometry, thickness and material. The support structure of the present disclosure is lightweight, and is suitable for cryogen-free or conduction-cooled magnets, since such magnets require fast cooling from room temperature on installation. The shield coil support20;60of the disclosure also has low material content, and correspondingly low mass, allowing easier siting of the magnet, shorter cooldown time and reduced shipping costs. The annular shield coil support structure20;60of the present disclosure is simple and cost effective. In exemplary embodiments, use of the conical support structure of the present disclosure allows access to the termination area of the magnet without obstruction.

The annular shield coil support structure of the present disclosure allows very lightweight magnet cold-masses to be mechanically suspended within a cryostat without excessive distortion, due to the mechanical rigidity imparted by the annular shield coil support.

In certain embodiments of the disclosure, the annular supports20;60form parts of a thermal radiation shield around the cold-mass which also provides a light tight isothermal volume in which to mount the termination components and magnet switch.

To enable those skilled in the art to better understand the solution of the present disclosure, the technical solution in the embodiments of the present disclosure is described clearly and completely below in conjunction with the drawings in the embodiments of the present disclosure. Obviously, the embodiments described are only some, not all, of the embodiments of the present disclosure. All other embodiments obtained by those skilled in the art on the basis of the embodiments in the present disclosure without any creative effort should fall within the scope of protection of the present disclosure.