Patent Description:
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 invention addresses this issue and aims to provide arrangements to support these shield coils in an efficient and cost-effective manner.

<FIG> shows a conventional superconducting magnet for use in an MRI system, as described in <CIT>. Self-supporting main magnet assembly <NUM> may comprise at least one annular superconducting main coil. A number of shield coils <NUM> of greater diameter than the main coils are provided, placed coaxially with the main coils about axis A-A (schematically represented). Intermediate coil support structures <NUM> are affixed to the self-supporting main magnet assembly <NUM> and to journals <NUM> retaining the shield coils <NUM>, to retain the main and shield coils in their correct respective positions.

Former-less structures <NUM> are 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 assembly <NUM> as shown in <FIG>, 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 <CIT>, 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 <NUM>, commonly about <NUM>. 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.

In <CIT>, a superconducting magnet arrangement, comprising a structurally self-supporting magnet structure of annular main coils, a number of shield coils of greater diameter than the main coils and intermediate coil support structures affixed to the self-supporting magnet structure of annular main coils is described.

The document <CIT> discloses an MRI apparatus having a plurality of gradient coils positioned about a bore of a magnet, and an RF transceiver system and an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly to acquire MR images.

In <CIT>, a cantilevered trapezoidal-toroidal structure is provided in an active magnetic shielding system for a superconducting magnet to support the bucking magnet coils in spaced relationship to the main magnet coils.

The present invention aims to provide thin, lightweight supports for mounting thin, lightweight shield coils onto a thin, lightweight main magnet assembly. The supports of the present invention 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 invention 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 invention 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.

The present invention accordingly provides apparatus as defined in the appended claims.

The above, and further, objects, characteristics and advantages of the present invention will become more apparent from review of the following description of certain embodiments thereof, in conjunction with the accompanying drawings, wherein:.

<FIG> shows a partial cross-section of an embodiment of the present invention, 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 in <FIG>, the invention provides one or more conical annular supports <NUM> to connect a thin and flexible inner magnet ("main magnet assembly") <NUM> to thin shield coils <NUM>. 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> shows a magnified view of certain features of <FIG>.

In <FIG>, a main magnet assembly <NUM>, not illustrated in detail, comprises at least one superconducting coil. Shield coils <NUM> are provided, of greater diameter than the main magnet assembly <NUM>. In the illustrated embodiment, an end-ring <NUM> is provided, bonded to the axially outer end of each respective shield coil <NUM>. This may be of a resin-impregnated fibre material.

According to a feature of the present invention, a conical annular support <NUM> is provided. A radially inner circumference <NUM> of this conical annular support <NUM> is attached to the main magnet assembly <NUM>. In the illustrated example, this is achieved by bolts <NUM> through an inner flange <NUM> in the material of the conical annular support <NUM>, and directed axially inwards from the radially inner circumference <NUM> of the annular support <NUM> either into a tapped hole in the material of the spacer <NUM>, or into a threaded insert itself retained within the material of the spacer <NUM>. A radially outer circumference <NUM> of the conical annular support <NUM> is attached to one of the shield coils <NUM>. In the illustrated embodiment, this is achieved by bolts <NUM> directed axially through a peripheral radially-extending outer flange <NUM> in the material of the conical annular support <NUM>, either into a tapped hole in the material of the end-ring <NUM>, or into a threaded insert itself retained within the material of the end-ring <NUM>.

In a preferred embodiment, a similar arrangement is provided at the other axial extremity of the main magnet assembly <NUM>, providing support to the axially outer extremity of the other shield coil.

In a preferred embodiment, the or each conical annular support <NUM> is formed from a thin metal sheet, for example <NUM> thick stainless steel, aluminium or aluminium alloy, although each conical annular support 20could be made of other material such as composite materials such as CFRP or GRP.

In some embodiments, such as illustrated in <FIG>, one or more suspension element, such as a pillar leg supports <NUM>, may be attached to the main magnet assembly <NUM>. As illustrated, and conveniently, the pillar leg support <NUM> may be attached to the main magnet assembly <NUM> by one or more of the bolts <NUM> which attach the conical annular support <NUM> to the main magnet assembly <NUM>. The other end of the pillar leg support <NUM> may be attached to an inner surface of an Outer Vacuum Container (OVC) (not illustrated in <FIG>) which defines a vacuum region around the magnet structure. In such a manner, the weight of the main magnet assembly <NUM> may be borne by the OVC through tension in the pillar leg supports <NUM>. The weight of the shield coils <NUM> may be borne by the OVC through tension in the pillar leg supports <NUM> and tension and compression in the conical annular supports <NUM>.

In use, shield coils <NUM> are subjected to axially outward forces of many tonnes, which may be sufficient to deform the or each conical annular support <NUM>. In the view shown in <FIG>, this deformation would appear in that the shield coil <NUM> rotates to some extent about the radially inner circumference <NUM> of the annular support <NUM> with deformation of the material of the conical annular support <NUM>. This in turn would mean that the radially-extending outer flange <NUM> is no longer radially-directed, and may lead to deformation of the shape of the shield coil(s) <NUM>. To resist such deformation, in certain embodiments of the present invention, axial rods <NUM> may 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 rods <NUM> provide additional stiffness in the axial direction and thus prevent excessive distortion of the conical annular support <NUM>. This additional stiffness provided by the axial rods <NUM> may allow a thinner material to be used to form the conical annular support <NUM>. In preferred embodiments, a conical annular support <NUM> is provided near each axial extremity of the main magnet assembly <NUM>, and the axial rods <NUM> extend between the two conical annular supports <NUM>. In the embodiment illustrated in <FIG>, flattened regions <NUM> are provided at circumferential intervals near to the outer flange <NUM>, and respective axial rods <NUM> are mounted to respective flattened regions <NUM> by bolting, or by attaching nuts to a threaded part of the axial rod <NUM> itself.

In the embodiment of <FIG>, each axial rod <NUM> is 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 supports <NUM>. Nuts are threaded onto the threaded regions either side of the conical annular support <NUM> and washers may be placed between each nut and the associated conical annular support <NUM>.

In an alternative arrangement, illustrated in <FIG>, the axial rod <NUM> is a hollow rod, with a threaded bolt <NUM> or similar mounted in each end, to enable it to be mounted to conical annular supports <NUM> in the manner of the axial rod <NUM> of <FIG>.

The presence of such axial rods prevents, or at least limits, deformation of the conical annular supports <NUM> and so reduces the tendancy of the shield coils <NUM> to deform or change position under electromagnetic loading.

The conical annular supports <NUM> are attached to the main magnet assembly <NUM> at least at intervals around the circumference of the main magnet assembly, for example by bolts <NUM>. In some embodiments, the conical annular supports <NUM> may be continuously attached to the main magnet assembly <NUM>, for example by a resin-impregnated glass band which overlaps the inner flange <NUM> and the main magnet assembly <NUM>. In alternative embodiments, the conical annular supports <NUM> are bonded to the main magnet assembly <NUM> or are clamped to the main magnet assembly <NUM> by a mechanical compression band. Discreet fixings such as the bolts <NUM> illustrated in the drawing simplify manufacture, but in embodiments where the conical annular supports <NUM> are of a resin-impregnated composite material, a bonded joint may be found advantageous.

In the radial direction, the conical annular supports <NUM> provide rigid support and retain the annular shape of the main magnet assembly <NUM>. This is required to enable adequate uniformity of the magnetic field.

The conical annular supports <NUM> are attached to the shield coils <NUM> at least at intervals around the circumference of the main magnet assembly, for example by bolts <NUM>. In some embodiments, the conical annular supports <NUM> may be continuously attached to the main magnet assembly <NUM>, for example by a resin-impregnated glass band which overlaps the inner flange <NUM> and the main magnet assembly. In the radial direction, the conical annular supports <NUM> provide rigid support and retain the annular shape of the shield coils <NUM>. This ensures that the shield coils remain accurately positioned with respect to each other and with respect to the main magnet assembly <NUM>. 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 supports <NUM> provide axial and radial stiffness to resist mechanical, thermal and electromagnetic loads.

The conical annular supports <NUM> may also provide mounting locations for a tensile suspension system, discussed below with reference to <FIG>, or a tension or compression suspension system such as using pillar supports <NUM> mounted to the inner flanges <NUM> of the conical annular supports <NUM> as discussed above with reference to <FIG>. The pillar supports <NUM> may attach to the conical annular supports <NUM>, as illustrated in <FIG> and <FIG>, but may alternatively be mounted to the main magnet assembly <NUM>, preferably near to a conical annular support <NUM>. The conical annular supports <NUM> retain the main magnet assembly <NUM> round, and thereby assist in preventing the pillar support from distorting the inner magnet.

The constraint of the main magnet assembly <NUM> and shield coils <NUM> provided by the conical annular supports <NUM> may be balanced against requirements for axial stiffness, by appropriately selecting parameters of the conical annular supports <NUM>, 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 rods <NUM>; <NUM>, where provided.

In certain embodiments of the invention, the conical annular supports <NUM> may 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 assembly <NUM> and the shield coils <NUM>. This will assist with thermal uniformity, particularly in conduction-cooled magnets.

Conventionally, a termination area is provided on a main magnet assembly <NUM> such as shown at <NUM> in <FIG>. 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 assembly <NUM>. The conical annular supports <NUM> of the present invention allow such region to be particularly accessible. The accessibility of such regions is improved yet further in case tension rods <NUM>, <NUM> are omitted, for example, in embodiments where the conical annular supports <NUM> themselves provide sufficient stiffness to locate and retain the shield coils.

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

<FIG> shows detail of an alternative embodiment of the invention, in which shield coils <NUM> are not provided with an end ring (<NUM>, <FIG>). Instead, an overbinding <NUM> of 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 coil <NUM>. The overbinding <NUM> protrudes axially beyond the axially outer end of the respective shield coil <NUM>. In the illustrated embodiment, the conical annular support <NUM> has a radially outer extremity turned axially towards the axial centre of the magnet, to form an outer axially-directed flange <NUM>. The overbinding <NUM> is attached to the outer axially-directed flange <NUM>, in the illustrated embodiment by fasteners such as rivets <NUM>. Such fasteners may be provided at intervals around the circumference of the shield coil <NUM>. The number of fasteners, and hence their spacing, should be determined in order to ensure that the shield coil <NUM> does not deform appreciably during normal use. That determination should also take into account the mechanical strength of the overbinding <NUM>, which will provide annular support to the shield coil.

Similarly to the embodiment of <FIG>, axial rods <NUM> or <NUM> are preferably attached to respective flattened regions <NUM> provided near to the outer flange <NUM>. In either embodiment, the flattened region may be provided axially inboard or outboard of the cone of the conical annular support <NUM>. In some embodiments, due to the preferred use of a relatively thin material for the conical annular support <NUM>, flattened regions <NUM> for attachment of axial rods <NUM> or <NUM> are provided with reinforcement such as plates of material, effectively locally increasing the thickness of the material of the conical annular support <NUM>, or pressed-in or spun-in features in the material of the conical annular support <NUM>.

In other embodiments, not specifically illustrated, the orientation of the conical annular support <NUM> is reversed, that is to say that the conical annular support <NUM> reaches the axially inner edges of the shield coils and slopes axially outwards towards the surface of the main magnet assembly <NUM>. In further embodiments, not specifically illustrated, each shield coil <NUM> may be provided on the opposite axial side of the conical annular support <NUM> to that proposed hitherto.

<FIG> illustrates a further embodiment of the present invention. In this embodiment, the conical annular support <NUM> is adapted to support the weight of the main magnet assembly <NUM> by support rods <NUM> braced against outer vacuum container (OVC) <NUM>. The support rods <NUM> preferably also support the weight of a thermal radiation shield <NUM>. OVC <NUM> provides an evacuated volume which encompasses the superconducting magnet. The thermal radiation shield <NUM> (at about <NUM>) must of course be retained and mechanically supported in a manner which ensures that it is thermally insulated both from the OVC (at about <NUM>) and the superconducting magnet (at about <NUM>).

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

<FIG> illustrates an embodiment of the present invention which enables such retention and mechanical support. As is conventional in itself, multi-layer insulation (MLI) <NUM> may 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 shield <NUM>. 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 of <FIG> shares many features with other embodiments of the present invention, and those features carry identical reference numbers to the reference numbers carried in earlier drawings.

A number of rod bosses <NUM> are introduced into the conical annular support <NUM>. For example, four such rod bosses may be provided distributed circumferentially around each conical annular support <NUM>. Each of the rod bosses is preferably of a material such as resin-impregnated glass fibre or carbon fibre; stainless steel; or titanium and includes a through-hole <NUM> directed radially, inclined to the axis A-A to accommodate a corresponding support rod <NUM>. Each rod boss <NUM> may be shaped, or otherwise arranged, to be retained in place mounted in the conical annular support <NUM>.

In a preferred embodiment, eight such bosses are provided, four on each of two conical annular supports <NUM>, located at respective ends of the superconducting magnet assembly. The rake angle β of each conical annular support preferably corresponds to the orientation of the support rods <NUM> mounted to the rod bosses <NUM>. Each of eight support rods <NUM>, each mounted to a respective one of the rod bosses <NUM>, is arranged at a compound angle between the OVC and the respective conical annular support <NUM> to provide mechanical support for the weight of the superconducting magnet structure, through the material of the conical annular support <NUM> to the OVC. By providing the tension rods <NUM> which follow the cone angle, torsion and bending of the material of the conical annular support <NUM> is 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 rod <NUM> are threaded. At the radially inner end of the support rod, the support rod <NUM> passes through the rod boss <NUM>. At the radially outer end of the support rod, the support rod passes through a hole or notch in a mounting point <NUM> attached to the inner surface of the OVC, by welding or some similar permanent attachment. Both threaded ends of the support rod <NUM> are respectively fastened in place with a nut and preferably also a washer. The support rods <NUM> are 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 preferred feature of the embodiment of <FIG> is that the weight of thermal radiation shield <NUM> is borne by the support rods <NUM>. To this end, a shield support <NUM> is mounted on the support rod <NUM>. At least one shield support <NUM> is mounted on at least one support rod, but preferably at least one shield support <NUM> is mounted on each support rod <NUM>. Each shield support <NUM> is 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 support <NUM> would 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 rod <NUM>. Shield support <NUM> is attached to the thermal radiation shield, so that the weight of the thermal radiation shield is at least partially borne by the shield support <NUM>.

In certain embodiments of the present invention, such as that illustrated in <FIG> by way of example, the conical annular supports <NUM>; <NUM> form parts of a "light-tight" thermal radiation shield. For example, an opaque, lightweight tube <NUM>, 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 supports <NUM>; <NUM> of any of the embodiments of the present invention, so as to form a radiation shield for the magnet cold mass comprising main magnet assembly <NUM> and shield coils <NUM>. In this manner, conical annular supports <NUM>; <NUM>, lightweight tube <NUM> and main magnet assembly <NUM> form an isothermal volume in which termination parts <NUM> can be accommodated without additional thermal shielding. The main magnet assembly <NUM> may be brought within the isothermal volume by addition of a thermally conductive layer <NUM>, 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 invention accordingly provides a superconducting magnet assembly comprising a main magnet assembly <NUM> comprising at least one annular superconducting coil arranged about an axis A-A, and at least one shield coil <NUM>, of greater diameter than the main magnet assembly <NUM>, arranged about the axis A-A, wherein at least one annular support is provided, attached to the shield coil <NUM> and to the main magnet assembly <NUM>.

The annular support may, in preferred embodiments, be conical; and in further preferred embodiments 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 invention 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 support <NUM>; <NUM> of the invention 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 structure <NUM>; <NUM> of the present invention is simple and cost effective. In preferred embodiments, use of the conical support structure of the present invention allows access to the termination area of the magnet without obstruction.

The annular shield coil support structure of the present invention 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.

Claim 1:
A superconducting magnet assembly comprising a main magnet assembly (<NUM>) comprising at least one annular coil (<NUM>) arranged about an axis, and at least one shield coil (<NUM>), of greater diameter than the main magnet assembly (<NUM>), arranged about the axis, wherein at least one conical annular support (<NUM>; <NUM>) is provided, attached to the shield coil (<NUM>) and to the main magnet assembly (<NUM>), characterized in that the main magnet assembly further comprises an end ring (<NUM>) bonded to an axially outer end of the shield coil (<NUM>), a radially outer circumference (<NUM>) of the material of the conical annular support being attached to the end ring.