GB2441795A

GB2441795A - Tubular support system for a superconducting magnet

Info

Publication number: GB2441795A
Application number: GB0618107A
Authority: GB
Inventors: Nicholas Mann; Marcel Jan Marie Kruip
Original assignee: Siemens Magnet Technology Ltd
Current assignee: Siemens Magnet Technology Ltd
Priority date: 2006-09-15
Filing date: 2006-09-15
Publication date: 2008-03-19
Anticipated expiration: 2026-09-15
Also published as: US20100265018A1; US8228147B2; GB2451214B; US8729990B2; JP5361724B2; GB0821235D0; CN101512687B; GB2441795B; GB2469203A; GB2451214A; GB2469203B; US20120238453A1; GB2451214B8; WO2008032110A1; JP2010503983A; GB201005531D0; GB0618107D0; CN101512687A

Abstract

A superconducting magnet support arrangement or a method of assembling such a magnet and support arrangement comprises a superconductive magnet coils 24 located within an outer vacuum container 28. A support structure 30 bears the weight of the magnet against a support surface 34. The support structure 30 comprises at least one substantially tubular support element 30A, 30B positioned between the magnet and the support surface 34 and a weight transmitting complementary interface 32 is formed between the cryogenic vessel 36 containing the magnet and the tubular support element 30A. A thermal shield 26 may also be supported by the tubular support 30 via a flange 38. The support surface 34 may form part of the outer vacuum container 28 and be arranged at the floor end of the support element 30B where the tubular element 30 has a vertical axis and the magnet coils 24 have a horizontal axis. The complementary surface 32 may be within a re-entrant portion of the cryogen vessel 36. The outer vacuum container 28 may be formed in parts which can be assembled round and attached to the tubular support element 30. The tubular support element 30 may have a non-circular cross-section which may be tapered and include a diameter which is larger than that of the cryogenic vessel.

Description

2441795

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SUSPENSION FOR A SUPPORTED SUPERCONDUCTING MAGNET

In order to maintain the necessary cryogenic temperature, superconducting magnets such as those used in MRI scanners must be 5 suspended inside a vacuum vessel. In order to fully constrain the suspended vessel under the loads encountered during operation and transportation, conventional designs employ multiple suspension elements. These elements are complicated and require multiple attachments to the vessels. The result is an expensive suspension system 10 which is time consuming to assemble and not well suited to volume production conditions.

Fig. 1 illustrates a cross-sectional view of a conventional solenoidal magnet arrangement for a nuclear magnetic resonance (NMR) or magnetic 15 resonance imaging (MRI) system. A number of coils of superconducting wire are wound onto a former 1. The resulting assembly is housed inside a cryogen vessel 2 which is at least partly filled with a liquid cryogen 2a at its boiling point. The coils are thereby held at a temperature below their critical point.

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Also illustrated in Fig. 1 are an outer vacuum container 4 and thermal shield 3. As is well known, these serve to thermally isolate the cryogen vessel 2, typically containing a liquid cryogen 2a, from the surrounding atmosphere. Solid insulation 5 may be placed inside the space between 25 the outer vacuum container 4 and the thermal shield 3. A central bore 4a is provided, of a certain dimension to allow access for a patient or other subject to be imaged.

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Conventionally, a number of supporting elements 7 are connected between the cryogen vessel 2 and the outer vacuum container 4 to bear the weight of the cryogen vessel. These may be tensile bands, tensile rods, straps, compression struts or any known element suitablefor the purpose.

5 The elements should have a very low thermal conductivity, in the case that a cryogen vessel is supported. This is important in order to minimise heat influx from the outer vacuum container 4, which is typically at ambient temperature, to the cryogen vessel 2. The suspension elements typically pass through holes in the thermal shield 3. Similar, or alternative, 10 suspension arrangements may be provided to retain thethermal shield 3.

The suspension elements must be of the minimum cross sectional area and maximum length in order to minimise the heat flow into the magnet. In conventional designs such as shown in Fig. 1, multiple tension-only 15 elements or combinations of tension and compression elements are employed. These may be made from high strength steel or advanced composite materials using glass, carbon or other suitable load-bearing fibres. Typically a minimum of eight elements are used for each vessel, giving a total of sixteen elements to support atypical magnet and radiation 20 shield system. Such a system will typically contain hundreds of individual parts, which must be individually assembled. Since MRI systems are commonly transported fully assembled and filled with cryogen, the suspension must be capable of resisting the high loads associated with transportation and handling. The suspended vessels must be accurately 25 constrained at all times, and when conventional elements are used this requires that the elements are pre-loaded to ensure than they remain tight under all the foreseeable design loads. Due to the need to achieve the smallest overall system dimensions, space for the suspension is limited, and access is further restricted by the insulation blankets that must be

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used. As a result, the assembly procedure is complicated and time consuming, and not well suited to high volume manufacture.

EP 1001438 describes a tube suspension arrangement for use in a 5 superconducting magnet, wherein the cryogenic vessel may move relative to the thermal shield and the OVC during cool down from ambient temperature to cryogenic temperature.

U S 6,358,583 describes an arrangement wherein a magnet structure is 10 supported on a thermal shield by a first tubular support, the thermal shield being supported on theOVCby asecond tubular support.

U S 5,530,413 describes a cooled solenoid magnet in a cryostat arrangement, wherein the magnet and thermal shield are supported by 15 tubular support structures coaxial with thesolenoidal magnet.

The present invention provides a supported solenoidal superconducting magnet having a substantially horizontal axis, in which the multiple suspension elements of conventional arrangements are replaced by a 20 single tubular suspension element arranged about a substantially vertical axis, substantially perpendicular to the axis of the magnet. The tubular suspension element is preferably capable of resisting loads in the directions of all anticipated forces to be experienced by the magnet, and is arranged to have the appropriate strength in appropriate directions such 25 as tension, compression and torsion. As a result, complexity of the suspension arrangement is substantially reduced, no pre-loading is required, and the assembly process is simplified.

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

The above, and further, objects, aims and characteristics of the present 5 invention will become more apparent from the following description of certain embodiments thereof, in conjunction with the following drawings, wherein:

Fig. 1 shows a conventional arrangement of suspension elements for 10 retaining a horizontally arranged solenoidal magnet in a cryogen vessel within an outer container;

Fig. 2 shows a suspension system according to an embodiment of the present invention;

Fig. 3 shows an enlarged portion of the embodiment of Fig. 2;

15 Fig. 4 shows a suspension system according to another embodiment of the present invention;

Fig. 5 illustrates a cross-section through the embodiment shown in Fig. 4; and

Fig.6 shows a suspension system according to afurther embodiment of the 20 present invention.

An embodiment of a suspension system according to the present invention is shown in Fig. 2. A cylindrical vessel 22 (the "cryogen vessel") containing the cryogenic magnet coils 24 is supported by a suspension system of the 25 present invention, bearing on a support surface, such as a floor. The cryogen vessel 22 is surrounded by a close fitting thermal radiation shield 26, and the whole assembly is contained within an outer vacuum container ("OVC") 28. A single tubular suspension element 30 is at one end 32 rigidly connected to the cryogen vessel 22 by way of appropriate interface

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means, and provides the floor mounting of the system at the other end 34. The OVC 28 is attached to the tubular suspension element at or near the floor end 34. A re-entrant section 36 is provided in the cryogen vessel to provide space for the tubular suspension element without increasing the 5 overall system height from the floor. This is made possible by the arrangement of magnet coils 24. This arrangement is conventional in itself, consisting essentially of a number of primary coils of similar radius, with a shield coil of significantly larger radius located at or near either end of the primary coils. The tubular suspension element tubular suspension 10 element retains the cryogen vessel through complementary interface surfaces on the tubular suspension element tubular suspension element and the cryogen vessel. In the simplest arrangement, the complementary surfaces comprise a cross-section through the support tube 20, perpendicular to an axis of the support tube, and a planar surface at the 15 end 32 of the re-entrant portion 36.

The thermal radiation shield 26 is preferably similarly supported from a flange 38 positioned part way up the tubular suspension element tubular suspension element 30. The tubular suspension element may serve to 20 retain the thermal radiation shield in a fixed position with reference to the outer vacuum container and the cryogen vessel by means of complementary interface surfaces arranged to transmit the weight of the shield to thetubular suspension element.

25 In order to achieve the required strength and thermal properties of a suspension system, yet in a compact configuration, the tubular suspension element 30 is preferably manufactured from one or more composite material composed of glass fibre, carbon fibre or other high performance fibres held in a suitable matrix material such as epoxy or similar resin.

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Using such materials, the orientation of the fibres can be optimised to provide the required combination of structural strength, thermal conduction and thermal contraction properties required to effectively support the loads of, and interface with, the suspended vessel 22, the floor 5 mounting 34, and thermal radiation shield 26.

The cryogen vessel suspension arrangement of Fig. 2 may be assembled in an assembly process whereby an assembly of magnet coils 24 and cryogen vessel 22 is first positioned on the tubular suspension element 30. The 10 radiation shield 26 is preferably manufactured from a number of parts which are assembled around the tubular suspension element 30 and attached to it. Finally, the OVC28 can be manufactured and installed in a similar manner. Accurate alignment of the vessels 22, 26, 28 is achieved by suitable features on the vessels and the tubular suspension element 15 tubular suspension element30, and no pre-loading or other adjustment is required. The OVC may be attached to a flange similar to 38, positioned nearer the floor. The tubular suspension element 30 must be closed by at least one closure surface 39, which effectively becomes part of the OVC when the parts of the OVC are assembled around the tubular suspension 20 element 30. Another closure surface may be provided, at about the height of the flange 38, to become part of the thermal radiation shield when the parts of the thermal radiation shield are assembled around the tubular suspension element 30.

25 Fig. 3 shows an enlarged part of Fig. 2, illustrating the tubular suspension element 30 (now at least notionally, and preferably also physically, divided into two sections 30A, 30B), the re-entrant portion 36, flange 38 and associated features in more detail. In order to provide complete thermal radiation shielding, thethermal radiation shield 26 iscontinued acrossthe

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inside of the tubular suspension element 30A, 30B by a closure surface, hereafter referred to as shield portion 26A. Shield portion 26A is preferably attached to the tubular suspension element 30 by an interior extension of the flange 38.

5

Similarly, in order to provide a complete OVC, the OVC is continued across the inside of the tubular suspension element 30A, 30Bby a closure surface, hereafter referred to as OVC portion 34. OVC portion 34 may be attached to the tubular suspension element 30 by an interior extension of a flange 10 connected to the remainder of the OVC. Alternatively, and more simply, the OVC portion 34 may be a continuous plate, forming the flange for the remainder of the OVC.

Refinement of the tubular suspension element is possible by dividing it 15 into two sections, for example having a lower section 30B placed between the OVC and the thermal radiation shield an upper section 30A placed between the thermal radiation shield and the cryogen vessel. This facilitates the use of different composite materials and constructions for each section. For example, materials may be selected whose thermal 20 conductivities are optimised for the temperature range to be encountered by each section when in use. That is, at cryogenic temperatures for the upper section 30A, and for higher temperatures for the lower section 30B. Further refinement may be achieved by selecting the material according to the mechanical load which must be supported by each section. The lower 25 section must support a somewhat higher mechanical load then the upper section, since the lower section must support the weight of the thermal radiation shield in addition to the load supported by the upper section. For example, appropriate thermal conductivity at each temperature may be achieved by using carbon fibre for upper section 30A, with glass fibres for

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lower section 30B, giving relatively low thermal conductivity at the temperatures of each vessel. The material should be provided of sufficient thickness, fibre orientation and other known parameters of such materials to ensure that the tubular support element is mechanically sufficiently 5 strong enough to support and retain the applied loads.

In a preferred embodiment of the invention, the thermal radiation shield mounting flange 38 is continuous between outer and inner surfaces of the tubular suspension element, with the separate sections 30A, 30B being 10 assembled to the flange to form a single tubular suspension element 30. The shield portion 26A which is preferably provided within the tubular suspension element serves also to strengthen the tubular suspension element against buckling. Thermal radiation shield such as 26 are typically "heat stationed" - cooled to an intermediate, cryogenic 15 temperature, typically in the range of 70-90K by active refrigeration. By providing flange 38 of a relatively conductive material such as aluminium or steel, the shield portion 26A may be cooled to the same temperature as the remainder of the shield.

20 In alternative embodiments, the tubular suspension element may be a continuous tube of composite material with flange 38 adhesively bonded to its surface. A similar flange may be adhesively bonded to the interior surface of the tubular suspension element. In such embodiments, alternative provision may need to be made for thermally linking the shield 25 portion 26A to the remainder of the thermal radiation shield, for example by way of a flexible copper braid link through a hole in one or other section 30A, 30Bof the tubular suspension element.

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Alternative arrangements of the tubular suspension are possible to suit different magnet configurations. Fig. 4 shows a design which eliminates the need for a re-entrant section in the cryogen vessel 22. In the arrangement of Fig. 4, a generally tubular suspension element 40 of 5 diameter somewhat greater than the axial length of the cryogen vessel 22 is arranged about a generally vertical axis, generally perpendicular to a generally horizontal axis of cylindrical cryogen vessel 22.

The tubular suspension element 40 is preferably not simply cylindrical, 10 but is shaped to suit the available space and support the cryogen vessel against translation and rotation. The support 40 must of course be sufficiently strong to support and retain the cryogen vessel 22 and its contents through the whole range of foreseen mechanical loads. Certain regions, such as at 42, of the support may be required to support a 15 relatively large portion of the weight of the cryogen vessel and its contents. Certain other regions, such as at 44, of the support may be required to support lower loads, but should match the thermal contraction characteristics of the cryogen vessel to which it is attached. By the use of suitable composite materials, fibre orientations and thicknesses, the 20 characteristics of the different sections of the support 40 can be optimised to fulfil these varying requirements. Compared to the arrangement of Figs. 2 and 3, the arrangement of Fig. 4 allows a greater tubular suspension element length to be achieved with consequent thermal performance advantages, and minimises the stresses in a thin-walled 25 cylindrical cryogen vessel.

It is intended that the cryogen vessel will be securely bonded to the tubular support element am all points of contact, to ensure that the support element retains the cryogen vessel against movement in all directions. On

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certain applications, it may be found sufficient that the cryogen vessel be cradled within the tubular support element, retained in position by its own weight.

5 Fig. 5 illustrates a cross-section through an arrangement such as shown in Fig. 4, within a complete cryostat arrangement comprising cryogen vessel 22 and support 40 of Fig. 4 within an OVC 28, having a thermal radiation shield 26 between the cryogen vessel 22 and the OVC 28, the shield being supported on the support 40. The support 40 may be arranged similarly to 10 any of the described variations of the support 30 of Fig. 3, with upper and lower parts of the support element optionally made of different materials, with amounting flange supporting the thermal radiation shield, the flange optionally extending through the material of the tubular support. A shield portion is preferably provided within the tubular support, to close the 15 shield. This is believed to be more important in embodiments such as that of Figs. 4, 5, since the tubular support element occupies a larger portion of the surface area of the thermal radiation shield than is the case in embodiments such asthat shown in Figs, 2, 3.

20 The support element may be mounted on a base 42, which serves as a flange for the mounting of OVC 28. Preferably, base 42 becomes part of the OVC.

The thermal radiation shield 26 and OVC 28 may respectively be 25 assembled as for the embodiment of Figs. 2, 3. An assembly of magnet coils and cryogen vessel 22 is first positioned on the tubular suspension element 40. The radiation shield 26 is preferably manufactured from a number of parts which are assembled around the tubular suspension element 40 and attached to it, preferably by way of a mounting flange.

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Finally, the OVC 28 is manufactured and installed in a similar manner. Accurate alignment of the vessels 22, 26, 28 is achieved by suitable features on the vessels and the tubular suspension element 40, and no preloading or other adjustment is required.

5

As illustrated in Fig. 5, it may be found convenient to produce the thermal radiation shield 26 and the OVC 28 in shapes other than the conventional cylinder shape, to better interface with the support 40. As also illustrated in Fig. 5, the support element 40 may be tapered to better restrain the 10 cryogen vessel.

In the arrangement of Figs. 4 and 5, the support 40 retains the cryogen vessel 22 through complementary interface surfaces, comprising the outer surface of the cryogen vessel and a suitably shaped portion of the inner 15 end surface of support 40.

Some cryogenically cooled equipment such as superconducting magnets are not housed within a cryogen vessel. They are directly cooled by a refrigerator, or by a cooling loop arrangement, well-known in the art. The 20 present invention may be applied to such equipment, modified at follows. For embodiments such as shown in Figs. 4, 5, the tubular suspension element 40 must be adapted to interface directly with the cooled equipment. Superconducting magnets are typically wound onto thermally conductive former, made of materials such as aluminium. The former 40 25 may be shaped to securely retain such former in directions of all expected forced, against rotation and translation. Care should be taken that the support element does not bear against any sensitive parts of the cooled equipment, such ascoilsof a superconducting magnet.

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Fig. 6 shows an embodiment of the present invention, similar to that shown in Figs. 2 and 3, adapted for cooled equipment which is not cooled within a cryogen vessel. The tubular suspension element 30 (30A, 30B) is 5 attached directly to a magnet coil support structure such as former 45. The absence of the cryogen vessel allows an increased length of tubular suspension element 30 with consequent thermal performance advantages. Other features and requirements of the design are similar to the arrangement shown in Fig. 3.

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The present invention accordingly provides a new superconducting magnet suspension arrangement, wherein a single tubular suspension element is used in place of the multiple suspension elements used in conventional systems. The tubular suspension element is arranged about 15 a generally vertical axis, and supports a solenoidal magnet structure which is arranged about a generally horizontal axis.

The present invention provides at least the following advantages over the conventional support system described with reference to Fig. 1. The 20 complexity and component parts count of the suspension arrangement is substantially reduced. The suspension arrangement requires only asingle connection to the outer vacuum container with consequent simplification of this structure. The overall assembly process is simplified, leading to simplification of high volume manufacture. In addition to these 25 manufacturing advantages, the suspension arrangement of the present invention offers increased precision of alignment of the suspended vessels.

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While the present invention has been described with reference to a limited number of particular embodiments, various modifications and variations of the present invention will be envisaged by those skilled in the art, within the scope of the present invention as defined by the appended 5 claims.

For example, while the present invention has been described as requiring only one tubular suspension element, more than one tubular suspension element may be employed. In such an arrangement, the tubular 10 suspension elements may be smaller than in the case a single tubular suspension element is used. In addition, the location of multiple tubular suspension elements may be chosen to provide optimal mechanical support for the outer vacuum container, the shields and the cryogen vessel and/or the magnet assembly. Indeed, separate tubular suspension 15 elements may be provided for the shields and for the cryogen vessel. Two or more of such tubular suspension elements may be concentrically arranged. Each tubular suspension element may be sized appropriate to the load it is to bear, such that a tubular suspension element for supporting a shield may be of lesser thickness than a tubular suspension 20 element for supporting a cryogen vessel with a magnet inside. Each tubular suspension element may be composed of one or more materials, chosen appropriately to the thermal end mechanical environment in which it is to serve.

Claims

CLAIMS: -14-

1. A supported superconducting magnet, comprising:

a superconducting magnet (24) arranged within an outer vacuum 5 container (OVC) (22); and asupport structure bearing theweight of the superconducting magnet against asupport surface,

wherein the support structure comprises agenerally tubular suspension element (30; 40) located between the magnet and the support surface, 10 characterised in that the tubular suspension element is arranged about a generally vertical axis, and supports a solenoidal magnet structure which is arrange about agenerally horizontal axis, and said tubular suspension element retains the magnet in a fixed relative position with reference to the outer vacuum container (22) by means of complementary interface 15 surfaces arranged to transmit theweight of the superconducting magnet to the support structure.

2. A supported superconducting magnet according to claim 1, wherein the superconducting magnet is retained within a cryogen vessel (22), and

20 the generally tubular suspension element supports the cryogen vessel, thereby bearing theweight of the superconducting magnet and the cryogen vessel against the support surface.

3. A supported superconducting magnet according to claim 2 wherein 25 one of the complementary interface surfaces is provided by are-entrant portion (36) of a wall of the cryogen vessel, and another of the complementary interface surfaces comprises an end of thegenerally tubular suspension element (30; 40).

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4. A supported superconducting magnet according to claim 3 wherein the superconducting magnet is retained within a tubular cryogen vessel (22) having a horizontal axis; thegenerally tubular suspension element having a radial diameter less than that of the cryogen vessel.

5

5. A supported superconducting magnet according to claim 2 wherein one of the complementary interface surfaces is provided by a wall of the cryogen vessel (22) and another of the complementary interface surfaces comprises appropriately formed surfaces of thegener ally tubular

10 suspension element (40).

6. A supported superconducting magnet according to claim 5 wherein the is retained within a tubular cryogen vessel (22) having a horizontal axis; thegener ally tubular suspension element having a radial diameter

15 greater than that of the cryogen vessel.

7. A supported superconducting magnet according to claim 6 wherein the generally tubular suspension element is non-circular in cross-section and is tapered.

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8. A supported superconducting magnet according to any of claims 5-7 wherein an upper and of thegenerally tubular suspension element is shaped to conform to the outer surface of the cylindrical cryogen vessel.

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9. A supported superconducting magnet according to any preceding claim, further comprising a radiation shield (26) located between the outer vacuum container and the cryogen vessel where provided, and between the outer vacuum container and the superconducting magnet where no cryogen vessel is provided, said radiation shield being supported on a

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shield support structure (38, 30; 40) comprising agenerally tubular suspension element located between the shield and the support surface, said agenerally tubular suspension element retaining theshield in afixed relative position with referenceto theouter vacuum container (22) by 5 means of complementary interface surfaces arranged to transmit the weight of the shield to the support structure.

10. A supported superconducting magnet according to claim 9 wherein the shield support structure (38, 30) comprises a flange (38) on the

10 generally tubular suspension element (30; 40) which bears the weight of the superconducting magnet.

11. A supported superconducting magnet according to claim 10 wherein the tubular suspension element comprises an upper part (30B) and a lower

15 part (30A), said upper and lower parts being joined at theshield support flange (38).

12. A supported superconducting magnet according to any of claims 10-11 wherein the flange is continuous through the wall of the generally

20 tubular suspension element, providing a mounting flange on the inside of the agenerally tubular suspension element, which retains a shield portion in position within the agenerally tubular suspension element.

13. A supported superconducting magnet according to any preceding 25 claim wherein the generally tubular suspension element has a closure portion (34) across its lower end, providing a flange for mounting the OVC, said closure portion becoming apart of theOVCwall once the remainder of the OVC is assembled thereto.

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14. A method of assembling a supported superconducting magnet (24) within an outer vacuum container (28), such superconducting magnet being supported on asupport surface, the method comprising the steps of:

assembling the magnet (24) within a cryogen vessel (22); 5 supporting the cryogen vessel on agenerally tubular suspension element (30; 40) located between the magnet and the support surface, said generally tubular suspension element retaining the magnet in a fixed position with reference to the support surface by means of complementary interface surfaces arranged to transmit theweight of the superconducting 10 magnet to thesupport structure;

manufacturing the outer vacuum container in a plurality of parts, to bejoined about thegener ally tubular suspension element; and assembling the outer vacuum container around the generally tubular suspension element (30) and attached to it.

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15. A method of assembling a supported superconducting magnet according to claim 14, further comprising, prior to thestep of assembling the outer vacuum container, the stepsof manufacturing a thermal radiation shield in a plurality of 20 parts, to bejoined about the generally tubular suspension element; and assembling the thermal radiation shield around the generally tubular suspension element (30) and attached to it.

16. A method of assembling a supported superconducting magnet 25 according to claim 14 or claim 15, wherein the tubular suspension element is arranged about a generally vertical axis, and supports a solenoidal magnet structure which is arrange about agenerally horizontal axis.

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17. A supported superconducting magnet substantially as described and/or as illustrated in Figs. 2-6 of the accompanying drawing.

18. A method of assembling a supported superconducting magnet 5 substantially asdescribed and/or as illustrated in Figs. 2-6 of the accompanying drawing.