GB2519811A

GB2519811A - Superconducting magnet assembly

Info

Publication number: GB2519811A
Application number: GB1319257.0A
Authority: GB
Inventors: Russell Peter Gore; Simon James Calvert
Original assignee: Siemens PLC
Current assignee: Siemens PLC
Priority date: 2013-10-31
Filing date: 2013-10-31
Publication date: 2015-05-06
Also published as: GB201319257D0

Abstract

A cylindrical electromagnet assembly 10 comprises a plurality of coils 14 and annular spacers 16, where the annular spacers 16 are placed between adjacent coils 14 to retain the coils in a fixed relative position and where the radial mid-point of each coil is axially aligned with part of the radial extent of an adjacent coil. The assembly may be symmetrical about an axis. The coils 14 and spacers 16 may be impregnated with a hardening material to form a monolithic structure. The spacers 16 may be formed of porous filler material or by solid extruded and anodised aluminium. The coils 14 may be bonded to the inside or outside of a tubular support. The assembly 10 may include shield coils with a greater radius than that of other coils in the assembly. Also disclosed is a method, suitable for producing solenoid magnets, comprising providing a plurality of coils, each formed of multiple turns of superconducting wire embedded within a thermosetting resin; providing annular spacers and bonding the annular spacers between the coils to retain the coils in position relative to one another.

Description

SUPERCONDUCTING MAGNET ASSEMBLY

The present invention relates to solenoidal magnets made up of several axially aligned coils, and methcds fcr their manufacture.

The present inventicn particularly relates tc solencidal

magnets fcr use as a magnetic field generatcr in a

Magnetic Resonance Imaging (MRI) system. In particular, ic the invention relates to such magnets formed of superconductive wire.

In known magnet arrangements, a solenoidal magnet typically comprises outer coils of relatively large number of turns, and hence cross-section and a number of inner coils of smaller number of turns and hence cross-section. Conventionally, an accurately machined former, such as an aluminium tube, is provided with appropriately shaped slots into which wire is wound to form the coils.

The coils may be impregnated with a thermosetting resin, either by wet-winding, in which a wire is passed through a bath of resin before being wound onto the former, or the coils may be wound dry, with the completed coils and former later being impregnated in a bath of resin.

Similar impregnation may be performed with a wax, but the present description will refer to "resin" only, for brevity.

Alternatively, arrangements of moulded coils are known.

In these arrangements, coils are wound into moulds, and the finished coil impregnated with resin within the mould. The resin is than cured, and a solid coil embedded in resin is produced. These moulded coils are then assembled into a magnet, for example by clamping onto a former or other meohanioal support structure.

A known compromise arrangement has the central coils, those toward the axial centre of the magnet, arranged on a former, with end coils moulded and mechanically attached to the former. The end coils tend to be larger in cross-section, and less critical in their placement.

This oompromise arrangement enables smaller, less ic expensive formers to be used, while maintaining accurate relative positioning between the central coils.

These kncwn arrangements suffer from certain drawbacks.

In use, magnet coils are subject to large forces, due to

interaction of the coils with the magnetic fields

prcduced. Some of these fcrces act axially, and urge the coil towards a wall of the former, while other forces act radially, tending to expand the coil to a larger diameter, or compress it onto the former. These forces may cause the coils no mcve relative tc the former. Such movement may cause heating of the coils, which in superconducting magnets may lead to a quench.

The forces acting on the coils may cause the former to flex. The former needs to be large, heavy and mechanically robust tc resist thcse forces. Due tc flexure in the former, the force reaction path resisting the coil forces then acts essentially at the inner edges of the coils, which is misaligned from the line of action of the coil body force, which may be considered to act in an axial direction, through the radial mid-point of the coil cross-section. This contributes to a tendency to flex the former. The forces are also borne by a limited surface area of the coils. This may cause deformation of the coils themselves, which may also lead to quench in a superconducting coil.

The majority of the force acts upon the magnet's end coils, and shield coils if present. Central coils are relatively lightly loaded, but are required to be the most accurately positioned in space to create a

homogeneous field as required for imaging.

An accurately-machined former, as conventionally used, is expensive, and is only available from a limited number of suppliers. Transport costs from the former factory to the magnet winding facility may be significant. Storage of the large former may be difficult and costly.

The present invention accordingly provides a new arrangement for the manufacture and retention of coils in their intended relative positions.

The present invention accordingly 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 consideration of the following description of certain embodiments thereof, in conjunction with the appended drawings, wherein: Fig. 1 shows a perspective view of a coil assembly according to an embodiment of the present invention; and Fig. 2 shows a part-axial sectional view of a coil assembly according to an embodiment of the present invention.

Fig. 1 shows a perspective view, and Fig. 2 shows an axial sectional view of a superconducting coil assembly 10 according to an embodiment of the present invention.

The coil assembly 10 is cylindrical, and is essentially symmetrical about axis 12.

As illustrated in the drawings, the superconducting coil ic assembly 10 comprises a plurality of superconducting coils 14, each formed of multiple turns of superconducting wire embedded within a thermoset resin.

Other impregnating materials may be used as appropriate for the size and intended use of the coils. Between the coils, annular spacers 16 are provided. These annular spacers serve to retain the coils in position relative to one another.

A radial mid-point of each coil 14 is axially aligned with a part of the radial extent of an axially inwardly-adjacent coil 14, and the annular spacers 16 are provided between adjacent coils, retaining the coils in fixed relative positions.

The coils 14 may be the central coils of a magnet. End coils may be separately formed, and assembled with the illustrated central coil assembly to form a completed magnet. Alternatively, the end coils may be formed and assembled in the same way as the central coils shown.

In the illustrated embodiment, the spacers 16 may be formed of a porous material, such as a glass-fibre preform, or glass fibre felt or cloth, or a granular loose material such as glass beads, impregnated with a thermoset resin or equivalent material. Preforms of glass-fibre may be sprayed with starch to improve the impregnation process during manufacture of the spacers.

Alternatively, the spacers may be formed of impregnated coils cf wire, or may be of sclid material such as PTFE or other suitable polymers, or other non-magnetic material, such as aluminium. The spacers may be formed from an extrusion, cut to length.

ic The coils 14 may be separately formed, for example, wound in a winding journal which is then enclosed in a mould so that the coil may be impregnated in a thermosetting resin. The separately formed coils 14 and spacers 16 may then be assembled together by bonding, for example using a thermosetting resin.

Alternatively, the spacers 16 may be formed by dry winding a filler material such as glassfibre cloth onto a bobbin or spigot carrying the coils 14, and thermcsetting impregnating resin applied to the structure, to impregnate the filler material to form the spacers and bond them to the coils.

The resulting structure is a single solid cylindrical structure which comprises coils 14 and annular spacers 16. The coil assembly so formed may be electrically and mechanically assembled into a cryostat to form part of an MRI imaging system, as will be apparent to those skilled in the art.

During the assembly step, the coils may be enclosed by a cylindrical closure plate to define an annular cylindrical moulding cavity. In the illustrated case, all coils have an equal cuter radius, as well as an equal inner radius, once assembly is complete. Other arrangements may be provided for, enabling the coils of the structure to have differing outer and inner radii.

While the present invention has been described with specific reference to certain non-limiting examples, the invention provides at least some of the following advantages.

ic The spacers 16 have only to react substantially axial loads. This enables lighter supporting structure to be used.

According to the present invention, use is made of a resin-impregnated ooil's structural strength: each impregnated coil 14 acts as an integral structural element in the coil assemblies of the present invention.

The inherent compressive strength of the resin-impregnated composite coils is utilised to transmit the coil body forces to an axial mid-plane of the magnet.

The large, precision-machined formers conventionally used for winding magnets are expensive in material and labour cost. They can only be sourced from a few locations worldwide. They are bulky and expensive to ship. The present invention may use spacers made of composite materials such as resin-impregnated glass fibre, formed in-situ during impregnation of the magnet coils. Such spacers are very inexpensive. When formed in situ, they do not require machining, obtaining from a third-party supplier, shipping or storage. Sufficient stock of the component parts -filler material, hardening material and hardener, if required -must be maintained.

Alternatively, annular spacers 16, for example of polymer or composite materials such as fibre-reinforced resin, may be used. These only need to be accurately dimensioned in the axial direction. They may be produced cheaply by any one of many manufacturers. They are lightweight and reguire very little space when shipping.

The present invention extends to other variations and modifications, as will be apparent to those skilled in the art, some examples of such modifications being described in the following paragraphs.

The coil assemblies provided by the present invention may be effectively cooled using a cryogen vessel containing a liquid cryogen. Alternatively, other cooling systems, such as conduction cooling or thermo-siphon cooling may be used with the coil assembly of the present invention, as the surfaces of the coils are readily accessible.

The coil assembly 10 of the present invention may be internally or externally bonded to a supporting structure -for example, a tubular non-magnetic structure -for improved stability of the resultant magnet. Outer shield coils of greater dimension may be provided, as is known by those skilled in the art.

The coil assembly of the present invention provides accurate relative positioning between the coils 14.

As rio former is provided, no guenches can be induced by relative movement of the coils on the former. There can be no flexure in a former, so that all reaction forces act in an axial direction, through the radial mid-point of the coil 14 and spacer 16 cross-section. This eliminates any tendency to flex the structure. The forces are also borne by a large surface area of the coils, which may reduce a tendency to quench.

Claims

CLAIMS1. A cylindrical magnet assembly (10) comprising a plurality of coils (14), wherein the radial mid-point of each coil is axially aligned with a part of the radial extent of an axially inwardly-adjacent coil (14), and annular spacers (16) are provided between adjacent coils, retaining the coils in fixed relative positions.
2. A magnet assembly according to claim 1 wherein the magnet assembly is essentially symmetrical about an axis (12).
3. A magnet assembly according to claim 1 wherein the is coils and the spacers are impregnated with a hardened material.
4. A magnet assembly according to claim 3 wherein the annular spacers (16) are formed of a composite material, itself formed of a porous filler material impregnated with the hardened material, and the coils and the annular spacers form a monolithic structure of the hardened material.
5. A magnet assembly according to claim 1 wherein the coils (14) are individually moulded and impregnated with a hardened material, then joined by solid annular spacers (16).
6. A magnet assembly according to claim 5 wherein the annular spacers (16) are adhesively bonded to the coils (14).
7. A magnet assembly according to claim 1 or claim 5 or claim 6, wherein the annular spacers are fcrmed by an extrusion, cut to length.
8. A magnet assembly according tc claim 1 cr claim 5 or claim 6 or claim 7, wherein the annular spacers are formed of aluminium and their surfaces are anodised.
9. A magnet assembly according to any preceding claim, ic wherein the coils are bonded to a tubular support structure (160)
10. A magnet assembly according to claim 9, wherein a radially outer surface of each of the coils is bonded to an inner surface of the tubular support structure.
11. A magnet assembly according to claim 9, wherein a radially inner surface of each of the coils is bonded to an outer surface of the tubular support structure.
12. A magnet assembly according to any preceding claim, further comprising shield coils of greater radius than the coil assembly.
13. A method for the production of solenoidal magnets made up of several axially aligned coils, comprising the steps of: -providing a plurality of superconducting coils (14) each formed of multiple turns of superconducting wire embedded within a thermoset resin; -providing annular spacers (16) -bonding the annular spacers between the coils to provide a coil assembly (10) which retain the coils in position relative to one another.
14. A method according to claim 13 further comprising providing end coils separately formed, and assembled with the coil assembly.
15. A method according to claim 13, wherein the spacers (16) are formed of a porous material impregnated with a thermoset resin or equivalent material.
16. A method according to claim 13, wherein the spacers (16) are formed of impregnated coils of wire.
17. A method according to claim 13, wherein the spacers (16) are of solid macerial such as lIFE or other suitable polymers, or other non-magnetic material, such as aluminium.
18. A method according to any of claims 14-17, wherein the spacers are formed from an extrusion, cut to length.
19. A method according to claim 13 wherein the coils (14) are separately formed, the assembled with the spacers (16) using a thermosetting resin.
20. A method according to claim 13 wherein the spacers (16) are formed by dry winding a filler material onto a bobbin or spigot carrying the coils (14), and applying thermosetting impregnating resin tc the structure, tc impregnate the filler material to form the spacers and bond them to the coils.