GB2596826A - Flexible thermal bus for superconducting coil - Google Patents

Abstract

A thermal bus 12, and a method of making a thermal bus, comprises: forming a number of turns of wire, or solid metal strips, wound around a component 10 to be cooled, where the turns are cut axially with the cut ends of the turns being partially peeled away from the component 10 to form a thermal bus 12 which is in thermal contact with the component 10. The thermal bus may be a flexible composite material arranged for the conductive cooling of a component 10, such as a superconducting magnet coil. The turns of the thermal bus 12 may be wound on the inside and/or outside surface of a coil 10. The turns may be impregnated with a resin or wax which may be removed from the cut end sections forming the thermal bus 12. The wire of the thermal bus 12 may be of the same dimension as the wire 14 of the superconducting coil 10. A protective layer may be arranged, prior to winding the thermal bus wire, to cover the coil wires 14 at a location where the thermal bus wires are to be cut and peeled. The thermal bus 12 may provide high thermal conductivity link, with a low thermal impedance joint and a matching thermal expansion coefficient, to a superconducting coil component 10.

Description

FLEXIBLE THERMAL BUS FOR SUPERCONDUCTING COIL

The present invention relates to cooling arrangements for superconducting coils. In particular, it relates to the provision of thermal buses which are thermally matched to the coil to be cooled. The invention also provides similar thermal buses for use in cooling other components, such as spacers and mandrels or formers.

Superconducting coils find applications in electromagnets such as used in MRI or NMR systems, energy storage systems, and high-strength magnetic field generators in multiple fields of application.

In the present description, coils are described as annular or cylindrical structures, for ease of description. References to "axial" directions are directions parallel to the axis of such annular or cylindrical structure. References to "radial" directions are directions perpendicular to that axis. References to "circumferential" directions are directions perpendicular both to that axis and to a radial direction.

Superconducting coils must be cryogenically cooled to below a transition temperature in order to become superconducting. In the past, it has been conventional to at least partially immerse superconducting coils in a liquid cryogen at its boiling point (e.g. helium, neon, nitrogen). Heat is removed from the superconducting coils as latent heat of evaporation as the cryogen liquid boils.

More recently, due to cost and scarcity of such cryogen liquids, it has been increasingly common to reduce the amount of liquid cryogen, or even to eliminate the use of liquid 35 cryogen entirely in the cooling of the superconducting coils.

In such arrangements, a cryogenic refrigerator is provided, and heat is transferred from superconducting coils, or other components of a system comprising superconducting coils, to the cryogenic refrigerator. This heat transfer may be achieved by a thermosyphon, which employs a small amount of cryogen liquid, or by heat transfer through a solid-state thermal bus. In case of a thermal bus, no liquid cryogen is required in this heat transfer, although the cryogenic refrigerator may itself incorporate some liquid cryogen, and liquid cryogen may be employed for other purposes within a system comprising superconducting coils. A thermal bus employed for such purpose typically comprises one or more conductors of a high purity metal of high thermal conductivity, such as copper or aluminium. The thermal bus may comprise one or more solid conductors, typically having a thickness in the order of 1-10mm and a width of 10-150mm; or, may comprise a flexible laminate made up of multiple layers of such a material, each layer having a thickness in the order of 0.1-1.0mm and a width of 10-300mm; or, may comprise a braid, made up of conductors of such a material, each conductor typically having a thickness in the order of 0.02-1.0mm.

While any such thermal bus must have a high conductivity in its own right in order to transfer heat over significant distances with minimal temperature drop along the thermal bus, it is important that there should be no significant thermal impedance introduced at joints linking the thermal bus to the cryogenic refrigerator, and to the superconducting coil, or other cooled component.

To achieve the required performance, the material used for the thermal bus must either have very high thermal conductivity, or have a large cross-sectional area, or both. Such solid state thermal buses have several disadvantages, for example in that high thermal conductivity material is expensive, and often only becomes high performing when cryogenic temperature is reached, meaning it is not particularly effective for initially cooling down a structure from room temperature; and that the buses must also be bonded to the components that are to be cooled, this introduces thermal interfaces which reduce performance, the thermal interfaces can be time-consuming to complete, for example one method for attaching a thermal bus involves cleaning the 5 respective parts and waiting for glue to cure, the construction of such interfaces may be subject to errors such as using wrong glue component ratios or disturbing joints before they are cured, alternatively there is a risk of damaging or affecting materials with excessive heat if the 10 thermal bus is joined to the superconducting coil or the refrigerator by soldering or welding.

Thermal buses may have a different coefficient of thermal expansion to the structure they are attached to, which means there is thermal stress developed between the structure and bus on cool-down. This can cause the bond between structure and bus to fail. This is particularly severe if the bus is made thick, for example to maintain a low temperature differential or to speed up initial cool-down. Thermal buses may be subject to unwanted interactions with the structure: for example, in respect of superconducting coils used in MRI systems, the thermal bus might suffer from induced eddy current heating from gradient coil stray field in an MRI scanner -known as gradient coil interaction (GCT). This heating leads to larger cryogenic load and can cause the magnet to quench. The eddy currents can also be a source of imaging artefacts because of their effect on the magnet field homogeneity.

The present invention aims to provide a thermal bus which has high thermal conductivity in its own right and a very low thermal impedance at its joint with a superconducting coil. The present invention also aims to provide such a thermal bus which has a coefficient of thermal expansion matched to that of a structure it is attached to.

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

The above, and further, objects, characteristics and advantages of the present invention will become more apparent from the following description of certain embodiments thereof, with reference to the accompanying drawings, wherein: Fig. 1 shows an example of a superconducting coil provided with a thermal bus according to an embodiment of the present invention; Figs. 2-5 show stages in a process, according to an embodiment of the present invention, for manufacture of a superconducting coil provided with a thermal bus such as illustrated in Fig. 1; Fig. 6 shows an embodiment of the present invention, in which 15 a plurality of superconducting coils, each provided with a thermal bus, as illustrated in Fig. 1, is shown, thermally linked to a cryogen vessel; and Fig. 7 shows an alternative example of a superconducting coil provided with a thermal bus according to another embodiment 20 of the present invention.

The present invention provides a thermal bus which is made up of a plurality of parallel wires, each in thermal contact with an object to be cooled. The thermal bus may be constructed by winding wire around the object, then cutting through the wires at a certain circumferential location of the windings, and partially peeling the cut windings away from the object thereby to form the thermal bus.

In a preferred embodiment, a thermal bus that is suitable for conduction-cooling a superconducting magnet coil is formed of turns of the same wire that makes up the superconducting magnet coil itself. Alternatively, the thermal bus may be made up of wire which is similar in dimensions and materials, but which lacks the superconducting filaments. Such wire may be expected to be cheaper and more thermally conductive than the superconducting wire of which the superconducting magnet coil is wound.

Fig. 1 illustrates an example embodiment of the present 5 invention, in which a superconducting coil 10 is provided with a thermal bus 12 according to an embodiment of the present invention.

In this embodiment, as is conventional in itself, coil 10 is 10 prepared by winding superconducting wire 14 onto a spool. In some conventional methods, that spool is placed into a mould, which is then closed, evacuated and a resin or wax impregnant introduced into the mould. The resin or wax is caused or allowed to harden, the resulting structure removed from the 15 mould and the spool and excess resin or wax is removed.

According to an embodiment of the present invention, one or more layers of wire windings 14 are cut through In an axial direction, and are partially peeled away from the remainder of the coil 10, to form thermal bus 12. The resin or wax may be removed over part or all of the peeled-away windings, to improve the flexibility of the thermal bus.

Although not clearly visible in Fig. 1, the windings used to 25 form thermal bus 12 are electrically separate from the remainder of the coil itself.

An example process for manufacturing the structure of Fig. 1 30 may proceed as follows, with reference to Figs. 2-5.

With reference to Fig. 2, the superconducting magnet coil 10 has been wound to the required size, with a defined number of turns of windings 14 to provide the required magnetic field.

A protection plate 16 is applied at a certain position on the coil's outer periphery, covering the axial extent of the outer surface of the coil. The protection plate may for example be a stainless steel plate lcm wide. The protection plate should be coated with a release material such as PTFE to prevent bonding to resin or wax used in impregnation of the coil. Similarly, a release layer 18, for example a PTFEcoated cloth, a sheet of PTFE or other suitable material is placed to cover the radially outer surface of the coil over an arc of its circumference corresponding to the required length of the thermal bus.

Further turns of windings are wound around the coil 10, over the protection plate 16 and release layer 18. Preferably, at least one complete layer of further turns is provided, extending the axial width of the radially outer surface of the coil 10. The resulting coil assembly may be wet-wound, or may be potted or impregnated with resin or wax as is conventional. Once the resin or wax has set, excess resin or wax is removed, as is conventional.

Fig. 3 shows the structure at this stage. An axially-20 directed cut 20 is made through the further turns of windings, at a location corresponding to the position of the protection plate 16. The cut may be performed with a suitable saw, and the protection plate 16 ensures that the underlying windings 14 of the coil 10 are not damaged by the action of the saw. In an alternative method, the further turns may be applied after formation of the coil 10 and may be cut as they are wound to remove the need to use a protection plate. In such an example, an end of a wire or strip of material may be attached to a radial surface of the coil 10, and the wire or strip wound around the coil, cutting the wire strip at circumferential locations corresponding to the desired location of axially-directed cut 20 as turns of the wire or strip are wound.

As illustrated in Fig. 4, a cut edge 22 of the further windings is peeled away from the protection plate 16 and the release layer 18. The further windings may be wet-wound, or impregnated in resin or wax, as described above, but the release layer 18 and the coating of release material on the protection plate 16 ensure that the further windings do not bond to the remainder of the coil over the area of the release layer 18. The further windings are peeled away from the cut edge 22 over the area of the release layer. The release layer 18 and protection plate 16 may be removed at this stage.

As shown in Fig. 5, the peeled section 24 of the further windings may be bent away and formed into a thermal bus, according to the present invention. The resin or wax impregnation may be removed from the further windings, to improve their flexibility. The wire making up the windings may be cleaned near the cut edge 22 to improve thermal contact and optimise efficiency as a thermal bus. The wires may be soldered together to improve thermal contact.

The free end of the peeled section may then be thermally linked to a cryogenic refrigerator, a cooling loop, a thermal 20 bus bar or any other thermal path, as desired.

Preferably, the area around the protection plate 16 where the cutting and initial peeling take place is masked, for example with putty or tape, to exclude excess resin. This may be 25 found to facilitate the cutting and start of peeling.

The further windings, which eventually make up the thermal bus bar 24, are electrically separate from the superconducting magnet coil. The further windings may accordingly be composed of the same type of wire as the superconducting magnet coil. In such embodiments, the same type of wire and the same type of impregnant are used in the thermal bus as in the coil, and so the thermal bus and the coil should have identical thermal expansion and contraction.

Alternatively, wire may be used for the further windings which has identical dimensions to the wire of the superconducting magnet coil but which does not include superconducting filaments. Such wire may provide a thermal bus having thermal expansion and contraction Practically identical to that of the superconducting magnet coil, but at less cost. Further, a different type of wire may be used for the further windings, as may be preferred for financial or process considerations. Such embodiments may provide a thermal bus having thermal expansion and contraction which differ somewhat from that of the superconducting magnet coil.

The layer(s) of further windings are either wet-wound, or 10 impregnated with resin or wax, as with the windings of the superconducting magnet coil, and so have a structure virtually identical to that of the superconducting magnet coil. This means that the superconducting magnet coil and the thermal bus should have identical thermal expansion and contraction. This means that there should be no thermally-induced stresses at the interface between the thermal bus and the superconducting magnet coil. In other embodiments of the present invention, the layer(s) of further windings may be interlayered with 'prepreg', a cloth which is impregnated with uncured resin, such that the windings press resin out of the cloth into spaces between the wires, effectively forming a resin-impregnated final structure.

In certain embodiments of the invention, a crack arrestor may be placed at or near circumferential ends of the release layer 18. This may be any appropriate structure, for example a length of glass fibre cord or a small GRP wedge or similar, extending axially across the release layer, to resist the peeling of the layer(s) of further windings circumferentially beyond the extent of the release layer. In the absence of such crack arrestor, the layer(s) of further windings may delaminate from the coil, either during peel-back of the thermal bus, or during magnet cool-down, or magnet energisation, or magnet quench.

In embodiments in which multiple layers of further windings are provided, release layers similar to release layer 18 shown in the drawings may be provided between the layers of further windings, so that the layers may be separated as they are peeled away. The overall bus may thereby remain flexible. Where multiple layers ae provided, and are impregnated together, the bus may not have the required 5 flexibility.

As may be seen in Figs. 1 and 5, the thermal interface between the thermal bus of the present invention and the superconducting magnet coil extends over the axial width of the further windings, which is preferably the whole width of the superconducting magnet coil, and around the circumference of the superconducting magnet coil, other than for the part of the further windings which has been peeled away. Such a large contact area means a minimal thermal interface resistance between the thermal bus and the superconducting magnet coil, meaning in turn that the temperature drop across the interface is low. Since the contact area of the thermal bus preferably extends across the whole axial width of the superconducting magnet coil, the cooling effect is even and efficient. As the thermal bus is made up of mul7iple wires in parallel, extending in the direction of thermal flux, the thermal bus will have a low thermal resistance along its length.

While the wires making up the thermal bus bar 24 preferably extend across the full width of the coil 10, more than one layer of such wires may be provided, to allow a thicker, and so more thermally conductive bus bar if required.

Typically, superconducting wire such as used to make superconducting magnet coils 10 comprises a number of superconducting filaments encased within a stabiliser material, which is typically a high-RRR (Residual Resistivity Ratio) copper or aluminium. The wire used to form the thermal bus 24 may be of wire of the same dimensions and of the same stabiliser material, without the superconducting filaments. In such an embodiment, the thermal performance is good, but the wire used is readily available and economical in cost. Most of the thermal matching properties will be achieved if the wire used is of the same dimensions as the wire used for the superconducting magnet coil. Further, a different type of wire may be used for the further windings, as may be preferred for financial or process considerations. Such embodiments may provide a thermal bus having thermal expansion and contraction which differ somewhat from that of the superconducting magnet coil.

The further windings, which make up the thermal bus 24, are wound at the same time as the superconducting magnet coil 10 is formed, and in a similar manner, so no particular steps are required to form a thermal interface between The thermal bus and the superconducting magnet coil. As the further windings, which make up the thermal bus 24, are wound in the same manner as the superconducting magnet coil, there is less scope for manufacturing errors in forming the thermal bus.

As the thermal bus 24 is made of several wires, it may be highly flexible and can be routed as required, for example to a cooling station, as schematically represented by Fig. 5. The flexibility of the thermal bus also means that it allows relative movement between the cooling station and the superconducting magnet coil. The flexibility of the thermal bus of the present invention may also limit the transmission of vibrations from cooling station to superconducting magnet coil.

The flexibility of the thermal bus means that it may be simply soldered or bonded to the cooling station, ensuring a simple, reliable and rapid installation. The one-piece construction of the thermal bus also minimises the number of thermal joints that must be created.

During the manufacturing process outlined above, the further windings are cut 20 axially. Once this cut is made, the further windings do not form any complete loops and so have a much reduced tendancy to coupling to the magnetic field of

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the superconducting magnet when ramping up or down or during quench.

The thermal bus 24 is esselmially immune to GCT (gradient 5 coil induced) eddy currents because the turns are insulated from each other, providing a very high electrical resistance in the axial direction.

In other embodiments, similar thermal bus structures may be formed on other components of a superconducting magnet or other cooled component. For example, components such as a magnet former or spacer may have wire wound over its radially outer surface, which is then cut and partially peeled back, in the manner described above, to form a flexible thermal bus for cooling such components. Of course, the advantage of matched thermal expansion and contraction will not be provided in embodiments in which the cooled component is not constructed of a wire similar to that used to construct the thermal bus.

Fig. 6 shows an arrangement according to an embodiment of the present invention in which two superconducting magnet coils 10, 10' are provided, separated by a spacer 30. Each of the superconducting magnet coils 10, 10' is provided with a thermal bus 24 as described in relation to Fig. S. The cut ends 22 and adjacent parts of each thermal bus 24 are peeled away from the respective coil 10, 10' and flexed to make thermal contact with cryogen vessel 32. The thermal buses 24 may be bonded to the exterior surface of the cryogen vessel 32 by soldering, by thermally conductive resin, or by mechanical clamping. The cryogen vessel itself 32 may be cooled by boiling of liquid cryogen, operation of a cryogenic cooler, either directly or by way of a thermosiphon cooling loop, or other known cooling method.

The present invention accordingly provides a thermal bus formed from a number of turns of wire, wound around a component to be cooled, the cut turns of wire being cut axially and the turns of wire being partially peeled away from the component, thereby to form a thermal bus in thermal contact with the component.

In alternate embodiments of the present invention, the thermal bus may be formed of a number of solid metal strips. Each may be wrapped around the coil, and impregnated with resin or wax in a similar manner to the turns of wire forming the thermal bus in the examples described above. This may be found simpler to construct than layers of wire windings, and may simplify later processing steps, although the thermal matching may not be as precise as with the wire-wound thermal buses. However, a close matching may be achieved by selection of alternative materials: for example, strips of 15 aluminium may be used which provide expansion and contraction similar to that of copper wire impregnated in resin.

In further alternative embodiments, such as shown in Fig. 7, the further layer(s) of wire, or the solid strips may be 20 applied at the start of the winding, such that they are located on the radially inner surface of the finished coil or other annular component 10 to be cooled. In such embodiments, the further layer(s) of wire or strips intended to make up the thermal bus are wound first; then release layer 18 and protective layer 16; then the windings making up the coil itself are applied. The assembly may be impregnated with resin or wax; or wet wound or provided with prepreg to provide impregnation. Once the impregnation has cured or hardened, the further layer(s) of wire or strips are cut axially at the location of the protective layer 16, and the cut end 22 of the further layer(s) or strips are peeled from the radially inner surface of the coil to provide the thermal bus 12. Such embodiments may be particularly appropriate in applications where external space it at a premium, for example in the case of shield coils for magnets for MRI apparatus.

Claims (13)

1. CLAIMS1. A thermal bus (12) formed from a number of turns of wire, or solid metal strips, wound around a component (10) to be cooled, the turns of wire, or solid metal strips, being out axially and a cut end (22) of the turns of wire being partially peeled away from the component, thereby to form the thermal bus in thermal contact with the component.
2. 2. A thermal bus (12) formed from a number of turns of wire, or solid metal strips, on a radially inner surface inside an annular component (10) to be cooled, the turns of wire, or solid metal strips, being cut axially and a cut end (22) of the turns of wire being partially peeled away from the component, thereby to form the thermal bus in thermal contact with the component.
3. 3. A thermal bus according to claim 1 or claim 2, wherein the turns of wire are impregnated with a resin or wax.
4. 4. A thermal bus according to claim 3, wherein the resin or wax is removed in the vicinity of the cut end (22).
5. 5. A thermal bus according to any preceding claim, formed in thermal contact with a superconducting magnet coil.
6. 6. A thermal bus according to claim 5, wherein wire forming the thermal bus is of same dimensions as wire forming the superconducting magnet coil.
7. 7. A method for forming a thermal bus (12) on a component (10) to be cooled, comprising the steps of: i. winding a number of turns of wire, or solid metal strips, around a component (10) to be cooled; impregnating the turns of wire, or solid metal strips, with a resin or wax; cutting the turns of wire, or solid metal strips, axially; and iv. partially peeling a cut end (22) of the turns of wire, or solid metal strips, away from the component, thereby to form the thermal bus (12) in thermal contact with the component.
8. 8. A method according to claim 7, further comprising the steps of: placing a protective layer (18) over a circumferential part of the component, prior to winding the turns of wire; -in step (iii), cutting the turns of wire axially over a part of the protective layer; and - in step (iv), peeling the cut end away from the component over an extent of the protective layer.
9. 9. A method according to claim 7 or claim 8, wherein, a protection mask (16) is placed on the component prior to winding the turns of wire, and wherein the cutting of the turns of step (iii) is performed over the protection mask, whereby the protection mask protects the underlying component from damage during the cutting process.
10. 10.A method according to any of claims 7-9, wherein the component is a superconducting magnet coil and wherein the superconducting magnet coil is impregnated with resin or wax in step (ii).
11. 11. A method according to claim 10 wherein the superconducting magnet coil comprises turns of wire, the wire used in the thermal bus being of same dimensions as wire used in the superconducting magnet coil.
12. 12.A method for forming a thermal bus (12) on a component (10) to be cooled, comprising the steps of: v. providing a number of turns of wire, or solid metal strips, on a radially inner surface of an annular component (10) to be cooled; vi. impregnating the turns of wire, or solid metal strips, with a resin or wax; vii. cutting the turns of wire, or solid metal strips, axially; and viii. partially peeling a cut end (22) of the turns of wire, or solid metal strips, away from the component, thereby to form the thermal bus (12) in thermal contact with the component.
13. 13.A method according to claim 12, further comprising the steps of: - placing a protective layer (18) between a circumferential part of the annular component to be cooled and the turns of wire, or solid metal strips; -in step (iii), cutting the turns of wire, or solid metal strips, axially over a part of the protective layer; and - in step (iv), peeling the cut end (22) away from the component over an extent of the protective layer. 25 14.A method according to claim 12 or claim 13, wherein, a protection mask (16) is placed between the component and the turns of wire, or solid metal strips, and wherein the cutting of the turns of step (iii) is performed over the protection mask, whereby the protection mask protects the underlying component from damage during the cutting process.15.A method according to any of claims 12-14, wherein the component is a superconducting magnet coil and wherein the superconducting magnet coil is impregnated with resin or wax in step (ii).16. A method according to claim 15 wherein the superconducting magnet coil comprises turns of wire, the wire used in the thermal bus being of same dimensions as wire used in the superconducting magnet coil.AMWNDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS:CLAIMS1 A cylindrical superconducting magnet coil with a thermal bus (12) in thermal contact therewith, said thermal bus formed from a number of turns of wire, or solid metal strips, wound around the cylindrical superconducting magnet coil over a protective layer (18) which partially covers an outer circumferential surface of the cylindrical superconducting magnet coil, the turns of wire, or solid metal strips, being cut in a direction parallel to the axis of the cylindrical superconducting magnet coil over the protective layer, a cut end (22) of the turns of wire being partially peeled away from the protective layer, thereby to form the thermal bus in thermal contact with the cylindrical CD superconducting magnet coil.C\I 2 A cylindrical superconducting magnet coil with a thermal bus (12) in thermal contact therewith, said --20 thermal bus formed from a number of turns of wire, or (t) solid metal strips, on a radially inner surface of the -- cylindrical superconducting magnet coil, with a protective layer (18) which partially covers the radially inner surface of the cylindrical superconducting magnet coil, located between the cylindrical superconducting magnet coil and the turns of wire, or solid metal strips, said the turns of wire, or solid metal strips, being cut in a direction parallel to the axis of the cylindrical superconducting magnet coil over the protective layer, and a cut end (22) of the turns of wire, or solid metal strips, being partially peeled away from the cylindrical superconducting magnet coil, thereby to form the thermal bus in thermal contact with the component.3. A cylindrical superconducting magnet coil with a thermal bus, according to claim 1 or claim 2, wherein the turns of wire, or solid metal strips, are impregnated with a resin or wax.4. A cylindrical superconducting magnet coil with a thermal bus, according to claim 3, wherein the resin or wax is removed in the vicinity of the cut end (22).S. A cylindrical superconducting magnet coil with a thermal bus according to claim 1, wherein wire forming the thermal bus is of same dimensions as wire forming the superconducting magnet coil.6. A method for forming a thermal bus (12) on and in thermal contact with a cylindrical superconducting magnet coil, comprising the steps of: i. placing a protective layer (16) over an outer circumferential part of the cylindrical superconducting magnet coil; winding a number of turns of wire, or solid metal strips, around the cylindrical superconducting magnet coil and over the protective layer; impregnating the turns of wire, or solid metal strips, with a resin or wax; iv. cutting the turns of wire, or solid metal strips, in a direction parallel to the axis of the cylindrical superconducting magnet coil, over the protective layer; and v. partially peeling a cut end (22) of the turns of wire, or solid metal strips, away from the cylindrical superconducting magnet coil over an extent of the protective layer (18), thereby to form the thermal bus (12) in thermal contact with the cylindrical superconducting magnet coil.7. A method according to claim 6, wherein, a protection mask (16) is placed on the cylindrical superconductingCD C)J C)J1--(t) 20 -- 25 magnet coil prior to winding the turns of wire, or solid metal strips, and wherein the cutting of the turns of step (iv) is performed over the protection mask, whereby the protection mask protects the underlying cylindrical superconducting magnet coil from damage during the cutting process.8. A method according to any of claims 6-7, wherein the superconducting magnet coil is impregnated with resin or wax in step (ii).9. A method according to claim 8 wherein the superconducting magnet coil comprises turns of wire, the wire used in the thermal bus being of same dimensions as wire used in the superconducting magnet coil.10.A method for forming a thermal bus (12) on and in thermal contact with a cylindrical superconducting magnet coil, comprising the steps of: vi. placing a protective layer (16) over an inner circumferential part of the cylindrical superconducting magnet coil; vii. providing a number of turns of wire, or solid metal strips, on a radially inner surface of the cylindrical superconducting magnet coil and over the protective layer; viii. impregnating the turns of wire, or solid metal strips, with a resin or wax; ix. cutting the turns of wire, or solid metal strips, in a direction parallel to the axis of the cylindrical superconducting magnet coil, over the protective layer; and x. partially peeling a cut end (22) of the turns of wire, or solid metal strips, away from the cylindrical superconducting magnet coil over an extent of the protective layer, thereby to form the thermal bus (12) in thermal contact with the cylindrical superconducting magnet coil.11.A method according to claim 10, wherein, a protection mask (16) is placed between the cylindrical superconducting magnet coil and the turns of wire, or solid metal strips, and wherein the cutting of the turns of step (ix) is performed over the protection mask, whereby the protection mask protects the underlying cylindrical superconducting magnet coil from damage during the cutting process.12.A method according to any of claims 10-11, wherein the cylindrical superconducting magnet coil is impregnated CD with resin or wax in step (viii).C\I 13. A method according to claim 12 wherein the C\I superconducting magnet coil comprises turns of wire, --20 the wire used in the thermal bus being of same (C) dimensions as wire used in the cylindrical -- superconducting magnet coil.