GB2586493A - Method and apparatus for shimming a superconducting magnet - Google Patents

Abstract

Ferromagnetic material e.g. iron, deposited on a support surface of a superconducting magnet correcting magnetic field inhomogeneities. Iron may be welded to a bore of the magnet using the methods of Metal Inert Gas (MIG) 30 or rotary friction welding. The regions of deposited ferromagnetic material may comprise weld pools 34. The iron may be contained within a thermosetting resin. Iron may also be deposited using adhesive containing iron or pieces of iron can be bonded to the support surface. The support surface on which the iron is deposited may be the inside or outside of a bore tube 12a of an outer vacuum container, or on the outside of a bore tube thermal radiation shield or cryogen vessel. The ferromagnetic material may be applied in a pattern using a robot 32. Instructions for location and quantity of ferromagnetic material may be obtained from a computer algorithm determining where the material needs to be applied to correct for the non-uniform magnetic field.

Description

METHOD AND APPARATUS FOR SHIMMING A SUPERCONDUCTING MAGNET

The present invention relates to superconducting magnets, particularly superconducting magnets which are required to 5 produce a magnetic field of high homogeneity.

In particular, it relates to methods and apparatus for improving the homogeneity of a static magnetic field produced by a superconducting magnet.

The present invention will be particularly described with reference to a cylindrical magnet suitable for an MRI (Magnetic Resonance Imaging) system, but the present invention may be applied to other superconducting magnets.

Fig. 1 shows an axial cross-section of an example conventional superconducting magnet assembly. The assembly is essentially symmetrical about axis A-A. Directions parallel to axis A-A will be referred to herein as "axial", 20 while directions perpendicular to an axial direction will be referred to as "radial".

Main magnet coils 10 produce a static magnetic field of high strength. They are enclosed within a vacuum vessel 12 known as an Outer Vacuum Container (OVC). A thermal radiation shield 14 is interposed between the main magnet coils 10 and the OVC, to reduce the amount of thermal radiation from the OVC (at a temperature of approximately 300K) which reaches the main magnet coils (at a temperature which may be as low as typically 4K). Magnet shield coils 16 are typically also provided, radially outside of the main magnet coils 10. The magnet shield coils 16 reduce the stray magnetic field and complement the main magnet coils 10 to produce a strong homogeneous field.

The OVC is made up of an inner cylindrical wall (bore tube) 12a, an outer cylindrical wall 12b and two annular, end walls 12c, joining the bore tube 12a to the outer cylindrical wall 5 12b to form an enclosed hollow cylindrical structure.

The bore tube 12a defines a cylindrical bore 18 within which a patient may be received for MRI imaging. Also within the bore 16 is a gradient and shimming assembly 20. In the illustrated example, the gradient and shimming assembly comprises main gradient coils 22, gradient shield coils 24 and a shim assembly 26.

Main gradient coils 22 typically comprise a number of 15 resistive coils, arranged to generate time-variant magnetic fields in three orthogonal planes. They usually comprise suitably-shaped coils of copper or aluminium wire embedded in thermosetting resin.

Gradient shield coils 24 are located radially outside of main gradient coils 22 and perform a function analogous to that of the magnet shield coils 16. They complement the main gradient coils 22 to produce the required pattern of time-variant gradient fields within the bore 14a and reduce the amount of stray gradient fields which may reach the OVC bore tube 12a.

Conventionally, shim assembly 26 is located between the main gradient coils 22 and the gradient shield coils 24. It is preferable for the gradient shield coils 24 to be located radially some distance from the main gradient coils 22 to be more effective.

Superconducting magnets for MRI or other analysis techniques that need very high levels of magnetic field homogeneity require a method of compensating for magnet build tolerances which inevitably lead to a reduction in the as-built field homogeneity. Such tolerances may result in coils being a different size compared to their design size, caused for example by variations in wire size, wire lay-up, winding tension, or by coils being in a different position compared to their design positions, caused for example by machining tolerances of the coil support structure, or by inaccurate alignment of the coils during magnet assembly, or other factors.

To mitigate this problem, a shim assembly 26 is typically provided. Typically, a magnet is provided with a unique set of passive shim plates applied within the inner bore 18 of the magnet. This may be within the gradient coil and shim assembly 20 as illustrated in Fig. 1.

Shim assembly 26 conventionally contains shims, discrete flat pieces of planar ferromagnetic material, retained in position by a structure such as shim trays, as is conventional and well known in the art. Use of such shim material may be known as passive shimming.

In passive shimming, a fixed set of locations is defined, usually within the gradient coil assembly, between main gradient coils 22 and shield gradient coils 24. Each of these locations is designed to accommodate a number of ferromagnetic shim plates. The shim plates become magnetized and provide a correction effect to the inhomogeneous magnetic field generated by the superconducting magnets. A computer algorithm calculates where and how many ferromagnetic plates are required. The passive shimming solution is typically done on-site but it is possible to provide a correction for the as-built harmonics in the factory. Correction for the as-built harmonics in the factory may be known as "balancing" the magnet.

Alternatively, or in addition, shim coils may be provided to provide an electromagnetic shim solution. Superconducting shim coils may be located within the OVC with the main coils 10. Resistive shim coils may be provided external to the OVC, for example incorporated within the gradient coils 22, or gradient shield coils 24. Use of such shim coils may be known as active shimming.

The process of balancing and shimming has two distinct purposes and may be performed in two correspondingly distinct steps. Inhomogeneities may be introduced into the field of the magnet either due to variation in the manufacture of the magnet, resulting in an as-built inhomogeneity which may be corrected for in the factory, either by balancing the magnet: adjusting the locations and possibly even the turn-count of the coils and/or by active or passive shimming to compensate for remaining inhomogeneity; or due to interference with the magnetic field due to magnetic materials in the vicinity of the magnet when installed. This latter type of shimming is typically carried out on the installation site, by methods of active or passive shimming.

To perform balancing, typically a relatively small electrical current is applied to the magnet coils 10 and magnet shield coils 16. A magnetic field homogeneity is then measured, typically over a spherical volume representing an imaging volume, at the axial and radial centre of the magnet. As this is carried out with the magnet coils at room temperature, it may be referred to as "Room Temperature Plot" (RTP). The results of the measurement are fed to a computer algorithm, known in itself, which provides recommendations for balancing operations which may consist of geometric changes to the magnet itself -either turns balancing the coils, by adding 5 or subtracting individual turns or by coil displacement, moving the coils individually along their axis. Once such a balancing step has been performed, passive shimming may be applied. In a passive shimming step, shim pieces are added, as recommended by the algorithm, and the process may 10 optionally be repeated, more than once if necessary, until a satisfactory magnetic field homogeneity is achieved.

The above shimming operations may be carried out in the factory, to correct as-built inhomogeneities, and repeated 15 on-site once the magnet has been installed, to correct for inhomogeneities caused by magnetic material in the vicinity of the magnet. It may be preferred to perform factory shimming to correct as-built inhomogeity by passive shimming, then perform on-site shimming by active shimming.

In active shimming, a set of electric shim coils is provided, which may be superconducting or resistive. They are designed either to create specific harmonics (e.g. 11, 12, X2-Y2) or to create a more generic field shape, the sums of which can provide harmonic correction. The coils may be driven individually by an attached, optionally detachable, power supply to create a set of correcting field harmonics. The coils are typically resident inside the OVC 12 for superconducting coils, or inside the bore 18 for room temperature resistive coils. The current required for each coil can be either calculated by a suitable algorithm or it can be manually adjusted by one skilled in the art to provide correction harmonics.

To perform active shimming, a magnetic field homogeneity is measured, typically over a spherical volume representing an imaging volume, at the axial and radial centre of the magnet when at field. The results of the measurement are fed to a computer algorithm, known in itself, which provides recommendations for application of electrical currents to the shim coils. Electrical currents may then be introduced into the shim coils as recommended by the algorithm, and the process may optionally be repeated, more than once if necessary, until a satisfactory magnetic field homogeneity is achieved.

Both conventional shimming methods discussed above have limitations as to the number of harmonic terms they can correct for. Shimming coils may be designed to correct particular harmonics and are only capable of correcting those specific harmonics and no more. To correct further harmonic orders requires one extra set of coils for each harmonic to be corrected. Alternatively, shimming coils working in a matrix arrangement can only provide an approximate correction for a set number of harmonics since they do not create truly accurate, orthogonal field shapes themselves. Passive shims can usually correct for a higher number of harmonics but are limited by the number of locations in which a shim plate can be positioned and the number of shim plates that can be co-located. The discrete nature of the shim plates, and the discrete locations in which they can be positioned are limiting for the precision with which inhomogeneities can be 30 corrected by passive shimming.

Both described conventional shimming methods require physical space within the magnet system to be specifically allocated to them. Even though superconducting coils can be located inside of the magnet itself they may not provide a high enough number of harmonic correction terms and are typically accompanied by either a room temperature resistive coil, or passive shim arrangement. The room temperature resistive shim coil and the passive shim solutions both require physical space to be allocated within the bore 18. The passive shim solution 26 often uses a space between the main gradient coils 22 and the gradient shield cols 24, but this then provides limitations as to the structure of the gradient coil and that space might otherwise be used for other purposes, such as for mechanically stiffening the gradient coil assembly or providing extra cooling to the gradient coils.

Wherever they may be located, the passive shim plates and any associated supporting structure take up valuable space within the bore 18 that could otherwise be used for other purposes. A passively shimmed superconducting magnet that has very poor as-built field homogeneity may require a very large number of shim plates to be used, reducing the opportunity for shimming the magnet on-site if the optimal shim plate locations for shimming site harmonics are already occupied by shim plates for as-built harmonics.

The present invention addresses some of the drawbacks of the 25 known arrangements and methods for shimming superconducting magnets and provides methods and/or equipment as defined in the appended claims.

In preferred embodiments of the present invention, a method is provided for adjusting the "as built" magnetic field homogeneity achieved during the balancing step that can be applied during the build process, which is not constrained by the discrete nature of the size and position of conventional passive shims. This method ensures improved flexibility in passive shimming on site installation. The method of the present invention may also be used for shimming after assembly of the magnet structure. Preferred embodiments of the present invention also provide shim solutions with much reduced space requirements, increasing space available for other purposes.

The present invention provides both fine resolution of shim material to precisely correct inhomogeneity, which is not limited to selection of discrete shim mass or position and uses far less volume either inside the magnet structure or in the magnet warm bore, allowing for more shimming opportunities and extra space available for magnet and system design.

The above, and further, objects, characteristics and advantages of the present invention will become more apparent from the following description of certain embodiments thereof, taken in conjunction with the accompanying drawing, wherein: Fig. 1 shows, in schematic axial cross-section, an example conventional superconducting magnet assembly; Fig. 2 schematically illustrates a step in a method according 25 to the present invention; and Fig. 3 schematically illustrates a step in a method according to the present invention, being an alternative or supplementary step to the step illustrated in Fig. 2.

The invention provides magnet field shimming by applying ferromagnetic material to a suitable structure within the magnet assembly. Preferably, the added ferromagnetic material is applied in a non-discrete form, so that a precise required amount may be applied in required locations, rather than being limited to an amount which corresponds to the mass of one or more predefined shim plates, as was the case in the prior art.

In example embodiments, the ferromagnetic material for shimming may be applied to an inner or outer cylindrical surface of an OVC bore tube 12a such as illustrated in Fig. 1.

In a preferred embodiment, illustrated schematically in Fig. 2, a MIG (metal-inert gas) welding tool 30 is mounted to a robot equipment 32. The robot equipment 32 is capable of moving the MIG welding tool 30 to positions along the axis and around the circumference of the bore tube 12a. At any position, the MIG welding tool 30 may be operated to deposit an amount of ferromagnetic material at a chosen location. In an alternative arrangement, instead of a MIG welding tool, a nozzle may be provided to deposit a controlled amount of an iron-laden resin at a chosen location on the bore tube 12a.

Fig. 4 shows a yet further alternative. In the illustrated embodiment, shimming is performed by attaching pieces 42 of ferromagnetic material to a radially outer surface of bore tube 12a. The pieces 42 of ferromagnetic material may be approximately the size of a small coin: 5-10mm in diameter, and 0.5-1.5 mm in thickness. The pieces 42 may be disc shaped, as illustrated, or may be square plates, hexagonal or any appropriate shape. They may be attached to the bore tube 12a by adhesive bonding, welding, brazing, soldering or other appropriate method. In a preferred embodiment, the pieces 42 are applied by a rotary chuck performing rotary friction welding. A piece 42 is held in place on chuck 50 by an appropriate arrangement, such as magnetic attraction or clamping. Force is applied by the chuck 50 to press the piece 42 into contact with the surface of the bore tube 12a. The chuck is then rotated at speed and the friction between piece 42 and bore tube 12a causes heating at both surfaces. By appropriate selection of the force and the speed of rotation, the contacting surfaces of the piece 42 and the bore tube 12a will soften or melt. By stopping the rotation of the chuck and allowing the structure to cool, rotary friction welding of the piece 42 onto the bore tube 12a may be performed in this manner. Alternative methods of friction welding may be used, for example in which the contacting surfaces are moved linearly with respect to one another. A number of pieces 42 of ferromagnetic material may be welded, or bonded or otherwise attached to the support structure, for example bore tube 12a.

Each embodiment described above case represents an additive manufacturing technique to apply low profile ferromagnetic correction pieces, e.g. pieces 42, weld material 34, iron-loaded resin or adhesive, to a suitable structure.

Preferred embodiments of a method incorporating an embodiment of the present invention may proceed as follows: As is conventional in itself, within the factory or elsewhere, the magnetic field of the as-built magnet is measured over a spherical measurement surface. In the case of superconducting magnets for MR1 or other imaging systems, this spherical surface represents an imaging volume. This measurement allows the as-built inhomogeneity to be measured, and the corresponding harmonics to be deduced.

Some balancing adjustments may be made to the structure of the magnet, but some inhomogeneities are likely to remain. The above magnetic field measurement should be performed after any such balancing operations are completed.

II

- The harmonics representing the inhomogeneity are provided to an algorithm which creates a proposed solution consisting of a set of instructions as to where and how much ferrous material should be applied to the chosen structure, such as the OVC bore tube 12a. The calculation will take account of the various harmonics which make up the measured field inhomogeneity and the magnetic properties of the material which is to be used for shimming.

- Corresponding instructions for location and quantity of ferromagnetic material to be deposited are fed into the control algorithm for a device which applies an appropriate amount of ferromagnetic material to the surface of the chosen support structure, such that a pattern of deposited ferromagnetic material is built up on the structure which will provide adequate cancellation of the contaminant harmonics representing the magnetic field inhomogeneity. Whichever method of ferromagnetic deposition is chosen, there is a limit to the quantity which can be deposited in any given location. Of course, where optimal correction would require a large quantity of deposited ferromagnetic material in a particular location, some of that ferromagnetic material may be deposited in adjacent locations in order to provide the same shimming effect, as may be deduced by the algorithm if programmed with the appropriate maximum ferromagnetic mass in any given position.

- The structure, along with the ferromagnetic material applied thereto is then included into the magnet build to ensure that the build tolerance harmonics are corrected for.

In a preferred embodiment of a method according to the present invention, such as illustrated in Fig. 2, an additive manufacturing device is used, in the form of a computer controlled MIG (Metal Inert Gas) welding tool 30 connected to a computer-controlled robot 32. The robot 32, as controlled by the computer controller (not illustrated) moves the MIG welding tool over the radially outer surface of the OVC bore tube 12a, the MIG welding tool 30 adds a weld pool 34 of a calculated mass of ferromagnetic material at selected locations. The minimum size of weld pool 34 at any particular location is determined by the MIG wire speed and the accuracy of the computer control. The maximum size of weld pool 34 at any particular location is determined by the maximum allowable weld distortion that can be tolerated by the bore tube 12a or other selected structure, the maximum allowable height of the added weld pool, determined by the available space within the finished magnet assembly and the effects on mechanical properties of the bore tube 12a or other structure, for example in consideration of addition of a ferrous weld material to a stainless steel OVC bore tube 12a.

The weld distortion of the structure can be minimized by allowing the computer controller to make multiple short time length passes over the same area, building up the weld pool 34 at each pass until a suitable amount of weld has been applied.

The addition of ferromagnetic material according to the present invention can be done on the outer cylindrical surface of the OVC bore tube 12a as illustrated in Fig. 2 to reduce the amount of volume taken up within the OVC. The maximum allowable height of added ferromagnetic material must be defined to ensure that there is minimal impact on flexible layered insulation which is typically wrapped on the outer cylindrical surface of the OVC bore tube.

In an alternative method and arrangement, illustrated in Fig. 3, the added ferromagnetic material may be applied to a radially inner surface of the bore tube 12a, in which case the added ferromagnetic material, for example applied as a weld pool 34 will occupy an amount of volume within the bore 18.

In the example embodiments illustrated in Figs. 2 and 3, a MIG welding tool 30 is attached to a robot 32 under computer control. In such arrangements, the robot may provide axial movement 40 of the MIG welding tool 30 along the length of the bore tube. The bore tube 12a may be under rotational control from the same computer, providing circumferential motion 42. This combination provides full access to all parts of the inner and outer surfaces of the bore tube 12a. The MIG welding tool 30 is preferably itself controlled by the same computer which controls motion of the robot 32 and the rotational control, such that switch on and switch off times of the MIG welding tool 30 are controlled by the requirements for where added ferrous material needs to be deposited on to the bore tube.

Alternative embodiments would replace the MIG welder with a device that applies and adheres small pieces of ferrous material to the bore tube 12a. The size of such pieces should be chosen that they are large enough to be manageable but small enough to provide fine resolution for precise shimming.

The adherence requires a quick setting adhesive, or friction welding such as rotary friction welding as described above with reference to Fig. 4. This approach would eliminate the risk of adding ferrous weld material to the usually stainless-steel bore tube. This approach would also work with a non-ferrous bore tube. Another alternative is to replace the MIG welding tool 30 with an arrangement for applying an iron-laden thermosetting resin or iron-laden adhesive to the bore tube 12a.

In all of these alternative embodiments, a robot 32 under computer control may be used to apply the added ferromagnetic material. In such arrangements, the robot may provide axial movement 40 of an appropriate tool along the length of the bore tube. The bore tube 12a may be under rotational control from the same computer, providing circumferential motion 42. This combination provides full access to all parts of the inner and outer surfaces of the bore tube 12a. The appropriate tool is preferably itself controlled by the same computer which controls motion of the robot 32 and the rotational control, such that switch on and switch off times of the appropriate tool are controlled by the requirements for where added ferrous material needs to be deposited on to the bore tube 12a.

In further embodiments, the added ferromagnetic material is applied to an alternative component rather than the (WC bore tube 12a. For example, the ferromagnetic material for shimming may be applied to a bore tube of a cryogen vessel, in superconducting magnets where such a vessel is provided, or to a thermal radiation shield such as shown at 14 in Fig. 1, or other suitable surface.

As illustrated, the computer-controlled robot 32 and the computer controlled MIG welding tool 30, or other alternative equipment for depositing ferromagnetic material, is controlled to deposit calculated amounts of ferromagnetic material as weld pool 34 at calculated locations on the surface of the support, and to leave other locations on the surface of the support free from deposited ferromagnetic material.

In some embodiments, ferromagnetic material may be deposited on both radially inner and radially outer surfaces of the support.

In the examples illustrated in Fig. 2 and Fig. 3, the robot 32 may provide motion of the MIG welding tool 30 or other equipment for depositing ferromagnetic material 32 in an axial direction 40 only, while circumferential motion 42 of the support is provided by a rotary support (not illustrated) upon which the bore tube 12a may be mounted.

In yet other embodiments, robot 32 may not be employed. MIG welding tool 30 or other equipment for depositing ferromagnetic material 32 may be held in a static position, with both axial 40 and circumferential 42 motion provided by a rotary and translatory support (not illustrated) upon which the bore tube 12a may be mounted.

The computer-controlled robot positioning of the MIG welding tool 30 allows practically infinite variability in the position and size of the weld pool 34, therefore it can provide shimming correction for all harmonics of measured inhomogeneity in the star_ic magnetic field of the superconducting magnet.

The described MIG welding or equivalent alternative deposition of ferromagnetic material can be achieved on either the radially outer cylindrical surface of the bore tube 12a before the OVC 12 is assembled, or on the radially inner cylindrical surface of the bore tube 12a, either before or after assembly of the OVC.

In the former case, where ferromagnetic material is applied to the radially outer cylindrical surface of the bore tube 12a, the MIG welding or other process for applying ferromagnetic material would be done after measurement of the magnetic field inhomogeneities, for example en a Room Temperature Plot (RTP, described above) and before final assembly of the magnet coils into the OVC. In the latter case, where ferromagnetic material is applied to the radially inner cylindrical surface of the bore tube 12a, the MIG welding or other process for applying ferromagnetic material may be done after assembly of the magnet coils ineo the OVC, and testing. The latter case would take up some of the volume within the bore 16 which might otherwise be used to accommodate other equipment such as the gradient coil but this is likely to be a very small volume loss, being only the maximum thickness of deposited shim material since the shim plate support structure of conventional arrangements is not required.

Practically, and if the welding parameters are carefully controlled, it would be possible to apply the corrective shimming by MIG welding according to an embodiment of the invention on a cold magnet, that is, one in which the superconducting magnet coils 10, 16 have been cooled to operating temperature, such as 4K. Spot welding or friction welding may be used to apply pieces of shim material to the surface of the OVC bore tube, or elsewhere.

The present invention accordingly provides methods for shimming of superconducting magnets, either before or after assembly into an outer vacuum chamber OVC. The methods of the present invention provide for deposition of ferromagnetic materials in quantities and locations determined by plotting the magnetic field homogeneity of the particular magnet in question. Example steps for deposition of ferromagnetic material include MIG welding, deposition of iron-loaded resin, or iron-laden adhesive, or bonding of small ferromagnetic pieces, to a support surface. Such methods enable a practically infinitely variable magnetic field correction entity using very little occupied volume.

The present invention also provides bore tubes 12a carrying ferromagnetic material field correction devices as described, and superconducting magnets including bore tubes 12a carrying ferromagnetic material field correction devices as described.

In certain embodiments of the invention, the field inhomogeneity measurement, such as RTP discussed above, may automatically give rise to calculated required shim material distribution which is automatically used to direct machinery to deposit appropriate quantities of ferromagnetic material in appropriate locations on the support surface. This may be used in the factory to correct as-built inhomogeneity using an automated process. In some embodiments, the volume conventionally occupied by a passive shim assembly, 26 in Fig. 1, may be used for a conventional passive shimming arrangement to correct for site harmonics, that is to say field inhomogeneity due to effects of nearby ferromagnetic materials, rather than such conventional passive shimming arrangement also having to cope with the magnet harmonics, the field inhomogeneity caused by tolerance in the magnet build itself. The site harmonics are usually small compared to the magnet harmonics so in certain embodiments the present invention improves the opportunity for providing a shim-atfield solution. Since only the site inhomogeneity will need to be corrected, it may be found possible to perform this operation with the magnet at field using a relatively small shim mass.

Superconducting magnets built and shimmed at the factory according to an embodiment of the present invention may be described as "an agnostic site solution" in that the magnet finished and shimmed at the factory according to an embodiment of the present invention generates a magnetic field of high homogeneity, and each such magnet will act in a similar manner when installed at a given site. Any field inhomogeneity caused by ferromagnetic material on site may be dealt with by a separate shimming process. It should then be possible to site any such superconducting magnet on any site since all, or at least most, of the as-built harmonics will be corrected in the factory by a method according to the present invention.

The time spent to shim on site will be reduced to simply correcting for the site harmonics, rather than having to also correct magnet harmonics, which is the case in some conventional methods.

Claims (19)

1. CLAIMS: 1. A method for shimming field inhomogeneities of a superconducting magnet, comprising the steps of depositing ferromagnetic material on a support surface to provide a correction effect to an inhomogeneous magnetic field generated by the superconducting magnet.
2. 2. A method according to claim 1, wherein the 10 superconducting magnet is cylindrical, and the support surface is located within a bore (18) of the superconducting magnet.
3. 3. A method according to any preceding claim, wherein the superconducting magnet is located within an outer vacuum container (OVC), and the support surface is on a bore tube (12a) of the OVC.
4. 4. A method according to any preceding claim, wherein the superconducting magnet is located within an outer vacuum container (OVC), and the support surface is on a bore tube of a cryogen vessel.
5. S. A method according to any preceding claim, wherein the superconducting magnet is located within an outer vacuum container (OVC), and the support surface is on a bore tube of a thermal radiation shield (14).
6. 6. A method according to any preceding claim wherein the ferromagnetic material is deposited in a MIG welding step.
7. 7. A method according to any of claims 1-5 wherein the ferromagnetic material is deposited by deposition of an iron-laden thermosetting resin.
8. 8. A method according to any of claims 1-5 wherein the ferromagnetic material is deposited by deposition of an iron-laden adhesive.
9. 9. A method according to any of claims 1-5 wherein the ferromagnetic material is deposited by bonding of pieces of ferromagnetic material to the support surface.
10. 10. A method according to claim 9 wherein the bonding step 10 is performed by rotary friction welding.
11. 11. A method according to any preceding claim, comprising the steps of: - measuring the magnetic field of the superconducting magnet 15 to calculate an as-built inhomogeneity; - calculating a set of instructions as to where and how much ferrous material to apply to the support surface; - providing corresponding instructions for location and quantity of ferromagnetic material to a control algorithm for 20 an additive manufacturing device; - depositing the ferromagnetic material according to the instructions, such that a pattern of deposited ferromagnetic material is built up on the support surface.
12. 12. A method according to any preceding claim wherein equipment for depositing ferromagnetic material is moved to required locations for deposit of ferromagnetic material on the support surface by a robot (32).
13. 13. A method according to any of claims 1-11 wherein the support surface is cylindrical, and wherein equipment for depositing ferromagnetic material is moved axially to required axial locations for deposit of ferromagnetic material on the support surface by a robot (32), while the cylindrical support surface is rotated by a rotary support to bring a required position of the support surface into proximity to the equipment for depositing ferromagnetic material.
14. 14. A method according to any of claims 1-11 wherein the support surface is cylindrical, and wherein the cylindrical support surface is rotated and translated by a rotary and translatory support to bring a required position of the support surface into proximity to the equipment for depositing ferromagnetic material.
15. 15. A ferromagnetic material field correction device comprising a support surface for shimming a superconducting 15 magnet comprising regions of deposited ferromagnetic material on at least one surface thereof.
16. 16. A ferromagnetic material field correction device according to claim 15, wherein the regions of deposited 20 ferromagnetic material comprise weld pools (34).
17. 17. A ferromagnetic material field correction device according to claim 15 or claim 16, being cylindrical.
18. 18. A ferromagnetic material field correction device according to any of claims 15-17, being a bore tube of an outer vacuum chamber (OVC), a bore tube of a cryogen vessel or a bore tube of a thermal radiation shield (14).
19. 19. A superconducting magnet assembly comprising a ferromagnetic material field correction device according to any of claims 15-18.