Dipole field magnet

Abstract

A dipole field magnet comprising an inner volume shielded from the magnetic fields of the magnet due to the current flowing in the magnet. The dipole field magnet may comprise a plurality of ring-shaped superconductor coils configured to shield an inner volume between the ring-shaped superconductor coils from the magnetic field of the dipole field magnet. The dipole field magnet may form part of a fusion reactor and / or a magnetorquer and / or other dipole field magnet application. In some cases the dipole field magnet may comprise a plurality of ring-shaped superconductor coils configured to shield an inner volume between the coils from magnetic fields and / or the dipole field magnet may create a low field region in the inner volume.

Description

Dipole Field MagnetTECHNICAL FIELD

[0001] The present disclosure generally relates to methods of shielding a volume in a dipole field magnet from magnetic fields. In part it relates to a dipole field magnet for a fusion reactor, in which an inner volume can be shielded to allow a current driving device to operate within the inner volume.BACKGROUND

[0002] A levitated dipole reactor uses a levitating superconducting magnet to create a high magnetic field surrounding the magnet. The high magnetic field is configured to confine a plasma. The concept was first theorized by Akira Hasegawa in 1987. Under the right conditions the magnet can confine a plasma hot enough to enable nuclear fusion, constituting a fusion reactor.

[0003] Massachusetts Institute of Technology (MIT) and Columbia University co-led the Levitated Dipole Experiment (LDX) to develop a prototype of a levitated dipole reactor using low-temperature superconductor (LTS) (Nb3Sn LTS). On multiple occasions, they demonstrated successful levitation of their dipole field magnet and characterised the plasma it confined. The device used a free-floating dipole field magnet shaped as a toroid, or donut. The magnet originally had two counter-wound LTS superconductor coils, later multiple coils sandwiched together into a single ring-shaped winding, configured to carry current when superconducting. The magnet was placed in a vacuum chamber, with optional Helmholtz coils used at the edges of the chamber to produce and / or control the magnetic field. The magnet was charged using induction and raised to the centre of the chamber using physical supports. An external field was then used to suspend the dipole field magnet, and an external microwave system used to generate the plasma around the dipole field magnet. The levitated dipole experiment and their LTS superconductor are not suitable for fusion relevant fields, or high temperature superconductors (HTS) due to the superconductor materials used and system structure.

[0004] The University of Tokyo used the Ring Trap-1 project to further the understanding of dipole confinement and used HTS (Bi-2223 HTS) to improve performance of levitated dipoles. However, the Ring Trap-1 project was also not scalable to fusion reactors due to the HTS materials used.

[0005] Figure 1 shows a cross-section of one side of the magnet used in the LDX system. A helium vessel, configured to be cooled to LTS operating temperatures, is supported inside a small vacuum vessel. Inside the helium pressure vessel, a magnet winding pack formed by a plurality of coils sandwiched together into a winding pack is formed as a volume of LTS extending around the magnet. In use, the magnet winding pack would be superconducting and carry a current of up to 2300 A.

[0006] The previous levitating dipole reactors have not been further developed. One challenge is powering or driving the current within a high temperature superconducting magnet. It is the intention of the present invention to describe a dipole field magnet, for example for a levitating dipole reactor, that substantially mitigates the above product limitations or at least provides a choice.SUMMARY

[0007] The prior art provides dipole field magnets. However, it is advantageous to improve the dipole field magnet to allow their use to scale to fusion reactor environments and operate with suitable superconductor materials and / or to improve the overall performance of the dipole field magnet.

[0008] In a first aspect the invention may broadly be said to consist in a dipole field magnet comprising: a plurality of ring-shaped superconductor coils configured to shield an inner volume between the ring-shaped superconductor coils from the magnetic field of the dipole field magnet. The ring-shaped superconductor coils may be located or arranged relative to one another so that current running through the superconducting coils results in a low field region in the inner volume, effectively shielding the inner volume from the magnetic field.

[0009] In some cases the inner volume is ring-shaped. In some cases the inner volume is toroidal. In some cases the superconductor coils are co-axial. In some cases the superconductor coils are axially spaced. In some cases the inner volume extends between a coil with a smaller diameter and a coil with a greater diameter. In some cases at least two of the coils have the same cross-section dimensions. In some cases the plurality of coils are the same height. In some cases the plurality of coils are symmetric above and below a radial plane. In some cases a greater volume of coils are disposed towards the inner perimeter of the magnet than the outer perimeter of the magnet. In some cases the plurality of coils comprises at least four coils. In some cases the superconductor coils comprise resistive joints or connections.

[0010] In some cases at the plurality of coils are configured to carry current in the same direction. In some cases the magnetic field of the dipole field magnet is produced by current flowing in the ring-shaped superconductor coils. In some cases the coils are disposed and / or configured by the positions and / or orientations of the coils. In some cases the plurality of coils form a substantially circular and / or elliptical cross-section. In some cases at least two of the plurality of coils are spaced apart. In some cases the coils are spaced apart by a spacer. In some cases the superposition of currents in each of the plurality of coils shields or reduces the magnetic field region in the inner volume.

[0011] In some cases each of the plurality of coils comprises a plurality of layers of superconductor. In some cases each of the plurality of coils comprises a plurality of turns, each turn comprising a plurality of layers of superconductor. In some cases each of the plurality of turns and / or layers is arranged in the same orientation. In some cases the turns and / or layers are orientated radially. In some cases the turns and / or layers are separated by spacer layers. In some cases the turns and / or layers are separated by support layers. In some cases the thickness of the spacer layers and / or support layers is variable between different coils. In some cases the superconductor comprises at least one of: superconductor tape and a superconductor wire. In some cases the superconductor comprises a high temperature superconductor. In some cases each of the plurality of coils is substantially rectangular in cross-section.

[0012] In some cases comprising a device disposed within the inner volume. In some cases the device is a device for driving current in the plurality of coils. In some cases the device inductively charges the superconductor coils. In some cases the device comprises a flux pump. In some cases comprising a magnetic field shield between the plurality of coils and the inner volume. In some cases the magnetic field shield comprises a ferromagnetic material, optionally iron. In some cases the magnetic field shield is configured to reduce inductive coupling between coils on opposing sides of the magnetic field shield. In some cases the magnetic field shield has a variable thickness, optionally the thickness configured to be more think in areas of high field, in use. In some cases the magnetic field shield and the plurality of coils are configured to work in conjunction to shield the inner volume between the ring-shaped superconductor coils from the magnetic field of the dipole field magnet.

[0013] In some cases comprising a housing surrounding the plurality of coils. In some cases the device comprises a magnetically susceptible portion. In some cases the devicecomprises an iron core. In some cases the shielding is sufficient to prevent saturation of at least one of the magnetically susceptible portion and the iron core.

[0014] In a further aspect the invention may broadly be said to consist in a reactor chamber comprising a dipole field magnet as described herein and / or in the other aspects.

[0015] In some cases comprising: a vacuum vessel; and a levitation device for levitating the dipole field magnet. In some cases the reactor chamber is configured to produce a plasma between the outer perimeter of the magnet and an inner wall of the reactor chamber.

[0016] In a further aspect the invention may broadly be said to consist in a dipole field magnet comprising: a plurality of ring-shaped superconductor coils disposed to create an inner volume between them, and a device within the inner volume.

[0017] In some cases the device is a device for driving current in the plurality of coils. In some cases the device inductively charges the superconductor coils. In some cases the device comprises a flux pump. In some cases comprising a magnetic field shield between the plurality of coils and the inner volume. In some cases comprising a housing surrounding the plurality of coils.

[0018] In a further aspect the invention may broadly be said to consist in a dipole field magnet comprising an inner volume disposed within a ring-shaped superconductor, the ringshaped superconductor configured to shield the inner volume from the magnetic field of the dipole field magnet.

[0019] In some cases the ring-shaped superconductor comprises a plurality of ringshaped coils. In some cases comprising a device within the inner volume. In some cases the inner volume is toroidal. In some cases the ring-shaped superconductor is a circular and / or substantially circular and / or elliptical and / or substantially elliptical cross-section.

[0020] In a further aspect the invention may broadly be said to consist in a dipole field magnet comprising: a plurality of ring-shaped superconductor coils configured to provide a lower magnetic field region between the ring-shaped superconductor coils than outside of the ring-shaped superconductor coils when current flows in the ring-shaped superconductor coils.

[0021] In a further aspect the invention may broadly be said to consist in a dipole field magnet comprising: an inner volume disposed within superconductor coils, the superconductor coils configured to provide a lower magnetic field region within the inner volume than outside of the superconductor coils when current flows in the superconductor coils.

[0022] In a further aspect the invention may broadly be said to consist in a ring-shaped superconductor surrounding an inner volume and configured to shield the inner volume from a magnetic field.

[0023] In some cases the ring-shaped superconductor comprises a plurality of ringshaped coils. In some cases comprising a device within the inner volume. In some cases the inner volume is toroidal. In some cases the ring-shaped superconductor is a circular and / or substantially circular and / or elliptical and / or substantially elliptical cross-section. In some cases the shielding of the inner volume is dependent on and / or proportional to a current flowing in the superconductor coils. In some cases the inner volume is ring-shaped.

[0024] In a further aspect the invention may broadly be said to consist in a levitating dipole reactor comprising: a dipole field magnet configured to levitate, the dipole field magnet comprising an internal current driving device for driving in the superconductor dipole magnet. In some cases the current driving device is surrounded by a plurality of ring-shaped superconductor coils.

[0025] In a further aspect the invention may broadly be said to consist in a dipole field magnet comprising: a plurality of ring-shaped superconductor coils disposed about an inner volume and configured to provide a low magnetic field region in the inner volume relative to outside of the dipole field magnet.

[0026] In some cases the inner volume is formed between the inner diameter of the largest diameter superconductor coil and the outer diameter of the smaller diameter superconductor coil. In some cases the inner volume is formed between the highest superconductor coil and the lowest superconductor coil along the axial axis.

[0027] In a further aspect the invention may broadly be said to consist in a dipole field magnet comprising a plurality of ring-shaped superconductor coils configured to shield an inner volume between the coils from magnetic fields generated by the dipole field coil.

[0028] In a further aspect the invention may broadly be said to consist in a superconductor comprising: a plurality of ring-shaped superconductor coils disposed about an inner volume, wherein the superconductor coils are configured to shield the innervolume from magnetic fields when current is flowing in the superconductor coils. In some cases the coils are ring-shaped. In some cases comprising a current driving device for driving current in the superconductor coils

[0029] Features from one or more embodiments or configurations may be combined with features of one or more other embodiments or configurations. Additionally, more than one embodiment or configuration may be used together in a dipole field magnet, or other system requiring or using a dipole field.

[0030] As used herein the term "(s)" following a noun means the plural and / or singular form of that noun.

[0031] As used herein the term "and / or" means "and" or "or", or where the context allows both.

[0032] The term "comprising" as used in this specification means "consisting at least in part of". When interpreting each statement in this specification that includes the term "comprising", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "comprises" are to be interpreted in the same manner.

[0033] It is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example, 1, 1.1 , 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7) and, therefore, all sub-ranges of all ranges expressly disclosed herein are hereby expressly disclosed. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner.

[0034] This disclosure may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more said parts, elements or features.

[0035] Where specific integers are mentioned herein which have known equivalents in the art to which this disclosure relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

[0036] The disclosure consists in the foregoing and also envisages constructions of which the following gives examples only.BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Specific embodiments and modifications thereof will become apparent to those skilled in the art from the detailed description herein having reference to the figures that follow, of which:

[0038] Figure 1 shows a prior art dipole field magnet in cross section.

[0039] Figure 2 shows a dipole field magnet levitating in a vacuum vessel, in cross section.

[0040] Figure 3 shows a magnetic field around a dipole field magnet.

[0041] Figure 4 shows magnetic flux around a dipole field magnet.

[0042] Figure 5 shows a dipole field magnet with a plurality of coils

[0043] Figure 6 shows a magnetic field around the dipole field magnet of Figure 5.

[0044] Figure 7 shows a magnetic flux around the dipole field magnet of Figure 6.

[0045] Figure 8 shows a cut-away view of the dipole field magnet of Figure 7, including structural support.

[0046] Figure 9 shows the dipole field magnet of Figure 8.

[0047] Figure 10 shows the dipole field magnet in a housing, with reference axes shown.

[0048] Figure 11 shows a selection of coils from the dipole field magnet of Figure 8 and an enlarged view of the winding of a coil.

[0049] Figure 12 shows the magnetic field of an alternative dipole field magnet in cross section.

[0050] Figure 13 shows the flux of an alternative dipole field magnet in cross section.

[0051] Figure 14 shows a plan view of a dipole field magnet in cross-section with devices in portions of the inner volume.

[0052] Figure 15 shows an inner volume of a dipole field magnet comprising a single pocket.

[0053] Figure 16 shows an inner volume of a dipole field magnet with a plurality of adjacent pockets.

[0054] Figure 17 shows a dipole field magnet with a plurality of coils and an example shield.DETAILED DESCRIPTION

[0055] Figure 2 shows a dipole field magnet 1 in a levitated position inside a vacuum vessel 26. A levitation device 21 , such as a fixed coil comprising an electromagnet may be arranged above the dipole field magnet 1 to support the dipole field magnet 1 in position. The vacuum vessel 26 may be arranged on supports 25 such as legs to allow access below the vacuum vessel 26. The vacuum vessel 26 may have an opening 27, such as a portal, to allow the dipole field magnet 1 to be inserted and / or removed from the vacuum vessel 26. Although shown in the base of the vacuum vessel 26 it may alternatively be located on the side or top of the vacuum vessel 26. In some cases, the vacuum vessel 26 has at least one magnet support (not shown). The magnet support provides the dipole field magnet 1 with a resting position when not levitating, so as the dipole field magnet 1 does not drop to the bottom of the vacuum vessel 26. This may be achieved by passing through the middle opening of the magnet 1. The magnet support(s) may also restrict upwards and / or sidewards movement of the dipole field magnet 1.

[0056] The dipole field magnet 1 comprises superconductor coils 50. The superconductor coils 50 are configured to carry a current while superconducting so as to create a dipole field, shown by magnetic flux lines 24 around the dipole field magnet 1. The superconductor coils 50 are orientated to extend radially about a central axis of the magnet 1 . The dipole field 24 can be used to constrain a plasma, for example for nuclear fusion. In a complete fusion reactor additional equipment may be used. For example: plasma sources excitement equipment, such as resonant microwave heating; and energy capture devices. The energy capture device may comprise a heat transfer system in or near the wall of the reactor chamber. The heat transfer system is configured to capture the neutrons generated by the fusion reactor and convert their energy into heat which can then, for example in a steam turbine, be converted into power. Additional coils or windings (not shown), such as Helmholtz windings may be used to shape the magnetic flux lines 24.

[0057] Figure 3 shows the magnetic field 32 around a coil 30 inside the housing 31 of a dipole field magnet 1 . Figure 4 shows the flux lines centralized on the coil 30 and expanding outwards radially. The view is shown in cross section with the origin 33 representing the centre of the toroid formed by the dipole field magnet 1. During operation of the dipole field magnet 1 a high magnetic field 32 is produced. This high field will affect all magnetic and / or magnetically susceptible objects at or near the dipole field magnet 1 , both inside and outside of the housing 31. As illustrated in Figure 3 the magnetic field has a strength of at least 0.4Tesla surrounding the dipole field magnet 1, although the particular strength will depend on size and nature of the system.

[0058] The high magnetic field can cause deleterious effects to devices at or near the magnet 1. For example, an iron core will saturate in a high magnetic field, limiting the ability to use transformers. Similarly, any electronic device may be affected, particularly with dynamic fields but even static fields will require heavy shielding, cause flux loss in transformers and inductors and trigger relays, for example. While heavy shielding can prevent or ameliorate some of the effects of the high fields the extra weight makes the use in many systems, such as a levitating dipole, difficult. Moreover, the amount of shielding required may not scale well with increasing size.

[0059] Figure 5 shows a cross section example dipole field magnet 1 where a plurality of coils 50 are used. Although shown in cross section each coil 50 is ring-shaped (as the cross section is rotated about the origin 33). The use of a plurality of coils 50 allows an inner volume 51 to be formed within the coils 50. The innervolume 51 may have a physical boundary marker such as a shield 52, optionally made of iron, or other housing. However, this shield or housing is not required.

[0060] Arrangements of the ring-shaped superconductor coils 50 around the inner volume 51 may shield the inner volume 51 from at least one of, or both of, the magnetic fields produced by the superconductor coils 50 and external magnetic fields. Creating a low field region within the magnet 1 , by separating the magnet into a plurality of ring-shaped superconductor coils 50 and spacing the coils 50 about an inner volume 51, allows devices to be located in a low field region formed in the innervolume 51, even when a substantial external magnetic field is present. The inner volume 51 , which may be referred to as a low field region, has a low magnetic field in comparison to the magnetic field outside the ring-shaped coils 50 or the magnet 1 itself, when current is present in the coils 50. The low magnet magnetic field may be less than ten percent of the external field, less than 1 percent of the external field, or below a field strength required for devices within the magnet 1 .

[0061] The number and arrangement of rings of superconductor coil 50 is variable. In some cases, an electromagnetic simulation may be required to determine a suitable arrangement to provide enough shielding to a large enough inner volume 51. COMSOL Multiphysics™ developed by COMSOL Inc. is an example of a suitable simulation program. In a simplest arrangement two ring-shaped superconductor coils 50 might be used, arrangedclose enough together to create a small inner volume 51 directly between them in which a low magnetic field could form, shieled by the two coils 50. However, the addition of further ringshaped coils 50 will improve the shielding from external magnetic fields and / or increase the size of the inner volume 51 able to be shielded.

[0062] In some cases, the dipole field magnet 1 may be formed by a single ring, or homogenous superconductor structure which is shaped to provide an inner volume 51. For example, the inside of a unitary or homogenous superconductor toroid could be hollowed out, or a superconductor toroid could be manufactured to surround an inner volume 51. Alternatively, a single coil 50 could be spiralled to form a path around the magnet, for example tracing the perimeter of a toroid. In these cases, the inner volume 51 remains substantially surrounded by superconductor, but the superconductor is not formed from a plurality of separable parts.

[0063] Where ring or ring-shaped is used herein it refers to the coil 50 forming a loop around a centre 33 of the magnet 1. While shown in the Figures as substantially circular the coils 50 may have different geometry and be symmetric or asymmetric. Forming a loop around the centre 33 allows the dipole field to form and allow current to flow continuously when superconducting in the coils 50.

[0064] The selection and position of coils 50 may be a compromise between the amount of superconductor required and the quality or extent of the inner volume 51 shielded. The shielding effect may be considered analogous to protecting the inner volume 51 from a flexible sheet: Considerthe external magnetic field as the flexible sheet attempting to penetrate into the inner volume 51. The coils 50 prevent the penetration of the flexible sheet. A continuous layer of superconductor around the internal volume would prevent any penetration of the flexible sheet. However, this would use a large amount of superconductor and be difficult to construct. For example a plurality of abutting ring-shaped superconductor coils 50 is possible but would increase cost and weight of the superconductor used. Increasing the spacing between and / or location of adjacent coils 50 reduces the amount of superconductor required. With careful selection of coil 50 positioning the spacing and / or location can be configured to still protect the inner volume 51 from the penetration of the flexible sheet. Following this analogy the arrangement of the coils 50 around the inner volume 51 can be configured to provide a low field region in the inner volume 51 because of the shielding provided by the coils 50.

[0065] Current flowing in the ring-shaped superconductor coils 50 generates a magnetic field, as well as shielding the inner volume 51 from external fields. Magnetic fields obey the superposition principle, and as such, opposing magnetic fields can cancel each other out. When a current is flowing in the plurality of coils 50 the coils will each produce a magnetic field. Due to the arrangement of the coils around the inner volume 51 the magnetic fields produced by the plurality of coils 50 may superimpose on each other in the inner volume 51 to create a region of low magnetic field. This leads to the coils 50 shielding the inner volume 51 from the magnetic field, producing a relatively low field region. As the shielding is caused by the current flow in the coils the shielding is dependent on and / or proportional to the current. This means that a greater current may result in greater shielding, allowing the system to scale effectively.

[0066] The current in the superconductor coils 50 may flow in the same direction in each, or at least a majority of the coils 50. The direction is about the centre 33 of the coils. However, in some circumstances shim coils, or coils with current flowing in the opposite direction may also be present, for example to help control the field or correct for homogeneity in the magnetic fields.

[0067] The plurality of coils 50 are configured to shield the inner volume 51 from magnetic fields but are able to shield much greater amounts of field than, for example, a physical shield such as iron and may be significantly lighter. The amount of shielding attained may be dependent on the amount of current flowing in the coils 50, meaning that the shielding effect can scale with the size of the magnet. 1.

[0068] An advantage of this system is that, because the inner volume 51 is shielded from the magnetic field 32, it has a relatively low magnetic field with respect to outside of the magnet 1. This enables devices, such as electronic devices, magnetically susceptible devices, and / or current driving devices to be placed inside the inner volume 51 and to operate without being affected by the potentially large magnetic fields generated by the dipole field magnet 1. This means that iron, such as any transformer cores, present within the magnet 1 will be able to operate without saturation due to external magnetic fields. This is particularly relevant for fusion reactors due to the field strengths required when scaling to fusion reactors.

[0069] In some cases, a device 130 for driving current in the superconductor coils 50 is positioned in the inner volume 51. The device 130 may be a current driving or charging device and may use induction to transfer power from a power source to the superconductor coils 50.The flux pump may have a first portion comprising an energy source inductively coupled to a superconducting portion. The superconducting portion is attached or attachable to the superconductor coils 50. The current driving device may be a flux pump or superconductor current driving devices configured to drive current into the superconductor coils 50. Alternative devices 130 such as energy storage and / or control devices may also be stored in this inner volume 51. Having the device 130 or current driving or pumping device close to the coils 50 allow simple and efficient driving current into the coils of and / or connections to the coils 50, while the current flow through the coils 50 causes shielding of the inner volume 51 from magnetic fields, resulting in the magnetic field within the inner volume 51 to be reduced, relative to the magnetic field outside of the superconductor coils 50. Having a device 130 for driving current in the superconductor coils 50 allows for a wider variety of superconductors to be used. For example, HTSs often require resistive joints between coils or coil sections. AN example is HTS tapes made with rare-earth elements (ReBCO) The presence of resistance leads to reduction in current in the coils 50. This can require maintenance or ramping of the current. Driving additional current in the coil(s) 50 by using, for example, a flux pump, allows the current (and therefore the dipole field) to be maintained for longer. Advantageously the present system allows the current driver, such as a flux pump, to be stored on the magnet in a low- field region, allowing easier operation and / or reducing connections to the levitating magnet 1 .

[0070] Figure 6 shows the magnetic field (shown by magnetic field levels 32) surrounding the magnet 1 of Figure 5 when current is flowing through the coils 50. Figure 7 shows the corresponding magnetic flux (shown by flux lines 40). Figures 6 and 7 show that inner volume 51 has been created within the ring-shaped coils 50 in which there is substantially no magnetic field. The inner volume 51 may be considered as forming a ring-shape in the centre of the coils 50 - between the smallest and largest diameter coils. This inner volume 51 extends in a ring-shaped or toroidal path around the centre 33 of the magnet 1. Figure 6 shows the inner perimeter 61 and outer perimeter 62 of the magnet 1 .

[0071] The particular shape of the inner volume 51 will depend on the arrangement of the ring-shaped coils 50. In the Example of Figure 6 and 7 fourteen coils 50 are used, however the number of coils 50 may be varied. For example, depending on the size and shielding required in the inner volume 51 . There may be at least four coils 50, at least six coils 50, at least eight coils 50 or at least ten coils 50. A greater volume or number of coils 50 may be arranged towards the inner perimeter 61 of the magnet 1 than the outer perimeter 62 of the magnet, asthe inner perimeter is where the majority of the current will flow in the coils 50 due to the toroidal shape. The ring-shape of the superconductor coils 50 means that a greater current density will occur on the inner portion of each coil 50, and the inner perimeter 61 of the magnet 1 as a whole. By increasing the volume of superconductor in the inner coils 50 (or inner perimeter 61 of the magnet) a greater current can be supported and the magnetic shielding of the inner volume 51 is improved. In some cases the magnet 1 may comprise at least one coil arranged in each thirty degree segment, in each twenty degree segment, or in each ten degree segment. In some cases there may be wider gap in at least one segment of the magnet, so as the coils 50 form a 'C' shape in cross-section instead of a circle or ellipse.

[0072] Figures 6 and 7 show this higher density of coils 50 in the inner perimeter 61 of the magnet 1 and then spaced apart coils 50 around the outer perimeter 62 of the magnet 1 . The size and / or spacing of the coils 50 may depend on the required shielding, with larger spacing between the coils 50 reducing the shielding. Spacing the coils 50 apart from one another reduces the amount of superconductor required to form the magnet 1 and allows the shape of the inner volume 51 to be controlled. The spacing creates gaps in the axial and / or radial directions. The spacing may be a trade-off between reducing the amount of superconductor required and ensuring adequate shielding of the inner volume 51.

[0073] Figure 8 shows a cutaway view of a 3D model of the magnet 1 shown in Figures 6 and 7. The inner volume 51 is shown with an additional shield 52 to reduce any remaining magnetic fields. The shield 52 may be made of iron, a ferromagnetic material, or other material configured to reduce magnetic fields. The shape of the shield 52 may follow the shape of the device or devices within the inner volume 51 . In other cases, the shield 52 may be replaced by a housing for the device(s) 80 within the inner volume 51 .

[0074] Figure 17 shows an alternative example of the shield 50. Again the coils 50 substantially surround the shield 52. One or more devices are protected within the inner volume 51 , which may be at least partially or fully defined by the shield 50. The inner volume may sit within the shield 50. The shield is shown with a variable thickness. The thickness may be varied so as to have a greater reduction of magnetic flux in areas where the magnetic field is greater during use of the magnet 1 . However, because the field strength at which the shield 52 saturates is independent of thickness the shield material must be chosen to have a sufficient saturation level. Figure 17 shows a thicker region 522 at the top of the shield 52. A thicker region 521 is also shown on the inner circumference of the shield 52. The thicker regions maybe otherwise positioned, depending on the high flux regions. The thicker region 521 is shown as a connection between two pieces of the shield. However, the shield may be substantially continuous, orthe pieces may be connected in a different location. The location may be chosen for ease of assembly, for example.

[0075] In some cases the shield 52 may provide a substantial field reduction. For example the coils 50 could be reduced or repositioned, leaving a larger field in the inner region 51 than desirable. However the shield 52 could be used to attenuate this larger field to a suitable level. The shield 52 would need to be thick enough to reduce the level of magnetic flux within the shield 52 over the operation cycle. In some cases, this example could balance a weight of the shield 52 against a complexity or amount of superconductor required in the coils 50. In such an example the low field region could be provided by a combination of the coils 50 and the shield 52. In some cases a thicker (or more absorbent, or more absorbent material) shield 52 could balance fewer coils 50. To provide a shielding effect the shield 52 material is chosen to have a saturation point above the expected maximum field of inner volume 51 .

[0076] Additionally the use of the shield 52 also maintains the field shape during charging of some coils 50. In particular, where coils 50 are non-insulated. Non-insulated coils do not have insulation between layers, or each layer of superconductor. Non-insulated coils 50 allow radial currents through the turns of the windings in coils 50. This additional conduction path allows the coils 50 to sustain an error current superimposed onto the operating current of the magnet 1. The error current can be sustained because of the additional current paths between the turns of the windings as well as along the windings. The operating current of the magnet is parallel to the winding of each of the coils 50, independent of the currents present in other coils. The error current can be induced by changing field conditions, for example during charging of the magnet 1. The error current in a coil 50 can cause the coil 50 to operate far outside of its design specification which in turn may increase the field strength in the low field region. Therefore, there is the potential of damaging the devices 130 within the inner volume 51 or affecting the performance of the devices 130 during these transient periods such as during charging.

[0077] The presence of a shield 52 of ferromagnetic material (such as iron, which saturates at about 1 -2 Tesla), or at least a field absorbing media within or about the inner region 51 can reduce these error currents. This is because the shield 52 redirects a portion of the field generated by the coils so reducing coupling (mutual inductance) between coils onopposing sides of the magnet. The reduction in mutual coupling means there are smaller error currents in the coils and therefore, less overall field in the inner volume 51. Alternatively an insulated coil 50 could be used. However, insulated coils 50 are less mechanically stable and are less resilient to superconducting 'quench' events. The mechanical stability may be reduced because of the filling medium (typically wax or epoxy) is often weaker than the other coil materials. In some cases a shield 52 may be located at least between antipodal coils arranged symmetrically on either side of a radial plane, as shown in Figure 5. Although a continuous shield 52 is shown in Figure 5 and Figure 17 in some cases the shield is comprises one or more openings. In some cases the shield 52 may be arranged to be thicker near the locations of coils 50 to better ameliorate the field from the coils 50.

[0078] Supports 81, 82, 84 are used to position the coils 50 in place and secure them, particularly due to forces when the magnet is operating. In this case the supports comprise metal sections 81 for strength (which may comprise steel), insulating sections 82 to control temperatures or thermal flow. The insulating sections 82 may comprise fiberglass, such as G10. Electrically conductive interconnects 84, which may comprise copper, are also present to connect coils 50. Figure 8 shows these layers 81, 82, 84 may be connected, such as by using fasteners such as bolts 83. The supports 81 , 82, 84 should be designed to tolerate the expected forces on the coils 50 during operation. These may include forces pushing out from the centre of the magnet, as well as forces pushing vertically to crush the coils 50 together. A superconductor current pump 80 is shown within the inner volume 51 . Other devices 130 may also or alternatively be located in the inner volume 51 . The devices may extend around the full inner volume 51, but typically it will only occupy a segment of the inner volume 51. Further devices 130 or equipment may be placed in the remaining inner volume 51 .

[0079] Figure 9 shows the magnet 1. The current pump 80 is visible in a segment of the ring-shaped magnet 1. Although shown as a circular ring the magnet 1 may have an alternative shape. For example, the coils 50 could be oval, an ellipse, more square or substantially square, depending, for example, on the application of the magnet 1. In some cases, the magnet 1 has a variable diameter: where the diameter expands to create a large inner volume 51 to hold the device(s), then reduces in other segments where no devices are required. The diameter reduction may be of the inner perimeter 61 , the outer perimeter 62 or both.

[0080] Figure 10 shows that, in use, the magnet may be arranged in a housing 31 surrounding the magnet 1. The housing may be used to contain a coolant within the magnet 1 and / or prevent damage to the magnet 1 . Figure 9 also shows reference axes for the magnet 1 , with the axial or height axis z through the centre of the magnet, a radial axis R extending horizontally away from central axis and an azimuth or angular axis cp.

[0081] Figure 11 shows a close-up cross section of the coils 50 on the inner side of the magnet 1 shown in Figures 6 and 7. The coils 50 may be formed by any suitable method. In this example they are formed by turns 110 of superconductor, each turn comprising at least one layers 1 14 of superconductor (e.g., HTS) tape, which is wound to form the ring-shaped coils 50. Spacer layers 1 11 , for example made of metal such as brass or stainless steel, may be arranged between adjacent superconductor layers 1 14 to control the characteristics of the coil 50. In this example the two innermost coils 50 have a greater spacer layer thickness than the remaining coils 50, however this is not necessary. The use of a tape may lead to the coils 50 having substantially the same height, as shown in Figure 1 1 . The coils may have a border or housing 1 17. The housing 117 may be electrically conductive, for example made of copper. The housing 117 may electrically connect the individual turns 1 10 and / or layers 1 14 of the coil. Support layers 115 may separate adjacent layers of superconductor tape to mechanically stabilise the coil 50. Support layers 115 may be made of epoxy resin, other electrically insulating material, solder or other electrically conducting materials. Using an electrically conducting material such as solder for the support layers 115 reduces the turn-to-turn contact resistance of the coils compared to, for example, epoxy resin, and / or provides optional current paths around damaged superconductor, if present.

[0082] Other arrangements for forming the coils are possible including: "Dry-wound", where there is no material between the layers 114 and the adjacent superconductor layers 114 (optionally coated) are directly in contact; "Metal-insulated", where a metal strip is co-wound between the superconductor layers 1 14; and conductive epoxy. The superconductor tape may comprise superconducting material (for example YBCO) sandwiched between multiple layers of material. The multiple layers of material may include copper on both sides of the tape. An insulated coil 50 would add one or more insulating layers between each superconductor layer 1 14 or between each turn 1 10 of superconductor layer. Insulating layers may comprise wax or epoxy, for example. Other superconductor tape arrangements or constructions may also be used.

[0083] In some cases, the length of each coil 50 is configured to shape and / or size the inner volume 51. Alternatively, different heights of tape and / or coils 50 may be used. The choice of coil height may be based on the tape height (e.g. 12mm). However, in other cases the height of each coil may be configured to shape and / or size the inner volume 51 . Alternative materials may be used to form the coils 50, for example superconductive tape, or wire or other superconductive material. The turns 1 10 and / or layers 114 of the coil are shown having a constant orientation or angle. The illustrated orientation arranges each layer radially. In some cases, the coils 50 may be curved or shaped so as the turns 1 10 and / or layers 1 14 vary in orientation. This may be used to shape the innervolume 51. The use of tape, in these examples, leads to substantially rectangular coils 50 in cross section, however different shapes of coils 50 may be used such as ellipsoidal or circular.

[0084] Figure 12 shows the magnetic field 32 of an alternative example magnet 1 using ring shaped coils 50. Figure 13 shows the magnetic flux lines 40 of the same alternative magnet 1. In this case a half cross section is shown, with the lower half being a mirror image. This example uses ten coils 50 per half (20 total coils) to produce a greater amount of shielding, but also use more superconductor than the previous example. Similar to the previous example there is a greater amount of superconductor towards the inner perimeter of the magnet 2. At least some of the superconductor coils 50 overlap. That is to say that the inner radius of a coil is smaller than the outer radius of an adjacent coil. For example coils 501 and 502 overlap as the inner radius of coil 501 is smaller (i.e., closer to the centre 33) than the outer radius of coil 502. In some cases the overlap may also or alternatively be along the z-axis (i.e., vertically).. In some cases, additional superconducting coils 50 could be added to the magnet, each filling one of the remaining gaps of Figure 12 and 13 to further surround the inner volume 51 and / or provide additional shielding. As shown in Figure 8 at least some of the coils 50 may abut or substantially abut one another, forming a layered or sandwiched configuration. The coils 50 may be separated a distance less than their height. The separation distance may be less than a quarter or a half of their height. The magnet 1 may comprises a plurality of spaced apart coils 50 and a plurality of substantially abutting coils 50.

[0085] Figure 6, 7 and 12 and 13 show cross sections of uniform annular coils 50 with circular inner and outer perimeters. However, the ring-shaped coils 50 are not limited to this shape. In some cases, the coils 50 may be oval or elliptical. The ring-shaped coils 50 may be squarer, or substantially square. The ability to vary the shape of the coils 50 will depend on theuse of the magnet 1- where symmetry and balance is required the annular version shown may be advantageous. Similarly, the coils 50 may not be vertically symmetrical; the coils 50 in the upper half of the magnet 1 may be different, for example in number or arrangement from the coils 50 in the lower half of the magnet 1 . In some cases the magnet 1 is a toroid or toroidal.

[0086] The use of different coils 50 may result in a magnet 1 that is not substantially toroidal. That is, the circumferences of the toroid may not be circular and / or the cross section of the body of the toroid may vary radially. Where a toroid is discussed herein it should be understood as referring to the broad concept of a ring-shaped object, such as a tube extending around a central axis. The tube is hollow or partially hollow to form the inner volume 51 . The inner volume 51 formed by the plurality of coils 50 should also be understood to be central with respect to being within the plurality of coils 50, so within the toroidal structure. It is surrounded vertically by coils 50 highest and lowest on the axial axis (z) and horizontally by the smallest and largest radius coils 50, or further separated on the radial axis (R).

[0087] Figure 14 shows an outline of the innervolume 51 of the magnet 1 as an annular ring or toroidal volume in cross section. Figure 14 is a plan view looking down from above the magnet 1 showing only the inner volume 51. The inner radius 134 and outer radius 135 are shown measured from the centre 33 of the magnet 1. The devices 130, such as a current driving device 80 are arranged within this inner volume 51 . In some cases, they extend around the full inner volume 51. However, as shown in Figure 13 there may be a plurality of devices 80, 130 each arranged separately in the innervolume 51. For example, devices such as batteries, current driving devices, and control devices may be within the inner volume 51 .

[0088] Figure 15 shows an alternative arrangement wherein the diameter of the inner volume 51 is variable about the circumference of the magnet 1. Figure 15 is a plan view looking down from above the magnet 1 about centre 33. In this case there is a wider pocket 151 in the innervolume 51 configured to hold the devices, with the innervolume 51 being reduced and / or variable for the remaining circumference of the magnet 1 . In some cases the inner volume 51 may disappear (i.e., have zero width) for portions of the circumference. This may reduce the required size of the magnet 1 and / or the amount of superconduct used, for example.

[0089] Figure 16 shows a further alternative where a plurality of pockets 151 are arranged around the circumference of the magnet 1. Figure 16 is a plan view looking down from above the magnet 1. The pockets 151 are shown linked to one another, although they may be spaces apart. In some cases, the pockets 151 may each contain one or more devices.The use of coils 50 of variable diameter allows the shape of the inner volume 51 to be varied about the radius of the magnet 1. In some cases, only the inner 134 or outer 135 radius of the inner volume 51 is varied, or both radii and / or diameters may be varied.

[0090] The examples herein use I ring-shaped coils 50 aligned or centred on the same axis so as to be coaxial. The common axis is the z-axis of the toroidal magnet 1 (as shown in Figure 9). Variation of the radius of the ring-shaped coils 50, and / or the spacing of the ringshaped coils 50 along the z-axis allows the inner volume 51 to be formed between the ringshaped coils 50. In some cases, the ring-shaped coils 50 may be offset from the axis, for example to create a variable sized inner volume 51 , for manufacturing reasons, or to provide a different field shape.

[0091] The examples describe the use of a plurality of independent ring-shaped coils50. However, it may also be possible to use a single coil with multiple loops about the magnet 1 to form an inner volume between a single coil. For example, the ends of each of the ringshaped coils 50 could be connected to the adjacent coil 50 to form a single winding. Alternatively, a single coil could be spirally wound about the surface of the inner volume 51. In some cases, at least some of the plurality of coils 50 have identical or substantially identical characteristics. This may simplify modification or repair of the magnet 1 as a set of known coils 50 can be used for replacements.

[0092] The examples show superconductor coils 50 with constant height and horizontal alignment. Variation of the superconductor properties of the coils 50 is achieved through modifying the type of superconductor, the spacing between turns 1 10 and / or layers 114 and / or the length of the coils 50. This allows for simpler design and manufacture of the magnet. In some cases, the coils 50 may have other characteristics varied to control performance. For example, the angle of at least one or each coil may be configured to shape the inner volume51 . For example, consider the coils 50 of Figure 7, the angles of the coil cross-sections (i.e. the cross sections visible in Figure 5) could be configured to be substantially parallel and / or perpendicular to the flux lines. In some cases, the cross-section angles could be adjusted to reduce the spacing between adjacent coils 50. The height of the superconductor coils 50 could also be adjusted, for example by using different superconductor tape widths. The position of the coils 50 within the vessel 26 could also be adjusted, for example to minimise the size of the vessel 26 and or balance the magnet.

[0093] The examples herein have been described using a superconductor which is a high temperature superconductor. However, the technique may be applied to other superconductors. A high temperature superconductor may refer to a superconductor able to reach a superconducting state above a temperature of 30K, or where it is able to superconduct when cooled only by liquid nitrogen. Low temperature superconductors typically require temperatures below 30K to superconduct, requiring liquid helium for cooling.Flux pump

[0094] The device within the inner volume 51 may be a superconductor current pump, also known as a flux pump. A current pump is configured to provide a stable high current source to drive current into a superconductor circuit, such as the ring-shaped coils 50. Different types of current pumps are known including dynamo flux pumps and rectifier flux pumps. For example, WO2016024214A1 , incorporated herein by reference, shows examples of a dynamo flux pump. For example, WO2022164330A1 , incorporated herein by reference shows examples of a rectifier flux pump. These current pumps have an iron core or yoke which saturates in high magnetic field. Current pumps connected across a superconductor load will allow incremental increase of flux in a loop, increasing the current in the superconductor. In a simplified circuit model of a rectifier flux pump, a high current source (typically provided by the output of a stepdown transformer) is connected to a superconductor load in parallel with a switch. An AC current is induced in the circuit loop between the high current source and the switch, and then rectified into a DC voltage in the loop between the switch and the superconductor load. The switch and / or rectifier portion may be more complicated, involving multiple switches or other rectification techniques, however the general mode of operation is the same. A transformer is used to transfer power from a power source to a superconducting portion of the current pump, and a rectifier is used in the superconducting portion to create a DC current to feed into the coils 50.

[0095] Alternative technologies and devices may be used to charge the superconductor coils 50 and / or disposed in the inner volume 51 surrounded by the superconductor coils 50. A flux pump provides a light, relatively small, current driving device compatible with the inner volume 51 within the ring-shaped coils 50 and capable of levitating within the magnet 1. Alternative current driving devices 80 may be used.

[0096] The dipole field magnet 1 has been discussed with regard to a fusion reactor. However, the magnet 1 may be applied in alternative applications. For example, it may be usedto provide an improved magnetorquer, for movement of objects in space. A dipole field magnet arranged on a gyroscope would align its magnetic field with Earth's magnetic field. The ability to store, for example, a current pump or current driving device within the magnet avoids external contact being required, improving the ability to align the dipole field magnet. By aligning with Earth's magnetic field, the dipole magnet allows surrounding equipment to more accurately measure and / or use the magnetic field for directional control because the dipole magnet can be configured to produce a substantially stronger magnet field than that of Earth. For example, a series of solenoids arranged about the magnet could interact with the magnet (which is following Earth's magnetic field). In this way the magnet provides an improved local device representing Earth's magnetic field, instead of requiring direct use of Earth's magnetic field. Further applications of shielding equipment in a dipole field magnet may also use the present techniques.

[0097] Although certain embodiments and examples are disclosed herein, inventive subject matter extends beyond the specifically disclosed embodiments to other alternative embodiments and / or uses, and to modifications and equivalents thereof. Thus, the scope of the claims or embodiments appended hereto is not limited by any of the particular embodiments described herein. For example, in any method or process disclosed herein, the acts or operations of the method or process may be performed in any suitable sequence and are not necessarily limited to any particular disclosed sequence. Various operations may be described as multiple discrete operations in turn, in a manner that may be helpful in understanding certain embodiments; however, the order of description should not be construed to imply that these operations are order dependent. Additionally, some structures described herein may be embodied as integrated components or as separate components. For purposes of comparing various embodiments, certain aspects and advantages of these embodiments are described. Not necessarily all such aspects or advantages are achieved by any particular embodiment. Thus, for example, various embodiments may be carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other aspects or advantages as may also be taught or suggested herein.

[0098] It should be emphasized that many variations and modifications may be made to the embodiments described herein, the elements of which are to be understood as being among other acceptable examples. All such modifications and variations are intended to beincluded herein within the scope of this disclosure and protected by the following claims. Further, nothing in the foregoing disclosure is intended to imply that any particular component, characteristic or process step is necessary or essential.

Claims

CLAIMS1. A dipole field magnet comprising: a plurality of ring-shaped superconductor coils configured to shield an inner volume between the ring-shaped superconductor coils from the magnetic field of the dipole field magnet.

2. The dipole field magnet of claim 1 wherein the inner volume is ring-shaped.

3. The dipole field magnet of any one of the preceding claims wherein the inner volume is toroidal.

4. The dipole field magnet of any one of the preceding claims wherein the superconductor coils are co-axial.

5. The dipole field magnet of any one of the preceding claims wherein the superconductor coils are axially spaced.

6. The dipole field magnet of any one of the preceding claims wherein at least two of the coils have the same cross-section dimensions.

7. The dipole field magnet of any one of the preceding claims wherein the plurality of coils are symmetric above and below a radial plane.

8. The dipole field magnet of any one of the preceding claims wherein a greater volume of coils are disposed towards the inner perimeter of the magnet than the outer perimeter of the magnet.

9. The dipole field magnet of any one of the preceding claims wherein the plurality of coils comprises at least four coils.

10. The dipole field magnet of any one of the preceding claims wherein the magnetic field of the dipole field magnet is produced by current flowing in the ring-shaped superconductor coils.

11. The dipole field magnet of any one of the preceding claims wherein the plurality of coils form one or more of a circular substantially circular, elliptical, or substantially elliptical cross-section.

12. The dipole field magnet of any one of the preceding claims wherein at least two of the plurality of coils are spaced apart.

13. The dipole field magnet of any one of the preceding claims wherein each of the plurality of coils comprises a plurality of turns and / or layers of superconductor.

14. The dipole field magnet of claim 14 wherein each of the plurality of turns and / or layers is arranged in the same orientation.

15. The dipole field magnet of any one of the preceding claims wherein the superconductor comprises at least one of: superconductor tape and a superconductor wire.

16. The dipole field magnet of any one of claims 1 to 18 comprising a device disposed within the inner volume.

17. The dipole field magnet of claim 19 wherein the device is a device for driving current in the plurality of coils.

18. The dipole field magnet of claim 20 wherein the device inductively charges the superconductor coils.

19. The dipole field magnet of any one of the preceding claims comprising a magnetic field shield between the plurality of coils and the inner volume.

20. The dipole field magnet of claim 19 wherein the magnetic field shield is configured to reduce inductive coupling between coils on either side of the magnetic field shield.

21. The dipole field magnet of any one of claims 19 or 20 wherein the magnetic field shield has a variable thickness, optionally the thickness configured to be more think in areas of high field, in use.

22. The dipole field magnet of any one of the preceding claims comprising a housing surrounding the plurality of coils.

23. A reactor chamber comprising a dipole field magnet as claimed in claim 1 to 22.

24. The reactor chamber of claim 23 comprising:A vacuum vessel; andA levitation device for levitating the dipole field magnet.

25. A dipole field magnet comprising: a plurality of ring-shaped superconductor coils disposed to create an inner volume between them, and a device within the inner volume.

26. A levitating dipole reactor comprising: a dipole field magnet configured to levitate, the dipole field magnet comprising an internal current driving device for driving in the superconductor dipole magnet.

27. A dipole field magnet comprising a plurality of ring-shaped superconductor coils configured to shield an inner volume between the coils from the magnetic fields generated by the dipole field coil.

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