Superconducting electromagnets and method of forming

Abstract

There is provided a superconducting electromagnet in a form similar to a Bitter magnet. In certain forms, the electromagnet may comprise a plurality of discs formed from superconducting material. Each disc may have a central through-hole and a slit between the through-hole and an outer circumference. The discs may be arranged in a coaxial stack with the slits being azimuthally offset. The discs may be conductively connected by a conducting joint aligned azimuthally between slits of adjacent discs so that the coaxial stack forms a helical conducting path. The discs may be formed from a crystalline ceramic superconductor, for example a cuprate or (RE)BCO.

Description

[0001] SUPERCONDUCTING ELECTROMAGNETS AND METHOD OF FORMING

[0002] 1. FIELD OF THE TECHNOLOGY

[0003] The field of the technology relates to superconducting electromagnets. More particularly, the field of the technology relates to superconducting electromagnets in a form similar to a Bitter magnet, and methods of forming such superconducting electromagnets.

[0004] 2. BACKGROUND TO THE TECHNOLOGY

[0005] Electromagnets have application in numerous fields. They are often advantageous compared to permanent magnets because their magnetic field can be controlled by controlling the amount of current in the winding, and this includes turning the electromagnet on and off. In addition, electromagnets are able to produce a strong magnetic field.

[0006] A Bitter magnet is a type of normal-conducting electromagnet (named after Francis Bitter) that is capable of achieving relatively high magnetic field strengths. A Bitter magnet consists of a stack of alternating conducting discs and insulation layers. The conducting discs (typically formed from copper) have slits in them and are arranged to form a substantially helical conducting path. Holes are typically drilled through the discs for coolant to flow through to carry away the large amount of heat produced by resistive heating from the large currents. One advantage of discs being used in a Bitter magnet is that the resultant coil is approximately homogeneous in the radial direction, which results in reduced hoop stress. However, even with the large cross-sectional area afforded by copper discs compared to wire, the current levels required to generate a large magnetic field strength (typically multiple kA) generate substantial heating. The need to cool the magnets largely precludes their use outside of dedicated facilities.

[0007] There is a need to produce magnets capable of higher magnetic field strengths. There is also a need for high-field magnets to be made smaller and / or lighter, and consequently more portable. This would have application in numerous areas, including (but not limited to): magnetic separation devices; magnetic energy storage systems; nuclear magnetic resonance (NMR) systems and magnetic resonance imaging (MRI) systems (e.g. for medical applications); non-destructive materials inspection; and aerospace propulsion (e.g. magnetofluid-dynamic thrusters). 3. OBJECT OF THE TECHNOLOGY

[0008] It is an object of the technology to provide an improved electromagnet, in particular a superconducting electromagnet. Alternatively, it is an object of the technology to provide an improved method of forming a electromagnet, in particular a superconducting electromagnet. Alternatively, it is an object of the technology to at least provide the public with a useful choice.

[0009] 4. SUMMARY OF THE TECHNOLOGY

[0010] According to one aspect of the technology there is provided an electromagnet for providing a magnetic field. In examples, there is provided a superconducting electromagnet for providing a relatively high magnetic field. The superconducting electromagnet may be smaller and / or lighter, and consequently more portable, than other electromagnets capable of producing similar magnetic field strengths.

[0011] According to one aspect of the technology, there is provided a superconducting electromagnet in a form similar to a Bitter magnet. In certain forms, the superconducting electromagnet may comprise a plurality of discs formed from superconducting material. Each of the discs may have formed therein a central through-hole and a slit extending through the disc between the through-hole and an outer circumference of the disc. The discs may be arranged in a coaxial stack with each disc adjacent to the adjacent disc(s) and with the slit of each disc being azimuthally offset from the slit of each adjacent disc. Each pair of adjacent discs may be conductively connected by a conducting joint aligned azimuthally between the respective slits of the respective pair of adjacent discs so that the coaxial stack of discs forms a substantially helical conducting path.

[0012] According to another aspect of the technology, there is provided a method of forming an electromagnet for providing a magnetic field. In some forms, there is provided a method of forming a superconducting electromagnet for providing a relatively high magnetic field.

[0013] According to one aspect of the technology, there is provided a method of forming a superconducting electromagnet in a form similar to a Bitter magnet. In certain forms, the method may comprise forming a plurality of discs from a superconducting material. Each of the discs may have formed therein a central through-hole and a slit extending through the disc between the through-hole and an outer circumference of the disc. The method may further comprise arranging the discs in a coaxial stack with each disc adjacent to the adjacent disc(s) and with the slit of each disc being azimuthally offset from the slit of each adjacent disc. The method may further comprise conductively connecting each pair of adjacent discs by forming a conducting joint aligned azimuthally between the respective slits of the respective pair of adjacent discs so that the coaxial stack of discs forms a substantially helical conducting path.

[0014] According to one aspect of the technology, there is provided a superconducting electromagnet. The superconducting electromagnet may comprise a plurality of discs formed from a superconducting material. Each of the discs may have formed therein a central through-hole and a slit extending through the disc between the through-hole and an outer circumference of the disc. The discs may be arranged in a coaxial stack with each disc immediately adjacent to the adjacent disc(s) and with the slit of each disc being azimuthally offset from the slit of each adjacent disc. Each pair of adjacent discs may be conductively connected by a conducting joint aligned azimuthally between the respective slits of the respective pair of adjacent discs so that the coaxial stack of discs forms a substantially helical conducting path.

[0015] In some forms, the superconducting material may be a crystalline ceramic superconductor. For example, the superconducting material may be a cuprate superconductor such as crystalline rare-earth barium copper oxide ("(RE)-Ba-Cu-O" or simply "(RE)BCO"), where the rare-earth element ("RE") corresponds to any rare earth element, such as lanthanum ("La"), samarium ("Sm"), gadolinium ("Gd"), or yttrium ("Y").

[0016] In some forms, a crystal axis of the superconducting material in each disc may be substantially parallel to a longitudinal axis of the coaxial stack.

[0017] In some forms, the discs may be conductively connected so that, when a current flows along the helical conducting path, the current flows into a first face of each disc and out of a second face of each disc, the second face being opposite to the first face.

[0018] In some forms, the conducting joint may comprise a conducting joint member positioned between the respective pair of adjacent discs and aligned azimuthally between the respective slits of the respective pair of adjacent discs. In some forms, each conducting joint member may be diffusion bonded to the respective pair of adjacent discs to form a conducting joint. In some forms, the conducting joint may comprise a normal conducting joint member. In other forms, the conducting joint may comprise a superconducting joint member.

[0019] In some forms, the slit may extend through each disc in a substantially radial direction.

[0020] In some forms, the superconducting electromagnet may further comprise a first conducting terminal at a first end of the coaxial stack and a second conducting terminal at a second end of the coaxial stack, the first end being opposite the second end. Each of the first terminal and the second terminal may comprise a terminal disc positioned coaxially with the discs of the coaxial stack. Each of the first terminal and the second terminal may be conductively connected to the disc at the respective end of the coaxial stack.

[0021] In some forms, the superconducting electromagnet may further comprise a housing to contain the coaxial stack. An internal volume of the housing that is not occupied by the coaxial stack may be substantially filled with a filler material.

[0022] According to one aspect of the technology, there is provided a superconducting electromagnet. The superconducting electromagnet may comprise a plurality of discs formed from a crystalline ceramic superconducting material. Each of the discs may have formed therein a central through-hole and a slit extending through the disc between the through-hole and an outer circumference of the disc. The discs may be arranged in a coaxial stack with each disc adjacent to the adjacent disc(s) and with the slit of each disc being azimuthally offset from the slit of each adjacent disc. Each pair of adjacent discs may be conductively connected by a conducting joint aligned azimuthally between the respective slits of the respective pair of adjacent discs so that the coaxial stack of discs forms a substantially helical conducting path.

[0023] In some forms, each pair of adjacent discs may be conductively connected by a conducting joint member positioned between the respective pair of adjacent discs and aligned azimuthally between the respective slits of the respective pair of adjacent discs so that the coaxial stack of discs forms the substantially helical conducting path. In some forms, the conducting joint member may be conductively connected to the pair of adjacent discs by heating. In some forms, the crystalline ceramic superconducting material may be a cuprate superconductor. For example, the crystalline ceramic superconducting material may be a quasi-single-crystal rare-earth barium copper oxide ((RE)-Ba-Cu-O) superconductor.

[0024] In some forms, a crystal axis, for example the c-axis, of the quasi-single-crystal (RE)-Ba-Cu-O superconducting material in each disc may be substantially parallel to a longitudinal axis of the coaxial stack.

[0025] In some forms, the discs may be conductively connected so that, when a current flows along the helical conducting path, the current flows into a first face of each disc and out of a second face of each disc, the second face being opposite to the first face.

[0026] In some forms, each conducting joint member may comprise a normal conducting joint member. In some forms, each normal conducting joint member may be diffusion bonded to the respective pair of adjacent discs to form a normal conducting joint. In some forms, each normal conducting joint member may be diffusion bonded to the respective pair of adjacent discs to form a superconducting joint.

[0027] In some forms, each conducting joint member may comprise a superconducting joint member.

[0028] In some forms, the superconducting electromagnet may further comprise a non-superconducting layer between each pair of adjacent discs.

[0029] In some forms, the slit may extend through each disc in a substantially radial direction.

[0030] In some forms, the superconducting electromagnet may further comprise a first conducting terminal at a first end of the coaxial stack and a second conducting terminal at a second end of the coaxial stack, the first end being opposite the second end. Each of the first conducting terminal and the second conducting terminal may comprise a terminal disc positioned coaxially with the discs of the coaxial stack. Each of the first conducting terminal and the second conducting terminal may be conductively connected to the disc at the respective end of the coaxial stack. In some forms, the superconducting electromagnet may further comprise a housing to contain the coaxial stack. An internal volume of the housing that is not occupied by the coaxial stack may be substantially filled with a filler material.

[0031] According to one aspect of the technology, there is provided a method of forming a superconducting electromagnet. The method may comprise forming a plurality of discs from a crystalline ceramic superconducting material. Each of the discs may have formed therein a central through-hole and a slit extending through the disc between the through-hole and an outer circumference of the disc. The method may further comprise applying a coating to at least one face of one or more of the discs. The coating may comprise a granular form of an insulating material. The method may further comprise arranging the discs in a coaxial stack with each disc immediately adjacent to the adjacent disc(s) and with the slit of each disc being azimuthally offset from the slit of each adjacent disc. The method may further comprise conductively connecting each pair of adjacent discs by forming a conducting joint aligned azimuthally between the respective slits of the respective pair of adjacent discs so that the coaxial stack of discs forms a substantially helical conducting path.

[0032] In some forms, the coating may be applied to the face of each disc as a granular form of the insulating material in a suspension. In some forms, the insulating material may be an oxide ceramic.

[0033] According to one aspect of the technology, there is provided a method of forming a superconducting electromagnet. The method may comprise forming a plurality of discs from a crystalline ceramic superconducting material. Each of the discs may have formed therein a central through-hole and a slit extending through the disc between the through-hole and an outer circumference of the disc. The method may further comprise arranging the discs in a coaxial stack with each disc immediately adjacent to the adjacent disc(s) and with the slit of each disc being azimuthally offset from the slit of each adjacent disc. The method may further comprise heating the coaxial stack to conductively join each pair of adjacent discs with a conducting joint aligned azimuthally between the respective slits of the respective pair of adjacent discs so that the coaxial stack of discs forms a substantially helical conducting path.

[0034] In some forms, the method may comprise conductively joining each pair of adjacent discs using diffusion bonding. In some forms, the method may further comprise placing one of a plurality of conducting joint members between each pair of adjacent discs, the respective conducting joint members being aligned azimuthally between the respective slits of the respective pair of adjacent discs, and the step of heating comprises heating the conducting joint members to form the conducting joints.

[0035] In some forms, the method may comprise heating the discs and joint members to a temperature below the peritectic temperature of the crystalline ceramic superconducting material so that the conducting joint members are conductively joined to adjacent discs in a normal conducting joint.

[0036] In some forms, the method may comprise heating the discs and joint members to a temperature at or proximate the peritectic temperature of the crystalline ceramic superconducting material so that the conducting joint members are conductively joined to adjacent discs in a superconducting joint.

[0037] In some forms, the method may further comprise forming a non-superconducting layer between each pair of adjacent discs.

[0038] According to another aspect of the technology, there is provided a superconducting electromagnet formed using a method of another aspect of the technology.

[0039] According to one aspect of the technology, there is provided a superconducting electromagnet. The superconducting electromagnet may comprise a plurality of inner discs formed from superconducting material. Each of the inner discs may have formed therein a central through-hole and a slit extending through the inner disc between the through-hole and an outer circumference of the inner disc. The inner discs may be arranged in a coaxial inner stack with the slit of each inner disc being azimuthally offset from the slit of each adjacent inner disc. Each pair of adjacent inner discs may be conductively connected by a conducting joint aligned azimuthally between the respective slits of the respective pair of adjacent inner discs so that the coaxial inner stack of inner discs forms a substantially helical conducting path. The superconducting electromagnet may further comprise a plurality of outer discs formed from superconducting material. Each of the outer discs may have formed therein a central through-hole and a slit extending through the outer disc between the through-hole and an outer circumference of the outer disc. The outer discs may be arranged in a coaxial outer stack with the central through-holes of each of the outer discs forming an axial bore and with the slit of each outer disc being azimuthally offset from the slit of each adjacent outer disc. Each pair of adjacent outer discs may be conductively connected by a conducting joint aligned azimuthally between the respective slits of the respective pair of adjacent outer discs so that the coaxial outer stack of outer discs forms a substantially helical conducting path. The inner stack and the outer stack may be arranged substantially coaxially with the inner stack positioned inside the axial bore of the outer stack. The superconducting electromagnet may further comprise one or more conductors connecting the inner stack and the outer stack in series so that, when current passes through the inner stack and the outer stack, the azimuthal direction of current flow is in the same direction in both the inner stack and the outer stack. The inner stack and the outer stack may be configured and arranged so that, as an amount of current flowing through the inner stack and the outer stack increases, the inner stack transitions from a low-resistance state to a higher- resistance state at substantially the same amount of current as the outer stack.

[0040] In some forms, the one or more conductors may comprise an end conductor forming a conducting path between a first end inner disc of the plurality of inner discs and a first end outer disc of the plurality of outer discs. The first end inner disc and the first end outer disc may be at the same ends of the inner stack and the outer stack respectively. The substantially helical conducting path may be formed by the inner stack of inner discs has a first chirality and the substantially helical conducting path formed by the outer stack of outer discs has a second chirality, wherein the second chirality is different from the first chirality.

[0041] In some forms, the superconducting electromagnet may further comprise a first conducting terminal conductively connected to a second end inner disc of the plurality of inner discs. The second end inner disc may be at the opposite end of the inner stack from the first end inner disc. The superconducting electromagnet may further comprise a second conducting terminal conductively connected to a second end outer disc of the plurality of outer discs. The second end outer disc may be at the opposite end of the outer stack from the first end outer disc.

[0042] In some forms, the radial width of each outer disc between the through-hole and the outer circumference of the outer disc may be less than the radial width of each inner disc between the through-hole and the outer circumference of the inner disc.

[0043] In some forms, each of the inner discs in the inner stack may have a respective critical current and one of the discs of the inner discs has a lowest critical current. Each of the outer discs in the outer stack may have a respective critical current and one of the discs of the outer discs has a lowest critical current. The critical current of the inner disc having the lowest critical current may be substantially the same as the critical current of the outer disc having the lowest critical current.

[0044] According to one aspect of the technology, there is provided a superconducting electromagnet. The superconducting electromagnet may comprise a plurality of discs formed from superconducting material. Each of the discs may have formed therein a central through-hole and a slit extending through the disc between the through-hole and an outer circumference of the disc. The discs may be arranged in a coaxial stack with the slit of each disc being azimuthally offset from the slit of each adjacent disc. Each of the discs may have a different peritectic temperature to the adjacent disc(s). Each pair of adjacent discs may be conductively connected by a conducting joint aligned azimuthally between the respective slits of the respective pair of adjacent discs so that the coaxial stack of discs forms a substantially helical conducting path. The superconducting electromagnet may further comprise a plurality of non- superconducting layers. Each pair of adjacent discs may be separated by one of the non- superconducting layers other than at the conducting joints.

[0045] In some forms, peritectic temperatures of the discs alternates along the stack.

[0046] In some forms, the conducting joint may be a superconducting joint. In some forms, the superconducting joint may be formed directly between each pair of adjacent discs.

[0047] According to one aspect of the technology, there is provided a superconducting electromagnet. The superconducting electromagnet may comprise a plurality of discs formed from a superconducting material. Each of the discs may have formed therein a central through-hole and a slit extending through the disc between the through-hole and an outer circumference of the disc. The discs may be arranged in a coaxial stack with the slit of each disc being azimuthally offset from the slit of each adjacent disc. The superconducting electromagnet may further comprise a plurality of superconducting joint members. The superconducting electromagnet may further comprise a plurality of non-superconducting layers. Each pair of adjacent discs may be conductively connected by one of the superconducting joints members aligned azimuthally between the respective slits of the respective pair of adjacent discs so that the coaxial stack of discs forms a substantially helical conducting path. Each pair of adjacent discs may be separated by one of the non-superconducting layers other than at the conducting joints. In some forms, each of the superconducting joint members may have a different peritectic temperature from the discs adjacent to the respective superconducting joint member.

[0048] In some forms, the plurality of discs may be formed from a superconducting material with substantially the same peritectic temperature as each other and the plurality of superconducting joint members may be formed from a superconducting material with substantially the same peritectic temperature as each other, the peritectic temperature of the plurality of discs being different from the peritectic temperature of the plurality of superconducting joint members.

[0049] In some forms, the superconducting material may a crystalline ceramic high temperature superconductor. For example, the superconducting material may be a cuprate superconductor. For example, the superconducting material may be a quasi-single-crystal rare-earth barium copper oxide ((RE)-Ba-Cu-O) superconductor.

[0050] In some forms, a crystal axis of the quasi-single-crystal superconducting material in each disc may be substantially parallel to a longitudinal axis of the coaxial stack.

[0051] In some forms, the discs may be conductively connected so that, when a current flows along the helical conducting path, the current flows into a first face of each disc and out of a second face of each disc, the second face being opposite to the first face.

[0052] According to one aspect of the technology, there is provided a superconducting electromagnet. The superconducting electromagnet may comprise a plurality of discs formed from superconducting material. Each of the discs may have formed therein a central through-hole and a slit extending through the disc between the through-hole and an outer circumference of the disc. The discs may be arranged in a coaxial stack with the slit of each disc being azimuthally offset from the slit of each adjacent disc. Each pair of adjacent discs may be conductively connected by a conducting joint aligned azimuthally between the respective slits of the respective pair of adjacent discs so that the coaxial stack of discs forms a substantially helical conducting path. At least one of the discs may have a different thickness in the longitudinal direction to an adjacent disc.

[0053] In some forms, the discs at or proximate each end of the stack may be thicker than a disc at or proximate a middle of the stack. In some forms, the plurality of discs may comprise discs having a variety of longitudinal thicknesses. The discs may be arranged in the stack so that the longitudinal thickness of the discs increases towards the ends of the stack.

[0054] In some forms, the superconducting electromagnet may further comprise a plurality of non- superconducting layers. Each pair of adjacent discs may be separated by one of the non- superconducting layers.

[0055] According to one aspect of the technology, there is provided a superconducting electromagnet system. The superconducting electromagnet system may comprise a superconducting electromagnet according to another aspect of the technology. The superconducting electromagnet system may further comprise a current supply for suppling current to the superconducting electromagnet. In some forms, the current supply may comprise a flux pump to wirelessly induce current supplied to the superconducting electromagnet. In other forms, the current supply may be electrically connected to the superconducting electromagnet via a plurality of current leads.

[0056] Further aspects of the technology, which should be considered in all its novel aspects, will become apparent to those skilled in the art upon reading of the following description which provides at least one example of a practical application of the technology.

[0057] 5. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] One or more embodiments of the technology will be described below by way of example only, and without intending to be limiting, with reference to the following drawings, in which:

[0059] Figure 1 illustrates a plot depicting the internal electric field versus current curve for an exemplary high-temperature superconductor;

[0060] Figure 2 is an illustration of the relationship between electric field and current for an exemplary superconducting material when three external magnetic fields of different magnitude are applied;

[0061] Figure 3A is an illustration of a disc for an electromagnet according to one exemplary form of the technology; Figure 3B is an illustration two discs for an electromagnet according to one exemplary form of the technology;

[0062] Figure 3C is an illustration of a coaxial stack of discs for an electromagnet according to one exemplary form of the technology;

[0063] Figure 3D is an illustration of an electromagnet according to one exemplary form of the technology;

[0064] Figure 4 is an exploded view of an electromagnet according to another exemplary form of the technology;

[0065] Figure 5 is an illustration of an electromagnet according to another exemplary form of the technology;

[0066] Figure 6 is an illustration of a part of an electromagnet according to another exemplary form of the technology;

[0067] Figures 7A-7D are illustrations modelling normalised current density in an electromagnet according to an exemplary form of the technology;

[0068] Figure 8 is an illustration of part of an electromagnet according to another exemplary form of the technology;

[0069] Figure 9A is an illustration of an electromagnet according to another exemplary form of the technology;

[0070] Figure 9B is an exploded view illustration of an electromagnet according to another exemplary form of the technology;

[0071] Figures 10A-10D are illustrations modelling normalised current density in an electromagnet according to an exemplary form of the technology;

[0072] Figures 11A-11D are illustrations modelling current density in an electromagnet according to an exemplary form of the technology;

[0073] Figure 12A is an illustration of an electromagnet according to another exemplary form of the technology;

[0074] Figure 12B is an illustration of an electromagnet according to another exemplary form of the technology; and

[0075] Figure 13 is an illustration of the crystal structure of (RE)-Ba-Cu-O in the superconducting RE-123 phase. 6. DETAILED DESCRIPTION OF EXEMPLARY FORMS OF THE TECHNOLOGY

[0076] 6.1. Superconductivity Principles

[0077] A superconductor or superconducting material is a material that exhibits zero electrical resistance below a certain temperature known as the critical temperature, Tc. This zero electrical resistance state is often referred to as a superconducting state.

[0078] There are two types of superconductors, named type I and type II. Type I superconductors are typically pure metals and exhibit a phenomenon known as the Meissner effect whereby magnetic flux is completely expelled from the interior of the superconductor. Type I superconductors may not be suitable for the construction of high-field electromagnets as they typically cannot conduct high electrical currents whilst in the superconducting state. Type II superconductors behave differently. Under applied magnetic fields below a certain magnitude (the first critical field Hci) they expel flux in a similar manner to type I superconductors, however, they also display a second critical field (HC2) which is always greater than Hd. At fields between Hciand HC2, magnetic field can penetrate type II superconductors in the form of Abrikosov vortices. This creates an intermediate phase where the material can exhibit a finite small resistance while remaining fundamentally superconducting. Because of this behaviour, type II superconductors can carry much more current in an applied magnetic field than type I superconductors, making them useful for practical applications.

[0079] 6.1.1. High Temperature Superconductors

[0080] The critical temperature for a superconductor is conventionally defined as the temperature below which the resistivity of the superconductor drops to zero or near zero. In other words, a superconductor is said to be in its superconducting state when the temperature of the superconductor is below the critical temperature and in a non-superconducting state when the temperature is above the critical temperature. Many superconductors have a critical temperature which is near absolute zero; for example, mercury is known to have a critical temperature of 4. IK. It is however also known that some materials can have much higher critical temperatures; for example, magnesium diboride has a critical temperature of approximately 39K, while yttrium barium copper oxide (YBCO) has a critical temperature of approximately 92K. These superconductors are often generally referred to as high-temperature superconductors (HTSs). HTSs are type II superconductors. 6.1.2. Crystalline Ceramic HTS Materials

[0081] Many HTSs are crystalline ceramic materials which are typically brittle and can exhibit anisotropic properties. Examples include the rare-earth barium cuprates and bismuth strontium calcium copper oxide.

[0082] The rare-earth barium cuprates have the general form (RE)-Ba-Cu-O where RE = rare-earth element, B = barium, C = copper and O = oxygen. The material may contain one or more rare-earth elements in combination. An illustration of the general crystal structure for this class of materials is shown in Figure 13, which shows the crystal structure of (RE)-Ba-Cu-O in the superconducting RE-123 phase, with a unit cell marked with a dashed box. Directions of the crystal axis are denoted by the crystal axes a, b and c. The crystal may be highly anisotropic such that the critical current density for current flowing in the c- axis direction is much lower than for current flowing in the a-b plane. Polycrystalline samples include grain boundaries which can severely depress the local critical current density. For this reason significant effort has been made over a period of years to grow bulk samples that will be referred to as 'quasi- single-crystal' bulk samples in this specification. This term should be understood to refer to bulk samples of crystalline ceramic superconducting materials that comprise a single grain with substantially uniform crystallographic orientation of at least one crystal axis. This may be achieved via templated growth methods which can include, but are not limited to, top-seeded melt growth (TSMG), infiltration growth and single domain melt growth. These methods typically result in the solidification growth of a bulk crystalline superconductor which exhibits substantially uniform orientation of one or more crystallographic axis throughout its extent, although these superconducting bulks can also include voids and inclusions of non-superconducting phases interspersed within the bulk volume. These impurities may be beneficial as they may provide additional pinning centres that increase the overall critical current density (Jc). In some cases the voids can be filled via infiltration with a nonreactive metal, such as silver, which consolidates the bulk volume and increases mechanical strength and electrical connectivity within the bulk. Typical applications of these crystalline bulk superconductors involve their magnetisation in a strong external magnetic field which generates circulating supercurrents within the bulk volume, thus trapping magnetic flux so as to form a type of persistent magnet.

[0083] In other forms, in which other types of crystalline materials are used, the crystal structure of the material may be different from the structure illustrated in Figure 13. In some forms, the structure may be less anisotropic than in the case of the (RE)-Ba-Cu-O illustrated in Figure 13. In general, any crystal structure defines multiple crystal axes.

[0084] 6.1.3. Critical Current Density

[0085] The total electrical current / flowing across any surface within a conducting body can be calculated from the equation: where J is the local current density expressed as a vector, A is the surface area and dr is a unit vector perpendicular to the surface at every point.

[0086] A superconducting material possesses a critical current density. The critical current density of a high- temperature superconducting material is conventionally defined as the magnitude of current density flowing in the superconductor which results in an electric field drop of 100 pV / m (= 1 pV / cm) along the direction of the current flow. The critical current density is a function of the superconducting material used, its temperature and any applied magnetic field.

[0087] The critical current / cthat may flow across a surface within a superconducting body is defined as:

[0088] The critical current is often taken as the maximum upper limit for the operating current that may be injected into a superconducting body of cross-sectional area A. In this specification, reference to the critical current of the superconductor / superconducting material may be made interchangeably with the critical current density for simplicity.

[0089] In a superconductor, if the current density J is approximately equal to the critical current density Jc, the resistance of the superconductor is non-zero, but small. However, if J is much larger than the critical current density Jc, then the local resistivity of the superconductor will become sufficiently large to cause heat dissipation, which may heat the superconductor to a temperature above its critical temperature, which in turn will cause it to no longer be superconducting. This condition is sometimes referred to as a "quench" and can be damaging to the superconductor itself. Figure 1 shows an exemplary plot depicting the internal electric field E versus current density J flowing in a high-temperature superconductor. In Figure 1 it can be seen that the electric field strength in the superconductor is substantially zero below the critical current density Jcfor the superconductor. As the current in the superconductor approaches the critical current, the electric-field in the superconductor starts to increase. At the critical current, the electric-field in the superconductor is 100 pV / m. Further increasing the current in the superconductor above the critical current results in rapid increases in the electric-field strength in the conductor.

[0090] The transition from the superconducting state to the resistive state in HTS materials, such as is shown in Figure 1, can be described by an empirical law known as the E-J power law: where E is the electric field in the conductor, J is the current density, and n is an experimentally defined unitless parameter which governs the steepness of the transition. In most superconductors, n has a value between 25-30. The critical current density Jcis defined by an arbitrarily chosen threshold field Eo, which may be 100 pV / m (= 1 pV / cm) as explained above.

[0091] In describing the technology in this specification, material and components comprising the material are referred to as "superconducting". This term is commonly used in the art for such materials and should not be taken to mean that the relevant material is always in a superconducting state. Under certain conditions the material and components comprising the material may not be in a superconducting state. The material may alternatively be described as being "superconductive".

[0092] 6.1.3.1. Magnetic-Field Suppression of Critical Current

[0093] The critical current density in a superconducting body is dependent on the local magnetic field strength, and consequently any external magnetic field applied to the superconductor. In general, the critical current density decreases at higher local magnetic field strengths. This relationship is shown in Figure 2, which is an illustration of graphs of electric field against current flowing along a length of superconducting material when three external magnetic fields of different magnitude are applied. The highest magnitude of external magnetic field, Bappi, results in the lowest critical current, I . The direction in which the magnetic field is applied can alter the extent to which the critical current density is suppressed within the superconductor. For example, in rare-earth cuprate materials, a magnetic field applied parallel to the c-axis of the crystal will suppress the critical current density to a greater extent than an equivalent magnetic field strength applied parallel to the a-b plane of the crystal.

[0094] For all superconductors, the critical current drops off sharply with only a small applied magnetic field. This means that a small change in the applied magnetic field can result in a large change in the critical current. This relationship is dependent on the superconducting material and the presence and nature of any crystal defects or contaminants within that material.

[0095] 6.2. Bitter Magnet

[0096] Forms of the technology provide a superconducting electromagnet 100 in a form similar to a Bitter magnet, such as shown in the examples of Figures 3A-D, 4, 5 and 12A. Each electromagnet 100 is formed from a plurality of discs 110. Certain forms of the technology may not be limited by the number of discs but, in certain forms, approximately 10 to 50 discs may be used, for example. Each of the discs 110 is formed from a superconducting material, examples of which are described later. An exemplary disc 110 is illustrated in Figure 3A. The exemplary disc 110 is a substantially flat body of superconducting material with a width significantly greater than its thickness. The disc 110 may be circular in plan view, although in other forms, the disc 110 may have another shape, for example oval, hexagonal or other polygon.

[0097] In view of this shape of the disc and the manner of stacking the discs in a stack, which forms a cylinder if the discs are circular in plan (as described below), terms referencing a cylindrical co-ordinate system will be used throughout this specification. That is, an axis is considered to pass through the centre of the disc 110 perpendicular to the plane of the disc. A direction parallel to this axis is the "longitudinal" direction. The term "radial" refers to a direction directly towards or away from the axis, i.e. in the direction of a radius of the circular disc. The term "azimuthal" refers to a direction around the axis, perpendicular to the radial direction.

[0098] In the example of Figure 3A, the disc 110 has a through-hole 112 at its centre. The central through-hole 112 passes all the way through the thickness of the disc 110 in the longitudinal direction. The through- hole 112 may be circular in shape, although in other forms, the through-hole 112 may have another shape, for example oval, hexagonal or other polygon. The centre of the through-hole 112 may be concentric with the centre of the disc 110, i.e. the longitudinal axis. In the example of Figure 3A, in which the disc 110 is circular and has a circular through-hole 112 therein, the disc 110 may be described as being an annular disc. The annular disc 110 has an inner radius, which is the radius of the through- hole 112 and the distance of the inner circumference of the disc from the centre, and an outer radius, which is the radius of the disc 110 and the distance of the outer circumference of the disc from the centre.

[0099] Furthermore, in the example of Figure 3A, the disc 110 has a slit 114. The slit 114 extends all the way through the thickness of the disc 110 and extends between the through-hole 112 and the outer circumference of the disc 110, i.e. from the inner circumference to the outer circumference of the disc 110. In the form shown, the slit 114 extends in a substantially radial direction, although in other forms the slit 114 may extend between the through-hole 112 and the outer circumference of the disc 110 in a different direction, including a straight line in a non-radial direction, and not in a straight line.

[0100] As stated above, each electromagnet 100 comprises a plurality of discs 110. The other discs in the electromagnet 100 may be formed substantially identically to the first disc as described above, for example as shown in Figure 3A. In some forms, for example as shown in Figure 4, some of the discs, for example the end discs 110 when the discs are arranged in a stack, may not have a slit 114.

[0101] As shown in Figure 3B, to form the electromagnet 100, one disc 110 may be placed adjacent to another disc 110. That is, the discs are placed in coaxial alignment with the face of one disc abutting against the face of another disc, or with the faces of the discs closely spaced apart (these options are discussed further below). One of the discs 110 may be placed in an orientation that is rotated with respect to the adjacent disc, i.e. the slit 114 of one disc 110 is azimuthally offset from the slit 114 of the adjacent disc 110. The amount of azimuthal offset may be relatively small, i.e. the azimuthal distance between the slits 114 may be small compared to the outer circumference of each disc 110, for example less than 20%, for example 10% or 5%, or less.

[0102] The two discs 110 may be conductively connected by a conducting joint 116. The joint 116 may be positioned so that it is aligned azimuthally between the slits 114 of the two adjacent discs 110. In some forms, the azimuthal extent of the conducting joints 116 may correlate to the azimuthal offset between the slits 114 and be a small percentage of the total circumference of the discs 110, for example less than 20%, for example 10% or 5%, or less. In some forms, the azimuthal extent of the conducting joints 116 may be such that the area of each conducting joint 116 is significantly larger than the cross-sectional area of each disc 110 (i.e. the cross-section of a slice cut through the disc parallel to the longitudinal axis of the stack 118). The joint 116 may conductively connect the discs 110 so that, when a current flows through the discs, the current flows in a substantially longitudinal direction into a first face of each disc 110 and out of a second face of each disc 110, the second face being opposite to the first face. In certain forms, the resistance of the conducting joint 116 is low, for example sufficiently low that heat is not significantly dissipated, for example less than approximately 100 nO / m, for example less than approximately 10 nO / m. In some forms, as will be explained further below, the conducting joint 116 may be superconducting. Further details of the conducting joint 116 and exemplary methods of forming the joint are provided below.

[0103] As shown in Figure 3C, additional discs 110 are provided in a similar manner to form a coaxial stack 118. That is, each disc 110 in the stack is aligned coaxially so that the through-holes 112 of the discs form an axial bore 113 located centrally in the coaxial stack 118. The slit 114 of each disc 110 is azimuthally offset from the slits of adjacent discs, with the azimuthal direction of the offset between the slits of adjacent discs being the same moving along the stack 118, i.e. so that the slits 114 spiral around the stack 118. In addition, adjacent discs 110 are conductively joined by a conducting joint 116 aligned azimuthally between the slits 114 of the respective discs 110. In this way, the coaxial stack 118 forms a conducting path that is substantially helical in shape: the path moves upwards from one disc 110 to the next through the conducting joint 116, and then azimuthally around the next disc until the joint with the next disc, which is positioned not quite the entire way around the circumference, and then up to the next disc, and so on. Consequently, the stack 118 amounts to a conducting coil, which generates a magnetic field when a current passes through the coil.

[0104] Electromagnets 100 according to the forms of the technology that have been described may be advantageous in permitting very high current densities in the coil with very low heating losses. Consequently, such electromagnets 100 may be compact, lightweight and capable of producing very high magnetic field strengths compared to conventional electromagnets of a similar size. Another advantage of using HTS discs to carry large currents in an electromagnet may be a reduction in coil inductance compared to a wired coil with a similar number of turns. A reduction in inductance reduces the required charging voltage and time. The electromagnet 100 may further comprise a first conducting terminal 142 and a second conducting terminal 144, for example as shown in Figures 3D, 4, 5 and 12A. It should be appreciated that, throughout this specification, unless the context clearly indicates otherwise, the term "conducting" refers to electrically conducting. In this form, each of the first and second terminals 142 and 144 are conductively connected to the discs 110 at respective ends of the coaxial stack 118 and allow electrical connection to other components, for example a current supply. The first and second terminals 142 and 144 may be provided in the form of terminal discs that are positioned coaxially with the discs 110 of the stack 118. The terminal discs of the first and second terminals 142 and 144 may have the same or similar diameter as the discs 110 of the coaxial stack 118, such as shown in Figure 3D, or the diameter of the terminal discs of the first and second terminals 142 and 144 may be greater than that of the discs 110 of the coaxial stack 118, such as shown in Figure 4. The first and second terminals 142 and 144 may also comprise a central through-hole, coaxially aligned with the central through-holes 112 of the discs 110 of the coaxial stack 118.

[0105] 6.3. Superconducting Material

[0106] In certain forms of the technology, the discs 110 of the electromagnet 100 may be formed of a crystalline ceramic superconductor material, for example a high-temperature superconducting (HTS) material.

[0107] In certain forms of the technology, the discs 110 are formed from a "bulk" superconducting material, for example a bulk high-temperature superconducting material. The bulk superconducting material used in certain forms of the technology may be a crystalline ceramic material, for example a quasi-single-crystal ceramic superconductor (referring to the term defined earlier in this specification). Exemplary crystalline ceramic HTS materials suitable that may be used in certain forms of technology include cuprate or copper-oxide superconductors, for example a rare-earth barium copper oxide ((RE)-Ba-Cu-O) such as yttrium barium copper oxide (YBCO), gadolinium barium copper oxide (GdBCO), europium barium copper oxide (EuBCO) or bismuth strontium calcium copper oxide (BSCCO) superconductors, and ironbased superconductors. In other exemplary forms, the material may be a pnictide or magnesium diboride (MgBz).

[0108] In certain forms, the discs 110 may be cut as wafers from the bulk material substantially normal to a crystal axis of the material, which may mean that the crystal axis is substantially parallel to a longitudinal axis of the stack 118 formed form the discs 110. In particular, if the material is anisotropic, the direction in which the material exhibits the lowest critical current density may be aligned substantially parallel to a longitudinal axis of the stack 118. For example, in the case of a (RE)-Ba-Cu-O material, the discs may be cut as wafers from the bulk material substantially normal to the c-axis, or parallel to the a-b plane (i.e. with the normal to the faces of the discs 110, which aligns with the longitudinal axis of the coaxial stack 118, being substantially parallel to the c-axis). This may be desirable due to the anisotropy of the critical current in the crystal so that the azimuthal currents in the discs 110, which flow through a relatively small cross-sectional area, experience a higher critical current than the longitudinal currents flowing between the discs 110, which flow through the joints which have a relatively larger cross-sectional area. There may be tolerance to some misalignment of the c-axis of the crystalline material with the longitudinal axis of the coaxial stack 118.

[0109] As explained earlier, typical crystalline ceramic superconducting materials may include voids. It may be desirable to select materials in which the voids are as small as possible, although some voids may be tolerated. In certain forms, the size of the voids may be such that a representative dimension of the voids is less than half the thickness of the disc 110 formed from the material. In certain forms, the representative dimension of the voids may be less than 10% of the thickness of the disc 110 formed from the material.

[0110] In certain forms of the technology, the body of the crystalline ceramic superconductor forming each disc 110 may typically be cylindrical, but may also be formed in another shape, for example a hexagonal, rectangular or square prism. The body may have a size in the order of several centimetres in size in any dimension.

[0111] In certain forms of the technology, the coaxial stack 118 may be formed by first forming the plurality of discs 110 from a cylindrically shaped body of bulk superconducting material, for example a bulk crystalline ceramic superconducting material, such as (RE)-Ba-Cu-O. The discs 110 may be formed from the body of bulk material by a) boring the central through-hole 112, b) cutting the slit 114 and c) cutting the disc from the bulk material. These three steps may, in different forms, occur in any order, although in certain forms of the technology they may occur in the recited order, i.e. a), b) then c). For example, a cylindrically shaped body of bulk superconducting material may be provided (the bulk superconducting material may first be formed into a cylindrical body using known manufacturing techniques) and a through-hole bored through the longitudinal axis of the cylindrical body. A boring machine, for example a drill, may be used to perform this step. The radius of the bore may be made relatively small by direct boring of the bulk superconducting material. A bore with a relatively small radius may be advantageous for increasing the volume of superconducting material in the coaxial stack 118, and consequently, the possible engineering current density that may be achieved in use. The slit 114 may be cut through the resulting cylindrical body between the central through-hole and the outer circumference using a suitable cutting device, for example a diamond wire saw. In some forms, the slit 114 may be cut in a direction that is substantially parallel to a crystal axis of the crystal lattice, for example the c-axis in the case of a (RE)-Ba-Cu-O. Next, the bored, slitted cylindrical body may be cut into discs 110. This may be achieved by using a suitable cutting device, for example a diamond wire saw, to cut through the cylindrical body in a direction substantially perpendicular to the longitudinal axis a plurality of times. The spacing between cuts may be selected to correspond to the desired thickness of the discs 110, i.e. their height in the longitudinal direction. In certain forms of the technology, the thickness of the discs may be selected from the range of approximately 0.2 to 10 mm, for example 0.5 to 3 mm. The thickness of the discs 110 may be selected dependent on the desired trade-off between mechanical strength and magnetic field strength; thicker discs being mechanically stronger while thinner discs produce a stronger magnetic field. As may be apparent from the foregoing description, in certain forms the discs 110 may be formed from one superconducting material as a monolithic body. In other forms, the discs 110 may be formed from multiple pieces of a type of superconducting material, or from multiple superconducting materials.

[0112] It may be useful to smooth the faces of the discs 110 using a polishing device, for example silicon carbide (SiC) paper. In some forms, a coarser grained paper (e.g. P180) may be used for the initial polishing process and increasingly fine paper may subsequently used (up to, e.g. R4000) to achieve as smooth a finish as possible.

[0113] 6.4. Stack Arrangements

[0114] Once the discs 110 are formed, they may be arranged into a coaxial stack 118 such as the forms described above. Some further specifics of coaxial stacks 118 of exemplary forms of electromagnet 100 will now be described.

[0115] In certain forms of the technology, for example as shown in Figures 3C and 3D, the discs 110 are arranged to be immediately adjacent to each other. That is, there is no component or medium separating the discs 110, other than the conducting joint 116 that conductively connects adjacent discs 110, or the medium in which the electromagnet 110 is positioned (e.g. air, other gas, liquid cryogen, filler material (e.g. epoxy - see discussion later) or vacuum) that may exist in small pockets between the discs 110. In such forms, except for the parts of the discs 110 that are conductively connected to the joints 116, one or both faces of each disc 110 abuts against the face of an adjacent disc (one of the faces of the end discs abut against another disc while both faces of non-end discs abut against other discs). In these forms, despite there being nothing separating, and there being contact between, most of the faces of the discs 110, current does not flow between the discs, other than through the conducting joint 116, because of surface interfacial resistance of the superconducting material that forms the discs 110. That is, no supercurrent may be able to flow through between the faces of the discs 110 unless there is a conducting joint 116. An advantage of this arrangement is that the lack of intermediary layers between discs reduces the overall size of the electromagnet 100 compared to an electromagnet with the same size and number of discs where an intermediary layer is present, making the electromagnet more compact. Put another way, for the same size electromagnet 100, the engineering current density is larger in forms where there are no intermediary layers compared to where there are, and therefore higher magnetic fields are achievable for the same size electromagnet 100. The coaxial stack 118 may also be easier to assembly in such forms of the technology compared to forms in which intermediary layers are present.

[0116] Another form of electromagnet 100 according to the technology is illustrated in Figure 5. In this form, the electromagnet 100 is broadly similar to the form shown in Figure 3D, but the electromagnet 100 further comprises a plurality of non-superconducting layers 130. Each pair of adjacent discs 110 is separated by one of the non-superconducting layers 130. Like the form of Figure 3D, the electromagnet in Figure 5 comprises conducting joints 116 connecting the discs 110 into a substantially helical conducting path therefore each non-superconducting layer 130 may comprise a gap which is positioned to allow the conducting joint 116 to connect the adjacent discs through the non-superconducting layer 130. The non-superconducting layers 130 act to electrically insulate most of the faces of the discs 110 from each other so that current only flows between the discs through the conducting joint 116. As explained earlier, this may not be necessary in some forms of the technology, but may be desirable in other forms. For example, where the conducting joint 116 between discs 110 is formed through a process that causes localised melting of the discs 110 (such as explained below), the presence of an insulating layer between the discs 110 may help to avoid a conductive connection being formed between adjacent discs 110 at a part of the face other than where the conducting joint 116 is positioned, which may lead to a turn of the substantially helical conducting path being short-circuited.

[0117] Further details of exemplary non-superconducting layers 130 are explained below.

[0118] It should be understood that, where the term "adjacent" in the context of the discs 110 of a coaxial stack 118 is not qualified as "immediately adjacent", forms of coaxial stack 118 described in this way may include forms where there is an intervening layer of some other material, for example a non- superconducting layer 130. In such forms, the term "adjacent" refers to the subsequent disc 110 in the stack, even if there is an intervening layer between these discs. In contrast, unless the context clearly indicates otherwise, the term "immediately adjacent" is reserved for stack arrangements where there is no component or medium separating the discs 110, other than the conducting joint 116 that conductively connects adjacent discs 110, or air, other gas, liquid cryogen, filler material or vacuum that may exist in small pockets between the discs 110.

[0119] Another exemplary form of the technology is illustrated in Figure 6, which illustrates a subset of discs 110 of an exemplary stack 118. In a practical application this form would usually comprise more discs. Similar to the form shown in Figure 5, the discs 110 are arranged in a coaxial stack 118 with the discs adjacent to each other but separated by non-superconducting layers 130 other than at the joints 116 that are formed through the gaps in the non-superconducting layers 130, as explained above. In the form of Figure 6, each of the discs 110 may have a different peritectic temperature to the disc or discs that are adjacent to it. It should be understood that reference to the peritectic temperature of a disc 110 may be reference to the peritectic temperature of the superconducting material from which the disc 110 is formed. The peritectic temperature may be understood to be the temperature at which an initial solid phase and a liquid phase react together to form a second solid phase at a particular composition. For example, in the case of forms of the technology in which the discs 110 are formed from a RE-123 superconductor, all rare earth cuprate-based superconducting materials of the RE-123 type exhibit a peritectic temperature at which the superconducting RE-123 compound (specifically ReBa2Cu3O7-deita, where RE is one of a number of rare earths) decomposes into a liquid (of approximate composition [3BaCuO2+2CuO]) and RE-211 (specifically Re2BaCuO5) with some evolution of oxygen. This property is an intrinsic feature of their phase diagram and a physical manifestation of the variation with temperature of the Gibbs free energy associated with the different phases. The peritectic reaction occurs at the peritectic temperature. A peritectic reaction is the type of phase reaction in which, on heating, a single phase solid transforms at a constant temperature into a mixture of a solid phase and a liquid phase (and vice versa on cooling). The peritectic temperature is singular, as by definition a peritectic phase reaction occurs at a single temperature. The exact peritectic temperature in any specific case is affected by the choice of rare earth or by the inclusion of other dopants, such as silver. In the case of some common (RE)-Ba-Cu-O materials, for example, exemplary peritectic temperatures may be: YBCO: 1005°C; GdBCO: 1040°C; EuBCO: 1046°C; YBCO-Ag: 989°C; GdBCO-Ag: 1034°C; EuBCO-Ag: 1040°C; NdBCO: 1068°C; and SmBCO: 1054°C. One advantage of discs with different peritectic temperatures being adjacent to each other arises in the way in which the conducting joints 116 between discs 110 may be formed. As will be described in more detail later, in some such forms, the joints 116 may be able to be formed by direct joining (e.g. diffusion bonding together) of the discs 110. In some forms, the peritectic temperatures of the discs 110 alternates along the stack 118. In other forms, the stack 118 may comprise discs 110 having three or more different peritectic temperatures, with the discs arranged so that no two discs with substantially the same peritectic temperature are adjacent to each other. In one example, a stack 118 is comprised of discs formed of EuBCO alternating with discs formed of GdBCO.

[0120] 6.4.1. Stack with Discs of Different Thickness

[0121] Figures 7A to 7D show the results of numerical modelling an electromagnet 100 similar to the form of technology shown in Figure 3D and described earlier. The model was constructed using a 2D axisymmetric geometry for simplicity. In the model, the electromagnet 100 comprises a stack 118 of ten discs 110 in which each disc has a thickness of 1 mm, an inner diameter of 6 mm and an outer diameter of 28 mm. Data for the field and temperature dependent critical current density, Jc(B, T) measured for a GdBCO bulk high-temperature superconductor (HTS) was taken from M. D. Ainslie et al., 2016, Supercond. Sci. Technol. 29 074003, M. D. Ainslie et al., 2019, Supercond. Sci. Technol. 32 034002. This data assumes that the applied magnetic field is substantially parallel to the c-axis of the bulk and ignores any angular dependence of Jc. Consideration of angular dependence would increase the modelled Jc in the stack, as the field experienced by the majority of the stack will be off-perpendicular, however currently there is no available data due to the difficulty in measuring the angular dependence of Jcin a high current HTS bulk. The model ignores losses from the joints between discs. In reality, ohmic heating will raise the temperature of the superconductor surrounding any normal conducting joints, which will suppress the local value of Jc- However, this assumption was considered acceptable, as when using silver diffusion joints (as described below), a resistance of only 72 nO is expected for the whole stack, equating to very low dissipative heating (72 mW at 1 kA or 7.2 W at 10 kA). Forms of the technology comprising superconducting joints are also described below. There is very little n-value data currently available for HTS bulks, so a constant value of n = 20 is used in the model, which is typical of HTS tapes. Figures 7A to 7D show the ratio of the modelled current density to the critical current density at points within a 2D model of the electromagnet 100 for different values of total injected current into the stack. The temperature is assumed to be 77K, which is approximately the boiling point of liquid nitrogen and therefore may be considered to be a temperature to which the HTS material may be cooled with liquid nitrogen. It should be appreciated that performance of the electromagnet 100 will increase at lower temperatures. In Figure 7A, the total current flowing through the stack 118 is modelled as 1000 A, in Figure 7B it is 1500 A, in Figure 7C it is 2000 A and in Figure 7D it is 2700 A, which is a current just lower than the point at which the superconducting discs are modelled to transition to a higher-resistance, or non-superconducting, state. It will be understood that the critical current density of each disc 110 varies due to the differing magnitude of magnetic field applied to the discs 110 from the current flowing through the entire stack 118, which creates a conducting coil.

[0122] The modelling illustrated in Figures 7A to 7D shows that the highest magnetic field generated by the electromagnet 100 is achieved with the highest achievable current passing through the discs, which field strength is modelled to be approximately 1.75 T in the example. The modelling also illustrates that, in each disc 110, the radially outer portion of the disc has a lower ratio of current density to critical current density than the radially inner portion of the disc. In addition, this ratio in the end discs is higher than in the central discs. Those regions in which the ratio remains low at relatively high currents may be said to be shielded from the critical current suppression effects of the magnetic field generated by the stack 118. As the current in the discs 110 continues to increase in the model, the end discs 110 transition to a higher-resistance state at lower currents than the middle discs in the stack 118.

[0123] In certain forms, it may be beneficial to reduce this effect in order that the ratio of current density to critical current density in the stack 118 is more uniform and thereby the discs 110 transition to a higher- resistance state at a more similar, and higher, current density. This may allow higher magnetic field strengths to be achieved. In some forms, the critical current of each disc 110 may be a function of its thickness (i.e. dimension in the direction of the longitudinal axis), with the azimuthal critical current of a thicker disc being higher than that of a thinner disc that is the same in all other ways when the applied magnetic field is the same or similar. Consequently, in some forms, the electromagnet 100 may comprise discs 100 having differing thicknesses. 1

[0124] More specifically, in some forms of electromagnet 100, at least one of the discs 110 in the coaxial stack 118 has a different thickness in this longitudinal direction to one of the other discs 110 in the stack and, in some forms, a different thickness to an adjacent disc 110. In certain forms, discs 110 that are proximate each end of the coaxial stack 118 are thicker in the longitudinal direction than one or all of the discs at or proximate a middle region of the stack 118. In certain forms, discs 110 that are at each end of the coaxial stack 118 are thicker in the longitudinal direction than one or all of the discs at or proximate a middle region of the stack 118. In certain forms, the discs 110 of the stack 118 may have a variety of longitudinal thicknesses and the discs may be arranged so that the thickness of the discs is lowest in a middle region of the stack and increases towards the ends of the stack. In some forms, the discs 110 may be arranged so that the thicknesses of the discs are symmetrical on either side of the middle of the stack, e.g. with the two end discs having the same thickness and so on. In some forms, the thickest discs 110 may be approximately 5-30% thicker than the thinnest discs, for example 10-20% thicker. For example, in a form of electromagnet 100 where the central disc has a thickness of 1 mm, the end discs may have a thickness of 0.85 mm.

[0125] 6.5. Conducting Joint

[0126] As described earlier, electromagnets 100 according to certain forms of the technology comprise discs 110, that are conductively connected into a substantially helical conducting path through conducting joints 116. Details of conducting joints 116 according to exemplary forms of the technology will now be described.

[0127] 6.5.1. Normal Conducting Joint

[0128] In some forms of the technology the conducting joints 116 are normal conducting joints, i.e. they conduct current without exhibiting superconductivity. In examples, the electromagnet 100 may comprise a plurality of conducting joint members 117. Each conducting joint member 117 may be positioned between a pair of the discs 110 and conductively connect the discs to form one of the joints 116. Each conducting joint member 117 may be aligned azimuthally between the respective slits 114 of the pair of discs 110 in order to form the substantially helical conducting path as described earlier.

[0129] In certain forms of the technology, the conducting joint members 117 may be formed from an electrically conducting material, for example a metal. In particular, metals that are non-reactive, and therefore are resistant to oxidisation, may be advantageous, for example silver, gold, platinum, indium, or any of the noble metals. Silver has a very low resistance and has been demonstrated in trials to operate without damaging superconducting material (e.g. (RE)-Ba-Cu-O) so conducting joint members 117 formed from silver may be particularly advantageous. In some forms, the resistance of each conducting joint 116 may be configured to be less than approximately 100 nO, or the joints may be configured to produce a heat dissipation of less than approximately 0.5 W at an operating current of 3000 A, for example.

[0130] One exemplary manner of forming a normal conducting joint 116 between discs 110 in an electromagnet 100 according to certain forms of the technology will now be described. For the purposes of this description, silver will be referred to as the material used for the conducting joint members 117, but it should be understood that other materials such as described above may be used in other forms. A silver member, for example a flat piece of silver is placed between discs 110. For example, as shown in Figure 8, a silver foil member may be placed on a face of one of the discs 110, adjacent the slit 114. The silver foil member may be shaped as a segment of an annulus, as shown in Figure 8. A second disc 110 may then be placed on top of the first disc, with the slit 114 of the second disc oriented so that the silver foil is positioned azimuthally between the slits of the first and second discs. Further pieces of silver and discs are subsequently added in a similar manner to form a coaxial stack 118 such has been described earlier. Next, the coaxial stack 118 may be heated. Heating may soften the silver and cause it to conductively connect to the discs 110, i.e. annealing the silver. In some forms, a diffusion bonding technique may be used, for example in addition to heating the coaxial stack 118, the discs 110 may be urged together in the longitudinal direction for a sufficient period of time to cause solid state diffusion between the materials forming the discs 110 and the intervening silver. The discs 110 may be urged together by placing the coaxial stack 118 in a press or placing a weight on top of the stack, for example. Such a process may form a conducting joint 116 that is a substantially continuous atomically bonded region.

[0131] In some forms, the coaxial stack 118 is heated to a temperature that is sufficiently high to diffusion bond the discs 110 to the conducting joint members 117 but is not so high so as to cause any bonding between adjacent discs 110. This may be a particularly important consideration in forms where there are no insulating non-superconducting layers 130 between the discs 110 so that bonding of the discs together is avoided, for example through localised melting, which would short-circuit the substantially helical conducting path. In some examples, the coaxial stack 118 is heated to a temperature sufficiently close to the melting point of the conducting joint member 117 to enable diffusion bonding of the conducting joint member 117 with the discs 110, but the temperature is not close to the peritectic temperature of the high-temperature superconducting material forming the discs 110. For example, in the case of conducting joint members 117 formed of silver (melting point of approximately 960°C) between discs 110 formed of EuBCO-Ag (peritectic temperature of approximately 980°C), the coaxial stack 118 may be heated to approximately 850°C for approximately an hour.

[0132] 6.5.2. Superconducting Joint

[0133] In some forms of the technology the conducting joints 116 are superconducting joints. Eliminating resistance from the conducting joints reduces energy loss through heating, which may heat the electromagnet 100 and may ultimately lead to a quench. It may also allow high-current output flux pumps to power the electromagnet 100. In examples, the electromagnet 100 may comprise a plurality of conducting joint members 117. Each conducting joint member 117 may be positioned between a pair of the discs 110 and conductively connect the discs to form one of the joints 116. Each conducting joint member 117 may be aligned azimuthally between the respective slits 114 of the pair of discs 110 in order to form the substantially helical conducting path as described earlier.

[0134] In certain forms of the technology, a superconducting joint 116 may be formed using a normal conducting joint member 117. As before, the example of a piece of silver will be used, but it should be understood that other materials such as described above may be used in other forms. A plurality of pieces of silver foil may be placed between discs 110 to form a coaxial stack 118, and the stack 118 may be heated and optionally also subject to pressure in the longitudinal direction to urge the discs 110 together, as described in the example of the normal conducting joint above. In this superconducting form, however, the stack 118 is heated to a temperature at or proximate, for example slightly below, the peritectic temperature of the HTS used to form the discs 110. The stack 118 may be held at this temperature for a sufficient duration, for example approximately an hour in some forms, to cause the silver to diffuse into the discs 110, which lowers the local peritectic temperature of the discs 110 and causes localised melting. After the heating step has been completed and the heat is removed, the stack 118 cools and the molten material recrystallises with the same crystal structure as present in the nonmelted parts of the discs 110, creating superconducting joints 116 between the discs 110. In certain forms, the stack 118 may be heated to approximately 900°C or above, with the upper limit of the peak temperature being the peritectic temperature of the disc 110 or joint member 117 having the highest peritectic temperature. If this temperature is exceeded, the relevant part may become polycrystalline, which may degrade its ability to carry current at elevated magnetic fields.

[0135] In these forms, since the stack is heated to a temperature at or close to the peritectic temperature of the discs 110, there may be some localised melting of the discs 110, so the stack 118 may comprise non- superconducting layers 130 between the discs to prevent short-circuiting between the turns of the substantially helical conducting path. Examples of non-superconducting layers 130 are described further below.

[0136] In other forms of the technology in which the joint 116 is a superconducting joint, the conducting joint members 117 may be formed from a superconducting material. One such exemplary form is shown in Figure 5. In this example, the joint members 117 may be formed from a first type of superconducting material, for example a high-temperature superconductor such as GdBCO-Ag. The discs 110 in this form may be formed from a second type of superconducting material, for example another high-temperature superconductor such as EuBCO-Ag. The first and second type of superconducting material may have different peritectic temperatures. When forming the electromagnet 100 in such forms, the stack 118 may be heated to a temperature at or proximate the lowest peritectic temperature of the two types of HTS. Consequently, the discs 110 and / or joint members 117 (whichever is formed from the material having the lower peritectic temperature) undergo localised melting during the heating process and, upon cooling, the molten material recrystallises with the same crystal structure as present in the nonmelted superconducting material (i.e. the other of the discs 110 or joint members 117), forming a strong superconducting joint 116. In some forms, insulation may be placed between the discs 110, e.g. non- superconducting layers 130 to avoid additional joints being formed between the discs 110, which may short-circuit the helical paths in use.

[0137] In other forms of the technology in which the joint 116 is a superconducting joint, superconducting joints 116 may be formed directly between the discs 110 through heating the stack 118 and conductively connecting adjacent discs to each other, e.g. through diffusion bonding. A part of a stack 118 in one such exemplary form of the technology is shown in Figure 6. This form of the technology has already been described above. To form the electromagnet 100 in this form, the components of the stack 118 may first be assembled, and then the stack 118 may be heated and may optionally be placed under pressure in the longitudinal direction. As with the previous example, the stack 118 may be heated to a temperature at or proximate the lowest peritectic temperature of the two types of HTS material that form the alternate disc layers. The discs 110 having the lower peritectic temperature undergo localised melting and bond with the adjacent discs through the gaps in the non-superconducting layers 130 to form a superconducting joint 116. One advantage of these forms of the technology is that the non- superconducting layers 130 may be formed to be relatively thin (compared to the layers in other forms) and therefore the engineering current density of the electromagnet may be increased, providing the advantages of a relatively high engineering current density as described earlier.

[0138] In certain forms, the electromagnet 100 is configured so that the critical current of each superconducting joint 116 is greater than the critical current of the discs 110. This may be achieved through suitable selection of the material used to form the superconducting joint 116, for example.

[0139] 6.6. Non-Superconducting Layers

[0140] It has already been explained that, in some forms of the technology, the electromagnet 100 may comprise a plurality of non-superconducting layers 130 that provide electrical insulation between the faces of adjacent discs 110 so that current only flows between the discs 110 through the joints 116. In some forms, the non-superconducting layers 130 may comprise a single body of material, while in other forms, each non-superconducting layer 130 may comprise a plurality of insulating bodies arranged across the width of the electromagnet 100 to separate all parts of the faces of adjacent discs 110. It will be appreciated that the non-superconducting layers 130 do not extend between adjacent discs 110 at the location of the joints 116. That is, each non-superconducting layer 130 may comprise a gap through which the helical conducting path may pass between adjacent discs 110 through the joint 116.

[0141] The non-superconducting layers 130 may be formed from any non-superconducting material, or electrically insulating material, that provide a suitable level of insulation between the faces of the discs 110. In some forms, each non-superconducting layer 130 may comprise a coating that is applied to one or more of the faces of the discs 110, such as is explained in more detail below. In some forms, each non-superconducting layer 130 may be formed from a ceramic, for example an oxide ceramic such as zirconium oxide (zirconia), aluminium oxide (alumina), titanium oxide (titania), magnesium oxide (magnesia) and silicon oxide (silica) or mixtures thereof. In some forms, each non-superconducting layer 130 may be formed from the granular form of an insulating material, such as those previously mentioned, in a suspension. In some forms, the granular form may be a powder. In other forms, the non-superconducting layers 130 may be formed from another type of insulator, for example Kapton, paper or sapphire.

[0142] In one exemplary method of forming an electromagnet 100 with non-superconducting layers 130 positioned between discs 110, a coating is applied to one or both faces of each disc 110, for example by painting or otherwise applying the coating to the faces. The coating may comprise an insulating material in granular form, which may in some forms be a powder, in a suspension. Once applied to the face of each disc 110, the medium in which the insulating material is suspended (which may be referred to as the solvent) may be permitted to evaporate, for example by leaving the coating to dry or by heating, so that the insulating material is left as a residue on the face of the disc 110. In some forms, the insulating material in the coating may be any of the ceramics described above, and the solvent may be any suitable liquid carrier that may be evaporated at achievable temperatures, for example a volatile organic solvent (e.g. ethanol, methanol, acetone, isopropanol, etc) or water. This method may be particularly desirable where the electromagnet 100 comprises superconducting joints 116 between discs 110 and where the process of forming the joints 116 comprises heating the stack 118 to a temperature at or proximate the peritectic temperature of the HTS used to form the discs 110.

[0143] In some forms, the areas of the faces of the discs 110 that will form the conducting joints 116 may be masked when the coating is applied to the faces. For example, masking tape or some other masking material may be used, which is later removed after the coating has been applied and before the conducting joints 116 are formed.

[0144] 6.7. Nested Stacks

[0145] In certain forms of the technology, the electromagnet 100 may be formed of two nested coaxial stacks of superconductor discs, with one stack placed inside the other. Exemplary electromagnets 100 according to such forms of the technology are illustrated in schematic view in Figures 9A and 12B and exploded view in Figure 9B.

[0146] In these forms, the electromagnet 100 comprises an inner stack 118a and an outer stack 118b. Each stack is formed from a plurality of discs, 110a and 110b respectively, arranged in a coaxial stack in any manner described according to earlier forms of the technology. The through-holes of the discs 110b of the outer stack 118b are aligned and form an axial bore along the longitudinal axis. The inner stack 118a is positioned inside the axial bore formed by the outer stack 118b and the two stacks are arranged coaxially. Consequently, the inner diameter of the outer discs 110b is larger than the outer diameter of the inner discs 110b and there is an annular gap between the two stacks. The two stacks may also be substantially longitudinally aligned, i.e. with the ends of each stack substantially aligned with each other.

[0147] In some forms, both the inner and outer stacks may comprise the same number of discs. The discs of both stacks may be formed from the same superconducting material. Both stacks may comprise conducting joints formed in the same way. Both stacks may also comprise the same form of non- superconducting layer 130, or both may lack a non-super conducting layer 130. In other forms, any one or more of these properties may differ between the inner and outer stacks.

[0148] The discs 110a and 110b of each stack are conductively joined into substantially helical conducting paths at joints (not shown in Figures 9A, 9B and 12B), for example in any of the arrangements and using any of the techniques explained earlier for conducting joints according to forms of the technology.

[0149] Furthermore, the inner stack 118a may be conductively connected in series to the outer stack 118b, and the electromagnet 100 may comprise one or more conductors electrically connected to the inner and outer stacks to achieve this series connection. The conductors may be superconducting and / or normal conducting. The series connection between the inner stack 118a and the outer stack 118b may be configured so that, when current passes through the stacks, the azimuthal direction of current flow is in the same direction in both the inner stack 118a and the outer stack 118b, e.g. both clockwise or both anti-clockwise. This means that the magnetic fields produced by the conducting coils in the stacks are in substantially the same direction and consequently add together. In some forms, this arrangement may therefore produce a higher strength magnetic field than an electromagnet of similar parameters comprising only a single helical conducting path.

[0150] In some forms, the azimuthal direction of current flow may be made the same in the two stacks by arranging the discs in each stack so that the chirality of the substantially helical conducting paths formed by each stack is different, and passing current in longitudinally different directions through the stacks. For example, the inner stack 118a may have a first chirality, for example it may form a right-handed helix, and the outer stack 118b may have a second chirality, different from the first chirality, for example it may form a left-handed helix. In other forms, the right-handedness and left-handedness of the inner and outer stacks may be reversed. Figure 9B illustrates how this differing chirality may be achieved in the arrangement of the discs in each stack. In the inner stack 118a, the orientation of the slits 114a in each adjacent disc 110a of the stack may change through rotation of one disc relative to the slit in the adjacent disc in one direction sequentially along the discs in the stack, for example this change may be anti-clockwise moving down the discs in the inner stack 118a as shown in Figure 9B. In the outer stack 118b, the orientation of the slits 114b in each adjacent disc 110b of the stack may change through rotation of one disc relative to the slit in the adjacent disc in the other, opposite direction sequentially along the discs in the stack, for example this change may be clockwise moving down the discs in the outer stack 118b as shown in Figure 9B.

[0151] Another property of such a form to achieve the aligned magnetic fields of the two stacks is that the current may pass longitudinally through the stacks in opposite longitudinal directions, i.e. current may pass through the inner stack 118a in a first longitudinal direction and the current may pass longitudinally through the outer stack 118b in a second direction, opposite to the first direction. This may be achieved by connecting the inner and outer stack in series using an end conductor 140 forming a conducting path between a first end inner disc of the plurality of inner discs in the inner stack 118a and a first end outer disc of the plurality of outer discs in the outer stack 118b, the first end inner disc and the first end outer disc being at the same ends of the inner stack 118a and the outer stack 118b respectively. Such an arrangement is shown in Figures 9A and 9B, where end conductor 140 comprises a conducting plate mounted at one end of the electromagnet 100 and with one of its faces conductively connected to the outer-facing faces of the end discs of both the inner stack 118a and the outer stack 118b. The end conductor may be formed from a bulk HTS material, for example a crystalline ceramic HTS, like the discs 110.

[0152] In these exemplary forms of the technology, such as shown in Figures 9A, 9B and 12B, the electromagnet 100 may comprise a first terminal 142 conductively connected to a second end inner disc of the plurality of inner discs 110a, wherein the second end inner disc is at the opposite end of the inner stack from the first end inner disc. The electromagnet 100 may further comprise a second terminal 144 conductively connected to a second end outer disc of the plurality of outer discs 110b, wherein the second end outer disc is at the opposite end of the outer stack from the first end outer disc. The first and second terminals allow an external current supply to provide current to the electromagnet 100.

[0153] In other forms, the azimuthal direction of current flow may be made the same in the two stacks by arranging the discs in each stack to have the same chirality and connecting the stacks in series so that current passes longitudinally in the same direction in each stack. In such a form, the angular change between the slits 114 of the discs 110 in each stack is in the same direction sequentially moving from one disc to the other in the same direction in each stack. In addition, the stacks are conductively connected in series so that an end inner disc at one end of the plurality of inner discs in the inner stack 118a is connected to an end outer disc at the other, opposite end of the plurality of outer discs in the outer stack 118b. In these other exemplary forms, the electromagnet 100 may comprise a first terminal 142 conductively connected to the end inner disc of the plurality of inner discs 110a at the opposite end of the inner stack 118a to which the conductive connection between the stacks is connected. The electromagnet 100 may further comprise a second terminal 144 conductively connected to an end outer disc of the plurality of outer discs 110b at the opposite end of the outer stack 118b to which the conductive connection between the stacks is connected, and at an opposite end of the electromagnet to the first terminal 142.

[0154] While the forms of the technology described above comprise two stacks of discs, one inner stack nested inside an outer stack, in other forms of the technology, an electromagnet 100 may comprise additional stacks in a similar nested arrangement, for example three, four or more nested stacks. As with the examples described above, in such an arrangement, the stacks may all be substantially coaxially arranged and current is configured to flow through the stacks in the same azimuthal direction so that the magnetic fields of each stack add together, therefore increasing the overall magnetic field strength generated by the electromagnet.

[0155] Figures 10A to 10D show the results of modelling an electromagnet 100 with inner and outer nested stacks of discs 110 similar to the form of technology shown in Figures 9A and 9B and described earlier. The model was constructed using a 2D axisymmetric geometry for simplicity. In the model, the electromagnet 100 comprises two stacks of ten discs each in which each disc has a thickness of 1 mm, The inner stack has an inner diameter of 6 mm and an outer diameter of 28 mm, and the outer stack has an inner diameter of 32 mm and an outer diameter of 56 mm. Other assumptions made in the modelling shown in Figures 10A to 10D are the same as those explained above for Figures 7A to 7D. Figures 10A to 10D show the ratio of the current density with the critical current density at points within a 2D model of the electromagnet 100. The temperature is assumed to again be 77K, which is approximately the boiling point of liquid nitrogen and therefore may be considered to be a temperature to which the HTS material may be cooled with liquid nitrogen. In Figure 10A, the current flowing through the stacks is modelled as 1000 A, in Figure 10B it is 1500 A, in Figure 10C it is 2000 A and in Figure 10D it is 2300 A, which is a current just lower than the point at which the superconducting discs are modelled to transition to a higher-resistance, or non-superconducting, state. Again it will be understood that the critical current density of each disc 110 varies due to the differing magnitude of magnetic field applied to the discs 110 from the current flowing through the stacks.

[0156] The modelling illustrated in Figures 10A to 10D illustrates that the highest magnetic field generated by the electromagnet 100 is achieved with the highest achievable current passing through the discs, which field strength is modelled to be approximately 2.1 T in the example. The modelling also illustrates that, for the electromagnet 100 with the parameters stated above, the discs 110a of the inner stack 118a transition to the higher-resistance, or non-superconducting, state while a large region of the outer stack 118b remains shielded. This is because the magnetic field strength applied to the inner stack 118a is much stronger, causing increased suppression of the critical current in the inner discs 110a.

[0157] To achieve the highest possible, or at least a relatively high, maximum magnetic field strength produced by the electromagnet 100, the electromagnet 100 may be configured so that both the inner stack 118a and the outer stack 118b saturate at the same transport current. That is, in certain forms of the technology, the inner stack 118a and the outer stack 118b are configured and arranged so that, as an amount of current flowing through the inner stack 118a and the outer stack 118b increases, the inner stack 118a transitions from a low-resistance state to a higher-resistance state (e.g. a non- superconducting state) at substantially the same amount of current as the outer stack 118b. Put another way, the critical current of the disc with the lowest critical current in each stack may be substantially equal to the critical current of the disc with the lowest critical current in each other stack.

[0158] In these nested forms of the technology there are numerous parameters that may be independently varied between forms, including but not limited to: the number of stacks; the number of discs in each stack; the inner and outer diameters of each stack; the total height of each stack; the height of each disc; the material used to form each disc; and the method used to form conducting joints in each stack. Any one or more of these parameters may be varied in order to achieve an electromagnet 100 in which the inner and outer stacks saturate at the same transport current, and the skilled addressee will appreciate how to experiment with some variation of these parameters in order to achieve this in an electromagnet 100 suitable for particular applications.

[0159] Figures 11A to 11C illustrate the effects of changing one of these parameters, specifically the width of the outer stack 118b, in an exemplary electromagnet 100 which comprises two nested stacks of superconducting discs. These figures illustrate the current density modelled to flow through discs of nested stacks in an electromagnet similar to that modelled in Figures 10A to 10D only in Figure 11A, the outer diameter of the outer stack 118b is 45 mm, in Figure 11B, the outer diameter of the outer stack 118b is 52 mm, and in Figure 11C, the outer diameter of the outer stack 118b is 56 mm. The other parameters of the electromagnet 100 are unchanged between these examples so the parameter that is changing is the outer diameter of the outer stack 118b, or the radial width of each outer disc 110b in the outer stack 118b, i.e. the radial distance between the inner circumference of the outer stack around the through-hole and the outer circumference of the outer stack 118b.

[0160] In Figure 11A, which shows an electromagnet 100 with relatively narrow outer discs 110b, the outer stack 118b is modelled to saturate first at 1.7 kA, and the top and bottom end discs 110b stop shielding at this point. The current density distribution just before saturation is shown in Figure 11A, in which the inner stack 118a is largely unsaturated and remains shielding. The maximum magnetic flux density at the centre of the electromagnet 100 is 1.81 T. Increasing the outer diameter of the outer discs 110b to 52 mm, as per the form shown in Figure 11B, the inner stack 118a is now modelled to saturate first at 2.32 kA, and the middle discs 118a of the inner stack stop shielding at this point. The current density distribution just before saturation is shown in Figure 11B. This time both the inner and outer stacks are almost saturated at the same time. The maximum magnetic flux density at the centre is 2.18 T. This is significantly (20.4%) higher than the value for the electromagnet of Figure 11A because the inner stack 118a (which contributes most of the magnetic field) is no longer being limited by the outer stack 118b. If the outer diameter of the outer stack 118b is increased further to 56 mm, as shown in Figure 11C, the inner stack 118a is modelled to still saturate first, also at approximately 2.32 kA (+ / - 20 A). The current density distribution just before saturation is shown in Figure 11C. In this example, the outer stack 118b is still largely shielded and the maximum magnetic flux density at the centre is 2.15 T. This central magnetic field strength is close to the example of Figure 11B, but is slightly lower because the current in the outer stack 118b isn't circulating as close to the inner radius on average.

[0161] From the above, it can be seen that it may be preferable to configure the electromagnet 100 so that the outer stack 118b does not limit the inner stack 118a to achieve high fields. In the example shown, this is achieved by the radial width of each outer disc 110b between the through-hole and the outer circumference of the outer disc 110b being less than the radial width of each inner disc 110a between the through-hole and the outer circumference of the inner disc 110a. Although the magnetic field strength in the case of Figure 11C, where the radial width of the outer discs is slightly larger than the radial width of the inner discs, is similar to that of Figure 11B, by increasing the radial width of the outer discs compared to the case of Figure 11B, the overall size of the electromagnet 100 is smaller, in this case 15% smaller in volume compared to the example of Figure 11C. This makes the electromagnet 100 of Figure 11B cheaper, as the constituent rare-earth elements and silver are expensive, and more compact. Another advantage of using smaller diameter discs 110b for the outer stack 118b is that the quality of bulk HTS material tends to drop off with size, and more defects are likely to form in a larger piece.

[0162] While this example shows the effect of altering the width of the outer stack 118b on the relative saturation points of the inner and outer stacks, and the maximum possible magnetic field strength achievable for a given electromagnet configuration, it will be understood that other parameters may be varied and the skilled addressee may be able to determine, for example through experimentation, a suitable configuration to achieve the desired magnetic field strength in a given application. For example, in certain forms of the technology, the achievable magnetic field strength may be able to be increased by reducing the diameter of the through-hole in the inner stack 118a (i.e. the inner diameter of the inner discs 110a). In other forms, reducing the gap between the inner stack 118a and the outer stack 118b may also increase the maximum magnetic field strength that can be achieved. While it may be desirable to minimise the diameter of the through-hole in the inner stack 118a (i.e. the central bore) and the gap between the inner and outer stacks from a performance perspective, there are also practical limitations to the minimum sizes of these gaps. For example, the diameter of the central bore depends on the intended use of the electromagnet 100, for example in some applications it may need to contain an object to which a magnetic field needs to be applied. The gap between stacks may be sufficiently large to enable the stacks to be positioned relative to each other without damaging any of the discs, for example the gap may be 1 to 2 mm in some exemplary forms.

[0163] 6.8. Housing

[0164] In certain forms of the technology, the electromagnet 100 may comprise a housing 150 to contain the one or more coaxial stacks 118. The housing may comprise an outer wall 152, which may be cylindrical in shape, positioned radially outside the outer circumference of the coaxial stack 118. In some forms, the housing may further comprise an inner wall 154, which may also be cylindrical in shape. The inner wall 154 may be positioned radially inside the inner circumference of the coaxial stack 118, for example inside the central axial bore 113. Examples of forms of two forms of the technology comprise housings

[0165] 150 are illustrated in Figures 12A and 12B.

[0166] The electromagnet 100 may further comprise a positioning and stabilising structure in order to position and stabilise the coaxial stack 119 within the housing. For example, in some forms, the internal volume of the housing that is not occupied by the coaxial stack(s) 118 may be substantially filled with a filler material 160, which may solidify after application in order to maintain the stack in position. The filler material 160 may be, for example, an epoxy resin or other thermosetting polymer. The housing and / or filler material may also act to absorb any mechanical stress exerted on the electromagnet 100 in order to protect the stack(s). In the example of Figure 12A, the filler material 160 fills the space, which may be in the shape of a cylindrical annulus, between the outer wall 142 and the outer circumference of the coaxial stack 118. In the example of Figure 12B, the filler material 160 fills the space, which may be in the shape of a cylindrical annulus, between the outer wall 142 and the outer circumference of the coaxial stack 118, and may also fill the space, which may also be in the shape of a cylindrical annulus, between the inner stack 118a and the outer stack 118b. In some forms of the technology, the filler material may also fill, or partly fill the axial bore 113 in the centre of the electromagnet 100. In other forms, the bore 113 may be unoccupied so that an object may be located inside the electromagnet 100, for example so that the magnetic field generated by the electromagnet 100 may be applied to that object.

[0167] 6.9. Current Supply

[0168] In certain forms of the technology, the electromagnet 100 may comprise a current supply for supplying current to the discs 110 of the coaxial stack(s) 118. In other forms, the electromagnet 100 may not be considered to itself comprise such a current supply, but be configured to receive current from a suitable current supply.

[0169] It has already been explained that, in the case of forms of the technology in which the electromagnet 100 comprises nested stacks 118, the electromagnet may comprise a first terminal 142 and a second terminal 144 to receive an external current supply. Other forms of electromagnet 100 may also comprise such first and second terminals. In forms where the electromagnet 100 comprises a housing, such as described above, the housing may be configured to permit the first terminal 142 and second terminal 144 to pass through one or more walls of the housing in order to permit the current supply to enter the housing. For example, conductors such as current leads may pass through one or more openings in the housing.

[0170] In alternative forms, the electromagnet 100 may be configured so that current may be wirelessly induced in the electromagnet 100, for example through the use of a flux pump. This may reduce the size of current source required to input large currents to the electromagnet 100, and may allow for relatively high magnetic fields to be produced by a relatively compact electromagnet.

[0171] 6.10. Cryogenic Cooler

[0172] It will be appreciated that, in order for the discs 110 to adopt a low-resistance superconducting state, they must be held at a temperature that is below the critical temperature for the superconducting material. In certain forms of the technology, the electromagnet 100 is maintained at such superconducting temperatures by a cryogenic cooler. For example, in some forms, the electromagnet 100 is positioned inside a cryostat. In other forms, the electromagnet 100 may be considered to comprise the cryostat and the coaxial stack 118 is positioned inside the cryostat.

[0173] 6.11. Other Remarks

[0174] Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like, are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense, that is to say, in the sense of "including, but not limited to".

[0175] The entire disclosures of all applications, patents and publications cited above and below, if any, are herein incorporated by reference.

[0176] Reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that that prior art forms part of the common general knowledge in the field of endeavour in any country in the world. The technology 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, in any or all combinations of two or more of said parts, elements or features. Where in the foregoing description reference has been made to integers or components having known equivalents thereof, those integers are herein incorporated as if individually set forth.

[0177] It should be noted that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the spirit and scope of the technology and without diminishing its attendant advantages. It is therefore intended that such changes and modifications be included within the present technology.

Claims

7. CLAIMS1. A superconducting electromagnet comprising: a plurality of discs formed from a superconducting material, wherein each of the discs has formed therein a central through-hole and a slit extending through the disc between the through-hole and an outer circumference of the disc, wherein the discs are arranged in a coaxial stack with each disc immediately adjacent to the adjacent disc(s) and with the slit of each disc being azimuthally offset from the slit of each adjacent disc, and wherein each pair of adjacent discs are conductively connected by a conducting joint aligned azimuthally between the respective slits of the respective pair of adjacent discs so that the coaxial stack of discs forms a substantially helical conducting path.

2. A superconducting electromagnet as claimed in claim 1, wherein the superconducting material is a crystalline ceramic superconductor.

3. A superconducting electromagnet as claimed in claim 2, wherein the superconducting material is a cuprate superconductor.

4. A superconducting electromagnet as claimed in claim 3, wherein the superconducting material is a rare-earth barium copper oxide ((RE)-Ba-Cu-O) superconductor.

5. A superconducting electromagnet as claimed in any one of claims 2-4, wherein a crystal axis of the superconducting material in each disc is substantially parallel to a longitudinal axis of the coaxial stack.

6. A superconducting electromagnet as claimed in any one of claims 1-5, wherein the discs are conductively connected so that, when a current flows along the helical conducting path, the current flows into a first face of each disc and out of a second face of each disc, the second face being opposite to the first face.

7. A superconducting electromagnet as claimed in any one of claims 1-6, wherein the conducting joint comprises a conducting joint member positioned between the respective pair of adjacent discs and aligned azimuthally between the respective slits of the respective pair of adjacent discs.

8. A superconducting electromagnet as claimed in claim 7 , wherein each-conducting joint member is diffusion bonded to the respective pair of adjacent discs to form a conducting joint.

9. A superconducting electromagnet as claimed in any one of claims 1-8, wherein the slit extends through each disc in a substantially radial direction.

10. A superconducting electromagnet as claimed in any one of claims 1-9, wherein the superconducting electromagnet further comprises a first conducting terminal at a first end of the coaxial stack and a second conducting terminal at a second end of the coaxial stack, the first end being opposite the second end, wherein each of the first terminal and the second terminal comprises a terminal disc positioned coaxially with the discs of the coaxial stack, and wherein each of the first terminal and the second terminal is conductively connected to the disc at the respective end of the coaxial stack.

11. A superconducting electromagnet as claimed in any one of claims 1-10, wherein the superconducting electromagnet further comprises a housing to contain the coaxial stack, wherein an internal volume of the housing that is not occupied by the coaxial stack is substantially filled with a filler material.

12. A superconducting electromagnet comprising: a plurality of discs formed from a crystalline ceramic superconducting material, wherein each of the discs has formed therein a central through-hole and a slit extending through the disc between the through-hole and an outer circumference of the disc; wherein the discs are arranged in a coaxial stack with each disc adjacent to the adjacent disc(s) and with the slit of each disc being azimuthally offset from the slit of each adjacent disc; andwherein each pair of adjacent discs are conductively connected by a conducting joint member positioned between the respective pair of adjacent discs and aligned azimuthally between the respective slits of the respective pair of adjacent discs so that the coaxial stack of discs forms a substantially helical conducting path, and wherein the conducting joint member is conductively connected to the pair of adjacent discs by heating.

13. A superconducting electromagnet as claimed in claim 12, wherein the crystalline ceramic superconducting material is a cuprate superconductor.

14. A superconducting electromagnet as claimed in claim 13, wherein the crystalline ceramic superconducting material is a quasi-single-crystal rare-earth barium copper oxide ((RE)-Ba-Cu-O) superconductor.

15. A superconducting electromagnet as claimed in any one of claims 12-14, wherein the c-axis of the quasi-single-crystal (RE)-Ba-Cu-O superconducting material in each disc is substantially parallel to a longitudinal axis of the coaxial stack.

16. A superconducting electromagnet as claimed in any one of claims 12-15, wherein the discs are conductively connected so that, when a current flows along the helical conducting path, the current flows into a first face of each disc and out of a second face of each disc, the second face being opposite to the first face.

17. A superconducting electromagnet as claimed in any one of claims 12-16, wherein each conducting joint member comprises a normal conducting joint member.

18. A superconducting electromagnet as claimed in any one of claims 12-17, wherein each conducting joint member is diffusion bonded to the respective pair of adjacent discs to form a normal conducting joint.

19. A superconducting electromagnet as claimed in any one of claims 12-17, wherein each conducting joint member is diffusion bonded to the respective pair of adjacent discs to form a superconducting joint.

20. A superconducting electromagnet as claimed in any one of claims 12-16, wherein each conducting joint comprises a superconducting joint member.

21. A superconducting electromagnet as claimed in any one of claims 12-20, wherein the superconducting electromagnet further comprises a non-superconducting layer between each pair of adjacent discs.

22. A superconducting electromagnet as claimed in any one of claims 12-21, wherein the slit extends through each disc in a substantially radial direction.

23. A superconducting electromagnet as claimed in any one of claims 12-22, wherein the superconducting electromagnet further comprises a first conducting terminal at a first end of the coaxial stack and a second conducting terminal at a second end of the coaxial stack, the first end being opposite the second end, wherein each of the first conducting terminal and the second conducting terminal comprises a terminal disc positioned coaxially with the discs of the coaxial stack, and wherein each of the first conducting terminal and the second conducting terminal is conductively connected to the disc at the respective end of the coaxial stack.

24. A superconducting electromagnet as claimed in any one of claims 12-23, wherein the superconducting electromagnet further comprises a housing to contain the coaxial stack, wherein an internal volume of the housing that is not occupied by the coaxial stack is substantially filled with a filler material.

25. A method of forming a superconducting electromagnet, the method comprising: forming a plurality of discs from a crystalline ceramic superconducting material, wherein each of the discs has formed therein a central through-hole and a slit extending through the disc between the through-hole and an outer circumference of the disc; applying a coating to at least one face of one or more of the discs, wherein the coating comprises a granular form of an insulating material; arranging the discs in a coaxial stack with each disc immediately adjacent to the adjacent disc(s) and with the slit of each disc being azimuthally offset from the slit of each adjacent disc; andconductively connecting each pair of adjacent discs by forming a conducting joint aligned azimuthally between the respective slits of the respective pair of adjacent discs so that the coaxial stack of discs forms a substantially helical conducting path.

26. A method of forming a superconducting electromagnet as claimed in claim 25, wherein the coating is applied to the face of each disc as a granular form of the insulating material in a suspension.

27. A method of forming a superconducting electromagnet as claimed in any one of claims 25-26, wherein the insulating material is an oxide ceramic.

28. A method of forming a superconducting electromagnet, the method comprising: forming a plurality of discs from a crystalline ceramic superconducting material, wherein each of the discs has formed therein a central through-hole and a slit extending through the disc between the through-hole and an outer circumference of the disc; arranging the discs in a coaxial stack with each disc immediately adjacent to the adjacent disc(s) and with the slit of each disc being azimuthally offset from the slit of each adjacent disc; and heating the coaxial stack to conductively join each pair of adjacent discs with a conducting joint aligned azimuthally between the respective slits of the respective pair of adjacent discs so that the coaxial stack of discs forms a substantially helical conducting path.

29. A method of forming a superconducting electromagnet as claimed in claim 28, wherein the method comprises conductively joining each pair of adjacent discs using diffusion bonding.

30. A method of forming a superconducting electromagnet as claimed in any one of claims 28-29, wherein the method further comprises placing one of a plurality of conducting joint members between each pair of adjacent discs, the respective conducting joint members being aligned azimuthally between the respective slits of the respective pair of adjacent discs, and the step of heating comprises heating the conducting joint members to form the conducting joints.

31. A method of forming a superconducting electromagnet as claimed in any one of claims 28-30, wherein the method comprises heating the discs and joint members to a temperature below theperitectic temperature of the crystalline ceramic superconducting material so that the conducting joint members are conductively joined to adjacent discs in a normal conducting joint.

32. A method of forming a superconducting electromagnet as claimed in any one of claims 28-30, wherein the method comprises heating the discs and joint members to a temperature at or proximate the peritectic temperature of the crystalline ceramic superconducting material so that the conducting joint members are conductively joined to adjacent discs in a superconducting joint.

33. A method of forming a superconducting electromagnet as claimed in any one of claims 28-32, wherein the method further comprises forming a non-superconducting layer between each pair of adjacent discs.

34. A superconducting electromagnet formed using the method of any one of claims 25-33.

35. A superconducting electromagnet comprising: a plurality of inner discs formed from superconducting material, wherein each of the inner discs has formed therein a central through-hole and a slit extending through the inner disc between the through-hole and an outer circumference of the inner disc, wherein the inner discs are arranged in a coaxial inner stack with the slit of each inner disc being azimuthally offset from the slit of each adjacent inner disc, and wherein each pair of adjacent inner discs are conductively connected by a conducting joint aligned azimuthally between the respective slits of the respective pair of adjacent inner discs so that the coaxial inner stack of inner discs forms a substantially helical conducting path; wherein the superconducting electromagnet further comprises: a plurality of outer discs formed from superconducting material, wherein each of the outer discs has formed therein a central through-hole and a slit extending through the outer disc between the through-hole and an outer circumference of the outer disc,wherein the outer discs are arranged in a coaxial outer stack with the central through-holes of each of the outer discs forming an axial bore and with the slit of each outer disc being azimuthally offset from the slit of each adjacent outer disc, and wherein each pair of adjacent outer discs are conductively connected by a conducting joint aligned azimuthally between the respective slits of the respective pair of adjacent outer discs so that the coaxial outer stack of outer discs forms a substantially helical conducting path, wherein the inner stack and the outer stack are arranged substantially coaxially with the inner stack positioned inside the axial bore of the outer stack, wherein the superconducting electromagnet further comprises one or more conductors connecting the inner stack and the outer stack in series so that, when current passes through the inner stack and the outer stack, the azimuthal direction of current flow is in the same direction in both the inner stack and the outer stack, and wherein the inner stack and the outer stack are configured and arranged so that, as an amount of current flowing through the inner stack and the outer stack increases, the inner stack transitions from a low-resistance state to a higher-resistance state at substantially the same amount of current as the outer stack.

36. A superconducting electromagnet as claimed in claim 35, wherein the one or more conductors comprise an end conductor forming a conducting path between a first end inner disc of the plurality of inner discs and a first end outer disc of the plurality of outer discs, wherein the first end inner disc and the first end outer disc are at the same ends of the inner stack and the outer stack respectively, and wherein the substantially helical conducting path formed by the inner stack of inner discs has a first chirality and the substantially helical conducting path formed by the outer stack of outer discs has a second chirality, wherein the second chirality is different from the first chirality.

37. A superconducting electromagnet as claimed in claim 36, wherein the electromagnet further comprises: a first conducting terminal conductively connected to a second end inner disc of the plurality of inner discs, wherein the second end inner disc is at the opposite end of the inner stack from the first end inner disc; anda second conducting terminal conductively connected to a second end outer disc of the plurality of outer discs, wherein the second end outer disc is at the opposite end of the outer stack from the first end outer disc.

38. A superconducting electromagnet as claimed in any one of claims 35-37, wherein the radial width of each outer disc between the through-hole and the outer circumference of the outer disc is less than the radial width of each inner disc between the through-hole and the outer circumference of the inner disc.

39. A superconducting electromagnet as claimed in any one of claims 35-38, wherein each of the inner discs in the inner stack has a respective critical current and one of the discs of the inner discs has a lowest critical current, wherein each of the outer discs in the outer stack has a respective critical current and one of the discs of the outer discs has a lowest critical current, and wherein the critical current of the inner disc having the lowest critical current is substantially the same as the critical current of the outer disc having the lowest critical current.

40. A superconducting electromagnet comprising: a plurality of discs formed from superconducting material, wherein each of the discs has formed therein a central through-hole and a slit extending through the disc between the through-hole and an outer circumference of the disc, wherein the discs are arranged in a coaxial stack with the slit of each disc being azimuthally offset from the slit of each adjacent disc, wherein each of the discs has a different peritectic temperature to the adjacent disc(s), and wherein each pair of adjacent discs are conductively connected by a conducting joint aligned azimuthally between the respective slits of the respective pair of adjacent discs so that the coaxial stack of discs forms a substantially helical conducting path, wherein the superconducting electromagnet further comprises: a plurality of non-superconducting layers, wherein each pair of adjacent discs is separated by one of the non-superconducting layers other than at the conducting joints.

41. A superconducting electromagnet as claimed in claim 40, wherein peritectic temperatures of the discs alternates along the stack.

42. A superconducting electromagnet as claimed in any one of claims 40-41, wherein the conducting joint is a superconducting joint.

43. A superconducting electromagnet as claimed in claim 42, wherein the superconducting joint is formed directly between each pair of adjacent discs.

44. A superconducting electromagnet comprising: a plurality of discs formed from a superconducting material, wherein each of the discs has formed therein a central through-hole and a slit extending through the disc between the through-hole and an outer circumference of the disc, wherein the discs are arranged in a coaxial stack with the slit of each disc being azimuthally offset from the slit of each adjacent disc, wherein the superconducting electromagnet further comprises: a plurality of superconducting joint members; and a plurality of non-superconducting layers, wherein each pair of adjacent discs are conductively connected by one of the superconducting joints members aligned azimuthally between the respective slits of the respective pair of adjacent discs so that the coaxial stack of discs forms a substantially helical conducting path, and wherein each pair of adjacent discs is separated by one of the non-superconducting layers other than at the conducting joints.

45. A superconducting member as claimed in claim 44, wherein each of the superconducting joint members have a different peritectic temperature from the discs adjacent to the respective superconducting joint member.

46. A superconducting member as claimed in any one of claims 44-45, wherein the plurality of discs are formed from a superconducting material with substantially the same peritectic temperature as each other and the plurality of superconducting joint members are formed from asuperconducting material with substantially the same peritectic temperature as each other, the peritectic temperature of the plurality of discs being different from the peritectic temperature of the plurality of superconducting joint members.

47. A superconducting electromagnet as claimed in any one of claims 44-46, wherein the superconducting material is a crystalline ceramic high temperature superconductor.

48. A superconducting electromagnet as claimed in claim 47, wherein the superconducting material is a cuprate superconductor.

49. A superconducting electromagnet as claimed in claim 48, wherein the superconducting material is a quasi-single-crystal rare-earth barium copper oxide ((RE)-Ba-Cu-O) superconductor.

50. A superconducting electromagnet as claimed in any one of claims 44-49, wherein a crystal axis of a quasi-single-crystal superconducting material in each disc is substantially parallel to a longitudinal axis of the coaxial stack.

51. A superconducting electromagnet as claimed in any one of claims 44-50, wherein the discs are conductively connected so that, when a current flows along the helical conducting path, the current flows into a first face of each disc and out of a second face of each disc, the second face being opposite to the first face.

52. A superconducting electromagnet system comprising a superconducting electromagnet as claimed in any one of claims 1-51 and a current supply for suppling current to the superconducting electromagnet.

53. A superconducting electromagnet as claimed in claim 52, wherein the current supply comprises a flux pump to wirelessly induce current supplied to the superconducting electromagnet.

54. A superconducting electromagnet as claimed in claim 52, wherein the current supply is electrically connected to the superconducting electromagnet via a plurality of current leads.

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