Conductor for superconducting magnets

The invention relates to high temperature superconductor for winding into coils suitable for creating superconducting electromagnets. The invention is particularly suited to manufacture of large magnets or magnet systems for magnetic resonance imaging (MRI) and in some of its currently most preferred embodiments is in the form of a flat tape.

Presently, most large superconducting magnets use low temperature

superconducting wires in their coils. The main reason for this is that a large magnet requires a large length of conductor for its coils and requires that a large current can be passed through those coils. Low temperature superconductors can be manufactured sufficiently reliably to produce such coils.

One of the main disadvantages to low temperature superconductors is the requirement to cool them to extremely low temperatures in order to achieve a superconducting state. This requires expensive cryogenic equipment using liquid helium. Helium is an expensive commodity and is not available in abundance. The scarcity of helium as a resource drives up the price and puts limits on the applications of large superconducting magnets. High temperature superconducting (HTS) materials are available with

superconductivity transition temperatures that can be achieved through cooling with liquid nitrogen, a much cheaper and more plentiful resource. However, limitations in the production methods of these materials (described further below) limits the usable lengths of good quality HTS conductors that can be produced and thus limits their applicability in the manufacture of large superconducting magnets. It is highly desirable to be able to use HTS materials in large superconducting magnets, but this has not previously been successfully achieved. By way of further background to the invention, particularly in relation to thin films of HTS, the defect density in currently produced HTS thin films limits the volume of intrinsic superconductivity as follows. HTS thin films can carry high density superconducting currents at temperatures 50 to 100°K above the boiling point of liquid Helium at atmospheric pressure. This property is potentially useful technologically to replace low temperature

superconductors used in superconducting magnets that require cooling using liquid Helium. Liquid Helium cryogenic equipment may be considered a restriction on magnet design and function, as well as using an increasingly scarce and expensive resource.

However, for HTS thin films to have useful superconducting properties, particularly current capacities, they need to be prepared with exact phase chemistry within complex phase diagrams and crystallographic properties typical of a single crystal, while also containing flux pinning sites, typically crystal dislocation clusters. In order to create HTS thin films, the formation of multi metal oxide components with the correct degree of oxidation on a substrate suitable for support of the film involves non equilibrium reaction conditions. The growth of films is difficult to control at elevated temperatures on activated intermediate buffer layers, themselves requiring specific physical chemical properties.

There are many ways to deposit HTS thin film on prepared substrates, usually involving vapour phase deposition or in-situ oxidation of precursor chemicals from solution. However, it has proved universally difficult to deposit thin film HTS conductors in volumes useful for technological applications, where the film has the required properties throughout the volume of the film.

It has also been found widely that local volumes of film exhibit departures from optimum phase chemistry and crystallographic structure. Where such volumes of thin film exist, superconducting performance is reduced relative to the best possible properties. Superconducting properties, such as critical field, critical temperature, and critical current are reduced in the "imperfect" local volume of film. Additionally, local thinning of the film layer also acts as a defect by reducing the cross-section available for current flow. ln order to form linear conductors based on HTS thin film, such as a coated tape intended to carry a super-current in the manner of a wire, every effort is made to optimise deposition processes and to control chemical and physical processes of thin film growth. Laboratory studies have created a basis for control of the formation and growth of thin films, and have identified means for delivering volumes of good thin film. The importance of experimental variables has been widely reported and the sensitivity of superconductivity to various types of defect for given films can be found in the literature. Nevertheless, each film deposition and growth process seems to have an intrinsic instability that provides defects in a given film volume at a process typical density. This results in a distribution throughout the volume of a film of poor areas of superconductivity, where crucially, the absolute number of defects produced in a given film deposition and growth operation increases in proportion to the volume of film grown.

In a linear conductor, like a tape, the statistical incidence of defects means that the critical current measured from end to end of the tape, apparently reduces as the length of tape is increased because longer tapes have an increased probability of containing worse defects. It has been found that defects in HTS thin film due to less than optimum deposition and growth of the film can have a volume such that reduced superconducting properties extend throughout the thickness and width of a typical tape, over a length of several centimetres. Historically, this has applied to any continuous film manufacturing operation. There are also point defects, such as micro-cracks induced by strains in film growth and use.

The length of HTS thin film tape which can be made on a continuous basis by reaction and deposition ultimately depends on the scale of processing equipment and the storage, plus delivery of reactant feed stock. For most processes where a stock of material is stored inside the process plant, such as target material for PLD, or a process, such as chemical vapour deposition, wherein reactants are supplied from sources external to the reaction equipment, the scale of production of HTS thin film tape now exceeds the thin film volume that can be produced, guaranteed free of defects. Some HTS thin film tape manufacturing plants produce tape on a multi stage basis with a core substrate passing in sequence through process stages on a common line (e.g. see US2007/1 16860). This can in principle provide a technologically useful volume of HTS thin film conductor at an economically attractive rate for some applications; e.g. insert magnet coils. However, for a traditional, low temperature superconductor wire or tape to be economic for MRI magnet production, a typical current capacity of 600 Amperes at 4.2°K & 6 Tesla would be needed in a conductor cross section of 2 to 4 mm ² . Typical lengths of HTS thin film tape with a guaranteed economic value of critical current, say 100 to 250 Amperes per centimetre width at 77°K and an applied field of 4 to 6 Tesla, are limited by the statistics of defect distribution, at the present date, to about 100 to 300 m. Historically, typically maximum economic lengths have been produced in 10m lengths in year 2000 and 100m lengths in year 2005.

Commercial HTS thin film tape manufacturers conventionally identify defects in superconductivity and splice over defect regions (See e.g. WO2012/128954). Similarly, where the length of tape is limited by production difficulties, longer lengths of tape are made by joining two or more lengths of tape by a spliced joint (See e.g. WO2012/037231). This typically comprises a short length of HTS thin film tape overlaid across the gap between two lengths of tape. Additional efforts to bridge regions in HTS thin film tape with reduced superconductivity or to join limited lengths of conductor curtailed in production by process induced defects have been described, wherein poor superconducting regions are bridged by double splices on each side of the conductor (WO2012/037231). Also normal metal shunts have been inserted in bridged sections (US2009/0298696), and dual tapes have been soldered together before inspection of their defect distribution, then de-soldered and splice bridged at defect zones discovered in coil testing (US2005/0016759), as well as inclusion of substantial shunts of normal metal in dual bonded tapes to

cryogenically stabilise the duplex composite (US2010/0173784).

It has also been proposed to laminate two HTS thin film coated tape like substrates together by a normal metal bond between substrates along the total length of two equal length tapes, where the first tape is in a first plane, and a second tape in a second plane (WO2007/016492). The two coated substrates are enclosed to a degree in normal metal cladding to provide a transverse shunt between the superconducting paths. The intention is for current to share between the two parallel HTS thin film paths, and for current to transfer from a reduced region of critical current at a defect in one HTS thin film coating to the second HTS thin film coating in a second plane. A similar proposal has been made to laminate two HTS thin film coated tape like substrates together by a normal metal bond between the HTS thin films along the total length of two equal length tapes (US6828507). The intended capability is for current sharing between HTS thin film coated layers, as above, but to reduce the length of the sharing path, so reducing the resistance of the sharing path.

It has also been proposed to physically divide HTS thin film coatings after deposition into longitudinal filaments to reduce AC losses (AU2001200987). In this case the resulting filaments in a layer would all be adversely attenuated by deposition defects extending through the given cross section of HTS thin film coating on a tape like substrate in respect of their superconducting properties. Multiplex current sharing between filaments would be possible between two substrates in parallel planes, but such current sharing would occur anyway within the single plane of a HTS thin film layer where point defects less than the width of a tape existed. It should be noted that it has also been proposed to use normal metal shunts along the length of a composite, on the edges of a composite comprised of two HTS thin film tape like substrates, otherwise bonded together by a normal metal between the substrate in a first plane and a second substrate in a second parallel plane. Using edge registration of the normal metal shunt and a small lateral off set, pairs of dual tape composites can be bonded together at their edges with HTS thin film conducting layers in a first and second plane. This approach has a significant disadvantage in that current sharing paths are highly anisotropic in respect of bonded contact areas, comparing contact between parallel planes and on the edge of tapes. Super-current sharing would be unstable in this case in that the resistance of current sharing path on composite edges would be substantially greater than the resistance of sharing between planes, so causing undesirable local heating of the composite in high current coil windings. All of these methods are unreliable and uneconomical, because of the difficulty of identifying defects as they are relevant to the design of a coil and its intended operating conditions. These approaches have a number of difficulties in respect of conductor yield. Splice joints only return the critical current of the tape to a value for single tape at the location of the splice, and do not deal with unidentified defects or defects that arise in operation of the coil. Also, current sharing across normal metal bonding of only two HTS thin films in two parallel planes limits the critical current of the composite, in the worst case of a zero critical current defect, to the critical current of one HTS thin film coated substrate, thus requiring twice as much thin film per unit current in the composite, and so providing a poor production yield.

The prior art can be characterized as follows. A first HTS thin film tape like conductor has a critical current id , determined in a short sample test as is well known. In such tests the critical current is only determined in the isolated conditions of the test, not in a magnet winding where other stresses and fields can alter the critical current. At some determined point along the conductor's length there is a defect region with critical current id, where id > id. A second conductor tape continues the conducting path, and has a critical current ic2, where ic1 ~ ic2. A current bridge is made over the known defect region using a short length of superconducting tape with a critical current icb, where icb ~ id ~ ic2. The objective is to obtain what is effectively a long length of superconductor tape which from end to end, including the bridge section has approximately the same critical current as the first length of tape. Typically a coil may have an operating current io which is 70% of the critical current ic. In the case of a dual tape laminate without superconducting splice regions, one can substitute approximately for the critical current of the bridge section icb, the resistive current through the normal shunt ir. There will be a resistance developed in the coil wound from such a current sharing dual tape composite due to sharing current through the normal conductive bonded region around a defect. As described in more detail below, the deleterious effects of defect regions in HTS thin film can only be determined under the exact operating conditions of a magnet coil in respect to the simultaneous effects of local magnetic fields, stresses, and temperature. Some means of providing a flexible and near isotropic accommodation of defects is highly desirable as an "automatic" and integrated maintenance of operating current (as a percentage of average critical current) in a composite coil winding, e.g. for MRI.

US 5,189,260 describes a method of constructing a multi-filament superconductor in which short (1 cm) filaments of a superconductor precursor material are arranged in a non-superconducting metal matrix and subsequently heat treated to transform the precursor filaments into a superconducting state. As the relative position of the filaments is determined prior to the heat treatment process, any errors or irregularities in the heat treatment process are likely to affect all filaments in a given region and therefore it is likely that superconductor defects will be formed in all (or a significant number) of filaments in the same location which will seriously degrade the current carrying capacity of the multi-filament conductor as a whole.

According to one aspect of the invention, there is provided an elongate

superconductor suitable for winding into coils to form an electromagnet, the superconductor comprising three or more strands of high temperature

superconductor arranged in parallel with each other and being electrically connected substantially along their entire length so as to allow current to transfer laterally between strands. Providing parallel, electrically interconnected strands of high temperature superconductor allows current sharing between strands. Thus, in the event that one strand has a defect which either wholly or partially restricts the current carrying capacity of that strand, some or all of the current carried on that strand can transfer to the other two strands. This arrangement mitigates the negative impact of such defective areas in the high temperature superconductor strands so that the elongate superconductor as a whole can tolerate the presence of such defects. In this manner an elongate superconductor can be formed from high temperature superconducting material with an overall length sufficient to be useful in the formation of large superconducting magnets, in particular those useful for MRI. The defects that currently arise in such long lengths of high temperature

superconducting strands can thus be tolerated without undue loss of performance. The small loss in performance that arises from current transfers between strands when part of the transfer path has a normal metal resistance is more than compensated by the savings made in being able to obtain a superconducting state at high temperatures (e.g. via savings on cryogenic equipment and resources).

As each strand is a superconductor prior to being electrically joined to the other strands, there is no need for a subsequent treatment process to attain the superconducting state. This avoids the risk of introducing further defects that are associated with such processes, in particular the risk of introducing defects at approximately the same location in all parallel strands that could block or degrade performance at that location and therefore in the conductor as a whole.

Multiplexed composites thus provide for attaining more reliably high operating current in a single composite HTS conductor than is possible with a set of spliced joints on a single current path, or with dual stacking of tapes. In both of these cases transfer of operating current between the superconducting strands is in only, or mainly, in one dimension, whereas according to the invention current transfer can occur between conductor strands in two dimensions, namely the two dimensions perpendicular to the overall current transfer direction of the composite conductor. The overall current transfer direction is the longest dimension of the strand, i.e. the direction in which it is elongate. This dimension is typically larger than the other two dimensions by a factor of ten, preferably by a factor of a hundred, more preferably by a factor of a thousand.

At the same time, the superconducting material is used efficiently to obtain a given current carrying capacity. For example, use of two parallel strands allows current sharing to avoid defects, but requires twice as much superconducting material as would be required for a single strand with the required carrying capacity. Each of the two strands has to be capable of carrying all of the required current (in case of a complete break in one strand). By contrast, the arrangement with three or more parallel strands allows each strand to be smaller. Each strand is only required to carry a fraction of the total required current (in the event that one strand has a complete break). It can safely be assumed that two strands will not have defects at the same location as the manufacturing processes currently available can produce strands with a statistical distribution of defects (i.e. a low enough number of defects per unit length) that makes such events of negligible likelihood. Thus, for example, three strands each capable of carrying half the total carrying capacity requires only one and a half times the amount of high temperature superconducting material compared with twice as much material for the case of only two parallel conductors described above. Thus the invention makes for more efficient usage of the superconducting material and consequently reduces cost.

Three parallel strands is the minimum number of strands required to put the invention into effect. In many preferred embodiments exactly three strands will be used as this will provide adequate current carrying capacity and adequate resilience in the event of local failures in one strand. However, in other embodiments more than three strands may be used, for example, four, five or six parallel strands may be combined together to provide higher resilience to breaks (i.e. the elongate superconductor as a whole is capable of maintaining the required current even if two or more strands have parallel defects) and higher efficiency in terms of superconducting material usage. Purely as an example, seven parallel strands could be rated as resistant to two parallel faults while still only requiring 1.4 times the material that would be required for a single conductor of that current carrying capacity.

Any defect region in one HTS conductor with a poor critical super-current can be assumed to be adjacent to good superconducting regions in at least two neighbouring HTS conductors. By use of multiples of such conductors in a lateral direction, more than one parallel conductor defect region can be accommodated.

By this means a near continuous superconducting path is provided in the composite conductor, despite the fact that a given distribution of deleterious defects in critical super-current values exists in the HTS conductors. It will be appreciated that it is not necessary to provide a non-superconducting bond from each strand to each other strand. It is sufficient that each strand is bonded to one or more other strands in the composite elongate conductor such that a current path exists from any strand to any other strand. Those current paths can pass through multiple superconducting strands and multiple non-superconducting bonds.

The invention can be implemented with any shape of superconducting strand, for example wires of circular or square (or other) cross-section. These could be solid wires of HTS material or they could be wires coated with HTS material. However, in particularly preferred embodiments the strands are in the form of flat tapes. Flat tapes are easy to bond together and to form into long lengths for winding into large magnet coils. Current methods of forming high temperature superconductor also lend themselves to formation of flat conductors as the material tends to be deposited onto a substrate by various thin film deposition processes. Preferably therefore each strand comprises a flat substrate with a thin film of high temperature superconductor formed thereon. One or more oxide layers may be formed between the substrate and the HTS layer. In some preferred embodiments HTS thin film superconductor coating is bonded to a buffer oxide layer coating, which itself comprises one or more oxides in sub-layers bonded onto a substrate which is a ductile metal, like stainless steel, that defines the overall dimensions of the substrate and the HTS thin film layer. The strands may be connected via normal metal face to face bonding across their width and along their length, so that shared current paths do not pass through the buffer oxides, but pass from HTS thin film in one strand to HTS thin film in a second strand through normal metal.

In preferred embodiments two strands of the elongate conductor are arranged side by side in a first plane and a third strand is arranged in a second plane parallel to the first plane such that it overlaps both of said two strands in the first plane.

This arrangement provides good electrical contact between the third strand and the first and second strands as it provides a large contact area between them, thus minimising the resistance presented to any current flowing between conductor strands.

In some embodiments the broad faces of each strand (each coated substrate), the broad face being defined by the strand's width and length, may be face to face and the HTS thin film layers may be bonded by normal metal and in electrical contact.

Preferably said two strands arranged side by side are of equal width and the third strand is symmetrically arranged so that it overlaps said two other strands by equal amounts. Keeping the overlap equal equalises the resistance of the two current sharing paths presented to current flowing from the third conductor strand to each of the first and second strands. In the event of a defect causing current sharing, this ensures equal sharing of current between the strands, thus minimising the maximum load presented to any individual strand. In some embodiments the third strand is equal in width to each of said two other strands. This has the advantage that all strands are equal in terms of current carrying capacity and makes the best efficiency of superconducting material for a given total current carrying capacity.

It will be appreciated that where more than three conductor strands are provided, the preferred arrangements described above can be extended simply by adding the further conductor strands side by side with the other conductor strands so that either each plane has the same number of conductor strands (if there is an even number of strands) or one plane has one more strand than the other plane (if there is an odd number of strands). Each additional strand may be added in overlapping arrangement with a strand in the opposite plane and be conductively bonded thereto to provide an electrical current sharing path substantially along its entire length. This side by side arrangement has the additional advantage of maintaining the flat structure of a tape which makes for easy storage and winding into coils.

In such arrangements, there may be portions of the conductor strands at the widthwise edges of the composite elongate conductor which are not bonded to a conductor strand in the opposite plane due to the offsetting of the strands in one plane relative to those in the other plane. Preferably therefore the elongate conductor further comprising conductive shunts at the widthwise edges of the elongate superconductor opposite non-overlapped portions of superconductor strands. These conductive shunts fill the spaces that would otherwise be left in the structure so as to create a generally rectangular cross-section. This increases the stability of the tape structure as well as providing a relatively large quantity of conductive material to carry current in the event of loss of the superconductive state during operation.

In alternative embodiments, the third strand may be wider than the two other strands. It is preferably still symmetrically arranged with respect to them. In some particularly preferred embodiments, the third strand may be twice the width of said two other strands. This arrangement has the advantage of the three conductors together forming a rectangular cross section with no need to provide additional conductive shunts for support. The design is not as efficient from the point of view of material usage as the two smaller strands must still be able to accommodate any current flowing in the larger strand in the event of a break in the larger strand. However, this may be outweighed by the structural and/or practical benefits of the arrangement. Due to the way thin film HTS tapes are manufactured currently, the

superconducting layer is generally conductively exposed on one side of the tape, but insulated from the other side of the tape. This is mostly due to the oxide layers formed between the substrate and the HTS layer (described later). Therefore in preferred embodiments, each tape comprises a thin film of high temperature superconducting material formed on a substrate and each tape is preferably arranged in either the first plane or second plane such that its superconducting material is closer to the other plane than its substrate. In this way two opposing tapes are arranged face to face so that the two conductive faces are adjacent to each other and a conductive path can be formed between the HTS layers in one plane and the HTS layers on the other plane.

In some embodiments, a further layer of conductive material (e.g. copper or silver) may be arranged on top of the HTS layer to protect the HTS layer while still providing a conductive path to one side of the tape. This additional conductive layer can also provide a better surface for soldering or otherwise electrically bonding two tapes together. As with the conductive side shunts described above, these conductive layers can also provide a large conductive path for current flow in the event that the superconducting state of the HTS layer is lost. The various strands of the elongate superconductor may be formed as single piece conductors, extending the full distance between the two ends of the elongate superconductor. However, due to manufacturing processes, that may be inconvenient. As the invention provides redundancy of current carrying capacity in the event of a break in one strand, a break can be introduced intentionally in the form of a join between two separate sub-lengths of HTS superconductor. In other words two separate elongate pieces of HTS conductor can be placed end to end so that each piece forms a sub-length of a longer strand. When such strands are used in a composite (multi-strand) elongate conductor as described above, the join simply acts as a break in strand and the current sharing ability of the composite conductor allows current to flow around the join via the other strands. The length of each sub-length of conductor is less than the length of the composite conductor. The sum of the lengths of the sub-lengths of each individual strand adds up to the length of the composite conductor. In preferred embodiments, each strand of superconductor in the elongate superconductor comprises a plurality of sub-lengths of superconductor arranged end to end and the join between any two sub-lengths of any strand is separated in the lengthwise direction from all other such joins in other strands. By ensuring that no two joins are formed at the same lengthwise position of the elongate

superconductor, it is ensured that the conductor is not restricted to a single strand capable of current carrying. As above, current methods of manufacturing thin film HTS conductors can produce conductors with sufficient quality that the chances of a defect occurring in one conductor strand at the same time as an intentional break in another conductor strand is negligible. In cases where the probability of a defect aligning with an intentional break (or another defect) is considered too high, further parallel strands can be added into the probability reduces to an acceptable level.

In some preferred embodiments the three or more strands of the composite conductor are taken from different production runs. As defects in the HTS material are often caused by variations in the processing conditions (e.g. deposition processes), strands taken from the same production run have a greater probability of having HTS defects at the same or similar locations along the length of the conductor. It is desirable to avoid two parallel strands having defects at the same location as this reduces the current sharing capability of the composite conductor as a whole. Therefore, by taking the different strands from different production batches, the likelihood of parallel defects is reduced.

In preferred embodiments, each individual strand or sub-length of strand (where appropriate) is at least 10 centimetres in length, more preferably at least 1 metre in length, yet more preferably at least 10 metres in length. Longer lengths of superconductor are preferred where these can be reliably formed without significant probability of defects. Fewer sub-lengths of superconductor means fewer breaks and therefore fewer requirements for current sharing. Longer lengths are also easier to join together in terms of manufacturing processes. Superconducting magnets can be made in different ways. However, in some preferred embodiments an array of coils is provided, each coil in the array enclosing an operating space such that their internal fields combine within the operating space. In such embodiments, the operating space, i.e. the space where the intended field is created, e.g. the patient space for a MRI magnet, is inside the coils, i.e. the coils of the each winding loop around the operating space. An array of coils allows better control over the homogeneity of the magnetic field within the operating space. In preferred embodiments, an outer pair of coils and an inner pair of coils are used, with different field strengths (obtained e.g. by different numbers of turns on the windings) to create a desired magnetic field within the operating space. For MRI applications it is desirable to produce a field that is homogeneous to within a few parts per million.

The coils in the array may all be wound from the same length of composite conductor, each coil being electrically connected to the next and thus all coils carrying the same current. However, an advantage of the use of HTS materials is that external connections (i.e. connections to external to the cryostat) which introduce a loss of energy from the system through heat transfer and associated boil off of refrigerant are less costly than with low temperature helium based systems. As such losses can be tolerated, the composite conductor of the invention can be used in systems where each coil in the array is separately powered and can carry a different current. This allows the relative field

contributions of different coils within the array to be adjusted during operation in ways that would not be possible where all the coils are permanently connected. In such systems the relative field contributions from each coil are fixed by the number of turns on each coil, which cannot be adjusted during operation.

In other embodiments, the coils in the array may be arranged such that together they enclose an operating space such that their external fields combine within the operating space. In such arrangements each coil itself does not surround the operating space and thus each coil can be made smaller. Smaller units are easier to replace in case of fault. When an array of such coils is arranged into a loop, the net effect is the same as two concentric loops, one at the inner envelope of the array and one at the outer envelope of the array. Again, all coils in an array may be connected together and commonly driven, or subsets of the array or indeed individual coils may be driven separately to provide greater flexibility of control of the operating space during operation.

The or each coil may be wound in a flat pancake coil. Alternatively, the or each coil may be wound in a helical coil. Other coil formations may be used according to the intended usage.

According to another aspect of the invention there is provided a method of manufacturing an elongate superconductor comprising: arranging three or more strands of high temperature superconductor in parallel with each other; and electrically connecting said strands substantially along their entire length so as to allow current transfer laterally between strands.

In preferred embodiments the step of electrically connecting said strands comprises applying solder between said strands and causing said solder to electrically bond to said strands. This step may be part of a continuous formation process. Therefore said three or more strands may be continuously brought together onto a rotating former and said step of electrically connecting said strands may comprise continually bonding said strands while said former rotates.

The former may be a permanent structure for shaping the winding or it may be a temporary structure for storing composite conductor for storage and later use. Formers of different shapes are known and may be used, e.g. cylindrical formers for forming flat spiral coils or helical coils with either circular or square cross-section.

In a continuous formation process, a solder bonding station may be arranged adjacent to the former so that individual strands are brought together onto the former as the former is rotated and the solder bonding takes place as the strands are brought together.

The preferred features described above in relation to the composite elongate superconductor apply also to the method of manufacturing it. Therefore preferably the three or more strands are in the form of flat tapes. Preferably two strands are brought together side by side in a first plane and a third strand is brought into contact with said two strands in a second plane parallel to the first plane such that it overlaps both of said two strands in the first plane. Preferably the two strands arranged side by side are of equal width and the third strand is symmetrically arranged so that it overlaps said two other strands by equal amounts. The third strand may be equal in width to each of said two other strands.

Conductive shunts may be arranged at the widthwise edges of the elongate superconductor opposite non-overlapped portions of superconductor strands.

Third strand may be wider than the two other strands. The third strand may be twice the width of the two other strands.

Each tape may comprise a thin film of high temperature superconducting material formed on a substrate and each tape may be arranged in either the first plane or second plane such that its superconducting material is closer to the other plane than its substrate.

Each strand of superconductor may comprise a plurality of sub-lengths of superconductor and the method may comprise arranging said sub-lengths end to end such that the join between any two sub-lengths of any strand is separated in the lengthwise direction from all other such joins in other strands.

The three or more strands are preferably from different production runs.

According to a further aspect, the invention provides a method of making a superconducting magnet comprising winding one or more coils of elongate superconductor as claimed in any of claims 1 to 17 to form a magnet winding. Preferably the magnet is an MRI magnet.

The method may comprise forming an array of coils, each coil in the array enclosing an operating space such that their internal fields combine within the operating space. Alternatively or additionally, the method may comprise forming an array of coils, the coils in the array being arranged such that together they enclose an operating space such that their external fields combine within the operating space. The method may comprise winding the or each coil in a flat pancake coil.

Alternatively the method may comprise winding the or each coil in a helical coil. Mixtures of pancake coils and helical coils within an array are also contemplated. In the above descriptions, the terms length or lengthwise are used to refer to the main direction of current flow in the conductor, i.e. the direction in which the conductor is most elongated. The terms width or widthwise are used with respect to tapes to refer to a direction perpendicular to the length direction, but also parallel to the plane of the thin film of HTS material in such tapes. The terms "height" and "thickness" refer to the direction perpendicular to both the length and width.

Preferred embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 a shows a typical construction of a tape like conductor;

Figure 1 b shows the co-ordinate system used to describe current sharing paths;

Figure 2 shows a section through two tape like conductors bonded face to face;

Figure 3 shows a section through two tape like conductors, bonded face to face, one having a poor conductivity region;

Figure 4 shows a section through two tape like conductors, bonded face to face, one having a zero conductivity region;

Figure 5a shows an isometric projection of a three fold tape bonded composite with equal width tapes, and current sharing around a zero conductivity region;

Figure 5b shows an isometric projection of a three fold tape bonded composite with two equal width tapes in a first plane and a third tape in a second plane, double the width of the tapes in the first plane, with current sharing around a zero conductivity region;

Figure 6a shows a plan view of a tape bonded composite with seven strands, with current sharing around a zero conductivity region;

Figure 6b shows a section view of the composite of Figure 6a;

Figure 7 schematically shows a current sharing between six tape like conductors in a bonded composite between equi-potential current blocks;

Figure 8a shows schematically a three fold tape bonded composite being formed into a spiral cylindrical winding; Figure 8b shows schematically in section a three fold tape bonded composite being formed into a spiral cylindrical winding via a solder bonding station;

Figure 9a illustrates a magnet arrangement in which the internal fields of four large coils are combined; and

Figure 9b illustrates a magnet arrangement in which the external fields of 32 small coils are combined.

Figure 1 a shows a typical prior art high temperature superconductor flat tape. The tape is formed from a number of different layers. The lowermost layer is a substrate 1 , typically formed from Nickel alloy or polished stainless steel. The substrate may be textured to facilitate growth of other layers deposited onto the substrate. For example, a RABiTS (Rolling Assisted Biaxially Textured Substrate) substrate may be used. Further oxide layers may be deposited on the substrate to provide keys for epitaxial growth of the superconductor layer. In Fig. 1 a, a 1.5 micron yttria stabilised zirconia (YSZ) layer 2 is deposited on the substrate 1 Alternating Beam Assisted Deposition (IBAD) and a 0.05 micron Cerium oxide layer 3 is deposited on the YSZ layer 2 via Pulsed Laser Deposition (PLD). Next, the 1 micron thick Yttrium Barium Cupper Oxide (YBCO) high temperature superconductor layer 4 is deposited by PLD and finally a 20 micron copper shunt layer 5 is deposited by Physical Vapour Deposition (PVD). This uppermost layer 5 acts to protect the superconductor layer 4 below. Other layer structures and materials may also be used. Figure 1 b shows the co-ordinate system used in this document to describe current sharing paths. Z is the axial (lengthwise) direction for average current flow in a tape like substrate or composite, X is the transverse (height or thickness) current sharing direction for current transfer between tape layers and between tape like conductors in different planes of a composite superconductor, Y is the lateral direction for current sharing within tape layers or between conductors in the same plane of a composite superconductor.

Figure 2 shows two tapes, each with the construction shown in Figure 1 , bonded face to face by a layer of indium/tin/lead solder 6 to form a composite (multi-strand) superconductor. The white arrows 30 in Figure 2 show the normal current paths through the YBCO superconducting layer 4.

Figure 3 shows the same structure as Figure 2, but with a poor conductivity region 7 in the YBCO layer 4 of the lower tape. This poor conductivity region cannot carry as much current as the rest of the HTS layer 4. In Figure 3, white arrows 30 show the original current flow (as shown in Fig. 2). Arrow 31 shows the reduced current flowing through the poor conductivity region 7. Arrow 32 shows current transfer from the lower HTS layer 4, through the copper layer 5 of the lower tape, the bonding solder layer 6 and the copper layer 5 of the upper tape into the HTS layer 4 of the upper tape. Arrow 33 shows this shared current flowing through HTS layer 4 of the upper tape alongside the original current indicated by arrow 30. After the obstruction 7 has been passed, arrow 34 shows the shared current flowing back through the intervening layers 5, 6 back to the HTS layer 4 of the lower tape.

Figure 4 is similar to Figure 3, but instead of poor conductivity region 7 there is a zero conductivity region 8 which may be caused by a crack or break in the lower tape or may be a join between two lengths of tape. As shown by arrows 32, 33, 34, the current sharing operates in the same manner as described in relation to Figure 3 except that all of the current from the lower HTS layer 4 has to cross into the upper tape and back again to circumvent the break 8. The arrows 30 and 33 in Figure 4 are the same size to indicate that the amount of current shared across from the lower tape to the upper tape is the same as the current that was already flowing in the upper tape. Thus the upper tape in this situation has to be able to carry twice as much current as is usually required of it. For example of the composite conductor (the two tapes and the bonding layer 6) is designed to carry 100 Amps, this would normally be shared equally between the two component tapes, each carrying 50 Amps. However each tape must be capable of carrying the full 100 Amps (i.e. it must have a critical current of at least 100 Amps) so as to permit all of the current to be carried around a zero conductivity region without exceeding the critical current and losing the superconductive state. Figure 5a shows a three fold tape according to an embodiment of the invention and illustrating the principles of the invention. In this and subsequent figures, the details of all the layers in each tape are not shown for clarity. In Figure 5a, a first tape conductor 9 and a second tape conductor 10 lie parallel to one another in a first plane. A third tape conductor 12 lies in a second plane, parallel to and above the first plane. The third tape conductor overlaps both the first tape conductor 9 and second tape conductor 10 and is conductively bonded to both of them by solder layer 6 via ultrasonic hot air diffusion. As with the structures shown in Figures 2, 3 and 4, the third tape 12 is bonded face to face with the first and second tapes 9, 10 so that current may flow between the first tape 9 and third tape 12 and current may flow between the second tape 10 and third tape 12.

Each of the three tapes 9, 10, 12 in this arrangement is a component conductor or a single strand conductor. The components together form a composite conductor or multi-strand conductor.

A zero conductivity region 8 is formed between second tape 10 and a fourth tape conductor 1 1. Fourth tape 1 1 is effectively a continuation of second tape 10, but with a small gap which interrupts the conductivity. White arrows in Figure 5a show how the current is shared between the conductors 9, 10, 1 1 , 12 to flow around the zero conductivity region 8. Current that is flowing in the first conductor 9 and third conductor 12 can still flow in those tapes. However the current that was flowing in second tape 10 needs to transfer into the first tape 9 and third 12. Current can flow directly from second tape 10 into third tape 12 via the solder bond 6. Current can flow from the second tape 10 into the first tape 9 via the solder bond 6 and the third tape 12. Current first flows from second tape 10 into third tape 12 via solder bond 6, then flows through third tape 12 and down into first tape 9, again via solder bond 6.

In Figure 5a there is a conductive path via just the solder bond 6, not entering the third tape 12, but this path as wholly non-superconducting so current will tend to favour the lowest resistance path via the superconducting layer 4 of third tape 12. Once past the obstruction (zero conductivity region 8 in this case), three parallel conductors are once again available to carry current (first conductor 9, third conductor 12 and fourth conductor 1 1) and so current will flow back from the first and third conductors 9, 12 into fourth conductor 11 to redistribute the load.

The current sharing that takes place within this composite conductor is in both the widthwise direction and the thickness direction, i.e. it is two dimensional current sharing. Sharing current between conductors in the same plane (e.g. the first conductor 9 and the second conductor 10) is widthwise current sharing and sharing current between conductors in different planes sharing in the thickness direction (e.g. between the first conductor 9 and the third conductor 12). As described above, the current sharing path between any two conductors may have

components in both these directions. For example, sharing between the first conductor 9 and the second conductor 10 involves transfer between planes to the third conductor 12 as part of the path. By contrast, in the two-fold composite conductor shown in Fig. 2, there is only current sharing in the thickness direction. There is no sharing between different conductors in the widthwise direction.

With the arrangement of three parallel conductors all connected together conductively, a defect (low conductivity or zero conductivity) in one component conductor can be bypassed via the other two component conductors. No prior knowledge of the location or nature of the defect needs to be known as the current sharing will happen automatically. There is therefore a reduced burden on the manufacturer with regard to testing the component tapes for defects. Also, several shorter lengths of tape can be connected together lengthwise to form a longer conductor. Each shorter length can be considered a sub-length of a single long conductor. The sub-lengths can simply be placed end to end, creating a conductivity defect which is then automatically circumvented by the current sharing described above. When forming a composite conductor from several component conductors, each component conductor comprising several sub-lengths, the joins between sub-lengths should be arranged so that no two components have a join at the same axial (lengthwise) position in the composite conductor, thus ensuring that there are always at least two other component conductors to share the current with. An additional benefit of this arrangement is that each component conductor need only be designed to carry its own share of the total current plus a fraction of (not the whole) of another conductor's share of the total current. Thus, for a given total current, less HTS material needs to be used in the composite conductor as the component conductors can be made smaller than with the two-conductor arrangement. Or to view it another way, each component conductor can be operated closer to its critical current, so for a given size of component conductor (a given amount of HTS material), the composite conductor can be operated at a higher current with this arrangement than with a two-conductor arrangement.

In the embodiment of Figure 5a, all three conductors are equal in width. The third conductor 12 is symmetrically arranged with respect to the first conductor 9 and second conductor 10 so that it overlaps half of each of them. This provides balanced (equal resistance) current paths so that when current is shared, it is shared equally between alternative current paths, thus minimising the overall resistance.

In the embodiment of Figure 5b, instead of all three conductors being equal in width, the third conductor 12 is twice the width of the first conductor 9 and second conductor 10. The double width third conductor 12 provides a rectangular cross section to the composite conductor as a whole which increases its stability. In addition, such coils have a rectangular cross section which makes it possible to wind the tape round on top of itself to form a flat spiral coil. Current sharing takes place in the same manner as described above with regard to Fig. 5a. The larger upper strand 12 will also be better able to tolerate some local defects, e.g. those with limited extent and which do not extend across the full width of the conductor.

Figures 6a and 6b illustrate the manner by which the three-fold composite conductor arrangement of Fig. 5a can be extended to have more than three component conductors. Figure 6a is a plan view and Figure 6b a cross-section.

Further component conductors 40, 41 , 42, 43 can simply be added to the first and second planes alongside the first three conductors 9, 10, 12. Each conductor component other than the two at either side (9, 43) overlaps two other conductor components thus providing a shared current path that extends across the full width of the composite conductor, passing up and down between the first plane and the second plane as it moves from one side to the other.

It should be noted that Figure 6 only shows small sections of the component conductors for simplicity. In practice the component conductors would each extend a long distance. Figure 6 also shows gaps between adjacent component conductors, but this is only for clarity. Although such an arrangement would work, in practice the conductor components would be placed tightly adjacent to each other so as to maximise the area of overlap and thus decrease the resistance to current transferring between conductors.

The white arrows 13 in Fig. 6a show the general direction of current flow in the conductors. The black arrows 14 indicate current sharing paths between conductors in one plane and conductors in the other plane. A zero conductivity region 8 is shown with the current sharing paths that carry current around it.

Extra copper shunts 15 are provided at the sides of the composite conductor tape in the plane opposite the side conductor components 9, 43. These shunts 15 fill the gap that is formed by the lack of another overlapping component. The shunts 15 are half the width of the conductor components. The shunts 15 provide a conduction path for current in the event that the superconducting material loses its superconducting state during use, thus reducing the chances of damage caused by arcing which may otherwise occur. The shunts 15 also provide mechanical stability to the composite tape which is particularly important for multi-layer windings when forming the tape into a coil as the copper shunts 15 help to support the next layer of tape.

In another embodiment, not shown here, the conductor components at the sides of the composite which overlap two components in the opposite layer (i.e. tapes 12 and 42 in Figs. 6a and 6b) can be extended to overlap fully the edge tape in the opposite layer as well as half the adjacent tape so as to provide the same structural benefits, but without the copper shunts 15. In Figs. 6a and 6b, tape 12 would fully overlap tape 9 and half overlap tape 10, while tape 42 would fully overlap tape 43 and half overlap tape 41. The black arrows 14 in Figure 6b show the general direction of the current sharing paths within the composite. It can be seen that these are two dimensional, i.e. transferring current across the width of the tape as well as through its thickness from one plane to the other.

Three-fold composite tapes (or indeed n-fold composite tapes) with equal width component tapes can be tessellated to form wider tape composites. For example a three-fold composite tape can be inverted and bonded to another three-fold tape to form a six-fold tape. A further three-fold tape could then be bonded to this six-fold tape to make a nine-fold tape. In this manner, composite conductors with larger current carrying capacity can be easily built from smaller composite conductors.

In an alternative method of bonding two three-fold composite conductors together into a wider composite conductor is to lay the two three-fold conductors side by side so that one plane (first plane) contains four component strands and the opposite plane (second plane) contains two component strands with a gap between them. The two three-fold composite conductors can then be bonded together by applying a further single conductor tape across the join between the two three-fold composite tapes, in the gap in the second plane. The composite conductor thus formed is a seven-fold composite conductor.

The skilled person will appreciate that the above combinations are not restricted to three-fold composites, but could be used to bond any n-fold composite to any other n-fold composite.

Figure 7 illustrates how the current sharing scheme circumnavigates various defects throughout a long length of composite superconductor according to the invention. Figure 7 schematically shows six conductor components in parallel, extending between two equipotential current junction blocks 19 that apply the same emf to all six conductors in parallel, Each of the component conductors has a zero current defect 20 (i.e. one that blocks current flow in that conductor component completely, thus requiring full transfer of all current in that component to the other components). The black arrows in Fig. 7 show how the current in the blocked component is shared between the other five non-blocked components until past the defect whereupon the blocked component can once again take up current flow. It can be seen that with six parallel component conductors, five components are available to share the additional temporary load. Therefore each component only needs to have extra capacity for one fifth of its own load and can thus be operated at (or close to) 5/6 of its critical current.

Figure 7 also illustrates how joints should be separated in an elongate composite conductor where each strand (component conductor) is formed from more than one sub-length of conductor. The defects 20 would arise from the join between two sub-lengths within a conductor strand. It can be seen that joins are not formed in two strands at the same place, but instead are separated along the length of the composite conductor. This ensures that the maximum number of other strands are available to take up load sharing around the joins. Figures 8a and 8b illustrate winding processes for creating a multi-strand composite conductor and winding it onto a mandrel 16. First tape conductor 9 and second tape conductor 10 are continuously fed in parallel in one plane while third conductor 12 is continuously fed and brought into face to face contact in overlapping relationship with them from above with solder 6 being provided in between to create a conductive bond. An ultrasonic hot air diffusion process is used to melt the solder and create the bond. As the mandrel 16 is rotated, the bonded composite tape is wound into a coil and can thus be used as an electromagnet.

Figure 8b shows the process from a side view and additionally shows a soldering bonding station 17 and an insulating tape 50 (such as a flexible polymer e.g. Mylar) being wound between successive coils. Alternatively, the solder bonding station could be used with additional apparatus to apply a dipped-on or spray-on insulator to the back side of the tape substrates 1 to achieve the same effect. Figure 9a shows a magnet arrangement in which the internal fields of four large coils 90, 91 , 92, 93 are combined to produce a homogeneous field region 95 suitable for MRI around the origin 96 of the indicated coordinate system. The coils 90 and 93 form a pair of outer coils, while the coils 91 and 92 form a pair of inner coils. In typical set ups the outer coils 90, 93 have more windings than the inner coils 91 , 92. In a low temperature superconducting magnet, these four coils would normally be electrically connected together so that they all carry the same current (although the outer coils would produce more Ampere turns). The same could be done with HTS conductors, but due to the lower cost of refrigerant, the coils could be separate and individually driven to increase control over the field in field region 95. With such arrangements the field could even be adjustable during operation of the magnet (i.e. by varying the current, without adjusting the number of turns of the windings). Each coil 90, 91 , 92, 93 is large and essentially surrounds the field region 95. In a typical medical MRI machine, these coils would each pass around the patient during measurements.

Figure 9b shows an alternative magnet arrangement in which the external fields of 32 small coils 100 are combined at the field region 95 at the origin 96 of the illustrated coordinate system. The coils 100 are smaller than those of Figure 9a and no coil individually surrounds the field region 95 (and would not surround a patient during measurement in a medical MRI machine). However, the coils 100 are arranged into four circular manifolds 101 , 102, 103, 104. Each manifold 101 , 102, 103, 104 is made up of 8 small coils 100 disposed at regular intervals around its circular circumference. Each manifold 101 , 102, 103, 104 is essentially equivalent to one of the large coils 90, 91 , 92, 93 of the embodiment of Fig. 9a and would surround a patient during measurements in a medical MRI machine.

However, the individual coils are smaller and thus it is their external fields that combine at the origin 96 to produce the MRI field region 95. As with Fig. 9a, the outer two manifolds 101 , 104 form an outer pair while the inner two manifolds 102, 103 form an inner pair. The coils 100 of the outer manifolds 101 , 104 again typically have more turns than the coils 100 of the inner manifolds 102, 103. As with Fig. 9a, the coils 100 can all be connected together so that they all carry the same current. Alternatively different subsets of coils 100 (e.g. all coils in a manifold or a subset of coils in one or more manifolds) may be connected together, or all coils 100 may be driven independently (and can carry different currents) so as to provide greater control over the field region 95.

Further details, advantages and benefits of preferred embodiments and uses of the invention will now be described. The preferred embodiments of the invention mitigate the most serious

consequences of current reduction in linear HTS thin film coated conductors which arise due to the distribution of local defects in HTS thin films, where the HTS thin film is intended for use in technologically useful linear superconductors. In preferred embodiment the concept combines in composite form, three (or preferably a greater number of) tape like substrates coated separately with HTS thin film, using a composite matrix of a normal conducting metal applied as a continuous bonding thin layer between HTS layers. The tape like substrates are preferably arranged in the composite with their broad faces parallel to each other. Thus the composite preferably has a rectangular cross section, and is many hundreds of times longer than the dimensions of the cross section.

In preferred embodiments it is intended to use similar lengths of tape as are available now. Three or more such lengths are combined to form a composite conductor which can be wound to form an electromagnet. Current inhibiting defects are accommodated in the magnet windings by self adjusting of current sharing paths in the composite winding as discussed above. The symmetry of the basic composite HTS thin film coated substrates provides naturally for substantially similar geometry for current sharing paths, irrespective of which of the three superconductor tapes has a defect zone.

In some preferred embodiments, a composite conductor of three or more HTS thin film coated substrates can readily be wound into a coil where a very small coil resistance is achieved in current sharing around defects due to the short length of current sharing paths and the duplication of such paths in two lateral dimensions of the composite. This maximises the composite critical current available. Preferably the HTS thin film layers are close to the neutral bending plane of the composite, thus improving the mechanical and electrical stability of the winding and therefore the available MRI field from a given volume of deposited HTS thin film. The neutral bending plane is the plane in the flat (unbent) composite tape that will experience no (or minimal) longitudinal stress or strain when the tape is bent. The position of the HTS thin films with respect to the rest of the composite conductor can be adjusted by varying the thickness of the substrates used for the component conductors in the two planes. The position of the HTS thin films also depends on the thickness of conductive shunt material (e.g. copper) in the layer 5 (see Fig. 1). Reducing the thickness of layer 5 will move the HTS thin films closer to the geometrical centre and thus closer to the neutral bending plane. However, reducing the thickness of layer 5 may increase resistance or create a risk of damage to the conductors in case the superconductive state is lost. In some embodiments the shunt layer 5 may be omitted altogether and the two HTS thin film layers from opposite conductor strands may be bonded directly together.

It should be noted that if HTS thin film process methods improve to reduce inherent defect density, but not to zero defect density, longer lengths of tape composite can be used between joints so reducing the current sharing resistance per unit length, and thus enabling economic winding of the largest MRI magnets with an even smaller operating voltage penalty.

There will be a small resistance in the current transfer zone arising from the thin matrix layer of normal metal. As explained below, the apparent resistance per unit length of a composite conductor of three or more layers can be adjusted in design and production of current sharing HTS thin film composite. Design parameters are chiefly a consideration of the number of layers of HTS thin film in the composite, and the manner in which the normal matrix metal is bonded to the HTS thin film. For the purpose of building a MRI magnet from coils wound from composite HTS thin film layers, the small current sharing resistance of the composite can be dealt with by using a low voltage power supply to overcome the current sharing resistance. As will be explained below, composite HTS thin film conductors are likely to have resistance in the range of a few milliohms per hundred metres when operated near the nominal critical current value of the composite. However, it is desirable to maintain the voltage created by current sharing at as low a value as possible to maximise the value of MRI field created from a given volume of HTS thin film. Also, the production process can be managed to reduce the composite end to end resistance by monitoring the location of the most severe defects along each HTS thin film coated substrate prior to bonding the HTS current paths. When the location of severe defect regions (or for economic benefit, joints between tapes along a given cross section location) in each HTS thin film layer is known, optimum positioning of HTS thin film layers can be selected when constructing multiplexed current paths in the composite conductor. The identified defects do not need to be removed or otherwise bypassed, but merely need to be placed adjacent to good quality sections in the other parallel strands of the composite conductor.

The cross section area and length of the current paths through the normal metal separating HTS layers are substantially similar in values for all typical defects, regardless of where the defects are located in HTS thin film layer along the length of coated substrate. The composite can be said to share current between separately prepared HTS thin films. The current sharing paths have a low resistance because the major part of the current sharing path is via superconductor in the proposed three-fold overlap arrangement (Fig. 5).

Economic benefits of composite HTS thin film coatings in construction of MRI magnets. At this time, production of linear tapes comprised of HTS thin film coated substrates has a practical upper limit of piece weights of typically 200 Amperes over 300 m at 77°K, in zero applied magnetic field. This is for reason of the extreme difficulty and cost of controlling, at the atomic level, HTS thin film deposition and formation via non-equilibrium processes over large film volumes. Several defect regions of poor critical current can be expected on average in production of, say, 1000 m lengths of HTS thin film coated substrate. The technological process yield is poor as only occasional defect free lengths of tape in the typical range 50 to 100 m are available for commercial use as conductor for coil winding. It should be noted that the HTS thin film will enter its non superconducting state with a high resistance to passing normal electric currents if an attempt is made to exceed the critical super-current by application of a high voltage. The relatively high power dissipated in the HTS thin film under such conditions will rapidly cause failure of the conducting thin film. The use of composite conductors with symmetric current sharing paths between HTS thin film coated substrates, continuously bonded along their length by normal metal, makes use of the fact that at the start and end of a composite conductor winding, the electromotive force providing current will be from equi-potential surfaces of connection blocks, e.g. as shown in Figure 7. This provides that the operating current in each component tape will be substantially the same in each component tape at the start and end point of the composite conductor. It should be noted that the fundamental properties of the HTS thin film superconductor depend on how appropriately the films are processed, and in particular, the critical current density Jc determines the available critical current ic for engineering purposes; ic = Jc multiplied by the cross section area of conductor. The critical current density Jc in a superconductor depends on how the

superconducting state is established in the material of the superconductor, related to phase composition and crystallographic structure, as well as to the

microstructure of the material and how the magnetic flux entering the material interacts with heterogeneous features in the microstructure.

For practical purposes Jc is a linear scalar property of a given superconductor. It is influenced by the temperature of the superconductor and the applied magnetic field intensity. Thus a substrate coated with HTS thin film will carry a current proportional to the cross section of the coating normal to the direction of current flow. Making a HTS conducting layer thicker or of greater width will increase the critical current ic in proportion to the increase in cross section. Thus in discussing current sharing in relation to Figure 5b, a tape conductor in a three fold composite in a first plane being twice the width of the two tape conductors in a second plane will have twice the critical current ic of each of the two conductors in the second plan. This assumes the same thickness and Jc of the HTS thin film coatings. In this discussion, the fact that HTS thin films can have in practice critical current densities Jc which are a function of film thickness is neglected.

For comparison, MRI magnets of the conventional type need some 600 Amperes over 20 to 30 km of low temperature co-processed composite (with liquid Helium operation at 4.2°K STP). It is essential to recognise that at 4.2°K current sharing composites are not practicable at significant values of current transfer. This is because at this temperature, small amounts of heat generated by sharing a few percent of the transport current can raise the temperature of the low temperature composite superconductor such that it becomes unstable, eventually losing the superconducting condition. In principle, at 4.2°K the specific heat of materials is so low that even a few millijoules of heat released inside the composite per cubic millimetre will affect some 1 to 2°K temperature rise. Typically, conventional unitary coils will be wound from eight to ten individually co- processed lengths of low temperature superconductor and joined externally of the unitary winding in special well cooled, low field regions of the magnet chamber. To emulate the Ampere turns of low temperature superconductor unitary MRI windings with HTS thin film tape in 100 m lengths, using only defect free material, is impossibly costly at the present time.

In the foreseeable future, production capability for HTS thin film tape coatings will not allow for direct emulation of low temperature MRI magnet construction.

However, composites in accordance with the preferred embodiments of the invention comprised of multiple HTS thin film coated tape like substrates, each typically of some 500 to 1000 m in length with statistically a few defects along their length, will allow for current sharing. It will then be possible to have a high current capacity over almost unlimited lengths of HTS composite.

Since current sharing is the main operational characteristic of multiplexed current paths between HTS thin film coated substrates bonded together by a normal metal, it is beneficial to quantify to a degree the form of the current paths adopted in current sharing regions of a composite conductor. This is relevant to the design of composite conductor structures. It can be expected that operating current sharing composites at 77°K will allow technologically useful values of current sharing because of the low resistance of current sharing paths (these being predominantly in superconductor and having only short normal path components) and the high specific heat of composites at 77°K. Estimates of the resistance of current sharing paths are given below. It should be noted that all production routes for HTS thin film coated conductor on tape like substrates create defect regions with a spectrum of reduced current capacity and volumes. The amount of current shared from a defect region in one HTS thin film layer with good quality thin film in adjacent layers will vary along the composite conductor due to the occurrence of different types of defect.

With the current sharing ability of multistrand composite high temperature superconductors as described herein, it becomes possible to produce unlimited lengths of HTS composite conductor. A small resistance at the milliohms level increases with the length of the composite conductor, but of vital economic importance in the construction of MRI HTS magnets, this small resistance can be accommodated by current control using a low voltage, current stabilised power supply. The composite conductor does not become unstable when carrying a

technologically useful transport current. This is because at temperatures above about 20°K where specific heats of metals are significant, the heat generated by current sharing to circumvent joints or defects can be extracted without incurring destabilising local temperature increases. Therefore, present day HTS

manufacturing processes, with their inherent defect densities, can be used to economically manufacture technologically useful lengths of conductor of many kilometres. However, it is important to provide the maximum possible cross section area (i.e. the area of normal metal bonding the two superconductors together) and minimum path length for current transfer, wherever a zero current defect exists, so that sharing voltages are similar in value throughout a coil winding.

Some indication by example will now be given of the extent to which a HTS thin film composite with multiplexed, current sharing paths can enable the construction of MRI magnets. Preferred embodiments of the invention can be described by a fundamental arrangement of three HTS thin film tapes. Figure 1 a shows a typical HTS thin film tape, comprising a substrate for mechanical integrity and various layers of metal oxides necessary to grow HTS thin films with good

superconductivity. Figure 1 b defines the orthogonal co-ordinates for current flow, where the technologically useful current flows along the longest axis of the tape composite.

In the three-fold (or n-fold with n > 3) tape arrangement of preferred embodiments of the invention, two tapes are arranged in the same plane with their long axes parallel to each other. A third tape in a plane above that of the first two overlaps each of the first two tapes, by half of the width of each tape if all three tapes have the same width. This arrangement is shown in Figure 5a. Importantly, this three fold tape arrangement allows for current sharing paths in both transverse direction (height or thickness direction of the tape) and lateral directions (widthwise direction), so that the path cross section area and length are of substantially similar values whichever of the three tapes has reduced critical current. That is, current can transfer from a tape that carries a super-current towards a defect or joint block region into a tape in the same plane (lateral transfer). This is achieved by following a path that first goes transversely into the overlapping tape (or possibly plural tapes in n-fold arrangements with n > 3) in the plane above the blocked tape (see Figures 3 & 4 and 6a), then follows a sideways path back into a tape (or tapes) in the first plane, beyond the defect or block. The three-fold composite arrangement permits multiplexing of many current paths in the region of a defect or block in any one path; see Figures 6a, 6b, 7. Throughout this document, unless specifically indicated to the contrary, the term

"three-fold" is used to include exactly three-fold and more than three-fold (e.g. fourfold, five-fold, six-fold, n-fold, etc.) arrangements, i.e. it is used to mean n-fold greater than two-fold. It may be convenient in some coil windings to wind a HTS thin film composite restricted to a minimum cross section dimension, such as when small diameter coils are to be constructed. In this case, a double width upper tape may be bonded to two tapes in the lower plane, (see Figure 5b). This results in transverse current sharing at a defect or joint in a first, lower tape, with a double width upper tape, with the advantage that additional capacity exists in the broader current sharing paths due to some lateral current sharing being possible. With the double width tape there is also lower resistance in the current sharing paths as there is a greater contact area between the lower and upper tapes. The double width geometry can be extended to n-fold arrangements by having one full width tape in one plane and (n-1) equal width tapes side by side in the other plane. However, if the large tape and the smaller tapes have the same thickness then the upper tape will carry half the current of the composite and therefore failure of the upper tape (which is likely to be failure across the whole of its width) will require transfer of half the current into the smaller tapes. In arrangements with only three strands, this does not compare too unfavourably with the transfer of one third of the total composite current which occurs where the strands are all of equal size, but if the arrangement is extended to larger numbers of conductors (one large opposite n-1 small), the advantage of having a large number of conductors is not as great due to the possible failure of the one big tape. Although such arrangements are also not as efficient in terms of material usage as arrangements with all equal width tapes, they may be easier to manufacture and they may have better stability without requiring the addition of the side shunts 15 shown in Figs. 6a and 6b. It is also the case that the one double width tape is better able to pass current around local defects which partially interrupt the good superconductor cross section within its own HTS thin film layer.

In other embodiments, the two smaller tapes opposite one double width tape may have twice the thickness of the double width tape. This means that all three conductor strands have the same cross-section and thus have equal current shares. Thus in the event of the larger strand failing, the current that needs to be shared is still only one third of the total composite current. Needless to say, this could be extended to greater than three strands (i.e. one large strand with n-1 smaller strands) all with the same cross-section area, but there is a corresponding increase in overall thickness which may cause a decline in some mechanical properties of the tape (e.g. resistance to bending strain).

Multiple HTS thin film coated tape like substrates as composite windings for MRI magnets.

One of the main uses of three-fold HTS current sharing composites will be winding magnet coils. Several winding methods are well known and will not be discussed in detail here. However, as an example, spiral pancake coils can be wound where the composite overlaps itself on radius for each turn. This is illustrated in figures 8a & 8b for a single three-fold composite conductor. A solder diffusion bonding process is undertaken to continuously join the HTS thin film tape like substrates immediately prior to conforming the current sharing composite into a winding. On line soldering methods for good diffusion bonding of normal metals are well known (See e.g. WO2008/007141), as are tape splicing methods (WO2012/037231). It should be noted that the solder bond between HTS thin film coated tape like substrates can be most simply achieved by applying solder and appropriate heat to cause diffusion bonding face to face of the copper claddings usually applied to HTS thin film tapes; see Figure 1 a. Continuous bonding of two substrates, coated with HTS thin film has been done (WO2007/016492) to achieve a composite conductor comprised of two HTS thin film coated substrates. Similarly, continuously bonded dual tape composites have been proposed for the tape equivalent of a graded winding, wherein each HTS thin film layer has different superconducting properties. In this type of HTS thin film composite the average composite current is maintained at a useful level in the internal field profile of coil windings, although such a composite will not accommodate defects in either HTS type of thin film (WO2008/036073) Preferred embodiments of the invention achieve self regulating current sharing between three or more HTS thin film coated substrates in electrical contact through normal metal between two parallel planes, in axial, transverse, and lateral directions. In this case the resistance of a current sharing path, say around and overlapping a joint in one tape like substrate, will include the resistance of the copper cladding. This is technically acceptable as the path resistance will typically be in the nano-ohm range because of the relatively large cross section area of the multiplexed current paths. However, if required, a lower resistance joint path can be achieved by removing the copper cladding on one side of each tape prior to feeding into the on line soldering zone. In this process a preferred solder bonding process would involve ultrasonic soldering to insure a good diffusion bond directly to each HTS thin film layer. Insulation between coil turns of HTS thin film composite can also be co-wound within dedicated composite bonding and winding operations, again using well known methods, as schematically indicated in figure 8b. It should be noted, that by using well known in line winding methods, bending stress and resulting strain of the current sharing HTS thin film composite can be controlled during winding such that critical stress strain failure values are not exceeded. By using automated and dedicated winding equipment, current sharing HTS thin film composites can be wound into helical coils. An important benefit of the three fold HTS thin film tape like composites (Fig 6a) with lateral overlap is the provision of current sharing paths with a substantial component of the path in the lateral direction. This permits the total average critical current io to be set to high values, suitable for MRI magnets without increasing the bending strain of winding a given coil diameter. Other attempts to increase current carrying capacity have involved stacking multiple tapes on top of one another. However, as well as not allowing current sharing, such arrangements increase the thickness of the composite tape which has a collateral effect of increasing the bending strain across the thickness of the composite when it is wound into a magnet winding. In the case of a helical coil winding, the average winding strain ε on the outside edge of a current sharing composite is proportional to the width w of the composite and depends on the pitch angle Φ of the tape like composite when the winding pitch is reversed at the coil ends to make a multi-layer coil (see equation 1). The pitch angle decreases for a given number of turns per unit length inversely in proportion to the coil winding radius. Thus it is straight forward to wind large diameter MRI coils that enclose the MRI region with multiple three fold composite HTS thin film tapes. £ ~ w tan 20 (1)

Both flat spiral (pancake) coils and helical coils may be wound on cylindrical formers with end cheeks as is well known to stabilise the spiral winding against electromagnetic forces caused by radial field components through the winding.

Performance improvements for MRI magnets delivered from HTS thin film near isotropic current sharing composites

The preferred embodiments of the invention, whereby it is practicable to fabricate HTS composite conductors in lengths needed for MRI magnets, can be applied to the manufacture of magnet coils that are used in arrays of coils. This includes arrays where each coil individually encloses the MRI volume as well as arrays where the coils collectively enclose the MRI volume, (e.g. see WO2010/100433). The geometries of MRI magnet coil arrays are well known.

An industry typical whole body MRI array, providing a shielded 1.5 Tesla imaging region in which the internal field of each coil is used in combination with its neighbouring coils, requires some 30 km of low temperature superconductor in total, usually deploying the conductor in 1 to 2 km lengths. Such solenoid magnet forms create MRI regions usually on the common axis of rotation of the coils, at or close to the origin of the array, where for field values in the range 0.5 to 3.0 Tesla the field uniformity is between 10 - 100 ppm over 1/3 of the bore diameter of the solenoid (see EP0251342). Conductor joints are typically made between conductor lengths in special well cooled regions of the magnet chamber in a low background magnetic field to avoid instability in the low temperature conductor for the reasons given above. Until now, it has not been possible to manufacture HTS thin film tape conductors of sufficient quality and reliability in lengths sufficient to directly emulate low temperature superconductors in typical MRI solenoid arrays. Smaller magnet coils may be wound and used in arrays wherein the external fields of the coils are combined in a whole body MRI region only enclosed by the manifold geometry of the coil arrays; that is none of the coils individually encloses the MRI region within its own winding. Coil arrays for MRI have been described for example in US 4985679 and WO 2010/100433. The use of three dimensional arrays of coils allows for a wide range of MRI field profiles and geometries of MRI magnet.

Typically, as explained in WO 2010/100433 external arrays can provide open access whole body MRI magnets and are therefore attractive to the MRI user community. Clearly individual smaller coils require shorter conductor lengths than coils that enclose a MRI region within their winding, when whole body MRI is considered. Nevertheless, as shown below, it is advantageous to create magnet coils in such an external array each comprising a substantial number of Ampere turns and requiring conductor lengths of the order of 500 m to 1 km. Thus, composite current sharing conductors according to preferred embodiments of the invention make MRI arrays economical in that they accommodate joints between good quality HTS thin film coated tape like substrates, such good quality tape being limited to lengths of typically 300 m by HTS thin film conductor processes.

For a comparison of conductor length in MRI magnets for similar field intensity and uniformity for magnet arrays the following two examples are considered. In each case, the field profile can be calculated using well known methods, such as by using the Opera software for Electromagnetic design by Cobham Technical Services: Vector Fields Software. The field uniformity is approximately 100 ppm error over a spherical MRI region 40 cm in diameter. Example 1

A magnet is constructed to make use of the internal field of its windings, each winding enclosing the patient access volume where the field due to each winding is combined at the origin of the coordinate system used to describe the set of coils to create a MRI volume at the same origin. Four coils are provided on a common axis, the coils being axially spaced from one another. The four coils form two pairs, an inner pair which are adjacent to each other and an outer pair which enclose the inner pair. The coils of the inner pair have the same number of turns and the coils of the outer pair have the same number of turns, but the coils of the outer pair will generally have more turns than the inner pair. Such an arrangement is illustrated in Fig. 9a and a similar arrangement is described in US 4985679. The following table shows the dimensions, turns and current that can be carried by the coils and the field generated by them in the MRI volume.

Table 1-1

a1 = inner coil radius (cm),

a2 = outer coil radius (cm)

b1 = inner distance of coil from origin (cm)

b2 = outer distance of coil from origin (cm)

N = number of turns of conductor carrying current I (Amperes)

Length = Total length of conductor per coil (metres)

MRI field = Flux density (Tesla) of Nl Ampere turns contributed to the MRI

Note that b1 and b2 are distances measured from the origin of the coordinate system (at the centre of the arrangement - see Fig. 9a) such that (b2 - b1) gives the width of an individual coil winding.

a1 a2 b1 b2 N I Length MRI field

Inner 60.4 61.4 12.4 17.4 1000 272 3826 0.26 pair

Outer 60.0 61.4 53.5 61.5 2240 272 8543 0.24 pair

Total fie d 0.5

Example 2

A magnet is constructed to make use of the external field of its windings, the external fields being combined at the origin of the coordinate system used to describe the set of coils to create a MRI volume at the same origin. The individual coil windings do not enclose the patient volume, but rather sets of coils arranged on particular manifolds enclose the patient access volume.

Four circular manifolds are provided. On each manifold, sixteen coils are provided, all with parallel axes, and arranged around the circumference of the circle. This circle also has an axis parallel to each of the coils. The four circular manifolds are coaxial and axially spaced from each other. Such an arrangement is described in WO 2010/100433. A similar arrangement is depicted in Fig. 9b, although in Fig. 9b only 8 coils are provided per manifold rather than 16. The following table shows the dimensions, turns and current that can be carried by the coils and the field generated by them in the MRI volume.

Each of the sixty-four basic coils:

Table 1-2-1

a1 a2 b1 b2 N I Length Self

Field

Basic 7.0 1 1.0 0 7 1 120 400 633 2.44 coil NB In Table 1-2-1 , the values for a1 , a2, b1 and b2 are not measured from the same origin as is used for the values a and b in Table 1-2-2 below, but they serve to indicate the size of the basic coils. The four circular manifolds (each with 16 basic coils) are arranged in an inner pair and an outer pair as in example 1 above.

Table 1-2-2

a = mean manifold radius (cm)

b = mean manifold half length (cm)

N = number of turns of conductor carrying current I (Amperes)

Length = Total length of conductor per manifold pair (metres)

MRI field = Flux density (Tesla) of Nl Ampere turns contributed to the MRI zone

Semi quantitative assessment of means by which HTS thin film composites enable and enhance the performance of MRI magnets. The dimensions and current capacity of HTS thin film tapes are typically standardised for optimum deposition and growth of the thin film. It is an economic benefit to manufacture a relatively few standard types of HTS thin film tape.

However, and conversely, the most economic cost of building a MRI magnet is achieved with the necessary properties of the superconductor determined from the dimensions and performance of the magnet windings as these deliver the required MRI magnet field profile. It is also the case that the overall geometry of the MRI magnet is chosen both to provide the target MRI field profile, as well as to support key aspects of the medical application involving patient examination. These factors are most effectively handled in design by working with a wide range of conductor types. Composite HTS thin film conductors utilising more than one tape like conductor offer a means of uniting the needs of economic conductor production and freedom of choice in magnet design. The following magnet performance criteria are discussed in relation to how composite HTS thin film conductors, with

superconductor arranged in two parallel layers, contribute to optimised MRI magnet cost and performance.

Defect effects & improved yield The use of three-fold HTS thin film tape like composites as shown e.g. in Figures 5 and 6, improves the effective yield of deposition processes employed to produce superconductor for MRI magnets. In general, the probability of a departure from ideal film deposition and growth conditions is the same at any time during a deposition run of a single tape like substrate through film coating equipment. Thus for any given process, there is, generally, a constant probability of a defect per unit length, and therefore the longer the deposition run, the greater the probability of the best critical current of ideal thin film being reduced somewhere in the total length of tape. This manifests itself as a reduced critical current ic in the total tape length. By using a current sharing two-fold composite, e.g. as shown in Figure 3, the integrated critical current, Ic of the composite is guaranteed to be, at the least, equal to the best ic of one tape of ideal film for any length of composite. This assumes defective regions do not lie parallel to each other in the composite. More generally using three-fold (or n-fold, n > 3) composite, e.g. as shown in Figures 5, 6 and 7, the composite critical current is reduced by defects in proportion to the defect cross section area and the amount by which critical current is reduced in the defect cross section. Of significant technological and economic advantage, n-fold composites with n greater than 3 can accommodate more than one HTS thin film tape like conductor having a defect in parallel with a neighbouring tape in the composite. A pro-rata reduction in composite Ic results is obtained.

As an example, for a three-fold composite, with three HTS thin film tape like conductors each of ideal critical current ic, the composite critical current Ic is 3ic. With current sharing, any defect giving zero current capacity, or a deliberate joint, over a short length of one tape-like conductor will reduce the composite critical current by 1/3 to result in 2/3 of the maximum possible composite Ic. In general, for n-fold conductor with n greater than 3, e.g. as shown in Figure 6, the reduction in composite critical current will simply be proportional to the reciprocal of the number of individual tape like conductors in the composite. In Figure 6 this will be 1/7. Of course, if the defect region in one tape like conductor has some critical current per unit width, the reduction in composite critical current will be less. For example, say one tape like conductor has a defect which halves its ic, then the composite lc shown in Figure 6 will be reduced by 1/14.

A simple composite effectiveness expression can be written as:- ic = [ io + D * ( io * f / (N - D) )], lo = N * io (2) where:

lo = the total operating current of the composite from one end to the other. ic = the best critical current of the HTS thin film processed on each tape like substrate.

(for illustration purposes ic is given the value of lOOAmperes)

N = the number of identical HTS thin film tape conductors in the composite.

D = the number of conductors with defects at a given longitudinal location. f = the transport current fraction reduced by the defect (f=1 for 100% current stop, f~ 0 for -0% current stop)

io = operating current per tape averaged over all tapes as substantially the same value after current sharing, measured at equi-potential contact points of the composite.

is = average shared current between tape with defect and each "good" tape at the longitudinal location of the defect.

lo = operating current of the composite determined by all defects at the longitudinal location.

Table 2 Composite critical current from start to end of a given length for one longitudinal defect region

The following points should be noted in interpretation of HTS thin film conductor yield. It is taken for the model that all defects in a common longitudinal location, and located in a common transverse plane, have the same current reduction effect. This would not be so in practice. It has been assumed for simplicity that current shared (is) has the same value for each path, and that without defects anywhere in the composite the operating current in each tape io is equal to the critical current ic, where in practice io < ic. Each parallel conductor has the same io because equi- potential start and end connections connect the composite to the source of emf.

Note that critical current values in practice will be expressed for a given

temperature and applied magnetic field, which condition in this simple illustration of yield, will be considered to apply to the total composite conductor length. The critical current of the composite HTS thin film conductor, as determined from start to end of the composite, will be that of the worst section of the composite. That is, the maximum current capacity of the composite is determined solely by the most inefficient combination section of HTS thin film tapes where defect or joint densities are greatest. In practice, it is likely that the critical current of the composite after accounting for defects will be averaged across the number of tape like conductors in the composite, so that there is spare current capacity in the good tapes at a region where a defect exists in one tape, see Figure 7. For example, with reference to table 2, a three-fold composite, each tape having ic = 100 Amperes, with a zero current defect in one tape has a total Io of 200 Amperes. Each tape like conductor at the start of the composite would carry an average current io of some 200 A / 3 = 66.7 Amperes, so at the zero current defect, 33.3 Amperes transfers into each of the good tapes, so that their ic is reached; i.e. 66.7 A + 33.3 A = 100 Amps. The yield value in the example of Figure 6 can be viewed thus: comparing a current sharing composite of seven unit lengths of tape like conductor, any tapes can have defect zones at a longitudinal location which reduces the operating current Io of the composite by up to 1/7 (that is, the operating current Io is 6/7 of the ideal, defect free critical current 7 * ic for the composite, or Io = 6/7 * (7 * ic) = 6 * ic). If the unit length of composite provides for one turn about a coil mandrel, then there are one times Io = 6 * ic Ampere turns creating field in the centre of the turn. If it were possible to manufacture a single length of tape, seven times as long as the unit length of the composite, then the same production cost of HTS thin film would be incurred as in the composite case, yielding seven turns about the coil mandrel. If this single tape were perfect (no defects), it could provide 7ic Ampere turns. However, the ampere turns available for field creation would be seven times zero if a single zero current fault existed anywhere along the length of the conductor (as ic would then be equal to zero). The actual ampere turns in the single tape would be determined by the lowest critical current in any section of the tape caused by the worst defect zone in the single tape conductor for the same thin film production cost.

It should be mentioned that in principle, it is possible to test on a production line each unit length of HTS thin film tape like conductor for some indication of the location of any defect regions. Usually, inductive measurements of critical current are made in tape locally cooled and run through a test station. However, it is only practicable to test for gross defects. It is not possible economically to test for defects along the length of a tape under exact operating conditions in a magnet, even if the field profile of the magnet design is known. The critical current ic of a HTS thin film tape like conductor (such as shown in Figure 1) is a function of operating temperature, local magnetic field strength and orientation relative to thin film crystallography, and to local forces applied to the thin film. Thus it is only possible to continually repeat deposition runs for coating single tape like substrates as a sequential manufacturing operation until one length is achieved of sufficient length without a gross defect, in this example equal to seven unit lengths. As the target length of conductor required is increased, then a law of diminishing returns is encountered if an attempt is made to coat a single HTS thin film tape to deliver the same number of ampere turns on a given coil mandrel radius. Economic benefits of distributions of HTS thin film conductors in coil windings for MRI magnets.

It is a technological advantage in the design and construction of MRI whole body magnets to have the freedom to choose the values of a number of electromagnetic parameters. By optimising the value set, the best field profile for a target MRI application can be economically obtained.

In some embodiments, it is particularly desirable to specify the operating currents of the magnet so that, as the magnetic field in the windings is a design variable, conductor critical current is adjusted to be appropriate to the field values applied to the conductor winding. Where the field on the windings is highest, the critical current of the conductor can be arranged to be suitably high. In regions of coil windings where the applied field is low, conductor with considerably less than the maximum lc can be selected. This avoids unnecessary extra cost otherwise arising from excess HTS thin film adding un-required current capacity in low field regions of the winding. Such a winding is traditionally said to be graded in critical current with respect to internal field profile. This applies to individual coils in their own self generated field, or to the net field in a winding created by the winding and the integrated external field of neighbouring coils of an array of windings. Note, Critical current ic at a point is approximately inversely proportional to the applied magnetic field at the point.

It is also desirable to operate MRI magnets at high critical currents, usually limited practically by the current value that can be conveniently supplied to the magnet. This permits the Ampere turns required for the MRI field intensity to be obtained with the smallest number of turns possible in the winding. Crucially, as the number of turns is increased for a given MRI field value, the inductance of the coil system comprising the MRI magnet is increased. Large inductances are undesirable in that high voltages can be generated within the winding when the field is changed.

Circumstances such as a rapid de-energising of the magnet to either remove the field in an application emergency or in the event of a major volume of conductor becoming normal can arise in service. Such voltages can cause internal arcing and local destruction of the winding. It is also well known that windings of many turns are more likely to be mechanically unstable in the presence of shear forces generated electromagnetically. In the case of low temperature superconductors, unstable windings return to normal state (quench) at low operating fields, and in the case of brittle HTS thin films, shear strain can irreversibly create local defects in current capacity. A further design requirement for a MRI magnet is to adjust stresses that arise in the composite HTS thin film conductor from electromagnetic forces. For a given composite operating current, the cross section area of the conductor can be varied by altering the number of three-fold HTS thin film tape like substrates in the cross section of the composite. The greater the cross section area of composite, on average, the smaller the value of principal stresses in the conductor for a given average composite operating lo. As shown in Figures 6a & 6b, passive tape like elements (shunts 15) can be added to the composite during the bonding assembly step to increase the cross section area without deploying costly HTS thin film. In this example, copper tape is added to provide average reduction in tensile stress in the composite and a normal current path to shunt major amounts of field energy in the event of catastrophic failure of the magnet coil winding. Force patterns directing forces towards the edge of a tape like conductor, for example from radial field components through the winding, can cause buckling of single HTS thin film tapes. Composites with larger edge dimensions than single tapes in relation to their width have improved mechanical stability. As a further note, the possibility of adding elements to composites for mechanical stability offer the magnet designer a general tool to control complex stress patterns in HTS thin film windings, and as is well known, develop desirable means to accommodate shear stress patterns within windings having a rotation of principal field directions in the windings.

Buried joints in windings versus joints in current supply looms.

Composites with three-fold HTS thin film tapes allow for joints between tape like substrates, as shown in Figure 6a and 6b. Windings of any length may be fabricated from elemental lengths of tape by allowing for an average number of joints per unit length of composite, and by designing the coils of the magnet allowing for the effects those joints will have on the properties of the winding. It can be said that the joints are buried in the winding. The most important aspect of buried joints is that the small amount of heat generated by current sharing around the joint can be extracted locally within the winding as a whole. Simple estimates of thermal stability of a buried joint are given below.

Given that the small voltage along the composite (nano-volts range) can be balanced by a drive voltage applied to the MRI magnet as a whole, it is possible to manufacture MRI coil arrays in ideally dimensioned units. For example, the bulk of a winding designed for optimised operating properties (field profile, mechanical stability, de-energising performance) can be realised by winding without the cost and operating compromises otherwise dictated by running conductor out of the winding to achieve all joints externally. External joints can compromise the freedom to design involved arrays of magnet coils, such as are needed for open access MRI magnets. It should be noted that the runs of leads to external joint stations are difficult to achieve with HTS thin film tape like conductors as the tape is limited to bending about its edges by virtue of the small intrinsic strain to failure of HTS thin films, (about 0.1 %). It can also be problematic to ensure joint lead in and corresponding lead outs from windings follow a non inductive path, so avoiding the creation of complex distortions of the MRI field. As shown in Table 1 , traditional designs of MRI magnet coils which enclose the MRI volume typically require lengths of tens of kilometres and operating currents of hundreds of Amperes.

Attempting to fabricate such arrays of enclosing windings from single tape lengths, typically a few hundred metres in length, would require hundreds of external joints and, thus, require a complex and costly winding loom and a significant departure from optimum winding geometry of the coils. A similar problem arises with arrays that combine the external fields of a large number of coils, such as are desirable for open access magnets (e.g. WO2010/100433). In this type of situation, such as illustrated in Table 1 , very high internal fields are required in each basic coil in order that the combined external fields of coils add up to a useful value for MRI. Thus, it is advantageous to magnet stability to construct the coil winding as one, unitary conductor section, and not disturb the mechanical integrity of the coil with frequent in and out lead runs. In the case of external field arrays, preferred lengths of conductor may be 600 to 1000 m in each basic coil winding.

As shown in consideration of the thermal stability of joints, buried joints with transverse and lateral current sharing paths have the benefit of substantial cross section of current transfer paths, and therefore, low current sharing resistance. It is not always practicable to include sufficient space for external joints for a current supply loom where each joint has a long overlap region and a large cross section area, particularly, again, when considering tapes which cannot be easily bent on edge. It should also be remembered that current looms experience substantial electromagnetic forces in the vicinity of coil arrays, and must be well supported mechanically. Buried joints in composite HTS thin film windings are readily supported by virtue of the winding of a constant cross section conductor, as represented by the composite (see Figure 6a and 6b). External joints of current conductors at the start and end of optimised windings, where buried joints exist in the optimised windings, are preferably reserved for major connections to the electrical supply circuit of the coil array. It should be noted that, given power supplies can be economically kept permanently connected to HTS thin film magnets due to the relatively high operating temperature of the HTS MRI magnet, there are operating advantages in some MRI applications for being able to change coil current during use of the magnet e.g. hyper-polarisation imaging.

Current sharing around defects and joints, and coil drive voltage.

While the maximum critical current in a composite of HTS thin film tape like conductors in a given length is determined by the worst set of combined critical currents in the parallel tapes, the power wasted in the conductor depends on the total defect density in the composite. Simple modelling again allows technical and economic estimates of HTS composite benefits for MRI. Estimates of composite current sharing voltages are obtained by considering a HTS thin film coated tape like substrate as shown in Figure 1. The illustration of a solder bonded composite in the simple case of two face to face HTS layers is shown in Figures 2 & 3. For two good layers (Figure 2), a current of two times ic, the best critical current of a unit lengths of one tape, (see Table 2) flows in the composite. Note, that tapes with a single HTS thin film coating on one side of a tape like substrate are considered throughout this invention because the oxide buffer layers required for the chemical and physical integrity of the thin film (shown in Figure 1), are in effect electrical insulators. Thus, as described, low resistance current sharing defect avoidance is only practicable transversely (in the thickness direction), and with a substantial cross sectional area path of short length, between two layers in two planes. Given the boundaries of the shared current path as described between two layers, the joint resistance is dominated by two parameters. The first is the transverse resistivity (X direction of Figure 1 b) and the second is the transfer length (Z direction in Figure 1 b) over which current is shared in the axial conduction direction. The transverse resistivity is determined approximately by the thickness of the solder bonded layer and the resistivity of the solder material, plus any inter-metallic layers formed in the diffusion bonding process and normal shunt metal e.g. copper. HTS thin film may be diffusion bonded with a variety of means. Commercial HTS thin film coated tape like substrates are usually finished with copper coating, typically applied by a vapour deposition process to chemically clean HTS thin film during in line deposition. Solder bonded copper joints are well known, and resistance values for the joints, solder / copper / HTS interface have been published, see Table 3. Another well known method, used with low temperature superconducting Niobium titanium alloys, is to use ultrasonic soldering with Indium alloys on bare

superconductor which has first been chemically cleaned and removed from its co processed copper matrix. It is possible to implement this in this invention by removing copper cladding on HTS thin film tape and directly solder bonding ultrasonically one layer of HTS thin film to another. This can be achieved typically with Indium based solder. In this event very low resistance joints can be achieved in the sub-nanoohm range. It is worth noting that in the case of low temperature superconductors, in low values of magnetic field, joints made by ultrasonic means behave as superconducting joints, assisted by the long coherence lengths in such superconductors. Table 3 lists some values of resistance to current relevant to assembling HTS thin film composites, where such values are subsequently considered in regard to current sharing transfer lengths automatically avoiding tape defects.

Table 3 Values of resistivity relevant to composite assembly Solder resistivity:

Sn62 Pb36 Ag1.4 : 14.5E-6 ohm cm at Room Temperature (RT), reduced to 3E-7 ohm cm at 77°K

In90 Ag10 7.8E-6 ohm cm at RT

Order of magnitude Resistance of a solder lap joint in copper / niobium titanium wire: 6E-9 ohms and power dissipation 6E-5 watts at 4.2°K for 100 Amperes Resistance scarf or lap joint HTS thin film tapes (one HTS thin film face to another through copper cladding): 40-50 nanoohms for two tapes 1 cm width for 100 mm overlap

Resistance of "superconducting" indium soldered Niobium Titanium estimated from time constants of low temperature MRI magnets: For a current change of ΔΙ = 1 E-7 in 3600 sec (e.g. for standard MRI magnet decay), typical inductance is about 1000 Henries and Magnet equivalent resistance is about 3E-8 ohms (typically 3E-9 ohms per joint).

Resistivity of Hastelloy (Ni Cr Fe Mo) typical tape substrate: 125 microohm cm

Magneto resistivity of Cu = 2E-8 ohm cm at 2 Tesla, and 8E-8 ohm cm at 10 Tesla, measured at a temperature of 4.2°K ;

Basic resistivity of Cu measured at room temperature and zero applied magnetic field = 2E-6 ohm cm.

It should be noted that joints between HTS thin film tapes using current transfer paths afforded by adjacent tapes, where all tapes of the composite have a rectangular cross section, (as in Figures 1 a & 6b), retains an overall composite rectangular cross section through the joint region. This means it is possible to bury joint sections in a coil winding without creating local distortions of dimensions within the winding, where such distortions could otherwise create local damaging strain. As commented on above, HTS coils may be wound from effectively unlimited lengths of continuous conductor. Round section low temperature superconductor wire conductors can typically also be used with buried joints in windings. The same would apply to solid wires of HTS material or to circular or square cross-section (or other cross-section) wires coated with a thin film of HTS material.

There has always been interest in jointing low temperature superconductors in coil winding to overcome the limitations of available wire lengths manufactured from composite batch processing. This interest has been summarised in the work of the Rutherford laboratory UK and other US and European laboratories on current transfer in low temperature composite superconductors. Current transfer in low temperature superconducting composites is relevant to this invention. Here, continuously bonded HTS thin film tapes in three-fold conductor sets, arranged in two HTS face to face layers on two parallel planes, use lateral and transverse current transfer paths to automatically circumvent thin film defects in separately processed HTS tapes.

Methods have been considered for estimating current transfer lengths along the conducting length of the composite. In this invention, given that all defect avoiding paths contribute to the overall resistance of a length of composite conductor, the behaviour of current sharing will affect the apparent resistance of a coil. This resistance is likely to increase non-linearly as the average composite operating current lo approaches the value at which all sharing paths (is) have reached their local maximum critical current ic. For a single, zero current capable defect or axial gap in a tape, (Figures 5a and 5b), relevant models show the development of a transverse voltage to "shift" current from one HTS layer into the adjacent HTS layer. The axial length of composite conductor of unit width needed for transfer of the shared current increases in proportion to the square root of the product of transverse path length and ratio of transverse to axial resistivities (for more information see "Superconducting magnets" by MN Wilson, ISBN 0-19-854805-2, page 235). On this basis, using composite arrangements of the form in Figures 1 a and 5, and considering commercial HTS thin film tape like conductors 1 cm width (Table 3), current sharing transfer via two HTS thin film layers facing each other should require a length of about twice times one to five centimetres (i.e. 2 to 10 cm). Using resistivity values from Table 3, and a typical solder layer thickness from the wetting and diffusion bonding continuous solder process of 0.05 cm, the resistance of a current transfer path is of the order of 6E-9 ohms. Published results from Table 3 indicate a similar figure for typical lap joints using HTS thin film tape like conductors.

Current transfer around defects blocking current flow in one of the tapes in a composite HTS thin film tape like conductor, at a resistance of the order of six nanoohms, for a typical shared current (is) of 200 Amperes per cm unit tape width results in a power dissipation of 0.00024 Watts. This power is dissipated in some 0.1 cm ^A 3 , i.e. about 1 gram of material, so increasing the joint temperature adiabatically by 0.0024°K per second (specific heat 0.1 J /g/°K). In models allowing for heat conduction from conductors to typical cryogenic sinks, such power generation in windings may be neglected in general as the local temperature rise in the conductor is less than 1/100 of Tc, which for HTS thin film materials will be less than 1/1000 of Tc. Actual measured joint resistances can probably be an order of magnitude higher due to the formation of intermetallic compounds at interfaces between components (Table 3), but still, the temperature rise in the joint region will be negligible. For complete transfer of current at (is) of 200 Amps per tape like conductor, such transfer paths would present a need for a coil drive voltage of about one microvolt. For example, in a coil of many kilometres of conductor, some 1000 complete current sharing transfer paths would only need a magnet power supply of a few millivolts. Given HTS thin film materials exhibit flux flow resistance at operating currents io over approximately 2/3 ic, it is useful to have a low voltage current stabilised drive available even when a constant magnetic field is required from the magnet using the maximum possible operating current.

Therefore, it is acceptable to bury zero current defects, or current blocking joints in composite HTS thin film comprised of HTS thin film coated tape like substrates as the composite will be stable in current sharing zones. By virtue of arranging current sharing paths to have a low resistance, it is possible to operate current sharing composites at high currents, and, in particular at low temperatures down to approximately 10°K, where instabilities typical of conventional low temperature Niobium titanium superconductors are likely. Thus, the lower is the typical current sharing resistance, the lower is the defect power generation, and the lower can be the minimum coil stable operating temperature. This in turn means that the critical current of each HTS thin film coated substrate will be increased by virtue of lowering the operating temperature, and, therefore, low current sharing path resistances mean greater MRI field intensities are possible.

An indication of the economic benefits of using composite HTS thin film tape like conductors for MRI magnet arrays can be obtained by considering the published values of critical current in commercially manufactured single HTS thin film tapes. There are two figures of merit to consider. The first is that single tapes are supplied reliably in lengths not greater than 300 m. This determines the typical number of zero current blocks, i.e. buried joints, in a given HTS thin film tape like substrate as a conducting element in a composite conductor, and therefore the number of current sharing regions buried in a magnet coil for an MRI array. The voltage created per buried joint will be that due to the estimated resistance, commented on above, and consideration of the average shared current (is) of all the tape conductors in the composite.

The second figure of merit, capable of approximate estimation of an average value is the amount of current shared per defect region of reduced ic. For simplicity, the amount of current shared is taken as passing through the same path resistance as used for buried joints. Commercial tape manufacturers publish values of critical current measured locally at regular intervals along HTS thin film tapes. Typically, a tape with an average critical current of 400 Amperes will have a spread in ic along its length of +/- 50 Amperes, taking measurements every few cm. However, in addition, there will be more significant departures in ic value, typically every 100 m or so, where ic may be as low as 240 Amperes, or tape ic is about 60% of average. These latter events seriously limit magnet performance. The preferred

embodiments of the invention seek to average out their reduction in conductor performance, at the expense of creating a resistive voltage when a composite HTS thin film tape like conductor is operated at transport current io values close to the critical current ic in all tape conductors. An estimate of MRI magnet coil typical drive voltage can be made as follows, shown in Table 4.

Table 4 Estimates of MRI coil voltage for a three-fold HTS thin film composite operating near maximum composite current.

Composite three-fold conductors: ic each conductor 100 Amperes;

200 Amperes composite

Buried joint in one conductor: Resistance current transfer path = 45E-9 ohms (Table 3)

Composite current 200 Amps (3-1 tapes) = 66.7 Amps per tape at start of composite.

Current shared at joint 1 tape into 2 tapes = 66.7A / 2 = 33.3 Amps

Voltage current sharing path per tape to tape E = l*R = 33.3 * 45E-9 = 1.5E- 6 x2off

Defect zones with reduced ic: take shared current 20 Amp in 100 Amperes; Composite current 280 Amps (3-1 (80%) tapes) = 90 Amps per tape at start of composite.

Current shared at defect 1 tape into 2 tapes = 20 A / 2 = 10 Amps

Voltage current sharing path per tape to tape E = l*R = 10 * 45E-9 = 0.5E-6 x2off

Voltage per 100 m at maximum composite I operation; combined tape ic. Allow one buried joint and 100 reduced ic point defects:

Volts (1* 3E-6) + (100 * 1.0E-6) ~ 103 microvolts / 100 m, Composite = 200 Amps

Conductor length per MRI array of magnets 12 - 40 km at 270 - 500 Amperes (Table 1)

MRI magnet array = 0.017 - 0.103 volts range

MRI magnet array power = 4.5 - 51.5 Watts range

As referred to above, this invention of combining tapes in a solder bonded composite allows the composite operating current lo to approach the sum of the critical currents ic of all tape like conductors in the composite. As the operating current of the composite is raised, a coil voltage will appear when current sharing is initiated. This will occur at a point in the composite were the average tape conductor current io just exceeds the critical current of a defect zone in one tape, the worst defect zone in the coil. For further increases in operating current, more current sharing will take place and the necessary coil drive voltage will increase, generally following the form of an exponential function volts versus current. By permitting MRI magnets to be driven on a power supply, a gain is made in MRI field value for a given volume of HTS thin film coating.

The yield value considering a drive voltage can be viewed thus: consider a current sharing composite of three unit lengths of tape like conductors as shown in Figures 5a and 5b. From the example in Table 2, a three fold composite with tape critical current ic = 100 A each and one defect with 50% current stop will have a maximum operating current of 240 Amperes, with current sharing. However, if the same three HTS thin film tapes, one with a 50% current stop defect, were powered in parallel from the same equi-potential current source and electrically insulted from each other, then they are all limited to the same, lowest current if overloading and destruction of the tape with a defect is to be avoided. Note that the model is based on the fact that before the coil is operated, the location of a defect is not known. Thus the composite with isolated tapes has a maximum operating current lo of 3x50 Amperes = 150 Amperes.

Thus, the effective yield with current sharing in this example is improved by 60% for a given volume of HTS thin film coated on tape like substrates by bonding the tapes in a composite. The dispersed power estimate for a large coil array of 51 Watts is an acceptable heat load at 77°K for a liquid nitrogen liquefier, although much beyond economic refrigeration costs if collected at liquid Helium temperature.

A general point concerning voltages arising when HTS thin film MRI magnets are energised is that power dissipated in the coil winding is reduced by the three fold symmetry of HTS thin film coated substrates in the composite conductor. When a MRI magnet is energised by the application of a drive voltage to increase the composite current and overcome the inductive reactance of the coil, there will be hysteretic power dissipation due to self field effects in the composite conductor. These can be minimised within the composite conductor by reducing self field effects between individual tape like conductors, which is achieved by displacing the conductors laterally, so that they edges of individual component conductors stacked on top of each other do not line up. If the edges of conductors which are stacked are in line, magnetic fields are concentrated at the radii of conductor edges and field hysteretically penetrates the thin film from the edges producing a power loss. This is avoided by the preferred embodiments of the invention as the edges of HTS thin film coated substrates in the composite do not line up (substrates in the first plane are offset from those in the second plane of the composite). (Ref

US20030178653A1) It is an objective of at least the preferred embodiments of the present invention to provide three or more HTS thin film tape like substrates connected electrically by normal metal bonding along their total length, so that current can be shared by transfer from any one tape to all of its neighbours at defects, there being a similar number of tapes in a first plane and a second parallel plane. This means that at any selected region along the composite of three or more tapes, the critical current of the composite is the average of the critical currents of the HTS thin film coated tape like substrates at that region (Table 2). So, for example, three HTS thin film coated tape like substrates with ic1 ~ic2~ic3 in general form a composite with total critical current lc = id + ic2 + ic3 ~ 3ic1. However, at a defect region where, say ic3=0, then lc = id + ic2 Thus, in the present invention, the composite total critical current will be less than that of prior art using bridge sections to pass a current around known defects. However, and of importance, as shown below, the composite using multiplex current paths always provides a total critical current which is averaged, and the best possible for the multiple HTS thin film tape like substrates, without necessarily knowing what the defect density and critical current values are locally. It is also the case that by the use of current sharing in both lateral and transverse current flow directions the current sharing paths have substantially similar resistance because the greatest fraction of the current sharing path length is comprised of HTS thin film superconductor. The value of composite critical current lc tends efficiently towards the value of the sum of the critical current used in spliced composites. This effect is improved as the number of HTS thin film tape like conductors in the composite is increased.

It is also an objective of the preferred embodiments of this invention to provide multiplexed current sharing paths, characterised in that the geometric profile of the current sharing paths, between two parallel planes, follows substantially lateral direction in the composite, as well as the transverse direction (between the two planes) of current sharing of prior art. This has the beneficial effect of greatly reducing the resistance of current sharing by all multiplex paths, and, therefore reducing the voltage generated across the terminals of a MRI coil winding. In turn, this reduces power loss in a HTS thin film MRI magnet and improves the stability of the coil. Further, reduced power losses in the winding in the vicinity of defects permits the coil to be operated at temperatures well below 77°K, thus accessing higher critical currents in the HTS thin film, so to deliver improved field performance from a HTS MRI magnet. The power loss at defects tends to similar values through out a coil winding, and therefore leads to predictable coil performance.

Inspection of the concept of multiplex current paths shows that current sharing by averaging transport currents between a set of HTS thin film tape like conductors, bonded together with a normal metal, will allow unlimited lengths of superconducting composite to be fabricated for MRI coil windings. This overcomes the limitations of all presently known methods of fabricating linear HTS thin film tape like conductors. Known tape fabrication processes are all limited to characteristic degrees in respect of critical current ic in the HTS thin film over a unit length, whereby defects in composition or structure of the film limit ic locally. Such local defects in a linear conductor present weak points that determine the maximum operating current of the tape, and therefore limit magnetic field values created in coils. Generally, because HTS thin film deposition and growth processes need to be carried out under non-equilibrium conditions, the complete avoidance of defects has hitherto proved impossible. It may be that the control of deposition and growth of

HTS thin films on tape will improve in the future so that longer continuous lengths of tape can be economically processed. However, this invention of multiplexed tape composites ensures that unlimited lengths of conductor can be realised in order to wind large MRI magnet coils. In addition, the concept of current sharing composites will always offer economic benefits in that the yield from tape processing will be increased as the presence of any defect distribution in a tape will not prevent the use of such tape in a composite, as the effects of defects are mitigated.

It is preferred that known processes for deposition and growth of HTS thin film are employed to fabricate linear conductors that operate at temperatures close to the critical temperature of such typical HTS materials. Some of the preferred embodiments of this invention place at least three HTS thin film layers face to face in two parallel planes using at least three separately processed tapes. This provides a symmetry such that current sharing paths that bypass defects have substantially the same resistance and power dissipation in a coil winding, irrespective of which HTS thin film has a defective region. The HTS thin film layers are coatings on one side of a buffered substrate. Preferably, and with increased economic benefit, multiple tapes are placed with their HTS thin film layers face to face, such that in a lower plane there are placed a series of tapes with their HTS layers facing, and, for every two adjacent tapes, the HTS thin film layer of a third tape is placed in an upper plane. Advantageously, each three-fold group can share current paths between the planes transversely between layers, and laterally between tapes, so that the current sharing path length is mainly through superconductor material, thus providing substantially similar resistance in sharing paths. Two-fold composites share current only transversely between layers. In the case of three-fold tapes, the upper tape preferably overlaps equally the two lower tapes in the direction lateral to the long axis of composite conduction.

Another benefit of using the three-fold tape arrangement in a bonded composite, where all tapes are of the same width, is that three-fold composites themselves can be arranged on parallel planes in the lateral direction. Each three fold tape arrangement in a multiple lateral group is inverted relative to its neighbours; i.e. starting with two tapes on the lower plane and one equally overlapping on the top plane, the next three fold arrangement has two tapes occupying the upper plane layer and one the lower plane layer with the same overlap arrangement. This provides for desirable high composite critical currents, not significantly limited by the ic of a given tape type of process, and low power dissipation in MRI coils. The power dissipation for a given defect is substantially the same wherever a defect in HTS thin film layers occurs in a given coil winding, so enhancing coil operating reliability.

It is considered that the electric field in composites, created under current sharing conditions to circumvent HTS thin film defects which inhibit super-currents, gives acceptably low drive voltages when multiplex composites are used to wind MRI magnet coils. The symmetric averaging of shared current (is) in the composite between sets of three-fold tapes means that, for a small coil drive voltage, all tapes in a composite can be operated near their critical current. This effect increases again the equivalent yield of HTS tape processing. It is proposed that known methods of continuous on line solder bonding are used. These methods may be applied directly to industry typical HTS thin film copper clad tape, wherein copper is included in the current transfer paths. It is also proposed, that for the lowest resistance current transfer paths, resembling superconducting joints as manufactured for low Tc superconductors, copper cladding may be continuously removed from commercial tapes as part of the solder bonding process. Solder bonding may then be achieved directly between HTS thin film layers. This has the desirable effect of allowing HTS thin film tape composites to achieve a combined operating current which depends on each tape operating near its best critical current value, thus further improving material yield in MRI. A further benefit of using composites with multiplexed current sharing at the level of path resistance assessed for the tape arrangements disclosed is that physical joints between tapes occupying any position in cross section in a three-fold composite may be buried in a MRI coil winding. As is the case for HTS thin film defects, buried joints present acceptably small drive voltage requirements for MRI coils. Because composite HTS thin film conductors retain the rectangular cross section geometry of a tape, buried joints do not give rise to distortions of unitary coil windings, which are generally to be avoided for the sake of coil mechanical stability. It is also the case that the composite is proposed as being solder bonded during coil winding and that dedicated equipment is set up to handle the formation of spiral windings, attending to control of handling issues to avoid exceeding critical conductor stress strain values. Both layer insulation and additional copper conductor for electrical protection of coils is preferably delivered by dedicated equipment into the winding operation. Straight forward spiral pancake coils may be wound from composite conductor, but given the design of composite structure is considered as part of MRI coil fabrication, helical spirals may also be wound on large diameter coil formers with adequate control of edge bending strain. The invention of composite HTS thin film multiplexed current sharing conductors is intended to deliver magnet coils for MRI where the coils themselves enclose the target MRI volume, or the coils are arranged in multiples on manifold supports which enclose the MRI volume. In the latter case, the external fields of coils are combined in the MRI volume. In both cases the invention makes it possible to obtain values of Ampere turns necessary for MRI magnets by delivering high currents and long lengths of HTS conductor, well beyond the current values and lengths of good quality HTS thin film practicably obtainable from single tape coating processes.

Although the above description is largely focused on thin film HTS tapes, many of the ideas clearly also apply where the individual component conductors (the strands that make up the composite conductor) are solid wires of HTS material or wires coated with a thin film of HTS material, providing such component conductors are bonded together for current sharing as described. A skilled person will readily be able to adapt the above examples for non-tape-like HTS conductors that are also useful in generating long lengths of HTS composite conductor which is tolerant of defects in individual component strands and is efficient in the use of HTS material and thus highly suited to the production of large superconducting magnets such as MRI magnets.