METHOD The present invention relates to the manufacture of electrically conducting cables of the type used for numerous applications that require the conduction of high currents with accurate position of each element within the cable and for use in the generation of, for example, high quality magnetic fields. One type of relatively well know conducting cable of this type is a continuously transposed cable, known as a Roebel cable. Such a cable has a number of flat tapes of conductive material that are laid and transposed such that they are geometrically aligned along their cross-section. In some cases the material forming the tapes is selected to be superconducting at the operating temperatures of the cable. Often the tapes are formed from a core of high temperature superconducting (HTS) material with a coating such as a metallised outer surface. Sometimes the "core" is represented by a metallic tape coated with HTS layer. Such cables have significant benefits in terms of providing, in principle, a high level of control of electromagnetic fields during their operation and can be employed in superconducting spools, windings, motor generator coils, magnets, transformer cables and current leads, for example. Roebel cable reduces ac losses that occur by transporting of ac current. Being based on high temperature superconducting (HTS) tape, the Roebel cable allows an outstanding performance regarding engineering current density at very high (> 20T) magnetic fields. Favourable properties of the Roebel cable are at least in part due to a transposition of meander shaped tapes (strands) within the cable.

Realization of the Roebel cable made of HTS coated tapes is described in [WO 2011 159176 A1] and [Wilfried Goldacker et al 2014 Supercond. Sci. Technol. 27 no. 9, p 093001]. According to this art the manufacturing of the Roebel cable comprises a deposition of a HTS superconducting layer onto a buffered metallic tape playing a role of substrate. Buffer layers on the substrate tape are aimed to form a biaxially textured template with a sufficient degree of crystallinity which should be finally "transferred" to the HTS layer. The HTS coated tape undergoes a metallization which is required for electrical stabilization of the HTS tape in case of a local quench. Higher degree of electrical protection may be achieved via electro-plating or lamination of the HTS tape with a thick (20-40 μιη) copper layer. In order to enable Roebel cable the tape is formed into meander shape that allows an assembling of the Roebel cable with a pre-designed transposition period (transposition length). The forming of the meander shape is usually performed by punching. However, there are issues associated with the manufacture of such cables which means that they can be difficult to produce in a manner which ensures there is no deterioration of their superconducting properties over the lifetime of the cable. For example, these techniques for manufacturing such cables can require punching of the tapes which form the cables which can create open edges on the tapes with gaps in the metallised exterior of the tapes. These openings can provide an interface for penetration of water and other contaminants into the cable. In the short term this may not be an issue, but over the long term it can result in deterioration of the entire cable because of diffusion of such contaminants.

Furthermore, what the applicants have realised is that such "openings" and other defects formed during manufacture can also lead to problems in adhesion of the layers of tape which can cause problems in controlling the galvanic "transparency" of the structure and therefore increase losses that are preferably avoided. In terms of superconductivity, the critical current should not degrade during processing and afterwards.

Recently a method for manufacturing of Roebel cable was disclosed which comprises a copper plating performed after the forming of the meander shape. This allowed to improve Roebel cable stability determined a as a number of heating /cooling cycling between 77 and 300 K. Nevertheless this does not provide a stability at liquid Helium temperature (4.2 K), especially at superfluid liquid Helium (<4 K) by where a "ballooning" of the tapes causes typically a deterioration after several cooling-heating cycles.

Accordingly, the present invention seeks provide a method which overcomes some of the problems caused by actual manufacturing technology for such transposed cables, as well as providing a cable formed by such a method.

The present invention provides a method for producing HTS tape and a cable formed from the produced tape as described in the accompanying claims.

A tape and corresponding cable produced in accordance with this method has a longer usable life and is capable of operating even after a large number of cooling and heating cycles with none of the bubbling effects as seen in prior art arrangements.

An example of the present invention will now be described with reference to the accompanying drawings, in which:

Fig. 1 is a schematic cross-sectional view of the HTS tape ready for assembling of the Roebel cable; Fig. 2 is a view of a HTS tape processed in accordance with suggested method;

Fig. 3 is a schematic view of a layout of the vacuum deposition installation for the sheath deposition; Fig. 4 is a schematic view of HTS tape torsion by installation in by the sheath deposition, an outer surface of the HTS tape is turned around after several windings towards the position of its "inner" surface; Fig. 5 is a schematic view of HTS tape torsion by winding on a drum for the sheath deposition;

Fig. 6 is a schematic view of tape stack employed in the sheath deposition;

Fig. 7 is a schematic view of reel-to-reel tape transport in "stack" modus with synchronizing of tape windings for the sheath deposition;

Fig. 8 is a schematic cross-sectional view of the HTS tape with two side deposited sheath layer ready for assembling of the Roebel cable, and;

Fig. 9 is a schematic cross-sectional view of the HTS tape with two side deposited sheath layer ready for assembling of the Roebel cable, and also schematically illustrates de-burring and forced smoothening at the HTS tape before the sheath deposition.

Referring to Figure 1 , the objective of the method of the present invention is to produce a time-stable cable formed from tape of the type shown in this figure. The cable has groups of tape 2 which are formed from metallised HTS material, the tapes 2 being of generally rectangular or square cross-section. In general terms the objective is to provide a stable cable that operates in a high current regime in the range of around 10KA to around 50KA and yet which has a sufficiently low value of AC loss for the application to which it is being applied. Such a cable is normally formed by producing the tapes, metalising them, forming a meander in the tapes, and then assembling them through weaving or other process generally in accordance with the teaching of prior art methods such as those disclosed in the citations above. As mentioned above however, what the applicants have realised is that such a process can leave the end product, the cable, prone to deterioration over time. The reason of this deterioration is not known in fact.

There are many various factors that may contribute to this effect such as (i) chemical diffusion of moisture and acids into "superconducting core" which is comprises a very sensitive to foreign elements HTS layer, (ii) "physical" penetration of liquid helium, especially super-fluid helium, that in warming up phase causes ballooning due to rapid evaporation in semi-closed environment, (iii) mechanical defects (as cracks or punctures for example) that develop in the course of cooling/heating cycling; they may finally lead to an effective reduction of cross-section of HTS layer and reduce performance. This is particularly the case in terms of the forming of the meander in the tapes 2 to enable weaving and the appropriate end product configuration. To overcome these issues the applicants have developed the processes described in detail below. In the processes of the present invention tapes are initially formed and provided with a minimal layer of metallisation coating on the core HTS material. In this process there is a first stage of metallising the tapes of HTS material in a manner similar to that in the prior art, but with a thickness of metallisation which is much reduced when compared to the prior art. In the present invention the metallisation is generally provided with a thickness in the range of 0.05 to 2μιη, dependent upon the material being used in the metallisation process. For example, using Pt may have a thickness of around 0.05μιη, Au 0.15μιη, Ag 0.2μιη and Cu up to 2μιη. Once the metallisation has been performed the tapes are punched to form the appropriate meanders as in the prior art processes. After this a deposition of a metallic sheath exhibiting a tight, voidless bonding to the tape surface is performed. The deposition of the metallic sheath has a basic difference with simple deposition of a metallic layer: the metallic sheath should not only eliminated voids that may be formed on the interface punched (and therefore, not smooth, and even sometimes not clean) edges but also the sheath should be free of voids in its interior. Furthermore, the sheath is assumed to have a good adhesion to all surfaces of the tape covered with it. In addition, it should provide excellent current exchange between all surface elements coated with this sheath. In this sense, the aim of the sheath is to provide a full tape protection in all chemical, electrical and mechanical aspects. The metallic sheath may cover possibly only a part of tape surface. It is clear therefore that the metallic sheath has to be deposited onto several surfaces of the tape, with preferential deposition to the punched sides and edges. Because of such multifunctional requirements the metallic sheath may have a multi-layered architecture in which for example one layer is responsible for adhesion, other layer provides tightness and some further layer yields electrical conductivity. Furthermore, the metallic sheath may comprise a mechanical impregnation that improves capability of the sheath to withstand cooling/heating cycling. Such impregnation may be based on specific sublayers with different thermal expansion coefficient or even discontinuous layer after deposition of further sheath sub-layers cause a formation of embedded islands of metal or even dielectric (e.g. Si0 ₂ , Cr ₂ 03) which are distributed within a "matrix" formed by other sheath sublayers.

In the metallic sheath, the layer or layers are in the region of 0.5 to 50μιη thickness, again dependent upon the metallisation material. In this deposition of the metallic sheath, the material may be the same metal as used in the prior metallisation step, although there may be a different metal applied or indeed, in the metallic sheath deposition may be different layers of different metals applied in sequence in sub-steps to form the overall metallisation. The resulting tapes are then annealed using a neutral annealing process that may be in an oxygen rich atmosphere, after which the tapes are assembled into the end cable.

In the metallisation process it will be appreciated that there may be steps, particularly in the second stage of metallisation, in which heat treatment is performed at certain thicknesses in between sub-steps, for example in the second metallisation step at a thickness of 3 to 5μιη before there is further metallisation to improve metallisation and reduce deterioration of the resulting cable.

It will also be appreciated that deburring may be performed before the deposition of the metallic sheath to improve the consistency of metallisation, that the deposition of the metallic sheath can be performed in a way that it enables coverage of the entire surface of each tape, including rear and edge surfaces to optimise coating, and may be performed such there is preferential deposition onto edge areas of the tape. During the processing of the tapes metallisation may be performed on plural tape elements that are of similar structure at the same time during the metallisation stage to improve efficiency of the manufacturing process. With such an approach the tape elements may be superimposed to provide a stacking structure and with spacing by interleaving tape.

It will be appreciated that it is preferable not to have a significant time interval between any deburring step that happens following primary metallisation and any second metallisation step that should provide stability of the entire metallic sheath and in this way to avoid deterioration of the tapes. The applicants have determined that the time period should not be greater than five hours to optimise deterioration prevention. It was experimentally shown that with such means applied in given sequence it is possible to avoid deterioration of the superconducting properties of the HTS coated tape in Roebel cable. The deterioration appears sometimes not in the beginning of use Roebel cable but after longer time of cooling in liquid helium (>100 hours) or after multiple cycles of cooling/heating of the Roebel cable. In the course of our experiments on present invention we found out that the weakest "element" in the Roebel cable is a side surface of the meander shaped HTS coated tape. We assume that two factors may be important with this respect: (i) mechanical load applied to the side surface of the HTS coated tape during cooling/heating cycles and (ii) a mechanical friction of different HTS coated tapes during cooling/heating cycles and cable bending. On the other hand this side area of the HTS coated tape represents possibly a "gate" for penetration of foreign materials into an "interior" of the HTS coated tape. This may lead to a local "ballooning" (delamination) effect that cause the deterioration after long exposition of Helium coolant. The deterioration results in lowering (by 5-60%) of critical current in the HTS coated tape, and finally in the entire Roebel cable. Finally, we found out that present inventive method allows to suppress completely such deterioration effect. Stabilization of the inventive method described above may be achieved via the following processing steps and conditions:

The minimal thickness of the metallization may vary from 0.05 to 3 μιη. It depends on applied metal and used deposition technique. The sheath deposition is performed by means of physical vacuum deposition. It is performed in a way which besides a preferential coverage of the area that comprises the side area and the edge area, a partial coverage of the entire surface of the tape including a rear surface of the HTS tape is provided. To improve stability (adhesion and homogeneity) of the metallic sheath an ion cleaning of the surface to be coated is performed prior to the main step of metallic sheath deposition. Alternatively, the ion assisted deposition where the ion treatment is performed during deposition may be also employed using ions with 1 -150 eV energy. A thickness of the sheath metallic layer may vary from 0.5 to 4 μιη. The sheath metallic layer comprises silver, gold or/and platinum or an alloy based on at least some of these metals. The sheath metallic layer may comprises a first sublayer and a second sublayer which comprise a metallic components or alloys of different art. For example the first sublayer may be based on very thin, say 0.05 μιη thick, silver-gold alloy that provides good adhesion, mechanical stability and "transparency" for oxygen diffusion but still not sufficient for the electrical or chemical protection of HTS material. Therefore the second based on e.g. silver or different silver alloys, 1-2 μιη thick, may be applied as the second sublayer. Furthermore, in accordance with the inventive method the heat treatment is performed in an oxygen atmosphere may be performed while an additional heat treatment in oxygen atmosphere may be performed after the metallization.

The sheath deposition is performed simultaneously onto a multitude of different tape areas corresponding to a different meander periods of the meander shape of the HTS tape. The different meander periods are shifted one against other in order to provide a simultaneous exposition of the side area and the edge area of the HTS tape to the incoming material. In the multitude of different tape areas the different meander periods are superimposed in a way that the HTS tape forms a stack in a deposition area where the sheath deposition is performed. HTS tape in the stack is spaced by an interleaf tape. Moreover, a deburring is performed with in interval which does not exceed 15 hours prior to the sheath deposition. This allows to gain a tape throughput and thus to suppress contaminations (introduced during deposition) of the sheath metallic layer and, as a result, to improve temperature/cycling stability of the Roebel cable. A "side positive effect" in this case is a saving of the precious metal because of higher filling factor of the deposition zone during the physical vacuum deposition. Figure 9 is a schematic view of de-burring and forced smoothening at the HTS tape before the sheath deposition. It may comprise the steps of smoothening via polishing, cleaning, blowing and/or etching, surface activation (ion etching), pre- metallization and then plating, e.g. with copper. We will now describe an embodiment based on sheath deposition of a silver sheath layer.

The method comprises of a number of processing steps listed below. A polished Cr-Ni stainless steel substrate tape is coated with yttria-stabilized zirconia (YSZ) buffer layer which is bi-axially textured. The bi-axial texture is provided by alternating ion assisted deposition (ABAD) which enables a high (<10° of full- width at half maximum) in-plane texture. The buffered substrate tape serves as a template for deposition of a further buffer cap layer of cerium oxide (CeO ₂ ) and finally a high temperature superconducting layer of YBa ₂ Cu30 ₇ - _x (YBCO). Both of these layers are deposited via pulsed laser deposition in one vacuum deposition run where the buffered substrate is heated to 750°C. During the PLD process a variation of target composition results in variation of deposited layer composition. In this way a layer of Ce0 ₂ and a layer of YBCO are deposited. Typical thickness of YSZ, CeO ₂ and YBCO layers corresponds to 2.0, 0.060 and 1.5 μιη respectively. After deposition of the layer system the HTS tape undergoes a metallization. Material used for metallization in this example is silver which was deposited via physical vacuum deposition based on thermal vacuum evaporation. A minimal thickness of the metallization (i.e. the thickness of the silver layer) of 0.5 μιη was employed in the tape structure. This thickness is quite sufficient to enable a structural uniformity the metallic layer over the tape surface, and thus to achieve a chemical and mechanical protection which is needed for the tape during the next step of forming of the meander shape and following quality control of electrical parameters.

Nevertheless we have shown in different tests that chemical protection may be achieved with much thinner, with a thickness of 0.05 μιη, metallic layer, which by room temperature physical vapour deposition forms a structural, uniform and continuous layer without interruptions and minimal (<1 volume %) voids. Such thin metallic layers are also sufficient for electrical stabilizing of the HTS tape during measurements of critical current because the electrical conductance of this layer allows to suppress hot spots which form in the superconductor before quench and may cause local overheating and burning out of the HTS layer. Mechanical protection is less available by such thin metallic layers. Thus a more accurate operation that includes soft/elastic materials employed for fixing the HTS tape during the forming of the meander shape that may be performed via punching, water jet cutting, electrical erosion, cutting of the HTS tape by a laser beam or, furthermore, via chemical / electrochemical /ion etching. Some of the later methods in this list are more expensive but more secure against possible mechanical damage. These alternative ways of forming of the meander shape may bring even a cost efficient technological step because the costs of the HTS tape itself plays a dominating role in overall cost.

During the metallization step, it is less important how far the edge and side areas as well as a rear surface of the HTS tape are coated with the metallic layer. The main requirement here is to provide a layer uniformly above the HTS layer.

The forming of the meander shape was performed in the present example by mechanical punching (see Fig. 2).

After the forming of the meander shape, the HTS tape undergoes a sheath deposition of a sheath metallic layer 5, which was a silver layer in given example. Prior to the deposition of silver the surface of the tape was cleaned by fine ion etching in the vacuum chamber. The cleaning was performed by negatively charged oxygen ions and molecules with characteristic energy of 0.5- 30 eV at oxygen pressure of 0.1 mbar under ion density of 1 mA/cm ² for 4 min. This step of cleaning/etching determines adhesion and uniformity degree of the sheath layer and thus represents an important factor in improving the long-term stability of the Roebel cable.

The sheath deposition was performed in the same chamber via a tangential flow of an incoming material in vacuum of 10 ^"5 mbar. Fig. 3 reveals one of possible layouts of the vacuum deposition technique. The HTS tape windings 10 which are shown in a cross-sectional view move in perpendicular to the plane of the figure. They move together or independently along a base 1 1. The base may be a wall of a drum on which the HTS tape is wound, with the rotation axis of the drum positioned above the HTS tape windings 10. A deposition source 12, 14 is represented by a boats 12 that are heated resistively, and material (silver in this case) 14 aimed for evaporation. The geometry of the installation enables a shorter distance between the deposition source and the base plane 11 than the distance between the deposition source and the nearest tape winding. Due to such geometry tangential flow 15 of the incoming material (silver vapour) is provided for both deposition sources. In reality the number of deposition sources 12, 14 (i.e. sources of tangential flows 15) may be higher than 2. Using the tangential flow deposition of the sheath deposition is performed onto a side area 16 and an edge area 17 of the HTS tape. Sometimes an additional masking technique that reduces deposition onto the rest of the open surface of the tape 10 may be used. In order to perform a symmetric sheath deposition, the tape, after several windings, should be turned around its longer axis by 180°. This is schematically shown in Fig. 4. Otherwise the sheath deposition has to be repeated one time more after turning around of a complete tape.

An example of tape torsion is clarified in Fig. 5 where the tape 10a is turned around (during winding on drum 11 , 19) to an orientation 10b that enables the sheath deposition onto a second (rear) surface of the HTS tape. In a further example twisting tape during reel-to-reel winding process is possible. In this example, the sheath deposition was performed about 30 hours after the forming of the meander shape, i.e. after punching in this case. After the deposition of the sheath silver layer the HTS tape was heated to 450°C for 1 hour in oxygen atmosphere with an ambient pressure. Cooling of the HTS tape was performed within 2 hours with a rate of 2°C/min. The following deposition of a shunt layer was performed by employing galvanic plating in a water solution of CuS0 ₄ . The HTS tape was translated from reel-to-reel with a speed of 1.5 m/hour. This regime resulted in a shunt layer thickness of 20 μιη per each side. After plating the tape was washed in water, after which the droplets of water were removed by blowing with dry gaseous nitrogen.

Assembling of the Roebel cable represents the last step in manufacture, This procedure is handled so as not to introduce additional mechanical load to the edge and side areas of the tape during cooling of the Roebel cable to the operation temperature of 4.2 K.

HTS tapes fabricated in accordance with given example exhibit outstanding mechanical and thermo-cycling stability. The tape withstands (without any signs of deterioration) more than 100 cycles of cooling to 4.2 K and heating to 300 K (instead of 3-5 cycles in the known state of the art). Similar stable behaviour was found for a cycled bending of the HTS tape between radii of 300 and 20 mm. The same was found for the Roebel cable assembled from the HTS tape. Thus an improvement of mechanical stability of the cable was also confirmed via repeatable bending and temperature cycling tests.

In some cases a sheath deposition of the sheath layer comprises two or more sublayers. In this example, we consider a sheath deposition which comprises two depositions of two sheath sublayers. A first sub layer deposition is aimed to deposit a gold-silver alloy with a 5% content of gold. This deposition is performed via flash thermal vacuum evaporation of mixed grains of silver and gold. Fig. 8 is a schematic cross-sectional view of the HTS tape with two side deposited sheath layer ready for assembling of the Roebel cable. Fig. 8 shows a substrate 80, multilayer with HTS layer shown generally at 81 , and an Ag envelope 82. The HTS tape has preferably been de-burred.

As will be appreciated from the above, the method of the present invention improves significantly the usable lifetime of cables produced by the method without incurring significantly large increases in the number of manufacturing steps or cost.