Sonja I. Schlachter

Roebel cables from REBCO coated conductors: a one-century-old concept for the superconductivity of the future

Energy applications employing high-temperature superconductors (HTS), such as motors/generators, transformers, transmission lines and fault current limiters, are usually operated in the alternate current (AC) regime. In order to be efficient, the HTS devices need to have a sufficiently low value of AC loss, in addition to the necessary current-carrying capacity. Most applications are operated with currents beyond the current capacity of single conductors and consequently require cabled conductor solutions with much higher current carrying capacity, from a few kA to up to 20-30 kA for large hydro-generators. A century ago, in 1914, Ludwig Roebel invented a low-loss cable design for copper cables, which was successively named after him. The main idea behind Roebel cables is to separate the current in different strands and to provide a full transposition of the strands along the cable direction. Nowadays, these cables are commonly used in the stator of large generators. Based on the same design concept of their conventional material counterparts, HTS Roebel cables from REBCO coated conductors were first manufactured at the Karlsruhe Institute of Technology (KIT) and have been successively developed in a number of varieties that provide all the required technical features such as fully transposed strands, high transport currents and low AC losses, yet retaining enough flexibility for a specific cable design. In the past few years a large number of scientific papers have been published on the concept, manufacturing and characterization of such cables. Times are therefore mature for a review of those results. The goal is to provide an overview and a succinct and easy-to-consult guide for users, developers, and manufacturers of this kind of HTS cables.

Giant pressure effect in oxygen deficient YBa2Cu3Ox

Abstract The pressure effect on T c of polycrystalline and single crystalline YBa 2 Cu 3 O x investigated as a function of oxygen content x by ac-susceptibility measurements under helium pressure. In the overdoped region x > 6.93 the single crystals show a negative d T c /d p , as expected from the charge transfer model. For optimally doped samples with x = 6.93 we find d T c /d p = 0.4 K/GPa which points to pressure effects on T c aside from charge transfer. In the underdoped region x T c /d p values obtained from the experiment depend strongly on the storage temperature of the sample during the experiment. When the samples are stored at temperatures well below 240 K throughout the entire experiment including pressure application and pressure release, d T c /d p increases to approx. 7 K/GPa at x = 6.7 but with a further decrease of the oxygen content the d T c /d p drops to approx. 2 K/GPa at x = 6.4. These effects are intrinsic to the YBa 2 Cu 3 O x structure and can be explained by considering the anisotropic structure of YBa 2 Cu 3 O x . The decrease of the c -axis lattice parameter results in a charge transfer to the CuO 2 -planes mainly [1], whereas the compression of the a - and b -axis lattice parameter is known to produce different pressure effects which are responsible for the peak in d T c /d p at x = 6.7 [2]. When pressure is changed at room temperature oxygen ordering effects occur which cause a relaxation of T c to the equilibrium value T c ( p ) at this pressure with a time constant depending on the oxygen content x . A decrease x results in a peak effect in d T c /d p at x = 6.7 again, which is enhanced to approx. 12 K/GPa. If the oxygen content is decreased further, d T c /d p first drops to 5 K/GPa at x = 6.6, but the increases to values of more than 20 K/GPa for x T c increase (d T c /d p ) O due to pressure induced oxygen ordering via oxygen motion between unit cells.

Coated Conductor Rutherford Cables (CCRC) for High-Current Applications: Concept and Properties

In the past few years the Roebel technique has been established as a method for cabling of coated conductor tapes to achieve high current carrying capabilities for low ac-loss applications. We have successfully developed Roebel cables consisting of up to 50 tapes and with current carrying capabilities up to 2.6 kA (77 K, self-field). However, for applications like busbars or fusion magnets current carrying capabilities of more than 10 kA are required. Such high current carrying capabilities cannot be reached by a simple scale-up with additional tapes. We present a concept for Coated Conductor Rutherford Cables (CCRCs) for currents exceeding 10 kA using Roebel subcables as strands. The first steps towards a short subsize CCRC demonstrator cable with electrical and mechanical characterization of commercial coated conductor tapes, strands and a first Roebel strand have been performed and will be discussed.

Investigation of a Rutherford cable using coated conductor Roebel cables as strands

Coated conductor applications such as fusion magnets, particle accelerator magnets and generator windings require high current-carrying capabilities. This requirement can be fulfilled by various cable concepts using commercial long length REBCO coated conductors with high current-carrying performance. In the past few years, our group has successfully developed the Roebel cable concept for coated conductors. The design advantages of such a cable are high current-carrying capability and low alternating current (AC) losses. Unfortunately, for large-scale applications, the possibilities of a simple scale-up of the Roebel geometry are limited and additional design ideas are needed. One way to reach the required high currents is the Rutherford cable concept. In this concept a conductor is wound with transposition on a flat metal former. In order to design the former, the bending properties of the Roebel assembled coated conductor cables (RACC) must be measured and characterized. This allows the identification of a destruction-free interval for the Roebel cable, in terms of bending angle and transposition length. In this work we designed and assembled a demonstrator of a coated conductor Rutherford cable (CCRC) with three RACC cables. We measured the critical current and the AC losses of the cable demonstrator. Our results show that, despite still needing efforts in terms of reproducibility of the assembly process and of AC loss reduction, this design is a promising and viable solution for high current-capacity cables made of coated conductors.

Overview of Progress on the EU DEMO Reactor Magnet System Design

The DEMO reactor is expected to be the first application of fusion for electricity generation in the near future. To this aim, conceptual design activities are progressing in Europe (EU) under the lead of the EUROfusion Consortium in order to drive on the development of the major tokamak systems. In 2014, the activities carried out by the magnet system project team were focused on the toroidal field (TF) magnet system design and demonstrated major achievements in terms of concept proposals and of consolidated evaluations against design criteria. Several magnet system R&D activities were conducted in parallel, together with broad investigations on high temperature superconductor (HTS) technologies. In this paper, we present the outcomes of the work conducted in two areas in the 2014 magnet work program: 1) the EU inductive reactor (called DEMO1) 2014 configuration (power plant operating under inductive regime) was the basis of conceptual design activities, including further optimizations; and 2) the HTS R&D activities building upon the consolidated knowledge acquired over the past years.

Reduction of coupling current losses by internal resistive barrier structures in multifilamentary Bi(2223) tapes in external magnetic fields

The AC losses in multifilamentary Bi(2223) tapes in external magnetic fields are dominated by the hysteresis losses in the superconducting filaments and by coupling current losses in the normal conducting matrix. In the last years great effort has been made in the development of low loss barrier tapes with internal interfilamentary resistive barriers, which reduce the coupling currents. The recent improvements of these conductors are presented. 37-filamentary tapes with a reduced barrier thickness, enhanced superconductor content as well as overall critical current densities were developed. Different barrier materials were used to study the effect of different barrier hardnesses on the deformation, barrier homogeneity, matrix resistivity and critical current density. The tapes were twisted with a twist length down to lp ¼ 5 mm. The parameters of filament twist and the AC losses of the tapes were investigated and are discussed in comparison to former conductors to find an optimum conductor design. 2002 Elsevier Science B.V. All rights reserved.

Influence of Ni and Cu contamination on the superconducting properties of MgB2 filaments

Technical MgB2 wires usually have a sheath composite consisting of different metals. For the inner sheath with direct contact to the superconducting filament, chemically inert Nb may be used as a reaction barrier and thermal stabilization is provided by a highly conductive metal like Cu. A mechanical reinforcement can be achieved by the addition of stainless steel. In order to illuminate the influence of defects in the reaction barrier, monofilament in situ wires with direct contact between the MgB2 filament and frequently applied reactive sheath metals like Cu, Ni or Monel are studied. Reactions of Mg and B with a Cu-containing sheath lead to Cu-based by-products penetrating the whole filament. Reactions with Ni-containing sheaths lead to Ni-based by-products which tend to remain at the filament–sheath interface. Cu and/or Ni contamination of the filament lowers the MgB2-forming temperature due to the eutectic reaction between Mg, Ni and Cu. Thus, for the samples heat-treated at low temperatures JC and (partly) TC are increased compared to stainless-steel-sheathed wires. At high heat treatment temperatures uncontaminated filaments lead to the highest JC values. From the point of view of broken reaction barriers in real wires, the contamination of the filament with Cu and/or Ni does not necessarily constrain the superconductivity; it may even improve the properties of the wire, depending on the desired application.

Bending strain investigations on BSCCO(2223) tapes at 77 K applying a new bending technique

A new bending device was developed which allows continuous change of the bending radius of the BSCCO tape sample at 77 K and a simultaneous measurement of the critical currents. The samples are mounted free between two clamps. The special geometry of the arrangement insures over the whole range a sample shape with homogeneous curvature on a circle line without the help of mechanical parts. This bending strain rig avoids effects from additional thermal stresses due to the cooling from room temperature (bending) to 77 K (critical current measurement). We mainly present measurements on different Ag/AgMg sheathed standard conductors. The results obtained with the new method will be compared for some selected samples with the bending strain behavior measured in the conventional way, applying the bent at RT. The potential of the method being suitable for standardized bending experiments will be discussed.

The effect of chemical doping and hydrostatic pressure on Tc of Y1−yCayBa2Cu3Ox single crystals

We performed susceptibility measurements on Y1-yCayBa2Cu3Ox single crystals under high He pressure. For each Ca content various samples with different oxygen contents have been prepared to probe the influence of Ca on Tc(x), dTc/dp(x) and Tc,max. Starting from the parabolic Tc(nh) behavior we calculated nh values from Tc and Tc,max for each sample. It is shown that in the overdoped region dTc/dp can be described by a pressure induced charge transfer with dnh/dp ≈ 3.7⋅10 -3 GPa -1 and a dTc,max/dp value of 0.8 K/GPa, irrespective of the Ca content. In the underdoped region additional pressure effects lead to a peak in dTc/dp at ≈0.11 holes/CuO2 plane. However, with increasing Ca content this peak is strongly depressed. This is explained in terms of an increasing disorder in the CuO chain system due to doping. Deviations in dTc/dp at very low nh values can be assigned to the ortho II ordering in the CuO chain system.

HTS CroCo: A Stacked HTS Conductor Optimized for High Currents and Long-Length Production

In order to build high-current cables from second-generation high-temperature superconductor (HTS) rare-earth barium-copper-oxide (REBCO) tapes, numerous approaches were studied, such as conductor on round core, Roebel cable, and several versions of twisted stacked-tape cable types. Based on the work of Takayasu et al. and Uglietti et al., we developed and tested a modified type of stacked HTS tape arrangement optimized for high engineering critical current density. Key aspects were the implementation of a simple and reliable continuous fabrication routine for production of application-relevant lengths of twisted conductors in the range of 100 m and above and the subsequent enveloping by a seamless copper tube. Several samples of this HTS Cross-Conductor (HTS CroCo) were prepared successfully, both partially and fully equipped with REBCO tapes in untwisted and twisted configurations. The critical current of the samples was measured at T = 77 K and self-field conditions. The measurements showed the expected critical currents calculated from the individual tape values if taking into account the enhanced self-field in the tape arrangement. Furthermore, one untwisted partially REBCO-equipped sample was tested in the FBI facility at KIT at T = 4.2 K and in magnetic fields up to B = 12 T, showing good performance with no degradation even at high Lorentz forces. In addition, the mechanical performance of this sample was studied under tensile loads. No degradation could be observed, and the strain dependence was equal to that of single REBCO tapes. Due to the combination of excellent mechanical and electrical performance, the HTS CroCo is a promising candidate as a strand for long-length high-current cables, for example, with Ic(4.2 K, self-field) ≈ 30 kA for power transmission or with Ic(4.5 K, 14 T) ≈ 8 kA as a strand for high-current cables targeting large high-field magnets.