Anna Kario

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.

Accelerator-Quality HTS Dipole Magnet Demonstrator Designs for the EuCARD-2 5-T 40-mm Clear Aperture Magnet

Future high-energy accelerators will need very high magnetic fields in the range of 20 T. The Enhanced European Coordination for Accelerator Research and Development (EuCARD-2) Work Package 10 is a collaborative push to take high-temperature superconductor (HTS) materials into an accelerator-quality demonstrator magnet. The demonstrator will produce 5 T stand alone and between 17 and 20 T when inserted into the 100-mm aperture of a Fresca-2 high-field outsert magnet. The HTS magnet will demonstrate the field strength and the field quality that can be achieved. An effective quench detection and protection system will have to be developed to operate with the HTS superconducting materials. This paper presents a ReBCO magnet design using a multistrand Roebel cable that develops a stand-alone field of 5 T in a 40-mm clear aperture and discusses the challenges associated with a good field quality using this type of material. A selection of magnet designs is presented as the result of the first phase of development.

The EuCARD-2 Future Magnets European Collaboration for Accelerator-Quality HTS Magnets

EuCARD-2 is a project supported by FP7-European Commission that includes, inter alia, a work-package (WP10) called “Future Magnets.” This project is part of the long term development that CERN is launching to explore magnet technology at 16 T to 20 T dipole operating field, within the scope of a study on Future Circular Colliders. The EuCARD2 collaboration is closely liaising with similar programs for high field accelerator magnets in the USA and Japan. The main focus of EuCARD2 WP10 is the development of a 10 kA-class superconducting, high current density cable suitable for accelerator magnets, The cable will be used to wind a stand-alone magnet 500 mm long and with an aperture of 40 mm. This magnet should yield 5 T, when stand-alone, and will enable to reach a 15 to 18 T dipole field by placing it in a large bore background dipole of 12-15 T. REBCO based Roebel cables is the baseline. Various magnet configurations with HTS tapes are under investigation and also use of Bi-2212 round wire based cables is considered. The paper presents the structure of the collaboration and describes the main choices made in the first year of the program, which has a breadth of five to six years of which four are covered by the FP7 frame.

Critical current density enhancement in strongly reactive ex?situ? MgB2 bulk and tapes prepared by high energy milling

The ex?situ technique is promising for its ease of preparation, low material shrinkage during annealing and the fact that it is already superconducting without annealing. However the critical current density (Jc) needs to be enhanced. To improve the Jc values of commercial powder high energy milling was used?a method commonly use in industry and easy to apply. It was found that high energy milling is successful in improving Jc in bulk and tapes. The reason for the improvement is reduced crystallite size and the large fraction of fresh grain surfaces. In this paper we also experimentally investigate the problem of decomposition of MgB2?ex?situ during heat treatment.

Self-Field Effects and AC Losses in Pancake Coils Assembled From Coated Conductor Roebel Cables

In this contribution, we develop a refined numerical model of pancake coils assembled from a coated conductor Roebel cable, which includes the angular dependence of the critical current density Jc on the magnetic field and the actual (three-dimensional) shape of the current lead used to inject the current. Previous works of ours indicate that this latter has an important influence on the measured value of the AC losses. For the simulation of the superconductor, we used two alternative models based on different descriptions of the superconductors properties and implemented in different mathematical schemes. For the simulation of the current lead we use a full three-dimensional finite-element model. The results of the simulation are compared with measurements and the main issues related to the modeling and the measurement of Roebel coils are discussed in detail.

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.

A self-consistent model for estimating the critical current of superconducting devices

Nowadays, there is growing interest in using superconducting wires or tapes for the design and manufacture of devices such as cables, coils, rotating machinery, transformers and fault current limiters among others. Their high current capacity has made them the candidates of choice for manufacturing compact and light cables and coils that can be used in the large scale power applications described above. However, the performance of these cables and coils is limited by their critical current, which is determined by several factors, including the conductors material properties and the geometric layout of the device itself. In this work we present a self-consistent model for estimating the critical current of superconducting devices. This is of large importance when the operating conditions are such that the self-field produced by the current is comparable to the overall background field. The model is based on the asymptotic limit when time approaches infinity of Faradays equation written in terms of the magnetic vector potential. It uses a continuous E-J relationship and takes the angular dependence of the critical current density on the magnetic flux density into account. The proposed model is used to estimate the critical current of superconducting devices such as cables, coils, and coils made of transposed cables with very high accuracy. The high computing speed of this model makes it an ideal candidate for design optimization.

Influence of laser striations on the properties of coated conductors

Due to their high current carrying capability, coated conductors are regarded as the most promising high-temperature superconductor tapes for power applications. However, their high aspect ratio causes too high magnetization losses. To reduce the ac loss, one way is to striate the wide tapes into filaments. We used a picosecond laser for the structuring of (RE)BCO coated conductors. The laser allows to burn 18 μm to 21 μm wide grooves (Ag-cap) with a depth between 0.5 μm to more than 100 μm into the coated conductors, with negligible heat effects at the edges of the structures. Different numbers of filament were structured in Cu-and Ag-cap coated conductors. Patterns with up to 120 parallel filaments in 12 mm wide conductor were made. The critical current and the total ac-magnetization loss were measured as a function of the filament count. With an increasing number of filaments Ic degradation occurs. This current reduction has two contributions, the removed HTS material and current inhomogeneities within the superconductor for instance defects along the tape causing secondary phases. For 120 filaments Ag-cap tapes the hysteresis loss reduction is about two orders of magnitude, as expected. The observation of some remaining filament coupling was investigated.

AC Losses of Pancake Coils Made of Roebel Cable

Roebel cables are a promising solution for high-current, low ac loss conductors for various applications, including magnets, rotating machines, and transformers, which generally require the cable to be wound in a coil. We recently assembled and characterized a 5 m long sample and wound it into pancake coils. In this contribution, we investigate the ac loss behavior of such pancake coils by means of numerical simulations based on two complementary models: the finite-element model based on the H-formulation and the minimum magnetic energy variation method based on the critical state. These two numerical models take into account the axis-symmetric geometry of the coil and its detailed structure, simulating each strand composing the cable. The local current density and magnetic field distributions are shown and the ac losses for various current amplitudes are computed. The influence of the number of turns and of their separation on the ac losses is investigated. The results of the computations are compared with the measurements and the main reasons for the observed discrepancy are discussed.