This manuscript reports on the recent progress and the remaining materials challenges in the development of coated conductors (CCs) for power applications and magnets, with a particular emphasis on the different initiatives being active at present in Europe. We first summarize the scientific and technological scope where CCs have been raised as a complex technology product and then we show that there exists still much room for performance improvement. The objectives and CC architectures being explored in the scope of the European project EUROTAPES are widely described and their potential in generating novel breakthroughs emphasized. The overall goal of this project is to create synergy among academic and industrial partners to go well beyond the state of the art in several scientific issues related to CCs’ enhanced performances and to develop nanoengineered CCs with reduced costs, using high throughput manufacturing processes which incorporate quality control tools and so lead to higher yields. Three general application targets are considered which will require different conductor architectures and performances and so the strategy is to combine vacuum and chemical solution deposition approaches to achieve the targeted goals. A few examples of such approaches are described related to defining new conductor architectures and shapes, as well as vortex pinning enhancement through novel paths towards nanostructure generation. Particular emphasis is made on solution chemistry approaches. We also describe the efforts being made in transforming the CCs into assembled conductors and cables which achieve appealing mechanical and electromagnetic performances for power systems. Finally, we briefly mention some outstanding superconducting power application projects being active at present, in Europe and worldwide, to exemplify the strong advances in reaching the demands to integrate them in a new electrical engineering paradigm.

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Coated conductors for power applications: materials challenges

Cyanocomplexes with one-dimensional structures: preparations, crystal structures and magnetic properties

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.

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

An efficient two dimensional T-A formulation based approach is proposed to calculate the electromagnetic characteristics of tape stacks and coils made of second generation high temperature superconductors. In the approach, a thin strip approximation of the superconductor is used in which the superconducting layer is modeled as a 1-dimensional domain. The formulation is mainly based on the calculation of the current vector potential T in the superconductor layer and the calculation of the magnetic vector potential A in the whole space, which are coupled together in the model. Compared with previous T-based models, the proposed model is innovative in terms of magnetic vector potential A solving, which is achieved by using the differential method, instead of the integral method. To validate the T-A formulation model, it is used to simulate racetrack coils made of second generation high temperature superconducting (2G HTS) tape, and the results are compared with the experimentally obtained data on the AC loss. The results show that the T-A formulation is accurate and efficient in calculating 2G HTS coils, including magnetic field distribution, current density distribution, and AC loss. Finally, the proposed model is used for simulating a 2000 turn coil to demonstrate its effectiveness and efficiency in simulating large-scale 2G HTS coils.

A finite element model for simulating second generation high temperature superconducting coils/stacks with large number of turns

A series of coordination polymers containing Cu(II) and [Au(CN)2]- units has been prepared. Most of their structures incorporate attractive gold−gold interactions, thus illustrating that such “aurophilic” interactions can be powerful tools for increasing structural dimensionality in supramolecular systems. [Cu(tren)Au(CN)2][Au(CN)2] (1, tren = tris(2-ethylamino)amine) forms a cation/anion pair, which is weakly linked by hydrogen bonds but not by aurophilic interactions. [Cu(en)2Au(CN)2][Au(CN)2] (2-Au, en = ethylenediamine) is a 2-D system composed of a chain of [Au(CN)2]- anions and another chain of [(en)2Cu−NCAuCN]+ cations; short Au−Au bonds of 3.1405(2) A connect the anions. This bond is shorter than that observed in the analogous silver(I) structure, 2-Ag. The average M−C bond lengths of 1.984(8) A in 2-Au are significantly shorter than those found in 2-Ag, suggesting that Au(I) is smaller than Ag(I). Cu(dien)[Au(CN)2]2 (3, dien = diethylenetriamine) forms a 1-D chain of tetranuclear [Au(CN)2]- units...

Gold−Gold Interactions as Crystal Engineering Design Elements in Heterobimetallic Coordination Polymers

Many applications of ReBCO coated conductors contain stacks of pancake coils. In order to reduce their high AC loss, it is necessary to understand the loss mechanisms. In this article, we measure and simulate the AC loss and the critical current, I_c, in stacks of pancake coils ("pancakes"). We construct stacks of up to 4 pancakes and we measure them by electrical means. We also obtain the anisotropic field dependence of J_c from I_c measurements of the tape. This J_c is the only input to the simulations, together with the coil dimensions. After validating our computations with the measurements, we simulate stacks of many pancakes, up to 32. We found that the AC loss in a stack of (four) pancakes is very high, two orders of magnitude larger than for a single tape. A double pancake behaves as a single one with double width but a stack of more pancakes is very different. Finally, we found that a 2-strand Roebel cable reduces the AC loss in a stack of pancakes but not in a single pancake. In conclusion, the AC loss in stacks of pancakes is too high. However, our simulations are useful to predict the AC loss and optimise the coil design, reducing the AC loss.

AC loss in ReBCO pancake coils and stacks of them: modelling and measurement

In this work we demonstrate that the use of striated tapes from coated conductors (CCs) significantly reduces the dissipation of a cable made of tapes wound helically on a round core when it is exposed to AC magnetic field The coupling loss can vanish provided that the striations ensure electrical insulation between filaments and the cable length corresponds to an entire number of lay pitches In our study we compare the magnetization loss in two cable models exposed to magnetic field perpendicular to their longitudinal axis The overall geometry of the models was identical: each consisted of three tapes 4?mm wide that were placed with a pitch of 50?mm in a single layer on the 8?mm diameter round core The cable length was designed to reach two complete tape pitches In the first cable (the reference cable) tapes without striation were used; the second cable was prepared using similar tapes but striated to five filaments by laser processing The AC loss was measured for cables without terminations as well as with low resistance terminations; this latter configuration simulates the conditions in a magnet winding Our experiments have clearly shown the loss behavior expected in the regime of uncoupled filaments In particular, at AC fields of 01?T amplitude the loss in the cable from striated tapes is five times lower than in the reference cable Numerical models have explained the experimentally observed cable behavior in the whole range of AC fields

https://iopscience.iop.org/article/10.1088/0953-2048/26/7/075020/pdf

Low AC loss cable produced from transposed striated CC tapes

Many applications of ReBCO-coated conductors contain stacks of pancake coils. In order to reduce their high AC loss, it is necessary to understand the loss mechanisms. In this paper, we measure and simulate the AC loss and the critical current, Ic, in stacks of pancake coils (‘pancakes’). We construct stacks of up to four pancakes and we measure them by electrical means. We also obtain the anisotropic field dependence of Jc from Ic measurements of the tape. This Jc is the only input to the simulations, together with the coil dimensions. After validating our computations with the measurements, we simulate stacks of many pancakes, up to 32. We found that the AC loss in a stack of (four) pancakes is very high, two orders of magnitude larger than for a single tape. A double pancake behaves as a single one with double the width but a stack of more pancakes is very different. Finally, we found that a two-strand Roebel cable reduces the AC loss in a stack of pancakes but not in a single pancake. In conclusion, our simulations are useful to predict the AC loss of stacks of coated conductor pancake coils and to reduce the AC loss by optimizing the coil design.

https://iopscience.iop.org/article/10.1088/0953-2048/25/3/035003/pdf

Chain-like crystal structure and magnetic properties of [catena-bis(1,2- diaminoethane)copper(II)-μ-dicyano-argentate]-dicyanoargentate

Several applications contain windings operating in DC submitted to changing magnetic fields. Their design needs to take the AC loss into account because of cryogenic and energy efficiency issues. This requires either measurements in demonstrators or computer simulations. In this article, we measure and numerically simulate the AC loss in a small racetrack coil made of ReBCO coated conductor. In particular, we study the separate contributions from the magnetization and transport loss. These provide additional information than only the total loss, such as the dynamic magneto-resistance effect. The measurements are done by electrical means and the simulations take a 2D approximation. We found that the total AC loss increases with the DC. However, the magnetization loss presents a peak. Simulations reveal that, contrary to the pure AC situation, the transient state can expand over a few cycles, even when the superconductor presents no flux creep. In conclusion, electrical measurements are useful to determine the AC loss under AC magnetic fields and DC transport current. These can be applied to any operating temperature, in opposition to boil-off methods. Finally, 2D simulation methods can reasonably predict the AC loss in racetrack coils with long straight segments.

Power Loss in ReBCO Racetrack Coils Under AC Applied Magnetic Field and DC Current

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Papers

AC loss in ReBCO pancake coils and stacks of them: modelling and measurement

Low AC loss cable produced from transposed striated CC tapes

AC loss in ReBCO pancake coils and stacks of them: modelling and measurement

Chain-like crystal structure and magnetic properties of [catena-bis(1,2- diaminoethane)copper(II)-μ-dicyano-argentate]-dicyanoargentate

Power Loss in ReBCO Racetrack Coils Under AC Applied Magnetic Field and DC Current