This paper uses both experimental and numerical approaches to revisit the concept of current transfer length (CTL) in second-generation high-temperature superconductor coated conductors with a current flow diverter (CFD) architecture. The CFD architecture has been implemented on eight commercial coated conductors samples from THEVA. In order to measure the 2-D current distribution in the silver stabilizer layer of the samples, we first used a custom-made array of 120 voltage taps to measure the surface potential distribution. Then, the so-called "static" CTL ($λ_s$) was extracted using a semi-analytical model that fitted well the experimental data. As defined in this paper, the static CTL on a 2-D domain is a generalization of the definition commonly used in literature. In addition, we used a 3-D finite element model to simulate the normal zone propagation in our CFD samples, in order to quantify their "dynamic" CTL ($λ_d$), a new concept introduced in this paper and defined as the CTL observed during the propagation of a quenched region. The results show that, for a CFD architecture, $λ_d$ is always larger than $λ_s$, whereas $λ_d = λ_s$ when the interfacial resistance between the stabilizer and the superconductor layers is the same everywhere. We proved that the cause of these different behaviors is related to the shape of the normal zone, which is curved for the CFD architecture, and rectangular otherwise. Finally, we showed that the NZPV is proportional to $λ_d$, not with $λ_s$, which suggests that the dynamic CTL $λ_d$ is the most general definition of the CTL and should always be used when current crowding and non-uniform heat generation occurs around a normal zone.
Striating HTS coated conductor (CC) tapes into narrow filaments offers the possibility of reducing the tapes' magnetization losses without unreasonably decreasing their current-carrying capability. However, realizing well-separated striations presents technological challenges, especially if the number of filaments is large and/or if a thick layer of metallic stabilizer is present. In these situations, the filaments can be easily coupled and their effectiveness to reduce magnetization losses is strongly diminished or eliminated. While the onset of coupling is well visible from magnetization los…
In this work we present a modeling tool designed to estimate the hysteretic losses in the coils of an electric generator with coils made of coated conductor tapes during transient operation. The model is based on a two-stage segregated model approach that allows simulating the electric generator and the current distribution in the superconducting coils using a one-way coupling from the generator to the HTS coils model. The model has two inputs: the rotational speed and the electric load signal. A homogeneous anisotropic bulk model for the coils allows computing the current distribution in the …
Numerical models are powerful tools to predict the electromagnetic behavior of superconductors. In recent years, a variety of models have been successfully developed to simulate high-temperature-superconducting (HTS) coated conductor tapes. While the models work well for the simulation of individual tapes or relatively small assemblies, their direct applicability to devices involving hundreds or thousands of tapes, as for example coils used in electrical machines, is questionable. Indeed the simulation time and memory requirement can quickly become prohibitive. In this article, we develop and …
High temperature superconducting (HTS) stacks of coated conductors (CCs) can work as strong trapped field magnets (TFMs) and show potential in electrical applications. Pulsed field magnetization (PFM) is a practical method to magnetize such TFMs, but due to heat generation during the dynamic process, it cannot achieve a trapped field as high as field cooling can. In this work, we construct a 2D electromagnetic-thermal coupled model to simulate stacks of HTS CCs with realistic laminated structures magnetized by PFM. The model considers temperature and anisotropic magnetic field dependent Jc of …
Modelling high temperature superconductor (HTS) motors remains challenging mainly due to the high aspect ratio of these conductors but also because of the properties of the magnetic materials. This paper presents a 2D time dependent model to assess the AC losses of superconducting motors based on the new T-A formulation, which by using Finite Element Methods (FEM), allows its implementation in commercial software. The T-A formulation computes the magnetic flux density with different Maxwell's equations depending on the areas of the motor and makes it possible to use the thin strip approximatio…
The ability of high-temperature superconductors (HTSs) to carry very large currents with almost no dissipation makes them irreplaceable for high-power applications. The development and further improvement of HTS-based cables requires an in-depth understanding of the superconducting vortex dynamics in presence of complex pinning landscapes. We present a critical current analysis of a real HTS sample in a magnetic field by combining state-of-the-art large-scale Ginzburg-Landau simulations with reconstructive three-dimensional scanning transmission electron microscopy tomography of the pinning la…
Direct current (DC) characterization of high temperature superconducting (HTS) coils is important for HTS applications, such as electric machines, superconducting magnetic energy storage (SMES) and transformers. In this paper, DC characterization of a circular, epoxy-impregnated HTS coil made from YBCO coated conductor for use as a prototype axial flux HTS electric machine is presented. Multiple voltage taps were utilized within the coil during measurement to help provide further detailed information on its DC behavior as a function of length. Based on the experimental results, there exist reg…
In the last 10-15 years, the most common strategy in the development of High Temperature Superconducting (HTS) cable for magnets has been to imitate Low Temperature Superconducting (LTS) cable designs. However, requirements for LTS materials are not valid for HTS materials, which are extremely stable. For example, non-twisted multifilamentary Bi-2223 tapes have been successfully used in several magnets. This paper review stability and analyse inductance and AC losses in non-twisted stack of HTS tapes. Numerical calculations show that twisting has negligible effect on inductance variations in a…
Roebel cables made of HTS coated conductors can carry high currents with a compact design and reduced AC losses. They are therefore good candidates for manufacturing coils for HTS applications such as motors and generators. In this paper we present the experimental DC and AC characterization of several coils assembled from a 5 meter long Roebel cable built at KIT, which differ in the number of turns and turn-to-turn spacing. Our experiments, supported by finite-element method (FEM) calculations, show that a more tightly wound Roebel coil, despite having a lower critical (and therefore operatin…
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 hyd…
A compact HTS cable that is able to carry large current density is crucial for developing high field accelerator magnets. We are reporting a novel HTS cable (named X-cable) that could achieve a high current density as the Roebel cable, but is implemented by in-plane bending stacked HTS tapes directly to realize the transposition. The cable is jointly developed with an industrial company on a production line: ready for large scale production from the beginning. Recently, a prototype cable with REBCO coated conductor has been successfully fabricated. Test results show no significant degradation,…
The electromagnetic properties of a pancake coil in AC regime as a function of the number of turns is studied theoretically and experimentally. Specifically, the AC loss, the coil critical current and the voltage signal are discussed. The coils are made of Bi2Sr2Ca2Cu3O10/Ag (BiSCCO) tape, although the main qualitative results are also applicable to other kinds of superconducting tapes, such as coated conductors. The AC loss and the voltage signal are electrically measured using different pick up coils with the help of a transformer. One of them avoids dealing with the huge coil inductance. Be…
We aim to demonstrate an efficient method to calculate the AC losses in multi-pancake coils. This method extends Yuan front-track model into several pancake application and investigates the detail parameters in comparison with established H-formula implemented in COMSOL and minimization of energy method. We use the front-track idea to analyze stacked pancakes assuming the current fronts are straight lines and using the critical state model. The current distribution is solved by two means: minimization of the perpendicular magnetic field in the subcritical region, as Clem and Yuan proposed; min…
The application of HTS coated conductors in future fusion or accelerator magnets is currently of increasing interest. High Lorentz forces and therefore high hoop stresses act on the conductors in large coils. The conductor is furthermore exposed to neutron radiation in fusion or accelerator magnets. The expected neutron fluence over the desired lifetime of such magnets can be simulated by irradiation experiments in a fission reactor. The coated conductors were characterized in the pristine state and after irradiation to the ITER design fluence. The sensitivity of the critical currents to appli…
Low AC loss high transport current HTS cables (>1 kA) are required for application in transformers, generators and are considered for future generations of fusion reactors coils. 2G coated conductors are suitable candidates for high field application at quite high operation temperatures of 50-77 K, which is crucial precondition for economical cooling costs. As a feasibility study we present the first ROEBEL bar cable of approx. 35 cm length made from industrial DyBCO coated conductor (THEVA GmbH, Germany). Meander shaped ROEBEL strands of 4 mm width with a twist pitch of 180 mm were cut from 1…
Scaling relations describing the electromagnetic behaviour of coated conductors (CCs) greatly simplify the design of REBCO-based devices. The performance of REBCO CCs is strongly influenced by fabrication route, conductor architecture and materials, and these parameters vary from one manufacturer to the others. In the present work we have examined the critical surface for the current density, Jc(T,B,$θ$), of coated conductors from six different manufacturers: American Superconductor Co. (US), Bruker HTS GmbH (Germany), Fujikura Ltd. (Japan), SuNAM Co. Ltd. (Korea), SuperOx ZAO (Russia) and Sup…
Rare-earth-barium-copper-oxide (REBCO) tapes are now available from several industrial manufacturers and are very promising conductors in high field applications. Due to diverging materials and deposition processes, these manufacturers' tapes can be expected to differ in their electro-mechanical and mechanical properties. For magnets designers, these are together with the conductors' in-field critical current performance of the highest importance in choosing a suitable conductor. In this work, the strain and stress dependence of the current carrying capabilities as well as the stress and strai…
The current-voltage characteristics of superconductors are commonly determined by means of a standard 4-probe DC measurement technique. In coated conductors, however, one often encounters problems due to weak or no stabilization and in such case, when the risk of burning the tape is high, the 4-probe approach can fail. In other cases such as in HTS cable structures, besides vulnerability and high risk of damage, it may also be difficult to achieve a high critical current level with a power supply that is normally used to characterize single strands. We therefore investigated an alternative met…
Numerical models for computing the effective critical current of devices made of HTS tapes require the knowledge of the Jc(B,theta) dependence, i.e. of the way the critical current density Jc depends on the magnetic flux density B and its orientation theta with respect to the tape. In this paper we present a numerical model based on the critical state with angular field dependence of Jc to extract the Jc(B,theta) relation from experimental data. The model takes into account the self-field created by the tape, which gives an important contribution when the field applied in the experiments is lo…
The high-Tc superconducting (HTS) dynamo is a promising device that can inject large DC supercurrents into a closed superconducting circuit. This is particularly attractive to energise HTS coils in NMR/MRI magnets and superconducting rotating machines without the need for connection to a power supply via current leads. It is only very recently that quantitatively accurate, predictive models have been developed which are capable of analysing HTS dynamos and explain their underlying physical mechanism. In this work, we propose to use the HTS dynamo as a new benchmark problem for the HTS modellin…
Striation of HTS coated conductors (CCs) as a way to reduce their magnetization AC losses has been the subject of intense research in the past years by several groups. While the principle of this approach is well understood, its practical application on commercial material to be used in power application is still far to be implemented due to manufacturing and technological constraints. Recent advances in tape quality and striation technology are now enabling systematic investigations of the influence of the number of filaments on AC loss reduction with a consistency that was not available in t…
We propose a magnetic dual of the Coulomb blockade effect for quantum nucleation of flux vortex pairs in high-Tc superconducting (HTS) films and grain boundaries in zero applied field. The magnetic blockade instability occurs at θ = π, where θ is the "vacuum" or theta angle. The θ term has recently been discussed in the context of several other systems, including charge and spin density waves, topological insulators, the quantum Hall effect, and spontaneous CP violation. Our model predicts a sharp pair creation threshold current at θ = π, analogous to the Coulomb blockade voltage of a tunnel j…
High temperature superconducting (HTS) bulks or stacks of coated conductors (CCs) can be magnetized to become trapped field magnets (TFMs). The magnetic fields of such TFMs can break the limitation of conventional magnets (<2 T), so they show potential for improving the performance of many electrical applications that use permanent magnets like rotating machines. Towards practical or commercial use of TFMs, effective in situ magnetization is one of the key issues. The pulsed field magnetization (PFM) is among the most promising magnetization methods in virtue of its compactness, mobility and l…
For an accurate estimation of the AC losses of superconducting triaxial cables, we present a 2D model capable to provide a global assessment of multi-layer triaxial cables, validated against the AC-losses on single-phase cables provided by the VNIIKP Cable Institute. Four models are presented, the first being a single-phase cable of 50 tapes and the others being three triaxial cables made of up to 135 coated conductors distributed in up to 9 layers. A systematic study is devised, where the number of layers per phase increases from 1 to 3, with at least 14 tapes distributed across each layer of…
HTS are feasible for hight power applications because of the smaller consumption demand of the cooling system in comparison to the whole power device. Real industral devices contain superconductors with 2D and 3D geometry (coated conductors tapes represent an example of 2D geometry, since the superconducting layer is very thin). In order to know the feasibility and the optimum design of a certain device, there is a need of software tools. These Numerical tools have hight requirements like fast computation, a physical model for any E(J) relation of the superconductor and any complex geometry su…
