Ján Šouc

Tapes made from high-temperature superconducting ceramic REBCO compounds, commonly known as coated conductors (CC), offer a promising solution for future magnetic systems in particle accelerators, nuclear fusion reactors, and electrical machines. Nevertheless, under dynamic operations, dissipation known as AC loss represents a critical factor restricting their applicability. The introduction of the filamentized CC (FCC), wherein the superconducting layer is striated into parallel strips called filaments, has emerged as a game changer. Proportional to the number of filaments, this measure suppresses the hysteresis loss caused by supercurrents circulating exclusively inside the superconductor. However, providing a metallic connection between the filaments is necessary despite the introduction of a new loss mechanism called the coupling loss. In our AC loss study, a series of FCC prepared by the two-level-undercut–profile-substrate (2LUPS) process, differing in the filament width (0.25–2 mm) and in the dimensions of metallic layer filling the gaps between the filaments structure, was investigated. We found that the analytical formulas for computing both primary AC loss components (hysteresis and coupling) yield relevant predictions aligned with experimental findings. Such an approach simplifies parametric studies of loss dependence on tape architecture, which is crucial for the rigorous understanding of the AC loss issue. This phenomenon is illustrated here by presenting the correlations between selected parameters (filament width, transverse resistivity) and the AC loss behavior observed in experiments.

Reducing alternating current (AC) losses arising in high-temperature superconducting (HTS) tapes is crucial for their efficient operation in environments with alternating or varying magnetic fields. This study focuses on developing multifilamentary HTS tapes with reduced AC losses by introducing striations through a photolithography process. Tapes with varying numbers of striations were fabricated and tested to assess their impact on critical currents and AC loss performance. The most promising design was further evaluated in a cable-like configuration by winding onto a non-conducting core. The quality of the filaments was characterized using scanning electron microscopy (SEM), confirming complete disruption of the REBCO (rare-earth barium copper oxide) layer within the grooves and ensuring proper filament isolation. Critical current and AC loss measurements were conducted to comprehensively evaluate the overall performance of the tapes. Striation into 10 filaments reduced AC losses by approximately 7–19 times compared to an unstriated tape, depending on the applied magnetic field amplitude. This improvement, however, was accompanied by an approximately 50% reduction in critical current, which may limit potential applications. Tapes with fewer filaments offered a more balanced compromise, achieving significant AC loss reduction while retaining a higher fraction of the critical current. Furthermore, winding of the tape did not substantially lower the critical current but provided an additional reduction in AC losses, highlighting the suitability of this approach for cable fabrication. Overall, the photolithography method proved to be effective and reproducible approach for fabricating multifilamentary structures.

Rare-earth barium copper oxide (REBCO) coated conductors are the suitable candidate materials for superconducting tapes on round tubes (TORTs) cables due to their excellent mechanical strength, high current-carrying capacity, and magnetic strength. It is commonly known that AC losses can be decreased when TORT cables are constructed using REBCO multi-filamentary tapes. However, the mechanical strength of these geometrically adjusted tapes is just as important when winding them into cables. The objective of this study was to understand the effects of the mechanical loading on the I _c (critical current) of the striated tapes with different filament geometries and, therefore, determine the safe strain limits. This work involved the preparation of various types of REBCO striated tapes through chemo-mechanical striating, as well as the deposition of stabilization multilayers. Subsequently, the striated tapes were bent at a lay angle of 45° over bender diameter from 13 mm to 1 mm, and current-carrying performance under self-field conditions at 77 K was measured. We observed that striated tapes with narrower gaps remained without I _c reduction to a bender diameter of 2 mm, while for non-striated tape there was a more significant decrease at 3 mm. Finally, the effect of filament geometrical irregularities on the mechanical performance of striated REBCO tapes were supported by finite element analysis and correlated with experimental results.

Frequent task in the numerical modeling of superconducting magnets generating AC or pulsing fields is the evaluation of AC loss. Commonly, after solving the electromagnetic problem, the loss is calculated by integrating in time and volume the density of dissipated power, obtained locally as the product of electrical field and current density. However, the loss can also be determined from the product of current and voltage detected at the device terminations, in a way analogous to electrical measurements. Fortunately, this voltage signal can be easily determined in the frequently used T–A formulation. It can be further used for more detailed comparison with experimentin particular, the creation of magnetic hysteresis loops relating the flux linkage to the coil current. Methodology is illustrated on benchmark #3 problem published by the HTS modeling workgroup.

Use of coated conductor tapes from high-temperature superconductors is a possible way for better energy efficiency in large magnetic systems for particle accelerators or fusion tokamaks. Because the hysteresis loss in a time varying magnetic field is proportional to the tape width, a substantial gain is expected by splitting of the REBCO layer into parallel filaments. Usually, a metal layer is added to facilitate stable operation, albeit this conductive connection between filaments introduces a new kind of loss, known as coupling loss. When analyzing the experimental data of samples produced by the scalable technology, where the buffer and the superconductor layer are deposited via standard industrial procedures on a 3D patterned Hastelloy substrate, we have found that the existing models for coupling loss do not provide satisfactory quantitative predictions. This motivated us to develop an improved model for low- and mid-frequency behavior. While preserving the well-known qualitative properties foreseen by previous models, our new model enables quantitative predictions at frequencies up to 1000 Hz, without the need for adjusting the results by fitting the geometric parameters.

Variation of the critical current along the conductor length is a feature commonly encountered in industrially produced REBCO tapes called coated conductors (CCs). A reduction of the critical current exceeding several percent in portions a few millimetres long can be observed in the data obtained by reel-to-reel characterization provided by manufacturers. Metallic layers in the CC architecture can take over some current in such a ‘weak spot’, and help to keep the local temperature stable, preventing its thermal runaway and conversion to a ‘hot spot’. Understanding this phenomenon is particularly crucial when space and weight limitations do not allow for the addition of a metallic layer with a thickness that would be sufficient to match the lost transport capability of superconductors. For this purpose, we studied a set of samples, representing both standard as well as infrequent profiles of weak spots identified in direct transport experiments. Analytical theory is then utilized as a basic tool for recovering the properties serving as input for numerical modelling. Temperature profiles and current redistribution at weak-spot locations were found, and the effect of cooling conditions and metallic layer thickness on the weak-spot resistance against thermal runaway was analyzed. Quantitative assessment of the possibility to improve the performance of CC tape by adding a Cu stabilizing layer or improving cooling settings could help to optimize the architecture of CCs intended for use in electrical transport.

REBCO (rare-earth barium copper oxide) high-temperature superconducting tapes, have a high potential for winding of large magnet coils. Tapes on round tube (TORT) cables represent a promising option for achieving a conductor suitable for the winding of magnet coils. However, certain applications, such as accelerator magnets, require the use of superconducting cables with low magnetization alternative current (AC) losses. There are several methods to reduce AC losses in TORT cables. Our first approach was to get rid of eddy currents by replacing the copper former with dielectric materials based on polymers and composites, i.e. polyethylene terephthalate glycol-modified reinforced with carbon fibers. Additional reduction of hysteretic loss was achieved by striating of copper coated REBCO tapes. We employed chemo-mechanical striating, for these objectives. However, the superconductor is exposed during the striating process, which may lead to later moisture-related degradation. Hence multilayers based on Ti/AlN were deposited using magnetron sputtering in order to protect the superconductor immediately after the striating process from water and atmospheric moisture corrosion. Subsequently, striated tapes as well as the non-striated tapes were then wound onto formers with diameters of 10 mm, 7 mm and 5.5 mm, and then on the short TORT cables bending tests were performed. After each technological step, direct current measurements were performed on the samples and finally the AC losses were measured.