Simona Hulačová

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.

Alternating current (AC) losses in REBCO (RE – rare earth metal, B – barium, C – copper, O – oxygen) high-temperature superconductors, can be reduced by several orders of magnitudes by the tape striation. Various methods were investigated so far for the decrease of the superconductor's width-to-thickness ratio, and new methods (or novel combination of previously reported methods) are still developed. A reason for the latter efforts is to reduce the superconductor's degradation, which cannot be completely avoided, and it is inherent to each method. Unfortunately, the degradation also seems to depend on the architecture (i.e., on the thicknesses of layers, deposition techniques used, chemical composition, etc.) We thus compared the ac losses of REBCO tapes striated by a chemo-mechanical method, by photolithography, and by laser ablation approach. The ac loss measurements and transport properties of Ag-stabilized superconducting tapes from SuperPower are complemented by the analysis of their structure. All methods resulted in a substantial decrease in normalized ac loss (more than 9 times at 36 Hz for tape with 15 striations in compare with non-striated tape. The photolithography method exhibited the highest decrease of ac loss, but also led to a notable decline in the critical current.

This study aimed to enhance the performance of Ag-stabilized high-temperature superconducting (HTS) tapes with a focus on reducing magnetization losses. Two approaches were employed: dividing the tapes into narrower widths and introducing striation at the level of the superconducting layer. The process of laser ablation proved to be an effective method for implementing these modifications. The quality of the cut edges and grooves was assessed using scanning electron microscopy. To evaluate the electrical properties, measurements were conducted on the critical current and magnetization loss in samples at different stages: in their initial state, after cutting, and after the striation process. Of the two modifications, the striation process more effectively reduced the AC losses in the HTS tapes, approximately by one order of magnitude. The retention of critical current remained high after cutting, but varied with the number of created filaments after the striation process. Subsequently, a short cable was wound from the cut and striated HTS tape. This cable demonstrated a remarkable sixfold reduction in AC losses compared to the initial HTS tape.

TORT (Tapes on Round Tube) cables, which implement high-temperature superconducting (HTS) tapes, offer additional benefit over other HTS cables, which is the internal cooling in the former itself. Up to now, the superconducting tapes of existing TORT cables have been wound on a copper tube of various diameters. However, in the alternating current (AC) mode, the high conductivity of the former causes undesirable eddy currents. Therefore, it is necessary to replace the copper with a less electrically conductive material. We designed a former from polyethylene terephthalate glycol-modified and reinforced with carbon fibers (PETG CF), polycarbonate (PC) and polycarbonate reinforced with carbon fibers (PC CF) materials in order to reduce the AC losses in TORT cables, using 3D printing technology. Because of the low thermal conductivity of polymers, the former must be specially designed to improve the cooling of the HTS tapes inside the former. Moreover, thermal deformation of the former during cooling from room temperature to liquid nitrogen temperature and resilience against low-diameter bending is another important issue that was taken into account. Detailed structural finite-element (FE) analysis was carried out using the FE modeling software ANSYS, in prototyping of the former. Based on the numerical models, formers differing in the materials used were fabricated, and subsequently a short TORT cable with wound HTS tape was electromagnetically characterized.