Yusuke Sogabe

Distributed fiber-optic sensing is a promising approach for quench detection and protection of high-temperature superconducting (HTS) systems, offering immunity to electromagnetic interference and scalability to large cable and magnet systems. In this work, we demonstrate the response of continuous fiber Bragg grating (cFBG) sensors integrated on the outer layer of a two-layer spiral-wound REBCO cable. The cable consists of three 2 mm-wide coated conductors per layer wound around a 3 mm brass core, with the optical fiber wound helically on the outer surface at a 20 mm pitch. We present results from two quench scenarios: heater-induced and over-current induced quench. In the heater experiment, an optical response is observed before any measurable voltage is generated, demonstrating the potential of temperature-based detection to provide early warning ahead of voltage-based methods. In the over-current experiment, a measurable optical response is observed within tenths of a second of the onset of rapid voltage increase. These results demonstrate the feasibility of cFBG sensing in spiral-wound HTS cables and support continued development of fiber-optic monitoring systems for HTS applications.

We experimentally measured the tape-to-tape contact resistance distribution in two-layer spiral-coated-conductor cables. Current was injected at one end of the outer layer and extracted from the opposite end of the inner layer. In this case, the entire contact resistance of the cable can be measured using a simple four probe method, based on the voltage between the two coated conductors and current. Besides, we measured the current in each layer, distributed along the sample, using arrays of Hall sensors installed in the core of the cable. By combining the voltage between layers and the layer currents distributed along the cable, we determined the local contact resistance distributed along the cable. We also studied the influence of several fabrication conditions on contact resistance. Three samples were fabricated for each condition to investigate the variability.

Current sharing between filaments is necessary in multifilament coated conductors to bypass local defects arising from the fabrication process and to impede current transport in each filament. We propose a novel cable concept, termed the Spiral Copper-plated Striated Coated conductor with Inter-Filament Bridges cable (SCSC-IFB cable). In this design, copper-plated multifilament (striated) coated conductors incorporating Inter-Filament Bridges (IFBs)—non-striated sections between filaments—are helically wound around a central core. In contrast to current sharing through copper plating, i.e., the current meandering through it to avoid local defects on filaments, which generates a small but finite voltage, current sharing through IFBs produces no voltage and thereby does not affect the electric field–current characteristics of multifilament coated conductors. Similar to twisting in conventional low Tc superconductors, helical cabling decouples filaments electromagnetically against a transverse alternating current (AC) magnetic field. We experimentally confirmed that IFBs improve the electric field–current characteristics, i.e., the n-values, and that the helical cabling does not affect them, by using 2 mm-wide copper-plated 10-filament coated conductors with IFBs. Using the same coated conductors, we wound short pieces of the monolayer SCSC-IFB cables and examined their AC loss characteristics under a transverse AC magnetic field at 77 K. Importantly, the bridge intervals were designed to exceed half the cabling pitch, preventing superconducting loop currents through two IFBs that could otherwise couple the filaments. The measured coupling time constant was 19 s, which is sufficiently small for a wide range of applications. Furthermore, the magnitudes of the measured magnetisation losses above the penetration field of each filament are nearly identical to the theoretical values predicted for cables composed of multifilament coated conductors with electromagnetically decoupled filaments.

Spiral Copper-plated Striated Coated-conductor (SCSC) cables are anticipated to be a game-changer for ac applications of high-temperature superconducting technology due to their low ac losses and high robustness. Experiments and numerical electromagnetic field analyses have confirmed that the structure of SCSC cables is effective in reducing dynamic losses and dynamic resistances, which is a problem in flux pumps and rotating machines. However, these results were confirmed for a single copper-plated multifilament coated conductor in a spiral configuration. It remains unclear whether they hold true for SCSC cables with the multilayer structure required for practical current-carrying capacity. Particularly, there is concern that the effect of reducing the dynamic resistance of SCSC cables may be lost due to tape-to-tape interactions caused by an increase in the number of layers or the number of tapes within a layer. We evaluated this using numerical electromagnetic field analyses.

We measured the frequency-dependent magnetization losses of twisted multifilament MgB₂ wires subjected to ac transverse magnetic fields with a small amplitude, under which coupling loss dominates magnetization loss, and then, we determined their coupling time constants as well as their geometry factors of coupling losses at various temperatures. We also measured the hysteresis loops of their magnetizations using a vibrating sample magnetometer, and then, we calculated the hysteresis losses at various magnetic field amplitudes and at various temperatures. Note that such calculated values represent the hysteresis losses in completely decoupled filaments in a twisted multifilament MgB₂ wire. Using these values obtained experimentally, we calculated the magnetization losses of twisted multifilament MgB₂ wires under practical operating conditions, i.e. at various magnetic field amplitudes, at various magnetic field frequencies, and at various temperatures, considering the saturation of the filamentary region in the wire, which occurs at magnetic fields with high frequencies and large amplitudes.

Fully superconducting machine typologies allow maximizing the advantages of superconductors in rotating electrical machines and might be a key in overcoming the power density limitations of current technologies. However, their realization is facing a challenge since superconductors generate non negligible AC losses when supplied by AC currents or subjected to AC fields. Deviating the magnetic field from the wide surface of the conductor and striation are proven methods to significantly reduce magnetization losses in coated conductors. However, striation is only effective when filaments are decoupled electromagnetically. Winding a copper-plated striated coated conductor spirally around a core is a novel method to decouple filaments. In this work, cables consisting of such multifilament coated conductors are applied in an axial flux synchronous machine model and their losses are compared to those of superconducting tapes with magnetic leakage flux parallel to their wide surfaces. Cables’ models are first verified by comparison with analytical and experimental results. Then, these are applied in a single slot model previously developed by the authors to analyze and compare loss results to those of single tapes carrying the same current when exposed to the same surrounding magnetic fields.

We fabricated a multi-layer coil with multiple turns per layer using a spiral-coated-conductor cable and measured its ac losses. Additionally, we measured the transport & magnetization losses of a short sample of the cable and integrated them to estimate the losses in the entire coil. The measured ac loss of the coil was significantly larger than the integrated transport loss of the short sample, likely because the ac loss in a turn of the coil was dominated by the transverse magnetic field generated by other turns. Furthermore, the measured coil loss was larger than the sum of the integrated transport and magnetization losses. The interaction between the transverse magnetic field and the azimuthal self-magnetic field of each turn might cause this difference.

Dynamic resistances appear in superconductors carrying dc currents subjected to ac magnetic fields. In coated conductors, multifilament structure is considered effective in reducing the dynamic resistance. However, in practical use, the coated conductors are plated with copper for improvement of stability and protection, so that coupling currents are induced under ac magnetic fields and deteriorate the effect of multifilamentation. The Spiral Copper-plated Coated-conductor cable (SCSC cable), which we developed to reduce as losses, might be effective to solve the problem. We conducted electromagnetic field analyses of four different copper-plated coated conductors to prove the effect; straight monofilament, straight multifilament, spiral monofilament, and straight multifilament coated conductors. We show the region where dynamic loss is generated (dynamic region) and then, discuss the difference of dynamic loss and hysteresis loss among the coated conductors. Also, the dynamic resistances in the analyzed coated conductors are compared.

Theoretically, it has been shown that the dynamic resistances of coated conductors can be reduced by decreasing their effective width through multifilamentation. In the case of copper-plated multifilament coated conductors, coupling currents are expected to worsen the effect of multifilamentation in reducing dynamic resistance. Therefore, the purpose of this study is to experimentally investigate the dynamic resistances of spiral copper-plated multifilament coated conductors, which are expected to reduce the coupling time constant in a manner similar to twisted low-T _c superconducting wires. We measured the dynamic resistance of four different samples—straight monofilament, straight multifilament, spiral monofilament, and spiral multifilament coated conductors—using the four-terminal method. We discuss the dynamic resistivity characteristics of the copper-plated multifilament coated conductors by comparing the magnetic field dependence of the dynamic resistivity, normalized by the critical current of each sample.

We experimentally studied the influence of current sharing among filaments on the initiation of thermal runaway and protection against it in a multifilament coated conductor wound spirally on a round core. We artificially introduced a local defect near one edge of a 4-mm-wide multifilament coated conductor as well as a monofilament coated conductor by using a fine drill bit. We wound each coated conductor on a glass fiber-reinforced plastic (GFRP) core and set several voltage taps on their both edges to measure the voltages in longitudinal (both edges) and transverse directions. We compared thermal runaway current, above which thermal runaway initiates, of multifilament coated conductor with that of monofilament coated conductor. We also compared their time constants of current ramping down, below which the coated conductors are protected against degradation after thermal runaway.