Abstract 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.

Abstract This paper presents the full-process fabrication and validation of a 16 m-long, six-slot REBCO twisted stacked-tape cable-in-conduit conductor (TSTC-CICC), a significant step toward compact, high-field fusion magnets. The conductor design incorporates a copper core with helical slots containing stacks of REBCO tapes, with vacuum pressure impregnation (VPI) of solder employed to achieve superior structural integrity and electrical stabilization. Characterization at 77 K demonstrated excellent longitudinal uniformity (maximum deviation 5%) in critical current and an average n -value of 15, indicating no performance degradation from the bending and impregnation processes. Experimental data were used to validate a finite-element H-formulation model, which subsequently predicts an engineering current density of 228.387 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">A</mml:mi> <mml:mstyle scriptlevel="0"/> <mml:msup> <mml:mi>mm</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> at 80 kA under operational conditions of 20 K and 20 T. These findings confirm the viability of both the proposed TSTC-CICC architecture and the VPI process for long-length high-temperature superconducting conductors, thereby underpinning their future engineering application in next-generation fusion devices.

Abstract Research linking surface hydrides to Q-disease, and the subsequent development of methods to eliminate surface hydrides, is one of the great successes of SRF cavity R&amp;D. We use time-dependent Ginzburg-Landau to extend the theory of hydride dissipation to sub-surface hydrides. Just as surface hydrides cause Q-disease behavior, we show that sub-surface hydrides cause high-field Qslope (HFQS) behavior. We find that the abrupt onset of HFQS is due to a transition from a vortex-free state to a vortex-penetration state. We show that controlling hydride size and depth through impurity doping can eliminate HFQS.

Abstract High-temperature superconducting (HTS) maglev offers non-contact and passively stable levitation, yet its large-scale deployment is hindered by the high cost and material utilization efficiency of permanent magnet guideways (PMGs). This paper proposes a discretization-based inverse design framework where the PMG cross-section is meshed into elemental units, treating the magnetization direction of each unit as a design variable to maximize levitation force. Case studies on two engineering PMGs demonstrate that the algorithm robustly converges to consistent optimal physical configurations regardless of initialization, with the primary variation observed only in computational convergence rates. To ensure engineering manufacturability, the framework incorporates fabrication constraints, including discrete angles, block-merging algorithms, and symmetry rules. Furthermore, the strategy is extended to a V-shaped PMG to verify its universality for complex geometries. Finally, the optimization is validated via levitation force experiments on 3D-printed scaled-down prototypes. The results demonstrate an 10% improvement in maximum levitation force over a initial scaled-down V-shaped Halbach baseline with an identical cross-sectional area. These findings confirm the feasibility and effectiveness of the proposed strategy, providing both theoretical insights and practical guidelines for next-generation HTS maglev systems.

Abstract In the application of high-field magnets, the quench behavior of no-insulation (NI) REBCO high-temperature superconducting coils can easily induce mechanical damage, which has become a key bottleneck restricting their development. In this paper, based on the improved T – A formulation with Neumann boundary conditions, a coupled electromagnetic–force–thermal multi-physical field model is established to systematically study the quenching characteristics of superconducting coils and their induced delamination failure behavior under the local thermal disturbances. A radial strain rate-based failure criterion is then proposed to investigate the correlation between the radial strain rate and the failure characteristics. A cohesive force model cohesive zone model is introduced to simulate the interface damage evolution of NI coils under localized thermal disturbances. The results show that the NI structure significantly improves the quench propagation velocity and enhances the thermal stability through the metal layer shunting effect. Concurrently, the strain rate field has a sensitive response to the temperature change and mechanical state during the quench process. The positive and negative values of the strain rate can clearly distinguish the quench recovery and persistence state, and the mutation peak can accurately locate the quench front and the stress concentration area, which are highly consistent with the high temperature gradient distribution. High working currents significantly increase the peak value of thermal strain rate and aggravate the thermal mechanical load. In addition, the location of thermal disturbance has a key influence on the damage mode. This study reveals the key role of strain rate in assessing the degree of quench and predicting mechanical failure, and provides a theoretical basis for the design and management of quench protection for NI coils with coupled multi-physical fields.

Abstract Understanding the tolerance of REBa 2 Cu 3 O <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi/> <mml:mrow> <mml:mn>7</mml:mn> <mml:mo>−</mml:mo> <mml:mi>x</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> (REBCO) high-temperature superconductors to neutron damage is essential for compact fusion reactor design. Here we report the first in situ measurements of the superconducting performance of REBCO coated conductor (CC) during neutron irradiation. The CC was exposed to 14 MeV neutrons at fluxes up to 7.3 × 10 7 n cm −2 s −1 while carrying current at 40 K. In situ voltage measurements revealed no instantaneous disruption of superconductivity, but the critical current ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>I</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">c</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ) decreased linearly with fluence at 0.03 A per 10 12 n cm −2 . Careful analysis excluded thermal effects as the origin of this degradation. Consistent with a point-defect damage mechanism, superconducting performance was recovered fully following room temperature annealing. This observed onset of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>I</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">c</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> reduction occurred several orders of magnitude below the critical neutron fluences anticipated by previous ex situ studies, indicating that service lifetimes of REBCO-based magnets may be substantially shorter than current projections.

Abstract Understanding and modelling heat transfer coefficient is essential for optimizing the performance of High-Temperature Superconducting (HTS) devices. The heat transfer coefficient exhibits highly non-linear behaviour with respect to temperature and is typically inferred from the properties of the cooling fluid. However, when the device is fully immersed in a liquid, the coefficient can deviate significantly from conventional estimations, especially if the operating conditions do not involve an extremely rapid temperature increase. This study presents a practical method for estimating the heat transfer coefficient of HTS coils immersed in liquid nitrogen, based on an experimental investigation of a prototype superconducting coil where precise temperature measurements are not guaranteed. By analysing transient temperature evolution and material properties, the coefficient is determined and validated through the comparison of measured and simulated device voltage and current. The proposed approach only relies on experimental voltage and current measurements, combined with known temperature-dependent material properties, to infer a global heat transfer coefficient. The methodology is tested on helical HTS coils under transient light over-current conditions and assumes an homogenised temperature rise along the winding. The estimated coefficient may lead to curves that slightly deviate from classical models while preserving their overall trend and physical significance. The proposed experimentally-based approach is conceived as a relatively straightforward and experimentally robust alternative to the computational burden of FEM simulations requiring reliable parameters, while overcoming the need for direct temperature measurements, within the limits of compact windings immersed in liquid nitrogen and subjected to mild transient conditions leading to distributed quench phenomena.

Abstract Iron-based superconductors (IBS) such as FeSe0.5Te0.5 (Fe(Se,Te)) are highly promising for applications in high magnetic fields and low temperatures, allowing their adoption in liquid helium at fields exceeding 20 T. In these regimes, while high-temperature superconductors (HTS) exhibit superior performance, IBS materials offer advantages, including a large critical grain-boundary angle that reduces the need for strict texturing, making them strong candidates for coated conductors. Fe(Se,Te) films can be deposited under less extreme conditions compared to HTS, simplifying manufacturing and making Fe(Se,Te)-based coated conductors a cost-effective alternative for large-scale production.&amp;#xD;Recently, we demonstrated that a thin TiN film serves as an effective buffer layer, enabling the oriented growth of superconducting Fe(Se,Te) on biaxially textured Ni-W. This leads to a simple coated conductor architecture, fabricated using Pulsed Laser Deposition (PLD), based on a single electrically conducting buffer layer. The resulting tape exhibits normal-state resistivity comparable to that of the substrate, proving the electrical connection that allows the substrate itself to function as an intrinsic stabiliser. &amp;#xD;In this work, we analyse the microstructure of the TiN-based tape via TEM, elucidating the Fe(Se,Te) growth mechanism on the mismatched TiN buffer layer and investigating the micro- and nanostructure of the superconducting film. Depending on deposition conditions, the formation of a nanostructured hexagonal secondary phase was detected, contributing an effective pinning landscape. This results in high critical current densities ( J C ), exceeding 0.1 MA/cm² at 4.2 K in magnetic fields up to 17.5 T and around 3.2 MA/cm² at 4.2 K in self-field. The angular dependence of the critical current density reveals a quasi-isotropic behaviour, attributed to the presence of the nanosized secondary phase, contributing to uniformly distributed pinning centres within the superconducting matrix.&amp;#xD;These findings represent significant advancements in process simplification and performance of Fe(Se,Te)-based coated conductors, further reinforcing the potential of IBS-coated conductors for applications.&amp;#xD;

Abstract We report a study of the transport critical current in the single-crystal nanobridges along the c -axis made of BaFe 2 (As,P) 2 superconductors. The magnitude of the critical current density J c as a function of the doping concentration shows a broad peak near the optimal doping, similar to a dome of the superconducting transition temperature T c . This shows a sharp contrast to the doping dependence of J c , determined by the depinning of vortices, which shows an abrupt peak near the optimal doping. The doping and temperature dependences of the transport J c are successfully explained by the microscopic theory of the depairing current density. The occurrence of phase slips is argued as an obstructive factor in the depairing current measurements at low temperatures.