Self-field Conditions Research Articles

Temperature evolution and thermoelastic behavior of epoxy-impregnated REBCO HTS composite coils studied using a heat-force bi-directional coupling quench model

Quench and mechanical behavior are the key issues for the safe operation of epoxy-impregnated (Re)Ba2Cu3Ox (REBCO) pancake coil. The contribution of strain energy to the temperature field during heat propagation and the effect of temperature changes on the mechanical behaviour in the force field are considered. In the paper, the bi-directional coupling differential equations of heat conduction and elastic equilibrium are proposed, and their bi-directional coupling solution is realized using finite element method. The model is not based on homogenization assumption considering separately the anisotropic characteristics of each layer in a REBCO composite tape. In addition, the REBCO tape after quench is defined as a parallel circuit model in the finite element model. During the quench, the current is distributed between the superconducting (REBCO) and metal (copper, silver and Hastelloy) layers according to their resistance. The results show that for the self-field conditions, the strain-energy contribution that is taken into account in the heat-force bi-directional coupling model causes a 3 % smaller overall temperature of the coil, which is still necessary to consider for the quench behavior of highly sensitive superconducting coils. Another important result is that thermal stresses are the major contribution to the internal stress state of the coil in self-field, whereas under the background magnetic field of 5 T, the radial stress inside the coil has reached the strength range of peeling delamination.

Effect of different core diameters on the current-carrying performance of CORC cables with REBCO multi-filamentary tapes

The second generation of high temperature superconductivity (HTS) is one of the candidate materials for future superconducting cables (such as CORC), CICC conductors and high field magnets due to its high current carrying performance and excellent mechanical strength. Currently, it has been demonstrated that the utilization of REBCO multi-filamentary tapes in the fabrication of CORC cables can further reduce AC losses. This study involved the preparation of various types of REBCO multi-filamentary tapes through reel-to-reel ultraviolet picosecond laser cutting technology, as well as the manual winding of multiple single-layer CORC cable samples. This work focused on the impact of core diameter on the current-carrying performance of REBCO multi-filamentary tapes under self-field conditions at 77 K. The results indicate that the critical current (IC) of REBCO multi-filamentary tapes decreases as the core diameter decreases. Furthermore, the decrease in IC becomes more significant with an increasing number of cores. When the cable core diameter is 4.6 mm, the critical current of the 3-filament tape is 180.49 A with an n-value of 24.84, which is only a 3.1 % recession compared to the critical current of the commercialized tape. The conclusion of this paper will provide a certain data reference for the subsequent selection of CORC cable core size prepared by REBCO multi-filamentary tapes.

AC transport loss analysis of HTS stack busbars for all-electric aircraft with harmonics and DC offset considerations

This paper presents a study of the current carrying capacity and AC loss of high-temperature superconducting (HTS) stacks to be used in busbar applications for all-electric aircraft. A 2D model was developed using COMSOL Multiphysics with a T-A formulation for detailed analysis. The study began by applying a stable 20 kA DC offset current to the HTS stacks to simulate practical operating conditions. Firstly, the behaviour of the critical current was studied under self-field conditions for stacks with different numbers of HTS tapes and spacing. Secondly, AC ripple currents were introduced with DC offsets, and the effects of 3rd and 5th harmonic distortions (HD) were studied. The results show that configurations with 40 tapes and gaps of more than 2 mm are considered suitable for safe current transport under DC conditions. On the other hand, increasing the tape spacing leads to an increase in the safe transport current ripple due to the reduced magnetic field interaction within the stack. In addition, the transport loss decreases as the air gap increases due to the reduction in the self-field, whereas it increases as the number of strips increases. The influence of the 3rd HD on the transport loss is minimal at a ripple current of 1% and slightly noticeable at 2%. However, it becomes more obvious as the ripple current approaches the critical value. Remarkably, even cases with equivalent total HD show significantly higher transport losses when characterised by higher 5th HD than their counterparts with 3rd HD. This comprehensive analysis provides valuable information on the performance characteristics of HTS stacks in all-electric aircraft busbar applications and offers important insights for the development and optimisation of these systems in practical aerospace applications.

Performance study of REBCO multi-filament tapes/CORC cables prepared by reel-to-reel mechanical incision

REBCO high-temperature superconductor (HTS) has widely used in high magnetic field applications, because of the excellent critical current properties and high critical temperature. However, REBCO tape has an extremely large width-to-thickness ratio (typically in the range of 1000–10000) to cause too high power dissipation in the applications. In order to reduce the shielding current effect and AC loss generated during the application of REBCO HTS tapes, a roll-to-roll mechanical incision device was developed for the preparation of REBCO multi-filaments tapes. The microscopic morphology, critical current IC, and hysteresis loss of the tapes before and after incision were comparatively characterized using optical microscope, standard four-probe method, and physical property measurement system (PPMS) and other equipment and methods. The results show that the developed device prepares multi-filaments tapes with incision grooves depths of ∼ 30 μm and grooves widths of ∼ 20 μm on the superconducting layer, and the hysteresis loss of the 3-filaments is reduced by up to 48.6 % compared to that of the non-striated tapes under different temperature conditions. In addition, the IC of the short samples of the 3-filaments tapes was reduced by ∼ 18 % compared with that of the non-striated tapes under 77 K and self-field test conditions, whereas the samples of the 3-filaments tapes and the non-striated tapes were hand-wound respectively onto a metal core with a diameter of 5.25 mm to make a single-layer CORC cable, and there was no significant reduction in the IC of either of them.

Screening-current-induced magnetic fields and strains in a compact REBCO coil in self field and background field

REBa2Cu3O7−x (REBCO) coated conductors, owing to its high tensile strength and current-carrying ability in a background field, are widely regarded a promising candidate in high-field applications. Despite the great potentials, recent studies have highlighted the challenges posed by screening currents, which are featured by a highly nonuniform current distribution in the superconducting layer. In this paper, we report a comprehensive study on the behaviors of screening currents in a compact REBCO coil, specifically the screening-current-induced magnetic fields and strains. Experiments were carried out in the self-generated magnetic field and a background field, respectively. In the self-field condition, the full hysteresis of the magnetic field was obtained by applying current sweeps with repeatedly reversed polarity, as the nominal center field reached 9.17 T with a maximum peak current of 350 A. In a background field of 23.15 T, the insert coil generated a center field of 4.17 T with an applied current of 170 A. Ultimately, a total center field of 32.58 T was achieved before quench. Both the sequential model and the coupled model considering the perpendicular field modification due to conductor deformation are applied. The comparative study shows that, for this coil, the electromagnetic–mechanical coupling plays a trivial role in self-field conditions up to 9 T. In contrast, with a high axial field dominated by the background field, the coupling effect has a stronger influence on the predicted current and strain distributions. Further discussions regarding the role of background field on the strains in the insert suggest potential design strategies to maximize the total center field.

Superconducting properties and vortex pinning in intermetallic BaBi3 single crystals: A magnetization study

Hysteresis and magnetic relaxation measurements were performed on single crystals of BaBi3, which is an intermetallic compound exhibiting superconductivity with a critical temperature (Tc) of 5.9 K. The London penetration depth (λ) was determined to be 130 nm, and the coherence length (ξ) was estimated to be 13 nm. The critical current density (Jc) was found remarkably low, measuring 20,000 A/cm2 under self-field conditions at 1.8 K. Analysis of the pinning forces suggests vortex pinning by crystalline defects associated with volume Δk (variations in the Ginzburg-Landau parameter). The relaxation rate follows the Anderson-Kim model, and the pinning energies range from 1600 K at 0.02 T to 500 K at 0.5 T. Our study aims to advance our understanding of the connection between intrinsic superconducting properties and vortex behavior in intermetallic materials characterized by low thermal fluctuations.

Preliminary Electromagnetic and Structural Analyses of the Conductors and Clamps of the DTT Current Feeders

The Divertor Tokamak Test facility (DTT) is being constructed in the Research Centre of ENEA Frascati. This nuclear fusion device is devoted to tackle the power exhaust issue in the view of future fusion power plant. The conceptual design and the performances of the DTT superconducting magnet feeders, with NbTi cables, are evaluated based on coupled electromagnetic and structural analyses. We here present a preliminary structural assessment of the feeder clamps. We have considered the self-field condition and the higher force per unit length on feeder paths as evaluated from a 3D EM simulation. The stress state is evaluated by means of a 2D model with plane stress with thickness formulation. Further, the bolt dimension and class, together with the distance between two clamps, is provided in both self-field and maximum force per unit length conditions.

Nanodiamond Influence on the Nucleation and Growth of YBCO Superconducting Film Deposited by Metal-Organic Decomposition.

It was recently shown that the introduction of nanodiamond (ND) into a superconducting metal-organic deposited YBa2Cu3O7-δ (YBCO) film produces an increase in critical current density in self-field conditions (B = 0 T). Such improvement appears to be due to the formation of denser and smoother films than the samples deposited without ND. This paper presents the work done to understand the role of ND during YBCO nucleation and growth. A detailed study on YBCO+ND films quenched at different temperatures of the crystallization process was carried out. Results showed that the reaction responsible for YBCO production appeared effectively affected by ND. In particular, ND stabilizes one of the YBCO precursors, BaF2(1-x)Ox, whose conversion into YBCO requires a prolonged time. Therefore, the YBCO nucleation is slowed down by ND and begins when the experimental conditions favor both thermodynamically and kinetically the formation of YBCO along the c-axis. This effect has important implications because the growth of a highly epitaxial c-axis YBCO film enables excellent superconducting performance.

Thermal stability of 6-filament MgB2 wire with resistive CuNi sheath cooled by liquid He and water ice

In this paper, thermal stability of two 6-filament MgB2 wires with resistive CuNi sheath has been examined by DC transport measurements at the temperature of liquid He and variable external magnetic fields, and at variable temperatures of water ice under self-field conditions. Current-voltage characteristics up to the quench were performed and the operation limits of both wires under different cooling are analyzed and compared. The obtained results show that only 6 % of well conducting copper core allows to improve the thermal stability considerably, and it also limits the wire heating after the quench. MgB2 wires showed more stable operation after the quench under subcooled water ice in comparison to traditional Helium bath cooling, which offer a cheap and safe cooling mode for future devices.

The influence of HTS single-pancake coil with double strips winding structure on transmission loss under different self-field conditions

The transport loss generated by a high temperature superconducting coil (HTS) in an AC transmission process will affect the operational stability of the superconducting power equipment. However, under different self-field conditions, the influence of the oscillation waveform parameters and coil structure parameters on the transport loss still needs to be clarified. This article establishes a two-dimensional axisymmetric model with a single-pancake coil with a double strip winding as the research object. For four different oscillation waveforms—sine waves, square waves, triangular waves and sawtooth waves—the influence of four variables, namely, the transmission current amplitude, the coil radius, the number of turns of the coil, and the turn-to-turn spacing of the coil, on the transport loss of HTS coils is studied in detail. The results show that under the exact parameters of the coil, for the generated transport loss, the square wave is the largest, the sine wave is the second largest, and the sawtooth and triangular wave are the smallest. Under the same self-field condition, the transport loss can be reduced by increasing the coil radius, the number of turns of the coil, and the turn-to-turn spacing of the coil, in which if the number of turns of the single-pancake coil with a double strip winding is less than 14, the transport loss can be obviously reduced by increasing the number of turns of the coil. When the number of turns exceeds 14, it does little to reduce the transport loss. Research findings can confirm the investigation of the operational stability and design of HTS coils.

Bending Strain Sensitivity of Critical Current in REBCO Coated Conductor Winding for Coil Application at 77 K

High-temperature superconducting (RE)Ba₂Cu₃ O_7-x (REBCO)-coated conductor (CC) tapes are known for their capacity to tolerate high tensile stress under uniaxial tension, which is typically in the range of 600-700 MPa at 77 K. However, when used in a coil, there is a limit to the allowed strain before degradation of the critical current (<i>I</i>_c) occurs. This is attributed to the combined strain due to bending and axial tension which was caused during fabrication, cool down, and operation of coils and magnets and corresponds to the irreversible strain limit of <i>I</i>_c degradation. Applied strain along the CC tape&#x0027;s edgewise direction (hard bending strain) also affects the irreversible strain limits of <i>I</i>_c considerably. With such conditions, degradation of the REBCO coil performance can be expected. Therefore, the current study aims to systematically investigate the mechanism for such <i>I</i>_c degradation in REBCO coils. Linear superposition of strains induced in different modes of easy and hard bending were studied analytically and experimentally. The <i>I</i>_c degradation behaviors were examined in IBAD&#x002F;REBCO CC tapes in windings with different pitches on mandrels with different diameters. The investigation was done at 77 K and in self-field conditions using an apparatus designed for coil testing. The bend strain parameters in coils were analyzed, including the radius of curvature and pitch between CC tape turns. Through the linear superposition of strains induced in the REBCO coils, an appropriate approach is suggested to effectively handle the <i>I</i>_c degradation under winding including edgewise bending.

Subscale HTS Fusion Conductor Fabrication and Testing in High Magnetic Background Field

In actual or future fusion machines like ITER, JT-60SA, W7-X or the present EU-DEMO design, low temperature superconductor (LTS) cables are used or planned to be used in the magnet system to create the strong magnetic field necessary for plasma confinement. High temperature superconductors (HTS) offer the potential to increase the magnetic field strength in fusion magnet systems even further, allowing to build more compact fusion machines or to realize higher flux swings in central solenoids. In this work, two HTS conductors were fabricated and characterized by high current testing at 77 K in self-field conditions and at 4.2 K with an external magnetic field up to 12 T. Both samples offer the same amount of superconducting HTS tapes but differ in the assembly process and termination type. These tests were intended to validate the current KIT design of a HTS CroCo triplet conductor and to qualify a low degradation cable manufacturing process.

Water ice-cooled MgB2 coil made by wind and react process

A wind and react (W&R) coil of inner diameter 53 mm has been made from multi-core MgB2/Nb/CuNi wire manufactured by the internal magnesium diffusion (IMD) process. The W&R coil is wound from non-insulated rectangular wire of 1 mm2 with only 5 µm thick stainless steel foil used for interlayer insulation. The transport current performance of the coil and short wire samples was measured in a liquid He bath at external magnetic fields of 4.5–8.5 T and also in self-field conditions in sub-cooled water ice at temperatures between 33 K and 38 K. The presented MgB2 coil exhibits stable behavior at water ice cooling, and its high space factor allows a high current density of winding in comparison to the data from the already published MgB2 coils. The presented results demonstrate that MgB2 windings can be used safely in He-free conditions inside sub-cooled water ice, and this technique can be further optimized and used for future MgB2 coils.

The Influence of Hts Single-Pancake Coil with Double Strips Winding Structure on Transmission Loss Under Different Self-Field Conditions

The Influence of Hts Single-Pancake Coil with Double Strips Winding Structure on Transmission Loss Under Different Self-Field Conditions

Anisotropy of Critical Current in QIS-CICC HTS Conductors

Based on existing quasi-isotropic strand (QIS) conductor and twisted stacked tape cable–cable in conduit conductor (TSTC-CICC), this paper proposed a novel QIS-CICC. The conductor was numerically modeled and the anisotropy of critical currents was analyzed. The simulations showed that although the critical currents of the QIS-CICC were worse than those of the TSTC-CICC for the self-field condition, the current decay of the QIS-CICC was far less than that of the TSTC-CICC under a 1 T external field, and the anisotropies of the two conductors were almost the same.

A wide range E − J constitutive law for simulating REBCO tapes above their critical current

When modeling superconducting devices based on rare earth barium copper oxide (REBCO) tapes and operating near or above the critical current value I c, the power-law is not always accurate. In our previous works, we proposed the overcritical current constitutive law, based on a combination of fast pulsed current measurements and finite element analysis. The overcritical current constitutive law was provided in the form of look-up tables and was validated experimentally. We showed that the overcritical current constitutive law could better reproduce experimental measurements than the power-law, and that the power-law predicts a faster quench than the overcritical current constitutive law. In this contribution we use a mathematical expression based on the collective pinning model to analytically describe the overcritical current regime of REBCO tapes based on measurements performed between 77 and 90 K in self-field conditions. The wide-range constitutive law is verified by comparing DC fault measurements with the results of numerical simulations using the overcritical current constitutive law to represent the electrical resistivity of the superconducting layer of REBCO tapes.

Impact of Bending on the Critical Current of HTS CrossConductors

Fusion applications require high magnetic fields for plasma confinement, which results in magnet coils with high operational currents. To achieve these high currents, high temperature superconductor (HTS) strands like the HTS CrossConductor (HTS CroCo) are an option. However, to reach the required high currents, multiple HTS CroCo strands have to be assembled as cable. These cabling processes as well as the winding of the magnet coil induce mechanical strain in the HTS CroCo. In this work the results of modelling critical currents of HTS CroCos at different bending radii are discussed and validated by experiments. The simulation uses the finite element method and is capable to calculate the critical current of the ideal HTS CroCo at different bending radii as well as predicting the possible range of the critical current degradation due to production tolerances of the HTS CroCos. For the experimental tests the examined samples were bent to radius at room temperature, afterwards the critical current measurement took place at T = 77 K and self-field conditions. Each sample was characterized at different bending radii in decreasing order. After the sequence of measurements, cross-sectional images of the samples were prepared to analyze macroscopic damage of the HTS CroCo.

Below 1 µV cm−1: determining the geometrically-saturated critical transport current of a superconducting tape

In the critical state model, the critical current, , of a superconductor defines the upper limit of dissipation-free current flow. However conventional transport measurements of practical superconductors rely on the detection of a voltage drop along the length of the conductor. This requires that the superconductor has entered the dissipative regime, and hence inherently over-estimates the current at which geometric saturation occurs. Nonetheless, the convenience of the 1 µV cm−1 criterion means that it has become the widely adopted definition of transport . Here, we present an alternative definition for the transport critical current of a superconducting tape under self-field conditions, which is based on the concept of geometric current saturation across the full cross-sectional area. This saturation threshold can be experimentally determined through simple Hall sensor measurements of the evolving perpendicular magnetic field at the tape surface. The surface field exhibits a signature transition when the transport current increases beyond the current-saturation threshold. We present an analytical model which describes this effect and defines the critical saturated current as a function of . This definition is first validated using finite element (FE) modelling, and then experimentally demonstrated through measurements on a variety of commercial REBCO and Bi-2223 tapes of differing widths. It is found that the 1 µV cm−1 criterion consistently leads to an overestimation of the saturated critical current by ∼15% in REBCO tapes, and up to 30% in a Bi-2223 tape. FE modelling indicates that this overestimate is most prominent in superconductors exhibiting a low n-value (i.e. n ≲ 20). A key advantage of the measurement approach presented here is that it allows the unambiguous measurement of a transport value to be completed at lower currents, without entering the dissipative region in which sample damage can occur. There are also implications as to the correct choice of value which should be employed within the well-known Norris and Brandt equations for AC loss.

Cooper pair trajectories in superconducting slab at self-field conditions

Dissipative-free electric current flow is one of the most fascinating and practically important properties of superconductors. Theoretical consideration of the charge carriers flow in infinitely long rectangular slab of superconductor in the absence of external magnetic field (so called, self-field) is based on an assumption that the charge carriers have rectilinear trajectories in the direction of the current flow whereas the current density and magnetic flux density are decaying towards superconducting slab with London penetration depth as characteristic length. Here, we calculate charge particle trajectories (as single electron/hole, as Cooper pair) at self-field conditions and find that charge carriers do not follow intuitive rectilinear trajectories along the slab surface, but instead ones have meander shape trajectories cross the whole thickness of the slab. Moreover, if the particle velocity is below some value, the charge moves in opposite direction to nominal current flow. This disturbance of the canonical magnetic flux density distribution and backward movement of Cooper pairs can be entire mechanism for power dissipation in superconductors.

Resistivity of REBCO tapes in overcritical current regime: impact on superconducting fault current limiter modeling

A detailed knowledge of the resistivity of high-temperature superconductors in the overcritical current regime is important to achieve reliable numerical simulations of applications such as superconducting fault current limiters. We have previously shown that the combination of fast pulsed current measurements and finite element analysis allows accounting for heating effects occurring during the current pulses. We demonstrated that it is possible to retrieve the correct current and temperature dependence of the resistivity data points of the superconductor material. In this contribution, we apply this method to characterize the resistivity vs. current and temperature of commercial REBCO tapes in the overcritical current regime, between 77 and 90 K and in self-field conditions. The self-consistency of the overcritical resistivity model is verified by comparing DC fault measurements with the results of numerical simulations using this model as input. We then analyze by numerical simulation to what extent using the model instead of the widely used power-law model affects the thermal and electrical performance of the tapes in the practical case of a superconducting fault current limiter. A remarkable difference is observed between the measured overcritical current resistivity model and the power-law resistivity model . In particular, the simulations using the power-law model show that the device quenches faster than with the overcritical resistivity model. This information can be used to optimize the architecture of the stabilizer in superconducting fault current limiters.