Critical Current Degradation Research Articles

Reliability Evaluation Procedure of Electromechanical Properties in GdBCO CC Tapes Obtained by Uniaxial Tension and Fatigue Tests at 77 K

In the design of 2G superconducting magnets and coils, both the yield strength and the irreversible stress limit for critical current ( I c ) degradation of coated conductor (CC) tapes under various environmental conditions have been investigated. Equally important and yet often discounted are the reliability tests to evaluate the I c degradation induced by the fatigue loading expected during operation. Because I c degradation at the stress level below the reversible stress limit of the CC tapes is possible (e.g., under a static fatigue load), in this study, the procedures for evaluating the reliability of 2G CC tapes using characteristic limit values obtained through a series of electromechanical properties evaluations are introduced. I c behaviors were obtained at 77 K through specified loading against the elapsed time in a static fatigue test, while electromechanical fatigue limits of CC tapes were determined at a stress ratio (R) of 0.1 in a high-cycle fatigue test. The determination of electromechanical properties through uniaxial tensile and fatigue tests was established, and its long-term reliability was examined.

Stress, strain and electromechanical analyses of (RE)Ba2Cu3Ox conductors using three-dimensional/two-dimensional mixed-dimensional modeling: fabrication, cooling and tensile behavior

High temperature superconducting (HTS) conductors, represented by Rare Earth-Barium-Copper-Oxide (REBCO) conductors, are promising for high energy and high field superconducting applications. In practical applications, however, the HTS conductors experience different stresses and strains, including residual stresses due to thermal mismatch and tensile stresses due to Lorentz forces, resulting in some circumstances to a reduction in the load-carrying capacity as well as the risk of degradation in conductor critical current. In this study a mixed-dimensional high-aspect-ratio laminated composite finite element model for REBCO conductor is developed for stress and strain analyses in the processes of fabricating and cooling, as well as tensile testing. The model includes all the major constituent layers of a typical REBCO conductor and is experimentally validated. First, the thermal residual stresses and strains accumulated during the fabrication and cooling processes are analyzed by a multi-step modeling method that emulates the manufacturing process. Then, with the residual stresses and strains as initial stresses and strains, the mechanical behavior under a tensile load is studied. Lastly, a phenomenological critical current-strain model based on the Ekin power-law formula and the Weibull distribution function is combined with the mixed-dimensional conductor model to predict the strain dependence behavior of critical current in the reversible and irreversible degradation strain ranges. Simulation results show that the multi-step modeling is an effective method for stress and strain analyses of REBCO conductors during the fabrication and cooling processes and under and tensile loads. Compressive thermal residual stress generated on the REBCO layer during fabrication and cooling strongly affects the subsequent mechanical and current-carrying properties. Stress–strain curves generated by tensile loads are analyzed and experimentally validated at both the conductor and constituent-layer levels. Simulation results for the strain dependence of critical current are in good agreement with experiment data in both the reversible and irreversible degradation stages.

Delamination and current-carrying degradation behavior of epoxy-impregnated superconducting coil winding with 2G HTS tape caused by thermal stress

A thermo-mechanical-electromagnetic model is developed for estimating the delamination and current-carrying degradation of epoxy-impregnated pancake coils. The mixed-mode traction–separation law and the Weibull distribution of delamination strength are considered in simulating the onset and extension of the delamination caused by thermal stress. Because of the considerable differences in thermal shrinkage between the epoxy resin, insulation tape, and second-generation high temperature superconducting (2G HTS) tape, the accumulated radial thermal stress locally exceeds the electro-mechanical delamination strength and even the mechanical delamination strength. The electro-mechanical delamination strength of the 2G HTS tape is the transverse tensile or shear stress level corresponding to an abrupt irreversible critical current degradation. The mechanical delamination strength is the transverse tensile or shear stress limit corresponding to a structural failure. After injecting current into the coil, we obtained the E–I curve of the coil. The critical current and n-value of the superconducting coil indicate a strong degradation after epoxy-impregnation. The current-carrying degradation precedes delamination because the electro-mechanical delamination strength is smaller than the mechanical delamination strength. The degradation is more obvious in large than in small superconducting coils because the radial thermal stress is larger. The onset of degradation depends on the minimum delamination strength, suggesting that caution is required in screening the 2G HTS tape before winding the coil. The simulation results indicate that reducing the thickness of the insulation tape and the amount of epoxy resin effectively reduces the degradation of epoxy-impregnated pancake coils.

AC losses in the development of superconducting magnetic energy storage devices

Abstract AC losses are inevitable to be considered for effective design of Superconducting Magnetic Energy Storage (SMES) devices using High Temperature Superconductors. Various analytical techniques are available to estimate these AC losses however not sufficient to accurately predict the same. Hence, computational methods are being widely used in the prediction of AC losses as the experimental techniques are complicated to be implemented. YBCO (Tc = 90 K @ 0T) and BSCCO (Tc = 110 K @ 0T) are generally used in the construction of SMES. However, the critical temperature reduces as the external magnetic field is experienced by the tapes increases giving lower temperature margin for cooling purposes. In the present work, Computational Electrodynamics is used to predict the AC losses in Second Generation HTS tapes used in constructing the SMES. These losses which are dissipated as heat must be balanced by the cooling the requirements. The effect of external magnetic field on the degradation of critical current is examined. Finite Difference Time Domain approach is used for discretization of governing equations in solving for the AC losses. Further, electric field distribution in coated conductors due to the applied magnetic field is investigated. Hence, the results of the present analysis guide the thermohydraulic design of SMES devices.

A new method for avoiding critical current degradation of YBCO coils using ice impregnation

For mechanical reinforcement against the huge Lorentz forces in high magnetic fields, epoxy resins are commonly used to impregnate high temperature superconductor (HTS) coils. Due to the different thermal expansion coefficients between the epoxy resin and the HTS tape, the contraction of the epoxy resin causes critical current degradation in the HTS coils during the cooling process. Alternative techniques, such as coils with no insulation, can avoid current degradation, however their structure has no reinforcement between layers. In view of this, we tried to apply the ice impregnation technique of pulsed magnets to HTS coils for the first time. HTS coils impregnated with ice could avoid critical current degradation after thermal cycles. Also, local heat pulse tests and overcurrent tests showed that the ice impregnated coil had better thermal stability than the epoxy impregnated coil. Moreover, the fabrication process of the ice impregnated coil is simple and economical. Meanwhile HTS tapes can be reused after impregnation. The ice impregnation technique is a promising prospect for high temperature superconducting coils.

Design and Experimental Research on Self-Shielding DC HTS Cable Model With Large Current Capacity

DC high-temperature superconducting (HTS) cable has advantages over the ac HTS cable in the power transmission due to its zero Joule loss, higher current capacity, and controllability. To minimize the magnetic field influence and critical current degradation in dc HTS cable, we have ever proposed a new structure of dc HTS cable that is, self-shielding dc HTS cable which has interleaved winding configuration. In this paper, in order to solve the difficulty of current lead connection with HTS layer in the cable terminal with multilayers, we put forward a new kind of terminal configuration which can make the current in each layer being controlled individually and avoid soldering in terminal as well as be convenient for installing and disassembly. With this kind of terminal connection method, a dc HTS model cable with eight layers and 1900 mm in length is fabricated and tested in liquid nitrogen temperature. Uniform current distribution among layers is realized by adjustable resistance connecting with each layer. The experimental results show that the critical current degradation of self-shielding dc HTS cable is only 7% compared with the critical current in self-field while the traditional structure of dc HTS cable is about 20%, which indicates that the self-shielding cable can promote the current capacity effectively with lower critical current degradation and save more HTS tapes in the application of power transmission. So, this kind of cable with a thinner insulation layer is suitable for mid- and low-voltage situations, especially for industrial and ship situations where large current and low voltage are required.

Investigation on I C Degradation of MgB2 Rutherford Cables by Deformation During Cabling Process

MgB2 superconductors are promising candidates for application to devices such as Superconducting Magnetic Energy Storage, and generators. To apply MgB2 conductors to such devices, the current capacity of a conductor must be in the kilo-ampere range. Meanwhile, because the current capacity of a MgB2 conductor is typically approximately 100 A at 5 T and 4.2 K, multiple stranded cables are required. One candidate is the Rutherford-type cable. During the fabrication of Rutherford cables, strands are deformed by large bending strains at edge corners and indented at flat parts from the pressure of roller dies to maintain the cable shape. It is important to understand how critical current degrades during the fabrication of Rutherford-type cables. To optimize the strand transposition length, three types of Rutherford cables were fabricated and the critical current degradation depending on bending and indented strains was measured. Moreover, to investigate the degradation, inner structures of the strands were observed using micro-focused X-ray computed tomography and an electron probe micro analyzer.

Comparison of Heat Assisted Lap Joints of High-Temperature Superconducting Tapes With Inserted Indium Foils

Easy and simple fabrication of a lap joint of high-temperature superconducting (HTS) tapes is desirable for various HTS applications such as power cables and current feeders. We have developed mechanical joints with a low-temperature heat treatment at around 370 K where indium foils are inserted between joint surfaces. Though the joint showed a good performance, it took over 10 min to fabricate one joint and ideally the fabrication time should be further shortened. In this study, we proposed to use ultrasonic welding (UW) to assist a heat generation at the joint for lap joints with indium foil, which can be completed within 1 s. We prepared various lap joint samples of HTS tapes: REBCO (REBa 2 Cu 3 O 7 , RE: rare-earth) or BSCCO (Bi 2 Sr 2 Ca 2 Cu 3 O 10 ) tapes were pressed together with an indium foil using a UW machine or pressing machine with cartridge heaters. Then, the current-voltage curve was evaluated at 77 K and the joint resistance characteristics were discussed depending on the joint methods and HTS tapes. The samples with copper-stabilized REBCO tapes all showed joint resistivities of 31.6-35.6 nΩcm 2 without critical current (I C ) degradation for both joint methods. The samples with silver-sheathed BSCCO tapes showed joint resistivities of 18.6-46.5 nΩcm 2 for both joint methods; however, some of those fabricated by the UW showed I C degradation. The degradation was able to be prevented by optimizing UW conditions.

Impact of a REBCO coated conductor stabilization layer on the fault current limiting functionality

Reliability is one of the crucial requirements of conductors used in resistive superconducting fault current limiters. Possible critical current degradation of a REBCO coated conductor during the limiter operation restrains broader application of devices based on these conductors. In this article the impact of an electrical stabilization layer on the conductor protection against overheating as well as its effect on electrical current evolution during the limitation period are investigated. For the experimental part of the study, a commercial copper stabilized REBCO coated conductor and a modified conductor without copper stabilization are compared. Extensive experimental work complemented by electro-thermal numerical modeling paved the way for studying the electrical and thermal effects separately. The numerical model assuming adiabatic conditions is sufficient to reproduce experimental results and predict the peak temperature for conductors with various stabilization layers in realistic conditions. Reduction of the conductor critical current caused by multiple current limitation pulses was studied using pulses with various durations. It was observed that the degradation due to temperature rise is gradual with the rate depending solely on the peak temperature. It is this quantity through which other parameters like the pulse duration, the thickness of stabilization and the peak current cause the tape damaging.

Design optimization and evaluation of the 3 kA MgB2 cable at 4.3 K for the superconducting link project at CERN

High-current superconducting links (SC links) are being developed at CERN for powering the superconducting magnets of the High-Luminosity Large Hadron Collider upgrade. The links contain MgB2 cables, each about 100 m long, rated at currents of up to 18 kA. The assembly of the cables is carried out with reacted wires; cable geometry and cabling processes take into account the mechanical properties of the reacted MgB2 conductor. The production of the cables as well as their final installation requires good mechanical stability, flexibility and electrical performance. The superconductor should have good mechanical strength in view of the stresses during cabling (tension and bending), operation at cryogenic temperatures (Lorentz forces and thermal contraction) and handling installation (tension and bending) at room temperature. Moreover, the electrical integrity of the MgB2 wire (IC degradation ≤5%) should be ensured after the cabling process for the SC links. Therefore, the study of the mechanical properties of the MgB2 wire and the definition of the cable design parameters are crucial for the project. In the present work we report on the optimization and validation of the design of the 18-strand, 3 kA MgB2 cable by assessing the electromechanical behaviour of 3 kA MgB2 superconducting cables produced with different cabling parameters. The critical current (IC) of the MgB2 wires was measured after triple bending and axial tensile load, and strands extracted from the cables were tested to quantify the critical current degradation. Electrical measurements of two, 2 m long, 18-strand MgB2 cables were carried out in the FRESCA test facility at CERN and no degradation was observed. Cabling parameters such as the bending radius and the twist pitch were evaluated, and the most appropriate geometries for the MgB2 18-strand cables have been selected. Numerical simulations were performed to allow a detailed study of the strains developed in the MgB2 strands due to the mechanical loading. The present results have been successfully transferred to industry for the first industrial production in the framework of the SC Link project.

Critical currents in REBaCuO superconducting tapes in response to neutron irradiation

Electromagnetic performance of superconducting REBaCuO tapes from several manufacturers was investigated in response to neutron irradiation. The aim was to test the feasibility of using these tapes for wiring high-Tc superconducting magnets for fusion reactors. Magnetic induction was employed to determine critical currents. In non-irradiated SuNAM, SuperOx, and SuperPower tapes without additional pinning centers, the normalized pinning force density as a function of the normalized magnetic field, h, exhibited the commonly observed single peak at about h = 0.2 in the whole temperature range 10 K–80 K. In the non-irradiated AP (advanced pinning) SuperPower tapes this dependence developed an extraordinary shape below 70 K. This feature changed with increasing neutron fluence. The effect of neutron irradiation on critical currents was found to be rather complex. In all samples, neutron irradiation by a total fluence of 8.03 × 1022 m−2 caused a critical current degradation at low magnetic fields in the temperature range 10 K–77 K. At high magnetic fields, the effect varied depending on the tape’s initial pinning structure, temperature, and neutron fluence. At 77 K, the current decreased and the irreversibility field was reduced in all the samples. At intermediate temperatures the critical current nearly did not change, while at temperatures below 30 K the critical current increased.

Effect of resin impregnation on the transverse pressure dependence of the critical current in ReBCO Roebel cables

ReBCO Roebel cables are being developed as one of the promising routes towards much higher-field magnets for future particle accelerators. The Lorentz forces generated in such magnets lead to sizeable transverse pressure on the superconducting cables, in the present 15–20 T magnet designs of the order of 100–150 MPa. Contrary to single ReBCO coated conductors, unprotected Roebel cable due to their uneven surface, start to degrade already with 40 MPa as shown in earlier work. Previous publications also showed that proper impregnation with epoxy resin can dramatically improve the transverse pressure tolerance of a relatively simple model cable with 10 ReBCO strands, assembled from SuperPower tape. However, the epoxy resins used until now are not suitable for large-scale magnet vacuum impregnation. For this reason, an alternative impregnation method using CTD-101K with S2 glass fibers has been implemented. The present paper expands further on the effect of impregnation on the transverse pressure susceptibility of Roebel cables. Three cables, with an architecture that is directly relevant for the so-called EuCARD-2 accelerator demonstrator magnet, were investigated with a variable transverse mechanical load at 4.2 K in a 10.5 T perpendicular magnetic field. The new cables feature a relatively long transposition length of 226 mm and comprise 15 strands of either SuperPower or Bruker coated conductor. For reference, one of the new SuperPower tape based cables was impregnated with the same resin as the one used in the earlier publications. The other cables were impregnated using the new method. The critical current at 4.2 K of the Bruker tape Roebel cable is 2.5 to 3 times higher than that of the SuperPower tape ones with similar architecture. Remarkably no critical current degradation was observed for transverse pressure up to 440 MPa in the SuperPower tape cables and up to 370 MPa in the Bruker tape cable. These values by far satisfy the design requirements of presently envisaged 20 T class accelerator demonstrator magnets.

Experimental and numerical study of the frequency-dependent transport ac losses of the YBa2Cu3O7−δ coil with and without flux diverters

This paper focuses on the study of the frequency-dependent transport ac loss of YBa2Cu3O7−δ (YBCO) coils and how it is influenced by flux diverters. A single pancake coil is wound by AMSC 8501 tape and its transport ac losses (in the frequency range of 50–1000 Hz) are investigated through the use of both experimental and finite element methods. The finite element model is based on the H-formulation and E-J power law, and it takes both the magnetic field dependences of critical current density and degradation of critical current density close to the edges into account. Different loss components (namely hysteresis losses in HTS layers, ferromagnetic losses in weakly magnetic substrates, eddy current losses in conventional metal and ferromagnetic losses in flux diverters) of YBCO-NiW coils at different frequencies are calculated by using the numerical model. The calculated ac losses accord well with the measured ones, which validates the numerical model. The influence of the flux diverter on different loss components at low/high frequencies is analyzed numerically. It is found that the flux diverter greatly decreases the hysteresis loss per cycle, but causes an enhancement of both eddy current and ferromagnetic losses per cycle. We propose a new parameter to clarify the region where flux diverters can effectively decrease the transport ac loss per cycle of the YBCO-NiW coil.

WC contamination or addition effect on the superconducting properties in ex situ processed MgB2 tapes using unmilled and ball milled filling powders

Effect of WC addition or contamination to MgB2 on the superconducting properties of ex situ processed sintered powders and tapes was studied systematically. Impurities such as MgB4 start to appear clearly in the mixtures of MgB2 and WC powders in the unmilled and milled states when sintered at 900 °C and 700 °C, respectively. Sintering of both unmilled and milled mixtures do not bring about carbon substitution in MgB2. The critical temperature (Tc) reduction, however, is clearly observed for tapes using the milled mixtures with WC addition amount of not less than 10 mass%, although tapes using the unmilled mixtures do not show appreciable reduction at all. The transport critical current (Jc) property degradation is clearly observed only for tapes using the milled mixtures with the addition amount of not less than 10 mass%. This degradation is attributed to the Tc reduction of those tapes.

CORC-like cable production and characterization of the solenoid made from it

A new cabling machine was constructed for conductor on round core (CORC)-like cable production. Its design allowed us to produce a cable with two layers in one run, each comprising four superconducting tapes. The cable of length of 40 m was made at a production rate of 6 m h−1 by this machine. It consisted of eight Furukawa-SuperPower tapes of width 4 mm and length 50 m wound in two layers on Cu tube with outer diameter 6.35 mm. To detect the influence of the cable winding technology on the superconducting tapes’ properties their critical currents were measured individually on the as-produced cable in the form of a one-layer coil. Afterwards the critical current of the one-layer coil was measured by monitoring the voltages on all tapes while increasing the coil current. The average of these voltages was utilized for coil critical current determination. Then the solenoid with four layers each containing 20 turns was created from the cable and the same measurement procedure was performed to check the influence of cable bending on the solenoid form. The results of these characterizations exclude critical current degradation for any tape of the cable used for solenoid preparation. The measured solenoid critical current of 622.8 A is lower than the simple sum 1240 A of the individual tapes’ critical currents due to the dependence of the tape critical current on the magnetic field. For all measurements the investigated object was immersed into a liquid nitrogen bath. In the case of the solenoid the bath was also cooled down to 66 K.

Effect of transverse compressive monotonic and cyclic loading on the performance of superconducting CORC® cables and wires

Superconducting CORC® cables and wires have become practical conductors for use in high-field magnets for fusion machines and particle accelerators by demonstrating their ability to carry very high currents in background magnetic fields of up to 20 T. The high mechanical stresses that develop on the CORC® conductor during such operation could result in permanent degradation of the conductor critical current. Transverse compressive stress is one of the predominant mechanical stresses when CORC® cables or wires are bundled into CICCs that allow fusion and detector magnets to operate at currents as high as 100 kA. The effect of transverse compressive load on the critical current of CORC® cables and wires has been investigated at 76 K to determine their irreversible load limit under monotonic loading and load cycling up to 100 000 cycles. The results show a clear effect of the CORC® conductor layout with respect to gap spacing between tapes, thickness of the copper plating surrounding the tapes and thickness of the former onto which the tapes are wound. CORC® cables and wires have demonstrated a remarkable resilience to transverse compressive load cycling where their critical current decreased by no more than a few percent after 100 000 load cycles at peak loads that resulted in an initial decrease in critical current of less than 5%. The results indicate that no significant degradation of CORC® cable and wire performance due to transverse compressive load is expected in large magnets after 100 000 load cycles, as long as the peak load on the conductor does not exceed the irreversible load limit defined at 95% retention in critical current. The irreversible load limit of CORC® conductors could be increased further by increasing the size or hardness of the former that makes up the conductor’s core.

The winding mechanical behavior of conductor on round core cables

In the process of the HTS cable structure cabling, the HTS tape is subjected to the combined deformation caused by tension, bending, and torsion. The electro-mechanical behaviors and the degradation mechanism of HTS cable are remain elusive. In the present paper, a numerical model is built in ABAQUS software to analyze the winding mechanical behavior of the CORC cable. Then, the critical current degradation is calculated and the effects of the tape width, substrate thickness, pre-stress, winding pitch and helix angle are investigated. The results show that it should be avoid selecting a roller with a much small radius to produce a large strain. The critical current degradation of the cable is minimal when the helix angle equals to 45°, which is the best choice to winding the tape to form the CORC cable. A smaller winding radius can be choose when the thickness of the substrate is in the small strain region of 10–20 µm. This study helps understand the deformation of the HTS tape during the winding process and provides a winding optimization for the CORC cable.

Critical Current Degradation of Coated Conductors Under Soldering Conditions

The critical current degradation of coated conductors was investigated at 77 K s.f., after heating the samples in a solder bath to temperatures between 200 °C and 300 °C for durations between 30 s and up to 2 h to simulate possible soldering conditions. We found that the critical current degradation is strongly dependent on temperature and time, but independent from the number of thermal cycles. At 225 °C for 10 min a degradation of about 5% was measured. At 300 °C for 10 min the degradation was about 45%. Slight variations among different manufacturers were observed. Across production batches of the same manufacturer no variations were found for tapes of the same structure. Slight variations of tapes with different stabilizer thicknesses were detected after heat treatment.

Computation of the Reversible Critical Current Degradation in Nb $_{3}$Sn Rutherford Cables for Particle Accelerator Magnets

Superconducting coils limit current is usually measured on unloaded short samples of strands extracted from the same cable used to wound the coil. However, the critical current density of Nb 3 Sn strands is strongly dependent on the applied strain. This dependence is well reproduced by the so-called exponential scaling laws on strands subject to uniaxial loading. Unfortunately, no standard procedure exists to apply these laws under the complex stress condition experienced by the strands inside Rutherford cables for particle accelerator magnets. As a consequence, an empirical limit from magnet tests was set in the past: 150-200 MPa of maximum equivalent stress within the coil, considered as a block of uniform material. It is not clear how this number relates with the actual stress/strain state in the strands and if it is possible in magnet applications to infer the coil degradation from single strands tests. In this context, this paper presents a model to estimate the critical current degradation of superconducting coils as a function of the applied strain. The model results were compared against the critical current degradation measured on impregnated cable stacks subject to transverse pressure.

Study on the Nb3Sn Rutherford Cable for High-Energy Accelerators

This paper studies the development of the Nb 3 Sn Rutherford cable for high-energy accelerators of Western Superconducting Technologies Company Ltd. (WST). In the first stage of cable development of WST, the cable is made of 18 Nb 3 Sn strands with a strand diameter of 0.818 mm. The experiment focuses on the cable parameters (filling factor of 83-91%, and a pitch length of 45-60 mm) to achieve both a minimal critical current degradation of the strand and a good mechanical stability of the cable for coil winding. Furthermore, the cable parameters laws are proposed. After cabling, the J c degradation is smaller than 5% and RRR (273 K/20 K) is more than 200. Last but not least, the results of the critical current degradation, the cabling parameters, and the microstructure of Rutherford Cable are discussed.