Stacked Tapes Research Articles

Evaluation AC Losses in Large-Scale Conductors Consisting of Stacked REBCO Tapes

Evaluation AC Losses in Large-Scale Conductors Consisting of Stacked REBCO Tapes

Rotating magnetization method for inspection of local defect in HTS conductor

High temperature superconducting (HTS) magnet used for a accelerator for high energy physics or a large-scale magnetic confinement system for nuclear fusion requires large current capacity HTS conductor in order to dump the stored magnetic energy quickly to protect the coil when quench has been occurred. To realize the HTS conductor, superimposed or stacked HTS tapes conductors are invented. However, local degradation of the critical current has been observed in the production of HTS conductors. Therefore, to confirm the soundness of the HTS conductor, it is necessary to establish the non-destructive detection method for the observation of degraded HTS tapes. In this study, we have investigated to inspect the degradation position in stacked HTS tapes by adopting rotating magnetization measurement method. This measurement method is measuring magnetization signal by rotating and transporting stacked HTS tapes in a static magnetic field. As a result, intentionally introduced buckling defect positions were detected by this measurement method.

Analysis of a Possibility to Use Parallel Non-Twisted Stacks of HTS Tapes as Cable in High Current Conductor of Tokamak Toroidal Field Coils

Development of high current cable-in-conduit conductors (CICC) based on HTS tapes aimed for fusion magnets has started recently. The majority of the suggested CICC use as a HTS cable either, twisted stacks of HTS tapes (stacked technology), or some layers of tapes wrapped around a cylindrical core (CORC technology). An idea to use parallel non-twisted stacks of HTS tapes in a cable design is received skeptically by the fusion community, although such a cable design for tokamak toroidal field (TF) coils has some advantages over twisted ones. The twofold benefits is determined by parallel (deviation- θ <; 5°) orientation of toroidal field vector to surface of HTS tape and mutual orientation of background field and operating current vectors along conductor. Due to such orientation the critical current of tape-Ic(B;T) is considerably (in 5-6 times) higher, due to an anisotropy of HTS tape electro-physical properties. In addition a compressive force is oriented in the most beneficial direction. The main arguments against the non-twisted stacks in the cable design is a concern that the favorable condition described above are not the same in every location of TF coil, or that they will not be favorable in all modes of tokamak operation and the time of TF magnet charging is unreasonably long because of the screening current. The analysis carried out shows that the negative effect of screening currents decreases significantly due to a wavelike magnetic field distribution along conductor and that suggested design of HTS CICC is expected to be successful at any tokamak operation scenario.

YBCO Coated Conductor Interlayer Electrical Contact Resistance Measured From 77 K to 4 K Under Applied Pressures up to 9.4 MPa

The inter-tape resistance is a key parameter for REBCO coated conductors, cables, and coils for various applications. We explored the transverse resistance for sets of REBCO tape stacks, measured in a “through-the-stack” configuration. For the first measurements (Rig 1), the stacks were cooled to LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> temperature and the tape stack resistance was measured as a function of applied pressure (i.e., the pressure was applied during the measurement) up to 417 kPa. We analyzed these direct measurements to examine the contributions of the tape surfaces vs the internal tape contributions, and the results were translated into a contact efficiency, η, defined as η = R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> *A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> (the contact resistance * contact area). It was found that the electrical contact efficiency, η, for an un-pressurized stack was ≅ 0.934 Ω·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , but was reduced by a factor of 8 under 417 kPa of pressure to η ≅ 0.111 Ω·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . A second test rig was used to explore tape stack resistances from 4.2 K - 77 K. This rig allowed pressures of up to 9.94 MPa to be applied. Here, the pressure was first applied, and then the sample was cooled. This difference between Rig 1 and Rig 2 allowed us to compare the protocols of fixed pressure (Rig 1) and fixed constraint (Rig 2). The contact efficiency reached 178 μΩ·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 4.2 K and 232 μΩ·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 77 K.

The Magnetization of HTSC Tapes Stack in the Flux Pump Regime

In the present paper, the possibility of magnetization of HTSC tape by means of pumping by a local magnetic field is considered. Within the framework of the critical state model, the calculation of the density of the induced currents and the magnetization of a square (12 × 12 mm) fragment of a second-generation HTS tape (YBCO) was made. The model considers real dependence of the critical current density on the magnetic field and has been successfully tested previously. In the sample, the currents were induced by the impact of a local magnetic field. The size of the localized area of the external magnetic field to be considered was much smaller than the size of the HTS sample. It is showed that the repeated cyclical impact of the local outer field leads to the accumulation of the total magnetic moment in the sample up to a magnitude of 94% of the theoretical magnetization limit for this sample. The results obtained make it possible to optimize the mode of magnetization of a square fragment of a tape (or a stack of tapes) when implementing a magnetic flux pump. The experimental studies showed results comprisable well with theoretical ones.

Spectral fusing Gabor domain optical coherence microscopy based on FPGA processing

High-resolution imaging using high numerical aperture imaging optics is commonly known to cause a narrow depth of focus, which limits the depth of field in optical coherence tomography (OCT). To achieve semi-invariant high resolution in all directions, Gabor domain optical coherence microscopy (GD-OCM) combines the in-focus regions of multiple cross-sectional images that are acquired while shifting the focal plane of the objective lens. As a result, GD-OCM requires additional processes for in-focus extraction and fusion, leading to longer processing times, as compared with conventional frequency domain OCT (FD-OCT). We previously proposed a method of spectral domain Gabor fusion that has been proven to improve the processing speed of GD-OCM. To investigate the full potential of the spectral domain Gabor fusion technique, we present the implementation of the spectral domain Gabor fusion algorithm using field programmable gate arrays (FPGAs) in a spectral acquisition hardware device. All filtering processes are now performed in an acquisition device as opposed to the post-processing of the original GD-OCM, which reduces the amount of data transfer between the image acquisition device and the processing host. To clearly demonstrate the imaging performance of the implemented system, we performed GD-OCM imaging of a stack of polymeric tapes. GD-OCM imaging was performed over seven focus zones. The results showed that the processing time for linear wavenumber calibration and spectral Gabor filtering can be improved with FPGA implementation. The total processing time was improved by about 35%.

Measurements of self-field and voltage for the REBCO stacked tapes assembled in rigid structure (STARS) conductor at 77 K

The measurements of self-fields and voltages for the stacked tapes assembled in rigid structure (STARS) conductor were carried out when the conductor immersed in liquid nitrogen was energized. In the measurements, a 3 m straight line shaped STARS conductor, which is composed of 45 REBCO tapes (15 tapes × 3 rows) embedded in a copper stabilizer, was used as a conductor sample. The measurement results indicate that the ramp rate at the conductor excitation and the one-time thermal cycle of the conductor do not affect the self-fields but the voltages.The current center position in the conductor cross-section during the conductor excitation was analyzed from the measurement results of the self-fields. The current center position always maintains the same position in the middle of the cross-section during the excitation. The analysis indicates that the current distribution of the STARS conductor is stable during the excitation. In addition, two-dimensional magnetic field calculations were conducted using the various models for the cross-sectional configuration of the STARS conductor. As a result, the calculation results agree with the measurement results regarding the self-fields.

The influence of temperature on levitation properties of CC-tape stacks

This article presents the study of the temperature effect on the levitation characteristics of stacks of CC-tapes from various manufacturers SuperOx, Theva and Sunam. Tapes differ in the values of the critical current density, architecture, and parameters of the metal substrate. Stacks of tapes were assembled from 12 by 12 mm square pieces of CC-tape. The number of elements in the stacks varied from 5 to 70. The dependences of the levitation force Fz (the vertical component of the force in the system permanent magnet—HTSC stack) on temperature were obtained both in zero field cooling (ZFC) and in field cooling (FC) modes. The measurements were carried out in the temperature range of 32–100 K. We have established that, down to the minimum operating temperatures, a decrease in temperature leads to a continuous increase in levitation force in ZFC mode. In FC mode, the Fz value tends to saturate after the initial increase in force. Cooling below the boiling point of liquid nitrogen up to 65 K gives a levitation force gain of 5%–10%. At the same time, cooling to 35 K gives an increase in levitation force by 7%–23%. It was found that an increase in the number of elements in the stack from 5 to 70 leads to an increase in the levitation force with a tendency to saturation for all temperatures studied. Excellent agreement between the experimental and calculated dependences of the levitation force on temperature and the number of elements in the stacks is demonstrated. Calculations show that for Sunam tapes, 90% of the levitation force is achieved with a stack thickness of 100 tapes at the boiling point of liquid nitrogen, and with 70 tapes at lower temperatures. The distributions of the magnetic field during the approach/removal of magnets, as well as the distribution of the trapped magnetic flux at different temperatures and numbers of elements in the stacks are modelled.

Conformal REBCO Windings for the Dipoles of an Ultimate-Energy Hadron Collider

REBCO tape has remarkable performance for superconducting technology, but it is extremely expensive and it is a strongly anisotropic superconductor. The design of a 3.5 T collider dipole is presented in which all turns of the body winding are oriented so that all the turns of REBCO tape operate with maximum current density. Each turn within the winding contains a stack of Cu-clad REBCO tapes; all tapes within the turn are compressed to provide low-resistance Cu-Cu contact, but the successive turns are electrically isolated. Each turn is oriented so that the local magnetic field at the tapes is closely parallel to the tape surface. Sextupole is controlled using a current-programmed turn to provide excellent homogeneity over a 20:1 dynamic range. Issues of current re-distribution during ramping, quench stability, AC losses, synchrotron radiation, and cryogenics have been considered. Two examples are evaluated: the 3.5 T dipole for a 500 TeV Collider-in-the-Sea, and a REBCO insert for an 18 T dipole for a 100 TeV FCC collider.

Experimental and numerical studies on current distribution in stacks of HTS tapes for cable-in-conduit-conductors

Cable-in-conduit conductors comprised of twisted stacks of high-temperature superconducting (HTS) tapes constitute a very promising technology by virtue of their easy manufacturing process, flexibility capabilities, and high current densities. In a cable, the current distribution among tapes is one of the key parameters affecting the cable performances. The distribution of current is affected mainly by the self-field configuration (ultimately related to the cable layout) and the termination resistances. In this paper we present a 2D finite element (FE) model, based on the T-A formulation, which computes the magnetic field and current distribution in stacked tapes. This model has been used to describe the experimental V–I results obtained in cables in which different current distributions among tapes are expected. The first case refers to V–I curves of stacks of HTS tapes inserted into ducts formed in the extruded aluminium cylindrical core for a straight cable. The excellent agreement between the experimental findings and the simulation results can be explained in terms of uniform current distribution within the tapes stack, up to the superconducting to normal transition. The second sample, an Al-slotted core Cable-In-Conduit-Conductor, has been bent down to a radius of 0.15 m, and from the measured V–I characteristic of each individual tape, a different tape degradation depending on the tape position within the stack was recorded. The model is able to reconstruct the V–I of the stacks from the characteristic curves of the individual tapes with a satisfactory agreement. The finite element analysis reveals non-uniform current distribution among the tapes, which could expose the cable to a potentially irreversible damage during operation. The proposed FE model constitutes a useful tool for the analysis and predictions of HTS CIC conductor performances and represents a suitable basis for the implementation of more complex models aimed at the design of specific and large applications of this conductor in the next future.

Levitation properties of pre-magnetized HTS tape stacks

This study presents new results on investigation of the levitation force between a permanent magnet and pre-magnetized HTS tape stacks containing from 20 to 100 tapes of 12 mm × 12 mm. Stacks were magnetized in superconducting magnet capable of creating a field of up to 8 T and placed in a levitation force measurement system. Experimental data on levitation force value and the effect of lateral displacement on it for stacks of tapes with different values of the captured flux and thickness have been obtained. Levitation force decay during the lateral displacements above a permanent magnet was observed for different stacks.

Estimation Method for AC Loss of Perpendicularly Stacked REBa2Cu3Oy Superconducting Tapes under Magnetic Field

The AC loss occurred in REBa2Cu3Oy (REBCO, RE = rare earth elements, Y, Eu, Gd) superconductor is serious problem in AC applications because it may cause a severe temperature rise of superconducting systems. Therefore, it is necessary to understand the AC loss property of REBCO superconducting tape in detail and to reduce that. As AC application such as motor, the tape is mostly stacked into multi-layer. Thus, understanding the relation between the number of the stacked tapes and the AC loss is one of the most important subjects. So far, we confirmed that the AC loss of the n-layer stacked REBCO superconducting tapes decreases to 1/n of that of 1-layer for the magnitude of the applied external field, Bm, below the penetration field, Bp, and such property can be explained from the viewpoint of the demagnetizing coefficient. However, the result in that report was only at 50 K and such property was not systematically confirmed at other temperatures. In this study, the AC loss decreasing property owing to stacking tapes for Bm below Bp was investigated via experiments. Furthermore, such property was considered theoretically from the viewpoint of the demagnetizing coefficient of the stacked REBCO superconducting tapes. Consequently, it was cleared, from theoretical consideration, that the n dependence of the AC loss can be explained quantitatively by the demagnetizing coefficient, and, in order to apply the theory to the experimental results, the stacked tapes have to satisfy the restrictions which are D/w < 0.1 and t/w << 1. Here, D, w and t are the distance between the neighboring superconducting layers, the width and the thickness of the superconducting layer. Moreover, from such AC loss property, it was revealed that the n dependence of Bp can be expressed as a power function. Using the results obtained in this study, the AC losses of the stacked tapes can be estimated easily from that of 1-layer tape.

(Invited) All-Solid-State Batteries Using Li7La3Zr2O12 Garnet Electrolyte Framework

All-solid-state batteries (ASSB) using LLZO (variants of Li7La3Zr2O12) garnet solid electrolyte with lithium metal anode potentially offer higher energy density and improved safety. Rational design of cell architecture as well as manufacturing scalability are key aspects to consider as the technology readiness level advances. A thick composite cathode layer attached to a thin solid electrolyte layer is desirable to obtain superior energy densities. Furthermore, the architecture within the composite cathode may be engineered to contain directional conduction paths of lithium ions or electrons for enhanced rate capabilities.Here, we demonstrate a functioning bulk-type LLZO based all-solid-state battery with a practical form factor incorporating the above described design concepts. Freeze-tape-casting (FTC), a scalable and environmentally friendly ceramic processing method, is used to construct 3D porous LLZO scaffolds composed of vertical arrays of LLZO walls. The thin solid-electrolyte layer is fabricated by tape-casting (TC). By sintering the stacks of FTC and TC green tapes, porous/dense bilayers and porous/dense/porous trilayers of LLZO frameworks are obtained. An ASSB was constructed using a porous/dense bilayer by infiltrating LiNi0.6Mn0.2Co0.2O2 cathode powder and carbon black into the porous layer and adhering lithium metal foil to the dense side. We find it crucial to introduce a plastic crystal based soft solid electrolyte to the porous layer to electrochemically connect all cathode components, obviating the need for co-sintering to establish contact. The soft nature of the plastic crystal based solid-electrolyte may be able to accommodate the volume change of the cathode material as it cycles. In this device, which contained no liquid component, the initial discharge capacities were similar to those observed in a lithium ion battery configuration using the same cathode powder at C/10 rates.

Magnetization Current Simulation of High-Temperature Bulk Superconductors Using the ANSYS Iterative Algorithm Method

The A-V formulation based iterative algorithm method (IAM), developed at Paul Scherrer Institute, shows excellent performance in modeling the critical state magnetization-current in the ReBCO tape stacks after field-cooling magnetization. This article upgrades the function of the IAM, making it realistic to take into account the widely used E-J power law, as well as to model the magnetization-current in bulk high-temperature superconductors under a zero-field-cooling cycle, having both the magnetization and demagnetization stages. The simulation results obtained from the ANSYS-IAM are validated with the well-known H-formulation method for both the critical state model and the flux creep model. The computation times from using the two different numerical methods are compared and discussed. The influence factors, including the iteration steps, the initial superconductor resistivity, the ramping time, and the reservation coefficient, are studied in detail.

Semianalytical Modeling of AC Losses in HTS Stacks Near Ferromagnetic Parts

This article presents a semianalytical modeling approach for ac losses computation in high-temperature superconducting (HTS) stacks and coils near ferromagnetic parts. The Brandt's model is combined with the equivalent current model to take into account of the proximity of ferromagnetic materials with finite relative permeability. Large stacks of HTS tapes are considered. A homogenization is applied to reduce the computing time. The obtained results are compared with finite element analysis. Very good agreements are obtained on both local and global quantities.

Magnetic flux in stacks of superconducting tapes of different architecture

Stacks of superconducting tapes nowadays have multiple applications and many new modifications are currently considered to enhance their beneficial properties. We have studied the field trapping in stacks of commercial superconducting tapes with different configurations. Experimental and numerical analyses were performed. The superconducting stacks were magnetized to act as powerful permanent magnets using pulsed field magnetization and field cooling at 77 K. The configurations include a basic stack made of layered tapes, a stack interlayered with ferromagnetic material, a sectioned stack made of thin tapes and a shielded basic stack.The present study shows that, in terms of total trapped flux, the basic stack performs best, closely followed by the shielded stack. No significant positive effect due to the presence of ferromagnetic layers was found in the studied configuration. The sectioned stack is the worst according to every analysed criterion. The possibility of application of the analysed modification is discussed.

Testing of Surface Mounted Superconducting Stacks as Trapped-Flux Magnets in a Synchronous Machine

Stacks of high-temperature superconducting tapes may offer a technically affordable solution for the application of superconducting materials as trapped-flux magnets in the rotor of synchronous electrical machines. Nevertheless, several concerns must be first addressed, among others, the optimal procedure to induce the current vortexes previously to operation (magnetization), the survivability of these vortexes in the electromagnetic environment present in an electrical machine and the accuracy of recently developed numerical models. With the aim of exploring such practicalities, this article presents a modified synchronous machine to test under liquid nitrogen conditions thin stacks of superconducting tapes. The machine is run under realistic conditions: currents in the stacks are induced from the stator, then the shaft is rotated, and finally, the machine is connected to a load, working as a generator. The results confirm previous numerical and experimental studies and establish a procedure for assessing the behavior of stacks in their actual operational environment.

Thermal-Hydraulic Analysis on Quench Behavior of Indium-Tin Soldered REBCO Composite Conductor

The second-generation high-temperature superconductor REBCO is one of the most valuable candidates that meet the requirements of future magnetic confinement fusion device. Quench behavior of an improved high-current REBCO composite conductor is simulated by a 1-D finite element method. Indium-tin alloy foil placed in the gap between stabilizer, jacket, and superconducting tape stacks can significantly reduce thermal contact resistance, which leads to an increase on the minimum quench energy of composite conductor. Short perturbations have a better response to this improvement than long perturbations, due to the low normal zone propagation velocity (NZPV) of REBCO conductor. For short perturbation (perturbation length (l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> = 0.1 m), the peak temperature in cable and helium drops 12.76 and 17.99%, respectively, after soldered. Due to the change of heat transfer in a composite conductor caused by helium wave propagation, the development and maximum length of normal zone significantly decrease from 13.06 to 4.78 m under long perturbation (l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> = 10 m) circumstance. High current REBCO composite conductor with proper indium-tin alloy soldered can enhance the stability and safety of composite conductors under severe working conditions, but it also slows down the NZPV and hinders the quench detection of high-temperature superconducting magnet.

Effect of Defects on the Quench Properties of Stacked REBCO Tapes

The local thermal disturbance will produce a joule heat. When the joule heat is larger than the minimum quench energy (MQE), it will lead to the phenomenon of a quench. The same problem exists in stacked superconducting tapes. In this article, the defects in REBCO tapes were created using the bending method. The voltage and temperature traces were measured during the quench process. The quench characteristics of the MQE and normal zone propagation velocity (NZPV) of the stacked REBCO tapes were investigated of different stacked REBCO tapes in the following configurations: two defects-free tapes; one defect-free tape and one tape with a defect; and both tapes with defects. The MQE of the stacked tapes with the defect is smaller than that of the tapes without defects, while the NZPV of the stacked tapes with the defect is larger than that of the tapes without defects.

Thermal Behavior of Quasi-isotropic Strand and Stacked-Tape Conductor

In this study, the thermal processes of a quasi-isotropic conductor in response to a heat disturbance are studied in detail, by combining simulations and experiments. A simulation model considering the anisotropy in the heat transfer coefficient of high-temperature superconducting (HTS) materials was established. The quasi-isotropic superconductor (QIS) was tested from 20 to 77 K, and the stacked tapes conductor (STC) was tested synchronously for the convenience of comparison. Compared with STC, the internal temperature distribution of QIS exhibits high isotropy and lower temperature rise. The thermal process following local tapes quenching is analyzed through simulation. The results show that the temperature rise of QIS is more uniform and lower than that of STC, which brings better thermal stability response to a heat disturbance.