Cross Section Of Tape Research Articles

Analyses of the plastic deformation of coated conductors deconstructed from ultra-high field test coils

REBCO coated conductors are now being used for building very high-field magnets with large electromagnetic stresses, both expected ones due to transport current × stresses and additional stresses resulting from the large screening currents inherent in wide tapes. Post mortem analyses of several recent test coils operated above 40 T show that significant conductor plastic deformation occurs, even for JBR stresses well below the ∼1 GPa yield of the Hastelloy substrate of the conductor. To investigate these deformation mechanisms, conductors were unwound after coil test and carefully examined with respect to their length-wise Ic which revealed many areas of local damage. Regions of interest were examined by metallographic cross-section, Hall microscopy, magneto-optic imaging and scanning electronic microscopy. Important damage frequently occurred to the outer edges of pancakes in the coil ends, which were often plastically deformed over the whole turn circumference, especially when this outer edge was a slit edge. Internal conductor damage was also seen, especially delamination between the buffer and REBCO layers at slit edges. Careful sectioning of the tape at ∼10 mm intervals showed that the plastic deformation of the turns was complex and variable around the turn circumference, with tape cross-sections that exhibited continuous shape change in the outer turns. The bending center line of tapes often shifted from the tape center line toward the edge closest to the coil center, indicating asymmetric effects of transport and screening current stresses across the conductor width. A surprising and vital result is that damage was prevalent when the slit edge was also the edge at which transport current flowed. This damage was absent when the transport current flowed at the not-slit edge, implying great sensitivity of the effect of screening current stresses to localized conductor damage.

Analysis of twisted tape solutions for cooling of the residual ion dump of the ITER HNB

The ITER HNB residual ion dump is exposed to a heat load about 17MW on the dump panels with a peak power density of 7MW/m2. Water flows through cooling channels, 2m long and 14mm diameter, realized by double side deep drilling. Unavoidable manufacturing deviations could generate a discontinuity at the channel length center. It is necessary to verify the influence of issues such as cavitation, fluid stagnation, low boiling margins, among others, in the cooling performance.Assuming worst case conditions, analytical and CFD methods showed a subcooled boiling operation with high safety margins to the water critical heat flux. Additionally, by analysing several thermo-hydraulic parameters, the twisted tape cross sections were optimized. Per cooling channel, two twisted tapes are inserted from the sides of the panels, thus, a study of a separation gap between them at the channel length center presented an optimal distance.This paper demonstrates that common machining techniques and drilling tolerances allow the manufacturing of panels able to withstand safely the required beam operation heat loads, even under worst case operation scenarios.

Estimation of hysteretic losses for MgB2 tapes under the operating conditions of a generator

Hysteretic losses in the MgB2 wound superconducting coils of a 550 kW synchronous hybrid scaled generator were estimated as part of the European project SUPRAPOWER led by the Spanish Fundación Tecnalia Research & Innovation. Particular interest was given to the losses caused by the magnetic flux ripples in the rotor coils originating from the conventional stator during nominal operation. To compute these losses, a 2D finite element analysis was conducted and Maxwell’s equations written in the H-formulation were solved considering the nonlinear material properties of the conductor materials. The modeled tapes are made of multiple MgB2 filaments embedded in a Ni matrix and soldered to a high purity copper strip and insulated with Dacron braid. Three geometrical models of single tape cross sections of decreasing complexity were studied: (1) the first model reproduced closely the actual cross section obtained from tape micrographs. (2) The second model was obtained from the computed elasto-plastic deformation of a round Ni wire. (3) The third model was based on a simplified cross section with the superconducting filaments bundled in a single elliptical bulky structure. The last geometry allowed the validation of the modeling technique by comparing numerical losses with results from well-established analytical expressions. Additionally, the following cases of filament transpositions of the multi-filamentary tape were studied: no transposition, partial and full transposition; thereby improving understanding of the relevance of the tape fabrication process on the magnitude of the determination of ac losses. Finally, choosing the right level of geometrical detail, the following operational regimes of the machine and its impact on individual superconducting tape losses in the rotor were studied: bias-dc current, ramping current under ramping background field and magnetic flux ripples under dc background current and field.

REBCO FCL Modelling: Influence of Local Critical Current Non-Uniformities on Overall Behavior for Various Tape Architectures

REBCO Coated Conductors with very good characteristics are now easily available in hundreds of meter lengths. Their use for resistive fault current limiter applications is constrained by the variations of the critical current density along the length, which may lead to the appearance of destructive hotspot under ohmic fault conditions. In this paper, we present a light 1D model of Coated Conductor FCL that take into account the critical current density variations along the length, assuming a homogenous temperature in the tape cross section. This assumption is discussed by comparing simulation results for a case of quench propagation under constant current with those obtained with a more refined 2D model. The new 1D model is then compared to small scale experiments of DC FCL with very short transient. Finally it is used to study the influence of tape architecture on the behavior of a MW-class DC Fault Current Limiter with critical current non-uniformities and select a suitable candidate architecture for the realization of such device.

Simulating tape resin infiltration during thermoset pultrusion process

The thermoset tape pultrusion is a widely adopted manufacturing process to produce long, constant cross-section composite structural parts. For high volume production, low cost can be achieved by maximizing the production rate which is a function of the material and process parameters, more specifically the rate of resin infiltration and resin cure. During resin infiltration, the resin saturates the dry reinforcement either under positive pressure in the pressure chamber, or, by the action of capillary and surface forces, within the resin bath. In either case, the saturation must be completed as the tape is squeezed into the final cross-sectional form at the entrance of the heated mold where the resin will be cured to form the composite part.This paper models the resin infiltration process during pultrusion, by modifying the pre-existing simulation tool for liquid molding processes. The formulated capability can be used not only to optimize the impregnation dynamics within the pressure chamber, but can also be used to predict the required forces for the selected pulling rate. The proposed model does allow one to handle a variety of tape cross-sections, not just rectangular prisms.

Ybco Superconductor Characterization under Shear Strain

YBCO based high temperature superconductors in practical applications are subjected to shear strain. `Critical current characteristics’ of such high temperature superconductors are known to get degraded in strain state. In this work, shear stresses resulting from the finite twisting of HTS tape having varying widths have been modeled using FEA analysis for different twisting angles. Supporting the findings of this model, an American Superconductor Corporation (AMSC) 2G YBCO tape of a given width has been twisted and experimentally investigated in self field for a given current ramp-rate. Under uniform twist of the YBCO tape at 77 K, the degradation in the current carrying ability up to 30% was observed. The irreversibility in the current carrying ability of HTS tape was also observed beyond the twisting angle per unit length of 25 degree/cm. The superconductor to resistive transition index, `n’ is found to behave in an identical manner to the critical current as a function of twisting angle. Such degradation is largely attributed to the torsional shear strain resulting from the twisting. High temperature second generation (2G) commercial grade YBCO coated conductors are available now-a-days in long length for extensive uses in electrical power cable (Rutherford cable) in the form of stacked twisted HTS conductors [1-3]. Even the proposal of coated conductor in conduit cable (CCICC) employs the idea of stacked HTS conductor twisted over different transposition lengths [4]. In such applications individual HTS tape subjects to twist induced shear stress/strain. The strain beyond a certain limit can critically affect the transport property as well as can modify the super-current carrying path in the coated conductor [5]. Hence, it has become necessary to ensure the maximum tolerable shear stress/strain applied to HTS which could induce recoverable damage. Previous works on the torsoinal strain dependence of Bi-2223, YBCO coated conductor shows experimental findings on Ic degradation behavior. Finite element analyses along with theoretical co-relation also have been surveyed on torsion experiments of high temperature superconducting tapes [6-8]. In case of pure torsion, the strain induced on the tape cross-section is non-uniform and complex. However, the maximum torsional shear strain could be determined at the midpoint of the width side of the cross-section using the formula (et = tθ/L) [8-9]. In the present study, FEA modeling for shear stress developed on 2G YBCO tape of different widths has been considered. Further, the shear strain dependence of the critical current of coated conductor has been investigated experimentally for a high current ramp-rate. In this paper, the FEA analysis is discussed in section-II. The experimental results and conclusion are discussed in section-III & section-IV respectively.

Improvement of coil performance by optimal use of multifilamentary superconducting tapes

It was theoretically confirmed that improvement of superconducting-coil performance can be obtained by making optimum use of multifilamentary tapes with high aspect-ratio cross-sections. Superconducting tapes, such as NbTi and MgB2 multifilamentary tapes, are known to have anisotropic electromagnetic properties, i.e., critical current densities and ac losses; under external transverse magnetic fields which are parallel to the tape face, the critical current density is higher than that of the multifilamentary round wire, but the ac loss is lower. For tapes with a high aspect-ratio cross-section, the anisotropic properties are still more remarkable, and therefore it is predicted that such a tape with superior electromagnetic properties will prove valuable as the windings of high performance coils for practical applications. In this paper, some examples of coil arrangements such as single or plural solenoid coil systems are shown in order to highlight the usefulness of this superior tape performance. The effect of the high aspect-ratio of tape cross-section on the electromagnetic coil performance was also discussed, taking the cooling condition of coil windings into account.

A new numerical approach to find current distribution and AC losses in coaxial assembly of twisted HTS tapes in single layer arrangement

This paper presents a novel technique for evaluating AC losses and current distribution in single layer assemblies of coaxially wound thin conductors, such as YBCO coated conductors. The proposed approach takes into account the twisted geometry of the individual superconducting tapes by considering the integral relation between the magnetic vector potential and the current density in the tapes (Biot-Savart formula). The integrals are solved numerically and semi-analytically, and the results are used to generate a discretized system of equations based on the magnetic flux diffusion equation (eddy current problem). The latter is solved using an efficient time transient solver (DASPK). It is assumed that, due to the helical symmetry of the problem, it is sufficient to solve for the current distribution in half of a single tape cross-section, even if many tapes are present, which allows a drastic reduction of the 3-D problem to a simple 1-D domain. The method was used to evaluate the AC losses of a HTS cable made of coated conductors, and it was observed that for a given radius of the former and number of tapes, twisted tapes with smaller pitch have lower AC losses.

Finite element models for thermal stress, axial tension, bending, and transversalcompression of filamentary Bi-2223 tapes

Most filamentary superconductors have soft matrix metal and brittle filaments, whichmake them very vulnerable during mechanical loading. Therefore, estimatingthe impacts of loading in the conductors is important in designing applications.Finite element models provide a way to study the stress and strain distributionsin a tape, taking into account the details of the tape cross section. This paperpresents finite element models for some basic loading conditions met, for example, inmagnets. The cases investigated include thermal stresses, axial tension, transversalcompression, and bending of a tape. The focus is on Bi-2223 tapes, although similarmodels could also be applied to other conductor types. To evaluate the validityof the models, the results are compared to experimental data. The results ofthe models showed the same trends as the experiments, but some inaccuratelyknown properties of the modelled tapes caused uncertainty in the models. Forexample, the stiffness and fracture strength of the filament material are essentialparameters to pay attention to when the initiation of critical current is predicted.

Stability of Bi-2223/Ag multifilamentary tapes with oxide barriers—a numerical simulation

The thermal profile of a Bi-2223/Ag tape with 55 filaments surrounded by oxide barriers was studied by a numerical simulation. The temperature distribution within the tape cross-section was calculated by commercial finite element method software. The simulations were performed when the tape was immersed in liquid nitrogen bath and carried a steady-state over-critical current with power law current–voltage characteristic in zero external magnetic field.

DC and AC self-magnetic field profiles of (6+1) filamentary BSCCO tapes with different filament arrangements measured by Hall probe magnetometry

The magnetic field distribution in the vicinity of (BiPb) 2Sr 2Ca 2Cu 3O 10+ x tapes carrying a transport current was measured with AC/DC Hall probe scanning magnetometer. Magnetic field profiles of multifilamentary BSCCO tapes supplied by DC transport current were compared with those of tapes supplied by AC transport current at frequency 90 Hz. The samples had seven untwisted filaments arranged in columns, in slanted columns or evenly distributed across the tape cross-section. Areas of filamentary regions and the aspect ratios of the samples were nearly identical. We compared calculated magnetic field profiles of each geometry in the regime of full penetration with the measured ones. It was found that in the case of evenly distributed filaments the shapes of DC and AC profiles were very similar, however the tapes with filaments stacked in columns and in slanted columns had very different shapes of DC and AC profiles at the same current amplitudes, especially in low current region. This effect can be attributed to the differences in static and dynamic penetration of flux into the interior of multifilamentary structure of the conductor.

Effect of core inhomogeneity on strain tolerance of Bi-2223 composite conductors

Effects of bend strains on monocore Bi-2223 tapes were studied by transport measurements, magneto-optical imaging and electron microscopy. The strain tolerance of tapes that were sintered for 100 (uniform and well-textured microstructure with high Ic) and 50 h (nonuniform microstructure with low Ic) was evaluated. The strain tolerance of the 100-h sintered tape was superior to that of the 50-h sintered tape. We have shown that the observed strain tolerance is largely determined by the interplay of nonuniform strains and microstructural variation across the tape cross section, leading to nonuniform Ic values. An explanation that accounts for an observed concentration of the critical-current density at the Ag/BSCCO interface was presented for the measured strain dependence of the retained Ic.

Jc distribution in the Bi-2223/Ag–AgMg tapes

Bismuth-based, silver-sheathed tapes are considered to have great possibilities for electric power applications. Remarkable progress in Bi-2223/Ag tapes has been made in the world. We also have been developing Bi-2223/Ag tape since 1989, and we can fabricate 1 km length tape now. One of the important properties for J c of Bi-2223/Ag tape is the distribution in the cross-section of tape, as well as that of longitudinal distribution. We measured J c distribution in the cross-section of tapes that were cut in the constant period from the edge of tape by using laser beam. It was cleared that J c increased periodically toward the center of the tape but J c slightly decreased at the center. We consider that J c distribution in the tape has good correlation with the density of filament. By this result, we succeeded in manufacturing the long Bi-2223/Ag–AgMg tape of J c =31 000 A/cm 2 and J e=8000 A/cm 2. Furthermore, we achieved I c=50 A in Bi-2223/Ag–AgMg twisted long tape.

A new experimental method to determine the local critical current density in high-temperature superconducting tapes

The lateral critical current density ( J c) distribution in high-temperature superconducting tapes is determined with a new experimental method. With this method it is possible to determine a J c-profile in a non-destructive manner. The experiment utilizes the strong suppression that occurs in the critical current ( I c) when a perpendicular magnetic field is applied to a Bi-2223 tape. A perpendicular magnetic field with a strong gradient that passes through zero is applied to a tape, to select a zone where the perpendicular field is small. This magnetically selected zone then determines the I c of the entire tape. By moving this field gradient along the tape it is possible to observe the spatial variations in J c. For a quantitative analysis of the critical current density a deconvolution of the measured I c-profile with the system response is required. The local superconducting area is obtained from an optical micrograph of the tape cross-section. Finally a position dependent J c is determined for two different types of superconducting tape.

Influence of longitudinal magnetic field on current distribution and AC loss in twisted multifilamentary high Tc superconducting tapes

The AC longitudinal magnetic field that is parallel to the tape axis couples with the twisted filament bundle in multifilamentary high T c superconducting tapes to change their current distribution. The temporal evolution of the current distribution in the tape cross-section was calculated numerically to study the influence of the longitudinal magnetic field on AC loss characteristics. The change in the current distribution by the longitudinal magnetic field results in the increase in the AC loss. The AC loss due to the longitudinal magnetic field increases with decreasing twist pitch, while the AC loss due to the transverse magnetic field decreases with decreasing twist pitch. In practical conditions where high T c superconducting tapes carry their AC transport current in an AC external magnetic field with transverse and longitudinal components, the optimum twist pitch to reduce the total AC loss must be selected.

Transport ac losses in (Bi,Pb)SrCaCuO-2223/Ag multifilamentary tapes with different filament arrangements

Transport ac losses in multifilamentary (Bi,Pb)SrCaCuO-2223/Ag tapes with different filament arrangements have been measured. The samples were untwisted and had seven filaments, which were arranged in columns, in slanted columns, or evenly distributed across the tape cross-section. The aspect ratio of the samples as well as their filamentary regions was the same. Special care during processing ensured that the filaments had the same orientation or arrangement along the sample length and that filament bridging was avoided. The ac losses were measured in the frequency range 40 Hz–120 Hz. Two different methods of determining the critical current were used: from dc current–voltage characteristics and from the saturation value of ac losses. The experimental results were compared with theoretical models. The normalized ac losses approximately follow the theoretical dependence of a tape with rectangular cross-section of similar aspect-ratio. It was found that the sample with filaments stacked in columns had slightly lower normalized losses at low normalized currents ( I o/ I c<0.2) compared with other filament arrangements. There was no difference in the loss data for the three different filament geometries in the range I o/ I c>0.2. It was also observed that tapes containing bridged filaments, but having similar critical currents, had variable normalized ac loss.

Computational comparison of magnetization losses in HTS solenoids wound of tape conductors having different aspect ratios

In this paper a computational model for magnetization losses in HTS magnets is presented. The model is based on a semianalytical expression derived for the hysteretic energy loss density per cycle in HTS tapes due to changing external magnetic field. The real magnetic field dependent electric field-current density characteristic of HTS material, the geometry of the tape cross-section and the orientation of magnetic field are taken into account. Using the model the dependence of hysteresis losses on the orientation and amplitude of external magnetic field are studied. Furthermore, the model is applied to five solenoidal HTS coils which are wound of tape conductors having different aspect ratios. Special attention is paid to the differences in energy loss distributions between coils. The results show that in the coils wound of tape having aspect ratio higher than 3 the loss maxima are mainly caused by the radial component of magnetic field. Averaged losses in the magnet are investigated as a function of ramping frequency and the peak value of the operation current of coils.

Effect of strain on ac power loss of Bi-2223/Ag superconducting tapes

The ac power losses of monofilament and multifilament Bi-2223/Ag composite tapes were investigated to determine the effect of mechanical strain on the loss and to identify the loss mechanisms. Measurement of the self-field losses was performed at 77 K and 60 Hz as a function of ac current amplitude (0–100 A) for tapes in their as-prepared or virgin state and after being subjected to applied strain by temperature cycling or bending to small radii of curvature ( R=1–50 mm). For good quality virgin tapes the experimental data are well described by the Norris equation P∼ I m n for the dependence of the power loss P on the amplitude I m of the ac transport current, with n=3 for monofilament and n=4 for multifilament tapes. Applied strain causes the loss to increase by several orders of magnitude and the exponent n to decrease below the Norris values. At strains very much greater than the irreversible strain limit, the loss becomes purely ohmic (i.e., I 2 R loss) and n=2. Examination by SEM of the transverse cross-section of tapes reveals a variety of strain-induced structural defects including transverse cracks that sever the filaments and extended regions where the superconductor core has delaminated from the silver sheath.

A new structural powder/wire-in-tube (PWIT) Ag-sheathed multifilamentary Bi-2223 tape and its superconducting properties

A new design of macrostructural multifilamentary tapes has been developed by parking powder and composite wires into the silver tube (PWIT) at the second stage of the powder-in-tube (PIT) process. The critical current J c and the strain tolerance for this PWIT multifilamentary tape were significantly improved in comparison with the PIT multifilamentary tapes. The enhancement of both J c and the strain tolerance are attributable to the increased interface between Ag and oxide superconductor. We have introduced a simple concept of the periphery length of the Ag/superconductor interface per unit area in the cross-section of tapes ( E) and have compared its properties in PWIT and PIT tapes. Both J c and the strain tolerance were improved with increasing E. This was attributed to the improved grain alignment and crack elimination at the Ag/superconductor interface. The J c– B properties and the J c-strain characteristic of two tapes have been compared.

The dynamics of current density distribution in Ag-sheathed TBCCO tape in changing gradient field

The dynamics of current density distribution across the TBCCO tape width was studied under steady and also varying ambient gradient field applied to tape with transport current. Self-field map values were extracted from local magnetic field mapping in vicinity of the tape and used in procedure of inverse problem calculation to get linear current density distribution within edges of a selected tape cross-section. It was suggested that inhomogeneity and asymmetry of current density distribution across the tape width can be determined in gradient field even without field mapping by comparing I/sub c/(B, grad B, x) and I/sub c/(B, -grad B, -x) critical current-position curves measured during step-wise shift of the tape in direction x of local gradient (x/spl perp/j and x/spl perp/B) in the plane of tape. The symmetry of curves achieved by entering the 'left-side' gradient (i.e. x 0) of the same ambient field, after rotating the tape 180/spl deg/ in its plane, proves the symmetry in current density distribution across the tape width and vice versa. The role of history can be suppressed and virgin conditions preserved by self-heating of the tape after transition to normal state prior to further measurement step.