Current Redistribution Effects Research Articles

Current redistribution model of anomalous resistance behaviour in superconductor-topological insulator heterostructures

Superconductor-topological insulator (TI) heterostructures are interesting due to induced p-wave superconductivity on the TI surface states near the interface. Transport studies carried out in this work on the TI surface of superconductor-TI (NbN-Bi1.95Sb0.05Se3) heterostructures have revealed anomalous resistance upturn and downturn at millimetre length scales away from the interface. Magnetotransport measurements have indicated that the anomaly is caused due to the superconducting transition of the NbN layer. The strength of the jump in resistance has been found to be strongest at the edges and the sign of the jump found to be opposite along opposite edges. Such resistance upturns and downturns have been previously reported in literature, wherein the upturns have been attributed to the antagonistic ground states of p-wave Cooper pairing and the spin-polarized TI states, and the downturns attributed to induced long-range proximity effects. However, the possibility of long-range superconducting proximity effect has been ruled out in this study through the observation of similar anomaly in NbN-Au and NbN-Al heterostructures. The present study demonstrates that the anomalies in resistance occur due to current redistribution (CRD) effects at the superconducting transition due to the geometry of the heterostructure. Results obtained from finite element analysis using COMSOL software have validated the proposed CRD model of long-range resistance anomalies in superconductor-TI and superconductor-metal heterostructures.

Current dependence of the red boundary of superconducting single-photon detectors in the modified hot-spot model

We find the relation between the energy of the absorbed photon and the threshold current at which the resistive state appears in the current-carrying superconducting film with probability about unity. In our calculations we use the modified hot-spot model, which assumes a different strength of suppression of the superconducting order parameter in the finite area of the film around the place where the photon is absorbed. To find the threshold current we solve the Ginzburg-Landau equation for the superconducting order parameter, which automatically includes the current continuity equation and it allows us to consider the back effect of current redistribution near the hot spot on the stability of the superconducting state. We find quantitative agreement with recent experiments, where we use the single fitting parameter which describes what part of the energy of the photon goes for the local destruction of the superconductivity in the film.

Status of high transport current ROEBEL assembled coated conductor cables

Assembling coated conductors (CC) into flat ROEBEL bars (RACC cable) was introducedin 2005 by the authors as a practicable method of reaching high transport currents in a lowAC loss cable, which is a cable design suited for application in windings. The transportcurrent of 1.02 kA in self-field at 77 K achieved so far, however, is still too lowfor several applications in electrical machinery such as larger transformers andgenerators/motors. A new cable concept for further increased currents was presented justrecently. The goal of the new design was primarily to demonstrate the possibilityof strongly increased transport currents without changing the important cablefeatures for low AC losses. such as, for example, the transposition length of thestrands. We present detailed investigations of the properties of this progressed cabledesign, which has threefold layered strands, an unchanged transposition pitchof 18.8 cm and finally the application of 45 coated conductors in the cable. A1.1 m long sample (equivalent to six transposition lengths) was prepared fromcommercial Cu stabilized coated conductors purchased from Superpower. Themeasured new record DC transport current of the cable was 2628 A at 77 K in self-field(5 µV cm−1 criterion). The use of three slightly different current carrying batches of strand material(± 10%) was a special feature of the cable, which allowed for interesting investigations of currentredistribution effects in the cable, by monitoring a representative strand of each batchduring the critical current measurement. Although current redistribution effects showed acomplex situation, the behaviour of the cable was found to be absolutely stable underall operational conditions, even above the critical current. The high self-fielddegradation of the critical current reached the order of 60% at 77 K, and couldbe modelled satisfactory with calculations based on a proven Biot–Savart-lawapproach, adapted to the specific boundary conditions given in this new cable design.

Inter-Strand Resistance in the Termination of ITER Conductors

In large, cabled superconductors, a non-negligible level of current unbalance among the strands is unavoidable in dc operation due to non-homogeneous contact resistance distribution at the electrical connections. When the overloaded strands saturate their current carrying capability and start to develop current sharing voltage at the high field section, the excess current must be transferred to less loaded strands. The inter-strand current re-distribution takes likely place at the electrical connections, where the inter-strand resistance is lower. Two samples of electrical termination, made of large cable-in-conduit conductors, were formerly prepared and tested for contact resistance distribution at CRPP. One of these samples is now re-arranged to measure the inter-strand resistance in different combination. A discussion of the results allows ruling out speculations about the impact of current re-distribution effects on the voltage-current and voltage-temperature characteristics in short length conductor test.

Simulation of electromagnetic and thermal processes in Rutherford superconducting cables during the initiation of a quench

Thermal stability for superconducting fast-cycling dipoles will play a vital role. The coupled numerical simulation of electromagnetic and thermal processes in Rutherford superconducting cables during the initiation of a quench was carried out. The network model has been combined with thermal analysis, which allows one to model quench dynamics, including the effects of current redistribution in strands, discontinuities and inhomogeneity, the initial heating in strand, and as results occasional quench recovery or runaway quench propagations.

Current redistribution effects on superconducting d.c, and microwave measurements

In the last two decades, non conventional behavior of the d.c. transport properties of superconductors, with the appearance of anomalous peaks at the transition, have been investigated and interpreted in different ways. In several cases it was recognized that the behavior can be due to current redistribution effects related to the non-homogeneous nature of the measured superconducting sample.In this paper we will briefly review and discuss these effects and, referring to simple concentrated constant equivalent circuits, we will show that sample non-homogeneity can produce the observed features. Then, in the same framework, by performing specific simulations on planar resonators, we will show that anomalous peaks in temperature dependence of the resonant frequency and of the extracted surface reactance can occur at the transition temperature of minority, lower Tc, superconducting phases.

Current redistribution effects in superconducting microwave measurements

Microwave measurements, performed on high temperature superconducting(HTS) films, have evidenced anomalous features in the temperaturedependence of some fundamental parameters such as the surface impedance(ZS), the penetrationdepth (λ) and thecomplex conductivity (σ). In particular, anomalous peaks have been observed in the temperature region just below the criticaltemperature Tc. In high temperature superconductors (HTSs), these behaviours have been interpreted interms of different theoretical models, concerning the nature of superconductingquasi-particles and their intrinsic pairing mechanisms. The aim of this paper is tostress the importance of current redistribution phenomena which take place inmicrowave measurements and are able to give rise to similar anomalous effects.By performing specific simulations on planar resonators and following standardprocedures in the data extraction, unconventional temperature dependences ofZS,λ and σ have been reproduced.

Frequency Characteristics of High Speed Power Distribution Grids

The variation of inductance with frequency in high performance power distribution grids is discussed in this paper. The impedance characteristics of the power grid need to be well understood for the design of efficient and robust high performance power distribution grids. The physical mechanisms underlying the dependence of inductance on frequency are discussed. The variation of inductance with frequency in three types of power grids is analyzed in terms of these mechanisms. The inductance of power distribution grids decreases with signal frequency. The decrease in inductance in non-interdigitated grids is primarily due to current redistribution in multiple forward and return current paths. In interdigitated grids, the variation of inductance with frequency is fairly small, typically less than 10% because both proximity and multi-path current redistribution effects are minimal. In paired grids, the relative decrease in inductance with frequency is larger as compared to interdigitated grids. This behavior is due to significant proximity effects. The smaller the separation between the power and ground lines and the wider the lines, the more significant proximity effects become and the greater the relative decrease in inductance with frequency.

Critical current degradation in jointed area of Ag-sheathed BSCCO tapes

Usually, the critical current of jointed BSCCO-2223 tapes degrades quite a lot compared to the normal tape regions. Besides the material characteristics such as thermal and mechanical properties, it has been thought that the current redistribution around the jointed region is one of the main causes of the critical current degradation. Analyzing the experimentally obtained voltage-current characteristics around the jointed tape region, we found linear voltage components around the jointed area, especially in the transition region, which suggests that the current redistribution around the transition area is one of the main causes of critical current degradation. In this paper, we present the experimental results and analysis procedure for both mono- and multi-filamentary tapes, and describe the current redistribution effects on the critical current degradation, especially for a mono-filamentary tape case.

Frequency response characteristics of reference plane effective inductance and resistance [IC packaging

A method which uses the partial element equivalent circuit (PEEC) method and electrical network theory to solve for the effective impedance matrix of reference planes is presented. The convergence and accuracy of the method are checked. The frequency responses of the effective inductance (L/sub eff/(f)) and resistance (R/sub eff/(f)) of reference plane are discussed. The effects of current redistribution and the skin effect on L/sub eff/(f) and R/sub eff/(f) are discussed. The effect of number of sinks and sources is examined.

Design of high frequency inductors based on magnetic films

Magnetically coated stripe inductors with conductor insulated from the magnetic film and with conductor in direct electrical contact with the magnetic film are analyzed. A simple equivalent circuit model shows that for the structure without insulation, regardless of the fact that the dc resistivity of the magnetic film is much higher than that of the conductor, most of the driving current flows in the magnetic film at frequencies where the impedance of the device is dominated by inductance, i.e., where the quality factor Q>1. This effect of current re-distribution significantly lowers the performance of the device. A design is proposed having insulation layers to avoid current flow in the magnetic film, which is broken up in sections along the conductor stripe to avoid displacement currents in the system.

Effects of the current redistribution within Nb/sub 3/Sn compacted-strand cable on its stability

The effects of current redistribution on the stability have been studied using two kinds of Nb/sub 3/Sn compacted-strand tables with chrome coating and without any coating. The minimum quench energy (MQE) has been measured in various external magnetic field and with various transport current. The MQE of the cable with chrome coating was slightly larger than that of the cable without coating when the transport/critical current ratio, I/I/sub c/, is small. The effects of the different surface condition of the strands on the stability becomes smaller, when the transport current approaches the critical current. The normal zone propagation process of these cables were also different. The initiation of the normal zone propagation in the cable without coating was slower than that in the cable with coating.

Magnetic stimulation of axons in a nerve bundle: effects of current redistribution in the bundle.

Recently, we developed a model of magnetic stimulation of a concentric axon in an anisotropic nerve bundle. In that earlier paper, we considered a single axon surrounded by a nerve bundle represented as a homogeneous anisotropic monodomain medium. In this paper we extend our previous calculations to examine excitation of axons within a nerve bundle without neglecting the presence of other axons in the nerve bundle. A three-dimensional axial symmetry volume conductor model is used to determine the transmembrane potential response along an axon due to induced electric fields produced by a toroidal coil. Our principal objective is to examine the effect of current redistribution to other axons in the bundle on excitation characteristics. We derive the transmembrane potential along an axon for two currently available models of current redistribution: the biodomain model and the spatial--frequency monodomain model. Results indicate that a reduction in the transmembrane potential along an axon due to the presence of other nerve fibers in the bundle is observed. Axons located at the periphery of a nerve bundle have lower thresholds and different excitation sites compared with axons located near the center of a nerve bundle.

Three-dimensional eddy current solution of a polyphase machine test model (abstract)

This abstract describes a three-dimensional (3D) finite element solution of a test model that has been reported in the literature. The model is a basis for calculating the current redistribution effects in the end windings of turbogenerators. The aim of the study is to see whether the analytical results of the test model can be found using a general purpose finite element package, thus indicating that the finite element model is accurate enough to treat real end winding problems. The real end winding problems cannot be solved analytically, as the geometry is far too complicated. The model consists of a polyphase coil set, containing 44 individual coils. This set generates a two pole mmf distribution on a cylindrical surface. The rotating field causes eddy currents to flow in the inner massive and conducting rotor. In the analytical solution a perfect sinusoidal mmf distribution is put forward. The finite element model contains 85824 tetrahedra and 16451 nodes. A complex single scalar potential representation is used in the nonconducting parts. The computation time required was 3 h and 42 min. The flux plots show that the field distribution is acceptable. Furthermore, the induced currents are calculated and compared with the values found from the analytical solution. The distribution of the eddy currents is very close to the distribution of the analytical solution. The most important results are the losses, both local and global. The value of the overall losses is less than 2% away from those of the analytical solution. Also the local distribution of the losses is at any given point less than 7% away from the analytical solution. The deviations of the results are acceptable and are partially due to the fact that the sinusoidal mmf distribution was not modeled perfectly in the finite element method.

An integrated injection logic (I/sup 2/L) macromodel including lateral and current redistribution effects

An integrated logic (I/SUP 2/L) macromodel for computer simulation of logical configurations of I/SUP 2/L gates is presented. The macromodel is synthesized from the familiar Ebers-Moll equivalent circuit which permits compatibility with numerous presently available circuit simulators. Measurement procedures are described for the complete and self-consistent set of electrical parameters required for macromodel definition. A five-stage ring oscillator is computer simulated to demonstrate the application of the macromodel. Lateral current transfer (LCT) between adjacent gates and injector current redistribution (ICR) effects are shown to reduce gate propagation delay times. When both effects are included, the macromodel produces an agreement between computer simulated and experimental results of better than 10 percent. A ring oscillator example illustrates the use of the macromodel to provide physical insight into the layout sensitivity of I/SUP 2/L.