Homogeneous Current Distribution Research Articles

Analysis of current distribution in a large superconductor

An imbalanced current distribution which is often observed in cable-in-conduit (CIC) superconductors composed of multistaged, triplet type sub-cables, can deteriorate the performance of the coils. It is, hence very important to analyze the current distribution in a superconductor and find out methods to realize a homogeneous current distribution in the conductor. We apply magnetic flux conservation in a loop contoured by electric center lines of filaments in two arbitrary strands located on adjacent layers in a coaxial multilayer superconductor, and thereby analyze the current distribution in the conductor. A generalized formula governing the current distribution can be described as explicit functions of the superconductor construction parameters, such as twist pitch, twist direction and radius of individual layer. We numerically analyze a homogeneous current distribution as a function of the twist pitches of layers, using the fundamental formula. Moreover, it is demonstrated that we can control current distribution in the coaxial superconductor.

Properties of SNS Josephson junctions fabricated by 200 keV oxygen implantation into YBa/sub 2/Cu/sub 3/O/sub 7-δ/

The properties of SNS Josephson junctions in YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films have been investigated, which were fabricated by oxygen irradiation at 200 keV through a 50 nm wide slit in an implantation mask. After annealing the irradiated microbridges at 500/spl deg/C in an oxygen atmosphere, the implanted region has a reduced but finite transition temperature, allowing Josephson coupling in a temperature window of <15 K. Close to the coupling temperature the critical current shows a nearly complete modulation in an applied magnetic field. This indicates a homogeneous current distribution in the junctions and therefore a homogeneous defect distribution throughout the implanted and subsequently annealed region of the superconducting microbridges. Over the entire temperature range of Josephson coupling, the junctions exhibit resistively shunted junction like I-V characteristics with additional excess current. Furthermore, the exponential dependence of the critical current on temperature is in good agreement with conventional superconductor-normal-superconductor proximity effect theory.

Electrical field and current distributions in electrochemotherapy

Electrochemotherapy is a method for cancer treatment consisting in combining intratumor injection of cytotoxic agent with the application of intensive electrical stimuli. Thus transient cell membranes permeabilization is created, allowing the agent to better exercise its destroying effect. Positive results have been published in the treatment of cutaneous malignant formations and other types of cancer are under consideration. The electrode configurations presently used are based mainly on empirical treatment results. In vivo imaging of stimulation currents was attempted in animal models. A preliminary study revealed that having in view the relatively high voltages and currents, there was a virtually resistive load to the electrical source. Assuming a homogeneous medium, potential and current distributions were modeled and studied. The results could help in selection of specific electrode designs, depending on tumor size and location. An optimization of the voltages and/or currents by different electrode arrays can lead to obtaining desired field distribution.

YBa 2Cu 3O 7−δ Josephson junctions fabricated by oxygen implantation

We report Josephson junctions in YBa 2Cu 3O 7− δ thin films, fabricated by oxygen irradiation at 200 keV through a 50 nm wide slit in an implantation mask. After annealing the irradiated microbridges at 500°C in an oxygen atmosphere, the implanted region has a reduced but finite transition temperature, allowing Josephson coupling in a temperature window of <15 K. Close to the coupling temperature the critical current shows a nearly complete modulation in an applied magnetic field. This indicates a homogeneous current distribution in the junction. Over the entire temperature range of Josephson coupling, the junctions exhibit resistively shunted junction like I– V characteristics with additional excess current. Furthermore, the exponential dependence of the critical current on temperature is in good agreement with conventional superconductor-normal-superconductor proximity effect theory.

Hexagon structures in a two-dimensional dc-driven gas discharge system

In a dc-driven planar gas discharge semiconductor system, self-organization phenomena have been investigated experimentally. The spatial structures we focus on appear as a spatial modulation of the transversal distribution of the electric current density. These patterns have been observed and recorded through a transparent electrode with spatial resolution via their corresponding light emission. When choosing appropriate experimental conditions, the initially homogeneous current density distribution undertakes a transition to a periodic hexagon and then to a stripe pattern. This behavior of the experimental system is investigated quantitatively and is discussed in the frame of a phenomenological model.

Stochastic Coulomb interactions in ion projection lithography systems with aberration-broadened crossover

The allowable current in ion beam projection lithography is limited by stochastic Coulomb interactions. Theories for the dependence of these interactions on the parameters of the system, i.e., length, beam energy, print size, crossover diameter, etc. are thus important to guide the design of future machines. All existing theories on blur from stochastic Coulomb interaction assume a homogeneous current distribution in the crossover, where a large part of the blur is produced. However, in actual lithography machines, the crossover may be substantially broadened by the spherical aberration of the ion lenses, thus giving a very inhomogeneous current distribution in the crossover. In most analytical theories, the stochastic blur is independent of the radius of the homogeneously field crossover rc for very small crossovers, and relates to rc as (1/rc)1/2 or (1/rc)1/3 for larger ones. If aberration broadening could do the same, the effect of broadening the crossover from rc=1.5 to 50 μm would decrease the blur by a factor as much as 6, without the negative effect of increasing the curvature-of-field blur. We find physical arguments that such a large factor is impossible. Monte Carlo simulations first of all show that the blur reduction is larger near the edges of the image field than in the center, near the optical axis. For the simulated model system, the reduction on axis can be increased to a factor 2 at very large aberration coefficients.

Phase formation and critical current density in Bi,Pb(2223) tapes

The various fabrication and reaction processes leading to multifilamentary Bi,Pb(2223) tapes with high transport are described. In air, the Bi,Pb(2223) phase in pressed powders is found to be formed by nucleation, following the complete decomposition of the Bi,Pb(2212) intermediate phase. In this work, the precise formation conditions of the Pb-free phase Bi(2223) are described for the first time. It is shown that the presence of Pb leads to a lowering of the melting temperature from 879 to and of that of the `transient liquid' . The latter is formed after decomposition of the 2-layer compound and is necessary to the formation of the 3-layer compound. Thus Pb is found to have no influence on the formation mechanism of the 3-layer compound, which always starts by a nucleation process. Nucleation processes are also found to occur in the first phase of reaction inside Ag sheathed tapes, intercalation being possible at later stages. The highest values of the critical current density in Bi,Pb(2223) tapes reported at the present day are compared, taking into account the analysed tape lengths. The variation of in mono- and multifilamentary Bi,Pb(2223) tapes as a function of the applied magnetic field at 77 and at 4.2 K is described. A new process including deformation by a four-roll machine is presented, and the distribution of inside various tape configurations with similar critical current densities is analysed: for monofilamentary tapes, a maximum of is found at the borders, the ratio being 2.4, while for conventional multifilamentary tapes the maximum is found at the tape centre (, ratio 1.6). For the new four-rolled tapes, a ratio of only 1.1 was measured, indicating a more homogeneous current distribution. In order to lower the AC losses due to the highly conductive Ag matrix, the new concept of `oxide barriers' is introduced. A barrier of surrounds each filament, thus leading to an enhanced radial resistivity, which in turn reduces AC losses. Finally, the current transport mechanism inside Bi,Pb(2223) tapes and the various current limiting mechanisms are discussed.

Investigation of dc Josephson current distribution in double-barrier three-terminal devices with a thin middle superconducting layer

Nb/Al–AlOx–Nb/Al–AlOx–Nb double-barrier three-terminal Josephson devices with a thin middle Nb/Al bilayer, which have potential for use as switching and amplifying elements in superconducting electronics, have been investigated in the stationary state by means of low-temperature scanning electron microscopy. For the devices with lateral sizes comparable with the effective Josephson penetration depth, we observed nearly homogeneous current distribution over the region common to the top and bottom junctions when the devices were biased across both the barriers in the absence of an external magnetic field. When the two junctions are biased separately, the current is concentrated at the junction edges in accordance with the behavior characteristic of distributed junctions. In an applied magnetic field, the vortex structure is confined to the area shared by both junctions when the device is biased as a whole. The experiment gives an indication that in the stationary state the spatial phase difference distribution of the two junctions coincides in the shared region at least in some interval of the external magnetic field.

Electromagnetic properties of Bi-2223/Ag concentric tapes

It is well established that near the silver interface highly textured Bi-2223 layers compared to the inner ceramic core may be obtained. Thus Bi-2223 multilayered concentric tapes with silver matrix are very promising in increasing transport critical current densities. In the present work we report on the electromagnetic characterization of short tapes having in their cross-section a very thin HTS flattened ring with Ag inside and outside of it. The samples were prepared by the powder in tube method and have self-field critical current densities J/sub c/ of the order of 3/spl times/10/sup 4/ A/cm/sup 2/ at 77 K. Transport and SQUID magnetization measurements revealed weak link nature of the samples in low magnetic fields. Large transport J/sub c/(B) hysteresis was observed at 4.2 K and magnetic fields up to 20 T. Magnetic field profiles measurements with miniature Hall sensors are in qualitative accordance with model calculations supposing homogeneous current density distribution across the superconducting core.

Performance predictions for electromagnetic launching with multi-fibre solid brush armatures and resistively layered rail accelerators

In the literature on EM launcher research, number of proposals have been made to suppress the negative influence of the velocity skin effect on the performance of solid armatures during electromagnetic launch. In this paper, the results of a study of two of these methods, i.e. the application of multi-fibre solid brush armatures and the use of accelerator rails with a resistive layer, are presented. This study is performed by means of two-dimensional finite element computer simulation of the electrothermal behaviour of the armature and the rails during the launch process. A description of the electrothermal mode used in the simulation is given. Here, the fibre armatures are regarded as made of materials with an anisotropic electrical and thermal conductivity. The results for the current distribution in a rectangular multi-fibre solid brush armature obtained from the simulations agree with the results of an analytical method. It is shown that fibre armatures have a more homogeneous current distribution during the acceleration process than monobloc armatures. U-shaped molybdenum multi-fibre solid brush armatures are good candidates for arc erosion-free launching if the electrical insulation of the fibres can be maintained at increasing temperature. Simulations also show that the skin depth in monobloc and fibre armatures can be increased by using rails with resistive layer with suitable material properties.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A new calibration technique for the determination of magnetic flux density with the magneto-optical Faraday effect

Using the high resolution magneto-optical Faraday effect the magnetic flux density distribution of circular shaped Y 1Ba 2Cu 3O 7−x thin films is investigated. A new calibration technique is developed to determine the z-component B z(x,y) of the local magnetic flux density B (x,y) in the x,y-surface of the films with high accuracy. The observed flux density profiles agree well with calculations of B ( r ) using a simple model with homogeneous current distribution. The appearance of a negative magnetic flux and reversed flux lines at the edge of the films in the remanent state is caused by stray fields of the trapped flux. For a determination of the local critical current density j c, the local pinning forces p v and the magnetization of thin films from the observed magnetic flux density distribution B (x,y) the curvature of the magnetic field ∂B z/∂r and ∂B r/∂z, respectively, has to be taken into account.

Nonequilibrium distribution of edge and bulk current in a quantum Hall conductor.

A quantitative model is presented that accounts for the experimental observation that four-terminal resistances of a high-mobility quantum Hall conductor cannot be related directly to a single resistivity tensor. The key ingredient is that the highest (partly occupied) Landau level is completely decoupled from the other levels except at the contacts. The current carried by the top level alone is related through a resistivity tensor to the longitudinal and the transverse electric-field components. Deviations from a homogeneous current distribution close to the contacts are taken into account. For the other (fully occupied) Landau levels the usual edge-channel description is used. It is shown how the electric field can be incorporated into this description in order to obtain a consistent model. A mechanism for the onset of nonlinear behavior is given.

A delayed rectifier potassium current in Xenopus oocytes

A delayed voltage-dependent K+ current endogenous to Xenopus oocytes has been investigated by the voltage-clamp technique. Both activation and inactivation of the K+ current are voltage-dependent processes. The K+ currents were activated when membrane potential was depolarized from a holding potential of -90 to -50 mV. The peak current was reached within 150 ms at membrane potential of +30 mV. Voltage-dependent inactivation of the current was observed by depolarizing the membrane potential from -50 to 0 mV at 10-mV increments. Voltage-dependent inactivation was a slow process with a time constant of 16.5 s at -10 mV. Removal of Ca2+ from the bath has no effect on current amplitudes, which indicates that the current is Ca2+)-insensitive. Tail current analysis showed that reversal potentials were shifted by changing external K+ concentration, as would be expected for a K(+)-selective channel. The current was sensitive to quinine, a K+ channel blocker, with a Ki of 35 microM. The blockade of quinine is voltage-independent in the range of -20 to +60 mV. Whereas oocytes from the same animal have a relatively homogeneous current distribution, average amplitude of the K+ current varied among oocytes from different animals from 30 to 400 nA at membrane potential of +30 mV. Our results indicate the presence of the endogenous K+ current in Xenopus oocytes with characteristics of the delayed rectifier found in some nerve and muscle cells.

Transvenous and Subcutaneous Electrode System for an Implantable Defibrillator, Improved on Large Pigs

The currently required surgical procedure for implantable defibrillator implantation is a limiting factor and several groups are therefore investigating transvenous approaches. Our electrode system consists of a nondistal right ventricular catheter electrode and two or three subcutaneously (SC) placed electrodes. An optimal location for these SC electrodes is important to obtain the lowest possible defibrillation threshold (DFT) by allowing a more homogeneous current distribution within the thorax. An empirical approach consists of placing randomly the SC electrodes to find out the lowest possible DFT. A mathematical approach is to calculate the SC electrode locations for an optimal electric field distribution by using magnetic resonance images of thorax cross-sections and a specially designed computer program. Our recent experimental results are based on a series of 15 pigs weighing between 60 and 102 Kg. DFT ranged between 10 and 26 joules. We conclude that an electrode system with a right ventricular electrode and two or three subcutaneous electrodes can be optimized to reach a DFT for pigs with human-near body weights which is compatible with the energy capabilities of our implantable device.

On the Reconstruction of Local Homogeneous Neutron Flux and Current Distributions of Light Water Reactors from Nodal Schemes

For nodal light water reactor calculations, the reconstruction of local multigroup neutron flux and current distributions within a node is discussed. Interpolation schemes that exploit the typical behavior of multigroup neutron spectra are proposed. These spectral interpolation schemes will be seen to be more accurate than the usual polynomial approaches. They reduce the error of pin power interpolation to that of fast flux interpolation. Thus the accuracy of one node per assembly calculations is considerably enhanced. The improvement is demonstrated by several benchmark problems.

Experimental investigation of the current density distribution in a simulated MHD generator.

In a magnetohydrodynamic generator with segmented electrodes, the Hall effect prevents a homogeneous current distribution in the region of the electrodes. The results of calculations of the current density distribution in an inert gas alkali metal plasma using simplifying assumptions were compared with measurements made in a simulated argon-potassium generator. The gas in the generator channel had a temperature of about 2000°K and a pressure of 1.1 atm. The potassium content was 0.22% by weight. The gas velocity did not exceed 130 m/sec and the magnetic-field strength was held below 4500 gauss. The emf was simulated by applying an electric field of about 15 v across each electrode pair. Measurements of the intensity distribution of the potassium resonance lines permitted conclusions to be drawn with respect to the distribution of electron temperature and thus current density. In general, qualitative agreement with theory was found. The most pronounced deviation from theory occurred near the electrodes, particularly along the insulator between electrodes. The potential distribution measured along the insulator wall was consistent with light-intensity measurements.

Field and Angular Dependence of Critical Currents inNb3Sn

Measurements have been made on the critical current of ${\mathrm{Nb}}_{3}$Sn vapor-deposited strips on ceramic material. The ${\mathrm{Nb}}_{3}$Sn was obtained from vapor deposition and is single phase without any preferred orientation in the plane of the strip. Critical currents were obtained at 4.2\ifmmode^\circ\else\textdegree\fi{}K as a function of field up to 20 000 G, and as a function of the angle between the field and the current axis. Below 10 000 G a sharp rise in critical current by about a factor of two was observed for longitudinal fields, and a gradual decrease in critical current for transverse fields. Above 10 000 G the field and angular dependence is in good agreement with the predictions of the Lorentz force model of Kim et al. Below 10 000 G, the field dependence can be associated with a transition from an inhomogeneous to homogeneous current distribution with consequent lower local critical current densities. Experimental results are also reported for field shielding measurements on similar material in cylindrical form. Excellent agreement is obtained in both magnitude and field dependence with the strip data.