Asymmetric Current Research Articles

Inferring bottom circulation based on sediment pattern distribution in the San José Gulf, Patagonia Argentina

The goal of this work was to infer the spatio-temporal patterns of bottom circulation of San José Gulf (SJG), Patagonia, Argentina. The SJG is mainly affected by tidal circulation and presents two hydrographic domains. The Western Hydrographic Domain (WHD) presents vertically mixed waters year-round contrary to the seasonal stratified Eastern Hydrographic Domain (EHD), resulting in the formation of a thermal front between domains. Samples of sediments were collected in spring 2016 and summer 2017 at several locations to analyze the grain-size composition by means of laser diffractometry. A conceptual model of bottom hydrodynamics was inferred based on sediment's grain-size trends. Asymmetric dominant tidal currents affect bottom sediment distribution. A general northward sediment transport is dominant at the WHD, while at the EHD there is a seasonal shift in the transport direction. There, a net northward transport with a counterclockwise gyre dominates bottom hydrodynamics at the end of the thermal front formation, conditioned by residual currents. In contrast, a net southward circulation likely driven by strong westerly winds throughout autumn and winter prevails at the beginning of the water column stratification. The inferred sediment transport suggests a net loss of material that has to be recovered by tidal currents entering through the mouth and coastal sources as runoff, mudslides, and/or aeolian dust.

Resistive switching device with highly asymmetric current–voltage characteristics: a solution to backward sneak current in passive crossbar arrays

With the view towards future non-volatile random access memories that can be integrated at a large scale, extensive study on resistive switching (RS) devices arranged in a crossbar array is currently underway. Although the crossbar array architecture offers relatively simple and acceptable scalability, the presence of sneak current is recognized as a critical issue that needs to be resolved at device level. In addressing this issue, we demonstrate a new type of RS device fabricated by combining graphene oxide (G–O) and zinc oxide (ZnO) with highly asymmetric current–voltage (I–V) characteristics depending on the polarity of bias voltage. The distinctive highly asymmetric I–V characteristics result from the presence of a hetero-junction interface formed between the G–O and ZnO layers. This hetero-junction manifests resistance in the range of GΩ under both forward and reverse bias voltage when the device is in the OFF state, in contrast, when the device is in the ON state, it exhibits resistance in the range of MΩ or kΩ under forward bias and GΩ under reverse bias. We propose to employ demonstrated RS devices with highly asymmetric I–V characteristics to mitigate adverse effects of the sneak current.

Three-Dimensional Space Vector Modulation Strategy for Capacitor Balancing in Split Inductor Neutral-Point Clamped Multilevel Inverters

Capacitor imbalance is one of the major drawbacks of a neutral-point clamped multilevel inverter (NPC-MLI). The capacitor imbalance rises due to the nonuniform switching, nonideal DC link capacitors, improper commutation, and various asymmetrical phase currents in switching states. The imbalance can be minimized by using proper switching using a redundancy switching method and avoiding the usage of medium vectors. The computational cost of this system is to be decided by the level of the inverter system. This paper presents a comprehensive three-dimensional space vector modulation (3D-SVM) strategy to eliminate the imbalance in the DC link capacitor voltage, which is across the applied input DC source. The technique is easy to implement without using any trigonometric functions, lookup tables, or angle determinations. This proposed scheme has been verified using MATLAB Simulink and authenticated using field-programmable gate array (FPGA) controller.

Electrical characteristics of etched ion-tracks in polyimide filled with silver nanoparticles

ABSTRACTSilicon ions, of energy 150 MeV and fluence ∼1012 ions/cm2, were used to register latent tracks in 40 µm thick polyimide samples. Different sizes of tracks were obtained by etching the ion irradiated polyimide samples, in chemical solutions, by varying the temperature and etching period. Silver nanoparticles were diffused into the etched tracks by immersing the polyimide samples in silver solution and then irradiating with 6.5 MeV electrons at different fluences varying from 1 × 1015 to 5 × 1015 cm−2. Results of morphological and elemental analysis, carried out by Scanning Electron Microscopy and Energy Dispersive X-ray. Analysis revealed that the conical tracks could be fully filled with silver nanoparticles at electron fluence of 5 × 1015 cm−2. The minimum d. c. resistance of an array of tracks, filled with silver nanoparticles and measured across the polyimide film, was orders of magnitude higher as compared to that of silver wires of equivalent sizes connected in parallel. In addition, these silver nanoparticles filled tracks exhibited rectifying I–V behavior and frequency dependent a. c. resistance, characteristic of metal–polymer nano-composites. Possible mechanisms have been discussed, which can justify the asymmetric current–voltage characteristics in such nano-composites.

Active Power Line Conditioner with Impulse Asymmetric Current Battery Charging System

A scheme of an active power line conditioner with a battery charge system with impulse asymmetric current battery charging system is presented. Numerous publications point to the expediency of constructing a system that would realize both functions of compensation of reactive power and the provision of guaranteed power supply. The existing nonlinearity of loads affects the reduction of the power factor and the increase of the content of higher harmonics of consumpted current, which leads to distortion of form of network voltage. Many loads require high quality power supply, and their high-quality work can only be achieved with low content of higher harmonics and a shape factor that corresponds to the ideal sinusoidal voltage. In addition, the deviation of the power factor from one and the corresponding current distortion result in energy losses in the networks and equipment, which in turn is one of the reasons for additional heating and a decrease in the life of the devices. From the analysis of battery charge methods, it follows that the method of impulse asymmetric current battery charging exceeds the quality of the other and provides a reduction in the charge: the memory effect, peak values of temperature and internal pressure of the battery, which can increase the life of the battery in 1.5-3 times. The main disadvantage of this method of battery charge is a relatively low efficiency, but with the use of circuit solutions that allow using the energy of a bit pulse repeatedly to charge the battery, the efficiency is increased to 80-90% and above. Total increase in the efficiency compared with existing devices reaches 12-34%. The purpose of the work is to calculate the reactive elements and the main parameters of the filter-compensating converter when used together with the system for the sake of the battery with impulse asymmetric current. Time diagrams describing the main operating modes of the device, as well as equivalent circuits of the converter, that describes its operation at the appropriate time intervals are given. Using the balance of energy and the method of calculation on average currents and voltages, expressions for calculating the basic parameters of the compensator, provided the consumed additional energy for charging the battery are derived. It is determined that the capacity of the storage capacitor increases in proportion to the additional energy required to charge the battery, with the constant value of the inductance of the smoothing inductor.Ref. 10, fig. 3, tabl. 1.

Research on Unbalanced Current Compensation Control Strategy Based on Neutral Point Clamp Converter

The use of distributed generation systems has led to serious power quality problems in low-voltage distribution networks. Low voltage distribuion network power quality needs to be valued. In this paper, a multi-function power converter (MPC) device with balanced three-phase asymmetrical current, reactive power compensation, power factor improvement and harmonic elimination is proposed. The paper introduces a MPC topology which has three-level inverter with a neutral point clamped (NPC) and a voltage balancer (VBC). A new control method combining the network current reference algorithm and PQ decoupling is given in this paper. Finally, the 100kW simulation system is built to verify the effectiveness of the proposed control strategy, which can effectively solve the power quality problems in the low voltage distribution network by using MPC. Some simulation and experimental results are illustrated to show the prototype device’s performance.

Evaluation of Low-Power Instrument Transformers for Generator Differential Protection

In hydroelectric power plants, which are characterized by particularly high short-circuit current levels and high time constants, conventional current transformer specifications can result in increased equipment dimensions, thus complicating their installation, especially during refurbishment projects. This paper presents a study to assess the potential of various low-power instrument transformers (LPITs) for generator protection applications. Two types of LPITs were evaluated: the optical current transformer (OCT) and the stand alone merging unit (SAMU). To test the OCT, a 65-kA-peak test source was developed using a real-time simulator. This source was used to inject symmetrical and asymmetrical currents into LPITs for the assessment of protection accuracy class. Low-current tests were also performed to assess their metering classes. This paper presents the design of a high-current test bench, measurement uncertainty analysis, and analysis algorithms used for device evaluation. Analysis results are then presented and discussed in terms of their applicability to generator differential protection using SAMUs and OCTs.

Size dependence of the spin-orbit torque induced magnetic reversal in W/CoFeB/MgO nanostructures

The spin-orbit torque (SOT) induced magnetic switching in structures such as Hall bars cannot be well explained with the macrospin model. The switching process is affected by the domain wall (DW) dynamics. In previous studies, some observed phenomena, such as intermediate states appearing during the magnetic switching of the Hall bar structure and asymmetric switching currents in two directions, were not well explained. In this letter, by studying the SOT induced magnetic switching in W/CoFeB/MgO nanostructures with different size, these phenomena are demonstrated to be governed by the DW propagations in nanowires and asymmetric DW pinnings at the Hall cross. The size dependence of the switching current is observed and explained with the DW depinning model. These studies provide an approach to detect the properties of the structure, such as the quantification of the spin Hall angle in the heavy metal layer.

Noninvasive electrical plasma monitoring method using reactor substrates as alternative current-sensing electrodes

The authors propose a noninvasive electrical plasma monitoring method that uses two initially present metal reactor substrates, without the probe insertion. When a small sinusoidal voltage is applied between these two substrates, harmonic currents flow in a closed-loop circuit through the plasma. Assuming that the plasma exhibits nonlocal electron kinetics, the electron temperature and plasma density are determined based on an asymmetric double probe harmonic currents analysis. Experimental demonstrations were conducted in an inductively coupled plasma reactor, in which a grounded substrate and a bias electrode were used as the current-sensing electrodes. The electron temperature and plasma density measured with the proposed method agree well with measurements from a floating-type planar probe. This method can be applied to processing reactors that have no available port for electrical probe installation.

Scrape-off-layer current loops and floating potential in limited tokamak plasmas

We investigate the question of how plasma currents circulate and close in the scrape-off-layer (SOL) of convection-limited tokamak plasmas. A simplified two-fluid model describes how currents must evacuate charge at the sheaths due to cross-field currents that are not divergence-free. These include turbulence-driven polarization currents and poloidally asymmetric equilibrium diamagnetic currents. The theory provides an estimate for the radial profile of the floating potential, which reveals a dipolar structure like the one observed experimentally. Simulations with a fluid turbulence code provide evidence for the predicted behaviour of currents and floating potential.

Fast analytical calculation method for eddy current induced by gradient fields in an MRI system

PurposeEddy currents are inevitable in magnetic resonance imaging (MRI) systems. These currents are mainly induced by gradient fields. This study aims to propose a fast analytical method to calculate eddy currents induced by frequently switching gradient fields in a traditional C-shape MRI system.Design/methodology/approachFourier decomposition and magnetic vector potentials were used to calculate the eddy currents. Calculations with the proposed analytical method revealed the spatial distribution and temporal evolution of eddy currents.FindingsCalculation and Maxwell simulation results were consistent. The agreement between calculation and simulation results indicates that increasingly sophisticated structures could be developed. The calculated results could guide the design of improved gradient coils.Originality/valueEddy currents induced by gradient current are decomposed into currents induced by each time-harmonic component, and then adding them together to obtain complete contribution of the eddy current. The analytical method was used to characterize the properties of symmetric and asymmetric eddy currents induced by gradient coils in MRI systems. The analytical method can be used to improve the gradient shield during the design of the gradient coil in the MRI system.

Investigation of the I – V characteristics asymmetry in semiconducting Y–Ba–Cu–O diodes

Thin films of YBa2Cu3O7–x (YBCO) oxygen-deficient superconductor were used to manufacture asymmetric microdiodes and investigate their asymmetric non-linear current–voltage (I –V) dependences at temperatures T > Tc, where Tc is the temperature of the onset of superconductivity in the material. The diodes are based on being asymmetrically narrowed down to a 10-μm-wide–neck mesa exhibit voltage asymmetry depending on the current bias conditions, the mesa's temperature, and a residual oxygen content in its neck region. The intrinsic electric field in the biased diode causes free-carrier drift velocity saturation, initiated in the mesa's neck and following its asymmetric enlarging towards the mesa's electrodes depending on the biasing pulsed current amplitude, polarity, and rise time/fall time of the pulse's leading/trailing edges. The variation of the I –V characteristics asymmetry has been associated with change in the mesa's electric resistivity, affected by the electric field's asymmetric distribution in the YBCO mesa. It has been also demonstrated that our asymmetric diodes with the reduced oxygen content are sensitive detectors of microwave radiation.

Dynamics-dependent synchronization in on-chip coupled semiconductor lasers.

Synchronization properties of chaotic dynamics in two mutually coupled semiconductor lasers with optical feedback embedded in a photonic integrated circuit are investigated from the point of view of their dynamical content. A phenomenon in which the two lasers can show qualitatively different synchronization properties according to the frequency range of investigation and their nonlinear dynamics is identified and termed dynamics-dependent synchronization. In-phase synchronization is observed for original signals and antiphase synchronization is observed for low-pass filtered signals in the case where one of the lasers shows chaotic oscillations while the other laser exhibits low-frequency fluctuations dynamics. The experimental conditions causing the synchronization states to vary according to the considered frequency interval are studied and the key roles of asymmetric coupling strength and injection currents are clarified.

Tunnelling characteristics of Stone–Wales defects in monolayers of Sn and group-V elements

Topological defects in ultrathin layers are often formed during synthesis and processing, thereby strongly influencing the electronic properties of layered systems. For the monolayers of Sn and group-V elements, we report the results based on density functional theory determining the role of Stone–Wales (SW) defects in modifying their electronic properties. The calculated results find the electronic properties of the Sn monolayer to be strongly dependent on the concentration of SW defects, e.g. defective stanene has nearly zero band gap (≈0.03 eV) for the defect concentration of 2.2 × 1013 cm−2 which opens up to 0.2 eV for the defect concentration of 3.7 × 1013 cm−2. In contrast, SW defects appear to induce conduction states in the semiconducting monolayers of group-V elements. These conduction states act as channels for electron tunnelling, and the calculated tunnelling characteristics show the highest differential conductance for the negative bias with the asymmetric current–voltage characteristics. On the other hand, the highest differential conductance was found for the positive bias in stanene. Simulated STM topographical images of stanene and group-V monolayers show distinctly different features in terms of their cross-sectional views and distance-height profiles. These distinctive features can serve as fingerprints to identify the topological defects in experiments for the monolayers of group-IV and group-V elements.

Piezotronic and Piezophototronic Properties of Orthorhombic ZnSnN2 Fabricated Using Zn–Sn3N4 Composition Spreads through Combinatorial Reactive Sputtering

This paper presents a combinatorial methodology for fabricating orthorhombic ZnSnN2 (ZTN) by using Zn–Sn3N4 composition spreads. This study is the first to verify the substantial piezotronic and piezophototronic features of ZTN on the basis of asymmetric current–voltage (I–V) characteristics. Regarding the piezophototronic effect, at a −5 V bias, the current density was enhanced up to 2.5 times when the applied pressure increased from 0.625 to 2.5 GPa. Schottky barrier height variations at S1 and S2 were calculated under a pressure of 2.5 GPa and were observed to have increased and decreased by approximately 1.0 and 22 mV, respectively. The results clarified the I–V behavior and also supported the proposed energy-band structure evolution of piezotronic and piezophototronic ZTN. In addition, ZTN formation was verified through X-ray photoelectron spectroscopy and X-ray diffraction. A deconvolution algorithm was employed to validate the ratio of orthorhombic ZTN (Pna21) (approximately 30%). In addition, UV–v...

Measurement of scrape-off-layer current dynamics during MHD activity and disruptions in HBT-EP

We report scrape-off layer (SOL) current measurements during magnetohydrodynamic (MHD) mode activity, resonant magnetic perturbations (RMPs), and disruptions in the High Beta Tokamak—Extended Pulse (HBT-EP) device. Currents are measured via segmented plasma current Rogowski coils, jumpers running toroidally between otherwise-isolated vessel sections, and a grounded electrode in the scrape-off layer. These currents strongly depend on the plasma’s major radius, and amplitude and phase of non-axisymmetric field components. SOL currents connecting through the vessel are seen to reach ∼ of the plasma current during typical kink activity and RMPs. Plasma current asymmetries and scrape-off-layer currents generated during disruptions, which are commonly called halo currents, reach ∼ of Ip. Asymmetric toroidal currents between vessel sections rotate at tens of kHz through most of the current quench, then symmetrize once Ip reaches ∼ of its pre-disruptive value. Toroidal jumper currents oscillate between co- and counter-Ip, with co-Ip being dominant on average during disruptions. A relative increase in local plasma current measured by a segmented Ip Rogowski coil correlates with counter-Ip current in the nearest toroidal jumper. Measurements are interpreted in the context of two models that produce contrary predictions for the toroidal vessel current polarity during disruptions. Plasma current asymmetry measurements are consistent with both models, and SOL currents scale with plasma displacement toward the vessel wall. The design of an upcoming SOL current diagnostic and control upgrade is also briefly presented.

Artificial synaptic characteristics with strong analog memristive switching in a Pt/CeO2/Pt structure

Artificial synaptic potentiation and depression characteristics were demonstrated with Pt/CeO2/Pt devices exhibiting polarity-dependent analog memristive switching. The strong and sequential resistance change with its maximum to minimum ratio >105, imperatively essential for stable operation, as repeating voltage application, emulated the potentiation and depression motion of a synapse with variable synaptic weight. The synaptic weight change could be controlled by the amplitude, width, and number of repeated voltage pulses. The voltage polarity-dependent and asymmetric current–voltage characteristics and consequential resistance change are thought to be due to local inhomogeneity of electrical and physical states of CeO2 such as charging at interface states, valence changes of Ce cations, and so on. These results revealed that the CeO2 layer could be a promising material for analog memristive switching elements with strong resistance change, as an artificial synapse in neuromorphic systems.

A Study on the YBCO-Coated Conductor Current Lead With Asymmetric Structure Tape Considering Temperature Distribution

A current lead is a connecting device between the superconducting magnet and the power supply. A large amount of heat transfer ocurrs through the current lead because the cold-end in a superconducting system is thermally linked with the warm-end part. Therefore, high-Tc superconductor (HTS) is used in the current lead for a tokamak system. However, it is hard to consider the dependance of HTS's critical current with respect to temperature because the conventional HTS current leads consist of the HTS tapes with the same width and thickness, and their critical currents can differ from their temperature. For this reason, cold-end of the HTS section in current lead is designed excessively than necessary specifications. This generates the additional heat transfer between cold-end and warm-end within the HTS current lead. In this paper, a study on the HTS current leads with asymmetric structure is conducted to reduce the heat transfer by reducing the cross-sectional area of the HTS current lead. Asymmetric current leads are fabricated considering the design parameters about heat transfer within the HTS current lead. Current transporting tests on the conventional, symmetrical, and asymmetrical HTS current leads are performed to confirm the feasibility of the suggested design. And then, heat load of the asymmetric current lead is compared with the symmetric current lead. From this study, it is found that heat load of the current lead is reduced by using the asymmetric HTS current lead.

Polarization modulation resistive switching in a lead-free ferroelectric Pt/Bi0.5Na0.5TiO3/La0.67Sr0.33MnO3 sandwiched heterostructure

High-quality lead-free Bi0.5Na0.5TiO3 (BNT)/La0.67Sr0.33MnO3 (LSMO) bilayer was grown on SrTiO3 crystal by pulsed laser deposition method. Ferroelectric and electrical properties of the Pt/BNT/LSMO heterostructure have been investigated. Interestingly, an asymmetric current–voltage hysteresis exhibits a ferroelectric BNT modulated resistive switching characteristics in BNT/LSMO heterostructure without a forming process. The On/Off ratio of resistance switching behaviors is higher than 103 and retention time is longer than 105 s. The relationship between potential barriers and polarization charges at the Pt/BNT and BNI/LSMO interfaces has been discussed. The Pt/BNT/LSMO cell with both high On/Off ratio and long retention properties exhibits potentials in lead-free nonvolatile memory applications.

Analysis of asymmetric magnetization current and reactive power demand of power transformers due to GIC

A comprehensive analysis of the asymmetric magnetization current of single phase power transformers due to geomagnetically induced currents, GICs, is presented in this paper. The aim of this work is to understand the main characteristic of this magnetization current such as peak current and harmonic contents. Usually, computation of the magnetization waveform requires a transient analysis in FEM or EMTP software. Also, the characteristic of the transformer and network parameters are demanded. We introduced a simple and accurate formula for estimation of the main characteristic of the magnetization current. Only general information of the transformer and the network is enough for this estimation. According to the result of this work, also a method for prediction of the reactive power demand is presented.The achievements of this paper are useful for designers of transformers in the calculation of losses due to GICs. Furthermore, the grid owners can have a good prediction of increasing reactive power demands and injected harmonics to the network due to a GIC event.