Large DC Currents Research Articles

Quantum Graphene Asymmetric Devices for Harvesting Electromagnetic Energy.

We present here the fabrication at the wafer level and the electrical performance of two types of graphene diodes: ballistic trapezoidal-shaped graphene diodes and lateral tunneling graphene diodes. In the case of the ballistic trapezoidal-shaped graphene diode, we observe a large DC current of 200 µA at a DC bias voltage of ±2 V and a large voltage responsivity of 2000 v/w, while in the case of the lateral tunneling graphene diodes, we obtain a DC current of 1.5 mA at a DC bias voltage of ±2 V, with a voltage responsivity of 3000 v/w. An extended analysis of the defects produced during the fabrication process and their influences on the graphene diode performance is also presented.

Nonvolatile magnetization switching in a single-layer magnetic topological insulator

Magnetization in a ferromagnetic layer could be manipulated by the spin-orbit torque whose generation commonly relies on the spin-orbit coupling from the adjacent heavy-metal layer within the bilayer. The fact that the magnetic topological insulator possesses both the ferromagnetic order with perpendicular anisotropy and inherent spin-orbit coupling inspires to realize such a torque-induced magnetization switching without forming any heterostructure with other materials. Here, only using a single layer of magnetically-doped topological insulator Cr:(Bi,Sb)2Te3, we realize a magnetization switching only by applying a large dc current. Assisted by the magnetic history, such a switching behaves nonvolatile under zero field but becomes volatile otherwise, as consistently shown by magnetoelectric transports and magneto-optical Kerr effect measurements. Static and quasistatic current are found to be equivalent for the switching. We propose that this switching may associate with the torque resulted from the spin-orbit coupling and the compositional asymmetry in the Cr-profile of the single layer.

High-current contactless ferromagnetic converters for multi-profile monitoring and control systems

The article shows that the undesirability of breaking the current circuit for the temporary inclusion of electrical measuring devices, the presence of large power losses on shunts, the impossibility of breaking the circuit under the conditions of the technological process, as well as safety requirements caused contactless conversion and measurement of large DC currents in circuits without breaking them in metallurgy, electrochemical industry, railway transport, in laser technology, renewable energy sources, land reclamation, irrigation and in general in agriculture. Lightweight detachable wide-band stationary and portable non-destructive contactless converters and meters of large direct currents are not yet mass-produced worldwide. Therefore, it is very important to develop and study contactless converters and meters of large direct currents, having an extended range of converted currents with small dimensions and weight and increased accuracy and sensitivity, a simplified and technologically advanced design with low material consumption and cost, and the ability to convert both constant and variable high currents and fixed sensitivity control. All these requirements are met by the effective multifunctional magneto-modulating contactless converters of large direct currents developed by us. The results of the development of one of them are given in the work. The issues of its dynamics are considered. The results of his research are shown. It was found that the transition time for him is 0.025 of the period of the industrial frequency. Therefore, it can be considered practically inertia-free. The developed effective multifunctional magneto-modulating contactless converter can be widely used in monitoring and control systems in water supply, land reclamation, and irrigation, industry, railway transport, metallurgy, science, technology for contactless monitoring of constant and alternating currents, as well as for checking electric meters at the place of their installation.

DC current generation and power feature in strongly driven Floquet-Bloch systems

We study the DC current generation in a periodically driven Bloch system connected to a heat bath. Under a relaxation time approximation, the density matrix for such a system is obtained, which is related to two equilibria: a Floquet quasiequilibrium where the density matrix is diagonal under the Floquet-Bloch eigenbasis and an instantaneous Bloch thermal equilibrium. Then, the current responses and their power features, i.e., the power input behavior, are discussed in a unified manner, which reveals that there exist an intrinsic current and an extrinsic correction. Remarkably, the intrinsic part consumes no energy and corresponds to the Floquet quasiequilibrium, while the extrinsic part needs a sustained energy input and originates from a shift between two equilibrium ensembles. We further investigate the role of the external driving field strength finding that large DC currents can be generated under a relatively strong but not too strong driving field.

3D modeling of a superconducting dynamo-type flux pump

High temperature superconducting (HTS) dynamos are promising devices that can inject large DC currents into the winding of superconducting machines or magnets in a contactless way. Thanks to this, troublesome brushes in HTS machines or bulky currents leads with high thermal losses will be no longer required. The working mechanism of HTS dynamo has been controversial during the recent years and several explanations and models have been proposed to elucidate its performance. In this paper, we present the first three-dimensional (3D) model of an HTS flux pump, which has good agreement with experiments. This model can be beneficial to clarify the mechanism of the dynamo and pinpoint its unnoticed characteristics. Employing this model, we delved into the screening current and electric field distribution across the tape surface in several crucial time steps. This is important, since the overcritical screening current has been shown to be the reason for flux pumping. In addition, we analyzed the impact of both components of electric field and screening current on voltage generation, which was not possible in previous 2D models. We also explored the necessary distance of voltage taps at different airgaps for precise measurement of the voltage across the tape in the dynamo.

Fast solution of the superconducting dynamo benchmark problem

A model of high-temperature superconducting dynamos, a promising type of flux pump capable of wireless injection of a large DC current into a superconducting circuit, has recently been chosen as an applied superconductivity benchmark problem and solved using ten different numerical methods (Ainslie et al 2020 Supercond. Sci. Technol. 33 105009). Using expansions in Chebyshev polynomials for approximation in space and the method of lines for integration in time we derive a simple and accurate numerical method which is much faster. The proposed numerical method was applied also to problems with transport current and a field-dependent sheet critical current density.

Efficient and Robust Nonvolatile Computing-In-Memory Based on Voltage Division in 2T2R RRAM With Input-Dependent Sensing Control

Resistive memory (RRAM) provides an ideal platform to develop embedded non-volatile computing-in-memory (nvCIM). However, it faces several critical challenges ranging from device non-idealities, large DC currents, and small signal margins. To address these issues, we propose voltage-division (VD) based computing approach and its circuit implementation in two-transistor-two-resistor (2T2R) RRAM cell arrays, which can realize energy-efficient, sign-aware, and robust deep neural network (DNN) processing. A readout technique, namely the input-dependent sensing control (IDSC) scheme, is also introduced for power saving. On this basis, a 400kb VD-based RRAM nvCIM is silicon verified. It achieves 2.54 times power reduction compared to that of the ones rely on conventional weighted-current summation (WCS) mechanism, a peak energy-efficiency of 42.6 TOPS/W and a minimum latency of 15.98 ns.

Modelling the Frequency Dependence of the Open-Circuit Voltage of a High-T c Superconducting Dynamo

A high-T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> superconducting (HTS) dynamo enables the injection of large DC currents into a superconducting circuit, without the requirement for current leads. In this work, we attempt to explain the frequency dependence of such dynamos/flux pumps reported in the literature, where it is observed that the rate at which the open-circuit DC voltage increases reduces with increasing frequency, in contrast to the expected linear behaviour. Heat generated in the HTS wire has been the common explanation given to date for this phenomenon. Here we offer an alternative explanation: the interaction between and current flow in the different layers of the HTS wire as the frequency of the dynamo increases. Our claim is based on numerical analysis using a segregated H-formulation finite-element model of the HTS dynamo benchmark problem that is extended to include the full HTS wire architecture and coupled with a thermal model. This framework enables us to efficiently model the relative movement between the rotating room-temperature permanent magnet and the stationary HTS wire and to study the impact of the frequency of rotation and temperature on the open-circuit DC voltage of the dynamo.

Two-Dimensional Model of a High-T c Superconducting Dynamo

High temperature superconducting dynamos are capable of low loss contactless pumping large dc currents into superconducting magnet coils. We present a model of such devices and use efficient numerical methods for computing the two-dimensional loop currents and electric fields induced in the thin superconducting stator strip by a dynamo rotor mounted permanent magnet. We find also the voltages generated in the stator under the open circuit conditions. It is shown that if the length of a permanent magnet is comparable to or smaller than the strip width, the one-dimensional model employed in the previous works can be inaccurate.

Mechanism of the High- Tc Superconducting Dynamo: Models and Experiment

High-${T}_{c}$ superconducting (HTS) dynamos are experimentally proven devices that can produce large (more than a kiloamp) dc currents in superconducting circuits, without the thermal leak associated with copper current leads. However, these dc currents are theoretically controversial, as it is not immediately apparent why a device that is topologically identical to an ac alternator should give a dc output at all. Here, we present a finite-element model and a comparison of it with experiment that fully explain this effect. It is shown that the dc output arises naturally from Maxwell's laws when time-varying overcritical eddy currents are induced to circulate in a HTS sheet. We first show that our finite-element model replicates all of the experimental electrical behavior reported so far for these devices, including the dc output characteristics and transient electrical waveforms. Direct experimental evidence for the presence of circulating eddy currents is also obtained through measurements of the transient magnetic field profile across the HTS tape, using a linear Hall array. These results are also found to agree closely with predictions from the finite-element model. Following this experimental validation, calculated sheet current densities and the associated local electric fields are examined for a range of frequencies and net transport currents. We find that the electrical output from a HTS dynamo is governed by the competition between transport and eddy currents induced as the magnet transits across the HTS tape. The eddy currents are significantly higher (approximately 1.5 times) than the local critical current density, and hence experience a highly nonlinear local resistivity. This nonlinearity breaks the symmetry observed in a normal ohmic material, which usually requires the net transport current to vary linearly with the average electric field. The interplay between local current densities and nonlinear resistivities (which both vary in time and space) is shown to systematically give rise to the key observed parameters of experimental HTS dynamo devices: the open-circuit voltage, the internal resistance, and the short-circuit current. Finally, we identify that the spatial boundaries formed by each edge of the HTS stator tape play a vital role in determining the total dc output. This offers the potential to develop alternative designs for HTS dynamo devices, in which the internal resistance is greatly reduced and the short-circuit current is substantially increased.

New Small-Volume High-Precision TMR Busbar DC Current Sensor

Large dc current is typically measured by an open-loop sensor without a magnetic core. The widely used configuration uses a differential magnetic sensor inserted into the busbar hole or a differential magnetic sensor placed on the upper and lower surfaces of the busbar. The use of a differential sensor has the advantage of being resistant to external magnetic fields. In this article, a new differential ultra-small magnetic sensor with the size of 23.2 mm × 16.5 mm × 7.2 mm is designed. The sensor is based on tunneling magnetoresistance (TMR) and is placed on the upper surface edge of the busbar. The supply voltage is 5 V, the rated current measurement range is ±150 A, and the peak current measurement range is ±250 A, corresponding to 2.5 ± 2 V output at ±250 A dc peak current. The relative accuracy is less than ±1% at 25 °C and is less than ±2% over the full temperature range from -40 °C to 105 °C.

Design and testing of a system for measuring high-frequency AC losses in superconducting wires and coils carrying DC and AC currents

Development of high-power superconducting applications requires the accurate estimation of AC losses in the superconductor. In applications such as superconducting magnetic energy storage, the charge/discharge/persistent switching frequency of the coil, resulting from pulse width modulation control algorithms, is usually in the kilohertz regime. Therefore, a thorough investigation of the losses in the kilohertz regime of AC currents superimposed on large DC currents is essential in order to ensure the device stable operation at a predefined temperature. We describe here a unique experimental setup designed and built for characterizing AC losses in superconducting wires and coils under such special conditions. To minimize the eddy currents induced in the apparatus, a cryostat vacuum vessel was made of Delrin, an insulating synthetic polymer. The measurement setup allows driving DC currents up to 150 A and superimposed AC currents with amplitudes up to 10 Arms and frequencies up to 18 kHz. The system utilizes conduction cooling to reach a wide range of temperatures between 6 and 100 K and allows measurements of 10 cm long superconducting wires and coils with a diameter of 40 cm. The loss is measured by the electrical method, i.e., by direct voltage and current waveform measurement, achieving a resolution better than 100 nW. The system described here will assist in developing superconducting wires and coils for high-power applications.

Spin injection with large DC currents in YIG/Pt nanostructures

Through the simple electrical measurement, the spin injection to magnetic insulators via large DC currents applied in heavy metals is reported. The measured nonlocal voltages are divided into two types: the electrical-generated magnons (Vodd) and the thermal-generated magnons (Veven) according to the different symmetry with respect to the applied current. With the current increasing, a nonlinear behavior of Vodd is observed. The dependences of Vodd on the current and temperature are compared, excluding the effect of current-induced heating on the nonlinear transport. The field dependence of the nonlocal voltages is also systematically studied which indicates the frequency-dependent mechanism needs to be introduced to explain the nonlinearity. It is possible that the frequency change of the dominate magnons when the current is large enough to drive the magnetization out of thermal equilibrium induces the deviation from the linear relationship in the low current region.

Miniature orthogonal fluxgate sensor in rotation magnetization mode: Modeling and characterization

Orthogonal fluxgates operated with a dc bias can achieve lower noise levels by suppressing Barkhausen jumps. We constructed and mathematically modeled a small orthogonal fluxgate consisting of a NiFe electroplated Cu wire, surrounded by a pick-up solenoid. A large dc current was applied through the wire to keep the core in constant saturation. This ensures that the main permeability modulation mechanism is magnetization rotation. Several output parameters were analyzed: sensitivity, noise and the perming error amplitude, all with respect to two input parameters, constant and alternating excitation field component. A good agreement between the experimental data and the theoretical model was found, with higher deviations appearing when the excitation field was not high enough to saturate the core and domain wall movement becomes relevant. Measured sensitivities were highly dependent on the excitation amplitudes, but mostly in the hundreds of V/T, and noise levels of about 0.75 nT in the 0.1–10 Hz range were found. As for the perming, the sensor zero readout can vary about 20 μT if subjected to magnetic shocks of up to 4.8 mT.

Output During Continuous Frequency Ramping of a Dynamo-Type HTS Flux Pump

Flux pumps can inject large dc currents into superconducting coils, without requiring physical connections between the cryogenic superconducting circuit and the room-temperature environment, thus significantly lowering the heat load on the cryogenic cooling system. Here, we study a dynamo-type flux pump employing a 2G ReBCO coated conductor wire at 77 K with multiple spinning Nd–Fe–B magnets and a soft ferromagnetic iron yoke, with all moving parts situated outside of the cryogenic environment. Dynamo-type flux pumps can be modeled as a dc voltage source with an internal resistance, where all output parameters are functions of the speed (frequency) of the spinning magnets. In this work, we focus on the frequency dependence of the dc output voltage. From instantaneous traces of the generated voltage waveform, we observe three distinct frequency regimes of behavior. We classify them into regions of “low,” “mid,” and “high” magnet frequency, depending on the magnitude of the dc open-circuit voltage and the shape of the voltage waveform. The voltage-generation mechanism is qualitatively understood at “low” magnet frequencies, where the dc voltage increases proportional to frequency. However, at “mid” and “high” magnet frequencies, the dynamo output deviates from this accepted model. In the “mid” frequency region, the dc output voltage is roughly constant with frequency, but is unstable. At “high” frequencies, the dc voltage drops with increasing frequency following an approximately 1/ f dependence. We have also studied the effects of ramping acceleration rate, noting a high degree of reproducibility in the low- and high-frequency regions, but complex unstable behavior in the intermediate region. We discuss our results in the context of self-heating due to EMF-driven eddy currents, leading to partial-quenching of the coated conductor wire. This becomes the dominant limiting factor in flux pump output at high frequencies, eventually suppressing the dc output altogether.

Impact of Magnet Geometry on Output of a Dynamo-Type HTS Flux Pump

A superconducting flux pump based on a high-temperature superconducting (HTS) dynamo has been demonstrated which employs rotating permanent magnets and a ReBCO conductor stator wire. This device is capable of injecting large dc currents (>100 A) into industrial superconducting magnet coils. Dynamic resistance, due to the interaction of the dc current with the ac magnetic field at the stator, provides a performance limit for this type of HTS dynamo exciter. It has been shown for a given device, the output of the HTS dynamo can be described by a simple circuit model linking short-circuit current (I sc ), dynamic resistance (R d ), and open-circuit voltage (V oc ). I sc represents the maximum dc current that can be delivered by the dynamo to a series-connected superconducting circuit and is found to be independent of rotational frequency for a fixed device geometry. We have recently demonstrated that this dynamo effect can be applied to excite a closed superconducting circuit through the cryostat wall, such that all active components of the dynamo are located outside of the cryogenic envelope. Here, we report experimental results which explore the relationship between rotor magnet geometry and ReBCO stator wire width on the output performance of a simple HTS dynamo. We characterize the effect of magnet orientation, aspect ratio and shape, at fixed flux gap, on I sc , R d , and V oc .

Novel neurobiological properties of elements in the escape circuitry of the shrimp.

Escape behaviors in penaeid shrimp are mediated by large myelinated medial giant fibers which course from the brain to the last abdominal ganglion in the ventral nerve cord. In each abdominal segment, the medial giant axons make synaptic connections with paired myelinated motor giant axons that excite the abdominal deep flexor muscles and drive the tailflips that constitute the escape behavior. I examined (1) anatomical features of the abdominal motor giant fibers and (2) electrical properties of both the medial and motor giant axons in the pink shrimp, Farfantepenaeus duorarum The motor giant axons in the paired third roots of shrimp abdominal ganglia emerge from a single fused neurite that originates from two clusters of cell bodies within the ganglion. Injection of large positive currents into the abdominal medial giant fibers generates action potentials that are transmitted to the opposite medial giant axon through putative collateral synapses within the ganglia. Transmission across the medial-to-motor giant synapse is fast and resistant to fatigue, with synaptic delays equal to or less than those previously documented at the lateral-to-motor giant electrical synapse in crayfish. Transmission was found to be extremely reliable even with presynaptic spike frequencies as high as 250 Hz. While action potentials within the medial giant fibers are transmitted across the medial-to-motor giant synapse with a large safety factor, neither prolonged positive nor prolonged negative currents pass through the synaptic nexus, irrespective of the site of injection. The lack of DC current passage along with the inability of neurobiotin or biocytin to spread through the synaptic nexus raises the possibility that the synaptic mechanism may be capacitative.

Current-Induced Pinwheel Oscillations in Perpendicular Magnetic Anisotropy Spin Valve Nanopillars

Nanopillar spin valve devices are typically comprised of two ferromagnetic layers: a reference layer and a free layer whose magnetic orientation can be changed by both an external magnetic field and through the introduction of spin-polarized electric current. Here we report the continuous repeated switching behavior of both the reference and free layers of a perpendicular spin valve made of Co/Pd and Co/Ni multilayers that arises for sufficiently large DC currents. This periodic switching of the two layers produces an oscillating signal in the MHz regime but is only observed for one sign of the applied current. The observed behavior agrees well with micromagnetic simulations.

A Fluxgate Current Sensor With an Amphitheater Busbar

Large dc and ac electric currents are often measured by open-loop sensors without a magnetic yoke. A widely used configuration uses a differential magnetic sensor inserted into a hole in a flat busbar. The use of a differential sensor offers the advantage of partial suppression of fields coming from external currents. Hall sensors and AMR sensors are currently used in this application. In this paper, we present a current sensor of this type that uses novel integrated fluxgate sensors, which offer a greater range than magnetoresistors and better stability than Hall sensors. The frequency response of this type of current sensor is limited due to the eddy currents in the solid busbar. We present a novel amphitheater geometry of the hole in the busbar of the sensor, which reduces the frequency dependence from 15% error at 1 kHz to 9%.

Development of a brushless HTS exciter for a 10 kW HTS synchronous generator

HTS synchronous generators, in which the rotor coils are wound from high-Tc superconducting wire, are exciting attention due to their potential to deliver very high torque and power densities. However, injection of the large DC currents required by the HTS rotor coils presents a technical challenge. In this paper we discuss the development of a brushless HTS exciter which operates across the cryostat wall to inject a superconducting DC current into the rotor coil circuit. This approach fundamentally alters the thermal load upon the cryogenic system by removing the need for thermally inefficient normal-conducting current leads. We report results from an experimental laboratory device and show that it operates as a constant voltage source with an effective internal resistance. We then discuss the design of a prototype HTS-PM exciter based on our experimental device, and describe its integration with a demonstration HTS generator. This 200 RPM, 10 kW synchronous generator comprises eight double pancake HTS rotor coils which are operated at 30 K, and are energised to 1.5 T field through the injection of 85 A per pole. We show how this excitation can be achieved using an HTS-PM exciter consisting of 12 stator poles of 12 mm YBCO coated-conductor wire and an external permanent magnet rotor. We demonstrate that such an exciter can excite the rotor windings of this generator without forming a thermal-bridge across the cryostat wall. Finally, we provide estimates of the thermal load imposed by our prototype HTS-PM exciter on the rotor cryostat. We show that duty cycle operation of the device ensures that this heat load can be minimised, and that it is substantially lower than that of equivalently-rated conventional current leads.