Shuki Wolfus - Profile on Academia.edu (original) (raw)

Papers by Shuki Wolfus

Energy Storing and Fault Current Limiting in a Unified Superconducting Magnetic Device

IEEE Transactions on Applied Superconductivity, Aug 1, 2023

Review of Scientific Instruments, Jun 1, 2019

Performance analysis of valveless piezoelectric pump with dome composite structures Review of Sci... more Performance analysis of valveless piezoelectric pump with dome composite structures Review of Scientific Instruments 90, 065002 (2019);

IEEE Transactions on Applied Superconductivity, Dec 1, 2018

AC losses for a wide range of ac amplitudes and frequencies have been studied in magnesium dibori... more AC losses for a wide range of ac amplitudes and frequencies have been studied in magnesium diboride (MgB 2) superconducting wire with 36 filaments and Monel sheath at different temperatures and dc current levels. The results show a strong nonlinear frequency dependence below 1 kHz, which crosses over to a more moderate linear behavior at frequencies up to 18 kHz. Surprisingly, the introduction of dc current causes a significant reduction in the ac losses. Finite element simulations yield ac losses consistent with that observed experimentally. The simulations show that the magnetic Monel sheath is a dominant source for ac losses in zero dc current and that nonzero dc current saturates the magnetization, thus reducing the ac losses. Index Terms-AC losses, finite element method (FEM), magnesium diboride (MgB 2), superconducting magnetic energy storage (SMES), superconducting filaments and wires.

Cryogenics, Apr 1, 2016

Current leads in cryogenic systems are a major heat source which eventually affects the entire sy... more Current leads in cryogenic systems are a major heat source which eventually affects the entire system. It has been shown in recent years that Peltier elements are useful in reducing incoming heat into the cold system. In this article we present a new tapered cone-like configuration of the Peltier Current Leads which increases the power saving. This configuration is compared to the standard cylindrical configuration utilizing advanced ANSYS simulations. The simulations show an additional power saving of 4% when using the tapered lead configuration.

We explore the effects of various receiver coil dimensions and configurations on power transfer e... more We explore the effects of various receiver coil dimensions and configurations on power transfer efficiency and cost of operation, using advanced simulation tools. We demonstrate that the spatial distribution of the magnetic field leads to a non-monotonic dependence of the coupling coefficient on coil size. Thus, an optimal coil size, where the coupling coefficient peaks, should be regarded a crucial design parameter which affects the entire system performances. The incorporation of our findings into a multi-objective optimization algorithm is also discussed.

Cryogenics, 2016

Saturated Cores FCL—A New Approach

The saturated cores FCL exhibits several attractive technological advantages: inherent fail-safe ... more The saturated cores FCL exhibits several attractive technological advantages: inherent fail-safe and selectivity design, superconductivity is maintained during both nominal and fault states, the limiting process as well as the recovery after fault are passive and immediate, operation in limiting state is not time-limited, and the superconducting bias coil is made of wires available as commercial shelf-product. Despite these advantages, saturated cores FCL did not make it to commercial phase because of the large volume and heavy weight associated with its realization, a coupling problem between the AC and bias coils while in limiting state, and non-optimal limitation resulting from the presence of the bias field during fault. This work presents a novel, improved saturated cores FCL concept that overcomes the above difficulties and reopens the possibility for commercialization. Unique design topography reduces the cores volume and at the same time reduces the AC and DC magnetic coupling to about 2%. In addition, a control circuit, triggered by voltage drop across the FCL terminals, is added and disconnects the bias coil during a fault for increased limiting performances. All above-mentioned advantages of the saturated cores concept are maintained in this new design. First, a 4.2 kVA laboratory scale FCL has been designed built and studied proving the feasibility of the new design. Then, an up-scaled, 120 kVA model has been designed, built and tested at the testing laboratory of the Israel Electric Company. The prospective short current in the test bed was 5000 A, successfully limited to 2400 A. The 120 kVA model is a single phase FCL designed for 400 V, 300 A nominal conditions. Core losses and AC coils losses are 0.09% and 0.18%, respectively.

Physical Review Letters, 1997

Local magnetic measurements in a highly anisotropic Nd-Ce-Cu-O crystal reveal a sharp onset of an... more Local magnetic measurements in a highly anisotropic Nd-Ce-Cu-O crystal reveal a sharp onset of an anomalous magnetization peak at a temperature-dependent field B on. The same field marks a change in the field profiles across the sample, from profiles dominated by geometrical barriers below B on to Bean-like profiles above it. The temperature dependence of B on and the flux distribution above and below B on imply a disorder-induced transition at B on from a relatively ordered vortex lattice to a highly disordered, entangled vortex solid. Local magnetic relaxation measurements above B on show evidence for plastic vortex creep associated with the motion of dislocations in the entangled vortex structure.

PLoS Computational Biology, 2011

Transcranial magnetic stimulation (TMS) is a stimulation method in which a magnetic coil generate... more Transcranial magnetic stimulation (TMS) is a stimulation method in which a magnetic coil generates a magnetic field in an area of interest in the brain. This magnetic field induces an electric field that modulates neuronal activity. The spatial distribution of the induced electric field is determined by the geometry and location of the coil relative to the brain. Although TMS has been used for several decades, the biophysical basis underlying the stimulation of neurons in the central nervous system (CNS) is still unknown. To address this problem we developed a numerical scheme enabling us to combine realistic magnetic stimulation (MS) with compartmental modeling of neurons with arbitrary morphology. The induced electric field for each location in space was combined with standard compartmental modeling software to calculate the membrane current generated by the electromagnetic field for each segment of the neuron. In agreement with previous studies, the simulations suggested that peripheral axons were excited by the spatial gradients of the induced electric field. In both peripheral and central neurons, MS amplitude required for action potential generation was inversely proportional to the square of the diameter of the stimulated compartment. Due to the importance of the fiber's diameter, magnetic stimulation of CNS neurons depolarized the soma followed by initiation of an action potential in the initial segment of the axon. Passive dendrites affect this process primarily as current sinks, not sources. The simulations predict that neurons with low current threshold are more susceptible to magnetic stimulation. Moreover, they suggest that MS does not directly trigger dendritic regenerative mechanisms. These insights into the mechanism of MS may be relevant for the design of multi-intensity TMS protocols, may facilitate the construction of magnetic stimulators, and may aid the interpretation of results of TMS of the CNS.

Physical Review Letters, 1996

Local magnetic relaxation measurements in YBa 2 Cu 3 O 7−x crystals show evidence for plastic vor... more Local magnetic relaxation measurements in YBa 2 Cu 3 O 7−x crystals show evidence for plastic vortex-creep associated with the motion of dislocations in the vortex lattice. This creep mechanism governs the vortex dynamics in a wide range of temperatures and fields below the melting line and above the field corresponding to the peak in the "fishtail" magnetization. In this range the

IEEE Transactions on Appiled Superconductivity, 2003

We have developed and tested a laboratory scale High-Superconducting Magnetic Energy Storage (HT-... more We have developed and tested a laboratory scale High-Superconducting Magnetic Energy Storage (HT-SMES) system with storage capacity of up to 1.2 kJ. It was designed to improve the power quality for a consumer supplied by 3-phase standard commercial electric power grid at a consumer power of up to 20 kW. This SMES is based on a high-superconducting coil with a ferromagnetic core, immersed in liquid nitrogen at 65 K to provide efficient thermal contact with the coolant. We also developed a cryogenic DC-DC converter based on low resistance power MOSFET transistors, providing low losses in the stored energy and high operational efficiency. The power conditioning capability of our HT-SMES was proved, and compensation of voltage drops in the electric grid was successfully demonstrated.

Skin Research and Technology, 2010

Background/purpose: Magnetic therapy has been popular for ages, but its therapeutic abilities rem... more Background/purpose: Magnetic therapy has been popular for ages, but its therapeutic abilities remain to be demonstrated. We aimed to develop a homogeneous, stable dispersion of magnetic nanoparticles in a skin-care preparation, as a tool to analyze the biological and physiological effects of superficial magnetism in skin. Methods: SrFe 12 O 19 nanoparticles were generated by ultrasound, dispersed in glycerol, stabilized in Dermudt cream and permanently magnetized. The magnetic cream was applied on the epidermis of human skin organ cultures. The effects on UV-induced cell toxicity, apoptosis and inflammatory cytokine expression were analyzed. A clinical test was performed to check skin moisturization. Results: Nanomagnets were found to be homogenously and stably dispersed. After magnetization, the preparation generated a magnetic field of 1-2 G. Upon cream application , no cytotoxicity and no impairment of cellular vitality were found after 24 and 48 h, respectively. The antiapoptotic and anti-inflammatory properties of Dermudt were not modified, but its long-term effect on moisturization in vivo was slightly increased. Conclusion: Nanomagnetic Dermudt cream can be used as a tool to analyze the biological effects of nanomagnets dispersed on the skin surface at the cellular and molecular levels, thus allowing to explore the possible therapeutic uses of superficial magnetism for skin care.

2017 Electric Vehicles International Conference (EV), 2017

Optimizing the efficiency of primary and secondary coil configurations for Dynamic Wireless Power... more Optimizing the efficiency of primary and secondary coil configurations for Dynamic Wireless Power Transfer (DWPT) in Electric Vehicles (EVs) requires means for accurate calculation of the mutual inductance in an array of coils. Based on finite element simulation method, we present a quick and accurate method for calculating the energy transfer capabilities of a given DWPT array. By consecutively switching on and off every coil and driving them with a constant current ramp rate, mutual inductances and a coupling coefficient matrix of the whole configuration is easily calculated. This method allows for relatively easy optimization and up-scaling of DWPT systems to multiple arrays of primary and secondary coils as well as implementing various coil designs and configurations. The data acquired using this method may also be used during real-time applications providing indications of vehicle relative alignment.

Energies

Designs of saturated-cores fault current limiters (FCLs) usually implement conducting or supercon... more Designs of saturated-cores fault current limiters (FCLs) usually implement conducting or superconducting DC coils serving to saturate the magnetic cores during nominal grid performance. The use of coils adds significantly to the operational cost of the system, consuming energy, and requiring maintenance. A derivative of the saturated-cores FCL is a design implementing permanent magnets as an alternative to the DC coils, eliminating practically all maintenance due to its entirely passive components. There are, however, various challenges such as the need to reach deep saturation with the currently available permanent magnets as well as the complications involved in the assembly process due to very powerful magnetic forces between the magnets and the cores. This paper presents several concepts, achieved by extensive magnetic simulations and verified experimentally, that help in maximizing the core saturation of the PMFCL (Permanent Magnet FCL), including optimization of the permanent ...

Frontiers in Cellular Neuroscience, 2014

Although transcranial magnetic stimulation (TMS) is a popular tool for both basic research and cl... more Although transcranial magnetic stimulation (TMS) is a popular tool for both basic research and clinical applications, its actions on nerve cells are only partially understood. We have previously predicted, using compartmental modeling, that magnetic stimulation of central nervous system neurons depolarized the soma followed by initiation of an action potential in the initial segment of the axon. The simulations also predict that neurons with low current threshold are more susceptible to magnetic stimulation. Here we tested these theoretical predictions by combining in vitro patch-clamp recordings from rat brain slices with magnetic stimulation and compartmental modeling. In agreement with the modeling, our recordings demonstrate the dependence of magnetic stimulation-triggered action potentials on the type and state of the neuron and its orientation within the magnetic field. Our results suggest that the observed effects of TMS are deeply rooted in the biophysical properties of single neurons in the central nervous system and provide a framework both for interpreting existing TMS data and developing new simulation-based tools and therapies.

Chinese Physics Letters, 1997

Superconductor Science and Technology, 2013

A saturated core fault current limiter (SCFCL) is a non-linear core-reactor where the core is sat... more A saturated core fault current limiter (SCFCL) is a non-linear core-reactor where the core is saturated by an external superconducting DC bias source to achieve a low core permeability at nominal AC currents. Fault current levels in the AC coils de-saturate the core and transform it to a higher permeability state, hence limiting the fault current. In this work we describe the transition between saturated and de-saturated states in three SCFCL configurations. The 'effective core length', L eff , of the SCFCL, defined as the length of the de-saturated AC core limb, is introduced for exploring this transition as a function of the current, I, in the AC coil. Practically, L eff allows one to see the SCFCL as an inductor with a variable core length, allowing calculations of the impedance of the SCFCL over the whole range of operating currents. The L eff (I) curve is further used to calculate the dynamics of the demagnetization factor in a SCFCL. We show that the strong change in the magnetic induction of a SCFCL at high current is the result of both increasing the effective core length and decreasing the demagnetization factor. The method and results presented here serve as an important tool for comparing between various SCFCL concepts not only by comparing their impedance values at the extreme fault and nominal current conditions but also by providing an insight into the full de-saturation process.

Journal of Materials Processing Technology, 2005

Since mid-1990s there was an impressive progress in manufacturing high-temperature superconductin... more Since mid-1990s there was an impressive progress in manufacturing high-temperature superconducting (HTS) wires. However neither their prices nor properties still do not allow manufacturing efficient superconducting magnetic energy storage devices (SMES's). One of the ways to increase the stored energy of the coil and reduce the cost of SMES lies in the use of a magnetic core. We found that the so-called pot-core configuration simultaneously minimizes a volume of the core and magnetic field on the winding. Two laboratory models of an SMES's with a ferromagnetic core were constructed and tested. We also developed a method for analytical calculation of the optimal air gap of magnetic core necessary for maximizing stored energy, where we used power approximation of the B-H curve of the ferromagnetic material. This method is valid for the calculation of the dependence of the stored energy on the value of air gap of the core. One of these models was used as an example of stored energy calculations. The experimental stored energy values are close to the calculated values.

Superconductor Science and Technology

Finite element method (FEM) analysis is employed to study and compare AC losses in a wide frequen... more Finite element method (FEM) analysis is employed to study and compare AC losses in a wide frequency range in two MgB 2 superconducting wires in self-field and in the presence of external AC field. The modelled wires, of the same external dimensions, are mono-and 36-superconducting filaments embedded in either magnetic Monel or a nonmagnetic metallic wire sheath. We demonstrate that in a multifilamentary wire in self-field the Monel sheath serves as a 'pole piece' at the filament outer surface and alters local magnetic fields, current flow and AC losses distribution within the filament. In comparison with the nonmagnetic sheath with the same electrical conductivity, AC current in the wire with the magnetic sheath penetrates significantly deeper into the filaments and AC losses in the filament and in the magnetic sheath increase significantly. In contrast, the symmetry of the monofilament wire makes the current and loss distributions in the filament practically indifferent to the sheath composition. Still, losses in the magnetic sheath are much higher than in the nonmagnetic sheath due to increased flux dynamics. The application of DC current, on which the AC current is superimposed, sharply reduces the AC losses in the magnetic sheath material due to the drop in its permeability. Filament losses are also reduced in the presence of DC current, but to a much lesser extent. Results also show that in the kHz frequency range, the magnetic permeability of the sheath increases the skin effect in both the wire and filaments complex. As a result, at such frequencies, a significant portion of the current is carried by the metallic part of the wire instead of the superconductor, contributing to a further increase in losses. The analysis also shows that in the presence of external AC magnetic field, the Monel can provide magnetic shielding for inner filaments, thus reducing coupling effects between filaments. However, if magnetically saturated by the DC current, the Monel behaves quite similarly to a nonmagnetic sheath.

The effect of substituting potassium for barium on the superconductivity of RBa2Cu3O7 (R=Y and Eu)

Physica C: Superconductivity, 1988