arXiv subjects

Vladimir Sokolovsky

Publications and source records attributed to Vladimir Sokolovsky.

28 records · Page 2Linked to original sources

arXiv2011

Computing AC losses in stacks of high-temperature superconducting tapes

Roebel cables and superconducting tape coils are often modeled as stacks of parallel superconducting tapes carrying the same transport current. We solved, in the infinitely thin approximation, the transport current and magnetization problems for such stacks using an efficient numerical scheme based on a variational formulation of the Kim critical-state model. We also refined the anisotropic bulk approximation, introduced by Clem et al. in order to simplify AC loss estimates for densely packed stacks of many tapes; this was achieved by removing the simplifying assumptions on the current sheet density in the subcritical zone and the shape of this zone boundary. Finally, we studied convergence of stack problem solutions to the solution of the modified bulk problem. It was shown that, due to the fast convergence to the anisotropic bulk limit, accurate AC loss estimates for stacks of hundreds of tapes can usually be obtained also using a properly rescaled model of a stack containing only ten-twenty tapes.

cond-mat.supr-con↗

arXiv2010

Entanglement in nuclear quadrupole resonance

Entangled quantum states are an important element of quantum information techniques. We determine the requirements for states of quadrupolar nuclei with spins >1/2 to be entangled. It was shown that entanglement is achieved at low temperature by applying a magnetic field to a quadrupolar nuclei possess quadrupole moments, which interacts with the electricfield gradient produced by the charge distribution in their surroundings.

quant-ph↗

arXiv2010

An improved design of an inductive fault current limiter based on a superconducting cylinder

The paper deals with basic designs of a fault current limiter of the transformer type which differ each other by the mutual location of a primary winding and a superconducting short-circuited cylinder. Theoretical study of the main parameters of the different designs is performed in the framework of the critical state model and shows that the most effective is a design in which the primary winding is divided to two sections with equal turn numbers. The sections are placed inside and outside of the cylinder and connected in series. Such arrangement of the windings leads to a substantial reduction of AC losses in the superconducting cylinder, an increase of the activation current and a decrease of the inductive reactance in the normal regime of a protected circuit. The experimental results obtained on the laboratory model with a BSSCO cylinder confirm the theoretical predictions.

cond-mat.supr-con↗

arXiv2010

Higher borides and oxygen-enriched Mg-B-O inclusions as possible pinning centers in nanostructural magnesium diboride and the influence of additives on their formation

The study of high pressure (2 GPa) synthesized MgB2-based materials allows us to conclude that higher borides (with near MgB12 stoichiometry) and oxygen-enriched Mg-B-O inclusions can be pinning centers in nanostructural magnesium diboride matrix (with average grain sizes of 15-37 nm). It has been established that additions of Ti or SiC as well as manufacturing temperature can affect the size, amount and distribution of these inclusions in the material structure and thus, influence critical current density. The superconducting behavior of materials with near MgB12 stoichiometry of matrix is discussed.

cond-mat.supr-con↗

arXiv2008

Losses in Superconductors under Non-Sinusoidal Currents and Magnetic Fields

Study of AC losses in superconducting wires and tapes is usually restricted by consideration of applied sinusoidal currents and/or magnetic fields. However, currents in electric power systems contain a wide variety of harmonics. The currents become strongly non-sinusoidal at the operation of converters, non-linear reactors, and during transient and overload conditions. We report the results of the analysis of the influence of higher harmonics of the current and magnetic field on AC losses in superconducting slabs, strips and coated conductors. Analytical expressions are obtained in the framework of Bean's critical state model; the power law voltage-current characteristics are treated numerically. It is shown that the contribution of higher harmonics to AC losses in superconducting elements can be tens times larger than in normal metals and the 5% harmonic can increases the losses by up to 20%.

cond-mat.supr-con↗

arXiv2008

Superconducting atom chips: advantages and challenges

Superconductors are considered in view of applications to atom chip devices. The main features of magnetic traps based on superconducting wires in the Meissner and mixed states are discussed. The former state may mainly be interesting for improved atom optics, while in the latter, cold atoms may provide a probe of superconductor phenomena. The properties of a magnetic side guide based on a single superconducting strip wire placed in an external magnetic field are calculated analytically and numerically. In the mixed state of type II superconductors, inhomogeneous trapped magnetic flux, relaxation processes and noise caused by vortex motion are posing specific challenges for atom trapping.

quant-ph↗

arXiv2007

Thermal regimes of HTS cylinders operating in devices for fault current limitation

We reveal obstacles related to the application of HTS cylinders in current limiting devices based on the superconducting - normal state transition. It is shown that, at the critical current density achieved presently in bulk materials, and especially in BSCCO-2212, the required thickness of the cylinder wall in a full-scale inductive device achieves several centimeters. A simple mathematical model of the operation of an inductive fault current limiter (FCL) is used to show that such cylinders cannot be cooled in admissible time after a fault clearing and, hence, the inductive FCLs and current-limiting transformers employing BSCCO cylinders do not return to the normal operation in the time required. For the recovery even with a non-current pause in the circuit, the cylinders are needed with the critical current density by an order higher than the existed ones.

cond-mat.supr-con↗

arXiv2006

Calculation principles for a superconducting inductive FCL and a current-limiting transformer

We present general principles of the parameters calculation and application of two power devices: an inductive fault current limiter and a current-limiting transformer. Both the devices are based on the superconducting - normal state transition in a superconducting switching element magnetically coupled with a primary winding. The relationships between the design parameters of the devices and the limited current, losses and temperature in the superconducting element were investigated. It is shown that, in spite of similarity, the devices have distinctive features manifested in different depths of the fault current limitation. The case of performance of the superconducting winding in the form of HTS BSCCO hollow cylinders is analyzed. Basic parameters of the devices are evaluated for their application in 45 MVA power circuit. Problems of the transition into the normal state and recovery of the superconducting state in the switching element are discussed.

cond-mat.supr-con↗

arXiv2006

Comparison of a Self-Limiting Transformer and a Transformer Type FCL with HTS Elements

A superconducting fault current limiter of the transformer type (inductive FCL) based on magnetic coupling between a superconducting element and a protected circuit has been investigated by many authors for various parameters and performances of a superconducting element. Another design of the device preventing high short-circuit currents is a self-limiting transformer combining the functions of a usual power transformer with the functions of a current limiter. In the presented work we compare the parameters, operation and application of these two devices. The operation of the devices is investigated experimentally on small models fabricated using the same superconducting element. The parameters of the full-scale devices are evaluated. It is shown that the requirements to superconducting elements are practically the same for both devices.

cond-mat.supr-con↗

arXiv2003

AC losses in type-II superconductors induced by nonuniform fluctuations of external magnetic field

Magnetic field fluctuations are inevitable in practical applications of superconductors and it is often necessary to estimate the AC losses these fluctuations induce. If the fluctuation wavelength is greater than the size of a superconductor, known estimates for an alternating uniform external magnetic field can be employed. Here we consider the opposite case and analyze, using a model critical-state problem, penetration of spatially nonuniform fluctuations into type-II superconductors. Numerical simulation is based on a variational formulation of the Bean model. The analytical solutions, found in a weak penetration limit, are used to evaluate AC losses for two types of fluctuations: the running and standing waves. It is shown that for spatially nonuniform fluctuations the losses are better characterized by the fluctuation penetration depth than by the fluctuation amplitude. The results can be used to estimate the AC losses in flywheels, electric motors, magnetic shields, etc.

cond-mat.supr-con↗