Quasi-static Field Research Articles

Systematic Analysis of the Improvements in Magnetic Resonance Microscopy with Ferroelectric Composite Ceramics.

The spatial resolution and signal-to-noise ratio (SNR) attainable in magnetic resonance microscopy (MRM) are limited by intrinsic probe losses and probe-sample interactions. In this work, the possibility to exceed the SNR of a standard solenoid coil by more than a factor-of-two is demonstrated theoretically and experimentally. This improvement is achieved by exciting the first transverse electric mode of a low-loss ceramic resonator instead of using the quasi-static field of the metal-wire solenoid coil. Based on theoretical considerations, a new probe for microscopy at 17 T is developed as a dielectric ring resonator made of ferroelectric/dielectric low-loss composite ceramics precisely tunable via temperature control. Besides the twofold increase in SNR, compared with the solenoid probe, the proposed ceramic probe does not cause static-field inhomogeneity and related image distortion.

Homogenization of multi-turn coil with elliptic cross-section using complex permeability

Purpose The tensor complex permeability of a multi-turn coil with elliptic cross-section is analytically expressed. In field analysis, a multi-turn coil can be modeled by the uniform material that has the present tensor complex permeability. It is shown that the frequency characteristic of the present tensor complex permeability is in good agreement with that evaluated by finite element method applied to a unit cell of the multi-turn coil region. Design/methodology/approach The authors introduce a new method to evaluate the complex permeability of a multi-turn rectangular coil. To obtain the complex permeability of a rectangular coil in a closed form, it is approximated as an elliptic coil. Because the rectangular coil has different complex permeabilities in the vertical and horizontal directions, the complex permeability have to be defined in a tensor form. It suffices to discretize the coil region into rather coarse finite elements without considering the skin depth in contrast to the conventional finite element method. Findings The proposed method is shown to give the impedance of multi-turn coils which is in good agreement with results obtained by the conventional finite element (FE) analysis. By extending the proposed approach, the authors can easily perform 3D FE analysis without difficulty in discretization of the coil region with fairly fine finite elements. Moreover, they found that the approximation of rectangular coils as the elliptic coils is valid for analysis of quasi-static fields using this homogenization method. Originality/value The novelty of this study is in the approximation of the rectangular coils with elliptic coils, and the complex permeability for them is formulated here in a closed form. The proposed formula includes that for the round coils. Using the present method, the authors analyze the rectangular coils without fine discretization.

S83. Mood and Anxiety Disorders Affect Brain Temporal Dynamics Evidence From EEG Microstates

EEG microstates (EEG-ms) reflect spatio-temporal dynamic brain neuronal activity, with quasi-stationary EEG global field power topographies that are typically classified into four distinct classes (coined EEG-ms A through D). EEG-ms classes have been associated with large-scale fMRI brain networks, suggesting that examining how EEG-ms characteristics relate to mental disorders may reveal how mental disorders affect brain function and dynamics.

Determination of Far Fields of Wire Antennas on a PEC Sphere Using Spherical Harmonic Expansion

Far-field patterns of wire antennas on a PEC sphere are calculated from quasi-static near fields using spherical harmonic expansion. Three cases of the wire antennas—F-antenna, dipole, and resonant loop on the PEC sphere—are considered. With the assumed sinusoidal current distribution on the antenna, the method of images is employed to obtain currents and charges on the antenna's image against the sphere. Only from the charge density and current distributions of the wire antenna and its image, ignoring the sphere, the quasi-static near fields are calculated using Coulomb's and Biot–Savart's laws. The fields in a source-free region are expressed in spherical harmonic expansion with the coefficients obtained from the radial components of the calculated quasi-static electric and magnetic near fields. The far fields thus obtained from the expansion match well with that of the simulated patterns. Deviations between simulation and theory are discussed and explained.

Tilted-pulse-front excitation of strong quasistatic precursors.

It was recently predicted [M. I. Bakunov, A. V. Maslov, and M. V. Tsarev, Phys. Rev. A95, 063817 (2017)10.1103/PhysRevA.95.063817] that concurrent processes of optical rectification and multiphoton absorption of an ultrashort laser pulse in an electro-optic crystal can generate a quasistatic electromagnetic precursor propagating ahead of the laser pulse. The electric and magnetic fields in the precursor can exceed the fields in the ordinary terahertz wave generated behind the laser pulse. We propose a way to enhance the precursor's magnitude tremendously, by at least two orders of magnitude, by using tilted-pulse-front excitation technique and higher orders of multiphoton absorption. In particular, we show that a pulse of 500 fs duration and 70 GW/cm2 peak intensity from a Yb-doped laser amplifier can generate in a 5-mm-thick LiNbO3 crystal a 0.5-mm-long precursor with the strengths of the electric and magnetic fields as high as 0.4 MV/cm and 0.13 T, respectively. Strong quasistatic (subterahertz) fields can be a useful tool for particle acceleration, molecular orientation, ultrafast control of magnetic order in matter, and in terahertz streaking techniques.

On Material Transport by Shelfbreak Eddies

AbstractThe Lagrangian motion in the eddy field produced from an unstable retrograde jet along the shelf break is studied from idealized numerical experiments with a primitive equation model. The jet is initially in thermal wind balance with a cross-isobath density gradient and is not subjected to any atmospheric forcing. Over the course of the model integration, the jet becomes unstable and produces a quasi-stationary eddy field over a 2-month period. During this period, the cross-slope flow at the shelf break is characterized by along-slope correlation scales of O(10) km and temporal correlation scales of a few days. The relative dispersion of parcels across isobaths is found to increase with time as tb, where 1 < b < 2. This mixed diffusive–ballistic regime appears to reflect the combined effects of (i) the short length scales of velocity correlation at the shelf break and (ii) the seaward excursion of monopolar and dipolar vortices. Cross-slope dispersion is greater offshore of the front than inshore of the front, as offshore parcels are both subducted onshore below density surfaces and translated offshore with eddies. Nonetheless, the exchange of parcels across the jet remains very limited on the monthly time scale. Particles originating from the bottom experience upward displacements of a few tens of meters and seaward displacements of O(100) km, suggesting that the eddy activity engendered by an unstable along-slope jet provides another mechanism for bottom boundary layer detachment near the shelf edge.

Ultrafast dynamics of magnetic vortices and pulse collapse in a laser-under dense plasma interaction

The energy of an intense ultrashort laser pulse interacting with high density (still under dense) plasma is typically transformed into electron heating and in excitation of nonlinear coherent structures such as magnetic vortices, solitons, or post-solitons. Using 33 TW 30 fs laser pulses and a high-density nitrogen gas jet, we experimentally investigated magnetic vortices in the laser pulse collapse region where electrons are efficiently accelerated and heated. Those vortices, which are associated with rapidly decaying magnetic fields, are found to be immobile and persist for several picoseconds. A collisionless plasma was formed due to the quasi-static field ionization of the gas associated with the hot and fast electron currents. The evolution dynamics of such nonlinear plasma phenomena have been monitored by using a 30-fs probe laser beam through employing the polarimetric and shadowgraphic techniques. Our experimental results are also supported by particle-in-cell simulations.

All-optical magnetization switching by counterpropagataion or two-frequency pulses using the plasmon-induced inverse Faraday effect in magnetoplasmonic structures

In this Letter we study the generation of quasi-static magnetic fields by the plasmon-induced inverse Faraday effect and propose a magneto-optical waveguide structure for achieving magnetization switching at sub-ps time in a nano-confined magneto-optical structure. In particular we show that the direction of the generated quasi-static field in a magneto-optical dielectric cavity side-coupled to a metal-insulator-metal (MIM) waveguide depends sensitively on the wavelength of the surface plasmon polaritions (SPP). This phenomenon could open up a new energy-efficient ultrafast method for nano-confined all-optical magnetization switching by two-frequency pulses.

Mathematical model of heat transfer in roll caliber

A physical model of the thermal process in the roll caliber during the rolling of the tape on a two-roll rolling mill was constructed. A mathematical model of the temperature field of a rolling hollow roll of a rolling state of a cylindrical shape rotating about its axis with constant angular velocity is proposed. The mathematical model takes into account different conditions of heat exchange of the inner and outer surfaces of the roll with the belt and its surrounding environment. The temperature field of a hollow roll of a rolling mill is considered as an initial boundary-value problem for a homogeneous non-stationary heat equation with inhomogeneous, nonlinear boundary conditions, which also depend on the angle of rotation of the roll around its axis. The equation describes the temperature field of the rolls during uncontrolled heat transfer during rolling. It significantly depends on the time and number of revolutions around its axis. With a large number of revolutions of the roll around its axis, a quasi-stationary temperature distribution occurs. Therefore, the simplified problem of determining a quasistationary temperature field, which is associated with a thermal process that is time-independent, is considered further in the work. In this case, the temperature field is described using the boundary value problem in a ring for a homogeneous stationary heat equation with inhomogeneous boundary conditions and heat transfer conditions outside the ring, which lie from the angular coordinate. After the averaging operation, the solution of this problem is reduced to solving the equivalent integral equation of Hammerstein type with a kernel in the form of the Green's function. The Mathcad computer mathematical system builds the temperature distribution of the roll surface. An algorithm for solving a inhomogeneous problem was developed and the temperature distribution of the roll was constructed.

Учет влияния полей остаточных деформаций в современных физико-механических технологиях обработки конструкционных материалов

AbstractA strict determination of the mechanisms of redistribution of previously accumulated irreversible strains as a result of additional elastic shock actions on the material is given for a nonlinear gradient model of large elastic–plastic strain. It is shown that this redistribution is limited by rigid transport and rotation of the plastic strain tensor. Formulas for a change in the initial components of the plastic strain tensor in elastic waves are derived. It is shown that the preliminary plastic field affects the dynamics of further reversible strain as one of the factors of formation of the initial quasi-static elastic field, which cannot be obtained in a purely elastic process.

Suppression of near-field coupling in plasmonic antennas on epsilon-near-zero substrates

Epsilon-near-zero (ENZ) media are an emerging class of nanophotonic materials that engender electromagnetic fields with small phase variation due to their approximately zero permittivity. These quasi-static fields facilitate several unique optical properties, such as subwavelength confinement, arbitrary wavefront control, and enhanced light–matter interactions, which make ENZ materials promising platforms for nanophotonic and plasmonic systems. Here, we report our analysis of single and dimer nanoantennas deposited on an aluminum-doped zinc oxide layer with an ENZ wavelength around 1.5 μm. Using near-field microscopy, far-field spectroscopy, finite-element numerical simulations, and a semi-analytic Fabry–Perot (FP) model, we show that single nanoantennas support highly dispersive plasmonic modes with less than unity effective mode index at wavelengths greater than the ENZ wavelength, which consequently fixes the resonance near the ENZ wavelength of the substrate. Furthermore, we observe a strong reduction in the near-field coupling between dimer nanoantennas via measurements of the resonance shift as a function of gap size. This reduction of near-field coupling allows one to design arrays of independently operating antennas with higher densities and thereby significantly improve the array characteristics, especially when targeting gradient metasurface implementations. Our results demonstrate the use of ENZ materials for increasing the versatility and functionality of plasmonic structures and provide foundational insight into this exotic material phenomenon.

Stochastic electron heating in the laser and quasi-static electric and magnetic fields

The dynamics of relativistic electrons in the intense laser radiation and quasi-static electromagnetic fields both along and across the laser propagating direction are studied in the 3/2 dimensional (3/2D) Hamiltonian framework. It is shown that the unperturbed oscillations of the relativistic electron in these electric fields could exhibit a long tail of the amplitude of harmonics which makes an onset of stochastic electron motion be a primary candidate for electron heating. Chirikov-like mappings which describe the recurrence relations of electron energy and time passing through zero canonical momentum plane are derived, and then, the criteria for instability are obtained. It follows that for both transverse and longitudinal electric fields, there exist upper limits of the stochastic electron energy depending on the laser intensity and electric field strength. These maximum energies could be increased by a weak electric field. However, the maximum energy is reduced for the superluminal phase velocity in both cases. The impacts of the magnetic fields on the electron dynamics are different for these two cases and discussed qualitatively. These analytic results are confirmed by the numerical simulations of solving the 3/2D Hamiltonian equations directly.

Consideration of the Influence of Residual Strain Fields in Modern Physicomechanical Technologies of Treating Structural Materials

A strict determination of the mechanisms of redistribution of previously accumulated irreversible strains as a result of additional elastic shock actions on the material is given for a nonlinear gradient model of large elastic–plastic strain. It is shown that this redistribution is limited by rigid transport and rotation of the plastic strain tensor. Formulas for a change in the initial components of the plastic strain tensor in elastic waves are derived. It is shown that the preliminary plastic field affects the dynamics of further reversible strain as one of the factors of formation of the initial quasi-static elastic field, which cannot be obtained in a purely elastic process.

Flat Bandwidth Expansion for Spin-Valve GMR Magnetic Field Sensors

Flat Bandwidth Expansion for Spin-Valve GMR Magnetic Field Sensors

Recent advances in the evolution of interfaces: thermodynamics, upscaling, and universality

We consider the evolution of interfaces in binary mixtures permeating strongly heterogeneous systems such as porous media. To this end, we first review available thermodynamic formulations for binary mixtures based on general reversible-irreversible couplings and the associated mathematical attempts to formulate a non-equilibrium variational principle in which these non-equilibrium couplings can be identified as minimizers.Based on this, we investigate two microscopic binary mixture formulations fully resolving heterogeneous/perforated domains: (a) a flux-driven immiscible fluid formulation without fluid flow; (b) a momentum-driven formulation for quasi-static and incompressible velocity fields. In both cases we state two novel, reliably upscaled equations for binary mixtures/multiphase fluids in strongly heterogeneous systems by systematically taking thermodynamic features such as free energies into account as well as the system’s heterogeneity defined on the microscale such as geometry and materials (e.g. wetting properties). In the context of (a), we unravel a universality with respect to the coarsening rate due to its independence of the system’s heterogeneity, i.e. the well-known O(t1/3)-behaviour for homogeneous systems holds also for perforated domains.Finally, the versatility of phase field equations and their thermodynamic foundation relying on free energies, make the collected recent developments here highly promising for scientific, engineering and industrial applications for which we provide an example for lithium batteries.

Frequency domain soil parameters inversion of horizontally multilayered earth model with considering high‐frequency field

A method to estimate soil parameters of horizontally multilayered earth model in frequency domain is developed with considering high-frequency field. The theoretical formula of complex apparent resistivity with considering high-frequency field is derived from Green's function. To avoid time-consuming numerical integration, complex image method is introduced to solve Sommerfeld integral. Simulated annealing algorithm is applied to optimise soil parameters. The accuracy of this method is confirmed by interpreting field data under the DC field. As the inversion results of this method are different from other literature's results in a four-layer model, the posterior analysis is made. Then this method is applied to the inversion of frequency domain soil parameters under both quasi-static field and high-frequency field. The differences of complex apparent resistivity distribution between quasi-static field and high-frequency field are compared at different frequencies.

Theory and simulation of spectral line broadening by exoplanetary atmospheric haze

Atmospheric haze is the leading candidate for the flattening of expolanetary spectra, as it's also an important source of opacity in the atmospheres of solar system planets, satellites, and comets. Exoplanetary transmission spectra, which carry information about how the planetary atmospheres become opaque to stellar light in transit, show broad featureless absorption in the region of wavelengths corresponding to spectral lines of sodium, potassium and water. We develop a detailed atomistic model, describing interactions of atomic or molecular radiators with dust and atmospheric haze particulates. This model incorporates a realistic structure of haze particulates from small nano-size seed particles up to sub-micron irregularly shaped aggregates, accounting for both pairwise collisions between the radiator and haze perturbers, and quasi-static mean field shift of levels in haze environments. This formalism can explain large flattening of absorption and emission spectra in haze atmospheres and shows how the radiator - haze particle interaction affects the absorption spectral shape in the wings of spectral lines and near their centers. The theory can account for nearly all realistic structure, size and chemical composition of haze particulates and predict their influence on absorption and emission spectra in hazy environments. We illustrate the utility of the method by computing shift and broadening of the emission spectra of the sodium D line in an argon haze. The simplicity, elegance and generality of the proposed model should make it amenable to a broad community of users in astrophysics and chemistry.

A mathematical model of quasistationary electric field penetration from ground to the ionosphere with inclined magnetic field

A quasi stationary model of the electric fields and currents in the conductor that includes the Earth's atmosphere and ionosphere is developed. The role of the inclination of the magnetic field is considered. A mathematical simulation has shown that the resulting electric field in the ionosphere decreases. The penetration of the electric field and current from the ground to the ionosphere can be a physical process which creates observed ionospheric precursors of earthquakes only if the electric field strength and conductivity of the air near ground as well as the size of the pre-earthquake perturbations zone can simultaneously reach extreme possible values. Additional experiments are necessary to verify such a possibility. Other models are analyzed and compared to the main outcomes of our investigations.

Generation of propagating quasistatic fields using ultrashort laser pulses in electro-optic crystals

We explore the effect of quasistatic field generation in a nonstationary medium. The nonstationarity is provided by time-dependent density of free carriers created by an ultrashort laser pulse, a situation which is common in electro-optic schemes of THz generation. The quasistatic fields can serve as indicators of ultrafast carrier dynamics and can be used for particle acceleration, control of magnetic materials, streaking techniques, and spectroscopy.

Experimental Studies of an Eddy Current Transducer with Spatially Periodic Fields

The purpose of this study is to perform a comparative analysis of the experimentally obtained and calculated values of the measuring windings total EMF amplitudes of the eddy current transducer with spatially periodic fields when magnetic and nonmagnetic cylindrical product are placed in it. To realize this goal in the work, a transducer with translational symmetry of excitation magnetic field has been developed.
 Translation symmetry typical example is the field of a long current conductor. The paper considers quasi-static electromagnetic field with the wavelength that exceeds characteristic transverse dimensions of conductor and cylinder.
 As the study result it is seen a satisfactory coincidence of the calculated and experimentally obtained values of the EMF of the transducer output signal in the cases of the product absence and with variety of products. As for example for measuring windings with angular coordinates φ = 30° and 60° an error of voltage values difference is less than 10%.
 The study shows that the advantage of such transducers while operating on one fixed frequency is capability to perform the multi-parameter testing on account of processing of certain amount of some spatial harmonics of excitation field.