Measuring the voltage of a train of high-tension rectangular pulses

Conclusions The above formulas serve to evaluate the voltage and the number of pulses required for charging the measuring capacitor of an amplitude diode voltmeter with a capacitive potential divider and electrostatic instrument up to a voltage which approaches that corresponding to the ratio of the divider. The formulas can also serve to select the parameters of the voltmeter circuit which meet the desired settling time of the measured voltage.     

Diffraction of femtosecond pulses; nonparaxial regime

We present a systematic study of linear propagation of ultrashort laser pulses in media with dispersion, dispersionless media, and vacuum. The applied method of amplitude envelopes makes it possible to estimate the limits of the slowly varying amplitude approximation and to describe an amplitude integrodifferential equation governing propagation of optical pulses in the single-cycle regime in solids. The well-known slowly varying amplitude equation and the amplitude equation for the vacuum case are written in dimensionless form. Three parameters are obtained defining different linear regimes of optical pulse evolution. In contrast to previous studies we demonstrate that in the femtosecond region the nonparaxial terms are not small and can dominate over the transverse Laplacian. The normalized amplitude nonparaxial equations are solved using the method of Fourier transforms. Fundamental solutions with spectral kernels different from those according to Fresnel are found. Exact unidirectional analytical solution of the nonparaxial amplitude equations and the 3D wave equations with initial conditions compatible with Gaussian light bullets are obtained also. One unexpected new result is the relative stability of light bullets (pulses with spherical and spheroidal spatial form) when we compare their transverse enlargement with paraxial diffraction of light beams in air. It is important to emphasize here the case of light disks, i.e., pulses whose longitudinal size is small with respect to the transverse one, which in some partial cases are practically diffractionless over distances of a thousand kilometers. A new formula that calculates the diffraction length of optical pulses is suggested. Finally, propagation of single-cycle pulses in air and vacuum was investigated, and a coronal (semispherical) form of diffraction at short distances was observed.     

Diffraction properties of double-layer rectangular phase gratings

Abstract

We theoretically present diffraction properties of double-layer rectangular phase gratings (DLRPGs). Computer simulations show that these DLRPGs can realize several functions such as optical filtering, optical switching, optical amplitude modulating, beam splitting with variable output beam numbers and wavelength division multiplexing, and so on. As a test of the above diffraction properties, we fabricate a set of DLRPGs in a double-layer light-sensitive material by the contact printing method. Based on the DLRPGs of the material, a single beam splitter with experimental results of splitting of an incident beam into 2, 3 or 5 output beams and of higher than 64% total diffraction efficiency are realized when the incident angle of the readout beam is tuned during the reading process.     

Stability of compound superconductors under localized heat pulses

Superconducting state stability of commercial Nb-Ti superconductors under heat pulses, generated at different lengths of superconductor, was investigated. The case of a one-dimensional superconductor without heat transfer to liquid helium was examined. It was found that due to dissipation of heat along the superconductor, the energy of irreversible quench may exceed considerably the energy needed for its adiabatic heating to critical temperature T_c (at given magnetic field and current). Such overheating, as it follows from the simplified analytical model and from experiments, depends on current density, magnetic field, the length of the heat generation and on the properties of matrix, but it is in broad limits independent of the form and duration of the pulse. Possible reasons of quantitative disagreement between calculations and experimental results are discussed.     

Graphical simulation of superluminal acoustic localized wave pulses

This paper investigates, graphically, a simulation of the launching of recently obtained superluminal localized wave solutions of the homogeneous wave equation. These solutions represent focused interference patterns that travel with speed greater than c in the direction of propagation. Graphical simulation indicates that approximations to these solutions may be launched from a finite array of point sources, and the superluminal nature of the launched pulses is maintained in the near field of the array     

Analytical modeling of liquid sloshing in a two-dimensional rectangular tank with a slat screen

Potential-flow theory is employed with linear free-surface conditions, multimodal method, and a screen-averaged pressure-drop condition to derive an analytical modal model describing the two-dimensional resonant liquid motions in a rectangular tank with a vertical slat-type screen in the tank middle. The tank is horizontally excited in a frequency range covering the two lowest natural sloshing frequencies. The model consists of a system of linear ordinary differential [modal] equations responsible for liquid sloshing in compartments, as well as a nonlinear ordinary differential equation describing the liquid flow between the compartments. New experimental model tests on steady-state wave elevations near the tank wall are reported for the solidity ratios 0.328 ?? Sn ?? 0.963 where Sn is the ratio between the solid area and the full area of the screen. The experiments generally support the applicability of the model. The discrepancy can be explained by the free-surface nonlinearity. The screen acts as a damping mechanism for low and intermediate solidity ratios, but it causes an increase in the lowest resonant sloshing frequency at higher solidity ratios as if the screen had been replaced by an unperforated wall.     

Diffraction characteristics of a Fresnel zone plate illuminated by 10 fs laser pulses

Results of experimental and theoretical work performed to compare diffraction patterns and focal distributions of a Fresnel zone plate illuminated by ultrashort 10 fs pulsed and cw laser beams are presented. It is shown that the foci intensities of 10 fs pulses are considerably lower than those of cw beams while the focal widths in the axial and radial directions are broadened. Calculations also indicate the spectral modulation along the center of the diffraction patterns. These phenomena are explained by the coherent superposition of the composing frequency content.     

Diffraction of electromagnetic waves by periodic arrays of rectangular cylinders

Reflection, transmission, and absorption of electromagnetic waves by periodic arrays of conducting or dielectric rectangular cylinders are studied by a finite-difference time-domain technique. Truncated gratings made of lossless and lossy conducting and dielectric elements are considered. Results for surface current density, transmission, and reflection coefficients are calculated and compared with corresponding results in the literature, which are obtained by approximate or rigorous methods applicable only to idealized infinite models. An excellent agreement is observed in all cases, which demonstrates the accuracy and efficacy of our proposed analysis technique. Additionally, this numerical method easily analyzes practical gratings that contain a finite number of elements made of lossless, lossy, or even inhomogeneous materials. The results rapidly approach those for the idealized infinite arrays as the number of elements is increased. The method can also solve nested gratings, stacked gratings, and holographic gratings with little analytical or computational effort.     

Diffraction characteristics of 10 fs laser pulses passing through an aperture

Results are presented on experimental and theoretical work performed to compare diffraction phenomena for ultrashort 10 fs pulses and continuous-wave propagation modes illuminating different-sized pinholes and slits. Results demonstrate that 10 fs pulses do not produce high-frequency diffraction like that produced with continuous-wave illumination. The diffraction through a 1 mm pinhole of temporally stretched pulses obtained by using fused silica plates whose frequency spectrum remains the same is compared with those of 10 fs pulses. The overall diffraction intensity profiles are, however, nearly identical in this case. The simulations of diffraction patterns for 100 fs, 10 fs, and 1 fs incident pulse were compared theoretically for different aperture sizes and frequencies. Calculations indicate that the lack of high-frequency diffraction for the mode-locked case is due to the broadband nature of the ultrashort laser pulses; i.e., the distribution of the frequency contained in the pulse ends up washing out when objects are illuminated with pulses of broad frequency content. The results of this work have important application in biomedical imaging and remote imaging applications, to name only a few.     

Diffraction model of short-range longitudinal wakefields in circular and rectangular structures

Diffraction models based on the Green function technique and ‘image’ field concept were developed for analytical calculation of high-frequency radiation in rectangular iris-loaded structures. The models for single-cell and periodic structures were considered as an extension of the Lawson and Sessler-Vainshtein approaches and were applied to circular, infinitely wide planar and ‘muffin-tin’ geometry. The calculated spectral radiation losses are in good agreement with existing formulae. For a 2.5D muffin-tin structure a special semi-analytical frequency domain model was built to compare the diffraction model with the matched field technique based on eigenmodes calculations.     

Turbulent biomagnetic fluid flow in a rectangular channel under the action of a localized magnetic field

The fundamental problem of the turbulent flow of a biomagnetic fluid (blood) between two parallel plates under the action of a localized magnetic field is studied. The blood is considered to be an electrically conducting, incompressible and Newtonian fluid and its flow is steady, two-dimensional and turbulent. The turbulent flow is described by the Reynolds averaged Navier–Stokes (RANS) equations. For the numerical solution of the problem under consideration, which is described by a coupled and non-linear system of PDEs, with appropriate boundary conditions, the stream function–vorticity formulation is used. For the eddy-kinematic viscosity, the low Reynolds number k–ε turbulence model is adopted. The solution of the problem, for different values of the dimensionless parameter entering into it, is obtained by developing and applying an efficient numerical technique based on finite differences scheme. Results concerning the velocity and temperature field, skin friction and rate of heat transfer, indicate that the presence of the localized magnetic field, appreciable influences the turbulent flow field. A comparison is also made with the corresponding laminar flow, indicating that the influence of the magnetic field decreases in the presence of turbulence.     

Elasto-plastic analysis for a finite thickness rectangular plate containing a through-thickness central crack

The objective of this work is to present a three-dimensional analysis of the elasto-plastic response of a center-cracked rectangular panel by using an alternate method, the method of lines (MOL). The problem of a finite thickness rectangular plate containing a through-thickness central crack under uniaxial tension is studied in detail. An incremental procedure is used to seek the 3-D elasto-plastic stress and strain distribution. The Prandtl-Reuss equations and the associated von Mises flow rule are employed. The material is assumed to be elastic-plastic with nonlinear work-hardening. Effective stress and effective strain concepts are used. The MOL is used to reduce the governing equations on displacements to a coupled set of ordinary differential equations (ODE) along x, y, z lines. These are integrated by Peano-Baker method. Since the ODE are of constant coefficient the coefficient matrix is diagonalized and the matrizon is easily evaluated. Numerical results on stresses, displacements, the growth of plastic zone and the crack surface opening displacements are reported and their variation along the thickness are presented in graphs. The variation of the stress intensity factor K ₁, along the thickness direction is given. It is found that the computation time needed for determining the distribution of stresses and strains is much less than what has been reported by using other methods. A comparison of results on SIF with those obtained by other methods is also given.

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Résumé L'objet du travail est de présenter une analyse tridimentionnelle de la réponse élastoplastique d'un panneau rectangulaire présentant une fissure centrale, et utilisant une méthode alternative, la méthode des lignes MOL. On étudie dans le détail le problème de la plaque rectangulaire d'épaisseur finie comportant une fissure centrale traversante et soumise à la contrainte uniaxiale. Une procédure incrémentale est utilisée en vue de rechercher la distribution des contraintes et des déformations élastoplastiques suivant les trois dimensions. On utilise les équations de Prandtl-Reuss et la règle d'écoulement de von Mises qui lui est associée. On suppose que le matériau est élastoplastique et qu'il présente un vieillissement non linéaire. On recourt au concept des contraintes effectives et des déformations effectives. Le MOL est utilisé en vue de ramener les équations gouvernant les déplacements à un ensemble d'équations différentielles ordinaires suivant les axes X, Y, Z; on fait appel à la méthode de Peano-Baker pour leur intégration. Comme les équations différentielles ont un coéfficient constant, la matrice de coéfficient peut être résolue par diagonales et le matriciel peut être aisément évalué. Les résultats numériques relatifs aux contraintes, aux déplacements, à la croissance de la zône plastique et aux déplacements d'ouverture de la surface de la fissure sont présentés, et leur variation sur l'épaisseur sont mises en graphiques. On fournit la variation du facteur d'intensité de contrainte K suivant la direction de l'épaisseur. On trouve que le temps de calcul nécessaire à la détermination de la distribution des contraintes et déformations est beaucoup moindre que celui nécessaire par l'utilisation d'autres méthodes. Une comparaison des résultats relatifs aux facteurs d'intensité de contraintes avec ceux obtenus par d'autres méthodes est également fournie.

     

二维矩形月池内流体自振特性研究

研究二维矩形月池流体的自振特性。基于线性势流理论,采用Galerkin展开,将问题转化为月池自振频率特征值问题,求解自振频率及速度势函数的系数矩阵,获得二维矩形月池固有频率及振型的半解析解,分析月池几何参数对月池流体自振特性影响。研究表明,由于内部与外流连通,月池流体呈垂向活塞(piston)振动及晃荡(sloshing)运动两种形式。垂向活塞振动模态频率变化主要受液面高度影响,且随液面高度增加而减小;晃荡模态频率主要受月池宽度影响,随月池宽度增加而减小。活塞振动模态振型在液面较浅时出现波峰,随液面高度增加而消失。利用CFD方法计算月池运动,结果与半解析解中振型曲线拟合良好。     

Probing of a random phase screen by a focused spatially modulated laser beam. Diffraction at a large number of inhomogeneities

The contrast of the Young’s interference fringes formed in the diffraction field when a dynamic random phase screen is illuminated by a focused, spatially modulated laser beam is obtained analytically as a function of the statistical parameters of the screen. A threshold relative bleaching effect is established for a highly dispersive medium when a low-divergence illuminating beam is used. Pis’ma Zh. Tekh. Fiz. 23, 47–53 (October 12, 1997)     

On the passage of ultrasonic pulses through a waveguide with a specimen containing charged defects

We discovered a slight increase in the velocity of ultrasonic wavesC and other parameters associated withC. We studied materials with charged defects and show that the physical quantity under consideration usually denoted byC may, in some cases, become so high that it loses its physical meaning. It is demonstrated that the discovered effects are similar to the variations of heat conductivity of liquid helium. We also discuss mechanisms of the anomalous behavior of the investigated quantity.Vladimir Pedagogical University, Vladimir, Russia. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 6, pp. 93–99, November–December, 1995.     

Diffraction of two optical beams by a SAW in an isotropic solid

This paper for the first time explores the diffraction of two spatially separated parallel light beams by reflection from a SAW, propagating in isotropic semi-space. In the analysis the attenuation and dispersion of SAW, the interference of light beams and scattering by different sites of acoustic wave are considered. The analytical expressions predict low-frequency oscillation of the resulting diffracted fields as observed in previous experiments. Using measurements of period oscillation of diffracted intensity vs. SAW frequency, the SAW dispersion curve and phase and group velocities were determined     

基于二维长方晶格柱状光子晶体的衍射抑制

通过等频图分析并结合时域有限差分法模拟,基于光子晶体自准直机制可以实现电磁能量高度局域的无衍射光束传播。实验上加工出柱状长方晶格光子晶体,利用红外CCD,在1540nm-1570nm波段范围内观测到0.4mm的自准直传播,通过粒子散射光亮度估算出光子晶体链上传输损耗为17dB/mm。该结构表现出来的宽频性能以及毫米量级的传输距离,使其具有应用在光互连中的潜力。     

Inter-pulse interval between rectangular voltage pulses affects electroporation threshold of artificial lipid bilayers

Describes experiments that determine how the inter-pulse interval between rectangular pulses in a train of pulses alters the threshold of electroporation of 1-pamitoyl 2-oleoyl phosphatidycholine bilayer lipid membranes. The bilayers were exposed to a train of sixteen 100-/spl mu/s duration pulses. Threshold voltage and the sequence number of the pulse in the train, where onset of the electroporation occurred, were recorded for six inter-pulse intervals (/spl infin/, 1000 /spl mu/s, 100 /spl mu/s, 10 /spl mu/s, 1 /spl mu/s, 0 /spl mu/s). The threshold voltage of the pulse train decreased linearly with the logarithm of the inter-pulse interval. When the inter-pulse interval was 1 /spl mu/m, electroporation threshold dropped to that of a single pulse with duration 1600 /spl mu/s (equal to the sum of all pulse durations in the train). In this case, the occurrence of bilayer rupture was almost equally frequent for all pulses in the train. When the inter-pulse interval between the pulses exceeded 1 /spl mu/s, the influence of the previous pulse on the response to the following pulse declined. It became more likely that the bilayer ruptured during the first half of the train. These experimental observations suggest that a train of pulses applied with short inter-pulse interval (less than 1 ms) can lower the electroporation threshold of bilayer lipid membranes.