bluuebunny/crossref_metadata_2025 huggingface dataset:
The stability of stationary rotation of a regular vortex polygon
Authors: Kurakin L. G. , Yudovich V. I.
Published: August 2002

This paper is devoted to the Lord Kelvin’s (1878) problem on stability of the stationary rotation of the system of n equal vortices located in the vertices of a regular n-gon. During the last decades this problem again became actual in connection with the investigation of point vortices in liquid helium and electron columns in plasma physics. This regime is described by the explicit solution of the Kirchhoff equations. The corresponding eigenvalue problem for the linearization matrix can be also decided explicitly. This was used in the works of Thomson (1883) and Havelock (1931) to obtain exhaustive results on the linear stability. Kurakin (1994) proved that for n⩽6 also the nonlinear orbital stability takes place. The case n=7 was doubtful—one can find in the literature statements about both stability and instability with incomplete or erroneous proofs. In this paper we prove that for n=7 the nonlinear stability still takes place. Thus the full answer to Kelvin’s question is that the regular vortex n-gon is stable at n⩽7, while at n⩾8 it is unstable. We also present the general theory of stationary motions of a dynamical system with symmetry group. The definitions of stability and instability are necessary to modify in the specific case of stationary regimes. We do not assume that the system is conservative. Thus, the results can be applied not only to various stationary regimes of an ideal fluid flows but, for instance, also to motions of viscous fluids.
Wave front fragmentation due to ventricular geometry in a model of the rabbit heart
Authors: Rogers Jack M.
Published: August 2002

The role of the heart’s complex shape in causing the fragmentation of activation wave fronts characteristic of ventricular fibrillation (VF) has not been well studied. We used a finite element model of cardiac propagation capable of simulating functional reentry on curved two-dimensional surfaces to test the hypothesis that uneven surface curvature can cause local propagation block leading to proliferation of reentrant wave fronts. We found that when reentry was induced on a flat sheet, it rotated in a repeatable meander pattern without breaking up. However, when a model of the rabbit ventricles was formed from the same medium, reentrant wave fronts followed complex, nonrepeating trajectories. Local propagation block often occurred when wave fronts propagated across regions where the Gaussian curvature of the surface changed rapidly. This type of block did not occur every time wave fronts crossed such a region; rather, it only occurred when the wave front was very close behind the previous wave in the cycle and was therefore propagating into relatively inexcitable tissue. Close wave front spacing resulted from nonstationary reentrant propagation. Thus, uneven surface curvature and nonstationary reentrant propagation worked in concert to produce wave front fragmentation and complex activation patterns. None of the factors previously thought to be necessary for local propagation block (e.g., heterogeneous refractory period, steep action potential duration restitution) were present. We conclude that the complex geometry of the heart may be an important determinant of VF activation patterns.
Onset of chaotic dynamics in a ball mill: Attractors merging and crisis induced intermittency
Authors: Manai G. , Delogu F. , Rustici M.
Published: August 2002

In mechanical treatment carried out by ball milling, powder particles are subjected to repeated high-energy mechanical loads which induce heavy plastic deformations together with fracturing and cold-welding events. Owing to the continuous defect accumulation and interface renewal, both structural and chemical transformations occur. The nature and the rate of such transformations have been shown to depend on variables, such as impact velocity and collision frequency that depend, in turn, on the whole dynamics of the system. The characterization of the ball dynamics under different impact conditions is then to be considered a necessary step in order to gain a satisfactory control of the experimental set up. In this paper we investigate the motion of a ball in a milling device. Since the ball motion is governed by impulsive forces acting during each collision, no analytical expression for the complete ball trajectory can be obtained. In addition, mechanical systems exhibiting impacts are strongly nonlinear due to sudden changes of velocities at the instant of impact. Many different types of periodic and chaotic impact motions exist indeed even for simple systems with external periodic excitation forces. We present results of the analysis on the ball trajectory, obtained from a suitable numerical model, under growing degree of impact elasticity. A route to high dimensional chaos is obtained. Crisis and attractors merging are also found.
Dissociative recombination of D+(D2O)2 water cluster ions with free electrons
Authors: Någård M. B. , Pettersson J. B. C. , Derkatch A. M. , Al Khalili A. , Neau A. , Rosén S. , Larsson M. , Semaniak J. , Danared H. , Källberg A. , Österdahl F. , Ugglas M. af.
Published: August 2002

Dissociative recombination (DR) of the water cluster ion D+(D2O)2 has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). Cluster ions were injected into the ring and accelerated to an energy of 2.28 MeV. The stored ion beam was merged with an almost monoenergetic electron beam, and neutral fragments produced by DR were detected by an energy-sensitive surface barrier detector. The first experimental determinations of the absolute DR cross section and branching ratios for a cluster ion are reported. The cross section for the process D+(D2O)2+e− is large and reaches 6⋅10−12 cm2 at a low center-of-mass collision energy of 0.001 eV. The cross section has an E−1.19±0.02 dependence in the energy range 0.001–0.0052 eV, and a steeper slope with an E−1.70±0.12 dependence for E=0.052–0.324 eV. The general trends are similar to the results for previously studied molecular ions, but the cross section is higher in absolute numbers for the cluster ion. Thermal rate coefficients for electron temperatures of 50–2000 K are deduced from the cross section data and the rate coefficients are consequently also large. Branching ratios for the product channels are determined with a grid technique. Break-up into 2D2O+D is the dominating dissociation channel with a probability of 0.94±0.04. The channel resulting in the fragments D2O+OD+D2 has a probability of 0.04±0.02, and the probability for formation of D3O+D2O is 0.02±0.03. The results are compared with data for molecular ions, and the cluster dissociation dynamics are discussed.
Investigating nonlinear dynamics from time series: The influence of symmetries and the choice of observables
Authors: Letellier Christophe , Aguirre Luis A.
Published: August 2002

When a dynamical system is investigated from a time series, one of the most challenging problems is to obtain a model that reproduces the underlying dynamics. Many papers have been devoted to this problem but very few have considered the influence of symmetries in the original system and the choice of the observable. Indeed, it is well known that there are usually some variables that provide a better representation of the underlying dynamics and, consequently, a global model can be obtained with less difficulties starting from such variables. This is connected to the problem of observing the dynamical system from a single time series. The roots of the nonequivalence between the dynamical variables will be investigated in a more systematic way using previously defined observability indices. It turns out that there are two important ingredients which are the complexity of the coupling between the dynamical variables and the symmetry properties of the original system. As will be mentioned, symmetries and the choice of observables also has important consequences in other problems such as synchronization of nonlinear oscillators.
Chaos control by using Motor Maps
Authors: Arena Paolo , Fortuna Luigi , Frasca Mattia
Published: August 2002

In this paper a new method for chaos control is proposed, consisting of an unsupervised neural network, namely a Motor Map. In particular a feedback entrainment scheme is adopted: a chaotic system with a given parameter set generates the reference trajectory for another chaotic system with different parameters to be controlled: the Motor Map is required to provide the appropriate time-varying gain value for the feedback signal. The state of the controlled system is considered as input to the Motor Map. Particular efforts have been paid to the feasibility of the implementation. Indeed, the simulations performed have been oriented to design a Motor Map suitable for an hardware realization, thus some restrictive hypotheses, such as for example a low number of neurons, have been assumed. A huge number of simulations has been carried out by considering as system to be controlled a Double Scroll Chua Attractor as well as other chaotic attractors. Several reference trajectories have also been considered: a limit cycle generated by a Chua’s circuit with different parameters values, a double scroll Chua attractor, a chaotic attractor of the family of the Chua’s circuit attractors. In all the simulations instead of controlling the whole state space, only two state variables have been fed back. Good results in terms of settling time (namely, the period in which the map learns the control task) and steady state errors have been obtained with a few neurons. The Motor Map based adaptive controller offers high performances, specially in the case when the reference trajectory is switched into another one. In this case, a specialization of the neurons constituting the Motor Map is observed: while a group of neurons learns the appropriate control law for a reference trajectory, another group specializes itself to control the system when the other trajectory is used as a reference. A discrete components electronic realization of the Motor Map is presented and experimental results confirming the simulation results are shown.
Anatomic determinants of atrial arrhythmias: New insights from three-dimensional mapping
Authors: Stein Kenneth M. , Markowitz Steven M. , Mittal Suneet , Slotwiner David J. , Iwai Sei , Lerman Bruce B.
Published: August 2002

The recent development of new technologies for in vivo three-dimensional mapping of arrhythmias has enabled a better understanding of the richness of intracardiac anatomy and the relationship between anatomy and arrhythmogenesis. In the present manuscript we review two new technologies for in vivo mapping of atrial arrhythmias and explores the degree to which the anatomic complexity they reveal is important in determining the physiology of both focal and macroreentrant atrial tachycardias. These observations highlight the importance of including sufficient anatomic detail in modeling studies aimed at elucidating the pathophysiology of atrial arrhythmias.
Transversal homoclinic orbits in a transiently chaotic neural network
Authors: Chen Shyan-Shiou , Shih Chih-Wen
Published: August 2002

We study the existence of snap-back repellers, hence the existence of transversal homoclinic orbits in a discrete-time neural network. Chaotic behaviors for the network system in the sense of Li and Yorke or Marotto can then be concluded. The result is established by analyzing the structures of the system and allocating suitable parameters in constructing the fixed points and their pre-images for the system. The investigation provides a theoretical confirmation on the scenario of transient chaos for the system. All the parameter conditions for the theory can be examined numerically. The numerical ranges for the parameters which yield chaotic dynamics and convergent dynamics provide significant information in the annealing process in solving combinatorial optimization problems using this transiently chaotic neural network.
Radiation damage and its recovery in focused ion beam fabricated ferroelectric capacitors
Authors: Stanishevsky A. , Nagaraj B. , Melngailis J. , Ramesh R. , Khriachtchev L. , McDaniel E.
Published: August 2002

We studied the effect of ion damage on the properties of 50 keV Ga+ focused ion beam fabricated lead-zirconate-titanate capacitors as a function of the ion dose. We observed significant modification in the chemical composition of the damaged layer due to loss of lead and oxygen, and gallium impurity accumulation. The 5–10 nm thick damaged layer becomes dielectric after annealing and does not recover its ferroelectric properties. This dielectric layer substantially reduces the actual volume of the ferroelectric material in sub-100 nm structures, and can affect their performance.
H-mode access and performance in the Mega-Amp Spherical Tokamak
Authors: Akers R. J. , Ahn J. W. , Appel L. C. , Arends E. R. , Axon K. B. , Buttery R. J. , Byrom C. , Carolan P. G. , Counsell G. F. , Cunningham G. , Ciric D. , Conway N. J. , Dowling J. , Field A. R. , Fielding S. J. , Gee S. , Gryaznevich M. P. , Hole M. , Kirk A. , Lehane I. , Lloyd B. , Manhood S. , Martin R. , McArdle G. , McGrath M. , Meyer H. , Morris A. W. , Nightingale M. P. S. , Pinfold T. , Price M. , Ribeiro C. , Shevchenko V. , Shibaev S. , Sykes A. , Tabasso A. , Taylor D. , Tournianski M. R. , Valovič M. , Walsh M. J. , Warder S. E. V. , Watkins J. R. , Wilson H. R. , You S. ,   MAST Team,   NBI Team
Published: August 2002

Spontaneous transitions from the low “L-mode” to high “H-mode” of tokamak plasma confinement, first observed during neutral beam heating experiments on ASDEX, are now routinely achieved in many tokamak experiments. The H-mode regime is attractive as it offers the possibility of enhanced confinement, and thus a route towards a more “compact” and cost-efficient fusion power-plant. Transition to H-mode is now routinely achievable in the Mega-Amp Spherical Tokamak (MAST) [A. C. Darke et al., Fusion Technology 1994 (Elsevier, Amsterdam, 1995), Vol. 1, p. 799] for both Ohmically and neutral beam injection (NBI) heated plasmas (PNBI∼0.5–1.7 MW). H-mode plasmas can be either center stack limited or X-point diverted, exhibiting regular Type III edge localized modes (ELMs). Global confinement in H-mode with low frequency ELMs is consistent with the international IPB(y,2) scaling and exceeds the scaling by a factor ∼1.5–2.0 for high performance discharges. Confinement degrades with increasing ELM frequency (which in turn scales with power and density) as for conventional tokamaks. Densities above the Greenwald limit (G∼1) have been achieved for plasma currents up to 0.8 MA using gas-fueling, and up to 0.9 MA using a low field side multi-pellet injector. High field side fueling, on the other hand, can be supplied via a gas-feed located at the center-column mid-plane, this technique having been found to dramatically enhance H-mode accessibility and quality. When combined with Connected Double Null plasma topology, a significant reduction in Ohmic L–H power threshold can be achieved; as a result, power threshold data are now in broad agreement with a number of the latest scaling law predictions. Following the transition to H-mode, power crossing the inner separatrix remains low, resulting in a high recycling scrape-off layer (compared with partial detachment in L-mode). To date, with NBI power limited to 1.7 MW, H-mode MAST plasmas have shown no evidence of having approached a beta limit (βN∼4.5li). High performance H-mode discharges are at sufficient poloidal beta (βp), however, to enable the first studies of the Neoclassical Tearing Mode, the MHD instability responsible for limiting the achievable beta in conventional tokamaks.
Single crystal Si layers on glass formed by ion cutting
Authors: Cai M. , Qiao D. , Yu L. S. , Lau S. S. , Li C. P. , Hung L. S. , Haynes Tony E. , Henttinen K. , Suni Ilkka , Poon V. M. C. , Marek T. , Mayer J. W.
Published: August 2002

The process of ion cutting was used to integrate single crystalline Si layers on glass for potential active matrix flat panel display and other applications. It was found that p-Si wafers implanted at 100–150 °C with H with a dose in the order of a few times 1016 cm−2 could be readily bonded to glass substrates when both of the surfaces were properly treated and activated. The as-implanted Si wafer surface was converted from p type to n type. Upon bonding at room temperature, annealing (300 °C) and exfoliation (450 °C), the transferred Si layer on glass and the as-exfoliated surface of the implanted Si wafer remained n type. A highly defective region was observed near the top of the Si layer on glass, however the crystalline quality was nearly defect free in the deeper region of the layer. Annealing at sequentially higher temperatures led to the recovery of p type conductivity at ∼600–650 °C. The type conversion and the subsequent annealing behavior observed on the samples were rationalized in terms of ion enhanced oxygen diffusion and the presence of H-related shallow donors in the Si.
Stability of active systems with a spatially periodic activity: Analysis of a simple model and application to the boiling crisis problem
Authors: Pumir Alain , Barelko Viktor V.
Published: August 2002

We investigate theoretically the possibility to control the transition between two metastable states in reactive systems by imposing a spatial modulation. In particular, we consider the technologically very important case of the transition between the low temperature (nucleate boiling) and the high temperature (film boiling) phases of boiling of a liquid over a heat generating element, also known as the boiling crisis. With the help of a simplified model, we demonstrate that the dangerous regime where the high temperature phase invades the whole system requires a larger heat power in a periodically spatially modulated system, than in a uniform system. The possibility that a local perturbation, such as a small gas bubble, may induce locally a transition to the film boiling state is also considered. We show that the transition to the film boiling regime is hindered in a spatially periodic system.
Fast heat pulse propagation with a nonlocal flux-gradient model
Authors: Kundu M. , Deshpande S. P. , Kaw P. K.
Published: August 2002

Time dependent solutions of the heat diffusion equation in a slab geometry are studied for a spatially nonlocal flux-gradient model of thermal diffusivity (χ) proposed by Taylor et al. [J. B. Taylor, J. W. Connor, and P. Helander, Phys. Plasmas 5, 3065 (1998)]. This model is distinguished by the presence of a parameter α in the expression for χ, which is related to the strength of velocity shear in the plasma. It is shown that when α is nonzero one can obtain a rapid propagation of heat pulse towards the core due to the cooling of the edge, exhibiting similar features as in “edge-cooling core-heating” experiments. A rapidly increasing α(t) also gives encouraging qualitative match with observed nonlocal signatures of current ramp-up in tokamak experiments. General issues of linear and nonlinear stability have also been presented.
The Kelvin–Helmholtz instability, differential rotation, and three-dimensional, localized, magnetic reconnection
Authors: Knoll D. A. , Brackbill J. U.
Published: August 2002

Results are presented from a study of three-dimensional magnetic reconnection caused by a Kelvin–Helmholtz instability and differential rotation. Specifically, subsonic and sub-Alfvénic flow is considered, which is Kelvin–Helmholtz stable in the direction of the magnetic field, but unstable perpendicular to the magnetic field. The flow is modeled by the resistive magnetohydrodynamics equations in three dimensions with constant resistivity. As a result of differential rotation (a gradient in vorticity parallel to the initial field), localized transient reconnection is observed on the Kelvin–Helmholtz time scale. Current amplification is observed along with the generation of parallel current. Results indicate that the observed transient reconnection rate is insensitive to resistivity (even with a constant resistivity model), but is sensitive to the initial flow shear.
Sensitivity of ray travel times
Authors: Smirnov I. P. , Virovlyansky A. L. , Zaslavsky G. M.
Published: August 2002

Ray in a waveguide can be considered as a trajectory of the corresponding Hamiltonian system, which appears to be chaotic in a nonuniform environment. From the experimental and practical viewpoints, the ray travel time is an important characteristic that, in some way, involves an information about the waveguide condition. It is shown that the ray travel time as a function of the initial momentum and propagation range in the unperturbed waveguide displays a scaling law. Some properties of the ray travel time predicted by this law still persist in periodically nonuniform waveguides with chaotic ray trajectories. As examples we consider few models with special attention to the underwater acoustic waveguide. It is demonstrated for a deep ocean propagation model that even under conditions of ray chaos the ray travel time is determined, to a considerable extent, by the coordinates of the ray endpoints and the number of turning points, i.e., by a topology of the ray path. We show how the closeness of travel times for rays with equal numbers of turning points reveals itself in ray travel time dependencies on the starting momentum and on the depth of the observation point. It has been shown that the same effect is associated with the appearance of the gap between travel times of chaotic and regular rays. The manifestation of the stickiness (the presence of such parts in a chaotic trajectory where the latter exhibits an almost regular behavior) in ray travel times is discussed.
Cookbook asymptotics for spiral and scroll waves in excitable media
Authors: Margerit Daniel , Barkley Dwight
Published: August 2002

Algebraic formulas predicting the frequencies and shapes of waves in a reaction–diffusion model of excitable media are presented in the form of four recipes. The formulas themselves are based on a detailed asymptotic analysis (published elsewhere) of the model equations at leading order and first order in the asymptotic parameter. The importance of the first order contribution is stressed throughout, beginning with a discussion of the Fife limit, Fife scaling, and Fife regime. Recipes are given for spiral waves and detailed comparisons are presented between the asymptotic predictions and the solutions of the full reaction–diffusion equations. Recipes for twisted scroll waves with straight filaments are given and again comparisons are shown. The connection between the asymptotic results and filament dynamics is discussed, and one of the previously unknown coefficients in the theory of filament dynamics is evaluated in terms of its asymptotic expansion.
Sound waves in two-ion plasmas
Authors: Kozlov M. V. , McKinstrie C. J.
Published: August 2002

The dispersive and nonlinear characteristics of sound waves in plasmas that consist of warm electron and two different ion fluids are studied in detail. Analytical solutions are used to illustrate the characteristics of sound waves and to validate a numerical scheme that solves fluid and Poisson equations. This scheme can be used to model sound waves that participate in stimulated Brillouin scattering in two-ion plasmas.
Fully nonlinear dust kinetic Alfvén waves
Authors: Mahmood M. Ansar , Mirza Arshad M. , Sakanaka P. H. , Murtaza G.
Published: August 2002

Exact localized nonlinear dust kinetic Alfvén waves are investigated by retaining the complete dust nonlinearity and by invoking small but finite-β effects in a collisionless plasma, whose constituents are electrons, ions and negatively charged dust grains. Sub-Alfvénic, as well as super-Alfvénic, solitary waves consisting of smooth density humps or dips are found to exist. Further, super-Alfvénic kink type solitons are also found to occur. The soliton amplitude depends upon various plasma parameters along with the soliton propagation speed. The results of the present investigation may be useful in the diagnostics of dust in magnetized plasmas and in understanding the formation of coherent soliton structures in space and laboratory plasmas.
Nanocrystalline order of zinc oxide thin films grown on optical fibers
Authors: Lee E. Y. M. , Tran N. , Russell J. , Lamb R. N.
Published: August 2002

The crystallographic orientation of polycrystalline zinc oxide films grown on optical fibers using single-source chemical vapor deposition (SS CVD) of basic zinc acetate have been studied. The films have been characterized using near-edge x-ray absorption fine structure. For the SS CVD ZnO growth on planar substrates, the film orientation can be varied from randomly oriented to highly c-axis oriented. In contrast, the films grown on optical fibers were either randomly oriented or a,b-axis oriented, depending on growth conditions. The correlations between growth conditions and the crystallographic properties of the films on fibers were discussed. The results suggest that factors such as curvature may have an effect on the crystallinity of film growth.
Laser spectroscopy of LaS: Hyperfine structure in the B 2Σ+–X 2Σ+ (0,0) band
Authors: He S. G. , Tam W. S. , Leung J. W-H. , Cheung A. S-C.
Published: September 2002

The (0,0) band of the B 2Σ+–X 2Σ+ transition of LaS near 726 nm has been studied at high resolution using laser vaporization/reaction supersonic free jet expansion and laser induced fluorescence spectroscopy. Spectra taken at a resolution of about 50 MHz show resolved hyperfine structure which is caused principally by the unpaired 6sσ electron in the ground state interacting with large magnetic moment of the  57139La nucleus with nuclear spin I=7/2. The X 2Σ+ state of LaS is in good Hund’s case (bβs) coupling, however, the upper state, B 2Σ+, is close to case (bβJ) coupling. A total of 1850 rotational-hyperfine transition lines have been assigned and measured. Accurate rotational, spin and hyperfine parameters for the B 2Σ+ and X 2Σ+ states have been obtained, which give information about the electron distribution in the molecule.
The self-guiding of radially asymmetric relativistically intense laser beams
Authors: Garuchava D. P. , Mikeladze S. V. , Paverman V. S. , Sigua K. I. , Shatashvili N. L. , Tsintsadze N. L. , Shukla P. K.
Published: August 2002

The evolution of initially sufficiently asymmetric super-strong laser pulse propagating in underdense cold plasma is studied. The effects of the relativistic electron mass increase and the repelling of electrons by the ponderomotive force from the central regions of laser beam propagation are taken into account. The beams with spot sizes much smaller than the pulse length are investigated. The beam, as well as plasma, characteristic parameters are obtained in the overcritical propagation regime, ignoring ion motion when the pulse self-focusing, self-guiding and filamentation take place. The role of the striction effect in the electron cavitation and ion channel formation phenomena is investigated. It is shown that, for specific parameters, the strongly asymmetric beam transforms to a symmetric one and later propagates in the plasma for a long distance.
Fast and slow plasma waves excitation by counterpropagating lasers in a hot plasma
Authors: Liu C. S. , Tripathi V. K.
Published: August 2002

Two lasers with frequency difference Δω0∼2ωp, counterpropagating in a hot plasma of plasma frequency ωp, drive a nonresonant space charge beat wave at frequency Δω0 and wave number k⃗b≈2k⃗0, where k⃗0 is the wave number of one of the lasers of frequency ω0. The driven beat wave excites, parametrically, a pair of copropagating fast and slow plasma waves. For Δω0=2ωP+2k02vth2/ωp, where vth is the electron thermal speed, the fast wave has phase velocity close to c, and is suitable for electron acceleration. However, when Δω0 deviates from this value by 2ω0 vth2/c2, the fast wave is too slow to be useful for electron acceleration. The finite spot size of the lasers adversely affects the parametric instability.
Resistive tearing mode instability in partially ionized dusty plasmas
Authors: Shukla P. K.
Published: August 2002

The resistive tearing mode instability in a partially ionized, self-gravitating dusty magnetoplasma is theoretically investigated, taking into account the dust grain dynamics. It is found that the latter effect leads to a growth rate that differs significantly from that of the classical case of an electron-ion plasma. This modification of the tearing instability may be responsible for magnetic field line reconnection in astrophysical environments.
Nucleation rate and primary particle size distribution
Authors: McClurg Richard B.
Published: August 2002

An analytic model for the steady-state homogeneous nucleation rate and primary particle size distribution is presented. It is a solution of the steady-state cluster concentration evolution equations, including terms for particle transport from the nucleation zone. In the model, the nucleation rate and primary particle size distribution are calculated self-consistently. The classical homogeneous nucleation rate expression is a special case of this nucleation zone with transport model in the limit of negligible particle transport. Asymptotic functions suitable for fitting to data are also provided.
Sub-100 nm SrBi2Ta2O9 film with ultrathin BiTaO4 capping layer for 3 V or lower-voltage ferroelectric memory operation
Authors: Lim M. , Joshi V. , Narayan S. , Celinska J. , Karasawa J.
Published: August 2002

This letter proposes an enhanced 700 °C process to improve the dielectric breakdown strength and surface roughness of sub-100 nm SrBi2Ta2O9 (SBT) film with a thin (<20 nm) BiTaO4 (BT) layer for the 3 V or lower voltage ferroelectric memory. The process temperature for rapid thermal annealing and furnace annealing is performed at or below 700 °C. The BT layer is used as a capping layer on top of SBT film. It improves the dielectric breakdown strength (1.2 MV/cm) of SBT film without sacrificing other ferroelectric properties.
On drift instabilities in magnetized target fusion devices
Authors: Ryutov D. D.
Published: August 2002

Some versions of magnetized target fusion (MTF) devices will be using a high beta plasma, with local beta exceeding 1. Drift instabilities in such a plasma are electromagnetic and are quite different from the analogous instabilities in a low beta plasma. In a collisionless limit they have been analyzed by El Nadi and Rosenbluth [Phys. Fluids 16, 2036 (1973)] who have shown that the cross-field transport coefficients in such a plasma may exceed a Bohm value. On the other hand, high-density plasma in MTF systems is usually strongly collisional in the sense that the drift frequency for the most dangerous large-scale perturbations is smaller than the ion–ion collision frequency, and the particle mean free path is shorter than the parallel wavelength. This regime is studied in the present paper. It is shown that transport coefficients in the MTF plasma are usually smaller than the Bohm diffusion coefficient.
Artifacts, assumptions, and ambiguity: Pitfalls in comparing experimental results to numerical simulations when studying electrical stimulation of the heart
Authors: Roth Bradley J.
Published: August 2002

Insidious experimental artifacts and invalid theoretical assumptions complicate the comparison of numerical predictions and observed data. Such difficulties are particularly troublesome when studying electrical stimulation of the heart. During unipolar stimulation of cardiac tissue, the artifacts include nonlinearity of membrane dyes, optical signals blocked by the stimulating electrode, averaging of optical signals with depth, lateral averaging of optical signals, limitations of the current source, and the use of excitation–contraction uncouplers. The assumptions involve electroporation, membrane models, electrode size, the perfusing bath, incorrect model parameters, the applicability of a continuum model, and tissue damage. Comparisons of theory and experiment during far-field stimulation are limited by many of these same factors, plus artifacts from plunge and epicardial recording electrodes and assumptions about the fiber angle at an insulating boundary. These pitfalls must be overcome in order to understand quantitatively how the heart responds to an electrical stimulus.
Collective chaos synchronization of pairs of modes in a chaotic three-mode laser
Authors: Otsuka Kenju , Ohtomo Takayuki , Yoshioka Atsushi , Ko Jing-Yuan
Published: August 2002

We study chaos synchronization experimentally in a modulated globally coupled three-mode laser with different modal gains subjected to self-mixing Doppler-shifted feedback, which can apply the loss modulation to individual modes at Doppler-shift frequencies. Depending on the pump power, different forms of collective chaos synchronizations were found to appear when the laser was modulated at the highest relaxation oscillation frequency, reflecting the change in cross-saturation coefficient among modes. In the present experiment, each pair of modes exhibited phase, lag, or generalized synchronization collectively according to the inherent antiphase dynamics, where these types of synchronization have already been demonstrated in two coupled chaotic oscillators in different physical systems. Information flows among oscillating modes which are established in different forms of collective chaos synchronizations were characterized by information-circulation analysis of the experimental time series.
Laser wakefield structure in a plasma column created in capillary tubes
Authors: Andreev N. E. , Cros B. , Gorbunov L. M. , Matthieussent G. , Mora P. , Ramazashvili R. R.
Published: August 2002

The structure of the wakefield is studied in a plasma column, created by a monomode laser pulse propagating in a capillary tube, filled with gas affected by tunneling ionization. Linear analytical considerations as well as self-consistent numerical simulations show that in the central bulk part of a plasma column where the laser intensity exceeds the ionization threshold, the wakefield structure is similar to that of an infinite homogeneous plasma. Near the wall of the capillary tube, where the laser intensity decreases below the ionization threshold and where the plasma density falls to zero, the curvature of the plasma wave phase front increases with the distance from the laser pulse, resulting in small-scale radial electric field which may undergo phase mixing.
Neoclassical calculation of poloidal rotation and poloidal density asymmetries in tokamaks
Authors: Stacey Weston M.
Published: August 2002

A previous model for the calculation of poloidal velocities and poloidal density asymmetries in the core of a tokamak plasma is refined and extended by the inclusion of terms which are important for the calculation of these quantities in the plasma edge. Agreement of predictions of the model with experiment is demonstrated. The effects of edge pressure gradient, collisionality, neutral density and up–down flux surface asymmetry on the edge poloidal rotation velocities are illustrated by a series of model problem calculations.
A simple system to measure magnetostriction in soft magnetic materials
Authors: Claassen J. H. , Willard Matthew A. , Harris Vincent G.
Published: August 2002

A direct measurement of strain in magnetostrictive materials yields a meaningful result only if the sample remains saturated and the magnetization is rotated between directions parallel and perpendicular to the measurement direction. A simple way to accomplish this is to use small permanent magnets mounted on a rotating shaft. Within the constraints of the limited available space for a sample, we found it most convenient to use a strain gage to detect small changes in length. With commonly available laboratory equipment we were able to achieve a sensitivity ∼0.03 ppm. There is a loading effect that reduces the apparent magnetostriction if the sample is of comparable size to the strain gage. We showed that for a typical nanocrystalline ribbon sample this correction could be experimentally estimated and was of order 20%.
High-order maps with acceleration for optimization of electrostatic and radio-frequency ion-optical elements
Authors: Geraci Andrew A. , Barlow Teresa A. , Portillo Mauricio , Nolen Jerry A. , Shepard Kenneth W. , Makino Kyoko , Berz Martin
Published: August 2002

A method has been developed to calculate accurate high-order ion-optical maps for electrostatic and radio-frequency accelerating elements. The method has been incorporated into the arbitrary-order map-based beam optics code COSY Infinity. The treatment is restricted to the case of negligible magnetic fields, as is typical of heavy-ion accelerating cavities, and does not include space charge. For validation purposes, the beam dynamics calculated for these elements is compared against ray tracing for typical beam and cavity parameters. Different from the ray-tracing approach, parameter changes of individual components typically require only recalculation of the maps of the particular components and not the entire system, and thus the method is particularly suitable for optimization. The approach developed for accurate analytical representation of the on- and off-axis electric fields of cylindrically symmetric electrostatic lenses and radio-frequency cavities is described. Some of the many possible applications for using accurate high-order map representations of Einzel lenses, electrostatic accelerating gaps, and radio-frequency accelerating structures are discussed.
A complete digital radio-frequency source for nuclear magnetic resonance spectroscopy
Authors: Yun Jiang , Yu Jiang , Hongyan Tao , Gengying Li
Published: August 2002

A complete digital radio-frequency source suitable for broadband nuclear magnetic resonance applications is described. The use of a complete direct digital synthesis chip makes the design not only as a frequency- and phase-continuous tunable radio-frequency source, but also as an arbitrary wave form generator. This rf source is capable of generating a signal with frequency from dc up to 135 MHz directly. The whole design is mounted on a printed circuit board that can be inserted into one of the industry standard architecture slots in the personal computer and is controlled by the software. Details about the implementation and characteristics of the whole design are discussed in this article.
A system for the inspection and quality control of glass slabs
Authors: Rovati Luigi , Pollonini Luca , Docchio Franco
Published: August 2002

This article describes an innovative interferometric system for the inspection and quality control of glass slabs. The instrument exploits a self-mixing superluminescent diode scheme to improve the interferometric signal and to reject typical noise and electromagnetic interference existing in industrial environments. Various instrumental aspects and performance issues are discussed. Experimental activities demonstrate a system measuring range of 4.5 mm with a relative measurement uncertainty of less than 2%. Whereas, the measuring stability was measured to be about 2.5 μm over 20 min.
High-energy photon radiography system using laser-Compton scattering for inspection of bulk materials
Authors: Toyokawa Hiroyuki , Ohgaki Hideaki , Mikado Tomohisa , Yamada Kawakatsu
Published: August 2002

Transmission radiography of bulk materials using continuously energy-tunable quasimonochromatic photon beams has been performed with the photons of a few tens MeV. The photon beam is generated with the laser-Compton scattering of the 300–800 MeV electrons in the electron storage ring “TERAS” with laser lights. The present radiography system is applicable to an inspection of bulk materials and to nondestructive testing of large industrial products. The effectiveness of the method has been examined using metals, ceramics, and concrete blocks. The spatial resolution of the radiography system was measured to be 650 μm using a 1 mm collimator system.
Development of a dilatometer based on diffractometry
Authors: Ledesma S. , Goyanes S. N. , Duplaá C.
Published: August 2002

In this article we present a new method to determine the thermal expansion coefficient of polymeric materials. The method is based on the measurement of the diffraction patterns produced by a slit. This slit is produced using two edges, one that remains fixed and the other one attached to the sample, so that the slit modifies its width as the sample temperature is changed. For each temperature the sample length expansion is obtained by using the Fraunhofer diffraction theory. Depending on which parameters of the experimental setup are chosen, this technique can be used to measure expansions of tens of microns or hundreds of microns with a resolution of tenths of microns or a few microns, respectively. As an example, we have determined the coefficient of thermal expansion and the glass transition temperature Tg of a commercial sample of polymethylmethacrylate (PMMA). The obtained results were compared with those reported previously in a sample of the same commercial material using a mechanical dilatometer. Moreover, our results agree with those reported in the literature for different samples of PMMA both for the low temperature TTg regions.
Development of a computer algorithm for the detection of phase singularities and initial application to analyze simulations of atrial fibrillation
Authors: Zou Renqiang , Kneller James , Leon L. Joshua , Nattel Stanley
Published: August 2002

Atrial fibrillation (AF) is a common cardiac arrhythmia, but its mechanisms are incompletely understood. The identification of phase singularities (PSs) has been used to define spiral waves involved in maintaining the arrhythmia, as well as daughter wavelets. In the past, PSs have often been identified manually. Automated PS detection algorithms have been described previously, but when we attempted to apply a previously developed algorithm we experienced problems with false positives that made the results difficult to use directly. We therefore developed a tool for PS identification that uses multiple strategies incorporating both image analysis and mathematical convolution for automated detection with optimized sensitivity and specificity, followed by manual verification. The tool was then applied to analyze PS behavior in simulations of AF maintained in the presence of spatially distributed acetylcholine effects in cell grids of varying size. These analyses indicated that in almost all cases, a single PS lasted throughout the simulation, corresponding to the central-core tip of a single spiral wave that maintained AF. The sustained PS always localized to an area of low acetylcholine concentration. When the grid became very small and no area of low acetylcholine concentration was surrounded by zones of higher concentration, AF could not be sustained. The behavior of PSs and the mechanisms of AF were qualitatively constant over an 11.1-fold range of atrial grid size, suggesting that the classical emphasis on tissue size as a primary determinant of fibrillatory behavior may be overstated.
Liquid-to-crystal nucleation: A new generation lag-time apparatus
Authors: Heneghan A. F. , Haymet A. D. J.
Published: August 2002

The statistics of liquid-to-crystal nucleation are examined using a new generation automated lag-time apparatus denoted ALTA 4. Two modes of operation are described and implemented. A single 200 μl sample of distilled water is cooled repeatedly at a constant rate, α, until the sample freezes. This cycle is repeated many hundreds of times, to generate the reliable and reproducible statistics for nucleation. From a single experiment, under a single testable approximation, we extract the functional form which characterizes the average lag-time, 〈τ〉, as a function of the degree of supercooling ΔT below the equilibrium freezing temperature, Tf. These data and the analysis yield, from a single experiment, a profile for the average lag-time, 〈τ〉, as a function of change in temperature, ΔT. Identical results are obtained over more than an order of magnitude of cooling rates α, from 0.6 to 7.5 K min−1 (0.01–0.125 K s−1).
Interface properties and bubble nucleation in compressible mixtures containing polymers
Authors: Müller M. , MacDowell L. G. , Virnau P. , Binder K.
Published: August 2002

Using self-consistent field theory we investigate the properties of interfaces and of bubbles that nucleate in response to a pressure change. We use a simple phenomenological equation of state for a compressible mixture of two polymers. The parameters are chosen as to mimic the behavior of a polymer in a supercritical solvent and the phase behavior in the bulk corresponds to class III in the classification of Konynenburg and Scott. At low pressure, the density of the volatile solvent is small and the interface and nucleation properties are similar to a one-component fluid. At higher pressure, however, there is a triple point at which the polymer coexists with a vapor of the solvent and a mixed solvent-rich liquid. The vicinity of the triple point alters the interface and nucleation behavior: There is a thick wetting layer of the (metastable) solvent-rich liquid at the interface between polymer and vapor, and the solvent condenses into a solvent-rich liquid inside small bubbles. We explore the dependence of the nucleation barrier on temperature, pressure and molecular weight dispersity of the polymer and relate our findings to the binodal and spinodal of the bulk.
Ab initio, kinetics, and dynamics study of Cl+CH4→HCl+CH3
Authors: Troya Diego , Millán Judith , Baños Irene , González Miguel
Published: September 2002

The Cl+CH4→HCl+CH3 reaction has been studied using different levels of the chemical reaction theory. Ab initio calculations at the fourth-order Møller–Plesset perturbation theory//second-order Møller–Plesset perturbation theory level, employing the 6-311G(2df,2pd) basis set, satisfactorily describe the system stationary points. A kinetics study using variational transition state theory has been accomplished, using ab initio information along the minimum energy reaction path. The agreement with experimental rate constants in the 200–500 K range is the best reported up to date. Reduced-dimensionality quasiclassical trajectory (QCT) calculations of the reaction dynamics have been performed on an analytical many-body potential energy surface, which is fitted to ab initio calculations of the system. The various experiments showing the absence of energy release to the CH3 group has led us to invoke a pseudotriatomic approximation, in which the CH3 moiety is treated as a single particle, so as to carry out dynamics calculations. Our QCT calculations give particular attention to the comparison with the numerous experiments available on this reaction. These include internal state distributions of the nascent diatomic molecule and state specific angular distributions. Further insight into the state specific scattering distributions is provided through analysis of opacity functions. The satisfactory reproduction of a great variety of experimental results by the theoretical study presented here is consistent with the idea that the CH3 modes are weakly coupled to the reaction coordinate, thus validating the model adopted in this work.
Novel concepts of vacuum microelectronic microwave devices with field emitter cathode arrays
Authors: Rozhnev A. G. , Ryskin N. M. , Sokolov D. V. , Trubetskov D. I. , Han S. T. , Kim J. I. , Park G. S.
Published: August 2002

Field emitter arrays (FEA) are promising electron sources for vacuum microelectronic microwave amplifiers and oscillators. In this paper, microelectronic analogs of “classic” microwave devices are considered. Traveling wave klystrons, klystrodes and cross-field amplifiers with FEA are described. Basic output parameters (gain, efficiency, etc.) of these devices are calculated. A model of a self-excited vacuum microtriode oscillator is developed. The oscillator is studied numerically, as well as by experimental investigation of the circuit analog model. Chaotic generation in the oscillator under external harmonic driving is analyzed. The rf interaction with the electron beam modulated by the field emission in a twystrode is investigated using a particle-in-cell code and a one-dimensional nonlinear code.