bluuebunny/crossref_metadata_2025 huggingface dataset:
Dispersive nonlinear geometric optics
Authors: Donnat Phillipe , Rauch Jeffrey
Published: July 2002

We construct infinitely accurate approximate solutions to systems of hyperbolic partial differential equations which model short wavelength dispersive nonlinear phenomena. The principal themes are the following. (1) The natural framework for the study of dispersion is wavelength ε solutions of systems of partial differential operators in ε∂. The natural ε-characteristic equation and ε-eikonal equations are not homogeneous. This corresponds exactly to the fact that the speeds of propagation, which are called group velocities, depend on the length of the wave number. (2) The basic dynamic equations are expressed in terms of the operator ε∂t. As a result growth or decay tends to occur at the catastrophic rate ect/ε. The analysis is limited to conservative or nearly conservative models. (3) If a phase φ(x)/ε satisfies the natural ε-eikonal equation, the natural harmonic phases, nφ(x)/ε, generally do not. One needs to impose a coherence hypothesis for the harmonics. (4) In typical examples the set of harmonics which are eikonal is finite. The fact that high harmonics are not eikonal suppresses the wave steepening which is characteristic of quasilinear wave equations. It also explains why a variety of monochromatic models are appropriate in nonlinear settings where harmonics would normally be expected to appear. (5) We study wavelength ε solutions of nonlinear equations in ε∂ for times O(1). For a given system, there is a critical exponent p so that for amplitudes O(εp), one has simultaneously smooth existence for t=O(1), and nonlinear behavior in the principal term of the approximate solutions. This is the amplitude for which the time scale of nonlinear interaction is O(1). (6) The approximate solutions have residual each of whose derivatives is O(εn) for all n>0. In addition, we prove that there are exact solutions of the partial differential equations, that is with zero residual, so that the difference between the exact solution and the approximate solutions is infinitely small. This is a stability result for the approximate solutions.
The role of information measures in the determination of the maximum entropy-minimum norm solution of the generalized inverse problem
Authors: Plastino A. R. , Miller H. G. , Plastino A. , Yen G. D.
Published: July 2002

We discuss here the role of different information measures vis-a-vis the maximum entropy-minimum norm method of Baker Jarvis et al. (BJ) for dealing with under-determined inverse linear transforms. It is shown that the structure of the approximate solutions provided by the BJ approach does not depend upon the functional form of the information measure.
New conformal gauging and the electromagnetic theory of Weyl
Authors: Wheeler James T.
Published: July 2002

A new eight-dimensional conformal gauging solves the auxiliary field problem and eliminates unphysical size change from Weyl’s electromagnetic theory. We derive the Maurer–Cartan structure equations and find the zero curvature solutions for the conformal connection. By showing that every one-particle Hamiltonian generates the structure equations we establish a correspondence between phase space and the eight-dimensional base space, and between the action and the integral of the Weyl vector. Applying the correspondence to generic flat solutions yields the Lorentz force law, the form and gauge dependence of the electromagnetic vector potential and minimal coupling. The dynamics found for these flat solutions applies locally in generic spaces. We then provide necessary and sufficient curvature constraints for general curved eight-dimensional geometries to be in 1–1 correspondence with four-dimensional Einstein–Maxwell space–times, based on a vector space isomorphism between the extra four dimensions and the Riemannian tangent space. Despite part of the Weyl vector serving as the electromagnetic vector potential, the entire class of geometries has vanishing dilation, thereby providing a consistent unified geometric theory of gravitation and electromagnetism. In concluding, we give a concise discussion of observability of the extra dimensions.
The general decomposition theory of SU(2) gauge potential, topological structure and bifurcation of SU(2) Chern density
Authors: Duan Yishi , Fu Libin
Published: July 2002

By means of the geometric algebra the general decomposition of SU(2) gauge potential on the sphere bundle of a compact and oriented four-dimensional manifold is given. Using this decomposition theory the SU(2) Chern density has been studied in detail. It shows that the SU(2) Chern density can be expressed in terms of the δ-function δ(φ). One can also find that the zero points of the vector fields φ are essential to the topological properties of a manifold. It is shown that there exists the crucial case of branch process at the zero points. Based on the implicit function theorem and the Taylor expansion, the bifurcation of the Chern density is detailed in the neighborhoods of the bifurcation points of φ. It is pointed out that, since the Chern density is a topological invariant, the sum topological chargers of the branches will remain constant during the bifurcation process.
Gauge transformation in Einstein–Yang–Mills theories
Authors: Pons J. M. , Salisbury D. C. , Shepley L. C.
Published: July 2002

We discuss the relation between space–time diffeomorphisms and gauge transformations in theories of the Yang–Mills type coupled with Einstein’s general relativity. We show that local symmetries of the Hamiltonian and Lagrangian formalisms of these generally covariant gauge systems are equivalent when gauge transformations are required to induce transformations which are projectable under the Legendre map. Although pure Yang–Mills gauge transformations are projectable by themselves, diffeomorphisms are not. Instead, the projectable symmetry group arises from infinitesimal diffeomorphism-inducing transformations which must depend on the lapse function and shift vector of the space–time metric plus associated gauge transformations. Our results are generalizations of earlier results by ourselves and by Salisbury and Sundermeyer.
Dynamics and stability of localized modes in nonlinear media with point defects
Authors: Bogdan M. M. , Kovalev A. S. , Gerasimchuk I. V.
Published: July 2002

The soliton states localized at a point defects are investigated by using the nonlinear Schrödinger equation for various signs of the nonlinearity and for different types of defects. The quantum interpretation of these nonlinear localized modes is given in terms of bound states of a large number of Bose particles. The dynamic properties and stability of these states for different types of interaction between elementary excitations with one another and with the defect are investigated. The boundaries of the region of existence and stability of “impurity” solitons are determined depending on the “intensity” of the defect, and the frequency of small oscillations of a soliton near the defect is calculated.
Emission Mössbauer study of CMR manganite La0.8Ca0.2MnO3. I. Anomalous ferromagnetism
Authors: Chechersky V. , Nath A. , Ju H. , Greene R. L.
Published: July 2002

Using Co57 emission Mössbauer technique, we present clear evidence that in Ca-doped manganite, the magnetic and paramagnetic phases coexist below TC, with the abundance of the latter increasing with temperature. In contrast with the regular ferromagnetic materials, the variation of the hyperfine internal magnetic field Hint with temperature deviates considerably from the Brillouin relation, and exhibits an abrupt drop at TC. These features characterize the magnetic transition as a first-order transition. The non-Brillouin behavior of Hint(T) and the temperature dependence of the shape of the magnetically split sextet indicate the presence of spin fluctuations in this material well below TC.
The bifurcation of a hot round jet to limit-cycle oscillations
Authors: Raghu Surya , Monkewitz Peter A.
Published: July 2002

The onset of self-excited or limit-cycle oscillations in an axisymmetric hot jet is studied experimentally as the jet density is systematically decreased below the value at which absolute instability first appears in the potential core region. By transient measurements at different supercritical jet densities, this bifurcation is demonstrated to be of the Hopf type. That is, close to the critical density, the oscillation amplitude at a fixed point in space is shown to be described by a Landau equation. All coefficients of the equation, i.e., linear temporal growth rate, linear frequency, and the Landau constant, are determined.
A proposed statistical-dynamic closure method for the linear or nonlinear subgrid-scale stresses
Authors: Wong V. C.
Published: July 2002

Following the dynamic subgrid-scale (SGS) modeling developed by Germano et al. [Phys. Fluids A 3, 1760 (1991)] and Lilly [Phys. Fluids A 4, 633 (1992)], a multidimensional statistical-dynamic closure method for the linear or nonlinear anisotropic representation of the SGS stresses is proposed. This method converts all previously prescribed SGS model coefficients for turbulent flows to nonsingular and self-determined parameters, and in principle it does not share most deficiencies of the Smagorinsky formulation for SGS eddy viscosity. During large-eddy simulations, the SGS representation will be locally and dynamically adjusted to match the statistical structure of the resolved turbulent eddies.
The effects of inertia on the viscoelastic Dean and Taylor–Couette flow instabilities with application to coating flows
Authors: Joo Yong Lak , Shaqfeh Eric S. G.
Published: July 2002

The effects of inertia on the elastic instabilities in Dean and Taylor–Couette flows are investigated through a linear stability analysis. The critical conditions and the structure of the vortex flow at the onset of these instabilities are presented. The results reveal that the purely elastic Dean flow is destabilized by inertial effects. It is also found that inertia destabilizes elastic Taylor–Couette flow if the rotation of the inner cylinder is the flow driving force, while it stabilizes the flow driven by rotation of the outer cylinder. The mechanism of destabilization or stabilization of these viscoelastic instabilities is investigated through an examination of the disturbance-energy equation. It is shown that Dean flow is destabilized by two separate mechanisms: a purely elastic mechanism discussed previously (i.e., energy production due to the coupling of a perturbation velocity to the polymeric stress gradient in the base state) [see Phys. Fluids A 3, 1691 (1991)] and a purely inertial mechanism discussed by Dean [Proc. R. Soc. London Ser. A 121, 402 (1928)] (i.e., energy production from Reynolds stresses). It is also shown that, when rotation of the inner cylinder drives Taylor–Couette flow, the Reynolds stresses produce energy, and thus are destablizing, while for the flow driven by the rotation of the outer cylinder alone, the Reynolds stresses dissipate energy, thus stabilizing the flow. The elastic forces remain destabilizing in both modes of operation. In a second study, a pressure-driven viscoelastic coating flow over a curved surface is examined. The results demonstrate the existence of a purely elastic stationary instability in the coating flow on a concave wall which is very similar to that which occurs in viscoelastic Dean flow. It is demonstrated that the mechanisms of instability in Dean flow and the coating flow are the same, again through an examination of the disturbance-energy equation.
Interaction of turbulence with a detonation wave
Authors: Jackson T. L. , Hussaini M. Y. , Ribner H. S.
Published: July 2002

This paper addresses a specific reactive-flow configuration, namely, the interaction of a detonation wave with convected homogeneous isotropic weak turbulence (which can be constructed by a Fourier synthesis of small-amplitude vorticity waves). The effect of chemical heat release on the rms fluctuations downstream of the detonation is presented as a function of Mach number. In addition, for the particular case of the von Kármán spectrum, the one-dimensional power spectra of these flow quantities are given.
The n=1 rotational instability in field-reversed configurations
Authors: Tuszewski M. , Barnes G. A. , Baron M. H. , Chrien R. E. , Hugrass W. N. , Klingner P. L. , Ng Chun , Rej D. J. , Taggart D. P. , Siemon R. E. , Wright B. L.
Published: July 2002

Data from an array of external probes provide the first clear evidence for the n=1 rotational instability in field-reversed configurations. The time evolution and the axial structure of this instability are clarified.
Collision and interpenetration of plasmas created by laser-illuminated disks
Authors: Bosch R. A. , Berger R. L. , Failor B. H. , Delamater N. D. , Charatis G. , Kauffman R. L.
Published: July 2002

Supersonic, counterstreaming plasmas were produced by ablating plasma from the inside surfaces of two parallel disks made of aluminum and magnesium, respectively, with a 0.53 μm laser at an intensity of 1014 W/cm2 for 1.3 nsec. Diagnostics included holographic interferometry, a time-integrated x-ray pinhole camera and a gated x-ray crystal spectrograph with imaging slits. The plasmas interpenetrate for the first half of the laser pulse but stagnate once the electron density exceeds 5×1020 cm−3. Spectroscopic measurements suggest a coronal electron temperature of ∼800 eV and an ion temperature of ∼15 keV in the stagnated plasma. The observations are in good agreement with a two ion fluid model of interpenetrating plasmas in which the dominant slowing down process is ion–ion collisions.
Film flow on a rotating disk
Authors: Higgins Brian G.
Published: July 2002

Unsteady liquid film flow on a rotating disk is analyzed by asymptotic methods for low and high Reynolds numbers. The analysis elucidates how a film of uniform thickness thins when the disk is set in steady rotation. In the low Reynolds number analysis two time scales for the thinning film are identified. The long-time-scale analysis ignores the initial acceleration of the fluid layer and hence is singular at the onset of rotation. The singularity is removed by matching the long-time-scale expansion for the transient film thickness with a short-time-scale expansion that accounts for fluid acceleration during spinup. The leading order term in the long-time-scale solution for the transient film thickness is shown to be a lower bound for film thickness for all time. A short-time analysis that accounts for boundary layer growth at the disk surface is also presented for arbitrary Reynolds number. The analysis becomes invalid either when the boundary layer has a thickness comparable to that of the thinning film, or when nonlinear effects become important.
Confinement of translated field-reversed configurations
Authors: Tuszewski M. , Armstrong W. T. , Chrien R. E. , Klingner P. L. , McKenna K. F. , Rej D. J. , Sherwood E. G. , Siemon R. E.
Published: July 2002

The confinement properties of translating field-reversed configurations (FRC) in the FRX-C/T device [Phys. Fluids 29, ▪ ▪ ▪ ▪ (1986)] are analyzed and compared to previous data without translation and to available theory. Translation dynamics do not appear to appreciably modify the FRC confinement. Some empirical scaling laws with respect to various plasma parameters are extracted from the data. These are qualitatively similar to those obtained in the TRX-1 device [Phys. Fluids 28, 888 (1985)] without translation and with a different formation method. Translation with a static gas fill offers new opportunities such as improved particle confinement or refueling of the FRC particle inventory.
Nonlinear dynamics of the m=1 kink-tearing instability in a modified magnetohydrodynamic model
Authors: Wang Xiaogang , Bhattacharjee A.
Published: July 2002

A theory is given for the nonlinear dynamical evolution of the collisionless m=1 kink-tearing instability, including the effects of electron inertia and electron pressure gradient in a generalized Ohm’s law. It is demonstrated that electron pressure gradients can cause near-explosive growth in the nonlinear regime of a thin m=1 island. This near-explosive phase is followed by a rapid decay phase as the island width becomes comparable to the radius of the sawtooth region. An island equation is derived for the entire nonlinear evolution of the instability, extending recent work on the subject [X. Wang and A. Bhattacharjee, Phys. Rev. Lett. 70, 1627 (1993)] to include the effects of both electron inertia and electron pressure gradient. Comparisons are made with experimental data from present-day tokamaks. It is suggested that the present model not only accounts for fast sawtooth crashes, but also provides possible explanations for the problems of sudden onset and incomplete reconnection that have been, heretofore, unexplained features of observations.
Stimulated processes and self-modulation of a short intense laser pulse in the laser wake-field accelerator
Authors: Andreev N. E. , Kirsanov V. I. , Gorbunov L. M.
Published: July 2002

The basic equations for self-consistent pulse evolution taking into account stimulated Raman backward and near-backward scattering are formulated. These equations are used to study the three-dimensional (3-D) axisymmetrical self-consistent laser pulse evolution analytically and numerically. Special attention is paid to the case of the pulse self-modulation. The spectra and intensity of backscattered radiation are obtained in both the strong and weak coupling limits. A simple criterion to ignore the action of stimulated Raman backscattering on the pulse evolution is derived. The possibility of using a backscattered radiation spectrum for diagnostics of both the laser-pulse and generated wake-field evolution is discussed. Triggering of the laser-pulse self-modulation by the relativistic self-focusing and by a second frequency-shifted weak-intensity laser pulse is discussed. Basing on the obtained results, a new configuration of stimulation and maintaining a strong wake-field excitation is proposed. This configuration makes it possible to obtain acceleration of electrons up to giga-electron-volt energies in the field of the excited plasma wave by using the laser technology that is presently available.
X-ray emission from plasmas created by smoothed KrF laser irradiation
Authors: Aglitskiy Y. , Lehecka T. , Deniz A. , Hardgrove J. , Seely J. , Brown C. , Feldman U. , Pawley C. , Gerber K. , Bodner S. , Obenschain S. , Lehmberg R. , McLean E. , Pronko M. , Sethian J. , Stamper J. , Schmitt A. , Sullivan C. , Holland G. , Laming M.
Published: July 2002

The x-ray emission from plasmas created by the Naval Research Laboratory Nike KrF laser [Phys. Plasmas 3, 2098 (1996) ] was characterized using imaging and spectroscopic instruments. The laser wavelength was 1/4 μm, and the beams were smoothed by induced spatial incoherence (ISI). The targets were thin foils of CH, aluminum, titanium, and cobalt and were irradiated by laser energies in the range 100–1500 J. A multilayer mirror microscope operating at an energy of 95 eV recorded images of the plasma with a spatial resolution of 2 μm. The variation of the 95 eV emission across the 800 μm focal spot was 1.3% rms. Using a curved crystal imager operating in the 1–2 keV x-ray region, the density, temperature, and opacity of aluminum plasmas were determined with a spatial resolution of 10 μm perpendicular to the target surface. The spectral line ratios indicated that the aluminum plasmas were relatively dense, cool, and optically thick near the target surface. The absolute radiation flux was determined at 95 eV and in x-ray bandpasses covering the 1–8 keV region. The electron temperature inferred from the slope of the x-ray flux versus energy data in the 5–8 keV region was 900 eV for an incident laser energy of 200 J and an intensity of ≊1013 W/cm2.
Magnetic reconnection and the topology of interacting twisted flux tubes
Authors: Lau Y.-T. , Finn J. M.
Published: July 2002

The self-consistent evolution of a pair of initially straight and either parallel or antiparallel magnetic flux tubes with prescribed boundary twist is studied using fully compressible three-dimensional (3-D) resistive magnetohydrodynamics (MHD). 3-D visualization techniques specially designed for divergence free vector fields are employed to investigate topological changes in the field lines and current lines associated with 3-D reconnection in the system. Four cases are studied, corresponding to either parallel or antiparallel initial magnetic fields and to the same or opposite sign of footpoint twist. It is found that in the case with antiparallel field and opposite twist, so that the currents are parallel, the evolution proceeds in two phases. In the first phase, a series of topological changes involving magnetic nulls (where B=0) create an X-type closed field line. In the second phase, the X-type line serves as the separator for reconnection, allowing field lines from the two tubes to merge and form loops. The magnetic field lines exhibit spatial chaos and chaotic scattering. The observed reconnection involves the X-type closed field line with evident current sheets. Later in time, the X-type line changes to an O-type closed field line, surrounded by a ring of toroidal flux surfaces. Reconnection continues until there emerges a final steady state having two reconnected loops and a toroidal ring of flux surfaces in between. The torus of magnetic surfaces has zero current in steady state because it is not connected by field lines to the twist imposed at the boundary. It is discussed how it is possible that such a region of zero current density can exist. The other three cases involve breaking of the ideal MHD flux constraint and changes in topology, but without localized current sheets, i.e., without reconnection. Implications for coronal loop interaction are discussed.
A study on ion energy distribution functions and plasma potentials in helicon wave plasmas
Authors: Kim Jung-Hyung , Chang Hong-Young
Published: July 2002

The characteristics of high-density helicon wave plasma generated in a quartz tube of 10 cm in diameter have been studied. The optimum conditions for efficiently exciting helicon wave plasma have been investigated. It is also observed that plasma fully ionized by helicon waves has ion pumping effect. Whether the discharge is inductive or capacitive, the plasma potential is important, since it determines the energy of the ion incident on the wall. It is investigated theoretically and experimentally that the fluctuation of the plasma potentials influences the ion energy distribution functions monitored by the retarding field energy analyzer. It is also observed that the plasma potential fluctuates with peak-to-peak voltage Vp–p in the low-density mode. The radio-frequency modulation to the plasma potential is weaker in the helicon mode than in the low mode.
Nonlocal particle loss effects on the electron kinetics in a direct current helium diffusion-controlled positive column
Authors: Sirghi L. , Ohe K. , Kimura T.
Published: July 2002

The electron energy distribution function (EEDF) in a positive column of low-pressure and discharge current is determined not only by the local collision processes and the axial electric field action, but also by the transport phenomenon, the radial ambipolar diffusion due to the gradient of plasma density. Thus, to completely determine the EEDF, the Boltzmann equation including radial inhomogeneity terms has to be solved. The present work proposes a simplified method to account for the radial inhomogeneity, when the electron kinetics in the central part of the positive column can be reduced to be one energy-dimensional. The radial diffusion of electrons is taken into account via a wall loss term. A greatly simplified kinetic equation is obtained and its numerical solutions agree well with the EEDF determined from Langmuir probe measurements in a helium dc discharge positive column. Also, a comparison of the present method with local and nonlocal approach theories is made. A discrepancy is observed, especially at high energies, where either local or nonlocal approach theories predict too-large values of EEDF.
Particle and heat transport in a low-frequency inductively coupled plasma
Authors: Tuszewski M.
Published: July 2002

The radial profiles of induced radio frequency magnetic fields, of plasma density, and of electron temperatures are measured in low-frequency inductively coupled plasmas. Oxygen and argon data are compared to one-dimensional fluid calculations that include time-averaged induced magnetic fields. These comparisons suggest that the induced magnetic fields broaden the profiles of input radio frequency power, somewhat flatten the density profiles, and produce an electron temperature increase in the edge region of oxygen discharges.
Preheating study by reflectivity measurements in laser-driven shocks
Authors: Benuzzi A. , Koenig M. , Faral B. , Krishnan J. , Pisani F. , Batani D. , Bossi S. , Beretta D. , Hall T. , Ellwi S. , Hüller S. , Honrubia J. , Grandjouan N.
Published: July 2002

A study on preheating effects in laser-driven shock waves is presented. Two different diagnostics were used: the color temperature measurement deduced by recording the target rear side emissivity in two spectral bands, and the rear surface reflectivity measurement by using a probe beam. In order to test the response of the two diagnostics to the preheating, three types of targets characterized by different radiative properties were used. The greater sensitivity of the second diagnostic compared with the first was demonstrated. A model which calculates the reflectivity using a one-dimensional hydrodynamic code data was developed. In this model, the wave propagation equations in the expanding plasma using an appropriate model for the electron–ion collision frequency applicable to the cold solid-hot plasma transition were solved. The comparison between the calculated and measured reflectivities allows us to estimate the preheating process.
The structure of an expanding hydrogen arcjet plasma
Authors: Hargus William A. , Cappelli Mark A.
Published: July 2002

The structure of an expanding low-power hydrogen arcjet plasma is investigated through probe-based measurements of impact pressure and mass flux. Comparisons are made to direct simulation Monte Carlo models (DSMC) of the nonignited flow (cold flow) and continuum magnetohydrodynamic (MHD) models of the arc-heated flow. While general agreement with previous spectroscopic data and DSMC calculations are obtained, the cold flow impact pressures are shown to exhibit features that may be due to probe–flow interactions and rarefied gas effects. The ability to identify shocks in the plume, predicted by the DSMC model and previously observed by Raman spectroscopy, shows that the cold flow probes were capable of resolving major flow features. The impact pressure measurements of arc-heated flow also agree quite well with the results calculated from the MHD model, and it is demonstrated that integrated thrust densities derived from impact pressure measurements are in agreement with direct measurements of thrust using an inverted pendulum thrust stand. Cold flow mass flux measurements appear to agree well with the DSMC model in the core portion of the flow. Using velocity and temperature data obtained from previous laser-induced fluorescence measurements; density, pressure, and thrust profiles were derived from the measured mass flux profiles. The density profile indicates that the dissociation fraction at the exit plane was within the uncertainty of the only direct spectroscopic molecular hydrogen density measurements available in the literature.
The orbital-motion-limited regime of cylindrical Langmuir probes
Authors: Sanmartı́n J. R. , Estes R. D.
Published: July 2002

An asymptotic analysis of electron collection at high bias Φp serves to determine the domain of validity of the orbital-motion-limited (OML) regime of cylindrical Langmuir probes, which is basic for the workings of conductive bare tethers. The radius of a wire collecting OML current in an unmagnetized plasma at rest cannot exceed a value, Rmax, which is found to exhibit a minimum as a function of Φp; at Φp values of interest, Rmax is already increasing and is larger than the electron Debye length λDe. The breakdown of the regime relates to conditions far from the probe, at electron energies comparable to the ion thermal energy, kTi; Rmax is found to increase with Ti. It is also found that (1) the maximum width of a thin tape, if used instead of a wire, is 4Rmax; (2) the electron thermal gyroradius must be larger than both R and λDe for magnetic effects to be negligible; and (3) conditions applying to the tether case are such that trapped-orbit effects are negligible.
Observation and poling of ferroelectric domains in KTiOPO4
Authors: Bierlein John D. , Ahmed Fakhruddin
Published: July 2002

Ferroelectric domains have been observed and characterized in hydrothermally grown KTiOPO4 and a poling process developed. The electro-optic, pyroelectric, and second-harmonic generation characteristics of multidomain crystals before and after poling are described and compared to single domain crystals.
Impact ionization in AlxGa1−xAs for x=0.1–0.4
Authors: Robbins V. M. , Smith S. C. , Stillman G. E.
Published: July 2002

The impact ionization coefficients in AlxGa1−xAs have been experimentally determined for compositions x≤0.4. These data are necessary for accurate device design and modeling. Both α, the electron ionization coefficient, and β, the hole ionization coefficient, decrease with increasing x, and there is no appreciable change in the ratio α/β over the range of compositions and electric fields studied.
Symmetry study of the Mn-Si-Al octagonal quasicrystal by convergent beam electron diffraction
Authors: Wang N. , Fung K. K. , Kuo K. H.
Published: July 2002

The addition of a few percent of Al to the Mn4Si alloy improved the perfection of the octagonal quasicrystal and made it possible for us to determine its point group symmetry as being 8/m or 8/mmm by convergent beam electron diffraction.
Temporal binding via cortical coincidence detection of specific and nonspecific thalamocortical inputs: A voltage-dependent dye-imaging study in mouse brain slices
Authors: Llinás Rodolfo R. , Leznik Elena , Urbano Francisco J.
Published: July 2002

Voltage-sensitive dye imaging of mouse thalamocortical slices demonstrated that electrical stimulation of the centrolateral intralaminar thalamic nucleus (CL) resulted in the specific activation of thalamic reticular nucleus, striatum/putamen, and cortical layers 5, 6, and 1. By contrast, ventrobasal (VB) thalamic stimulation, while activating the reticular and basal ganglia nuclei, also activated directly layers 4 and deep 5 of the cortex. Conjoined stimulation of the VB and CL nuclei resulted in supralinear summation of the two inputs at cortical output layer 5, demonstrating coincidence detection along the apical dendrites. This supralinear summation was also noticed at gamma band stimulus frequency (≈40 Hz). Direct stimulation of cortical layer 1, after a radial section of the cortex that spared only that layer, was shown to sum supralinearly with the cortical activation triggered by VB stimulation, providing a second demonstration for coincidence detection. Coincidence detection by coactivation of the specific (VB) and nonspecific (CL) thalamic nuclei has been proposed as the basis for the temporal conjunction that supports cognitive binding in the brain.
Plasminogen activator inhibitor-1 is a major stress-regulated gene: Implications for stress-induced thrombosis in aged individuals
Authors: Yamamoto Koji , Takeshita Kyosuke , Shimokawa Takayoshi , Yi Hong , Isobe Ken-ichi , Loskutoff David J. , Saito Hidehiko
Published: July 2002

Plasminogen activator inhibitor-1 (PAI-1) is one of the primary inhibitors of the fibrinolytic system and has been implicated in a variety of thrombotic disorders. In this report, stress-induced changes in murine PAI-1 gene expression were investigated to study the role of this inhibitor in the development of stress-induced hypercoagulability. Restraint stress led to a dramatic induction of plasma PAI-1 antigen and of tissue PAI-1 mRNA with maximum induction in adipose tissues.
In situ
hybridization analysis of the stressed mice revealed that strong signals for PAI-1 mRNA were localized to hepatocytes, renal tubular epithelial cells, adrenomedullar chromaffin cells, neural cells in the paraaortic sympathetic ganglion, vascular smooth muscle cells, and adipocytes, but not to endothelial cells. These observations indicate that the stress induces the PAI-1 gene expression in a tissue-specific and cell type-specific manner. The induction of PAI-1 mRNA by restraint stress was greater than that observed for heat shock protein, a typical stress protein, suggesting that PAI-1 is one of the most highly induced stress proteins. Importantly, the magnitude of induction of PAI-1 mRNA by stress increased markedly with age, and this increase in PAI-1 correlated with tissue thrombosis in the older stressed mice. Moreover, much less tissue thrombosis was induced by restraint stress in young and aged PAI-1-deficient mice compared with age-matched wild-type mice. These results suggest that the large induction of PAI-1 by stress increases the risk for thrombosis in the older populations, and that the adipose tissue may be involved.
Overhauser enhanced magnetic resonance imaging for tumor oximetry: Coregistration of tumor anatomy and tissue oxygen concentration
Authors: Krishna Murali C. , English Sean , Yamada Kenichi , Yoo John , Murugesan Ramachandran , Devasahayam Nallathamby , Cook John A. , Golman Klaes , Ardenkjaer-Larsen Jan Henrik , Subramanian Sankaran , Mitchell James B.
Published: July 2002

An efficient noninvasive method for
in vivo
imaging of tumor oxygenation by using a low-field magnetic resonance scanner and a paramagnetic contrast agent is described. The methodology is based on Overhauser enhanced magnetic resonance imaging (OMRI), a functional imaging technique. OMRI experiments were performed on tumor-bearing mice (squamous cell carcinoma) by i.v. administration of the contrast agent Oxo63 (a highly derivatized triarylmethyl radical) at nontoxic doses in the range of 2–7 mmol/kg either as a bolus or as a continuous infusion. Spatially resolved pO
2
(oxygen concentration) images from OMRI experiments of tumor-bearing mice exhibited heterogeneous oxygenation profiles and revealed regions of hypoxia in tumors (<10 mmHg; 1 mmHg = 133 Pa). Oxygenation of tumors was enhanced on carbogen (95% O
2
/5% CO
2
) inhalation. The pO
2
measurements from OMRI were found to be in agreement with those obtained by independent polarographic measurements using a pO
2
Eppendorf electrode. This work illustrates that anatomically coregistered pO
2
maps of tumors can be readily obtained by combining the good anatomical resolution of water proton-based MRI, and the superior pO
2
sensitivity of EPR. OMRI affords the opportunity to perform noninvasive and repeated pO
2
measurements of the same animal with useful spatial (≈1 mm) and temporal (2 min) resolution, making this method a powerful imaging modality for small animal research to understand tumor physiology and potentially for human applications.
Toward optimized carbohydrate-based anticancer vaccines: Epitope clustering, carrier structure, and adjuvant all influence antibody responses to Lewisyconjugates in mice
Authors: Kudryashov Valery , Glunz Peter W. , Williams Lawrence J. , Hintermann Samuel , Danishefsky Samuel J. , Lloyd Kenneth O.
Published: July 2002

The feasibility of using carbohydrate-based vaccines for the immunotherapy of cancer is being actively explored at the present time. Although a number of clinical trials have already been conducted with glycoconjugate vaccines, the optimal design and composition of the vaccines has yet to be determined. Among the candidate antigens being examined is Lewisy(Ley), a blood group-related antigen that is overexpressed on the majority of human carcinomas. Using Leyas a model for specificity, we have examined the role of epitope clustering, carrier structure, and adjuvant on the immunogenicity of Leyconjugates in mice. A glycolipopeptide containing a cluster of three contiguous Ley-serine epitopes and the Pam3Cys immunostimulating moiety was found to be superior to a similar construct containing only one Ley-serine epitope in eliciting antitumor cell antibodies. Because only IgM antibodies were produced by this vaccine, the effect on immunogenicity of coupling the glycopeptide to keyhole limpet hemocyanin was examined; although both IgM and IgG antibodies were formed, the antibodies reacted only with the immunizing structure. Reexamination of the clustered Ley-serine Pam3Cys conjugate with the adjuvant QS-21 resulted in the identification of both IgG and IgM antibodies reacting with tumor cells, thus demonstrating the feasibility of an entirely synthetic carbohydrate-based anticancer vaccine in an animal model.
Identification of a protein essential for a major pathway used by human cells to avoid UV- induced DNA damage
Authors: Li Ziqiang , Xiao Wei , McCormick J. Justin , Maher Veronica M.
Published: July 2002

When DNA replication stalls at a fork-blocking lesion, cells use damage tolerance pathways to continue replication. One pathway, “translesion synthesis,” involves specialized DNA polymerases that can use damaged DNA as a template. Translesion synthesis can result in mutations (i.e., can be error-prone), but it can also be error-free. An alternative pathway has been hypothesized (sometimes called “damage avoidance”), by which cells make temporary use of an undamaged copy of the blocked sequence as a template, i.e., the newly synthesized daughter strand of the sister duplex or the allelic copy. This pathway is error-free. Evidence of the use of the daughter strand of the sister duplex as a template in intact mammalian cells has not been available heretofore. To determine whether hMms2, a ubiquitin-conjugating enzyme-like protein, plays a critical role in such damage avoidance, a human fibroblast cell strain in which both error-prone translesion synthesis and error-free damage avoidance can be detected and quantified simultaneously, and several derivative strains in which expression of hMms2 protein had been eliminated or greatly decreased, were compared for their ability to avoid translesion synthesis past UV
254nm
-induced DNA photoproducts. Loss of hMms2 protein eliminated the ability of the latter strains to use an allelic copy of a target gene for damage avoidance, i.e., to produce a wild-type gene from two nonfunctional allelic copies of that gene. Molecular analysis of the wild-type gene showed that this process involves gene conversion unassociated with crossing-over. That the loss of hMms2 also eliminated use of the daughter strand of the sister duplex as a template for damage avoidance could be inferred from the fact that the frequency of mutations induced by UV in the single copy
HPRT
gene of the derivative strains was significantly higher than that observed in the parental strain. These data indicate that
hMMS2
is essential for human cells to carry out damage avoidance by using either type of homolog, and that damage avoidance and translesion synthesis are alternative pathways for tolerating fork-blocking photoproducts.
Functional asymmetry of photosystem II D1 and D2 peripheral chlorophyll mutants of
Chlamydomonas reinhardtii
Authors: Wang Jun , Gosztola David , Ruffle Stuart V. , Hemann Craig , Seibert Michael , Wasielewski Michael R. , Hille Russ , Gustafson Terry L. , Sayre Richard T.
Published: July 2002

The peripheral accessory chlorophylls (Chls) of the photosystem II (PSII) reaction center (RC) are coordinated by a pair of symmetry-related histidine residues (D1-H118 and D2-H117). These Chls participate in energy transfer from the proximal antennae complexes (CP43 and CP47) to the RC core chromophores. In addition, one or both of the peripheral Chls are redox-active and participate in a low-quantum-yield electron transfer cycle around PSII. We demonstrate that conservative mutations of the D2-H117 residue result in decreased Chl fluorescence quenching efficiency attributed to reduced accumulation of the peripheral accessory Chl cation, Chl

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{Z}^{+}}}\end{equation*}\end{document}

. In contrast, identical symmetry-related mutations at residue D1-H118 had no effect on Chl fluorescence yield or quenching kinetics. Mutagenesis of the D2-H117 residue also altered the line width of the Chl

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{Z}^{+}}}\end{equation*}\end{document}

EPR signal, but the line shape of the D1-H118Q mutant remained unchanged. The D1-H118 and D2-H117 mutations also altered energy transfer properties in PSII RCs. Unlike wild type or the D1-H118Q mutant, D2-H117N RCs exhibited a reduced CD doublet in the red region of Chl absorbance band, indicative of reduced energetic coupling between P680 and the peripheral accessory Chl. In addition, transient absorption measurements of D2-H117N RCs, excited on the blue side of the Chl absorbance band, exhibited a (≈400 fs) pheophytin Q
X
band bleach lifetime component not seen in wild-type or D1-H118Q RCs. The origin of this component may be related to delayed fast-energy equilibration of the excited state between the core pigments of this mutant.
Adenoviral vectors can impair adrenocortical steroidogenesis: Clinical implications for natural infections and gene therapy
Authors: Alesci Salvatore , Ramsey Walter J. , Bornstein Stefan R. , Chrousos George P. , Hornsby Peter J. , Benvenga Salvatore , Trimarchi Francesco , Ehrhart-Bornstein Monika
Published: July 2002

Recombinant adenoviral vectors are effective in transferring foreign genes to a variety of cells and tissue types, both
in vitro
and
in vivo
. However, during the gene transfer, they may alter the principal function and local environment of transfected cells. Increasing evidence exists for a selective adrenotropism of adenovirus during infections and gene transfer. Therefore, using bovine adrenocortical cells in primary culture, we analyzed the influence of different adenoviral deletion mutants on cell morphology and physiology. Transfection of cells with an E1/E3-deleted adenoviral vector, engineered to express a modified form of the
Aequorea victoria
green fluorescent protein, was highly efficient, as documented by fluorescent microscopy. Ultrastructural analysis, however, demonstrated nuclear fragmentation and mitochondrial alterations in addition to intranuclear viral particles. Basal secretion of 17-OH-progesterone, 11-deoxycortisol, and cortisol was significantly increased by E1/E3-deleted vectors; yet, the corticotropin-stimulated release of these steroids was decreased. Interestingly, neither purified viral capsids nor E3/E4-deleted adenoviral mutants altered basal and stimulated steroidogenesis of adrenocortical cells. An intact adrenal response is crucial for adaptation to stress and survival. Therefore, the implications of our findings need to be considered in patients with adenoviral infections and those undergoing clinical studies using adenoviral gene transfer. At the same time, the high level of transfection in adrenocortical cells might make appropriately modified adenoviral vectors suitable for gene therapy of adrenocortical carcinomas with poor prognosis.
A causal link between respiration and senescence in
Podospora anserina
Authors: Dufour Eric , Boulay Joceline , Rincheval Vincent , Sainsard-Chanet Annie
Published: July 2002

Senescence, a progressive degenerative process leading to age-related increase in mortality, is found in most eukaryotes. However, the molecular events underlying aging remain largely unknown. Understanding how longevity is regulated is a fundamental problem. Here we demonstrate that the respiratory function is a key factor that contributes to shortening lifespan of the filamentous fungus
Podospora anserina
. In this organism, senescence is systematically associated with mitochondrial DNA instabilities. We show that inactivation of the nuclear
COX5
gene encoding subunit V of the cytochrome
c
oxidase complex leads to the exclusive use of the alternative respiratory pathway and to a decrease in production of reactive oxygen species. This inactivation results in a striking increase of longevity associated with stabilization of the mitochondrial chromosome. Moreover, accumulation of several senescence-specific mitochondrial DNA molecules is prevented in this nuclear mutant. These findings provide direct evidence of a causal link between mitochondrial metabolism and longevity in
Podospora anserina
.
A protein subunit of human RNase P, Rpp14, and its interacting partner, OIP2, have 3′→5′ exoribonuclease activity
Authors: Jiang Taijiao , Altman Sidney
Published: July 2002

The processing of precursor tRNAs at their 5′ and 3′ termini is a fundamental event in the biosynthesis of tRNA. RNase P is generally responsible for endonucleolytic removal of a leader sequence of precursor tRNA to generate the mature 5′ terminus. However, much less is known about the RNase P counterparts or other proteins that are active at the tRNA 3′ terminus. Here we show that one of the human RNase P subunits, Rpp14, together with one of its interacting protein partners, OIP2, is a 3′→5′ exoribonuclease with a phosphorolytic activity that processes the 3′ terminus of precursor tRNA. Immunoprecipitates of a crude human RNase P complex can process both ends of precursor tRNA by hydrolysis, but purified RNase P has no exonuclease activity. Rpp14 and OIP2 may be part of an exosome activity.
Hermaphroditic, demasculinized frogs after exposure to the herbicide atrazine at low ecologically relevant doses
Authors: Hayes Tyrone B. , Collins Atif , Lee Melissa , Mendoza Magdelena , Noriega Nigel , Stuart A. Ali , Vonk Aaron
Published: July 2002

Atrazine is the most commonly used herbicide in the U.S. and probably the world. It can be present at several parts per million in agricultural runoff and can reach 40 parts per billion (ppb) in precipitation. We examined the effects of atrazine on sexual development in African clawed frogs (
Xenopus
laevis
). Larvae were exposed to atrazine (0.01–200 ppb) by immersion throughout larval development, and we examined gonadal histology and laryngeal size at metamorphosis. Atrazine (≥0.1 ppb) induced hermaphroditism and demasculinized the larynges of exposed males (≥1.0 ppb). In addition, we examined plasma testosterone levels in sexually mature males. Male
X. laevis
suffered a 10-fold decrease in testosterone levels when exposed to 25 ppb atrazine. We hypothesize that atrazine induces aromatase and promotes the conversion of testosterone to estrogen. This disruption in steroidogenesis likely explains the demasculinization of the male larynx and the production of hermaphrodites. The effective levels reported in the current study are realistic exposures that suggest that other amphibian species exposed to atrazine in the wild could be at risk of impaired sexual development. This widespread compound and other environmental endocrine disruptors may be a factor in global amphibian declines.