bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of direct and resonant scattering for H2+CN(A 2Π): Collisional energy transfer versus predissociation of CN(A)–H2 complexes
Authors: Chen Yaling , Heaven Michael C.
Published: July 2002

Electronic predissociation of CN(A 2Π)–H2 was examined by characterizing action spectra and product state distributions. Both spin-orbit [CN(A 2Π1/2)–H2→CN(A 2Π3/2)+H2] and internal conversion [CN(A 2Π3/2)–H2→CN(X 2Σ)+H2] decay channels were observed. For comparison with the predissociation data, CN(A)+H2 collisional energy transfer was examined at temperatures near 10 K. The product state distributions resulting from collisions showed symmetry preferences that were not evident in the distributions resulting from predissociation. It is argued that the lack of symmetry preferences in the predissociation dynamics is indicative of a resonant scattering process. Qualitative differences in the predissociation and collisional transfer dynamics can be explained by considering the symmetry properties of the intermolecular potential energy surfaces, and the regions of these surfaces that are sampled by each type of event. Most predissociation data was recorded for ortho CN(A)–H2(J=1) complexes. A new spectral feature of the complex, tentatively assigned to para CN(A)–H2(J=0), was observed in this study. The predissociation dynamics of this state were significantly different from those of the ortho complex.
CN spectroscopy and physico–chemistry in the boundary layer of a C/SiC tile in a low pressure nitrogen/carbon dioxide plasma flow
Authors: Boubert P. , Vervisch P.
Published: July 2002

The boundary layer of a C/SiC tile in a low pressure nitrogen/carbon dioxide plasma flow is investigated by numerous measurement techniques. CN emission is especially studied through its red and violet systems. Radial and axial profiles of CN relative density are obtained and absolute values are estimated by calibration, thanks to a tungsten lamp. Rotational and vibrational temperatures are determined and compared with wall, heavy particle, and electron temperatures. High vibronic bands should be taken into account in order to determine accurate vibrational temperatures. Δv=0 sequence of the violet system is examined in detail and differences between calculations made using existing sets of spectroscopic constants and experimental spectra are pointed out. Inadequacy of very accurate Fourier transform spectroscopy (FTS) sets of constants in high temperature calculations is underlined. CN and NO density increases in the boundary layer are explained through a dissociative adsorption of N2(v) or N2(A 3Σ+). Such a process implies a significant action of nitrogen in surface erosion. Excitation of CN and NO by electrons is rejected and an excitation transfer with vibrationally excited and metastable states of N2 and CO is proposed.
Effects of the polarizability and water density constraint on the structure of water near charged surfaces: Molecular dynamics simulations
Authors: Yeh In-Chul , Berkowitz Max L.
Published: July 2002

To understand how the inclusion of explicit polarizability into water interaction potential changes the structure of water next to a charged metallic surface we compared the results from the simulations using polarizable point charge model and simple point charge model. To study the effect of density constraints we also performed simulations of water next to hydrophobic walls and metal walls. In these simulations the water density was not predetermined, but regulated itself during the run.
Rotationally resolved spectroscopy of a librational fundamental band of hydrogen fluoride tetramer
Authors: Blake Thomas A. , Sharpe Steven W. , Xantheas Sotiris S.
Published: July 2002

The rotationally resolved spectrum of a fundamental band of hydrogen fluoride tetramer has been recorded using a pulsed slit-jet, diode laser spectrometer. The band has a parallel rotational structure and is assigned as the H–F out-of-plane libration fundamental with Au symmetry. Ninety-five ground state combination differences were fit to a symmetric top Hamiltonian to give the following ground state rotational constants: B″=0.132 081(7) cm−1, DJ″=7.1(7)×10−7 cm−1, DJK″=−9(2)×10−7 cm−1, HJJJ″=6(2)×10−10 cm−1, HJJK″=9(7)×10−10 cm−1, HJKK″=−1.3(8)×10−10 cm−1. A total of 190 transitions were fit to determine the upper state spectroscopic constants: v4=714.7849(1) cm−1, B′=0.129 634(5) cm−1, Δ(C−B)=0.001 344 cm−1, DJ′=6.4(5)×10−7 cm−1, DJK′=−4.5(6)×10−7 cm−1, ΔDK=2.92(8)×10−6 cm−1, HJJJ′=3(1)×10−10 cm−1, HJKK′=−1.55(6)×10−8 cm−1; ΔHKKK=−4.65(6)×10−8 cm−1. Furthermore, a perpendicular band centered at 752.7 cm−1 was observed. The band has a rotational line spacing that gives an approximate B″ value of 0.132 cm−1; it has been assigned as the Eu symmetry, H–F in-plane libration fundamental of the HF tetramer. Finally, a parallel band was observed at 741.0 cm−1 with B″=0.076 cm−1 and has been assigned as the A″ symmetry, H–F out-of-plane libration fundamental of the HF pentamer. Structural parameters and harmonic vibrational frequencies are estimated from first-principles, correlated MP2 and CCSD(T) calculations. These are the largest calculations performed to date for this system with respect to both orbital basis set and level of electron correlation. The CCSD(T) harmonic frequencies are, in particular, the first reported for the tetramer at this level of theory. Based on our results, we suggest “best estimates” of RFF=2.51 Å, rHF=0.947 Å, and θHFF=9.7° for the structural parameters and a range for De of 27.4 to 28.1 kcal/mol (D0=19.7 to 20.4 kcal/mol) for the parameters and for the energy of the tetramer dissociating into four monomers.
Symmetries of two-mode squeezed states
Authors: Han D. , Kim Y. S. , Noz Marilyn E. , Yeh Leehwa
Published: July 2002

It is known that the symmetry of two-mode squeezed states is governed by the group Sp(4) which is locally isomorphic to the O(3,2) de Sitter group. It is shown that this complicated ten-parameter group can be regarded as a product of two three-parameter Sp(2) groups. It is shown also that two coupled harmonic oscillators serve as a physical basis for the symmetry decomposition. It is shown further that the concept of entropy is needed when one of the two modes is not observed. The entropy is zero when the system is uncoupled. The system reaches thermal equilibrium when the entropy becomes maximal.
Relativistic hydrogen atom: Wave equation in Whittaker form
Authors: Cohen Jeffrey M. , Kuharetz Boris
Published: July 2002

For the hydrogen atom, a single relativistic wave equation in Whittaker form was derived which yields the energy level spectrum, the same as that given by the Dirac (coupled system of) equations. Unlike the usual treatments of the Dirac equation, the solutions presented here do not diverge at infinity, in fact they approach zero. Also, for Z<119, it is shown that the usual energy levels follow; however, for 119<Z<137 the spectrum depends on a free parameter.
(2+1)-dimensional models of relativistic particles with curvature and torsion
Authors: Kuznetsov Yu. A. , Plyushchay M. S.
Published: July 2002

(2+1)-dimensional models of relativistic particles with the action depending arbitrarily on the world-trajectory curvature and torsion are investigated. A special class of models, described by the action with a maximal symmetry allowing only spin internal degrees of freedom, is singled out. A classical analysis of the systems from this special class is carried out, and the quantization procedure for them is described. Within this class, tachyon-free models are pointed out and the systems having classical and quantum spectra with only massive spin states (whose number in the quantum case may be both finite and infinite) are considered as concrete examples of such models.
The stress tensor conformal anomaly and analytic regularizations
Authors: Svaiter B. F. , Svaiter N. F.
Published: July 2002

In this paper the stress tensor conformal anomaly of a conformally coupled scalar field defined in a d dimensional Riemannian manifold is derived with or without a boundary, using the zeta function analytic regularization method. It is found that the scalar anomaly is independent of any massless limit. In an odd dimensional manifold with a boundary, the anomaly is not zero owing the boundary condition dependent term. After briefly discussing the cutoff method, it is proven that the zeta function method is equivalent to the cutoff method with a subtraction of the polar terms.
Random walk and the heat equation on superspace and anyspace
Authors: Majid S. , Rodríguez-Plaza M. J.
Published: July 2002

Random walks are used to study diffusion on anyspace. Anyspace is characterized by coordinate ξ with ξN=0 and statistics ξξ′=e2πi/Nξ′ξ between independent copies. Anyonic integration and anyonic Dirac δ functions are introduced, and reduced to familiar results for supersymmetry when N=2. These ingredients are then used to formulate and solve the resulting anyonic diffusion equation.
Coherent states over Grassmann manifolds and the WKB exactness in path integral
Authors: Fujii Kazuyuki , Kashiwa Taro , Sakoda Seiji
Published: July 2002

U(N) coherent states over Grassmann manifold, GN,n≂U(N)/(U(n)×U(N−n)), are formulated to be able to argue the WKB exactness in the path integral representation of a character formula. The phenomena is the so-called localization of Duistermaat–Heckman. The exponent in the path integral formula is proportional to an integer k labeling the U(N) representation. Thus, when k→∞ a usual semiclassical approximation, by regarding k∼1/ℏ, can be performed to yield a desired conclusion. The mechanism of the localization is uncovered by the help of the (generalized) Schwinger boson technique. The discussion on the Feynman kernel is also presented.
On integrable potential perturbations of the billiard system within an ellipsoid
Authors: Dragović V. , Jovanović B.
Published: July 2002

Generalizing Kozlov’s method of integrable potential perturbations for systems with three degrees of freedom, new integrable billiard systems within ellipsoid in R3 are constructed. Two countable families of the basic solutions in the class of the Laurent polynomials are obtained. Explicit formulas are given.
Time-dependent plane wave and multipole expansions of the electromagnetic field
Authors: Marengo Edwin A. , Devaney Anthony J.
Published: July 2002

A new and conceptually simple derivation is presented of the time-dependent multipole expansion of the electromagnetic field radiated by a time-varying, localized, volume charge-current distribution. The analysis is based on a new time-dependent plane wave representation of the electromagnetic field (i.e., an “angular spectrum expansion” in the time domain), also derived in the paper. Expressions are given for the time-dependent plane wave spectra in terms of a fourfold Radon transform representation of the transverse current distribution. Two alternative expressions for the time-dependent multipole moments are derived; the first gives them in terms of the spectral amplitude vectors of the corresponding time-dependent plane wave representation while the second gives them in terms of a weighted radial-temporal average of the current distribution. Thus the analysis also sheds light on the relationship between the time-dependent plane wave and multipole expansions in their common domains of validity.
Continuous wavelet transforms with Abelian dilation groups
Authors: Führ Hartmut
Published: July 2002

This paper is devoted to the construction of wavelet (or coherent state) systems arising from the action of certain semidirect products G=Rk⋊H on L2(Rk). For this purpose previous results which guarantee the existence of inversion formulas are applied to the special case where H is Abelian. The questions of systematic construction and conjugacy of such groups are completely resolved by setting up a correspondence to unit groups of commutative associative algebras. As an application the numbers of conjugacy classes of possible Abelian groups are computed for k=2,3,4. For k⩾7, there are uncountably many conjugacy classes. We then compute the admissibility conditions belonging to Abelian groups. The final section contains a characterization of Abelian matrix groups acting ergodically on some subset of Rk. This result ensures that the approach via associative algebras yields all possible groups.
Almost-complex and almost-product Einstein manifolds from a variational principle
Authors: Borowiec Andrzej , Ferraris Marco , Francaviglia Mauro , Volovich Igor
Published: July 2002

It is shown that the first-order (Palatini) variational principle for a generic nonlinear metric-affine Lagrangian depending on the (symmetrized) Ricci square invariant leads to an almost-product Einstein structure or to an almost-complex anti-Hermitian Einstein structure on a manifold. It is proved that a real anti-Hermitian metric on a complex manifold satisfies the Kähler condition on the same manifold treated as a real manifold if and only if the metric is the real part of a holomorphic metric. A characterization of anti-Kähler Einstein manifolds and almost-product Einstein manifolds is obtained. Examples of such manifolds are considered.
Perturbation growth in shear flow exhibits universality
Authors: Farrell Brian F. , Ioannou Petros J.
Published: July 2002

Disturbance structures that achieve maximum growth over a specified interval of time have recently been obtained for unbounded constant shear flow making use of closed-form solutions. Optimal perturbations have also been obtained for the canonical bounded shear flows, the Couette, and plane Poiseuille flows, using numerical solution of the linearized Navier–Stokes equations. In this note it is shown that these optimal perturbations have similar spectra and structure indicating an underlying universality of shear flow dynamics that is not revealed by traditional methods based on modal analysis.
Degrees of locality of energy transfer in the inertial range
Authors: Zhou Ye
Published: July 2002

Recent interpretations of direct numerical simulation (DNS) measurements have led some authors to suggest that energy is largely transferred downscale locally, supporting a basic concept of the Kolmogorov phenomenology that leads to the universal inertial subrange. However, these authors conclude that the local energy transfer results from nonlocal triad interactions. This claim brings into question the validity of the assumption of the statistical independence of the large- and small-scale motions in the Kolmogorov universal theory of turbulence. In this Letter, the measured raw transfer interactions have been summed in a way that directly indicates the scale disparity (s) of contributions to the net energy flux across the spectrum. It is found that the dependence upon s closely follows the s−4/3 form predicted by classical arguments. As a result, it is concluded that DNS measurements, in fact, lend support to the classical Kolmogorov phenomenology of local interactions and local transfer in an inertial range.
Structure of the velocity field associated with the spanwise vorticity in the wall region of a turbulent boundary layer
Authors: Rajagopalan S. , Antonia R. A.
Published: July 2002

Measurements of the spanwise vorticity fluctuation ωz were made in a low Reynolds number turbulent boundary layer with a four-hot-wire probe. Measurements of the longitudinal velocity fluctuation up were also made with a single hot wire located at a fixed position near the edge of the viscous sublayer. The simultaneously sampled data are used to gain some insight into the vortical structure of the flow in the wall region. The large negative correlation coefficient between up and ωz near the wall may be due partly to a sweeplike motion associated with negative ωz and partly to the association between low-speed fluid and positive ωz. Spanwise vorticity fluctuations near the wall are, in general, negatively correlated with those in the outer part of the buffer region. This result is consistent with the effect of the wall on the velocity field induced by relatively strong vortices of the same sign as the mean shear. It is also consistent with the arrival in the wall region of eddies which rotate in the opposite direction to the mean shear. Conditional averages of the velocity field associated with both positive and negative ωz detections are compared with those obtained from detections of various different features of the organized motion in the wall region. These averages support the conclusions inferred from the correlation between up and ωz.
Fast magnetization of a high-to-low-beta plasma beam
Authors: Song J. J. , Wessel F. J. , Yur G. , Rahman H. U. , Rostoker N. , White R. S.
Published: July 2002

The magnetization of a high-beta (plasma energy density/magnetic field energy density≡β≥1) hydrogen-plasma beam injected into a vacuum transverse magnetic field is studied experimentally. Nominal parameters were Ti≊1 eV, Te≊5 eV, n≤3×1013 cm−3, vi≤7×106 cm/sec, tpulse<70 μsec, Bz≤300 G. Plasma characteristics were measured for a wide beam, a/ρi≤35, and a downstream distance, x≤300ρi, where a is the beam radius, x is the downstream distance, and ρi is the ion gyroradius. A brief initial state of diamagnetic propagation is observed, followed by E×B (magnetized) propagation; E×B propagation is accompanied by beam compression transverse to B with as much as a factor of 4 increase in density and a slight drift of the beam in the ion Lorentz force direction. For Bz=200–300 G the observed magnetization time is much faster than calculated from classical Spitzer conductivity and is more of the order of the magnetization time based on Hall conductivity.
A model for the simulation of nonequilibrium line transfer in laboratory plasmas
Authors: Peyrusse Olivier
Published: July 2002

A numerical model for the simulation of emission spectra of laboratory plasmas is presented. A special attention has been focused on the coupling between the line transfer and the ionic populations. This coupling is carried out by an iterative procedure, where, given the temperature, density, and velocity profiles of a plasma, the populations are obtained by linearization while the line transfer is computed within the core saturation approximation and with complete frequency redistribution. This procedure has been developed along with a collisional-radiative model for K-shell spectroscopy. As an application, the emission of a laser-created aluminum plasma was calculated, and the spectra of radiation emerging from different emission zones were accordingly computed. It is found that an accurate evaluation of line intensities involves a computation of line transfer, taking into account the macroscopic Doppler shift.
Numerical simulation of a low-density plasma erosion opening switch
Authors: Grossmann J. M. , Ottinger P. F. , Neri J. M. , Drobot A. T.
Published: July 2002

Current conduction through a low density (∼1012 cm−3) collisionless plasma injected between two coaxial conducting cylinders is simulated using a 2 (1)/(2) -D, electromagnetic particle-in-cell code. Plasma is injected through the anode towards the cathode with flow velocity, VF, and is assumed to be azimuthally symmetric. Current is driven through the plasma so that the 100 kA level is reached in ≂5 nsec. The opening process, when current is diverted to a load, is also treated. Electrons are found to carry current in a narrow current channel across the plasma by E×B drift. A large electric field is established by charge separation in the plasma in order to provide the drift. The motion of the anode end of the current channel controls the time of opening and is found to be independent of VF and to depend strongly on density and length.
On the structure of dynamical systems describing the evolution of coherent structures in a convective boundary layer
Authors: Rempfer D.
Published: July 2002

By Galerkin projection of the Navier–Stokes equations onto a system of empirical eigenfunctions, as obtained using the POD method, systems of ODEs have been derived that model the dynamics of coherent structures in a transitional flat-plate boundary layer. These ODEs are found to approximately exhibit the structure of systems of linear oscillators that are nonlinearly coupled via quadratic interactions. Investigations for different regions of the boundary layer (at different downstream positions) show distinct changes in the eigenfrequencies of these linear oscillators, eventually leading to low-dimensional chaos at the onset of turbulence.
The persistence of trailing vortices: A modeling study
Authors: Zeman O.
Published: July 2002

The principal subject of this paper is analysis and modeling of turbulent wing tip vortex flows in a far-field region of the vortex evolution. The choice of a Reynolds stress closure (RSC) to model the vortex turbulence is shown to be indispensable for representation of the flow rotation effects on turbulence. The principal result reported is the model–experiment comparison of the vortex growth rates for different vortex Reynolds numbers. The mean vortical flow generated by the wing tip very effectively suppresses the Reynolds shear stress, which mediates the extraction of energy from the mean flow by turbulence. In consequence, the vortex-core growth rate is controlled only by molecular viscosity and the vortex turbulence decays since the turbulence production rate is very nearly zero. This rather unexpected result is shown to be supported by experiments. Finally, it is shown that the computed turbulence structure is consistent with experimental data at the NASA Ames Research Center.
Dynamic Smagorinsky model on anisotropic grids
Authors: Scotti A. , Meneveau C. , Fatica M.
Published: July 2002

To examine the performance of the dynamic Smagorinsky model in a controlled fashion on anisotropic grids, simulations of forced isotropic turbulence are performed using highly anisotropic discretizations. The resulting model coefficients are compared with an earlier prediction. Two extreme cases are considered: pancake-like grids, for which two directions are poorly resolved compared to the third, and pencil-like grids, where one direction is poorly resolved when compared to the other two.
A wavelet-packet census algorithm for calculating vortex statistics
Authors: Siegel Andrew , Weiss Jeffrey B.
Published: July 2002

A generalized wavelet-packet based technique for decomposing signals into coherent and noncoherent parts is presented. The method is tested on the vorticity field of numerical simulations of weakly decaying two-dimensional turbulence. The easily recognizable coherent vortex structures that emerge are systematically filtered from the solution. Once extracted, various properties of the vortices, such as their number, size, circulation, and peak value are computed. The results compare well with a similar study [J. Fluid Mech. 219, 361 (1990); Phys. Fluids 5, 608 (1993)], which employs a complex pattern recognition technique based exclusively on a priori knowledge of the properties of the solution—that is, the features typical of the resulting vortex structures. The similarity of the results is encouraging, suggesting that the wavelet packet technique, by absorbing much of the complexity into the mathematical features of the transform itself, can provide an efficient, standardized tool that is readily extendible to more complex problems in two and three dimensions.
Azimuthal mode measurements of elliptic jets
Authors: Kinzie Kevin W. , McLaughlin Dennis K.
Published: July 2002

A procedure to quantitatively measure the relative amplitudes of azimuthal modes in the acoustic field of an elliptic jet is presented. The work describes how the azimuthal modes in an elliptic jet can be represented by a linear combination of Mathieu function modes and how the amplitude coefficients of each individual mode can be determined through an orthogonal decomposition based on Mathieu functions. The modal decomposition is performed in an elliptic cylindrical coordinate system natural to the elliptic jet geometry. The procedure is first tested on an artificially excited, perfectly expanded Mach 1.5 elliptic jet with preferential varicose and flapping mode excitation of discrete frequencies. The excitation was provided with a four electrode glow discharge system with phase control of the individual electrodes. Following that, the procedure was applied to naturally excited Mach 1.5 jets with both air and a helium/air mixture as the jet working gas. The helium/air jets simulate the higher jet velocity and lower jet density of heated jets. The modal decomposition technique is verified by experiment, allowing significant differences to be identified in the azimuthal modal content as the jet simulated temperature is increased.
Stochastic theory of compressible turbulent fluid transport
Authors: Dukowicz John K. , Smith Richard D.
Published: July 2002

We develop a stochastic model for the turbulent transport of passive scalars based on the Fokker–Planck equation for the probability density distribution of the displacements of infinitesimal fluid parcels (“particles”) in random turbulent motion. Such a theory is the microscopic basis behind semiempirical models of turbulent diffusion which apparently have been developed only for incompressible flow. Here, we specifically develop the theory so that it applies to compressible flow. We then apply it to the particular case of stratified mesoscale turbulent transport of tracers in the ocean, and we find that it generalizes the recent parametrization of Gent and McWilliams [J. Phys. Oceanogr. 20, 150 (1990)].
Non-linear spirals in the Taylor–Couette problem
Authors: Antonijoan J. , Marquès F. , Sánchez J.
Published: July 2002

We examine non-linear spiral flow in the Taylor–Couette problem for a wide gap with axially periodic conditions. We present a highly efficient computational method adapted to this problem, based on continuation methods applied to a pseudospectral discretization of the Navier–Stokes equations in a rotating frame of reference. The spiral flow is computed in a wide range of parameters, and different features are explored in detail: domain of existence of the flow, behavior for high Reynolds number, appearance of axial flows, dependency on parameters, and stability against helical disturbances. A first integral is obtained and used to describe the particle trajectories in the fluid. This description shows that the axial and radial motion of the particles is mainly confined within an internal boundary layer.
Observations of shear-induced particle migration for oscillatory flow of a suspension within a tube
Authors: Butler Jason E. , Majors Paul D. , Bonnecaze Roger T.
Published: July 2002

Suspensions of noncolloidal, neutrally buoyant, spherical particles were subjected to oscillating displacements at low Reynolds number along the axis of a circular tube. Using nuclear magnetic resonance imaging (NMRI), the phase distribution of a suspension with a particle volume fraction 0.4 was assessed for a variety of conditions. The variables studied included ratio of particle to tube diameter, amplitude of oscillation, and number of oscillations. Consistent with macroscopic theories of shear-induced particle migration, the particles preferentially moved away from the walls and to the center of the pipe for amplitudes of oscillation much greater than the particle diameter when the ratios of particle radius to tube radius were 6.4×10−3 and 1.48×10−2. However, for a ratio of particle radius to tube radius of 6.4×10−3, the images showed that the suspension was not uniform along the tube length for an amplitude of oscillation equivalent to one pipe diameter. For a larger ratio of particle radius to tube radius of 1.48×10−2, the suspension remained uniform along the pipe for similar conditions. For the smaller ratio of particle to tube radius of 6.4×10−3 and an amplitude of oscillation of five particle radii, the particles migrated to the wall of the pipe as predicted by the Stokesian dynamics simulations of Morris [“Anomalous particle migration in oscillatory pressure-driven suspension flow,” presented at the 1997 Annual Meeting of the AICHE (unpublished)]. These phenomena, which have not previously been observed experimentally, are not described by any existing theories of shear-induced particle migration.
A nonlinear mechanism for receptivity of free-stream disturbances
Authors: Berlin S. , Henningson D. S.
Published: July 2002

Numerical experiments on the interaction of simple vortical free-stream disturbances with a laminar boundary layer are presented. Both spatial and temporal direct numerical simulations (DNS) have been performed for two types of free-stream disturbances. A linear and a nonlinear receptivity mechanism were identified. The nonlinear mechanism was found to force streaks inside the boundary layer similar to those found in experiments on free-stream turbulence and it performed equally well for disturbances elongated in the streamwise direction as for and oblique free-stream disturbances. The boundary layer response caused by the nonlinear mechanism was, depending on the initial disturbance energy, comparable to that of the linear mechanism, which was only efficient for free-stream streamwise vortices. A parameter study revealed that the wall normal velocity component of the free-stream disturbances is more important for the investigated receptivity mechanisms than the streamwise component. The identified boundary layer receptivity mechanism, in which three-dimensional disturbances in the free-stream continuously force streaks inside the boundary layer, may explain differences between experimental results and previously suggested theories for the origin of streaks in boundary layers subjected to free-stream turbulence.
Numerical bifurcation study of a nonlinear current sheet model
Authors: Schröer A. , Neukirch T. , Kiessling M. , Hesse M. , Schindler K.
Published: July 2002

Results are presented of a numerical investigation of the strongly nonlinear bifurcation properties of an isothermal current sheet model which has previously been investigated in the weakly nonlinear regime by analytical methods. With the method used, the linear stability of the bifurcating branch is also checked. The results have applications in the stability of plasma systems with current sheets, e.g., the Earth’s magnetotail or solar coronal arcades.
Lower hybrid dissipative cavitons and ion heating in the auroral ionosphere
Authors: Shapiro V. D. , Soloviev G. I. , Dawson J. M. , Bingham R.
Published: July 2002

In this paper numerical solutions of Zakharov-type equations for lower-hybrid (LH) waves, including pumping at the long wavelengths and dissipation at short wavelengths in the form of dissipative cavitons are described. The caviton is a quasistationary structure undergoing many sequences of collapse due to dissipation, created by ion–wave interactions, which is compensated for by constant pump action. The possibility of trapping of short-wavelength LH oscillations by much broader density cavitons is investigated both analytically and numerically. Analytic self-similar solutions corresponding to collapse of such cavitons are constructed and demonstrate cascading to shorter wavelengths, which develops faster than the three-dimensional (3-D) quasiclassical cavity contraction. Numerical solutions show the development of deep caviton modulation due to the instability of quasiclassical collapse. Results of the numerical and analytical investigation are used to explain the recent observations of cavity formation in the auroral ionosphere, and show that the measured structures could indeed arise from quasiclassical LH collapse.
Extension of the ray equations of geometric optics to include diffraction effects
Authors: Peeters A. G.
Published: July 2002

The paper discusses an extension of the ray equations of geometric optics to include diffraction effects for a wave beam propagating in a dispersive anisotropic medium. The diffraction effects are introduced through a complex eikonal function, where the complex part describes the electric field profile of the beam. The ray equations are derived using a formalism that allows for a complex wave vector but yields trajectories of wave propagation in real space. The wavelength, width of the beam, and length scale over which the plasma parameters change are ordered 1: δ −1:δ −2, with δ being a small parameter. A consistent treatment of this ordering yields additional terms in the ray equations when compared with expressions in the literature, that arise from corrections to the dispersion relation. It is discussed to what accuracy the rays represent the flux of wave energy. An approximated set of equations that describe the propagation of a Gaussian beam is derived.
Effects of orbit squeezing on transport fluxes
Authors: Shaing K. C. , Zarnstorff M. C.
Published: July 2002

When the ratio μ1i/μ1e is of the order of unity, electron transport fluxes such as bootstrap current can be affected by the ion orbit squeezing. Here, μ1e is the electron viscous coefficient, and μ1i is the ion viscous coefficient with the effects of ion orbit squeezing included. The condition μ1i/μ1e∼1 can be satisfied for plasmas, such as those in enhanced reversed shear (ERS) mode, with ion temperature greater than electron temperature and orbit squeezing factor S≫1. Transport fluxes for both standard squeezed banana orbits and for squeezed orbits close to the magnetic axis are calculated for a large aspect ratio tokamak. It is found that ion heat flux and bootstrap current are reduced in ERS-like plasmas. Furthermore, the bootstrap current density does not vanish on the magnetic axis for a parabolic pressure profile in minor radius r due to the finite value of the fraction of trapped particles. This makes it possible to operate tokamaks in steady state.
Titanium K-shell x-ray production from high velocity wire array implosions on the 20-MA Z accelerator
Authors: Deeney C. , Coverdale C. A. , Douglas M. R. , Nash T. J. , Spielman R. B. , Struve K. W. , Whitney K. G. , Thornhill J. W. , Apruzese J. P. , Clark R. W. , Davis J. , Beg F. N. , Ruiz-Camacho J.
Published: July 2002

The advent of the 20-MA Z accelerator [R. B. Spielman, C. Deeney, G. A. Chandler et al., Phys. Plasmas 5, 2105 (1997)] has enabled implosions of large diameter, high-wire-number arrays of titanium to begin testing Z pinch K-shell scaling theories. The 2 cm long titanium arrays, which were mounted on a 40 mm diameter, produced between 75±15 to 125±20 kJ of K-shell x rays. A mass scan indicates that, as predicted, higher velocity implosions in the series produced higher x-ray yields. Spectroscopic analyses indicate that these high velocity implosions achieved peak electron temperatures from 2.7±0.1 to 3.2±0.2 keV and obtained a K-shell emission mass participation of up to 12%.
Correlation between cohesive energy and mixing rate in ion mixing of metallic bilayers
Authors: Van Rossum M. , Cheng Y-T. , Nicolet M-A. , Johnson W. L.
Published: July 2002

We have compared the mixing rate of several 5d-4d metal bilayers which form ideal solutions. We observe a strong correlation between the mixing rate and the average cohesive energy of each bilayer. A model based on the thermal spike concept is proposed to explain this behavior. The model leads to a general expression describing mixing rates in metallic bilayers.
Dislocation energies and hardness of semiconductors
Authors: Sher A. , Chen A.-B. , Spicer W. E.
Published: July 2002

The dislocation energies and hardness of semiconductors are calculated by an extension of Harrison’s method. It is demonstrated in agreement with experiment that dislocation energies per unit length are proportional to d−3– d−9, where d is the bond length and hardness is proportional to d−5– d−11. The hardness is related to the interaction energies among dislocations. It is argued that dislocation densities of semiconductors will be reduced if they are alloyed with a second constituent that has a shorter bond length. Experimental evidence supporting this strategy is noted.
Phase formation and reaction kinetics in the thin-film Co/GaAs system
Authors: Yu A. J. , Galvin G. J. , Palmstro/m C. J. , Mayer J. W.
Published: July 2002

The solid state reaction between a Co thin film and (100) GaAs was investigated using Rutherford backscattering spectrometry. At 400 °C a reacted layer of composition Co:Ga:As=2:1:1 was observed to form. This reaction product obeys diffusion controlled growth kinetics and maintains its composition for annealing (400 °C) from 15 min up to 24 h.
Polarized shear waves using laser line sources and electromagnetic acoustic transducer detection
Authors: Hutchins D. A. , Wilkins D. E.
Published: July 2002

The polarization features of ultrasonic shear waves from a pulsed laser line source have been investigated, using detection by an electromagnetic acoustic transducer. The result is a means for undertaking polarized shear wave experiments over a wide bandwidth.
Intracavity chirp compensation in a colliding pulse mode-locked laser using thin-film interferometers
Authors: Heppner J. , Kuhl J.
Published: July 2002

Dielectric multilayer mirrors designed as Gires–Tournois interferometers have been used in a colliding pulse mode-locked dye laser to study the influence of intracavity group velocity dispersion on the pulse formation. Simple angle tuning of the interferometric mirrors which is equivalent to a change of the net cavity dispersion resulted in a shift of the laser wavelength between 627 and 614 nm. Within this range stable operation with pulse durations of 130 fs or less and average output powers between 30 and 80 mW was obtained. The shortest pulse of 80 fs duration was found at 626 nm.
Particle accelerators powered by modulated intense relativistic electron beams
Authors: Friedman M. , Serlin V.
Published: July 2002

The development of high current particle accelerators powered by modulated intense relativistic electron beams is addressed in this paper. It is shown that when a modulated electron beam of power ≥109 W traverses metallic structures it leaves behind an electromagnetic field which generates a voltage gradient ≳100 MV/m along a path through which electrons or ions can be accelerated.
Glow-discharge-implanted, thermally annealed, oxide-passivated silicon solar cells of 19% efficiency
Authors: Westbrook R. D. , Wood R. F. , Jellison G. E.
Published: July 2002

Single-crystal silicon solar cells with efficiencies as high as 19.3% AM1.5 have been fabricated using simple, glow-discharge ion implantation followed by conventional furnace annealing during which an oxide layer was grown for surface passivation. Fine-line (∼5 μm) photolithography was used for metallization to reduce carrier recombination at the metal-silicon interface and optimized ZnS/MgF2 antireflection coatings were applied. The efficiencies of these cells are the highest reported to date for ion-implanted cells.
Strain-induced lateral confinement of excitons in GaAs-AlGaAs quantum well microstructures
Authors: Kash K. , Worlock J. M. , Sturge M. D. , Grabbe P. , Harbison J. P. , Scherer A. , Lin P. S. D.
Published: July 2002

We report evidence for lateral confinement of excitons within a continuous two-dimensional GaAs-AlGaAs quantum well. The confinement to ‘‘wires’’ within the well was produced by partially etching a pattern through the upper AlGaAs barrier. We propose a new mechanism, that of patterned strain, for lateral quantum confinement of carriers in semiconductor microstructures, to explain our results.
Folding and activity of circularly permuted forms of a polytopic membrane protein
Authors: Beutler Rudolf , Ruggiero Francesco , Erni Bernhard
Published: July 2002

The transmembrane subunit of the Glc transporter (IICB
Glc
), which mediates uptake and concomitant phosphorylation of glucose, spans the membrane eight times. Variants of IICB
Glc
with the native N and C termini joined and new N and C termini in the periplasmic and cytoplasmic surface loops were expressed in
Escherichia coli
.
In vivo
transport/
in vitro
phosphotransferase activities of the circularly permuted variants with the termini in the periplasmic loops 1 to 4 were 35/58, 32/37, 0/3, and 0/0% of wild type, respectively. The activities of the variants with the termini in the cytoplasmic loops 1 to 3 were 0/25, 0/4 and 24/70, respectively. Fusion of alkaline phosphatase to the periplasmic C termini stabilized membrane integration and increased uptake and/or phosphorylation activities. These results suggest that internal signal anchor and stop transfer sequences can function as N-terminal signal sequences in a circularly permuted α-helical bundle protein and that the orientation of transmembrane segments is determined by the amino acid sequence and not by the sequential appearance during translation. Of the four IICB
Glc
variants with new termini in periplasmic loops, only the one with the discontinuity in loop 4 is inactive. The sequences of loop 4 and of the adjacent TM7 and TM8 are conserved in all phospho
enol
pyruvate-dependent carbohydrate:phosphotransferase system transporters of the glucose family.
Benzo[
a
]pyrene diol epoxide adducts in DNA are potent suppressors of a normal topoisomerase I cleavage site and powerful inducers of other topoisomerase I cleavages
Authors: Pommier Yves , Kohlhagen Glenda , Pourquier Philippe , Sayer Jane M. , Kroth Heiko , Jerina Donald M.
Published: July 2002

The catalytic intermediates of DNA topoisomerase I (top1) are cleavage complexes that can relax DNA supercoiling (intramolecular reaction) or mediate recombinations (intermolecular religation). We report here that DNA adducts formed from benzo[
a
]pyrene bay-region diol epoxides can markedly affect top1 activity. Four oligonucleotide 22-mers of the same sequence were synthesized, each of which contained a stereoisomerically unique benzo[
a
]pyrene 7,8-diol 9,10-epoxide adduct at the 2-amino group of a central 2′-deoxyguanosine residue. These four adducts correspond to either cis or trans opening at C-10 of the (+)-(7
R
, 8
S
, 9
S
, 10
R
)- or (−)-(7
S
, 8
R
, 9
R
, 10
S
)-7,8-diol 9,10-epoxides. Their solution conformations in duplex DNA (intercalated and minor-groove bound for the cis and trans opened adducts respectively) can be deduced from previous NMR studies. All four adducts completely suppress top1 cleavage activity at the alkylation site and induce the formation of new top1cleavage complexes on both strands of the DNA 3–6 bases away from the alkylation site. The trans opened adduct from the highly carcinogenic (+)-diol epoxide is the most active in inducing top1 cleavage independently of camptothecin, demonstrating that minor groove alkylation can efficiently poison top1. We also found that this isomer of the diol epoxide induces the formation of top1-DNA complexes in mammalian cells, which suggests a possible relationship between induction of top1 cleavage complexes and carcinogenic activity of benzo[
a
]pyrene diol epoxides.
DnaK dependence of mutant ethanol oxidoreductases evolved for aerobic function and protective role of the chaperone against protein oxidative damage in
Escherichia coli
Authors: Echave Pedro , Esparza-Cerón M. Angel , Cabiscol Elisa , Tamarit Jordi , Ros Joaquim , Membrillo-Hernández Jorge , Lin E. C. C.
Published: July 2002

The
adhE
gene of
Escherichia coli
encodes a multifunctional ethanol oxidoreductase (AdhE) that catalyzes successive reductions of acetyl-CoA to acetaldehyde and then to ethanol reversibly at the expense of NADH. Mutant JE52, serially selected for acquired and improved ability to grow aerobically on ethanol, synthesized an AdhE
A267T/E568K
with two amino acid substitutions that sequentially conferred improved catalytic properties and stability. Here we show that the aerobic growth ability on ethanol depends also on protection of the mutant AdhE against metal-catalyzed oxidation by the chaperone DnaK (a member of the Hsp70 family). No DnaK protection of the enzyme is evident during anaerobic growth on glucose. Synthesis of DnaK also protected
E. coli
from H
2
O
2
killing under conditions when functional AdhE is not required. Our results therefore suggest that, in addition to the known role of protecting cells against heat stress, DnaK also protects numerous kinds of proteins from oxidative damage.
The role of the third β strand in gp120 conformation and neutralization sensitivity of the HIV-1 primary isolate DH012
Authors: Zhu Chong-Bin , Zhu Lei , Holz-Smith Sonia , Matthews Thomas J. , Chen Chin Ho
Published: July 2002

Neutralization of HIV-1 primary isolates has been a tremendous
challenge for AIDS vaccine development. Here, we identify a
single amino acid change (T198P) in gp120 that alters the
neutralization sensitivity of the primary isolate DH012 to antibodies
against multiple neutralization epitopes that include the V3,
CD4-induced, and CD4 binding sites in gp120. This mutation is located
in the V1/V2 stem region that forms the third β strand (β3) of
the bridging sheet of gp120. The conformation of variable loops,
especially V1/V2 and V3, was proposed to regulate the accessibility
of these neutralization epitopes. The results of this study indicate a
direct association between the V1/V2 and V3 loops of DH012 gp120. The
single amino acid mutation T198P in the β3 severely compromises the
interaction between the V1/V2 and V3 loops. These results suggest
that interaction of V1/V2 and V3 can mask the neutralization epitopes
and that the β3 plays a critical role in determining the
neutralization sensitivity by modulating the interaction. This
study provides an insight into why primary isolates are relatively
resistant to antibody neutralization and might facilitate the
development of anti-HIV strategies against HIV-1 infection.
Environmental change and hedonic cost functions for automobiles
Authors: Berry Steven , Kortum Samuel , Pakes Ariel
Published: July 2002

This paper focuses on how changes in the economic and regulatory
environment have affected production costs and product characteristics
in the automobile industry. We estimate “hedonic cost functions”
that relate product-level costs to their characteristics. Then we
examine how this cost surface has changed over time and how these
changes relate to changes in gas prices and in emission standard
regulations. We also briefly consider the related questions of how
changes in automobile characteristics, and in the rate of patenting,
are related to regulations and gas prices.
Animal fatty acid synthase: Functional mapping and cloning and expression of the domain I constituent activities
Authors: Chirala Subrahmanyam S. , Huang Wei-Yong , Jayakumar Arumugam , Sakai Keiko , Wakil Salih J.
Published: July 2002

Animal fatty acid synthase (FAS; EC
2.3.1.85
) is a homodimer of a multifunctional subunit protein and catalyzes the synthesis of palmitate from acetyl-CoA, malonyl-CoA, and NADPH. The subunit (
M
r
≈270,000) carries seven distinct component activities and a site for the prosthetic group 4′-phosphopantetheine (acyl carrier protein). Based on proteolytic mapping, the organization of the activity domains along the subunit polypeptide from the N terminus is as follows: β-ketoacyl synthase, acetyl and malonyl transacylases, β-hydroxyacyl dehydratase, enoyl reductase, β-ketoacyl reductase, acyl carrier protein, and thioesterase. By comparing the amino acid sequences of the chicken, rat, and human synthases, we found that kallikrein cleavage sites occur in the least conserved regions of the FAS polypeptide subunit. Determining the amino acid sequences of the N-terminal end of the major kallikrein cleavage peptides helped delineate the most likely boundaries of the component activities in the cDNA-derived amino acid sequence. To confirm this organization, we cloned the chicken FAS cDNA coding for domain I and expressed it in
Escherichia coli
as a maltose-binding fusion protein. The isolated recombinant protein contained the activities of the acetyl and malonyl transacylases and the β-hydroxyacyl dehydratase. Based on the boundaries of the acetyl and malonyl transacylases and the β-hydroxyacyl dehydratase, we also cloned the appropriate cDNA fragments encoding the domains that contain the transacylases and the dehydratase in pET vectors and expressed them in
E. coli
as thioredoxin-6xHis fusion proteins. The purified recombinant proteins contained, respectively, the activities of the acetyl and malonyl transacylases and the dehydratase. These results not only confirmed the order of the component activities in domain I, but also paved the way for successful expression and characterization of the remaining activities.