bluuebunny/crossref_metadata_2025 huggingface dataset:
Periodic boundary conditions and the fast multipole method
Authors: Challacombe Matt , White Chris , Head-Gordon Martin
Published: July 2002

The seminal work of Nijboer and De Wette [Physica 23, 309 (1957)] enables the calculation of lattice sums of spherical harmonics, but has long been overlooked. In this article, their central result is recast in a simplified form suitable for modern multipole algorithms that employ the solid harmonics. This formulation makes possible the imposition of periodic boundary conditions within modern versions of the fast multipole method, and other fast N-body methods. The distinction between the extrinsic values obtained with the lattice sums ℳ of the multipole interaction tensors, and the intrinsic values associated with Taylor’s expansion of the Ewald formulas, is made. The central constants, ℳ, are computed to 32 digit accuracy using extended precision arithmetic. Timings and corresponding errors obtained with a periodic version of the fast multipole method are presented for particle numbers spanning [103,106], and a range of expansion orders. A qualitative comparison is made between the present implementation, other periodic versions of the fast multipole method, and fast Ewald methods.
The photoabsorption spectrum of Na⋯FH van der Waals molecule: Comparison of theory and experiment for a harpooning reaction studied by transition state spectroscopy
Authors: Topaler Maria S. , Truhlar Donald G. , Chang Xiao Yan , Piecuch Piotr , Polanyi John C.
Published: July 2002

The photodissociation of Na⋯FH van der Waals molecules in the 1.5–2.3 eV energy region is a very interesting system for transition state spectroscopy, because the potential energy surfaces for electronically excited states funnel the system down to the ground electronic state in a critical region where detailed features of the potential energy surfaces may be important in determining the branching probability between the harpooning reaction to form NaF or the dissociative E→V energy transfer process to form vibrationally excited HF. We used an analytic representation, reported earlier, of the potential surfaces for the two lowest A′ electronic states of NaFH as well as separable fits for two higher excited potential surfaces to simulate the experimental photodepletion spectrum of the Na⋯FH van der Waals molecule. Franck–Condon analysis was performed for the X̃ 2A′→Ã 2A′, X̃ 2A′→B̃ 2A″, and X̃ 2A′→B̃′ 2A′ transitions to predissociative states of the exciplex by making a separable approximation in Jacobi coordinates. Theoretical simulation based on ab initio energies and transition dipole moments produced an excitation spectrum that is in good agreement with the experimental data. Including the dependence of the transition dipole moment on nuclear geometry had only a small quantitative effect on the calculated photoabsorption spectrum. The present calculation, in spite of the approximations involved, provides a semiquantitative description of the experimental spectrum of the resonance states in the funnel and allows us to explain all the main features of the spectrum.
Electron impact ionization of the hydroxyl radical
Authors: Tarnovsky V. , Deutsch H. , Becker K.
Published: July 2002

We employed the fast-neutral-beam technique in a measurement of absolute partial cross sections for the electron-impact ionization and dissociative ionization of the hydroxyl free radical from threshold to 200 eV. The deuterated OD radical rather than the protonated OH radical was used as a target in our studies in order to allow a better separation of the various product ions in our apparatus. The total (single) OD ionization cross section was found to have a value of slightly less than 2.0×10−16 cm2 at 70 eV. The ionization of OD is dominated by the formation of parent ions with a parent ionization cross section of 1.85×10−16 cm2 at this energy. A comparison of the experimentally determined total single OD ionization cross section with a calculated OH cross section using a modified additivity rule showed good agreement in terms of the absolute value and the cross section shape (at least above 50 eV). In the course of this work, we also measured the partial ionization cross sections for the D2O molecule and found good agreement between our cross sections and the most recent measurements of Straub et al. [J. Chem. Phys. 108, 109 (1998)] as well as with recent calculations.
Layer-by-layer heteroepitaxial growth process of a BaO layer on SrTiO3(001) as investigated by molecular dynamics
Authors: Kubo Momoji , Oumi Yasunori , Miura Ryuji , Stirling Andras , Miyamoto Akira , Kawasaki Masashi , Yoshimoto Mamoru , Koinuma Hideomi
Published: July 2002

Layer-by-layer heteroepitaxial growth processes of a BaO layer on SrTiO3(001) were simulated in order to predict an appropriate buffer layer for a YBa2Cu3O7−x(YBCO)/SrTiO3(001) heterojunction by using our crystal growth molecular dynamics (MD) simulation code. The SrTiO3(001) terminated by a TiO2 atomic plane was employed as the substrate in the present simulations. BaO molecules were continuously deposited on the SrTiO3(001) one by one, and finally a two-dimensional and epitaxial growth of a BaO layer was observed at 700 K. Moreover, the constructed BaO layer was atomically flat and smooth without defects, retaining a NaCl-type structure and (001) oriented configuration. However, the stress of the BaO/SrTiO3(001) heterojunction gradually increased and finally reached to approximately 1.2 GPa during the epitaxial growth process. It is expected that the large stress disturbs the subsequent fabrication of the uniform YBCO/SrTiO3(001) heterojunction. We also simulated the epitaxial growth process of a BaO layer on a [single SrO layer/SrTiO3(001)]. An atomically flat and smooth BaO layer without defects was also obtained at 700 K. Surprisingly, the stress of the BaO/[single SrO layer/SrTiO3(001)] heterojunction was almost 0.0 GPa after epitaxial growth. We have already suggested that [BaO layers/single SrO layer] are suitable buffer layers for the YBCO/SrTiO3(001) heterojunction on the basis of regular MD simulations [M. Kubo et al., Phys. Rev. B 56, 13535 (1997)]. From the present crystal growth simulations, we confirmed that the above atomically uniform and smooth BaO/SrO/SrTiO3(001) can be fabricated and almost no heterointerface stress was induced after the epitaxial growth. Moreover, the effect of substrate temperature on the heteroepitaxial growth process of the BaO layer on the SrO/SrTiO3(001) was discussed.
Cluster KN formation by Rydberg collision complex stabilization during scattering of a K beam off zirconia surfaces
Authors: Wang Jiaxi , Engvall Klas , Holmlid Leif
Published: July 2002

The molecular beam scattering of a K atom beam off a zirconia surface at 1100 K is studied with four different detection techniques: field ionization, which is sensitive only to field ionizable Rydberg species, in this case, with principal quantum number n>29; ion collection, which is sensitive only to positive ions; ion multiplier detection, which will give a response for both positive ions and Rydberg species; and finally, surface ionization detection, which will give a signal proportional to the flux of all forms of K, including excited K* species and clusters KN. Combining all these methods, the different scattering processes can be disentangled. A condensation scattering process is observed between a K beam atom and an electronically excited cluster KN* at the surface. This is seen in the angular distributions as several sharp peaks in the angular directions of the center-of-mass motion for the complexes formed. Electronically excited species K* and KN* are formed by thermal excitation due to mechanisms which have recently been investigated in detail for the K–graphite surface interaction. Electronically excited clusters KN* with N at least up to 4 exist at the surface. Specular scattering is observed, which indicates an excitation energy exchange and scattering of a K* atom in a repulsive state interaction with the surface. Backwards ion formation scattering is also observed, due to cluster decomposition. The processes giving scattering of excited species are very sensitive to the external electric-field strength.
Molecular simulation of the phase behavior of noble gases using accurate two-body and three-body intermolecular potentials
Authors: Marcelli Gianluca , Sadus Richard J.
Published: July 2002

Gibbs ensemble Monte Carlo simulations are reported for the vapor–liquid phase coexistence of argon, krypton, and xenon. The calculations employ accurate two-body potentials in addition to contributions from three-body dispersion interactions resulting from third-order triple-dipole, dipole–dipole–quadrupole, dipole–quadrupole–quadrupole, quadrupole–quadrupole–quadrupole, and fourth-order triple-dipole terms. It is shown that vapor–liquid equilibria are affected substantially by three-body interactions. The addition of three-body interactions results in good overall agreement of theory with experimental data. In particular, the subcritical liquid-phase densities are predicted accurately.
A numerical model for simulating mechanical behavior of flexible fibers
Authors: Ning Zemin , Melrose John R.
Published: July 2002

A numerical method is developed for simulating the mechanical behavior of flexible fibers. A circular crossed fiber is represented by a number of cylindrical segments linked by spring dash-pot systems. Segments are lined up and bonded to each neighbor. Each bond can be stretched or compressed by changing the bond distance. Bending deflection and twist movement occur, respectively, in the bending and torsion planes. While the bending angle is determined by the positions of two neighboring bonds, a reference twist vector is introduced to record the torsion motion along the segment chain. Fluid drag forces are calculated based on the Stokes’ Law, where a free draining assumption is made. The motion of the fiber is determined by solving the translational and rotational equations of individual segments. Computer simulation has been conducted to verify the single fiber model with elastic theory and excellent agreements have been found between the simulation results and the theory in various situations such as beam deflection under static loads, vibrating cantilevers, and dynamics of helical shaped fibers. Examining orientations of rigid fibers in a viscous shear flow, simulation results suggest that the rotational time is sensitive to the fluid drag torque which is related to the shape of the segments. For highly flexible fibers, the effect of bending deformation on the period of rotation and the rotation orbits is also investigated. This numerical model for single flexible fibers linked by discrete segments provides a framework in the future studies on fibrous assemblies.
Separation of variables on n-dimensional Riemannian manifolds. I. The n-sphere S
n and Euclidean n-space R
n
Authors: Kalnins E. G. , Miller W.
Published: July 2002

The following problem is solved: What are all the ‘‘different’’ separable coordinate systems for the Laplace–Beltrami eigenvalue equation on the n-sphere Sn and Euclidean n-space Rn and how are they constructed? This is achieved through a combination of differential geometric and group theoretic methods. A graphical procedure for construction of these systems is developed that generalizes Vilenkin’s construction of polyspherical coordinates. The significance of these results for exactly soluble dynamical systems on these manifolds is pointed out. The results are also of importance for the analysis of the special functions appearing in the separable solutions of the Laplace–Beltrami eigenvalue equation on these manifolds.
A bidirectional traveling plane wave representation of exact solutions of the scalar wave equation
Authors: Besieris Ioannis M. , Shaarawi Amr M. , Ziolkowski Richard W.
Published: July 2002

A new decomposition of exact solutions to the scalar wave equation into bidirectional, forward and backward, traveling plane wave solutions is described. The resulting representation is a natural basis for synthesizing pulse solutions that can be tailored to give directed energy transfer in space. The development of known free-space solutions, such as the focus wave modes, the electromagnetic directed energy pulse trains, the spinor splash pulses, and the Bessel beams, in terms of this decomposition will be given. The efficacy of this representation in geometries with boundaries, such as a propagation in a circular waveguide, will also be demonstrated.
On a basic theorem of quaternion modules
Authors: Razon Aharon , Horwitz L. P. , Biedenharn L. C.
Published: July 2002

A complete proof of the equivalence of the star operation in the operator algebra isomorphic to quaternions and the adjoint operation in a quaternion Hilbert module is given.
A Fokker–Planck equation of fractional order with respect to time
Authors: Jumarie Guy
Published: July 2002

By combining the maximum entropy principle with some considerations related to derivatives of fractional order, one is led to suggest a Fokker–Planck of fractional order with respect to time, which could be related to dynamical systems subject to fractional Brownian motion. The relation with the process associated with the equation ∂p/∂t=(−1)n+1∂2np/∂x2n is exhibited.
An eigenvalue criterion for matrices transforming Stokes parameters
Authors: van der Mee C. V. M.
Published: July 2002

Simple eigenvalue tests are given to ascertain that a given real 4×4 matrix transforms the four-vector of Stokes parameters of a beam of light into the four-vector of Stokes parameters of another beam of light, and to determine whether a given 4×4 matrix is a weighted sum of pure Mueller matrices. The latter result is derived for matrices satisfying a certain symmetry condition. To derive these results indefinite inner products are applied.
Rational solutions of the Kadomtsev–Petviashvili hierarchy and the dynamics of their poles. I. New form of a general rational solution
Authors: Pelinovsky Dmitry
Published: July 2002

A new approach to the construction of the rational solutions to the hierarchy of the Kadomtsev–Petviashvili equation is presented. The generalization of the ‘‘superposition formula’’ is found to permit the construction of a new general solution from some partial ones. The features of the general polynomial factorization and the types of scattering in the many-body Calogero–Moser problem are investigated.
Positivity properties of the Bel–Robinson tensor
Authors: Bergqvist Göran
Published: July 2002

The Bel–Robinson tensor is a rank 4 tensor constructed from the Weyl curvature tensor. It is well known that when it is contracted with four future-pointing vectors a non-negative value is always obtained. In this paper we investigate various cases in which the value zero is attained and show that it occurs in some situations where space–time is of an algebraically special type. This is of importance for expressions of gravitational energy related to the Bel–Robinson tensor. Some new invariant ways of classifying the Petrov type of space–times using the Bel–Robinson tensor are also presented.
Bäcklund transformation of partial differential equations from the Painlevé–Gambier classification. I. Kaup–Kupershmidt equation
Authors: Musette Micheline , Conte Robert
Published: July 2002

Among the 50 nonlinear second-order differential equations of Painlevé and Gambier, those which are linearizable provide a natural scheme for deriving the Lax pair and the Darboux transformation of a nonlinear partial differential equation when the order of the scattering problem is three. This new method allows one to obtain, from singularity analysis only, the Bäcklund transformation of the Kaup–Kupershmidt equation, a result which was missing until now.
Anomalous diffusion of tracer in convection rolls
Authors: Young W. , Pumir A. , Pomeau Y.
Published: July 2002

The dispersion of a passive tracer in a two-dimensional, spatially periodic stationary flow, such as convection rolls, is studied in the large Peclet number limit. In the case where injection, at time t=0, is localized in one roll, two regimes exist. First, there is an anomalous diffusion regime in which the number of invaded rolls grows like t1/3. This regime is due to the presence of separatrices between rolls that induce trapping of tracer within each roll. At a later time, when t≫Td (the diffusion time within a roll), the usual diffusion regime is recovered, yet with an effective diffusive coefficient κeff that is greater than the molecular diffusivity κ by a factor proportional to the square root of the Peclet number.
Some comments on turbulence
Authors: Lumley J. L.
Published: July 2002

A few remarks are made about the average dissipation of energy in unsteady turbulent flows, and the way in which it is modeled. Some suggestions are made that place the customary models on a more rational basis, and which lead to an improved model. Sample calculations with the improved model are presented. From time to time, philosophical comments are made about turbulence as a field, theoreticians, and other subjects.
Asymptotic solution of transonic nozzle flows with homogeneous condensation. I. Subcritical flows
Authors: Delale Can F. , Schnerr Günter H. , Zierep Jürgen
Published: July 2002

The one-dimensional (1-D) asymptotic solution of subcritical transonic nozzle flows with nonequilibrium homogeneous condensation is presented. An algorithm based on a local iterative scheme that exhibits the asymptotic solution in distinct condensation zones is developed for transonic moist air expansions under atmospheric supply conditions. Two models that characterize the state of the condensed phase as water drops or ice crystals are employed, together with the classical nucleation theory and Hertz–Knudsen droplet growth law. It is shown that the 1-D asymptotic predictions are in good agreement with the recent static pressure measurements of moist air expansions in relatively slender nozzles when the condensed phase is assumed to consist purely of water drops.
Suppression of magnetic islands by energetic ions in toroidal plasmas
Authors: Hegna Chris C. , Bhattacharjee A.
Published: July 2002

This is a study of the effect of an energetic ion population injected from a neutral beam source on the nonlinear stability of magnetic islands. A kinetic theory is used to obtain the island current, which includes previously known modifications resulting from bootstrap currents and resistive interchanges. The island current thus obtained is used in an asymptotic treatment of the perturbed Ampère’s law, which yields an evolution equation for the island width. It is shown that the island width saturates at a very small amplitude in tokamaks (with Δ′>0) when the energetic ion density profile peaks just outside the rational surface. Energetic ions can also be used to modify the island width in three-dimensional equilibria. It is found that in stellarator equilibria with large vacuum islands, energetic ions can be injected to reduce the widths of equilibrium islands.
Simulation of phase space hole growth and the development of intermittent plasma turbulence
Authors: Berman Robert H. , Tetreault David J. , Dupree Thomas H.
Published: July 2002

An unstable, highly intermittent state of turbulence is observed to evolve from a quiescent homogeneous plasma that is linearly stable. This intermittent state, which consists of isolated phase space density holes, produces a pronounced non-Gaussian distribution of fluctuation amplitudes. The skewness becomes increasingly more negative with time. The plasma is nonlinearly unstable because the holes grow in amplitude by accelerating to regions of higher average phase space density. The instability can be interpreted as a collection of colliding, growing holes and, in its early stages, is consistent with theoretical predictions for the clump instability. A series of runs with a single isolated hole indicates that an isolated hole grows for any finite electron-ion drift velocity. The isolated hole growth rate is consistent with theoretical predictions. The implications of the instability to laboratory and space plasma are discussed.
Three-drift-wave interaction at finite parallel wavelength: Bifurcations and transition to turbulence
Authors: Biskamp Dieter , Kaifen He
Published: July 2002

The generic properties of the nonlinear interaction of three drift waves with finite k∥ are investigated. The different types of stationary or quasistationary states are characterized by the bifurcation diagram in γ1, κ parameter space, where γ1 measures the mode excitation and κ the parallel wavenumber. The transition to turbulence corresponds exactly to the Ruelle–Takens picture: steady state→periodic solution→doubly periodic solution→turbulence, in contrast to the period-doubling route usually observed in low-dimensional dynamic systems. The transition to k∥=0, the model of Horton and Terry [Phys. Fluids 25, 491 (1982)], occurs at very small values of k∥.
Electromagnetic instabilities and gyrophase-bunched particles
Authors: Gary S. Peter , Thomsen Michelle F. , Fuselier Stephen A.
Published: July 2002

The linear theory of electromagnetic instabilities at propagation parallel or antiparallel to a uniform magnetic field B(0) is used to calculate the relative phase angle ψj between the fluctuating velocity vector of the  jth component and the fluctuating magnetic field. A criterion that determines when this phase relationship leads to observable gyrophase bunching during the linear growth phase of an instability is also derived. The theory is applied to bunching of ion components by ion beam instabilities. Among the electromagnetic cool ion beam instabilities, it is found that the right-hand resonant mode is most likely to cause observable gyrophase bunching of an ion beam during linear growth, and that for sufficiently large beam–core relative drift speeds, ψb ≂90° at maximum growth for instability propagation parallel to B(0).
Theory for resonant ion acceleration by nonlinear magnetosonic fast and slow waves in finite beta plasmas
Authors: Ohsawa Yukiharu
Published: July 2002

A Korteweg–de Vries equation that is applicable to both the nonlinear magnetosonic fast and slow waves is derived from a two-fluid model with finite ion and electron pressures. As in the cold plasma theory, the fast wave has a critical angle θc. For propagation angles greater than θc (quasiperpendicular propagation), the fast wave has a positive soliton, whereas for angles smaller than θc, it has a negative soliton. Finite  β effects decrease the value of θc. The slow wave has a positive soliton for all angles of propagation. The magnitude of resonant ion acceleration (the vp×B acceleration) by the nonlinear fast and slow waves is evaluated. In the fast wave, the electron pressure makes the acceleration stronger for all propagation angles. The decrease in θc resulting from finite  β effects results in broadening of the region of strong acceleration. It is also found that fairly strong ion acceleration can occur in the nonlinear slow wave in high  β plasmas. The possibility of unlimited acceleration of ions by quasiperpendicular magnetosonic fast waves is discussed.
The location of a shock in rimming flow
Authors: O’Brien S. B. G. , Gath E. G.
Published: July 2002

In rimming flow, a thin film of viscous liquid is entrained on the inside of a horizontally rotating cylinder. We give an explicit criterion for determining whether or not shock solutions occur and show that the location and height of these shocks can be determined using a simple lubrication theory.
Measurements of a confined jet
Authors: Chua L. P. , Lua A. C.
Published: July 2002

In this paper we study the flow characteristics of an air jet issuing from a rectangular nozzle of aspect ratio 6.0 into a confined chamber. Hot-wire measurements of mean and root mean square velocities were made in the mixing layers and interaction region of the jet, at two Reynolds numbers. The results showed that the mean velocity profiles achieved approximate self-preservation almost immediately after the end of potential core. However, the self-similarity of root mean square velocity profiles was established only after a distance of 12 nozzle widths downstream of the nozzle exit. The decay rate of the confined jet was found to be almost three times less than that of square and circular free jets but was greater than that of plane free jets by 10–30% with laminar initial conditions and similar Reynolds number (<8000). The spreading rate of the confined jet was found to be quite similar to that of the plane free jets but higher than the circular and square free jets. It is deduced from the results obtained that although the confined jet is three-dimensional flow in nature, the decay rate of the confined jet follows more closely to that of a two-dimensional type of flow rather than that for a three-dimensional flow.
Relativistic shock waves in an electron–positron plasma
Authors: Tsintsadze Levan N.
Published: July 2002

The equations describing the detailed structure of radiation electromagnetic hydrodynamics for a relativistically hot electron–positron plasma are derived. Various discontinuities are studied by these equations. It is shown that the dependence of the electron (positron) mass on the temperature changes the structure of discontinuities, including shock waves, both qualitatively and quantitatively. Steady radiative shocks are considered, which can arise in steady flows, and which also can be used to describe the propagation of shocks when the shock thickness is very small as compared to the characteristic length over which the ambient medium changes significantly. First, the magnetohydrodynamic shock wave is treated as a discontinuity and jump relations, which relate the equilibrium states of the upstream and downstream plasma far from the front, are derived. Then the structure of the front itself is considered and tangential, contact (or entropy) and rotational discontinuities are investigated.
On the phase velocity of plasma waves in a self-modulated laser wake-field accelerator
Authors: Andreev N. E. , Kirsanov V. I. , Sakharov A. S. , van Amersfoort P. W. , Goloviznin V. V.
Published: July 2002

The properties of the wake field excited by a flattop laser pulse with a sharp leading edge and a power below the critical one for relativistic self-focusing are studied analytically and numerically with emphasis on the phase velocity of the plasma wave. The paraxial model describing modulation of the pulse spot size is shown to provide a good quantitative description, including preexponential factors, of the wake field during the linear stage of the resonant self-modulation instability. Though the phase velocity of the excited plasma wave is shown to be close to the group velocity of the laser pulse, the relativistic γ-factor corresponding to the phase velocity, which is a crucial parameter for particle acceleration, can be substantially less than that of the group velocity. Fortunately, for the considered three-dimensional regime of self-modulation, the noticeable difference between these two γ-factors takes place only in the initial stage of the instability and vanishes in the later stages.
Theory of fast ion transport induced by sawtooth oscillations: Overview and new results
Authors: Kolesnichenko Ya. I. , Lutsenko V. V. , Yakovenko Yu. V. , Kamelander G.
Published: July 2002

The work contains both an overview of recent theories and new results on the influence of sawtooth oscillations on the superthermal ions in a tokamak plasma. In particular, new results of numerical simulations of the sawtooth-crash-induced redistribution of fast ions are presented. The results are based on the approach suggested by the authors earlier [Nucl. Fusion 36, 159 (1996)]. Peculiarities of the particle motion during the crash are revealed. Dependence of the behavior of fast ions on their parameters, as well as on tokamak parameters and features of sawteeth, is analyzed. Based on this analysis, a simple picture showing the different effects of sawtooth oscillations on various groups of particles is suggested.
Sheared slab ηi instability in tokamak plasma with negative magnetic shear
Authors: Li Jiquan , Huang Lin , Qu Wenxiao
Published: July 2002

The sheared slab ηi instability is reconsidered in tokamak plasma with negative magnetic shear. A modified sheared slab model is presented to include both the magnetic shear and its variation with the magnetic surface. The results show that the slow variation of magnetic shear can aggravate the sheared slab ηi instability in the region near the minimum-q magnetic surface and that it has a weak stabilizing role in the plasma core near the axis. However, when the effect of the variation of magnetic shear increases, it can give rise to a stronger slab ηi instability. In addition, a linear mode coupling mechanism could be mediated by the variation of magnetic shear with a magnetic surface.
Two-fluid magnetohydrodynamic simulation of confinement of pellet-produced hydrogen clouds in hot magnetized plasmas
Authors: Kristof G. , Lengyel L.
Published: July 2002

The structure of dense particle clouds surrounding ablating hydrogen isotope pellets is investigated, with particular emphasis on the B-perpendicular expansion, ionization, and deceleration dynamics. A time-dependent single-temperature two-fluid one and one-half-dimensional Lagrangian model is used in which the neutral and ionized fluid components are allowed to move with different velocities. The expansion of the Lagrangian cells along the magnetic field lines is also taken into account. For given ablation rates, the confinement radii and the radial distributions of the velocities, particle densities, magnetic field strength, temperature, and other flow and field properties are calculated as functions of the background plasma parameters. The results show that collisional coupling between the neutral and ionized components is strong enough to change the initially spherically symmetric expansion of the neutral particles to a field-aligned, funneled flow pattern, in full agreement with experimental observations. The calculated cloud parameters are compared with measured data and results stemming from earlier single-velocity models.
Magnetic field line transport in three dimensional turbulence: Lévy random walk and spectrum models
Authors: Pommois P. , Zimbardo G. , Veltri P.
Published: July 2002

The transport of magnetic field lines is investigated numerically by summing an isotropic spectrum of randomly phased magnetic fluctuations to a uniform background field. The aim of this study is to clarify the dependence of transport properties on the fluctuation correlation length lc, which ideally should be much smaller than the periodicity scale L, and to eliminate the effects due to periodicity. Two types of spectra are considered: in the first, the longest turbulence wavelength equals L, and lc is varied in order to describe either the injection zone or the inertial range of turbulence. In this case, strong anisotropy of transport and, for moderate fluctuation levels, Lévy random walk in the y direction only is found. When lc is decreased, both transport and anisotropy are reduced. In the second type of spectrum the longest wavelengths in the spectrum are excluded in order to have lc≪L and to reduce the influence of periodicity of the magnetic field model. In this case transport is isotropic for all fluctuation levels, and the Lévy random walk is independent of periodicity, since it is found even when the transverse displacement Δr is within the periodicity box, lc<Δr<L. On the other hand this superdiffusive regime is limited to very small fluctuation levels. Because of the efficient reduction of anisotropy and periodicity effects, the second type of spectrum appears to be more appropriate to investigate the transport properties.
Magnetohydrodynamic effects on pellet injection in tokamaks
Authors: Strauss H. R. , Park W.
Published: July 2002

The location at which pellets are injected into a plasma can have a significant effect on what fraction of the pellet mass remains in the plasma for refueling purposes. Magnetohydrodynamic (MHD) simulations presented here, confirm the results of pellet injection experiments: toroidal curvature makes it favorable to inject pellets from the inboard side or from the top or bottom, rather than from the outboard side. Sufficiently large pellets injected at the inboard edge can reach the plasma center, and in the process drive magnetic reconnection to produce negative central shear. Injection at the top (or bottom) of the tokamak causes relatively little displacement of the pellet. A scaling law is obtained for pellet displacement which agrees well with the simulations. The MHD simulations were carried out with a new unstructured mesh finite element version of the MH3D full MHD code.
Stabilization of neoclassical tearing modes using a continuous localized current drive
Authors: Pletzer A. , Perkins F. W.
Published: July 2002

The stability of a test equilibrium relevant to the International Thermonuclear Experimental Reactor [Fusion Eng. Des. 36, 9 (1997)] has been studied within the framework of the neoclassical island theory. The most unstable modes, with the most positive matching index Δ′, have been found by solving the toroidal stability equation to have 3/2, 2/1, and 4/3 helicities. Quasilinear effects resulting from the flattening of the current profile as the island develops are important and stabilizing. Large saturated islands are predicted to arise from a combination of strong bootstrap current drive and weakly negative Δ′. The island size can be significantly reduced by applying a continuous current drive at the unstable rational surfaces. The so-obtained reduction of island width is approximately proportional to the current drive and inversely proportional to the square of the current channel. This stabilization relies on the removal of the free energy in the outer region and can thus be regarded as a Δ′ effect.
Density fluctuation phenomena in the scrape-off layer and edge plasma of the Wendelstein 7-AS stellarator
Authors: Zoletnik S. , Anton M. , Endler M. , Fiedler S. , Hirsch M. , McCormick K. , Schweinzer J. ,   the W7-AS Team
Published: July 2002

The fluctuation of electron density in the scrape-off layer (SOL) and edge plasma regions of the Wendelstein 7-AS stellarator [H. Renner et al., Plasma Phys Controlled Fusion 31, 1579 (1989)] is investigated by the beam emission spectroscopy technique, observing fluctuations in the Li(2p) line emission intensity of a 50 keV accelerated Li beam. A recently developed numerical technique enables correct reconstruction of correlation functions (and as a consequence, fluctuation amplitudes as well) of the electron density fluctuations from correlations of beam light fluctuations. Depending on the plasma electron density, the radial range of the measurement is either limited to r/a>0.9 (at the highest densities) or can extend to about r/a>0.3 at the lowest densities. As the technique is nonperturbing to the plasma, several hundred shots were analyzed and a catalog of the observed phenomena was established. The results always show a change in the properties of the fluctuations at approximately the last closed flux surface. This observation might indicate differences in the turbulence drive in the SOL and the edge plasma. However, one cannot exclude that the same turbulence phenomenon results in different density fluctuations in the two regions due to the different magnetic topologies. The relevance of the observed phenomena to anomalous transport is discussed.
Enhanced power absorption in planar microwave discharges
Authors: Yasaka Y. , Hojo H.
Published: July 2002

The wave propagation and absorption in planar microwave discharges without magnetic fields are calculated with nonuniformity in plasma density taken into account. When the wave is incident in the direction oblique to the density gradient, the field amplitude is strongly peaked at a position where the wave frequency coincides with the plasma frequency. The absorption coefficient of the wave for the oblique incidence is calculated to be much larger than for the parallel incidence. A comparative experiment of the plasma production is performed to find that the power coupling efficiency is larger for the oblique incidence antenna than for the parallel incidence antenna, especially in low collisionality regimes. The comparison between the calculation and the experimental result reveals that the mechanism of the efficient production of overdense plasmas in planar microwave discharges is due to the enhanced electric field near the plasma resonance and subsequent acceleration of electrons in the case of oblique incidence.
Light propagation and acousto-optic interaction in a LiNbO3 spherical waveguide
Authors: Li Q. , Tsai C. S. , Sottini S. , Lee C. C.
Published: July 2002

Propagation, guiding, collimation, and focusing of a light beam at 6328-Å wavelength in a LiNbO3 spherical waveguide as well as acousto-optic diffraction in such a waveguide using the surface acoustic wave centering at 245 MHz have been accomplished for the first time. Near diffraction-limited characteristics were measured in the incident light and the diffracted light was shown to preserve this desirable characteristic. Light beam scanning and various rf signal processing experiments including spectral analysis, correlation, and convolution were carried out with encouraging results. The potential advantages of the resultant spherical waveguide acousto-optic signal processor modules over the planar waveguide counterparts are also discussed.
Laser pulse width dependent surface ripples on silicon
Authors: Jost D. , Lüthy W. , Weber H. P. , Salathé R. P.
Published: July 2002

Scanned irradiation of a Si (111) crystal with a focused cw mode-locked argon or neodymium:yttrium aluminum garnet laser to its melting threshold has generated a type of surface morphology, ripples, with a periodicity which is dependent on the laser pulse width. We interpret these ripples as being a thermoelastically generated surface acoustic wave frozen out on the crystal surface.
Photoacoustic optical and thermal characterization of Si and GaAs ion implanted layers
Authors: Zammit U. , Marinelli M. , Scudieri F. , Martellucci S.
Published: July 2002

The values of the thermal conductivity and of the optical absorption coefficient of ion implanted Si and GaAs have been measured in a nondestructive way by photoacoustic measurements. The values obtained for Si are in good agreement with those found in literature, while no data have been found for ion implanted GaAs as a function of the implanted ions dose. The thermal conductivity value for the implanted layers is more than two orders of magnitude lower than the ones for the respective crystalline material. The implanted ion dose dependence of the two parameters is reported for GaAs.
High transition temperature superconductor LaBa2Cu3O7−y with zero resistance at 92 K
Authors: Wada T. , Suzuki N. , Maeda T. , Maeda A. , Uchida S. , Uchinokura K. , Tanaka S.
Published: July 2002

Samples of LaBa2 Cu3 O7−y have been synthesized, which show a superconducting transition above 90 K. One of the samples shows a superconducting transition with an onset at 93 K and a zero resistivity at 92 K. These temperatures are the highest among those reported so far on this system and even higher than those in YBa2 Cu3 O7−y. The magnetic susceptibility measurement also shows that it has a superconducting transition with onset at around 93 K. To synthesize such good LaBa2 Cu3 O7−y requires two important procedures. First is sintering above 950 °C in N2 gas atmosphere and the second is annealing at 300 °C in dried O2 gas atmosphere.
Effect of post-oxidation anneal temperature on radiation-induced charge trapping in metal-oxide-semiconductor devices
Authors: Schwank J. R. , Fleetwood D. M.
Published: July 2002

Polycrystalline silicon-gate metal-oxide-semiconductor (MOS) capacitors have been fabricated with high-temperature anneals from 800 to 950 °C after gate oxidation and polycrystalline silicon deposition. Temperatures from 800 to 875 °C are found to have very little effect on the radiation response of these devices. However, a rapid increase in radiation-induced oxide-trapped charge, ΔVot, is observed for anneal temperatures above 875 °C. This increase in ΔVot coincides with an experimentally observed change in the polycrystalline silicon grain structure. The anneal temperature was found to have a much smaller effect on radiation-induced interface-trap charge. The correlation of these results to other properties, e.g., stress, is discussed.
Lesions of the nucleus basalis magnocellularis induced by 192 IgG-saporin block memory enhancement with posttraining norepinephrine in the basolateral amygdala
Authors: Power Ann E. , Thal Leon J. , McGaugh James L.
Published: July 2002

Extensive evidence indicates that drugs and stress hormones act in the basolateral amygdala (BLA) to modulate memory consolidation. The BLA projects to the nucleus basalis magnocellularis (NBM), which sends broad cholinergic projections to the neocortex. NBM-cortex projections have been implicated in learning, memory storage, and plasticity. The current study investigated whether the cholinergic NBM-cortex projections are involved in BLA-mediated modulation of memory consolidation. Bilateral cholinergic cell lesions of the NBM were induced in rats with infusions of 192 IgG-saporin (0.1 μg/0.5 μl per side). Additionally, cannulae were implanted bilaterally in the BLA. One week after surgery, the rats were trained in an inhibitory avoidance task and, immediately after training, norepinephrine (0.3 μg, 1.0 μg, or 3.0 μg in 0.2 μl) or vehicle (PBS) was infused bilaterally into the BLA. Norepinephrine infusions produced a dose-dependent enhancement of 48-h retention (0.3 μg and 1.0 μg doses enhanced) in nonlesioned rats but did not affect retention in NBM-lesioned rats. Choline acetyltransferase assays of frontal and occipital cortices confirmed the NBM lesions. These findings indicate that cholinergic NBM-cortex projections are required for BLA-mediated modulation of memory consolidation.
Homologous recombination at the border: Insertion-deletions and the trapping of foreign DNA in
Streptococcus pneumoniae
Authors: Prudhomme Marc , Libante Virginie , Claverys Jean-Pierre
Published: July 2002

Integration of foreign DNA was observed in the Gram-positive human pathogen
Streptococcus pneumoniae
(pneumococcus) after transformation with DNA from a recombinant
Escherichia coli
bacteriophage λ carrying a pneumococcal insert. Segments of λ DNA replaced chromosomal sequences adjacent to the region homologous with the pneumococcal insert, whence the name insertion-deletion. Here we report that a pneumococcal insert was absolutely required for insertion-deletion formation, but could be as short as 153 bp; that the sizes of foreign DNA insertions (289–2,474 bp) and concomitant chromosomal deletions (45–1,485 bp) were not obviously correlated; that novel joints clustered preferentially within segments of high GC content; and that the crossovers in 29 independent novel joints were located 1 bp from the border or within short (3–10 nt long) stretches of identity (microhomology) between resident and foreign DNA. The data are consistent with a model in which the insert serving as a homologous recombination anchor favors interaction and subsequent illegitimate recombination events at microhomologies between foreign and resident sequences. The potential of homology- directed illegitimate recombination for genome evolution was illustrated by the trapping of functional heterologous genes.
Notch
signaling directly controls cell proliferation in the
Drosophila wing
disc
Authors: Baonza A. , Garcia-Bellido A.
Published: July 2002

Notch
signaling is involved in cell differentiation
and patterning during morphogenesis. In the
Drosophila
wing,
Notch
activity regulates the expression of several
genes at the dorsal/ventral boundary, and this is thought to elicit
wing-cell proliferation. In this work, we show the effect of clones of
cells expressing different forms of several members of the
Notch
signaling pathway, which result in an alteration
of
Notch
activity. The ectopic expression in clones of
activated forms of
Notch
or of its ligands
(
Delta
or
Serrate
) in the wing causes
outgrowths associated with the appearance of ectopic wing margins.
These outgrowths consist of mutant territories and of surrounding
wild-type cells. However, the ectopic expression of
Delta
, at low levels in ventral clones, causes large
outgrowths that are associated neither with the generation of wing
margin structures nor with the expression of genes characteristic of
the dorsal/ventral boundary. These results suggest that
Notch
activity is directly involved in cell
proliferation, independently of its role in the formation of the
dorsal/ventral boundary. We propose that the nonautonomous effects
(induction of extraproliferation and vein differentiation in the
surrounding wild-type cells) result from pattern accommodation to
positional values caused by the ectopic expression of
Notch
.
Identification of GIT1/Cat-1 as a substrate molecule of protein tyrosine phosphatase ζ/β by the yeast substrate-trapping system
Authors: Kawachi Hiroyuki , Fujikawa Akihiro , Maeda Nobuaki , Noda Masaharu
Published: July 2002

We used a genetic method, the yeast substrate-trapping system, to
identify substrates for protein tyrosine phosphatases ζ
(PTPζ/RPTPβ). This method is based on the yeast two-hybrid
system, with two essential modifications: conditional expression of
protein tyrosine kinase v-src (active src) to tyrosine-phosphorylate
the prey proteins and screening by using a substrate-trap mutant of
PTPζ (PTPζ-D1902A) as bait. By using this system, several substrate
candidates for PTPζ were isolated. Among them, GIT1/Cat-1 (G
protein-coupled receptor kinase-interactor 1/Cool-associated,
tyrosine-phosphorylated 1) was examined further. GIT1/Cat-1 bound to
PTPζ-D1902A dependent on the substrate tyrosine phosphorylation.
Tyrosine-phosphorylated GIT1/Cat-1 was dephosphorylated by PTPζ
in vitro
. Immunoprecipitation experiments indicated that
PTPζ-D1902A and GIT1/Cat-1 form a stable complex also in mammalian
cells. Immunohistochemical analyses revealed that PTPζ and
GIT1/Cat-1 were colocalized in the processes of pyramidal cells in
the hippocampus and neocortex in rat brain. Subcellular colocalization
was further verified in the growth cones of mossy fibers from pontine
explants and in the ruffling membranes and processes of B103
neuroblastoma cells. Moreover, pleiotrophin, a ligand for PTPζ,
increased tyrosine phosphorylation of GIT1/Cat-1 in B103 cells. All
these results indicate that GIT1/Cat-1 is a substrate molecule of
PTPζ.