bluuebunny/crossref_metadata_2025 huggingface dataset:
Dipolar couplings and internuclear distances by double-quantum nuclear magnetic resonance spectroscopy of solids
Authors: Graf R. , Demco D. E. , Gottwald J. , Hafner S. , Spiess H. W.
Published: July 2002

The analysis of high-resolution double-quantum nuclear magnetic resonance spinning sidebands for measuring dipolar couplings and internuclear distances in dipolar solids is described. For this purpose, the response of a dipolar-coupled spin system in a rigid solid is investigated with respect to high-resolution multiple-quantum experiments using rotor-frequency synchronized pulse sequences. For isolated, magnetically equivalent spin-1/2 pairs, exact expressions for the double-quantum spinning-sideband patterns are derived. These patterns show spinning sidebands only at odd numbers of the rotor frequency. For longer excitation/reconversion cycles, the double-quantum spinning sidebands are sensitive to changes in the internuclear distances. Using this technique, the dipolar couplings for C13 spin pairs in double-labeled polyethylene were measured in crystalline and amorphous domains, respectively. In the former the dipolar coupling reflects the carbon–carbon distance, in the latter it is reduced due to molecular dynamics. The possibility to use multiple-quantum pulse sequences as a dipolar filter for the rigid domains is also shown.
Optical detection of yttrium dicarbide, a “T-shaped” molecule
Authors: Steimle T. C. , Marr A. J. , Xin J. , Merer A. J. , Athanassenas K. , Gillett D.
Published: July 2002

The yttrium dicarbide molecule, YC2, has been identified in the reactions of laser-ablated yttrium with methane and other small hydrocarbons under supersonic jet-cooled expansion conditions. A parallel-polarized electronic band system with origin at 12 889.5 cm−1 has been tentatively identified as an A1–X̃ 2A1 transition of a “T-shaped’’ isomer where the Y atom is bonded to the side of a C2 molecule. Weak vibronically induced perpendicular bands, representing Δυ3=odd transitions, are also present. Extensive vibrational progressions in the excitation and the resolved fluorescence spectra have been analyzed to give the two lowest vibrational frequencies of the two electronic states: X̃ 2A1, ν2 (a1)=561.1(±2.8 cm−1), ν3 (b2)=369.6 (±1.7 cm−1); excited A1, ν2=494.5(±5.0 cm−1), ν3=331.6(±2.2 cm−1). No bands involving the C–C stretching vibration (ν1,a1) have been found. The frequency of the ν3 vibration in the ground state indicates that the barrier to internal rotation of the C2 group against the Y atom is quite high, but large anharmonic coupling between the ν2 and ν3 vibrations suggests that internal rotation is facilitated by excitation of the Y–C2 stretching vibration.
Femtosecond photoelectron spectroscopy of the I2− anion: A semiclassical molecular dynamics simulation method
Authors: Batista Victor S. , Zanni Martin T. , Greenblatt B. Jefferys , Neumark Daniel M. , Miller William H.
Published: July 2002

In this paper we describe a new semiclassical method for simulating femtosecond pump–probe photoelectron spectroscopy, and its implementation to study the excited state photodissociation dynamics of the I2− anion. Our algorithm involves a forward–backward (FB) semiclassical (SC) initial value representation (IVR) method for calculating the time dependent photodetachment spectrum P(ε,Δt) as a function of the kinetic energy ε of the photodetached electron and the delay time Δt between the pump and probe pulses. We describe the radiation-chromophore interaction perturbatively to first order in both pulse fields, assuming the Condon approximation for the electronic transition dipole moments. Our computed spectra are in excellent agreement with full quantum mechanical simulations.
Adsorption geometries of CO on Cu (211)
Authors: Radnik J. , Ernst H.-J.
Published: July 2002

The adsorption of CO on Cu (211), serving as model system for the adsorption of CO in stepped surfaces, has been investigated using electron energy loss spectroscopy (EELS). At a surface temperature of 110 K, CO adsorbs nondissociatively and is drawn exclusively to descending step sites in this system. On-top and bridge sites are sequentially populated with the CO axis being perpendicular to the surface. In addition, at low coverage, a molecular state is observed, for which the CO axis is strongly inclined.
High-resolution ion mobility measurements for silicon cluster anions and cations
Authors: Hudgins Robert R. , Imai Motoharu , Jarrold Martin F. , Dugourd Philippe
Published: July 2002

High-resolution ion mobility measurements have been performed for silicon cluster anions and cations, Sin− and Sin+, n=6–55. New isomers have been resolved for every cluster size larger than Si20. The results for the anions and the cations have the same global features. However, changing the charge often causes a shift in the isomer distribution, or causes new isomers to emerge. For example, the transition from prolate geometries to more-spherical ones is shifted to larger cluster sizes for the anions than for the cations. The mobilities of the anions are systematically smaller than those of the cations, presumably because of differences in the exterior electron densities.
Electrostatic attraction and phase separation in solutions of like-charged colloidal particles
Authors: Linse Per , Lobaskin Vladimir
Published: July 2002

Asymmetric electrolytes consisting of highly charged spherical macroions and small ions representing an aqueous solution of ionic surfactant micelles have been studied at different macroion concentrations by means of Monte Carlo simulations. The model system comprises macroions with 60 elementary charges and either monovalent, divalent, or trivalent counterions interacting solely through hard-core and Coulomb forces. Thermodynamic and structural properties are examined, and the effects of the counterion valency are discussed. For the lowest electrostatic macroion–counterion coupling (monovalent counterions), the macroions are well separated and an effective repulsive potential is acting between them. At stronger electrostatic coupling (divalent counterions), the double-layer repulsion between the macroions is strongly reduced and at short separations the attractive force becomes comparable to the double-layer repulsion. At even stronger coupling (trivalent counterions), the attractive correlation force between the macroions dominates and causes the solution to separate into two fluid phases of highly different density of the electrolyte. Our results differ quantitatively from the mean-field Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, one of the cornerstones of colloid science, which predicts a purely repulsive electrostatic force among like-charged colloidal particles under all conditions. At the same time, our results are consistent with similar finding of an attraction of electrostatic origin between two similarly charged planar surfaces at sufficiently large electrostatic coupling. A detailed analysis of the counterion distribution in the neighborhood of two macroions close to each other has also been performed for divalent counterions. Finally, the effect of salt addition has also been examined.
Vibrations of the carbon dioxide dimer
Authors: Chen Hua , Light J. C.
Published: July 2002

Fully coupled four-dimensional quantum-mechanical calculations are presented for intermolecular vibrational states of rigid carbon dioxide dimer for J=0. The Hamiltonian operator is given in collision coordinates. The Hamiltonian matrix elements are evaluated using symmetrized products of spherical harmonics for angles and a potential optimized discrete variable representation (PO-DVR) for the intermolecular distance. The lowest ten or so states of each symmetry are reported for the potential energy surface (PES) given by Bukowski et al. [J. Chem. Phys. 110, 3785 (1999)]. Due to symmetries, there is no interconversion tunneling splitting for the ground state. Our calculations show that there is no tunneling shift of the ground state within our computation precision (0.01 cm−1). Analysis of the wave functions shows that only the ground states of each symmetry are nearly harmonic. The van der Waals frequencies and symmetry adapted force constants are found and compared to available experimental values. Strong coupling between the stretching coordinates and the bending coordinates are found for vibrationally excited states. The interconversion tunneling shifts are discussed for the vibrationally excited states.
A density functional theory study of CO oxidation on Ru(0001) at low coverage
Authors: Zhang C. J. , Hu P. , Alavi A.
Published: July 2002

We have performed ab initio density functional theory calculations with the generalized gradient approximation to investigate CO oxidation on Ru(0001). Several reaction pathways and transition states are identified. A much higher reaction barrier compared to that on Pt(111) is determined, confirming that the Ru is very inactive for CO oxidation under UHV conditions. The origin of the reaction barrier was analyzed. It is found that in the transition state the chemisorbed O atom sits in an unfavorable bonding site and a significant competition for bonding with the same substrate atoms occurs between the CO and the chemisorbed O, resulting in the high barrier. Ab initio molecular dynamics calculations show that the activation of the chemisorbed O atom from the initial hcp hollow site (the most stable site) to the bridge site is the crucial step for the reaction. The CO oxidation on Ru(0001) via the Eley–Rideal mechanism has also been investigated. A comparison with previous theoretical work has been made.
Extended Hilbert space approach to few-body problems
Authors: Kuperin Yu. A. , Makarov K. A. , Merkuriev S. P. , Motovilov A. K. , Pavlov B. S.
Published: July 2002

A general formulation of the quantum scattering theory for a system of few particles, which have an internal structure, is given. Due to freezing out the internal degrees of freedom in the external channels, a certain class of energy-dependent potentials is generated. By means of potential theory, a modified Faddeev equation is derived both in external and internal channels. The Fredholmity of these equations is proven and this is what provides a sound basis for solving the addressed scattering problem.
Krein’s formula approach to the multisolenoid Aharonov–Bohm effect
Authors: Šťovíček P.
Published: July 2002

Two approaches are applied when investigating the Green’s function for the multisolenoid Aharonov–Bohm effect in the idealized setup. The first one relies on Krein’s formula, the second one is based on Schulman’s ansatz. These two methods mutually complete each other and lead to the same result.
On the evanescent fields and the causality of the focus wave modes
Authors: Shaarawi Amr M. , Ziolkowski Richard W. , Besieris Ioannis M.
Published: July 2002

The diverging and converging field components of the source-free focus wave modes are studied within the framework of both the Whittaker and Weyl plane wave expansions. It is shown that, in the Weyl picture, the evanescent fields associated with the diverging and converging components of the focus wave mode solution cancel each other identically. The source-free focus wave modes are, hence, composed solely of backward and forward propagating components of the Whittaker type. It will also be shown that no evanescent fields are associated with the causal excitation of an aperture by an initial focus wave mode field. The diverging field, however, is composed solely of causal components that propagate away from the aperture. With a specific choice of parameters, the field generated by the aperture is a very good approximation to the source-free solution. Under the same conditions, the original focus wave mode solution is composed predominantly of causal forward propagating fields.
Generalization of the Coleman–Mandula theorem to higher dimension
Authors: Pelc Oskar , Horwitz L. P.
Published: July 2002

The Coleman–Mandula theorem, which states that space–time and internal symmetries cannot be combined in any but a trivial way, is generalized to an arbitrarily higher spacelike dimension. Prospects for further generalizations of the theorem (spacelike representations, larger timelike dimension, infinite number of particle types) are also discussed. The original proof relied heavily on the Dirac formalism, which was not well defined mathematically at that time. The proof given here is based on the rigorous version of the Dirac formalism, based on the theory of distributions. This work also serves to demonstrate the suitability of this formalism for practical applications.
Canonically relativistic quantum mechanics: Representations of the unitary semidirect Heisenberg group, U(1,3)⊗sH(1,3)
Authors: Low Stephen G.
Published: July 2002

Born proposed a unification of special relativity and quantum mechanics that placed position, time, energy and momentum on equal footing through a reciprocity principle and extended the usual position-time and energy-momentum line elements to this space by combining them through a new fundamental constant. Requiring also invariance of the symplectic metric yields U(1,3) as the invariance group, the inhomogeneous counterpart of which is the canonically relativistic group CR(1,3)=U(1,3)⊗sH(1,3), where H(1,3) is the Heisenberg group in four dimensions. This is the counterpart in this theory of the Poincaré group and reduces in the appropriate limits to the expected special relativity and classical Hamiltonian mechanics transformation equations. This group has the Poincaré group as a subgroup and is intrinsically quantum with the position, time, energy, and momentum operators satisfying the Heisenberg algebra. The representations of the algebra are studied and Casimir invariants are computed. Like the Poincaré group, it has a Little Group for a massive rest frame and a null frame. The former is U(3) which clearly contains SU(3) and the latter is Os(2) which contains SU(2)⊗U(1).
Separability, positivity, and minimum uncertainty in time–frequency energy distributions
Authors: Flandrin Patrick
Published: July 2002

Gaussian signals play a very special role in classical time–frequency analysis because they are solutions of apparently unrelated problems such as minimum uncertainty, and positivity and separability of Wigner–Ville distributions. We investigate here some of the logical connections which exist between these different features, and we discuss some examples and counterexamples of their extension to more general joint distributions within Cohen’s class and the affine class.
Zero-helicity Lagrangian kinematics of three-dimensional advection
Authors: Holm Darryl D. , Kimura Yoshifumi
Published: July 2002

The toroidal–poloidal decomposition for divergenceless vector fields in three dimensions is used to classify incompressible steady velocity fields in three dimensions according to whether they have zero, or nonzero helicity, and whether they are integrable, or nonintegrable as dynamical systems. Linearized steady Rayleigh–Bénard convection flows provide examples from each class. Computational techniques that preserve volume and helicity are developed and used to visualize the Lagrangian particle trajectories of three-dimensional advection for two Rayleigh–Bénard flows having zero helicity in a periodic domain.
Evolution of a thermal mixing layer in uniformly sheared turbulent flow
Authors: Ferchichi M. , Tavoularis S.
Published: July 2002

The present study is concerned with scalar mixing in uniformly sheared turbulent flow, in which Reynolds stresses and integral length scales grow nearly exponentially. For this purpose, the high-velocity half of a uniformly sheared, turbulent air stream was heated electrically to a nearly uniform temperature, slightly above that in the other half, thus generating a thermal mixing layer between the heated and unheated streams. Measurements show that the mean temperature field reached a quasiasymptotic state, in which, to a first approximation, it maintained an error-function-like profile with a width that grew almost linearly within the range of measurements. The effects of shear were, however, manifested by an asymmetry of the mean temperature profile, indicating deeper penetration of the layer into the lower velocity side. Measurements of the evolution of rms temperature fluctuations, heat fluxes, third-order covariances, and length scales are also reported. Simple theoretical predictions of the evolution of the thermal mixing layer width and of temperature fluctuation statistics are also presented and shown to agree well with the measurements.
Experimental verification of the Kolmogorov refined similarity hypothesis
Authors: Praskovsky Alexander A.
Published: July 2002

Kolmogorov’s refined similarity hypothesis (RSH) was challenged by Hosokawa and Yamamoto [Phys. Fluids A 4, 457 (1992)] on the basis of a direct numerical simulation data set at Rλ<100. The authors considered that their results were ‘‘a clear case of violation of the RSH.’’ However, the hypothesis was proposed for very high Reynolds number flows and is not expected to apply at the low Reynolds numbers typical of current simulations. The Hosokawa and Yamamoto tests have been repeated with a high Reynolds number (Rλ≊3.2×103) experimental data set. The results are in good agreement with the Kolmogorov RSH.
A scalar imaging velocimetry technique for fully resolved four-dimensional vector velocity field measurements in turbulent flows
Authors: Dahm Werner J. A. , Su Lester K. , Southerland Kenneth B.
Published: July 2002

This paper presents an experimental technique for obtaining fully resolved measurements of the vector velocity field u(x,t) throughout a four-dimensional spatiotemporal region in a turbulent flow. The method uses fully resolved four-dimensional scalar field imaging measurements in turbulent flows [Phys. Fluids A 3, 1115 (1991)] to extract the underlying velocity field from the exact conserved scalar transport equation. A procedure for accomplishing this is described, and results from a series of test cases are presented. These involve synthetically generated scalar fields as well as actual measured turbulent flow scalar fields advected numerically by various imposed flow fields. The imposed velocity fields are exactly known, allowing a careful validation of the technique and its potential accuracy. Results obtained from a zeroth iteration of the technique are found to be very close to the exact underlying vector velocity field. Further results show that successive iterations bring the velocity field from the zeroth iteration even closer to the exact result. It is also shown that the comparatively dense velocity field information that this technique provides is well suited for accurate extraction of the more dynamically insightful strain rate and vorticity fields ε(x,t) and ω(x,t).
Characterization of vortex tubes and sheets
Authors: Tanaka Mitsuru , Kida Shigeo
Published: July 2002

Vortex structure is characterized in terms of the magnitude of the vorticity and strain rate. High vorticity with relatively low strain rate represents the vortex tube, and high vorticity with comparable strain rate the vortex sheet. The regions of high Laplacian of pressure and of high strain rate also represent the tubelike and sheetlike structures, respectively.
A simple derivation of the Pease–Braginskiĭ current
Authors: Pereira Nino R.
Published: July 2002

The Pease–Braginskiĭ current is rederived for a uniform single species Z pinch. The result contains the Alfvén current and the ratio between the collisional and bremsstrahlung cross sections.
A review of results from the HBTX reversed field pinch
Authors: Alper B.
Published: July 2002

The series of experiments carried out in recent years on the HBTX reversed field pinch [Plasma Phys. Controlled Fusion 30, 843 (1988)] have highlighted the importance of minimizing the flux that penetrates the plasma–wall boundary. This flux penetration, whether from equilibrium field errors, the presence of limiters, or plasma magnetic activity from the growth of thin shell modes, leads to an increase in the loop voltage. The magnitude of this increase can be understood in terms of magnetic helicity balance. Feedback stabilization of the (m,n)=(1,2) thin shell mode has been successfully demonstrated, though with little effect on global plasma properties. A new design is being developed for HBTX aimed at minimizing the flux that can penetrate the first wall.
Modifications in turbulence and edge electric fields at the L–H transition in the DIII-D tokamak
Authors: Doyle E. J. , Groebner R. J. , Burrell K. H. , Gohil P. , Lehecka T. , Luhmann N. C. , Matsumoto H. , Osborne T. H. , Peebles W. A. , Philipona R.
Published: July 2002

The L to H transition in the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (IAEA, Vienna, 1987), Vol. I, p. 159] is associated with two clear signatures: edge density fluctuations are abruptly suppressed (in ≊100 μsec), while the edge poloidal rotation velocity vθ increases, implying that the radial electric field Er becomes more negative. Detailed new spectroscopic profile measurements show that the changes in vθ and Er generate a region of sheared electric field and poloidal flow of width ≊3–5 cm. This region develops simultaneously with, and has the same spatial extent as, the edge fluctuation suppression zone as measured using a reflectometer system. Furthermore, the radial extent of the shear and fluctuation suppression zones encompass the location of the H-mode edge transport barrier. These observations are consistent with recent theoretical models of the L–H transition, and a comparison with these theories is presented. Data are also presented on the evolution of edge parameters and density fluctuations after the transition: the shear and fluctuation suppression layers are maintained for the duration of the quiescent H-mode phase, while relative density fluctuation levels decrease and interior plasma confinement gradually improves. Precursors to several different types of edge localized mode (ELMs) are also discussed.
Development and applications of compact high-intensity lasers
Authors: Mourou G. , Umstadter D.
Published: July 2002

The development of compact high-intensity lasers, made possible by the technique of chirped pulse amplification, is reviewed. This includes the complexities of high-power laser implementation, such as the generation of short pulses, pulse cleaning, wide-bandwidth amplification, temporal stretching and compression, and the requirements for high-average powers. Details of specific solid-state laser systems are given. Some applications of these lasers to short-pulse coherent short-wavelength [x-ray ultraviolet (XUV)] sources are also reviewed. This includes several nonlinear effects observed by focusing a subpicosecond laser into a gas; namely, an anomalous scaling of harmonic generation in atomic media, an upper limit on the conversion efficiency of relativistic harmonics in a plasma, and the observation of short-pulse self-focusing and multifoci formation. Finally, the effects of large ponderomotive pressures (100 Mbars) in short-pulse high-intensity laser–plasma interactions are discussed, with relevance both to recombination x-ray lasers and a novel method of igniting thermonuclear fusion.
Transport description of the rise time of sawtooth oscillations in reversed-field pinches
Authors: Werley K. A. , Nebel R. A. , Wurden G. A.
Published: July 2002

High-Θ (Θ≡Bϑ(a)/〈Bφ〉) toroidal reversed-field pinch discharges in the ZT-40M device are characterized by soft x-ray sawtooth oscillations. It is demonstrated that a one-dimensional transport description is sufficient to model the rise time of sawtooth oscillations, provided the resistivity profile has significant gradients. This result suggests that the toroidal magnetic flux generation (dynamo) observed in the reversed-field pinch (RFP) may be restricted to the m=1 tearing events associated with the sawtooth crash (decrease in the soft x-ray flux).
The effects of inertia on the buoyancy-driven convection flow in settling vessels having inclined walls
Authors: Shaqfeh E. S. G. , Acrivos A.
Published: July 2002

The bulk-averaged equations of motion, which describe the buoyancy driven flow in particle settlers having inclined walls, are solved over a wide range of the relevant parameter space. The solution assumes that the flow is laminar, that the particle Reynolds number is small, that the suspension is monodisperse, and that the settling occurs in a parallel plate vessel whose length to width ratio is not too large. It is shown that, in the asymptotic limit, Λ→∞, where Λ is the ratio of a sedimentation Grashof number to a sedimentation Reynolds number, R, inertial effects in the flow are O(ξ1/6), where ξ is given explicitly in terms of R, Λ, the inclination angle, θ, and the dimensionless distance from the vessel bottom. Using regular and singular perturbation techniques, the asymptotic form of the equations for Λ≫1 are then solved over the entire range of ξ and the solutions are shown to reduce to those given by Acrivos and Herbolzheimer [J. Fluid Mech. 92, 435 (1979)] and by Schneider [J. Fluid Mech. 120, 323 (1982)] in the limits ξ→0 and ξ→∞, respectively. Since typically in practice Λ∼O(106–109), the present solutions give expressions for the velocity profiles and the thickness of the clear-fluid slit that forms underneath the downward facing vessel wall, which are valid for a wide class of systems of practical interest.
Enhanced sedimentation in vessels with inclined walls: Experimental observations
Authors: Shaqfeh E. S. G. , Acrivos A.
Published: July 2002

Observations made in various batch and continuously operated inclined settlers are reported. The systems considered are the so-called ‘‘low aspect ratio’’ devices in which the width of the vessel is large enough such that the presence of the enclosing walls does not affect the dynamic phenomena. The principal aim of these experiments was to test the predictions of the recent theoretical papers of Shaqfeh and Acrivos [Phys. Fluids 29, 3935 (1986); 30, 960 (1987)] concerning the laminar base state convective flow and accompanying instability wave growth under conditions in which inertial effects play an important role. The observations confirm that the theory predicts various characteristics of the laminar flow both quantitatively and qualitatively and that, within the range of experimental conditions, the more recent linear stability theory qualitatively describes the salient features of the instability wave growth in these systems.
The hydrodynamic stability of an evaporating liquid
Authors: Higuera F. J.
Published: July 2002

A linear stability analysis of the rapid evaporation of a superheated liquid is presented. Due to the small value of the vapor-to-liquid density ratio, the evolution time for the perturbations that can grow is found to be much shorter than that of the basic planar configuration and the normal mode analysis for steady basic solutions is applicable as an approximation. Depending on the values of the surface tension, the effective gravity acceleration, and the instantaneous vaporizing mass flux, an initial perturbation of a given wavelength can grow directly or give rise to an oscillatory state. In the second case, the slow evolution of the oscillations depends on the liquid viscosity and on temperature sensitivities of the evaporating mass flux and the surface tension.
Kelvin–Helmholtz instability and vortices in magnetized plasma
Authors: Horton W. , Tajima T. , Kamimura T.
Published: July 2002

Analytic theory and implicit particle simulations are used to describe the evolution of the Kelvin–Helmholtz instability and the plasma vortices driven by a nonuniform E×B flow velocity. Formulas for plasma convection arising from the self-consistent plasma electric field give the rate of anomalous momentum transport across the magnetic field. The momentum transport is shown to be controlled by the tilting angle of the elliptical vortex with respect to the direction of the parallel flow. Three stages of evolution are investigated and formulas for the final vortex state are given.
Hamiltonian four-field model for nonlinear tokamak dynamics
Authors: Hazeltine R. D. , Hsu C. T. , Morrison P. J.
Published: July 2002

The Hamiltonian four-field model is a simplified description of nonlinear tokamak dynamics that allows for finite ion Larmor radius physics, as well as other effects related to compressibility and electron adiabaticity. Much simpler than some previous descriptions of the same physics, it still preserves essential features of the underlying exact dynamics. In particular, because it is a Hamiltonian dynamical system it conserves the appropriate Casimir invariants, as well as avoiding implicit, unphysical dissipation. Here the model is derived and interpreted, its Hamiltonian nature is demonstrated, and its constants of motion are extracted.
On the mode development in the developing region of a plane jet
Authors: Huang Jiann-Min , Hsiao Fei-Bin
Published: July 2002

The development of the mode arrangement in the developing region of an acoustically excited plane jet is extensively studied using hot-wire measurements. Two single-wire probes are located within the two shear layers of the jet to detect the flow structure patterns simultaneously. The jet is excited at its fundamental frequency in either varicose mode (m=0) or sinuous mode (m=1) condition. Experimental results show that different excitation mode conditions lead to different spreading rates and velocity fluctuation distributions in the developing flow field. The frequency spectral distributions for the m=1 excitation exhibit double peaks of fa and fb, rather than a single subharmonic f0/2 for the m=0 excitation. These three components constitute a relationship of (fa+fb)/2=f0/2. The mode switching phenomenon is found to be prominent under the m=0 case, while less pronounced for the m=1 case. The mode development is mainly governed by the evolution of the primary instabilities in the jet shear layers.
Local pressure-transport structure in a convective atmospheric boundary layer
Authors: Lin Ching-Long
Published: July 2002

Local pressure-transport structure in a convective atmospheric boundary layer is studied through large-eddy simulation and a conditional sampling technique. Two cases are simulated: A free-convection boundary layer and a sheared convective boundary layer with −zi/L≈17, where zi is the boundary layer height and L is the Monin–Obukhov length. Results show that pressure-transport flux tends to increase turbulent kinetic energy in the lower part of the sheared convective boundary layer. Furthermore, the root-mean-square resolved pressure fluctuation and the resolved negative pressure fluctuation due to −u1,2ru2,1r become much stronger in the sheared case. Flow visualization demonstrates that strong pressure transport is physically correlated with vortical structure embedded within large-scale updrafts. A conditional sampling technique is applied to study statistical characteristics of resolved fields surrounding strong pressure transport events. The conditional field reveals a boundary-layer-scale roll circulation with a large-scale thermal located at its center and characterized by a negative pressure minimum. Conditional pressure transport is a gain in the lower part of the pressure minimum and a loss in the upper part. The conditional vorticity lines converge to four distinct regions relative to the thermal: Large-scale horseshoe-shaped vorticity lines are wrapped around the thermal; small-scale arch-shaped vorticity lines drag behind the thermal; helical vorticity lines originate in the thermal core; and converging vorticity lines are found above the neck of the large-scale horseshoe-shaped vorticity lines. These regions roughly coincide with conditional negative momentum fluxes. We thus conclude that local pressure-transport structures are spatially associated with localized low pressure regions and strong vertical vorticity fluctuations, being embedded within thermals and advected along with large-scale convective rolls.
Important plasma problems in astrophysics
Authors: Kulsrud Russell M.
Published: July 2002

In astrophysics, plasmas occur under very extreme conditions. For example, there are ultrastrong magnetic fields in neutron stars, relativistic plasmas around black holes and in jets, extremely energetic particles such as cosmic rays in the interstellar medium, extremely dense plasmas in accretion disks, and extremely large magnetic Reynolds numbers in the interstellar medium. These extreme limits for astrophysical plasmas make plasma phenomena much simpler to analyze in astrophysics than in the laboratory. An understanding of such phenomena often results in an interesting way, by simply taking the extreme limiting case of a known plasma theory. The author will describe one of the more exciting examples and will attempt to convey the excitement he felt when he was first exposed to it. However, not all plasma astrophysical phenomena are so simple. There are certain important plasma phenomena in astrophysics that have not been so easily resolved. In fact, a resolution of them is blocking significant progress in astrophysical research. They have not yet yielded to attacks by theoretical astrophysicists nor to extensive numerical simulation. The author will attempt to describe one of the more important of these plasma–astrophysical problems, and discuss why its resolution is so important to astrophysics. This significant example is fast, magnetic reconnection. Another significant example is the large-magnetic-Reynolds number magnetohydrodynamics (MHD) dynamos.
Generalized magnetohydrodynamic equations for partially ionized dusty magnetoplasmas: Derivation and applications
Authors: Birk G. T. , Kopp A. , Shukla P. K.
Published: July 2002

A comprehensive investigation of electromagnetic wave and instability phenomena in partially ionized, magnetized dusty plasmas has been carried out. By employing the multi-fluid balance equations along with the Maxwell equations, a compact set of coupled field equations for the cases in which the dust grains are either robust (vd=0) or dynamic is derived. These systems of partial differential equations are used to study wave phenomena and resistive tearing mode instabilities analytically as well as by means of numerical simulations. For robust dust grains, it is shown that coupled sound-Alfvén waves can appear even in the absence of ion-neutral collisions. The unstable tearing modes are coupled to convective drift modes, if the dust number density is inhomogeneous. In the induction equation two new source terms for self-generation of magnetic fields can be identified. In parameter regimes that are characterized by dynamic dust grains, the low-frequency phenomena develop on timescales that are governed by the dust particle inertia rather than the ion inertia, as it is the case in dust-free plasmas. The results of this investigation should be useful in understanding the properties of low-frequency electromagnetic wave phenomena and the formation of coherent structures in dusty magnetoplasmas whose main constituents are negatively charged dust grains, singly charged positive ions, and neutrals.
Measurement of the dynamo effect in a plasma
Authors: Ji H. , Prager S. C. , Almagri A. F. , Sarff J. S. , Yagi Y. , Hirano Y. , Hattori K. , Toyama H.
Published: July 2002

A series of detailed experiments has been conducted in three laboratory plasma devices to measure the dynamo electric field along the equilibrium field line (the α effect) arising from the correlation between the fluctuating flow velocity and magnetic field. The fluctuating flow velocity is obtained from probe measurement of the fluctuating E×B drift and electron diamagnetic drift. The three major findings are the following: (1) The α effect accounts for the dynamo current generation, even in the time dependence through a ‘‘sawtooth’’ cycle; (2) at low collisionality the dynamo is explained primarily by the widely studied pressureless magnetohydrodynamic (MHD) model, i.e., the fluctuating velocity is dominated by the E×B drift; (3) at high collisionality, a new ‘‘diamagnetic dynamo’’ is observed, in which the fluctuating velocity is dominated by the electron diamagnetic drift. In addition, direct measurements of the helicity flux indicate that the dynamo activity transports magnetic helicity from one part of the plasma to another, but the total helicity is roughly conserved, verifying Taylor’s [Phys. Rev. Lett. 33, 1139 (1974); Rev. Mod. Phys. 58, 741 (1986)] conjecture.
Scaling of magnetic turbulence with Lundquist number in relaxed state devices
Authors: Mattor Nathan
Published: July 2002

The scaling of magnetic turbulence with Lundquist number, B̃(S), as generated by the dynamo in reversed-field pinches and spheromaks, is addressed. A theoretical framework is described, showing how the fluctuations arise from the dynamo and what dynamics determine the B̃(S) scaling. There are two limits of the dynamo. For a discrete (sawtoothing) dynamo, the time average of the magnetic fluctuations is given by B̃∝S0, but it is argued that the averaged flux surface destroying magnetic field fluctuations scale as B̃br∝S−1/2. For a continuous dynamo, with a steady-state saturated turbulent spectrum, the magnetic field perturbations scale as B̃∝S−1/4. Previous theories of S scaling are reviewed.
Stochastic particle transport in a magnetic island due to electrostatic drift waves
Authors: Nishimura Y. , Azumi M.
Published: July 2002

The effect of poloidally mode coupled, ballooning type electrostatic drift waves on a magnetic island has been studied both analytically and numerically. It has been shown quantitatively that particle orbits become stochastic and their behavior can be a possible candidate for the radial plasma transport across a magnetic island of a tokamak. The transport is significant in that it takes place even when the flux surface is not destroyed. The mechanism of the stochasticity generation is understood as an overlapping of secondary islands caused by resonance between periodic particle motions in the magnetic island and Fourier modes of E×B drift due to the electrostatic drift waves. The diffusion process perpendicular to magnetic surface has been analyzed by approximating the distribution to the Gaussian type. In addition, local diffusion process in the vicinity of Kolmogorov, Arnold, and Moser surfaces has been discussed.
Signatures of target performance and mixing in titanium-doped, laser-driven target implosions
Authors: Yaakobi B. , Marshall F. J. , Bradley D. K. , Delettrez J. A. , Craxton R. S. , Epstein R.
Published: July 2002

Absorption-spectroscopy measurements using an embedded titanium layer have been performed on laser implosions of spherical shells. Predicted absorption features formed by the titanium layer were observed: absorption lines due to 1s–2p transitions in titanium ions of incomplete L shell, as well as K-edge absorption in cold titanium. Also observed were oscillations due to EXAFS (Extended X-Ray Absorption Fine Structure) above the Ti K edge. The core temperature and shell areal density and temperature are derived from the measurements and are found to fall short of the values predicted by one-dimensional code simulations. This shortfall is attributed to the lack of irradiation symmetry, which leads to hydrodynamic instability and mixing (smoothing by phase plates was not used in this experiment). Signatures of mixing include the emission of the He-α line of Ti+20 due to titanium migrating to the core, as well as the EXAFS spectrum indicating cold titanium close to peak compression.
Computational modeling of direct-drive fusion pellets and KrF-driven foil experiments
Authors: Gardner John H. , Schmitt Andrew J. , Dahlburg Jill P. , Pawley Carl J. , Bodner Stephen E. , Obenschain Stephen P. , Serlin Victor , Aglitskiy Yefim
Published: July 2002

FAST is a radiation transport hydrodynamics code that simulates laser matter interactions of relevance to direct-drive laser fusion target design. FAST solves the Euler equations of compressible flow using the Flux-Corrected Transport finite volume method. The advection algorithm provides accurate computation of flows from nearly incompressible vortical flows to those that are highly compressible and dominated by strong pressure and density gradients. In this paper we describe the numerical techniques and physics packages. FAST has also been benchmarked with Nike laser facility experiments in which linearly perturbed, low adiabat planar plastic targets are ablatively accelerated to velocities approaching 107 cm/s. Over a range of perturbation wavelengths, the code results agree with the measured Rayleigh–Taylor growth from the linear through the deeply nonlinear regimes. FAST has been applied to the two-dimensional spherical simulation design to provide surface finish and laser bandwidth tolerances for a promising new direct-drive pellet that uses a foam ablator.
Locking of multiple resonant mode structures in the reversed-field pinch
Authors: Hansen A. K. , Almagri A. F. , Den Hartog D. J. , Prager S. C. , Sarff J. S.
Published: July 2002

Locking of a rotating mode by applying a resonant magnetic perturbation having the same helicity has been observed on various devices. Experiments have been carried out on the Madison Symmetric Torus reversed-field pinch (RFP) [Dexter et al., Fusion Technol. 19, 131 (1991)] which show that an externally applied magnetic perturbation can cause locking of the dominant magnetic modes (poloidal mode number m=1, toroidal mode numbers n=5–10) when the perturbation is resonant with them. A perturbation which is not resonant (m=0 or 2) produces no such effect. Thus, resonant torques may lock a stochastic magnetic structure arising from several modes, as likely exists in the RFP, as well as a distinct island as exists in tokamaks, although the details of the interaction are likely to be different.
Calibration of one-dimensional boosted kinetic codes for modeling high-intensity laser–solid interactions
Authors: Gibbon P. , Andreev A. , Lefebvre E. , Bonnaud G. , Ruhl H. , Delettrez J. , Bell A. R.
Published: July 2002

An efficient means of performing kinetic simulations of oblique-incidence laser–plasma interaction via a relativistic Lorentz transformation is described in detail. Comparisons are made between one-dimensional boost codes and previous two-dimensional particle-in-cell (PIC) simulations in an effort to define benchmarks for modeling high-intensity, short-pulse interactions. Apparent discrepancies between results obtained using PIC and Vlasov codes are resolved, and some pitfalls involved with both techniques are identified.
A new eigenvalue problem associated with the two-dimensional Newcomb equation without continuous spectra
Authors: Tokuda Shinji , Watanabe Tomoko
Published: July 2002

A new eigenvalue problem associated with the two-dimensional Newcomb equation in an axisymmetric toroidal plasma, such as a tokamak, has been posed and solved numerically by using a finite element method. In the formulation of the eigenvalue problem, the weight functions (the kinetic energy integral) and the boundary conditions at rational surfaces are chosen such that the spectra of the eigenvalue problem are comprised of only the real and denumerable eigenvalues (point spectra) without continuous spectra. Applications to the ideal m=1 mode have verified that this formulation is able to identify stable states as well as unstable states, and that the numerically obtained eigenfunctions show the singular behavior predicted by the theory at rational surfaces.
Kelvin–Helmholtz instabilities in tokamak edge plasmas
Authors: Garbet X. , Fenzi C. , Capes H. , Devynck P. , Antar G.
Published: July 2002

The parallel Kelvin–Helmholtz instability is investigated as a possible explanation for poloidal asymmetries of density fluctuations which reverse with the plasma current direction. It is shown that these modes are localized around the position where the radial gradient of parallel velocity is maximum. Two mechanisms lead to unstable Kelvin–Helmholtz modes; the acceleration of ions in a presheath and the anomalous Stringer spin-up due to asymmetries of the particle flux. Up–down asymmetries are explained by combining these two effects. Depending on the limiter configuration, the Stringer effect amplifies or weakens the flow due to presheath acceleration. This type of asymmetry reverses with the plasma current direction.
Three-dimensional particle simulation of plasma instabilities and collisionless reconnection in a current sheet
Authors: Horiuchi Ritoku , Sato Tetsuya
Published: July 2002

Generation of anomalous resistivity and dynamical development of collisionless reconnection in the vicinity of a magnetically neutral sheet are investigated by means of a three-dimensional particle simulation. For no external driving source, two different types of plasma instabilities are excited in the current layer. The lower hybrid drift instability (LHDI) is observed to grow in the periphery of current layer in an early period, while a drift kink instability (DKI) is triggered at the neutral sheet in a late period as a result of the nonlinear deformation of the current sheet by the LHDI. A reconnection electric field grows at the neutral sheet in accordance with the excitation of the DKI. When an external driving field exists, the convective electric field penetrates into the current layer through the particle kinetic effect and collisionless reconnection is triggered by the convective electric field earlier than the DKI is excited. It is also found that the anisotropic ion distribution is formed through the anomalous ion heating by the DKI.
Dust lattice waves in a plasma crystal
Authors: Farokhi B. , Shukla P. K. , Tsintsadze N. L. , Tskhakaya D. D.
Published: July 2002

The dust lattice wave in dusty plasma crystals is reexamined, taking into account the dependence of the dust grain charge on the grain potential. The Poisson equation for small grain potentials then assumes the form of the Schrödinger equation. The spatial distribution of the potential in the lattice includes the effect of the whole system of dust particles. Such a self-consistent description gives the dispersion relation for the dust lattice wave, which is different from the expression found earlier. The case of large grain charge is also considered. The frequency of the lattice oscillation increases considerably for large grain charges. Furthermore, it is noted that an ideal lattice can only exist if the dusty plasma parameters satisfy a definite relation between the dusty plasma Debye radius, the intergrain separation, and the grain size.
Vertical stability in a current-carrying stellarator
Authors: Fu G. Y.
Published: July 2002

An analytic stability criterion is derived for the vertical mode in a large aspect ratio stellarator with uniform current density profile. The effects of vacuum magnetic field generated by helical coils are shown to be stabilizing due to enhancement of field line bending energy. For a wall at infinite distance from the plasma, the amount of external poloidal flux needed for stabilization is given by f=(κ2−κ)/(κ2+1), where κ is the axisymmetric elongation and f is the ratio of vacuum rotational transform to the total transform.
Dust acoustic surface waves on a dusty plasma slab
Authors: Stenflo L. , Shukla P. K. , Yu M. Y.
Published: July 2002

The propagation of dust acoustic surface waves on a thin dusty plasma slab is investigated. The dispersion relation for the waves is found to differ significantly from that for infinite plasmas.
Electrical properties of Li-doped Hg1−xCdxTe(100) by molecular beam epitaxy
Authors: Wijewarnasuriya P. S. , Sou I. K. , Kim Y. J. , Mahavadi K. K. , Sivananthan S. , Boukerche M. , Faurie J. P.
Published: July 2002

p-type doping of HgCdTe(100) layers with lithium during growth by molecular beam epitaxy is reported. Hall measurements have been performed on these layers between 300 and 30 K. The Li concentration is found to increase with the Li cell temperature. Li-doped HgCdTe layers are estimated to have very shallow acceptor levels. Acceptor concentrations as high as 8×1018 cm−3 have been achieved. At low doping levels, due to residual donors, layers show compensation. Incorporation coefficient of Li close to 1 and almost 100% electrical efficiency for Li in molecular beam epitaxy HgCdTe layers were observed. However, Li is found to diffuse rapidly in HgCdTe layers grown by molecular beam epitaxy.