bluuebunny/crossref_metadata_2025 huggingface dataset:
New mechanisms of droplet coarsening in phase-separating fluid mixtures
Authors: Tanaka Hajime
Published: July 2002

We propose here a new mechanism of droplet coarsening in phase-separating fluid mixtures. In contrast to the conventional understanding that there are no interactions between droplets in the late stage of spinodal decomposition, we demonstrate the existence of interactions between droplets that is caused by the coupling between diffuse concentration change around droplets. We show the possibility that this mechanism plays an important role in droplet phase separation together with Brownian-coagulation mechanism. We also discuss the coupling between hydrodynamic and diffusion modes, namely, “collision-induced collision” phenomena.
Molecular and dissociative chemisorption of NO on palladium and rhodium (100) and (111) surfaces: A density-functional periodic study
Authors: Loffreda D. , Simon D. , Sautet P.
Published: July 2002

The efforts to reduce NOx pollutants have stimulated a large interest in the understanding of the elementary processes for NO transformation on transition metal surfaces. Periodic density-functional calculations have been performed for the molecular and dissociative chemisorption of NO on Pd and Rh(100) and (111) surfaces, with generalized gradient approximation exchange-correlation functionals. The periodic systems are modeled by two-dimensional palladium or rhodium slabs with frozen geometry, on which a NO, N, O, or (N+O) adlayer is set. On Pd and Rh(100) at a coverage of 0.5 monolayer (ML), the bridge site is the most stable one with respective binding energies of −1.54 and −2.18 eV. On the (111) surfaces, at a coverage of 0.33 ML, the threefold hollow sites are favored with binding energies of −2.0 eV for Pd(111) and −2.18 eV for Rh(111). For the dissociated structures, the mixed coadsorption of N and O is favored in most cases compared to separated domains. The chemisorption of NO, N, or O is stronger on Rh surfaces than on Pd ones but the stability gain is larger for the atomic chemisorption. The absolute values of binding energies decrease with the coverage. The NO dissociation is exothermic only for Rh at low coverage, while it is endothermic on Pd due to smaller atomic binding energies. This reaction becomes more endothermic when the coverage increases.
Hyperspherical surface functions for nonzero total angular momentum. I. Eckart singularities
Authors: Kendrick Brian K. , Pack Russell T , Walker Robert B. , Hayes Edward F.
Published: July 2002

A new hybrid numerical technique which utilizes both the DVR (Discrete Variable Representation) and FBR (Finite Basis Representation) to solve for the full 5D surface functions in the three-atom problem in hyperspherical coordinates for nonzero total angular momentum (J≠0) is reported. This method accurately treats the Eckart singularities in the kinetic energy operator which occur at both the north pole and equator of the 2D hypersphere in body-frame coordinates. The effects of the Eckart singularities on the surface function energies for HD2 are investigated and it is shown that an accurate treatment of these singularities is crucial in order to obtain the correct results. An improper treatment of the Eckart singularities could be a source for some of the discrepancies between recent experimental results and theory for the reaction H+D2→HD+D.
Infrared spectroelectrochemical investigations on the doping of soluble poly(isothianaphthene methine) (PIM)
Authors: Neugebauer H. , Kvarnström C. , Brabec C. , Sariciftci N. S. , Kiebooms R. , Wudl F. , Luzzati S.
Published: July 2002

Attenuated total reflection Fourier transform infrared (ATR-FTIR) investigations on the doping processes in substituted poly(isothianaphtene methine) (PIM), a new low band-gap conjugated polymer, are reported. The doping was studied in situ during chemical p-doping (oxidation) by iodine and during electrochemical p-doping (oxidation) and n-doping (reduction). During both signs of doping, infrared active vibrational (IRAV) bands due to strong coupling of the electrons to lattice vibrations are observed. The results are compared to FT-Raman spectra. Electrochemical p-doping shows two different doping regimes depending on the electrochemical potential. The narrow linewidth and low absorption intensities of the IRAV bands indicate a strong localization of the doping induced charge carriers, which is ascribed to a tilted geometry of the conjugated backbone due to steric repulsion effects.
Second- and third-order spin-orbit contributions to nuclear shielding tensors
Authors: Vaara Juha , Ruud Kenneth , Vahtras Olav
Published: July 2002

We present analytical calculations of the electronic spin–orbit interaction contribution to nuclear magnetic shielding tensors using linear and quadratic response theory. The effects of the Fermi contact and the spin-dipole interactions with both the one- and two-electron spin–orbit Hamiltonians, included as first-order perturbations, are studied for the H2X (X=O, S, Se, and Te), HX (X=F, Cl, Br, and I), and CH3X (X=F, Cl, Br, and I) systems using nonrelativistic multiconfiguration self-consistent field reference states. We also present the first correlated study of the spin–orbit-induced contributions to shielding tensors arising from the magnetic field dependence of the spin–orbit Hamiltonian. While the terms usually considered are formally calculated using third-order perturbation theory, the magnetic-field dependent spin-orbit Hamiltonian requires a second-order calculation only. For the hydrogen chalcogenides, we show that contributions often neglected in studies of spin–orbit effects on nuclear shieldings, the spin-dipole coupling mechanism and the coupling of the two-electron spin–orbit Hamiltonian to the Fermi-contact operator, are important for the spin–orbit effect on the heavy-atom shielding, adding up to about half the value of the one-electron spin–orbit interaction with the Fermi-contact contribution. Whereas the second-order spin-orbit-induced shieldings of light ligands are small, the effect is larger for the heavy nuclei themselves and of opposite sign compared to the third-order contribution.
Topological analysis of the molecular electrostatic potential
Authors: Leboeuf Martin , Köster Andreas M. , Jug Karl , Salahub Dennis R.
Published: July 2002

The topology of the molecular electrostatic potential of 18 molecules, calculated in the framework of Kohn–Sham density functional theory, is studied. For the location of the different kinds of critical points a newly developed search algorithm is applied. A chemical interpretation of the critical points in terms of lone pairs, π bonds, hybrid orbitals and other electronic structure elements is suggested. A Poincaré–Hopf relationship for the molecular electrostatic potential is derived, connecting electronic structure elements and electrostatic reactivity via the topology of the molecular electrostatic potential.
Raman-spectroscopy of oligomeric SiO species isolated in solid methane
Authors: Friesen Markus , Junker Markus , Zumbusch Andreas , Schnöckel Hansgeorg
Published: July 2002

From the IR-spectra of matrix isolated SiO species a D2h-structure has been postulated for the dimer and a D3h-structure for the trimer. High quality Raman-spectra—necessary for the complete characterization—were missing so far. Here we report the Raman-spectra especially of the totally symmetric vibrations for Si2O2 and Si3O3 and their O16/18O isotopomers isolated in solid methane. We also detect the most intense A1-vibration of Si4O4 and can assign it with its O16/18O isotopic splitting. Ab initio calculations for all oligomers are presented in order to support the assignment of the spectra and to obtain geometric and energetic information about the oligomeric species which have been detected experimentally.
Quantum calculations on the vibrational predissociation of NeBr2: Evidence for continuum resonances
Authors: Stephenson Thomas A. , Halberstadt Nadine
Published: July 2002

Quantum mechanical calculations on the vibrational predissociation dynamics of NeBr2 in the B electronic state have been performed and the results compared with both experimental data and other computational studies. For vibrational levels with v⩽20 we find that the vibrational state dependence of the predissociation lifetimes is in qualitative agreement with experimental measurements, as are the calculated Br2 fragment rotational distributions. For higher vibrational levels, the B←X excitation profiles are well represented by a sum of two Lorentzian line shapes. We attribute this result to the presence of long-lived resonances in the dissociative continuum that are reminiscent of long-lived dissociative trajectories in previous classical studies of NeBr2.
A theoretical study of the 1B2u and 1B1u vibronic bands in benzene
Authors: Bernhardsson Anders , Forsberg Niclas , Malmqvist Per-Åke , Roos Björn O. , Serrano-Andrés Luis
Published: July 2002

The two lowest bands, 1B2u and 1B1u, of the electronic spectrum of the benzene molecule have been studied theoretically using a new method to compute vibronic excitation energies and intensities. The complete active space (CAS) self-contained field (SCF) method (with six active π-orbitals) was used to compute harmonic force field for the ground state and the 1B2u and 1B1u electronic states. A linear approximation has been used for the transition dipole as a function of the nuclear displacement coordinates. Derivatives of the transition dipole were computed using a variant of the CASSCF state interaction method. Multiconfigurational second-order perturbation theory (CASPT2) was used to obtain absolute excitation energies (12 active π-orbitals). The results show that the approach works well. Vibrational progressions are well described in both bands and intensities, and energies are in agreement with experiment, in particular when CASPT2 derived geometries are used. One interesting result is that computed vertical energies fall about 0.1 eV on the high energy side of the band maximum.
Anisotropic spheres admitting a one-parameter group of conformal motions
Authors: Herrera L. , Ponce de León J.
Published: July 2002

The Einstein equations for spherically symmetric distributions of anisotropic matter (principal stresses unequal), are solved, assuming the existence of a one-parameter group of conformal motions. All solutions can be matched with the Schwarzschild exterior metric on the boundary of matter. Two families of solutions represent, respectively, expanding and contracting spheres which asymptotically tend to a static sphere with a surface potential equal to (1)/(3) . A third family of solutions describes ‘‘oscillating black holes.’’ All solutions possess a positive energy density larger than the stresses everywhere.
On the Kolmogorov’s condition for a special case of the Kirchhoff top
Authors: Christov Ognyan
Published: July 2002

In this article the classical Kirchhoff case of motion of a rigid body in an infinite ideal fluid is considered. Then for the corresponding Hamiltonian system on the zero integral level, the Kolmogorov’s condition which is important for Kolmogorov–Arnold–Moser theory is checked. In contrast to known similar results, there exists a curve in the bifurcation diagram along which the Kolmogorov’s condition vanishes for certain values of the parameters.
On the Casimir energy for a 2N-piece relativistic string
Authors: Brevik I. , Sollie R.
Published: July 2002

The Casimir energy for the transverse oscillations of a piecewise uniform closed string is calculated. The string consists of 2N pieces of equal length, of alternating type I and type II material, and is taken to be relativistic in the sense that the velocity of sound always equals the velocity of light. By means of a new recursion formula we manage to calculate the Casimir energy for arbitrary integers N. Agreement with results obtained in earlier works on the string is found in all special cases. As basic regularization method we use the contour integration method. As a check, agreement is found with results obtained from the ζ function method (the Hurwitz function) in the case of low N(N=1–4). The Casimir energy is generally negative, and the more so the larger is the value of N. We illustrate the results graphically in some cases. The generalization to finite temperature theory is also given.
Effect algebras and statistical physical theories
Authors: Beltrametti E. G. , Bugajski S.
Published: July 2002

The dichotomic physical quantities of a physical system can be naturally hosted in a mathematical structure, called effect algebra, of which orthomodular posets and Boolean algebras are particular examples. We examine how effect algebras arise inside statistical physical theories and, conversely, we study to what extent an effect algebra can be taken as a primitive structure on which a satisfactory statistical physical model equipped with a convex set of states can be constructed.
Majorana spinors on unoriented surfaces
Authors: Kalogeropoulos Nikolaos
Published: July 2002

In this paper we find the conditions for the existence of Majorana spinors on unoriented surfaces. We prove that a Majorana spinor exists on these surfaces if and only if its genus is an odd integer. We enumerate the different real spin structures and we comment on their parity classification. Potentially interesting applications are pointed out.
Singular boundary perturbations for some eigenvalue problems
Authors: Zhevandrov P. , Alcántar E.
Published: July 2002

By means of two examples arising from physics we show that in contrast to a small perturbation of a regular boundary point, a small displacement of a singular boundary is singular in the sense that the expansions of the perturbed eigenvalues contain not only the integer powers of the small parameter involved, but also powers of the logarithm of this parameter. Examples considered are the Schrödinger equation for a hydrogenlike atom with nucleus of finite small size and the linearized shallow-water equation describing water waves trapped by a sloping beach.
Twist fields and broken supersymmetry
Authors: Grandjean Olivier , Jaffe Arthur
Published: July 2002

A twist field on a cylindrical space–time has the defining property that translation about a spatial circle results in multiplying the field by a phase. In this paper we investigate how such multivalued twist fields fit into the framework of constructive quantum field theory. Twisted theories have an interest in their own right; the twists also serve as infrared regulators that partially preserve the underlying symmetries of the Hamiltonian. The main focus of this paper is to investigate the extent that boson–fermion twist-field systems are compatible with the Lie symmetry and with the N=2 supersymmetry that one expects in the same examples without twists. We consider free systems and nonlinear boson–fermion interactions that arise from a holomorphic, quasihomogeneous, polynomial superpotential. We choose the twisting angles to lie on a chosen line in twist parameter space (leaving one free twist parameter). Doing this, we can obtain Lie symmetry and half the number of supersymmetry generators that one expects in our examples without the twists. We also show that the Hamiltonians for scalar twist fields yield twisted, positive-temperature expectations with the “twist-positivity” property. This is important because it justifies the existence of a functional integral representation for twisted, positive-temperature trace functionals. We regularize these systems in a way that preserves symmetry to the maximal extent. We pursue elsewhere other aspects and applications of this method, including bounding the extent of supersymmetry breaking.
Constructive field theory and applications: Perspectives and open problems
Authors: Rivasseau V.
Published: July 2002

In this paper we review many interesting open problems in mathematical physics which may be attacked with the help of tools from constructive field theory. They could give work for future mathematical physicists trained with constructive methods well into the 21st century.
Critical properties of the model of antiferromagnet Cr2O3
Authors: Murtazaev A. K.
Published: July 2002

The critical properties of the model of real antiferromagnet Cr2O3 are investigated by the Monte Carlo method. The static critical exponents α, β, and γ are calculated for systems with periodic boundary conditions containing from 500 to 4000 spins. It is shown that the values of critical exponents, the type of critical behavior, and crossover effects are determined to a considerable extent by the relation between the interaction and anisotropy constants. The results are compared with the theoretical data and with the results of laboratory investigations of Cr2O3. The critical parameters calculated by approximating the data by traditional exponential functions are compared with the values obtained on the basis of the finite-size scaling theory.
Effect of plastic deformation on the shape and parameters of the low-temperature peak of internal friction in niobium
Authors: Natsik V. D. , Pal-Val P. P. , Pal-Val L. N. , Semerenko Yu. A.
Published: July 2002

Temperature dependences of the decrement of longitudinal vibrations are studied at moderately low temperatures in polycrystalline niobium with the residual resistivity ratio RRR=60 at frequencies 78 and 363 kHz. A peak of internal friction is detected in the vicinity of 200 K. The height, width, and temperature of the peak change significantly upon a variation of vibrational frequency and as a result of changes in the defect structure of the sample under thermocycling, plastic deformation, or prolonged low-temperature recovery. It is shown that the absorption peak is due to the interaction of elastic vibrations of the sample with a system of identical thermally activated relaxators with an activation energy of 0.15 eV and an attack frequency of 1×1010 s−1 in a nearly perfect crystal. A theory is proposed for describing the variation of the shape and parameters of the internal friction peak due to statistical dispersion of the values of activation energy of the relaxators.
Localization of vibrations near impurity atoms in semi-infinite and infinite linear chains
Authors: Mamalui M. A. , Syrkin E. S. , Fedosev S. B.
Published: July 2002

Exact analytic expressions for local densities of atoms of the system as functions of the distance to a defect as well as defect parameters (variations of mass and force constants) are derived for an semi-infinite linear chain with an impurity at the end as well as for an infinite linear chain with an interstitial or substitutional impurity (a triatomic defect cluster in the general case). The dependences of the conditions of formation and frequencies of local vibrations and of the amplitudes of vibrations of these frequencies (i.e., the form of their damping) on defect parameters are determined for different atoms of the system. Exact analytic expressions determining the threshold values of the defect parameters as functions of the distance between and impurity atom and the end of the chain are derived and analyzed.
Proximity phenomena in double-barrier structure NbZr/NbOx/Al/AlOy/NbZr
Authors: Plecenik A. , Gaži Š. , Zuzčak M. , Beňačka Š. , Shaternik V. , Rudenko E.
Published: July 2002

A tunneling structure NbZr/NbOx/Al/AlOy/NbZr with a thin barrier in the NbZr/NbOx/Al junction and 4 to 6-nm-thick Al interlayer was prepared and studied experimentally. A proximity effect between NbZr and Al through NbOx barrier has been observed. An electrical voltage was generated in the NbOx barrier and a coexistence of the proximity effect and applied voltage in the junction NbZr/NbOx/Al has been observed. This experiment could be described on the basis of a model for coherent charge transport in superconducting/normal proximity structures.
On the mechanism of transformation of icosahedral rare-gas clusters into fcc aggregations
Authors: Kovalenko S. I. , Solnyshkin D. D. , Verkhovtseva É. T.
Published: July 2002

The experimental diffraction patterns from Kr cluster beams are compared with the diffraction functions calculated for rare-gas clusters with dimensions of 3×103 and 1×104 atoms/cluster. The experimental results are found to correlate well with the calculation if it is assumed that the clusters have a face-centered cubic structure with a constant number of intersecting stacking faults. Additional confirmation is obtained for the decisive role of the kinetic factor in the formation of the crystalline phase of the clusters. Conjectures are offered concerning the possible reasons why the densitometer traces presented by M-F. de Feraudy, G. Torchet, and B. W. van de Wall at the conference ISSPIC (1998) showed no substantial changes when the cluster size was increased by more than an order of magnitude.
Flow similitude and vortex lock-on in bluff body near wakes
Authors: Griffin Owen M.
Published: July 2002

Some years ago a universal Strouhal number for bluff body wakes was introduced, which was based upon the Strouhal frequency fs0 of the vortex shedding, the measured wake width d′ at the end of the vortex formation region, and the mean velocity Ub at the edge of the separated boundary layer on the body. This universal parameter was shown to collapse these characteristic scales of bluff body flows onto a single curve for wake Reynolds numbers between 200 and 107. The results of more recent experiments show the concept of universal wake similitude to be even more general than was previously supposed. In this paper these similarity relationships are applied to the case of vortex lock-on in oscillatory flow. A full understanding of the macroscopic averaged properties in the wake in this traditional manner is an important precursor to studying, in detail, the microscale wake properties using the most modern experimental measurements and diagnostic techniques and theoretical concepts such as the absolute-convective stability of shear flows.
Correspondence principle for turbulence: Application to the Chicago experiments on high Rayleigh number Bénard convection
Authors: Yakhot Victor
Published: July 2002

The correspondence principle postulated for the description of hydrodynamic turbulence [Phys. Rev. Lett. 57, 1722 (1986)] combined with the theory of thermal boundary layer [B. Castaing et al. (private communication)] is applied to high Rayleigh number convection in a Bénard cell. Quantitative interpretation of recent experimental data [B. Castaing et al. (private communication)] is presented. The predicted intermittency exponent following from comparison of the theory with experiment is 0.175<μ<0.275. A crucial experimental test of the renormalization group theory of turbulence is proposed.
Static shape and instability of an acoustically levitated liquid drop
Authors: Lee C. P. , Anilkumar A. V. , Wang T. G.
Published: July 2002

There have been observations that an intense sound field can break up a liquid drop in levitation by flattening it drastically through radiation pressure. Using high-speed photography, it is observed that, for a low-viscosity liquid, at a high sound intensity, ripples appear on the central membrane of the drop. At a higher intensity, the membrane may atomize by emitting satellite drops from its unstable ripples. For a general viscosity, it might also buckle upward like an umbrella and shatter, or might simply expand horizontally like a sheet and shatter. Using a disklike model for the flattened drop, the phenomenon was studied and good qualitative agreement with the observations was found. It is believed that at low viscosity, the ripples are capillary waves generated by the parametric instability excited by the membrane vibration, which is driven by the sound pressure. Atomization occurs whenever the membrane becomes so thin that the vibration is sufficiently intense. For any viscosity, the vibration leads to a Bernoulli correction in the static pressure, which is destabilizing. Buckling occurs when an existent equilibrium is unstable to a radial oscillation of the membrane because of the Bernoulli effect. Besides, the radiation stress at the rim of the flattened drop, being a suction stress, is also destabilizing, leading to the horizontal expansion and the subsequent breakup.
Structure in turbulent thermal convection
Authors: Balachandar S.
Published: July 2002

Small-scale features of vorticity, strain rate, and temperature gradients are considered in a Rayleigh–Bénard convection. The results reported are from a direct numerical simulation of turbulent convection performed in a rectangular box of aspect ratio 2√2 at a Rayleigh number of 6.5×106 and a Prandtl number of 0.72. In agreement with earlier results [Ashurst et al., Phys. Fluids 30, 2343 (1987) and Ruetsch and Maxey, Phys. Fluids A 3, 1587 (1991)], the intermediate strain rate is on an average positive, but the ratio of alpha, beta, and gamma strain rates are measured to be 5.3:1.0:−6.3. This result differs from the earlier result of 3:1:−4 obtained in homogeneous isotropic and shear turbulences. Buoyancy-induced vorticity production makes significant contribution to the overall enstrophy balance, especially close to the boundaries. Vorticity production by buoyancy is exclusively in the horizontal direction and is balanced by preferred production by stretching and tilting in the vertical direction, due to the preferred alignment of extensional alpha strain rate with the vertical direction. Such directional alignment of vorticity, strain rate, and scalar gradient is explained on the basis of preferred spatial orientation of coherent structures in thermal turbulence.
The effect of Reynolds number on vortex asymmetry about slender bodies
Authors: Bernhardt J. E. , Williams D. R.
Published: July 2002

The response of the sectional side force coefficient to localized suction has been studied on a slender body placed at 55° angle of attack. A strong dependence in the behavior of the asymmetric vortex system on Reynolds number is shown, which resolves discrepancies observed by different investigators.
Mode conversion and dissipation of the fast Alfvén wave in ion cyclotron heating
Authors: Cairns R. A. , Fuchs V.
Published: July 2002

Recent work on fast-wave propagation in the ion cyclotron range of frequencies has shown that the fast-wave transmission and reflection coefficients can be obtained from a second-order equation in which the ion Bernstein mode is treated as a driven response to the fast wave. Although this yields the total power lost from the fast wave, it does not enable one to say how much power is damped locally because of cyclotron damping and how much propagates away in the Bernstein mode. In the present paper attention is turned to this problem and a coupling model is developed in which a Bernstein wave-energy equation is derived and the power absorbed from the fast wave is used as a source for it. Information about the Bernstein wave amplitude is thus lost, but for both low- and high-field incidence it is shown how the problem of fast-wave and Bernstein wave-energy propagation can be treated by a series of equations of no higher than second order that pose no numerical problems and can be integrated together as a single initial value problem. This yields a simple and efficient numerical technique, the results of which compare well with more complicated procedures.
An integral equation formulation of electromagnetic modes in finite ion temperature plasmas: Application to the collisionless tearing mode
Authors: Kim Jin-Yong , Hong Bong-Gun , Choi Duk-In
Published: July 2002

The set of coupled integrodifferential equations describing the low frequency electromagnetic perturbation in a finite ion temperature plasma of slab geometry is reduced to a single simple integral equation. The equation is described in terms of a localized variable, and the boundary condition is clear and easy to implement, particularly for the tearing mode. As an application of the formalism, collisionless tearing modes are studied numerically, and the results are compared with previous analytic works.
Computer simulations of finite plasmas convected across magnetized plasmas
Authors: Galvez Miguel , Gisler Galen , Barnes Christopher
Published: July 2002

A two-dimensional electrostatic particle-in-cell code is used to simulate the convection of a plasma stream across a uniform magnetized plasma in the plane perpendicular to a given magnetic field B. These simulations show that the stream is convected by the E×B drift, where E is the electric field produced by polarization charge layers at the edges of the stream. The E×B velocity decreases as the background plasma density increases and with the distance traveled by the stream. This slowing of the stream is due to a short circuiting of the electric field by the background plasma within the stream that also polarizes and E×B drifts with the plasma stream. The simulations also show that the short circuiting of the electric field reduces the erosion of the plasma stream and the background plasma outside the stream is pushed away from the stream path by the fringe electric field.
Some recent developments in geodynamo theory
Authors: Braginsky S. I.
Published: July 2002

A geodynamo of model-Z type is presented. A characteristic feature of such a model is that, even if core–mantle coupling is weak, it has a significant effect. A nonstationary solution was obtained in which the axial dipole strength varies periodically. By comparing the period of this oscillation with the archeomagnetically observed period, the electrical conductivity of the Earth’s core was estimated.
Fast electron transport in lower-hybrid current drive
Authors: Kupfer K. , Bers A.
Published: July 2002

The quasilinear Fokker–Planck formulation is generalized for lower-hybrid current drive to include the wave-induced radial transport of fast electrons. Toroidal ray tracing shows that the wave fields in the plasma develop a large poloidal component associated with the upshift in k∥ and the filling of the ‘‘spectral gap.’’ These fields lead to an enhanced radial E×B drift of resonant electrons. Two types of radial flows are obtained: an outward convective flow driven by the asymmetry in the poloidal wave spectrum, and a diffusive flow proportional to the width of the poloidal spectrum. Numerical results relevant to Alcator C [Phys. Rev. Lett. 53, 450 (1984)] and JT-60 [Nucl. Fusion 29, 1815 (1989)] are presented; they show that the radial convection velocity has a broad maximum of nearly 1 m/sec and is independent of the amplitude of fields. In both cases, the radial diffusion is found to be highly localized near the magnetic axis. For JT-60, the peak of the diffusion profile can be quite large, nearly 1 m2/sec.
Slow equations of motion for gyrotron and peniotron interactions
Authors: Brand G. F.
Published: July 2002

In this paper, a new and more general set of slow equations of motion for computing gyrotron (including high-harmonic gyrotron) and peniotron (including both peniotrons, where the orbit is centered on the axis and gyropeniotrons) interactions is derived. These slow equations are convenient to use and allow the interactions to be investigated numerically. They reduce to the correct expressions in the small-signal limit to yield starting currents. Small-signal and large-signal calculations yield results that agree with those of other workers. The equations are used to investigate the gyropeniotron.
Far field and energy flux caused by a radiating source in an anisotropic medium
Authors: Lai H. M. , Chan Pui King
Published: July 2002

The Lighthill–Giles [Philos. Trans. R. Soc. London Ser. A 252, 397 (1960); J. Plasma Phys. 19, 201 (1978)] method is extended to evaluate the far field resulting from a moving radiating source in an anisotropic and dispersive medium, and the result thus obtained has a retardation interpretation. The radiation energy flux derived from the J⋅E method, only after a modification resulting from anisotropy of the medium and a correction factor resulting from retardation, is found to be compatible with that directly calculated from the far field, provided interference may be neglected. While the interference, which arises from the mixed product of any two far-field terms of different wave vectors, is demonstrated to be completely null in a uniaxial medium for a stationary source, it is shown to exist in a gyrotropic magnetoplasma and the total Poynting vector could be substantially nonradial in certain special cases.
Hot-electron instability in mirror geometry
Authors: Lichtenberg A. J. , Meuth H.
Published: July 2002

Plasmas with variable ratios of hot electrons to colder background plasma, but without significant neutral gas, are investigated in the multiple mirror experiment (MMX) [Phys. Fluids 29, 1208 (1986)]. The device permits, sequentially, the injection of plasma (H2 or He) into a stable mirror cell, and electron-cyclotron resonance heating (ECRH) to an electron temperature of several keV. A second source then provides the cold-electron component with arbitrary nc/nh≤50. At any point in time, the field coils that provide magnetohydrodynamic (MHD) stability for the hot electrons may be pulsed off for times short in comparison to the time required for cold plasma density to change. Instantaneous destabilization, even for plasma parameters well in excess of the value of nc/nh for which a hot-electron drift approximation would predict stability, was observed. This result suggests that a mechanism in addition to a cold plasma background is required to stabilize a hot-electron distribution. Comparison is made with recent theoretical work.
Anomalous current penetration and oscillating current drive in tokamaks
Authors: Finn John M. , Antonsen Thomas M.
Published: July 2002

A current drive scheme for tokamaks, involving oscillating the Ohmic heating (OH) circuit, is proposed. This scheme is based upon the experimentally observed anomalous skin penetration during the start-up phase of tokamaks. Analytic and numerical results indicating feasibility are shown and scaling of steady-state current and power dissipated with frequency and plasma resistivity is presented. An explanation is given of why this scheme should work, in spite of the fact that the external circuit supplies no magnetic helicity. This scheme is potentially more efficient in terms of current generated per unit power dissipated than other current drive schemes. It is shown that a resistive magnetohydrodynamic (MHD) activity responsible for anomalous current penetration, and therefore required for the success of this current drive scheme, must be fully three dimensional. Finally, a comparison of the effectiveness of this scheme with that of F-Θ pumping in reversed-field pinches (RFP’s) is presented.
Eigenmode electric field profiles in cylindrical plasmas
Authors: Litwin C. , Hershkowitz N.
Published: July 2002

Electric field profiles of plasma column eigenmodes in the ion-cyclotron range of frequencies are discussed. Step and parabolic density profiles are compared. The role of temperature and Alfvén resonance is analyzed.
Collisionless electron temperature gradient instability
Authors: Lee Y. C. , Dong J. Q. , Guzdar P. N. , Liu C. S.
Published: July 2002

The stability of electron temperature gradient driven modes in a collisionless plasma with magnetic shear has been studied. Two coupled integral equations for the electrostatic and vector potentials have been solved numerically. The properties of these modes derived on the basis of numerical investigation have been utilized to estimate the anomalous electron transport induced by them.
Higher harmonic generation in the induced resonance electron cyclotron maser
Authors: Riyopoulos S. , Tang C. M. , Sprangle P. , Levush B.
Published: July 2002

Operation of the induced resonance electron cyclotron (IREC) maser [Appl. Phys. Lett. 49, 1154 (1986); Nucl. Instrum. Methods Phys. A 250, 36 (1986)] at Doppler upshifted cyclotron harmonics is studied. A set of fast-time averaged nonlinear equations of motion is derived for the particle motion near an arbitrary harmonic at any index of refraction. The small signal efficiency is computed analytically and the minimum current to start the cavity oscillations is obtained. The nonlinear equations of motion are integrated numerically. The interaction efficiency at the first few harmonics is found to be comparable to the efficiency at the fundamental. The sensitivity of the efficiency to beam thermal spreads is minimized by proper selection of the index of refraction.
Comment on ‘‘Plasma kinetic theory in action-angle variables’’ [Phys. Fluids 2
8, 3538 (1985)]
Authors: Cary John R. , Wong H. Vernon , Mahajan S. M.
Published: July 2002

Kaufman’s formula [Phys. Fluids 15, 1063 (1972)] for the first-order distribution function obtained via the action-angle formalism is shown to be correct. The proof requires that it be noted that the transformation from canonical coordinates to physical variables has terms of all orders. Neglect of this fact has led to an incorrect formula for the linear susceptibility in previous work.
Comments on the numerical investigation of Rayleigh and Marangoni convection in a vertical circular cylinder
Authors: Wagner Claus , Friedrich Rainer , Narayanan Ranga
Published: July 2002

Convection in a cylindrical container was simulated with a three-dimensional, time-dependent code. For the case of purely Rayleigh convection, a completely rigid cylinder with adiabatic vertical walls and conducting horizontal walls was considered. The calculations showed that the individual velocity components could be in a transient state, while the total heat transfer was steady. This occurred in cases where the maximum azimuthal component of velocity was very small in magnitude, in comparison to the other two components, and this component decreased in time. On the other hand, the total kinetic energy along with the heat transfer reached a steady value. Consequently the present results have been shown to be at variance with the calculations of Neumann [J. Fluid Mech. 214, 559 (1990)]. Marangoni convection was modeled with a free flat surface on the upper side, assuming the superimposed second layer to be passive. The numerically obtained critical Marangoni numbers and flow patterns were compared favorably to earlier results from linearized stability. In addition, flow structural changes for supercritical Marangoni numbers were illustrated. Interestingly, axisymmetric disturbances led to nonsymmetric bifurcation diagrams, but three-dimensional disturbance calculations led to symmetry in the bifurcation plots. Another very interesting result was the observed transition from three-dimensional to two-dimensional patterns as the Marangoni number was increased. Large computational requirements precluded a detailed parametric study. The special case of Prandtl number equal to 6.7 (corresponding to water), and in the case of Marangoni convection, a surface Biot number of unity was assumed.
The structure of the axisymmetric high-Reynolds number flow around an ellipsoidal bubble of fixed shape
Authors: Blanco Armando , Magnaudet Jacques
Published: July 2002

The structure of the flow around an oblate ellipsoidal bubble of fixed shape is studied by means of direct numerical simulation for Reynolds numbers Re up to 103. In agreement with a previous study by Dandy and Leal [Phys. Fluids 29, 1360 (1986)] the computations demonstrate that if the bubble aspect ratio χ is high enough a standing eddy can exist at the rear of the bubble in an intermediate range of Re. This eddy disappears beyond a certain Reynolds number and it is shown that its existence is governed by the competition between accumulation and evacuation of the vorticity in the flow. The range of Re where the eddy exists increases very rapidly with χ meaning that this structure is certainly present in many experimental situations. The evolution of the drag coefficient with Re reveals that the oblateness has a dramatic influence on the minimum value of Re beyond which Moore’s theory [J. Fluid Mech. 23, 749 (1965)] can be used to predict the rise velocity of a bubble of fixed shape. In contrast, owing to the shape of the vorticity distribution at the surface of the bubble, no noticeable influence of the standing eddy on the drag is found. A quantitative comparison between the present results and those of previous authors shows that the computational description of the boundary layer around curved free surfaces is not a trivial matter since a strong influence of the numerical method is observed.
Linear bioconvection in a suspension of randomly swimming, gyrotactic micro-organisms
Authors: Bees M. A. , Hill N. A.
Published: July 2002

We have analyzed the initiation of pattern formation in a layer of finite depth for Pedley and Kessler’s new model [J. Fluid Mech. 212, 155 (1990)] of bioconvection. This is the first analysis of bioconvection in a realistic geometry using a model that deals with random swimming in a rational manner. We have considered the effects of a distribution of swimming speeds, which has not previously received attention in theoretical papers and find that it is important in calculating the diffusivity. Our predictions of initial pattern wavelengths are reasonably close to the observed ones but better experimental measurements of key parameters are needed for a proper comparison.
Direct excitation of small-scale motions in free shear flows
Authors: Wiltse John M. , Glezer Ari
Published: July 2002

The conventional approach to small-scale mixing enhancement in free shear flows by the manipulation of global flow instabilities and the ensuing large-scale vortical structures depends on the classical cascading mechanism to transfer the control influence to the scales at which molecular mixing takes place. Thus the manipulation of mixing at the smallest scales is indirect and only weakly coupled to the control input. The present work focuses on direct excitation of the small scales within the dissipation range of a free shear flow. This approach is demonstrated in a shear layer segment of an air jet emanating from a square conduit. The flow is forced at a frequency that is approximately an order of magnitude lower than the passage frequency of eddies at the Kolmogorov scale using cantilevered piezoelectric actuators. Cross-stream distributions of the streamwise velocity component are measured at a number of streamwise stations downstream of the actuator using hot wire anemometry. Direct small scale excitation results in enhanced energy transfer from the large to the small scales and in a substantial increase in the dissipation and in the decay rate of turbulent kinetic energy.
Inviscid instability of compressible swirling mixing layers
Authors: Lu Ganyu , Lele Sanjiva K.
Published: July 2002

In this paper, we examine the inviscid instability of a swirling mixing layer, a circular mixing layer with an additional swirl component which is present only within the shear layer. The mean flow is solved numerically using the boundary layer equations in cylindrical coordinates. Spatial inviscid stability characteristics of the base flow are studied. The results show that swirl significantly enhances the maximum amplification rate for incompressible mixing layers. This enhancement sustains under the compressible conditions. For swirling mixing layers, a significant amount of disturbance energy is extracted from the shear in the swirl component.
Firespread through fuel beds: Modeling of wind-aided fires and induced hydrodynamics
Authors: Porterie B. , Morvan D. , Loraud J. C. , Larini M.
Published: July 2002

The propagation of wind-aided line fires through fuel beds is simulated by using a multiphase approach. In this approach, a gas phase flows through N solid phases which constitute an idealized reproduction of the heterogeneous combustible medium. A set of time-dependent equations is obtained for each phase and the coupling between the gas phase and the solid phases is rendered through exchange terms of mass, momentum, and energy. Turbulence is approached by using a RNG k−ε statistical model constructed from the Favre averaging method. The radiative transfer equation extended to multiphase media is solved using the discrete ordinates method (DOM). Soot formation is taken into account for the evaluation of the absorption coefficient of the soot/fuel particles/combustion products mixture using the gray gas assumption. First-order kinetics is incorporated to describe water vaporization, pyrolysis, and char combustion processes. The solution is performed numerically by a finite-volume method including a high-order upwind convective scheme and a flux limiter strategy along with a projection method for the pressure–velocity coupling. This model has been applied to describe the unsteady behavior of wind-aided fires spreading through a litter of dead pine needles and the induced hydrodynamics. The numerical results obtained from our model are presented and compared to measurements and predictions from other laboratory-based models.
High confinement and high density with stationary plasma energy and strong edge radiation cooling in the upgraded Torus Experiment for Technology Oriented Research (TEXTOR-94)
Authors: Messiaen A. M. , Ongena J. , Unterberg B. , Boedo J. , Fuchs G. , Jaspers R. , Konen L. , Koslowski H. R. , Mank G. , Rapp J. , Samm U. , Vandenplas P. E. , Van Oost G. , Van Wassenhove G. , Waidmann G. , Weynants R. R. , Wolf G. H. , Bertschinger G. , Bonheure G. , Brix M. , Dumortier P. , Durodie F. , Finken K. H. , Giesen B. , Hillis D. , Hutteman P. , Koch R. , Kramer-Flecken A. , Lyssoivan A. , Mertens Ph. , Pospieszczyk A. , Post-Zwicker A. , Sauer M. , Schweer B. , Schwelberger J. , Telesca G. , Tokar M. Z. , Uhlemann R. , Vervier M. , Winter J.
Published: July 2002

An overview of the results obtained so far for the radiative I-mode regime on the upgraded Torus Experiment for Technology Oriented Research (TEXTOR-94) [Proceedings of the 16th IEEE Symposium on Fusion Engineering (Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1995), Vol. 1, p. 470] is given. This regime is obtained under quasistationary conditions with edge neon seeding in a pumped limiter tokamak with circular cross section. It combines high confinement and high β (up to a normalized beta, βn=2) with low edge q values (down to qa=2.8) and high density even above the Greenwald limit together with dominant edge radiative heat exhaust, and therefore shows promise for the future of fusion research. Bulk and edge properties of these discharges are described, and a detailed account is given of the energy and particle confinement and their scaling. Energy confinement scales linearly with density as for the nonsaturated Ohmic Neo-Alcator scaling, but the usual degradation with total power remains. No deleterious effects of the neon seeding on fusion reactivity and plasma stability have been observed.
Nonequilibrium and current sheet formation in line-tied magnetic fields
Authors: Ng C. S. , Bhattacharjee A.
Published: July 2002

Parker’s model of coronal heating [E. N. Parker, Astrophys. J. 174, 499 (1972)] is considered within the framework of ideal reduced magnetohydrodynamics. It is shown that there can be at most one smooth magnetostatic equilibrium for a given smooth footpoint mapping between two end plates to which field lines are line-tied. If such a smooth equilibrium is deformed continuously by further footpoint motion so that it becomes unstable, there is no other smooth equilibrium for the plasma to relax to, and the system tends to a nonequilibrium state containing singular currents (“current sheets”). It is shown that this process can occur as the system relaxes asymptotically to a state of minimum energy (possibly in infinite time). Numerical simulations that begin from smooth initial conditions containing current layers are presented. As the current layers become increasingly intense due to footpoint motion and eventually cross a threshold for instability, the magnetic relaxation observed in the simulation shows a tendency to form nonequilibrium states with current sheets. A necessary geometrical criterion that determines the sites of current sheet formation in models without nulls or closed field lines is given. According to this criterion, the rate of velocity amplification, analogous to the Lyapunov exponent in nonlinear dynamics, becomes unbounded at singularities.