bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular dynamics simulations of a fully hydrated dimyristoylphosphatidylcholine membrane in liquid-crystalline phase
Authors: Zubrzycki Igor Z. , Xu Yan , Madrid Marcela , Tang Pei
Published: July 2002

Molecular dynamics (MD) simulations were performed to investigate the structure of a fully hydrated 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayer in liquid-crystalline (fluid) phase at 30 °C. The bilayer consists of 200 DMPC lipid molecules with nw=27.4 water molecules per lipid. The membrane was built with reference to the coordinates of a previously published 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membrane patch. A four-step dynamic procedure (110 ps) with Berendsen pressure rescaling (P=0 and 1 bar), applied in all three directions, was used to rapidly prepare the bilayer. This system was then subjected to two separate constant pressure and temperature simulations at 1 bar and 30 °C for ∼380 ps, using the Nosé–Hoover NPT method with periodical boundaries and Berendsen temperature and pressure rescaling method, respectively. The resultant bilayer has an area per lipid of 59.2 Å2 and a head-to-head thickness (DHH) of 36.3 Å. These values are in good agreement with the x-ray diffraction data of 59.7 Å2 and 34.4 Å, respectively, for DMPC at 30 °C with nw of 25.7 [H. I. Petrache, S. Tristram-Nagle, and J. F. Nagle, Chem. Phys. Lipids 95, 83 (1998)]. The fractions of trans and gauche bonds in the hydrocarbon chains, averaged for the last 94 ps of simulation, are 81.7% and 18.3%, respectively, suggesting a fluid phase of the membrane. The electron density profile resembles closely that measured by x-ray diffraction. Water density profile suggests a significant penetration of water molecules into the bilayer head region to as deep as the carbonyl groups, with phosphate groups being strongly hydrated.
Measurement and analysis of molecular hyperpolarizability in the two-photon resonance regime
Authors: Berkovic G. , Meshulam G. , Kotler Z.
Published: July 2002

The frequency dependent hyperpolarizability of typical donor–acceptor organic nonlinear optical molecules is commonly represented by a nonresonant two-level model, first presented by Oudar and Chemla. We discuss how this model can be extended into the resonant regime, including cases where the molecular transition is described by an inhomogeneously broadened peak. A resonant measurement of hyperpolarizability by electric field induced second harmonic generation (EFISH) is demonstrated, as well as the more conventional off-resonance EFISH. The theoretical model correctly predicts both the amplitude and phase of the resonant hyperpolarizability measured by EFISH. We also show that both on-resonance and off-resonance EFISH yield the same hyperpolarizability extrapolated to the zero frequency limit.
Computing coupled-cluster wave functions with arbitrary excitations
Authors: Kállay Mihály , Surján Péter R.
Published: July 2002

An algorithm is presented for solving coupled-cluster (CC) equations by successive diagonalization of 2×2 matrices. It is more expensive than usual procedures, but it is capable of solving a CC problem where any arbitrary excitation is included in the cluster operator. Equation-of-motion coupled-cluster (EOMCC) excitation energies can also be determined by this method regardless of the type of excitations in the cluster operator and the space where the effective Hamiltonian is diagonalized. The algorithm is applied to the study of the convergence of CC and EOMCC series in some small bases.
The weak-Painlevé property as a criterion for the integrability of dynamical systems
Authors: Rañada A. F. , Ramani A. , Dorizzi B. , Grammaticos B.
Published: July 2002

We investigate the validity of the weak-Painlevé property as an integrability criterion. We present an example of a time-dependent Hamiltonian system which possesses a weak-Painlevé type expansion, while presenting a chaotic behavior. However, this system presents also critical fixed singularities. The importance of the latter, as far as integrability is concerned, is discussed here.
Discrete quantum mechanics
Authors: Gudder Stanley P.
Published: July 2002

A discrete model for quantum mechanics is presented. First a discrete phase space S is formed by coupling vertices and edges of a graph. The dynamics is developed by introducing paths or discrete trajectories in S. An amplitude function is used to compute probabilities of quantum events and a discrete Feynman path integral is presented. Many of the results can be formulated in terms of transition probabilities and unitary operators on a Hilbert space l2(S).
Cylindrical group and massless particles
Authors: Kim Y. S. , Wigner E. P.
Published: July 2002

It is shown that the representation of the E(2)-like little group for photons can be reduced to the coordinate transformation matrix of the cylindrical group, which describes movement of a point on a cylindrical surface. The cylindrical group is isomorphic to the two-dimensional Euclidean group. As in the case of E(2), the cylindrical group can be regarded as a contraction of the three-dimensional rotation group. It is pointed out that the E(2)-like little group is the Lorentz-boosted O(3)-like little group for massive particles in the infinite-momentum/zero-mass limit. This limiting process is shown to be identical to that of the contraction of O(3) to the cylindrical group. Gauge transformations for free massless particles can thus be regarded as Lorentz-boosted rotations.
The Pauli matrices in n dimensions and finest gradings of simple Lie algebras of type A
n−1
Authors: Patera J. , Zassenhaus H.
Published: July 2002

Properties of the Lie algebra gl(n,C) are described for a basis which is a generalization of the 2×2 Pauli matrices. The 3×3 case is described in detail. The remarkable properties of that basis are the grading of the Lie algebra it offers (each grading subspace is one dimensional) and the matrix group it generates [it is a finite group with the center of SL(n,C) as its commutator group].
Heat kernel expansion on a generalized cone
Authors: Dowker J. S.
Published: July 2002

It is shown that the short-time expansion of the integrated heat kernel on a locally flat generalized cone C(N), as defined by Cheeger, consists of just the Weyl volume term and the constant term. This latter is explicitly evaluated when N is a lens space, Sd/Zm (for odd d), elliptic space Sd/Z2 (for all d), and any of the three-dimensional, homogeneous space forms S3/Γ. Agreement is found with the corresponding expansion on the orbifold version, T4/Z2, of the K3 surface, and, in fact, with all TD/Z2.
Spherically symmetric static SU(2) Einstein–Yang–Mills fields
Authors: Künzle H. P. , Masood-ul-Alam A. K. M.
Published: July 2002

The discrete family of global solutions of the static spherically symmetric SU(2) Einstein–Yang–Mills equations that were recently numerically obtained by Bartnik and McKinnon [Phys. Rev. Lett. 61, 141 (1988)] is studied in greater detail, both numerically and analytically. A similar discrete sequence of numerical solutions outside a regular event horizon is shown to exist for every radius of the horizon.
Thomas–Fermi theory with an external magnetic field
Authors: Goldstein Jerome A. , Rieder Gisèle Ruiz
Published: July 2002

Of concern is a rigorous Thomas–Fermi theory of ground state electron densities for quantum mechanical systems in an external magnetic field. The energy functional takes the form ℰ(ρ1,ρ2)=∑2i=1ci ∫R3ρi (x)5/3 dx + (1)/(2) ∫R3∫R3[ρ(x)ρ(y)/‖x−y‖]dx dy +∫R3V(x)ρ(x)dx +∫R3(B(x)(ρ1(x)−ρ2(x))dx; here ci is a positive constant, ρ1 [resp. ρ2] is the density of spin-up [resp. spin-down] electrons, ρ=ρ1+ρ2 is the total electron density, V is a given potential (typically a Coulomb potential describing electron–nuclear attraction), and B describes the effect of the external magnetic field. Let Ni=∫R3ρi(x)dx be the number of spin-up and spin-down electrons for i=1,2, and let N=N1+N2 be the total number of electrons. Specifying N and minimizing ℰ(ρ1,ρ2) generally leads to a spin polarized system. For example, if B≤0 and B■0, then ρ1≥ρ2 and N1>N2. This and a number of related results are proved.
Affine conformal vectors in space-time
Authors: Coley A. A. , Tupper B. O. J.
Published: July 2002

All space-times admitting a proper affine conformal vector (ACV) are found. By using a theorem of Hall and da Costa, it is shown that such space-times either (i) admit a covariantly constant vector (timelike, spacelike, or null) and the ACV is the sum of a proper affine vector and a conformal Killing vector or (ii) the space-time is 2+2 decomposable, in which case it is shown that no ACV can exist (unless the space-time decomposes further). Furthermore, it is proved that all space-times admitting an ACV and a null covariantly constant vector (which are necessarily generalized pp-wave space-times) must have Ricci tensor of Segré type {2,(1,1)}. It follows that, among space-times admitting proper ACV, the Einstein static universe is the only perfect fluid space-time, there are no non-null Einstein–Maxwell space-times, and only the pp-wave space-times are representative of null Einstein–Maxwell solutions. Otherwise, the space-times can represent anisotropic fluids and viscous heat-conducting fluids, but only with restricted equations of state in each case.
Direct sum decompositions and indecomposable TQFTs
Authors: Sawin Stephen
Published: July 2002

The decomposition of an arbitrary axiomatic topological quantum field theory or TQFT into indecomposable theories is given. In particular, unitary TQFTs in arbitrary dimensions are shown to decompose into a sum of theories in which the Hilbert space of the sphere is one-dimensional, and indecomposable two-dimensional theories are classified.
Spectrum of the Dirac operator on Gr2(Cm+2)
Authors: Milhorat Jean-Louis
Published: July 2002

The spectrum of the Dirac operator, acting on the quaternion-Kähler spin symmetric space Gr2(Cm+2), is explicitly computed by harmonic analysis methods: in particular “branching rules” for irreducible representations of the Lie group SU(m+2) and its subgroup S(U(m)×U(2)), are given.
Principal vertex operator representations for toroidal Lie algebras
Authors: Billig Yuly
Published: July 2002

We introduce the principal vertex operator representations for the toroidal Lie algebras generalizing the construction for the affine Kac–Moody algebras. We also represent the derivations of the toroidal algebras and introduce analogs of the Sugawara operators.
Exact periodic solutions of the complex Ginzburg–Landau equation
Authors: Porubov A. V. , Velarde M. G.
Published: July 2002

Three new exact periodic solutions of the complex Ginzburg–Landau equation are obtained in terms of the Weierstrass elliptic function ℘. Furthermore, the new periodic solutions and other shock solutions appear as their bounded limits (along the real axis) for particular relationships between the coefficients in the equation. In the general case, bounded limits are nothing but the already known pulse, hole, and shock solutions. It is also shown that the shapes of the solutions are quite different from the shape of the usual envelope wave solution. In particular, the spatial structure of the new bounded periodic solutions varies in time, while the pulse solution may exhibit breather-like behavior.
Froude number dependence of the flow separation line on a sphere towed in a stratified fluid
Authors: Chomaz J. M. , Bonneton P. , Butet A. , Perrier M. , Hopfinger E. J.
Published: July 2002

In this paper experimental results on the near field of the flow past a sphere in a linearly stratified medium are presented. Emphasis is placed on the variation of the flow separation line with internal Froude number F=U/NR and also with Reynolds number Re=2RU/ν, where U and R are, respectively, the velocity and the radius of the sphere, N is the Brünt–Väisälä frequency (rad sec−1 ), and ν is the cinematic viscosity. It is shown that in the Reynolds number range 200<Re<30 000 the flow is primarily conditioned by the Froude number when F≤1. The condition F=1 defines a resonance state between the sphere and the internal wave field. In this case the waves create a strong depression behind the sphere that keeps the flow from separating. When F<0.8 the flow is two dimensional in a layer confined between the upper and the lower wave. When F>1.5 the flow starts to recover its three-dimensionality.
Relaxation in two dimensions and the ‘‘sinh-Poisson’’ equation
Authors: Montgomery D. , Matthaeus W. H. , Stribling W. T. , Martinez D. , Oughton S.
Published: July 2002

Long-time states of a turbulent, decaying, two-dimensional, Navier–Stokes flow are shown numerically to relax toward maximum-entropy configurations, as defined by the ‘‘sinh-Poisson’’ equation. The large-scale Reynolds number is about 14 000, the spatial resolution is (512)2, the boundary conditions are spatially periodic, and the evolution takes place over nearly 400 large-scale eddy-turnover times.
Stationary probability density functions: An exact result
Authors: Pope S. B. , Ching Emily S. C.
Published: July 2002

An exact expression is obtained for the probability density function (pdf) of any quantity measured in a general stationary process, in terms of conditional expectations of time derivatives of the signal. This expression indicates that the conditional expectations of both the time derivative squared and of the second time derivative influence the shape of the pdf, including its tails. A previous result of Ching [Phys. Rev. Lett. 70, 283 (1993)] for temperature measurements in turbulent flows corresponds to the particular case when the latter quantity is linear.
Efficiency of caviton formation as a function of plasma density gradient
Authors: Bauer B. S. , Wong A. Y. , Scurry L. , Decyk V. K.
Published: July 2002

The effect of a zeroth-order density gradient on the development of cavitons has been investigated experimentally and numerically. The cavitons were produced via excitation of electron plasma waves (EPW) with a modest (E2/4πnTe≪1) resonant radio-frequency pump. The location of the resonance, on an inverse-parabolic density profile, was varied, with all other parameters being held constant. The depth of the caviton, and the strength of its associated trapped electric fields, are found to depend strongly on the density gradient scale length at the critical layer, with a maximum occurring when this length is infinite, at the flat top of the density profile. The results are accounted for by the dependence on the density gradient of the EPW convection rate and wave-breaking time. The study helps illuminate recent large-scale ionospheric density modification experiments.
Electron–ion hybrid instabilities driven by velocity shear in a magnetized plasma
Authors: Romero H. , Ganguli G. , Lee Y. C. , Palmadesso P. J.
Published: July 2002

The stability of a magnetized plasma is investigated in which a sheared electron flow channel is present. The flow’s peak velocity and shear scale length are denoted by V and L, respectively. If the velocity channel is perpendicular to the confining magnetic field and L≤ ρi (ρi is the ion Larmor radius) an electrostatic instability develops whose frequency is on the order of the lower hybrid frequency. For V/(ΩeL) ≳ 0.02 (Ωe denotes the electron cyclotron frequency), the peak growth rate is on the order of the lower hybrid frequency when k∥ = 0 (in here, k∥ is the wave number along the magnetic field). For V/(ΩeL) ≳ 0.1 and k∥ = 0, the spectrum peaks when kyL ∼ 1, where ky is the wave number in the direction of the flow velocity. For this mode it is shown that (i) a net cross-field current is not required for the onset of instability and (ii) the growth rate is not reduced by a velocity profile with no net flow (spatially averaged). Hence we conclude that velocity shear is the only source of free energy. Further, it is shown that density gradients do not stabilize this mode. It follows that the mode presented in this work cannot be identified with the well-known modified two-stream instability. If the velocity channel is parallel to the confining magnetic field and the plasma is weakly magnetized, an instability driven by velocity shear is shown to exist, provided that V/(ωpeL) ≳ 0.32, where ωpe is the electron plasma frequency. It is shown that a net plasma current is not required in order for this instability to be excited.
Locking of kink modes in a reversed-field pinch
Authors: Greene Philip , Robertson Scott
Published: July 2002

Experiments have been performed on the Reversatron II [IEEE Trans. Plasma Sci. PS-16, 667 (1988)] reversed-field pinch to study phase locking of kink modes in discharges with thick and thin shells. In the thick shell discharges, internal modes which are resonant or nearly resonant on axis are found to phase lock at random locations in toroidal angle. Externally nonresonant modes which lock tend to do so at the shell gap. In the thin shell discharges, the internally resonant modes and the externally nonresonant modes both tend to lock at the shell gap. Phase locking is not observed in every shot and the duration of the locking varies. The position of locking does not rotate.
Anomalous momentum transport from drift wave turbulence
Authors: Dominguez R. R. , Staebler G. M.
Published: July 2002

A sheared slab magnetic field model B=B0[ẑ+(x/Ls)ŷ], with inhomogeneous flows in the ŷ and ẑ directions, is used to perform a fully kinetic stability analysis of the ion temperature gradient (ITG) and dissipative trapped electron (DTE) modes. The concomitant quasilinear stress components that couple to the local perpendicular (y component) and parallel (z component) momentum transport are also calculated and the anomalous perpendicular and parallel viscous stresses obtained. A breakdown of the ITG-induced viscous stresses are generally observed at moderate values of the sheared perpendicular flow. Even in the absence of external momentum sources, ion diamagnetic effects can generate an inhomogeneous radial electric field which gives rise to a sheared perpendicular flow which can sustain a sheared parallel flow. The effect of the perpendicular stress component in the momentum balance equations is generally small while the parallel stress component, which is primarily determined by the perpendicular flow shear, can dominate the usual neoclassical viscous stress terms. The large anomalous effect suggests that the neoclassical explanation of poloidal flows in tokamaks may be incorrect. The present results are in general agreement with existing experimental observations on momentum transport in tokamaks.
Experimental investigation of three-dimensional magnetic reconnection by use of two colliding spheromaks
Authors: Ono Y. , Morita A. , Katsurai M. , Yamada M.
Published: July 2002

Experimental investigation of three-dimensional (3-D) effects of magnetic reconnection dynamics has been extended by use of axially colliding spheromaks [M. Yamada et al., Phys. Fluids B 3, 2379 (1991)]. The two toroidal shape spheromak plasmas with major radii of 15–20 cm and with parallel toroidal currents of up to 30 kA collide to merge in an external equilibrium field. It is important to note that the present experimental setup allows one to investigate magnetic reconnection comprehensively from both local and global points of view. Reconnection angle θ between the merging field lines is varied by changing the polarity of the internal toroidal field and the magnitude of an external toroidal field. It is observed that the speed of counterhelicity merging with θ∼180° is about three times faster than that of cohelicity merging with θ∼90°. This suggests the significance of a 3-D effect on the reconnection process. This difference is attributed to the property of the neutral current sheets with and without the magnetic field component parallel to the reconnection (X) line. In the counterhelicity merging, the neutral current sheet is compressed in much shorter time than in the cohelicity merging, resulting in much higher current density and subsequent faster decay of the current sheet. This induces a faster magnetic reconnection. The reconnection speed increases proportionally with the initial approaching speed of the spheromaks, suggesting that a compressible driven reconnection model is consistent with the present reconnection experimental results.
Stimulated Raman sidescattering with the effects of oblique incidence
Authors: Afeyan B. B. , Williams E. A.
Published: July 2002

A straightforward analytic theory is presented for the stimulated Raman sidescattering instability in an inhomogeneous plasma. The Liu–Rosenbluth perturbation technique in k space [Phys. Fluids 19, 967 (1976)] is used. The effects of oblique incidence of the light wave, the dispersion of the plasma waves, and the damping of the daughter waves are included. Possible experimental signatures are discussed.
The effect of a third diffusing component on the onset of convection in a horizontal porous layer
Authors: Poulikakos Dimos
Published: July 2002

This Brief Communication is concerned with establishing the effect of the presence of a third diffusing component on the onset of convection in a horizontal fluid-saturated porous layer. The linear stability analysis procedures used in double diffusion are extended to take into account a third diffusing constituent. As a result, general expressions for the various stability boundaries are obtained. The presence of a third diffusing component with small diffusivity can seriously alter the nature of the convective instabilities in the system. For the most part, the findings of the present study for a porous layer exhibit similar behavior to the results reported by Griffiths [J. Fluid Mech. 92, 659 (1979)] for triple diffusion in a layer of classical fluid. However, when heat is one of the three diffusing components, the porosity of the porous medium has a considerable effect on the marginal stability boundary for oscillatory convection.
Atomization of a liquid jet
Authors: Lin S. P. , Kang D. J.
Published: July 2002

Generation of ripples by wind blowing over a viscous fluid was investigated by G. I. Taylor [The Scientific Papers of G. I. Taylor (Cambridge U. P., Cambridge, 1963), Vol. 3, No. 25] with linear stability analysis. Taylor considered the case of temporally growing disturbances in a low density gas and applied his results to explain the process of atomization of a liquid jet injected into a low density gas. Taylor’s analysis is extended here to investigate the case of a spatially growing disturbance in a dense gas. Taylor showed that temporal disturbances of wavelength shorter than the capillary length are stable. The same is found for the spatial disturbances. Each type of disturbance possesses a maximum growth rate with a specific wavelength and frequency. The atomized droplet size corresponding to the maximum growth rate is shown in both theories to decrease inversely as the square of the jet velocity. While the maximum growth rate increases as the square root of the gas-to-liquid density ratio when A2 exceeds 1 for the temporal disturbances, the same dependence on the density ratio does not hold for spatial disturbances until A2 exceeds 100, where A2 is a flow parameter representing the ratio of surface force to the viscous force. When A2 exceeds 100 the growth rates predicted by two theories deviate significantly only at air pressure higher than 10 atm for most liquids at room temperature.

However, for all parameters, the spray angle changes along the jet axis according to the spatial theory, but remains constant according to the temporal theory. It is shown that the viscous force in the liquid may be increased relative to the surface tension force to the point that no discernable spray angle may be observed in practice. Then an intact jet without atomization may result. It is shown that the onset of atomization is primarily caused by the pressure fluctuation which resonates the capillary waves. The results on the interfacial amplification rate suggest that a sufficiently large initial amplitude at the nozzle exit is essential for the onset of atomization.
The transport of test ions in a quiet plasma
Authors: McWilliams Roger , Okubo Mason
Published: July 2002

Experimental measurements of cross-field transport of test ions in a quiet plasma are reported. In a plasma with density fluctuations below 〈(δn/n0)2〉1/2≊0.002, the experimental results agree with simple predictions of test ion diffusion resulting from collisional processes for the range of parameters tested. The observed transport may be attributed to Coulomb collisions and the results disagree with predictions based solely on Bohm diffusion or turbulent processes.
Parametric growth of space charge waves using a self-biased electrostatic wiggler
Authors: Anselmo A. , Nation J. A.
Published: July 2002

An investigation is reported on the use of parametric scattering from an electrostatic wiggler for the growth of space charge waves. This investigation extends previous work on slow space charge wave generation and propagation in inhomogeneous waveguides. Observations show that the scattering technique may be used for the growth of large amplitude waves and that this technique permits wave growth at all values of interest for the ratio of beam to limiting current.
Alpha loss-cone Alfvén-wave instabilities in a sharp-boundary model of a tandem-mirror central cell
Authors: Ho S. K. , Nevins William McCay , Smith Gary R. , Miley G. H.
Published: July 2002

The Alfvén waves of a cylindrical, sharp-boundary plasma, which models the plasma in a tandem-mirror central cell, have been analyzed for instabilities driven by the loss cone of the alpha-particle distribution function. Improved methods for treating the waves, the distribution function, and the instability have been developed and used. The many normal modes of the cylindrical plasma can be found reliably. The diffusion-front method for calculation of the distribution function allows accurate and rapid computation of instability growth rates and marginal-stability boundaries. Present tandem-mirror reactor designs have densities and temperatures that lead to instability. Consequences of the instability should be assessed by studying the quasilinear evolution of the alpha particles.
The accumulation and dispersion of heavy particles in forced two-dimensional mixing layers. I. The fundamental and subharmonic cases
Authors: Martin J. E. , Meiburg E.
Published: July 2002

This paper presents detailed computational results for the dispersion of heavy particles in transitional mixing layers forced at both the fundamental and subharmonic frequencies. The results confirm earlier observations of particle streaks forming in the braid region between successive vortices. A scaling argument based on the idealization of the spatially periodic mixing layer as a row of point vortices shows that the formation of these concentrated particle streaks proceeds with optimum efficiency for St≂1. It thereby provides a quantitative basis for experimental and numerical observations of preferential particle dispersion at Stokes numbers of order unity. Both the model and full simulation furthermore exhibit oscillatory particle motion, as well as the formation of two bands of high particle concentrations, for larger Stokes numbers. The particle dispersion as a function of time and the Stokes number is quantified by means of two different integral scales. These show that the number of dispersed particles does not reach a maximum for intermediate Stokes number. However, when the distance is weighted, optimum dispersion is observed for Stokes numbers around unity. By tracing the dispersed particles backwards in time, they are found to originate in inclined, narrow bands that initially stretch from the braid region into the seeded free stream. This suggests that particle dispersion can be optimized by phase coupling the injection device with the forcing signal for the continuous phase. In the presence of a subharmonic perturbation, enhanced particle dispersion is observed as a result of the motion of the vortices, whereby a larger part of the flow field is swept out.
Free-flight mixing and deposition of aerosols
Authors: Brooke John W. , Hanratty T. J. , McLaughlin J. B.
Published: July 2002

The mechanism for the deposition of spherical aerosols on a wall is investigated in a high resolution direct numerical simulation of low Reynolds number turbulent flow in a vertical channel. A Stokes drag force is used in the equation of motion for the aerosol and the particles are assumed to have no influence on the flow field. A particle deposits when it is one radius from the wall. This occurs mainly by an inertial-free flight from distances outside the viscous sublayer. Particle concentration profiles are presented as a function of time and as a function of distance from the channel walls. It is shown that particles accumulate in the viscous sublayer by turbophoresis and by free flights that do not carry particles to the wall. A new method for identifying free-flight particles is presented and a prediction of free-flight deposition is made using fluid velocity distributions.
A spectral model applied to homogeneous turbulence
Authors: Clark Timothy T. , Zemach Charles
Published: July 2002

Because a spectral model describes distributions of turbulent energy and stress in wave-number space or, equivalently, in terms of a distribution of length scales, it can account for the variation of evolution rates with length scale. A spectral turbulence model adapted from a model introduced by Besnard, Rauenzahn, Harlow, and Zemach is applied here to homogeneous turbulent flows driven by constant mean-flow gradients and to free decay of such flows. To the extent permitted by the experimental data, initial turbulent spectra are inferred, and their evolutions in time are computed to obtain detailed quantitative predictions of the spectra, relaxation times to self-similarity, self-similar spectrum shapes, growth rates, and power-law time dependence of turbulent energies and dominant-eddy sizes, and integral data, such as the components of the Reynolds stress tensor and the Reynolds stress anisotropy tensor. The match to experimental data, within the limits of experimental uncertainties, is good. Some qualifications on the limits of validity of the model are noted. Among phenomena encountered for which the spectral description provides quantitative understanding are the convergence of the anisotropy tensor to a nonzero limit under conditions of free decay (i.e., incomplete return to isotropy, implying a Rotta constant of unity) and the apparent ‘‘return to anisotropy,’’ observed after an anisotropy tensor vanishes due to a temporary cancellation of positive and negative parts of a spectrum, which evolve at different rates.
Vapor flows caused by evaporation and condensation on two parallel plane surfaces: Effect of the presence of a noncondensable gas
Authors: Aoki Kazuo , Takata Shigeru , Kosuge Shingo
Published: July 2002

A vapor in a gap between two parallel plane surfaces of its condensed phase, on which evaporation or condensation may take place, is considered in the case where another gas that neither evaporates nor condenses on the surfaces (say, a noncondensable gas) is also contained in the gap. The steady flow of the vapor caused by evaporation on one surface and condensation on the other and the behavior of the noncondensable gas are investigated on the basis of kinetic theory. First, fundamental features of the flow field are clarified for small values of the Knudsen number (associated with vapor–vapor collisions) by a systematic asymptotic analysis of the Boltzmann equation. Then, the problem is analyzed numerically by means of the direct simulation Monte Carlo method, and the steady behavior of the vapor and of the noncondensable gas (e.g., the spatial distributions of the macroscopic quantities) is clarified for a wide range of the Knudsen number. In particular, it is shown that, in the limit as the Knudsen number tends to zero (the continuum limit with respect to the vapor), there are two different types of the limiting behavior depending on the amount of the noncondensable gas, and evaporation and condensation can take place only when the average density of the noncondensable gas is vanishingly small in comparison with that of the vapor.
Filamentary magnetohydrodynamic plasmas
Authors: Kinney R. , Tajima T. , McWilliams J. C. , Petviashvili N.
Published: July 2002

A filamentary construct of magnetohydrodynamical plasma dynamics based on the Elsässer variables is developed. This approach is modeled after discrete vortex models of hydrodynamical turbulence, which cannot be expected in general to produce results identical to those based on a Fourier decomposition of the fields. In a highly intermittent plasma, the induction force is small compared to the convective motion, and when this force is neglected, the plasma vortex system is described by a Hamiltonian. A statistical treatment of a collection of discrete current-vorticity concentrations is given. Canonical and microcanonical statistical calculations show that both the vorticity and the current spectra are peaked at long wavelengths, and the expected states revert to known hydrodynamical states as the magnetic field vanishes. These results differ from previous Fourier-based statistical theories, but it is found that when the filament calculation is expanded to include the inductive force, the results approach the Fourier equilibria in the low-temperature limit, and the previous Hamiltonian plasma vortex results in the high-temperature limit. Numerical simulations of a large number of filaments are carried out and support the theory. A three-dimensional vortex model is presented as well, which is also Hamiltonian when the inductive force is neglected. A statistical calculation in the canonical ensemble and numerical simulations show that a nonzero large-scale magnetic field is statistically favored, and that the preferred shape of this field is a long, thin tube of flux. Possible applications to a variety of physical phenomena are suggested.
Transverse ion acceleration and ion conic formation in a divergent-field laboratory plasma
Authors: Zintl M. , McWilliams R. , Wolf N.
Published: July 2002

The results of laboratory experiments at the University of California at Irvine are presented in which multidimensional ion velocity distributions in the presence of radio-frequency (RF) waves and a spatially divergent external magnetic field are observed. A plasma volume is subjected to either local or nonlocal electrostatic turbulence, which in turn is responsible for accelerating the ions transverse to the confining magnetic field. The ions flow away from the source of turbulence in a spatially decreasing magnetic field, where the μ ∇B force and magnetic-moment conservation work to distort the heated distribution. Laser-induced-fluorescence (LIF) signals, measured downstream from the plasma source with the aid of optical tomography techniques, reveal substantial ion heating and conic formation.
Ripple-induced energetic particle loss in tokamaks
Authors: White R. B. , Goldston R. J. , Redi M. H. , Budny R. V.
Published: July 2002

The threshold for stochastic transport of high energy trapped particles in a tokamak due to toroidal field ripple is calculated by explicit construction of primary resonances, and a numerical examination of the route to chaos. Critical field ripple amplitude is determined for loss. The expression is given in magnetic coordinates and makes no assumptions regarding shape or up-down symmetry. An algorithm is developed including the effects of prompt axisymmetic orbit loss, ripple trapping, convective banana flow, and stochastic ripple loss, which gives accurate ripple loss predictions for representative Tokamak Fusion Test Reactor [R. Hawryluk, Plasma Phys. Controlled Fusion 33, 1509 (1991)] and International Thermonuclear Experimental Reactor [K. Tomabechi, Proceedings of the 12th International Conference on Plasma Physics and Controlled Nuclear Fusion Research (International Atomic Energy Agency, Vienna, 1989), Vol. 3, p. 214] equilibria. The algorithm is extended to include the effects of collisions and drag, allowing rapid estimation of alpha particle loss in tokamaks.
Direct-drive laser-fusion experiments with the OMEGA, 60-beam, >40 kJ, ultraviolet laser system
Authors: Soures J. M. , McCrory R. L. , Verdon C. P. , Babushkin A. , Bahr R. E. , Boehly T. R. , Boni R. , Bradley D. K. , Brown D. L. , Craxton R. S. , Delettrez J. A. , Donaldson W. R. , Epstein R. , Jaanimagi P. A. , Jacobs S. D. , Kearney K. , Keck R. L. , Kelly J. H. , Kessler T. J. , Kremens R. L. , Knauer J. P. , Kumpan S. A. , Letzring S. A. , Lonobile D. J. , Loucks S. J. , Lund L. D. , Marshall F. J. , McKenty P. W. , Meyerhofer D. D. , Morse S. F. B. , Okishev A. , Papernov S. , Pien G. , Seka W. , Short R. , Shoup M. J. , Skeldon M. , Skupsky S. , Schmid A. W. , Smith D. J. , Swales S. , Wittman M. , Yaakobi B.
Published: July 2002

OMEGA, a 60-beam, 351 nm, Nd:glass laser with an on-target energy capability of more than 40 kJ, is a flexible facility that can be used for both direct- and indirect-drive targets and is designed to ultimately achieve irradiation uniformity of 1% on direct-drive capsules with shaped laser pulses (dynamic range ≳400:1). The OMEGA program for the next five years includes plasma physics experiments to investigate laser–matter interaction physics at temperatures, densities, and scale lengths approaching those of direct-drive capsules designed for the 1.8 MJ National Ignition Facility (NIF); experiments to characterize and mitigate the deleterious effects of hydrodynamic instabilities; and implosion experiments with capsules that are hydrodynamically equivalent to high-gain, direct-drive capsules. Details are presented of the OMEGA direct-drive experimental program and initial data from direct-drive implosion experiments that have achieved the highest thermonuclear yield (1014 DT neutrons) and yield efficiency (1% of scientific breakeven) ever attained in laser-fusion experiments.
The effect of the ion temperature on large amplitude ion-acoustic waves in an electron–positron–ion plasma
Authors: Nejoh Y. N.
Published: July 2002

The effect of the ion temperature on large amplitude ion-acoustic waves is studied in an electron–positron–ion plasma. The region of the existence of the ion-acoustic waves is presented, by the analysis of the structure of the pseudopotential. It is found that the ion temperature increases the maximum Mach number and decreases the amplitude of the ion-acoustic waves. The region of the existence sensitively depends on the ion temperature and the positron density and temperature. It is shown that the region of the existence of the ion-acoustic wave spreads as the ion temperature decreases. The present theory is applicable to analyzing large amplitude ion-acoustic waves in an electron–positron–ion plasma associated with the ion temperature which may occur in space.
Plasma recombination and molecular effects in tokamak divertors and divertor simulators
Authors: Krasheninnikov S. I. , Pigarov A. Yu. , Knoll D. A. , LaBombard B. , Lipschultz B. , Sigmar D. J. , Soboleva T. K. , Terry J. L. , Wising F.
Published: July 2002

Analysis of the experimental data from tokamaks and linear divertor simulators leads to the conclusion that plasma recombination is a crucial element of plasma detachment. Different mechanisms of plasma recombination relevant to the experimental conditions of the tokamak scrape-off layer (SOL) and divertor simulators are considered. The physics of Molecular Activated Recombination (MAR) involving vibrationally excited molecular hydrogen are discussed. Although conventional Electron–Ion Recombination (EIR) alone can strongly alter the plasma parameters, MAR impact can be substantial for both tokamak SOL plasma and divertor simulators. Investigation of the effects of EIR on the plasma flow in divertor simulators shows that due to the balances of (a) energy transport and electron cooling, and (b) the plasma flow and recombination, that EIR extinguishes the simulator plasma at an electron temperature about 0.15 eV.
Enhanced harmonic radiation from a dual-harmonic wiggler
Authors: Yang Yunyuan , Ding Wu
Published: July 2002

A dual-harmonic wiggler scheme is proposed in this paper. Formulas of coupling factors and small signal gain on each harmonic are derived. Both theoretical analysis and numerical simulation show that the third harmonic radiation is effectively enhanced in this new model, and under given conditions the radiation power on the third harmonic can be higher than that of the fundamental.
Velocity shear-driven instabilities in a rotating plasma layer
Authors: Peñano J. R. , Ganguli G. , Amatucci W. E. , Walker D. N. , Gavrishchaka V.
Published: July 2002

The linear stability of a radially localized layer rotating about the cylindrical axis in a magnetized plasma is investigated using an eigenvalue analysis. The eigenvalue equation is solved numerically in a parameter regime characteristic of the Space Physics Simulation Chamber (SPSC) experiments [Amatucci et al., Phys. Rev. Lett. 77, 1978 (1996)] at the Naval Research Laboratory (NRL). Four types of instabilities are predicted. They are type-A and type-B Kelvin-Helmholtz instabilities, a transverse current-driven instability, and the inhomogeneous energy density driven instability (IEDDI). A quantitative comparison between theory and experiment indicates that an experimentally observed fluctuation in a rotating plasma layer is an IEDDI.
Improved confinement and transport studies in the reversed field experiment (RFX)
Authors: Bartiromo R. , Antoni V. , Bolzonella T. , Buffa A. , Marrelli L. , Martin P. , Martines E. , Martini S. , Pasqualotto R.
Published: July 2002

This paper presents results obtained on the Reversed Field eXperiment, RFX [G. Rostagni, Fusion Engineering and Design 25, 301 (1995)], on the study of local transport properties of the RFP configuration both in the standard regime and during improved confinement phases obtained with poloidal current drive. The plasma core is dominated by magnetic turbulence, as indicated by both energy and particle transport analysis. Diffusivity decreases toward the edge and is reduced in the core at high Lundquist number. The edge region is apparently not affected by dynamo mode turbulence and determines global confinement in standard operation. Particle and heat diffusivity in this region show a favorable dependence with increasing density. The most interesting feature of the edge region of RFX consists of a sheared radial electric field with shearing rate comparable to that observed in tokamaks. Indications of turbulence reduction in the shear layer have been obtained. Enhanced energy confinement has been produced by pulsed poloidal current drive (PPCD). A clear reduction of the dynamo activity has been observed with a decrease of core thermal diffusivity to level comparable to the edge and global confinement improvement up to a factor of 3.
Mode-locking phenomena in the TPE-RX reversed-field pinch plasma
Authors: Yagi Yasuyuki , Koguchi Haruhisa , Sakakita Hajime , Sekine Shigeyuki , Maejima Yoshiki , Nilsson Jenny-Ann B. , Bolzonella Tommaso , Zanca Paolo
Published: July 2002

The characteristics of the phase- and wall-locked mode found in a large-sized reversed-field pinch (RFP) machine TPE-RX [Y. Yagi et al. Plasma Phys. Controlled Fusion 41, 255 (1999)] are described in detail. The toroidally-localized radial magnetic field starts to grow after the setup of the RFP configuration in a current rising phase, reaching up to 2% of the poloidal magnetic field at the plasma surface, and stays at the same toroidal location throughout the discharge. The mode frequently locks to the thick shell gap position with a 20%–30% probability. The plasma–wall interaction is enhanced at the locked position where the thermal wall load is peaked by a factor of about 3 on average. The locked mode disappears in some experimental conditions. The probability for the locked mode to appear depends on the experimental conditions, especially on the filling pressure of the fueling gas and on the rise time of the plasma current. Possible causes of the locked mode are discussed from the braking effect of the eddy current as well as from the halo current in the vacuum vessel.
A sandpile model with dual scaling regimes for laboratory, space and astrophysical plasmas
Authors: Chapman S. C. , Dendy R. O. , Rowlands G.
Published: July 2002

There is increasing evidence that the Earth’s magnetosphere, like other macroscopic confined plasma systems (magnetic fusion plasmas, astrophysical accretion discs), displays sandpile-type phenomenology so that energy dissipation is by means of avalanches which do not have an intrinsic scale. This may in turn imply that these systems evolve via self-organized criticality (SOC). For example, the power law dependence of the power spectrum of auroral indices, and in situ magnetic field observations in the Earth’s geotail, indicate that the coupled solar wind-magnetospheric system can to some extent be described by an avalanche model. However, substorm statistics exhibit probability distributions with characteristic scales. In this paper a simple sandpile model is discussed which yields for energy discharges due to internal reorganization a probability distribution that is a power law, implying SOC, whereas systemwide discharges (flow of “sand” out of the system) form a distinct group whose probability distribution has a well defined mean. When the model is analyzed over its full dynamic range, two regimes having different inverse power law statistics emerge. These correspond to reconfigurations on two distinct length scales: short length scales sensitive to the discrete nature of the sandpile model, and long length scales up to the system size which correspond to the continuous limit of the model. These are anticipated to correspond regimes accessible to both laboratory and astrophysical plasmas. The relevance of the emergence of distinct self-organized confinement regimes in space, astrophysical, and magnetic fusion plasmas is discussed. Since the energy inflow may be highly variable, the response of the sandpile model is examined under strong or variable loading.
The effect of divertor baffling on Alcator C-Mod discharges
Authors: Pitcher C. S. , Boswell C. J. , Goetz J. A. , LaBombard B. , Lipschultz B. , Rice J. E. , Terry J. L.
Published: July 2002

The effect of divertor baffling on Alcator C-Mod [Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] discharges has been investigated using a novel divertor bypass. The bypass allows the in situ variation of the mechanical conductance from the divertor plenum to the main chamber between or even during discharges. The results indicate a strong dependence on baffling for the compression of deuterium and recycling impurity gases in the divertor plenum in a variety of C-Mod discharges, indicating that tight baffling would be desirable in a future tokamak reactor. There is no apparent effect of the divertor baffling for the Ohmic, L-mode and EDA (Enhanced Dα) H-mode discharges in this study on main chamber neutral particle populations, scrape-off-layer flows, nonrecycling impurity screening, global energy confinement or H-mode threshold powers.