bluuebunny/crossref_metadata_2025 huggingface dataset:
Near-singular collapse and local intensification of a ‘‘Lissajous-elliptic’’ vortex ring: Nonmonotonic behavior and zero-approaching local energy densities
Authors: Fernandez Victor M. , Zabusky Norman J. , Gryanik Vladimir M.
Published: July 2002

This Letter observes collapse and intensification of the two parameter (b,c) ‘‘Lissajous-elliptic’’ vortex ring. Laboratory and direct numerical studies of this ring are proposed to elucidate near-singular and intermittent fluid phenomena at very high Reynolds number. Quantifications of single filament Biot–Savart numerical simulations with various core ‘‘radii’’ show that collapse may be nonmonotonic in time. In the collapsing region, the largest positive strain-rate eigenvalue, α, is off the filament and exhibits self-similar, singular-like behavior. A signature of collapse is found in the local approach to zero of the filament energy density in the collapsing regions.
Large-scale flow and pattern drift in finite amplitude convection
Authors: Massaguer J. M.
Published: July 2002

The connection between the large-scale flows observed in thermal convection and the drift of the roll structures in large aspect ratios is examined. The analysis is based on a perturbative expansion of slightly deformed, finite-amplitude straight rolls. The derivation assumes that the aspect ratio of the container is large, so that the flow is translation invariant. A scale separation technique is used to smooth out the small-scale dynamics. If the unperturbed rolls cannot support a mean velocity field, the dynamics of the perturbations is diffusive and the mean flow is driven by roll bending, stretching, or pinching. If the unperturbed state supports a mean velocity field, the perturbations in the roll structure can be advected or even can propagate as a dispersive wave. In the present formulation, pattern defects arise from resonance and multivaluedness, with the implication that they can be advected or even travel.
Characteristics of asymmetric turbulent near wakes
Authors: Nakayama Akihiko , Kreplin Hans-Peter
Published: July 2002

The mean-velocity and turbulence quantities in a few qualitatively different types of asymmetric two-dimensional turbulent near wakes in nearly zero-pressure gradient have been experimentally studied. In all cases, the log-law similarity of the boundary layers was found to continue into the initial part of the wake, with the same similarity variables as in the boundary layers on the same side, provided the origin of the normal distance is taken at the smooth extension of the respective surfaces. The Reynolds stress profiles show sharp peaks just downstream of the trailing edge, and the magnitudes of the peaks are found to be related to the values of the surface friction at the trailing edge. These peaks grow into the full width of the wake to form the far wake profiles. In this process, the turbulent diffusions of the Reynolds stresses are very important, and when the size asymmetry is severe, is not clearly of the gradient-transport type.
Long nonlinear waves in an unbounded rotating jet or rotating two-fluid flow
Authors: Sun S. M.
Published: July 2002

The objective of this paper is to study weakly nonlinear waves in an infinitely long rotating jet and a rotating two-fluid flow bounded by an infinitely long rigid cylinder with surface tension at the interface. The critical values for Rossby number, a nondimensional wave speed, are found. When the Rossby number is near one of the critical values, nonlinear theory is developed under long-wave approximation and the well-known Korteweg–de Vries (KdV) equations for the free surface and free interface are obtained. Then the solitary wave solutions are given as the first-order approximations of the solutions of the equations governing the motion of the flows. The analogy between the rotating fluid flows and a two-dimensional flow with density stratification is discussed.
Nonlinearly coupled inertial and Rossby waves
Authors: Pokhotelov O. A. , McKenzie J. F. , Shukla P. K. , Stenflo L.
Published: July 2002

The nonlinear coupling between inertial and Rossby waves is considered accounting for the action of the low-frequency nonlinear force associated with the inertial waves. It is found that this interaction is governed by a pair of equations, which can be useful for studying the modulational instability of a constant amplitude inertial wave as well as the dynamics of nonlinearly coupled inertial and Rossby waves.
Simulation of early transition in compressible rotating Couette flow: Subharmonic development
Authors: Hatay Ferhat F. , Biringen Sedat
Published: July 2002

The nonlinear temporal evolution of disturbances in compressible flow between infinitely long, concentric cylinders is investigated through direct numerical simulations of the full, three-dimensional Navier–Stokes and energy equations. Counter-rotating cylinders separated by wide gaps are considered with supersonic velocities of the inner cylinder. Initially, the primary disturbance grows exponentially, in accordance with linear stability theory. As the disturbances evolve, higher harmonics and subharmonics are generated in a cascading order, eventually reaching a saturation state. Subsequent highly nonlinear stages of the evolution are governed by the interaction of the disturbance modes, particularly the axial subharmonics. Nonlinear evolution of the disturbance field is characterized by the formation of high-shear layers, extending from the inner cylinder toward the center of the gap in the form of jets similar to the ejection events in transitional and turbulent wall-bounded shear flows.
Wavelet based model for small-scale turbulence
Authors: Zimin Valery , Hussain Fazle
Published: July 2002

Using a vector wavelet decomposition of the velocity field a simple model for locally isotropic turbulence has been derived from the Navier–Stokes equation (NSE). This model, which involves no empirical or ad hoc parameter, incorporates nonlocal interscale interactions, reveals backscatter and can be applied to represent small-scale turbulence in LES schemes. The model gives a stationary solution which corresponds to k−5/3 Kolmogorov spectrum. Moreover, the model equations produce k4 infrared spectrum from an initial energy peak at intermediate scales.
Solutions near the hydraulic control point in a gravity current model
Authors: Alendal G.
Published: July 2002

A steady-state gravity current model that incorporates entrainment and friction is used to describe large-scale gravity currents and channel flows. When the model includes pressure effects from varying current thickness, critical points occur when the current velocity is equal to the phase velocity of waves on the interface. Some solutions have the possibility to pass from super to subcritical flow, or vice versa. These solutions pass through a hydraulic control point and the objective is to analyze the behavior of the solutions in the vicinity of such points. Using a phase space in which the hydraulic control points occur as equilibrium points, and performing Taylor expansion to the first order, the result is a system of autonomous differential equations with constant coefficients that can describe the behavior of the solutions for different parameter regimes near a hydraulic control point. If an equilibrium point in phase space represents a saddle point, it is distinguished between three different solution classes; solutions that approach the critical velocity but never reach it, solutions that reach the critical velocity and obtain infinitely large derivative, and the solutions (one from subcritical and one from supercritical) that reach the critical velocity exactly in the equilibrium point. The method gives a way to tell whether hydraulically controlled solutions exist and in special cases an algorithm for finding these solutions.
Internal structure of extreme standing waves on deep water
Authors: Smith D. H. , Roberts A. J.
Published: July 2002

A special procedure is applied to remove the non-physical distortion that always appears when numerically calculating velocity and related quantities in the close vicinity of a discretized vortex sheet. The procedure is used in conjunction with a semi-Lagrangian boundary integral method to study the flowfield beneath free surface standing water waves. Pressure, velocity and acceleration are calculated below the wave and some aspects of crest formation are investigated. At large depths results confirm the existence of an unattenuated oscillatory pressure component, giving behavior consistent with previously known theory.
Convective and absolute instabilities of a buoyancy-induced flow in a thermally stratified medium
Authors: Krizhevsky Larisa , Cohen Jacob , Tanny Josef
Published: July 2002

Convective and absolute instabilities of the buoyancy-induced boundary layer adjacent to an isothermal vertical flat plate immersed in a linear ambient thermal stratification are investigated. Because of the ambient stratification, the mean temperature and velocity similarity profiles possess a region of temperature deficit and flow reversal. Linear stability analysis reveals the existence of convective and absolute instabilities, which are studied in terms of the governing nondimensional parameters of the problem, namely, the Prandtl number (Pr) and a modified Grashof number (G). The critical value of G=Gc at which transition to convective instability occurs has a maximum between Pr=2 and 3; for larger values, Gc is a weak function of Pr. At higher G=Ga, the flow becomes absolutely unstable. It is found that Ga increases with Pr. This tendency is related to the existence of the reverse flow due to the ambient stratification. It is shown that as the reverse flow becomes relatively more dominant, the flow is more susceptible to absolute instability.
Hierarchical order in wall-bounded shear turbulence
Authors: Carbone Fernando , Aubry Nadine
Published: July 2002

Since turbulence at realistic Reynolds numbers, such as those occurring in the atmosphere or in the ocean, involve a high number of modes that cannot be resolved computationally in the foreseeable future, there is a strong motivation for finding techniques which drastically decrease the number of such required modes, particularly under inhomogeneous conditions. The significance of this work is to show that wall-bounded shear turbulence, in its strongly inhomogeneous direction (normal to the wall), can be decomposed into one (or a few) space–time mother mode(s), with each mother generating a whole family of modes by stretching symmetry. In other words, the generated modes are similar, dilated copies of their mother. In addition, we show that the nature of all previous modes strongly depends on the symmetry itself. These findings constitute the first scaling theory of inhomogeneous turbulence.
Propagation of grid turbulence in homogeneous fluids
Authors: Voropayev S. I. , Fernando H. J. S.
Published: July 2002

The purpose of this paper is to present the results of a laboratory experiment dealing with one-dimensional propagation of a turbulent front induced by an oscillating grid in a homogeneous fluid. The symmetrical quadrupolar mode of the flow, induced by the grid elements, was studied in the range KC=4–45, S=1–20 (KC is the Keulegan–Carpenter number, S is the Stokes number). It is shown that the depth H of the turbulent layer increases with time t as H∝t1/2. Experimental data for different grid parameters and oscillation amplitudes and frequencies are presented in the nondimensional form h=const τ1/2, using nondimensional depth h of the turbulent layer and nondimensional time τ. A semiempirical model is presented to explain this behavior. This model is based on the properties of a flow induced in a visous fluid by localized forcing of a line force dipole (force doublet).
Simulation of extraction of velocity from passive scalar data in a two-dimensional diverging channel flow
Authors: Carpenter Bonnie N. , Pearlstein Arne J.
Published: July 2002

A technique for determining n-dimensional (n=2,3) velocity fields from measurement of n−1 passive or reactive scalars has recently been developed [Phys. Fluids 7, 754 (1995)]. The method utilizes n linear, first-order, uncoupled hyperbolic equations for n velocity components, derived from transport equations for n−1 scalars and conservation of mass. The velocity is determined by integration along the characteristic curves defined by the hyperbolic equations. Here we present the results of a computational proof-of-concept study for a steady, two-dimensional, diverging channel flow. We consider the effects of the resolution of the grid (and the shape of its elements) on which the scalar is known and its derivatives are approximated, the integration step size used to calculate the velocity components along characteristic curves, and the effects of multiplicative noise introduced into the scalar field. The results show that extraction of the velocity by integration of the equations along characteristics is stable, and that the techniques proposed for removal of singularities are effective. For steady flows, we show that noise in the scalar field measurements can be dealt with by extracting the velocity from the mean of several noisy scalar fields.
Instantaneous viscous flow in a corner bounded by free surfaces
Authors: Betelú S. , Diez J. , Gratton R. , Thomas L.
Published: July 2002

We study the instantaneous Stokes flow near the apex of a corner of angle 2α formed by two plane stress free surfaces. The fluid is under the action of gravity with g↘ parallel to the bisecting plane, and surface tension is neglected. For 2α≳126.28°, the dominant term of the solution as the distance r to the apex tends to zero does not depend on gravity and has the character of a self-similar solution of the second kind; the exponent of r cannot be obtained on dimensional grounds and the scale of the coefficient depends on the far flow field. Within this angular domain, the instantaneous flow is deeply related to the (steady) flow in a rigid corner known since Moffatt [J. Fluid Mech. 18, 1 (1964)] and, as in that case, there may be eddies in the flow. The situation is substantially different for 2α<126.28°, as the dominant term is related to gravity and not to the far flow. It has the character of a self-similar solution of the first kind, with the exponent of r being given by dimensional analysis. The solution cannot be continued in time since it leads to the curling of the boundaries. Nevertheless, it provides information on how such a cornered contour may evolve. When 2α<180°, the corner angle does not vary as the flow develops; on the other hand, if 2α≳180° the corner must round or tend to a narrow cusp, depending on the far flow. These predictions are supported by simple experiments.
Investigation of a lattice gas model for surface gravity waves
Authors: Buick J. M. , Greated C. A. , Easson W. J.
Published: July 2002

A recently proposed lattice gas model for simulating surface waves at a free boundary is described in detail. Simulated waves are compared to linear theory and are seen to compare well, provided an additional parameter ε is introduced. This parameter, ε, is investigated and found to be required due to the density gradient produced across the fluid by the gravitational interaction. Its value and the values of the other model parameters are found for a range of gravitational strengths.
Dynamics of air bearing sliders
Authors: Witelski T. P.
Published: July 2002

In this paper we present new results for the dynamics of a problem for the interaction of a compressible gas flow with a movable rigid surface. Compressible lubrication theory is applied to describe the dynamics of the vertical motion of air bearing sliders used in computer hard disk drives. In the limit of large bearing number we show this problem can be reduced to a nonlinear integrodifferential equation. Linear stability analysis and perturbation methods show that over the range of possible slider positions there is an infinite sequence of Hopf bifurcations yielding stable large amplitude periodic orbits. The dynamics of near-crash behavior and interaction with a moving disk surface are also addressed.
Modeling vibrational energy exchange of diatomic molecules using the Morse interatomic potential
Authors: Lord R. G.
Published: July 2002

A model of dissociation of diatomic molecules in intermolecular collisions, for use in Direct Simulation Monte Carlo (DSMC) calculations, has been proposed by Lord. This model, is a development of the exact available energy or vibrationally linked chemistry model of Bird, which models dissociation and chemical reactions directly via Borgnakke–Larsen type energy redistributions between translation and internal modes, dissociation being assumed to occur whenever the vibrational energy of a molecule exceeds its dissociation energy. The model employs the correct energy level scheme of a Morse oscillator and has been shown to yield equilibrium dissociation rate constants for N2 which are in excellent agreement with published values. A shortcoming of the model, however, is that it is purely classical, the molecular vibrational energy being assumed to be continuously distributed. Although this is not a serious defect for the prediction of dissociation, since the dissociation energy is normally many times the largest vibrational energy level spacing, it means that the model is unsuitable for describing vibrational energy exchange at lower collision energies where quantization effects are significant and must be taken into account in a way which satisfies detailed balance at equilibrium. The present paper describes a quantized extension of the earlier model which satisfies this requirement and can therefore be used in conjunction with it. It is shown that the two models become identical at high collision energies.
The effect of viscosity on the spherical stability of oscillating gas bubbles
Authors: Hao Y. , Prosperetti A.
Published: July 2002

Gas bubbles driven in radial oscillations are subject to an instability of the spherical shape that is opposed by surface tension and viscosity. An exact linear formulation for the study of the phenomenon has been available for many years, but its complexity has discouraged a detailed investigation. With the recent theory of sonoluminescence of Lohse and co-workers [Hilgenfeldt et al., Phys. Fluids, 8, 2808 (1996)], there has been a renewed interest in the problem and new data have become available. This paper presents a numerical method for the solution of the pertinent equations and compares the theory with these new data. The coupling of the strong nonlinearity of the bubble radial oscillations with the parametric mechanism of the surface instability results in a very complex structure for the stability boundary. Nevertheless, a good agreement between theory and data is found. A comparison with earlier approximate models is also made.
Fully nonlinear three-dimensional convection in a rapidly rotating layer
Authors: Julien Keith , Knobloch Edgar
Published: July 2002

Fully nonlinear three-dimensional convection in a rotating layer is studied for large Taylor numbers. In this regime, the leading order nonlinearity arises from the distortion of the horizontally averaged temperature profile. As a result, steady rolls, squares, hexagons, triangles, and a pattern called patchwork quilt all have identical Nusselt numbers. A similar degeneracy is present in overstable convection with six patterns having identical time-averaged Nusselt numbers and oscillation frequencies. These results are obtained via an asymptotic expansion in the Taylor number that determines, for each Rayleigh number, the time-averaged Nusselt number and oscillation frequency from the solution of a nonlinear eigenvalue problem for the vertical temperature profile. A number of other patterns are determined by a weakly nonlinear analysis that cannot be extended into the fully nonlinear regime by the present methods, but these patterns are necessarily unstable.
Instabilities of shear flows between two coaxial differentially rotating cones
Authors: Hoffmann N. P. , Busse F. H.
Published: July 2002

The instabilities of fluid flows between coaxial differentially rotating cones are considered in the limit of the small gap approximation as function of the cone angle. As in the experimental study of Wimmer (presented at the 10th International Couette-Taylor Workshop, Paris, 15–17 July 1997), a transition from Taylor vortex instabilities to Ekman-type instabilities is found at a cone angle of about 45°.
Anisotropy of turbulence in stably stratified mixing layers
Authors: Smyth William D. , Moum James N.
Published: July 2002

Direct numerical simulations of turbulence resulting from Kelvin–Helmholtz instability in stably stratified shear flow are used to study sources of anisotropy in various spectral ranges. The set of simulations includes various values of the initial Richardson and Reynolds numbers, as well as Prandtl numbers ranging from 1 to 7. We demonstrate that small-scale anisotropy is determined almost entirely by the spectral separation between the small scales and the larger scales on which background shear and stratification act, as quantified by the buoyancy Reynolds number. Extrapolation of our results suggests that the dissipation range becomes isotropic at buoyancy Reynolds numbers of order 105, although we cannot rule out the possibility that small-scale anisotropy persists at arbitrarily high Reynolds numbers, as some investigators have suggested. Correlation-coefficient spectra reveal the existence of anisotropic flux reversals in the dissipation subrange whose magnitude decreases with increasing Reynolds number. The scalar concentration field tends to be more anisotropic than the velocity field. Estimates of the dissipation rates of kinetic energy and scalar variance based on the assumption of isotropy are shown to be accurate for buoyancy Reynolds numbers greater than O(102). Such estimates are therefore reliable for use in the interpretation of most geophysical turbulence data, but may give misleading results when applied to smaller-scale flows.
Mode cooperation in a submillimeter wave gyrotron
Authors: Idehara T. , Shimizu Y.
Published: July 2002

The phase bunching of a gyrating electron beam under the second harmonic operation reduces the starting current for the fundamental operation, in a submillimeter wave gyrotron. Such mode cooperation observed experimentally is shown and considered on the basis of the computer simulation results. The cooperation really reduces the starting current of the fundamental operation and increases total output power, which results in increasing the beam efficiency.
Relaxed plasmas in external magnetic fields
Authors: Spies G. O. , Li J.
Published: July 2002

The extension of the theory of relaxed plasmas to external magnetic fields whose field lines intersect the wall is concisely formulated and then applied to the Extrap experiment [J. R. Drake, Plasma Phys. Controlled Fusion 26, 387 (1984)]. It is found that the external octupole field, though not affecting the phenomenon of current saturation, inhibits field reversal at parts of the wall if it is sufficiently strong to generate magnetic x points within the plasma.
Toroidal coupling of drift-type modes in the long-wavelength regime
Authors: Kim J. Y. , Wakatani M.
Published: July 2002

Toroidal coupling in the long-wavelength regime is shown to be weak or absent for drift-type modes if there are no trapped ions. This is in contrast to the case of interchange-type modes for which strong coupling can occur in all wavelength regimes, even when there are no trapped ions.
Nonlinear wave evolution in bounded plasmas
Authors: Stenflo L. , Yu M. Y.
Published: July 2002

The evolution equations governing large amplitude waves in cold plasma columns bounded by deformable dielectrics are presented and discussed.
Measurements of the angular and temporal structure of second-harmonic emission from laser-produced plasmas
Authors: Drake R. P. , Bauer B. S. , Baker K. L. , Mizuno K. , Sleaford B. , Rubenchik A. M. , Estabrook Kent , Labaune C. , Baton S. , Jalinaud T. , Schifano E. , Goldman M. V. , Newman D. L.
Published: July 2002

We have measured and analyzed the second harmonic emission, both in the plane of the laser electric field and perpendicular to it, at several angles near 135° from the laser wave vector. The experiments used from 1 to 80 J of 1.053 μm light to irradiate carbon–hydrogen (CH) targets with a 550 ps pulse. A random phase plate was used, producing characteristic intensities in the range of 1013–1014 W/cm2. This was sufficient to drive the Ion Acoustic Decay Instability, producing Stokes emission well-separated from the emission spike at the second harmonic of the laser frequency. The spectral structure of the Stokes emission was qualitatively similar for all intensities and angles of observation. The duration of the signals showed trends anticipated from linear theory. To explain the scaling of the signal strength and spectral width requires nonlinear theory.
High-frequency electrostatic modes in non-neutral plasmas
Authors: Book David L.
Published: July 2002

A fluid description is employed to derive the dispersion relation for cyclotron modes in a cylindrical non-neutral plasma of radius R confined by a uniform magnetic field B=B0ez inside a chamber with conducting walls of radius R0. In contrast to the theory of Gould and LaPointe [Phys. Rev. Lett. 67, 3685 (1991); Phys. Fluids B 4, 2038 (1992)] the model includes the diamagnetic drift but omits finite Larmor radius effects. The density and the temperature of the unperturbed state are taken to be n(r)=n0(1−r2/R2) and T(r)=T0(1−r2/R2)γ−1, where γ is the adiabatic index, so that the r-dependent slow rotation frequency is ωr≊−[δ(1−r2/2R2)+ε(1−r2/R2)γ−2]Ω/2, where Ω=qB0/Mc is the ion gyrofrequency, δ=4πn0q2/MΩ2, and ε=4γT0/MΩ2R2. For the linearized fluid equations together with the Poisson equation the eigenvalue problem is solved in the limit δ≪1, ε≪1. The eigenfrequencies for high-frequency electrostatic modes with wave vectors satisfying k⋅B=0 (Bernstein modes) are found in the form ω=−Ω+Δω, where Δω/Ω contains terms proportional to δ and ε. Solutions are obtained and compared with experiment and the theory of Gould and LaPointe. The present theory predicts that at a given T0 modes with m≳1 propagate only when the density is less than a critical value that increases with m, and that Δω normalized by the diocotron frequency depends only on the ratio of the Debye length to the plasma radius and hence is independent of B and M. The predictions of the two theories differ in several other respects; future observations may serve to decide between them.
Computer simulation of a magnetohydrodynamic dynamo. II
Authors: Kageyama Akira , Sato Tetsuya ,   the Complexity Simulation Groupa)
Published: July 2002

A computer simulation of a magnetohydrodynamic dynamo in a rapidly rotating spherical shell is performed. Extensive parameter runs are carried out changing electrical resistivity. When resistivity is sufficiently small, total magnetic energy can grow more than ten times larger than total kinetic energy of convection motion which is driven by an unlimited external energy source. When resistivity is relatively large and magnetic energy is comparable or smaller than kinetic energy, the convection motion maintains its well-organized structure. However, when resistivity is small and magnetic energy becomes larger than kinetic energy, the well-organized convection motion is highly irregular. The magnetic field is organized in two ways. One is the concentration of component parallel to the rotation axis and the other is the concentration of perpendicular component. The parallel component tends to be confined inside anticyclonic columnar convection cells, while the perpendicular component is confined outside convection cells.
On the statistical mechanics of self-organized profiles
Authors: Medvedev M. V. , Diamond P. H. , Carreras B. A.
Published: July 2002

The radial structure of tokamak profiles determined by anomalous transport is elucidated by studying the statistical mechanics of a sand pile automaton for which the toppling conditions depend on local gradient, alone. In this representation, the sand pile dynamics is Markovian, and spatial profiles may be obtained from calculated expectation values of the local gradient. The Markovian structure of the dynamics is exploited to analytically derive a local gradient probability distribution function from a generalized kinetic equation. For homogeneous, weak noise, the calculated expectation value of the gradient is well below the marginally stable state. In the over-driven limit (i.e., strong homogeneous noise), a region of super-critical gradient is shown to form near the bottom of the pile. For the case of localized noise, the mean self-organized profile is always sub-critical. These results are consistent with numerical studies of simple automata. Their relevance to and implications for tokamak confinement are discussed.
Relaxation to steady state in neutral-beam-injected mirrors
Authors: Mirnov V. , Demokan O.
Published: July 2002

The time-dependent Vlasov–Boltzmann equation is analytically studied in mirror machines with perpendicular injection. The atomic and collisional processes are taken into account, with their explicit, rigorous forms. A uniform model is assumed by considering a square-well magnetic field configuration, and the mirrors are represented by related boundary conditions on the ion distribution function. It is shown that, the ion density increases from the initial value, drops by a certain amount, then increases again up to its final value, around which it performs damped oscillations.
Leaky electromagnetic wave resonances of a plasma sphere
Authors: Ghanashev I. , Zhelyazkov I. , Stoykov Sv.
Published: July 2002

In the paper we consider theoretically electromagnetic modes in a homogeneous cold plasma sphere. We find, in addition to the surface transverse magnetic modes, known from the electrostatic analysis of the same structure, two new families of weakly leaky modes (one of them consisting of transverse electric modes). A classification of the modes is proposed. An approximate analytic expression for the wave damping rate of the surface modes due to energy leakage is given, making it possible to draw conclusions on the behavior of more complex three-dimensional plasmas.
Stochastic ion behavior in subharmonic and superharmonic electrostatic waves
Authors: Chia Ping-Kun , Schmitz L. , Conn R. W.
Published: July 2002

The dynamics of a charged particle in a magnetic field perturbed by electrostatic waves propagating transverse to the magnetic field has been investigated. Depending on the number of perturbing waves, the magnitude of the perturbation and the perturbation frequency (ν=r/s=ω/ω0, where r and s are relatively prime integers, and ω0 is the cyclotron frequency), the particle motion can exhibit either a small deviation from simple cyclotron motion or stochastic motion over phase space. The latter case is found to correspond to particle heating and anomalous transport. In the case of cyclotron harmonic perturbations (superharmonic case, s=1), the phase space of the particle is covered by a stochastic web, inside of which the particle motion is chaotic. The particle can, in principle, be heated to very high energy. However, if the wave frequency is some fraction of the ion cyclotron frequency (subharmonic case, s≥2), the heating range is limited.
Intermittent self-organization: Nonlinear responses of twisting multiple flux tubes
Authors: Takamaru Hisanori , Sato Tetsuya
Published: July 2002

A new type of self-organization is presented where a system evolves intermittently and undergoes self-adaptively local maxima and minima of energy state. Numerical study of nonlinear interactions of twisting multiple flux tubes have shown that each flux tube suffers from a helical kink instability, resulting in the formation of a knotted structure. The knotted deformation stimulates reconnection with a neighboring flux tube whereby kinetic and thermal energies are impulsively and markedly released. Repeating reconnection intermittently with surrounding field lines, the whole structure returns to the more or less originally separated flux tubes and thereafter repeats an intermittent and recursive evolution. All these results lead to a working hypothesis that in an open complex nonlinear system where energy is externally and continuously supplied, the system exhibits an intermittent self-organization, self-adapting local maxima and minima of energy state alternatively in temporal evolution. The present work will also forecast this as some essential concept to an energy conversion problem such as solar flares.
Numerical simulation of a negative ion plasma expansion into vacuum
Authors: Garcia L. G. , Goedert J. , Figua H. , Fijalkow E. , Feix M. R.
Published: July 2002

The expansion into vacuum of a one-dimensional, collisionless, negative ion plasma is investigated in the framework of the Vlasov–Poisson model. The basic equations are written in a “new time space” by use of a rescaling transformation and, subsequently, solved numerically through a fully Eulerian code. As in the case of a two species plasma, the time-asymptotic regime is found to be self-similar with the temperature decreasing as t−2. The numerical results exhibit clearly the physically expected effects produced by the variation of parameters such as initial temperatures, mass ratios and charge of the negative ions.
Measuring Δ′ from electron temperature fluctuations in the Tokamak Fusion Test Reactor
Authors: Ren C. , Callen J. D. , Gianakon T. A. , Hegna C. C. , Chang Z. , Fredrickson E. D. , McGuire K. M. , Taylor G. , Zarnstorff M. C.
Published: July 2002

A method is developed for determining directly from experimental data the classical tearing mode stability parameter Δ′. Specifically, an analytical fit function is derived for the electron temperature fluctuations (T̃e) in the vicinity of a magnetic island. Values of Δ′ determined from the fit function parameters for m/n=2/1, 3/2 and 4/3 modes (m and n are poloidal and toroidal mode numbers) are obtained using the high resolution T̃e profile data from major radius shift (“jog”) experiments on the Tokamak Fusion Test Reactor (TFTR) [D. Meade et al., Proceedings of the International Conference on Plasma Physics and Controlled Nuclear Fusion. Washington, District of Columbia, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. I, pp. 9–24]. It is found that the n⩾2 modes have Δ′<0.
Poloidal magnetic field around a tokamak magnetic surface
Authors: Martı́n P. , Haines M. G.
Published: July 2002

In this paper a differential equation is obtained for the variation of the poloidal magnetic field along the cross section of the magnetic field surfaces in a tokamak. Toroidal axisymmetry is assumed. The coefficients of the differential equation depend on the curvature of the family of curves orthogonal to the magnetic field lines. Several shapes of magnetic field surfaces have been analyzed in detail as families of nested curves of circular, elliptic, and triangular elliptic shapes. Closed analytic solutions have been obtained in some cases.
Kinetic theory of plasma adiabatic major radius compression in tokamaks
Authors: Gorelenkova M. V. , Gorelenkov N. N. , Azizov E. A. , Romannikov A. N. , Herrmann H. W.
Published: July 2002

In order to understand the individual charged particle behavior as well as plasma macroparameters (temperature, density, etc.) during the adiabatic major radius compression (R-compression) in a tokamak, a kinetic approach is used. The perpendicular electric field from the Ohm’s law at zero resistivity is made use of in order to describe particle motion during the R-compression. Expressions for both passing and trapped particle energy and pitch angle change are derived for a plasma with high aspect ratio and circular magnetic surfaces. The particle behavior near the passing trapped boundary during the compression is studied to simulate the compression-induced collisional losses of alpha particles. Qualitative agreement is obtained with the alphas loss measurements in deuterium-tritium (D-T) experiments in the Tokamak Fusion Test Reactor (TFTR) [World Survey of Activities in Controlled Fusion Research [Nucl. Fusion special supplement (1991)] (International Atomic Energy Agency, Vienna, 1991)]. The plasma macroparameters evolution at the R-compression is calculated by solving the gyroaveraged drift kinetic equation.
On the continuous plug potential profile in a tandem mirror
Authors: Katanuma I. , Kiwamoto Y. , Tatematsu Y. , Ishii K. , Saito T. , Tamano T. , Yatsu K.
Published: July 2002

The plug potential is formed experimentally, even without high energy sloshing ion population in the end-mirror cells of a tandem mirror. It has been shown analytically that the plug potential can be formed in the present experimental conditions by using the plausible distribution functions of ions and electrons and a small ionization source. In this paper, the required ionization rate is estimated for the continuous potential profile around the plug by using Emmert’s source.
Physical processes in extragalactic radio sources
Authors: Carilli C. L. , Perley R. , Harris D. E. , Barthel P. D.
Published: July 2002

This paper summarizes extensive observational studies of the closest ultraluminous radio galaxy Cygnus A. These data are used to test jet theory for powering the double-lobed radio emitting structures. Issues addressed include: (i) jet stability, confinement, composition, and velocity, (ii) the double shock structure for the jet terminus and the origin of multiple radio hotspots, (iii) the nature of filamentary structure in the radio lobes, and (iv) the hydrodynamic evolution of the radio lobes within a hot cluster atmosphere. These data are also used to constrain models for relativistic particle acceleration and energy losses (Fermi acceleration and synchrotron aging), as well as to determine magnetic field strengths and morphologies in the radio source and in the surrounding intracluster medium.
A high brightness, X-band photoinjector for the production of coherent synchrotron radiation
Authors: Le Sage G. P. , Bennett C. V. , White W. E. , Landahl E. C. , Laurent L. L. , Luhmann N. C. , Hartemann F. V. , Ho C. H. , Lau W. K. , Yang T. T.
Published: July 2002

Linear colliders, future electron acceleration concepts, and short pulse, ultrawideband millimeter-wave sources all require bright electron beams. Photoinjectors have demonstrated the ability to produce relativistic electron beams with low emittance and energy spread. The system described herein combines state-of-the-art capabilities in the laser and rf systems, advanced photocathode materials, and new concepts for synchronization. Phase jitter has been measured in detail, and schemes for alleviating this problem have undergone initial proof-of-principle testing. Direct mode locking of a multiple quantum well Al:GaAs solid-state laser oscillator by an rf signal sampled from within a high-power rf accelerator cavity was demonstrated for the first time. Characterization of the electron beam produced by the system is presented. The linear electron accelerator system is comprised of a 1.5 cell side-wall coupled standing wave accelerator structure, driven by a 20 MW Stanford Linear Accelerator Center (SLAC) Klystron operating at 8.548 GHz, a Ti:sapphire laser oscillator, and a chirped pulse Ti:sapphire laser amplifier.
Longitudinal space-charge waves and instabilities in intense beams
Authors: Wang J. G. , Reiser M.
Published: July 2002

Space-charge effects and collective behavior play an important role in the dynamics of intense beams for accelerator applications. One of the research issues is the longitudinal space-charge waves and instabilities in high current accelerators. At the University of Maryland, an experimental program has been conducted to systematically study space-charge waves and longitudinal instabilities in space-charge dominated electron beams. This includes the generation of localized space-charge waves in an active device; the propagation and end reflection of space-charge waves in a conducting transport channel; and the longitudinal instabilities in a resistive-wall channel. This paper reports and discuss our experimental results.
Generation of hot spots by fast electrons in lower hybrid grills
Authors: Rantamäki K. M. , Pättikangas T. J. H. , Karttunen S. J. , Litaudon X. , Moreau D.
Published: July 2002

Generation of hot spots on plasma facing components magnetically connected to the grill is a main limiting factor in high-power operation of a lower hybrid system. A possible explanation for the hot spots is the sputtering caused by fast electrons generated by parasitic absorption of lower hybrid power near the grill mouth. The behavior of the edge plasma near the grill mouth is investigated with a new tool in this context: the self-consistent particle-in-cell (PIC) code XPDP2 [V. Vahedi et al., Phys. Fluids B 5, 2719 (1993)]. The PIC simulations provide the key parameters of the problem: the absorbed power, the radial deposition profiles, and the energy spectrum of the accelerated particles close to the grill.
A computational investigation of divertor plasma scaling laws
Authors: Knoll D. A. , Catto Peter J. , Krasheninnikov S. I.
Published: July 2002

Usually, tokamak core scaling laws are written in terms of dimensionless geometrical quantities and parameters corresponding to Coulomb collisionality, gyro-motion, and plasma beta. However, Lackner [K. Lackner, Comments Plasma Phys. Controlled Fusion 15, 359 (1994)] observed that the temperature profiles also must be the same to obtain the same atomic physics in the divertor region of similar discharges. He obtained a scaling indicating that none of the present tokamaks could be made similar to the International Thermonuclear Experimental Reactor (ITER) [G. Janeschitz et al., J. Nucl. Mater. 220–222, 73 (1995)], but implicitly retained only two body interactions. Subsequent work [P. J. Catto et al., Phys. Plasmas 3, 3191 (1996)] demonstrated that non-two-body effects (multistep radiation, excitation, and ionization processes as well as three body recombination) cannot be ignored for plasma densities above 1019 m−3; the regime in which the ITER divertor must operate. In this reactor relevant regime, scaling law information must be obtained experimentally and by complex numerical simulations. To retain and quantify non-two-body effects on scaling laws we employ numerical simulations from a two dimensional box geometry version of the UEDGE code [D. A. Knoll et al., Phys. Plasmas 3, 293 (1996)] which includes a coupled plasma and neutral fluid description retaining non-two-body effects. Results are presented from a numerical investigation into the upstream parallel heat flux divided by upstream pressure scaling, as well as collisionality scaling, of the tokamak divertor target heat flux and ion saturation current.
Magnetohydrodynamic turbulence and enhanced atomic processes in astrophysical plasmas
Authors: Spangler Steven R.
Published: July 2002

This article discusses a way in which enhanced atomic physics processes, including radiative energy losses, may occur in an astrophysical plasma containing magnetohydrodynamic turbulence. Two-dimensional (2D) magnetohydrodynamics (MHD) is adopted as a model. A major characteristic feature of 2D MHD turbulence is the development of strong current sheets on a dynamical time scale L/V0 where L is the spatial scale of the turbulent fluid and V0 is the scale of the velocity fluctuations. The current contained in the sheets will be carried by an electron drift relative to the ions. The case of a plasma containing minority atoms or ions with an excited state accessible to collisions from the tail of the electron distribution is considered. In the current carrying sheets or filaments, the electron distribution function will be perturbed such that collisional excitations will be enhanced relative to the current-free plasma. Subsequent radiative de-excitation of the atoms or ions removes energy from the turbulence. Expressions are presented for the electron drift velocity arising in 2D turbulence, the enhancement of collisional excitations of a trace atom or ion, and the energy lost to the plasma turbulence by radiative de-excitation of these atoms or ions. The mechanism would be most pronounced in plasmas for which the magnitude of the magnetic field is large, the outer scale of the turbulence is small, and the electron density and temperature are low. A brief discussion of the relevance of this mechanism to some specific astrophysical plasmas is given.
Two-dimensional instability of fast ionization waves propagating in an external electric field
Authors: Rutkevich I.
Published: July 2002

Stability of fast planar ionization waves (IW) propagating in a cold electropositive gas in the presence of constant applied electric field E0 has been investigated. The stationary IW are one-dimensional (1D) nonlinear solutions depending on the variable ξ=x−Vt and connecting two different steady states with constant parameters. The basic system of equations contains the rate equations for the concentrations of electrons and positive ions and the Poisson equation for the electric field. There is a continuum of possible velocities of stationary anode-directed (E0<0) and cathode-directed (E0>0) IW. All stationary IW are stable to small 1D perturbations. It has been shown that in a wide range of the parameters E0/p and V/Ve, where p is the gas pressure and Ve is the electron drift velocity ahead of the wave front, both anode- and cathode-directed IW are unstable to two-dimensional (2D) perturbations having the form f(ξ)exp(iky+λt). The 2D instability is displayed in the form of increasing corrugation of the planar IW. The integral representation of the electric potential perturbation has been employed for calculation of the eigenmodes and the growth rates in the long-wave limit kΔ≪1 where Δ is the characteristic thickness of a stationary IW.
Reconnection of coalescing magnetic islands
Authors: Marliani C. , Strauss H. R.
Published: July 2002

The dynamics of magnetic reconnection in the framework of two-dimensional incompressible magnetohydrodynamics is studied numerically. The case of a doubly periodic array of magnetic islands where coalescence of neighboring islands occurs due to self-consistent magnetic forces is investigated. To adequately resolve the current sheets which evolve in between two islands adaptive structured mesh methods are applied. At the onset of the reconnection process the kinetic energy rises and drops rapidly and afterward settles into an oscillatory phase. The time scale of the magnetic reconnection process is not affected by these fast events but consistent with the Sweet–Parker model of stationary reconnection. When the spatial extension of the system is enlarged it undergoes a sequence of merging processes and unstable equilibria towards a large-scale pattern of magnetic islands. During this process the frequency of the oscillations in the kinetic energy is found to scale with the island size.
Response to “Comment on ‘Parametric decays of a circularly polarized electromagnetic wave in an electron–positron plasma’ ” [Phys. Plasmas 6, 633 (1999)]
Authors: Gomberoff L. , Muñoz V.
Published: July 2002

In this response we shall rebut what we consider to be misleading statements in the comment by Shukla and Stenflo (hereafter referred to as SS).