bluuebunny/crossref_metadata_2025 huggingface dataset:
Two-phase Rayleigh–Bénard instabilities
Authors: Girasole T. , Darrigo R. , Gouesbet G. , Roze C.
Published: July 2002

The influence of particles on the first Rayleigh–Bénard bifurcation from the motionless to the convective state has been studied in a suspension layer heated from below and cooled from above, and effective physical properties have been measured. The particle parameters are the diameter and the concentration. The experimental setup allows one to measure accurately the temperature and the heat flux and to computerize the data recording. The suspensions consisted of polystyrene particles (diameters 0.1, 0.2, 3, and 100 μm and solid volume fraction from 1% to 10%) in water with the addition of surfactants and sodium sulphate for particles greater than 1 μm. The onset of convection was detected by a change of the slope of the curves showing the heat flux versus the temperature difference. The experimental results show (i) a good agreement between the experimental values of effective thermal conductivity and the theoretical formulas, (ii) an increase of the convective threshold with a volume fraction that will be attributed to the stabilizing effects of the particles on the fluid (growth of the thermal relaxation time and decrease of the buoyancy time), and (iii) a nonmonotonous subcritical bifurcation for particles smaller than 1 μm and supercritical bifurcation otherwise.
The steady boundary layer due to a fast vortex
Authors: Bernoff Andrew J. , van Dongen Harald J. H. M. , Lichter Seth
Published: July 2002

A point vortex located above and convected parallel to a wall is an important model of the process by which a boundary layer becomes unstable due to external disturbances. Often it has been assumed that the boundary layer due to the passage of the vortex is inherently unsteady. Here we show that for a vortex convected by a uniform shear flow, there is a steady solution when the speed of the vortex cv is sufficiently fast. The existence of the steady solution is demonstrated analytically in the limit of large vortex velocity (cv→∞) and numerically at more moderate speeds. This solution may provide a useful base state about which to investigate the stability of a boundary layer induced by external disturbances.
Uniform mean scalar gradient in grid turbulence: Asymptotic probability distribution of a passive scalar
Authors: Cai X. D. , O’Brien E. E. , Ladeinde F.
Published: July 2002

An asymptotic self-similar solution is obtained for the one-point probability density function (pdf) equation, of a passive scalar with uniform mean gradient in incompressible homogeneous turbulence. It is argued that the same solution should be a valid approximation when turbulence is generated in a high-quality wind tunnel. The asymptotic pdf shape is a unique function of the conditional expectation of the normalized scalar dissipation rate. The mean scalar gradient modifies the scalar pdf shape only if the conditional expected velocity component in the direction of the mean gradient is a nonlinear function of scalar fluctuation value. Experimental data from wind tunnel studies are consistent with the sign and scale of the changes produced by these nonlinearities.
A boundary integral method applied to the 3D water coning problem
Authors: Lucas S. K. , Kucera A.
Published: July 2002

Often in oil reservoirs a layer of water lies under the layer of oil. The suction pressure due to a distribution of oil wells will cause the oil-water interface to rise up towards the wells. A three-dimensional boundary integral formulation is presented for calculating the steady interface shape when the oil wells are represented by point sinks. Sophisticated integration techniques are implemented in an effort to obtain accurate results. In particular, the efficiency of various integration methods are compared for this problem, including QUADPACK routines, adaptive methods based on the IMT rule, the Kronrod rule, the method of degenerate quadrilaterals, and the Gauss-Rational rule for infinite integrals. Numerical results for various general multi-sink distributions are discussed, as are some further results for the axisymmetric single well problem.
Early bifurcation in rotating fluid flow with free surface studied by axisymmetric numerical simulations
Authors: Santos M. B. L. , So/rensen J. N.
Published: July 2002

Simulations of a fluid rotating inside a partially open cylindrical cavity, performed by numerical solution of the unsteady axisymmetric Navier–Stokes equations, are presented. The configuration consists of a cylindrical vessel holding the fluid, which is entrained into motion by a rotating lid. This one is a coaxial disk in contact with the fluid surface but without covering it entirely. The study focuses on the occurrence of time-dependent flow, more specifically, the first transition to unsteadiness, by considering cavity cases with different amounts of free surface, for a fixed aspect ratio. By following the time evolution of a few arbitrarily chosen dynamical variables as a function of the Reynolds number, the location of this first Hopf bifurcation is obtained for a collection of cavity cases. Results show a rather strong influence of the free surface both on the onset of the unsteadiness and on the dynamical features of the flow.
Numerical evaluation of a vortex-breakdown criterion
Authors: Watson J. P. , Neitzel G. P.
Published: July 2002

Numerical experimentation is used to examine the onset of axisymmetric vortex breakdown in a completely closed, cylindrical environment. Modification of the boundary motion allows one to compute a state of incipient vortex breakdown. The numerically determined states are used to test a recently posed hypothesis for the occurrence of vortex breakdown.
A new net-like structure formed by a metal/oil electrorheological fluid
Authors: Wen Weijia , Lu Kunquan
Published: July 2002

A new kind of net-like structure formed by the metal spheres suspended in a metal/oil electrorheological (ER) fluid is reported for the first time in this Letter. The experimental results show that this structure is totally different from that formed by dielectric particles. After comparing the two formation patterns, it is found that the dielectric particles in certain ER fluids align themselves into chains or columns in the direction of the external electric field. The metal particles in a metal/oil ER fluid, on the other hand, form a net-like structure of chains and no column could be observed even when the ER fluid has been exposed to a high electric field. We attribute the net-like structure of the metal ER fluid to the strong interaction between dipole fields induced on the metal particles under the external electric field. Finally, the dielectric properties of the different structures were measured and are discussed in this paper.
The Lagrangian spectral relaxation model of the scalar dissipation in homogeneous turbulence
Authors: Fox R. O.
Published: July 2002

Lagrangian pdf methods are employed to extend the spectral relaxation (SR) model of the scalar dissipation of an inert, passive scalar (1⩽Sc) in homogeneous turbulence. The Lagrangian spectral relaxation (LSR) model divides wavenumber space into a finite number (the total number depending on the Taylor-scale Reynolds number Rλ and the Schmidt number Sc) of wavenumber bands whose time constants are determined from the mean turbulent kinetic energy and instantaneous turbulent energy dissipation rate. The LSR model accounts for the evolution of the scalar spectrum (viz., pdf) from an arbitrary initial shape to its fully developed form. The effect of turbulent-frequency fluctuations on the instantaneous scalar dissipation rate following a Kolmogorov-scale fluid particle is incorporated into the LSR model through a Lagrangian pdf model for the turbulent frequency. Model results are compared with DNS data for passive scalar mixing in stationary, isotropic turbulence. Two distinct causes of non-Gaussian scalar statistics are investigated: small-scale intermittency due to scalar-dissipation fluctuations at scales near the Kolmogorov scale, and transient large-scale inhomogeneities due to the form of the initial scalar spectrum at scales near the integral scale. Despite the absence of fitting parameters, the LSR model shows satisfactory agreement with available DNS data for both types of flows.
Diffusion flame in a two-dimensional, accelerating mixing layer
Authors: Sirignano William A. , Kim Inchul
Published: July 2002

A theoretical and computational study of a laminar, two-dimensional, compressible, mixing, reacting layer with a pressure gradient that accelerates the flow in the direction of the primary stream is performed. One objective is to analyze the problems of a new technology related to combustion occurring in an accelerating transonic flow. Potential exists for reduction in nitric oxide formation and improvement in engine efficiency and/or power/weight. A similarity solution is found that reduces the partial differential equations to a system of ordinary differential equations. The solution is found in terms of a new acceleration parameter for compressible flows. The parameter is also useful for nonreacting mixing layers and for reacting or nonreacting wall boundary layers. For a low Mach number, the parameter reduces to the classical incompressible parameter. A numerical solution to these equations was performed. In the presence of exothermic reaction and accelerating pressure gradients, there are nonmonotonic velocity and density variations across the layer. That is, a peak velocity and a minimum density occur near the middle of the layer. The Mach number is minimized in the flame region sometimes resulting in a subsonic local flow even with supersonic freestreams. This indicates the occurrence of large strain and large vorticity (created by the cross product of pressure gradient and density gradient). Temperature and mass fractions across the layer are also obtained as a function of the acceleration parameter and the freestream conditions. The temperature profile is nonsimilar; the acceleration results in decreasing flame temperature with downstream distance indicating the possibilities for extinction or NOx reduction.
Influence functions for edge wave propagation over a nonplanar bottom bathymetry
Authors: Putrevu Uday , Oltman-Shay Joan
Published: July 2002

The problem of edge waves propagating over a nonplanar bottom bathymetry is examined. Assuming that the variation from the planar case is small, we examine the problem using a perturbation expansion. This assumption allows an analytical estimate of how nonplanar features change the frequency and spatial structure of an edge wave with a given wavenumber. We find that these changes can be conveniently expressed in terms of “influence functions.” For example, the change in frequency, σ1,n, of a given edge-wave mode, n, can be expressed as σ1,n/σ0,n=∫0∞h1Iσdx where x is the cross-shore coordinate, h1(x) is the deviation from the planar topography, σ0,n is the frequency of the edge wave on a plane beach, and Iσ is the influence function. Similar results are also derived for the spatial structure of the edge wave. The results show that 1) in general, the spatial structure of the edge wave is more sensitive to bottom perturbations than the frequency, and 2) at a given wavenumber, the higher modes are more sensitive to shoreline features than the lower modes.
Correlations and conditional statistics in differential diffusion: Scalars with uniform mean gradients
Authors: Yeung P. K.
Published: July 2002

A number of statistical measures of the differential diffusion of multiple scalars driven by uniform mean gradients are studied by direct numerical simulation (DNS), in isotropic turbulence using up to 5123 grid points. Consistent with a gradient correlation given by a function of the molecular diffusivities alone, the coherency spectrum is found to be insensitive to the Reynolds number when scaled by Kolmogorov variables in wave number space. Analytical results based on joint Gaussianity assumptions are derived for important unclosed terms in the probability density function (PDF) and conditional moment closure (CMC) approaches, and shown to be successful in describing the behavior of conditional statistics extracted from DNS. The multiscalar conditional diffusion in the joint PDF equation is linear in the sample space variables and specified by the two-scalar correlation coefficients. A linear result is also derived for the term “ey,” which represents the transport of conditional fluctuations in the CMC method. The functional form obtained indicates that closure for this quantity remains an important issue at high Reynolds numbers. The transport of conditional fluctuations due to unsteady, convective and diffusive effects are analyzed separately; results for conditioning on the more diffusive versus less diffusive scalar differ in a manner traceable to the fact that less diffusive scalar fields carry more small-scale spectral content.
Two-fluid Taylor–Couette flow: Experiments and linear theory for immiscible liquids between corotating cylinders
Authors: Baier Gretchen , Graham Michael D.
Published: July 2002

We investigate the centrifugal instability of a pair of radially stratified immiscible liquids in the annular gap between concentric, corotating cylinders: two-fluid Taylor–Couette flow. Experiments show that a two-layer flow with a well-defined interface and Taylor vortices in each phase can be obtained. The experimental results are in good agreement with predictions of inviscid arguments based on a two-phase extension of Rayleigh’s criterion, as well as with detailed linear stability calculations. For a given geometry, the most stable configuration occurs for fluids of roughly (exactly in the inviscid limit) equal dynamic viscosities. For fluids with sufficiently low viscosity, we also experimentally observe another instability that is evidently the two-fluid analog of the instability of a thin film of liquid coating the inside of a rotating drum.
Analysis and simulation of a turbulent, compressible starting vortex
Authors: Grasso F. , Pirozzoli S. , Gatski T. B.
Published: July 2002

The numerical simulation of a shock induced, turbulent compressible vortex is analyzed. The vortex is generated by the movement of an (unsteady) shock over the trailing-edge of a two-dimensional airfoil at different incidence angle. A theoretical analysis is performed for the case of a turbulent line vortex, at a low (vortex) Mach number and high Reynolds number, as a basis of comparison for the dynamic structure of the starting vortex. This theoretical analysis confirms the existence of an equilibrium structure of the isolated vortex similar to the laminar case. In the numerical computations, both an isotropic eddy viscosity two-equation turbulence model and an algebraic stress model are employed to assess the influence of turbulence model on the predictive capabilities of such a flow including the realization of the self-similar behavior found in the isolated vortex case. The results are also compared to theoretical estimates of the production-to-dissipation rate ratio for both types of turbulence models, and to recent experiments on such flows.
Domain of convergence of perturbative solutions for Hele-Shaw flow near interface collapse
Authors: Pesci Adriana I. , Goldstein Raymond E. , Shelley Michael J.
Published: July 2002

Recent work [Phys. Fluids 10, 2701 (1998)] has shown that for Hele-Shaw flows sufficiently near a finite-time pinching singularity, there is a breakdown of the leading-order solutions perturbative in a small parameter ε controlling the large-scale dynamics. To elucidate the nature of this breakdown we study the structure of these solutions at higher order. We find a finite radius of convergence that yields a new length scale exponentially small in ε. That length scale defines a ball in space and time, centered around the incipient singularity, inside of which perturbation theory fails. Implications of these results for a possible matching of outer solutions to inner scaling solutions are discussed.
New instability modes of a diffusion flame near extinction
Authors: Papas Paul , Monkewitz Peter A. , Tomboulides Ananias G.
Published: July 2002

The linear temporal stability of a diffusion flame, modeled with a simplified one-dimensional approach, in a two-dimensional mixing layer is investigated. In addition to the well-known “shear” or Kelvin–Helmholtz mode, which is also present in nonreacting flows, the current analysis reveals new “heat release” or combustion modes for flames near the extinction limit, i.e., when the Damköhler number is low. For conditions near extinction, the shear mode is found to be damped and only the combustion modes are unstable. The fundamental destabilizing mechanism of these combustion modes is elucidated.
The wall-normal position in pipe and channel flows at which viscous and turbulent shear stresses are equal
Authors: Sahay Anupam , Sreenivasan Katepalli R.
Published: July 2002

The Reynolds number dependence is estimated for the wall-normal position at which the turbulent and viscous shear stresses are equal. For large Reynolds numbers, this position coincides with the peak location of the mean momentum transport terms due to turbulent and viscous action. Experimental data are used to corroborate the results and assess some quantitative details.
The inhomogeneous structure of a bidisperse sedimenting gas–solid suspension
Authors: Valiveti Prabhu , Koch Donald L.
Published: July 2002

We consider a model of a bidisperse gas–solid suspension in which the particles are subject to gravitational and Stokes drag forces and undergo elastic solid-body collisions. Dynamic simulations of many interacting particles in a unit cell with periodic boundary conditions indicate that the suspension has an inhomogeneous structure on the length scale of the cell. A linear stability analysis of averaged equations of motion for the particulate phase is used to predict the values of the Stokes number, particle volume fraction, and unit cell length for which the homogeneous suspension is unstable and these results are compared with the numerical simulations. The suspension is subject to long horizontal wave instabilities at sufficiently high particle volume fractions and low Stokes numbers. The mechanism of instability involves a coupling between the shear flow induced by particle volume fraction variations and the collisional exchange of momentum between the particles. Solutions of the averaged equations successfully capture the particle velocity fields induced by the inhomogeneous structure in the unstable suspensions. These velocity fields are characterized by the mean and variance of the particle velocity and by momentum-density correlation functions. When the total particle volume fraction is small, the simulated suspensions are stable but still exhibit long-range structure. This structure may be attributed to a pair probability, corresponding to an excess of neighbors of the same species, and a deficit of neighbors of the other species, which decays like 1/r with radial distance r.
Experimental and numerical study of anomalous thermocapillary convection in liquid gallium
Authors: Priede Janis , Cramer Andreas , Bojarevics Andris , Gelfgat Alexander Yu. , Bar-Yoseph Pinhas Z. , Yarin Alexander L. , Gerbeth Gunter
Published: July 2002

Thermocapillary Marangoni convection of liquid gallium was studied experimentally and numerically. A specially designed experimental setup ensured an oxide-free surface of the liquid gallium for a very long time. The convective flow at the free surface was found to be directed opposite to both buoyancy-driven and ordinary thermocapillary convection. The anomalous direction of the thermocapillary flow was explained by the presence of a small amount of a surface-active contaminant—lead adsorbed at the free surface. Two different approaches were used to describe the observed phenomenon. First, the flow was treated as a pure thermocapillary convection with a modified dependence of the surface tension on temperature so that to reproduce the measured velocity distribution. Second, a novel physical model was devised for the flow driven by the gradient of the surface tension induced by the temperature dependence of the concentration of the adsorbed layer of contaminant. In contrast to the ordinary thermocapillary convection in low-Prandtl-number liquids, there is a strong coupling between the flow and the driving force in the proposed model resulting in velocity profiles very similar to those observed in the experiment.
Absolute and convective nature of the Eckhaus and zigzag instability with throughflow
Authors: Chomaz J.-M. , Couairon A. , Julien S.
Published: July 2002

The nature of the Eckhaus and of the zigzag instability is investigated for a periodic basic “flow” (a y-periodic Stokes solution) in the presence of a transverse or a longitudinal mean flow using the two-dimensional extension of the absolute instability criterion. For each flow orientation, stability diagrams are obtained numerically and analytically for a simple amplitude-equation model considering both the Eckhaus and the zigzag instability. Analytical results extend and correct a previous analysis by Müller and Tveitereid. In particular, for a longitudinal flow, the Eckhaus instability is convective near its instability threshold and the absolute destabilization occurs at a finite wave number. Similar results hold for the zigzag instability for a transverse throughflow which is convective near threshold. In the presence of an arbitrarily oriented mean flow, the absolute threshold for the Eckhaus instability is also numerically determined. Implications of these results for real experiments are discussed.
On the weakly nonlinear development of the elliptic instability
Authors: Lebovitz Norman R. , Saldanha Kenneth I.
Published: July 2002

Results are presented for the outcome of the elliptic instability, investigated by methods of dynamical-systems theory. Finite-dimensional nonlinear systems are obtained through Galerkin truncation using a systematic truncation criterion that exactly captures any fluid behavior that can also be captured via amplitude expansions. Six different regions of parameter space are explored, corresponding to linear instabilities of different symmetries and temporal types (“steady” or “oscillatory”). Four different kinds of bifurcation behavior are found among the six cases considered. One of these equilibrates at small amplitude, and the others do not, but depart significantly from the unstable equilibrium solution.
Anomalous scaling from controlled closure in a shell model of turbulence
Authors: L’vov Victor S. , Pierotti Daniela , Pomyalov Anna , Procaccia Itamar
Published: July 2002

We present a model of hydrodynamic turbulence for which the program of computing the scaling exponents from first principles can be developed in a controlled fashion. The model consists of N suitably coupled copies of the “Sabra” shell model of turbulence. The couplings are chosen to include two components: random and deterministic, with a relative importance that is characterized by a parameter called ε. It is demonstrated, using numerical simulations of up to 25 copies and 28 shells that in the N→∞ limit but for 0<ε⩽1 this model exhibits correlation functions whose scaling exponents are anomalous. The theoretical calculation of the scaling exponents follows verbatim the closure procedure suggested recently for the Navier–Stokes problem, with the additional advantage that in the N→∞ limit the parameter ε can be used to regularize the closure procedure. The main result of this paper is a finite and closed set of scale-invariant equations for the 2nd and 3rd order statistical objects of the theory. This set of equations takes into account terms up to order ε4 and neglects terms of order ε6. Preliminary analysis of this set of equations indicates a K41 normal scaling at ε=0, with a birth of anomalous exponents at larger values of ε, in agreement with the numerical simulations.
Nonlinear dynamo in ABC flow: The Hall effect
Authors: Galanti B. , Kleeorin N. , Rogachevskii I.
Published: July 2002

Nonlinear evolution of the magnetic field generated by a prescribed deterministic flow of a conducting fluid in form of the Arnold–Beltrami–Childress (ABC) flow is studied numerically. The nonlinearity is caused by the Hall effect. After the linear regime, the Hall term in the induction equation becomes important, leading to saturation of the magnetic field. The oscillations of the magnetic field which characterize the linear regime fade away into a steady state regime. The structure of the magnetic field can be viewed as a sum of two components: a field of the integral scale and a small-scale field. The large-scale field contains most of the energy of the system, whereas the energy of the small-scale field is very small. These results demonstrate significant difference between the actions of two types of nonlinearity in terms of the magnetic field: the Ampère force and the Hall effect.
Analysis of transient and asymptotic behavior in relativistic Landau damping
Authors: Sartori C. , Coppa G. G. M.
Published: July 2002

Relativistic electrostatic oscillations in a collisionless nonmagnetized plasma are considered. An integral form of the Vlasov–Poisson problem is solved numerically and both the transient and the asymptotic behavior of the electric field are calculated in a very accurate way. The existence of a critical value for the wave number, under which no Landau damping occurs, has been verified, and the asymptotic behavior of the oscillations in the one-dimensional kinetic model has been evaluated. Some errors have been found in previously published analytical expressions. A new expression is proposed and verified numerically. An analytical expression of the asymptotic behavior for full three-dimensional kinetic model is deduced and numerically tested.
Neoclassical rotation of tokamak plasmas in the plateau regime
Authors: Novakovskii S. V. , Galeev A. A. , Liu C. S. , Sagdeev R. Z. , Hassam A. B.
Published: July 2002

The poloidal rotation of tokamak plasmas is studied in the plateau regime. It is shown that the relaxation rate is given by vT/(qR)ν̂1/3O(1), while the inertia enhancement in this regime is 1+q2ν̂−1/3O(1), resulting from the time-dependent parallel viscosity, where q is the safety factor, ν̂=ε3/2ν* is the plateau collisionality parameter (ε3/2<ν̂<1), ε is the inverse aspect ratio, and ν* is the standard neoclassical collisionality parameter. An evolutionary equation for the radial electric field is derived.
Excitation of electromagnetic waves in a relativistic backward wave oscillator with end reflectors
Authors: Choyal Y. , Maheshwari K. P.
Published: July 2002

An investigation of the boundary effects on the operation of a relativistic backward wave oscillator (BWO) is presented. Analytical expressions for dispersion relation, global instability, and start oscillation current are obtained for the case of weak ripple of the slow wave structure. Numerical results show a periodic behavior of the start oscillation current with the length of the device. Results are also obtained depicting the variation of the start current with reflection coefficient and beam energy. A comparison of the results is made with those of earlier work [Phys. Fluids B 4, 2293 (1992)].
Simulation studies of nonlinear magnetosonic waves in a two-ion-species plasma
Authors: Toida Mieko , Ohsawa Yukiharu , Jyounouchi Tadamasa
Published: July 2002

Magnetosonic waves propagating perpendicular to a magnetic field in a plasma containing two ion species is studied. In a two-ion-species plasma there are two kinds of magnetosonic waves; low- and high-frequency modes. First, linear dispersion relations and Korteweg–de Vries (KdV) equations for these modes are briefly described. The KdV equation for the low-frequency mode is valid when the amplitudes ε are quite small, while that for the high-frequency mode is valid when (me/mi)1/2≪ε≪1, where me/mi is a measure of electron-to-ion mass ratios. Then, a one-dimensional, electromagnetic simulation code based on a three-fluid model is used to study evolution of a large-amplitude low-frequency mode and a small-amplitude high-frequency mode, which the KdV theory cannot deal with. It is found that, from a profile of a large-amplitude solitary wave of the low-frequency mode, short-wavelength solitary waves of the high-frequency mode can be generated. A small-amplitude solitary wave of the high-frequency mode produces long-wavelength perturbations behind the pulse. It leads to the gradual damping of the original pulse.
The collisionless coalescence instability with two-species and in-plane-current effects
Authors: Pritchett P. L.
Published: July 2002

The investigation by means of particle simulations of the process whereby magnetic island structures tend to merge into larger units is extended to include the effects of electron dynamics and the in-plane components of the currents. With the ions marginally unmagnetized (ρi/λ∼1, where ρi is the asymptotic ion gyroradius and λ is the current sheet half-thickness), the electrons reduce the linear growth rate by some 20%, but they do not alter the basic coalescence physics. During the nonlinear stage of the pairwise merging process, a quadrupole out-of-the-plane magnetic field structure with peak amplitude ∼0.1B0 is formed, but this field collapses after the merging is completed. In the parameter regime accessible to the simulations [flux bundle size L/(c/ωpi)≳3 and mass ratio Mi/me≤36], there is no indication of any significant change in the reconnection rate that could be associated with whistler-mediated reconnection. The sequential merging of 4 islands into one large island leads to the formation of a vacuum X point and substantial particle energization.
Ion velocity distribution function observations in a multipolar argon discharge
Authors: Bachet G. , Chérigier L. , Doveil F.
Published: July 2002

Observations of ion velocity distribution functions in two different configurations of a multipolar device with a laser induced fluorescence diagnostic in argon plasma are presented. The first experiment, carried out in the magnetic field closing the two ends of the device, shows the influence of this field on the ion trajectories. In the second experiment performed without the magnetic field closing the two ends of the device, the influence of the electrostatic boundary potential of the plasma is observed. Longitudinal ion velocity distribution functions have been recorded until inside the plasma sheath. The shapes of these distribution functions are in qualitative agreement with the results of a theoretical model and give an experimental justification for the ‘‘flux’’ shape of the ion velocity distribution source function retained in this model. However, to reach this qualitative agreement, it is necessary to adjust the ionic temperature for each distribution function suggesting that, for the authors’ weakly ionized plasma, another heating mechanism is missing in this theory.
Electron beam pulses produced by helicon-wave excitation
Authors: Ellingboe A. R. , Boswell R. W. , Booth J. P. , Sadeghi N.
Published: July 2002

The 443 nm Ar+ line emission intensity in a cylindrical argon magnetoplasma excited by a 13.56 MHz helicon antenna is found to be strongly modulated at the excitation frequency. The peak in optical emission propagates in the axial direction at a velocity corresponding to that of helicon waves launched by the antenna. The spatiotemporal modulation is consistent with pulses of electrons produced by acceleration under the antenna and subsequent entrainment of these electrons over half of a trapping period in the axial electric field of the helicon wave.
Kinetic effects on the electron thermal transport in ignition target design
Authors: Honda M. , Nishiguchi A. , Takabe H. , Azechi H. , Mima K.
Published: July 2002

Preheating is one of the most critical issues in laser fusion, because of significant reduction of volume compression. The nonlocal heat transport in an ablative plasma is found to play an important role in the preheating under high intensity laser irradiation. Namely, the electron heat transport should be described by the Fokker–Planck (FP) equation in the fluid implosion code. The Spitzer–Härm (SH) thermal conduction model is not applicable because the electron mean free path is comparable to the temperature scale length. The numerical simulations of the implosion with the FP heat transport have been carried out for the fast (high entropy) implosion mode in which the implosion velocity reaches as high as 6×107 cm/s. In the fast implosion, the required laser energy for ignition can be reduced. It is found in the simulation that the isentrope in the FP simulation code is higher by two to four times than that in the flux limited SH simulation.
Some remarks on two-dimensional incompressible stationary reconnection
Authors: Neukirch T. , Priest E. R.
Published: July 2002

An alternative formulation of the problem of two-dimensional incompressible stationary resistive flows is presented. This formulation is used to (a) investigate the conditions under which a recently established anti-reconnection theorem may be overcome for stationary resistive flows with vanishing viscosity and (b) to rederive a reconnective annihilation solution and discuss its properties in the light of the anti-reconnection theorem.
Spherical stellarator with plasma current
Authors: Moroz Paul E.
Published: July 2002

Recently proposed novel concept of a spherical stellarator (P. E. Moroz, ‘‘Spherical stellarator configuration,’’ to appear in Phys. Rev. Lett) is enhanced by adding the plasma current to the otherwise pure stellarator system. The coil configuration of this ultra low aspect ratio system differs from that of a spherical tokamak by inclination of external parts of the toroidal field coils. It is shown that the configuration considered possesses many attractive properties, including: wide flexibility of operating regimes, compact design and coil simplicity, good access to the plasma, closed vacuum flux surfaces with large enclosed volume, significant external rotational transform, strong magnetic well, and a high plasma β [β(0) in excess of 30%] equilibrium. It is shown that the bootstrap effect in a spherical stellarator, in principle, can supply the full plasma current required for the high-β equilibrium.
Asymptotic electron trajectories and an adiabatic invariant for a helical-wiggler free electron laser with weak self-fields
Authors: Wang L. , Kevorkian J.
Published: July 2002

The dynamics of a relativistic electron in the field configuration consisting of a constant-amplitude helical-wiggler magnetic field, a uniform axial magnetic field, and the equilibrium self-fields is described by a near-integrable three-degree-of-freedom Hamiltonian system. The system is solved asymptotically for small ε by the method of averaging, where ε measures the strength of the self-fields. Because the Hamiltonian does not depend on one of the coordinates, it immediately reduces to a two-degree-of-freedom system. For ε=0, this reduced system is integrable, but is not in standard form. The action-angle transformation to standard form is derived explicitly in terms of elliptic functions, thus enabling the application of the averaging procedure. For almost all regular electron trajectories the solution is explicitly derived in asymptotic form and an adiabatic invariant is constructed, both results are in a form that remains uniformly valid over the time interval for electrons to transit the laser. The analytical results are verified by numerical calculations for an example problem.
Creation of a resonant diocotron mode
Authors: Kaup D. J. , Thomas Gary E.
Published: July 2002

The modal structure for linearized waves in a magnetron or a crossed-field amplifier (CFA) is shown to be very sensitive to the profile of the electron density at the edge of the electron sheath. As is well known, with the classical Brillouin density profile, no propagating diocotron mode can exist. Furthermore, in the parameter regime at which these devices operate (ω=kvd where ω is the frequency, k is the wave vector and vd is the drift velocity at the top of the sheath), there are no unstable modes. However if we replace the Brillouin density profile with a ‘‘ramped’’ density profile (where the discontinuity is replaced by a finite, but large, negative density gradient), then we can show that any mode in the operating regime will become weakly unstable. However these weakly unstable modes in the presence of the strong density gradient at the edge of the ramped density profile combine to generate a quasilinear diffusion of order unity. Thus, after a time on the order of a few cyclotron periods, the original density profile will become modified. This modification is expected to generate a plateau on the edge of the sheath at which time a resonant propagating diocotron mode can exist. Implications of these results and their predictions for magnetron and CFA operation are discussed.
Stopping power of ions in a strongly magnetized plasma
Authors: Boine-Frankenheim O. , D’Avanzo J.
Published: July 2002

The energy loss of heavy ions in a strongly magnetized plasma is studied by means of Vlasov and Particle-in-Cell (PIC) simulations. The topic is of importance for many practical applications, as for example in the electron cooling of heavy ion beams. Measurements of the longitudinal cooling force show strong deviations from the well known linear theories. Moreover, the possibility of heavy ions to reach high charge states Zp increases the interest in understanding the plasma regime with coupling parameter Zp/ND≳1 (ND is the number of electrons in a Debye sphere), when nonlinear phenomena are important. In this regime a linearization of the drift kinetic Vlasov–Poisson system is not possible, so that a fully nonlinear numerical approach is unavoidable. Comparisons between the two numerical schemes are made for the stopping power and the potential created in the plasma by a moving ion, together with the dielectric theory of ion stopping. Our results show strong deviations from the dielectric theory, and our investigations show the importance of the role played by the electrons trapped in the potential troughs excited by the ion in the stopping process.
Exact solutions for steady-state, planar, magnetic reconnection in an incompressible viscous plasma
Authors: Fabling R. B. , Craig I. J. D.
Published: July 2002

The exact planar reconnection analysis of Craig and Henton [Astrophys. J. 450, 280 (1995)] is extended to include the finite viscosity of the fluid and the presence of nonplanar components in the magnetic and velocity fields. It is shown that fast reconnection can be achieved for sufficiently small values of the kinematic viscosity. In particular, the dissipation rate is sustained by the strong amplification of planar magnetic field components advected toward the neutral point. By contrast, nonplanar field components are advected without amplification and so dissipate energy at the slow Sweet–Parker rate.
Detonation: A mechanism for the explosive release of energy in a plasma
Authors: Cowley Steven , Artun Mehmet , Albright Brian
Published: July 2002

The explosive release of energy by a plasma is a common and striking occurrence. A novel explosive scenario is demonstrated in a simple system that becomes unstable to a line tied Rayleigh–Taylor–Parker instability. The system crosses the linear instability threshold in a narrow region. Initially the instability grows in the narrow region. Two nonlinear terms become important at low amplitude. One nonlinearity causes the mode to grow explosively and narrow into finger-like structures. The other nonlinearity broadens the mode into the linearly stable region. The explosive nonlinearity makes the linearly stable region metastable. The combination of the two nonlinearities causes the mode to progressively destabilize the metastable region. Since this process is reminiscent of the detonation of chemical explosives, it is called detonation. Without dissipation the kinetic energy released is proportional to (t0−t)−6.4.
Stable ellipticity-induced Alfvén eigenmodes in the Joint European Torus
Authors: Heidbrink W. W. , Fasoli A. , Borba D. , Jaun A.
Published: July 2002

An external antenna excites stable eigenmodes in elongated Ohmically heated plasmas in the Joint European Torus (JET) [P.-H. Rebut, R. J. Bickerton, and B. E. Keen, Nucl. Fusion 25, 1011 (1985)]. The frequency of the modes (240−290 kHz) falls in the gap in the magnetohydrodynamic (MHD) continuum that is produced by ellipticity. Some modes are very weakly damped (γ/ω<10−3).
Description of contained mode solutions to the relevant magnetosonic-whistler wave equations
Authors: Penn G. , Riconda C. , Rubini F.
Published: July 2002

The generalization of magnetosonic-whistler modes having a dispersion relation of the form ω2=k⊥2(vA2+DH2k‖2), where vA is the Alfvén velocity and DH≡B/μ0ne, to the case of an inhomogeneous plasma leads to localized eigenmode solutions. These solutions, which are referred to as contained modes, are tightly confined in the direction of inhomogeneity. In this paper, the contained modes are analyzed for frequencies ω≳Ωi and for k‖≪k⊥. In a cylindrical or toroidal configuration, these modes propagate mostly in the poloidal direction and are localized within a narrow radial shell. Contained modes have been proposed as an underlying mechanism for anomalous ion cyclotron emission (ICE) in plasmas with an energetic ion population. Analytic expressions for the characteristics of these modes are obtained using an expansion in high poloidal mode number and are found to agree with numerical results. For typical plasma profiles, the contained modes are localized towards the edge of the plasma column. It is found that the key parameter which regulates the behavior of the mode is di0/a=DH/avA≪1. This quantity is related to the Hall term which is kept as the first significant correction to Ohm’s law for the frequencies being considered. The Hall term introduces to the eigenmode equation a dependence on the sign of the poloidal phase velocity, and when di0/a is sufficiently large contained solutions only exist for modes whose poloidal phase velocity has the same orientation as the ion cyclotron motion. The dependence of the mode characteristics on finite k‖ is examined in detail. In particular, the quantity R0q/di0 determines the poloidal mode number above which k‖ effects, such as a shift in the region of localization of the mode, become significant.
On the viscous boundary layer near the center of the resistive reconnection region
Authors: Uzdensky Dmitri A. , Kulsrud Russell M.
Published: July 2002

This paper studies the behavior of the magnetic field near the center of the reconnection layer in the framework of two-dimensional incompressible resistive magnetohydrodynamics with uniform resistivity in a steady state. Priest and Cowley [J. Plasma Phys. 14, 271 (1975)] have presented an argument showing that when the viscosity ν is zero, the magnetic separatrices do not cross at a finite angle but osculate at the X-point. In the present paper it is shown that this conclusion is in fact not correct. First, some results of numerical simulations of the reconnection layer are presented. These results contradict the conclusions of Priest and Cowley. To explain this contradiction, an analytical theory for the neighborhood of the X-point is developed. When viscosity ν is small, a narrow boundary layer develops near the neutral point. Some of the higher derivatives of the stream function Φ become very large near the X-point, leading to a nonzero angle between the separatrices. As ν→0, the boundary layer shrinks and nonanalytic logarithmic terms emerge in the expansion of Φ in the outer region. This makes the Taylor expansion of Priest and Cowley invalid in the case ν=0. The results of our boundary layer analysis are in good agreement with the numerical simulations.
Self-modulation of surface waves at a plasma–metal interface
Authors: Ostrikov K. N. , Yu M. Y. , Azarenkov N. A.
Published: July 2002

The ponderomotive force effects on surface waves at a plasma–metal interface are studied. The waves propagate across an external magnetic field parallel to the interface. It is shown that the account of the ponderomotive force can lead to the appearance of solitons, which are not possible when the second-harmonic and magnetic nonlinearities are concerned.
Center manifold approach to the reduced magnetohydrodynamic bifurcations with diffusive magnetic field lines
Authors: Blüming M. , Spatschek K. H. , Grauer R.
Published: July 2002

Bifurcations in plasmas are investigated on the basis of a reduced dissipative magnetohydrodynamic (MHD) model. In contrast to previous investigations, the diffusivity of magnetic field lines is taken into account. Making use of the center manifold theory for the first bifurcations, and Galerkin approximations for higher bifurcations, it is shown that the diffusion of magnetic field lines affects the transitions in the transients. When the mode which resembles the so-called high confinement mode becomes unstable via a Hopf bifurcation, the changes in the oscillation frequencies are calculated. It is demonstrated that over a wide range of parameter values the so called electrostatic approximation is quite good. The strength of the generated magnetic field fluctuations is calculated, and the influence of the latter on a possible magnetic braiding is estimated.
Influence of finite particle temperature on a density distribution in beams
Authors: Karimov A. R. , Aliev Yu. M.
Published: July 2002

Based on the gas dynamics description, the structural dynamics of a non-neutralized charged-particle beam is examined within limited spaces with reference to Lagrangian coordinates. The spatial behavior of density for beams with a finite particle temperature is analyzed.
Mean magnetic field renormalization and Kolmogorov’s energy spectrum in magnetohydrodynamic turbulence
Authors: Verma Mahendra K.
Published: July 2002

A self-consistent renormalization group procedure has been constructed for magnetohydrodynamic turbulence in which small wave number modes are averaged out, and the effective mean magnetic field at large wave numbers is obtained self consistently. In this renormalization group scheme, it is found that an E(k) proportional to k−5/3 (Kolmogorov’s spectrum) is a self-consistent solution, and the procedure yields a self-consistent effective mean magnetic field proportional to k−1/3. It is also deduced from the formalism that the magnitude of the cascade rate decreases as the strength of the mean magnetic field is increased.
Generalized gyrotron theory with inclusion of electron velocity and energy spreads
Authors: Dumbrajs O. , Koponen J. P. T.
Published: July 2002

The influence of the electron velocity and energy spreads on gyrotron operation is studied by generalizing the well-known η⊥(μ,F) plots by means of the Monte Carlo techniques. Such contour plots may be useful for a quick quantitative estimation of the detrimental effect of the spreads on the efficiency of a gyrotron.
Longitudinal and transversal structure functions in two-dimensional electron magnetohydrodynamic flows
Authors: Germaschewski Kai , Grauer Rainer
Published: July 2002

Electron magnetohydrodynamic flows have recently attracted considerable interest not only in the field of collisionless reconnection, but also as the first two-dimensional turbulent system showing Kolmogorov-type scaling for the energy spectrum. In this paper, longitudinal and transversal structure functions are calculated for varying Reynolds numbers. The simulations show that the differences between longitudinal and transversal structure functions are finite size effects for this type of flow and vanish in the limit of high Reynolds numbers. In addition, the scaling of structure functions for velocity and current density could be described by assuming a log-Poisson statistics with two atoms, as recently proposed for passive scalar advection.