bluuebunny/crossref_metadata_2025 huggingface dataset:
The influence of laser frequency bandwidth on the time and space resolved structures of the 2ω harmonic generation
Authors: Weihan Tan , Zunqi Lin , Min Gu , Aing Shi , Wenyan Yu , Ximing Deng
Published: July 2002

Reported here are 2ω0 harmonic space and time resolved structures taken by a streak camera at an angle of 90° with respect to the laser optical axis. The evident differences, as shown between the broadband and narrowband laser shooting, are analyzed using the theory of plasma soliton generation. The results obtained are consistent with other experiments.
Angular dispersion of material lines in isotropic turbulence
Authors: Lumley John L. , Sarkar Sutanu
Published: July 2002

The existing body of literature on particle transport by turbulent flow has concentrated on the behavior of spherical particles. However, the dispersed phase in several suspensions of industrial interest consists of nonspherical particles, in particular, flexible slender bodies or threads. The present work considers the problem of disorientation of initially aligned material lines in isotropic turbulence, an idealization which serves to test a general model for particle orientation effects in the transport of threads in turbulent flow. Results obtained from the model agree well with data.
Asymptotic solutions of steady magneto-fluid-dynamic motion between two rotating disks with a small gap
Authors: Xu J. J. , Woo J. T.
Published: July 2002

The steady-state flow of a conducting fluid between two coaxial rotating disks in the presence of an axial magnetic field is considered for the following conditions: (1) the gap d between two disks is very small compared with the radial extension of the disks R; (2) the angular velocity of the disks is not too high, so that the thickness of the Eckman layer δ is still larger than the gap d, (d/δ)1/4≪1; and (3) the magnetic field B is moderate so that the corresponding Hartman number M≪R2/d2. Under these conditions asymptotic solutions to the problem are obtained in terms of the small parameter Ε=d/R. The results show that to the lowest-order approximation, the electric properties of the disks are not important to the flow field, while the magnitude of the magnetic field plays an important role in the equilibrium flow profile.
Interspecies transfer of momentum and energy in disparate-mass gas mixtures
Authors: Riesco-Chueca P. , Fernandez-Feria R. , Fernandez de la Mora J.
Published: July 2002

The collision integrals describing the rate of exchange of momentum and tensorial energy between the components in a binary mixture of neutral gases with very different atomic masses are determined for arbitrary values of their two temperatures and velocities, for realistic intermolecular potentials, and allowing for large departures of the heavy gas from equilibrium conditions. In the range of interest where the system is perturbed within times of the order of the slow relaxation time characterizing the transfer of energy between unlike molecules, the light gas distribution function is Maxwellian to lowest order, with corrections given asymptotically in powers of the small parameter m/mp formed with the ratio of the species molecular masses. Also, provided that the ratio Tp/T between the temperatures of the two gases remains much smaller than mp/m, the desired collision integrals may be evaluated asymptotically in powers of m/mp in all generality. The computation is carried out in detail for the case when the interaction between atoms is described by a Lennard–Jones potential. A combination of numerical computations with optimal matching of analytical expressions valid for large and small slip velocities leads to a set of compact formulas which hold for the limits of high and low temperatures and to a general approximate expression for all temperatures.
Transport scaling in the collisionless-detrapping regime in stellarators
Authors: Crume E. C. , Shaing K. C. , Hirshman S. P. , van Rij W. I.
Published: July 2002

Stellarator transport scalings with electric field, geometry, and collision frequency in the reactor-relevant collisionless-detrapping regime are determined from numerical solutions of the drift kinetic equation. A new geometrical scaling, proportional to ε3/2t rather than εtε1/2h, is found, where εt is the inverse aspect ratio and εh is the helical ripple. With the new scaling, no reduction in energy confinement time is associated with large helical ripple, which provides design flexibility. Integral expressions for the particle and heat fluxes that are useful for transport simulations are given.
Model functions of Reynolds stress models
Authors: Janicka Johannes
Published: July 2002

A Reynolds stress (Re-stress) model for the prediction of shear flows is presented. Special attention is paid to the determination of the model functions. First, the general modeling assumptions are discussed and the modeling of the equations is then described in detail. It is shown that by successive analysis of different types of flows a set of model functions with a high level of universality can be deduced. Finally, prediction/measurement comparisons in homogeneous shear flows, round jets, plane jets, and wakes are presented.
Relative velocity fluctuations in turbulent flows at moderate Reynolds numbers. I. Experimental
Authors: Tong P. , Goldburg W. I.
Published: July 2002

Turbulent pipe flow and grid flow have been explored by the scattering of light from small particles suspended in a fluid. Laser Doppler velocimetry and visual observation were used to characterize the gross features of the flows. However, novel information came from the homodyne correlation function g(t), which was measured as a function of the Reynolds number, the photon momentum transfer, and the size of the scattering volume. In terms of these control variables, g(t) was found to be of scaling form. Using such measurements one can deduce from the probability distribution function, P(V,R), that two particles, separated by a distance R, have velocity difference V(R,t). For small-velocity fluctuations, the scaling behavior of g(t) implies that P(V,R) has the form Q[V/ū(R)]/ū(R). This self-similarity in P(V,R) is seen only when Re exceeds a transition Reynolds number Rec. The measured scaling velocity ū(R) has the form ū(R)∼Rζ, with ζ increasing from 0 at Re=Rec to ∼ (1)/(3) at the maximum attainable levels of turbulence. This scaling behavior was seen in both the grid and pipe flows. By measuring g(t) at very small t, one can also obtain information about the large-velocity fluctuations. It is found that P(V,R) is well approximated by the product of a Lorentzian and a Gaussian function with characteristic velocities ū(R) and u(R), respectively. Here u(R) identifies the large-velocity fluctuations.
Anomalous dispersion in a dipole flow geometry
Authors: Kurowski P. , Ippolito I. , Hulin J. P. , Koplik J. , Hinch E. J.
Published: July 2002

The dispersion of a passive tracer in fluid flowing between a source and a sink in a Hele–Shaw geometry, characteristic of field scale flows in a layer or fracture, is considered. A combination of analytic and numerical techniques and complementary experimental measurements are employed, leading to a consistent picture. This dispersion process is found to be characterized by a power-law decay in time of the tracer concentration, with an exponential cutoff at very long times, in strong contrast to the Gaussian behavior associated with the widely used quasi-one-dimensional (1-D) models.
Simplified modeling of transition to detonation in porous energetic materials
Authors: Stewart D. Scott , Asay Blaine W. , Prasad Kuldeep
Published: July 2002

A simplified model that can predict the transitions from compaction to detonation and shock to detonation is given with the aim of describing experiments in beds of porous HMX. In the case of compaction to detonation, the energy of early impact generates a slowly moving, convective-reactive deflagration that expands near the piston face and evolves in a manner that is characteristic of confined deflagration to detonation transition. A single-phase state variable theory is adopted in contrast to a two-phase axiomatic mixture theory. The ability of the porous material to compact is treated as an endothermic process. Reaction is treated as an exothermic process. The algebraic (Rankine–Hugoniot) steady wave analysis is given for inert compaction waves and steady detonation waves in a piston supported configuration, typical of the experiments carried out in porous HMX. A structure analysis of the steady compaction wave is given. Numerical simulations of deflagration to detonation are carried out for parameters that describe an HMX-like material and compared with the experiments. The simple model predicts the high density plug that is observed in the experiments and suggests that the leading front of the plug is a secondary compaction wave. A shock to detonation transition is also numerically simulated.
An improved analytic solution for analysis of particle trajectories in fibrous, two-dimensional filters
Authors: Marshall H. , Sahraoui M. , Kaviany M.
Published: July 2002

The Kuwabara solution for creeping fluid flow through periodic arrangement of cylinders is widely used in analytic and numerical studies of fibrous filters. Numerical solutions have shown that the Kuwabara solution has systematic errors, and when used for the particle trajectories in filters it results in some error in the predicted filter efficiency. The numerical solutions, although accurate, preclude further analytic treatments, and are not as compact and convenient to use as the Kuwabara solution. By reexamining the outer boundary conditions of the Kuwabara solution, a correction term to the Kuwabara solution has been derived to obtain an extended solution that is more accurate and improves prediction of the filter efficiency. By comparison with the numerical solutions, it is shown that the Kuwabara solution is the high porosity asymptote, and that the extended solution has an improved porosity dependence. A rectification is explained that can make particle collection less efficient for periodic, in-line arrangements of fibers with particle diffusion or body force. This rectification also results in the alignment of particles with inertia (i.e., high Stokes number particles).
Absolute and convective instabilities of fluidized beds
Authors: Nicolas Maxime , Chomaz Jean-Marc , Guazzelli Elisabeth
Published: July 2002

The stability of a fluidized bed is investigated with respect to spatially growing disturbances. A general linearized model is derived from the theories of Anderson and Jackson and of Batchelor. The absolute and convective nature of the instability is analyzed using the mathematical framework of the open flow linear stability theory. The results of the analysis provide the domains of absolute and convective instabilities.
Homoclinic bifurcation in Blasius boundary-layer flow
Authors: Ehrenstein Uwe , Koch Werner
Published: July 2002

In an attempt to elucidate the laminar/turbulent transition mechanism in a Blasius boundary-layer flow, a nonsemisimple resonance of phase-locked secondary instability modes is investigated. The local nonlinear behavior is described by means of a center manifold reduction. The numerically computed normal form is of the symmetric Takens–Bogdanov type and predicts a homoclinic orbit which is possibly related to a physical bursting process. A global continuation procedure for equilibrated three-dimensional (3-D) waves in the full Navier–Stokes system validates some of the local predictions and very closely outlines the experimentally observed skin friction domain including subcritical transition.
Magnetic resonance imaging study of sedimenting suspensions of noncolloidal spheres
Authors: Turney Michael A. , Cheung Man Ken , McCarthy Michael J. , Powell Robert L.
Published: July 2002

Batch sedimentation experiments were conducted with suspensions of noncolloidal spherical particles. Using nuclear magnetic resonance imaging (NMRI), the time evolution of the volume fraction versus height profile was measured for initial suspension volume fractions, φi, ranging from 0.08 to 0.44. NMRI clearly delineates the clear fluid layer at the top of the suspension, below which there is a transition to a region having the initial mean particle concentration. The hindered settling function determined from these data corresponds well with previous results. The spreading of the interface in excess of that expected from the combined effects of polydispersity and self-sharpening was analyzed as a diffusion process. The measured values of the self-induced hydrodynamic diffusivity agreed with those reported previously, and they decreased sharply for φi≳0.15. The concentration profile was also measured through the fan region into the sediment, where the volume fraction is near maximum packing, and determined the fan thickness as a function of time. The fan thickness is found to increase as the initial suspension volume fraction is increased.
Asymmetry and Hopf bifurcation in spherical Couette flow
Authors: Mamun Chowdhury K. , Tuckerman Laurette S.
Published: July 2002

Spherical Couette flow is studied with a view to elucidating the transitions between various axisymmetric steady-state flow configurations. A stable, equatorially asymmetric state discovered by Bühler [Acta Mech. 81, 3 (1990)] consists of two Taylor vortices, one slightly larger than the other and straddling the equator. By adapting a pseudospectral time-stepping formulation to enable stable and unstable steady states to be computed (by Newton’s method) and linear stability analysis to be conducted (by Arnoldi’s method), the bifurcation-theoretic genesis of the asymmetric state is analyzed. It is found that the asymmetric branch originates from a pitchfork bifurcation; its stabilization, however, occurs via a subsequent subcritical Hopf bifurcation.
Symmetry and instability in rotating hydrodynamic and magnetohydrodynamic flows
Authors: Knobloch Edgar
Published: July 2002

The effect of symmetries on instabilitites in rotating hydrodynamic and magnetohydrodynamic flows is considered. Examples are described in which models of such flows or special cases have O(2) symmetry instead of the expected SO(2) symmetry. This change in symmetry has a profound effect on the nature of any instabilities and on their subsequent evolution. Both primary and secondary instabilities in systems with O(2) symmetry are discussed, and the effects of restoring SO(2) symmetry are analyzed. Implications of the results for convection in a rotating annulus, magnetohydrodynamic instabilities in accretion disks and a two-layer baroclinic instability model are described.
Response of the Blasius boundary layer to free-stream vorticity
Authors: Bertolotti F. P.
Published: July 2002

Two- and three-dimensional vortical modes that solve the linearized Navier-Stokes equations in the free stream are used in the present theory to represent some of the key features of low-level turbulence. Excluding the leading edge, the effect of these modes on the Blasius boundary layer is investigated using the parabolized stability equations (PSE). When the vortical modes are steady, or have low frequencies, the PSE analysis is started at a location x0 from the solution to a new set of ordinary differential equations. This solution is able to satisfy the linearized Navier-Stokes equations in a rather large neighborhood of x0. When the vortical modes have frequencies equal to those of unstable Tollmien-Schlichting waves, the scattering of the vortical modes by surface undulation produces only a weak response in the boundary layer, in agreement with other investigations. However, when steady and low-frequency vortical modes are considered, the analysis yields results that successfully reproduce a number of the experimental measurements of Kendall [AIAA Paper 90-1504 (1990)] on streaky structures, known as Klebanoff modes, that cause a periodic spanwise modulation of the streamwise velocity.
Differentially diffusing scalars in turbulence
Authors: Nilsen Vebjo/rn , Kosály George
Published: July 2002

Direct numerical simulation (DNS) data are presented to study differentially diffusing conserved scalars in isotropic turbulence. The two scalars are initially identical but subsequently decorrelate because one is diffusing faster than the other. The main focus of the paper is the study of the time evolution and possible modeling of quantities that characterize differential diffusion. The Reynolds number dependence of the mean square of the difference of the conserved scalars (α=Z−Zf, 〈α2〉) is investigated and compared to the theoretical scaling predicted by Kerstein et al. [Phys. Fluids A 7, 1999 (1995)]. The behavior and possible modeling of the conditional average of α is studied. The recent closure method of Kronenburg and Bilger [Phys. Fluids A 5, 1435 (1977)] is further corroborated and a novel approach to the prediction of the conditional average of α is discussed.
Suppression of coalescence and of wetting: The shape of the interstitial film
Authors: Dell’Aversana P. , Tontodonato V. , Carotenuto L.
Published: July 2002

The shape of the interstitial air film between noncoalescing liquids and between a drop and a solid surface is detected by means of laser interferometry. Alternative optical techniques are exploited to reveal the film profile and absolute thickness and its dynamical behavior. In particular, the thickness of the film is measured, by means of an angle shift method, in the case of a drop resting on a solid surface; further experiments, based upon reflectometry, show that stable noncoalescence may be realized both under steady and time-dependent conditions of the interfaces. A light deflection technique, coupled with image analysis, is exploited to detect the features of the film unsteadiness. The related Fourier spectra are explicitly calculated. The procedure for the determination of the film thickness is explained with the aid of a specific numerical code that is able to simulate the interference fringe patterns generated by thin films. The results presented herewith are explained in terms of theory of lubrication, exploiting the experimental data as boundary conditions that allow one to uncouple the stress–balance equations at the interfaces and the lubrication equations and solve only the latter ones.
Stability of viscoelastic dynamic contact lines: An experimental study
Authors: Spaid M. A. , Homsy G. M.
Published: July 2002

An experimental study of the rivulet instability associated with spin coating a circular drop of fluid is conducted to examine the effect of elasticity on the onset and evolution of the instability. The spin coating experiments are conducted with viscoelastic drops consisting of a high molecular weight polystyrene in tricresyl phosphate (TCP), as well as the Newtonian solvent TCP. Results show an unequivocal delay in the onset of the instability when the appropriate Weissenberg number is sufficiently large, resulting in a larger coated area and more finger arms relative to Newtonian results. Experiments performed with the viscoelastic fluid at low Weissenberg number exhibit similar behavior to those performed with the Newtonian solvent as expected. Additionally, the growth rate of the instability is reduced for experiments in which the elastic forces are important, in agreement with the perturbation theory of Spaid and Homsy [Phys. Fluids 8, 460 (1996)], demonstrating that elastic forces have a stabilizing influence on the contact line instability.
Convection related phenomena in granular dynamics simulations of vibrated beds
Authors: Lan Yidan , Rosato Anthony D.
Published: July 2002

Three-dimensional discrete element simulations are carried out to investigate the behavior of a shallow bed of inelastic, frictional spheres (of uniform diameter d), which are energized by vertical sinusoidal oscillations of a plane floor at amplitude a and frequency ω=2πf. We investigate the long-term and instantaneous velocity fields as well as the evolution of the pressure tensor. Results show that the onset of convection reported in the literature is not only determined by the floor acceleration, but also the ratio a/d. In a wide bed (L/d∼100) narrow persistent vortices appear near vertical sidewalls, while no distinct pattern is found within the central region. A large sphere within the bed is convected upward to the surface and either “segregates” itself from the bulk, or becomes reentrained, depending on the width of the downward velocity field near the wall relative to the sphere size. An inspection of the bed microstructure reveals internal vortex-like cells spanning its width giving rise to arching observed in recent experiments and other simulations. Computations of the potential constituent of the pressure tensor revealed high values in collision-dominated regions of the bed and a trend that repeated every two oscillations of the floor.
Oscillating crescent-shaped water wave patterns
Authors: Collard F. , Caulliez G.
Published: July 2002

A new type of persistent three-dimensional crescent-shaped water wave patterns was found experimentally in the laboratory. The particular features of the patterns are their oscillating character and the regular orthogonal grid the crescent crests form in the plane view. They emerge from a steep initially two-dimensional paddle-generated wave amplified by wind. The patterns are composed of the basic wave and two nonsymmetric oblique satellites with longitudinal components of wavevector equal to that of the basic wave and twice this wave number. The main mechanism of their formation was identified to be a five-wave resonant interaction process. However, the mechanism of the sharp selection of these particular harmonics was found not to be explained within the framework of existing theories.
Direct numerical simulation of differential diffusion with Schmidt numbers up to 4.0
Authors: Yeung P. K. , Sykes Michelle C. , Vedula Prakash
Published: July 2002

Highly resolved direct numerical simulations are used to study the differential diffusion of passive scalars of above-unity Schmidt number in low Reynolds number turbulence. Both the coherency and the spectrum of the two-scalar difference obey Batchelor scaling based on the parameters of the less diffusive scalar. A reverse spectral transfer effect due to velocity modes in the far dissipation range is identified, which may be important at very high Schmidt numbers.
Stability theorem for a single species plasma in a toroidal magnetic configuration
Authors: O’Neil T. M. , Smith R. A.
Published: July 2002

A stability theorem is developed for a single species plasma that is confined by a purely toroidal magnetic field. A toroidal conductor is assumed to bound the confinement region, and frequencies are ordered so that the cyclotron action and the toroidal action for each particle are good adiabatic invariants. The cross-field motion is described by toroidal-average drift dynamics. In this situation, it is possible to find plasma equilibria for which the energy is a maximum, as compared to all neighboring states that are accessible under general constraints on the dynamics. Since the energy is conserved, such states must be stable to small-amplitude perturbations. This theorem is developed formally using Arnold’s method, and examples of stable equilibria are obtained.
Theory of ion Bernstein wave induced shear suppression of turbulence
Authors: Craddock G. G. , Diamond P. H. , Ono M. , Biglari H.
Published: July 2002

The theory of radio frequency induced ion Bernstein wave- (IBW) driven shear flow in the edge is examined, with the goal of application of shear suppression of fluctuations. This work is motivated by the observed confinement improvement on IBW heated tokamaks [Phys. Fluids B 5, 241 (1993)], and by previous low-frequency work on RF-driven shear flows [Phys. Rev. Lett. 67, 1535 (1991)]. It is found that the poloidal shear flow is driven electrostatically by both Reynolds stress and a direct ion momentum source, analogous to the concepts of helicity injection and electron momentum input in current drive, respectively. Flow drive by the former does not necessarily require momentum input to the plasma to induce a shear flow. For IBW, the direct ion momentum can be represented by direct electron momentum input, and a charge separation induced stress that imparts little momentum to the plasma. The derived Er profile due to IBW predominantly points inward, with little possibility of direction change, unlike low-frequency Alfvénic RF drive. The profile scale is set by the edge density gradient and electron dissipation. Due to the electrostatic nature of ion Bernstein waves, the poloidal flow contribution dominates in Er. Finally, the necessary edge power absorbed for shear suppression on Princeton Beta Experiment-Modified (PBX-M) [9th Topical Conference on Radio Frequency Power in Plasmas, Charleston, SC, 1991 (American Institute of Physics, New York, 1991), p. 129] is estimated to be 100 kW distributed over 5 cm.
Self-generated magnetic field in an amplitude modulated laser filament in a plasma
Authors: Tripathi V. K. , Liu C. S.
Published: July 2002

A short pulse laser of finite spot size is modeled as an amplitude modulation in time and transverse dimension. The laser exerts a time-dependent curl-free ponderomotive force Fp on the electrons, imparting to them a transverse velocity. When equilibrium plasma density has a gradient ∇n0⊥Fp the electron current density is irrotational, producing a quasistatic magnetic field. The relevance of the process to plasma accelerators and moving ionization fronts is discussed.
Comment on ‘‘Alfvén ‘resonance’ reconsidered: Exact equations for wave propagation across a cold inhomogeneous plasma’’ [Phys. Plasmas 1, 3523 (1994)]
Authors: Ruderman M. S. , Goossens M. , Zhelyazkov I.
Published: July 2002

The author’s content that the free electron laser instability is not modify and the operation frequency of the FEL is only slightly affected by the modification suggested by Shouric in the dispersion relation of the pump wave. (AIP)
Pulsed currents carried by whistlers. III. Magnetic fields and currents excited by an electrode
Authors: Urrutia J. M. , Stenzel R. L. , Rousculp C. L.
Published: July 2002

Detailed measurements and analysis of electromagnetic fields asociated with pulsed plasma currents are reported. The objective is to demonstrate the properties of plasma currents in the electron magnetohydrodynamic regime and their relation to low frequency whistler waves. Short current pulses (fce−1≪Δt≪fci−1) are injected from an electrode into a large, uniform magnetoplasma. The dynamic fields, B(r,t), are measured with probes in three-dimensional space and time, and are observed to propagate as wave packets predominantly along the guide magnetic field, B0. Four-dimensional fast Fourier transformation of B(r,t) to B(k,ω) verifies that the wave fields fall on the dispersion surface of low-frequency oblique whistlers. The magnetic field topology of the packets consists of linked toroidal and solenoidal contributions in force-free configurations. The wave magnetic helicity is obtained quantitatively. Similarly, the topology of the current density field, J=∇×B/μ0, is explained by its components, characteristic field lines, and helicity, ∫J⋅BdV. It is shown, both theoretically and experimentally, that A, B, and J are nearly parallel to one another. Field energy and helicity decay at the same rate without change in topology.
Modeling of a stimulated Brillouin scattering experiment with statistical distribution of speckles
Authors: Tikhonchuk V. T. , Labaune C. , Baldis H. A.
Published: July 2002

The results of experimental studies of stimulated Brillouin scattering (SBS) from a laser beam smoothed with a random phase plate (RPP) in a preformed expanding plasma are compared with the statistical theory of SBS from an inhomogeneous plasma. Many observed features can be explained assuming that SBS occurs in randomly distributed speckles. This hypothesis naturally combines observed pump depletion effects within a speckle, with relatively minor SBS reflectivity of the whole interaction beam.
Pulsed currents carried by whistlers. VII. Helicity and transport in heat pulses
Authors: Stenzel R. L. , Urrutia J. M.
Published: July 2002

In a uniform magnetoplasma (n≂1011 cm−3, kTe≥0.5 eV, B0≥15 G, 1 m × 2.5 m), electrons are heated locally and temporally by applying a short current pulse to a loop antenna or disk electrode. Electron magnetohydrodynamics characterize the experimental conditions. After the end of the applied current pulse and whistler wave transients, a current system driven by temperature gradients remains embedded in the plasma. The current system exhibits helicity. The associated electron drifts convect heat out of the flux tube. From diamagnetic field measurements, the decay of the electron temperature is obtained with high sensitivity (ΔkTe≂0.001 eV). The heat transport is inferred from the space–time dependence of the electron temperature. The temperature enhancement is confined to a channel whose length depends on heat input since the transport coefficients are temperature-dependent.
Critical nonlinear phenomena for kinetic instabilities near threshold
Authors: Breizman B. N. , Berk H. L. , Pekker M. S. , Porcelli F. , Stupakov G. V. , Wong K. L.
Published: July 2002

A universal integral equation has been derived and solved for the nonlinear evolution of collective modes driven by kinetic wave particle resonances just above the threshold for instability. The dominant nonlinearity stems from the dynamics of resonant particles that can be treated perturbatively near the marginal state of the system. With a resonant particle source and classical relaxation processes included, the new equation allows the determination of conditions for a soft nonlinear regime, where the saturation level is proportional to the increment above threshold, or a hard nonlinear regime, characterized by explosive behavior, where the saturation level is independent of the closeness to threshold. In the hard regime, rapid oscillations typically arise that lead to large frequency shifts in a fully developed nonlinear stage. The universality of the approach suggests that the theory applies to many types of resonant particle driven instabilities, and several specific cases, viz. energetic particle driven Alfvén wave excitation, the fishbone oscillation, and a collective mode in particle accelerators, are discussed.
Particle simulation of a magnetized plasma contacting the wall
Authors: Daube Th. , Riemann K.-U. , Schmitz H.
Published: July 2002

One-dimensional particle simulations are performed to study the influence of a strong magnetic field on the plasma boundary layer in front of a completely absorbing wall. The magnetic field lines are parallel to the wall, and the ion transport is provided by charge exchange collisions with cold neutrals. The Debye length is small compared with the ion gyroradius and the electrons are Boltzmann distributed. A modified Particle-in-Cell Monte-Carlo-Collision (PIC-MCC) code is developed to avoid the problem of different time scales of electrons and ions. The self-consistent steady-state simulation is performed for a system with one spatial coordinate and two velocity components (1d, 2v). The results are compared with corresponding results of a self-consistent stationary solution of the ion Boltzmann equation. Although the potential and density profiles are essentially confirmed, the ion velocity distribution functions disagree with analytic solutions in certain singular regions unless certain pertubations in the electric field are suppressed. It is shown that this is due to a microscopic instability of the stationary solution.
Alpha particle-driven toroidal Alfvén eigenmodes in Tokamak Fusion Test Reactor deuterium–tritium plasmas: Theory and experiments
Authors: Fu G. Y. , Nazikian R. , Budny R. , Chang Z.
Published: July 2002

The toroidal Alfvén eigenmodes (TAE) in the Tokamak Fusion Test Reactor [K. Young et al., Plasma Phys. Controlled Fusion 26, 11 (1984)] deuterium–tritium plasmas are analyzed using the NOVA-K code [C. Z. Cheng, Phys. Rep. 211, 1 (1992)]. The theoretical results are compared with the experimental measurements in detail. In most cases, the theory agrees with the observations in terms of mode frequency, mode structure and mode stability. However, one mode with toroidal mode number n=2 is observed to be poloidally localized on the high field side of the magnetic axis with a mode frequency substantially below the TAE frequency.
Drift-Alfvén fluctuations associated with a narrow pressure striation
Authors: Peñano J. R. , Morales G. J. , Maggs J. E.
Published: July 2002

This analytical and numerical study illustrates the linear stability properties of low frequency electromagnetic eigenmodes driven by field-aligned pressure striations whose scale transverse to the confining magnetic field is on the order of the electron skin-depth. A full electromagnetic formulation is given in terms of the coupling of the fluctuating axial fields (Ẽz,B̃z) and incorporates shear and compressional Alfvén waves, drift waves, and ion acoustic waves. The kinetic response of the electrons includes pitch-angle scattering (Lorentz model) and the ions are treated as a magnetized, cold fluid. Detailed quantitative comparisons of the theoretical predictions are made with laboratory observations of fluctuations generated in controlled pressure depletions [J. E. Maggs and G. J. Morales, Phys. Plasmas 4, 290 (1997)] and in narrow temperature plumes [A. T. Burke, J. E. Maggs, and G. J. Morales, Phys. Rev. Lett. 81, 3659 (1998)].
Ion momentum driven by a short intense laser pulse in an underdense plasma
Authors: Gorbunov L. M. , Mora P. , Ramazashvili R. R. , Solodov A. A.
Published: July 2002

The analysis of a one-dimensional relativistic two-fluid hydrodynamical model shows that a short powerful laser pulse propagating in underdense plasmas generates an averaged momentum of plasma ions. Analytical results are in excellent agreement with numerical simulations performed with a particle code.
Transverse electric field and density gradient induced instabilities in a cylindrical magnetron discharge
Authors: van der Straaten T. A. , Cramer N. F.
Published: July 2002

Recent one-dimensional particle-in-cell simulations of a cylindrical postcathode direct current magnetron discharge have shown a transition from the usual positive space charge mode, dominated by a strong cathode fall, to a higher impedance negative space charge mode, characterized by a broad anode fall, as either the pressure is decreased or the magnetic field is increased. Such behavior is consistent with a steady decrease in the cross-field electron transport coefficients as calculated from classical fluid theory. However, experimental measurements of the distribution of electric potential between the electrodes do not exhibit any systematic variation with changes in magnetic field or pressure. This suggests that anomalous transport mechanisms increase the electron transport above the level predicted by classical transport theory, so maintaining the positive space charge mode at low pressures. It is shown using a fluid model that electrostatic oscillations with ω≳νi, where νi is the ion-neutral collision frequency, propagating in the E×B direction are unstable with growth rates comparable to the charged particle transit times. Such oscillations, which cannot be modeled in a one-dimensional particle-in-cell model, may contribute to the anomalous transport observed in experiments.
Experimental study of classical heat transport in a magnetized plasma
Authors: Burke A. T. , Maggs J. E. , Morales G. J.
Published: July 2002

A systematic study is made of the axial and transverse heat transport that arises when a microscopic temperature filament is generated, under controlled conditions, in a magnetized plasma of large dimensions. For early times and relatively small temperature gradients the study conclusively demonstrates the two-dimensional pattern characteristic of the classical theory based on Coulomb collisions. The full nonlinear dependence of the transverse and axial electron heat conductivities is sampled through temperature changes in the range δTe/Te∼1–10. The dependence on the confining magnetic field is explored over a factor of 3 (∼ factor of 10 in transverse conductivity). It is found that under quiescent conditions, the observed behavior agrees with classical theory within the experimental uncertainties. However, over long times and/or for steep temperature gradients, fluctuations develop spontaneously and cause a significant departure from the predictions of the classical theory.
Confinement and manipulation of non-neutral plasmas using rotating wall electric fields
Authors: Hollmann E. M. , Anderegg F. , Driscoll C. F.
Published: July 2002

A “rotating wall” perturbation technique enables confinement of up to 3×109 electrons or 109 ions in Penning–Malmberg traps for periods of weeks. These rotating wall electric fields transfer torque to the particles by exciting Trivelpiece–Gould plasma modes with kz≠0 and mθ=1 or 2. Modes that rotate faster than the plasma column provide a positive torque that counteracts the background drags, resulting in radial plasma compression or steady-state confinement in near-thermal equilibrium states. Conversely, modes that rotate slower than the plasma provide a negative torque, and enhanced plasma expansion is observed. The observed Trivelpiece–Gould mode frequencies are well predicted by linear, infinite-length, guiding-center theory.
On the possibility of electromagnetically induced transparency in a plasma. II. Bounded plasma
Authors: Gordon D. F. , Mori W. B. , Joshi C.
Published: July 2002

The theory of electromagnetically induced transparency (EIT) in a plasma [S. E. Harris, Phys. Rev. Lett. 77, 5357 (1996)] is examined in the context of a bounded system. It is found via particle simulations that an overdense plasma slab reflects short pulses even in the presence of an EIT passband. However, a two-frequency laser, or “beatwave,” will cascade into a Stokes satellite with a frequency below the cut-off frequency. This can lead to an apparent EIT signature if a specific set of parameters are chosen.
Pulse compression in optical fiber filters
Authors: Winful Herbert G.
Published: July 2002

A method is described for compressing optical pulses at any wavelength in an optical fiber. It uses the negative dispersion property of permanent phase gratings created within the fiber. No external grating pair is required.
Monolithic integration of a laser diode and an optical waveguide modulator having a GaAs/AlGaAs quantum well double heterostructure
Authors: Tarucha Seigo , Okamoto Hiroshi
Published: July 2002

This letter describes the monolithic integration of a laser diode and an optical waveguide modulator that have the same GaAs/AlGaAs quantum well double heterostructure grown on the same substrate. The intensity of the light emitted from the laser diode is modulated by the modulator, utilizing the effect of an electric field on exciton absorption. The absorption loss coefficient of the modulator waveguide with no applied voltage was estimated to be 60 cm−1. This value is much smaller than the previously reported corresponding value of a veguide having a conventional GaAs double heterostructure. The modulation depth achieved was 7 dB for a driving voltage of 2.3 V; the cut-off modulation frequency was 0.88 GHz. A prospect for improving these characteristics is also presented.
Pulse generation with an acousto-optic frequency shifter in a passive cavity
Authors: Kowalski F. V. , Squier J. A. , Pinckney J. T.
Published: July 2002

A train of pulses is generated by coupling the cw output of a single frequency He-Ne laser into a passive ring ‘‘cavity’’ which contains an acousto-optic frequency shifter. Each cavity pass shifts the frequency of the wave by 80 MHz. This results in an intensity pattern which is an Airy function in time. For a single frequency input, the output pulse width is 2 ns with a repetition rate of 80 MHz. Data are also presented on the output of this device for multimode cw input.
Enhancement of keV x-ray emission in laser-produced plasmas by a weak prepulse laser
Authors: Kodama R. , Mochizuki T. , Tanaka K. A. , Yamanaka C.
Published: July 2002

X-ray conversion efficiency in a 0.53-μm laser-produced copper plasma is studied in the 1.5–5.0 keV range as a function of laser pulse duration (τL) with a laser intensity of 1×1014 W/cm2. The efficiency increases as τ1.3L at pulse lengths of less than 400 ps. For a 200-ps pulse duration, an enhancement of the conversion efficiency is observed with the use of a prepulse. The efficiency is found to be proportional to the scale length of a preformed plasma. Enhancement of a factor 3 is observed for the shots with a prepulse.
Analysis of screening effects on interband Auger processes in GaSb
Authors: Yevick David , Bardyszewski Witold
Published: July 2002

We analyze the dependence of the Auger coefficient of GaSb on the assumed inverse screening length λ. We demonstrate that current experimental values can only be reproduced if λ≂0. We also derive this result theoretically with a general nonequilibrium Green’s function technique and provide a simple intuitive justification. Our accompanying numerical calculations are based on a Monte Carlo integration procedure which enables us to avoid approximations to the Kane-model overlap matrix elements and to Fermi and phase-space factors.
Low-threshold room-temperature cw operation of (AlGaAs)m(GaAs)n superlattice quantum well lasers emitting at ∼680 nm
Authors: Hayakawa T. , Suyama T. , Takahashi K. , Kondo M. , Yamamoto S. , Hijikata T.
Published: July 2002

Low cw threshold current of 45 mA and high differential quantum efficiency of 78% have been achieved at room temperature in an (Al0.6Ga0.4As)2(GaAs)2 superlattice quantum well laser emitting at 681 nm. The quantum well structure has been optimized to minimize the threshold current density at 680 nm. The new ridge-waveguide structure with a current-blocking supporting region is employed to reduce the thermal resistance.
Observation of a resonant enhancement of microwave radiation from a gas-filled backward wave oscillator
Authors: Minami K. , Lou W. R. , Destler W. W. , Kehs R. A. , Granatstein V. L. , Carmel Y.
Published: July 2002

High-power microwave radiation at 8 GHz from a gas-filled backward wave oscillator driven by an intense relativistic electron beam has been measured. It has been observed that an order of magnitude resonant increase in microwave output power occurs at a particular pressure of the background gas. This resonance has also been observed to be strongly dependent on the value of applied axial magnetic field. Possible explanations for the observed results are discussed.
Immortalization of a primate bipotent epithelial liver stem cell
Authors: Allain Jean-Etienne , Dagher Ibrahim , Mahieu-Caputo Dominique , Loux Nathalie , Andreoletti Marion , Westerman Karen , Briand Pascale , Franco Dominique , Leboulch Philippe , Weber Anne
Published: July 2002

Liver regeneration after partial hepatectomy results primarily from the simple division of mature hepatocytes. However, during embryonic and fetal development or in circumstances under which postnatal hepatocytes are injured, organ regeneration is believed to occur from a compartment of epithelial liver stem or progenitor cells with biliary and hepatocytic bipotentiality. The ability to identify, isolate, and transplant epithelial liver stem cells from fetal liver would greatly facilitate the treatment of hepatic diseases currently requiring orthotopic liver transplantation. Here we report the identification and immortalization by retrovirus-mediated transfer of the simian virus 40 large T antigen gene of primate fetal epithelial liver cells with a dual hepatocytic biliary phenotype. These cells grow indefinitely
in vitro
and express the liver epithelial cell markers cytokeratins 8/18, the hepatocyte-specific markers albumin and α-fetoprotein, and the biliary-specific markers cytokeratins 7 and 19. Bipotentiality of gene expression was confirmed by clonal analysis initiated from single cells. Endogenous telomerase also is expressed constitutively. After orthotopic transplantation via the portal vein, ≈50% of the injected cells integrated into the liver parenchyma of athymic mice without tumorigenicity. Three weeks after transplantation, cells having seeded in the liver parenchyma expressed both albumin and α-fetoprotein but had lost expression of cytokeratin 19. These results provide strong evidence for the existence of a bipotent epithelial liver stem cell in nonhuman primates. This unlimited source of donor cells also should enable the establishment of a model of allogenic liver cell transplantation in a large animal closely related to humans and shed light on important questions related to liver organogenesis and differentiation.
Factors affecting the threading of axle molecules through macrocycles: Binding constants for semirotaxane formation
Authors: Clifford Thomas , Abushamleh Ahmad , Busch Daryle H.
Published: July 2002

The threading of more or less linear axle molecules through macrocyclic molecules, a fundamental process relating to the formation of interlocked molecular structures, has been investigated through the study in acetone of the equilibrium constants for the formation of pseudorotaxanes by NMR methods. The 30 new axle molecules have in common a secondary ammonium group, present as the thiocyanate salt, and an anthracen-9-ylmethyl group, but are rendered unique by the second amine substituent. All rotaxanes involve the well known polyether macrocycle, benzo[24]crown-8. The constants for the binding of axles having linear groups ranging from 2 to 18 carbon atoms show little variation in binding constant but are divided into two groups by their equilibration rates. Those with less than five methylene groups react rapidly on the NMR timescale, whereas those having more than five methylene groups are slow. Branching inhibits binding, but the effect decreases as the branch is moved away from the amine. Phenyl groups weaken binding when close to the amine but strengthen binding when more remote. Some functional groups decrease pseudorotaxane stability (alcohol functions), whereas others increase binding (carboxylic acid groups).
Supercool or dehydrate? An experimental analysis of overwintering strategies in small permeable arctic invertebrates
Authors: Holmstrup Martin , Bayley Mark , Ramløv Hans
Published: July 2002

Soil invertebrate survival in freezing temperatures has generally been considered in the light of the physiological adaptations seen in surface living insects. These adaptations, notably the ability to supercool, have evolved in concert with surface invertebrates' ability to retain body water in a dry environment. However, most soil invertebrates are orders of magnitude less resistant to desiccation than these truly terrestrial insects, opening the possibility that the mechanisms involved in their cold-hardiness are also of a radically different nature. Permeable soil invertebrates dehydrate when exposed in frozen soil. This dehydration occurs because the water vapor pressure of supercooled water is higher than that of ice at the same temperature. The force of this vapor pressure difference is so large that even a few degrees of supercooling will result in substantial water loss, continuing until the vapor pressure of body fluids equals that of the surrounding ice. At this stage, the risk of tissue ice formation has been eliminated, and subzero survival is ensured. Here we show that these soil invertebrates do not base their winter survival on supercooling, as do many other ectotherms, but instead dehydrate and equilibrate their body-fluid melting point to the ambient temperature. They can achieve this equilibration even at the extreme cooling rates seen in polar soils.