bluuebunny/crossref_metadata_2025 huggingface dataset:
Joukovskii’s model for a rising bubble
Authors: Vanden-Broeck Jean-Marc
Published: July 2002

Joukovskii’s model [J. Russian Physico. Chem. Soc. 22, 19 (1891)] for a two-dimensional bubble rising in an unbounded fluid is considered. This model approximates the wake behind the bubble by a region bounded by vertical walls. By using an inverse method, Joukovskii obtained an exact solution characterized by a Froude number F=U/(gL)1/2= (2π)−1/2. Here U is the velocity at which the bubble rises, g is the acceleration of gravity, and L is the distance between the vertical walls. It is shown numerically that Joukovskii’s solution is not unique. When surface tension is neglected there is a solution for each value of 0<FFc, and an isolated solution with a 120° at the apex for F=Fc. When surface tension is taken into account there is a discrete set of solutions. Each of these solutions corresponds to a different value of F. As the surface tension tends to zero, all these solutions approach a unique solution. The numerical results indicate that this limiting solution is Joukovskii’s exact solution.
Thermocapillary migration of an attached drop on a solid surface
Authors: Ford Mark L. , Nadim Ali
Published: July 2002

Migration of a small drop of liquid, initially at rest on a level, solid surface, can be induced by means of thermocapillary forces. If a temperature gradient is imposed across the solid, it has the effect of diminishing the surface tension on the warmer side of the droplet. Consequently, migration manifests itself as the difference in surface tension preferentially draws the droplet toward the cooler region of the solid. The study describes the behavior of a droplet modeled as an infinitely long strip of finite width and arbitrary height profile, subject to a uniform temperature gradient imposed along the base. Lubrication theory is employed to determine the velocity and pressure fields within the drop, as well as the net migration velocity of the droplet as a whole. The role of the dynamic boundary condition in the vicinity of the contact lines (including the allowance for slip) on the migration velocity is highlighted.
A note on measures of disturbance size for spatially evolving flows
Authors: Henningson Dan S. , Schmid Peter J.
Published: July 2002

The problem of defining a proper measure for the size of disturbances that evolve spatially is assessed. A model problem serves as a demonstration that the disturbance energy, which is almost exclusively used in a temporal setting, is generally not sufficient as a measure of the disturbance size in the spatial case. It is shown that the specific form of the ‘‘energy’’ transferred between Fourier components in the spatial case is not unique, but rather depends on the formulation of the spatial evolution problem. In addition to the model problem, suggestions on how to evaluate spatial ‘‘energy’’ transfer in the Navier–Stokes equations are presented.
The three-dimensional vortex structure of an impacting water drop
Authors: Peck Bill , Sigurdson Lorenz
Published: July 2002

Observations are reported for the three-dimensional vortex structure created by a dyed water drop impacting a pool of water. The structure and evolution of the vorticity is studied for a Weber number of 22–25 and a Froude number of 25–28. The drop and pool do not make first contact at the bottom of the drop but at latitudes away from the bottom pole of the drop. This traps a thin, curved, pancake-shaped air bubble beneath the drop which rapidly contracts into a sphere. As the drop impacts the pool its impulse produces vorticity which rolls up into a primary vortex ring. As the vortex ring travels down through the pool, vortex filaments extend from the central axis of the vortex ring to form a ‘‘stalk.’’ This reaches from the primary ring to another ring of vorticity which has formed in the now reversing free surface impact crater. As the primary ring convects downward some vortex filaments undergo an azimuthal instability which grows until the filaments escape the trapped orbits of the primary vortex ring and are ‘‘shed.’’ This results in three to five loops or ‘‘petals’’ left behind the primary ring. A three-dimensional vortex skeleton of this structure is presented. The results confirm the hypothesis that the structure is topologically similar to that of the separation bubble on a blunt flat plate. The structure’s appearance is also strikingly similar to the appearance of an aboveground atomic blast.
The effect of hydrodynamic interactions on the orientation distribution in a fiber suspension subject to simple shear flow
Authors: Rahnama Mani , Koch Donald L. , Shaqfeh Eric S. G.
Published: July 2002

A single, non-Brownian fiber suspended in a viscous, Newtonian fluid undergoing simple shear flow rotates in one of a set of closed orbits known as Jeffery orbits. In a fiber suspension, the hydrodynamic interactions among the fibers determine the distribution of fibers among these orbits. The hydrodynamic interactions in dilute and semidilute suspensions have been studied using slender-body theory. Hydrodynamic, orientational diffusivities were obtained from an ensemble average of the fiber–fiber interactions. The steady-state fiber orientation distribution is controlled by the anisotropy and orientation dependence of the diffusivities. The steady-state and transient fiber orientation distributions are derived using a perturbation analysis for weak hydrodynamic orientational diffusion that is an extension of the work of Leal and Hinch [J. Fluid Mech. 46, 685 (1972)] for weak, isotropic, rotary Brownian motion. In the dilute regime, the steady-state experimental distributions of Anczurowski and Mason [J. Colloid Interface Science 23, 522 (1967)] do not agree with the theoretical predictions. An explanation for these discrepancies accompanied with new experimental results is presented in this work. The theoretical predictions for the steady-state orientation distribution, and the temporal orbit constant correlation function in the semidilute regime are in good agreement with the experimental results of Stover et al. [J. Fluid Mech. 238, 277 (1992)]. The correlation time for the fiber orientation is approximately inversely proportional to fiber concentration in both the dilute and semidilute regimes.
Growth characteristics downstream of a shallow bump: Computation and experiment
Authors: Joslin Ronald D. , Grosch Chester E.
Published: July 2002

Measurements of the velocity field created by a shallow bump on a wall revealed that an energy peak in the spanwise spectrum associated with the driver decays and an initially small-amplitude secondary mode rapidly grows with distance downstream of the bump. Linear theories could not provide an explanation for this growing mode. The present Navier–Stokes simulation replicates and confirms the experimental results. Insight into the structure of the flow was obtained from a study of the results of the calculations and is presented.
Qualitative analysis of the Navier–Stokes equations for evaporation–condensation problems
Authors: Bobylev A. V. , O/stmo S. , Ytrehus T.
Published: July 2002

The methods of qualitative theory of dynamical systems are used to provide new information of the Navier–Stokes solutions for gas flows driven by evaporation and condensation at interphase surfaces. It is shown that these solutions correspond to separatrixes of the saddle point in the (u,T) plane. The classification of solutions is given, and some special cases are studied in detail. The qualitative methods are applied to the problem of evaporation/condensation between two plates. It is shown that the topology of the saddle point implies the following situation: one of the functions u(x) (velocity) or T(x) (temperature) is always non-monotone for sufficiently small Knudsen number. This explains some previously published numerical results. The case of finite Reynolds number is considered separately, and relationship with the kinetic-theory results is discussed throughout.
Scalar intermittency and the ground state of periodic Schrödinger equations
Authors: Bronski Jared C. , McLaughlin Richard M.
Published: July 2002

Recent studies of a passive scalar diffusing in a rapidly fluctuating Gaussian distributed linear shear layer have demonstrated intermittency in the form of broad tails and non-symmetric limiting probability distribution functions. In this paper the authors explore similar issues within the context of a large class of rapidly fluctuating bounded periodic shear layers. We compute the evolution of the moments by analogy to an N dimensional quantum mechanics problem. By direct comparison of an appropriate system of interacting and non-interacting quantum particles, we illustrate that the role of interaction is to induce a lowering of the ground state energy, which implies that the scalar PDF will have broader than Gaussian tails for all large, but finite times. We demonstrate for the case of Gaussian random wave initial data involving a zero spatial mean, that the effect of this energy shift is to induce diverging normalized flatness factors indicative of very broad tails. For the more general case with Gaussian random initial data involving a non-zero spatial mean, the distribution must approach that of a Gaussian at infinite times, as required by homogenization theory, but we show that the approach is highly non-uniform. In particular our calculation shows that the time required for the system to approach Gaussian statistics grows like the square of the moment number.
On the self-similarity assumption in dynamic models for large eddy simulations
Authors: Carati Daniele , Vanden Eijnden Eric
Published: July 2002

The consistency between dynamic models for large eddy simulations and their underlying self-similarity assumption is discussed. The interpretation of the resolved field is shown to be fully determined by the choice of the test-filter. Consequences for comparison to direct numerical simulations and experimental results are presented.
Particle collision rate in fluid flows
Authors: Hu Kevin C. , Mei Renwei
Published: July 2002

The classical result of Smoluchowski [Z. Phys. Chem. 92, 129 (1917)] for the collision rate of monodisperse particles in a laminar shear flow is shown to be inaccurate due to the inclusion of the self-collision. In the present work we extend Smoluchowski’s result by excluding the self-collision in the counting of collision pairs. A numerical simulation for particle collisions in a laminar shear flow at very low concentration is carried out to validate the extended result of Smoluchowski. Good agreement for the collision rate between the numerical simulation and the prediction based on the extended expression is obtained.
Pressure driven disturbances on a thin viscous film
Authors: Kriegsmann James J. , Miksis Michael J. , Vanden-Broeck Jean-Marc
Published: July 2002

Here we investigate the effect of a steady pressure disturbance moving at a constant velocity along the interface of a thin viscous film flowing down an inclined plane. Both steady and unsteady solutions are determined for the interface. We find that it is possible for the steady solutions to have surface waves, either preceding the disturbance or behind it, depending on the values of the Bond and capillary numbers. In addition, for fixed Bond number, disturbance amplitude, and constant film heights both far upstream and downstream, there exists a finite range of capillary numbers for which no steady solutions exist. Within this range of capillary numbers, the transient solution develops a shock like profile which adjusts the heights upstream and downstream of the disturbance and grows in width.
Coherent structure and heat transfer in geostrophic flow under density stratification
Authors: Iida O. , Nagano Y.
Published: July 2002

The mechanism of heat transport in geostrophic flows under various density stratifications has been studied by using both direct numerical simulation and rapid distortion theory. It is found that in cases with rotation, the iso-surfaces of the temperature fluctuations are aligned in the direction of the rotational axis, and they become very close to two-dimensional. Under stable stratification with rotation, the velocity and temperature fluctuations tend to oscillate with the Brunt–Väisälä frequency, while they tend to oscillate with a longer period in cases without rotation. Under unstable stratification with rotation, on the other hand, vortex columns are formed in the direction parallel to the axis of rotation owing to the effects of nonlinear interactions.
High-frequency waves in a plasma waveguide
Authors: Maraghechi B. , Willett J. E. , Mehdian H. , Aktas Y.
Published: July 2002

An analysis of the azimuthally symmetrical, high-frequency eigenmodes of a cylindrical metallic waveguide partially filled with a magnetized plasma is presented. Equations that permit calculation of the dispersion curves for four families of electromagnetic and electrostatic modes are derived. Numerical solutions are presented to facilitate the development of devices for generation of high-power electromagnetic radiation, charged particle acceleration, and other applications of plasma waveguides. The dependence of the cutoff frequencies, and dispersion curves of various modes on the ratio of the plasma radius a to the waveguide radius R is studied in detail. Space-charge modes are found to be strongly dependent on the radius ratio a/R. The coupling of the dispersion curves of different modes and the variation of the cutoff frequencies of HE waveguide and cyclotron modes with cyclotron frequency are illustrated for the partially filled waveguide.
A model cylindrical magnetron Vlasov distribution function
Authors: Kaup D. J. , Choudhury S. Roy
Published: July 2002

The analysis of the planar magnetron Vlasov distribution function [Phys. Fluids 31, 2362 (1988)] is extended to the cylindrical case. In momentum space, the model distribution function is f(w,pθ) =Ne−βwwe−(Ωβθ/4p0)(pθ−p0)2 where w(pθ) is the single particle energy (angular momentum), βw(βθ) is the inverse of the thermal energy associated with variations in w(pθ), p0 is the angular momentum at the cathode, and Ω is the electron cyclotron frequency (=eB0/mc). The problem is shown to be too ‘‘stiff’’ numerically to permit a pure numerical solution even using very high accuracy and state-of-the-art numerical schemes. It is shown that one may use a global singular perturbation expansion, similar to, but significantly more complex than the one used in the planar case, to solve the resulting nonlinear ordinary differential equation for the spatial dependence of the distribution function, density, electrostatic potential, and drift velocity.
Study of nonlinear Alfvén waves in an electron–positron plasma with a three-dimensional electromagnetic particle code
Authors: Zhao J. , Nishikawa K. I. , Sakai J. I. , Neubert T.
Published: July 2002

Results from three-dimensional (3-D) electromagnetic particle simulations of Alfvén waves generated by an electron beam in a nonrelativistic electron–positron plasma are presented. The results show that electrostatic modes are excited due to the beam instability. The bunches of the particles (electrons and positrons) caused by electrostatic waves are directly involved in the generation of Alfvén waves. The Alfvén waves propagate along the beam as damped solitons accelerating the background particles. The simulation results are in good agreement with theoretical analysis.
The energy of perturbations for Vlasov plasmas*,a)
Authors: Morrison P. J.
Published: July 2002

The energy content of electrostatic perturbations about homogeneous equilibria is discussed. The calculation leading to the well-known dielectric (or as it is sometimes called, the wave) energy is revisited and interpreted in light of Vlasov theory. It is argued that this quantity is deficient because resonant particles are not correctly handled. A linear integral transform is presented that solves the linear Vlasov–Poisson equation. This solution, together with the Kruskal–Oberman energy [Phys. Fluids 1, 275 (1958)], is used to obtain an energy expression in terms of the electric field [Phys. Fluids B 4, 3038 (1992)]. It is described how the integral transform amounts to a change to normal coordinates in an infinite-dimensional Hamiltonian system.
Partition in power transfer and dissipation rate in self-sustained turbulence
Authors: Itoh S.-I. , Itoh K. , Fukuyama A. , Yagi M. , Azumi M. , Nishikawa K.
Published: July 2002

Power partition and energy dissipation rates are examined for a self-sustained stationary turbulence of a high-n ballooning mode in a tokamak plasma. It is found that the power to excite fluctuations is almost equally transferred to perpendicular ion motion and to parallel electron motion. The ratio of the thermalized power, which excites and sustains the turbulence, to the total power lost by energy diffusion is found to be of the order of the broken symmetry parameter, i.e., the inverse aspect ratio, a/R. The dissipation rates of the fluctuations due to the thermal conductivity, the electron viscosity, and the ion viscosity are also calculated separately. The dissipation is dominated by that associated with the thermal conductivity. The relation between the induced global flux and the microscopic dissipation is also derived. It is found that a fractional part of order a/R of the dissipated power is effective in sustaining the turbulent modes.
Edge energy balance and temperature scalings in a tokamak
Authors: Martinell Julio J.
Published: July 2002

An analysis of the tokamak edge energy balance is performed in order to study the thermal equilibrium states in the presence of poloidal rotation and a sheared radial electric field. The processes considered in the energy balance are those relevant to the edge region, especially the term of viscous heating, which may be important in a plasma with poloidal rotation by the effect of magnetic pumping. The conditions that determine a stable equilibrium are obtained using the local values of the temperature and its radial gradient as parameters. Usually, there is only one possible such equilibrium, but for steep density gradients another one appears for a given temperature. For some special forms of the thermal diffusivity that follow from models that reduce the heat flux by electric-drift-shear suppression of fluctuations, an additional stable equilibrium with a high-temperature gradient may appear. It is found that the inclusion of radiation losses is important in determining the possible profile shapes, but they are also dependent on the temperature scaling of the radiation. Viscous heating affects the allowed values of the edge temperature and gradient, preventing them from being too large. Some examples of equilibrium states are given using parameters typical of DIII-D [Fusion Technol. 8, 441 (1985)]. The present study also reveals a preference for certain temperature scalings of the thermal diffusivity for the appearance of multiple equilibria, which might point to a particular class of transport models for the edge.
Free electron laser saturation: An analytical description
Authors: Dattoli G. , Giannessi L. , Ottaviani P. L. , Segreto A.
Published: July 2002

An analytical description of the free electron laser (FEL) strong-signal regime is developed by means of simple formulas obtained using the theory of Padé approximants. The analysis includes inhomogeneous broadening effects and its validity is tested with two different numerical procedures. The dependence of the saturation intensity on the detuning parameter and on the e-beam relative energy spread is also analyzed.
Convective threshold of stimulated Brillouin scattering in a two-ion plasma
Authors: Vu H. X. , Wallace J. M. , Bezzerides B.
Published: July 2002

The convective threshold of stimulated Brillouin backscatter (SBS) in a laser-irradiated plasma containing two distinct ion species is investigated over a wide range of plasma conditions and laser intensities. This investigation, carried out in the context of kinetic theory, is a generalization of a previous investigation of ion acoustic waves in a two-ion plasma, i.e., the unpumped system. The totality of SBS modes in the two-ion system is obtained using a numerical sweep through the complex-ω plane. The coupling of the electromagnetic pump wave to the plasma produces only one mode, in addition to the ion acoustic modes. Because this additional mode is not a normal mode of the plasma in the absence of the pump wave, it is called a quasimode. The SBS modes fall into three classes: (a) at least one and, at most, two modes that are weakly damped for sufficiently low laser intensity, reducing to the weakly damped ion acoustic modes as the pump strength vanishes; (b) an infinity of critically damped modes, most of which are relatively insensitive to the laser intensity; and (c) a single mode (quasimode), which may exhibit either growth or damping as laser intensity increases. The threshold laser intensity for convective SBS growth, the smallest intensity, where one of the modes ceases to be damped, ωI=0, may be associated with modes of classes (a), (b), or (c). The present study shows that the growing mode is not necessarily associated with the SBS modes obtained from fluid theory. A practical numerical procedure is presented for quickly determining the threshold intensity for SBS in a two-ion system for a given set of plasma parameters. The method is quite efficient and may be readily incorporated into large-scale hydrodynamic simulation codes to provide an assessment of SBS growth over the numerical mesh of the computation.
Comment on ‘‘Landau-fluid simulation of laser filamentation’’ [Phys. Plasmas 1, 1287 (1994)]
Authors: Maximov A. V. , Silin V. P.
Published: July 2002

The expression for the heat flux Eq. (26) of Ref. 1 is inappropriate for studying the ion sound damping on electron in weakly collisional fully ionized plasmas. (AIP)
Influence of poloidal rotation on the toroidally coupled tearing mode
Authors: Li Ding , Wang Chuanbing
Published: July 2002

Poloidal rotation and growth of the toroidally coupled tearing mode are analyzed based on the dispersion relation obtained by Li and Huo [Phys. Fluids B 5, 3737 (1993)]. The dominant (subdominant) branch is most (least) unstable when the frequencies of both unstable harmonics are equal. The poloidal rotation has a stabilizing effect on the subdominant branch through the toroidal coupling with the dominant branch. The stable branch cannot be excited by the poloidal rotation when one or both harmonics are stable. The rotation frequencies of both branches can be mutually locked to the average of, but not one of, both harmonic frequencies when the growth rates of both harmonics are equal.
Semiclassical transition probabilities for the electron-impact excitation of hydrogenic ions in dense plasmas
Authors: Jung Young-Dae
Published: July 2002

Plasma-screening effects are investigated on semiclassical transition probabilities for electron-impact excitation of hydrogenic ions in dense plasmas. The electron–ion interaction potential is obtained by a nonspherical Debye–Hückel model of the screened Coulomb interaction for the interesting domain of the Debye length Λ/aZ≥10. Semiclassical straight-line trajectory method is used to describe the behavior of the projectile electron. The screening effect is increased with increasing the projectile energy. The plasma-screening effect on the optically allowed (dipole) transition (1s→2p) is found to be more effective than that on the optically forbidden transition (1s→2s). For 1s→2s transition, the maximum position of the transition probability is almost unchanged with increasing or decreasing the projectile energy and plasma-screening effect. However, for 1s→2p transition, the maximum position of the transition probability has receded from the nucleus as the projectile energy increases.
Scattering of electromagnetic waves by counter-rotating vortex streets in plasmas
Authors: Guerra R. , Mendonça J. T. , Dendy R. O. , Shukla P. K.
Published: July 2002

The scattering of electromagnetic waves from counter-rotating vortex streets associated with nonlinear convective cells in uniform plasmas has been considered. The vortex street solution of the Navier–Stokes or the Hasegawa–Mima (and of the ‘‘sinh-Poisson’’) equation is adopted as a scatterer. Assuming arbitrary polarization and profile function for the incident electromagnetic field, a compact expression for the scattering cross section has been obtained. Specific results for the differential cross section are obtained for the case in which the incident beam has a Gaussian profile and propagates as an ordinary mode. The results show that when the characteristic wavelength of the vortex street (λv=2π/a) is larger than that of the incident electromagnetic wave (λi=2π/ki), the differential cross section dσ/dΩ has a very well-defined angular periodicity; in fact, it is a collection of Gaussians varying as exp[−f(kiw)2], where w is the waist and f is a function expressing a kind of ‘‘Bragg condition.’’ On the other hand, for λi≳λv the incident electromagnetic beam is unable to distinguish the periodic structure of the vortex street. The effects of the vortex street as well as the incident beam parameters on the scattering cross section are examined.
Sawteeth-induced impurity transport in tokamak plasmas
Authors: Ödblom A. , Anderson D. , Eriksson L.-G. , Lisak M.
Published: July 2002

A consistent picture of sawteeth-induced impurity transport is presented based on a recently given model for plasma redistribution by sawtooth crashes. The redistribution model is based on ideal magnetohydrodynamic conservation laws and assumes that the magnetic flux and the volume of the layers that reconnect during the crash are conserved. The post-crash redistributed impurity density is found in terms of the impurity density before the crash and the profiles of the safety factor before and after the crash. Comparisons with experimental results show good qualitative agreement.
Linear-conversion theory of energetic minority-ion Bernstein-wave propagation across gyroresonance in nonuniform magnetic field
Authors: Brizard A. J. , Kaufman A. N.
Published: July 2002

An energetic minority-ion population, such as neonatal fusion alphas, can support a Bernstein wave whose frequency is a harmonic of their gyrofrequency. The sign of the wave energy depends on the local wave vector, whose rate of change is proportional to the magnetic-field gradient. As a result of the field nonuniformity, the wave crosses the gyroresonance layer and its energy flips sign. This results in energy transfer to a gyroballistic mode (which exists only in the resonance layer), with a conversion coefficient exactly equal to 2.
The controlling parameters and the laser expression for a wiggler-free free-electron laser
Authors: Gou S. K.
Published: July 2002

Using the canonical transformations in Bambini–Renieri frame, the basic equations governing a wiggler-free free-electron laser are presented. It is found that the evolution of the system is determined by two controlling parameters. The condition, under which the exponentially growing exists, is obtained. Furthermore, starting from an arbitrary initial state of the system, the laser expression is derived. Finally, the possibility of employing this kind of system to give 300 (or 400) μm radiation is discussed using the Free Electron Laser for Infrared eXperiments (FELIX) parameters [P. W. van Amersfoort et al., Nucl. Instrum. Methods Phys. Res. A 318, 42 (1992)].
A model of ions interacting with neutrals in high electric field and the application to presheath formations
Authors: Senda Ikuo
Published: July 2002

A model of interactions between ions and neutrals in the high electric field is proposed and the exact solution to the Boltzmann equation with the interaction terms is derived. It is shown that the solution produces the known dependence of the ion drift velocity and the mobility on the parameters of the plasma and neutrals in the high electric field. Experimental results on mobilities of N2+ ions in the parent gas are studied by using the formula of the drift velocity obtained in our model. As an application, the presheath formation at the plasma–wall boundary is discussed and a comparison with experimental measurements of sheath potential profiles has been performed.
Radiative transfer in anisotropic, weakly inhomogeneous media with internal sources
Authors: Bellotti U. , Bornatici M.
Published: July 2002

The transport of radiation in anisotropic, weakly inhomogeneous and spatially dispersive media with internal sources is considered on the basis of the relevant geometric optics far-field as a function of position and frequency. The second rank radiation intensity tensor is obtained directly from the autocorrelation of the electric far-field in terms of a second rank emissivity tensor and accounts for coherent as well as incoherent emission, along with the effects of generalized Faraday rotation. The case of Čerenkov radiation is considered in detail.
Nonlinear theory of slow dissipative layers in anisotropic plasmas
Authors: Ballai I. , Ruderman M. S. , Erdélyi R.
Published: July 2002

The solar coronal plasma is a well-known example of a plasma with strongly anisotropic dissipative coefficients. The main dissipative processes in the solar corona are strongly anisotropic thermal conductivity and viscosity. Ruderman and Goossens [Astrophys. J. 471, 1015 (1996)] developed a linear theory of driven slow resonant waves in plasmas with strongly anisotropic viscosity and thermal conductivity. Linear theory shows that in the slow dissipative layer the amplitudes of oscillations become very large for high Reynolds and Pecklet numbers, so that nonlinearity may be important. In the present paper the nonlinear behavior of driven magnetohydrodynamic waves in the slow dissipative layer in plasmas with strongly anisotropic viscosity and thermal conductivity is studied. The nonlinear governing equation for wave variables in the dissipative layer is derived. The nonlinear connection formulae, which are extensions of the linear connection formulae first introduced in the theory of resonant magnetohydrodynamic waves by Sakurai, Goossens, and Hollweg [Solar Phys. 133, 127 (1991)], are derived.
Predictive simulations of tokamak plasmas with a model for ion-temperature-gradient-driven turbulence
Authors: Redd Aaron J. , Kritz Arnold H. , Bateman Glenn , Horton Wendell
Published: July 2002

A drift wave transport model, recently developed by Ottaviani, Horton and Erba (OHE) [Ottaviani et al., Plasma Phys. Controlled Fusion 39, 1461 (1997)], has been implemented and tested in a time-dependent predictive transport code. This OHE model assumes that anomalous transport is due to turbulence driven by ion temperature gradients and that the fully developed turbulence will extend into linearly stable regions, as described in the reference cited above. A multiplicative elongation factor is introduced in the OHE model and simulations are carried out for 12 discharges from major tokamak experiments, including both L- and H-modes (low- and high-confinement modes) and both circular and elongated discharges. Good agreement is found between the OHE model predictions and experiment. This OHE model is also used to describe the performance of the International Thermonuclear Experimental Reactor (ITER) [Putvinski et al., in Proceedings of the 16th IAEA Fusion Energy Conference, Montréal, Canada, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 2, p. 737.] A second version of the OHE model, in which the turbulent transport is not allowed to penetrate into linearly stable regions, has also been implemented and tested. In simulations utilizing this version of the model, the linear stability of the plasma core eliminates the anomalous thermal transport near the magnetic axis, resulting in an increase in the core temperatures to well above the experimental values.
Neutrinos generating inhomogeneities and magnetic fields in the early universe
Authors: Shukla P. K. , Bingham R. , Mendonça J. T. , Stenflo L.
Published: July 2002

It is found that an ensemble of random phase incoherent neutrinos can generate both the inhomogeneities and magnetic fields in the early universe. Hence, neutrinos can be considered as building blocks of the two most important astrophysical phenomena that are central to modern astroparticle physics and cosmology.
Viscous instability in a non-neutral plasma
Authors: Goswami Priyanka , Bhattacharyya S. N. , Sen A. , Maheshwari K. P.
Published: July 2002

The effect of viscosity on the stability of a non-neutral plasma is studied. Stability boundaries are obtained for two-dimensional electrostatic perturbations of long wavelengths. It is shown that a configuration with a monotone decreasing number density profile can be unstable when the plasma has viscosity.
Nonlinear interaction of a high-power electromagnetic beam in a dusty plasma: Two-dimensional effects
Authors: Sambandan G. , Tripathi V. K. , Parashar J. , Bharuthram R.
Published: July 2002

A large-amplitude Gaussian electromagnetic beam, propagating through a dusty plasma, heats the electrons nonuniformly. As the electron temperature rises, the rate of electron attachment to dust particles changes, modifying dust charge and free electron density. Further, the ambipolar diffusion of the plasma under thermal pressure gradient creates a plasma channel that guides the electromagnetic beam. At powers exceeding a threshold value, the beam becomes self-focused.
Dynamo efficiency and Beltrami flows in rectangular cells
Authors: Kageyama Akira , Sato Tetsuya
Published: July 2002

Numerical simulations are performed for a kinematic dynamo problem in a rectangular cell geometry having dynamo laboratory experiments in mind. It is shown that the maximum growth rate of the magnetic field is given by a Beltrami flow with extremum flow helicity. This relation is so accurate that a slight change of the flow always gives a lower growth rate, at least in the low magnetic Reynolds number limit. It is also pointed out that two Beltrami flows with exactly the same kinetic energy and the helicity give very different dynamo growth rates. This indicates that the helicity extremum or the Beltrami condition alone is not sufficient to tell the most efficient dynamo flow.
Scrape-off layer intermittency in the Castor tokamak
Authors: Heller M. V. A. P. , Brasilio Z. A. , Caldas I. L. , Stöckel J. , Petrzilka J.
Published: July 2002

Spatial–temporal intermittency of floating potential and ion saturation current fluctuations is analyzed by using data obtained from two probes arrays in the scrape-off layer of the CASTOR tokamak [Proceedings of the 1996 International Conference on Plasma Physics (Nagoya) (International Atomic Energy Agency, Vienna, 1997), Vol. I, p. 322]. For these ion saturation current fluctuations with non-Gaussian probability density functions, a conditional averaging analysis shows coherent structures with correlation lengths and lifetimes larger for larger amplitude conditions. Nevertheless, there is no evidence of such large structures in the potential fluctuations. Furthermore, wavelet transforms are used to analyze these nonstationary fluctuations and obtain details not observed with the Fourier technique. So, examining wavelet power and coherence spectra, strong intermittency is found for both kinds of fluctuations, in a time scale two orders of magnitude higher than that observed in the conditional analysis. Moreover, during the discharges the observed wavelet quadratic coupling alters intermittently for some low-frequency components.
Direct losses of highly energetic trapped particles in torsatrons and heliotrons with a vertically asymmetric helical magnetic-field ripple
Authors: Smirnova M. S.
Published: July 2002

Direct losses of highly energetic helically trapped particles, especially of superbananas, are analytically investigated in torsatrons and heliotrons [Uo, J. Phys. Soc. Jpn. 16, 1380 (1961)] with a vertically asymmetric helical field ripple. It is found that the vertically asymmetric helical ripple causes an additional net radial drift of suprathermal superbananas, leading to convective losses of these particles from the confinement volume mainly in the vertical direction. For a vertically asymmetric rippled torsatron, a location of the particle loss region in the coordinate space and a dominated direction of the particle escape from the confinement volume is predicted on the basis of the probabilities of the particle trapping/detrapping in/from the helical ripple wells. Analytical predictions are confirmed by numerical computations of direct losses of highly energetic helically trapped particles for a number of torsatrons/heliotrons of today.
Chaos in the parallel sheared plasma flow driven electromagnetic turbulence in nonuniform magnetoplasmas
Authors: Mirza Arshad M. , Rafiq Tariq , Murtaza G. , Shukla P. K. , Faria R. T.
Published: July 2002

By employing the two-fluid model, a system of nonlinear equations for low-frequency electromagnetic waves in nonuniform collisional magnetoplasmas has been derived. The plasma contains both the equilibrium density gradient and sheared flows. In the linear limit, a local dispersion relation has been obtained and analyzed in several interesting limiting cases. It is found that equilibrium sheared plasma flows cause instabilities of Alfvén-type waves even in the absence of the density gradient. The numerical results also show a large growth rate of electromagnetic parallel velocity shear (PVS) mode compared to the electrostatic mode for some ionospheric parameters. For this case, the temporal nonlinear behavior of the relevant governing mode coupling equations is governed by six coupled equations, which are a generalization of the Lorenz–Stenflo equations and which admit chaotic trajectories. The results of this investigation should be useful for understanding the linear and nonlinear properties of electromagnetic waves that are generated by sheared plasma flows in magnetized plasmas.
Saturation of stimulated Raman backscatter in strongly turbulent plasmas
Authors: Mourenas D.
Published: July 2002

The nonlinear behavior of stimulated Raman backscattering in a large, homogeneous and hot plasma slab at subcritical density is investigated analytically in the limit of moderate to strong linear and nonlinear ion acoustic damping. It is found that the Langmuir waves damping might be reduced to the collisional one, leading to a highly incoherent interaction for moderate to strong laser pump power. Scaling laws for the saturated reflectivity are obtained in the presence of parametric decay cascade and strong Langmuir turbulence, and compared to recent experimental observations.
Resonant interactions between nonlinear Alfvén waves in electron–positron plasmas
Authors: Ciurea-Borcia Rodica , Ignat Margareta
Published: July 2002

This paper deals with resonant interactions between four circularly polarized Alfvén waves propagating through relativistic electron–positron plasmas. The nonlinear coupling equations for the case under consideration are derived. These ones are numerically solved to obtain the steady-state spatial profile of the waves excited at the boundary z=0. The present results might be useful to explain some astrophysical observations.
Magnetohydrodynamic mechanisms of electric-field transport suppression and plasma-rotation generation, with special reference to tokamak’s reversed-shear confinement
Authors: Yoshizawa Akira , Yokoi Nobumitsu , Itoh Sanae-I. , Itoh Kimitaka
Published: July 2002

A magnetohydrodynamic analysis is made of the mechanisms of electric-field transport suppression and plasma-flow generation in the context of tokamak’s reversed-shear (RS) confinement. The turbulent transport of heat is shown to be suppressed through the combined effect of radial electric field and charge inhomogeneity. The similarity is pointed out between the internal transport barrier in RS modes and the edge counterpart in high-confinement modes. A concave electric-current profile is shown to be a cause of the global plasma rotation, and the latter contributes to the occurrence of a negative radial electric field that plays a key role in the former suppression mechanism.
Three-dimensional Fokker–Planck equation for trapped fast ions in a Tokamak with weak toroidal field ripples
Authors: Yavorskij V. A. , Andrushchenko Zh. N. , Edenstrasser J. W. , Goloborod’ko V. Ya
Published: July 2002

The five-dimensional (5D) drift kinetic Fokker–Planck equation for fast charged particles confined in a tokamak with a toroidal field (TF) ripple magnitude below the Goldston–White–Boozer stochasticity threshold is averaged over the banana and superbanana timescales. As a result, a three-dimensional (3D) Fokker–Planck equation in the constants of motion (COM) space describing the collisional transport of charged high-energy particles is obtained. Toroidally trapped particles with the toroidal precession being in resonance with the ripple perturbations are shown to yield the main contribution to the ripple induced transport. It is found that the rates of ripple superbanana diffusion and convection in the radial coordinate significantly exceed the corresponding rates of the bananas in the axisymmetric limit. The superbanana diffusion and convection shown to be dominant in the MeV energy range may be responsible for the loss of partially thermalized fusion products observed in the Tokamak fusion test reactor (TFTR) [S. J. Zweben, R. L. Boivin, C.-S. Chang et al., Nucl. Fusion 31, 2219 (1991); H. W. Herrmann, S. J. Zweben, D. S. Darrow et al., ibid. 37, 1437 (1997)].
Undercooling of molten silicon
Authors: Devaud G. , Turnbull D.
Published: July 2002

Droplets of uncoated molten Si (0.4–0.8 mm diameter) have been undercooled 250 °C. Ge droplets of similar size have been undercooled 280 °C in a B2O3 flux. The observed nucleation onset temperatures of both Si and Ge droplets are at or below the predicted amorphous phase melting temperatures Ta1. The solidified structures were polycrystalline. The nucleation frequency I, calculated from the Si data (2×104/cm3 s at 240±20 °C undercooling), should be an upper limit of the homogeneous nucleation frequency of the crystal phase, since we did not establish that nucleation was homogeneous in our experiments. However, this limiting I for Si indicates that homogeneous nucleation of crystal would not become appreciable in laser pulsing experiments until the liquid Si is undercooled to well below Ta1.
Optoelectronic properties of Cd1−xZnxTe films grown by molecular beam epitaxy on GaAs substrates
Authors: Olego D. J. , Faurie J. P. , Sivananthan S. , Raccah P. M.
Published: July 2002

Photoluminescence (PL) experiments were carried out at 300 and 12 K to investigate the electro-optical properties of Cd1−xZnxTe grown by molecular beam epitaxy on GaAs substrates. The compositional dependence of the band-gap energy was determined. It has a quadratic dependence on x. The near band edge PL spectra at 12 K show free and bound exciton lines for x=0 and 1 and only broadened bound exciton peaks for other compositions. The bound exciton broadenings are quantitatively explained based on the compositional fluctuations of the cations. The PL line shapes give indications of the high quality of the layers.
Room-temperature operation of a para-H2 rotational Raman laser
Authors: Midorikawa K. , Tashiro H. , Aoki Y. , Nagasaka K. , Toyoda K. , Namba S.
Published: July 2002

We have successfully demonstrated high-power operation of a para-H2 rotational Raman laser at room temperature. A photon conversion efficiency of 60% was obtained with a 10.6-μm CO2 pump energy of 4.3 J, giving 0.85 J in the first Stokes pulse at 17.0 μm. Transient effects observed under room-temperature conditions are also reported.
Hydrogen passivation of the oxygen-related thermal-donor defect in silicon
Authors: Johnson N. M. , Hahn S. K.
Published: July 2002

It is demonstrated with both spreading resistance measurements and deep level transient spectroscopy that hydrogenation neutralizes the oxygen-related thermal-donor defect in n-type silicon. Hydrogen neutralization of shallow-donor dopants (e.g., phosphorus) is also illustrated. A model which accounts for shallow-donor passivation is applied to explain the interaction between hydrogen and thermal donors.
Direct measurements of liquid/solid interface kinetics during pulsed-laser-induced melting of aluminum
Authors: Tsao J. Y. , Picraux S. T. , Peercy P. S. , Thompson Michael O.
Published: July 2002

We report time-resolved electrical-resistance measurements obtained during pulsed-laser melting of a metal. Through heat-flow calculations and solute-diffusion measurements, the measured resistances are correlated with the thresholds for partial and full melting of a thin film of aluminum. Furthermore, simultaneous time-resolved reflectance measurements establish that, in this geometry, melting and solidification proceed via the motion of a well-defined, planar liquid/solid interface, whose position can be deduced from the resistance measurements. These measurements permit, for the first time, real-time determinations of melt-depth histories in fundamental studies of rapid solidification processing of metals.
Material-dependent infrared transmittance of mineral wool
Authors: Ljungdahl G. , Ribbing C-G.
Published: July 2002

The specular infrared transmittance of mineral wool is reported and compared to the optical properties of the raw material, a silicate glass. A transmittance peak at 8.3 μm for the wool, not previously reported, is correlated to the reflectance minimum at the same wavelength for the bulk glass. It is concluded that the radiative behavior of this fibrous insulation cannot be described by gray models, neglecting the dispersion of the raw material. It is also demonstrated that the transmittance peak values are much higher than could be expected from the absorption of the glass. This is tentatively explained as an effect of forward scattering through very small angles, thus increasing the specular transmittance.
Femtosecond carrier relaxation in semiconductor-doped glasses
Authors: Nuss M. C. , Zinth W. , Kaiser W.
Published: July 2002

Light induced changes of absorption in semiconductor-doped glasses are studied on the femtosecond and picosecond time scale. Extremely rapid absorption recovery times of 200 fs are found when carriers are excited with large excess energy (500 meV) above the absorption edge of the semiconductor-doped glasses. Excitation close to the band edge results in a slower absorption recovery with a time constant of τ≂100 ps.
Second quantized state lasing of a current pumped single quantum well laser
Authors: Mittelstein Michael , Arakawa Yasuhiko , Larsson Anders , Yariv Amnon
Published: July 2002

Newly observed features of quantum well lasers are presented and explained with the aid of a simple model. These involve lasing with gain contributions not only from the fundamental (n=1) state, but simultaneously from the second quantized (n=2) state as well. Experimental data for current pumped GaAlAs/GaAs single quantum well lasers are presented. Very high resonator losses (≳100 cm−1) force the lasers to augment their gain with major contributions from the second quantized state. The main signature of n=2 lasing, a sudden and large increase in the lasing photon energy, is observed and explained by the theory.
Erbium doping of molecular beam epitaxial GaAs
Authors: Smith R. S. , Müller H. D. , Ennen H. , Wennekers P. , Maier M.
Published: July 2002

The doping of molecular beam epitaxial GaAs with erbium up to a concentration of 2×1019 cm−3 has been successfully demonstrated. Up to a concentration of about 5×1018 cm−3 the surface morphology remained good but for higher doping levels the surface became structured. Hall and profile measurements indicate that erbium doping gives rise to a trapping level capable of compensating silicon-doped layers. For the first time photoluminescence from a rare earth element incorporated in a III-V semiconductor has been observed at room temperature.