bluuebunny/crossref_metadata_2025 huggingface dataset:
Particle simulation of the ion-extraction process from a plasma by a static electric field
Authors: Watanabe Junko , Okano Kunihiko
Published: July 2002

Ion extraction from a plasma plays important roles in various technology fields, such as ion plantation, ion beam generation, and laser isotope separation. In this paper, the behavior of ions during the extraction process is investigated in detail using two types of particle simulation codes, and the ion-extraction time is compared with the results of an analytical model. A two-dimensional code, in which the electrons are assumed to be distributed, in accordance with the Boltzmann relation, while the ion behavior is calculated by the particle model, results in essentially the same values for ion-extraction time, as with the analytical model. On the other hand, the results of the one-dimensional full particle code partly differ from that of the two-dimensional code and the analytical model. The use of the Boltzmann distribution of the electrons tends to overestimate the ion-extraction time up to a factor of approximately 2 at its worst point.
Wave collapse at the lower-hybrid resonance
Authors: Shapiro V. D. , Shevchenko V. I. , Solov’ev G. I. , Kalinin V. P. , Bingham R. , Sagdeev R. Z. , Ashour-Abdalla M. , Dawson J. , Su J. J.
Published: July 2002

The modulational instability and collapse of waves in the vicinity of the lower-hybrid resonance including both magnetosonic and lower-hybrid waves are investigated by analytical and numerical methods. The mechanism leading to the modulational instability is the nonlinear coupling of lower-hybrid waves with the much lower-frequency quasineutral density perturbations via the ponderomotive force. The result is a filamentation of the high-frequency field producing elongated, cigar-shaped nonlinear wave packets aligned along the magnetic field with the plasma expelled outside (cavities). The analytical self-similar solutions describing cavity collapse are obtained and compared with the results of numerical simulation for both two- and three-dimensional cavity geometries. It is shown that in three-dimensional solutions the transverse, with respect to the magnetic field, contraction remains prevailing. The possibility of ion acceleration as the result of the lower-hybrid collapse is discussed and detailed comparison is made with the observations of the phenomena in the auroral ionosphere.
Theory of dissipative density-gradient-driven turbulence in the tokamak edge
Authors: Terry P. W. , Diamond P. H.
Published: July 2002

A theory of resistive, density-gradient-driven turbulence is presented and compared with tokamak edge fluctuation measurements. In addition to linear driving, the theory accounts for relaxation of the density gradient through a nonlinear process associated with emission from localized density fluctuation elements. From a fluid model for isothermal electrons in toroidal geometry, equations are obtained and solved analytically, retaining both coherent and incoherent contributions. The effect of collisions on the density blobs is treated. A Reynolds number parameterizes the magnitude of the turbulent scattering relative to the collisional viscous diffusion. The analytic results indicate that the spectrum is characterized by linewidths which increase as a function of the Reynolds number and may reach Δω/ω≳1. Energy lies predominantly in the small wavenumbers (k⊥ ρs∼0.1). For larger wavenumbers and frequency, the spectrum decays as k−17/6 and ω−2. The fluctuation level scales as 1/k⊥Ln and may reach −30% for parameters typical of the pretext edge. Particle diffusion is Bohm-like in magnitude but does not follow Bohm scaling, going instead as n2/3T1/6e. The density fluctuations exhibit nonadiabatic character caused by the incoherent mode coupling. An expression for the departure from adiabaticity is given.
Elastohydrodynamic collision and rebound of spheres: Experimental verification
Authors: Barnocky Guy , Davis Robert H.
Published: July 2002

Experiments were performed in order to delineate the conditions under which small metal and plastic spheres rebound, as opposed to stick, when dropped upon a smooth quartz surface overlaid with a thin layer of a viscous fluid. The parameters that were varied include the fluid layer thickness and viscosity, and the ball size, density, and elastic properties. The minimum drop height that allowed the ball to rebound out of the fluid layer was determined. The results are in very good agreement with the recent elastohydrodynamic theory of Davis et al. [J. Fluid Mech. 163, 479 (1986)]. Additional experiments were performed for which the quartz surface was made artificially rough by adhering fine glass spheres to it. For these experiments, the resistance to rebound caused by the fluid layer was significantly reduced, in close agreement with the recent theory for rough surfaces developed by Davis [Phys. Chem. Hydro. 9, 41 (1987)].
A model for orientational diffusion in fiber suspensions
Authors: Koch Donald L.
Published: July 2002

A model for the orientational diffusion resulting from hydrodynamic fiber–fiber interactions in a general linear shear flow is presented. The constants appearing in the expression for the diffusivity are determined from calculations of orientational diffusion in pure extensional flows. The model is tested against experimental and theoretical studies of orientational dynamics in simple shear flows.
Estimation of the Kolmogorov constant (C) for the Lagrangian structure function, using a second-order Lagrangian model of grid turbulence
Authors: Du Shuming , Sawford Brian L. , Wilson John D. , Wilson David J.
Published: July 2002

We review Sawford’s [Phys. Fluids A 3, 1577 (1991)] second-order Lagrangian stochastic model for particle trajectories in low Reynolds number turbulence, showing that it satisfies a well-mixed constraint for the (hypothetical) case of stationary, homogeneous, isotropic turbulence in which the joint probability density function for the fixed-point velocity and acceleration is Gaussian. We then extend the model to decaying homogeneous turbulence and, by optimizing model agreement with the measured spread of tracers in grid turbulence, estimate that Kolmogorov’s universal constant (C0) for the Lagrangian velocity structure function has the value of 3.0±0.5.
Advection by polytropic compressible turbulence
Authors: Ladeinde F. , O’Brien E. E. , Cai X. , Liu W.
Published: July 2002

Direct numerical simulation (DNS) is used to examine scalar correlation in low Mach number, polytropic, homogeneous, two-dimensional turbulence (Ms≤0.7) for which the initial conditions, Reynolds, and Mach numbers have been chosen to produce three types of flow suggested by theory: (a) nearly incompressible flow dominated by vorticity, (b) nearly pure acoustic turbulence dominated by compression, and (c) nearly statistical equipartition of vorticity and compressions. Turbulent flows typical of each of these cases have been generated and a passive scalar field imbedded in them. The results show that a finite-difference based computer program is capable of producing results that are in reasonable agreement with pseudospectral calculations. Scalar correlations have been calculated from the DNS results and the relative magnitudes of terms in low-order scalar moment equations determined. It is shown that the scalar equation terms with explicit compressibility are negligible on a long time-averaged basis. A physical-space EDQNM model has been adapted to provide another estimate of scalar correlation evolution in these same two-dimensional, compressible cases. The use of the solenoidal component of turbulence energy, rather than total turbulence energy, in the EDQNM model gives results closer to those from DNS in all cases.
A dynamical model for turbulence. I. General formalism
Authors: Canuto V. M. , Dubovikov M. S.
Published: July 2002

We propose new dynamical equations to describe fully developed turbulence. We begin with the Wyld equations (WE), which are exact solutions of the NSE. The WE, and their Langevin-like representation, show that nonlinearities induce a turbulent force ft(k) and a turbulent viscosity νt(k), which are given by an infinite series of Wyld diagrams. The series for νt(k) is renormalizable, and its sum can be found using RNG methods. The result, Eq. (2a), holds for stirring forces fext with an arbitrary correlation function φ and generalizes previous RNG results, which neglected ft and were limited to power law φ∼k1−2ε. To recover Kolmogorov law, these earlier RNG-based theories were forced to introduce an ad hoc stirring force with a prescribed φ∼k−3. By contrast, we show that ∼k−3 belongs to φ̃, which is the correlation function of ft, and that in the inertial range ft≫fext. The series for φ̃ cannot be summed because of a nonrenormalizable infrared divergence (IR) with an infinite number of divergent irreducible diagrams. To overcome this difficulty, we use the well-accepted notion of local energy transfer and we derive an expression for the energy flux Π(k), Eq. (2d), as well as a dynamical equation for the energy spectrum E(k), Eq. (2b). We also construct the dynamical equations for Reynolds stress spectra (solved in papers II and III). An analogous approach is developed for the temperature field. The model contains no free parameters. Some of its predictions are Kolmogorov spectrum E(k)∼k−5/3 with Ko=5/3, in agreement with recent data; temperature spectrum in the inertial-convective region Eθ∼Ba ε̄−1/3εθk−5/3, in agreement with the data; Batchelor constant Ba=σt Ko. In addition, in papers II and III we carry out extensive comparisons with the laboratory, DNS, LES data, and phenomenological models. The model can be used to construct a subgrid model for LES calculations.
Coherent structures in two uniformly distorted plane turbulent wakes
Authors: Kopp G. A. , Keffer J. F.
Published: July 2002

A pattern-recognition analysis was performed on two uniformly distorted plane turbulent wakes in the far region: the first was a wake generated by a high solidity mesh strip and the second was a wake generated by a solid flat plate. We observed that the coherent structures in the porous wake are significantly different from those in the solid wake. The porous wake structures are characterized by ‘‘ejections’’ of turbulent fluid from the center of the wake. The solid wake has nearly two-dimensional and quasi-periodic structures with strong circulation patterns in the vertical plane. We interpret the results as showing that there is a deficit of spanwise vorticity, relative to lateral or streamwise vorticity, in the porous wake structures, as compared to the solid wake structures. It is shown that these differences have a marked effect on the mixing and entrainment in the two flows.
Observations of high Reynolds number bubbles interacting with a rigid wall
Authors: Tsao Heng-Kwong , Koch Donald L.
Published: July 2002

The behavior of bubbles with radii of 0.5–0.7 mm rising through water in the presence of a solid boundary were observed using a high-speed video camera. Fluid inertia and surface tension cause a bubble to bounce several times from a horizontal wall before viscosity dissipates the energy. An energy balance involving the kinetic energy of the fluid motion, the surface energy of the air–water interface, and the gravitational potential energy aids in the interpretation of the dynamics of the collision. We also observed the motion of a bubble rising under an oblique wall with an angle of 10°–85° to the horizontal. When the angle was less than about 55° corresponding to We<0.4, the bubble slid steadily along the wall. At steeper angles the bubble was observed to bounce repeatedly from the inclined wall without any apparent loss of amplitude. It was also determined that the critical Weber number of coalescence of a bubble rising toward a stationary bubble is 1.6. At Weber numbers below this critical value, the two bubbles coalesce on impact while bubbles bounce at higher Weber numbers.
Thermocapillary motion of deformable drops at finite Reynolds and Marangoni numbers
Authors: Haj-Hariri H. , Shi Q. , Borhan A.
Published: July 2002

We present the results of numerical simulations of the three-dimensional thermocapillary motion of deformable viscous drops under the influence of a constant temperature gradient within a second liquid medium. In particular, we examine the effects of shape deformations and convective transport of momentum and energy on the migration velocity of the drop. A numerical method based on a continuum model for the fluid–fluid interface is used to account for finite drop deformations. An oct-tree adaptive grid refinement scheme is integrated into the numerical method in order to track the interface without the need for interface reconstruction. Interface deformations arising from the convection of energy at small Reynolds numbers are found to be negligible. On the other hand, deformations of the drop shape due to inertial effects, though small in magnitude, are found to retard the motion of the drop. The steady drop shapes are found to resemble oblate or prolate spheroids without fore and aft symmetry, with the direction of elongation of the drop depending on the value of the density ratio between the two phases. As in the case of a gas bubble, convection of energy is shown to retard the thermocapillary motion of a viscous drop, as the isotherms get wrapped around the front surface of the drop and effectively reduce the surface temperature gradient which drives the motion. The effect of inertia on the mobility of viscous drops is found to be weaker than that in the case of gas bubbles.
Combined thermocapillary-buoyancy convection in a cavity. Part II. An experimental study
Authors: Braunsfurth M. G. , Homsy G. M.
Published: July 2002

The problem of buoyant-thermocapillary convection in cavities is governed by a relatively large number of nondimensional parameters, and there is consequently a large number of different types of flow that can be found in this system. Previous results give disjoint glimpses of a wide variety of qualitatively and quantitatively different results in widely different parts of parameter space. In this study, we report experiments on the primary and secondary instabilities for acetone as the working fluid with a Prandtl number of 4.44, and in a geometry with equal aspect ratios in the range from 1 to 8 in both the direction along and perpendicular to the applied temperature gradient. We thus complement previous work that mostly involved either fluid layers of large extent in both directions, or consisted of investigations of strictly two-dimensional disturbances. We investigate the qualitative and quantitative features of the fluid velocity field by flow visualization and particle tracking techniques. We observe the primary transition from an essentially two-dimensional flow to steady three-dimensional longitudinal rolls. The critical Marangoni number for this first transition is found to depend on the aspect ratios of the system, and varies from 4.6×105 at aspect ratio 2.0 to 5.5×104 at aspect ratio 3.5. The structure of the steady three-dimensional flow far above the transition is found to involve a nonintuitive reverse flow against the temperature driving due to the strongly nonlinear three-dimensional flow associated with the longitudinal rolls. Further, we have investigated the stability of this three-dimensional flow at larger Marangoni numbers, and find a novel oscillatory flow at critical Marangoni numbers of the order of 6×105. We suggest possible mechanisms that give rise to the oscillations.
Self-similar solutions in gas dynamics with exponential time dependence
Authors: Simonsen V. , Meyer-ter-Vehn J.
Published: July 2002

Self-similar gas flow with similarity coordinate ξ=re∓t, exponential in time, is investigated for plane, cylindrical and spherical geometry. The underlying Lie group symmetry is pointed out and also how it is obtained from flow with power-law self-similarity (ξ=r/|t|α) in the limit α→∞. Six distinct types of solutions are derived and plotted in r−t coordinates. They are characterized by steep density gradients. Also, solutions with strong shock boundaries are discussed. A completely analytical solution is presented, related to Sedov’s solution of a strong point explosion.
Depth of penetration of bubbles entrained by a plunging water jet
Authors: Clanet Christophe , Lasheras Juan C.
Published: July 2002

A model is proposed to predict the depth of penetration of the air bubbles entrained by a round water jet impacting into a flat, liquid pool. This depth is shown to be determined only by the initial jet momentum and by the non-monotonic nature of the bubble terminal velocities as a function of their size. The model is shown to be in excellent agreement with measurements of the depth and width of penetration of the bubbles performed over a wide range of jet diameters, velocities, and plunging angles.
Pressure and shear stress measurements at the wall in a turbulent boundary layer on a cylinder
Authors: Nepomuceno Henry G. , Lueptow Richard M.
Published: July 2002

The fluctuating wall shear stress, wall pressure, and streamwise velocity were measured simultaneously in a cylindrical boundary layer at a momentum thickness Reynolds number of Reθ=2160 and a boundary layer thickness to cylinder radius ratio of δ/a=5 using a hot wire wall shear stress probe mounted just upstream of a hearing aid microphone and a hot wire velocity probe. Variable Interval Time Averaging (VITA) event detection on streamwise velocity indicates that streamwise accelerations are associated with positive wall pressure peaks and sudden increases in wall shear stress. Likewise, positive pressure peak events are associated with streamwise accelerations and sudden increases in wall shear stress. VITA detection on wall shear stress reveals that increasing wall shear stress corresponds to streamwise accelerations and small-amplitude pressure rises, not distinct intense pressure peaks. Detection of strong adverse and favorable instantaneous pressure gradients indicates that a shear layer at y+=13 coincides with a positive peak in the wall pressure, suggesting that a positive wall pressure peak event is the key wall pressure signature associated with the burst cycle. Measurements of the cross-correlation indicate that the pressure–shear stress relationship is about two times weaker than the pressure–velocity relation and about ten times weaker than the shear stress–velocity relation. Thus, a strong relationship exists between wall pressure and streamwise velocity as well as between wall shear stress and streamwise velocity, but the relationship between wall shear stress and wall pressure is quite weak. Because of the similarity of the near-wall structure of all wall-bounded turbulent flows, regardless of transverse curvature, these conclusions should be applicable to planar boundary layers.
Coherent structures in a round, spatially evolving, unforced, homogeneous jet at low Reynolds numbers
Authors: Danaila Ionut , Dušek Jan , Anselmet Fabien
Published: July 2002

Three–dimensional direct numerical simulations of unforced, incompressible, free, spatially evolving round jets are used to investigate the onset of instability at low diametral Reynolds numbers (Re⩽500). Compact, coherent structures are identified by means of iso-surfaces of vorticity and pressure fields and shown to be synonymous with instability modes. Once the inflow velocity profile is fixed, as the Reynolds number increases from 200 to 500, the most amplified unstable mode switches from the helical mode to the axisymmetric one, as expected from the predictions of the viscous linear stability theory analysis and from experimental observations [J. Fluid Mech. 77, 511 (1976); Prog. Aerosp. Sci. 21, 159 (1984)] [ J. Fluid Mech. 48, 547 (1971)]. At the upper limit of the investigated range of Reynolds numbers, the present simulations are consistent with the widely accepted scenario of the space time development of the round jet instability. This scenario is analyzed in detail. The appearance of pairs of axially counter-rotating vortex filaments is found (for the first time, to our knowledge, in unforced, spatial numerical simulations) to characterize the destabilization of initial axisymmetric vortical structures. The spatial evolution of these structures is investigated and their role in vortex rings reconnection is evidenced. For lower Reynolds numbers, a superposition of symmetry-breaking (helical) modes is shown to characterize the instability of the round jet. The Fourier decomposition of the fluctuating flow field allows the extraction of the helical modes and the identification of the flow patterns resulting from their interactions. The attractor is shown to be a limit torus very close to the onset of the instability.
The mechanism of surface wave suppression in film flow down a vertical plane
Authors: Lin S. P. , Chen J. N.
Published: July 2002

The equation relating the average rate of change of disturbance kinetic energy to the rates of work done by the surface tension, the shear stress, the Reynolds stress, and the rate of mechanical energy dissipation through viscosity in a falling liquid film flow down an oscillating vertical plate is obtained. Each term in the equation is evaluated in various regions of parameter space to elucidate the physical mechanism of stabilizing an inherently unstable vertical film flow by use of plate oscillations [Lin, Chen, and Woods, Phys. Fluids 8, 3247 (1996); Lin and Chen, Phys. Fluids 9, 3926 (1997)].
Cascade of structures in long-wavelength Marangoni instability
Authors: Boos W. , Thess A.
Published: July 2002

The long-wavelength instability for thermocapillary-driven convection in two dimensions is studied numerically. The system under consideration consists of a horizontal periodical liquid layer bounded from below by a rigid wall and from above by a deformable free surface. The liquid is heated from the bottom wall and cooled from above. The problem can be approximated by the Stokes equation and has two dimensionless parameters. One parameter is the dynamic Bond number which is the ratio between gravity and thermocapillary force. The other is the static Bond number, which describes the ratio between gravity and the surface tension. Using the boundary integral method we present full-scale direct numerical simulations of the long-wavelength Marangoni instability in two dimensions. The time evolution of the free surface leads to the formation of drained regions (so-called “dry spots”). The simulations demonstrate a remarkable complexity of the touchdown process, involving a deep cascade from large to increasingly small structures. In the behavior of the minimum height of the interface hmin at large time simple scaling dependence on time was not observed. Extrapolation of hmin exhibits infinite-time singularity. The dependence of the size of drained region on the parameters is discussed.
Scott Russell’s wave generator
Authors: Monaghan J. J. , Kos A.
Published: July 2002

In this paper we study the dynamics of a system consisting of a heavy box sinking vertically into water. The classic configuration is due to Scott Russell who used the sinking box in 1844 to illustrate the formation of a solitary wave in a long rectangular tank. We use a combination of computer simulation and experiment to clarify details of the wave formation and the dynamics of the sinking box. We find that as the box sinks the water is heaved up to form both the solitary wave and a reverse plunging wave which forms a vortex. This vortex follows the wave down the tank. The computer simulation uses the particle method smoothed particle hydrodynamics (SPH) which allows us to follow the formation of the waves and the dynamics of the box. The simulation results are in satisfactory agreement with the experiments. We derive scaling relations for the velocity of the box and for the amplitude of the solitary wave. These scaling relations are in reasonable agreement with the experiments and the simulations.
The migration of a drop in a uniform temperature gradient at large Marangoni numbers
Authors: Balasubramaniam R. , Subramanian R. Shankar
Published: July 2002

The steady thermocapillary motion of a spherical drop in a uniform temperature gradient is treated in the situation where convective transport of energy is predominant in the drop phase as well as in the continuous phase, i.e., when the Marangoni numbers are large. It is assumed that the Reynolds numbers in both phases are large as well; to leading order, the velocity fields are given by a potential flow field in the continuous phase and Hill’s vortex inside the drop. The migration velocity of the drop is obtained by equating the rate at which work is done by the thermocapillary stress to the rate of viscous dissipation of energy. The analysis deals with an asymptotic situation wherein convective transport of energy dominates with conduction playing a role only where essential. This leads to thin thermal boundary layers both outside and within the drop. The method of matched asymptotic expansions is employed to solve the conjugate heat transfer problem in the two phases. It is shown that the demand for energy within the drop, necessary to increase its temperature at a steady rate as it moves into warmer surroundings, results in a large temperature difference between the surface of the drop and its interior. The variation of temperature over the drop surface is large as well, and leads to a linear increase of the migration velocity of the drop with increasing Marangoni number. This result is strikingly different from that for the limiting case when the viscosity and thermal conductivity inside the drop become negligible compared to the corresponding properties in the continuous phase. This limit, which holds for a gas bubble, is recovered correctly from the analysis.
The kinetic energy spectrum of the two-dimensional enstrophy turbulence cascade
Authors: Lindborg Erik , Alvelius Krister
Published: July 2002

A direct numerical simulation of forced two-dimensional turbulence with hyperviscosity is performed at resolution 40962. A stage is reached at which the flux of enstrophy from large to small scales is approximately constant in time. The cubic and quintic relations for the third-order velocity structure function derived by Lindborg [J. Fluid Mech. 388, 259 (1999)] are verified. The calculated kinetic energy spectrum in the constant enstrophy flux range has the form E(k)=Kεω2/3k−3, where εω is the enstrophy dissipation. This is in accordance with the prediction of Kraichnan [Phys. Fluids 10, 1417 (1970)] and Batchelor [Phys. Fluids 12, II233 (1969)]. The logarithmic correction, suggested by Kraichnan [J. Fluid Mech. 47, 525 (1970)], is not present in the calculated spectrum.
Gap formation processes in a high-density plasma opening switch
Authors: Grossmann J. M. , Swanekamp S. B. , Ottinger P. F. , Commisso R. J. , Hinshelwood D. D. , Weber B. V.
Published: July 2002

A gap opening process in plasma opening switches (POS) is examined with the aid of numerical simulations. In these simulations, a high density (ne=1014–5×1015 cm−3) uniform plasma initially bridges a small section of the coaxial transmission line of an inductive energy storage generator. A short section of vacuum transmission line connects the POS to a short circuit load. The results presented here extend previous simulations in the ne=1012–1013 cm−3 density regime. The simulations show that a two-dimensional (2-D) sheath forms in the plasma near a cathode. This sheath is positively charged, and electrostatic sheath potentials that are large compared to the anode–cathode voltage develop. Initially, the 2-D sheath is located at the generator edge of the plasma. As ions are accelerated out of the sheath, it retains its original 2-D structure, but migrates axially toward the load creating a magnetically insulated gap in its wake. When the sheath reaches the load edge of the POS, the POS stops conducting current and the load current increases rapidly. At the end of the conduction phase a gap exists in the POS whose size is determined by the radial dimensions of the 2-D sheath. Simulations at various plasma densities and current levels show that the radial size of the gap scales roughly as B/ne, where B is the magnetic field. The results of this work are discussed in the context of long-conduction-time POS physics, but exhibit the same physical gap formation mechanisms as earlier lower density simulations more relevant to short-conduction-time POS.
Effect of toroidal plasma flow and flow shear on global magnetohydrodynamic MHD modes
Authors: Chu M. S. , Greene J. M. , Jensen T. H. , Miller R. L. , Bondeson A. , Johnson R. W. , Mauel M. E.
Published: July 2002

The effect of a subsonic toroidal flow on the linear magnetohydrodynamic stability of a tokamak plasma surrounded by an external resistive wall is studied. A complex non-self-adjoint eigenvalue problem for the stability of general kink and tearing modes is formulated, solved numerically, and applied to high β tokamaks. Results indicate that toroidal plasma flow, in conjunction with dissipation in the plasma, can open a window of stability for the position of the external wall. In this window, stable plasma beta values can significantly exceed those predicted by the Troyon scaling law with no wall. Computations utilizing experimental data indicate good agreement with observations.
Plasma-screening effects on direct 1s→2p± electron–ion collisional excitation rates in dense plasmas
Authors: Hong Woopyo , Jung Young-Dae
Published: July 2002

Plasma-screening effects are investigated on the 1s→2p− (m=−1) and 1s→2p+ (m=+1) electron–ion collisional excitation rates in dense plasmas. The excitation rates are calculated using the semiclassical excitation cross sections obtained by the classical straight-line trajectory approximation with the nonspherical Debye–Hückel potential. The results show that the ratio of the 1s→2p+ to 1s→2p− excitation rates is found to be a slowly varying function of the temperature. Including the plasma-screening effects for hydrogenic ions, the ratio is found to be a monotonically increasing function of the parameter (Z2Ry/kT)1/2. However, without the plasma-screening effects, the ratio becomes a monotonically decreasing function. The plasma-screening effects appreciably enhance the 1s→2p− excitation rates and reduce the 1s→2p+ excitation rates.
Theory of self-organized critical transport in tokamak plasmas
Authors: Kishimoto Y. , Tajima T. , Horton W. , LeBrun M. J. , Kim J. Y.
Published: July 2002

A theoretical and computational study of the ion temperature gradient (ITG) and ηi instabilities in tokamak plasmas has been carried out. In a toroidal geometry the modes have a radially extended structure and their eigenfrequencies are constant over many rational surfaces that are coupled through toroidicity. These nonlocal properties of the ITG modes impose a strong constraint on the drift mode fluctuations and the associated transport, showing self-organized criticality. As any significant deviation away from marginal stability causes rapid temperature relaxation and intermittent bursts, the modes hover near marginality and exhibit strong kinetic characteristics. As a result of this, the temperature relaxation is self-similar and nonlocal, leading to radially increasing heat diffusivity. The nonlocal transport leads to Bohm-like diffusion scaling. Heat input regulates the deviation of the temperature gradient away from marginality. We present a critical gradient transport model that describes such a self-organized relaxed state. Some of the important aspects in tokamak transport like Bohm diffusion, near marginal stability, radially increasing fluctuation energy and heat diffusivity, intermittency of the wave excitation, and resilient tendency of the plasma profile can be described by this model, and these prominent features are found to belong to one physical category that originates from the radially extended nonlocal drift modes. The obtained transport properties and scalings are globally consistent with experimental observations of low confinement mode (L-mode) discharges. The nonlocal modes can be disintegrated into smaller radial islands by a poloidal shear flow, suggesting that the transport changes from Bohm-like to near gyro-Bohm.
Statistical description and transport in stochastic magnetic fields
Authors: Vanden Eijnden E. , Balescu R.
Published: July 2002

The statistical description of particle motion in a stochastic magnetic field is presented. Starting form the stochastic Liouville equation (or, hybrid kinetic equation) associated with the equations of motion of a test particle, the probability distribution function of the system is obtained for various magnetic fields and collisional processes. The influence of these two ingredients on the statistics of the particle dynamics is stressed. In all cases, transport properties of the system are discussed.
Scaling laws of the Rayleigh–Taylor ablation front mixing zone evolution in inertial confinement fusion
Authors: Oron D. , Alon U. , Shvarts D.
Published: July 2002

A theoretical model for the ablatively driven Rayleigh–Taylor (RT) instability single-mode and multimode mixing fronts is presented. The effect of ablation is approximately included in a Layzer-type potential flow model, yielding the description of both the single-mode evolution and the two-bubble nonlinear competition. The reduction factor of the linear growth rate due to ablative stabilization obtained by the model is similar to the Takabe formula. The single-bubble terminal velocity is found to be similarly reduced by ablation, in good agreement with numerical simulations. Two-bubble competition is calculated, and a statistical mechanics model for multimode fronts is presented. The asymptotic ablation correction to the classical RT αgt2 mixing zone growth law is derived. The effect of ablative stabilization on the allowed in-flight aspect ratio of inertial confinement fusion pellets is estimated using the results of the statistical mechanics model.
Mathematical models of magnetic insulation
Authors: Ben Abdallah Naoufel , Degond Pierre , Méhats Florian
Published: July 2002

The problem of magnetic insulation in a plane diode is discussed. Starting from the Child–Langmuir asymptotics of the stationary Vlasov–Maxwell system, various limit problems are exhibited and solved. Explicit formulas for the magnetic field at the cathode and for the electron sheath width are provided. The analysis allows one to recover and to complete the various known results related to magnetic insulation. In addition, an ensemble of intermediate models ranging from the quasilaminar to the laminar model, and involving the formation of virtual cathodes, is exhibited. A stability analysis is then performed to determine which of these models are stable. The virtual cathode models are shown to be unstable under dissipation effects, and most of them are shown to be unstable under geometry effects (i.e., when replacing the planar geometry by the cylindrical one). The stability argument shows that in some situations, the laminar model is unstable while in other situations the quasilaminar one is unstable. This gives some indication for the choice of the appropriate magnetic insulation model.
Improvement of surface properties by modification and alloying with high-power ion beams
Authors: Renk T. J. , Buchheit R. G. , Sorensen N. R. , Cowell Senft D. , Thompson M. O. , Grabowski K. S.
Published: July 2002

Surface treatment and alloying experiments with Al, Fe, and Ti-based metals as well as Si wafers were conducted on the Repetitive High Energy Pulsed Power-I (RHEPP-I) accelerator [0.8 MV, 20 Ω, 80 ns full width at half maximum (FWHM) pulse width, up to 1 Hz repetition rate] at Sandia National Laboratories. Ions are generated by the magnetically confined anode plasma (MAP) gas-breakdown active anode, which can yield a number of different beam species including H, C, N, O, Kr, and Xe, depending upon the injected gas. Enhanced hardness and wear resistance have been produced by treatment of 440C stainless steel, by the mixing of a thin-film Pt coating into Ti-6Al-4V alloy, and of a Si coating into Al 6061-T6 alloy (Al-1.0Mg-0.6Si). Mixing of a thin-film Hf layer into Al 6061-T6 has improved its corrosion resistance by as much as four orders of magnitude in electrochemical testing, compared with untreated and uncoated Al6061. Processing of Si was used to validate simulation codes. When treated with nitrogen ions, melt and full epitaxial growth was observed to depths of 4–6 μm. Experiments are ongoing to further understand the microstructural basis for these surface improvements.
Three-dimensional particle-in-cell simulation study of a relativistic magnetron
Authors: Lemke R. W. , Genoni T. C. , Spencer T. A.
Published: July 2002

The A6 (six cavity) magnetron operated at relativistic voltages is investigated using three-dimensional particle-in-cell simulation. Previous experiments [A. Palevsky and G. Bekefi, Phys. Fluids 22, 986 (1979)] are repeated numerically for the purpose of ascertaining fidelity of code results in addition to obtaining insight into magnetron operation at high voltage. Simulations produce all qualitative behavior observed in experiments, with good quantitative agreement in most cases. This numerical study also reveals details concerning mode control, the cause and prevention of leakage current, and voltage scaling in the A6. It is shown that: (1) leakage current substantially reduces efficiency, (2) elimination of leakage current using cathode endcaps substantially increases efficiency, and (3) the functional dependence of operating voltage on applied magnetic field may depart from conventional scaling at very high voltage.
Kinetic modes in a hot magnetized and weakly collisional plasma
Authors: De Souza-Machado S. , Sarfaty M. , Skiff F.
Published: July 2002

Velocity space perturbations associated with low-frequency waves launched in a weakly collisional plasma are shown to consist of a discrete spectrum of modes. Collisions are modeled using an energy and momentum conserving one-dimensional Fokker–Planck operator. The linearized Vlasov–Poisson–Fokker–Planck system of equations is solved by expanding the perturbed ion-distribution function in terms of Hermite polynomials, from which an eigenvalue problem is set up. The eigenvalues and eigenvectors yield the ion acoustic mode that is weakly damped [J. Dougherty, Physics of Fluids 7, 1788 (1964)], as well as a discrete spectrum of kinetic modes.
High gain free electron laser amplifiers starting from coherent and incoherent spontaneous emission
Authors: Piovella Nicola
Published: July 2002

Self-amplification of coherent spontaneous emission and shot noise in high gain single pass free electron lasers is investigated using a multifrequency model. This approach includes intrinsically the coherent spontaneous emission generated by the components of the Fourier transform of electron current that are close to the resonant frequency. The shot noise is modeled by adding random fluctuations to the initial electron phases. The complete model of equations is derived and analytical and numerical results are presented. The main features of the emitted radiation starting from coherent spontaneous emission and shot noise are discussed.
Ionization equilibrium and equation of state in strongly coupled plasmas
Authors: Rogers Forrest J.
Published: July 2002

Calculation of the physical properties of reacting plasmas depends on knowing the state of ionization and/or the state occupation numbers. Simple methods have often been used to estimate ionization balance in plasmas, but they are not adequate for understanding a variety of new experimental and observational measurements. Theoretical methods to determine the ionization state of partially ionized plasmas must confront the effects of density on bound states and strong ion coupling. These methods can be separated into two categories. Chemical picture methods consider the system to be composed of distinct chemical species. Consequently, it is necessary to assert the effect of the plasma environment on internal states of these species. On the other hand, physical picture methods view the plasma in terms of its fundamental constituents; i.e., electrons and nuclei, so that plasma effects on bound states are a basic component of the theory. A discussion of some work representative of both of these philosophies will be given. Some comparisons between theories and with recent helioseismic observations and shock experiments will also be given.
Transverse dynamics of a relativistic electron beam in an underdense plasma channel
Authors: Geraci Andrew A. , Whittum David H.
Published: July 2002

The transverse dynamics of a relativistic electron beam in a plasma less dense than the beam is analyzed, with particular attention to the electron-hose instability as it appears in a plasma channel of finite radius. A comparison is made between analytic asymptotic forms, linearized “beam break-up” simulations, and particle-in-cell simulations. The analysis provides a framework for the interpretation of projected and time-resolved diagnostics in such a system. Examples used for illustration correspond to parameters of an experiment in progress, employing a 30 GeV electron beam in a 1.5 m plasma cell, with the amplification of beam-centroid offsets on the order of 100–102×, depending on experimental settings.
Reversibility and stability of tellurium alloys for optical data storage applications
Authors: Chen M. , Rubin K. A. , Marrello V. , Gerber U. G. , Jipson V. B.
Published: July 2002

We demonstrate for the first time that Te87Ge8Sn5 films, which are amorphous as deposited, can be optically switched between the crystalline and amorphous states more than 106 times. The measured reflectivity changed from 40% to 60% and the transmission changed from 3% to 1.5%, respectively, between the amorphous and crystalline states. The crystallization temperature of the cycled spots is ≊75 °C and these spots are observable after >20 weeks. It is found that the crystallization temperature of cycled spots is typically about 20 °C lower than that of the unwritten film. Increasing the Ge concentration leads to increased crystallization temperature and increased minimum crystallization time without affecting reversibility.
Ultrafast carrier and grating lifetimes in semiconductor-doped glasses
Authors: Yao S. S. , Karaguleff C. , Gabel A. , Fortenberry R. , Seaton C. T. , Stegeman G. I.
Published: July 2002

The time dependence of both the photoluminescence due to carrier recombination and the gratings created by degenerate four-wave mixing were measured in semiconductor-doped color-filter glasses. Values ranging from 80 to less than 16 ps (laser pulse width limited) are measured in several different samples at various excitation levels. A slower mechanism, believed to be thermal in nature, is also observed with a lifetime in excess of 9 ns.
Band alignments of coherently strained GexSi1−x/Si heterostructures on 〈001〉 GeySi1−y substrates
Authors: People R. , Bean J. C.
Published: July 2002

The self-consistent ab initio pseudopotential results of C. G. Van de Walle and R. M. Martin [J. Vac. Sci. Technol. B 3, 1256 (1985)] have been combined with a phenomenological deformation potential theory to estimate the band gap and band offsets for coherently strained multilayers of GexSi1−x/Si for growth on 〈001〉 GeySi1−y substrates. It is found that ΔEc is negligible and the narrower GexSi1−x gap falls within the wider Si gap (type I band alignment) if the Si in the multilayers is cubic, whereas ΔEc can be appreciable and the GexSi1−x conduction-band edge tends to be higher in energy than the Si conduction-band edge(type II band alignment) if both the Si and the GexSi1−x are strained. In particular, the present results resolve the seeming paradox which arose from interpretations of modulation doping experiments using heterojunctions grown either on Si〈001〉 substrates or on an unstrained alloy buffer layer.
Emission wavelength of AlGaAs-GaAs multiple quantum well lasers
Authors: Blood P. , Fletcher E. D. , Hulyer P. J. , Smowton P. M.
Published: July 2002

We have recorded spontaneous emission spectra from multiple quantum well lasers grown by molecular beam epitaxy with 25-Å-wide GaAs wells by opening a window in the top contact stripe. These spectra have a low-energy tail and consequently the gain spectra derived from them show that laser emission occurs at a lower photon energy than the lowest energy confined particle transition. The observed laser wavelength and threshold current are consistent with the position of the peak in the gain spectrum.
Dislocation reduction in epitaxial GaAs on Si(100)
Authors: Fischer R. , Neuman D. , Zabel H. , Morkoç H. , Choi C. , Otsuka N.
Published: July 2002

We have studied the nucleation and propagation of threading dislocations in GaAs on Si epitaxial layers, and have found several techniques which are effective in reducing their density. The use of substrates properly tilted off (100) reduces the dislocation density as the presence of steps helps create perfect edge dislocations with their Burgers vector parallel to the interface and thus do not propagate into the bulk epitaxial layer. Cross sections by transmission electron microscopy show that the incorporation of an InGaAs/GaAs strained-layer superlattice reduces the density of threading dislocations above it by a factor of 10, clearly demonstrating the effectiveness of this technique. These methods lead to a dislocation density of 103 cm−2 near the surface of 2 μm layers which is five orders of magnitude lower than what has been obtained previously. We have also found that the density of oval defects is much lower for GaAs on Si than for GaAs on GaAs.
Development and origin of conical structures on XeCl laser ablated polyimide
Authors: Dyer P. E. , Jenkins S. D. , Sidhu J.
Published: July 2002

Stable, well-defined conical structures have been observed to develop on the surface of excimer laser etched polyimide. Deliberate seeding shows that these are induced by the shielding effect of particulate impurities and indicates means for controlling the surface microstructure.
High resolution electron microscopy of misfit dislocations in the GaAs/Si epitaxial interface
Authors: Otsuka N. , Choi C. , Nakamura Y. , Nagakura S. , Fischer R. , Peng C. K. , Morkoç H.
Published: July 2002

Recent studies have shown that high quality GaAs films can be grown by molecular beam epitaxy on Si substrates whose surfaces are slightly tilted from the (100) plane. In order to investigate the effect of the tilting of substrate surfaces on the formation of defects, the atomic structure in the GaAs/Si epitaxial interface has been studied with a 1-MV ultrahigh vacuum, high voltage electron microscope. Two types of misfit dislocations, one with Burgers vectors parallel to the interface and the other with Burgers vectors inclined from the interface by 45°, were found in the epitaxial interface. The observation of two cross-sectional samples perpendicular to each other has shown that the tilting of the substrate surface directly influences the formation of these two types of misfit dislocations. A possible mechanism of the reduction of threading dislocations by tilting the substrate surface is discussed based on these observations.
Dynamics and tunability of a small optically pumped cw far-infrared laser
Authors: Everitt Henry O. , Skatrud David D. , DeLucia Frank C.
Published: July 2002

We report the development of an ultrasmall, optically pumped cw far-infrared (FIR) laser that provides substantial tunability. This laser operates at pressures significantly higher than the maximum allowed by currently accepted theory. We also report the development of a new theoretical model for diffusion limited optically pumped FIR lasers which accounts for this behavior. It is shown that the consideration of additional higher energy vibrational states, along with appropriate energy transfer mechanisms, fundamentally alters the behavior of the system in the high pressure, high pump intensity regime. Although 13CH3F is used for both the experimental demonstration and the theoretical model, the concept is general and should apply to all diffusion relaxed FIR lasers.