bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron vortex orbits and merger
Authors: Mitchell T. B. , Driscoll C. F.
Published: July 2002

Pure electron plasma columns are contained inside hollow conducting cylinders in an axial magnetic field. In the 2D E×B drift approximation, an electron column is a vortex evolving in (r,θ) according to the Euler equation. First the center-of-mass orbits of two vortices sufficiently well-separated to be stable to merger are characterized. Equilibria are observed in which the vortices orbit about the center of the cylinder, with either oscillations about stable equilibria or exponential divergence away from unstable equilibria. The equilibrium positions, oscillation frequencies, and instability rates for these spatially extended vortices agree well with the predictions of point vortex theory, apparently because surface waves and shape distortions do not couple significantly to the center-of-mass motion. Next, the merger of two vortices with unequal radii is quantified. Merger is accompanied by the formation of filamentary arms, and results ultimately in an axisymmetric central core surrounded by a lower density halo. The self-energy of the merged core is found to be roughly the sum of the self-energy of the merging vortices. The fraction of the total circulation entrained into the core varies from 70% to 90% as the ratio of the initial vortex radii is varied from 1:1 to 2:1. The point-like vortex dynamics and the circulation loss with merger are both consistent with the ‘‘punctuated Hamiltonian’’ models of decaying turbulence.
Intrinsic convection in a settling suspension
Authors: Bruneau D. , Feuillebois F. , Anthore R. , Hinch E. J.
Published: July 2002

The intrinsic convection in a settling suspension is shown to have its origins in the buoyancy of the particle-depleted layer next to the side walls. The convection is calculated using a simple model in which the particles are represented by an excluded volume and by point forces at their centres. A boundary layer approximation is developed for vessels which are much wider than the particle diameter. It is shown that the direction of the convection depends critically on assumptions about the formation of the particle-depleted layer during the placement of the suspension into the vessel.
Modeling of the dynamic subgrid-scale viscosity in large eddy simulation
Authors: Yoshizawa Akira , Tsubokura Makoto , Kobayashi Toshio , Taniguchi Nobuyuki
Published: July 2002

The dynamic subgrid-scale (SGS) viscosity is reexamined on the basis of the production process of the SGS Reynolds stress. A new model for the SGS viscosity is proposed in place of the Smagorinsky-type model of mixing-length form. The model is applied to a channel flow, and the comparison with other models is made.
Experiments on the oscillatory behavior of buoyant plumes of helium and helium-air mixtures
Authors: Cetegen Baki M. , Kasper Kent D.
Published: July 2002

Experiments on the oscillatory behavior of axisymmetric buoyant plumes of helium and helium-air mixtures are reported for a range of nozzle diameters (3.6 cm<d<20 cm), source velocities, and plume densities. Measurements include pulsation frequencies as determined from total pressure fluctuations along the plume centerline in addition to the phase resolved laser Doppler velocity measurements. These nonreacting buoyant plumes are found to exhibit periodic oscillations of plume boundaries which subsequently evolve into toroidal vortices within one-half diameter above the nozzle exit. These oscillations and vortices are similar to those observed in pool fires, although their frequency scaling is somewhat different. The frequency relationship is well represented by the expression S=0.8Ri0.381, where the Strouhal number is S=fd/V0 and the Richardson number is defined as Ri1=[(ρ∞−ρp)gd]/ρ∞V20. Parameters f, V0, ρ are frequency, source velocity, and density and subscripts p and ∞ refer to the plume fluid and ambient, respectively. Between Ri1=100 and 500, a transition in the frequency scaling is observed as evidenced by more turbulent and vigorously mixing plumes beyond this transition. In this region, S=2.1Ri0.281. This change in scaling can be explained by the effect of turbulent mixing on local plume density and the resulting modification of the convection speed of the toroidal vortices. These results provide a consistent basis for the mechanism of the observed instability which is quite different than other types of flow instabilities. Additionally, the phase resolved velocity field of a pulsating buoyant plume reveals a strong buoyant acceleration along the plume centerline followed by a deceleration in the region of the toroidal vortex formation. The strong upward acceleration is also accompanied by significant radial inflow toward the plume centerline determining the entrainment characteristics of these pulsating buoyant plumes.
Direct numerical simulation of a passive scalar with imposed mean gradient in isotropic turbulence
Authors: Overholt M. R. , Pope S. B.
Published: July 2002

Mixing of a passive scalar in statistically homogeneous, isotropic, and stationary turbulence with a mean scalar gradient is investigated via direct numerical simulation, for Taylor-scale Reynolds numbers, Rλ, from 28 to 185. Multiple independent simulations are performed to get confidence intervals, and local regression smoothing is used to further reduce statistical fluctuations. The scalar fluctuation field, φ(x,t), is initially zero, and develops to a statistically stationary state after about four eddy turnover times. Quantities investigated include the dissipation of scalar flux, which is found to be significant; probability density functions (pdfs) and joint-pdfs of the scalar, its derivatives, scalar dissipation, and mechanical dissipation; and conditional expectations of scalar mixing, ∇2φ. A linear model for scalar mixing jointly conditioned on the scalar and v-velocity is developed, and reproduces the data quite well. Also considered is scalar mixing jointly conditioned on the scalar and scalar dissipation. Terms appearing in the balance equation for the pdf of φ are examined. From a solution of the scalar pdf equation two sufficient conditions arise for the scalar pdf to be Gaussian. These are shown to be well satisfied for moderate values of the scalar, and approximately so for large fluctuations. Many correlations are also presented, including ρ(v,φ), which changes during the evolution of the scalar from a value of unity when initialized to the stationary value of 0.5–0.6.
Absolute instability of Rayleigh-Bénard convection in a time-periodic shear flow
Authors: Li Hsi-shang , Kelly Robert E. , Hall Philip
Published: July 2002

The onset of thermal convection in an oscillatory shear flow is considered as an initial-value problem. For the case with zero mean shear, numerical results indicate that the instability is absolute in nature, just as for the case without any shear. If a mean flow component also exists, then the instability tends to have the same nature as that which occurs in the flow when no oscillation is present.
Steady streaming in oscillatory viscous flow under a transverse magnetic field
Authors: Cuevas Sergio , Ramos Eduardo
Published: July 2002

In this paper we deal with the oscillatory laminar boundary layer flow of an electrically conducting fluid near an insulating solid body under a transverse, uniform magnetic field. It is assumed that the two-dimensional flow is produced by an external stream velocity which varies periodically and that the magnetic field induced in the fluid can be neglected. The solution of the boundary layer equations is found by an expansion on the small parameter ε (the inverse of the Strouhal number). Both the primary oscillatory flow and the secondary flow which is composed of an oscillatory motion and a steady contribution, i.e. the steady streaming, are analytically determined. The magnetohydrodynamic (MHD) flow correctly reduces to the hydrodynamic limit when the magnetic field vanishes. However, differently from the hydrodynamic case, the MHD second order steady solution satisfies the vanishing of the steady streaming motion as the distance from the body tends to infinity. This result is a consequence of the suppression of the only vorticity component, which is perpendicular to the magnetic field. Further, when the magnetic interaction parameter is sufficiently high, the streaming motion can be completely suppressed.
Reconstructing the flow in the wall region from wall sensors
Authors: Podvin Bérengère , Lumley John
Published: July 2002

In this paper we examine how the flow in the wall region can be predicted from realistic (partial) measurements. The underlying motivation of this study is the improvement of control schemes for near-wall flows. We propose a method based on the proper orthogonal decomposition which provides estimated amplitudes for the coherent structures (i.e., the large scales) of the flow from wall measurements. The method is tested for the direct numerical simulation of a minimal flow unit. The large scales obtained by reconstruction from wall data are compared to those of the velocity field in the wall layer. The dominant structures—the streamwise streaks—are well recovered, the cross-stream motions less so since they are associated with higher-order structures unaccounted for in our truncation. We defined “rescaled” eigenfunctions to try to improve the representation of the cross-stream components of the flow. Aliasing effects due to realistic (large) sensor spacings were examined. We find that the spanwise spacing is the limiting factor for the estimation, so that a realistic grid will affect the reconstruction by at least 20% compared to full wall information.
On the alignment of strain, vorticity and scalar gradient in turbulent, buoyant, nonpremixed flames
Authors: Boratav O. N. , Elghobashi S. E. , Zhong R.
Published: July 2002

The alignment of vorticity and scalar gradient with the eigendirections of the rate of strain tensor is investigated in turbulent buoyant nonpremixed horizontal and vertical flames. The uniqueness of a buoyant nonpremixed flame is that it contains regions with distinct alignment characteristics. The strain-enstrophy angle Ψ is used to identify these regions. Examination of the vorticity field and the vorticity production in these different regions indicates that Ψ and consequently the alignment properties near the flame surface identified by the mixture fraction band F≈Fst differ from those in the fuel region, F>Fst and the oxidizer region, FFst (and F<Fst for the vertical flame) band(s) show(s) vorticity/β alignment. The implication of this result is that the scalar dissipation, εF, attains its maximum value always near F≈Fst. These results are also discussed within the framework of recent dynamical results [Galanti et al., Nonlinearity 10, 1675 (1997)] suggesting that the Navier-Stokes equations evolve towards an attracting solution. It is shown that the properties of such an attracting solution are also consistent with our results of buoyant turbulent nonpremixed flames.
Scaling laws for the turbulent mixing of a passive scalar in the wake of a cylinder
Authors: Lévêque Emmanuel , Ruiz-Chavarria Gerardo , Baudet Christophe , Ciliberto Sergio
Published: July 2002

Turbulence is triggered by an air flow passing around a cylinder and heated by an array of hot wires parallel to the cylinder axis. Simultaneous velocity and passive temperature fluctuations are measured locally on the centerline of the cylinder wake. The focus is on the scaling properties of the mixed scalar-velocity structure functions 〈|δu(r)δθ(r)2|p/3〉. It is observed that 〈|δu(r)δθ(r)2|p/3〉 displays the same scaling behavior as 〈|δu(r)δθ(r)2|〉Ψp/Ψ3 over an extended range of scales. This yields accurate measurements for the relative scaling exponents Ψp/Ψ3. It is further observed that the moments of the coarse-grained scalar energy diffusion rate 〈N(r)p〉 satisfy a closure condition as proposed by She and Lévêque [Phys. Rev. Lett. 72, 336 (1994)]. Otherwise it is proposed that 〈|δu(r)δθ(r)2|p/3〉 scales as 〈N(r)p/3〉〈|δu(r)δθ(r)2|〉p/3. This generalized similarity relation is well supported by our measurements over a range of scales which greatly exceeds the inertial range. A theoretical prediction is finally proposed, Ψp=p/9+(10/9)(1−(2/5)p/3), in good agreement with our experimental data.
Nonuniform two-dimensional fluid flow from a point source
Authors: Putkaradze Vachtang , Dimon Peter
Published: July 2002

We study exact solutions describing two-dimensional fluid flow from a point source. We find that the solutions have a k-fold symmetry and that they exist only if the Reynolds number is less than Rc(k)=π(k2−4). We also address the stability of these solutions.
Laminar vortex shedding from a trapezoidal cylinder with different height ratios
Authors: Chung Y. J. , Kang S.-H.
Published: July 2002

The Strouhal number from trapezoidal cylinders depends on the Reynolds number as well as the height ratio, which is the ratio of the height of the rear face to that of the front face. For Re=100 and 150, distributions of the Strouhal numbers show their minimum values at a height ratio of 0.7 and 0.85, respectively. However, for Re=200, the variation of Strouhal number increases as the height ratio from a square section decreases. After showing the maximum value at a height ratio of 0.7, it decreases as the height ratio decreases. It then increases again for the smaller height ratios. Movement of the flow separation point from the rear to front corners and change of secondary vortex strength are important factors in determining the shedding structure for Re=100 and 150. However, for Re=200, flow separation always occurs at the front corners. The change of secondary vortex strength and the interaction between the primary and secondary vortices influence the variation of shedding.
Eulerian action principles for linearized reduced dynamical equations
Authors: Brizard Alain
Published: July 2002

New Eulerian action principles for the linearized gyrokinetic Maxwell–Vlasov equations and the linearized kinetic-magnetohydrodynamic (kinetic-MHD) equations are presented. The variational fields for the linearized gyrokinetic Vlasov–Maxwell equations are the perturbed electromagnetic potentials (φ1,A1) and the gyroangle-independent gyrocenter (gy) function Sgy, while the variational fields for the linearized kinetic-MHD equations are the ideal MHD fluid displacement ξ and the gyroangle-independent drift-kinetic (dk) function Sdk (defined as the drift-kinetic limit of Sgy). According to the Lie-transform approach to Vlasov perturbation theory, Sgy generates first-order perturbations in the gyrocenter distribution F1≡{Sgy, F0}gc, where F1 satisfies the linearized gyrokinetic Vlasov equation and {, }gc denotes the unperturbed guiding-center (gc) Poisson bracket. Previous quadratic variational forms were constructed ad hoc from the linearized equations, and required the linearized gyrokinetic (or drift-kinetic) Vlasov equation to be solved a priori (e.g., by integration along an unperturbed guiding-center orbit) through the use of the normal-mode and ballooning-mode representations. The presented action principles ignore these requirements and, thus, apply to more general perturbations.
Nonlinear electromagnetic gyrokinetic equations for rotating axisymmetric plasmas
Authors: Artun M. , Tang W. M.
Published: July 2002

The influence of sheared equilibrium flows on the confinement properties of tokamak plasmas is a topic of much current interest. A proper theoretical foundation for the systematic kinetic analysis of this important problem has been provided here by presenting the derivation of a set of nonlinear electromagnetic gyrokinetic equations applicable to low-frequency microinstabilities in a rotating axisymmetric plasma. The subsonic rotation velocity considered is in the direction of symmetry, with the angular rotation frequency being a function of the equilibrium magnetic flux surface. In accordance with experimental observations, the rotation profile is chosen to scale with the ion temperature. The results obtained represent the shear flow generalization of the earlier analysis by Frieman and Chen [Phys. Fluids 25, 502 (1982)], where such flows were not taken into account. In order to make it readily applicable to gyrokinetic particle simulations, this set of equations is cast in a phase-space-conserving continuity equation form.
Instability due to axial shear and surface impedance
Authors: Tsidulko Yu. A. , Berk H. L. , Cohen Ronald H.
Published: July 2002

The stability of plasma flow in the scrape-off layer of a tokamak, taking into account the surface sheath impedance and the axial shear in the E×B flow is analyzed. An interesting stability problem arises in the limit that end plates are sufficiently far apart, so that stability can be analyzed when the plasma is taken to interact with a single end plate. As parameters are varied, windows of instability are found, and it is shown that growth rates are maximized for an insulating end plate and are also quite sensitive to the ratio of the ion diamagnetic and E×B drift frequencies. Mixing-length estimates of the diffusivity are comparable to experimentally observed values.
Rayleigh–Taylor eigenmodes of a thin layer in the nonlinear regime
Authors: Basko Mikhail M.
Published: July 2002

In the long-wavelength limit, many aspects of the Rayleigh–Taylor (RT) instability of accelerated fluid shells can be explored by using the thin sheet approximation. For two-dimensional (2-D) planar eigenmodes, analytic nonlinear solutions [E. Ott, Phys. Rev. Lett. 29, 1429 (1972)] are available. Comparing the simplest of them for the nonconstant acceleration, g∝t−2, with Ott’s solution for constant g, the applicability of nonlinear results obtained for constant g to situations with variable acceleration is analyzed. Nonlinear three-dimensional (3-D) effects are investigated by comparing the numerical solutions for axisymmetric Bessel eigenmodes with Ott’s solution for 2-D modes. It is shown that there is a qualitative difference between 2-D and 3-D bubbles in the way they rupture a RT unstable fluid shell: In contrast to the exponential thinning of 2-D bubbles, mass is fully eroded from the top of an axisymmetric 3-D bubble within a finite time of (1.1–1.2)γ−1 after the onset of the free-fall stage; γ is the RT growth rate.
Anomalous perturbative transport in tokamaks due to drift-Alfvén-wave turbulence
Authors: Thoul Anne A. , Similon P. L. , Sudan R. N.
Published: July 2002

The method developed in Thoul, Similon, and Sudan [Phys. Plasmas 1, 579 (1994)] is used to calculate the transport due to drift-Alfvén-wave turbulence, in which electromagnetic effects such as the fluttering of the magnetic field lines are important. Explicit expressions are obtained for all coefficients of the anomalous transport matrix relating particle and heat fluxes to density and temperature gradients in the plasma. Although the magnetic terms leave the transport by trapped electrons unaffected, they are important for the transport by circulating electrons.
Magnetic field integrability and the current closure condition
Authors: Hanson James D. , Cary John R.
Published: July 2002

Magnetic fields with translational symmetry, axisymmetry, and helical symmetry are integrable. That is, they have good flux surfaces. The current closure condition, that ∮dl/B is a flux surface quantity, also holds. It is possible to construct magnetic fields with broken symmetry, which have nearly integrable magnetic fields. It is shown that for nearly integrable nonsymmetric fields, the current closure condition need not hold.
Two-stage electron acceleration by simultaneous stimulated Raman backward and forward scattering
Authors: Bertrand P. , Ghizzo A. , Karttunen S. J. , Pättikangas T. J. H. , Salomaa R. R. E. , Shoucri M.
Published: July 2002

The coexistence of stimulated Raman forward and backward scattering of intense electromagnetic radiation, which can occur, for instance, in laser fusion plasmas, is investigated. The simultaneous Raman forward and backward scattering is shown to create an electrostatic field structure which is exceptionally efficient in producing highly relativistic electrons. The mechanism of the electron acceleration is analyzed both by Vlasov–Maxwell simulations with self-consistent fields and by test particle calculations with prescribed electrostatic fields. The Vlasov–Maxwell simulations reveal that the two plasma waves generated by the backward and forward scattering are spatially separated, and thus form a two-stage electron ‘‘accelerator.’’
Measurable signatures of relativistic self-focusing in underdense plasmas
Authors: Gibbon P. , Monot P. , Auguste T. , Mainfray G.
Published: July 2002

The propagation of intense, picosecond laser pulses in tenuous plasmas is studied for conditions close to those required for relativistic self-focusing. Solutions of the steady-state wave-envelope equations are used to obtain experimentally measurable quantities such as the Thomson-scattered light at 90°, and the far-field radiation pattern. It is demonstrated that in the presence of electron cavitation, the Thomson signal has a longitudinal structure which is generally the converse of the intensity pattern. Furthermore, beam collimation in the far-field depends on where the laser is focused in the plasma, and may actually be degraded if self-channeling occurs.
An investigation into the role that a transverse magnetic field plays in the formation of large anode sheath potentials
Authors: Foster J. E. , Gallimore A. D.
Published: July 2002

A 9.25 A low-pressure (45–55 mTorr) hollow cathode arc discharge has been used to simulate plasma processes that occur at the anode of magnetoplasmadynamic accelerators used for space propulsion applications. The interest in the near-anode region is related to findings of past research, which indicate that large anode sheath potentials can drive as much as 70% of the input electrical power into the anode, thus degrading thrust efficiency. Presented here are results that essentially characterize the behavior of the near-anode plasma as a function of a transverse magnetic field. Plasma diagnostics included single Langmuir probe techniques, emission spectroscopy, and water calorimetry for anode heat flux measurements. Phenomenological arguments based on measurements taken suggest that observed changes in anode fall voltage are related to variations in the measured local electron number density as the magnetic field is varied. This behavior is attributed to the variations in the measured ionization rate, which is shown to be a nonlinear function of transverse magnetic field.
Vlasov gyrokinetic simulations of ion-temperature-gradient driven instabilities
Authors: Manfredi G. , Shoucri M. , Dendy R. O. , Ghizzo A. , Bertrand P.
Published: July 2002

An Eulerian code that solves the gyrokinetic Vlasov equation in slab geometry is presented. It takes into account the E×B and polarization drifts in the plane perpendicular to the magnetic field, and kinetic effects in the parallel direction. The finite Larmor radius is modelled by a convolution operator. The relation is established between this model and others proposed previously, and they are shown to be equivalent in the limit of long wavelengths and small Larmor radii. The code is applied to investigate ion-temperature-gradient modes in the quasi-neutral regime, with adiabatic electrons. Numerical results are reported for a wide range of parameters, including density and temperature profiles, magnetic field strength, and ion to electron temperature ratio. Normally the plasma evolves towards long wavelength structures, although in some cases (when Landau damping is very weak) more strongly turbulent regimes are observed. Test particles are used to compute diffusion coefficients both in real space and velocity space. For the most strongly turbulent regimes, particle diffusion coefficients are of order 20 m2 s−1. The saturation mechanism is also investigated. Many previous numerical results obtained with particle codes are confirmed, but the Vlasov Eulerian technique allows a much finer resolution of structures both in real space and velocity space.
A toroidal boundary-value problem in resistive magnetohydrodynamics
Authors: Bates Jason W. , Lewis H. Ralph
Published: July 2002

Under a standard set of assumptions, the magnetic flux function for a static, resistive magnetofluid is determined in axisymmetric toroidal geometry for two different scalar-conductivity profiles; in one case the conductivity is spatially uniform, and in the other it is proportional to the square of the distance from the toroidal axis. Exact analytic expressions are found in both cases for a magnetofluid that is surrounded by a perfectly conducting toroidal shell with a circular cross section.
Radially global gyrokinetic simulation studies of transport barriers
Authors: Parker S. E. , Mynick H. E. , Artun M. , Cummings J. C. , Decyk V. , Kepner J. V. , Lee W. W. , Tang W. M.
Published: July 2002

Improvements in tokamak transport have recently been obtained in a variety of operational modes through the formation of transport barriers, or good confinement radial zones. Here global nonlinear three-dimensional toroidal gyrokinetic simulation is used to study three effects that are linearly stabilizing and may cause the formation of transport barriers, namely, sheared toroidal rotation, reversed magnetic shear, and peaked density profiles. The effect of toroidal shear flow on ion heat diffusivity is found to be relatively weak compared to mixing-length expectations based on linear calculations. In contrast, it is found that weak or negative magnetic shear (s<1/2) in combination with a peaked density profile relative to the temperature profile greatly suppresses ion-temperature-gradient-driven turbulence in the central region of global nonlinear simulations. Similar features are seen experimentally in reversed magnetic shear tokamak plasmas. There is some nonlocal penetration (∼20–30ρi) of the turbulence into the subcritical core region.
Influence of resonant magnetic perturbations on plasma edge turbulence
Authors: Castro R. M. , Heller M. V. A. P. , Caldas I. L. , Brası́lio Z. A. , da Silva R. P. , Nascimento I. C.
Published: July 2002

This work reports alterations on plasma-edge equilibrium profiles and on edge turbulence, and anomalous transport induced by resonant perturbing magnetic fields in the TBR (Brazilian Tokamak) tokamak [J. Fusion Energy 12, 295 (1993)]. Thus, these perturbations have reduced the equilibrium parameters and the spectral power of the fluctuations and enhanced their phase velocity. They have also reduced the particle flux at the plasma edge. All these electrostatic edge parameters have been computed by taking into account temperature fluctuation corrections. Although the perturbation slightly affected just the linear correlation between electrostatic fluctuations, its bispectral analysis shows a reduction of the quadratic mode coupling. Furthermore, the energy transferred between different spectral components, with and without the magnetic perturbation, did not have the same direction for all fluctuations. Finally, the normal probability distribution functions of the fluctuations show significant non-Gaussian features, although the fluctuating potential distribution became near Gaussian with the magnetic perturbation.
Numerical study of the propagation of ionization processes in an oxygen Z-pinch plasma
Authors: Kim Seong Ho , Lee Ki-Tae , Kim Dong-Eon , Lee Tong Nyong
Published: July 2002

The dynamics of the ionization processes in an oxygen Z-pinch plasma has been numerically studied. Ionization balance equations incorporated in single-fluid magnetohydrodynamics (MHD) equations were solved. The focus was on the imploding phase before a shock wave was fully developed. It was shown that the propagation speed of an ionization stage is larger than the fluid speed. For example, the propagation speed of O III ionization stage is faster than the fluid speed at the position of the peak density of O III particles. The larger propagation speeds of ionization stages than the fluid speed result from the propagation of Joule heating power.
Expansion of the relativistic Fokker-Planck equation including non-linear terms and a non-Maxwellian background
Authors: Shkarofsky I. P.
Published: July 2002

The relativistic Fokker-Planck collision term in Braams and Karney [Phys. Fluids B 1, 1355 (1989)] is expanded using Cartesian tensors (equivalent to associated Legendre spherical harmonics) retaining all non-linear terms and an arbitrary zeroth order distribution background. Expressions are given for collision terms between all harmonics and the background distribution in terms of the j and y functions in Braams and Karney. The results reduce to Braams and Karney for the first order harmonic term with a Maxwellian background and to those given by Shkarofsky [Can. J. Phys. 41, 1753 (1963)] in the non-relativistic limit. Expressions for the energy and momentum transfer associated with relativistic Coulomb collisions are given. The fast two dimensional Fokker-Planck solver in Shoucri and Shkarofsky [Comput. Phys. Commun. 82, 287 (1994)] has been extended to include the second order harmonic term.
Radiative relativistic shock adiabate
Authors: Tsintsadze Levan N. , Nishikawa Kyoji
Published: July 2002

The influences of thermal radiation on the state equation of shock waves, derived in the previous paper [L. N. Tsintsadze, Phys. Plasmas 2, 4462 (1995)], are studied and a series of relations of thermodynamic quantities that hold for shock waves are derived. It is shown that the presence of radiation can strongly change the compressibility of the plasma. It is well known that for polytropic gases the compressibility cannot change more than four times the initial value in the case of nonrelativistic temperatures. The numerical calculations show that there are no such restrictions, when the radiation energy exceeds the kinetic energy of the plasma. The ultrarelativistic temperature range is also covered in our numerical calculations. Also studied are the influences of the radiation on the PT and the TV diagrams. A significant modification due to radiation is found in every case studied.
Hydrodynamic waves and correlation functions in dusty plasmas
Authors: Wang Xiaogang , Bhattacharjee A.
Published: July 2002

A hydrodynamic description of strongly coupled dusty plasmas is given when physical quantities vary slowly in space and time and the system can be assumed to be in local thermodynamic equilibrium. The linear waves in such a system are analyzed. In particular, a dispersion equation is derived for low-frequency dust acoustic waves, including collisional damping effects, and compared with experimental results. The linear response of the system is calculated from the fluctuation-dissipation theorem and the hydrodynamic equations. The requirement that these two calculations coincide constrains the particle correlation function for slowly varying perturbations. It is shown that in the presence of weakly damped, long-wavelength dust-acoustic waves, the dust autocorrelation function is of the Debye–Hückel form and the characteristic shielding distance is the dust Debye length.
Warm-fluid description of intense beam equilibrium and electrostatic stability properties
Authors: Lund Steven M. , Davidson Ronald C.
Published: July 2002

A nonrelativistic warm-fluid model is employed in the electrostatic approximation to investigate the equilibrium and stability properties of an unbunched, continuously focused intense ion beam. A closed macroscopic model is obtained by truncating the hierarchy of moment equations by the assumption of negligible heat flow. Equations describing self-consistent fluid equilibria are derived and elucidated with examples corresponding to thermal equilibrium, the Kapchinskij–Vladimirskij (KV) equilibrium, and the waterbag equilibrium. Linearized fluid equations are derived that describe the evolution of small-amplitude perturbations about an arbitrary equilibrium. Electrostatic stability properties are analyzed in detail for a cold beam with step-function density profile, and then for axisymmetric flute perturbations with ∂/∂θ=0 and ∂/∂z=0 about a warm-fluid KV beam equilibrium. The radial eigenfunction describing axisymmetric flute perturbations about the KV equilibrium is found to be identical to the eigenfunction derived in a full kinetic treatment. However, in contrast to the kinetic treatment, the warm-fluid model predicts stable oscillations. None of the instabilities that are present in a kinetic description are obtained in the fluid model. A careful comparison of the mode oscillation frequencies associated with the fluid and kinetic models is made in order to delineate which stability features of a KV beam are model-dependent and which may have general applicability.
Particle simulation on x-ray emissions from ultra-intense laser produced plasmas
Authors: Sentoku Y. , Mima K. , Taguchi T. , Miyamoto S. , Kishimoto Y.
Published: July 2002

The interaction of ultra-short intense laser with a thin foil has been studied by one- and two-dimensional particle simulations, which include the collisional absorption and the hard x-ray bremsstrahlung processes. The bremsstrahlung processes are modeled in a fully relativistic way in the frame work of quantum electrodynamics and integrated by the Monte Carlo method in particle-in-cell code. For carbon thin foils in which electron density is 200 times critical density, a laser pulse with an intensity greater than 1018 W/cm2 irradiates the foil normally to accelerate electrons on the surface along the laser propagation direction by the oscillating ponderomotive force. As a result, hard x rays over the range of 100 keV are emitted strongly toward the laser direction. The spectrum of the photon energy and scaling of the hard x-ray emission with respect to the various laser intensity are investigated by the newly developed simulation code.
Influence of magnetohydrodynamic Rayleigh–Taylor instability on radiation of imploded heavy ion plasmas
Authors: Benattar R. , Zakharov S. V. , Nikiforov A. F. , Novikov V. G. , Gasilov V. A. , Krukovskii A. Yu. , Zakharov V. S.
Published: July 2002

The multicharged plasma implosion stability with respect to Rayleigh–Taylor axial modes and its modification by the electromagnetic field diffusion and radiation cooling is considered. The exterior and the interior parts of an imploded plasma shell are examined and stability and conditions for magnetohydrodynamic Rayleigh–Taylor instability are obtained. The external surface is always unstable. The interior instability appears, as a rule, to be under a significant degree of compression near the final stage of implosion. Theoretical results and numerical simulations using the two-dimensional ZETA code are [R. Benattar et al., 4th International Conference on Dense Z pinches, Vancouver (American Institute of Physics, Woodbury, 1997), p. 211] compared. The modeling of the implosion of wire arrays and nested tungsten wire arrays on the Z generator by the two-dimensional magnetohydrodynamic code ZETA, including radiation transport with local thermodynamic equilibrium (LTE)—nonequilibrium (non-LTE) approximation, is performed in order to study the influence of instability level on high-Z plasma radiation and to reproduce the experimental results. It is shown that it is possible to fit the experimental results if a 10% level of initial mass perturbation of the tungsten wire arrays is imposed. The dynamics of the implosion and the development of Rayleigh–Taylor instability are discussed. The plasma of the Z pinch is shown to be in a non-LTE regime.
Magnetohydrodynamics problems in the interstellar medium
Authors: Zweibel Ellen G.
Published: July 2002

Many defining features of galaxies and their evolution are controlled by their interstellar gas. From 20% to 50% of the interstellar gas in the Milky Way is in molecular clouds, which are also the sites of star formation. Molecular clouds are turbulent, and magnetic fields are strong enough that the turbulence is believed to be magnetohydrodynamic in character. Analytical and numerical estimates suggest that the turbulence would be dissipated in less than the lifetime of the cloud if it were not driven. A small fraction of the mass in molecular clouds fails to be supported against self-gravity and collapses to form stars; star formation also is strongly influenced by magnetohydrodynamic effects.
Parallel current in anisotropic high-beta extremely low aspect ratio tokamak plasmas
Authors: Shaing K. C.
Published: July 2002

Parallel current in an anisotropic high-beta extremely low aspect ratio tokamak plasma is calculated from the linearized drift kinetic equation. It is found that it depends on the radial gradients of the parallel plasma pressure and the magnetic field strength. It can be expressed as the sum of the bootstrap current and the Pfirsch–Schlüter current.
Ion heating in the HELIX helicon plasma source
Authors: Kline J. L. , Scime E. E. , Keiter P. A. , Balkey M. M. , Boivin R. F.
Published: July 2002

Efficient ion heating in a steady-state helicon plasma source is observed with two external loop antennae just above the ion cyclotron frequency. The ion velocity space distribution is measured by laser induced fluorescence in an argon plasma. The measured bulk ion heating is highly anisotropic (the perpendicular temperature increase is ten times the parallel temperature increase) even though the plasma is moderately collisional. Measurements of the perturbed distribution function with laser induced fluorescence suggest that an electrostatic ion cyclotron wave is launched.
Excitation and damping of a self-modulated laser wakefield
Authors: Chen S.-Y. , Krishnan M. , Maksimchuk A. , Umstadter D.
Published: July 2002

Spatially, temporally, and angularly resolved collinear collective Thomson scattering was used to diagnose the excitation and damping of a relativistic-phase-velocity self-modulated laser wakefield. The excitation of the electron plasma wave was observed to be driven by Raman-type instabilities. The damping is believed to originate from both electron beam loading and modulational instability. The collective Thomson scattering of a probe pulse from the ion acoustic waves, resulting from modulational instability, allows us to measure the temporal evolution of the plasma temperature. The latter was found to be consistent with the damping of the electron plasma wave.
Effect of dust charge fluctuations on Kelvin–Helmholtz instability in a cold dust plasma
Authors: Li Lu , Zhong-Yuan Li , Zhen-Xing Liu
Published: July 2002

Effect of dust charge fluctuations on Kelvin–Helmholtz (K–H) instability driven by sheared dust flow with both perpendicular and parallel components to magnetic field is investigated in magnetized three-component cold dust plasma. It is found that K–H instabilities are mainly driven by the parallel component of the sheared dust flow. The perpendicular component of the sheared dust flow influences the extent of the oblique propagation of the unstable perturbed wave with respect to the ambient magnetic field. For a dense dust plasma, as the perturbed frequency is much higher than the dust charging frequency, the maximum damping rate of the perturbed wave due to dust charge fluctuations will reach the order of the dust charging frequency. In this case, the critical shear for the parallel K–H instability increases rapidly because of the charging processes in the long-wavelength perturbation.
GaAs bipolar transistors grown on (100) Si substrates by molecular beam epitaxy
Authors: Fischer R. , Chand N. , Kopp W. , Morkoç H. , Erickson L. P. , Youngman R.
Published: July 2002

We have investigated the properties of GaAs homojunction bipolar transistors grown on (100) oriented Si substrates by molecular beam epitaxy. In a structure with a base thickness of 0.2 μm, a small-signal common emitter current gain β of about 10 at a current density of 10 kA/cm2 has been obtained. Current densities as high as 105 000 A/cm2 were obtained in these devices without degradation demonstrating the excellent stability of this material. Since the minority-carrier lifetime is quite sensitive to defects in the base region, these measurements demonstrate the high quality of GaAs on Si substrates. From the collector current dependence of current gain, an ideality factor n=1.5 for the emitter junction was obtained, indicating that space-charge recombination is an important mechanism in these devices. We also demonstrate that the entire GaAs on Si wafer is phase ordered by observing the orientation effect in the mesa etching. These results show that excellent minority-carrier properties can be obtained in GaAs on Si epitaxial layers despite the somewhat large dislocation density due to lattice mismatch.
Laser-stimulable transparent CsI:Na film for a high quality x-ray imaging sensor
Authors: Kano Tsuyoshi , Takahashi Tetsuhiko , Okajima Kenichi , Umetani Keiji , Ataka Saburo , Yokouchi Hisatake , Suzuki Ryuichi
Published: July 2002

Transparent films, which can be stimulated by laser beams after x-ray irradiation, have been searched to improve the spatial resolution of digital x-ray imaging sensors. As a result, evaporated CsI:Na films are found to be efficiently laser stimulable around 77 K. The modulation transfer function (MTF) of the film is evaluated using a scanning Ga1−xAlxAs semiconductor laser. The high MTF value (57% at 2 1p/mm), strong x-ray absorption, and high stimulation efficiency of the film ensure a high quality sensor for digital radiography.
Quantum transport calculation of the small-signal response of a resonant tunneling diode
Authors: Frensley William R.
Published: July 2002

The linear and lowest order nonlinear response of a quantum well resonant tunneling diode is evaluated using quantum transport theory. The calculations show that the negative conductance persists up to about 5 THz, although parasitic circuit elements will limit the maximum oscillation frequency to a much lower value. The nonlinear response (rectification) remains significant to frequencies near 10 THz and shows a resonant peak near 4 THz. These calculations support the interpretation of the experimental data of T. C. L. G. Sollner, W. D. Goodhue, P. E. Tannenwald, C. D. Parker, and D. D. Peck [Appl. Phys. Lett. 43, 588 (1983)] that rectification at 2.5 THz was observed in their devices.
Nanometer scale structure fabrication with the scanning tunneling microscope
Authors: Staufer U. , Wiesendanger R. , Eng L. , Rosenthaler L. , Hidber H. R. , Güntherodt H.-J. , Garcia N.
Published: July 2002

Nanometer scale structures have been produced on atomically flat surfaces of metallic glasses using the scanning tunneling microscope in the tunneling mode with enhanced local current densities and strong electric fields. Depending on the current and the electric field enhanced diffusion, local crystallization of the glassy state or Taylor cone formation of the locally molten surface can occur. These structures provide a potential means of studying the behavior of matter at nanometer dimensions.
Crystal growth and characterization of 4-(N,N-dimethylamino)-3-acetamidonitrobenzene, a new organic material for nonlinear optics
Authors: Baumert J.-C. , Twieg R. J. , Bjorklund G. C. , Logan J. A. , Dirk C. W.
Published: July 2002

The linear and nonlinear optical properties have been determined for a new organic nonlinear material 4-(N,N-dimethylamino)-3-acetamidonitrobenzene. Room-temperature angularly tuned type I phase matching for doubling of 1.064 μm radiation was achieved with a deff =27 pm/V. Favorable crystal growth habits were observed.
Effect of sodium sulfide treatment on band bending in GaAs
Authors: Besser R. S. , Helms C. R.
Published: July 2002

Recent evidence suggests that sodium sulfide treatment of GaAs surfaces results in the reduction of surface recombination and in Fermi level unpinning. We have used a surface conductivity technique to measure the Fermi level position of samples with and without sodium sulfide treatment. This method has specific advantages over spectroscopic techniques for studying thin films on GaAs. Photoluminescence (PL) measurements were used to qualitatively evaluate surface recombination. We conclude that while the reduction of surface recombination is attained, the surface is not unpinned.
Electro-optic phase modulation and optical second-harmonic generation in corona-poled polymer films
Authors: Singer K. D. , Kuzyk M. G. , Holland W. R. , Sohn J. E. , Lalama S. J. , Comizzoli R. B. , Katz H. E. , Schilling M. L.
Published: July 2002

Electro-optic phase modulation was measured along with optical second-harmonic generation in thin films of a new copolymer containing a dicyanovinyl-terminated azo dye side chain. Orientational order was imparted to these films by poling with a corona discharge. Details of the electro-optic measurement technique, in which the real part of the electro-optic coefficient can be determined directly, are presented. Taking advantage of the increased orientation imparted by corona poling and the hindered motion of the nonlinear optical moiety in the side chain of the polymer leads to substantial improvements in both the magnitude and stability of nonlinear optical susceptibilities compared to guest-host polymer systems ordered by electrode poling.
High quality ultrathin InAs/GaAs quantum wells grown by standard and low-temperature modulated-fluxes molecular beam epitaxy
Authors: Gerard J. M. , Marzin J. Y.
Published: July 2002

We report on the optical study of ultrathin (1–3 monolayers) InAs quantum wells in GaAs. The samples have been grown either by standard molecular beam epitaxy (MBE) or low-temperature (350 °C) modulated-fluxes MBE. The latter technique enlarges the range of pseudomorphic deposition of strained InAs layers on GaAs. Both types of samples display sharp (down to 6 meV spectral width) and intense low-temperature excitonic photoluminescence, and a high in-plane homogeneity. Higher energy optical transitions have been observed in both absorption and photoluminescence excitation experiments, performed on multiple quantum well structures.
Direct gating by retinoic acid of retinal electrical synapses
Authors: Zhang Dao-Qi , McMahon Douglas G.
Published: July 2002

Retinoic acid (RA), a signaling molecule derived from vitamin A,
controls growth and differentiation of a variety of cell types through
regulation of gene transcription. In the vertebrate retina, RA also
regulates gap junction-mediated physiological coupling of retinal
neurons through a nontranscriptional mechanism. Here we report that RA
rapidly and specifically modulates synaptic transmission at electrical
synapses of cultured retinal horizontal cells through an external
RAR
β
/γ
-like binding site, the action of
which is independent of second messenger cascades. External application
of all-trans retinoic acid (at-RA) reversibly reduced the amplitude of
gap junctional conductance in a dose-dependent manner, but failed to
affect non-gap-junctional channels, including glutamate receptors. In
contrast, internal dialysis with at-RA was ineffective, indicating an
external site of action. Selective RAR
β
/γ
ligands, but not an RAR
α
-selective agonist, mimicked the
action of at-RA, suggesting that gating of gap junctional channels is
mediated through an RAR
β
/γ
-like binding
site. At-RA did not act on gap junctional conductance by lowering
[pH]
i
or by increasing [Ca
2+
]
i
.
A G protein inhibitor and protein kinase inhibitors did not block at-RA
uncoupling effects indicating no second messenger systems were
involved. Direct action of at-RA on gap junction channels was further
supported by its equivalent action on whole-cell hemi-gap-junctional
currents and on cell-free excised patch hemichannel currents. At-RA
significantly reduced single-channel open probability but did not
change unitary conductance. Overall, the results indicate that RA
modulates horizontal cell electrical synapses by activation of novel
nonnuclear RAR
β
/γ
-like sites either
directly on, or intimately associated with, gap junction channels.