bluuebunny/crossref_metadata_2025 huggingface dataset:
Traveling wave instability in helical coil flow
Authors: Webster D. R. , Humphrey J. A. C.
Published: July 2002

Complementary flow visualization photographs and numerical calculations are presented for the transitional state between the laminar and turbulent flow regimes in a helically coiled pipe. The flow visualization covers a Reynolds number range from 3800 to 8650 (890<De<2030, where De is the Dean number). Estimates of the wavelength and wave speed of a traveling wave instability are made from photographs and video recordings at Re=5060 and 5480 (De=1190 and 1280). The unsteady three-dimensional finite difference approximations of the Navier–Stokes equations formulated for the toroidal coordinate system are solved numerically. The calculations are performed in a curved pipe with a radius of curvature to pipe radius ratio equal to 18.2 and Re=5480 (De=1280). These test conditions match the flow visualization and previously reported laser Doppler velocimetry measurements. The calculations reveal a complex interaction between the centrifugal force and the cross-stream velocity, hence explaining the mechanism for maintaining the traveling wave. An analogy is made with known centrifugal instabilities to explain the character of the motion observed in the inner half of the pipe along planes defined by the radial and streamwise coordinate directions. Simple considerations show that the cross-stream flow has the potential for a centrifugal instability.
Nonpremixed spontaneous ignition in the laminar wake of a splitter plate
Authors: Sánchez Antonio L.
Published: July 2002

Ignition in the laminar wake that forms at the trailing edge of a thin splitter plate separating two parallel streams of fuel and oxidizer is studied in the limit of large activation energy. The analysis presented covers ignition events ocurring in the Rott–Hakkinen and Goldstein regions, where self-similar solutions for the different frozen flow variables are available. Because of the strong exponential sensitivity of the reaction rate on the temperature, ignition becomes a self-accelerating phenomenon on the temperature increment that leads to a thermal runaway a finite distance downstream from the splitter plate. The dependence of the ignition distance on the values of the velocity and temperature gradients that exist at the trailing edge of the splitter plate is investigated. In particular, it is seen that, for sufficiently large values of the activation energy, the small temperature variations that exist across the wake must be taken into account in calculating the ignition distance. As the transverse gradient of temperature at the trailing edge increases, the reaction zone becomes thinner and migrates towards the hotter side of the mixing layer, where convection becomes the dominant transport mechanism, causing the thermal runaway distance to be determined in the first approximation by a convective-reactive balance across the thin reaction layer. Ignition in the wake when one of the streams is initially stagnant is also addressed. The character of the resulting solution is seen to depend strongly on the Lewis number of the reactant supplied by the colder stream, yielding three distinct ignition regimes that are analyzed separately.
The onset of Rayleigh–Bénard instability in molecular radiating gases
Authors: Bdéoui F. , Soufiani A.
Published: July 2002

Radiation effect on the onset of Rayleigh–Bénard instability is studied theoretically in the case of real molecular emitting and absorbing gases. Radiative transfer is treated using the full integro-differential formulation without recourse to the differential approximation or to the asymptotic optically thin or optically thick limiting cases. The marginal stability is determined using a combined Galerkin-collocation method based on Chebyshev polynomials. A modified correlated-k (CK) model is developed for NH3, H2O and CO2 in order to take into account the fine spectral structure of the absorption coefficient and of its derivative with respect to temperature. No restrictions are made concerning the nature of the bounding surfaces and their radiative properties. The study of gray media allows us to discuss first the limitations of the differential approximation and the validity range of the optically thin and thick limits. The principle of exchange of stability is found to hold for all the studied cases. For real radiating gases, a reasonable agreement with the few available experimental data which are related to NH3 is first shown and then a parametric study for H2O and CO2 is reported. It is shown that neither radiation effect on the static temperature profile nor the radiative damping of thermal disturbances can be neglected. Radiation effects on the delay of instability onset increase with temperature, layer depth, and decrease with boundary emissivities. Pressure effects depend on the level of saturation of gas active absorption bands.
Monte Carlo simulation and Navier–Stokes finite difference calculation of unsteady-state rarefied gas flows
Authors: Stefanov Stefan , Gospodinov Peter , Cercignani Carlo
Published: July 2002

A comparative analysis of two approaches (molecular and continuum) for three one-dimensional unsteady-state rarefied gas flows with small Knudsen number is presented. Numerical results have been obtained by using Direct Simulation Monte Carlo (DSMC) method for the molecular and a finite difference method for the continuum approaches, respectively.
Soret coefficient of nanoparticles in ferrofluids in the presence of a magnetic field
Authors: Blums Elmars , Odenbach Stefan , Mezulis Ansis , Maiorov Michail
Published: July 2002

Experiments on a nonstationary separation of nanometer-sized Mn0.5Zn0.5Fe2O particles of hydrocarbon-based ferrocolloids in a flat vertical thermal diffusion column are performed. By using a modified separation theory which accounts for a one-dimensional mixed (thermal and concentration) convection in the column, the Soret coefficient of lyophilized nanoparticles from the separation curves are calculated. It is shown that in a zero magnetic field particles are transferring toward decreasing temperatures. The thermal diffusion ratio αT reaches a value αT≈+20. A significant influence of a uniform magnetic field B on particle separation is observed. If B is oriented along the temperature gradient ∇T, a strong decrease in thermal diffusion coefficient takes place whereas the transversal field B⊥∇T causes an intensification of particle thermophoretic transfer. Both effects qualitatively well agree with theoretical predictions based on a hydrodynamic theory of particle thermomagnetophoretic motion.
Numerical investigation of thermocapillary flow around a bubble
Authors: Arlabosse P. , Lock N. , Medale M. , Jaeger M.
Published: July 2002

The thermocapillary flow induced by a gas bubble in a Newtonian liquid layer subjected to a stable temperature stratification is investigated. This flow is analyzed for a special configuration when the surface tension and buoyant forces oppose one another. The driving mechanism is the surface tension gradient related to the Marangoni number whereas the stabilizing effects are the viscous and buoyant forces related to the Prandtl and the Rayleigh numbers. In a previous work, this flow has been investigated experimentally for a few combinations of these three parameters. In order to make a more systematic study of the influence of these parameters, numerical simulations are used as a decisive tool. Indeed, it allows the contribution of the different mechanisms to be evaluated. To validate the finite element model, developed for this purpose, the numerical results are first compared to experimental ones. Then, the influence of these three dimensionless parameters on the flow pattern and the magnitude of the flow is analyzed. This sensitivity study is supplemented by a convergence study. It appears that the Rayleigh number modifies the flow pattern but has little influence on the strength of the primary vortex induced by the bubble. On the other hand, the Marangoni and Prandtl numbers induce little change in the flow pattern but they respectively enhance and reduce the strength of the primary vortex. This systematic analysis leads us to propose an empirical relationship for the strength of the flow.
Spanwise structure of wall pressure on a cylinder in axial flow
Authors: Bokde Arun L. W. , Lueptow Richard M. , Abraham Bruce
Published: July 2002

The spanwise structure of wall pressure fluctuations was measured in an axisymmetric turbulent boundary layer on a cylinder parallel to the mean flow at a momentum thickness Reynolds number of 2530 and a boundary layer thickness to cylinder radius ratio of 4.81. The measurements were made using miniature hearing aid type condenser microphones with spanwise separations of 0°, 10°, 20°, 30°, 60°, and 90°. An improved wall pressure power spectrum was obtained at low frequencies by utilizing a two-point subtraction method to remove low frequency acoustic background noise of the wind tunnel. The spanwise correlations indicate that the spanwise coherent length of the wall pressure is 30° (78ν/uτ or 0.11δ). The spanwise coherence is weak and concentrated in a frequency band that is substantially lower than the most energetic frequency band of the wall pressure spectrum. A mode number–frequency decomposition of the wall pressure spectrum indicates that the greatest quantity of energy is in the circumferential modes nearest zero. Modes −4 to 4 contain most of the wall pressure energy. Conditional sampling by pressure peak and VITA detection schemes (where VITA was applied to wall pressure to detect strong pressure gradient events) indicate that the spanwise extent of the high pressure peaks and high wall pressure gradients is 60° (156ν/uτ or 0.22δ).
A weakly nonlinear analysis of the dynamics of a viscous flow in a symmetric channel with a sudden expansion
Authors: Rusak Zvi , Hawa Takumi
Published: July 2002

A weakly nonlinear analysis of the dynamics of a two-dimensional, laminar flow in a long channel with a sudden expansion and the transition from symmetric to asymmetric states is presented. The asymptotic analysis is based on a study of the unsteady Navier–Stokes equations around the critical Reynolds number, Rec, where a bifurcation occurs. It explores the special nonlinear interactions between the unsteady, convective, and viscous effects. The analysis results in an ordinary, nonlinear, first-order differential equation (similar to the Landau equation) which describes the evolution of the perturbation’s amplitude as function of Re near Rec. The analytical solution shows that when Re<Rec the symmetric state is stable. However, when Re⩾Rec the symmetric state in the channel loses its stability and evolves into an asymmetric state. The flow evolution, as described by the nonlinear model, shows agreement with time history plots from simulations using the unsteady Navier–Stokes equations. The linear stability characteristics of both the symmetric and asymmetric states are also found from the nonlinear approach and match with the previous results. The analysis provides new insight into the previous experimental and numerical results and sheds light on the nonlinear transition of a viscous flow in an expanding channel.
Thermocapillary migration of long bubbles in cylindrical capillary tubes
Authors: Mazouchi Ali , Homsy G. M.
Published: July 2002

We consider the problem of the migration of a long bubble in a tube with a prescribed axial temperature gradient. The resulting thermocapillary stress moves the bubble toward hotter regions and we are interested in determining the speed of the bubble. Assuming small Peclet, Reynolds, Bond, and capillary numbers, Ca allows the uncoupling of the temperature field from the flow field, the use of creeping flow and lubrication theory, the assumption of axisymmetry, and the use of matched expansions in Ca, respectively. The structure of the solution is that of a constant thickness film bounded by constant curvature cap regions, with transition layers in between. A modified Landau–Levich equation governing the film profile in the transition regions is solved numerically, establishing the relationship between the unknown film thickness and the unknown bubble speed. A global mass conservation relation is then used to complete the solution and relate the bubble speed to the thermophysical properties. The solution is a function of a single dimensionless parameter, Δσ*=γTβa/σ where β is the temperature gradient, a the tube radius, σ the mean surface tension, and γT the temperature coefficient of surface tension. It is found that the speed increases with the temperature gradient as expected, and has a nonlinear dependence on Δσ* that is ultimately connected with the nonlinearity of the fluid dynamics in the transition regions.
Mixing of viscous polymer liquids
Authors: Stokes Jason R. , Boger David V.
Published: July 2002

A torsionally driven cavity is proposed as a suitable mixing device for viscoelastic fluids. “Chaotic” behavior occurs in the secondary flow plane using polyacrylamide Boger fluids due to the inducement of elastic flow instabilities in situations where inertial forces are small. The torsionally driven cavity is also a suitable well-defined geometry for testing the ability of non-Newtonian constitutive models to describe the behavior of elastic fluids in three-dimensional flow as a prelude to solving more difficult mixing problems.
Electrohydrodynamic effects on the deformation and orientation of a liquid capsule in a linear flow
Authors: Ha Jong-Wook , Yang Seung-Man
Published: July 2002

The role of a uniform electric field on the deformation and orientation of a liquid capsule with a viscoelastic membrane is considered analytically in the small deformation limit. The capsule is freely suspended either in a quiescent fluid or in a shear flow. The viscoelasticity of the membrane is taken into account by the Kelvin–Voigt model and the electrohydrodynamic flow is analyzed on the basis of the leaky dielectric model. In this article, we consider three different prototype models of capsules; viz., a neo-Hookean (incompressible isotropic) membrane, a red blood cell-type (area-preserving) membrane, and an interfacial-tension droplet. The deformed capsule shape from its initial sphericity and its orientaion are determined from the linearized governing equations and boundary conditions in the limit of small deformations. The asymptotic theory shows that the degree of capsule deformation induced by a uniform electric field alone is independent of the surface viscosity of the capsule as well as the viscosity ratio between the two fluids inside and outside the capsule. Meanwhile, in the presence of an imposed shear flow, the degree of deformation depends on the surface viscosity with preserving still the independence of the viscosity ratio. For an illustrative purpose, experimental results for the role of a uniform electric field on the orientation of an interfacial-tension droplet in a shear flow are discussed briefly.
Demonstration of a one-way flow of a rarefied gas induced through a pipe without average pressure and temperature gradients
Authors: Sone Yoshio , Sato Ko
Published: July 2002

A simple experiment is performed to demonstrate that a one-way flow of a rarefied gas can be induced through a pipe without average pressure and temperature gradients by devising the shape of the pipe. A one-way flow is found to be induced through a pipe consisting of two circular pipes of different diameters connected and heated at their joint in the middle part of the pipe. The flow is from the thinner pipe to the thicker. In a pipe of a uniform diameter, no one-way flow is induced even when it is heated at a position away from the middle of the pipe.
Electron cyclotron absorption and emission by superthermal electrons in tokamak plasmas
Authors: Vézard D.
Published: July 2002

Electron cyclotron wave absorption and emission by mildly relativistic electrons is investigated. The linear theory is developed for a small fraction of fast electrons in order to determine the dynamics of such particles by using transmission measurements in tokamaks. Simple expressions for the dielectric tensor in the presence of a superthermal electron parallel-momentum distribution are obtained by taking the perpendicular energy of this population into account. It is shown that electron cyclotron wave absorption is weakly modified by the shape of the perpendicular-momentum distribution. On the other hand, wave attenuation at the fundamental resonance is significantly affected by the average perpendicular energy for low values of the parallel wave refractive index. A new diagnostic method is proposed to measure the perpendicular electron temperature and the parallel-momentum distribution simultaneously, by using the ordinary and extraordinary modes of polarization.
Nonlinear entropy production operators for magnetohydrodynamic plasmas
Authors: Siregar E. , Ghosh S. , Goldstein M. L.
Published: July 2002

A method for constructing closure relations based on the invariants of the tensors representing nonequilibrium thermodynamic forcing within the plasma is presented. This approach leads to closure relations that describe all higher-order forcing effects contained within the continuum description. Nonlinear convective-momentum transport and nonlinear momentum-exchange operators are constructed as applications of the method. Closure is achieved by relating the pressure tensor to invariants of the rate of strain tensor, and the momentum-exchange operator to invariants of the gradient of magnetic field tensor. These operators lead to positive definite viscous and Joule entropy production and enhance high wave number dissipative couplings over all other dissipative couplings. The nonlinear dissipative action is localized in physical space, where velocity and magnetic gradients are large, while allowing nearly ideal behavior elsewhere. The operators are computationally tested against the standard magnetohydrodynamic (MHD) operators using three-dimensional configurations that lead to vortex street formation and magnetic reconnection. The nonlinear operators allow greater spatial structure and have flatter modal energy spectra than the standard MHD dissipation operators. Closures that describe the plasma response to nonequilibrium thermodynamic forcing of all orders can be constructed using this approach.
Comment on ‘‘Alfvén ‘resonance’ reconsidered: Exact equations for wave propagation across a cold inhomogeneous plasma’’ [Phys. Plasmas 1, 3523 (1994)]
Authors: Goedbloed J. P. , Lifschitz A.
Published: July 2002

In a recent paper Bellan [P. M. Bellan, Phys. Plasmas 1, 3523 (1994)] criticizes magnetohydrodynamics for not being an accurate model of plasma behavior at low frequencies and leading to the incorrect prediction of singularities associated with the Alfvén wave resonances. Grave errors of the paper having to do with misunderstanding of what is a physical model and with misrepresentation of the original contributions to the field are exposed.
Collisionless m=1 reconnection in tokamaks
Authors: Rogers B. , Zakharov L.
Published: July 2002

In high temperature tokamak regimes, the m=1 reconnecting mode is governed by two fluid effects associated with the electron skin-depth de=c/ωpe and the ion sound radius ρs=cs/Ωci. To clarify the early nonlinear dynamics of this mode, numerical simulations of a two fluid model are presented. To simulate physically reasonable parameters in a tokamak geometry, an efficient approach is used in which only a narrow layer surrounding the q=1 surface and m=1 magnetic island must be simulated. In the usual tokamak case ρs≳de, as well as the limit ρs=0, the early nonlinear growth of the mode is faster than exponential due a broadening of the flow layer. This broadening appears to result from tension in the magnetic field lines, which acts to pull the layer open as the island grows.
Theory of pulse–particle interaction in one dimension
Authors: Akimoto K.
Published: July 2002

Interaction of charged particles with various types of one-dimensional pulses is investigated systematically. Gaussian pulses of arbitrary velocity and polarization, both electrostatic and electromagnetic, are considered. The pulses range from ordinary wave packets to impulses. Analytic expressions are obtained for velocities of charged particles interacting with those traveling pulses, and are compared with numerical counterparts. Dependence of the interaction on pulse size, initial particle velocity, and external magnetic field is investigated. For many nonrelativistic pulses traveling along an external magnetic field, the interaction can be divided into two types, i.e., transit-time acceleration and reflection in the wave frame. In the absence of the magnetic field, transit-time acceleration is significant only for impulses with sizes of less than one wavelength. On the other hand, with a magnetic field (transit-time) cyclotron acceleration becomes possible for pulses of sizes greater than one wavelength. With larger amplitudes the wave-frame reflection becomes increasingly important, typically occurring at the pulse head or tail. However, it tends to be suppressed by external magnetic fields. Compact expressions are obtained for final velocities of the particles for the two types of interaction. Both types of acceleration mechanism may be important in dissipating relatively large-amplitude waves.
Modified self-consistent nonlinear simulation of cyclotron autoresonance maser amplifiers
Authors: Dawn T. Y. , Chien H. C. , Chen J. M. , Chen S. H.
Published: July 2002

Since the electron cyclotron auto-resonance maser (CARM) is operated with the wave refractive index being very close to unity, a little variation of the refractive index in the CARM calculations might be crucial. Therefore, a modified self-consistent one-dimensional nonlinear model was developed by exactly setting the evolving wave refractive index and keeping all second-order terms to examine the inadequacy induced by the simplification of the earlier one-dimensional models. Comparisons of the numerical results obtained by the earlier models and the modified model for a CARM amplifier reveal that the inadequate relaxation of the limited azimuthal and axial bunching region for electrons to lose energy in the earlier models was the main reason for the overestimate of the linear gain and the saturated efficiency for the fast (the slow) wave branch with initial refractive index n0<1 (n0>1). However, the earlier model with fixed n0=1 is an excellent approximation whatever linear or saturated state is considered. The efficiency dependence of a CARM amplifier on some basic operating parameters was reexamined by the modified formulations.
Kelvin–Helmholtz instability of magnetized plasma with polytropic pressure laws
Authors: Sharma P. K. , Chhajlani R. K.
Published: July 2002

The Kelvin–Helmholtz (K-H) instability of two fluids of a plasma, streaming in opposite directions with the same velocity and in the presence of an external magnetic field, is investigated. The external magnetic field in both fluids are in different directions. The usual magnetohydrodynamic (MHD) equations with anisotropic pressures are considered. The generalized pressure relation is used and two equations of state for two pressures are taken up in the problem. The equations are linearized and initially two different flow velocities are taken for the system. The problem is solved and a dispersion relation is obtained. It is found that the instability condition for the static configuration depends on the polytropic index of the pressure relations. The condition of instability is further obtained for MHD and Chew–Goldberger–Low (CGL) systems. For the nonstatic streaming configuration it is also found that growth rate of K-H instability depends on various polytropic indices and magnetic field.
Nonlinear dynamics of magnetic buoyancy instabilities in a sheared magnetic field
Authors: Kusano K. , Moriyama K. , Miyoshi T.
Published: July 2002

Nonlinear dynamics of magnetic buoyancy instabilities for the Parker-type and interchange-type modes are numerically investigated with the aim of understanding the emerging mechanism of solar coronal magnetic loops. Two-dimensional nonlinear magnetohydrodynamic (MHD) simulations of the magnetic buoyancy instabilities are performed in a highly stratified domain consisting of two temperature regions corresponding to the corona and the chromosphere, respectively. First, it is revealed that the angle between the wave vector and the magnetic field at the top interface of the magnetic layer is a crucial parameter controlling the basic properties of the instability. Second, it is found that the magnetic buoyancy instability in the sheared field grows nonlinearly larger than in the shearless field, although the magnetic shear reduces the linear growth rate. This is due to the Parker-type mode being destabilized by the nonlinear effect of the interchange-type instabilities. Third, it is shown that the magnetic buoyancy instability can drive magnetic reconnections which create isolated flux regions.
Wave excitation in a modified double plasma device
Authors: Honzawa Tadao
Published: July 2002

A modified double plasma (MDP) device is newly developed and shown to be useful for basic plasma wave experiments. Further, the mechanism of wave excitation in the MDP device is experimentally clarified. From the results the peculiarity of the device is found, that ion-acoustic waves can be excited in plasmas essentially freely from the effects of stationary beam ions and other modes, such as ion-beam modes as far as linear waves are concerned. For nonlinear waves, however, ion-acoustic solitons or shocks with reflected ions ahead and slower wave-like signals behind are possibly excited just as in a conventional DP device.
Ion oscillation modes in inhomogeneous plasmas
Authors: Duan Wen-shan , Lu Ke-pu
Published: July 2002

Using the reductive perturbation method, the carrier wave modulation of ion acoustics waves in inhomogeneous plasma is investigated. It is shown that such a process can be described by a Nonlinear Schrödinger-type equation (NLS-type) equation.
Absolute instability in a traveling wave tube model
Authors: Ang L. K. , Lau Y. Y.
Published: July 2002

A model is constructed to evaluate absolute instability which may lead to bandedge oscillations in a traveling wave tube. Under the assumptions (a) that all modes have forward group velocities, and (b) that the slow wave structure has a parabolic dispersion relation in the ω-k plane, the threshold coupling constant (Pierce’s parameter C) is calculated for the onset of absolute instability. The effect of distributed resistive loss in the circuit is included. The axial wave number and the characteristic frequency of the oscillation at the onset are given.
Wavelength limits on isobaricity of perturbations in a thermally unstable radiatively cooling medium
Authors: Kovalenko I. G. , Shchekinov Yu. A.
Published: July 2002

Nonlinear evolution of one-dimensional planar perturbations in an optically thin, radiatively cooling medium in the long-wavelength limit is studied numerically. The accepted cooling function generates, in thermal equilibrium, a bistable equation of state P(ρ). The unperturbed state is taken close to the upper (low-density) unstable state with infinite compressibility (dP/dρ=0). The evolution is shown to proceed in three different stages. At the first stage, pressure and density set in the equilibrium equation of state, and velocity profile steepens gradually, as in the case of pressure-free flows. At the second stage, those regions of the flow where anomalous pressure (i.e., with negative compressibility) holds create a velocity profile sharper than in the pressure-free case, which in turn results in formation of a very narrow (short-wavelength) region where gas separates the equilibrium equation of state and pressure equilibrium sets in rapidly. At this stage, the variation in pressure between the narrow dense region and the extended environment does not exceed more than 0.01 of the unperturbed value. At the third stage, gas in the short-wavelength region reaches the second (high-density) stable state, and pressure balance establishes through the flow, with pressure equal to the one in the unperturbed state. In external (long-wavelength) regions, gas forms slow isobaric inflow toward the short-wavelength layer. The duration of these stages decreases when the ratio of the acoustic time to the radiative cooling time increases. The limits in which nonlinear evolution of thermally unstable long-wavelength perturbations develops in isobaric regime are obtained.
Eikonal elastic scattering cross section for electron–dust grain collisions in dusty plasmas
Authors: Jung Young-Dae , Tawara Hiro
Published: July 2002

Plasma screening effects on electron–dust grain elastic collisions in dusty plasmas are investigated using the eikonal method. Attractive interaction between the projectile electron and its own image charge inside the dust grain is included to obtain the total interaction potential between the projectile electron and the dust grain. The semiclassical straight-line trajectory method is applied to the path of the projectile electron in order to investigate the variation of the eikonal phase as a function of the impact parameter, dust charge, and Debye length. The eikonal elastic cross section substantially decreases with a decrease of the Debye length. It is also found that the position of the maximum cross section is shifted to a higher energy region with increasing the dust charge.
One-dimensional hybrid simulations of the imploding large Larmor radius Z pinch
Authors: Channon S. W. , Coppins M. , Arber T. D.
Published: July 2002

The interaction of an imploding plasma–vacuum boundary (PVB) with a collisionless Z pinch for the case where the ions have orbits comparable to the radius is studied. Two principal results are found. First, the development of sheared axial ion flow occurs as a result of large Larmor radius (LLR) ions interacting with the moving PVB. This contrasts strongly with the equivalent case in ideal magnetohydrodynamics (MHD). The second result is a modification to current density profiles. These are smoother (due to LLR effects) than those from ideal MHD.
Excitation of current-driven electrostatic ion-cyclotron waves in presence of a transverse direct current electric fields in a magnetized plasma
Authors: Sharma Suresh C. , Sugawa M. , Jain V. K.
Published: July 2002

The temporal evolution of the current-driven electrostatic ion–cyclotron (CDEIC) instability is investigated in the presence of a transverse dc electric field in a collisional magnetized plasma. It was found that the inclusion of a transverse dc electric field in addition to the magnetic field changes the dispersion characteristics of the ion cyclotron waves. The growth rate of the instability increases with the mode frequency and has the largest value at the mode frequencies 43 kHz (for ω≫kzvte) and 7.53 kHz (for kzvti≪ω≪kzvte) when the electron drift velocity is less than the critical value for the CDEIC instability. The growth rate also increases with the guide magnetic field and has the largest value at the magnetic fields 1.0 kG (kyρi∼0.35) for ω≫kzvte and 0.1 kG (kyρi∼0.2) for kzvti≪ω≪kzvte. The growth rate is a sensitive function of electron collision frequency. The results of the theory are applied to explain some of the experimental observations of Koepke et al. [IEEE Trans. Plasma Sci. 20, 631 (1992)].
On the continua in two-dimensional nonadiabatic magnetohydrodynamic spectra
Authors: De Ploey A. , Van der Linden R. A. M. , Beliën A. J. C.
Published: July 2002

The equations for the continuous subspectra of the linear magnetohydrodynamic (MHD) normal modes spectrum of two-dimensional (2D) plasmas are derived in general curvilinear coordinates, taking nonadiabatic effects in the energy equation into account. Previously published derivations of continuous spectra in the 2D ideal MHD case and in 1D nonadiabatic MHD are thus complemented with calculations for 2D nonadiabatic MHD spectra. The nonadiabatic MHD spectrum contains a thermal continuum in addition to the well-known Alfvén and slow continua. These thermal continuum modes are exponentially decaying or growing in time. The latter unstable modes are widely accepted as an explanation for the onset of marfes in tokamaks and the most favored hypothesis for prominence formation in the solar corona. A numerical code has been developed to calculate the continuum modes. An investigation of a typical ‘pre-MARFE’ (multifaceted asymmetric vadiation from the edge) equilibrium is presented.
Shock structure of Beltrami magnetic fields in plasmas
Authors: Ohsaki Shuichi , Yoshida Zensho
Published: July 2002

A jump discontinuity in the twist (pitch) of magnetic fields can be generated by a shock wave propagating in a plasma. The corresponding Rankine–Hugoniot condition can be solved independently of the other jump conditions. The structure of the magnetic field is represented by the Beltrami eigenfunction of the curl operator.
Dielectric Cherenkov maser with a magnetically confined plasma column in a dielectric lined slow-wave waveguide
Authors: Shokri B. , Ghomi H. , Latifi H.
Published: July 2002

The effect of the strong external magnetic field is studied on the dielectric Cherenkov maser, which is included with an axial thin annular relativistic electron beam and a plasma column in a dielectric lined slow-wave waveguide. The dispersion equations of the beam-wave interaction are derived. The growth rates of the wave are obtained, and the effects of the accelerating voltage, beam current and the background plasma density on the growth rate of the wave are numerically calculated, discussed, and compared in the presence and absence of external magnetic field.
Dielectric response function and stopping power of dense magnetized plasma
Authors: Cereceda Carlo , Deutsch Claude , Peretti Michel De , Sabatier Michel , Nersisyan Hrachya B.
Published: July 2002

Using a kinetic-theoretic approach to Fokker–Planck equilibrium of thermonuclear α particles in dense and magnetized plasmas, the corresponding longitudinal dielectric function is investigated at length. It is used to evaluate the energy loss of the αs′ through the excitation of collective plasma modes. Specific attention was paid to the case of extreme magnetization, as well as to the parallel stopping of α particles in dense and hot plasmas of magnetized target fusion (MTF) interest. Maximum stopping is shown to be strongly dependent on magnetic field intensity.
Positive voltage spikes in runaway tokamak discharges
Authors: El Chamaa Neto I. , Kuznetsov Yu. K. , Nascimento I. C. , Galvão R. M. O. , Tsypin V. S.
Published: July 2002

Positive voltage spikes caused by a fast decrease of the plasma current are observed in runaway dominated discharges in the small Brazilian Tokamak TBR-1[Kuznetsov et al., Phys. Plasmas 6, 4002 (1999)]. Comparison of the measured voltage spike with the value given by the solution of the one-dimensional diffusion equation for the toroidal electric field permits one to infer the plasma conductivity and initial electric field distribution. An additional runaway generation due to voltage spikes can be important. In this case, both the decay time of the voltage spike and the total toroidal current perturbation can decrease substantially.
Series in vector spherical harmonics: An efficient tool for solution of nonlinear problems in spherical plasmas
Authors: Xu S. , Ostrikov K. N.
Published: July 2002

The series expansion of the plasma fields and currents in vector spherical harmonics has been demonstrated to be an efficient technique for solution of nonlinear problems in spherically bounded plasmas. Using this technique, it is possible to describe the nonlinear plasma response to the rotating high-frequency magnetic field applied to the magnetically confined plasma sphere. The effect of the external magnetic field on the current drive and field configuration is studied. The results obtained are important for continuous current drive experiments in compact toruses.
A novel method for detection of strong magnetic fields in femtosecond laser–matter interaction
Authors: Bésuelle E. , Teychenné D. , Salomaa R. R. E.
Published: July 2002

Femtosecond laser–matter interaction can produce strongly magnetized dense plasmas via ultrafast ionization process. This paper proposes a diagnostic—which is not yet existing as far as we know—to prove the existence and to measure the strength of the quasistatic magnetic field in overdense plasma where the Faraday rotation method is not applicable. The method is based on the analysis of the propagation of a quasi-longitudinal right circular polarized electromagnetic probe wave, i.e., a whistler wave. The method is applicable when the quasistatic magnetic field is strong enough (some tens of megagauss) and the overdense plasma electron density not too high (ne⩽5×1022 cm−3).
Decay of the daughter electromagnetic wave driven by stimulated Raman scattering into two Langmuir waves
Authors: Baker K. L.
Published: July 2002

Much attention has been focused on the saturation of stimulated Raman scattering by the decay of its daughter waves. This research has almost exclusively dealt with the decay of the daughter Langmuir wave into secondary Langmuir waves and ion acoustic waves. In this Letter, the decay of the Raman driven electromagnetic wave into two Langmuir waves, two-plasmon decay, is discussed for the first time. It is shown that based on the stimulated Raman scattering reflectivity levels measured in experiments, this instability should be present in current laser–plasma experiments. In addition, experimental signatures of this process are discussed.
Photocurrent response of GaInAs/InP multiple quantum well detectors grown by gas source molecular beam epitaxy
Authors: Temkin H. , Panish M. B. , Logan R. A.
Published: July 2002

We have studied properties of mesa and waveguide p-i-n detectors fabricated from multiple quantum well structures grown by gas source molecular beam epitaxy. With 50 InGaAs wells, each approximately 120 Å thick, mesa detectors show quantum efficiency of up to 77% at 1.56 μm and a capacitance limited pulse response of 200 ps. The photocurrent response of waveguide structures is strongly TE polarized and enhanced at the n=1 exciton. These devices could be useful as wavelength selective, polarization sensitive detectors.
Defect structure of epitaxial CdTe layers grown on {100} and {111}B GaAs and on {111}B CdTe by metalorganic chemical vapor deposition
Authors: Brown P. D. , Hails J. E. , Russell G. J. , Woods J.
Published: July 2002

The technique of cross-sectional transmission electron microscopy has been used to investigate the defect content of epitaxial CdTe layers grown on {100} and {111}B GaAs and on {111}B CdTe by metalorganic chemical vapor deposition. Growth on {111} oriented substrates invaribly gave rise to layers of {111} orientation and these contained a large number of thin (100–1000 Å) lamella twins lying parallel to the interface. In contrast, layers grown on {100} GaAs substrates were found to exhibit either {100} or {111} orientation. Epilayers with the former alignment contained arrays of misfit dislocations at the interface, whereas those with the latter orientation exhibited a density and distribution of lamella twins which were comparable with those of layers grown on {111} substrates. The presence of these defects in homoepitaxially grown CdTe, where the effects of lattice mismatch do not arise, clearly indicates that twinning in {111}B CdTe epilayers is a growth phenomenon.
Photolytic decomposition of adsorbed tellurium and cadmium alkyl species at 295 K upon 193 nm photon irradiation
Authors: Stinespring C. D. , Freedman A.
Published: July 2002

The photolytic decomposition of adspecies formed by the adsorption of tellurium and cadmium alkyls at 295 K under ultrahigh-vacuum conditions has been studied using x-ray photoelectron spectroscopy. Dimethyl tellurium adsorbed at submonolayer coverages on a polycrystalline gold substrate has been observed to undergo nearly quantitative photolytic decomposition at 193 nm to form metallic tellurium. The hydrocarbon photofragments produced in the decomposition lead to negligible carbon contamination on the gold surface. Dimethyl cadmium adsorbed on amorphous SiO2 both desorbs and decomposes to form the metal adspecies. In this case, most of the carbon remains as hydrocarbon and carbidic contaminants. Monomethyl adspecies of both metals formed on Si(100) and GaAs(100) surfaces are inactive with respect to decomposition at the low fluences (0.25 mJ cm−2) used in these experiments; however, substantial desorption is observed.
Electron transport in heterostructure hot-electron diodes
Authors: Arnold D. , Hess K. , Iafrate G. J.
Published: July 2002

Vertical electron transport in GaAs/AlxGa1−xAs heterostructures is investigated through Monte Carlo simulations of the heterostructure hot-electron diode (HHED). A fully self-consistent ensemble Monte Carlo algorithm with a realistic numerical band structure is employed. Results show that transport in the HHED is dominated by the influence of the heterointerface closest to the emitting contact. Two distinct modes of conduction are observed. One is a low-conductivity regime where transport is predominantly by gamma valley tunneling. The other is a high-conductivity regime where transport is mainly due to thermionic emission of electrons from the GaAs to the AlxGa1−xAs. In the latter most of the electrons are in the L and X valleys when they transfer to the wide-gap material. In these valleys the propagation from GaAs to AlxGa1−xAs is virtually unimpeded as the band-edge discontinuities are small in contrast to the situation for gamma electrons.
The Pythagorean Theorem: I. The finite case
Authors: Kadison Richard V.
Published: July 2002

The Pythagorean Theorem and variants of it are studied. The variations evolve to a formulation in terms of noncommutative, conditional expectations on von Neumann algebras that displays the theorem as the basic result of noncommutative, metric, Euclidean Geometry. The emphasis in the present article is finite dimensionality, both “discrete” and “continuous.”
Neurotrophic factor intervention restores auditory function in deafened animals
Authors: Shinohara Takayuki , Bredberg Göran , Ulfendahl Mats , Pyykkö Ilmari , Olivius N. Petri , Kaksonen Risto , Lindström Bo , Altschuler Richard , Miller Josef M.
Published: July 2002

A primary cause of deafness is damage of receptor cells in the inner ear. Clinically, it has been demonstrated that effective functionality can be provided by electrical stimulation of the auditory nerve, thus bypassing damaged receptor cells. However, subsequent to sensory cell loss there is a secondary degeneration of the afferent nerve fibers, resulting in reduced effectiveness of such cochlear prostheses. The effects of neurotrophic factors were tested in a guinea pig cochlear prosthesis model. After chemical deafening to mimic the clinical situation, the neurotrophic factors brain-derived neurotrophic factor and an analogue of ciliary neurotrophic factor were infused directly into the cochlea of the inner ear for 26 days by using an osmotic pump system. An electrode introduced into the cochlea was used to elicit auditory responses just as in patients implanted with cochlear prostheses. Intervention with brain-derived neurotrophic factor and the ciliary neurotrophic factor analogue not only increased the survival of auditory spiral ganglion neurons, but significantly enhanced the functional responsiveness of the auditory system as measured by using electrically evoked auditory brainstem responses. This demonstration that neurotrophin intervention enhances threshold sensitivity within the auditory system will have great clinical importance for the treatment of deaf patients with cochlear prostheses. The findings have direct implications for the enhancement of responsiveness in deafferented peripheral nerves.
Identification of
CDK4
as a target of c-MYC
Authors: Hermeking Heiko , Rago Carlo , Schuhmacher Marino , Li Qing , Barrett John F. , Obaya Alvaro J. , O'Connell Brenda C. , Mateyak Maria K. , Tam Wanny , Kohlhuber Franz , Dang Chi V. , Sedivy John M. , Eick Dirk , Vogelstein Bert , Kinzler Kenneth W.
Published: July 2002

The prototypic oncogene c-
MYC
encodes a transcription factor that can drive proliferation by promoting cell-cycle reentry. However, the mechanisms through which c-MYC achieves these effects have been unclear. Using serial analysis of gene expression, we have identified the cyclin-dependent kinase 4 (
CDK4
) gene as a transcriptional target of c-MYC. c-MYC induced a rapid increase in
CDK4
mRNA levels through four highly conserved c-MYC binding sites within the
CDK4
promoter. Cell-cycle progression is delayed in c-
MYC
-deficient RAT1 cells, and this delay was associated with a defect in CDK4 induction. Ectopic expression of
CDK4
in these cells partially alleviated the growth defect. Thus,
CDK4
provides a direct link between the oncogenic effects of c-
MYC
and cell-cycle regulation.
Activation of antitumor cytotoxic T lymphocytes by fusions of human dendritic cells and breast carcinoma cells
Authors: Gong Jianlin , Avigan David , Chen Dongshu , Wu Zekui , Koido Shigeo , Kashiwaba Masahiro , Kufe Donald
Published: July 2002

We have reported that fusions of murine dendritic cells (DCs) and murine carcinoma cells reverse unresponsiveness to tumor-associated antigens and induce the rejection of established metastases. In the present study, fusions were generated with primary human breast carcinoma cells and autologous DCs. Fusion cells coexpressed tumor-associated antigens and DC-derived costimulatory molecules. The fusion cells also retained the functional potency of DCs and stimulated autologous T cell proliferation. Significantly, the results show that autologous T cells are primed by the fusion cells to induce MHC class I-dependent lysis of autologous breast tumor cells. These findings demonstrate that fusions of human breast cancer cells and DCs activate T cell responses against autologous tumors.
Hydrogen bond-directed aggregation of diazadibenzoperylene dyes in low-polarity solvents and the solid state
Authors: Sautter Armin , Thalacker Christoph , Heise Bernd , Würthner Frank
Published: July 2002

The formation of complex superstructures via hydrogen bonding of two ditopic building blocks, diazadibenzoperylenes 1a,b and isophthalic acid 2, has been investigated. It was found that only the phenoxy-substituted diazadibenzoperylene 1a forms extended assemblies with 2 in complexes of a 1:1 stoichiometry, whereas for the 4-
tert
-butylphenoxy-substituted analogue 1b, no indications for superstructure formation with 2 were found. The different behavior is explained by the presence of additional π–π interactions, which are only observed for [1a⋅2], as revealed by concentration-dependent optical absorption and fluorescence spectroscopy. Based on variable temperature x-ray diffraction studies, a lamellar structure for [1a⋅2] is proposed that takes into account the concept of microphase-segregation.
Retroviral membrane display of apoptotic effector molecules
Authors: Strehlow David , Jodo Satoshi , Ju Shyr-Te
Published: July 2002

Retroviruses have been widely used in gene transmission studies. In this paper, we show that nonviral apoptotic proteins can be displayed on viral membrane surfaces and that the displayed proteins can execute their normal effector functions. We introduced the genes encoding the apoptosis effector proteins, human CD95 ligand (hFasL) or human tumor necrosis factor-related apoptosis-inducing ligand (hTRAIL), into a cell line that packages Moloney murine leukemia virus vectors. Retrovirus preparations from these lines killed target cells efficiently, and target killing was prevented by Fas-Ig fusion protein or soluble TRAIL receptor (sDR5), respectively. We show that the virus preparation exhibiting Fas-specific cytotoxicity has the same density as a retrovirus, contains full-length FasL protein, and can be depleted of infectivity by immunoadsorption with anti-FasL antibody. This novel property of retroviruses—the display of functional effector proteins—may allow the custom design of reagents whose normal function requires their being embedded in a membrane.