bluuebunny/crossref_metadata_2025 huggingface dataset:
The NASA/JPL program in microgravity fundamental physics
Authors: Israelsson U. E. , Lee M. C.
Published: July 2002

The National Aeronautics and Space Administration (NASA) has been supporting research in microgravity low temperature physics for about 20 years. In the last 10 years, the program has seen significant growth in the number of funded investigations and in the breadth of the research activities being pursued. Currently, flight experiments are being performed exclusively on the Space Shuttle. For the future, a cryogenic Space Station facility is being developed by the Jet Propulsion Laboratory and industrial partners to support the microgravity needs of the international scientific community. The facility will be attached to the Japanese Experiments Module’s Exposed Facility and will operate with a 6 month helium cryogen lifetime. Flights of the facility are planned at 2 year intervals starting in 2003 with each flight accommodating multiple scientific experiments. Capabilities, conceptual designs and development plans for the facility are discussed along with a summary of potential near term flight candidate experiments.
Effect of impurities and external fields on second-harmonic generation in ceramics YBa2Cu3O7−δ exposed to laser radiation
Authors: Dovhyj Ya. O. , Kityk I. V. , Lutciv R. V. , Malynych S. Z.
Published: July 2002

A single-mode picosecond YAG-Nd3+ laser (P=30 MW, λ=1.06 μm) is used to measure the reflected second-harmonic generation (SHG) in cerium-doped YBa2Cu3O7−δ ceramics at low temperatures. An enhancement of the SHG signal is observed in the vicinity of Tc. Concentration variations of the effect are associated with the splitting near the van Hove singularities. The SHG signal is found to depend on the magnitude and direction of the external magnetic field.
Heat capacity of reentrant ferrimagnets Li0.5Fe2.5−xGaxO4 with x=0.9 and 1.2
Authors: Efimova N. N. , Kufterina S. R. , Anders A. G. , Startsev S. V. , Gurevich A. M. , Eropkin V. N.
Published: July 2002

The specific heat Cm of the magnetic subsystem of reentrant ferrimagnets Li0.5Fe2.5−xGaxO4 with x=0.9 and 1.2 is investigated in the temperature range 1.9–22 K. It is found that, in the low-temperature region of this interval, Cm is a linear function of temperature, while the dependence Cm∼T3/2 is observed for T>11 K. Possible mechanisms responsible for such a behavior are discussed.
Study of matrix isolation of nitrogen atoms in solid N2
Authors: Dmitriev Yu. A. , Zhitnikov R. A.
Published: July 2002

The process of matrix isolation of nitrogen atoms in solid N2 is studied by the method of condensation from the gaseous phase. In order to vary the flow rates of nitrogen atoms and molecules at the sample surface over a wide range, N2–He mixtures of various compositions have been passed through the gas discharge zone. It is found that the EPR line width for N atoms in N2 increases upon a decrease in the nitrogen flow rate from the discharge zone to a cold substrate. It is shown that this increase is due to an increase in the concentration of atomic nitrogen in the matrix leading to an enhancement of the dipole–dipole interaction between atoms. It is established that a decrease in the intensity of recombination of atoms in the course of diffusion at the sample surface due to a decrease in the surface density of these atoms plays a decisive role in the growth of the concentration of matrix-isolated atoms in the experiments. It is found that for a constant flow rate of atoms from the discharge, their concentration in the matrix increases and attains saturation upon an increase in the discharge intensity. The dependence of concentration on the discharge intensity is attributed to a change in the degree of dissociation of molecular nitrogen in the discharge.
On anomalies of optical exciton reflection spectra of crystals at low temperatures
Authors: Kosobutskii P. S.
Published: July 2002

The anomalies in the optical exciton reflection spectra of crystals at low temperatures are interpreted. It is shown that the amplitude of the reflection minima increases upon an increase in damping.
On spin wave scattering by a soliton in a two-dimensional isotropic ferromagnet
Authors: Ivanov B. A. , Muravyov V. M.
Published: July 2002

Scattering of magnons by a two-dimensional topological Belavin–Polyakov soliton in an isotropic ferromagnet is studied analytically. It is shown that the problem of spin wave scattering by a soliton with an arbitrary value of the topological charge ν can be analyzed completely in the longwave limit. General principles of the soliton–magnon interaction are studied, especially the relation between scattering and the behavior of the mode as the magnon wave vector k approaches zero. It is found that the scattering intensity has its maximum value for partial waves with the azimuthal number m=0,±1,±2 (m=ν−1). Although the mode with the maximum scattering always passes to a local mode for k→0 according to the general law, this fact is not crucial for the scattering intensity. In particular, the scattering intensity is stronger for a partial wave with m=−1 for ν=1 (there is no local mode for k→0) than for a partial wave with m=+1 (a local mode exists for k→0).
EPR investigation of the fluctuating valence compound YbB12
Authors: Altshuler A. E. , Altshuler T. S. , Rameev B. S. , Khaimovich E. P.
Published: July 2002

The compound YbB12 having a fluctuating valence is investigated by the EPR method. It is suggested that the valence fluctuation frequency of ytterbium ions ν>1010 Hz. The electron excitation spectra reveal a gap of the order of 6–7 meV.
Low-temperature field reconstruction of atomic nanoclusters at tungsten surface
Authors: Mazilova T. I.
Published: July 2002

Low-temperature field reconstruction of atomic clusters at a closely packed face of a metal is observed for the first time by the method of field ion microscopy. It is found that compact clusters disintegrate into non-closely packed zigzag atomic chains.
Attenuation of low-frequency acoustic energy in Bi-based superconductors
Authors: Sergeeva G. G.
Published: July 2002

The attenuation of low-frequency acoustic energy in stripes of the low-temperature tetragonal phase of Bi-2212 ceramics is studied taking into account the dx2−y2-symmetry of the superconducting order parameter (SOP) and the dynamic analog of phase separation. The contribution to the energy density of the strain–order parameter coupling leads to a finite value of attenuation in superconductors with a peculiar SOP. The frequency and temperature dependences of attenuation are analyzed and compared with the available experimental results. It is shown that these data are an indirect evidence of the SOP symmetry: the dx2−y2-symmetry for the Bi-2212 phase and the s-symmetry for the Bi–2223 phase. The existence of an ordered low-temperature state responsible for the low-frequency attenuation peak and other anomalies in the properties of Bi-based ceramics in the temperature range 20–40 K is proposed.
Study of stimulated dynamics and pinning of Abrikosov vortex lattice in type II superconductors in an a.c. magnetic field
Authors: Chigvinadze J. G. , Mamniashvili G. I.
Published: July 2002

A dynamic mechanical method using a weak low-frequency magnetic field is proposed for investigation of stimulated dynamics and pinning in the Abrikosov vortex lattice in type II superconductors. It is shown that the application of a weak a.c. field facilitates depinning of Abrikosov vortices and additional dissipation of energy by liberated vortices. As the frequency of the field increases, a mode in which the vortices virtually do not oscillate is attained, i.e., dissipation ceases, while pinning is extremely effective. The observed effects can be successfully explained on the basis of the Gor’kov–Kopnin theory.
Did Ogg really observe high-temperature superconductivity in metal-ammonia solutions?
Authors: Pashitskiı̆ É. A.
Published: July 2002

An empirical analysis of the results of time- and temperature dependences of the electrical resistance R of metal-ammonia solutions NH3:Na with different Na concentrations quenched in liquid nitrogen point towards an anomalously high but finite conductivity rather than superconductivity. In particular, the time dependence R(t) of solutions NH3:Na quenched rapidly at T<80 K can be described correctly by a “logistic curve” with a nonzero initial value R(0)≠0 (at t=0).
Collective electromagnetic mode in layered conductors
Authors: Gokhfeld V. M. , Peschansky V. G.
Published: July 2002

The transverse zero-sound wave in a one-component charged Fermi liquid with quasi-two-dimensional electron energy spectrum is considered in the framework of the Landau theory. In contrast to conventional metals, the electromagnetic wave propagation in such media along the weak conductivity direction is possible even for low intensities of the Fermi-liquid interaction. The field distribution in the sample under unilateral excitation is determined, the wave impedance is calculated, and the possibility of observing the effect in the pulse mode is considered.
Structure and lattice parameters of thin C60 films
Authors: Pugachev A. T. , Churakova N. P. , Gorbenko N. I. , Saadly H. , Solodovnik A. A.
Published: July 2002

The structure and lattice parameters of C60 fullerite films evaporated in vacuum on the (100) cleavage plane of NaCl at the substrate temperature 290–400 K are investigated in the temperature range 300-5 K by electron-optical methods. Fullerite films have an fcc lattice at room temperature. The film structure changes with the temperature of condensation from epitaxial with the (111) orientation to a disordered and highly nanodisperse structure with a grain size of 4–5 nm. The crystallographic conditions of conjugation of the (100) surface of NaCl and epitaxial C60 fullerite films are determined, and the four-position type of their structure is established. The fcc–sc transition temperature and the observed jump in the lattice parameter are close to the corresponding characteristics of bulk fullerite. The temperature dependence of the lattice parameter in the temperature range 100–260 K is used to determine the average linear thermal expansion coefficient α of the films. An increase in α for small thicknesses is a size effect associated with a considerable influence of the surface. A mechanism of formation of the structure of condensed C60 fullerite films is proposed on the basis of the obtained results.
Size effect in impuriton gas of He3–He4 superfluid mixture
Authors: Adamenko I. N. , Bortnik L. N. , Chervanyov A. I.
Published: July 2002

The flow of quasiparticle gas under the action of gradients of thermodynamic quantities in a volume filled with a powder (crocus) is considered. The exact solution of the kinetic equation is obtained in terms of the matrix elements of the collision integral for quasiparticles and partial cross sections of quasiparticles scattering by crocus. The condition describing a steady nonequilibrium state of the quasiparticle gas in a volume filled with a porous material is determined. The obtained results are valid for arbitrary relations between the frequencies of collisions of quasiparticles with one another and with powder particles. The Knudsen effect in a degenerate quantum gas is studied, and the transition from the Knudsen to hydrodynamic flow of quasiparticle gas is analyzed. The steady nonequilibrium state of the impuriton gas in the He3-He4 superfluid mixture is investigated in a confined geometry.
Kinetic properties of the HTSC compound Ag–Bi2223
Authors: Merisov B. A. , Khadjai G. Ya. , Obolensky M. A. , Cherpak N. T. , Haldar P. , Hazelton D.
Published: July 2002

Thermal conductivity λ(T) and resistivity ρ(T) of the HTSC compound Ag–Bi2223 (Tc≃107 K; ΔTc≃2 K) produced by the Intermagnetics General Corporation (USA) were measured in the temperature intervals 4.2–300 K and Tc–300 K respectively. Away from the SC transition, the values of λ(T) and ρ(T) are determined by the silver content. For T>60 K, irregular thermal conductivity oscillations are observed in the vicinity of the superconducting transition against the background of the dependence λ(T) typical of silver. The position and amplitude of the oscillations are not affected by temperature cycling. A sharp minimum on the λ(T) dependence, whose depth is much larger than the estimated contribution of thermal conductivity of Bi2223 to the thermal conductivity of the system, is observed at Tc.
Optical generation of the first and second sounds in superfluid He3–4He solutions
Authors: Salikhov T. Kh.
Published: July 2002

An optical method of generation of the first and second sounds in superfluid He3–4He solutions is proposed. The method is based on illumination by amplitude-modulated laser beam with wavelength corresponding to a region of absorption. Numerical calculations of pressure and temperature as a function of time are carried out. It was found that the presence of He3 isotopes in the solution provides strong coupling between these parameters. As a result, the waves of the first sound involve oscillations of the temperature, while the waves of the second sound involve oscillations of the pressure.
Effect of anomalously small dose of fast neutron radiation on the surface impedance of a Pb single crystal near Hc3
Authors: Jorjishvili L. I.
Published: July 2002

It is shown experimentally that the exposure of a Pb single crystal to an anomalously small dose of fast neutrons (∼4×106 neutrons/cm2) leads to suppression of cyclotron resonance and surface superconductivity. This effect is explained by geometrical factors.
On the self-consistent theory of Josephson effect in ballistic superconducting microconstrictions
Authors: Zareyan M. , Kolesnichenko Yu. A. , Omelyanchouk A. N.
Published: July 2002

The microscopic theory of current-carrying states in the ballistic superconducting microchannel is presented. The effects of the contact length L on the Josephson current are investigated. For the temperatures T close to the critical temperature Tc the problem is treated self-consistently, with allowance for the distribution of the order parameter Δ(r) inside the contact. The closed integral equation for Δ in strongly inhomogeneous microcontact geometry (L≲ξ0, where ξ0 is the coherence length at T=0) replaces the differential Ginzburg–Landau equation. The critical current Ic(L) is expressed in terms of the solution of this integral equation. The limiting cases of L≪ξ0 and L≫ξ0 are considered. With increasing length L, the critical current decreases, although the ballistic Sharvin resistance of the contact remains the same as at L=0. For ultrashort channels with L≲aD (aD∼υF/ωD, where ωD is the Debye frequency) the corrections for the value of the critical current Ic (L=0) are sensitive to the strong-coupling effects.
Dynamics and drag of a vortex in type II superconductor with weak inhomogeneities
Authors: Galkin A. Yu. , Ivanov B. A.
Published: July 2002

The dynamics of the Abrikosov vortex lattice and a single vortex in a type II superconductor with defects is studied taking into account inertial as well as gyroscopic (Hall) properties. The spectrum of normal modes in the absence of defects has two branches. In the limit of small k, one of the branches is gapless and has a quadratic dispersion relation, while the other branch has a finite gap. In the limit of large k, the dispersion relations for both modes become linear (acoustic). It is shown that the interaction with defects in a moving vortex or a vortex lattice excites oscillations corresponding to these modes. This creates an additional energy dissipation channel for translation motion of the vortex. In the case of a single vortex, the corresponding drag force diverges as V−1/2 for V→0, i.e., prevails over the regular force of viscous friction for small velocities.
Magnetically stimulated inhomogeneity of conductivity and nonlocal transport phenomena in metals
Authors: Sobol V. R. , Mazurenko O. N. , Drozd A. A.
Published: July 2002

Charge transport in a conducting medium with a magnetically stimulated inhomogeneity of kinetic coefficients along the direction of transport is investigated both experimentally and analytically. Measurements were made on samples in the form of high-purity polycrystalline aluminum plates whose conductivity inhomogeneity was simulated by the technique of curving of current lines so that the local normal component of the applied magnetic field varies according to an exponential or quadratic law. The relations describing the tensor connection between the electric field and charge flux density are used to calculate spatial dependence of the potential. The sign reversal of the electric field is described as the result of competition between the potential contributions from the current along the transverse magnetic field gradient and the Hall current at right angles to it.
Characteristic energy losses of electrons in a two-dimensional electron gas in a magnetic field
Authors: Ermolaev A. M. , Haghighi Babak
Published: July 2002

The electron energy loss function is calculated in the random phase approximation for a two-dimensional electron gas in a quantizing magnetic field. Local states of electrons at impurity atoms are taken into consideration. The energy losses due to one-particle and collective excitations of two-dimensional electrons are determined. The activation of electrons localized at impurities leads to the emergence of steps on the dependence of loss function on the energy of an incident electron. Cerenkov losses associated with emission of magnetoplasmons appear starting from a threshold velocity of the electron. When the velocity exceeds the threshold value significantly, the losses are due only to spontaneous emission of magnetoplasmons. The corresponding loss function decreases in inverse proportion to the electron velocity.
Low-temperature nonlinear lattices in ferroelectrics with protonic conductivity
Authors: Belonenko M. B. , Koleganova E. Yu.
Published: July 2002

An S=1 pseudospin formalism is constructed for ferroelectrics with protonic conductivity, the change carriers in which are protons moving along a network of hydrogen bonds. The Hamiltonian is written in the pseudospin form and in the second-quantization operator representation. A system of equations describing the state of the ferroelectric cell is obtained. A numerical analysis of this system reveals the presence of traveling nonlinear lattices that can affect the dynamical properties of the ferroelectric.
Low-temperature thermopower in quasiamorphous carbons
Authors: Matsuı̆ L. Yu. , Vovchenko L. L. , Ovsienko I. V.
Published: July 2002

Results are presented from a study of the thermopower of quasiamorphous carbons in the temperature interval from 20 to 200 K. It is shown that the thermopower of quasiamorphous carbons is described satisfactorily in a model based on the series connection of regions with metallic conductivity, hopping conductivity with a variable hopping length, and hopping conductivity with a constant hopping length. It is found that when considering the thermopower of the regions with the metallic conductivity, it is necessary to take into account the temperature dependence of the main mechanisms of carrier scattering. The parameters of the electronic structure of quasiamorphous carbon are calculated using experimental data on the temperature dependence of the electrical resistivity and thermopower.
The Second Chernogolovka Workshop on Low Temperature Physics in Microgravity Environment (CWS-99), July 28–August 2, 1999
Authors: Mezhov-Deglin L. P.
Published: July 2002

The International Seminar CWS-99, devoted to a discussion of the current status and prospects for development of fundamental research in the field of low-temperature physics in a microgravity environment, was held in at the Research Center of the Russian Academy of Sciences (RCC RAN) in Chernogolovka, Moscow District on July 28–August 2, 1999. This seminar was organized by the Institute of Solid State Physics and the Space Materials Science section of the Space Council of the Russian Academy of Sciences with the financial support of the Russian Fund for Fundamental Research, the Ministry of Science and Technology of the Russian Federation, The Russian Aviation & Space Agency, and the Council on Low Temperature Physics of the Russian Academy of Sciences. The CWS-99 Seminar preceded the International Conference on Low Temperature Physics LT-22, and, as one of the satellite activities of LT-22 conducted in the framework of international scientific cooperation, it received substantial support from the Organizing Committee of LT-22 and from the OLMSA Division of the National Aeronoutics and Space Administration (NASA), USA.
Properties and structure of fluid hydrogen near the line of crystallization
Authors: Yakub E. S.
Published: July 2002

The thermodynamic properties and distribution functions of highly compressed molecular hydrogen are investigated in a nonempirical atom–atom approximation. Quantum corrections are introduced on the basis of the Feynman variational approach. The pressure, energy, compressibility, thermal expansion, heat capacity, and sound velocity are calculated by the Monte Carlo method at temperatures of 200–500 K and pressures up to 5 GPa. The results are compared with the available experimental data. It is shown that effects due to incompressibility of the molecules and quantum effects must be taken into account in calculating the contributions to the thermodynamic functions from inter- and intramolecular forces. The behavior of the properties of fluid hydrogen near the line of crystallization outside the investigated region is predicted, and an estimate is made for the jump in volume upon crystallization of hydrogen at T=300 K.
Saturation effect in the problem of microwave energy absorption by two-dimensional electrons on a helium film
Authors: Shikin V. B.
Published: July 2002

The details of the absorption of an rf electromagnetic field by two-dimensional (2D) electrons on a thin film of helium in relation to the confining electric field are discussed. The connection between this problem and the problem of saturation in a 2D electron system is noted. A special study is made for the case of cylindrical geometry, which corresponds qualitatively to the conditions of the experiment of B. Lehndorff and K. Dransfeld, J. Phys. (Paris) 50, 2579 (1989). It is shown that the data obtained on the field dependence of the absorption of rf field energy by 2D electrons contains information about the supersaturated electronic states in the cell.
On the magnetoelectric effect in LiNiPO4
Authors: Chupis I. E.
Published: July 2002

It is shown that the “butterfly” magnetoelectric hysteresis loop usually associated with the presence of a spontaneous magnetic moment in a crystal can also be observed in the case of an “angled cross” type of antiferromagnetic ordering of the spins. The magnetoelectric hysteresis observed in LiNiPO4 is explained by the cross-shaped antiferromagnetic ordering of the spins in this compound. It is assumed that the jump in the electric polarization near the Néel temperature in a high magnetic field is the result of a first-order phase transition from the antiferromagnetic cross-shaped to a weak ferromagnetic state.
Log-corrected energy spectrum and dimension of attractor in two-dimensional turbulence
Authors: Ohkitani Koji
Published: July 2002

The dimension of the attractor in two-dimensional turbulence is estimated qualitatively by using Kraichnan’s log-corrected energy spectrum. It is found to be identical to the leading order with rigorous upper bounds, obtained by Constantin, Foias, and Temam [Physica D 30, 284 (1988)], which also include log factors.
Surface waves in closed basins under principal and autoparametric resonances
Authors: Nayfeh A. H. , Nayfeh J. F.
Published: July 2002

The method of multiple scales is used to analyze the nonlinear response of the surface of a liquid in a cylindrical container to a principal parametric resonant excitation in the presence of a two-to-one internal (autoparametric) resonance. Four autonomous first-order ordinary-differential equations are derived for the modulation of the amplitudes and phases of the two modes involved in the internal resonance when the higher mode is being excited by a principal parametric resonance. The modulation equations are used to determine the periodic oscillations and their stability. The force-response curves exhibit the jump and saturation phenomena as well as a Hopf bifurcation, whereas the frequency-response curves exhibit the jump phenomenon and supercritical and subcritical Hopf bifurcations. Limit-cycle solutions of the modulation equations are found between the Hopf frequencies; they correspond to aperiodic motions of the liquid surface. All limit cycles deform and lose stability by either pitchfork or cyclic-fold bifurcations as the excitation frequency or amplitude is varied. The pitchfork bifurcation breaks the symmetry of the limit cycles whereas the cyclic-fold bifurcation causes cyclic jumps, which may result in a transition to chaos. Period-three motions are found in a very narrow range of the excitation frequency.
The effect of nonuniform density on the absolute instability of two-dimensional inertial jets and wakes
Authors: Yu Ming-Huei , Monkewitz Peter A.
Published: July 2002

The boundary between absolute and convective (linear) instability of two-dimensional inertial jets and wakes is determined as a function of the ratio of jet/wake to ambient density, as well as the ratio of mixing layer thickness to jet/wake width, the velocity ratio, and the Reynolds number. For this, a viscous, heat-conducting ideal gas is taken as the fluid, a zero Mach number, no buoyancy and a parallel basic flow are assumed, and the density variation is achieved by specifying a mean temperature profile similar to the velocity profile. Considering both ‘‘varicose’’ and ‘‘sinuous’’ disturbances, results are obtained for the inviscid top-hat jet/wake bounded by two vortex sheets, the inviscid jet with continuous velocity and density profiles, and the viscous wake. For the latter, both constant and temperature-dependent viscosity are investigated. In all the cases it is found that low density of the high-speed fluid promotes absolute instability, while low density of the low-speed fluid has the opposite effect. By comparison with experiments it is shown that the present results provide useful information about the parameter range in which flow oscillations are self-excited.
On the effect of riblets in fully developed laminar channel flows
Authors: Choi Haecheon , Moin Parviz , Kim John
Published: July 2002

The effect of longitudinal riblet surfaces on viscous drag in fully developed laminar channel flows was investigated. Unlike turbulent flows, drag reduction was not obtained in the laminar flows. Results were independent of Reynolds number. Wall-shear rates on most regions of the cross-sectional perimeter of riblets were smaller than that of corresponding plane channel flow even though the net drag was increased.
Dispersion in an unconsolidated porous medium
Authors: Young W. R. , Jones Scott W.
Published: July 2002

The dispersion of passive scalar by Stokes flow through a dilute suspension of solid spheres fixed at random points in space is a basic model of mixing and transport in an unconsolidated porous medium. Earlier investigators have used a general theory of macroscopic transport to calculate the Lagrangian velocity autocorrelation function and the effective diffusivity of this system. If the tracer does not diffuse molecularly then the autocorrelation function has a ‘‘long tail,’’ proportional to t−1. Small molecular diffusivity provides a cutoff at long times and implies an effective diffusivity that increases as the logarithm of the Péclet number. In the present work the same results are obtained including the constants of proportionality, using an elementary Lagrangian argument that avoids the formalism of the macrotransport theory.
Double-diffusive flow in enclosures
Authors: Shyy Wei , Chen Ming-Hsiung
Published: July 2002

Numerical calculations are conducted for two-dimensional steady-state double-diffusive flow in enclosures, where both the temperature and solute gradients are imposed horizontally, and the two buoyancy effects can either augment or counteract each other. Important controlling parameters including the thermal and solutal Grashof numbers, Schmidt number, and Prandtl number, are varied and new insights into the solutions of both the augmenting and counteracting modes gained. Even though the relative directions of the two buoyancy mechanisms reverse between the augmenting and counteracting modes, qualitative similarities in convection characteristics emerge. For both modes, with fixed thermal and solutal Grashof numbers, convection strength increases as the ratio between the Schmidt and Prandtl numbers, i.e., the Lewis number, becomes higher. With appropriate combinations of the Grashof number and buoyancy ratio, multiple-cell flow patterns also appear in both modes, where the length scale disparity due to a large Lewis number causes the convection to be largely controlled by the temperature field within the primary convection cell, and by the solute field either in the region across the cell interface or in the secondary cells. Effects of irregular geometry of the enclosure on the transport processes are also assessed.
Effects of horizontal gradients on thermohaline instabilities in a thick porous layer
Authors: Sarkar Alok , Phillips O. M.
Published: July 2002

Linear stability analysis is used to examine the effects of horizontal gradients of temperature and salinity on characteristics of thermohaline instabilities in a saturated, homogeneous, isotropic, horizontal porous layer. The emphasis is on obtaining analytical results valid in the limit of large Rayleigh numbers. The present results are compared with an earlier infinite medium study (Sarkar and Phillips, to appear in J. Fluid Mech.) to understand how horizontal impervious boundaries affect the dynamics. It appears that the geometric constraints imposed by the boundaries tend to eliminate the most effective of unstable modes. Thus the boundaries have a stabilizing effect. Necessary conditions for the boundaries to not have an appreciable effect on the overall dynamics of instabilities are also obtained. The properties of both neutrally stable and supercritical disturbances are described. The supercritical convection cells are similar to salt fingers or horizontal interleaving depending on whether the vertical Rayleigh number is positive or negative, respectively. This is in contrast to the critical modes (i.e., neutrally stable disturbances corresponding to the minimum vertical Rayleigh number) where the convection cells are almost horizontal and are about as thick as the porous layer.
Lower-hybrid current drive experiments in TORE SUPRA
Authors: Moreau D. ,   the TORE SUPRA Team‡
Published: July 2002

Lower-hybrid current drive (LHCD) experiments performed in TORE SUPRA [Plasma Physics and Controlled Nuclear Fusion Research, 1988 (IAEA, Vienna, 1989), Vol. 1, p. 9] are reported. Two large ‘‘multijunction’’ launchers have allowed to couple up to 6 MW to the plasma with a maximum power density of 45 MW/m2 and reflection coefficients lower than 3%. The current drive efficiency was about 2×1019 Am−2/W with LH power alone at a volume-averaged electron temperature 〈Te〉=1.4 keV, and a 22 sec long quasistationary discharge could be sustained by applying 2.8 MW during an 18 sec/1.6 MA current flattop at a line-averaged density n̄e=3×1019 m−3. Stable LH current ramp-up assist was achieved, thus reducing the resistive flux consumption with an efficiency of 0.7×1019 Wb m−1/MJ. Experiments with combined LHCD and ion cyclotron resonant heating allowed to inject up to 7.5 MW into the plasma. The electron energy content followed fairly well the Rebut–Lallia scaling law [Plasma Physics and Controlled Nuclear Fusion Research, 1988 (IAEA, Vienna, 1989), Vol. 2, p. 191]. At n̄e=1.5×1019 m−3, sawteeth were suppressed and m=1 MHD (magnetohydrodynamics) oscillations appeared. The central electron temperature then reached 8 keV for 3.6 MW injected.

Lower-hybrid power modulation experiments performed at n̄e=4×1019 m−3 showed a delayed central electron heating despite the off-axis creation of suprathermal electrons, thus ruling out the possibility of direct heating through central wave absorption. Successful pellet fueling of a partially LH-driven plasma was obtained, in which 28 successive pellets could penetrate almost to half radius as in Ohmic discharges, with 50% to 80% of the pellet content deposited in the plasma. First attempts to combine LHCD with ergodic divertor discharges showed that, when the plasma edge was subject to a radial magnetic perturbation smaller than the ergodicity threshold, a strong stationary radiation (MARFE) was triggered, locked near the inner wall. The radiated power then amounted to 90% of the total input power with no indication of a radiative collapse.
The Bohm criterion for radio-frequency discharges: A numerical verification based on Poisson’s equation
Authors: Meijer P. M. , Goedheer W. J.
Published: July 2002

Recently it was shown that, by using the analysis of electrostatic waves entering the plasma–sheath edge, the direct-current (dc) Bohm criterion also holds for discharges under radio-frequency (rf) conditions. In this paper, the influence of Bohm’s criterion on the sheath characteristics for generator frequencies much higher than the ion plasma frequency has been examined by means of an analysis which is based directly on the numerical solution of the Poisson equation. The present calculations indicate that for high rf voltages the time-dependent potential profile does not always increase monotonically, even if Bohm’s criterion is fulfilled. However, as this nonmonotonic behavior does not lead to a more stringent Bohm criterion, the statement that the original dc Bohm criterion also holds in the high-frequency regime is confirmed. The calculations show further that the time-dependent sheath potential is almost completely modulated in the major part of the sheath and that the modulation drops abruptly to zero close to the plasma–sheath edge.
Thermosolutal convection during directional solidification. II. Flow transitions
Authors: McFadden G. B. , Coriell S. R.
Published: July 2002

The effect of thermosolutal convection on the solute segregation in crystals grown by vertical directional solidification of binary metallic alloys or semiconductors is considered. Numerical results are obtained using finite differences in a two-dimensional, time-dependent model that assumes a planar crystal–melt interface. The configuration is assumed to be periodic in the horizontal direction with a given period, and the possibility of multiple flow states sharing the same period is examined. The results are summarized in bifurcation diagrams of the nonlinear states associated with the critical points of linear theory. The use of a time-dependent numerical scheme results in gaps in the bifurcation diagram where presumed unstable states exist that cannot be computed by this procedure. As the solutal Rayleigh number is varied, multiple steady states, time-periodic states, and quasiperiodic states may occur. This case is compared to the simpler case of thermosolutal convection with linear vertical gradients and stress-free boundaries, for which a rather complete numerical treatment is possible through the use of a simple spectral representation of the nonlinear solution. Retaining a finite number of terms in the expansion results in a set of coupled nonlinear algebraic equations for the Fourier coefficients, which is solved by a quasi-Newton method. The linear stability of the nonlinear solutions is also determined from a numerical computation of the eigenvalues of the Jacobian matrix. The resulting bifurcation diagram shows qualitative similarity to the bifurcation diagram computed for the solidification system.
Negative ion-acoustic solitons in a two-component magnetized plasma
Authors: Song L. T. , Lee L. C. , Huang L.
Published: July 2002

Ion-acoustic solitons in a two-component low-beta plasma are studied. It is found that under the quasineutrality condition only positive ion-acoustic solitons can exist in such a plasma. When the effect of charge separation is included, both positive and negative solitons can be found for Cs cos θ<Vp<Cs. Here θ is the angle between the wave vector and the magnetic field, Cs is the ion-acoustic velocity, and Vp is the phase speed. The amplitudes of the positive and negative solitons as a function of θ and Vp/Cs are also calculated.
Laplace pressure driven drop spreading
Authors: Diez J. A. , Gratton R. , Thomas L. P. , Marino B.
Published: July 2002

This work concerns the spreading of viscous droplets on a smooth rigid horizontal surface, under the condition of complete wetting (spreading parameter S≳0) with the Laplace pressure as the dominant force. Owing to the self-similar character foreseeable for this flow, a self-similar solution is built up by numerical integration from the center of symmetry to the front position to be determined, defined as the point where the free-surface slope becomes zero. Mass and energy conservation are invoked as the only further conditions to determine the flow. The resulting fluid thickness at the front is a small but finite (≊10−7) fraction of the height at the center. By comparison with experimental results the regime is determined in which the spreading can be described by this solution with good accuracy. Moreover, even within this regime, small but systematic deviations from the predictions of the theory were observed, showing the need to add terms modifying the Laplace pressure force.
Power-law decay of homogeneous turbulence at low Reynolds numbers
Authors: Huang Mei-Jiau , Leonard Anthony
Published: July 2002

The decay of nominally isotropic, homogeneous incompressible turbulence is studied by direct numerical simulations for Reλ in the range (5–50) with 2563 spectral coefficients. A power-law decay of the turbulent energy is observed with exponents approximately equal to 1.5 and 1.25, apparently dependent on Reλ. A new complete similarity form for the double and triple velocity correlation functions, f(r,t) and k(r,t), is proposed for low to intermediate Reλ that is consistent with the Kármán–Howarth equation and the results of the numerical experiments. The results are also consistent with Saffman’s proposed asymptotic behavior of f(r,t) for large separation r for runs with a decay exponent of 1.5. The so-called final period of decay is not observed.
The growth of wind waves at the crests and troughs of a low amplitude swell
Authors: Morland L. C.
Published: July 2002

If the interface between the fluids is initially planar, air blowing over water is subject to an inviscid instability, which acts to generate wind waves. Conditions in nature motivate studying the influence of a longer wave (a swell) on the instability. In the absence of waves, the wind and its associated wind-drift current are taken to be plane, parallel flows. Linear theory is used to produce a first approximation to a wind and current modified by the presence of a low amplitude swell, and the stability of the modified wind and current to much shorter wavelength disturbances (the wind waves) is examined. In the neighborhood of a crest or trough of the swell, where the fluid flow is nearly parallel, the linearized equations are closely related to those obtained in the absence of a swell. The amplitude of the swell appears as a parameter, and because of the large separation in wavelength assumed between the swell and the wind waves, the phase of the swell may be approximated as a parameter. It is found that, at both crests and troughs, the growth rates of the wind waves ultimately decrease rapidly with increasing swell amplitude, indicating that the swell has a stabilizing influence. This result is consistent with the experimentally observed suppression of wind waves by a swell propagating in the direction of the wind.
Sphere theorem for the Stokes flow
Authors: Hasimoto Hidenori
Published: July 2002

The sphere theorem for general three-dimension Stokes flow is presented in a simple vector form. The perturbation pressure and velocity due to a sphere introduced into an unlimited viscous fluid of given pressure and velocity is given directly from the original field. For this purpose a single harmonic function is derived from the original flow. The proof and the interpretation of the theorem are made easily. Extension to the interior problem is possible with a slight modification if special consideration is taken on the Stokeslet behavior of the given field at infinity. As illustrations, the case of a single asymmetric Stokeslet is treated in the Appendix.
The effect of bulk viscosity on temperature relaxation near the critical point
Authors: Carlès Pierre
Published: July 2002

The heat transfer near the critical point is governed not only by diffusion, convection, and radiation, but also by a thermomechanical coupling called the Piston Effect. This fourth mode of heat transfer is responsible for the so-called critical speeding up, which contradicts the first expectation of a critical slowing down of the heat diffusion. So far, the viscosity has been neglected in all the existing theoretical models of the Piston Effect. The aim of this paper is to present a comprehensive model of the Piston Effect, written for a real-fluid equation of state and including the critical divergence of the bulk viscosity. It is shown in particular that when the critical point is neared, the heat transfer goes faster and faster, until a point is reached where viscous stresses are no longer negligible. When going closer to the critical point, the heat transfer then slows down again; a regime of critical slowing down is entered. This phenomenon should happen sufficiently far from the critical temperature to allow experimental checks. Moreover, it could be used as an indirect way of measuring the critical divergence of the bulk viscosity.
Measurement of shock structure and shock–vortex interaction in underexpanded jets using Rayleigh scattering
Authors: Panda J. , Seasholtz R. G.
Published: July 2002

The density field of underexpanded supersonic free jets issuing from a choked circular nozzle was measured using a Rayleigh scattering-based technique. This reliable and nonintrusive technique is particularly suitable for high-speed flows and is fundamentally superior to the intrusive probes and particle-based techniques such as laser Doppler velocimetry. A continuous wave laser and photon counting electronics were employed for time and phase-averaged density measurements. The use of dust-free air for the entrained flow allowed measurements in the shear layer region. The free jets were produced in the plenum to ambient pressure ratio range of 1.88–5.75, which corresponded to a fully expanded Mach number range of 0.99⩽Mj⩽1.8. A comparative study of schlieren photographs and time-averaged density data provided insight into the shock-cell structures. The radial profiles obtained at various axial stations covering a downstream distance of 10 jet diameters show the development of the jet shear layer and the decay of the shock–cells. The supersonic free jets produced screech sound. A phase-averaged photon counting technique, using the screech tone as the trigger source, was used to measure the unsteady density variation. The phase-averaged density data show the evolution of the large-scale turbulent vortices that are found to be modulated periodically along the flow direction. A comparison with previously obtained data showing near-field pressure fluctuation and convective speed of the organized vortices reveals many interesting dynamics. All quantities show regular spatial modulation. The locations of local maxima in density fluctuations are found to coincide with the high convective speed and the antinode points in the near-field pressure fluctuation. Interestingly, the periodicity of modulation is found to be somewhat different from the shock spacing. Instead it shows that the standing wave system, known to exist in the near-field pressure fluctuation, extends into the jet shear layer. The standing wave is formed between the downstream moving Kelvin–Helmholtz instability waves and the upstream propagating part of sound waves. A detailed field measurement of the unsteady density fluctuation was conducted for the Mj=1.19 and 1.42 jets for which the near-field pressure fluctuation data were obtained previously. The phase-matched, combined plots of the density fluctuation present inside the jet flow, and the pressure fluctuation present just outside the jet boundary provide a charming insight into the shock–vortex interaction leading to the sound wave generation.
Effect of ion temperature on large-amplitude ion-acoustic solitary waves in relativistic plasma
Authors: Chatterjee Prasanta , Roychoudhury Rajkumar
Published: July 2002

Effect of ion temperature on the conditions for existence of solitary waves in a relativistic plasma is studied using Sagdeev’s pseudopotential approach. It is shown that the ion temperature puts a restriction on the values of V, the soliton velocity. It is also shown that for small amplitude and cold ions, the present results agree with the existing published results. Numerical solutions of the equation of motion derived from the pseudopotential are obtained to see the effect of ion temperature on the width and amplitude of the ion-acoustic solitary waves.
Periodic magnetic focusing of sheet electron beams*
Authors: Booske J. H. , Basten M. A. , Kumbasar A. H. , Antonsen T. M. , Bidwell S. W. , Carmel Y. , Destler W. W. , Granatstein V. L. , Radack D. J.
Published: July 2002

Sheet electron beams focused by periodically cusped magnetic (PCM) fields are stable against low-frequency velocity-shear instabilities (such as the diocotron mode). This is in contrast to the more familiar unstable behavior in uniform solenoidal magnetic fields. A period-averaged analytic model shows that a PCM-focused beam is stabilized by ponderomotive forces for short PCM periods. Numerical particle simulations for a semi-infinite sheet beam verify this prediction and also indicate diocotron stability for long PCM periods is less constraining than providing for space-charge confinement and trajectory stability in the PCM focusing system. In this article the issue of beam matching and side focusing for sheet beams of finite width is also discussed. A review of past and present theoretical and experimental investigations of sheet-beam transport is presented.
Electromagnetic instability and anomalous resistivity in a magnetic neutral sheet
Authors: Ozaki Masao , Sato Tetsuya , Horiuchi Ritoku ,   the Complexity Simulation Group
Published: July 2002

An electromagnetic instability in a magnetic neutral sheet is investigated by means of a two-and-one-half dimensional (21/2-D) semi-implicit particle simulation code. Electromagnetic waves are excited slowly in a field null region after saturation of the lower hybrid drift waves excited in an early time on both sides of the neutral sheet. This instability is found to be a new instability, independent of the lower hybrid drift instability. Examination of its characteristic properties indicates that the new instability is highly related to the meandering motions of ions in the neutral sheet. The growth of the instability gives rise to anomalous resistivity in the neutral sheet current.