bluuebunny/crossref_metadata_2025 huggingface dataset:
A Monte Carlo-quantum mechanics study of the solvatochromic shifts of the lowest transition of benzene
Authors: Coutinho Kaline , Canuto Sylvio , Zerner M. C.
Published: July 2002

We examine the spectroscopic red shifts that occur when benzene is dissolved in (liquid) benzene, in cyclohexane, in carbon tetrachloride, and in water. For this we develop a mixed classical/quantum model in which uncorrelated structures are obtained from Monte Carlo simulation, and these structures are then used for quantum chemical calculations including the chromophore and all solvent molecules within the first radial distribution maxima. We discuss the effects of different sampling techniques and the inclusion of more, or less, solvent molecules in the quantum chemical supermolecule calculation. We obtain shifts of −306 cm−1, −268 cm−1, −456 cm−1, and −122 cm−1, in excellent agreement with the experimentally observed shifts of −332 cm−1, −308 cm−1, −458 cm−1, and −143 cm−1, respectively. We note that the larger shift observed in carbon tetrachloride that is not expected on the basis of larger dielectric constant results from small contributions of the charge transfer type from solvent to solute.
Nonlinear corrections to the Schrödinger equation from geometric quantum mechanics
Authors: Castro Carlos
Published: July 2002

The nonlinear corrections to the Schrödinger equation based on the results of geometric quantum mechanics (GQM) are derived. Such theory derives quantum mechanics from the underlying Weyl space-time geometry associated with the classical ensemble of particle paths given by the solutions of the Hamilton–Jacobi equation.
An analytic solution to the Thomas–Fermi equation
Authors: Laurenzi Bernard J.
Published: July 2002

A perturbative procedure due to Bender et al. (here referred to as the BMPS procedure) [J. Math. Phys. 30, 1447 (1989)] and useful in solving difficult nonlinear problems, has been used here to solve the Thomas–Fermi (T–F) equation. The present work attempts to balance the ease of the ensuing analysis with the use of an analytic, zero-order function that already contains a good deal of the nonlinearity of the T–F equation. The initial slope of the T–F potential is computed with 0.35% error in a second-order application of the theory.
Wavelet spectra compared to Fourier spectra
Authors: Perrier Valérie , Philipovitch Thierry , Basdevant Claude
Published: July 2002

The relation between Fourier spectra and spectra obtained from wavelet analysis is established. Small scale asymptotic analysis shows that the wavelet spectrum is meaningful only when the analyzing wavelet has enough vanishing moments. These results are related to regularity theorems in Besov spaces. For the analysis of infinitely regular signals, a new wavelet, with an infinite number of cancellations is proposed.
Soliton solutions, Liouville integrability and gauge equivalence of Sasa Satsuma equation
Authors: Ghosh Sasanka , Kundu Anjan , Nandy Sudipta
Published: July 2002

Exact integrability of the Sasa Satsuma equation (SSE) in the Liouville sense is established by showing the existence of an infinite set of conservation laws. The explicit form of the conserved quantities in terms of the fields are obtained by solving the Riccati equation for the associated 3×3 Lax operator. The soliton solutions, in particular, one and two soliton solutions, are constructed by the Hirota’s bilinear method. The one soliton solution is also compared with that found through the inverse scattering method. The gauge equivalence of the SSE with a generalized Landau Lifshitz equation is established with the explicit construction of the new equivalent Lax pair.
Optimal cloning of pure states, testing single clones
Authors: Keyl M. , Werner R. F.
Published: July 2002

We consider quantum devices for turning a finite number N of d-level quantum systems in the same unknown pure state σ into M>N systems of the same kind, in an approximation of the M-fold tensor product of the state σ. In a previous paper it was shown that this problem has a unique optimal solution, when the quality of the output is judged by arbitrary measurements, involving also the correlations between the clones. We show in this paper, that if the quality judgment is based solely on measurements of single output clones, there is again a unique optimal cloning device, which coincides with the one found previously.
Stochastic backscatter in a subgrid-scale model: Plane shear mixing layer
Authors: Leith C. E.
Published: July 2002

The traditional Smagorinsky subgrid-scale viscosity (CSλ)2S has been supplemented by the addition of stochastic backscatter. The random acceleration is derived from a vector potential Cb‖S δt‖3/2(λ/δt)2g. Here S is the local strain rate, λ is the grid resolution length scale, δt is the time step, and g is a unit random Gaussian. It is found that values CS=0.2 for the Smagorinsky constant and Cb=0.4 for the backscatter constant give a robust calculation of the two-dimensional shear mixing layer with the observed growth rate and with realistic emergence of random coherent eddy structures.
Theory of H− formation in a weakly collisional plasma
Authors: Brynolf J. , Persson H. , Ågren O.
Published: July 2002

A kinetic plasma model for the formation of negative hydrogen ions is developed, with particular emphasis on conditions prevailing in the NIBLI experiment [Nucl. Instrum. Methods A 243, 255 (1986)]. The overall picture of the plasma in the discharge and values of various parameters have been taken from experimental data, notably a series of electrostatic probe measurements. Physically reasonable electron and ion distributions have been assumed. The magnetic field has a mirror inhomogeneity, and analysis of the quasineutral steady states of isotropic ions with electrons having high perpendicular energy, due to gyroresonance, shows that the plasma is divided into two compartments, one with energetic electrons where vibrationally excited hydrogen molecules can be produced, the other with low-energy electrons where negative ions can be produced and maintained. The mutual interaction between the electric field necessary for quasineutrality and the H− formation from electrons and vibrationally excited hydrogen molecules has been studied. The position is determined as a function of the potential by solving an integrodifferential equation. Quasineutral solutions exist only for a limited range of position and potential, which may be related to transitions between subsonic and supersonic motion. Substantial produced fluxes of 10–100 mA/cm2 of H− are found. The influence of various factors on the flux, the magnitude of the potential, and the size of the structure is discussed, as well as a possible autocatalytic effect from the reaction-induced electric field, and the extraction of the negative ions without simultaneous depletion of the plasma in which they are formed.
Saturation of a single mode driven by an energetic injected beam. II. Electrostatic ‘‘universal’’ destabilization mechanism
Authors: Berk H. L. , Breizman B. N.
Published: July 2002

The formalism to describe the saturation of a discrete mode that is destabilized by hot particles fed by neutral beam injection is extended. The destabilization mechanism described in this work arises from the density gradient in the distribution function formed from a spatially inhomogeneous source. Energetic particles are injected at a fixed speed and collisionally relax through drag and pitch-angle scattering with the background plasma. The distribution formed is solved self-consistently in the presence of a finite amplitude wave in a sheared magnetic field. Three regimes of collisionality are found and the expressions for the nonlinear wave–particle power transfer is determined in each regime. With the dissipation processes of the background plasma given, the wave saturation level is then determined. When pitch-angle scattering is sufficiently weak, particles trapped in a wave convect across the magnetic field as they slow down, a phenomenon similar to the Ware pinch.
Electromagnetic energy density and stress tensor in a warm and collisionless plasma
Authors: Kang Kyeong-Ok , Lee Nam-Cheol
Published: July 2002

Expressions for the electromagnetic energy density and stress tensor for a warm, magnetized, and collisionless plasma are obtained by averaging the two-fluid energy equation over the high-frequency fluctuation. The results are compared to the existing expressions and are shown to be more general, having terms explicitly representing effects of spatiotemporal dispersion and temperature of the plasma.
Pulsed-plasma-based x-ray source and new x-ray optics
Authors: Rudakov L. I. , Baigarin K. A. , Kalinin Y. G. , Korolev V. D. , Kumachov M. A.
Published: July 2002

In this review, the problems under investigation in the I. V. Kurchatov Institute of Atomic Energy are discussed concerning x-ray generation in the range of 1–50 keV by pulsed power and new x-ray capillary optics for x-ray lithography, medical diagnostics, and science. A new x-ray tube driven by pulsed power generator (I>100 kA, V>1 MV, T<10−7 sec) has been demonstrated. Because of the strong magnetic field self-focusing in diode, enhanced Kα emission per atom of anode foil takes place. In experiments with an imploding plasma cylindrical shell (liner), a new approach to x-ray generation was investigated. Liner kinetic energy was used for plasma heating by compression to temperature at which heat transport to a small plate at the end of the liner begins to be important in the energy balance. As a result, a significant part of the liner energy can be converted to line radiation of high-Z plasma. In this way, three-dimensional compression of the energy can be realized and a 1–2 mm size x-ray source in a 1–10 keV range can be created. Together with a lens made from thousands of glass capillaries up to 6 μm inner diameter, in the near future it will be possible to have an x-ray projector with a brightness of 1025 photons/cm2 sec strd in the 20–30 keV range and with divergency of 10−3 rad for high-space resolution diagnostics in science and medicine. The new x-ray capillary optics are based on the principle of total external reflection from the inner glass tube wall. High efficiency of photo transmission along capillaries in the energy range 10–50 keV and capability to bend the capillary and photon flax by an angle of 0.1 rad has been demonstrated.
Lower-hybrid quasimode decay in a plasma cylinder
Authors: Praburam G.
Published: July 2002

In the presence of an azimuthally symmetric lower-hybrid pump wave, a low-frequency density perturbation of frequency ω≊ωci having finite azimuthal mode number, generates a nonlinear current driving a lower-hybrid sideband wave. The sideband couples with the pump to produce a parallel ponderomotive force driving the low-frequency perturbation. For strongly ion cyclotron damped perturbation, the mode structure equation for the sideband wave has been solved using perturbation technique. The growth rate is strongly sensitive to the density and temperature of the plasma.
Flux generation in ultra-low-q tokamak discharges
Authors: Schmid Peter , Barrick Greg , Greene Philip , Robertson Scott
Published: July 2002

Spontaneous generation of toroidal flux is observed in a reversed-field pinch device operated as an ultra-low-q tokamak with a safety factor on axis of 1/4. An initial toroidal bias field of 150 G is increased to 600 G on axis in 300 μsec while the field at the wall is held nearly constant. Sawteeth are observed, which indicate cyclic magnetic reconnection.
Three-dimensional hybrid gyrokinetic-magnetohydrodynamics simulation
Authors: Park W. , Parker S. , Biglari H. , Chance M. , Chen L. , Cheng C. Z. , Hahm T. S. , Lee W. W. , Kulsrud R. , Monticello D. , Sugiyama L. , White R.
Published: July 2002

A three-dimensional (3-D) hybrid gyrokinetic-MHD (magnetohydrodynamic) simulation scheme is presented. To the 3-D toroidal MHD code, MH3D-K the energetic particle component is added as gyrokinetic particles. The resulting code, mh3d-k, is used to study the nonlinear behavior of energetic particle effects in tokamaks, such as the energetic particle stabilization of sawteeth, fishbone oscillations, and alpha-particle-driven toroidal Alfvén eigenmode (TAE) modes.
Double tearing instability with shear flow
Authors: Ofman L.
Published: July 2002

The linear evolution of the double tearing mode with parallel to the magnetic field equilibrium shear flow and viscosity is investigated numerically. Numerically obtained growth rates are found to agree with the solutions of the double tearing dispersion relation in the parameter range of validity. Solutions of the incompressible, time-dependent, linearized, viscoresistive magnetohydrodynamic equations for the double tearing mode with parallel flow are found for wide relevant parameter ranges. Large (weakly coupled) and small (strongly coupled) rational surface separation ys are investigated. The magnetic Reynolds number S is varied up to 108, and ambient flow velocities up to 0.57 of the Alfvén speed VA far from the tearing layer are considered. The normalized wave number α is 0.05 (long wavelength) and 0.5 (short wavelength). Spatial variations of the perturbed magnetic field ψ and flow W indicate the ‘‘nonconstant-ψ’’ effects for small ys. Shear flow decouples the rational surfaces, reduces the growth rate, and transforms the instability to the standard tearing mode. Overstable modes are found from the solutions of the dispersion relation and in the numerical computations, and their frequencies are not affected by the value of viscosity. The temporal oscillations of the solutions increase with the flow at the resonant surfaces at a rate slower than that of the Doppler shift. For viscous Reynolds number Sv comparable to or larger than the magnetic Reynolds number a stabilizing effect was found, and in the presence of large flow the real growth rate γR scaling approaches the standard tearing mode scaling γR∼Sv1/6.
Rotational stabilization of drift tearing modes
Authors: Strauss H. R.
Published: July 2002

Drift tearing modes have a threshold Δ′ for instability, which is increased by sheared poloidal rotation. Sheared rotation becomes important for v′θ≥ω*. The critical Δ′ is approximately linear in ‖v′θ‖. The linear stability of tearing modes is relevant to tokamak edge conditions at the L- to H-mode transition.
Electron-cyclotron heating in the presence of a dc electric field in tokamak plasmas
Authors: Meyer R. L. , Fidone I. , Giruzzi G. , Granata G.
Published: July 2002

The combined effects of the dc electric field and electron-cyclotron wave absorption in tokamak plasmas are investigated. The case of the ordinary mode for nearly normal propagation from the low magnetic field side is studied by a two-dimensional Fokker–Planck code. It is found that the small fraction of the wave energy that goes into the superthermal electrons enhances the effect of the continuous pushing of the tokamak electric field.
Effect of Coulomb collisions on universal mode stability
Authors: Beasley C. O. , van Rij W. I. , Molvig K.
Published: July 2002

The effect of electron–ion Coulomb collisions on the universal mode is studied using a Fokker–Planck collision operator in the kinetic equation for the electron distribution f∥e(x,v∥,t). The expected smoothing of the v∥ structure of f∥e is found, and the character of the mode is little changed when νei→ω*. It is also confirmed that a collisionality on the order of that of present tokamaks is sufficient to provide for destabilization of the mode even in the absence of nonlinear orbit stochastic diffusion.
Observation of helical sawtooth oscillations in a tokamak
Authors: Brickhouse N. S. , Osborne T. H. , Dexter R. N. , Prager S. C.
Published: July 2002

In the Tokapole II tokamak [Nucl. Fusion 19, 1509 (1979)] the three-dimensional structure of the rapid sawtooth crash that accompanies internal disruptions has been observed. The sawtooth crash is helical, i.e., the decrease in soft x-ray intensity occurs along a q=1 helical contour, in accord with the standard reconnection model. The poloidally asymmetric radial expansion of the crash inward from the q=1 surface provides a measure of the reconnection time consistent with single helicity reconnection from a growing, nonrotating magnetic island.
An infrared method for viscous fluids. I. Point vortices and vortex sheets in two dimensions
Authors: Fried H. M.
Published: July 2002

A method of extracting the behavior of velocity/vorticity fields caused by the low-frequency, or infrared (IR), portion of nonlinear interactions in arbitrary spatial dimensions is transcribed from the Schwinger/Fradkin representation of Green’s functions in quantum field theory to problems of viscous Navier–Stokes fluids. The general IR formalism is developed and applied to certain simple, two-dimensional situations involving point vortices and vortex sheets. As an illustration point vortices inserted into a viscous fluid are shown to perform a finite number of revolutions about each other before they dissipate and disappear.
Mixed convection flow over localized multiple thermal sources on a vertical surface
Authors: Jaluria Y.
Published: July 2002

A theoretical study of the effect of an aligned, aiding, externally induced free stream on the buoyancy-induced flow adjacent to a vertical surface, on which isolated thermal sources are located, is carried out. The thermal sources are taken as long planar sources of finite height and the resulting two-dimensional flow is numerically studied. The nature and the basic characteristics of the mixed convection flow that arises are investigated. Of particular interest is the interaction of the wakes generated by the sources and the dependence of this interaction on the thermal energy input and the distance between the sources. The distance beyond which the two sources may be treated as independent is investigated. The governing parameters are varied over wide ranges and their effect on the flow and on the transport mechanisms is studied in detail. The flow over and heat transfer from a thermal source located in the wake of an upstream source are determined. The parametric range for which a boundary-layer treatment is satisfactory is determined by solving the full elliptic equations. Several other important features are obtained and considered in terms of the underlying mechanisms.
Hamiltonian structure and Lyapunov stability of a hyperbolic system of two-phase flow equations including surface tension
Authors: Holm Darryl D. , Kupershmidt Boris A.
Published: July 2002

This paper: (1) provides the Hamiltonian formulation of the hyperbolic system of two-phase flow equations for separated flows including surface tension; (2) shows that equilibrium solutions of this system are critical states of a certain sum of conserved functionals; and (3) determines sufficient conditions for Lyapunov stability of the equilibrium solutions in two dimensions. The method used to establish the Lyapunov stability conditions incorporates recent advances in the Hamiltonian theory of continuum systems.
The magnetic field structure in a modular stellarator
Authors: Doerner R. P. , Anderson D. T. , Anderson F. S. B. , Probert P. H. , Shohet J. L. , Talmadge J. N.
Published: July 2002

The numerically predicted magnetic surface structure within the separatrix of the interchangeable module stellarator (IMS) [IEEE Trans. Plasma Sci. PS-9, 212 (1981)] has been measured experimentally with the use of an electron beam. The results show nested, well-formed surfaces with a rotational transform profile corresponding to the predicted profile. In the area outside the separatrix, regions of localized magnetic flux emergence from the coil volume (modular divertors) have been seen using an electron beam as well as during plasma operation, and correspond to regions of localized particle flux emergence from between the modular coils.
Modulated spontaneous magnetic field in a spherically symmetric laser plasma
Authors: Wenda Shen , Jie Yuan , Shitong Zhu
Published: July 2002

The spontaneous magnetic field in a spherically symmetric laser plasma is discussed and the expression for the magnetic field caused by a transverse electric wave is derived. This solution is contrasted with results for an s-polarized wave in a planar plasma. By means of these theoretical results, the performance of the spontaneous magnetic field observed in recent experiments can be explained.
Detection of low-frequency instability through the satellites of a hydrogenlike atom
Authors: Amini Behrouz
Published: July 2002

The time-dependent Stark effect in a hydrogenlike atom is used to detect a low-frequency (close to the ion plasma frequency) instability in a theta-pinch plasma. Detection required the observation of a sequence of harmonics emitted by the 4686 Å line of Heii. The instability is believed to be a two-stream instability between the ions and electrons which develops during the imploding stage of the theta pinch.
A limit where the equations of magnetohydrodynamics are invariant under conformal mappings
Authors: Jansons Kalvis M.
Published: July 2002

The steady two-dimensional equations of magnetohydrodynamics are preserved under conformal mapping in a limit where the fluid motion is incompressible, inviscid, and irrotational.
Comments on ‘‘Magnetic surface wave instabilities in plasmas’’ [Phys. Fluids 2
8, 3447 (1985)]
Authors: Amendt Peter , Rahman H. U. , Strauss M.
Published: July 2002

It is shown that the Yu–Stenflo [Phys. Fluids 28, 3447 (1985)] treatment of magnetic surface waves in an inertial reference frame is (1) in significant disagreement with Jones’ [Phys. Rev. Lett. 51, 1269 (1983)] surface wave results; (2) does not give rise to a new instability; and (3) is unable, in principle, to recover the accelerating plasma surface magnetic instability [Phys. Rev. Lett. 53, 1226 (1984)].
A set of model cross sections for the Monte Carlo simulation of rarefied real gases: Atom–diatom collisions
Authors: Koura Katsuhisa
Published: July 2002

A set of model cross sections for atom–diatom collisions is presented for use in the Monte Carlo simulation of rarefied real gases and in the master relaxation equation. The elastic and rotationally inelastic cross sections are taken as the variable-soft-sphere and statistical-inelastic-cross-section models, respectively. The vibrationally inelastic and dissociation cross sections are derived by combining the improvement-to-forced-oscillator-impulsive-transfer-semiclassical vibrational transition probability and the weak-vibrational-bias dissociation probability, respectively, with the line-of-centers cross section. The recombination is modeled by the two-step binary collision process with two cross sections for the formation and stabilization of an orbiting pair. Assessment of reality and feasibility of the model cross sections is made by comparing the master-relaxation-equation and Monte Carlo results with available experimental data of the vibrational relaxation time, dissociation rate constant and induction time, and recombination rate constant of oxygen diluted in argon.
Analytic expression for Taylor–Couette stability boundary
Authors: Esser Alexander , Grossmann Siegfried
Published: July 2002

We analyze the mechanism that determines the boundary of stability in Taylor–Couette flow. By simple physical argument we derive an analytic expression to approximate the stability line for all radius ratios and all speed ratios, for co- and counterrotating cylinders. The expression includes viscosity and so generalizes Rayleigh’s criterion. We achieve agreement with linear stability theory and with experiments in the whole parameter space. Explicit formulae are given for limiting cases.
Monte Carlo modeling of nitric oxide formation based on quasi-classical trajectory calculations
Authors: Boyd Iain D. , Bose Deepak , Candler Graham V.
Published: July 2002

A new chemistry model is developed for the direct simulation Monte Carlo method (DSMC). This model explicitly includes separate biasing of the reaction cross section to the translational, rotational, and vibrational energies of each collision. The multiple parameter model is calibrated using detailed information on the formation of nitric oxide based on quasi-classical trajectory calculations. The trajectory analysis provides reaction cross sections, and the energy distributions of reactants and products. In the DSMC approach, product energies are assigned by the widely used Borgnakke–Larsen approach. The performance of the new DSMC model is evaluated in terms of reaction cross sections, energy distributions of reacting molecules, energy distributions of nitric oxide molecules formed in the reaction, and overall reaction rate coefficient. In all cases, the new chemistry model gives favorable agreement with the trajectory calculations. The excellent agreement obtained for product energy distributions indicates that the simple Borgnakke–Larsen energy partitioning scheme is a valid approach for this reaction. The new Monte Carlo chemistry model is applied to a hypersonic, low-density, reacting flow of air. By comparison with a previous chemistry model, the new model predicts significantly higher concentrations of nitric oxide. It is also found that nitric oxide molecules are formed in highly nonequilibrium states. Both of these findings are supported by experimental observations.
Stokes flow in the presence of a planar interface covered with incompressible surfactant
Authors: Bławzdziewicz Jerzy , Cristini Vittorio , Loewenberg Michael
Published: July 2002

The Lorentz solution for Stokes flow in the presence of a plane wall is generalized to a surfactant-covered interface, and the Stokeslet solution is derived. The result is used to describe the motion of a small particle in the presence of the interface. The surfactant is insoluble and nondiffusing. The effects of surface viscosity are included. Small variations in surfactant concentration are assumed; this assumption usually holds under small capillary number conditions.
Direct numerical simulation of bifurcating jets
Authors: Danaila Ionut , Boersma Bendiks Jan
Published: July 2002

The mechanisms leading to bifurcating jets are investigated by means of direct numerical simulation. Two distinct types of jets were obtained by applying a bi-modal perturbation at the nozzle. When forcing simultaneously the counter-rotating helical modes (flapping mode) with the same amplitude and the same frequency, the jet splits into two branches, taking a distinct Y-shape. A different evolution of the jet (Ψ-shape) is observed when superposing the axisymmetric mode at the most amplified unstable frequency on the flapping mode with the same amplitude, but subharmonic frequency. In both cases a spectacular increase of the jet spreading is observed.
Statistics of wind direction and its increments
Authors: van Doorn Eric , Dhruva Brindesh , Sreenivasan Katepalli R. , Cassella Victor
Published: July 2002

We study some elementary statistics of wind direction fluctuations in the atmosphere for a wide range of time scales (10−4 sec to 1 h), and in both vertical and horizontal planes. In the plane parallel to the ground surface, the direction time series consists of two parts: a constant drift due to large weather systems moving with the mean wind speed, and fluctuations about this drift. The statistics of the direction fluctuations show a rough similarity to Brownian motion but depend, in detail, on the wind speed. This dependence manifests itself quite clearly in the statistics of wind-direction increments over various intervals of time. These increments are intermittent during periods of low wind speeds but Gaussian-like during periods of high wind speeds.
Hybrid magnetohydrodynamic-gyrokinetic simulation of toroidal Alfvén modes
Authors: Briguglio S. , Vlad G. , Zonca F. , Kar C.
Published: July 2002

Resonant energetic particles play a major role in determining the stability of toroidal Alfvén eigenmodes (TAE’s) by yielding the well-known driving mechanism for the instability and by producing an effective dissipation, which removes the singular character of local oscillations of the shear-Alfvén continuum and gives discrete kinetic Alfvén waves (KAW’s). Toroidal coupling of two counterpropagating KAW’s generates the kinetic analog of the TAE, the KTAE (kinetic TAE). The nonperturbative character of this phenomenon and of the coupling between TAE and KAW’s, and the relevance of finite drift-orbit effects limit the effectiveness of the analytical approach to asymptotic regimes, which are difficult to compare with realistic situations. A three-dimensional hybrid fluid-particle initial-value code for the numerical simulation of the linear and nonlinear evolution of toroidal modes of the Alfvén branch has been developed. It is shown that for typical parameters the KTAE is, indeed, more unstable than the TAE.
Lattice waves in dust plasma crystals
Authors: Melandso/ Frank
Published: July 2002

Techniques previously known from solid state physics are used to look at linear and weak non-linear wave propagation in dust lattices. These expansion techniques include only electrostatic interactions between neighbor particles in addition to assuming small vibrations in the dust lattice. As a simple model for the dust lattice, a one-dimensional Bravais lattice is considered. For this particular lattice, expressions for the linear phase velocity are compared to a quasi-particle simulation. The word quasi here means that only the dust particles are represented as diffuse objects, while the plasma is treated as a fluid. The simulation is also used to study the breakdown of the analytical theory and to investigate non-linear dust lattice waves. A very good agreement is found between the analytical expressions and the particle simulations, for cases where the average dust separation a is of the order of or larger than the plasma Debye length λD. This is a condition which very often applies to dust crystal in laboratory experiments. Application of this wave theory is therefore discussed with respect to recent laboratory experiments where dust lattice waves are excited.
Beta limits in long-pulse tokamak discharges
Authors: Sauter O. , La Haye R. J. , Chang Z. , Gates D. A. , Kamada Y. , Zohm H. , Bondeson A. , Boucher D. , Callen J. D. , Chu M. S. , Gianakon T. A. , Gruber O. , Harvey R. W. , Hegna C. C. , Lao L. L. , Monticello D. A. , Perkins F. , Pletzer A. , Reiman A. H. , Rosenbluth M. , Strait E. J. , Taylor T. S. , Turnbull A. D. , Waelbroeck F. , Wesley J. C. , Wilson H. R. , Yoshino R.
Published: July 2002

The maximum normalized beta achieved in long-pulse tokamak discharges at low collisionality falls significantly below both that observed in short pulse discharges and that predicted by the ideal MHD theory. Recent long-pulse experiments, in particular those simulating the International Thermonuclear Experimental Reactor (ITER) [M. Rosenbluth et al., Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1995), Vol. 2, p. 517] scenarios with low collisionality νe*, are often limited by low-m/n nonideal magnetohydrodynamic (MHD) modes. The effect of saturated MHD modes is a reduction of the confinement time by 10%–20%, depending on the island size and location, and can lead to a disruption. Recent theories on neoclassical destabilization of tearing modes, including the effects of a perturbed helical bootstrap current, are successful in explaining the qualitative behavior of the resistive modes and recent results are consistent with the size of the saturated islands. Also, a strong correlation is observed between the onset of these low-m/n modes with sawteeth, edge localized modes (ELM), or fishbone events, consistent with the seed island required by the theory. We will focus on a quantitative comparison between both the conventional resistive and neoclassical theories, and the experimental results of several machines, which have all observed these low-m/n nonideal modes. This enables us to single out the key issues in projecting the long-pulse beta limits of ITER-size tokamaks and also to discuss possible plasma control methods that can increase the soft β limit, decrease the seed perturbations, and/or diminish the effects on confinement.
Plasma transport near the separatrix of a magnetic island
Authors: Hazeltine R. D. , Helander P. , Catto Peter J.
Published: July 2002

The simplest nontrivial model of transport across a magnetic island chain in the presence of collisionless streaming along the magnetic field is solved by a Wiener–Hopf procedure. The solution found is valid provided the boundary layer about the island separatrix is narrow compared to the island width. The result demonstrates that when this assumption is satisfied the flattened profile region is reduced by the boundary layer width. The calculation is similar to the recent work by Fitzpatrick [Phys. Plasmas 2, 825 (1995)] but is carried out in the collisionless, rather than the collisional, limit of parallel transport, and determines the plasma parameters on the separatrix self-consistently.
Linear and nonlinear particle-magnetohydrodynamic simulations of the toroidal Alfvén eigenmode
Authors: Todo Y. , Sato T.
Published: July 2002

Linear and nonlinear particle-magnetohydrodynamic (MHD) simulation codes are developed to study interactions between energetic ions and MHD modes. Energetic alpha particles with the slowing-down distribution are considered and the behavior of n=2 toroidal Alfvén eigenmodes (TAE modes) is investigated with the parameters pertinent to the present large tokamaks. The linear simulation reveals the resonance condition between alpha particles and TAE mode. In the nonlinear simulation, two n=2 TAE modes are destabilized and alpha particle losses induced thereby are observed. Counterpassing particles are lost when they cross the passing-trapped boundary. They are the major part of lost particles, but trapped particles are also lost appreciably.
Magnetohydrodynamic flow physics of magnetically nozzled plasma accelerators with applications to advanced manufacturing
Authors: Schoenberg Kurt F. , Gerwin Richard A. , Moses Ronald W. , Scheuer Jay T. , Wagner Henri P.
Published: July 2002

The coaxial plasma accelerator is a simple, compact, and mechanically robust device that utilizes the Lorentz J×B force to accelerate plasma to high velocity. Originally developed in the 1950s for the purpose of providing energetic plasmas for fusion energy experiments, coaxial plasma accelerators are presently being investigated as an environmentally sound and economical means of materials processing and advanced manufacturing. While commercial applications of this technology are already on line, future commercial applications will require improving accelerator reproducibility and efficiency, better controlling the accelerated plasma flow velocity or energy, and better controlling the distribution of directed energy or power on target. In this paper, the magnetohydrodynamic flow physics of magnetically nozzled plasma accelerators is presented with a view to achieving the accelerator control necessary for future industrial applications. Included is a fundamental description of plasma production, acceleration, and flow in a magnetic nozzle.
Hybrid magnetohydrodynamic-particle simulation of linear and nonlinear evolution of Alfvén modes in tokamaks
Authors: Briguglio S. , Zonca F. , Vlad G.
Published: July 2002

Linear and nonlinear properties of moderate-toroidal-number (n) shear-Alfvén modes in tokamaks are investigated by using a hybrid MHD-particle simulation code, which solves the coupled set of MHD (magnetohydrodynamic) equations for the electromagnetic fields and gyrocenter Vlasov equation for a population of energetic ions. The existence of unstable toroidal Alfvén eigenmodes (TAE’s) and their kinetic counterpart is shown for low values of the energetic-ion pressure gradient. Above a certain threshold value, the energetic particle continuum mode (EPM) is destabilized, with growth rate fast increasing with increasing energetic-particle pressure gradient. The threshold shows an inverse dependence on n. High-n EPM’s could then be unstable in realistic plasma conditions. Neglecting MHD nonlinearities, for the sake of simplicity, it is shown that nonlinear TAE saturation appears to be due to the trapping of resonant energetic ions in the potential well of the wave. Saturation of the EPM occurs instead because of a macroscopic outward displacement of the energetic-ion population, with potentially dramatic consequences on α-particle confinement; such conclusions are not modified by the inclusion of MHD nonlinearities.
A unified model of acoustic and lattice waves in a one-dimensional strongly coupled dusty plasma
Authors: Otani Niels F. , Bhattacharjee A. , Wang Xiaogang
Published: July 2002

An exact dispersion relation is obtained for linear dust-compressional waves in a one-dimensional plasma crystal consisting of cold dust particles located at lattice points uniformly spaced in equilibrium. This general dispersion relation reduces in asymptotic limits to the dust-acoustic and dust-lattice wave dispersion relations. Implications for experiments are discussed.
Two-dimensional electric fields and drifts near the magnetic separatrix in divertor tokamaks
Authors: Rognlien T. D. , Ryutov D. D. , Mattor N. , Porter G. D.
Published: July 2002

A two-dimensional calculation is presented for the transport of plasma in the edge region of a divertor tokamak solving continuity, momentum, and energy balance fluid equations. The model uses classical processes of parallel transport along the magnetic field and cross-field drifts together with anomalous radial diffusion, including perpendicular ion viscosity. The self-consistent electrostatic potential is calculated on both sides of the magnetic separatrix via quasineutrality and current continuity. Outside the separatrix, the model extends to material divertor plates where the incident plasma is recycled as neutral gas and where the plate sheath and parallel currents dominate the potential structure. Inside the separatrix, various radial current terms—from anomalous viscosity, collisional damping, inertia, and ∇B drifts—contribute to determining the potential. The model rigorously enforces cancellation of gyroviscous and magnetization terms from the transport equations. The results emphasize the importance of E×B particle flow under the X-point which depends on the sign of the toroidal magnetic field. Radial electric field profiles at the outer midplane show strong variation with the magnitude of the anomalous diffusion coefficients and the core toroidal rotation velocity, indicating that shear stabilization of edge turbulence can likewise be sensitive to these parameters.
On the electron energy distribution function in a Hall-type thruster
Authors: Fedotov V. Yu. , Ivanov A. A. , Guerrini G. , Vesselovzorov A. N. , Bacal M.
Published: July 2002

The electron energy distribution function measured at the channel exit of a small Hall-type thruster is interpreted as a beam–plasma electron distribution function with electron beam energy of several tens of electronvolts. Though unstable, the distribution function does not correspond to the completely quasilinear relaxed beam function. Comparison with the measured function in the vicinity of the channel exit shows that the electron beam energy is about 30 eV for the total voltage applied ∼200 V. The contribution of the electron beam is shown to be essential in the ionization of the working gas, even when only the single ionization processes are considered.
Transverse electrostatic modes in a one-dimensional strongly coupled dusty plasma
Authors: Wang Xiaogang , Bhattacharjee A.
Published: July 2002

A unified dispersion relation is derived for linear transverse electrostatic modes in a one-dimensional dust plasma crystal consisting of cold dust particles located at lattice points uniformly spaced in equilibrium. In special limits, this dispersion relation reduces to the transverse shear and lattice modes predicted by earlier generalized hydrodynamic and lattice models in the strong-coupling regime.
Disruption of estrogen receptor β gene impairs spatial learning in female mice
Authors: Rissman Emilie F. , Heck Amy L. , Leonard Julie E. , Shupnik Margaret A. , Gustafsson Jan-Åke
Published: July 2002

Here we provide the first evidence, to our knowledge, that estradiol (E
2
) affects learning and memory via the newly discovered estrogen receptor β (ERβ). In this study, ERβ knockout (ERβKO) and wild-type littermates were tested for spatial learning in the Morris water maze after ovariectomy, appropriate control treatment, or one of two physiological doses of E
2
. Regardless of treatment, all wild-type females displayed significant learning. However, ERβKOs given the low dose of E
2
were delayed in learning acquisition, and ERβKOs administered the higher dose of E
2
failed to learn the task. These data show that ERβ is required for optimal spatial learning and may have implications for hormone replacement therapy in women.
Genome-wide analysis of clustered Dorsal binding sites identifies putative target genes in the
Drosophila
embryo
Authors: Markstein Michele , Markstein Peter , Markstein Vicky , Levine Michael S.
Published: July 2002

Metazoan genomes contain vast tracts of cis-regulatory DNA that have been identified typically through tedious functional assays. As a result, it has not been possible to uncover a cis-regulatory code that links primary DNA sequences to gene expression patterns. In an initial effort to determine whether coordinately regulated genes share a common “grammar,” we have examined the distribution of Dorsal recognition sequences in the
Drosophila
genome. Dorsal is one of the best-characterized sequence-specific transcription factors in
Drosophila
. The homeobox gene zerknullt (
zen
) is repressed directly by Dorsal, and this repression is mediated by a 600-bp silencer, the ventral repression element (VRE), which contains four optimal Dorsal binding sites. The arrangement and sequence of the Dorsal recognition sequences in the VRE were used to develop a computational algorithm to search the
Drosophila
genome for clusters of optimal Dorsal binding sites. There are 15 regions in the genome that contain three or more optimal sites within a span of 400 bp or less. Three of these regions are associated with known Dorsal target genes:
sog
,
zen
, and
Brinker
. The Dorsal binding cluster in
sog
is shown to mediate lateral stripes of gene expression in response to low levels of the Dorsal gradient. Two of the remaining 12 clusters are shown to be associated with genes that exhibit asymmetric patterns of expression across the dorsoventral axis. These results suggest that bioinformatics can be used to identify novel target genes and associated regulatory DNAs in a gene network.
A small RNA regulates the expression of genes involved in iron metabolism in
Escherichia
coli
Authors: Massé Eric , Gottesman Susan
Published: July 2002

A small RNA, RyhB, was found as part of a genomewide search for novel small RNAs in
Escherichia coli
. The RyhB 90-nt RNA down-regulates a set of iron-storage and iron-using proteins when iron is limiting; it is itself negatively regulated by the ferric uptake repressor protein, Fur (
F
erric
u
ptake
r
egulator). RyhB RNA levels are inversely correlated with mRNA levels for the
sdhCDAB
operon, encoding succinate dehydrogenase, as well as five other genes previously shown to be positively regulated by Fur by an unknown mechanism. These include two other genes encoding enzymes in the tricarboxylic acid cycle,
acnA
and
fumA
, two ferritin genes,
ftnA
and
bfr
, and a gene for superoxide dismutase,
sodB
. Fur positive regulation of all these genes is fully reversed in an
ryhB
mutant. Our results explain the previously observed inability of
fur
mutants to grow on succinate. RyhB requires the RNA-binding protein, Hfq, for activity. Sequences within RyhB are complementary to regions within each of the target genes, suggesting that RyhB acts as an antisense RNA. In
sdhCDAB
, the complementary region is at the end of the first gene of the
sdhCDAB
operon; full-length
sdhCDAB
message disappears and a truncated message, equivalent in size to the region upstream of the complementarity, is detected when RyhB is expressed. RyhB provides a mechanism for the cell to down-regulate iron-storage proteins and nonessential ironcontaining proteins when iron is limiting, thus modulating intracellular iron usage to supplement mechanisms for iron uptake directly regulated by Fur.
Redundancy, antiredundancy, and the robustness of genomes
Authors: Krakauer David C. , Plotkin Joshua B.
Published: July 2002

Genetic mutations that lead to undetectable or minimal changes in phenotypes are said to reveal redundant functions. Redundancy is common among phenotypes of higher organisms that experience low mutation rates and small population sizes. Redundancy is less common among organisms with high mutation rates and large populations, or among the rapidly dividing cells of multicellular organisms. In these cases, one even observes the opposite tendency: a hypersensitivity to mutation, which we refer to as antiredundancy. In this paper we analyze the evolutionary dynamics of redundancy and antiredundancy. Assuming a cost of redundancy, we find that large populations will evolve antiredundant mechanisms for removing mutants and thereby bolster the robustness of wild-type genomes; whereas small populations will evolve redundancy to ensure that all individuals have a high chance of survival. We propose that antiredundancy is as important for developmental robustness as redundancy, and is an essential mechanism for ensuring tissue-level stability in complex multicellular organisms. We suggest that antiredundancy deserves greater attention in relation to cancer, mitochondrial disease, and virus infection.
Solution structure of the antiapoptotic protein bcl-2
Authors: Petros Andrew M. , Medek Ales , Nettesheim David G. , Kim Daniel H. , Yoon Ho Sup , Swift Kerry , Matayoshi Edmund D. , Oltersdorf Tilman , Fesik Stephen W.
Published: July 2002

The structures of two isoforms of Bcl-2 that differ by two amino
acids have been determined by NMR spectroscopy. Because wild-type Bcl-2
behaved poorly in solution, the structures were determined by using
Bcl-2/Bcl-x
L
chimeras in which part of the putative
unstructured loop of Bcl-2 was replaced with a shortened loop from
Bcl-x
L
. These chimeric proteins have a low pI compared with
the wild-type protein and are soluble. The structures of the two Bcl-2
isoforms consist of 6 α-helices with a hydrophobic groove on the
surface similar to that observed for the homologous protein,
Bcl-x
L
. Comparison of the Bcl-2 structures to that of
Bcl-x
L
shows that although the overall fold is the same,
there are differences in the structural topology and electrostatic
potential of the binding groove. Although the structures of the two
isoforms of Bcl-2 are virtually identical, differences were observed in
the ability of the proteins to bind to a 25-residue peptide from the
proapoptotic Bad protein and a 16-residue peptide from the
proapoptotic Bak protein. These results suggest that there are
subtle differences in the hydrophobic binding groove in Bcl-2 that may
translate into differences in antiapoptotic activity for the
two isoforms.