bluuebunny/crossref_metadata_2025 huggingface dataset:
Stochastic particle motion in laminar flows
Authors: Aref Hassan
Published: July 2002

The practical actions referred to as stirring, mixing, blending, and so on rely on our ability to produce stochastic trajectories of particles within given flow fields. There are time-honored ways of achieving stochastic particle motion within a fluid, in particular, by exploiting molecular diffusion and through advection by a turbulent flow. There are, however, also other methods, particularly useful for laminar flows, that rely on the notion of chaos in a low-dimensional dynamical system for their explanation and elucidation. Indeed, the theory of dynamical systems is one of the few places where the term ‘‘mixing’’ has a precise technical meaning. The paper reviews several of these concepts, many of them taken from ergodic theory, and attempts to explain their application to the fluid mechanical problems. All these developments rely on a correspondence between the ‘‘phase space’’ of the general dynamical system, and the real-space motion of the fluid in the stirring/mixing problem. Other aspects of theoretical physics that appear useful are the kinetic theory of Brownian motion and equilibrium statistical mechanics. Again, pertinent concepts, definitions, and results are stated. The theoretical ideas presented offer some measure of general understanding for simple problems of stirring and mixing.
Secondary instabilities in compressible boundary layers
Authors: Ng Lian L. , Erlebacher Gordon
Published: July 2002

The secondary instability mechanisms in a two-dimensional (2-D) compressible boundary layer over a flat plate have been examined for a range of Mach numbers up to 4.5. For subsonic and low supersonic flows, fundamental resonance dominates when the amplitude of the (2-D) primary disturbance is high, while subharmonic resonance prevails in an environment with a low primary disturbance. At a high supersonic Mach number of 4.5, the secondary instability of the second-mode primary is stronger than that of the first-mode primary. Further, the subharmonic and the combination resonance modes, which are slightly detuned from the subharmonic, are the dominant instabilities. The influence of the propagation direction of the primary disturbance on secondary instabilities is investigated at Mach 1.6. Whereas the fundamental and the subharmonic disturbances propagate synchronously with a 2-D primary wave, this is not true for an oblique primary. A subset of the secondary instability results is verified against direct numerical simulations. Some comparisons between spatial and temporal secondary instabilities have been made at Mach 1.6.
Research on pulsed and steady-state plasma guns and their applications in the Troitsk branch of Kurchatov Institute of Atomic Energy
Authors: Skvortsov Yu. V.
Published: July 2002

The parameters of plasma guns developed and plasma streams generated are presented. The streams were used for the studies of plasma dynamics in a magnetic field, for high-temperature plasma production, for plasma injection into open magnetic traps, as well as for industrial applications. The main results in these areas of studies are presented.
Effect of current variation on the dynamics of railgun plasma armatures
Authors: Rolader Glenn E. , Batteh Jad H. , Powell John D.
Published: July 2002

The effect of current variations on the behavior of railgun plasma armatures is investigated with a one-dimensional, time-dependent model. Three typical current profiles are considered: two inductive profiles, one with a time constant of 0.5 msec and the other with a time constant of 2.0 msec, and a capacitive profile. All three calculations are for an armature in a 5 cm square-bore railgun. It is observed that current variations create oscillations in the armature length, projectile acceleration, and projectile jerk. In addition, the current profile significantly affects the axial distribution of the plasma velocity, current density, and total pressure. The significance of these transients on the design of projectiles and the interpretation of diagnostic data is discussed, and suggestions for future calculations are outlined.
Reply to comments of Nordman and Weiland on Horton et al. on ‘‘Ion-temperature-gradient-driven transport in a density modification experiment on the Tokamak Fusion Test Reactor’’ [Phys. Fluids B 4, 953 (1992)]
Authors: Horton W. , Lindberg D. , Kim J. Y. , Dong J. Q. , Hammett G. W. , Scott S. D. , Zarnstorff M. C. , Hamaguchi S.
Published: July 2002

The instability growth rates on the tokamak fusion test reactors are commented upon. Comments by Nordman and Weiland 1,3,4 on mode stability and transport are discussed. (AIP)
Electromagnetic effect on the toroidal ion temperature gradient mode
Authors: Kim J. Y. , Horton W. , Dong J. Q.
Published: July 2002

A systematic study of the electromagnetic effects on the toroidal ion temperature gradient mode is presented using the local and nonlocal theories with the full kinetic terms. For the nonlocal study, a numerical code is developed to solve the electromagnetic gyrokinetic equation in the ballooning space. The electromagnetic coupling to the shear Alfvén mode is shown to give a stabilization of the toroidal temperature gradient mode at almost the same plasma pressure as that at which the kinetically modified magnetohydrodynamic (MHD) ballooning mode becomes destabilized. The transitional β value is shown to be lower in the full kinetic description than in the fluid theory. Possible correlations of these stability results with experimental observations are discussed.
The enhancement of edge turbulence in tokamaks by a limiter current
Authors: Nedospasov A. V.
Published: July 2002

The results indicating that enhanced electrostatic potential fluctuations had been observed in the scrape-off layer, due to biasing, were published recently. The qualitative explanation of these experimental results as an effect of current owing to the flute instability with the dissipation into the sheath layer near the surfaces is presented in this paper. The possibility of controlling the edge turbulence by a limiter current is shown.
Intense relativistic electron-beam steady flows
Authors: Muscato O.
Published: July 2002

A relativistic collisionless electron beam is described through moments on the Vlasov equation. The closure is obtained both in the case of local thermodynamical equilibrium and for a warm plasma. Radial steady solutions are studied in the two closure approaches and compared: In particular, the generalization of the Bennett pinch solution is found in the warm closure.
Gyrofluid turbulence models with kinetic effects
Authors: Dorland W. , Hammett G. W.
Published: July 2002

Nonlinear gyrofluid equations are derived by taking moments of the nonlinear, electrostatic gyrokinetic equation. The principal model presented includes evolution equations for the guiding center n, u∥, T∥, and T⊥ along with an equation expressing the quasineutrality constraint. Additional evolution equations for higher moments are derived that may be used if greater accuracy is desired. The moment hierarchy is closed with a Landau damping model [G. W. Hammett and F. W. Perkins, Phys. Rev. Lett. 64, 3019 (1990)], which is equivalent to a multipole approximation to the plasma dispersion function, extended to include finite Larmor radius effects (FLR). In particular, new dissipative, nonlinear terms are found that model the perpendicular phase mixing of the distribution function along contours of constant electrostatic potential. These ‘‘FLR phase-mixing’’ terms introduce a hyperviscositylike damping ∝k⊥2‖Φkk×k′‖, which should provide a physics-based damping mechanism at high k⊥ρ which is potentially as important as the usual polarization drift nonlinearity. The moments are taken in guiding center space to pick up the correct nonlinear FLR terms and the gyroaveraging of the shear. The equations are solved with a nonlinear, three-dimensional initial value code. Linear results are presented, showing excellent agreement with linear gyrokinetic theory.
Low-n shear Alfvén spectra in axisymmetric toroidal plasmas
Authors: Cheng C. Z. , Chance M. S.
Published: July 2002

In toroidal plasmas, the toroidal magnetic field is nonuniform over a magnetic surface and causes coupling of different poloidal harmonics. It is shown both analytically and numerically that the toroidicity not only breaks up the shear Alfvén continuous spectrum, but also creates new, discrete, toroidicity-induced shear Alfvén eigenmodes with frequencies inside the continuum gaps. Potential applications of the low-n toroidicity-induced shear Alfvén eigenmodes on plasma heating and instabilities are addressed.
Warm relativistic electron fluid. II. Small-m/e limit
Authors: Newcomb William A.
Published: July 2002

It is shown that the generalized equation of state of a warm relativistic electron fluid reduces in the small-m/e limit to the well-known double adiabatic law of collisionless magnetohydrodynamics.
The drag coefficient for a spherical bubble in a uniform streaming flow
Authors: Kang I. S. , Leal L. G.
Published: July 2002

The drag coefficient CD=48/R for a spherical bubble in a uniform streaming flow at high Reynolds number, which was first obtained via a dissipation method by Levich [Zh. Eksp. Teor. Fiz. 19, 18 (1949)], is rederived here by direct integration of the normal stress over the bubble surface. The present study also shows that the drag coefficient up to O(R−1) depends only on the O(1) vorticity distribution right on the bubble surface, and is independent of the vorticity distribution in the fluid. Therefore the drag coefficient up to O(R−1) is completely determined by the irrotational flow solution, which is perfectly consistent with the dissipation method.
Toroidal electron temperature gradient driven drift modes
Authors: Horton W. , Hong B. G. , Tang W. M.
Published: July 2002

The electron temperature gradient in tokamak geometry is shown to drive a short wavelength lower hybrid drift wave turbulence resulting from the unfavorable magnetic curvature on the outside of the torus. Ballooning mode theory is used to determine the stability regimes and the complex eigenfrequencies. At wavelengths of the order of the electron gyroradius, the polarization is electrostatic and the growth rate is greater than the electron transit time around the torus. At longer wavelengths of the order of the collisionless skin depth, the polarization is electromagnetic with electromagnetic vortices producing the dominant transport. The small scale electrostatic component of the turbulence produces a small, of order (me/mi)1/2, drift wave anomalous transport of both the trapped and passing electrons while the c/ωpe scale turbulence produces a neo-Alcator [Nucl. Fusion 25, 1127 (1985)] type transport from the stochastic diffusion of the trapped electrons.
The combined effects of hydrodynamic interactions and Brownian motion on the orientation of particles flowing through fixed beds
Authors: Shaqfeh Eric S. G. , Koch Donald L.
Published: July 2002

In a previous publication [Phys. Fluids 31, 728 (1988)], the evolution of the orientation distribution in a dilute suspension of axisymmetric ‘‘tracer’’ particles flowing through a bed of fixed fibers or spheres was determined under conditions in which hydrodynamic interactions played a dominant role in determining particle orientation. In this paper, the previous results are extended to account for the effects of Brownian motion on the tracer particles and for different configuration statistics of the force in the fixed bed. In the former case, orientational evolution equations are developed and solutions obtained that are valid for large values of the rotary Peclet number (the parameter regime of greatest interest). In the latter case, it is demonstrated that, when the disturbance flow ceases to have fore–aft symmetry, a small orientational drift and dispersion are introduced into the resulting evolution equations. The latter effects are particularly important since they will occur in all fixed beds because of interactions between bed particles.
The structure of (linearly) stable double diffusive flow patterns in a laterally heated stratified liquid
Authors: Kranenborg E. Jurjen , Dijkstra Henk A.
Published: July 2002

Layered double diffusive flow patterns in a laterally heated stably stratified liquid are considered in a configuration which allows for steady states to exist. For the heat/salt system, these flows are characterized by the thermal and solutal Rayleigh numbers RaT and RaS, or equivalently by RaT and the buoyancy ratio Rρ. The bifurcation structure of steady patterns with respect to RaT is computed for two cases: fixed RaS and fixed Rρ. For the first case, results in N. Tsitverblit and E. Kit [Phys. Fluids A 5, 1062 (1993)], are computed and extended, and it is shown that many of the previously found flow patterns are unstable; only in a small interval of RaT, multiple (linearly) stable steady states exist. For the second case, the physical relevance of the unstable steady states with respect to the evolution of the flow toward a stable steady state is demonstrated.
Disturbance growth triggered by steady heating of a jet’s nozzle exit boundary layer
Authors: Cornelius David , Raman Ganesh
Published: July 2002

Experiments were conducted to investigate how heating only the nozzle exit boundary layer of an axisymmetric jet, with an unheated potential core, affects disturbance growth in the initial shear layer. The exit boundary layer had a minimum density ratio of 0.74, was laminar, and had a constant momentum thickness (θ) for all levels of nozzle heating used in this study. The fluctuating velocity (u′) and temperature (t′) in the exit boundary layer increased monotonically with increasing nozzle temperature. Low-amplitude acoustic excitation produced a more rapid growth of coherent velocity fluctuations for the heated case than for the unheated.
Formation producibility and fractional flow curves from radial resistivity variation caused by drilling fluid invasion
Authors: Ramakrishnan T. S. , Wilkinson D. J.
Published: July 2002

In order to characterize conductivity profiles due to drilling fluid invasion into a hydrocarbon formation, a model for radial fluid transport is presented. The model assumes a water-based mud and accounts for the convective movement of oil, water, and salt. A mathematical analysis of the model using the method of characteristics is given. An equivalent graphical construction is also provided. Computations of radial conductivity profiles for specified formation characteristics and total filtrate loss are given. For typical fractional flow curves, it is shown that three qualitatively different profiles may occur depending on the initial water saturation. These results are compared with numerical simulations that include capillary pressure and gravity segregation. Next, the important issue of the inverse problem is analyzed. It is shown that from a single snapshot of the conductivity profile, an exact calculation of filtrate loss and formation fractional flow curve is possible. The calculation is valid provided there is a resistivity contrast between the drilling fluid filtrate and the formation water. For practical application, we show the limits of applicability of this result with complete numerical calculations. Thus, if wireline logs of resistivity variation away from the wellbore are available, formation oil producibility and water cut at native conditions may be estimated.
Onset of convection in a porous layer with net through-flow and internal heat generation
Authors: Khalili A. , Shivakumara I. S.
Published: July 2002

Onset of convection in a horizontal porous layer is investigated including the effects of through-flow and a uniformly distributed internal heat generation for different types of hydrodynamic boundary conditions. The instability parameter is either a Darcy-external or Darcy-internal Rayleigh number which has been determined numerically using Galerkin technique. When the boundary conditions at the top and bottom are not identical, it is found that a small amount of through-flow in one particular direction destabilizes the system in the absence of heat generation. However, in the presence of an internal heat generation, through-flow in one direction destabilizes the system even if the boundary conditions at the two boundaries are of the same type and the destabilization is found to be more with the increase in internal Rayleigh number.
The breakdown of large-scale flows in rotating, stratified fluids
Authors: McWilliams James C. , Yavneh Irad , Cullen Michael J. P. , Gent Peter R.
Published: July 2002

Flows under the influences of environmental rotation and stable density stratification often exhibit an approximate force balance and a consequently slow rate of evolution at large Reynolds number. Such flows are typically anisotropic in their velocity field. This regime is relevant to large-scale motions in the Earth’s atmosphere and ocean, as well as many other planetary and astrophysical systems. The Balance Equations are usually an accurate approximate model for this regime. However, they have solvability limits associated with a change of type in their time-integration operator. In this paper we derive these limiting conditions for the conservative Balance Equations in isentropic coordinates, show that the least familiar of these conditions coincides with loss of convexity of the streamfunction for horizontal velocity in the inertial reference frame, and identify these conditions with the general conditions for symmetric loss of stability for circular and parallel flows as well as for the three-dimensional loss of stability for elliptical flows. We then conjecture that the identified limits of balance coincide generally with the boundary between the distinctive nonlinear dynamical behaviors (i.e., their turbulent cascade and dissipation rates) associated with the large- and small-scale regimes in geophysical and astrophysical flows.
Shock-induced flow resonance in supersonic jets of complex geometry
Authors: Raman Ganesh
Published: July 2002

Jets with complex shock-cell structures exist in numerous technological applications. This paper describes a fundamental study of shock-induced flow resonance (commonly referred to as “jet screech”) in supersonic jets with spanwise nonuniform shock-cell structures. Experiments that involve flow visualization and detailed mapping of the near field reveal unsteady aspects of shock-induced flow resonances, mode transitions, and directivity of the radiated noise. The following important results about the role of spanwise nonuniform shock-cells emerged: (1) It is possible to have two coexisting, independent feedback loops at nonharmonically related frequencies and different spanwise modes. (2) The same type of spanwise asymmetric mode was produced by two entirely different source configurations. (3) Nozzle geometry significantly altered the intensity and directivity of screech and broadband shock noise. The results presented here provide considerable insight into the fluid dynamics and acoustics of jets with spanwise oblique shock-cell structures and provide grounds for believing that shock-induced noise can be controlled by tailoring nozzle geometry.
Self-similar dynamics of a viscous wedge of fluid
Authors: Miksis Michael J. , Vanden-Broeck Jean-Marc
Published: July 2002

The 2D (two dimensional) motion of two symmetric wedges of viscous fluid is determined. The evolution of the fluid is governed by the Stokes equations plus the interfacial boundary conditions including surface tension. A similarity solution is determined and these equations are solved numerically by using a boundary integral method. Solutions for different wedge angles are found.
Bubble size distribution resulting from the breakup of an air cavity injected into a turbulent water jet
Authors: Martı́nez-Bazán Carlos , Montañés José L. , Lasheras Juan C.
Published: July 2002

We investigated experimentally the shape of the final size PDF(D) resulting from the breakup of an air bubble injected into the fully developed region of a high Reynolds number turbulent water jet. It is shown that the PDF(D̂) of the normalized bubble size D̂=D/D32, where D32 is the Sauter mean diameter of the distribution, has a universal single shape independent of the value of the turbulent kinetic energy of the water jet at the bubble injection point and of the air void fraction, α. The shape of the exponential tails characterizing each PDF(D) is shown to be only a function of the initial bubble size D0 and the critical bubble size Dc, defined as Dc=(1.46σ/ρ)3/5ε−2/5, where ε is the value of the dissipation rate of turbulent kinetic energy per unit mass at the air injection point.
Spatially correlated and inhomogeneous random advection
Authors: Ngan K. , Pierrehumbert R. T.
Published: July 2002

We study the equilibrium statistics of an idealized model of random advection. Theoretical results for large-scale homogeneous forcing—notably the shapes of the PDFs—are recovered; they are compared with results for various inhomogeneous and intermittent forcings. The correlation times of the forcings are varied. The PDFs are interpreted in terms of the underlying mixing time scales. Conditional statistics of the tracer field—the conditional diffusion in particular—are computed; they are used to elucidate the influence of small-scale tracer–field correlations. We contrast results for spatially smooth and spatially rough velocity fields.
Solution of the drift-kinetic equation for global plasma modes and finite particle orbit widths
Authors: Porcelli F. , Stankiewicz R. , Kerner W. , Berk H. L.
Published: July 2002

The response of a collisionless plasma to global electromagnetic perturbations of an axisymmetric toroidal equilibrium is derived. By adopting a variational formulation for guiding center motion, the perturbed distribution function is expressed in terms of the linearized guiding center Lagrangian. Finite orbit widths are retained. In particular, the high particle energy limit where mirror-trapped banana orbits are distorted into ‘‘potato-shaped’’ orbits is considered. In this limit, the time scales associated with the drift and bounce motions of a mirror-trapped orbit become comparable, yielding important consequences on plasma stability. Quadratic forms are constructed in the context of kinetic-magnetohydrodynamic (MHD) models of plasmas composed of a thermal component obeying fluid-like equations and a high-energy component described in terms of the collisionless drift-kinetic equation. Relevant applications include improved modeling of energetic ion effects on toroidicity-induced Alfvén gap modes and internal kinks.
The simulation of hard x-ray images obtained during lower hybrid current drive on the Princeton Beta Experiment Modification
Authors: von Goeler S. , Fishman H. , Ignat D. , Jones S. , Roney P. , Stevens J. , Bernabei S. , Davis W. , Kaita R. , Paoletti F. , Petravich G. , Rimini F.
Published: July 2002

During lower hybrid current drive on the Princeton Beta Experiment Modification PBX-M [Bell et al., Phys. Fluids B 2, 1271 (1990)], suprathermal electrons in the 30–150 keV range are generated. These electrons emit hard x-ray bremsstrahlung in collisions with plasma ions; the radiation creates images in a hard x-ray pinhole camera. In order to interpret the hard x-ray images, a computer simulation code has been written; the ‘‘pbxray’’ code, which is described in this paper. The code represents an extension of the Stevens code [Stevens et al., Nucl. Fusion 25, 1529 (1985)], which calculates the free–free and free–bound radiation for non-Maxwellian relativistic electron tail distributions. The pbxray code provides the chord integration in the bean-shaped plasma geometry on the PBX-M tokamak and integrates over photon energy. The simulations show that the location of the suprathermal electrons can be determined with an accuracy of approximately 2 cm in the plasma. In particular, discharges are analyzed whose characteristic ‘‘hollow’’ images indicate off-axis lower hybrid current drive. A comparison of images taken with different absorber foils reveals that the suprathermal electrons have less than 150 keV parallel energy for the hollow discharges.
Deuterium–tritium high confinement (H-mode) studies in the Tokamak Fusion Test Reactor
Authors: Bush C. E. , Sabbagh S. A. , Zweben S. J. , Bell R. E. , Synakowski E. J. , Taylor G. , Batha S. , Bell M. , Bitter M. , Bretz N. L. , Budny R. , Chang Z. , Darrow D. S. , Efthimion P. C. , Ernst D. , Fredrickson E. , Hanson G. R. , Johnson L. C. , Kesner J. , LeBlanc B. , Levinton F. M. , Mansfield D. , Mauel M. E. , Mazzucato E. , McCune D. , Murakami M. , Nazikian R. , Navratil G. A. , Park H. , Paul S. F. , Phillips C. K. , Redi M. H. , Schivell J. , Scott S. D. , Skinner C. H. , Towner H. H. , Wilgen J. B. , Zarnstorff M. C. ,   the TFTR Group
Published: July 2002

High or enhanced confinement (H-mode) plasmas have been obtained for the first time with nearly equal concentrations of deuterium and tritium in high-temperature, high poloidal beta plasmas in the Tokamak Fusion Test Reactor (TFTR) [McGuire, Phys. Plasmas 2, 2176 (1995)]. Tritium fueling was provided mainly through high-power neutral beam injection (NBI) with powers up to 31 MW and beam energies of 90–110 keV. A transition to a circular limiter H-mode configuration has been obtained, following a programmed rapid decrease of the plasma current. Isotope effects, due to the presence of tritium, led to different behavior for deuterium–deuterium (DD) and deuterium–tritium (DT) H-modes relative to confinement, edge localized magnetohydrodynamic modes (ELMs), and ELM effects on fusion products. However, the threshold power for the H-mode transition was the same in DD and DT. Some of the highest values of the global energy confinement time, τE, have been achieved on TFTR during the ELM-free phase of DT H-mode plasmas. Enhancements of τE greater than four times the L-mode have been attained.
Changes of plasma potential induced by ion-beam injection
Authors: Honzawa Tadao , Saito Ryosuke , Saitou Yoshifumi
Published: July 2002

It is experimentally clarified that injection of a stationary ion beam always changes the background plasma potential in a beam–plasma system. In this experiment, a technique of ion energy analysis is adopted to determine the plasma potential in the system. Results show that the plasma potential changes sensitively depending on the beam-to-plasma density ratio, even if other parameters are not allowed to vary. Finally, all the results obtained here are tentatively explained with the help of a Boltzmann relation.
Characteristics of high-confinement modes in Alcator C Mod
Authors: Snipes J. A. , Boivin R. L. , Christensen C. , Fiore C. , Garnier D. , Goetz J. , Golovato S. N. , Graf M. , Granetz R. S. , Greenwald M. , Hubbard A. , Hutchinson I. H. , Irby J. , LaBombard B. , Marmar E. S. , Niemczewski A. , O’Shea P. , Porkolab M. , Stek P. , Takase Y. , Terry J. L. , Umansky M. , Wolfe S. M.
Published: July 2002

The regime of high particle and energy confinement known as the H mode [Phys. Rev. Lett. 49, 1408 (1982)] has been extended to a unique range of operation for divertor tokamaks up to toroidal fields of nearly 8 T, line-averaged electron densities of 3×1020 m−3, and surface power densities of nearly 0.6 MW/m2 in the compact high-field tokamak Alcator C Mod [Phys. Plasmas 1, 1511 (1994)]. H modes are achieved in Alcator C Mod with Ion Cyclotron Resonant Frequency (ICRF) heating and with Ohmic heating alone without boronization of the all molybdenum tiled first wall. Large increases in charge exchange flux are observed during the H mode over the entire range of energies from 2 to 10 keV. There appears to be an upper limit to the midplane neutral pressure, of about 0.08 Pa above which no H modes have been observed. The plasmas with the best energy confinement have the lowest midplane neutral pressures, below 0.01 Pa. There is an edge electron temperature threshold such that Te≥280 eV ±40 eV for sustaining the H mode, which is equal at L–H and H–L transitions. The hysteresis in the threshold power between L–H and H–L transitions is less than 25% on average. Both core and edge particle confinement improve by a factor of 2–4 from L mode to H mode. Energy confinement also improves by up to a factor of 2 over L mode.
Observations of the parametric decay instability of nonlinear magnetohydrodynamic waves
Authors: Spangler Steven R. , Leckband James A. , Cairns Iver H.
Published: July 2002

One of the most important nonlinear processes for Alfvén and fast magnetosonic waves is the decay instability, in which a forward propagating magnetohydrodynamic (MHD) wave is converted into a forward propagating ion acoustic wave and a backward propagating MHD wave. Despite an extensive theoretical literature and numerous computer simulations of the process, there is minimal experimental or observational evidence for its existence. In this paper we report an extensive search for evidence of the decay instability in the MHD wave field upstream of the Earth’s bow shock. Twenty intervals of spacecraft magnetometer and density data with durations between 21 and 168 min were examined. The observational signature of the decay instability sought was a quasi-monochromatic feature in the density power spectrum, attributable to the daughter ion acoustic wave, at a frequency higher than the main wave features in the magnetic power spectra. Such a feature was in fact observed for the interval in which the theoretically predicted instability growth rate was highest, as well as in a second interval for which the instability was permitted with a slower growth rate. However, the data set also contains three long intervals of data in which the “decay line’’ signature is not seen, although theoretically permitted. The decay line is also absent in four shorter intervals in which the plasma β is less than unity, and the instability accordingly facilitated. Possible reasons for the absence of the instability in these intervals are discussed, such as a finite bandwidth for the parent wave field and plasma kinetic effects.
A gyro-Landau-fluid transport model
Authors: Waltz R. E. , Staebler G. M. , Dorland W. , Hammett G. W. , Kotschenreuther M. , Konings J. A.
Published: July 2002

A physically comprehensive and theoretically based transport model tuned to three-dimensional (3-D) ballooning mode gyrokinetic instabilities and gyrofluid nonlinear turbulence simulations is formulated with global and local magnetic shear stabilization and E×B rotational shear stabilization. Taking no fit coefficients from experiment, the model is tested against a large transport profile database with good agreement. This model is capable of describing enhanced core confinement transport barriers in negative central shear discharges based on rotational shear stabilization. The model is used to make ignition projections from relative gyroradius scaling discharges.
Bifurcated states of a rotating tokamak plasma in the presence of a static error-field
Authors: Fitzpatrick Richard
Published: July 2002

The bifurcated states of a rotating tokamak plasma in the presence of a static, resonant, error-field are strongly analogous to the bifurcated states of a conventional induction motor. The two plasma states are the “unreconnected” state, in which the plasma rotates and error-field-driven magnetic reconnection is suppressed, and the “fully reconnected” state, in which the plasma rotation at the rational surface is arrested and driven magnetic reconnection proceeds without hindrance. The response regime of a rotating tokamak plasma in the vicinity of the rational surface to a static, resonant, error-field is determined by three parameters: the normalized plasma viscosity, P, the normalized plasma rotation, Q0, and the normalized plasma resistivity, R. There are 11 distinguishable response regimes. The extents of these regimes are calculated in P–Q0–R space. In addition, an expression for the critical error-field amplitude required to trigger a bifurcation from the “unreconnected” to the “fully reconnected” state is obtained in each regime. The appropriate response regime for low-density, ohmically heated, tokamak plasmas is found to be the nonlinear constant-ψ regime for small tokamaks, and the linear constant-ψ regime for large tokamaks. The critical error-field amplitude required to trigger error-field-driven magnetic reconnection in such plasmas is a rapidly decreasing function of machine size, indicating that particular care may be needed to be taken to reduce resonant error-fields in a reactor-sized tokamak.
Neutral diffusion and anomalous effects on collisional ion flow shear in tokamaks
Authors: Fülöp T. , Catto Peter J. , Helander P.
Published: July 2002

Ion plasma flow and flow shear just inside the last closed flux surface of a tokamak can be strongly altered by neutral atoms and anomalous effects. For a collisional edge, neutrals modify the standard Pfirsch–Schlüter expression for the parallel ion flow through the strong coupling provided by ion–neutral collisions. Even for rather small neutral to plasma density ratios, the large diffusivity of the neutrals in combination with the ion–neutral coupling can directly modify the ion distribution function as well as cause neutral diffusion modifications to the parallel momentum constraint that determines the parallel ion flow. Direct modification of the ion distribution function only dominates at order unity aspect ratios, and was unimportant in an earlier plateau evaluation of the effects of neutrals on ion flow. Like the earlier work, anomalous effects are retained to maintain a steady state and demonstrate that large anomalous transport can alter neoclassical collisional ion flows.
Stochastic heating in field-reversed low pressure discharges
Authors: Kostyukov I. Yu. , Rax J. M.
Published: July 2002

Capacitive and inductive stochastic heating rely on the repeated interaction of electrons with a localized oscillating electromagnetic structure, such as an electric sheath or a magnetic skin depth. The periodic interaction with a third type of localized structure, the zero line of a linear magnetic field, also provides an efficient means to sustain radio frequency low pressure discharges. In order to calculate the stochastic heating rate, the relevant variables are identified and the electron orbits are classified. Then the kinetic theory of collisionless heating in this type of magnetized discharge is developed. The stochasticity threshold and the quasilinear equation are derived to evaluate the collisionless absorption. The analytical scaling of the heating threshold and the collisionless power absorption are expressed and discussed as a function of the discharge parameters.
Isentropic compression experiments on the Sandia Z accelerator
Authors: Hall Clint A.
Published: July 2002

A long-standing goal of the equation of state (EOS) community has been the development of a loading capability for direct measurement of material properties along an isentrope. Previous efforts on smooth bore launchers have been somewhat successful, but quite difficult to accurately reproduce, had pressure limitations, or tended to be a series of small shocks as opposed to a smoothly increasing pressure load. A technique has recently been developed on the Sandia National Laboratories Z accelerator which makes use of the high current densities and magnetic fields available to produce nearly isentropic compression of samples that are approximately 1 mm in thickness over approximately 120 ns. Velocity interferometry is used to measure the rear surface motion of these samples. The resulting time resolved velocity profiles from multiple sample thicknesses provide information about mechanical response under isentropic loading conditions and phase transition kinetics. Feasibility experiments have been performed to pressures of approximately 130 kbar in copper and 300 kbar in iron with effects of the α–ε phase change kinetics in iron clearly observed. Work is in progress to achieve 1%–2% accuracy in P–v space along an isentrope, provide uniaxial strain, and to eliminate magnetic field and current diffusion within the sample of interest.
Dimensionless scaling of the critical beta for onset of a neoclassical tearing mode
Authors: La Haye R. J. , Buttery R. J. , Guenter S. , Huysmans G. T. A. , Maraschek M. , Wilson H. R.
Published: July 2002

The islands from tearing modes driven unstable and sustained by the helically perturbed neoclassical bootstrap current often provide the practical limit to long-pulse, high confinement tokamak operation. The destabilization of such “metastable” plasmas depends on a “seed” island exceeding a threshold. A database from similar regimes [high confinement H-mode with periodic edge localized modes (ELMs) and periodic central sawteeth] was compiled from the tokamaks ASDEX Upgrade (AUG) [Plasma Phys. Controlled Fusion 41, 767 (1999)], DIII-D [Nucl. Fusion 38, 987 (1998)], and JET (Joint European Torus) [Plasma Phys. Controlled Fusion 41, B1 (1999)]. A comparison is made of the measured critical beta for onset of the m/n=3/2 mode (m and n being the poloidal and toroidal Fourier harmonics, respectively) to a model in terms of dimensionless parameters for the seed and threshold islands. This modeling is then used for extrapolation to a reactor-grade tokamak design such as ITER/FDR (International Thermonuclear Experimental Reactor/Final Design Report) [Nucl. Fusion 39, 2137 (1999)]; this indicates that the seed island from sawteeth could be too small to sufficiently disturb the metastable plasma and excite the m/n=3/2 neoclassical tearing mode.
Perpendicular magnetic anisotropy in Pd/Co thin film layered structures
Authors: Carcia P. F. , Meinhaldt A. D. , Suna A.
Published: July 2002

This letter presents the results of magnetic studies of thin film, periodic, Pd/Co layered structures with ultrathin cobalt (4–13 Å). We show that films with Co thicknesses less than 8 Å are easy to magnetize along a direction normal to the film surface. The best films have a saturation magnetization of about 500 emu/cm3 and a coercivity of 550 Oe, and thus they are candidates for a vertical magnetic recording medium. We attribute the perpendicular magnetic easy axis in these films to surface anisotropy at Pd/Co interfaces and strain in thin Co layers.
Evidence for the existence of an ordered state in Ga0.5In0.5P grown by metalorganic vapor phase epitaxy and its relation to band-gap energy
Authors: Gomyo A. , Suzuki T. , Kobayashi K. , Kawata S. , Hino I. , Yuasa T.
Published: July 2002

The band-gap energy (Eg) of metalorganic vapor phase epitaxially (MOVPE) grown Ga0.5In0.5P lattice matched to (001) GaAs is presented as a function of a wide range of V/III ratios and growth temperatures. Photoluminescence, Raman scattering spectroscopy, transmission electron microscopy, and impurity diffusion were used to investigate this functional relationship. Two pieces of evidence are shown which demonstrate that MOVPE Ga0.5In0.5P epitaxial layers with ‘‘abnormal’’ Eg∼1.85 eV and ‘‘normal’’ Eg∼1.9 eV correspond to an ordered and a random (Ga,In) distribution on column III sublattices, respectively. In an ordered state, a sequence of (110) planes...GaGaInInGaGaInIn...in the [110] direction is the most probable distribution.
A recombinant rabies virus expressing vesicular stomatitis virus glycoprotein fails to protect against rabies virus infection
Authors: Foley Heather D. , McGettigan James P. , Siler Catherine A. , Dietzschold Bernhard , Schnell Matthias J.
Published: July 2002

To investigate the importance of the rabies virus (RV) glycoprotein (G) in protection against rabies, we constructed a recombinant RV (rRV) in which the RV G ecto- and transmembrane domains were replaced with the corresponding regions of vesicular stomatitis virus (VSV) glycoprotein (rRV-VSV-G). We were able to recover rRV-VSV-G and found that particle production was equal to rRV. However, the budding of the chimeric virus was delayed and infectious titers were reduced 10-fold compared with the parental rRV strain containing RV G. Biochemical analysis showed equal replication rates of both viruses, and similar amounts of wild-type and chimeric G were present in the respective viral particles. Additional studies were performed to determine whether the immune response against rRV-VSV-G was sufficient to protect against rabies. Mice were primed with rRV or rRV-VSV-G and challenged with a pathogenic strain of RV 12 days later. Similar immune responses against the internal viral proteins of both viruses indicated successful infection. All mice receiving the rRV vaccine survived the challenge, whereas immunization with rRV-VSV-G did not induce protection. The results confirm the crucial role of RV G in an RV vaccine.
Phylogenetic relationships among domesticated and wild species of
Cucurbita
(Cucurbitaceae) inferred from a mitochondrial gene: Implications for crop plant evolution and areas of origin
Authors: Sanjur Oris I. , Piperno Dolores R. , Andres Thomas C. , Wessel-Beaver Linda
Published: July 2002

We have investigated the phylogenetic relationships
among six wild and six domesticated taxa of
Cucurbita
using as a marker an intron region from the mitochondrial
nad1
gene. Our study represents one of the first
successful uses of a mtDNA gene in resolving inter- and intraspecific
taxonomic relationships in Angiosperms and yields several important
insights into the origins of domesticated
Cucurbita
.
First, our data suggest at least six independent domestication events
from distinct wild ancestors. Second,
Cucurbita
argyrosperma
likely was domesticated from a wild Mexican gourd,
Cucurbita sororia
, probably in the same region of
southwest Mexico that gave rise to maize. Third, the wild ancestor of
Cucurbita moschata
is still unknown, but mtDNA data
combined with other sources of information suggest that it will
probably be found in lowland northern South America. Fourth,
Cucurbita andreana
is supported as the wild progenitor
of
Cucurbita maxima
, but humid lowland regions of
Bolivia in addition to warmer temperate zones in South America from
where
C. andreana
was originally described should
possibly be considered as an area of origin for
C.
maxima
. Fifth, our data support other molecular results that
indicate two separate domestications in the
Cucurbita
pepo
complex. The potential zone of domestication for one of
the domesticated subspecies,
C. pepo
subsp.
ovifera
, includes eastern North America and should be
extended to northeastern Mexico. The wild ancestor of the other
domesticated subspecies,
C. pepo
subsp.
pepo
, is undiscovered but is closely related to
C. pepo
subsp.
fraterna
and possibly will
be found in southern Mexico.
Targeted gene expression in transgenic
Xenopus
using the binary Gal4-UAS system
Authors: Hartley Katharine O. , Nutt Stephen L. , Amaya Enrique
Published: July 2002

The transgenic technique in
Xenopus
allows one to misexpress genes in a temporally and spatially controlled manner. However, this system suffers from two experimental limitations. First, the restriction enzyme-mediated integration procedure relies on chromosomal damage, resulting in a percentage of embryos failing to develop normally. Second, every transgenic embryo has unique sites of integration and unique transgene copy number, resulting in variable transgene expression levels and variable phenotypes. For these reasons, we have adapted the Gal4-UAS method for targeted gene expression to
Xenopus
. This technique relies on the generation of transgenic lines that carry “activator” or “effector” constructs. Activator lines express the yeast transcription factor, Gal4, under the control of a desired promoter, whereas effector lines contain DNA-binding motifs for Gal4-(UAS) linked to the gene of interest. We show that on intercrossing of these lines, the effector gene is transcribed in the temporal and spatial manner of the activator's promoter. Furthermore, we use the Gal4-UAS system to misexpress Xvent-2, a transcriptional target of bone morphogenetic protein 4 (BMP4) signaling during early embryogenesis. Embryos inheriting both the Gal4 activator and Xvent-2 effector transgenes display a consistent microcephalic phenotype. Finally, we exploit this system to characterize the neural and mesodermal defects obtained from early misexpression of Xvent-2. These results emphasize the potential of this system for the controlled analyses of gene function in
Xenopus
.
Guiding the study of brain dynamics by using first-person data: Synchrony patterns correlate with ongoing conscious states during a simple visual task
Authors: Lutz Antoine , Lachaux Jean-Philippe , Martinerie Jacques , Varela Francisco J.
Published: July 2002

Even during well-calibrated cognitive tasks, successive brain responses to repeated identical stimulations are highly variable. The source of this variability is believed to reside mainly in fluctuations of the subject's cognitive “context” defined by his/her attentive state, spontaneous thought process, strategy to carry out the task, and so on … As these factors are hard to manipulate precisely, they are usually not controlled, and the variability is discarded by averaging techniques. We combined first-person data and the analysis of neural processes to reduce such noise. We presented the subjects with a three-dimensional illusion and recorded their electrical brain activity and their own report about their cognitive context. Trials were clustered according to these first-person data, and separate dynamical analyses were conducted for each cluster. We found that (
i
) characteristic patterns of endogenous synchrony appeared in frontal electrodes before stimulation. These patterns depended on the degree of preparation and the immediacy of perception as verbally reported. (
ii
) These patterns were stable for several recordings. (
iii
) Preparatory states modulate both the behavioral performance and the evoked and induced synchronous patterns that follow. (
iv
) These results indicated that first-person data can be used to detect and interpret neural processes.
Estrogen up-regulates Bcl-2 and blocks tolerance induction of naïve B cells
Authors: Bynoe Margaret S. , Grimaldi Christine M. , Diamond Betty
Published: July 2002

Sex hormones are presumed to contribute to sexual dimorphism in the immune system. Estrogen, in particular, has been suggested to predispose women to systemic lupus erythematosus. We report here that estradiol (E2) can break B cell tolerance and induce a lupus-like phenotype in nonautoimmune mice transgenic for the heavy chain of a pathogenic anti-DNA antibody. E2treatment resulted in a rise in anti-DNA serum titers and in Ig deposition in renal glomeruli. ELISPOT analysis confirmed a significant increase in the number of high-affinity anti-DNA antibody-secreting B cells in the spleens of E2-treated mice. Hybridomas generated from E2-treated mice express high-affinity, unmutated anti-DNA antibodies, indicating that naïve B cells that are normally deleted or anergized are rescued from tolerance induction. Finally, immunohistochemical studies revealed increased Bcl-2 expression in splenic B cells of E2-treated mice. These data demonstrate that estrogen interferes with tolerance induction of naïve autoreactive B cells and that the presence of these B cells in the periphery is associated with up-regulation of Bcl-2.