bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic surface breaking in three-dimensional l=2 heliotron/torsatron equilibria
Authors: Hayashi Takaya , Takei Akira , Sato Tetsuya
Published: July 2002

Magnetic surface breaking due to finite pressure effect is analyzed for three-dimensional magnetohydrodynamic equilibria of l=2 heliotron/torsatron configuration. Characteristics of magnetic island formation are numerically studied in detail. Main results are as follows: (i) There exists a clear-cut boundary, which practically determines the position of the outermost closed surface. The boundary position shifts as beta increases, though not affected by the external axisymmetric fields. (ii) The clear-cut boundary is maintained for a higher beta plasma as the aspect ratio of the configuration increases. For a low aspect ratio configuration, however, high beta equilibria with clear surfaces can be obtained by controlling the external poloidal field, especially the vertical field. (iii) The external l=1 field is particularly viable in controlling the breaking of magnetic surfaces.
Drift wave vortices in nonuniform plasmas with sheared magnetic fields
Authors: Su X. N. , Horton W. , Morrison P. J.
Published: July 2002

Nonlinear coherent structures governed by the coupled drift wave–ion-acoustic mode equations in nonuniform plasmas with sheared magnetic fields are studied analytically and numerically. A solitary vortex equation that includes the effects of density and temperature gradients and magnetic shear is derived and analyzed. The analytic and numerical studies show that for a plasma in a sheared magnetic field, even without the temperature and drift velocity gradients, solitary vortex solutions are possible; however, these solutions are not exponentially localized due to the presence of a nonstructurally stable perturbative tail that connects to the core of the vortex. The new coherent vortex structures are dipolelike in their symmetry, but are not the modons of Larichev and Reznik. In the presence of a small temperature or drift velocity gradient, the new shear-induced dipole cannot survive and will separate into monopoles, like the case of the modon in a sheared drift velocity as studied in Su et al. [Phys. Fluids B 3, 921 (1991)]. The solitary solutions are found from the nonlinear eigenvalue problem for the effective potential in a quasi-one-dimensional approximation. The numerical simulations are performed in two dimensions with the coupled vorticity and parallel mass flow equations.
Theory and simulation of stimulated Raman scatter at near-forward angles
Authors: Wilks S. C. , Kruer W. L. , Estabrook K. , Langdon A. B.
Published: July 2002

Computer simulations in two dimensions of Raman near-forward scatter are presented to show the linear and nonlinear behavior of this instability in a high-temperature, low-density plasma. Modifications to one-dimensional theory are discussed, as is the angular dependence of the spatial growth rate. In the simulations, the amount of laser energy scattered into various angles is examined and the subsequent heating of the electrons is discussed. Results with mobile ions are presented as well. It is found that, for the strongly driven parameter regime examined, some stimulated Brillouin scattering is observed early in time until the ions heat up and quench the instability. However, the ion motion does not substantially change the level of stimulated Raman scattered light in these simulations.
Backscattered spectra for stimulated Raman scattering in an inhomogeneous plasma
Authors: Bonnaud Guy , Pesme Denis , Pellat René
Published: July 2002

Using one-dimensional wave-coupling simulations, Raman scattering is investigated in a laser-irradiated plasma with a linear density profile. The quarter-critical density appears to have a dominant role in the Raman spectrum. In the case of no density modulation, a spectral broadening is observed and found to result from a coupling between the convective and absolute Raman, via pump depletion. The modulation of the plasma density by Raman and Brillouin driven ion waves appears to give rise to a multiple blue-shifted peak spectrum.
Parametric excitation of electromagnetic waves by electron Bernstein waves
Authors: Kuo S. P.
Published: July 2002

A parametric instability involving the decay of a standing electron Bernstein pump into electromagnetic sidebands and lower-hybrid decay waves is studied. A general dispersion relation is derived and analyzed. Threshold fields and growth rates are obtained for the two cases that the electron Bernstein pump is introduced near the X-mode cutoff layer or introduced in the region between the upper-hybrid resonance layer and the O-mode cutoff layer. Applications of these results to the recent observation [P. Stubbe and H. Kopka, Phys. Rev. Lett. 65, 183 (1990)] of stimulated electromagnetic emission (SEE) with a broad symmetrical structure (BSS) in the ionospheric modifications by powerful high-frequency (HF) wave are discussed.
Electron beam radiation by collective Compton boosting of strong turbulence
Authors: Benford Gregory , Weatherall James C.
Published: July 2002

Several strong relativistic beam–plasma experiments have found powerful broadband microwave emission far above the plasma line. To model these experiments, consider scattering of electrons by cavitons of plasma turbulence of size D. This yields radiation Compton boosted to frequencies ∼2γ2c/D and relativistically beamed. To attain the observed powers (up to 108 W) the beam must be bunched. Several models for beam density fluctuation statistics can give the observed power, but only broad distributions, such as a Gaussian electrostatic spectrum in k, can yield a qualitatively wide radiation spectrum. Power scalings can be checked in future experiments.
Resistive instability of periodic current sheets
Authors: Otto A. , Birk G. T.
Published: July 2002

The dispersion relation of the resistive tearing instability is derived and numerically solved for configurations characterized by periodic current sheets where a parameter-dependent resistivity can be included. Two unstable modes of different symmetry properties and different growth rates are identified, where the growth rates depend on the distance between the current layers. The results are relevant to a number of space and laboratory plasmas and the corresponding numerical simulations.
Self-similar expansion of dusts in a plasma
Authors: Luo H. , Yu M. Y.
Published: July 2002

The self-similar expansion of two species of dust particles in an equilibrium plasma is investigated by means of fluid as well as Vlasov theories. It is found that under certain conditions the density of the dust with the smaller charge-to-mass ratio can vanish at a finite value of the self-similar variable, while the density of the remaining dust species attains a plateau. The kinetic theory predicts a secondary decay in which the latter density eventually also vanishes.
Measurements in a nitrogen plasma boundary layer: The boundary conditions of charged species
Authors: Robin L. , Cheron B. G. , Vervisch P.
Published: July 2002

The recombination of the major charged species on a metallic wall has been analyzed in a low-pressure and highly dissociated nitrogen plasma jet. The electron temperature and density in the boundary layer have been measured by electrostatic probes. Information about the temperature and density of some excited species has been deduced from emission spectroscopic measurements. It is shown that the metallic surface is not completely catalytic. The recombination rate has been determined and follows a 0.5 reaction order near the leading edge. The experimental results show that the Bohm criterion overestimates the value of the ion drift velocity at the sheath entrance and confirm the recent value proposed by Zawaideh [Phys. Fluids B 2, 647 (1990)]. At last, a strong enhancement of the N2+(B) emission has been observed near the wall.
Small-scale magnetic islands in collisionless plasmas
Authors: Smolyakov A. I. , Hirose A.
Published: July 2002

The existence of small-scale magnetic islands having a characteristic width of the order of the skin depth is discussed in terms of nonlinear hydrodynamic equations for a nonuniform plasma in a sheared magnetic field. A resultant electron thermal diffusivity has sensitive dependence on the electron temperature gradient, ηe.
Sheath dynamics induced by ion-acoustic rarefaction wave
Authors: Murakami M. , Nishihara K.
Published: July 2002

A simple analytical model is presented to describe the sheath dynamics, when a stationary slab plasma with a uniform density n0 faces a negatively biased wall with a voltage φ0. The dynamics strongly depends on the sheath thickness s. If s is less than a well-defined distance se, which is given by n0, φ0, and the electron temperature Te, then the sheath front advances supersonically. On the contrary, if s is greater than se, the sheath front moves subsonically, and a rarefaction wave is launched ahead of the sheath. In this case, the ion sheath and the rarefaction flow are continuously matched throughout the process. The model is applied to evaluate the ion collection time, and gives a good agreement with experiments over a wide region of plasma density. Rapid heating of electrons up to temperatures in the order of 10 eV is required for applications to atomic vapor laser isotope separation (AVLIS).
Transient ion resonance instability
Authors: Fajans J.
Published: July 2002

A trapped pure electron plasma column can be unstable if the plasma column is contaminated by a small number of oppositely charged ions. This instability was analyzed by Levy, Daugherty, and Buneman [Phys. Fluids, 12, 2616 (1969)] for the case of trapped ions. The instability is analyzed here for the case of untrapped ions. The analysis is inherently nonlinear, and growth results from averaging over the ion initial conditions. Several significant differences exist between the two cases, including, for the latter, growth over a much broader parameter region and linear (secular) rather than exponential growth. The predictions of the analysis are compared to the results from a recent experiment on the instability.
A Fokker–Planck operator for the emission and absorption of electron plasma waves in a magnetized plasma
Authors: Ware Alan A.
Published: July 2002

For slab geometry the perturbation of the electrostatic wake of a superthermal test electron in a magnetized plasma (ωce ≫ ωpe) due to moderate magnetic shear is determined. Allowing for the spherical symmetry of the surfaces of constant phase to the rear of the test electron, the ‘‘resonant’’ field electrons causing the damping of the wave in a magnetic surface at a distance x from the test electron are those with parallel velocity v′∥ = v∥cosβ/cos(β + γ). Here β is the angle between the emitted ray and B(0), γ is the angle between B(0) and B(x) and v∥ is the velocity of the test electron. As a result the damping in the WKB approximation for the wave emission is a function of both the angle of emission and γ. A Fokker–Planck equation is derived for the rate of change of the electron distribution function (f) due to the emission and absorption of the waves under these conditions; f is assumed approximately Maxwellian for v∥ ≲ vT but with an arbitrary tail for v∥ ≳ vT.
Investigation of ballooning modes in high poloidal beta plasmas in the Tokamak Fusion Test Reactor*
Authors: Nagayama Y. , Yamada M. , Sabbagh S. A. , Fredrickson E. D. , Manickam J. , Bell M. , Budny R. V. , Cavallo A. , Janos A. C. , Mauel M. E. , McGuire K. M. , Navratil G. A. , Taylor G.
Published: July 2002

Medium-n (toroidal mode number) instabilities with ballooning characteristics were investigated using x-ray and electron-cyclotron emission (ECE) imaging techniques in the Tokamak Fusion Test Reactor (TFTR) [R. J. Hawryluk et al., Plasma Phys. Controlled Fusion 33, 1509 (1991)]. The poloidal mode numbers were determined by fitting the data to soft x-ray signals simulated with a wave-packet model, and the toroidal mode numbers were determined from the phase difference between the soft x-ray and ECE signals. The modes are identified as ideal ballooning modes from the observation that the identified mode numbers are much higher than that of the usual kink or tearing modes, the mode has a strong ballooning characteristic, the growth rate is consistent with the theoretical prediction, and the equilibrium is theoretically predicted to be marginal to the infinite n ballooning mode. The ballooning mode accompanies a β collapse which happens near the Troyon limit in TFTR. The ballooning mode has also been observed just before a major disruption in TFTR.
Helium, iron, and electron particle transport and energy transport studies on the Tokamak Fusion Test Reactor
Authors: Synakowski E. J. , Efthimion P. C. , Rewoldt G. , Stratton B. C. , Tang W. M. , Grek B. , Hill K. W. , Hulse R. A. , Johnson D. W. , Kissick M. W. , Mansfield D. K. , McCune D. , Mikkelsen D. R. , Park H. K. , Ramsey A. T. , Redi M. H. , Scott S. D. , Taylor G. , Timberlake J. , Zarnstorff M. C.
Published: July 2002

Results from helium, iron, and electron transport studies on the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Nucl. Fusion Res. 26, 11 (1984)] in L-mode and supershot deuterium plasmas with the same toroidal field, plasma current, and neutral beam heating power are presented. They are compared to results from thermal transport analysis based on power balance. Particle diffusivities and thermal conductivities are radially hollow and larger than neoclassical values, except possibly near the magnetic axis. The ion channel dominates over the electron channel in both particle and thermal diffusion. A peaked helium profile, supported by inward convection that is stronger than predicted by neoclassical theory, is measured in the supershot. The helium profile shape is consistent with predictions from quasilinear electrostatic drift-wave theory. While the perturbative particle diffusion coefficients of all three species are similar in the supershot, differences are found in the L mode. Quasilinear theory calculations of the ratios of impurity diffusivities are in good accord with measurements. Theory estimates indicate that the ion heat flux should be larger than the electron heat flux, consistent with power balance analysis. However, theoretical values of the ratio of the ion to electron heat flux can be more than a factor of 3 larger than experimental values. A correlation between helium diffusion and ion thermal transport is observed and has favorable implications for sustained ignition of a tokamak fusion reactor.
Observation and analysis of a two-parallel-temperature electron tail during lower-hybrid current-drive on a tokamak
Authors: Colborn J. A. , Porkolab M. , Squire J. P. , Villaseñor J. , Skiff F. , Boyd D. A. , Harvey R. W.
Published: July 2002

A ‘‘two-parallel-temperature’’ feature of the fast-electron distribution function has been observed during lower-hybrid current-drive experiments on the Versator-II tokamak (B. Richards, Ph.D. thesis, Massachusetts Institute of Technology, 1981). Computational modeling predicts this feature and indicates that the colder tail is initiated by low-power, high-N∥ waves from the sidelobes of the antenna N∥ spectrum. The modeling indicates these waves bridge the ‘‘spectral gap,’’ enabling current drive even though the wave parallel phase velocity greatly exceeds the electron thermal velocity for the majority of the wave power.
Simulation of collisionless electrostatic velocity-shear-driven instabilities
Authors: Pritchett P. L.
Published: July 2002

The properties of electrostatic instabilities in velocity shear layers in collisionless plasmas are investigated by means of two-dimensional particle simulations for the case where the ion gyroradius is comparable to the scale length of the velocity shear. For modes exactly perpendicular to the magnetic field, the Kelvin–Helmholtz instability dominates the evolution of the system producing eφ/Te≳1; the observed growth rates show no reduction compared to the hydromagnetic limit. To obtain this result for the case of negative shear (v0y′/Ωi<0), it is necessary to include the kinetic modifications to the structure of the shear layer equilibrium. For finite k∥ in the range 0<k∥/k<0.04, the shorter-wavelength inhomogeneous-energy-density-driven instability cannot be identified in the simulations, and the upper limit on its excitation is eφ/Te≲2×10−3.
Ion emission from anode foils during multistage acceleration of intense ion beams
Authors: Slutz S. A.
Published: July 2002

Steady-state calculations are presented that show that the ion current density extracted off an anode foil in a multistage ion diode is a monotonically decreasing function of the injected current density J, going to zero as J goes to a limiting value J1. However, J1 can be quite large, going to infinity as the diode voltage approaches the limiting voltage V1 from below. Thus, it may not always be practical to inject a beam of current density exceeding J1. The results presented in this paper can be used to determine the fraction of (typically unwanted) ions extracted from the anode foil when J≤J1.
Micro- to macroscale perspectives on space plasmas*
Authors: Eastman Timothy E.
Published: July 2002

The Earth’s magnetosphere is the most accessible of natural collisionless plasma environments; an astrophysical plasma ‘‘laboratory.’’ Magnetospheric physics has been in an exploration phase since its origin 35 years ago but new coordinated, multipoint observations, theory, modeling, and simulations are moving this highly interdisciplinary field of plasma science into a new phase of synthesis and understanding. Plasma systems are ones in which binary collisions are relatively negligible and collective behavior beyond the microscale emerges. Most readily accessible natural plasma systems are collisional and nearest-neighbor classical interactions compete with longer-range plasma effects. Except for stars, most space plasmas are collisionless, however, and the effects of electrodynamic coupling dominate. Basic physical processes in such collisionless plasmas occur at micro-, meso-, and macroscales that are not merely reducible to each other in certain crucial ways as illustrated for the global coupling of the Earth’s magnetosphere and for the nonlinear dynamics of charged particle motion in the magnetotail. Such global coupling and coherence makes the geospace environment, the domain of solar-terrestrial science, the most highly coupled of all physical geospheres.
Resonant absorption in a steep density profile
Authors: Colunga Mario , Mora P. , Pellat R.
Published: July 2002

The electric field pattern is studied in the capacitor model of resonance absorption. Hydrodynamic and kinetic theory are used. The electric field is calculated for L≳10λD, where L is the density gradient length and λD the Debye length. The effect of collisional damping is studied. One obtains three different regimes. In the intermediate regime, the electric field amplitude is determined by the thermal convection, while the energy absorption is mainly caused by collisional damping.
Search for coherent structure within tokamak plasma turbulence
Authors: Zweben S. J.
Published: July 2002

Two-dimensional tokamak edge density turbulence data are examined for possible coherent or organized structure. The spatial patterns of density fluctuations ñ appear to consist of localized ‘‘blobs’’ of relatively high or low density which can move irregularly both radially and poloidally through the edge region. However, a statistical analysis of the lifetime, area, direction, speed, and amplitude of these blobs does not as yet suggest any organized structure associated with the blobs beyond that which can be described by time-averaged correlation functions.
Variance in the sedimentation speed of a suspension
Authors: Caflisch Russel E. , Luke Jonathan H. C.
Published: July 2002

The variance in the sedimentation speed for a homogeneous suspension of solid spheres in a Stokes fluid is calculated for a particular choice of the distribution function of the spheres. In the infinite particle number limit, the variance is found to be infinite.
Strong Langmuir turbulence in a magnetic field
Authors: Rowland H. L.
Published: July 2002

A series of fluid simulations are reported on strong Langmuir turbulence in two dimensions in the presence of a magnetic field. Starting with a group of Langmuir waves such as would be generated by an electron beam, the strong turbulence is initially one-dimensional, forming planar solitons. With no transverse wavenumber spread in the initial waves (Δk⊥=0), the magnetic field leads to more pancake-shaped solitons by reducing k*⊥ (the shortest unstable mode), but has little effect on the collapse. For an initial wave packet, if k*⊥<Δk⊥, the transverse collapse does not occur.
Particle simulations of the low-α Pierce diode
Authors: Crystal T. L. , Kuhn S.
Published: July 2002

The evolution of small initial perturbations of the uniform equilibrium of the ‘‘classical’’ Pierce diode [J. Pierce, J. Appl. Phys. 15, 721 (1944)] is studied using particle simulations. These simulations have been performed with the new bounded-plasma code PDW1 [Wm. S. Lawson (private communication)] and cover the parameter range 0<α<3π, where α=ωpL/v0. In the linear regime, three stages (initial transit, adjustment, and dominant eigenmode) are distinguished; oscillation frequencies, growth/damping rates, and potential profiles of the dominant eigenmode as well as oscillation frequencies of the next-to-dominant eigenmode are recovered and shown to agree quantitatively with recent analytical results. In the linearly unstable cases, the system evolves nonlinearly to a final state which may be either a new, nonuniform dc equilibrium, or a state of large-amplitude oscillations. In particular, for α=1.5π the character of the final state is found to depend on the details of the initial conditions.
Raman scattering in experiments with planar Au targets irradiated with 0.53 μm laser light
Authors: Shepard C. L. , Tarvin J. A. , Berger R. L. , Busch Gar. E. , Johnson R. R. , Schroeder R. J.
Published: July 2002

Stimulated Raman scattering (SRS) was studied in experiments with laser-irradiated Au disk targets. Typically the laser energy was 150 J in a 1.0 nsec pulse at a wavelength of 526 nm. Laser intensity at the target was ∼7×1014 W/cm2. The spectrum, angular distribution, onset time, duration, and intensity of the Raman light were measured. In addition the electron density distribution was determined by holographic interferometry and images of the Raman emission were obtained. These data show that the Raman light is primarily backscattered and that scattering occurs at electron plasma densities 0.01 nc ≤n≤0.15nc. SRS occurred at laser intensities which are below theoretical thresholds, given the plasma conditions. Otherwise, most of the data support the present understanding of Raman scattering.
Electromagnetic solitary vortices in a rotating plasma
Authors: Liu Jixing , Horton Wendell
Published: July 2002

The nonlinear equations describing drift-Alfvén solitary vortices in a low β, rotating plasma are derived. Two types of solitary vortex solutions along with their corresponding nonlinear dispersion relations are obtained. Both solutions have the localized coherent dipolar structure. The first type of solution belongs to the family of the usual Rossby or drift wave vortex, while the second type of solution is intrinsic to the electromagnetic perturbation in a magnetized plasma and is a complicated structure. While the first type of vortex is a solution to a second-order differential equation the second one is the solution of a fourth-order differential equation intrinsic to the electromagnetic problem. The fourth-order vortex solution has two intrinsic space scales in contrast to the single space scale of the previous drift vortex solution. With the second short scale length the parallel current density at the vortex interface becomes continuous. As special cases the rotational electron drift vortex and the rotational ballooning vortex also are given.
Electromagnetic stability of high-power ion diodes
Authors: Chang C. L. , Chernin D. P. , Drobot A. T. , Ott E. , Antonsen T. M.
Published: July 2002

The stability of high-power magnetically insulated ion diodes is investigated. Interactions between the fields, the electron flow, and the ion beam in the diode gap lead to instabilities. These instabilities are (i) low-frequency (below electron plasma frequency) instabilities that include a two-stream type instability at ω≊0 and ion transit time instabilities at finite frequency, and (ii) high-frequency instabilities (also called ‘‘magnetron instabilities’’) at a frequency above the electron plasma frequency. Under certain experimental conditions, one of the ion transit time instabilities is found to be broadbanded and may become absolutely unstable. Analysis based on realistic parameters shows the elimination of this broadband instability at diode operation close to insulation threshold, and implications of this finding are discussed.
Reynolds shear stress and heat flux measurements in a cylinder wake
Authors: Browne L. W. B. , Antonia R. A.
Published: July 2002

Measurements of Reynolds shear stress profiles and lateral heat flux profiles in the nearly self-preserving region of the turbulent wake of a heated circular cylinder are compared with calculations obtained from the mean momentum and mean enthalpy equations. Two types of self-preserving scales are used. The first uses the free-stream velocity U1 and the length scale [(x−x0)d]1/2, where x is the distance from the cylinder, x0 is the virtual origin, and d the cylinder diameter. The second uses the centerline velocity defect U0 and the velocity defect half-width L. Consistent results are obtained with both types of scales. In particular, the comparisons do not support the need for determining a new virtual origin, as suggested by Hariri et al. [Phys. Fluids 25, 1964 (1982)].
External inductance of an axisymmetric plasma
Authors: Hirshman S. P. , Neilson G. H.
Published: July 2002

The external inductance of an axisymmetric toroidal plasma with an arbitrary aspect ratio and cross section is obtained using a Green’s function method. By varying an equivalent skin current density over the plasma surface, while keeping the total toroidal current fixed, the plasma boundary is made to coincide with a magnetic surface. Numerical computations of the self-inductance and mutual inductance as functions of aspect ratio and elongation are fitted to simple analytic formulas. The effect of distributed plasma current on the volt-seconds required to reach a prescribed net current is considered.
Interchange stabilization of an axisymmetric single cell mirror using high-frequency electric fields
Authors: Ferron J. R. , Golovato S. N. , Hershkowitz N. , Goulding R.
Published: July 2002

Stabilization of interchange instabilities in the Phaedrus tandem mirror [Phys. Rev. Lett. 51, 1955 (1983)] through the use of externally applied rf at a single frequency is demonstrated over a wide range of frequency (1.03Ωi <ω<4.1Ωi). Here rf stabilized plasmas are reported with ion temperature up to 550 eV and beta up to 8%. Calculations of the rf electric fields show that the dominant mode is the m=+1 fast wave. When the rf frequency is near Ωi the calculated ponderomotive forces on ions and electrons are shown to be comparable. For ω>1.3Ωi it is shown that the force acts primarily on electrons. Stable plasmas are only achieved when the net radial ponderomotive force is calculated to be stabilizing and comparable to the curvature force. Results are also reported for rf applied simultaneously at two frequencies, one which is stabilizing and one which is destabilizing. Abrupt changes in stability are observed as the rf power is increased whenever one of the rf frequencies is effective at plasma heating.
Analytical theories of turbulence and the ε expansion
Authors: Dannevik William P. , Yakhot Victor , Orszag Steven A.
Published: July 2002

The fixed-point form of hydrodynamic equations emerging from renormalization group analysis of strong turbulence is analyzed using perturbation expansion in powers of the renormalized coupling constant (Reynolds number) Re*∝ε1/2. In the second order of ε expansion, transport equations are derived for energy and variance of a passive scalar which are identical to the inertial-range form of the eddy-damped quasinormal Markovian approximation (EDQNM), but without adjustable parameters which must be determined from experiment. The energy and scalar variance spectra are predicted to be Ek=1.617ε̄2/3k−5/3, and Θk=1.146N̄ ε̄−1/3k−5/3, where ε̄ and N̄ are, respectively, the dissipation rate of energy and scalar variance. This level of agreement with experiment has in the past proved difficult to obtain from renormalized perturbation theory applied to the bare hydrodynamic equations, without introduction of parameters that must be adjusted to different values for different dynamical fields. The fidelity of the present method, which emphasizes in a particular way the distant interactions in k space, may be related to the degree of nonlocality of the bare dynamics as manifested by possible generic Beltramization tendencies in strong turbulence.
Magnetic vortices in nonuniform plasmas
Authors: Nycander J. , Pavlenko V. P. , Stenflo L.
Published: July 2002

A localized generation of magnetic fields is found to occur in an unmagnetized nonuniform plasma. The process is described in terms of dipole and monopole vortex solutions of the equations governing the electron motion.
Access to the second stability region in a high-shear, low-aspect-ratio tokamak
Authors: Gerver M. J. , Kesner J. , Ramos J. J.
Published: July 2002

It is known that tokamaks display a second region of stability to ideal magnetohydrodynamic (MHD) internal modes. An important determining factor for MHD properties is the radial profile of toroidal current. Here it is shown that in a low-aspect-ratio tokamak with high on-axis safety factor (q0≂2) and high shear, a path to high beta can be obtained that remains completely stable against ideal MHD modes. By maintaining high shear this scenario avoids fixed boundary instabilities for both high and low toroidal mode numbers for beta values well above the Troyon limit (stability was tested up to εβp=1.4, β=10.8%). For a close fitting wall (awall/aplasma≂1.2) this configuration is also stable to low toroidal mode number balloon-kink modes.
An experimental study of electron distributions and wave potentials in ion-acoustic and pseudoshocks
Authors: Honzawa Tadao
Published: July 2002

Distribution functions of trapped and untrapped electrons in ion-acoustic shocks have been measured separately. Using these data, the trapped-to-untrapped electron density ratios are determined as a function of the shock potential. Further, shock potentials estimated from the distributions are compared with the directly measured values. In addition, experimental results of electron distributions and shock potentials in pseudoshocks are provided. Finally, all the results are compared with those expected from existing theory.
Observation of forward Raman scattering enhanced by backward Raman scattering in a laser plasma
Authors: Villeneuve D. M. , Baldis H. A.
Published: July 2002

The coupling between backward and forward Raman scattering was studied experimentally in a preformed underdense parabolic shaped plasma irradiated with a CO2 laser. The forward and backward Raman spectra were simultaneously recorded, and together with interferometry indicated that stimulated Raman scattering was occurring at the top of the profile. The forward Raman was observed well below its parabolic threshold, and it is proposed that plasma waves from backward Raman scattering act as an enhanced noise source for the forward Raman process. A model, whereby the forward and backward Raman processes are coupled together to produce an absolute instability, based on the Koch and Williams coupled Raman model [Phys. Fluids 27, 2346 (1984)], is shown to explain many of the observed features. This is shown to be very similar to a model of two backward Raman amplifiers displaced from the top of the plasma profile.
Self-consistent nonlinear evolution of the cyclotron autoresonance maser
Authors: Lee J. K. , Bard W. D. , Chiu S. C. , Davidson R. C. , Goforth R. R.
Published: July 2002

The nonlinear efficiency of the cyclotron autoresonance maser (CARM) amplifier is calculated numerically following the self-consistent evolution of the amplitude and phase of the wave. As the beam density is increased, the nonlinear efficiency and linear growth rate of the instability increase, and the interaction length for maximum efficiency decreases. Increasing the beam density also plays a favorable role in reducing the sensitivity of the instability to adverse velocity-spread effects.
Two-dimensional electrostatic simulations of plasma propagation perpendicular to a magnetic field
Authors: Galvez Miguel , Barnes Christopher
Published: July 2002

A two-dimensional electrostatic particle-in-cell code is used to simulate a finite-width plasma streaming across a uniform magnetic field. The simulations show that the plasma polarizes, and non-neutral charge layers develop along its edges. In an electron–ion plasma, the charge layers are asymmetric and the electron layer is unstable to the diocotron mode. The simulations show that this instability has smaller growth rate for plasma streams that are relatively less dense and wider. For a positive/negative ion plasma with equal mass ions the charge layers are symmetric and the plasma is stable to the diocotron mode. The results show that the diocotron instability leads to vortex structure when the plasma width is greater than the ion gyroradius, but this instability disrupts the entire plasma when the plasma width is of the order of or smaller than the ion gyroradius.
Path-integral formulation of ion heating
Authors: Crew G. B. , Chang Tom
Published: July 2002

A description of the generation and evolution of ionospheric oxygen ion conic distributions by electromagnetic ion cyclotron resonance heating is formulated in terms of a path integral. All of the relevant physics is contained in this path integral, which may be used to calculate measurable properties of the conic distribution. Although the presentation is applied to this specific ionospheric context, the treatment may be generalized to treat other diffusion problems of interest.
Leakage losses from a hydraulic fracture and fracture propagation
Authors: Johnson Robert E. , Gustafson Craig W.
Published: July 2002

The fluid mechanics of viscous fluid injection into a fracture embedded in a permeable rock formation is studied. Coupling between flow in the fracture and flow in the rock is retained. The analysis is based on a perturbation scheme that assumes the depth of penetration of the fluid into the rock is small compared to the characteristic length w30/k, where w0 is the characteristic crack width and k is the permeability. This restriction, however, is shown to be minor. The spatial dependence of the leakage rate per unit length from the fracture is found to be linear, decreasing from the well bore to the fracture tip where it vanishes. The magnitude of the leakage rate per unit length is found to decay in time as t−1/3 if the injection rate at the well bore is constant, and as t−1/2 if the well bore pressure is held constant. The results cast considerable doubt on the validity of Carter’s well-known leakage formula (Drilling Prod. Prac. API 1957, 261) derived from a one-dimensional theory. Using the simple fracture propagation model made popular by Carter, the present work also predicts that the fracture grows at a rate proportional to t1/3 for a fixed well bore injection rate and a rate proportional to t1/4 for a fixed well bore pressure.
Measurements of the free-stream fluctuations above a turbulent boundary layer
Authors: Wood D. H. , Westphal R. V.
Published: July 2002

In this paper an investigation of the velocity fluctuations in the free stream above an incompressible turbulent boundary layer developing at constant pressure is described. It is assumed that the fluctuations receive contributions from three statistically independent sources: (i) one-dimensional unsteadiness, (ii) free-stream turbulence, and (iii) the irrotational motion induced by the turbulent boundary layer. Measurements were made in a wind tunnel with a root-mean-square level of the axial velocity fluctuations of about 0.2%. All three velocity components were measured using an X-wire probe. The unsteadiness was determined from the spanwise covariance of the axial velocity fluctuations, measured using two single-wire probes. The results show that it is possible to separate the contributions to the rms level of the velocity fluctuations without resorting to the dubious technique of high-pass filtering. This separation could be extended to the spectral densities of the contributions if measurements of sufficient accuracy were available.
Stability of the O(2)-symmetric flow past a sphere in a pipe
Authors: Tavener S. J.
Published: July 2002

The linear stability of the O(2)-symmetric laminar flow past a centrally located sphere in a pipe for a range of sphere sizes is examined. For all values of the blockage ratio studied, the first instability observed as the flow rate increased, occurred at a steady symmetry-breaking bifurcation point. A transformation technique was used to solve the generalized eigenvalue problem which arises when using a mixed finite-element method to determine linear stability. The present results support the conclusion of Natarajan and Acrivos [J. Fluid Mech. 254, 323 (1993)] who have computed the stability of the flow past a sphere in an unbounded domain. The primary instability in three dimensions differs qualitatively from that of the analogous two-dimensional flow past a centrally located cylinder in a channel, which Chen et al. (to appear in J. Fluid Mech.) have shown to occur at a Z2 symmetry breaking Hopf bifurcation point.
On using Taylor’s hypothesis for three-dimensional mixing layers
Authors: LeBoeuf Richard L. , Mehta Rabindra D.
Published: July 2002

In the present study, errors in using Taylor’s hypothesis to transform measurements obtained in a temporal (or phase) frame onto a spatial one were evaluated. For the first time, phase-averaged (‘‘real’’) spanwise and streamwise vorticity data measured on a three-dimensional grid were compared directly to those obtained using Taylor’s hypothesis. The results show that even the qualitative features of the spanwise and streamwise vorticity distributions given by the two techniques can be very different. This is particularly true in the region of the spanwise roller pairing. The phase-averaged spanwise and streamwise peak vorticity levels given by Taylor’s hypothesis are typically lower (by up to 40%) compared to the real measurements.
Rayleigh convection in a closed cylinder—Experiments and a three-dimensional model with temperature-dependent viscosity effects
Authors: Zhao A. X. , Moates F. C. , Narayanan R.
Published: July 2002

In this paper our most recent research results on natural convection in a closed cylinder, where our interest focuses on pattern structure dependence on aspect ratio and on temperature-dependent viscosity, are summarized. The main results are (a) the experiments on the onset pattern and conditions for pure Rayleigh convection in circular cylinders compare favorably with linearized stability results of Hardin et al. [Int. J. Num. Methods Fluids 10, 79 (1990)], as well as three-dimensional nonlinear calculations made by us; and (b) experiments and nonlinear calculations indicate a variation of the patterns at and near the codimension two points when large temperature differences are introduced, so as to cause a substantial change in viscosity.
Heat transfer in a strongly nonlinear spatially developing longitudinal vorticity system
Authors: Liu Joseph T. C. , Lee Kiseok
Published: July 2002

In the present paper, the heat-transfer problem in a nonlinearly developing longitudinal vorticity system that arose from upstream weak Görtler vortices is considered. The general situation in which the Prandtl number (Pr) is different from unity is studied. The heat transport equation must be integrated for each Pr. The present paper presents results for gaseous (Pr<1) and liquid [Pr=O(10)] media, with the downstream integration carried out for three-dimensional advecting velocities in a spatially developing longitudinal vorticity system [Phys. Fluids A 4, 95 (1992)]. The behavior of the gaseous system is not significantly different from the Pr=1 situation; for a liquid system the isotemperature structures develop into thin, palm-tree-like figures in the cross-sectional plane. It is shown that for larger Pr, local surface heat-transfer rates are significantly enhanced over the local Blasius–Pohlhausen flat-plate value (by about 400% in the example computed for an initially amplified Görtler vortex for Pr=7.07 and about 340% for Pr=0.72), with an accompanying similar (about 350%) but considered moderate increase in the skin friction relative to the Blasius value. Detailed analyses of the heat transport and conversion mechanisms before and after Reynolds averaging are presented. For a fixed flow-parameter range, it is shown that the initial velocity perturbation level, not the initial scalar perturbation level, is an important controlling parameter for heat-transfer enhancement. The well-known analogy between heat and mass transfer is used to interpret flow visualization studies; in particular, the isoconcentration lines are typified by palm-tree-like structures in water, for which the Schmidt number Sc≫1.
Transition from spiral- to bubble-type vortex breakdown
Authors: Spall Robert E.
Published: July 2002

Numerical calculations employing the fully three-dimensional, time-dependent Navier–Stokes equations have been utilized to compute a transition from spiral- to bubble-type vortex breakdown for an unconfined longitudinal vortex. The transition was initiated through a small increase in the magnitude of the free-stream axial velocity deceleration. The resulting bubble structure, which consisted of a single toroidal recirculation cell, ultimately grew unstable. Transition back to a stronger spiral breakdown followed. Results are in qualitative agreement with experimental observations.
Comparison between the sum of second-order velocity structure functions and the second-order temperature structure function
Authors: Antonia R. A. , Zhu Y. , Anselmet F. , Ould-Rouis M.
Published: July 2002

The previously established similarity between the temperature spectrum and the spectrum corresponding to the mean turbulent energy in a wide variety of turbulent (shear) flows is re-examined within the framework of second-order velocity and temperature structure functions. Measurements in a turbulent wake indicate that Dq, the sum of the three second-order velocity structure functions bears close similarity to Dθ, the second-order temperature structure function, when Dq and Dθ are normalized by the mean turbulent energy and temperature variance, respectively. This similarity also applies to other flows. In the limit of small separations, the Kolmogorov-normalized structure functions differ only by the value of the molecular Prandtl number. In the inertial range, the Obukhov–Corrsin constant differs from the Dq Kolmogorov constant by a factor equal to the dissipation time scale ratio. This ratio is typically about 0.5.
On explicit solutions of the free-surface Euler equations in the presence of gravity
Authors: Daboussy D. , Dias F. , Vanden-Broeck J.-M.
Published: July 2002

There are very few explicit solutions of the free-surface Euler equations in the presence of gravity. These include Stokes’ corner flow, Richardson’s flow, and Joukovski’s flow. In this paper, it is shown how these solutions appear as particular solutions of a more general family of solutions of the free-surface Euler equations in the presence of gravity, which can only be computed numerically. The numerical method is based on series truncation. The physical relevance of these solutions is discussed as well.
Shock wave focusing using geometrical shock dynamics
Authors: Cates J. E. , Sturtevant B.
Published: July 2002

A finite-difference numerical method for geometrical shock dynamics has been developed based on the analogy between the nonlinear ray equations and the supersonic potential equation. The method has proven to be an efficient and inexpensive tool for approximately analyzing the focusing of weak shock waves, where complex nonlinear wave interactions occur over a large range of physical scales. The numerical results exhibit the qualitative behavior of strong, moderate, and weak shock focusing observed experimentally. The physical mechanisms that are influenced by aperture angle and shock strength are properly represented by geometrical shock dynamics. Comparison with experimental measurements of the location at which maximum shock pressure occurs shows good agreement, but the maximum pressure at focus is overestimated by about 60%. This error, though large, is acceptable when the speed and low cost of the method is taken into consideration. The error is primarily due to the under prediction of disturbance speed on weak shock fronts. Adequate resolution of the focal region proves to be particularly important to properly judge the validity of shock dynamics theory, under-resolution leading to overly optimistic conclusions.
Flow structure and mixing in a low Reynolds number forced wake inside a confined channel
Authors: MacKinnon C. G. , Koochesfahani M. M.
Published: July 2002

The spanwise structure and scalar mixing are documented in a confined low Reynolds number wake forced at the Kármán shedding frequency. Results show that the extent of three-dimensionality is dependent on the forcing amplitude. Data show evidence of streamwise vorticity initially in the form of a counter-rotating vortex pair near the side walls which evolves into a much more complex spatial structure farther downstream. The amount of mixed fluid can be significantly increased at high forcing amplitudes; the mixed fluid is estimated to occupy about 41% of the test section cross-sectional area at the farthest downstream station. The large mixing increase in this case is contained within the central portion of the span while the regions near the side walls are practically devoid of mixed fluid.
Intensification of magnetic field by concentrate-and-stretch of magnetic flux lines
Authors: Kitauchi Hideaki , Kida Shigeo
Published: July 2002

The mechanism of generation of magnetic field by thermal convection is investigated by analyzing the numerical solution to the Boussinesq magneto-hydrodynamic equations in a rotating spherical shell. The magnetic flux lines are intensified by concentrate-and-stretch around sinks in the outer boundary layer, on the equatorial plane and in the inner boundary layer, and the strong magnetic field is generated downstream of them. The magnetic flux lines are being stretched there irrespective of whether fluid elements accelerate or decelerate.