bluuebunny/crossref_metadata_2025 huggingface dataset:
Intrinsic optical bistablility of an ultrathin film consisting of oriented linear aggregates
Authors: Malyshev V. A. , Glaeske H. , Feller K.-H.
Published: July 2002

A theoretical study of transmittivity of an ultrathin film (of thickness smaller than an optical wavelength) consisting of oriented linear aggregates is carried out. We show that within a certain domain of the incident field amplitude, optical transitions of a single disordered aggregate, the eigenstates of which are represented by the Frenkel exciton states, can be treated in the framework of a two-level approximation. This allows us to claim that an ensemble of linear aggregates is analogous to that comprised of inhomogeneously broadened two-level systems. Based on this analogy, we show that the transmittivity of the film can manifest a bistable behavior originating from the coupling of the aggregates via the emission (far-zone) field. This mechanism of bistability differs from that occurring in a single perfect aggregate. In the latter case, bistability results from the population-dependent resonance frequency shift of an individual aggregate, originating from the coupling of the molecules via the dipole–dipole (near-zone) field. Estimates of the driving parameters show that real candidates, to realize the type of behavior we found, are such well-studied objects as J-aggregates of polymethine dyes or thin films of thiophene oligomers.
Transitioning model potentials to real systems. II. Application to molecular oxygen
Authors: Bembenek Scott D. , Rice Betsy M.
Published: July 2002

Recently, we introduced a novel computer simulation technique to determine the optimal set of parameters of an interaction potential for a simple monatomic liquid. This technique was used to obtain interaction potentials of the Lennard-Jones form that accurately describe argon over its entire liquid phase at a fixed pressure [S. D. Bembenek and B. M. Rice, Mol. Phys. 97, 1085 (1999)]. Here, we extend this technique to a homonuclear diatomic molecular system, liquid oxygen. This technique was first applied to a system in which the oxygen molecules were treated as point masses interacting through a modified Lennard-Jones potential. Simulations using the resulting optimal set of potential parameters of this system predict densities that are within 0.25% of experiment over the entire liquid range of oxygen at a fixed pressure. However, the errors in the internal energy and the enthalpy were as large as 9.8%. The technique was then used to determine the optimal parameters for a system of harmonic molecules, in which each molecule has two interaction sites centered at the atomic nuclei. The intermolecular interaction is the sum of all site–site interactions described by a modified Lennard-Jones potential. Simulations using these parameters reproduce experimental densities with an error no greater than 0.80%. The predictions of the internal energy and enthalpy differ from experiment by no more than 3.0% for temperatures below 90 K; predictions at 90 K differ from experiment by no more than 4.11%. These results seem to suggest that our method for determining parameters for an interaction potential is also applicable to simple molecular systems.
Dynamic structure and embedded representation in physics: The group theory of the adiabatic approximation
Authors: Rowe D. J. , Rochford P. , Repka J.
Published: July 2002

The group theoretical concepts of embedded representations and dynamical structure groups, distinct from dynamical symmetry groups, are introduced in order to describe the common physical situation in which collective bands of states of a many-body system are well described by an algebraic collective model even though the states may not span an invariant subspace of the many-body Hilbert space.
Relativistic quantum chaos in Robertson–Walker cosmologies
Authors: Tomaschitz Roman
Published: July 2002

Open Robertson–Walker cosmologies of multiple spatial connectivity provide a challenging example for the possible influence of the global topological structure of space-time on the laws of microscopic motion. Free geodesic motion is investigated in such cosmologies in the context of first quantization. A unique localized wave field, a solution of the Klein–Gordon equation, is found as a consequence of the topological structure of the spacelike slices t=const of the manifold. This solution is closely related to the collection of the bounded chaotic trajectories. The link is provided by the quasi-self-similar limit set of the group of covering transformations on the boundary of the universal covering space of the spacelike sections. It is this fractal set from which the covering geodesics of the bounded trajectories emerge, its Hausdorff measure and dimension determine the localized wave field.
Maximum-entropy approach to classical hard-sphere and hard-disk equations of state
Authors: Wang D. , Mead L. R. , de Llano M.
Published: July 2002

The maximum-entropy procedure for extrapolating series is applied to sum the known virial series for the hard-disk and hard-sphere fluids. This procedure is found to produce numerical results more accurate than Padé approximants, and it agrees with Monte Carlo simulations of these systems. The maximum-entropy results extrapolate the series smoothly to densities arbitrarily near to maximum close packing, and predict no singularities whatsoever.
Constant potential solutions of the Yang–Mills equation
Authors: Schimming R. , Mundt E.
Published: July 2002

The sourceless Yang–Mills equation reduces to an algebraic problem if the potential A has constant components Aα in a Lie algebra L with respect to some gauge and choice of coordinates. For some types of L there exist nontrivial solutions, for other types they do not. It is decided this in the Euclidean case for all Lie algebras with a dimension not greater than 5. A constant Yang–Mills potential on a Euclidean or Minkowski space with values in a compact Lie algebra is shown to be trivial.
Laguerre ensembles of random matrices: Nonuniversal correlation functions
Authors: Nagao Taro , Slevin Keith
Published: July 2002

The eigenvalue correlation functions of the Laguerre ensembles (exponential ensembles) of real symmetric and self-dual quaternion random matrices are calculated. It is shown that in certain intervals of the spectrum the renormalized correlations are not universal. A physical example in the transmission spectrum of a disordered conductor where such nonuniversality occurs is given.
On pseudosymmetric space–times
Authors: Defever Filip , Deszcz Ryszard , Verstraelen Leopold , Vrancken Luc
Published: July 2002

An algebraic classification of four-dimensional pseudosymmetric Einstein space–times is given and more examples of pseudosymmetric space–times are provided.
From geometric quantization to Moyal quantization
Authors: Gracia-Bondía José M. , Várilly Joseph C.
Published: July 2002

It is shown how the Moyal product of phase-space functions, and the Weyl correspondence between symbols and operator kernels, may be obtained directly using the procedures of geometric quantization, applied to the symplectic groupoid constructed by ‘‘doubling’’ the phase space.
Exact solutions for the Tikekar superdense star
Authors: Maharaj S. D. , Leach P. G. L.
Published: July 2002

We analyze a relativistic model for superdense stars proposed by Tikekar [R. Tikekar, J. Math. Phys. 31, 2454 (1990)]. In this model the hypersurfaces generated by {t=const} have the geometry of the 3-spheroid which gives the solutions a clear geometrical characterization. The solution of the Einstein field equations is reduced to integrating a second order ordinary differential equation. New classes of solutions are presented by restricting the choice of the spheroidal parameter K so that polynomial solutions are admitted for the first solution and then we find that there exists another solution which is a product of polynomials and algebraic functions. A remarkable feature of the class of solutions generated is that they are expressible completely in terms of polynomials and algebraic functions. Some physical aspects of the solutions are briefly considered. We regain the Tikekar solution as a special case when K=−7. As another example we explicitly present the form of the solution when K=−2.
Complex Hamiltonian evolution equations and field theory
Authors: Bodurov Theodore
Published: July 2002

The form of the complex Hamiltonian evolution equations (linear and nonlinear) is deduced here from that of the classical Hamiltonian systems. Their properties and those of the associated continuous Poisson bracket are studied. It is shown that if any space-localized solution of the above type equations is expanded in any Hilbert space basis the expansion coefficients are canonically conjugate variables and obey Hamiltonian systems of complex equations. The last is proved also for sets of “canonically conjugate” functionals of general type. As an application, commutation relations which are equivalent in form to Dirac’s quantization rules (the constant in place of ℏ is undetermined) are obtained. The question of how a nonlinear field theory may contain quantum mechanics as a special case is discussed in view of this paper’s results.
Functional and graphical methods for classical statistical dynamics. I. A formulation of the Martin–Siggia–Rose method
Authors: Andersen Hans C.
Published: July 2002

A formulation of the Martin–Siggia–Rose (MSR) method for describing the statistical dynamics of classical systems is presented. The present formulation is very similar in structure to the original MSR “operator” formalism and very different from the alternative functional integral formalism of Janssen, de Dominicis, Peliti, and others. The need for imposing certain boundary conditions in the MSR formalism, as pointed out by Deker, is clarified. The basic results of this paper include: a construction of the MSR formalism in a way that demonstrates its internal consistency; a definition of a functional whose functional derivatives give all the correlation functions and response functions of an ensemble of mechanical systems; a graphical expression for the correlation functions and response functions; a graphical expression for the Legendre transform of the functional and of the resulting vertex functions; and a graphical derivation of the appropriate Dyson equation. The present formulation is applicable to systems with highly non-Gaussian statistics, including systems of classical particles described in terms of the particle density in single-particle phase space. In this paper, we consider only the case of ensembles of systems whose coordinates are continuous and whose time evolution is described by deterministic first order differential equations that are local in time. The method is easily extended to systems whose dynamics is governed by stochastic differential equations and to spin systems.
Bose gas with nontrivial particle interaction and semiclassical interpretation of exotic solitons
Authors: Kovalev A. S. , Gvozdikova M. V.
Published: July 2002

Many-particle bound states in a Bose gas with a complex pair interaction between particles are considered. It is shown that the combination of attraction and repulsion between particles leads to the emergence of bound states with peculiar physical properties. In the limit of a large number of bound states in the Hartree approximation, these states are close in the structure and properties to exotic solitons (compactons and peakons) in classical systems with weak spatial dispersion. Examples of exotic solitons of various types in magnetically ordered media are considered.
Hysteretic phenomena and switching effects under phase transitions in external field
Authors: Stefanovich L. I.
Published: July 2002

The field dependence of the order parameter is investigated theoretically, using a statistical approach within the framework of the Ginzburg–Landau model with respect to the ferroelectric phase transition. It is found that, along with the existence of ordinary non-switching hysteresis is not too high fields ε (ε≪εc, where εc is the coercive field), hysteresis of a switching type can be observed at the fields in excess of the coercive one.
Piston-driven, unsteady separation at a sudden expansion in a tube: Flow visualization and LDA measurements
Authors: Durst F. , Maxworthy T. , Pereira J. C. F.
Published: July 2002

Flow visualization studies and laser-Doppler anemometry (LDA) measurements have been performed on the flow field generated at a sudden expansion in a cylindrical pipe during the intake stroke of a piston. The range of piston Reynolds numbers investigated, based on piston diameter and velocity, was 18–125 and covered a laminar and a transition-to-turbulence regime. A detailed study of the parameters influencing the structure of the flow, i.e., the clearance volume and the piston velocity, was carried out and is presented, and the mechanism for the creation of flow instabilities at low Reynolds numbers is outlined. One of the striking features of the flow relates to the formation of vortex structures during the intake flow. It is found that under some circumstances these structures become unstable, despite the relatively low Reynolds number of the flow.
Thermocapillary convection in liquid bridges: Solution structure and eddy motions
Authors: Davis A. M. J.
Published: July 2002

The steady thermocapillary convection in a cylindrical liquid bridge is considered in the absence of gravitational forces. The field equations are simplified by setting the Marangoni and Reynolds numbers equal to zero and the relative change in the surface tension is assumed to be very small, i.e., the appropriate capillary number is much less than unity. For suitable aspect ratios, there is a transition from a pattern of vortices determined by the applied surface temperature gradient to those of Moffatt type in the interior of the fluid. If the bridge is rotated, the equations can be modified by Coriolis terms but the parameter values are such that rotational effects tend to dominate the motion.
Experimental investigation of fluctuating forces exerted on a cylindrical tube (Reynolds numbers from 3000 to 30 000)
Authors: Tadrist H. , Martin R. , Tadrist L. , Seguin P.
Published: July 2002

In this paper, an experimental investigation of variations of the global lift coefficient as a function of the Reynolds number is discussed. The experimental unit was capable of measuring the lift coefficient at Reynolds numbers ranging from 3000 to 30 000. Since the intensity of the unsteady lift forces was of the order of few millinewtons, a sensor and signal-conditioning circuit was specially designed for measurement. The velocity field and the flow turbulence rate were measured upstream from the cylinder using laser-Doppler anemometry. Tube aspect ratio, blockage, and ends effects are discussed. The evolution of the Strouhal number was observed as the Reynolds number passed from 3000 to 30 000. When measured as a function of the Reynolds number, the global lift coefficient displayed considerable variation for Reynolds numbers between 3000 and 10 000.
On unsteady flows through two sequentially connected Laval nozzles
Authors: Rein Martin , Ehrenfried Klaus
Published: July 2002

The flow through two sequentially connected Laval nozzles depends on the ratio of the two throat cross sections. If the second throat is wider than the first one, two different cases are distinguishable. In steady flows solutions with a single shock can occur if the second throat is relatively wide. Within a certain range of exit pressures, two different one-shock solutions are possible. Under these conditions the shock position depends hysteretically on the exit pressure. If the second throat is only slightly wider than the first one, either a one- or a two-shock solution can be present for the same exit pressure. The unsteady transition between these solutions that can be caused by a variation of the exit pressure with time, is investigated.
Convection and mixing in the oceans and the Earth
Authors: Turner J. S.
Published: July 2002

Mixing processes are important in many natural flows, and they occur over a wide range of scales. The terms mixing and stirring are used to describe diverse physical processes, and the aim of this survey is to compare and contrast various phenomena included by this description. Motions driven by buoyant convection in the ocean and the Earth’s mantle are emphasized, and related observations, laboratory experiments, and theoretical studies of natural processes are discussed. The flows covered range from the slow creeping motions in the ‘‘solid’’ Earth, which can be treated as a viscous fluid in this context, to fully turbulent convective flows in liquid magmas and the surface layers of the ocean. Another important class of problem treated is penetrative convection, and the mechanisms of mixing across density interfaces. The related transports between ‘‘double-diffusive’’ layers have a profound effect on the rate of vertical mixing in the ocean and in various geological contexts.
Response of a confined gas to volumetric heating in the absence of gravity. I: Slow transients
Authors: Herczynski A. , Kassoy D. R.
Published: July 2002

A one-dimensional model for bulk motion induced by a transient volumetric heat source in a confined gas at zero gravity is considered. Rational approximation methods are used to derive a quantitative theory for the gas response to a spatially distributed, time-dependent internal power deposition. The resulting low Mach number compressible flow equations are solved by using perturbation methods. Solutions are given for a conduction-free core and thin conductive boundary layers adjacent to the end walls. It is found that any spatially nonuniform power deposition will cause fluid motion. Net mass transport in the closed container will occur for certain spatially distributed heating. The model mimics the thermal effects of an exothermic gas phase reaction in vapor transport experiments conducted in space. The solutions demonstrate that thermally induced mass transport can be as large as diffusive mass transport in a typical experiment.
Thermal effects on axisymmetric vortex breakdown in a spherical gap
Authors: Arkadyev A. , Bar-Yoseph P. , Solan A. , Roesner K. G.
Published: July 2002

The thermal effects on vortex breakdown in a Boussinesq fluid contained within a rotating concentric spherical annulus are examined numerically for the case in which the outer sphere is stationary and hot while the inner sphere rotates and is at a lower temperature. An axial gravitational field is assumed. The investigation was done by means of a steady-state axisymmetric finite-element code. The method of solution was verified by analyzing the axisymmetric vortex breakdown with and without thermal effects in a closed circular-cylindrical container with a rotating lid. Comparisons of the flow patterns with numerical results in the literature show good agreement. Solutions have been obtained for a spherical gap of radii ratio 0.5, Reynolds number 0≤Re≤6000, and Rayleigh number 0≤Ra≤1.5×106, at Prandtl number Pr=0.7. The results show that depending on the relative strength of the imposed temperature difference and rate of rotation, the flow field may be a large meridional circulation (‘‘base flow’’) with a disturbed symmetry about the equator, a bubble-type vortex breakdown on the polar axis, similar to that in the isothermal case, or a vortex ring centered on the polar axis.
Origin of turbulence-producing eddies in a channel flow
Authors: Brooke John W. , Hanratty T. J.
Published: July 2002

A principal theoretical problem in understanding wall turbulence is the determination of how turbulence is created and sustained, i.e., the explanation of how energy is transferred from the mean flow to the turbulence. Flow-oriented vortical eddies have been associated with large Reynolds stresses and with the production of turbulence in the viscous region close to the wall. Their creation and evolution are investigated in a high-resolution direct numerical simulation of turbulent flow in a channel. An important finding is that they regenerate themselves by a process that appears to be weakly dependent on the outer flow. This involves the enhancement of streamwise vorticity at the wall, of opposite sign, at a location where a stress-producing eddy lifts from the wall.
Note on secondary flows in oscillating-grid, mixing-box experiments
Authors: Fernando H. J. S. , De Silva I. P. D.
Published: July 2002

Oscillating-grid induced turbulence in confined geometries (tanks) is commonly used in the study of turbulence with zero-mean shear. It is demonstrated that the mean secondary circulation generated during such experiments can be reduced by selecting conditions that lessen the Reynolds-stress gradients within the fluid. A simple power law for the spatial decay of turbulent velocity fluctuations is realized only in the absence of such mean circulation.
Experimental dynamics of an annulus of vorticity in a pure electron plasma
Authors: Peurrung A. J. , Fajans J.
Published: July 2002

The dynamics of hollow, cylindrically shaped pure electron plasmas are studied. Since these plasmas behave like two-dimensional (2-D) fluids with low viscosity, their evolution parallels the evolution of 2-D fluid vorticity annuli inside frictionless circular containers. Growth of the Kelvin–Helmholtz instability in circular geometry is observed. The formation and subsequent interaction of vortices are studied as a function of ring thickness. Small initial perturbations can dramatically increase the symmetry and repeatability of the dynamics.
Bootstrap current and parallel viscosity in the low collisionality regime in toroidal plasmas
Authors: Shaing K. C. , Crume E. C. , Tolliver J. S. , Hirshman S. P. , van Rij W. I.
Published: July 2002

Bootstrap currents in the low collisionality regime in nonaxisymmetric toroidal plasmas are calculated analytically using a model drift kinetic equation. The results obtained using the analytic expression are compared with those calculated from the numerical solutions of the drift kinetic equation. Qualitatively and quantitively, the results are in reasonable agreement.
One-dimensional calculation of a turbulent dynamo model for reversed field pinches
Authors: Hamba Fujihiro
Published: July 2002

A one-dimensional simulation of reversed field pinches is performed using a turbulent dynamo model. Steady states with field reversal are obtained. Effects of the boundary condition at the wall are investigated to show the importance of injection of the turbulent energy. The dependence on the magnetic Reynolds number is examined to derive the scaling of magnetic fluctuation level. The oscillating current field drive is computed by applying a time-varying electric field.
Anisotropic distribution function of minority tail ions generated by strong ion-cyclotron resonance heating
Authors: Chang C. S. , Colestock P.
Published: July 2002

The highly anisotropic particle distribution function of minority tail ions driven by ion-cyclotron resonance heating at the fundamental harmonic is calculated in a two-dimensional velocity space. It is assumed that the heating is strong enough to drive most of the resonant ions above the ion–electron critical slowing-down energy. Simple analytic expressions for the tail distribution are obtained for the cases when the bounce-averaged quasilinear heating coefficient Db is assumed to be constant in pitch angle and energy, when the Doppler effect is sufficiently large to flatten the sharp pitch angle dependence in Db but the energy dependence is allowed, and when the sharp pitch angle dependence is retained. It is found that a simple constant-Db solution can be used instead of the more complicated velocity-dependent-Db solutions for many analytic purposes.
Magnetohydrodynamic modeling of the solar corona
Authors: Mikić Zoran
Published: July 2002

The ideal and resistive magnetohydrodynamic (MHD) model is used to examine the dynamics and structure of the solar corona. When the coronal magnetic field is deformed by photospheric flow it can evolve to states that become unstable to ideal MHD modes. The nonlinear evolution of these instabilities can lead to the generation of current sheets, field line reconnection, and energy release. The disruption of an arcade field and the kinking of coronal loops is described. The braiding of the large-scale coronal field by convective photospheric motions develops fine-scale structure in the magnetic field and leads to the development of intense current filaments. The resistive dissipation of these currents can provide an efficient coronal heating mechanism.
Stimulated Brillouin scattering off nonlinear ion acoustic waves
Authors: Hüller S.
Published: July 2002

The influence of nonlinear ion acoustic waves on the stimulated Brillouin backscattering process is investigated with the help of the hydrodynamic description of the ion fluid. Numerical calculations were systematically performed for a wide range of relevant parameters in the presence of an ideal plasma. It is shown that the scaling of steady-state solutions due to spatially amplified scattering deviates significantly from the theory where only linearized ion acoustic waves are considered. An analytic approach using harmonic expansion confirms the similarity parameters found numerically. The significance of kinetic effects, such as anomalous heat transport due to fast ions and ion trapping in the presence of weakly damped ion waves, is also studied using particle simulations.
Stimulated Raman scattering: Action evolution and particle trapping via Euler–Vlasov fluid simulation
Authors: Johnston T. W. , Bertrand P. , Ghizzo A. , Shoucri M. , Fijalkow E. , Feix M. R.
Published: July 2002

The Manley–Rowe action relations and trapped-electron orbit theory have been applied to the problem of stimulated Raman scattering (SRS) in the forward direction, as simulated by an Euler–Vlasov fluid (EVF) code in periodic geometry. After a slight initial delay, the Vlasov results agree well with three-oscillator theory (but with damping larger than the Landau linear value). The action sum of the pump and the scattered SRS wave was well conserved, while the action loss between the pump and plasma wave was shown to be well accounted for in detail by examining the electron energy for electrons above the lower trapping boundary (separatrix) in x-px phase space. However, unlike the three-oscillator model, the action sum of the pump and plasma wave was observed to actually increase for a short time because of retrieval of apparently lost action from the trapped-electron effect in regenerating the plasma wave. Thus the utility was clearly demonstrated of using action conservation to separate the effects of three-wave interaction from those due to nonlinear particle–wave interaction. The EVF code also shows clearly the fine details of layering and folding seen in x-px phase space due to electron trapping, detrapping, and retrapping. [These details would have been invisible in the usual particle-in-cell (PIC) code because of graininess at low phase space densities.]
Analytic expressions for mode conversion in a plasma with a linear density profile
Authors: Hinkel-Lipsker D. E. , Fried B. D. , Morales G. J.
Published: July 2002

The transformation of electromagnetic waves into Langmuir oscillations (and vice versa) is explicitly examined in the vicinity where the wave frequency matches the electron plasma frequency in an inhomogeneous plasma. For an unmagnetized plasma with a linear density profile of scale length L, closed-form, analytic expressions are derived, in terms of Airy functions, for the reflection and mode conversion coefficients of Langmuir and electromagnetic waves utilizing a source approximation that is valid when the electromagnetic field scale length is large compared to that of the electrostatic field. The technique developed to determine the energy flux coefficients and the fields is general enough to apply to a plasma with a profile other than a linear one, and should prove useful in other problems where a scale length separation is valid. The reflection coefficient for the ‘‘direct’’ problem (incident electromagnetic wave) is equal in magnitude to that of the ‘‘inverse’’ problem (incident electrostatic wave) and the corresponding mode conversion coefficients satisfy energy conservation. The mode conversion coefficient of the warm, collisionless problem is independent of electron temperature, T, in the limit of small T, and is equal in magnitude to the mode conversion coefficient of the cold, collisional problem, which is likewise independent of the collision frequency. It also depends on the angle of incidence θ and the vacuum scale length k0L, and for T/mc2≪1, agrees with earlier, numerical calculations.
Anisotropic Z-pinch equilibria and their stability
Authors: Coppins M. , Scheffel J.
Published: July 2002

The Chew–Goldberger–Low equations are used to study the effect of pressure anisotropy on Z-pinches operating in the collisionless regime. The limitations on the form of accessible equilibria are investigated. The effect on the m=0 instability is asessed both by means of the energy principle and by direct solution of the eigenvalue equation for a variety of anisotropic equilibria. The results indicate that in the small Larmor radius limit pressure anisotropy offers a rather slight enhancement of the linear stability of the Z-pinch.
Experimental study of a megawatt 200–300 GHz gyrotron oscillator
Authors: Grimm T. L. , Kreischer K. E. , Temkin R. J.
Published: July 2002

A detailed experimental study is presented of a pulsed megawatt gyrotron oscillator operating in the 200–300 GHz range, whose design is consistent with continuous operation for electron cyclotron resonance heating (ECRH) of fusion plasmas. Two different radii beams produced by magnetron injection guns (MIG’s) were used to excite the cylindrical waveguide cavity. The emission was found experimentally to be single mode, single frequency with a single rotation, which can be mode converted for transmission. The highest power reached with the larger radius electron beam was 1.2 MW at 230 GHz in the TE34,6 mode with an efficiency of 20% and beam parameters of 59 A and 100 kV. The highest power reached with the smaller radius electron beam was 0.78 MW at 280 GHz in the TE25,13 mode with an efficiency of 17% and beam parameters of 51 A and 92 kV. The smaller radius beam gave a peak efficiency of 18% at 0.72 MW, 290 GHz in the TE25,14 mode. Efficiencies obtained in this experiment are about half that of less highly overmoded gyrotrons. Analysis of the experiment suggests that the low efficiency is primarily caused by azimuthal mode competition, in agreement with multimode theory for a tapered cavity. These experimental results show that megawatt power levels can be generated in continuous wave (cw) gyrotron oscillators at 200–300 GHz with efficiencies approaching 20%.
Pure electron plasmas in asymmetric traps*
Authors: Chu R. , Wurtele J. S. , Notte J. , Peurrung A. J. , Fajans J.
Published: July 2002

Pure electron plasmas are routinely confined within cylindrically symmetric Penning traps. In this paper the static and dynamic properties of plasmas confined in traps with applied electric field asymmetries are investigated. Simple analytical theories are derived and used to predict the shapes of the stable noncircular plasma equilibria observed in experiments. Both analytical and experimental results agree with those of a vortex-in-cell simulation. For an ℓ=1 diocotron mode in a cylindrically symmetric trap, the plasma rotates as a rigid column in a circular orbit. In contrast, plasmas in systems with electric field asymmetries are shown to have an analog to the ℓ=1 mode in which the shape of the plasma changes as it rotates in a noncircular orbit. These bulk plasma features are studied with a Hamiltonian model. It is seen that, for a small plasma, the area enclosed by the orbit of the center of charge is an invariant when electric field perturbations are applied adiabatically. This invariant has been observed experimentally. The breaking of the invariant is also studied. The dynamic Hamiltonian model is also used to predict the shape and frequency of the large amplitude ℓ=1 and ℓ=2 diocotron modes in symmetric traps.
Eigenvalue solution for the ion-collisional effects on ion-acoustic and entropy waves
Authors: Tracy M. D. , Williams E. A. , Estabrook K. G. , De Groot J. S. , Cameron S. M.
Published: July 2002

The linearized ion Fokker–Planck equation is solved as an eigenvalue problem under the condition of collisionless electrons in the quasineutral limit (φ=0) for ionization-temperature ratios, ZTe/Ti=2, 4, and 8 for entropy waves and ionization-temperature ratios, ZTe/Ti=4, 8, 16, 32, 48, 64, and 80 for ion-acoustic waves. The perturbed ion distribution function for the ion-acoustic and entropy waves is formed from a Legendre polynomial expansion of eigenvectors and can be used to calculate collisionally dependent macroscopic quantities in the plasma such as gamma (Γ=Cp/Cv), the ratio of specific heats, and the ion thermal conductivity (κi).
A stochastic model for lower hybrid wave scattering by density fluctuations
Authors: Grossmann William , Spigler Renato
Published: July 2002

Recent tokamak experiments have shown the presence of drift-wave-induced random density fluctuations localized near the plasma edge. These fluctuations may be treated as a random perturbation of the refractive index that appears in the scalar wave equation obeyed by the electrostatic potential. The treatment here includes the additional effect on density gradient fluctuations through a term, kyM, appearing in the wave equation, where ky is the poloidal wavenumber of the incident wave and M≡〈ω2pe〉/ωΩce . This additional term is shown to have non-negligible consequences in several cases of physical interest. By invoking the modern theory of stochastic differential equations, various statistics of particular functionals of the potential may be computed. An explicit calculation of the fractional power Pt transmitted through the scattering region is given as a function of the usual parameter ε2LS, where ε is the relative density fluctuation amplitude, L is the scale width of the scattering region, and S is the power spectral density of the fluctuations. For typical tokamak parameters (ε,L/a∼0.01–0.1, where a is the plasma radius; ky/k∥ ∼0.5–1.0) the present theory predicts a Pt∼10%—50% which favorably agrees with Andrews and Perkins’ [P. L. Andrews and F. W. Perkins, Phys. Fluids 26, 2537 (1983)] radiative transfer model.
Slow radial instability of large-orbit rings trapped in a mirror field with resistive walls
Authors: Fleischmann H. H. , Kupfer K. , Kribel R. E.
Published: July 2002

In this paper, the radial stability of strong fast-particle rings trapped in a magnetic mirror field in the presence of resistive tank walls is investigated theoretically. The results reconfirm that the normal high-frequency precessional mode will occur whenever ηm+ηim<0. Here ηm=(R/Bz)(∂Bz/∂r)R is the (negative) radial field index of the applied mirror field and ηim=ηs+ηd denotes the (positive) total image index with the effective ‘‘static’’ index ηs related to the resistively decaying (m=0) component of the image fields and the ‘‘dynamic’’ image index ηd related to the (m=1) component of the image currents. However, the present analysis predicts full stability only for ηm+ηs>0; in the intermediate case where ηm+ηs<0 but ηm+ηs+ηd>0, the normal precessional mode will be stable, but a slow radial instability is predicted to drive the rings into the wall without much precession. It appears that these effects will similarly occur in other cases where stabilization by cylindrical walls is anticipated.
Rapid magnetic reconnection caused by finite amplitude fluctuations
Authors: Matthaeus W. H. , Lamkin S. L.
Published: July 2002

The nonlinear dynamics of the magnetohydrodynamic sheet pinch have been investigated as an unforced initial value problem for large scale Reynolds numbers up to 1000. Reconnection is triggered by adding to the sheet pinch a small but finite level of broadband random perturbations. Effects of turbulence in the solutions include the production of reconnected magnetic islands at rates that are insensitive to resistivity at early times. This is explained by noting that electric field fluctuations near the X point produce irregularities in the vector potential, sometimes taking the form of ‘‘magnetic bubbles,’’ which allow rapid change of field topology.
Magnetohydrodynamic stability of a resistive fluid limited by a nonideal boundary
Authors: Nalesso G. F.
Published: July 2002

A new set of boundary conditions for a resistive plasma that takes into account the effect of a nonideal rigid wall of finite conductivity and thickness, is derived. These conditions, in cylindrical geometry, take the form of a system of integrodifferential equations involving the perturbed plasma variables v1, B1, T1, at the inner surface of the rigid wall. This system of integrodifferential equations is numerically solved, using a numerically stable scheme, and is implemented in an existing linear one-dimensional resistive initial value code, the Ripple IVA, developed by Dibiase and Killeen [Comput. Phys. 24, 158 (1977)]. An approximate solution of the problem, valid in a neighborhood of the time origin, is also derived. Attention is then focused on the linear behavior of resistive tearing and interchange modes as a function of the ratio of plasma and wall conductivity as well as the magnetic Lundquist number S, especially for high-beta equilibria of the reversed field pinch (RFP) type.
Hot-electron ring formation in tokamak plasmas
Authors: Hsu J. Y. , Liu C. S. , Prater R. , Lin S. H.
Published: July 2002

Energetic particles of tokamak plasmas may be preferentially heated by the second harmonic X mode. Their orbits collapse toward the low magnetic field side, where the resonance layer is placed, until the relativistic detuning moves the resonance layer outside the particle orbit. The time evolution of the heating process and the ring structure are presented.
Movement of liquid inclusions in soluble solids: An inverse Stokes’ law
Authors: Yih Chia-Shun
Published: July 2002

The temperature distribution in an ellipsoidal liquid inclusion in a soluble solid, with a constant gradient far away from the liquid, and the movement of the liquid inclusion as a whole, which results as a consequence, are investigated. Since the solid is soluble and its concentration in solution is temperature dependent, any temperature variation in the liquid induces a concentration variation, which will transfer mass by diffusion, eroding the wall where the temperature is high and depositing solid material at the wall where the temperature is lower. This erosion or deposition will cause the liquid inclusion to move, and will, through absorption or release of latent heat, in turn affect the temperature distribution. From the result obtained for the general ellipsoid, specific results for prolate and oblate ellipsoids of revolution, the sphere, and circular and elliptic cylinders are obtained.
Gallery of fluid motion
Authors: Reed Helen L.
Published: July 2002

This article displays winning photographs from the third annual fluid-mechanics photo contest that was held at the November 1985 meeting of the American Physical Society, Division of Fluid Dynamics, Tucson, Arizona.
Isochronous containers
Authors: Weidman Patrick D.
Published: July 2002

The problem of isochronous fluid oscillation in two- and three-dimensional containers is studied for the case where both gravitational and surface tension restoring forces are present. For the problems considered, the container shapes are identical to those found previously in which gravity was the only restoring mechanism. The oscillation frequencies, however, are modified by surface tension, fluid density, and container geometry. A new isochronous container of rectangular section for fundamental cross-mode oscillations is also presented.
Unstable-magnetic-drift waves in a plasma with temperature anisotropy
Authors: Wu C. S. , Shi B. R. , Zhou G. C.
Published: July 2002

A theory of unstable magnetic-drift waves was first discussed by Krall and Rosenbluth [Phys. Fluids 6, 254 (1963)]. However, the growth rate is exceedingly small and, therefore, the instability seems to be unimportant. It is found in the present analysis that a weak electron temperature anisotropy T⊥<T∥ can lead to a much stronger instability. When there is no temperature gradient, the conditions of the new instability are (c2k2/ω2e+1)(T⊥/T∥)−1 <0, εn/εB>0, where εn=(1/n)(∂n/∂x) and εB=(1/B)(∂B/∂x). In this case, the instability is attributed to wave–electron interactions, whereas the case discussed by Krall and Rosenbluth [Phys. Fluids 6, 254 (1963)] attributes the instability to wave–ion interactions. A general discussion including a temperature gradient is presented with the usual local approximation which is justifiable when kεn>1.
Pointed bubbles rising in a two-dimensional tube
Authors: Vanden-Broeck Jean-Marc
Published: July 2002

Pointed bubbles with a 120° angle at the apex rising in a two-dimensional tube are considered. A solution is found at a unique Froude number of 0.36.
Stability of symmetric m=1 island equilibria
Authors: Jensen Torkil H. , McClain Fred W.
Published: July 2002

It is shown that magnetohydrodynamic (MHD) equilibria with reflectional symmetry and an internal m=1 island are either marginally stable or unstable if they possess a helical symmetry with a pitch length much larger than their radius.
Imploding spherical and cylindrical shocks
Authors: Yousaf M.
Published: July 2002

In this paper it is shown that the value of the similarity exponent α derived analytically by Fujimoto and Mishkin [J. Fluid Mech. 89, 61 (1978); Phys. Fluids 21, 1933 (1978)] is exactly the same as that found by Stanyukovich [Unsteady Motion of Continuous Media, (Academic, New York, 1960)]. Since the result found by Stanyukovich is an approximation to α, Fujimoto and Mishkin’s claim to have an exact expression of α is false. The two methods are outlined and Stanyukovich’s result is simplified to show its equivalence to the work of Fujimoto and Mishkin.
Isodynamic magnetohydrodynamic equilibria
Authors: Channell Paul J.
Published: July 2002

The class of isodynamic magnetohydrodynamic (MHD) equilibria in three dimensions is studied. A reduction of the problem to that of finding a force-free equilibrium is carried out, and the resulting force-free problem transformed, by means of a three-dimensional hodograph technique, to a simple and elegant form. The flux surface equation in a potentially very useful form is split off in a further representation.
Long-scale thermodiffusional convection
Authors: Hefer D. , Pismen L. M.
Published: July 2002

Thermodiffusional convection in the presence of the Soret effect is considered under conditions of poor heat exchange when convective patterns with large aspect ratio are formed beyond the instability threshold. Three distinct long-scale equations are obtained for the description of long-scale patterns associated with stationary and oscillatory convection. The equation of stationary convection coincides, up to coefficients, with the respective equation derived for thermal convection in one-component fluids. Long-scale oscillatory instability is found to obey the Schrödinger equation with the amplitude modulated on a longer time scale. An equation retaining the widest variety of behavior and describing transitions to both stationary and oscillatory convection patterns is obtained in the vicinity of the degenerate bifurcation at double zero eigenvalue.
Maximum growth rates for packets of waves in water of finite depth
Authors: Blaha R. , Laedke E. W. , Spatschek K. H.
Published: July 2002

The stability of two-dimensional infinitesimal disturbances of soliton solutions of the two-dimensional Davey–Stewartson equations [Proc. R. Soc. London Ser. A 338, 101 (1974)] is considered. It is shown that a previously developed generalized variational method, which allows the calculation of the maximum growth rates close to the cutoff, is applicable here. A numerical evaluation of the variational principle is presented.
Dynamic centering of liquid shells
Authors: Tsamopoulos John A. , Brown Robert A.
Published: July 2002

The moderate-amplitude axisymmetric oscillations of an inviscid liquid shell surrounding an incompressible gas bubble are calculated by a multiple-time-scale expansion for initial deformations composed of two-lobed perturbations of the shell and a displacement of the bubble from the center of mass of the liquid. Two types of small-amplitude motion are identified and lead to very different nonlinear dynamic interactions, as described by the results valid up to second order in the amplitude of the initial deformation. In the ‘‘bubble mode,’’ the oscillations of the captive bubble and the liquid shell are exactly in phase and the bubble vibrates about its initial eccentric location. The bubble moves toward the center of the drop when the shell is perturbed into a ‘‘sloshing mode’’ of oscillation where both interfaces move out of phase. These results explain the centering of liquid shells observed in several experiments.
The progressive wave approach analyzing the decay of a sawtooth profile in magnetogasdynamics
Authors: Sharma V. D. , Singh L. P. , Ram Rishi
Published: July 2002

An asymptotic approach is used to analyze the main features of weakly nonlinear waves propagating through an electrically conducting gas permeated by a transverse magnetic field. The analysis leads to an evolution equation, which characterizes the wave process in the high-frequency domain. The growth equation for an acceleration front is recovered as a special case. The influence of the magnetic field on the decay behavior of a sawtooth profile, which is headed by a weak shock front and ends with a magnetosonic disturbance, is investigated. A remarkable difference between the plane and cylindrical wave profiles is noted; for instance, when the adiabatic index γ is 2, the field does not affect the decay behavior of plane waves but does affect cylindrical waves.
The effects of sloshing energetic particles on ballooning modes in tokamaks
Authors: Stotler D. P. , Berk H. L.
Published: July 2002

Distributions that give rise to energetic trapped particle pressures peaked in the ‘‘good curvature’’ region of a tokamak (sloshing distributions) are examined in an attempt to find stable regimes for both the magnetohydrodynamic (MHD) and precessional modes. It is the precessional drift destabilization of ballooning modes that inhibits bridging the unstable gap to second stability by the use of deeply trapped energetic particles unless the hot particles have an extremely large energy (∼0.35 MeV for a tokamak like PDX [Phys. Rev. Lett. 49, 326 (1982)]). Unfortunately, our calculations indicate that the sloshing particles do not have a significant stabilizing effect. An analytic treatment shows that stability for the precessional mode can be found only if the sign of the energetic particle magnetic drift frequency can be reversed from its value in vacuum bad curvature without hot species diamagnetism. This is difficult to do in a tokamak because of the destabilizing contribution of the geodesic curvature to the drift frequency. Furthermore, for each of the two sloshing distributions employed (one contains only trapped particles; the other includes trapped and passing particles), a new ‘‘continuum instability’’ (where asymptotically along the field line the mode is a propagating plane wave) is found to be driven by geodesic curvature. These results indicate that energetic sloshing particles are not able to bridge the unstable gap to second stability.
Stochasticity and the m=1 mode in tokamaks
Authors: Izzo R. , Monticello D. A. , Stodiek W. , Park W.
Published: July 2002

It has recently been proposed that stochasticity resulting from toroidal coupling could lead to a saturation of the m=1 internal mode in tokamaks. Results are presented from the nonlinear evolution of the m=1 mode with toroidal coupling that show that stochasticity is not enough to cause saturation of the m=1 mode.