bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetic effects of electron thermal conduction on implosion hydrodynamics
Authors: Nishiguchi A. , Mima K. , Azechi H. , Miyanaga N. , Nakai S.
Published: July 2002

The kinetic effects of electron heat transport in an imploding plasma are investigated by using the one-dimensional hydrodynamics implosion code with the Fokker–Planck equation. The Fokker–Planck equation is solved in the implosion simulation by reducing it to the multigroup transport equations. The implosion time, mass ablation rates, and compressed core plasma parameters of the simulations are compared with those of the flux-limited Spitzer–Härm model simulations and the experimental results of high-density compression. As a result, it is shown that the Fokker–Planck simulation results agree well with the experiments, in which the random phase plate is employed for the uniform implosion.
Conditioning electron beams in the ion-focused regime
Authors: Fernsler R. F. , Hubbard R. F. , Slinker S. P.
Published: July 2002

Relativistic electron beams propagating through dense gas are subject to the resistive hose instability, a virulent kink instability that restricts the effective range of high-current beams. Previous studies have shown that the instability can be suppressed by centering the beam and tailoring its emittance prior to injection into the gas. One means of centering and tailoring a beam is to use short ‘‘conditioning’’ cells that operate in the low-pressure, ion-focused regime. In this paper, analytic models are developed to understand and assess the performance of such cells.
Ion collisionality and L–H transition in tokamaks
Authors: Shaing K. C. , Christenson P. J.
Published: July 2002

When the previous H-mode theory [K. C. Shaing and E. C. Crume, Jr., Phys. Rev. Lett. 63, 2369 (1989)] is extended to include a bi-Maxwellian ion distribution with both cold and hot ion components to model the effects of the non-Maxwellian distribution, the collisionality of the cold ions ν*ic becomes an irrelevant bifurcation parameter, as it is for the electrode-induced H mode. Thus, the observation that the L–H transition can occur even when ν*ic≫1 is consistent with the fundamental physics of the ion orbit loss model, including a hot ion component.
Diffuse plasma effects on the ion-hose instability
Authors: Welch Dale R. , Hughes Thomas P.
Published: July 2002

The transverse stability of a relativistic electron beam focused by an ion channel in the presence of a diffuse background plasma is investigated. The linear behavior of the ion-hose and electron two-stream instabilities is treated analytically using a spread-mass model for the beam and ion channel and a cold-fluid model for the plasma. The electron two-stream instability is found to be quite weak. As the plasma neutralization radius approaches the beam radius, the ion-hose growth rate is reduced up to 50% before the model’s assumptions break down. Particle-in-cell simulations confirm the linear analytic theory and show that the electron two-stream instability can saturate nonlinearly with little beam emittance growth.
Nonlinear interaction of Rayleigh–Taylor and shear instabilities
Authors: Finn John M.
Published: July 2002

Results on the nonlinear behavior of the Rayleigh–Taylor instability and consequent development of shear flow by the shear instability [Phys. Fluids B 4, 488 (1992)] are presented. It is found that the shear flow is generated at sufficient amplitude to reduce greatly the convective transport. For high viscosity, the time-asymptotic state consists of an equilibrium with shear flow and vortex flow (with islands, or ‘‘cat’s eyes’’), or a relaxation oscillation involving an interplay between the shear instability and the Rayleigh–Taylor instability in the presence of shear. For low viscosity, the dominant feature is a high-frequency nonlinear standing wave consisting of convective vortices localized near the top and bottom boundaries. The localization of these vortices is due to the smaller shear near the boundary regions. The convective transport is largest around these convective vortices near the boundary and there is a region of good confinement near the center. The possible relevance of this behavior to the H mode and edge-localized modes (ELM’s) in the tokamak edge region is discussed.
Particle flux across a stochastic magnetic layer
Authors: Samain A. , Capes H. , Ghendrih Ph. , Nguyen F.
Published: July 2002

Radial particle transport is considered in a plasma with ergodic and stochastic flux lines. The self-consistent electric field is neglected and the particle flux is due to a succession of mass convections along flux lines, in series with very small-scale-length transverse diffusion between neighboring flux lines. The dependence of the average radial flux on the thermodynamic forces, namely the pressure gradient that controls the parallel convection, and the density gradient that controls the small-scale transverse diffusion is investigated. The conditions for which the average radial flux is driven by the average pressure or density gradient are derived.
Stimulated scattering instabilities of electromagnetic waves in collisional plasmas
Authors: Shukla P. K.
Published: July 2002

The effects of nonlocal heat transport on stimulated scattering instabilities of electromagnetic waves are investigated employing the recently derived expression for the electron number density perturbations that are driven by the ponderomotive and thermal forces in high Zi collisional plasmas. The growth rates and the threshold conditions for stimulated Brillouin and stimulated Compton scattering, as well as modulational instabilities, are obtained. It is found that the growth rates of the scattering instabilities of electromagnetic waves are considerably enhanced due to nonlocal electron heat conduction in collisional plasmas.
Plasma autobiasing during Ohmic H-mode in the STOR-M tokamak
Authors: Zhang W. , Xiao C. , Hirose A.
Published: July 2002

Application of a short current pulse on a nominal Ohmic discharge in the STOR-M tokamak (Saskatchewan Torus-Modified) [Phys. Fluids B 4, 3277 (1992)] triggers the Ohmic H-mode characterized by reduced Hα radiation, increased electron density, and reduced edge density/magnetic fluctuations. Measurements of plasma floating potential at the plasma edge and in the scrape-off layer indicate that the Ohmic H-mode is accompanied by negative plasma autobiasing, which leads to a steeper radial electric field profile at the edge. Since the duration of the current pulse (≤20 kA, 100 μsec) is shorter than the resistive skin time (≂1 msec), preferential edge heating is expected, which is believed to be responsible for changes in the edge discharge condition favorable for inducing the Ohmic H-mode. The electron density profile becomes steeper at the edge during the H-mode, and clear formation of a density pedestal has been seen. The evolution of the density profile suggests the presence of particle pinch. An improved confinement phase (ICP) is induced by external negative electrode biasing. The ICP reveals some similarities as compared to the current pulse induced H-mode. It is found that the electrostatic modes are dominant in the scrape-off layer while electromagnetic modes dominate in the plasma edge during the normal Ohmic discharges.
Shear flow effects on the nonlinear evolution of thermal instabilities
Authors: Leboeuf J.-N. , Charlton L. A. , Carreras B. A.
Published: July 2002

In the weak radiation drive regime, the coupling between the thermal instability driven by impurity radiation and the self-consistent flow profile modification leads to a simple dynamical system that can be approximated by the Volterra–Lotka equations. In this system the shear flow acts as a predator and the temperature fluctuations act as prey. The solutions are oscillatory, and their behavior resembles that of edge-localized modes (ELM’s). The solutions of the simplified model are compared with the three-dimensional and two-dimensional nonlinear numerical results for this instability.
A comparison of edge turbulence in tokamaks, stellarators, and reversed-field pinches*
Authors: Tsui H. Y. W. , Wootton A. J. , Bell J. D. , Bengston R. D. , Diebold D. , Harris J. H. , Hershkowitz N. , Hidalgo C. , Ingraham J. C. , Kilpatrick S. J. , Li G. X. , Lin H. , Manos D. M. , Meier M. A. , Miller G. M. , Munson C. P. , Pew J. , Prager S. C. , Ritz Ch. P. , Rudyj A. , Schoenberg K. F. , Sorensen J. , Tanaka T. , Uckan T. , Weber P. G.
Published: July 2002

Edge equilibrium, turbulence and transport related plasma parameters from the Advanced Toroidal Facility (ATF) [Fusion Technol. 10, 179 (1986)] torsatron, the ZT-40M [Fusion Technol. 8, 1571 (1985)] reversed-field pinch, the Phaedrus-T [Nucl. Fusion 32, 2040 (1992)] tokamak, the Texas Experimental Tokamak (TEXT) [Nucl. Technol. Fusion 1, 479 (1981)], and the Tokamak Fusion Test Reactor (TFTR) [in Plasma Physics and Controlled Nuclear Fusion Research, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 1, p. 9] have been obtained using a standardized Langmuir probe array and a consistent set of data analysis packages. Additional data from some other devices have also been furnished via private communications and incorporated from published results. Experimental results over a wide range of parameters are compared and the turbulence contribution to edge transport are assessed. Certain physical properties that are relevant to the modeling of edge turbulence are identified: namely, shear decorrelation of turbulence, the role of resistive dissipation and electron parallel thermal conduction, radial mode structure in sheared magnetic field, and electromagnetic contribution to the parallel Ohm’s law.
Alfvén wave experiments in the Phaedrus-T tokamak*
Authors: Majeski R. , Probert P. , Moroz P. , Intrator T. , Breun R. , Brouchous D. , Che H. Y. , DeKock J. R. , Diebold D. , Doczy M. , Fonck R. , Hershkowitz N. , Johnson R. D. , Kishinevsky M. , McKee G. , Meyer J. , Nonn P. , Oliva S. P. , Pew J. , Sorensen J. , Tanaka T. , Vukovic M. , Winz G.
Published: July 2002

Heating in the Alfvén resonant regime has been demonstrated in the Phaedrus-T tokamak [Fusion Technol. 19, 1327 (1991)]. Electron heating during injection of radio-frequency (rf) power is indicated by a 30%–40% drop in loop voltage and modifications in sawtooth activity. Heating was observed at a frequency ωrf≊0.7Ωi on axis, using a two-strap fast wave antenna operated at 7 and 9.2 MHz with 180° phasing (N∥∼100). Numerical modeling with the fast wave code fastwa [Plasma Phys. Controlled Fusion 33, 417 (1991)] indicates that for Phaedrus-T parameters the kinetic Alfvén wave is excited via mode conversion from a surface fast wave at the Alfvén resonance and is subsequently damped on electrons.
Nonlinear ideal magnetohydrodynamics instabilities
Authors: Pfirsch D. , Sudan R. N.
Published: July 2002

Explosive phenomena such as internal disruptions in toroidal discharges and solar flares are difficult to explain in terms of linear instabilities. A plasma approaching a linear stability limit can, however, become nonlinearly and explosively unstable, with noninfinitesimal perturbations even before the marginal state is reached. For such investigations, a nonlinear extension of the usual MHD (magnetohydrodynamic) energy principle is helpful. (This was obtained by Merkel and Schlüter, Sitzungsberichted. Bayer. Akad. Wiss., Munich, 1976, No. 7, for Cartesian coordinate systems.) A coordinate system independent Eulerian formulation for the Lagrangian allowing for equilibria with flow and with built-in conservation laws for mass, magnetic flux, and entropy is developed in this paper which is similar to Newcomb’s Lagrangian method of 1962 [Nucl. Fusion, Suppl., Pt. II, 452 (1962)]. For static equilibria nonlinear stability is completely determined by the potential energy. For a potential energy which contains second- and nth order or some more general contributions only, it is shown in full generality that linearly unstable and marginally stable systems are explosively unstable even for infinitesimal perturbations; linearly absolutely stable systems require finite initial perturbations. For equilibria with Abelian symmetries symmetry breaking initial perturbations are needed, which should be observed in numerical simulations. Nonlinear stability is proved for two simple examples, m=0 perturbations of a Bennet Z-pinch and z-independent perturbations of a θ pinch. The algebra for treating these cases reduces considerably if symmetries are taken into account from the outset, as suggested by M. N. Rosenbluth (private communication, 1992).
Measurement of Alfvén ion-cyclotron wave using both X- and O-mode reflectometers
Authors: Mase A. , Ichimura M. , Satake H. , Katsumata R. , Tokuzawa T. , Ito Y. , Hojo H. , Doyle E. J. , Itakura A. , Inutake M. , Tamano T.
Published: July 2002

Alfvén ion-cyclotron wave excited in the central-cell plasma of the GAMMA 10 tandem mirror is investigated by a reflectometer system. The Fourier amplitude of the instability strongly depends on the anisotropy of the ion velocity distribution function and the plasma β value. The ratio 0.2–0.3 between O-mode and X-mode reflectometer signals suggests that a magnetic fluctuation level similar to the density fluctuation level is excited in the plasma due to the instability.
Two-dimensional studies of stimulated Brillouin scattering, filamentation, and self-focusing instabilities of laser light in plasmas
Authors: Amin M. R. , Capjack C. E. , Frycz P. , Rozmus W. , Tikhonchuk V. T.
Published: July 2002

The parametric interaction of an intense laser beam with ion plasma modes in a two-dimensional Cartesian geometry has been studied for the first time by avoiding the paraxial optics approximation. This model allows investigation of the competition between forward, sideward, and backward stimulated Brillouin scattering (SBS) along with filamentation and self-focusing instabilities. It is shown that the SBS saturation level, the angular distribution of transmitted and scattered light, and their temporal dependence are governed mainly by two control parameters: the backward SBS gain coefficient G, and the ratio αsf of the incident beam power to its critical value for the onset of self-focusing. In the case of large values of G≳15, backward SBS dominates and prevents both self-focusing and forward SBS. For smaller values of G, the interaction exhibits a complex oscillatory behavior, which corresponds to the competition between backward and forward SBS for αsf≲1, and involves also self-focusing for higher beam intensities.
Coherent corrections to the synchrotron radiation spectrum of a relativistic electron beam
Authors: Kaltchev D. I. , Perelstein E. A.
Published: July 2002

The coherent corrections to the radiation spectrum of a relativistic electron beam, propagating along an external magnetic field are investigated. In the beam reference frame the cyclotron radiation from a hot magnetoplasma is calculated taking into account the electron correlations. The fluctuation-dissipation theorem for the case of an equilibrium plasma is used to find the spectrum of current-density fluctuations. At harmonics of the cyclotron frequency ≥2, the collective distortion of this spectrum decreases the radiation intensity compared to the spontaneous one. The coherent effects manifest themselves at wavelengths of the order of the Debye shielding length and become important when the electronic density in the laboratory frame is higher than 1013 cm−3 for typical experimental beam parameters.
Reaction and diffusion in dense nonideal plasmas
Authors: Kremp D. , Schlanges M. , Bonitz M. , Bornath T.
Published: July 2002

The nonequilibrium properties of strongly coupled plasmas are investigated taking into account reaction and diffusion processes. The starting point is quantum kinetic equations for systems with chemical reactions involving many-body effects like screening, self-energy, and medium-dependent scattering. The influence of these effects on the kinetics of reaction and diffusion processes is discussed. Generalized expressions for the coefficients of impact ionization and diffusion are derived exhibiting a strong density dependence due to the many-body effects. Solving the reaction-diffusion equation (RDE) for a dense hydrogen plasma, nonlinear phenomena such as bistability, running ionization fronts, and droplet formation are obtained.
Effects of density scale length on backward and forward stimulated Raman scattering in 0.53 μm laser–plasma interaction
Authors: Rousseaux C. , Amiranoff F. , Labaune C. , Matthieussent G.
Published: July 2002

Forward and backward stimulated Raman scattering (SRS) have been investigated in three different plasma geometries: in exponential density profile, in Gaussian density profile, and in quasihomogeneous preformed plasmas. The experiments were performed with the same irradiation setup at 0.53 μm wavelength and at a laser intensity ∼1015 W/cm2. Time-resolved spectra and energy rates of this instability have been measured in the three experiments. A strong increase of the backward SRS energy rate—up to 1%—with the increasing length of the plasma was observed. From data analysis, backward SRS is explained in terms of absolute instability. On the other hand, forward SRS can be interpreted by considering linear scattering of the incident laser light on the plasma wave driven by backward SRS. This assumption is adequately supported both by the spectral characteristics and by the energy rates of the forward emission light.
Transit-time interactions in magnetized plasmas
Authors: Melatos A. , Robinson P. A.
Published: July 2002

The theory of transit-time interactions between particles and coherent wave packets is generalized to include the effects of ambient and induced magnetic fields. Using a Born approximation, the wave–particle energy transfer is calculated analytically to second order in the perturbing fields, which is sufficient to treat both particle scattering and wave damping. The results possess the correct unmagnetized limit, and can be approximated in a simple form in the small gyroradius regime.
Quasistatic heat front and delocalized heat flux
Authors: Luciani J. F. , Mora P. , Pellat R.
Published: July 2002

New results concerning the mathematical properties of the Fokker–Planck equation describing the electron distribution function are presented. The validity of the approximations obtained by using a finite number of Legendre polynomials to describe the electron distribution function is discussed. It is shown that, due to the Landau form of the electron-ion collision operator, it is sufficient to use two or three Legendre polynomials in problems of interest. The theory is applied to the classical albedo problem as a test, and is also applied to determine the distribution and the heat flux in a heat front typical of laser plasma experiments. It is shown that the heat flux can be expressed as a sort of convolution of the Spitzer–Härm heat flux by a delocalization function. The convolution formula leads in a physically relevant way to the saturation and the delocalization of the heat flux.
Parametric instabilities in underdense plasma and stochastic heating of electrons
Authors: Rozmus W. , Tighe W. , Offenberger A. A. , Estabrook Kent
Published: July 2002

Several aspects of ion-acoustic decay (IAD) and oscillating two-stream (OTS) instabilities are studied by theory and particle simulations in homogeneous plasma for densities 0.5≤ne/ nc ≤0.8. Linear theory for IAD and OTS is reviewed and compared with particle-in-cell simulations. Nonlinear aspects of IAD such as the appearance of ion-acoustic second harmonic are observed and theoretically investigated. A saturation mechanism for IAD in which the second harmonic plays an important role is proposed. A simple model in which particles move in the fields of coherent waves related to IAD (OTS) is proposed in order to explain heating of the electron distribution function. If the wave amplitudes exceed a certain threshold, large scale stochasticity (LSS) develops allowing appreciable momentum transfer to particles. The appearance of a characteristic Maxwellian tail in the electron distribution function is related to LSS in particle motion. The method presented here should also be useful for analyzing hot particle production in other parametric instabilities.
Monte Carlo studies of transport in stellarators
Authors: Fowler R. H. , Rome J. A. , Lyon J. F.
Published: July 2002

Transport is studied in toroidal geometry by integrating the guiding-center equations in magnetic coordinates and simulating collisions with a Monte Carlo collision operator. The effects of the ambipolar electric field on diffusion losses are determined for model magnetic fields and the correct magnetic field of the Advanced Toroidal Facility (ATF-1) stellarator. Comparisons are made of the computed diffusion coefficients and the theoretically predicted values.
Resistive ballooning modes in an axisymmetric toroidal plasma with long mean free path
Authors: Connor J. W. , Chen Liu
Published: July 2002

Tokamak devices normally operate at such high temperatures that the resistive fluid description is inappropriate. In particular, the collision frequency may be low enough for trapped particles to exist. However, on account of the high conductivity of such plasmas, one can identify two separate scale lengths when discussing resistive ballooning modes. By describing plasma motion on one of these, the connection length, in terms of kinetic theory the dynamics of trapped particles can be incorporated. On the resistive scale length, this leads to a description in terms of modified fluid equations in which trapped particle effects appear. The resulting equations are analyzed and the presence of trapped particles is found to modify the stability properties qualitatively.
Spheromak global instabilities and stabilization by nearby conductors
Authors: Hayashi Takaya , Sato Tetsuya
Published: July 2002

By means of a three-dimensional (3-D) magnetohydrodynamic (MHD) simulation, the stabilization effects of conducting walls on the tilting instability of spheromaks created by the Princeton induction method are extensively studied. The nonlinear development and deformation of the spheromak resulting from global instabilities with toroidal modes n=1 (tilting), 2, and 3 are also studied in detail. Simulation results have shown that while the induction effect of the wall current is insufficient, the line-tying effect is much more powerful for the suppression of the strong tilting instability. For a perfect line-tying stabilization, a certain amount of surface magnetic flux around the spheromak should be tied to the conducting wall. Next, the influence of the shape of the cylindrical vacuum vessel on the stabilization is studied. It is found that the created spheromak assumes a similar figure irrespective of the vessel shape; thus we conclude that controlling the spheromak shape by changing the vacuum vessel is ineffective. Also investigated is the tilting behavior when the magnetic index is changed. The result is not conclusive in the sense that a well-developed oblate spheromak could not be created when the magnetic index was close to 1. Simulation runs in which n=2 and n=3 perturbations are applied, in addition to the tilt mode (n=1), have shown that the spheromak with a large aspect ratio is unstable to all of these modes. Interestingly, it is found that as the n=2 mode develops sufficiently, the spheromak torus is severely folded and twisted; consequently, the folded arms undergo reconnection internally.
Ion viscosity stabilization of resistive internal kink modes
Authors: Porcelli Francesco , Migliuolo Stefano
Published: July 2002

The linear stability theory of resistive m0=1 internal kink modes is presented, including the effects of finite electron drift frequency, finite ion Larmor radius, and ion viscosity. Analytic and numerical computations are used to show that moderate values of the ion viscosity parameter are sufficient to stabilize the resistive internal kink mode.
Turbulent relaxation processes in magnetohydrodynamics
Authors: Ting Antonio C. , Matthaeus William H. , Montgomery David
Published: July 2002

Competing processes previously called ‘‘selective decay’’ and ‘‘dynamic alignment’’ are studied numerically for two-dimensional magnetohydrodynamic turbulence. In selective decay, the energy decays relatively to mean-square vector potential, and in dynamic alignment, the energy decays relatively to cross helicity. In the former case, the kinetic (fluid) energy decays to zero and the magnetic energy occupies the largest scales allowed by the boundary conditions. In the latter case, the velocity field and magnetic field become aligned and energetically equipartitioned. An extensive study of the initial value problem, with viscous and resistive dissipation, indicates that four distinct regimes of behavior are possible: a magnetically dominated regime, a velocity dominated regime, a dynamic alignment dominated regime, and a transition regime separating the others in parameter space. An analytical variational problem predicts several features seen in the computations, including geometrical alignment of velocity and magnetic fields, constraints on global quantities in the long-time limit, and in some cases, condensation to long wavelength modes.
Hot electron stabilization of a helically symmetric plasma
Authors: Miller R. L.
Published: July 2002

Furth and Boozer (private communication; Proceedings of the Advanced Bumpy Torus Concepts Workshop, CONF-830758, Oak Ridge National Laboratory, Oak Ridge, Tennessee, 1983, p. 161) have suggested the use of relativistic electrons to achieve the second stability regime in a helical axis stellarator (Heliac). The hot electrons would only be required until the background plasma reached the second stability regime; the heating power maintaining the hot electron layer would then be turned off. The basic correctness of Furth and Boozer’s suggestion is confirmed numerically by a localized stability analysis of helically symmetric plasma equilibria, with anisotropic pressure profiles. Stability is evaluated using the localized interchange criterion in which the hot electrons, because of their large drift speeds, are treated as rigid. A hot electron pressure profile is exhibited; it provides a stable path to the second stability regime for the background plasma.
Ion-acoustic double layers in a magnetized plasma
Authors: Goswami K. S. , Bujarbarua S.
Published: July 2002

Solutions for double layers (DL), propagating obliquely to the magnetic field with a speed near the slow ion-acoustic velocity are presented. Using fluid equations for the ions and an arbitrary equation of state for the electrons, it is shown that in such a system, DL solutions exist for the two-electron Boltzmann model. No DL exists when the electrons are isothermal, and when the free and reflected electrons are described by Maxwell distributions.
The effect of slab width on the stability of natural convection in confined saturated porous media
Authors: Chelghoum D. E. , Weidman P. D. , Kassoy D. R.
Published: July 2002

Stability boundaries separating different states of bimodal convection in a box of saturated porous material with impermeable bounding faces are determined. Two opposing vertical end walls are always insulated, while the thermal conditions on the other set of opposing vertical side walls range from insulated to perfectly conducting, as measured by the Biot number based on box height. The temperature difference between the hot lower plate and the cooler upper plate provides the mechanism for instability. The eigenvalue problem for the critical Rayleigh number is solved numerically over a range of box sizes and side-wall heat transfer conditions. For small values of the distance H2 between conducting side walls, agreement with previous asymptotic analysis is obtained. New results for boxes with planform dimensions both comparable to the box height show the existence of isolated regions rich in bimodal structure. These islands are separated by broad regions having modal properties identical to those found for conducting side walls in the limit H2→0. The numerical results for O(1) box dimensions continue to exhibit the stabilizing effect of side-wall heat transfer as previously observed for thin slabs.
Operation of a reversed field pinch without a conducting shell
Authors: Robertson Scott , Schmid Peter
Published: July 2002

The Reversatron reversed field pinch (R/a=50 cm/8 cm) [IEEE Trans. Plasma Sci. PS-14, 267 (1986)] has been operated with and without an aluminum shell surrounding the resistive toroidal vacuum chamber. Discharges without the shell have the same density, fluctuation level, and dynamo activity as seen in discharges with the shell; however, the plasma current and discharge duration are reduced. With zero reversal of the toroidal field the reversed field pinch (RFP) becomes a spheromak of high aspect ratio. Discharges without the shell in this mode are also of lower current and shorter duration. The early terminations may be caused by the tilt instability. In all cases discharges in helium were of longer duration than those in hydrogen.
Direct simulation of high-vorticity gas flows
Authors: Bird G. A.
Published: July 2002

The computational limitations associated with the molecular dynamics (MD) method and the direct simulation Monte Carlo (DSMC) method are reviewed in the context of the computation of dilute gas flows with high vorticity. It is concluded that the MD method is generally limited to the dense gas case in which the molecular diameter is one-tenth or more of the mean free path. It is shown that the cell size in DSMC calculations should be small in comparison with the mean free path, and that this may be facilitated by a new subcell procedure for the selection of collision partners.
Conditions for the validity of unmagnetized-plasma theory in describing weakly magnetized plasmas
Authors: Robinson P. A.
Published: July 2002

An investigation is made of the conditions under which the expression for the dielectric tensor from magnetized-plasma theory may be approximated by that from unmagnetized-plasma theory. Surprisingly, the conditions usually quoted are shown to be inaccurate chiefly because they make no reference to the form of the plasma distribution function. Corrected conditions depend strongly on the distribution function and their application is thus highly problem-dependent. For completely arbitrary distributions these conditions are generally stronger than the commonly quoted ones for weakly damped waves. By contrast, the conditions applicable to axisymmetric distributions can be weaker than some of those commonly quoted. A geometric interpretation of the revised conditions for axisymmetric distributions is given in terms of the resonance ellipses of cyclotron maser theory and is illustrated with reference to dispersion and instability of Langmuir and Bernstein waves in weakly magnetized plasmas. These results apply to all weakly magnetized plasmas that can be described by the linearized Vlasov equations and imply that some investigations which have used the commonly quoted conditions to justify use of unmagnetized-plasma theory may be in error on this point.
Energetic particle stabilization of ballooning modes in finite-aspect-ratio tokamaks
Authors: Wang X.-H. , Bhattacharjee A. , Mauel M. E. , Van Dam J. W.
Published: July 2002

The theory of energetic particle stabilization of ballooning modes in tokamaks is revisited. When the energetic particles are drift-reversed, a region of ballooning instability, akin to that which causes the Van Dam–Lee–Nelson limit [in Proceedings of the Workshop on EBT Ring Physics (Oak Ridge National Laboratory, Oak Ridge, TN, 1980), p. 471; Phys. Fluids 23, 1850 (1980)] in magnetic mirrors, is identified. At higher values of core poloidal beta a ‘‘third’’ region of stability is conjectured to occur. The inclusion of finite-aspect-ratio effects can eliminate ballooning instability completely when the energetic particles are non-drift-reversed, but the region of instability for drift-reversed particles persists. Requirements for energetic particle stabilization in large- and small-aspect-ratio tokamak experiments and reactors are discussed.
The effect of the time-dependent self-consistent electrostatic field on gyrotron operation
Authors: Kleva Robert G. , Antonsen Thomas M. , Levush Baruch
Published: July 2002

The time-dependent self-consistent electrostatic field is shown to have a deleterious effect on gyrotron operation. As the electron beam density increases, the nonlinear efficiency is degraded seriously by the self-electrostatic field. Time-dependent multimode simulations demonstrate that at sufficiently large beam densities the electron cyclotron instability is quenched and the oscillation will not start. Contrary to previous investigations, electrostatic effects do not necessarily increase the linear growth rate of the electromagnetic cavity mode and, depending on the beam density, electrostatic effects can actually stabilize the mode. Typically, however, considerations of linear theory are not important in an overmoded, open resonator gyrotron because the system evolves nonlinearly into a state consisting of a single mode which is linearly stable, but nonlinearly the most efficient mode.
Stability loss and sensitivity in hollow fiber drawing
Authors: Yarin A. L. , Gospodinov P. , Roussinov V. I.
Published: July 2002

Stability of hollow fiber drawing is studied, based on the quasi-one-dimensional equations of thin film dynamics. It is shown that the model isothermal drawing process is unstable when the draw ratio E (that of output to input velocities) exceeds the critical value E*=20.22 when the viscosity force is dominating. Under stable regimes with E<E*, the response of the as-drawn fiber to external perturbations is studied (the sensitivity problem). In unstable situations with E≳E*, onset of the so-called draw resonance regime with self-sustained oscillations is predicted by using numerical simulation. The effects of the inertia, gravity, surface tension, and gas pressure differential are considered.
Experimental observations and marginal stability calculations for counterflowing streams with swirl
Authors: Wu C. Y. , Didwania A. K. , Goddard J. D.
Published: July 2002

In this paper reports are given on laser-Doppler velocimeter measurements on counterflowing water streams with swirl. The experiments lend qualitative support to previous theory and computations [Goddard, Didwania, and Wu, J. Fluid Mech. 251, 149 (1993)] of the effects of swirl and confinement on the hydrodynamic stability of this flow in the high-Re regime. In the present paper a linear stability analysis and computations of the marginal stability curve for self-similar, radially unconfined flows are also given. This and the above-cited work serve to define the practical operating limits on an associated flow device for generating a uniformly accessible interface of mass or heat transfer between counterflowing fluid streams.
Reply to ‘‘Observations regarding ‘Coherence and chaos in a model of turbulent boundary layer’ by X. Zhou and L. Sirovich [Phys. Fluids A 4, 2855 (1992)]’’
Authors: Sirovich L. , Zhou X.
Published: July 2002

In view of the ‘‘observations’’ of the Cornell group, reassessment of their and our models for wall-bounded turbulence has been made. Wide ranging evidence is presented for the existence and key role of propagating modes (streamwise dependent modes), absent in the original Cornell model but present in some of their later models. Evidence that the heteroclinic orbit (the bursting mechanisms) found in the original Cornell model is most likely an artifact of their Galerkin projection is presented. A thorough discussion detailing the physical and mathematical soundness, as well as the universality, of our models is presented.
Role of coherent structures in an isothermally reacting mixing layer
Authors: Park Kye-Hong , Metcalfe Ralph W. , Hussain Fazle
Published: July 2002

The effects of coherent structures on mixing and chemical reaction in a temporally growing plane mixing layer are studied by direct numerical simulations of an incompressible flow with binary, irreversible, isothermal reactions. ‘‘Rolls’’ and ‘‘ribs’’ (counter-rotating longitudinal vortices connecting the nominally spanwise rolls) give rise to two distinctively different shapes for probability density functions (PDFs) of species concentration. It is found that roll stirring causes stationary or reverse marching PDF peaks, whereas rib stirring produces marching peaks. Deviation of the mean-mixed fluid (MMF) concentration from the mean species concentration can be caused by transverse transport of species by roll stirring. The shape of the MMF curve can be determined by the counteracting effects of the ribs and rolls. The ribs can cause significant reaction enhancement: product generation in the braid can vary by a factor of 3 compared to that in the roll for the Schmidt numbers (0.07≤Sc≤0.7) and (first) Damköhler numbers (0≤DI≤∞) studied. A high product generation rate in the braid—the case of a chemically active braid—occurs for Pe ≲ 10DI′, where Pe is a Péclet number, and DI′ is DI based on the species separation thickness. The effect of ribs on local product generation becomes more significant as DI and, more importantly, Sc increase. The reaction rate is strongly dominated by the roll dynamics when SR≲10MR, where SR is the reactive stirring rate in the ribs, and MR is the species mixing rate. The coherent structure effects found in this study help explain the experimentally observed variations in species concentration PDFs and MMF curves in both the pre- and post-mixing transition states, and suggest some additional mechanisms for reaction enhancement.
Similarity states of passive scalar transport in isotropic turbulence
Authors: Chasnov J. R.
Published: July 2002

By simple analytical and large-eddy simulations, the time evolution of the kinetic energy and scalar variance in decaying isotropic turbulence transporting passive scalars are determined. The evolution of a passive scalar field with and without a uniform mean gradient is considered. First, similarity states of the flow during the final period of decay are discussed. Exact analytical solutions may be obtained, and these depend only on the form of the energy and scalar-variance spectra at low wave numbers, and the molecular transport coefficients. The solutions for a passive scalar field with mean-scalar gradient are of special interest, and we find that the scalar variance may grow or decay asymptotically in the final period, depending on the initial velocity distribution. Second, similarity states of the flow at high Reynolds and Péclet numbers are considered. Here it is assumed that the solutions also depend on the low-wave-number spectral coefficients, but not on the molecular transport coefficients. This results in a nonlinear dependence of the kinetic energy and scalar variance on the spectral coefficients, in contrast to the final period results. The analytical results obtained may be exact when the similarity solutions depend only on spectral coefficients that are time invariant. The present analysis also leads directly to a similarity state for a passive scalar field with uniform mean scalar gradient. Last, large-eddy simulations of the flow field are performed to test the theoretical results. Asymptotic similarity states at large times in the simulations are obtained and found to be in good agreement with predictions of the analysis. Several dimensionless quantities are also determined, which compare favorably to earlier experimental results. An argument for the inertial subrange scaling of the scalar-flux spectrum is presented, which yields a spectrum proportional to the scalar gradient and decaying as k−7/3. This result is partially supported by the small-scale statistics of the large-eddy simulations.
Linear oscillations of weakly dissipative axisymmetric liquid bridges
Authors: Higuera María , Nicolás José A. , Vega José M.
Published: July 2002

Linear oscillations of axisymmetric capillary bridges are analyzed for large values of the modified Reynolds number C−1. There are two kinds of oscillating modes. For nearly inviscid modes (the flow being potential, except in boundary layers), it is seen that the damping rate −ΩR and the frequency ΩI are of the form ΩR=ω1C1/2+ω2C+𝒪(C3/2) and ΩI=ω0+ω1C1/2+𝒪(C3/2), where the coefficients ω0≳0, ω1<0, and ω2<0 depend on the aspect ratio of the bridge and the mode being excited. This result compares well with numerical results if C≲0.01, while the leading term in the expansion of the damping rate (that was already known) gives a bad approximation, except for unrealistically large values of the modified Reynolds number (C≲10−6). Viscous modes (involving a nonvanishing vorticity distribution everywhere in the liquid bridge), providing damping rates of the order of C, are also considered.
Distributed acoustic receptivity in laminar flow control configurations
Authors: Choudhari Meelan
Published: July 2002

A model problem related to distributed receptivity to free-stream acoustic waves in laminar flow control (LFC) configurations is studied within the Orr–Sommerfeld framework, by developing a suitable extension of the Goldstein–Ruban theory for receptivity due to localized disturbances on the airfoil surface. One advantage of the proposed method is that it easily permits the prediction of receptivity due to a wide class of noncompact distributions for surface nonuniformities, such as small-amplitude variations in suction velocity and surface admittance. The results, thus, complement the earlier work on the receptivity produced by local variations in these two quantities. It is shown that the cumulative effect of distributed receptivity can be substantially larger than that of a single, isolated suction strip or slot. Furthermore, even if the receptivity is spread out over very large distances, the most effective contributions come from a relatively short region in the vicinity of the lower branch of the neutral stability curve. Finally, it is found that the receptivity is effectively dominated by a narrow band of Fourier components from the wall-suction and admittance distributions, roughly corresponding to a detuning of less than 10% with respect to the neutral instability wave number at the frequency under consideration. The results also indicate that the drop-off in receptivity magnitudes away from the resonant wave number is nearly independent of the frequency parameter. Differences in receptivity due to the concentration of wall suction in different parts of the airfoil chord are also investigated.
Resonant triads of capillary–gravity waves in the presence of a current
Authors: Morland L. C.
Published: July 2002

Resonant triads of capillary–gravity waves propagating on the surface of inviscid fluid of infinite depth, in which the basic state is a plane, parallel current of the wind-drift type, are examined in the case that one of the waves propagates parallel to the current. The velocity profiles chosen for the current have nonconstant vorticity, necessitating numerical computation of the dispersion relation and the coefficients of the amplitude equations. A wave propagating in the direction of the current is found to participate in two families of triads, corresponding to those described by Simmons [Proc. R. Soc. London Ser. A 309, 551 (1969)] for a stagnant basic state. Only triads from one of the families will be excited, and this family only exists when the original wave has a sufficiently short wavelength. The presence of a current increases the maximum wavelength at which this family of resonances occurs and increases the maximum possible angle between the original wave and an oblique, excited, resonant wave. When a wave propagates against the current the maximum wavelength and the maximum angle are decreased by the presence of the current. The amplitude equations governing temporal modulation are derived, and in the cases examined, the coefficients are found to be of differing sign, which excludes the possibility of explosive instability.
A multiple translational temperature gas dynamics model
Authors: Candler Graham V. , Nijhawan Sandeep , Bose Deepak , Boyd Iain D.
Published: July 2002

Within shock waves, the translational motion of the gas is more energetic in the direction perpendicular to the shock than in the direction parallel to the shock. To represent this translational nonequilibrium, new continuum conservation equations are developed. These equations are derived by solving the Boltzmann equation with a first-order Chapman–Enskog expansion of an anisotropic velocity distribution function. This results in a gas model with anisotropic pressure, temperature, and speed of sound. The governing equations are solved numerically for one-dimensional steady shock waves in a Maxwellian gas. The numerical results are compared to those obtained using the direct simulation Monte Carlo method. The new continuum model captures many of the features of shock waves. In particular, this paper finds that translational nonequilibrium is present at all Mach numbers. For Mach numbers greater than 1.5, the perpendicular temperature overshoots the post-shock temperature. At the point where this temperature reaches a maximum, the model predicts that for any shock wave, the square of the perpendicular-direction Mach number is one-third; this is substantiated by the DSMC results.
Time-resolved velocity gradient and optical anisotropy in linear flow by photon correlation spectroscopy
Authors: Wang James J. , Yavich Dmitry , Leal L. Gary
Published: July 2002

A time-resolved dynamic light scattering technique is presented for measuring the velocity gradient in transient but repeatable flows. The experimental technique is verified via measurements for a Newtonian fluid undergoing a known time-dependent flow. The method is then applied to the creeping flow of a Newtonian fluid in a corotating two-roll mill. It is demonstrated that the flow near the stagnation point can be accurately described by an analytical creeping flow solution for a two-roll mill in an unbounded fluid. The time dependence of the velocity gradient for a concentrated polymer solution in the startup of the two-roll mill has also been measured, it is believed, for the first time. The measurement provides direct evidence of the modification of the flow for the viscoelastic polymer liquid, and will ultimately lead to significant insights into the polymer dynamics for concentrated solutions in strong, extension-like flows. A second significant feature of the dynamic light scattering experiment is that the initial magnitude of the correlation function is related to the degree of optical anisotropy of the polymer molecules, i.e., to the geometric configuration of the polymer chains. Thus, it yields information on the time-dependent degree of polymer orientation and stretch that is equivalent to birefringence, but is obtained at the ‘‘point’’ occupied by the scattering volume rather than as a two-dimensional average across the whole fluid as in birefringence. This measurement of polymer configuration is compared with birefringence data for the exact same flow.
Fluid cusps at the molecular scale
Authors: Koplik Joel , Banavar Jayanth R.
Published: July 2002

Recent experiments and calculations have exhibited apparent steady cusps in certain fluid free surfaces driven by a converging subsurface flow. Molecular dynamics simulations are used to elucidate some of the issues raised by this phenomenon, with emphasis on the behavior of the fluid on very small scales. The similar but different situation of two immiscible fluids of equal viscosity driven by solid rollers is considered. Molecular scale simulations of the evolution of the separating interface exhibit a gradual increase of curvature with rotation rate. However, high curvature interfaces do not reach a steady state, but instead drops of the fluid above the free surface are detached. In no case does a true cusp form, and the stress tensor is never unusually large in the near-cusp region.