bluuebunny/crossref_metadata_2025 huggingface dataset:
Reply to the comments of Riemann and van den Berg
Authors: Zawaideh Emad , Kim N. S. , Najmabadi Farrokh
Published: July 2002

The comments of Riemann and van den Berg represent a misinterpretation of what was presented by Zawaideh et al. [Phys. Fluids B 2, 647 (1990)]. Namely, they claim that the oscillations in the electric field in the presheath region are a result of incorrect boundary conditions. To disprove their claim, both the sheath and presheath region with no assumption about quasineutrality have been solved self-consistently. To eliminate the issue of incorrect boundary conditions, a symmetrically confined plasma between two floating electrodes was considered. From the self-consistent solution, it is shown that, for subsonic flow, the oscillations in the electric field do exist in the absence of external particle sources and are not due to an incorrect boundary condition.
Experimental simulation of a gaseous divertor: Measurements of neutral density inside the plasma
Authors: Fiksel G. , Kishinevsky M. , Hershkowitz N.
Published: July 2002

Direct measurements of the neutral density in the core of hydrogen plasma with a density of 3–4×1014 cm−3 and electron temperature of 15–20 eV in a magnetic field of 0.2 T, injected into hydrogen neutral gas at a pressure of 0.1–2 Torr are performed with plasma emission spectroscopy. The data are in agreement with the results of measured plasma decay [Phys. Fluids B 2, 837 (1990)] and can be explained by radial pressure balance between fast neutrals inside the plasma and cold background gas.
The influence of some atomic processes on the compression of microballoons
Authors: Mostacci D. , Dinguirard J.-P. , Montierth L. , Marcerou P.
Published: July 2002

One-dimensional computer simulations have been performed to investigate the effect that preheating of the filling gas by x rays coming from the shell has on the compression of inertial confinement fusion microballoons, and the dependence of this effect on the equation of state describing the filling gas. The targets considered here are glass microballoons filled with D–T mixtures at an initial pressure of 10 bars. Nonlocal thermal equilibrium ionization is explicitly calculated and along with inverse bremsstrahlung, photoionization is taken into account, both as an absorption mechanism and for its effect on the ionization state. The results clearly indicate that radiative preheating energy has a non-negligible adverse effect on compression, as evidenced by the Lawson criterion, and the effect of photoionization and out of equilibrium ionization proves important.
The influence of subsidiary Langmuir decay on the spectrum of stimulated Raman scattering
Authors: Drake R. P. , Batha S. H.
Published: July 2002

The saturation of stimulated Raman scattering (SRS) from plasmas produced by visible or ultraviolet (UV) lasers is discussed. A correspondence is reported between relatively strong plasma-wave damping and relatively large spectral intensity in the observed SRS spectra. This observation is interpreted by the hypothesis that SRS saturates at roughly the level of marginal stability for Langmuir decay.
Gyroinvariant high-order orbit theory: General treatment
Authors: Martin Pablo , Haines Malcolm
Published: July 2002

The new approach to calculating gyroinvariant drift velocities, recently published [Phys. Fluids B 2, 11 (1990)], is generalized to the case of nonuniform and time variable electric and magnetic fields. As before, a continuous averaging is done over a suitable quasiperiodic function. However, now the nonanalytic complex functions are related to a rotating frame. Successive integration by parts is done as before to obtain higher-order drift velocities, in contrast to the Hamiltonian or Lagrangian treatment of Littlejohn and other authors. The results in this paper are gyroinvariant as in the earlier treatment and the second-order drift velocities found are more compact and simpler than previous ones. The guiding center position is also determined to second order using algebraic procedures.
Study of plasma edge turbulence in tokamaks
Authors: Garbet X. , Laurent L. , Mourgues F. , Roubin J. P. , Samain A.
Published: July 2002

The plasma edge in tokamaks is known to be very turbulent. The nonlinear stability of a test mode is investigated in the presence of a helical potential perturbation, i.e., a ‘‘pump mode,’’ which simulates the plasma turbulence. The particle trajectories in this perturbed equilibrium are derived using a Hamiltonian formalism. The electrons appear to have trapped trajectories in the potential well of the pump mode, while the ions experience a large convective motion. These two effects have a large influence on the test mode stability. First, nonlinearly trapped electrons supply an energy source for the test mode. Second, the ion convective motion introduces a radial scale of the test mode larger than the ion Larmor radius, in agreement with experimental data. These two phenomena allow a bifurcation in the turbulence level and therefore provide an explanation for the L–H transition.
Resolution of zero-frequency modes into finite-frequency modes
Authors: Ryu Chang-Mo
Published: July 2002

A two and one-half dimensional (21/2-D) particle simulation study showing the resolution of the zero-frequency modes into finite-frequency modes is presented. The resolution of the electrostatic mode at a nonzero frequency indicates a strong coupling between the longitudinal electric field fluctuation and the magnetic field fluctuation.
Effects of magnetic geometry, fluctuations, and electric fields on confinement in the Advanced Toroidal Facility
Authors: Isler R. C. , Aceto S. , Baylor L. R. , Bigelow T. S. , Bell G. L. , Bell J. D. , Carreras B. A. , Colchin R. J. , Crume E. C. , Dominguez N. , Dory R. A. , Dunlap J. L. , Dyer G. R. , England A. C. , Gandy R. F. , Glowienka J. C. , Hanson G. R. , Harris J. H. , Hiroe S. , Horton L. D. , Jernigan T. C. , Ji H. , Langley R. A. , Lee D. K. , Likin K. M. , Lyon J. F. , Ma C. H. , Morimoto S. , Murakami M. , Okada H. , Qualls A. L. , Rasmussen D. A. , Rome J. A. , Sato M. , Schwelberger J. G. , Shats M. G. , Simpkins J. E. , Thomas C. E. , Uckan T. , Wade M. R. , Wilgen J. B. , Wing W. R. , Yamada H. , Zielinski J. J.
Published: July 2002

Recent experiments in the Advanced Toroidal Facility (ATF) [Fusion Technol. 10, 179 (1986)] have been directed toward investigations of the basic physics mechanisms that control confinement in this device. Measurements of the density fluctuations throughout the plasma volume have provided indications for the existence of theoretically predicted dissipative trapped electron and resistive interchange instabilities. These identifications are supported by results of dynamic configuration scans of the magnetic fields during which the magnetic well volume, shear, and fraction of confined trapped particles are changed continuously. The influence of magnetic islands on the global confinement has been studied by deliberately applying error fields which strongly perturb the nested flux-surface geometry, and the effects of electric fields have been investigated by means of biased limiter experiments.
Toroidal effects on the nonlinearly saturated m=1 island in tokamaks
Authors: Avinash K. , Haas F. A. , Thyagaraja A.
Published: July 2002

This Brief Communication investigates the influence of toroidal effects (due to the coupling of various poloidal harmonics) on the nonlinear saturation of the m=1 island. Bounds are obtained relating the aspect ratio, the shear at the q=1 surface, and the saturated island width. Provided these bounds are satisfied, it is then found that the cylindrical m=1 island theory of Thyagaraja and Haas [Phys. Fluids B 3, 580 (1991)] is valid for toroidal geometry.
Comment on the stability theorem of Davidson and Lund
Authors: Smith Ralph A. , O’Neil Thomas M. , Lund Steven M. , Ramos Jesus J. , Davidson Ronald C.
Published: July 2002

A convexity argument is used to show that an equilibrium in the nonlinear stability of cold electron plasma must be axisymmetric. (AIP)
Stabilization of the m=1 tearing mode by resonance detuning
Authors: Thyagaraja A. , Hazeltine R. D. , Aydemir A. Y.
Published: July 2002

The possibility of stabilizing the m=1 tearing mode by rapidly oscillating the resonance point rs(t) about its mean position is examined. The calculations are carried out for both externally controlled, coherent oscillations of rs(t), as well as those resulting from turbulent plasma motions. Complete stabilization is possible in the coherent case, while turbulent fluctuations may yield substantial reduction in the growth rate. The technique seems to apply to any linear mode that depends upon local properties around a resonant surface.
The exact and drift Hamiltonian
Authors: Yao Qun , Boozer Allen H.
Published: July 2002

A canonical Hamiltonian treatment for the motion of the guiding center of a charged particle in a magnetic field of a toroidal configuration is given. The Hamiltonian of the exact trajectory, first- and second-order corrections to the standard drift Hamiltonian are given in the Boozer coordinates for a vacuum magnetic field with magnetic surfaces. The exact Hamiltonian depends on both the field strength and the shape of the magnetic surfaces (the metric of the magnetic coordinates) while the standard drift Hamiltonian depends only on the field strength. The first-order correction, in gyroradius to system size, to the standard drift Hamiltonian depends in a generic way on the metric of the magnetic coordinates, as does the exact Hamiltonian.
Heat flux due to poloidal electric field in the banana regime
Authors: Taguchi M.
Published: July 2002

The heat flux due to poloidally varying electrostatic potential is calculated in the banana regime. This electrostatic potential determined self-consistently from charge neutrality is shown to increase the electron heat flux by a factor √mi/me compared with that when this potential is neglected, where me and mi are the masses of electron and ion, respectively.
Potential well formation by injection of electrons with various energy distributions into a sphere or a slab
Authors: Wong S. Kai , Krall Nicholas A.
Published: July 2002

The details of the potential well structure produced by the injection of electrons from high-energy electron guns is studied as a function of the energy, velocity spread, and density of injected electrons. The effect of geometry, slab versus sphere, is also calculated. A motivation for the study is that some devices that attempt to produce fusion reactions from the acceleration of ions by an electric potential use energetic injected electrons to produce the potential.
Plasma potential enhancement by waves in the ion cyclotron range of frequencies
Authors: Nelson B. A. , Hershkowitz N.
Published: July 2002

Measured ion-confining potentials in a tandem mirror (using only power in the ion cyclotron range of frequencies to form and sustain the plasma) are found to be larger than those predicted by the Boltzmann relation. The behavior of the potentials is explained as a balance of electron collisional inscattering with electron pumping by time-varying axial electric fields. A single-particle following code is used to model the effect of the time-varying axial electric field on electrostatic/magnetic confinement. Magnitudes of the electric fields required to sustain these potentials agree with values predicted by the available plasma wave-field solving codes. Measured scaling of the ion-confining potential is in qualitative agreement with scaling predicted by the model.
Ion orbit loss and poloidal plasma rotation in tokamaks
Authors: Shaing K. C.
Published: July 2002

A momentum balance equation including the effect of ion orbit loss is employed to discuss the poloidal plasma rotation in tokamaks. It is found that in a steady state, the high-energy collisionless ions leave the system through an ion orbit loss mechanism and the low-energy collisional ions convect into the system as viscosity-driven flux in such a way that there is no net flux-surface-averaged radial current across the flux surface. The value of poloidal rotation necessary to maintain steady-state ambipolarity is different from that of the conventional neoclassical theory. This confirms the previous results [Phys. Rev. Lett. 63, 2369 (1989)]. It is also shown, by considering the effects of incompressible flow in the phase space described by the orbit-averaged Coulomb collision operator, that the ion orbit loss flux is intimately related to the plasma viscosity.
Holographic interferograms from laser fusion code simulations
Authors: Prasad M. K. , Estabrook K. G. , Harte J. A. , Craxton R. S. , Bosch R. A. , Busch Gar. E. , Kollin J. S.
Published: July 2002

The simulation of interferograms for laser fusion-type targets is described. Three-dimensional ray-tracing postprocessors to the two-dimensional hydrodynamic codes lasnex and sage are used for this purpose. Simulated interferograms can then be directly compared with experimentally obtained interferograms, thus providing insight into various aspects of target physics, such as anisotropic thermal transport and details of the hydrodynamic flow. The method is illustrated for two specific KMS microdisk target shots diagnosed with double-exposure holographic interferometry.
Atomic physics effects on tokamak edge drift-tearing modes
Authors: Hahm T. S.
Published: July 2002

The effects of ionization and charge exchange on the linear stability of drift-tearing modes are analytically investigated. In particular, the linear instability threshold ΔTh, produced by ion sound wave coupling [Phys. Rev. Lett. 40, 1500 (1978)] is modified. In the strongly collisional regime, the ionization breaks up the near cancellation of the perturbed electric field and the pressure gradient along the magnetic field, and increases the threshold. In the semicollisional regime, both ionization and charge exchange act as drag on the ion parallel velocity [Phys. Fluids B 4, 2567 (1992)], and consequently decrease the threshold by reducing the effectiveness of ion sound wave propagation.
Magnetic field penetration due to the Hall field in (almost) collisionless plasmas*
Authors: Fruchtman A. , Gomberoff K.
Published: July 2002

The fast penetration of magnetic fields into plasmas due to the Hall field is described. The penetration occurs in nonuniform plasmas of a characteristic length smaller than the ion skin depth, it is much faster than the ion motion, and its rate is independent of the resistivity. Some previous results are described: a shock penetration of the magnetic field accompanied by a large energy dissipation, and field expulsion from an initially magnetized plasma. It is then shown how the Hall field can enhance the penetration into a plasma surrounded by vacuum. Finally, it is demonstrated how the evolution of the magnetic field in a plasma that conducts current between electrodes depends crucially on its evolution near the electrodes, when a realistic density profile is taken into account.
Spontaneous and driven perpendicular rotation in tokamaks
Authors: Hassam A. B. , Antonsen T. M. , Drake J. F. , Guzdar P. N. , Liu C. S. , McCarthy D. R. , Waelbroeck F. L.
Published: July 2002

Recent theoretical work pertaining to spontaneously generated or forced plasma rotation in tokamaks is discussed. A description of the spontaneous poloidal spin-up of tokamaks from the Stringer effect is given, highlighting the necessary condition of poloidally asymmetric particle accumulation. The possibility of inducing poloidal rotation using the Stringer effect by poloidally asymmetric particle fueling is suggested. The linear theory of E×B velocity shear stabilization of tokamak microinstabilities is discussed with an emphasis on the general features of the theory and some nonlinear concerns. It is argued that the critical velocity shear for stabilization of microinstabilities in tokamaks required by linear theory may be, in order of magnitude, a universal frequency. The feasibility of driving perpendicular rotation in tokamaks by neutral beam injection, to suppress microturbulence, is assessed for both toroidal and poloidal injection schemes.
Effects of non-Maxwellian electron distributions on charge-state populations in laser-produced plasmas
Authors: Lee Y. T. , Lamoureux M.
Published: July 2002

The effects of a non-Maxwellian electron distribution on the charge-state populations in a plasma with the distribution characterized by the function f(v)=Fm exp[−(v/vm)m] with 2≤m≤5 are investigated. In the underdense region of a laser produced plasma, the parameter m would depend on the electron temperature, electron density, and the average ionization state of the plasma in addition to the optical laser intensity and wavelength. The ion populations are obtained by solving the rate equations in which the coefficients are evaluated by integrating the cross sections over the non-Maxwellian electron distributions. The scaling of m with column density and optical laser intensity in laser exploding foils is obtained. The effects of a non-Maxwellian electron distribution on the charge-state populations in both selenium and molybdenum foils, similar to those used to model recent x-ray laser experiments, are calculated. The effects on the dominant populations are found to be small, less than a dozen of percents.
The effect of an applied magnetic field on ion generation processes in a rarefied gas
Authors: Kuperman V. Yu. , Friedman G.
Published: July 2002

An equilibrium under which ions’ escape from a generated plasma due to their own electric field occurs in the presence of an external uniform magnetic field is studied analytically. It is found that for an arbitrary source of ionization there is a limiting value (threshold) of the magnetic field after which no balance between birth and collisionless escape of ions is possible and, therefore, no stationary process of ion generation can exist. An experimental setup allowing the observation of the threshold is discussed.
Kinetic theory of a nonequilibrium plasma: Evaluation of the vectorized collisional Boltzmann equation
Authors: Morgenthaler Ann W. , Hagelstein Peter L.
Published: July 2002

Cartesian velocity moments of the Boltzmann equation are evaluated using modal solutions to the spherical harmonic oscillator as a basis set. The nonlinear collision matrix describing the interaction between any two modes is evaluated analytically for the Landau collision operator, and matrix elements describing collisions between identical particles are calculated for some pairs of azimuthally symmetric modes. First-order linear transport coefficients calculated directly from collision matrix elements are shown to agree exactly with previously published results; coefficients of thermal conductivity and viscosity are computed much more accurately by trivially extending this calculation. Relaxation times for self-collisions in a two-dimensional linearized plasma are also computed, indicating that the plasma equilibrates in roughly one to ten times the Spitzer self-collision time. The results obtained in this paper are useful for both analytic and numerical simulations of nonequilibrium plasmas and an explicit six-moment model for a one-component azimuthally symmetric plasma is given.
Theory of multiple-helicity interactions in dissipative trapped-electron drift wave turbulence
Authors: Liang Y.-M. , Diamond P. H.
Published: July 2002

The two-nonlinearity model of dissipative trapped-electron drift wave turbulence [Y.-M. Liang et al., Phys. Fluids B 5, 1128 (1993)] is generalized to include the effects of a sheared magnetic field. Because of the coupling of drift waves with the damped ion-acoustic modes, the eigenmode k has a radial structure centered on its rational surface characterized by a mode width Δk. In this work, both the linear properties of the model, and the multiple-helicity nonlinear dynamics, including the interactions of the two nonlinearities, i.e., the E×B drift and the polarization drift nonlinearities, are analyzed in detail. In particular, a one-point renormalization is performed to investigate the nonlinear eigenmode properties and the saturation of the system in the multiple-helicity limit. It is shown that, for the ultrastrong magnetic shear limit where Δk≤2ρs, the polarization drift nonlinearity is the dominant nonlinear transfer mechanism. However, for the moderate-to-weak magnetic shear limit where Δk≫2ρs, both nonlinearities and cross-coupling effects will affect the nonlinear transfer processes. An analytic explanation of previous computational observation of the suppression of magnetic shear damping by turbulence [D. Biskamp and M. Walter, Phys. Lett. A 109, 34 (1985)] is also given.
Two-dimensional numerical simulation of trapped ion mode turbulence in a tokamak
Authors: Kingsbury O. T. , Waltz R. E.
Published: July 2002

A two-dimensional k-space simulation of long-wavelength trapped ion mode turbulence is used to investigate Bohm (macroscopic) versus gyro-Bohm (microscopic) scaling behavior in tokamaks. A nonlinear two-field model of dissipative trapped ion turbulence evolving trapped ion and trapped electron density fluctuations shows gyro-Bohm-like scaling in contrast to earlier simulation work by Saison et al. [Plasma Phys. 20, 1 (1978)]. In fact, nearly all features of trapped ion mode turbulence and transport originally posited by Kadomtsev and Pogutse [Reviews of Plasma Physics (Consultants Bureau, New York, 1970), Vol. 5, p. 379] are recovered. However, the widely used Kadomtsev–Pogutse one-field approximation to the two-field dissipative trapped ion model is unexpectedly found to be unrepresentative with much different fluctuation levels and diffusion. Transformation to a new field basis diagonalizing the linear part of the two-field equations clarifies the importance of the damped branch not contained in the one-field equation. The two-field collisionless trapped ion mode equations are transformed into a parameterless single equation for a complex field providing a perfect scaling relation for the dependence on the driving curvature drifts.
Confinement effects of electron heating in the thermal barrier of a tandem mirror
Authors: Carrera R. , Catto Peter J.
Published: July 2002

In order to assess the possible benefit of applying low levels of electron-cyclotron resonance heating (ECRH) in the thermal barrier region of a tandem mirror a simple thermal barrier model is considered. The collisional fluxes of the hot, barrier electrons diffusing into the central cell and plug, the warm electrons escaping from the plug, and the cooler, central-cell electrons passing over the thermal barrier and becoming trapped, are evaluated.
Nonlinear propagation of electromagnetic ion-cyclotron Alfvén waves
Authors: Shukla P. K. , Stenflo L.
Published: July 2002

The nonlinear interaction of an electron-ion plasma with an electromagnetic ion-cyclotron Alfvén wave propagating along an external magnetic field is considered. It is found that the radiation pressure of the electromagnetic wave can excite field-aligned electrostatic density perturbations. Localized wave packets are shown to exist. The relevance of our investigation to laboratory and astrophysical plasmas is discussed.
Azimuthal electric fields and ambipolarity in a multiple-helicity torsatron
Authors: Hastings D. E. , Shaing K. C.
Published: July 2002

In a torsatron there are multiple solutions to the ambipolarity relationship for the electric field. If the electric field is small over some region of space then the self-consistent poloidal electric field can be important and lead to potential islands. If the plasma is in the superbanana plateau regime, then slow resonant particles limit the rate of change of the electric field and, hence, give a minimum width for the spatial zone where the plasma is changing roots of the ambipolarity relationship.
Reply to Comments of Mynick and Hitchon
Authors: Shaing K. C. , Rome J. A. , Fowler R. H.
Published: July 2002

A reply to the comment on charged particle transport associated with the collisionless detrapping/retrapping orbits in a nonaxisymmetric torus is presented. (AIP)
Confinement and transport in stellarators
Authors: Betancourt O. , Garabedian P.
Published: July 2002

A model of particle transport has been incorporated in the equilibrium and stability code beta. From the Clebsch potentials of Boozer’s theory a first-order partial differential equation with constant coefficients is derived relating a guiding center step size λ∥/‖∇s‖ to the parallel gyroradius ρ∥. In terms of the Fourier coefficients of ρ∥, a formal solution for λ∥ is found that exhibits the resonance at rational values of the rotational transform explicitly. Phenomenological reasoning leads to a transport coefficient for both circulating and trapped particles that involves integrals of λ2∥ analogous to a term in the Mercier stability criterion depending on the square of the Pfirsch–Schlüter current. Good correlation has been established between plateau values of this transport coefficient and Monte Carlo calculations performed by H. Wobig for the Wendelstein VII-AS experiment, for a typical Heliac, and for reference l=2 stellarators.
Finite Larmor radius diocotron instability
Authors: Kleva Robert G. , Ott Edward , Manheimer Wallace M.
Published: July 2002

The diocotron instability of an electron layer in which the electron Larmor radius is of the order of the layer thickness is studied. Remarkably, exact analytical solutions are obtainable in a nontrivial special case. These results allow an examination of the effects of finite Larmor radius for arbitrary ratios of Larmor radius to wavelength and of Larmor radius to layer thickness. In addition, an energy principle which yields a necessary and sufficient condition for instability for general distribution functions is derived.
Multiwavelength laser solid–target interaction at 10 microns
Authors: Lavigne P. , Johnston T. W. , Pascale D. , Pépin H. , Piché M. , Martin F. , Décoste R. , Estabrook Kent
Published: July 2002

Some aspects of multiple-wavelength interaction on solid targets have been experimentally studied at CO2-laser wavelengths with 1-nsec pulses. Contrary to predictions, the use of a two-line (10.6- and 9.6-μm) source does not reduce the back reflection. This result could be explained by a very short Brillouin interaction length or/and a very low saturation level. The hot-electron production is also not significantly modified by multiline illumination. A signal at the frequency sum indicates that, even at Iλ2 as low as 5×1013 W cm−2 μm2, the critical surface is not well defined and that both the 9.6- and 10.6-μm light interact in the same resonant zone. Finally, significant sidebands were measured at 8.7 and 12 μm. Some 1 (1)/(2) -dimensional simulations with mobile ions indicate that those sidebands probably originate from the beat frequency resonant density at 0.0123nc.
Wave absorption at the second harmonic of the electron-cyclotron frequency in a tokamak plasma
Authors: McDermott F. S. , Bekefi G. , England A. C. , Attenberger S. E. , Batchelor D. B. , Edmonds P. H. , Goldfinger R. C. , Kindsfather R. R. , Lazarus E. A. , Murakami M. , Neilson G. H. , Wilgen J. B. , Wootton A. J.
Published: July 2002

Wave absorption in the ISX-B tokamak [Phys. Rev. Lett. 44, 647 (1980)] at the second harmonic (ω=2ωce) of the electron-cyclotron frequency is reported. Measurements of the absorption of a wave polarized in the extraordinary mode and propagating perpendicular to the toroidal magnetic field are in agreement with computations.
Electrostatic radiation in a hot magnetoplasma: Intrinsic diffraction or interference effects?
Authors: de Feraudy H. , Lembege B.
Published: July 2002

Several experiments have evidenced oscillatory patterns of the electrostatic potential radiated by a small antenna in a hot magnetoplasma. Several previous interpretations based on analytical calculations and numerical computations are discussed; they are found not to be exhaustive or completely satisfactory. A new mechanism responsible for these structures is proposed. It consists of a filtering by the plasma of the wave vectors of the radiated waves. This mechanism is caused by the existence of a restricted range of undamped wave vectors surrounded by highly damped ones. This phenomenon, presently named ‘‘intrinsic diffraction,’’ is used to interpret both the spatial oscillations of the potential around the resonance cone when ω<min(ωp, ωc) and around the direction perpendicular to the magnetostatic field when nωc<ω<(n+1)ωc; ω, ωp, ωc are, respectively, the wave, the plasma, and the electron-cyclotron frequencies, and n is any positive integer. The results of both analytical calculations and numerical computations are found to be in good agreement within the two frequency ranges. The location and the amplitude of the oscillatory structure are simply related to the width of the domain of unattenuated wave vectors. New diagnosis methods for the plasma temperature are suggested from this model. Additional directivity and curvature effects related to the topology of the wavenumber surfaces are discussed. The generalization of the use of the intrinsic diffraction picture is proposed.
Linear versus energy stability for time-periodic flows
Authors: Herron Isom H.
Published: July 2002

An upper bound is calculated for the real parts of the Floquet exponents for the equation governing the linearized stability of time-periodic flows. The bound is given by the average over a period of the first energy eigenvalue. In particular, this shows that for modulated Taylor flow, mean energy stability implies linear stability, while linear instability implies mean energy instability at the same Taylor number.
Electrostatic ion cyclotron instability near threshold
Authors: McWilliams R. , Okubo M. K. , Wolf N. S.
Published: July 2002

By drawing an electron current through a plasma to a variably biased conducting button, the electrostatic ion cyclotron instability is excited, consistent with predictions of Drummond and Rosenbluth [Phys. Fluids 5, 1507 (1962)]. Recent results show that a nonuniform steady-state electric field reaches axially into the plasma. Our results indicate that electrons are accelerated along magnetic field lines and decelerated by collisions. The point in space where the electron drift speed exceeds a critical speed (vc∼0.2vte) determines the instability amplitude and frequency. This initiation point moved further into the plasma as the externally controlled electric field was raised. This experiment demonstrates the importance of applying biases so that eφ/T≲1, near threshold, to observe the linear electrostatic ion cyclotron instability.
Free streamline model for a rising bubble
Authors: Vanden-Broeck Jean-Marc
Published: July 2002

The motion of a two-dimensional bubble rising at a constant velocity in an unbounded fluid is solved by series truncation. It is assumed that there is a wake of stagnant liquid extending to infinity below the bubble. Both the effects of gravity g and surface tension T are taken into account. It is shown that the problem is characterized by a continuum of solutions for T=0 and by a discrete set of solutions when T>0. In addition a unique solution is obtained in the limit as the surface tension approaches zero. The corresponding profile of the bubble is found to be in good agreement with experimental data.
Nonlinear electron transport in magnetized laser plasmas
Authors: Kho T. H. , Haines M. G.
Published: July 2002

Electron transport in a magnetized plasma heated by inverse bremsstrahlung is studied numerically using a nonlinear Fokker–Planck model with self-consistent E and B fields. The numerical scheme is described. Nonlocal transport is found to alter many of the transport coefficients derived from linear transport theory, in particular, the Nernst and Righi–Leduc effects, in addition to the perpendicular heat flux q⊥, are substantially reduced near critical surface. The magnetic field, however, remains strongly coupled to the nonlinear q⊥ and, as has been found in hydrosimulations, convective amplification of the magnetic field occurs in the overdense plasma.
A simple model for the average local entrainment rate
Authors: Dancey C. L.
Published: July 2002

A new expression for the mean local entrainment rate in a turbulent intermittent flow is obtained. This expression is used to obtain a simple model for the entrainment rate assuming that an indicator function can be defined for the flow, that the interface defined by the indicator function is homogeneous in two directions, and that the turbulent Reynolds number is very large. A particularly simple form is obtained if the intermittency is in a scalar imbedded in a turbulence field and the correlation coefficient for the indicator function is self-similar. The final expression compares favorably with the limited existing data and with other model expressions. Recommendations for experimental verification are presented.
Evolution of a non-self-preserving thermal mixing layer
Authors: Lumley J. L.
Published: July 2002

In an attempt to explain apparently anomalous measurements, a first integral of the equation relating mean temperature and heat flux is obtained using a Gram–Charlier representation. This permits relating moments of the heat flux distribution to derivatives of coefficients in the mean temperature representation. The same sort of analysis is applied to the temperature variance equation, giving other relations among coefficients. It is concluded that the measurements are consistent with a very small and rapidly decaying fourth-order component which has a surprisingly large effect on the maximum value of the heat flux. It is found that the evolution of the layer certainly cannot be described by an eddy viscosity. Several relations (e.g., connecting the integral of the heat flux with the growth of the length scale, and another giving an estimate for the temperature variance) are thought to be new, are not limited by assumptions of self-preservation, and should prove useful in analyzing other data.
Analytical solutions to the general problem of oblique wave growth and damping
Authors: Thorne Richard M. , Summers Danny
Published: July 2002

Analytical solutions are presented for the linear growth or damping rate caused by resonant particle interactions with plasma waves propagating in any arbitrary direction relative to the ambient magnetic field. For the case of three realistic and widely adopted representations for the particle distribution function, namely for a bi-Maxwellian, a bi-Lorenzian, and a loss-cone distribution, the angular dependence of the net growth rate can be expressed in terms of simple, smoothly varying functions involving modified Bessel functions of the first and second kind. Furthermore, in the limits of both quasilongitudinal and quasitransverse wave propagation, the analytical results reduce to simple algebraic expressions that may readily be used to compare the contributions from any specific harmonic resonance. These analytical solutions eliminate the need for costly and time-consuming numerical integration that hitherto was the standard procedure for obtaining growth rates for oblique waves.
Transition of a tokamak plasma from a state with low- to high-poloidal beta
Authors: Deniz A. V. , Marshall T. C. , Ivers T. , Li X. , Mauel M.
Published: July 2002

A non-flux-conserving transition of tokamak plasma from a state with low-poloidal beta to a state with high-poloidal beta is reported, using data obtained from the Columbia high-beta tokamak machine [Nucl. Fusion 25, 1109 (1985)]. The plasma survives the transition without any low-mode number kink instability or disruption, possibly because of an increasing plasma safety factor.
Motion of an electron bunch through a plasma
Authors: Shoucri M. M. , Storey L. R. O.
Published: July 2002

The motion of a tenuous bunch of fast electrons through an isotropic plasma has been studied using a one-dimensional Vlasov code [J. Comp. Phys. 27, 315 (1978)]. Given a fairly large initial perturbation from equilibrium, the moving bunch sheds a train of nonlinear plasma oscillations, which appear to be sustained by a mild instability.
Richardson criteria for stratified vortex motions under gravity
Authors: Fung Y. T.
Published: July 2002

Three sufficient conditions for stability are established for a general class of rotating flows with the velocity and the density distribution varying in both the axial and radial directions. Two are the classical Richardson criteria in the axial and radial directions. The third, measured by a newly defined Richardson number, acts as a constraint on the other two. The newly defined Richardson number is a ratio between the interaction of the density variations with the force fields and the interaction of the velocity gradients in two directions. The former interaction is a result of the pressure restraint condition and is measured by a new Brunt–Väisälä frequency. The latter interaction determines whether the velocity gradients in the second direction strengthen or weaken the resultant shear effect. Because of the generality of the flow profiles being considered, the criteria established in this investigation are valid for a wide range of problems in oceanographic and atmospheric studies.
Instability of compound vortex layers and wakes
Authors: Pozrikidis C. , Higdon J. J. L.
Published: July 2002

The stability of two adjoining vortex layers with constant vorticity of opposite sign is analyzed. Linear stability analysis shows that two families of instability may exist, depending on the relative strength of the vortex layers. The first type for moderate and long waves produces simultaneous growth of disturbances on both layers, while the second for short waves is associated primarily with the weaker layer. Numerical calculations show that the nonlinear growth of the various types of instability leads to asymptotic states of different character. For wake-like flows with equal but opposite vorticity distributions, the fastest growing eigenmodes lead to the formation of a classic vortex street with an aspect ratio of 0.345. Longer waves lead to the break up of the layer into a number of small vortex regions producing disorganized motion and a general dispersal of the wake vorticity. For compound shear layers with unequal strength, the fastest growing modes show a progression from wake-like behavior to pure shear-layer behavior as the strength of the second layer diminishes. In addition, there is a new type of instability associated with short-wavelength disturbances on the weaker layer. In this case, the shear layer ejects the opposite signed vorticity along with an equal quantity of its own circulation. The ejected vorticity propagates away from the layer at a 45° angle in the form of neutral vortex pairs. The remaining vorticity forms a simpler shear layer of reduced strength.
A velocity-biased turbulent mixing model for passive scalars in homogeneous turbulence
Authors: Song J. C.
Published: July 2002

In applications to turbulent reactive flows, the probability density function approach requires a model for molecular diffusivity. The objective of the present work is to develop a consistent model for passive scalars in turbulent flows, improving the calculation of the velocity-scalar correlation coefficient. This is done by using the Langevin model for the velocity and by adjusting two model parameters of a velocity-biased mixing model for the scalar, which are determined by reference to homogeneous isotropic turbulent flows with a uniform mean cross-stream scalar gradient. Calculated and measured profiles are in good agreement.
A study of ignition and burn propagation in reactor-size inertial fusion targets using a three-temperature plasma simulation model
Authors: Tahir Naeem A. , Long Keith A.
Published: July 2002

In a previous paper by Tahir and Long [Phys. Fluids 29, 1282 (1986)] it was shown that radiation energy transport can compensate for the range shortening effects in ion-beam inertial confinement fusion (ICF) plasma. Implosion results of a high-gain, ion-beam driven, reactor-size ICF target including radiation effects were also reported. These implosion simulations were carried out under an assumption of a complete thermodynamic equilibrium between the radiation field and the electrons. This assumption, however, is only applicable in optically thick media and is thus valid in the compression phase of ICF targets only. In the burn phase the radiation field is totally decoupled from the electrons and the above approximation breaks down. A three-temperature model described in another paper by Tahir et al. [J. Appl. Phys. 60, 898 (1986)] is a much better model for such a regime. Using this three-temperature model, the ignition and burn of the target reported in the Tahir–Long paper have been recalculated and these improved results are presented here. The importance of the inverse Compton effect and the α-particle transport in inertial fusion burn plasmas is also discussed.
Thermocapillary effects in liquid bridges. I. Thermocapillary convection
Authors: Rybicki A. , Floryan J. M.
Published: July 2002

The steady thermocapillary convection in an axisymmetric liquid bridge in the absence of gravitational forces is considered. The relative change in the surface tension is assumed to be very small, i.e., the appropriate capillary number is much less than 1. A detailed analysis of the flow field is carried out for cylindrical bridges. The field equations are simplified by neglecting convective effects (Marangoni number M=0, Reynolds number Re=0). The effects of the bridge aspect ratio, differential heating of the side walls, and arbitrary heating of the free surface are analyzed numerically. The results show that flow patterns consisting of one, two, three, or more columns of vortices, or of their superposition, may exist in the liquid, depending on the type of heating. Decrease of the aspect ratio (long bridges) results in an emergence of the core zones of almost parallel flow. Increase of the aspect ratio (short bridges) leads to the emergence of several layers of vortices, with the strength of each layer decreasing approximately exponentially with the distance from the surface. It is noted that an asymptotic flow pattern around the interface emerges rapidly with the decrease of the length of the bridge.
Impinging stagnation flows
Authors: Wang C. Y.
Published: July 2002

Jets of different fluid properties impinge head-on. The region near the stagnation point is investigated. Similarity solutions of the Navier–Stokes equations are matched at the interface. It is found that the flow field depends heavily on the ratios of viscosity and density.
A comparison of the implementation and performance of the Nanbu and Bird direct simulation Monte Carlo methods
Authors: Boyd I. D. , Stark J. P. W.
Published: July 2002

Since Bird [Phys. Fluids 6, 1518 (1963)] first proposed the idea of direct simulation Monte Carlo (DSMC) techniques in order to model rarefied gas flows, a concern has been that his method does not provide a solution to the Boltzmann equation. Several simulation schemes have now been developed and, in a review of these methods, Nanbu [J. Phys. Soc. Jpn. 52, 3382 (1983)] showed that only his method [J. Phys. Soc. Jpn. 49, 2042 (1980)] gave a true solution to the Boltzmann equation. However, Nanbu’s simulation method is also the most computationally expensive scheme and in his review paper he recommended the use of the Bird technique for complicated engineering problems. Nanbu’s method also suffers from the apparent disadvantage that momentum and energy are not conserved at each collision. With the advent of the supercomputer the relative expense of the methods will inevitably be of lesser importance so that the testing of the Nanbu method in an engineering context is desirable. A detailed study of the implementation and performance of the Bird and Nanbu simulation schemes has therefore been carried out with reference to the expansion of a gas into vacuum. The results reported herein show, however, that in addition to increased computational overheads, the Nanbu method suffers from further defects that make its implementation more difficult. It is concluded that the Nanbu method is generally less flexible than that of Bird and that greater care must be exercised if meaningful results are to be obtained using the Nanbu approach.
Rippling mode in tokamaks
Authors: Hirose A. , Kroeker T. L.
Published: July 2002

It is shown that toroidicity has stabilizing effects on the rippling mode in tokamaks. For typical tokamak edge parameters, the instability as predicted in slab geometry is unlikely to exist.
A new angle on G. I. Taylor’s scraper
Authors: Jansons Kalvis M.
Published: July 2002

An extension of G. I. Taylor’s [G. I. Taylor: Scientific Papers, edited by G. K. Batchelor (Cambridge U.P., Cambridge, 1960), Vol. 4, p. 410] scraper problem (i.e., the displacement of a viscous fluid on a flat solid surface by a flat solid scraper) is investigated. Consider an infinite strip of viscous fluid of uniform width and height on a horizontal surface with a scraper that spans the full width. If the speed of the scraper U is given, what should the scraper’s orientation be to the strip to minimize the component of force in the direction of motion that must be applied to the scraper?
Experimental investigation of a coherent flute instability using a heavy ion beam probe
Authors: Glowienka J. C. , Jennings W. C. , Hickok R. L.
Published: July 2002

A coherent, low-frequency instability found in a cylindrical, hollow cathode arc plasma has been investigated by using a heavy ion beam probe (HIBP). The energy density of the plasma was high enough to render it inaccessible to Langmuir probes, but the HIBP was able to provide measurements throughout the plasma cross section. The data clearly show that azimuthal symmetry does not exist. Radial profiles of steady-state density and space potential and of simultaneous ñ, φ̃ amplitude and phase were obtained to allow detailed comparison between theory and experiment. Predictions from a cylindrically symmetric, small-perturbation theoretical model provide reasonably conclusive identification of the instability as a Kelvin–Helmholtz flute driven by and localized in a region of fluid shear. The most serious discrepancy was with regard to the oscillation frequency, which was consistently predicted to be three to four times lower than that observed experimentally. The reason for the discrepancy is not understood, but it is probably related to inadequacies in the theory caused by assumptions of azimuthal symmetry and of small linear perturbations.
Nonlinear coupling of waves in the ion cyclotron range of frequencies to tokamak trapped electron modes
Authors: McBride John B.
Published: July 2002

The results of a theoretical study of nonlinear coupling of waves in the ion cyclotron range of frequencies (ICRF) to tokamak dissipative trapped electron modes (DTEM) via sidebands of the ICRF source wave are presented. These interactions can affect the stability of the DTEM spectrum and thus tokamak energy confinement. Explicit expressions are derived for the nonlinear growth rates of DTEM both for fast magnetosonic and ion Bernstein source waves. Fast wave coupling is found to be destabilizing for frequencies in the ICRF at low plasma ion beta, but can be stabilizing for wave frequencies well above the ion gyrofrequency (ω0≳ωciβ−1/2i). Ion Bernstein coupling can also be stabilizing or destabilizing depending on details of the source wave. Waves with ω0>ωci, k0>k, and k0 ρi≳1 appear better suited for stabilization (k0, k are the wavenumbers of the source wave, DTEM). For example, if these conditions are met and 2k0y<k (k0y is the poloidal source wavenumber), it is shown that the whole DTEM spectrum for kρi <1 can be stabilized. Analytic expressions for the ICRF wave electric field strengths for DTEM stabilization are given.