bluuebunny/crossref_metadata_2025 huggingface dataset:
Low-speed streaks in drag-reduced turbulent flow
Authors: Hetsroni G. , Zakin J. L. , Mosyak A.
Published: July 2002

The effect of a surfactant drag-reducing additive (530 ppm Habon G solution) on the structure of wall turbulence, both in a flume and in pipe flow, was investigated experimentally. Real-time infrared thermography was used for flow visualization and measurements of the spanwise spacing between the thermal streaks. The experiments were carried out over a broad range of friction velocities, i.e., up*=0.51–3.27 cm/s. With wall shear velocities 1.54⩽up*⩽3.27 cm/s drag reduction of 82%–85% was achieved in a tube flow, well below the predictions of the Virk maximum drag reduction asymptote proposed for high polymers. The results of spanwise streak spacing indicate that wall shear velocity may be an appropriate parameter for describing nondimensional streak spacing behavior in drag reducing flows. A hypothesis, based on the average spanwise streak spacing λ+, can be applied to describe the mean velocity profiles of Habon G solutions. The ratio (λp+−100)/100 was applied to describe mean velocity profiles with 530 ppm Habon G solution.
Steady buoyant droplets with circulation
Authors: Kao Shin-Shin , Caflisch Russel E.
Published: July 2002

Numerical solutions are presented for the steady flow corresponding to a two-dimensional moving droplet with circulation. Differences in the density of the droplet and surrounding fluid result in a buoyancy force which is balanced by a lift force due to the Magnus effect. The droplet is assumed to have constant vorticity in its interior, and its boundary may be a vortex sheet, as in a Prandtl–Batchelor flow. Only symmetric solutions are calculated. For Atwood number A=0 (no density difference) the droplet is a circle. As the Atwood number is increased, the droplet shape begins to resemble a circular cap with a dimpled base. There is a critical Atwood number Alim at which the droplet develops two corners. For 0⩽A<Alim, the solution is smooth; while for Alim<A, we do not find a solution.
Flow patterns of natural convection in an air-filled vertical cavity
Authors: Wakitani Shunichi
Published: July 2002

Flow patterns of two-dimensional natural convection in a vertical air-filled tall cavity with differentially heated sidewalls are investigated. Numerical simulations based on a finite difference method are carried out for a wide range of Rayleigh numbers and aspect ratios from the onset of the steady multicellular flow, through the reverse transition to the unicellular pattern, to the unsteady multicellular flow. For aspect ratios (height/width) from 10 to 24, the various cellular structures characterized by the number of secondary cells are clarified from the simulations by means of gradually increasing Rayleigh number to 106. Unsteady multicellular solutions are found in some region of Rayleigh numbers less than those at which the reverse transition has occurred.
The curvature of material lines in a three-dimensional chaotic flow
Authors: Hobbs D. M. , Muzzio F. J.
Published: July 2002

Folding of material filaments was examined computationally in the three-dimensional flow in a cylindrical duct with helical deflectors by tracking the curvature of line elements in the flow. Two geometries were analyzed: a configuration in which the flow is globally chaotic, and an alternative geometry which has a mixture of chaotic and regular motion. The behavior of the curvature field in this complex flow geometry was in agreement with that previously observed for much simpler two-dimensional model flows [Phys. Fluids 8, 75 (1996)]. Curvature profiles along individual element trajectories indicate that an inverse relationship exists between the rates of stretching and curvature. Material elements are compressed when they are folded. After an initial transient, the mean curvature oscillates within a finite range with a periodicity matching that of the flow geometry. The spatial structure of the curvature field becomes period-independent, and the probability density functions of curvature computed for different numbers of periods collapse to an invariant, self-similar distribution without the need for scaling.
A weakly nonlinear theory of the spatial evolution of disturbances in a shear flow with a parallel magnetic field
Authors: Shukhman I. G.
Published: July 2002

A study is made of the spatial downstream evolution of a weakly unstable disturbance excited by an external source of a frequency ω close to the frequency of a marginally stable (neutral) mode, in a mixing layer of conducting, nearly inviscid fluid with a uniform parallel magnetic field. The nonlinear dynamics of such a disturbance is governed largely by two critical layers, i.e., by two narrow regions on the flow profile vx=u(y) near critical levels y=yj (j=±1), on which the resonance condition u(y±1)=c±cA is satisfied (c being the wave’s phase velocity corresponding to the neutral mode, and cA is the Alfven velocity). A nonlinear integrodifferential evolution equation is derived for a complex amplitude of the disturbance, and its solutions are analyzed (numerically and analytically) with different relationships between problem parameters. It is shown that the nonlinearity can play both a stabilizing and destabilizing role depending on the magnitude of the magnetic field and on the degree of supercriticality of the wave. With not too large a magnetic field (cA<cA*), the future of a disturbance depends only on the relationship between the reciprocal of the Reynolds number ν and the wave’s linear spatial growth rate γL (provided that the magnetic Prandtl number is of order unity). With a sufficiently small growth rate (γL≲ν1/3), the critical layer regime throughout the evolution remains a viscous one, and the wave amplitude, upon reaching a very smooth maximum, goes very slowly to zero. At larger values of γL(γL≳ν1/3), when the nonlinearity threshold is attained, a peculiar kind of self-maintaining unsteady regime of critical layers is established, and the amplitude grows explosively, |A|∝(x0−x)−2 (and ultimately reaches values where weakly nonlinear theory becomes invalid). With a sufficiently large magnetic field (cA>cA*, but cA<12Δu, where Δu is the velocity difference), the nonlinearity leads to an “explosion” with any relationship between γL and ν1/3.
Particle aggregation in a turbulent Keplerian flow
Authors: Bracco A. , Chavanis P. H. , Provenzale A. , Spiegel E. A.
Published: July 2002

For the problem of planetary formation one seeks a mechanism to gather small dust particles together into larger solid objects. Here we describe a scenario in which turbulence mediates this process by aggregating particles into anticyclonic regions. If, as our simulations suggest, anticyclonic vortices form as long-lived coherent structures, the process becomes more powerful because such vortices trap particles effectively. Even if the turbulence is decaying, following the upheaval that formed the disk, there is enough time to make the dust distribution quite lumpy.
Resonant thermocapillary and buoyant flows with finite frequency gravity modulation
Authors: Suresh V. A. , Christov C. I. , Homsy G. M.
Published: July 2002

Interaction between a base thermocapillary flow and a time-dependent buoyant force is studied for a slot geometry. A temperature gradient applied along a fluid-filled slot with thermocapillarity at a free surface produces a base parallel flow. The system is subjected to streamwise gravitational acceleration that varies harmonically in time. Grassia and Homsy [Phys. Fluids. 10, 1273 (1998)] have shown that in the limit of zero frequency modulation, coupling of the thermocapillary flow with long wave convective modes leads to singularities at critical points corresponding to the Rayleigh–Bénard eigenvalues. In the case of small but finite frequency modulation studied here, inertial effects moderate the singularities which are replaced by a response that scales exponentially with the inverse of the dimensionless modulation frequency. An O(1) delay is observed in the onset of the resonant response even for small modulation frequencies. The response is also found to scale exponentially with the inverse Prandtl number for large Prandtl numbers and to be independent of Prandtl number for small Prandtl numbers. Relaxation oscillations are observed in certain parameter ranges as a result of the coupling between the fluid and thermal fields. A Galerkin approximation is used to reduce the problem to an equivalent dynamical system, the analysis of which gives analytical support to and insight into the numerical results.
Experimental study of turbulent Poiseuille–Couette flow
Authors: Thurlow Ernest M. , Klewicki Joseph C.
Published: July 2002

Results from the experimental study of fully developed turbulent plane Poiseuille–Couette flow (PCF) are presented. This study provides information that improves the physical descriptions of previous experimental works and sheds light on turbulence phenomena reported at low to moderate Reynolds numbers. Molecular tagging velocimetry (MTV) is employed for the first time on gas PCFs over the Poiseuille and Couette flow Reynolds number ranges of 3500<Red<9700 and −1700<Rec<+1700, respectively. MTV has allowed the measurement of instantaneous velocity profiles at a much higher resolution than previous single point techniques. Results include a locally negative production of streamwise turbulence near the forward moving wall, and a noticeable effect on intensities in the core flow region and near the moving wall. It has been found that the so-called “geometry effect” does play a role in the exchange of fluid motions between the channel walls at low Red. This effect is amplified when one of the walls is put into motion. Results indicate that the exchange of disturbances is mainly driven by the more energetic motions or “active” motions originating at the high stress wall. These disturbances are convected towards the low stress wall and impinge on its surface. This can lead to a localized region of negative turbulence energy production.
Instabilities in a laterally heated liquid layer
Authors: Mancho A. M. , Herrero H.
Published: July 2002

We study a convection problem in a free-surface container with lateral walls heated at different temperatures. The effects of buoyancy and thermocapillarity are taken into account. A basic convective state appears as soon as a temperature gradient with nonzero horizontal component is applied. This state bifurcates to new convective solutions for further values on the imposed temperature gradient. Our main contribution is to consider this situation in a container finite not only in the vertical coordinate, but also in the direction of the gradient. The third dimension is kept infinite. We determine the basic state, compare it with the usual one of parallel flow approach, and study its stability. When the lateral heating walls are considered new results are found. The boundary conditions on the top surface are no longer restricted to those that allow analytical solutions for the basic state, and we have considered for the heat interchange with the atmosphere the Newton law with constant ambient temperature. Due to this boundary condition, two control parameters related to the temperature field appear. One is the temperature difference between lateral walls as in previous research, and the new one is the temperature difference between the atmosphere and the cold wall. After a stationary bifurcation a three-dimensional structure which along the infinite direction consists of longitudinal rolls grows. On the vertical plane along the gradient direction this structure is nonhomogeneous but located near the hot side. These features coincide with observations of recent experiments.
Zero-absolute-vorticity state in a rotating turbulent shear flow
Authors: Tanaka Mitsuru , Kida Shigeo , Yanase Shinichiro , Kawahara Genta
Published: July 2002

Stability characteristics of a rotating simple shear flow modulated periodically in the shear direction is investigated by direct numerical simulation. The temporal evolution and the final profile of the mean flow are examined, starting with a random perturbation imposed on it under the assumption that the velocity and pressure fields are uniform in the mean flow direction. It is found that the mean velocity profile changes due to the shear-Coriolis instability. If there is a locally unstable region in a stable rotating simple shear, the mean velocity profile there is deformed into a linear one with nearly-zero absolute vorticity. An analogy holds between rotating uniformly sheared turbulence and thermally convective turbulence that the region of nearly-zero absolute vorticity in the former corresponds to that of nearly-uniform temperature in the latter.
Long time mass fraction statistics in stationary compressible isotropic turbulence at supercritical pressure
Authors: Miller Richard S.
Published: July 2002

Direct numerical simulations are used to investigate the “long time” distribution of mass fraction fluctuations in stationary compressible isotropic turbulent binary nitrogen–hydrocarbon mixtures under supercritical pressure conditions. The governing equations are the compressible Navier–Stokes equations together with the cubic Peng–Robinson real gas state equation, and generalized heat and mass diffusion derived from nonequilibrium thermodynamics. A highly efficient procedure is presented which allows for the solution of all thermodynamic quantities without iterations or interpolation tables. The simulations consider equal mass binary mixtures of various combinations of nitrogen, heptane, dodecane, and 3-methylhexane, having molecular weight ratios in the range 1⩽MB/MA⩽6.08. It is shown that temperature and pressure-gradient-induced Soret mass diffusion results in statistically stationary mass fraction distributions at long times. The results reveal that the mass diffusion term due to the pressure gradient acts as a production mechanism in the Favre averaged scalar variance transport equation, and is balanced by Fickian dissipation to produce the stationary states. The resulting scalar probability density function is characterized by a larger than Gaussian flatness factor, and is asymmetric due to the mass fraction dependence of the partial molar volume. The stationary scalar variance amplitude increases both with increasing turbulence Mach number and molecular weight ratio of the species, but is inversely related to the turbulence Reynolds number. The scalar energy spectra exhibit peak values at wave numbers corresponding to the peak in the velocity dissipation spectra.
Particle-in-cell (PIC) simulations of electromagnetic emissions from plasma turbulence
Authors: Goodman Simon , Usui Hideyuki , Matsumoto Hiroshi
Published: July 2002

Particle-in-cell (PIC) simulations of electromagnetic (EM) pump wave propagation into an overdense, strongly magnetized plasma with a linear density gradient are investigated. A localized three-wave interaction is observed involving the pump decaying into upper and lower hybrid waves. The upper hybrid (UH) wave converts on pump induced density depletions into an electromagnetic wave which exits the plasma. When the pump wave frequency equals an electron cyclotron harmonic plus lower hybrid (LH) frequency, all excited waves vanish, implying a magnetic field strength remote diagnostic capability. These results are relevant to stimulated electromagnetic emissions (SEE) and also to electron heating in a strongly magnetized plasma.
Very high-current propagation in the ion-focused to collision-dominated regime
Authors: Sanford T. W. L. , Welch D. R. , Mock R. C.
Published: July 2002

Measurements and analysis show that the 13 TW, Hermes-III [J. J. Ramirez et al., Digest of Technical Papers, 6th IEEE Pulsed Power Conference (Institute of Electrical and Electronic Engineers, New York, 1987), p. 294], pulsed, electron beam has two windows of stable transport in long drift cells filled with N2 gas terminated by a bremsstrahlung producing target: a low-pressure window (between ∼1 and ∼100 mTorr) that is dominated by propagation in the semicollisionless ion-focused regime (IFR), and a high-pressure window (between ∼1 and ∼100 Torr) that is dominated by propagation in the resistive collisional regime. In the transition region between the two windows, beam plasma–electron instabilities significantly disrupt propagation. Propagation in both regimes (the IFR at early time and the collisional at later time) is observed from ∼5 to ∼100 mTorr, which produces two distinct bremsstrahlung pulses from the single injected beam pulse. As the pressure increases, two-stream instabilities terminate IFR propagation and the associated bremsstrahlung pulse earlier and earlier in time. Above 5 mTorr, the instability is sufficiently quenched by gas collisions that propagation in the collisional regime back in the beam body occurs, leading to a second propagation and associated bremsstrahlung pulse. Above 200 mTorr, the gas breaks down too rapidly for a significant IFR pulse to form, and for higher pressures only a single pulse in the collisional regime is propagated. Reasonable stability in the collisional regime is not achieved until pressures exceed 1 Torr.
Drift wave propagation as a source of plasma edge turbulence: Slab theory
Authors: Mattor Nathan , Diamond Patrick H.
Published: July 2002

In a recent work [N. Mattor and P. H. Diamond, Phys. Rev. Lett. 62, 486 (1994)], it was suggested that large amplitude turbulence in edge plasmas [ñ/n0∼O(1)] could originate from the core, reaching large amplitude when encountering regions of low density and temperature. Here the slab model of that theory is elaborated on and generalized to the case of nonadiabatic electron dynamics. It is shown that a drift wave launched with ñ/n0≪1 in a dense region can propagate into a sparse region and amplify. It is shown that such waves cannot propagate into regions with a large increase of velocity, suggesting a simple explanation of the observed correlation between edge shear flows and high confinement (‘‘H’’ modes). It is shown that a wave launched in the adiabatic regime can become nonadiabatic when propagating into a low density and temperature edge region, suggesting a possible explanation for probe measurements.
Plasma self-oscillations in the temperature-limited current regime of a hot cathode discharge
Authors: Arnas Capeau C. , Bachet G. , Doveil F.
Published: July 2002

Experimental observations of self-oscillations occurring in the so-called ‘‘temperature-limited current regime’’ of a hot cathode discharge are presented. Their frequency and amplitude are strongly dependent on the discharge parameters. The scaling laws of their variation and an example of a period-doubling route to chaos are reported. A two probe experiment showing that the plasma behavior is closely related to the hot cathode sheath stability is also reported.
Weakly three-dimensional model of spherical contactors in unmagnetized plasmas
Authors: Ahedo E. , Lapuerta V.
Published: July 2002

A model of steady spherical contactors that takes into account small transverse gradients in plasma variables is presented. The effects of non-uniform boundary conditions on the electric potential profile, the collection of current, and the position of the double layer that confines the emitted plasma cloud, are studied. Appropriate jump conditions across the double layer are derived. Plasma profiles and parameters are calculated for different values of Legendre index ℓ, dimensionless contactor potential χR, and dimensionless collected current je0. For high contactor potentials, small non-uniformities in the plasma emission can substantially affect the approximate spherical symmetry of the plasma. Also, for definite values of index ℓ and double layer mean position, the symmetrical boundary-value problem can sustain free, stationary, non-symmetrical modes. The nature of these modes and its relevancy to the global stability of the plasma field are discussed.
Drift waves with parallel flow revisited
Authors: Sen S. , Rusbridge M. G.
Published: July 2002

Earlier studies indicate that shear in the parallel flow (V∥′(x)) is destabilizing for drift-like modes. Here, it is shown that taking the flow curvature (V∥″(x)) effect into account can change the picture altogether. Depending on its sign, parallel flow curvature can be both stabilizing and destabilizing. For a negative value in the curvature [usually found during the neutral beam injection (NBI) heating and the high (H)-mode edges] parallel flow has a stabilizing effect on drift waves and the mode width gets elongated. Velocity shear, on the other hand, plays an insignificant role both in determining the mode stability and its radial width. As a result, parallel flow must be considered as a viable agent in drift-like wave stabilization for achieving high confinement operation of fusion devices.
Charged-neutral collision models in the presence of drifts
Authors: Pocobelli G.
Published: July 2002

A charged-neutral collision model is presented for application to the kinetic theory of weakly ionized plasmas with immobile neutrals. The model is a modification of the well-known Bhatnagar-Gross-Krook model, which has been the main model used in space plasmas so far. In comparison, the new model produces self-consistently correct zero order drifts, and properly behaved zero order evolutions, while otherwise leaving identically unchanged all the kinetic dispersion equations of space plasma instabilities derived by the old model. Also, the present model allows to use directly the zero order κ-distributions recently introduced by Summers and Thorne [Phys. Fluids B 3, 1835 (1991)] for space plasmas, as input parameters, to which the actual distribution relaxes in time, as well as their generalized plasma dispersion function Zκ*(z) for the solution of dispersion equations. The new model is compared throughout to a previous model by Mikhailovskii and Pogutse [Sov. Phys. Tech. Phys. 11, 153 (1966)], which also produces the correct zero order drifts, but predicts autonomously the zero order distribution.
Convective gain of stimulated Brillouin scattering in long-scale length, two-ion-component plasmas
Authors: Bezzerides B. , Vu H. X. , Wallace J. M.
Published: July 2002

The linear kinetic theory is developed for the convective amplification of stimulated Brillouin scattering in a plasma containing two distinct ion species. A computationally tractable expression for the gain coefficient Q is obtained by, first, restricting consideration to growth from the two possible weakly damped ion modes in the two-ion-species plasma and, second, invoking the two-mode approximation for ε−1, the ion response function, in the plasma. Furthermore, a practical procedure is presented for efficiently obtaining Q over the mesh of a large-scale hydrodynamic simulation of a laser-irradiated target. The theory and its application are demonstrated in the simulation of a simple gas target.
A magnetohydrodynamic model of kinetic Alfvén waves with finite ion gyroradius
Authors: Marchenko Victor A. , Denton Richard E. , Hudson Mary K.
Published: July 2002

Low-frequency magnetohydrodynamic equations including the effects of finite ion Larmor radius are derived. These equations contain all the physical terms necessary to describe kinetic Alfvén waves. Among other applications, these waves are used to interpret ultra-low-frequency wave phenomena in the Earth’s magnetosphere. The finite ion Larmor radius effects are usually disregarded; however, under magnetospheric conditions, the ion temperature cannot be considered negligible since Ti≥Te. The finite ion gyroradius is accounted for by the ion stress tensor calculated in this Brief Communication. The nonlinear one-fluid system of equations is shown to adequately describe linear kinetic Alfvén waves in a low-β(β≪1) homogeneous plasma. It is demonstrated that energy of the plasma system is conserved.
Study of Ne- and Ni-like x-ray lasers using the prepulse technique
Authors: Li Yuelin , Pretzler Georg , Lu Peixiang , Fill Ernst E. , Nilsen Joseph
Published: July 2002

Recent studies of lasing in Ne- and Ni-like ions on the Asterix IV iodine laser [H. Baumhacker et al. Appl. Phys. B 61, 325 (1995)] using the prepulse technique are reviewed. Experimental evidence shows that beam refraction is the main factor for the lack of lasing in low-Z elements, as well as the J=0−1 vs J=2−1 anomaly in Ne-like ion lasers when there is no prepulse. It is shown that the role of the prepulse in enhancing the J=0−1 lasing line in Ne-like ion is to produce a larger and more homogeneous plasma. The measurement of lasing on the J=0−1, 3p−3s transition in Ne-like Mn, V, Sc, Ca, K, Cl, S, and Si using the prepulse technique is reviewed. Wavelengths of these lasers range from 22 to 87 nm with gain lengths between 7 and 12. The drive energy for S was scaled down to 20 J. The experiment demonstrating the 12 nm lasing on the J=0−1, 4d−4p transition in Ni-like Sn is also reviewed.
Electric field distribution in plasma during ion extraction by a radio frequency resonance method
Authors: Matsui Tetsuya , Tsuda Shinji , Tsuchida Kazuki , Suzuki Kazumichi , Shoji Tatsuo
Published: July 2002

An ion extraction method using plasma-sheath resonance in a weak magnetic field has been employed to measure the electric field distribution and the time-resolved potential distribution experimentally. The Xe discharge plasma is sandwiched by parallel plate electrodes, which are set parallel to the magnetic field. When resonance occurs at 10 MHz, the electric field perpendicular to the magnetic field has its peak at the midpoint of the electrodes’ length. Therefore, a standing wave seems to be induced in the electrodes. When measuring the time-resolved potential distribution perpendicular to the magnetic field at the midpoint of the electrodes’ length, potential gradients are formed and their directions are reversed in one rf period. It is, therefore, verified that the rf electric field penetrates to the plasma. Moreover, simulation results show that the electric field strength in the resonance increases, but nonlinearly, with the applied voltage.
Frequency dependence of the plasma density for helicon plasmas
Authors: Kwak J. G. , Choi H. D. , Bak H. I. , Cho Suwon , Bak J. G. , Kim S. K.
Published: July 2002

Variations of plasma density are investigated as a function of frequency of rf power in a helicon plasma source. Abrupt, almost step-like changes in the plasma density are observed during the frequency scans under various conditions of the input rf power, the argon gas pressure, and the magnetic field. It is found that the transition frequencies shift to the lower value region as the input rf power and/or the argon gas pressure is increased, and to the higher value region as the magnetic field is increased. The observed density transitions are compared with semianalytical calculations based on the power balance relation and it has been shown that the results are in good agreement.
Neoclassical ion thermal conductivity modified by finite banana effects in a tokamak plasma
Authors: Chang C. S.
Published: July 2002

A finite-banana-width correction to the neoclassical ion thermal conductivity is obtained in a tokamak plasma under the conventional assumption that the particle flow parallel to magnetic-field lines dominates the trapped particle’s orbital dynamics. It is found that the finite-banana-width effect makes ion thermal conductivity itself be a function of radial plasma density gradient and magnetic shear. Negative radial gradients in plasma density and/or safety factor can reduce the neoclassical ion thermal conductivity when the banana width is a significant fraction of the gradient scale length.
Transverse collapse of Alfvén wave-trains with small dispersion
Authors: Champeaux S. , Passot T. , Sulem P. L.
Published: July 2002

While in the presence of dispersion the circular polarization of a weakly nonlinear Alfvén wave-train is preserved, and the envelope dynamics governed by the usual scalar nonlinear Schrödinger equation, this is no longer the case in the small dispersion limit. It is shown that the wave amplitude then obeys a vector Schrödinger equation with an anisotropic diffraction term. The transverse collapse developing in three space dimensions then leads to the formation of thin layers of intense gradients instead of the foci characteristic of the dispersive case. The transition between the two regimes when the dispersion is decreased, is also analyzed.
Collisionless plasma modeling in an arbitrary potential energy distribution
Authors: Liemohn M. W. , Khazanov G. V.
Published: July 2002

A new technique for calculating a collisionless plasma along a field line is presented. The primary feature of this new ion-exospheric model is that it can handle an arbitrary (including nonmonotonic) potential energy distribution. This was one of the limiting constraints on the existing models in this class, and these constraints are generalized for an arbitrary potential energy composition. The formulation for relating current density to the field-aligned potential as well as formulas for density, temperature, and energy flux calculations are presented for several distribution functions, ranging from a bi-Lorentzian with a loss cone to an isotropic Maxwellian. A comparison of these results with previous models shows that the formulation reduces to the earlier models under similar assumptions.
Analysis of the tokamak ripple-blocked ion distribution with abrupt changes of a radial electric field
Authors: Heikkinen J. A. , Kurki-Suonio T.
Published: July 2002

Convective transport of non-thermal, ripple-blocked ions in the presence of a rapidly varying radial electric field (Er) is investigated by numerically solving a relevant kinetic equation in tokamak geometry. Near the plasma periphery, at small poloidal angles, a strong suppression in the deeply ripple-blocked ion distribution is observed in the absence of a radial electric field for particle energies exceeding a collisional threshold. The deficit of the ripple-blocked ions is found to be rapidly filled by the onset of an inward radial electric field of a sufficient magnitude. The time scale for this filling can be explained by the different blocked-ion drift orbit topologies generated by the radial electric field, and it is determined by the convective drift time of ripple-blocked ions from the inner, well-filled ripple region to the depleted region along these orbits. Consequently, the time scale can be much faster than the collisional time scale, and the blocked ion distribution function should faithfully follow the changes in the radial electric field. Thus the present mechanism gives a physical basis for the earlier interpretation of experimental results [W. Herrmann and ASDEX Upgrade Team, Phys. Rev. Lett. 75, 4401 (1995)] that the measured changes in the neutral fluxes from charge exchange processes with ripple-blocked ions are caused by changes in the radial electric field. The dependence of the neutral flux changes on collisionality as well as on the width and magnitude of a radial electric field with a Gaussian potential profile are studied.
Resistivity profile and instability of the plane sheet pinch
Authors: Seehafer N. , Schumacher J.
Published: July 2002

The stability of the quiescent ground state of an incompressible, viscous and electrically conducting fluid sheet, bounded by stress-free parallel planes and driven by an external electric field tangential to the boundaries, is studied numerically. The electrical conductivity varies as cosh−2(x1/a), where x1 is the cross-sheet coordinate and a is the half width of a current layer centered about the midplane of the sheet. For a≲0.4L, where L is the distance between the boundary planes, the ground state is unstable to disturbances whose wavelengths parallel to the sheet lie between lower and upper bounds depending on the value of a and on the Hartmann number. Asymmetry of the configuration with respect to the midplane of the sheet, modelled by the addition of an externally imposed constant magnetic field to a symmetric equilibrium field, acts as a stabilizing factor.
Growth and saturation of toroidal Alfvén eigenmode modes destabilized by ion cyclotron range of frequency produced tails
Authors: Wong H. Vernon , Berk H. L.
Published: July 2002

The linear growth rates of TAE (toroidal Alfvén eigenmode) modes destabilized by ICRF (ion cyclotron range of frequency) heating are calculated over a range of plasma parameters. Nonlinear saturation of a single unstable mode is investigated both analytically and numerically when wave–particle trapping is the dominant saturation mechanism. A numerical code has been developed based on a reduced resonance description of the wave–particle interaction (using a Hamiltonian formalism). A delta-f algorithm was incorporated to allow a low-noise description of mode evolution with particle sources and sinks present. The numerically observed saturation amplitudes correlate well with theoretical predictions to within 20%. Self-excited frequency sweeping resulting from the excitation of many simultaneous wave–particle resonances at different energies is demonstrated and explained as an extension of previous published theory [Berk et al., Phys. Lett. A 234, 213 (1997)].
Magnetohydrodynamic flow instability in the presence of resistive wall
Authors: Wesson J. A.
Published: July 2002

A conducting fluid flowing along a magnetic field parallel to a conducting wall is unstable if the flow velocity exceeds a critical value.
Mode conversion in a weakly magnetized plasma with a longitudinal density profile
Authors: Yin L. , Ashour-Abdalla M.
Published: July 2002

Motivated by conditions in the terrestrial foreshock, the mode conversion process in the inner region for a weakly magnetized plasma with a longitudinal linear density profile is examined. Coupled differential equations are employed to describe the wave energy coupling and the mode structure of electrostatic Langmuir and upper-hybrid branches, and electromagnetic O, X, and Z modes. For an incident electrostatic wave (the “inverse” problem), the coupled equations are solved using Green’s function method, and the mode conversion coefficient is determined from the asymptotic solutions. In the limit of a vanishing magnetic field, solutions converge to previous unmagnetized solutions. It is found that the magnetic field narrows the window of conversion and reduces the angles of propagation of electromagnetic waves with respect to the magnetic field, in agreement with results from previous solutions (of the “direct” problem) for a magnetized cold plasma. The narrowing of the window occurs at angles of propagation larger than the angle corresponding to the maximum conversion.
Electron acceleration to ultrarelativistic energies in a collisionless oblique shock wave
Authors: Bessho Naoki , Ohsawa Yukiharu
Published: July 2002

Electron motion in an oblique shock wave is studied by means of a one-dimensional, relativistic, electromagnetic, particle simulation code with full ion and electron dynamics. It is found that an oblique shock can produce electrons with ultrarelativistic energies; Lorentz factors with γ≳100 have been observed in our simulations. The physical mechanisms for the reflection and acceleration are discussed, and the maximum energy is estimated. If the electron reflection occurs near the end of a large-amplitude pulse, those particles will then be trapped in the pulse and gain a great deal of energy. The theory predicts that the electron energies can become especially high at certain propagation angles. This is verified by the simulations.
Fusion performance analysis of plasmas with reversed magnetic shear in the Tokamak Fusion Test Reactor
Authors: Ruskov E. , Bell M. , Budny R. V. , McCune D. C. , Medley S. S. , Nazikian R. , Synakowski E. J. , von Goeler S. , White R. B. , Zweben S. J.
Published: July 2002

A case for substantial loss of fast ions degrading the performance of tokamak fusion test reactor plasmas [Phys. Plasmas 2, 2176 (1995)] with reversed magnetic shear (RS) is presented. The principal evidence is obtained from an experiment with short (40–70 ms) tritium beam pulses injected into deuterium beam heated RS plasmas [Phys. Rev. Lett. 82, 924 (1999)]. Modeling of this experiment indicates that up to 40% beam power is lost on a time scale much shorter than the beam–ion slowing down time. Critical parameters which connect modeling and experiment are: The total 14 MeV neutron emission, its radial profile, and the transverse stored energy. The fusion performance of some plasmas with internal transport barriers is further deteriorated by impurity accumulation in the plasma core.
Observation of spatial intermittency in Tokamak plasma turbulence
Authors: Wang Guiding , Liu Wandong , Yu Chang-Xuan , Wen Yizhi , Wang Chao , Pan Gesheng , Zhuang Ge , Zhai Kan , Xu Yuhong , Wang Cheng , Wan Shude
Published: July 2002

A sharp variation at some radial positions superimposed on a slow change in the profiles of the fluctuation levels, fluctuation-driven particle and energy fluxes, which is referred as spatial intermittency, is observed in the core plasma of the Keda Tokamak-5C (KT-5C) [World Survey of Activities in Controlled Fusion Research, Nuclear Fusion Special Supplement (International Atomic Energy Agency, Vienna, 1991), p. 190.]. The peaks in the profiles are located in the vicinity of low-q rational surfaces, and fluctuation spectra perpendicular to the magnetic field become more anisotropy there. The intermittency may be related to the radial variations in the nonlinear mode couplings near the low-q resonant surfaces.
Radiation temperature scaling in indirect-drive hohlraums
Authors: Pei Wenbing , Chang Tieqiang , Wang Guangyu , Zhang Xinghong , Sui Chengzhi , Zhang Jun
Published: July 2002

The hohlraum radiation temperature is studied for Gaussian laser pulses of order of 1 ns. The simulation results show that for this kind of pulse, the self-similar law of the radiation hydrodynamics is also valid in its time-rising phase. The scaling of the hohlraum radiation temperature as a function of laser parameter, hohlraum size, and x-ray conversion efficiency is obtained and the effect of laser entrance hole is included.
Dust shielding and correlation function for dusty plasmas
Authors: Xie B. S. , He K. F. , Huang Z. Q. , Yu M. Y.
Published: July 2002

Dust shielding and correlation function are investigated using a viscoelastic fluid theory, which allows for internal energy relaxation. The corresponding dispersion relation for dust-acoustic waves is derived. The dust static structure factor is calculated using the fluctuation-dissipation theorem. It is found that when the shear viscosity and relaxation time satisfy a certain condition the static structure factor of the strongly coupled system retains the Debye–Hückel form. In general, the characteristic shielding of the dust is found to be determined by a combination of the dust and plasma Debye lengths.
Numerical simulations of self-pinched transport of intense ion beams in low-pressure gases
Authors: Rose D. V. , Ottinger P. F. , Welch D. R. , Oliver B. V. , Olson C. L.
Published: July 2002

The self-pinched transport of intense ion beams in low-pressure background gases is studied using numerical simulations and theoretical analysis. The simulations are carried out in a parameter regime that is similar to proton beam experiments being fielded on the Gamble II pulsed power generator [J. D. Shipman, Jr., IEEE Trans. Nucl. Sci. NS-18, 243 (1971)] at the Naval Research Laboratory. Simulation parameter variations provide information on scaling with background gas species, gas pressure, beam current, beam energy, injection angles, and boundaries. The simulation results compare well with simple analytic scaling arguments for the gas pressure at which the effective net current should peak and with estimates for the required confinement current. The analysis indicates that the self-pinched transport of intense proton beams produced on Gamble II (1.5 MeV, 100 kA, 3 cm radius) is expected to occur at gas pressures between 30 and 80 mTorr of He or between 3 and 10 mTorr of Ar. The significance of these results to ion-driven inertial confinement fusion is discussed.
Measurement of the frequency and spectral width of the Langmuir wave spectrum driven by stimulated Raman scattering
Authors: Baker K. L. , Drake R. P. , Estabrook K. G. , Sleaford Brad , Prasad M. K. , La Fontaine B. , Villeneuve D. M.
Published: July 2002

Thomson scattering was used to measure the spectrum of Langmuir waves, in both frequency and wave number, driven below quarter-critical density by a laser beam. These measurements were capable of detecting and identifying waves driven by stimulated Raman scattering (SRS) and also of detecting waves driven by other effects such as the bump-on-tail instability postulated by the enhanced Thomson scattering model of Raman emission. The observed Langmuir waves were consistent with SRS and not with other possible sources. The width in k-space of the measured Thomson scattering signals also has implications for the saturation amplitude of the Langmuir waves.
Pitch-angle diffusion of relativistic electrons due to resonant interactions with whistler waves
Authors: Ciurea-Borcia Rodica , Matthieussent Gilles , Solomon Jacques , Le Bel Edouard , Simonet Françoise
Published: July 2002

Pitch-angle diffusion phenomenon caused by resonant interactions between parallel whistler waves and relativistic electrons is examined for the Van Allen radiation belts. Thus, the quasilinear pitch-angle diffusion equation for the relativistic case is derived. From this equation, in stationary conditions, one computes the wave spectrum, the distribution function, the particle lifetime, and the trapped electron flux between the magnetic mirrors. The present results may explain phenomena occurring in Earth’s magnetosphere.
Geometric effects on drift wave stability in advanced stellarators
Authors: Kendl A. , Wobig H.
Published: July 2002

The linear stability of electrostatic drift waves in three-dimensional helical advanced stellarator (Helias) configurations is investigated. Effects of local geometrical properties and optimization due to reduction of Pfirsch–Schlüter currents on drift waves are discussed in a comparative numerical study for various configurations of Helias stellarators. The ballooning mode representation is used both for a basic approach with arbitrary nonadiabaticity and perturbatively for the more realistic dissipative trapped electron regime. Mode number spectra and linear growth rates are obtained with an eigenvalue code for three-dimensional equilibria. Local shear properties are found to have considerable influence on stability.
Self-pinched transport of an intense proton beam
Authors: Ottinger P. F. , Young F. C. , Stephanakis S. J. , Rose D. V. , Neri J. M. , Weber B. V. , Myers M. C. , Hinshelwood D. D. , Mosher D. , Olson C. L. , Welch D. R.
Published: July 2002

Ion beam self-pinched transport (SPT) experiments have been carried out using a 1.1-MeV, 100-kA proton beam. A Rutherford scattering diagnostic and a LiF nuclear activation diagnostic measured the number of protons within a 5 cm radius at 50 cm into the transport region that was filled with low-pressure helium. Time-integrated signals from both diagnostics indicate self-pinching of the ion beam in a helium pressure window between 35 and 80 mTorr. Signals from these two diagnostics are consistent with ballistic transport at pressures above and below this SPT pressure window. Interferometric measurements of electron densities during beam injection into vacuum are consistent with ballistic transport with co-moving electrons. Interferometric measurements for beam injection into helium show that the electron density increases quadratically with pressure through the SPT window and roughly linearly with pressure above the SPT window. The ionization fraction of the helium plateaus at about 1.5% for pressures above 80 mTorr. In the SPT window, the electron density is 3 to 20 times the beam density. Numerical simulations of these beam transport experiments produce results that are in qualitative agreement with the experimental measurements.
Interaction of a shear Alfvén wave with a filamentary density perturbation in a low-β plasma
Authors: Drozdenko T. , Morales G. J.
Published: July 2002

This analytic study investigates the interaction between a large-scale shear Alfvén wave propagating through a low-β plasma and a pre-existing density perturbation of small transverse scale. The interaction forms an in situ antenna that self-consistently generates two field-aligned current channels of opposite polarity. The expansion of the current channels across the confining field is bounded by the cone trajectories of small-scale inertial Alfvén waves. The spatial patterns of the radiated fields are obtained, and the magnitude of the parallel electric field and its effective phase velocity are assessed. An effective cross section that varies with the parallel and transverse scale lengths of the density perturbation summarizes the efficiency of the direct conversion process.
Kinetic and fluid calculations for the periodically oscillating plasma sphere
Authors: Nebel R. A. , Finn J. M.
Published: July 2002

Previous work [D. C. Barnes and R. A. Nebel, Phys. Plasmas 5, 2498 (1998)] has demonstrated the existence of a one-dimensional self-similar oscillating ion solution which remains in local thermodynamic equilibrium at all times during an oscillation in a harmonic oscillator potential. Here it is shown that all spherically symmetric distributions, in which x, y, and z are independent, are of this form. However, in a real device the density profile will be truncated due to the presence of a wall or conductor. Particle simulations of these truncated profiles are presented and compared with the idealized solutions in the proper limits. Results are also interpreted in terms of rigid rotor rotation in phase space as is appropriate for a harmonic oscillator. Next, it is demonstrated that the deviations from Maxwellian velocity distributions that are observed when the plasma contracts will be quickly rethermalized during the expansion phase. Energy throughput resulting from this rethermalization is discussed.
Ionization-recombination instability and multifaceted asymmetric radiation from the edge in magnetic fusion devices
Authors: Simakov A. N. , Krasheninnikov S. I.
Published: July 2002

Recent experimental studies of MARFEs (Multifaceted Asymmetric Radiation From the Edge) in the Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] and TEXTOR-94 (upgraded Torus Experiment for Technology Oriented Research) [A. M. Messiaen et al., Phys. Plasmas 4, 1690 (1997)] tokamaks reveal low plasma temperature T∼0.7 eV, strong hydrogen radiation, and plasma recombination in the MARFE region. These results contradict a well-accepted model, based on low-Z impurity radiation, and can be better explained by a nonlinear stage of an ionization-recombination instability, discussed recently in the literature. In this paper, a qualitative analysis of the instability is performed, and its growth rate γ and the range of wavelengths of unstable perturbations are estimated. It is found that the instability exists for plasma temperatures near the wall T≲10 eV; and for lower temperatures, T∼1 eV, volumetric recombination becomes important and can increase γ considerably compared to the case without the volumetric recombination. The results of the qualitative analysis are confirmed by a numerical solution of the linearized fluid equations with recombination and ionization describing MARFEs.