bluuebunny/crossref_metadata_2025 huggingface dataset:
The instability of long fingers in Hele–Shaw flows
Authors: Park C. W. , Homsy G. M.
Published: July 2002

Experiments on steady fingers and their stability in Hele–Shaw cells, are reported. It is shown that the shape of steady fingers scales with a modified capillary number, Ca′, as suggested by McLean and Saffman [J. Fluid. Mech. 102, 455 (1981) and our previous analysis [J. Fluid Mech. 139, 291 (1984)]. The behavior at large Ca′ is investigated by using a wide Hele–Shaw cell. It is observed that such fingers are unstable for Ca′>100, in agreement with the prediction by Taylor and Saffman (second symposium on naval hydrodynamics, 1958, p. 277) of instability as Ca′→∞. The mechanism is identified as one of tip-splitting, which occurs periodically in the weakly supercritical regime, and in a more complex fashion for large Ca′.
Thermal structures on resonance cones in a weakly collisional plasma
Authors: Piel A. , Oelerich G.
Published: July 2002

Resonance cones in the range of the lower-hybrid frequency are investigated experimentally and theoretically. Two competing models, which alternatively relate the cone structure to the curvature of the dispersion branch or to the onset of damping are considered. In an extended model the superposition of both effects is studied by numerical integration of the potential. The cone structures related to the second effect turn out to be sensitive to collisional damping.
Superbanana plateau regime transport in a multiple-helicity torsatron and a bumpy torus
Authors: Hastings D. E. , Shaing K. C.
Published: July 2002

An analytic expression for the neoclassical flux valid in both a multiple-helicity torsatron and a bumpy torus in the superbanana plateau regime is obtained. The expression is valid for arbitrary values of Φ′, the radial electric field, ∂εH/∂r with εH the effective helical modulation or bumpiness, and ∂εT/∂r with εT the effective toroidal modulation. When small mirror force terms are included in the flux, a nonlinear first-order ordinary differential equation in Φ′ is obtained from the ambipolarity relationship.
Experimental control of the spheromak tilting instability
Authors: Munson C. , Janos A. , Wysocki F. , Yamada M.
Published: July 2002

The magnetohydrodynamic (MHD) tilting instability of the spheromak configuration has been experimentally investigated by recording the time evolution of the magnetic field with small magnetic probes. Passive coil systems such as the figure-eight and saddle coils have been proven experimentally effective against this instability.
Reduced equations for finite beta tearing modes in tokamaks
Authors: Izzo R. , Monticello D. A. , DeLucia J. , Park W. , Ryu C. M.
Published: July 2002

The equations of resistive magnetohydrodynamics (MHD) are recast in a form that is useful for studying the evolution of those toroidal systems where the fast magnetosonic wave plays no important role. The equations are exact and have ∇ ⋅ B=0 satisfied explicitly. From this set of equations it is a simple matter to derive the equations of reduced MHD to any order in the inverse aspect ratio ε of the torus and for β∼ε or smaller. This is demonstrated by deriving a reduced set of MHD equations that are correct to fifth order in ε. These equations contain the exact equilibrium relation and, as such, can be used to find three-dimensional stellarator equilibria. In addition, if a subsidiary ordering in η, the resistivity, is made, the equations of Glasser, Greene, and Johnson [Phys. Fluids 8, 875 (1967); 19, 567 (1967)] are recovered. This set of reduced equations has been coded by extending the initial value code hilo [Phys. Fluids 26, 3066 (1983)]. Results obtained for both ideal and resistive linear stability from the reduced equations are compared with those obtained by solving the full set of MHD equations in a cylinder. Good agreement is shown for both zero and finite-beta calculations. Comparisons are also made with analytic theory illuminating the present limitations of the latter.
Turbulent normal velocity fluctuations close to a wall
Authors: Finnicum Douglas S. , Hanratty Thomas J.
Published: July 2002

Measurements of the normal velocity fluctuations in the immediate vicinity of a wall are made using a technique which does not interfere with the flow. It is found that (∼(v+2))1/2 =0.005y+2 for y+→0. Values of the normal intensity obtained by extrapolating measurements made with thermal probes are shown to be in considerable error.
Angular and momentum distribution dependence of electron cyclotron absorption and amplification in mirror-confined plasmas
Authors: Ziebell L. F. , Granata G.
Published: July 2002

The absorption and amplification of fundamental electron cyclotron radiation in inhomogeneous, weakly relativistic, loss-cone and partially filled loss-cone plasmas, are studied for both perpendicular and oblique propagation, and the extraordinary mode is numerically analyzed. It is shown that there are situations where small changes in the distribution function may strongly affect the wave vector, and an explanation is given, pointing to the eventual relevance of the use of the full momentum distribution, along with relativistic effects, in the evaluation of both the real and imaginary part of the dielectric tensor. A model that could describe plasmas in actual situations like the thermal barriers in tandem mirror devices is presented and numerically investigated, and it is shown that particles scattered into the loss cone may play a relevant role.
Computer simulation of a plasma streaming across a magnetic field
Authors: Galvez Miguel
Published: July 2002

A one-dimensional electrostatic particle simulation code has been used to study a plasma streaming across a uniform magnetic field in vacuum. It is found that the plasma may be convected across the magnetic field even if the density is less than the critical density nc necessary to maintain steady propagation. For all densities studied, polarization charge layers are formed, but they do not become steady for plasmas with n/nc<1. For plasma densities at n<nc, the electric field keeps oscillating with the upper hybrid frequency, but for nc<n, the amplitude of the oscillations are damped at later times. The mechanism that produces this damping is associated with a nonlinear wave–particle interaction. For unequal mass species, the simulations show that the heavier particles are convected faster than the lighter ones; thus the positive particles move in front and the negatives move behind creating a virtual cathode. The polarization charge layers are asymmetric, with the thicker layer corresponding to the heavier particles. This paper also includes simulations of plasma streams with different initial widths. The results of these simulations are that the electric field oscillation shows an earlier damping for narrow plasmas, the charge layer thickness decreases for wider plasmas, and the drift velocity is not sensitive to the plasma width.
The two-dimensional stability of periodic and solitary wave trains on a water surface over arbitrary depth
Authors: Infeld E. , Ziemkiewicz J. , Rowlands G.
Published: July 2002

The two-surface dimensional dynamics of a periodically or soliton-shaped modulated wave train on a water surface is investigated, using the Davey–Stewartson model. The depth h is taken to be uniform. The results obtained are twofold: the general behavior of the Benjamin–Feir (BF) instability is found for an arbitrary stationary envelope profile, thus generalizing Hayes’ analysis for uniform wave trains, and a new ‘‘K degeneracy’’ instability is found. The instability is always limited to ∼45° around the direction of propagation of the basic wave train and this critical angle decreases as the modulation becomes stronger. The new instability covers a narrow range of acute angles for 2πh/λ≤1.363, where λ is the wavelength of the carrier wave, and all angles for 2πh/λ≥1.363. Thus our results add new significance to the famous critical number 1.363. A simple explanation of how the new instability comes about concludes the paper. Agreement with previous work is demonstrated.
Stability of miscible displacements in porous media: Radial source flow
Authors: Tan C. T. , Homsy G. M.
Published: July 2002

A linear stability analysis of miscible displacement for a radial source flow in porous media is presented. Since there is no characteristic time or length scale for the system, it is shown that solutions to the stability equations depend only upon a similarity variable, with disturbances growing algebraically in time. Two parameters, the mobility ratio and a Peclet number based upon the source strength, determine the stability. Results for the growth constant as a function of mobility ratio and Peclet number are given. It is shown that there is a critical Peclet number Pec above which displacement becomes unstable. For Pe>Pec, there is always a cutoff scale attributable to dispersion, and a most dangerous mode, with the two corresponding wavenumbers increasing with Pe. The growth constant increases with Pe as well. The effect of mobility ratio is also studied. The result indicates that, as expected, increasing mobility ratio destabilizes the displacement. Asymptotic results for the growth rate, cutoff, and preferred scales that hold as Pe→∞ are given, and are found to be in good agreement with the numerical results.
Filamentation in a magnetized plasma
Authors: Boyd T. J. M. , Coutts G. A. , Marks D. C.
Published: July 2002

Filamentation has been examined in laser-produced plasmas in which magnetic fields are present, using a two-fluid model of the plasma. Configurations in which the low-frequency perturbation is in turn parallel and perpendicular to the magnetic field have been considered and the instability examined in the two cases. In the latter it is shown that the instability is quenched by large enough fields, while for the ion-acoustic perturbation the behavior is more complex.
Feasibility of numerical tracking of material lines and surfaces in chaotic flows
Authors: Franjione J. G. , Ottino J. M.
Published: July 2002

The evolution of material lines and surfaces is important in mixing involving fast reactions between fluids. It is shown that direct numerical tracking of the interface presents formidable computational problems in even the simplest chaotic flows because of storage requirements. The results indicate the need for a building block approach for the analysis of mixing problems involving chaotic flows.
Investigation of collective acceleration using an ion beam probe diagnostic method and time resolved mass spectroscopy
Authors: Markovits M. , Blaugrund A. E.
Published: July 2002

Collective acceleration of ions by a pinched relativistic electron beam was investigated. Two experimental methods were employed: probing the potential well held responsible for the ion acceleration with an externally generated ion beam probe, and time resolved magnetic spectrometer measurements of the collectively accelerated ions. The results obtained shed light on the temporal and spatial characteristics of the potential well and on the origin and energy spectra of the collectively accelerated ions. The properties and behavior of the potential well are described by a multiparameter model. The measurements indicate that the acceleration of ions is not caused by a moving well but probably by fast variations in its depth and position.
Nonuniform target illumination in the deflagration regime: Thermal smoothing
Authors: Sanz J. , Nicolás J. A. , Sanmartín J. R. , Hilario J.
Published: July 2002

Thermal smoothing in the plasma ablated from a laser target under weakly nonuniform irradiation is analyzed, assuming absorption at nc and a deflagration regime (conduction restricted to a thin quasisteady layer next to the target). Magnetic generation effects are included and found to be weak. Differences from results available in the literature are explained; the importance of the character of the underdense flow at uniform irradiation is emphasized.
Gasdynamic agglomeration of aerosols. I. Acoustic waves
Authors: Temkin S.
Published: July 2002

In this paper acoustic agglomeration of aerosols is studied in its simplest form, using a time-averaged form of the coalescence equations. An acoustic coagulation kernel is introduced that neglects all nonlinear effects, as well as particle interactions. Based on this model, it is shown that an acoustic frequency exists that optimizes the coalescence process. This optimum frequency does not have a unique value for all distributions, but for size distributions that are not too wide, it is given by ωopt=1/τ*, where τ* is the relaxation time of a particle having a diameter corresponding to the mode count diameter of the initial size distribution function. The paper also includes the result of a numerical integration of the coalescence equations for typical aerosol distributions, which show their evolution under the influence of a sound wave.
Image scanning ellipsometry for measuring the transient, film thickness profiles of draining liquids
Authors: Liu An-Hong , Wayner Peter C. , Plawsky Joel L.
Published: July 2002

Image Scanning Ellipsometry, a technique to measure the two-dimensional thickness profile of a nonuniform, thin, liquid film, from several nanometers up to tens of microns, in the steady and transient states, was developed and tested. The ability of this full-field imaging technique to map every point on the surface simultaneously was demonstrated by measuring the thickness profiles of very thin, draining, liquid films in the interfacial, transition, hydrodynamic, and capillary regions. Depending on the relative size of the intermolecular, gravitational, and capillary forces, four flow regions were identified. Using a simple model for the transient film thickness profiles of a completely wetting, draining film of FC-70, the experimental results were successfully analyzed in the interfacial, transition, and hydrodynamic regions. A diffusion coefficient for the junction line between the interfacial and transition regions was theoretically and experimentally evaluated.
On the modulational stability of traveling and standing water waves
Authors: Pierce R. D. , Knobloch E.
Published: July 2002

Asymptotically exact evolution equations are derived for trains of small amplitude counterpropagating water waves over finite depth. Surface tension is included. The resulting equations are nonlocal and generalize the equations derived by Davey and Stewartson for unidirectional wave trains. The stability properties of stationary standing and quasiperiodic waves are determined as a function of surface tension and fluid depth for both long wavelength longitudinal and transverse perturbations.
Transient growth in circular pipe flow. I. Linear disturbances
Authors: O’Sullivan P. L. , Breuer K. S.
Published: July 2002

The behavior of very low-amplitude disturbances in a circular pipe is considered. Direct simulation of the Navier–Stokes equations is used to compute the evolution of two- and three-dimensional waves and the results are found to be in good agreement with solutions to the Orr–Sommerfeld equation for Hagen–Poiseuille flow. Transient growth mechanisms are also investigated computationally, in which case it is found that the growth of disturbances with large but finite streamwise wavelength exhibits a very rich structure of temporal evolution depending on the particular initial condition chosen. Comparison with recent results reported by Bergström on optimal disturbances is also given. In Part II of this study these findings will be extended to the nonlinear development of like disturbances.
A phenomenological treatment of rotating turbulence
Authors: Zhou Ye
Published: July 2002

In this Brief Communication, we first note the strong similarity between the magnetohydrodynamic (MHD) turbulence and initially isotropic turbulence subject to rotation. We then applied the MHD phenomenologies of Kraichnan [Phys. Fluids 8, 1385 (1965)] and Matthaeus and Zhou [Phys. Fluids B 1, 1929 (1989)] to rotating turbulence. We deduced a ‘‘rule’’ that relates spectral transfer time to the eddy turnover time and the time scale for decay of the triple correlations. Our hypothesis on the triple correlation decay rate leads to the spectral law, which varies between the ‘‘−5/3’’ (without rotation) and ‘‘−2’’ laws (with strong rotation). For intermediate rotation rates, the spectrum varies according to the value of a dimensionless parameter that measures the strength of the rotation wave number kΩ=(Ω3/ε)1/2 relative to the wave number k. The eddy viscosity is derived with an explicitly dependence on the rotation rate.
The passive scalar spectrum and the Obukhov–Corrsin constant
Authors: Sreenivasan Katepalli R.
Published: July 2002

It is pointed out that, for microscale Reynolds numbers less than about 1000, the passive scalar spectrum in turbulent shear flows is less steep than anticipated and that the Obukhov–Corrsin constant can be defined only if the microscale Reynolds number exceeds this value. In flows where the large-scale velocity field is essentially isotropic (as in grid turbulence), the expected 5/3 scaling is observed even at modest Reynolds numbers. All known data on the Obukhov–Corrsin constant are collected. The support for the notion of a ‘‘universal’’ constant is shown to be reasonable. Its value is about 0.4.
Intermittency and Reynolds number
Authors: Kahalerras H. , Malécot Y. , Gagne Y. , Castaing B.
Published: July 2002

Hot wire measurements of longitudinal and transverse increments are performed in three different types of flows on a large range of Reynolds numbers (100≲Rλ≲3000). An improved technique based on cumulant expansion of velocity structure functions is used to estimate the spreading of the pdfs and to study their scaling properties in the inertial range. Thus, the rate of intermittency depth through the scales of flow, called here β(Rλ), is experimentally introduced, and it is shown that β(Rλ) has a universal behavior on a very large Reynolds numbers range.
The subgrid-scale estimation model in the physical space representation
Authors: Domaradzki J. Andrzej , Loh Kuo-Chieh
Published: July 2002

The subgrid-scale estimation procedure for large eddy simulations developed previously in the spectral (Fourier) representation is extended to the physical space representation. The procedure provides an estimate of the unfiltered velocity field appearing in the definition of the subgridscale stress tensor and consists of two steps. In the deconvolution step an approximate inversion of the filtering operation is performed. Subsequently, the nonlinear step is used to generate a range of subgrid scales on a mesh two times smaller than the mesh employed for a discretization of the resolved quantities. The modeling procedure is evaluated by comparing results of large eddy simulations of turbulent channel flow with the corresponding results of direct numerical simulations, experiments, and other large eddy simulations.
Nonlinear dynamics and transient growth of driven contact lines
Authors: Kondic L. , Bertozzi A. L.
Published: July 2002

We consider the theory of driven contact lines in a complete wetting scenario and examine the effect of small scale localized surface disturbances on the global shape of the film profile. We compute how the nonlinear amplification scales with the precursor thickness of the film and the characteristic width of the surface pattern. Nonlinear disturbances of the film profile are connected to “transient growth” in the linear stability analysis [Phys. Fluids 9, 530 (1997)].
Asymptotic study of the convective parametric instability in Hele-Shaw cell
Authors: Aniss S. , Souhar M. , Belhaq M.
Published: July 2002

This paper concerns a linear study of the convective parametric instability in the case of a Newtonian fluid confined in a Hele-Shaw cell and submitted to a vertical periodic motion. The gradient of temperature, applied to the fluid layer, is either in the same direction that gravity or in the opposite one. An asymptotic analysis shows that the Hele-Shaw approximation leads to two linear formulations depending on the order of magnitude of the Prandtl number. For these two asymptotic cases, the convective threshold is determined. It turns out that in the Hele-Shaw geometrical configuration, parametric oscillations have no influence on the criterion of stability when the Prandtl number is in the order of the unity or very superior to the unity. However, when the Prandtl number is small than unity, the parametric oscillations can affect the convective instability threshold.
Stabilization of electrically conducting capillary bridges using feedback control of radial electrostatic stresses and the shapes of extended bridges
Authors: Marr-Lyon Mark J. , Thiessen David B. , Blonigen Florian J. , Marston Philip L.
Published: July 2002

Electrically conducting, cylindrical liquid bridges in a density-matched, electrically insulating bath were stabilized beyond the Rayleigh–Plateau (RP) limit using electrostatic stresses applied by concentric ring electrodes. A circular liquid cylinder of length L and radius R in real or simulated zero gravity becomes unstable when the slenderness S=L/2R exceeds π. The initial instability involves the growth of the so-called (2, 0) mode of the bridge in which one side becomes thin and the other side rotund. A mode-sensing optical system detects the growth of the (2, 0) mode and an analog feedback system applies the appropriate voltages to a pair of concentric ring electrodes positioned near the ends of the bridge in order to counter the growth of the (2, 0) mode and prevent breakup of the bridge. The conducting bridge is formed between metal disks which are grounded. Three feedback algorithms were tested and each found capable of stabilizing a bridge well beyond the RP limit. All three algorithms stabilized bridges having S as great as 4.3 and the extended bridges broke immediately when feedback was terminated. One algorithm was suitable for stabilization approaching S=4.493… where the (3, 0) mode is predicted to become unstable for cylindrical bridges. For that algorithm the equilibrium shapes of bridges that were slightly under or over inflated corresponded to solutions of the Young–Laplace equation with negligible electrostatic stresses. The electrical conductivity of the bridge liquid need not be large. The conductivity was associated with salt added to the aqueous bridge liquid.
A model for the turbulent Hartmann layer
Authors: Alboussière T. , Lingwood R. J.
Published: July 2002

Here we study the Hartmann layer, which forms at the boundary of any electrically-conducting fluid flow under a steady magnetic field at high Hartmann number provided the magnetic field is not parallel to the wall. The Hartmann layer has a well-known form when laminar. In this paper we develop a model for the turbulent Hartmann layer based on Prandtl’s mixing-length model without adding arbitrary parameters, other than those already included in the log-law. We find an exact expression for the displacement thickness of the turbulent Hartmann layer [also given by Tennekes, Phys. Fluids 9, 1876 (1966)], which supports our assertion that a fully-developed turbulent Hartmann layer of finite extent exists. Leading from this expression, we show that the interaction parameter is small compared with unity and that therefore the Lorentz force is negligible compared with inertia. Hence, we suggest that the turbulence present in the Hartmann layer is of classical type and not affected by the imposed magnetic field, so justifying use of a Prandtl model. A major result is a simple implicit relationship between the Reynolds number and the friction coefficient for the turbulent Hartmann layer in the limit of large Reynolds number. By considering the distance over which the stress decays, we find a condition for the two opposite Hartmann layers in duct flows to be isolated (nonoverlapping).
Fluid equations for a partially ionized plasma
Authors: Helander P. , Krasheninnikov S. I. , Catto P. J.
Published: July 2002

Transport coefficients and fluid equations in the limit of short mean free path are derived for a partially ionized plasma undergoing charge exchange, ionization, and recombination. The electron distribution is assumed to be unaffected by these processes and by collisions with neutral particles, and collisions among the neutrals are neglected. The calculated heat flux and viscosity contain terms purely related to charge exchange, and terms proportional to their usual counterparts in a completely ionized plasma with an additional proportionality factor equal to the inverse square of the fraction of ions in the plasma. The reason for this latter behavior is that charge exchange does not randomize particle trajectories, so that heavy particles suffer an ‘‘identity crisis’’ by existing alternately as neutrals and ions. The use of a closer approximation to the charge-exchange cross section than previously employed gives rise to new terms in the neutral momentum and energy equations, representing the thermal force and the diffusion thermoeffect. The latter contributes significantly to the total heat flux in the plasma.
Self-consistent model of the Rayleigh–Taylor instability in ablatively accelerated laser plasma
Authors: Bychkov V. V. , Golberg S. M. , Liberman M. A.
Published: July 2002

A self-consistent approach to the problem of the growth rate of the Rayleigh–Taylor instability in laser accelerated targets is developed. The analytical solution of the problem is obtained by solving the complete system of the hydrodynamical equations which include both thermal conductivity and energy release due to absorption of the laser light. The developed theory provides a rigorous justification for the supplementary boundary condition in the limiting case of the discontinuity model. An analysis of the suppression of the Rayleigh–Taylor instability by the ablation flow is done and it is found that there is a good agreement between the obtained solution and the approximate formula σ = 0.9√gk − 3u1k, where g is the acceleration, u1 is the ablation velocity. This paper discusses different regimes of the ablative stabilization and compares them with previous analytical and numerical works.
Plasma rotation and electrostatic potential formation at the edge due to suprathermal particle losses*
Authors: Yushmanov P. N. , Dong J. Q. , Horton W. , Su X. N. , Krasheninnikov S. I.
Published: July 2002

Generation of the edge radial electric field and corresponding plasma rotation is studied for toroidal confinement systems with a separatrix configuration of magnetic surfaces. It is shown that classical acceleration of plasma by ion orbital losses can explain the electric field and the plasma rotation observed near walls when plasma flow to the neutralizing wall is included.
Relativistic and ponderomotive self-focusing of a laser beam in a radially inhomogeneous plasma. II. Beyond the paraxial approximation
Authors: Bonnaud G. , Brandi H. S. , Manus C. , Mainfray G. , Lehner T.
Published: July 2002

The propagation in a plasma of a high-intensity electromagnetic wave inducing both relativistic mass increase and ponderomotive expulsion of electrons is analyzed via two-dimensional simulations. The time/space evolution of the wave is modeled by an axisymmetric scalar wave equation in which the plasma frequency is an instantaneous and local function of the wave energy; the incident irradiance is assumed to be constant in time. The specific features of relativistic focusing are first discussed. The ponderomotive effect enforces the focusing process by expelling the plasma electrons, creating density bumps and sharp density gradient on the edge of the light beam; the nonlinear focusing is faster and stronger confirming the paraxial/Gaussian beam core analysis presented in Part I [Phys. Fluids B 5, 3539 (1993)]. In contrast to Part I, the light is guided in a sharp-edged density channel. The influence of the radial density inhomogeneity is then examined by using both convex (basin shape) and concave (bump shape) profiles. The self-focusing threshold power is increased for concave profiles. For convex profiles, the natural refraction helps the self-focusing observation but weakens the light-guiding trend previously observed. Finally, new features characterizing wave self-focusing, such as self-steepening and light reflection, are shown.
Thermal and nonthermal electron–ion bremsstrahlung spectrum from high-temperature plasmas
Authors: Jung Young-Dae
Published: July 2002

Electron–ion bremsstrahlung radiation from high-temperature plasmas is investigated. The first- and second-order Coulomb corrections in the nonrelativistic bremsstrahlung radiation power are obtained by the Elwert–Sommerfeld factor. In this paper, two cases of the electron distributions, the thermal and nonthermal power-law distributions, are considered. The inclusion of Coulomb corrections is necessary in deducing correctly the electron distribution function from radiation data. These results provide the correct information of electron distributions in high-temperature plasmas, such as in inertial confinement fusion plasmas and in the astrophysical hot thermal and nonthermal x-ray sources.
Plasma-loaded free-electron laser with an electromagnetic wave wiggler and axial guide field
Authors: Qian Bao-Liang , Liu Yong-Gui , Li Chuan-Lu
Published: July 2002

A fluid model is used to study the effects of background plasma in an electromagnetically pumped free-electron laser with an axial guide field. The dispersion relation is derived, and the growth rate is formulated for the Raman regime. Numerical calculations are also carried out for both fast electromagnetic wave and whistler wigglers propagating opposite to the relativistic electron beam. In the case of a fast wave wiggler, the background plasma can enhance the growth rate considerably, and a much larger growth rate can be obtained around cyclotron resonance ωi∼Ω0, where ωi is the angular frequency of pump wave, and Ω0 represents the cyclotron frequency of the background plasma electrons. However, in the case of a whistler wiggler, the growth rate decreases rapidly as the plasma density increases, and a considerable decrease also occurs when ωi is near Ω0.
Self-consistent theory of ion acoustic waves in a dusty plasma
Authors: Ma Jin-Xiu , Yu M. Y.
Published: July 2002

From a self-consistent theory for dust-charging, the linear dispersion relation for ion acoustic waves is derived. It is shown that the waves are damped because of energy exchange with the dust-charging process.
Wave dispersion and growth analysis of low-voltage grating Čerenkov amplifiers
Authors: Joe J. , Scharer J. , Booske J. , McVey B.
Published: July 2002

A theoretical and computational investigation of an electron sheet beam propagating over a grating structure in a rectangular waveguide is carried out. Regimes for low voltage Čerenkov amplifier operation are sought by examining the complex dispersion relation for hybrid waveguide modes in the slow wave structure, which includes sheet beam space-charge effects. A computer code is developed to examine the complex dispersion relation and growth rates for the wave modes. Mode competition is considered and methods to reduce it are presented. Briggs’ criteria is utilized to examine absolute and convective wave growth for the forward wave, backward wave, and transition mode regimes of operation as a function of the beam, hybrid mode, and slow wave grating characteristics. An examination of the effects of beam spread on absolute and convective wave growth to determine regimes for amplifier operation is carried out. A modest Maxwellian beam spread is found to yield a regime of effective backward convective amplification, in addition to the purely convective growth characteristics for the forward wave mode case.
Laser beam propagation and channel formation in underdense plasmas
Authors: Young P. E. , Hammer J. H. , Wilks S. C. , Kruer W. L.
Published: July 2002

Experimental results are presented which show the formation of density channels in a preformed plasma by a 100 ps laser beam focused with different f/number lenses. The density channels are diagnosed by an interferometer. The experiments are made with both line foci and circular foci up to intensities of 5×1016 W/cm2. The experimental channel size and density perturbation compare favorably with the predictions of two-dimensional theoretical models. The limited axial extent of the channels is shown to be due to the onset of the filamentation instability.
The effect of lower hybrid instabilities on plasma confinement in fiber Z pinches
Authors: Chittenden J. P.
Published: July 2002

Lower hybrid microinstabilities are found to have an adverse effect upon radial pressure balance in fiber Z-pinch plasmas. Magnetohydrodynamic simulations show that the anomalous resistivity resulting from these microinstabilities causes the formation of a low-density, highly resistive, coronal plasma. This corona penetrates farther and at a faster rate into the plasma for higher pinch currents and lower line densities. If the resistive corona penetrates to the pinch axis then continuous, global expansion ensues. Simulations of the ionization phase of both carbon and hydrogen fiber Z-pinch experiments show substantial expansion of the plasma column due to these microinstabilities during this phase. These microinstabilities also make radiative collapse more difficult to achieve, particularly in low line density hydrogen pinches. This reduces the peak density that can be achieved by this process.
Kinetic description of the influence of electron-beam misalignment on the performance of a coaxial-cavity gyrotron
Authors: Zhang Shi-Chang , Thumm Manfred
Published: July 2002

Based on the gyrokinetics of free-electron masers, the influence of the electron-beam misalignment on the energy-transfer rate and starting current of a coaxial cavity gyrotron oscillator is studied. As a practical example, a numerical analysis is given to the 1.5 MW/140 GHz coaxial-cavity gyrotron that is being constructed at the Forschungszentrum Karlsruhe (FZK) [M. Thumm, Fusion Eng. Design 30, 139 (1995)]. Results show that the electron-beam misalignment will decrease the energy-transfer rate and increase the starting current, and the dependence of the output power on the magnetic field may become more sensitive due to a poor alignment of the electron-beam.
Fast rotational equilibrium in a toroidal non-neutral plasma
Authors: Bhattacharyya S. N.
Published: July 2002

In a toroidally confined non-neutral plasma, in certain parameter regimes it is shown that a fast rotational equilibrium can exist in addition to the slow rotational equilibrium. For a low density plasma, the fast rotational equilibrium is shown to have a much larger inward shift of the streamlines of flow compared to the slow rotational equilibrium.
Numerical study on the dynamics of Z-pinch carbon plasma
Authors: Lee K. T. , Kim S. H. , Kim D. , Lee T. N.
Published: July 2002

The dynamics of Z-pinch carbon plasma has been investigated using one-dimensional Lagrangian code. This code calculates the single-fluid, two-temperature magnetohydrodynamic (MHD) equations coupled with an ionization balance equation. The motion of plasma column and shock front is studied in comparison with the analytical models such as the snowplow and the slug model. The energy flow during the pinch is also studied. During the pinch phase, the temperature increases due to shock heating and adiabatic heating. After the pinch the plasma is cooled down rapidly due to adiabatic expansion which can lead to an adequate condition for recombination Extreme-Ultra-Violet (XUV) lasers. The effect of the radiative trapping of resonance line on hydrodynamics and population kinetics is also investigated. The calculation shows that there can exist a high gain on hydrogen-like C VI Balmer-α line (18.2 nm).
Richtmyer–Meshkov instability with strong radiatively driven shocks
Authors: Dimonte Guy , Frerking C. Eric , Schneider Marilyn , Remington Bruce
Published: July 2002

The Richtmyer–Meshkov instability is investigated with strong radiatively driven shocks (Mach≳20, 5×compression) using the Nova laser. The target consists of a solid density ablator and lower-density plastics (Atwood number A<0) in planar geometry to facilitate in-flight radiographic diagnosis. Perturbations η=η0 cos kx are imposed at the interface to seed the instability. The experiments agree with full hydrodynamic simulations over a wide variety of conditions. For small initial amplitudes ‖A‖kη0<1, the growth rate agrees with a linear impulsive model using the average of the pre- and post-shock initial amplitudes. For ‖A‖kη0≳1, the growth rate is limited to the difference between the transmitted shock speed and the interface speed.
Action at distance and Bohm scaling of turbulence in tokamaks
Authors: Garbet X. , Waltz R. E.
Published: July 2002

Recent transport experiments in tokamaks have suggested the concept of ‘‘action at distance’’ in which the local turbulence depends on gradients at a distance larger than the correlation length. Furthermore, the scaling of the ion thermal diffusivity is not always consistent with local gyro-Bohm-like transport but rather scales worse than Bohm-like. This work is an attempt to reconcile these observations with simplified numerical simulations of toroidal ion temperature gradient (ITG) mode turbulence using a fast two-dimensional (2-D) inhomogeneous full radius turbulence code. It is found that action at a distance is possible, but only at weak damping rates, since the propagation range is given simply by the curvature drift group velocity divided by the average damping rate. The correlation lengths always scale linearly with the gyroradius. It is found that Bohm scaling or worse is possible when the gradients are close to the ITG threshold and the radial modes keep the turbulence level small enough to avoid destroying the slow to form global eigenmodes. In contrast to local ITG ballooning modes, the global eigenmode growth rates decrease with increasing gyroradius from the effect of larger diamagnetic rotational shear. This behavior results in an increase of the correlation time with increasing gyroradius, when the gradients are close to the thresholds. Thus, at sufficiently large relative gyroradius, the breaking of gyro-Bohm scaling can result from increased stability not mixing length.
Alfvén surface waves in a magnetized dusty plasma
Authors: Cramer N. F. , Vladimirov S. V.
Published: July 2002

Surface wave propagation in a dusty magnetized plasma at frequencies below and of the order of the ion-cyclotron frequency, but well above the dust cyclotron frequency, is considered. The dust grains are assumed to be stationary, but to carry a proportion of the negative charge of the plasma. The dispersion relation for surface waves propagating on an interface between a dusty plasma and a vacuum is derived and discussed. The damping of the waves due to Alfvén resonance absorption in a narrow but non-zero width interface is derived.
Cross field diffusion and transport in a relativistic and beam-driven magnetoplasma: Collisional kinetics
Authors: Mohanty J. N. , Baral K. C.
Published: July 2002

Streaming or beam-driven kinetic theory is formulated for a relativistic and collisional plasma diffusing across magnetic field lines, including the small density and temperature gradient. Explicit formulas for modified transport coefficients are presented in various thermal regimes of interest. Their dependence on streaming velocity (V0) is discussed qualitatively.
Kinetic modeling of intense, short laser pulses propagating in tenuous plasmas
Authors: Mora Patrick , Antonsen, Jr. Thomas M.
Published: July 2002

Fast time averaged equations are derived for the motion of particles and the generation of electromagnetic wake fields under the action of the ponderomotive potential of an ultraintense laser pulse propagating through a tenuous plasma. Based on these averaged equations, a new particle code is designed which calculates the particle trajectories on the plasma period time scale. The regime of total cavitation of the plasma is investigated. It is found that stable propagation over a long distance is possible in this regime, and that energetic electrons are produced with a simple characteristic dependence of their angle of deflection on energy. This new code allows for computationally efficient modeling of pulse propagation over great distances.
Radial electric field shear generation by drift-thermal turbulence
Authors: Peng X. D. , Zhang J. H. , Gao Q. D. , Han Z. J.
Published: July 2002

The self-consistent radial electric field shear generated by a drift-thermal turbulence is studied in some detail. A set of new dynamical equations is derived. In this model, there exist three kinds of equilibrium states. The first is the zero shear state of the radial electric field; the second is the positive shear state of the radial electric field; and the third is the negative shear state of the radial electric field. But only the negative shear state may be stable. Two kinds of stable states with the negative shear radial electric field are relevant to the behavior such as that exhibited by high (H) mode and by edge localized modes (ELMs), respectively.
Simulation of α-particle redistribution due to sawteeth on the Tokamak Fusion Test Reactor
Authors: Zhao Yi , White Roscoe B.
Published: July 2002

In recent deuterium–tritium experiments on the Tokamak Fusion Test Reactor (TFTR), both the pellet charge exchange (PCX) [Phys. Rev. Lett. 75, 846 (1995); Nucl. Fusion 35, 1437 (1995)] and the alpha charge exchange recombination spectroscopy (α-CHERS) [Phys. Rev. Lett. 75, 649 (1995)] diagnostics indicate that sawtooth oscillations can cause significant broadening of the fusion alpha radial density profile. Investigation of this sawtooth mixing phenomenon was performed by applying a Hamiltonian guiding center approach. A model of time evolution of the Kadomtsev-type sawtooth [Sov. J. Plasma Phys. 1, 389 (1976)] is constructed. The presence of more than one mode in the nonlinear stage of the sawtooth crash is necessary to cause significant broadening of the alpha density profile. Use of numerical equilibria allows us to perform detailed comparisons with TFTR experimental data. The simulation results are in reasonable agreement with α-CHERS and show a broadening of alpha particles similar to that seen in PCX measurements.
Effects of negative magnetic shear on toroidicity induced eigenmode instability in tokamaks
Authors: Rajendran K. , Dong J. Q. , Mahajan S. M.
Published: July 2002

The effects of negative magnetic shear on the toroidicity induced (TI) eigenmode instability are studied with a numerical WKB shooting scheme. The ion temperature gradient (ITG or ηi) and the parallel velocity shear (PVS) of the ions are included. It is found that for a given set of plasma parameters, the damping mechanism from a negative magnetic shear is much stronger than it is from a positive shear of equal magnitude. The effects of PVS on TI modes are demonstrated to be destabilizing.
A model for particle acceleration in lower hybrid collapse
Authors: Retterer John M.
Published: July 2002

A model for particle acceleration during the nonlinear collapse of lower hybrid waves is described. Using the Musher-Sturman wave equation to describe the effects of nonlinear processes and a velocity diffusion equation for the particle velocity distribution, the model self-consistently describes the exchange of energy between the fields and the particles in the local plasma. Two-dimensional solutions are presented for the modulational instability of a plane wave and the collapse of a cylindrical wave packet. These calculations were motivated by sounding rocket observations in the vicinity of auroral arcs in the Earth’s ionosphere, which have revealed the existence of large-amplitude lower-hybrid wave packets associated with ions accelerated to energies of 100 eV. The scaling of the sizes of these wave packets is consistent with the theory of lower-hybrid collapse and the observed lower-hybrid field amplitudes are adequate to accelerate the ionospheric ions to the observed energies.
Nonlinear behavior of radiative modes in noncoronal equilibrium
Authors: McCarthy D. R. , Booth A. E. , Hutchinson J. E. J. , Krasheninnikov S. I.
Published: July 2002

The nonlinear evolution of radiative modes with the effects of finite recombination time were studied in a uniformly magnetized quasi two-dimensional plasma. In the one-dimensional limit, it was found that a plasma in noncoronal equilibrium (impurities evolve at a slower rate than the temperature), radiative modes were modestly more stable and evolved at a slower rate. A new unstable state was found in which the plasma initially cooled, only to grow unstable to the radiative condensation mode. This nonlinearly unstable regime has a stability boundary an order of magnitude higher than that predicted by linear theory. For a parameter regime appropriate to the tokamak edge, the nonlinear condensation mode proved to be more unstable in noncoronal equilibrium than in coronal equilibrium. When two-dimensional effects were added, little difference was found in the behavior of the mode in coronal equilibrium, but in noncoronal equilibrium, a new state evolved in which the condensation initially grew, but then flattened as the plasma continued to heat at a constant rate. The results provide insight as to why condensations are observed in tokamaks when they are seemingly stable to condensation modes.
Enhanced reverse shear bifurcation in tokamak
Authors: Avinash K. , Kaw P. K. , Singh R.
Published: July 2002

A transport bifurcation model for enhanced reverse shear (ERS) mode recently observed in tokamaks is given. The model includes effects due to evolution of magnetic shear and bootstrap current. It is shown that these effects substantially reduce the power threshold for ERS transition.
Effects of orbit squeezing on ion transport processes close to magnetic axis
Authors: Shaing K. C. , Hazeltine R. D. , Zarnstorff M. C.
Published: July 2002

It is shown that ion thermal conductivity close to the magnetic axis in tokamaks is reduced by a factor of |S|5/3 if (Mi/Me)2/3(Te/Ti)4/3/|S|5/3≫1. Here, S is the orbit squeezing factor, Mi(Me) is the ion (electron) mass, and Ti(Te) is the ion (electron) temperature. The reduction reflects both the increase of the fraction of trapped particles by a factor of |S|1/3, and the decrease of the orbit size in units of the poloidal flux ψ by a factor of |S|2/3.
Theory of plasma confinement in devices with pure toroidal field
Authors: Mahajan Sangeeta , Singh R. , Avinash K.
Published: July 2002

Equilibrium, stability, and confinement in currentless toroidal device is studied in the terms of the flow-fluctuation cycle. In the initial seed equilibrium provided by the limiter, Rayleigh–Taylor (RT) fluctuations grow appreciably. These fluctuations are additional source of rotational transform in two ways. First, they directly drive a poloidal flow via Reynolds stress which improves the equilibrium. Second, the flow modifies the rms level profile in such a way that the ponderomotive force due to the fluctuations impedes the free fall. Detailed linear theory of Rayleigh–Taylor fluctuations with poloidal flow is presented and criteria for flow stabilization are identified. Using the exact eigenfunction of fluctuations, an exact ordinary differential equation for poloidal flow is derived and solved using an ansatz. Finally, the relevance of this analysis to the recently proposed low to high confinement mode transition theories is discussed.
Wavelet analysis of fusion plasma transients
Authors: Dose V. , Venus G. , Zohm H.
Published: July 2002

Analysis of transient signals in the diagnostic of fusion plasmas often requires the simultaneous consideration of their time and frequency information. The newly emerging technique of wavelet analysis contains both time and frequency domains. Therefore it can be a valuable tool for the analysis of transients. In this paper the basic method of wavelet analysis is described. As an example, wavelet analysis is applied to the well-known phenomena of mode locking and fishbone instability. The results quantify the current qualitative understanding of these events in terms of instantaneous frequencies and amplitudes and encourage applications of the method to other problems.