bluuebunny/crossref_metadata_2025 huggingface dataset:
The asymptotic motion of an accelerating, thick layer of inviscid liquid
Authors: Baker Greg , Nie Qing
Published: July 2002

Most studies of a gravitationally unstable interface between a liquid and a gas by boundary integral techniques prescribe the motion of the liquid in the far field. The mean gas pressure at the interface is then irrelevant in its motion. On the other hand, when a pressure jump is applied to a liquid column in a vertical duct, its acceleration is determined by the pressure jump no matter how tall the column. Previous studies of accelerating liquid layers [G. R. Baker, R. L. McCrory, C. P. Verdon, and S. A. Orszag, “Rayleigh–Taylor instability of fluid layers,” J. Fluid Mech. 178, 161 (1987)] show that the motion of the gravitationally unstable interface depends on the reciprocal of the mean layer thickness H. In this paper, we derive an asymptotic boundary integral method that captures the O(1/H) effects on the motion of the unstable interface with a correction that is exponentially small in H. The validity of the asymptotic approach is confirmed by comparison with numerical simulations of the liquid layer. The success of the approach relies on expansions of the kernels in the boundary integrals, indicating that the procedure for deriving the asymptotic equations is more general than just for vertical ducts or periodic geometry. In a subsequent paper, we use our approach to derive the equations for the formation of a bubble at a submerged orifice that is driven by an increase in gas pressure.
Forces on ellipsoidal bubbles in a turbulent shear layer
Authors: Ford Barry , Loth Eric
Published: July 2002

The objective of this research was to gain fundamental knowledge of the drag and lift forces on ellipsoidal air bubbles in water in a turbulent flow. This was accomplished by employing a cinematic two-phase particle image velocimetry (PIV) system to evaluate bubbly flow in a two-stream, turbulent, planar free shear layer of filtered tap water. Ellipsoidal air bubbles with nominal diameters from 1.5 to 4.5 mm were injected directly into the shear layer through a single slender tube. The cinematic PIV allowed for high resolution of the unsteady liquid velocity vector field. Triple-pulsed bubble images were obtained in a temporal sequence, such that the bubble size and bubble trajectory could be accurately determined. The bubble’s oscillation characteristics, velocity, acceleration, and buoyancy force were obtained from the trajectory data. A bubble dynamic equation was then applied to allow determination of the time-evolving lift and drag forces acting upon bubbles within the shear layer. The results indicate that for a fixed bubble diameter (and fixed Bond and Morton numbers), the drag coefficient decreases for an increasing Reynolds number. This is fundamentally different than the increasing drag coefficient trend seen for ellipsoidal bubbles rising in quiescent baths for increasing diameter (and increasing Bond number), but is qualitatively consistent with the trend for spherical bubbles. A new empirical expression for the dependence of the drag coefficient on Reynolds number for air bubbles in tap water for both quiescent and turbulent flows is constructed herein. Finally, the instantaneous side forces measured in this study were dominated by the inherent deformation-induced vortex shedding of the bubble wake rather than the inviscid lift force based on the background fluid vorticity.
The formation and evolution of synthetic jets
Authors: Smith Barton L. , Glezer Ari
Published: July 2002

A nominally plane turbulent jet is synthesized by the interactions of a train of counter-rotating vortex pairs that are formed at the edge of an orifice by the time-periodic motion of a flexible diaphragm in a sealed cavity. Even though the jet is formed without net mass injection, the hydrodynamic impulse of the ejected fluid and thus the momentum of the ensuing jet are nonzero. Successive vortex pairs are not subjected to pairing or other subharmonic interactions. Each vortex of the pair develops a spanwise instability and ultimately undergoes transition to turbulence, slows down, loses its coherence and becomes indistinguishable from the mean jet flow. The trajectories of vortex pairs at a given formation frequency scale with the length of the ejected fluid slug regardless of the magnitude of the formation impulse and, near the jet exit plane, their celerity decreases monotonically with streamwise distance while the local mean velocity of the ensuing jet increases. In the far field, the synthetic jet is similar to conventional 2D jets in that cross-stream distributions of the time-averaged velocity and the corresponding rms fluctuations appear to collapse when plotted in the usual similarity coordinates. However, compared to conventional 2D jets, the streamwise decrease of the mean centerline velocity of the synthetic jet is somewhat higher (∼x−0.58), and the streamwise increase of its width and volume flow rate is lower (∼x0.88 and ∼x0.33, respectively). This departure from conventional self-similarity is consistent with the streamwise decrease in the jet’s momentum flux as a result of an adverse streamwise pressure gradient near its orifice.
Stability of a stratified viscous shear flow in a tilted tube
Authors: Defina Andrea , Lanzoni Stefano , Susin Francesca M.
Published: July 2002

The present investigation is concerned with the effects of viscosity on the stability of a bounded stratified shear flow with Prandtl number Pr≫1. Theoretical results obtained from the solution of the Orr–Sommerfeld equation extended to stratified fluids are compared with experiments performed in a tilting tube filled with water and brine. Theoretical analysis shows that a complete stabilization of the flow field with respect to infinitesimal disturbances is attained, irrespective of the Richardson number J, as the Reynolds number Re decreases below 75. This damping action of viscosity is shown to appreciably reduce the critical Richardson number Jc with respect to the inviscid limit Jc=0.25, even at moderately high Re. On the other hand, the destabilizing action enhanced by viscosity through the diffusion of momentum leads to a viscous mode of instability that may develop if J decreases below a threshold value. An extensive series of experiments has been carried out in a long tilting tube in order to verify theoretical results. The agreement between observations and theory is quite satisfactory. Kelvin–Helmholtz waves grow whenever theoretical unstable conditions are attained. The values of measured wavelengths well correspond to maximum growth rate wave numbers. The comparison between theoretical and experimental results also shows that acceleration plays a stabilizing action.
Planar visualizations of large-scale turbulent structures in axisymmetric supersonic separated flows
Authors: Bourdon Christopher J. , Dutton J. Craig
Published: July 2002

The spatial evolution of large-scale turbulent structures in the shear layer of an axisymmetric, supersonic separated flow has been investigated. The experimental diagnostic used was planar visualization of condensed ethanol droplets that were suspended in the supersonic free stream. Spatial correlation analyses of large ensembles of images show that the mean side-view structure is highly strained and elliptical in shape and is inclined toward the local free stream direction. It is also shown that the effect of lateral streamline convergence for this axisymmetric case causes a reduction in side-view structure size and eccentricity at the reattachment point as compared to the planar case. End-view structures are wedge shaped, wider on the free-stream side than on the recirculation region or developing wake side. It is concluded that the wedge shape is caused by the axisymmetric confinement of the shear layer as it approaches the wake centerline. The average number of structures present in the end-view plane decreases significantly from 10–14 at recompression to 4–5 in the developing wake region. Evidence of an amalgamation of end-view structures in the images at the reattachment point illustrates one of the mechanisms responsible for this reduction.
A geometric/kinematic interpretation of scalar mixing
Authors: Dopazo César , Calvo Pablo , Pétriz Felipe
Published: July 2002

Scalar mixing is interpreted in terms of the growth or decay with time of an infinitesimal spatial volume located between two neighboring iso-scalar surfaces. The divergence of the velocity of propagation of iso-scalar surfaces relative to the fluid and perpendicular to them yields a variable which may be interpreted as the inverse of a scalar characteristic mixing time. Two contributions, the planar iso-scalar surface part and the one due to its curvature, are distinguished. The former is directly linked to the diffusive and chemical mechanisms contributing to the evolution of the scalar dissipation rate. The second rank symmetric tensor, ∂2C/∂xi∂xj, seems to play an important role in these definitions. Direct numerical simulations (DNS) help to identify possible dependences of ∂2C/∂xi∂xj on its invariants.
Three-dimensional bubbles in Rayleigh–Taylor instability
Authors: Oparin A. , Abarzhi S.
Published: July 2002

We study the highly nonlinear stages of the Rayleigh–Taylor instability (RTI) for three-dimensional flow. The proposed numerical and analytical methods are original approaches to the problem. They validate each other and the obtained results agree well.
Intermittency and coherent structures in a swirling flow: A wavelet analysis of joint pressure and velocity measurements
Authors: Chainais Pierre , Abry Patrice , Pinton Jean-François
Published: July 2002

This work is a contribution to the analysis of intermittency in turbulence, and more precisely of its relation with the existence of coherent structures in the flow. We perform joint in time and space measurements of velocity and pressure in a swirling flow. We detect strong pressure drops on the pressure time series, and study the statistics of the interarrivals of the drops. We then perform a statistical analysis of the pressure and velocity signals, conditioned to the list of detected pressure drops. This allows us to show that the detected drops can be associated with the existence of coherent structures in the flow, whose time patterns, in pressure and velocity, can be deeply connected to these of a filamentary vortex. The wavelet transform allows us to separate the velocity signal into a background phase and a filament phase. We analyze intermittency in terms of log-infinitely divisible cascade models. To do so we rewrite the structure functions, the extended self-similarity ansatz and the log-infinitely divisible cascades formalism in the wavelet framework and discuss the relations between these approaches and intermittency. We show that both the complete velocity signal and its filament or background phases are well described by a log-infinitely divisible cascade. Filaments do influence intermittency: They act on the way the cascade develops along the scales but they do not modify the elementary step of the cascade.
Stability of the shape of a surfactant-laden drop translating at low Reynolds number
Authors: Johnson Robert A. , Borhan Ali
Published: July 2002

We examine the effect of surfactants on the evolution of the shape of an initially nonspherical drop translating in an otherwise quiescent fluid at low Reynolds number. A combination of the boundary-integral method and a finite-difference scheme is used to solve the coupled fluid dynamics and surfactant transport problems, in conjunction with the Frumkin adsorption framework to account for the effects of monolayer saturation and nonideal surfactant interactions. For sufficiently small Bond numbers, the drop achieves a nonspherical steady shape. For large initial deformations or Bond numbers, however, the drop deforms continuously, and eventually breaks up through either the formation of an elongated tail or the development of a re-entrant cavity at the trailing end, similar to the mechanisms of drop breakup reported by Koh and Leal [Phys. Fluids A 1, 8 (1989)], and Pozrikidis [J. Fluid Mech. 210, 1 (1990)], for surfactant-free drops. Surfactants are found to have a destabilizing effect on the shape of translating drops. The destabilizing effect is mitigated by the presence of strongly-cohesive surfactant interactions, and by surfactant transport between the bulk and the interface.
The pseudospectrum of the resistive magnetohydrodynamics operator: Resolving the resistive Alfvén paradox
Authors: Borba D. , Riedel K. S. , Kerner W. , Huysmans G. T. A. , Ottaviani M. , Schmid P. J.
Published: July 2002

The ‘‘Alfvén paradox’’ is that as resistivity decreases, the discrete eigenmodes do not converge to the generalized eigenmodes of the ideal Alfvén continuum. To resolve the paradox, the ε-pseudospectrum of the resistive magnetohydrodynamic (RMHD) operator is considered. It is proven that for any ε, the ε-pseudospectrum contains the Alfvén continuum for sufficiently small resistivity. Formal ε-pseudoeigenmodes are constructed using the formal Wentzel–Kramers–Brillouin–Jeffreys solutions, and it is shown that the entire stable half-annulus of complex frequencies with ρ‖ω‖2=‖k⋅B(x)‖2 is resonant to order ε, i.e., belongs to the ε-pseudospectrum. The resistive eigenmodes are exponentially ill-conditioned as a basis and the condition number is proportional to exp(R1/2M), where RM is the magnetic Reynolds number.
Dynamics behavior of self-generated magnetic fields
Authors: Li Xiao-qing , Ma Yue-hua
Published: July 2002

It is analytically proved that the self-generated magnetic fields in the static limit have the self-collapse behavior. Numerical results show that in the nonstatic limit magnetic fields self-generated by transverse plasmons with frequency ωP≊ωpe may collapse.
Ignition and high gain with ultrapowerful lasers*
Authors: Tabak Max , Hammer James , Glinsky Michael E. , Kruer William L. , Wilks Scott C. , Woodworth John , Campbell E. Michael , Perry Michael D. , Mason Rodney J.
Published: July 2002

Ultrahigh intensity lasers can potentially be used in conjunction with conventional fusion lasers to ignite inertial confinement fusion (ICF) capsules with a total energy of a few tens of kilojoules of laser light, and can possibly lead to high gain with as little as 100 kJ. A scheme is proposed with three phases. First, a capsule is imploded as in the conventional approach to inertial fusion to assemble a high-density fuel configuration. Second, a hole is bored through the capsule corona composed of ablated material, as the critical density is pushed close to the high-density core of the capsule by the ponderomotive force associated with high-intensity laser light. Finally, the fuel is ignited by suprathermal electrons, produced in the high-intensity laser–plasma interactions, which then propagate from critical density to this high-density core. This new scheme also drastically reduces the difficulty of the implosion, and thereby allows lower quality fabrication and less stringent beam quality and symmetry requirements from the implosion driver. The difficulty of the fusion scheme is transferred to the technological difficulty of producing the ultrahigh-intensity laser and of transporting this energy to the fuel.
Elimination of the sideband instability in variable-parameter free-electron lasers and inverse free-electron lasers
Authors: Pilla Ravi P. , Bhattacharjee A.
Published: July 2002

A one-dimensional theory is given for the suppression of the sideband instability in a variable-parameter free-electron laser amplifier operating in the Compton regime. It is demonstrated that strong tapering can reduce drastically the intensity of sideband radiation because the frequency of the most unstable sideband mode varies continuously along the wiggler axis. Due to this continuous detuning effect, no single sideband mode can grow at a fixed maximal rate. Furthermore, the peak growth rate decreases gradually along the wiggler. Hence, only a suppressed but broadened sideband spectrum is obtained. In a particular numerical example, high efficiency (∼70%) with sideband intensities suppressed by eight orders of magnitude with respect to the intensity of the primary signal is obtained. The theory is also applied to an inverse free-electron laser in which sidebands are eliminated.
Experimental study of the ponderomotive effects of a large amplitude microwave field on a magnetized electron beam
Authors: Hadjoudj Y. , Pierre Th. , Heuraux S. , Leclert G.
Published: July 2002

Charged particles propagating in an inhomogeneous large amplitude microwave field experience a nonlinear force called the ponderomotive force. This effect can be theoretically evaluated either from the single particle or the fluid theories. For magnetized particles, these two descriptions predict quite different behaviors. A previous experiment [J. Vaclavik et al., Phys. Fluids 29, 2034 (1986)] has shown results in agreement with the fluid theory. Here, an experiment with a magnetized electron beam is described, which brings out a ponderomotive deflection in agreement with the single particle theory.
Comment on ‘‘Phase space evolution in linear instabilities’’ [Phys. Plasmas 1, 3784 (1994)]
Authors: Woodcock J. P.
Published: July 2002

In their paper (Ref. 1), Pantellini et al. presented a method to investigate the particle distribution function in kinetic instabilities in a collisionless plasma. A minor error in the expression they derived is reported. (AIP)
New parallel velocity shear instability
Authors: Finn John M.
Published: July 2002

Analytic and numerical results on the linear theory of a new instability driven by shear in the parallel velocity v∥′ will be presented. This instability exists in the presence of parallel velocity shear and field line curvature. It also requires either parallel viscosity—the full Braginskii stress tensor [S. I. Braginskii, in Reviews of Plasma Physics, edited by M. A. Leontovich (Consultants Bureau, New York, 1965), Vol. 1, p. 205]—or parallel compression. The mode exists in an electromagnetic version, where it can enhance the growth rate of an unstable resistive magnetohydrodynamic (MHD) mode or cause an otherwise stable resistive mode to grow. In its electromagnetic form it is global (matches to an ideal MHD outer region) and can influence modes responsible for disruptions; it can also lead to anomalous transport by stochastic field lines as well as by E×B advection. The instability also exists in an electrostatic form for short wavelengths. For most reasonable edge plasma parameters the effect of parallel compression dominates that of parallel viscosity. The properties of this mode, specifically its growth rate and localization width, are compared with those of the usual v∥′ mode [N. D’Angelo, Phys. Fluids 8, 1748 (1965)]. The importance of this mode in edge fluctuations and on more global resistive MHD is discussed.
On the dispersion relation of longitudinal waves in equilibrium plasmas
Authors: Schlickeiser Reinhard , Mause Hartmut
Published: July 2002

The dispersion relation of subluminal longitudinal waves in thermal non-degenerated isotropic equilibrium plasmas is investigated. The dispersion relation is derived from the relativistically correct form of the kinetic plasma equations and the appropriate Maxwell–Boltzmann–Jüttner plasma equilibrium distribution function. A simple analytic expression for the real part of the dispersion relation is obtained, which holds for all values of the plasma temperature and the index of refraction and improves earlier infinite series approximations. The new form of the dispersion function allows the derivation in implicit form of the wavenumber, real part and imaginary part (for weak damping) of all possible subluminal longitudinal plasma modes. The relativistically correct analysis leads to markedly different result than previous non-relativistic work. The results are illustrated for a one-component electron plasma and a two-component electron-ion plasma. In both cases it is found that Langmuir waves and sound waves are asymptotic solutions of the same longitudinal plasma mode and that they only occur for wavelengths above the electron thermal Debye length. While superluminal Langmuir waves have no Landau damping, subluminal Langmuir waves undergo weak Landau damping. The derived damping rate of subluminal Langmuir waves differs by orders of magnitude from the decrement derived by Landau and corrected by Jackson on the basis of non-relativistic equations.
Resistive drift-wave turbulence
Authors: Camargo Suzana J. , Biskamp Dieter , Scott Bruce D.
Published: July 2002

Two-dimensional resistive drift-wave turbulence is studied by high-resolution numerical simulations in the limit of small viscosity. Density and potential fluctuations are cross-coupled by resistive dissipation, proportional to the adiabaticity parameter, 𝒞, which determines the character of the system: adiabatic (𝒞≫1) or hydrodynamic (𝒞≪1). Various cases are computed for 0.1≤𝒞≤5. Energy spectra exhibit a maximum at some wave number k0(𝒞) and an inertial range behavior for k≳k0. The transfer of energy and vorticity is directly computed and confirms the persistence of local cascade dynamics in all regimes: the familiar dual cascade for the E×B flow eddies, and the direct cascade to small scales for the density as it is advected by the eddies. Inertial range spectral power laws agree surprisingly well with simple scaling predictions. No prominent large-scale long-lived coherent structures are observed, an absence that is consistent with the statistical properties, which are found to be perfectly Gaussian for k≲k0, but exhibit the non-Gaussian behavior, typical for small-scale intermittency, in the inertial range.
Stability of vortex flows in magnetized plasmas. I. Non-Boltzmann vortices
Authors: Chakrabarti Nikhil , Das Amita , Kaw Predhiman , Singh Raghvendra
Published: July 2002

In this paper the stability of long scale vortex flows to secondary perturbations with short parallel wavelength is analyzed. It is demonstrated that flow ellipticity can drive secondary instabilities of ion-acoustic and shear Alfvén waves provided certain resonance conditions between the rotation speed and the Doppler shifted secondary wave frequency are satisfied. Such secondary processes can act as a major sink of energy and may help in our understanding of nonlinear saturation of low frequency instabilities.
Hydrodynamic description of Alfvén and fast wave current drive in weakly collisional plasmas of magnetic traps
Authors: Tsypin V. S. , Elfimov A. G. , de Azevedo C. A. , de Assis A. S.
Published: July 2002

In this paper, general aspects of the Alfvén and fast wave nonresonant and resonant current-drive problem are considered both for collisional and weakly collisional plasmas. The magnetohydrodynamic approach is used for all collisional regimes and Landau damping is taken into account in the weakly collisional plasma case. Averaged in time and over magnetic surface, the general form of current-drive forces are derived, including the viscosity and collision frequency perturbation terms. As an example, a weakly collisional plasma cylinder case is discussed and some disagreements on this problem between different authors results are eliminated.
Erosion and evaporation theory in ion-focused electron-beam transport
Authors: Mostrom Michael A. , Mitrovich Dushan , Welch Dale R. , Campbell Mark M.
Published: July 2002

Three physical mechanisms that degrade the energy transport by electron beams in the ion focused regime are examined. One of these, magnetic evaporation, affects the whole beam pulse and reduces the current propagated. It occurs when a transverse magnetic field enables beam electrons with sufficient transverse energy to escape from the beam. A simple model for this process is developed, and its predictions are shown to be closely matched by simulation results. The other two degrading mechanisms cause a steady erosion of the beam head, thereby shortening the pulse. When propagating along the plasma channel, beam electrons cause electrostatic ejection of plasma channel electrons. The energetic cost of this process is the slowing of beam head electrons, which are eventually lost from the beam, i.e., eroded away. Inductive erosion is the cylindrically symmetric aspect that takes place when there is no transverse magnetic field. An analytic model for this is developed and compared with particle-in-cell code simulations and with experiments. A transverse magnetic field makes the electron displacement process asymmetric, with channel and beam electrons leaving on opposite sides of the channel. No reliable analytic model for this is available, so simulation results are used to obtain an approximate dependence of erosion rate on parameters. A simple argument is presented for dealing with the transition between inductive and magnetic erosion. The predictions correlate well with experimental results.
Operation of a quasi-optical gyrotron with a Gaussian output coupler
Authors: Hogge J. P. , Tran T. M. , Paris P. J. , Tran M. Q.
Published: July 2002

The operation of a 92 GHz quasi-optical gyrotron having a resonator formed by a spherical mirror and a diffraction grating placed in −1 order Littrow mount is presented. A power of 150 kW with a Gaussian output pattern was measured. The Gaussian content in the output was 98% with less than 1% of depolarization. By optimizing the magnetic field at fixed frequency, a maximum efficiency of 15% was reached.
Analytic theory of sheath expansion into a cylindrical bore
Authors: Sheridan T. E.
Published: July 2002

A simple analytic theory describing the expansion of the collisionless, pulsed, plasma sheath into a cylindrical bore is presented. An approximate expression for the Child–Langmuir law is derived, allowing the sheath speed and sheath width to be predicted for quasistatic expansion. The dependence of the ion impact energy on sheath speed is estimated. Finally, the dependence of the maximum ion impact energy on the radius of the bore is calculated from the sheath speed dependence of the ion impact energy and the minimum calculated sheath speed. The theory agrees well with numerical solutions of a time-dependent fluid model.
Even and odd modes driven by the trapped ion temperature gradient over the secular scale
Authors: Li Jiquan , Ding Houchang , Huang Lin
Published: July 2002

In the long wavelength limit, the trapped ion temperature gradient driven modes on the secular scale are investigated by considering the lowest order effects of the finite banana orbit width and using the two-scale expansion. Attention is focused on the effects of the wave–particle interaction over the secular scale. Also, the effects of the collisionless trapped electron dynamics are considered. The eigenmode equation derived is in the form of a modified Bessel equation. The even modes are almost not influenced by the effects of the wave–particle interaction. But for odd modes, the wave–particle interaction can give rise to instabilities of the ion branches. These odd modes are similar to the trapped ion scattering modes pointed out first by Coppi and Rewoldt [Advances in Plasma Physics, edited by A. Simon and W. Thompson (Wiley, New York, 1976), Vol. 6, p. 421]. The dependence of the growth rates on the ion temperature gradient is analyzed numerically. The typical eigenfunction curves of both the even and odd modes driven by the trapped ion temperature gradient over the secular scale are displayed.
Properties of thermal flare plasmas (3×106–3×107 K): Observational results
Authors: Feldman Uri
Published: July 2002

One of the most violent while best observed phenomena occurring in the solar upper atmosphere is flare emission in the 3×106 to 3×107 K temperature range. This emission, commonly called thermal flare emission, can vary in intensity by more than five orders of magnitude, and exhibits regular and predictable properties. A wealth of observational data regarding thermal solar flares has been collected. Through these data the morphological properties of thermal flares have been determined. Plasma properties such as electron temperatures, electron densities, mass motions, and variations in elemental abundances during the course of the flare are well established. Observational data have also been used to determine relationships between peak fluxes and maximum flare temperatures as well as general properties of the light curve of flares.
Analyses of three shock interactions in magnetohydrodynamics: Aligned-field case
Authors: Ogawa Hiroyuki , Fujiwara Toshi
Published: July 2002

The paper deals with the confluence of three shock waves at a point, in a magnetohydrodynamic (MHD) fluid. Based on the three-shock theory, the equations governing the flow field in the vicinity of the intersection point are obtained. The three shock confluences in field-aligned cases are studied here using shock polars, revealing that only seven combinations of three shock types are possible. The relations among (a) the combinations of incident and reflected shock types, (b) the angle between incident and reflected shocks, and (c) the streamline deflection angle across the reflected shock are shown. As an example of application, the flow field induced by a supersonic MHD flow over a concave double wedge is studied both analytically and numerically.
A plasma filled gyrotron-pumped free-electron laser
Authors: Sharma Anamika , Tripathi V. K.
Published: July 2002

The introduction of magnetized plasma medium in the interaction region of a gyrotron-pumped free-electron laser (FEL) offers the possibility of generating short radiation wavelengths using moderate energy beams. The plasma slows down the electromagnetic wave produced by a high-power gyrotron that can now act as a wiggler for infrared free-electron laser. The wiggler period can be tuned by varying the plasma density and/or ambient magnetic field. The wiggler wave vector and its field amplitude are sensitive to the choice of wave frequency as this slow electromagnetic wave is dispersive in nature. The free-electron laser (FEL) operation for wiggler frequency close to electron cyclotron frequency is examined. The FEL efficiency and the gain are estimated.
The Nike KrF laser facility: Performance and initial target experiments
Authors: Obenschain S. P. , Bodner S. E. , Colombant D. , Gerber K. , Lehmberg R. H. , McLean E. A. , Mostovych A. N. , Pronko M. S. , Pawley C. J. , Schmitt A. J. , Sethian J. D. , Serlin V. , Stamper J. A. , Sullivan C. A. , Dahlburg J. P. , Gardner J. H. , Chan Y. , Deniz A. V. , Hardgrove J. , Lehecka T. , Klapisch M.
Published: July 2002

Krypton-fluoride (KrF) lasers are of interest to laser fusion because they have both the large bandwidth capability (≳THz) desired for rapid beam smoothing and the short laser wavelength (1/4 μm) needed for good laser–target coupling. Nike is a recently completed 56-beam KrF laser and target facility at the Naval Research Laboratory. Because of its bandwidth of 1 THz FWHM (full width at half-maximum), Nike produces more uniform focal distributions than any other high-energy ultraviolet laser. Nike was designed to study the hydrodynamic instability of ablatively accelerated planar targets. First results show that Nike has spatially uniform ablation pressures (Δp/p<2%). Targets have been accelerated for distances sufficient to study hydrodynamic instability while maintaining good planarity. In this review we present the performance of the Nike laser in producing uniform illumination, and its performance in correspondingly uniform acceleration of targets.
Resistive wall mode stabilization by plasma rotation in advanced tokamaks
Authors: Eriksson G.
Published: July 2002

By combining previous results of Betti and Freidberg [Phys. Rev. Lett. 74, 2949 (1995)] and Eriksson [Phys. Plasmas 2, 3095 (1995)], a fully analytical description is obtained for the stabilizing effect of toroidal plasma rotation in a large aspect ratio tokamak surrounded by a resistive wall. As in advanced tokamak configurations with a large fraction of bootstrap current, it is assumed that the current gradient near the plasma edge is large. This assumption enables an analytical analysis of external kink modes with low poloidal mode numbers. An expression is obtained, showing explicitly how the window of stable wall distances depends on the current profile.
Strongly anomalous diffusion in sheared magnetic configurations
Authors: Vanden Eijnden E. , Balescu R.
Published: July 2002

The statistical behavior of magnetic lines in a sheared magnetic configuration with reference surface x=0 is investigated within the framework of the kinetic theory. A Liouville equation is associated with the equations of motion of the stochastic magnetic lines. After averaging over an ensemble of realizations, it yields a convection-diffusion equation within the quasilinear approximation. The diffusion coefficients are space dependent and peaked around the reference surface x=0. Due to the shear, the diffusion of lines away from the reference surface is slowed down. The behavior of the lines is asymptotically strongly non-Gaussian. The reference surface acts like an attractor around which the magnetic lines spread with an effective subdiffusive behavior. Comparison is also made with more usual treatments based on the study of the first two moments equations. For sheared systems, it is explicitly shown that the Corrsin approximation assumed in the latter approach is no longer valid. It is also concluded that the diffusion coefficients cannot be derived from the mean square displacement of the magnetic lines in an inhomogeneous medium.
Nonlinear interaction of tearing modes in an infinite-aspect-ratio tokamak
Authors: Barbulla W. , Rebhan E.
Published: July 2002

The nonlinear interaction of tearing modes centered on different resonant surfaces in a low-β tokamak is studied. Center manifold theory is applied to the reduced equations of resistive magnetohydrodynamics, including the electron energy equation. For the equilibrium current profile a polynomial parametrized by the values of shear and safety factor at several given radii is chosen. Amplitude equations describing the asymptotic time behavior of interacting modes are obtained. The coefficients of these equations depend on the mode numbers of the modes involved and on the parameters of the problem. They were calculated for the interaction of the (2,1)- and the (3,2)-tearing mode. The dependence of the corresponding solutions on equilibrium parameters and transport coefficients is investigated.
Simulation of Z-pinch dynamics in a high-current plasma lens with a new one-dimensional model including finite plasma conductivity
Authors: Kumpf C. , Kowalewicz R. , Stetter M. , Tkotz R.
Published: July 2002

The dynamics of a fast Z-pinch plasma lens was simulated with a one-dimensional model for the phase of constriction. The calculation resulted in a realistic description—compared to experiment—of pinch dynamics and current density distribution up to the time of minimum plasma radius, which is important to evaluate the focus quality of the lens. Equations of the inverse skin effect and the momentum conservation equation of magnetohydrodynamics were used. Due to the consideration of a finite plasma conductivity a finite current density distribution was computed, reaching 16 kA/cm2 at its maximum, which corresponds to a total current of about 280 kA at pinch time. The plasma is compressed from an initial radius of 100 mm to a pinch radius of about 14 mm within 3.33 μs. Thus, an ion density of 1.6×1018 cm−3 and a temperature of 30 eV is calculated. Although no shock wave is implemented in the model explicitly, a sharp radial density increase appears in the contraction phase and moves toward the axis with a velocity of up to 8×104 m/s.
Low-frequency current drive utilizing nonlinear wave mixing
Authors: Rauf S. , Tataronis J. A.
Published: July 2002

In this paper we present a self-consistent analysis of steady-state current generation by nonlinear mixing of Alfvén waves in a resistive incompressible plasma slab. In the model, the low-frequency waves are excited inside the plasma using external electric and magnetic current sheet antennas. Since the plasma medium is nonlinear, the first-order primary waves beat together with each other, and generate a second-order steady-state longitudinal current. The steady-state current density is computed self-consistently in the analysis using Ampère’s law. Several antenna configurations and current production schemes are explored. It is shown that it is possible to drive reasonably large steady-state currents in the plasma using waves with a modest amplitude. Methods for controlling the current characteristics by means of the antenna parameters are also studied.
Gyrokinetic-magnetohydrodynamic hybrid simulation of the transition from toroidal Alfvén eigenmodes to kinetic ballooning modes in tokamaks
Authors: Santoro R. A. , Chen Liu
Published: July 2002

A gyrokinetic-magnetohydrodynamic (MHD) hybrid simulation code has been developed in order to study high-n (where n is the toroidal mode number) MHD instabilities driven by energetic particles in finite-β tokamaks. Here β is the ratio between plasma and magnetic pressures. Specifically, it is observed that as the core plasma β value increases, there is a corresponding transition from the toroidal Alfvén eigenmode (TAE) to the kinetic ballooning mode (KBM). The energetic particle mode (EPM) branches of both the toroidal Alfvén mode (TAM) and KBM are shown to be important in this transition. KBM are preferentially excited when the energetic particle velocity is small compared to the Alfvén velocity.
Attenuation of energetic helium beams in fusion plasmas
Authors: Korotkov A. A. , Janev R. K.
Published: July 2002

The efficiency of plasma heating and diagnostics based on using atomic beams or neutralized ion fluxes depends crucially on the atomic processes involved in the stopping of the atoms. These processes have been studied in detail for helium atoms in the energy range from 25 keV/u to 2.5 MeV/u. The processes considered include both the ground state and the excited states of the atoms, thus allowing for the multistep electron loss in collisions with the plasma constituents. Special attention is paid to electron loss from the atoms in metastable states. The effective beam stopping cross section and attenuation factor have been calculated for a wide variety of beam and plasma parameters. The atomic database necessary for these calculations has been documented using the best data available at present. The results are also given in terms of convenient analytic fits. Special calculations have been carried out for the helium probing beam and neutralized alpha particles in International Thermonuclear Experimental Reactor (ITER) [Kikuchi et al., Fusion Eng. Design 16, 253 (1991)] plasmas.
Contribution of Yushmanov-trapped electrons to a thermal dike
Authors: Tatematsu Y. , Kiwamoto Y. , Saito T. , Yoshimura Y. , Katanuma I. , Tamano T.
Published: July 2002

A substantial amount of Yushmanov-trapped electrons are generated as a result of electron cyclotron resonance heating employed for generating a thermal dike in an open magnetic field configuration. The production rate and the energy spectrum of the trapped electrons are calculated by using a heating response function. The calculated energy spectrum of the end-loss electrons driven from the Yushmanov-trapped region indicates a reduction in power efficiency of the thermal dike. The potential profile modified by the Yushmanov-trapped particles tends to reduce their production rate, leading to a stationary state. Their significance to a thermal dike basic experiment in the end region of the GAMMA 10 tandem mirror [Plasma Physics and Controlled Nuclear Fusion Research 1992 (International Atomic Energy Agency, Vienna, 1993), Vol. 2, p. 651] is discussed.
Relativistic effects in energy extraction from alpha particles
Authors: Fruchtman A. , Fisch N. J. , Valeo E. J.
Published: July 2002

The use in a tokamak of the recently reported relativistic two-gyrostream instability is investigated. The concept is evaluated with respect to the extraction of energy from relativistic ions in an inhomogeneous magnetized plasma by means of an electrostatic wave. For application to energetic alpha particle channeling in a tokamak fusion reactor, the relativistic two-gyrostream instability effects turn out to be relatively minor.
Particle simulations of divergent and convergent radial electron flows in cylindrical Pierce diodes
Authors: Alves M. Virgínia , Gratton Fausto T. , Gnavi Graciela , Moreno Cesar H.
Published: July 2002

A special class of Pierce diodes, that consists of a pair of grounded coaxial cylindrical electrodes, with electrons streaming radially between them, is studied. The full set of roots of the dispersion relation of the Pierce instability is obtained by solving the dispersion relation numerically. The nonlinear behaviour of the system is then investigated using particle simulation. It is shown that the behaviour of cylindrical systems depends on two nondimensional parameters. Linear and nonlinear properties are different for convergent and divergent configurations. Several remarkable differences between cylindrical and planar Pierce diodes are reported.
Is dust acoustic wave a new plasma acoustic mode?
Authors: Dwivedi C. B.
Published: July 2002

In this Brief Communication, the claim of the novelty of the dust acoustic wave in a dusty plasma within the constant dust charge model is questioned. Conceptual lacunas behind the claim have been highlighted and appropriate physical arguments have been forwarded against the claim. It is demonstrated that the so-called dust acoustic wave could better be termed as a general acoustic fluctuation response with a dominant characteristic feature of the acoustic-like mode (ALM) fluctuation response reported by Dwivedi et al. [J. Plasma Phys. 41, 219 (1989)]. It is suggested that both correct and more usable nomenclature of the ALM should be the so-called acoustic mode.
Near-forward stimulated Brillouin scattering
Authors: McKinstrie C. J. , Li J. S. , Kanaev A. V.
Published: July 2002

The spatiotemporal evolution of near-forward stimulated Brillouin scattering (SBS) is studied in detail. For large scattering angles SBS grows and saturates as a three-wave instability. For small scattering angles SBS begins to grow as a three-wave instability, then continues to grow and saturates as a four-wave instability. Expressions for the saturation time and steady-state gain exponent of SBS are derived for large and small scattering angles.
Instability and turbulent mixing of ablatively accelerated thin layer
Authors: Neuvazhayev V. E. , Lykov V. A. , Ljagina E. L. , Murashkina V. A. , Podkorytova A. F. , Frolov V. D. , Shushlebin A. N.
Published: July 2002

Two-dimensional calculations by the TIGR code [Izv. Sib. Otd. Akad. Nauk SSSR 8, 74 (1967); Vop. Atom. Nauk Tekh. Met. Prog. Chisl. Resh. Zad. Mat Fiz. 3, 34, (1984)] have verified the main result of the theoretical research on the limitation of perturbation growth at the ablation front, which appeared in the planar foils irradiated by laser flow in the regime of accelerated motion of the ablation front. It is shown by calculations that the growth rate γ, characterizing the perturbation growth, achieves its maximal value γm at some wave number km, which is defined by initial parameters of the problem—acceleration at the ablation front g and velocity of ablation ua. It is in good agreement qualitatively and quantitatively, with theoretical results obtained earlier by some authors. It is shown by calculations that for k>km, as it proceeds from the theory, the growth rate decreases down to zero. Simulations of turbulent mixing on the basis of the Kε model is implemented. For the thin foil case considered, an analytic estimate of the effect of turbulent mix, which included Takabe’s formula, showed ablation reduced in the coefficient in the mixing equation by a factor of 25. Theoretical and numerical research on the influence of turbulent mixing on the performance of the compressed part of the plate has been conducted.
Explicit reduction of N-body dynamics to self-consistent particle–wave interaction
Authors: Antoni Mickaël , Elskens Yves , Escande Dominique F.
Published: July 2002

The one-dimensional (1-D) spatially periodic system of N classical particles, interacting via a Coulomb-like repulsive long-range force, is studied using classical mechanics. The usual Bohm–Gross dispersion relation for the collective modes is obtained in the absence of quasiresonant particles. In the presence of R quasiresonant particles, the evolution equations for M long-wavelength modes are coupled to the particles’ motion through a self-consistent wave–particle Hamiltonian. The wave–particle Lagrangian is derived from the full N-body Lagrangian. The derivation relies on an explicit scale separation argument and avoids the use of kinetic theory and continuous medium formalism.
Kinetic description of cyclotron-range oscillations of a non-neutral plasma column
Authors: Neu S. C. , Morales G. J.
Published: July 2002

The kinetic analysis introduced by Prasad, Morales, and Fried [Prasad et al., Phys. Fluids 30, 3093 (1987)] is used to derive damping conditions and a differential equation for azimuthally propagating waves in a non-neutral plasma column in the limits rl/L≪1 and krl≪1 (where rl is the Larmor radius, k is the wave number, and L is the density scale length). The predictions of the kinetic analysis are verified using a two-dimensional particle-in-cell simulation of Bernstein modes in a thermal rigid-rotor equilibrium. Differences between modes in a strongly magnetized limit and near the Brillouin limit are studied in the simulation.
Stationary modes in a non-neutral plasma
Authors: Goswami Priyanka , Bhattacharyya S. N. , Sen A. , Maheshwari K. P.
Published: July 2002

In a non-neutral plasma column of constant density it is shown that, for certain parameter values, an infinitely degenerate internal mode, which has no variation in the axial direction, can be stationary in the laboratory frame. However, this mode cannot be driven resonantly by stationary electric or magnetic field asymmetries. When the plasma density is nonuniform the possibility of resonance of stationary field asymmetries with the continuous spectrum is pointed out.
Constraints on finite-time current sheet formation at null points in two-dimensional ideal incompressible magnetohydrodynamics
Authors: Klapper I.
Published: July 2002

It is shown rigorously that, under the conditions of two-dimensional ideal incompressible magnetohydrodynamics, finite-time singularity formation (including finite-time collapse to a current sheet) cannot occur at a magnetic null point of any type unless driven by a pressure singularity occurring outside a neighborhood of the null point. The proof is based on the depletion of nonlinearity at a two-dimensional magnetic null point.
Momentum–space structure of relativistic runaway electrons
Authors: Martı́n-Solı́s J. R. , Alvarez J. D. , Sánchez R. , Esposito B.
Published: July 2002

The dynamics of relativistic runaway electrons in tokamak plasmas is analyzed using a test particle description that includes acceleration in the toroidal electric field, collisions with the plasma particles, and deceleration due to synchrotron radiation losses. The region of momentum space in which electron runaway takes place is determined. It is found that relativistic and synchrotron radiation effects lead to a critical electric field ER>(kTe/mec2)ED, below which no runaways are generated. In addition, the trajectories of the test electrons in momentum space show a stable equilibrium point that sets a limit on the energy that the runaway electrons can reach. Analytical expressions are given for this energy limit as a function of the toroidal electric field and plasma parameters. The dominant radiative mechanisms limiting the runaway electron energy are identified in the whole range of electric field values.