bluuebunny/crossref_metadata_2025 huggingface dataset:
Instability of a liquid jet emerging from a droplet upon collision with a solid surface
Authors: Kim H.-Y. , Feng Z. C. , Chun J.-H.
Published: July 2002

A linear perturbation theory is developed to investigate the interface instabilities of a radially-expanding, liquid jet in cylindrical geometries. The theory is applied to rapidly spreading droplets upon collision with solid surfaces as the fundamental mechanism behind splashing. The analysis is based on the observation that the instability of the liquid sheet, i.e., the formation of the fingers at the spreading front, develops in the extremely early stages of droplet impact. The shape evolution of the interface in the very early stages of spreading is numerically simulated based on the axisymmetric solutions obtained by a theoretical model. The effects that factors such as the transient profile of an interface radius, the perturbation onset time, and the Weber number have on the analysis results are examined. This study shows that a large impact inertia, associated with a high Weber number, promotes interface instability, and prefers high wave number for maximum instability. The numbers of fingers at the spreading front of droplets predicted by the model agree well with those experimentally observed.
On the existence of two-dimensional Euler flows satisfying energy-Casimir stability criteria
Authors: Wirosoetisno Djoko , Shepherd Theodore G.
Published: July 2002

The energy-Casimir stability method, also known as the Arnold stability method, has been widely used in fluid dynamical applications to derive sufficient conditions for nonlinear stability. The most commonly studied system is two-dimensional Euler flow. It is shown that the set of two-dimensional Euler flows satisfying the energy-Casimir stability criteria is empty for two important cases: (i) domains having the topology of the sphere, and (ii) simply-connected bounded domains with zero net vorticity. The results apply to both the first and the second of Arnold’s stability theorems. In the spirit of Andrews’ theorem, this puts a further limitation on the applicability of the method.
Chemical transport associated with discharge of contaminated fine particles to a steady open-channel flow
Authors: Ng Chiu-On
Published: July 2002

In this paper, an analytical study on the advective-dispersive transport of a chemical contaminant resulting from the discharge of contaminated fine solid particles into a two-dimensional, steady and uniform turbulent open-channel flow is presented. Because of sorptive exchange, the transport of the chemical cloud is affected by that of the suspended particulates. Such a relationship has so far not been explicitly established by intuitive arguments. The effective transport equations are formally derived by an extended method of homogenization. It is found that over a long time scale the fall velocity will delay the sediment advection, and the advection velocity and dispersion coefficient for the chemical transport will change with space and time according to the local sediment concentration. Numerical results confirm that the centers of mass of the sediment and dissolved phase clouds are not advancing at the same speed, and the dispersion of the chemical is enhanced by the local retardation factor.
Two-fluid Taylor-Couette flow with countercurrent axial flow: Linear theory for immiscible liquids between corotating cylinders
Authors: Baier Gretchen , Graham Michael D.
Published: July 2002

We computationally investigate the stability of a pair of radially stratified immiscible liquids undergoing countercurrent axial flow in the annular gap between rapidly corotating coaxial cylinders: two-fluid Taylor-Couette flow with counterflow. A simple analysis determines conditions under which a nearly cylindrical interface is maintained in the presence of counterflow (i.e., axial pressure gradients). Stability analysis reveals that for small axial Reynolds numbers, the flow is slightly stabilized against Taylor-Couette instability, consistent with results for a single phase. At axial Reynolds numbers greater than about ten, however, the flow is susceptible to a (generally nonaxisymmetric) Kelvin-Helmholtz instability, which precedes the Taylor-Couette mode. Furthermore, new results are presented for the case without axial flow. A bifurcation to vortices that corotate with their counterparts in the other phase is found. Finally, limitations of the generalized Rayleigh criterion developed in our earlier work are elucidated. In particular, we show how it fails if one of the fluid layers is very thin.
Plasma emission via a beam instability with density modulation
Authors: Yoon Peter H. , Wu C. S.
Published: July 2002

In this article, a new emission process for radiation near the plasma frequency in an unmagnetized plasma (i.e., the plasma emission) is discussed. The process involves a tenuous beam of energetic electrons and low-frequency ion waves. The key point is that the ion waves can modulate the thermal electron density so that the condition for beam–plasma interaction (ω=k⋅V, where V is the average beam velocity) is Doppler shifted in wave vector space in accordance with the wavelength that characterizes the ion density fluctuations [ω+ω′=(k+k′)⋅V, where ω′ and k′ are the characteristic frequency and wave vector associated with the ion wave; in the present discussion, let set ω′=0]. As a result, the unstable electrostatic beam–plasma mode becomes coupled with the fast electromagnetic mode. Consequently in an inhomogeneous plasma the amplified waves can naturally change to electromagnetic mode and escape from the source region.
On the stability of shear-Alfvén vortices
Authors: Jovanović D. , Horton W.
Published: July 2002

Linear stability of shear-Alfvén vortices is studied analytically, using the Lyapunov method. Vortices belonging to the drift mode, which is a generalization of the standard Hasegawa–Mima vortex to the case of large parallel phase velocities, are proved to be unstable. In the case of the convective-cell mode, short perpendicular-wavelength perturbations are stable for a broad class of vortices. Eventually, instability of convective-cell vortices may occur on the perpendicular scale comparable with the vortex size, but it is followed by a simultaneous excitation of coherent structures with a better localization than the original vortex.
Two-fluid equations for singular modes with frequency comparable to the parallel ion acoustic frequency
Authors: Zheng L.-J. , Tessarotto Massimo
Published: July 2002

Braginskii’s two-fluid equations, with collisional effects neglected and electron fluid assumed to be isothermal along the magnetic field lines, are employed to obtain the singular mode equations. While the ordering analysis in the paper indicates that, in the intermediate frequency regime (i.e., that with the frequency of the modes higher than the ion acoustic frequency along the magnetic field lines, but lower than the electron one), the stabilizing finite Larmor radius (FLR) effect on the modes dominates over other collisionless, two-fluid effects, the paper is devoted to the investigation of the comparable frequency regime (i.e., that with the frequency of the modes comparable to the ion acoustic frequency along the field lines). It is found that, in such a regime, the parallel electric field is excited and the plasma behaves nonadiabatically. The singular mode equations consist of two coupled second-order partial differential equations, in which the FLR, the plasma compression, and the parallel electric field effects are present. The results are discussed, and a comparison with the ideal magnetohydrodynamic formalism is given.
Interaction of an ultrashort, relativistically strong laser pulse with an overdense plasma
Authors: Bulanov S. V. , Naumova N. M. , Pegoraro F.
Published: July 2002

The results of an analytical description and of a particle-in-cell simulation of the interaction of an ultrashort, relativistically intense laser pulse, obliquely incident on a nonuniform overdense plasma, are presented and several novel features are identified. The absorption and reflection of the ultraintense electromagnetic laser radiation from a sharp-boundary plasma, high harmonic generation, and the transformation into low-frequency radiation are discussed. In the case of weak plasma nonuniformity the excitation of nonlinear Langmuir oscillations in the plasma resonance region and the resulting electron acceleration are investigated. The vacuum heating of the electrons and the self-intersection of the electron trajectories are also studied. In the case of a sharp-boundary plasma, part of the energy of the laser pulse is found to be converted into a localized, relativistically strong, nonlinear electromagnetic pulse propagating into the plasma. The expansion of the hot electron cloud into the vacuum region and the action of the ponderomotive force of the laser pulse in the localized longitudinal electric field of the Langmuir oscillations lead to ion acceleration. The energy increase of a minority population of multicharged ions is found to be much greater than that of the ambient ions.
Local modes of field-reversed configurations
Authors: Ishida A. , Shibata N. , Steinhauer L. C.
Published: July 2002

The local stability of field-reversed configurations (FRC) is analyzed using hydrodynamic stability theory. The equation of state includes both compressibility and double-adiabatic effects. For the first time, eigenmodes of the linearized equations of motion have been computed. The most unstable modes have fast growth rates, comparable to the Alfvén transit time across the FRC radius; i.e., somewhat faster than the frequency (or growth rate) of global modes. In realistic equilibria, the most unstable local modes concentrate, ballooning-mode style, in the high curvature region of magnetic flux lines. The familiar interchange stability criterion is irrelevant for FRCs, since the actual eigenmodes differ markedly from interchange, both in structure and stability. The appearance of fast local modes raises the possibility that they may regulate FRC equilibria. However, surprisingly, equilibria with realistic internal structure (i.e. resembling experiments) are more unstable to ideal local modes than less realistic equilibria, as have often been studied theoretically. Thus, a nonideal theory will be needed to explain the equilibria observed in experiments.
Microwave signal amplification and Pierce instability on radial electron flows in cylindrical and spherical diodes
Authors: Gnavi Graciela , Gratton Fausto T.
Published: July 2002

Linear space charge perturbations of focused electron beams flowing between cylindrical and spherical electrodes on convergent or divergent trajectories are studied, and the amplification of high-frequency signals when the flow is modulated at one electrode is computed. It is shown that divergent beams give the largest amplification effect. The instability of electron beams drifting through grounded grids (Pierce instability in cylindrical or spherical diodes) is also considered. The instability threshold occurs at higher critical currents when the curvature of the electrodes is large. Results for planar electrodes are recovered in the limit of zero curvature devices. Spherical configurations have better signal amplification and stability properties than similar planar or cylindrical systems.
Diocotron instability of a relativistic sheet electron beam propagating through a rectangular conducting wall
Authors: Uhm Han S. , Ben-Menahem Shahar , Yu David
Published: July 2002

Properties of the diocotron instability in a relativistic sheet electron beam propagating through a rectangular conducting wall are investigated within the framework of a macroscopic cold fluid model. The electron beam is assumed to be partially neutralized by the positive immobile ions with the fractional charge neutralization f. The eigenvalue equation is obtained for low-frequency perturbations in standing waves. The dispersion relation of the diocotron instability is derived and used to investigate stability properties for a broad range of system parameters including the ratio a/d of the beam thickness (2a) to the conductor gap (2d) and the charge neutralization f. The dispersion relation indicates that the system is stabilized by increasing the neutralization f to 1/γ2b, where γb is the characteristic value of the beam relativistic factor. It is also shown that the diocotron perturbations are completely stabilized by increasing the beam thickness to more than one-half the conductor gap (i.e., a/d≳0.5) for f=0. The growth rate of instability is a substantial fraction of the diocotron frequency if the system is unstable.
Variational equilibrium of relativistic electron beams
Authors: Ganesh R. , Avinash K.
Published: July 2002

The problem of equilibrium of relativistic electron beams is formulated in terms of a variational principle. The equilibrium is determined in terms of spatially averaged conditions at the cathode. For a shielded cathode, variational solution reduces to Reiser–Diament solution. In the case of imperfect shielding, the variational solution shows a degradation of limiting current.
Time dependent parallel viscosity and relaxation rate of poloidal rotation in the banana regime
Authors: Hsu C. T. , Shaing K. C. , Gormley R.
Published: July 2002

Time dependent ion parallel viscous force in the banana regime with arbitrary inverse aspect ratio ε is calculated using the eigenfunction approach. The flux surface averaged viscosity is then used to study the relaxation process of the poloidal rotation which leads to oscillatory relaxation behavior. The relaxation rate νp is found approximately proportional to νii/ε (where νii is the ion collision frequency and ε is the inverse aspect ratio).
‘‘Phase mixing’’ of bunches and decrease of negative-mass instability increments in cyclotron resonance masers
Authors: Bratman V. L. , Savilov A. V.
Published: July 2002

Negative-mass (space-charge) instability in magnetized electron beams is considered for a flat, thin (having a thickness about the cyclotron diameter) beam of weakly relativistic electrons oscillators. Pitch angle and energy spreads of particles, as well as magnetic tapering are investigated on the base of averaged differential-integral equations of electron motion. For the energy spread, the phenomenon of ‘‘phase mixing’’ of particles rotating with different frequencies limits the number of cyclotron harmonics taking part in the instability and significantly decreases the increments. For the magnetic tapering, this effect leads to linear saturation at the length comparable with the characteristic scale of the tapering.
An analytical solution of finite-amplitude solitary kinetic Alfvén waves
Authors: Wu De-Jin , Wang De-Yu , Fälthammar Carl-Gunne
Published: July 2002

An analytical solution of finite-amplitude solitary kinetic Alfvén waves (SKAWs) in a low-β (β≪me/mi≪1) plasma is presented. This solution has been compared with the solution of the Korteweg–de Vries (KdV) equation in the small-amplitude limit. It is found that the KdV soliton solution is valid only for the maximum relative density perturbation Nm<0.1. For the larger Nm, the exact analytical solution shows that the SKAWs have a much wider structure and much stronger perturbed fields than the KdV solitons with the same Nm. Moreover, the relations between the width and the amplitude of SKAWs are also considerably different from that of the KdV solitons. In addition, the possibility for applying these results to some events observed from the Freja scientific satellite is discussed. (The Freja is a Swedish–German scientific project for the investigation of ionospheric and magnetospheric plasmas, and the Freja satellite was launched on a Long-March II rocket of China on October 6, 1992.)
Return current instability in laser heated plasmas
Authors: Tikhonchuk V. T. , Rozmus W. , Bychenkov V. Yu. , Capjack C. E. , Epperlein E.
Published: July 2002

The localized heating of an underdense plasma by a focused laser beam has been studied with a kinetic Fokker–Planck code. Simulations show an inhibition of the electron heat flux in the region where temperature gradients are maximized. A time analysis of electron distribution function demonstrates that the heat flux inhibition does not interfere with the excitation of the return current ion acoustic instability. The conditions for onset of the instability and its possible effect on plasma transport are also discussed.
Reduced description of strong Langmuir turbulence from kinetic theory
Authors: DuBois D. F. , Russell David , Rose Harvey A.
Published: July 2002

A reduced description of strong Langmuir turbulence is derived from the Vlasov–Poisson equations by using an oscillation center transformation that decomposes the distribution function into envelope components which vary slowly on the plasma frequency time scale. The familiar high-frequency response is identified in the context of a basic ordering of small quantities. A generalization of the ion acoustic wave equation to the regime of equal electron and ion temperatures which is valid in this ordering is derived using a three-pole approximant for the collisionless ion susceptibility. The resulting extended Zakharov model is solved numerically in two dimensions for parameters relevant to radio-frequency heating of the ionosphere, producing energy and power spectra of fluctuations that are qualitatively similar to experimental observations. A new local quasilinear equation for the spatial and temporal evolution of the slowly varying part of the oscillation center distribution function is proposed.
Detached scrape-off layer tokamak plasmas
Authors: Kesner J.
Published: July 2002

The equilibrium and stability of scrape-off layer plasmas are considered using a one-dimensional treatment of coupled heat conduction and pressure balance equations. It is found that, for sufficiently low-temperature and high neutral density, a region of greatly reduced power flux to the endplate can be achieved. The plasma in the vicinity of the end wall is characterized by a sharp plasma pressure gradient and a relatively low temperature, 1<T0<10 eV.
Mapping closure for probability distribution function in low frequency magnetized plasma turbulence
Authors: Das Amita , Kaw Predhiman
Published: July 2002

Recent numerical studies on the Hasegawa–Mima equation and its variants describing low frequency magnetized plasma turbulence indicate that the potential fluctuations have a Gaussian character whereas the vorticity exhibits non-Gaussian features. A theoretical interpretation for this observation using the recently developed mapping closure technique [Chen, Chen, and Kraichnan, Phys. Rev. Lett. 63, 2657 (1989)] has been provided here. It has been shown that non-Gaussian statistics for the vorticity arises because of a competition between nonlinear straining and diffusive damping whereas the Gaussianity of the statistics of φ arises because the only significant nonlinearity is associated with divergence free convection, which produces no strain terms.
A δf Monte Carlo method to calculate plasma currents
Authors: Sasinowski Maciek , Boozer Allen H.
Published: July 2002

A Monte Carlo code has been developed which efficiently calculates plasma currents by finding the deviation, f̂, of the exact distribution function, f, from the Maxwellian, FM(ψ,H), with ψ the toroidal magnetic flux enclosed by a pressure surface and H the Hamiltonian. The particles in the simulation are followed with a traditional Monte Carlo scheme consisting of an orbit step in which new values for the positions and momenta are obtained, and a collision step in which a Monte Carlo equivalent of the Lorentz operator is applied to change the pitch of each particle. The δf code was used to study the aspect ratio and collisionality dependence of the bootstrap and two Fourier components of the Pfirsch–Schlüter current. The δf code is about 104 times as efficient as other Monte Carlo techniques used to calculate currents in plasmas.
Stabilization of the resistive wall mode using a fake rotating shell
Authors: Fitzpatrick Richard , Jensen Torkil H.
Published: July 2002

Tokamak plasma performance can, in theory, be greatly improved if the so-called ‘‘resistive wall mode’’ is stabilized. This can be achieved by spinning the plasma rapidly, but such a scheme would be difficult to implement in a reactor setting. A more promising approach is to apply external feedback in order to make a resistive shell placed around the plasma act like a perfect conductor. A scheme is outlined by which a network of feedback controlled conductors surrounding the plasma can be made to act like a rotating shell. This fake rotating shell combined with a stationary conventional shell (e.g., the vacuum vessel) can completely stabilize the resistive wall mode. The gain, bandwidth, current, and power requirements of the feedback amplifiers are extremely modest. A previously proposed stabilization scheme (the intelligent shell) is also investigated, and is compared with the fake rotating shell concept. The main disadvantage of the former scheme is that it requires a high gain.
Dielectric current in a plasma
Authors: Cole K. D.
Published: July 2002

The ‘‘dielectric’’ current driven by a spatially dependent, suddenly applied, electric field E orthogonal (⊥) to the magnetic induction B in a collisionless plasma is investigated from first principles. Both E and B are assumed in fixed directions. The current is qualitatively different in two conditions, whether ∇E<B2q/mc2 or ∇E≳B2q/mc2 [K. D. Cole, Planet. Space Sci. 24, 515 (1976)], where q and m are the charge and mass, respectively, of charged particles. Here the current is verified in the first condition by a new argument. Nonadiabatically (NA) established uniform electric field orthogonal to a magnetic field causes an additional time-averaged pressure in the plasma, given by ρmc2E2/2B2, where ρm is the mass density of the plasma, and a consequent accompanying additional magnetic moment per unit volume, given by −ρmc2E2/2B3. When the electric field has a gradient parallel to the field itself, there is produced a dielectric current which is balanced by a grad p current due to the pressure caused by the electric field. This analysis of the NA case leads to the identification of div D⊥, rather than div E⊥, with the net charge density, consistent with the Jean’s theorem of the equivalence of free particle orbit theory with the collisionless Boltzmann equation. In turn, this identification leads to the integral relation, along a line orthogonal to B in the direction of E, whether the latter is adiabatically or nonadiabatically established, p⊥+(B2/8π)−(ρmc2E2/2B2)=const, instead of the conventional relation in which the electric field term is missing.
Fast local mode properties in field-reversed configurations
Authors: Ishida A. , Shibata N. , Steinhauer L. C.
Published: July 2002

Local eigenmodes of field-reversed configurations (FRCs) were previously computed using ideal magnetohydrodynamics, including compressibility and double adiabaticity. Here the eigenmodes are compared with earlier analytic models. In equilibria, as initially generated in θ pinches, the rigid displacements of the analytic models are similar to actual eigenmodes in structure and growth rate; moreover, the growth rates are similar to those of global modes. In equilibria that naturally arise later in the quiescent FRC, the analytic models fail to predict features of the eigenmode behavior: ballooning-like structure, and much faster growth rate than global modes. This suggests explanations for the difficulty of forming large FRCs in θ pinches, and for the appearance of characteristic profiles in quiescent FRCs.
Modal model for the nonlinear multimode Rayleigh–Taylor instability
Authors: Ofer D. , Alon U. , Shvarts D. , McCrory R. L. , Verdon C. P.
Published: July 2002

A modal model for the Rayleigh–Taylor (RT) instability, applicable at all stages of the flow, is introduced. The model includes a description of nonlinear low-order mode coupling, mode growth saturation, and post-saturation mode coupling. It is shown to significantly extend the range of applicability of a previous model proposed by Haan, to cases where nonlinear mode generation is important. Using the new modal model, we study the relative importance of mode coupling at late nonlinear stages and resolve the difference between cases in which mode generation assumes a dominant role, leading to the late time inverse cascade of modes and loss of memory of initial conditions, and cases where mode generation is not important and memory of initial conditions is retained. Effects of finite density ratios (Atwood number A<1) are also included in the model and the difference between various measures of the mixing zone penetration depth for A<1 is discussed.
Effects of laser beam smoothing on stimulated Raman scattering in exploding foil plasmas
Authors: Montgomery D. S. , Moody J. D. , Baldis H. A. , Afeyan B. B. , Berger R. L. , Estabrook K. G. , Lasinski B. F. , Williams E. A. , Labaune C.
Published: July 2002

Time-resolved spectra of backward stimulated Raman scattering (SRS) were measured from the interaction of a 527 nm laser with a preformed plasma. The effect of laser smoothing by spectral dispersion (SSD) was studied using laser bandwidth (Δλ/λ=0.1%) and varying the laser intensity (2–20×1014 W/cm2). A broad SRS spectrum extending to short wavelengths was observed for the high-intensity, Δλ/λ=0 case. Narrow spectra corresponding to the peak plasma density were observed for cases with either high intensity and Δλ/λ∼0.1%, or with low intensity and Δλ/λ=0. Simulations of the filamentation process were performed for the conditions of these experiments. The simulations show that laser smoothing stabilizes filamentation for high-intensity interactions, and that filaments are stable without smoothing for low intensity. The predicted onset of filamentation without smoothing correlates with the growth of short wavelength SRS. These experimental results are presented and models are discussed that may help explain the data.
Center manifold theory for the dynamics of the L–H-transition
Authors: Beyer P. , Spatschek K. H.
Published: July 2002

The transition of a tokamak plasma from low (L) to high (H) confinement is investigated. The spatio-temporal behavior is analyzed using a model which is based on the reduced magnetohydrodynamic (MHD) description. The latter leads to a set of partial differential equations (PDE’s) which are solved numerically. Analytical insight into the dynamical behavior is obtained by applying the center manifold theory (in the case of one dominating marginal mode) and an inertial manifold approximation (for several unstable modes). These procedures suggest to include an effective heating source into the equations in order to qualitatively understand the appearance of dithering cycles and edge localized modes (ELM’s). The numerical and analytical predictions are compared with recent experimental observations.
Alpha particle losses from Tokamak Fusion Test Reactor deuterium–tritium plasmas
Authors: Darrow D. S. , Zweben S. J. , Batha S. , Budny R. V. , Bush C. E. , Chang Z. , Cheng C. Z. , Duong H. H. , Fang J. , Fisch N. J. , Fischer R. , Fredrickson E. D. , Fu G. Y. , Heeter R. F. , Heidbrink W. W. , Herrmann H. W. , Herrmann M. C. , Hill K. , Jaeger E. F. , James R. , Majeski R. , Medley S. S. , Murakami M. , Petrov M. , Phillips C. K. , Redi M. H. , Ruskov E. , Spong D. A. , Strait E. J. , Taylor G. , White R. B. , Wilson J. R. , Wong K.-L. , Zarnstorff M. C.
Published: July 2002

Because alpha particle losses can have a significant influence on tokamak reactor viability, the loss of deuterium–tritium alpha particles from the Tokamak Fusion Test Reactor (TFTR) [K. M. McGuire et al., Phys. Plasmas 2, 2176 (1995)] has been measured under a wide range of conditions. In TFTR, first orbit loss and stochastic toroidal field ripple diffusion are always present. Other losses can arise due to magnetohydrodynamic instabilities or due to waves in the ion cyclotron range of frequencies. No alpha particle losses have yet been seen due to collective instabilities driven by alphas. Ion Bernstein waves can drive large losses of fast ions from TFTR, and details of those losses support one element of the alpha energy channeling scenario.
Relativistic theory of nonlinear oscillations in magnetoactive and transient plasmas or jets
Authors: Mohanty J. N. , Naik Antaryami
Published: July 2002

Explicit results pertaining to nonlinear oscillation and triggering of chaos in a relativistic magnetoactive beam–plasma system, which shows transient or streaming phenomena, are analytically presented. The phase-space mappings and their striking features, both in strong streaming and nonstreaming situations, are shown. A new feature concerning streaming plasma analysis is revealed in the form of frequency modulation, especially at the onset of chaos, including varying kz pertaining to an ensemble of electrostatic waves. It is further shown that the modulation frequency is governed by marked cutoff values, corresponding to varying magnetic field lines, and that it approaches a steady value with the rise in the streaming parameter. Our findings may have important implications for astrophysical settings where there beam–plasma interactions exist.
Fluctuations associated with a filamentary density depletion
Authors: Maggs J. E. , Morales G. J.
Published: July 2002

Density and magnetic fluctuations arising spontaneously in a narrow field-aligned density striation in a magnetized discharge He plasma are found to exhibit a radial eigenmode structure. The nature of the fluctuations depends upon the electron plasma beta, βe. For βe greater than the electron to ion mass ratio (βe>m/M) the frequency spectrum exhibits sharply peaked eigenfrequencies with the density and magnetic fluctuations strongly coupled so that the growing mode is identified as the drift-Alfvén wave. For βe less than the mass ratio (βe<m/M) the density and magnetic fluctuations separate in frequency and broadband magnetic shear Alfvén wave turbulence develops. The driving source for the fluctuations is the cross-field density and temperature gradients in the edge of the striation which have scale lengths on the order of the electron skin depth. The fluctuations associated with the striation are compared to the edge fluctuations of the plasma column which are found to exhibit a universal exponential frequency spectrum.
Investigation of lower hybrid wave damping in the PBX-M tokamak
Authors: Bernabei S. , Cardinali A. , Giruzzi G. , Hoang G. T. , Ignat D. , Kaita, R. , Okabayashi M. , Paoletti F. , von Goeler S.
Published: July 2002

Lower hybrid (LH) current drive experiments on the Princeton Beta Experiment-Modified (PBX-M) [Phys. Fluids B 2, 1271 (1990)] have shown that the current profile can be changed by varying the phase velocity of the waves. The radial profile of the current carrying electrons was deduced from two-dimensional hard x-ray tomography. For a certain range of phase velocities, there is a correlation between the peak of the fast electron profile and the launched wave spectrum, despite the presence of a wide spectral gap between the phase velocity and the thermal electron energy distribution. A new model is proposed to explain how first-pass wave damping is possible in such plasmas. The rf power can form a tail of energetic electrons, and subsequently waves with moderate phase velocity can damp on them. For waves with very fast phase velocity, there must be an upshift of the n∥ spectrum for any damping to occur. These hypotheses are supported by ray tracing results which are coupled to relativistic Fokker–Planck calculations of the electron distribution function.
Beam dynamics studies with the heavy-ion linear induction accelerator MBE-4
Authors: Fawley W. M. , Garvey T. , Eylon S. , Henestroza E. , Faltens A. , Fessenden T. J. , Hahn K. , Smith L. , Grote D. P.
Published: July 2002

Current amplification of heavy-ion beams is an integral feature of the induction linac approach to heavy-ion fusion (HIF). In this paper we report on amplification experiments conducted on a single beam of the Multiple Beam Experiment (MBE-4), a heavy-ion (Cs+) induction linac. Earlier MBE-4 experiments [H. Meuth et al., Nucl. Instrum. Methods Phys. Res. A 278, 153 (1989)] had demonstrated up-to-9× current amplification but had been accompanied by an up-to-2× increase of normalized transverse emittance. Experiments to pinpoint the causes of this emittance growth indicated various factors were responsible, including focusing aberrations and mismatch difficulties between the injector diode and the accelerator transport lattice, a localized quadrupole misalignment problem, and the interaction of transversely large beams with the nonlinear elements of the focusing lattice. Following ameliorative measures, new current amplification experiments, both with and without acceleration, showed that current amplifications of up to 3× and line charge density increases of up to ≈2× could be achieved without increasing the beam’s normalized transverse emittance. Finally, both the transverse beam dynamics, and beam current and energy measurements were accurately modeled by numerical particle-in-cell simulations and longitudinal dynamics codes, respectively.
Magnetic field dissipation and fractal model of current sheets
Authors: Yankov V. V.
Published: July 2002

A model of magnetic field dissipation is suggested, which possesses features of both the Kolmogorov model of turbulence of ideal fluid and the Sweet–Parker model of magnetic reconnection. It is suggested that current sheets are smooth along the magnetic lines but have fractal structure in the transverse dimension. The fractal dimension d=2.5 of the current sheet was found instead of d=3 in Kolmogorov-like models and d=2 in the Sweet–Parker model. The reconnection time is the geometrical mean of the Alfvén time and the Sweet–Parker time. The magnetic energy dissipation time can be even shorter, up to the Alfvén time.
Magnetohydrodynamic simulation on co- and counter-helicity merging of spheromaks and driven magnetic reconnection
Authors: Watanabe T.-H. , Sato T. , Hayashi T.
Published: July 2002

A magnetohydrodynamic relaxation process of spheromak merging is studied by means of an axisymmetric numerical simulation. As a result of counter-helicity merging, a field-reversed configuration is obtained in the final state, while a larger spheromak is formed after co-helicity merging. In the counter-helicity case, a clear pressure profile of which iso-surfaces coincide with flux surfaces is generated by thermal transport of a poloidal flow induced by driven reconnection. It is also found that a sharp pressure gradient formed in the vicinity of a reconnection point causes a bouncing motion of spheromaks. According to the bounce motion, the reconnection rate changes repeatedly. As shown by the Tokyo University Spherical Torus No. 3 (TS-3) experiments [M. Yamada, et al., Phys. Rev. Lett. 65, 721 (1990)], furthermore, strong acceleration of a toroidal flow and reversal of a toroidal field in the counter-helicity merging were observed.
Technical noise in gyroklystrons and phase-locked gyrotron oscillators
Authors: Nusinovich G. S. , Dumbrajs O.
Published: July 2002

The formalism describing the effect of fluctuations in the technical parameters and in the driving signal on the radiation linewidth in gyroklystrons and phase locked gyrotron oscillators is developed. The phase locking by direct injection of the input signal into the resonator and by prebunching of electrons in the input cavity followed by the drift section is considered. In the first case the results of analysis of the free running gyrotron oscillator given in a companion paper [Phys. Plasmas 4, 1413 (1997)] can be used in a straightforward manner. For gyroklystrons and phase-locked gyrotron oscillators with prebunched electron beams, the numerical study is carried out. In particular, the effect of the detuning of the signal frequency from the eigenfrequency of the resonator on the linewidth of radiation in a gyrodevice with a given level of technical noise is analyzed.
Fivefold confinement time increase in the Madison Symmetric Torus using inductive poloidal current drive
Authors: Stoneking M. R. , Lanier N. E. , Prager S. C. , Sarff J. S. , Sinitsyn D.
Published: July 2002

Current profile control is employed in the Madison Symmetric Torus [R. N. Dexter et al., Fusion Technol. 19, 131 (1991)] reversed field pinch to reduce the magnetic fluctuations responsible for anomalous transport. An inductive poloidal electric-field pulse is applied in the sense to flatten the parallel current profile, reducing the dynamo fluctuation amplitude required to sustain the equilibrium. This technique demonstrates a substantial reduction in fluctuation amplitude (as much as 50%), and improvement in energy confinement (from 1 to 5 ms); a record low fluctuation (0.8%) and record high temperature (615 eV) for this device were observed simultaneously during current drive experiments. Plasma beta increases by 50% and the Ohmic input power is three times lower. Particle confinement improves and plasma impurity contamination is reduced. The results of the transient current drive experiments provide motivation for continuing development of steady-state current profile control strategies for the reversed field pinch.
Anomalous electron precipitation near the Earth’s auroral zone induced by the wave–particle interaction
Authors: Potapenko I. F. , Elfimov A. G. , de Assis A. S. , de Azevedo C. A.
Published: July 2002

The results of a numerical simulation of anomalous electron precipitation in the Earth’s auroral zone induced by waves are discussed. In order to study the plasma electron–wave interaction, a two-dimensional quasilinear code has been used. A quasilinear operator models the electron Landau damping of any plasma eigenmodes: kinetic Alfvén waves, whistlers, lower hybrid waves, etc. The distribution function structure was analyzed. The electron current induced by waves, the density, and the plasma energy decay, which stimulates the electron precipitation, have also been investigated.
Submarginal profiles and turbulent transport: An exactly solvable model
Authors: Krommes John A.
Published: July 2002

The possibility that linearly stable (“submarginal”) profiles can support turbulent transport in the absence of external noise is considered in the context of a one-dimensional, realizable stochastic model of “almost passive” advection that includes a stability threshold dependent on a critical gradient. The important limits of zero and infinite Kubo number (dimensionless autocorrelation time) are solved analytically. When the stability threshold is linear, it is proven rigorously that, within the context of the model, submarginal profiles do not carry any turbulent flux. A generalization to include a nonlinear stability threshold reminiscent of a subcritical bifurcation supports submarginal profiles.
An alternative method for linearized plasma kinetics and its application to drift resonances
Authors: Nakamura Tadas K.
Published: July 2002

A method different from the conventional method of characteristics (integration over unperturbed orbits) is proposed to solve the linearized Vlasov equation. This method can solve problems for which the conventional method is not applicable. Also, it can give a much simpler calculation for problems that can be solved by the conventional method. The effectiveness of this method is demonstrated with its application to drift resonances.
The shear Alfvén continuum in an asymmetric magnetohydrodynamic equilibrium
Authors: Salat A. , Tataronis J. A.
Published: July 2002

The Alfvén continuum of an asymmetric plasma configuration that is an exact solution of the equilibrium equations of ideal magnetohydrodynamics is treated. The equilibrium is parallel to an infinite straight magnetic axis about which the magnetic lines of force form closed magnetic surfaces. Properties of the governing continuum equation for small plasma beta are explored, and the associated spectrum is determined. The continuum has a localized component characterized by modes that decay to zero along magnetic field lines, and a non-localized component characterized by modes that approach non-zero constants at the plasma ends. The localized component is restricted to real frequencies, while the non-localized component covers the entire complex frequency plane. The non-localized modes can be joined generally with those on neighboring field lines to form modes that smoothly cover the entire magnetic surface. The localized modes do not occur in symmetric plasmas.
The effect of a partial resistive shell on the magnetohydrodynamical stability of tokamak plasmas
Authors: Fitzpatrick Richard
Published: July 2002

A comprehensive theory is developed to determine the effect of a partial resistive shell on the growth rate of the external kink mode in a low-β, large aspect-ratio, circular flux-surface tokamak. In most cases, it is possible to replace a partial shell by a complete “effective shell” of somewhat larger radius. In fact, the radius of the effective shell can be used to parametrize the ability of a partial shell to moderate the growth of the external kink mode. It is necessary to draw a distinction between “resonant shells,” for which the eddy currents excited in the shell are able to flow in unidirectional continuous loops around the plasma, and “nonresonant shells,” for which this is not possible. As a general rule, resonant shells perform better than similar nonresonant shells. The theory is used to derive some general rules regarding the design of incomplete passive stabilizing shells. The theory is also employed to determine the effectiveness of two realistic feedback stabilization schemes for the resistive shell mode, both of which only require a relatively small number of independent feedback controlled conductors external to the plasma.
Cusp and Y-type magnetic structures and velocity fields at the endpoint of the reconnection layer
Authors: Uzdensky Dmitri A. , Kulsrud Russell M.
Published: July 2002

The two-dimensional global scale magnetic field structure for a system of two merging cylindrical plasmas in a steady state is studied. In the limit of very large magnetic Reynolds numbers the reconnection process is slow, and the plasma almost everywhere finds itself in magnetostatic equilibrium. It is shown that under certain conditions the classical Syrovatskii-type Y-point configuration, with surface current concentrated only in the reconnection layer, is not possible. Instead, a cusp configuration is formed, with finite surface current in the separatrix. The equilibrium condition, together with constraints on the volume per flux, enables one to determine the shape of the separatrix and the magnetic field in the vicinity of the cusp point. The solution is characterized by a singular power law dependence of current density on the flux coordinate Ψ near the separatrix: j(Ψ)∼|Ψ|−1/2. This solution gives the boundary conditions that are needed to find the flow in the reconnection and the separatrix regions.
Ion transport in a partially ionized impure edge plasma
Authors: Helander P. , Wising F. , Catto Peter J. , Hazeltine R. D.
Published: July 2002

The kinetic theory of transport in an impure, partially ionized, edge plasma is developed. It is found that the thermal force between bulk ions and impurities is enhanced by the presence of neutral atoms, but the dynamical friction is not affected by the neutrals. When the neutral viscosity is large, an additional force on the impurities also arises. This force is parallel to the magnetic field, and is proportional to the shear of the parallel plasma velocity and the perpendicular ion density and temperature gradients.
Space charge effects in plasma-filled traveling-wave tubes
Authors: Nusinovich G. S. , Mitin L. A. , Vlasov A. N.
Published: July 2002

A theory describing the ac space charge effects in plasma-filled traveling-wave tubes (TWT) is presented. It is shown that in the case of operation at frequencies between the plasma frequency and the upper-hybrid frequency, the space charge forces can significantly enhance the efficiency. The analysis of a concrete, plasma-filled, coupled-cavity TWT shows that over a 20% bandwidth the calculated efficiency can exceed 40%.
Scaling studies of spheromak formation and equilibrium
Authors: Geddes C. G. R. , Kornack T. W. , Brown M. R.
Published: July 2002

Formation and equilibrium studies have been performed on the Swarthmore Spheromak Experiment (SSX). Spheromaks are formed with a magnetized coaxial plasma gun and equilibrium is established in both small (dsmall=0.16 m) and large (dlarge=3dsmall=0.50 m) copper flux conservers. Using magnetic probe arrays it has been verified that spheromak formation is governed solely by gun physics (in particular the ratio of gun current to flux, μ0Igun/Φgun) and is independent of the flux conserver dimensions. It has also been verified that equilibrium is well described by the force free condition ∇×B=λB (λ=constant), particularly early in decay. Departures from the force-free state are due to current profile effects described by a quadratic function λ=λ(ψ). Force-free SSX spheromaks will be merged to study magnetic reconnection in simple magnetofluid structures.
Propagation of a small-amplitude shock wave associated with the ion acoustic-like mode in a dusty plasma
Authors: Das N. , Goswami K. S.
Published: July 2002

The reductive perturbation method is used to investigate the existence of a shock wave in the presence of dust charge fluctuation in a dusty plasma. A Burgers-type equation is obtained for the first-order perturbed potential. The potential and velocity of the shock are obtained. The conditions for the existence of the shock solutions are discussed.
Dynamic plasma screening effects on electron–ion collisional excitations in a generalized Lorenzian (kappa) distribution plasma
Authors: Yoon Jung-Sik , Jung Young-Dae
Published: July 2002

Dynamic plasma screening effects are investigated on 1s→2p dipole transition probabilities for electron-impact excitation of hydrogenic ions in a generalized Lorenzian (kappa) velocity distribution plasmas. The electron–ion interaction potential is obtained by introduction of the plasma dielectric function. A semiclassical straight-line trajectory method is applied to the path of the projectile electron in order to visualize the dynamic plasma screening effects on the semiclassical transition probabilities as a function of the impact parameter and the collision energy. The transition probability including the dynamic plasma screening effect is found to be always greater than that including the static plasma screening effects. It is also found that the dynamic plasma screening effects on the electron–ion collisional excitations in the Lorenzian velocity distribution plasmas are more effective than those in the Maxwellian velocity distribution plasmas.
Picosecond optical probing of ultrafast energy transport in short pulse laser solid target interaction experiments
Authors: Gumbrell E. T. , Smith R. A. , Ditmire T. , Djaoui A. , Rose S. J. , Hutchinson M. H. R.
Published: July 2002

This paper reports observations of rapid energy transport effects resulting from high intensity laser heating of fused silica targets. Picosecond optical probing of these interactions provides information on the kinematics of supersonic ionization fronts driven into the targets. Studies have been conducted as a function of laser intensity, wavelength, and target angle. Additionally, targets with metallic surface layers have been investigated. Characterization of the laser absorption has enabled plasma and radiation hydrodynamics energy transport simulations to be implemented. Although consideration has been given to several energy transport mechanisms, including thermal and suprathermal electron transport, the kinematics are best explained with a radiation transport model. This is confirmed by angled and high and medium Z coated target experiments.