Volume 13, Issue 3, March 2006
 LETTERS


Investigation of laser ion acceleration inside irradiated solid targets by neutron spectroscopy
View Description Hide DescriptionOrigins and acceleration directions of accelerated ions inside solid LiF, CHLiF, and LiFCH targets irradiated by a , , laser at an intensity of have been investigated by neutron spectroscopy. The irradiated targets generate neutrons through the reaction between acceleratedprotons and background ions inside the target. The produced neutron spectra observed from two different observation angles 20° and 120° to the target rearside normal. From the measured and calculated spectra, by threedimensional Monte Carlo code, the maximum energy, the total number, and the slope temperature of the accelerated ions are investigated. The results indicate that ions are not only accelerated from the front surface toward the rear surface, but also from the rear surface toward the front surface with comparable maximum energy and higher number.

Study of proton acceleration at the target front surface in lasersolid interactions by neutron spectroscopy
View Description Hide DescriptionProtonacceleration inside solid LiF and CH–LiF targets irradiated by a , , laser at an intensity of has been studied via neutron spectroscopy.Neutron spectra produced through the reaction that occurs between acceleratedprotons, at the front surface, and background ions inside the target. From measured and calculated spectra, by three–dimensional Monte Carlo code, the maximum energy, total number, and slope temperature of the acceleratedprotons are investigated. The study indicates that protons originate at the front surface and are accelerated to a maximum energy that is reasonably consistent with the calculated one due to the ponderomotive force.

Bending modes in the hexagonal dustplasma crystal
View Description Hide DescriptionWe study modes of particle oscillations associated with outofplane motions propagating in a twodimensional hexagonal lattice of charged solid particles in a plasma. The analytical dispersion relation explicitly containing dependencies on the main plasma parameters and the direction of the mode propagation is obtained. The characteristics of the modes, stability of equilibria, and a critical dependence on the dust and plasma parameters for the realistic experimental conditions are investigated.
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Drift wave instability in the Helimak experiment
View Description Hide DescriptionElectrostatic drift wave linear stability analysis is carried out for the Helimak configuration and compared against experimental data. Density fluctuation and crossspectrum measurements show evidence of a coherent mode propagating perpendicular to the magnetic field which becomes unstable at . By comparing the experimental results with the wave characteristic of linear twofluid theory, this mode is identified as an unstable resistive drift wave driven by the density gradient and magnetic grad/curvature present in an otherwise magnetohydrodynamic stable steadystate equilibrium.

Plasmasheath effects on the Debye screening problem
View Description Hide DescriptionThe classical DebyeHückel screening effect of the electrostatic field generated by isolated charged particles immersed in a plasma is reviewed. The validity of the underlying mathematical model, and particularly of the weakfield approximation, is analyzed. It is shown that the presence of the plasma sheath around test particles and the resulting effect of charge screening are essential for the description of plasmas that are strongly coupled.

Influence of a finite initial ion density gradient on plasma expansion into a vacuum
View Description Hide DescriptionThe influence of a finite initial ion density gradient on a plasma expansion into a vacuum is studied with a numerical model that takes into account the chargeseparation effects and assumes a Boltzmann equilibrium for the electrons. The cases of a semiinfinite plasma and of a finite plasma slab are treated. In both cases it is shown that the finite initial ion density gradient of the plasma surface leads to two phases in the plasma expansion, separated by a wave breaking of the ion flow. An ion front forms after the wave breaking and, in the semiinfinite plasma case, the plasma expansion becomes closer and closer to the initially sharp boundary case, the maximum ion velocity increasing logarithmically with time. In the finite plasma slab case, the energy conservation has to be taken into account, the thermal electron energy being progressively converted into the kinetic energy of the ions. When the initial ion density scale length is larger than a few percent of the total plasma slab width, the final maximum ion velocity decreases with .

The ionization instability and resonant acoustic modes suppression by charge space effects in a dusty plasma
View Description Hide DescriptionThe large wavenumber suppression of unstable modes by space charge effects of the ionizationinstability in a weakly ionized and unmagnetized dusty plasma is investigated. The charge losses in the initial equilibrium state are balanced by electron impact ionizations originated by both the thermal electron populations and an additional monoenergetic electron beam. The multifluid dimensionless equations are deduced by using the time and length scales for elastic collisions between ions and neutral atoms and the Poisson equation relates the plasma potential fluctuations with charged particle densities instead of the quasineutral approximation. A general dimensionless dispersion relation is obtained from the linearized transport equations, where the ratios between the characteristic velocities, as the dust ion acoustic (IA), dust acoustic (DA), ion sound, and thermal speeds permits us to evaluate the weight of the different terms. In the long wavelength limit the results obtained using the quasineutral approximation are recovered. The differences found between roots of both dispersion equations are discussed, as well as those of previous models. The unstable mode of the linear ionizationinstability is originated by the imbalance between ion and electron densities in the rest state caused by the negative charging of dust grains. Contrary to dust free plasmas, the unstable mode exists, even in the absence of the ionizing electron beam. The numerical calculations of the roots of the full dispersion equation present a maximum unstable wavenumber not predicted by the quasineutral approximation, which is related with the minimum allowed length for space charge fluctuations within a fluid model. This upper limit of unstable wave numbers hinders the predicted resonant coupling in the long wavenumber regime between the DA and DIA waves.

Phenomenological theory of the kink instability in a slender plasma column
View Description Hide DescriptionIn this paper we are concerned with the kink instability of a currentcarrying plasma column whose radius is much smaller than its length . In the limit , one can consider the column as a thin filament whose kinking can be adequately described simply by a two dimensional 2D displacement vector, ; . Details of the internal structure of the column such as the radial distribution of the current, density, and axial flow can be lumped into some phenomenological parameters. This approach is particularly efficient in the problems with nonideal (sheath)boundary conditions (BC) at the end electrodes, with the finite plasma resistivity, and with a substantial axial flow. With the sheath BC imposed at one of the endplates, we find instability in the domain well below the classical KruskalShafranov limit. The presence of an axial flow causes the onset of rotation of the kink and strong axial “skewness” of the eigenfunction, with the perturbation amplitude increasing in the flow direction. The limitations of the phenomenological approach are analyzed and are related to the steepness with which the plasma resistivity increases at the plasma boundary with vacuum.

Laser method of heating monolayer dusty plasmas
View Description Hide DescriptionA method has been developed to heat and control temperature in a twodimensional monolayerdusty plasma. A monolayer of highly charged polymer microspheres was suspended in a plasma sheath. The microspheres interacted with a Yukawa potential and formed a triangular lattice. Laser manipulation was used to apply random kicks to the particles. Two focused laser beams were moved rapidly around drawing Lissajous figures in the monolayer. The kinetic temperature of the particles increased with the laser power applied, and above a threshold a melting transition was observed. Characteristics of a thermal equilibrium of the laserheated dusty plasma in solid and liquid states are discussed.

DriftAlfvén eigenmodes in inhomogeneous plasma
View Description Hide DescriptionA set of three nonlinear equations describing driftAlfvén waves in a nonuniform magnetized plasma is derived and discussed both in linear and nonlinear limits. In the case of a cylindric radially bounded plasma with a Gaussian density distribution in the radial direction the linearized equations are solved exactly yielding general solutions for modes with quantized frequencies and with radially dependent amplitudes. The full set of nonlinear equations is also solved yielding particular solutions in the form of rotating radially limited structures. The results should be applicable to the description of electromagnetic perturbations in solar magnetic structures and in astrophysical columnlike objects including cosmic tornados.

On collisionless energy absorption in plasmas: Theory and experiment in spherical geometry
View Description Hide DescriptionAn investigation of the rf impedance characteristics of a small spherical probe immersed in a laboratory plasma is ongoing in the large Space Physics Simulation Chamber [D. N. Walker et al., Rev. Sci. Instrum.65, 661 (1994)] at the Naval Research Laboratory. The data taken are from network analyzer measurements of the reflection coefficient obtained when applying a low level rf signal to the probe which is either near floating potential or negatively dc biased in a low pressure plasma. As is well known, sheaths form around objects placed inside plasmas. The electron density is smaller inside the sheath, and the reduction in density alters the plasma impedance. Surprisingly, the impedance becomes “resistive,” even though the plasma is effectively collisionless, at frequencies below the bulk plasma frequency, thus leading to collisionless energy absorption. This behavior comes directly from Maxwell's equations along with the cold fluid equations. The solutions obtained indicate that this form of plasma resistance is inversely proportional to the plasma density gradient evaluated at the location where the plasma frequency is equal to the applied frequency. The sphere for this work is typically near plasma potential or biased more negatively into the ion collection regime. Applying a supplemental, negative dc bias to the sphere thickens the sheath and tends to raise its resistance as the density gradient is softened. Much of the earlier work in the area of collisionless resistance concentrated primarily on planar probes as opposed to the present work which is concerned with spheres. Interpreting the results is simpler for a sphere and the results obtained agree well with theory as described. For comparison to the theory we use only the parameter outputs (or reflection coefficients) of the network analyzer in the experimental series. Significant energy absorption is observed at frequencies generally near onehalf the plasma frequency. One result of this work is that the most efficient transfer of power to the plasma occurs not unexpectedly when there is impedance matching between input impedance and output (collisionless) impedance. This paper is an exposition of these ideas along with data results and a comparison to theory for the spherical probe which has not been published in this form.
 Nonlinear Phenomena, Turbulence, Transport

Further investigation of energy principle for model current sheets
View Description Hide DescriptionIn the present paper we revisit the energy principle associated with magnetic reconnection. In the earlier approach the boundary of the system was freely adjusted to always contain one full period of the magnetic islands. In view of the fact that in simulations of reconnection such an adjustment of the boundary size is not feasible, in the present paper we reconsider a fixed boundary problem. It is shown that the energy conversion is almost negligible in such a situation. It is thus concluded that the choice of appropriate boundary conditions is critically important in the study of the energy conversion by magnetic reconnection. The present finding also points to the likelihood that the study of reconnection in two dimensions in a fixed boundary system may not reveal any appreciable magnetic fieldtoparticle energy conversion in an overall sense, although the present idealized theoretical approach cannot address the possibility of local heating and acceleration. This does not necessarily mean that the energy conversion by reconnection is not an efficient source of particle energization, for in nature the system is not limited by any artificial boundaries. The present finding instead only points to the possible limitation of studying the reconnection by a twodimensional fixedboundary system.

Multiscale interaction of a tearing mode with drift wave turbulence: A minimal selfconsistent model
View Description Hide DescriptionA minimal selfconsistent model of the multiscale interaction of a tearing mode with drift waveturbulence is presented. A tearing instability in a cylindrical plasma interacting with electrostaticdrift waves is considered, for reasons of simplicity. Wave kinetics and adiabatic theory are used to treat the feedback of tearing mode flows on the drift waves via shearing and radial advection. The stresses exerted by the selfconsistently evolved drift wave population density on the tearing mode are calculated by mean field methods. The principal effect of the drift waves is to pump the resonant low mode via a negative viscosity, consistent with the classical notion of an inverse cascade in quasitwodimensional turbulence. This process can occur alone or in synergy with current gradient drive of the low mode. Speculations of the relation of this multiscale process to the more general issue of the fate of energy transferred to large scales by an inverse cascade are presented. The existence of nonlinearly driven vortices pinned to low surfaces as a class of highly anisotropic dissipative structures which terminate the inverse cascade is proposed. The evolution of a finite size magnetic island is discussed.

Effect of axial magnetic field variations on asymmetryinduced transport in a nonneutral plasma trap
View Description Hide DescriptionIt has been suggested that magnetically trapped particles play a role in the asymmetryinduced radial transport observed in the Occidental nonneutral plasma trap. This magnetic trapping would occur due to a small increase in magnetic field at the center of our solenoid and would keep low velocity particles confined to the ends of the trap. To test this suggestion, three coils of additional windings have been added to the trap solenoid thus allowing adjustment of the axial field variation . The effect of these adjustments on typical radial flux resonances is investigated. Making as uniform as possible reduces by a factor of 5.9, but this produces little change in the transport. Varying over the broader range from to 9.5% gives variations of 20%–90% in the magnitude, peak frequency, and width of the flux resonances, but these variations do not match the predictions of a simple model of trapped particle transport based on isotropic particle distributions.

Largeeddy simulation of magnetohydrodynamic turbulence in compressible fluid
View Description Hide DescriptionIn the present article, the large eddy simulation(LES) technique for the study of compressible magnetohydrodynamicturbulence is developed. The filtered equations of magnetohydrodynamics of compressible fluid are obtained with the use of a massweighted filtering procedure (Favre filtering). Favrefiltered equations for largescale components of turbulence include subgridscale terms describing subgrid phenomena. Different models for closure of subgrid terms are suggested. In this work numerical simulation of filtered magnetohydrodynamic equations and an analysis of the received characteristics of turbulent flow is carried out. The obtained results of numerical computations for different LESmodels are compared with the results of direct numerical simulation.

Theory of dust selforganized convection in cylindrical discharges. I. The model and stationary nonlinear dust structures
View Description Hide DescriptionThe convection of dust particles in a plasma is related to the spatial gradients of dust charge distributions existing due to different plasma conditions in different parts of dusty structures. For many experiments, the convection appears as a result of convective perturbations of basic nonlinear selforganized states of the dust structures. Here, the set of nonlinear equations is derived suitable for the study of basic stationary dust structures as well as their convective perturbations. On its basis, the stationary nonlinear states of selforganized cylindrical dusty structures in a plasma are investigated. It is demonstrated that there is the broad range of plasma and dust parameters where the selforganized states exist. The characteristics of these basic structures are determined by a limited number of the controlling parameters such as the ion number density in the center of the structure and the plasma ionization level. The derived set of equations also forms the basis for further investigation of the dust convection.

Theory of dust selforganized convection in cylindrical discharges. II. Dust convective structures
View Description Hide DescriptionThe convection of dust particles in a plasma is related to the spatial gradients of dust charge distributions existing due to different plasma conditions in different parts of dusty structures. Here, the set of nonlinear equations is derived to describe the dust convective structures appearing as perturbations of the basic nonlinear structures obtained in Part I. On its basis, various cases of the dust convection in cylindrical dusty structures in a plasma are obtained. It is demonstrated that there is the broad range of plasma and dust parameters where the selforganized dust convection exist.

Coalescence of magnetic islands, sloshing, and the pressure problem
View Description Hide DescriptionThe coalescence of magnetic islands in the high Lundquist number regime is studied. Within the resistivemagnetohydrodynamics model, the coalescence rate is known to stall (sloshing) in the limit of high Lundquist number. Previously, this stalling has been associated with the presence of a secondary tearing mode. Here it is shown that this stalling results from high magnetic pressure gradients, which result from thin current sheets at high Lundquist number. This phenomenon is frequently referred to as the “pressure problem” in flux pileup reconnection studies. Sensitivity of the dynamic solution is presented, over a wide range of resistivities.

Alfvén multistability: Transient and intermittent dynamics induced by noise
View Description Hide DescriptionThe effects of noise in the dynamics of Alfvén waves described by the derivative nonlinear Schrödinger equation are investigated. In a complex region of the parameter space, where multistability is observed, an external stochastic source can effectively destroy attractors present in the noisefree system, as well as induce chaotic transients and extrinsic intermittency. In the intermittent regime, the Alfvén wave exhibits random qualitative changes in its behavior as a result of a competition between three attractors and a chaotic saddle embedded in the fractal basin boundary.

Fluid simulation studies of the dynamical behavior of onedimensional relativistic electromagnetic solitons
View Description Hide DescriptionA numerical fluid simulation investigation of the temporal evolution of a special class of traveling wavesolutions of the onedimensional relativistic cold plasmamodel is reported. The solutions consist of coupled electromagnetic and plasma waves in a solitary pulse shape [Sov. Phys. JETP49, 75 (1979); Phys. Rev. Lett.68, 3172 (1992); Phys. Plasmas9, 1820 (2002)]. Issues pertaining to their stability, mutual collisional interactions, and propagation in an inhomogeneous plasma medium are addressed. It is found that solitary pulses that consist of a single light peak trapped in a modulated density structure are longlived whereas structures with multiple peaks of trapped light develop an instability at the trailing edge. The interaction properties of two single peak structures show interesting dependencies on their relative amplitudes and propagation speeds and can be understood in terms of their propagation characteristics in an inhomogeneous plasma medium.