Volume 13, Issue 8, August 2006
 LETTERS


Selforganization in electron temperature gradient driven turbulence
View Description Hide DescriptionBased on first principle gyrokinetic calculations, a zonal flow generation mechanism in the slab electron temperature gradient driven (ETG) turbulence with weak magnetic shear is identified as selforganization via the turbulent spectral cascade in the two dimensional rotating fluid turbulence. The inverse energy cascade and the scaling of a zonal flow wavenumber, which is consistent with the Rhines scale length, are confirmed. An impact of the scaling, which depends on the density gradient, on the turbulent structure and transport is demonstrated for the slab ETG turbulence.

Observation of a quasimonoenergetic electron beam from a femtosecond prepulseexploded foil
View Description Hide DescriptionWe report the first observation of a quasimonoenergetic electron beam emitted from a femtosecond prepulseexploded foil. At earlier, a thick plastic foil is irradiated by a copropagating femtosecond prepulse of modest intensity , which then explodes to become an underdense foil plasma. When a 70fs highintensity pulse arrives and interacts with it, a collimated electron beam of divergence angle is emitted in the forward direction. The energy spectrum of the electron beam shows a quasimonoenergetic peak at of energy spread , within which about pC of charge is contained.

Perturbation evolution started by RichtmyerMeshkov instability in planar laser targets
View Description Hide DescriptionThe first observations of the interaction of the RichtmyerMeshkov (RM)instability with reflected shock and rarefaction waves in laserdriven targets are reported. The RM growth is started by a shock wave incident upon a rippled interface between lowdensity foam and solid plastic. The subsequent interaction of secondary rarefaction and/or shock waves arriving from the ablation front and the rear surface of the target with the RMunstable interface stops the perturbation growth and reverses its direction. The ensuing exponential RayleighTaylor growth thus can sometimes proceed with an inverted phase.
 Top

 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Dispersive electromagnetic drift modes in nonuniform quantum magnetoplasmas
View Description Hide DescriptionThe existence of new electromagnetic drift modes in nonuniform quantum magnetoplasmas is predicted. For this purpose, new dispersion relations are derived by employing the quantum magnetohydrodynamic equation for plasmas without and with ion motions. The effects of electron corrections due to quantum fluctuations, density gradients, and external magnetic field strengths on the angular wave frequencies and growth/damping rates are examined. The present analytical and numerical investigations are relevant to dense astrophysical objects.

The VlasovPoisson model and the validity of a numerical approach
View Description Hide DescriptionOur aim in this work is to show that the final macroscopic state of a noncollisional plasma system, computed through numerical simulations, depends on artificial small scale effects induced by the used numerical scheme and/or grid discretization. By using the continuous, Hamiltonian VlasovPoisson model, we found significant differences in the nonlinear dynamics when varying the importance of dissipative and/or dispersive (numerical) effects. In particular, such artificial processes are crucial during phase space vortex generation and vortex merging dynamics leading to different irreversible asymptotic states. These results are obtained for numerical grid scale lengths much smaller than any noncollisional physical scale length.

Semianalytical model for fluxpileuplimited, dynamically reconnecting systems in resistive magnetohydrodynamics
View Description Hide DescriptionA simple zerodimensional model is proposed that synthesizes the complex dynamics of driven, twodimensional, magnetically reconnecting systems within the resistivemagnetohydrodynamics(MHD) framework. The model applies to the socalled asymptotic regime of the wellknown magnetic island coalescence problem, where wellseparated macroscopic “ideal” and microscopic “resistive” regions are assumed to exist. The dynamics of the ideal region is described in terms of the Lorentz and pressure gradient forces on current filaments, and conservation of magnetic flux; that of the resistive region is described in terms of plasma flow and magnetic field magnitudes at the current sheet boundaries. Matching of the two regions provides the evolution equation for the current sheet thickness. The results for the Opoint position versus time, reconnection rate, current sheet thickness, etc., at different resistivities obtained from the model agree well with those from the fully nonlinear twodimensional reduced MHD simulation.

Soliton reflection in a negative ion containing plasma: Effect of magnetic field and ion temperature
View Description Hide DescriptionConsidering an inhomogeneous plasma having negative ions, positive ions, and electrons, relevant modified Kortewegde Vries equations are derived and solved for obtaining the expressions of amplitudes and widths of the incident and reflected solitons together with the reflection coefficient under the combined effect of magnetic field, obliqueness (angle between the magnetic field and the direction of wave propagation) and ion temperature. A limit is found on the obliqueness for the reflection of incident soliton, which shows a dependence on the density, temperature, and drift velocity of both the ions. The incident solitons are observed to reflect more strongly under the effects of weak magnetic field, higher negative ion density, higher ion temperature, stronger density gradient and smaller angle . It is inferred that whereas the incident solitons show faster change in their characteristics with the ion density, the reflected solitons are more sensitive to the ion temperature.

Nonlinear electromagnetic plasma eigenmodes and their stability to stimulated Raman scattering
View Description Hide DescriptionTransverse mode structure of nonlinear laser eigenmodes in underdense and overdense plasmas has been obtained by numerically solving the wave equation under relativistic and ponderomotive nonlinearities. The mode structure closely resembles a Lorentzian with half width scaling inversely as the axial intensity of the laser. The threshold condition for laser penetration in an overdense plasma turns out to be , where is the equilibrium electron density, is the critical density at laser frequency, and is the electron Lorentz factor due to the laser of normalized axial intensity . The nonlinear laser eigenmode, in a low density plasma, is unstable to stimulated Raman backscattering off a copropagating space charge reactive quasimode. The growth rate increases with laser intensity as rises up to . Beyond this value, growth rate decreases with , due to the enhancement of electron mass and depletion of electrons from the axial region. Geometrical effects also reduce the growth rate.

Simulation of highenergy particle production through sausage and kink instabilities in pinched plasma discharges
View Description Hide DescriptionIn an experimental plasma, highenergy particles were observed by using a plasma focus device, to obtain energies of a few hundred keV for electrons, up to MeV for ions. In order to study the mechanism of highenergy particle production in pinched plasma discharges, a numerical simulation was introduced. By use of a threedimensional relativistic and fully electromagnetic particleincell code, the dynamics of a Zpinchplasma, thought to be unstable against sausage and kink instabilities, are investigated. In this work, the development of sausage and kink instabilities and subsequent highenergy particle production are shown. In the model used here, cylindrically distributed electrons and ions are driven by an external electric field. The driven particles spontaneously produce a current, which begins to pinch by the Lorentz force. Initially the pinched current is unstable against a sausage instability, and then becomes unstable against a kink instability. As a result highenergy particles are observed.

Influences of the dust size and the dust charge variations to the lowfrequency wave modes in a dusty plasma
View Description Hide DescriptionThe effects of both the dust size distribution and the adiabatic dust charge variation in unmagnetized dusty plasmas have been studied. Both the analytical and the numerical results are given to compare the differences between the dustplasma with a powerlaw distribution and the monosized dusty plasma. The comparisons are also given between the dusty plasmas with or without charge fluctuations. The present analytical results show that only rarefactive solitary waves exist in this system, which is similar to that without dust charge variation. However, it is shown that the presence of adiabatic dust charge variations and the dust size distribution can significantly influence the characteristic of lowfrequency wave modes. This investigation can be relevant to the dust acoustic waves in various space plasma environments, such as Jupiter’s ring and interstellar dusty clouds.

Electrostatic potentials and energy loss due to a projectile propagating through a nonMaxwellian dusty plasma
View Description Hide DescriptionThe electrostatic potentials (Debye and wake) and energy loss due to a charged projectile propagating through an unmagnetized collisionless dusty plasma are derived employing kappa and generalized velocity distributions for the dust acoustic wave. It is found that these quantities in general differ from their Maxwellian counterparts and are sensitive to the values of spectral index, in the case of kappa distribution and to , in the case of generalized distribution. The amplitudes of these quantities are less for small values of the spectral index (, , ) but approach the Maxwellian in the limit (for kappa distribution) and for , [for generalized distribution]. For any nonzero value of , the potential and the energy loss grow beyond the Maxwellian results. The effect of kappa and generalized distributions on potential and energy loss is also studied numerically and the results are compared with those of the Maxwellian distribution.

Oblique electromagnetic instabilities for a hot relativistic beam interacting with a hot and magnetized plasma
View Description Hide DescriptionThe temperaturedependentfluid model from Phys. Plasmas13, 042106 (2006) is expanded in order to explore the oblique electromagnetic instabilities, which are driven by a hot relativistic electron beam that is interpenetrating a hot and magnetized plasma. The beam velocity vector is parallel to the magneticfield direction. The results are restricted to nonrelativistic temperatures. The growth rates of all instabilities but the twostream instability can be reduced by a strong magnetic field so that the distribution of unstable waves becomes almost one dimensional. For high beam densities, highly unstable oblique modes dominate the spectrum of unstable waves as long as , where is the electron gyrofrequency, is the electron plasma frequency, and is the relativistic factor of the beam. A uniform stabilization over the entire space cannot be achieved.

Nonstationarity of diffusioncontrolled surfacewavesustained discharges in a cw regime
View Description Hide DescriptionThe theory of the cw regime of maintenance of surfacewavesustained gas discharges developed up to now is for discharge production at a given frequency. A generalization of this theory is presented here by considering discharge maintenance by a narrowband signal, the actual situation in the experiments. Discharge production in a diffusioncontrolled regime is treated within the fluid plasma theory. The set of the equations is for the interrelated variations along the discharge length of the timedependent envelope of the electric field maintaining the plasma and for the timedependent plasmacharacteristics(plasma density, electron temperature, power absorbed on average by an electron). The numerical results presented for the timespace variations of wavefield amplitude and plasma density describe the nonstationary state of the cw regime of discharge maintenance. Detailed analysis with deterministic signals (Gaussian and superGaussian pulses) superimposed on a stationary cwpower level demonstrate different types of nonlinear effects—selfsteepening and spontaneous modulation near the pulse edge—which are reminiscent of phenomena known from the nonlinear optics. The results for discharge maintenance by a highfrequency power modulated by a narrowband lowamplitude Gaussian noise are discussed in terms of discharge stability/instability. Relevance to experimental finding in surfacewavesustained discharge is commented on.

Solitary electrostatic waves are possible in unmagnetized symmetric pair plasmas
View Description Hide DescriptionA possibility of stationary solitary electrostatic waves with large amplitude in symmetric unmagnetized symmetric pair plasmas (plasma,plasma or plasma) is proven. The main idea of the work is a thermodynamic unequilibrium of plasma species which may be created in lowdensity ideal pair plasmas. Ranges of parameters (Mach number and a nonequilibrium degree ) which lead to the possibility of solitary waves are found.

Hydrodynamics of shock waves with reflected particles. I. RankineHugoniot relations and stationary solutions
View Description Hide DescriptionIn this work we investigate how reflected particles modify the RankineHugoniot (RH) relations in a simple hydrodynamical framework. It is assumed that the ions are specularly reflected by the crossshock potential. For simplicity, an exactly perpendicular shock is assumed, thus other reflection mechanisms, such as magnetic mirroring, can be neglected. Momentum and energy terms are introduced to model reflected particles at the shock and the RH conditions are examined using a geometrical entropy condition to distinguish the physically relevant states. Although such shocks have some common features with combustion shocks within a narrow range of reflection parameters, for a wide range of reflection parameters, particularly for highly oblique shocks, ChapmanJouguet solutions do not exist. It is conjectured that these shocks comprise a distinct class. Decelerated solutions of the RH conditions are shown to exist only under specific conditions for shocks with reflected particles. Velocity flows both parallel and oblique to the perpendicular shock (with respect to an upstream magnetic field) are considered and found to be strongly sheared.
 Nonlinear Phenomena, Turbulence, Transport

Existence of bulk acoustic modes in pair plasmas
View Description Hide DescriptionIn view of applications to electronpositron and fullerene pair plasmas, a thorough discussion is given of the dispersion of linear electrostatic waves, before finding the parameter ranges in which stationary nonlinear electrostatic modes can exist, when there is a thermodynamic asymmetry between both constituents. Arguments for such an asymmetry can be found in the observed dispersion in fullerene pair plasma experiments, but point to small asymmetries. The existence of solitary modes is first discussed in general terms for various polytropic pressuredensity relations, showing that the solitons are always compressive in both pair components. For large thermal asymmetries, there is at most a doubling of the densities, but small asymmetries lead to weakly nonlinear amplitudes, described by familiar Kortewegde Vries solitons. Observations of an intermediate wave in the theoretical gap between the acoustic and Langmuir branches of the linear dispersion diagram might be accounted for, at least partially, by a train of weak solitons, as the difference between weak solitary modes and linear harmonic waves is rather difficult to ascertain experimentally.

Nonlinear dynamics of electrostatic iontemperaturegradient modes in a dustcontaminated plasma with variable charge and sheared ion flows
View Description Hide DescriptionBy employing Braginskii transportequations for ions and Boltzmann distribution for electrons in a dustcontaminated plasma with equilibrium density, temperature, and magnetic field gradients, the nonlinear set of equations are derived. New iontemperaturegradient driven modes are obtained and various limiting cases are discussed. It is shown that the iontemperaturegradient driven mode of driftwaves are attenuated in the presence of dustcharge fluctuations. It has been found that dust charging is always dissipative and the growth rate of various modes are damped. Furthermore, the possible stationary solution of the nonlinear mode coupling equations can be represented in the form of dipolar and vortex chains type solutions. The results of the present investigation should be helpful in understanding the fluctuations and transport phenomena in magnetically confined dustcontaminated tokamakplasma.

Evolution of nonlinear ionacoustic solitary wave propagation in rotating plasma
View Description Hide DescriptionA simple unmagnetized plasma rotating around an axis at an angle with the propagation direction of the acoustic mode has been taken. The nonlinear wave mode has been derived as an equivalent Sagdeev potential equation. A special procedure, known as the tanh method, has been developed to study the nonlinear wave propagation in plasma dynamics. Further, under small amplitude approximation, the nonlinear plasma acoustic mode has been exploited to study the evolution of soliton propagation in the plasma. The main emphasis has been given to the interaction of Coriolis force on the changes of coherent structure of the soliton. The solitary wave solution finds the different nature of solitons called compressive and rarefactive solitons as well as its explosions or collapses along with solitondynamics and these have been showing exciting observations in exhibiting a narrow wave packet with the generation of high electric pressure and the growth of high energy which, in turn, yields the phenomena of radiating soliton in dynamics.

Anomalous resistivity of currentdriven isothermal plasmas due to phase space structuring
View Description Hide DescriptionThe anomalous electric resistivity of collisionless plasmas is an important issue in the physics of hot plasmas, e.g., in the context of auroral particle acceleration and of reconnection in the solar corona. The linear stability theory of isothermal current driven space plasmas predicts an ionacoustic instability if the relative drift velocity of the current carrying particles exceeds a certain threshold, which, generally, depends on the plasma parameters. The spectrum of waves, excited by a marginal instability, is very narrow. Hence, the wave power at saturation and the corresponding electric resistivity due to waveparticle interaction cannot be obtained by means of a quasilinear, weak turbulence approach and the nonlinear single mode theory provides too small saturation amplitudes. To solve the nonlinear problem a newly developed unsplit conservative Eulerian Vlasov code is applied to simulate a strongly magnetized current driven plasma, which can be considered in 1D1V (one spatial, one velocity space direction). Instead of periodic boundary conditions, usually used as they are simpler to treat, open boundaries are implemented which allow to maintain a constant current flow. Simulated is a typical almost isothermal hot space plasma for the real mass ratio . The initial spontaneous instability is followed by a threestage nonlinear evolution: First electron trapping leads to the formation of electron phase space holes. Due to a steepening of the leading edges of the potential wells the electron phase space holes gradually become asymmetric, they grow in size and deepen. The phase space holes accelerate until they move much faster than the initial ionacoustic waves. The interaction of the current carriers with the asymmetric potential wells and causes a nonvanishing net momentum transfer between the particles and the selfgenerated electric field. After a few ion plasma periods ion trapping starts until, finally, an electrostaticdouble layer arises. It is found that the nonlinear saturated state of the system is dominated by the particle interaction with coherent phase space structures. The corresponding anomalous resistivity is slightly modulated with an oscillation period ). For a macroscopic description its major part can be parameterized by means of an effective collision rate of the order of , where is the electron and the ion plasma frequency.

Quasilinear calculation of Langmuir wave generation and beam propagation in the presence of density fluctuations
View Description Hide DescriptionThe generation of beamdriven Langmuir waves and the propagation of an electron beam in the presence of ambient density fluctuations are numerically studied using quasilinear calculations in one spatial dimension. The random spatiotemporal density fluctuations are driven externally as ionsoundlike turbulence. The effects of Langmuir wavescattering off density inhomogeneities in three spatial dimensions are represented through effective damping of the Langmuir waves, and are included in the quasilinear model. The numerical results are explored for illustrative parameters, and Langmuir wave field statistics are compared with stochastic growth theory (SGT) predictions. Due to the combined effects of quasilinear interaction with the beam and scattering off density fluctuations, the Langmuir waves show burstiness and the levels are generally lower than when the density is homogeneous, qualitatively consistent with previous predictions. Apart from early evolution, the average beam speed is approximately the same as in the homogeneous case, but relaxation of the beam is significantly retarded. Both features are in qualitative agreement with earlier predictions. Moreover, the beam distribution function displays relatively smooth variations, which implies that the burstiness in the wave levels originates predominantly from the randomness in the damping rate due to density perturbations, rather than from the stochasticity in the beam growth rate. The statistics of the Langmuir wave field show good agreement with SGT predictions, thus indicating the beamLangmuir wave system is in a SGT state. Furthermore, variations of the density fluctuation parameters are found to affect the evolution of both beam and Langmuir waves.