Volume 13, Issue 12, December 2006
 LETTERS


Wire dynamics model of the implosion of nested and planar wire arrays
View Description Hide DescriptionThis paper presents the wire dynamics model (WDM), which can effectively replace the generic 0D (zerodimensional) model in simulation of the implosions of arbitrary shaped wire arrays, including highwirenumber nested and planar array loads at multiMA generators. Fast and inexpensive WDM modeling can predict the array implosion time and the rate of thermalization of the kinetic energy, and can estimate the timing of the xray pulse. Besides serving the purposes of the design and optimization of the wire array loads of complex configurations, the WDM reproduces the specific features of the wire array implosion dynamics due to the inductive current transfer, which makes the WDM a valuable amplification of the magnetohydrodynamicmodels.

Large density variation predicted along the magnetic axis for cold electron plasmas in the Columbia Nonneutral Torus (CNT)
View Description Hide DescriptionCold pure electron plasmas confined in PenningMalmberg traps with mirror fields are known to exhibit density variations along field lines, such that the density is roughly proportional to the magnetic field strength, . The Columbia Nonneutral Torus (CNT) is the first stellarator designed to study pure electron plasmas, and exhibits substantial mirroring, with . However, results of a threedimensional equilibrium solver, presented in this Letter, predict a factor of 5.3 increase in density from the minimumfield cross section to the maximumfield cross section along the magnetic axis, for a Debye length plasma ( for CNT). In this Letter, it is shown that the density variation of electron plasmas in mirror traps can be significantly enhanced in a device that has a cross section that varies from cylinderlike to slablike, such as the CNT. A simple analytic expression is derived that describes the axial density variation in such a device, and it is found to agree well with the computational predictions for CNT.

dimensional generalized Johnson model for cosmic dustionacoustic nebulons with symbolic computation
View Description Hide DescriptionIn a cosmic dusty plasma, both azimuthal and height perturbations of a nonplanar cylindrical geometry are considered. For dustionacoustic waves and with symbolic computation,dimensional generalized Johnson model is derived and analytic solutions are constructed. Supernovashelltyped expanding bright nebulons and SaturnFringtype expanding dark nebulons are both pictured and discussed. Essential difference of this letter from the existing literature is pointed out, with the relevant, possibly observable nebulonic structures for the future cosmic experiments proposed.
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Experiments and theory of an upstream ionization instability excited by an accelerated electron beam through a currentfree double layer
View Description Hide DescriptionA lowfrequency instability varying from 10 to has been discovered in the presence of a currentfree double layer (DL) in a lowpressure expanding helicon plasma. The instability is observed using various electrostatic probes, such as Langmuir probes floating or biased to ion saturation and emissive probes measuring the plasma potential. A retarding field energy analyzer measuring the ion energy distribution function downstream of the double layer is used together with the LP to simultaneously observe the DL and the instability, confirming their coexistence. The frequency of the instability decreases with increasing neutral pressure, increases with increasing magnetic field in the source and increases with increasing rf power. A theory for an upstream ionizationinstability has been developed, in which electrons accelerated through the DL increase the ionization upstream and are responsible for the observed instability. The theory is in good agreement with the experimental results and shows that the frequency increases with the potential drop of the double layer and with decreasing chamber radius.

Electromagnetic dustlowerhybrid and dustmagnetosonic waves and their instabilities in a dusty magnetoplasma
View Description Hide DescriptionThe electromagnetic waves below the ioncyclotron frequency have been examined in a collisionless and homogeneous dusty plasma in the presence of a dust beam parallel to the direction of the external magnetic field. The lowfrequency mixed electromagnetic dustlowerhybrid and purely transverse magnetosonic waves become unstable for the sheared flow of dust grains and grow in amplitude when the drift velocity of the dust grains exceeds the parallel phase velocity of the waves. The growth rate depends dominantly upon the thermal velocity and density of the electrons.

Fluid modeling of the electron flow driven ion acoustic mode in a collisional plasma with magnetized electrons
View Description Hide DescriptionA fluid analysis is presented of the ion sound mode in a weakly ionized collisional plasma. The ionneutral collision frequency exceeds the ion gyrofrequency while the electrons remain magnetized. Under these conditions, an ion sound wave can propagate at arbitrary angles with respect to the direction of the magnetic field. In the presence of an electron flow along the magnetic lines the sound mode can grow. Due to the electron collisions the mode is unstable while ion collisions cause an angle dependent instability threshold which is such that the mode is most easily excited at very large angles. Hot ion effects are included in the study by means of an effective viscosity which effectively describes the ion Landau dampingeffect. In the presence of an additional light ion specie, the mode frequency and increment in a certain parameter range are increased.

Properties of linear and nonlinear ion thermal waves in a pair ion plasma containing charged dust impurities
View Description Hide DescriptionProperties of linear and nonlinear ion thermal waves (ITWs) in pair ion plasmas containing a fraction of stationary charged (positive or negative) dust grains are investigated. For this purpose, a linear dispersion relation, a Kortewegde Vries equation and (an energy integral of a classical potential) for linear (nonlinear) ITWs are derived from the ion continuity and momentum equations together with the Poisson equation. It is found that both the ITW frequency and the profile of the ion thermal solitary waves are significantly affected by the presence of positively/negatively charged dust grains. The present results should be useful in understanding the salient features of finite amplitude localized ion thermal solitary pulses in a pair ion plasma containing charged dust impurities.

Hall assisted forced magnetic reconnection
View Description Hide DescriptionThe role of the Hall effect in forced magnetic reconnection is investigated analytically for the socalled Taylor problem. In the latter, a tearing stable slab plasma equilibrium, which is chosen here to be a simple magnetic field reversal, is subjected to a smallamplitude boundary deformation that drives magnetic reconnection (hence the adjective “forced” ) at the neutral surface within the plasma. It is shown that such reconnection becomes substantially accelerated by the Hall effect when the nondimensional parameter exceeds . Here, is the ion inertial skin depth, is the width of the plasma slab, and is the Lundquist number of a highly conducting plasma. Two different types of external perturbation are considered. In the case of continuous quasistatic driving, with a frequency such that , being the Alfvén transit time, various reconnection regimes are identified. The corresponding heating rates, which are determined by the parameters , , and , are derived. In the case of a “oneoff” reconnection event, we demonstrate when and how the transition from the Hall regime to the magnetohydrodynamic regime occurs in the course of the reconnection process. It is found that the peak instantaneous reconnection rate scales as , where is the reconnectedmagnetic flux, is the magnetic field strength, and is the amplitude of the boundary deformation.

Physics of the dusty Hall plasmas
View Description Hide DescriptionThe presence of the immobile charged dust in the plasma modifies the scale over which the Hall effect becomes important. For a positively charged dusty background this scale can become arbitrarily large. It is shown that the emergence of the Hall effect in an immobile charged background is related to the presence of an electric field that operates over the plasma gyration period. The generalized flux, which is a combination of the magnetic and fluid vortex flux, can decay due to the presence of the charge or the density inhomogeneities. The normal mode behavior of such a dusty plasma could be very different for positively and negatively charged grains. Whereas for negatively charged grains the usual magnetohydrodynamic(MHD) modes are present in the system, for positively charged grains, the Alfvén mode may not exist if , where is the charge of the dust and are the dust (electron) number densities. In the presence of the inhomogeneities, inertialess dusty plasma is subject to the Hall instability. It is shown that the growth rate of the Hall instability is proportional to the whistler frequency. Since Hall drift is nondissipative in nature, this instability can play important role in redistributing the magnetic energy from the large to small scales.

Dissipation in magnetic reconnection with a guide magnetic field
View Description Hide DescriptionA combination of numerical simulation results and analytical theory is applied to the problem of magnetic reconnection in a guide magnetic field. An investigation of electron distribution functions within the electron diffusion region leads to a picture of mixing of particles with different acceleration histories on electron Larmor scales. Based on an apparent average loss of accelerated particles by fieldaligned and transport, it is proposed that the role of the reconnectionelectric field is to replenish this loss by acceleration of particles that enter the electron diffusion region. Analytic theory is employed to verify this model, and an equation is derived, which balances the average electric field force density by a diffusion term applied to the electron momentum density. The diffusion coefficient contains explicitly the electron Larmor spatial scale and a poloidal transport time scale.
 Nonlinear Phenomena, Turbulence, Transport

Stabilization of the fan instability: Electron flux relaxation
View Description Hide DescriptionThis paper presents some relevant simulation results on the interaction between electrostatic waves and suprathermal electron fluxes at anomalous cyclotron and Landau resonances. In particular, the case of a dense and continuous wave spectrum is studied. It is shown that, after the waves excited by the fan instability at anomalous cyclotron resonances have reached a first saturation stage due to particle trapping, the process of “dynamical resonance merging” takes place, which leads to a strong amplification of the waves’ amplitudes. The Landau resonances do not play an essential role in the total energy exchange between the particles and the waves, as they mainly help to smooth the peaks rising during the evolution of the electron parallel velocity distribution and contribute to damping. Moreover, the paper shows that at the asymptotic stage of the interaction, when the waves’ amplitudes are saturated and the electron flux is relaxed, some physical features clearly do not fit the predictions of the wellknown quasilinear theory. The careful examination of a huge number of trajectories of particles moving in the effective field of the wave packet allows to state that most of the particles involved in the resonantinteractions are trapped by several waves simultaneously. In this socalled “multitrapping” process, the particles perform complex oscillatory motions which are far from what is expected from the quasilinear theory, where the diffusive behavior of the particles in the velocity space results from small successive random steps.

Equilibrium statistical mechanics for single waves and wave spectra in Langmuir waveparticle interaction
View Description Hide DescriptionUnder the conditions of weak Langmuir turbulence, a selfconsistent waveparticle Hamiltonian models the effective nonlinear interaction of a spectrum of waves with resonant outofequilibrium tail electrons. In order to address its intrinsically nonlinear timeasymptotic behavior, a Monte Carlo code was built to estimate its equilibrium statistical mechanics in both the canonical and microcanonical ensembles. First, the single wavemodel is considered in the cold beamplasma instability and in the O’Neil setting for nonlinear Landau damping. O’Neil’s threshold, which separates nonzero timeasymptotic wave amplitude states from zero ones, is associated with a secondorder phase transition. These two studies provide both a testbed for the Monte Carlo canonical and microcanonical codes, with the comparison with exact canonical results, and an opportunity to propose quantitative results to longstanding issues in basic nonlinear plasma physics. Then, the properly speaking weak turbulence framework is considered through the case of a large spectrum of waves. Focusing on the small coupling limit as a benchmark for the statistical mechanics of weak Langmuir turbulence, it is shown that Monte Carlo microcanonical results fully agree with an exact microcanonical derivation. The wave spectrum is predicted to collapse towards small wavelengths together with the escape of initially resonant particles towards low bulk plasma thermal speeds. This study reveals the fundamental discrepancy between the longtime dynamics of single waves, which can support finite amplitude steady states, and of wave spectra, which cannot.

Radial propagation of structures in drift wave turbulence
View Description Hide DescriptionThe formation and propagation of spatiotemporal fluctuation structures in weakly developed driftwave turbulence in a linearly magnetized helicon device is investigated. Turbulent density fluctuations in the far edge plasma display an intermittent character with largeamplitude positive density bursts. Their peak amplitudes correspond to the timeaveraged density in the maximum radial plasma pressure gradient. The conditional average technique is applied to reconstruct the dynamics of turbulent coherent structures in the azimuthal plane. The formation of turbulent structures is closely linked to a quasicoherent drift wave mode, which is generally observed in the radial density gradient region in the weakly developed turbulent state. It is demonstrated that every positive high amplitude density burst in the plasma edge is due to the radial propagation of a turbulent structure. The typical scale size of the turbulent structures is and their lifetime exceeds the eddy turnover time by orders of magnitude, thereby characterizing them as coherent structures. Although the turbulent structures propagate mainly azimuthally in the direction of the drift they are observed to have a radial velocity, which is typically 10% of the ion sound speed.

Nonlinear modulated dust lattice wave packets in twodimensional hexagonal dust crystals
View Description Hide DescriptionThe amplitude modulation of dust lattice waves(DLWs) propagating in a twodimensional hexagonal dust crystal is investigated in a continuum approximation, accounting for the effect of dust charge polarization (dressed interactions). A dusty plasma crystalline configuration with constant dust grain charge and mass is considered. The dispersion relation and the group velocity for DLWs are determined for wave propagation in both longitudinal and transverse directions. The reductive perturbation method is used to derive a dimensional nonlinear Schrödinger equation (NLSE). New expressions for the coefficients of the NLSE are derived and compared, for a Yukawatype potential energy and for a “dressed” potential energy, taking into account interaction and geometric nonlinearities.

Experimental evidence of mode coupling in drift wave intermittent turbulence using a wave number bicoherence analysis
View Description Hide DescriptionSpatiotemporal mode coupling is studied experimentally in a cylindrical plasma device. For that purpose, a bicoherence analysis is applied to spatially resolved measurements of drift wavefluctuations in order to study nonlinear coupling in the wave number spectrum. The use of the bicoherence is shown to be much more accurate and straightforward than one of the bicoherence, revealing bicoherence bursts with a characteristic duration shorter than the characteristic period of the signals. It is demonstrated that intermittent structures can be produced during these events.

Characterizing electron temperature gradient turbulence via numerical simulation
View Description Hide DescriptionNumerical simulations of electron temperature gradient (ETG) turbulence are presented that characterize the ETG fluctuation spectrum, establish limits to the validity of the adiabatic ion model often employed in studying ETG turbulence, and support the tentative conclusion that plasmaoperating regimes exist in which ETG turbulence produces sufficient electron heat transport to be experimentally relevant. We resolve prior controversies regarding simulation techniques and convergence by benchmarking simulations of ETG turbulence from four microturbulence codes, demonstrating agreement on the electron heat flux, correlation functions,fluctuation intensity, and flow shear at fixed simulation cross section and resolution in the plane perpendicular to the magnetic field. Excellent convergence of both continuum and particleincell codes with time step and velocityspace resolution is demonstrated, while numerical issues relating to perpendicular (to the magnetic field) simulation dimensions and resolution are discussed. A parameter scan in the magnetic shear, , demonstrates that the adiabatic ion model is valid at small values of ( for the parameters used in this scan) but breaks down at higher magnetic shear. A proper treatment employing gyrokinetic ions reveals a steady increase in the electron heat transport with increasing magnetic shear, reaching electron heat transport rates consistent with analyses of experimental tokamak discharges.

The dynamics of an isolated plasma filament at the edge of a toroidal device
View Description Hide DescriptionThe dynamics of an isolated plasma filament (an isolated blob) in the far scrapeoff layer (SOL) of a toroidal device is described, with a proper averaging of the geometrical parameters as well as plasma parameters along the filament. The analysis is limited to the magnetohydrodynamic description. The effects of the electrical contact of the filament end with the limiter and of the finite plasma resistivity are also discussed.

Coupled fluidflow and magneticfield simulation of the Riga dynamo experiment
View Description Hide DescriptionMagnetic fields of planets, stars, and galaxies result from selfexcitation in moving electroconducting fluids, also known as the dynamo effect. This phenomenon was recently experimentally confirmed in the Riga dynamo experiment [A. Gailitis et al., Phys. Rev. Lett.84, 4365 (2000); A. Gailitis et al., Physics of Plasmas11, 2838 (2004)], consisting of a helical motion of sodium in a long pipe followed by a straight backflow in a surrounding annular passage, which provided adequate conditions for magneticfield selfexcitation. In this paper, a first attempt to simulate computationally the Riga experiment is reported. The velocity and turbulence fields are modeled by a finitevolume NavierStokes solver using a ReynoldsaveragedNavierStokes turbulencemodel. The magnetic field is computed by an AdamsBashforth finitedifference solver. The coupling of the two computational codes, although performed sequentially, provides an improved understanding of the interaction between the fluid velocity and magnetic fields in the saturation regime of the Riga dynamo experiment under realistic working conditions.

Estimation of higherorder contribution to viscosity of hydrogen plasmas including electronically excited states
View Description Hide DescriptionWithin the framework of the ChapmanEnskog method, the influence of electronically excited states on viscosity and its higherorder contributions has been investigated for a partially ionized thermal hydrogen plasma. A strong dependence of viscosity and its higherorder contributions on the presence of electronically excited states (especially at high pressure) has been observed. In the present work, a simplified relationship has been suggested for estimating higherorder contribution to viscosity, which avoids the cumbersome computational procedure involved in its exact evaluation. The results thus obtained agree with the exact contributions satisfactorily.

Oblique modulation of electrostatic modes and envelope excitations in pairion and electronpositron plasmas
View Description Hide DescriptionThe nonlinear amplitude modulation of electrostatic waves propagating in a collisionless twocomponent plasma consisting of negative and positive species of equal mass and absolute charge is investigated. Pairion (e.g., fullerene) and electronpositron () plasmas (neglecting recombination) are covered by this description. Amplitude perturbation oblique to the direction of propagation of the wave has been considered. Two distinct linear electrostatic modes exist, namely an acoustic lower mode and Langmuirtype optictype upper one. The behavior of each of these modes is examined from the modulational stability point of view. The stability criteria are investigated, depending on the electrostatic carrier wave number, the angle between the modulation and propagation directions, and the positrontoelectron temperature ratio . The analysis shows that modulated electrostatic wavepackets associated to the lower (acoustic) mode are unstable, for small values of carrier wave number (i.e., for large wavelength ) and for finite (small) values of the angle (yet stable for higher ), while those related to the upper (opticlike) mode are stable for large values of the angle only, in the same limit, yet nearly for all values of . These results are of relevance in astrophysical contexts (e.g., in pulsar environments), where plasmas are encountered, or in pair fullereneion plasmas, in laboratory.