Volume 12, Issue 6, June 2005
 LETTERS


Temperature and melting of lasershocked iron releasing into an LiF window
View Description Hide DescriptionAbsolute reflectivity and selfemission diagnostics are used to determine the graybody equivalent temperature of lasershocked iron partially releasing into a lithium fluoride window. Pressure and reflectivity are measured simultaneously by means of velocityinterferometer system for any reflector interferometers. In the temperaturepressure plane, a temperature plateau in the release is observed which is attributed to iron’s melting line. Extrapolation of data leads to a meltingtemperature at Earth’s innerouter core boundary of , in good agreement with previous works based on dynamic compression. Shock temperatures were calculated and found to be in the liquid phase.
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Invariant imbedding theory of mode conversion in inhomogeneous plasmas. I. Exact calculation of the mode conversion coefficient in cold, unmagnetized plasmas
View Description Hide DescriptionThis is the first of a series of papers devoted to the development of the invariant imbedding theory of mode conversion in inhomogeneous plasmas. A new version of the invariant imbedding theory of wave propagation in inhomogeneous media allows one to solve a wide variety of coupled wave equations exactly and efficiently, even in the cases where the material parameters change discontinuously at the boundaries and inside the inhomogeneous medium. In this paper, the invariant imbedding method is applied to the mode conversion of the simplest kind, that is, the conversion of polarized electromagnetic waves into electrostatic modes in cold, unmagnetized plasmas. The mode conversion coefficient and the field distribution are calculated exactly for linear and parabolic plasma density profiles and compared quantitatively with previous results.

Exact orbital motion theory of the shielding potential around an emitting, spherical body
View Description Hide DescriptionA kinetic theory for the equilibrium of an unmagnetized plasma consisting of electrons and ions surrounding a spherical body emitting electrons (due to thermionic emission,photoemission, or secondary emission) is presented. The theory is valid for positively charged bodies, neglects collisions of the plasma particles, and is formulated for profiles of the shielding potential presenting an attractive well. Particleincell simulations are shown to be in good agreement with the theory. An approximated criterion is derived to determine the presence of the potential well.

Absolute and convective instabilities of parallel propagating circularly polarized Alfvén waves: Beat instability
View Description Hide DescriptionRuderman and Simpson [Phys. Plasmas11, 4178 (2004)] studied the absolute and convective decay instabilities of parallel propagating circularly polarized Alfvén waves in plasmas where the sound speed is smaller than the Alfvén speed . We extend their analysis for the beat instability which occurs in plasmas with . We assume that the dimensionless amplitude of the circularly polarized Alfvén wave (pump wave),, is small. Applying Briggs’ method we study the problem analytically using expansions in power series with respect to . It is shown that the pump wave is absolutely unstable in a reference frame moving with the velocity with respect to the rest plasma if , where and . When or , the instability is convective. The signaling problem is studied in a reference frame where the pump wave is convectively unstable. It is shown that the spatially amplifying waves exist only when the signaling frequency is in two narrow symmetric frequency bands with the widths of the order of . These results enable us to extend for the case when the conclusions, previously made for the case when , that circularly polarized Alfvén waves propagating in the solar wind are convectively unstable in a reference frame of any spacecraft moving with the velocity not exceeding a few tens of km/s in the solar reference frame. The characteristic scale of spatial amplification for these waves exceeds

Transition from flute modes to drift waves in a magnetized plasma column
View Description Hide DescriptionRecent experiments performed on the low plasma device Mirabelle [T. Pierre, G. Leclert, and F. Braun, Rev. Sci. Instrum.58, 6 (1987)] using a limiter have shown that transitions between various gradient driven instabilities occurred on increasing the magnetic field strength. New thorough measurements allow to identify unambiguously three instability regimes. At low magnetic field the strong velocity shear drives a Kelvin–Helmholtz instability, whereas at high magnetic fielddrift waves are only observed. A centrifugal (Rayleigh–Taylor) instability is also observed in between when the velocity is shearless and strong enough. A close connection is made between the ratio of the drift parameter to the radial density gradient length and each instability regime.

Tailoring of ion species composition in complex plasmas with charge exchange collisions
View Description Hide DescriptionA generic approach towards tailoring of ion species composition in reactive plasmas used for nanofabrication of various functional nanofilms and nanoassemblies, based on a simplified model of a parallelplate rf discharge, is proposed. The model includes an idealized reactive plasma containing two neutral and two ionic species interacting via charge exchangecollisions in the presence of a microdispersed solid component. It is shown that the number densities of the desired ionic species can be efficiently managed by adjusting the dilution of the working gas in a buffer gas, rates of electron impact ionization, losses of plasma species on the discharge walls, and surfaces of fine particles, charge exchange rates, and efficiency of threebody recombination processes in the plasma bulk. The results are relevant to the plasmaaided nanomanufacturing of ordered patterns of carbon nanotip and nanopyramid microemitters.

Experimental verification of the dielectric constant of a magnetized rotating plasma
View Description Hide DescriptionDirect measurements confirm that the magnetized plasma perpendicular dielectric constant has a linear dependence on the plasma density for fixed magnetic field, as predicted by magnetohydrodynamic(MHD) theory. In experiments performed on the Maryland Centrifugal Experiment (MCX) [R. F. Ellis, A. B. Hassam, S. Messer, and B. R. Osborn, Phys. Plasmas, 8, 2057 (2001)], lineaveraged hydrogen plasma electron density is measured using a Mach–Zehnder interferometer. For small rotational Alfven Mach numbers, the measured size of the perpendicular plasmadielectric constant is also in agreement with MHD theory. Plasma density in the range of and relative perpendicular plasmadielectric constant of the order of are measured.

Dustlattice waves: Role of charge variations and anisotropy of dustdust interaction
View Description Hide DescriptionDustlattice waves are studied in the framework of the onedimensional particle string model. The dustdust interaction potential is assumed to have an arbitrary dependence on the vertical and horizontal coordinates, which allows to take into account the wake field effects. Both the vertical and horizontal charge variations are also included into the model. The model yields the coupling between the vertical and horizontal (longitudinal) modes: the coupling coefficient is the sum of six terms, each caused by a different physical mechanism. It is shown that the coupling can trigger the resonance oscillatory instability, which has been already observed in experiments. It is also shown that a nonoscillatory instability can appear at small wave numbers due to the coupling.

Effect of charge fluctuations and collisions on the energy loss of projectiles for a generalized Lorentzian dusty plasma
View Description Hide DescriptionThe shielded potential and the energy loss by projectiles passing through a collisional dustcontaminated plasma with dustcharge fluctuations and grainsize distribution are presented. Analytical general expressions are obtained for the shielded potential and for the energy loss by considering twobody correlation effects. An interference contribution of these projectiles to the potential and energy loss is observed which depends upon their orientation and separation distance. The dustcharge fluctuation produces a potential well instead of Coulombtype potential for a slowly moving test charge with slow charge relaxation rate and energy is gained by the charged projectiles. However, fast charge relaxation enhances the energy loss and some peaks are observed showing the excitation of some electrostatic modes. On the other hand, the dust neutral collisions also enhance the energy loss for projectile velocities greater than the dust acoustic speed for a Maxwellianplasma (for a large value of the spectral index ).

The effect of ion drift on the sheath, presheath, and ioncurrent collection for cylinders in a collisionless plasma
View Description Hide DescriptionA calculation is presented of the behavior of the sheath and presheath surrounding an infinite cylindrical conducting object, representing a spacecraft or electrostatic probe, which is moving transversely through a collisionless plasma, such as is encountered in the ionosphere. The calculation is done by solving the coupled Vlasov (collisionless Boltzmann) and Poisson equations in an iterative manner. The results show that for some ratios of probe radius to electron Debye length, the ion current collected by the probe in a drifting plasma can be less than that collected in a nondrifting plasma. These changes in the currentcollection behavior can be linked to changes that occur in the sheath and presheath with plasma drift, including at large enough drift speeds the disappearance or “collapse” of the presheath.

Nonlinear dynamic of lowfrequency Buneman instability of a currentdriven plasma
View Description Hide DescriptionTheoretical investigation of the nonlinear dynamic of the lowfrequency Buneman instability of a currentdriven plasma is presented. In the nonlinear stage, this instability is described by the diffusion equation with a negative nonlinear diffusion coefficient. As a result, the plasma density profile reaches a sharp peak and is accompanied by the breakdown of quasineutrality and establishment of stationary selffocusing structures.

Polarization evolution of radiation in hot magnetized plasma with dissipation
View Description Hide DescriptionA formalism is presented for the analysis of polarization evolution in a magnetized plasma with dissipation due to kinetic effects. Such a plasma in addition to the Faraday and CottonMouton effects also presents dichroism, namely anisotropic absorption. As expected this effect is significant near the cyclotron harmonics.

Numerical studies of driven, chirped Bernstein, Greene, and Kruskal modes
View Description Hide DescriptionRecent experiments showed the possibility of creating longlived, nonlinear kinetic structures in a pureelectron plasma. These structures, responsible for largeamplitude periodic density fluctuations, were induced by driving the plasma with a weak oscillating drive, whose frequency was adiabatically decreased in time [W. Bertsche, J. Fajans, and L. Friedland, Phys. Rev. Lett.91, 265003 (2003)]. A onedimensional analytical model of the system was developed [L. Friedland, F. Peinetti, W. Bertsche, J. Fajans, and J. Wurtele, Phys. Plasmas11, 4305 (2004)], which pointed out the phenomenon responsible for the modifications induced by the weak drive in the phasespace distribution of the plasma (initially Maxwellian). In order to validate the theory and to perform quantitative comparisons with the experiments, a more accurate description of the system is developed and presented here. The new detailed analysis of the geometry under consideration allows for more precise simulations of the excitation process, in which important physical and geometrical parameters (such as the length of the plasma column) are evaluated accurately. The numerical investigations probe properties and features of the modes not accessible to direct measurement. Due to the presence of two distinct time scales (because of the adiabatic chirp of the drive frequency), a fully twodimensional numerical study of the system is expected to be rather time consuming. This becomes particularly important when, as here, a large number of comparisons (covering a wide range of drive parameters) are performed. For this reason, a coupled onedimensional, radially averaged model is derived and implemented in a particleincell code.

Waves and instabilities in weak inhomogeneous ideal Hallmagnetohydrodynamic plasmas
View Description Hide DescriptionThe twofluid magnetohydrodynamic equations in which the Hall term is taken into account in Faraday’s law (Hall magnetohydrodynamic) for lowdensity magnetized inhomogeneous plasmas with finite pressure, and accounting for the inertial electron and ion dynamics responses are considered for slab geometry. Waves that propagate in the direction perpendicular to the density gradient and close to the direction of the background magnetic field are considered and linear analysis is employed in order to derive a local dispersion relation. It is shown that both the ioncyclotron acoustic mode as well as the magnetosound mode are susceptible to an instability that is driven by the spatial inhomogeneity of the plasma density. The condition under which unstable waves appear and the growth rate of the unstable modes are obtained for collisionless plasmas. The results of this theoretical investigation can be relevant to instabilities in lowbeta fusion plasma devices as well as in various space applications.

Spin waves in dusty plasmas
View Description Hide DescriptionTaking into account the spin of dust particles in a dusty plasma, the bulk and surface spin oscillations in an isotropic one are studied. The dispersion relation and frequency spectra for bulk and surface spin waves are obtained.

Control of the charge and the nonlinear oscillation of dust particles by alternating current voltage superposition on the cathode in a direct current discharge
View Description Hide DescriptionExperimental and theoretical studies were conducted to investigate the control of charge and modification of nonlinear oscillations of externally injected dust particles in a dc discharge. The superposition of ac voltage on a dc cathode led to plasma density modulation, which brought about a drastic change of particle oscillation characteristics. Examples of the changes include disappearance of the subharmonic resonance peak and hysteresis as the ac superposition voltage was increased, which is attributed to the fact that the ac superposition made sheath structure less nonlinear and less parametrically resonant. In addition, as the ac frequency decreased from at the same ac voltage , the subharmonic peak became weakened along with its frequency. This result demonstrates that the dust charge is the main parameter in determining occurrence of the subharmonic resonance peak. We consequently expect that modification of the oscillationdynamics of dust particles and furthermore the separate control of the charge may be possible by the ac modulation of the dc biased cathode.

Effect of frequency variation on electromagnetic pulse interaction with charges and plasma
View Description Hide DescriptionThe effect of frequency variation (chirp) in an electromagnetic (EM) pulse on the pulse interaction with a charged particle and plasma is studied. Various types of chirp and pulse envelopes are considered. In vacuum, a charged particle receives a kick in the polarization direction after interaction with a chirped EM pulse. Interaction of a onedimensional chirped pulse with uniform plasma is considered. We found that the amplitude of the wake wave generated in plasma by an EM pulse can be significantly higher when the pulse is chirped.

A magnetohydrodynamic model for quantum plasmas
View Description Hide DescriptionThe quantum hydrodynamic model for charged particle systems is extended to the cases of nonzero magnetic fields. In this way, quantum corrections to magnetohydrodynamics are obtained starting from the quantum hydrodynamical model with magnetic fields. The importance of the quantum corrections is described by a parameter which can be significant in dense astrophysical plasmas. The quantum magnetohydrodynamic model is analyzed in the infinite conductivity limit. The conditions for equilibrium in ideal quantum magnetohydrodynamics are established. Translationally invariant exact equilibrium solutions are obtained in the case of the ideal quantum magnetohydrodynamic model.
 Nonlinear Phenomena, Turbulence, Transport

Onset and saturation of guidefield magnetic reconnection
View Description Hide DescriptionThe onset and saturation of collisionless magnetic reconnection in the presence of a guide field are investigated using twodimensional particleincell simulations in which the reconnection evolves out of the initial thermal noise in the current sheet and in which the resolution is sufficient to resolve the electron singular layer. The simulations show that reconnection does not abate when the island width exceeds either the electron singular layer or the initial current sheet width. Instead, reconnection proceeds through an explosive stage which appears to be limited only by the spatial size of the system. The guidefield reconnection dynamics is dominated by the formation of an asymmetric configuration with a deep density cavity along one pair of separatrix arms. In this cavity an electron beam feature is formed which excites the Buneman instability. Near the line the reconnectionelectric field is supported by a combination of quasiviscous and bulk inertia effects for the electrons. Around the island perimeter, intense Debyescale, predominantly perpendicular, electric field structures are formed.

Nonlinear gyrokinetic turbulence simulations of shear quenching of transport
View Description Hide DescriptionThe effects of velocity shear have been investigated in nonliner gyrokineticturbulence simulations with and without kinetic electrons. The impact of shear stabilization in electrostatic fluxtube simulations is well modeled by a simple quench rule with the turbulent diffusivity scaling like , where is the shear rate, is maximum linear growth rate without shear, and is a multiplier. The quench rule was originally deduced from adiabatic electronion temperature gradient(ITG) simulations where it was found that . The results presented in this paper show that the quench rule also applies in the presence of kinetic electrons for longwavelength transport down to the ion gyroradius scale. Without parallel velocity shear, the electron and ion transport is quenched near . When the destabilizing effect of parallel velocity shear is included in the simulations, consistent with purely toroidal rotation, the transport may not be completely quenched by any level of shear because the Kelvin–Helmholtz drive increases faster than increases. Both ITGturbulence with added trapped electron drive and electrondirected and curvaturedriven trapped electron mode turbulence are considered.