Volume 12, Issue 5, May 2005
 LETTERS


Explicit threshold of the toroidal ion temperature gradient mode instability
View Description Hide DescriptionThe explicit stability threshold of the toroidalion temperature gradient modeinstability is analytically derived using the standard reactive fluid model. It is shown that in the peak density region, the threshold gets significantly smaller due to finite ion Larmor radius effects, and the marginal unstable modes acquire finite wavelengths.

The origin of the long time correlations of the density fluctuations in the scrapeoff layer of the Tore Supra Tokamak
View Description Hide DescriptionIt is shown that intermittent density bursts observed in the scrapeoff layer of Tore Supra [J. Jacquinot, Nucl. Fusion43, 1583 (2003)] are detected in packs on the probe. In such a pack, typically two to three bursts are separated by time intervals smaller than the mean separation time. The long tails above observed on the autocorrelation function of the density fluctuations are found to be the temporal correlation between the individual bursts within their pack. Packs of density bursts can be detected in two limiting states of the turbulence: when the coupling between density and potential is strong and large density bursts split during their radial propagation or at the opposite when the coupling is weak so that different density bursts can propagate radially along the potential valleys. The lack of spatial resolution of the diagnostic does not allow to discriminate between the two mechanisms.

Excitation of the toroidicityinduced shear Alfvén eigenmode by toroidal iontemperaturegradient mode turbulence
View Description Hide DescriptionIt is shown that the toroidicityinduced shear Alfvén eigenmode (TAE) with low mode numbers can be excited as a result of the modulational instability of the short wavelength toroidaliontemperaturegradient mode turbulence. This instability seems to be responsible for the TAE excitation in Ohmically heated discharges at ASDEX Upgrade tokamak [M. Maraschek, S. Günter, T. Kass, B. Scott, and H. Zohm, Phys. Rev. Lett.79, 4186 (1997)].
 Top

 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Spatial Landau damping in plasmas with threedimensional distributions
View Description Hide DescriptionThe increase in linear Landau damping in distributed plasmas compared to thermal equilibrium plasmas is studied by solving a boundary value problem for the spatially damped plasma waves generated by a planar grid electrode with an applied time harmonic potential. Solutions are computed for the plasma potential versus the distance from the electrode for different values of the parameter (kappa). The velocity parameter of the distribution function is chosen so that, as the parameter varies, the kinetic temperature of the plasma remains constant. The exact solutions of this problem are also compared to approximate solutions derived from the theory of normal modes, that is, from the roots of the dispersion relation. This model problem demonstrates the significant increase in Landau damping by electrons which occurs for small values of the parameter .

Fluid formalism for collisionless magnetized plasmas
View Description Hide DescriptionA comprehensive analysis of the finiteLarmorradius (FLR) fluid moment equations for collisionless magnetized plasmas is presented. It is based on perturbative but otherwise general solutions for the second and third rank fluid moments (the stress and stress flux tensors), with closure conditions still to be specified on the fourth rank moment. The single expansion parameter is the ratio between the largest among the gyroradii and any other characteristic length, which is assumed to be small but finite in a magnetized medium. This formalism allows a complete account of the gyroviscous stress, the pressure anisotropy, and the anisotropic heat fluxes, and is valid for arbitrary magnetic geometry, arbitrary plasma pressure, and fully electromagnetic nonlinear dynamics. As the result, very general yet notably compact perturbative systems of FLR collisionless fluid equations, applicable to either fast (sonic or Alfvénic) or slow (diamagnetic) motions, are obtained.

Electron inertia effect on small amplitude solitons in a weakly relativistic twofluid plasma
View Description Hide DescriptionOnedimensional evolution of solitons in a twofluid plasma having weakly relativistic streaming ions and electrons is studied through usual Korteweg–de Vries equation under the effect of electron inertia. Although fast and slow ion acoustic modes are possible in such a plasma, only the fast mode corresponds to the soliton propagation for a particular range of velocity difference of ions and electrons. This range depends upon the ratios of mass and temperature of the ions and electrons. The effect of electron inertia on the propagation characteristics of the soliton is studied for typical values of the speed and temperature of the ions and electrons and it is found that this effect is dominant over the relativistic effect and the effect of ion temperature.

Surface waves in anisotropic Maxwellian plasmas
View Description Hide DescriptionDispersion relation of electromagneticsurface waves propagating on the interface between a vacuum and anisotropicMaxwellian plasmas is derived from Vlasov–Maxwell equations. By taking the limit of infinite speed of light in the electromagneticdispersion relation, we derived dispersion relation of electrostaticsurface waves, and we find that plasmas with equilibrium distribution (; electrons and ions), where the direction is normal to the interface, can be unstable to perturbations of ionacoustic surface wave type.

Effects of finite sized charge on downstream wake patterns
View Description Hide DescriptionFinite sized charged particles introduced into streaming dusty plasmas produce wake patterns in the downstream region. The structure of the wake potential is found to depend on values of the charge size and Mach number, where is the ratio of the flow speed to the dust acoustic speed.

Cylindrical and spherical ion acoustic waves in a plasma with nonthermal electrons and warm ions
View Description Hide DescriptionUsing the reductive perturbation technique, nonlinear cylindrical and spherical Korteweg–de Vries (KdV) and modified KdV equations are derived for ion acoustic waves in an unmagnetized plasma consisting of warm adiabatic ions and nonthermal electrons. The effects of nonthermally distributed electrons on cylindrical and spherical ion acoustic waves are investigated. It is found that the nonthermality has a very significant effect on the nature of ion acoustic waves.

Eigenmode response to driven magnetic reconnection in a collisionless plasma
View Description Hide DescriptionThe collisionless plasma of the Versatile Toroidal Facility experiment [J. Egedal, A. Fasoli, M. Porkolab, and D. Tarkowski, Rev. Sci. Instrum.71, 3351 (2000)], applied to the study of magnetic reconnection, exhibits a global eigenmode response during which the reconnection rate and the current channel oscillate. The present paper describes experiments tailored for investigating this eigenmode response. It is found that the oscillatory plasma behavior is linked to ionpolarization currents and associated electron currents that flow to maintain quasineutrality within the active reconnection region. A theoretical model is developed which describes the eigenmode response and accounts for the temporal evolution of the measured profiles of key plasma parameters.

Measurements of classical transport of fast ions
View Description Hide DescriptionTo study the fastion transport in a well controlled background plasma, a 3cm diameter rf ion gun launches a pulsed, ribbon shaped argon ion beam parallel to or at 15° to the magnetic field in the Large Plasma Device (LAPD) [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum.62, 2875 (1991)] at UCLA. The parallel energy of the beam is measured by a twogrid energy analyzer at two axial locations ( and ) from the ion gun in LAPD. The calculated ion beam slowingdown time is consistent to within 10% with the prediction of classical Coulomb collision theory using the LAPD plasma parameters measured by a Langmuir probe. To measure crossfield transport, the beam is launched at 15° to the magnetic field. The beam then is focused periodically by the magnetic field to avoid geometrical spreading. The radial beam profile measurements are performed at different axial locations where the ion beam is periodically focused. The measured crossfield transport is in agreement to within 15% with the analytical classical collision theory and the solution to the Fokker–Planck kinetic equation. Collisions with neutrals have a negligible effect on the beamtransportmeasurement but do attenuate the beam current.

Effects of dustcharge fluctuation on the damping of Alfvén waves in dusty plasmas
View Description Hide DescriptionUsing a completely kinetic description to analyze wave propagation in dusty plasmas, the case of propagation of waves exactly parallel to the external magnetic field and Maxwellian distributions for electrons and ions in the equilibrium is considered. A model for the charging process of dust particles which depends on the frequency of inelastic collisions between dust particles and electrons and ions is used. The dispersion relation and damping rates for Alfvén waves are obtained. For the numerical solutions, the average value of the inelastic collision frequency is used as an approximation. The results show that the presence of dust particles with variable charge in the plasma produces significant additional damping of the Alfvén wave. A novel process of mode coupling of lowfrequency waves is demonstrated to occur due to the presence of dust particles.

Velocity shear induced transition of magnetohydrodynamic to kinetic Alfvén waves
View Description Hide DescriptionIt is found that in the linear shear flow after a certain time the noncompressive magnetohydrodynamic Alfvén wave with only develops large values of and becomes the kinetic Alfvén wave with considerable compressibility, and velocity and magnetic field perturbations in the parallel direction (to the background magnetic field). It is found that the perturbation amplitudes of the above kinetic quantities increase continuously with time. The amplitudes are mostly determined by the ratios of the Alfvénic to ion gyrofrequency and of the thermal to magnetic pressure for a specified shear flow.

Even harmonics generation of high frequency radiation in currentcarrying plasmas
View Description Hide DescriptionGeneration of high frequency radiation harmonics in a currentcarrying plasma is studied. The physical mechanism responsible for harmonics generation is provided by electronion collisions. The current in the plasma is sustained by a constant electric field. It is shown that the electron distribution function anisotropy due to the static field yields generation of even harmonics. As a result, the radiation spectrum emitted by the currentcarrying plasma contains both even and odd harmonics, the latter being attributed to currentless plasma. For a broad range of plasma and high frequency radiation parameters, a detailed analysis of the even harmonics properties is reported.

Electromagnetic instability of an axially inhomogeneous plasma
View Description Hide DescriptionThe electromagnetic drift instability leading to the excitation of the Alfvén waves in an axially inhomogeneous plasma is investigated. The instability is driven by the axial shear of the drift velocity maintained by the localized density gradient. Analytical dispersion relation has been derived in the short wavelength (WKB) limit. The eigenvalue problem has also been solved numerically by using the recently developed algebraic method [D. I. Tolstikhin, V. N. Ostrovsky, and H. Nakamura, Phys. Rev. A58, 2077 (1998)].

Properties of low and medium frequency modes in twofluid plasma
View Description Hide DescriptionBased on a twofluid plasma model where the electron mass and the displacement current are neglected, the eigenmodeproperties such as the frequency and the compressibility are studied. It is found that these properties strongly depend on the twofluid parameter , where is the wave number of a mode and is the ion skin depth. Especially it is found that as the twofluid parameter increases beyond unity, the Alfvén wave, which is an incompressible mode in the magnetohydrodynamics(MHD) limit, becomes compressible and its phase velocity approaches to the acoustic speed. The slow magnetosonic wave, which is compressible in the MHD limit, becomes incompressible as increases. Implications of these results are also discussed.

Linear theory for fast collisionless magnetic reconnection in the lowerhybrid frequency range
View Description Hide DescriptionA linear theory is presented for the interplay between the fast collisionless magnetic reconnection and the lowerhybrid waves that has been observed in recent computer simulations [J. F. Drake, M. Swisdak, C. Cattell et al., Science299, 873 (2003)]. In plasma configurations with a strong guide field and anisotropic electron temperature, the electron dynamics is described within the framework of standard electron magnetohydrodynamic equations, accounting also for the effects of the electron polarization and ion motions in the presence of perpendicular electric fields. In the linear phase, we find two types of instabilities of a thin current sheet with steep edges, corresponding to its filamentation (or tearing) and bending. Using a surfacewave formalism for the perturbations whose wavelength is larger than the thickness of the current sheet, the corresponding growth rates are calculated as the contributions of singularities in the plasma dispersion function. These are governed by the electron inertia and the linear coupling of the reconnectingmagnetic field with local plasma modes propagating in the perpendicular direction that are subject to the Buneman instability. The linear surface waveinstability may be particularly important as a secondary instability, dissipating the thin current sheets that develop in the course of the fast reconnection in the shearAlfvén and kineticAlfvén regimes, and providing the anomalous resistivity for the growth of magnetic islands beyond the shearAlfvén and kineticAlfvén scales.

Effects of Landau quantization on the equations of state in intense laser plasma interactions with strong magnetic fields
View Description Hide DescriptionRecently, magnetic fields of gigaGauss (GG) have been observed in the laboratory in laser plasma interactions. From scaling arguments, it appears that a few gigaGauss magnetic fields may be within reach of existing petawatt lasers. In this paper, the equations of state(EOS) are calculated in the presence of these very large magnetic fields. The appropriate domain for electron degeneracy and for Landau quantization is calculated for the densitytemperature domain relevant to laser plasma interactions. The conditions for a strong Landau quantization, for a magnetic field in the domain of 1–10 GG, are obtained. The role of this paper is to formulate the EOS in terms of those that can potentially be realized in laboratory plasmas. By doing so, it is intended to alert the experimental laserplasma physics community to the potential of realizing Landau quantization in the laboratory for the first time since the theory was first formulated.

Magnetohydrodynamic shock wave formation: Effect of area and density variation
View Description Hide DescriptionThe nonlinear steepening of finite amplitude magnetohydrodynamic(MHD)waves propagating perpendicular to the magnetic field is investigated. The nonlinear evolution of a planar fast magnetosonic wave in a homentropic flow field is understood well through simple waves. However, in situations where the wave is moving through a variable area duct or when the flow field is nonhomentropic, the concept of simple waves cannot be used. In the present paper, the quasionedimensional MHD equations that include the effect of area variation and density gradients are solved using the wave front expansion technique. The analysis is performed for a perfectly conducting fluid and also for a weakly conducting fluid. Closed form solutions are obtained for the nonlinear evolution of the slope of the wave front in the limits of infinitely large and small conductivity. A general criterion for a compression wave to steepen into a shock is obtained. An analytical expression for the location of shock formation is derived. The effect of area variation and density gradient on shock formation is studied and examples highlighting the same are presented.

Stability of dust voids
View Description Hide DescriptionDust voids are frequently observed as dustfree regions in a dusty plasma. Experiments demonstrate a variety of dust void structures such as stable or unstable voids, global “heatbeat” modes of oscillations of voids, dust voids in the center of the chamber or near its walls. Theory shows that a dust void generally results from the balance of the electrostatic and the plasma (such as the ion drag) forces acting on a dust particle. Here, the stability theory of a void is developed and the void behavior is modeled. It is shown that sequences of stable and unstable void sizes can exist. The dynamics of dust in a plasma follows these stability characteristics leading to various stable and/or unstable dust void structures.