Volume 7, Issue 2, February 2000
Index of content:
 LETTERS


The effect of insulating coatings on exploding wire plasma formation
View Description Hide DescriptionSubstantial increases are reported in the expansion rates of exploding, dense wire cores under conditions simulating the prepulse phase of wire array zpinch experiments [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] using wires with insulating coatings. The insulation apparently allows additional wire heating by delaying the formation of plasma around the wires. Once plasma is formed it terminates significant current flow in the residual wire cores. This effect is demonstrated for 25μm diameter W and 25μm diameter Ag wires.

On finite β stabilization of the toroidal ion temperature gradient mode
View Description Hide DescriptionIt is shown that finite beta stabilization of the toroidal ion temperature gradient(ITG)mode is attributable to the electron ballooning parameter (normalized electron pressure gradient) A modest much smaller than that required from the drift reversal α (normalized total pressure gradient) where q is the safety factor, can effectively stabilize the ITGmode.

Inboard/outboard asymmetry of poloidal flow observed in the Compact Helical System
View Description Hide DescriptionThe poloidal flow velocity and the density of fully ionized carbon were measured in the Compact Helical System [K. Ida et al., Phys. Rev. Lett. 67, 58 (1991)] using the charge exchange spectroscopy with the bidirectional viewing. The poloidal asymmetry of the poloidal flow velocity observed in the outward shifted plasma is due to the electrostatic potential being constant on the magnetic flux surfaces. The poloidal asymmetry of the ion density can be explained by the conservation of the poloidal ion flux under the strong toroidal viscosity.

Stationary laser beam filaments in a semicollisional plasma
View Description Hide DescriptionThe cumulative effect of the ponderomotive force, inverse Bremsstrahlung heating and nonlocal electron thermal transport on the formation of steady state laser channels in an underdense weakly collisional plasma is studied for the first time. The nonlocal effects change both the distribution of the laser intensity and the density profile in the channel. Compared to the ponderomotive case, the addition of the nonlocal thermal effects results in density channels that are wider and deeper, and the power required for their formation is significantly decreased. The widespread density depletion wings of filaments created by nonlocal thermal effects can be important in the interaction of speckled laser beams with a plasma.

Intermittency and selfsimilarity in plasma edge fluctuations
View Description Hide DescriptionThe intermittency of the floating potential fluctuations as measured at the edge of a plasma of interest for controlled thermonuclear fusion research is investigated. The probability distribution functions of fluctuations are not scale invariant, that is the wings of these functions are more important at the smallest scales, a classical signature of intermittency. Selfsimilarity is recovered at scales greater than about 20 μs.

 ARTICLES


Basic Plasma Phenomena, Waves, Instabilities

Simulations of electron/electron instabilities: Electromagnetic fluctuations
View Description Hide DescriptionElectron/electron instabilities arise in collisionless plasmas when the electron velocity distribution consists of two distinct components with a sufficiently large relative drift speed between them. If the less dense beam component is not too tenuous and sufficiently fast, the electron/electron beam instability is excited over a relatively broad range of frequencies. This instability is often studied in the electrostatic limit, which is appropriate at where is the electron plasma frequency and is the electron cyclotron frequency, but is not necessarily valid at Here linear Vlasov dispersion theory has been used and fully electromagneticparticleincell simulations have been run in a spatially homogeneous, magnetized plasma model at and 0.5 Theory and simulations (run to times of order of the electron/electron beam instability show the growth of appreciable magnetic fluctuations at these waves bear righthand elliptical magnetic polarization. The simulations reproduce the wellknown slowing and heating of the beam; at this heating is predominantly parallel to the background magnetic field, but as becomes greater than unity the perpendicular heating of the beam increases. The simulations also demonstrate that, for electromagneticfluctuations impart to the more dense electron core component significant heating perpendicular to the background magnetic field.

Excitation of currentdriven electrostatic ioncyclotron waves in presence of a transverse direct current electric fields in a magnetized plasma
View Description Hide DescriptionThe temporal evolution of the currentdriven electrostatic ion–cyclotron (CDEIC) instability is investigated in the presence of a transverse dcelectric field in a collisional magnetized plasma. It was found that the inclusion of a transverse dcelectric field in addition to the magnetic field changes the dispersion characteristics of the ion cyclotron waves. The growth rate of the instability increases with the mode frequency and has the largest value at the mode frequencies 43 kHz (for and 7.53 kHz (for when the electron drift velocity is less than the critical value for the CDEIC instability. The growth rate also increases with the guide magnetic field and has the largest value at the magnetic fields 1.0 kG for and 0.1 kG for The growth rate is a sensitive function of electron collision frequency. The results of the theory are applied to explain some of the experimental observations of Koepke et al. [IEEE Trans. Plasma Sci. 20, 631 (1992)].

Lowfrequency dispersion properties of plasmas with variablecharge impurities
View Description Hide DescriptionA theory of lowfrequency dustacoustic waves in lowtemperature collisional plasmas containing variablecharge impurities is presented. Physical processes such as dustcharge relaxation, ionizationrecombination of the electrons and ions, electron and ion elastic collisions with neutrals and dusts, as well as charging collisions with the dusts, are taken into account. Inclusion of these processes allows a balance of the plasma particles and thus a selfconsistent determination of the stationary state of the unperturbed plasma. The generalized dispersion relation describing the propagation and damping of the dust acoustic waves is derived and analyzed.

Gyrokinetic field theory
View Description Hide DescriptionThe Lagrangian formulation of the gyrokinetic theory is generalized in order to describe the particles’ dynamics, as well as the selfconsistent behavior of the electromagnetic fields. The gyrokineticequation for the particle distribution function and the gyrokineticMaxwell’sequations, for the electromagnetic fields, are both derived from the variational principle for the Lagrangian consisting of the parts of particles, fields, and their interaction. In this generalized Lagrangian formulation, the energy conservation property for the total nonlinear gyrokineticsystem of equations is directly shown from Noether’s theorem. This formulation can be utilized in order to derive the nonlinear gyrokineticsystem of equations and the rigorously conserved total energy for fluctuations with arbitrary frequencies. Simplified gyrokineticsystems of equations with the conserved energy are obtained from the Lagrangian with the small electron gyroradii, quasineutrality, and linear polarization–magnetization approximations.

Kinetic longitudinal eigenmodes for an inhomogeneous plasma diode with resolved sheaths using a Landau analogy
View Description Hide DescriptionElectrostatic waves in strongly inhomogeneous systems are analyzed using initial value formulations of the kinetic equations. The method resolves the gradual reflection in a trapped electron population, the successive acceleration of a kinetic beam, and it includes the outer circuit. The regularity of the electric field in the sheaths turns out to be crucial. At the turning points for closed trajectories, representing particles with a circulation time that is an integer number of inverse wave frequencies, it is shown that weak oscillating double charge layers can form. This phenomenon does not exist for homogeneous systems, since the structures rest on particle reflection. The final equations are solved numerically to illustrate the technique. Resonant damping and the singular space charge structures in the sheaths are the main differences compared with inhomogeneous fluid theory.

Kinetic model of threecomponent, weakly ionized, collisional plasma with a beam of neutral particles
View Description Hide DescriptionKinetic model of three component, weakly ionized, collisional plasma with a beam of neutral particles is developed. New dispersion relations for linear perturbations are derived and analyzed in various limiting cases.

Nonlinear Phenomena, Turbulence, Transport

Ponderomotive force in a moving warm twofluid plasma
View Description Hide DescriptionA microscopic method is used to obtain an expression for the ponderomotive force of high frequency electromagnetic waves in a warm twofluid collisionless plasma with nonzero fluid velocity. The result contains new terms which explicitly depend on the fluid velocity and its gradients, in addition to the familiar expressions of earlier studies. It is shown that the new terms are of the same order as the Abraham force in the smallness parameter that measures the magnitude of the flow velocity of the plasma relative to the phase velocity of the wave. The stress tensor and the momentum density vector are also derived from the newly found ponderomotive force.

Particle pinch in collisionless driftwave turbulence
View Description Hide DescriptionNonlinear numerical simulations show that the particle flux from collisionless driftwave turbulence is directed upgradient for sufficiently high values of This “particle pinch” results from the completely different perpendicular dynamics of slow (resonant) and fast (nonresonant) electrons, making it a genuinely kinetic effect which cannot easily be described by fluid models. Moreover, the linearly stable system selfsustains its turbulent state through a finiteamplitude (nonlinear) instability. Therefore, quasilinear estimates of turbulenttransport caused by collisionless drift waves are practically useless and have to be replaced by nonlinear kinetic simulations like the present one.

Laboratory studies of magnetic vortices. III. Collisions of electron magnetohydrodynamic vortices
View Description Hide DescriptionMagnetic vortices in the parameter regime of electron magnetohydrodynamics are studied in a large laboratory plasma. The vortices consist of magnetic field perturbations, which propagate in the whistler mode along a uniform dc magnetic field. The magnetic selfhelicity of the spheromaklike field perturbations depends on the direction of propagation. Vortices with opposite toroidal or poloidal fields are launched from two antennas and propagated through each other. The vortices collide and propagate through one another without an exchange of momentum, energy, and helicity. The absence of nonlinear interactions is explained by the forcefree fields of electron magnetohydrodynamic (EMHD) vortices.

Enhanced acceleration of energetic ions by oblique shock waves
View Description Hide DescriptionInteractions of energetic ions with a shock wave propagating obliquely to a magnetic field are studied by means of a onedimensional, relativistic, electromagnetic, particle simulation code with full ion and electron dynamics. It is found that energetic ions that barely enter the shock region can be accelerated by the transverse electric field. At the same time, their parallel momenta are increased by the magnetic field. Because of this, after the encounter with the shock wave, some of the energetic ions can move with it for long periods of time during which they undergo the acceleration repeatedly. These particles eventually go away from the shock wave to the upstream region, and their acceleration ceases.

Relativistic motion of a charged particle in a superposition of circularly polarized plane electromagnetic waves and a uniform magnetic field
View Description Hide DescriptionThe relativistic motion of a charged particle in a superposition of circularly polarized plane electromagnetic waves and a uniform magnetic field is studied by deriving an exact solution to the Lorentz force equation of the charged particle. All of the circularly polarized plane electromagnetic waves propagate parallel to the uniform magnetic field. The explicit expressions of the charged particle position, velocity, and energy are obtained for arbitrary initial conditions, and the behavior of the particle motion is studied both analytically and numerically. It is found that for certain initial conditions, the particle gains energy from the waves and the energy gain could reach a maximum value during the time evolution of the charged particle motion in the nonresonance case. In addition, the particle could be accelerated to a much higher energy at the cyclotron resonance, and, in comparison with the situation of a single wave, the use of the superposition wave that consists of a group of circularly polarized plane electromagnetic waves with different frequencies increases the chance of the occurrence of cyclotron resonance for a charged particle, making it easier for the particle to be accelerated. It has also been observed that the interaction of the charged particle with the superposition of electromagnetic waves can be improved significantly at cyclotron resonance when the frequency and phase differences of the waves remain so small that the phases of the waves are in the same quadrant. The results of the present paper are of interest to particle acceleration and heating applications, as well as to basic plasma processes.

Experimental study of classical heat transport in a magnetized plasma
View Description Hide DescriptionA systematic study is made of the axial and transverse heat transport that arises when a microscopic temperature filament is generated, under controlled conditions, in a magnetized plasma of large dimensions. For early times and relatively small temperature gradients the study conclusively demonstrates the twodimensional pattern characteristic of the classical theory based on Coulomb collisions. The full nonlinear dependence of the transverse and axial electron heat conductivities is sampled through temperature changes in the range The dependence on the confining magnetic field is explored over a factor of 3 factor of 10 in transverse conductivity). It is found that under quiescent conditions, the observed behavior agrees with classical theory within the experimental uncertainties. However, over long times and/or for steep temperature gradients, fluctuations develop spontaneously and cause a significant departure from the predictions of the classical theory.

Kinetic theory of dusty plasmas II. Dust–plasma particle collision integrals
View Description Hide DescriptionThe recent results of the kinetic theory of dusty plasmas [V. N. Tsytovich and U. de Angelis, Phys. Plasmas6, 1093 (1999)] are analyzed to assess the importance of new qualitative effects: Inelasticity of dust–plasma particle collisions, deviations from Debye screening and from the dust equilibrium charge in the interactions, the existence of a friction force on plasma particles, and their diffusion in energy. These collective effects depend on the plasmadielectric constant modified by dust charging effects as well as on two new types of dusty plasma responses, related to the fluctuations of plasma particle currents to dust particles. Analytic expressions for the responses are derived and numerical results are given to assess their importance and dependence on dusty plasma parameters. The rate of energy and momentum transfer from plasma particles to dust particles is calculated analytically and numerical results are presented.

Ion flow driven by waves in the ion cyclotron frequency range
View Description Hide DescriptionAn ion kinetic equation is solved by expansion in a parameter corresponding to the small parameter of neoclassical transporttheory and the small parameter of geometrical optics. A slab model with perpendicular stratification of the magnetic field is assumed, and a given rf (radio frequency) field is present with a frequency corresponding to a modest multiple of the ion cyclotron frequency. The rf induced ion flow perpendicular to the magnetic field is calculated. The parallel flow may be large, but it cannot be calculated without an analysis of the electron transport.

Whistlerization and anisotropy in twodimensional electron magnetohydrodynamic turbulence
View Description Hide DescriptionA detailed numerical simulation to understand the turbulent state of the decaying twodimensional electron magnetohydrodynamics is presented. It is observed that the evolved spectrum is comprised of a collection of random eddies and a gas of whistler waves, the latter constituting the normal oscillatory modes of such a model. The whistlerization of the turbulent spectra has been quantified by novel diagnostics. In this work, results are presented only in the regime where the spatial excitation scales are longer than the electron skin depth. Simulations suggest that spectra at short scales are comparatively more whistlerized. The long scale field merely acts as the ambient field along which whistler waves propagate. It is also observed that, in the presence of an external magnetic field, the power spectrum acquires a distinct directional dependence. This anisotropy is dominant at short scales. It is shown that such an anisotropy at short scales results from a cascade mechanism governed by the interacting whistlers waves.
