Volume 10, Issue 5, May 2003
 LETTERS


Hysteresislike effects in gyrotron oscillators
View Description Hide DescriptionSpecial experiments devoted to studying hysteresis in gyrotronoscillators have been performed for the first time. Clear hysteresislike effects with respect to variation of the cathode voltage have been observed in the mode competition scenario of the Forschungszentrum Karlsruhe coaxial gyrotron [B. Piosczyk et al., IEEE Trans. Plasma Sci. 30, 818 (2002)] and with respect to variation of the magnetic field and voltage in a singlemode operation of the Fukui IV gyrotron [T. Idehara et al., Int. J. Infrared Millim. Waves19, 793 (1998)]. The observed phenomena are explained theoretically.

Unexpected axial asymmetry in radiated power from hightemperature dynamichohlraum xray sources
View Description Hide DescriptionRadiation from the interior of a dynamic hohlraum within a wirearray Z pinch is used to generate highpower xray pulses in both the up and down axial directions through radiation exit holes (REHs) in the anode and cathode, respectively. Despite a concerted effort to ensure a symmetrical updown configuration, the measured peak top radiated power remained about twice that of the bottom (with similar total radiated energies from each REH), as compared to current simulations that predict equal powers. This large asymmetry suggests the need for improved physics models and simulation capabilities.

Observations of low frequency oscillations due to transverse sheared flows
View Description Hide DescriptionThis Letter details observations of low frequency (i.e., where is the ion cyclotron frequency), coherent instabilities in the Auburn Linear Experiment for Instability Studies (ALEXIS). In the ALEXIS device, which is a 1.8m long magnetized plasma column, the observed instabilities are excited by the presence of sheared flows in the plasma that are transverse to the axial magnetic field. The instabilities have long azimuthal wavelengths and are localized in the plasma to regions where the sheared flow is maximized and are anticorrelated to both density gradients and field aligned currents.

Kinetic ballooning stability of internal transport barriers in tokamaks
View Description Hide DescriptionSteep plasma pressure gradient can be stably sustained in tokamaks if the magnetic shear is weak, either positive or negative. A fully kinetic integral equation code has been developed to investigate stability of the drift and ballooning modes in tokamaks. For small shear where the magnetohydrodynamicballooning mode is known to be stable, the kinetic ballooning mode is stable only if the pressure gradient exceeds a threshold.
 Top

 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Dustacoustic wave instability at the diffuse edge of radio frequency inductive lowpressure gas discharge plasma
View Description Hide DescriptionA spontaneous excitation of a grain density wave in a dusty cloud of monodisperse particles suspended at the diffuse edge of an rf inductive gas discharge has been discovered. The main physical parameters of this wave (phase velocity, wavelength, and growth rate) and of the background plasma (distributions of the electron density, electron temperature, and space potential) were measured. A theoreticalmodel of the observable phenomenon based on the theory of dust acoustic waves in a collisional dusty plasma correlates well with the experimental data in a broad range of experimental conditions. The influence of a varying dust grain charge on the development of the observed dusty plasmainstability has been analyzed. It is shown that the necessary condition for the instability excitation is the availability of a permanent electrical field ⩾3 V/cm) in the dusty cloud region.

Nonneutral plasma equilibria, trapping, separatrices, and separatrix crossing in magnetic mirrors
View Description Hide DescriptionThe equilibria of nonneutral plasmas confined in Penning–Malmberg traps with axial varying (mirror)magnetic fields exhibit numerous unusual features, including potential differences along field lines, plasma density variations, trapped particles in both the high and low field regions, and unusual separatrices between trapped and untrapped particles. Mirror fields play prominent roles in a number of recent experiments, and overly simplistic models of the equilibria can lead to errors in the interpretation of experimental results.

Nonthermal effects on occurrence scattering time in generalized Lorentzian distribution plasmas
View Description Hide DescriptionNonthermal effects on the occurrence time advance for the elastic electron–ion collisions in generalized Lorentzian (kappa) distribution plasmas are investigated using the firstorder eikonal analysis. The electron–ion interaction potential in the Lorentzian plasmas is obtained by the introduction of the plasma dielectric function The semiclassical straightline trajectory method is applied to the path of the projectile electron in order to obtain the eikonal phase and the scattering amplitude as functions of the impact parameter, Debye length, and spectral index (κ), and collision energy. The occurrence time advance is found to be increased with the increasing scattering angle. It is found that the occurrence time advance has the maximum value for the case of the thermal Maxwellian distribution plasmas. It is also found that the occurrence time advance decreases with the decreasing spectral index, i.e., increasing the nonthermal character of the plasma.

Equilibrium molecular dynamics simulations of the transport coefficients of the Yukawa one component plasma
View Description Hide DescriptionEquilibrium moleculardynamics simulations in the microcanonical ensemble have been performed to obtain the thermal conductivity and the two viscosities of the Yukawa onecomponent plasma from the Kubo formulas. The expressions of the Kubo currents (pressure tensor and energy current) which enter these formulas are derived in terms of Ewald sums. The simulation results for the transport coefficients are compared with the predictions of the Chapman–Enskog theory which has been solved numerically.

Decay of trappedparticle asymmetry modes in nonneutral plasmas in a Malmberg–Penning trap
View Description Hide DescriptionThe mechanism for the strong damping of diocotronlike azimuthal trappedparticle asymmetry modes in a Malmberg–Penning trap is investigated with a detailed threedimensional particleincellcomputer simulation. The modes are created by a voltage squeeze from a middetector ring followed by a displacement of trapped particles in opposite directions on either side of the ring. The voltage squeeze creates a population of particles confined to half the trap length (trapped) and a population of particles that move longitudinally along the full length of the cylinder (untrapped). The damping of the modes is found to be the result of radial transport relative to the mode (charge) center caused by transitions of particles from untrappedtotrapped states induced by diffusion of the particles in velocity space. The transport is the immediate consequence of a difference in dynamical orbits for trapped and untrapped particles. The random walk in velocity space results in particles repeatedly changing state from trapped to untrapped and back. The dependence of the mode frequency and the exponential decay constant are explored as a function of squeeze voltage, magnetic field, and temperature in order to establish scaling behavior.

Simple modes of thin oblate nonneutral plasmas
View Description Hide DescriptionLagrangian variables are used to describe linear and nonlinear oscillations of a magnetized nonneutral plasma slab in a harmonic trap, for slab width larger or comparable to the Debye length. The plasma particles move along the magnetic field lines, so that the oscillations are onedimensional. The oscillation spectrum is found analytically, and the thermal corrections to the frequencies are calculated in a nonperturbative manner. Simple exact nonlinear solutions for the loworder modes are also obtained.

Currents and shear Alfvén wave radiation generated by an exploding laserproduced plasma: Perpendicular incidence
View Description Hide DescriptionExamples of one plasma expanding into another and the consequent radiation of wave energy are abundant in both nature and the laboratory. This work is an experimental study of the expansion of a dense laserproduced plasma (initially, into a magnetized background plasma capable of supporting Alfvén waves. The experiments are carried out on the upgraded Large Plasma Device (LAPD) at UCLA [W. Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)]. It has been observed that the presence of a background plasma allows laserplasma charge separation to occur that would otherwise be limited by large ambipolar fields. This charge separation results in the creation of current structures which radiate shear Alfvén waves. The waves propagate away from the target and are observed to become plasma column resonances. Conditions for increased current amplitude and wave coupling are investigated.

Quasiperpendicular magnetosonic waves in a multiionspecies plasma
View Description Hide DescriptionPropagation of oblique magnetosonic waves in a multiionspecies plasma is studied with theory and threefluid simulations. First, the linear dispersion relation of the highfrequency mode, the upper branch of magnetosonic waves, is examined in detail. Next, nonlinear evolution equation for this mode is derived. This mode can propagate as compressive or rarefactive solitary pulses, depending on the propagation angle. Ion acceleration due to these pulses and resultant pulse damping are analytically discussed. These processes are then demonstrated by fluid simulations for a plasma consisting of H, He, and electrons; either rarefactive or compressive pulses are gradually damped owing to the slight ion acceleration.

Stability analysis of hollow electron columns including compressional and thermal effects: Initial value treatment
View Description Hide DescriptionThe diocotron spectrum for a simplified fluid model of Malmberg–Penning traps that includes compressional effects due to end curvature with finite temperature is investigated analytically. The general initial value treatment of the mode is performed and the algebraic growth proportional to is recovered when the plasma length profile is the integrable one as introduced by Delzanno et al. [Phys. Plasmas9, 4863 (2002)]. Then, nonintegrable length profiles slightly different from the integrable one are considered (the difference being characterized by ε). It is shown that complex discrete eigenfrequencies appear near the edge of the continuous spectrum of the mode. Depending on the sign of ε, these discrete eigenfrequencies may or may not lead to exponential instability. The discrete eigenfrequency scales as with respect to the upper edge of the continuum. This confirms and explains the numerical observations of Finn et al. [Phys. Plasmas6, 3744 (1999)] and Delzanno et al. [Phys. Plasmas9, 4863 (2002)] and proves that the scaling law is a generic property of the modified driftPoisson model near the edge of the continuum. The same general treatment is also applied to the diocotron spectrum in a cylindrical Malmberg–Penning trap with an additional coaxial inner conductor of radius

Measurement of the ion drag force on falling dust particles and its relation to the void formation in complex (dusty) plasmas
View Description Hide DescriptionExperiments on the quantitative determination of the weaker forces (ion drag, thermophoresis, and electric field force) on freefalling dust particles in a rf discharge tube are presented. The strongest force, gravity, is balanced by gas friction and the weaker forces are investigated in the radial (horizontal) plane. Under most discharge conditions, the particles are found to be expelled from the central plasma region. A transition to a situation where the falling particles are focused into the plasma center is observed at low gas pressures using small particles. These investigations allow a quantitative understanding of the mechanism of unwanted dustfree areas (socalled voids) in dusty plasmas under microgravity. Good quantitative agreement with standard models of the ion drag is found.
 Nonlinear Phenomena, Turbulence, Transport

Periodic driving of plasma turbulence
View Description Hide DescriptionTools to characterize three important characteristics of turbulence are proposed: Structureswithinstructures, intermittent amplitude bursting, and turbulence complexity. These tools are applied to show that the injection of a rf wave into the plasma confined on the Tokamak Chauffage Alfvén Bresilién (TCABR) [R. M. O. Galvao, V. Bellintani, Jr., R. D. Bengtson et al., Plasma Phys. Controlled Fusion43, A299 (2001)] decreases plasma edge turbulence, although not completely destroy it, by destroying the only two types of time structures found in the data. Both structures present multiscaling spectra, with infinitely many possible scalings. So, according to this analysis, complexity of this turbulence is mainly due to the multiscaling character of the oscillations.

Global extended magnetohydrodynamic studies of fast magnetic reconnection
View Description Hide DescriptionRecent experimental and theoretical results have led to two lines of thought regarding the physical processes underlying fast magnetic reconnection. One is based on the traditional Sweet–Parker model but replaces the Spitzer resistivity with an enhanced resistivity caused by electron scattering by ion acoustic turbulence. The other includes the finite gyroradius effects that enter Ohm’s law through the Hall and electron pressure gradient terms. A twodimensional numerical study, conducted with a new implicit parallel twofluid code, has helped to clarify the similarities and differences in predictions between these two models. The former yields resistivitydependentreconnection with a thick, moderateaspectratio current sheet. If the sheet thickness is less than or comparable to the ion skin depth, it is verified that the Hall effect will predominate [Shay et al., Geophys. Res. Lett. 26, 2163 (1999)], producing true fast reconnection with a microscopic current sheet of unit aspect ratio and a distorted outofplane magnetic field [Mandt et al., Geophys. Res. Lett. 21, 73 (1994)].

Kinetic instabilities of thin current sheets: Results of twoandonehalfdimensional Vlasov code simulations
View Description Hide DescriptionNonlinear triggering of the instability of thin current sheets is investigated by twoandonehalf dimensional Vlasov code simulations. A global driftresonant instability (DRI) is found, which results from the lowerhybriddrift waves penetrating from the current sheet edges to the center where they resonantly interact with unmagnetized ions. This resonant nonlinear instability grows faster than a Kelvin–Helmholtz instability obtained in previous studies. The DRI is either asymmetric or symmetric mode or a combination of the two, depending on the relative phase of the lowerhybriddrift waves at the edges of the current sheet. With increasing particle mass ratio the wavenumber of the fastestgrowing mode increases as and the growth rate of the DRI saturates at a finite level.

Frequency dependence of asymmetryinduced transport in a nonneutral plasma trap
View Description Hide DescriptionA key prediction of the theory of asymmetryinduced transport is that the particle flux will be dominated by particles that move in resonance with the asymmetry. For the case of a timevarying asymmetry, the resonance condition is where is the axial velocity, is the plasma length, is the rotation frequency, and ω, and are the asymmetry frequency, azimuthal wavenumber, and axial wavenumber, respectively. Data are presented from experiments on a low density trap in which ω, and are varied and the resulting radial particle flux is measured. The experiments show a resonance in the flux similar to that predicted by theory. The peak frequency of this resonance increases with and varies with in qualitative agreement with theory, but quantitative comparisons between experiment and theory show serious discrepancies.

Simulations and theories of relativistic ion cyclotron instabilities driven by MeV alpha particles in thermal deuterium plasmas
View Description Hide DescriptionThe harmonic relativistic ion cyclotron instabilities driven by MeV alpha particles in magnetized thermal deuterium plasmas are studied with a perturbation theory, a kinetic theory, and particleincell simulations. Due to the mass deficit of alphas, the resonantharmonic cyclotron frequency of the alphas is not smaller than that of the deuterons such that the low harmonics are linearly stabilized by the deuterons. However, the thermal deuterons behave as a coldgyrostream plasma at high harmonics so that, by including relativistic streaming in gyrophase, the alpha particles can drive the high harmonic electrostatic ion cyclotron instability, that is in a quadratic form, although their Lorentz factor is very close to unity (e.g., for 3.5 MeV alphas). The dielectric of the first deuteron cyclotron harmonic determines the alpha harmonic number threshold for instability; the quadratic instability occurs in the lowerhybrid frequency regime. As in a sharp contrast to the nonresistive thermalization of MeV protons due to the twogyrostream instability dominating at low harmonics, the high harmonicinteraction between the unstable waves and the alphas becomes selective; only alpha particles with its perpendicular momentum above a threshold are involved and the interaction stops when they are slowed down to the threshold. The resultant energy spectrum is in a shape close to the profile of the theoretical interaction strength between the alpha particles and the dominating wave mode; the alpha particles almost loss no net energy. Newborn alpha particles experience a similar selective gyrobroadening process in a shorter time scale. A simple explanation for the selective gyrobroadening based on the wave–alpha interaction calculated from the perturbation theory is given.

Hamiltonian canonical formulation of Hallmagnetohydrodynamics: Toward an application to weak turbulence theory
View Description Hide DescriptionThe different levels of description of fluid media [e.g., magnetohydrodynamics(MHD), Hallmagnetohydrodynamics, bifluid,…] are commonly known under the form of Newtonian systems of equations. Nevertheless, this form proves to be illsuited to derive a fully analytical weak turbulence theory of these media, due to the wellknown complexity of the calculations implied. For such studies, therefore, a more appropriate mathematical frame needs to be found and this is shown to be the Hamiltonian formalism, even though it can often appear difficult to handle. The goal of this paper is to look for Hamiltonian formulations for the different levels of the fluid description of a plasma using the variational principle. Starting from the bifluid system, it is shown that such a formulation can be obtained by combining the Lagrangians already used for describing: (i) the motion of a charged particle in an electromagnetic field; (ii) the evolution of an electromagnetic field in presence of sources; (iii) the motion of a neutral fluid (Clebsch variables). The equivalence of the obtained description in terms of the generalizedClebsch variables to the familiar Newtonian formulation is discussed. It is shown that each solution of the Hamiltonian system is also a solution for the Newtonian one, but that the converse is not true. The origin and the implication of this restriction are discussed. Reducing the Hamiltonian formulation obtained for the bifluid system to lower orders of the fluid approximations is then shown to be mandatory when one tries to obtain analytical results for linear waves and nonlinear wave–wave couplings. It is shown that this goal can be reached in two steps. The first one leads to a “reduced bifluid” system, which is identical to the bifluid one when the displacement current is neglected but the electron inertia is still working. The number of linear modes then goes down from six to three. The second step, leading to the HallMHD system, consists in neglecting the electron mass. It is demonstrated that the only four generalized Clebsch variables are sufficient to describe the full HallMHD dynamics. Some future applications of such a powerful formalism are outlined.