Volume 18, Issue 6, June 2011
Index of content:

A blobfilament (or simply “blob”) is a magneticfieldaligned plasma structure which is considerably denser than the surrounding background plasma and highly localized in the directions perpendicular to the equilibrium magnetic field B. In experiments and simulations, these intermittent filaments are often formed near the boundary between open and closed field lines, and seem to arise in theory from the saturation process for the dominant edge instabilities and turbulence.Blobs become chargepolarized under the action of an external force which causes unequal drifts on ions and electrons; the resulting polarizationinduced E × B drift moves the blobs radially outwards across the scrapeofflayer (SOL). Since confined plasmas generally are subject to radial or outwards expansion forces (e.g., curvature and B forces in toroidalplasmas),blobtransport is a general phenomenon occurring in nearly all plasmas. This paper reviews the relationship between the experimental and theoretical results on blob formation, dynamics and transport and assesses the degree to which blob theory and simulations can be compared and validated against experiments.
 REVIEW ARTICLE


Convective transport by intermittent blobfilaments: Comparison of theory and experiment
View Description Hide DescriptionA blobfilament (or simply “blob”) is a magneticfieldaligned plasma structure which is considerably denser than the surrounding background plasma and highly localized in the directions perpendicular to the equilibrium magnetic field B. In experiments and simulations, these intermittent filaments are often formed near the boundary between open and closed field lines, and seem to arise in theory from the saturation process for the dominant edge instabilities and turbulence.Blobs become chargepolarized under the action of an external force which causes unequal drifts on ions and electrons; the resulting polarizationinduced E × B drift moves the blobs radially outwards across the scrapeofflayer (SOL). Since confined plasmas generally are subject to radial or outwards expansion forces (e.g., curvature and B forces in toroidalplasmas),blobtransport is a general phenomenon occurring in nearly all plasmas. This paper reviews the relationship between the experimental and theoretical results on blob formation, dynamics and transport and assesses the degree to which blob theory and simulations can be compared and validated against experiments.

 ARTICLES


Basic Plasma Phenomena, Waves, Instabilities

Wave kinetics of relativistic quantum plasmas
View Description Hide DescriptionA quantum kinetic equation, valid for relativistic unmagnetized plasmas, is derived here. This equation describes the evolution of a quantum quasidistribution, which is the Wigner function for relativistic spinless charged particles in a plasma, and it is exactly equivalent to a KleinGordon equation. Our quantum kinetic equation reduces to the Vlasov equation in the classical limit, where the Wigner function is replaced by a classical distribution function. An approximate form of the quantum kinetic equation is also derived, which includes first order quantum corrections. This is applied to electron plasma waves, for which a new dispersion relation is obtained. It is shown that quantum recoil effects contribute to the electron Landau damping with a third order derivative term. The case of high frequency electromagnetic waves is also considered. Its dispersion relation is shown to be insensitive to quantum recoil effects for equilibrium plasma distributions.

Debye shielding in a nonextensive plasma
View Description Hide DescriptionThe phenomenon of Debye Shielding is revisited within the theoretical framework of the Tsallis statistical mechanics. The plasma consists of nonextensive electrons and ions. Both the effective Debye length and the falloff of the electrostatic potential are considered and a parameter study conducted. Owing to electron nonextensivity, the critical Mach number derived from the modified Bohm sheath criterion may become less than unity allowing therefore ions with speed less than ionacoustic speed to enter the sheath from the main body of the plasma. Considering the wide relevance of collective processes, our analysis may be viewed as a first step toward a more comprehensive Debye shielding and electrostaticplasma sheath in nonequilibrium plasmas.

Renormalized nonmodal theory of the kinetic drift instability of plasma shear flows
View Description Hide DescriptionThe linear and renormalized nonlinear kinetic theory of drift instability of plasma shear flow across the magnetic field, which has the Kelvin’s method of shearing modes or the socalled nonmodal approach as its foundation, is developed. The developed theory proves that the timedependent effect of the finite ion Larmor radius is the key effect, which is responsible for the suppression of drift turbulence in an inhomogeneous electric field. This effect leads to the nonmodal decrease of the frequency and growth rate of the unstable drift perturbations with time. We find that turbulent scattering of the ion gyrophase is the dominant effect which determines the extremely rapid suppression of drift turbulence in shear flow.

Shear flow instability in a partiallyionized plasma sheath around a fastmoving vehicle
View Description Hide DescriptionThe stability of ion acoustic waves in a shearedflow, partiallyionized compressible plasma sheath around a fastmoving vehicle in the upper atmosphere, is described and evaluated for different flow profiles. In a compressible plasma with shear flow,instability occurs for any velocity profile, not just for profiles with an inflection point. A secondorder differential equation for the electrostatic potential of excited ion acoustic waves in the presence of electron and ion collisions with neutrals is derived and solved numerically using a shooting method with boundary conditions appropriate for a finite thickness sheath in contact with the vehicle. We consider three different velocityflow profiles and find that in all cases that neutral collisions can completely suppress the instability.

Gyromap for a twodimensional Hamiltonian fluid model derived from Braginskii’s closure for magnetized plasmas
View Description Hide DescriptionWe consider a plasma described by means of a twodimensional fluid model across a constant but nonuniform magnetic field. The dynamical evolution of the density and the vorticity takes into account the interchange instability and magnetic field inhomogeneities. First, in order to describe the finite Larmor radius effects, we apply the gyromap to build a Hamiltonian model with ion temperature from a coldion model. Second, we show that the gyromap is justified using Braginskii’s closure for the stress tensor as well as an apt ordering on the fluctuating quantities.

A theory of nonlocal linear drift wave transport
View Description Hide DescriptionTransport events in turbulent tokamak plasmas often exhibit nonlocal or nondiffusive action at a distance features that so far have eluded a conclusive theoretical description. In this paper a theory of nonlocal transport is investigated through a FokkerPlanck equation with fractional velocity derivatives. A dispersion relation for density gradient driven linear drift modes is derived including the effects of the fractional velocity derivative in the FokkerPlanck equation. It is found that a small deviation (a few percent) from the Maxwelliandistribution function alters the dispersion relation such that the growth rates are substantially increased and thereby may cause enhanced levels of transport.

Phase space coherent structure of charged particles system
View Description Hide DescriptionA class of 3D numerical solutions of VlasovMaxwell equation set is obtained from standard power series solution. Such a class of 3D numerical solutions corresponds to some phase space coherent structures in electron distribution function, which are more complicated than the wellknown hole structure. Based on these solutions, various profiles of related physical quantities are also calculated.

Influence of real gas effects on ablative RayleighTaylor instability in plastic target
View Description Hide DescriptionIn this research, real gas effects on ablative RayleighTaylor instability are investigated in a plastic target. The real gas effects are included by adopting the quotidian equation of state (QEOS) model. Theoretical solutions for both QEOS and ideal gas EOS are obtained and compared, based on a same set of ablation parameters. It is found that when real gas effects are considered, the density gradient becomes less steep than that of ideal gas assumption, even though this cannot be used directly to draw a stabilization conclusion for the real gas effects. Further analysis shows that when real gas effects are considered, lower ∂p/∂T in the dense shell region has the effect of stabilization, whereas the dependence of the internal energy on the density, lower specific heat (at constant volume) in the dense shell region, and higher specific heat in the lowdensity ablation region contribute to stronger destabilization effects. Overall, when real gas effects are considered, the destabilization effects are dominant for long wavelength perturbations, and the growth rates become much higher than the results of ideal gas assumption. In our specific case, the maximum relative error reaches 18%.

Nonlinear evolution of two fastparticledriven modes near the linear stability threshold
View Description Hide DescriptionA system of two coupled integrodifferential equations is derived and solved for the nonlinear evolution of two waves excited by the resonant interaction with fast ions just above the linear instability threshold. The effects of a resonant particle source and classical relaxation processes represented by the Krook, diffusion, and dynamical friction collision operators are included in the model, which exhibits different nonlinear evolution regimes, mainly depending on the type of relaxation process that restores the unstable distribution function of fast ions. When the Krook collisions or diffusion dominate, the wave amplitude evolution is characterized by modulation and saturation. However, when the dynamical friction dominates, the wave amplitude is in the explosive regime. In addition, it is found that the finite separation in the phase velocities of the two modes weakens the interaction strength between the modes.

The effects of strong temperature anisotropy on the kinetic structure of collisionless slow shocks and reconnection exhausts. I. Particleincell simulations
View Description Hide DescriptionA 2D Riemann problem is designed to study the development and dynamics of the slow shocks that are thought to form at the boundaries of reconnection exhausts. Simulations are carried out for varying ratios of normal magnetic field to the transverse upstream magnetic field (i.e., propagation angle with respect to the upstream magnetic field). When the angle is sufficiently oblique, the simulations reveal a large firehosesense temperatureanisotropy in the downstream region, accompanied by a transition from a coplanar slow shock to a noncoplanar rotational mode. In the downstream region the firehose stability parameter tends to plateau at 0.25. This balance arises from the competition between counterstreaming ions, which drive ɛ down, and the scattering due to ion inertial scale waves, which are driven unstable by the downstream rotational wave. At very oblique propagating angles, 2D turbulence also develops in the downstream region.

Discontinuous Galerkin particleincell simulation of longitudinal plasma wave damping and comparison to the Landau approximation and the exact solution of the dispersion relation
View Description Hide DescriptionWe present results showing the measured Landau damping rate using a highorder discontinuous Galerkin particleincell (DGPIC) [G. B. Jacobs and J. S. Hesthaven, J. Comput. Phys. 214, 96 (2006)] method. We show that typical damping rates measured in particleincell(PIC) simulations can differ significantly from the linearized Landau damping coefficient and propose a simple numerical method to solve the plasma dispersion function exactly for moderate to high damping rates. Simulation results show a high degree of agreement between the highorder PIC results and this calculated theoretical damping rate.

Electronrich sheath dynamics. I. Transient currents and sheathplasma instabilities
View Description Hide DescriptionThe evolution of an electronrich sheath on a plane electrode has been investigated experimentally. A rapidly rising voltage is applied to a plane gridded electrode in a weakly ionized, low temperature, and fieldfree discharge plasma. Transient currents during the transition from ionrich to electronrich sheath are explained including the current closure. Timeresolved currentvoltage characteristics of the electrode are presented. The time scale for the formation of an electronrich sheath is determined by the ion dynamics and takes about an ion plasma period. When the ions have been expelled from the sheath a highfrequency sheathplasma instability grows. The electric field contracts into the electronrich sheath which implies that the potential outside the sheath drops. It occurs abruptly and creates a large current pulse on the electrode which is not a conduction but a displacement current. The expulsion of ions from the vicinity of the electrode lowers the electron density, electrodecurrent, and the frequency of the sheathplasma oscillations. Electron energization in the sheath creates ionization which reduces the space charge density, hence sheathelectric field. The sheathplasma instability is weakened or vanishes. The ionization rate decreases, and the sheathelectric field recovers. A relaxation instability with repeated current transients can arise which is presented in a companion paper. Only for voltages below the ionization potential a quiescent electron richsheath is observed.

Electronrich sheath dynamics. II. Sheath ionization and relaxation instabilities
View Description Hide DescriptionInstabilities in an electronrich sheath on a plane electrode in a discharge plasma have been investigated experimentally. The highfrequency sheathplasma instability near the electron plasma frequency is observed. With increasing dc voltage, the instability exhibits bursty amplitude and frequency jumps. The electrodecurrent shows spikes and jumps, and the plasma potential near the electrode shows large fluctuations below the ion plasma frequency. Sheathionization has been identified as the cause for these low frequency instabilities. Electrons energized in the sheath produce ions which reduce the space charge in the sheath and the electric field and the ionization rate. Ions are ejected from the sheath which increases the charge density, electric field, and ionization rate. The positive feedback between these processes leads to a relaxation instability whose time scale is determined by ion inertia and ionization rates. The associated density and potential fluctuations affect the amplitude and frequency of the sheathplasma instability. When the sheathionization rate exceeds the ion losses, the sheath expands into an anode plasma or “fireball.” The potential drop across the sheath decreases and the sheathplasma instability vanishes. The electrode currentvoltage characteristics develop a region of negative conductance. For short grid voltage pulses, the ionization effects can be avoided.

Neoclassical transport and plasma mode damping caused by collisionless scattering across an asymmetric separatrix
View Description Hide DescriptionPlasma loss due to apparatus asymmetries is a ubiquitous phenomenon in magnetic plasma confinement. When the plasma equilibrium has locallytrapped particle populations partitioned by a separatrix from one another and from passing particles, the asymmetry transport is enhanced. The trapped and passing particle populations react differently to the asymmetries, leading to the standard and transport regimes of superbanana orbit theory as particles collisionally scatter from one orbit type to another. However, when the separatrix is itself asymmetric, particles can collisionlessly transit from trapped to passing and back, leading to the enhanced diffusion and mobility that is calculated here. The effect of this collisionless scattering across an asymmetric separatrix on the damping rate of trapped particle diocotron modes is also considered.

Nonlinear Phenomena, Turbulence, Transport

Threedimensional electromagnetic strong turbulence. I. Scalings, spectra, and field statistics
View Description Hide DescriptionThe first fully threedimensional (3D) simulations of largescale electromagnetic strong turbulence (EMST) are performed by numerically solving the electromagnetic Zakharov equations for electron thermal speeds with . The results of these simulations are presented, focusing on scaling behavior, energy density spectra, and field statistics of the Langmuir (longitudinal) and transverse components of the electric fields during steadystate strong turbulence, where multiple wave packets collapse simultaneously and the system is approximately statistically steady in time. It is shown that for strong turbulence is approximately electrostatic and can be explained using the electrostatic twocomponent model. For the powerlaw behaviors of the scalings, spectra, and field statistics differ from the electrostatic predictions and results because _{ e }/c is sufficiently high to allow transverse modes to become trapped in density wells. The results are compared with those of past 3D electrostatic strong turbulence (ESST) simulations and 2D EMST simulations. For number density perturbations, the scaling behavior, spectra, and field statistics are shown to be only weakly dependent on _{ e }/c, whereas the Langmuir and transverse scalings, spectra, and field statistics are shown to be strongly dependent on _{ e }/c. Threedimensional EMST is shown to have features in common with 2D EMST, such as a twocomponent structure and trapping of transverse modes which are dependent on _{ e }/c.

Impact of a shear flow on double tearing nonlinear dynamics
View Description Hide DescriptionThe dynamics of global reconnection in the presence of a poloidal shear flow located in between magnetic islands is investigated. Different linear and nonlinear regimes are identified depending on the resistivity, the equilibrium velocity amplitude, and the distance between the loworder resonant surfaces. It is found that nonlinearly, the shear flow can significantly delay DTM generation and global reconnection. It is shown that this delay is linked to a symmetry breaking imposed by the shear flow and the generation of mean poloidal flows in the resistive layers. It is also found that turbulence can be generated by KelvinHelmholtz instability in between the resonance layers and enhance magnetic reconnection processes.

Parameteric studies of nonlinear oblique magnetosonic waves in twoionspecies plasmas
View Description Hide DescriptionThe study of the effects of ion composition on perpendicular magnetosonic waves in twoionspecies plasmas [M. Toida, H. Higashino, and Y. Ohsawa, J. Phys. Soc. Jpn. 76, 104052 (2007)] is extended to include oblique waves. First, the conditions necessary for KdV equations for low and highfrequency modes to be valid are analytically obtained. The upper limit of the amplitude of the lowfrequencymode pulse is expressed as a function of the propagation angle , density ratio, and cyclotron frequency ratio of the two ion species. Next, with electromagnetic particle simulations, the nonlinear evolution of a longwavelength lowfrequencymode disturbance is examined for various s in two plasmas with different ion densities and cyclotron frequency ratios, and the theory for the lowfrequencymode pulse is confirmed. It is also shown that if the pulse amplitude exceeds the theoretical value of the upper limit of the amplitude, then shorterwavelength low and highfrequencymode waves are generated.

Effect of nonthermal electrons on the propagation characteristics and stability of twodimensional nonlinear electrostatic coherent structures in relativistic electron positron ion plasmas
View Description Hide DescriptionTwodimensional propagation of nonlinear ion acoustic shock and solitary waves in an unmagnetized plasma consisting of nonthermal electrons, Boltzmannian positrons, and singly charged hot ions streaming with relativistic velocities are investigated. The system of fluid equations is reduced to KadomtsevPetviashviliBurgers and KadomtsevPetviashvili (KP) equations in the limit of small amplitude perturbation. The dependence of the ion acoustic shock and solitary waves on various plasma parameters are explored in detail. Interestingly, it is observed that increasing the nonthermal electron population increases the wave dispersion which enervates the strength of the ion acoustic shock wave; however, the same effect leads to an enhancement of the soliton amplitude due to the absence of dissipation in the KP equation. The present investigation may be useful to understand the twodimensional propagation characteristics of small but finite amplitude localized shock and solitary structures in planetary magnetospheres and auroral plasmas where nonthermal populations of electrons have been observed by several satellite missions.

Propagation of threedimensional electronacoustic solitary waves
View Description Hide DescriptionTheoretical investigation is carried out for understanding the properties of threedimensional electronacoustic waves propagating in magnetized plasma whose constituents are cold magnetized electron fluid, hot electrons obeying nonthermal distribution, and stationary ions. For this purpose, the hydrodynamic equations for the cold magnetized electron fluid, nonthermal electron density distribution, and the Poisson equation are used to derive the corresponding nonlinear evolution equation, Zkharov–Kuznetsov (ZK) equation, in the small but finite amplitude regime. The ZK equation is solved analytically and it is found that it supports both solitary and blowup solutions. It is found that rarefactive electronacoustic solitary waves strongly depend on the density and temperature ratios of the hottocold electron species as well as the nonthermal electron parameter. Furthermore, there is a critical value for the nonthermal electron parameter, which decides whether the electronacoustic solitary wave’s amplitude is decreased or increased by changing various plasma parameters. Importantly, the change of the propagation angles leads to miss the balance between the nonlinearity and dispersion; hence, the localized pulses convert to explosive/blowup pulses. The relevance of this study to the nonlinear electronacoustic structures in the dayside auroral zone in the light of Vikingsatellite observations is discussed.
