Volume 19, Issue 4, April 2012
Index of content:

Ion temperature gradientturbulenttransport in the large helical device(LHD) is investigated by means of gyrokinetic simulations in comparison with the experimental density fluctuation measurements of ionscale turbulence. The local gyrokinetic Vlasov simulations are carried out incorporating full geometrical effects of the LHD configuration, and reproduce the turbulenttransport levels comparable to the experimental results. Reasonable agreements are also found in the poloidal wavenumber spectra of the density fluctuations obtained from the simulation and the experiment. Numerical analysis of the spectra of the turbulent potential fluctuations on the twodimensional wavenumber space perpendicular to the magnetic field clarifies the spectral transfer into a high radial wavenumber region which correlates with the regulation of the turbulenttransport due to the zonal flows. The resultant transport levels at different flux surfaces are expressed in terms of a simple linear relation between the transport coefficient and the ratio of the squared turbulent potential fluctuation to the averaged zonal flow amplitude.
 ARTICLES


Basic Plasma Phenomena, Waves, Instabilities

Particleincell simulations of beamdriven electrostatic waves in a plasma
View Description Hide DescriptionUsing a particleincell simulation, the characteristics of electrostatic waves are investigated in a threeelectron component plasma including an electron beam. A Maxwellian distribution is used to describe the electron velocities. Three electrostatic modes are excited, namely electron plasma, electron acoustic, and beamdriven waves. These modes have a broad frequency spectrum and have been associated with intense broadband electrostatic noise observed in the Earth’s auroral zone. The simulation results compare well with analytical dispersion and growth rate relations. This agreement serves to validate the simulation technique.

A simulation approach of highfrequency electrostatic waves found in Saturn’s magnetosphere
View Description Hide DescriptionUsing a particleincell simulation, the characteristics of electron plasma and electron acoustic waves are investigated in plasmas containing an ion and two electron components. The electron velocities are modeled by a combination of two distributions. The model applies to the extended plasma sheet region in Saturn’smagnetosphere where the cool and hot electronvelocities are found to have low indices, and . For such low values of and , the electron plasma and electron acoustic waves are coupled. The model predicts weakly damped electron plasma waves while electron acoustic waves should also be observable, although less prominent.

Numerical investigation of a compressible gyrofluid model for collisionless magnetic reconnection
View Description Hide DescriptionIon Larmor radius effects on collisionless magnetic reconnection in the presence of a guide field are investigated by means of numerical simulations based on a gyrofluid model for compressible plasmas. Compressibility along the magnetic field is seen to favour the distribution of ion guiding center density along the neutral line, rather than along the separatrices, unlike the electron density. On the other hand, increasing ion temperature reduces the intensity of localized ion guiding center flows that develop in the direction parallel to the guide field. Numerical simulations suggest that the width of these barshaped velocity layers scale linearly with the ion Larmor radius. The increase of ion temperature radius causes also a reduction of the electron parallel velocity. As a consequence, it is found that the cusplike current profiles distinctive of nondissipative reconnection are strongly attenuated. The field structures are interpreted in terms of the behavior of the four topological invariants of the system. Two of these are seen to behave similarly to invariants of simpler models that do not account for parallel ion flow. The other two exhibit different structures, partly as a consequence of the small electron/ion mass ratio. The origin of these invariants at the gyrokinetic level is also discussed. The investigation of the field structures is complemented by an analysis of the energetics of the system.

Theoretical and numerical studies of wavepacket propagation in tokamak plasmas
View Description Hide DescriptionTheoretical and numerical studies of wavepacket propagation are presented to analyze the time varying 2D mode structures of electrostatic fluctuations in tokamak plasmas, using general flux coordinates. Instead of solving the 2D wave equations directly, the solution of the initial value problem is used to obtain the 2D mode structure, following the propagation of wavepackets generated by a source and reconstructing the time varying field. As application, the 2D WKB method is applied to investigate the shaping effects (elongation and triangularity) of tokamak geometry on the lower hybrid wave propagation and absorption. Meanwhile, the mode structure decomposition (MSD) method is used to handle the boundary conditions and simplify the 2D problem to two nested 1D problems. The MSD method is related to that discussed earlier by Zonca and Chen [Phys. Fluids B 5, 3668 (1993)] and reduces to the wellknown “ballooning formalism” [J. W. Connor et al., Phys. Rev. Lett. 40, 396 (1978)], when spatial scale separation applies. This method is used to investigate the time varying 2D electrostatic ion temperature gradient(ITG)mode structure with a mixed WKBfullwave technique. The time varying field pattern is reconstructed, and the time asymptotic structure of the wavepacket propagation gives the 2D eigenmode and the corresponding eigenvalue. As a general approach to investigate 2D mode structures in tokamak plasmas, our method also applies for electromagnetic waves with general source/sink terms either by an internal/external antenna or a nonlinear wave interaction with zonal structures.

Formation and dynamics of electrostatic solitary waves associated with relativistic electron beam
View Description Hide DescriptionProperties of nonlinear electrostatic solitary waves in a plasma are analyzed by using the hydrodynamic model for electrons,positrons, and relativistic electron beam. For this purpose, the Kadomtsev–Petviashvili (KP) equation has been derived and its analytical solution is presented. It is found that the nonlinear solitary structures can propagate as slow and fast modes. The dependence of these modes on the plasma parameters is defined numerically. Furthermore, positive and negative electrostatic solitary structures can exist. In order to show that the characteristics of the solitary wave profile are influenced by the plasma parameters, the relevant numerical analysis of the KP equation is obtained. The electrostatic solitary waves, as predicted here, may be associated with the nonlinear structures caused by the interaction of relativistic jets with plasma medium, such as in the active galactic nuclei and in the magnetosphere of collapsing stars.

Parametric decay of laser into two electromagnetic waves in a rippled density plasma
View Description Hide DescriptionThe presence of a density ripple in an unmagnetized plasma is shown to allow parametric decay of an electromagnetic wave into two electromagnetic waves, which is otherwise not allowed due to wave number mismatch between the decay waves. The static density ripple accounts for the mismatch. The decay occurs at plasma densities below the quarter critical density and the decay electromagnetic waves propagate at angles to the pump laser. The requisite ripple wave number q increases with the increase in pump wave frequency. However, as the ripple orientation with respect to the pump increases, q decreases. The growth rate for the parametric instability initially decreases with the frequency of the lower frequency electromagnetic wave, attains a minimum and then increases. The growth rate is higher for lower values of .

Short scale electrostatic vortices driven by electrons sheared flow parallel to external magnetic field in heavier ion plasmas
View Description Hide DescriptionNonlinear equations for an electrostatic perturbation in hybrid frequency range are derived in a magnetized heavier ion plasma assuming that the electrons are flowing with a sheared velocity along the initial external constant magnetic field. As a result of this current, the total zeroorder magnetic field becomes sheared as . Such a system can give rise to unstable electrostatic waves under certain conditions. The solutions of the nonlinear equations are obtained in the form of dipolar vortices, which can play an important role in plasma transport across field lines. This work can be useful for the future experiments on sheared electron flows.

Twodimensional simulations of nonlinear beamplasma interaction in isotropic and magnetized plasmas
View Description Hide DescriptionNonlinear interaction of a low density electron beam with an uniform plasma is studied using twodimensional particleincell simulations. We focus on formation of coherent phase space structures in the case, when a wide twodimensional wave spectrum is driven unstable, and we also study how nonlinear evolution of these structures is affected by the external magnetic field. In the case of isotropic plasma, nonlinear buildup of filamentation modes due to the combined effects of twostream and oblique instabilities is found to exist and growth mechanisms of secondary instabilities destroying the BernsteinGreenKruskal–type nonlinear wave are identified. In the weak magnetic field, the energy of beamexcited plasma waves at the nonlinear stage of beamplasma interaction goes predominantly to the shortwavelength upperhybrid waves propagating parallel to the magnetic field, whereas in the strong magnetic field, the spectral energy is transferred to the electrostatic whistlers with oblique propagation.

Microinstabilities at perpendicular collisionless shocks: A comparison of full particle simulations with different ion to electron mass ratio
View Description Hide DescriptionA full particle simulation study is carried out for studying microinstabilities generated at the shock front of perpendicular collisionless shocks. The structure and dynamics of shock waves are determined by Alfvén Mach number and plasma beta, while microinstabilities are controlled by the ratio of the upstream bulk velocity to the electron thermal velocity and the plasmatocyclotron frequency. Thus, growth rates of microinstabilities are changed by the iontoelectron mass ratio, even with the same Mach number and plasma beta. The present twodimensional simulations show that the electron cyclotron drift instability is dominant for a lower mass ratio, and electrostatic electron cyclotron harmonicwaves are excited. For a higher mass ratio, the modified twostream instability is dominant and oblique electromagneticwhistler waves are excited, which can affect the structure and dynamics of collisionless shocks by modifying shock magnetic fields.

Numerical simulations of separatrix instabilities in collisionless magnetic reconnection
View Description Hide DescriptionElectron scale dynamics of magnetic reconnection separatrix jets is studied in this paper. Instabilities developing in directions both parallel and perpendicular to the magnetic field are investigated. Implicit particleincell simulations with realistic electrontoion mass ratio are complemented by a set of small scale high resolution runs having the separatrix force balance as the initial condition. A special numerical procedure is developed to introduce the force balance into the small scale runs. Simulations show the development of streaming instabilities and consequent formation of electron holes in the parallel direction. A new electron jet instability develops in the perpendicular direction. The instability is closely related to the electron MHD KelvinHelmholtz mode and is destabilized by a flow, perpendicular to magnetic field at the separatrix. Tearing instability of the separatrix electron jet is modulated strongly by the electron MHD KelvinHelmholtz mode.

Kinetic simulations of the structures of magnetic island in multiple X line guide field reconnection
View Description Hide DescriptionMagnetic reconnection is one of the most important processes in astrophysical, space, and laboratory plasmas, and magnetic island is an important feature in reconnection. Therefore, identifying the structures of magnetic island is crucial to improving our understanding of magnetic reconnection. Using twodimensional (2D) particleincell(PIC) simulations, we demonstrate that the outofplane magnetic field has a dip in the center of magnetic island, which is formed during multiple X line guide field reconnection. Such structures are considered to be produced by the current system in the magnetic island. At the edge of the magnetic island, there exists a current antiparallel to the inplane magnetic field, while the current is parallel to the inplane magnetic field inside the magnetic island. Such a dualring current system, which is attributed to the electron dynamics in the magnetic island, leads to the dip of the outofplane magnetic field in the center of the island. The relevance between our simulations and crater flux transfer events (CFTEs) is also discussed.

Filamentation instability of quantum magnetized plasma in the presence of an external periodic magnetic field
View Description Hide DescriptionThis paper investigates the filamentation instability of a nonrelativistic electron beam passing through a periodic longitudinal magnetic field and an infinite uniform magnetized plasma. The linearized fluidMaxwell equations are used to derive an equation for the dispersion relation of quantum magnetized plasma. The resulting dispersion equation is analyzed numerically over a wide range of system parameters. It is found that the growth rate of the filamentation instabilities is strongly affected by the strength of the periodic magnetic field.

Large amplitude doublelayers in a dusty plasma with a qnonextensive electron velocity distribution and twotemperature isothermal ions
View Description Hide DescriptionArbitrary amplitude dustacoustic (DA) doublelayers in a plasma with nonextensive electrons, twotemperature thermal ions, and warm drifting dust grains are addressed. It is shown that DA doublelayer structures, the onset of which depends sensitively on the plasma parameters, can exist. In particular, it may be noted that the electron nonextensivity may affect drastically the existence of these localized structures. In view of recent observation, our results should assist in the interpretation of the nonlinear doublelayers observed in the downward current region of the aurora.

Nonlinear Phenomena, Turbulence, Transport

Synergy between ion temperature gradient turbulence and neoclassical processes in global gyrokinetic particleincell simulations
View Description Hide DescriptionBased on the CYCLONE case, simulations of collisional electrostatic ion temperature gradient(ITG) microturbulence carried out with the global gyrokineticparticleincell(PIC) code ORB5 are presented. Considering adiabatic electrons, an increase in ion heat transport over the collisionless turbulent case due to ionion collisions is found to exceed the neoclassical contribution. This synergetic effect is due to interaction of collisions, turbulence, and zonal flows. When going from a collisionless to a collisional ITGturbulence simulation, a moderate reduction of the average zonal flow level is observed. The collisional zonal flow level turns out to be roughly independent of the finite collisionality considered. The Dimits shift softening by collisions [Z. Lin et al., Phys. Rev. Lett. 83, 3645 (1999)] is further characterized, and the shearing rate saturation mechanism is emphasized. Turbulence simulations start from a neoclassical equilibrium [T. Vernay et al., Phys. Plasmas 17, 122301 (2010)] and are carried out over significant turbulence times and several collision times thanks to a coarsegraining procedure, ensuring a sufficient signal/noise ratio even at late times in the simulation. The relevance of the Lorentz approximation for ionion collisions, compared to a linearized Landau selfcollision operator, is finally addressed in the frame of both neoclassical and turbulence studies.

Role of external torque in the formation of ion thermal internal transport barriers
View Description Hide DescriptionWe present an analytic study of the impact of external torque on the formation of ion internal transport barriers(ITBs). A simple analytic relation representing the effect of low external torque on transport bifurcations is derived based on a two field transport model of pressure and toroidal momentum density. It is found that the application of an external torque can either facilitate or hamper bifurcation in heat flux driven plasmas depending on its sign relative to the direction of intrinsic torque. The ratio between radially integrated momentum (i.e., external torque) density to power input is shown to be a key macroscopic control parameter governing the characteristics of bifurcation.

Magnetic reconnection and stochastic plasmoid chains in highLundquistnumber plasmas
View Description Hide DescriptionA numerical study of magnetic reconnection in the largeLundquistnumber (S), plasmoiddominated regime is carried out for S up to . The theoretical model of Uzdensky et al. [Phys. Rev. Lett. 105, 235002 (2010)] is confirmed and partially amended. The normalized reconnection rate is independently of S for . The plasmoid flux () and halfwidth () distribution functions scale as and . The joint distribution of and shows that plasmoids populate a triangular region , where is the reconnecting field. It is argued that this feature is due to plasmoid coalescence. Macroscopic “monster” plasmoids with of the system size are shown to emerge in just a few Alfvén times, independently of S, suggesting that large disruptive events are an inevitable feature of largeSreconnection.

Dust ion acoustic soliton in pairion plasmas with nonisothermal electrons
View Description Hide DescriptionDust ion acoustic (DIA) solitons in an unmagnetized pairion (PI) plasmas with adiabatic pairions, nonisothermal electrons, and negatively charged background dust are investigated, using both small and arbitrary amplitude techniques. An energy integral equation involving the Sagdeev potential is derived, and basic properties of the large amplitude solitary structures are investigated. The effects of dust concentration, resonant electrons, and ion temperatures on the profiles of the Sagdeev potential and corresponding solitary waves are studied. The related SchamelKortewegde Vries (SKdV) equation with mixednonlinearity is derived by expanding the Sagdeev potential. Asymptotic solutions for different orders of nonlinearity are discussed for DIA solitary waves. The present work is applicable to understand the wave phenomena and associated nonlinear electrostatic perturbations in the doped pair ion plasmas, not completely filtered e.g., pair ionelectron plasmas, enriched with an extra massive charged component (e.g., dust defects), which may be academic for the moment but might be of interest for forthcoming experiments in laboratory (space) plasmas.

Effect of dynamical friction on interchange motion of plasma filaments
View Description Hide DescriptionTheory and numerical simulations are presented for interchange motion of plasma filaments in the presence of dynamical friction and allowing large relative filament amplitudes. When friction is negligible, the filament velocity is proportional to the square root of gravity and its crossfield size. For strong friction, the filament velocity is independent of the crossfield size, proportional to gravity, and inversely proportional to the friction coefficient. In this frictional regime, the filament moves a large distance with nearly constant velocity and shape. The transition between these velocity scaling regimes and the amplitude dependence are revealed. The results presented here complement previous theories for irregularities in the equatorial ionosphere and are in excellent agreement with recent experiments on simply magnetized toroidal plasmas. The relevance to bloblike structures in the scrapeoff layer of magnetically confined plasmas is also discussed.

Vacuum polarization and magnetization effects in ultraintense laser pulsepair plasmas
View Description Hide DescriptionThe nonlinear effects associated with the vacuum polarization and magnetization in the propagation of ultraintense linearly polarized laser pulse in electronpositron plasmas are investigated. Using the slowly varying envelope approximation, a modified nonlinear Schrödinger equation describing the evolution of the pulse envelope is derived based on the Maxwell equations which include the vacuum polarization and magnetization effects. The analytical and numerical analysis show that the number density of electronpositron plasmas can enhance the vacuum polarization and magnetization effects, and due to the vacuum polarization and magnetization nonlinearity, a onedimensional laser pulse envelope soliton can be formed. The evolution of an initially Gaussian laser pulse is also discussed by numerical analysis.

Two sources of asymmetryinduced transport
View Description Hide DescriptionA singleparticle computer code with collisional effects is used to study asymmetryinduced radial transport of a nonneutral plasma in a coaxial MalmbergPenning trap. Following the time variation of the mean change and mean square change in radial position allows for the calculation of the radial drift velocity and the diffusion coefficient D as defined by the radial flux equation. For asymmetries of the form and periodic boundary conditions, the transport coefficients obtained match those predicted by resonant particle transport theory where the transport is produced by particles with velocities near , with being the azimuthal rotation frequency. For asymmetries of the form and low collision frequency, there is a second contribution to the transport produced by low velocity particles axially trapped in the asymmetry potential. These produce a stronger variation of D with with a peak at . The width of the peak increases with center conductor bias and decreases with radius, while the height shows the opposite behavior. The transport due to axially trapped particles is typically comparable to or larger than that from resonant particles. This second contribution to the transport may explain the discrepancies between experiments and resonant particle theory.
