Volume 19, Issue 10, October 2012

Flux ropes are ubiquitous in space and solar plasmas. Multiple adjacent flux ropes are commonly observed both in the solar corona and in the earth's magnetotail. The interaction of adjacent flux ropes is often dynamic and can lead to magnetic reconnection. In this paper, the interaction of three flux ropes is studied in a low background laboratory magnetoplasma. The magnetic structure of the flux rope is produced by the poloidal field of a fieldaligned finite sized current which adds to the guide magnetic field and creates the typical helical field line structure. Each rope produces magnetic fields on the order of a few percent of the guide field. Volumetric magnetic field data were acquired and the magnetic field structure and dynamics of the flux ropes can thus be reconstructed. The flux ropes are found to propagate at the Alfvén speed. Merging and bouncing of the flux ropes have been observed. The ropes twist and writhe as they propagate through the plasma. They are line tied and clearly separate at the cathode end but further away they merge into one extended rope. The steady state of the flux ropes is characterized by a rotation of the three flux ropes as a whole. At the same time, the flux ropes are twisting around each other. Time resolved density and temperature measurements from Langmuir probe data exhibit the same dynamics as the magnetic field data.
 LETTERS


Formation of jetlike spikes from the ablative RayleighTaylor instability
View Description Hide DescriptionThe mechanism of jetlike spike formation from the ablative RayleighTaylor instability (ARTI) in the presence of preheating is reported. It is found that the preheating plays an essential role in the formation of the jetlike spikes. In the early stage, the preheating significantly increases the plasma density gradient, which can reduce the linear growth of ARTI and suppress its harmonics. In the middle stage, the preheating can markedly increase the vorticityconvection and effectively reduce the vorticity intensity resulting in a broadened velocity shear layer near the spikes. Then the growth of ablative KelvinHelmholtz instability is dramatically suppressed and the ARTI remains dominant. In the late stage, nonlinear bubbleacceleration further elongates the bubblespike amplitude and eventually leads to the formation of jetlike spikes.
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Implications for the electron distribution from the stationary hydrodynamic model of a onedimensional plasma expansion into vacuum
View Description Hide DescriptionIt is shown that the hydrodynamic model of a onedimensional collisionless plasma expansion is contained in the kinetic description as a special case. This belongs to a specific choice for the electron distribution function. Moreover, the consequences of the use of the hydrodynamic approach regarding the temporal evolution of the electron phase space density are investigated. It turns out that only the case of a hydrodynamic description with the adiabatic constant is physically selfconsistent. Numerical simulations confirm this argumentation. The analysis for the case is extended to the kinetics of a relativistic electron gas.

Morphology and dynamics of three interacting kinkunstable flux ropes in a laboratory magnetoplasma
View Description Hide DescriptionFlux ropes are ubiquitous in space and solar plasmas. Multiple adjacent flux ropes are commonly observed both in the solar corona and in the earth's magnetotail. The interaction of adjacent flux ropes is often dynamic and can lead to magnetic reconnection. In this paper, the interaction of three flux ropes is studied in a low background laboratory magnetoplasma. The magnetic structure of the flux rope is produced by the poloidal field of a fieldaligned finite sized current which adds to the guide magnetic field and creates the typical helical field line structure. Each rope produces magnetic fields on the order of a few percent of the guide field. Volumetric magnetic field data were acquired and the magnetic field structure and dynamics of the flux ropes can thus be reconstructed. The flux ropes are found to propagate at the Alfvén speed. Merging and bouncing of the flux ropes have been observed. The ropes twist and writhe as they propagate through the plasma. They are line tied and clearly separate at the cathode end but further away they merge into one extended rope. The steady state of the flux ropes is characterized by a rotation of the three flux ropes as a whole. At the same time, the flux ropes are twisting around each other. Time resolved density and temperature measurements from Langmuir probe data exhibit the same dynamics as the magnetic field data.

Multitemperature representation of electron velocity distribution functions. I. Fits to numerical results
View Description Hide DescriptionElectron energy distribution functions are expressed as a sum of 6–12 Maxwellians or a sum of 3, but each multiplied by a finite series of generalized Laguerre polynomials. We fitted several distribution functions obtained from the finite difference FokkerPlanck code “FPI” [Matte and Virmont, Phys. Rev. Lett. 49, 1936 (1982)] to these forms, by matching the moments, and showed that they can represent very well the coexistence of hot and cold populations, with a temperature ratio as high as 1000. This was performed for two types of problems: (1) the collisional relaxation of a minority hot component in a uniform plasma and (2) electron heat flow down steep temperature gradients, from a hot to a much colder plasma. We find that the multiMaxwellian representation is particularly good if we accept complex temperatures and coefficients, and it is always better than the representation with generalized Laguerre polynomials for an equal number of moments. For the electron heat flow problem, the method was modified to also fit the first order anisotropy, again with excellent results. We conclude that this multiMaxwellian representation can provide a viable alternative to the finite difference speed or energy grid in kinetic codes.

Low frequency electromagnetic oscillations in dense degenerate electronpositron pair plasma, with and without ions
View Description Hide DescriptionQuantum plasma oscillations are studied in a strongly magnetized, ultradense plasma with degenerate electrons and positrons. The dispersive role of electron and positronquantum effects on low frequency (in comparison to electron cyclotron frequency) shear electromagnetic wave is investigated by employing hydrodynamic formulation. In the presence of ions, the density balance changes, and the electromagnetic wave (with frequency lower than the ion cyclotron frequency) is shown to couple with electrostatic ion mode under certain conditions. For such low frequency waves, it is also seen that the contribution of electron and positron degeneracy pressure is dominant as compared to their diffraction effects. The results are analyzed numerically for illustrative purpose pointing out their relevance to the dense laboratory (e.g., superintense laserdense matter interactions) and astrophysical plasmas.

Ionacoustic solitary waves and their multidimensional instability in a magnetized degenerate plasma
View Description Hide DescriptionA rigorous theoretical investigation has been made on ZakharovKuznetsov (ZK) equation of ionacoustic (IA) solitary waves (SWs) and their multidimensional instability in a magnetized degenerate plasma which consists of inertialess electrons, inertial ions, negatively, and positively charged stationary heavy ions. The ZK equation is derived by the reductive perturbation method, and multidimensional instability of these solitary structures is also studied by the smallk (long wavelength plane wave)perturbation expansion technique. The effects of the external magnetic field are found to significantly modify the basic properties of small but finiteamplitude IA SWs. The external magnetic field and the propagation directions of both the nonlinear waves and their perturbation modes are found to play a very important role in changing the instability criterion and the growth rate of the unstable IA SWs. The basic features (viz., amplitude, width, instability, etc.) and the underlying physics of the IA SWs, which are relevant to space and laboratory plasma situations, are briefly discussed.

Quantum ring solitons and nonlocal effects in plasma wake field excitations
View Description Hide DescriptionA theoretical investigation of the quantum transverse beam motion for a cold relativistic charged particle beam travelling in a cold, collisionless, strongly magnetized plasma is carried out. This is done by taking into account both the individual quantum nature of the beam particles (singleparticle uncertainty relations and spin) and the self consistent interaction generated by the plasma wake field excitation. By adopting a fluid model of a strongly magnetized plasma, the analysis is carried out in the overdense regime (dilute beams) and in the long beam limit. It is shown that the quantum description of the collective transverse beam dynamics is provided by a pair of coupled nonlinear governing equations. It comprises a Poissonlike equation for the plasma wake potential (driven by the beam density) and a 2D spinorial Schrödinger equation for the wave function, whose squared modulus is proportional to the beam density, that is obtained in the Hartree's mean field approximation, after disregarding the exchange interactions. The analysis of this pair of equations, which in general exhibits a strong nonlocal character, is carried out analytically as well as numerically in both the linear and the nonlinear regimes, showing the existence of the quantum beam vortices in the form of LaguerreGauss modes and ring envelope solitons, respectively. In particular, when the relation between the plasma wake field response and the beam probability density is strictly local, the pair of the governing equations is reduced to the 2D GrossPitaevskii equation that allows one to establish the conditions for the self focusing and collapse. These conditions include the quantum nature of the beam particles. Finally, when the relation between the plasma wake field response and the beam probability density is moderately nonlocal, the above pair of equations permits to follow the spatiotemporal evolution of a quantum ring envelope soliton. Such a structure exhibits small or violent breathing, but it remains very stable for long time.

Geometric phases of the Faraday rotation of electromagnetic waves in magnetized plasmas
View Description Hide DescriptionGeometric phases of circularly polarized electromagnetic waves in nonuniform magnetized plasmas is studied theoretically. The variation of the propagation direction of circularly polarized waves results in a geometric phase, which also contributes to the Faraday rotation, in addition to the standard dynamical phase. The origin and properties of the geometric phase are investigated. The influence of the geometric phase to plasma diagnostics using the Faraday rotation is discussed as an application of the theory.

The positive ion temperature effect in magnetized electronegative plasma sheath with two species of positive ions
View Description Hide DescriptionThe properties of a magnetized multicomponent (two species of positive ions, negative ions and electrons) plasma sheath with finite positive ion temperature are studied. By using three fluid hydrodynamic model and some dimensionless variables, the ion (both lighter and heavier positive ions, and negative ions) densities, the ion (only for positive ions) velocities, and electric potential inside the sheath are investigated. In addition, the absence and presence of magnetic field and the orientation of magnetic field are considered. It is noticed that, with increase of positive ion temperature, the lighter positive ion density peaks increase only at the sheath edge and shift towards the sheath edge for both absence and presence of magnetic field. For heavier positive ions, in the absence of magnetic field, the density peaks increase at the sheath edge. But in the presence of magnetic field, the density fluctuations increase at the sheath edge. For both the cases, the density peaks shift towards the sheath edge.

Spatiotemporal evolution and breaking of double layers: A description using Lagrangian hydrodynamics
View Description Hide DescriptionThe nonlinear development and collapse (breaking) of double layers in the long scale length limit is well described by equations for the cold ion fluid with quasineutrality. It is shown that electron dynamics is responsible for giving an “equation of state” with negative ratio of specific heats to this fluid. Introducing a transformation for the density variable, the governing equation for the transformed quantity in terms of Lagrange variables turns out exactly to be a linear partial differential equation. This equation has been analyzed in various limits of interest. Nonlinear development of double layers with a sinusoidal initial disturbance and collapse of double layers with an initial perturbation in the form of a density void are analytically investigated.

Time growth rate and field profiles of hybrid modes excited by a relativistic elliptical electron beam in an elliptical metallic waveguide with dielectric rod
View Description Hide DescriptionThe dispersion relation of guided electromagnetic waves propagating in an elliptical metallic waveguide with a dielectric rod driven by relativistic elliptical electron beam (REEB) is investigated. The electric field profiles and the growth rates of the waves are numerically calculated by using Mathieu functions. The effects of relative permittivity constant of dielectric rod, accelerating voltage, and current density of REEB on the growth rate are presented.

The cylindrical magnetic RayleighTaylor instability for viscous fluids
View Description Hide DescriptionThis paper considers a cylindrical RayleighTaylor instability, in which a heavy fluid surrounds a light fluid, and gravity is directed radially inwards. A massive object is located at the centre of the light fluid, and it behaves like a line dipole both for fluid flow and magnetic field strength. The initially circular interface between the two conducting fluids evolves into plumes, dependent on the magnetic and fluid dipole strengths and the nature of the initial disturbance to the interface. A spectral method is presented to solve the timedependent interface shapes, and results are presented and discussed. Bipolar solutions are possible, and these are of particular relevance to astrophysics. The solutions obtained resemble structures of some HII regions and nebulae.

Computational fluid dynamics and frequencydependent finitedifference timedomain method coupling for the interaction between microwaves and plasma in rocket plumes
View Description Hide DescriptionUnder certain conditions during rocket flights, ionized exhaust plumes from solid rocket motors may interfere with radio frequency transmissions. To understand the relevant physical processes involved in this phenomenon and establish a prediction process for inflight attenuation levels, we attempted to measure microwave attenuation caused by rocket exhaust plumes in a sealevel static firing test for a fullscale solid propellant rocket motor. The microwave attenuation level was calculated by a coupling simulation of the inviscidfrozenflow computational fluid dynamics of an exhaust plume and detailed analysis of microwave transmissions by applying a frequencydependent finitedifference timedomain method with the Drude dispersion model. The calculated microwave attenuation level agreed well with the experimental results, except in the case of interference downstream the Mach disk in the exhaust plume. It was concluded that the coupling estimation method based on the physics of the frozen plasma flow with Drude dispersion would be suitable for actual flight conditions, although the mixing and afterburning in the plume should be considered depending on the flow condition.

Investigations on loop antenna excited whistler waves in a cylindrical plasma based on laboratory experiments and simulations
View Description Hide DescriptionExperimental investigations and simulations of loop antenna excited whistler waves in a cylindrical low temperature plasma are presented. Experiments are performed in the VINETA [Franck et al., Plasma Sources Sci. Technol. 14, 226 (2005)] device and simulations are generated using IPFFD3D [C. Lechte, IEEE Trans. Plasma Sci. 37, 1099 (2009)], an implementation of the Yeealgorithm. A good agreement of the experiment and the simulation is found. The simulation is used to predict the spatial structure of the wave, the dominant source of the electric field, the relative contribution of electron, and displacement current as well as parallel and perpendicular currents to the magnetic field of the wave.
 Nonlinear Phenomena, Turbulence, Transport

Effect of parallel currents on driftinterchange turbulence: Comparison of simulation and experiment
View Description Hide DescriptionTwodimensional (2D) turbulence simulations are reported in which the balancing of the parallel and perpendicular currents is modified by changing the axial boundary condition (BC) to vary the sheath conductivity. The simulations are carried out using the 2D scrapeofflayer turbulence (SOLT) code. The results are compared with recent experiments on the controlled shear decorrelation experiment (CSDX) in which the axial BC was modified by changing the composition of the end plate. Reasonable qualitative agreement is found between the simulations and the experiment. When an insulating axial BC is used, broadband turbulence is obtained and an inverse cascade occurs down to low frequencies and long spatial scales. Robust sheared flows are obtained. By contrast, employing a conducting BC at the plate resulted in coherent (drift wave) modes rather than broadband turbulence, with weaker inverse cascade, and smaller zonal flows. The dependence of the two instability mechanisms (rotationally driven interchange mode and drift waves) on the axial BC is also discussed.

Wave breaking phenomenon of lowerhybrid oscillations induced by a background inhomogeneous magnetic field
View Description Hide DescriptionIn a fluid description, we study spacetime evolution of lower hybrid modes in a cold quasineutral homogeneous plasma in presence of a background inhomogeneous magnetic field. Within a linear analysis, a dispersion relation with inhomogeneous magnetic field shows “phase mixing” of such oscillations. A manifestation of “phase mixing” is shown in “mode coupling.” By using Lagrangian variables, an exact solution is presented in parametric form of this nonlinear time dependent problem. It is demonstrated that initially excited lower hybrid modes always break via phase mixing phenomenon in presence of an inhomogeneous magnetic field. Breaking of such oscillations is revealed by the appearance of spikes in the plasma density profile.

Electromagnetic weak turbulence theory revisited
View Description Hide DescriptionThe statistical mechanical reformulation of weak turbulence theory for unmagnetized plasmas including fully electromagnetic effects was carried out by Yoon [Phys. Plasmas 13, 022302 (2006)]. However, the wave kinetic equation for the transverse wave ignores the nonlinear threewave interaction that involves two transverse waves and a Langmuir wave, the incoherent analogue of the socalled Raman scattering process, which may account for the third and higherharmonic plasma emissions. The present paper extends the previous formalism by including such a term.

Observation of a strong correlation between electromagnetic soliton formation and relativistic selffocusing for ultrashort laser pulses propagating through an underdense plasma
View Description Hide DescriptionA strong correlation is observed between the formation of electromagneticsolitons, generated during the interaction of a short intense laser pulse (30 fs, ∼10^{18} W/cm^{2}) with a rarefied (<0.1n_{c} ) plasma, and pulse selffocusing. Pulse defocusing, which occurs after soliton generation, results in laserpulse energy depletion. The role of stimulated Raman scattering in soliton generation is analyzed from 2D particleincell simulations. An observed relationship between initial plasma density and soliton generation is presented that might have relevance to wakefield accelerators.

Plasma transport induced by kinetic Alfvén wave turbulence
View Description Hide DescriptionAt the Earth's magnetopause that separates the hottenuous magnetospheric plasma from the cold dense solar wind plasma, often seen is a boundary layer where plasmas of both origins coexist. Plasma diffusions of various forms have been considered as the cause of this plasma mixing. Here, we investigate the plasma transport induced by waveparticle interaction in kinetic Alfvén wave (KAW) turbulence, which is one of the candidate processes. We clarify that the physical origin of the KAWinduced crossfield diffusion is the drift motions of those particles that are in Cerenkov resonance with the wave: E×Blike drift that emerges in the presence of nonzero parallel electric field component and gradB drift due to compressional magnetic fluctuations. We find that KAW turbulence, which has a spectral breakpoint at which an MHD inertial range transits to a dissipation range, causes selective transport for particles whose parallel velocities are specified by the local Alfvén velocity and the parallel phase velocity at the spectral breakpoint. This finding leads us to propose a new data analysis method for identifying whether or not a mixed plasma in the boundary layer is a consequence of KAWinduced transport across the magnetopause. The method refers to the velocity space distribution function data obtained by a spacecraft that performs in situ observations and, in principle, is applicable to currently available dataset such as that provided by the NASA's THEMIS mission.

Nonlinear saturation mechanism of electron temperature gradient modes
View Description Hide DescriptionThe electron temperature gradient (ETG) mode is a very plausible candidate to explain the large electron particle transport and thermal conduction. Production and identification of slab ETG modes and measurement electron transport have been already reported [X. Wei, V. Sokolov, and A. K. Sen, Phys. Plasmas 17, 042108 (2010); V. Sokolov and A. K. Sen, Phys. Rev. Lett. (2011)]. Now, we develop a theoretical model of nonlinear saturation mechanism of ETG mode based on the three wave coupling of an unstable high frequency ETG mode with a damped ETG radial harmonic and a damped ion acoustic (IA) mode. Bicoherence analysis of Columbia linear machine (CLM) data show coupling between ETG modes (∼2.4 MHz) and a low frequency mode (∼50 kHz). The large damping drive of the ETG radial harmonic accompanied by the smaller but finite damping of the IA mode presents an energy sink for the unstable ETG mode, thus causing saturation. This model predicts a saturation level of ∼10% and agrees with the observed levels of ETG modes in the CLM.