Volume 17, Issue 5, May 2010

Experimental and computational analyses of jet formation at the boundary of a decaying pulse discharge in an ambient quiescent air at 1 atm are presented. High velocity jets are observed attributed to the channel curvature set during the initial breakdown phase. The general convextoconcave jet direction is explained, and the mechanisms of jets formation are discussed.
 LETTERS


Spheromak formation and sustainment by tangential boundary flows
View Description Hide DescriptionThe nonlinear, resistive, threedimensional magnetohydrodynamic equations are solved numerically to demonstrate the possibility of forming and sustaining a spheromak by forcing tangential flows at the plasma boundary. The method can be explained in terms of helicity injection. Several features previously observed in dc helicity injection experiments are reproduced and analyzed.
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Connection between the two branches of the quantum twostream instability across the space
View Description Hide DescriptionThe stability of two quantum counterstreaming electron beams is investigated within the quantum plasmafluid equations for arbitrarily oriented wave vectors . The analysis reveals that the two quantum twostream unstable branches are indeed connected by a continuum of unstable modes with oblique wave vectors. Using the longitudinal approximation, the stability domain for any is analytically explained, together with the growth rate.

Thermal convective and rotational instability in dissipative magnetohydrodynamics
View Description Hide DescriptionThe thermal convective and magnetorotational instability is investigated by means of magnetohydrodynamic equations including anisotropicviscosity and resistivity dissipative effects. Magnetic force lines are assumed to be initially isothermal and the heat is restricted to being primarily transported along the magnetic force lines. To obtain the analytic expressions for the growth rate and instability criteria, we neglect the crossfield resistivity by applying our result to the weakly ionized environment. Under this assumption, the general dispersion relation describing the local thermal convective and magnetorotational instability is derived. The effects on the dispersion relation due to anisotropicresistivity and viscosity are discussed. Both the resistivity and viscosity show stabilizing effect on the thermal convective and rotational instability but do not affect the instability criterion. The analytic expression governing the growth rate is presented for Prandtl number case.

Structures of diffusion regions in collisionless magnetic reconnection
View Description Hide DescriptionDetailed structures of diffusion regions in twodimensional collisionless magnetic reconnection are studied by using an electromagnetic Vlasov simulation. It has been well known that plasma number density decreases near the Xpoint of the reconnection. However, numerical thermal fluctuations exist in particleincell simulations, and there is a possibility that detailed structures near the Xpoint diffuse numerically when the number of particles per cell is not enough. In the present study, a highresolution twodimensional Vlasov simulation is performed. It is found that electron number density in the electron diffusion region decreases to a hundredth of the initial value. Structures of electron diffusion region are determined by the local electron inertial length.

Numerical study of ioncyclotron resonant interaction via hybridVlasov simulations
View Description Hide DescriptionHybrid Vlasov–Maxwell numerical simulations are used to investigate the collisionless resonantinteraction of ions with ioncyclotron waves in parallel propagation with respect to a background magnetic field. In linear regime, analytical results on wave damping, obtained by integrating the linearized Vlasov equation through the wellknown characteristics method, are compared with the numerical results. Then, the ion heating process and the consequent generation of temperatureanisotropy in the direction perpendicular to the background magnetic field are investigated numerically in detail. In nonlinear regime, the numerical results show that the distribution of particle velocities is strongly distorted due to the resonant ioncyclotron interaction with the formation of diffusive plateaus in the longitudinal direction (with respect to the ambient field) and significantly departs from the Maxwellian equilibrium. These results are relevant in many plasma physics environments, such as the solar wind, where the process of ioncyclotron heating and the generation of temperatureanisotropy and nonMaxwellian velocity distributions are routinely recovered in many in situ measurements, or the laboratory plasmas, where the resonantinteraction of ions with ioncyclotron waves is the primary source of auxiliary heating in the confining apparatus.

Langmuir wave dispersion relation in nonMaxwellian plasmas
View Description Hide DescriptionThe Langmuir wavedispersion relation is derived in partially ionized plasmas, where free electrons are confined to move in a nearest neighbor ions’ potential well. The equilibrium velocity distribution function experiences then, a departure from Maxwelldistribution function. The effect of the nonMaxwellian character of the distribution function on the Langmuir phase and group velocities as well as the phase matching conditions and the nonlinear growth rate of decay instability is investigated. The proposed Langmuir wavedispersion relation is relevant to dense and cryogenic plasmas.

Effect of two ion species on the propagation of shear Alfvén waves of small transverse scale
View Description Hide DescriptionThe results of a theoretical modeling study and experimental investigation of the propagation properties of shear Alfvén waves of small transverse scale in a plasma with two ion species are reported. In the two ion plasma, depending on the mass of the heavier species, ion kinetic effects can become prominent, and significant parallel electric fields result in electron acceleration. The theory predicts the appearance of frequency propagation gaps at the ionion hybrid frequency and between harmonics of the lower cyclotron frequency. Within these frequency bands spatial structures arise that mix the conepropagation characteristics of Alfvén waves with radially expanding ion Bernstein modes. The experiments, performed at the Basic Plasma Science Facility (BaPSF) at UCLA, consist of the spatial mapping of shear waves launched by a loop antenna. Although a variety of two ionspecies combinations were explored, only results from a heliumneon mix are reported. A clear signature of a shear wave propagation gap, as well as propagation between multiple harmonics, is found for this gas combination. The evanescence of shear waves beyond the reflection point at the ionion hybrid frequency in the presence of an axial magnetic field gradient is also documented.

The lower hybrid wave cutoff: A case study in eikonal methods
View Description Hide DescriptionEikonal, or ray tracing, methods are commonly used to estimate the propagation of radio frequency fields in plasmas. While the information gained from the rays is quite useful, an approximate solution for the fields would also be valuable, e.g., for comparison to full wave simulations. Such approximations are often difficult to perform numerically because of the special care which must be taken to correctly reconstruct the fields near reflection and focusing caustics. In this paper, we compare the standard eikonal method for approximating fields to a method based on the dynamics of wave packets. We compare the approximations resulting from these two methods to the analytical solution for a lower hybridwave reflecting from a cutoff. The algorithm based on wave packets has the advantage that it can correctly deal with caustics, without any special treatment.

Scaling of asymmetric reconnection in compressible plasmas
View Description Hide DescriptionThe scaling of the reconnection rate with external parameters is reconsidered for antiparallel reconnection in a singlefluid magnetohydrodynamic(MHD) model, allowing for compressibility as well as asymmetry between the plasmas and magnetic fields in the two inflow regions. The results show a modest dependence of the reconnection rate on the plasma beta (ratio of plasma to magnetic pressure) in the inflow regions and demonstrate the importance of the conversion of magnetic energy to enthalpy flux (that is, convected thermal energy) in the outflow regions. The conversion of incoming magnetic to outgoing thermal energy flux remains finite even in the limit of incompressibility, while the scaling of the reconnection rate obtained earlier [P. A. Cassak and M. A. Shay, Phys. Plasmas14, 102114 (2007)] is recovered. The assumptions entering the scaling estimates are critically investigated on the basis of twodimensional resistiveMHD simulations, confirming and even strengthening the importance of the enthalpy flux in the outflow from the reconnection site.

Linear plasmoid instability of thin current sheets with shear flow
View Description Hide DescriptionThis paper presents linear analytical and numerical studies of plasmoid instabilities in the presence of shear flow in highLundquistnumber plasmas. Analysis demonstrates that the stability problem becomes essentially two dimensional as the stabilizing effects of shear flow become more prominent. Scaling results are presented for the twodimensional instabilities. An approximate criterion is given for the critical aspect ratio of thin current sheets at which the plasmoid instability is triggered.
 Nonlinear Phenomena, Turbulence, Transport

Envelope ionacoustic solitary waves in a plasma with positivenegative ions and nonthermal electrons
View Description Hide DescriptionModulation instability of ionacoustic waves is investigated in a plasma composed of positive and negative ions as well as nonthermal electrons. For this purpose, a linear dispersion relation and a nonlinear Schrödinger equation are derived. The latter admits localized envelope solitary wave solutions of bright(pulses) and dark(holes, voids) type. The envelope soliton depends on the intrinsic plasma parameters. It is found that modulation instability of ionacoustic waves is significantly affected by the presence of nonthermal electrons. The present model is used to investigate the solitary excitations in the and plasmas, where they are presented in the Dregion and Fregion of the Earth’s ionosphere. The findings of this investigation should be useful in understanding the stable electrostatic wave packet acceleration mechanisms in positivenegative ion plasmas, and also enhance our knowledge on the occurrence of instability associated to the propagation of the envelope ionacoustic solitary waves in space and in laboratory plasmas where two distinct groups of ions and nonBoltzmann distributed electrons are present.

Dressed electrostatic solitary waves in quantum dusty pair plasmas
View Description Hide DescriptionQuantumhydrodynamics model is applied to investigate the nonlinear propagation of electrostatic solitary excitations in a quantum dusty pair plasma. A Korteweg de Vries evolution equation is obtained using reductive perturbation technique and the highernonlinearity effects are derived by solving the linear inhomogeneous differential equation analytically using Kodama–Taniuti renormalizing method. The possibility of propagation of bright and darktype solitary excitations is examined. It is shown that a critical value of quantum diffraction parameter exists, on either side of which, only one type of solitary propagation is possible. It is also found that unlike for the firstorder amplitude component, the variation of parameter dominantly affects the soliton amplitude in higherorder approximation. The effect of fractional quantum number density on compressive and rarefactive soliton dynamics is also discussed.

Large amplitude relativistic plasma waves
View Description Hide DescriptionRelativistic, longitudinal plasma oscillations are studied for the case of a simple water bag distribution of electrons having cylindrical symmetry in momentum space with the axis of the cylinder parallel to the velocity of wave propagation. The plasma is required to obey the relativistic Vlasov–Poisson equations, and solutions are sought in the wave frame. An exact solution for the plasma density as a function of the electrostatic field is derived. The maximum electric field is presented in terms of an integral over the known density. It is shown that when the perpendicular momentum is neglected, the maximum electric field approaches infinity as the wave phase velocity approaches the speed of light. It is also shown that for any nonzero perpendicular momentum, the maximum electric field will remain finite as the wave phase velocity approaches the speed of light. The relationship to previously published solutions is discussed as is some recent controversy regarding the proper modeling of large amplitude relativistic plasmawaves.

Test particle analysis in L and Hmode simulations
View Description Hide DescriptionIn this work, the radial transport of tracers in an Hmode run in the CUTIE code [A. Thyagaraja et al., Phys. Plasmas12, 090907 (2005)] is analyzed globally. Several techniques are applied to the study of the trajectories performed by the tracers, measuring the degree of selfsimilarity in the motion and searching for long range spatial and temporal correlations. The results are compared to those of an Lmode run [G. Sánchez Burillo et al., Phys. Plasmas16, 042319 (2009)] in order to highlight the changes between L and H. The analysis of selfsimilarity parameters of the motion reveals that changes, if any, are slight, although the reliability of the results is limited. Nevertheless, the study of the mean step size indicates that transport is more local (or rather less global) and the anomalous diffusion contribution is less dominant. Namely, the variance of the radial distribution of tracers is smaller in Hmode and the strong asymmetry in the positive/negative steps performed by the tracers vanishes.

Interface width effect on the classical Rayleigh–Taylor instability in the weakly nonlinear regime
View Description Hide DescriptionIn this paper, the interface width effects (i.e., the density gradient effects or the density transition layer effects) on the Rayleigh–Taylor instability(RTI) in the weakly nonlinear (WN) regime are investigated by numerical simulation (NS). It is found that the interface width effects dramatically influence the linear growth rate in the linear growth regime and the mode coupling process in the WN growth regime. First, the interface width effects decrease the linear growth rate of the RTI, particularly for the short perturbation wavelengths. Second, the interface width effects suppress (reduce) the thirdorder feedback to the fundamental mode, which induces the nonlinear saturation amplitude (NSA) to exceed the classical prediction, . The wider the density transition layer is, the larger the NSA is. The NSA in our NS can reach a half of its perturbation wavelength. Finally, the interface width effects suppress the generation and the growth of the second and the third harmonics. The ability to suppress the harmonics’ growth increases with the interface width but decreases with the perturbation wavelength. On the whole, in the WN regime, the interface width effects stabilize the RTI, except for an enhancement of the NSA, which is expected to improve the understanding of the formation mechanism for the astrophysical jets, and for the jetlike long spikes in the high energy density physics.

Nonlinear studies of fast electron current pulse propagation in a two dimensional inhomogeneous plasma
View Description Hide DescriptionThe evolution of fast current and magnetic field pulse structures through an inhomogeneous plasma medium was reported in recent publications [Yadav et al., Phys. Plasmas15, 062308 (2008); Yadav et al., Phys. Plasmas16, 040701 (2009)]. The evolution characteristics have been investigated here in further detail. Following specific issues have been addressed; (i) characterization of the phenomena of trapping versus transmission of the current pulse structures through a high density plasma region, (ii) interaction of the current pulse with plasma density inhomogeneity at various incidence angles, and (iii) destabilization of sharp sheared current layers resulting from the interaction with an elongated plasma density inhomogeneity. It is also illustrated that the destabilization of the current pulse trapped in an elongated high density plasma region forms a novel stable coherent nonlinear pattern of alternating signed vortices arranged as beads along the density profile.

The thirdorder law for magnetohydrodynamic turbulence with shear: Numerical investigation
View Description Hide DescriptionThe scaling laws of thirdorder structure functions for isotropic, homogeneous, and incompressible magnetohydrodynamic(MHD)turbulence relate the observable structure function with the energy dissipation rate. Recently [Wan et al.Phys. Plasmas16, 090703 (2009)], the theory was extended to the case in which a constant velocity shear is present, motivated by the application of the thirdorder law to the solar wind. We use direct numerical simulations of twodimensional MHD with shear to confirm this new generalization of the theory. The presence of the shear effect broadens the circumstances in which the law can be applied. Important implications for laboratory and space plasmas are discussed.

Perpendicular propagating electromagnetic envelope solitons in electronpositronion plasma
View Description Hide DescriptionThe nonlinear amplitude modulation of electromagnetic waves propagating perpendicular to the direction of ambient magnetic field in a uniform collisionless magnetized electronpositronion plasma is studied. The Krylov–Bogoliubov–Mitropolsky perturbation method is employed to derive nonlinear Schrödinger equation, which describes the amplitude dynamics of perturbed magnetic field. The modulation instability criterion reveals that the low frequency mode is always stable, whereas the high frequency mode becomes modulationally unstable for certain ranges of wave number and positrontoelectron density ratio. Furthermore, the positrontoelectron density ratio as well as the strength of ambient magnetic field is found to have significant effect on the solitary wave solutions of the nonlinear Schrödinger equation, namely, dark and bright envelope solitons.

Study of the change of electron temperature inside magnetic island caused by localized radio frequency heating
View Description Hide DescriptionThe change in the electron temperature inside magnetic island caused by localized radio frequency (rf)heating is studied numerically by solving the twodimensional energy transport equation, to investigate the dependence of the temperature change on the location and width of the rf power deposition along the minor radius and the helical angle, the island width, and the ratio between the parallel and the perpendicular heat conductivity. Based on obtained numerical results, suggestions for optimizing the island stabilization by localized rf heating are made.

Three dimensional character of whistler turbulence
View Description Hide DescriptionIt is shown that the dominant nonlinear effect makes the evolution of whistlerturbulence essentially three dimensional in character. Induced nonlinear scattering due to slow density perturbation resulting from ponderomotive force triggers energy flux toward lower frequency. Anisotropic wave vector spectrum is generated by large angle scatterings from thermal plasma particles, in which the wave propagation angle is substantially altered but the frequency spectrum changes a little. As a consequence, the wave vector spectrum does not indicate the trajectory of the energy flux. There can be conversion of quasielectrostatic waves into electromagnetic waves with large group velocity, enabling convection of energy away from the region. We use a twodimensional electromagnetic particleincell model with the ambient magnetic field out of the simulation plane to generate the essential threedimensional nonlinear effects.