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May 2012

Volume 19, Issue 5, Articles (05xxxx)

Issue Cover Spotlight Figure

Phys. Plasmas 19, 056314 (2012); http://dx.doi.org/10.1063/1.4718594 (12 pages)

I. V. Igumenshchev, W. Seka, D. H. Edgell, D. T. Michel, D. H. Froula, V. N. Goncharov, R. S. Craxton, L. Divol, R. Epstein, R. Follett, J. H. Kelly, T. Z. Kosc, A. V. Maximov, R. L. McCrory, D. D. Meyerhofer, et al.
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back to top Nonlinear Phenomena, Turbulence, Transport

Electron kappa distribution and steady-state Langmuir turbulence

Peter H. Yoon

Phys. Plasmas 19, 052301 (2012); http://dx.doi.org/10.1063/1.4710515 (6 pages) | Cited 3 times

Online Publication Date: 4 May 2012

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In a recent pair of papers, the present author discussed a self-consistent theory of asymptotically steady-state electron distribution function and Langmuir turbulence intensity in one [P. H. Yoon, Phys. Plasmas 18, 122303 (2011)] and three [P. H. Yoon, Phys. Plasmas 19, 012304 (2012)] dimensions. The resulting electron distribution function is a type of kappa distribution that features a non-Maxwellian energetic tail component. However, while the one-dimensional solution is rigorously correct, the three-dimensional solution, which was obtained using the cylindrical coordinate representation, contains two features that may be inconsistent for field-free plasmas. One is the assumption that the resonance condition can be approximated by ω-k·v ≈ ω-k∥v∥. Needless to say, this is not the most general condition. The second inconsistency is that while the electron distribution is isotropic in velocity, the Langmuir turbulence intensity depends on the wave propagation direction. While these features may not be too unrealistic in the presence of an implicit ambient magnetic field, they certainly cannot be correct if the plasma is genuinely unmagnetized. In the present paper, we rectify such shortcomings by properly reformulating the problem using a spherical coordinate system in a truly free-field plasma.
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52.25.Fi Transport properties
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Ra Plasma turbulence

Effect of nonthermal electrons on oblique electrostatic excitations in a magnetized electron-positron-ion plasma

H. Alinejad

Phys. Plasmas 19, 052302 (2012); http://dx.doi.org/10.1063/1.4714609 (6 pages) | Cited 4 times

Online Publication Date: 14 May 2012

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The linear and nonlinear propagation of ion-acoustic waves are investigated in a magnetized electron-positron-ion (e-p-i) plasma with nonthermal electrons. In the linear regime, the propagation of two possible modes and their evolution are studied via a dispersion relation. In the cases of parallel and perpendicular propagation, it is shown that these two possible modes are always stable. Then, the Korteweg-de Vries equation describing the dynamics of ion-acoustic solitary waves is derived from a weakly nonlinear analysis. The influence on the solitary wave characteristics of relevant physical parameters such as nonthermal electrons, magnetic field, obliqueness, positron concentration, and temperature ratio is examined. It is observed that the increasing nonthermal electrons parameter makes the solitary structures much taller and narrower. Also, it is revealed that the magnetic field strength makes the solitary waves more spiky. The present investigation contributes to the physics of the nonlinear electrostatic ion-acoustic waves in space and laboratory e-p-i plasmas in which wave damping produces an electron tail.
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52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Sb Solitons; BGK modes
02.10.-v Logic, set theory, and algebra
52.25.-b Plasma properties

Tripolar vortex formation in dense quantum plasma with ion-temperature-gradients

Anisa Qamar, Ata-ur-Rahman, and Arshad M. Mirza

Phys. Plasmas 19, 052303 (2012); http://dx.doi.org/10.1063/1.4714648 (5 pages) | Cited 1 time

Online Publication Date: 16 May 2012

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We have derived system of nonlinear equations governing the dynamics of low-frequency electrostatic toroidal ion-temperature-gradient mode for dense quantum magnetoplasma. For some specific profiles of the equilibrium density, temperature, and ion velocity gradients, the nonlinear equations admit a stationary solution in the form of a tripolar vortex. These results are relevant to understand nonlinear structure formation in dense quantum plasmas in the presence of equilibrium ion-temperature and density gradients.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.25.Fi Transport properties

Hexagonal superlattice pattern consisting of colliding filament pairs in a dielectric barrier discharge

Lifang Dong, Ben Li, Ning Lu, Xinchun Li, and Zhongkai Shen

Phys. Plasmas 19, 052304 (2012); http://dx.doi.org/10.1063/1.4717466 (5 pages) | Cited 2 times

Online Publication Date: 17 May 2012

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Colliding-pairs hexagonal superlattice pattern (CPHSP) is studied in a dielectric barrier discharge system. The evolution of CPHSP bifurcating from a hexagonal pattern to chaos is shown. The phase diagrams of CPHSP as a function of discharge parameters are given. From a series of pictures taken by a high speed video camera, collisions between two spots are observed and the superposition of many collisions results in each big spot presenting four small spots on long time scales. Measurements of the correlation between filaments indicate that the pattern is an interleaving of four different transient hexagonal sublattices. Depending on the discharging sequence, the forces exerted on one colliding spot are discussed briefly.
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52.20.-j Elementary processes in plasmas
52.25.Gj Fluctuation and chaos phenomena
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.80.-s Electric discharges

Energy spectrum, dissipation, and spatial structures in reduced Hall magnetohydrodynamic

L. N. Martin, P. Dmitruk, and D. O. Gomez

Phys. Plasmas 19, 052305 (2012); http://dx.doi.org/10.1063/1.4717728 (6 pages) | Cited 1 time

Online Publication Date: 18 May 2012

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We analyze the effect of the Hall term in the magnetohydrodynamic turbulence under a strong externally supported magnetic field, seeing how this changes the energy cascade, the characteristic scales of the flow, and the dynamics of global magnitudes, with particular interest in the dissipation. Numerical simulations of freely evolving three-dimensional reduced magnetohydrodynamics are performed, for different values of the Hall parameter (the ratio of the ion skin depth to the macroscopic scale of the turbulence) controlling the impact of the Hall term. The Hall effect modifies the transfer of energy across scales, slowing down the transfer of energy from the large scales up to the Hall scale (ion skin depth) and carrying faster the energy from the Hall scale to smaller scales. The final outcome is an effective shift of the dissipation scale to larger scales but also a development of smaller scales. Current sheets (fundamental structures for energy dissipation) are affected in two ways by increasing the Hall effect, with a widening but at the same time generating an internal structure within them. In the case where the Hall term is sufficiently intense, the current sheet is fully delocalized. The effect appears to reduce impulsive effects in the flow, making it less intermittent.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Ra Plasma turbulence
52.65.-y Plasma simulation
02.60.Cb Numerical simulation; solution of equations
52.25.Fi Transport properties

Quasi-periodic behavior of ion acoustic solitary waves in electron-ion quantum plasma

Biswajit Sahu, Swarup Poria, Uday Narayan Ghosh, and Rajkumar Roychoudhury

Phys. Plasmas 19, 052306 (2012); http://dx.doi.org/10.1063/1.4714804 (6 pages)

Online Publication Date: 30 May 2012

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The ion acoustic solitary waves are investigated in an unmagnetized electron-ion quantum plasmas. The one dimensional quantum hydrodynamic model is used to study small as well as arbitrary amplitude ion acoustic waves in quantum plasmas. It is shown that ion temperature plays a critical role in the dynamics of quantum electron ion plasma, especially for arbitrary amplitude nonlinear waves. In the small amplitude region Korteweg-de Vries equation describes the solitonic nature of the waves. However, for arbitrary amplitude waves, in the fully nonlinear regime, the system exhibits possible existence of quasi-periodic behavior for small values of ion temperature.
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52.35.Sb Solitons; BGK modes
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)

Effect of poloidal asymmetries on impurity peaking in tokamaks

A. Mollén, I. Pusztai, T. Fülöp, Ye. O. Kazakov, and S. Moradi

Phys. Plasmas 19, 052307 (2012); http://dx.doi.org/10.1063/1.4719711 (11 pages) | Cited 6 times

Online Publication Date: 30 May 2012

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Poloidal impurity asymmetries are frequently observed in tokamaks. In this paper, the effect of poloidal asymmetry on electrostatic turbulent transport is studied, including the effect of the E×B drift. Collisions are modeled by a Lorentz operator, and the gyrokinetic equation is solved with a variational approach. The impurity transport is shown to be sensitive to the magnetic shear and changes sign for s≳0.5 in the presence of inboard accumulation. The zero-flux impurity density gradient (peaking factor) is shown to be rather insensitive to collisions in both ion temperature gradient and trapped electron mode driven cases. Our results suggest that the asymmetry (both the location of its maximum and its strength) and the magnetic shear are the two most important parameters that affect the impurity peaking.
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52.55.Fa Tokamaks, spherical tokamaks
52.25.Vy Impurities in plasmas
52.35.Ra Plasma turbulence
52.25.Fi Transport properties
52.25.Dg Plasma kinetic equations
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
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Evidence of critical balance in kinetic Alfvén wave turbulence simulations

J. M. TenBarge and G. G. Howes

Phys. Plasmas 19, 055901 (2012); http://dx.doi.org/10.1063/1.3693974 (8 pages) | Cited 4 times

Online Publication Date: 21 March 2012

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A numerical simulation of kinetic plasma turbulence is performed to assess the applicability of critical balance to kinetic, dissipation scale turbulence. The analysis is performed in the frequency domain to obviate complications inherent in performing a local analysis of turbulence. A theoretical model of dissipation scale critical balance is constructed and compared to simulation results, and excellent agreement is found. This result constitutes the first evidence of critical balance in a kinetic turbulence simulation and provides evidence of an anisotropic turbulence cascade extending into the dissipation range. We also perform an Eulerian frequency analysis of the simulation data and compare it to the results of a previous study of magnetohydrodynamic turbulence simulations.
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52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Ra Plasma turbulence
47.27.E- Turbulence simulation and modeling

Numerical simulations of strong incompressible magnetohydrodynamic turbulence

J. Mason, J. C. Perez, S. Boldyrev, and F. Cattaneo

Phys. Plasmas 19, 055902 (2012); http://dx.doi.org/10.1063/1.3694123 (7 pages) | Cited 2 times

Online Publication Date: 21 March 2012

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Magnetised plasma turbulence pervades the universe and is likely to play an important role in a variety of astrophysical settings. Magnetohydrodynamics (MHD) provides the simplest theoretical framework in which phenomenological models for the turbulent dynamics can be built. Numerical simulations of MHD turbulence are widely used to guide and test the theoretical predictions; however, simulating MHD turbulence and accurately measuring its scaling properties is far from straightforward. Computational power limits the calculations to moderate Reynolds numbers and often simplifying assumptions are made in order that a wider range of scales can be accessed. After describing the theoretical predictions and the numerical approaches that are often employed in studying strong incompressible MHD turbulence, we present the findings of a series of high-resolution direct numerical simulations. We discuss the effects that insufficiencies in the computational approach can have on the solution and its physical interpretation.
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52.35.Ra Plasma turbulence
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.65.Kj Magnetohydrodynamic and fluid equation
95.30.Qd Magnetohydrodynamics and plasmas

Impact of resonant magnetic perturbations on nonlinearly driven modes in drift-wave turbulence

M. Leconte and P. H. Diamond

Phys. Plasmas 19, 055903 (2012); http://dx.doi.org/10.1063/1.3694675 (11 pages) | Cited 1 time

Online Publication Date: 23 March 2012

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In this work, we study the effects of resonant magnetic perturbations (RMPs) on turbulence, flows, and confinement in the framework of resistive drift wave turbulence. We extend the Hasegawa-Wakatani model to include RMP fields. The effect of the RMPs is to induce a linear coupling between the zonal electric field and the zonal density gradient, which drives the system to a state of electron radial force balance for large math. Both the vorticity flux (Reynolds stress) and particle flux are modulated. We derive an extended predator prey model which couples zonal potential and density dynamics to the evolution of turbulence intensity. This model has both turbulence drive and RMP amplitude as control parameters and predicts a novel type of transport bifurcation in the presence of RMPs. We find states that are similar to the ZF-dominated state of the standard predator-prey model, but for which the power threshold is now a function of the RMP strength. For small RMP amplitude, the energy of zonal flows decreases and the turbulence energy increases with math, corresponding to a damping of zonal flows.
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52.35.Kt Drift waves
52.35.Ra Plasma turbulence
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.25.Fi Transport properties

Thermal plasma and fast ion transport in electrostatic turbulence in the large plasma device

Shu Zhou, W. W. Heidbrink, H. Boehmer, R. McWilliams, T. A. Carter, S. Vincena, S. K. P. Tripathi, and B. Van Compernolle

Phys. Plasmas 19, 055904 (2012); http://dx.doi.org/10.1063/1.3695341 (8 pages) | Cited 1 time

Online Publication Date: 28 March 2012

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The transport of thermal plasma and fast ions in electrostatic microturbulence is studied. Strong density and potential fluctuations (δn/n ∼ δφ/kTe ∼ 0.5, f ∼ 5–50 kHz) are observed in the large plasma device (LAPD) [W. Gekelman, H. Pfister, Z. Lucky et al., Rev. Sci. Instrum. 62, 2875 (1991)] in density gradient regions produced by obstacles with slab or cylindrical geometry. Wave characteristics and the associated plasma transport are modified by driving sheared E × B drift through biasing the obstacle and by modification of the axial magnetic fields (Bz) and the plasma species. Cross-field plasma transport is suppressed with small bias and large Bz and is enhanced with large bias and small Bz. The transition in thermal plasma confinement is well explained by the cross-phase between density and potential fluctuations. Large gyroradius lithium fast ion beam (ρfast/ρs ∼ 10) orbits through the turbulent region. Scans with a collimated analyzer give detailed profiles of the fast ion spatial-temporal distribution. Fast-ion transport decreases rapidly with increasing fast-ion energy and gyroradius. Background waves with different scale lengths also alter the fast ion transport. Experimental results agree well with gyro-averaging theory. When the fast ion interacts with the wave for most of a wave period, a transition from super-diffusive to sub-diffusive transport is observed, as predicted by diffusion theory. Besides turbulent-wave-induced fast-ion transport, the static radial electric field (Er) from biasing the obstacle leads to drift of the fast-ion beam centroid. The drift and broadening of the beam due to static Er are evaluated both analytically and numerically. Simulation results indicate that the Er induced transport is predominately convective.
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52.25.Fi Transport properties
52.35.Ra Plasma turbulence
52.35.Kt Drift waves
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.65.-y Plasma simulation
52.25.Gj Fluctuation and chaos phenomena

First-order finite-Larmor-radius fluid modeling of tearing and relaxation in a plasma pinch

J. R. King, C. R. Sovinec, and V. V. Mirnov

Phys. Plasmas 19, 055905 (2012); http://dx.doi.org/10.1063/1.3695346 (11 pages) | Cited 2 times

Online Publication Date: 29 March 2012

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Drift and Hall effects on magnetic tearing, island evolution, and relaxation in pinch configurations are investigated using a non-reduced first-order finite-Larmor-radius (FLR) fluid model with the nonideal magnetohydrodynamics (MHD) with rotation, open discussion (NIMROD) code [C.R. Sovinec and J. R. King, J. Comput. Phys. 229, 5803 (2010)]. An unexpected result with a uniform pressure profile is a drift effect that reduces the growth rate when the ion sound gyroradius (ρs) is smaller than the tearing-layer width. This drift is present only with warm-ion FLR modeling, and analytics show that it arises from ∇B and poloidal curvature represented in the Braginskii gyroviscous stress. Nonlinear single-helicity computations with experimentally relevant ρs values show that the warm-ion gyroviscous effects reduce saturated-island widths. Computations with multiple nonlinearly interacting tearing fluctuations find that m = 1 core-resonant-fluctuation amplitudes are reduced by a factor of two relative to single-fluid modeling by the warm-ion effects. These reduced core-resonant-fluctuation amplitudes compare favorably to edge coil measurements in the Madison Symmetric Torus (MST) reversed-field pinch [R. N. Dexter et al., Fusion Technol. 19, 131 (1991)]. The computations demonstrate that fluctuations induce both MHD- and Hall-dynamo emfs during relaxation events. The presence of a Hall-dynamo emf implies a fluctuation-induced Maxwell stress, and the simulation results show net transport of parallel momentum. The computed magnitude of force densities from the Maxwell and competing Reynolds stresses, and changes in the parallel flow profile, are qualitatively and semi-quantitatively similar to measurements during relaxation in MST.
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52.58.Lq Z-pinches, plasma focus, and other pinch devices
52.25.Fi Transport properties
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.25.Gj Fluctuation and chaos phenomena
52.65.Kj Magnetohydrodynamic and fluid equation

Dissipation range turbulent cascades in plasmas

P. W. Terry, A. F. Almagri, G. Fiksel, C. B. Forest, D. R. Hatch, F. Jenko, M. D. Nornberg, S. C. Prager, K. Rahbarnia, Y. Ren, and J. S. Sarff

Phys. Plasmas 19, 055906 (2012); http://dx.doi.org/10.1063/1.3698309 (10 pages)

Online Publication Date: 2 April 2012

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Dissipation range cascades in plasma turbulence are described and spectra are formulated from the scaled attenuation in wavenumber space of the spectral energy transfer rate. This yields spectra characterized by the product of a power law and exponential fall-off, applicable to all scales. Spectral indices of the power law and exponential fall-off depend on the scaling of the dissipation, the strength of the nonlinearity, and nonlocal effects when dissipation rates of multiple fluctuation fields are different. The theory is used to derive spectra for MHD turbulence with magnetic Prandtl number greater than unity, extending previous work. The theory is also applied to generic plasma turbulence by considering the spectrum from damping with arbitrary wavenumber scaling. The latter is relevant to ion temperature gradient turbulence modeled by gyrokinetics. The spectrum in this case has an exponential component that becomes weaker at small scale, giving a power law asymptotically. Results from the theory are compared to three very different types of turbulence. These include the magnetic plasma turbulence of the Madison Symmetric Torus, the MHD turbulence of liquid metal in the Madison Dynamo Experiment, and gyrokinetic simulation of ion temperature gradient turbulence.
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52.35.Ra Plasma turbulence
52.65.Tt Gyrofluid and gyrokinetic simulations
52.25.Gj Fluctuation and chaos phenomena

Gyrokinetic prediction of microtearing turbulence in standard tokamaks

H. Doerk, F. Jenko, T. Görler, D. Told, M. J. Pueschel, and D. R. Hatch

Phys. Plasmas 19, 055907 (2012); http://dx.doi.org/10.1063/1.3694663 (12 pages) | Cited 4 times

Online Publication Date: 27 April 2012

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First global gyrokinetic simulations of microtearing instabilities in ASDEX Upgrade geometry provide increasing evidence for the existence of these modes in standard tokamaks. It is found that even in only moderately large devices, nonlocal effects like profile shearing are negligible, supporting the use of an efficient flux-tube approach. Nonlinear gyrokinetic simulations show that the resulting level of magnetic electron heat flux can be experimentally relevant.
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52.55.Fa Tokamaks, spherical tokamaks
52.35.Ra Plasma turbulence
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.25.Fi Transport properties
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.65.Tt Gyrofluid and gyrokinetic simulations

Tokamak-edge toroidal rotation due to inhomogeneous transport and geodesic curvature

T. Stoltzfus-Dueck

Phys. Plasmas 19, 055908 (2012); http://dx.doi.org/10.1063/1.4718335 (16 pages)

Online Publication Date: 31 May 2012

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In a model kinetic ion transport equation for the pedestal and scrape-off layer, passing-ion drift orbit excursions interact with spatially inhomogeneous but purely diffusive transport to cause the orbit-averaged diffusivities to depend on the sign of ν∥, preferentially transporting counter-current ions for realistic parameter values. The resulting pedestal-top intrinsic rotation is typically co-current, reaches experimentally relevant values, and is proportional to pedestal-top ion temperature Ti|pt over plasma current Ip, as observed in experiment. The rotation drive is independent of the toroidal velocity and its radial gradient, representing a residual stress. Co-current spin-up at the L-H transition is expected due to increasing Ti|pt and a steepening of the turbulence intensity gradient. A more inboard (outboard) X-point leads to additional co- (counter-) current rotation drive. Beyond intrinsic rotation, comparison of heat and momentum transport reveals that neutral beam injection must be significantly unbalanced in the counter-current direction to cause zero toroidal rotation at the pedestal top.
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52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Ra Plasma turbulence
52.40.Hf Plasma-material interactions; boundary layer effects
52.50.Gj Plasma heating by particle beams
52.55.Fa Tokamaks, spherical tokamaks
52.25.Fi Transport properties
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