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Oct 2008

Volume 15, Issue 10, Articles (10xxxx)

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Phys. Plasmas 15, 103501 (2008); http://dx.doi.org/10.1063/1.2988781 (8 pages)

I. Levchenko, K. Ostrikov, J. Khachan, and S. V. Vladimirov
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Wave localization and density bunching in pair ion plasmas

Swadesh M. Mahajan and Nana L. Shatashvili

Phys. Plasmas 15, 100701 (2008); http://dx.doi.org/10.1063/1.3005382 (4 pages) | Cited 9 times

Online Publication Date: 27 October 2008

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By investigating the nonlinear propagation of high intensity electromagnetic (EM) waves in a pair ion plasma, whose symmetry is broken via contamination by a small fraction of high mass immobile ions, it is shown that this new and interesting state of (laboratory created) matter is capable of supporting structures that strongly localize and bunch the EM radiation with density excess in the region of localization. Testing of this prediction in controlled laboratory experiments can lend credence, inter alia, to conjectures on structure formation (via the same mechanism) in the MEV era of the early universe.
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52.35.-g Waves, oscillations, and instabilities in plasmas and intense beams
52.25.-b Plasma properties
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back to top Basic Plasma Phenomena, Waves, Instabilities

Landau collision operators and general moment equations for an electron-ion plasma

Jeong-Young Ji and Eric D. Held

Phys. Plasmas 15, 102101 (2008); http://dx.doi.org/10.1063/1.2977983 (13 pages) | Cited 2 times

Online Publication Date: 3 October 2008

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The general moment equations for an electron-ion plasma are established. The distribution functions for electrons and ions are expanded in terms of orthogonal polynomials of random velocity variables in contrast to the total velocity variables [ J.-Y. Ji and E. D. Held, Phys. Plasmas 13, 102103 (2006) ]. The moments of the streaming part of the kinetic equation are explicitly written with simple formulas. A simple version of the exact linearized Coulomb collision integrals is presented for like species. The electron-ion and ion-electron operators that conserve momentum and energy are also calculated with a small mass-ratio approximation. It is shown in the relaxation theory that the Lorentz operator, as a replacement of the like-species operator, is acceptable only for the high-order harmonic moments.
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52.20.Fs Electron collisions
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.25.Dg Plasma kinetic equations

Numerical and theoretical study of Bernstein modes in a magnetized quantum plasma

Bengt Eliasson and Padma K. Shukla

Phys. Plasmas 15, 102102 (2008); http://dx.doi.org/10.1063/1.2994723 (5 pages) | Cited 4 times

Online Publication Date: 7 October 2008

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A numerical and theoretical study is presented for the propagation of electron Bernstein modes in a magnetized quantum plasma. The dispersion relation for electrostatic waves is derived, using a semiclassical Vlasov model for Fermi–Dirac distributed electrons. The dispersion relation is checked numerically with direct Vlasov simulations, where the wave energy is concentrated to the Bernstein modes as well as to the zero-frequency convective mode. Dispersion relations in the long wavelength limit are derived, including the upper hybrid dispersion relation for a degenerate quantum plasma.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.65.Ff Fokker-Planck and Vlasov equation
05.30.Fk Fermion systems and electron gas

Linear wave propagation in relativistic magnetohydrodynamics

R. Keppens and Z. Meliani

Phys. Plasmas 15, 102103 (2008); http://dx.doi.org/10.1063/1.2991408 (11 pages) | Cited 4 times

Online Publication Date: 8 October 2008

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The properties of linear Alfvén, slow, and fast magnetoacoustic waves for uniform plasmas in relativistic magnetohydrodynamics (MHD) are discussed, augmenting the well-known expressions for their phase speeds with knowledge on the group speed. A 3+1 formalism is purposely adopted to make direct comparison with the Newtonian MHD limits easier and to stress the graphical representation of their anisotropic linear wave properties using the phase and group speed diagrams. By drawing these for both the fluid rest frame and for a laboratory Lorentzian frame which sees the plasma move with a three-velocity having an arbitrary orientation with respect to the magnetic field, a graphical view of the relativistic aberration effects is obtained for all three MHD wave families. Moreover, it is confirmed that the classical Huygens construction relates the phase and group speed diagram in the usual way, even for the lab frame viewpoint. Since the group speed diagrams correspond to exact solutions for initial conditions corresponding to a localized point perturbation, their formulae and geometrical construction can serve to benchmark current high-resolution algorithms for numerical relativistic MHD.
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47.35.Tv Magnetohydrodynamic waves
47.75.+f Relativistic fluid dynamics
52.27.Ny Relativistic plasmas
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)

Multidimensional, autoresonant three-wave interactions

O. Yaakobi and L. Friedland

Phys. Plasmas 15, 102104 (2008); http://dx.doi.org/10.1063/1.2992529 (9 pages) | Cited 2 times

Online Publication Date: 8 October 2008

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The theory of autoresonant three-wave interactions is generalized to more than one space and/or time variation of the background medium. In the most general case, the three waves propagate in a four-dimensional (4D) slowly space-time varying background, with an embedded 3D linear resonance hypersurface, where the linear frequency and wave-vector matching conditions of the three waves are satisfied exactly. The autoresonance in the system is the result of weak nonlinear frequency shifts and nonuniformity in the problem and is manifested by satisfaction of the nonlinear resonance conditions in an extended region of space-time adjacent to the resonance surface despite the variation of the background. The threshold condition for autoresonance is found and further discussed in application to stimulated Raman scattering in a 1D, time-dependent plasma case. Asymptotic description of the autoresonant waves far away from the resonance surface is obtained. The theory is illustrated and tested in 2D numerical simulations.
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52.35.-g Waves, oscillations, and instabilities in plasmas and intense beams
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)

Energetic electron acceleration during multi-island coalescence

P. L. Pritchett

Phys. Plasmas 15, 102105 (2008); http://dx.doi.org/10.1063/1.2996321 (9 pages) | Cited 16 times

Online Publication Date: 8 October 2008

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The mechanisms for the production of relativistic electrons associated with the coalescence/reconnection of multiple magnetic islands are investigated using two-dimensional particle-in-cell simulations for the case where the initial island half width L is comparable to the ion inertia length. Configurations without and with a uniform magnetic guide field are considered. Significant energization occurs only when the number of islands is reduced to 2 or 3 with wavelength satisfying kxL≲0.2. The energization proceeds in two distinct stages. In the first stage, a small number of electrons are accelerated to relativistic energies at the X-line by the inductive electric field, corresponding to perpendicular acceleration in the absence of the guide field and parallel/anti-parallel acceleration with a guide field. The second stage is associated with the final coalescence into one large island and produces a considerably larger number of relativistic electrons. With a guide field, this stage is dominated by the formation of elongated density cavities along one pair of separatrices and continued direct acceleration at the X-line. Without the guide field, the direct X-line acceleration becomes unimportant, and the acceleration is localized in the flux pile-up regions and results from the curvature drift interacting with the localized inductive electric field. Typically, some 15%–20% of the decrease in magnetic field energy is transferred to the electrons, with a few percent appearing in relativistic (E/mec2>0.3) electrons.
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52.25.Fi Transport properties
52.35.Vd Magnetic reconnection
52.65.Rr Particle-in-cell method

The scaling of forced collisionless reconnection

Brian P. Sullivan and Barrett N. Rogers

Phys. Plasmas 15, 102106 (2008); http://dx.doi.org/10.1063/1.2992136 (9 pages) | Cited 2 times

Online Publication Date: 9 October 2008

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This paper presents two-fluid simulations of forced magnetic reconnection with finite electron inertia in a collisionless three-dimensional (3D) cube with periodic boundaries in all three directions. Comparisons are made to analogous two-dimensional (2D) simulations. Reconnection in this system is driven by a spatially localized forcing function that is added to the ion momentum equation inside the computational domain. Consistent with previous results in similar, but larger forced 2D simulations [ B. Sullivan, B. N. Rogers, and M. A. Shay, Phys. Plasmas 12, 122312 (2005) ], for sufficiently strong forcing the reconnection process is found to become Alfvénic in both 2D and 3D, i.e., the inflow velocity scales roughly like some fraction of the Alfvén speed based on the upstream reconnecting magnetic field, and the system takes on a stable configuration with a dissipation region aspect ratio on the order of 0.15.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Fi Transport properties
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.65.Kj Magnetohydrodynamic and fluid equation

Two-dimensional fully kinetic simulations of driven magnetic reconnection with boundary conditions relevant to the Magnetic Reconnection Experiment

S. Dorfman, W. Daughton, V. Roytershteyn, H. Ji, Y. Ren, and M. Yamada

Phys. Plasmas 15, 102107 (2008); http://dx.doi.org/10.1063/1.2991361 (14 pages) | Cited 7 times

Online Publication Date: 17 October 2008

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Two-dimensional fully kinetic simulations are performed using global boundary conditions relevant to model the Magnetic Reconnection Experiment (MRX) [ M. Yamada et al., Phys Plasmas 4, 1936 (1997) ]. The geometry is scaled in terms of the ion kinetic scales in the experiment, and a reconnection layer is created by reducing the toroidal current in the flux cores in a manner similar to the actual experiment. The ion-scale features in these kinetic simulations are in remarkable agreement with those observed in MRX, including the reconnection inflow rate and quadrupole field structure. In contrast, there are significant discrepancies in the simulated structure of the electron layer that remain unexplained. In particular, the measured thickness of the electron layers is 3–5 times thicker in MRX than in the kinetic simulations. The layer length is highly sensitive to downstream boundary conditions as well as the time over which the simulation is driven. However, for a fixed set of chosen boundary conditions, an extrapolation of the scaling with the ion to electron mass ratio implies that at realistic mass ratio both the length and width will be too small compared to the experiment. This discrepancy implies that the basic electron layer physics may differ significantly between MRX and the two-dimensional, collisionless simulations. The two leading possibilities to explain the discrepancy are weak Coulomb collisions and three-dimensional effects that are present in the experiment but not included in the simulation model.
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52.35.Vd Magnetic reconnection
52.65.Kj Magnetohydrodynamic and fluid equation

Electrostatic drift modes in quantum pair plasmas

Hai Jun Ren, Jintao Cao, and Zhengwei Wu

Phys. Plasmas 15, 102108 (2008); http://dx.doi.org/10.1063/1.3000358 (7 pages) | Cited 5 times

Online Publication Date: 24 October 2008

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Electrostatic drift waves in a nonuniform quantum magnetized electron-positron (pair) plasma are investigated. An explicit and straightforward analytical expression of the fluctuation frequency is presented. The effects induced by quantum fluctuations, density gradients, and magnetic field inhomogeneity on the wave frequencies are discussed and a purely quantum drift mode appears. The present analytical investigations are relevant to dense astrophysical objects as well as laboratory ultracold plasmas.
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52.27.Ep Electron-positron plasmas
03.65.-w Quantum mechanics
52.25.Xz Magnetized plasmas
52.35.Kt Drift waves

Equilibrium and magnetic properties of a rotating plasma annulus

Zhehui Wang, Jiahe Si, Wei Liu, and Hui Li

Phys. Plasmas 15, 102109 (2008); http://dx.doi.org/10.1063/1.3002395 (11 pages) | Cited 6 times

Online Publication Date: 24 October 2008

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Local linear analysis shows that magneto-rotational instability can be excited in laboratory rotating plasmas with a density of 1019 m−3, a temperature on the order of 10 eV, and a magnetic field on the order of 100 G. A laboratory plasma annulus experiment with a dimension of ∼ 1 m, and rotation at ∼ 0.5 sound speed is described. Correspondingly, magnetic Reynolds number of these plasmas is ∼ 1000, and magnetic Prandtl number ranges from about one to a few hundred. A radial equilibrium, ρUθ2/r = d(p+Bz2/2μ0)/dr = K0, with K0 being a nonzero constant, is proposed for the experimental data. Plasma rotation is observed to drive a quasisteady diamagnetic electrical current (rotational current drive) in a high-β plasma annulus. The rotational energy depends on the direction and the magnitude of the externally applied magnetic field. Radial current (Jr) is produced through biasing the center rod at a negative electric potential relative to the outer wall. Jr×Bz torque generates and sustains the plasma rotation. Rotational current drive can reverse the direction of vacuum magnetic field, satisfying a necessary condition for self-generated closed magnetic flux surfaces inside plasmas. The Hall term is found to be substantial and therefore needs to be included in the Ohm’s law for the plasmas. Azimuthal magnetic field (Bθ) is found to be comparable with the externally applied vacuum magnetic field Bz, and mainly caused by the electric current flowing in the center cylinder; thus, Bθ∝r−1. Magnetic fluctuations are anisotropic, radial-dependent, and contain many Fourier modes below the ion cyclotron frequency. Further theoretical analysis reflecting these observations is needed to interpret the magnetic fluctuations.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.25.Gj Fluctuation and chaos phenomena

Mode conversion of Langmuir to electromagnetic waves at magnetic field-aligned density inhomogeneities: Simulations, theory, and applications to the solar wind and the corona

Eun-Hwa Kim, Iver H. Cairns, and Peter A. Robinson

Phys. Plasmas 15, 102110 (2008); http://dx.doi.org/10.1063/1.2994719 (19 pages) | Cited 2 times

Online Publication Date: 27 October 2008

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Linear mode conversion of Langmuir waves to radiation near the plasma frequency at density gradients is potentially relevant to multiple solar radio emissions, ionospheric radar experiments, laboratory plasma devices, and pulsars. Here we study mode conversion in warm magnetized plasmas using a numerical electron fluid simulation code with the density gradient parallel to the ambient magnetic field B0 for a range of incident Langmuir wavevectors. Our results include: (1) both o- and x-mode waves are produced for Ω = (ωL/c)1/3(ωc/ω)≲1, contrary to previous ideas. Only the o mode is produced for Ω≳1.5. Here ωc is the (angular) electron cyclotron frequency, ω is the angular wave frequency, L is the length scale of the (linear) density gradient, and c is the speed of light. A WKB-style analysis accounts semiquantitatively for the production and relative conversion efficiencies of the o and x modes in the simulations. (2) In the unmagnetized limit, equal amounts of o- and x-mode radiation are produced. (3) The mode conversion window narrows as Ω increases. (4) As Ω increases the total electromagnetic field changes from linear to circular polarization, with the o- and x-mode signals remaining circularly polarized. (5) The conversion efficiency to the x mode decreases monotonically as Ω increases while the o-mode conversion efficiency oscillates due to an interference phenomenon between incoming and reflected Langmuir/z modes. (6) The maximum total conversion efficiencies for wave power from the Langmuir/z mode to radiation are of order 50%–70%. They depend strongly on the wave frequency when close to the background plasma frequency but weakly on the electron temperature T0 and β = T0/mc2. The corresponding energy conversion efficiencies are favored since they allow separation into o and x modes, use directly measured experimental quantities, and generalize easily for wave packets. The total energy conversion efficiency differs from the power conversion efficiency by the ratio of the group speeds for each mode, is less than 10% for the value of β = 0.01 simulated, and decreases linearly with β. Since β ≈ 10−5–10−4 in the solar wind and corona, this β dependence is important in applications. (7) The interference effect and the disappearance of the x mode at Ω≳1 can be accounted for semiquantitatively using a WKB-type analysis. (8) Constraints on density turbulence are developed for the x mode to be generated and be able to propagate from the source. (9) Standard parameters for the corona and the solar wind near 1 AU suggest that linear mode conversion should produce both o- and x-mode radiation for solar and interplanetary radio bursts. It is therefore possible that linear mode conversion under these conditions might explain the weak total circular polarizations of type II and III solar radio bursts.
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52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
52.25.Xz Magnetized plasmas
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
96.60.Vg Particle emission, solar wind
96.60.P- Corona

Multidimensional instability of electron-acoustic solitary waves in a magnetized plasma with vortexlike electron distribution

M. G. M. Anowar and A. A. Mamun

Phys. Plasmas 15, 102111 (2008); http://dx.doi.org/10.1063/1.3006087 (6 pages) | Cited 2 times

Online Publication Date: 29 October 2008

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The basic features of obliquely propagating electron-acoustic (EA) solitary waves and their multidimensional instability in a magnetized plasma containing cold electrons, hot electrons obeying a vortexlike distribution, and stationary ions have been theoretically investigated by the reductive perturbation method and small-k perturbation expansion technique. The combined effects of external magnetic field (obliqueness) and trapped electron distribution, which are found to significantly modify the basic properties (amplitude and width) of small but finite-amplitude EA solitary waves, are explicitly examined. It is also found that the instability criterion and the growth rate are significantly modified by the external magnetic field and the propagation directions of both the nonlinear waves and their perturbation modes. The implications of our results in space plasmas are briefly discussed.
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52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.35.Sb Solitons; BGK modes
52.35.We Plasma vorticity

Observation of fast-ion Doppler-shifted cyclotron resonance with shear Alfvén waves

Yang Zhang, W. W. Heidbrink, H. Boehmer, R. McWilliams, S. Vincena, T. A. Carter, W. Gekelman, D. Leneman, and P. Pribyl

Phys. Plasmas 15, 102112 (2008); http://dx.doi.org/10.1063/1.2996323 (16 pages) | Cited 4 times

Online Publication Date: 30 October 2008

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The Doppler-shifted cyclotron resonance (ω−kzvz = Ωf) between fast ions and shear Alfvén waves is experimentally investigated (ω, wave frequency; kz, axial wavenumber; vz, fast-ion axial speed; Ωf, fast-ion cyclotron frequency). A test particle beam of fast ions is launched by a Li+ source in the helium plasma of the LArge Plasma Device (LAPD) [ W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991) ], with shear Alfvén waves (SAW) (amplitude δ B/B up to 1%) launched by a loop antenna. A collimated fast-ion energy analyzer measures the nonclassical spreading of the beam, which is proportional to the resonance with the wave. A resonance spectrum is observed by launching SAWs at 0.3–0.8ωci. Both the magnitude and frequency dependence of the beam-spreading are in agreement with the theoretical prediction using a Monte Carlo Lorentz code that launches fast ions with an initial spread in real/velocity space and random phases relative to the wave. Measured wave magnetic field data are used in the simulation.
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52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)

Double layer in an expanding plasma: Simultaneous upstream and downstream measurements

H. S. Byhring, C. Charles, Å. Fredriksen, and R. W. Boswell

Phys. Plasmas 15, 102113 (2008); http://dx.doi.org/10.1063/1.3002396 (6 pages) | Cited 11 times

Online Publication Date: 30 October 2008

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Ion energy measurements were taken simultaneously using one retarding field energy analyzer placed at the open end of the plasma source, and one in the plasma diffusion region of an expanding low pressure argon plasma. An electric double layer was found, which is well separated from the region of high magnetic field and which is downstream of the maximum in the magnetic field gradient. An axially movable analyzer was used to determine the position of the double layer. It appears to be more closely connected to the rapid change in diameter from the source to the diffusion chamber, but still has a radial dimension close to that of the source diameter. These results suggest that the double layer forms, not as much as a result of a magnetic nozzle, but rather as a reaction to a dramatic change in boundary conditions. Still, a magnetic field of at least a few tens of Gauss in the double layer region is necessary for its spontaneous formation.
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52.40.Kh Plasma sheaths
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.70.-m Plasma diagnostic techniques and instrumentation
back to top Nonlinear Phenomena, Turbulence, Transport

Self-focusing of electromagnetic pulsed beams in collisional plasmas

Mohammad Faisal, M. P. Verma, and Mahendra Singh Sodha

Phys. Plasmas 15, 102301 (2008); http://dx.doi.org/10.1063/1.2991360 (6 pages) | Cited 2 times

Online Publication Date: 2 October 2008

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In this paper, the self-focusing of an electromagnetic pulsed beam in a collisional plasma has been investigated in the paraxial approximation, following the formalism developed by Akhmanov. The energy balance equation for electrons, the equation expressing the equality of pressure gradient (of electrons and ions) to the force due to space charge field, and the equation for the beam width parameter f (obtained by following Akhmanov’s approach) have been simultaneously solved for given initial (z = 0) time profile of the pulse to obtain f as a function of ξ (cz/ωr02) and t′ = t−z/Vg, where Vg is the group velocity. Both Gaussian and sine time profiles of the pulse have been investigated.
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52.40.Mj Particle beam interactions in plasmas
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation

Evolutions of non-steady-state magnetic reconnection

Weigang Wan and Giovanni Lapenta

Phys. Plasmas 15, 102302 (2008); http://dx.doi.org/10.1063/1.2991406 (10 pages) | Cited 6 times

Online Publication Date: 3 October 2008

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The full evolutions of collisionless non-steady-state magnetic reconnection are studied with full kinetic particle-in-cell simulations. There are different stages of reconnection: the onset or early growing stage when the out-of-plane electric field (Ey) structure is a monopole at the X-point, the bipolar stage when the Ey structure is bipolar and the outer electron diffusion region (EDR) is being elongated over time, and the possible final steady-state stage when Ey is uniform in the reconnection plane. We find the change of reconnection rate is not empowered or dependent on the length of the EDR. During the early growing stage, the EDR is elongated while the reconnection rate is growing. During the later stage, the reconnection rate may significantly decrease but the length of the inner EDR is largely stable. The results indicate that reconnection is not controlled by the downstream physics, but rather by the availability of plasma inflows from upstream. The physical mechanism of the EDR elongation is studied. The Hall current induced by the quadrupole magnetic field (By) is discovered to play an important role in this process. The condition of forming an extended electron super-Alfvénic outflow jet structure in nature is discussed. The jet structure could be elongated during the bipolar stage, and remains stable during steady state. The sufficiency of the electron inflow is crucial for the elongation. Open boundary conditions are applied in the outflow direction.
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52.35.Vd Magnetic reconnection
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.65.Rr Particle-in-cell method

Hall magnetohydrodynamics in a strong magnetic field

Daniel O. Gómez, Swadesh M. Mahajan, and Pablo Dmitruk

Phys. Plasmas 15, 102303 (2008); http://dx.doi.org/10.1063/1.2991395 (6 pages) | Cited 7 times

Online Publication Date: 7 October 2008

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For a plasma embedded in a strong external magnetic field, the spatial structures tend to develop fine scales preferentially across the field, rather than along the parallel direction. This feature, which allowed a major simplification in the theoretical structure of one-fluid magnetohydrodynamics (leading to reduced magnetohydrodynamics), is exploited here to derive what may be called the reduced Hall magnetohydrodynamic equations (RHMHD) reflecting two-fluid effects such as the Hall current and the electron pressure. These physical effects, which can be relevant in astrophysical environments and also in fusion plasmas, allow for the propagation of circularly polarized normal modes such as whistlers and shear/ion-cyclotron waves. In this paper, the RHMHD system of equations is integrated numerically, to investigate externally driven turbulence.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Ra Plasma turbulence

Blob dynamics in an inhomogeneous plasma

K. Bodi, S. I. Krasheninnikov, and A. I. Smolyakov

Phys. Plasmas 15, 102304 (2008); http://dx.doi.org/10.1063/1.2993211 (5 pages) | Cited 1 time

Online Publication Date: 8 October 2008

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Convective blob propagation in the scrape-off-layer and/or limiter shadow region is analyzed analytically and numerically with emphasis on effects of a gradient in the equilibrium plasma density. The gradient of the equilibrium plasma density is taken into account beyond the Boussineque approximation. It is shown that the vorticity modification due to the plasma density gradient leads to the acceleration for the blobs propagating into the region of lower density and de-acceleration for the blobs propagating toward the regions of higher density. Analytical estimates are corroborated by direct numerical simulations.
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52.25.Fi Transport properties
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.65.-y Plasma simulation

Whistler turbulence: Particle-in-cell simulations

Shinji Saito, S. Peter Gary, Hui Li, and Yasuhito Narita

Phys. Plasmas 15, 102305 (2008); http://dx.doi.org/10.1063/1.2997339 (8 pages) | Cited 37 times

Online Publication Date: 15 October 2008

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Two-dimensional electromagnetic particle-in-cell simulations in a magnetized, homogeneous, collisionless electron-proton plasma demonstrate the forward cascade of whistler turbulence. The simulations represent decaying turbulence, in which an initial, narrowband spectrum of fluctuations at wavenumbers kc/ωe ≃ 0.1 cascades toward increased damping at kc/ωe ≃ 1.0, where c/ωe is the electron inertial length. The turbulence displays magnetic energy spectra that are relatively steep functions of wavenumber and are anisotropic with more energy in directions relatively perpendicular to the background magnetic field Bo = mathBo than at the same wavenumbers parallel to Bo. In the weak turbulence regime, the primary new results of the simulations are as follows: (1) Magnetic spectra of the cascading fluctuations become more anisotropic with increasing fluctuation energy; (2) the wavevector dependence of the three magnetic energy ratios, ∣δBj∣2/∣δB∣2 with j = x,y,z, show good agreement with linear dispersion theory for whistler fluctuations; (3) the magnetic compressibility summed over the cascading modes satisfies 0.3≲∣δBx∣2/∣δB∣2≲0.6; and (4) the turbulence heats electrons in directions both parallel and perpendicular to Bo, with stronger heating in the parallel direction.
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52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
52.35.Ra Plasma turbulence
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Gj Fluctuation and chaos phenomena
52.65.Rr Particle-in-cell method

Gyrokinetic microinstabilities in ASDEX Upgrade edge plasmas

D. Told, F. Jenko, P. Xanthopoulos, L. D. Horton, E. Wolfrum, and ASDEX Upgrade Team

Phys. Plasmas 15, 102306 (2008); http://dx.doi.org/10.1063/1.3000132 (11 pages) | Cited 9 times

Online Publication Date: 17 October 2008

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Results of linear gyrokinetic simulations of ASDEX Upgrade [ O. Gruber et al., Nucl. Fusion 39, 1321 (1999) ] edge plasmas, with experimentally determined geometry and input parameters, are presented. It is found that in the near-edge region, microtearing modes can exist under conditions found in conventional tokamaks. As one enters the steep-gradient region, the growth rate spectrum is dominated—down to very low wavenumbers—by electron temperature gradient modes. The latter tend to peak near the X-point(s) and possess properties which may explain the ratios of the density and temperature gradient scale lengths that have been observed in various experiments over the last decade.
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52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.65.Tt Gyrofluid and gyrokinetic simulations
52.40.Hf Plasma-material interactions; boundary layer effects
52.55.Fa Tokamaks, spherical tokamaks

Dynamics of Alfvén solitons in inhomogeneous plasmas

Tao Xu, Bo Tian, Li-Li Li, Xing Lü, and Cheng Zhang

Phys. Plasmas 15, 102307 (2008); http://dx.doi.org/10.1063/1.2997340 (6 pages) | Cited 25 times

Online Publication Date: 22 October 2008

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To provide an analytical scheme for the dynamical behavior of nonlinear Alfvén waves in inhomogeneous plasmas, this paper investigates a generalized variable-coefficient derivative nonlinear Schrödinger equation. In the sense of admitting the Lax pair and infinitely many conservation laws, the integrability of this equation is established under certain coefficient constraint which suggests which inhomogeneities support stable Alfvén solitons. The Hirota method is adopted to construct the one- and multi-Alfvén-soliton solutions. The inhomogeneous soliton features are also discussed through analyzing some important physical quantities. A sample model is treated with our results, and graphical illustration presents two energy-radiating Alfvén soliton structures.
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05.45.Yv Solitons
94.05.Fg Solitons and solitary waves
02.30.Ik Integrable systems

Stable and unstable invariant manifolds in a partially chaotic magnetic configuration generated by nonlinear reconnection

D. Borgogno, D. Grasso, F. Pegoraro, and T. J. Schep

Phys. Plasmas 15, 102308 (2008); http://dx.doi.org/10.1063/1.2999539 (7 pages) | Cited 7 times

Online Publication Date: 22 October 2008

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A numerical contour dynamics code has been employed to calculate the stable and unstable manifolds related to two interacting magnetic island chains. The magnetic configuration is generated by a nonlinear reconnection process described in D. Borgogno et al. [Phys. Plasmas. 12, 032309 (2005) ]. The appearance of the first homoclinic and heteroclinic intersections of the dominant manifolds are shown and one of the associated uniformly hyperbolic orbits is given. The stickiness of the field lines around the island and the eventual development of global stochasticity are discussed. The basic geometry of the magnetic configuration is periodic so that the structure of the manifolds may be compared with the one obtained with Poincaré plots.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Gj Fluctuation and chaos phenomena

Finite Larmor radius effects on nondiffusive tracer transport in a zonal flow

K. Gustafson, D. del-Castillo-Negrete, and W. Dorland

Phys. Plasmas 15, 102309 (2008); http://dx.doi.org/10.1063/1.3003072 (13 pages) | Cited 5 times

Online Publication Date: 28 October 2008

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Finite Larmor radius (FLR) effects on nondiffusive transport in a prototypical zonal flow with drift waves are studied in the context of a simplified chaotic transport model. The model consists of a superposition of drift waves from the linearized Hasegawa–Mima equation and a zonal shear flow perpendicular to the density gradient. High frequency FLR effects are incorporated by gyroaveraging the E×B velocity. Transport in the direction of the density gradient is negligible and we therefore focus on transport parallel to the zonal flows. A prescribed asymmetry produces strongly asymmetric non-Gaussian probability distribution functions (PDFs) of particle displacements, with Lévy flights in one direction only. For k⊥ρth = 0, where k⊥ is the characteristic wavelength of the flow and ρth is the thermal Larmor radius, a transition is observed in the scaling of the second moment of particle displacements: σ2 ∼ tγ. The transition separates ballistic motion (γ ≈ 2) at intermediate times from superdiffusion (γ = 1.6) at larger times. This change of scaling is accompanied by the transition of the PDF of particle displacements from algebraic decay to exponential decay. However, FLR effects seem to eliminate this transition. In all cases, the Lagrangian velocity autocorrelation function exhibits nondiffusive algebraic decay, C ∼ τ−κ, with κ = 2−γ to a good approximation. The PDFs of trapping and flight events show clear evidence of algebraic scaling with decay exponents depending on the value of k⊥ρth. The shape and spatiotemporal self-similar anomalous scaling of the PDFs of particle displacements are reproduced accurately with a neutral (α = β), asymmetric, effective fractional diffusion model, where α and β are the orders of the spatial and temporal fractional derivatives, respectively.
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52.25.Gj Fluctuation and chaos phenomena
52.35.Kt Drift waves
52.65.Cc Particle orbit and trajectory
05.40.Fb Random walks and Levy flights
05.45.Pq Numerical simulations of chaotic systems
52.25.Fi Transport properties
52.65.-y Plasma simulation

Gyrokinetic turbulence simulations at high plasma beta

M. J. Pueschel, M. Kammerer, and F. Jenko

Phys. Plasmas 15, 102310 (2008); http://dx.doi.org/10.1063/1.3005380 (10 pages) | Cited 17 times

Online Publication Date: 29 October 2008

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Electromagnetic gyrokinetic turbulence simulations employing Cyclone Base Case parameters are presented for β values up to and beyond the kinetic ballooning threshold. The β scaling of the turbulent transport is found to be linked to a complex interplay of linear and nonlinear effects. Linear investigation of the kinetic ballooning mode is performed in detail, while nonlinearly, it is found to dominate the turbulence only in a fairly narrow range of β values just below the respective ideal limit. The magnetic transport scales like β2 and is well described by a Rechester–Rosenbluth-type ansatz.
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52.35.Ra Plasma turbulence
52.25.Fi Transport properties
52.65.-y Plasma simulation
52.59.-f Intense particle beams and radiation sources
52.75.-d Plasma devices
back to top Magnetically Confined Plasmas, Heating, Confinement

Far-field sheaths due to fast waves incident on material boundaries

D. A. D’Ippolito, J. R. Myra, E. F. Jaeger, and L. A. Berry

Phys. Plasmas 15, 102501 (2008); http://dx.doi.org/10.1063/1.2990025 (12 pages) | Cited 5 times

Online Publication Date: 6 October 2008

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The problem of “far-field” sheath formation is studied with a new quantitative one-dimensional model. These radio-frequency (rf) sheaths occur when unabsorbed fast waves in the ion cyclotron range of frequencies are incident on a conducting surface not aligned with a flux surface. Use of a nonlinear sheath boundary condition gives self-consistent solutions for the wave fields and sheath characteristics, and it introduces a sheath-plasma-wave resonance which can enhance the sheath potential. The model is used to compute the parametric dependence of the far-field sheath potential. Its application to post-process the rf fields computed by a full-wave code for a typical D(H) minority heating scenario is also discussed. This work shows that two-dimensional effects (included heuristically) are essential in determining whether far-field sheath potentials are strong enough to cause significant edge interactions, such as impurity generation and reduced heating efficiency.
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52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.40.Kh Plasma sheaths
52.50.Qt Plasma heating by radio-frequency fields; ICR, ICP, helicons
52.55.Fa Tokamaks, spherical tokamaks
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