POP Nameplate
  • Volume/Page
  • Keyword
  • DOI
  • Citation
  • Advanced
   
 
 
 

Flickr Twitter UniPHY Group iResearch App Facebook

BROWSE VOLUMES

Year Range: 
Search Issue | RSS Feeds RSS
Previous Issue

Feb 2012

Volume 19, Issue 2 (partial)

Page 1 of 3 Pages Next Page | Jump to Page
back to top
RSS Feeds
FREE

Cnoidal electron hole propagation: Trapping, the forgotten nonlinearity in plasma and fluid dynamics

Hans Schamel

Phys. Plasmas 19, 020501 (2012); http://dx.doi.org/10.1063/1.3682047 (17 pages)

Online Publication Date: 15 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
In this review a plaidoyer is held for a specific form of nonlinearity, the trapping nonlinearity (TN), which arises due to a capture of particles and/or fluid elements in an excited coherent structure. This is of some importance since it appears that TN has not yet taken roots hitherto, neither in turbulence nor in anomalous transport models. The present state of knowledge about wave excitation, seen numerically and experimentally, especially at space craft, however, speaks a different language suggesting that current wave models are constructed too narrowly to reflect reality. The focus is on traveling cnoidal electron holes (CEHs) in electrostatically driven plasmas and the physical world associated with these. As a result a new wave concept emerges, in which the low amplitude dynamics is nonlinearly controlled by TN.
Show PACS
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.25.Dg Plasma kinetic equations
52.30.-q Plasma dynamics and flow
47.32.-y Vortex dynamics; rotating fluids
52.35.Sb Solitons; BGK modes
52.35.Ra Plasma turbulence
back to top
RSS Feeds

Analysis on the exclusiveness of turbulence suppression between static and time-varying shear flow

Y. Z. Zhang, T. Xie, and S. M. Mahajan

Phys. Plasmas 19, 020701 (2012); http://dx.doi.org/10.1063/1.3676597 (4 pages)

Online Publication Date: 3 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The analytical theory of turbulence suppression by shear flow [Y. Z. Zhang and S. M. Mahajan, Phys. Fluids B 4, 1385 (1992)] is extended to analyze the combined actions of flows that have time-varying as well as static components. It is found that each component, appearing alone, may yield the same suppression level. However, when both components co-exist, either tends to diminish the suppression caused by the other in certain parameter ranges—a conclusion that agrees with recently published simulation results by Maeyama et al. [Phys. Plasmas 17, 062305 (2010)]. In particular, the mutual exclusiveness is maximized as the strengths of the two components become comparable. The adopted averaging method of the asymptotic theory reveals that it is the coupling between the time-varying shear flow and the induced time-varying relative orbit motion that causes the asymmetry of the two components in turbulence suppression. The numerical results based on a Floquet analysis are also presented for comparison. The implications of the theory to L-H transition on tokamaks are discussed, especially, regarding experimental observations of the disappearance of the geodesic acoustic mode in H phases.
Show PACS
52.35.Ra Plasma turbulence
52.55.Fa Tokamaks, spherical tokamaks
52.65.-y Plasma simulation
52.30.-q Plasma dynamics and flow

Relativistic spherical plasma waves

S. S. Bulanov, A. Maksimchuk, C. B. Schroeder, A. G. Zhidkov, E. Esarey, and W. P. Leemans

Phys. Plasmas 19, 020702 (2012); http://dx.doi.org/10.1063/1.3683001 (4 pages)

Online Publication Date: 14 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Tightly focused laser pulses that diverge or converge in underdense plasma can generate wake waves, having local structures that are spherical waves. Here we study theoretically and numerically relativistic spherical wake waves and their properties, including wave breaking.
Show PACS
52.27.Ny Relativistic plasmas
52.38.Dx Laser light absorption in plasmas (collisional, parametric, etc.)
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
02.60.-x Numerical approximation and analysis

An optical analysis tool for avoiding dust formation in very-high frequency hydrogen diluted silane plasmas at low substrate temperatures

M. M. de Jong, J. de Koning, J. K. Rath, and R. E. I. Schropp

Phys. Plasmas 19, 020703 (2012); http://dx.doi.org/10.1063/1.3683559 (4 pages)

Online Publication Date: 14 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Control of the formation of dust particles in a silane deposition plasma is very important for avoiding electrical shunts in devices, such as thin film silicon solar cells. In this work we present a noninvasive in situ method for identification of the plasma regime, based on optical emission spectroscopy (OES), which can be applied to silane/hydrogen plasmas at low substrate temperatures. By monitoring the OES spectra as a function of the position perpendicular to the plasma electrodes we developed a method to identify the transition of a plasma from the dust free to a dusty regime, which was confirmed by TEM images of layers deposited in both regimes. Using this technique we mapped this transition as a function of applied forward very-high frequency (VHF) power and hydrogen dilution at different substrate temperatures. The advantage of this technique is that the experiment is insensitive to optical transmission loss at the viewport due to deposition of silicon films. As the transition from the dust free to the dusty regime is substrate temperature dependent and the transition from amorphous to nanocrystalline growth mainly depends on hydrogen dilution, a limited parameter window has been defined in which dust-free amorphous silicon can be deposited at low substrate temperatures. A single simple OES technique can be used for in situ monitoring of amorphous to nanocrystalline transition as well as the onset of the dusty regime in a thin film silicon cell fabrication process.
Show PACS
52.27.Lw Dusty or complex plasmas; plasma crystals
52.70.Ds Electric and magnetic measurements
52.77.Dq Plasma-based ion implantation and deposition
52.25.Vy Impurities in plasmas

High-frequency devices with weakly relativistic hollow thin-wall electron beams

V. L. Bratman, A. E. Fedotov, and P. B. Makhalov

Phys. Plasmas 19, 020704 (2012); http://dx.doi.org/10.1063/1.3686138 (4 pages)

Online Publication Date: 15 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Slow-wave devices with hollow electron beams and azimuthally symmetric corrugated operating waveguides can be very effective not only for relativistic but also weakly relativistic particle energies. In the weakly relativistic case, the use of hollow beams permits a significant increase in the diameter of the beam channel and, simultaneously, a drastic decrease in the required current density and heat load at the interaction structure wall in comparison with the conventional devices, which basically exploit thin pencil-like beams. Advantages of the hollow beams in the achievement of continuous wave (CW) and long-pulse generation can manifest themselves in a wide range from gigahertz to terahertz frequencies. As an example of the concept, a W-band oscillator (orotron) with kilowatt output power in CW regime is discussed in detail. Modification of the microwave system makes it possible to implement high-power frequency-tunable BWOs, klystron or TWT amplifiers, and many types of hybrid devices.
Show PACS
84.40.Fe Microwave tubes (e.g., klystrons, magnetrons, traveling-wave, backward-wave tubes, etc.)
84.40.Az Waveguides, transmission lines, striplines
back to top
RSS Feeds
back to top Basic Plasma Phenomena, Waves, Instabilities

Reduced magnetohydrodynamic theory of oblique plasmoid instabilities

S. D. Baalrud, A. Bhattacharjee, and Y.-M. Huang

Phys. Plasmas 19, 022101 (2012); http://dx.doi.org/10.1063/1.3678211 (8 pages)

Online Publication Date: 3 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The three-dimensional nature of plasmoid instabilities is studied using the reduced magnetohydrodynamic equations. For a Harris equilibrium with guide field, represented by Bo = Bpotanh(x/λ)math+Bzomath, a spectrum of modes are unstable at multiple resonant surfaces in the current sheet, rather than just the null surface of the poloidal field Byo(x) = Bpotanh(x/λ), which is the only resonant surface in 2D or in the absence of a guide field. Here, Bpo is the asymptotic value of the equilibrium poloidal field, Bzo is the constant equilibrium guide field, and λ is the current sheet width. Plasmoids on each resonant surface have a unique angle of obliquity θ ≡ arctan(kz/ky). The resonant surface location for angle θ is xs = λarctanh(μ), where μ = tanθBzo/Bpo and the existence of a resonant surface requires |θ|<arctan(Bpo/Bzo). The most unstable angle is oblique, i.e., θ ≠ 0 and xs ≠ 0, in the constant-ψ regime, but parallel, i.e., θ = 0 and xs = 0, in the nonconstant-ψ regime. For a fixed angle of obliquity, the most unstable wavenumber lies at the intersection of the constant-ψ and nonconstant-ψ regimes. The growth rate of this mode is γmax/Γo ≃ SL1/4(1-μ4)1/2, in which Γo = VA/L, VA is the Alfvén speed, L is the current sheet length, and SL is the Lundquist number. The number of plasmoids scales as N~SL3/8(1-μ2)-1/4(1+μ2)3/4.
Show PACS
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Fi Transport properties

Fully kinetic description of the linear excitation and nonlinear saturation of fast-ion-driven geodesic acoustic mode instability

D. Zarzoso, X. Garbet, Y. Sarazin, R. Dumont, and V. Grandgirard

Phys. Plasmas 19, 022102 (2012); http://dx.doi.org/10.1063/1.3680633 (13 pages)

Online Publication Date: 8 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
We show in this paper that geodesic acoustic modes (GAMs) can be efficiently excited by a population of fast ions even when Landau damping on thermal ions is accounted for. We report in particular fully kinetic calculations of the GAM dispersion relation and its complete solution. Written under a variational form, the quasi-neutrality condition, together with the kinetic Vlasov equation, leads to the density of exchanged energy between particles and the mode. In particular, a linear threshold for the GAMs excitation is derived. Two examples of fast ion distribution have been discussed analytically. It turns out that particles with high perpendicular energy compared to the parallel resonance energy are most responsible for the excitation of the mode. Subsequent numerical simulations of circular plasmas using gysela code have been carried out. In particular, the linear kinetic threshold has been reproduced during the excitation phase, and a nonlinear saturation has been observed. Analysis in the phase space of the evolution of the equilibrium distribution function is presented and the saturation level quantified.
Show PACS
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.65.Ff Fokker-Planck and Vlasov equation
52.25.Dg Plasma kinetic equations
52.25.Fi Transport properties
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)

Enhancement of omnidirectional photonic band gaps in one-dimensional dielectric plasma photonic crystals with a matching layer

Hai-Feng Zhang, Shao-Bin Liu, Xiang-Kun Kong, Liang Zou, Chun-Zao Li, and Wu-shu Qing

Phys. Plasmas 19, 022103 (2012); http://dx.doi.org/10.1063/1.3680628 (7 pages)

Online Publication Date: 14 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
In this paper, we demonstrate by theoretical analysis a novel way to enhance the omnidirectional photonic band gap (OBG) in a type of photonic structure made of dielectric and plasma one-dimensional (1D) photonic crystals (1D PCs) by introducing a matching layer. Simulations by the transfer matrix method (TMM) show that such an OBG is insensitive to the incident angle and the polarization of electromagnetic (EM) wave; the frequency range and central frequency of OBG are significantly enlarged by introducing a matching layer in the heterostructure compared to 1D conventional binary dielectric photonic crystals (DPCs). The photonic band gap (PBG) of both polarizations also can be obviously enlarged as the incident angle is relatively small. The OBG originates from a Bragg gap in contrast to zero-math gap or single negative (negative permittivity or negative permeability) gap. From the numerical results, it has been shown that introducing a matching layer in such a heterostructure has a superior feature in the enhancement relative bandwidth of OBG compared with the conventional ternary plasma photonic crystals (PPCs); the frequency range of OBG can be notably enlarged by increasing the thickness and density of plasma layer.
Show PACS
52.27.Lw Dusty or complex plasmas; plasma crystals
52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma
52.40.Hf Plasma-material interactions; boundary layer effects
52.65.-y Plasma simulation
52.25.Mq Dielectric properties

2.5D magnetohydrodynamic simulation of the Kelvin-Helmholtz instability around Venus—Comparison of the influence of gravity and density increase

M. Zellinger, U. V. Möstl, N. V. Erkaev, and H. K. Biernat

Phys. Plasmas 19, 022104 (2012); http://dx.doi.org/10.1063/1.3682039 (8 pages)

Online Publication Date: 14 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
We present a numerical study of the 2.5D Kelvin-Helmholtz instability and its vortices, where an initial plasma configuration appropriate for the situation around unmagnetized planets is assumed. We solve the set of ideal magnetohydrodynamic equations numerically with the total variation diminishing Lax-Friedrichs algorithm. Our density profile is such that the mass density increases toward the planet. A high density leads to smaller growth rates of the instability and, thus, has a stabilizing effect for the boundary layer. Moreover, we include source terms in the equations, enabling us to study the influence of gravity. Our results show that gravity affects the evolution of the Kelvin-Helmholtz instability. However, the effect is not very significant. We thus conclude that the density increase toward the planet stabilizes the boundary layer around Venus more than gravity does.
Show PACS
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.65.Kj Magnetohydrodynamic and fluid equation
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
96.30.Ea Venus
95.30.Qd Magnetohydrodynamics and plasmas
52.35.We Plasma vorticity

Spontaneous electromagnetic fluctuations in unmagnetized plasmas I: General theory and nonrelativistic limit

R. Schlickeiser and P. H. Yoon

Phys. Plasmas 19, 022105 (2012); http://dx.doi.org/10.1063/1.3682985 (9 pages)

Online Publication Date: 15 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Using the system of the Klimontovich and Maxwell equations, general expressions for the electromagnetic fluctuation spectra (electric and magnetic field, charge and current densities) from uncorrelated plasma particles are derived, which are covariantly correct within the theory of special relativity. The general expressions hold for arbitrary momentum dependences of the plasma particle distribution functions and for collective and non-collective fluctuations. In this first paper of a series, the results are illustrated for the important special case of nonrelativistic isotropic Maxwellian particle distribution functions providing in particular the thermal fluctuations of weakly amplified modes and aperiodic modes.
Show PACS
52.25.Gj Fluctuation and chaos phenomena
52.25.Fi Transport properties
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma

Thermal force drift wave

C. P. Hung and A. B. Hassam

Phys. Plasmas 19, 022106 (2012); http://dx.doi.org/10.1063/1.3684027 (9 pages)

Online Publication Date: 15 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
A drift instability of a collisional magnetized plasma, unstable due to the Braginskii thermal force but not requiring any direct dissipation such as resistivity or electron inertia, is examined. Unlike conventional drift-modes, the maximum growth rate of the thermal force drift wave (TFDW) is of order the drift frequency, making for a strongly turbulent nonlinear state. A 3D, magnetized two-fluid code is developed to allow the study of both ideal MHD modes as well as lower frequency drift modes. The governing equations are essentially the ideal MHD equations with the inclusion of Hall and thermal force terms in Ohm’s law. This set of equations is reduced in a finite β, long parallel wavelength, and small but significant Larmor radius ordering and tested for shear Alfven waves, parallel sound waves, and drift modes. The code is employed to recover the TFDW instability, to verify the code against the mode’s analytic linear characteristics, and to study the nonlinear behavior of the TFDW. The TFDW growth is strongly suppressed by parallel thermal conduction and thus this mode is more likely to be observed in low temperature plasmas.
Show PACS
52.35.Kt Drift waves
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.25.Xz Magnetized plasmas
52.35.Ra Plasma turbulence

Modeling the Parker instability in a rotating plasma screw pinch

I. V. Khalzov, B. P. Brown, N. Katz, and C. B. Forest

Phys. Plasmas 19, 022107 (2012); http://dx.doi.org/10.1063/1.3684240 (10 pages)

Online Publication Date: 15 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
We analytically and numerically study the analogue of the Parker (magnetic buoyancy) instability in a uniformly rotating plasma screw pinch confined in a cylinder. Uniform plasma rotation is imposed to create a centrifugal acceleration, which mimics the gravity required for the classical Parker instability. The goal of this study is to determine how the Parker instability could be unambiguously identified in a weakly magnetized, rapidly rotating screw pinch, in which the rotation provides an effective gravity and a radially varying azimuthal field is controlled to give conditions for which the plasma is magnetically buoyant to inward motion. We show that an axial magnetic field is also required to circumvent conventional current driven magnetohydrodynamic (MHD) instabilities such as the sausage and kink modes that would obscure the Parker instability. These conditions can be realized in the Madison plasma Couette experiment (MPCX). Simulations are performed using the extended MHD code NIMROD for an isothermal compressible plasma model. Both linear and nonlinear regimes of the instability are studied, and the results obtained for the linear regime are compared with analytical results from a slab geometry. Based on this comparison, it is found that in a cylindrical pinch, the magnetic buoyancy mechanism dominates at relatively large Mach numbers (M > 5), while at low Mach numbers (M < 1), the instability is due to the curvature of magnetic field lines. At intermediate values of Mach number (1 < M < 5), the Coriolis force has a strong stabilizing effect on the plasma. A possible scenario for experimental demonstration of the Parker instability in MPCX is discussed.
Show PACS
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.55.Ez Theta pinch
52.65.Kj Magnetohydrodynamic and fluid equation
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)
02.60.-x Numerical approximation and analysis

Kinetic equilibrium for an asymmetric tangential layer

G. Belmont, N. Aunai, and R. Smets

Phys. Plasmas 19, 022108 (2012); http://dx.doi.org/10.1063/1.3685707 (10 pages)

Online Publication Date: 21 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Finding kinetic (Vlasov) equilibria for tangential current layers is a long standing problem, especially in the context of reconnection studies, when the magnetic field reverses. Its solution is of pivotal interest for both theoretical and technical reasons when such layers must be used for initializing kinetic simulations. The famous Harris equilibrium is known to be limited to symmetric layers surrounded by vacuum, with constant ion and electron flow velocities, and with current variation purely dependent on density variation. It is clearly not suited for the “magnetopause-like” layers, which separate two plasmas of different densities and temperatures, and for which the localization of the current density j = nδv is due to the localization of the electron-to-ion velocity difference δv and not of the density n. We present here a practical method for constructing a Vlasov stationary solution in the asymmetric case, extending the standard theoretical methods based on the particle motion invariants. We show that, in the case investigated of a coplanar reversal of the magnetic field without electrostatic field, the distribution function must necessarily be a multi-valued function of the invariants to get asymmetric profiles for the plasma parameters together with a symmetric current profile. We show also how the concept of “accessibility” makes these multi-valued functions possible, due to the particle excursion inside the layer being limited by the Larmor radius. In the presented method, the current profile across the layer is chosen as an input, while the ion density and temperature profiles in between the two asymptotic imposed values are a result of the calculation. It is shown that, assuming the distribution is continuous along the layer normal, these profiles have always a more complex profile than the profile of the current density and extends on a larger thickness. The different components of the pressure tensor are also outputs of the calculation and some conclusions concerning the symmetries of this tensor are pointed out.
Show PACS
52.25.Dg Plasma kinetic equations
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Fi Transport properties
52.65.Ff Fokker-Planck and Vlasov equation
52.35.Vd Magnetic reconnection

Pinching of ablation streams via magnetic field curvature in wire-array Z-pinches

I. C. Blesener, J. B. Greenly, B. R. Kusse, K. S. Blesener, C. E. Seyler, and D. A. Hammer

Phys. Plasmas 19, 022109 (2012); http://dx.doi.org/10.1063/1.3685726 (5 pages)

Online Publication Date: 21 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
In this paper, the shapes of the ablation streams in non-imploding cylindrical wire-array Z-pinches are investigated. Experimental observations using axial X pinch imaging show an azimuthal pinching of the streams that appear to depend on the topology of the global magnetic field. With fewer wires and increased interwire spacing, the radial component of the global field is increased; resulting in a stronger pinching of the streams. Computer simulations are used to model the magnetic field development and show that the sparser array has a significantly stronger azimuthal math×math force.
Show PACS
52.59.Qy Wire array Z-pinches
52.58.Lq Z-pinches, plasma focus, and other pinch devices
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.65.Kj Magnetohydrodynamic and fluid equation
52.70.La X-ray and γ-ray measurements
back to top Nonlinear Phenomena, Turbulence, Transport

Experimental investigation of geodesic acoustic mode spatial structure, intermittency, and interaction with turbulence in the DIII-D tokamak

J. C. Hillesheim, W. A. Peebles, T. A. Carter, L. Schmitz, and T. L. Rhodes

Phys. Plasmas 19, 022301 (2012); http://dx.doi.org/10.1063/1.3678210 (19 pages)

Online Publication Date: 1 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Geodesic acoustic modes (GAMs) and zonal flows are nonlinearly driven, axisymmetric (m = 0andn = 0) E×B flows, which are thought to play an important role in establishing the saturated level of turbulence in tokamaks. Results are presented showing the GAM’s observed spatial scales, temporal scales, and nonlinear interaction characteristics, which may have implications for the assumptions underpinning turbulence models towards the tokamak edge (r/a>rsim0.75). Measurements in the DIII-D tokamak [Luxon, Nucl. Fusion 42, 614 (2002)] have been made with multichannel Doppler backscattering systems at toroidal locations separated by 180∘; analysis reveals that the GAM is highly coherent between the toroidally separated systems (γ>0.8) and that measurements are consistent with the expected m = 0andn = 0 structure. Observations show that the GAM in L-mode plasmas with ~2.5-4.5 MW auxiliary heating occurs as a radially coherent eigenmode, rather than as a continuum of frequencies as occurs in lower temperature discharges; this is consistent with theoretical expectations when finite ion Larmor radius effects are included. The intermittency of the GAM has been quantified, revealing that its autocorrelation time is fairly short, ranging from about 4 to about 15 GAM periods in cases examined, a difference that is accompanied by a modification to the probability distribution function of the E×B velocity at the GAM frequency. Conditionally-averaged bispectral analysis shows the strength of the nonlinear interaction of the GAM with broadband turbulence can vary with the magnitude of the GAM. Data also indicate a wavenumber dependence to the GAM’s interaction with turbulence.
Show PACS
52.55.Fa Tokamaks, spherical tokamaks
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)
52.40.Hf Plasma-material interactions; boundary layer effects
52.25.Os Emission, absorption, and scattering of electromagnetic radiation

Simulation studies of positron acceleration in a shock wave in a nonuniform external magnetic field

Takashi Iwata, Seiichi Takahashi, and Yukiharu Ohsawa

Phys. Plasmas 19, 022302 (2012); http://dx.doi.org/10.1063/1.3676632 (8 pages)

Online Publication Date: 3 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Positron acceleration in a shock wave in an electron-positron-ion plasma is studied with one-dimensional, fully kinetic, electromagnetic particle simulations, with particular attention paid to the effect of inhomogeneity of external magnetic field B0. First, acceleration to γ ∼ 104, where γ is the Lorentz factor, is demonstrated for a shock wave in a uniform B0 with the shock speed νsh close to c cos θ, where c is the speed of light and θ is the angle between B0 and the wave normal. The acceleration is not saturated till the end of the simulation run. Then, the effect of nonuniformity of B0 is investigated: Comparisons are made between the case in which the difference (νsh − c cos θ) at the shock front changes from negative to positive values as the shock wave propagates and the case with this relation reversed. The latter is found to create a greater number of high-energy particles than the former.
Show PACS
52.35.Tc Shock waves and discontinuities
52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma
52.65.-y Plasma simulation
52.25.Dg Plasma kinetic equations
52.25.Fi Transport properties

Nonlinear entropy transfer via zonal flows in gyrokinetic plasma turbulence

M. Nakata, T.-H. Watanabe, and H. Sugama

Phys. Plasmas 19, 022303 (2012); http://dx.doi.org/10.1063/1.3675855 (14 pages)

Online Publication Date: 6 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Nonlinear entropy transfer processes in toroidal ion temperature gradient (ITG) and electron temperature gradient (ETG) driven turbulence are investigated based on the gyrokinetic entropy balance relations for zonal and non-zonal modes, which are coupled through the entropy transfer function regarded as a kinetic extension of the zonal-flow production due to the Reynolds stress. Spectral analyses of the “triad” entropy transfer function introduced in this study reveal not only the nonlinear interactions among the zonal and non-zonal modes, but also their effects on the turbulent transport level. Different types of the entropy transfer processes between the ITG and ETG turbulence are found: the entropy transfer from non-zonal to zonal modes is substantial in the saturation phase of the ITG instability, while, once the strong zonal flow is generated, the entropy transfer to the zonal modes becomes quite weak in the steady turbulence state. Instead, the zonal flows mediate the entropy transfer from non-zonal modes with low radial-wavenumbers (with contribution to the heat flux) to the other non-zonal modes with higher radial-wavenumbers (but with less contribution to the heat flux) through the triad interaction. The successive entropy transfer processes to the higher radial-wavenumber modes are associated with transport regulation in the steady turbulence state. In contrast, in both the instability-saturation and steady phases of the ETG turbulence, the entropy transfer processes among low-wavenumber non-zonal modes are dominant rather than the transfer via zonal modes.
Show PACS
52.35.Ra Plasma turbulence
52.25.Fi Transport properties
52.25.Kn Thermodynamics of plasmas
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)

Nonplanar electron acoustic shock waves in a plasma with electrons featuring Tsallis distribution

Biswajit Sahu and Mouloud Tribeche

Phys. Plasmas 19, 022304 (2012); http://dx.doi.org/10.1063/1.3684234 (5 pages)

Online Publication Date: 9 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The properties of cylindrical and spherical electron acoustic shock waves (EASWs) in an unmagnetized plasma consisting of cold electrons, immobile ions, and hot electrons featuring Tsallis statistics are investigated by employing the reductive perturbation technique. A Korteweg-de Vries Burgers (KdVB) equation is derived and its numerical solution is obtained. The effects of electron nonextensivity and electron kinematic viscosity on the basic features of EA shock waves are discussed in nonplanar geometry. It is found that nonextensive nonplanar EA shock waves behave quite differently from their planar counterpart. Deviations from a pure planar geometry are significant only for times shorter that the inverse of the cold electron plasma frequency. Given that the hot electron dynamics is the most interesting one, and that in many astrophysical scenarios the cold electrons can be significantly rarefied, this restriction is not too limiting for the applicability of our model.
Show PACS
52.35.Tc Shock waves and discontinuities
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.25.Fi Transport properties
02.30.Jr Partial differential equations

Kinetic cascade beyond magnetohydrodynamics of solar wind turbulence in two-dimensional hybrid simulations

D. Verscharen, E. Marsch, U. Motschmann, and J. Müller

Phys. Plasmas 19, 022305 (2012); http://dx.doi.org/10.1063/1.3682960 (8 pages)

Online Publication Date: 16 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The nature of solar wind turbulence in the dissipation range at scales much smaller than the large magnetohydrodynamic (MHD) scales remains under debate. Here, a two-dimensional model based on the hybrid code abbreviated as A.I.K.E.F. is presented, which treats massive ions as particles obeying the kinetic Vlasov equation and massless electrons as a neutralizing fluid. Up to a certain wavenumber in the MHD regime, the numerical system is initialized by assuming a superposition of isotropic Alfvén waves with amplitudes that follow the empirically confirmed spectral law of Kolmogorov. Then, turbulence develops and energy cascades into the dispersive spectral range, where also dissipative effects occur. Under typical solar wind conditions, weak turbulence develops as a superposition of normal modes in the kinetic regime. Spectral analysis in the direction parallel to the background magnetic field reveals a cascade of left-handed Alfvén/ion-cyclotron waves up to wave vectors where their resonant absorption sets in, as well as a continuing cascade of right-handed fast-mode and whistler waves. Perpendicular to the background field, a broad turbulent spectrum is found to be built up of fluctuations having a strong compressive component. Ion-Bernstein waves seem to be possible normal modes in this propagation direction for lower driving amplitudes. Also, signatures of short-scale pressure-balanced structures (very oblique slow-mode waves) are found.
Show PACS
96.20.Br Origin and evolution
95.30.Qd Magnetohydrodynamics and plasmas

Three-dimensional electromagnetic strong turbulence: Dependence of the statistics and dynamics of strong turbulence on the electron to ion temperature ratio

D. B. Graham, Iver H. Cairns, O. Skjaeraasen, and P. A. Robinson

Phys. Plasmas 19, 022306 (2012); http://dx.doi.org/10.1063/1.3684672 (15 pages)

Online Publication Date: 17 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The temperature ratio Ti/Te of ions to electrons affects both the ion-damping rate and the ion-acoustic speed in plasmas. The effects of changing the ion-damping rate and ion-acoustic speed are investigated for electrostatic strong turbulence and electromagnetic strong turbulence in three dimensions. When ion damping is strong, density wells relax in place and act as nucleation sites for the formation of new wave packets. In this case, the density perturbations are primarily density wells supported by the ponderomotive force. For weak ion damping, corresponding to low Ti/Te, ion-acoustic waves are launched radially outwards when wave packets dissipate at burnout, thereby increasing the level of density perturbations in the system and thus raising the level of scattering of Langmuir waves off density perturbations. Density wells no longer relax in place so renucleation at recent collapse sites no longer occurs, instead wave packets form in background low density regions, such as superpositions of troughs of propagating ion-acoustic waves. This transition is found to occur at Ti/Te ≈ 0.1. The change in behavior with Ti/Te is shown to change the bulk statistical properties, scaling behavior, spectra, and field statistics of strong turbulence. For Ti/Te>rsim0.1, the electrostatic results approach the predictions of the two-component model of Robinson and Newman, and good agreement is found for Ti/Te>rsim0.15.
Show PACS
52.35.Ra Plasma turbulence
52.65.-y Plasma simulation
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.25.-b Plasma properties
back to top Magnetically Confined Plasmas, Heating, Confinement

Effects of dust particles on the dynamics of blobs in the scrape off layer II

D. Jovanović and U. de Angelis

Phys. Plasmas 19, 022501 (2012); http://dx.doi.org/10.1063/1.3680610 (9 pages)

Online Publication Date: 16 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
A detailed analysis is performed of the results of numerical simulations of the dynamics of plasma blobs, obtained within the model of the nonlinear interchange mode, including the effects of the dissipation by dust, in tokamak scrape-off layer plasmas. The maximum distances that a plasma blob may travel in the radial direction are calculated under physical conditions characteristic for several large tokamaks that are presently in operation, and it is found that a relatively small amount of 30 nm carbon dust particulates, corresponding to only a few grams of dust in the entire scrape-off layer, may dissipate the blobs and prevent them from reaching the tokamak wall.
Show PACS
52.27.Lw Dusty or complex plasmas; plasma crystals
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.40.Hf Plasma-material interactions; boundary layer effects
52.55.Fa Tokamaks, spherical tokamaks
52.65.-y Plasma simulation
52.25.Fi Transport properties

Neoclassical theory inside transport barriers in tokamaks

K. C. Shaing and C. T. Hsu

Phys. Plasmas 19, 022502 (2012); http://dx.doi.org/10.1063/1.3682044 (13 pages)

Online Publication Date: 17 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Inside the transport barriers in tokamaks, ion energy losses sometimes are smaller than the value predicted by the standard neoclassical theory. This improvement can be understood in terms of the orbit squeezing theory in addition to the sonic poloidal E×B Mach number Up,m that pushes the tips of the trapped particles to the higher energy. In general, Up,m also includes the poloidal component of the parallel mass flow speed. These physics mechanisms are the corner stones for the transition theory of the low confinement mode (L-mode) to the high confinement mode (H-mode) in tokamaks. Here, detailed transport fluxes in the banana regime are presented using the parallel viscous forces calculated earlier. It is found, as expected, that effects of orbit squeezing and the sonic Up,m reduce the ion heat conductivity. The former reduces it by a factor of |S|3/2 and the later by a factor of R(Up,m2)exp(-Up,m2) with R(Up,m2), a rational function. Here, S is the orbit squeezing factor.
Show PACS
52.20.Dq Particle orbits
52.55.Fa Tokamaks, spherical tokamaks
52.55.Pi Fusion products effects (e.g., alpha-particles, etc.), fast particle effects
52.35.Tc Shock waves and discontinuities
52.25.Fi Transport properties

Hybrid-like 2/1 flux-pumping and magnetic island evolution due to edge localized mode-neoclassical tearing mode coupling in DIII-D

J. D. King, R. J. La Haye, C. C. Petty, T. H. Osborne, C. J. Lasnier, R. J. Groebner, F. A. Volpe, M. J. Lanctot, M. A. Makowski, C. T. Holcomb, W. M. Solomon, S. L. Allen, T. C. Luce, M. E. Austin, W. H. Meyer, et al.

Phys. Plasmas 19, 022503 (2012); http://dx.doi.org/10.1063/1.3684648 (8 pages)

Online Publication Date: 17 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Direct analysis of internal magnetic field pitch angles measured using the motional Stark effect diagnostic shows m/n = 2/1 neoclassical tearing modes exhibit stronger poloidal magnetic flux-pumping than typical hybrids containing m/n = 3/2 modes. This flux-pumping causes the avoidance of sawteeth, and is present during partial electron cyclotron current drive suppression of the tearing mode. This finding could lead to hybrid discharges with higher normalized fusion performance at lower q95. The degree of edge localized mode-neoclassical tearing mode (ELM-NTM) coupling and the strength of flux-pumping increase with beta and the proximity of the modes to the ELMing pedestal. Flux-pumping appears independent of magnetic island width. Individual ELM-NTM coupling events show a rapid timescale drop in the island width followed by a resistive recovery that is successfully modeled using the modified Rutherford equation. The fast transient drop in island width increases with ELM size.
Show PACS
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.40.Hf Plasma-material interactions; boundary layer effects
52.55.Fa Tokamaks, spherical tokamaks
52.65.-y Plasma simulation

Gyrokinetic equations for strong-gradient regions

Andris M. Dimits

Phys. Plasmas 19, 022504 (2012); http://dx.doi.org/10.1063/1.3683000 (9 pages)

Online Publication Date: 21 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
A gyrokinetic theory is developed under a set of orderings applicable to the edge region of tokamaks and other magnetic confinement devices, as well as to internal transport barriers. The result is a practical set equations that is valid for large perturbation amplitudes [qδψ/T = O(1), where δψ = δφ-ν∥δA∥/c], which is straightforward to implement numerically, and which has straightforward expressions for its conservation properties. Here, δφ and δA∥ are the perturbed electrostatic and parallel magnetic potentials, ν∥ is the particle velocity, c is the speed of light, and T is the temperature. The derivation is based on the quantity ɛ ≡ (ρ/λ⊥)qδψ/T≪1 as the small expansion parameter, where ρ is the gyroradius and λ⊥ is the perpendicular wavelength. Physically, this ordering requires that the E×B velocity and the component of the parallel velocity perpendicular to the equilibrium magnetic field are small compared to the thermal velocity. For nonlinear fluctuations saturated at “mixing-length” levels (i.e., at a level such that driving gradients in profile quantities are locally flattened), ɛ is of the order ρ/Lp, where Lp is the equilibrium profile scale length, for all scales λ⊥ ranging from ρ to Lp. This is true even though qδψ/T = O(1) for λ⊥ ∼ Lp. Significant additional simplifications result from ordering Lp/LB = O(ɛ), where LB is the spatial scale of variation of the magnetic field. We argue that these orderings are well satisfied in strong-gradient regions, such as edge and scrapeoff layer regions and internal transport barriers in tokamaks, and anticipate that our equations will be useful as a basis for simulation models for these regions.
Show PACS
52.25.Dg Plasma kinetic equations
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.55.Fa Tokamaks, spherical tokamaks
52.25.Fi Transport properties
52.25.Gj Fluctuation and chaos phenomena
52.40.Hf Plasma-material interactions; boundary layer effects

On the mechanism for edge localized mode mitigation by supersonic molecular beam injection

T. Rhee, J. M. Kwon, P. H. Diamond, and W. W. Xiao

Phys. Plasmas 19, 022505 (2012); http://dx.doi.org/10.1063/1.3685720 (10 pages)

Online Publication Date: 21 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
We construct a diffusive, bi-stable cellular automata model to elucidate the physical mechanisms underlying observed edge localized mode (ELM) mitigation by supersonic molecular beam injection (SMBI). The extended cellular automata model reproduces key qualitative features of ELM mitigation experiments, most significantly the increase in frequency of grain ejection events (ELMs), and the decrease in the number of grains ejected by these transport events. The basic mechanism of mitigation is the triggering of small scale pedestal avalanches by additional grain injection directly into the H-mode pedestal. The small scale avalanches prevent the gradient from building-up to marginality throughout the pedestal, thus avoiding large scale transport events which span the full extent of that region. We explore different grain injection parameters to find an optimal SMBI scenario. We show that shallow SMBI deposition is sufficient for ELM mitigation.
Show PACS
52.25.Fi Transport properties
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.40.Hf Plasma-material interactions; boundary layer effects
Page 1 of 3 Pages Next Page | Jump to Page
Close

Google Calendar

ADVERTISEMENT

Go to AIP Home
Copyright © American Institute of Physics
close