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Dec 2007

Volume 14, Issue 12, Articles (12xxxx)

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Experimental observations of transport of picosecond laser generated electrons in a nail-like target

J. Pasley, M. Wei, E. Shipton, S. Chen, T. Ma, F. N. Beg, N. Alexander, R. Stephens, A. G. MacPhee, D. Hey, S. Le Pape, P. Patel, A. Mackinnon, M. Key, D. Offermann, et al.

Phys. Plasmas 14, 120701 (2007); http://dx.doi.org/10.1063/1.2815790 (4 pages) | Cited 5 times

Online Publication Date: 6 December 2007

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The transport of relativistic electrons, generated by the interaction of a high intensity (2×1020 W/cm2) laser, has been studied in a nail-like target comprised of a 20 μm diameter solid copper wire, coated with ∼ 2 μm of titanium, with an 80 μm diameter hemispherical termination. A ∼ 500 fs, ∼ 200 J pulse of 1.053 μm laser light produced by the Titan Laser at Lawrence Livermore National Laboratory was focused to a ∼ 20 μm diameter spot centered on the flat face of the hemisphere. Kα fluorescence from the Cu and Ti regions was imaged together with extreme ultraviolet (XUV) emission at 68 and 256 eV. Results showed a quasiexponential decline in Kα emission along the wire over a distance of a few hundred microns from the laser focus, consistent with bulk Ohmic inhibition of the relativistic electron transport. Weaker Kα and XUV emission on a longer scale length showed limb brightening suggesting a transition to enhanced transport at the surface of the wire.
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52.25.Fi Transport properties
52.27.Ny Relativistic plasmas
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.70.La X-ray and γ-ray measurements
52.25.Os Emission, absorption, and scattering of electromagnetic radiation

Is the “out-of-plane” magnetic perturbation always a quadrupole in the Hall-mediated magnetic reconnection?

N. H. Bian and G. Vekstein

Phys. Plasmas 14, 120702 (2007); http://dx.doi.org/10.1063/1.2820904 (4 pages) | Cited 8 times

Online Publication Date: 11 December 2007

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The structure of the “out-of-plane” component of the magnetic field is studied from the two-fluid description of magnetic reconnection via resistive tearing instability in a force-free field. Odd parity of the out-of-plane field component signifies a quadrupolar structure, which is usually taken as a signature of the Hall-mediated magnetic reconnection. Here we argue that the structure of the out-of-plane magnetic perturbation is not uniquely specified by the regime of reconnection. Thus, it can be quadrupolar in the standard magnetohydrodynamics (MHD) regime of magnetic reconnection, while being nonquadrupolar in the Hall-MHD regimes.
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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.)

Tomographic phase-space mapping of intense particle beams using solenoids

D. Stratakis, K. Tian, R. A. Kishek, I. Haber, M. Reiser, and P. G. O’Shea

Phys. Plasmas 14, 120703 (2007); http://dx.doi.org/10.1063/1.2823037 (4 pages) | Cited 8 times

Online Publication Date: 27 December 2007

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Tomography based on quadrupole scans has been successfully applied to reconstruct the phase space of intense charged particle beams. This letter develops a tomographic technique based on solenoid scans, which is advantageous for solenoidal systems and injectors. The technique is generalized to the diagnosis of beams that are not axisymmetric, and validated through simulation. Solenoidal tomography is applied experimentally to a system with a nonequilibrium initial distribution, demonstrating its detailed evolution in phase space.
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29.25.-t Particle sources and targets
52.40.Mj Particle beam interactions in plasmas
52.70.Ds Electric and magnetic measurements
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back to top Basic Plasma Phenomena, Waves, Instabilities

Cylindrical stationary striations in surface wave produced plasma columns of argon

Rajneesh Kumar, Sanjay V. Kulkarni, and Dhiraj Bora

Phys. Plasmas 14, 122101 (2007); http://dx.doi.org/10.1063/1.2816438 (8 pages) | Cited 4 times

Online Publication Date: 4 December 2007

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Striations are a good example of manifestation of a glow discharge. In the present investigation, stationary striations in the surface wave produced plasma column are formed. Physical parameters (length, number, etc.) of such striations can be controlled by operating parameters. With the help of bifurcation theory, experimental results are explained by considering two-step ionization in the surface wave discharge mechanism in argon gas. It is also observed that the bifurcation parameter is a function of input power, working pressure, and tube radius.
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02.30.Oz Bifurcation theory
52.80.Hc Glow; corona
52.50.-b Plasma production and heating
52.35.-g Waves, oscillations, and instabilities in plasmas and intense beams

Resistive stability of a plasma with runaway electrons

P. Helander, D. Grasso, R. J. Hastie, and A. Perona

Phys. Plasmas 14, 122102 (2007); http://dx.doi.org/10.1063/1.2817016 (9 pages) | Cited 2 times

Online Publication Date: 5 December 2007

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In tokamak disruptions the Ohmic current is often replaced by a current of runaway electrons, which is likely to be more peaked in the center of the discharge than the predisruption current. This raises the question of the resistive stability of the postdisruption plasma, where the equilibrium current is entirely carried by the runaway electrons while the cold ( ∼ 10 eV) background plasma is relatively resistive. It is found that the linear properties of the classical tearing mode are essentially determined by the cold bulk plasma, and the growth rate is approximately the same as in a plasma without runaways but with the same current profile. The nonlinear saturation amplitude is different, however. In a symmetric plasma slab, the saturated island size is larger when the current is carried by runaway electrons than in the Ohmic case, and undergoes a nonlinear bifurcation when the stability index Δ′ exceeds a critical value.
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
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.)
52.55.Fa Tokamaks, spherical tokamaks

Self-consistent Langmuir waves in resonantly driven thermal plasmas

R. R. Lindberg, A. E. Charman, and J. S. Wurtele

Phys. Plasmas 14, 122103 (2007); http://dx.doi.org/10.1063/1.2801714 (12 pages) | Cited 12 times

Online Publication Date: 6 December 2007

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The longitudinal dynamics of a resonantly driven Langmuir wave are analyzed in the limit that the growth of the electrostatic wave is slow compared to the bounce frequency. Using simple physical arguments, the nonlinear distribution function is shown to be nearly invariant in the canonical particle action, provided both a spatially uniform term and higher-order spatial harmonics are included along with the fundamental in the longitudinal electric field. Requirements of self-consistency with the electrostatic potential yield the basic properties of the nonlinear distribution function, including a frequency shift that agrees closely with driven, electrostatic particle simulations over a range of temperatures. This extends earlier work on nonlinear Langmuir waves by Morales and O’Neil [ G. J. Morales and T. M. O’Neil, Phys. Rev. Lett. 28, 417 (1972) ] and Dewar [ R. L. Dewar, Phys. Plasmas 15, 712 (1972) ], and could form the basis of a reduced kinetic treatment of plasma dynamics for accelerator applications or Raman backscatter.
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52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Sb Solitons; BGK modes

A sufficient condition for the linear stability of magnetohydrodynamic equilibria with field aligned incompressible flows

G. N. Throumoulopoulos and H. Tasso

Phys. Plasmas 14, 122104 (2007); http://dx.doi.org/10.1063/1.2817957 (4 pages) | Cited 8 times

Online Publication Date: 6 December 2007

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A sufficient condition for the linear stability of three dimensional equilibria with incompressible flows parallel to the magnetic field is derived. The condition refers to internal modes and involves physically interpretable terms related to the magnetic shear and the flow shear.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)

Weakly relativistic dielectric tensor for arbitrary wavenumbers

Francesco Volpe

Phys. Plasmas 14, 122105 (2007); http://dx.doi.org/10.1063/1.2812707 (13 pages) | Cited 1 time

Online Publication Date: 12 December 2007

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The validity of Shkarofsky’s dielectric tensor is extended by taking the strictly weakly relativistic limit and removing, when possible, assumptions on the wavenumbers along and across the ambient magnetic field, k∥ and k⊥. An approximation of the time integral is retained, but is shown to be valid under more benign assumptions than those of quasiperpendicular incidence and small Larmor radius. The increased generality with respect to k∥ permits to handle cases of comparable Doppler and relativistic widths of electron cyclotron resonances. The tensor also suits Bernstein waves, as it captures both their natural large k⊥ and the finite k∥ that is typical of some mode conversions, or acquired as a consequence of the large k⊥ when propagating in curved magnetic fields. Finally, relativistic corrections to the optimal angle for the ordinary-extraordinary Bernstein mode conversion are presented.
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52.27.Ny Relativistic plasmas
52.25.Mq Dielectric properties
02.10.Ud Linear algebra
52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)

Nonlinear trapping and self-guiding of magnetized Langmuir waves due to thermal plasma filamentation

Vladimir V. Nazarov, Mikhail V. Starodubtsev, and Alexander V. Kostrov

Phys. Plasmas 14, 122106 (2007); http://dx.doi.org/10.1063/1.2822161 (10 pages) | Cited 3 times

Online Publication Date: 19 December 2007

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Nonlinear interaction of Langmuir waves with a laboratory magnetoplasma has been studied under the conditions relevant to the ionospheric heating experiments. Self-guiding of magnetized Langmuir waves is observed at critical plasma density (ω = ωp): Langmuir waves are trapped inside a narrow, magnetic-field-aligned plasma density depletion region, which is formed by trapped waves due to thermal plasma nonlinearity, i.e., due to local plasma heating and consequent thermodiffusion. Magnetized Langmuir waves are trapped inside the depletion region through their specific dispersion properties; this fact has been shown using the kinetically modified dispersion relation. The threshold of the nonlinear wave trapping exhibits significant growth in the vicinity of harmonics of the electron gyrofrequency.
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52.50.-b Plasma production and heating
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)

Oblique modulation of electron-acoustic waves in a Fermi electron-ion plasma

C. Bhowmik, A. P. Misra, and P. K. Shukla

Phys. Plasmas 14, 122107 (2007); http://dx.doi.org/10.1063/1.2822158 (7 pages) | Cited 13 times

Online Publication Date: 20 December 2007

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The oblique modulational instability (MI) of electron-acoustic waves (EAWs) in a quantum plasma whose components are two distinct groups of electrons (one inertial cold electrons and other inertialess hot electrons) and immobile ions is investigated, by using a quantum hydrodynamic model. The analysis is carried out through the derivation of the nonlinear Schrödinger equation for the modulated EAW packets. The effects of obliqueness, the quantum diffraction (H), and the equilibrium density ratio of the cold to hot electron component (δ) on the MI of EAWs are numerically examined. At quantum scales, these parameters are found to significantly modify the MI domain in the plane of wave number and the angle (θ) between the modulation and the propagation direction. The relevance of our results in astrophysical environments, as well as in intense laser-solid density plasma interaction experiments is discussed.
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52.40.-w Plasma interactions (nonlaser)
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)

Diffusion of an annular plasma in positron acceleration

Francis F. Chen

Phys. Plasmas 14, 122108 (2007); http://dx.doi.org/10.1063/1.2824995 (4 pages) | Cited 1 time

Online Publication Date: 20 December 2007

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Electron acceleration to multi-GeV energies has been demonstrated using plasma wakefields in a tunnel-ionized plasma. However, coherent wakefields for positron acceleration may require hollow plasmas pre-ionized by a laser beam. The lifetime of such a plasma is determined by an unusual diffusion problem in which the diffusion rate varies by an order of magnitude inside the hole. The problem is solved by numerical differentiation without using a particle-in-cell code. The densities assumed in this work match those in existing positron experiments and are low compared with those in electron experiments. Future positron experiments at higher densities will not exhibit the nonlinear diffusion treated here because they will be dominated by recombination and tunneling ionization by the beam.
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52.38.Kd Laser-plasma acceleration of electrons and ions
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)

Reflection of Alfvén waves from boundaries with different conductivities

D. Leneman

Phys. Plasmas 14, 122109 (2007); http://dx.doi.org/10.1063/1.2813459 (11 pages)

Online Publication Date: 20 December 2007

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The reflection of Alfvén waves from the ionosphere plays a crucial role because the reflected wave can reduce or enhance the electric field pattern of the incident wave. The ionosphere is typically treated as a conducting surface, which has a height integrated Pederson conductivity. This approximation is appropriate in considering the reflection of Alfvén waves because the wavelengths along the magnetic field are large compared to the height of the ionosphere. Shear Alfvén wave reflection experiments have been performed in the large plasma device [ W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. of Sci. Instrum. 62, 2875 (1991) ] at the University of California, Los Angeles. A single frequency wave is launched from an antenna and reflects from a large plate inserted into the plasma column. By alternatively using a conducting and an insulating plate, the two extremes of conductivity relative to the Alfvén conductivity, 1/(μovA) are tested. The data are compared with the expected theoretical behavior of the interference pattern of incident and reflected waves. Perhaps due to experimental effects, the conducting reflector is found to behave in much the same fashion as the insulator.
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52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.40.Hf Plasma-material interactions; boundary layer effects
94.05.Rx Experimental techniques and laboratory studies
94.20.wf Plasma waves and instabilities

Parametric instabilities of parallel propagating incoherent Alfvén waves in a finite ion beta plasma

Y. Nariyuki, T. Hada, and K. Tsubouchi

Phys. Plasmas 14, 122110 (2007); http://dx.doi.org/10.1063/1.2824986 (9 pages) | Cited 9 times

Online Publication Date: 26 December 2007

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Large amplitude, low-frequency Alfvén waves constitute one of the most essential elements of magnetohydrodynamic (MHD) turbulence in the fast solar wind. Due to small collisionless dissipation rates, the waves can propagate long distances and efficiently convey such macroscopic quantities as momentum, energy, and helicity. Since loading of such quantities is completed when the waves damp away, it is important to examine how the waves can dissipate in the solar wind. Among various possible dissipation processes of the Alfvén waves, parametric instabilities have been believed to be important. In this paper, we numerically discuss the parametric instabilities of coherent/incoherent Alfvén waves in a finite ion beta plasma using a one-dimensional hybrid (superparticle ions plus an electron massless fluid) simulation, in order to explain local production of sunward propagating Alfvén waves, as suggested by Helios/Ulysses observation results. Parameter studies clarify the dependence of parametric instabilities of coherent/incoherent Alfvén waves on the ion and electron beta ratio. Parametric instabilities of coherent Alfvén waves in a finite ion beta plasma are vastly different from those in the cold ions (i.e., MHD and/or Hall-MHD systems), even if the collisionless damping of the Alfvén waves are neglected. Further, “nonlinearly driven” modulational instability is important for the dissipation of incoherent Alfvén waves in a finite ion beta plasma regardless of their polarization, since the ion kinetic effects let both the right-hand and left-hand polarized waves become unstable to the modulational instability. The present results suggest that, although the antisunward propagating dispersive Alfvén waves are efficiently dissipated through the parametric instabilities in a finite ion beta plasma, these instabilities hardly produce the sunward propagating waves.
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52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
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)
52.35.Ra Plasma turbulence

Spatiotemporal correlation functions in beam-driven plasmas with fluctuations

Yu. Tyshetskiy, J. A. Roberts, P. A. Robinson, I. H. Cairns, and B. Li

Phys. Plasmas 14, 122111 (2007); http://dx.doi.org/10.1063/1.2819678 (16 pages) | Cited 2 times

Online Publication Date: 27 December 2007

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Using a reduced-parameter model of wave-particle interactions in a beam-driven plasma, the linear spatiotemporal correlation functions of wave and particle quantities are derived. These are found to have an oscillatory structure with characteristic spatial and temporal scales reflecting the dynamics of energy exchange between particles and waves. The effects of various system parameters on these characteristic scales and the correlation functions are investigated. The correlation scales are shown to diverge in some limiting cases, implying the possibility of criticality in the system. A comparison with fully nonlinear numerical simulations is carried out, and the criterion for validity of the linear correlation functions is derived and verified. The nonlinear simulation results are shown to converge to the linear prediction in appropriate limits dictated by this criterion. The correlation functions obtained provide a useful tool for studying dynamical properties of beam-driven plasma-wave systems with fluctuating parameters.
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52.25.Gj Fluctuation and chaos phenomena
52.65.-y Plasma simulation
52.35.-g Waves, oscillations, and instabilities in plasmas and intense beams
back to top Nonlinear Phenomena, Turbulence, Transport

Characteristics of geodesic acoustic mode zonal flow and ambient turbulence at the edge of the HL-2A tokamak plasmas

K. J. Zhao, J. Q. Dong, L. W. Yan, W. Y. Hong, T. Lan, A. D. Liu, J. Qian, J. Cheng, D. L. Yu, Y. Huang, H. D. He, Yi. Liu, Q. W. Yang, X. R. Duan, X. M. Song, et al.

Phys. Plasmas 14, 122301 (2007); http://dx.doi.org/10.1063/1.2817047 (8 pages) | Cited 11 times

Online Publication Date: 5 December 2007

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The three-dimensional characteristics of the geodesic acoustic mode zonal flows (GAMZFs) and the ambient turbulence (AT) at the edge of the HuanLiuqi-2A tokamak [ Y. Liu et al., Nucl. Fusion 45, S203 (2005) ] are investigated with Langmuir probe arrays and the results are presented in detail. The toroidal and poloidal symmetries, and the radial scale of the GAMZFs are simultaneously identified. The envelopes of the high frequency components of the AT in the presence of the GAMZFs are analyzed. The GAM frequency components (GAMFCs) of the coherent envelopes are also shown to have poloidal and toroidal symmetries, and similar radial scales as the GAMZF does. The correlation between the GAMFCs of the envelopes and the GAMs is high, with phase shifts between π/2 to π, indicating that the GAMZFs may regulate the AT and the regulation is embodied in the envelopes. Three-wave coupling between GAM and AT is found to be a plausible formation mechanism for the former, which acts on the whole spectra of the latter within its scale length. The temporal evolutions of the total fluctuation power, the GAM and the AT powers show that the AT power decreases when GAM power increases and vice versa, indicating possible regulating effects of the latter on the former.
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52.35.Ra Plasma turbulence
52.25.Fi Transport properties
52.55.Fa Tokamaks, spherical tokamaks
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)

Loss-cone instability: Wave saturation by particle trapping

A. Zaslavsky, C. Krafft, and A. Volokitin

Phys. Plasmas 14, 122302 (2007); http://dx.doi.org/10.1063/1.2799621 (12 pages) | Cited 1 time

Online Publication Date: 5 December 2007

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The nonlinear mechanisms governing the interactions between whistler or lower hybrid waves and loss-cone type particles’ distributions in magnetized plasmas are of great importance if one considers the major role that waves of frequency below the electron cyclotron frequency play in space and thermonuclear fusion plasmas. Up to now, most of the numerical simulations have been devoted to study the nonlinear processes at work when the plasma is weakly relativistic and when the anisotropy of the particles’ distributions leads to the so-called maser instability. However, in many interesting cases, the particles’ energies are sufficiently weak to ensure the validity of the nonrelativistic approximation. In this framework, the paper studies the interaction at normal cyclotron resonances between lower hybrid waves and electron distributions presenting loss-cone like features. A theoretical Hamiltonian model and a corresponding numerical symplectic code are used to evidence and to explain the nonlinear mechanisms at work at the saturation stage of the loss-cone instability. Moreover, simple analytical expressions and scaling laws have been derived for the linear growth rates and the wave amplitude at saturation.
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52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.65.Kj Magnetohydrodynamic and fluid equation

Statistics of passive tracers in three-dimensional magnetohydrodynamic turbulence

Angela Busse, Wolf-Christian Müller, Holger Homann, and Rainer Grauer

Phys. Plasmas 14, 122303 (2007); http://dx.doi.org/10.1063/1.2818770 (12 pages)

Online Publication Date: 13 December 2007

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Magnetohydrodynamic (MHD) turbulence is studied from the Lagrangian viewpoint by following fluid particle tracers in high resolution direct numerical simulations. Results regarding turbulent diffusion and dispersion as well as Lagrangian structure functions are presented. Whereas turbulent single-particle diffusion exhibits essentially the same behavior in Navier-Stokes and MHD turbulence, two-particle relative dispersion in the MHD case differs significantly from the Navier-Stokes behavior. This observation is linked to the local anisotropy of MHD turbulence which is clearly reflected by quantities measured in a Lagrangian frame of reference. In the MHD case the Lagrangian structure functions display a lower level of intermittency as compared to the Navier-Stokes case contrasting Eulerian results. This is not only true for short time increments [ H. Homann, R. Grauer, A. Busse, and W.-C. Müller, J. Plasma Phys. 73, 821 (2007) ] but also holds for increments up to the order of the integral time scale. The apparent discrepancy can be explained by the difference in the characteristic shapes of fluid particle trajectories in the vicinity of most singular dissipative structures.
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47.27.ek Direct numerical simulations
47.27.eb Statistical theories and models
47.65.-d Magnetohydrodynamics and electrohydrodynamics

Excitation of ion-acoustic perturbations by incoherent kinetic Alfvén waves in plasmas

J. T. Mendonça and P. K. Shukla

Phys. Plasmas 14, 122304 (2007); http://dx.doi.org/10.1063/1.2822157 (4 pages) | Cited 4 times

Online Publication Date: 20 December 2007

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The dispersion relation for ion-acoustic perturbations (IAPs) in the presence of incoherent kinetic Alfvén waves (KAWs) in plasmas is derived. The wave-kinetic-approach is used to study the nonlinear interactions between an ensemble of random phase KAWs and IAPs. It is found that incoherent KAW spectrum is unstable against IAPs. The instability growth rates for particular cases are obtained. The present instability offers the possibility of heating ions in a turbulent magnetoplasma composed of incoherent KAWs.
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52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.65.Kj Magnetohydrodynamic and fluid equation
52.35.Ra Plasma turbulence

Front propagation and critical gradient transport models

X. Garbet, Y. Sarazin, F. Imbeaux, P. Ghendrih, C. Bourdelle, Ö. D. Gürcan, and P. H. Diamond

Phys. Plasmas 14, 122305 (2007); http://dx.doi.org/10.1063/1.2824375 (12 pages) | Cited 13 times

Online Publication Date: 26 December 2007

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This paper analyzes the properties of a two-field critical gradient model that couples a heat equation to an evolution equation for the turbulence intensity. It is shown that the dynamics of a perturbation is ballistic or diffusive depending on the shape of the pulse and also on the distance of the temperature gradient to the instability threshold. This dual character appears in the linear response of this model for a wave packet. It is recovered when investigating the nonlinear solutions of this system. Both self-similar diffusive fronts and ballistic fronts are shown to exist. When the propagation is ballistic, it is found that the front velocity is the geometric mean between the turbulent diffusion coefficient and a microinstability growth rate.
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52.35.Ra Plasma turbulence
52.25.Fi Transport properties
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.)

Resonance enhanced turbulent transport

Andrew P. L. Newton and Eun-jin Kim

Phys. Plasmas 14, 122306 (2007); http://dx.doi.org/10.1063/1.2821246 (8 pages) | Cited 5 times

Online Publication Date: 27 December 2007

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The effect of oscillatory shear flows on turbulent transport of passive scalar fields is studied by numerical computations based on the results provided by E. Kim [Physics of Plasmas 13, 022308 (2006)] . Turbulent diffusion is found to depend crucially on the competition between suppression due to shearing and enhancement due to resonances, depending on the characteristic time and length scales of shear flow and turbulence. Enhancements in transport occur for turbulence with finite memory time either due to Doppler or parametric resonances. Scalings of turbulence amplitude and transport are provided in different parameter spaces. The results suggest that oscillatory shear flows are not only less efficient in regulating turbulence, but also can enhance the value of turbulent diffusion, accelerating turbulent transport.
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47.27.-i Turbulent flows
back to top Magnetically Confined Plasmas, Heating, Confinement

Kinetic theory of radial angular momentum flux of collisional plasmas in an axisymmetric magnetic field

S. K. Wong and V. S. Chan

Phys. Plasmas 14, 122501 (2007); http://dx.doi.org/10.1063/1.2813456 (10 pages) | Cited 6 times

Online Publication Date: 4 December 2007

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A recent calculation [ P. J. Catto and A. N. Simakov, Phys. Plasmas 12, 012501 (2005) ] of the radial angular momentum flux for collisional plasmas with small toroidal flows using fluid equations is at variance with an existing result [ R. D. Hazeltine, Phys. Fluids 17, 961 (1974) ] based on a drift kinetic approach. The difference has been ascribed to the incompleteness of the drift kinetic equation when second order accuracy in the ratio of gyroradius over scale length is required. This paper reformulates the problem starting from the full kinetic equation that includes gyromotion and performs an expansion in the same ratio. The drift kinetic equation being used is recovered by keeping leading order terms in an additional expansion of poloidal to toroidal magnetic fields. Solving the equation in the Pfirsch–Schlüter regime leads to an agreement with the fluid approach, after correcting an error in the existing drift kinetic calculation.
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52.55.Jd Magnetic mirrors, gas dynamic traps
52.20.-j Elementary processes in plasmas

Hypersonic drift-tearing magnetic islands in tokamak plasmas

R. Fitzpatrick and F. L. Waelbroeck

Phys. Plasmas 14, 122502 (2007); http://dx.doi.org/10.1063/1.2811928 (11 pages) | Cited 2 times

Online Publication Date: 6 December 2007

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A two-fluid theory of long wavelength, hypersonic, drift-tearing magnetic islands in low-collisionality, low-β plasmas possessing relatively weak magnetic shear is developed. The model assumes both slab geometry and cold ions, and neglects electron temperature and equilibrium current gradient effects. The problem is solved in three asymptotically matched regions. The “inner region” contains the island. However, the island emits electrostatic drift-acoustic waves that propagate into the surrounding “intermediate region,” where they are absorbed by the plasma. Since the waves carry momentum, the inner region exerts a net force on the intermediate region, and vice versa, giving rise to strong velocity shear in the region immediately surrounding the island. The intermediate region is matched to the surrounding “outer region,” in which ideal magnetohydrodynamic holds. Isolated hypersonic islands propagate with a velocity that lies between those of the unperturbed local ion and electron fluids, but is much closer to the latter. The ion polarization current is stabilizing, and increases with increasing island width. Finally, the hypersonic branch of isolated island solutions ceases to exist above a certain critical island width. Hypersonic islands whose widths exceed the critical width are hypothesized to bifurcate to the so-called “sonic” solution branch.
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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)
52.55.Fa Tokamaks, spherical tokamaks
52.35.Kt Drift waves
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Dm Sound waves

High-beta plasma effects in a low-pressure helicon plasma

C. S. Corr and R. W. Boswell

Phys. Plasmas 14, 122503 (2007); http://dx.doi.org/10.1063/1.2802080 (7 pages) | Cited 6 times

Online Publication Date: 7 December 2007

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In this work, high-beta plasma effects are investigated in a low-pressure helicon plasma source attached to a large volume diffusion chamber. When operating above an input power of 900 W and a magnetic field of 30 G a narrow column of bright blue light (due to Ar II radiation) is observed along the axis of the diffusion chamber. With this blue mode, the plasma density is axially very uniform in the diffusion chamber; however, the radial profiles are not, suggesting that a large diamagnetic current might be induced. The diamagnetic behavior of the plasma has been investigated by measuring the temporal evolution of the magnetic field (Bz) and the plasma kinetic pressure when operating in a pulsed discharge mode. It is found that although the electron pressure can exceed the magnetic field pressure by a factor of 2, a complete expulsion of the magnetic field from the plasma interior is not observed. In fact, under our operating conditions with magnetized ions, the maximum diamagnetism observed is ∼ 2%. It is observed that the magnetic field displays the strongest change at the plasma centre, which corresponds to the maximum in the plasma kinetic pressure. These results suggest that the magnetic field diffuses into the plasma sufficiently quickly that on a long time scale only a slight perturbation of the magnetic field is ever observed.
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52.25.-b Plasma properties
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Dg Plasma kinetic equations

Experimentally inferred thermal diffusivities in the edge pedestal between edge-localized modes in DIII-D

W. M. Stacey and R. J. Groebner

Phys. Plasmas 14, 122504 (2007); http://dx.doi.org/10.1063/1.2817969 (9 pages) | Cited 8 times

Online Publication Date: 11 December 2007

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Using temperature and density profiles averaged over the same subinterval of several successive inter-edge-localized-mode (ELM) periods, the ion and electron thermal diffusivities in the edge pedestal were inferred between ELMs for two DIII-D [ J. Luxon, Nucl. Fusion 42, 614 (2002) ] discharges. The inference procedure took into account the effects of plasma reheating and density buildup between ELMs, radiation and atomic physics cooling, neutral beam heating and ion-electron equilibration, and recycling neutral and beam ionization particle sources in determining the conductive heat flux profiles used to infer the thermal diffusivities in the edge pedestal. Comparison of the inferred thermal diffusivities with theoretical formulas based on various transport mechanisms was inconclusive insofar as identifying likely transport mechanisms.
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52.55.Fa Tokamaks, spherical tokamaks
52.25.Fi Transport properties
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)

Zonal flows and ion temperature gradient instabilities in multiple-helicity magnetic fields

S. Ferrando-Margalet, H. Sugama, and T.-H. Watanabe

Phys. Plasmas 14, 122505 (2007); http://dx.doi.org/10.1063/1.2813182 (10 pages) | Cited 11 times

Online Publication Date: 12 December 2007

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The effects of multiple-helicity magnetic fields on the ion temperature gradient (ITG) instability and on the zonal flow (ZF) evolution are studied with the linear gyrokinetic Vlasov code GKV [ T.-H. Watanabe and H. Sugama, Nucl. Fusion 46, 24 (2006) ]. The model helical fields corresponding to the standard and inward-shifted axis configurations of the Large Helical Device [ O. Motojima, N. Ohyabu, A. Komori et al., Nucl. Fusion 43, 1674 (2003) ] are used to investigate how ITG mode properties and ZF evolution response to a given source are influenced by the field geometry. It is shown that, in the inward-shifted configuration, the ITG mode growth rate increases slightly while the ZF is sustained for a longer time. In addition, velocity-space structures of the ion perturbed distribution function are numerically obtained which illustrate the validity of the analytical prediction that the plasma inward shift retards the radial drift of the helically trapped particles leading to the enhancement of the ZF response. This supports the conjecture that anomalous transport can be reduced by the ZF generated in the configurations optimized to decrease the neoclassical transport.
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.25.Dg Plasma kinetic equations
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.55.Jd Magnetic mirrors, gas dynamic traps
52.25.Fi Transport properties
52.65.Ff Fokker-Planck and Vlasov equation
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