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Top 20 Most Read Articles
August 2009
The 20 articles with the most full-text downloads during the month, in descending order.
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Simplified model of nonlinear Landau damping Phys. Plasmas 16, 072104 (2009); http://dx.doi.org/10.1063/1.3160604 (8 pages) Online Publication Date: 8 July 2009
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The nonlinear interaction of a plasma wave with resonant electrons results in a plateau in the electron distribution function close to the phase velocity of the plasma wave. As a result, Landau damping of the plasma wave vanishes and the resonant frequency of the plasma wave downshifts. However, this simple picture is invalid when the external driving force changes the plasma wave fast enough so that the plateau cannot be fully developed. A new model to describe amplification of the plasma wave including the saturation of Landau damping and the nonlinear frequency shift is proposed. The proposed model takes into account the change of the plasma wave amplitude and describes saturation of the Landau damping rate in terms of a single fluid equation, which simplifies the description of the inherently kinetic nature of Landau damping. A proposed fluid model, incorporating these simplifications, is verified numerically using a kinetic Vlasov code.
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Reconnection in semicollisional, low-β plasmas Phys. Plasmas 16, 072302 (2009); http://dx.doi.org/10.1063/1.3155453 (10 pages) Online Publication Date: 9 July 2009
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Reconnection of semicollisional, low-β plasmas is studied numerically for two model problems using a two-field description of the plasma including electron pressure effects (and hence kinetic Alfvén-wave dynamics). The tearing unstable Harris sheet, with the global parameters of the Geospace Environment Modeling-challenge case, shows a linear growth of the peak reconnection rate with the drift parameter ρs when this scale is significantly larger than the resistive skin depth, and the island is smaller than the Harris sheet current layer width. As exemplary for a driven, rather than a spontaneous reconnection situation we study as second model system two coalescing islands, starting from a nonequilibrium situation. The peak reconnection rate again increases initially linearly with ρs but saturates and becomes ρs independent for larger values. In this saturated regime, no flux pileup occurs, and the reconnection is limited by the rate of approach of the two coalescing islands. The qualitative differences between spontaneous and driven reconnection cases and their scaling behavior are best understood by considering the reconnection rate as a triple product of outflow Mach number, outflow to inflow channel width ratio, and magnetic energy density at a height ρs above the X point.
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Nonlinear electromagnetic wave equations for superdense magnetized plasmas Phys. Plasmas 16, 072114 (2009); http://dx.doi.org/10.1063/1.3184571 (4 pages) Online Publication Date: 21 July 2009
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By using the quantum hydrodynamic and Maxwell equations, we derive the generalized nonlinear electron magnetohydrodynamic, the generalized nonlinear Hall-MHD (HMHD), and the generalized nonlinear dust HMHD equations in a self-gravitating dense magnetoplasma. Our nonlinear equations include the self-gravitating, the electromagnetic, the quantum statistical electron pressure, as well as the quantum electron tunneling and electron spin forces. They are useful for investigating a number of wave phenomena including linear and nonlinear electromagnetic waves, as well as three-dimensional electromagnetic wave turbulence spectra and structures arising from mode coupling processes at nanoscales in dense quantum magnetoplasmas.
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Electron trajectories and betatron oscillation in the wake bubble in laser-plasma interaction Phys. Plasmas 16, 073108 (2009); http://dx.doi.org/10.1063/1.3184576 (5 pages) Online Publication Date: 27 July 2009
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The trajectories of electrons originating from different initial locations in the unperturbed plasma during the interaction of an ultraintense laser with underdense plasma in the bubble regime are followed by particle-in-cell simulation. It is found that plasma electrons initially aligned with the rim of the laser focal spot contribute most to the bow wave in front of the bubble and those aligned with the lateral bubble sheath edge contribute most to the self-injection at the back of the bubble. A scaling law between the transverse electric and magnetic fields for the case where there are many electrons in the bubble is given and discussed in terms of betatron oscillations of the injected electrons.
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Jeans instability in quantum magnetoplasma with resistive effects Phys. Plasmas 16, 072101 (2009); http://dx.doi.org/10.1063/1.3168612 (5 pages) Online Publication Date: 2 July 2009
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The Jeans instability in dense quantum plasmas is investigated in the presence of two dimensional magnetic fields and resistive effects. The resistive effects are shown to introduce instability whether the perturbation is stable or not in the ideal magnetohydrodynamic model. The analytical expressions of the growth rate of Jeans instability are obtained for both the finite and remarkable resistive effects cases. The results are relevant to dense astrophysical objects, e.g., neutron stars and the interior of white dwarfs, as well as low-temperature laboratory plasmas.
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An assessment of full wave effects on the propagation and absorption of lower hybrid waves Phys. Plasmas 16, 072502 (2009); http://dx.doi.org/10.1063/1.3166137 (7 pages) Online Publication Date: 8 July 2009
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Lower hybrid (LH) waves (Ωci⪡ω⪡Ωce, where Ωi,e ≡ Zi,eeB/mi,ec) have the attractive property of damping strongly via electron Landau resonance on relatively fast tail electrons and consequently are well-suited to driving current. Established modeling techniques use Wentzel–Kramers–Brillouin (WKB) expansions with self-consistent non-Maxwellian distributions. Higher order WKB expansions have shown some effects on the parallel wave number evolution and consequently on the damping due to diffraction [
G. Pereverzev, Nucl. Fusion 32, 1091 (1991)
]. A massively parallel version of the TORIC full wave electromagnetic field solver valid in the LH range of frequencies has been developed [
J. C. Wright et al., Comm. Comp. Phys. 4, 545 (2008)
] and coupled to an electron Fokker–Planck solver CQL3D [
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Nonlinear interaction of electron plasma waves with electron acoustic waves in plasmas Phys. Plasmas 16, 072311 (2009); http://dx.doi.org/10.1063/1.3191722 (4 pages) Online Publication Date: 28 July 2009
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An analysis of interaction between two temperature electron species in the presence of static neutralizing ion background is presented. It is shown that electron plasma waves can nonlinearly interact with electron acoustic wave in a time scale much longer than ωp−1, where ωp is electron plasma frequency. A set of coupled nonlinear differential equations is shown to exist in such a scenario. Propagating soliton solutions are demonstrated from these equations.
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Transverse oscillations in a single-layer dusty plasma under microgravity Phys. Plasmas 16, 083703 (2009); http://dx.doi.org/10.1063/1.3204638 (10 pages) Online Publication Date: 13 August 2009
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A single-layer suspension of microparticles was formed in a plasma under microgravity conditions. This single layer is confined at a void boundary by a balance of ion drag and electric forces, where the ion flow velocity is much slower than in the sheath of laboratory plasmas. Using a high-resolution camera that allows measurements of velocities at a low level, the microparticle kinetic temperature was found to be close to that of the neutral gas. The random motion transverse to the single layer was found to have oscillations of the form expected for harmonic oscillators driven by white noise. The driving of the oscillation is mostly attributed to the Brownian motion of neutral atoms, while the damping is mostly due to neutral gas friction. An observed resonance frequency of 25 s−1 allows us to quantify the electric and ion drag forces as being in the range of 0.2–0.4mpg, where mp is the microparticle mass and g is the acceleration of gravity on Earth’s surface. No signature of wave dispersion was detected for this experiment with neon at a pressure of 0.12 Torr.
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Proposed hole-target for improving maximum proton energy driven by a short intense laser pulse Phys. Plasmas 16, 073106 (2009); http://dx.doi.org/10.1063/1.3174434 (5 pages) Online Publication Date: 15 July 2009
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By using particle-in-cell simulations, a new method for energetic collimated proton generation via intense short pulse laser-thin foil interactions is presented. To enhance the electron heating efficiency, a small hole is bored at the center of a thin foil target. The small hole combines target heating mechanisms effectively, which results in a high proton maximum energy. While an ultraintense, ultrashort laser pulse propagates through a small hole (diameter<laser spot size), the laser pulse drives electrons pulled out from the hole inner wall effectively inside the hole. When these electrons leave the target, a strong sheath field is formed between the electrons and the target rear surface and this accelerates protons from the rear surface of the target. The effective combination of the laser longitudinal ponderomotive force with the transverse heating (by E field) mechanism results in highly efficient electron heating of the hole target. When the rear part of the hole is filled with a proton-electron contamination layer, energetic collimated protons are produced. The scaling of the maximum proton energy of a hole target over a wide range of laser pulse intensities is presented and compared with that of a simple planar target.
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Surface energy transport following relativistic laser-solid interaction Phys. Plasmas 16, 072702 (2009); http://dx.doi.org/10.1063/1.3158950 (8 pages) Online Publication Date: 6 July 2009
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A planar Al target is excited by a 25 fs laser pulse focused to intensity up to 3×1018 W/cm2 in a ∼ 1 μm radius spot; subsequent heat propagation along the target surface, imaged by a delayed probe pulse, appears as a roughly circular area of reduced reflectivity centered on the pump spot, that expands to as much as 12±3 μm in radius within 500 fs. We present a semiempirical model in which the pump laser pulse drives hot electrons into the target via collisionless interactions. A return current heats the target and, above a critical temperature, includes runaway electrons that return to the surface before dissipating their energy. Ultrafast radial expansion of the heated surface layer is explained by lateral diffusive motion of returning runaway electrons oscillating across the target surface layer confined by space charge. Isotropy of the observed expansion is consistent with dominance of resonance absorption over j×B heating, indicating prepulse heating is important.
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Strongly coupled plasma physics and high energy-density matter Phys. Plasmas 11, 2964 (2004); http://dx.doi.org/10.1063/1.1652853 (8 pages) Online Publication Date: 23 April 2004
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High energy–density matter—matter with pressures in excess of a megabar—covers a wide range of parameter space. Many laboratory experiments span a large portion of this parameter space as they evolve from a liquid or solid phase through the strongly coupled plasma phase to a hot plasma phase. This tutorial will introduce the basic physics of the intermediate, strongly coupled plasma phase from a very general point of view, including a discussion of experiments, such as laser-cooled ions, dusty plasmas, and white dwarfs. Basic definitions and results will be given for simple strongly coupled plasmas in the context of concepts familiar from weakly coupled plasma physics, to the extent possible. Definitions relevant to high energy–density physics are then introduced before focusing on dense plasmas, which form the overlap between the strongly coupled and high energy–density regions. © 2004 American Institute of Physics. |
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Intense laser-plasma interactions: New frontiers in high energy density physics Phys. Plasmas 16, 041002 (2009); http://dx.doi.org/10.1063/1.3101813 (15 pages) Online Publication Date: 22 April 2009
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A review is presented here of a number of invited papers presented at the 2008 American Physical Society April meeting [held jointly with High Energy Density Physics/High Energy Density Laboratory Astrophysics (HEDP/HEDLA) Conference] devoted to intense laser-matter interactions. They include new insights gained from wave-kinetic theory into laser-wakefield accelerators and drift wave turbulence interacting with zonal flows in magnetized plasmas; interactions with cluster media for the generation of radiative blast waves; fast electron energy transport in cone-wire targets; numerical investigations into Weibel instability in electron-positron-ion plasmas and the generation of gigabar pressures with thin foil interactions.
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Radial propagation of geodesic acoustic modes Phys. Plasmas 16, 072503 (2009); http://dx.doi.org/10.1063/1.3155106 (8 pages) Online Publication Date: 9 July 2009
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The GAM group velocity is estimated from the ratio of the radial free energy flux to the total free energy applying gyrokinetic and two-fluid theory. This method is much more robust than approaches that calculate the group velocity directly and can be generalized to include additional physics, e.g., magnetic geometry. The results are verified with the gyrokinetic code GYRO [
J. Candy and R. E. Waltz, J. Comput. Phys. 186, 545 (2003)
], the two-fluid code NLET [
K. Hallatschek and A. Zeiler, Phys. Plasmas 7, 2554 (2000)
], and analytical calculations. GAM propagation must be kept in mind when discussing the windows of GAM activity observed experimentally and the match between linear theory and experimental GAM frequencies.
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Phys. Plasmas 16, 072113 (2009); http://dx.doi.org/10.1063/1.3179807 (9 pages) Online Publication Date: 20 July 2009
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A simplified derivation of the relationship between the dispersion function for a plasma with a kappa velocity distribution and the Gauss hypergeometric function is presented. This derivation relies on only a few standard integrals. It naturally leads to a new integral representation for the dispersion function, which readily yields the power and Laurent series for it. The new integral representation is shown to be closely related to the Gordeyev integral for a kappa distribution.
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Influence of Coulomb collisions on the structure of reconnection layers Phys. Plasmas 16, 072117 (2009); http://dx.doi.org/10.1063/1.3191718 (16 pages) Online Publication Date: 30 July 2009
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The influence of Coulomb collisions on the structure of reconnection layers is examined in neutral sheet geometry using fully kinetic simulations with a Monte Carlo treatment of the Fokker–Planck operator. The algorithm is first carefully benchmarked against key predictions from transport theory, including the parallel and perpendicular resistivities as well as the thermal force. The results demonstrate that the collisionality is accurately specified, thus allowing the initial Lundquist number to be chosen as desired. For modest Lundquist numbers S≲1000, the classic Sweet–Parker solution is recovered. Furthermore, a distinct transition to a faster kinetic regime is observed when the thickness of the resistive layer δSP falls below the ion inertial length di. For higher Lundquist numbers S≳1000, plasmoids (secondary islands) are observed within the elongated resistive layers. These plasmoids give rise to a measurable increase in the reconnection rate and for certain cases induce a transition to kinetic regimes sooner than expected from the δSP ≈ di condition. During this transition, the reconnection electric field exceeds the runaway limit, leading to electron scale current layers in which the nonideal electric field is supported predominantly by off-diagonal components in the electron pressure tensor, along with a residual contribution from electron-ion momentum exchange. These weakly collisional electron layers are also unstable to the formation of new plasmoids.
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Discharge ignition characteristics of pulsed radio-frequency glow discharges in atmospheric helium Phys. Plasmas 16, 070702 (2009); http://dx.doi.org/10.1063/1.3184824 (4 pages) Online Publication Date: 17 July 2009
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An experimental study of radio-frequency (15 MHz) glow discharges in atmospheric helium modulated by pulses with repetition frequency of 500 kHz and duty cycle of 6% and 8% is presented in this paper. In each discharge burst, the discharge is restricted to operate in ignition phase with duration of one or two radio-frequency cycles. The ignition characteristics in terms of spatial-temporal evolution of discharge interelectrode structure and optical emission intensity are investigated by time resolved imaging. Optical emission intensities at lines of 706 and 777 nm are used to capture clearly the temporal evolution of energetic electrons and active specie of atom oxygen generated in discharge.
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Experimental study of a fourth-harmonic gyromultiplier Phys. Plasmas 16, 070701 (2009); http://dx.doi.org/10.1063/1.3179805 (3 pages) Online Publication Date: 14 July 2009
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Simultaneous generation at the second and fourth cyclotron harmonics has been obtained from a single-cavity self-excited gyromultiplier. Output power of the short-wavelength radiation amounts to 100 W at a frequency of 75 GHz. The proposed scheme seems to be promising for the terahertz frequency range.
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Phys. Plasmas 16, 073504 (2009); http://dx.doi.org/10.1063/1.3155097 (11 pages) Online Publication Date: 16 July 2009
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Space- and time-correlated measurements of floating and plasma potential are made in the near field, external flow cathode region of a coaxial Hall plasma discharge using an emissive probe synchronized to quasicoherent fluctuations in discharge current. The luminous axial feature frequently observed in the near field of operating plasma accelerators is found to be concomitant with a spike in the plasma potential (and electron temperature). The structure of the plasma potential allows for multiple avenues for back-streaming ions to accelerate toward the discharge front pole and may pull some classes of ions toward the central axis. The fluctuations in plasma properties exhibit a complex structure at frequencies on the order of the so-called “breathing mode” ionization instability often seen in these types of discharges. Most notably, the plasma potential appears to fluctuate in a helical fashion, resembling tilted drift waves rotating about the central axis. A simple analysis of these waves draws attention to the possible role that they may play in driving anomalous cross-field electron transport in the near field region.
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Perspectives on high-energy-density physics Phys. Plasmas 16, 055501 (2009); http://dx.doi.org/10.1063/1.3078101 (7 pages) Online Publication Date: 6 March 2009
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Much of 21st century plasma physics will involve work to produce, understand, control, and exploit very nontraditional plasmas. High-energy-density (HED) plasmas are often examples, variously involving strong Coulomb interactions and ⪡1 particles per Debye sphere, dominant radiation effects, and strongly relativistic or strongly quantum-mechanical behavior. Indeed, these and other modern plasma systems often fall outside the early standard theoretical definitions of “plasma.” Here the specific ways in which HED plasmas differ from traditional plasmas are discussed. This is first done by comparison of important physical quantities across the parameter regime accessible by existing or contemplated experimental facilities. A specific discussion of some illustrative cases follows, including strongly radiative shocks and the production of relativistic, quasimonoenergetic beams of accelerated electrons.
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Plasma equilibrium in a magnetic field with stochastic regions Phys. Plasmas 16, 072308 (2009); http://dx.doi.org/10.1063/1.3159607 (26 pages) Online Publication Date: 16 July 2009
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The nature of plasma equilibrium in a magnetic field with stochastic regions is examined. It is shown that the magnetic differential equation that determines the equilibrium Pfirsch–Schlüter currents can be cast in a form similar to various nonlinear equations for a turbulent plasma, allowing application of the mathematical methods of statistical turbulence theory. An analytically tractable model, previously studied in the context of resonance-broadening theory, is applied with particular attention paid to the periodicity constraints required in toroidal configurations. It is shown that even a very weak radial diffusion of the magnetic field lines can have a significant effect on the equilibrium in the neighborhood of the rational surfaces, strongly modifying the near-resonant Pfirsch–Schlüter currents. Implications for the numerical calculation of three-dimensional equilibria are discussed.
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