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Top 20 Most Read Articles
July 2008
The 20 articles with the most full-text downloads during the month, in descending order.
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Hard x-ray radiography for density measurement in shock compressed matter Phys. Plasmas 15, 060701 (2008); http://dx.doi.org/10.1063/1.2928156 (4 pages) Online Publication Date: 10 June 2008
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In this letter we report on the direct density measurement in a shock compressed aluminum target using hard x-ray radiography. Experimental data employing a molybdenum Kα source at 17.5 keV, generated with a short pulse laser are presented. High spatial resolution was obtained thanks to a new design for the backlighter geometry. Density values deduced from radiography are compared to predictions from hydrodynamic simulations, which have been calibrated in order to reproduce shock velocities measured from a rear-side self-emission diagnostic. Our results reveal the great potential of this technique as a diagnostic tool for direct density measurements in dense high-Z opaque materials.
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Betatron radiation from density tailored plasmas Phys. Plasmas 15, 063102 (2008); http://dx.doi.org/10.1063/1.2918657 (10 pages) Online Publication Date: 9 June 2008
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In laser wakefield accelerators, electron motion is driven by intense forces that depend on the plasma density. Transverse oscillations in the accelerated electron orbits produce betatron radiation. The electron motion and the resulting betatron radiation spectrum can therefore be controlled by shaping the plasma density along the orbit of the electrons. Here, a method based on the use of a plasma with a longitudinal density variation (density depression or step) is proposed to increase the transverse oscillation amplitude and the energy of the electrons accelerated in a wakefield cavity. For fixed laser parameters, by appropriately tailoring the plasma profile, the betatron radiation emitted by these electrons is significantly increased in both flux and energy.
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Phys. Plasmas 15, 063104 (2008); http://dx.doi.org/10.1063/1.2937817 (8 pages) Online Publication Date: 26 June 2008
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The interaction of an ultraintense laser pulse with parabolic concave target is studied using two-dimensional particle-in-cell simulation. It is found that ions at the back of the concave target can be accelerated more effectively than that with a plane target because of a large electrostatic field on the propagation axis induced by focusing of the laser expelled electrons. A highly collimated ion bunch with high particle and current densities appears. The dependence of the quality of ion acceleration and bunching on the width of the concave cavity is examined. It is found that the maximum ion energy is greatly enhanced when the width of the cavity attains a threshold, and an optimum width for generating a high-quality ion bunch exists. The results should be useful in the design of targets used in producing highly collimated energetic ions in high-energy-density physics.
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A note on dust wave excitation in a plasma with warm dust: Comparison with experiment Phys. Plasmas 15, 073701 (2008); http://dx.doi.org/10.1063/1.2943218 (5 pages) Online Publication Date: 8 July 2008
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Ion-dust streaming instability in a plasma containing dust grains with large thermal speeds is considered using kinetic theory. The results are compared with experimental measurements of the dispersion relation of dust acoustic waves performed at the University of Iowa and Auburn University.
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Time reversal duality of magnetohydrodynamic shocks Phys. Plasmas 15, 062101 (2008); http://dx.doi.org/10.1063/1.2919795 (19 pages) Online Publication Date: 3 June 2008
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The shock conditions in magnetohydrodynamics (MHD) are reduced to their most concise, three-parameter, distilled form by consistent use of the scale independence of the MHD equations and of the de Hoffmann–Teller transformation. They then exhibit a distinct time reversal duality between entropy-allowed shocks and entropy-forbidden jumps. This yields a new classification of MHD shocks by means of the monotonicity properties with respect to upstream and downstream Alfvén Mach numbers, it exhibits the central role of intermediate discontinuities, and permits straightforward construction of all relevant dimensionless quantities of the shocks. An exhaustive overview is presented of solutions in the different parameter regimes.
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Theory of current-free double layers in plasmas Phys. Plasmas 15, 062111 (2008); http://dx.doi.org/10.1063/1.2937153 (6 pages) Online Publication Date: 26 June 2008
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The existence of current-free double layers in unmagnetized plasma is studied by means of the quasipotential method applied to the Vlasov–Poisson system. Crucial for its existence are trapped particle populations that are characterized by notches (dips) in the velocity distribution functions at resonant velocity becoming flat at large amplitude limit. The potential drop across the double layer, or its amplitude ψ, can be arbitrarily strong covering the whole range 0<ψ<∞. Both the small and large amplitude limit are worked out explicitly, inclusively effective kinetic temperatures and pressures. It is, hence, the effective electron (ion) temperature increase (decrease) with increasing potential, caused by the trapped particles, which is responsible for the existence of this two-parameter family of solutions.
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Phys. Plasmas 15, 074701 (2008); http://dx.doi.org/10.1063/1.2955787 (3 pages) Online Publication Date: 16 July 2008
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Short-pulse space-charge-limited electron flows in a drift space Phys. Plasmas 15, 063105 (2008); http://dx.doi.org/10.1063/1.2941490 (5 pages) Online Publication Date: 30 June 2008
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In this paper, the space-charge-limited (SCL) electron flows in a drift space is studied by including the effect of finite electron pulse length, which is smaller than the gap transit time. Analytical formulas are derived to calculate the maximum SCL current density that can be transported across a drift space under the short-pulse injection condition. For a given voltage or injection energy, the maximum current density that can be transported is enhanced by a large factor (as compared to the long-pulse or steady-state case), and the enhancement is inversely proportional to the electron pulse length. In drift space, the effect of pulse expansion is important at very short-pulse length, and the short-pulse enhancement factor is smaller as compared to a diode. The enhancement factor will be suppressed when the injection energy is larger than the electron rest mass, and effect of pulse expansion is less critical at relativistic energy. The analytical formulas have been verified by performing a particle-in-cell simulation in the electrostatic mode.
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Hot electron generation from intense laser irradiation of microtipped cone and wedge targets Phys. Plasmas 15, 052701 (2008); http://dx.doi.org/10.1063/1.2912457 (6 pages) Online Publication Date: 8 May 2008
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X-ray production from the interaction of femtosecond laser pulses focused to relativistic intensity into re-entrant targets etched into silicon has been investigated. Kα and hard x-ray yields were compared when the laser was focused into pyramidal shaped cone targets and wedge shaped targets. Hot electron production is highest in the wedge targets irradiated with transverse polarization, though Kα is maximized with wedge targets and parallel polarization. These results are explained with particle-in-cell simulations.
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Modulational instabilities of electromagnetic electron cyclotron waves in a dense magnetized plasma Phys. Plasmas 15, 074501 (2008); http://dx.doi.org/10.1063/1.2947196 (3 pages) Online Publication Date: 2 July 2008
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Modulational instabilities of electromagnetic electron-cyclotron waves in a dense magnetized plasma are investigated. Dispersion relations for the modulational instabilities are derived and analyzed. The effects of quantum statistical pressure and quantum electron tunneling are discussed.
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Fluid model for relativistic, magnetized plasmas Phys. Plasmas 15, 062112 (2008); http://dx.doi.org/10.1063/1.2937123 (12 pages) Online Publication Date: 27 June 2008
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Many astrophysical plasmas and some laboratory plasmas are relativistic: Either the thermal speed or the local bulk flow in some frame approaches the speed of light. Often, such plasmas are magnetized in the sense that the Larmor radius is smaller than any gradient scale length of interest. Conventionally, relativistic magnetohydrodynamics (MHD) is employed to treat relativistic, magnetized plasmas. However, MHD requires the collision time to be shorter than any other time scale in the system. Thus, MHD employs the thermodynamic equilibrium form of the stress tensor, neglecting pressure anisotropy and heat flow parallel to the magnetic field. Recent work has attempted to remedy these shortcomings. This paper re-examines the closure question and finds a more complete theory, which yields a more physical and self-consistent closure. Beginning with exact moments of the kinetic equation, we derive a closed set of Lorentz-covariant fluid equations for a magnetized plasma allowing for pressure and heat flow anisotropy. Basic predictions of the model, especially of the dispersion relation’s dependence upon relativistic temperature, are examined.
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Modeling of two-dimensional effects in hot spot relaxation in laser-produced plasmas Phys. Plasmas 15, 062701 (2008); http://dx.doi.org/10.1063/1.2919791 (12 pages) Online Publication Date: 5 June 2008
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Two-dimensional numerical simulations of plasma heating and temperature hot spots relaxation are presented in the domain where the diffusive approximation for heat transport fails. Under relevant conditions for laser plasma interactions, the effects of the nonlocality of heat transport on the plasma response are studied comparing the Spitzer–Härm model with several frequently used nonlocal models. The importance of using a high-order numerical scheme to correctly model nonlocal effects is discussed. A significant increase of the temperature relaxation time due to nonlocal heat transport is observed, accompanied by enhanced density perturbations. Applications to plasma-induced smoothing of laser beams are considered.
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Anomalous transport of particles in plasma flow with strong inhomogeneous velocity shear Phys. Plasmas 15, 072102 (2008); http://dx.doi.org/10.1063/1.2943703 (5 pages) Online Publication Date: 2 July 2008
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The temporal evolution of the drift modes and resulting anomalous transport are considered under the conditions of strong inhomogeneous flow shear [flow shear parameter dv0(r)/dr is greater or comparable to the drift frequency] on the ground of the nonmodal approach with application to boundary regions of tokamaks. The nonmodal linear analysis of the effect of flow shear nonuniformity on the temporal evolution of the drift modes, performed on the base of the Hasegava–Wakatani model, has shown, that terms reflecting velocity profile curvature decay more rapidly with time, as compared with those containing only velocity shearing rate. Therefore, the linear effect of the flow shear nonuniformity appears to be subdominant and the long-time evolution of the drift modes is determined by more slowly damped shear rate contained terms. The anomalous transport of particles in shear flow due to nonmodal drift perturbations exhibits a subdiffusive behavior with the diffusion coefficient reducing in time as t−3.
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Drift instabilities in current sheets with guide field Phys. Plasmas 15, 072101 (2008); http://dx.doi.org/10.1063/1.2938386 (7 pages) Online Publication Date: 1 July 2008
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Drift instabilities in current sheets with or without the guide field are investigated with a newly developed improved electrostatic dispersion relation. Traditional (local) theories of lower-hybrid drift instability typically assumes small electron drift speed, and expand the electron distribution function in Taylor series. This approximate treatment is removed in this paper. The resulting formalism is uniformly valid for an arbitrary magnitude of relative ion and electron drift speeds, and is valid for an arbitrary strength of the guide field.
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Propagation of electron magnetohydrodynamic structures in a two-dimensional inhomogeneous plasma Phys. Plasmas 15, 062308 (2008); http://dx.doi.org/10.1063/1.2943693 (9 pages) Online Publication Date: 30 June 2008
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The fully three-dimensional governing equations in the electron magnetohydrodynamic (EMHD) regime for a plasma with inhomogeneous density are obtained. These equations in the two-dimensional limit can be cast in terms of the evolution of two coupled scalar fields. The nonlinear simulations for the two-dimensional case are carried out to understand the propagation of EMHD magnetic structures in the presence of inhomogeneity. A novel effect related to the trapping of dipolar magnetic structures in the high density plasma region in the EMHD regime is observed. The interpretation of this phenomena as well as its relevance to the problem of hot spot generation in the context of fast ignition is presented.
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Characterization of intermittency of impurity turbulent transport in tokamak edge plasmas Phys. Plasmas 15, 072506 (2008); http://dx.doi.org/10.1063/1.2947027 (8 pages) Online Publication Date: 11 July 2008
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The statistical properties of impurity transport of a tokamak edge plasma embedded in a dissipative drift-wave turbulence are investigated using structure function analysis. The impurities are considered as a passive scalar advected by the plasma flow. Two cases of impurity advection are studied and compared: A decaying impurities case (given by a diffusion-advection equation) and a driven case (forced by a mean scalar gradient). The use of extended self-similarity enables us to show that the relative scaling exponent of structure functions of impurity density and vorticity exhibit similar multifractal scaling in the decaying case and follows the She–Lévêque model. However, this property is invalidated for the impurity driven advection case. For both cases, potential fluctuations are self-similar and exhibit a monofractal scaling in agreement with Kolmogorov–Kraichnan theory for two-dimensional turbulence. These results obtained with a passive scalar model agree also with test-particle simulations.
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Streaming instabilities in multicomponent interstellar clouds Phys. Plasmas 15, 072904 (2008); http://dx.doi.org/10.1063/1.2936268 (9 pages) Online Publication Date: 2 July 2008
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Streaming instabilities in interstellar clouds have been investigated. It is pointed out that the motion of a lighter dust species relative to a plasma containing a heavier dust species can give rise to low frequency electrostatic fluctuations. The dust ion acoustic and dust acoustic waves can become unstable in such plasmas. The instability regions are influenced by the presence of background dust. Since interstellar clouds have relative motions in many cases and contain dust species, therefore the present results indicate the presence of an electrostatic dust ion acoustic wave and dust acoustic wave in such systems. The results have been applied to interstellar medium using particular data and comparisons have been made with previous works.
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Eigenmode analysis of geodesic acoustic modes Phys. Plasmas 15, 072511 (2008); http://dx.doi.org/10.1063/1.2956993 (9 pages) Online Publication Date: 17 July 2008
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Geodesic acoustic modes (GAMs) are studied as plasma eigenmodes when an electrostatic potential nearly constant around a magnetic surface is applied to collisionless toroidal plasmas. Besides the standard GAM, a branch of low frequency mode and an infinite series of ion sound wavelike modes are identified. Eigenfrequencies of these modes are obtained analytically and numerically from a linear gyrokinetic model. The finite gyroradius effect is found to enhance the collisionless damping of the standard GAM, while this enhancement is not monotonic as the safety factor varies. Moreover, additional damping due to higher-harmonic resonances becomes important when the safety factor increases. The mode structure of the GAM is also discussed.
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Nonlinear transport processes in tokamak plasmas. I. The collisional regimes Phys. Plasmas 15, 062309 (2008); http://dx.doi.org/10.1063/1.2939377 (23 pages) Online Publication Date: 30 June 2008
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An application of the thermodynamic field theory (TFT) to transport processes in L-mode tokamak plasmas is presented. The nonlinear corrections to the linear (“Onsager”) transport coefficients in the collisional regimes are derived. A quite encouraging result is the appearance of an asymmetry between the Pfirsch–Schlüter (P-S) ion and electron transport coefficients: the latter presents a nonlinear correction, which is absent for the ions, and makes the radial electron coefficients much larger than the former. Explicit calculations and comparisons between the neoclassical results and the TFT predictions for Joint European Torus (JET) plasmas are also reported. It is found that the nonlinear electron P-S transport coefficients exceed the values provided by neoclassical theory by a factor that may be of the order 102. The nonlinear classical coefficients exceed the neoclassical ones by a factor that may be of order 2. For JET, the discrepancy between experimental and theoretical results for the electron losses is therefore significantly reduced by a factor 102 when the nonlinear contributions are duly taken into account but, there is still a factor of 102 to be explained. This is most likely due to turbulence. The expressions of the ion transport coefficients, determined by the neoclassical theory in these two regimes, remain unaltered. The low-collisional regimes, i.e., the plateau and the banana regimes, are analyzed in the second part of this work.
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A particle simulation of current sheet instabilities under finite guide field Phys. Plasmas 15, 072103 (2008); http://dx.doi.org/10.1063/1.2938732 (13 pages) Online Publication Date: 3 July 2008
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The instability of a Harris current sheet under a broad range of finite guide field (BG) is investigated using a linearized (δf) gyrokinetic electron and fully kinetic ion particle simulation code. The simulation is carried out in the two-dimensional plane containing the guide field along y and the current sheet normal along z. In this particle model, the rapid electron cyclotron motion is removed, while the realistic mass ratio mi/me, finite electron Larmor radii, and wave-particle interactions are kept. It is found that for a finite BG/Bx0 ⩽ 1, where Bx0 is the asymptotic antiparallel component of magnetic field, three unstable modes, i.e., modes A, B, and C, can be excited in the current sheet. Modes A and C, appearing to be quasielectrostatic modified two-stream instability/whistler mode, are located mainly on the edge of the current sheet. Mode B, on the other hand, is confined in the current sheet center and carries a compressional magnetic field (δBy) perturbation along the direction of electron drift velocity. Our new finding suggests that mode B may contribute directly to the electron anomalous resistivity in magnetic reconnection. In the cases with extremely large BG/Bx0⪢1, the wave modes evolve to a globally propagating instability. The simulation shows that the presence of finite BG modifies the physics of the current sheet significantly.
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