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Top 20 Most Read Articles
September 2006
The 20 articles with the most full-text downloads during the month, in descending order.
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Phys. Plasmas 13, 093501 (2006); http://dx.doi.org/10.1063/1.2338282 (7 pages) Online Publication Date: 6 September 2006
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A novel spectroscopic method is proposed for the measurement of electron density and temperature in atmospheric pressure dielectric barrier discharges using nitrogen gas. Simplified collisional-radiative models for the electronic and the vibrational states yield two separate continuity equations as a function of the electron density and the temperature with the coefficients expressed in terms of rotational temperature, vibrational temperature, and emission intensity ratio between the first positive system and the second positive system of nitrogen molecules. The electron density and the temperature in nonequilibrium atmospheric pressure plasmas can be determined by solving the continuity equations with the coefficients estimated from the spectroscopic measurements. It was confirmed by applying to a high power dielectric barrier discharge, where the measured plasma parameters were in good agreement with the estimation by using the electron conductivity of the discharge.
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Gyro-kinetic simulation of global turbulent transport properties in tokamak experiments Phys. Plasmas 13, 092505 (2006); http://dx.doi.org/10.1063/1.2338775 (12 pages) Online Publication Date: 11 September 2006
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A general geometry gyro-kinetic model for particle simulation of plasma turbulence in tokamak experiments is described. It incorporates the comprehensive influence of noncircular cross section, realistic plasma profiles, plasma rotation, neoclassical (equilibrium) electric fields, and Coulomb collisions. An interesting result of global turbulence development in a shaped tokamak plasma is presented with regard to nonlinear turbulence spreading into the linearly stable region. The mutual interaction between turbulence and zonal flows in collisionless plasmas is studied with a focus on identifying possible nonlinear saturation mechanisms for zonal flows. A bursting temporal behavior with a period longer than the geodesic acoustic oscillation period is observed even in a collisionless system. Our simulation results suggest that the zonal flows can drive turbulence. However, this process is too weak to be an effective zonal flow saturation mechanism.
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A laboratory produced extremely large beta plasma Phys. Plasmas 13, 092503 (2006); http://dx.doi.org/10.1063/1.2338022 (12 pages) Online Publication Date: 8 September 2006
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This paper discusses laboratory production of an extremely large-beta (β ∼ 103) plasma, termed ELB plasma, over an extended volume of 0.5 m3. The plasma β can be varied from <1 to values as large as 103, thereby allowing us to study physical processes through a large range of β. Desired β values are achieved by tailoring the ambient magnetic field by compensating fields over time scales far less than the plasma confinement time. We observe that during this compensation, the magnetic field penetration from the wall to the plasma occurs at Whistler speed. The Whistler mode is accompanied by excitation of electrostatic ion wave. We have shown that the diamagnetism of the ELB plasma is considerably modified essentially through the effect of finiteness of electron Larmor radius (rLe) in the force equilibrium. Application of these results to the space plasma is pointed out.
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Self-organization in electron temperature gradient driven turbulence Phys. Plasmas 13, 080701 (2006); http://dx.doi.org/10.1063/1.2335821 (4 pages) Online Publication Date: 3 August 2006
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Based on first principle gyrokinetic calculations, a zonal flow generation mechanism in the slab electron temperature gradient driven (ETG) turbulence with weak magnetic shear is identified as self-organization via the turbulent spectral cascade in the two dimensional rotating fluid turbulence. The inverse energy cascade and the scaling of a zonal flow wavenumber, which is consistent with the Rhines scale length, are confirmed. An impact of the scaling, which depends on the density gradient, on the turbulent structure and transport is demonstrated for the slab ETG turbulence.
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The Vlasov-Poisson model and the validity of a numerical approach Phys. Plasmas 13, 082102 (2006); http://dx.doi.org/10.1063/1.2215596 (7 pages) Online Publication Date: 16 August 2006
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Our aim in this work is to show that the final macroscopic state of a noncollisional plasma system, computed through numerical simulations, depends on artificial small scale effects induced by the used numerical scheme and/or grid discretization. By using the continuous, Hamiltonian Vlasov-Poisson model, we found significant differences in the nonlinear dynamics when varying the importance of dissipative and/or dispersive (numerical) effects. In particular, such artificial processes are crucial during phase space vortex generation and vortex merging dynamics leading to different irreversible asymptotic states. These results are obtained for numerical grid scale lengths much smaller than any noncollisional physical scale length.
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Long plasma channels in segmented capillary discharges Phys. Plasmas 13, 083108 (2006); http://dx.doi.org/10.1063/1.2261853 (5 pages) Online Publication Date: 31 August 2006
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Guided propagation of ultrashort (100 fs) high intensity (1016 W cm−2) laser pulses over distances up to 12.6 cm using optimized segmented capillary is reported. A new diagnostic technique is presented in which the transport of a guided laser pulse at different delay times from the initiation of the discharge is sampled on a single discharge shot. The current waveform was optimized to obtain a long lasting, deep radial profile. Radial profiles with the maximal electron density from 4×1017 to 2×1018 cm−3 and up to 25% deep were obtained thereby, whereas longitudinal profiles were found to be remarkably uniform. The potential application of these long channels to the laser wake field accelerator is discussed.
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Phys. Plasmas 13, 092101 (2006); http://dx.doi.org/10.1063/1.2236277 (12 pages) Online Publication Date: 1 September 2006
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A general coordinate-independent expression for Braginskii’s form of the ion gyroviscosity in the two-dimensional potential field representation is presented, and is implemented in a full two-dimensional, two-fluid extended magnetohydrodynamic (MHD) numerical model. The expression for the gyroviscous force requires no field to be differentiated more than twice, and thus is appropriate for finite elements with first derivatives continuous across element boundaries (C1 finite elements). From the extended MHD model, which includes the full gyroviscous stress, are derived linear dispersion relations of a homogeneous equilibrium and of an inverted-density profile in the presence of gravity. The treatment of the gravitational instability presented here extends previous work on the subject
[M. N. Rosenbluth, N. A. Krall, and N. Rostoker, Nucl. Fusion Suppl. 1, 143 (1962);
K. V. Roberts and J. B. Taylor, Phys. Rev. Lett. 8, 197 (1962)].
Linear and nonlinear simulations of the gravitational instability are presented. Simulations are shown to agree closely with the derived dispersion relations in the linear regime. The “gyroviscous cancellation” effect is demonstrated, and some limitations of the
 * approximation are discussed. |
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Relating parallel and perpendicular flows of particles and heat in a magnetized toroidal plasma Phys. Plasmas 13, 094501 (2006); http://dx.doi.org/10.1063/1.2338821 (4 pages) Online Publication Date: 7 September 2006
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The purpose of this Brief Communication is to emphasize the existence of a general relation between the parallel flows of heat and particles within flux surfaces and the transport of heat and particles across those flux surfaces predicted by neoclassical theory. The essential ingredients are a perspective that promotes the heat flow to the status of a fully independent dynamical variable and a unified treatment that makes no restriction regarding collisionality. Applied to well-known expressions from the literature, this approach provides a simple and explicit relation between parallel and radial flows that applies in all collisionality regimes.
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Electron energy distribution functions in low-pressure inductively coupled bounded plasmas Phys. Plasmas 13, 092104 (2006); http://dx.doi.org/10.1063/1.2339024 (5 pages) Online Publication Date: 7 September 2006
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The electron energy distribution function (EEDF) in a low-pressure inductively coupled plasma confined between two infinite plates separated by 10 cm is investigated using a one-dimensional particle-in-cell simulation including Monte Carlo collisions. At low pressure, where the electron mean free path is of the order of or greater than the system length, the EEDF is close to Maxwellian, except for its tail, depleted at high energy. We give clear evidence that this depletion is mostly due to the high-energy electrons escaping to the walls. As a result of the EEDF nonlocality, the break energy, for which the depletion of the Maxwellian starts, is found to track the plasma potential. At a higher pressure, the electron mean free paths of the various elastic and inelastic collisions become shorter than the system length, resulting in a loss of nonlocality and the break energy of the distribution function moves to energies lower than the plasma potential.
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m = 1 ideal internal kink modes in a line-tied screw pinch Phys. Plasmas 13, 092102 (2006); http://dx.doi.org/10.1063/1.2336506 (15 pages) Online Publication Date: 1 September 2006
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It is well known that the radial displacement of the m = 1 internal kink mode in a periodic screw pinch has a steep jump at the resonant surface where k∙B = 0 [
Rosenbluth, Dagazian, and Rutherford, Phys. Fluids 16, 1894 (1973)
]. In a line-tied system, relevant to solar and astrophysical plasmas, the resonant surface is no longer a valid concept. It is then of interest to see how line-tying alters the aforementioned result for a periodic system. If the line-tied kink also produces a steep gradient, corresponding to a thin current layer, it may lead to strong resistive effects even with weak dissipation. Numerical solution of the eigenmode equations shows that the fastest growing kink mode in a line-tied system still possesses a jump in the radial displacement at the location coincident with the resonant surface of the fastest growing mode in the periodic counterpart. However, line-tying thickens the inner layer and slows down the growth rate. As the system length L approaches infinity, both the inner layer thickness and the growth rate approach the periodic values. In the limit of small ϵ ∼ Bϕ/Bz, the critical length for instability Lc ∼ ϵ−3. The relative increase in the inner layer thickness due to line-tying scales as ϵ−1(Lc/L)2.5.
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Low frequency instabilities during dust particle growth in a radio-frequency plasma Phys. Plasmas 13, 092103 (2006); http://dx.doi.org/10.1063/1.2337793 (8 pages) Online Publication Date: 6 September 2006
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In this paper, instabilities appearing in a dusty plasma are experimentally investigated. These low frequency self-excited instabilities appear during dust particle growth and are characterized by a frequency spectrum evolving during this process. The onset, the time evolution and the main characteristics of these instabilities are investigated thanks to electrical and optical measurements. Both signals show a clear evolution scheme with a well-defined succession of phases. From the beginning to the end of this scheme, regular oscillations and/or chaotic regimes are observed. Finally, instabilities stop when the dust particle size reaches a few hundreds of nanometers and a stable three-dimensional dust cloud is obtained. A dust-free region called void is then usually observed in the plasma center.
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New approach for the study of linear Vlasov stability of inhomogeneous systems Phys. Plasmas 13, 092110 (2006); http://dx.doi.org/10.1063/1.2345358 (23 pages) Online Publication Date: 14 September 2006
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This paper presents an alternative technique for solving the linearized Vlasov-Maxwell set of equations, in which the velocity dependence of the perturbed distribution function is described by means of an infinite series of orthogonal functions, chosen as Hermite polynomials. The orthogonality properties of such functions allow us to decompose the Vlasov equation into a set of infinite coupled linear equations. With a suitable truncation relation, the problem is transformed in an eigenvalue problem. This technique is based on solid but easy concepts, not attempting to evaluate the integration over the unperturbed trajectories and can be applied to any equilibrium. Although the solutions are approximate, because they neglect contributions of higher order coefficients of the series, the physical meaning of the low-order coefficients is clear. Furthermore the accuracy of the solution, which depends on the number of terms taken into account in the Hermite series, appears to be merely a problem of computational power. The method has been tested for a 1D Harris equilibrium, known to give rise to several instabilities like tearing, drift kink, and lower hybrid. The results are shown in agreement with those obtained by Daughton with a traditional technique based on the integration over unperturbed orbits.
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Kinetic theory of electromagnetic ion waves in relativistic plasmas Phys. Plasmas 13, 094503 (2006); http://dx.doi.org/10.1063/1.2348086 (3 pages) Online Publication Date: 11 September 2006
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A kinetic theory for electromagnetic ion waves in a cold relativistic plasma is derived. The kinetic equation for the broadband electromagnetic ion waves is coupled to the slow density response via an acoustic equation driven by a ponderomotive force-like term linear in the electromagnetic field amplitude. The modulational instability growth rate is derived for an arbitrary spectrum of waves. The monochromatic and random phase cases are studied.
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Limits for light intensification by reflection from relativistic plasma mirrors Phys. Plasmas 13, 093102 (2006); http://dx.doi.org/10.1063/1.2348087 (5 pages) Online Publication Date: 15 September 2006
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Ultraintense laser pulses might be capable of producing plasma mirrors through ultrarelativistic oscillating electrons. Two kinds of such mirrors with relativistic factors γ ∼ 100–300 were contemplated recently as a tool for producing huge frequency upshifts 4γ2 of reflected laser pulses as well as the compressing and focusing of those pulses. The combination of these effects would result in dramatic light intensification toward the vacuum breakdown intensities (Schwinger limit) [
S. V. Bulanov et al., Phys. Rev. Lett. 91, 085001 (2003)
and
S. Gordienko et al., Phys. Rev. Lett. 94, 103903 (2005)
]. The analysis performed in these publications was limited, however, to idealized situations of cold uniform plasmas and uniform laser intensities. The analysis here of effects of electron thermal motion and random inhomogeneities in the plasma density or laser intensity indicates that the largest relativistic factors allowed within these schemes are much smaller than those assumed in the idealized models, unless essentially new physical mechanisms are adduced in addition to those already considered.
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Prepulse effect on intense femtosecond laser pulse propagation in gas Phys. Plasmas 13, 093103 (2006); http://dx.doi.org/10.1063/1.2351961 (6 pages) Online Publication Date: 15 September 2006
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The propagation of an ultrashort laser pulse can be affected by the light reaching the medium before the pulse. This can cause a serious drawback to possible applications. The propagation in He of an intense 60-fs pulse delivered by a Ti:sapphire laser in the chirped pulse amplification (CPA) mode has been investigated in conditions of interest for laser-plasma acceleration of electrons. The effects of both nanosecond amplified spontaneous emission and picosecond pedestals have been clearly identified. There is evidence that such effects are basically of refractive nature and that they are not detrimental for the propagation of a CPA pulse focused to moderately relativistic intensity. The observations are fully consistent with numerical simulations and can contribute to the search of a stable regime for laser acceleration.
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Vlasov simulation of amplitude-modulated Langmuir waves Phys. Plasmas 13, 092304 (2006); http://dx.doi.org/10.1063/1.2348088 (5 pages) Online Publication Date: 11 September 2006
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Mechanisms for the generation of Langmuir wave packets are studied by performing a one-dimensional electrostatic Vlasov simulation. The present simulation of a weak-electron-beam instability without ion dynamics suggests two new processes for the amplitude modulation of Langmuir waves. The beam instability excites Langmuir modes over a wide wave number range, but the saturation of the most unstable Langmuir mode “filters” the growth of sideband modes. Specific upper and lower sideband modes linearly grow to a high saturation level. Then the primary Langmuir mode is amplified and strongly modulated through interaction with the sideband modes.
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Observation of a quasimonoenergetic electron beam from a femtosecond prepulse-exploded foil Phys. Plasmas 13, 080702 (2006); http://dx.doi.org/10.1063/1.2336589 (4 pages) Online Publication Date: 9 August 2006
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We report the first observation of a quasimonoenergetic electron beam emitted from a femtosecond prepulse-exploded foil. At 5 ns earlier, a 7.5-μm-thick plastic foil is irradiated by a copropagating femtosecond prepulse of modest intensity (1.5×1017 W/cm2), which then explodes to become an underdense foil plasma. When a 70-fs high-intensity (2.5×1018 W/cm2) pulse arrives and interacts with it, a collimated electron beam of divergence angle ≲4° is emitted in the forward direction. The energy spectrum of the electron beam shows a quasimonoenergetic peak at 0.63 MeV of energy spread ∼ 21%, within which about 50 pC of charge is contained.
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Comparison of hybrid Hall thruster model to experimental measurements Phys. Plasmas 13, 083505 (2006); http://dx.doi.org/10.1063/1.2336186 (12 pages) Online Publication Date: 31 August 2006
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A two-dimensional hybrid particle-in-cell numerical model has been constructed in the radial-axial plane with the intent of examining the physics governing Hall thruster operation. The electrons are treated as a magnetized quasi-one-dimensional fluid and the ions are treated as collisionless, unmagnetized discrete particles. The anomalously high electron conductivity experimentally observed in Hall thrusters is accounted for using experimental measurements of electron mobility in the Stanford Hall Thruster. While an experimental mobility results in improved simulation of electron temperature and electric potential relative to a Bohm-type model, results suggest that energy losses due to electron wall interactions may also be an important factor in accurately simulating plasma properties. Using a simplified electron wall damping model modified to produce general agreement with experimental measurements, an evaluation is made of differing treatments of electron mobility, background gas, neutral wall interactions, and charge exchange collisions. Although background gas results in two populations of neutrals, the increased neutral density has little effect on other plasma properties. Diffuse neutral wall interactions are in better agreement with experimental measurements than specular scattering. Also, charge exchange collisions result in an increase in average neutral velocity of 11% and a decrease in average ion velocity of 4% near the exit plane. The momentum exchange that occurs during charge exchange collisions is found to be negligible.
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A nonlinear global model of a dual frequency capacitive discharge Phys. Plasmas 13, 083501 (2006); http://dx.doi.org/10.1063/1.2244525 (6 pages) Online Publication Date: 3 August 2006
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The behavior of dual frequency capacitively coupled plasmas is investigated. Assuming a realistic reactor configuration represented by effective geometry factors and taking into account two separate sinusoidal voltage sources operating at different frequencies, an ordinary differential equation is derived which describes the nonlinear dynamics of such discharges. An exact analytical solution of the equation is presented and employed for a parameter study of the discharge current characteristics. Simulation results for various gas pressures (=various electron-neutral collision rates), various amplitude ratios of the two independent rf sources, and various integer frequency ratios are shown. When the two frequencies are comparable, surprising nonlinear effects are observed. Particular under study is the heating at the plasma series resonance, either by direct excitation or via the nonlinear electron resonance heating mechanism.
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Internal plasma potential measurements of a Hall thruster using xenon and krypton propellant Phys. Plasmas 13, 093502 (2006); http://dx.doi.org/10.1063/1.2335820 (10 pages) Online Publication Date: 11 September 2006
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For krypton to become a realistic option for Hall thruster operation, it is necessary to understand the performance gap between xenon and krypton and what can be done to reduce it. A floating emissive probe is used with the Plasmadynamics and Electric Propulsion Laboratory’s High-speed Axial Reciprocating Probe system to map the internal plasma potential structure of the NASA-173Mv1 Hall thruster [
R. R. Hofer, R. S. Jankovsky, and A. D. Gallimore, J. Propulsion Power 22, 721 (2006);
R. R. Hofer, R. S. Jankovsky, and A. D. Gallimore, J. Propulsion Power22, 732 (2006)
] using xenon and krypton propellant. Measurements are taken for both propellants at discharge voltages of 500 and 600 V. Electron temperatures and electric fields are also reported. The acceleration zone and equipotential lines are found to be strongly linked to the magnetic-field lines. The electrostatic plasma lens of the NASA-173Mv1 Hall thruster strongly focuses the xenon ions toward the center of the discharge channel, whereas the krypton ions are defocused. Krypton is also found to have a longer acceleration zone than the xenon cases. These results explain the large beam divergence observed with krypton operation. Krypton and xenon have similar maximum electron temperatures and similar lengths of the high electron temperature zone, although the high electron temperature zone is located farther downstream in the krypton case.
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