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Top 20 Most Read Articles
September 2008
The 20 articles with the most full-text downloads during the month, in descending order.
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Sizing up plasmas using dimensionless parameters Phys. Plasmas 15, 080501 (2008); http://dx.doi.org/10.1063/1.2961043 (19 pages) Online Publication Date: 27 August 2008
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This paper reviews the application to magnetic-confinement fusion experiments of dimensional analysis, which holds that the behavior of physical systems can be determined from the scaling of phenomena with the set of dimensionless parameters that embody the governing physics. This paper begins by explaining the two most well-known approaches to dimensional analysis, and shows that the principle of similarity has been demonstrated in high-temperature plasmas of different physical size. Next, the measured dependences of cross-magnetic-field transport and edge plasma characteristics on dimensionless parameters are examined. These dimensionless parameter scans are generally in good agreement with drift wave models of turbulent transport (i.e., microturbulence), although some discrepancies remain. Finally, the benefits of incorporating dimensional analysis into the extrapolation of plasma behavior from present-day experiments to future burning plasma devices are discussed. The experiments reviewed in this paper have greatly improved our understanding of the underlying physics of many plasma phenomena.
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Phys. Plasmas 2, 3933 (1995); http://dx.doi.org/10.1063/1.871025 (92 pages)
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Inertial confinement fusion (ICF) is an approach to fusion that relies on the inertia of the fuel mass to provide confinement. To achieve conditions under which inertial confinement is sufficient for efficient thermonuclear burn, a capsule (generally a spherical shell) containing thermonuclear fuel is compressed in an implosion process to conditions of high density and temperature. ICF capsules rely on either electron conduction (direct drive) or x rays (indirect drive) for energy transport to drive an implosion. In direct drive, the laser beams (or charged particle beams) are aimed directly at a target. The laser energy is transferred to electrons by means of inverse bremsstrahlung or a variety of plasma collective processes. In indirect drive, the driver energy (from laser beams or ion beams) is first absorbed in a high‐Z enclosure (a hohlraum), which surrounds the capsule. The material heated by the driver emits x rays, which drive the capsule implosion. For optimally designed targets, 70%–80% of the driver energy can be converted to x rays. The optimal hohlraum geometry depends on the driver. Because of relaxed requirements on laser beam uniformity, and reduced sensitivity to hydrodynamic instabilities, the U.S. ICF Program has concentrated most of its effort since 1976 on the x‐ray or indirect‐drive approach to ICF. As a result of years of experiments and modeling, we are building an increasingly strong case for achieving ignition by indirect drive on the proposed National Ignition Facility (NIF). The ignition target requirements for hohlraum energetics, radiation symmetry, hydrodynamic instabilities and mix, laser plasma interaction, pulse shaping, and ignition requirements are all consistent with experiments. The NIF laser design, at 1.8 MJ and 500 TW, has the margin to cover uncertainties in the baseline ignition targets. In addition, data from the NIF will provide a solid database for ion‐beam‐driven hohlraums being considered for future energy applications. In this paper we analyze the requirements for indirect drive ICF and review the theoretical and experimental basis for these requirements. Although significant parts of the discussion apply to both direct and indirect drive, the principal focus is on indirect drive. © 1995 American Institute of Physics. |
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Laser proton acceleration in a water spray target Phys. Plasmas 15, 083106 (2008); http://dx.doi.org/10.1063/1.2968456 (8 pages) Online Publication Date: 13 August 2008
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Studies of interaction of a cloud of submicrometer water droplets with ultrashort (40 fs) and intense ( ∼ 2×1019 W/cm2) laser pulses demonstrate an efficient acceleration of protons and oxygen ions. Due to a high ratio of the volume to the enveloping surface of a single droplet and a large number of droplets in a focal volume, efficient laser pulse absorption is enabled, which provides high electron temperatures and ion acceleration to high energies. The generation of ions with energies more than 1 MeV per nucleon is demonstrated. The observed quasi-monoenergetic feature in the proton spectrum is discussed with the thermal expansion–Coulomb explosion model and numerical simulations.
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Phys. Plasmas 15, 092701 (2008); http://dx.doi.org/10.1063/1.2966115 (7 pages) Online Publication Date: 10 September 2008
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Measurements of the bidirectional plasma jets that form at the surface of a solid target during a laser-generated driven magnetic reconnection are presented. Resistivity enhancement of at least 25× the classical Spitzer value is required when applying the Sweet–Parker model of reconnection to reconcile the experimentally observed reconnection time scale. Analytic calculations show that a fast reconnection model, which includes a priori the effects of microturbulent resistivity enhancement, better reproduces the experimental observations.
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Particle-in-cell simulation of collisionless reconnection with open outflow boundaries Phys. Plasmas 15, 082102 (2008); http://dx.doi.org/10.1063/1.2965826 (9 pages) Online Publication Date: 4 August 2008
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A new method for applying open boundary conditions in particle-in-cell (PIC) simulations is utilized to study magnetic reconnection. Particle distributions are assumed to have zero normal derivatives at the boundaries. Advantages and possible limitations of this method for PIC simulations are discussed. Results from a reconnection simulation study are presented. For the purpose of this investigation, a 2 ½-dimensional electromagnetic PIC simulation using open conditions at the outflow boundaries and simple reflecting boundaries to the inflow regions is discussed. The electron diffusion region is defined as that region where the out-of-plane electron inertial electric field is positive indicating acceleration and flux transfer; the evolution of this region is analyzed. It is found that this region varies in the range 2.5–4 local electron inertial lengths in total width and in the range 10–15 local electron inertial lengths in total length for the mass ratio 25. The reconnection rate is investigated in terms of the aspect ratio of the electron diffusion region plus inflow and outflow measures at its boundaries. It is shown that a properly measured aspect ratio predicts the flux transfer rate, scaled to account for the decline in field strength and electron density at the inflow boundary to the electron diffusion region. It is concluded that this electron diffusion region either adjusts its aspect ratio for compatibility with the flux transfer rate that is set elsewhere, as in the Hall reconnection model, or that it is this region that controls the reconnection flux transfer rate.
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A fluid model for ion heating due to ionization in a plasma flow Phys. Plasmas 15, 093501 (2008); http://dx.doi.org/10.1063/1.2976367 (6 pages) Online Publication Date: 5 September 2008
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A mechanism of ion heating due to local ionization in a plasma flow with hot electrons and initially cold ions is described. It is shown that the ion temperature can increase significantly as the ions are accelerated to the ion-acoustic speed, provided there is significant ionization in the acceleration region. The fluid model describing ionization ion heating includes particle, momentum, and energy balance equations for ion species. Using this model with parameters characteristic of gas-dynamic electron cyclotron resonance (ECR) ion source SMIS 37 yields much higher effective ion temperature than can be attributed to the electron-ion collisional energy transfer, typically considered for classical ECR ion sources. This theoretical result is found to be in agreement with findings of recent experiments carried out in SMIS 37.
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Modulation instability of ion thermal waves in a pair-ion plasma containing charged dust impurities Phys. Plasmas 15, 092101 (2008); http://dx.doi.org/10.1063/1.2976169 (10 pages) Online Publication Date: 2 September 2008
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Modulation instability of ion thermal waves (ITWs) is investigated in a plasma composed of positive and negative ions as well as a fraction of stationary charged (positive or negative) dust impurities. For this purpose, a linear dispersion relation and a nonlinear Schrödinger equation are derived. The latter admits localized envelope solitary wave solutions of bright (pulses) and dark (holes, voids) type. The envelope soliton depends on the intrinsic plasma parameters. It is found that modulation instability of ITWs is significantly affected by the presence of positively/negatively charged dust grains. The findings of this investigation should be useful in understanding the stable electrostatic wave packet acceleration mechanisms in pair-ion plasma, and also enhances our knowledge on the occurrence of instability associated to the existence of charged dust impurities in pair-ion plasmas. Our results should be of relevance for laboratory plasmas.
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Hall currents in a current sheet: Structure and dynamics Phys. Plasmas 15, 092102 (2008); http://dx.doi.org/10.1063/1.2972158 (10 pages) Online Publication Date: 4 September 2008
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Experimental results are presented from the study of the structure and time evolution of the Hall currents in the current sheets produced in the two-dimensional magnetic fields with the null line of the X type, in plasmas with heavy ions. Three-component magnetic fields generated by plasma currents were measured, and particular emphasis was placed on the out-of-plane magnetic field component aligned with the null line. The temporal evolution and spatial structure of the out-of-plane magnetic field and its dependence on the ion mass made us conclude that this field is produced by the Hall currents. The out-of-plane magnetic field is of the quadrupole structure, being directed oppositely on the opposite sides of the current sheet symmetry planes. The out-of-plane field exists at the initial stage of the sheet evolution, in a limited time interval, which is more prolonged for the sheets formed in plasmas with heavier ions. We revealed that the Hall currents of the opposite directions exist inside the current sheet, while the basic current has only one direction. Near the sheet middle plane the Hall currents flow from the peripheral regions toward the null line, whereas at larger distances from the middle plane the Hall currents become reversed. The Hall currents in both directions are localized only in the regions, where the basic current exists. At every time moment the oppositely directed Hall currents practically cancel each other and form four closed current circuits in the (x,y) plane, which produce the out-of-plane quadrupole magnetic field.
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Phys. Plasmas 15, 083101 (2008); http://dx.doi.org/10.1063/1.2965149 (7 pages) Online Publication Date: 1 August 2008
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The results of a numerical study of high-intensity short-pulse laser interaction with wire targets are presented. Fast electron production and transport in solid density plasma is modeled using the implicit hybrid particle-in-cell code LSP [
D. R. Welch et al., Phys. Plasmas 13, 063105 (2006)
]. These simulations were performed with realistic target size and laser parameters and over times much longer than the laser pulse. Nonlinear interaction processes, i.e., microchanneling and density steepening, have been observed. The spectrum of the relativistic electrons produced has a reduced slope temperature compared to that predicted by ponderomotive scaling. Preformed underdense plasma has been found to bottleneck fast electrons due to the intense magnetic fields generated near the critical surface. In a thin long wire target, the overall propagation length of the fast electrons is about 160 μm; however, surface fields guide a small fraction of electrons to longer distances. These results are in good agreement with the experiments and have demonstrated that the modeling of electron transport relevant to fast ignition can be pursued in an integrated manner.
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Influence of prepulse plasma formation on neutron production from the laser–target interaction Phys. Plasmas 15, 083107 (2008); http://dx.doi.org/10.1063/1.2969437 (9 pages) Online Publication Date: 14 August 2008
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The interaction of an intense ultrashort pulse laser with a planar uniform target was studied with a two-dimensional relativistic electromagnetic particle-in-cell method to determine the acceleration of deuterons and production of neutrons. A Au-CD2 double-layer planar target with thickness of ∼ 1 μm and a preplasma of variable length was used to generate high-energy deuterons as a precursor for neutron production. The deuteron energy and angular distributions and the neutron production from D(d,n)–3He nuclear fusion reactions were studied as a function of the preplasma scale length and target thickness. For very thin (submicron) targets the preplasma increases the neutron yield only marginally, but for realistic targets with thickness of a few microns the preplasma enhances the neutron yield by two orders of magnitude. Both the average deuteron energy and neutron yield peak at an optimum preplasma scale length Lpopt ≈ 1/k0 (k0 laser wave vector), which is of the order of one inverse laser wave vector.
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Phys. Plasmas 15, 093106 (2008); http://dx.doi.org/10.1063/1.2977485 (6 pages) Online Publication Date: 19 September 2008
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Ion acceleration by ultrashort circularly polarized laser pulse in a solid-density target is investigated using two-dimensional particle-in-cell simulation. The ions are accelerated and compressed by the continuously extending space-charge field created by the evacuation and compression of the target electrons by the laser light pressure. For a sufficiently thin target, the accelerated and compressed ions can reach and exit from the rear surface as a high-density high-energy ion bunch. The peak ion energy depends on the target thickness and reaches maximum when the compressed ion layer can just reach the rear target surface. The compressed ion layer exhibits lateral striation which can be suppressed by using a sharp-rising laser pulse.
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Kinetic and collisional effects on the linear evolution of fast ignition relevant beam instabilities Phys. Plasmas 15, 082108 (2008); http://dx.doi.org/10.1063/1.2953816 (12 pages) Online Publication Date: 14 August 2008
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The fast ignition scheme will involve the generation and transport of a relativistic electron beam, which may be subject to a number of instabilities that act to inhibit energy transport. This study will address the effects of collisions and the initial electron beam distribution on the linear evolution of these instabilities for theoretical distributions including the relativistic waterbag, the relativistic Maxwellian (Jüttner), and the saddle point (low temperature) approximation of the relativistic Maxwellian. It will then be shown that a more physical distribution obtained from a 2D explicit particle-in-cell simulation of the laser-plasma interaction can be best modeled with a Jüttner distribution, but well-approximated with a relativistic waterbag distribution. In sum, for all distributions of interest, collisions were found to have the ability to both suppress and enhance growth for the filamentary instability, while they only suppress growth for the two-stream instability.
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Phys. Plasmas 15, 082703 (2008); http://dx.doi.org/10.1063/1.2967899 (7 pages) Online Publication Date: 20 August 2008
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Direct-drive, Rayleigh–Taylor (RT) growth experiments were performed using planar plastic targets on the OMEGA Laser Facility [
T. R. Boehly et al., Opt. Commun. 133, 495 (1997)
] at laser intensities between ∼ 2×1014 and ∼ 1.5×1015 W/cm2. The primary purpose of the experiments was to test fundamental physics in hydrocodes at the range of drive intensities relevant to ignition designs. The target acceleration was measured with a streak camera using side-on, x-ray radiography, while RT growth was measured with a framing camera using face-on radiography. In a laser-intensity range from 2 to 5×1014 W/cm2, the measured RT growth agrees well with two-dimensional simulations, based on a local model of thermal-electron transport. The RT growth at drive intensities above ∼ 1.0×1015 W/cm2 was strongly stabilized compared to the local model predictions. The experiments demonstrate that standard simulations, based on a local model of electron thermal transport, break down at peak intensities of ignition designs, although they work well at lower intensities. These results also imply that direct-drive ignition targets are significantly more stable than previously calculated using local electron-transport models at peak intensities of ignition designs. The preheating effects by nonlocal electron transport and hot electrons were identified as some of the stabilizing mechanisms.
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On the Debye and transient sheaths in a discharge plasma Phys. Plasmas 15, 093505 (2008); http://dx.doi.org/10.1063/1.2978163 (3 pages) Online Publication Date: 12 September 2008
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Using just the mathematical properties of Poisson’s equation, we obtain a general solution for the spatial variation of the electric potential surrounding an electrode that is inserted in a laboratory discharge plasma. The Debye sheath and the transient or ion matrix sheath in front of an electrode are obtained as special limiting cases.
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Phys. Plasmas 15, 094501 (2008); http://dx.doi.org/10.1063/1.2977486 (4 pages) Online Publication Date: 3 September 2008
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A coupled linear dispersion relation for the basic electrostatic and electromagnetic waves in the ultracold nonuniform magnetized dense plasmas has been obtained which interestingly is analogous to the classical case. The scales of macroscopic phenomena and the interparticle quantum interactions are discussed. It is important to point out that hydrodynamic models cannot take into account strong quantum effects and they are not applicable to very dense plasmas. The analysis is presented with applications to dense plasmas which are relevant to both laboratory and astrophysical environments.
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Efficient multi-keV X-ray sources from laser-exploded metallic thin foils Phys. Plasmas 15, 092702 (2008); http://dx.doi.org/10.1063/1.2973480 (15 pages) Online Publication Date: 17 September 2008
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A set of materials—titanium, copper, and germanium—has been experimented with at the OMEGA laser facility [
Boehly, Opt. Commun. 133, 495 (1997)
] by irradiating thin foils with a prepulse prior to a main pulse with variable delay, in order to design efficient x-ray laser-sources for backlighting, material testing, and code validation. This concept led to increasing factors from 2 to 4 comparing to cases without prepulse, in the experimental conditions. As a result, high multi-keV x-ray conversion rates have been obtained: 9% for titanium around 4 keV, 1% for copper around 8 keV, and 2.5 to 3% for germanium around 10 keV, which places these pre-exploded metallic targets close to the gas with respect to their performance, with wider energy range. A good agreement with hydroradiative code FCI2 [
Schurtz, Phys. Plasmas 7, 4238 (2000)
] calculations is found for titanium and copper on all diagnostics, with nonlocal-thermal-equilibrium atomic physics and, either nonlocal thermal conduction taking self-generated B-fields into account, or limited thermal conduction with intensity-dependent factor f. The results for germanium indicate that dielectronic processes could play a more significant role when higher irradiation intensity on higher Z material.
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Quasi-static magnetic field generation due to a linearly polarized laser beam propagating in plasma Phys. Plasmas 15, 093101 (2008); http://dx.doi.org/10.1063/1.2977491 (4 pages) Online Publication Date: 5 September 2008
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Quasi-static magnetic field generation due to propagation of a linearly polarized laser beam in homogeneous, underdense plasma, is presented. The magnetic field is generated due to the interaction of a narrow laser beam with plasma, for which the ponderomotive force dominates over space charge force between electrons and ions. In addition, for narrow beams, the longitudinal component of the laser field becomes significant and therefore contributes to the generation of the quasi-static magnetic field.
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Collisional energy transfer in two-component plasmas Phys. Plasmas 15, 092107 (2008); http://dx.doi.org/10.1063/1.2977987 (5 pages) Online Publication Date: 11 September 2008
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The friction in plasmas consisting of two species with different temperatures is discussed together with the consequent energy transfer. It is shown that the friction between the two species has no effect on the ion acoustic mode in a quasineutral plasma. Using the Poisson equation instead of the quasineutrality reveals the possibility for an instability driven by the collisional energy transfer. However, the different starting temperatures of the two species imply an evolving background. It is shown that the relaxation time of the background electron-ion plasma is, in fact, always shorter than the growth rate time. Therefore the instability is unlikely to develop. The results obtained here should contribute to the definite clarification of some contradictory results obtained in the past.
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A simple diffusion model showing anomalous scaling Phys. Plasmas 15, 082308 (2008); http://dx.doi.org/10.1063/1.2969429 (7 pages) Online Publication Date: 13 August 2008
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A number of iterated maps and one flow, which show chaotic behavior, have been studied numerically and their time evolution expressed in terms of higher-order moments Mm(t). All the cases show anomalous behavior with Mm(t) ∼ tg(m), with g(m) ≠ αm. A simple analytic treatment is given based on an effective diffusion that is dependent on both space and time. This leads to a form for g(m)/m = a−b/m, which is in good agreement with numerical results. This behavior is attributed to the presence of convective motion superimposed on the background diffusion, and hence this behavior is expected in a wide variety of maps and flows.
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High-resolution 17–75 keV backlighters for high energy density experiments Phys. Plasmas 15, 072705 (2008); http://dx.doi.org/10.1063/1.2957918 (9 pages) Online Publication Date: 23 July 2008
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17–75 keV one- and two-dimensional high-resolution (<10 μm) radiography has been developed using high-intensity short pulse lasers. High energy Kα sources are created by fluorescence from hot electrons interacting in the target material after irradiation by lasers with intensity IL>1017 W/cm2. High-resolution point projection one- and two-dimensional radiography has been achieved using microfoil and microwire targets attached to low-Z substrate materials. The microwire size was 10 μm×10 μm×300 μm on a 300 μm×300 μm×5 μm polystyrene substrate. The radiography experiments were performed using the Titan laser at Lawrence Livermore National Laboratory. The results show that the resolution is dominated by the microwire target size and there is very little degradation from the plasma plume, implying that the high-energy x-ray photons are generated mostly within the microwire volume. There are enough Kα photons created with a 300 J, 1-ω, 40 ps pulse laser from these small volume targets, and that the signal-to-noise ratio is sufficiently high, for single shot radiography experiments. This unique technique will be used on future high energy density experiments at many new high-power laser facilities.
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