bluuebunny/crossref_metadata_2025 huggingface dataset:
Privatization, Marketization, and Neoliberalism—The Political Dynamics of Post-Katrina New Orleans
Authors: Schneider Aaron
Published: August 2012

Hurricane Katrina was a “disaster” both “natural” and “social.” The storm destroyed a major American city that, like most American cities, was already the site of great inequality and vulnerability. It also dramatically put to the test both the logistical capabilities and the political responsibilities of national, state, and local governmental institutions. The Neoliberal Deluge: Hurricane Katrina, Late Capitalism, and the Remaking of New Orleans is an important collection of essays on the dynamics of “remaking New Orleans” and the limits of that effort. We have thus asked a diverse group of political scientists to review the book, and at the same time to treat it as an opportunity to reflect on two related questions: 1) What are the most important economic, cultural, and political dimensions of the crisis precipitated by Katrina, both for New Orleans and for US cities more generally? 2) What resources does political science as a discipline possess to help us understand these issues, and can political science as a discipline do a better job on this score?—Jeffrey C. Issac, Editor
American Cities and American Political Science—a Look at Thick Injustice
Authors: Passavant Paul A.
Published: August 2012

The back cover of Clarissa Rile Hayward and Todd Swanstrom'sJustice and the American Metropolisconcisely lays out a central challenge of contemporary politics: “Today's American cities and suburbs are the sites of ‘thick injustice’—unjust power relations that are deeply and densely concentrated as well as opaque and seemingly intractable. Thick injustice is hard to see, to assign responsibility for, and to change.” The fact that the topic of “urban politics” is not a major theme of political science scholarship both reflects and exacerbates this challenge. And so we have decided to invite a diverse group of social scientists to discuss the book in light of the very big question that it poses: How do American cities look when assessed in terms of their “justice” (or “injustice”), and how might they look if they were assessed in these terms more seriously? In considering this question, discussants have also been asked to consider a related question: How does Americanpolitical sciencelook when assessed in terms of the extent to which it takes the question of urban justice and injustice seriously?—Jeffrey C. Isaac, Editor
American Cities and American Political Science—a Look at Thick Injustice
Authors: Pinderhughes Dianne
Published: August 2012

The back cover of Clarissa Rile Hayward and Todd Swanstrom's Justice and the American Metropolis concisely lays out a central challenge of contemporary politics: “Today's American cities and suburbs are the sites of ‘thick injustice’—unjust power relations that are deeply and densely concentrated as well as opaque and seemingly intractable. Thick injustice is hard to see, to assign responsibility for, and to change.” The fact that the topic of “urban politics” is not a major theme of political science scholarship both reflects and exacerbates this challenge. And so we have decided to invite a diverse group of social scientists to discuss the book in light of the very big question that it poses: How do American cities look when assessed in terms of their “justice” (or “injustice”), and how might they look if they were assessed in these terms more seriously? In considering this question, discussants have also been asked to consider a related question: How does American political science look when assessed in terms of the extent to which it takes the question of urban justice and injustice seriously?—Jeffrey C. Isaac, Editor
American Cities and American Political Science—a Look at Thick Injustice
Authors: Blitz Mark
Published: August 2012

The back cover of Clarissa Rile Hayward and Todd Swanstrom's Justice and the American Metropolis concisely lays out a central challenge of contemporary politics: “Today's American cities and suburbs are the sites of ‘thick injustice’—unjust power relations that are deeply and densely concentrated as well as opaque and seemingly intractable. Thick injustice is hard to see, to assign responsibility for, and to change.” The fact that the topic of “urban politics” is not a major theme of political science scholarship both reflects and exacerbates this challenge. And so we have decided to invite a diverse group of social scientists to discuss the book in light of the very big question that it poses: How do American cities look when assessed in terms of their “justice” (or “injustice”), and how might they look if they were assessed in these terms more seriously? In considering this question, discussants have also been asked to consider a related question: How does American political science look when assessed in terms of the extent to which it takes the question of urban justice and injustice seriously?—Jeffrey C. Isaac, Editor
Race in America
Authors: Davis Heath Fogg
Published: August 2012

Both the plight of African American young people and their feelings and thoughts about this plight are major issues of concern in U.S. politics. In 2003, the Black Youth Project was launched, with funding by the Ford Foundation and the Robert Wood Johnson Foundation, to promote both social scientific analysis and public understanding of these issues (the project has an innovative and engaging Website that can be accessed at http://www.blackyouthproject.com/). Cathy J. Cohen is the principal investigator of the project and, in Democracy Remixed, she draws upon a new national survey of black youth to offer a mixed-method empirical description and theoretical analysis of “black youth and the future of American politics.” In this symposium, a diverse group of political and social scientists have been asked to critically assess the book's account and to comment more broadly on the importance of black youth to the future of American politics.—Jeffrey C. Isaac, Editor
Race in America
Authors: Levine Peter
Published: August 2012

Both the plight of African American young people and their feelings and thoughts about this plight are major issues of concern in U.S. politics. In 2003, the Black Youth Project was launched, with funding by the Ford Foundation and the Robert Wood Johnson Foundation, to promote both social scientific analysis and public understanding of these issues (the project has an innovative and engaging Website that can be accessed at http://www.blackyouthproject.com/). Cathy J. Cohen is the principal investigator of the project and, in Democracy Remixed, she draws upon a new national survey of black youth to offer a mixed-method empirical description and theoretical analysis of “black youth and the future of American politics.” In this symposium, a diverse group of political and social scientists have been asked to critically assess the book's account and to comment more broadly on the importance of black youth to the future of American politics.—Jeffrey C. Isaac, Editor
Jazz and the Boundaries of Race
Authors: Hersch Charles B.
Published: August 2012

What does racial identity mean in twenty-first-century America? Some say we live in a “postracial” world, and increasing numbers of Americans have multiethnic backgrounds. We academics recognize that race is a social construction, yet Americans remain attached to traditional racial categories. In 2008, approximately 15% of all marriages in the United States were interracial, and beginning with the 2000 census, Americans have been allowed to check more than one racial category. Yet 97% of Americans in 2010 reported only one race. We are proud of electing our first “black president” even though his mother was white and he grew up barely knowing his African father.
Does the US Media Have a Liberal Bias?
Authors: McCarty Nolan
Published: August 2012

In Left Turn: How Liberal Bias Distorts The American Mind, Tim Groseclose argues that media effects play a crucial role in American politics. His case rests on three arguments: (1) that journalists tend overwhelmingly to be liberal rather than conservative; (2) that their innate political bias slants their views in empirically measurable ways; and (3) that this bias fundamentally shapes American politics, by bringing US citizens further to the left than they would naturally be. According to Groseclose, in a world where media bias did not exist, American citizens would on average hold views close to those of Ben Stein or Bill O'Reilly. In such a world, John McCain would have defeated Barack Obama by a popular vote margin of 56%—42% in the 2008 presidential election.

In making these claims, Groseclose draws on his own research, and on recent media scholarship by both political scientists and economists, making the broader claim that peer-reviewed social science—which seeks to deal with problems such as endogeneity and selection bias—should be the starting point for public arguments about the role of the media. His book, then, is clearly an effort to bring social scientific arguments into mainstream debates. Groseclose makes no secret of his conservative political leanings—but recent books from left-leaning political scientists such as Jacob Hacker and Paul Pierson are equally unapologetic. It is at least plausible that political scientists' typical unwillingness to engage directly in political arguments has weakened the discipline's capacity for public engagement.

In this symposium a diverse group of contributors have been invited to engage with Groseclose's arguments in ways that bring together specific empirical and/or theoretical points and arguments aimed at the broader “political science public sphere” that Perspectives on Politics seeks to nurture. Contributors were asked to consider these five questions: (1): How do we best measure media effects? (2): If media bias exists, what are its plausible sources? (3): Can one use work on media effects to determine what people's views would be in the absence of such bias? (4): Do you agree that American politics is insufficiently representative, and if so what do you consider the primary sources of this problem? (5): What kinds of political and/or media institutions or practices might enhance democratic discourse?—Henry Farrell, Associate Editor
The Pacific lithosphere‐asthenosphere boundary: Seismic imaging and anisotropic constraints from SS waveforms
Authors: Rychert Catherine A. , Schmerr Nicholas , Harmon Nicholas
Published: August 2012

The lithosphere‐asthenosphere boundary (LAB) separating the rigid lid from the underlying weaker, convecting asthenosphere is a fundamental interface in mantle dynamics and plate tectonics. However, the exact depth and defining mechanism of the LAB interface remain poorly understood. The ocean plates are ideal for testing hypotheses regarding the nature of a plate since they make up 70% of Earth's surface area and have a relatively simple geological history. Seismically imaging the oceanic LAB at high resolution has proved challenging. Yet, several studies have recently increased resolution with provocative results. We summarize recent imaging of discontinuity structure beneath much of the Pacific using receiver functions from ocean floor borehole seismometers and land stations located at ocean‐continent margins, SS precursors, and waveform modeling of multiple phases including multiple bounce S waves, ScS reverberations, and surface waves. Overall, there is much agreement among these different approaches about the reported depth of a negative discontinuity that occurs near the expected depth of the LAB. Some of the apparent discrepancies in depth are explained by the variation in sensitivity of seismic waves that sample structure at different wavelengths. Yet, when the results are considered together, no single age‐depth relationship is illuminated. There are also puzzling discrepancies in where the discontinuity is detected, which again suggests greater complexity. Here we test the possibility that discrepant detection of a strong sharp discontinuity is caused by anisotropic structure. We stack SS waveforms with bounce points in the central Pacific into azimuthal bins. We use two methods, one that inverts for discontinuity structure based on subtle variations in the character of the SS waveform, and another that considers SS at higher frequency. We find azimuthal variation in the amplitude of the waveform, including a polarity reversal. We suggest that anisotropy is an important factor in imaging and constraining discontinuity structure of the oceanic plate, and must be carefully considered to constrain the age‐depth dependence and defining mechanism of the oceanic lithosphere.
Simulation of ionospheric disturbances created by Alfvén waves
Authors: Sydorenko D. , Rankin R.
Published: August 2012

A two‐dimensional numerical model of low‐altitude auroral flux tubes has been developed for simulation of coupling between the magnetosphere and the ionosphere. The model considers a realistic ionosphere with multiple ion and neutral species, collisions, cooling and heating processes, as well as the North‐South electric field due to the global azimuthal plasma convection. In the paper, a detailed description of the model is given. A representative simulation where the ionosphere is perturbed by an Alfvén wave is discussed. The model demonstrates formation of intense parallel electric fields, radiation of Alfvén waves by density perturbations in the convective flow, and electron and ion heating.
Effect of Perceptual Load on Semantic Access by Speech in Children
Authors: Jerger Susan , Damian Markus F. , Mills Candice , Bartlett James , Tye-Murray Nancy , Abdi Hervé
Published: August 2012

Purpose

To examine whether semantic access by speech requires attention in children.

Method

Children (N= 200) named pictures and ignored distractors on a cross-modal (distractors: auditory–no face) or multimodal (distractors: auditory–static face and audiovisual–dynamic face) picture word task. The cross-modal task had a low load, and the multimodal task had a high load (i.e., respectively naming pictures displayed on a blank screen vs. below the talker's face on his T-shirt). Semantic content of distractors was manipulated to be related vs. unrelated to the picture (e.g., picture “dog” with distractors “bear” vs. “cheese”). If irrelevant semantic content manipulation influences naming times on both tasks despite variations in loads, Lavie's (2005) perceptual load model proposes that semantic access is independent of capacity-limited attentional resources; if, however, irrelevant content influences naming only on the cross-modal task (low load), the perceptual load model proposes that semantic access is dependent on attentional resources exhausted by the higher load task.

Results

Irrelevant semantic content affected performance for both tasks in 6- to 9-year-olds but only on the cross-modal task in 4- to 5-year-olds. The addition of visual speech did not influence results on the multimodal task.

Conclusion

Younger and older children differ in dependence on attentional resources for semantic access by speech.
Alternative Tense and Agreement Morpheme Measures for Assessing Grammatical Deficits During the Preschool Period
Authors: Gladfelter Allison , Leonard Laurence B.
Published: August 2012

Purpose

P. A. Hadley and H. Short (2005) developed a set of measures designed to assess the emerging diversity and productivity of tense and agreement (T/A) morpheme use by 2-year-olds. The authors extended 2 of these measures to the preschool years to evaluate their utility in distinguishing children with specific language impairment (SLI) from their typically developing (TD) peers.

Method

Spontaneous speech samples from 55 children (25 with SLI, 30 TD) at 2 different age levels (4;0–4;6 [years;months] and 5;0–5;6) were analyzed, using a traditional T/A morphology composite that assessed accuracy, and the Hadley and Short measures of Tense Marker Total (assessing diversity of T/A morpheme use) and Productivity Score (assessing productivity of major T/A categories).

Results

All 3 measures showed acceptable levels of sensitivity and specificity. In addition, similar differences in levels of productivity across T/A categories were seen in the TD and SLI groups.

Conclusion

The Tense Marker Total and Productivity Score measures seem to have considerable utility for preschool-age children, in that they provide information about specific T/A morphemes and major T/A categories that are not distinguished using the traditional composite measure. The findings are discussed within the framework of the gradual morphosyntactic learning account.
Dynamic modelling and simulation of a polymer electrolyte membrane fuel cell used in vehicle considering heat transfer effects
Authors: Hosseini S. M. , Shamekhi A. H. , Yazdani A.
Published: August 2012

Fuel cell technology is recently becoming one of the most interesting fields for the car companies to invest in. This interest is because of their high efficiency and zero environmental pollution. Polymer electrolyte membrane fuel cells are the most appropriate type of fuel cells for use in vehicles due to their low performance temperature and high power density. Air and fuel mass flow rate and partial pressure, fuel cell stack temperature, relative humidity of fuel cell membrane, and heat and water management are the effective parameters of fuel cell power systems. Good transient behavior is one of the important factors that affect the success of fuel cell vehicles. In order to avoid stack voltage drop during transient condition, the control system of fuel cell vehicle is required to preserve optimal temperature, membrane hydration, and partial pressure of reactants across the membrane. In this paper, we developed a dynamic model for fuel cell power system. The compressor dynamic, supply and return manifold filling dynamics (anode and cathode), cooling system dynamic, membrane hydration, and time-evolving reactant partial pressure are the most significant parameters in transient and steady state of system. The effects of membrane humidity, varying inlet air pressure, and compressor performance condition on the generated power are studied in this paper.
Parallel resistivity and ohmic heating of laboratory dipole plasmas
Authors: Fox W.
Published: August 2012

The parallel resistivity is calculated in the long-mean-free-path regime for the dipole plasma geometry; this is shown to be a neoclassical transport problem in the limit of a small number of circulating electrons. In this regime, the resistivity is substantially higher than the Spitzer resistivity due to the magnetic trapping of a majority of the electrons. This suggests that heating the outer flux surfaces of the plasma with low-frequency parallel electric fields can be substantially more efficient than might be naively estimated. Such a skin-current heating scheme is analyzed by deriving an equation for diffusion of skin currents into the plasma, from which quantities such as the resistive skin-depth, lumped-circuit impedance, and power deposited in the plasma can be estimated. Numerical estimates indicate that this may be a simple and efficient way to couple power into experiments in this geometry.
Designs for highly nonlinear ablative Rayleigh-Taylor experiments on the National Ignition Facility
Authors: Casner A. , Smalyuk V. A. , Masse L. , Igumenshchev I. , Liberatore S. , Jacquet L. , Chicanne C. , Loiseau P. , Poujade O. , Bradley D. K. , Park H. S. , Remington B. A.
Published: August 2012

We present two designs relevant to ablative Rayleigh-Taylor instability in transition from weakly nonlinear to highly nonlinear regimes at the National Ignition Facility [E. I. Moses, J. Phys.: Conf. Ser. 112, 012003 (2008)]. The sensitivity of nonlinear Rayleigh-Taylor instability physics to ablation velocity is addressed with targets driven by indirect drive, with stronger ablative stabilization, and by direct drive, with weaker ablative stabilization. The indirect drive design demonstrates the potential to reach a two-dimensional bubble-merger regime with a 20 ns duration drive at moderate radiation temperature. The direct drive design achieves a 3 to 5 times increased acceleration distance for the sample in comparison to previous experiments allowing at least 2 more bubble generations when starting from a three-dimensional broadband spectrum.
A protection system for the JET ITER-like wall based on imaging diagnostics
Authors: Arnoux G. , Devaux S. , Alves D. , Balboa I. , Balorin C. , Balshaw N. , Beldishevski M. , Carvalho P. , Clever M. , Cramp S. , de Pablos J.-L. , de la Cal E. , Falie D. , Garcia-Sanchez P. , Felton R. , Gervaise V. , Goodyear A. , Horton A. , Jachmich S. , Huber A. , Jouve M. , Kinna D. , Kruezi U. , Manzanares A. , Martin V. , McCullen P. , Moncada V. , Obrejan K. , Patel K. , Lomas P. J. , Neto A. , Rimini F. , Ruset C. , Schweer B. , Sergienko G. , Sieglin B. , Soleto A. , Stamp M. , Stephen A. , Thomas P. D. , Valcárcel D. F. , Williams J. , Wilson J. , Zastrow K.-D. ,   JET-EFDA Contributors
Published: August 2012

The new JET ITER-like wall (made of beryllium and tungsten) is more fragile than the former carbon fiber composite wall and requires active protection to prevent excessive heat loads on the plasma facing components (PFC). Analog CCD cameras operating in the near infrared wavelength are used to measure surface temperature of the PFCs. Region of interest (ROI) analysis is performed in real time and the maximum temperature measured in each ROI is sent to the vessel thermal map. The protection of the ITER-like wall system started in October 2011 and has already successfully led to a safe landing of the plasma when hot spots were observed on the Be main chamber PFCs. Divertor protection is more of a challenge due to dust deposits that often generate false hot spots. In this contribution we describe the camera, data capture and real time processing systems. We discuss the calibration strategy for the temperature measurements with cross validation with thermal IR cameras and bi-color pyrometers. Most importantly, we demonstrate that a protection system based on CCD cameras can work and show examples of hot spot detections that stop the plasma pulse. The limits of such a design and the associated constraints on the operations are also presented.
Energy extrapolation schemes for adaptive multi-scale molecular dynamics simulations
Authors: Fleurat-Lessard Paul , Michel Carine , Bulo Rosa E.
Published: August 2012

This paper evaluates simple schemes to extrapolate potential energy values using the set of energies and forces extracted from a molecular dynamics trajectory. In general, such a scheme affords the maximum amount of information about a molecular system at minimal computational cost. More specifically, schemes like this are very important in the field of adaptive multi-scale molecular dynamics simulations. In this field, often the computation of potential energy values at certain trajectory points is not required for the simulation itself, but solely for the a posteriori analysis of the simulation data. Extrapolating the values at these points from the available data can save considerable computational time. A set of extrapolation schemes are employed based on Taylor series and central finite difference approximations. The schemes are first tested on the trajectories of molecular systems of varying sizes, obtained at MM and QM level using velocity-Verlet integration with standard simulation time steps. Remarkably good accuracy was obtained with some of the approximations, while the failure of others can be explained in terms of the distinct features of a molecular dynamics trajectory. We have found that, for a Taylor expansion of the potential energy, both a first and a second order truncation exhibit errors that grow with system size. In contrast, the second order central finite difference approximation displays an accuracy that is independent of the size of the system, while giving a very good estimate of the energy, and costing as little as a first order truncation of the Taylor series. A fourth order central finite difference approximation requires more input data, which is not always available in adaptive multi-scale simulations. Furthermore, this approximation gives errors of similar magnitude or larger than its second order counterpart, at standard simulation time steps. This leads to the conclusion that a second order central finite difference approximation is the optimal choice for energy extrapolation from molecular dynamics trajectories. This finding is confirmed in a final application to the analysis of an adaptive multi-scale simulation.
A soft x-ray transmission grating imaging-spectrometer for the National Ignition Facility
Authors: Moore A. S. , Guymer T. M. , Kline J. L. , Morton J. , Taccetti M. , Lanier N. E. , Bentley C. , Workman J. , Peterson B. , Mussack K. , Cowan J. , Prasad R. , Richardson M. , Burns S. , Kalantar D. H. , Benedetti L. R. , Bell P. , Bradley D. , Hsing W. , Stevenson M.
Published: August 2012

A soft x-ray transmission grating spectrometer has been designed for use on high energy-density physics experiments at the National Ignition Facility (NIF); coupled to one of the NIF gated x-ray detectors it records 16 time-gated spectra between 250 and 1000 eV with 100 ps temporal resolution. The trade-off between spectral and spatial resolution leads to an optimized design for measurement of emission around the peak of a 100–300 eV blackbody spectrum. Performance qualification results from the NIF, the Trident Laser Facility and vacuum ultraviolet beamline at the National Synchrotron Light Source, evidence a <100 μm spatial resolution in combination with a source-size limited spectral resolution that is <10 eV at photon energies of 300 eV.
Isochoric heating of foamed metal using pulsed power discharge as a making technique of warm dense matter
Authors: Amano Yusuke , Miki Yasutoshi , Takahashi Takuya , Sasaki Toru , Kikuchi Takashi , Harada Nob
Published: August 2012

To generate well-defined warm dense state for evaluating electrical conductivity by using pulsed-power discharge, we have proposed an isochoric heating of foamed metal. Isochoric heating can be achieved by surrounding the foamed metal with a rigid-walled sapphire capillary. We evaluate the temperature and electrical conductivity of the foam/plasma based on the line-pair method of the foam/plasma emission and on the voltage-current waveforms. The electrical conductivity observed agrees with previous experiments and predictions. Thus, the proposed technique yields the electrical conductivity of warm dense matter with a well-defined temperature.
Layout and results from the initial operation of the high-resolution x-ray imaging crystal spectrometer on the Large Helical Device
Authors: Pablant N. A. , Bitter M. , Delgado-Aparicio L. , Goto M. , Hill K. W. , Lazerson S. , Morita S. , Roquemore A. L. , Gates D. , Monticello D. , Nielson H. , Reiman A. , Reinke M. , Rice J. E. , Yamada H.
Published: August 2012

First results of ion and electron temperature profile measurements from the x-ray imaging crystal spectrometer (XICS) diagnostic on the Large Helical Device (LHD) are presented. This diagnostic system has been operational since the beginning of the 2011 LHD experimental campaign and is the first application of the XICS diagnostic technique to helical plasma geometry. The XICS diagnostic provides measurements of ion and electron temperature profiles in LHD with a spatial resolution of 2 cm and a maximum time resolution of 5 ms (typically 20 ms). Ion temperature profiles from the XICS diagnostic are possible under conditions where charge exchange recombination spectroscopy (CXRS) is not possible (high density) or is perturbative to the plasma (low density or radio frequency heated plasmas). Measurements are made by using a spherically bent crystal to provide a spectrally resolved 1D image of the plasma from line integrated emission of helium-like Ar16 +. The final hardware design and configuration are detailed along with the calibration procedures. Line-integrated ion and electron temperature measurements are presented, and the measurement accuracy is discussed. Finally central temperature measurements from the XICS system are compared to measurements from the Thomson scattering and CXRS systems, showing excellent agreement.
Inertial particle acceleration statistics in turbulence: Effects of filtering, biased sampling, and flow topology
Authors: Salazar Juan P. L. C. , Collins Lance R.
Published: August 2012

In this study, we investigate the effect of “biased sampling,” i.e., the clustering of inertial particles in regions of the flow with low vorticity, and “filtering,” i.e., the tendency of inertial particles to attenuate the fluid velocity fluctuations, on the probability density function of inertial particle accelerations. In particular, we find that the concept of “biased filtering” introduced by Ayyalasomayajula et al. [“Modeling inertial particle acceleration statistics in isotropic turbulence,” Phys. Fluids 20, 0945104 (2008)10.1063/1.2976174], in which particles filter stronger acceleration events more than weaker ones, is relevant to the higher order moments of acceleration. Flow topology and its connection to acceleration is explored through invariants of the velocity-gradient, strain-rate, and rotation-rate tensors. A semi-quantitative analysis is performed where we assess the contribution of specific flow topologies to acceleration moments. Our findings show that the contributions of regions of high vorticity and low strain decrease significantly with Stokes number, a non-dimensional measure of particle inertia. The contribution from regions of low vorticity and high strain exhibits a peak at a Stokes number of approximately 0.2. Following the methodology of Ooi et al. [“A study of the evolution and characteristics of the invariants of the velocity-gradient tensor in isotropic turbulence,” J. Fluid Mech. 381, 141 (1999)10.1017/S0022112098003681], we compute mean conditional trajectories in planes formed by pairs of tensor invariants in time. Among the interesting findings is the existence of a stable focus in the plane formed by the second invariants of the strain-rate and rotation-rate tensors. Contradicting the results of Ooi et al., we find a stable focus in the plane formed by the second and third invariants of the strain-rate tensor for fluid tracers. We confirm, at an even higher Reynolds number, the conjecture of Collins and Keswani [“Reynolds number scaling of particle clustering in turbulent aerosols,” New J. Phys. 6, 119 (2004)10.1088/1367-2630/6/1/119] that inertial particle clustering saturates at large Reynolds numbers. The result is supported by the theory presented in Chun et al. [“Clustering of aerosol particles in isotropic turbulence,” J. Fluid Mech. 536, 219 (2005)10.1017/S0022112005004568].
Thermodynamics and kinetics of bubble nucleation: Simulation methodology
Authors: Meadley Stacey L. , Escobedo Fernando A.
Published: August 2012

The simulation of homogeneous liquid to vapor nucleation is investigated using three rare-event algorithms, boxed molecular dynamics, hybrid umbrella sampling Monte Carlo, and forward flux sampling. Using novel implementations of these methods for efficient use in the isothermal-isobaric ensemble, the free energy barrier to nucleation and the kinetic rate are obtained for a Lennard-Jones fluid at stretched and at superheated conditions. From the free energy surface mapped as a function of two order parameters, the global density and largest bubble volume, we find that the free energy barrier height is larger when projected over bubble volume. Using a regression analysis of forward flux sampling results, we show that bubble volume is a more ideal reaction coordinate than global density to quantify the progression of the metastable liquid toward the stable vapor phase and the intervening free energy barrier. Contrary to the assumptions of theoretical approaches, we find that the bubble takes on cohesive non-spherical shapes with irregular and (sometimes highly) undulating surfaces. Overall, the resulting free energy barriers and rates agree well between the methods, providing a set of complementary algorithms useful for studies of different types of nucleation events.
The interaction of quasi-monoenergetic protons with pre-compressed inertial fusion fuels
Authors: Mahdavi M. , Koohrokhi T. , Azadifar R.
Published: August 2012

The interaction of a quasi-monoenergetic proton beam with a pre-compressed plasma is studied in the context of inertial fusion fast ignition (FI). Based on fundamental principles, a kinetic model is developed by considering hard collisions, nuclear scattering, and the contribution due to collective processes. The penetration depth, longitudinal straggling, and the transverse blooming are evaluated by solving the Boltzmann transport equation using the multiple scattering theory. The stopping power, transport scattering cross sections, and convenient expressions for the angular moments of the proton distribution function have been used in modeling the collisional proton transport in a three-dimensional (3D) Monte Carlo code. The transport of a proton beam with a quasi-monoenergetic energy ⟨E⟩=10 MeV is studied for pre-compressed deuterium-tritium plasma with an average density of ρ=400 g cm−3 and temperatures T=1 keV, 5 keV, and 10 keV. The net effects of multiple scattering are to reduce the penetration from 1.028 to 0.828 g cm−2 with range straggling ρΣR=0.044 g cm−2 and beam blooming ρΣB=0.272 g cm−2, for 10 MeV protons in a ρ=400 g cm−3 plasma at T = 5 keV. This model can be used for quantitatively assessing ignition requirements for proton fast ignition.
Potential of a plasma bound between two biased walls
Authors: Loizu J. , Dominski J. , Ricci P. , Theiler C.
Published: August 2012

An analytical study is presented for an one-dimensional, steady-state plasma bound between two perfectly absorbing walls that are biased with respect to each other. Starting from a description of the plasma sheaths formed at both walls, an expression relating the bulk plasma potential to the wall currents is derived, showing that the plasma potential undergoes an abrupt transition when currents cross a critical value. This result is confirmed by numerical simulations performed with a particle-in-cell code.
Paraxial Wentzel–Kramers–Brillouin method applied to the lower hybrid wave propagation
Authors: Bertelli N. , Maj O. , Poli E. , Harvey R. , Wright J. C. , Bonoli P. T. , Phillips C. K. , Smirnov A. P. , Valeo E. , Wilson J. R.
Published: August 2012

The paraxial Wentzel–Kramers–Brillouin (pWKB) approximation, also called beam tracing method, has been employed in order to study the propagation of lower hybrid waves in a tokamak plasma. Analogous to the well-know ray tracing method, this approach reduces Maxwell’s equations to a set of ordinary differential equations, while, in addition, retains the effects of the finite beam cross-section, and, thus, the effects of diffraction. A new code, LHBEAM (lower hybrid BEAM tracing), is presented, which solves the pWKB equations in tokamak geometry for arbitrary launching conditions and for analytic and experimental plasma equilibria. In addition, LHBEAM includes linear electron Landau damping for the evaluation of the absorbed power density and the reconstruction of the wave electric field in both the physical and Fourier space. Illustrative LHBEAM calculations are presented along with a comparison with the ray tracing code GENRAY and the full wave solver TORIC-LH.
Stability of the Landau state in square two-dimensional magnetic nanorings
Authors: Mamica S. , Lévy J.-C. S. , Krawczyk M. , Depondt Ph.
Published: August 2012

We use a microscopic theory taking into account dipolar and nearest-neighbour exchange interactions to explore spin-wave excitations in two-dimensional square-shaped magnetic nanorings with the Landau state assumed as a magnetic state. From the spin-wave spectra, we determine the range of the dipolar-to-exchange interaction ratio in which the assumed state is stable. Various types of localized spin waves prove responsible for the transition to a new magnetic configuration. We found the transition forced by predominating exchange interactions size-independent in a wide range of both external and internal size of the ring.
Collisionless inter-species energy transfer and turbulent heating in drift wave turbulence
Authors: Zhao L. , Diamond P. H.
Published: August 2012

We reconsider the classic problems of calculating “turbulent heating” and collisionless inter-species transfer of energy in drift wave turbulence. These issues are of interest for low collisionality, electron heated plasmas, such as ITER, where collisionless energy transfer from electrons to ions is likely to be significant. From the wave Poynting theorem at steady state, a volume integral over an annulus r1<r<r2, gives the net heating as ∫r1r2dr〈Ẽ·J̃〉=−Sr|r1r2≠0. Here Sr is the wave energy density flux in the radial direction. Thus, a wave energy flux differential across an annular region indeed gives rise to a net heating, in contrast to previous predictions. This heating is related to the Reynolds work by the zonal flow, since Sr is directly linked to the zonal flow drive. In addition to net heating, there is inter-species heat transfer. For collisionless electron drift waves, the total turbulent energy source for collisionless heat transfer is due to quasilinear electron cooling. Subsequent quasilinear ion heating occurs through linear ion Landau damping. In addition, perpendicular heating via ion polarization currents contributes to ion heating. Since at steady state, Reynolds work of the turbulence on the zonal flow must balance zonal flow frictional damping (∼νii〈Vθ〉2∼|eϕ̃T|4), it is no surprise that zonal flow friction appears as an important channel for ion heating. This process of energy transfer via zonal flow has not previously been accounted for in analyses of energy transfer. As an application, we compare the rate of turbulent energy transfer in a low collisionality plasma with the rate of the energy transfer by collisions. The result shows that the collisionless turbulent energy transfer is a significant energy coupling process for ITER plasma.
Twisted dust acoustic waves in dusty plasmas
Authors: Shukla P. K.
Published: August 2012

We examine linear dust acoustic waves (DAWs) in a dusty plasma with strongly correlated dust grains, and discuss possibility of a twisted DA vortex beam carrying orbital angular momentum (OAM). For our purposes, we use the Boltzmann distributed electron and ion density perturbations, the dust continuity and generalized viscoelastic dust momentum equations, and Poisson’s equation to obtain a dispersion relation for the modified DAWs. The effects of the polarization force, strong dust couplings, and dust charge fluctuations on the DAW spectrum are examined. Furthermore, we demonstrate that the DAW can propagate as a twisted vortex beam carrying OAM. A twisted DA vortex structure can trap and transport dust particles in dusty plasmas.
Indirect optical absorption in silicon via thin-film surface plasmon
Authors: Trolle Mads L. , Pedersen Thomas G.
Published: August 2012

Optical excitations in semiconductors have been extensively studied in the dipole limit, neglecting the spatial variation of the exciting field. This is a very good approximation in most cases. However, when approaching the regime of nano optics and plasmonics, optical fields of extreme spatial confinement is found, e.g., surface plasmon polaritons (SPPs). Here we investigate the optical absorption of a SPP bound to an Ag thin-film embedded in Si at photon energies below the direct band gap of Si. In this regime, all absorbtion processes in Si must be assisted by a source of crystal momentum to overcome the momentum mismatch of the indirect transition. While phonon-assisted processes are the obvious example, the optical wave vector can also supply the needed momentum, provided the optical field displays sufficient spatial variation. We apply a simple model for the electric field and a band theoretical model response function for Si, including both frequency and wave vector dependence, and find that absorption in the Si medium relative to the Ag film may be increased significantly due to indirect transitions facilitated by the optical wave vector.
The influence of the Franz-Keldysh effect on the electron diffusion length in p-type GaN determined using the spectral photocurrent technique
Authors: Wee D. , Parish G. , Nener B. D.
Published: August 2012

The zero-bias spectral response of a p-type GaN Schottky diode with an acceptor doping density of 1019 cm−3 was measured, and the electron diffusion length was calculated using the standard theoretical equation that relates this parameter to the measured photocurrent. Here it is shown that the minor but perceptible spectral variation of the diffusion length calculated can be explained by taking into account the Franz-Keldysh effect. Consideration of this effect, through the use of an equation that describes the electric field and spectral dependence of the absorption coefficient, leads to a constant diffusion length value of 14 nm. By also taking into account the inherent inaccuracy of the theoretical equation, a final value of 10 nm was estimated for the electron diffusion length.
Advanced experimental applications for x-ray transmission gratings spectroscopy using a novel grating fabrication method
Authors: Hurvitz G. , Ehrlich Y. , Strum G. , Shpilman Z. , Levy I. , Fraenkel M.
Published: August 2012

A novel fabrication method for soft x-ray transmission grating and other optical elements is presented. The method uses focused-ion-beam technology to fabricate high-quality free standing grating bars on transmission electron microscopy grids. High quality transmission gratings are obtained with superb accuracy and versatility. Using these gratings and back-illuminated CCD camera, absolutely calibrated x-ray spectra can be acquired for soft x-ray source diagnostics in the 100−3000 eV spectral range. Double grating combinations of identical or different parameters are easily fabricated, allowing advanced one-shot application of transmission grating spectroscopy. These applications include spectroscopy with different spectral resolutions, bandwidths, dynamic ranges, and may serve for identification of high-order contribution, and spectral calibrations of various x-ray optical elements.
Relation between charge carrier density and lifetime in polymer-fullerene solar cells
Authors: Thakur Anil K. , Baboz Henri , Wantz Guillaume , Hodgkiss Justin , Hirsch Lionel
Published: August 2012

The recombination kinetics of photo-generated charge carriers in polymer-fullerene solar cells has been studied. The transition between bimolecular to monomolecular (trap-assisted) recombination has been investigated by transient open-circuit voltage decay and intensity dependence of open-circuit voltage at different temperatures to quantify the trap delimitation energy. It has been observed that the critical parameter underpinning the transition between bimolecular to monomolecular recombination is the density of photo-generated charge carriers and their lifetime. The correspondence between carrier lifetime and density of photo-carriers has been experimentally observed for the first time.
Fluid elasticity can enable propulsion at low Reynolds number
Authors: Keim Nathan C. , Garcia Mike , Arratia Paulo E.
Published: August 2012

Conventionally, a microscopic particle that performs a reciprocal stroke cannot move through its environment. This is because at small scales, the response of simple Newtonian fluids is purely viscous and flows are time-reversible. We show that by contrast, fluid elasticity enables propulsion by reciprocal forcing that is otherwise impossible. We present experiments on rigid objects actuated reciprocally in viscous fluids, demonstrating for the first time a purely elastic propulsion set by the object's shape and boundary conditions. We describe two different artificial “swimmers” that experimentally realize this principle.
Existence and stability of standing waves for nonlinear fractional Schrödinger equations
Authors: Guo Boling , Huang Daiwen
Published: August 2012

In this paper, we consider the nonlinear fractional Schrödinger equations. By studying constrained minimization problems and applying the method of concentration-compactness and commutator estimates, we obtain the existence of standing waves for nonlinear fractional Schrödinger equations under some assumptions. Moreover, we prove that the set of minimizers is a stable set for the initial value problem of the equations, in the sense that a solution which starts near the set will remain near it for all time.
Efficient proton beam generation from a foam-carbon foil target using an intense circularly polarized laser
Authors: Gao Lingyuan , Wang Hongyong , Lin Chen , Zou Yubin , Yan Xueqing
Published: August 2012

Energetic proton acceleration from interaction of intense circularly polarized laser pulse with a foam-carbon foil target is investigated using two-dimensional particle-in-cell simulation. It is found that protons from this target are accelerated to much higher energy in comparison with double-layer targets and simple flat targets. This result can be attributed to an enhancement of energy conversion efficiency from laser to electrons inside foam region. When a large number of energetic electrons are generated from the nearcritical plasma, they transport through the foil and form a strong backside sheath field which accelerates protons more efficiently.
Parallel transport of long mean-free-path plasmas along open magnetic field lines: Plasma profile variation
Authors: Guo Zehua , Tang Xian-Zhu
Published: August 2012

Parallel transport of long mean-free-path plasma along an open magnetic field line is characterized by strong temperature anisotropy, which is driven by two effects. The first is magnetic moment conservation in a non-uniform magnetic field, which can transfer energy between parallel and perpendicular degrees of freedom. The second is decompressional cooling of the parallel temperature due to parallel flow acceleration by conventional presheath electric field which is associated with the sheath condition near the wall surface where the open magnetic field line intercepts the discharge chamber. To the leading order in gyroradius to system gradient length scale expansion, the parallel transport can be understood via the Chew-Goldbeger-Low (CGL) model which retains two components of the parallel heat flux, i.e., qn associated with the parallel thermal energy and qs related to perpendicular thermal energy. It is shown that in addition to the effect of magnetic field strength (B) modulation, the two components (qn and qs) of the parallel heat flux play decisive roles in the parallel variation of the plasma profile, which includes the plasma density (n), parallel flow (u), parallel and perpendicular temperatures (T∥ and T⊥), and the ambipolar potential (ϕ). Both their profile (qn/B and qs/B2) and the upstream values of the ratio of the conductive and convective thermal flux (qn/nuT∥ and qs/nuT⊥) provide the controlling physics, in addition to B modulation. The physics described by the CGL model are contrasted with those of the double-adiabatic laws and further elucidated by comparison with the first-principles kinetic simulation for a specific but representative flux expander case.
Zero-bias anomaly in one-dimensional ultrathin metallic nanowires
Authors: Yan Hui , Xu Rui , Hong Xun , Sun Yi , Feng Lei , Nie Jia-Cai , He Lin
Published: August 2012

The electronic properties of ultrathin bimetallic Au/Ag nanowires with 2-3 nm in diameter were explored by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) at liquid-nitrogen temperature. A pronounced dip around zero-bias, followed by a series of discrete single electron tunneling peaks at both negative and positive bias, is observed in the tunneling spectra. Several possible mechanisms, such as exchange interaction and Luttinger-liquid behavior, for the presence of the dip around zero-bias in the Au/Ag nanowires are excluded. Our experimental results and analysis indicate that the disorder of the bimetallic nanowire as the origin of the dip around zero-bias.