bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement of areal density in the ablators of inertial-confinement-fusion capsules via detection of ablator (n, n′γ) gamma-ray emission
Authors: Hoffman N. M. , Herrmann H. W. , Kim Y. H. , Hsu H. H. , Horsfield C. J. , Rubery M. S. , Miller E. K. , Grafil E. , Stoeffl W. , Church J. A. , Young C. S. , Mack J. M. , Wilson D. C. , Langenbrunner J. R. , Evans S. C. , Sedillo T. J. , Glebov V. Yu. , Duffy T.
Published: April 2013

We report the first gamma-ray-based measurements of the areal density of ablators in inertial-confinement-fusion capsule implosions. The measurements, made at the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)], used observations of gamma rays arising from inelastic scattering of 14.1-MeV deuterium-tritium (DT) neutrons on 12C nuclei in the compressed plastic ablators. The emission of 12C(n,n′γ) gamma rays from the capsules is detected using the Gamma Reaction History instrument [H. W. Herrmann et al., J. Phys.: Conf. Ser. 244, 032047 (2010)] operating at OMEGA. From the ratio of a capsule's 12C(n,n′γ) emission to the emission from the same processes in an in situ reference graphite “puck” of known mass and geometry [N. M. Hoffman et al., in IFSA 2011 proceedings (submitted)], we determine the time-averaged areal density of 12C in the capsule's compressed ablator. Measured values of total ablator areal density for thirteen imploded capsules, in the range 23 ± 10 to 58 ± 14 mg/cm2, are comparable to values calculated in 1D radiation-hydrodynamic simulations, and measured by charged-particle techniques.
Comment on “Species separation in inertial confinement fusion fuels” [Phys. Plasmas 20, 012701 (2013)]
Authors: Larroche O.
Published: April 2013

A recent paper presents numerical simulations of shock waves in a two-ion-component plasma, investigating how species separation occurring in the latter can affect the nuclear fusion yield of inertial confinement fusion targets. Here, it is shown that an important physical mechanism has obviously been omitted in those calculations, which thus lead to significantly overestimated results.
Response to “Comment on ‘Species separation in inertial confinement fusion fuels’” [Phys. Plasmas 20, 044701 (2013)]
Authors: Bellei C. , Amendt P. A. , Wilks S. C. , Casey D. T. , Li C. K. , Petrasso R. , Welch D. R.
Published: April 2013

The claims made in the preceding Comment are categorically refuted. Further evidence to support the conclusions of our original paper is herein provided.
Improving the homogeneity of alternating current-drive atmospheric pressure dielectric barrier discharges in helium with an additional low-amplitude radio frequency power source: A numerical study
Authors: Wang Qi , Sun Jizhong , Zhang Jianhong , Liu Liying , Wang Dezhen
Published: April 2013

It was proposed in this paper that the homogeneity of the atmospheric pressure discharge driven by an ac power source could be improved by applying an auxiliary low-amplitude rf power source. To verify the idea, a two-dimensional fluid model then was applied to study the atmospheric discharges in helium driven by ac power, low-amplitude rf power, and combined ac and low–amplitude rf power, respectively. Simulation results confirmed that an auxiliary rf power could improve the homogeneity of a discharge driven by an ac power source. It was further found that there existed a threshold voltage of the rf power source leading to the transition from inhomogeneous to homogeneous discharge. As the frequency of the rf power source increased from 2 to 22 MHz, the magnitude of the threshold voltage dropped first rapidly and then to a constant value. When the frequency was over 13.56 MHz, the magnitude of the threshold voltage was smaller than one-sixth of the ac voltage amplitude under the simulated discharge parameters.
A pendulum in a flowing soap film
Authors: Bandi M. M. , Concha A. , Wood R. , Mahadevan L.
Published: April 2013

We consider the dynamics of a pendulum made of a rigid ring attached to an elastic filament immersed in a flowing soap film. The system shows an oscillatory instability whose onset is a function of the flow speed, length of the supporting string, the ring mass, and ring radius. We characterize this system and show that there are different regimes where the frequency is dependent or independent of the pendulum length depending on the relative magnitude of the added-mass. Although the system is an infinite-dimensional, we can explain many of our results in terms of a one degree-of-freedom system corresponding to a forced pendulum. Indeed, using the vorticity measured via particle imaging velocimetry allows us to make the model quantitative, and a comparison with our experimental results shows we can capture the basic phenomenology of this system.
Effect of strongly coupled plasma on the magnetic dipolar and quadrupolar transitions of two-electron ions
Authors: Saha Jayanta K. , Mukherjee T. K. , Mukherjee P. K. , Fricke B.
Published: April 2013

Effect of strongly coupled plasma on the excitation energies and transition probabilities for the respective transitions 1s2:1Se → 1sns:3Se (n = 2, 3, 4) and 1s2:1Se → 1snp:3Po (n = 2, 3, 4) allowed by magnetic dipolar and quadrupolar excitations have been analyzed for the first time for the two-electron ions C4+, O6+, Ne8+, Mg10+, Si12+, and S14+. Time dependent Hatree-Fock theory within variational approach has been adopted for such a study. The effect of surrounding plasma has been treated through the standard Ion-Sphere (IS) model of the plasma where the plasma density is varied systematically from a low value to a pretty high value such that the respective excited states go over to continuum due to such a confinement. The effect of external pressure generated due to plasma confinement on the estimated spectral properties has been analyzed systematically.
Closure and transport theory for high-collisionality electron-ion plasmas
Authors: Ji Jeong-Young , Held Eric D.
Published: April 2013

Systems of algebraic equations for a high-collisionality electron-ion plasma are constructed from the general moment equations with linearized collision operators [J.-Y. Ji and E. D. Held, Phys. Plasmas 13, 102103 (2006) and J.-Y. Ji and E. D. Held, Phys. Plasmas 15, 102101 (2008)]. A systematic geometric method is invented and applied to solve the system of equations to find closure and transport relations. It is known that some closure coefficients of Braginskii [S. I. Braginskii, Reviews of Plasma Physics (Consultants Bureau, New York, 1965), Vol. 1] are in error up to 65% for some finite values of x (cyclotron frequency × electron-ion collision time) and have significant error in the large-x limit [E. M. Epperlein and M. G. Haines, Phys. Fluids 29, 1029 (1986)]. In this work, fitting formulas for electron coefficients are obtained from the 160 moment (Laguerre polynomial) solution, which converges with increasing moments for x≤100 and from the asymptotic solution for large x-values. The new fitting formulas are practically exact (less than 1% error) for arbitrary x and Z (the ion charge number, checked up to Z = 100). The ion coefficients for equal electron and ion temperatures are moderately modified by including the ion-electron collision operator. When the ion temperature is higher than the electron temperature, the ion-electron collision and the temperature change terms in the moment equations must be kept. The ion coefficient formulas from 3 moment (Laguerre polynomial) calculations, precise to less than 0.4% error from the convergent values, are explicitly written.
Stability of compressible reduced magnetohydrodynamic equilibria—Analogy with magnetorotational instability
Authors: Morrison P. J. , Tassi E. , Tronko N.
Published: April 2013

Stability analyses for equilibria of the compressible reduced magnetohydrodynamics (CRMHD) model are carried out by means of the Energy-Casimir (EC) method. Stability results are compared with those obtained for ideal magnetohydrodynamics (MHD) from the classical δW criterion. An identification of the terms in the second variation of the free energy functional for CRMHD with those of δW is made: two destabilizing effects present for CRMHD turn out to correspond to the kink and interchange instabilities in usual MHD, while the stabilizing roles of field line bending and compressibility are also identified in the reduced model. Also, using the EC method, stability conditions in the presence of toroidal flow are obtained. A formal analogy between CRMHD and a reduced incompressible model for magnetized rotating disks, due to Julien and Knobloch [EAS Pub. Series, 21, 81 (2006)], is discovered. In light of this analogy, energy stability analysis shows that the condition for magnetorotational instability (MRI) for the latter model corresponds to the condition for interchange instability in CRMHD, with the Coriolis term and shear velocity playing the roles of the curvature term and pressure gradient, respectively. Using the EC method, stability conditions for the rotating disk model, for a large class of equilibria with possible non-uniform magnetic fields, are obtained. In particular, this shows it is possible for the MRI system to undergo, in addition to the MRI, another instability that is analogous to the kink instability. For vanishing magnetic field, the Rayleigh hydrodynamical stability condition is recovered.
An improved model to determine the inception of positive upward leader–streamer system considering the leader propagation during dark period
Authors: Xie Shijun , He Junjia , Chen Weijiang
Published: April 2013

Stem–leader transition and front-streamer inception are two essential conditions for the inception of positive upward leader–streamer system (LSS). Previous models have not considered the initial-leader propagation during dark period and have not been verified systematically. In this paper, a series of positive upward discharge simulation experiments was designed and carried out. Characteristic parameters of the discharge process related to the inception of positive upward LSS, namely, the first-corona inception voltage, the first-corona charge, the dark period, and the LSS inception voltage, were obtained. By comparing these experiment results with simulation results calculated using previous models, it was found that it is improper to assume that the length of the initial leader is a fixed value. Finally, an improved inception model of positive upward LSS considering the leader propagation during dark period was developed and verified with experiment results.
Stability of phases of a square-well fluid within superposition approximation
Authors: Piasecki Jarosław , Szymczak Piotr , Kozak John J.
Published: April 2013

The analytic and numerical methods introduced previously to study the phase behavior of hard sphere fluids starting from the Yvon-Born-Green (YBG) equation under the Kirkwood superposition approximation (KSA) are adapted to the square-well fluid. We are able to show conclusively that the YBG equation under the KSA closure when applied to the square-well fluid: (i) predicts the existence of an absolute stability limit corresponding to freezing where undamped oscillations appear in the long-distance behavior of correlations, (ii) in accordance with earlier studies reveals the existence of a liquid-vapor transition by the appearance of a “near-critical region” where monotonically decaying correlations acquire very long range, although the system never loses stability.
Parametric study of non-relativistic electrostatic shocks and the structure of their transition layer
Authors: Dieckmann M. E. , Ahmed H. , Sarri G. , Doria D. , Kourakis I. , Romagnani L. , Pohl M. , Borghesi M.
Published: April 2013

Nonrelativistic electrostatic unmagnetized shocks are frequently observed in laboratory plasmas and they are likely to exist in astrophysical plasmas. Their maximum speed, expressed in units of the ion acoustic speed far upstream of the shock, depends only on the electron-to-ion temperature ratio if binary collisions are absent. The formation and evolution of such shocks is examined here for a wide range of shock speeds with particle-in-cell simulations. The initial temperatures of the electrons and the 400 times heavier ions are equal. Shocks form on electron time scales at Mach numbers between 1.7 and 2.2. Shocks with Mach numbers up to 2.5 form after tens of inverse ion plasma frequencies. The density of the shock-reflected ion beam increases and the number of ions crossing the shock thus decreases with an increasing Mach number, causing a slower expansion of the downstream region in its rest frame. The interval occupied by this ion beam is on a positive potential relative to the far upstream. This potential pre-heats the electrons ahead of the shock even in the absence of beam instabilities and decouples the electron temperature in the foreshock ahead of the shock from the one in the far upstream plasma. The effective Mach number of the shock is reduced by this electron heating. This effect can potentially stabilize nonrelativistic electrostatic shocks moving as fast as supernova remnant shocks.
Visible imaging and spectroscopy of disruption runaway electrons in DIII-D
Authors: Yu J. H. , Hollmann E. M. , Commaux N. , Eidietis N. W. , Humphreys D. A. , James A. N. , Jernigan T. C. , Moyer R. A.
Published: April 2013

The first visible light images of synchrotron emission from disruption runaway electrons are presented. The forward-detected continuum radiation from runaways is identified as synchrotron emission by comparing two survey spectrometers and two visible fast cameras viewing in opposite toroidal directions. Analysis of the elongation of 2D synchrotron images of oval-shaped runaway beams indicates that the velocity pitch angle v⊥/v|| ranges from 0.1 to 0.2 for the detected electrons, with energies above 25 MeV. Analysis of synchrotron intensity from a camera indicates that the tail of the runaway energy distribution reaches energies up to 60 MeV, which agrees with 0D modeling of electron acceleration in the toroidal electric field generated during the current quench. A visible spectrometer provides an independent estimate of the upper limit of runaway electron energy which is roughly consistent with energy determined from camera data. Synchrotron spectra reveal that approximately 1% of the total post-thermal quench plasma current is carried by the detected high-energy runaway population with energies in the range of 25–60 MeV; the bulk of the plasma current thus appears to be carried by relativistic electrons with energy less than 25 MeV. In addition to stable oval shapes, runaway beams with other shapes and internal structure are sometimes observed.
On the estimation of cooperativity in ion channel kinetics: Activation free energy and kinetic mechanism of Shaker K+ channel
Authors: Banerjee Kinshuk , Das Biswajit , Gangopadhyay Gautam
Published: April 2013

In this paper, we have explored generic criteria of cooperative behavior in ion channel kinetics treating it on the same footing with multistate receptor-ligand binding in a compact theoretical framework. We have shown that the characterization of cooperativity of ion channels in terms of the Hill coefficient violates the standard Hill criteria defined for allosteric cooperativity of ligand binding. To resolve the issue, an alternative measure of cooperativity is proposed here in terms of the cooperativity index that sets a unified criteria for both the systems. More importantly, for ion channel this index can be very useful to describe the cooperative kinetics as it can be readily determined from the experimentally measured ionic current combined with theoretical modelling. We have analyzed the correlation between the voltage value and slope of the voltage-activation curve at the half-activation point and consequently determined the standard free energy of activation of the ion channel using two well-established mechanisms of cooperativity, namely, Koshland-Nemethy-Filmer (KNF) and Monod-Wyman-Changeux (MWC) models. Comparison of the theoretical results for both the models with appropriate experimental data of mutational perturbation of Shaker \documentclass[12pt]{minimal}\begin{document}$\rm K^+$\end{document}K+ channel supports the experimental fact that the KNF model is more suitable to describe the cooperative behavior of this class of ion channels, whereas the performance of the MWC model is unsatisfactory. We have also estimated the mechanistic performance through standard free energy of channel activation for both the models and proposed a possible functional disadvantage in the MWC scheme.
Friction law and turbulent properties in a laboratory Ekman boundary layer
Authors: Sous D. , Sommeria J. , Boyer D.
Published: April 2013

We use spin-up/spin-down laboratory experiments to study the neutrally stratified Ekman boundary layer. The experiments are performed in the 13 m diameter, 1 m deep Coriolis rotating tank of the LEGI in Grenoble, France. A global flow rotation is produced by an initial change in the tank rotation speed. It then slowly decays under the effect of Ekman friction, evolving from the turbulent state to the laminar state. It is checked that the Ekman layer itself remains in a quasi-steady state during this decay. The velocity is measured by Particle Imaging Velocimetry (PIV) at two scales: the global rotation in a horizontal plane, and the vertical profile inside the boundary layer, where the three velocity components are obtained by stereoscopic PIV. The friction law is obtained by relating the decay rate of the bulk velocity to the velocity itself. This method is justified by the fact that this bulk velocity is independent of height beyond the top of the boundary layer (a few cm), as expected from the Taylor-Proudman theorem for rotating fluids. The local measurements inside the boundary layer provide profiles of the mean velocity and Reynolds stress components, in particular the cross-isobar angle between the interior and near surface velocities. In the laminar regime, good agreement is obtained with the classical Ekman's theory, which validates the method. In the turbulent regime, the results are found consistent with the classical Atmospheric Boundary Layer (ABL) model based on the von Karman logarithmic layer. Our experiments therefore indicate that this theory, in principle valid for very large Reynolds numbers, is already relevant close to the transitional regimes. A fit of the empirical coefficients A and B appearing in this theory yields A = 3.3 and B = 3.0. Extrapolating the results to the atmospheric case gives a friction velocity u* about 12% higher than the traditional fit for the ABL. We may safely deduce that for the oceanic bottom boundary layer, corresponding to lower Reynolds numbers than the atmosphere, our result provides a correct estimate within 10%. The previous laboratory results of Caldwell et al. [“A laboratory study of the turbulent Ekman layer,” Geophys. Fluid Dyn. 3, 125–160 (1972)10.1080/03091927208236078] provided frictions velocities about 20% higher than in our experiments, and slightly higher cross-isobar angles. We attribute this difference to the higher vortical Rossby number Rot in those experiments, and maybe also to roughness effects. We take into account the effect of this vortical Rossby number within the framework of the Ekman layer (Rot → 0) by replacing the tank rotation rate by the fluid rotation rate.
Scattering of radiation in collisionless dusty plasmas
Authors: Tolias P. , Ratynskaia S.
Published: April 2013

Scattering of electromagnetic waves in collisionless dusty plasmas is studied in the framework of a multi-component kinetic model. The investigation focuses on the spectral distribution of the scattered radiation. Pronounced dust signatures are identified in the coherent spectrum due to scattering from the shielding cloud around the dust grains, dust acoustic waves, and dust-ion acoustic waves. The magnitude and shape of the scattered signal near these spectral regions are determined with the aid of analytical expressions and its dependence on the dust parameters is investigated. The use of radiation scattering as a potential diagnostic tool for dust detection is discussed.
Fluid modeling of radio frequency and direct currents in a biased magnetized plasma
Authors: Faudot E. , Heuraux S. , Kubic M. , Gunn J. , Colas L.
Published: April 2013

This model aims at simulating a magnetized plasma column connected on one side to a probe and on the other side to an ICRH (Ion Cyclotron Resonant Heating) antenna. This is a double probe modeling of a plasma flux tube exchanging perpendicular currents: rf polarization current and dc currents (inertia, viscous, and friction) perpendicular to the magnetic field. A self consistent solution for the rectified potential and the density is obtained under the assumptions of flute hypothesis, inertialess electrons, and no collision in parallel direction. The main effect of rf biasing on the antenna side is to shift the IV characteristic so that the floating potential can be increased up to ln(I0(eVrf/(kBTe))), with I0 the modified Bessel function of the first kind. On the contrary, the effect of dc currents is to decrease the plasma potential and the sheath potential which can be lower than 3kBTe/e or even be negative. Experimental characteristics are well matched by the 1D fluid code and exhibit very high negative currents (more than 30 ji the ion saturation current) for high positive biasing of the probe and for long flux tube (10 m). The non-saturation of the electron current is here due to high transverse transport enhanced by convective fluxes and dc currents able to bring an amount of density around the biased flux tube. During comparisons with experiments, the floating potential measured by a reciprocating probe is recovered by the code revealing that for a 120 V measured peak potential on the probe, the rf potential on the ICRH antenna is twice this value. Finally, the density profile can be flattened or steepened as a function of the transverse dc current direction.
Strong asymptotics for Cauchy biorthogonal polynomials with application to the Cauchy two-matrix model
Authors: Bertola M. , Gekhtman M. , Szmigielski J.
Published: April 2013

We apply the nonlinear steepest descent method to a class of 3 × 3 Riemann-Hilbert problems introduced in connection with the Cauchy two-matrix random model. The general case of two equilibrium measures supported on an arbitrary number of intervals is considered. In this case, we solve the Riemann-Hilbert problem for the outer parametrix in terms of sections of a spinorial line bundle on a three-sheeted Riemann surface of arbitrary genus and establish strong asymptotic results for the Cauchy biorthogonal polynomials.
Comment on “A model for internal photoemission at high-k oxide/silicon energy barriers” [J. Appl. Phys. 112, 064115 (2012)]
Authors: Afanas'ev V. V.
Published: April 2013

The conclusions of the recently proposed model for internal photoemission of electrons at the interfaces between silicon and high-k oxides [O. Engström, J. Appl. Phys. 112, 064115 (2012)] have been compared to some of the available experimental results. In this Comment, it is shown that this model cannot fully account for a number of significant observations and, therefore, is limited in its use for analysis of internal photoemission spectra.
A high-speed magnetic tweezer beyond 10,000 frames per second
Authors: Lansdorp Bob M. , Tabrizi Shawn J. , Dittmore Andrew , Saleh Omar A.
Published: April 2013

The magnetic tweezer is a single-molecule instrument that can apply a constant force to a biomolecule over a range of extensions, and is therefore an ideal tool to study biomolecules and their interactions. However, the video-based tracking inherent to most magnetic single-molecule instruments has traditionally limited the instrumental resolution to a few nanometers, above the length scale of single DNA base-pairs. Here we have introduced superluminescent diode illumination and high-speed camera detection to the magnetic tweezer, with graphics processing unit-accelerated particle tracking for high-speed analysis of video files. We have demonstrated the ability of the high-speed magnetic tweezer to resolve particle position to within 1 Å at 100 Hz, and to measure the extension of a 1566 bp DNA with 1 nm precision at 100 Hz in the presence of thermal noise.
Flow bioreactor design for quantitative measurements over endothelial cells using micro-particle image velocimetry
Authors: Leong Chia Min , Voorhees Abram , Nackman Gary B. , Wei Timothy
Published: April 2013

Mechanotransduction in endothelial cells (ECs) is a highly complex process through which cells respond to changes in hemodynamic loading by generating biochemical signals involving gene and protein expression. To study the effects of mechanical loading on ECs in a controlled fashion, different in vitro devices have been designed to simulate or replicate various aspects of these physiological phenomena. This paper describes the design, use, and validation of a flow chamber which allows for spatially and temporally resolved micro-particle image velocimetry measurements of endothelial surface topography and stresses over living ECs immersed in pulsatile flow. This flow chamber also allows the study of co-cultures (i.e., ECs and smooth muscle cells) and the effect of different substrates (i.e., coverslip and/or polyethylene terepthalate (PET) membrane) on cellular response. In this report, the results of steady and pulsatile flow on fixed endothelial cells seeded on PET membrane and coverslip, respectively, are presented. Surface topography of ECs is computed from multiple two-dimensional flow measurements. The distributions of shear stress and wall pressure on each individual cell are also determined and the importance of both types of stress in cell remodeling is highlighted.
Creation of quantum-degenerate gases of ytterbium in a compact 2D-/3D-magneto-optical trap setup
Authors: Dörscher Sören , Thobe Alexander , Hundt Bastian , Kochanke André , Le Targat Rodolphe , Windpassinger Patrick , Becker Christoph , Sengstock Klaus
Published: April 2013

We report on the first experimental setup based on a 2D-/3D-magneto-optical trap (MOT) scheme to create both Bose-Einstein condensates and degenerate Fermi gases of several ytterbium isotopes. Our setup does not require a Zeeman slower and offers the flexibility to simultaneously produce ultracold samples of other atomic species. Furthermore, the extraordinary optical access favors future experiments in optical lattices. A 2D-MOT on the strong 1S0 → 1P1 transition captures ytterbium directly from a dispenser of atoms and loads a 3D-MOT on the narrow 1S0 → 3P1 intercombination transition. Subsequently, atoms are transferred to a crossed optical dipole trap and cooled evaporatively to quantum degeneracy.
Measurements with magnetic field in the National Spherical Torus Experiment using the motional Stark effect with laser induced fluorescence diagnostic
Authors: Foley E. L. , Levinton F. M.
Published: April 2013

The motional Stark effect with laser-induced fluorescence diagnostic (MSE-LIF) has been installed and tested on the National Spherical Torus Experiment (NSTX) at the Princeton Plasma Physics Lab. The MSE-LIF diagnostic will be capable of measuring radially resolved profiles of magnetic field magnitude or pitch angle in NSTX plasmas. The system includes a diagnostic neutral hydrogen beam and a laser which excites the n = 2 to n = 3 transition. A viewing system has been implemented which will support up to 38 channels from the plasma edge to past the magnetic axis. First measurements of MSE-LIF signals in the presence of small applied magnetic fields in neutral gas are reported.
Investigation of the electron trajectories and gain regimes of the whistler pumped free-electron laser
Authors: Jafarinia F. , Jafari S. , Mehdian H.
Published: April 2013

A free-electron laser (FEL) scheme, which employs the whistler wave as a slow electromagnetic wave wiggler, was studied theoretically. Subjected to the transverse fields of whistler wave wiggler, the beam electrons are the source of the energy needed to produce electromagnetic radiation. The strength and the period of the wiggler field depend on the parameters of the magnetoplasma medium. This configuration has a higher tunability by controlling the plasma density, on top of the γ-tunability of the conventional FELs. The theory of linear gain and electron trajectories was presented and four groups (I, II, III, and IV) of electron orbits were found in the presence of an axial guide magnetic field. Using perturbation analysis, it is found that these groups of orbits were stable except small regions of group I and IV orbits. The function Φ which determines the rate of change of axial velocity with beam energy was also derived. In the case in which Φ<0 represents a negative-mass regime in which the axial velocity accelerates as the electrons lose energy. Numerical solutions showed that by increasing the cyclotron frequency, the gain for group I and III orbits increased, while a gain decrement was obtained for group II and IV orbits.
Measurements of collective fuel velocities in deuterium-tritium exploding pusher and cryogenically layered deuterium-tritium implosions on the NIF
Authors: Gatu Johnson M. , Casey D. T. , Frenje J. A. , Li C.-K. , Séguin F. H. , Petrasso R. D. , Ashabranner R. , Bionta R. , LePape S. , McKernan M. , Mackinnon A. , Kilkenny J. D. , Knauer J. , Sangster T. C.
Published: April 2013

For the first time, quantitative measurements of collective fuel velocities in Inertial Confinement Fusion implosions at the National Ignition Facility are reported. Velocities along the line-of-sight (LOS) of the Magnetic Recoil neutron Spectrometer (MRS), positioned close to the equator (73°–324°), were inferred from the measured mean energy of the deuterium-tritium (DT)-primary neutron peak. Substantial mean energy shifts up to 113 ± 16 keV were observed in DT gas-filled exploding-pusher implosions, driven in a polar-direct drive configuration, which corresponds to bulk fuel velocities up to 210 ± 30 km/s. In contrast, only marginal bulk fuel velocities along the MRS LOS were observed in cryogenically layered DT implosions. Integrated analysis of data from a large number of cryogenically layered implosions has recently identified a deficit in achieved hot-spot energy of ∼3 kJ for these implosions [C. Cerjan et al., Phys. Plasmas (2013)]. One hypothesis that could explain this missing energy is a collective, directional fuel velocity of ∼190 km/s. As only marginal bulk fuel velocities are observed in the MRS data, this might indicate that turbulent or radial flows would be a likely explanation for the missing energy. However, a directional velocity close to perpendicular to the MRS LOS cannot be ruled out.
Enhancing the efficiency of slow-wave electron cyclotron masers with the tapered refractive index
Authors: Kong Ling-Bao , Hou Zhi-Ling , Jing Jian , Jin Hai-Bo , Du Chao-Hai
Published: April 2013

The nonlinear analysis of slow-wave electron cyclotron masers (ECM) based on anomalous Doppler effect in a slab waveguide is presented. A method of tapered refractive index (TRI) is proposed to enhance the efficiency of slow-wave ECM. The numerical calculations show that the TRI method can significantly enhance the efficiency of slow-wave ECM with the frequency ranging from the microwave to terahertz band. The effect of beam velocity spread on the efficiency has also been studied. Although the velocity spread suppresses the efficiency significantly, a great enhancement of efficiency can still be introduced by the TRI method.
Linear permittivity tapering in a Cerenkov microwave source with a pre-bunched beam
Authors: Poole B. R. , Harris J. R.
Published: April 2013

Cerenkov microwave sources use a dielectric-lined waveguide to reduce the velocity of the electromagnetic wave and provide efficient energy transfer between the wave and the driving electron beam. Tapering the permittivity of the dielectric to maintain synchronism between the beam and the wave as the beam loses energy can increase the efficiency of these devices. Here, we consider such a structure driven by an electron beam with a harmonic density perturbation. Particle-In-Cell (PIC) simulations and a macro-particle model based on the slowly varying envelope approximation are first used to examine an un-tapered baseline case. PIC simulations of the source with linear tapers over the entire amplifier length as well as over only a section of the amplifier where the beam executes synchrotron oscillations are examined. The efficiency for the baseline un-tapered source is 18%, while efficiencies up to approximately 48% are found using a taper in dielectric permittivity. Results of the best performing cases are presented. Detailed examination of longitudinal phase space, particle energy distributions, evolution of longitudinal wavenumber, and phase dynamics are presented from the PIC simulations.
Plasmoid instability in high-Lundquist-number magnetic reconnection
Authors: Huang Yi-Min , Bhattacharjee A.
Published: April 2013

Our understanding of magnetic reconnection in resistive magnetohydrodynamics has gone through a fundamental change in recent years. The conventional wisdom is that magnetic reconnection mediated by resistivity is slow in laminar high Lundquist (S) plasmas, constrained by the scaling of the reconnection rate predicted by Sweet-Parker theory. However, recent studies have shown that when S exceeds a critical value ∼104, the Sweet-Parker current sheet is unstable to a super-Alfvénic plasmoid instability, with a linear growth rate that scales as S1/4. In the fully developed statistical steady state of two-dimensional resistive magnetohydrodynamic simulations, the normalized average reconnection rate is approximately 0.01, nearly independent of S, and the distribution function f(ψ) of plasmoid magnetic flux ψ follows a power law f(ψ)∼ψ−1. When Hall effects are included, the plasmoid instability may trigger onset of Hall reconnection even when the conventional criterion for onset is not satisfied. The rich variety of possible reconnection dynamics is organized in the framework of a phase diagram.
Note: Solid state photon counters with sub-picosecond timing stability
Authors: Prochazka Ivan , Kodet Jan , Blazej Josef
Published: April 2013

We are reporting on a design, construction, and performance of photon counting detector packages based on solid state sensors. These photon counting devices have been optimized for extremely high stability of their detection delay. The detectors have been designed for applications in fundamental metrology and optical time transfer. The single photon avalanche diode structure manufactured on silicon using the K14 technology is used as a sensor. The active area of the sensor is circular with a diameter of 100 or 200 μm. The sensor is operated in an active quenching and gating mode. The photon detection efficiency exceeds 40% in a wavelength range spanning from 500 to 800 nm. The timing resolution is better than 20 ps rms. Its detection delay is stable within ±600 fs over several days of operation, in a sense of time deviation the detection delay stability of 150 fs has been achieved. The temperature change of the detection delay is as low as 280 fs/K. This timing performance is preserved even under extremely high background photon fluxes exceeding 100 Mc/s. The detectors have been qualified for operation in space missions.
Enhanced detection of NO2 with recessed AlGaN/GaN open gate structures
Authors: Vitushinsky R. , Crego-Calama M. , Brongersma S. H. , Offermans P.
Published: April 2013

Recessing of the open gate region of AlGaN/GaN heterostructures is shown to dramatically enhance the sensitivity of these devices to surface interactions. This is demonstrated by NO2 exposure under humid conditions. NO2 adsorption at the open gate region leads to a decrease in two-dimensional electron gas conductivity, which we attribute to the interaction of NO2 with surface donor states. We find that the recessing enhances the sensitivity to NO2 by almost three orders of magnitude. This may also be used to improve the sensitivity of other (bio)chemical sensors based on AlGaN/GaN. Furthermore, the effect of humidity on the NO2 response is investigated.
Quantized conductance and field-effect topological quantum transistor in silicene nanoribbons
Authors: Ezawa Motohiko
Published: April 2013

Silicene is a quantum spin-Hall insulator, which undergoes a topological phase transition into other insulators by applying external fields. We investigate transport properties of silicene nanoribbons based on the Landauer formalism. We propose to determine topological phase transitions by measuring the density of states and conductance. The conductance is quantized and changes its value when the system transforms into different phases. We show that a silicene nanoribbon near the zero energy acts as a field-effect transistor. This transistor is robust since the zero-energy edge states are topologically protected. Our findings open a way to future topological quantum devices.
Influence of the absorber layer thickness and rod length on the performance of three-dimensional nanorods thin film hydrogenated amorphous silicon solar cells
Authors: Ho Chung-I , Liang Wei-Chieh , Yeh Dan-Ju , Su Vin-Cent , Yang Po-Chuan , Chen Shih-Yen , Yang Tsai-Ting , Lee Jeng-Han , Kuan Chieh-Hsiung , Cheng I-Chun , Lee Si-Chen
Published: April 2013

Performance of substrate-configured hydrogenated amorphous silicon solar cells based on ZnO nanorod arrays prepared by hydrothermal method has been investigated. The light harvest ability of three-dimensional nanorods solar cells is a compromise between the absorber layer thickness and the nanorods geometry. By optimizing the intrinsic a-Si:H absorber layer thickness from 75 to 250 nm and varying the length of the nanorods from 600 to 1800 nm, the highest energy conversion efficiency of 6.07% is obtained for the nanorods solar cell having thin absorber layer thickness of 200 nm with the rod length of 600 nm. This represents up to 28% enhanced efficiency compared to the conventional flat reference cell with similar absorber layer thickness.
Percolation transport and filament formation in nanocrystalline silicon nanowires
Authors: Fischer S. , Osorio C. , Williams N. E. , Ayas S. , Silva H. , Gokirmak A.
Published: April 2013

Under sufficient electrical bias, electron percolation in nanocrystalline silicon can lead to rapid self-heating and formation of highly conductive, molten filaments, and local crystallization upon cooling. Self-heated nanocrystalline silicon nanowires are modeled using 2-D finite element simulations using temperature dependent material parameters. Nanocrystalline silicon is modeled as randomly distributed isolated crystalline grains embedded in an amorphous matrix. Highly conductive, nanometer-width molten filaments form from percolation paths at the beginning of nanosecond voltage pulses. In a short period of time, the most conductive filament starts drawing all the current (with current densities > 100 MA/cm2) while the remaining percolation paths resolidify.
Impact of rapid thermal processing on oxygen precipitation in heavily arsenic and antimony doped Czochralski silicon
Authors: Zhang Xinpeng , Gao Chao , Fu Maosen , Ma Xiangyang , Vanhellemont Jan , Yang Deren
Published: April 2013

A comparative investigation is performed on the effects of vacancies induced by rapid thermal processing on oxygen precipitation behavior in heavily arsenic- and antimony-doped Czochralski silicon wafers. It is experimentally found that vacancy-assisted oxide precipitate nucleation occurs at 800, 900, and 1000  °C in the Sb-doped wafers, while it only occurs at 800  °C in the As-doped ones. Density functional theory calculations indicate that it is energetically favorable to form AsVO and SbVO complexes in As- and Sb-doped silicon crystals, respectively. These complexes might act as precursors for oxide precipitate nucleation under appropriate conditions. The difference between the effects of rapid thermal processing -induced vacancies on oxide precipitate nucleation in the heavily As- and Sb-doped Cz silicon crystals is tentatively elucidated based on density functional theory calculations revealing the difference in binding energies of AsVO and SbVO complexes.
Weakly nonlinear incompressible Rayleigh-Taylor instability growth at cylindrically convergent interfaces
Authors: Wang L. F. , Wu J. F. , Ye W. H. , Zhang W. Y. , He X. T.
Published: April 2013

A weakly nonlinear (WN) model has been developed for the incompressible Rayleigh-Taylor instability (RTI) in cylindrical geometry. The transition from linear to nonlinear growth is analytically investigated via a third-order solutions for the cylindrical RTI initiated by a single-mode velocity perturbation. The third-order solutions can depict the early stage of the interface asymmetry due to the bubble-spike formation, as well as the saturation of the linear (exponential) growth of the fundamental mode. The WN results in planar RTI [Wang et al., Phys. Plasmas 19, 112706 (2012)] are recovered in the limit of high-mode number perturbations. The difference between the WN growth of the RTI in cylindrical geometry and in planar geometry is discussed. It is found that the interface of the inward (outward) development spike/bubble is extruded (stretched) by the additional inertial force in cylindrical geometry compared with that in planar geometry. For interfaces with small density ratios, the inward growth bubble can grow fast than the outward growth spike in cylindrical RTI. Moreover, a reduced formula is proposed to describe the WN growth of the RTI in cylindrical geometry with an acceptable precision, especially for small-amplitude perturbations. Using the reduced formula, the nonlinear saturation amplitude of the fundamental mode and the phases of the Fourier harmonics are studied. Thus, it should be included in applications where converging geometry effects play an important role, such as the supernova explosions and inertial confinement fusion implosions.
Low-frequency locally resonant band-gaps in phononic crystal plates with periodic spiral resonators
Authors: Zhang Siwen , Hui Wu Jiu , Hu Zhiping
Published: April 2013

In this paper, low-frequency band-gaps (BGs) in a phononic crystal (PC) thin plate with periodic spiral resonators are investigated numerically and experimentally. The formation mechanisms of the BGs in the proposed structure are explained based on the modal analysis. We find that the interaction between the local resonances and the traveling wave modes in the plate is responsible for the formation of the BG in low-frequency range. This interaction strength greatly affects the bandwidth of the BG, of which the lower edge depends on the corresponding local resonance frequency. It is shown that the out-of-plane BG can be modulated by changing the geometrical parameters. The proposed PC plate is demonstrated to possess a broad out-of-plane BG in low-frequency range from 42 Hz to 150 Hz, by combining the numerical calculations with experimental measurements. The structure design and its results provide an effective way for phononic crystals to obtain broad BGs in low-frequency range, which has potential applications in the low-frequency vibration and noise reduction.
Orientation dependent band alignment for p-NiO/n-ZnO heterojunctions
Authors: Ma M. J. , Lu B. , Zhou T. T. , Ye Z. Z. , Lu J. G. , Pan X. H.
Published: April 2013

Nonpolor a-plane and polar c-plane ZnO thin films were prepared on r-plane sapphire and quartz substrates, respectively. The electronic structure of the interface between subsequently fabricated NiO/ZnO heterojunctions has been investigated by x-ray photoelectron spectroscopy measurements and the band offsets are determined together with information yielded from UV-vis transition spectra. It is found that a type-II band alignment forms at the interface for both the samples. The revealed ZnO-orientation dependent band offsets are analyzed and are attributed mainly due to the variations in internal electric field arose from spontaneous polarization effect. The accurate determination of the band alignment is important for the design and application of NiO/ZnO based hybrid devices.
Modeling of thin-film solar thermoelectric generators
Authors: Weinstein L. A. , McEnaney K. , Chen G.
Published: April 2013

Recent advances in solar thermoelectric generator (STEG) performance have raised their prospect as a potential technology to convert solar energy into electricity. This paper presents an analysis of thin-film STEGs. Properties and geometries of the devices are lumped into two parameters which are optimized to guide device design. The predicted efficiencies of thin-film STEGs are comparable to those of existing STEG configurations built on bulk materials.
Temperature-induced ductile-to-brittle transition of bulk metallic glasses
Authors: Li G. , Jiang M. Q. , Jiang F. , He L. , Sun J.
Published: April 2013

Uniaxial tensile and uniaxial compressive tests for Zr-based bulk metallic glasses (BMGs) were conducted at room and cryogenic temperatures, respectively. It was observed that both the change of macroscopic fracture mode from ductile shear fracture to brittle normal tensile fracture and microscopic fracture feature from micron-scaled vein patterns to nano-scaled dimples with decreasing test temperatures were identified, indicating a significant ductile-to-brittle transition (DBT) behavior. The mechanism of DBT behavior was revealed by the competition between the intrinsic critical shear strength τ0 and critical tensile strength σ0 at different temperatures.
Pre-breakdown evaluation of gas discharge mechanisms in microgaps
Authors: Semnani Abbas , Venkattraman Ayyaswamy , Alexeenko Alina A. , Peroulis Dimitrios
Published: April 2013

The individual contributions of various gas discharge mechanisms to total pre-breakdown current in microgaps are quantified numerically. The variation of contributions of field emission and secondary electron emission with increasing electric field shows contrasting behavior even for a given gap size. The total current near breakdown decreases rapidly with gap size indicating that microscale discharges operate in a high-current, low-voltage regime. This study provides the first such analysis of breakdown mechanisms and aids in the formulation of physics-based theories for microscale breakdown.
Voltage-controlled spin injection with an endohedral fullerene Co@C60 dimer
Authors: Saffarzadeh Alireza , Kirczenow George
Published: April 2013

Spin-dependent transport through an endohedral fullerene Co@C60 dimer with gold electrodes is explored theoretically using density functional and extended Hückel theory. Density of states spin polarizations up to 95%, due to spin-splitting of Co 3d orbitals, are found by varying the gate and/or bias voltage. The current-voltage characteristics and strong (up to 100%) spin polarization of the current indicate that the device can be utilized for highly efficient spin injection into nonmagnetic conductors. This finding opens the way to the realization of electrostatically tuned spintronic nano devices less than 2 nm in size, without ferromagnetic electrodes.
Effective increasing of optical absorption of TiO2 by introducing trivalent titanium
Authors: Zhao Yanfei , Hou Tingjun , Li Youyong , Chan K. S. , Lee Shuit-Tong
Published: April 2013

We investigate the role of hydrogenation on titanium dioxide (TiO2) by using DFT + U calculations. We find that hydrogenation on oxygen is more favorable than on titanium, which reduces Ti4+ into Ti3+ on the surface and introduces mid-gap state into TiO2 for enhanced optical absorption.