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At least 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Coulomb acceleration of light ions from homogeneous and layered targets

As applied to the problem of laser-induced ion acceleration, we have developed a theory for the expansion of a completely ionized thin two-ion-species target in the regime of a Coulomb explosion typical of the interaction of relativistically intense femtosecond laser pulses with nanofoils. Based on this theory and a simple numerical model, we have studied the generation of bunches of quasi-monoenergetic light ions and found their characteristics as a function of target parameters. We have performed a comparative analysis of the acceleration of light ions uniformly distributed in the target and concentrated in the form of a thin layer. For the production of high-quality accelerated ion beams, we have found optimal conditions that were qualitatively confirmed by our three-dimensional kinetic simulations of the laser pulse-thin target interaction.

43 PARTICLE ACCELERATORS↗
DOE OSTI2012

Kinetics and energy states of nanoclusters in the initial stage of homogeneous condensation at high supersaturation degrees

The condensation of metal vapor in an inert gas is studied by the molecular dynamics method. Two condensation regimes are investigated: with maintenance of partial pressure of the metal vapor and with a fixed number of metal atoms in the system. The main focus is the study of the cluster energy distribution over the degrees of freedom and mechanisms of the establishment of thermal equilibrium. It is shown that the internal temperature of a cluster considerably exceeds the buffer gas temperature and the thermal balance is established for a time considerably exceeding the nucleation time. It is found that, when the metal vapor concentration exceeds 0.1 of the argon concentration, the growth of clusters with the highest possible internal energy occurs, the condensation rate being determined only by the rate of heat removal from clusters.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

On the calculation of thermodynamic quantities in the Holstein model for homogeneous polynucleotides

The dynamics of a system for different types of polarons, i.e., in polythymine nucleotides (large-radius polaron), in polyadenine fragments (small-radius polaron), and in polyguanine DNA (intermediate case) at different thermostat temperatures are calculated using the semi-classical Holstein model. The temperature dependences of the thermodynamic equilibrium values of the total energy, the energy of an excess charge, and the electronic heat capacity have been obtained. For all polaron types, the peak of the electronic heat capacity dependence on temperature separates two modes (polaron and delocalized state). The electronic part of the energy is estimated in the high-temperature limit. In all cases, the electron heat capacity at high temperatures decreases in inverse proportion to the square of the temperature.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Study of the homogeneity of the current distribution in a dielectric barrier discharge in air by means of a segmented electrode

The current distribution in a dielectric barrier discharge in atmospheric-pressure air at a natural humidity of 40–60% was studied experimentally with a time resolution of 200 ps. The experimental results are interpreted by means of numerically simulating the discharge electric circuit. The obtained results indicate that the discharge operating in the volumetric mode develops simultaneously over the entire transverse cross section of the discharge gap.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Effect of Bi isovalent dopants on the formation of homogeneous coherently strained InAs quantum dots in GaAs matrices

The distribution of hydrostatic strains in Bi{sup 3+}-doped InAs quantum dots embedded in a GaAs matrix are calculated in the context of the deformation-potential model. The dependences of strains in the material of spherical InAs quantum dots with substitutional (Bi {yields} As) and interstitial (Bi) impurities on the quantum-dot size are derived. The qualitative correlation of the model with the experiment is discussed. The data on the effect of doping on the morphology of self-assembled InAs:Bi quantum dots in a GaAs matrix are obtained.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Investigation of the Far-IR Reflection Spectra of SmS Single Crystals and Polycrystals in the Homogeneity Range

The far- and mid-IR reflection spectra of Sm{sub 1+} {sub x}S (x = 0–0.17) samples are recorded and analyzed, as well as their electrical and structural parameters at a temperature of T = 300 K. The bond ionicity in SmS is shown to fall with a decrease in the area of the X-ray coherent scattering region and an increase in the concentration of donor impurities and, consequently, conduction electron concentration. The electrical conductivity of stoichiometric SmS single crystals and polycrystals can be determined with an error of 10% from the IR reflection spectra. Due to the low structural quality of the samples, the electrical conductivity cannot be determined in the case of deviation from stoichiometry.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Ferroelectrics of homogeneously deformed rare-earth garnet crystals, excited by elastic wave propagation

The effect of deformation on the electric properties in rare-earth garnet compounds is elucidated. It is shown that inhomogeneous deformation causes the emergence of electric polarization in garnet crystals on account of nonequivalent low-symmetry sites of rare-earth ions in the cubic structure of these crystals, whose symmetry of the environment has no inversion center. The polarization of a rare-earth ion subsystem in garnet crystals is studied upon the elastic wave propagation therein.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Nonlinearity of Volt–Ampere Characteristics of Homogeneous Compensated Detector GaAs Structures

The results of a study of carrier transfer and recharging of deep levels in semiconductor structures for ionizing radiation detectors are presented. Resistive-type arsenide–gallium structures with Schottky barriers and uniform distribution of deep chromium acceptors and donor EL2 centers have been studied. Solving the continuity and Poisson equations using the commercial design package, the effect of the volume depletion of detector structures by electrons with an increase in the applied voltage has been observed. It is established that the nonlinearity of the volt-ampere characteristics of structures occurs due to a change in the type of conductivity upon transition from an equilibrium to a nonequilibrium state. In this case, structures with initial (equilibrium) hole-type conductivity have close to linear volt–ampere characteristics.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

Analysis and optimization of gyrokinetic toroidal simulations on homogenous and heterogenous platforms

The Gyrokinetic Toroidal Code (GTC) uses the particle-in-cell method to efficiently simulate plasma microturbulence. This paper presents novel analysis and optimization techniques to enhance the performance of GTC on large-scale machines. We introduce cell access analysis to better manage locality vs. synchronization tradeoffs on CPU and GPU-based architectures. Finally, our optimized hybrid parallel implementation of GTC uses MPI, OpenMP, and NVIDIA CUDA, achieves up to a 2× speedup over the reference Fortran version on multiple parallel systems, and scales efficiently to tens of thousands of cores.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2010

Tunable double photonic bandgaps in a homogeneous atomic medium

Double photonic bandgaps (PBGs) can simultaneously appear when double dark resonances in uniform cold atoms are spatially modulated by a resonance standing-wave. Theoretical calculations show that variable and efficient coherent optical control of the PBGs can be achieved by modulating the coupling field and standing-wave. The structures of double PBGs induced by the atomic coherence effect are better than those obtained in the photonic crystal heterostructures. We anticipate that this scheme has potential applications in optical networks for dual-channel all-optical switching or a dual-frequency optical Bragg reflector.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2016

Plant Growth Environments with Programmable Relative Humidity and Homogeneous Nutrient Availability

We describe the design, characterization, and use of “programmable”, sterile growth environments for individual (or small sets of) plants. The specific relative humidities and nutrient availability experienced by the plant is established (RH between 15% and 95%; nutrient concentration as desired) during the setup of the growth environment, which takes about 5 minutes and <1$ in disposable cost. These systems maintain these environmental parameters constant for at least 14 days with minimal intervention (one minute every two days). The design is composed entirely of off-the-shelf components (e.g., LEGO1 bricks) and is characterized by (i) a separation of root and shoot environment (which is physiologically relevant and facilitates imposing specific conditions on the root system, e.g., darkness), (ii) the development of the root system on a flat surface, where the root enjoys constant contact with nutrient solution and air, (iii) a compatibility with root phenotyping. We demonstrate phenotyping by characterizing root systems of Brassica rapa plants growing in different relative humidities (55%, 75%, and 95%). While most phenotypes were found to be sensitive to these environmental changes, a phenotype tightly associated with root system topology– the size distribution of the areas encircled by roots–appeared to be remarkably and counterintuitively insensitive to humidity changes. These setups combine many of the advantages of hydroponics conditions (e.g., root phenotyping, complete control over nutrient composition, scalability) and soil conditions (e.g., aeration of roots, shading of roots), while being comparable in cost and setup time to Magenta1 boxes.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2017

Crystal structure, homogeneity range and electronic structure of rhombohedral γ-Mn 5 Al 8

Here, the γ-region of the Mn–Al phase diagram between 45 and 70 at.% Al was re-investigated by a combination of powder and single crystal X-ray diffraction as well as EDS analysis to establish the distribution of Mn and Al atoms. Single crystals of γ-Mn 5–x Al 8+x were grown using Sn-flux at 650 °C. The crystal structure, atomic coordinates and site occupancy parameters of γ-Mn 5–x Al 8+x phases were refined from single crystal X-ray data. The γ-Mn 5-x Al 8+x phase adopts the rhombohedral Cr 5 Al 8 -type structure rather than a cubic γ-brass structure. The refined compositions from two crystals extracted from the Al-rich and Mn-rich sides are, respectively, Mn 4.76 Al 8.24(2) (I) and Mn 6.32 Al 6.68(2) (II). The structure was refined in the acentric R3m space group (No.160, Z=6), in order to compare with other reported rhombohedral γ-brasses. In addition, according to X-ray powder diffraction analysis, at the Al-rich side the γ-phase coexists with LT–Mn 4 Al 11 and, at the Mn-rich side, with a hitherto unknown phase. The refined lattice parameters from powder patterns fall in the range a=12.6814(7)–12.6012(5) Å and c=7.9444(2)–7.9311(2) Å from Al-rich to Mn-rich loadings, and the corresponding rhombohedral angles distorted from a pseudo-cubic cell were found to be 89.1(1)°–88.9(1)°. Magnetic susceptibility and magnetization studies of Mn 4.92 Al 8.08(2) are consistent with moment bearing Mn and suggest a spin glass state below 27 K. Tight-binding electronic structure calculations (LMTO-ASA with LSDA) showed that the calculated Fermi level for γ-“Mn 5 Al 8 ” falls within a pseudogap of the density of states, a result which is in accordance with a Hume-Rothery stabilization mechanism γ-brass type phases.

36 MATERIALS SCIENCE↗