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Results for “effective diffusivity”

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At least 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Spectral shape of a signal in light-induced diffusive pulling (pushing) of particles into a light beam

We study theoretically how the dependences of transport collision frequencies ν{sub i}, collision broadening γ and collision shift Δ of the levels on the velocity v of resonant particles influence lightinduced diffusive pulling (pushing) (LDP) effects in the framework of a generalised model of strong collisions in the case of velocitydependent collision rates (so-called kangaroo model). It is found that allowance for the dependences ν{sub i}(v), γ(v) and Δ(v) does not change the spectral shape of an LDP signal. In particular, in the case of low-intensity radiation, the spectral dependence of the LDP signal coincides with the absorption line shape. It is shown that the magnitude of the LDP effect is proportional to the difference between the diffusion coefficients of particles in the excited and ground states. It is found that the spectral anomalies previously predicted in the LDP effect [Gel'mukhanov F.Kh. JETP Lett., 55, 214 (1992)] for an idealised model of the Lorentz gas (the limiting case of heavy buffer particles), which arise due to the dependences ν{sub i}(v), γ(v) and Δ(v), are typical only for this gas. At a realistic ratio of the masses of absorbing and buffer particles, spectral anomalies do not occur in the LDP effect. (nonlinear optical phenomena)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Quantifying Aerosol Direct Effects from Broadband Irradiance and Spectral Aerosol Optical Depth Observations

We outline a methodology using broadband and spectral irradiances to quantify aerosol direct effects on the surface diffuse shortwave (SW) irradiance. Best Estimate Flux data span a 13 year timeframe at the Department of Energy Atmospheric Radiation Measurement Program’s Southern Great Plains (SGP) site. Screened clear-sky irradiances and aerosol optical depth (AOD), for solar zenith angles ≤ 65°, are used to estimate clear-sky diffuse irradiances. We validate against detected clear-sky observations from SGP’s Basic Radiation System (BRS). BRS diffuse irradiances were in accordance with estimates, producing a root-mean-square error and mean bias errors of 4.0 W/m2 and -1.4 W/m2, respectively. Absolute differences show 99% of estimates within ±10 W/m2 (10%) of the mean BRS observations. Clear-sky diffuse estimates are used to derive quantitative estimates of aerosol radiative effects, represented as the aerosol diffuse irradiance (ADI). ADI is the contribution of diffuse SW to global SW, attributable to scattering of atmospheric transmission by natural plus anthropogenic aerosols. Estimated slope for the ADI as a function of AOD indicates an increase of ~22 W/m2 in diffuse SW for every 0.1 increase in AOD. Such significant increases in the diffuse fraction could possibly increase photosynthesis. Annual mean ADI is 28.2 W/m2, and heavy aerosol loading at SGP provides up to a maximum increase of 120 W/m2 in diffuse SW over background conditions. With regard to seasonal variation, the mean diffuse forcings are 17.2, 33.3, 39.0, and 23.6 W/m2 for winter, spring, summer, and fall, respectively.

broadband↗
DOE OSTI2014

Coulomb collision effects on linear Landau damping

Coulomb collisions at rate ν produce slightly probabilistic rather than fully deterministic charged particle trajectories in weakly collisional plasmas. Their diffusive velocity scattering effects on the response to a wave yield an effective collision rate ν{sub eff} ≫ ν and a narrow dissipative boundary layer for particles with velocities near the wave phase velocity. These dissipative effects produce temporal irreversibility for times t ≳ 1/ν{sub eff} during Landau damping of a small amplitude Langmuir wave.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Anomalous diffusion of Ga and As from semi-insulating GaAs substrate into MOCVD grown ZnO films as a function of annealing temperature and its effect on charge compensation

The diffusion behavior of arsenic (As) and gallium (Ga) atoms from semi-insulating GaAs (SI-GaAs) into ZnO films upon post-growth annealing vis-à-vis the resulting charge compensation was investigated with the help of x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy. The films, annealed at 600 ºC and 700 ºC showed p-type conductivity with a hole concentration of 1.1 × 10{sup 18} cm{sup −3} and 2.8 × 10{sup 19} cm{sup −3} respectively, whereas those annealed at 800 ºC showed n-type conductivity with a carrier concentration of 6.5 × 10{sup 16} cm{sup −3}. It is observed that at lower temperatures, large fraction of As atoms diffused from the SI-GaAs substrates into ZnO and formed acceptor related complex, (As{sub Zn}–2V{sub Zn}), by substituting Zn atoms (As{sub Zn}) and thereby creating two zinc vacancies (V{sub Zn}). Thus as-grown ZnO which was supposed to be n-type due to nonstoichiometric nature showed p-type behavior. On further increasing the annealing temperature to 800 ºC, Ga atoms diffused more than As atoms and substitute Zn atoms thereby forming shallow donor complex, Ga{sub Zn}. Electrons from donor levels then compensate the p-type carriers and the material reverts back to n-type. Thus the conversion of carrier type took place due to charge compensation between the donors and acceptors in ZnO and this compensation is the possible origin of anomalous conduction in wide band gap materials.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Systematic characterization of protein folding pathways using diffusion maps: Application to Trp-cage miniprotein

Understanding the mechanisms by which proteins fold from disordered amino-acid chains to spatially ordered structures remains an area of active inquiry. Molecular simulations can provide atomistic details of the folding dynamics which complement experimental findings. Conventional order parameters, such as root-mean-square deviation and radius of gyration, provide structural information but fail to capture the underlying dynamics of the protein folding process. It is therefore advantageous to adopt a method that can systematically analyze simulation data to extract relevant structural as well as dynamical information. The nonlinear dimensionality reduction technique known as diffusion maps automatically embeds the high-dimensional folding trajectories in a lower-dimensional space from which one can more easily visualize folding pathways, assuming the data lie approximately on a lower-dimensional manifold. The eigenvectors that parametrize the low-dimensional space, furthermore, are determined systematically, rather than chosen heuristically, as is done with phenomenological order parameters. We demonstrate that diffusion maps can effectively characterize the folding process of a Trp-cage miniprotein. By embedding molecular dynamics simulation trajectories of Trp-cage folding in diffusion maps space, we identify two folding pathways and intermediate structures that are consistent with the previous studies, demonstrating that this technique can be employed as an effective way of analyzing and constructing protein folding pathways from molecular simulations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

THE IMPLICIT CONTRIBUTION OF SLAB MODES TO THE PERPENDICULAR DIFFUSION COEFFICIENT OF PARTICLES INTERACTING WITH TWO-COMPONENT TURBULENCE

We explore the transport of energetic particles in two-component turbulence in which the stochastic magnetic field is assumed to be a superposition of slab and two-dimensional modes. It is known that in magnetostatic slab turbulence, the motion of particles across the mean magnetic field is subdiffusive. If a two-dimensional component is added, diffusion is recovered. It was also shown before that in two-component turbulence, the slab modes do not explicitly contribute to the perpendicular diffusion coefficient. In the current paper, the implicit contribution of slab modes is explored and it is shown that this contribution leads to a reduction of the perpendicular diffusion coefficient. This effect improves the agreement between simulations and analytical theory. Furthermore, the obtained results are relevant for investigations of diffusive shock acceleration.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Efficient reactive Brownian dynamics

We develop a Split Reactive Brownian Dynamics (SRBD) algorithm for particle simulations of reaction-diffusion systems based on the Doi or volume reactivity model, in which pairs of particles react with a specified Poisson rate if they are closer than a chosen reactive distance. In our Doi model, we ensure that the microscopic reaction rules for various association and dissociation reactions are consistent with detailed balance (time reversibility) at thermodynamic equilibrium. The SRBD algorithm uses Strang splitting in time to separate reaction and diffusion and solves both the diffusion-only and reaction-only subproblems exactly, even at high packing densities. To efficiently process reactions without uncontrolled approximations, SRBD employs an event-driven algorithm that processes reactions in a time-ordered sequence over the duration of the time step. A grid of cells with size larger than all of the reactive distances is used to schedule and process the reactions, but unlike traditional grid-based methods such as reaction-diffusion master equation algorithms, the results of SRBD are statistically independent of the size of the grid used to accelerate the processing of reactions. We use the SRBD algorithm to compute the effective macroscopic reaction rate for both reaction-limited and diffusion-limited irreversible association in three dimensions and compare to existing theoretical predictions at low and moderate densities. We also study long-time tails in the time correlation functions for reversible association at thermodynamic equilibrium and compare to recent theoretical predictions. Finally, we compare different particle and continuum methods on a model exhibiting a Turing-like instability and pattern formation. Our studies reinforce the common finding that microscopic mechanisms and correlations matter for diffusion-limited systems, making continuum and even mesoscopic modeling of such systems difficult or impossible. We also find that for models in which particles diffuse off lattice, such as the Doi model, reactions lead to a spurious enhancement of the effective diffusion coefficients.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2014

A numerical simulation of solar energetic particle dropouts during impulsive events

This paper investigates the conditions for producing rapid variations of solar energetic particle (SEP) intensity commonly known as 'dropouts'. In particular, we use numerical model simulations based on solving the focused transport equation in the three-dimensional Parker interplanetary magnetic field to put constraints on the properties of particle transport coefficients in both directions perpendicular and parallel to the magnetic field. Our calculations of the temporal intensity profile of 0.5 and 5 MeV protons at the Earth show that the perpendicular diffusion must be small while the parallel mean free path is long in order to reproduce the phenomenon of SEP dropouts. When the parallel mean free path is a fraction of 1 AU and the observer is located at 1 AU, the perpendicular to parallel diffusion ratio must be below 10{sup –5} if we want to see the particle flux dropping by at least several times within 3 hr. When the observer is located at a larger solar radial distance, the perpendicular to parallel diffusion ratio for reproducing the dropouts should be even lower than that in the case of 1 AU distance. A shorter parallel mean free path or a larger radial distance from the source to observer will cause the particles to arrive later, making the effects of perpendicular diffusion more prominent and SEP dropouts disappear. All of these effects require the magnetic turbulence that resonates with the particles to be low everywhere in the inner heliosphere.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

WE-FG-BRA-09: Using Graphene Oxide Nano Flakes During Image Guided Radiotherapy to Minimize the Potential of Cancer Recurrence Or Metastasis

Purpose: An increasing number of studies show that cancer stem cells (CSCs) become more invasive (metastatic) and may escape into the blood stream and lymph nodes during radiotherapy (RT), before they have received a lethal dose during RT. Other Studies have shown that Graphene oxide (GO) can selectively inhibit the proliferative expansion of CSCs across multiplicative tumor types. In this study we investigate the feasibility of using GO during radiotherapy (RT) to minimize the escape of CSCs towards preventing cancer metastasis or recurrence. Methods: We hypothesize that sufficient amount of GO nano-flakes (GONFs) released from new design radiotherapy biomaterials (fiducials or spacers) loaded with the GONFs can reach all tumor cells within typical times of 14 or 21days before the beginning of image-guided radiotherapy (IGRT) following implantation. To test this hypothesis, the space-time diffusion of the GONFs was investigated. Knudsen’s and Cunningham’s numbers were calculated to get the Stokes’ velocities and mobility values, according to these values, diffusion coefficients were calculated. In a previous study it was shown that GONFs concentration of 50 µg/ml were effective. In the diffusion study, 100 µg/ml was chosen as an initial concentration because it has been shown to be relatively non-toxic. Results: The 50 µg/ml concentration in a 2 cm diameter volume of lung tumor could be only achieved using 2 nm and 6 nm GONFs with respective diffusion times of 14 and 21 days. As expected, increased nanoflake size requires longer times to achieve the target 50 µg/ml concentration. Conclusion: The preliminary results indicate the potential of using GONFs delivered via new design radiotherapy biomaterials (e.g. fiducials) to inhibit the proliferative expansion of CSCs. The study avails ongoing in-vivo studies on using GONFs to enhance treatment outcomes for cancer patients.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2022

Using θ' interfaces as templates for planar L1 2 precipitation in AlCuMnZr alloys

Controlled Mn and Zr additions to Al-Cu alloys have allowed for the improved retention of mechanical properties after extended 350°C exposures by stabilizing the main strengthening θ' (Al 2 Cu) phase. Ultimately, θ'/L1 2 (Al 3 Zr) co-precipitate formation stabilizes θ' most effectively; however, Zr diffuses sluggishly and has low solubility in aluminum castings. Increasing the Zr segregation rate would allow for faster and more effective θ'/L1 2 co-precipitation. It is demonstrated that the Zr segregation rate is faster when the Zr matrix content is higher. A much higher Zr matrix content was achieved by rapid cooling during additive manufacturing (AM) that produces θ'/L1 2 co-precipitation faster, which is shown by scanning transmission electron microscopy and atom probe tomography experiments. It was also found that Zr continuously segregates to θ' interfaces up to the most aggressive heat treatment studied such that planar L1 2 precipitates remain after the metastable θ' dissolves. In this manner, we demonstrate that θ' coherent interfaces serve as perfect templates to form stable planar L1 2 precipitates that can provide strength at higher temperatures than traditional θ' strengthened AlCu alloys. This work introduces an alloy design strategy that uses metastable precipitates to quickly nucleate and grow co-precipitates with a desired geometry that contain slow diffusing elements. These ideas can be applied to engineer more heat resistant alloys by taking advantage of high solute matrix contents enabled by rapid cooling during additive manufacturing.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Effect of the Maximum Dose on White Matter Fiber Bundles Using Longitudinal Diffusion Tensor Imaging

Purpose: Previous efforts to decrease neurocognitive effects of radiation focused on sparing isolated cortical structures. We hypothesize that understanding temporal, spatial, and dosimetric patterns of radiation damage to whole-brain white matter (WM) after partial-brain irradiation might also be important. Therefore, we carried out a study to develop the methodology to assess radiation therapy (RT)–induced damage to whole-brain WM bundles. Methods and Materials: An atlas-based, automated WM tractography analysis was implemented to quantify longitudinal changes in indices of diffusion tensor imaging (DTI) of 22 major WM fibers in 33 patients with predominantly low-grade or benign brain tumors treated by RT. Six DTI scans per patient were performed from before RT to 18 months after RT. The DTI indices and planned doses (maximum and mean doses) were mapped onto profiles of each of 22 WM bundles. A multivariate linear regression was performed to determine the main dose effect as well as the influence of other clinical factors on longitudinal percentage changes in axial diffusivity (AD) and radial diffusivity (RD) from before RT. Results: Among 22 fiber bundles, AD or RD changes in 12 bundles were affected significantly by doses (P<.05), as the effect was progressive over time. In 9 elongated tracts, decreased AD or RD was significantly related to maximum doses received, consistent with a serial structure. In individual bundles, AD changes were up to 11.5% at the maximum dose locations 18 months after RT. The dose effect on WM was greater in older female patients than younger male patients. Conclusions: Our study demonstrates for the first time that the maximum dose to the elongated WM bundles causes post-RT damage in WM. Validation and correlative studies are necessary to determine the ability and impact of sparing these bundles on preserving neurocognitive function after RT.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2018

Hygrothermal Effect on Composites Under In-Plane Fatigue at Stress Ratios of R = −1 and R = 0.1: An Analysis of Quasi-Isotropic Stitched Carbon Fibers

The aeronautic structures normally operate under high levels of hygroscopic moisture from the surrounding environment at different temperature ranges while in service. Under such conditions, the behavior of laminate composite submitted to cyclic or static loadings can change drastically. In order to understand those effects in stitched fabrics, fatigue tests with open-hole specimens were carried out with a stress ratio of R = −1 and R = 0.1. The specimens were fatigue-tested as provided (environmental conditions) and after exposed to hygrothermal weathering conditions. Based on evidences from recent studies available in the open literature, e.g., effect of water diffusion on epoxy matrix, the overall results indicated a significant reduction in stiffness after the specimens are exposed to hygrothermal effects. The reduction in matrix stiffness, in this case, enhanced the fatigue strength in tension–tension load (R = 0.1) when compared to the specimens in normal conditions. The opposite occurs for the specimens loaded with stress ratio of R = −1, in which the delamination mechanisms changed during the loading reversion from tension to compression that promoted early delamination. Therefore, this process reduced the fatigue life of the specimens under hygrothermal condition. Then, by fractographic investigation, it was verified fracture patterns that regard to mode II damage in R = −1, in which mode II fracture toughness is known to decrease in the presence of water molecules.

36 MATERIALS SCIENCE↗
DOE OSTI2017

The Great Solar Active Region NOAA 12192: Helicity Transport, Filament Formation, and Impact on the Polar Field

The solar active region (AR), NOAA 12192, appeared in 2014 October as the largest AR in 24 years. Here we examine the counterintuitive nature of two diffusion-driven processes in the region: the role of helicity buildup in the formation of a major filament, and the relationship between the effects of supergranular diffusion and meridional flow on the AR and on the polar field. Quantitatively, calculations of current helicity and magnetic twist from Helioseismic and Magnetic Imager (HMI) vector magnetograms indicate that, though AR 12192 emerged with negative helicity, positive helicity from subsequent flux emergence, consistent with the hemispheric sign-preference of helicity, increased over time within large-scale, weak-field regions such as those near the polarity inversion line (PIL). Morphologically, Atmospheric Imaging Assembly observations of filament barbs, sigmoidal patterns, and bases of Fe xii stalks initially exhibited signatures of negative helicity, and the long filament that subsequently formed had a strong positive helicity consistent with the helicity buildup along the PIL. We find from full-disk HMI magnetograms that AR 12192's leading positive flux was initially closer to the equator but, owing either to the region’s magnetic surroundings or to its asymmetric flux density distribution, was transported poleward more quickly on average than its trailing negative flux, contrary to the canonical pattern of bipole flux transport. This behavior caused the AR to have a smaller effect on the polar fields than expected and enabled the formation of the very long neutral line where the filament formed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Shock-driven kinetic and diffusive mix in high-Z pusher ICF designs

Revolver and Double Shell Inertial Confinement Fusion capsule designs hope to achieve a robust volumetric thermonuclear burn via the use of a high-Z pusher shell filled with a cryogenic D–T fuel. Unfortunately, mix of the pusher material into the fuel (gas) may adversely impact the burn performance. Hydrodynamic instability of the metal/gas interface as the mix source is an obvious concern, but 1D effects may also be detrimental. Such effects include plasma diffusion at material interfaces, which has been the subject of numerous theoretical, computational, and experimental investigations. However, other 1D mix mechanisms may exist, which have yet to be thoroughly explored. In particular, plasma kinetic effects may drive the mix when a shock breaks out of the metal/gas interface. Using the state-of-the-art, hybrid (kinetic-ion/fluid electron), multi-ion Vlasov–Fokker–Planck code, iFP, we show herein that shock-driven kinetic effects can reconfigure the interface and the interfacial width subsequently grows diffusively. Finally, we consider any implications for high-Z pusher designs.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Effect of Radiation on Chromospheric Magnetic Reconnection: Reactive and Collisional Multi-fluid Simulations

We study magnetic reconnection under chromospheric conditions in five different ionization levels from 0.5% to 50% using a self-consistent two-fluid (ions + neutrals) model that accounts for compressibility, collisional effects, chemical inequilibrium, and anisotropic heat conduction. Results with and without radiation are compared, using two models for the radiative losses: an optically thin radiation loss function, and an approximation of the radiative losses of a plasma with photospheric abundances. The results without radiation show that reconnection occurs faster for the weakly ionized cases as a result of the effect of ambipolar diffusion and fast recombination. The tearing mode instability appears earlier in the low ionized cases and grows rapidly. We find that radiative losses have a stronger effect than was found in previous results as the cooling changes the plasma pressure and the concentration of ions inside the current sheet. This affects the ambipolar diffusion and the chemical equilibrium, resulting in thin current sheets and enhanced reconnection. The results quantify this complex nonlinear interaction by showing that a strong cooling produces faster reconnections than have been found in models without radiation. The results accounting for radiation show timescales and outflows comparable to spicules and chromospheric jets.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2026

Effect of Magnetic Islands on Neoclassical Heat Diffusivity in a Global Tokamak Simulation

Resonant magnetic perturbations (RMPs) alter magnetic field topology via island formation and can modify core transport in toroidal fusion devices. Using the global gyrokinetic particle‐in‐cell code XGC‐S, originally developed for stellarator geometries, we quantify how island topology affects neoclassical heat transport in a circular tokamak with ($m,n$) = (5,2) RMP‐induced islands. Electron radial heat diffusivity exhibits a two‐peak structure with a dominant enhancement near the O/X‐point region and a secondary peak at the outer island boundary, whereas ion transport remains close to the neoclassical prediction. The observed electron diffusivity is strongly enhanced over the neoclassical baseline but remains well below the Rechester–Rosenbluth estimate, consistent with the non‐ergodic character of the island topology. The density dependence of diffusivity is weaker inside the islands than outside, indicating the presence of topology‐driven transport channels that do not depend strongly on collisionality. Two‐dimensional heat flux maps reveal an up–down antisymmetric structure at the island separatrices consistent with magnetic gradient and curvature drift effects, in contrast to the symmetric temperature flattening inside the islands. These results identify topology‐dependent avenues for electron heat transport in RMP‐perturbed cores and suggest regimes in which island geometry, rather than collisions alone, governs radial transport.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Diffusion of alloying and fission atoms in α-U investigated by first-principles calculation

Stability and diffusion of small defect clusters containing alloying or fission atoms (“F” is used to represent both atoms) in an α-uranium (U) crystal are investigated using first-principles calculations. Here, results indicate that different stable defect structures are observed for an interstitial defect and a cluster (F n V m ) containing different alloying or fission atoms and vacancies. An interstitial defect is found to be most stable either at the center of a pyramid pentahedron or in a 〈010〉 mixed dumbbell configuration and the latter one is reported for the first time. Formation energy of a small F n V m cluster depends on the distribution of vacancies. Based on these stable defects, new migration energies and paths have been explored in this work. Anisotropic migration of an interstitial defect by jumping between nearest centers of two pyramid pentahedrons or jumping between two mixed U-Zr dumbbells has been suggested. Furthermore, a new migration-rotation mechanism has been explored for the first time for a FV 2 cluster, resulting in a 3D diffusion. Finally, the effect of substitutional Pu atoms on the migration of a FV 2 cluster suggests that optimizing the Pu concentration may prohibit the diffusion of some alloying or fission atoms, increasing the safe performance of metallic fuels.

3D diffusion↗