A simple reaction–diffusion system as a possible model for the origin of chemotaxis
Not Available
Gong, Yishu↗
SEARCH · Engineering Papers
Results for “Diffusion models”
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Electron kinetics in a positive column of AC discharges in a dynamic regime
Abstract We have performed hybrid kinetic-fluid simulations of a positive column in alternating current (AC) argon discharges over a range of driving frequencies f and gas pressure p for the conditions when the spatial nonlocality of the electron energy distribution function (EEDF) is substantial. Our simulations confirmed that the most efficient conditions of plasma maintenance are observed in the dynamic regime when time modulations of mean electron energy (temperature) are substantial. The minimal values of the root mean square electric field and the electron temperature have been observed at f/p values of about 3 kHz Torr −1 in a tube of radius R = 1 cm. The ionization rate and plasma density reached maximal values under these conditions. The numerical solution of a kinetic equation allowed accounting for the kinetic effects associated with spatial and temporal nonlocality of the EEDF. Using the kinetic energy of electrons as an independent variable, we solved an anisotropic tensor diffusion equation in phase space. We clarified the role of different flux components during electron diffusion in phase space over surfaces of constant total energy. We have shown that the kinetic theory uncovers a more exciting and rich physics than the classical ambipolar diffusion (Schottky) model. Non-monotonic radial distributions of excitation rates, metastable densities, and plasma density have been observed in our simulations at pR > 6 Torr cm. The predicted off-axis plasma density peak in the dynamic regime has never been observed in experiments so far. We hope our results stimulate further experimental studies of the AC positive column. The kinetic analysis could help uncover new physics even for such a well-known plasma object as a positive column in noble gases.
Physics↗
Using convolutional neural networks to accelerate three-dimensional coherent synchrotron radiation computations
Calculating the effects of coherent synchrotron radiation (CSR) is one of the most computationally expensive tasks in accelerator physics. Here, we use convolutional neural networks (CNNs), along with a latent conditional diffusion (LCD) model, trained on physics-based simulations to speed up calculations. Specifically, we produce the 3D CSR wakefields generated by electron bunches in circular orbit in the steady-state condition. Two datasets are used for training and testing the models: wakefields generated by three-dimensional Gaussian electron distributions and wakefields from a sum of up to 25 three-dimensional Gaussian distributions. The CNNs are able to accurately produce the 3D wakefields ∼250–1000 times faster than the numerical calculations, while the LCD achieves a gain of a factor of ∼34. We also test the extrapolation and out-of-distribution generalization ability of the models. They generalize well on distributions with larger spreads than what they were trained on but struggle with smaller spreads.
43 PARTICLE ACCELERATORS↗
Localized basis set for plutonium
The implementation of optimal strictly localized atomic orbitals basis for plutonium (Pu) using norm-conserving pseudopotential density-functional theory (DFT) is presented. The basis set was applied to the α, β, γ, δ, δ', and ε phases of Pu, δ-Pu surface, and δ-PuGa alloys. The computed properties of the Pu phases and δ-Pu surface were in good agreement with both available experimental data and prior DFT calculations based on plane-wave methodologies. Results for the δ-PuGa alloys were also in good agreement with experimental data. Finally, the reliability of the basis set was further demonstrated by using ab initio molecular dynamics to model the diffusion coefficient and activation barrier for atomic diffusion in a δ-PuGa alloy.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Atomistic modeling of the structure and diffusion processes at Al(110)/Si(001) interphase boundaries obtained by vapor deposition
Not Available
Li, Yang (ORCID:0000000311710852)↗
Millisecond timescale reactions observed via X-ray spectroscopy in a 3D microfabricated fused silica mixer
Determination of electronic structures during chemical reactions remains challenging in studies which involve reactions in the millisecond timescale, toxic chemicals, and/or anaerobic conditions. Here, a three-dimensionally (3D) microfabricated microfluidic mixer platform that is compatible with time-resolved X-ray absorption and emission spectroscopy (XAS and XES, respectively) is presented. This platform, to initiate reactions and study their progression, mixes a high flow rate (0.50–1.5 ml min –1 ) sheath stream with a low-flow-rate (5–90 µl min –1 ) sample stream within a monolithic fused silica chip. The chip geometry enables hydrodynamic focusing of the sample stream in 3D and sample widths as small as 5 µm. The chip is also connected to a polyimide capillary downstream to enable sample stream deceleration, expansion, and X-ray detection. In this capillary, sample widths of 50 µm are demonstrated. Further, convection–diffusion-reaction models of the mixer are presented. The models are experimentally validated using confocal epifluorescence microscopy and XAS/XES measurements of a ferricyanide and ascorbic acid reaction. The models additionally enable prediction of the residence time and residence time uncertainty of reactive species as well as mixing times. Residence times (from initiation of mixing to the point of X-ray detection) during sample stream expansion as small as 2.1 ± 0.3 ms are also demonstrated. Importantly, an exploration of the mixer operational space reveals a theoretical minimum mixing time of 0.91 ms. The proposed platform is applicable to the determination of the electronic structure of conventionally inaccessible reaction intermediates.
3D microfabrication↗
Lorentz factor for time-of-flight neutron Bragg and total scattering
We report the three fundamental origins of the Lorentz factor for neutron time-of-flight powder diffraction are revisited. A detailed derivation of the Lorentz factor is presented in the context of diffuse scattering modelling in reciprocal space when perfect periodicity is assumed, and the total scattering pattern is constructed in its discrete form – the factor in this case becomes 1/Q 2 (or d 2 ). Discussion is also presented with respect to practical data reduction where a vanadium measurement is usually taken as the normalization factor (to account for various factors such as detector efficiency), and it is shown that the existence of the Lorentz factor is independent of such a normalization process.
36 MATERIALS SCIENCE↗
Diffusion-MBIR: 3D sparse view CT reconstruction from 2D diffusion
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3D reconstruction↗
SU-D-210-03: Limited-View Multi-Source Quantitative Photoacoustic Tomography
Purpose: This work is to investigate a novel limited-view multi-source acquisition scheme for the direct and simultaneous reconstruction of optical coefficients in quantitative photoacoustic tomography (QPAT), which has potentially improved signal-to-noise ratio and reduced data acquisition time. Methods: Conventional QPAT is often considered in two steps: first to reconstruct the initial acoustic pressure from the full-view ultrasonic data after each optical illumination, and then to quantitatively reconstruct optical coefficients (e.g., absorption and scattering coefficients) from the initial acoustic pressure, using multi-source or multi-wavelength scheme.Based on a novel limited-view multi-source scheme here, We have to consider the direct reconstruction of optical coefficients from the ultrasonic data, since the initial acoustic pressure can no longer be reconstructed as an intermediate variable due to the incomplete acoustic data in the proposed limited-view scheme. In this work, based on a coupled photo-acoustic forward model combining diffusion approximation and wave equation, we develop a limited-memory Quasi-Newton method (LBFGS) for image reconstruction that utilizes the adjoint forward problem for fast computation of gradients. Furthermore, the tensor framelet sparsity is utilized to improve the image reconstruction which is solved by Alternative Direction Method of Multipliers (ADMM). Results: The simulation was performed on a modified Shepp-Logan phantom to validate the feasibility of the proposed limited-view scheme and its corresponding image reconstruction algorithms. Conclusion: A limited-view multi-source QPAT scheme is proposed, i.e., the partial-view acoustic data acquisition accompanying each optical illumination, and then the simultaneous rotations of both optical sources and ultrasonic detectors for next optical illumination. Moreover, LBFGS and ADMM algorithms are developed for the direct reconstruction of optical coefficients from the acoustic data. Jing Feng and Hao Gao were partially supported by the NSFC (#11405105), the 973 Program (#2015CB856000) and the Shanghai Pujiang Talent Program (#14PJ1404500)
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Zircon age-temperature-compositional spectra in plutonic rocks
We present that geochronology can resolve dispersed zircon dates in plutonic rocks when magma cooling time scales exceed the temporal precision of individual U-Pb analyses; such age heterogeneity may indicate protracted crystallization between the temperatures of zircon saturation (T sat ) and rock solidification (T solid ). Diffusive growth models predict asymmetric distributions of zircon dates and crystallization temperatures in a cooling magma, with volumetrically abundant old, hot crystallization at T sat decreasing continuously to volumetrically minor young, cold crystallization at T solid . We present integrated geochronological and geochemical data from Bergell Intrusion tonalites (Central Alps, Europe) that document zircon compositional change over hundreds of thousands of years at the hand-sample scale, indicating melt compositional evolution during solidification. Ti-in-zircon thermometry, crystallization simulation using MELTS software, and U-Pb dates produce zircon mass-temperature-time distributions that are in excellent agreement with zircon growth models. In conclusion, these findings provide the first quantitative validation of longstanding expectations from zircon saturation theory by direct geochronological investigation, underscoring zircon’s capacity to quantify supersolidus cooling rates in magmas and resolve dynamic differentiation histories in the plutonic rock record.
58 GEOSCIENCES↗
Analysis of Poloidal Asymmetric Density Behaviors in SOL Induced by 4.6-GHz Lower Hybrid Launcher Power in EAST
On the experimental advanced superconducting tokamak (EAST), a series of striations, including a few strong emissivity striations and several low emissivity striations, were observed in front of the 4.6-GHz lower hybrid (LH) launcher with the visible video camera for the LH power discharge. These striations indicate that LH may create significant poloidal scrape-off layer (SOL) density profile asymmetries in front of the LH launcher. These poloidal asymmetric density behaviors are further confirmed with the edge density measured by two Langmuir probes installed at the top and bottom of the LH launcher. The measured density depends on LH power injection and magnetic field direction. A 2D diffusive convective model was used to study the mechanisms of the observed striations and poloidal asymmetric density. The simulation results qualitatively match with the measured density, indicating these poloidal asymmetric effects are ascribed to the LHW-induced E{sub LH} × B{sub t} drift.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Nonisothermal Free-Molecular Flow of Gas in an Elliptic Channel with a Circular Cylindrical Element Inside
The linearized problem of free-molecular gas flow in a long elliptic channel with a circular cylindrical element inside has been solved in the kinetic approximation. The flow in the channel is caused by temperature and pressure drops between its ends. The Boltzmann kinetic equation for collisionless gas has been used as a basic equation, and the boundary condition has been set in terms of the diffuse reflection model. The distribution of the mass velocity of the gas over the cross section of the channel has been obtained. The mass flow rate of the gas in the channel versus the temperature and pressure drops between its ends has been calculated. It has been found that the mass flow of the gas substantially depends on the radius of the inner cylinder.
42 ENGINEERING↗
Avoiding Pitfalls in the Determination of Reliable Electrochemical Kinetics Parameters for the Cu 2+ → Cu 1+ Reduction Reaction in Deep Eutectic Solvents
Deep eutectic solvents (DES), on account of their low-cost, non-flammability and electrochemical stability, are attracting attention for their potential use in applications such as electrodeposition and energy storage. In these applications, knowledge of the transport and electrochemical kinetics properties of DES is critically important. To date, attempts to measure the kinetics parameters of the Cu 2+ + e ⇌ Cu 1+ reaction in ethaline DES have yielded a cathodic charge transfer coefficient (α) in the range of 0.2–0.3 suggesting an unexpected asymmetric polarization behavior. In the present work, we pursued a comprehensive study of the kinetics and transport properties of the aforementioned reaction. Using steady-state and transient polarization measurements on RDE and microelectrodes combined with diffusion-reaction modeling, we demonstrate that the Cu 2+ /Cu 1+ transition exhibits a charge transfer coefficient in the range of 0.49–0.54 and an exchange current density in the range 1.72–1.88 mA/cm 2 . Recommendations are provided for avoiding pitfalls in kinetics analysis of highly resistive DES electrolytes. These recommendations include the application of accurate $IR_Ω$ correction and the use of electrode configurations where the secondary current distribution is relatively uniform.
25 ENERGY STORAGE↗
Deep phylogenomics of a tandem-repeat galectin regulating appendicular skeletal pattern formation
Background A multiscale network of two galectins Galectin-1 (Gal-1) and Galectin-8 (Gal-8) patterns the avian limb skeleton. Among vertebrates with paired appendages, chondrichthyan fins typically have one or more cartilage plates and many repeating parallel endoskeletal elements, actinopterygian fins have more varied patterns of nodules, bars and plates, while tetrapod limbs exhibit tandem arrays of few, proximodistally increasing numbers of elements. We applied a comparative genomic and protein evolution approach to understand the origin of the galectin patterning network. Having previously observed a phylogenetic constraint on Gal-1 structure across vertebrates, we asked whether evolutionary changes of Gal-8 could have critically contributed to the origin of the tetrapod pattern. Results Translocations, duplications, and losses of Gal-8 genes in Actinopterygii established them in different genomic locations from those that the Sarcopterygii (including the tetrapods) share with chondrichthyans. The sarcopterygian Gal-8 genes acquired a potentially regulatory non-coding motif and underwent purifying selection. The actinopterygian Gal-8 genes, in contrast, did not acquire the non-coding motif and underwent positive selection. Conclusion These observations interpreted through the lens of a reaction-diffusion-adhesion model based on avian experimental findings can account for the distinct endoskeletal patterns of cartilaginous, ray-finned, and lobe-finned fishes, and the stereotypical limb skeletons of tetrapods.
Evolutionary Biology↗
FY2022 Quarter 3 Report, Tools for assessing performance project
Updates on the following two topics are covered: Diffusive Wake Model and QUIC Integration into API.
42 ENGINEERING↗
Tools for Assessing Performance (TAP) Project: FY2023 Quarter 2 Report
LANL personnel presented TAP-sponsored work at the 103 rd Annual Meeting of the American Meteorological Society, which was held in Denver, CO in January 2023. There were four different presentations related to TAP-sponsored work that were given by LANL staff and studentsat this meeting: (1) Gloeckler, T., M. Nelson, H. Shah, M. Deshler, L. Wedell, R. Linn, N. Duboc, S. Pol, P. Bieringer, A. Annunzio, B. Martin, and H. Tinnesand, 2023. Testing Validity and Limitations of a Low-Order Diffusive Wake Model. 15 th Symposium on the Urban Environment, January 8-12, 2023, Denver, CO, U.S.A. (2) Nelson, M., H. Shah, T. Gloeckler, M. Deshler, L. Wedell, A. Ortega, R. Linn, N. Duboc, P. Conry, S. Pol, and H. Tinnesand, 2023. The Isolated Building Wake Experiment. Paper 1A.1. 24 th Symposium on Boundary Layers and Turbulence, January 8-12, 2023. Denver, CO, U.S.A. (3) Shah, H., M. Nelson, S. Pol, R. Linn, N. Duboc, and H. Tinnesand, 2023. Detailed Measurement of a Building Wake Under Real-World Atmospheric Conditions. Poster 102. 24 th Symposium on Boundary Layers and Turbulence, January 8-12, 2023, Denver, CO, U.S.A. (4) Wedell, L., M. Nelson, H. Shah, T. Gloeckler, M. Deshler, R. Linn, S. Pol, H. Tinnesand, 2023. The Isolated Building Wake Experiment: Using a Rotating Building to Mulitply the Number of Wake Measurement Locations without Moving the Instruments. Paper 1A.4A. 24 th Symposium on Boundary Layers and Turbulence, January 8-12, 2023, Denver, CO, U.S.A.
54 ENVIRONMENTAL SCIENCES↗
An ab initio Driven Model for the Trapping and Diffusion of Hydrogen in Fe Cr Ni Alloys
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Thomas, Patrick R.↗
Diffusion Behavior of Oversized Fission Products in bcc Fe Cladding: A First-Principles Study
Fuel-Cladding Chemical Interaction (FCCI) poses significant challenges in nuclear reactors, where fission products from nuclear fuel interact with Fe-based cladding materials, potentially compromising their structural integrity. This study investigates the diffusion behavior of oversized fission products, Pr, Nd, Ce, and La, within bcc Fe cladding using density functional theory (DFT), nudged elastic band (NEB) method, and self-consistent mean field (SCMF) theory. Our results reveal significant long-range vacancy binding energies, particularly up to the 6th nearest neighbor, with La exhibiting the strongest binding affinity, followed by Nd, Ce, and Pr. The NEB calculations indicate significant high barriers for the dissociation of 1nn vacancy-solute pairs for all fission products. The tracer diffusion coefficients of these fission products was derived in Arrhenius form. The significant trapping effect of vacancies by a very dilute amount of fission products reduces vacancy mobility, leading to an oversaturation of point defects, void nucleation, and swelling. These are critical issues for irradiated cladding materials. The tracer diffusion coefficients indicate that Nd diffuses the fastest, followed by La, Ce, and Pr. This study provides essential insights for developing advanced cladding materials and design strategies to mitigate FCCI, ultimately enhancing nuclear reactor safety and performance.
11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗