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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,369 records · Page 76
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Ultrafast photoinduced charge carrier dynamics of L-cysteine and oleylamine stabilized CsPbBr3 perovskite quantum dots coupled with gold nanoparticles

We have synthesized L-cysteine and oleylamine stabilized CsPbBr3 perovskite quantum dots (PQDs) and coupled them with gold nanoparticles (AuNPs). The PQDs and AuNPs, as well as their hybrid nanostructures (HNS), were characterized using UV–visible (UV–vis) and photoluminescence (PL) spectroscopy. The UV–vis spectra show absorption bands of the HNS at 503 and 520 nm, attributed mainly to PQDs and AuNPs, respectively. The PQDs show a strong excitonic PL band peaked at 513 nm from PQDs. The HR-TEM results show the formation of hybrid structures between PQDs and AuNPs, which is also supported by the PL quenching of the PQDs by the coupled AuNPs. Ultrafast dynamics of the exciton and charge carriers in the HNS and pristine PQD were studied using femtosecond transient absorption. Multiexponential fitting of the dynamic data revealed the existence of shallow and deep trap states in pristine PQDs and ultrafast electron transfer from PQDs to AuNPs in the HNS. A kinetic model was proposed to account for the key dynamic processes involved and to extract the time for electron transfer from PQDs to AuNPs in the HNS, found to be ∼2 ps. Dynamic processes in pristine PQDs are largely unchanged by HNS formation with AuNPs.

Chemistry↗
DOE OSTI2025

Characterization of laser-accelerated proton beams from a 0.5 kJ sub-picosecond laser for radiography applications

Laser-accelerated ion beams show promise for many applications, including high-resolution flash imaging of static or dynamic objects in next-generation radiography to probe materials and plasmas in extreme environments and inertial confinement fusion. To scale up ion beam production for radiography applications, we conducted experiments using sub-picosecond lasers up to 0.5 kJ at the OMEGA-EP facility to characterize proton beams from solid targets, primarily CH/CD sub-micron thin films from which ion beams were also used for static and dynamic radiography for the first time. For standalone sub-micron thin CH films, the highest detected proton energy is in the range of 72–97 MeV. Proton beams with highest energy near or above 60 MeV at full laser energy and similar beam profiles are also measured from low-density CD foams and flat CH foil target of micrometer-scale thickness. The ~ 700–800 nm CH/CD foils achieve the highest ion yield among the targets tested. For sub-micron thin films, the laser prepulse can expand the target and lead to complex interactions, which is simulated using coupled hydrodynamic and two-step kinetic models. Simulations suggest the presence of a micrometer-scale preplasma plateau with near-critical density and further indicate that target normal sheath acceleration, electron heating from Relativistic transparency in the preplasma plateau, and background proton reflection from carbon ion front at the rear side contribute to the resulting proton spectrum from these sub-micron thin targets at various stages. These proton beams show strong potential for radiography and for production of secondary sources.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Negative Cooperativity in the Nitrogenase Fe Protein Electron Delivery Cycle

Nitrogenase catalyzes the ATP-dependent reduction of dinitrogen (N2) to two ammonia (NH3) molecules. Catalysis involves the participation of two protein components designated the MoFe and Fe proteins. Electron transfer (ET) from the Fe protein to the catalytic MoFe protein involves a series of synchronized events requiring the transient association of one Fe protein with each aß half of the MoFe protein. This process is referred to as the Fe protein cycle and includes: binding of two ATP to an Fe protein, association of an Fe protein with the MoFe protein, ET from the Fe protein to the MoFe protein, hydrolysis of two ATP to two ADP and two Pi for each ET, Pi release, and dissociation of oxidized Fe protein-(ADP)2 from the MoFe protein. Because the MoFe protein tetramer has two separate aß active units, it has the capacity to participate in two independent Fe protein cycles. The present work examines whether the two catalytic halves of an individual MoFe protein function independently or, instead, communicate with each other during catalysis. Kinetic analyses of ET, ATP hydrolysis, and Pi release during the pre-steady state phase indicate that the two halves of the complex in fact cooperate. A global fit of the data is consistent with a two-branch negative-cooperativity kinetic model. A possible mechanism for communication between the two halves of nitrogenase is suggested by normal-mode calculations suggesting correlated and anti-correlated motions between the two nitrogenase halves.

Danyal, Karamatullah↗
DOE OSTI2019

Derivation of the Two-Exponential Probability Density Function for Rossi-Alpha Measurements of Reflected Assemblies and Validation for the Special Case of Shielded Measurements

Rossi-alpha measurements of fissionable assemblies are used to estimate the prompt neutron decay constant, α, of a system, which can be used to infer reactivity. Previous works have demonstrated that two-exponential fits for Rossi-alpha measurements of reflected assemblies are better than one exponential fits; however, the two-exponential probability density function that is needed to obtain α from the fit parameters has not been derived. This paper derives the two-region point kinetics model (two-exponential fit) for Rossi-alpha measurements, a generalization of the current, one region model (one-exponential fit). The new model is validated with measurements of reflected, fissionable assemblies (highly enriched uranium with k eff ≈ 0.95 and weapons-grade plutonium with k eff > 0.77) with α held constant. Here, the results show that the twoexponential model can (i) predict the constant α, and (ii) deconvolve α and the time a neutron spends in the reflector, neither of which is possible with the one-exponential model.

42 ENGINEERING↗
DOE OSTI2020

Electrochemical characterization of the plasma-water interface

Nonthermal atmospheric pressure plasmas generated using noble gases act as gaseous electrodes capable of promoting electrochemical reactions when in contact with aqueous media. Despite overwhelming evidence in favor of the interfacial region between the plasma and solution being a reducing environment, the plasma-liquid interface has not been evaluated from an electrochemical perspective. Herein, we present an electrochemical method to measure the overpotential at the plasma-liquid interface of solutions containing organic redox couples. Negative overpotentials indicate that the interfacial region is a reducing environment, which is consistent with the results of colorimetric assays using redox indicators. The reaction rates are related to the magnitude of the overpotential, as expected from electrochemical kinetic models. The ability to characterize the overpotential of plasma-liquid interfaces adds to the plasma electrochemistry toolkit and contributes to efforts being made to tune gaseous electrodes for the rational control of redox reactions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

NO PLIF flow visualization and time-resolved temperature distributions in laser induced breakdown plumes

NO planar laser induced fluorescence (PLIF) is used to obtain images of laser-induced breakdown plasma plumes in NO-seeded nitrogen and dry air at near atmospheric pressure. Single-shot PLIF-images show that the plume development 5–50 μs after the breakdown pulse is fairly reproducible shot-to-shot, although the plume becomes increasingly stochastic on longer timescales, 100–500 μs. The stochastic behavior of the plume is quantified using probability distributions of the loci of the plume boundary. Analysis of the single-shot images indicates that the mixing of the plume with ambient gas on sub-ms time scale is insignificant. The induced flow velocity in the plume is fairly low, up to 30 m s –1 , suggesting that laser breakdowns are ineffective for mixing enhancement in high speed flows. The ensemble-averaged PLIF images indicate the evolution of the plume from an initially elongated shape to near-spherical to toroidal shape, with a subsequent radial expansion and formation of an axial jet in the center. Temperature distributions in the plume in air are obtained from the NO PLIF images, using two rotational transitions in the NO(X, v' = 0 → A, v'' = 0) band, J'' = 6.5 and 12.5 of the Q R 12 + Q 2 branch. The results indicate that the temperature in the plume remains high, above 1000 K, for approximately 100 μs, after which it decays gradually, to below 500 K at 500 μs. The residual NO fraction in the plume is ~0.1%, indicating that repetitive laser-assisted ignition may result in significant NO-generation. Furthermore, these measured temperature and velocity distributions can be used for detailed validation of kinetic models of laser-induced breakdown and assessment of their predictive capability.

42 ENGINEERING↗
DOE OSTI2022

Mixed stochastic-deterministic time-dependent density functional theory: application to stopping power of warm dense carbon

Warm dense matter (WDM) describes an intermediate phase, between condensed matter and classical plasmas, found in natural and man-made systems. In a laboratory setting, WDM is often created dynamically. It is typically laser or pulse-power generated and can be difficult to characterize experimentally. Measuring the energy loss of high energy ions, caused by a WDM target, is both a promising diagnostic and of fundamental importance to inertial confinement fusion research. However, electron coupling, degeneracy, and quantum effects limit the accuracy of easily calculable kinetic models for stopping power, while high temperatures make the traditional tools of condensed matter, e.g. time-dependent density functional theory (TD-DFT), often intractable. In this study, we have developed a mixed stochastic-deterministic approach to TD-DFT which provides more efficient computation while maintaining the required precision for model discrimination. Recently, this approach showed significant improvement compared to models when compared to experimental energy loss measurements in WDM carbon. Here, we describe this approach and demonstrate its application to warm dense carbon stopping across a range of projectile velocities. We compare direct stopping-power calculation to approaches based on combining homogeneous electron gas response with bound electrons, with parameters extracted from our TD-DFT calculations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Nonequilibrium plasma in water vapor

A kinetic model of a non-equilibrium water vapor plasma created by a nanosecond pulsed discharge at elevated temperatures and at low pressure is presented. The model includes equations for electron, vibrational and gas temperatures combined with equations for neutral, charged, electronically and vibrationally excited species. Simulation results are compared with the available experimental data and show good agreement in dynamics of the plasma decay and electron drift velocities. It is shown that the plasma decay in pure water vapor is controlled by the dissociative recombination. Under the conditions simulated, the concentration of electronically excited species is low, but we observe a formation of vibrationally excited species. Finally, a significant decrease in the electrical conductivity of water plasma with time, which depends on the level of initial ionization, is shown.

42 ENGINEERING↗
DOE OSTI2023

2D-imaging of absolute OH and H 2 O 2 profiles in a He–H 2 O nanosecond pulsed dielectric barrier discharge by photo-fragmentation laser-induced fluorescence

We report pulsed dielectric barrier discharges (DBD) in He–H 2 O and He–H 2 O–O 2 mixtures are studied in near atmospheric conditions using temporally and spatially resolved quantitative 2D imaging of the hydroxyl radical (OH) and hydrogen peroxide (H 2 O 2 ). The primary goal was to detect and quantify the production of these strongly oxidative species in water-laden helium discharges in a DBD jet configuration, which is of interest for biomedical applications such as disinfection of surfaces and treatment of biological samples. Hydroxyl profiles are obtained by laser-induced fluorescence (LIF) measurements using 282 nm laser excitation. Hydrogen peroxide profiles are measured by photo-fragmentation LIF (PF-LIF), which involves photo-dissociating H 2 O 2 into OH with a 212.8 nm laser sheet and detecting the OH fragments by LIF. The H 2 O 2 profiles are calibrated by measuring PF-LIF profiles in a reference mixture of He seeded with a known amount of H 2 O 2 . OH profiles are calibrated by measuring OH-radical decay times and comparing these with predictions from a chemical kinetics model. Two different burst discharge modes with five and ten pulses per burst are studied, both with a burst repetition rate of 50 Hz. In both cases, dynamics of OH and H 2 O 2 distributions in the afterglow of the discharge are investigated. Gas temperatures determined from the OH-LIF spectra indicate that gas heating due to the plasma is insignificant. The addition of 5% O 2 in the He admixture decreases the OH densities and increases the H 2 O 2 densities. The increased coupled energy in the ten-pulse discharge increases OH and H 2 O 2 mole fractions, except for the H 2 O 2 in the He–H 2 O–O 2 mixture which is relatively insensitive to the additional pulses.

hydrogen peroxide↗
DOE OSTI2023

Rotating spokes, potential hump and modulated ionization in radio frequency magnetron discharges

Here, in this work, the transition from the gradient drift instability (GDI) into an m = 1 rotating spoke in the radio frequency magnetron discharge was studied by means of the two-dimensional axial-azimuthal (z–y) particle-in-cell/Monte Carlo collision method. The kinetic model combined with the linear analysis of the perturbation revealed that the cathode sheath (axial) electric field E z triggers the GDI, deforming the local potential until the instability condition is not fulfilled and the fluctuation growth stops in which moment the instability becomes saturated. The potential deformation consequently leads to the formation of the potential hump, surrounding which the azimuthal electric field E y is present. The saturation level of E y was found to be synchronized with and proportional to the time-changing voltage applied at the cathode, resulting in the RF-modulation of the electron heating in the E y due to $\nabla B$ drift. In the saturated stage of the instability, it was shown that the rotation velocity and direction of the spoke present in the simulations agree well with the experimental observation (Panjan 2019 J. Appl. Phys.125 203303). In the instability linear stage, the instability mode wavelength and the growth rate were also found to be in good agreement with the prediction of the GDI linear fluid theory.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Spatio-temporal electric field distributions in an atmospheric plasma jet impinging on a microchannel array surface

The electric field distribution in the ionization waves (IWs) propagating over a microchannel array dielectric surface, with the channels either empty or filled with distilled water, is measured by ps electric field induced second harmonic generation. The surface IW is initiated by the atmospheric pressure N 2 –Ar plasma jet impinging on the surface vertically and powered by ns pulse discharge bursts. The results show that the electric field inside the microchannels, specifically its horizontal component, is enhanced by up to a factor of 2. The field enhancement region is localized within the channels. The vertical electric field inside the channels lags in time compared to the field measured at the ridges, indicating the transient reversal of the IW propagation direction across the channels (toward the jet). This is consistent with the phase-locked plasma emission images and confirmed by the kinetic modeling predictions, which show that the IW ‘jumps’ over the empty channels and propagates into the channels only after the jump between the adjacent ridges. When the channels are filled with water, the wave speed increases by up to 50%, due to the higher effective dielectric constant of the surface. No evidence of a significant electric field enhancement near the dielectric surface (ceramic or water) has been detected, within the spatial resolution of the present diagnostic, ~100 μ m.

plasmas and liquids↗
DOE OSTI2017

The effect of Electron Cyclotron Heating on density fluctuations at ion and electron scales in ITER Baseline Scenario discharges on the DIII-D tokamak

Experiments simulating the ITER Baseline Scenario on the DIII-D tokamak show that torque-free pure electron heating, when coupled to plasmas subject to a net co-current beam torque, affects density fluctuations at electron scales on a sub-confinement time scale, whereas fluctuations at ion scales change only after profiles have evolved to a new stationary state. Modifications to the density fluctuations measured by the Phase Contrast Imaging diagnostic (PCI) are assessed by analyzing the time evolution following the switch-off of Electron Cyclotron Heating (ECH), thus going from mixed beam/ECH to pure neutral beam heating at fixed β N . Within 20 ms after turning off ECH, the intensity of fluctuations is observed to increase at frequencies higher than 200 kHz; in contrast, fluctuations at lower frequency are seen to decrease in intensity on a longer time scale, after other equilibrium quantities have evolved. Non-linear gyro-kinetic modeling at ion and electron scales scales suggest that, while the low frequency response of the diagnostic is consistent with the dominant ITG modes being weakened by the slow-time increase in flow shear, the high frequency response is due to prompt changes to the electron temperature profile that enhance electron modes and generate a larger heat flux and an inward particle pinch. Furthermore, these results suggest that electron heated regimes in ITER will feature multi-scale fluctuations that might affect fusion performance via modifications to profiles.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Rate constants for the formation of SiO by radiative association

Accurate molecular data for the low-lying states of SiO are computed and used here to calculate rate constants for radiative association (RA) of Si and O. Einstein A-coefficients are also calculated for transitions between all of the bound and quasi-bound levels for each molecular state. The radiative widths are used together with elastic tunnelling widths to define effective RA rate constants which include both direct and indirect (inverse pre-dissociation) formation processes. The indirect process is evaluated for two kinetic models which represent limiting cases for astrophysical environments. The first case scenario assumes an equilibrium distribution of quasi-bound states and would be applicable whenever collisional and/or radiative excitation mechanisms are able to maintain the population. The second case scenario assumes that no excitation mechanisms are available which corresponds to the limit of zero radiation temperature and zero atomic density. Rate constants for SiO formation in realistic astrophysical environments would presumably lie between these two limiting cases.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

A broad-host-range event detector: expanding and quantifying performance between Escherichia coli and Pseudomonas species

Modern microbial biodesign relies on the principle that well-characterized genetic parts can be reused and reconfigured for different functions. However, this paradigm has only been successful in a limited set of hosts, mostly comprised from common lab strains of Escherichia coli . It is clear that new applications such as chemical sensing and event logging in complex environments will benefit from new host chassis. This study quantitatively compared how the same chemical event logger performed across four strains and three different microbial species. An integrase-based sensor and memory device was operated by two representative soil Pseudomonads— Pseudomonas fluorescens SBW25 and Pseudomonas putida DSM 291. Quantitative comparisons were made between these two non-traditional hosts and two benchmark E. coli chassis including the probiotic Nissle 1917 and common cloning strain DH5α. The performance of sensor and memory components changed according to each host, such that a clear chassis effect was observed and quantified. These results were obtained via fluorescence from reporter proteins that were transcriptionally fused to the integrase and downstream recombinant region and via data-driven kinetic models. The Pseudomonads proved to be acceptable chassis for the operation of this event logger, which outperformed the common E. coli DH5α in many ways. This study advances an emerging frontier in synthetic biology that aims to build broad-host-range devices and understand the context by which different species can execute programmable genetic operations.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2025

Implementation of Stochastic Gradient Descent in an Automated Glow Peak Identification Software for Multiple Thermoluminescent Dosimeter Types

A glow-curve analysis code was previously developed in C++ to analyze thermoluminescent dosimeter glow curves using automated peak detection while applying a first-order kinetics model. A newer version of this code was implemented to improve the automated peak detection and curve fitting models. The Stochastic Gradient Descent Algorithm was introduced to replace the prior approach of taking first and second-order derivatives for peak detection. Additionally, early stopping mechanisms were invoked to improve the previously used Levenberg-Marquardt Algorithm employed for curve fitting. The two software versions were compared through glow curve analysis of different thermoluminescent dosimeter materials and calculation of the corresponding figures of merit. Altogether improvements were shown, namely an increase in the number of peaks detected and a reduction of the mean figure of merit by approximately 46%.

137Cs↗
DOE OSTI2022

Phase transitions in high-purity zirconium under dynamic compression

In this work, we present results from ramp compression experiments on high-purity Zr that show the α → ω, ω → β, as well as reverse β → ω phase transitions. Simulations with a multi-phase equation of state and phenomenological kinetic model match the experimental wave profiles well. While the dynamic α ! ! transition occurs ~ 9 GPa above the equilibrium phase boundary, the ω → β transition occurs within 0.9 GPa of equilibrium. We estimate that the dynamic compression path intersects the equilibrium ω – β line at P = 29:2 GPa, and T = 490 K. The thermodynamic path in the interior of the sample lies ~ 100 K above the isentrope at the point of the ω → β transition. Approximately half of this dissipative temperature rise is due to plastic work, and half is due to the non-equilibrium α → ω transition. The inferred rate of the α → ω transition is several orders of magnitude higher than that measured in dynamic diamond anvil cell (DDAC) experiments in an overlapping pressure range. We discuss a model for the influence of shear stress on the nucleation rate. We find that the shear stress s ji has the same effect on the nucleation rate as a pressure increase δP = cϵ ij s ji /(ΔV/V); where c is a geometric constant ~ 1 and, ϵ ij are the transformation shear strains. The small fractional volume change ΔV/V ≈ 0:1 at the α → ω transition amplifies the effect of shear stress, and we estimate that for this case δP is in the range of several GPa. Correcting our transition rate to a hydrostatic rate brings it approximately into line with the DDAC results, suggesting that shear stress plays a significant role in the transformation rate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Energy transfer dynamics in strongly inhomogeneous hot-dense-matter systems

Direct measurements of energy transfer across steep density and temperature gradients in a hot-dense-matter system are presented. Hot dense plasma conditions were generated by high-intensity laser irradiation of a thin-foil target containing a buried metal layer. Energy transfer to the layer was measured using picosecond time-resolved x-ray emission spectroscopy. Here, the data show two x-ray flashes in time. Fully explicit, coupled particle-in-cell and collisional-radiative atomic kinetics model predictions reproduce these observations, connecting the two x-ray flashes with staged radial energy transfer within the target.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Universal molecular-kinetic scaling relation for slip of a simple fluid at a solid boundary

Using the observation that slip in simple fluids at low and moderate shear rates is a thermally activated process driven by the shear stress in the fluid close to the solid boundary, we develop a molecular-kinetic model for simple fluid slip at solid boundaries. Here, the proposed model, which is in the form of a universal scaling relation that connects slip and shear rate, reduces to the well-known Navier slip condition under low shear conditions, providing a direct connection between molecular parameters and the slip length. Molecular-dynamics simulations are in very good agreement with the predicted dependence of slip on system parameters, including the temperature and fluid-solid interaction strength. Connections between our model and previous work, as well as simulation and experimental results, are explored and discussed.

42 ENGINEERING↗