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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 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Bubble explosion induced melt pool instabilities in electron beam melting of aluminum alloy

Electron beam melting (EBM) is an additive manufacturing technology that can process materials and manufacture components otherwise impossible or uneconomical. However, defects, including porosity and surface irregularities, are widely reported in EBM-built components, and their formation mechanisms are not fully understood. Here, using in-situ high-speed synchrotron X-ray imaging, we reveal that bubble explosions in Al6061 during EBM induce melt pool instabilities contributing to defect formation. The melt pool and keyhole evolve through three stages: (1) initial formation of a melt pool, (2) subsurface bubble formation and explosion, and (3) periodic keyhole oscillation. During scanning, periodic bubble explosions can eject molten liquid as spatters and disturb the vapor depression and melt pool, contributing to surface humping, that may trigger lack-of-fusion defects in subsequent layers. The physical insights we report could provide guidance for EBM machine development, process innovation, alloy design and model development.

36 MATERIALS SCIENCE↗
DOE OSTI2022

X-ray multiphoton-induced Coulomb explosion images complex single molecules

Following structural dynamics in real time is a fundamental goal towards a better understanding of chemical reactions. Recording snapshots of individual molecules with ultrashort exposure times is a key ingredient towards this goal, as atoms move on femtosecond (10 -15 s) timescales. For condensed-phase samples, ultrafast, atomically resolved structure determination has been demonstrated using X-ray and electron diffraction. Pioneering experiments have also started addressing gaseous samples. However, they face the problem of low target densities, low scattering cross sections and random spatial orientation of the molecules. Therefore, obtaining images of entire, isolated molecules capturing all constituents, including hydrogen atoms, remains challenging. Here we demonstrate that intense femtosecond pulses from an X-ray free-electron laser trigger rapid and complete Coulomb explosions of 2-iodopyridine and 2-iodopyrazine molecules. We obtain intriguingly clear momentum images depicting ten or eleven atoms, including all the hydrogens, and thus overcome a so-far impregnable barrier for complete Coulomb explosion imaging—its limitation on molecules consisting of three to five atoms. In combination with state-of-the-art multi-coincidence techniques and elaborate theoretical modelling, this allows tracing ultrafast hydrogen emission and obtaining information on the result of intramolecular electron rearrangement. Our work represents an important step towards imaging femtosecond chemistry via Coulomb explosion.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Investigating the electronic structure of high explosives with X-ray Raman spectroscopy

Abstract We investigate the sensitivity and potential of a synergistic experiment-theory X-ray Raman spectroscopy (XRS) methodology on revealing and following the static and dynamic electronic structure of high explosive molecular materials. We show that advanced ab-initio theoretical calculations accounting for the core-hole effect based on the Bethe-Salpeter Equation (BSE) approximation are critical for accurately predicting the shape and the energy position of the spectral features of C and N core-level spectra. Moreover, the incident X-ray dose typical XRS experiments require can induce, in certain unstable structures, a prominent radiation damage at room temperature. Upon developing a compatible cryostat module for enabling cryogenic temperatures ( $$\approx$$ ≈ 10 K) we suppress the radiation damage and enable the acquisition of reliable experimental spectra in excellent agreement with the theory. Overall, we demonstrate the high sensitivity of the recently available state-of-the-art X-ray Raman spectroscopy capabilities in characterizing the electronic structure of high explosives. At the same time, the high accuracy of the theoretical approach may enable reliable identification of intermediate structures upon rapid chemical decomposition during detonation. Considering the increasing availability of X-ray free-electron lasers, such a combined experiment-theory approach paves the way for time-resolved dynamic studies of high explosives under detonation conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Standing shock prevents propagation of sparks in supersonic explosive flows

Abstract Volcanic jet flows in explosive eruptions emit radio frequency signatures, indicative of their fluid dynamic and electrostatic conditions. The emissions originate from sparks supported by an electric field built up by the ejected charged volcanic particles. When shock-defined, low-pressure regions confine the sparks, the signatures may be limited to high-frequency content corresponding to the early components of the avalanche-streamer-leader hierarchy. Here, we image sparks and a standing shock together in a transient supersonic jet of micro-diamonds entrained in argon. Fluid dynamic and kinetic simulations of the experiment demonstrate that the observed sparks originate upstream of the standing shock. The sparks are initiated in the rarefaction region, and cut off at the shock, which would limit their radio frequency emissions to a tell-tale high-frequency regime. We show that sparks transmit an impression of the explosive flow, and open the way for novel instrumentation to diagnose currently inaccessible explosive phenomena.

58 GEOSCIENCES↗
DOE OSTI2018

Constraints on core-collapse supernova progenitors from explosion site integral field spectroscopy

Observationally, supernovae (SNe) are divided into subclasses according to their distinct characteristics. This diversity naturally reflects the diversity in the progenitor stars. It is not entirely clear, however, how different evolutionary paths leading massive stars to become an SN are governed by fundamental parameters such as progenitor initial mass and metallicity. Furthermore, this paper places constraints on progenitor initial mass and metallicity in distinct core-collapse SN subclasses through a study of the parent stellar populations at the explosion sites. Integral field spectroscopy (IFS) of 83 nearby SN explosion sites with a median distance of 18 Mpc has been collected and analysed, enabling detection and spectral extraction of the parent stellar population of SN progenitors. From the parent stellar population spectrum, the initial mass and metallicity of the coeval progenitor are derived by means of comparison to simple stellar population models and strong-line methods. Additionally, near-infrared IFS was employed to characterise the star formation history at the explosion sites. No significant metallicity differences are observed among distinct SN types. The typical progenitor mass is found to be highest for SN type Ic, followed by type Ib, then types IIb and II. Type IIn is the least associated with young stellar populations and thus massive progenitors. However, statistically significant differences in progenitor initial mass are observed only when comparing SNe IIn with other subclasses. Stripped-envelope SN progenitors with initial mass estimates lower than 25 M ⊙ are found; they are thought to be the result of binary progenitors. Confirming previous studies, these results support the notion that core-collapse SN progenitors cannot arise from single-star channels only, and both single and binary channels are at play in the production of core-collapse SNe. Near-infrared IFS suggests that multiple stellar populations with different ages may be present in some of the SN sites. As a consequence, there could be a non-negligible amount of contamination from old populations, and therefore the individual age estimates are effectively lower limits.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Molecular dynamics simulation of Coulomb explosion, melting and shock wave creation in silicon after an ionization pulse

Strong electronic stopping power of swift ions in a semiconducting or insulating substrate can lead to localized electron stripping. The subsequent repulsive interactions among charged target atoms can cause Coulomb explosion. Using molecular dynamics simulation, we simulate Coulomb explosion in silicon by introducing an ionization pulse lasting for different periods, and at different substrate temperatures. We find that the longer the pulse period, the larger the melting radius. The observation can be explained by a critical energy density model assuming that melting required thermal energy density is a constant value and the total thermal energy gained from Coulomb explosion is linearly proportional to the ionization period. Our studies also show that melting radius is larger at higher substrate temperatures. The temperature effect is explained due to a longer structural relaxation above the melting temperature at original ionization boundary due to lower heat dissipation rates. Furthermore, simulations show the formation of shock waves, created due to the compression from the melting core.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Influence of emission threshold of explosive emission cathodes on current waveform in foilless diodes

The emission threshold of explosive emission cathodes (EECs) is an important factor for beam quality. It can affect the explosive emission delay time, the plasma expansion process on the cathode surface, and even the current amplitude when the current is not fully space-charge-limited. This paper researches the influence of the emission threshold of an annular EEC on the current waveform in a foilless diode when the current is measured by a Rogowski coil. The particle-in-cell simulation which is performed under some tolerable and necessary simplifications shows that the long explosive emission delay time of high-threshold cathodes may leave an apparent peak of displacement current on the rise edge of the current waveform, and this will occur only when the electron emission starts after this peak. The experimental researches, which are performed under a diode voltage of 1 MV and a repetitive frequency of 20 Hz, demonstrate that the graphite cathode has a lower emission threshold and a longer lifetime than the stainless steel cathode according to the variation of the peak of displacement current on the rise edge of the current waveform.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Radiation-stimulated explosive evaporation and burning of hydrogen droplets in hot aerosol mixtures

We present results of analytical and numerical investigation of explosive evaporation and burning scenarios of hydrogen droplets in hydrogen/oxygen aerosols. The following two scenarios have been elucidated. The first scenario, corresponding to sufficiently large droplets, is characterized by three stages: (i) an essentially homogeneous heating of a droplet to a near-critical temperature by IR radiation from the hot gas; (ii) explosive evaporation; and (iii) burning of hydrogen cloud formed by evaporation. The second scenario, corresponding to small droplets, differs in that a droplet is heated mainly by thermal conduction from the hot gas. The heating is accompanied by evaporation which can become explosive at the final stage of evaporation. The crossover droplet size separating the two scenarios is calculated. Conservative finite-difference numerical analysis is used to explore the predicted scenarios and verify analytical estimates.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Embedded fiber Bragg grating pressure measurement during thermal ignition of a high explosive

A high-speed fiber Bragg grating based pressure-only measurement is reported for the high explosive PBXN-9 under thermal initiation conditions. During exothermic thermal runaway, an explosion rise time of 500 μs reaching a peak pressure of 660 MPa is measured. Lastly, the approach offers a direct measure pressure diagnostic useful for quantifying reaction violence for high explosive chemistry.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Explosive attractor solutions to a universal cubic delay equation

New explosive attractor solutions have been found in a universal cubic delay equation that has been studied in both the plasma and the fluid mechanics literature. Through computational simulations and analytic approximations, it is found that the oscillatory component of the explosive mode amplitude solutions are described through multi-frequency Fourier expansions with respect to a pseudo-time variable. The spectral dependence of these solutions as a function of a system parameter, ϕ, is studied. The mode amplitude that is described in the explosive regime has two main features: a well-known envelope (t0−t)−5/2, with t0 the blow-up time of the amplitude, and a spectrum of discrete oscillations with ever-increasing frequencies, which may give experimental information about the properties of a system's equilibrium.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Photodissociation of aligned CH3I and C6H3F2I molecules probed with time-resolved Coulomb explosion imaging by site-selective extreme ultraviolet ionization

We explore time-resolved Coulomb explosion induced by intense, extreme ultraviolet (XUV) femtosecond pulses from a free-electron laser as a method to image photo-induced molecular dynamics in two molecules, iodomethane and 2,6-difluoroiodobenzene. At an excitation wavelength of 267 nm, the dominant reaction pathway in both molecules is neutral dissociation via cleavage of the carbon–iodine bond. This allows investigating the influence of the molecular environment on the absorption of an intense, femtosecond XUV pulse and the subsequent Coulomb explosion process. We find that the XUV probe pulse induces local inner-shell ionization of atomic iodine in dissociating iodomethane, in contrast to non-selective ionization of all photofragments in difluoroiodobenzene. The results reveal evidence of electron transfer from methyl and phenyl moieties to a multiply charged iodine ion. In addition, indications for ultrafast charge rearrangement on the phenyl radical are found, suggesting that time-resolved Coulomb explosion imaging is sensitive to the localization of charge in extended molecules.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2017

Internal sub-sonic burning during an explosion viewed via dynamic X-ray radiography

In this study, we observe internal convective and conductive burn front propagation and solid consumption subsequent to thermal ignition for plastic bonded formulations of the solid organic secondary explosives octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine and 1,3,5-triamino-2,4,6-trinitrobenzene. This work describes x-ray radiographic diagnostics enabling the study of solid density in a fully encased explosive during internal burning subsequent to ignition. Finally, the result of this study is the ability to directly observe and measure rates of energy release during a thermal explosion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Multi-dimensional simulations of core-collapse supernova explosions with CHIMERA

Unraveling the core-collapse supernova (CCSN) mechanism is a problem that remains essentially unsolved despite more than four decades of effort. Spherically symmetric models with otherwise high physical fidelity generally fail to produce explosions, and it is widely accepted that CCSNe are inherently multi-dimensional. Progress in realistic modeling has occurred recently through the availability of petascale platforms and the increasing sophistication of supernova codes. We will discuss our most recent work on understanding neutrino-driven CCSN explosions employing multi-dimensional neutrino-radiation hydrodynamics simulations with the Chimera code. We discuss the inputs and resulting outputs from these simulations, the role of neutrino radiation transport, and the importance of multi-dimensional fluid flows in shaping the explosions. We also highlight the production of 48Ca in long-running Chimera simulations.

Messer II, Bronson↗
DOE OSTI2018

Coulomb explosion imaging of CH 3 I and CH 2 ClI photodissociation dynamics

The photodissociation dynamics of CH 3 I and CH 2 ClI at 272 nm were investigated by time-resolved Coulomb explosion imaging, with an intense non-resonant 815 nm probe pulse. Fragment ion momenta over a wide m/z range were recorded simultaneously by coupling a velocity map imaging spectrometer with a pixel imaging mass spectrometry camera. For both molecules, delay-dependent pump-probe features were assigned to ultraviolet-induced carbon-iodine bond cleavage followed by Coulomb explosion. Multi-mass imaging also allowed the sequential cleavage of both carbon-halogen bonds in CH 2 ClI to be investigated. Furthermore, delay-dependent relative fragment momenta of a pair of ions were directly determined using recoil-frame covariance analysis. These results are complementary to conventional velocity map imaging experiments and demonstrate the application of time-resolved Coulomb explosion imaging to photoinduced real-time molecular motion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Use of synchrotron-based radiography to diagnose pulsed power driven wire explosion experiments

We describe the first use of synchrotron radiation to probe pulsed power driven high energy density physics (HEDP) experiments. Multi-frame x-ray radiography with interframe spacing of 704 ns and temporal resolution of < 100 ps was used to diagnose the electrical explosion of different wire configurations in water including single copper and tungsten wires, parallel copper wire pairs and copper x-pinches. Such experiments are of great interest to a variety of areas including equation of state studies and high pressure materials research, but the optical diagnostics that are usually employed in these experiments are unable to probe areas behind the shock wave generated in the water, as well as the internal structure of the exploding material. The x-ray radiography presented here, performed at beamline ID19 at ESRF, was able to image both sides of the shock to a resolution of up to 8 µm, and phase contrast imaging (PCI) allowed fine details of the wire structure during the current driven explosion and the shock waves to be clearly observed. These results demonstrate the feasibility of pulsed power operated in conjunction with synchrotron facilities, as well as an effective technique in the study of shock waves and wire explosion dynamics.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Explosion dynamics of thin flat foils at high current density

This paper presents characteristic features of the explosion of thin flat foils for currents and pulse risetimes ranging from 8 kA at 350 ns to 1000 kA at ~100 ns. Foils made of aluminum, copper, nickel, and titanium with thicknesses of 1–100 µm are tested. Various diagnostics in the optical, UV, and x-ray spectral ranges are used to image the exploding foils from initial breakdown to complete destruction or pinching. It is shown that foil explosion is a complex process that depends on many factors, but features common to all foils are found that do not depend on the parameters of the generators or, accordingly, on the energy deposited in the foil: for example, the breakdown of flat foils under different conditions occurs at the edges of the foil. For the first time, the formation of a precursor over the central part of the foil is shown, which significantly changes the dynamics of the foil explosion.

36 MATERIALS SCIENCE↗
DOE OSTI2023

The reactive flow evolution of the polymer-bonded explosive PBX 9502: Experiments and model validation in extreme pressure regimes

The shock-to-detonation transition properties of the triaminotrinitrobenzene based PBX 9502 high explosive (HE) are experimentally and computationally explored in extremely high input pressure conditions. These include both slightly sub-Chapman–Jouguet and overdriven input pressure conditions, namely, ∼25 and ∼31 GPa, respectively. Our experiments capture the transient buildup of a shock-induced reaction via measurement of HE and polymethyl methacrylate window interface particle velocity profiles for a variety of sample thicknesses for this insensitive HE. These observations necessitate extremely thin explosive samples, and the high rates of reaction provide a considerable challenge to optical diagnostics. Samples at these thicknesses also provide an opportunity for evaluation of potential micro-structure effects on the resulting shock-to-detonation-transition measurements. To address this, the thin samples are also characterized via x-ray micro-computed tomography. Finally, a pair of previously established continuum-level detonation performance modeling approaches for PBX 9502 were used to analyze the experiments. The employed model variants crucially differ in their definition of each model’s empirical reaction rate functional form, utilization of shock state quantities, and local flow variable dependencies. As a result, the present experiments provide a novel platform to evaluate the quantitative and qualitative consequences stemming from these modeling choices in a challenging initiation scenario, largely beyond the chosen calibration range of either model. This new experimental information will provide a platform for both improved physics and model parameterizations for this well-studied explosive.

36 MATERIALS SCIENCE↗
DOE OSTI2022

High temperature and pressure regime soot: Physical, optical and chemical signatures from high explosive detonations

Submicron particles are formed during the detonation and deflagration of carbon-rich energetic materials known as high explosives (HE) that can be lofted and transported long distances in the atmosphere. Particles include refractory species like soot, a.k.a. black carbon (BC), and have impacts to climate and human health at local, regional, and global scales. Here, we contrast detonation soot aerosol physical, optical and chemical properties that are formed more efficiently in terms of emission factors and ratios with traditional combustion sources from biomass burning and fossil fuels. We utilize ambient aerosol techniques for the first time to identify soot from two well-known HE's (1) Comp B-3: 40% trinitrotoluene (TNT) and 60% 1,3,5-trinitro-1,3,5-triazinane (RDX) and (2) PBX 9501: 95% 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (HMX) with 2.5% Estane and a 2.5% mixture of bis(2,2-dinitropropyl)-acetal (BDNPA) and bis(2,2-dinitropropyl)-formal (BDNPF). Size distributions, optical and chemical properties are presented. Aggregate soot had small mobility diameters (<100 nm) and unique optical and chemical signatures depending on the explosive composite composition, detonation atmosphere (e.g., air, argon) and pressure. Single-scattering albedos (SSA) from 0.3 to 0.8 and absorption Angstrom exponents (AAE) from 0.5 to 2.0 measured across 405–870 nm wavelengths depended on the explosive composite composition and detonation conditions. Comp B-3 detonation soot was similar to other combustion soot sources, whereas PBX 9501 soot had high scattering relative to absorption. Carbonaceous chemical signatures included the presence of fullerenes that were distinct from ambient and laboratory-generated soot surrogates and the detection of trace metals, including aluminum, copper, iron, and zinc.

36 MATERIALS SCIENCE↗