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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

On The Deflagration to Detonation Transition in 1.65 g cm -3 Pentaerythritol Tetranitrate

The LANL Weapons Response Group is charged with making determinations for a wide range of scenarios regarding the probability that inadvertent ignition of the high explosive (HE) components might cause nuclear yield, or dispersal of special nuclear material, during assembly/disassembly operations at the Pantex Plant. These scenarios are categorized based on the nature of the ignition causing stimulus, i.e. impact, electrostatic discharge, pressure-shear, etc. However, for all of these scenarios, regardless of how ignition is achieved, there must also be a credible path for the burning HE to transition to detonation for yield or dispersal to occur. If it can be determined that there is no feasible path for deflagration-to-detonation transition (DDT), the scenario space that requires mitigative action is greatly reduced. The strategy to support Weapons Response determinations is to use experimentation and observation to evaluate HE response in severe “overtest” configurations. To increase with confidence in the margins of HE response, these overtests are designed, by controlling parameters known to enhance HE response violence, to be more severe than what is ever encountered in the actual operations environment at the Plant. This study is focused on DDT in pentaerythritol tetranitrate (PETN), a high explosive that is used in LANL’s detonators. PETN is known to be able to DDT at certain densities, and this represents a safety concern. The PETN in LANL’s detonators is commonly a density of 1.65 g cm -3 . However, DDT has never been observed at this density in PETN. In this work, we studied the DDT reaction in 1.65 g cm -3 PETN to determine whether DDT is possible at this density, and if so, what length of PETN is required for the transition to occur. In any of the tests conducted, we did not observe DDT in 1.65 g cm -3 PETN. It is possible that the DDT length for 1.65 g cm -3 PETN is greater than 6” (the available run length in the experiments), but this length is simply not available in a detonator, and is therefore not a concern.

45 MILITARY TECHNOLOGY, WEAPONRY, AND NATIONAL DEF↗
DOE OSTI2020

Study of the ${{^{25}}}$Mg(d,p)${{^{26}}}$Mg reaction to constrain the ${{^{25}}}$Al(p,${\gamma }$)${{^{26}}}$Si resonant reaction rates in nova burning conditions

The rate of the 25 Al( p,γ) 26 Si reaction is one of the few key remaining nuclear uncertainties required for predicting the production of the cosmic γ-ray emitter 26 Al in explosive burning in novae. This reaction rate is dominated by three key resonances (J π = 0 + , 1 + and 3 + ) in 26 Si. Only the 3 + resonance strength has been directly constrained by experiment. A high resolution measurement of the 25 Mg(d, p) reaction was used to determine spectroscopic factors for analog states in the mirror nucleus, 26 Mg. A first spectroscopic factor value is reported for the 0 + state at 6.256 MeV, and a strict upper limit is set on the value for the 1 + state at 5.691 MeV, that is incompatible with an earlier ( 4 He, 3 He) study. These results are used to estimate proton partial widths, and resonance strengths of analog states in 26 Si contributing to the 25 Al( p,γ) 26 Si reaction rate in nova burning conditions.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2013

TAKING THE ''UN'' OUT OF ''UNNOVAE''

It has long been expected that some massive stars produce stellar mass black holes (BHs) upon death. Unfortunately, the observational signature of such events has been unclear. It has even been suggested that the result may be an ''unnova,'' in which the formation of a BH is marked by the disappearance of a star rather than an electromagnetic outburst. I argue that when the progenitor is a red supergiant, evidence for BH creation may instead be a Almost-Equal-To 3-10 day optical transient with a peak luminosity of Almost-Equal-To 10{sup 40}-10{sup 41} erg s{sup -1}, a temperature of Almost-Equal-To 10{sup 4} K, slow ejection speeds of Almost-Equal-To 200 km s{sup -1}, and a spectrum devoid of the nucleosynthetic products associated with explosive burning. This signal is the breakout of a shock generated by the hydrodynamic response of a massive stellar envelope when the protoneutron star loses {approx}few Multiplication-Sign 0.1 M{sub Sun} to neutrino emission prior to collapse to a BH. Current and future wide-field, high-cadence optical surveys make this an ideal time to discover and study these events. Motivated by the unique parameter space probed by this scenario, I discuss more broadly the range of properties expected for shock breakout flashes, with an emphasis on progenitors with large radii and/or small shock energies. This may have application in a wider diversity of explosive events, from pair instability supernovae to newly discovered but yet to be understood transients.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Laser propagation in a subcritical foam: Ion and electron heating

We develop a model for laser propagation and heating in a subcritical foam (homogeneous electron density as a fraction of critical n e,0 /n c <1). Our model describes the partition of energy between ions and electrons in an expanding foam element irradiated by a laser, and we solve for the plasma conditions generated by burning down the foam microstructure. We find that a substantial fraction of laser energy goes into breaking down and homogenizing the foam microstructure, which slows down the laser heat front. We also find that the ion temperature in a plasma generated by burning down a foam can exceed the electron temperature. This is because laser energy is deposited into the expansion kinetic energy of ions as the foam microstructure burns down explosively. The higher ion temperature increases ion-acoustic wave damping which reduces stimulated Brillouin scattering (SBS). We test our model against data from an experiment that shot a subcritical foam with a high intensity laser pulse on the Janus laser facility at LLNL. We find that by modeling the effect of the foam microstructure, we can explain both the experimentally measured laser propagation velocity and the backscattered SBS power.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Freely Expanding Knots of X-Ray-emitting Ejecta in Kepler’s Supernova Remnant

We report measurements of proper motion, radial velocity, and elemental composition for 14 compact X-ray-bright knots in Kepler’s supernova remnant (SNR) using archival Chandra data. The knots with the highest speed show both large proper motions ( μ ∼ 0.″11–0.″14 yr{sup −1}) and high radial velocities ( v ∼ 8700–10,020 km s{sup −1}). For these knots the estimated space velocities (9100 km s{sup −1} ≲ v {sub 3D} ≲ 10,400 km s{sup −1}) are similar to the typical Si velocity seen in supernovae (SNe) Ia near maximum light. High-speed ejecta knots appear only in specific locations and are morphologically and kinematically distinct from the rest of the ejecta. The proper motions of five knots extrapolate back over the age of Kepler’s SNR to a consistent central position. This new kinematic center agrees well with previous determinations, but is less subject to systematic errors and denotes a location about which several prominent structures in the remnant display a high degree of symmetry. These five knots are expanding at close to the free expansion rate (expansion indices of 0.75 ≲ m ≲ 1.0), which we argue indicates either that they were formed in the explosion with a high density contrast (more than 100 times the ambient density) or that they have propagated through regions of relatively low density ( n {sub H} < 0.1 cm{sup −3}) in the ambient medium. X-ray spectral analysis shows that the undecelerated knots have high Si and S abundances, a lower Fe abundance, and very low O abundance, pointing to an origin in the partial Si-burning zone, which occurs in the outer layer of the exploding white dwarf for models of SNe Ia. Other knots show lower speeds and expansion indices consistent with decelerated ejecta knots or features in the ambient medium overrun by the forward shock. Our new accurate location for the explosion site has well-defined positional uncertainties, allowing for a great reduction in the area to be searched for faint surviving donor stars under non-traditional single-degenerate SNe Ia scenarios; because of the lack of bright stars in the search area the traditional scenario remains ruled out.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Thermonuclear supernovae: probing magnetic fields by positrons and late-time IR line profiles

We show the importance of γ and positron transport for the formation of late-time spectra in Type Ia supernovae (SNe Ia). The goal is to study the imprint of magnetic fields (B) on late-time IR line profiles, particularly the [Fe II] feature at 1.644 μm, which becomes prominent two to three months after the explosion. As a benchmark, we use the explosion of a Chandrasekhar mass (M {sub Ch}) white dwarf (WD) and, specifically, a delayed detonation model that can reproduce the light curves and spectra for a Branch-normal SN Ia. We assume WDs with initial magnetic surface fields between 1 and 10{sup 9} G. We discuss large-scale dipole and small-scale magnetic fields. We show that positron transport effects must be taken into account for the interpretation of emission features starting at about one to two years after maximum light, depending on the size of B. The [Fe II] line profile and its evolution with time can be understood in terms of the overall energy input by radioactive decay and the transition from a γ-ray to a positron-dominated regime. We find that the [Fe II] line at 1.644 μm can be used to analyze the overall chemical and density structure of the exploding WD up to day 200 without considering B. At later times, positron transport and magnetic field effects become important. After about day 300, the line profile allows one to probe the size of the B-field. The profile becomes sensitive to the morphology of B at about day 500. In the presence of a large-scale dipole field, a broad line is produced in M {sub Ch} mass explosions that may appear flat-topped or rounded depending on the inclination at which the SN is observed. Small or no directional dependence of the spectra is found for small-scale B. We note that narrow-line profiles require central {sup 56}Ni as shown in our previous studies. Persistent broad-line, flat-topped profiles require high-density burning, which is the signature of a WD close to M {sub Ch}. Good time coverage is required to separate the effects of optical depth, the size and morphology of B, and the aspect angle of the observer. The spectra require a resolution of about 500 km s{sup –1} and a signal-to-noise ratio of about 20%. Two other strong near-IR spectral features at about 1.5 and 1.8 μm are used to demonstrate the importance of line blending, which may invalidate a kinematic interpretation of emission lines. Flat-topped line profiles between 300 and 400 days have been observed and reported in the literature. They lend support for M {sub Ch} mass explosions in at least some cases and require magnetic fields equal to or in excess of 10{sup 6} G. We briefly discuss the effects of the size and morphology of B on light curves, as well as limitations. We argue that line profiles are a more direct measurement of B than light curves because they measure both the distribution of {sup 56}Ni and the redistribution of the energy input by positrons rather than the total energy input. Finally, we discuss possible mechanisms for the formation of high B-fields and the limitations of our analysis.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1980

Synthesis of Al-26 in explosive hydrogen burning

The possibility of Al-26 synthesis during the explosive processing of hydrogen-rich material in the outer layers of a supernova or in nova envelopes is investigated. It is found that in the peak temperature range of 1-3 x 10 to the 8th deg K and for expansion time scales of the order of 1-1000 s, values of (Al-26)/(Al-27) as high as 0.1-1 can be obtained for values of 0.001-100,000 for the product of the peak density in g/cu cm and the proton mass fraction. Such a level of Al-26 production is considerably higher than that of recent carbon/neon burning nucleosynthesis models, and is sufficient to account for the magnesium anomalies detected in certain inclusions from the Leoville and Allende meteorites. Al abundances resulting from a hydrodynamical calculation performed on the grounds of a 25 solar mass presupernova model are also presented, and the influence of uncertainties in the input physics is discussed; in particular, the rates of the (Mg-25)(p,gamma)(Al-26) and (Al-26)(p,gamma)(Si-27) key reactions are studied.

Arnould, M.↗
DOE OSTI2022

Ignition of Carbon Burning from Nuclear Fission in Compact Stars

Abstract Type Ia supernovae (SN Ia) are powerful stellar explosions that provide important distance indicators in cosmology. Recently, we proposed a new SN Ia mechanism that involves a nuclear fission chain reaction in an isolated white dwarf (WD). The first solids that form as a WD starts to freeze are actinide rich and potentially support a fission chain reaction. In this Letter, we explore thermonuclear ignition from fission heating. We perform thermal diffusion simulations and find at high densities, above about 7 × 10 8 g cm −3 , that fission heating can ignite carbon burning. This could produce an SN Ia or another kind of astrophysical transient.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Development and Execution of a Large-scale DDT Tube Test for IHE Material Qualification

Insensitive High Explosive (IHE) Materials are defined in Chapter IX of the DOE Explosive Safety Standard (DOE-STD-1212-2012) as being materials that are massdetonable explosives that are so insensitive that the probability of accidental initiation or transition from burning to detonation is negligible1. There are currently a number of tests included in the standard that are required to qualify a material as IHE, however, none of the tests directly evaluate for the transition from burning to detonation (aka deflagration-to-detonation transition, DDT). Currently, there is a DOE complex-wide effort to revisit the IHE definition in DOE-STD-1212-2012 and change the qualification requirements. The proposal lays out a new approach, requiring fewer, but more appropriate tests, for IHE Material qualification. One of these new tests is the Deflagration-to-Detonation Test. According to the redefinition proposal, the purpose of the new deflagration-todetonation test is “to demonstrate that an IHE material will not undergo deflagration-to-detonation under stockpile relevant conditions of scale, confinement, and material condition. Inherent in this test design is the assumption that ignition does occur, with onset of deflagration. The test design will incorporate large margins and replicates to account for the stochastic nature of DDT events.” In short, the philosophy behind this approach is that if a material fails to undergo DDT in a significant over-test, then it is extremely unlikely to do so in realistic conditions. This effort will be valuable for the B61 LEP to satisfy their need qualify the new production lots of PBX 9502. The work described in this report is intended as a preliminary investigation to support the proposed design of an overly conservative, easily fielded DDT test for updated IHE Material Qualification standard. Specifically, we evaluated the aspects of confinement, geometry, material morphology and temperature. We also developed and tested a thermally robust igniter system.

36 MATERIALS SCIENCE↗
NASA NTRS1994

Optical Detection Of Flameout In A Combustor

Fuel supply shut down in time to prevent explosion. Optical flameout detector designed to signal control system of facility to cut off supply of fuel into combustion chamber if flame goes out. Combustor which optical flameout detector designed burns methane in air to provide hot gases for 8-ft high-temperature test chamber. Acoustical flameout detector for same combustor described in "Acoustical Detection of Flameout in Combustor" (LAR-14900). Fiber optic probes mounted to fuel-spray bar upstream of flame. No focusing optics used, and probes aimed across flow of gases at spot on combustion chamber wall downstream from spray bar. Arrangement enables flameout detection system to respond quickly to potential loss of flame since it detects movement of flame front away from spray bar face. Overall response time of detection system under 10 milliseconds.

Borg, Stephen E.↗
DOE OSTI2014

Near-infrared line identification in type Ia supernovae during the transitional phase

We present near-infrared synthetic spectra of a delayed-detonation hydrodynamical model and compare them to observed spectra of four normal Type Ia supernovae ranging from day +56.5 to day +85. This is the epoch during which supernovae are believed to be undergoing the transition from the photospheric phase, where spectra are characterized by line scattering above an optically thick photosphere, to the nebular phase, where spectra consist of optically thin emission from forbidden lines. We find that most spectral features in the near-infrared can be accounted for by permitted lines of Fe II and Co II. In addition, we find that [Ni II] fits the emission feature near 1.98 μm, suggesting that a substantial mass of {sup 58}Ni exists near the center of the ejecta in these objects, arising from nuclear burning at high density.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

HERMES Model Modifications and Applications 2012

HERMES was developed to meet requirements for a model to describe explosive response to impacts that do not produce prompt detonation, but can include delayed detonations such as DDT and XDT. Such response is termed High Explosive Violent Response (HEVR). The HERMES model includes a constitutive description of strength as a function of pressure, strain-rate, strain, and ambient temperature; the development of damage including fragmentation and porosity; the conditions leading to ignition; the propagation of a burning reaction through the volume of damaged explosive; the burn-up of the damaged and undamaged explosive; and the development of detonation using the Computational Reaction Evolution dependent on Entropy (S) and Time (CREST) detonation model. CREST can distinguish gradual pressure increases from shock waves, and also predict the increase in shock sensitivity to detonation that results from increased porosity. This year HERMES has been implemented as a User Defined Function in ALE 3D. As a result, modifications to the model can be implemented in a timely fashion asynchronously from the ALE 3D distribution process. There are differences between the implementation of HERMES in ALE 3D and the implementation in LS DYNA. The major difference is the propagation of an ignition front from an element that has started to burn. In LS DYNA, the time at which an element begins to burn is based on the minimum line-of-sight distance between the element and any previously ignited element. In ALE 3D the time to burn is based on a level-set method that propagates the ignition front from all elements that were previously ignited. The ALE 3D implementation modifies the time of ignition when element deformation changes the distance from the element to the source. The LS DYNA implementation does not. There are also differences in the implementation of artificial viscosity, which is important for CREST since the artificial viscosity increases the element entropy in a shock. In general, however, the differences have proven minor. In collaboration with the Atomic Weapons Establishment, UK (AWE), we have characterized two additional explosives – PETN and Composition B, a mixture of 60% RDX and 40% TNT by weight. Applications of HERMES to these explosives are described below.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1994

Thermonuclear runaways in nova outbursts. 2: Effect of strong, instantaneous, local fluctuations

In an attempt to understand the manner in which nova outbursts are initiated on the surface of a white dwarf, we investigate the effects fluctuations have on the evolution of a thermonuclear runaway. Fluctuations in temperature density, or the composition of material in the burning shell may arise due to the chaotic flow field generated by convection when it occurs, or by the accretion process itself. With the aid of two-dimensional reactive flow calculations, we consider cases where a strong fluctutation in temperature arises during the early, quiescent accretion phase or during the later, more dynamic, explosion phase. In all cases we find that an instantaneous, local temperature fluctuation causes the affected material to become Rayleigh-Taylor unstable. The rapid rise and subsequent expansion of matter immediately cools the hot blob, which prevents the lateral propagation of burning. This suggests that local temperature fluctuations do not play a significant role in directly initiating the runaway, especially during the early stages. However, they may provide an efficient mechanism of mixing core material into the envelope (thereby pre-enriching the fuel for subsequent episodes of explosive hydrogen burning) and of mixing substantial amounts of the radioactive nucleus N-13 into the surface layers, making novae potential gamma-ray sources. This suggests that it is the global not the local, evolution of the core-envelope interface to high temperatures which dominates the development of the runaway. We also present a possible new scenario for the initiation of nova outbursts based on our results.

Shankar, Anurag↗
DOE OSTI2021

Progenitor and close-in circumstellar medium of type II supernova 2020fqv from high-cadence photometry and ultra-rapid UV spectroscopy

ABSTRACT We present observations of SN 2020fqv, a Virgo-cluster type II core-collapse supernova (CCSN) with a high temporal resolution light curve from the Transiting Exoplanet Survey Satellite (TESS) covering the time of explosion; ultraviolet (UV) spectroscopy from the Hubble Space Telescope (HST) starting 3.3 d post-explosion; ground-based spectroscopic observations starting 1.1 d post-explosion; along with extensive photometric observations. Massive stars have complicated mass-loss histories leading up to their death as CCSNe, creating circumstellar medium (CSM) with which the SNe interact. Observations during the first few days post-explosion can provide important information about the mass-loss rate during the late stages of stellar evolution. Model fits to the quasi-bolometric light curve of SN 2020fqv reveal 0.23 M⊙ of CSM confined within 1450 R⊙ (1014 cm) from its progenitor star. Early spectra (<4 d post-explosion), both from HST and ground-based observatories, show emission features from high-ionization metal species from the outer, optically thin part of this CSM. We find that the CSM is consistent with an eruption caused by the injection of ∼5 × 1046 erg into the stellar envelope ∼300 d pre-explosion, potentially from a nuclear burning instability at the onset of oxygen burning. Light-curve fitting, nebular spectroscopy, and pre-explosion HST imaging consistently point to a red supergiant (RSG) progenitor with $M_{\rm ZAMS}\approx 13.5\!-\!15 \, \mathrm{M}_{\odot }$, typical for SN II progenitor stars. This finding demonstrates that a typical RSG, like the progenitor of SN 2020fqv, has a complicated mass-loss history immediately before core collapse.

Tinyanont, Samaporn (ORCID:0000000214814676)↗
NASA NTRS1980

Sympathetic reaction tests and analyses

The critical separation distances for explosive reactions of a solid rocket propellant were measured. Explosive reactions included low order explosion, low order detonation, and high order detonation. The effects of sample size, shape, damage and temperature on sympathetic reaction were determined experimentally. The sympathetic detonation of small cubes of solid rocket propellant was modelled numerically, using the Eulerian reactive hydrodynamic code 2DE with Forest Fire burn rates. The model was applied to cubes of 2.54 - 7.62 cm (1 - 3 in.), with agreement between calculated and experimental results.

Ricardson, D. E.↗
DOE OSTI2016

Discovery of stationary operation of quiescent H-mode plasmas with net-zero neutral beam injection torque and high energy confinement on DIII-D

Recent experiments in DIII-D [J. L. Luxon et al., in Plasma Physics and Controlled Nuclear Fusion Research 1996 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] have led to the discovery of a means of modifying edge turbulence to achieve stationary, high confinement operation without Edge Localized Mode (ELM) instabilities and with no net external torque input. Eliminating the ELM-induced heat bursts and controlling plasma stability at low rotation represent two of the great challenges for fusion energy. By exploiting edge turbulence in a novel manner, we achieved excellent tokamak performance, well above the H{sub 98y2} international tokamak energy confinement scaling (H{sub 98y2} = 1.25), thus meeting an additional confinement challenge that is usually difficult at low torque. The new regime is triggered in double null plasmas by ramping the injected torque to zero and then maintaining it there. This lowers E × B rotation shear in the plasma edge, allowing low-k, broadband, electromagnetic turbulence to increase. In the H-mode edge, a narrow transport barrier usually grows until MHD instability (a peeling ballooning mode) leads to the ELM heat burst. However, the increased turbulence reduces the pressure gradient, allowing the development of a broader and thus higher transport barrier. A 60% increase in pedestal pressure and 40% increase in energy confinement result. An increase in the E × B shearing rate inside of the edge pedestal is a key factor in the confinement increase. Strong double-null plasma shaping raises the threshold for the ELM instability, allowing the plasma to reach a transport-limited state near but below the explosive ELM stability boundary. The resulting plasmas have burning-plasma-relevant β{sub N} = 1.6–1.8 and run without the need for extra torque from 3D magnetic fields. To date, stationary conditions have been produced for 2 s or 12 energy confinement times, limited only by external hardware constraints. Stationary operation with improved pedestal conditions is highly significant for future burning plasma devices, since operation without ELMs at low rotation and good confinement is key for fusion energy production.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

THE ROLE OF TURBULENCE IN NEUTRINO-DRIVEN CORE-COLLAPSE SUPERNOVA EXPLOSIONS

The neutrino-heated ''gain layer'' immediately behind the stalled shock in a core-collapse supernova is unstable to high-Reynolds-number turbulent convection. We carry out and analyze a new set of 19 high-resolution three-dimensional (3D) simulations with a three-species neutrino leakage/heating scheme and compare with spherically symmetric (one-dimensional, 1D) and axisymmetric (two-dimensional, 2D) simulations carried out with the same methods. We study the postbounce supernova evolution in a 15 M {sub ☉} progenitor star and vary the local neutrino heating rate, the magnitude and spatial dependence of asphericity from convective burning in the Si/O shell, and spatial resolution. Our simulations suggest that there is a direct correlation between the strength of turbulence in the gain layer and the susceptibility to explosion. 2D and 3D simulations explode at much lower neutrino heating rates than 1D simulations. This is commonly explained by the fact that nonradial dynamics allows accreting material to stay longer in the gain layer. We show that this explanation is incomplete. Our results indicate that the effective turbulent ram pressure exerted on the shock plays a crucial role by allowing multi-dimensional models to explode at a lower postshock thermal pressure and thus with less neutrino heating than 1D models. We connect the turbulent ram pressure with turbulent energy at large scales and in this way explain why 2D simulations are erroneously exploding more easily than 3D simulations.

79 ASTRONOMY AND ASTROPHYSICS↗