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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Multielectronic and multiatomic effects in the U O 4,5 x-ray absorption spectroscopy of uranium dioxide

In order to explain all of the spectral features observed in the U O 4,5 x-ray absorption spectrum of uranium dioxide (UO 2 ), it is necessary to include both multielectron effects and multiatomic effects. The 5d (core hole)-5f (electron) angular momentum coupling that gives rise to the giant resonance has been treated within ligand field density functional theory, and the electron scattering that generates the extended x-ray absorption fine structure has been included via the spectral simulation program FEFF: both within a UO 8 fluorite cluster picture. An atomic model is insufficient to explain all of the observed spectral features.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Image Rotation in Plasmas

Because of the speed of light compared to material motion, the dragging of light is difficult to observe under laboratory conditions. Furthermore we report on the first observation of image rotation, i. e. a dragging by the medium of the wave’s transverse structure, of Alfv´en waves in plasmas. Exploiting the naturally slow group velocity of these waves, significant wave rotation is achieved for modest angular frequency. Control over the rotation of the wave’s structure is demonstrated through the plasma rotation imposed by biased electrodes. Remarkably, experimental results are well reproduced by light dragging theory derived for isotropic media, even if magnetized plasmas are anisotropic. In addition to offering new insights into the fundamental issue of angular momentum coupling between waves and media, these findings also open possibilities for new remote rotation sensing tools.

Alfvén waves↗
DOE OSTI2020

Three-body Förster resonance of a new type in Rydberg atoms

The three-body Förster resonances 3 × nP{sub 3/2}(|M|) →nS{sub 1/2} + (n + 1)S{sub 1/2} + nP{sub 3/2}(|M*|), controlled by a constant electric field, were realised earlier by the authors in an ensemble of several cold Rydberg Rb atoms. One of the drawbacks of such resonances for potential application in three-qubit quantum gates is the proximity of the two-body Förster resonance 2 × nP{sub 3/2} → nS{sub 1/2} + (n + 1)S{sub 1/2}, as well as the possibility of their implementation only for states with values of the principal quantum numbers n ⩽38. A three-body resonance of a new type, 3 × nP{sub 3/2} → nS{sub 1/2} + (n + 1)S{sub 1/2} + nP{sub 1/2}, which can be realised for arbitrary n, is proposed and analysed. Its specific feature is also that the third atom transits into a state with a different total angular momentum J = 1/2, which has no Stark structure, so that the two-body resonance is completely absent. Numerical calculations showed that for not too strong interaction, it is possible to observe coherent three-body oscillations of the populations of collective states, which is of interest for developing new schemes of three-qubit quantum gates controlled by an electric field. (paper)

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

Suhl instabilities in nanoscopic spheroids

We simulate the magnetization dynamics of a permalloy spheroid of nanoscopic size in zero external field, such that both dipolar and exchange interactions are important. Low excitation power is used to obtain the frequencies and mode patterns of many normal modes. At higher power, non-linear three and four mode couplings between magnons carrying orbital angular momentum are observed to give rise to Suhl instabilities. Suhl’s analysis of the selection rules governing the allowed processes is extended to initial states other than uniform precession. These rules are studied and confirmed by the simulations. Both down- and up-conversion are seen as well as three and four-mode processes. As a result, general trends are inferred for preferred instabilities among those that are allowed, although the thresholds for some instabilities appear to be very high.

3D nanomagnetism↗
DOE OSTI2023

Exact solutions for the electromagnetic fields of a flying focus

The intensity peak of a “flying” focus travels at a programmable velocity over many Rayleigh ranges while maintaining a near-constant profile. Assessing the extent to which these features can enhance laser-based applications requires an accurate description of the electromagnetic fields. Here we present exact analytical solutions to Maxwell’s equations for the electromagnetic fields of a constant-velocity flying focus, generalized for arbitrary polarization and orbital angular momentum. Furthermore, the approach combines the complex source-point method, which transforms multipole solutions into beam-like solutions, with the Lorentz invariance of Maxwell’s equations. Propagating the fields backward in space reveals the space-time profile that an optical assembly must produce to realize these fields in the laboratory. Comparisons with simpler paraxial solutions provide conditions for their reliable use when modeling a flying focus.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Machine Learning for Neutron Resonance Evaluations [Slides]

The performance of nuclear reactors and other nuclear systems depends on a precise understanding of the neutron interaction cross sections for materials used in these systems. These cross sections exhibit a resonance structure whose shape is determined in part by the angular momentum quantum numbers of the resonances. The correct assignment of the quantum numbers of neutron resonances is therefore of paramount importance. In this presentation, we describe the application of machine learning to automate the quantum number assignments. Scikit-learn classifiers were trained on simulated resonance data whose statistical properties were chosen to mimic real data. We explored the use of several physics (and random matrix theory)-motivated features for training the classifiers, including the nearest neighbor spacing distribution, cumulative level distribution, and channel width distributions. Initial results demonstrated that we can determine resonance spin groups somewhat reliably. We are now investigating the application of our approach to 52 Cr resonance data.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Lyapunov and Clearing Timescales in Planetary Chaotic Zones

Debris disks in planetary systems are known to possess gaps “opened” by planets in their close-to-coorbital neighborhoods; the gaps are free from low-mass material such as planetesimals. We consider the escape process of the initially zero-eccentric particles from the gap zone in a scenario consisting of two basic consequent stages: (1) the particle orbit's eccentricity is inflated, while its specific energy is relatively constant, and (2) the particle orbit's semimajor axis is inflated, while its specific angular momentum is relatively constant. During the first stage, the chaotic transport (diffusion) proceeds along the “staircase” of (p + 1):p (with integer p ≫ 1) overlapping particle–planet resonances, and during the second stage, it proceeds along the p:1 particle–planet resonance staircase. A specialized two-dimensional area-preserving map (a “tokamap” version) is derived to describe the first stage, while a multiharmonic Kepler map theory is used to describe the second stage. We derive the μ (mass parameter) scaling for the clearing timescale in the planetary chaotic zone. At small μ, it turns out to be inverse-quadratic in μ. The μ scaling for the Lyapunov timescale inside the gap is also derived. The derived scalings are in qualitative agreement with available numerical-experimental data.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Spatially Resolved Velocity Structures in Jets of DF Tau and UY Aur A

Young stars accrete mass and angular momentum from their circumstellar disks. Some of them also drive outflows, which can be distinguished in optical forbidden emission lines. We analyze a sample of binary T Tauri stars observed with long-slit spectroscopy by the Hubble Space Telescope between 1998 and 2000, searching for spatially resolved outflows. We detect resolved [O i] emission in 2 cases out of 21. In DF Tau we resolve high- and medium-velocity outflows in a jet and counterjet out to 60 au. The outflows are accelerated within the inner 12 au and retain a constant speed thereafter. In UY Aur, we detect a blue- and a redshifted outflow from UY Aur A, as well as a blueshifted jet from UY Aur B. All of these features have been seen in [Fe ii] with data taken 10 years apart indicating that the underlying outflow pattern is stable on these timescales.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Dynamical Instabilities in Systems of Multiple Short-period Planets Are Likely Driven by Secular Chaos: A Case Study of Kepler-102

We investigated the dynamical stability of high-multiplicity Kepler and K2 planetary systems. Our numerical simulations find instabilities in ∼20% of the cases on a wide range of timescales (up to 5 × 10{sup 9} orbits) and over an unexpectedly wide range of initial dynamical spacings. To identify the triggers of long-term instability in multiplanet systems, we investigated in detail the five-planet Kepler-102 system. Despite having several near-resonant period ratios, we find that mean-motion resonances are unlikely to directly cause instability for plausible planet masses in this system. Instead, we find strong evidence that slow inward transfer of angular momentum deficit (AMD) via secular chaos excites the eccentricity of the innermost planet, Kepler-102 b, eventually leading to planet–planet collisions in ∼80% of Kepler-102 simulations. Kepler-102 b likely needs a mass ≳0.1 M {sub ⊕}, hence a bulk density exceeding about half Earth’s, in order to avoid dynamical instability. To investigate the role of secular chaos in our wider set of simulations, we characterize each planetary system’s AMD evolution with a “spectral fraction” calculated from the power spectrum of short integrations (∼5 × 10{sup 6} orbits). We find that small spectral fractions (≲0.01) are strongly associated with dynamical stability on long timescales (5 × 10{sup 9} orbits) and that the median time to instability decreases with increasing spectral fraction. Our results support the hypothesis that secular chaos is the driver of instabilities in many nonresonant multiplanet systems and also demonstrate that the spectral analysis method is an efficient numerical tool to diagnose long-term (in)stability of multiplanet systems from short simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Kinetic-energy dissipation and fluctuations in strongly damped heavy-ion collisions within the stochastic mean-field approach

Microscopic mean-field approaches have been successful in describing the most probable reaction outcomes in low-energy heavy-ion reactions. However, those approaches are known to severely underestimate dispersions of observables around the average values that has limited their applicability. Recently it has been shown that a quantal transport approach based on the stochastic mean-field (SMF) theory significantly improves the description, while its application has been limited so far to fragment mass and charge dispersions. In this work, we extend the quantal transport approach based on the SMF theory for relative kinetic energy dissipation and angular momentum transfer in low-energy heavy-ion reactions. Based on the SMF concept, analytical expressions are derived for the radial and tangential friction and associated diffusion coefficients. Those quantal transport coefficients are calculated microscopically in terms of single-particle orbitals within the time-dependent Hartree-Fock (TDHF) approach. As the first application of the proposed formalism, we consider the radial linear momentum dispersion, neglecting the coupling between radial and angular momenta. We analyze the total kinetic energy (TKE) distribution of binary reaction products in the 136 Xe + 208 Pb reaction at E c.m. =526 MeV and compare with experimental data. From time evolution of single-particle orbitals in TDHF, the radial diffusion coefficient is computed on a microscopic basis, while a phenomenological treatment is introduced for the radial friction coefficient. By solving the quantal diffusion equation for the radial linear momentum, the dispersion of the radial linear momentum is obtained, from which one can construct the TKE distribution. We find that the calculations provide a good description of the TKE distribution for strongly damped events with large energy losses, TKEL ≳ 150 MeV. However, the calculations underestimate the TKE distribution for smaller energy losses. Further studies are needed to improve the technical details of calculations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Machine learning applied to classifying neutron resonances

The performance of nuclear reactors and other nuclear systems depends on a precise understanding of the neutron interaction cross sections for materials used in these systems. These cross sections exhibit resonance structure whose shape is determined in part by the angular momentum quantum numbers of the resonances. The correct assignment of the quantum numbers of neutron resonances is therefore of paramount importance. In this project, we apply a machine learning technique, namely decision trees, to automate the quantum number assignments. The tree is trained from simulated data generated to mimic the errors found in real data. We explore the use of several physics-motivated features for training our trees, including the nearest neighbor spacing distribution, cumulative level distribution, and channel width distributions. Initial results using random matrix theory motivated fits which demonstrated that we can determine resonance spin groups somewhat reliably. If we use these fits as features in our trees, we can train them to spot outliers corresponding to misassigned resonances. We found that with the large number of features used in this project that the decision tree tended to over t training data resulting in poor performance with respect to the test data. By reducing the number of features, we can achieve nearly perfect assignment of quantum numbers with our training data.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

The Detection and Characterization of Be+sdO Binaries from HST/STIS FUV Spectroscopy

The B emission-line stars are rapid rotators that were probably spun up by mass and angular momentum accretion through mass transfer in an interacting binary. Mass transfer will strip the donor star of its envelope to create a small and hot subdwarf remnant. Here we report on Hubble Space Telescope/STIS far-ultraviolet spectroscopy of a sample of Be stars that reveals the presence of the hot sdO companion through the calculation of cross-correlation functions of the observed and model spectra. We clearly detect the spectral signature of the sdO star in 10 of the 13 stars in the sample, and the spectral signals indicate that the sdO stars are hot, relatively faint, and slowly rotating as predicted by models. A comparison of their temperatures and radii with evolutionary tracks indicates that the sdO stars occupy the relatively long-lived, He-core burning stage. Only 1 of the 10 detections was a known binary prior to this investigation, which emphasizes the difficulty of finding such Be+sdO binaries through optical spectroscopy. However, these results and others indicate that many Be stars probably host hot subdwarf companions.

47 OTHER INSTRUMENTATION↗
DOE OSTI2024

Advances in laser-plasma interactions using intense vortex laser beams

Low-intensity light beams carrying orbital angular momentum (OAM), commonly known as vortex beams, have garnered significant attention due to promising applications in areas ranging from optical trapping to communication. In recent years, there has been a surge in global research exploring the potential of high-intensity vortex laser beams and specifically their interactions with plasmas. Here, this paper provides a comprehensive review of recent advances in this area. Compared with conventional laser beams, intense vortex beams exhibit unique properties such as twisted phase fronts, OAM delivery, hollow intensity distribution, and spatially isolated longitudinal fields. These distinct characteristics give rise to a multitude of rich phenomena, profoundly influencing laser-plasma interactions and offering diverse applications. The paper also discusses future prospects and identifies promising general research areas involving vortex beams. These areas include low-divergence particle acceleration, instability suppression, high-energy photon delivery with OAM, and the generation of strong magnetic fields. With growing scientific interest and application potential, the study of intense vortex lasers is poised for rapid development in the coming years.

high energy density science↗
DOE OSTI2021

Combining Astrometry and Elemental Abundances: The Case of the Candidate Pre-Gaia Halo Moving Groups G03-37, G18-39, and G21-22

While most moving groups are young and nearby, a small number have been identified in the Galactic halo. Understanding the origin and evolution of these groups is an important piece of reconstructing the formation history of the halo. Here we report on our analysis of three putative halo moving groups: G03-37, G18-39, and G21-22. Based on Gaia EDR3 data, the stars associated with each group show some scatter in velocity (e.g., Toomre diagram) and integrals of motion (energy, angular momentum) spaces, counter to expectations of moving-group stars. We choose the best candidate of the three groups, G21-22, for follow-up chemical analysis based on high-resolution spectroscopy of six presumptive members. Using a new Python code that uses a Bayesian method to self-consistently propagate uncertainties from stellar atmosphere solutions in calculating individual abundances and spectral synthesis, we derive the abundances of α- (Mg, Si, Ca, Ti), Fe-peak (Cr, Sc, Mn, Fe, Ni), odd-Z (Na, Al, V), and neutron-capture (Ba, Eu) elements for each star. We find that the G21-22 stars are not chemically homogeneous. Based on the kinematic analysis for all three groups and the chemical analysis for G21-22, we conclude the three are not genuine moving groups. The case for G21-22 demonstrates the benefit of combining kinematic and chemical information in identifying conatal populations when either alone may be insufficient. Comparing the integrals of motion and velocities of the six G21-22 stars with those of known structures in the halo, we tentatively associate them with the Gaia-Enceladus accretion event.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Combined multiplet theory and experiment for the Fe 2p and 3p XPS of FeO and Fe 2 O 3

The Al K alpha, 1486.6eV, based XPS of Fe 2p and Fe 3p for Fe(III) in Fe 2 O 3 and Fe(II) in FeO are compared with theoretical predictions based on ab initio wavefunctions that accurately treat the final, core-hole, multiplets. The principal objectives of this comparison are to understand the multiplet structure and to evaluate the use of both the 2p and 3p spectra in determining oxidation state. In order to properly interpret the features of these spectra and to use the XPS to provide atomistic insights as well as atomic composition, it is necessary to understand the origin of the multiplet energies and intensities. The theoretical treatment takes into account the ligand field and spin-orbit splittings, the covalent mixing of ligand and Fe 3d orbitals, and the angular momentum coupling of the open shell electrons. These effects lead to the distribution of XPS intensity into a large number of final, ionic, states that are only partly resolved with energies spread over a wide range of binding energies, BEs. For this reason, it is necessary to record the Fe 2p and 3p XPS spectra over a wide energy range which includes all the multiplets in the theoretical treatment as well as additional shake satellites. We also evaluate the effects of differing assumptions concerning the extrinsic background subtraction, to be sure our experimental spectrum may be fairly compared to the theory. In this work, we conclude that the Fe 3p XPS provides an additional means for distinguishing Fe(III) and Fe(II) oxidation states beyond just using the Fe 2p spectrum. In particular, with the use of the Fe 3p XPS, the depth of the material probed is about 1.5 times greater than for the Fe 2p XPS. In addition, a new type of atomic many-body effect that involves excitations into orbitals that have Fe f,$\ell$=3, symmetry has been shown to be important for the Fe 3p XPS.

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

The Volume-complete Sample of M Dwarfs with Masses 0.1 ≤ M/M{sub ⊙} ≤ 0.3 within 15 Parsecs

M dwarfs with masses 0.1 ≤ M/M {sub ⊙} ≤ 0.3 are under increasing scrutiny because these fully convective stars pose interesting astrophysical questions regarding their magnetic activity and angular momentum history. They also afford the most accessible near future opportunity to study the atmospheres of terrestrial planets. Because they are intrinsically low in luminosity, the identification of the nearest examples of these M dwarfs is essential for progress. We present the volume-complete, all-sky list of 512 M dwarfs with masses 0.1 ≤ M/M {sub ⊙} ≤ 0.3 and with trigonometric distances placing them within 15 pc (π {sub trig} ≥ 66.67 mas) from which we have created a sample of 413 M dwarfs for spectroscopic study. We present the mass function for these 512 M dwarfs, which increases with decreasing stellar mass in linear mass space, but is flat in logarithmic mass space. As part of this sample, we present new V {sub J} R {sub KC} I {sub KC} photometry for 17 targets, measured as a result of the RECONS group’s long-term work at the CTIO/SMARTS 0.9 m telescope. We also note the details of targets that are known to be members of multiple systems and find a preliminary multiplicity rate of 21% ± 2% for the primary M dwarfs in our sample when considering known stellar and brown dwarf companions at all separations from their primaries. We further find that 43 ± 2% of all M dwarfs with masses 0.1 ≤ M/M {sub ⊙} ≤ 0.3 are found in multiple systems with primary stars of all masses within 15 pc.

47 OTHER INSTRUMENTATION↗
DOE OSTI2023

Magnetoelectronic phenomena due to quantum magnetization fluctuations

The goal of the project was to identify techniques to study magnetoelectronic phenomena such as spin transfer (ST) that stem from the quantum-mechanical nature of magnetization, and utilize them to characterize these quantum phenomena. The first such technique, electronic measurements based on the dependence of resistance on the population of spin-wave quanta (magnons) in a magnetic nanostructure, was shown by detailed measurements to be significantly influenced by the current-driven generation of a highly nonequilibrium phonon distribution by electrical current. This previously unrecognized phenomenon adds to the understanding of current-induced heating phenomena, qualitatively changes the methodology of analysis of current-induced heating common in scientific studies and engineering of current-carrying nanostructures. It benefits the public by providing a new direction for mitigation of Joule heating effects in electronic devices. The project also included quantum simulations of current-induced effects in magnetic nanostructures, which revealed that linear momentum and energy conservation are important for ST. Only angular momentum conservation was previously believed to be important. This finding is important for the fundamental understanding of ST, and for the design of efficient magnetoelectronic devices taking advantage of these conservation laws. Simulations also showed that non-classical ST is dominant in antiferromagnets, which will transform the understanding of ST in these systems. Finally, experimental studies of spin-transfer in ultrathin transition metal films revealed very large non-classical effects which were traced to the quantum origins of magnetism associated in the studied systems with the formation of an orbital liquid state. These findings i) transform the understanding of "conventional" transition-metal magnetism, ii) open a new field of studies in magnetism and a new method for the electronic control of magnetic properties and magnetism itself, and iii) provide a new approach to the development of efficient magnetoelectronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Shaping Dynamical Casimir Photons

Temporal modulation of the quantum vacuum through fast motion of a neutral body or fast changes of its optical properties is known to promote virtual into real photons, the so-called dynamical Casimir effect. Empowering modulation protocols with spatial control could enable the shaping of spectral, spatial, spin, and entanglement properties of the emitted photon pairs. Space–time quantum metasurfaces have been proposed as a platform to realize this physics via modulation of their optical properties. Here, we report the mechanical analog of this phenomenon by considering systems in which the lattice structure undergoes modulation in space and in time. We develop a microscopic theory that applies both to moving mirrors with a modulated surface profile and atomic array meta-mirrors with perturbed lattice configuration. Spatiotemporal modulation enables motion-induced generation of co- and cross-polarized photon pairs that feature frequency-linear momentum entanglement as well as vortex photon pairs featuring frequency-angular momentum entanglement. The proposed space–time dynamical Casimir effect can be interpreted as induced dynamical asymmetry in the quantum vacuum.

74 ATOMIC AND MOLECULAR PHYSICS↗