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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 1,045 records · Page 58
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Plasma wave observations at comet Giacobini-Zinner

The plasma wave instrument on the International Cometary Explorer (ICE) detected strong ion acoustic waves together with electromagnetic whistlers and low-level electron plasma oscillations when the spacecraft was within two million km of the nucleus of comet Giacobini-Zinner. As ICE approached the anticipated bow-shock location, electromagnetic and electrostatic wave levels increased significantly, but even amidst this turbulence, the wave instrument detected structures with familiar bow shock characteristics that were correlated with observations of localized electron heating phenomena. Just beyond the visible coma, high-amplitude broadband waves were detected accounting for the significant electron heating observed in this region. Near closest approach, broadband electrostatic noise was detected together with a changing pattern of weak electron plasma oscillations that yielded a density profile for the outer layers of the cold plasma tail. Near the tail axis, the plasma wave instrument also detected a nonuniform flux of dust impacts, and a preliminary profile of the Giacobini-Zinner dust distribution for micrometer-sized particles is presented.

Scarf, F. L.↗
NASA NTRS1986

Magnetohydrodynamic solitons and radio knots in jets

Weakly nonlinear surface waves are examined in the context of the beam model for jetlike radio sources. By introducing a finite scale length, viz. the beam radius, geometrical dispersion can act to balance nonlinear wave growth and thereby produce solitons, localized wave packets of stable waveform. A method for obtaining a soliton equation from the MHD equations is presented and then applied to radio knots in jets.

Fiedler, R.↗
NASA NTRS1993

The development of a mixing layer under the action of weak streamwise vortices

The action of weak, streamwise vortices on a plane, incompressible, steady mixing layer is examined in the large Reynolds number limit. The outer, inviscid region is bounded by a vortex sheet to which the viscous region is confined. It is shown that the local linear analysis becomes invalid at streamwise distances O(epsilon sup -1), where (epsilon much less than 1) is the crossflow amplitude, and a new nonlinear analysis is constructed for this region. Numerical solutions of the nonlinear problem show that the vortex sheet undergoes an O(1) change in position and that the solution is ultimately terminated by a breakdown in the numerical procedure. The corresponding viscous layer shows downstream thickening, but appears to remain well behaved up to the terminal location.

Goldstein, M. E.↗
DOE OSTI2014

Water on BN doped benzene: A hard test for exchange-correlation functionals and the impact of exact exchange on weak binding

Density functional theory (DFT) studies of weakly interacting complexes have recently focused on the importance of van der Waals dispersion forces, whereas the role of exchange has received far less attention. Here, by exploiting the subtle binding between water and a boron and nitrogen doped benzene derivative (1,2-azaborine) we show how exact exchange can alter the binding conformation within a complex. Benchmark values have been calculated for three orientations of the water monomer on 1,2-azaborine from explicitly correlated quantum chemical methods, and we have also used diffusion quantum Monte Carlo. For a host of popular DFT exchange-correlation functionals we show that the lack of exact exchange leads to the wrong lowest energy orientation of water on 1,2-azaborine. As such, we suggest that a high proportion of exact exchange and the associated improvement in the electronic structure could be needed for the accurate prediction of physisorption sites on doped surfaces and in complex organic molecules. Meanwhile to predict correct absolute interaction energies an accurate description of exchange needs to be augmented by dispersion inclusive functionals, and certain non-local van der Waals functionals (optB88- and optB86b-vdW) perform very well for absolute interaction energies. Through a comparison with water on benzene and borazine (B{sub 3}N{sub 3}H{sub 6}) we show that these results could have implications for the interaction of water with doped graphene surfaces, and suggest a possible way of tuning the interaction energy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

A halo-independent lower bound on the dark matter capture rate in the Sun from a direct detection signal

We show that a positive signal in a dark matter (DM) direct detection experiment can be used to place a lower bound on the DM capture rate in the Sun, independent of the DM halo. For a given particle physics model and DM mass we obtain a lower bound on the capture rate independent of the local DM density, velocity distribution, galactic escape velocity, as well as the scattering cross section. We illustrate this lower bound on the capture rate by assuming that upcoming direct detection experiments will soon obtain a significant signal. When comparing the lower bound on the capture rate with limits on the high-energy neutrino flux from the Sun from neutrino telescopes, we can place upper limits on the branching fraction of DM annihilation channels leading to neutrinos. With current data from IceCube and Super-Kamiokande non-trivial limits can be obtained for spin-dependent interactions and direct annihilations into neutrinos. In some cases also annihilations into ττ or bb start getting constrained. For spin-independent interactions current constraints are weak, but they may become interesting for data from future neutrino telescopes.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Learning-based approach to plasticity in athermal sheared amorphous packings: Improving softness

The plasticity of amorphous solids undergoing shear is characterized by quasi-localized rearrangements of particles. While many models of plasticity exist, the precise relationship between the plastic dynamics and the structure of a particle’s local environment remains an open question. Previously, machine learning was used to identify a structural predictor of rearrangements called “softness.” Although softness has been shown to predict which particles will rearrange with high accuracy, the method can be difficult to implement in experiments where data are limited and the combinations of descriptors it identifies are often difficult to interpret physically. Here, we address both of these weaknesses, presenting two major improvements to the standard softness method. First, we present a natural representation of each particle’s observed mobility, allowing for the use of statistical models that are both simpler and provide greater accuracy in limited datasets. Second, we employ persistent homology as a systematic means of identifying simple, topologically informed, structural quantities that are easy to interpret and measure experimentally. We test our methods on two-dimensional athermal packings of soft spheres under quasi-static shear. We find that the same structural information that predicts small variations in the response is also predictive of where plastic events will localize. We also find that an excellent accuracy is achieved in athermal sheared packings using simply a particle’s species and the number of nearest neighbor contacts.

36 MATERIALS SCIENCE↗
DOE OSTI2017

The Effect of a Guide Field on Local Energy Conversion During Asymmetric Magnetic Reconnection: Particle-in-Cell Simulations

We use theory and simulations to study how the out-of-plane (guide) magnetic field strength modifies the location where the energy conversion rate between the electric field and the plasma is appreciable during asymmetric magnetic reconnection, motivated by observations (Genestreti et al., 2017). For weak guide fields, energy conversion is maximum on the magnetospheric side of the X line, midway between the X line and electron stagnation point. As the guide field increases, the electron stagnation point gets closer to the X line, and energy conversion occurs closer to the electron stagnation point. We motivate one possible nonrigorous approach to extend the theory of the stagnation point location to include a guidefield. The predictions are compared to two-dimensional particle-in-cell (PIC) simulations with vastly different guide fields. The simulations have upstream parameters corresponding to three events observed with Magnetospheric Multiscale (MMS). The predictions agree reasonably well with the simulation results,capturing trends with the guide field. The theory correctly predicts that the X line and stagnation points approach each other as the guide field increases. The results are compared to MMS observations, Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) observations of each event,and a global resistive-magnetohydrodynamics simulation of the 16 October 2015 event. The PIC simulation results agree well with the global observations and simulation but differ in the strong electric fields and energy conversion rates found in MMS observations. The observational, theoretical, and numerical resultssuggest that the strong electric fields observed by MMS do not represent a steady global reconnection rate.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

One possible source of mass-independent fractionation of sulfur isotopes in the Archean atmosphere of Earth

Energy transfer mechanism for recombination of two sulfur atoms into a diatomic molecule, S 2 , is studied theoretically and computationally to determine whether the rate coefficient of this process can be significantly affected by isotopic substitutions, and whether the resultant isotope effect is expected to be mass-dependent or mass-independent. This is one of sulfur polymerization processes thought to be important in the anoxic atmosphere of the Archean Earth and, potentially, relevant to mass-independent fractionation of sulfur isotopes. A simplified theoretical approach is employed, in which all properties of S 2 molecule are characterized rather accurately, whereas the process of stabilization of metastable S 2 * by bath gas collisions is described approximately. Properties of individual scattering resonances in S 2 are studied in detail, and it is found that most important contributions to the recombination process come from ro-vibrational states formed near the top of centrifugal barrier, and that the number of such states is about 50 (in 32 S 32 S). Absolute value of recombination rate coefficient is computed to be 1.22 x 10 -33 cm 6 /s (for 32 S 32 S at room temperature and atmospheric pressure), close to experimental result. Two distinct isotope effects are identified. One is a classical mass-dependent effect due to translational partition function, which leads to a weak, smooth, and negative mass-dependence of rate coefficient (4% decrease when the mass is raised from 32 S 32 S to 34 S 34 S). Second effect, due to quantized resonances, is two orders of magnitude stronger, but is local. In practice, due to presence of multiple individual resonances, this phenomenon leads to irregular mass-independent variations of rate coefficients in the ranges ±5%. It is also demonstrated that in real molecules this irregular behavior is expected to be somewhat smoother, and the isotope effect is somewhat smaller, due to dependence of stabilization cross section on properties of individual resonances (not described by present model). So additional calculations of stabilization cross sections are needed in order to give quantitative prediction of this mass-independent isotope effect, and to determine its relevance to mass-independent fractionation of sulfur isotopes in the Archean rock record.

58 GEOSCIENCES↗
DOE OSTI2024

Dedicated beam position monitor pair for model-independent lattice characterization at NSLS-II

This paper reports recent lattice characterization results obtained at the National Synchrotron Light Source II (NSLS-II) storage ring, conducted without reliance on a lattice model. A pair of beam position monitors (BPMs) with bunch-by-bunch (B$\times$B) resolution, were recently installed in a section of the storage ring free of magnetic fields. The new BPM pair measured the beam, or bunch’s transverse Poincaré map precisely after the beam was excited. Linear one-turn-matrices (OTM) were then derived, and from these, the 4-dimensional coupled Twiss parameters were extracted at the locations of the BPM pair. By normalizing beam oscillation amplitudes with the Twiss parameters, the global action-variables were obtained. Additionally, these action-variables facilitated the measurement of the local Twiss parameters observed by other BPMs independent on lattice model. This method is general, and particularly useful in certain scenarios such as a round beam mode in a diffraction-limited light source ring. We applied it to assess both weakly and strongly coupled lattices at the NSLS-II ring. Through analysis of the strongly coupled lattice, the quadrupole tilt errors were estimated to be less than 400 μrad. Utilizing the BPMs’ B$\times$B resolution, for the first time we observed the variations of the linear lattice along a long bunch-train.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Theory of exciton transfer and diffusion in conjugated polymers

We describe a theory of Förster-type exciton transfer between conjugated polymers. The theory is built on three assumptions. First, we assume that the low-lying excited states of conjugated polymers are Frenkel excitons coupled to local normal modes, and described by the Frenkel-Holstein model. Second, we assume that the relevant parameter regime is ℏω < J, i.e., the adiabatic regime, and thus the Born-Oppenheimer factorization of the electronic and nuclear degrees of freedom is generally applicable. Finally, we assume that the Condon approximation is valid, i.e., the exciton-polaron wavefunction is essentially independent of the normal modes. The resulting expression for the exciton transfer rate has a familiar form, being a function of the exciton transfer integral and the effective Franck-Condon factors. The effective Franck-Condon factors are functions of the effective Huang-Rhys parameters, which are inversely proportional to the chromophore size. The Born-Oppenheimer expressions were checked against DMRG calculations, and are found to be within 10% of the exact value for a tiny fraction of the computational cost. This theory of exciton transfer is then applied to model exciton migration in conformationally disordered poly(p-phenylene vinylene). Key to this modeling is the assumption that the donor and acceptor chromophores are defined by local exciton ground states (LEGSs). Since LEGSs are readily determined by the exciton center-of-mass wavefunction, this theory provides a quantitative link between polymer conformation and exciton migration. Our Monte Carlo simulations indicate that the exciton diffusion length depends weakly on the conformation of the polymer, with the diffusion length increasing slightly as the chromophores became straighter and longer. This is largely a geometrical effect: longer and straighter chromophores extend over larger distances. The calculated diffusion lengths of ∼10 nm are in good agreement with experiment. The spectral properties of the migrating excitons are also investigated. The emission intensity ratio of the 0-0 and 0-1 vibronic peaks is related to the effective Huang-Rhys parameter of the emitting state, which in turn is related to the chromophore size. The intensity ratios calculated from the effective Huang-Rhys parameters are in agreement with experimental spectra, and the time-resolved trend for the intensity ratio to decrease with time was also reproduced as the excitation migrates to shorter, lower energy chromophores as a function of time. In addition, the energy of the exciton state shows a logarithmic decrease with time, in agreement with experimental observations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Dedicated beam position monitor pair for model-independent lattice characterization at NSLS-II

This paper reports recent lattice characterization results obtained at the National Synchrotron Light Source II (NSLS-II) storage ring, conducted without reliance on a lattice model. A pair of beam position monitors (BPMs) with bunch-by-bunch (B×B) resolution, were recently installed in a section of the storage ring free of magnetic fields. The new BPM pair measured the beam, or bunch’s transverse Poincar´e map precisely after the beam was excited. Linear one-turn-matrices (OTM) were then derived, and from these, the 4-dimensional coupled Twiss parameters were extracted at the locations of the BPM pair. By normalizing beam oscillation amplitudes with the Twiss parameters, the global action-variables were obtained. These action-variables facilitated the measurement of the local Twiss parameters observed by other BPMs independent on lattice model. This method is general, and particularly useful in certain scenarios such as a round beam mode in a diffraction-limited light source ring. We applied it to assess both weakly and strongly coupled lattices at the NSLS-II ring. Through analysis of the strongly coupled lattice, the quadrupole tilt errors were estimated to be less than 400 µrad. Utilizing the BPMs’ B×B resolution, for the first time we observed the variations of the linear lattice along a long bunch-train.

43 PARTICLE ACCELERATORS↗
DOE OSTI2018

Electron density and gas density measurements in a millimeter-wave discharge

Electron density and neutral gas density have been measured in a non-equilibrium air breakdown plasma using optical emission spectroscopy and two-dimensional laser interferometry, respectively. A plasma was created with a focused high frequency microwave beam in air. Experiments were run with 110 GHz and 124.5 GHz microwaves at powers up to 1.2 MW. Microwave pulses were 3 microsec. long at 110 GHz and 2.2 microsec. long at 124.5 GHz. Electron density was measured over a pressure range of 25 to 700 Torr as the input microwave power was varied. Electron density was found to be close to the critical density over the pressure range studied and to vary weakly with input power. Neutral gas density was measured over a pressure range from 150 to 750 Torr at power levels high above the threshold for initiating breakdown. The two-dimensional structure of the neutral gas density was resolved. Intense, localized heating was found to occur hundreds of nanoseconds after visible plasma formed. This heating led to neutral gas density reductions of greater than 80% where peak plasma densities occurred. Spatial and temporal structure of gas heating at atmospheric pressure were found to agree well with numerical simulations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1984

Track damage and erosion of insulators by ion-induced electronic processes

Track damage and the associated ejection of atoms and molecules from insulators, which occur as a result of ion-induced electronic excitation, are of interest both in their own right and because of the mechanisms through which the energy in the excited electrons is transformed into atomic motion. In this paper an overview is given of the phenomena that are observed. We show that there is a remarkable similarity between the damage profile along the ion's track in the solid and the yield of ejected atoms at the energy that corresponds to each point on the track. It is also seen that the density of extended defects (or, correspondingly, the ejected particle yield) appears to have a 'universal' form that is weakly dependent on the type of material. In the model presented this is a consequence of the inner-shell ionization of light elements in the solid by the incident ion; the resulting Auger decay produces an intense ionization spike that locally triggers the track formation/erosion process. This model allows the estimation of erosion yields/damage profiles for different ions and materials.

Tombrello, T. A.↗
NASA NTRS1993

The development of a mixing layer under the action of weak streamwise vortices

The action of weak, streamwise vortices on a plane, incompressible, steady mixing layer is examined in the large Reynolds-number limit. The outer, inviscid region is bounded by a vortex sheet to which the viscous region is confined. It is shown that the local linear analysis becomes invalid at streamwise distances O(epsilon(sup -1)), where epsilon is much less than 1 is the cross flow amplitude, and a new nonlinear analysis is constructed for this region. Numerical solutions of the nonlinear problem show that the vortex sheet undergoes an O(1) change in position and that the solution is ultimately terminated by the appearance of a singularity. The corresponding viscous layer shows downstream thickening, but appears to remain well behaved up to the singular location.

Goldstein, Marvin E.↗
NASA NTRS1992

Mean winds and momentum fluxes over Jicamarca, Peru, during June and August 1987

Data from the mesosphere-stratosphere-troposphere radar at Jicamarca, Peru, together with other available data, are used to diagnose the mean structure of winds and gravity-wave momentum fluxes from the surface to 90 km during two-ten-day campaigns in June and August of 1987. In the stratosphere a layer of maximum eastward flow associated with the quasi-biennial oscillation (QBO) was seen to strengthen and descend rapidly from June to August, overlying persistent westward flow. A layer of enhanced signal return, suggestive of a turbulent layer, was observed just above the descending QBO eastward maximum. Notable zonal asymmetries were present during this transition and the local meridional circulation departed from zonal-mean QBO theory. A substantial northeastward momentum flux was found below 25 km, which may be related to topographic gravity waves excited by southeastward flow across the Andes. In the lower mesosphere a relatively weak 'second' mesopause semiannual oscillation is confirmed. In both the lower stratosphere and mesosphere, body forces were comparable in magnitude to inferred Coriolis torques.

Hitchman, Matthew H.↗
NASA NTRS1993

Point charge representation of multicenter multipole moments in calculation of electrostatic properties

Distributed Point Charge Models (PCM) for CO, (H2O)2, and HS-SH molecules have been computed from analytical expressions using multi-center multipole moments. The point charges (set of charges including both atomic and non-atomic positions) exactly reproduce both molecular and segmental multipole moments, thus constituting an accurate representation of the local anisotropy of electrostatic properties. In contrast to other known point charge models, PCM can be used to calculate not only intermolecular, but also intramolecular interactions. Comparison of these results with more accurate calculations demonstrated that PCM can correctly represent both weak and strong (intramolecular) interactions, thus indicating the merit of extending PCM to obtain improved potentials for molecular mechanics and molecular dynamics computational methods.

Non-NASA Center↗
DOE OSTI2024

LSZ in action: extracting form factors from correlators nonperturbatively in 2d ϕ4 theory

Abstract In this paper, we compute multiparticle form factors of local operators in 2dϕ 4 theory using a recently proposed method [1] for efficiently implementing the LSZ prescription with Hamiltonian Truncation methods, and we adopt Lightcone Conformal Truncation (LCT) in particular for our calculations. We perform various checks of our results at weak and strong coupling, and elucidate the parametric behavior of truncation errors. This opens up the possibility to compute S-matrix in various strongly coupled models using the LSZ method in LCT.

Physics↗