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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 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS2024

The Role of the Stratosphere in Teleconnections Arising From Fast and Slow MJO Episodes

The Madden-Julian Oscillation (MJO) can influence the extratropical circulation on timescales up to several weeks, with a dependence on the MJO characteristics: MJO episodes that propagate slowly across the Maritime Continent have a stronger impact on Euro-Atlantic weather than fast MJO episodes. While the tropospheric pathway for MJO teleconnections with varying phase speeds is well understood, in this study, we investigate the contribution of the Northern Hemisphere stratospheric pathway for fast versus slow MJO episodes. During slow MJO episodes, Phases 5–6 lead to increased upward wave propagation in the North Pacific sector, and subsequently enhanced heat flux at 100 hPa, leading to the weakening of the polar vortex. The results suggest a clear role of stratosphere-troposphere coupling for slow MJO episodes, which is proposed as a mechanism for anomalously strong positive polar cap height anomalies in MJO Phases 7–8.

Priyanka Yadav↗
DOE OSTI2012

Novel Slow Extraction Scheme for Proton Accelerators Using Pulsed Dipole Correctors and Crystals

Slow extraction of protons beams from circular accelerators is currently widely used for a variety of beam-based experiments. The method has some deficiencies including limited efficiency of extraction, radiation induced due to scattering on the electrostatic septa and limited beam pipe aperture, beam dynamics effects of space charge forces and magnet power supplies ripple. Here we present a novel slow extraction scheme employing a number of non-standard accelerator elements, such as Silicone crystal strips and pulsed stripline dipole correctors, and illustrate practicality of these examples at the 8 GeV proton Recycler Ring at Fermilab. The proposed method of non-resonant slow extraction of protons by bent crystals in combination with orbit fast deflectors shows great promise in simulations. We propose to initiate an R&D program in the Fermilab 8 GeV Recycler to address the key issues of the method: (a) feasibility of very short crystals - from few mm down to 0.2 mm; (b) their efficiency in the channelling and volume reflection regimes; (c) practical aspects of the fast deflectors.

43 PARTICLE ACCELERATORS↗
DOE OSTI2019

Slow liquid scintillator candidates for MeV-scale neutrino experiments

Slow liquid scintillator Cherenkov detectors have been proposed as part of several future neutrino experiments because they can provide both directionality and energy measurements. This feature is expected to enhance the sensitivities for MeV-scale neutrino physics, including solar physics, the search for supernova relic neutrino, and the study of geo-sciences. In this study, the characteristics of a slow liquid scintillator were investigated, along with the light yields and decay time constants for various combinations of linear alkylbenzene (LAB), 2,5-diphenyloxazole (PPO), and 1,4-bis (2-methylstyryl)-benzene (bis-MSB). Here, the results of our study indicated that LAB with 0.07 g/L of PPO and 13 mg/L of bis-MSB was the best candidate for an effective separation between Cherenkov and scintillation lights with a reasonably high light yield.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

Atomic force microscopy monitoring of subcritical slow crack growth in soda lime silicate glass

Prediction of brittle fracture in silicate glasses requires a fundamental understanding of slow crack growth rates, far below the critical stress intensity (K IC ) of the material. Here, in this study, atomic force microscopy (AFM) was used to characterize slow crack propagation in soda lime silicate glasses. Using large-sample AFM systems with specialized tips and scanners, fractures less than 20 nm wide were measured as a function of relative humidity (RH). The results were found to be sensitive to the type of AFM tip used and to the dryness of the surface. Continuous scanning and image capture over several days allowed the determination of the crack tip velocity. The dependance of crack tip velocity on RH was measured by enclosing the sample and AFM head in a humidity-controlled chamber. The crack tip velocity was measured as low 4.0×10 -12 m/s at 15 % RH and as high as 3.7×10 -10 m/s at 40 % RH. Overall crack growth was smooth during the AFM scan. Additionally, recession of the crack tip was noted as the sample dried, indicating crack tip closure may be occurring. Further studies on alternative compositions will provide details on subcritical fracture processes in a range of different environmental conditions.

Atomic force microscopy↗
DOE OSTI2018

An efficient and cost-effective microchannel plate detector for slow neutron radiography

We report a novel approach for efficiently imaging objects with slow neutrons in two dimensions is realized. Neutron sensitivity is achieved by use of a boron doped microchannel plate (MCP). The resulting electron avalanche is further amplified with a Z-stack MCP before being sensed by two orthogonally oriented wire planes. Coupling of the wire planes to delay lines efficiently encodes the position information as a time difference. To determine the position resolution, slow neutrons were used to illuminate a Cd-mask placed directly in front of the detector. Peaks in the resulting spectrum exhibited an average peak width of 329 μm FWHM, corresponding to an average intrinsic resolution of 216 μm. The center region of the detector exhibits a significantly better spatial resolution with an intrinsic resolution of <100 μm observed.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2025

Purcell-Enhanced Emissions from Diamond Color Centers in Slow Light Photonic Crystal Waveguides

Diamond color centers are promising candidates for optically addressable quantum memories, which motivates the development of efficient photonic interfaces, often using nanophotonic cavities with narrow spectral line widths and small mode volumes. However, they require perfect spectral and spatial overlap between the cavity mode and quantum emitter, which is challenging. This is especially true for solid-state quantum emitters that are often randomly positioned and suffer from inhomogeneous broadening. Another approach to enhance light-matter interaction across large optical bandwidths and areas is using slow light waveguides. Here, in this study, we demonstrate diamond slow light photonic crystal (PhC) waveguides optically coupled to embedded silicon-vacancy (SiV) color centers. We use the recently developed thin-film diamond approach to fabricate fully suspended two-dimensional PhC waveguides. We demonstrate waveguide modes with high group indices up to 70 and observe Purcell-enhanced emissions of the SiVs. Our approach represents a practical diamond platform for robust spin-photon interfaces with color centers.

2D photonic crystal waveguide↗
DOE OSTI2022

Slow light in a 2D semiconductor plasmonic structure

Spectrally narrow optical resonances can be used to generate slow light, i.e., a large reduction in the group velocity. In a previous work, we developed hybrid 2D semiconductor plasmonic structures, which consist of propagating optical frequency surface-plasmon polaritons interacting with excitons in a semiconductor monolayer. Here, we use coupled exciton-surface plasmon polaritons (E-SPPs) in monolayer WSe 2 to demonstrate slow light with a 1300 fold decrease of the SPP group velocity. Specifically, we use a high resolution two-color laser technique where the nonlinear E-SPP response gives rise to ultra-narrow coherent population oscillation (CPO) resonances, resulting in a group velocity on order of 105 m/s. Our work paves the way toward on-chip actively switched delay lines and optical buffers that utilize 2D semiconductors as active elements.

42 ENGINEERING↗
DOE OSTI2012

Three-port beam splitter for slow neutrons using holographic nanoparticle-polymer composite diffraction gratings

Diffraction of slow neutrons by nanoparticle-polymer composite gratings has been observed. By carefully choosing grating parameters such as grating thickness and spacing, a three-port beam splitter operation for slow neutrons - splitting the incident neutron intensity equally into the {+-}1st and the 0th diffraction orders - has been realized. As a possible application, a Zernike three-path interferometer is briefly discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Benchmark of Neutron Thermalization in Graphite Using a Pulsed Slowing-Down-Time Experiment

A benchmark has been developed using a pulsed slowing-down-time experiment to isolate the thermalization process in graphite. The experiment was conducted at the Oak Ridge Electron Linear Accelerator facility at Oak Ridge National Laboratory, and it measured the time spectrum of neutrons leaking from a graphite pile during slowing down and thermalization within graphite. Simulations of the benchmark experiment were performed using the MCNP6.1 Monte Carlo code and the ENDF/B-VII.1 and ENDF/B-VIII.0 cross-section databases. The benchmark provides a time spectrum (i.e., time-dependent counts in a detector) that allows for validation of the graphite thermal scattering libraries (TSLs). The impact on the simulations using a suite of graphite TSLs was compared with the experimental results. Given the density of nuclear graphite, the TSL corresponding to graphite with 30% porosity, as implemented in ENDF/B-VIII.0, was found to most accurately represent the measured time spectrum corresponding to the thermal energy range with an average deviation of ±1.7%.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

Slow light with photorefractive four-wave mixing

A slowing down of light pulses using backward-wave four-wave mixing is achieved in photorefractive crystals with different coupling strength. The delay and width of the output pulse are studied as a function of the input pulse width and pump intensity ratio for the amplified transmitted beam and for the phase-conjugated beam. The delay characteristics are compared with those of the two-beam coupling. It is demonstrated that the four-wave mixing process ensures a larger slowing down of short pulses (pulses with width shorter than the photorefractive response time) as compared to the photorefractive two-beam coupling scheme and guarantees the elimination of forerunners. The delay of long pulses in the four-wave mixing configuration is almost the same as the one that can be reached with two-beam coupling. The effect of absorption on the nonlinear shape transformation of the output pulses is discussed qualitatively.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2012

Cross-section uncertainty propagation in the multigroup slowing-down equations using probability table formalism

In the frame of neutral-particle (neutron, gamma) transport, the uncertainty propagation calculation regarding the uncertainties on cross sections is often carried out without explicitly taking into account their probabilistic distribution. We investigate a new uncertainty propagation formalism where the cross-section uncertainty distributions are represented by probability tables. This technical note develops this approach for the steady-state slowing-down equation without up-scattering and in an infinite medium. This work is based on a deterministic multiband formalism that takes into account multilevel probability tables for cross sections. The first level represents the variation of cross sections versus lethargy (or energy) in each group of the multigroup lethargy mesh and thus corresponds to the classical cross-section probability tables. The higher levels represent the uncertainties on each step of the first-level cross-section probability table. This method is validated against a Monte Carlo calculation in a case of neutron slowing down in a {sup 238}U-hydrogen homogeneous mixture, showing fully consistent numerical results. The main interest of the deterministic multilevel multiband formalism is that it gives not only the mean value and the variance but also a probabilistic distribution of the fluxes. In the near future, we plan to investigate more deeply the robustness of this new approach in relation to high values of cross-section uncertainties and to introduce cross-section uncertainty correlations as well. Meanwhile, the promise of this work is its extension to the general transport steady-state equation solved by the discrete ordinates (S(N)) or Monte Carlo methods. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
NASA NTRS2002

Thin filament diversity and physiological properties of fast and slow fiber types in astronaut leg muscles

Slow type I fibers in soleus and fast white (IIa/IIx, IIx), fast red (IIa), and slow red (I) fibers in gastrocnemius were examined electron microscopically and physiologically from pre- and postflight biopsies of four astronauts from the 17-day, Life and Microgravity Sciences Spacelab Shuttle Transport System-78 mission. At 2.5-microm sarcomere length, thick filament density is approximately 1,012 filaments/microm(2) in all fiber types and unchanged by spaceflight. In preflight aldehyde-fixed biopsies, gastrocnemius fibers possess higher percentages (approximately 23%) of short thin filaments than soleus (9%). In type I fibers, spaceflight increases short, thin filament content from 9 to 24% in soleus and from 26 to 31% in gastrocnemius. Thick and thin filament spacing is wider at short sarcomere lengths. The Z-band lattice is also expanded, except for soleus type I fibers with presumably stiffer Z bands. Thin filament packing density correlates directly with specific tension for gastrocnemius fibers but not soleus. Thin filament density is inversely related to shortening velocity in all fibers. Thin filament structural variation contributes to the functional diversity of normal and spaceflight-unloaded muscles.

Non-NASA Center↗
NASA NTRS1996

Properties of Slow-Mode Shocks in the Distant (>200 R(sub e)) Geomagnetic Tail

Two distant ISEE-3 geomagnetic tail passes have been examined to identify all slow-mode shocks present in the data. We find a total of 86 events from 439 plasmasheet/lobe crossings, using five criteria based on relations between the upstream lobe and the downstream plasmasheet magnetic field and plasma measurements. The statistical results of slow-mode shock parameters such as the angle between magnetic field and shock normal, Theta(sub bn), Alfven Mach number along the normal direction, M(sub an), and electron beta, Beta(sub e), are calculated and reported.

magnothermodynamic theory MHD geomagnetic tails sl↗
DOE OSTI2019

Machine learning accurately predicts slow slip in earthquakes

Machine-learning research has detected seismic signals accurately predicting the slow slipping of the Cascadia fault. It has also found similar signals predicting slow slip failure in Chile and New Zealand. Los Alamos National Laboratory researchers applied machine learning to analyze 12 years of historic Cascadia data. Cascadia’s constant tremors produce an acoustic signal, like sound waves. The key to Cascadia’s behavior was buried in that acoustic data.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2013

Modeling resonance interference by 0-D slowing-down solution with embedded self-shielding method

The resonance integral table based methods employing conventional multigroup structure for the resonance self-shielding calculation have a common difficulty on treating the resonance interference. The problem arises due to the lack of sufficient energy dependence of the resonance cross sections when the calculation is performed in the multigroup structure. To address this, a resonance interference factor model has been proposed to account for the interference effect by comparing the interfered and non-interfered effective cross sections obtained from 0-D homogeneous slowing-down solutions by continuous-energy cross sections. A rigorous homogeneous slowing-down solver is developed with two important features for reducing the calculation time and memory requirement for practical applications. The embedded self-shielding method (ESSM) is chosen as the multigroup resonance self-shielding solver as an integral component of the interference method. The interference method is implemented in the DeCART transport code. Verification results show that the code system provides more accurate effective cross sections and multiplication factors than the conventional interference method for UO{sub 2} and MOX fuel cases. The additional computing time and memory for the interference correction is acceptable for the test problems including a depletion case with 87 isotopes in the fuel region. (authors)

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2013

Development for fissile assay in recycled fuel using lead slowing down spectrometer

A future nuclear energy system is under development to turn spent fuels produced by PWRs into fuels for a SFR (Sodium Fast Reactor) through the pyrochemical process. The knowledge of the isotopic fissile content of the new fuel is very important for fuel safety. A lead slowing down spectrometer (LSDS) is under development to analyze the fissile material content (Pu{sup 239}, Pu{sup 241} and U{sup 235}) of the fuel. The LSDS requires a neutron source, the neutrons will be slowed down through their passage in a lead medium and will finally enter the fuel and will induce fission reactions that will be analysed and the isotopic content of the fuel will be then determined. The issue is that the spent fuel emits intense gamma rays and neutrons by spontaneous fission. The threshold fission detector screens the prompt fast fission neutrons and as a result the LSDS is not influenced by the high level radiation background. The energy resolution of LSDS is good in the range 0.1 eV to 1 keV. It is also the range in which the fission reaction is the most discriminating for the considered fissile isotopes. An electron accelerator has been chosen to produce neutrons with an adequate target through (e{sup -},γ)(γ,n) reactions.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2025

Analytic Sensitivity Coefficients for Bethe's Solution of the Neutron Slowing Down Equation

Neutron slowing down theory is used to derive expressions for the sensitivity coefficients of the neutron collision density in a hydrogenous infinite medium with respect to the fixed source, scattering probability, and macroscopic nuclear cross sections. Analytic expressions for Bethe’s solution of the neutron slowing down equation are derived for the constant cross section approximation with a point, uniform, and gamma lethargy spectrum. Analytic expressions for the corresponding sensitivity coefficients are derived and used to verify Monte Carlo neutron transport calculations.

Analytic Benchmark↗
DOE OSTI2016

Laboratory demonstration of slow rise to fast acceleration of arched magnetic flux ropes

In this paper, we demonstrate the slow rise to fast acceleration of an arched plasma-filled magnetic flux rope. The flux rope expansion is inhibited by an externally applied customizable strapping field. When the strapping field is not too strong and not too weak, expansion forces build up while the flux rope is in the strapping field region. When the flux rope moves to a critical height beyond the peak strapping field region, the plasma accelerates quickly corresponding to the observed slow rise to fast acceleration of solar eruptions. Finally, this behavior is in agreement with the predictions of the torus instability.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗