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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 1,603 records · Page 89
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Induced Radioactivity Analysis for the NSRL Linac in China Using Monte Carlo Simulations and Gamma Spectroscopy - Paper 141

The Synchrotron Radiation Facility of the National Synchrotron Radiation Laboratory (NSRL) located in Hefei, China consists of a 200-MeV electron linear accelerator and an 800-MeV electronic storage ring. Constructed in 1989, the 200-MeV Electron linac is one of the earliest high-energy electron linear accelerators in China. The NSRL is a large-scale experiment device, having been operated more than twenty years, so it is hard to give a historical recount about the injection of NSRL Linac. Lower-energy electrons emitted from the accelerator electron gun are progressively accelerated to about 200 MeV in five acceleration tubes, and then are injected into the electron storage ring, reaching the final energy of 800 MeV. The linear accelerator is a traveling wave linear accelerator, consisting of pre-injection, four 6-m acceleration segment, some beam sensing elements, and focusing inserts. The electrons are accelerated in the acceleration tubes to 32 MeV, 74 MeV, 116 MeV, 158 MeV, respectively, and the electronic energy reaches 200 MeV at the end of the acceleration trip. A scraper is used for beam collimation and is located at the end of each acceleration tube, with a length that increases with the energy to provide sufficient collimation. The NSRL Linac was retired in 2012 due to a plan to upgrade the facility. The decommissioning produced many real challenges related to the methods to handle the large size of the device and the time required to store structural materials until the radioactivity is reduced in the waste repository. These problems should be solved as soon as possible in order for the upgrade to proceed smoothly. At present, no prior decommissioning experience existed in China on such a high-energy electron linear accelerator. Therefore, the decommission project provided an opportunity to study and evaluate the challenges, in particular, involving the induced radioactivity for the purposes of radiation protection. For a well-shielded accelerator facility, the induced radioactivity is an important source of radiation for people who perform the decommissioning. In addition, information on the level of induced radioactivity is needed for disposing of the retired accelerator materials. Therefore it is significant to predict such radioactivity level accurately. However, the induced radioactivity study for such type high-energy electron linear accelerator is very seldom. Particle accelerators that operate at energy greater than 10 MeV will produce induced radioactivity due to well-known photonuclear interactions. However, the assessment of such as induced radioactivity of large-scale installations is a time-consuming process. Experimental methods are time-consuming and require special radiation safety measures to protection the investigators. On the other hand, empirical formula cannot yield sufficiently accurate results. Therefore, Monte Carlo simulation method is the primary means to study such problems. In the decommissioning process of the NSRL Linac, the induced radioactivity assessment will be based on Monte Carlo simulation method as the main auxiliary means to expand a series of studies. (authors)

43 PARTICLE ACCELERATORS↗
DOE OSTI2022

A parallel discrete dislocation dynamics/kinetic Monte Carlo method to study non-conservative plastic processes

Non-conservative processes play a fundamental role in plasticity and are behind important macroscopic phenomena such as creep, dynamic strain aging, loop raft formation, etc. In the most general case, vacancy-induced dislocation climb is the operating unit mechanism. While dislocation/vacancy interactions have been modeled in the literature using a variety of methods, the approaches developed rely on continuum descriptions of both the vacancy population and its fluxes. However, there are numerous situations in physics where point defect populations display heterogeneous concentrations and/or non-smooth kinetics. Here, a kinetic Monte Carlo (kMC) approach for modeling vacancy transport in response to arbitrary stress fields is used. Vacancies are treated as point particles and are coupled to the dislocation substructure representing a deformed material via an advection term defined by the local stress gradients. The stress fields and the dislocation substructure are evolved using a discrete dislocation dynamics (DDD) module. To extend the coupled model to the treatment of large systems, we have implemented it in the massively-parallel DDD code ParaDiS. To avoid numerical incompatibilities associated with merging deterministic (DDD) and stochastic (kMC) integration algorithms, we cast the entire elasto-plastic-diffusive problem within a single stochastic framework, taking advantage of a parallel kMC algorithm to evolve the system as a single event-driven process. The large-scale implementation enables the study of the evolution of a variety of dislocation-defect scenarios governed by non-conservative transport kinetics. After carrying out an exhaustive numerical and computational analysis of our parallel algorithm, we show results that emphasize situations where inhomogeneous vacancy dynamics are of relevance, and compare discrete kinetics to continuum solutions for several cases.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Computational techniques for the analysis of small signals in high-statistics neutrino oscillation experiments

The current and upcoming generation of Very Large Volume Neutrino Telescopes – collecting unprecedented quantities of neutrino events – can be used to explore subtle effects in oscillation physics, such as (but not restricted to) the neutrino mass ordering. The sensitivity of an experiment to these effects can be estimated from Monte Carlo simulations. With the high number of events that will be collected, there is a trade-off between the computational expense of running such simulations and the inherent statistical uncertainty in the determined values. In such a scenario, it becomes impractical to produce and use adequately-sized sets of simulated events with traditional methods, such as Monte Carlo weighting. In this work we present a staged approach to the generation of expected distributions of observables in order to overcome these challenges. By combining multiple integration and smoothing techniques which address limited statistics from simulation it arrives at reliable analysis results using modest computational resources.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
NASA NTRS1971

Production of polydisperse sprays.

A device which produces a spray with a controllable drop size distribution is described. This system is especially well suited for generating nonsmooth (e.g., bimodal) distributions, although smooth distributions can also be produced. Droplets of uniform size result from the rapid growth of an oscillatory disturbance in a free liquid jet. Simultaneous generation of droplets of various diameters by this method produces the polydisperse spray. The range of drop diameters is approximately 300-3000 microns.

Pierce, T. H.↗
NASA NTRS1975

Carbide coated fibers in graphite-aluminum composites

The NASA-supported program at the Los Alamos Scientific Laboratory (LASL) to develop carbon fiber-aluminum matrix composites is described. Chemical vapor deposition (CVD) was used to uniformly deposit thin, smooth, continuous coats of TiC on the fibers of graphite tows. Wet chemical coating of fibers, followed by high-temperature treatment, was also used, but showed little promise as an alternative coating method. Strength measurements on CVD coated fiber tows showed that thin carbide coats can add to fiber strength. The ability of aluminum alloys to wet TiC was successfully demonstrated using TiC-coated graphite surfaces. Pressure-infiltration of TiC- and ZrC-coated fiber tows with aluminum alloys was only partially successful. Experiments were performed to evaluate the effectiveness of carbide coats on carbon as barriers to prevent reaction between alluminum alloys and carbon. Initial results indicate that composites of aluminum and carbide-coated graphite are stable for long periods of time at temperatures near the alloy solidus.

Imprescia, R. J.↗
NASA NTRS1979

Laminar and turbulent flows over a spherically blunted cone with massive surface blowing

Numerical solutions are presented for the flow over a spherically blunted cone with massive surface blowing. Time-dependent viscous shock layer equations are used to describe the flowfield. The boundary conditions on the body surface include a prescribed blowing rate distribution. The governing equations are solved by a time-asymptotic finite-difference method. Results presented here are only for a perfect gas-type flow at zero angle of attack. Both laminar and turbulent flow solutions are obtained. It is found that the surface blowing smooths out the effect of the curvature discontinuity at the sphere-cone juncture point on the laminar flowfield and results in a negative pressure gradient over the body. The shock slope increases on the downstream portion of the body as the surface blowing rate is increased. The turbulent flow with surface blowing is found to redevelop a boundary-layer-like region near the surface. The effects of this boundary-layer-like region on the flowfield and heating rates are discussed.

Kumar, A.↗
NASA NTRS1986

Evidence for a low-density component in the broad-line region of Seyfert 1 galaxies

It is found that the forbidden 4959-5007 A broad O III lines, good candidates for the excess shelf emission, are found in the vicinity of H-beta in Seyfert 1 galaxy spectra. The shelf-study method used to remove Fe II assumes that the profiles of the contributors to the shelf are known, and that subtraction of these contributors in the correct amount will result in a fairly smooth H-beta profile. Data were obtained with the Ohio State University Image-Dissector Scanner with a 600 line/mm grating, on a 1.8-m reflector. The estimated cloud electron density of 1 billion/sq cm is at the low end of those used for BLR photoionization models. The presence of a broad residual emission feature in the neighborhood of 5050 A after subtraction of the shelf is thought to be the result of a weak broad emission in this range from yet another species.

Crenshaw, D. M.↗
NASA NTRS1953

On transonic flow past a wave-shaped wall

This report is an extension of a previous investigation (described in NACA rep. 1069) concerned with the solution of the nonlinear differential equation for transonic flow past a wavy wall. In the present work several new notions are introduced which permit the solution of the recursion formulas arising from the method of integration in series. In addition, a novel numerical tests of convergence, applied to the power series (in transonic similarity parameter) representing the local Mach number distribution at the boundary, indicates that smooth symmetrical potential flow past the wavy wall is no longer possible once the critical value of the stream Mach number has been exceeded.

Kaplan, Carl↗
NASA NTRS1993

Optimal open-loop and feedback control using single gimbal control moment gyroscopes

Methods for control of spacecraft maneuvers through the use of single gimbal control moment gyroscopes are developed. The development employs an integrated model of the spacecraft dynamics with the control moment gyroscope dynamics. Smooth and continuous open-loop control profiles are obtained which minimize a weighted function of maneuver time, magnitude of control effort, and proximity to singular gimbal configurations. Closed-loop state feedback control laws are derived by invoking Lyapunov stability theory. The schemes are presented for implementing the commanded state feedback: gimbal rate control and gimbal acceleration control. The appropriate handling of singular gimbal configurations is also discussed.

Hoelscher, Brian R.↗
NASA NTRS2003

Computational Aeroacoustics: An Overview

An overview of recent advances in computational aeroacoustics (CAA) is presented. CAA algorithms must not be dispersive and dissipative. It should propagate waves supported by the Euler equations with the correct group velocities. Computation domains are inevitably finite in size. To avoid the reflection of acoustic and other outgoing waves at the boundaries of the computation domain, it is required that special boundary conditions be imposed at the boundary region. These boundary conditions either absorb all the outgoing waves without reflection or allow the waves to exit smoothly. High-order schemes, invariably, supports spurious short waves. These spurious waves tend to pollute the numerical solution. They must be selectively damped or filtered out. All these issues and relevant computation methods are briefly reviewed. Jet screech tones are known to have caused structural fatigue in military combat aircrafts. Numerical simulation of the jet screech phenomenon is presented as an example of a successful application of CAA.

Tam, Christopher K. W.↗
NASA NTRS2024

Double Power-law Formation by Sequential Particle Acceleration

Spectral double power laws are common in solar high-energy phenomena such as flares and interplanetary energetic-electron events. However, the physical mechanism that produces the changes in power-law index within a single spectrum is unclear. We developed a fully analytical method of forming single power-law spectra from sequential acceleration of particles orbiting inside and hopping between simulated large-scale magnetic islands formed by flare reconnection. Here, we extend the analytical method to the formation of double power-law spectra by assuming sequential acceleration in two successive regions with different acceleration and particle-transport rates. The resulting spectral distribution is continuous and smooth, with a flattening at low energies, two power-law regions at mid-energies, and a steep rollover at high energies. The model provides analytical expressions for the spectral indices, all energy breaks, and normalization constants as functions of just three physical parameters of each acceleration region: (1) the energy gain in each accelerator, (2) the percentage of particles transferred between accelerators, and (3) the number of accelerators visited. One of the most salient predictions of our work is that the spectral index at high (low) energies is determined by the parameters of the first "seed" (second) acceleration region. By constructing the spectral distribution through an iterative analytical process, the evolution toward a double power law is easily characterized and explained. Our analytical model provides tools to interpret space- and ground-based observations from RHESSI, FOXSI, NuSTAR, Solar Orbiter/STIX, EOVSA, and future high-energy missions.

Solar flares↗
DOE OSTI2012

The Method of Manufactured Solutions for RattleSnake A SN Radiation Transport Solver Inside the MOOSE Framework

The Method of Manufactured Solutions (MMS) is an accepted technique to verify that a numerical discretization for the radiation transport equation has been implemented correctly. This technique offers a few advantages over other methods such as benchmark problems or analytical solutions. The solution can be manufactured such that properties for the angular flux are either stressed or preserved. For radiation transport, these properties can include desired smoothness, positiveness and arbitrary order of anisotropy in angle. Another advantage is that the angular flux solution can be manufactured for multidimensional problems where analytical solutions are difficult to obtain in general.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2021

Robust method for broadband efficiency enhancement of electron photocathodes using optical interferences

In this work, we demonstrate the key features of an interference cathode using both simulations and experiments. We deposit Cs 3 Sb photocathodes on Ag to produce an interference enhanced photocathode with 2–5× quantum efficiency (QE) enhancement using a robust procedure that requires only a smooth metal substrate and QE monitoring during growth. We grow both an interference cathode (Ag substrate) and a typical photocathode (Si reference substrate) simultaneously to confirm that the effects are due to optical interactions with the substrate rather than photocathode composition or surface electron affinity differences. Growing the cathodes until the QE converges shows both the characteristic interference peaks during growth and the identical limiting case where the cathode is “infinitely thick,” in agreement with simulations. We also grow a cathode until the QE on Ag peaks and then stop the growth, demonstrating broadband QE enhancement.

36 MATERIALS SCIENCE↗
DOE OSTI2026

An efficient level set method for tracking many materials

Here, we present an efficient level set method to track an arbitrary number of materials. The algorithm is optimal in the sense that it only needs to store a single unsigned distance-like function and a single integer indicator function, independent of the number of materials or distinct regions being tracked. Furthermore, for smooth velocity fields and smooth interface shape, arbitrarily high order solutions can be demonstrated. For interfaces that are or become kinked, the solution is limited to second-order convergence rates in the L 1 norm and first-order in the L ∞ norm.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2016

Influence of surface crystallinity on the surface roughness of different ceramic glazes

Surface smoothness is an important characteristic of ceramic glazes as it influences chemical resistivity, glossiness and stainability of glazes. Surface crystallinity of zirconia-based and titania-based glazes (as two common opaque glazes), calcium-zinc based matte glaze and also two transparent glazes with different compositions were studied in this work. Degree of surface crystallinity measured by stereological methods using scanning electron microscopy (SEM) images showed that transparent glaze has the minimum degree of crystallinity (2.1%) and the maximum amount (25.4%) was obtained for matte glaze. In addition, X-ray diffraction (XRD) was used to characterize the glaze mineralogy. Optical surface profiler showed that the smoothest surface belongs to titania-based opaque glaze (R{sub a} = 0.0157 µm) and transparent glaze (R{sub a} = 0.0168 µm). In contrast, the roughest glaze surface corresponded to the matte glaze (R{sub a} = 0.2772 µm). The results suggest that surface roughness is influenced by crystallinity of surface, but degree of surface crystallinity is not the only parameter that can influence the surface roughness. The most important factor is the morphology of the crystals grow and their protrusion from the surface. - Highlights: •Degree of crystallinity of glazed surfaces was measured using stereological methods. •Crystallinity of the glazed surfaces is not the main reason of surface roughness. •The morphology of crystal grow and its surface protrusion can affect the roughness.

36 MATERIALS SCIENCE↗
NASA NTRS1998

The Space-Time Conservation Element and Solution Element Method: A New High-Resolution and Genuinely Multidimensional Paradigm for Solving Conservation Laws: The Two Dimensional Time Marching Schemes - 1

A new high resolution and genuinely multidimensional numerical method for solving conservation laws is being, developed. It was designed to avoid the limitations of the traditional methods. and was built from round zero with extensive physics considerations. Nevertheless, its foundation is mathmatically simple enough that one can build from it a coherent, robust. efficient and accurate numerical framework. Two basic beliefs that set the new method apart from the established methods are at the core of its development. The first belief is that, in order to capture physics more efficiently and realistically, the modeling, focus should be placed on the original integral form of the physical conservation laws, rather than the differential form. The latter form follows from the integral form under the additional assumption that the physical solution is smooth, an assumption that is difficult to realize numerically in a region of rapid chance. such as a boundary layer or a shock. The second belief is that, with proper modeling of the integral and differential forms themselves, the resulting, numerical solution should automatically be consistent with the properties derived front the integral and differential forms, e.g., the jump conditions across a shock and the properties of characteristics. Therefore a much simpler and more robust method can be developed by not using the above derived properties explicitly.

Chang, Sin-Chung↗
DOE OSTI2026

HostSub_GP: Precise Galaxy Background Subtraction in Transient Long-slit Spectroscopy with Gaussian Processes

We present a novel host galaxy subtraction technique in long-slit spectroscopy for extragalactic transients. Unlike classic methods which generally estimate the background using simple interpolation of local galaxy flux in the 2D spectrum, our approach leverages multi-band archival images of the host galaxies to model the background emission from the galaxy in the 2D spectrum. Such imaging encodes the wavelength-dependent galaxy profile along the slit, and is readily accessible through wide-field imaging surveys. We construct a smooth prior for the 2D galaxy profile with a Gaussian process (GP) based on these reference images, and use another GP to model the correlated deviations from the prior in the observed spectrum. This enables accurate inference of the galaxy flux blended with the transient. On synthetic long-slit data of a spiral galaxy extracted from a Multi Unit Spectroscopic Explorer hyper-spectral cube, the GP method remains robust as long as the host galaxy is spatially resolved and consistently outperforms classic methods. We apply the method to archival Keck spectra of two real transients, SN 2019eix and AT 2019qiz, to further demonstrate how the method uniquely recovers weak spectral features amid strong galaxy contamination, enabling refined constraints on the properties of both transients. We have released the software implementation, HostSub_GP, a scalable toolkit that leverages JAX, with an MIT license.

79 ASTRONOMY AND ASTROPHYSICS↗