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Lu, W.

Publications and source records attributed to Lu, W..

At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Geometric consideration of nanostructures for energy storage systems

Battery performance and its fade are determined by various aspects such as the transport of ions and electrons through heterogeneous internal structures; kinetic reactions at the interfaces; and the corresponding interplay between mechanical, chemical, and thermal responses. The fundamental factor determining this complex multiscale and multiphysical nature of a battery is the geometry of active materials. In this work, we systematically consider the tradeoffs among a selection of limiting geometries of media designed to store ions or other species via a diffusion process. Specifically, we begin the investigation by considering diffusion in spheres, rods, and plates at the particle level, in order to assess the effects of geometry, diffusivity, and rate on capacity. Then, the study is extended to considering of the volume fraction and particle network, as well as kinetics at the interface with electrolyte. Our study suggests that, in terms of overall bulk level material performance, thin film batteries may generate the highest energy density with high power capability when they are implemented at nanoscales or with highly diffusion materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Introducing single-crystal scattering and optical potentials into MCNPX: Predicting neutron emission from a convoluted moderator

Neutron transport simulation codes are an indispensable tool used for the design and construction of modern neutron scattering facilities and instrumentation. It has become increasingly clear that some neutron instrumentation has started to exploit physics that is not well-modelled by the existing codes. Particularly, the transport of neutrons through single crystals and across interfaces in MCNP(X), Geant4 and other codes ignores scattering from oriented crystals and refractive effects, and yet these are essential ingredients for the performance of monochromators and ultra-cold neutron transport respectively (to mention but two examples). In light of these developments, we have extended the MCNPX code to include a single-crystal neutron scattering model and neutron reflection/refraction physics. Furthermore, we have also generated silicon scattering kernels for single crystals of definable orientation with respect to an incoming neutron beam. As a first test of these new tools, we have chosen to model the recently developed convoluted moderator concept, in which a moderating material is interleaved with layers of perfect crystals to provide an exit path for neutrons moderated to energies below the crystal s Bragg cut off at locations deep within the moderator. Studies of simple cylindrical convoluted moderator systems of 100 mm diameter and composed of polyethylene and single crystal silicon were performed with the upgraded MCNPX code and reproduced the magnitude of effects seen in experiments compared to homogeneous moderator systems. Applying different material properties for refraction and reflection, and by replacing the silicon in the models with voids, we show that the emission enhancements seen in recent experiments are primarily caused by the transparency of the silicon/void layers. Finally the convoluted moderator experiments described by Iverson et al. were simulated and we find satisfactory agreement between the measurement and the results of simulations performed using the tools we have developed.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

SECOND TARGET STATION MODERATOR PERFORMANCE WITH A ROTATING TARGET

Oak Ridge National Laboratory manages and operates the Spallation Neutron Source and the High Flux Isotope Reactor, two of the world's most advanced neutron scattering facilities. Both facilities are funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Science, and are available to researchers from all over the world. Delivering cutting edge science requires continuous improvements and development of the facilities and instruments. The SNS was designed from the outset to accommodate an additional target station, or Second Target Station (STS), and an upgraded accelerator feeding proton beams to STS and the existing First Target Station (FTS). Upgrade of the accelerator and the design and construction of STS are being proposed. The presently considered STS configuration is driven with short (<1 s) proton pulses at 10 Hz repetition rate and 467 kW proton beam power, and is optimized for high intensity and high resolution long wavelength neutron applications. STS will allow installation of 22 beamlines and will expand and complement the current national neutron scattering capabilities. In 2015 the STS studies were performed for a compact tungsten target; first a stationary tungsten plate target was analyzed to considerable details and then dropped in favor of a rotating target. For both target options the proton beam footprint as small as acceptable from mechanical and heat removal aspects is required to arrive at a compact-volume neutron production zone in the target, which is essential for tight coupling of target and moderators and for achieving high-intensity peak neutron fluxes. This paper will present recent STS work with the emphasis on neutronics and moderator performance.

long wavelength neutrons↗
DOE OSTI2015

Towards a Measurement of the Half-Life of {sup 60}Fe for Stellar and Early Solar System Models

Radioisotopes, produced in stars and ejected into the Interstellar Medium, are important for constraining stellar and early Solar System (ESS) models. In particular, the half-life of the radioisotope, Fe-60, can have an impact on calculations for the timing for ESS events, the distance to nearby Supernovae, and the brightness of individual, non-steady-state Fe gamma ray sources in the Galaxy. A half-life measurement has been undertaken at the University of Notre Dame and measurements of the Fe-60/Fe-56 concentration of our samples using Accelerator Mass Spectrometry has begun. This result will be coupled with an activity measurement of the isomeric decay in Co-60, which is the decay product of Fe. Preliminary half-life estimates of (2.53 +/- 0.24) x 10(6) years seem to confirm the recent measurement by Rugel et al. (2009). (C) 2015 Elsevier B.V. All rights reserved.

Accelerator Mass Spectrometry↗
DOE OSTI2015

Breakup branches of Borromean beryllium-9

The breakup reaction {sup 9}Be({sup 4}He, 3α)n was measured using an array of four double-sided silicon strip detectors at beam energies of 22 and 26 MeV. Excited states in {sup 9}Be up to 12 MeV were populated and reconstructed through the measurement of the charged reaction products. It is proposed that limits on the spins and parities of the states can be derived from the way that they decay. Various breakup paths for excited states in {sup 9}Be have been explored including the {sup 8}Be{sub g.s.} + n, {sup 8}Be{sub 2{sup +}} + n and {sup 5}He{sub g.s.} + {sup 4}He channels. By imposing the condition that the breakup proceeded via the {sup 8}Be ground state, clean excitation spectra for {sup 9}Be were reconstructed. The remaining two breakup channels were found to possess strongly-overlapping kinematic signatures and more sophisticated methods (referenced) are required to completely disentangle these other possibilities. Emphasis is placed on the development of the experimental analysis and the usefulness of Monte-Carlo simulations for this purpose.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Multi-gigaelectronvolt acceleration of positrons in a self-loaded plasma wakefield

New accelerator concepts must be developed to make future particle colliders more compact and affordable. The Plasma Wakefield Accelerator (PWFA) is one such concept, where the electric field of a plasma wake excited by a charged-particle bunch is used to accelerate a trailing bunch of particles. To apply plasma acceleration to particle colliders, it is imperative that both the electrons and their antimatter counterpart, the positrons, are efficiently accelerated at high fields using plasmas1. While substantial progress has recently been reported on high-field, high-efficiency acceleration of electrons in a PWFA powered by an electron bunch 2, such an electron-driven wake is unsuitable for the acceleration and focusing of a positron bunch. Here we demonstrate a new regime of PWFA where particles in the front of a single positron bunch transfer their energy to a substantial number of those in the rear of the same bunch by exciting a wakefield in the plasma. In the process, the accelerating field is altered – self-loaded – so that about a billion positrons gain five gigaelectronvolts (GeV) of energy with a narrow energy spread in a distance of just 1.3 meters. They extract about 30% of the wake’s energy and form a spectrally distinct bunch with as low as a 1.8% r.m.s. energy spread. This demonstrated ability of positron-driven plasma wakes to efficiently accelerate a significant number of positrons with a small energy spread may overcome the long-standing challenge of positron acceleration in plasma-based accelerators.

ACCPHY↗
DOE OSTI2015

Comparative study on the charge-trapping properties of TaAlO and ZrAlO high-k composites with designed band alignment

The charge-trapping memory (CTM) structures Pt/Al{sub 2}O{sub 3}/TaAlO/Al{sub 2}O{sub 3}/p-Si and Pt/Al{sub 2}O{sub 3}/ZrAlO/Al{sub 2}O{sub 3}/p-Si were fabricated by using rf-sputtering and atomic layer deposition techniques, in which the potentials at the bottom of the conduction band (PBCB) of high-k composites TaAlO and ZrAlO were specially designed. With a lower PBCB difference between TaAlO and p-Si than that between ZrAlO and p-Si, TaAlO CTM device shows a better charge-trapping performance. A density of trapped charges 2.88 × 10{sup 13}/cm{sup 2} at an applied voltage of ±7 V was obtained for TaAlO CTM device, and it could keep about 60% of initially trapped charges after 10 years. It was suggested that the PBCB difference between high-k composite and p-Si dominates their charge-trapping behaviors.

36 MATERIALS SCIENCE↗
DOE OSTI2015

First Direct Measurement of C 12 ( C 12 , n ) Mg 23 at Stellar Energies

Neutrons produced by the carbon fusion reaction 12 C( 12 C,n) 23 Mg play an important role in stellar nucleosynthesis. However, past studies have shown large discrepancies between experimental data and theory, leading to an uncertain cross section extrapolation at astrophysical energies. Here in this paper, we present the first direct measurement that extends deep into the astrophysical energy range along with a new and improved extrapolation technique based on experimental data from the mirror reaction 12 C( 12 C,p) 23 Na . The new reaction rate has been determined with a well-defined uncertainty that exceeds the precision required by astrophysics models. Using our constrained rate, we find that 12 C ( 12 C,n) 23 Mg is crucial to the production of Na and Al in pop-III pair instability supernovae. It also plays a nonnegligible role in the production of weak s -process elements, as well as in the production of the important galactic γ-ray emitter 60 Fe.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2015

Electric-field-induced strain effects on the magnetization of a Pr 0.67 Sr 0.33 MnO 3 film

The electric-field control of magnetic properties of Pr 0.67 Sr 0.33 MnO 3 (PSMO) film on piezoelectric Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (PMNT) substrate was investigated. The piezoelectric response of the PMNT substrate to the electric field produced strain that was coupled to the PSMO film. The in-plane compressive (tensile) strain increased (decreased) the magnetization. The change of magnetic moment was associated with the Mn ions. First principle simulations showed that the strain-induced electronic redistribution of the two e g orbitals (3d z 2 and 3d x 2 -y 2 ) of Mn ions was responsible for the change of magnetic moment. This work demonstrates that the magnetoelectric effect in manganite/piezoelectric hetero-structures originates from the change in eg orbital occupancy of Mn ions induced by strain rather than the interfacial effect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Implementation of a hybrid particle code with a PIC description in r–z and a gridless description in ϕ into OSIRIS

For many plasma physics problems, three-dimensional and kinetic effects are very important. However, such simulations are very computationally intensive. Fortunately, there is a class of problems for which there is nearly azimuthal symmetry and the dominant three-dimensional physics is captured by the inclusion of only a few azimuthal harmonics. Recently, it was proposed [1] to model one such problem, laser wakefield acceleration, by expanding the fields and currents in azimuthal harmonics and truncating the expansion. The complex amplitudes of the fundamental and first harmonic for the fields were solved on an r–z grid and a procedure for calculating the complex current amplitudes for each particle based on its motion in Cartesian geometry was presented using a Marder's correction to maintain the validity of Gauss's law. In this paper, we describe an implementation of this algorithm into OSIRIS using a rigorous charge conserving current deposition method to maintain the validity of Gauss's law. We show that this algorithm is a hybrid method which uses a particles-in-cell description in r–z and a gridless description in ϕ. We include the ability to keep an arbitrary number of harmonics and higher order particle shapes. Examples for laser wakefield acceleration, plasma wakefield acceleration, and beam loading are also presented and directions for future work are discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Technical Design Report, Second Target Station

The Second Target Station (STS) is a proposed upgrade for SNS. It includes a doubling of the accelerator power and an additional instrument hall. The new instrument hall will receive a 467 kW 10 Hz beam. The parameters and preliminary design aspects of the STS are presented for the accelerator, target systems, instrument hall, instruments and civil construction aspects.

43 PARTICLE ACCELERATORS↗
DOE OSTI2014

Transverse coupling property of beam from ECR ion sources

Experimental evidence of the property of transverse coupling of beam from Electron Cyclotron Resonance (ECR) ion source is presented. It is especially of interest for an ECR ion source, where the cross section of extracted beam is not round along transport path due to the magnetic confinement configuration. When the ions are extracted and accelerated through the descending axial magnetic field at the extraction region, the horizontal and vertical phase space strongly coupled. In this study, the coupling configuration between the transverse phase spaces of the beam from ECR ion source is achieved by beam back-tracking simulation based on the measurements. The reasonability of this coupling configuration has been proven by a series of subsequent simulations.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

High-efficiency acceleration of an electron beam in a plasma wakefield accelerator

High-efficiency acceleration of charged particle beams at high gradients of energy gain per unit length is necessary to achieve an affordable and compact high-energy collider. The plasma wakefield accelerator is one concept being developed for this purpose. In plasma wakefield acceleration, a charge-density wake with high accelerating fields is driven by the passage of an ultra-relativistic bunch of charged particles (the drive bunch) through a plasma. Furthermore, if a second bunch of relativistic electrons (the trailing bunch) with sufficient charge follows in the wake of the drive bunch at an appropriate distance, it can be efficiently accelerated to high energy.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗