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At least 1,153 records · Page 64

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DOE OSTI2013

A direct Vlasov code to study the non-stationary current collection by a cylindrical Langmuir probe

The time-dependent current collection by a cylindrical Langmuir probe, whose bias is suddenly changed from zero to a positive or negative finite value, is studied with a novel direct Vlasov code. The numerical algorithm is based on finite-difference formulas to approximate spatial and velocity derivatives and the time integration is carried out with an explicit Runge-Kutta method, or in the case of probe radius small compared with the Debye length, by using the unconditionally stable backward Euler scheme. Both electrons and ions are treated kinetically by the code, which implements initial and boundary conditions that are consistent with the presence of the probe. Within the considered parameter range, the plasma sheath around the probe exhibited an overshoot and it later recovered a steady state. Phase space diagrams of the particle trajectories revealed the presence of a trapped population of particles. The dependence of this population as a function of the probe radius is presented as well as a comparison with the stationary theory. The performance of the code and a comparison with previously used particle-in-cell algorithms are discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2013

Electron attachment to CF{sub 3} and CF{sub 3}Br at temperatures up to 890 K: Experimental test of the kinetic modeling approach

Thermal rate constants and product branching fractions for electron attachment to CF{sub 3}Br and the CF{sub 3} radical have been measured over the temperature range 300-890 K, the upper limit being restricted by thermal decomposition of CF{sub 3}Br. Both measurements were made in Flowing Afterglow Langmuir Probe apparatuses; the CF{sub 3}Br measurement was made using standard techniques, and the CF{sub 3} measurement using the Variable Electron and Neutral Density Attachment Mass Spectrometry technique. Attachment to CF{sub 3}Br proceeds exclusively by the dissociative channel yielding Br{sup -}, with a rate constant increasing from 1.1 Multiplication-Sign 10{sup -8} cm{sup 3} s{sup -1} at 300 K to 5.3 Multiplication-Sign 10{sup -8} cm{sup 3} s{sup -1} at 890 K, somewhat lower than previous data at temperatures up to 777 K. CF{sub 3} attachment proceeds through competition between associative attachment yielding CF{sub 3}{sup -} and dissociative attachment yielding F{sup -}. Prior data up to 600 K showed the rate constant monotonically increasing, with the partial rate constant of the dissociative channel following Arrhenius behavior; however, extrapolation of the data using a recently proposed kinetic modeling approach predicted the rate constant to turn over at higher temperatures, despite being only {approx}5% of the collision rate. The current data agree well with the previous kinetic modeling extrapolation, providing a demonstration of the predictive capabilities of the approach.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2013

Time Domain Simulations of Chemical Bonding Effects in Surface-Enhanced Spectroscopy

We employ the atom-centered density-matrix propagation method to illustrate how time-dependent structural and conformational changes affect the electronic structure and derived spectroscopic properties of a prototypical finite metal cluster-bound π-conjugated organic, namely the Ag7-benzenethiol complex. We find that the calculated spectroscopic properties are dictated by large amplitude motion which controls the coupling between the aromatic ring of the molecule and the metal cluster. The simulated vibrational spectra of Ag7-benzenethiol are in accord with previous experiments which probe Raman scattering from benzenethiol adsorbed on silver surfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2013

Robust relativistic electron mirrors in laser wakefields for enhanced Thomson backscattering

By adopting an up-ramp density profile, we propose to generate relativistic electron mirrors from laser-driven underdense plasma waves, which are insensitive to finite thermal temperature within a certain range. Along the density ramp, premature wavebreaking due to thermal effects is shown to be well mitigated. Under sufficiently high amplitudes of wake excitation, overcritical dense electron mirrors can pile up when approaching the end of the up-ramp. The consequent mirror speed can be stably driven to the group velocity of the laser propagating in a corresponding uniform plasma. Compared with using purely uniform but thermal plasmas, the present thermal-insensitive mirrors can provide enhanced scattering efficiency and spectral upshift for a counter-propagating probe pulse. These observations are confirmed by multi-dimensional particle-in-cell simulations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2014

Disentangling phonon and impurity interactions in δ-doped Si(001)

We present a study of the phonon and impurity interactions in a shallow two dimensional electron gas formed in Si(001). A highly conductive ultra-narrow n-type dopant δ-layer, which serves as a platform for quantum computation architecture, is formed and studied by angle resolved photoemission spectroscopy (ARPES) and temperature dependent nanoscale 4-point probe (4PP). The bandstructure of the δ-layer state is both measured and simulated. At 100 K, good agreement is only achieved by including interactions; electron-impurity scattering (W{sub 0} = 56 to 61 meV); and electron-phonon coupling (λ = 0.14 ± 0.04). These results are shown to be consistent with temperature dependent 4PP resistance measurements which indicate that at 100 K, ≈7∕8 of the measured resistance is due to impurity scattering with the remaining 1/8 coming from phonon interactions. In both resistance and bandstructure measurements, the impurity contribution exhibits a variability of ≈9% for nominally identical samples. The combination of ARPES and 4PP affords a thorough insight into the relevant contributions to electrical resistance in reduced dimensionality electronic platforms.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Effect of secondary electron emission on the plasma sheath

In this experiment, plasma sheath potential profiles are measured over boron nitride walls in argon plasma and the effect of secondary electron emission is observed. Results are compared to a kinetic model. Plasmas are generated with a number density of 3 × 10{sup 12} m{sup −3} at a pressure of 10{sup −4} Torr-Ar, with a 1%–16% fraction of energetic primary electrons. The sheath potential profile at the surface of each sample is measured with emissive probes. The electron number densities and temperatures are measured in the bulk plasma with a planar Langmuir probe. The plasma is non-Maxwellian, with isotropic and directed energetic electron populations from 50 to 200 eV and hot and cold Maxwellian populations from 3.6 to 6.4 eV and 0.3 to 1.3 eV, respectively. Plasma Debye lengths range from 4 to 7 mm and the ion-neutral mean free path is 0.8 m. Sheath thicknesses range from 20 to 50 mm, with the smaller thickness occurring near the critical secondary electron emission yield of the wall material. Measured floating potentials are within 16% of model predictions. Measured sheath potential profiles agree with model predictions within 5 V (∼1 T{sub e}), and in four out of six cases deviate less than the measurement uncertainty of 1 V.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2019

Radial transport modeling of high density deuterium plasmas in proto-MPEX with the B2.5-Eirene code

High density (≥6 × 10 19 m –3 ), low temperature (2–6 eV) helicon discharges in the Prototype Material Plasma Exposure eXperiment (Proto-MPEX) are analyzed with the coupled multifluid plasma, kinetic neutrals code B2.5-Eirene. The interpretative analyses are constrained by data from multiple diagnostics, including Langmuir probes, Mach probes, filterscopes, infrared TV system, Thomson scattering, and baratrons. The objectives of the transport simulations include: investigation of the effects of heating, fueling, and plasma production; pumping, and assumed radial transport models on the calculated density and temperature distributions; plasma flow profiles and power balance. The primary objective in this report is to investigate the effects of the radial transport model in full plasma (the entire length of the plasma column in Proto-MPEX) data-constrained simulations. Results from three assumed forms of the radial transport coefficients are presented, including spatially constant, radially decreasing, and Bohm (D,χ ~T e /|B|). The results from each of the three transport coefficient sets agree qualitatively with the core (near axis) data. With the implicit T e dependence, the Bohm coefficients tend to decrease as functions of radius, although not as strongly as the centrally peaked set. The axial variation in the Bohm coefficients is largely due to the axial structure of the magnetic field. Furthermore, the agreement of the simulations and the diagnostic data with the Bohm set indicates that transport in the plasma column of Proto-MPEX is dominated by Bohm diffusion.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2023

Slater transition methods for core-level electron binding energies

Methods for computing core-level ionization energies using self-consistent field (SCF) calculations are evaluated and benchmarked. These include a “full core hole” (or “ΔSCF”) approach that fully accounts for orbital relaxation upon ionization, but also methods based on Slater’s transition concept in which the binding energy is estimated from an orbital energy level that is obtained from a fractional-occupancy SCF calculation. A generalization that uses two different fractional-occupancy SCF calculations is also considered. The best of the Slater-type methods afford mean errors of 0.3–0.4 eV with respect to experiment for a dataset of K-shell ionization energies, a level of accuracy that is competitive with more expensive many-body techniques. An empirical shifting procedure with one adjustable parameter reduces the average error below 0.2 eV. Here, this shifted Slater transition method is a simple and practical way to compute core-level binding energies using only initial-state Kohn–Sham eigenvalues. It requires no more computational effort than ΔSCF and may be especially useful for simulating transient x-ray experiments where core-level spectroscopy is used to probe an excited electronic state, for which the ΔSCF approach requires a tedious state-by-state calculation of the spectrum. As an example, we use Slater-type methods to model x-ray emission spectroscopy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2015

TOWARD THE FORMATION OF CARBONACEOUS REFRACTORY MATTER IN HIGH TEMPERATURE HYDROCARBON-RICH ATMOSPHERES OF EXOPLANETS UPON MICROMETEOROID IMPACT

We report on laboratory simulation experiments mimicking the chemical processing of model atmospheres of exoplanets containing C3 and C4 hydrocarbons at moderate temperatures of 400 K upon interaction of catalytic surfaces of micrometeoroids. By utilizing an ultrasonic levitator device and heating singly levitated particles under simulated microgravity conditions, Raman spectroscopy is utilized as a non-invasive tool to probe on line and in situ the conversion of C3 and C4 hydrocarbons to refractory carbonaceous matter on the surfaces of levitated particles. Secondary Ion Mass Spectrometry and electron microscopic imaging were also conducted to gain further insight into the elementary composition and structures of the refractories formed. Our results provide compelling evidence that in the presence of a catalytic surface, which can be supplied in the form of micrometeoroids and atmospheric dust particles, hydrocarbon gases present in the atmospheres of exoplanets can be converted to refractory, carbon-rich carbonaceous matter of mainly graphitic structure with a carbon content of at least 90% at elevated temperatures. This finding might explain the low methane to carbon monoxide (CH{sub 4}–CO) ratio in the hot Neptune GJ 436b, where the abundant methane photochemically converts to higher order hydrocarbons and ultimately to refractory graphite-like carbon in the presence of a silicon surface.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2016

Probing plasmons in three dimensions by combining complementary spectroscopies in a scanning transmission electron microscope

The nanoscale optical response of surface plasmons in three-dimensional metallic nanostructures plays an important role in many nanotechnology applications, where precise spatial and spectral characteristics of plasmonic elements control device performance. Electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) within a scanning transmission electron microscope have proven to be valuable tools for studying plasmonics at the nanoscale. Each technique has been used separately, producing three-dimensional reconstructions through tomography, often aided by simulations for complete characterization. Here we demonstrate that the complementary nature of the two techniques, namely that EELS probes beam-induced electronic excitations while CL probes radiative decay, allows us to directly obtain a spatially- and spectrally-resolved picture of the plasmonic characteristics of nanostructures in three dimensions. Furthermore, the approach enables nanoparticle-by-nanoparticle plasmonic analysis in three dimensions to aid in the design of diverse nanoplasmonic applications.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Disconnected covariance of 2-point functions in large-scale structure

Optimal analyses using the 2-point functions of large-scale structure probes require accurate covariance matrices. A covariance matrix of the 2-point function comprises the disconnected part and the connected part. While the connected covariance only becomes important on small scales, the disconnected covariance is dominant on large scales, where the survey window has a significant impact. In this work, we develop an analytical method to compute the disconnected covariance, accounting for the window effect. Derived under the flat-sky approximation, our formalism is applicable to wide surveys by swapping in the curved-sky window functions. Our method works for both the power spectrum and the correlation function, and applies to the covariances of various probes including the multipoles and the wedges of 3D clustering, the angular and the projected statistics of clustering and shear, as well as the cross covariances between different probes. We verify the analytic covariance against the sample covariance from the galaxy mock simulations in two test cases: (1) the power spectrum multipole covariance, and (2) the joint covariance of the projected correlation function and the correlation function multipoles. Our method achieve good agreement with the mocks, while at a negligible computational cost. Unlike mocks, our analytic covariance is free of sampling noise, which often leads to numerical problems and the need to inflate the errors. In addition, our method can use the best-fit power spectrum as input, in contrast to the standard procedure of using a fiducial model that may deviate significantly from the truth. We also show that a naive diagonal power spectrum covariance underestimates the signal-to-noise ratio compared to our analytic covariance. The code that accompanies this paper is available at https://github.com/eelregit/covdisc.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2023

A riboswitch separated from its ribosome-binding site still regulates translation

Abstract Riboswitches regulate downstream gene expression by binding cellular metabolites. Regulation of translation initiation by riboswitches is posited to occur by metabolite-mediated sequestration of the Shine-Dalgarno sequence (SDS), causing bypass by the ribosome. Recently, we solved a co-crystal structure of a prequeuosine1-sensing riboswitch from Carnobacterium antarcticum that binds two metabolites in a single pocket. The structure revealed that the second nucleotide within the gene-regulatory SDS, G34, engages in a crystal contact, obscuring the molecular basis of gene regulation. Here, we report a co-crystal structure wherein C10 pairs with G34. However, molecular dynamics simulations reveal quick dissolution of the pair, which fails to reform. Functional and chemical probing assays inside live bacterial cells corroborate the dispensability of the C10–G34 pair in gene regulation, leading to the hypothesis that the compact pseudoknot fold is sufficient for translation attenuation. Remarkably, the C. antarcticum aptamer retained significant gene-regulatory activity when uncoupled from the SDS using unstructured spacers up to 10 nucleotides away from the riboswitch—akin to steric-blocking employed by sRNAs. Accordingly, our work reveals that the RNA fold regulates translation without SDS sequestration, expanding known riboswitch-mediated gene-regulatory mechanisms. The results infer that riboswitches exist wherein the SDS is not embedded inside a stable fold.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2024

Probing nuclear structure of heavy ions at energies available at the CERN Large Hadron Collider

We perform high-statistics simulations to study the impacts of nuclear structure on the ratios of anisotropic flow observables in 208 Pb + 208 Pb and 129 Xe + 129 Xe collisions at the Large Hadron Collider. Even with 40% difference in atomic numbers between 208 Pb and 129 Xe nuclei, the ratios of anisotropic flow in the same centrality class between the two collision systems are strongly affected by the nuclear structure inputs in the initial state. The ratios of v 2 ⁡{4}/v 2 ⁡{2} in these collisions are sensitive to the nuclear skin thickness of the colliding nuclei, providing indirect constraints on the nuclei's neutron skin. In conclusion, our model predictions serve as a benchmark to compare with experimental measurements.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2017

Machine Learning for Turbulence Model Development Using a High-Fidelity HPT Cascade Simulation

The validity of the Boussinesq approximation in the wake behind a high-pressure turbine blade is explored. We probe the mathematical assumptions of such a relationship by employing a least-squares technique. Next, we use an evolutionary algorithm to modify the anisotropy tensor a priori using highly resolved LES data. In the latter case we build a non-linear stress-strain relationship. Results show that the standard eddy-viscosity assumption underpredicts turbulent diffusion and is theoretically invalid. By increasing the coefficient of the linear term, the farwake prediction shows minor improvement. By using additional non-linear terms in the stress-strain coupling relationship, created by the evolutionary algorithm, the near-wake can also be improved upon. Terms created by the algorithm are scrutinized and the discussion is closed by suggesting a tentative non-linear expression for the Reynolds stress, suitable for the wake behind a high-pressure turbine blade.

Weatheritt, Jack↗

DOE OSTI2017

Probing of Zγγ production sensitivity to anomalous quartic gauge couplings at LHC experiments with √ s =13 TeV

Sensitivity to anomalous ZZγγ and Zγγγ couplings in Zγγ production was probed for the ATLAS experiment at Large Hadron Collider. Zγγ process with anomalous couplings simulation in ppcollisions with √ s = 13 TeV was performed using VBFNLO MC generator. The expected limits on the Effective Field Theory parameters f{sub T0}/Λ4, f{sub T5}/Λ4, f{sub T9}/Λ4, f{sub M2}/Λ4, f{sub M3}/Λ4 were extracted for 5 fb{sup −1} integral luminosity using the distribution on the invariant mass of Zγγ from the combination of charged leptonic decay channels of Z boson (Zγγ → μ{sup +}μ{sup −}γγ and Zγγ → e{sup +}e{sup −}γγ).

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2016

Xyce Parallel Electronic Simulator Reference Guide Version 6.6.

This document is a reference guide to the Xyce Parallel Electronic Simulator, and is a companion document to the Xyce Users' Guide [1] . The focus of this document is (to the extent possible) exhaustively list device parameters, solver options, parser options, and other usage details of Xyce . This document is not intended to be a tutorial. Users who are new to circuit simulation are better served by the Xyce Users' Guide [1] . The information herein is subject to change without notice. Copyright c 2002-2016 Sandia Corporation. All rights reserved. Acknowledgements The BSIM Group at the University of California, Berkeley developed the BSIM3, BSIM4, BSIM6, BSIM-CMG and BSIM-SOI models. The BSIM3 is Copyright c 1999, Regents of the University of California. The BSIM4 is Copyright c 2006, Regents of the University of California. The BSIM6 is Copyright c 2015, Regents of the University of California. The BSIM-CMG is Copyright c 2012 and 2016, Regents of the University of California. The BSIM-SOI is Copyright c 1990, Regents of the University of California. All rights reserved. The Mextram model has been developed by NXP Semiconductors until 2007, Delft University of Technology from 2007 to 2014, and Auburn University since April 2015. Copyrights c of Mextram are with Delft University of Technology, NXP Semiconductors and Auburn University. The MIT VS Model Research Group developed the MIT Virtual Source (MVS) model. Copyright c 2013 Massachusetts Institute of Technology (MIT). The EKV3 MOSFET model was developed by the EKV Team of the Electronics Laboratory-TUC of the Technical University of Crete. Trademarks Xyce TM Electronic Simulator and Xyce TM are trademarks of Sandia Corporation. Orcad, Orcad Capture, PSpice and Probe are registered trademarks of Cadence Design Systems, Inc. Microsoft, Windows and Windows 7 are registered trademarks of Microsoft Corporation. Medici, DaVinci and Taurus are registered trademarks of Synopsys Corporation. Amtec and TecPlot are trademarks of Amtec Engineering, Inc. All other trademarks are property of their respective owners. Contacts World Wide Web http://xyce.sandia.gov https://info.sandia.gov/xyce (Sandia only) Email xyce@sandia.gov (outside Sandia) xyce-sandia@sandia.gov (Sandia only) Bug Reports (Sandia only) http://joseki-vm.sandia.gov/bugzilla http://morannon.sandia.gov/bugzilla

47 OTHER INSTRUMENTATION↗

DOE OSTI2016

Xyce Parallel Electronic Simulator Users' Guide Version 6.6.

This manual describes the use of the Xyce Parallel Electronic Simulator. Xyce has been de- signed as a SPICE-compatible, high-performance analog circuit simulator, and has been written to support the simulation needs of the Sandia National Laboratories electrical designers. This development has focused on improving capability over the current state-of-the-art in the following areas: Capability to solve extremely large circuit problems by supporting large-scale parallel com- puting platforms (up to thousands of processors). This includes support for most popular parallel and serial computers. A differential-algebraic-equation (DAE) formulation, which better isolates the device model package from solver algorithms. This allows one to develop new types of analysis without requiring the implementation of analysis-specific device models. Device models that are specifically tailored to meet Sandia's needs, including some radiation- aware devices (for Sandia users only). Object-oriented code design and implementation using modern coding practices. Xyce is a parallel code in the most general sense of the phrase -- a message passing parallel implementation -- which allows it to run efficiently a wide range of computing platforms. These include serial, shared-memory and distributed-memory parallel platforms. Attention has been paid to the specific nature of circuit-simulation problems to ensure that optimal parallel efficiency is achieved as the number of processors grows. The information herein is subject to change without notice. Copyright c 2002-2016 Sandia Corporation. All rights reserved. Acknowledgements The BSIM Group at the University of California, Berkeley developed the BSIM3, BSIM4, BSIM6, BSIM-CMG and BSIM-SOI models. The BSIM3 is Copyright c 1999, Regents of the University of California. The BSIM4 is Copyright c 2006, Regents of the University of California. The BSIM6 is Copyright c 2015, Regents of the University of California. The BSIM-CMG is Copyright c 2012 and 2016, Regents of the University of California. The BSIM-SOI is Copyright c 1990, Regents of the University of California. All rights reserved. The Mextram model has been developed by NXP Semiconductors until 2007, Delft University of Technology from 2007 to 2014, and Auburn University since April 2015. Copyrights c of Mextram are with Delft University of Technology, NXP Semiconductors and Auburn University. The MIT VS Model Research Group developed the MIT Virtual Source (MVS) model. Copyright c 2013 Massachusetts Institute of Technology (MIT). The EKV3 MOSFET model was developed by the EKV Team of the Electronics Laboratory-TUC of the Technical University of Crete. Trademarks Xyce TM Electronic Simulator and Xyce TM are trademarks of Sandia Corporation. Orcad, Orcad Capture, PSpice and Probe are registered trademarks of Cadence Design Systems, Inc. Microsoft, Windows and Windows 7 are registered trademarks of Microsoft Corporation. Medici, DaVinci and Taurus are registered trademarks of Synopsys Corporation. Amtec and TecPlot are trademarks of Amtec Engineering, Inc. All other trademarks are property of their respective owners. Contacts World Wide Web http://xyce.sandia.gov https://info.sandia.gov/xyce (Sandia only) Email xyce@sandia.gov (outside Sandia) xyce-sandia@sandia.gov (Sandia only) Bug Reports (Sandia only) http://joseki-vm.sandia.gov/bugzilla http://morannon.sandia.gov/bugzilla

47 OTHER INSTRUMENTATION↗

DOE OSTI2017

Xyce Parallel Electronic Simulator Reference Guide Version 6.7.

97 MATHEMATICS AND COMPUTING↗