Oak Ridge Assoc Universities

Oak Ridge, TN, United States

Oak Ridge Assoc Universities

Oak Ridge, TN, United States
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Paz-Soldan C.,General Atomics | Cooper C.M.,Oak Ridge Assoc Universities | Aleynikov P.,Max Planck Institute for Plasma Physics (Greifswald) | Pace D.C.,General Atomics | And 8 more authors.
Physical Review Letters | Year: 2017

Novel spatial, temporal, and energetically resolved measurements of bremsstrahlung hard-x-ray (HXR) emission from runaway electron (RE) populations in tokamaks reveal nonmonotonic RE distribution functions whose properties depend on the interplay of electric field acceleration with collisional and synchrotron damping. Measurements are consistent with theoretical predictions of momentum-space attractors that accumulate runaway electrons. RE distribution functions are measured to shift to a higher energy when the synchrotron force is reduced by decreasing the toroidal magnetic field strength. Increasing the collisional damping by increasing the electron density (at a fixed magnetic and electric field) reduces the energy of the nonmonotonic feature and reduces the HXR growth rate at all energies. Higher-energy HXR growth rates extrapolate to zero at the expected threshold electric field for RE sustainment, while low-energy REs are anomalously lost. The compilation of HXR emission from different sight lines into the plasma yields energy and pitch-angle-resolved RE distributions and demonstrates increasing pitch-angle and radial gradients with energy. © 2017 American Physical Society.


Klahr B.M.,Oak Ridge Assoc Universities | Peterson D.R.,U.S. Department of Energy | Randolph K.L.,U.S. Department of Energy | Miller E.L.,U.S. Department of Energy
ECS Transactions | Year: 2016

The emergence of hydrogen and fuel cell technologies offers important and potentially transformative environmental and energy security benefits. In recent years, research supported by the U.S. Department of Energy's Fuel Cell Technologies Office has contributed substantially to the development and advancement of these technologies. The research investments to reduce costs in fuel cell stacks, for example, are clearly paying off, as commercial fuel-cell electric vehicles are being rolled out by major car manufacturers today. With increasing market penetration of FCEVs, enabling technologies for the affordable and widespread production of clean, renewable hydrogen is becoming increasingly important. Long term commercial viability of renewable hydrogen in the commercial marketplace will rely on continued materials research on several important fronts. Examples include the discovery and development of efficient, durable, and cost effective materials, devices and systems for hydrogen production based on: (1) low and high temperature advanced electrolysis powered by renewable electricity sources; (2) direct low-temperature photoelectrochemical water splitting powered by sunlight; and (3) direct high-temperature solar thermochemical water splitting. Research innovations in macro-, meso- and nano-scale materials are all needed for pushing forward the state-of-the-art in these areas. Such innovations are being facilitated by the DOE Energy Materials Network (EMN) initiative which aims to accelerate materials research and development (R&D) for clean energy applications through the streamlined utilization of advanced scientific resources in theory. computation, experimentation, analysis, and data informatics. The "HydroGEN" EMN consortium on Advanced Water Splitting Materials for renewable hydrogen production will be described and discussed. © The Electrochemical Society.


Pain S.D.,Oak Ridge National Laboratory | Bardayan D.W.,Oak Ridge National Laboratory | Bardayan D.W.,University of Notre Dame | Blackmon J.C.,Louisiana State University | And 23 more authors.
Physical Review Letters | Year: 2015

The Galactic 1.809-MeV γ-ray signature from the β decay of Al26g is a dominant target of γ-ray astronomy, of which a significant component is understood to originate from massive stars. The Al26g(p,γ)Si27 reaction is a major destruction pathway for Al26g at stellar temperatures, but the reaction rate is poorly constrained due to uncertainties in the strengths of low-lying resonances in Si27. The Al26g(d,p)Al27 reaction has been employed in inverse kinematics to determine the spectroscopic factors, and hence resonance strengths, of proton resonances in Si27 via mirror symmetry. The strength of the 127-keV resonance is found to be a factor of 4 higher than the previously adopted upper limit, and the upper limit for the 68-keV resonance has been reduced by an order of magnitude, considerably constraining the Al26g destruction rate at stellar temperatures. © 2015 American Physical Society.


Miller S.G.,NASA | Williams T.S.,NASA | Baker J.S.,Oak Ridge Assoc Universities | Sola F.,NASA | And 7 more authors.
ACS Applied Materials and Interfaces | Year: 2014

The inherent strength of individual carbon nanotubes (CNTs) offers considerable opportunity for the development of advanced, lightweight composite structures. Recent work in the fabrication and application of CNT forms such as yarns and sheets has addressed early nanocomposite limitations with respect to nanotube dispersion and loading and has pushed the technology toward structural composite applications. However, the high tensile strength of an individual CNT has not directly translated into that of sheets and yarns, where the bulk material strength is limited by intertube electrostatic attractions and slippage. The focus of this work was to assess postprocessing of CNT sheets and yarns to improve the macro-scale strength of these material forms. Both small-molecule functionalization and electron-beam irradiation were evaluated as means to enhance the tensile strength and Young's modulus of the bulk CNT materials. Mechanical testing revealed a 57% increase in tensile strength of CNT sheets upon functionalization compared with unfunctionalized sheets, while an additional 48% increase in tensile strength was observed when functionalized sheets were irradiated. Similarly, small-molecule functionalization increased tensile strength of yarn by up to 25%, whereas irradiation of the functionalized yarns pushed the tensile strength to 88% beyond that of the baseline yarn. © 2014 American Chemical Society.


Miernik K.,Oak Ridge National Laboratory | Miernik K.,University of Warsaw | Rykaczewski K.P.,Oak Ridge National Laboratory | Gross C.J.,Oak Ridge National Laboratory | And 21 more authors.
Physical Review Letters | Year: 2013

Beta decay of Ga86 was studied by means of β-neutron-γ spectroscopy. An isotopically pure Ga86 beam was produced at the Holifield Radioactive Ion Beam Facility using a resonance ionization laser ion source and high-resolution electromagnetic separation. The decay of Ga86 revealed a half-life of 43-15+21 ms and large β-delayed one-neutron and two-neutron branching ratios of P1n=60(10)% and P2n=20(10)%. The βγ decay of Ga86 populated a 527 keV transition that is interpreted as the deexcitation of the first 2+ state in the N=54 isotone Ge86 and suggests a quick onset of deformation in Ge isotopes beyond N=50. © 2013 American Physical Society.


Allmond J.M.,Oak Ridge National Laboratory | Brown B.A.,Michigan State University | Stuchbery A.E.,Australian National University | Galindo-Uribarri A.,Oak Ridge National Laboratory | And 9 more authors.
Physical Review C - Nuclear Physics | Year: 2014

High-precision reduced electric-quadrupole transition probabilities B(E2;01+→21+) have been measured from single-step Coulomb excitation of semi-magic Ni58,60,62,64 (Z=28) beams at 1.8 MeV per nucleon on a natural carbon target. The energy loss of the nickel beams through the carbon target were directly measured with a zero-degree Bragg detector and the absolute B(E2) values were normalized by Rutherford scattering. The B(E2) values disagree with recent lifetime studies that employed the Doppler-shift attenuation method. The present high-precision B(E2) values reveal an asymmetry about Ni62, midshell between N=28 and 40, with larger values towards Ni56 (Z=N=28). The experimental B(E2) values are compared with shell-model calculations in the full pf model space and the results indicate a soft Ni56 core. © 2014 American Physical Society.


Wingard L.A.,Oak Ridge Assoc Universities | Guzman P.E.,U.S. Army | Sabatini J.J.,U.S. Army
Organic Process Research and Development | Year: 2016

A new procedure for the synthesis and isolation of dichloroglyoxime is discussed. This material has historically been synthesized from glyoxime and elemental chlorine gas. Chlorine gas is difficult to handle and control in the laboratory and has a high toxicity profile. Our method for making dichloroglyoxime in high purity uses glyoxime and N-chlorosuccinimide in DMF, with a lithium chloride-based workup. Overall yields are comparable with those obtained using the procedure involving the use of chlorine gas. © 2016 American Chemical Society.


Chrystal C.,Oak Ridge Assoc Universities | Burrell K.H.,General Atomics | Grierson B.A.,Princeton Plasma Physics Laboratory | Pace D.C.,General Atomics
Review of Scientific Instruments | Year: 2015

Neutral beam injection is used in tokamaks to heat, apply torque, drive non-inductive current, and diagnose plasmas. Neutral beam diagnostics need accurate spatial calibrations to benefit from the measurement localization provided by the neutral beam. A new technique has been developed that uses in situ measurements of neutral beam emission to determine the spatial location of the beam and the associated diagnostic views. This technique was developed to improve the charge exchange recombination (CER) diagnostic at the DIII-D tokamak and uses measurements of the Doppler shift and Stark splitting of neutral beam emission made by that diagnostic. These measurements contain information about the geometric relation between the diagnostic views and the neutral beams when they are injecting power. This information is combined with standard spatial calibration measurements to create an integrated spatial calibration that provides a more complete description of the neutral beam-CER system. The integrated spatial calibration results are very similar to the standard calibration results and derived quantities from CER measurements are unchanged within their measurement errors. The methods developed to perform the integrated spatial calibration could be useful for tokamaks with limited physical access. © 2015 AIP Publishing LLC.


Materese C.K.,NASA | Materese C.K.,Oak Ridge Assoc Universities | Nuevo M.,NASA | Nuevo M.,Bay Area Environmental Research Institute | Sandford S.A.,NASA
Astrophysical Journal | Year: 2015

Aromatic heterocyclic molecules are an important class of molecules of astrophysical and biological significance that include pyridine, pyrimidine, and their derivatives. Such compounds are believed to exist in interstellar and circumstellar environments, though they have never been observed in the gas phase. Regardless of their presence in the gas phase in space, numerous heterocycles have been reported in carbonaceous meteorites, which indicates that they are formed under astrophysical conditions. The experimental work described here shows that N- and O-heterocyclic molecules can form from the ultraviolet (UV) irradiation of the homocyclic aromatic molecules benzene (C6H6) or naphthalene (C10H8) mixed in ices containing H2O and NH3. This represents an alternative way to generate aromatic heterocycles to those considered before and may have important implications for astrochemistry and astrobiology. © 2015. The American Astronomical Society. All rights reserved..


Tchonkova M.,Oak Ridge Assoc Universities
Engineering Computations (Swansea, Wales) | Year: 2015

Purpose - The purpose of this paper is to present an original mixed least squares method for the numerical solution of vector wave equations. Design/methodology/approach - The proposed approach involves two different types of unknowns: velocities and stresses. The approximate solution to the dynamic elasticity equations is obtained via a minimization of a least squares functional, consisting of two terms: a term, which includes the squared residual of a weak form of the time rate of the constitutive relationships, expressed in terms of velocities and stresses, and a term, which depends on the squared residual of the equations of motion. At each time step the functional is minimized with respect to the velocities and stresses, which for the purpose of this study, are approximated by equal order piece-wise linear polynomial functions. The time discretization is based upon the backward Euler scheme. The displacements are computed from the obtained velocities in terms of a finite difference interpolation. Findings - To test the performance of the method, it has been implemented in original computer codes, using object-oriented logic. One model problem has been solved: propagation of Rayleigh waves. The performed convergence study suggests that the method is convergent for both: velocities and stresses. The obtained results show excellent agreement between the exact and analytical solutions for displacement modes, velocities and stresses. It is observed that this method appears to be stable for the different mesh sizes and time step values. Originality/value - The mixed least squares formulation, described in this paper, serves as a basis for interesting future developments and applications. © Emerald Group Publishing Limited.

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