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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 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Entire plasmas can be restructured when electrons are emitted from the boundaries

It is well known that electron emission can restructure the thin sheaths at plasma-facing surfaces. But conventional models assume that the plasma's structure negligibly changes (the “presheath” is still thought to be governed by ion acceleration to the Bohm speed). Here, it is shown by theory and simulation that the presheath can take a fundamentally different structure where the emitted electrons entering the quasineutral region cause numerous changes. Gradients of total plasma density, ion and electron pressures, and electric potential throughout the “inverted” presheath can carry different magnitudes, and opposite signs, from Bohm presheaths.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Structure and electronic properties of Cu nanoclusters supported on Mo{sub 2}C(001) and MoC(001) surfaces

The atomic structure and electronic properties of Cu{sub n} nanoclusters (n = 4, 6, 7, and 10) supported on cubic nonpolar δ-MoC(001) and orthorhombic C- or Mo-terminated polar β-Mo{sub 2} C(001) surfaces have been investigated by means of periodic density functional theory based calculations. The electronic properties have been analyzed by means of the density of states, Bader charges, and electron localization function plots. The Cu nanoparticles supported on β-Mo{sub 2} C(001), either Mo- or C-terminated, tend to present a two-dimensional structure whereas a three-dimensional geometry is preferred when supported on δ-MoC(001), indicating that the Mo:C ratio and the surface polarity play a key role determining the structure of supported clusters. Nevertheless, calculations also reveal important differences between the C- and Mo-terminated β-Mo{sub 2} C(001) supports to the point that supported Cu particles exhibit different charge states, which opens a way to control the reactivity of these potential catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

First-principles equation of state and electronic properties of warm dense oxygen

We perform all-electron path integral Monte Carlo (PIMC) and density functional theory molecular dynamics (DFT-MD) calculations to explore warm dense matter states of oxygen. Our simulations cover a wide density-temperature range of 1–100 g cm{sup −3} and 10{sup 4}–10{sup 9} K. By combining results from PIMC and DFT-MD, we are able to compute pressures and internal energies from first-principles at all temperatures and provide a coherent equation of state. We compare our first-principles calculations with analytic equations of state, which tend to agree for temperatures above 8 × 10{sup 6} K. Pair-correlation functions and the electronic density of states reveal an evolving plasma structure and ionization process that is driven by temperature and density. As we increase the density at constant temperature, we find that the ionization fraction of the 1s state decreases while the other electronic states move towards the continuum. Finally, the computed shock Hugoniot curves show an increase in compression as the first and second shells are ionized.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Electronic properties of Mn-phthalocyanine–C{sub 60} bulk heterojunctions: Combining photoemission and electron energy-loss spectroscopy

The electronic properties of co-evaporated mixtures (blends) of manganese phthalocyanine and the fullerene C{sub 60} (MnPc:C{sub 60}) have been studied as a function of the concentration of the two constituents using two supplementary electron spectroscopic methods, photoemission spectroscopy (PES) and electron energy-loss spectroscopy (EELS) in transmission. Our PES measurements provide a detailed picture of the electronic structure measured with different excitation energies as well as different mixing ratios between MnPc and C{sub 60}. Besides a relative energy shift, the occupied electronic states of the two materials remain essentially unchanged. The observed energy level alignment is different compared to that of the related CuPc:C{sub 60} bulk heterojunction. Moreover, the results from our EELS investigations show that, despite the rather small interface interaction, the MnPc related electronic excitation spectrum changes significantly by admixing C{sub 60} to MnPc thin films.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Adjoint Fokker-Planck equation and runaway electron dynamics

The adjoint Fokker-Planck equation method is applied to study the runaway probability function and the expected slowing-down time for highly relativistic runaway electrons, including the loss of energy due to synchrotron radiation. In direct correspondence to Monte Carlo simulation methods, the runaway probability function has a smooth transition across the runaway separatrix, which can be attributed to effect of the pitch angle scattering term in the kinetic equation. However, for the same numerical accuracy, the adjoint method is more efficient than the Monte Carlo method. The expected slowing-down time gives a novel method to estimate the runaway current decay time in experiments. A new result from this work is that the decay rate of high energy electrons is very slow when E is close to the critical electric field. This effect contributes further to a hysteresis previously found in the runaway electron population.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Control of secondary electrons from ion beam impact using a positive potential electrode

Secondary electrons emitted when an ion beam impacts a detector can amplify the ion beam signal, but also introduce errors if electrons from one detector propagate to another. A potassium ion beam and a detector comprised of ten impact wires, four split-plates, and a pair of biased electrodes were used to demonstrate that a low-voltage, positive electrode can be used to maintain the beneficial amplification effect while greatly reducing the error introduced from the electrons traveling between detector elements.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

Revealing the role of electron-electron correlations by mapping dissociation of highly excited D 2 + using ultrashort XUV pulses

Understanding electron-electron correlations in matter ranging from atoms to solids represents a grand challenge for both experiment and theory. Here, these correlations occur on attosecond timescales and have only recently become experimentally accessible. In the case of highly excited systems, the task of understanding and probing correlated interactions is even greater. In this work, we combine state-of-the-art light sources and advanced detection techniques with ab initio calculations to unravel the role of electron-electron correlation in D 2 photoionization by mapping the dissociation of a highly excited D 2 + molecule. Correlations between the two electrons dictate the pathways along which the molecule dissociates and lead to a superposition of excited ionic states. Using 3D Coulomb explosion imaging and electron-ion coincidence techniques, we assess the relative contribution of competing parent ion states to the dissociation process for different orientations of the molecule with respect to the laser polarization, which is consistent with a shake-up ionization process. As a step toward observing coherent superposition experimentally, we map the relevant nuclear potentials using Coulomb explosion imaging and show theoretically that such an experiment could confirm this coherence via two-path interference.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Precision Control of the Electron Longitudinal Bunch Shape Using an Emittance-Exchange Beam Line

We report on the experimental generation of relativistic electron bunches with a tunable longitudinal bunch shape. A longitudinal bunch-shaping (LBS) beam line, consisting of a transverse mask followed by a transverse-to-longitudinal emittance exchange (EEX) beam line, is used to tailor the longitudinal bunch shape (or current profile) of the electron bunch. The mask shapes the bunch’s horizontal profile, and the EEX beam line converts it to a corresponding longitudinal profile. The Argonne wakefield accelerator rf photoinjector delivers electron bunches into a LBS beamline to generate a variety of longitudinal bunch shapes. The quality of the longitudinal bunch shape is limited by various perturbations in the exchange process. We develop a simple method, based on the incident slope of the bunch, to significantly suppress the perturbations

Ha, G.↗
DOE OSTI2015

Spatially resolved electron density and electron energy distribution function in Ar magnetron plasmas used for sputter-deposition of ZnO-based thin films

Langmuir probe and trace rare gases optical emission spectroscopy were used to analyze the spatial structure of the electron density and electron energy distribution function (EEDF) in a cylindrical Ar magnetron plasma reactor used for sputter-deposition of ZnO-based thin films. While a typical Bessel (zero order) diffusion profile was observed along the radial direction for the number density of charged particles at 21 cm from the ZnO target, a significant rise of these populations with respect to the Bessel function was seen in the center of the reactor at 4 cm from the magnetron surface. As for the EEDF, it was found to transform from a more or less Maxwellian far from the target to a two-temperature Maxwellian with a depletion of high-energy electrons where magnetic field confinement effects become important. No significant change in the behavior of the electron density and EEDF across a wide range of pressures (5–100 mTorr) and self-bias voltages (115–300 V) was observed during magnetron sputtering of Zn, ZnO, and In{sub 2}O{sub 3} targets. This indicates that sputtering of Zn, In, and O atoms do not play a very significant role on the electron particle balance and electron heating dynamics, at least over the range of experimental conditions investigated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Precise polarization measurements via detection of compton scattered electrons

The Qweak experiment at Jefferson Lab aims to make a 4% measurement of the parity-violating asymmetry in elastic scattering at very low Q{sup 2} of a longitudinally polarized electron beam off a proton target. One of the dominant experimental systematic uncertainties in Qweak will result from determining the beam polarization. A new Compton polarimeter was installed in the fall of 2010 to provide a non-invasive and continuous monitoring of the electron beam polarization in Hall C at Jefferson Lab. The Compton-scattered electrons are detected in four planes of diamond micro-strip detectors. We have achieved the design goals of <1% statistical uncertainty per hour and expect to achieve <1% systematic uncertainty.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

Simulation of Non-Uniform Electron Beams in the Gyrotron Electron-Optical System

New calculated data on the effect of emission inhomogeneities on the quality of the electron beam, which is formed in an electron-optical system of a gyrotron, have been obtained. The calculations were based on emission current density distributions, which were measured for the different cathodes in the gyrotron of Peter the Great St. Petersburg Polytechnic University. A satisfactory agreement between the experimental and calculated data on the influence of emission nonuniformities on the velocity spread of electrons has been shown. The necessity of considering the real distribution of the emission current density over the cathode surface to determine the main parameters of the electron beam—the velocity and energy spreads of the electrons, spatial structure of the beam, and coefficient of reflection of electrons from the magnetic mirror—has been demonstrated. The maximum level of emission inhomogeneities, which are permissible for effective work of gyrotrons, has been discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Relativistic Shear Flow between Electron–Ion and Electron–Positron Plasmas and Astrophysical Applications

We present particle-in-cell simulation results of relativistic shear boundary layers between electron–ion and electron–positron plasmas and discuss their potential applications to astrophysics. Specifically, we find that in the case of a fast electron–positron spine surrounded by a slow-moving or stationary electron–ion sheath, lepton acceleration proceeds in a highly anisotropic manner due to electromagnetic fields created at the shear interface. While the highest-energy leptons still produce a beaming pattern (as seen in the quasi-stationary frame of the sheath) of order 1/Γ, where Γ is the bulk Lorentz factor of the spine, for lower-energy particles, the beaming is much less pronounced. This is in stark contrast to the case of pure electron–ion shear layers, in which anisotropic particle acceleration leads to significantly narrower beaming patterns than 1/Γ for the highest-energy particles. In either case, shear-layer acceleration is expected to produce strongly angle-dependent lepton (hence, emanating radiation) spectra, with a significantly harder spectrum in the forward direction than viewed from larger off-axis angles, much beyond the regular Doppler boosting effect from a co-moving isotropic lepton distribution. This may solve the problem of the need for high (and apparently arbitrarily chosen) minimum Lorentz factors of radiating electrons, often plaguing current blazar and GRB jet modeling efforts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Electron tunneling spectroscopy study of electrically active traps in AlGaN/GaN high electron mobility transistors

We investigate the energy levels of electron traps in AlGaN/GaN high electron mobility transistors by the use of electron tunneling spectroscopy. Detailed analysis of a typical spectrum, obtained in a wide gate bias range and with both bias polarities, suggests the existence of electron traps both in the bulk of AlGaN and at the AlGaN/GaN interface. The energy levels of the electron traps have been determined to lie within a 0.5 eV band below the conduction band minimum of AlGaN, and there is strong evidence suggesting that these traps contribute to Frenkel-Poole conduction through the AlGaN barrier.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Nonlinear space charge dynamics in mixed ionic-electronic conductors: Resistive switching and ferroelectric-like hysteresis of electromechanical response

We performed self-consistent modelling of nonlinear electrotransport and electromechanical response of thin films of mixed ionic-electronic conductors (MIEC) allowing for steric effects of mobile charged defects (ions, protons, or vacancies), electron degeneration, and Vegard stresses. We establish correlations between the features of the nonlinear space-charge dynamics, current-voltage, and bending-voltage curves for different types of the film electrodes. A pronounced ferroelectric-like hysteresis of the bending-voltage loops and current maxima on the double hysteresis current-voltage loops appear for the electron-transport electrodes. The double hysteresis loop with pronounced humps indicates a memristor-type resistive switching. The switching occurs due to the strong nonlinear coupling between the electronic and ionic subsystems. A sharp meta-stable maximum of the electron density appears near one open electrode and moves to another one during the periodic change of applied voltage. Our results can explain the nonlinear nature and correlation of electrical and mechanical memory effects in thin MIEC films. The analytical expression proving that the electrically induced bending of MIEC films can be detected by interferometric methods is derived.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Differential cross sections for electron impact excitation of the electronic bands of phenol

We report results from a joint theoretical and experimental investigation into electron scattering from the important organic species phenol (C{sub 6}H{sub 5}OH). Specifically, differential cross sections (DCSs) have been measured and calculated for the electron-impact excitation of the electronic states of C{sub 6}H{sub 5}OH. The measurements were carried out at energies in the range 15–40 eV, and for scattered-electron angles between 10{sup ∘} and 90{sup ∘}. The energy resolution of those experiments was typically ∼80 meV. Corresponding Schwinger multichannel method with pseudo-potentials calculations, with and without Born-closure, were also performed for a sub-set of the excited electronic-states that were accessed in the measurements. Those calculations were conducted at the static exchange plus polarisation (SEP)-level using a minimum orbital basis for single configuration interaction (MOBSCI) approach. Agreement between the measured and calculated DCSs was typically fair, although to obtain quantitative accord, the theory would need to incorporate even more channels into the MOBSCI.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

The Coulomb, exchange, and correlation components of the electron-electron repulsion in harmonium atoms

Highly accurate Coulomb, exchange, and correlation components of the electron-electron repulsion energies of the three-electron harmonium atoms in the {sup 2}P{sub −} and {sup 4}P{sub +} states are obtained for 19 values of the confinement strength ω ranging from 10{sup −3} to 10{sup 3}. The computed data are consistent with their ω → 0 and ω → ∞ asymptotics that are given by closed-form algebraic expressions. Robust approximants that accurately reproduce the actual values of the energy components while strictly conforming to these limits are constructed, opening an avenue to stringent tests capable of predicting the performance of electronic structure methods for systems with varying extents of the dynamical and nondynamical electron correlation. The values of the correlation components, paired with the computed 1-matrices are expected to be particularly useful in the context of benchmarking of approximate density matrix functionals.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Electroluminescence of hot electrons in AlGaN/GaN high-electron-mobility transistors under radio frequency operation

Hot electrons in AlGaN/GaN high electron mobility transistors are studied during radio frequency (RF) and DC operation by means of electroluminescence (EL) microscopy and spectroscopy. The measured EL intensity is decreased under RF operation compared to DC at the same average current, indicating a lower hot electron density. This is explained by averaging the DC EL intensity over the measured load line used in RF measurements, giving reasonable agreement. In addition, the hot electron temperature is lower by up to 15% under RF compared to DC, again at least partially explainable by the weighted averaging along the specific load line. However, peak electron temperature under RF occurs at high V{sub DS} and low I{sub DS} where EL is insignificant suggesting that any wear-out differences between RF and DC stress of the devices will depend on the balance between hot-carrier and field driven degradation mechanisms.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Positive semidefinite tensor factorizations of the two-electron integral matrix for low-scaling ab initio electronic structure

Tensor factorization of the 2-electron integral matrix is a well-known technique for reducing the computational scaling of ab initio electronic structure methods toward that of Hartree-Fock and density functional theories. The simplest factorization that maintains the positive semidefinite character of the 2-electron integral matrix is the Cholesky factorization. In this paper, we introduce a family of positive semidefinite factorizations that generalize the Cholesky factorization. Using an implementation of the factorization within the parametric 2-RDM method [D. A. Mazziotti, Phys. Rev. Lett. 101, 253002 (2008)], we study several inorganic molecules, alkane chains, and potential energy curves and find that this generalized factorization retains the accuracy and size extensivity of the Cholesky factorization, even in the presence of multi-reference correlation. The generalized family of positive semidefinite factorizations has potential applications to low-scaling ab initio electronic structure methods that treat electron correlation with a computational cost approaching that of the Hartree-Fock method or density functional theory.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗