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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 1,765 records · Page 98
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Infalling ultra-faint dwarfs as emissaries of the Axiverse

Recent discoveries of ultra-faint dwarf galaxies (UFDs) infalling onto the Milky Way, namely Leo K & M at r ≃ 450 kpc, considerably strengthens the case that UFDs constitute a distinct galaxy class that is inherently smaller and fainter, and metal-poorer than the classical dwarf spheroidals (dSph). This distinction is at odds with the inherent continuity of galaxy halo masses formed under scale-free gravity for any standard dark-matter (DM) model. Here, we show that distinct galaxy classes do evolve in cosmological simulations of multiple light bosons representing the “Axiverse” proposal of string theory, where a discrete mass spectrum of axions is generically predicted to span many decades in mass. In this context, the observed UFD class we show corresponds to a relatively heavy boson of 3 × 10 −21 eV, including Leo K & M, whereas a lighter axion of 10 −22 eV comprises the bulk of DM in all larger galaxies including the dSphs. Although Leo M is larger in size than Leo K, we predict its velocity dispersion to be smaller ( ≃ 1.7 km/s) than that of Leo K ( ≃ 4.5 km/s) because of the inverse de Broglie scale dependence on momentum. This scenario can be definitively tested using millisecond pulsars close to the Galactic center, where the Compton frequencies of the heavy and light bosons imprint monotone timing residuals that may be detected by the Square Kilometre Array (SKA) on timescales of approximately one week and four months, respectively.

dark matter↗
DOE OSTI2011

Scanning tunneling microscopic studies of laser-induced modifications of Si(001)-(2 x 1) surface

Scanning tunneling microscopic studies of Si(001)-2 x 1 surfaces excited with 532-nm laser pulses of intensities below melting and ablation thresholds have revealed two different modes of structural modifications, strongly depending on the intensity of laser lights. The excitation below 100 mJ/cm{sup 2} causes bond rupture at individual dimer-sites leading to the formation of vacancies selectively on the outermost layer. The bond rupture, which shows a strongly site-sensitive rate, forms efficiently vacancy-strings elongated along the surface dimer-rows. Selective removal of surface dimers results in the exposure of flat and defect-less underlying layer as reported previously, which is resistive to the excitation at this range of intensity. At intensities above 100 mJ/cm{sup 2}, on the other hand, the excitation forms not only vacancies but also ad-dimers on terraces. The number density of ad-dimers is in proportion to the square of that for vacancies, indicating strongly that silicon atoms released by laser-induced bond rupture are associated with each other to form ad-dimers. The repeated irradiations at this range of intensities induce anisotropic growth of ad-dimer islands and of vacancy clusters on terrace regions, leading to multiply terraced structure. The primary processes of the structural modifications are discussed based on the quantitative analyses of the growth of vacancy and ad-dimer under excitation.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Process Options for Nominal 2-K Helium Refrigeration System Designs

Nominal 2-K helium refrigeration systems are frequently used for superconducting radio frequency and magnet string technologies used in accelerators. This paper examines the trade-offs and approximate performance of four basic types of processes used for the refrigeration of these technologies; direct vacuum pumping on a helium bath, direct vacuum pumping using full or partial refrigeration recovery, cold compression, and hybrid compression (i.e., a blend of cold and warm sub-atmospheric compression).

43 PARTICLE ACCELERATORS↗
DOE OSTI2012

A quantum generalization of intrinsic reaction coordinate using path integral centroid coordinates

We propose a generalization of the intrinsic reaction coordinate (IRC) for quantum many-body systems described in terms of the mass-weighted ring polymer centroids in the imaginary-time path integral theory. This novel kind of reaction coordinate, which may be called the ''centroid IRC,'' corresponds to the minimum free energy path connecting reactant and product states with a least amount of reversible work applied to the center of masses of the quantum nuclei, i.e., the centroids. We provide a numerical procedure to obtain the centroid IRC based on first principles by combining ab initio path integral simulation with the string method. This approach is applied to NH{sub 3} molecule and N{sub 2}H{sub 5}{sup -} ion as well as their deuterated isotopomers to study the importance of nuclear quantum effects in the intramolecular and intermolecular proton transfer reactions. We find that, in the intramolecular proton transfer (inversion) of NH{sub 3}, the free energy barrier for the centroid variables decreases with an amount of about 20% compared to the classical one at the room temperature. In the intermolecular proton transfer of N{sub 2}H{sub 5}{sup -}, the centroid IRC is largely deviated from the ''classical'' IRC, and the free energy barrier is reduced by the quantum effects even more drastically.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Model of magnetic reconnection in space and astrophysical plasmas

Maxwell's equations imply that exponentially smaller non-ideal effects than commonly assumed can give rapid magnetic reconnection in space and astrophysical plasmas. In an ideal evolution, magnetic field lines act as stretchable strings, which can become ever more entangled but cannot be cut. High entanglement makes the lines exponentially sensitive to small non-ideal changes in the magnetic field. The cause is well known in popular culture as the butterfly effect and in the theory of deterministic dynamical systems as a sensitive dependence on initial conditions, but the importance to magnetic reconnection is not generally recognized. Two-coordinate models are too constrained geometrically for the required entanglement, but otherwise the effect is general and can be studied in simple models. A simple model is introduced, which is periodic in the x and y Cartesian coordinates and bounded by perfectly conducting planes in z. Starting from a constant magnetic field in the z direction, reconnection is driven by a spatially smooth, bounded force. The model is complete and could be used to study the impulsive transfer of energy between the magnetic field and the ions and electrons using a kinetic plasma model.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Self-organization during growth of ZrN/SiN{sub x} multilayers by epitaxial lateral overgrowth

ZrN/SiN{sub x} nanoscale multilayers were deposited on ZrN seed layers grown on top of MgO(001) substrates by dc magnetron sputtering with a constant ZrN thickness of 40 Å and with an intended SiN{sub x} thickness of 2, 4, 6, 8, and 15 Å at a substrate temperature of 800 °C and 6 Å at 500 °C. The films were investigated by X-ray diffraction, high-resolution scanning transmission electron microscopy, and energy dispersive X-ray spectroscopy. The investigations show that the SiN{sub x} is amorphous and that the ZrN layers are crystalline. Growth of epitaxial cubic SiN{sub x}—known to take place on TiN(001)—on ZrN(001) is excluded to the monolayer resolution of this study. During the course of SiN{sub x} deposition, the material segregates to form surface precipitates in discontinuous layers for SiN{sub x} thicknesses ≤6 Å that coalesce into continuous layers for 8 and 15 Å thickness at 800 °C, and for 6 Å at 500 °C. The SiN{sub x} precipitates are aligned vertically. The ZrN layers in turn grow by epitaxial lateral overgrowth on the discontinuous SiN{sub x} in samples deposited at 800 °C with up to 6 Å thick SiN{sub x} layers. Effectively a self-organized nanostructure can be grown consisting of strings of 1–3 nm large SiN{sub x} precipitates along apparent column boundaries in the epitaxial ZrN.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Closure measures for coarse-graining of the tent map

We quantify the relationship between the dynamics of a time-discrete dynamical system, the tent map T and its iterations T{sup m}, and the induced dynamics at a symbolical level in information theoretical terms. The symbol dynamics, given by a binary string s of length m, is obtained by choosing a partition point α∈[0,1] and lumping together the points x∈[0,1] s.t. T{sup i}(x) concurs with the i − 1th digit of s—i.e., we apply a so called threshold crossing technique. Interpreting the original dynamics and the symbolic one as different levels, this allows us to quantitatively evaluate and compare various closure measures that have been proposed for identifying emergent macro-levels of a dynamical system. In particular, we can see how these measures depend on the choice of the partition point α. As main benefit of this new information theoretical approach, we get all Markov partitions with full support of the time-discrete dynamical system induced by the tent map. Furthermore, we could derive an example of a Markovian symbol dynamics whose underlying partition is not Markovian at all, and even a whole hierarchy of Markovian symbol dynamics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Simultaneous determination of the elastic modulus and density/thickness of ultrathin films utilizing micro-/nanoresonators under applied axial force

Thin films are widely used in microelectronics, optics, filters, and various sensing devices. We propose a method to simultaneously determine the elastic modulus and density or thickness of ultrathin films deposited on various substrate materials. This methodology utilizes measurement of the resonant frequencies of the micro-/nanoresonator under intentionally applied axial tension and, consequently, the beam to string transition. Elastic modulus and density/thickness of thin film are obtained from the ratio between the resonant frequencies of the nanoresonator with and without applied axial force.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Note: Fast imaging of DNA in atomic force microscopy enabled by a local raster scan algorithm

Approaches to high-speed atomic force microscopy typically involve some combination of novel mechanical design to increase the physical bandwidth and advanced controllers to take maximum advantage of the physical capabilities. For certain classes of samples, however, imaging time can be reduced on standard instruments by reducing the amount of measurement that is performed to image the sample. One such technique is the local raster scan algorithm, developed for imaging of string-like samples. Here we provide experimental results on the use of this technique to image DNA samples, demonstrating the efficacy of the scheme and illustrating the order-of-magnitude improvement in imaging time that it provides.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Dynamics of Johari-Goldstein β relaxation and its universal relation to α relaxation in bulk metallic glasses by mechanical spectroscopy

The dynamics of the Johari–Goldstein (JG) β relaxation and the α relaxation in bulk metallic glasses (MGs) has been investigated by using mechanical spectroscopy combined with the Coupling Model. The β relaxations of MGs exhibit different behaviors such as peaks, humps, and excess wings due to the different fluctuations of the chemical interactions among the constituting atoms. A universal correlation between the β relaxation and the α relaxation is generally found by their activation energies and relaxation times as well as the non-exponentiality parameter of the α relaxation, which can be predicted quantitatively from the Coupling Model. Based on the quasi-point defects theory, a correlation factor χ shows a broad peak along with the β relaxation, suggesting that the concentration and the correlation degree of the string-like configurations involved in the β relaxation vary with increasing temperature, which challenges the previous view that the system is in an iso-configuration state below T{sub g} and may shed new light on the nature of the JG β relaxation in metallic glasses.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Design of 95 GHz gyrotron based on continuous operation copper solenoid with water cooling

The design work for 2nd harmonic 95 GHz, 50 kW gyrotron based on continuous operation copper solenoid is presented. Thermionic magnetron injection gun specifications were calculated according to the linear trade off equation, and simulated with CST program. Numerical code is used for cavity design using the non-uniform string equation as well as particle motion in the “cold” cavity field. The mode TE02 with low Ohmic losses in the cavity walls was chosen as the operating mode. The Solenoid is designed to induce magnetic field of 1.8 T over a length of 40 mm in the interaction region with homogeneity of ±0.34%. The solenoid has six concentric cylindrical segments (and two correction segments) of copper foil windings separated by water channels for cooling. The predicted temperature in continuous operation is below 93 °C. The parameters of the design together with simulation results of the electromagnetic cavity field, magnetic field, electron trajectories, and thermal analyses are presented.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

PINGU sensitivity to neutrino mass hierarchy

Determination of the neutrino mass hierarchy (NMH) is among the most fundamental questions in particle physics. Recent measurements of 1) a large mixing angle between the first and the third neutrino mass eigenstates and 2) the first observation of atmospheric neutrino oscillations at tens of GeV with neutrino telescopes, open the intriguing new possibility to exploit matter effects in neutrino oscillation to determine the neutrino mass hierarchy. A further extension of IceCube/DeepCore called PINGU (Precision IceCube Next Generation Upgrade) has been recently envisioned with the ultimate goal to measure neutrino mass hierarchy. PINGU would consist of additional IceCube-like strings of detectors deployed in the deepest and cleanest ice in the center of IceCube. More densely deployed instrumentation would provide a threshold substantially below 10 GeV and enhance the sensitivity to the mass hierarchy signal in atmospheric neutrinos. Here we discuss an estimate of the PINGU sensitivity to the mass hierarchy determined using an approximation with an Asimov dataset and an oscillation parameter fit.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Low-energy neutrino search for dark matter in the Galactic Center with IceCube-DeepCore

The cubic-kilometer sized IceCube neutrino observatory, constructed in the glacial ice at the South Pole, offers new opportunities for neutrino physics to reach lower neutrino energies with its in-fill array 'DeepCore'. A set of analysis techniques where the outer layers of IceCube are used as a veto makes it possible to reject atmospheric muons and thereby allows low-energy neutrino searches to be performed above the horizon. With these techniques the Galactic Center, an important target in searches for self-annihilating dark matter, becomes accessible for IceCube. In this contribution, we present the status of the Galactic Center dark matter analysis, using more than 10 months of data taken with the 79-string configuration of IceCube-DeepCore, as well as the new veto techniques used in the analysis.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

A first calibration study for PINGU at DESY

PINGU, the Precision IceCube Next Generation Upgrade, aims at lowering the neutrino detection threshold of the IceCube neutrino telescope to a few GeV by adding new in-fill strings to the existing IceCube/DeepCore array. The main goal of the upgrade is to provide sensitivity to the neutrino mass hierarchy utilizing atmospheric neutrinos. Precision reconstruction of the neutrino direction and energy, which is a prerequisite for the determination of the mass hierarchy, requires accurate in-situ calibration and monitoring of the PINGU digital optical modules (PDOMs) and high-level control of systematic effects like those caused by uncertainties in the optical ice properties and sensitivity of the PDOM in ice. These proceedings present an early status of calibration studies for PINGU performed at DESY.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Development of a multi-PMT optical module prototype for PINGU

The multi-PMT digital optical module (mDOM) contains 41 three-inch{sup 2} PMTs in a cylindrical glass vessel. The design is based on the KM3NeT DOM. Among other advantages, this approach offers a significant increase of photocathode area per module and also per string length. As an R and D activity targeted at potential future extensions, this new type of optical modules will be tested in the framework of the PINGU project (Precision IceCube Next Generation Upgrade). PINGU will add denser instrumentation to the IceCube/DeepCore sub-volume aiming at a further decrease of the neutrino energy threshold (to a few GeV). The current status of the development and the prototype construction is presented.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Digital optical module design for PINGU

The Precision IceCube Next Generation Upgrade (PINGU) will require optical sensors with similar performance as the digital optical modules (DOMs) of IceCube, but implemented in a higher-density array. A new design for the PINGU DOM (PDOM) is being pursued that retains the proven mechanical elements of the IceCube DOM, yet takes advantage of recently commercialized high-speed digitizer technology. The main features of the proposed PDOM electronics are discussed, along with status and plans for development. Proposed modifications to the IceCube string architecture that will accommodate the smaller vertical PDOM spacing are presented.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Electrodynamics of massless charged particles

We derive the classical dynamics of massless charged particles in a rigorous way from first principles. Since due to ultraviolet divergences this dynamics does not follow from an action principle, we rely on (a) Maxwell’s equations, (b) Lorentz- and reparameterization-invariance, and (c) local conservation of energy and momentum. Despite the presence of pronounced singularities of the electromagnetic field along Dirac-like strings, we give a constructive proof of the existence of a unique distribution-valued energy-momentum tensor. Its conservation requires the particles to obey standard Lorentz equations and they experience, hence, no radiation reaction. Correspondingly, the dynamics of interacting classical massless charged particles can be consistently defined, although they do not emit bremsstrahlung end experience no self-interaction.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Control of the conformations of ion Coulomb crystals in a Penning trap

Ion Coulomb crystals containing small numbers of ions have been created and manipulated in a wide range of configurations in a Penning trap, from a linear string, through various three-dimensional conformations, to a planar crystal. We show that the dynamics of the system simplifies enormously in a frame which rotates at half the cyclotron frequency and we discuss the effect of the radial cooling laser beam in this frame. Simulations show that the crystal conformations can be reproduced by finding the minimum energy configuration in a frame whose radial potential is modified by the rotation of the ion crystal. The rotation frequency of the crystal deduced from the simulations is consistent with the known laser parameters. We also show that even though the number of ions in our system is small (typically less than 20), the system still behaves like a plasma and its static properties can be calculated using the standard model for a single-component plasma in a trap.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗