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Results for “Structural beams”

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 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Al and Ge simultaneous oxidation using neutral beam post-oxidation for formation of gate stack structures

To obtain a high-quality Germanium (Ge) metal–oxide–semiconductor structure, a Ge gate stacked structure was fabricated using neutral beam post-oxidation. After deposition of a 1-nm-thick Al metal film on a Ge substrate, simultaneous oxidation of Al and Ge was carried out at 300 °C, and a Ge oxide film with 29% GeO{sub 2} content was obtained by controlling the acceleration bias power of the neutral oxygen beam. In addition, the fabricated AlO{sub x}/GeO{sub x}/Ge structure achieved a low interface state density of less than 1 × 10{sup 11 }cm{sup −2 }eV{sup −1} near the midgap.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Gas Electric Discharge with a Developed Streamer Structure in a Quasi-Optical Microwave Beam

Experiments on initiating an electrical discharge with a developed streamer structure in atmospheric pressure air are described. The discharge was initiated in a quasi-optical linearly polarized microwave beam with a subcritical field. Air breakdown was initiated by an electromagnetic vibrator placed in free space, on the surface of a dielectric plate, and on the outer or inner surface of a quartz tube. From experimental data, the velocity of the discharge front propagation toward the wavevector of the electromagnetic wave has been estimated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Terahertz Chiral Metamaterials Enabling Broadband Polarization Conversion Using Polarization Guiding Effect

Polarization control plays a vital role in terahertz (THz) photonics, enabling a wide range of applications from imaging and spectroscopy to sensing and wireless communication. However, conventional polarization control methods at THz frequencies are limited by narrow operational bandwidths and excessive absorption losses. In this work, we present a broadband THz polarization control based on the polarization guiding effect. We design and fabricate a THz polarization rotator capable of rotating incident linear polarization by 90°. The device consists of 24 twistingly stacked silicon–air metagrating layers that introduce a large degree of form birefringence, with each metagrating layer being 280 µm thick and sequentially twisted by 3.75°. Numerical simulations using the Berreman 4 × 4 method predict broadband operation of the device. Experimental validation using THz time-domain spectroscopy confirms efficient and broadband 90° polarization rotation from 0.2 to 1.25 THz. Based on this concept, we further propose THz Q-plates for the generation of broadband structured vector beams with radial and azimuthal polarization states. This work offers a scalable, material-agnostic platform for advanced THz polarization manipulation and vector beam engineering, opening new pathways for next-generation THz photonic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of X-ray beam on the molecular–topological structure of the surface of kynar® polyvinylidene fluoride resin

Thermomechanical analysis of poly(vinylidene fluoride) (PVDF) film shows that, unlike the completely amorphous structure of the native polymer, after irradiation with X-rays, the weight fraction of the crystalline modification in the polymer is 0.5. The initial PVDF film had a linear thermal expansion coefficient of α 1 = 11.33х10 5 K -1 and a glass transition temperature of T g =256 K. An efficient conversion of the amorphous structures of the polymer pseudo-network into crystalline networks occurs under X-ray irradiation. The irradiated polymer exhibits amorphous ( T g =239 K) and crystalline topological blocks ( T m =385 K). The presence of polyenyl and alkyl radicals is found in the electron paramagnetic resonance (EPR) spectra of the irradiated polymer. Lines with a splitting of 3.3 m T can be observed at the edges of the EPR spectra, consistent with radical centers like -CF 2 -C • H-CF 2 - . At high doses of radiolysis, polyenyl radicals are the main observed radicals. The macromolecular chains contain all three (α, β, γ) morphologies of the polymer. The Raman spectra show that X-ray irradiation can lead to carbonization and the formation of unsaturated bonds. Electronic structure calculations were used to aid in the analysis of the observed spectra.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

In-line three-dimensional holography of nanocrystalline objects at atomic resolution

We report that resolution and sensitivity of the latest generation aberration-corrected transmission electron microscopes allow the vast majority of single atoms to be imaged with sub-Ångstrom resolution and their locations determined in an image plane with a precision that exceeds the 1.9-pm wavelength of 300 kV electrons. Such unprecedented performance allows expansion of electron microscopic investigations with atomic resolution into the third dimension. Here we show a general tomographic method to recover the three-dimensional shape of a crystalline particle from high-resolution images of a single projection without the need for sample rotation. The method is compatible with low dose rate electron microscopy, which improves on signal quality, while minimizing electron beam-induced structure modifications even for small particles or surfaces. Lastly, we apply it to germanium, gold and magnesium oxide particles, and achieve a depth resolution of 1–2 Å, which is smaller than inter-atomic distances.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Broadband polarization conversion with anisotropic plasmonic metasurfaces

Metasurfaces offer exciting opportunities that enable precise control of light propagation, optical intensity, phase and polarization. Plasmonic metasurface based quarter-wave plates have been recently studied to realize the conversion between linear polarization and circular polarization. However, it is still quite challenging to directly measure the birefringent phase retardation introduced by metasurface wave plates with a reliable technique. Here, we report a high-performance broadband metasurface quarter-wave plate made of anisotropic T-shaped plasmonic antennas in near-infrared wavelength range, where the achromatic nearly 90° transmitted phase retardation through the metasurface is precisely characterized with an optical vortex based interferometric approach. Based on the measured transmission amplitude and phase of two orthogonal linear polarization components, nearly unit degree of linear polarization is extracted from the Stokes parameters, indicating excellent broadband polarization conversion between linearly and circularly polarized light through the metasurface. Our results will be an important step forward in the advancement of integrated metasurface devices for polarization conversion and beam manipulation, structured light control, as well as new spectroscopic and interferometric techniques for metasurface characterization.

Science & Technology - Other Topics↗
DOE OSTI2014

Two-stream instability with time-dependent drift velocity

The classical two-stream instability driven by a constant relative drift velocity between two plasma components is extended to the case with time-dependent drift velocity. A solution method is developed to rigorously define and calculate the instability growth rate for linear perturbations relative to the time-dependent unperturbed two-stream motions. The stability diagrams for the oscillating two-stream instability are presented over a large region of parameter space. It is shown that the growth rate for the classical two-stream instability can be significantly reduced by adding an oscillatory component to the relative drift velocity.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Argon-ion-induced formation of nanoporous GaSb layer: Microstructure, infrared luminescence, and vibrational properties

Room temperature implantation of 60 keV Ar{sup +}-ions in GaSb to the fluences of 7 × 10{sup 16} to 3 × 10{sup 18} ions cm{sup −2} is carried out at two incidence angles, viz 0° and 60°, leading to formation of a nanoporous layer. As the ion fluence increases, patches grow on the porous layer under normal ion implantation, whereas the porous layer gradually becomes embedded under a rough top surface for oblique incidence of ions. Grazing incidence x-ray diffraction and cross-sectional transmission electron microscopy studies reveal the existence of nanocrystallites embedded in the ion-beam amorphized GaSb matrix up to the highest fluence used in our experiment. Oxidation of the nanoporous layers becomes obvious from x-ray photoelectron spectroscopy and Raman mapping. The correlation of ion-beam induced structural modification with photoluminescence signals in the infrared region has further been studied, showing defect induced emission of additional peaks near the band edge of GaSb.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

An algorithm to unveil the inner structure of objects concealed by beam divergence in radiographic image acquisition systems

Two main parameters rule the performance of an Image Acquisition System, namely, spatial resolution and contrast. For radiographic systems using cone beam arrangements, the farther the source, the better the resolution, but the contrast would diminish due to the lower statistics. A closer source would yield a higher contrast but it would no longer reproduce the attenuation map of the object, as the incoming beam flux would be reduced by unequal large divergences and attenuation factors. This work proposes a procedure to correct these effects when the object is comprised of a hull - or encased in it - possessing a shape capable to be described in analytical geometry terms. Such a description allows the construction of a matrix containing the attenuation factors undergone by the beam from the source until its final destination at each coordinate on the 2D detector. Each matrix element incorporates the attenuation suffered by the beam after its travel through the hull wall, as well as its reduction due to the square of distance to the source and the angle it hits the detector surface. When the pixel intensities of the original image are corrected by these factors, the image contrast, reduced by the overall attenuation in the exposure phase, are recovered, allowing one to see details otherwise concealed due to the low contrast. In order to verify the soundness of this approach, synthetic images of objects of different shapes, such as plates and tubes, incorporating defects and statistical fluctuation, have been generated, recorded for further comparison and afterwards processed to improve their contrast. The developed algorithm which, generates processes and plots the images has been written in Fortran 90 language. As the resulting final images exhibit the expected improvements, it therefore seemed worthwhile to carry out further tests with actual experimental radiographies.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Deviation from threshold model in ultrafast laser ablation of graphene at sub-micron scale

We investigate a method to measure ultrafast laser ablation threshold with respect to spot size. We use structured complex beams to generate a pattern of craters in CVD graphene with a single laser pulse. A direct comparison between beam profile and SEM characterization allows us to determine the dependence of ablation probability on spot-size, for crater diameters ranging between 700 nm and 2.5 μm. We report a drastic decrease of ablation probability when the crater diameter is below 1 μm which we interpret in terms of free-carrier diffusion.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

A chemical dynamics study on the gas-phase formation of triplet and singlet C 5 H 2 carbenes

Significance Carbenes represent key reactive intermediates in molecular mass growth processes leading to carbonaceous nanostructures in the interstellar medium and in combustion systems. However, due to their short lifetimes and tendency for dimerization, carbenes represent one of the foremost obscured classes of reactive intermediates, with the preparation of carbenes in the gas phase remaining largely elusive. By merging molecular beams with electronic structure and quasi-classical trajectory calculations and exploiting triplet pentadiynylidene (HCCCCCH) and singlet ethynylcyclopropenylidene (c-C 5 H 2 ) carbene as benchmarks, we present a versatile protocol to unravel the dynamics leading to exotic carbenes under single-collision conditions. These elementary mechanisms are of significance to our understanding of the fundamental processes leading to unsaturated organic transients in extreme, hydrocarbon-rich environments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Ionization detail parameters and cluster dose: a mathematical model for selection of nanodosimetric quantities for use in treatment planning in charged particle radiotherapy

Abstract Objective . To propose a mathematical model for applying ionization detail (ID), the detailed spatial distribution of ionization along a particle track, to proton and ion beam radiotherapy treatment planning (RTP). Approach . Our model provides for selection of preferred ID parameters ( I p ) for RTP, that associate closest to biological effects. Cluster dose is proposed to bridge the large gap between nanoscopic I p and macroscopic RTP. Selection of I p is demonstrated using published cell survival measurements for protons through argon, comparing results for nineteen I p : N k , k = 2, 3, …, 10, the number of ionizations in clusters of k or more per particle, and F k , k = 1, 2, …, 10, the number of clusters of k or more per particle. We then describe application of the model to ID-based RTP and propose a path to clinical translation. Main results . The preferred I p were N 4 and F 5 for aerobic cells, N 5 and F 7 for hypoxic cells. Significant differences were found in cell survival for beams having the same LET or the preferred N k . Conversely, there was no significant difference for F 5 for aerobic cells and F 7 for hypoxic cells, regardless of ion beam atomic number or energy. Further, cells irradiated with the same cluster dose for these I p had the same cell survival. Based on these preliminary results and other compelling results in nanodosimetry, it is reasonable to assert that I p exist that are more closely associated with biological effects than current LET-based approaches and microdosimetric RBE-based models used in particle RTP. However, more biological variables such as cell line and cycle phase, as well as ion beam pulse structure and rate still need investigation. Significance . Our model provides a practical means to select preferred I p from radiobiological data, and to convert I p to the macroscopic cluster dose for particle RTP.

Engineering↗
DOE OSTI2016

Dynamic Provisioning and Execution of HPC Workflows Using Python

High-performance computing (HPC) workflows over the last several decades have proven to assist in the understanding of scientific phenomena and the production of better products, more quickly, and at reduced cost. However, HPC workflows are difficult to implement and use for a variety of reasons. In this paper, we describe the development of the Python-based cumulus, which addresses many of these barriers. cumulus is a platform for the dynamic provisioning and execution of HPC workflows. cumulus provides the infrastructure needed to build applications that leverage traditional or Cloud-based HPC resources in their workflows. Lastly, we demonstrate the use of cumulus in both web and desktop simulation applications, as well as in an Apache Spark-based analysis application.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2022

Helmholtz vibrations in bowed strings

We report for almost 160 years, it has been known that Helmholtz oscillations, unique to vibrating strings in bowed instruments (violin, cello, etc.), have two distinct regimes: “slip” and “stick.” During the slip regime, the force at the bow-string interaction is attributed to friction between the sliding bow hair and the vibrating string, with a friction coefficient that decreases with increasing relative velocity. Yet the hair-string interaction during the stick regime is less understood. We propose that the interaction force during the stick regime is proportional to the product of the longitudinal acoustic impedance of the bow hair to the relative bow-string velocity. We validate this hypothesis by solving the string's differential equation of motion, including an enhanced formulation to avoid parasitic high-frequency oscillations. This physical model enables us to analyze, in real time, the characteristics of the Helmholtz oscillations, including the string shape, excitation of harmonics, Schelleng ripples, and string energy, showing that the bowed string gains energy during the stick regime and loses energy during the slip regime.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Mode Purity of Electron Cyclotron Waves after Their Passage through the Peripheral Plasma in the Large Helical Device

Plasma heating and current drive with Electron Cyclotron Waves (ECWs) require precise control over the polarization state of ECWs to ensure that the entire input power is deposited where intended. However, due to the magnetic shear in the peripheral plasma, the polarization state can change. This effect is particularly pronounced in the Large Helical Device (LHD), where the magnetic field is sheared strongly. Here, we present a new code PARADE (PAraxial RAy DEscription) that can simulate the evolution of the polarization state along the beam propagation without resorting to full-wave modeling. We apply PARADE to the LHD plasma and simulate the evolution of the beam transverse structure, including the local amplitudes of the two electromagnetic eigenmodes. Here, the results surpass those yielded by the code LHDGauss used in the past. Based on these new results, we discuss how to improve the mode purity of ECWs by controlling the initial polarization state. A remarkable improvement is predicted numerically.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗