1,173 research outputs found
Some inconsistencies of the finite element method as applied to inelastic response
The inadequacy of a two noded beam-column element with a linear axial and a cubic transverse displacement field for inelastic analysis is demonstrated. For complete equilibrium satisfaction in the linear elastic range a three noded beam-column element is shown to be consistent. Next, the sensitivity of the inelastic response to numerical solutions of the inelastic response of a cantilever beam resulting from approximate integration of strain energy are brought out and finally, consequences of this on the nonlinear transient response of structures are considered
Antenna pattern shaping, sensing, and steering study Final report
Design of steerable satellite antenna with beam pattern sensing syste
Ultracold Neutral Plasmas
Ultracold neutral plasmas are formed by photoionizing laser-cooled atoms near
the ionization threshold. Through the application of atomic physics techniques
and diagnostics, these experiments stretch the boundaries of traditional
neutral plasma physics. The electron temperature in these plasmas ranges from
1-1000 K and the ion temperature is around 1 K. The density can approach
cm. Fundamental interest stems from the possibility of
creating strongly-coupled plasmas, but recombination, collective modes, and
thermalization in these systems have also been studied. Optical absorption
images of a strontium plasma, using the Sr
transition at 422 nm, depict the density profile of the plasma, and probe
kinetics on a 50 ns time-scale. The Doppler-broadened ion absorption spectrum
measures the ion velocity distribution, which gives an accurate measure of the
ion dynamics in the first microsecond after photoionization.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,
Nice (France
Evolution of Ultracold, Neutral Plasmas
We present the first large-scale simulations of an ultracold, neutral plasma,
produced by photoionization of laser-cooled xenon atoms, from creation to
initial expansion, using classical molecular dynamics methods with open
boundary conditions. We reproduce many of the experimental findings such as the
trapping efficiency of electrons with increased ion number, a minimum electron
temperature achieved on approach to the photoionization threshold, and
recombination into Rydberg states of anomalously-low principal quantum number.
In addition, many of these effects establish themselves very early in the
plasma evolution ( ns) before present experimental observations begin.Comment: 4 pages, 3 figures, submitted to PR
Creation of Rydberg Polarons in a Bose Gas
We report spectroscopic observation of Rydberg polarons in an atomic Bose
gas. Polarons are created by excitation of Rydberg atoms as impurities in a
strontium Bose-Einstein condensate. They are distinguished from previously
studied polarons by macroscopic occupation of bound molecular states that arise
from scattering of the weakly bound Rydberg electron from ground-state atoms.
The absence of a -wave resonance in the low-energy electron-atom scattering
in Sr introduces a universal behavior in the Rydberg spectral lineshape and in
scaling of the spectral width (narrowing) with the Rydberg principal quantum
number, . Spectral features are described with a functional determinant
approach (FDA) that solves an extended Fr\"{o}hlich Hamiltonian for a mobile
impurity in a Bose gas. Excited states of polyatomic Rydberg molecules
(trimers, tetrameters, and pentamers) are experimentally resolved and
accurately reproduced with FDA.Comment: 5 pages, 3 figure
Theory of excitation of Rydberg polarons in an atomic quantum gas
We present a quantum many-body description of the excitation spectrum of
Rydberg polarons in a Bose gas. The many-body Hamiltonian is solved with
functional determinant theory, and we extend this technique to describe Rydberg
polarons of finite mass. Mean-field and classical descriptions of the spectrum
are derived as approximations of the many-body theory. The various approaches
are applied to experimental observations of polarons created by excitation of
Rydberg atoms in a strontium Bose-Einstein condensate.Comment: 14 pages, 9 figures. arXiv admin note: substantial text overlap with
arXiv:1706.0371
Spectroscopic determination of the s-wave scattering lengths of 86Sr and 88Sr
We report the use of photoassociative spectroscopy to determine the ground
state s-wave scattering lengths for the main bosonic isotopes of strontium,
86Sr and 88Sr. Photoassociative transitions are driven with a laser red-detuned
by up to 1400 GHz from the 1S0-1P1 atomic resonance at 461 nm. A minimum in the
transition amplitude for 86Sr at -494+/-5 GHz allows us to determine the
scattering lengths 610a0 < a86 < 2300a0 for 86Sr and a much smaller value of
-1a0 < a88 < 13a0 for 88Sr.Comment: 4 pages, 3 figures, submitted to Physical Review Letter
Controlling trapping potentials and stray electric fields in a microfabricated ion trap through design and compensation
Recent advances in quantum information processing with trapped ions have
demonstrated the need for new ion trap architectures capable of holding and
manipulating chains of many (>10) ions. Here we present the design and detailed
characterization of a new linear trap, microfabricated with scalable
complementary metal-oxide-semiconductor (CMOS) techniques, that is well-suited
to this challenge. Forty-four individually controlled DC electrodes provide the
many degrees of freedom required to construct anharmonic potential wells,
shuttle ions, merge and split ion chains, precisely tune secular mode
frequencies, and adjust the orientation of trap axes. Microfabricated
capacitors on DC electrodes suppress radio-frequency pickup and excess
micromotion, while a top-level ground layer simplifies modeling of electric
fields and protects trap structures underneath. A localized aperture in the
substrate provides access to the trapping region from an oven below, permitting
deterministic loading of particular isotopic/elemental sequences via
species-selective photoionization. The shapes of the aperture and
radio-frequency electrodes are optimized to minimize perturbation of the
trapping pseudopotential. Laboratory experiments verify simulated potentials
and characterize trapping lifetimes, stray electric fields, and ion heating
rates, while measurement and cancellation of spatially-varying stray electric
fields permits the formation of nearly-equally spaced ion chains.Comment: 17 pages (including references), 7 figure
Intercontinental spread of Asian-origin H7 avian influenza viruses by captive bird trade in 1990's
Wild bird migration and illegal trade of infected poultry, eggs, and poultry products have been associated with the spread of avian influenza viruses (AIV). During 1992â1996, H7N1 and H7N8 low pathogenic AIV (LPAIV) were identified from captive wild birds; such as Pekin robin (Leiothrix lutea), magpie robin (Copsychus saularis), flycatcher sp. (genus Empidonax), a species of softbill and parakeet, sun conure (Aratinga solstitialis), painted conure (Pyrrhura picta), fairy bluebird (Irena puella), and common iora (Aegithina tiphia), kept in aviaries or quarantine stations in England, The Netherlands, Singapore and the United States (U.S.). In this study, we sequenced these H7 viruses isolated from quarantine facilities and aviaries using next-generation sequencing and conducted a comparative phylogenetic analysis of complete genome sequences to elucidate spread patterns. The complete genome sequencing and phylogenetic analysis suggested that H7 viruses originated from a common source, even though they were obtained from birds in distant geographical regions. All H7N1 and H7N8 viruses were LPAIV, except a H7N1 highly pathogenic AIV (HPAIV), A/Pekin robin/California/30412/1994(H7N1) virus. Our results support the continued need for regulation of the captive wild bird trade to reduce the risk of introduction and dissemination of both LPAIV and HPAIV throughout the world
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