21,830 research outputs found
Modelling and Control of an Annular Momentum Control Device
The results of a modelling and control study for an advanced momentum storage device supported on magnetic bearings are documented. The control challenge posed by this device lies in its dynamics being such a strong function of flywheel rotational speed. At high rotational speed, this can lead to open loop instabilities, resulting in requirements for minimum and maximum control bandwidths and gains for the stabilizing controllers. Using recently developed analysis tools for systems described by complex coefficient differential equations, the closed properties of the controllers were analyzed and stability properties established. Various feedback controllers are investigated and discussed. Both translational and angular dynamics compensators are developed, and measures of system stability and robustness to plant and operational speed variations are presented
The Effect of varying Ground-state Aromaticity on the First Molecular Electronic Hyperpolarizabilites of Organic Donor-Acceptor Molecules
A series of compounds of the form 4-dimethylaminophenyl–polyene–acceptor, where the polyene ranges from nothing to all-trans-1,3,5-hexatriene and the acceptor is 2-nitrovinyl, formyl, or 2,2-dicyanovinyl has been prepared and their β values measured by solution electric-field-induced second-harmonic generation; these molecules, which lose only one aromatic resonance upon charge-transfer excitation, show enhanced β compared to bi-aromatic molecules with the same substitution and total conjugation length, such as 4-dimethylamino-4′-nitrostilbene (DANS), a well-known benchmark for high β organic molecules
The First Molecular Electronic Hyperpolarizabilities of Highly Polarizable Organic Molecules: 2,6-Di-tert-butylindoanilines
The first molecular hyperpolarizabilities (β) of a series of 2,6-di-tert-butylindoanilines, measured by electric-field-induced second harmonic generation are somewhat more sensitive to donor strength than was found for analogously substituted nitrostilbenes, and dimethylindoaniline has a β roughly twice that of its 2,6 di-tert-butylated analogue, measured in chloroform; solvatochromic measurements on the former compound suggest that this decrease in hyperpolarizability is consistent with a bound-solvent effect
Synthesis and First Hyperpolarizabilities of Acceptor-substituted β-apo-8’-Carotenal Derived Compounds
The synthesis and second-order nonlinear optical properties of acceptor-substituted biologically derived β-apo-8′-carotenal compounds are reported; electric field-induced second harmonic generation (EFISH) measurements give values of β(0) which are 2–6 times greater than for 4-N,N-dimethylamino-4′-nitrostilbene (DANS)
Running a distributed virtual observatory: US Virtual Astronomical Observatory operations
Operation of the US Virtual Astronomical Observatory shares some issues with
modern physical observatories, e.g., intimidating data volumes and rapid
technological change, and must also address unique concerns like the lack of
direct control of the underlying and scattered data resources, and the
distributed nature of the observatory itself. In this paper we discuss how the
VAO has addressed these challenges to provide the astronomical community with a
coherent set of science-enabling tools and services. The distributed nature of
our virtual observatory-with data and personnel spanning geographic,
institutional and regime boundaries-is simultaneously a major operational
headache and the primary science motivation for the VAO. Most astronomy today
uses data from many resources. Facilitation of matching heterogeneous datasets
is a fundamental reason for the virtual observatory. Key aspects of our
approach include continuous monitoring and validation of VAO and VO services
and the datasets provided by the community, monitoring of user requests to
optimize access, caching for large datasets, and providing distributed storage
services that allow user to collect results near large data repositories. Some
elements are now fully implemented, while others are planned for subsequent
years. The distributed nature of the VAO requires careful attention to what can
be a straightforward operation at a conventional observatory, e.g., the
organization of the web site or the collection and combined analysis of logs.
Many of these strategies use and extend protocols developed by the
international virtual observatory community.Comment: 7 pages with 2 figures included within PD
A Book Review: Anfara, V. A., & Mertz, N. T. (Eds.). (2006). Theoretical frameworks in qualitative research. Thousand Oaks, CA: Sage
This review describes a recent book on qualitative research by Anfara and Mertz concerning the important task facing academic researchers to properly relate their study to theoretical frameworks. Thanks to this collection of ten actual studies by respected academics, it becomes possible for novice researchers to see how the process of aligning theoretical frameworks with qualitative research endeavours should unfold. This will no doubt lessen angst about accomplishing their work. Readers will find at least one chapter in this book that is similar enough to their own chosen research topic to serve as a very useful guideline in choosing and working with theoretical frameworks
Search for bursts in air shower data
There have been reports in recent years of the possible observation of bursts in air shower data. If such events are truly of an astrophysical nature then, they represent an important new class of phemonenon since no other bursts have been observed above the MeV level. The spectra of conventional gamma ray bursts are unknown at higher energies but their observed spectra at MeV energies appear generally to exhibit a steepening in the higher MeV range and are thus unlikely to extrapolate to measurable fluxes at air shower energies. An attempt has been made to look for deviations from randomness in the arrival times of air showers above approx. 10 to the 14th power eV with a number of systems and results so far are presented here. This work will be continued for a substantial period of ime with a system capable of recording bursts with multiple events down to a spacing of 4 microns. Earlier data have also been searched for the possible association of air shower events with a glitch of the Vela pulsar
Measurements of the effect of collisions on transverse beam halo diffusion in the Tevatron and in the LHC
Beam-beam forces and collision optics can strongly affect beam lifetime,
dynamic aperture, and halo formation in particle colliders. Extensive
analytical and numerical simulations are carried out in the design and
operational stage of a machine to quantify these effects, but experimental data
is scarce. The technique of small-step collimator scans was applied to the
Fermilab Tevatron collider and to the CERN Large Hadron Collider to study the
effect of collisions on transverse beam halo dynamics. We describe the
technique and present a summary of the first results on the dependence of the
halo diffusion coefficient on betatron amplitude in the Tevatron and in the
LHC.Comment: 4 pages, 2 figures. Submitted to the Proceedings of the ICFA
Mini-Workshop on Beam-beam Effects in Hadron Colliders (BB2013), Geneva,
Switzerland, 18-22 March 201
Chemical Constraints on the Water and Total Oxygen Abundances in the Deep Atmosphere of Saturn
Thermochemical equilibrium and kinetic calculations for the trace gases CO,
PH3, and SiH4 give three independent constraints on the water and total oxygen
abundances of Saturn's deep atmosphere. A lower limit to the water abundance of
H2O/H2 > 1.7 x 10^-3 is given by CO chemistry while an upper limit of H2O/H2 <
5.5 x 10^-3 is given by PH3 chemistry. A combination of the CO and PH3
constraints indicates a water enrichment on Saturn of 1.9 to 6.1 times the
solar system abundance (H2O/H2 = 8.96 x 10^-4). The total oxygen abundance must
be at least 1.7 times the solar system abundance (O/H2 = 1.16 x 10^-3) in order
for the SiH4 to remain below a detection limit of SiH4/H2 < 2 x 10^-10. A
combination of the CO, PH3, and SiH4 constraints suggests that the total oxygen
abundance on Saturn is 3.2 to 6.4 times the solar system abundance. Our results
indicate that oxygen on Saturn is less enriched than other heavy elements (such
as C and P) relative to a solar system composition. This work was supported by
NASA NAG5-11958.Comment: 19 pages, 2 figures, accepted for publication in the Astrophysical
Journa
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