3,497 research outputs found
Space shuttle external tank performance improvements: The challenge
The external tank (ET) has been actively involved in performance improvements since the inception of the space shuttle program, primarily by weight savings. Weight savings were realized on the first block of flight articles (standard weight tank). With a need for further performance improvements, the ET Program Office was requested to develop a program to reduce tank weight an additional 6000 lb and schedule delivery of the first lightweight ET (LWT) for June 1982. The weight savings program was accomplished by: (1) a unique approach to use of factors of safety; (2) design optimization; and (3) redesign of structures with large margins of safety which resulted in an actual weight savings of 7294 lb. Additional studies have identified further weight savings which are to be implemented at appropriate times in production flow. Examples are an improved thermal protection system for the LH2 tank aft dome and reduction of slosh baffles in the LO2 tank based on flight data. All performance improvements were compared and selected based on non-recurring and recurring cost and technical risk
Formation of shock waves in a Bose-Einstein condensate
We consider propagation of density wave packets in a Bose-Einstein
condensate. We show that the shape of initially broad, laser-induced, density
perturbation changes in the course of free time evolution so that a shock wave
front finally forms. Our results are well beyond predictions of commonly used
zero-amplitude approach, so they can be useful in extraction of a speed of
sound from experimental data. We discuss a simple experimental setup for shock
propagation and point out possible limitations of the mean-field approach for
description of shock phenomena in a BEC.Comment: 8 pages & 6 figures, minor changes, more references, to appear in
Phys. Rev.
Theoretical X-Ray Absorption Debye-Waller Factors
An approach is presented for theoretical calculations of the Debye-Waller
factors in x-ray absorption spectra. These factors are represented in terms of
the cumulant expansion up to third order. They account respectively for the net
thermal expansion , the mean-square relative displacements
, and the asymmetry of the pair distribution function
. Similarly, we obtain Debye-Waller factors for x-ray and
neutron scattering in terms of the mean-square vibrational amplitudes .
Our method is based on density functional theory calculations of the dynamical
matrix, together with an efficient Lanczos algorithm for projected phonon
spectra within the quasi-harmonic approximation. Due to anharmonicity in the
interatomic forces, the results are highly sensitive to variations in the
equilibrium lattice constants, and hence to the choice of exchange-correlation
potential. In order to treat this sensitivity, we introduce two prescriptions:
one based on the local density approximation, and a second based on a modified
generalized gradient approximation. Illustrative results for the leading
cumulants are presented for several materials and compared with experiment and
with correlated Einstein and Debye models. We also obtain Born-von Karman
parameters and corrections due to perpendicular vibrations.Comment: 11 pages, 8 figure
Electromagnetically induced transparency in superconducting quantum circuits : Effects of decoherence, tunneling and multi-level cross-talk
We explore theoretically electromagnetically-induced transparency (EIT) in a
superconducting quantum circuit (SQC). The system is a persistent-current flux
qubit biased in a configuration. Previously [Phys. Rev. Lett. 93,
087003 (2004)], we showed that an ideally-prepared EIT system provides a
sensitive means to probe decoherence. Here, we extend this work by exploring
the effects of imperfect dark-state preparation and specific decoherence
mechanisms (population loss via tunneling, pure dephasing, and incoherent
population exchange). We find an initial, rapid population loss from the
system for an imperfectly prepared dark state. This is followed by a
slower population loss due to both the detuning of the microwave fields from
the EIT resonance and the existing decoherence mechanisms. We find analytic
expressions for the slow loss rate, with coefficients that depend on the
particular decoherence mechanisms, thereby providing a means to probe,
identify, and quantify various sources of decoherence with EIT. We go beyond
the rotating wave approximation to consider how strong microwave fields can
induce additional off-resonant transitions in the SQC, and we show how these
effects can be mitigated by compensation of the resulting AC Stark shifts
Towards a first principles description of phonons in NiPt disordered alloys: the role of relaxation
Using a combination of density-functional perturbation theory and the
itinerant coherent potential approximation, we study the effects of atomic
relaxation on the inelastic incoherent neutron scattering cross sections of
disordered NiPt alloys. We build on previous work, where
empirical force constants were adjusted {\it ad hoc} to agree with experiment.
After first relaxing all structural parameters within the local-density
approximation for ordered NiPt compounds, density-functional perturbation
theory is then used to compute phonon spectra, densities of states, and the
force constants. The resulting nearest-neighbor force constants are first
compared to those of other ordered structures of different stoichiometry, and
then used to generate the inelastic scattering cross sections within the
itinerant coherent potential approximation. We find that structural relaxation
substantially affects the computed force constants and resulting inelastic
cross sections, and that the effect is much more pronounced in random alloys
than in ordered alloys.Comment: 8 pages, 3 eps figures, uses revtex
Improving measurements of SF6 for the study of atmospheric transport and emissions
Sulfur hexafluoride (SF6) is a potent greenhouse gas and useful atmospheric tracer. Measurements of SF6 on global and regional scales are necessary to estimate emissions and to verify or examine the performance of atmospheric transport models. Typical precision for common gas chromatographic methods with electron capture detection (GC-ECD) is 1–2%. We have modified a common GC-ECD method to achieve measurement precision of 0.5% or better. Global mean SF6 measurements were used to examine changes in the growth rate of SF6 and corresponding SF6 emissions. Global emissions and mixing ratios from 2000–2008 are consistent with recently published work. More recent observations show a 10% decline in SF6 emissions in 2008–2009, which seems to coincide with a decrease in world economic output. This decline was short-lived, as the global SF6 growth rate has recently increased to near its 2007–2008 maximum value of 0.30±0.03 pmol mol−1 (ppt) yr−1 (95% C.L.)
OA13.01. Mind-Body Medicine Skills training for self-care and emotional well-being in medical students
Dominant ferromagnetism in the spin-1/2 half-twist ladder 334 compounds, Ba3Cu3In4O12 and Ba3Cu3Sc4O12
The magnetic properties of polycrystalline samples of Ba3Cu3In4O12 (In-334)
and Ba3Cu3Sc4O12 (Sc-334) are reported. Both 334 phases have a structure
derived from perovskite, with CuO4 squares interconnected to form half-twist
ladders along the c-axis. The Cu-O-Cu angles, ~ 90o, and the positive Weiss
temperatures indicate the presence of significant ferromagnetic (FM)
interactions along the Cu ladders. At low temperatures, T < 20 K, sharp
transitions in the magnetic susceptibility and heat capacity measurements
indicate three-dimensional (3D) antiferromagnetic (AFM) ordering at TN. TN is
suppressed on application of a field and a complex magnetic phase diagram with
three distinct magnetic regimes below the upper critical field can be inferred
from our measurements. The magnetic interactions are discussed in relation to a
modified spin-1/2 FM-AFM model and the 334 half-twist ladder is compared to
other 2-rung ladder spin-1/2 systems.Comment: 20 pages, 7 figure
Galaxy Masses
Galaxy masses play a fundamental role in our understanding of structure
formation models. This review addresses the variety and reliability of mass
estimators that pertain to stars, gas, and dark matter. The different sections
on masses from stellar populations, dynamical masses of gas-rich and gas-poor
galaxies, with some attention paid to our Milky Way, and masses from weak and
strong lensing methods, all provide review material on galaxy masses in a
self-consistent manner.Comment: 145 pages, 28 figures, to appear in Reviews of Modern Physics. Figure
22 is missing here, and Figs. 15, 26-28 are at low resolution. This version
has a slightly different title and some typos fixed in Chapter 5. For the
full review with figures, please consult:
http://www.astro.queensu.ca/~courteau/GalaxyMasses_28apr2014.pd
Statistical Basis for Predicting Technological Progress
Forecasting technological progress is of great interest to engineers, policy
makers, and private investors. Several models have been proposed for predicting
technological improvement, but how well do these models perform? An early
hypothesis made by Theodore Wright in 1936 is that cost decreases as a power
law of cumulative production. An alternative hypothesis is Moore's law, which
can be generalized to say that technologies improve exponentially with time.
Other alternatives were proposed by Goddard, Sinclair et al., and Nordhaus.
These hypotheses have not previously been rigorously tested. Using a new
database on the cost and production of 62 different technologies, which is the
most expansive of its kind, we test the ability of six different postulated
laws to predict future costs. Our approach involves hindcasting and developing
a statistical model to rank the performance of the postulated laws. Wright's
law produces the best forecasts, but Moore's law is not far behind. We discover
a previously unobserved regularity that production tends to increase
exponentially. A combination of an exponential decrease in cost and an
exponential increase in production would make Moore's law and Wright's law
indistinguishable, as originally pointed out by Sahal. We show for the first
time that these regularities are observed in data to such a degree that the
performance of these two laws is nearly tied. Our results show that
technological progress is forecastable, with the square root of the logarithmic
error growing linearly with the forecasting horizon at a typical rate of 2.5%
per year. These results have implications for theories of technological change,
and assessments of candidate technologies and policies for climate change
mitigation
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