5,089 research outputs found
Simulator test to study hot-flow problems related to a gas cooled reactor
An advance study of materials, fuel injection, and hot flow problems related to the gas core nuclear rocket is reported. The first task was to test a previously constructed induction heated plasma GCNR simulator above 300 kW. A number of tests are reported operating in the range of 300 kW at 10,000 cps. A second simulator was designed but not constructed for cold-hot visualization studies using louvered walls. A third task was a paper investigation of practical uranium feed systems, including a detailed discussion of related problems. The last assignment resulted in two designs for plasma nozzle test devices that could be operated at 200 atm on hydrogen
Matrix bandwidth and profile reduction
This program, REDUCE, reduces the bandwidth and profile of sparse symmetric matrices, using row and corresponding column permutations. It is a realization of the algorithm described by the authors elsewhere. It was extensively tested and compared with several other programs and was found to be considerably faster than the others, superior for bandwidth reduction and as satisfactory as any other for profile reduction
Magnetization dynamics in the single-molecule magnet Fe8 under pulsed microwave irradiation
We present measurements on the single molecule magnet Fe8 in the presence of
pulsed microwave radiation at 118 GHz. The spin dynamics is studied via time
resolved magnetization experiments using a Hall probe magnetometer. We
investigate the relaxation behavior of magnetization after the microwave pulse.
The analysis of the experimental data is performed in terms of different
contributions to the magnetization after-pulse relaxation. We find that the
phonon bottleneck with a characteristic relaxation time of 10 to 100 ms
strongly affects the magnetization dynamics. In addition, the spatial effect of
spin diffusion is evidenced by using samples of different sizes and different
ways of the sample's irradiation with microwaves.Comment: 14 pages, 12 figure
In-situ measurements of total reactive nitrogen, total water vapor, and aerosols in polar stratospheric clouds in the Antarctic stratosphere
Measurements of total reactive nitrogen, NOy, total water vapor, and aerosols were made as part of the Airborne Antarctic Ozone Experiment. The measurements were made using instruments located onboard the NASA ER-2 aircrafts which conducted twelve flights over the Antarctic continent reaching altitudes of 18 km at 72 S latitude. Each instrument utilized an ambient air sample and provided a measurement up to 1 Hz or every 200 m of flight path. The data presented focus on the flights of Aug. 17th and 18th during which Polar Stratospheric Clouds (PSCs) were encountered containing concentrations of 0.5 to 1.0 micron diameter aerosols greater than 1 cm/cu. The temperature pressure during these events ranged as low as 184 K near 75 mb pressure, with water values near 3.5 ppm by volume (ppmv). With the exception of two short periods, the PSC activity was observed at temperatures above the frost point of water over ice. The data gathered during these flights are analyzed and presented
Test of classical nucleation theory on deeply supercooled high-pressure simulated silica
We test classical nucleation theory (CNT) in the case of simulations of
deeply supercooled, high density liquid silica, as modelled by the BKS
potential. We find that at density ~g/cm, spontaneous nucleation
of crystalline stishovite occurs in conventional molecular dynamics simulations
at temperature T=3000 K, and we evaluate the nucleation rate J directly at this
T via "brute force" sampling of nucleation events. We then use parallel,
constrained Monte Carlo simulations to evaluate , the free energy
to form a crystalline embryo containing n silicon atoms, at T=3000, 3100, 3200
and 3300 K. We find that the prediction of CNT for the n-dependence of fits reasonably well to the data at all T studied, and at 3300 K yields a
chemical potential difference between liquid and stishovite that matches
independent calculation. We find that , the size of the critical nucleus,
is approximately 10 silicon atoms at T=3300 K. At 3000 K, decreases to
approximately 3, and at such small sizes methodological challenges arise in the
evaluation of when using standard techniques; indeed even the
thermodynamic stability of the supercooled liquid comes into question under
these conditions. We therefore present a modified approach that permits an
estimation of at 3000 K. Finally, we directly evaluate at T=3000
K the kinetic prefactors in the CNT expression for J, and find physically
reasonable values; e.g. the diffusion length that Si atoms must travel in order
to move from the liquid to the crystal embryo is approximately 0.2 nm. We are
thereby able to compare the results for J at 3000 K obtained both directly and
based on CNT, and find that they agree within an order of magnitude.Comment: corrected calculation, new figure, accepted in JC
Extinction and backscatter measurements of Antarctic PSC's, 1987: Implications for particle and vapor removal
The temperature dependence is examined of optical properties measured in the Antarctic during 1987 at the 70 mb level (near 18 km), a level chosen to correlate the results with in situ measurements made from the NASA-Ames ER-2 aircraft during the 1987 Airborne Antarctic Ozone Experiment (AAOE). The data set consists of extinction measurements by Sam 2 inside the Antarctic polar vortex from May to October 1987; and backscatter measurements by the UV-DIAL (Ultraviolet Differential Absorption Lidar) system aboard the Ames DC-8 aircraft during selected AAOE flights. Observed trends are compared with results from a revised version of Pole and McCormick's model to classify the PSC observations by Type (1 or 2) and infer the temporal behavior of the ambient aerosol and ambient vapor mixing ratios. The sample figures show monthly ensembles of the 70-mb Sam 2 extinction ratio (the ratio of aerosol or PSC extinction to molecule extinction) as a function of NMC temperature at the beginning (June) and (October) of the 1987 Antarctic winter. Both ensembles show two rather distinct clusters of points: one oriented in the near vertical direction which depicts the change with temperature of the ambient aerosol extinction ratio; and a second cluster oriented in the near horizontal direction whose position on the vertical scale marks a change in particle phase (i.e., PSC formation) and whose length (the extinction enhancement related to that of the ambient aerosol) is an indicator of PSC type
A measure of centrality based on the spectrum of the Laplacian
We introduce a family of new centralities, the k-spectral centralities.
k-Spectral centrality is a measurement of importance with respect to the
deformation of the graph Laplacian associated with the graph. Due to this
connection, k-spectral centralities have various interpretations in terms of
spectrally determined information.
We explore this centrality in the context of several examples. While for
sparse unweighted networks 1-spectral centrality behaves similarly to other
standard centralities, for dense weighted networks they show different
properties. In summary, the k-spectral centralities provide a novel and useful
measurement of relevance (for single network elements as well as whole
subnetworks) distinct from other known measures.Comment: 12 pages, 6 figures, 2 table
Polar ozone
The observation and interpretation of a large, unexpected ozone depletion over Antarctica has changed the international scientific view of stratospheric chemistry. The observations which show the veracity, seasonal nature, and vertical structure of the Antarctic ozone hole are presented. Evidence for Arctic and midlatitude ozone loss is also discussed. The chemical theory for Antarctic ozone depletion centers around the occurrence of polar stratospheric clouds (PSCs) in Antarctic winter and spring; the climatology and radiative properties of these clouds are presented. Lab studies of the physical properties of PSCs and the chemical processes that subsequently influence ozone depletion are discussed. Observations and interpretation of the chemical composition of the Antarctic stratosphere are described. It is shown that the observed, greatly enhanced abundances of chlorine monoxide in the lower stratosphere are sufficient to explain much if not all of the ozone decrease. The dynamic meteorology of both polar regions is given, interannual and interhemispheric variations in dynamical processes are outlined, and their likely roles in ozone loss are discussed
Free energy and configurational entropy of liquid silica: fragile-to-strong crossover and polyamorphism
Recent molecular dynamics (MD) simulations of liquid silica, using the
``BKS'' model [Van Beest, Kramer and van Santen, Phys. Rev. Lett. {\bf 64},
1955 (1990)], have demonstrated that the liquid undergoes a dynamical crossover
from super-Arrhenius, or ``fragile'' behavior, to Arrhenius, or ``strong''
behavior, as temperature is decreased. From extensive MD simulations, we
show that this fragile-to-strong crossover (FSC) can be connected to changes in
the properties of the potential energy landscape, or surface (PES), of the
liquid. To achieve this, we use thermodynamic integration to evaluate the
absolute free energy of the liquid over a wide range of density and . We use
this free energy data, along with the concept of ``inherent structures'' of the
PES, to evaluate the absolute configurational entropy of the liquid. We
find that the temperature dependence of the diffusion coefficient and of
are consistent with the prediction of Adam and Gibbs, including in the region
where we observe the FSC to occur. We find that the FSC is related to a change
in the properties of the PES explored by the liquid, specifically an inflection
in the dependence of the average inherent structure energy. In addition, we
find that the high behavior of suggests that the liquid entropy might
approach zero at finite , behavior associated with the so-called Kauzmann
paradox. However, we find that the change in the PES that underlies the FSC is
associated with a change in the dependence of that elucidates how the
Kauzmann paradox is avoided in this system. Finally, we also explore the
relation of the observed PES changes to the recently discussed possibility that
BKS silica exhibits a liquid-liquid phase transition, a behavior that has been
proposed to underlie the observed polyamorphism of amorphous solid silica.Comment: 14 pages, 18 figure
Minimax Current Density Coil Design
'Coil design' is an inverse problem in which arrangements of wire are
designed to generate a prescribed magnetic field when energized with electric
current. The design of gradient and shim coils for magnetic resonance imaging
(MRI) are important examples of coil design. The magnetic fields that these
coils generate are usually required to be both strong and accurate. Other
electromagnetic properties of the coils, such as inductance, may be considered
in the design process, which becomes an optimization problem. The maximum
current density is additionally optimized in this work and the resultant coils
are investigated for performance and practicality. Coils with minimax current
density were found to exhibit maximally spread wires and may help disperse
localized regions of Joule heating. They also produce the highest possible
magnetic field strength per unit current for any given surface and wire size.
Three different flavours of boundary element method that employ different basis
functions (triangular elements with uniform current, cylindrical elements with
sinusoidal current and conic section elements with sinusoidal-uniform current)
were used with this approach to illustrate its generality.Comment: 24 pages, 6 figures, 2 tables. To appear in Journal of Physics D:
Applied Physic
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