325,338 research outputs found
An evaluation plan of bus architectures and protocols using the NASA Ames intelligent redundant actuation system
Means for evaluating data bus architectures and protocols for highly integrated flight control system applications are needed. Described are the criteria and plans to do this by using the NASA/Ames Intelligent Redundant Actuation System (IRAS) experimental set-up. Candidate bus architectures differ from one another in terms of: topology, access control, message transfer schemes, message characteristics, initialization. data flow control, transmission rates, fault tolerance, and time synchronization. The evaluation criteria are developed relative to these features. A preliminary, analytical evaluation of four candidate busses (MIL-STD-1553B, DATAC, Ethernet, and HSIS) is described. A bus must be exercised in a real-time environment to evaluate its dynamic characteristics. A plan for real-time evaluation of these four busses using a combination of hardware and simulation techniques is presented
Nonsingular density profiles of dark matter halos and Strong gravitational lensing
We use the statistics of strong gravitational lenses to investigate whether
mass profiles with a flat density core are supported. The probability for
lensing by halos modeled by a nonsingular truncated isothermal sphere (NTIS)
with image separations greater than a certain value (ranging from zero to ten
arcseconds) is calculated. NTIS is an analytical model for the postcollapse
equilibrium structure of virialized objects derived by Shapiro, Iliev & Raga.
This profile has a soft core and matches quite well with the mass profiles of
dark matter-dominated dwarf galaxies deduced from their observed rotation
curves. It also agrees well with the NFW (Navarro-Frenk-White) profile at all
radii outside of a few NTIS core radii. Unfortunately, comparing the results
with those for singular lensing halos (NFW and SIS+NFW) and strong lensing
observations, the probabilities for lensing by NTIS halos are far too low. As
this result is valid for any other nonsingular density profiles (with a large
core radius), we conclude that nonsingular density profiles (with a large core
radius) for CDM halos are ruled out by statistics of strong gravitational
lenses.Comment: 17 pages, 4 figures, ApJ accepted. Final version matches the proofs.
A curve in figure 2 is corrected, conclusions unchange
Allowable silicon wafer thickness versus diameter for ingot rotation ID wafering
Inner diameter (ID) wafering of ingot rotation reduce the ID saw blade diameter was investigated. The blade thickness can be reduced, resulting in minimal kerf loss. However, significant breakage of wafers occurs during the rotation wafering as the wafer thickness decreases. Fracture mechanics was used to develop an equation relating wafer thickness, diameter and fracture behavior at the point of fracture by using a model of a wafer, supported by a center column and subjected to a cantilever force. It is indicated that the minimum allowable wafer thickness does not increase appreciably with increasing wafer diameter and that fracture through the thickness rather than through the center supporting column limits the minimum allowable wafer thickness. It is suggested that the minimum allowable wafer thickness can be reduced by using a vacuum chuck on the wafer surface to enhance cleavage fracture of the center core and by using 111 ingots
The Massive Disk Around OH 231.8+4.2
We have obtained 11.7 micron and 17.9 micron images at the Keck I telescope
of the circumstellar dust emission from OH 231.8+4.2, an evolved mass-losing
red giant with a well studied bipolar outflow. We detect both a central
unresolved point source and extended emission which is aligned with the bipolar
outflow seen on larger scales. We find that the unresolved central source can
be explained by an opaque, flared disk with an outer radius near 300 AU and an
outer temperature of about 130 K. One possible model to explain this flaring is
that the material in the disk is orbiting the central star and not simply
undergoing a radial expansion.Comment: ApJ, in pres
Phase diagram of QCD at finite temperature and chemical potential from lattice simulations with dynamical Wilson quarks
We present the first results for lattice QCD at finite temperature and
chemical potential with four flavors of Wilson quarks. The calculations
are performed using the imaginary chemical potential method at ,
0.001, 0.15, 0.165, 0.17 and 0.25, where is the hopping parameter,
related to the bare quark mass and lattice spacing by
. Such a method allows us to do large scale Monte Carlo
simulations at imaginary chemical potential . By analytic
continuation of the data with to real values of the chemical
potential, we expect at each , a transition line
on the plane, in a region relevant to the search for quark gluon
plasma in heavy-ion collision experiments. The transition is first order at
small or large quark mass, and becomes a crossover at intermediate quark mass.Comment: Published versio
A model of a dual-core matter-wave soliton laser
We propose a system which can generate a periodic array of solitary-wave
pulses from a finite reservoir of coherent Bose-Einstein condensate (BEC). The
system is built as a set of two parallel quasi-one-dimensional traps (the
reservoir proper and a pulse-generating cavity), which are linearly coupled by
the tunneling of atoms. The scattering length is tuned to be negative and small
in the absolute value in the cavity, and still smaller but positive in the
reservoir. Additionally, a parabolic potential profile is created around the
center of the cavity. Both edges of the reservoir and one edge of the cavity
are impenetrable. Solitons are released through the other cavity's edge, which
is semi-transparent. Two different regimes of the intrinsic operation of the
laser are identified: circulations of a narrow wave-function pulse in the
cavity, and oscillations of a broad standing pulse. The latter regime is
stable, readily providing for the generation of an array containing up to
10,000 permanent-shape pulses. The circulation regime provides for no more than
40 cycles, and then it transforms into the oscillation mode. The dependence of
the dynamical regime on parameters of the system is investigated in detail.Comment: Journal of Physics B, in pres
Dirty, Skewed, and Backwards: The Smectic - Phase Transition in Aerogel
We study the smectic AC transition in anisotropic and uniaxial disordered
environments, e.g., aerogel with an external field. We find very strange
behavior of translational correlations: the low-temperature, lower-symmetry
Smectic C phase is itless translationally ordered than the it high-temperature,
higher-symmetry Smectic A phase, with short-ranged and algebraic translational
correlations, respectively. Specifically, the A and C phase belong to the
quasi-long-ranged translationally ordered " XY Bragg glass '' and short-ranged
translationally ordered " m=1 Bragg glass '' phase, respectively. The AC phase
transition itself belongs to a new universality class, whose fixed points and
exponents we find in a d=5-epsilon expansion
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