15,214 research outputs found
Emergency relief venting of the infrared telescope liquid helium dewar
An analysis is made of the emergency relief venting of the liquid helium dewar of the Spacelab 2 infrared telescope experiment in the event of a massive failure of the dewar guard vacuum. Such a failure, resulting from a major accident, could cause rapid heating and pressurization of the liquid helium in the dewar and lead to relief venting through the emergency relief system. The heat input from an accident is estimated for various fluid conditions in the dewar and the relief process as it takes place through one or both of the emergency relief paths is considered. It is shown that under all reasonable circumstances the dewar will safely relieve itself, and the pressure will not exceed 85 percent of the proof pressure or 63 percent of the burst pressure
Emergency relief venting of the infrared telescope liquid helium dewar, second edition
An updated analysis is made of the emergency relief venting of the liquid helium dewar of the Spacelab 2 Infrared Telescope experiment in the event of a massive failure of the dewar guard vacuum. Such a failure, resulting from a major accident, could cause rapid heating and pressurization of the liquid helium in the dewar and lead to relief venting through the emergency relief system. The heat input from an accident is estimated for various fluid conditions in the dewar and the relief process considered as it takes place through one or both of the emergency relief paths. It was previously assumed that the burst diaphragms in the dewar relief paths would rupture at a pressure of 65 psi differential or 4.4 atmospheres. In fact, it has proved necessary to use burst diaphragms in the dewar which rupture at 115 psid or 7.8 atmospheres. An analysis of this case was carried out and shows that when the high pressure diaphragm rupture occurs, the dewar pressure falls within 8 s to below the 4.4 atmospheres for which the original analysis was performed, and thereafter it remains below that level
Possibility of "magic" trapping of three-level system for Rydberg blockade implementation
The Rydberg blockade mechanism has shown noteworthy promise for scalable
quantum computation with neutral atoms. Both qubit states and gate-mediating
Rydberg state belong to the same optically-trapped atom. The trapping fields,
while being essential, induce detrimental decoherence. Here we theoretically
demonstrate that this Stark-induced decoherence may be completely removed using
powerful concepts of "magic" optical traps. We analyze "magic" trapping of a
prototype three-level system: a Rydberg state along with two qubit states:
hyperfine states attached to a J=1/2 ground state. Our numerical results show
that, while such a "magic" trap for alkali metals would require prohibitively
large magnetic fields, the group IIIB metals such as Al are suitable
candidates.Comment: 5 pages, 3 figure
The Incidence of Debris Disks at 24 {\mu}m and 670 Myr
We use Spitzer Space Telescope 24 {\mu}m data to search for debris disks
among 122 AFGKM stars from the \sim 670 Myr clusters Hyades, Coma Ber, and
Praesepe, utilizing a number of advances in data reduction and determining the
intrinsic colors of main sequence stars. For our sample, the 1{\sigma}
dispersion about the main sequence V-K, K-[24] locus is approximately 3.1%. We
identify seven debris disks at 10% or more (\geq 3{\sigma} confidence level)
above the expected K-[24] for purely photospheric emission. The incidence of
excesses of 10% or greater in our sample at this age is 5.7 +3.1/-1.7%.
Combining with results from the literature, the rate is 7.8 +4.2/-2.1% for
early- type (B9 - F4) stars and 2.7 +3.3/-1.7% for solar-like (F5 - K9) stars.
Our primary sample has strict criteria for inclusion to allow comparison with
other work; when we relax these criteria, three additional debris disks are
detected. They are all around stars of solar-like type and hence reinforce our
conclusion that disks around such stars are still relatively common at 670 Myr
and are similar to the rate around early-type stars. The apparently small
difference in decay rates between early-type and solar-like stars is
inconsistent with the first order theoretical predictions that the later type
stellar disks would decay an order of magnitude more quickly than the earlier
type ones.Comment: 26 pages, 3 figures, accepted for publication in Ap
Comparing Tycho-2 Astrometry with UCAC1
The Tycho-2 Catalogue, released in February 2000, is based on the ESA
Hipparcos space mission data and various ground-based catalogs for proper
motions. An external comparison of the Tycho-2 astrometry is presented here
using the first U.S. Naval Observatory CCD Astrograph Catalog (UCAC1). The
UCAC1 data were obtained from observations performed at CTIO between February
1998 and November 1999, using the 206 mm aperture 5-element lens astrograph and
a 4k x 4k CCD. Only small systematic differences in position between Tycho-2
and UCAC1 up to 15 milliarcseconds (mas) are found, mainly as a function of
magnitude. The standard deviations of the distributions of the position
differences are in the 35 to 140 mas range, depending on magnitude. The
observed scatter in the position differences is about 30% larger than expected
from the combined formal, internal errors, also depending on magnitude. The
Tycho-2 Catalogue has the more precise positions for bright stars (V <= 10 mag)
while the UCAC1 positions are significantly better at the faint end (11 mag <=
V <= 12.5 mag) of the magnitude range in common. UCAC1 goes much fainter (to
R=16) than Tycho-2; however complete sky coverage is not expected before mid
2003.Comment: LaTeX, 8 pages, 3 PS figures, accepted by AJ (Aug 2000) see also
http://ad.usno.navy.mil/ad/ucac/ request for UCAC1 CD-ROM: e-mail to
[email protected] request for Tycho-2 CD-ROM: e-mail to
[email protected] or [email protected]
Temperature and finite-size effects in collective modes of superfluid Fermi gases
We study the effects of superfluidity on the monopole and quadrupole
collective excitations of a dilute ultra-cold Fermi gas with an attractive
interatomic interaction. The system is treated fully microscopically within the
Bogoliubov-de Gennes and quasiparticle random-phase approximation methods. The
dependence on the temperature and on the trap frequency is analyzed and
systematic comparisons with the corresponding hydrodynamic predictions are
presented in order to study the limits of validity of the semiclassical
approach.Comment: 9 pages, 4 figure
Brane Cosmology and KK Gravitinos
The cosmology of KK gravitinos in models with extra dimensions is considered.
The main result is that the production of such KK modes is not compatible with
an epoch of non--standard expansion after inflation. This is so because the BBN
constraint on the zero mode forces the reduced five dimensional Planck mass
down to values much smaller than the usual four dimensional one, but this
in turn implies many KK states available for a given temperature. Once these
states are taken into account one finds that there is no for which the
produced KK gravitinos satisfy BBN and overclosure constraints. This conclusion
holds for both flat and warped models in which only gravity propagates in the
full spacetime.Comment: 19 pages, references added, IoP styl
Premise Selection for Mathematics by Corpus Analysis and Kernel Methods
Smart premise selection is essential when using automated reasoning as a tool
for large-theory formal proof development. A good method for premise selection
in complex mathematical libraries is the application of machine learning to
large corpora of proofs. This work develops learning-based premise selection in
two ways. First, a newly available minimal dependency analysis of existing
high-level formal mathematical proofs is used to build a large knowledge base
of proof dependencies, providing precise data for ATP-based re-verification and
for training premise selection algorithms. Second, a new machine learning
algorithm for premise selection based on kernel methods is proposed and
implemented. To evaluate the impact of both techniques, a benchmark consisting
of 2078 large-theory mathematical problems is constructed,extending the older
MPTP Challenge benchmark. The combined effect of the techniques results in a
50% improvement on the benchmark over the Vampire/SInE state-of-the-art system
for automated reasoning in large theories.Comment: 26 page
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