9,291 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
Thermal performance evaluation of the infrared telescope dewar subsystem
Thermal performance evaluations (TPE) were conducted with the superfluid helium dewar of the Infrared Telescope (IRT) experiment from November 1981 to August 1982. Test included measuring key operating parameters, simulating operations with an attached instrument cryostat and validating servicing, operating and safety procedures. Test activities and results are summarized. All objectives are satisfied except for those involving transfer of low pressure liquid helium (LHe) from a supply dewar into the dewar subsystem
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
Numerical solution of the Boltzmann equation for the collective modes of trapped Fermi gases
We numerically solve the Boltzmann equation for trapped fermions in the
normal phase using the test-particle method. After discussing a couple of tests
in order to estimate the reliability of the method, we apply it to the
description of collective modes in a spherical harmonic trap. The numerical
results are compared with those obtained previously by taking moments of the
Boltzmann equation. We find that the general shape of the response function is
very similar in both methods, but the relaxation time obtained from the
simulation is significantly longer than that predicted by the method of
moments. It is shown that the result of the method of moments can be corrected
by including fourth-order moments in addition to the usual second-order ones
and that this method agrees very well with our numerical simulations.Comment: 13 pages, 8 figures, accepted for publication in Phys. Rev.
Theory of a Magnetically-Controlled Quantum-Dot Spin Transistor
We examine transport through a quantum dot coupled to three ferromagnetic
leads in the regime of weak tunnel coupling. A finite source-drain voltage
generates a nonequilibrium spin on the otherwise non-magnetic quantum dot. This
spin accumulation leads to magnetoresistance. A ferromagnetic but current-free
base electrode influences the quantum-dot spin via incoherent spin-flip
processes and coherent spin precession. As the dot spin determines the
conductance of the device, this allows for a purely magnetic transistor-like
operation. We analyze the effect of both types of processes on the electric
current in different geometries.Comment: 7 pages, 6 figure
Test results of Spacelab 2 infrared telescope focal plane
The small helium cooled infrared telescope for Spacelab 2 is designed for sensitive mapping of extended, low-surface-brightness celestial sources as well as highly sensitive investigations of the shuttle contamination environment (FPA) for this mission is described as well as the design for a thermally isolated, self-heated J-FET transimpedance amplifier. This amplifier is Johnson noise limited for feedback resistances from less than 10 to the 8th power Omega to greater than 2 x 10 to the 10th power Omega at T = 4.2K. Work on the focal plane array is complete. Performance testing for qualification of the flight hardware is discussed, and results are presented. All infrared data channels are measured to be background limited by the expected level of zodiacal emission
- …