34,826 research outputs found
Studies of molecular properties of polymeric materials
Aerospace environment effects (high energy electrons, thermal cycling, atomic oxygen, and aircraft fluids) on polymeric and composite materials considered for structural use in spacecraft and advanced aircraft are examined. These materials include Mylar, Ultem, and Kapton. In addition to providing information on the behavior of the materials, attempts are made to relate the measurements to the molecular processes occurring in the material. A summary and overview of the technical aspects are given along with a list of the papers that resulted from the studies. The actual papers are included in the appendices and a glossary of technical terms and definitions is included in the front matter
Single and Many Particle Correlation Functions and Uniform Phase Bases for Strongly Correlated Systems
The need for suitable many or infinite fermion correlation functions to
describe some low dimensional strongly correlated systems is discussed. This is
linked to the need for a correlated basis, in which the ground state may be
postive definite, and in which single particle correlations may suffice. A
particular trial basis is proposed, and applied to a certain quasi-1D model.
The model is a strip of the 2D square lattice wrapped around a cylinder, and is
related to the ladder geometries, but with periodic instead of open boundary
conditions along the edges. Analysis involves a novel mean-field approach and
exact diagonalisation. The model has a paramagnetic region and a Nagaoka
ferromagnetic region. The proposed basis is well suited to the model, and
single particle correlations in it have power law decay for the paramagnet,
where the charge motion is qualitatively hard core bosonic. The mean field also
leads to a BCS-type model with single particle long range order.Comment: 23 pages, in plain tex, 12 Postscript figures included. Accepted for
publication in J.Physics : Condensed Matte
A New Linear Inductive Voltage Adder Driver for the Saturn Accelerator
Saturn is a dual-purpose accelerator. It can be operated as a large-area
flash x-ray source for simulation testing or as a Z-pinch driver especially for
K-line x-ray production. In the first mode, the accelerator is fitted with
three concentric-ring 2-MV electron diodes, while in the Z-pinch mode the
current of all the modules is combined via a post-hole convolute arrangement
and driven through a cylindrical array of very fine wires. We present here a
point design for a new Saturn class driver based on a number of linear
inductive voltage adders connected in parallel. A technology recently
implemented at the Institute of High Current Electronics in Tomsk (Russia) is
being utilized[1].
In the present design we eliminate Marx generators and pulse-forming
networks. Each inductive voltage adder cavity is directly fed by a number of
fast 100-kV small-size capacitors arranged in a circular array around each
accelerating gap. The number of capacitors connected in parallel to each cavity
defines the total maximum current. By selecting low inductance switches,
voltage pulses as short as 30-50-ns FWHM can be directly achieved.Comment: 3 pages, 4 figures. This paper is submitted for the 20th Linear
Accelerator Conference LINAC2000, Monterey, C
Precipitation detector Patent
Precipitation detector and mechanism for stopping and restarting machinery at initiation and cessation of rai
Engineering handbook on the atmospheric environmental guidelines for use in wind turbine generator development
The guidelines are given in the form of design criteria relative to wind speed, wind shear, turbulence, wind direction, ice and snow loading, and other climatological parameters which include rain, hail, thermal effects, abrasive and corrosive effects, and humidity. This report is a presentation of design criteria in an engineering format which can be directly input to wind turbine generator design computations. Guidelines are also provided for developing specialized wind turbine generators or for designing wind turbine generators which are to be used in a special region of the United States
The magnetic structure of Gd_2Ti_2O_7
We attempt to solve the magnetic structure of the gadolinium analogue of
`spin-ice', using a mixture of experimental and theoretical assumptions. The
eventual predictions are essentially consistent with both the Mossbauer and
neutron measurements but are unrelated to previous proposals. We find two
possible distinct states, one of which is coplanar and the other is fully
three-dimensional. We predict that close to the initial transition the
preferred state is coplanar but that at the lowest temperature the ground-state
becomes fully three-dimensional. Unfortunately the energetics are consequently
complicated. There is a dominant nearest-neighbour Heisenberg interaction but
then a compromise solution for lifting the final degeneracy resulting from a
competition between longer-range Heisenberg interactions and direct dipolar
interactions on similar energy scales.Comment: 12 pages, 15 figure
Research program to develop a technology improvement program for closed die forging Final report
Upset forging tests on aluminum and titanium alloys and maraging steel using high temperature die
Evolution of cooperation in multilevel public goods games with community structures
In a community-structured population, public goods games (PGG) occur both
within and between communities. Such type of PGG is referred as multilevel
public goods games (MPGG). We propose a minimalist evolutionary model of the
MPGG and analytically study the evolution of cooperation. We demonstrate that
in the case of sufficiently large community size and community number, if the
imitation strength within community is weak, i.e., an individual imitates
another one in the same community almost randomly, cooperation as well as
punishment are more abundant than defection in the long run; if the imitation
strength between communities is strong, i.e., the more successful strategy in
two individuals from distinct communities is always imitated, cooperation and
punishment are also more abundant. However, when both of the two imitation
intensities are strong, defection becomes the most abundant strategy in the
population. Our model provides insight into the investigation of the
large-scale cooperation in public social dilemma among contemporary
communities.Comment: 6 pages, 4 figures, Accepted by EP
Propellant material compatibility program and results
The effects of long-term (up to 10 years) contact of inert materials with earth-storable propellants were studied for the purpose of designing chemical propulsion system components that can be used for current as well as future planetary spacecraft. The primary experimental work, and results to date are reported. Investigations include the following propellants: hydrazine, hydrazine-hydrazine nitrate blends, monomethyl-hydrazine, and nitrogen tetroxide. Materials include: aluminum alloys, corrosion-resistant steels, and titanium alloys. More than 700 test specimen capsules were placed in long-term storage testing at 43 C in the special material compatibility facility. Material ratings relative to the 10-year requirement have been assigned
Discovery of 15-second oscillations in Hubble Space Telescope observations of WZ Sagittae following the 2001 outburst
We report the discovery of 15-s oscillations in ultraviolet observations of
WZ Sge obtained with the Hubble Space Telescope approximately one month after
the peak of the 2001 outburst. This is the earliest detection of oscillations
in WZ Sge following an outburst and the first time that a signal near 15 s has
been seen to be dominant. The oscillations are quite strong (amplitude about
5%), but not particularly coherent. In one instance, the oscillation period
changed by 0.7 s between successive observations separated by less than 1 hour.
We have also found evidence for weaker signals with periods near 6.5 s in some
of our data. We discuss the implications of our results for the models that
have been proposed to account for the 28-s oscillations seen in quiescence. If
the periods of the 15-s oscillations can be identified with the periods of
revolution of material rotating about the white dwarf, the mass of the white
dwarf must satisfy M_WD > 0.71 M_sun. The corresponding limit for the 6.5-s
signals is M_WD > 1.03 M_sun.Comment: accepted for publication in ApJ Letters; 13 pages, 4 postscript
figures; new version corrects a few typos and matches version that will
appear in ApJ
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