8,854 research outputs found
Optimizing an array of antennas for cellular coverage from a high altitude platform
In a wireless communications network served by a high altitude platform (HAP) the cochannel interference is a function of the antenna beamwidth, angular separation and. sidelobe level. At the millimeter wave frequencies proposed for HAPs, an array of aperture type antennas on the platform is a practicable solution for serving the cells. We present a method for predicting cochannel interference based on curve-fit approximations for radiation patterns of elliptic beams which illuminate cell edges with optimum power, and a means of estimating optimum beamwidths for each cell of a regular hexagonal layout. The method is then applied to a 121 cell architecture. Where sidelobes are modeled As a flat floor at 40-dB below peak directivity, a cell cluster size of four yields carrier-to-interference ratios (CIRs), which vary from 15 dB at cell edges to 27 dB at cell centers. On adopting a cluster size of seven, these figures increase, respectively, to 19 and 30 dB. On reducing the sidelobe level, the. improvement in CIR can be quantified. The method also readily allows for regions of overlapping channel coverage to be shown
Soft Manifold Dynamics Behind Negative Thermal Expansion
Minimal models are developed to examine the origin of large negative thermal
expansion (NTE) in under-constrained systems. The dynamics of these models
reveals how underconstraint can organize a thermodynamically extensive manifold
of low-energy modes which not only drives NTE but extends across the Brillioun
zone. Mixing of twist and translation in the eigenvectors of these modes, for
which in ZrW2O8 there is evidence from infrared and neutron scattering
measurements, emerges naturally in our model as a signature of the dynamics of
underconstraint.Comment: 5 pages, 3 figure
Investigation of fatigue strength of multilayer advanced fiber composites
The analytical characterization of a multilayer fiber composite plate (without hole) was accomplished for both static and dynamic loading conditions using the finite difference technique. Thornel 300/5208 composites with and without holes were subjected to static and tensile fatigue testing. Five (5) fiber orientations were submitted to test. Tensile fatigue testing also included three (3) loading conditions and two (2) frequencies. The low-cycle test specimens demonstrated a shorter tensile fatigue life than the high-cycle test specimens. Failure surfaces demonstrated effect of testing conditions. Secondary failure mechanisms, such as: delamination, fiber breakage, and edge fiber delamination were present. Longitudinal delamination between plies also occurred in these specimens
Characterisation of the L-mode Scrape Off Layer in MAST: decay lengths
This work presents a detailed characterisation of the MAST Scrape Off Layer
in L-mode. Scans in line averaged density, plasma current and toroidal magnetic
field were performed. A comprehensive and integrated study of the SOL was
allowed by the use of a wide range of diagnostics. In agreement with previous
results, an increase of the line averaged density induced a broadening of the
midplane density profile.Comment: 30 pages, 11 figure
Understanding the effect resonant magnetic perturbations have on ELMs
All current estimations of the energy released by type I ELMs indicate that,
in order to ensure an adequate lifetime of the divertor targets on ITER, a
mechanism is required to decrease the amount of energy released by an ELM, or
to eliminate ELMs altogether. One such amelioration mechanism relies on
perturbing the magnetic field in the edge plasma region, either leading to more
frequent, smaller ELMs (ELM mitigation) or ELM suppression. This technique of
Resonant Magnetic Perturbations (RMPs) has been employed to suppress type I
ELMs at high collisionality/density on DIII-D, ASDEX Upgrade, KSTAR and JET and
at low collisionality on DIII-D. At ITER-like collisionality the RMPs enhance
the transport of particles or energy and keep the edge pressure gradient below
the 2D linear ideal MHD critical value that would trigger an ELM, whereas at
high collisionality/density the type I ELMs are replaced by small type II ELMs.
Although ELM suppression only occurs within limitied operational ranges, ELM
mitigation is much more easily achieved. The exact parameters that determine
the onset of ELM suppression are unknown but in all cases the magnetic
perturbations produce 3D distortions to the plasma and enhanced particle
transport. The incorporation of these 3D effects in codes will be essential in
order to make quantitative predictions for future devices.Comment: 32 pages, 9 figure
THE PSOCOPTERA OF THE KRAKATAU ISLANDS, INDONESIA
At least 80 species of Psocoptera are represented in recent collections from the Krakatau Islands, Sunda Strait. They represent 15 families, and 29 species are described tu new. Twenty four species are known only from the Krahataus but. others are widespread in nearby regions of Sumatra and Java and a few are known to be more broadly distributed. Numbers of species recorded from each of the Krahatau Islands are : Rakata 58, Panjang 48, Sertung 27, Anak Krakatau 27
Timing analysis techniques at large core distances for multi-TeV gamma ray astronomy
We present an analysis technique that uses the timing information of
Cherenkov images from extensive air showers (EAS). Our emphasis is on distant,
or large core distance gamma-ray induced showers at multi-TeV energies.
Specifically, combining pixel timing information with an improved direction
reconstruction algorithm, leads to improvements in angular and core resolution
as large as ~40% and ~30%, respectively, when compared with the same algorithm
without the use of timing. Above 10 TeV, this results in an angular resolution
approaching 0.05 degrees, together with a core resolution better than ~15 m.
The off-axis post-cut gamma-ray acceptance is energy dependent and its full
width at half maximum ranges from 4 degrees to 8 degrees. For shower directions
that are up to ~6 degrees off-axis, the angular resolution achieved by using
timing information is comparable, around 100 TeV, to the on-axis angular
resolution. The telescope specifications and layout we describe here are geared
towards energies above 10 TeV. However, the methods can in principle be applied
to other energies, given suitable telescope parameters. The 5-telescope cell
investigated in this study could initially pave the way for a larger array of
sparsely spaced telescopes in an effort to push the collection area to >10 km2.
These results highlight the potential of a `sparse array' approach in
effectively opening up the energy range above 10 TeV.Comment: Published in Astroparticle Physic
Why the idea of framework propositions cannot contribute to an understanding of delusions
One of the tasks that recent philosophy of psychiatry has taken upon itself is to extend the range of understanding to some of those aspects of psychopathology that Jaspers deemed beyond its limits. Given the fundamental difficulties of offering a literal interpretation of the contents of primary delusions, a number of alternative strategies have been put forward including regarding them as abnormal versions of framework propositions described by Wittgenstein in On Certainty. But although framework propositions share some of the apparent epistemic features of primary delusions, their role in partially constituting the sense of inquiry rules out their role in helping to understand delusions
- …