25,817 research outputs found
Mode-medium instability and its correction with a Gaussian reflectivity mirror
A high power CO2 laser beam is known to deteriorate after a few microseconds due to a mode-medium instability (MMI) which results from an intensity dependent heating rate related to the vibrational-to-translational decay of the upper and lower CO2 lasing levels. An iterative numerical technique is developed to model the time evolution of the beam as it is affected by the MMI. The technique is used to study the MMI in an unstable CO2 resonator with a hard-edge output mirror for different parameters like the Fresnel number and the gas density. The results show that the mode of the hard edge unstable resonator deteriorates because of the diffraction ripples in the mode. A Gaussian-reflectivity mirror was used to correct the MMI. This mirror produces a smoother intensity profile which significantly reduces the effects of the MMI. Quantitative results on peak density variation and beam quality are presented
Virtual Enriching Operators
We construct bounded linear operators that map conforming Lagrange
finite element spaces to conforming virtual element spaces in two and
three dimensions. These operators are useful for the analysis of nonstandard
finite element methods
Virtual Element Methods on Meshes with Small Edges or Faces
We consider a model Poisson problem in () and establish error
estimates for virtual element methods on polygonal or polyhedral meshes that
can contain small edges () or small faces ().Comment: 36 page
Land use/land cover mapping (1:25000) of Taiwan, Republic of China by automated multispectral interpretation of LANDSAT imagery
Three methods were tested for collection of the training sets needed to establish the spectral signatures of the land uses/land covers sought due to the difficulties of retrospective collection of representative ground control data. Computer preprocessing techniques applied to the digital images to improve the final classification results were geometric corrections, spectral band or image ratioing and statistical cleaning of the representative training sets. A minimal level of statistical verification was made based upon the comparisons between the airphoto estimates and the classification results. The verifications provided a further support to the selection of MSS band 5 and 7. It also indicated that the maximum likelihood ratioing technique can achieve more agreeable classification results with the airphoto estimates than the stepwise discriminant analysis
Scattering measurements on natural and model trees
The acoustical back scattering from a simple scale model of a tree has been experimentally measured. The model consisted of a trunk and six limbs, each with 4 branches; no foliage or twigs were included. The data from the anechoic chamber measurements were then mathematically combined to construct the effective back scattering from groups of trees. Also, initial measurements have been conducted out-of-doors on a single tree in an open field in order to characterize its acoustic scattering as a function of azimuth angle. These measurements were performed in the spring, prior to leaf development. The data support a statistical model of forest scattering; the scattered signal spectrum is highly irregular but with a remarkable general resemblance to the incident signal spectrum. Also, the scattered signal's spectra showed little dependence upon scattering angle
Internal Stresses and Formation of Switchable Nanowires at Thin Silica Film Edge
At vertical edges, thin films of silicon oxide (SiO_{2-x}) contain
semiconductive c-Si layered nanocrystals (Si NC) embedded in and supported by
an insulating g-SiO2 matrix. Tour et al. have shown that a trenched thin film
geometry enables the NC to form switchable nanowires (SNW) when trained by an
applied field. The field required to form SNW decreases rapidly within a few
cycles, or by annealing at 600 C in even fewer cycles, and is stable to 700C.
Here we describe the intrinsic evolution of Si NC and SNW in terms of the
competition between internal stresses and electro-osmosis. The analysis relies
heavily on experimental data from a wide range of thin film studies, and it
explains why a vertical edge across the planar Si-SiOx interface is necessary
to form SNW. The discussion also shows that the formation mechanisms of Si NC
and Si/SiO_{2-x} SNW are intrinsic and result from optimization of nanowire
conductivity in the presence of residual host misfit stresses
Queuing Theoretic Analysis of Power-performance Tradeoff in Power-efficient Computing
In this paper we study the power-performance relationship of power-efficient
computing from a queuing theoretic perspective. We investigate the interplay of
several system operations including processing speed, system on/off decisions,
and server farm size. We identify that there are oftentimes "sweet spots" in
power-efficient operations: there exist optimal combinations of processing
speed and system settings that maximize power efficiency. For the single server
case, a widely deployed threshold mechanism is studied. We show that there
exist optimal processing speed and threshold value pairs that minimize the
power consumption. This holds for the threshold mechanism with job batching.
For the multi-server case, it is shown that there exist best processing speed
and server farm size combinations.Comment: Paper published in CISS 201
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