4,562 research outputs found
Stereo electro-optical tracker study for the measurement of model deformations at the National Transonic Facility
The effects of model vibration, camera and window nonlinearities, and aerodynamic disturbances in the optical path on the measurement of target position is examined. Window distortion, temperature and pressure changes, laminar and turbulent boundary layers, shock waves, target intensity and, target vibration are also studied. A general computer program was developed to trace optical rays through these disturbances. The use of a charge injection device camera as an alternative to the image dissector camera was examined
Magnetization structure of a Bloch point singularity
Switching of magnetic vortex cores involves a topological transition
characterized by the presence of a magnetization singularity, a point where the
magnetization vanishes (Bloch point). We analytically derive the shape of the
Bloch point that is an extremum of the free energy with exchange, dipole and
the Landau terms for the determination of the local value of the magnetization
modulus.Comment: 4 pages, 2 figure
Projecting Supply and Demand for Land in the Long Run
The goal of this work is to investigate land-use change at the global scale over the long run particularly in the context of analyzing the fundamental drivers behind land-use related GHG emissions. For this purpose, we identify the most important drivers of supply and demand for land. On the demand side, we begin with a dynamic general equilibrium (GE) model that predicts economic growth in each region of the world, based on exogenous projections of population, skilled and unskilled labor and technical change. Economy-wide growth is, in turn, translated into consumer demand for specific products using an econometrically estimated, international cross-section, demand system that permits us to predict the pattern of future consumer demands across the development spectrum. This is particularly important in the fast-growing, developing countries, where the composition of consumer demand is changing rapidly. These countries also account for an increasing share of global economic growth and greenhouse gas emissions. Consumer demand is translated into derived demands for land through a set of sectoral production functions that differentiate the demand for land by Agro-Ecological Zone (AEZ). The paper devotes considerable attention to modeling the supply of land to different land-using activities in the economy. In order to represent the competition for land among different sectors in the model, we disaggregated the land endowment in each region into Agro-Ecological Zones, drawing on the data base of Lee et al. (2005), to reflect the fact that land is heterogeneous endowment. To further restrict land mobility across uses, land supply within an AEZ is constrained via a nested Constant Elasticity of Transformation (CET) frontier. In the nested structure, land owner of particular type of land (AEZ) first decides on the allocation of land between agriculture and forestry to maximize the total returns from land. Then, based on the relative returns to land in crop and livestock production, the land owner decides on the allocation of land between these two broad types of agricultural activities. A soft link between our GE model and an intertemporal forestry model is included for better representation of forestry sector in GE model. To reflect the real world fact that deforestation represents an important source of land supply in the face of high demand, we also introduce the possibility of conversion of unmanaged forest land to land used in production. This is treated as an investment decision whereby new land is accessed only when present value of returns on land in a given region is high enough to cover the costs of accessing the new land. In equilibrium, the supply of land to each land-using activity adjusts to meet the derived demand for land. A set of projections for the long run supply and demand for land obtained with this model is a useful input to improving our understanding of land-related GHG emissions in the future.land use, climate change policy, baseline, general equilibrium, agro-ecological zones, C68, R14, Q24, Land Economics/Use,
A Path to the Direct Detection of sub-GeV Dark Matter Using Calorimetric Readout of a Superfluid He Target
A promising technology concept for sub-GeV dark matter detection is
described, in which low-temperature microcalorimeters serve as the sensors and
superfluid He serves as the target material. A superfluid helium target has
several advantageous properties, including a light nuclear mass for better
kinematic matching with light dark matter particles, copious production of
scintillation light, extremely good intrinsic radiopurity, a high impedance to
external vibration noise, and a unique mechanism for observing phonon-like
modes via liberation of He atoms into a vacuum (`quantum evaporation'). In
this concept, both scintillation photons and triplet excimers are detected
using calorimeters, including calorimeters immersed in the superfluid. Kinetic
excitations of the superfluid medium (rotons and phonons) are detected using
quantum evaporation and subsequent atomic adsorption onto a microcalorimeter
suspended in vacuum above the target helium. The energy of adsorption amplifies
the phonon/roton signal before calorimetric sensing, producing a gain mechanism
that can reduce the techonology's recoil energy threshold below the calorimeter
energy threshold. We describe signal production and signal sensing
probabilities, and estimate electron recoil discrimination. We then simulate
radioactive backgrounds from gamma rays and neutrons. Dark matter - nucleon
elastic scattering cross-section sensitivities are projected, demonstrating
that even very small (sub-kg) target masses can probe wide regions of as-yet
untested dark matter parameter space
Electronic structure and dynamics of optically excited single-wall carbon nanotubes
We have studied the electronic structure and charge-carrier dynamics of
individual single-wall carbon nanotubes (SWNTs) and nanotube ropes using
optical and electron-spectroscopic techniques. The electronic structure of
semiconducting SWNTs in the band-gap region is analyzed using near-infrared
absorption spectroscopy. A semi-empirical expression for
transition energies, based on tight-binding calculations is found to give
striking agreement with experimental data. Time-resolved PL from dispersed
SWNT-micelles shows a decay with a time constant of about 15 ps. Using
time-resolved photoemission we also find that the electron-phonon ({\it e-ph})
coupling in metallic tubes is characterized by a very small {\it e-ph}
mass-enhancement of 0.0004. Ultrafast electron-electron scattering of
photo-excited carriers in nanotube ropes is finally found to lead to internal
thermalization of the electronic system within about 200 fs.Comment: 10 pages, 10 figures, submitted to Applied Physics
Poverty analysis using an international cross-country demand system
This paper proposes a new method for ex ante analysis of the poverty impacts arising from policy reforms. Three innovations underlie this approach. The first is the estimation of a global demand system using a combination of micro-data from household surveys and macro-data from the International Comparisons Project (ICP). Estimation is undertaken in a manner that reconciles these two sources of information, explicitly recognizing that per capita national demands are an aggregation of the disaggregated, individual household demands. The second innovation relates to a methodology for post-estimation calibration of the global demand system, giving rise to country-specific demand systems and an associated expenditure function which, when aggregated across the expenditure distribution, reproduce observed per capita budget shares exactly. This leads to the third innovation, which is the establishment of a unique poverty level of utility and an appropriately modified set of Foster-Greer-Thorbecke poverty measures. With these tools in hand, the authors are able to calculate the change in the head-count of poverty, poverty gap, and squared poverty gap arising from policy reforms, where the poverty measures are derived using a unique poverty level of utility, rather than an income or expenditure-based measure. They use these techniques with a demand system for food, other nondurables and services estimated using a combination of 1996 ICP data set and national expenditure distribution data. Calibration is demonstrated for three countries for which household survey expenditure data are used during estimation-Indonesia, the Philippines and Thailand. To show the usefulness of these calibrated models for policy analysis, the authors assess the effects of an assumed 5 percent food price rise as might be realized in the wake of a multilateral trade agreement. Results illustrate the important role of subsistence expenditures at lowest income levels, but of discretionary expenditure at higher income levels. The welfare analysis underscores the relatively large impact of the price hike on poorer households, while a modified Foster-Greer-Thorbecke poverty measure shows that the 5 percent price rise increases the incidence and intensity of poverty in all three cases, although the specific effects vary considerably by country.Markets and Market Access,Economic Theory&Research,Population Policies,Rural Poverty Reduction,Poverty Lines
Three-dimensional magnetic flux-closure patterns in mesoscopic Fe islands
We have investigated three-dimensional magnetization structures in numerous
mesoscopic Fe/Mo(110) islands by means of x-ray magnetic circular dichroism
combined with photoemission electron microscopy (XMCD-PEEM). The particles are
epitaxial islands with an elongated hexagonal shape with length of up to 2.5
micrometer and thickness of up to 250 nm. The XMCD-PEEM studies reveal
asymmetric magnetization distributions at the surface of these particles.
Micromagnetic simulations are in excellent agreement with the observed magnetic
structures and provide information on the internal structure of the
magnetization which is not accessible in the experiment. It is shown that the
magnetization is influenced mostly by the particle size and thickness rather
than by the details of its shape. Hence, these hexagonal samples can be
regarded as model systems for the study of the magnetization in thick,
mesoscopic ferromagnets.Comment: 12 pages, 11 figure
Biofuel Growth: Global Greenhouse Gas Emissions Impacts from Changes in Forest Carbon Stocks
There is significant policy interest in liquid biofuels with appealing prospects for energy security, farm security, poverty alleviation, and climate change. Large-scale commercial biofuel production could have far reaching implications for regional and global markets – particularly those related to energy and land use. As such, large-scale biofuels growth is likely to have significant impacts on global greenhouse gas (GHG) emissions. This paper utilizes a CGE model with explicit biofuel, land, and energy markets. The model is able to estimate the effects on the broad range of input and output markets potentially affected globally by biofuels policies. One of the most controversial issues within the biofuels debate is potential indirect changes in land use and, in particular, the resulting changes in forest carbon stocks. To uncover consequences of biofuel policies for forest carbon, we link our CGE model with a dynamic forward looking model of the forest sector. Within this framework, we evaluate the potential effects of US and multinational biofuels growth on changes in land use and emissions from changes in forest carbon stocks.land use change, biofuels, CGE model, forest carbon stocks, GHG emissions, Resource /Energy Economics and Policy,
The Effect of Carbon Payments on Deforestation Rates and Carbon Sequestration: Estimates from a Global Forestry and Agricultural Model.
Environmental Economics and Policy,
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