5,074 research outputs found
Toward 3D reconstruction of outdoor scenes using an MMW radar and a monocular vision sensor
International audienceIn this paper, we introduce a geometric method for 3D reconstruction of the exterior environment using a panoramic microwave radar and a camera. We rely on the complementarity of these two sensors considering the robustness to the environmental conditions and depth detection ability of the radar, on the one hand, and the high spatial resolution of a vision sensor, on the other. Firstly, geometric modeling of each sensor and of the entire system is presented. Secondly, we address the global calibration problem, which consists of finding the exact transformation between the sensors' coordinate systems. Two implementation methods are proposed and compared, based on the optimization of a non-linear criterion obtained from a set of radar-to-image target correspondences. Unlike existing methods, no special configuration of the 3D points is required for calibration. This makes the methods flexible and easy to use by a non-expert operator. Finally, we present a very simple, yet robust 3D reconstruction method based on the sensors' geometry. This method enables one to reconstruct observed features in 3D using one acquisition (static sensor), which is not always met in the state of the art for outdoor scene reconstruction.The proposed methods have been validated with synthetic and real data
Betatron emission as a diagnostic for injection and acceleration mechanisms in laser-plasma accelerators
Betatron x-ray emission in laser-plasma accelerators is a promising compact
source that may be an alternative to conventional x-ray sources, based on large
scale machines. In addition to its potential as a source, precise measurements
of betatron emission can reveal crucial information about relativistic
laser-plasma interaction. We show that the emission length and the position of
the x-ray emission can be obtained by placing an aperture mask close to the
source, and by measuring the beam profile of the betatron x-ray radiation far
from the aperture mask. The position of the x-ray emission gives information on
plasma wave breaking and hence on the laser non-linear propagation. Moreover,
the measurement of the longitudinal extension helps one to determine whether
the acceleration is limited by pump depletion or dephasing effects. In the case
of multiple injections, it is used to retrieve unambiguously the position in
the plasma of each injection. This technique is also used to study how, in a
capillary discharge, the variations of the delay between the discharge and the
laser pulse affect the interaction. The study reveals that, for a delay
appropriate for laser guiding, the x-ray emission only occurs in the second
half of the capillary: no electrons are injected and accelerated in the first
half.Comment: 8 pages, 6 figures. arXiv admin note: text overlap with
arXiv:1104.245
Zeeman slowers made simple with permanent magnets in a Halbach configuration
We describe a simple Zeeman slower design using permanent magnets. Contrary
to common wire-wound setups no electric power and water cooling are required.
In addition, the whole system can be assembled and disassembled at will. The
magnetic field is however transverse to the atomic motion and an extra repumper
laser is necessary. A Halbach configuration of the magnets produces a high
quality magnetic field and no further adjustment is needed. After optimization
of the laser parameters, the apparatus produces an intense beam of slow and
cold 87Rb atoms. With a typical flux of 1 - 5 \times 10^10 atoms/s at 30 ms^-1,
our apparatus efficiently loads a large magneto-optical trap with more than
10^10 atoms in one second, which is an ideal starting point for degenerate
quantum gases experiments.Comment: 8+6 pages (article + appendices: calculation details, probe and oven
description, pictures), 18 figures, supplementary material (movie,
Mathematica programs and technical drawings
Fault2SHA Central Apennines database and structuring active fault data for seismic hazard assessment
We present a database of field data for active faults in the central Apennines, Italy, including trace, fault and main fault locations with activity and location certainties, and slip-rate, slip-vector and surface geometry data. As advances occur in our capability to create more detailed fault-based hazard models, depending on the availability of primary data and observations, it is desirable that such data can be organized in a way that is easily understood and incorporated into present and future models. The database structure presented herein aims to assist this process. We recommend stating what observations have led to different location and activity certainty and presenting slip-rate data with point location coordinates of where the data were collected with the time periods over which they were calculated. Such data reporting allows more complete uncertainty analyses in hazard and risk modelling. The data and maps are available as kmz, kml, and geopackage files with the data presented in spreadsheet files and the map coordinates as txt files. The files are available at: https://doi.org/10.1594/PANGAEA.922582
Mapping the X-Ray Emission Region in a Laser-Plasma Accelerator
The x-ray emission in laser-plasma accelerators can be a powerful tool to
understand the physics of relativistic laser-plasma interaction. It is shown
here that the mapping of betatron x-ray radiation can be obtained from the
x-ray beam profile when an aperture mask is positioned just beyond the end of
the emission region. The influence of the plasma density on the position and
the longitudinal profile of the x-ray emission is investigated and compared to
particle-in-cell simulations. The measurement of the x-ray emission position
and length provides insight on the dynamics of the interaction, including the
electron self-injection region, possible multiple injection, and the role of
the electron beam driven wakefield.Comment: 5 pages, 4 figure
A complementary compact laser based neutron source
Several experiments of neutron generation using high intensity laser sources,
with a power exceeding 10^19W/cm^2 via TNSA (Target Normal Sheath Acceleration)
or other similar methods, have been performed in the past years in different
laboratories. However, so far there is no one running neutron source based on
such a technology. In the framework of the Conceptual Report Design of a new
accelerator in the Eupraxia project we are studying the possibility to have a
laser-based neutron source, not only by TNSA but also from self-injection
schemes. We focus our attention on the applications in cultural heritage
studies as well also on the complementary role that such a source can have in
the framework of large facilities devoted to radiation production.Comment: 4 pages, two figures, 3rd European Advanced Accelerators Concept
Mixed Quantum/Classical Approach for Description of Molecular Collisions in Astrophysical Environments
An efficient and accurate mixed quantum/classical theory approach for computational treatment of inelastic scattering is extended to describe collision of an atom with a general asymmetric-top rotor polyatomic molecule. Quantum mechanics, employed to describe transitions between the internal states of the molecule, and classical mechanics, employed for description of scattering of the atom, are used in a self-consistent manner. Such calculations for rotational excitation of HCOOCH3 in collisions with He produce accurate results at scattering energies above 15 cm–1, although resonances near threshold, below 5 cm–1, cannot be reproduced. Importantly, the method remains computationally affordable at high scattering energies (here up to 1000 cm–1), which enables calculations for larger molecules and at higher collision energies than was possible previously with the standard full-quantum approach. Theoretical prediction of inelastic cross sections for a number of complex organic molecules observed in space becomes feasible using this new computational tool
Gauging the dark matter fraction in a S0 galaxy at z=0.47 through gravitational lensing from deep HST/ACS imaging
We analyze a new gravitational lens, OAC-GL J1223-1239, serendipitously found
in a deep I-band image of the Hubble Space Telescope (HST) Advanced Camera for
Surveys (ACS). The lens is a L_*, edge-on S0 galaxy at z=0.4656. The
gravitational arc has a radius of 0.42 arcsec. We have determined the total
mass and the dark matter (DM) fraction within the Einstein radius as a function
of the lensed source redshift, which is presently unknown. For z ~ 1.3, which
is in the middle of the redshift range plausible for the source according to
some external constraints, we find the central velocity dispersion to be ~180
km/s. With this value, close to that obtained by means of the Faber-Jackson
relation at the lens redshift, we compute a 30% DM fraction within the Einstein
radius (given the uncertainty in the source redshift, the allowed range for the
DM fraction is 25-35 % in our lensing model). When compared with the galaxies
in the local Universe, the lensing galaxy, OAC-GL J1223-1239 seems to fall in
the transition regime between massive DM dominated galaxies and lower-mass, DM
deficient systems.Comment: 18 pages, 5 figures; accepted for publication in Ap
Herschel observations of the Herbig-Haro objects HH52-54
We are aiming at the observational estimation of the relative contribution to
the cooling by CO and H2O, as this provides decisive information for the
understanding of the oxygen chemistry behind interstellar shock waves. Methods.
The high sensitivity of HIFI, in combination with its high spectral resolution
capability, allows us to trace the H2O outflow wings at unprecedented
signal-to-noise. From the observation of spectrally resolved H2O and CO lines
in the HH52-54 system, both from space and from ground, we arrive at the
spatial and velocity distribution of the molecular outflow gas. Solving the
statistical equilibrium and non-LTE radiative transfer equations provides us
with estimates of the physical parameters of this gas, including the cooling
rate ratios of the species. The radiative transfer is based on an ALI code,
where we use the fact that variable shock strengths, distributed along the
front, are naturally implied by a curved surface. Based on observations of CO
and H2O spectral lines, we conclude that the emission is confined to the HH54
region. The quantitative analysis of our observations favours a ratio of the
CO-to-H2O-cooling-rate >> 1. From the best-fit model to the CO emission, we
arrive at an H2O abundance close to 1e-5. The line profiles exhibit two
components, one of which is triangular and another, which is a superposed,
additional feature. This additional feature likely originates from a region
smaller than the beam where the ortho-water abundance is smaller than in the
quiescent gas. Comparison with recent shock models indicate that a planar shock
can not easily explain the observed line strengths and triangular line
profiles.We conclude that the geometry can play an important role. Although
abundances support a scenario where J-type shocks are present, higher cooling
rate ratios than predicted by these type of shocks are derived.Comment: Accepted for publication in A&
H2O line mapping at high spatial and spectral resolution - Herschel observations of the VLA1623 outflow
Apart from being an important coolant, H2O is known to be a tracer of
high-velocity molecular gas. Recent models predict relatively high abundances
behind interstellar shockwaves. The dynamical and physical conditions of the
H2O emitting gas, however, are not fully understood yet. We aim to determine
the abundance and distribution of H2O, its kinematics and the physical
conditions of the gas responsible for the H2O emission. The observed line
profile shapes help us understand the dynamics in molecular outflows. We mapped
the VLA1623 outflow, in the ground-state transitions of o-H2O, with the HIFI
and PACS instruments. We also present observations of higher energy transitions
of o-H2O and p-H2O obtained with HIFI and PACS towards selected outflow
positions. From comparison with non-LTE radiative transfer calculations, we
estimate the physical parameters of the water emitting regions. The observed
water emission line profiles vary over the mapped area. Spectral features and
components, tracing gas in different excitation conditions, allow us to
constrain the density and temperature of the gas. The H2O emission originates
in a region where temperatures are comparable to that of the warm H2 gas
(T\gtrsim200K). Thus, the H2O emission traces a gas component significantly
warmer than the gas responsible for the low-J CO emission. The H2O column
densities at the CO peak positions are low, i.e. N(H2O) \simeq (0.03-10)x10e14
cm-2. The H2O abundance with respect to H2 in the extended outflow is estimated
at X(H2O)<1x10e-6, significantly lower than what would be expected from most
recent shock models. The H2O emission traces a gas component moving at
relatively high velocity compared to the low-J CO emitting gas. However, other
dynamical quantities such as the momentum rate, energy and mechanical
luminosity are estimated to be the same, independent of the molecular tracer
used, CO or H2O.Comment: 14 pages, 13 figures, 4 table
- …