5,499 research outputs found
Photon orbital angular momentum and torque metrics for single telescopes and interferometers
Context. Photon orbital angular momentum (POAM) is normally invoked in a
quantum mechanical context. It can, however, also be adapted to the classical
regime, which includes observational astronomy.
Aims. I explain why POAM quantities are excellent metrics for describing the
end-to-end behavior of astronomical systems. To demonstrate their utility, I
calculate POAM probabilities and torques from holography measurements of EVLA
antenna surfaces.
Methods. With previously defined concepts and calculi, I present generic
expressions for POAM spectra, total POAM, torque spectra, and total torque in
the image plane. I extend these functional forms to describe the specific POAM
behavior of single telescopes and interferometers.
Results. POAM probabilities of spatially uncorrelated astronomical sources
are symmetric in quantum number. Such objects have zero intrinsic total POAM on
the celestial sphere, which means that the total POAM in the image plane is
identical to the total torque induced by aberrations within propagation media &
instrumentation. The total torque can be divided into source- independent and
dependent components, and the latter can be written in terms of three
illustrative forms. For interferometers, complications arise from discrete
sampling of synthesized apertures, but they can be overcome. POAM also
manifests itself in the apodization of each telescope in an array. Holography
of EVLA antennas observing a point source indicate that ~ 10% of photons in the
n = 0 state are torqued to n != 0 states.
Conclusions. POAM quantities represent excellent metrics for characterizing
instruments because they are used to simultaneously describe amplitude and
phase aberrations. In contrast, Zernike polynomials are just solutions of a
differential equation that happen to ~ correspond to specific types of
aberrations and are typically employed to fit only phases
Towards realistic simulations of QED cascades: non-ideal laser and electron seeding effects
A number of analytical and numerical studies has been performed to
investigate the onset and the development of QED cascades in the collision of
two counterpropagating laser pulses as a function of the laser intensity.
However, it has been recently demonstrated [M. Tamburini et al., Sci. Rep. 7,
5694 (2017)] that the onset of QED cascades is also strongly influenced by the
structure of the laser pulses, such as the laser pulse waist radius. Here we
investigate how QED cascades are affected by: (a) the laser pulse duration, (b)
the presence of a relative delay for the peak of the laser pulses to reach the
focus, (c) the existence of a mismatch between the laser focal axis of the two
laser pulses. This is especially important as, in realistic laboratory
conditions, fluctuations may arise in the temporal and point stability of the
lasers.Comment: 10 pages, 5 figures, 2 tables. Accepted for publication in Physics of
Plasma
Urban Sensibility of Landscape Structures in Italy General Characteristics and Local Details
Recently the conservation policies in Europe are considering the problem of the urban increasing in terms of soil destroyed and ecosystem fragmentation effects. In Italy this phenomena are becoming particularly important if we consider it at national level, but also at regional level. The paper has the goal to show some data relative to the distribution and the impact of urban surfaces on the large landscape national units, comparing the values among the units kind. Moreover will be implemented the data relative to some regional situation (Lazio, Marche, Umbria) of the Italian peninsula for having the indication about different environmental conditions as, for example, coastal areas, mountain areas or hill areas or also flat areas and different morphological structures. These data will be compare with other territorial characteristics, as the protected areas distribution and the biopermeability areas distribution. The knowledge of these information is very important for the planning action because it is possible to obtain, by means particular GIS models, indications about the urban sensibility of the different land parts in the future.
Assessment of the Maltese environmental matrix to define the future monitoring strategy
Chapter 16As a member of the EU and of the international community, Malta has important
obligations to report on the state of the environment and the effectiveness of policy measures
addressing particular concerns, such as pollution (Regional Environmental Center, 2008).
Failure to collect reliable and up-to-date environmental data would make Malta exposed
to various environmental pressures due to poorly informed policy decisions, which could
eventually also lead to economic implications (European Commission, 2014a). Under this
context Malta is in the process of implementing the EU legislative framework regarding
the environmental themes of air, water, radiation and soil. Prior to the implementation
of the project “Development of environmental monitoring strategy and environmental
monitoring baseline surveys”, air and water data from the Maltese environmental network
were already available, though data coverage presented some lacunas, whereas radiation
and soil network system was not in place, hence minimum baseline data was available.
Therefore, the long term monitoring strategic proposal discussed in this article has been
designed as a primary tool to improve the state of the environment as well as to be in
compliance with the EU Directives related with nature conservation.peer-reviewe
Giant collimated gamma-ray flashes
Bright sources of high energy electromagnetic radiation are widely employed
in fundamental research as well as in industry and medicine. This steadily
growing interest motivated the construction of several facilities aiming at the
realisation of sources of intense X- and gamma-ray pulses. To date, free
electron lasers and synchrotrons provide intense sources of photons with
energies up to 10-100 keV. Facilities under construction based on incoherent
Compton back scattering of an optical laser pulse off an electron beam are
expected to yield photon beams with energy up to 19.5 MeV and peak brilliance
in the range 10-10 photons s mrad mm per
0.1% bandwidth. Here, we demonstrate a novel mechanism based on the strongly
amplified synchrotron emission which occurs when a sufficiently dense electron
beam interacts with a millimetre thickness solid target. For electron beam
densities exceeding approximately 3\times10^{19}\text{ cm^{-3}}
filamentation instability occurs with the self-generation of 10-10
gauss magnetic fields where the electrons of the beam are trapped. This results
into a giant amplification of synchrotron emission with the production of
collimated gamma-ray pulses with peak brilliance above photons
s mrad mm per 0.1% bandwidth and photon energies ranging
from 200 keV up to several hundreds MeV. These findings pave the way to
compact, high-repetition-rate (kHz) sources of short (30 fs), collimated (mrad)
and high flux ( photons/s) gamma-ray pulses.Comment: Full-text access to a view-only version of the published paper by the
following SharedIt link: https://rdcu.be/LGtC This is part of the Springer
Nature Content Sharing Initiative
(https://www.springernature.com/gp/researchers/sharedit). Enhanced PDF
features such as annotation tools, one-click supplements, citation file
exports and article metrics are freely availabl
On the (2,3)-generation of the finite symplectic groups
This paper is a new important step towards the complete classification of the
finite simple groups which are -generated. In fact, we prove that the
symplectic groups are -generated for all . Because
of the existing literature, this result implies that the groups
are -generated for all , with the exception of and
Tailoring Dielectric Properties of Multilayer Composites Using Spark Plasma Sintering
A straightforward and simple way to produce well-densified ferroelectric ceramic composites with a full control of both architecture and properties using spark plasma sintering (SPS) is proposed. SPS main outcome is indeed to obtain high densification at relatively low temperatures and short treatment times thus limiting interdiffusion in multimaterials. Ferroelectric/dielectric (BST64/MgO/BST64) multilayer ceramic densified at 97% was obtained, with unmodified Curie temperature, a stack dielectric constant reaching 600, and dielectric losses dropping down to 0.5%, at room-temperature. This result ascertains SPS as a relevant tool for the design of functional materials with tailored properties
- …