861 research outputs found
Dynamics and control of the expansion of finite-size plasmas produced in ultraintense laser-matter interactions
The strong influence of the electron dynamics provides the possibility of
controlling the expansion of laser-produced plasmas by appropriately shaping
the laser pulse. A simple irradiation scheme is proposed to tailor the
explosion of large deuterium clusters, inducing the formation of shock
structures, capable of driving nuclear fusion reactions. Such a scenario has
been thoroughly investigated, resorting to two- and three-dimensional
particle-in-cell simulations. Furthermore, the intricate dynamics of ions and
electrons during the collisionless expansion of spherical nanoplasmas has been
analyzed in detail using a self-consistent ergodic-kinetic model. This study
clarifies the transition from hydrodynamic-like to Coulomb-explosion regimes
All-optical trapping and acceleration of heavy particles
A scheme for fast, compact, and controllable acceleration of heavy particles
in vacuum is proposed, in which two counterpropagating lasers with variable
frequencies drive a beat-wave structure with variable phase velocity, thus
allowing for trapping and acceleration of heavy particles, such as ions or
muons. Fine control over the energy distribution and the total charge of the
beam is obtained via tuning of the frequency variation. The acceleration scheme
is described with a one-dimensional theory, providing the general conditions
for trapping and scaling laws for the relevant features of the particle beam.
Two-dimensional, electromagnetic particle-in-cell simulations confirm the
validity and the robustness of the physical mechanism.Comment: 10 pages, 3 figures, to appear in New Journal of Physic
Physiological Responses to Drought Stress in Jatropha curcas Seedlings
Aims: The aim of the study was to investigate the physiological mechanisms of Jatropha curcas seedlings exposed to drought and the possible influence of seedling age.
Study Design: A pot experiment was carried out using a completely randomized design with two seedling ages (2- and 3-month-old seedlings), two treatments per age (Watered: fully irrigated, and Unwatered: Not irrigated), six replicates (24 pots).
Place and Duration of Study: The experiment was performed in a greenhouse facility located at the Experimental Station “Mauro Deidda” (Department of Agriculture of University of Sassari) at Ottava (Sassari, Italy) between June and September 2011.
Methodology: To investigate the responses of 2- and 3-month-old J. curcas seedlings exposed to drought stress on 4th, 8th, 12th, 19th, and 26th day from treatment’s beginning, leaf and soil water content, biometric, gas exchange, and chlorophyll a fluorescence measurements were performed; on 26th day from treatment’s beginning, biometric destructive measurements were carried out.
Results: Results support the hypothesis that J. curcas is appropriate to be cultivated in areas with limited water availability or prolonged periods of drought and highlight that mechanisms of drought response are highly influenced by seedling age. J. curcas seedlings maintained a good leaf water status by means of an effective stomatal closure, associated with a reduced aboveground growth and an increased root:shoot ratio. Under drought stress, 2-month seedlings showed a higher allocation of resources to roots compared to 3-month seedlings. Drought resulted in more detrimental effects on the photosynthetic response of 3-month seedlings, inducing the reduction of stomata conductance and the loss of photosystem II integrity. 2-month seedlings were instead able to activate mechanisms of drought tolerance through the activation of excess energy dissipation mechanisms.
Conclusion: In the early stage of crop establishment, the transplanting of J. curcas 2-month seedlings proved to be more effective in order to avoid water stress related consequences
Patrimonio genetico delle pomacee in Sardegna: il pero: cenni storici e cultivar individuate
The cultivation of pomaceous varieties, especially the pear, has for many centuries been of great importance
and widely diffused in Sardinia. This is confirmed by many historical documents, a brief riview of which
is given in this paper.
Also reviewed are the principal pomological and qualitative characteristics of pear cultivars, either of local
origin or which were introduced in ancient times, and which can still be found on the island. For the most
part they are cultivars either forgotten or on the way to becoming extinct, but which present special characteristics,
both in form and taste, which justify plans being implemented for the recovery and exploitation.
In fact, this idea appears to represent one of the approaches to the problem of limiting the importantion
of fruit and vegetables into Sardinia: that of offering the consumer typical, high-quality produce with a «difference»
with respect to italian national standards
Efficient calculation of van der Waals dispersion coefficients with time-dependent density functional theory in real time: application to polycyclic aromatic hydrocarbons
The van der Waals dispersion coefficients of a set of polycyclic aromatic
hydrocarbons, ranging in size from the single-cycle benzene to circumovalene
(C66H20), are calculated with a real-time propagation approach to
time-dependent density functional theory (TDDFT). In the non-retarded regime,
the Casimir-Polder integral is employed to obtain C6, once the dynamic
polarizabilities have been computed at imaginary frequencies with TDDFT. On the
other hand, the numerical coefficient that characterizes the fully retarded
regime is obtained from the static polarizabilities. This ab initio strategy
has favorable scaling with the size of the system - as demonstrated by the size
of the reported molecules - and can be easily extended to obtain higher order
van der Waals coefficients.Comment: submitted to J. Chem. Phy
Direct Acceleration of Ions With Variable-frequency Lasers
A method is proposed for producing monoergetic, high-quality ion beams in
vacuum, via direct acceleration by the electromagnetic field of two
counterpropagating, variable-frequency lasers: ions are trapped and accelerated
by a beat-wave structure with variable phase velocity, allowing for fine
control over the energy and the charge of the beam via tuning of the frequency
variation. The physical mechanism is described with a one-dimensional theory,
providing the general conditions for trapping and scaling laws for the relevant
features of the ion beam. Two-dimensional, electromagnetic particle-in-cell
simulations, in which hydrogen gas is considered as an ion source, confirm the
validity and the robustness of the method.Comment: 15 pages, 6 figures, to appear in IEEE Transactions on Plasma
Science, special issue Laser & Plasma Accelerator
Polycyclic aromatic hydrocarbons with SPICA
Thanks to high sensitivity and angular resolution and broad spectral
coverage, SPICA will offer a unique opportunity to better characterize the
nature of polycyclic aromatic hydrocarbons (PAHs) and very small grains (VSGs),
to better use them as probes of astrophysical environments. The angular
resolution will enable to probe the chemical frontiers in the evolution process
from VSGs to neutral PAHs, to ionized PAHs and to "Grand-PAHs" in
photodissotiation regions and HII regions, as a function of G/n (UV
radiation field / density). High sensitivity will favor the detection of the
far-IR skeletal emission bands of PAHs, which provide specific fingerprints and
could lead to the identification of individual PAHs. This overall
characterization will allow to use PAH and VSG populations as tracers of
physical conditions in spatially resolved protoplanetary disks and nearby
galaxies (using mid-IR instruments), and in high redshift galaxies (using the
far-IR instrument), thanks to the broad spectral coverage SPICA provides. Based
on our previous experience with ISO and Spitzer we discuss how these goals can
be reached.Comment: To appear in Proc. Workshop "The Space Infrared Telescope for
Cosmology & Astrophysics: Revealing the Origins of Planets and Galaxies".
Eds. A.M. Heras, B. Swinyard, K. Isaak, and J.R. Goicoeche
The First Level Trigger of JEM-EUSO: Concept and tests
The trigger system of JEM-EUSO is designed to meet specific challenging requirements. These include managing a large number of pixels ( 3·10^5) and using a very fast, low power consuming, and radiation hard electronics. It must achieve a high signal-to-noise performance and flexibility and cope with the limited down-link transmission rate from the International Space Station (ISS) to Earth. The general overview of the First Level Trigger for cosmic ray detection is reviewed; tests that validate its performance are discussed
New insights on the systematics of echinoids belonging to the family Spatangidae Gray, 1825 using a combined approach based on morphology, morphometry, and genetics
Spatangoids are probably the least resolved group within echinoids, with known topological incongruencies between phylogenies derived from molecular (very scarce) and morphological data. The present work, based on the analysis of 270 specimens of Spatangidae (Echinoidea, Spatangoida) trawled in the Sardinian seas (Western Mediterranean), allowed us to verify the constancy of some characters that we consider to be diagnostic at the genus level —such as the path of the subanal fasciole and the relationship between labrum and adjacent ambulacral plates —and to distinguish two distinct forms within the studied material. Based on morphological characters, morphometrics, and molecular analyses (sequencing of two mitochondrial markers: cytochrome c oxidase subunit1 (COI) and 16S), most of the individuals were classified as morphotype A and attributed to the species Spatangus purpureus, the most common spatangoid in the Mediterranean Sea, while a few corresponded to a different morphotype (B), genetically close to the species Spatangus raschi. Preliminary morphological analyses seemed to indicate that morphotype B specimens from Sardinia are slightly different from S. raschi and from Spatangus subinermis individuals, the second species of the family known to occur in the Mediterranean Sea. On the basis of morpho-structural observations and molecular analyses, comparing Mediterranean living forms with species from other areas (Central Eastern Atlantic, North Sea and neighboring basins, South African Sea, Philippines and Indonesian Archipelago, New Zealand, and Hawaiian Islands), the clear distinction of S. purpureus from several other species classified as Spatangus was confirmed. Based on the morphological and genetic differences, we propose to maintain the genus Spatangus including in it only the type species S. purpureus among the living species and to establish the new genus Propespatagus nov. gen. to include several other species previously classified as Spatangus. The clear distinction among different genera was also detected in fossil forms of Spatangus, Propespatagus nov. gen., and Sardospatangus (†) from the European Oligo-Miocene sedimentary rocks of Germany; the Miocene of Ukraine, Italy, and North Africa; the Plio-Pleistocene of Italy; and the Mio-Pliocene of Florida (USA). The new data can help in addressing taxonomic ambiguities within echinoids, as well as in improving species identification, and hence biodiversity assessments in the Mediterranean region
- …