2,119 research outputs found
Business Interruption Insurance - A Business Perspective
Business interruption insurance policies are contracts of indemnity
Laser ion acceleration using a solid target coupled with a low density layer
We investigate by particle-in-cell simulations in two and three dimensions
the laser-plasma interaction and the proton acceleration in multilayer targets
where a low density "near-critical" layer of a few micron thickness is added on
the illuminated side of a thin, high density layer. This target design can be
obtained by depositing a "foam" layer on a thin metallic foil. The presence of
the near-critical plasma strongly increases both the conversion efficiency and
the energy of electrons and leads to enhanced acceleration of proton from a
rear side layer via the Target Normal Sheath Acceleration mechanism. The
electrons of the foam are strongly accelerated in the forward direction and
propagate on the rear side of the target building up a high electric field with
a relatively flat longitudinal profile. In these conditions the maximum proton
energy is up to three times higher than in the case of the bare solid target.Comment: 9 pages, 11 figures. Submitted to Physical Review
Electric field dynamics and ion acceleration in the self-channeling of a superintense laser pulse
The dynamics of electric field generation and radial acceleration of ions by
a laser pulse of relativistic intensity propagating in an underdense plasma has
been investigated using an one-dimensional electrostatic, ponderomotive model
developed to interpret experimental measurements of electric fields [S. Kar et
al, New J. Phys. *9*, 402 (2007)]. Ions are spatially focused at the edge of
the charge-displacement channel, leading to hydrodynamical breaking, which in
turns causes the heating of electrons and an "echo" effect in the electric
field. The onset of complete electron depletion in the central region of the
channel leads to a smooth transition to a "Coulomb explosion" regime and a
saturation of ion acceleration.Comment: 9 pages, 7 figures, final revised version, to appear on Plasma Phys.
Contr. Fus., special issue on "Laser and Plasma Accelerators", scheduled for
February, 200
Infrared spectra of crystalline and glassy silicates and application to interstellar dust
The infrared spectra of crystalline minerals predicted in theoretical condensation sequences do not match the astronomical observations. Since the astronomical spectra are a closer match to glassy silicates, the authors undertook a study to measure the infrared spectra of glassy silicates that have compositions similar to silicate minerals predicted in theoretical condensation sequences. The data should support observations aimed at elucidating condensation chemistry in dust forming regions. The authors measured the mass absorption coefficients, from 2.5 to 25 microns, of ground samples of olivine, diopside, and serpentine and also smoke samples that were prepared from these minerals. The smoke samples prepared in this way are predominantly glassy with nearly the same composition as the parent minerals. The crystalline samples consisted of pure olivine ((Fe(0.1)Mg(0.9))(2)SiO(4)), serpentine, diopside. Sample purity was confirmed by x ray diffraction. Each mineral was ground for 10 hours and a measured mass of the powder was mixed with KBr powder for absorption measurements using the method of Borghesi et a. (1985). The smoke samples were prepared from the same samples used for grinding by vaporizing the minerals using pulsed laser radiation in air. The smoke samples formed by condensation of the resulting vapor. The smoke settled onto infrared transparent KRS-5 substrates and onto a quartz crystal microbalance used to obtain mass measurements. A description of the preparation method is given in Stephens (1980). The glassy diopside showed only diffuse electron diffraction peaks and hence was nearly amorphous, while the serpentine smoke showed a weak diffraction pattern corresponding to MgO. The smoke from olivine showed a weak diffraction pattern corresponding to Fe2O3 and/or Fe3O4. The mass absorption coefficients, from 2.5 to 25 microns, of crystalline diopside, olivine, and serpentine and their corresponding smoke samples are shown in figures
Constrained Hardware Dimensioning for AI Algorithms
Given the diffusion of Artificial Intelligence (AI) in numerous domains,
experts and practitioners are faced with the challenge of finding the optimal hardware
(HW) resources and configuration (hardware dimensioning) under different con-
straints and objectives (e.g., budget, time, solution quality). To tackle this challenge,
we propose an automated tool for HArdware Dimensioning of (AI) Algorithms
(HADA), an approach relying on the integration of Machine Learning (ML) models
together into an optimization problem, where experts domain knowledge can be
injected as well. The ML models encapsulate the data-driven knowledge about
the relationships between HW requirements and AI algorithm performances. We
show how HADA can be employed to find the best HW configuration that respects
user-defined constraints in three different domains
Ion dynamics and coherent structure formation following laser pulse self-channeling
The propagation of a superintense laser pulse in an underdense, inhomogeneous
plasma has been studied numerically by two-dimensional particle-in-cell
simulations on a time scale extending up to several picoseconds. The effects of
the ion dynamics following the charge-displacement self-channeling of the laser
pulse have been addressed. Radial ion acceleration leads to the ``breaking'' of
the plasma channel walls, causing an inversion of the radial space-charge field
and the filamentation of the laser pulse. At later times a number of
long-lived, quasi-periodic field structures are observed and their dynamics is
characterized with high resolution. Inside the plasma channel, a pattern of
electric and magnetic fields resembling both soliton- and vortex-like
structures is observed.Comment: 10 pages, 5 figures (visit http://www.df.unipi.it/~macchi to download
a high-resolution version), to appear in Plasma Physics and Controlled Fusion
(Dec. 2007), special issue containing invited papers from the 34th EPS
Conference on Plasma Physics (Warsaw, July 2007
IR emission from circumstellar envelopes of C-rich stars
The reliability of a theoretical model that solves the radiative transfer equation in dust clouds surrounding a central star is checked. In particular, it is found that both classical scattering by dust and the back-heating effects are negligible in the radiative transfer when envelopes similar to IRC+10216 are taken into consideration. In addition, new fits of IRC+10216 spectra are presented which were obtained, when the source is in different luminosity phases, under the assumption that amorphous carbon grains are in the circumstellar envelope. The same model is currently used to simulate the emission from carbon-rich sources showing the silicon carbide feature at 11.3 microns
Application of novel techniques for interferogram analysis to laser-plasma femtosecond probing
Recently, two novel techniques for the extraction of the phase-shift map
(Tomassini {\it et.~al.}, Applied Optics {\bf 40} 35 (2001)) and the electronic
density map estimation (Tomassini P. and Giulietti A., Optics Communication
{\bf 199}, pp 143-148 (2001)) have been proposed. In this paper we apply both
methods to a sample laser-plasma interferogram obtained with femtoseconds probe
pulse, in an experimental setup devoted to laser particle acceleration studies.Comment: Submitted to Laser and Particle Beam
Shocks in unmagnetized plasma with a shear flow: Stability and magnetic field generation
A pair of curved shocks in a collisionless plasma is examined with a
two-dimensional particle-in-cell (PIC) simulation. The shocks are created by
the collision of two electron-ion clouds at a speed that exceeds everywhere the
threshold speed for shock formation. A variation of the collision speed along
the initially planar collision boundary, which is comparable to the ion
acoustic speed, yields a curvature of the shock that increases with time. The
spatially varying Mach number of the shocks results in a variation of the
downstream density in the direction along the shock boundary. This variation is
eventually equilibrated by the thermal diffusion of ions. The pair of shocks is
stable for tens of inverse ion plasma frequencies. The angle between the mean
flow velocity vector of the inflowing upstream plasma and the shock's
electrostatic field increases steadily during this time. The disalignment of
both vectors gives rise to a rotational electron flow, which yields the growth
of magnetic field patches that are coherent over tens of electron skin depths.Comment: 10 pages, 10 figures accepted for publication in Physics of Plasma
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