1,759 research outputs found
Shock creation and particle acceleration driven by plasma expansion into a rarefied medium
The expansion of a dense plasma through a more rarefied ionised medium is a
phenomenon of interest in various physics environments ranging from
astrophysics to high energy density laser- matter laboratory experiments. Here
this situation is modeled via a 1D Particle-In-Cell simulation; a jump in the
plasma density of a factor of 100 is introduced in the middle of an otherwise
equally dense electron-proton plasma with an uniform proton and electron
temperature of 10eV and 1keV respectively. The diffusion of the dense plasma,
through the rarified one, triggers the onset of different nonlinear phenomena
such as a strong ion-acoustic shock wave and a rarefaction wave. Secondary
structures are detected, some of which are driven by a drift instability of the
rarefaction wave. Efficient proton acceleration occurs ahead of the shock,
bringing the maximum proton velocity up to 60 times the initial ion thermal
speed
Control of fast electron propagation in foam target by high-Z doping
The influence of high-Z dopant (Bromine) in low-Z foam (polystyrene) target
on laser-driven fast electron propagation is studied by the 3D hybrid
particle-in-cell (PIC)/fluid code HEETS.It is found that the fast electrons are
better confined in doped targets due to the increasing resistivity of the
target, which induces a stronger resistive magnetic field which acts to
collimate the fast electron propagation.The energy deposition of fast electrons
into the background target is increased slightly in the doped target, which is
beneficial for applications requiring long distance propagation of fast
electrons, such as fast ignition
X-ray Raman compression via two-stream instability in dense plasmas
A Raman compression scheme suitable for x-rays, where the Langmuir wave is
created by an intense beam rather than the pondermotive potential between the
seed and pump pulses, is proposed.
The required intensity of the seed and pump pulses enabling the compression
could be mitigated by more than a factor of 100, compared to conventionally
available other Raman compression schemes. The relevant wavelength of x-rays
ranges from 1 to 10 nm
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
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
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
Election turnout statistics in many countries: similarities, differences, and a diffusive field model for decision-making
We study in details the turnout rate statistics for 77 elections in 11
different countries. We show that the empirical results established in a
previous paper for French elections appear to hold much more generally. We find
in particular that the spatial correlation of turnout rates decay
logarithmically with distance in all cases. This result is quantitatively
reproduced by a decision model that assumes that each voter makes his mind as a
result of three influence terms: one totally idiosyncratic component, one
city-specific term with short-ranged fluctuations in space, and one long-ranged
correlated field which propagates diffusively in space. A detailed analysis
reveals several interesting features: for example, different countries have
different degrees of local heterogeneities and seem to be characterized by a
different propensity for individuals to conform to the cultural norm. We
furthermore find clear signs of herding (i.e. strongly correlated decisions at
the individual level) in some countries, but not in others.Comment: 15 pages, 9 figures, 7 table
Dominance of Radiation Pressure in Ion Acceleration with Linearly Polarized Pulses at Intensities of
A novel regime is proposed where, employing linearly polarized laser pulses
at intensities as two order of magnitude lower than
earlier predicted [T. Esirkepov et al., Phys. Rev. Lett. 92, 175003 (2004)],
ions are dominantly accelerated from ultrathin foils by the radiation pressure,
and have monoenergetic spectra. In the regime, ions accelerated from the
hole-boring process quickly catch up with the ions accelerated by target normal
sheath acceleration (TNSA), and they then join in a single bunch, undergoing a
hybrid Light-Sail/TNSA acceleration. Under an appropriate coupling condition
between foil thickness, laser intensity and pulse duration, laser radiation
pressure can be dominant in this hybrid acceleration. Two-dimensional PIC
simulations show that quasimonoenergetic
beams are obtained by linearly polarized laser pulses at intensities of
.Comment: 6 pages, 4 figure
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