166 research outputs found
Origins of plateau formation in ion energy spectra under target normal sheath acceleration
Target normal sheath acceleration (TNSA) is a method employed in
laser--matter interaction experiments to accelerate light ions (usually
protons). Laser setups with durations of a few 10 fs and relatively low
intensity contrasts observe plateau regions in their ion energy spectra when
shooting on thin foil targets with thicknesses of order 10 m. In
this paper we identify a mechanism which explains this phenomenon using one
dimensional particle-in-cell simulations. Fast electrons generated from the
laser interaction recirculate back and forth through the target, giving rise to
time-oscillating charge and current densities at the target backside. Periodic
decreases in the electron density lead to transient disruptions of the TNSA
sheath field: peaks in the ion spectra form as a result, which are then spread
in energy from a modified potential driven by further electron recirculation.
The ratio between the laser pulse duration and the recirculation period
(dependent on the target thickness, including the portion of the pre-plasma
which is denser than the critical density) determines if a plateau forms in the
energy spectra.Comment: 11 pages, 12 figure
High-energy acceleration phenomena in extreme radiation-plasma interactions
We simulate, using a particle-in-cell code, the chain of acceleration
processes at work during the Compton-based interaction of a dilute electron-ion
plasma with an extreme-intensity, incoherent gamma-ray flux with a photon
density several orders of magnitude above the particle density. The plasma
electrons are initially accelerated in the radiative flux direction through
Compton scattering. In turn, the charge-separation field from the induced
current drives forward the plasma ions to near-relativistic speed and
accelerates backwards the non-scattered electrons to energies easily exceeding
those of the driving photons. The dynamics of those energized electrons is
determined by the interplay of electrostatic acceleration, bulk plasma motion,
inverse Compton scattering and deflections off the mobile magnetic fluctuations
generated by a Weibel-type instability. The latter Fermi-like effect notably
gives rise to a forward-directed suprathermal electron tail. We provide simple
analytical descriptions for most of those phenomena and examine numerically
their sensitivity to the parameters of the problem
Enhancement of laser-driven ion acceleration in non-periodic nanostructured targets
Using particle-in-cell simulations, we demonstrate an improvement of the
target normal sheath acceleration (TNSA) of protons in non-periodically
nanostructured targets with micron-scale thickness. Compared to standard flat
foils, an increase in the proton cutoff energy by up to a factor of two is
observed in foils coated with nanocones or perforated with nanoholes. The
latter nano-perforated foils yield the highest enhancement, which we show to be
robust over a broad range of foil thicknesses and hole diameters. The
improvement of TNSA performance results from more efficient hot-electron
generation, caused by a more complex laser-electron interaction geometry and
increased effective interaction area and duration. We show that TNSA is
optimized for a nanohole distribution of relatively low areal density and that
is not required to be periodic, thus relaxing the manufacturing constraints.Comment: 11 pages, 8 figure
Modeling target bulk heating resulting from ultra-intense short pulse laser irradiation of solid density targets
Isochoric heating of solid-density matter up to a few tens of eV is of interest for investigating astrophysical or inertial fusion scenarios. Such ultra-fast heating can be achieved via the energy deposition of short-pulse laser generated electrons. Here, we report on experimental measurements of this process by means of time-and space-resolved optical interferometry. Our results are found in reasonable agreement with a simple numerical model of fast electron-induced heating. (C) 2013 AIP Publishing LLC.</p
Consistency of safety and efficacy of new oral anticoagulants across subgroups of patients with atrial fibrillation.
AIMS: The well-known limitations of vitamin K antagonists (VKA) led to development of new oral anticoagulants (NOAC) in non-valvular atrial fibrillation (NVAF). The aim of this meta-analysis was to determine the consistency of treatment effects of NOAC irrespective of age, comorbidities, or prior VKA exposure.
METHODS AND RESULTS: All randomized, controlled phase III trials comparing NOAC to VKA up to October 2012 were eligible provided their results (stroke/systemic embolism (SSE) and major bleeding (MB)) were reported according to age (†or >75 years), renal function, CHADS2 score, presence of diabetes mellitus or heart failure, prior VKA use or previous cerebrovascular events. Interactions were considered significant at p <0.05. Three studies (50,578 patients) were included, respectively evaluating apixaban, rivaroxaban, and dabigatran versus warfarin. A trend towards interaction with heart failure (pâ=â0.08) was observed with respect to SSE reduction, this being greater in patients not presenting heart failure (RRâ=â0.76 [0.67-0.86]) than in those with heart failure (RRâ=â0.90 [0.78-1.04]); Significant interaction (pâ=â0.01) with CHADS2 score was observed, NOAC achieving a greater reduction in bleeding risk in patients with a score of 0-1 (RR 0.67 CI 0.57-0.79) than in those with a score â„2 (RR 0.85 CI 0.74-0.98). Comparison of MB in patients with (RR 0.97 CI 0.79-1.18) and without (RR 0.76 CI 0.65-0.88) diabetes mellitus showed a similar trend (pâ=â0.06). No other interactions were found. All subgroups derived benefit from NOA in terms of SSE or MB reduction.
CONCLUSIONS: NOAC appeared to be more effective and safer than VKA in reducing SSE or MB irrespective of patient comorbidities. Thromboembolism risk, evaluated by CHADS2 score and, to a lesser extent, diabetes mellitus modified the treatment effects of NOAC without complete loss of benefit with respect to MB reduction
- âŠ