9,455 research outputs found
A global simulation for laser driven MeV electrons in -diameter fast ignition targets
The results from 2.5-dimensional Particle-in-Cell simulations for the
interaction of a picosecond-long ignition laser pulse with a plasma pellet of
50- diameter and 40 critical density are presented. The high density
pellet is surrounded by an underdense corona and is isolated by a vacuum region
from the simulation box boundary. The laser pulse is shown to filament and
create density channels on the laser-plasma interface. The density channels
increase the laser absorption efficiency and help generate an energetic
electron distribution with a large angular spread. The combined distribution of
the forward-going energetic electrons and the induced return electrons is
marginally unstable to the current filament instability. The ions play an
important role in neutralizing the space charges induced by the the temperature
disparity between different electron groups. No global coalescing of the
current filaments resulted from the instability is observed, consistent with
the observed large angular spread of the energetic electrons.Comment: 9 pages, 6 figures, to appear in Physics of Plasmas (May 2006
Dynamic multiscale spatiotemporal models for Poisson data
We propose a new class of dynamic multiscale models for Poisson spatiotemporal processes. Specifically, we use a multiscale spatial Poisson factorization to decompose the Poisson process at each time point into spatiotemporal multiscale coefficients. We then connect these spatiotemporal multiscale coefficients through time with a novel Dirichlet evolution. Further, we propose a simulation-based full Bayesian posterior analysis. In particular, we develop filtering equations for updating of information forward in time and smoothing equations for integration of information backward in time, and use these equations to develop a forward filter backward sampler for the spatiotemporal multiscale coefficients. Because the multiscale coefficients are conditionally independent a posteriori, our full Bayesian posterior analysis is scalable, computationally efficient, and highly parallelizable. Moreover, the Dirichlet evolution of each spatiotemporal multiscale coefficient is parametrized by a discount factor that encodes the relevance of the temporal evolution of the spatiotemporal multiscale coefficient. Therefore, the analysis of discount factors provides a powerful way to identify regions with distinctive spatiotemporal dynamics. Finally, we illustrate the usefulness of our multiscale spatiotemporal Poisson methodology with two applications. The first application examines mortality ratios in the state of Missouri, and the second application considers tornado reports in the American Midwest
Beam loading in the nonlinear regime of plasma-based acceleration
A theory that describes how to load negative charge into a nonlinear,
three-dimensional plasma wakefield is presented. In this regime, a laser or an
electron beam blows out the plasma electrons and creates a nearly spherical ion
channel, which is modified by the presence of the beam load. Analytical
solutions for the fields and the shape of the ion channel are derived. It is
shown that very high beam-loading efficiency can be achieved, while the energy
spread of the bunch is conserved. The theoretical results are verified with the
Particle-In-Cell code OSIRIS.Comment: 5 pages, 2 figures, to appear in Physical Review Letter
Low energy n-\nuc{3}{H} scattering : a novel testground for nuclear interaction
The low energy n-\nuc{3}{H} elastic cross sections near the resonance peak
are calculated by solving the 4-nucleon problem with realistic NN interactions.
Three different methods -- Alt, Grassberger and Shandas (AGS), Hyperspherical
Harmonics and Faddeev-Yakubovsky -- have been used and their respective results
are compared. We conclude on a failure of the existing NN forces to reproduce
the n-\nuc{3}{H} total cross section.Comment: To be published in Phys. Rev.
Ion acceleration from laser-driven electrostatic shocks
Multi-dimensional particle-in-cell simulations are used to study the
generation of electrostatic shocks in plasma and the reflection of background
ions to produce high-quality and high-energy ion beams. Electrostatic shocks
are driven by the interaction of two plasmas with different density and/or
relative drift velocity. The energy and number of ions reflected by the shock
increase with increasing density ratio and relative drift velocity between the
two interacting plasmas. It is shown that the interaction of intense lasers
with tailored near-critical density plasmas allows for the efficient heating of
the plasma electrons and steepening of the plasma profile at the critical
density interface, leading to the generation of high-velocity shock structures
and high-energy ion beams. Our results indicate that high-quality 200 MeV
shock-accelerated ion beams required for medical applications may be obtained
with current laser systems.Comment: 33 pages, 12 figures, accepted for publication in Physics of Plasma
Lapse risk modelling in insurance: a Bayesian mixture approach
This paper focuses on modelling surrender time for policyholders in the
context of life insurance. In this setup, a large lapse rate at the first
months of a contract is often observed, with a decrease in this rate after some
months. The modelling of the time to cancellation must account for this
specific behaviour. Another stylised fact is that policies which are not
cancelled in the study period are considered censored. To account for both
censuring and heterogeneous lapse rates, this work assumes a Bayesian survival
model with a mixture of regressions. The inference is based on data
augmentation allowing for fast computations even for data sets of over a
million clients. Moreover, scalable point estimation based on EM algorithm is
also presented. An illustrative example emulates a typical behaviour for life
insurance contracts and a simulated study investigates the properties of the
proposed model. In particular, the observed censuring in the insurance context
might be up to 50% of the data, which is very unusual for survival models in
other fields such as epidemiology. This aspect is exploited in our simulated
study
Laser-driven shock acceleration of monoenergetic ion beams
We show that monoenergetic ion beams can be accelerated by moderate Mach
number collisionless, electrostatic shocks propagating in a long scale-length
exponentially decaying plasma profile. Strong plasma heating and density
steepening produced by an intense laser pulse near the critical density can
launch such shocks that propagate in the extended plasma at high velocities.
The generation of a monoenergetic ion beam is possible due to the small and
constant sheath electric field associated with the slowly decreasing density
profile. The conditions for the acceleration of high-quality, energetic ion
beams are identified through theory and multidimensional particle-in-cell
simulations. The scaling of the ion energy with laser intensity shows that it
is possible to generate MeV proton beams with state-of-the-art 100
TW class laser systems.Comment: 13 pages, 4 figures, accepted for publication in Physical Review
Letter
Photodisintegration of the triton with realistic potentials
The process is treated by means of three-body integral
equations employing in their kernel the W-Matrix representation of the
subsystem amplitudes. As compared to the plane wave (Born) approximation the
full solution of the integral equations, which takes into account the final
state interaction, shows at low energies a 24% enhancement. The calculations
are based on the semirealistic Malfliet-Tjon and the realistic Paris and Bonn B
potentials. For comparison with earlier calculations we also present results
for the Yamaguchi potential. In the low-energy region a remarkable potential
dependence is observed, which vanishes at higher energies.Comment: 16 pages REVTeX, 8 postscript figures included, uses epsfig.st
- …