7,013 research outputs found
Coulomb-Volkov approach of ionization by extreme ultraviolet laser pulses in the subfemtosecond regime
In conditions where the interaction betweeen an atom and a short
high-frequency extreme ultraviolet laser pulse is a perturbation, we show that
a simple theoretical approach, based on Coulomb-Volkov-type states, can make
reliable predictions for ionization. To avoid any additional approximation, we
consider here a standard case : the ionization of hydrogen atoms initially in
their ground state. For any field parameter, we show that the method provides
accurate energy spectra of ejected electrons, including many above threshold
ionization peaks, as long as the two following conditions are simultaneously
fulfilled : (i) the photon energy is greater than or equal to the ionization
potential ; (ii) the ionization process is not saturated. Thus, ionization of
atoms or molecules by the high order harmonic laser pulses which are generated
at present may be addressed through this Coulomb-Volkov treatment.Comment: 19 pages including 5 figures and figure caption
Modeling laser wakefield accelerators in a Lorentz boosted frame
Modeling of laser-plasma wakefield accelerators in an optimal frame of
reference \cite{VayPRL07} is shown to produce orders of magnitude speed-up of
calculations from first principles. Obtaining these speedups requires
mitigation of a high-frequency instability that otherwise limits effectiveness
in addition to solutions for handling data input and output in a
relativistically boosted frame of reference. The observed high-frequency
instability is mitigated using methods including an electromagnetic solver with
tunable coefficients, its extension to accomodate Perfectly Matched Layers and
Friedman's damping algorithms, as well as an efficient large bandwidth digital
filter. It is shown that choosing the frame of the wake as the frame of
reference allows for higher levels of filtering and damping than is possible in
other frames for the same accuracy. Detailed testing also revealed
serendipitously the existence of a singular time step at which the instability
level is minimized, independently of numerical dispersion, thus indicating that
the observed instability may not be due primarily to Numerical Cerenkov as has
been conjectured. The techniques developed for Cerenkov mitigation prove
nonetheless to be very efficient at controlling the instability. Using these
techniques, agreement at the percentage level is demonstrated between
simulations using different frames of reference, with speedups reaching two
orders of magnitude for a 0.1 GeV class stages. The method then allows direct
and efficient full-scale modeling of deeply depleted laser-plasma stages of 10
GeV-1 TeV for the first time, verifying the scaling of plasma accelerators to
very high energies. Over 4, 5 and 6 orders of magnitude speedup is achieved for
the modeling of 10 GeV, 100 GeV and 1 TeV class stages, respectively
Speeding up simulations of relativistic systems using an optimal boosted frame
It can be computationally advantageous to perform computer simulations in a
Lorentz boosted frame for a certain class of systems. However, even if the
computer model relies on a covariant set of equations, it has been pointed out
that algorithmic difficulties related to discretization errors may have to be
overcome in order to take full advantage of the potential speedup. We summarize
the findings, the difficulties and their solutions, and show that the technique
enables simulations important to several areas of accelerator physics that are
otherwise problematic, including self-consistent modeling in three-dimensions
of laser wakefield accelerator stages at energies of 10 GeV and above.Comment: To be published in the proceedings of DPF-2009, Detroit, MI, July
2009, eConf C09072
Effects of Hyperbolic Rotation in Minkowski Space on the Modeling of Plasma Accelerators in a Lorentz Boosted Frame
Laser driven plasma accelerators promise much shorter particle accelerators
but their development requires detailed simulations that challenge or exceed
current capabilities. We report the first direct simulations of stages up to 1
TeV from simulations using a Lorentz boosted calculation frame resulting in a
million times speedup, thanks to a frame boost as high as gamma=1300. Effects
of the hyperbolic rotation in Minkowski space resulting from the frame boost on
the laser propagation in the plasma is shown to be key in the mitigation of a
numerical instability that was limiting previous attempts
A Microstructure Sensitive Approach for the Prediction of the Creep Behaviour and Life under Complex Loading Paths
The prediction of the creep behaviour and life of components of aeronautic engines like high pressure turbine blades is still a challenging issue due to non-isothermal loadings. Indeed, certification procedures of turboshaft engines for helicopters consist of complex thermomechanical histories, sometimes including short and very high temperature excursions close to the γ’-solvus (T~1200°C) of the blade alloy. A better design of those components could be gained using a model that takes into account non-isothermal loadings inducing microstructural changes.
Most of the commonly used models consider only a nearly constant (or slowly evolving) microstructure, i.e. far from the rapid microstructure evolutions encountered during close γ’-solvus overheatings where a rapid dissolution/precipitation of the γ’-phase and fast recovery mechanisms were observed by Cormier et al. (2007b). A new constitutive modelling approach was hence recently proposed in a crystal viscoplasticity framework to capture the transient effects of such rapid microstructure evolutions on the creep behaviour and life (Cormier and Cailletaud (2010a)).
In this article, an updated version of this model is detailed. Special attention will be paid to (i) the effect of the accumulated plastic strain on the microstructure evolution, (ii) the introduction of an additional damage formulation, and (iii) the creep strain at failure. The performances of the model are illustrated on the basis of isothermal or complex non-isothermal creep experiments performed on nearly [001] oriented samples
On the gravitational production of superheavy dark matter
The dark matter in the universe can be in the form of a superheavy matter
species (WIMPZILLA). Several mechanisms have been proposed for the production
of WIMPZILLA particles during or immediately following the inflationary epoch.
Perhaps the most attractive mechanism is through gravitational particle
production, where particles are produced simply as a result of the expansion of
the universe. In this paper we present a detailed numerical calculation of
WIMPZILLA gravitational production in hybrid-inflation models and
natural-inflation models. Generalizing these findings, we also explore the
dependence of the gravitational production mechanism on various models of
inflation. We show that superheavy dark matter production seems to be robust,
with Omega_X h^2 ~ (M_X / (10^11 GeV))^2 (T_RH / (10^9 GeV)), so long as M_X <
H_I, where M_X is the WIMPZILLA mass, T_RH is the reheat temperature, and H_I
is the expansion rate of the universe during inflation.Comment: 26 pages, 7 figures; LaTeX; submitted to Physical Review D; minor
typographical error correcte
A Survey of Atomic Carbon [C I] in High-redshift Main-Sequence Galaxies
We present the first results of an ALMA survey of the lower fine structure
line of atomic carbon [C I](^3P_1\,-\,^{3}P_0) in far infrared-selected
galaxies on the main sequence at in the COSMOS field. We compare our
sample with a comprehensive compilation of data available in the literature for
local and high-redshift starbursting systems and quasars. We show that the [C
I]() luminosity correlates on global scales with the
infrared luminosity similarly to low- CO transitions. We report
a systematic variation of L'_{\rm [C\,I]^3P_1\,-\, ^3P_0}/ as a
function of the galaxy type, with the ratio being larger for main-sequence
galaxies than for starbursts and sub-millimeter galaxies at fixed .
The L'_{\rm [C\,I]^3P_1\,-\, ^3P_0}/ and / mass ratios are similar for main-sequence galaxies and for
local and high-redshift starbursts within a 0.2 dex intrinsic scatter,
suggesting that [C I] is a good tracer of molecular gas mass as CO and dust. We
derive a fraction of %
of the total carbon mass in the atomic neutral phase. Moreover, we estimate the
neutral atomic carbon abundance, the fundamental ingredient to calibrate [C I]
as a gas tracer, by comparing L'_{\rm [C\,I]^3P_1\,-\, ^3P_0} and available
gas masses from CO lines and dust emission. We find lower [C I] abundances in
main-sequence galaxies than in starbursting systems and sub-millimeter
galaxies, as a consequence of the canonical and gas-to-dust
conversion factors. This argues against the application to different galaxy
populations of a universal standard [C I] abundance derived from highly biased
samples.Comment: 14 pages + Appendix. Accepted for publication in ApJ. All the data
tables in Appendix will be also released in electronic forma
A plasma wakefield acceleration experiment using CLARA beam
We propose a Plasma Accelerator Research Station (PARS) based at proposed FEL
test facility CLARA (Compact Linear Accelerator for Research and Applications)
at Daresbury Laboratory. The idea is to use the relativistic electron beam from
CLARA, to investigate some key issues in electron beam transport and in
electron beam driven plasma wakefield acceleration, e.g. high gradient plasma
wakefield excitation driven by a relativistic electron bunch, two bunch
experiment for CLARA beam energy doubling, high transformer ratio, long bunch
self-modulation and some other advanced beam dynamics issues. This paper
presents the feasibility studies of electron beam transport to meet the
requirements for beam driven wakefield acceleration and presents the plasma
wakefield simulation results based on CLARA beam parameters. Other possible
experiments which can be conducted at the PARS beam line are also discussed
Time-dependent calculation of ionization in Potassium at mid-infrared wavelengths
We study the dynamics of the Potassium atom in the mid-infrared, high
intensity, short laser pulse regime. We ascertain numerical convergence by
comparing the results obtained by the direct expansion of the time-dependent
Schroedinger equation onto B-Splines, to those obtained by the eigenbasis
expansion method. We present ionization curves in the 12-, 13-, and 14-photon
ionization range for Potassium. The ionization curve of a scaled system, namely
Hydrogen starting from the 2s, is compared to the 12-photon results. In the
13-photon regime, a dynamic resonance is found and analyzed in some detail. The
results for all wavelengths and intensities, including Hydrogen, display a
clear plateau in the peak-heights of the low energy part of the Above Threshold
Ionization (ATI) spectrum, which scales with the ponderomotive energy Up, and
extends to 2.8 +- 0.5 Up.Comment: 15 two-column pages with 15 figures, 3 tables. Accepted for
publication in Phys. Rev A. Improved figures, language and punctuation, and
made minor corrections. We also added a comparison to the ADK theor
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