7,611 research outputs found
Two-Electron Effects in the Multiphoton Ionization of Magnesium with 400 nm 150 fs Pulses
The multiphoton ionization and photoelectron spectra of magnesium were
studied at laser intensities of up to 6x10^{13} Wcm^{-2} using 150 fs laser
pulses of a wavelength of 400 nm. The results indicated that a variety of
different ionization mechanisms played a role in both types of spectra. A
theoretical model describing the processes is presented and the routes to
ionization are identified. The work demonstrates the significance of the
two-electron nature of the atom in interpreting the experimental results.Comment: 14 pages, 9 figures, submitted to Physical Review
Interference effects in two-photon ATI by multiple orders high harmonics with random or locked phases
We numerically study 2-photon processes using a set of harmonics from a
Ti:Sapphire laser and in particular interference effects in the Above Threshold
Ionization spectra. We compare the situation where the harmonic phases are
assumed locked to the case where they have a random distribution. Suggestions
for possible experiments, using realistic parameters are discussed.Comment: 11 pages, 13 figures, LaTe
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
Photo-electron momentum spectra from minimal volumes: the time-dependent surface flux method
The time-dependent surface flux (t-SURFF) method is introduced for computing
of strong-field infrared photo-ionization spectra of atoms by numerically
solving the time-dependent Schr\"odinger equation on minimal simulation
volumes. The volumes only need to accommodate the electron quiver motion and
the relevant range of the atomic binding potential. Spectra are computed from
the electron flux through a surface, beyond which the outgoing flux is absorbed
by infinite range exterior complex scaling (irECS). Highly accurate infrared
photo-electron spectra are calculated in single active electron approximation
and compared to literature results. Detailed numerical evidence for performance
and accuracy is given. Extensions to multi-electron systems and double
ionization are discussed.Comment: 18 pages, 5 figure
A milestone toward understanding PDR properties in the extreme environment of LMC-30Dor
More complete knowledge of galaxy evolution requires understanding the
process of star formation and interaction between the interstellar radiation
field and the interstellar medium in galactic environments traversing a wide
range of physical parameter space. Here we focus on the impact of massive star
formation on the surrounding low metallicity ISM in 30 Doradus in the Large
Magellanic Cloud. A low metal abundance, as is the case of some galaxies of the
early universe, results in less ultra-violet shielding for the formation of the
molecular gas necessary for star formation to proceed. The half-solar
metallicity gas in this region is strongly irradiated by the super star cluster
R136, making it an ideal laboratory to study the structure of the ISM in an
extreme environment. Our spatially resolved study investigates the gas heating
and cooling mechanisms, particularly in the photo-dissociation regions where
the chemistry and thermal balance are regulated by far-ultraviolet photons (6
eV< h\nu <13.6 eV).
We present Herschel observations of far-infrared fine-structure lines
obtained with PACS and SPIRE/FTS. We have combined atomic fine-structure lines
from Herschel and Spitzer observations with ground-based CO data to provide
diagnostics on the properties and the structure of the gas by modeling it with
the Meudon PDR code. We derive the spatial distribution of the radiation field,
the pressure, the size, and the filling factor of the photodissociated gas and
molecular clouds. We find a range of pressure of ~ 10^5 - 1.7x10^6 cm^{-3} K
and a range of incident radiation field G_UV ~ 10^2 - 2.5x10^4 through PDR
modeling. Assuming a plane-parallel geometry and a uniform medium, we find a
total extinction of 1-3 mag , which correspond to a PDR cloud size of 0.2 to
3pc, with small CO depth scale of 0.06 to 0.5pc. We also determine the three
dimensional structure of the gas. (Abridged)Comment: 20 pages, 23 figures, accepted in A&
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