5,464 research outputs found
Stochastic simulation of charged particle transport on the massively parallel processor
Computations of cosmic-ray transport based upon finite-difference methods are afflicted by instabilities, inaccuracies, and artifacts. To avoid these problems, researchers developed a Monte Carlo formulation which is closely related not only to the finite-difference formulation, but also to the underlying physics of transport phenomena. Implementations of this approach are currently running on the Massively Parallel Processor at Goddard Space Flight Center, whose enormous computing power overcomes the poor statistical accuracy that usually limits the use of stochastic methods. These simulations have progressed to a stage where they provide a useful and realistic picture of solar energetic particle propagation in interplanetary space
Methods for Quantitative Local Structure Analysis of Crystalline Materials Employing High Performance Computing
A fundamental computational methodology was investigated to extract quantitative local structure information from single crystal diffuse scattering data. The principles of a highly efficient, parallelizable local structure analysis using massively parallel computing resources at Oak Ridge National Laboratory (ORNL) are demonstrated on an organic hydrocarbon compound containing stacking faults, Tris(bicyclo[2.1.1]hexeno)benzene. A probabilistic model of the stacking variations with a five layer interaction depth was developed. The final model structure motif statistics are verified using the steady state distribution of Markov matrix representing the four to five layer transitions. The computations revealed that highly parallelizable “structure-clones” could replace less computationally efficient “structure lots”. Further testing of the method is under way, using a new comprehensive modeling software suite ZODS (Zürich Oak Ridge Disorder Simulations) developed in Zürich, on synchrotron and lab X-Ray data of a highly efficient light-upconversion member of the NaLnF44 [Sodium Lanthanide tetra fluoride] family. Initially, a synchrotron data set was collected at the high resolution Swiss-Norwegian Beam Line at the European Synchrotron Radiation Facility and is being analyzed. High resolution neutron diffraction data were recently collected at the time-of-flight Laue single crystal diffractometer TOPAZ at the Spallation Neutron Source at ORNL using the newly available event-mode processing. Currently, exploration of the event-mode data treatment and event based corrections for data preparation are under way. Simultaneous massively parallel local structure simulations of NaLaF4 [Sodium Lanthanum tetra fluoride] using ZODS on the National Energy Research Scientific Computing Center are in progress. A step-wise modeling approach was adopted. The largest contributors to the X-Ray diffuse scattering, La2 [Lanthanum 2] and Na2 [Sodium 2] column neighbor interactions were modeled first, followed by F1 [Fluorine 1] shift from its average position toward La [Lanthanum] and away from Na [Sodium]. This work provides a basis for streamlining diffuse scattering analysis and yields a quantitative interpretation of the local atomic arrangement of crystalline materials, which may provide valuable information for interpreting their structure property relationships
Ray-based calculations of backscatter in laser fusion targets
A 1D, steady-state model for Brillouin and Raman backscatter from an
inhomogeneous plasma is presented. The daughter plasma waves are treated in the
strong damping limit, and have amplitudes given by the (linear) kinetic
response to the ponderomotive drive. Pump depletion, inverse-bremsstrahlung
damping, bremsstrahlung emission, Thomson scattering off density fluctuations,
and whole-beam focusing are included. The numerical code DEPLETE, which
implements this model, is described. The model is compared with traditional
linear gain calculations, as well as "plane-wave" simulations with the paraxial
propagation code pF3D. Comparisons with Brillouin-scattering experiments at the
OMEGA Laser Facility [T. R. Boehly et al., Opt. Commun. 133, p. 495 (1997)]
show that laser speckles greatly enhance the reflectivity over the DEPLETE
results. An approximate upper bound on this enhancement, motivated by phase
conjugation, is given by doubling the DEPLETE coupling coefficient. Analysis
with DEPLETE of an ignition design for the National Ignition Facility (NIF) [J.
A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, p. 755
(1994)], with a peak radiation temperature of 285 eV, shows encouragingly low
reflectivity. Re-absorption of Raman light is seen to be significant in this
design.Comment: 16 pages, 19 figure
PORTA: A three-dimensional multilevel radiative transfer code for modeling the intensity and polarization of spectral lines with massively parallel computers
The interpretation of the intensity and polarization of the spectral line
radiation produced in the atmosphere of the Sun and of other stars requires
solving a radiative transfer problem that can be very complex, especially when
the main interest lies in modeling the spectral line polarization produced by
scattering processes and the Hanle and Zeeman effects. One of the difficulties
is that the plasma of a stellar atmosphere can be highly inhomogeneous and
dynamic, which implies the need to solve the non-equilibrium problem of the
generation and transfer of polarized radiation in realistic three-dimensional
(3D) stellar atmospheric models. Here we present PORTA, an efficient multilevel
radiative transfer code we have developed for the simulation of the spectral
line polarization caused by scattering processes and the Hanle and Zeeman
effects in 3D models of stellar atmospheres. The numerical method of solution
is based on the non-linear multigrid iterative method and on a novel
short-characteristics formal solver of the Stokes-vector transfer equation
which uses monotonic B\'ezier interpolation. Therefore, with PORTA the
computing time needed to obtain at each spatial grid point the self-consistent
values of the atomic density matrix (which quantifies the excitation state of
the atomic system) scales linearly with the total number of grid points.
Another crucial feature of PORTA is its parallelization strategy, which allows
us to speed up the numerical solution of complicated 3D problems by several
orders of magnitude with respect to sequential radiative transfer approaches,
given its excellent linear scaling with the number of available processors. The
PORTA code can also be conveniently applied to solve the simpler 3D radiative
transfer problem of unpolarized radiation in multilevel systems.Comment: 15 pages, 15 figures, to appear in Astronomy and Astrophysic
Fermi acceleration at supernova remnant shocks
We investigate the physics of particle acceleration at non-relativistic
shocks exploiting two different and complementary approaches, namely a
semi-analytic modeling of cosmic-ray modified shocks and large hybrid (kinetic
protons/fluid electrons) simulations. The former technique allows us to extract
some information from the multi-wavelength observations of supernova remnants,
especially in the gamma-ray band, while the latter returns fundamental insights
into the details of particle injection and magnetic field amplification via
plasma instabilities. In particular, we present the results of large hybrid
simulations of non-relativistic shocks, discussing the properties of the
transition from the thermal to the non-thermal component, the spectrum of which
turns out to be the power-law predicted by first-order Fermi acceleration.
Along with a rather effective magnetic field amplification, we find that more
than 20% of the bulk energy is converted in non-thermal particles, altering
significantly the dynamics of the shock and leading to the formation of a
precursor.Comment: 4 pages, 1 figure - Proceedings of the 5th International Symposium on
High-Energy Gamma-Ray Astronomy - Heidelberg, Germany, July 9-13th, 201
Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
Quantum ESPRESSO is an integrated suite of computer codes for
electronic-structure calculations and materials modeling, based on
density-functional theory, plane waves, and pseudopotentials (norm-conserving,
ultrasoft, and projector-augmented wave). Quantum ESPRESSO stands for "opEn
Source Package for Research in Electronic Structure, Simulation, and
Optimization". It is freely available to researchers around the world under the
terms of the GNU General Public License. Quantum ESPRESSO builds upon
newly-restructured electronic-structure codes that have been developed and
tested by some of the original authors of novel electronic-structure algorithms
and applied in the last twenty years by some of the leading materials modeling
groups worldwide. Innovation and efficiency are still its main focus, with
special attention paid to massively-parallel architectures, and a great effort
being devoted to user friendliness. Quantum ESPRESSO is evolving towards a
distribution of independent and inter-operable codes in the spirit of an
open-source project, where researchers active in the field of
electronic-structure calculations are encouraged to participate in the project
by contributing their own codes or by implementing their own ideas into
existing codes.Comment: 36 pages, 5 figures, resubmitted to J.Phys.: Condens. Matte
Generation of Warm Dense Plasma on Solar Panel Infrastructure in Exo-Atmospheric Conditions
The use of a weaponized thermo-nuclear device in exo-atmospheric conditions would be of great impact on the material integrity of orbiting satellite infrastructure. Particular damage would occur to the multi-layered, solar cell components of such satellites. The rapid absorption of X-ray radiation originating from a nuclear blast into these layers occurs over a picosecond time scale and leads to the generation of Warm Dense Plasma (WDP). While incredibly difficult and costly to replicate in a laboratory setting, a collection of computational techniques and software libraries may be utilized to simulate the intricate atomic and subatomic physics characteristics of such an event. Use of the Monte Carlo sampling method within the Geant4 software library allows for the energy deposition and power density profiles by X-rays into this system to be determined. By understanding and modeling the different factors which can affect the absorption of thermonuclear X-ray radiation, specifically, “cold –X-ray radiation,” in the energy range of approximately 1 to 1.5 keV, the molecular dynamics modeling of WDP generation and evolution can be performed using the LAMMPS code library. One aspect modeled and utilized within this software is the Planck blackbody spectrum of X-rays, assumed to be emitted by the detonation. Another such factor explored is the effect of primary and secondary particle backscattering within the active solar cell layer. Ultimately, it was determined that the primary and secondary particle backscattering of photons and electrons occurs at such a relatively low rate that its effect on the properties of the generated WDP is negligible. Once the energy deposition and power density profiles are determined, LAMMPS is utilized in order to understand the spatio-temporal evolution of the WDP as well as the temperature, stress, and mass density distribution within the material, at its surface, and its immediate vacuum surroundings
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