343 research outputs found
Multi-GPU Acceleration of the iPIC3D Implicit Particle-in-Cell Code
iPIC3D is a widely used massively parallel Particle-in-Cell code for the
simulation of space plasmas. However, its current implementation does not
support execution on multiple GPUs. In this paper, we describe the porting of
iPIC3D particle mover to GPUs and the optimization steps to increase the
performance and parallel scaling on multiple GPUs. We analyze the strong
scaling of the mover on two GPU clusters and evaluate its performance and
acceleration. The optimized GPU version which uses pinned memory and
asynchronous data prefetching outperform their corresponding CPU versions by
5-10x on two different systems equipped with NVIDIA K80 and V100 GPUs.Comment: Accepted for publication in ICCS 201
A note on the O(n)-storage implementation of the GKO algorithm
We propose a new O(n)-space implementation of the GKO-Cauchy algorithm for
the solution of linear systems with Cauchy-like matrix. Despite its slightly
higher computational cost, this new algorithm makes a more efficient use of the
processor cache memory. Thus, for matrices of size larger than about 500-1000,
it outperforms the existing algorithms.
We present an applicative case of Cauchy-like matrices with
non-reconstructible main diagonal. In this special instance, the O(n) space
algorithms can be adapted nicely to provide an efficient implementation of
basic linear algebra operations in terms of the low displacement-rank
generators
Solar BaII 4554 A line as Doppler diagnostics: NLTE analysis in 3D hydrodynamical model
The aim of this paper is to analyse the validity of the Dopplergram and
lambda-meter techniques for the Doppler diagnostics of solar photospheric
velocities using the BaII 4554 A line. Both techniques are evaluated by means
of NLTE radiative transfer calculations of the BaII 4554 A line in a
three-dimensional hydrodynamical model of solar convection. We consider the
cases of spatially unsmeared profiles and the profiles smeared to the
resolution of ground-based observations. We find that: (i)
Speckle-reconstructed Dopplergram velocities reproduce the ``true'' velocities
well at heights around 300 km, except for intergranular lanes with strong
downflows where the velocity can be overestimated. (ii) The lambda-meter
velocities give a good representation of the ``true'' velocities through the
whole photosphere, both under the original and reduced spatial resolutions. The
velocities derived from the inner wing of smeared BaII 4554 A line profiles are
more reliable than those for the outer wing. Only under high spatial resolution
does the inner wing velocities calculated in intergranular regions give an
underestimate (or even a sign reversal) compared with the model velocities.
(iii) NLTE effects should be taken into account in modelling the BaII 4554 A
line profiles. Such effects are more pronounced in intergranular regions. Our
analysis supports the opinion that the Dopplergram technique applied to the
BaII 4554 A line is a valuable tool for the Doppler diagnostics of the middle
photosphere around 300 km. The \lambda-meter technique applied to this line
gives us a good opportunity to ``trace'' the non-thermal motions along the
whole photosphere up to the temperature minimum and lower chromosphere.Comment: accepted by Astronomy and Astrophysic
Electroweak Working Group Report
The report summarizes the results of the activities of the Working Group on
Precision Calculations for the Z Resonance at CERN during 1994.Comment: 158 Latex, including 37 figures (27 eps figures), 30 tables; The
report is part of: D. Bardin, W. Hollik, G. Passarino (eds.), Reports of the
working group on precision calculations for the Z resonance, CERN 95-03 (31
March 1995
Effects of fieldline topology on energy propagation in the corona
We study the effect of photospheric footpoint motions on magnetic field
structures containing magnetic nulls. The footpoint motions are prescribed on
the photospheric boundary as a velocity field which entangles the magnetic
field. We investigate the propagation of the injected energy, the conversion of
energy, emergence of current layers and other consequences of the non-trivial
magnetic field topology in this situation. These boundary motions lead
initially to an increase in magnetic and kinetic energy. Following this, the
energy input from the photosphere is partially dissipated and partially
transported out of the domain through the Poynting flux. The presence of
separatrix layers and magnetic null-points fundamentally alters the propagation
behavior of disturbances from the photosphere into the corona. Depending on the
field line topology close to the photosphere, the energy is either trapped or
free to propagate into the corona.Comment: 14 pages, 15 figure
Four-Fermion Production in Electron-Positron Collisions
This report summarises the results of the four-fermion working group of the
LEP2-MC workshop, held at CERN from 1999 to 2000. Recent developments in the
calculation of four-fermion processes in electron-positron collisions at LEP-2
centre-of-mass energies are presented, concentrating on predictions for four
main reactions: W-pair production, visible photons in four-fermion events,
single-W production and Z-pair production. Based on a comparison of results
derived within different approaches, theoretical uncertainties on these
predictions are established.Comment: 150 pages, 73 figures, 45 table
First Measurement of the Transverse Spin Asymmetries of the Deuteron in Semi-Inclusive Deep Inelastic Scattering
First measurements of the Collins and Sivers asymmetries of charged hadrons
produced in deep-inelastic scattering of muons on a transversely polarized
6-LiD target are presented. The data were taken in 2002 with the COMPASS
spectrometer using the muon beam of the CERN SPS at 160 GeV/c. The Collins
asymmetry turns out to be compatible with zero, as does the measured Sivers
asymmetry within the present statistical errors.Comment: 6 pages, 2 figure
Interplay among transversity induced asymmetries in hadron leptoproduction
In the fragmentation of a transversely polarized quark several left-right
asymmetries are possible for the hadrons in the jet. When only one unpolarized
hadron is selected, it exhibits an azimuthal modulation known as Collins
effect. When a pair of oppositely charged hadrons is observed, three
asymmetries can be considered, a di-hadron asymmetry and two single hadron
asymmetries. In lepton deep inelastic scattering on transversely polarized
nucleons all these asymmetries are coupled with the transversity distribution.
From the high statistics COMPASS data on oppositely charged hadron-pair
production we have investigated for the first time the dependence of these
three asymmetries on the difference of the azimuthal angles of the two hadrons.
The similarity of transversity induced single and di-hadron asymmetries is
discussed. A new analysis of the data allows to establish quantitative
relationships among them, providing for the first time strong experimental
indication that the underlying fragmentation mechanisms are all driven by a
common physical process.Comment: 6 figure
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