1,659 research outputs found
Particle Merging Algorithm for PIC Codes
Particle-in-cell merging algorithms aim to resample dynamically the
six-dimensional phase space occupied by particles without distorting
substantially the physical description of the system. Whereas various
approaches have been proposed in previous works, none of them seemed to be able
to conserve fully charge, momentum, energy and their associated distributions.
We describe here an alternative algorithm based on the coalescence of N massive
or massless particles, considered to be close enough in phase space, into two
new macro-particles. The local conservation of charge, momentum and energy are
ensured by the resolution of a system of scalar equations. Various simulation
comparisons have been carried out with and without the merging algorithm, from
classical plasma physics problems to extreme scenarios where quantum
electrodynamics is taken into account, showing in addition to the conservation
of local quantities, the good reproducibility of the particle distributions. In
case where the number of particles ought to increase exponentially in the
simulation box, the dynamical merging permits a considerable speedup, and
significant memory savings that otherwise would make the simulations impossible
to perform
Voronoi Particle Merging Algorithm for PIC Codes
We present a new particle-merging algorithm for the particle-in-cell method.
Based on the concept of the Voronoi diagram, the algorithm partitions the phase
space into smaller subsets, which consist of only particles that are in close
proximity in the phase space to each other. We show the performance of our
algorithm in the case of the two-stream instability and the magnetic shower.Comment: 11 figure
Load management strategy for Particle-In-Cell simulations in high energy particle acceleration
In the wake of the intense effort made for the experimental CILEX project,
numerical simulation cam- paigns have been carried out in order to finalize the
design of the facility and to identify optimal laser and plasma parameters.
These simulations bring, of course, important insight into the fundamental
physics at play. As a by-product, they also characterize the quality of our
theoretical and numerical models. In this paper, we compare the results given
by different codes and point out algorithmic lim- itations both in terms of
physical accuracy and computational performances. These limitations are illu-
strated in the context of electron laser wakefield acceleration (LWFA). The
main limitation we identify in state-of-the-art Particle-In-Cell (PIC) codes is
computational load imbalance. We propose an innovative algorithm to deal with
this specific issue as well as milestones towards a modern, accurate high-per-
formance PIC code for high energy particle acceleration
Apar-T: code, validation, and physical interpretation of particle-in-cell results
We present the parallel particle-in-cell (PIC) code Apar-T and, more
importantly, address the fundamental question of the relations between the PIC
model, the Vlasov-Maxwell theory, and real plasmas.
First, we present four validation tests: spectra from simulations of thermal
plasmas, linear growth rates of the relativistic tearing instability and of the
filamentation instability, and non-linear filamentation merging phase. For the
filamentation instability we show that the effective growth rates measured on
the total energy can differ by more than 50% from the linear cold predictions
and from the fastest modes of the simulation.
Second, we detail a new method for initial loading of Maxwell-J\"uttner
particle distributions with relativistic bulk velocity and relativistic
temperature, and explain why the traditional method with individual particle
boosting fails.
Third, we scrutinize the question of what description of physical plasmas is
obtained by PIC models. These models rely on two building blocks:
coarse-graining, i.e., grouping of the order of p~10^10 real particles into a
single computer superparticle, and field storage on a grid with its subsequent
finite superparticle size. We introduce the notion of coarse-graining dependent
quantities, i.e., quantities depending on p. They derive from the PIC plasma
parameter Lambda^{PIC}, which we show to scale as 1/p. We explore two
implications. One is that PIC collision- and fluctuation-induced thermalization
times are expected to scale with the number of superparticles per grid cell,
and thus to be a factor p~10^10 smaller than in real plasmas. The other is that
the level of electric field fluctuations scales as 1/Lambda^{PIC} ~ p. We
provide a corresponding exact expression.
Fourth, we compare the Vlasov-Maxwell theory, which describes a phase-space
fluid with infinite Lambda, to the PIC model and its relatively small Lambda.Comment: 24 pages, 14 figures, accepted in Astronomy & Astrophysic
An Arbitrary Curvilinear Coordinate Method for Particle-In-Cell Modeling
A new approach to the kinetic simulation of plasmas in complex geometries,
based on the Particle-in- Cell (PIC) simulation method, is explored. In the two
dimensional (2d) electrostatic version of our method, called the Arbitrary
Curvilinear Coordinate PIC (ACC-PIC) method, all essential PIC operations are
carried out in 2d on a uniform grid on the unit square logical domain, and
mapped to a nonuniform boundary-fitted grid on the physical domain. As the
resulting logical grid equations of motion are not separable, we have developed
an extension of the semi-implicit Modified Leapfrog (ML) integration technique
to preserve the symplectic nature of the logical grid particle mover. A
generalized, curvilinear coordinate formulation of Poisson's equations to solve
for the electrostatic fields on the uniform logical grid is also developed. By
our formulation, we compute the plasma charge density on the logical grid based
on the particles' positions on the logical domain. That is, the plasma
particles are weighted to the uniform logical grid and the self-consistent mean
electrostatic fields obtained from the solution of the logical grid Poisson
equation are interpolated to the particle positions on the logical grid. This
process eliminates the complexity associated with the weighting and
interpolation processes on the nonuniform physical grid and allows us to run
the PIC method on arbitrary boundary-fitted meshes.Comment: Submitted to Computational Science & Discovery December 201
Electron - positron cascades in multiple-laser optical traps
We present an analytical and numerical study of multiple-laser QED cascades
induced with linearly polarised laser pulses. We analyse different polarisation
orientations and propose a configuration that maximises the cascade
multiplicity and favours the laser absorption. We generalise the analytical
estimate for the cascade growth rate previously calculated in the field of two
colliding linearly polarised laser pulses and account for multiple laser
interaction. The estimate is verified by a comprehensive numerical study of
four-laser QED cascades across a range of different laser intensities with QED
PIC module of OSIRIS. We show that by using four linearly polarised 30 fs laser
pulses, one can convert more than 50 % of the total energy to gamma-rays
already at laser intensity . In this
configuration, the laser conversion efficiency is higher compared with the case
with two colliding lasers
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