5,623 research outputs found
Towards a unified linear kinetic transport model with the trace ion module for EIRENE
Linear kinetic Monte Carlo particle transport models are frequently employed
in fusion plasma simulations to quantify atomic and surface effects on the main
plasma flow dynamics. Separate codes are used for transport of neutral
particles (incl. radiation) and charged particles (trace impurity ions).
Integration of both modules into main plasma fluid solvers provides then self
consistent solutions, in principle. The required interfaces are far from
trivial, because rapid atomic processes in particular in the edge region of
fusion plasmas require either smoothing and resampling, or frequent transfer of
particles from one into the other Monte Carlo code. We propose a different
scheme here, in which despite the inherently different mathematical form of
kinetic equations for ions and neutrals (e.g. Fokker-Planck vs. Boltzmann
collision integrals) both types of particle orbits can be integrated into one
single code. We show that the approximations and shortcomings of this "single
sourcing" concept (e.g., restriction to explicit ion drift orbit integration)
can be fully tolerable in a wide range of typical fusion edge plasma
conditions, and be overcompensated by the code-system simplicity, as well as by
inherently ensured consistency in geometry (one single numerical grid only) and
(the common) atomic and surface process modulesComment: 15 pages, 7 figure
Linear Boltzmann dynamics in a strip with large reflective obstacles: stationary state and residence time
The presence of obstacles modify the way in which particles diffuse. In
cells, for instance, it is observed that, due to the presence of macromolecules
playing the role of obstacles, the mean square displacement ofbiomolecules
scales as a power law with exponent smaller than one. On the other hand,
different situations in grain and pedestrian dynamics in which the presence of
an obstacle accelerate the dynamics are known. We focus on the time, called
residence time, needed by particles to cross a strip assuming that the dynamics
inside the strip follows the linear Boltzmann dynamics. We find that the
residence time is not monotonic with the sizeand the location of the obstacles,
since the obstacle can force those particles that eventually cross the strip to
spend a smaller time in the strip itself. We focus on the case of a rectangular
strip with two open sides and two reflective sides and we consider reflective
obstaclea into the strip
Palindromic 3-stage splitting integrators, a roadmap
The implementation of multi-stage splitting integrators is essentially the
same as the implementation of the familiar Strang/Verlet method. Therefore
multi-stage formulas may be easily incorporated into software that now uses the
Strang/Verlet integrator. We study in detail the two-parameter family of
palindromic, three-stage splitting formulas and identify choices of parameters
that may outperform the Strang/Verlet method. One of these choices leads to a
method of effective order four suitable to integrate in time some partial
differential equations. Other choices may be seen as perturbations of the
Strang method that increase efficiency in molecular dynamics simulations and in
Hybrid Monte Carlo sampling.Comment: 20 pages, 8 figures, 2 table
Towards an ultra efficient kinetic scheme. Part I: basics on the BGK equation
In this paper we present a new ultra efficient numerical method for solving
kinetic equations. In this preliminary work, we present the scheme in the case
of the BGK relaxation operator. The scheme, being based on a splitting
technique between transport and collision, can be easily extended to other
collisional operators as the Boltzmann collision integral or to other kinetic
equations such as the Vlasov equation. The key idea, on which the method
relies, is to solve the collision part on a grid and then to solve exactly the
transport linear part by following the characteristics backward in time. The
main difference between the method proposed and semi-Lagrangian methods is that
here we do not need to reconstruct the distribution function at each time step.
This allows to tremendously reduce the computational cost of the method and it
permits for the first time, to the author's knowledge, to compute solutions of
full six dimensional kinetic equations on a single processor laptop machine.
Numerical examples, up to the full three dimensional case, are presented which
validate the method and assess its efficiency in 1D, 2D and 3D
Spatially hybrid computations for streamer discharges with generic features of pulled fronts: I. Planar fronts
Streamers are the first stage of sparks and lightning; they grow due to a
strongly enhanced electric field at their tips; this field is created by a thin
curved space charge layer. These multiple scales are already challenging when
the electrons are approximated by densities. However, electron density
fluctuations in the leading edge of the front and non-thermal stretched tails
of the electron energy distribution (as a cause of X-ray emissions) require a
particle model to follow the electron motion. As super-particle methods create
wrong statistics and numerical artifacts, modeling the individual electron
dynamics in streamers is limited to early stages where the total electron
number still is limited.
The method of choice is a hybrid computation in space where individual
electrons are followed in the region of high electric field and low density
while the bulk of the electrons is approximated by densities (or fluids). We
here develop the hybrid coupling for planar fronts. First, to obtain a
consistent flux at the interface between particle and fluid model in the hybrid
computation, the widely used classical fluid model is replaced by an extended
fluid model. Then the coupling algorithm and the numerical implementation of
the spatially hybrid model are presented in detail, in particular, the position
of the model interface and the construction of the buffer region. The method
carries generic features of pulled fronts that can be applied to similar
problems like large deviations in the leading edge of population fronts etc.Comment: 33 pages, 15 figures and 2 table
Coarse Molecular Dynamics of a Peptide Fragment: Free Energy, Kinetics, and Long-Time Dynamics Computations
We present a ``coarse molecular dynamics'' approach and apply it to studying
the kinetics and thermodynamics of a peptide fragment dissolved in water. Short
bursts of appropriately initialized simulations are used to infer the
deterministic and stochastic components of the peptide motion parametrized by
an appropriate set of coarse variables. Techniques from traditional numerical
analysis (Newton-Raphson, coarse projective integration) are thus enabled;
these techniques help analyze important features of the free-energy landscape
(coarse transition states, eigenvalues and eigenvectors, transition rates,
etc.). Reverse integration of (irreversible) expected coarse variables backward
in time can assist escape from free energy minima and trace low-dimensional
free energy surfaces. To illustrate the ``coarse molecular dynamics'' approach,
we combine multiple short (0.5-ps) replica simulations to map the free energy
surface of the ``alanine dipeptide'' in water, and to determine the ~ 1/(1000
ps) rate of interconversion between the two stable configurational basins
corresponding to the alpha-helical and extended minima.Comment: The article has been submitted to "The Journal of Chemical Physics.
Numerical approximation of BSDEs using local polynomial drivers and branching processes
We propose a new numerical scheme for Backward Stochastic Differential
Equations based on branching processes. We approximate an arbitrary (Lipschitz)
driver by local polynomials and then use a Picard iteration scheme. Each step
of the Picard iteration can be solved by using a representation in terms of
branching diffusion systems, thus avoiding the need for a fine time
discretization. In contrast to the previous literature on the numerical
resolution of BSDEs based on branching processes, we prove the convergence of
our numerical scheme without limitation on the time horizon. Numerical
simulations are provided to illustrate the performance of the algorithm.Comment: 28 page
Branching diffusion representation of semi-linear elliptic PDEs and estimation using Monte Carlo method
We study semi-linear elliptic PDEs with polynomial non-linearity and provide
a probabilistic representation of their solution using branching diffusion
processes. When the non-linearity involves the unknown function but not its
derivatives, we extend previous results in the literature by showing that our
probabilistic representation provides a solution to the PDE without assuming
its existence. In the general case, we derive a new representation of the
solution by using marked branching diffusion processes and automatic
differentiation formulas to account for the non-linear gradient term. In both
cases, we develop new theoretical tools to provide explicit sufficient
conditions under which our probabilistic representations hold. As an
application, we consider several examples including multi-dimensional
semi-linear elliptic PDEs and estimate their solution by using the Monte Carlo
method
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