991 research outputs found
Multiphysics simulations of collisionless plasmas
Collisionless plasmas, mostly present in astrophysical and space
environments, often require a kinetic treatment as given by the Vlasov
equation. Unfortunately, the six-dimensional Vlasov equation can only be solved
on very small parts of the considered spatial domain. However, in some cases,
e.g. magnetic reconnection, it is sufficient to solve the Vlasov equation in a
localized domain and solve the remaining domain by appropriate fluid models. In
this paper, we describe a hierarchical treatment of collisionless plasmas in
the following way. On the finest level of description, the Vlasov equation is
solved both for ions and electrons. The next courser description treats
electrons with a 10-moment fluid model incorporating a simplified treatment of
Landau damping. At the boundary between the electron kinetic and fluid region,
the central question is how the fluid moments influence the electron
distribution function. On the next coarser level of description the ions are
treated by an 10-moment fluid model as well. It may turn out that in some
spatial regions far away from the reconnection zone the temperature tensor in
the 10-moment description is nearly isotopic. In this case it is even possible
to switch to a 5-moment description. This change can be done separately for
ions and electrons. To test this multiphysics approach, we apply this full
physics-adaptive simulations to the Geospace Environmental Modeling (GEM)
challenge of magnetic reconnection.Comment: 13 pages, 5 figure
Simulating streamer discharges in 3D with the parallel adaptive Afivo framework
We present an open-source plasma fluid code for 2D, cylindrical and 3D
simulations of streamer discharges, based on the Afivo framework that features
adaptive mesh refinement, geometric multigrid methods for Poisson's equation,
and OpenMP parallelism. We describe the numerical implementation of a fluid
model of the drift-diffusion-reaction type, combined with the local field
approximation. Then we demonstrate its functionality with 3D simulations of
long positive streamers in nitrogen in undervolted gaps, using three examples.
The first example shows how a stochastic background density affects streamer
propagation and branching. The second one focuses on the interaction of a
streamer with preionized regions, and the third one investigates the
interaction between two streamers. The simulations run on up to grid
cells within less than a day. Without mesh refinement, they would require
grid cells
Coupled Vlasov and two-fluid codes on GPUs
We present a way to combine Vlasov and two-fluid codes for the simulation of
a collisionless plasma in large domains while keeping full information of the
velocity distribution in localized areas of interest. This is made possible by
solving the full Vlasov equation in one region while the remaining area is
treated by a 5-moment two-fluid code. In such a treatment, the main challenge
of coupling kinetic and fluid descriptions is the interchange of physically
correct boundary conditions between the different plasma models. In contrast to
other treatments, we do not rely on any specific form of the distribution
function, e.g. a Maxwellian type. Instead, we combine an extrapolation of the
distribution function and a correction of the moments based on the fluid data.
Thus, throughout the simulation both codes provide the necessary boundary
conditions for each other. A speed-up factor of around 20 is achieved by using
GPUs for the computationally expensive solution of the Vlasov equation and an
overall factor of at least 60 using the coupling strategy combined with the GPU
computation. The coupled codes were then tested on the GEM reconnection
challenge
The physics of streamer discharge phenomena
In this review we describe a transient type of gas discharge which is
commonly called a streamer discharge, as well as a few related phenomena in
pulsed discharges. Streamers are propagating ionization fronts with
self-organized field enhancement at their tips that can appear in gases at (or
close to) atmospheric pressure. They are the precursors of other discharges
like sparks and lightning, but they also occur in for example corona reactors
or plasma jets which are used for a variety of plasma chemical purposes. When
enough space is available, streamers can also form at much lower pressures,
like in the case of sprite discharges high up in the atmosphere.
We explain the structure and basic underlying physics of streamer discharges,
and how they scale with gas density. We discuss the chemistry and applications
of streamers, and describe their two main stages in detail: inception and
propagation. We also look at some other topics, like interaction with flow and
heat, related pulsed discharges, and electron runaway and high energy
radiation. Finally, we discuss streamer simulations and diagnostics in quite
some detail.
This review is written with two purposes in mind: First, we describe recent
results on the physics of streamer discharges, with a focus on the work
performed in our groups. We also describe recent developments in diagnostics
and simulations of streamers. Second, we provide background information on the
above-mentioned aspects of streamers. This review can therefore be used as a
tutorial by researchers starting to work in the field of streamer physics.Comment: 89 pages, 29 figure
Kinetic Solvers with Adaptive Mesh in Phase Space
An Adaptive Mesh in Phase Space (AMPS) methodology has been developed for
solving multi-dimensional kinetic equations by the discrete velocity method. A
Cartesian mesh for both configuration (r) and velocity (v) spaces is produced
using a tree of trees data structure. The mesh in r-space is automatically
generated around embedded boundaries and dynamically adapted to local solution
properties. The mesh in v-space is created on-the-fly for each cell in r-space.
Mappings between neighboring v-space trees implemented for the advection
operator in configuration space. We have developed new algorithms for solving
the full Boltzmann and linear Boltzmann equations with AMPS. Several recent
innovations were used to calculate the discrete Boltzmann collision integral
with dynamically adaptive mesh in velocity space: importance sampling,
multi-point projection method, and the variance reduction method. We have
developed an efficient algorithm for calculating the linear Boltzmann collision
integral for elastic and inelastic collisions in a Lorentz gas. New AMPS
technique has been demonstrated for simulations of hypersonic rarefied gas
flows, ion and electron kinetics in weakly ionized plasma, radiation and light
particle transport through thin films, and electron streaming in
semiconductors. We have shown that AMPS allows minimizing the number of cells
in phase space to reduce computational cost and memory usage for solving
challenging kinetic problems
A comparison of 3D particle, fluid and hybrid simulations for negative streamers
In the high field region at the head of a discharge streamer, the electron
energy distribution develops a long tail. In negative streamers, these
electrons can run away and contribute to energetic processes such as
terrestrial gamma-ray and electron flashes. Moreover, electron density
fluctuations can accelerate streamer branching. To track energies and locations
of single electrons in relevant regions, we have developed a 3D hybrid model
that couples a particle model in the region of high fields and low electron
densities with a fluid model in the rest of the domain. Here we validate our 3D
hybrid model on a 3D (super-)particle model for negative streamers in
overvolted gaps, and we show that it almost reaches the computational
efficiency of a 3D fluid model. We also show that the extended fluid model
approximates the particle and the hybrid model well until stochastic
fluctuations become important, while the classical fluid model underestimates
velocities and ionization densities. We compare density fluctuations and the
onset of branching between the models, and we compare the front velocities with
an analytical approximation
Hydrodynamic model for picosecond propagation of laser-created nanoplasmas
The interaction of a free-electron-laser pulse with a moderate or large size
cluster is known to create a quasi-neutral nanoplasma, which then expands on
hydrodynamic timescale, i.e., ps. To have a better understanding of ion
and electron data from experiments derived from laser-irradiated clusters, one
needs to simulate cluster dynamics on such long timescales for which the
molecular dynamics approach becomes inefficient. We therefore propose a
two-step Molecular Dynamics-Hydrodynamic scheme. In the first step we use
molecular dynamics code to follow the dynamics of an irradiated cluster until
all the photo-excitation and corresponding relaxation processes are finished
and a nanoplasma, consisting of ground-state ions and thermalized electrons, is
formed. In the second step we perform long-timescale propagation of this
nanoplasma with a computationally efficient hydrodynamic approach.
In the present paper we examine the feasibility of a hydrodynamic two-fluid
approach to follow the expansion of spherically symmetric nanoplasma, without
accounting for the impact ionization and three-body recombination processes at
this stage. We compare our results with the corresponding molecular dynamics
simulations. We show that all relevant information about the nanoplasma
propagation can be extracted from hydrodynamic simulations at a significantly
lower computational cost when compared to a molecular dynamics approach.
Finally, we comment on the accuracy and limitations of our present model and
discuss possible future developments of the two-step strategy.Comment: 14 pages, 6 figure
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