140 research outputs found
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
Afivo: a framework for quadtree/octree AMR with shared-memory parallelization and geometric multigrid methods
Afivo is a framework for simulations with adaptive mesh refinement (AMR) on
quadtree (2D) and octree (3D) grids. The framework comes with a geometric
multigrid solver, shared-memory (OpenMP) parallelism and it supports output in
Silo and VTK file formats. Afivo can be used to efficiently simulate AMR
problems with up to about unknowns on desktops, workstations or single
compute nodes. For larger problems, existing distributed-memory frameworks are
better suited. The framework has no built-in functionality for specific physics
applications, so users have to implement their own numerical methods. The
included multigrid solver can be used to efficiently solve elliptic partial
differential equations such as Poisson's equation. Afivo's design was kept
simple, which in combination with the shared-memory parallelism facilitates
modification and experimentation with AMR algorithms. The framework was already
used to perform 3D simulations of streamer discharges, which required tens of
millions of cells
Reply to comment on "Improvements for drift-diffusion plasma fluid models with explicit time integration"
This is a reply to the comment of Jiayong Zou on the paper "Improvements for
drift-diffusion plasma fluid models with explicit time integration". The
criticism in the comment, namely that the current-limited approach is
inconsistent with the underlying partial different equations, seems to be
invalid. However, this criticism raises an interesting question about the
behavior of the current-limited scheme for a given time step, which is
discussed in this reply
Improvements for drift-diffusion plasma fluid models with explicit time integration
Drift-diffusion plasma fluid models are commonly used to simulate electric
discharges. Such models can computationally be very efficient if they are
combined with explicit time integration. This paper deals with two issues that
often arise with such models. First, a high plasma conductivity can severely
limit the time step. A fully explicit method to overcome this limitation is
presented. This method is compared to the existing semi-implicit method, and it
is shown to have several advantages. A second issue is specific to models with
the local field approximation. Near strong density and electric field
gradients, electrons can diffuse parallel to the field, and unphysically
generate ionization. Existing and new approaches to correct this behavior are
compared. Details on the implementation of the models and the various
approaches are provided
A time scale for electrical screening in pulsed gas discharges
The Maxwell time is a typical time scale for the screening of an electric
field in a medium with a given conductivity. We introduce a generalization of
the Maxwell time that is valid for gas discharges: the \emph{ionization
screening time}, that takes the growth of the conductivity due to impact
ionization into account. We present an analytic estimate for this time scale,
assuming a planar geometry, and evaluate its accuracy by comparing with
numerical simulations in 1D and 3D. We investigate the minimum plasma density
required to prevent the growth of streamers with local field enhancement, and
we discuss the effects of photoionization and electron detachment on ionization
screening. Our results can help to understand the development of pulsed
discharges, for example nanosecond pulsed discharges at atmospheric pressure or
halo discharges in the lower ionosphere
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
The inception of pulsed discharges in air: simulations in background fields above and below breakdown
We investigate discharge inception in air, in uniform background electric
fields above and below the breakdown threshold. We perform 3D particle
simulations that include a natural level of background ionization in the form
of positive and O ions. When the electric field rises above the
breakdown and the detachment threshold, which are similar in air, electrons can
detach from O and start ionization avalanches. These avalanches
together create one large discharge, in contrast to the `double-headed'
streamers found in many fluid simulations.
On the other hand, in background fields below breakdown, something must
enhance the field sufficiently for a streamer to form. We use a strongly
ionized seed of electrons and positive ions for this, with which we observe the
growth of positive streamers. Negative streamers were not observed. Below
breakdown, the inclusion of electron detachment does not change the results
much, and we observe similar discharge development as in fluid simulations
Particle based 3D modeling of positive streamer inception
In this report we present a particle based 3D model for the study of streamer inception near positive electrodes in air. The particle code is of the PIC-MCC type and an electrode is included using the charge simulation method. An algorithm for the ada
A Computational Study of Negative Surface Discharges: Characteristics of Surface Streamers and Surface Charges
We investigate the dynamics of negative surface discharges in air through
numerical simulations with a 2D fluid model. A geometry consisting of a flat
dielectric embedded between parallel-plate electrodes is used. Compared to
negative streamers in bulk gas, negative surface streamers are observed to have
a higher electron density, a higher electric field and higher propagation
velocity. On the other hand, their maximum electric field and velocity are
lower than for positive surface streamers. In our simulations, negative surface
streamers are slower for larger relative permittivity. Negative charge
accumulates on a dielectric surface when a negative streamer propagates along
it, which can lead to a high electric field inside the dielectric. If we
initially put negative surface charge on the dielectric, the growth of negative
surface discharges is delayed or inhibited. Positive surface charge has the
opposite effect.Comment: 8 page
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