230 research outputs found
Probing photo-ionization: simulations of positive streamers in varying N2:O2 mixtures
Photo-ionization is the accepted mechanism for the propagation of positive
streamers in air though the parameters are not very well known; the efficiency
of this mechanism largely depends on the presence of both nitrogen and oxygen.
But experiments show that streamer propagation is amazingly robust against
changes of the gas composition; even for pure nitrogen with impurity levels
below 1 ppm streamers propagate essentially with the same velocity as in air,
but their minimal diameter is smaller, and they branch more frequently.
Additionally, they move more in a zigzag fashion and sometimes exhibit a
feathery structure. In our simulations, we test the relative importance of
photo-ionization and of the background ionization from pulsed repetitive
discharges, in air as well as in nitrogen with 1 ppm O2 . We also test
reasonable parameter changes of the photo-ionization model. We find that photo-
ionization dominates streamer propagation in air for repetition frequencies of
at least 1 kHz, while in nitrogen with 1 ppm O2 the effect of the repetition
frequency has to be included above 1 Hz. Finally, we explain the feather-like
structures around streamer channels that are observed in experiments in
nitrogen with high purity, but not in air.Comment: 12 figure
A new numerical strategy with space-time adaptivity and error control for multi-scale streamer discharge simulations
This paper presents a new resolution strategy for multi-scale streamer
discharge simulations based on a second order time adaptive integration and
space adaptive multiresolution. A classical fluid model is used to describe
plasma discharges, considering drift-diffusion equations and the computation of
electric field. The proposed numerical method provides a time-space accuracy
control of the solution, and thus, an effective accurate resolution independent
of the fastest physical time scale. An important improvement of the
computational efficiency is achieved whenever the required time steps go beyond
standard stability constraints associated with mesh size or source time scales
for the resolution of the drift-diffusion equations, whereas the stability
constraint related to the dielectric relaxation time scale is respected but
with a second order precision. Numerical illustrations show that the strategy
can be efficiently applied to simulate the propagation of highly nonlinear
ionizing waves as streamer discharges, as well as highly multi-scale nanosecond
repetitively pulsed discharges, describing consistently a broad spectrum of
space and time scales as well as different physical scenarios for consecutive
discharge/post-discharge phases, out of reach of standard non-adaptive methods.Comment: Support of Ecole Centrale Paris is gratefully acknowledged for
several month stay of Z. Bonaventura at Laboratory EM2C as visiting
Professor. Authors express special thanks to Christian Tenaud (LIMSI-CNRS)
for providing the basis of the multiresolution kernel of MR CHORUS, code
developed for compressible Navier-Stokes equations (D\'eclaration d'Invention
DI 03760-01). Accepted for publication; Journal of Computational Physics
(2011) 1-2
Probing background ionization: Positive streamers with varying pulse repetition rate and with a radioactive admixture
Positive streamers need a source of free electrons ahead of them to
propagate. A streamer can supply these electrons by itself through
photo-ionization, or the electrons can be present due to external background
ionization. Here we investigate the effects of background ionization on
streamer propagation and morphology by changing the gas composition and the
repetition rate of the voltage pulses, and by adding a small amount of
radioactive Krypton 85.
We find that the general morphology of a positive streamer discharge in high
purity nitrogen depends on background ionization: at lower background
ionization levels the streamers branch more and have a more feather-like
appearance. This is observed both when varying the repetition rate and when
adding Krypton 85, though side branches are longer with the radioactive
admixture. But velocities and minimal diameters of streamers are virtually
independent of the background ionization level. In air, the inception cloud
breaks up into streamers at a smaller radius when the repetition rate and
therefore the background ionization level is higher. When measuring the effects
of the pulse repetition rate and of the radioactive admixture on the discharge
morphology, we found that our estimates of background ionization levels are
consistent with these observations; this gives confidence in the estimates.
Streamer channels generally do not follow the paths of previous discharge
channels for repetition rates of up to 10 Hz. We estimate the effect of
recombination and diffusion of ions and free electrons from the previous
discharge and conclude that the old trail has largely disappeared at the moment
of the next voltage pulse; therefore the next streamers indeed cannot follow
the old trail.Comment: 30 pages, 13 figure
Positive and negative streamers in ambient air: modeling evolution and velocities
We simulate short positive and negative streamers in air at standard
temperature and pressure. They evolve in homogeneous electric fields or emerge
from needle electrodes with voltages of 10 to 20 kV. The streamer velocity at
given streamer length depends only weakly on the initial ionization seed,
except in the case of negative streamers in homogeneous fields. We characterize
the streamers by length, head radius, head charge and field enhancement. We
show that the velocity of positive streamers is mainly determined by their
radius and in quantitative agreement with recent experimental results both for
radius and velocity. The velocity of negative streamers is dominated by
electron drift in the enhanced field; in the low local fields of the present
simulations, it is little influenced by photo-ionization. Though negative
streamer fronts always move at least with the electron drift velocity in the
local field, this drift motion broadens the streamer head, decreases the field
enhancement and ultimately leads to slower propagation or even extinction of
the negative streamer.Comment: 18 pages, 10 figure
Spatially hybrid computations for streamer discharges: II. Fully 3D simulations
We recently have presented first physical predictions of a spatially hybrid
model that follows the evolution of a negative streamer discharge in full three
spatial dimensions; our spatially hybrid model couples a particle model in the
high field region ahead of the streamer with a fluid model in the streamer
interior where electron densities are high and fields are low. Therefore the
model is computationally efficient, while it also follows the dynamics of
single electrons including their possible run-away. Here we describe the
technical details of our computations, and present the next step in a
systematic development of the simulation code. First, new sets of transport
coefficients and reaction rates are obtained from particle swarm simulations in
air, nitrogen, oxygen and argon. These coefficients are implemented in an
extended fluid model to make the fluid approximation as consistent as possible
with the particle model, and to avoid discontinuities at the interface between
fluid and particle regions. Then two splitting methods are introduced and
compared for the location and motion of the fluid-particle-interface in three
spatial dimensions. Finally, we present first results of the 3D spatially
hybrid model for a negative streamer in air
Computer simulation in low-temperature plasma physics: future challenges
Computer simulations can be carried out with various aims. Perhaps the most challenging is prediction under conditions where experiments are difficult or inaccessible, especially when failure to predict adequately may have unhappy consequences. There is, probably, not much confidence at present in the capability of low-temperature plasma physics simulations in such a context. Other fields have attempted to meet this challenge using a collection of techniques collectively known as Verification and Validation, or V&V. These are methods for enhancing confidence in the correctness and fidelity of computer simulations. This paper surveys these techniques and discusses their application to improvements of simulation capability in low-temperature plasma physics
Controlling plasma properties under differing degrees of electronegativity using odd harmonic dual frequency excitation
International audienceThe charged particle dynamics in low-pressure oxygen plasmas excited by odd harmonic dual frequency waveforms (low frequency of 13.56 MHz and high frequency of 40.68 MHz) are investigated using a one-dimensional numerical simulation in regimes of both low and high electronegativity. In the low electronegativity regime, the time and space averaged electron and negative ion densities are approximately equal and plasma sustainment is dominated by ionisation at the sheath expansion for all combinations of low and high frequency and the phase shift between them. In the high electronegativity regime, the negative ion density is a factor of 15--20 greater than the low electronegativity cases. In these cases, plasma sustainment is dominated by ionisation inside the bulk plasma and at the collapsing sheath edge when the contribution of the high frequency to the overall voltage waveform is low. As the high frequency component contribution to the waveform increases, sheath expansion ionisation begins to dominate. It is found that the control of the average voltage drop across the plasma sheath and the average ion flux to the powered electrode are similar in both regimes of electronegativity, despite the differing electron dynamics using the considered dual frequency approach. This offers potential for similar control of ion dynamics under a range of process conditions, independent of the electronegativity. This is in contrast to ion control offered by electrically asymmetric waveforms where the relationship between the ion flux and ion bombardment energy is dependent upon the electronegativity
Concepts, Capabilities, and Limitations of Global Models : A Review
International audienceFor researchers wishing to generate an understanding of complex plasma systems, global models often present an attractive first step, mainly due to their ease of development and use. These volume averaged models are able to give descriptions of plasmas with complex chemical kinetics, and without the computationally intensive numerical methods required for spatially resolved models. This paper gives a tutorial on global modeling, including development and techniques, and provides a discussion on the issues and pitfalls that researchers should be aware of. Further discussion is provided in the form of two reviews on methods of extending global modeling techniques to encompass variations in either time or space
New Nanostructured Carbon Coating Inhibits Bacterial Growth, but Does Not Influence on Animal Cells
An electrospark technology has been developed for obtaining a colloidal solution containing nanosized amorphous carbon. The advantages of the technology are its low cost and high performance. The colloidal solution of nanosized carbon is highly stable. The coatings on its basis are nanostructured. They are characterized by high adhesion and hydrophobicity. It was found that the propagation of microorganisms on nanosized carbon coatings is significantly hindered. At the same time, eukaryotic animal cells grow and develop on nanosized carbon coatings, as well as on the nitinol medical alloy. The use of a colloidal solution as available, cheap and non-toxic nanomaterial for the creation of antibacterial coatings to prevent biofilm formation seems to be very promising for modern medicine, pharmaceutical and food industries
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