424 research outputs found
Positive streamers in ambient air and a N2:O2-mixture (99.8 : 0.2)
Photographs show distinct differences between positive streamers in air or in
a nitrogen-oxygen mixture (0.2% O2). The streamers in the mixture branch more
frequently, but the branches also extinguish more easily. Probably related to
that, the streamers in the mixture propagate more in a zigzag manner while they
are straighter in air. Furthermore, streamers in the mixture can become longer;
they are thinner and more intense.Comment: 2 pages, 4 figures, paper is accepted for IEEE Trans. Plasma Sci. and
scheduled to appear in June 200
Streamers in air splitting into three branches
We investigate the branching of positive streamers in air and present the
first systematic investigation of splitting into more than two branches. We
study discharges in 100 mbar artificial air that is exposed to voltage pulses
of 10 kV applied to a needle electrode 160 mm above a grounded plate. By
imaging the discharge with two cameras from three angles, we establish that
about every 200th branching event is a branching into three. Branching into
three occurs more frequently for the relatively thicker streamers. In fact, we
find that the surface of the total streamer cross-sections before and after a
branching event is roughly the same.Comment: 6 pages, 7 figure
Positive and negative streamers in ambient air: measuring diameter, velocity and dissipated energy
Positive and negative streamers are studied in ambient air at 1 bar; they
emerge from a needle electrode placed 40 mm above a planar electrode. The
amplitudes of the applied voltage pulses range from 5 to 96 kV; most pulses
have rise times of 30 ns or shorter. Diameters, velocities and energies of the
streamers are measured. Two regimes are identified; a low voltage regime where
only positive streamers appear and a high voltage regime where both positive
and negative streamers exist. Below 5 kV, no streamers emerge. In the range
from 5 to 40 kV, positive streamers form, while the negative discharges only
form a glowing cloud at the electrode tip, but no streamers. For 5 to 20 kV,
diameters and velocities of the positive streamers have the minimal values of
d=0.2 mm and v \approx 10^5 m/s. For 20 to 40 kV, their diameters increase by a
factor 6 while the voltage increases only by a factor 2. Above the transition
value of 40 kV, streamers of both polarities form; they strongly resemble each
other, though the positive ones propagate further; their diameters continue to
increase with applied voltage. For 96 kV, positive streamers attain diameters
of 3 mm and velocities of 4*10^6 m/s, negative streamers are about 20 % slower
and thinner. An empirical fit formula for the relation between velocity v and
diameter d is v=0.5 d^2/(mm ns) for both polarities. Streamers of both
polarities dissipate energies of the order of several mJ per streamer while
crossing the gap.Comment: 20 pages, 9 figures, accepted for J. Phys.
Positive streamers in air and nitrogen of varying density: experiments on similarity laws
Positive streamers in ambient air at pressures from 0.013 to 1 bar are
investigated experimentally. The voltage applied to the anode needle ranges
from 5 to 45 kV, the discharge gap from 1 to 16 cm. Using a "slow" voltage rise
time of 100 to 180 ns, the streamers are intentionally kept thin. For each
pressure p, we find a minimal diameter d_{min}. To test whether streamers at
different pressures are similar, the minimal streamer diameter d_{min} is
multiplied by its pressure p; we find this product to be well approximated by
p*d_{min}=0.20 \pm 0.02 mm*bar over two decades of air pressure at room
temperature. The value also fits diameters of sprite discharges above
thunderclouds at an altitude of 80 km when extrapolated to room temperature (as
air density rather than pressure determines the physical behavior). The minimal
velocity of streamers in our measurements is approximately 0.1 mm/ns = 10^5
m/s. The same minimal velocity has been reported for tendrils in sprites. We
also investigate the size of the initial ionization cloud at the electrode tip
from which the streamers emerge, and the streamer length between branching
events. The same quantities are also measured in nitrogen with a purity of
approximately 99.9 %. We characterize the essential differences with streamers
in air and find a minimal diameter of p*d_{min}=0.12 \pm 0.02 mm*bar in our
nitrogen.Comment: 24 pages, 11 figures, accepted for J. Phys.
Time-resolved characterization of a pulsed discharge in a stationary bubble
In recent years, plasma generation in water has been proposed for the application of water treatment. The process efficiency is believed to be improved by the introduction of bubbles in the plasma active region. For further optimization, the initiating and developmental mechanisms of plasma inside bubbles need to be understood to a greater extent. In order to meet this necessity, we investigated pulsed electrical discharge inside a stationary bubble in water. This paper deals with the evolution of the discharge and of the bubble shape during discharge, investigated by electrical characterization and fast imaging. Only several microseconds after the application of the voltage pulse, plasma light is observed. Different phases are observed during plasma formation. The plasma is strongest at the bubble surface, causing the surrounding water to evaporate. This leads to both the formation of propagating streamers into the water and the expansion and collapse of the bubble. These observations show that plasma inside a bubble has the strongest activity at the bubble surface, making it attractive for water treatment
Photoionization in negative streamers: fast computations and two propagation modes
Streamer discharges play a central role in electric breakdown of matter in pulsed electric fields, both in nature and in technology. Reliable and fast computations of the minimal model for negative streamers in simple gases like nitrogen have recently been developed. However, photoionization was not included; it is important in air and poses a major numerical challenge. We here introduce a fast and reliable method to include photoionization into our numerical scheme with adaptice grids, and we discuss its importance for negative streamers. In particular, we identify different propagation regimes where photoionization does or does not play a role
Power laws and self-similar behavior in negative ionization fronts
We study anode-directed ionization fronts in curved geometries. When the
magnetic effects can be neglected, an electric shielding factor determines the
behavior of the electric field and the charged particle densities. From a
minimal streamer model, a Burgers type equation which governs the dynamics of
the electric shielding factor is obtained. A Lagrangian formulation is then
derived to analyze the ionization fronts. Power laws for the velocity and the
amplitude of streamer fronts are observed numerically and calculated
analytically by using the shielding factor formulation. The phenomenon of
geometrical diffusion is explained and clarified, and a universal self-similar
asymptotic behavior is derived.Comment: 25 pages, 9 figure
Probing photo-ionization: Experiments on positive streamers in pure gasses and mixtures
Positive streamers are thought to propagate by photo-ionization whose
parameters depend on the nitrogen:oxygen ratio. Therefore we study streamers in
nitrogen with 20%, 0.2% and 0.01% oxygen and in pure nitrogen, as well as in
pure oxygen and argon. Our new experimental set-up guarantees contamination of
the pure gases to be well below 1 ppm. Streamers in oxygen are difficult to
measure as they emit considerably less light in the sensitivity range of our
fast ICCD camera than the other gasses. Streamers in pure nitrogen and in all
nitrogen/oxygen mixtures look generally similar, but become somewhat thinner
and branch more with decreasing oxygen content. In pure nitrogen the streamers
can branch so much that they resemble feathers. This feature is even more
pronounced in pure argon, with approximately 10^2 hair tips/cm^3 in the
feathers at 200 mbar; this density could be interpreted as the free electron
density creating avalanches towards the streamer stem. It is remarkable that
the streamer velocity is essentially the same for similar voltage and pressure
in all nitrogen/oxygen mixtures as well as in pure nitrogen, while the oxygen
concentration and therefore the photo-ionization lengths vary by more than five
orders of magnitude. Streamers in argon have essentially the same velocity as
well. The physical similarity of streamers at different pressures is confirmed
in all gases; the minimal diameters are smaller than in earlier measurements.Comment: 28 pages, 14 figures. Major differences with v1: - appendix and
spectra removed - subsection regarding effects of repetition frequency added
- many more smaller change
Inception and propagation of positive streamers in high-purity nitrogen: effects of the voltage rise-rate
Controlling streamer morphology is important for numerous applications. Up to
now, the effect of the voltage rise rate was only studied across a wide range.
Here we show that even slight variations in the voltage rise can have
significant effects. We have studied positive streamer discharges in a 16 cm
point-plane gap in high-purity nitrogen 6.0, created by 25 kV pulses with a
duration of 130 ns. The voltage rise varies by a rise rate from 1.9 kV/ns to
2.7 kV/ns and by the first peak voltage of 22 to 28 kV. A structural link is
found between smaller discharges with a larger inception cloud caused by a
faster rising voltage. This relation is explained by the greater stability of
the inception cloud due to a faster voltage rise, causing a delay in the
destabilisation. Time-resolved measurements show that the inception cloud
propagates slower than an earlier destabilised, more filamentary discharge.
This explains that the discharge with a faster rising voltage pulse ends up to
be shorter. Furthermore, the effect of remaining background ionisation in a
pulse sequence has been studied, showing that channel thickness and branching
rate are locally affected, depending on the covered volume of the previous
discharge.Comment: 16 pages, 9 figure
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