206 research outputs found
Simulation of transient energy distributions in sub-ns streamer formation
Breakdown and streamer formation is simulated in atmospheric pressure nitrogen for a 2D planar electrode system. A PIC code with multigrid potential solver is used to simulate the evolution of the non-equilibrium ionization front on sub-nanosecond timescales. The ion and electron energy distributions are computed, accounting for the inclusion of inelastic scattering of electrons, and collisionally excited metastable production and ionization. Of particular interest is the increased production of metastable and low-energy ions and electrons when the applied field is reversed during the progress of the ionization front, giving insight into the improved species yields in nanosecond pulsed systems
Spatial coupling of particle and fluid models for streamers: where nonlocality matters
Particle models for streamer ionization fronts contain correct electron
energy distributions, runaway effects and single electron statistics.
Conventional fluid models are computationally much more efficient for large
particle numbers, but create too low ionization densities in high fields. To
combine their respective advantages, we here show how to couple both models in
space. We confirm that the discrepancies between particle and fluid fronts
arise from the steep electron density gradients in the leading edge of the
fronts. We find the optimal position for the interface between models that
minimizes computational effort and reproduces the results of a pure particle
model.Comment: 4 pages, 5 figure
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
An investigation of CO2 splitting using nanosecond pulsed corona discharge: effect of argon addition on CO2 conversion and energy efficiency
The plasma chemical splitting of carbon dioxide (CO2) to produce carbon monoxide (CO) in a
pulsed corona discharge was investigated from both an experimental and a numerical standpoint.
High voltage nanosecond pulses were applied to a stream of pure CO2 and its mixture with argon,
and the gaseous products were identified using Fourier transform infrared spectroscopy. Due to the
shape of pulses, the process of CO2 splitting was found to proceed in two phases. The first phase is
dominated by ionization, which generates a high electron density. Then, during the second phase,
direct electron impact dissociation of CO2 contributes to a large portion of CO production.
Conversion and energy efficiency were calculated for the tested conditions. The conversions
achieved are comparable to those obtained using other high pressure non-thermal discharges, such as
dielectric barrier discharge. However, the energy efficiencies were considerably higher, which are
favorable to industrial applications that require atmospheric conditions and elevated gas flow rates
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
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