29 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
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
Effect of inhomogeneities on streamer propagation: I. Intersection with isolated bubbles and particles
Sub-micron proximal probe thermal desorption and laser mass spectrometry on painting cross-sections
Cross-sections containing organic dyes are used to demonstrate sub-micron atomic force microscopy thermal desorption (AFM-TD), followed by laser mass spectrometry
Development of a cathode-directed streamer discharge in air at different pressures : Experiment and comparison with direct numerical simulation
The results are given of an experimental investigation of a cathode-directed streamer discharge in synthetic air in the pressure range from 760 to 300 torr and their comparison with the results of direct numerical simulation in a 2D hydrodynamic approximation. The pattern of discharge branching upon variation of pressure is investigated experimentally. The results are given of comparison of the predicted and measured values of anode current, streamer propagation velocity, and channel diameter. It has been demonstrated that the electric field in the streamer head is hardly affected by the pressure decrease, while the electron concentration decreases with pressure by an order of magnitude. At the same time, production of chemical species in a cathode-directed streamer discharge varies at a rate of at least the second power of inverse pressure
Real-time breath analysis with active capillary plasma ionization-ambient mass spectrometry
ISSN:1752-7155ISSN:1752-716
Plasma Ionization Source for Atmospheric Pressure Mass Spectrometry Imaging Using Near-Field Optical Laser Ablation
Mass spectrometry imaging (MSI) at
ambient pressures with submicrometer
resolution is challenging, due to the very low amount of material
available for mass spectrometric analysis. In this work, we present
the development and characterization of a method for MSI based on
pulsed laser ablation via a scanning near-field optical microscopy
(SNOM) aperture tip. SNOM allows laser ablation of material from surfaces
with submicrometer spatial resolution, which can be ionized for further
chemical analysis with MS. Efficient ionization is realized here with
a custom-built capillary plasma ionization source. We show the applicability
of this setup for mass spectrometric analysis of three common MALDI
matrices, α-4-hydroxycyanocinnamic acid, 3-aminobenzoic acid,
and 2,5-dihydroxybenzoic acid. Although the ultimate goal has been
to optimize sensitivity for detecting material ablated from submicrometer
diameter craters, the effective lateral resolution is currently limited
by the sensitivity of the MS detection system. In our case, the sensitivity
of the MS was about 1 fmol, which allowed us to achieve a spatial
resolution of 2 ÎŒm. We also characterize the analytical figures
of merit of our method. In particular, we demonstrate good reproducibility,
a repetition rate in the range of only a few seconds, and we determined
the amount of substance required to achieve optimal resolution and
sensitivity. Moreover, the sample topography is available from SNOM
scans, a parameter that is missing in common MSI methods