The visual appearance of an expanding nitrogen plasma with or without oxygen is shown. The interaction of the plasma with a substrate leads to the appearance of additional light, which is ascribed to the formation of excited molecules by association of N and/or O atoms at the substrate.
"In particular, the effect of a substrate as well as the retraction or extension of a shutter in front of the a-C:H sample on the flow dynamics and the local plasma chemistry are investigated. As can be clearly seen in the photographic images by van Helden et al., a substrate deflects an expanding thermal plasma . The presence of a substrate in a plasma expansion may also lead to changes in the recirculation cells and thus affect where the diffusion of the background gas into the plasma occurs. "
[Show abstract][Hide abstract] ABSTRACT: The gas flow in a linear plasma reactor and the plasma chemistry during hydrogenated amorphous carbon and graphite etching are investigated via time and spatially resolved measurements of the ion density and CH emission. A convolution of the ion and hydrocarbon density shows the importance of charge transfer in the plasma chemistry which ultimately yields the CH emission. The spatially resolved measurements clearly visualize the plasma expansion in the reactor and its deflection on a substrate. A stagnation zone is furthermore formed in front of a substrate when placed inside an expanding thermal plasma. An increased ion density further upstream from the substrate is attributed to a reorganization of the gas recirculation cells in the background of the reactor. The movement of a shutter in and out of the plasma expansion likewise affects these recirculation cells. This movement consequently redirects the energy flow to and from the background, as is deduced from the variation in rotational temperature of the CH radical.
Journal of Physics D Applied Physics 08/2011; 44(35):355205. DOI:10.1088/0022-3727/44/35/355205 · 2.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The dielectric-barrier (DB) discharge is an important approach to generate uniform non-equilibrium atmospheric-pressure glow discharges. We report run-to-run variations, asymmetric pulse formation and long time-scale transient phenomena in these discharges. For similar DB discharge geometric and operating conditions, we observe significant run-to-run variations as manifested in the different voltage–current waveforms at the start of each new run. These run-to-run variations are also accompanied by asymmetric pulses at the start of each run. The variations are observed to drift to a repeatable true steady-state condition on time scales of order tens of minutes to hours. Asymmetric pulse waveforms drift to a symmetric pulse waveform at the true steady state. We explore reasons for these phenomena and rule out thermal drift during a discharge run and gas-phase impurity buildup as potential causes. The most plausible explanation appears to be variations in the surface characteristics of the DBs between two consecutive runs owing to varying inter-run environmental exposure and the conditioning of the dielectric surface during a run owing to plasma–surface interactions. We speculate that the dielectric surface state affects the secondary electron emission coefficient of the surface which in turn is manifested in the discharge properties. A zero-dimensional model of the discharge is used to explore the effect of secondary electron emission.
Journal of Physics D Applied Physics 05/2007; 40(10):3145. DOI:10.1088/0022-3727/40/10/018 · 2.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An analytic model to describe the heterogeneous recombination of a single atomic species on silicalike surfaces is developed. The theoretical investigation herein presented provides ready-to-use expressions for the surface atomic recombination probability gamma obtained as a function of surface characteristics such as the densities of adsorption sites and the activation energies for the different elementary surface processes. The model takes into account physisorption, chemisorption, thermal desorption, surface diffusion, and both Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) recombination mechanisms. The results are applied to the cases of nitrogen and oxygen recombination on silica and to oxygen recombination on Pyrex. However, since the derivation is kept in a very general form, it allows the exploration of several distinct limit cases and provides a deeper understanding of the underlaying surface kinetics. The dependence of the recombination probability with the wall temperature and with the gas pressure is studied in detail. It is found that gamma can have a complex nonmonotonic behavior with the wall temperature, as a result of the competition between E-R and L-H recombination processes. The transition from first- to second-order recombinations (and vice-versa) with pressure is studied and debated. [All rights reserved Elsevier].
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