Atmospheric pressure plasma jet (APPJ) can be generated in capillary tubes flowing with pure helium and with admixtures of oxygen into the pure helium. The jet exiting the tube can be used for a variety of applications through surface interaction. In this study, a twodimensional axi-symmetric model has been developed to provide insight into the evolution of capillary helium plasma jet with and without the presence of oxygen admixtures and its interaction with a dielectric surface placed normal to the jet axis. The model considers the gas mixing of helium and ambient air and the analytical chemistry between helium, nitrogen and oxygen species. Experiments were performed in similar conditions as the simulations in order to get qualitative agreement between them. The numerical and experimental results show that the evolution of the helium plasma jet is highly affected by the introduction of oxygen admixtures. In particular, it was observed that the addition of oxygen admixtures in the helium gas promotes plasma bullet propagation on the axis of symmetry of the tube (instead off axis propagation for the pure helium plasma jet). On the other hand, the presence of the dielectric surface (the slab placed in front of the tube exit) forces the plasma bullet to spread radially. Furthermore, the plasma bullet speed decreases when the helium plasma jet is operated in the presence of oxygen admixtures. The numerical results also showed that He/O2 plasma jets induced much higher electric fields on the dielectric surface in comparison to the pure helium plasma jet.
We report certain complex behaviors of a nanosecond Nd:YAG laser produced plasma on a Ni target. For high laser fluences, a split in the transversal expansion plane along with oscillatory regimes of the ablation plasma were observed and investigated. These complex phenomena, which take place at various interaction time scales, are described using a fractal theoretical model based on continuous but non-differentiable curves of particle movement.
In this study, a one-dimensional plasma fluid model is used to shed light into the evolution of a He/dry air (500 ppm, 79% N2 and 21% O2) dielectric barrier discharge (DBD) under different levels of water admixtures (20 to 2000 ppm). The model considers the analytical chemistry between helium, nitrogen, oxygen and water species and it is verified with experimental results to ensure its correctness. The simulation results show that water admixtures highly affect the discharge characteristics and the dominant ions in the mixture. In particular, it was observed that the increase of water in the mixture up to 600 ppm causes the reduction of the breakdown voltage, while above 600 ppm the breakdown voltage increases. Furthermore, the simulation results show that the most important positive ion in the mixture is H2O + for 20-100 ppm of water admixtures and H11O5 + for 100-2000 ppm of admixtures. The most abundant negative charged species is found to be electrons for the range of water admixtures considered in this study. To interpret these results and to get an insight into the discharge evolution the main reaction pathways of ion production are investigated and analyzed.
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