The effect of seed electrons on the repeatability of atmospheric pressure plasma plume propagation. II. Modeling

摘要

One of the distinguishable features of plasma jets compared with the traditional streamers is their repeatable propagation. As an initial objective, the effect of seed electrons on the repeatability of plasma plume propagation is investigated numerically. Besides residual electrons left from previous pulses, the electrons detached from O-2(-) ions could also be a significant source of the seed electrons to affect the repeatability of plasma plume propagation when an electronegative gas admixture is presented. In this investigation, a global plasma chemical kinetics model is developed to investigate the temporal evolution of the electron and O-2(-) ions in the afterglow of a plasma plume driven by microsecond pulse direct current voltages, at a total gas pressure of 2 x 10(4) Pa or 4 x 10(3) Pa in helium or helium-oxygen mixtures with an air impurity of 0.025%. In addition, a Monte Carlo technique has been applied to calculate the O-2(-) detachment rate coefficient. Accordingly, the seed electron density due to detachment from O-2(-) ions for different percentages of oxygen is obtained. Finally, the minimum seed electron density required for the plasma bullets to propagate in a repeatable mode is obtained according to the critical frequency from the experiments. It is found that the order of minimum seed electron number density required for repeatable propagation mode is independent of oxygen concentrations in the helium-oxygen mixture. It is 108 cm(-3) for 20 kPa and 107 cm(-3) for 4 kPa. Furthermore, for the helium with an air impurity of 0.025%, the residual electrons left over from previous discharges are the main source of seed electrons. On the other hand, when 0.5% of O-2 is added, the detachment of O-2(-) is the main source of the seed electrons. Published by AIP Publishing.