Effects of N2/O2 Additives on the Repeatability of the Dynamics of an Atmospheric-Pressure He Plasma Jet

To better understand the role of residual charges in the repeatability of an atmospheric-pressure He plasma plume, the effects of N-2/O-2 additives on the minimum pulse repetition frequency fmin to sustain the repeatable discharge are investigated for the first time. First, when the percentage of N-2 gas in He/N-2 mixture increases from 0% to 1.7%, f(min) increases from 130 to 900 Hz. While when the percentage of O-2 gas in He/O-2 mixture increases from 0.38% to 1.7%, fmin keeps almost constant at 300 Hz. Second, a charge collection system is used to measure the residual charge density. The results show that the residual charge density in the plasma core for He/(1%) O-2 mixture is much higher than that for He/(1%) N-2 mixture. Furthermore, the simulation results on the time-resolved distribution of residual charge density show that the residual electron density just before the following voltage pulse decreases with the increase in N-2 percentage, while the residual O-2 ion density is independent on the O-2 percentage. Finally, the residual charge density of 10(9) cm(-3) or higher is needed for the repeatable discharge mode.