Effect Of N₂ Mole Fraction On Plasma Bullets Propagation

Non-equilibrium atmospheric pressure plasma jet(NAPPJ) is in fact a phenomenon of neutrol gas flow discharge in open air. So far, it has been developed into an advanced technology which is closely related to and becomes indispensable to human beings. NAPPJs are well-known for their abundance of highly active chemical species generated at a near-room temperature in a stable and controllable way, and for the great advantage that they flow into the surrounding environment outside the confinement of the electrodes leaving no limit to the size of treated samples and keeping bio-materials safe. Therefore, NAPPJ has played an important role in material processing industries, and the near two decades of researches show that NAPPJ is highly promising in biomedicine. However, every coin has two sides. Our knowledge about the nature of NAPPJ is still limited with many unknown physic mechanism disturbing. So it seems full of theoretical and practical significance when we come to explore the basic problems of plasma jet propagation process.In this work, a computational modeling study on the mechanism of the acceleration behavior of a plasma bullet in needle-plane configuration is presented. Above all, in our model, two sub-models of time-dependent plasma dynamics and laminar flow are connected using one-way coupled method, and both the working gas and the surrounding gas around the plasma jet are assumed the same, which are premixed He/N2 gas. The mole fraction of N2(NMF) ingredient is set to be 0.01%, 0.1% and 1% in three cases, respectively. It is found that in each case, the plasma bullet accelerates with time to a peak velocity after it exits the nozzle and then decreases until getting to the treated surface, and that the velocity of the plasma bullet increases at each time moment with the peak value changing from 5 17.2 10 m s-? ? to 5 18.0 10 m s-? ?, but then drops more sharply when the NMF varies from 0.01% to 1%. Besides, the electron impact ionizations of helium neutrals and nitrogen molecules are found to have key influence on the propagation of plasma bullet instead of the penning ionization.