N₂ Mole Fraction Dependence of Plasma Bullet Propagation in Premixed He/N₂ Plasma Needle Discharge at Atmospheric Pressure

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 a one-way coupled method, and both the working gas and the surrounding gas around the plasma jet are assumed to be the same, which are premixed He/N₂ gas. The mole fractions of the N₂ (NMF) ingredient are 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 0.72×10⁶ m/s to 0.80×10⁶ 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 influences on the propagation of a plasma bullet instead of the penning ionization.