Numerical study on the effect of shielding gas on atmospheric pressure plasma jet interacting with target surface

Abstract This study investigates how N 2 /O 2 mixture shielding gas influences the interaction between a helium atmospheric pressure plasma jet (APPJ) and a target surface using a 2D axisymmetric fluid model. The ratio of N 2 /O 2 mixture is set at 1/0 (pure N 2 ), 0.79/0.21, 0.5/0.5 or 0/1 (pure O 2 ) in the model. The results reveal that the distribution of electron density along the radial axis within the ionization wave (IW) is influenced by the shielding gas. When the shielding gas is pure N 2 , the electron density remains relatively uniform along the radial axis of IW, maintaining a higher level near the center (defined as ‘solid shape’). As the O 2 ratio in shielding gas is increased, the distribution of electron density exhibits a different pattern, and the electron density at the edge is significantly higher than in the central (defined as ‘ring shape’). The ‘solid shape’ of IW results from using pure N 2 as a shielding gas and the target acting as a cathode, which together enhance the electric field of the IW at its center. The ‘ring shape’ of IW is due to the low ionization threshold of O 2 and its high Penning ionization rate, which together enhance the electric field of IW at its edge. When the APPJ strikes the target, charges are transferred to the target surface as the IW propagates downward. The distribution of surface charge density and ion flux intensity is influenced by the morphology of IW. Specifically, when the shielding gas is changed from pure N 2 to pure O 2 , the distributions of surface charge density and ion flux intensity change from a ‘solid’ shape to a ‘ring shape’. As the mixture ratio of N 2 is increased, the surface charges density and ion flux intensity at the central surface increased significantly, suggesting its considerable potential for various applications.