Numerical Investigation of a Helium Plasma Jet Impinging on Substrates: Effects of Relative Permittivity and Surface Morphology

Objective: Atmospheric pressure plasma jet is one of the promising plasma sources for biomedical applications. This work provides insight into the plasma dynamics near the substrate surface during the plasma treatment. Methods: Two-dimensional modeling of a helium plasma jet impinging on substrates with different relative permittivity and surface morphology is implemented. Results: When the tip of the plasma jet touches the substrate surface, there is a strong electric field and a high density of surface charge on the surface. The higher the substrate’s relative permittivity, the higher density of electrons and excited states near the surface are obtained, and the smaller area of the plasma on the surface is observed. For the wavy surface of the substrate, a surface ionization wave slowly propagating along the protrusions of the substrate is observed. It propagates along the protrusions (rather than the concaves), resulting in no plasma being generated directly inside the concaves. Detailed analysis shows that because the height and width of the concaves are smaller than the thickness of the plasma sheath, the narrow space of the concaves probably restricts the direct production of plasma inside it. Conclusion: The increase of substrate permittivity is beneficial to the production of electrons and chemical species but limits the radial propagation of the plasma along the surface. To generate the plasma directly inside the narrow concaves for the uneven surface of the substrate, the electron density or the width of the concaves should increase.