Effect of jet-to-substrate distance on luminous plume of pulsed atmospheric pressure plasma jet

The interaction between various substrates and atmospheric pressure plasma jet (APPJ) is of great interest to the generation and application of industrial non-thermal plasma. Quantitative measurement of plasma-substrate interaction is required to analyze mechanisms of plasma-related effect on surfaces and to optimize plasma parameters. This study presents the dependence of plume luminous intensity of He APPJ interacting with a dielectric surface on the jet-to-substrate distance. After quantitatively acquiring the spatial-temporal resolved dynamics of plasma plumes and surface charge evolution, we established the mechanistic relations between charge accumulation on substrates and plasma plume in vicinity. Results show that the plume luminous intensity can be enhanced by shortening jet-to-substrate distance d_tr to different degrees during the rising and falling edges of the voltage pulse. Although the peak density of deposited charge almost remains constant at distances of below 8 mm, the distribution diameter decreases from ∼16 to ∼2 mm as the d_tr rises from 2 to 12 mm, with the resultant total charge decreasing from ∼35 to ∼5 nC. On this basis, the work clarifies the mechanism of plume enhancement by interaction with a dielectric substrate. Two factors are considered to contribute to the enhancement effect: one is the more concentrated electric field in the vicinity of downstream substrate, which influences the plume dynamics at both the rising and falling edges. The other is the recombination between the deposited positive charges on dielectric and transferred negative charges at the falling edge. These results have provided a deep insight into the influence of treatment distance on the properties of APPJ processing, emphasizing the necessity for precise control over the jet-to-board distance due to its potential to induce significant difference in discharge intensity.