In situ measurement of dynamic surface charge on dielectrics interacted with plasma jet arrays

As a flexible tool to generate large-scale non-thermal plasma in ambient air, plasma jet arrays have attracted academic attention from multiple fields of biomedicine and material surface sciences. Optimization of plasma processing that heavily relies on trial-and-error experiments requires quantitative in situ diagnostics of plasma-surface interaction. This study focuses on the fundamental surface charge evolution in quartz dielectric deposited by plasma jet arrays. The thin quartz plate is attached to an electro-optic BSO crystal that is conductive and grounded in the back. These jet arrays are driven by a positive microsecond pulsed voltage with kHz frequency. Special attention is paid to the properties and mechanisms of the intriguing convergence of multiple adjacent surface charge patterns. The measured 1D and 2D jet arrays verify the trend of homopolar charge fusion at the plasma-surface interface, which was initially thought to be repulsive under electrostatic forces. The fusion of surface charges is considerably enhanced when more charges are transported onto targeted dielectrics, as well as at smaller separations. The simulation results reveal the mechanism of charge fusion that occurs when surface ionization waves (IWs) are in close proximity, and the high electric field between their heads provides a cluster of high-energy electrons that forces two IWs to merge until they connect. In the case of extremely small separations, the close-in IWs directly merge in bulk before touching the target. Quantitative in situ measurements of surface charges and the corresponding simulations provide fresh insight into the electrical interaction in large-scale atmospheric-pressure plasma jets. The properties of charge fusion can help with manufacturing and optimizing large-area uniform plasma jet sources for various dielectric materials.