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Pulse-width-dependent dynamic charge deposition on quartz dielectrics interacted with atmospheric pressure He plasma jet

Surface processing with atmospheric pressure plasma jet (APPJ) is attracting attention from both academia and industry for its capability of flexible plasma release onto targets. It is significant to understand the characteristics and mechanism of interfacial phenomenon in plasma surface interaction. This paper reports the fundamental surface charging phenomenon on quartz dielectrics by a He APPJ excited by μs-width pulses with duty ratios from 0.5% to 75%. The targeted quartz plate is attached to an electro-optic crystal grounded in the back for charge measurement. The evolution of surface charges is detected with a reflective-type optical measurement platform based on Pockels effect and the fast charge inversion on 2D-FTA. Results show that ionization waves and quartz dielectric interaction generates a round positive charge deposition, whose lifetime greatly depends on the pulse widths. Under a fixed pulse width, higher voltage magnitudes encourage more charges with a higher deposition rate. An interesting polarity reversal of surface charging occurs when the pulse width increases to over half-cycle, with a reduced positive charge on pulse-on and more negative charges on pulse-off. Efforts are devoted to exploring the mechanisms of charge maintenance on pulse-on and the polarity reversal under longer pulses of several hundred microseconds. These results reveal that the unchanged charge density in dark channel is due to the much lower rate of charge decay compared with the fast ionization wave propagation. The abrupt decrease at the falling edge results from the charge neutralization in the negative conductive channel formed by the secondary discharge. By measuring the initial evolution during the foremost 1000 periods, we prove that the accumulative difference in dissipated charges between pulse-on and pulse-off periods is the main cause of the occurrence of polarity reversal under long pulses. This study provides a more compatible tool for in-depth insight into the charging process of plasma interacting with multiple dielectrics, and exhibits a potential strategy of regulating surface charging by adjusting pulse parameters, which are of interest to the plasma community.