Quantitative analysis of atmospheric pressure pulsed discharge plasmoids via image processing

Atmospheric-pressure pulsed discharges above gas-liquid interfaces generate transient luminous plasmoids whose evolution remains poorly quantified. In this work, high-speed imaging combined with automated image processing is employed to characterize their spatiotemporal behavior under systematically varied voltage and solution conductivity. Grayscale conversion, adaptive thresholding, and contour extraction provide time-resolved measurements of plasma intensity, projected area, centroid height, and lifetime. The plasmoid evolution can be divided into three phenomenological stages: initiation, transition, and a post-discharge luminous stage, corresponding to streamer formation, vertical expansion/detachment, and delayed luminous decay. Higher applied voltage accelerates ionization and enlarges the luminous region, whereas higher conductivity increases current amplitude but reduces lifetime through faster charge dissipation. The plasmoid brightness and projected area reach their maxima at approximately 100–150 ms, and the detached plasmoid rises with an average upward velocity of ~ 2 m/s. The electrical excitation, however, is concentrated in the earlier breakdown stage. This delayed optical response indicates that visible plasmoid evolution is governed not only by the instantaneous power input but also by subsequent plasma expansion, gas heating, and post-discharge processes.