Some insights into the dynamics of long-lived atmospheric pressure plasmoids

Abstract This study investigates the detailed generation mechanism of ball plasmoids formed by millisecond-scale atmospheric discharges in contact with water. Using high-speed filming with a higher frame rate and shorter exposure time than in previous studies, we focus on capturing and analyzing the rapid processes that occur at the beginning of plasmoid formation. Particular attention is given to the initiation phase, in which an electrical breakdown within a cathode ceramic cup containing conductive liquid triggers a shockwave and the subsequent injection of the liquid. This critical initial event results in the formation of a mixture of water vapor and droplets, which is weakly ionized by the breakdown and provides the primary material for plasmoid development. We also demonstrate that the electrical arc formed after the breakdown evolves into a plasma jet that supplies plasma to the forming plasmoid. We then explore the mechanism by which plasma is formed and transported into the developing plasmoid. The post-breakdown electrical arc transitions into a plasma jet that dynamically accelerates and channels plasma upwards into the plasmoid region. This jet transports ionized matter and facilitates ionization through interactions with entrained vapor and droplets, establishing a continuous supply of plasma to the plasmoid. We obtained optical emission spectra from different parts of the surface discharge at different times. The presence of the Balmer series is characteristic of the jet, which weakens as the current decreases, allowing the atomic sodium doublet line to become dominant in the plasmoid spectra.