Insight into discharge mode transition in spatially confined pulsed bubbles-plasma system

Bubbles plasma (BP) has emerged as a promising technology for enhancing gas–liquid mass transfer through plasma–liquid interactions, with potential applications in water purification, biomedicine, and chemical synthesis. The discharge mode of BP is a critical factor influencing the mass transfer efficiency of reactive oxygen and nitrogen species (RONS) in plasma-activated water (PAW). This Letter presents a systematic investigation of the discharge mode transition of spatially confined BP driven by microsecond pulses in helium–air mixture under varying gas flow rates. The discharge modes are characterized using both experiment and simulation technologies. Two distinct modes are identified: Mode A, a dual-mode discharge resulting from the synergistic effect of uniform discharge (UD) and spark discharge (SD), and Mode B, a single-mode discharge sustained exclusively by UD without SD involvement. Both modes coexist at flow rates between 500 and 2500 sccm, while only Mode A persists above 3200 sccm. Compared to Mode B, Mode A exhibits a higher emission intensity, Trot, Tvib, and Te, but lower Ne. Meanwhile, to validate the proposed mechanism of mode transition, the concentrations of RONS in PAW and the associated energy efficiency with flow rates are also evaluated. Furthermore, numerical simulations reveal that the mode transition is governed by the interplay between hydrodynamics (airflow distribution) and electrodynamics (electric field distribution), resulting in a significantly enhanced electric field strength in Mode A. These findings provide valuable insights into discharge dynamics of BP and the effective regulation of PAW properties.