Fig. 1: Observation of the ring bubble induced by submerged sparker catapult.

A Schematic representation of the submerged sparker catapult producing a primary bubble, which subsequently forms a ring bubble that is propelled forward. The green and red arrow represent unidirectional motion and rotational motion, respectively. B Photographs and schematic, showing the measurements of the toroidal structure. Symbols are in Supplementary Table S1. C Illustration of the initial arrangement of submerged sparker catapult, the formation of the primary bubble, and the subsequent migration and oscillation of the ring. D Sequential high-speed images capturing the temporal evolution from plasma formation to the generation of the primary and ring bubbles at various time intervals (with 10000 fps frame rate at E_out = 1.4 J, t = 0 μs for the moment right when the underwater spark discharge initiates). E Schematic of the pulse power supply used to energize the submerged sparker, showing the input, charging, and energy storage circuits. The right side of the figure shows a cross-sectional view of the submerged sparker device. The central orange component represents the discharge electrode (Φ_e = 0.7 mm) and the attached high-voltage cable. The transparent hard interface surrounding it is a quartz tube (Φ_tube = 10 mm, T_tube = 2 mm, distance from electrode to tube opening = 12 mm). The outermost mesh serves as the grounding electrode. F Comparison of the migration velocities between the primary and ring bubbles as a function of translational distance, highlighting the enhanced stability and persistence of the ring bubble. G Comparative metrics of the primary and ring bubbles. The data represents the normalized proportion of individual bubbles and rings, demonstrating the superior endurance and distance coverage of the ring bubble relative to the primary bubble. Source data is provided as a Source Data file.