Initiation and propagation mechanisms of underwater streamers

To clarify the initiation process and the propagation mechanism of positive and negative underwater streamers, focusing on two different theories of the bubble theory and the direct ionization theory for positive streamers, and focusing on precise analysis of generated pressure waves for negative streamers.

The initiation process of the positive streamers visualized by a high-speed camera of 100 Mfps was related to the bubble theory because the streamer inception was observed from the tip of a protrusion on the surface of this bubble cluster. Regarding the secondary steamer propagation with a velocity of 32 km/s, the streak imaging showed that luminescence preceded gas channel generation, suggesting a mechanism of direct ionization in water. In addition, the streak imaging of primary streamer propagation with a velocity of 2.4 km/s revealed intermittent propagation, synchronized with repetitive pulsed currents. Shadowgraph imaging of streamers synchronized with the light emission signal indicated the possibility of direct ionization in water for primary and secondary streamer propagation.

For the propagation processes of the negative streamers, as negative streamers are much smaller and have weaker luminescence compared with the positive streamers, a new visualization optical system was newly developed to detect weak pressure waves using a pair of polarizing plates. The generation of pressure waves was observed at the same time as the pulsed currents accompanied by light emissions detected by a streak camera. Our results indicated that the initiation of the streamer generates pressure waves. Analysis of temporal resolution with nano-second order clarified that the streamer propagates intermittently with the distance of ~20 µm and the interval of ~20 ns, and also the branching phenomenon occurred at different times resulting in the branching streamer propagation in different directions.