Mechanisms of underwater discharge initiation and propagation are of a great interest in pulsed systems, water treatment, and biomedical applications. Nanosecond discharges allow the generation of nonthermal plasma without substantial water heating. Positive nanosecond discharge in water develops in two possible configurations: slow and weakly luminescent bush-like mode or fast and luminous tree-like mode with well-pronounced branching. Under our experimental conditions, transition between two modes occurs in the range between 8 and 15 kV of applied voltage pulse. Spectroscopic study reveals intensive emission of nitrogen species.
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