Modes of underwater discharge propagation in a series of nanosecond successive pulses

Initiation and development of nanosecond electric discharges in deionized water was studied by time-resolved shadowgraphy and emission imaging. Pulses of 4-10 kV in amplitude, 5 ns rise time and 30 ns duration were applied to a high-voltage (HV) pin-like electrode with the diameter of the pin tip about 2 mu m. Two distinct bush-like and tree-like modes of the discharge propagation were observed in the same conditions already at the rising slope of HV pulse. Both the modes are supersonic. The probability of finding one of them depends on applied voltage. Bush-mode propagates at 4 km s(-1) at the initial stage; the deposited energy is less than 0.05 mJ. For the tree-like mode, propagation velocity, number of branches and deposited energy show well-prononced dependence on applied voltage. It is suggested that the bush-like discharge ignites in the gaseous cavity, whereas the tree-like discharge propagates in bulk liquid. The role of electrostriciton in the initiation and propagation of two modes is discussed, and comparison with microsecond discharges is given. Transition from bush-like discharge to tree-like mode may occur in successive positive pulse coming 500 ns after initial pulse.