Effect of pulse width on streamer propagation of underwater corona discharge

The propagation of pulsed negative corona discharges in water as a function of electrical pulse width ranging from 100ns to 100s has been studied experimentally. Using point-to-plane electrodes, it is demonstrated that the propagation of streamers exhibits different behavior under excitations with different pulse width. For pulses with width shorter than 1 mu s, the average velocity stays relatively constant at about 1000ms(-1). For pulses with width longer than 1 mu s, the average velocity decreases dramatically with increasing pulse width. Numerical fitting shows that the relationship between the velocity and pulse width follows u (-0.72). Two different mechanisms are proposed to explain the different discharge characteristics observed at different time scales. At the initial stage before the generation of any significant heat, we show that the electrostatic force may play a major role in supplying the driving force. As the pulse width increases, the gas temperature rises, and the pressure caused by evaporation may become the dominant force to push the head of the filament to propagate. The transition time between two modes was estimated to be in the order of microseconds. Our quantitative predictions are in reasonable agreement with experimental results.