Imaging of a nanosecond pulse discharge in a quasi stationary bubble on time scales from nanoseconds to microseconds

Fast imaging and precise timing allows us to investigate the initiation and development of a nanosecond pulse discharge in deionized water with a quasi stationary gas bubble between two submerged electrodes. The applied voltage pulse is an unmatched 20 kV with a 7 ns rise time and ~30 ns duration of the first pulse followed by decaying oscillations lasting approximately 500 ns. When observed on a time scale of hundreds of nanoseconds or longer, the discharge appears to bridge the gap between the electrodes, while imaging on a nanosecond and ten nanosecond scale presents a different picture. Discharge initiates at the sharp positive electrode and then appears in the bubble on the positive side. The timing of the appearance of the discharge in the bubble depends on the distance between the bubble apex and the electrode tip. Imaging with delay times of 10 - 100 ns extending to the subsequent repeated voltage pulses captures repeated partial discharges in the same quasi stationary bubble. Extending the imaging to microseconds and longer reveals the formation of bubbles in the electrode region as well as the breakup of the original bubble.