Nanosecond pulsed discharge in liquid: Initiation mechanism and diagnostics

Summary form only given. Nanosecond high voltage pulsing has been recently found to enable plasma formation in liquid media without the traditionally expected “bubble” pre-initiation mechanism. In this work, we study the effect of a highly non-uniform electric field on liquid water, in the proximity of the electrode tip of a pin-to-plane electrode configuration. Nanosecond duration pulses with 4 ns rise time, 10ns duration and 5ns fall time are generated between submerged electrodes in a chamber with 50ml of deionized water. The inter-electrode gap is fixed at 1.5 mm in these experiments, and the electric field is varied by adjusting the voltage pulse amplitude between 7.2 kV and 15.5 kV. Schlieren imaging is then used to investigate density changes near the area of the pin electrode tip for under-voltage conditions for breakdown. The results show the propagation of a weak shockwave emanating from the electrode tip after the application of the voltage pulse, the perturbation size and energy being associated with the amplitude of the applied pulse. In experiments where conditions for breakdown are met, the plasma emission was too intense to resolve any shockwaves using the laser schlieren technique adopted. Preliminary results of optical emission spectroscopy are presented.