Time-resolved imaging of nanosecond discharges near the heptane-water interface : effects of gap distance on ignition and propagation of positive streamers

Abstract

Electrical discharges in liquid exhibit unique properties and are of great interest for fundamental investigations as well as for applications. Introducing an additional liquid to form a liquid–liquid interface has shown novel perspectives for in-liquid discharges. Here, we investigate the generation of millimeter-long spark discharges using a water–heptane with 20 kV–500 ns high voltage pulses. The use of 1 ns ICCD imaging allowed the detection of different phases during discharge propagation for gap distance (dgap) of 2, 4, and 6 mm, namely discharge ignition, streamer propagation, transition from streamer to spark, and discharge extinction. We developed a model based on Gaussian processes (GP) to determine streamer propagation velocity. We found that the velocity ranges between 20 and 30 km s−1 for dgap of 4 and 6 mm, and ∼50 km s−1 for dgap of 2 mm. The measurements in the different phases during propagation can be correlated with hypothetical geometric electric field intensity variations along the gap.