Prebreakdown currents and optical events in insulating liquids subjected to a voltage step in point-plane geometry

The simultaneous recordings of the transient currents and optical events following a rectangular wave voltage has been taken to do a systematic study of prebreakdown phenomena in dielectric liquids. A very simple optical method using a laser beam and a photodiode is used to measure the steamer velocity. Negative streamers produce a series of current pulses, while positive one produce only a single pulse during their transit from the point to the plane. Ion injection inducing liquid motion is shown to take place (at voltage lower than the streamer threshold voltage). Negative injection is higher than the positive one in cyclohexane, the field is much reduced at the point, which allows us to explain why streamer threshold voltages are higher for negative polarity.