Kinematical characteristics of the long spark in stages of final jump and return stroke

The paper presents results of high-speed optical studies carried out at the High Voltage Research Center (HVRC), Istra, Russia (2002-2007). Different measurements were conducted in rod-to-plane gap of 5.5-6 m in length under both polarities HV pulses of around 2 MV in altitude with waveform of 20/7500 and 100/7500 ¿s. The velocities of the long spark in stages of final jump and return stroke were directly measured using framing and streak modes. The highest velocities near the grounded plane or near the junction point have been found to be around 5-10 m/¿s that is much larger than usually reported. These results have been confirmed and ascertained with additional quantitative treatment of the streak photographs. The velocities have been compared with those in the natural lightning and in the short gap streamer discharges; simple explanations suggested.