Experimental study of the spatio-temporal development of metre-scale negative discharge in air

We study the development of a negative discharge of about 1 MV driven by a Marx generator in an air gap of 1–1.5 m, at standard temperature and pressure. We show the evolution of the discharge with nanosecond-fast photography together with the electrical characteristics. The negative discharge develops through four well-distinguished streamer bursts. The streamers have different velocities and life times in different bursts. The last burst triggers a positive inception cloud on the positive grounded electrode and a burst of positive counter-streamers emerge. The pre-discharge then bridges the gap and leaders grow from both electrodes. Finally a spark is formed. Looking closer into the pre-ionized zone near the cathode, we find isolated dots which are potential branching points. These dots act as starting points for positive streamers that move toward the high-voltage electrode. We also find phenomena such as space leaders and leader stepping in our laboratory sparks.