Fast fine-scale spark filamentation and its effect on the spark resistance

Formation of a millimeter-sized spark discharge in ambient air is traced on a nanosecond time scale using multi-frame laser probing with an exposure time of 70 ps and spatial resolution of 3-4 mu m. The discharge is initiated by a 25 kV voltage pulse with a rise time of 4 ns, with the pulse applied to the gap formed by a point cathode and flat anode. It is demonstrated that the gap breakdown is accompanied by the fast (similar to 1 ns) formation of a highly ionized homogeneous spark channel originating from the point cathode. We discover that the fast fine-scale filamentation of the homogeneous spark channel arises several nanoseconds after the breakdown and at some distance from the cathode, which results in a complex filamentary structure of the channel. We find that the growing spark channel, in fact, develops in the form of multiple (N greater than or similar to 10) rapidly-evolving filaments that constitute micron-sized (similar to 10-50 mu m) plasma channels with an electron density of n(e) similar to 10(19)-10(20) cm(-3) and subnanosecond characteristic evolution time. First filaments appear at the top of the developing homogenous spark channel. Further, the growing filaments are split themselves, and their number is increased over time up to several tens. Our findings indicate that the fast fine-scale filamentation is one of the important mechanisms governing the spark channel resistance after the breakdown.