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 μm. 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 (∼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  10) rapidlyevolving filaments that constitute micron-sized (∼10-50 μm) plasma channels with an electron density of -ñ 10 10 e 19 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.

By employing multi-frame laser interferometry, shadow, and schlieren imaging, we trace the formation of a nanosecond spark discharge in millimeter-sized air gaps formed by a point cathode and flat anode or vice-versa. We discover that the electrical breakdown of the discharge gap is associated with extremely fast (=1 ns) explosive formation of micron-sized cathode and anode spots. We find that the characteristic delay between the instants of the anode and cathode spot initiation can be much shorter than 1ns. The spots appear as highly ionized near-electrode plasmas with an electron density n e ∼10 19 -10 20 cm −3 . The spots then give rise to highly ionized spark channels with pronounced filamentary structures. Our findings indicate that the extremely fast formation of anode spots is associated with an ultrafast gap breakdown promoted by an ultrafast ionization wave (UFIW). The role of the UFIW governed by the rapidly evolving cathode spot is discussed as a fundamental mechanism of the breakdown.

The initial stage of the fast electrical breakdown of an air gap with a pin-to-plane electrode geometry is studied on a nanosecond time scale using multi-frame laser probing with an exposure time of 70ps and spatial resolution as high as 3-4μm. We find that the gap breakdown is associated with the fast (1 ns) formation of a micron-sized (∼10 μm) cathode spot that appears as a plasma with an electron density of » n 10 cm e 19

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