Microstructure of the current channels in a nanosecond spark discharge in atmospheric-pressure air in uniform and highly nonuniform electric fields

The microstructure of a nanosecond spark discharge in atmospheric-pressure air in uniform and highly nonuniform electric fields is investigated. It is found that an 0.1-to 0.4-mm spark channel consists of a large number (from 100 to 1000) of 5-to 10-μm-diameter microchannels distributed nearly uniformly over the channel cross section. The current amplitude in the spark is 1.5–3 kA, and the current density in a microchannel is 10⁷ A/cm². It is shown that the formation of the microstructure cannot be attributed to ionization-heating instability. The instability of the ionization wave front is suggested as a mechanism for the microstructure formation.