On the Nature of Nanosecond Diffusion-Channel Discharges in Air

For a discharge developing in a millimeter gap with a uniform electric field under a double, triple, or more severe overvoltage, a diffuse (volume) glow pierced by several plasma channels is observed. Thus, this type of discharge exhibits simultaneously the features of Townsend and streamer discharges. A streamer discharge has unusual properties. For instance, the avalanche critical length is much less than the gap spacing. An avalanche is initiated at cathode microprotrusions by field emission electrons. The electron drift velocity in an avalanche is many times greater than the velocity of the diffusion front; therefore, the ratio of the avalanche length to its radius is much greater than unity. This leads to a tenfold increase in the electric field in the avalanche head and to the generation of runaway electrons, which produce plasma and excite the gas molecules thus making the discharge diffuse in structure.