Characterization of Nanosecond Diffuse-Channel Discharges in Atmospheric Air

Abstract−

It is shown that, in addition to the well-known Townsend and streamer discharges in gas, there is a third type of discharge, namely nanosecond diffuse-channel discharge. It occurs in a highly overvolted gas gap. The study is carried out on the example of air at normal conditions in a uniform electric field. In this case, the ratio $d{\text{/}}{{x}_{{\text{c}}}} \gg 1$, where d is the gap spacing and x_c is the electron avalanche critical length. The electric field at the head of such an avalanche reaches 10⁶ V cm^–1 and higher, therefore, it emits runaway electrons, which create new electrons ahead of the old ones. An avalanche chain is formed, formally similar to a streamer but with low electrical conductivity. The runaway electrons and ultraviolet photoemission from the cathode contribute to the accumulation of secondary electrons in the gap. This leads to the appearance of a diffuse glow discharge, which then turns into a channel discharge and in an arc. The dependence of the overvoltage coefficient η on the product pd is calculated, where p is the gas pressure at d/x_c = 10. It is compared with the well-known curve that separates Townsend and streamer discharges in air.