Laser-induced undervoltage breakdown in atmospheric N2 correlated with time-resolved avalanches

Time-resolved avalanches in atmospheric N₂ were recorded in a parallel-plane arrangement operating under nonuniform field conditions. When the gap was irradiated with a UV laser pulse, it has been possible to produce breakdown at a value of the applied DC voltage considerably lower than the corresponding spontaneous value. At low field, the photoelectrons initially released at the plane facing the anode, drift in a decreasing geometrical field, as indicated by the negative signal slope over the transit time. When avalanches were time-resolved under weak probability of an undervoltage breakdown, the recorded waveforms have shown particular features. Despite growing in a geometrically decreasing field, the swarm exhibits an exponential growth, at least up to half the transit time. The exponential part of the current growth is followed by a depression, an under-exponential growth starting before the maximum current is reached. These observations provide supporting evidence for an active role of space-charge over a large part of the gap, reaching early in the development of the avalanche