Beaded Discharges Formed under Pulsed Breakdowns of Air and Nitrogen

The mode of a pulsed discharge in a nonuniform electric field is investigated at which bright plasma bunches with a beaded structure are generated in atmospheric-pressure air. Using an ICCD camera, it is found that, at centimeter gap lengths and a voltage pulse duration of ≈300 ns, the beaded structure can be observed with a probability close to 100% within time intervals from a few nanoseconds to several tens of nanoseconds. The beaded structure can also be observed in the time-integrated photographs of the discharge gap, but with a low probability. It is shown that individual beads arise in the point-to-plane gap after the diffuse stage of the discharge and start from the electrode with a small curvature radius. It is established that the spark channel bridges the gap by passing through the formed beads. The glowing beads are again observed in the final stage of the discharge, when the discharge current and, accordingly, the intensity of spark emission decrease.