Short time optical and electrical diagnostics of pulsed N₂ and SF₆ discharges

The discharge development in N₂ and SF₆ was investigated in the nanosecond range for impulse voltage stresses. The experiments were carried out using two cable pulse generators with 150-kV, 200-ns and 250-kV, 400-ns square-wave pulses, respectively. Measurements were made in free gas and at gas-solid interfaces. The processes in the test gap were registered with a high-speed framing camera from the predischarge regime until the formation of a highly conductive plasma channel was completed. It was found that after the formative time lag, impulse breakdown in SF₆ was initiated by a steep partial voltage drop followed by an intermediate stage of slow voltage drop. In the presence of an insulator surface no intermediate stage was observed, and the voltage drop completed in a few nanoseconds.