Anna Zahoranová

Current waveforms of first negative corona pulses have been measured in N₂-SF₆ mixtures over a pressure range extending from 30 to 60 kPa and various overvoltages. Effects of changing cathode secondary electron emission were studied using a copper cathode coated by CuI and graphite. For a given set of experimental conditions, it is concluded that in the mixtures containing less than approximately 10% of SF₆ the negative corona pulse is associated with the formation of a cathode-directed streamer-like ionizing wave in the immediate vicinity of the cathode. In the mixtures containing more than 10% of SF₆ the ionizing wave is quenched and, consequently, the discharge is governed by the Townsend ionization mechanism fed by cathode photoemission processes. The results indicate that using the graphite-coated cathode, field emission processes play an important role in the corona pulse formation, especially at higher concentrations of SF₆.