Control of corona and sparkover of coaxial systems by irradiation

The glow- and streamer-corona discharge forms occurring in air at the positive central conductor of a coaxial-cylinder system are examined. It is found that, for conductor diameters up to 1.2 cm, a glow discharge can be established, whereas for larger conductors streamer channels are always formed. For gaps preionised by irradiation from a 90Sr source of strength 0.4 mCi, the glow discharge is sustained for all conductor diameters. This produces a reduction in corona current of up to threefold and a decrease in acoustic noise. The critical diameter of the central conductor which gives a minimum sparkover voltage for the system is found to be dependent on whether alternating voltage or direct voltage is applied. This difference is interpreted in terms of the different corona development in the two cases, since the glow discharge suppresses streamer development and inhibits spark breakdown. Irradiation of the direct-voltage system thus increases the sparkover voltage for a positive central conductor; for a 1.27 cm conductor diameter the average field strength across the gap at sparkover is increased from 0.47 MV/m to 0.74 MV/m. No increase in sparkover voltage is found for alternating-voltage systems.