[en] The relative importance of atmospheric negative ions in corona formation and breakdown in a radial electric field has been investigated, with special reference to effects occurring at an insulating surface of a cycloaliphatic polyurethane with dolomite filler. The inception times, corona charge, light emission (by UV photography) and sparkover voltages under lightning impulses have been recorded under natural atmospheric conditions and under the influence of negative ions introduced from an auxiliary corona. The presence of excess negative ions on the surface is shown to increase both the charge injected and the mean radius of the corona, attributable to an augmentation of streamer discharges. In similar experiments in air, the excess ions cause a transition to a 'glow' discharge. Effects of negative ions on sparkover are not significant on the insulator surface, but they cause a small increase in air. Comparisons between the two cases lead to the conclusion that the most important effect of the ions on the surface is to provide 'seed' electrons for streamer propagation, following ion detachment in the field of advancing streamer tips

[en] A transition from a negative regime to positive streamers in a guiding field has been studied. The apparatus is a simple parallel plane electrode arrangement that provides the uniform guiding field. Negative streamers were generated by a pulsed negative voltage of magnitude ∼5 kV and duration ∼270 ns applied to a trigger pin electrode placed in an aperture at the centre of the grounded electrode and insulated from it by air. It has been shown that the velocities of streamers was between 4x10⁴ and 1.5x10⁵ m s^-1 over an external guiding field range of 5.5-6.3 kV cm^-1. Results have been presented showing the variation of field in the mid-gap region and the characteristics of streamers. Optical observations have shown the light emission from the developing discharge to be non-linear