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The development of surface corona on dielectric plates under negative impulse voltages has been observed with the dust figure technique in atmospheric air. The dielectric plate was inserted perpendicularly to the axis of a rod-to-plane electrode system and the thickness of the plate and the radius of the rod tip were changed to investigate the corona discharge mechanism at the respective corona onset voltages. Experiments show that the first negative corona along the plate surface with a thicker rod than 0.5 cm diameter forms a distribution of the negative space charge in ring shape and the diameter of the ring increases with the rod radius and the plate thickness.The origin of the ring formation and characteristics on the ring pattern are discussed on the basis of a numerical analysis on electron avalanche developed along the line of electric force around the rod electrode.

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