Using a special gap providing a needle protrusion in the anode, values of time lag τ from the start of UV irradiation to sparkover under DC voltages are measured. The dstribution of log τ for the applied voltage shows a slim and linear pattern over about 10% voltage range from the minimum value. The linear region in log τ-V diagram moves consistently depending on the gas temperature, gas flow and other conditions not electrical. From these results, it is considered that the sparkover in this gap under a lower voltage is only accomplished by forming up the condition for the streamer initiation through the accumulation of metastable molecules in front of the needle tip. The build up of metastable molecule concentration due to the successive injection of avalanches can increase the ionization rate through the energy enhancement of electrons by collisions of the second kind and by cumulative processes. Then the formative characteristics are observed in the sparkover time lag in this gap.
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