Delayed expansions and ionization instabilities observed in the current decay phase of a SF6 spark discharge

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In SF6 gas at 5–30 Torr, self-breakdown spark discharges starting in a constricted phase expanded spatially when the discharge current decayed to a critical value. The strength of the electric field increased from 400 to 2×103 Td in E/N, where 1 Td=10−21 V m2, where E is the electric field and N is the neutral gas number density. At the same time, the optical emission from the spark increased in strength and ionization instabilities started to grow. In the decay phase of SF6 spark discharge, negative ions strongly enhanced electron diffusion in the multipolar diffusion that resulted in the expansion and the excitation of ionization instabilities.