In SF6gas 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 2×103Td inE/N, where 1 Td=10−21V m2, whereEis the electric field andNis 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 SF6spark discharge, negative ions strongly enhanced electron diffusion in the multipolar diffusion that resulted in the expansion and the excitation of ionization instabilities.