The sheath electric field must remain reversed long enough for negative ions to escape.

The estimated sheath width, the time for a 500 Kelvin negative ion to cross that width, and the frequency corresponding to the sheath crossing time as a function of the ion density for discharges with Ne ~ Ni+. It is assumed that the sheath voltage is 20V, Te=2.5eV, and M(negative ion)=19 amu.