Flashover in a vacuum

It is shown that vacuum flashover is due to one or another process alternatively: (a) a thermal breakdown in the thin pre-surface layer of dielectric and (b) a streamer discharge in the desorbed gas cloud. The thermal flashover is observed in dielectrics with ρ v ≤ 10 11 -10 12 Ω cm under dc voltage. The desorption flashover exists under impulse voltage (and under dc voltage in insulators with ρ v > 10 13 Ω cm). Quantitative models are proposed to describe both the flashover mechanisms. The calculations are in good agreement with experimental data.