Applicability of simple expressions for electrical breakdown probability in vacuum

Simple analytical expressions for the law of breakdown probability are suggested. They are tested by comparison with experimental data obtained by impulse breakdown in 10-Pa vacuum for very small gaps (d<0.1 mm). It is concluded that the simplified approach is not appropriate for conditions of poor vacuum, because breakdown initiation occurs both on the electrodes and in the residual gas, so that the Weibull distribution assumed on the basis of just one breakdown mechanism is not applicable.