Probability of electrical breakdown: Evidence for a transition between the Townsend and streamer breakdown mechanisms

Spark-breakdown delay times were measured for ${\mathrm{N}}_2$, ${\mathrm{H}}_2$, Ar, ${\mathrm{SF}}_6$, and ${\mathrm{CCl}}_2$ ${\mathrm{F}}_2$ in a uniform field gap provided with a small current (∼${10}^{\mathrm{-}15}$ A) of free electrons by uv illumination of the cathode. Laue plots of the delay times yielded straight lines with slope iP, where i is the photocurrent and P is the breakdown probability. The dependence of the breakdown probability on voltage for ${\mathrm{N}}_2$, ${\mathrm{H}}_2$, and Ar was in good agreement with predictions of the Townsend breakdown mechanism. In ${\mathrm{SF}}_6$ and ${\mathrm{CCl}}_2$ ${\mathrm{F}}_2$, a transition was observed with increasing pressure from a dependence that agreed with the Townsend theory to a more gradual rise with voltage, characteristic of a streamer mechanism. This transition was ascribed to a decrease in the secondary-ionization coefficient with increasing pressure in ${\mathrm{SF}}_6$ and ${\mathrm{CCl}}_2$ ${\mathrm{F}}_2$, which resulted in an average electron-avalanche size at the static breakdown voltage that approached the critical value for streamer formation. A unified breakdown-probability theory, for which the Townsend and streamer mechanisms are limiting cases, was developed to account for the data over the full pressure range. The implications of these results for measurement of the static breakdown voltage and the secondary-ionization coefficient are discussed.