Formative Time Lags of Uniform Field Breakdown inN2

Formative time lags of spark breakdown in ${\mathrm{N}}_2$ have been measured in uniform fields as a function of percent overvoltage, pressure (150 to 700 mm Hg) and electrode separation (0.3 to 1.4 cm). For the range of variables studied the formative time lags are almost identical with values previously observed in air; the time lags vary from 100 μsec close to threshold to 1 μsec at a few percent overvoltage. The present data as well as the earlier measurements in air indicate a secondary mechanism of photoemission of electrons either from the cathode or from the gas near the cathode. This mechanism requires a large number of successive electron crossings. The dependence of the sparking potential on the number of initiating electrons has been determined; the sparking potential of pure ${\mathrm{N}}_2$ shows an enormous dependence on the value of the primary current.