Effect of Wavefront on Impulse Breakdown Voltages of Slightly Nonuniform Field Gaps in Nitrogen, Air and SF6

The influence of the wavefront on impulse breakdown voltages for enclosed slightly-nonuniform-field gaps is reported. A 25 mm sphere-plane gap and a 20/50 mm diameter coaxial electrode system were used to obtain different fielt nonuniformity factors and different critical volumes. The wavefronts used in this work were 0.45, 1.35, 20, 50 and 90 ¿s while the wavetail was around 450 , ¿s for all cases. A 30 W UV lamp and 2 mCi caesium isotope were employed as irradiation sources. Experimental results show that strongly wavefront-dependent breakdown characteristics for small enclosed gaps reported by some other investigators were probably due to insufficient irradiation conditions. On the other hand, for the coaxial cylinder gap having a large critical volume, artificial irradiation was not necessary.