LASER-INDUCED BREAKDOWN CHARACTERISTICS OF NONUNIFORM DC CORONA DISCHARGE FIELDS

Laser-induced breakdown of nonuniform field gaps in atmospheric air was caused by an XeCl excimer laser. This laser beam was focused on the center of a rod-to-plane, needle-to-plane or hemisphere-to-plane gap with dc voltage. The laser energy dependency of breakdown voltage was investigated for positive and negative applied.voltages and gap lengths of 15 to 25 mm. It was found that the positive breakdown voltage was higher than the negative one in the hemisphere-to-plane gaps or the rod-to-plane gaps with low voltages, where there is little or no space charge. However, the effect of polarity is reversed in the rod-to-plane gaps with large spacings and the needle-to-plane gaps. On the basis of the dependence of the discharge mode on this polarity effect, it can be seen that it was caused by the difference in development of positive and negative streamers in the corona discharge.