Streak observation of air breakdown triggered by a high-power pulsed microwave

Summary form only given. High-power microwaves with the peak power of 20 MW, frequency of 12 GHz and pulse duration of 14 ns, which are produced by a virtual cathode oscillation, are sent to a discharge electrode in a stainless steel vacuum vessel. A DC voltage less than a gap-breakdown voltage is applied to the discharge electrode, consisting of a pair of needle and plane electrodes. The gap separation is 1.7 cm. Time and spatial evolutions of breakdown luminosity are observed by a high-speed image converter camera. Microwave induced breakdowns occur at the pressure range less than 760 Torr for negative polarity and less than 600 Torr for positive polarity. An injection of microwaves at first leads to local luminescence at several spots between the gap. Each luminescence grows mainly toward the positive electrode. When these luminescences link together, the air breakdown occurs when the discharge current between the gaps grows suddenly.