Optical observation of gas discharge induced by high-power and short-pulsed microwave

The temporal and spatial evolution of gas discharge induced by a high-power , short-pulsed microwave (HPM) was observed using a high-speed image conver ter camera. The HPM with the peak power of 20 MW, frequency of 13 GHz, and pulse duration of 14 ns was irradiated uniformly into a gap between two sph erical electrodes biased to a DC voltage less than the gap breakdown voltag e. The streak images show two different breakdown mechanisms depending on a ir pressure. In the high-pressure region above about 300 Torr, the microwav e-induced discharge begins at several points on the gap axis. When these lo cal discharges increase sufficiently to link together with each other, the gap breakdown occurs. The gap breakdown is delayed by 30-300 ns depending o n gas pressures and applied DC voltages. In the low-pressure region less th an 300 Torr, on the other hand, a two-stage breakdown occurs. Ln the first stage, a single large discharge luminescence appears in the gap center at t he time of the microwave injection and is followed by another discharge bet ween the first discharge and two electrodes. After approximately 0.1-20 mic roseconds of the first stage discharge, the second-stage discharge appears followed by the gap breakdown.