STUDIES ON MICROSCOPIC STRUCTURES FOR PRODUCTION/DISAPPEARANCE MECHANISMS OF LASER-INDUCED PLASMA AND DYNAMICS DURING LASER-INDUCED DISCHARGE

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The aim of this research is to find the successful conditions for laser-induced lightning, by understanding (1) the mechanism of ionization by excimer alsers in pure gases and (2) the formation mechanism of plasma produced by CO_2 laser in atmospheric air and (3) processes during laser-induced dicharge. For (1), experiments were done in different kind of ambient gas (air, N_2, O_2 of 100-1000mb in pressure) under an uniform DC electric field using Rogowski electrodes. Through electrical and optical measurements, it was found that oxygen molecules play an important role in characteristics of a laser-induced discharge. For (2) and (3), flashover induction experiments were perfomed for two types of long-gap electrode configurations, plate to rod and plate to plate, with a 50 J TEA CO_2 laser. From the flashover voltage characteristics and interferometric optical measurements on laser-induced plasma, it was found that the delay time for minimum V_<50> defined as 50% flashover voltage, measured from a laser incidence exists in the range of 50 to 200 mus, in laser-induced discharge of plate to rod, the value of V_<50> for negatively biased on the plate are larger than the case of positively biased, and laser-induced discharge does not occur on the surface of shock wave but relates on the low density region in the plasma produced by the laser.