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

本研究的目的是通过了解（1）纯气体中准分子激光的电离机制;（2）大气中CO_2激光产生等离子体的形成机制;（3）激光诱导放电过程，寻找激光诱导闪电的成功条件。对于（1），在不同的环境气体（空气，N_2，O_2，压力100- 1000 mb）中，在均匀直流电场下，用Rogowski电极进行了实验。通过电学和光学测量，发现氧分子对激光诱导放电的特性起着重要的作用。对于（2）和（3），用50 JTEA CO_2激光器对两种长间隙电极结构（板-棒和板-板）进行了闪络感应实验。从激光诱导等离子体的闪络电压特性和干涉光学测量中发现，在激光入射下测量到的最小闪络电压V_（<50>定义为50%闪络电压）的延迟时间在50 ~ 200 μ s之间，在激光诱导板棒放电中，负偏压时的V_（定义为50%闪络电压）值<50>大于正偏压时的V_（定义为50%闪络电压）值，激光诱导放电不是发生在冲击波表面，而是发生在激光产生的等离子体中的低密度区域。

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.