Fundamental Study due to Quantitative Measurement of Light Emission from Induced Discharge in Cloud

本项目的目的是阐明激光产生的等离子体触发云中闪电的各种特性。结果如下：实验采用水蒸气作为等离子体产生的靶。在这种条件下，产生等离子体的激光阈值能量降低了1/3。利用由图像转换相机和CCD相机组成的放电光测量光学图像处理系统，成功地测量了激光等离子体光发射的空间分布及其时间变化。这些实验数据的分析程序是在常用的个人计算机和工程工作站上新编写的。根据分析结果定量评价了膨胀相和衰减相。一方面，对点爆炸产生的冲击波进行了数值分析，阐明了其物理机理；计算结果与实验结果吻合较好。等离子体在激光能量沉积后的相位行为可以用激波传播特性来描述。实验发展到光发射光谱分析。光的绝对强度是在时间和波长的二维空间中测量的。对SF_6空气中的短间隙放电进行了标定实验，验证了该测量系统的有效性。将测量结果应用于激光产生的等离子体，并对其密度和温度进行了估计。估计值与其他小组的报告相似。进一步的实验，即弱电场中感应电流的测量，已经开始了。通过对项目结果的综合分析，将获得更详细、更可靠的知识。

The purpose of this project is to clarify various characteristics of laser produced plasma to trigger lightning in cloud. The results are as follows.The experiment is performed by using water vapor as a target for the plasma production. The threshold laser energy for plasma production is reduced by 1/3 in this conditon. Using the optical image processing system for discharge light measurements, which consists of an image converter camera and a CCD camera, the spatial distributions of light emission from laser produced plasma and their temporal developments are successfully measured. The analyzing programs for these experimental data are newly coded on the usual personal computers and the engineering work station. The expansion and decay phases are quantitatively evaluated by the result of analysis. On one hand, the numerical analysis, in which the shock wave induced by a point explosion is treated, is performed to clarify the physical mechanism. The calculated results are well agree with the experimental results. The behavior of the plasma in the phase after the laser energy deposition can be described by the characteristics of shock wave propagation.The experiment is developed to spectrum analysis of the light emission. The absolute intensity of light is measured in two dimensional space of time and wavelength. The calibration experiments of the measurement system are carried out to short-gap discharge in air SF_6, which show the effectiveness of the system. The measurement is applied to the laser produced plasma, and its density and temperature are estimated. The estimated values are similar with a report by other groups.The further experiments, the measurement of induced current in the weak electric field, are already started. The more detailed and reliable knowledge will be obtained by the synthetic analysis with the result of this project.