Elongation And Branching Characteristics Of Stems In Long Air Gap Discharges

To understand the natural lightning,the physical mechanisms of the long air gap discharges is essential,where the process of streamer-leader is a key problem to characterize the leader initiation and development.It is extremely difficult to directly observe the characteristics of the stem,which leads to the lack of visualization results of the stem.At present,the research results mainly focus on numerical models.The insufficiency of the experimental results also leads to the failure to demonstrate the related characteristics of the stem in the model,which limits the further study of the process of streamer-leader.Therefore,based on the analysis of visualization conditions and methods,the stem of the second streamer burst was identified as the equivalent observation object,and the synchronous comprehensive observation platform was built to study the morphological characteristics of the stem.Before the initiation of the leader,the formation process of the stem is similar to that of the leader head.A large number of electrons are generated by the streamer into a common root to form the stem.To eliminate the effect of the streamer and leader luminosity,and increase the survival time of the stem,the characteristics of the stem were studied by combining the Schlieren technique with the first corona stem based on the discharge theory and indirect observation method.However,the first corona stem does not start on the basis of background ionization,and the equivalence with the leader head is not sufficient,while the third or forth corona stem is easy to exceed the shooting range.Therefore,the stem of the second streamer burst was selected as the equivalent object in this thesis.This new method was proposed for observing the stem directly,which makes up for the weakness of the time resolution,spatial resolution,and system sensitivity in the existing observation system.At the same time,using quantitative Schlieren technology as the observation method,the parameters of the optical lens,the auxiliary light source and the focal length were designed properly.The Schlieren images,traditional images,voltage,and current can be measured synchronously in combination with the current measurement system with the time resolution of 2 ns.Based on Schlieren image,it is found that the step size of the positive leader is not constant.The effects of the rising rate of the voltage and injected charge on the elongation were obtained.The charge required for elongation per unit length of the first streamer burst is compared with that of the second streamer burst.It is pointed out that the remaining active particles in the streamer zone are the important factors to affect the heating efficiency of the stem.At the same time,it is found that the stem is a treelike structure,and the characteristics of the main channel and branching channel were quite different.The value of the reduced electric field E/N becomes an important parameter to determine the development path of the subsequent channel.According to the equivalence of the leader head and the stem of the second streamer burst,it is suggested that the local concentration of the current of the streamer is the mechanism that causes the streamer to the leader.Based on this mechanism,it is preliminarily considered that the stem branch probably be one of the mechanisms affecting the subsequent leader branching.The study in this thesis is of great significance to the understanding of the morphological growth characteristics and mechanism of the stem.