No observations show evidence of ionization in long air gap discharge during dark periods. However, for the first time, weak channel illuminations are observed during the so‐called dark periods in this paper. The length of these faint bright channels is almost constant during each dark period and is nearly equal to that of the remaining low‐density channels created by the previous discharge. The phenomenon, that is, the dark period is not really dark, is more likely to occur at the time when there is a greater change rate of the applied voltage. These findings are based on the high‐speed direct imaging and Schlieren photography together, in which the former is used to visualize the bright channels and the latter to detect the low‐density area.
We have performed a comparative study on the luminosity of 12 steps in an upward positive leader and eight steps in an upward negative leader recorded under similar conditions. Through analyzing their so‐called differential luminosity profiles along leader channels, we found that the steps of positive leaders can be classified into two types. The type I step luminosity is similar to the luminosity produced by a negative leader step and is generally featured with a rapid increase near the leader tip and accompanied by a backward propagation along the leader channel. The type II step is featured with continuous luminosity variation along the leader channel. No apparent backward propagation has been identified in the type II step luminosity of the positive leader. Based on these observations, we have discussed how the two types of steps of positive leaders are produced.
Characteristics of the streamer stem are essential for the study on the stable leader inception. Experiments are conducted to observe the re-illumination of streamer stems under either rising or non-changing positive electric fields in long air gaps, preceding the inception of a stable leader. Based on the simultaneously measured applied voltage, discharge current, direct images, and schlieren images, two typical types of stem re-illuminations are identified and show great differences. For the stem re-illumination under the rising electric field, it is accompanied by an abrupt elongation of stem channel, intense optical radiation, and a current superposed on at least two asynchronous current pulses, while for that subjected to the nearly non-changing electric field, there are no elongation of stem channel, a much weaker light emission, and a single, double exponential current pulse. Moreover, the latter case shows obvious periodicity. A comparative analysis of experimental results shows that factors triggering these two types of stem re-illuminations may be also different in which not only the electric field but also its time derivatives play an important role. These findings can further deepen the understanding of the streamer stem before the leader inception.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.