Location of the space stem in long negative laboratory sparks

It is well known that the negative leader in the air develops in a stepwise manner, with each step originating from a space stem. However, the formation mechanism of a space stem remains unknown, leading to a blockage in the study on negative leader discharge. One of the main reasons is the lack of observations of space stems. For this reason, and as a continuation of our work, this paper focuses on the initiation location of the space stems. We have designed experimental conditions to enable space stem observation using a high-speed camera. Through processing the experimental images, it is found that the location where the space stems start to be formed may be located at the boundary of the previous corona streamer discharge, or inside the pre-corona streamer region, but at the edge of the intense discharge area. These findings may help to further Figure out the formation mechanism of a space stem in long negative sparks in the air.