On a Possible Mechanism of Space Stem Formation in Negative Long Sparks

To study the mechanism of space stem formation, the high‐speed direct imaging technique and Schlieren photography are used to simultaneously observe the space stems in a 1.35‐m air gap under the lightning impulse voltage. It is found that the average length and diameter of the space stem is approximately 5 and 0.5 mm, which is about 1/3 of the length and 1/10 of the diameter of the local luminous region respectively. Moreover, the space stem exhibits a non‐uniform distributed temperature with a maximum value of several hundred Kelvin for tens of microseconds. Accordingly, a possible mechanism is proposed that the space stem is formed at the primary corona streamer boundary as a result of the propagation of a secondary ionization wave from the HV electrode and survives long enough (to finish the polarization) due to the fast gas heating and production of atomic oxygen. Plain Language Summary It is well known that negative leaders in the air develop in a step‐wise manner, with each step originating from a space stem. However, the mechanism of space stem formation is still unknown, one of the main reasons being the lack of experimental observations. For this reason, this paper carries out laboratory research to observe the optical and thermal properties of space stems. The observations reveal that the local luminous region at the primary corona streamer boundary is not fully heated into the space stem, but only about 1/3 of its length. If based on conventional direct images, the measured length and diameter of space stems would be 2.5 and 10 times larger than the actual values, respectively. The reconstructed temperature field shows that the axial temperature and diameter of the space stem are not uniformly distributed, with higher temperatures and smaller diameters at the end away from the HV electrode. Finally, a new possible mechanism is proposed for the space stem formation, in which the secondary ionization wave from the HV electrode contributes to the initial formation of the space stem, and then the voltage changes help the space stem survive long enough to finish the polarization. Key Points Only a section of the local luminous region at the primary corona streamer boundary is heated to form a space stem, but not all of it The temperature field of a space stem is reconstructed for the first time and presents a non‐uniform distribution A secondary ionization wave may contribute to the space stem formation at the primary corona streamer burst boundary