Observation of isolated heating segments ahead of the discharge channel under positive and negative lightning impulses

Space stems are luminous and heated spots that appear ahead of the main discharge channel. They are very common in long negative discharges, while are rarely observed in long positive discharges. Recent numerical simulations show that isolated space stems in positive discharge may exist, but these are just too tiny to be observed. This paper attempts to demonstrate the above inference through an experimental study in the laboratory. A Schlieren system with a spatial resolution of 180 μm/pixel was established to visualize the isolated heating segment. A series of tests were carried out in a 1.35-m rod-plane air gap under both positive and negative lightning impulse, in which the applied voltage, discharge current, direct images and Schlieren images were synchronously recorded. The occurrence probability, length, position, and direction of the isolated heating segment are analyzed according to the experimental results. Moreover, the effects of the impulse voltage (i.e., polarity, rising time, amplitude) and the electrode size on the characteristics of the isolated heating segment is introduced. At last, we explore the issue of whether the observed isolated heating segment is the space stem. Observations in this paper may provide some references to better understanding the physical mechanism of space stem formation.