Observation of a suspected positive space leader in long air gaps

摘要

In order to get more details about the leader propagation in long positive sparks, experiments were conducted in a rod-plane air gap with a length of 74 cm. The low-density area was imaged using a Schlieren system with spatial and temporal resolutions of 70 μm/pixel and 5 μs. Moreover, the discharge current and applied switching impulse were recorded synchronously. A suspected space leader is observed for the first time. By analyzing typical experimental results, it is assumed that there may be four stages included in the incepting and developing processes of a positive space stem and a leader: (1) the first corona streamers is initiated around the electrode tip; (2) a bipolar system or a pilot is formed; (3) the space stem is transited to a space leader; (4) electrode leader and the space leader are connected to each other. Comparing the phenomenon in positive discharge with that in negative discharges, it can be found that: (1) the propagation sequence of the bipolar system in both polarities discharges is very similar; (2) the size of the bipolar system in positive discharges is smaller.