Observation of Spatial and Temporal Evolution of Stem near the Electrode in a 1-m Air Gap

Stem is a special short bright zone for steamer-to-leader transition in long air gap discharges, whose characteristics play an important role in leader inception and development. The corona streamer currents rooted in the stem region increase the thermal energy, resulting in an increase in the gas temperature due to Joule heating. In this work, we report the stem near the electrode in a 1-m air gap under positive impulse observed using a Mach-Zehnder interferometer with a time resolution of several microseconds, which provides a better diagnostic method for stems. The experimental observation shows that usually more than one stem appears near the electrode, which is important for corona streamer currents assignment to stem heating. One stem elongates at a speed of about 104 m/s, while other stems are suppressed. Usually one stem forks to two or more stems, and extends in a straight or curved form.