Numerical Investigation of Runaway Electrons during the Breakdown of Homogeneous Electric Field Air Gaps under Nanosecond Pulse Voltage

The development of discharge that starts near the cathode in a homogeneous electric field air gap under nanosecond pulse voltage is studied using a 2.5-D particle-in-cell/Monte Carlo collisional model. The simulation is first performed in the absence of photoionization reactions. It is found that runaway electrons are produced in the head of the streamer when the local electric field strength is sufficiently high. Runaway electrons ionize the area in front of the streamer tip while producing abundant preionization electrons, which significantly accelerate the rate of discharge propagation. Photoelectrons and runaway electrons perform an analogous role in preionizing gas, but the latter is more efficient because of their directionality. A more complete discharge process is simulated in the presence of photoionization reactions. Electrons with relatively high energy appear in the bipolar end of the discharge channel while the discharge continuously progresses toward the anode. During the later moment of discharge, the streamer expands to the anode at an extremely fast velocity as a result of the combined action of photoionization and runaway electrons located at the negative streamer tip. The simulation results reveal the discharge mechanisms of the air gap applied with a homogeneous electric field under a nanosecond voltage pulse and provide a comprehensive understanding of the fast breakdown of air gaps.