Energetic electrons and their contribution to the breakdown of a point-plane air gap with a positive nanosecond pulse

It is found that the polarity reversal phenomenon occurs in the nanosecond pulse breakdown experiment. Two-dimensional axisymmetric particle-in-cell/Monte Carlo collisions’ calculation is used to observe energetic electrons at positive nanosecond pulse voltage in atmospheric air and compared with previous calculation results under negative polarity in an attempt to explain the polarity reversal phenomenon. During the evolution of the positive polarity discharge, the difference in spatial net charge distribution leads to distortion of the electric field, which makes the electric field in the area of the ionization channel head very high, exceeding the threshold at which electrons can enter the high-energy state. The simulation results show that although electrons travel in the opposite direction to the ionization channel, energetic electrons can also be generated during the positive polarity discharge’s evolution, which may differ from what some researchers expected. However, it is also found that the maximum energy of energetic electrons under positive nanosecond pulse voltage is lower than that under negative nanosecond pulse voltage (only about 1/4). This may be mainly because in the case of positive polarity, the energetic electrons in the head of the ionization channel will move to the low-field intensity region inside the ionization channel and cannot be accelerated continuously. However, it must be pointed out that in the case of positive polarity, energetic electrons still contribute significantly to the rapid breakdown of the air gap. This study provides a deeper understanding of the physics of nanosecond pulse discharge.