The contribution of femtosecond laser filaments to positive and negative breakdown discharge in a long air gap

Discharges of different polarities develop in different modes, resulting in different guiding effects by femtosecond laser filaments. Knowledge of the contribution of laser filaments to positive and negative discharges is the basis of the laser-guided long-air-gap discharge technique. This study presents a direct comparison of the inception, propagation, and breakdown characteristics of discharges of both polarities. Long-air-gap discharge experiments under the switching impulses of both polarities are carried out under the same experimental conditions. Discharge modes and phases are also considered. The statistical results show that positive discharge inception voltages are transformed from a Weibull distribution into an exponential distribution under the influence of laser filaments, but there is little effect on the negative discharge inception voltage. The guidance probability of a positive discharge reaches 15% at most during the dark period stage, leading to little effect on the breakdown discharge probability. However, for negative discharges, the guidance probability can exceed 95%. An investigation of the filament contributions to both polarity discharges shows that the different migration directions of photoelectrons lead to a difference in the effects of laser filaments on inception voltages, and the difference in the connection of the two discharge passages leads to a difference in the guidance probability. Through the results of a simulation model, it is speculated that the connection condition for positive discharges is that the positive leader overlaps with the laser filaments, and, for negative discharges, the rod electrode is connected to the laser filaments through bi-directional discharge propagation.