Contribution of Femtosecond Laser Filaments to Positive and Negative Breakdown Discharge in 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 for the laser-guided long air-gap discharge technique. This study presented a direct comparison of the inception, propagation and breakdown characteristics of both polarity discharges. Long air-gap discharge experiments under the switching impulse of both polarities were carried out under the same experimental conditions. Discharge modes and phases were also considered. Statistical results showed that positive discharge inception voltages were transformed to exponential distribution from Weibull distribution under the influence of laser filaments, but it had little effect on the negative discharge inception voltage. The guidance probability of a positive discharge reached at most 15% during the dark period stage, leading to little effect on the breakdown discharge probability, whereas, for negative discharges, the guidance probability could exceed 95%. Discussion of the filament contribution to both polarity discharges found that the different migration directions of photoelectrons led to a difference in the effects of laser filaments on inception voltages, and the difference in the connection of the two discharge passages led to a difference in the guidance probability. Through a simulation model, it was speculated that the connection condition for positive discharges was that the positive leader superimposed the laser filaments, and, for negative discharges, the rod electrode was connected to the laser filaments through a bi-directional discharge propagation.