Negative discharge characteristics in a long air gap triggered by femtosecond laser filaments

Abstract

Femtosecond laser filamentation is a promising approach for active lightning protection, laser-triggered switches, and high-voltage gas discharges. For negative-polarity lightning, which dominates natural cloud-to-ground lightning, the strong stochasticity of negative-discharge inception and propagation remains a major obstacle to validating this approach. In this study, negative discharges in long air gaps were generated using a 4.8 MV impulse-voltage source, and the effects of gap geometry, laser peak power, and filament focal position were systematically investigated. The filament lowered the inception-voltage threshold of the large-curvature electrode (r = 200 mm) by more than 70% and narrowed its distribution. The observed voltage reduction supports an effective “virtual-tip” interpretation. Increasing Ppeak reduced the breakdown voltage and increased the guided discharge length and discharge probability. At 0.8 and 1.0 TW, the guided discharge lengths and discharge probabilities were comparable, indicating reduced sensitivity to laser peak power in the upper part of the tested range. The reduction in breakdown voltage also depended on gap distance because the unguided region constituted a larger fraction of longer gaps. Finally, varying the focal position revealed distinct spatial optima for the measured discharge characteristics. A focal position of df-e = 0.3 m yielded the lowest breakdown voltage and the highest discharge probability, whereas df-e = 0.7 m produced the longest guided discharge channel in the 2.5 m gap. These results characterize laser-guided negative discharges across coupled geometrical and laser conditions and provide reference data for active laser-induced lightning protection and laser-triggered switching.