Breakdown of atmospheric air by neodymium laser radiation forming large-diameter focusing spots

An investigation was made of the threshold power density w_th needed for breakdown in the case of largediameter focusing spots. It was found that w_th decreased from 70 to 2 GW/cm² when d increased from 0.2 to 2.0 mm, which was in conflict with the avalanche theory of breakdown, according to which w_th should be independent of d in the range d > 100 μ. Estimates indicated that this reduction in w_th could be explained by the influence of ion–molecule reactions.