BRIEF COMMUNICATIONS: Breakdown of atmospheric air by neodymium laser radiation forming large-diameter focusing spots
An investigation was made of the threshold power density wth needed for breakdown in the case of largediameter focusing spots. It was found that wth 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 wth should be independent of d in the range d > 100 μ. Estimates indicated that this reduction in wth could be explained by the influence of ion–molecule reactions.