Abstract:
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.