Nature of the size effect in laser breakdown of condensed media

An experimental investigation was made of the nature of the dependence of the laser breakdown threshold of condensed media on the size of the interaction region in the case when this does not result from the statistical nature of the breakdown. A mutual influence of two spatially separated laser beams was discovered for the first time. This consisted of a lowering of the breakdown threshold on reduction in the separation of the beams. This effect was of the same nature as the size dependence and was interpreted as a manifestation of self-defocusing. A method for direct observation of self-defocusing was developed and experiments were performed on ethyl alcohol at prebreakdown intensities.