Self-focusing of a laser beam as a whole in air

A study was made of the time dependence of the diameter of a neodymium laser beam in its focal plane and of the intensity distribution when it was focused in laboratory air. Self-focusing of the laser beam as a whole was observed. The collapse of the beam occurred in 3–8 nsec and it resulted in breakdown of air. Estimates deduced from the experimental results indicated that the critical self-focusing power was ~10⁴ W and the nonlinear coefficient was n₂~4×10^–12 (in absolute units).