Nonlinear propagation of a Bessel light beam of near-breakdown intensity in water

The shadow technique was used to reveal a change in the refractive index of water during propagation of a powerful Bessel light beam (λ = 1.06 μm). The investigation was carried out on a beam 12 cm long with the diameter of the caustic 10 μm. The heating radiation power was 2.5 — 250 MW. The axial region near the acoustic, 1 cm in diameter, was investigated. The region was probed parallel to the heating beam axis. When the heating radiation power exceeded 40 MW, the shadow photographs showed characteristic inhomogeneities of dimensions which increased with the power. These inhomogeneities were a system of fringes and they were attributed to diffraction at the boundary between illuminated and unilluminated regions of water. The refractive index step at this boundary was estimated to be –3 × 10^–6 when the electric field of the heating radiation was 10⁵ V cm^–1.