Generation of periodic inhomogeneities in optical breakdown by a Bessel laser beam

A model of the appearance of periodic structures in the plasma channel of an extended laser spark is considered. It is shown that sideways delivery of the heating radiation by a Bessel beam to the breakdown region and expansion of the heated plasma result in inhomogeneous refraction and, consequently, give rise to structures with a longitudinal period ~ λ/γ² (λ is the radiation wavelength and γ is the angle of inclination of the beam to the axis). This model is used to account for the results of experiments demonstrating that such structures are not observed in the case of a low-density gas (at pressures of 0.05 bar or less) and short (~ 0.1 ns) heating pulses.