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 a n extended laser spark is considered. It is shown that sideways delivery of the heating radiation by a Bessel beam to the breakdown region and expansi on of the heated plasma result in inhomogeneous refraction and, consequentl y give rise to structures with a longitudinal period similar to lambda/gamm a(2) (lambda is the radiation wavelength and gamma is the angle of inclinat ion 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 (sim ilar to 0.1 ns) heating pulses.