Optical breakdown of helium in Bessel laser radiation beams

Numerical simulation is used to investigate the dynamics of formation of a helium plasma in Bessel beams, shaped by an axicon and a phase converter fr om a laser radiation pulse with Gaussian temporal and radial intensity prof iles. The beam intensities at the breakdown threshold are determined asa fu nction of the pulse duration for various radial field distributions in a be am characterised by Bessel functions of order m (m = 0-5). It is shown that in the investigated range of parameters the threshold intensity is indepen dent of m. The temporal and spatial evolution of the resultant plasma, and the dependence of the plasma characteristics on the pulse parameters are co nsidered; Conditions are found for the formation of tabular plasma channels in beams of orders m greater than or equal to 1. The adopted model of the optical breakdown of helium is shown to be satisfactory because of a good a greement between the results of calculations of the moment of breakdown in a zeroth-order Bessel beam and experimental results.