Dynamics of periodic structures in an extended laser spark

The time scans of the intrinsic and scattered heating radiations of plasma foci, arising in the spark channel of the optical gas breakdown in a Bessel beam, were investigated. The experiments were carried out in argon at atmospheric pressure by using the heating radiation from a powerful nanosecond neodymium laser. The influence of independent preionisation and of the presence of aerosol particles was studied separately. The results indicate the appearance of bright foci against the background of the initially almost homogeneous plasma. The location of the foci in the longitudinal direction and the dynamics of the emitted radiation by them make it possible to assign the observed inhomogeneities to two quasi-periodic structures with periods differing almost by an order of magnitude. The above structures result from the operation of various mechanisms of the formation of inhomogeneities.