Characterization of laser-induced plasma during its expansion in air by optical emission spectroscopy: Observation of strong explosion self-similar behavior

We present measurements of the temporal evolution of temperature and electron density in laser-induced plasmas generated in air and on a glass sample at atmospheric pressure. The measurements are made over a wide range of time, from 35 ns to 6 μs, in order to study the evolution of the plasma. The procedure for selecting the lines suitable for characterization at each stage, based on the evolution of the densities of the ionization states, is discussed. At the initial stage of fast plasma expansion, the temperature and electron density evolve as t− 6/5, a result compatible with the self-similar theory for strong explosions in homogeneous atmosphere, according to which the plasma is bounded by a Sedov-Taylor shock wave propagating in the atmosphere. From the experimental results, it is deduced that a change of trend of the cooling mechanism of the plasma from expansion to radiation takes place when the plasma ends its expansion as a shock wave (~ 0.5 μs). The parameters of the plasmas generated in air and on the solid sample are compared.