Experimental verification of a radiative model of laser-induced plasma expanding into vacuum

A radiation dynamic model of the postbreakdown stage of laser-induced plasma solves a twofold task: first, the direct problem, it yields an analytical expression for the plasma radiation dynamics under arbitrarily chosen initial conditions allowing the computation of synthetic spectra; second, the inverse problem, it allows finding of the initial conditions by a direct comparison of calculated synthetic spectra with experimentally measured ones. In this work, we carry out experimental verification of the model, thus dealing with the inverse problem. We vary the initial parameters of the model (plasma initial temperature and the initial concentrations of species) until a close fit between the synthetic and the experimental spectrum is obtained. Some of the model inputs (e.g., the initial radius of the plasma) are measured and introduced into the model as fixed constants. Calculations and measurements are performed on a binary SiC system; on a series of multicomponent aluminum samples doped with Si, Mg, Cu, Zn, and Fe; and on pure iron, silicon, and carbon. From two to six elements and up to 500 spectral lines were involved in the calculations. The Monte Carlo optimization (the simulated annealing method) is used for finding initial plasma temperature and number densities. A reasonably good agreement is obtained between the computed and the experimental spectra. This approach can be considered as a valuable step towards the achievement of absolute analysis.