Diagnostics of nonuniform plasmas for elemental analysis via laser-induced breakdown spectroscopy: demonstration on carbon-based materials
Résumé
We investigate the plasma produced by the interaction of ultraviolet nanosecond laser pulses with a carbon fiber composite tile from the inner wall of a fusion reactor. The experiments are carried out in argon at a pressure of 5 x 10(4) Pa. Fast imaging is used to characterize the plume expansion dynamics and excited plasma species are followed by time-and space-resolved emission spectroscopy. The measurements show that ionized and highly excited plasma species are located in the front of the expanding plume, whereas neutral atoms and lower excited species dominate in the plasma core. By measuring the excitation temperature of metallic ions and neutral atoms, we evidence the existence of a temperature gradient that appears during the early expansion stage and remains up to delays typically used in material analysis via laser-induced breakdown spectroscopy. The knowledge of the spatial distribution of the plasma properties is then used to model the plasma emission spectrum. Assuming two plasma zones in local thermodynamic equilibrium of different temperatures and electron densities, we calculate the spectral radiance and compare it to the spatially integrated spectrum recorded with an Echelle spectrometer. From the best agreement between measured and computed spectra we deduce the elemental concentrations of carbon, hydrogen and metal impurities. The validity of the model is critically discussed and the measurement uncertainties are evaluated. The present approach is foreseen to improve the accuracy of analysis via laser-induced breakdown spectroscopy in many applications, in particular when materials of elements with significantly different ionization potentials are investigated.