ESCAMPIG XXI, Viana do Castelo, Portugal, July 10-14 2012 Topic number: 12 Determination of electron parameters in nascent aluminum laser- induced plasma by means of Thomson scattering Vincent Morel (*) , Arnaud Bultel UMR CNRS 6614 CORIA, Université de Rouen, 76801 Saint-Etienne du Rouvray, FRANCE (*) morel@coria.fr In the purpose of determining the electron density and temperature of a laser-induced plasma, a Thomson scattering experiment is performed. A 5 ns laser pulse at 532 nm is focused on an aluminum target. The resulting plasma is probed using a 5 ns laser pulse at 355 nm with an angle of 90° with respect to the pump laser direction. The Thomson scattering results are compared with the ones obtained by the study of the continuum radiation or spectral lines with known Stark parameters. The first results show that the electron temperature and density reach 15000 K and 10 25 m -3 respectively, at time t = 20 ns after the maximum of the pump laser pulse. Large intentions are dedicated to the laser-surface interaction since the development of short laser technology. Many research works have shown that the predominant parameters are the laser fluence F and the pulse time . The plasma results from the absorption of the laser light by the ejected matter. This so-formed plasma radiates lines which lead through emission spectroscopy analysis to the identification of the material elemental composition: the derived diagnostic method is called LIBS (Laser Induced Breakdown Spectroscopy). This analysis is based on the Local Thermodynamic Equilibrium (LTE) assumption. During the last years, different calibration free procedures have been elaborated [1] which illustrates the improvement of the LIBS technique. However, the Local Thermodynamic Equilibrium (LTE) seems to be systematically assumed in the purpose of deriving the elementary composition of the sample [2]. This hypothesis is still largely discussed [3] and conditions to obtain LTE have to be clearly understood to estimate its validity. The situation studied here concerns an interesting case of irreversible evolution and illustration of the second law of thermodynamics. The initial conditions at the moment of the laser pulse – sample interaction correspond indeed to a strong non-equilibrium between matter and radiation. The subsequent evolution leads the system from this initial situation until the final equilibrium according an irreversible evolution driven by radiative and collisional elementary processes. At each time of this evolution, the system presents a departure from equilibrium decreasing with time. The whole duration of the transient state until the final equilibrium depends strongly on the initial conditions. In this context, these initial conditions have to be perfectly identified. One of the main objectives of the work developed in our lab is to study thoroughly the early phase of the plasma, in other words the few nanoseconds corresponding to the laser pulse, when the light interacts with the sample. The aim of the study is the experimental characterization of the plasma just during and after its formation. Thomson scattering (TS) is a powerful and well known technique suitable for measuring the electron temperature T e and density n e when these parameters are sufficiently high. The estimate of these parameters has been obtained using other methods mainly based on emission spectroscopy. The continuum, ascribed to thermal Bremsstrahlung and radiative recombination, has a spectral radiance compatible with strong values of n e and T e . TS was therefore chosen to confirm the values thus obtained.