Experimental characterization of double pulse laser- induced plasmas on Aluminum and Tungsten targets

International audienceLaser-induced plasmas can be used to determine the multi-elemental composition of solid, liquid or gaseoussamples. Using a pulsed laser beam focused on the sample, the resulting absorption leads to temperature increase of several 10,000 K [1], therefore to a neutral phase to plasma transition. During the relaxation phase over which the recombination takes place, a continuum then lines radiation is emitted. Its analysis based on Optical Emission Spectroscopy (OES) may lead to the composition of the plasma, therefore to that of the sample if fundamental assumptions are fulfilled [2]. The most restrictive one is the achievement of the Local Thermodynamic Equilibrium (LTE). Indeed, since only OES is used, the excited states number density is determined. Then, the ground states number density is obtained if LTE is observed and if electron temperature T_e is known, the electron density n_e being enough high. The characterization of LTE is therefore one of the key points of this analysis method, also known as Laser-Induced Breakdown Spectroscopy (LIBS) [3].Using only one pulse may limit the capacity of LIBS to perform the complete analysis of the target. We think about the particular cases of low elemental mole fraction in the sample or species difficult to excite for instance. Using a second pulse may lead to significant improvements in terms of limit of detection. It consists in irradiating the plasma produced by a first laser pulse by a second one, whose characteristics (wavelength, duration, energy) may be different from the first laser pulse. The delay D_t between the two pulses is a key parameter of the experiment : it directly leads the efficiency of the plasma absorption (by Inverse Bremsstrahlung) of the second pulse. We have performed experiments in such configurations. The first pulse (1064 nm, 30 ps, 14 mJ) is used to produce plasmas on metallic samples (aluminum, tungsten) using a lens of 10 cm in focal length. The second pulse (532 nm, 6 ns, 65 mJ) is focused D_t later with a second lens of 15 cm in focal length on the plasma produced by the ps laser pulse. The beam directions are perpendicular to each other in order to avoid ablating the target with the second pulse. The characterization enables the time evolution of the total radiance, n_e and T_e by a thorough spectroscopic study. The experiments described are set to expose the advantages of a double pulse device