Diagnostics of nonuniform plasmas for elemental analysis via laser-induced breakdown spectroscopy: demonstration on carbon-based materials

Abstract

International audienceWe 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

Quantitative analysis by laser-induced breakdown spectroscopy based on generalized curves of growth

“…A procedure, described in Section 4, is used to eliminate from the Cσ graph the data for which (τ l Δλ L ) z exceeds the model limit previously established. The use of a quantitative model limit that allows discarding data in the analytical procedure is also a distinctive feature of our method from previous approaches [3,4,[6][7][8][9]. Following the idea expressed previously, it should be stressed again that, for elements with strongly different ionization energies, a different value of the model limit or an additional criterion might be required.…”

“…In a recent work [8], the same group has performed elemental analysis of glass, including oxygen, and determined all the main constituents of the samples, achieving a measurement precision of about 10%. Also, in [9], they have demonstrated the method on carbon-based materials from the inner wall of a fusion reactor. In [6][7][8], a model of uniform plasma is considered, whereas a plasma composed of two different uniform zones is used in [9] to account for the observed temperature gradient.…”

“…Also, in [9], they have demonstrated the method on carbon-based materials from the inner wall of a fusion reactor. In [6][7][8], a model of uniform plasma is considered, whereas a plasma composed of two different uniform zones is used in [9] to account for the observed temperature gradient. Thus, setting a suitable plasma model is probably the most relevant and difficult issue in standardless methods.…”

Laser induced breakdown spectroscopy application in joint European torus

“…Our laboratory LIBS experiments with different DL were also aimed to study how quantitative data from LIBS signal and its evolution may be extracted. The studies were made in CEA, France within the frames of EFDA programs and ITER project [5][6][7][8][9][10][11]. Feasibility of LIBS remote measurements was demonstrated with nanosecond Nd-YAG lasers (at 5-10 m distance, under a reduced pressure) with the samples from the European tokamaks (TORE SUPRA, France and TEXTOR, Germany) and with ITER-like samples.…”

“…To determine the optimal regime of our specific JET in situ measurements, preliminary laboratory LIBS studies (on laser/target interaction regime, collection optics, optical spectrometer features, ICCD-camera and detection regime, analytical wavelengths and spectral line intensities) were made with the tiles from TEXTOR (Jülich, Germany) and TORE Supra (Cadarache, France) tokamaks and with ITER-like samples [5][6][7][8][9][10][11]. The laboratory LIBS studies were made with the installation developed earlier in CEA for the Mars surface analysis (the NASA Mars Rover Curiosity mission, within the ChemCam project [29][30][31]).…”