Tomography of homogenized laser-induced plasma by Radon transform technique

Tomography of a laser-induced plasma in air is performed by inverse Radon transform of angle-resolved plasma images. Plasmas were induced by single laser pulses (SP), double pulses (DP) in collinear geometry, and by a combination of single laser pulses with pulsed arc discharges (SP-AD). Images of plasmas on metallurgical steel slags were taken at delay times suitable for calibration-free laser-induced breakdown spectroscopy (CF-LIBS). Delays ranged from few microseconds for SP and DP up to tens of microseconds for SP-AD excitation. The white-light and the spectrally resolved emissivity ε(x,y,z) was reconstructed for the three plasma excitation schemes. The electron number density N_e(x,y,z) and plasma temperature T_e(x,y,z) were determined from Mg and Mn emission lines in reconstructed spectra employing the Saha-Boltzmann plot method. The SP plasma revealed strongly inhomogeneous emissivity and plasma temperature. Re-excitation of plasma by a second laser pulse (DP) and by an arc discharge (SP-AD) homogenized the plasma and reduced the spatial variation of ε and T_e. The homogenization of a plasma is a promising approach to increase the accuracy of calibration-free LIBS analysis of complex materials. - Highlights: • Tomography of laser-induced plasma in air is performed by inverse Radon transform. • Spatial emissivity ε, electron density N_e, and temperature T_e are reconstructed. • ε(x,y,z) and T_e(x,y,z) of plasma induced by single laser pulses are inhomogeneous. • Re-excitation by 2nd laser pulse and pulsed arc discharge homogenizes the plasma.