Spatial characterization of ns-laser induced Tungsten plasma in air using laser induced breakdown spectroscopy

Abstract

The laser induced plasma (LIP) of tungsten is studied using space-resolved laser induced breakdown spectroscopy (SR-LIBS). In the present work a Q-switched Nd:YAG laser working at wavelength of 532 nm having maximum energy of 600 mJ and pulse duration of 7 ns with repetition rate of 1 Hz is used to generate the plasma onto the target sample surface. The LIP is generated at four different incident laser energy of 25, 50, 75 and 100 mJ. The spectra recorded from the different location of the LIP is analyzed to get the spatial distribution of plasma emission intensity of atomic and ionic transitions, plasma temperature and electron density. The plasma temperature is determined using the Boltzmann plot from the WI lines and that of the electron density estimated using Stark broadened profile of WI transition at 430.2 nm. The intensity of atomic lines as well as plasma parameters attain the maximum value at a distance ranging from 1.6 mm to 2.0 mm away from the target surface for the incident laser energy of 25 mJ and 50–100 mJ, respectively. The thermal conduction towards the solid target and radiative cooling of the plasma and conversion of thermal energy into kinetic energy is responsible for the decrease in plasma temperature towards the plasma edge away from the target surface whereas the direct ejection of low velocity neutrals around the target surface is responsible for lower temperature. Both these plasma parameters are also studied at incident laser energy of 25, 50, 75 and 100 mJ. The validity of the local thermodynamic equilibrium (LTE) is studied by Mc-Whirter criteria and diffusion length is estimated at different location of the plasma to understand the effect of inhomogeneous nature of LIP on the validity of LTE. The branching ratio method is employed to study optical thin condition of the LIP at different axial location as function of energy. From this an optimized spatial window is identified for the LIBS studies.