Topic/Type: 1.5 Low-temperature, dusty and nano-plasmas, Oral
S. H. Tavassoli, S. Mehrabian, M. Aghaei
Laser and Plasma Research Institute, Shahid Beheshti University G. C., Evin, Tehran, Iran
Effect of pulse energy on laser ablation of Aluminum target followed by plume expansion in 1 atm He and Ar background gas has been numerically investigated. A one dimensional, two-stage computational model, including generation and expansion of plume is considered. In the first stage, the laser pulse is absorbed in a thin boundary layer of the target. Absorbed energy is dissipated inside the target by heat conduction mechanism and results in heating, melting and evaporation of the sample. Evaporated matter is used as input for the next stage. In the second stage, the gas dynamic expansion is investigated by solving the hydrodynamic equations based on Reimann characteristics. The energy and mass balance in the target surface couple heat conduction and hydrodynamic equations in the sample and plume respectively. During the plume expansion, some part of the laser pulse is absorbed by the plume before reaching the target and accordingly, ionization is happen in the plume and plasma is formed in front of the target. Plasma absorbs more energy from the pulse due to the so called plasma shielding. Two mechanism of inverse bremsstrahlung and photoionization of excited atoms are dominant mechanisms of plasma shielding. On the other hand, the plasma loses some of its energy due to the bremsstrahlung and spectral emission. By assumption of local thermodynamic equilibrium (LTE), number density of ionized species, Al+, Al2+ and He+, are derived using Saha-Eggert equations. Heat conduction, hydrodynamic and Saha- Eggert Equations are solved coupledly to determine sample temperature, plume temperature, aluminum and background gas densities including ionized and neutral species, plume pressure and plume velocity as a function of time. In this paper the effects of pulse energy and ambient gas on plasma parameters are studied. Results are applicable in Laser Induced Breakdown Spectroscopy (LIBS) in which plasma emission is used to determine quantitative information on the composition of the sample.