Modeling of laser induced plasma expansion in the presence of non-Maxwellian electrons

Commonly, laser-induced plasma expansion is modeled by assuming local thermodynamic equilibrium conditions for all plasma components. In the present work, we investigate the case of plasma containing electrons obeying a non-Maxwellian distribution. To this purpose, a twofluid model is considered with charge quasi-neutrality assumption. The deduced set of nonlinear differential equations is solved numerically using a self-similar approach. It is found that the deviation of the electrons from Maxwellian distribution gives rise to an increasing of the electrostatic potential that accelerates ion motion. Introduction The present work deals with the problem of laser induced plasma expansion into vacuum in the regime where the expanding plasma consists of hot electrons and warm ions. The expansion is caused by electron pressure and serves as an energy transfer mechanism from electrons to ions. This process is often described under the assumption of Maxwellian electrons, which easily fails in the absence of collisions. Indeed, plasmas, both in the laboratory and in space, are often not in thermodynamic equilibrium, and the plasma electron distribution function (eedf) is accordingly non-Maxwellian. Then, caution must be taken during the temporal acquisition of data specially when one tries to develop a calibration free Laser Induced Breakdown Spectroscopy (LIBS) method [1] in pulsed laser deposition experiments, for instance.The classical approach of this technique and its validity are essentially based on the existence of local thermodynamic equilibrium (LTE) conditions and of optically thin plasma. But, in the conditions of the experiments, as a consequence of fast expansion, the plasma parameters can change in times shorter than those necessary for the establishment of elementary process balances, thus leading to non-equilibrium excitation and chemical processes [2]. Then, electron, ion and atom temperatures can essentially differ. The electron temperature in non-LTE plasmas is higher than that of ions because of insufficient collisions among like particles. So, the knowledge of the deviations from the LTE is really important to understand the limits of theory to be taken into account 36th EPS Conference on Plasma Phys. Sofia, June 29 July 3, 2009 ECA Vol.33E, P-4.046 (2009)