Investigation of the State of Local Thermodynamic Equilibrium of a Laser-Produced Aluminum Plasma

In this work, the assumption of local thermodynamic equilibrium (LTE) for a laser-induced plasma in ambient air is examined experimentally using two different laser systems, namely an infrared short-pulse Ti:Sapphire laser and an ultraviolet long-pulse XeCl excimer laser. The LTE assumption is investigated by examining the plasma produced at a laser fluence of 10 J/cm2 from aluminum targets containing iron and magnesium impurities. The excitation temperature is deduced from Boltzmann diagrams built from a large number of spatially integrated neutral iron lines distributed from 3.21 to 6.56 eV. It is shown that at any time after the end of the laser pulse, the neutral excited states are in excellent Boltzmann equilibrium. Detailed investigation of Boltzmann equilibrium further validates previous temperature measurements using less accurate diagrams. However, observations of ion lines provide some evidence that the ionized species do not obey Saha equilibrium, thereby indicating departure from LTE. This could be explained by the fact that the plasma cannot be considered as stationary for these species.