Spatial and temporal scaling and common apparent excitation temperature of laser-induced plasmas generated at constant irradiance with different pulse energies

The characteristics of laser-induced plasmas generated with different pulse energies E in the range of 50–200mJ at constant irradiance (40GW∕cm2) have been investigated. The plasmas have been generated in air at atmospheric pressure from Fe–Ni samples using an infrared Nd:YAG laser. Plasma characterization has been carried out by emission spectroscopy with three types of measurements as follows. (1) Time-resolved, space-integrated measurements have shown that a temporal scaling exists between the curves of temporal evolution of the apparent (population-averaged) temperature and electron density of the plasmas, with scaling factors proportional to E0.4±0.1. (2) Time- and space-integrated measurements have led to a common value of the plasma apparent temperature (7200±200K) for all the pulse energies and to a scaling of the line intensities as E1.6±0.2. The coincidence of apparent temperatures is explained by the hypothesis that laser-induced plasmas generated with different pulse energies at constant irradiance show a space and time scaling. (3) Time- and space-resolved measurements of the plasma temperature have been carried out to verify the scaling properties. A qualitative interpretation of the experimental results is provided supposing that a laser-supported radiation wave is responsible for the absorption of the laser energy during the formation of the plasma.