Modeling and experimental verification of plasmas induced by high-power nanosecond laser-aluminum interactions in air B. Wu, Y. C. Shin, H. Pakhal, N. M. Laurendeau, and R. P. Lucht School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907, USA Received 5 April 2007; published 31 August 2007 It has been generally believed in literature that in nanosecond laser ablation, the condensed substrate phase contributes mass to the plasma plume through surface evaporation across the sharp interface between the condensed phase and the vapor or plasma phase. However, this will not be true when laser intensity is sufficiently high. In this case, the target temperature can be greater than the critical temperature, so that the sharp interface between the condensed and gaseous phases disappears and is smeared into a macroscopic transition layer. The substrate should contribute mass to the plasma region mainly through hydrodynamic expansion instead of surface evaporation. Based on this physical mechanism, a numerical model has been developed by solving the one-dimensional hydrodynamic equations over the entire physical domain supple- mented by wide-range equations of state. It has been found that model predictions have good agreements with experimental measurement for plasma front location, temperature, and electron number density. This has provided further evidence at least in the indirect sense, besides the above theoretical analysis, that for nanosecond laser metal ablation in air at sufficiently high intensity, the dominant physical mechanism for mass transfer from the condensed phase to the plasma plume is hydrodynamic expansion instead of surface evapo- ration. The developed and verified numerical model provides useful means for the investigation of nanosecond laser-induced plasma at high intensities. DOI: 10.1103/PhysRevE.76.026405 PACS numbers: 52.38.Mf, 52.50.Jm I. INTRODUCTION Nanosecond pulsed-laser ablation of materials has been investigated for many years because of applications in mate- rial removal from solid parts or thin-film deposition 1. Un- derstanding the fundamental aspects of pulsed laser-matter interactions has also generated considerable interest in the research community. The irradiation of a target with a laser pulse having sufficiently high intensity produces a high- temperature plasma. The laser-induced plasma becomes an important part of the laser ablation process. Therefore, to understand laser-matter interactions, investigating the dy- namics of the plasma plume is essential. During nanosecond-pulsed laser ablation, if the target sur- face temperature does not closely approach or exceed the thermodynamic critical temperature that is, when the laser fluence is below a certain threshold, the process is charac- terized by the formation of a sharp liquid-vapor interface 2. All vapor molecules leaving the liquid-vapor interface during laser evaporation have velocity components in the direction away from the surface at the interface and develop an equi- librium velocity distribution within several mean free paths. The thin region adjacent to the interface, where the normal velocity of vapor molecules transforms from a nonequilib- rium to an equilibrium distribution is called the Knudsen Layer KL2. The vapor flow above KL can be considered to be a gas dynamic flow satisfying the continuum approxi- mation. In the numerical modeling of laser evaporation, KL is treated as a discontinuity and provides boundary condi- tions that couple the heat transfer equation for the target with gas dynamic equations for the vapor and ambient gas phase 2–6. When the laser intensity is sufficiently high, the surface evaporation process as discussed above should not be the dominant physical mechanism for mass transfer from the condensed phase to the gaseous phase. In this case, the target temperature can be greater than the critical temperature, so that the sharp interface between the condensed and gaseous phases disappears and is smeared into a macroscopic transi- tion layer 7. Under such conditions, the condensed phase should contribute mass to the plasma region mainly through hydrodynamic expansion, and the laser ablation process should be described by solving the hydrodynamic equations for the whole physical domain, supplemented with wide- range equations of state 7. The experimental investigation of laser ablation of aluminum in Ref. 8 indicates that the aluminum target surface temperature is already close to and may be even higher than the critical temperature when the laser irradiance approaches 0.5 GW/ cm 2 for laser-pulse du- ration in the order of around 10 ns. Therefore, for nanosec- ond pulses with irradiances of several GW/ cm 2 , the plasma induced by laser ablation of aluminum should be described through the hydrodynamic equations for the whole physical domain, where the condensed phase contributes mass to the plasma region through hydrodynamic expansion. Unfortu- nately, the physical mechanism discussed above, although it seems to be true based on theoretical analysis, has rarely been rigorously verified through experimental and numerical work in literature for common metals such as aluminum. This paper provides a confirmation of such a mechanism through both modeling and experiments. In this paper, a numerical model is developed for the plasma induced by the interaction of high-power nanosecond laser pulses with aluminum in air. The one-dimensional 1D hydrodynamic equations are solved numerically for the whole involved physical domain supplemented by wide- range equations of state EOS. Experimental measurements are also performed on the laser-induced plasma. Although much previous experimental PHYSICAL REVIEW E 76, 026405 2007 1539-3755/2007/762/0264058 ©2007 The American Physical Society 026405-1