Molecular Dynamics Simulations of Laser Ablation in Metals: Parameter Dependence, Extended Models and Double Pulses
Laser ablation has become a very useful tool in machining today. For example for drilling holes, welding, engraving or coating by deposition of laser-irradiated material. The opposite process, laser removal of material is in general called laser ablation
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1 Introduction Laser ablation has become a very useful tool in machining today. For example for drilling holes, welding, engraving or coating by deposition of laser-irradiated material. The opposite process, laser removal of material is in general called laser ablation and some aspects of this process shall be discussed here. If the target of laser ablation are metals, and the pulses applied have durations of a few femtoseconds, then the time evolution of the process can be described in the following way: The laser acts on the free electrons of the metal and excites them. The next step is a thermalization of the electrons which leads to an electron temperature different from the ordinary lattice temperature. Then the electron system and the lattice start to exchange heat which means especially heat conduction into the bulk by the electrons. Up to this point the process is dominated by the quantum nature of the material. The lattice is heated up by the energy obtained from the electrons, it melts, and finally ablation occurs. The latter processes now take place on the scale of several picoseconds and can thus be simulated by classical molecular dynamics simulations. Since we want to simulate large samples with millions to billions of atoms, we cannot use ab-initio-methods to study the quantum effects. Instead of that we apply a continuum model, the so-called two temperature model (TTM) [1, 3, 7], which consists of two coupled heat balance equations formulated for the electrons and lattice as a function of the temperatures mentioned above. The lattice equation will later be replaced by molecular dynamics (MD) simulations which allows us to obtain atomistic information about the ablation process. The combined model is called TTMCMD.
J. Roth () J. Karlin M. Sartison A. Krauß H.-R. -Trebin Institut f¨ur Theoretische und Angewandte Physik, Universit¨at Stuttgart, Stuttgart, Germany e-mail: [email protected] W.E. Nagel et al. (eds.), High Performance Computing in Science and Engineering ’12, DOI 10.1007/978-3-642-33374-3 10, © Springer-Verlag Berlin Heidelberg 2013
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The behavior of a material in classical MD simulations is given by the structure (initial conditions) and the interaction. The question is now how this behavior is driven by the coupling to the electron system whose quantum nature is described by the continuum model. In the case of metals there are three relevant parameters: electron heat conductivity, electron heat capacity and electron-phonon coupling. With respect to these parameters all metals can be divided into classes. Keeping the interaction and the crystal structure fixed we have varied the parameters within the experimentally observed range. In a first part of this work we will present results on this study. In Fourier’s law for the heat flow generated by a temperature gradient it is assumed that the temperature change is instantaneous. This might not be true in general for femtosecond pulses. Several people have generalized Fourier’s law to inc
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