David Autrique (Physics, Univ. of Kaiserslautern, Germany),
Harihar Khanal (Math, Embry-Riddle Aeronautical Univ., Daytona Beach FL)
Developing seamless, thermodynamically consistent, hydrodynamic models of target heating/melting/vaporization, and of plasma formation and evolution, in laser ablation (LA) of metals by nanosecond lasers.
Laser ablation: Several complex, tightly coupled physical processes occur in and just above the target. The target heats up, melts and vaporizes. In the Knudsen Layer, just above the target, the evaporated particles rapidly equilibrate by collisions, get ionized and form a plasma. The plasma absorbs laser energy, shielding the target, and attains very high temperatures, velocities, species densities, and pressures. As the plume cools, homogenous nucleation and recondensation result in nanosized particles. Recoil pressures on the melt can cause melt motion and melt expulsion forming larger particles.
Additional challenges in modeling laser ablation arise from: (a) extreme space and time scales; (b) extreme gradients: temperature may rise to thousands of degrees locally; (c) extreme variation in thermophysical properties; (d) the need for extensive thermophysical data: T-dependent density, heat capacity, thermal conductivity; phase diagram for solid, liquid, vapor over the range 300 K to critical temperature (8000 K for Cu); (e) the need for T-dependent and wavelength dependent optical data.
Approach: Our formulation is based on Equations of State of the form H = H(T, P, phase) consistent with the thermochemistry of the material. It allows full temperature (and/or pressure) dependence of thermophysical and of optical properties, and can use available EOS data, up to critical temperature. Thus, it can realistically describe all the phases of real materials.
No a priori assumptions are made regarding phase formation, thus simulations can reveal phenomena not expected a priori (e.g. bimodal temperature and pressure evolution that may induce re-condensation).
The conservation laws are discretized by finite volumes, and time-stepping can be explicit or implicit. For the Euler equations in the plume, high resolution numerical schemes are used to capture the (very) strong shocks.
Applications: occur in diverse fields, from archaeology, chemistry, and medicine, to environmental science, and, especially, in materials science.
Materials processing applications include: pulsed laser deposition, nanoparticle manufacturing, micromachining, and chemical microanalysis.